Chemistry: fischer-tropsch processes; or purification or recover – Group viii metal containing catalyst utilized for the...
Reexamination Certificate
2002-09-26
2004-11-30
Parsa, J. (Department: 1621)
Chemistry: fischer-tropsch processes; or purification or recover
Group viii metal containing catalyst utilized for the...
C518S700000
Reexamination Certificate
active
06825237
ABSTRACT:
The present invention relates to synthesising heavy hydrocarbons using the Fischer-Tropsch reaction, i.e., the production of hydrocarbons by reacting a mixture essentially containing carbon monoxide and hydrogen and possibly carbon dioxide. That mixture is also known as synthesis gas.
More particularly, the present invention relates to a process for synthesising hydrocarbons by reacting a mixture comprising at least carbon monoxide and hydrogen in the presence of a catalyst carried out in a three-phase reactor and in which the liquid Peclet number (Pe
l
) is in the range 0 (excluded) to about 10.
PRIOR ART
Synthesising hydrocarbons by the reaction known as the Fischer-Tropsch reaction is an industrial process that is well known for the production of hydrocarbons that are essentially paraffinic, such as naphtha or gas oil fractions or heavier fractions such as waxes (long chain paraffins). Such hydrocarbons can be converted into fuels (gas oil, kerosene) and/or into lubricants in a consecutive step such as isomerising hydrocracking.
The hydrocarbons can be produced catalytically by chemical conversion of synthesis gas that is rich in hydrogen and carbon monoxide, generally obtained from natural gas or coal. Synthesis gas can also contain carbon dioxide. The pressures used are generally about 5 to about 200 bars absolute, normally about 5 to about 80 bars absolute and usually about 10 to about 60 bars absolute (10 bars=1 MPa), and the reaction temperatures are normally about 130° C. to about 400° C., normally about 150° C. to about 350° C. and usually about 200° C. to about 300° C.
The catalysts used in the process, and the methods for producing these catalysts are well known to the skilled person. Such catalysts can be of a variety of natures, and usually contain at least one metal from group VIII of the periodic table (groups 8, 9 and 10 of the new periodic table), preferably dispersed on a support that is usually mineral. Frequently, the catalyst contains at least one metal selected in the group consisting of iron, cobalt and ruthenium and usually selected in the group consisting of iron and cobalt.
The support is generally a porous material and usually a porous inorganic refractory oxide. By way of example, the support can be selected in the group consisting of alumina, silica, titanium oxide, zirconia, rare earths or mixtures of at least two of these porous minerals. Typically, the quantity of metal present in the catalyst is about 1 to about 100 parts by weight per 100 parts by weight of support and usually about 5 to about 50 parts by weight per 100 parts by weight of support.
The catalyst can also contain promoters such as those cited in the following patents: British patent GB-A-2 291 819, European patents EP-A-0 581 619, EP-B-0 764 465, U.S. Pat. No. 5,783,607, French patent FR-A-2 782 319, cited by way of reference, the description of which should be considered to be included in the present description by dint of this citation.
Several types of reactor can be used for the Fischer-Tropsch reaction, the catalyst being used either in an entrained bed, or in a reactor of the slurry bubble column or bubble column reactor type in which a gas is brought into contact with a liquid/very finely divided solid mixture, or slurry. The term “slurry” will be used in the remainder of the description to designate a suspension of solid particles in a liquid. The very high heat of reaction is normally eliminated using a cooling exchanger that is generally inside the reactor.
Fischer-Tropsch synthesis facilities also comprise separation means to separate firstly liquid hydrocarbons and secondly gaseous products that are residual or formed as secondary products during the synthesis, mainly comprising inert compounds, light gaseous hydrocarbons and the unreacted fraction of the synthesis gas.
The desired products are generally separated substantially completely from the catalyst (for example until the amount of residual catalyst is of the order of 1 to a few parts per million (ppm)), to enable its use or treatment in subsequent steps.
Typically, the quantity of solid particles of catalyst in the slurry represents 10% to 65% by weight of the slurry. These particles usually have a mean diameter in the range about 10 to about 800 microns. Finer particles may be produced by attrition, i.e., fragmentation of the initial catalyst particles.
The Fischer-Tropsch synthesis is a synthesis reaction that aims to produce essentially paraffinic hydrocarbons essentially containing more than 5 carbon atoms per molecule (C
5
+
hydrocarbons). This reaction is exothermic. Further, the catalyst and operating conditions are usually selected so as to minimise the formation of methane, which is not a desired product. That reaction is particularly exothermic and has a higher activation energy than the principal C
5
+
paraffin formation reaction.
European patent application EP-A-0 450 861 describes the use of a Fischer-Tropsch catalyst based on cobalt dispersed on titanium oxide in a slurry bubble column type reactor. Further, EP-B-0 450 860 describes a method for operating that type of reactor in an optimal manner.
Those two documents indicate that the performance of the catalysts essentially depends on the concentration of gaseous reactant (synthesis gas) in the reactor, i.e., on the partial pressure of carbon monoxide and hydrogen in the reaction zone.
In hydrodynamics terms, those documents then indicate that in a perfectly mixed reactor, such as a fully back-mixed reactor or CSTR, the composition of gaseous reactants and liquid and gaseous products and the concentration of catalyst are the same at any point in the reactor. Thus, those perfectly mixed reactors lead to the highest selectivity for C
5
+
hydrocarbons, but to the detriment of productivity.
In contrast, in a plug flow reactor, the partial concentration of reactant decreases along the entire length of the reaction zone, and that type of reactor results in the highest productivities to the detriment of selectivity.
EP-B-0 450 860 indicates that Peclet numbers for the gas phase of more than 10, also known as “gas Peclet numbers or Pe
g
”, lead to a plug flow type operation regarding the gas phase, while gas Peclet numbers (Pe
g
) of less than 1 correspond to systems in which the gas phase is perfectly mixed or stirred. Ideal perfectly stirred systems correspond to Peclet numbers tending towards zero. This Peclet number is equal to Pe
g
=H u
g
/D
ax
, where H is the expansion height of the catalytic bed, u
g
is the space velocity of the gas and D
ax
is the axial dispersion coefficient of the gas phase.
The method that can produce an optimal slurry bubble column that is described in EP-B-0 450 860 comprises injecting gas at a mean superficial velocity such that the formation of slug flow is avoided, the gas superficial velocity being 0.2 (H/D
ax
) or more. A further condition applies to the superficial velocity of the liquid and the sedimentation rate of the solid (generally the catalyst) so that the solid is suitably fluidised in the liquid phase.
Those documents do not take thermal effects into account, nor the presence of an undesirable methanation reaction that has a large negative influence on the exothermicity and selectivity of the reaction. Too much exothermicity in the catalyst generally leads to an increase in the formation of methane, a product that is favoured by high temperatures, and a drop in activity, for example by sintering of the active phase (M. E. DRY, “Catalysis Science and Technology”, Volume 1, Anderson and Boudart, pages 175 and 198).
Thus, those phenomena result in a substantial reduction in the production of C
5
+
hydrocarbons, usually irreversibly.
SUMMARY OF THE INVENTION
The invention concerns a process for converting hydrocarbons by reacting a mixture comprising at least carbon monoxide and hydrogen in the presence of a catalyst, usually based on a group VIII metal, carried out in a three-phase reactor and in which the liquid Peclet number (Pe
l
) is in the range 0 (excluded) to abo
Galtier Pierre
Hugues Francois
Maretto Cristina
Schweitzer Jean-Marc
Institut Francais DuPetrole
Miller, White, Zelano & Branigan, P.C.
Parsa J.
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