Mar 21, 2016 - Chemical and Process Engineering focuses on adding value to raw ... in this section is the final evaporator in the process which is used to.
DEPARTMENT OF CHEMICAL AND PROCESS ENGINEERING UNIVERSITY OF MORATUWA CH 4033 – Comprehensive Design Project (Individual) Design of a basket type evaporator with thermal vapour recompression for a HFCS manufacturing plant
S.W.S Samarakoon
110491 M Submitted on 21st March 2016
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PREFACE Chemical and Process Engineering focuses on adding value to raw materials and converting them into useful, value added products. The ultimate goal of a chemical engineer should be to utilize the available resources in the most energy efficient and cost effective manner to scale up the laboratory scale processes to industrial scale. Therefore, as a chemical and process engineering undergraduate, it is vital task to have a balanced exposure to both theoretical and practical aspects. Comprehensive Design Project, group and individual modules are such modules which encourages the student to gain that competency. Under CH 4013, the group design project, our group was assigned to design a HFCS manufacturing plant. HFCS is a substitute for common table sugar made of corn which is more commonly known as maize. The capacity of the plant was decided to match with 10% of the annual sugar import to the country. The material balance and energy balances were performed and process flow diagram and economic and safety reviews were done. The designed plant contained various unit operations such as Steepening, Centrifugation, Liquefaction and Saccharification of starch, Ion exchange, Filtration, Isomeration and Evaporation. Evaporation is an essential unit operation to achieve the required composition of the end product. Under CH 4033, the individual design project, I was assigned to design an evaporator. The evaporator designed in this section is the final evaporator in the process which is used to obtain the correct composition of HFCS-55 by evaporating excess water. This report contains 10 chapters describing the detail design of the evaporator including the literature review, selection of the evaporator configuration, chemical design, detail mechanical design and drawings, pipe and instrumentation and P&I diagrams, safety and economic aspects.
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ACKNOWLEDGEMENT The successful completion of this report has everything to do with the support and the guidance given by many people. Among them, first and foremost I extend my heartfelt gratitude to Dr. P.G Rathnasiri, Head of the Department, Department of Chemical and Process Engineering. I must be thankful to Dr. Sanja Gunawardene, my supervisor for the support and the guidance provided. If not for the motivation provided by my supervisor, this report wouldn’t have been a reality. Also, I must be thankful to Dr. Olga Sumanapala, the module coordinator for always guiding and encouraging me to do my best. All the theories I applied to complete this report successfully were taught to me by the excellent panel of lecturers in my department. Therefore, I am forever in debt with them for providing me that wisdom to accomplish this task successfully. Last but not least, I am thankful to my fellow colleagues of my department for the friendly support given to complete this report successfully.
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EXECUTIVE SUMMARY HFCS-55 has a close resemblance to sugar. Therefore, import cost of sugar can be reduced by producing HFCS-55 locally. HFCS-55 production process has several unit operations. Evaporation is one of the most important unit operations in the process. The evaporator designed in this report is the final one in the process which is used to adjust the final composition of HFCS-55 which is 77% of solids. The feed to the evaporator has a solid composition of 72.5%. The designed evaporator is a single effect basket type evaporator with thermal vapor recompression. The internal diameter of the evaporator is 0.502m and the height is 2.51m. The calandria consists of 333 tubes each of length 0.9144m and external diameter 19.05mm. The operating pressure of the evaporator is 70kPa and motive steam pressure to the steam jet ejector is 500kPa. Exit vapor from the evaporator is compressed at a ratio of 2.5 to the steam jet ejector and the exit steam pressure which is the steam inlet pressure to the evaporator is 175kPa. The evaporator is constructed using SS 304 L with a design stress of 115 MPa. The thickness of the evaporator shell is around 4 mm and the thickness of the calandria shell is around 3 mm to resist both elastic and plastic failure. The evaporator is constructed with a hemispherical head to the top and a conical head for the bottom to facilitate easy product removal. Openings are constructed for the feed in, steam in, concentrate out, condensate out and vapor out with suitable dimensions and reinforcements are done. Combined loading calculations are done to verify the thickness is able to handle the stresses due to combined loading. A skirt support is used as the support. Pipes are designed for the feed flow, concentrate out, steam in, condensate out and vapor out using standard ASMI schedules. Plug valves are used as shut-off valves and globe and butterfly valves are used as control valves. Classical feedback control and cascade control structures are used to control flow, temperature, pressure and level. Use of PLC control is also suggested. Safety aspects of the operation procedure is inspected and a HAZOP study is performed. Economic aspects are being considered and the total capital cost investment is found as 129.907 million LKR. (Keywords: HFCS, Basket type evaporator, thermal vapor recompression)
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TABLE OF CONTENTS PREFACE ................................................................................................................................... i ACKNOWLEDGEMENT ......................................................................................................... ii EXECUTIVE SUMMARY ....................................................................................................... iii TABLE OF CONTENTS .......................................................................................................... iv LIST OF TABLES .................................................................................................................... ix LIST OF FIGURES .................................................................................................................... x CHAPTER 01 – INTRODUCTION .......................................................................................... 1 Comprehensive Design Project - Group ................................................................................. 1 Comprehensive Design Project – Individual .......................................................................... 3 CHAPTER 02 – LITERATURE SURVEY ............................................................................... 4 2.1 What is Evaporation?........................................................................................................ 4 2.2 Types of Evaporators ........................................................................................................ 5 2.2.1 Evaporators in which heating medium is confined by double walls, plates, coils or jackets. ........................................................................................................................... 6 2.2.2 Evaporators in which heating medium is brought into direct contact with evaporating liquid. .......................................................................................................... 10 2.2.3 Evaporators in which heating medium is separated from evaporating liquid by tubular surfaces. .............................................................................................................. 11 CHAPTER 03 – EVAPORATOR SELECTION ..................................................................... 18 3.1 Liquid characteristics which affect evaporator selection. .............................................. 18 3.1.1 Viscosity .................................................................................................................. 18 3.1.2 Concentration of the solution ................................................................................ 18 3.1.3 Temperature sensitivity ......................................................................................... 18 3.1.4 Salting...................................................................................................................... 19 3.1.5 Scaling ..................................................................................................................... 19 3.1.6 Fouling .................................................................................................................... 19
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3.1.7 Foaming .................................................................................................................. 19 3.2 Selecting the best evaporator type .................................................................................. 20 3.3 Basket Type Evaporator ................................................................................................. 21 3.4 Selecting the best configuration for energy conservation .............................................. 22 3.4.1 Multi-effect evaporation ........................................................................................ 22 3.4.2 Thermal Vapor Recompression (TVR) ................................................................ 22 3.4.3 Mechanical Vapor Recompression (MVR) .......................................................... 23 3.5 Evaporator selection summary ....................................................................................... 24 CHAPTER 04 – CHEMICAL DESIGN .................................................................................. 25 4.1 Nomenclature.................................................................................................................. 25 4.2 Description of the evaporator configuration ................................................................... 26 4.3 Material and Energy Balance ......................................................................................... 27 4.3.1 Material balance..................................................................................................... 27 4.3.2 Energy balance ....................................................................................................... 27 4.3.3 Material Balance – Summary ............................................................................... 31 4.3.4 Energy Balance – Summary .................................................................................. 31 4.4 Sizing of the evaporator and steam jet ejector ................................................................ 32 4.4.1 Determining the tube internal and external diameter and tube length ............ 32 4.4.2 Calculating the number of tubes required for the calandria ............................. 32 4.4.3 Determining the suitable tube sheet arrangement .............................................. 33 4.4.4 Calculating the bundle diameter or the calandria diameter .............................. 33 4.4.5 Calculating the evaporator shell internal diameter ............................................ 34 4.4.6 Calculating the height of the evaporator shell .................................................... 34 4.4.7 Sizing of the steam jet ejector ............................................................................... 35 4.5 Evaporator Performance ................................................................................................. 37 4.6 Parameter Calculation – Summary ................................................................................. 37 CHAPTER 05 – OVERVIEW TO DETAILED MECHANICAL DESIGN .................... 38
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5.1
Main geometry of the vessel .................................................................................... 38
5.2
Main dimensions of the vessel................................................................................. 38
5.3
Operating conditions ............................................................................................... 38
CHAPTER 06 – MATERIAL SELECTION FOR CONSTRUCTION ............................ 39 6.1 Material of construction for major components of the evaporator ......................... 39 6.2 Material of construction for calandria tubes ............................................................. 39 6.3 Welding Technique ....................................................................................................... 40 CHAPTER 07 – MECHANICAL DESIGN OF THE EVAPORATOR ........................... 41 7.1 Parameter calculation .................................................................................................. 41 7.1.1 Design Pressure ...................................................................................................... 41 7.1.2 Design Temperature .............................................................................................. 42 7.1.3 Design Stress ........................................................................................................... 42 7.2 Thickness calculations .................................................................................................. 43 7.2.1 Thickness of the evaporator shell ......................................................................... 43 7.2.2 Thickness of the calandria shell ............................................................................ 43 7.2.3 Thickness of the calandria tubes .......................................................................... 45 7.3 Design of top and bottom enclosures .......................................................................... 45 7.3.1 Thickness of the hemispherical head.................................................................... 46 7.3.2 Thickness of the conical head................................................................................ 46 7.4 Determining the safe thickness against elastic and plastic failure for the evaporator shell ...................................................................................................................................... 47 7.5 Openings and reinforcements ...................................................................................... 48 7.5.1 Determination of size of the openings .................................................................. 48 7.5.2 Reinforcements for openings ................................................................................ 49 7.6 Insulation thickness calculation .................................................................................. 54 7.7 Combined loading calculations.................................................................................... 55 7.7.1 Stresses due to pressure ......................................................................................... 55
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7.7.2 Stresses due to dead weight ................................................................................... 55 7.7.3 Stresses due to wind ............................................................................................... 57 7.7.4 Calculation of principle stresses and stress intensities. ...................................... 58 7.8 Design of supports ........................................................................................................ 59 7.8.1 Determination of the skirt wall thickness ............................................................ 59 7.8.2 Design of skirt bearing plate ................................................................................. 60 7.8.3 Design of anchor bolts ........................................................................................... 60 7.9 Description of fabrication ............................................................................................ 62 7.10 Mechanical drawings. ................................................................................................. 64 CHAPTER 08 – PIPES AND INSTRUMENTATION ....................................................... 65 8.1
Selection of pipes...................................................................................................... 65
8.2 Selection of valves ......................................................................................................... 65 8.2.1 Selection of shut-off valves .................................................................................... 66 8.2.2 Selection of control valves ..................................................................................... 66 8.3 Selection of pumps and other auxiliary items ............................................................ 68 8.4 Instrumentation ............................................................................................................ 69 8.4.1 Temperature measurement ................................................................................... 69 8.4.2 Pressure measurement........................................................................................... 70 8.4.3 Flow measurement ................................................................................................. 70 8.4.4 Level measurement ................................................................................................ 70 8.5 Process controlling aspects .......................................................................................... 71 8.5.1 PID controlling ....................................................................................................... 71 8.5.2 Programmable Logic Controllers (PLC) ............................................................. 74 8.5.3 Controlling of the steam jet ejector ...................................................................... 75 8.6 P&I Diagram................................................................................................................... 77 CHAPTER 09 – SAFETY ASPECTS................................................................................... 78 9.1 Startup procedure......................................................................................................... 78
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9.2 Shut down procedure ................................................................................................... 78 9.3 Maintenance procedure ............................................................................................... 79 9.4 Hazard and Operability Study (HAZOP) .................................................................. 79 CHAPTER 10 – ECONOMIC ASPECTS ........................................................................... 87 10.1 Estimation of the fixed capital cost ........................................................................... 87 10.2 Estimation of the working capital cost ..................................................................... 89 10.3 Total capital cost ......................................................................................................... 89 REFERENCES ......................................................................................................................... 90 APPENDIX .............................................................................................................................. xii Appendix A........................................................................................................................... xii
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LIST OF TABLES Table 3.1 Variation of viscosity of HFCS-55 with temperature [4]......................................... 18 Table 3.2 Guide to evaporator selection based on liquid characteristics [5]............................ 19 Table 4.1 Summary of Material Balance ................................................................................. 31 Table 4.2 Summary of Energy Balance ................................................................................... 31 Table 4.3 constants to be used in the correlation [5] ................................................................ 33 Table 4.4 Vapor density, liquid density and cross sectional area ............................................ 35 Table 4.5 Evaporator performance ........................................................................................... 37 Table 4.6 Parameter calculation summary ............................................................................... 37 Table 6.1 – SS 304 L properties ............................................................................................... 39 Table 6.2 – SB – 171 properties ............................................................................................... 40 Table 7.1 k and m values [12] .................................................................................................. 44 Table 7.2 volumetric flow rate determination .......................................................................... 48 Table 7.3 opening diameter calculation ................................................................................... 48 Table 7.4 pipe selection [18] .................................................................................................... 49 Table 7.5 list of drawings ......................................................................................................... 64 Table8.1 Pipe details ................................................................................................................ 65 Table 8.2 Valve details ............................................................................................................. 68 Table 9.1 HAZOP study for the evaporator ............................................................................. 80 Table10.1 typical factors for estimating the fixed capital cost [27] ......................................... 88
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LIST OF FIGURES Figure 1.1 – Process Flow Chart of the HFCS manufacturing plant .......................................... 2 Figure 2.1 Types of jackets [1]................................................................................................... 7 Figure 2.2 Stirrer evaporator [1] ............................................................................................... 7 Figure 2.3 Spiral Plate Evaporator [1] ....................................................................................... 8 Figure 2.4 a schematic diagram of a gasketed plate evaporator. [1] .......................................... 9 Figure 2.5 Patterned-Plate Evaporators [1] ................................................................................ 9 Figure 2.6 a schematic diagram of a submerged combustion evaporator [1] .......................... 10 Figure 2.7 a schematic diagram of a horizontal spray-film evaporator [1] .............................. 11 Figure 2.8 a typical short tube vertical evaporator [3] ............................................................. 12 Figure 2.9 a schematic diagram of an inclined tube evaporator [1] ......................................... 13 Figure 2.10 a typical long-tube vertical evaporator [1] ............................................................ 14 Figure 2.11 a schematic of a rising film evaporator [3] ........................................................... 15 Figure 2.12 a schematic diagram of a falling film evaporator [3] ............................................ 15 Figure 2.13 a rising/falling film evaporator [3]........................................................................ 16 Figure 2.14 a schematic diagram of an agitated thin film evaporator [3] ................................ 17 Figure 3.1 a basket type evaporator [1] .................................................................................... 21 Figure 3.2 a TVR system (Left) A comparison of the steam consumption when TVR is not used and when TVR is used 1, 2 and 3 effects respectively (Right) [2] .................................. 22 Figure 3.3 a schematic diagram of a MVR system [2]............................................................. 23 Figure 4.1 Tube arrangements with the demarcation of the tube pitch Pt [5] .......................... 33 Figure 4.2 Shell-bundle clearance [5] ...................................................................................... 34 Figure 4.3 a single fixed-nozzle steam jet ejector [1] .............................................................. 35 Figure 4.4 Sizing chart for steam jet ejectors [1] ..................................................................... 36 Figure 3.1 welded plate ............................................................................................................ 51 Figure 8.1 schematic diagram of a plug valve ......................................................................... 66 Figure 8.2 Schematic drawing of a butterfly valve .................................................................. 67 Figure 8.3 Schematic diagram of a globe valve ....................................................................... 67 Figure 8.4 Block diagram of the flow controlling system ........................................................ 72 Figure 8.5 Block diagram of the level controlling system ....................................................... 72 Figure 8.6 Block diagram for the pressure control system ...................................................... 72 Figure 8.7 Block diagram of the cascade loop ......................................................................... 73 Figure 8.8 Positioning of the two level sensors ....................................................................... 74 Figure 8.9 Controlling aspects of steam jet ejectors [24] ......................................................... 75
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Figure 8.10 sonic Vs sub-sonic design [24] ............................................................................. 76 Figure 10.1 Purchased equipment cost of a single stage evaporator made of stainless steel [26] .................................................................................................................................................. 87
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CHAPTER 01 – INTRODUCTION This chapter presents a brief description about the work carried out during the Comprehensive Design Group Project of designing a High Fructose Corn Syrup (HFCS) manufacturing plant. This also includes an introduction to the Comprehensive Design Individual Project. Comprehensive Design Project - Group As per the literature survey conducted during the Comprehensive Design Group Project, the annual sugar requirement of the country is around 650 000 MT and around 94% of that is imported spending a significant amount of valuable foreign exchange. HFCS-55 which is the main product of the plant, being a closer derivative to common sugar or sucrose is used as a substitute for table sugar. Thus, it is used as a sweetener in soft drinks, fruit juices and carbonated drinks. Therefore, the main goal of establishing the plant is to manufacture as much as HFCS-55 to account for 5% of the processed sugar imported annually. The annual production capacity of the plant is 30000 MT of HFCS-55 which accounts for around 100 MT per day of HFCS- 55. The plant yields around 8 MT per day of HFCS-42 and 2 MT per day of HFCS-90 as main by-products. In addition it yields around 13 MT of germ, 11 MT of fiber and 12 MT of gluten as other by-products which can be marketed as useful products such as feedstock for cattle. The payback period is 2.13 years and an Internal Rate of Return of 29% is achievable while maintaining a PI of 2.4. The manufacturing process used for HFCS production is known as the Wet Corn Milling Process, which involves numerous unit operations such as,
Steepening with SO2
Grinding
Centrifuging
Filtration
Enzymatic liquefaction and saccharification of starch
Enzymatic isomerization of glucose to sucrose
Ion exchange and carbon refining
Evaporation
Chromatographic separation and blending.
Fig 1.1 shows the process flow chart of the plant.
2 Figure 1.1 – Process Flow Chart of the HFCS manufacturing plant
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Comprehensive Design Project – Individual The purpose of the Comprehensive Design Project – Individual is to design the final evaporator denoted by FE-03 in Fig1.1. FE-03 is fed with HFCS-55 with accurate composition in dry basis. However, the water content is more than required and hence the purpose of FE-03 is to evaporate the excess water present in the feed to obtain the final specifications of the product to be sold. Thus, the evaporator is used to concentrate HFCS-55 from 72.5% solids to 77% solids. Chapters to come will concentrate on a literature survey on available evaporator technologies and the selection process of the most suitable evaporator configuration and finally the parameter calculation.
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CHAPTER 02 – LITERATURE SURVEY 2.1 What is Evaporation? Evaporation is the removal of solvent in terms of vapor from a solution or a slurry. As far as the industrial applications are being concerned, the solvent is often water. In most of the times, the objective of evaporation is to obtain a concentrated solution from a less concentrated feed, rather than to obtain the vaporized solvent as the main product. Therefore, depending on the process requirement, the vapor given out may or may not be recycled. Thus, evaporation is usually achieved by partially vaporizing a portion of the feed to obtain a concentrated slurry. [1] As a unit operation, evaporation can be compared with distillation, drying and crystallization. Evaporation is distinguished from distillation due to the fact that no effort is done in evaporation to fractionate the vapor into individual components. In evaporation, the final residue or the concentrate is always a liquid or a suspension of a solid in a liquid. The desired product may be a solid, but the heat transfer should be occurred in the evaporator from the heating medium to a solution or a suspension of the solid in a liquid. Thus, it differs from drying. Crystallization is utilized in order to facilitate crystal growth whereas evaporation is only used to concentrate a solution. Thus, in most industries such as sugar manufacturing crystallization is essentially a downstream process to evaporation. Evaporation is used extensively in processing foods, chemicals, pharmaceuticals, fruit juices, dairy products, paper and pulp, and both malt and grain beverages and it is a unit operation which, with the possible exception of distillation, is the most energy intensive. [2] Evaporator design consists of three principal elements: heat transfer, vapor-liquid separation and efficient utilization of energy. Mostly, heat is supplied by condensing steam and heat is transferred indirectly across metallic surfaces. In order to assert an efficient, reliable performance, evaporators should be equipped with some key basic features:
Transfer large amounts of heat to the solution with the minimum amount of metallic surface area. This plays the leading role in determining the type, size and the cost for the evaporator system. [1]
Achieve the specified vapor-liquid separation with the simplest devices available. Separation is a critical factor owing to several reasons: Ensuring the quality of the product, preventing pollution, eliminating fouling and corrosion of the same
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equipment or downstream equipment and avoiding inefficient performance due to unwanted recirculation. [1]
Utilize the energy in the most efficient manner. Evaporator performance is often rated in terms of Steam Economy, which is defined as the tons of vapor formed per ton of steam utilized. Numerous strategies such as using multiple effect systems and vapor recompression are implemented in order to increase the steam economy, which will be discussed in detail in chapter 03. [1]
Meet the conditions imposed by the solution being evaporated or the slurry being formed. This ensures that the operation of the evaporator does not degrade the quality, in terms of both nutritional as well as physical appearance. For example, in milk industry, the maximum allowable temperature is 70o C in order to prevent denature of proteins found in milk. Also, possibility of fouling, foaming should also be considered in designing the evaporator. [1]
While the design criteria for evaporators are the same regardless of the industry involved, two questions always exist: is this equipment best suited to the duty, and is the equipment arranged for the most efficient and economic use? [2] As a result, numerous evaporator types and arrangements have been developed to be used in specific applications. 2.2 Types of Evaporators Evaporators are often classified as follows: [1] 1. Heating medium separated from evaporating liquid by tubular heating surfaces 2. Heating medium confined by double walls, plates, coils or jackets 3. Heating medium brought into direct contact with evaporating liquid, and 4. Heating with solar radiation. Solar evaporation demands a large extend of land and only feasible in evaporating natural brines such as during the manufacture of table salt. [1] Evaporators with tubular heating surfaces are prominent in the industry. Circulation of the liquid past the surface may be induced in terms of boiling (natural circulation) or by using pumps (forced circulation). The latter may or may not involve boiling in the heating surface. Evaporators can be further categorized according to the mode of operation: Batch, semi-batch, continuous-batch and continuous.
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Batch evaporators are utilized for small quantities of product which require large residence times. Filling, evaporation and emptying of the vessel occur at three separate stages which generates unwanted changeover times. A batch evaporator should be large enough to handle the entire amount of the feed and the heating element should be located as low enough not to be uncovered when the volume is reduced to that of the product. [1] Semi-batch operation is more commonly used than batch operation where a continuous supply of feed is taking place in order to keep the liquid level a constant compensating for the loss of water due to evaporation. [1]However, product is withdrawn once only when the desired concentration has been achieved. Continuous-batch mode also has a continuous feed but unlike semi-batch, it has a continuous discharge over at least part of the cycle. The most widely used continuous evaporators have continuous feed and discharge. An evaporator may be operated as a once-through unit or a unit with recirculation. In a oncethrough unit, a single-pass is used and in recirculation units a multi-pass is used. Single-pass units are often used to handle heat sensitive liquids. In recirculation units, a pool of liquid is held within the equipment with which the recirculating liquid mixes which leads to extended residence times. Therefore, multi-pass systems are not suitable for handling heat sensitive liquids. [1] 2.2.1 Evaporators in which heating medium is confined by double walls, plates, coils or jackets. 2.2.1.1 Jacketed Vessels These are utilized in small scale operations. The rate of heat transfer is lower than other types of evaporators and a limited surface area is available for heat transfer. Agitation may be provided to improve heat transfer characteristics. Jackets may be of several types: conventional jackets made with another cylinder concentric to the evaporator, dimpled jackets and patterned plate jackets. Jacket evaporators are used when the product is very viscous and good mixing is required. [1]
7 Figure 2.1 Types of jackets [1]
2.2.1.2 Evaporators with coils The most common application is heating coils placed inside the evaporator with heating medium flowing inside and evaporation taking place outside the coil. Agitation can be used to improve the heat transfer characteristics. Fig 2.2 shows such an evaporator configuration. [1] Figure 2.2 Stirrer evaporator [1]
Coils are used instead of jackets due to the following reasons:
Small capacity
When the product is difficult to handle
High operating pressures for either process or heating fluid.
Spiral flow is used to increase heat transfer characteristics and reduce fouling.
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2.2.1.3 Plate evaporators Plate evaporators can be constructed from flat plates or corrugated plates. In those equipped with flat plates, each side of the flat plate can be alternatively used as the liquor side or the steam side. This facilitates the scale deposited during the liquor pass to be washed out by condensing steam during the steam pass. Plate evaporators are often used as an alternative design for tubular evaporators. [1] 2.2.1.3.1 Spiral Plate Evaporators They offer a number of advantages over conventional tubular evaporators. Some of them are:
Increased heat transfer due to centrifugal force
Compact design
Relative easiness in cleaning
Resistance to fouling
These curved flow units are particularly suitable to handle high viscous fluids containing solid particles. Figure 2.3 Spiral Plate Evaporator [1]
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2.2.1.3.2 Gasketed- Plate Evaporators This is also known as the plate and frame evaporator due to the resemblance with the plate and frame filter press. The design is similar to the design of the plate and frame filter press. The plates are gasketed and arranged to form narrow flow passages when a series of plates are clamped together in the frame. Fluids are directed through the adjacent layers between the plates. Fig 2.4 shows a schematic diagram of such an evaporator. [1] Figure 2.4 a schematic diagram of a gasketed plate evaporator. [1]
Some advantages of this type of evaporator are: ability to handle heat sensitive, viscous and foaming materials, faster start up and shut down times, compact size and lower head room requirement and easiness to clean and modification. However, the large gasket area and the possibility to rupture and mix the two fluids are some drawbacks. [1] 2.2.1.3.3 Patterned-Plate Evaporators Evaporators are constructed using patterned plates to serve as alternatives to tubular configurations. They are often less costly than tubular elements. Patterned plates can also be used inside tanks instead of coils. Figure 2.5 Patterned-Plate Evaporators [1]
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2.2.2 Evaporators in which heating medium is brought into direct contact with evaporating liquid. 2.2.2.1 Submerged Combustion Evaporator This is burning a fuel inside a specially designed burner under the surface of a liquid. The hot combusted gases bubble out from the liquid immediately transferring the heat of combustion to the liquid to be evaporated. A typical submerged combustion evaporator consists of a tank, a burner, a combustion distribution system and a combustion control unit. [1] A schematic diagram is shown in Fig 2.6 Figure 2.6 a schematic diagram of a submerged combustion evaporator [1]
Submerged combustion evaporators are widely used with corrosive, scaling, salting and highly viscous liquids having considerably high boiling points. [1]The depression in boiling point associated with bubbling can be controlled with the percentage of excess air supplied with fuel. However, the maximum excess air percentage tolerable is 20%. In practice, submerged combustion is utilized along with multiple effects technology to increase the energy efficiency. Thus, Direct-contact Multiple Effect Evaporators are being introduced. [1]
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2.2.3 Evaporators in which heating medium is separated from evaporating liquid by tubular surfaces. The most common type of evaporator configuration used in the industry. The investment cost of these type of evaporators are lower than that of plate type evaporators but the maintenance cost is higher. [2]There are two main types: natural circulation and forced circulation. 2.2.3.1 Horizontal Tube Evaporators The oldest model belonging to this category. The simplest design included a shell and a bundle of horizontal tubes which was not removable. The heating medium flowed inside the tube bundle and evaporation took place in the shell side. Some advantages of horizontal tube evaporators:
Relatively low cost in small capacity applications
Low headroom requirement
Large vapor-liquid disengaging area
Relatively good heat transfer with proper design
Some disadvantages of horizontal tube evaporators:
Cannot be used with heat sensitive liquids
Not suitable with scaling, salting liquids
2.2.3.2 Horizontal Spray-Film Evaporators This is a modification to the conventional horizontal tube evaporator. This is essentially a horizontal, falling film evaporator in which the liquid is distributed by recirculation through the spray system. [1] Figure 2.7 a schematic diagram of a horizontal spray-film evaporator [1]
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Some advantages of this type are:
Distribution of liquid is easily achieved
Vapor separation is easy
Chemical cleaning is easy
Reliable operation under scaling situations
Non condensable gases are more easily vented.
Some drawbacks of this type are:
Limited viscosity range
Tubes may be blocked by crystals
More floor space required
Limited application in once-through evaporation (not suitable for heat sensitive liquids)
2.2.3.3 Short-tube Vertical Evaporators This is a very common evaporator in the industry, it is often known as the standard evaporator. Fig 2.8 shows an illustration of a typical short tube vertical evaporator [3]. Figure 2.8 a typical short tube vertical evaporator [3]
In short-tube vertical evaporators, the boiling liquid is inside the tubes and heating medium outside the tubes. The circulation of boiling liquid past the heating surface is accomplished by natural circulation. Since the rate of circulation is many times that of the feed rate, down comers are essential to permit liquid flow from top of the tube sheet to the bottom.
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The advantages of this type of evaporators are as follows: [3]
High heat transfer rates for high temperature differences
Ease of cleaning
Low capital investment
Low headroom requirement
The disadvantages are: [3]
High floor space and weight
Relatively high liquid hold up
Poor heat transfer at low temperature differences or high viscosity
A special type of short-tube vertical evaporator called Basket Type Evaporator is used where the central down comer is replaced with an annular down comer which facilitates removal of the calandria and easy cleaning. [1] 2.2.3.4 Inclined Tube Evaporators In this type of evaporators, the tubes are inclined usually at an angle of 30o or 45o. These type of evaporators usually perform well in evaporating foaming liquids due to the sharp change in vapor flow direction. In addition, they are beneficial in treating heat sensitive liquids. [1] Figure 2.9 a schematic diagram of an inclined tube evaporator [1]
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2.2.3.5 Long-tube Vertical Evaporators These evaporators are more versatile and are the cheapest per unit. [3] These consist of tubes 1 to 2 inches in diameter and 12 to 30 feet in length. Fig 2.10 shows a sketch of a typical long-tube vertical evaporator. Figure 2.10 a typical long-tube vertical evaporator [1]
They may be operated as once-through systems or recirculated systems. Recirculated systems can be either batch wise or continuous. Circulation of liquid across heat transfer surface is accomplished by boiling. However, the temperature variation inside the long tubes is difficult to predict and depends heavily on the effects due to hydrostatic head. [1] There are three main variants of long-tube vertical evaporators which will be discussed next one by one. 2.2.3.5.1 Rising or Climbing Film Evaporators This is known as the first “modern” evaporator used in the industry. [3] The theory of the climbing film is that vapor traveling faster than the liquid flows in the core of the tube causing the liquid to rise up the tube in a film. This type of flow can occur only in a portion of the tube. When it occurs, the liquid film is highly turbulent and high heat transfer rates are realized. Residence time is also low permitting application for heat sensitive materials. [3] Some advantages of this type of evaporators are: reduced floor space requirement and ability to handle foamy liquids. Some disadvantages are: high head-room requirement, higher pressure drop through tubes than in a falling film evaporator and possibility of inclined boiling temperature due to the hydrostatic head at the bottom of the tubes which will affect heat sensitive material. [3]A schematic diagram is shown in Fig 2.11
15 Figure 2.11 a schematic of a rising film evaporator [3]
2.2.3.5.2 Falling Film Evaporators This is essentially an upside down rising film evaporator in which liquid flows down under gravity. Unlike in rising film evaporators, a distributor is required to evenly distribute the liquid to all the tubes. A vapor-liquid separation unit is situated at the bottom. Compared to rising film evaporators, these evaporators give higher heat transfer rates for a lower temperature difference with a short residence time. [3]
Figure 2.12 a schematic diagram of a falling film evaporator [3]
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Some advantages of falling film evaporators: [3]
Relatively low cost
Large heating surfaces in one body
Low product hold up
Small floor space requirement
Good heat transfer coefficients at significantly low temperature differences
Some disadvantages are: [3]
High head room requirement
Generally not suited for scaling or salting liquids
Recirculation is usually required.
2.2.3.5.3 Rising/Falling Film Evaporator Rising film evaporators and falling film evaporators are combined to make use of the advantages inherent to each type. The feed is initially supplied to the rising film pass. The boiled liquid rises up along with vapor formed and is introduced to the falling film pass. The presence of vapor formed at this instance is advantageous to handle high viscous liquids. [3] Fig 2.13 shows an illustration of a rising/falling film evaporator. Figure 2.13 a rising/falling film evaporator [3]
17
2.2.3.6 Forced circulation evaporators These are generally more expensive when compared with natural circulation systems. Therefore, they should be used only when the use of natural circulation systems is infeasible. There is always a tradeoff between the increased heat transfer rates due to increased pumping capacity and the increased energy cost. Normally in forced circulation units, boiling is suppressed and flashing is done. Typical advantages of forced circulation evaporators are: high rate of heat transfer, positive circulation, relative freedom from salting, scaling and fouling and ease of cleaning. High capital cost, high residence times and maintenance and operation cost associated with pumps are some notable disadvantages. [1] 2.2.3.7 Agitated Thin Film Evaporators One of the most useful type of evaporators to handle liquids otherwise impossible to handle using other evaporator types. The combination of short residence time, narrow residence time distribution, high turbulence and rapid surface renewal allows the agitated thin film evaporator to handle viscous, heat sensitive and fouling liquids. A standard agitated thin film evaporator is capable to process liquids with viscosities of 1-50 000 cP. However, special transporting rotor designs can be operated in the range of 50 000 – 20 million cP. [3]Fig 2.14 shows a schematic diagram of an agitated thin film evaporator. Figure 2.14 a schematic diagram of an agitated thin film evaporator [3]
Some advantages of agitated thin film evaporators are: high heat transfer coefficients due to the turbulence imparted by the rotor, plug flow with minimum back mixing and ability to handle viscous material. One notable disadvantage is the high capital cost and the impracticability for vapor recompression.
18
CHAPTER 03 – EVAPORATOR SELECTION This chapter includes the criteria for selecting the best evaporator type and the energy recovery mechanisms associated with it. 3.1 Liquid characteristics which affect evaporator selection. The practical application of evaporation technology is significantly affected by the properties of the solution being evaporated. Some of such important properties of evaporating liquids is discussed below in comparison to HFCS-55 [1]. 3.1.1 Viscosity Generally, more viscous the liquid, more difficult it will be to flow and handle using normal evaporators. Higher viscosities over 300 cP eliminates the use of falling film and rising film evaporators. The viscosity of the HFCS-55 feed entering the evaporator at 70oC was calculated using a correlation developed using the available viscosity data as follows. Temperature (oC)
Viscosity (cP)
21
1200
27
700
32
400
38
250
43
160
Table 3.1 Variation of viscosity of HFCS-55 with temperature [4]
Viscosity of HFCS-55 at the operating conditions was calculated as around 13 cP. 3.1.2 Concentration of the solution Increased concentration may lead to the formation of crystals which is a hindrance to the proper operation of evaporation. Since the maximum concentration of 77% by weight is achieved at elevated temperatures, there is no tendency to crystallization. 3.1.3 Temperature sensitivity Some compounds such as proteins in milk tend to denature when high temperatures and longer residence times are being used. Here, since vacuum evaporation is being done, high temperatures are not being involved.
19
3.1.4 Salting Salting refers to the growth of a material on the evaporator surface having a solubility which increases with the temperature. [1] Here, there is minimum possibility of salting. 3.1.5 Scaling Scaling means the growth or deposition of a material which is either insoluble or having a solubility which decreases with the temperature [1]. Here, scaling is mostly due to the presence of impurities such as ash. However, the scaling possibility is moderate. 3.1.6 Fouling Fouling is the formation of deposits due to reasons other than salting or scaling [1]. In HFCS manufacturing, there are two main fouling possibilities: Due to impurities carried out with steam and impurities found in HFCS. However, the possibility of fouling is also moderate. 3.1.7 Foaming Stable foaming might lead to excessive entrainment. There are many reasons for the occurrence of foams: Presence of dissolved gases in the liquor, presence of an air leak in the vessel and presence of surface-active agents or finely divided solid particles in the liquor [1]. Foaming can be suppressed by operating at low liquid heights, using inclined calandria and by using mechanical methods. Here, since the concentration of HFCS is quite high, there is a moderate possibility of foaming. The following table shows a guide to select an evaporator based on the liquid characteristics.
Table 3.2 Guide to evaporator selection based on liquid characteristics [5]
20
3.2 Selecting the best evaporator type As mentioned in section 3.1, the product involved in this operation has moderate scaling, fouling and foaming possibilities. Temperature sensitivity is not a problem due to vacuum operation, however since the operation is continuous, residence time should be minimized. Batch evaporators are not suitable because: the operation is continuous and large scale and batch evaporators have very high residence times and very low heat transfer characteristics. Therefore, continuous, once-through mode of operation should be selected. Forced circulation evaporators and agitated thin film evaporators are suited for high viscous, high scaling liquids which have tendency to form crystals. However, in this operation the liquid is having low viscosity, moderate scaling possibility and no tendency to form crystals. Therefore, those two types of evaporators can be eliminated. Submerged combustion evaporators are used with corrosive, highly viscous liquids. Therefore, they can also be eliminated. Jacketed and coiled evaporators are also not suitable due to the limited heat transfer. Plate type evaporators are not suitable to be used in this application because plate heat evaporators generally give higher efficiencies for small scale operation. Also, it should be assured that the maintenance which should be performed once in every week or every other week owing to the moderate scaling and fouling possibility does not interrupt the normal operation of the plant. Therefore, the most suitable type of evaporators are the natural circulation tube evaporators. Horizontal tube evaporators cannot be used due to the inefficiency as well as due to moderate possibility of scaling while horizontal spray film evaporators cannot be used due to infeasibility in once-through evaporation. Long-tube vertical evaporators are not suited due to the moderate scaling possibility which can cause severe operational drawbacks and difficulty in maintenance. Therefore, the most suitable type of evaporator for this application is the Short-tube Vertical Evaporator which has significant advantages in handling moderately scaling liquids and has significantly high heat transfer coefficients for higher temperature differences. Further, heat transfer characteristics can be improved by reducing the tube diameter and increasing the number of tubes. Entrainment separators are used to prevent entrainment.
21
A special type of short-tube vertical evaporator called Basket Type Evaporator is preferred as the type of evaporator to be used in this application. 3.3 Basket Type Evaporator Basket type evaporator is a special type of short-tube vertical evaporator in which an annular down comer is situated. This allows the calandria to be separated from the main shell during maintenance activities. Therefore, basket type evaporators are more economical and practically easy during maintenance activities compared to standard evaporators. Figure 3.1 a basket type evaporator [1]
The major components inside a basket type evaporator are the calandria, deflector and vapor head and the down comer. Calandria is the heat exchanger consisting of a bundle of tubes in which heat transfer occurs from steam to the boiling liquid. Deflector is used to assist entrainment separation. Vapor head is used to accumulate the vapor formed and also acts as an entrainment separator. However, due to the high possibility of entrainment, an umbrella type entrainment separator is used with the evaporator in which the working principle is based on difference in momentum. An annular down comer is required to allow the cool liquid to pass down, boil and move up through the tubes. The working pressure is 70 kPa which is a rough vacuum. Therefore, a steam jet ejector is used to provide the necessary pressure drop. In addition, a sight glass should be installed to inspect the evaporator interior during operation and a non-condensable gas vent should be located in calandria.
22
3.4 Selecting the best configuration for energy conservation As mentioned earlier, evaporation is one of the most energy intensive unit operation. Therefore efficient utilization of the available energy is a must in designing an evaporator. There are three main methods used in the industry to accomplish this task [2]. They are, 1. Multi-effect evaporation 2. Thermal vapor recompression 3. Mechanical vapor recompression This section will briefly identify features of each of the methods mentioned and adopt the most feasible method to the evaporator design. 3.4.1 Multi-effect evaporation Multi-effect systems utilize the vapor produced in one effect to provide heat to evaporate product in a second effect maintained at a lower pressure. Theoretically in a double effect system, the steam economy can be increased by a factor of two compared to a single effect system by using this technique. However, with the increasing number of effects, the capital cost also increases. Therefore, there is a tradeoff between the capital cost and the energy recovery. Generally, multi-effect systems are not economically feasible for evaporation rates lower than 1350 kg/h [2]. 3.4.2 Thermal Vapor Recompression (TVR) When steam is available at pressures exceeding 3 bars and preferably over 7 bars, TVR is possible [2]. In TVR, portion of the vapor formed is recompressed by using a flow of high pressure steam known as motive steam inside a steam jet venturi and used to heat up the liquid. A single TVR can be analogous to addition of another effect. However, thermocompressors are somewhat inflexible and do not operate well outside the design conditions. Therefore, TVR is not suited for liquids which foul severely and have significantly high boiling point elevation such as caustic solutions [2]. Figure 3.2 a TVR system (Left) A comparison of the steam consumption when TVR is not used and when TVR is used 1, 2 and 3 effects respectively (Right) [2]
23
3.4.3 Mechanical Vapor Recompression (MVR) This is the most efficient method available for energy conservation. Here, instead of thermal compression, mechanical compression is done in terms of a fan or a centrifugal compressor. MVR is operated usually with low temperature differences, thus requiring high heat transfer areas [2]. However, a single MVR unit is equivalent to an evaporator system with over 100 effects in terms of energy recovery. Owing to the capital cost required to purchase equipment like compressors and fans and the electricity cost, MVR systems are generally expensive than TVR systems. MVR is inappropriate for high fouling liquids with large boiling point elevation [2]. Generally MVR is not used for single effect systems with low evaporation rates. Figure 3.3 a schematic diagram of a MVR system [2].
One of the above three methods should be incorporated with the evaporator design to facilitate energy conservation. According to the results of the material balance done to FE-03 in Comprehensive Design Group Project, the evaporation rate is 6160 kg/day. Here, a scale up factor of 5 is used which makes the evaporation rate around 1283 kg/hr. Therefore, using multi-effects as an energy conservation mechanisms is infeasible. Hence, the evaporator should be a single-effect one. MVR is infeasible owing to the lower evaporation rate. Therefore, the possible configuration for energy conservation is using a single-effect evaporator with thermal vapor recompression.
24
3.5 Evaporator selection summary Selected type of evaporator was basket type evaporator due to the suitability in using with moderate fouling and scaling liquids and ease of maintenance. An umbrella type entrainment separator is used in the evaporator. A steam jet ejector is used to create vacuum inside the evaporator and a vacuum pump is used to generate vacuum during start up. The used configuration for energy conservation is single-effect with vapor recompression.
25
CHAPTER 04 – CHEMICAL DESIGN This chapter emphasizes on determining operating parameters and sizing of the evaporators. First, material and energy balances are carried out in order to determine the operating parameters and later sizing and performance evaluation is done. 4.1 Nomenclature T
Temperature (oC)
B.P.R
Boiling Point Rise (oC)
P
Absolute pressure (kPa)
X
Mass fraction of solids in the liquid stream
M
Flow rate (kgs-1 )
U
Overall heat transfer coefficient (kWm-2K-1)
PCF
Pressure Correction Factor
TCF
Temperature Correction Factor
Ra
Entrainment ratio
λ
Latent heat ( kJkg-1)
Cp
Specific heat capacity (kJkg-1K-1)
A
Area of the heat exchanger (m2)
Q
Heat load
Subscripts in
Inlet
f
Feed
o
Outlet
b
Boiling
v
Evaporator
ev
Entrained vapor
c
At the condenser
d
Evaporated steam
m
Motive steam
s
Compressed vapor
26
4.2 Description of the evaporator configuration
Steam jet ejector
Heating steam
Motive steam Entrained vapor Total vapor formed C
Basket type evaporator Inlet HFCS-55 Feed inlet
Condenser/feed pre-heater
Condensed vapor
Condensed motive steam
Product concentrate
72.5% HFCS-55 at 70o C enters the feed pre-heater. The function of the pre-heater is to heat the feed before being sent to the evaporator by using the enthalpy of the condensing vapor formed within the evaporator. This helps to increase the thermal efficiency of the evaporator system. Part of the vapor formed is entrained by the steam jet ejector to be recompressed. The rest of the vapor is utilized in two ways. Part of that is condensed inside the condenser/preheater and the remaining is sent through a cooler. (A boiler feed water heater). The entrained vapor is compressed with a supply of high pressure steam known as motive steam. Then, the compressed vapor is supplied back to the evaporator. 77% concentrated HFCS-55 is removed from the bottom of the evaporator.
27
4.3 Material and Energy Balance The following assumptions are made in carrying out the material balance.
Effects from non-sugar compounds and scaling is negligible
There are no leakages between the liquor side and steam side.
Steady state prevails at all instances.
All the heating steam is condensed inside the evaporator and the condenser
The following assumptions were made in carrying out the energy balance.
Steam and condensate are always considered to be saturated.
Energy loss to the environment as radiation is negligible.
Steam jet ejector operates within the normal operating limits
4.3.1 Material balance
However,
and 4.3.2 Energy balance Operating pressure
and
is calculated from steam tables as 89.08oC. B.P.R associated is calculated as 5.62oC [6]. Therefore, The outlet temperature from the pre-heater is determined as 10 degrees below the boiling temperature. Therefore, Pressure drop across the entrainment separator is neglected. Similarly pressure drop inside the condenser is also neglected. Therefore,
28
For the successful operation of the steam jet ejector, the compression ratio should be equal or above 1.81 [7]. Therefore, here a compression ratio of 2.5 is selected for the design. , Therefore, is calculated using steam tables as 116.06oC. Overall heat transfer coefficients for the evaporator and condenser are calculated using the correlations given in Appendix A *
(
*
(
)
(
)
)
(
(
)
) +
(
Then, the P.C.F and T.C.F are calculated using the following equations ( ( (
)
[7] )
)
(
[7] )
Then, the entrainment ratio .
/
( Therefore,
is calculated using the correlation given below [7]
)
) +
29
In order to find
, energy balance should be applied to the evaporator. The following
equation is obtained after the energy balance. (
)
However, since (
(
)
(
)
,
)
should be calculated for the temperature range from 84.7oC to 94.7oC for HFCS-55
Here,
solution. By using interpolation, it is calculated as 2.8985 kJ/kg.K [6] are calculated from steam tables as 2285.56 kJ/kg and 2213.55 kJ/kg respectively. Therefore, (
)
Therefore,
(
)
and
Therefore, Heat duty of the evaporator can be found by using the following equation (
)
(
)
Heat transfer area required for the evaporator can be calculated using the following equation
(
)
(
)
Energy available in Heat required to raise the temperature of (
)
from 70oC to 84.7oC can be found from
30
Here
should be calculated for the temperature range from 70oC to 84.7oC. The calculated
value using interpolation is 2.8368 kJ/kg.K. [6] Therefore, (
)
Therefore, it is clear the condensing vapor has excess energy than required for the pre-heater. Thus, condenser load In order to calculate the condenser heat transfer area required, LMTD across the condenser is calculated as follows
(
.
)
/
Therefore,
Assuming the efficiency of the condenser is 90%, Enthalpy of vapor remaining after condensation = 497.3379-256.3191/0.9 = 212.5389 kW If this heat is utilized in the secondary cooler to heat up boiler feed water from 30oC to 40oC, the flow rate of cooling water M in the cooler can be found out by applying energy balance to the cooler. Assuming 90% efficiency in the cooler, ( Therefore,
)
31
4.3.3 Material Balance – Summary MT/day 6.1466
531.066
5.7901
500.265
0.3565
30.802
0.3097
26.758
0.1389
12.001
0.2176
18.801
4.5784
395.574
Table 4.1 Summary of Material Balance
4.3.4 Energy Balance – Summary 992.9985 kW 256.3191 kW 18.1738 m2 12.8945 m2 Table 4.2 Summary of Energy Balance
32
4.4 Sizing of the evaporator and steam jet ejector This section focuses on determining the dimensions of the evaporator. It is done in several sub sections. They are,
Determining the tube internal and external diameters and tube length
Calculating the number of tubes required for the calandaria
Determining the suitable tube sheet arrangement
Calculating the bundle diameter or the calandria diameter
Calculating the evaporator shell internal diameter
Calculating the height of the evaporator shell
Sizing of the steam jet ejector.
4.4.1 Determining the tube internal and external diameter and tube length Tube diameters in the range 16mm to 50mm are being used in the industry [5]. Smaller diameters in the range 16mm to 25mm are used in most duties since they are more compact and cost effective [5]. Larger diameter tubes are chosen for higher capacities and heavily fouling fluids. Here, the capacity is small and there is moderate fouling possibility [5]. Therefore, tubes of outside diameter 19.05mm and inside diameter 14.83mm are selected for the design. The preferred tube lengths used in the industry are 3ft, 6ft, 8ft, 12ft, 16ft, 20ft and 24ft. Long tubes are used to reduce the shell diameter when the capacity is large. Therefore, here the chosen tube length is 3 ft (0.9144m). 4.4.2 Calculating the number of tubes required for the calandria
(
)
33
4.4.3 Determining the suitable tube sheet arrangement Tubes are arranged usually in equilateral triangular, square or rotated square arrangements. Triangular and rotated square patterns have high heat transfer rates. Square or rotated square pattern is used for heavily fouling liquids [5]. Figure 4.1 Tube arrangements with the demarcation of the tube pitch Pt [5]
Therefore, here equilateral triangular pattern is selected in which the pitch is given by, [5] 4.4.4 Calculating the bundle diameter or the calandria diameter In order to calculate the bundle diameter, the following correlation is used which is derived considering the available tube arrangements [5].
(
)
, (
), (
)
are constants which are specified in the correlation and should be found from Table 4.3
Table 4.3 constants to be used in the correlation [5]
34
By choosing the two constants for single pass, triangular pitch,
(
)
4.4.5 Calculating the evaporator shell internal diameter In order to calculate the shell internal diameter, a correlation is used which is shown in Fig 4.2 Figure 4.2 Shell-bundle clearance [5]
According to Fig 4.2, the shell-bundle clearance for fixed and u tube type is taken as 13mm.
4.4.6 Calculating the height of the evaporator shell Height of the vessel is generally taken as 2-5 times the internal diameter of the shell depending on the entrainment possibility [8]. Higher the entrainment possibility, larger the height of the vessel should be. The entrainment possibility should be evaluated in terms of the factor which is given by,
(√
(
)
)
35
A= evaporator internal cross section V= volumetric flow rate of vapor (m3/s) Vapor mass flowrate (kg/s)
0.3565
Density of saturated steam at 70 kPa (kg/m3) 0.4200 Vapor flow rate (m3/s)
0.8488
Density of HFCS-55 (kg/m3)
1312
Internal cross sectional area (m2)
0.1979
Table 4.4 Vapor density, liquid density and cross sectional area
Therefore, ( (√
)
)
Therefore, Since the value of
is high, the entrainment possibility is high. Therefore, the height of the
vessel is chosen as 5 times the internal diameter of the shell Therefore, 4.4.7 Sizing of the steam jet ejector The type steam jet ejector chosen is single-fixed nozzle type. The main reason to select this type is the compactness and the small capacity of steam involved. Figure 4.3 a single fixed-nozzle steam jet ejector [1]
36
In order to determine the size of the ejector, the following graph is used which is shown in Fig 4.4 Figure 4.4 Sizing chart for steam jet ejectors [1]
Therefore, the size of the steam jet ejector should be 4 inch
37
4.5 Evaporator Performance Motive steam supplied (MT/day)
26.578
Vapor generated (MT/day)
30.802
Steam economy
1.16
Table 4.5 Evaporator performance
4.6 Parameter Calculation – Summary Operating pressure (kPa)
70
Boiling temperature (oC)
94.7
Feed flow rate (MT/day)
531.066
Product flow rate (MT/day)
500.265
Motive steam supply rate (MT/day)
26.578
Evaporator heat duty (kW)
992.9985
Evaporator heat transfer area (m2)
18.1738
Condenser heat duty (kW)
256.3191
Condenser heat transfer area (m2)
12.8945
Tube external diameter (mm)
19.05
Tube internal diameter (mm)
14.83
Tube length (m)
0.9144
Number of tubes
333
Bundle / calandria diameter (m)
0.489
Evaporator shell diameter (m)
0.502
Evaporator height (m)
2.51
Steam economy
1.16
Table 4.6 Parameter calculation summary
38
CHAPTER 05 – OVERVIEW TO DETAILED MECHANICAL DESIGN The design code used here is the 2007 ASME Boiler and Pressure Vessel Code. Under that code, the following sub sections are used.
ASME BPVC Section II part A, B – Ferrous and non-ferrous material specifications
ASME BPVC Section V – Non-destructive examination
ASME BPVC Section VIII – Rules for construction of pressure vessels
ASME BPVC Section IX – Welding and brazing qualifications
The product being evaporated is HFCS which is a non-toxic and a non-lethal liquid of a moderate pH value around 7 at room temperature. The operating pressure of the evaporator is 70 kPa and the maximum operating temperature is 116 o C. Therefore, by considering product and operating conditions this vessel can be categorized as a light duty vessel (U-70) which is operating below 100 psi pressure and 250 F. The procedure for detail mechanical design of the evaporator unit excluding the steam jet ejector can be carried out step by step as follows.
Determining the main geometry of the vessel
Determining the main dimensions of the vessel
Determining the operating conditions
Selection of the material of fabrication based on the required properties
Determining the welding techniques and the design allowances
Determining the required thicknesses of various components
5.1 Main geometry of the vessel The main geometry of the vessel is a cylindrical shell with a hemispherical enclosure at the top and a conical enclosure at the bottom. 5.2 Main dimensions of the vessel Dimensions were calculated as follows, Diameter of the evaporator shell = 0.502 m Height of the evaporator shell
= 2.51m
5.3 Operating conditions Operating conditions were calculated as follows Evaporator operating pressure = 70 kPa Motive steam pressure
= 500 kPa
Entrainment ratio for the steam jet ejector = 2.23
39
CHAPTER 06 – MATERIAL SELECTION FOR CONSTRUCTION A common material is used for the construction of the evaporator unit including the evaporator shell, top and bottom enclosures, calandria tube sheets, supports, nuts and bolts and pipelines for steam and products. A separate material is used for the construction of the heat transfer system, which is the calandria. 6.1 Material of construction for major components of the evaporator The products in contact are HFCS, which is a non-oxidising, non-toxic, non-lethal, moderately-corrosive organic liquid and steam. The material of construction should be food grade, commonly available, cost effective and easy to fabricate. Therefore, considering the all the requirements Stainless Steel with the Unified Numbering System (UNS) identification number of SS30403 which commonly known as SS 304L was chosen as the material of construction for major components of the evaporator. Some properties of SS 304L are tabulated below Table 6.1 – SS 304 L properties
SS 304 L properties Minimum Tensile Strength (MPa)
485 [9]
Minimum Yield Strength (MPa)
170 [9]
Melting range of the alloy (oC)
1399 – 1454
Can be subjected to temperatures around 900oC in continuous use Has an excellent corrosion resistance over a wide range of industrial applications Can be welded by using common fusion and resistance techniques
6.2 Material of construction for calandria tubes For the construction of calandria tubes a material with good heat transfer characteristics and corrosion resistance should be chosen. The most widely used material for the construction of calandria tubes in evaporators is Brass. The following compositions are preferred in industrial applications [10]
Cu – 70% and Zn – 30%
Cu – 70%, Zn – 29% and Sn – 1%
Cu – 64% and Zn – 36%
However, the composition of Copper should be always kept over 60% to ensure there is no effect from the non-condensable gases. Therefore, considering all those requirements, Brass
40
with UNS identification number of C-44300, which is commonly known as SB-171 was chosen as the material of construction for the calandria tubes. Some properties of SB-171 are tabulated below Table 6.2 – SB – 171 properties
SB-171 properties Composition
Cu – 71%, Zn – 28%, Sn – 1% [11]
Minimum Tensile Strength (MPa)
310 [9]
Minimum Yield Strength (MPa)
100 [9]
6.3 Welding Technique Type of welding joint used will be type 1 joints which is butt joints and spot radiography will be used as the non-destructive testing method. Therefore, according to Section IX of ASME BPVC – 2007, the weld joint efficiency is taken as 0.85.
41
CHAPTER 07 – MECHANICAL DESIGN OF THE EVAPORATOR 7.1 Parameter calculation In this section, important parameters such as the Design Pressure, Design Temperature for the evaporator shell and calandria tubes and Design Stresses for SS 304 L and SB – 171 are calculated. 7.1.1 Design Pressure 7.1.1.1 Design Pressure for calandria tubes
The maximum possible pressure difference occurs while in operation and the hydrostatic head within the tubes is neglected. According to Section VIII of ASME BPVC 2007, this belongs to the category where, P external > P atmospheric and P internal absolute < P atmospheric Therefore, P design = P maximum external by guage + 10% extra + P max internal by gauge [12]
7.1.1.2 Design pressure for evaporator shell
However, here the effect of the hydrostatic head is considered. Considering the level of HFCS is maintained at 1m so that the calandria is totally immersed in the liquid and the density of HFCS is 1312 kg/m3
Since P hydrostatic < P maximum by gauge, Design pressure is based on the maximum pressure difference by gauge P design = P maximum operating by gauge + P atmospheric
42
7.1.2 Design Temperature The maximum temperature involved in the evaporator is the steam temperature which is 116 o
C. Since, non-direct heating is used,
T design = T maximum + 10 oC [12] o o
C
C
7.1.3 Design Stress Design Stress is calculated in terms of the maximum allowable stress at the design temperature. , [9]
43
7.2 Thickness calculations In this section, thickness of the evaporator shell, calandria shell and calandria tubes will be calculated. 7.2.1 Thickness of the evaporator shell In order to calculate the thickness of the evaporator shell, the following equation is used as given in Section VIII, ASME BPVC 2007.
In the above equation, all symbols used have their usual meanings. Since the material of construction is SS 304 L, corrosion allowance is taken as zero.
This thickness will be checked for elastic and plastic failure under external pressure later in proceeding sections. 7.2.2 Thickness of the calandria shell The following equation is used,
Here also, corrosion allowance is taken as zero due to corrosion resistance in SS 304 L
However, since the calandria shell is under external pressure, this thickness should be checked for elastic and plastic failure. 7.2.2.1 Safe thickness against elastic failure The following equation is used to determine the safe thickness against elastic failure. . / [12] Where all symbols have their usual meanings. [13] K and m should be found from the following table based on
ratio.
44
Here,
Table 7.1 k and m values [12]
By interpolation,
Finding the safe design pressure for the thickness calculated against elastic failure, (
)
The calculated value is much less than the actual design pressure, Therefore under a thickness of 0.28mm, the shell will undergo elastic deformation. In order to find safe thickness for elastic failure,
is replaced with the operating design
pressure and a new thickness is calculated. (
)
Now, this new thickness should be checked for plastic failure
45
7.2.2.2 Safe thickness against plastic failure The following equation is used to check for plastic failure, . /
*
+ [
[12]
]
Where all symbols have their usual meanings. U, which is the out of roundness is taken as 1.5%. Determining the safe design pressure for the calculated safe thickness for elastic failure, (
)
0
1 0
1
This is greater than the operating design pressure. Therefore, this thickness is safe against plastic failure. Therefore, the thickness of the calandria shell which is safe against both elastic and plastic failure is 2.06 mm. However, available thickness is 2.108 mm [13] Therefore, thickness of the calandria shell = 2.108 mm 7.2.3 Thickness of the calandria tubes The following equation is used to calculate the thickness required ,
-
Here, corrosion allowance is taken as zero due to the corrosion resistance in SB-171.
However, the external diameter of the chosen tube is 19.05mm. Therefore, thickness of the selected tube = 2.11 mm This is greater than the thickness calculated. Therefore, thickness of the tube = 2.11 mm 7.3 Design of top and bottom enclosures In order to facilitate better stress distribution, the top enclosure is chosen as a hemispherical head and the bottom enclosure is chosen as a conical head to facilitate removal of concentrated product.
46
7.3.1 Thickness of the hemispherical head The following equation is used to calculate the thickness of the hemispherical head, [12] Here all symbols have their usual meaning. For the case of a hemispherical head, Crown radius is the internal radius of the head which is equal to the internal radius of the shell to which it is fixed. Since the material of construction is SS 304 L, corrosion allowance = 0.
However, available thickness is 0.203 mm. [14]
7.3.2 Thickness of the conical head A conical bottom is selected with an apex angle of 50o. The thickness of the conical head is given by the following equation, (
[12]
)
Here also, corrosions allowance is taken as zero due to SS 304 L (
)
However, available thickness is 0.381 mm [14].
Additionally, it should be inspected whether additional reinforcement is required or not at the cone-cyndrical shell junction. For that, the following procedure is followed as mentioned in Section VIII, ASME BPVC 2007.
Calculate the value
Compare the value of the semi apex angle with
found using the following table
47
By interpolation, =14.78 which is less than the semi-apex angle. Therefore additional reinforcement is required.
If
reinforce the junction with an area given by, 0
1 [12] [
]
7.4 Determining the safe thickness against elastic and plastic failure for the evaporator shell Calculation procedure is similar as shown in 3.2.2.1 and 3.2.2.2 Here,
(
)
Therefore,
Therefore, safe design pressure to resist elastic failure is found as
This is much less than the operating design pressure. Therefore, a new thickness is calculated to resist elastic failure. Therefore,
Now, this thickness should be checked for plastic failure, Safe design pressure for this thickness against plastic failure is calculated as 341.2 kPa which is greater than the operating design pressure which makes the calculated thickness safe against plastic failure. Therefore thickness to resist both elastic and plastic failure = 3.56 mm However, available thickness = 3.81 mm [13] Therefore, thickness of the evaporator shell = 3.81 mm
48
7.5 Openings and reinforcements Openings should be constructed in the shell for product removal, raw material introduction, man holes and etc. Construction of opening degrades the strength of the shell which needs additional reinforcements to maintain the original strength. Here, a total of five openings are designed for Feed In, Thick Liquor Out, Steam In, Condensate Out and Vapour Out. A man hole is not constructed due to the small cross sectional area. Instead of a man hole, the top hemispherical head is flanged to the main shell so that it can be removed during maintenance activities. 7.5.1 Determination of size of the openings In order to calculate the size of the openings, the volumetric flow rates should be known. The following table depicts the solution approach to that. Table 7.2 volumetric flow rate determination
Stream
Density (kg/m3)
Mass flow (kg/s)
Volumetric flow (m3/s)
Feed In
1280
6.1466
0.00480
Steam In
0.996 [15]
0.4486
0.450
Condensate out
946.26 [15]
0.4486
0.00047
Liquor Out
1320
5.7901
0.00439
Vapour Out
0.423
0.3565
0.843
Generally steam and vapour flow rates are maintained between 80-100 ft/sec [16]and juice flow rates are maintained between 5-10 ft/sec [17]for safe and optimum operation. Therefore, calculated opening diameter assuming all openings are circular in shape are shown in the following table Table 7.3 opening diameter calculation
Stream
Volumetric flow 3
(m /s)
Opening cross
Opening
Opening
sectional area
diameter
diameter
(m2)
(m)
(inches)
Feed In
0.00480
0.0021
0.052
2.05
Steam In
0.450
0.016
0.143
5.63
Condensate out
0.00047
0.00021
0.016
0.63
Liquor Out
0.00439
0.00192
0.049
1.93
Vapour Out
0.843
0.031
0.199
7.83
49
However, the pipes are selected based on the ANSI Schedule for stainless steel pipes. The selected pipe dimensions are listed in the following table Table 7.4 pipe selection [18]
Stream
Opening
Selected nominal
Outer diameter
diameter
pipe size (inches)
(mm)
(inches) Feed In
2.05
2
60.3
Steam In
5.63
6
168.3
Condensate out
0.63
3/8
17.2
Liquor Out
1.93
1.5
48.3
Vapour Out
7.83
8
219.1
The suitable pipe schedules will be selected after the calculation of the theoretical wall thickness of the nozzles. The type of the nozzle is chosen such that a protruded nozzle is used for Feed In stream and External nozzles are used for all the other streams. 7.5.2 Reinforcements for openings Area needed for reinforcements will be calculated using the equal area method and it is assumed that the openings are well apart from each other so that there is no interaction between any openings. 7.5.2.1 Opening for the Feed In (
)
(
)
(
)
Where t theoretical is the calculated theoretical thickness of the shell However corrosion allowance is zero for SS 304 L, Therefore, (
) (
)
(
)
(
)
(
)
(
)
T actual = 0.00381mm Therefore,
Excess area in the nozzle should also be found out in order to calculate the area for reinforcements
50
First, the theoretical thickness for the nozzle should be calculated using,
Where all parameters are for SS 304 L and weld joint efficiency at the joint is assumed to be 1. Therefore,
However, available thickness is 1.65mm which is schedule 5s. Therefore, T actual = 1.65mm Next, excess area available in the nozzle is calculated using the equation,
Where Ao is the excess area in the external nozzle and Ai is the excess area in the internal nozzle. Considering the external protrusion H1 as 5 cm, The external boundary limit should be calculated using the equation √(
)
(
)
Therefore, )
√(
, which is lower than H1 actual
Therefore, H1 calculated should be used in the proceeding calculations ( (
)
(
)
)
Considering internal protrusion H2 as 3 cm, The internal boundary limit should be calculated using the equation, √( Therefore,
)
(
) )
√(
, which is lower than
actual. Therefore, H2 calculated should be used in the proceeding calculations. ( )
(
)
Note that, t theoretical is zero for internal nozzles since the design pressure is taken as zero. Therefore,
51
Therefore,
This area is supplied in terms of a welded plate as shown in Fig 3.1
Figure 3.1 welded plate
Select
Therefore,
But ( )
(
)
, where tr is the thickness of reinforcement
Therefore,
Thickness of reinforcement = 2.293 mm 7.5.2.2 Opening for the Steam In A similar procedure is carried out excluding the internal nozzle calculations.
Therefore, (
) (
)
However available thickness is 2.77mm which is schedule 5s according to ANSI Taking H1 as 5 cm , which is lower than 5cm, Therefore,
52
Therefore,
Considering
Therefore,
7.5.2.3 Opening for Thick Liquor Out Similarly,
Therefore, (
) (
)
However available thickness is 1.65 mm which is schedule 5s according to ANSI Taking H1 as 5 cm , which is lower than 5cm, Therefore,
Therefore,
Considering
Therefore,
7.5.2.4 Opening for the Condensate Out Similarly,
Therefore, (
) (
)
53
However available thickness is 1.66 mm which is schedule 10s according to ANSI Taking H1 as 5 cm , which is lower than 5cm, Therefore,
Therefore,
Considering
Therefore,
7.5.2.5 Opening for the Vapour Out Similarly,
Therefore, (
) (
)
However available thickness is 2.77 mm which is schedule 5s according to ANSI Taking H1 as 5 cm , which is lower than 5cm, Therefore,
Therefore,
Considering
Therefore,
54
7.6 Insulation thickness calculation In order to determine the insulation thickness required, the worst case scenario is considered. The following parameters are needed to do the calculation Ambient temperature is assumed as 30oC Therefore the maximum temperature difference occurring at the worst condition is 64.7oC. Polyurethane is chosen as the insulation material with a conductivity ( ) of 0.03 W/m.K [19] Conductivity of SS 304 L (ks) at the operating conditions is found as 16.2 W/m.K [20] (
) ( )
Overall heat transfer co-efficient based on the internal surface without insulation is calculated using the following equation . /
( )
Where r1 is the internal radius and r2 is the external radius of the shell
Therefore,
Applying heat transfer rate for a unit length of the shell, ̇ Now, considering the overall heat transfer coefficient based on the internal surface when an insulation layer is applied . /
. /
( )
Where r3-r2 is the insulation thickness Assuming that by applying insulation, heat loss can be prevented by 98%,
Therefore,
55
r3 is calculated as 0.4614m
Therefore,
7.7 Combined loading calculations A vessel should be designed to withstand not only stresses due to pressure, but also additional stresses exerted by dead weight, wind, seismic activity and eccentric loads on the vessel. In this section, calculations are done to inspect the stability of the evaporator for combined loadings. Here, the evaporator is assumed to be mounted on a skirt support which is 1m from the ground. Design calculations regarding the skirt support will be carried out later. 7.7.1 Stresses due to pressure Circumferential and longitudinal stresses are formed due to pressure.
Both circumferential and longitudinal stresses are way below the design stress. 7.7.2 Stresses due to dead weight The following components are considered in calculating the dead weight of the vessel
Weight of the evaporator shell
Weight of the two heads
Weight of the working volume of HFCS-55 in the evaporator
Weight of the calandria including the tubes
Weight of the insulation is neglected. 7.7.2.1 Weight of the evaporator shell (
)
Where all symbols have their usual meanings ( But density of SS 304L is 8000kg/m3 [13] Therefore,
)
56
7.7.2.2 Weight of the two heads Volume of the hemispherical head is calculated as 0.0001608 m3 Volume of the conical head is calculated as 0.000188 m3 Therefore,
7.7.2.3 Weight of the working volume of HFCS-55 The level of liquid inside the evaporator shell is taken as 1m and the density of HFCS-55 is taken as 1320 kg/m3 Therefore, Weight of the liquid inside the evaporator shell is calculated as 2.563kN 7.7.2.4 Weight of the calandria including the tubes (
)
Where all symbols have their usual meanings Therefore, 0.0342m3 The density of SB-171 is found as 8530 kg/m3 [11] Therefore,
Therefore,
Compressive stress due to total dead weight is given by the following equation, (
)
Where all symbols have their usual meanings ( Therefore,
)
57
7.7.3 Stresses due to wind Here the total height of the evaporator from the ground is 2.51+1, which is 3.51m. Since this is below 20m, wind calculations are done only to downwind section. Wind pressure is taken as 0.7 kN/m2 since the height is below 20m and the site location is near the coastal area. Since only the downwind part is calculated, a compressive stress is generated due to wind which is given by, (
)
Where
Here, H is the to the top of the vessel from the ground Pwind is calculated using the following equation.
is taken as 0.7 since the cylindrical surface is exposed to wind and
is taken as 2 assuming
is taken as 0.7 kN/m2
a natural period greater than 0.5 seconds.
is the height of the vessel from the foundation which is 1m in this case. Therefore,
Therefore,
Therefore, (
)
Therefore,
Stresses due to seismic loads have been neglected since earthquakes are not abundant in the site location.
58
7.7.4 Calculation of principle stresses and stress intensities. Resultant axial stress can be calculated by using the following equation, (Negative sign due to downwind case)
Therefore,
Resultant hoop stress is the circumferential pressure due to pressure. Therefore,
Now, the three principle stresses should be found out. (
)
√.
/
√.
/
Therefore,
(
)
Therefore,
Therefore,
Stress intensity distribution can be found out as
All the stress intensities are much lower than the design stress of SS 304 L in this case which is 115 MPa. Therefore, the calculated thickness in previous sections is stable against combined loading.
59
7.8 Design of supports Due to the comparatively large size of the vessel, a straight skirt support is used. Skirt support design can be carried out in 3 steps.
Determination of the skirt wall thickness
Design of skirt bearing plate
Design of anchor bolts
7.8.1 Determination of the skirt wall thickness In order to design the skirt, 15.875mm thick carbon steel (grade C) is used [12]. Load on the skirt support is due to dead weight and wind.
Where all symbols have their usual meanings
Therefore, (Compressive)
( ) Therefore,
Similarly, ( ) Therefore,
If Taking
, then the used thickness is okay for the skirt for a welded steel plate, , which is much greater than
safe.
. Therefore the used thickness is
60
7.8.2 Design of skirt bearing plate For safe design of the bearing plate,
However,
Where all symbols have their usual meanings and is taken as 4 MPa which is a common value. Considering the limiting case,
Therefore,
However,
Therefore,
Substituting this value instead of
the actual compressive stress should be calculated
Thickness of the bearing plate (tbp) should be such that, √ Therefore,
Therefore (
)
Therefore, the use of a steel rolled angle is recommended. 7.8.3 Design of anchor bolts First, the requirement for anchor bolts is checked
Where all symbols have their usual meanings
61
Therefore,
Since,
, anchor bolts should be designed.
The bolt area required ( 0
) is given by the following equation, 1 [21]
Where,
Considering the skirt inner diameter and the bearing plate width (Lb), a suitable bolt circle diameter is selected as 0.566m. Considering the minimum recommended bolt spacing as 600mm [21],
Since the minimum number of bolts should be 8 [21],
Now, bolt root diameter should be calculated in order find out the size of the anchor bolt required. Here, (
)
Therefore, 0
1
62
Therefore, √ But minimum diameter of bolts should be at least 25.4 mm (1 inch). Therefore,
7.9 Description of fabrication Austenitic steel type SS 304 L is used for the fabrication of the main evaporator shell, calandria shell and top and bottom heads. Cartridge brass SB-171 is used for the construction of tubes. SS 304 L is a very tough and a ductile metal which is hard to machine. Therefore, high performance machineries with high speeds, rigidity and heavy duty cuts are required for the machining. Common fabrication methods such as roll forming, deep drawing and bending are used. SS 304 L can be welded using the common fusion and resistance techniques, however special care should be taken to prevent “hot cracking”. Some measures to prevent this are maintaining the strain on the weld pool at a moderate level and preventing formation of narrow grains in the solidifying metal which blocks the passage of liquid metal to those areas. For fabrication of cylindrical shell of the evaporator 3.81mm SS 304 L sheets are used and for the fabrication of calandria shell 2.108 mm SS 304 L sheets are used. These sheets are rolled to get a cylindrical shape and welded using a double V grooved butt joint which is a category A type weld with efficiency 1 achieved with full radiography. The top and bottom tube sheets are welded to the calandria shell using single V grooved butt joint without a backing plate which is a category C type weld with efficiency 0.6. Cartridge brass tubes are welded to the tube sheets using full fillet lap joints. The top hemispherical head is formed by forging and heat treatment to SS 304 L and the bottom conical head is formed by rolling and welding using double V grooved butt joint. The top hemispherical head is flanged to the cylindrical shell and the bottom conical head is welded using single V grooved butt joint with a backing plate which is a category A weld with efficiency of 0.8. All nozzles are welded to the main shell using category D fillet welds and L support brackets are used to fix calandria to evaporator shell. Four L support couple brackets are used and fillet
63
supports are used to weld L supports to shells and each other. The skirt is welded to the main shell with a lap joint with efficiency 0.8 and fixed to the bearing plate using rolled ring plate.
64
7.10 Mechanical drawings. Table 7.5 list of drawings
Description
Drawing Number
Basket type evaporator
01
Calandria and tube bundle
02
Top hemispherical head
03
Bottom conical head
04
Sectional view of evaporator
05
Sectional view of calandria
06
Skirt support
07
65
CHAPTER 08 – PIPES AND INSTRUMENTATION In this section selection of suitable pipes, valves, pumps, other auxiliary items such as steam traps, instruments is done. In addition, processing controlling aspects are also being considered. 8.1 Selection of pipes Pipes are required for the transportation of liquid concentrate and steam and vapour. Pipes are required for the following purposes,
Transport of feed into the evaporator
Transport of concentrate from the evaporator
Transport of steam into the calandria
Transport of condensate away from the calandria
Transport of vapour from the evaporator
Owing to the corrosion risk and food grade clearance, SS 304 L is used as a material of construction for pipes. Pipe selection is based on the volumetric flow rates and the maximum allowable flow velocities. The following table depicts the details of the pipes selected. Table8.1 Pipe details
Stream
Pipe nominal
Pipe outer diameter
Pipe thickness (mm)
diameter (inches)
(mm)
Feed In
2
60.3
1.65 (5s)
Steam In
6
168.3
2.77 (5s)
Condensate Out
3/8
17.2
1.66 (10s)
Concentrate Out
1.5
48.3
1.65 (5s)
Vapour Out
8
219.1
2.775s)
8.2 Selection of valves Valves are an integral part of a vessel to facilitate flow regulation. There are different types of valves: shut-off valves, control valves and non-return valves. The requirements of valves for the evaporator are as follows,
Shut-off valve for feed line
Non-return valve for condensate out line
Shut-off valve for motive steam in line
Shut-off valve for the steam in line
Shut-off valve for concentrate out line
66
Shut off valve for the vapour leaving the steam jet ejector
Control valve to regulate feed flow
Control valve to regulate vapour flow from the evaporator
Control valve to regulate motive steam flow rate to the steam jet ejector
Control valve to regulate the input steam flow rate
Control valve to regulate the vapour flow rate to the condenser
Control valve to regulate concentrate flow rate
Drain valve for the evaporator
8.2.1 Selection of shut-off valves Shut-off valves should provide minimum resistance to flow when they are open and positive seal when they are closed. Therefore, based on the requirements of the process, plug valves made of SS 304 L are used since they are relatively cheaper and provide low resistance to flow in open condition and tight seal in close position.
Figure 8.1 schematic diagram of a plug valve
8.2.2 Selection of control valves Control valves should have a quick response time and should be reliable in operation. Here, two types of control valves are used to control liquid and vapour flows respectively. To control vapour flows, butterfly valves are used due to the following characteristics,
Most widely used in gas flow control
Light weight with a quick response time
Economical
Low resistance to flow
67
Figure 8.2 Schematic drawing of a butterfly valve
In order to control liquid flows, globe valves are used due to the following characteristic,
Ability to withstand high pressures
Full range operation
Precise flow regulation
Figure 8.3 Schematic diagram of a globe valve
68
The following table shows the details of the selected valves. Table 8.2 Valve details
Purpose of the valve
Type of valve
Notation
Shut-off valve for feed line
Plug valve
V1S
Non-return valve for condensate
Non-return type valve
V2NR
Plug valve
V3S
Shut-off valve for the steam in line
Plug valve
V4S
Shut-off valve for concentrate out
Plug valve
V5S
Plug valve
V6S
Control valve to regulate feed flow
Globe valve
V7C
Control valve to regulate vapour
Butterfly valve
V8C
Butterfly valve
V9C
Butterfly valve
V10C
Butterfly valve
V11C
Globe valve
V12C
Globe valve
V13C
Plug valve
V14S
out line Shut-off valve for motive steam in line
line Shut off valve for the vapour leaving the steam jet ejector
flow from the evaporator Control valve to regulate motive steam flow rate to the steam jet ejector Control valve to regulate the input steam flow rate Control valve to regulate the vapour flow rate to the condenser Control valve to regulate concentrate flow rate Control valve to regulate condensate flow Drain valve for the evaporator
8.3 Selection of pumps and other auxiliary items Evaporator is operating under a vacuum which is provided by the steam jet ejector. Therefore, there are no pumping requirements. A vacuum pump is used to provide the required vacuum at the start up.
69
One float steam trap is used for the steam supply to the evaporator. Continuous operability, good corrosion resistance, resistance to wear, fast response to condensate slugs are some promising features of this type of steam traps. 8.4 Instrumentation Instrumentation is a vital part in designing of an equipment. Instruments provide vital information that can be used for process controlling purpose. Some of the key processing controlling aspects facilitated by instrumentation are,
Temperature control
Flow control
Level control
Pressure control
8.4.1 Temperature measurement Temperature measurements are very critical for the operation of an evaporator. Lower the temperature, the required evaporation rate will not be achieved and higher the temperature, over evaporation will occur and product degradation can also occur. Therefore, temperature measurements should be obtained at the following locations of the evaporator,
Temperature of the inlet feed solution
Temperature inside the evaporator
Temperature inside the calandria
Temperature of the concentrate out
Temperature of the steam coming to the evaporator
Several factors should be considered in selecting a temperature sensor. Some of them are,
Operating temperature range
Sensitivity
Precision
Reliability
Accuracy
The most common type of temperature sensors used in the industry are thermocouples and resistance thermometers which are better known as Resistance Temperature Devices (RTD). Particular to this application, the sensor used should be guarded against attacks from corrosion. In that case, thermocouples show an increased time constant to a rapid change in temperature which is not favourable. However, in the case of RTDs, even when they are protected by enclosing in a sheath, very small time constants in the order of 0.4 seconds are
70
obtained [22]. In addition, out of the two, thermocouples show much lower temperatures upon aging. Therefore, RTDs are selected as the suitable temperature sensors. Since the maximum operating temperature is around 116oC and the minimum operating temperature which is the temperature of the inlet feed is 70oC, RTDs made of copper are selected which have an operating temperature range from -200oC-260oC [22]. 8.4.2 Pressure measurement Maintaining the internal pressure at the optimum level is a must for proper functioning of the vessel. If the internal pressure is higher than the required level, the boiling point will rise which changes the temperature and product degradation might occur. Also, a higher internal pressure reduces the level of vacuum. Then the natural pumping effect due to vacuum might be not sufficient to pump the feed into the evaporator. If the internal pressure is lower than the required level, boiling point is reduced and under evaporation occurs which affects the composition of the end product. Other than that, maintaining the inlet steam pressure at the required level is also important since it determines the steam temperature. Change of steam pressure changes the temperature driving force of the evaporator and affects the performance. Considering all these, pressure sensors should be located to measure the internal pressure of the vessel and the inlet steam pressure. For that, a diaphragm type pressure sensor is used. 8.4.3 Flow measurement In process controlling aspects, flow rate is used as the manipulated variable to control the pressure, temperature and level. Therefore, precise instrumentation should be there to measure both liquid and vapour flow rates. Orifice plate flow meters which can be used to measure both liquid and vapour flow rates are used in this case due to their applicability over a broad range and relatively low cost [22]. 8.4.4 Level measurement The liquid level should be maintained just above the top level of the calandria in the evaporator for optimum heat transfer. Lower the liquid level, maximum heat transfer is not occurred and higher the liquid level, steam economy will reduce. Owing to the food grade requirement of the product, non-contacting type of level sensors should be used in with respect to this application. Therefore, guided wave radar type level measuring device is used as the level sensor.
71
8.5 Process controlling aspects Temperature control, pressure control, level control and flow control are essential to facilitate an optimum behaviour for the evaporator. With regard to the evaporator, controlling of these aspects can be done by manipulating the flow rates. Different control strategies can be implemented and various control structures can be developed depending on the requirements. Some of the most widely used control strategies are PID controlling and PLC controlling. In this section, application of both PID and PLC controlling with regard to the application is discussed in terms of control structures. 8.5.1 PID controlling PID controllers are widely used in the process industry due to their versatility and ease of implementation. A PID controller can be implemented on a feedback loop to increase the stability of the loop. The controlling mechanism is error based controlling. The general representation of a PID controller is given below ∫ Here,
, is the control signal and
constant and
, is the proportional constant and
, is the integral time
, is the differential time constant. A P controller generates the control signal
proportional to the error signal which is the difference between the set point value and the actual value. However, a P type controller has an offset which is not favourable for industrial applications requiring very high accuracy. A PI controller generates the control signal based on both the error and the accumulation of error. Therefore, the offset is very much reduced and the accuracy is increased. A PID controller generates the control signal based on the error, accumulation of error and the rate of change of error. Therefore, the offset is further reduced and the accuracy is the highest. However, a noise filter should be included in the control loop when a PID controller is utilized. The following sections show some control structures that can be implemented for controlling temperature, pressure, flow rate and level in the evaporator using PID controllers. 8.5.1.1 Flow control of the feed into the evaporator The main elements of the control loop in this case are as follows Process – controlling the flow rate of the feed in Controller – PID controller Sensor – Orifice plate flow meter at feed in Final element – globe valve in the feed flow pipe The block diagram is shown below.
72
Figure 8.4 Block diagram of the flow controlling system
8.5.1.2 Liquid level control inside the evaporator The main elements of the control loop are as follows Process – Controlling the level Controller – PID controller Sensor – Guided wave radar level sensor Final element – globe valve of feed in The block diagram is shown below
Figure 8.5 Block diagram of the level controlling system
8.5.1.3 Pressure control inside the evaporator The main elements of the control loop are as follows Process – Controlling the pressure Controller – PID controller Sensor – Diaphragm pressure sensor Final element – Butterfly valve in steam in The block diagram is shown below,
Figure 8.6 Block diagram for the pressure control system
73
8.5.1.4 Temperature control in the evaporator using a cascade control structure The temperature inside the evaporator can be controlled by manipulating the flow rate of the steam entering the evaporator. However, in this simple control structure pressure of inlet steam is a disturbance. A sudden increment in pressure is not felt by the temperature controlling loop and eventually increases the temperature. If the temperature control mechanism can be integrated with a flow rate control mechanism, then such a problem will not occur. Therefore, a cascade control structure can be developed. In this case, flow controlling of the inlet steam flow is taken as the secondary loop and the temperature controlling is taken as the primary loop. The selection is such because for cascade controlling to be successful, the secondary loop should be faster than the primary loop [23]. Main elements of the primary and secondary control loops are shown below Primary control loop Process – controlling the temperature inside the evaporator Controller – PID controller Sensor – Copper RTD Final element – butterfly valve of the steam in pipe Secondary control loop Process – controlling the flow rate of the steam in Controller – P type controller at critical gain Sensor – Orifice plate flow meter at steam in pipe Final element – butterfly valve of the steam in pipe The block diagram of the cascade control structure is shown below
Figure 8.7 Block diagram of the cascade loop
74
8.5.2 Programmable Logic Controllers (PLC) PLC is a digital computer used for automation of electromechanical processes such as control of machinery on factory assembly lines, amusement rides or light fixtures. PLC can easily handle cases with multiple inputs and multiples outputs. Other than that PLCs can work in extended temperature ranges, have immunity to electrical noises and are resistant to vibration and impact. Therefore, most of the controlling aspects in the industry are carried out using PLCs. The most common brands of PLCs used are SiemensTM and Allan BradleyTM Here, a case when a PLC controller is applied to control the globe valve to the feed pipe is considered. Controlling of the globe valve is done in terms of two modes of operation: one to close it rapidly and the other to close it slowly. The inputs to the PLC is taken as the position of the output shut-off valve in the concentrate pipe and signals from two level sensors situated at two different levels in the evaporator.
Figure 8.8 Positioning of the two level sensors
The description of the functioning behavior of the system is as follows, “Whenever the output shut-off valve is closed and the upper maximum level is reached or whenever the output shut-off valve is closed and the lower maximum level is reached, the globe valve should be closed rapidly” “Whenever the output shut-off valve is opened and the upper maximum level is reached or whenever the output shut-off valve is opened and the lower maximum level is reached, the globe valve should be closed slowly”. The ladder diagrams shown are shown below for the two cases.
75
In the above diagram, 1 represents the output shut-off valve and it is considered as a normally open input which is activated by closing the valve. Notation 2 stands for the upper maximum level and it is also considered as a normally open input which is activated when the level is reached Notation 3 stands for the lower maximum level and it is also considered as a normally open input which is activated when the level is reached. The output is rapidly closing the valve. Similarly ladder diagram for the slow closing of the valve is shown below using the same notations
8.5.3 Controlling of the steam jet ejector In thermal vapor recompression, steam jet ejector plays a vital role. Controlling the flowrates and pressures of suction low pressure vapor, high pressure motive steam and intermediate pressure outlet steam is crucial. The following diagram shows some controlling aspects related to steam jet ejectors.
Figure 8.9 Controlling aspects of steam jet ejectors [24]
76
Type of the control method depends on whether the steam jet ejector is sonic or sub-sonic. If the compression ratio is larger than 1.8, the ejector is sonic [24]. In this case the compression ratio is taken as 2.5. Therefore, the steam jet ejector is sonic. Respective control methods for sonic and sub-sonic conditions are depicted below.
Figure 8.10 sonic Vs sub-sonic design [24]
Therefore, in this case, option 6 is used.
77
8.6 P&I Diagram
78
CHAPTER 09 – SAFETY ASPECTS 9.1 Startup procedure 1
Make sure all valves are closed and control loops are switched off and get familiarized with the pipe layout, instruments and valves.
2
Then open V1S to allow feed flow to the evaporator
3
Start the vacuum pump and allow the feed to flow into the evaporator under the generated vacuum until 40% of the evaporator height is filled.
4
Allow steam to flow by opening V3S, V4S and V6S and let the feed transferred in to boil
5
Open V2NR and V13C to control the condensate flow
6
Switch on the control valves V8C, V9C, V10C and V11C to control the steam flow rates and vapor flow rates
7
After the required evaporation is achieved, switch off the vacuum pump and switch on the steam jet ejector
8
After the required concentration of syrup is reached open V5S and V12C to obtain the end product
9
Switch on V7C to regulate the feed flow rate and reduce the liquid hold up to the required level of 1m.
10 Make sure all the control loops are functional and allow the evaporator to operate continuously
9.2 Shut down procedure 1
Switch off the steam supply to the evaporator
2
Then switch off the feed supply to the evaporator
3
Drain all the condensates and liquid held by opening V14S
4
Make sure all the valves are closed and control loops are properly switched off.
79
9.3 Maintenance procedure HFCS Manufacturing plant has weekly and monthly maintenance plans for each and every equipment. Evaporator maintenance is carried out under that protocol. Periodic maintenance of the evaporator is very critical for the optimum performance of it because scaling and fouling inside calandria tubes can greatly decrease the heat transfer coefficients and hence the heat transfer characteristics. Therefore, cleaning of the calandria tubes should be carried out. It can be done both mechanically and chemically. Chemical cleaning is comparatively easier but additional safety measures should be taken to ensure safety and health aspects of the worker. Mechanical cleaning of the tubes is recommended once in every week and chemical cleaning is to be done during monthly maintenance. During the maintenance process, the top hemispherical head which is flanged can be removed and the basket type tube bundle can be removed. Then, during mechanical cleaning the worker can use scrapers, wire brushes or rotary cleaners to clean the inside of the pipes. During chemical cleaning first boiling caustic solution should be used to wash the tube internals followed by HCl solution. The former ensures the removal of deposited oxalates, sulphates and silicates and the latter ensures the removal of sulphites, carbonates and phosphates. 9.4 Hazard and Operability Study (HAZOP) A Hazard and Operability Study is a procedure for the systematic, critical examination of the operability of a process. When applied to a process design, it indicates potential hazards that may arise from deviations in the intended design conditions [25]. A HAZOP study conducted for the evaporator unit is shown below.
80 Table 9.1 HAZOP study for the evaporator
Project Name : Production of HFCS
Date : 17.03.2016
Process : Evaporation of HFCS-55 solution to obtain required composition Section : Final evaporator FE-03 Study Node
1.Liquid feed
Process
Deviations
Parameters
(Guide words)
Flow
No
pipe
High
Possible causes
Possible consequences
Actions required
Closed shut off valve.
Temperature inside the
Regular maintenance of
Malfunctioning control valve.
evaporator rises.
valves and pipes.
Blockage in the pipe.
Damage to sensors and
Install filters to prevent
Steam jet ejector
internals.
any blockages.
malfunctioning.
Install high temperature
Failure of controller.
alarm system.
Failure of controller.
Drop in temperature and can Regular maintenance and
Failure of controller valve.
reduce the evaporation rate
monitoring of the
and eventually increases
controllers and controller
water content in the final
valves
product Low
Failure of controller.
Rise in temperature and
Regular maintenance and
Failure of controller valve.
increases the evaporation
monitoring of the
Partially blocked line.
rate and reduces the water
controllers and controller
content in the final product
valves.
81
Installing filters to the pipes and regular maintenance Composition
2. Steam Inlet
Flow
As well as
No
pipe
Unwanted impurities like ash,
Increased scaling and
Installing of filters to the
ions can be included in the
fouling which greatly
line.
flow due to malfunctioning
reduces the heat transfer
Regular maintenance of
ion exchange unit and carbon
coefficients and hence the
upstream processes.
filter
evaporation rate
Closed shut off valve.
With no steam supply
Regular maintenance of
Malfunctioning controller.
evaporation rate will reduce
valves and controllers.
Malfunctioning controller
and eventually stop
Regular maintenance of
valve.
steam jet ejector
Malfunctioning steam trap.
Regular maintenance of
Malfunctioning Steam jet
steam lines
ejector. No motive steam supply. Blocked steam line. High
Increased motive pressure.
Evaporation rate will
Regular maintenance of
Malfunctioning controller.
increase and change the
control valves and
Malfunctioning control valve.
desired composition.
controllers. Monitoring the motive
82
stream pressure and installing an alarming system in case of unacceptable levels. Low
Decreased motive pressure.
With low steam flow rate,
Regular maintenance of
Malfunctioning controller.
evaporation rate will reduce
controllers and control
Malfunctioning control valve.
and affect the composition
valves.
Partially blocked line.
of the final product.
Regular maintenance of
Malfunctioning steam traps
steam lines and steam traps for blockages.
3. Evaporator
Pressure
Low
Vacuum pump may be
When the pressure reduces,
Make sure vacuum pump
operating along with the
boiling point reduces and it
is operational only during
steam jet ejector.
reduces the evaporation rate
the startup.
Pressure controller
Regular maintenance for
malfunctioning.
controllers and sensors.
Pressure sensor malfunctioning. High
Malfunctioning in the steam
If there is a leak and air is
Regular maintenance of
jet ejector.
sucked in, contamination
the steam jet ejector and
A leak in the nozzles, valves
happens.
nozzle, valve and pipe
83
or pipes can suck air into the
Higher pressure increases
connections for possible
vessel
the boiling point and over
leakages.
evaporation occurs. Temperature
Low
Temperature controller
Temperature driving force is Regular maintenance of
malfunctioning.
increased and increases the
Temperature sensors
evaporation rate which can
malfunctioning.
affect the product
controllers and sensors.
composition High
Temperature controller
Temperature driving for is
Regular maintenance of
malfunctioning.
reduced and decreases the
controllers and sensors
Temperature sensors
evaporation rate which can
malfunctioning.
affect the product composition
Level of liquid
High
Level controller
increased hold up of liquid
Regular maintenance of
malfunctioning.
increases the retention time
controllers, valves and
Level sensor malfunctioning.
which can degrade the
sensors.
Control valve malfunctioning. quality of the end product Flow controller malfunctioning. Flow meter malfunctioning.
84
Low
Level controller
Decreased level of liquid
Regular maintenance of
malfunctioning.
reduces the evaporation rate
controllers, valves and
Level sensor malfunctioning.
and the production rate.
sensors.
High pressure in steam
Increased pressure can
Installing an alarm system
inflow.
increase the temperature and for increased pressure.
Pressure controller
hence increase the
Regular maintenance of
malfunctioning.
evaporation rate and affect
controllers, control valves
Pressure sensors
product composition.
and sensors.
malfunctioning.
Further increment in
Control valve malfunctioning
pressure may affect the
Control valve malfunctioning. Flow controller malfunctioning. Flow meter malfunctioning.
4.Calandria
Pressure
High
mechanical integrity of the tubes.
Low
Low pressure in steam
Decreased pressure reduces
Regular maintenance of
inflow.
the steam temperature and
controllers, control valves
Pressure controller
hence the driving force and
and sensors.
85
malfunctioning.
lowers the evaporation rate
Pressure sensors
affecting the product
malfunctioning.
composition.
Control valve malfunctioning 5. Product line
Flow
No
Closed shut off valve.
No production
Making sure shut off
Malfunctioning controller,
valve is always open
control valves and sensors.
during operation and
Blocked product line.
install locking system to prevent someone from accidently closing it. Regular maintenance and inspection of controllers, control valves and sensors and pipes. Installing filters to prevent blockages.
High
Malfunctioning controller.
Higher production rate
Regular maintenance of
Malfunctioning control valve
might be due to partially
controllers, control valves
or sensor.
evaporated liquid which can
and sensors.
degrade the product quality Low
Malfunctioning controller.
Affect the productivity.
Regular maintenance of
86
Composition
6. Condensate out pipe
Flow
As well as
No
Malfunctioning control valve
Liquid hold up can increase
controllers, control valves
or sensor.
the retention time and
and sensors and pipes for
Partially blocked product
degrading the product
blockages.
pipe.
quality.
Composition of the product
Presence of impurities
Ensuring all lubricants
may be changed and
vastly degrade the quality of
used are food grade.
impurities can be present due
the end product.
Installing metal detectors.
to oil leakages, worn out
Regular maintenance of
metal parts.
the vessel parts.
Non-return valve is closed.
Accumulation of condensate Regular maintenance and
Malfunctioning controller,
can affect the condensation
inspection of controllers,
control valve or flow meters.
process of steam and lessen
control valves and sensors
Blockage in the condensate
the steam economy
and condensate out pipe
line.
for blockages
87
CHAPTER 10 – ECONOMIC ASPECTS In this section, a rough estimate of the total capital cost investment required for the vessel is done. In order to calculate the total capital cost investment, fixed capital cost and the working capital cost is estimated. 10.1 Estimation of the fixed capital cost Most of the capital cost estimation methods are based on the total purchased cost for the equipment. Here, also a similar approach is made. In order to calculate the purchased equipment cost, order of magnitude method is used. For that, the following chart is used which shows the relationship between the purchased equipment cost and the heat transfer area of a single stage evaporator constructed from Stainless steel. However, the data belongs to 1987 and later, Chemical Engineering Plant Cost Indices (CEPCI) are used to convert that cost to the year of installation.
Figure 10.1 Purchased equipment cost of a single stage evaporator made of stainless steel [26]
88
Therefore according to the above chart taking the respective equipment cost for a short tube evaporator with a heat transfer area of 195.62 square feet,
Using the CEPCIs for 1987 and 2015, the equipment cost in 2015 was calculated using the following equation, [27] CEPCI for 1987 = 320 [26], CEPCI for 2015 as at May = 560.7 [28] Therefore,
(
)
Therefore, (
)
Then, the fixed capital cost is calculated based on this calculated purchased cost of equipment using the relations given in the following table Table10.1 typical factors for estimating the fixed capital cost [27]
89
Based on Table 3.1, evaporator falls under the category of fluids. Therefore, by using the factors given in the table, (
)
(
)
The combined cost for design and engineering, contractor’s fee and contingency (C) is calculated as follows,
Total fixed capital cost is given by,
Therefore,
10.2 Estimation of the working capital cost Usually working capital is estimated as 10-20% of the fixed capital cost. Therefore here, as an average value 15% is used. Therefore,
10.3 Total capital cost Total capital cost is given by,
Therefore, Total capital cost=112.963+16.944=129.907 million LKR Total capital cost = 129.907 million LKR
90
REFERENCES [1] P. E. Minton, Handook of evaporation technology, New Jersey: Noyes publications, 1986. [2] "Evaporator Handbook," SPX. [3] W. B. Glover, "Selecting evaporators for process applications," 2004. [4] "Typical data information for Cornsweet 55," ADM Corn Processing. [5] R. K. Sinnot, "Heat transfer equipment design," in Chemical Engineering Design-Volume 6, Elsevier, 2005, pp. 645-685. [6] "Thermodynamic data of sucrose solutions". [7] "Single Effect Evaporation_Vapor Compression," in Design of Evaporators, pp. 50-78. [8] "DESIGN PROCEDURE FOR STANDARD VERTICAL SHORT TUBE EVAPORATOR," 11 June 2013. [Online]. Available: http://www.chemavishkar.com/2013/06/design-procedure-forstandard-vertical.html. [Accessed 23 12 2015]. [9] ASME, "Section ii part D," in ASME Boiler and Pressure Vessel Codes, ASME, 2004, pp. 105106. [10] E.Hugot, "Construction of multiple effects," in Handbook of Cane Sugar Engineering, Elsevier, 1972, pp. 500-508. [11] "Brass alloy UNS 44300," [Online]. Available: http://www.azom.com/article.aspx?ArticleID=6371. [Accessed 14 03 2016]. [12] ASME, Section VIII BPVC 2007, ASME, 2007. [13] "AISI Type 304L Stainless Steel," [Online]. Available: http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MQ304L. [Accessed 14 03 2016]. [14] "304/304L Stainless Steel Sheet, Coil & Plate AMS 5513, AMS 5511 UNS S30400, UNS S30403," [Online]. Available: http://www.upmet.com/products/stainless-steel/304304l. [Accessed 14 March 2016]. [15] "Saturated Steam Tables by Pressure," [Online]. Available: http://www.tlv.com/global/TI/calculator/steam-table-pressure.html. [Accessed 15 03 2016]. [16] E.Hugot, "Circulation of Steam and Vapour," in Handbook of Cane Sugar Engineering, Elsevier, 1972, pp. 507-508. [17] E.Hugot, "Juice Circulation," in Handbook of Cane Sugar Engineering, Elsevier, 1972, pp. 524525. [18] "Dimensions, wall thickness and weights of stainless steel pipes according to ASME B36.19 Stainless Steel Pipe," [Online]. Available: http://www.engineeringtoolbox.com/ansi-stainlesssteel-pipes-d_247.html. [Accessed 15 03 2016]. [19] "Insulation materials_engineeering toolbox," [Online]. Available:
91 http://www.engineeringtoolbox.com/polyurethane-insulation-k-values-d_1174.html. [Accessed 15 03 2016]. [20] "Stainless Steel," AK Steel. [21] R. Sinnot, "Skirt supports," in Chemical Engineering Design Volume 6, Coulson and Richardson's, pp. 844-852. [22] A. E. Morris, "Resistance Temperature Devices," in Measurement and Instrumentation Principles, 2001, pp. 286-288. [23] B. W. Bequeete, Process Control, modelling and simulation. [24] "Steam Jet Thermocompressor," Spirax Sarco. [25] R.Sinnot, "Hazard and operability studies," in Chemical Engineering Design-Volume 6, Coulson and Richardson, pp. 380-385. [26] "Equipment cost estimates". [27] R.Sinnot, "Capital cost estimation," in Chemical Engineering Design, Coulson and Richardson, pp. 250-260. [28] "Economic indicators". [29] "Density of sugar solutions," [Online]. Available: http://www.sugartech.co.za/density/index.php. [Accessed 24 12 2015].
xii
APPENDIX Appendix A The overall heat transfer co-efficient of a short tube vertical evaporator and a condenser can be calculated using the following two correlations *
( )
( )
( ) +
is the temperature of the boiling liquor inside the evaporator. *
( )
is the temperature inside the condensor
( )
( )
