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.
Highlights
The Fischer-Tropsch reaction that was discovered in Germany nearly three quarters of a century ago has recently become a subject of renewed interest, in the context of the conversion of remote natural gas to liquid transportation fuels
A careful examination of the published literature [1,2,3,4,5,6] shows that the viable reactor choices for a commercial process aimed at the production of relatively heavy hydrocarbon products are the multi-tubular fixed bed operating in the trickle flow regime (Fig. 1) and the bubble column slurry reactor (Fig. 2)
The following important aspects of bubble column hydrodynamics have been emphasised in this review: – in the heterogeneous flow regime, the dispersion consists of a variety of bubble sizes, which can be simplified into two classes: “small” and “large”
Summary
The Fischer-Tropsch reaction that was discovered in Germany nearly three quarters of a century ago has recently become a subject of renewed interest, in the context of the conversion of remote natural gas to liquid transportation fuels. A careful examination of the published literature [1,2,3,4,5,6] shows that the viable reactor choices for a commercial process aimed at the production of relatively heavy hydrocarbon products are the multi-tubular fixed bed operating in the trickle flow regime (Fig. 1) and the bubble column slurry reactor (Fig. 2). These two reactor types can be built with substantially higher capacities (2500 bbl/d or higher) than the reactors developed before, during and shortly after World War II. In this review we try to develop procedures for estimating the required information, using more recently available experimental data, largely generated at the University of Amsterdam [9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38]
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