Prediction of Sedimentation in Glycol-Glycol Heat Exchanger by ...

World Applied Sciences Journal 17 (11): 1405-1415, 2012 ISSN 1818-4952 © IDOSI Publications, 2012

Prediction of Sedimentation in Glycol-Glycol Heat Exchanger by Computational Fluid Dynamics (CFD) 1

Behrouz Raei, 2Davood Abbaspour and 3Farhad Shahrakic

Chemical Engineering Faculty, Mahshahr Branch, Islamic Azad University, Mahshahr 63519, Iran 2 National Iranian Petrochemical Co., Mobin Petrochemical Co., Asalooyeh, Iran 3 Department of Chemical Engineering, Engineering Faculty, Sistan and Baluchestan University, Zahedan, Iran 1

Abstract: In this paper, creation of sedimentation in glycol - glycol heat exchanger of one of the Iran's refinery has been investigated. These kinds of heat exchangers are used for heat recovery from glycol of natural gas dehumidification tower and reduction of re-boiler heat load. According to environmental conditions, effects of sediment on flow in the hot side(shell) and then effect of flow patterns and temperature on heat distribution and heat exchanger efficiency with Fluent software have been investigated finally modeling conditions are compared with given experimental data from refinery. In the CFD numerical calculation, a small deviation was observed with about 10-18% of deviation between experimental and modeling. Both experimental observation and CFD prediction confirm that Sediment consists on the lower half of exchanger. Key words: Heat exchanger

Glycol

Computational fluid dynamics

INTRODUCTION Dehumidification process is one of the main parts of gas sweating process. In this process, water is absorbed from natural gas with absorbent or water adsorbent solvers. This process is done with glycol solvents in absorption towers or contactor towers. Other equipment are includes regenerator, glycol tanks, heat exchanger and some others equipments. In dehumidification process, typically a glycol family includes MEG (Mono Ethylene Glycol), DEG (Di ethylene glycol) and TEG (Tree ethylene glycol) and TREG (Tetra ethylene glycol) is used as water absorbent. However, for natural gas, TEG and DEG are used and more than others [1]. In this article, heat exchanger is used for contract between hot glycol (diluted with water) and cold glycol flow (water-rich) [1]. Since the cool flow must enter the regenerator and heat form re-boiler to lose its water as much as possible. It is also necessary to cool the regenerator hot glycol outlet flow to re-enter to absorption tower [2]. Required heat for increasing the solution temperature in regeneration tower is supplied with re-boiler. Evaporated water is vented from

Modeling

Simulation

regeneration tower in the form vapor. To reduce temperature of regenerated glycol flow, glycol / glycol heat exchanger is used as pre-heater to decrease heat duty in re-boiler [3]. MATERIALS AND METHODS Computational Fluid Dynamics is a theoretical method of computing for predicting and modeling fluid flow and heat transfer that is based on computer simulation. CFD analysis of fluid dynamic is creating a unique advantage in the analysis of complex fluid systems. After solving system equations, all calculated data such as temperature, concentration, speed and fluid pressure in all parts of the system, entered force into levels, level temperature, rate of surfaces heat transfer and storage and other details are presented into graphs for users [4]. Construction and Meshing of Glycol / Glycol in GAMBIT: In this article, a PC computer with 3.2 GHz processing speed profile and the 4GB memory is used for modeling. Also for reduce the computing time, attempts to

Corresponding Author: Behrouz Raei, Chemical Engineering Faculty, Mahshahr Branch, Islamic Azad University, Mahshahr 63519, Iran. Tel: +986522327070.

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Fig. 1: Schematic longitudinal and transverse cross of glycol / glycol exchanger instead of modeling entire exchanger, modeling is done for some parts as a sample and results will be generalized for the other sections. Geometric shape of exchanger is observing in Figure (1). In this article, GAMBIT software is used for preprocessor to generate the geometry and grids [5]. Figures 2, 3 and 4 display chosen elements of the exchanger (with actual scale) that built in GAMBIT program. After the construction of selected models, models meshed in GAMBIT with TGrid elements with Tet / Hybrid elements. Finally, geometric modeling and mesh classification with pre-processor software, each of the models was built into the programs and inputted to FLUNT simulation software.

Finite Volume Method: The general algebraic equations derived from differential equations (material, energy, momentum) for each control volume are solved as below [6, 7]: (

)t +∆t − ( ∆t

)t

∆V +

∑

f f V f A= f

faces

∑ Γ f (∇ )⊥, f A f + S ∆V

faces

(1)

Basic Physical Model: Mass, momentum, turbulence and energy equations are as below [8, 9]. Mass Conservation Equation: ∂ + div( U ) = 0 ∂t

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(2)

World Appl. Sci. J., 17 (11): 1405-1415, 2012

Fig. 2: Primary element of glycol / glycol exchanger

Fig. 3: Middle element of glycol / glycol exchanger

Fig. 4: End element of glycol / glycol exchanger Momentum Conservation Equation: Momentum equation of survival is Navir - Stokes equation that has been partitions in three main directions.  ∂( '2 ) ∂ ( u ' v ') ∂ ( u ' w ')  ∂( u) ∂P  + S mx + div( uU ) =− + div ( .grad .u ) +  − − − ∂t ∂x ∂x ∂y ∂z      ∂( u ' v ') ∂ ( v '2 ) ∂( u ' w ')  ∂ ( v) ∂P  + S my + div ( vU ) =− + div ( .grad .v ) +  − − − ∂t ∂y ∂x ∂y ∂z    

(3)

 ∂ ( u ' w ') ∂ ( w ' v ') ∂ ( w '2 )  ∂ ( w) ∂P  + S mz + div ( wU ) =− + div ( .grad .w) +  − − − ∂t ∂z ∂x ∂y ∂z    

Turbulence Equations: Turbulence equations are used for modeling turbulence fluid flow. These equations explain motion of fluid in the turbulence. In some circumstances when there is turbulence, turbulence equations are solved with momentum equations. Many models are presented to model turbulence. k - model is one of the most prestigious models for turbulence equation that below equations are presented for this model. eff

= + T,

T

 ∂( ) = + div ( kU ) div  ∂t 

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= C eff L

k2

 grad .k  + G − 

(4)

World Appl. Sci. J., 17 (11): 1405-1415, 2012 Table 1: Parameters for modeling System constituent parts

Selected options

Parameters

Shell fluid

Composition Turbulence Composition Turbulence 2 phase

62.3% glycol, 37.7% water Yes 51% glycol, 49% water Yes Phase 1 Phase 2 The mixing model 0.0001 to 0.001 m

Tube fluid Number of phases

Multi-phase model Particles approximately diameter (obtained from refinery)

 ∂( ) U ) div  + div (= ∂t 

eff L

 grad .  + G 2 k 

eff

Eij .Eij − C2

Di ethylene glycol and water. Particles

2

k

Energy Conservation Equation: The energy equation expressed of input, output and accumulation energy in one controlled volume. ∂( E ) ∂ ∂ + (ui ( E + P() = − − ∂t ∂xi ∂xi

∑ h j J j + u j ( ij )eff ] + S

(5)

j

Euler-Lagrange Two-Phase Flow Models: Implementation of CFD technique for simulation of sedimentation was used, where Euler-Lagrange model was used and the inter phase drag force was applied based on non-porous particle model [10]. In addition, conservation equation for particle phase volume fraction was solved in order to simulate batch sedimentation and viscous re-suspension [11]. On the other end, a direct numerical simulation of sedimentation of particle agglomerates by using Immersed Boundary Method, giving a deeper insight into hydrodynamics of agglomerates [12]. Generally, two approaches are possible for modeling two-phase flow in Fluent that are Euler -Lagrange and Euler Euler methods that Euler - Euler method is used in this study. Continuity and momentum equations are solved for Euler model as below: ∂ ( ∂t ∂ ( ∂t

q

 q vq ) + ∇.(

q

 q vq vq ) = −



q q ) + ∇.( q q vq )

q∇ p + ∇.

q+

n

∑

p =1

=

n

∑ m pq

(6)

p =1

  ( R pq + m pq v pq ) +

q

   q ( Fq + Flift , q + Fvm, q )

(7)

Table 1 is sediment formation parameters used for modeling. RESULTS AND DISCUSSION The results of heat exchanger modeling by Fluent are as bellow.

of outlet nozzles, temperature distribution is influenced by higher flow pattern temperature gradient. Figures 7 and 8 indicate more details about heat exchange and temperature profile.

Velocity and Temperature Distribution Input Elements: The initial section of this exchanger shell is containing diluted glycol (hot) outlet nozzle and glycol-rich (cold) tubes that are enter to exchanger from its head. Figure 5 illustrates cross sectional profiles of velocity at centerpiece of exchanger. Qualitative pattern of temperature distribution in the input element is shown in Figure 6. Figure 6 indicates that heat exchange on the right side of baffled (where the hot fluid enters the shell elements) is uniform and temperature gradient in this region is not significant but in the left of baffled and top

Velocity and Temperature Distribution in Middle Exchanger Elements: According to Figure 9 observed that shell hot fluid enters from right and top of baffle and impressed with the fluid flow pattern, exchange temperature with tubes and after passing under the baffled in the middle of the element to the next element. Number of these same elements are 10 that have similar velocity and temperature distribution patterns. Figures 10 and 11 indicate temperature distribution on the external surface of pipes and distribution of temperature in the longitudinal section passing through the element.

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Fig. 5: Velocity distribution in the longitudinal section passing through the element

Fig. 6: 3D temperature distribution throughout the initial element

Fig. 7: Temperature distribution on the external surface of pipes 1409

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Fig. 8: Temperature distribution in the longitudinal section passing through the element

Fig. 9: 3D temperature distribution across the middle element (longitudinal section)

Fig. 10: Temperature distribution on the external surface of pipes 1410

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Fig. 11: Temperature distribution in the longitudinal section passing through the element

Fig. 12: Distribution of velocity in the longitudinal section passing through the element For investigation of temperature distribution pattern in middle element of heat exchanger Figure 12 shows two-dimensional surfaces of the intermediate elements that could generalized to the entire length of the heat exchanger. Velocity and Temperature Distribution in the Final Element of Heat Exchanger: This section includes the head end and the U-shaped tubes. In this model, hot shell fluid entered to the system by a nozzle and the fluids inside the pipe are removed the elements after entering the element and passing the final bending element. Temperature and heat distribution in the three-dimensional model is shown in Figure 13. Figure 13 indicates that hot flows enters shell flow from the nozzle, directed down wards in the vicinity of the baffled and affected by flow movement leave the

element from the left side of the baffle. Convection heat transfer is increased by intensity of turbulent flow; the heat exchange is in charge element at the corners and borders. Figure 14 and 15 shows the temperature distribution on the outer surfaces of pipes and longitudinal section passing through the element. The fluid flow in shell is pushed upward through passing the baffle that fluid velocity between the tubes and shells is more than velocity of fluid at small spaces between the tubes. At the entrance of the nozzle, space between shell, baffle and wall (called baffle window) and the focal point of pipes (where pipes return) velocity has reached its maximum values due to reduction in surface area and compression of the lines. Figure 16 shows velocity distribution in cross section passing through the longitudinal axis.

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Fig. 13: 3D temperature distribution throughout the final element

Fig. 14: Temperature distribution on the outer surfaces of pipes

Fig. 15: Temperature distribution in the longitudinal section passing through the element 1412

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Fig. 16: Distribution of velocity in the longitudinal section passing through the element

Fig. 17: Bar graph of volume of sediment particles in the volume element

Fig. 18: Three-dimensional distribution of sediment over the element 1413

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Fig. 19: Distribution of velocity in the longitudinal section passing through the elements

Fig. 20: Temperature distribution in the longitudinal section passing through the elements Distribution of Sediment in Exchanger: Sediments are consists more on the half lower of elements and four lower rows of pipes and the lowest rate of sedimentation is on the outer surface of higher rows of tubes and the highest amount of sediments are on the inner surface of the shell at the floor which in the lower regions of exchanger elements have almost complete sediment of the flow path at shell. Figure 17 shows histograms (bar) of suspended particles in terms of volume fraction (Phase 2)and figure 18 is shown three-dimensional sediment distribution in entrance elements.

center of the elements (mode cleaning) to near the baffle and the flow path elements is observed on the sediment layers at shell crust. Temperature Distribution in Exchanger: Figure 20 shows temperature distribution which strongly affected by sediment and the velocity distribution which indicates that heterogeneous distribution of temperature and higher temperature gradient is due to resistance of sediment. CONCLUSION

Velocity Distribution in Exchanger: According to Figure 19, the flow pattern due to higher sediment and pressure is change from perpendicular of pipe in the

The predicted of sediment in glycol - glycol heat exchanger by CFD code FLUENT have a good relationship by experimental data and can be used in

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glycol - glycol heat exchanger design for any operating conditions. In the CFD numerical calculation, a small deviation was observed with about 10-18% of deviation between experimental and modeling. Both experimental observation and CFD prediction confirm that Sediment consists on the lower half of exchanger. This research illustrates that the sediment pattern depend on interaction between the exchanger surface and fluid. ACKNOWLEDGMENT This paper is a part of research which was supported financially by the research department of Islamic Azad University, Mahshahr branch. REFERENCES 1. 2. 3.

4.

Kohl, A. and R. Nielson, 2001. Gas Purification, Gulf Publishing, Fifth Edition, pp: 967-971. GPSA, 1987. GPSA Engineering Data Book, dehydration and treating, Chapter 20, (Tenth Edition). Ludwig, E.E., 1999. Applied process design for chemical and petrochemical plants, (Third Edition), Gulf Publishing Co., Huston, 2: 108-201. Vial, C., S. Poncin, G. Wild and N. Midoux, 2001. A simple method for regime identification and flow characterization, Chemical Engineering and Processing., 40: 135-151.

5.

FLUENT 6.1., Reading and Manipulating Grids. Manual Guide, Chapter 5, FLUENT International, (2006). 6. Patankar, S.V., 1980. Numerical heat transfer and fluid flow. Hemisphere Publishing Corporation, Taylor and Francis Group, New York, pp: 11-23. 7. Lomax, H., T.H. Pulliam and D.W. Zingg, 1999. Fundamentals of Computational Fluid Dynamics, pp: 71-83. 8. Joshi, J.B., 2001. Computational flow modeling and design of bubble column reactors, Chemical Engineering Sci., 65: 5893-5933. 9. Issa, R.I., A.D. Gosman and A.P. Watkins, 1986. The computational of compressible and incompressible recirculating flows, J. Comput. Phys., 62: 40-65. 10. Latsa, M., D. Assimacopoulos, A. Stamou and N. Markatos, 1999. Two phase modeling of batch sedimentation, Appl. Math. Model., 23: 881-897. 11. Rao, R., L. Mondy, A. Sun and S. Altobelli, 2002. A numerical and experimental study of batch sedimentation and viscous resuspension, Int. J. Num. Meth. Fluids., 39: 465-483. 12. Takeuchi, S., I. Morita and T. Kajishima, 2008. Motion of particle agglomerate involving inter particle force in dilute suspension, Powder Technol., 184: 232-240.

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