(454b) Experimental Capabilities and Hydrodynamic Data for Validation of Computational Hydrodynamics for Slurry Bubble Column Reactors

Slurry Bubble Column Reactors (SBCRs) are used by industry to manufacture liquid hydrocarbon transportation fuels via the Fischer Tropsch (FT) process. The FT process is fed with synthesis gas comprised of hydrogen and carbon monoxide sparged through a distributor into a suspension of liquid and solid catalyst particles, where it flows vertically upward through a cylindrical vessel. Although the process was developed in the 1920s, details concerning the processes occurring in the reactor are still poorly understood. The unsteady, multiphase fluid dynamics controls the fluid mixing and interphase transport processes, which in turn determine the reactor performance. Due to the complexity of the thermal fluids processes occurring in the SBCR, the process has traditionally been characterized empirically, rather than from a fundamental physical basis. Since empirical correlations are generally valid only for the parameter ranges over which they are generated, mechanistic models of the sub-processes occurring in a SBCR as necessary to optimize the process and scale laboratory data to industrial proportions. These mechanistic models will be used to develop closure relations for use with computational multiphase fluid dynamic (CMFD) software. The purpose of this study is to identify appropriate experimental data to validate CMFD models of gas-liquid-solid flows through SBCRs.

INTRODUCTION

Attractive features of a SBCR for FT synthesis include low pressure drop, no moving parts, simple construction, ease of operation, and excellent heat and mass transfer characteristics. Advantages of a SBCR over a multi-tubular fixed bed reactor are lower construction cost, flexibility to operate at higher and uniform temperatures, lower gas compression cost (due to lower pressure drop across reactor), relatively uniform catalyst distribution and liquid temperature, reduced catalyst consumption, and ease of online removal or addition of catalyst which reduces reactor down time. Disadvantages of SBCRs include difficulties with scale-up due to complex interaction among phases, separation of catalyst particles from the slurry, and the presence of local regions of recirculating flow, which may adversely affect chemical conversion and cause catalyst attrition (Forret, et al., 2006).

The configuration employed in industry is a vertical cylindrical vessel with co-current gas-liquid-solid flow. High throughput necessitates the use of large diameter reactor vessels (typically > 5 m). To economically operate the reactor and produce sufficient product yield, a high slurry concentration (typically 30-50 vol%) is utilized. To suspend such high catalyst loadings, promote mass transfer and remove the large exothermic heat of reaction, a high superficial gas velocity is required.

Due to the high superficial gas velocity, the reactor operates in the churn-turbulent flow regime. This flow regime can be considered as a transition condition between the slug and annular flow regimes (Kim, et al., 2004). The churn-turbulent flow regime is characterized by the presence of small bubbles along with larger, slug/cap bubbles, a wide bubble-size distribution, and local and gross liquid circulation. The gas dynamics dictate the fluid motion and mixing in the reactor.

The complexity in the prediction and design of gas-liquid-solid systems lies in the simultaneous existence of the three phases. The interface between the two phases can be distributed in many configurations (referred to as a flow regime), which is a very important feature of multiphase flows. Due to complicated interaction of phases (particularly in churn-turbulent flow regime), the hydrodynamics in such flow regime is not yet fully understood and hence, the reactor design and scale-up are still a challenging task. The use of CMFD, rather than empirical correlations, is in principle applicable to a wider range of applications. However this procedure calls for a solution of the continuity, momentum and energy equations for the two fluids. For problems of practical utility, the Reynolds-averaged form of the Navier-Stokes equations will be solved. This approach necessitates the use of constitutive equations to ?replace? the information that was lost through the averaging procedure. Mechanistic-based closure relations are currently being developed, but require validation with experimental data obtained at conditions applicable to the FT process.

The Idaho National Laboratory and Rensselaer Polytechnic Institute (RPI) have embarked on a joint effort to develop mechanistic models of the churn-turbulent flow regime for incorporation into the computational multiphase fluid dynamics (CMFD) code, NPHASE, developed at RPI. This paper outlines: (1) institutions with experimental facilities capable of providing validation data, (2) relevant experimental techniques, and (3) sources of validation data published in the open literature. Only sources of data for churn-turbulent flows are provided, since the hydrodynamics of the flow, as well as the flow mechanisms, change significantly from one flow regime to another (Chen, 2001).

REFERENCES

Chen, Y., ?Modeling Gas-Liquid Flow in Pipes: Flow Regime Transitions and Drift-Flux Modeling,? Stanford University, M.S. Thesis, June 2001.

Forret, A., Schweitzer, J.M., Gauthier, T., Drishna, R., and Schweich, D., ?Scale Up of Slurry Bubble Reactors,? Oil & Gas Science and Technology, Rev. IFP, 6(3), 2006, pp. 443-458.

Kim, S., et al., ?Interfacial Structures and Regime Transition in Co-Current Downward Bubbly Flow,? Journal of Fluids Engineering, July 2004, Vol. 126, p. 529-538.