Hydrodynamics and Thermal Characteristics of Reactor Modified by LPD Static Mixers

Transcription

1 2012 4th International Conference on Chemical, Biological and Environmental Engineering IPCBEE vol.43 (2012) (2012) IACSIT Press, Singapore DOI: /IPCBEE V Hydrodynamics and Thermal Characteristics of Reactor Modified by LPD Static Mixers Prayut Jiamrittiwong 1, Karn Pana-Suppamassadu 1, Phavanee Narataruksa 1, Sabaithip Tungkamani 2 and Nuwong Chollacoop 3 1 Department of Chemical Engineering, King Mongkut s University of Technology North Bangkok, 1518 Pibulsongkram Rd. Bangsue Bangkok,10800, Thailand 2 Department of Industrial Chemistry, King Mongkut s University of Technology North Bangkok, 1518 Pibulsongkram Rd. Bangsue Bangkok, 10800, Thailand 3 National Metal and Materials Technology Center, National Science and Technology Development Agency Ministry of Science and Technology, Klong Luang, Pathumthani, Thailand Abstract. Generally, an application of packed-bed reactor in Gas-to-Liquid or Biomass-to-Liquid technologies to produce liquid fuels can cause unfavorable effects e.g. high pressure drop, flow and temperature maldistributions, and deactivation of metal catalyst. Thus, major focus of this work was to adopt a computational approach (FEM) in order to characterize the hydrodynamics and involved heat transfers within a packed-bed reactor modified by Low-Pressure-Drop (LPD) static mixers in comparison with a conventional one. Since the liquid fuel production via Fischer Tropsch Synthesis (FTS) by feeding syngas i.e. CO and H 2 through the catalyst bed caused the temperature rise due to an exothermic enthalpy, the temperature of gas tended to be increased and so needed to be controlled by proper heat transfers. The simulated flow field indicated that the modified reactor could achieve better distribution of the gases velocity and temperature field. For the selected LPD static mixers, the flow fields were divided by the static mixers and the radial and tangential velocity components were induced. On the other hand, for the conventional packed-bed, the study showed that the syngas flew mainly in the axial direction and the flow maldistribution was observed in the vicinity of reactor wall. With static mixers, the temperature distributions related with the velocity field were also strongly influenced by the configuration of the static mixers. Furthermore, the better heat transfer could be achieved for the reactor installed with the static mixers, and the convective heat transfer coefficients were definitely higher. Keywords: Packed-Bed, Low-Pressure-Drop Static Mixer, Fischer Tropsch Synthesis 1. Introduction Fischer Tropsch Synthesis (FTS) is a series of hydrocarbon polymerization that converts syngas i.e. a mixture of carbon monoxide and hydrogen into long chain liquid hydrocarbons, and a packed-bed reactor is usually utilized [1]. The target liquid hydrocarbon fuel depends on the type of catalyst used e.g. bi-functional catalysts promote the yield and quality of the gasoline from FTS [2]. Calleja et al. [3] investigated the effects of process variables i.e., temperature, space velocity, CO:H 2 feed ratio and pressure on the activity of a Co/HZSM5 zeolite bifunctional catalyst. They reported that their catalyst exhibited higher selectivity and yield as compared with conventional catalysts. Besides the mentioned process variables, the type and configuration of the reactor used in FTS were also reported to affect the product liquid fuel. Since the hydrodynamics and heat and mass transfers played a key role in a performance of the FT reactors in many scales, for the last decade, computational fluid dynamics or CFD technique had been widely used as a powerful tool try to comprehend the complex transport phenomena even in the long standing fixedbed reactor [4-6]. So far, both the one- and two-dimensional heterogeneous models were adopted in order to investigate steady and unsteady behaviors of a fixed bed FTS reactor and report about the impact of feed 62

2 temperature, flow rate and the wall temperature etc [7-10]. One possibility to improve flow pattern of effluent gas was to use a static mixer to promote mixing pattern by successively dividing, rotating and redirecting the gas while keeping a pressure drop quite low [11]. It was also possible that this type of reactor might be a novel design to suppress a hot-spot formation in heterogeneous solid-gas or catalytic viscous liquid-phase reactions [12]. The present study aims to further investigate the hydrodynamics and heat transfer of gaseous phase within the packed-bed reactors with and without the low-pressure-drop static mixers by using the three dimensional CFD models. 2. Simulation Model and Methodology The reactor vessel and static mixers used in the simulations adopted from part of the on-going BTL research carried by the Research and Development Center for Chemical Engineering Unit Operations and Catalyst (RCC) at King Mongkut s University of Technology North Bangkok. The reactor of diameter approximately of 0.8 cm might be installed with static LPD mixers. As shown in Fig. 1 and 2, the single element of the static mixer was represented by a half-circle stainless steel sheet with 0.1-cm thickness and 0.8-cm diameter. The configurations of the mixers were arranged by varying pitch between elements. For the LPD1 and LPD2, the pitch was 1.2 cm and 0.6 cm, respectively. (a) (b) (c) Fig. 1: Reactor vessel: (a) without static mixer (b) with LPD1 static mixer (c) LPD2 static mixer Fig. 2: Structure of catalyst bed packed in the reactor. The catalyst pellets were represented by spheres of diameter of 600 μm and they were arranged in a tetrahedral configuration as packed-bed. With the same amount of catalyst packed, the total bed heights were slightly different among each bed. The resultant velocity and temperature fields in the packed-beds were obtained by applying the Finite Element schemes (COMSOL MULTIPHYSICS version 3.5a) to the three dimensional models. The hydrodynamics and heat transfer of the gases phase were governed by the incompressible Navier-Stokes equation coupled with the convection-conduction equation as indicated in Eq. (1) and (2), respectively: 63

3 u T ρ η( u + ( u) ) + ρ( u ) u + p = 0 t u = 0 (1) where η = dynamic viscosity, u = velocity vector, ρ = density of mixed gas, and p = pressure of mixed gas T ρ C p ( + u T ) = ( k T ) + Q (2) t where T = temperature of mixed gas, C = heat capacity of mixed gas, k = thermal conductivity of mixed p gas, and Q = heat source. The governing equations were discretized and solved numerically to provide the results in terms of the hydrodynamics and thermal parameters. In momentum subdomain setting, the mixed gas between CO and H 2 (at the operating temperature of 180 C) was assigned to the effective density of kg/m 3 and viscosity of 1.5x10-5 kg/m s. The mixed gas was fed uniformly into the catalyst bed with the volumetric flow rate of 40 ml/min (0.013 m/s). In thermal subdomain setting, the mixed gas was assigned to the effective thermal conductivity of W/m K and heat capacity of 1,186 J/kg K. In the simulation, the enthalpy of exothermic FT reaction was represented in term of the equivalent functions of temperature, and applied as one of the thermal boundary conditions. For comparison and design purposes, the velocity and temperature profiles were determined for both the conventional and modified catalyst bed under the operating condition of inlet velocity of m/s and heat removal flux at wall of 17,000 W/m 2 controlling the temperature within beds. 3. Simulation Result and Discussion The flow structures of mixed gas as a result of the interaction with the packed-beds were shown in Fig. 3. From Fig. 3(a), the mixed gas flew apparently in the axial direction through voids between catalyst pellets as indicated by the velocity vectors (red arrows). The gas velocity was higher in the region close to the reactor wall and quite low around the catalyst. The flow distribution was rather similar in each cross-section of different axial location. (a) (b) (c) Fig. 3: Flow distributions of mixed gas velocity vector and contour within packed beds: (a) without static mixer (b) with LPD1 static mixer (c) LPD2 static mixer. For the catalyst bed equipped with static-mixers, however, the pattern of flow distribution was rather predefined by the configuration of the static mixers as illustrated in Fig. 3(b) and (c). Overall, the presence of the static mixers induced appreciable radial and tangential velocity components at each axial plane. The strong induced flows were apparent especially in the regions near the surfaces of static mixer. These induced flows could force the mixed gas to flow pass catalyst pellets in an effective fashion i.e. catalyst surfaces exposed to the mixed (reactant) gas from all around, and so expected to promote the better chain growth and chain propagation during the FTS. The performance of FT reactor depended on the time duration that the surface of catalyst pellets exposed to the mixed gas which in turn depended on the true configuration of 64

4 catalyst bed structure. In case of packed-bed with the static mixer, the pitch of LPD1 and LPD2 determined the exposure time. Fig. 4 showed the temperature field of the mixed gas within packed beds for each reactor. For all packed beds, the temperature of mixed gas was highest on the surfaces of catalyst pellets due to the exothermic heat released from FT reaction, and was lower within the bulk flow. The temperature of mixed gas was lowest on the reactor wall caused by the heat removal flux applied. The strong coupling between the hydrodynamics and thermal behaviors could also be observed from Fig. 3 and 4 i.e. in the region of high temperature the flow velocity was increased according to continuity. (b) (a) (c) Fig. 4: Temperature distributions of mixed gas velocity vector and contour within packed beds: (a) without static mixer (b) with LPD1 static mixer (c) LPD2 static mixer. (a) (b) Fig. 5: Heat transfer coefficients: (a) local at different axial cross-sections (b) average of the entire bed. The effect of static mixer on the performance of packed-bed reactor could be more pronounced considering the heat transfer coefficients as demonstrated in Fig. 5. As shown in Fig. 5(a), the local heat transfer coefficient fluctuated between W/m2 K for the conventional packed bed whereas it fluctuated in a wider range from 600 to 850 W/m2 K for the modified bed. Thus, the average heat transfer coefficient was significantly higher in case of the modified beds as shown in Fig. 5(b). The superior heat transfers experienced in the modified beds were originated from the local turbulence induced by the static mixers. However, among the cases of packed beds using LPD1 and LPD2, the average heat transfer coefficients were approximately the same. 4. Conclusion 65

5 In this study, the simulation offered the hydrodynamics and thermal characteristics as the guideline for designing the FT packed-bed reactor. For the conventional packed-bed reactor, the velocity and temperature distributions exhibited higher nonuniformity as compared with those of the modified reactors. In term of heat transfer, the static mixers could provide the higher heat transfer coefficients that in turn better control the temperature of mixed gas. The performances of reactors with LPD1 and LPD2, however, were not noticeably different. 5. Acknowledgment The authors would like to express gratitude to the National Metal and Materials Technology Center (MTEC), and the Faculty of Engineering, King Mongkut s University of Technology North Bangkok, for supports on this research work. 6. References [1] A.P. Steynberg, M.E. Dry, B.H. Havis, B.B. Berman, Chapter 2 Fischer Tropsch reactors, Studies in Surface Science and Catalysis. 2004, 152: 64. [2] Pour Nakhaei, Y. Zamani, A. Tavasoli, S.M.K. Shahri, S.A. Taheri, Fuel. 2004, 87. [3] G. Calleja, A.D. Lucas, R.V. Grieken. Co/HZSM-5 catalyst for syngas conversion: influence of process variables. Fuel. 1995, 74 (3): [4] S.T. Sie & R. Krishna. Process development and scale up: II. Catalyst design strategy. Reviews in Chemical Engineering. 1998, 14: [5] E.H. Stitt. Reactor technology for syngas and hydrogen. In: E. Derouane, V. Parmon, F. Lemos and F. Ramoa- Ribiero, Editors. Sustainable Strategies for the Upgrading of Natural Gas: Fundamentals, Challenges and Opportunities, NATO Science Series II vol. 191, Kluwer Academic Publishers, Dordrecht. 2005, [6] S.P. Runo et al. Effect of the geometric characteristics of commercial catalysts for steam reforming. Chemical Engineering Journal. 1988, 39: [7] Q.S. Liu, Z.X. Zhang, J.L. Zhou. Steady state and dynamic behaviour of fixed bed catalytic reactor for Fiscer Tropsch senthesis. I. mathematical model and numerical method. Journal of Natural Gas Chemistry. 1999, 8 (2): [8] Q.S. Liu, Z.X. Zhang, J.L. Zhou. Steady state and dynamic behaviour of fixed bed catalytic reactor for Fiscer Tropsch senthesis. II. steady state and dynamic simulation results, Journal of Natural Gas Chemistry. 1999, 8 (3): [9] Y. Wang, Y. Xu, y. Li, Y. Zhao, B. Zhang. Heterogeneous modeling for fixed-bed Fischer Tropsch synthesis: reactor model and its applications. Chemical Engineering Science. 2003, 58 (3 6): [10] M.A. Marvast, M. Sohrabi, S. Zarrinpashneh, G. Baghmisheh, Chemical Engineering and Technology. 2005, 28 (1), [11] S. Andersson & N. SchLoLon. Industrial and Engineering Chemistry Research. 1993, 32: Khinast, G. J., et al. Mass transfer enhancement by static mixers in a wall-coated catalytic reactor. Chemical Engineering Science. 2003, 58: [12] G. J. Khinast et al. Mass transfer enhancement by static mixers in a wall-coated catalytic reactor. Chemical Engineering Science. 2003, 58: