Denver, Colorado-based Rentech Inc. has achieved a world first by developing three-dimensional CFD models to simulate bubble column hydrodynamics in large slurry reactors. This was done so that validated models of small-scale reactors could be easily scaled-up to commercial-size equipment. Rentech’s approach, based on FLUENT simulations, is an important step towards designing a full-scale reactor for gas-to-liquid (GTL) conversions using Fischer-Tropsch chemistry. This method has the potential for wide application in the conversion of refinery residues, an ever-increasing problem worldwide.

Gas volume fraction in the reactor vessel showing the gas / liquid interface

Rentech owns, licenses, and markets a proprietary and patented process that converts syngas, a mixture of hydrogen and carbon monoxide produced from any carbon containing materials, into valuable liquid hydrocarbons including diesel fuel, naphthas, and waxes. The Rentech process uses an iron-based catalyst in a slurry reactor to affect the GTL conversion. Studies suggest that petrochemical companies could expect millions of dollars of additional revenues each year in every refinery when the technique is fully commercialized and used to convert current refinery residues.

One of the drawbacks in pioneering this technology has been the prohibitively expensive capital cost of equipment for plant-scale prototype reactor testing. CFD modeling using FLUENT software is a promising alternative for scale-up studies of slurry reactor vessels. The reactor vessel contains catalyst suspended in liquid hydrocarbon and the synthesis gas is brought in at the bottom of the unit and bubbled up through the reactor body. As the bubbles contact the catalyst material, the Fischer-Tropsch reaction takes place. The key to the process is understanding the three-dimensional hydrodynamics in the vessel so that a high reaction rate for commercially-viable GTL conversion rates can be achieved. This is inevitably linked to a high gas bubble distribution and large gas/liquid interfacial area in the reactor. Gas hold-up is another important factor affecting reaction rate that needs to be maximized. Through transient three-dimensional CFD simulations using FLUENT software, Rentech has been able to determine this parameter for various vessel geometries, bubble release points, and bubble diameters.

Rentech found a strong correlation between these CFD simulations and results of experimental work, thereby gaining greater confidence in this technique. With potential savings in the millions of dollars for a single refinery, the technology offers great promise.

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