Chemistry: fischer-tropsch processes; or purification or recover – Liquid phase fischer-tropsch reaction
Reexamination Certificate
2002-09-03
2004-07-13
Parsa, J. (Department: 1621)
Chemistry: fischer-tropsch processes; or purification or recover
Liquid phase fischer-tropsch reaction
C518S715000
Reexamination Certificate
active
06762209
ABSTRACT:
TECHNICAL FIELD
The present invention relates to methods and devices for producing liquid hydrocarbon products, particularly heavier products such as waxes, from gaseous reactants in a reactor. More specifically, the invention relates to methods and devices for separating liquid hydrocarbon products produced by the Fischer-Tropsch reaction using a slurry bubble column reactor.
BACKGROUND OF THE INVENTION
As an alternative or supplement to refinement of fossil fuels, it is known to react synthesis gas or syngas (usually produced by steam reforming or partial oxidation of feedstocks such as natural gas), which comprises mainly CO and H
2
, with a catalyst such as Fe or Co to produce a wide range of hydrocarbons. This process, known as Fischer-Tropsch synthesis, is a well-known process for conversion of synthesis gas to synthetic fuels and raw materials for the chemical industry. The process is versatile in that it may use any type of coal, natural gas, or similar carbon-containing feedstock as raw material, and similarly the product distribution may be altered as desired. The product stream from known methods and devices employing Fischer-Tropsch synthesis includes, but is not limited to, naphtha, diesel, waxes, steam, water, and alcohols.
Various devices for conducting Fischer-Tropsch synthesis are known in the art, including packed bed reactors, slurry reactors such as stirred tank slurry reactors, and slurry bubble column reactors. At present, the slurry bubble column reactor is most applicable to processes utilizing Fischer-Tropsch synthesis to produce synthetic fuels and the like on a commercial basis. The slurry bubble column reactor is advantageous in comparison to the fixed or packed bed reactor system due to improved heat transfer and mass transfer, maintenance of an isothermal temperature profile, and comparatively low capital and operating costs.
In obtaining product from a slurry bubble column reactor via Fischer-Tropsch synthesis, it is necessary to separate the product from the slurry containing catalyst in order to recycle the slurry/catalyst phase into the reactor. Advantageously, in order to maximize efficiency of such a system the recycling of slurry through the slurry bubble column reactor should assume plug flow characteristics, i.e. the slurry should pass through the length of the system at a constant velocity. Prior art systems have successfully extracted product from a slurry bubble column reactor, but at the cost of maintenance of plug-flow kinetics (thereby adversely affecting efficiency of the reactor). These prior art systems further require complicated mechanisms for separating liquid products from catalyst/slurry phases in a slurry bubble column reactor.
Thus, there is a need in the art for methods and devices for separating hydrocarbon products from a slurry bubble column reactor which simply and efficiently separate the desired liquid product from the slurry/catalyst phase, maximizing separation while minimizing slurry hold-up and catalyst losses during separation. There is further a need in the art for such methods and devices which promote and enhance plug-flow characteristics of the slurry bubble column reactor, thereby maximizing efficiency and predictability of the system.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides, in a Fischer-Tropsch process for synthesizing a liquid hydrocarbon product from a gaseous reactant, a method for separating a substantially particle-free liquid hydrocarbon product from a slurry comprising a catalyst particle and a suspension liquid while substantially preventing depletion of catalyst particle from the slurry. The method comprises introducing the gaseous reactant into a reactor containing the slurry, and bubbling the gaseous reactant upwardly through the catalyst particle-containing slurry to form a reaction mixture comprising liquid and gaseous hydrocarbon product, catalyst particle-containing slurry, and unreacted gaseous reactant. The gaseous reactant may be introduced into the reactor at a flow rate of from about 1 to about 20 cm/s.
A gas distributor such as a sparger may be used to bubble the gaseous reactant, typically a synthesis gas, through the slurry. Any synthesis gas resulting from conventional processing may be utilized. Typically, the synthesis gas will comprise hydrogen and carbon monoxide in a ratio of from about 0.5 to about 3.0. Suitable catalysts for the present method are those known in the art for Fischer-Tropsch reactions, including iron-based, cobalt-based, zinc-based, ruthenium-based, any catalyst based on metals from Group 8 of the Periodic Table of the Elements, or any mixture thereof. Suitable catalyst particles will have a particle size of from about 1 to about 200 &mgr;m.
The reaction mixture is then passed reactor upwardly through one or more risers to discharge into a separator chamber. Typically, the separator chamber will be placed in a spaced vertical orientation with the reactor. Gaseous hydrocarbon product and unreacted gaseous reactant from the reaction mixture separate from the liquid hydrocarbon product and catalyst-containing slurry in the separator chamber, and may be removed from the separator chamber via a port and exit pipe. As is known in the art, a system of warm and cold traps may be included downstream of the exit pipe to remove wax and light oil products from the unreacted synthesis gas.
Advantageously, heavier liquid hydrocarbon products such as waxes and catalyst particle-containing slurry may be returned from the overhead separator chamber via a gravity feed to the reactor. It will be appreciated that the driving force for this recirculating flow is the difference in density between the fluid column in the riser, containing slurry and gas, and the fluid column returning to the reactor (slurry only). The liquid hydrocarbon product and slurry are returned to the reactor through at least one downcomer containing at least one cross-flow filtration element.
Typically, the cross-flow filtration element will be a device comprising a porous tube encapsulated within a shell, located within the downcomer. It will be appreciated that any suitable filter may be employed, such as a sintered metal filter, a ceramic filter, a fiber filter, a wire mesh filter, or any other suitable filtration material. Such cross-flow filtration devices are well known in the art (Kirk-Othmer Encyclopedia of Chemical Technology, 1993, Vol. 10, pages 841-847, incorporated herein by reference). The cross-flow filtration element may comprise a metal or ceramic sinter having a pore size of from about 0.05 &mgr;m to about 20 &mgr;m. In another embodiment, a wire mesh filter having multiple layers of mesh with variable mesh sizes (varying from coarse to finer mesh) may be used. Typically, a range of mesh sizes from about 20 to about 200 mesh is used.
Accordingly, substantially catalyst particle-free liquid hydrocarbon products (typically waxes) of the Fischer-Tropsch synthesis reaction employed herein may be axially withdrawn from the downcomer without interference with the recirculating flow described. Typically, the liquid hydrocarbon product and catalyst particle-containing slurry are passed through the downcomer at a velocity sufficient to prevent accumulation of catalyst particle on the filtration material due to the shear force provided by the slurry flow. Typically, the liquid hydrocarbon product and catalyst particle-containing slurry are passed through the downcomer at a velocity of from about 0.5 to about 100 M/min. The downcomer extends from a bottom of the separator chamber and discharges into the reactor, thereby returning slurry and catalyst to the reactor for continued use.
In another aspect, the present invention provides, in a process for synthesizing a liquid hydrocarbon product from a gaseous reactant by a Fischer-Tropsch reaction, a method for promoting plug-flow characteristics of a bubble column reactor system by establishing a natural convection loop. The method comprises essentially the steps summarized above. As noted, a recirculating flow is establi
Davis Burtron H.
Neathery James K.
King & Schickli PLLC
Parsa J.
University of Kentucky Research Foundation
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