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Multiphase CFD simulation of an F-T airlift external loop slurry reactor
I.A. Karimi and Rajagopalan Srinivasan (Editors), Proceedings of the 11th International Symposium on Process Systems Engineering, 15-19 July 2012, Singapore. © 2012 Elsevier B.V. All rights reserved.
Multiphase CFD simulation of an F-T airlift external loop slurry reactor Zhenxing ZhuˈJie YangˈQing Bian SINOPEC Research Institute of Petroleum Processing ˈNo.18, Xueyuan Road, Haidian District, Beijing, 100083ˈChina
Abstract F-T Synthesis is a hot spot in the development of new resource. Because of its simple structure and effective heat removal, Slurry bed becomes a most promising technology for F-T Synthesis. But because of its complex hydrodynamics, the reactor is difficult to design and enlarge. Based on the airlift external-loop slurry reactor’s cold model, CFD simulations were taken to optimize the gas distributor in a gas-liquid system. Then a study on a gas-liquid-solid system was carried out to observe the flow and distribution of each phase. Keywords: CFD, F-T Synthesis, Slurry bed, Multiphase Main Text
1. 1. Introduction As a route converting synthesis gas to liquid fuels or chemical feedstocks, FischerTropsch synthesis (FT Synthesis) has been an interesting topic in reactor design as well as process scale-up.[1] Understanding the flowing status in the designed reactor is of great importance for the design of a specified F-T process because of the complex multiphase flow in FT Synthesis reactor. The flowing status may dramatically change with operation condition and reactor structure.[2] It is well-known that the flow field (such as velocity, pressure, volume fraction of each phase) is closely related to the type of reactor. There are many types of reactors can be used in F-T Synthesis process, such as fixed bed reactor, fluidized bed reactor and slurry bed reactor, among which slurry bed reactor is superior to other beds in many fields, such as heat transfer performance, investment and product yield. Because of the simple structure and effective ability of heat removal, the airlift external-loop slurry bed reactor has been regarded as the most promising technology for F-T synthesis. The flow in an airlift external-loop slurry bed reactor is quite complicated, because there are three phases inside it, namely gas, liquid and solid. In such fluidization system, bubble dynamics plays a key role in dictating the transport phenomena and ultimately affects the overall rates of reactions. It has been recognized that the bubble wake, when it is present, is the dominant factor governing the system hydrodynamics (Fan and Tsuchiya, 1990).[3] A proper holdup and volume fraction distribution of gas, which is mostly depend on the structure of gas distributor, may be quite important to the process. On the other hand, the existence of solid may be an intractable influence on flowing behavior inside reactor. Unfortunately, the solid motion can not be described directly from experiments because of the lack of measuring instruments. There are various approaches to the mathematical and physical modeling of multiphase flows. The most widely used methods for CFD are the Eulerian-Eulerian[4] and the Eulerian-Lagrangian approach[5]. In the Eulerian method, both the continuous and dispersed phase(s) are mathematically modeled as interpenetrating fluids, represented by sets of mass, momentum and energy balances. In the Lagrangian approach, a large
Multiphase CFD simulation of an F-T airlift external loop slurry reactor
601
number of particles are tracked individually, while the liquid phase is treated as a continuum. The interaction between the particles and the liquid shows up as a source term in the momentum equations. The advantage of the Eulerian method above the Lagrangian way is the void fraction of the dispersed phase is high. While the computational time in the latter approach depends highly on the number of particle trajectories to be calculated, in the Eulerian method the number of equations to be solved remains the same. Interaction terms describing drag, virtual mass and elects of lift forces appear in momentum balances of both phases.[6] Turbulence modeling in three-phase flow is done using the same approach as in single-phase flow. Two CFD causes using Eulerian method was involved in this article, a simulation of gas-slurry system to optimize the gas distributor and a simulation of gas-liquid-solid system to gain the flowing field and volume fraction of each phase. Both of them was based on and had been verified with a cold model test.
2. 2. Setup and CFD-Approach In this paper, an Eulerian-Eulerian approach is presented to study the flow in a threephase external-loop airlift slurry bed reactor. The reactor is shown schematically in Fig. 1. The airlift consists of a riser that is a cylinder tower (H=3000mm, D=280mm) filled with water. A circulating tube (H=2000mm, D=70mm) create a down comer beside the riser. Gas is injected via a distributor on the bottom of the riser. At first, the system is treated as a pseudo-two-phase: the carrier-phase contains liquid with small solid particles, which is modeled as a pseudo single-phase to reduce the calculating time. The gas enters directly into the riser (see Fig. 1) as a dispersed phase. A Reynolds stress models, including source terms due to coupling of the phases, is used for the turbulence and solved simultaneously with the mass and momentum balances. The turbulent kinetic energy and dissipation rate of the dispersed phase are obtained by algebraic relation and are functions of turbulent kinetic energy and dissipation rate of the continuous phase. In the present simulations, interest is merely in steady-state solutions of the two-fluid formulations, throw which the gas distributor may be optimized and the velocity distribution of each phase can be obtained. Then, a gas-liquid-solid system can be simulated by using the information of the two-phase calculation.
Fig. 1 schematic diagram of external-loop airlift slurry bed reactor
602
Zhenxing Zhu et al.
3. 3. Optimization of gas distribution Through long term experiments, the gas should be better injected down into the reactor. The diameter and the open porosity of gas holes on the distributor were considered in this section. When the average volume fraction of solid is 0.2, the superficial velocity of gas is 0.144m/s, the diameter of gas hole is 2mm, the holdup and velocity distribution of gas at a horizontal plane which is 619mm above the distributor with deferent open porosity can be obtained (see Fig. 2). Under the same conditions, the holdup and velocity distribution of gas at the plane with deferent diameter of gas holes can be obtained by fixing the open porosity on 0.2 (see Fig. 3).
φ=0.18% φ=0.20% φ=0.24%
d0=2mm d0=3mm d0=4mm
(a) gas velocity
φ=0.18% φ=0.20% φ=0.24%
(a) gas velocity
d0=2mm d0=3mm d0=4mm
(b) holdup
(b) holdup
Fig. 2 Velocity and holdup distribution of gas with deferent open porosity
Fig. 3 Velocity and holdup distribution of gas with deferent diameter
It can be found from Fig. 2 and Fig. 3 that gas was distributed uniformly, when the open porosity was 0.2 and diameter is 2mm. It can be inferred that the slurry was also distributed uniformly under the same conditions. Furthermore, the variance of the flowing field of each phase at a horizontal plane which is 619mm above the distributor with deferent open porosity and deferent diameter of gas holes was involved to verify the conclusions (see Table 1 and Table 2). When the open porosity was 0.2 and diameter was 2mm, the variance of gas velocity, slurry velocity and holdup was minimum, which indicated that the most uniform flow was provided. Table 1 The variance of the flowing field with deferent open porosity Open porosity of gas holes Gas velocity Slurry velocity Holdup
φ=0.18% 4.78E-03 1.89E-02 2.81E-03
φ=0.20% 3.14E-03 3.25E-03 2.83E-03
φ=0.24% 3.71E-03 6.42E-03 2.88E-03
Multiphase CFD simulation of an F-T airlift external loop slurry reactor
603
Table 2 The variance of the flowing field with deferent diameter Diameter of gas holes Gas velocity Slurry velocity Holdup
d0=2mm 3.14E-03 3.25E-03 2.83E-03
d0=3mm 6.53E-03 1.32E-02 3.40E-03
d0=4mm 9.03E-03 4.25E-03 5.59E-03
4. 4. Gas-liquid-solid CFD simulation To describe the flow of each phase in details, a gas-liquid-solid CFD simulation was carried out with the results of former section, for example, the gas velocity via holes. The volume fraction of each phase in the reactor can be calculated (see Fig. 4), when the average volume fraction of solid is 0.1, the superficial velocity of gas is 0.054m/s.
˄a˅ ˅Holdup in ZX plane
˄b˅ ˅Holdup in ZY plane
˄c˅ ˅Volume fraction of liquid in ZX plane
˄d˅ ˅Volume fraction of liquid in ZY plane
˄e˅ ˅Volume fraction of solid in ZX plane ˄f˅ ˅Volume fraction of solid in ZY plane Fig. 4 The volume fraction distribution of each phase in reactor
604
Zhenxing Zhu et al.
$YHUDJHKROGXSRQWKH UDGLDOSODQH
It can be seen that the volume fraction distribution of solid and liquid showed a good agreement, a slurry had been formed by these two phases. All of the three phases distributed symmetrically in the view of ZY plane, by which the effects of gas distributor could be confirmed.
$[LDOKHLJKWRIUHDFWRUP &)'UHVXOWV
([SULPHQWUHVXOWV
Fig. 5 Average holdup along the axis of reactor The average gas holdup of radial planes along the axis of reactor was involved to verify the simulation through the results of a cold model test (see Fig. 5). There is a good agreement between simulation and experiment, by which the simulations was proved to be correct. The tiny errors may be cased by neglecting the Coalescence and breakup of bubbles, or gas expansion in reactor.
5. 5. Conclusion An Eulerian-Eulerian approach was presented to simulate the flow in a multiphase external-loop airlift slurry bed reactor. The gas distributor inside the reactor was optimized by a two-phase simulation. Then, a gas-liquid-solid CFD simulation was performed to observe the flow inside the reactor extensively. The simulation was reasonable through a comparison of the calculating results and the experimental results. Many useful information can be provided from the CFD simulations, according to which, the further researches on the design and scale up of F-T slurry beds must be stepped deeply.
References [1] R.Krishna, J. M. V.Baten, M. I.Urseanu, J.Ellenberger, 2001, Design and scale up of a bubble column slurry reactor for Fischer-Tropsch synthesis. Chem. Eng.Sci. 56, 537. [2] B. H. Davis, 2002, Overview of reactors for liquid phase Fischer-Tropsch synthesis. Catal. Today 71, 249. [3] L.S. Fan, K. Tsuchiya, , 1990. Bubble Wake Dynamics in Liquids and Liquid–Solid Suspensions. Butterworth-Heinemann, Stoneham, MA. [4] A. Sokolichin, , G. Eigenberger, 1999. Application of the standard k-ε model to the dynamic solution of bubble columns. Chemical Engineering Science, 54, 2273-2284. [5] S. Lain, S. D. Broder, M. Sommerfeld, 1999. Experimental and numerical studies of the hydrodynamics in a bubble column. Chemical Engineering Science, 54, 4913-4920. [6] R. T. Jr. Lahey, D. A. Drew, 1992. On the development of multidimensional two-fluid models for vapor/liquid two-phase flows. Chemical Engineering Committee, 118, 125-139.
Multiphase CFD simulation of an F-T airlift external loop slurry reactor Zhenxing ZhuˈJie YangˈQing Bian SINOPEC Research Institute of Petroleum Processing ˈNo.18, Xueyuan Road, Haidian District, Beijing, 100083ˈChina
Abstract F-T Synthesis is a hot spot in the development of new resource. Because of its simple structure and effective heat removal, Slurry bed becomes a most promising technology for F-T Synthesis. But because of its complex hydrodynamics, the reactor is difficult to design and enlarge. Based on the airlift external-loop slurry reactor’s cold model, CFD simulations were taken to optimize the gas distributor in a gas-liquid system. Then a study on a gas-liquid-solid system was carried out to observe the flow and distribution of each phase. Keywords: CFD, F-T Synthesis, Slurry bed, Multiphase Main Text
1. 1. Introduction As a route converting synthesis gas to liquid fuels or chemical feedstocks, FischerTropsch synthesis (FT Synthesis) has been an interesting topic in reactor design as well as process scale-up.[1] Understanding the flowing status in the designed reactor is of great importance for the design of a specified F-T process because of the complex multiphase flow in FT Synthesis reactor. The flowing status may dramatically change with operation condition and reactor structure.[2] It is well-known that the flow field (such as velocity, pressure, volume fraction of each phase) is closely related to the type of reactor. There are many types of reactors can be used in F-T Synthesis process, such as fixed bed reactor, fluidized bed reactor and slurry bed reactor, among which slurry bed reactor is superior to other beds in many fields, such as heat transfer performance, investment and product yield. Because of the simple structure and effective ability of heat removal, the airlift external-loop slurry bed reactor has been regarded as the most promising technology for F-T synthesis. The flow in an airlift external-loop slurry bed reactor is quite complicated, because there are three phases inside it, namely gas, liquid and solid. In such fluidization system, bubble dynamics plays a key role in dictating the transport phenomena and ultimately affects the overall rates of reactions. It has been recognized that the bubble wake, when it is present, is the dominant factor governing the system hydrodynamics (Fan and Tsuchiya, 1990).[3] A proper holdup and volume fraction distribution of gas, which is mostly depend on the structure of gas distributor, may be quite important to the process. On the other hand, the existence of solid may be an intractable influence on flowing behavior inside reactor. Unfortunately, the solid motion can not be described directly from experiments because of the lack of measuring instruments. There are various approaches to the mathematical and physical modeling of multiphase flows. The most widely used methods for CFD are the Eulerian-Eulerian[4] and the Eulerian-Lagrangian approach[5]. In the Eulerian method, both the continuous and dispersed phase(s) are mathematically modeled as interpenetrating fluids, represented by sets of mass, momentum and energy balances. In the Lagrangian approach, a large
Multiphase CFD simulation of an F-T airlift external loop slurry reactor
601
number of particles are tracked individually, while the liquid phase is treated as a continuum. The interaction between the particles and the liquid shows up as a source term in the momentum equations. The advantage of the Eulerian method above the Lagrangian way is the void fraction of the dispersed phase is high. While the computational time in the latter approach depends highly on the number of particle trajectories to be calculated, in the Eulerian method the number of equations to be solved remains the same. Interaction terms describing drag, virtual mass and elects of lift forces appear in momentum balances of both phases.[6] Turbulence modeling in three-phase flow is done using the same approach as in single-phase flow. Two CFD causes using Eulerian method was involved in this article, a simulation of gas-slurry system to optimize the gas distributor and a simulation of gas-liquid-solid system to gain the flowing field and volume fraction of each phase. Both of them was based on and had been verified with a cold model test.
2. 2. Setup and CFD-Approach In this paper, an Eulerian-Eulerian approach is presented to study the flow in a threephase external-loop airlift slurry bed reactor. The reactor is shown schematically in Fig. 1. The airlift consists of a riser that is a cylinder tower (H=3000mm, D=280mm) filled with water. A circulating tube (H=2000mm, D=70mm) create a down comer beside the riser. Gas is injected via a distributor on the bottom of the riser. At first, the system is treated as a pseudo-two-phase: the carrier-phase contains liquid with small solid particles, which is modeled as a pseudo single-phase to reduce the calculating time. The gas enters directly into the riser (see Fig. 1) as a dispersed phase. A Reynolds stress models, including source terms due to coupling of the phases, is used for the turbulence and solved simultaneously with the mass and momentum balances. The turbulent kinetic energy and dissipation rate of the dispersed phase are obtained by algebraic relation and are functions of turbulent kinetic energy and dissipation rate of the continuous phase. In the present simulations, interest is merely in steady-state solutions of the two-fluid formulations, throw which the gas distributor may be optimized and the velocity distribution of each phase can be obtained. Then, a gas-liquid-solid system can be simulated by using the information of the two-phase calculation.
Fig. 1 schematic diagram of external-loop airlift slurry bed reactor
602
Zhenxing Zhu et al.
3. 3. Optimization of gas distribution Through long term experiments, the gas should be better injected down into the reactor. The diameter and the open porosity of gas holes on the distributor were considered in this section. When the average volume fraction of solid is 0.2, the superficial velocity of gas is 0.144m/s, the diameter of gas hole is 2mm, the holdup and velocity distribution of gas at a horizontal plane which is 619mm above the distributor with deferent open porosity can be obtained (see Fig. 2). Under the same conditions, the holdup and velocity distribution of gas at the plane with deferent diameter of gas holes can be obtained by fixing the open porosity on 0.2 (see Fig. 3).
φ=0.18% φ=0.20% φ=0.24%
d0=2mm d0=3mm d0=4mm
(a) gas velocity
φ=0.18% φ=0.20% φ=0.24%
(a) gas velocity
d0=2mm d0=3mm d0=4mm
(b) holdup
(b) holdup
Fig. 2 Velocity and holdup distribution of gas with deferent open porosity
Fig. 3 Velocity and holdup distribution of gas with deferent diameter
It can be found from Fig. 2 and Fig. 3 that gas was distributed uniformly, when the open porosity was 0.2 and diameter is 2mm. It can be inferred that the slurry was also distributed uniformly under the same conditions. Furthermore, the variance of the flowing field of each phase at a horizontal plane which is 619mm above the distributor with deferent open porosity and deferent diameter of gas holes was involved to verify the conclusions (see Table 1 and Table 2). When the open porosity was 0.2 and diameter was 2mm, the variance of gas velocity, slurry velocity and holdup was minimum, which indicated that the most uniform flow was provided. Table 1 The variance of the flowing field with deferent open porosity Open porosity of gas holes Gas velocity Slurry velocity Holdup
φ=0.18% 4.78E-03 1.89E-02 2.81E-03
φ=0.20% 3.14E-03 3.25E-03 2.83E-03
φ=0.24% 3.71E-03 6.42E-03 2.88E-03
Multiphase CFD simulation of an F-T airlift external loop slurry reactor
603
Table 2 The variance of the flowing field with deferent diameter Diameter of gas holes Gas velocity Slurry velocity Holdup
d0=2mm 3.14E-03 3.25E-03 2.83E-03
d0=3mm 6.53E-03 1.32E-02 3.40E-03
d0=4mm 9.03E-03 4.25E-03 5.59E-03
4. 4. Gas-liquid-solid CFD simulation To describe the flow of each phase in details, a gas-liquid-solid CFD simulation was carried out with the results of former section, for example, the gas velocity via holes. The volume fraction of each phase in the reactor can be calculated (see Fig. 4), when the average volume fraction of solid is 0.1, the superficial velocity of gas is 0.054m/s.
˄a˅ ˅Holdup in ZX plane
˄b˅ ˅Holdup in ZY plane
˄c˅ ˅Volume fraction of liquid in ZX plane
˄d˅ ˅Volume fraction of liquid in ZY plane
˄e˅ ˅Volume fraction of solid in ZX plane ˄f˅ ˅Volume fraction of solid in ZY plane Fig. 4 The volume fraction distribution of each phase in reactor
604
Zhenxing Zhu et al.
$YHUDJHKROGXSRQWKH UDGLDOSODQH
It can be seen that the volume fraction distribution of solid and liquid showed a good agreement, a slurry had been formed by these two phases. All of the three phases distributed symmetrically in the view of ZY plane, by which the effects of gas distributor could be confirmed.
$[LDOKHLJKWRIUHDFWRUP &)'UHVXOWV
([SULPHQWUHVXOWV
Fig. 5 Average holdup along the axis of reactor The average gas holdup of radial planes along the axis of reactor was involved to verify the simulation through the results of a cold model test (see Fig. 5). There is a good agreement between simulation and experiment, by which the simulations was proved to be correct. The tiny errors may be cased by neglecting the Coalescence and breakup of bubbles, or gas expansion in reactor.
5. 5. Conclusion An Eulerian-Eulerian approach was presented to simulate the flow in a multiphase external-loop airlift slurry bed reactor. The gas distributor inside the reactor was optimized by a two-phase simulation. Then, a gas-liquid-solid CFD simulation was performed to observe the flow inside the reactor extensively. The simulation was reasonable through a comparison of the calculating results and the experimental results. Many useful information can be provided from the CFD simulations, according to which, the further researches on the design and scale up of F-T slurry beds must be stepped deeply.
References [1] R.Krishna, J. M. V.Baten, M. I.Urseanu, J.Ellenberger, 2001, Design and scale up of a bubble column slurry reactor for Fischer-Tropsch synthesis. Chem. Eng.Sci. 56, 537. [2] B. H. Davis, 2002, Overview of reactors for liquid phase Fischer-Tropsch synthesis. Catal. Today 71, 249. [3] L.S. Fan, K. Tsuchiya, , 1990. Bubble Wake Dynamics in Liquids and Liquid–Solid Suspensions. Butterworth-Heinemann, Stoneham, MA. [4] A. Sokolichin, , G. Eigenberger, 1999. Application of the standard k-ε model to the dynamic solution of bubble columns. Chemical Engineering Science, 54, 2273-2284. [5] S. Lain, S. D. Broder, M. Sommerfeld, 1999. Experimental and numerical studies of the hydrodynamics in a bubble column. Chemical Engineering Science, 54, 4913-4920. [6] R. T. Jr. Lahey, D. A. Drew, 1992. On the development of multidimensional two-fluid models for vapor/liquid two-phase flows. Chemical Engineering Committee, 118, 125-139.