Process for producing liquid and, optionally, gaseous...

Details Process for producing liquid and, optionally, gaseous... Process for producing liquid and, optionally, gaseous...

2001-07-25

2002-10-08

Parsa, Jafar (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Liquid phase fischer-tropsch reaction

C518S728000, C210S497010, C210S497100

Reexamination Certificate

active

06462098

ABSTRACT:

FIELD OF INVENTION
This invention relates to a process for producing liquid and, optionally, gaseous products from gaseous reactants.
SUMMARY OF INVENTION
According to a first aspect of the invention, there is provided a process for producing liquid and, optionally, gaseous products from gaseous reactants, which process comprises
feeding, at a low level, gaseous reactants into a slurry bed of solid catalyst particles suspended in a suspension liquid;
allowing the gaseous reactants to react as they pass upwardly through the slurry bed, thereby to form liquid and, optionally, gaseous products, with the reaction being catalyzed by the catalyst particles; and
separating liquid product from the catalyst particles by passing, in a filtration zone within the slurry bed, liquid product through a filtering medium having a plurality of openings through which the liquid passes, with the openings having a controlling dimension of x microns, and with the proportion of catalyst particles, which have a particle size smaller than x microns, in the slurry bed being less than 18% by volume based on the total volume of the catalyst in the slurry bed.
A low proportion, or even absence, of such fine catalyst particles in the slurry bed ensures a high solids separation efficiency across the filtering medium, and also ensures a low degree of catalyst contamination of the liquid product downstream of the filtering medium; however, as also described in more detail hereunder, it has surprisingly been found that high conversions of gaseous reactants to products are nevertheless obtainable in the process. Thus, a low proportion of less than 4 vol %, preferably less than 2 vol %, catalyst particles smaller than x microns, normally ensures that the liquid product has a catalyst content less than 10 ppm (by mass)
By ‘controlling dimension’ in respect of the filtering medium openings or filtering openings is meant the maximum dimension of the filtering openings through which the catalyst particles can pass. The controlling dimension may, for example, be obtained from the filter manufacturer's specification. Thus, it may be the upper tolerance level, or it may be the average gap size added to a factor of, e.g. three times, the gap size standard deviation.
The filtering openings may thus be of any desired shape. In one embodiment, the filtering medium openings, when seen in the direction of liquid flow through the openings, may be circular, with the controlling dimension of each opening thus being its diameter. Instead, in another embodiment, the filtering medium openings, when seen in the direction of liquid flow through the openings, may be more-or-less rectangular so that the width of each opening is shorter than its length, with the controlling dimension of each opening being its width.
Thus, for example, x may typically be 40 microns. The filtering medium may then, for example, be a wedge wire filtering medium comprising parallel wires which are spaced so as to provide openings whose average widths are 10 microns, i.e. the filtering medium has a nominal opening or gap size of 10 microns; the opening widths thus vary or deviate from 10 microns, with the average width being 10 microns, while the maximum width or gap size is 40 microns. The slurry bed will then contain less than 18% by volume (based on the volume of the total catalyst inventory of the bed) of catalyst particles having a diameter less than 40 microns, ie particles smaller than 40 microns.
The proportion of catalyst particles smaller than x microns in the slurry bed may thus be less than 18% by volume at the start of a catalyst run, ie when the catalyst is initially loaded in a slurry phase reactor or on initial formation of the slurry bed at the start of the ruin. However, during the course of the run, the proportion of catalyst particles smaller than x microns is worked down, through normal operation of the slurry bed, to less than 4 vol %, and preferably less than 2 vol %. Accordingly, the slurry bed may typically contain substantially no catalyst particles smaller than x microns for at least a major portion of the catalyst run, eg for substantially the entire run.
In a particular embodiment of the invention, the controlling dimension of the filtering medium openings may be their minimum dimension, with the proportion of catalyst particles, whose minimum dimension is less than x microns, in the slurry bed being less than 4% by volume based on the total volume of the catalyst in the slurry bed.
While the process can, at least in principle, have broader application, it is envisaged that the suspension liquid will normally, but thus not necessarily always, be the liquid product.
Furthermore, while it is also believed that, in principle, the process can have broader application, it is envisaged that it will have particular application in hydrocarbon synthesis where the gaseous reactants are capable of reacting catalytically in the slurry bed to form liquid hydrocarbon product and, optionally, gaseous hydrocarbon product. In particular, the hydrocarbon synthesis may be Fischer-Tropsch synthesis, with the gaseous reactants being in the form of a synthesis gas stream comprising mainly carbon monoxide and hydrogen, with both liquid and gaseous hydrocarbon products being produced, and with the catalyst particles thus being Fischer-Tropsch catalyst particles.
The slurry bed will thus be provided in a suitable vessel, eg a column, with unreacted reactants and gaseous product being withdrawn from the vessel above the slurry bed, and the separated liquid product also being withdrawn from the vessel. The vessel will be maintained at normal elevated pressure and temperature conditions associated with Fischer-Tropsch synthesis, eg a predetermined operating pressure in the range 10 to 50 bar, and at a predetermined temperature in the range 180° C. and 280° C., or even higher for the production of lower boiling point product.
Any suitable filtering medium can, at least in principle, be used, and the filtering medium may have differing opening or gap sizes. However, all the openings of the filtering medium will normally be of nominally the same size and have the same geometry. The filtering medium may be part of a filter cartridge or element mounted in the vessel, and may be of a type which is of elongate form, with the filtering medium being of cylindrical form and enclosing a filtrate collecting zone, and with a filtrate outlet for withdrawing filtrate, ie liquid product, being provided at one end thereof. While, in principle, the filtering medium can be any desired filtering medium having the desired opening size to prevent catalyst particles passing therethrough, it is preferably of a type or construction with which permanent clogging or impregnation thereof with the catalyst particles does not readily occur. Thus, the filtering medium can be a mesh, eg a woven mesh; a porous material such as a ceramic material; a perforated sheet; spiral wire wound, eg from wedge wire; or the like.
The maximum allowable controlling dimension of the filtering medium will thus be dictated by the portion of catalyst particle sizes smaller than the controlling dimension of the filter, present in the slurry bed. Although, in slurry phase reactions, catalyst breakup due to attrition normally takes place, resulting in a lowering of the minimum particle size, and a decrease in the average catalyst particle size, it has surprisingly been found that catalyst breakup by attrition or any other means of disintegration can be almost entirely avoided.
The catalyst particles can, at least in principle, be any desired supported Fischer-Tropsch catalyst, such as an iron-based catalyst, a cobalt-based catalyst, or any other Fischer-Tropsch catalyst. Supported catalysts, which are physically stronger than unsupported catalysts, are typically used, and supported cobalt catalysts are preferred. Preferably, the catalyst may be that obtained by a preparation method as described in ZA 96/2759//U.S Pat. No. 5,733,839 or ZA 99/1265//PCT/GB99/00527. Such a method is hereinafter also referred

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