A scale-up strategy for a commercial scale bubble column slurry reactor for Fischer-Tropsch synthesis

Abstract

Bubble column reactors are finding increasing use in industrial practice; this reactor technology figures prominently in processes for converting natural gas to liquid fuels and light olefins using Fischer-Tropsch synthesis. There are considerable reactor design and scale-up problems associated with the Fischer-Tropsch bubble column slurry reactor. Firstly, large gas throughputs are involved, necessitating the use of large diameter reactors, typically 5-8 m, often in parallel. Secondly, the process operates under high-pressure conditions, typically 40 bar. Thirdly, in order to obtain high conversion levels, large reactor heights, typically 30-40 m tall, are required along with the use of highly concentrated slurries, approaching 40 vol%. Finally, the process is exothermic in nature, requiring heat removal by means of cooling tubes inserted in the reactor. Successful commercialisation of this technology is crucially dependent on the proper understanding of the scaling-up principles of bubble columns for the above mentioned conditions which fall outside the purview of most published theory and correlations. In order to develop the proper scale-up rules for the bubble column slurry reactor we have undertaken a comprehensive program of investigation of the hydrodynamics (gas holdup, radial distribution of liquid velocities, backmixing of the liquid) in columns of diameters 0. 05, 0. 1, 0. 15, 0. 174, 0. 19, 0. 38 and 0. 63 m. A variety of liquids (water, tetradecane, paraffin oil, Tellus oil) were used as the liquid phase. Silica particles in concentrations up to about 40 vol% were added to paraffin oil in order to study slurry hydrodynamics. One column of 0. 15 m diameter was operated at pressures ranging from 0. 1 to 1. 3 MPa with the air-water system and the gas holdup and gas-liquid mass transfer were measured. Additionally, video imaging studies in a rectangular two-dimensional column were carried out to study the rise characteristics of single bubbles, bubble-bubble interactions and coalescence-breakup phenomena. Our experiments show that the hydrodynamics is significantly affected by column diameter, elevated system pressures, concentration of the slurry. These effects are not adequately described by published literature correlations. The extrapolation of data obtained in laboratory cold flow units to the commercial scale reactors requires a systematic approach based on the understanding of the scaling principles of bubble dynamics and of the behaviour of two-phase dispersions in large scale columns. We develop a multi-tiered approach to bubble column reactor scale-up, relying on a combination of experiments, backed by Computational Fluid Dynamics (CFD) simulations for physical understanding. This approach consists of the following steps:- description of single bubble morphology and rise dynamics; here both experiments and Volume of Fluid (VOF) simulations are used;- modelling of bubble-bubble interactions;- description of the behaviour of bubble swarms and of the development of the proper interfacial momentum exchange relations between the bubbles and the liquid;- CFD simulations in the Eulerian framework for extrapolation of laboratory scale information to large scale commercial reactors.