MODELLING AND SIMULATION OF CONVENTIONAL DISTILLATION, REACTIVE DISTILLATION COLUMN AND DIVIDED WALL DISTILLATION COLUMN FOR THE PRODUCTION OF METHYL-TERT-BUTYL-ETHER (MTBE) USING ASPEN PLUS Sanjay Singh2 , Hema Jha1 , Tanushree1 , Uma Babode1 and Lekhraj Malviya1 1
Department of Chemical Engineering, Laxmi Narain College of Technology, Bhopal, India E-mail:
[email protected]
Department of Chemical Engineering, Maulana Azad National Institute of India, Bhopal, India E-mail:
[email protected] Abstract ID: MS-069 Abstract
Distillation is one of the most important separation processes. Divided wall distillation column (DWC) and reactive distillation column (RDC) are two such technologies which have brought about a drastic change in the production quantity and quality and improved the energy efficiency and cost redundancy. The development of simulation technology is the main reason that these technologies evolved and got utilized in industries. ASPEN Plus is one such modeling and simulation package that is used for optimizing any unit operation. In this study ASPEN Plus was used to optimize the production of MTBE using CD, RDC and DWC technologies in terms of different variables. This study led to the conclusion that the production of MTBE was found to be the best in DWC and the heat required for heating purpose was much less which was -40 mm kcal/h while in case of CD and RDC, it was -47 and -44 mm kcal/ h at the stage number of 45.The purity of the product was best seen in DWC column which was found to be 98% and also the number of stages required was less as compared to the RDC and conventional column. The heat load on re-boiler was 230000kW in case of DWC column which was less than other two. Thus DWC is more efficient in terms of energy consumption and require less number of stages and this leads to reduced operating costs and higher product quality. KEYWORDS: ASPEN Plus, Simulation, Distillation column.
1. Introduction For the study of simulation of a process which includes distillation process, here we have chosen MTBE production technique. MTBE is an organic compound with molecular formula (CH3)3COCH3 and IUPAC nomenclature 2-methoxy 2-methyl propane. It is a volatile flammable and colorless liquid that is sparingly soluble in water. It is a gasoline additive used as an oxygenate to raise the octane number.
CHEMCON2015"ChemicalEngineering:FromLaboratoryToIndustr y"
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MTBE is manufactured via the chemical reaction of methanol and isobutylene. Methanol is derived from natural gas and isobutylene is deriving from butane obtained from crude oil or natural gas. MTBE is mostly used as a fuel for gasoline engines. It is one of a group of chemicals commonly known as oxygenates because they raise the oxygen content of gasoline. Oxygenates help gasoline burn more completely reducing tailpipe emissions from motor vehicles, dilutes or displaces gasoline components such as aromatics and sulphur and optimize the oxidation during combustion. This paper is study of simulation of different distillation column which includes conventional distillation column, reactive distillation column and divided wall distillation column for production of MTBE with the help of ASPEN Plus Software. In reactive distillation column, both reaction and separation takes place simultaneously while Divided Wall Distillation Column has the capacity of separating more than one component in a single distillation column. ASPEN PLUS software helps in extending steady-state simulation to dynamic simulation for safety and controllability studies. Properties analyses which are required during simulation are properties of pure components and mixtures that are enthalpy, density viscosity, heat capacity etc are done by ASPEN. ASPEN PLUS software is employed for mass and energy balances, physical chemistry, chemical engineering thermodynamics, chemical reaction engineering, unit operations, process design and process control. It uses a mathematical model to predict the performance of the process. These accurate modeling of thermodynamic properties are particularly important in the separation of non-ideal mixtures and ASPEN PLUS has a large data of regressed parameters. Overall it can simulate any of the unit operation. 2. Effective Distillation Columns with Aspen Implication A. Conventional Distillation Column CDCs are the general fractionating columns which work on the basis of different boiling point of different components. It consumes more energy and an inefficient way for the separation comparatively. ASPEN modeling for this is shown in fig.1.
Figure 1: Conventional Route for the Production of MTBE Using ASPEN PLUS Software
B. REACTIVE DISTILLATION COLUMN In this a lot of numerical problems are arisen in the modeling, design and optimization of the RDC which results into simpler and intensified processes with less recycle streams and decreasing waste handling and consequently lower investments and operating costs. RDC offers higher reaction rate and selectivity; prevent the performance of zoetrope’s, less energy consumption and solvent usage. In spite of all these advantages, the RDC has limited commercial applications; it is because of the control performance and
Figure 2 Flowsheet for the Production of MTBE using ASPEN PLUS Software in RDC the complexity in the operation of the RDC. For modeling we have assumed that it operates in adiabatic condition with liquid phase. There is no vapor hold up in any stage of the DC. For the simplification of the modeling complexities, there are no hydrodynamic effects in the modeling work. C. DIVIDED WALL DISTILLATION COLUMN It has achieved a greater attention in the chemical industry for the separation process and saves both energy and capital. The DWC technology is not confined to ternary separation only but it can also carry out azeotropic separations and reactive distillation. The feasibility of the DWC in the industry depends upon the thermodynamic properties, composition of the stream to be separated and the product requirements. DWC is more advantageous in those cases where the composition of one of the component is 60% to 70%.
Figure 3. Flowsheet for the production of MTBE Using ASPEN PLUS in DWC.
3. FEED SPECIFICATION Methanol Table 1: Input Specifications of Methanol Feed Temp
Pressure
Total Flow Rate
Mole Fraction
320K
1atm
711.3
0.64
Isobutylene Table 2: Input Specifications of Iso Butylene Feed Temp
Pressure
Total Flow Rate
Mole Fraction
350K
1atm
1965.8
0.36
Pump
1.Discharge Pressure – 11.7atm
2. Control Volume 1
3.Outlet Pressure – 11.5atm
4.Valid Phases – Liquid Only
Compressor
1. Type- Isentropic
2. Discharge Pressure- 11.5atm
3. Control Volume 2
4. Outlet Pressure -10.8atm
5. Valid Phases- Liquid only
6. Control Volume 3
7. Outlet Pressure-11.3atm
8. Valid Phases- Liquid only
Distillation Column Specification
1. Calculation Type- Equilibrium
2. Number of stages- 17
3. Condenser Type- Total
4. Reboiler Type- Kettle
5. Valid Phases- Vapor - Liquid
6. Convergence- Strongly Non-Ideal Liq.
a. Operating conditions 1. Bottom rate- 640.8 Kmol/h
2. Reflux Ratio- 7
b. Stream specifications i. Product stream 1 1. Vapor feed- Stage 11
2. Liquid feed- Stage 10
ii. Product stream 2 1. Distillate- Stage 1
2. Bottom- Stage 17
3. Pr. Stage1 or condenser pr. – 11atm
4. Pr. Drop for rest columns- 0.5atm
Reactions
1. Starting stage- 4
2. Ending stage-16
3. Reaction ID- R1
Sizing and rating specifications a. Packing section
1. Starting stage - 2
2. Ending Stage-16
3. Type- Raschig
b. Packing characteristics 1.Vendor - Raschig
2.Material- Standard
3.Sec. Dia.- 6m
4.HETP-1
5.Dimesional-35mm
c. Basic convergence 1. Algorithms- non-ideal
2. Maximum Iteration-200
Methods
1. Initialization method- standard
2. Damping level – none
3.Liq.-Liq. Phase splitting method-Gibbs
4. Solid Handling – overall
4.Salt precipitation handling - include Stoichiometric specification Reaction name 1 Type - kinetic
Reactants Table 3: Stoichiometry of reactants
Component
Coefficient
Exponent
MeOH
-1
-1
IB
-1
1
Products
Table 4: Stoichiometry of products Component
Coefficient
Exponent
MTBE
1
-
Reaction name 2 Type – kinetic
Reactants Table 5: Stoichiometry of reactants
Component
Coefficient
Exponent
MTBE
-1
1
Products Table 6: Stoichiometry of products
Component
Coefficient
Exponent
MeOH
1
2
IB
1
-
Kinetics MeOH + IB = MTBE Reacting phase – Liquid Power law kinetic expression
If T0 is specified then E 1 1 n RT T T T 0 Kinetic Factor = k e T0
1
If T0 is not specified then Kinetic Factor =
kT n e
E RT
2
Here, K = 3.67e+12 n=0 E = 92440 Basis – mole fraction MTBE = MeOH + IB Power law kinetic factors are same as for the above reaction and only the value of the k, n and E are changed. K = 2.67e+12 N=0 E = 134554 in kj/mol T0 = 0 Basis – mole fraction.
4. RESULTS A. Conventional distillation column Fig. 4 shows the effect of reflux ratio on product quality. At reflux ratio of 5.5, the product quality of MTBE is 68% which is the bottom product.
Figure 4: Product purity with changing reflux ratio for conventional distillation
Fig. 5 shows the effect of reflux ratio on reboiler duty. On increasing the reflux ratio up to 5.5, the heat load on reboiler duty increases upto 2600000 kW. If the reflux ratio is further increased then there is no significant effect of the reflux on reboiler duty is seen.
Figure 5: Reboiler duty with changing reflux ratio for conventional distillation Fig. 6 shows the effect of the number of the stages on the production rate of the product. The bottom product increases at a very slow rate. At the stage of 40, the production rate of the top product is minimum and the bottom product is maximum. Therefore the optimum number of the stage is 40.
Figure 6: Production rate with changing number of stages for conventional distillation B. Reactive distillation column Fig. 7 shows the effect of reflux ratio on product purity. At reflux ratio of 7, the purity of the bottom product increases up to 90%.
Figure 7: Product purity with changing reflux ratio for RDC
Fig. 8 shows effect of reflux ratio on reboiler duty. On increasing the reflux ratio of 5, the reboiler duty becomes 2700000 kW.
Figure 8: Reboiler duty with changing reflux ratio for RDC Fig. 9 shows the effect of the number of stages on the amount of the product. On increasing the number of stages, the production rate of the top product decreases while that of the bottom product increases. On stage number 30, the production of the top product is minimum and the production of the bottom product that is the desired product MTBE is maximum, so the optimum stage is 30.
Figure 9: Production rate with changing number of stages for RDC
C. Divided wall distillation column Fig. 10 shows effect of reflux ratio on product. On increasing reflux ratio, the product quality increases, it is because when the reflux comes in contact with the vapor coming towards the upper portion of the column then the mass transfer between the vapor and the reflux takes place as a result of this the concentration of the vapor to be condensed increases. At total reflux condition it becomes constant.
Figure 10: Product purity with changing reflux ratio for DWC Fig. 11 shows effect of reflux ratio on reboiler duty. In case of DWC column, the load on reboiler increases as the reflux ratio increases.
Figure 11: Reboiler duty with changing reflux ratio for DWC Fig. 12 shows effect of the number of stages on the amount of the product. At the stage number 20, the top product is minimum and the bottom product is maximum. Therefore the optimum number of stage is 20.
Figure 12: Production Rate with Number of Stages for DWC
5. CONCLUSION AND RECOMMENDATIONS Following conclusion were drawn from the simulation study: 1. The product purity increases with increase in the reflux ratio in case of the conventional distillation column, RDC and DWC column. In case of conventional distillation, there was no much effect of reflux ratio on product quality and found to be 60% at 5.5 reflux ratio and 90%and 98% for both RDC and DWC.
2. In case of conventional distillation, load on the re boiler was 2800000 kW at reflux ratio of 5.5 but in case of RDC and DWC column the Reboiler duty was 2500000 kW and 2300000 kW at reflux ratio of 5.5. 3. At the stage of 30, the desired production rate for conventional column was maximum 400kmol/h while for RDC and DWC; it was 600kmol/h and 700kmol/h. Thus the DWC column was better among the other two. 4. The Reboiler duty increases linearly with the number of stages in case of CDC; Reboiler duty is -47 mmkcal/h. While in case of RDC and DWC column, it was -44 and -40 mmkcal/h at the stage of 45. Future work includes the coupling of the heat pump with conventional distillation column, RDC and DWC. Implementation of the divided wall in the reactive distillation column and then simulate these columns for the production of MTBE in terms of product purity with reflux ratio, production rate with number of stages, temperature and pressure change with reflux ratio and number of stages. 6. REFERENCE 1. Taylor R, Krishna R. Modelling Reactive Distillation. Chemical Engineering Science. 55, 5183, 2000. 2. Affadala H.E., Al-Musleh E. Proceedings of the First Annual Gas Processing Symposium, 2009, Quatar. 3. Sangal V.K., Kumar V., Mishra I.M. Optimization of Structural and Operational Variables For Energy Efficiency of a Divided Wall Distillation Column. Computational Chemical Engineering, 40, 33-40, 2012. 4. Kiss A.A., Suszwalak D.J.-P.C. Innovative Di-methyl ether synthesis in a reactive dividing wall column. Computational Chemical Engineering. 38, 74-81, 2012. 5. Ignat R.M., Kiss A.A. Integrated Bioethanol separation and dehydration in a non-extractive DWC. Chem. Eng. Trans. 29, 619-624, 2012. 6. Wu Y.C., Hen-Chia P., Chien I.-L. Critical Assessment of the Energy Saving Potential of an Extractive Dividing Wall Column. Ind. Eng. Chem. Res. 52, 5384-5399, 2013. 7. Sudibyo; Murat, M.N.; Aziz, N., "Simulation studies of Methyl Tert-butyl Ether production in reactive distillation," Control System, Computing and Engineering (ICCSCE), 2011 IEEE International Conference, vol., no., PP.369,374, 25-27 Nov. 2011.
