Process for converting synthesis gas into higher hydrocarbons

Details Process for converting synthesis gas into higher hydrocarbons Process for converting synthesis gas into higher hydrocarbons

2003-12-19

2004-11-23

Parsa, J. (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Temperature control or regulation of the fischer-tropsch...

C518S700000, C518S705000, C518S715000

Reexamination Certificate

active

06822007

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a process for the conversion of carbon monoxide and hydrogen (synthesis gas) to liquid hydrocarbon products in the presence of a Fischer-Tropsch catalyst.
In the Fischer-Tropsch reaction a gaseous mixture of carbon monoxide and hydrogen is reacted in the presence of a heterogeneous catalyst to give a hydrocarbon mixture having a relatively broad molecular weight distribution. This product is predominantly straight chain, saturated hydrocarbons which typically have a chain length of more than 5 carbon atoms. The reaction is highly exothermic and therefore heat removal is one of the primary constraints of all Fischer-Tropsch processes. This has directed commercial processes away from fixed bed operation to slurry systems. Such slurry systems employ a suspension of catalyst particles in a liquid medium thereby allowing both the gross temperature control and the local temperature control (in the vicinity of individual catalyst particles) to be significantly improved compared with fixed bed operation.
Fischer-Tropsch processes are known which employ slurry bubble columns in which the catalyst is primarily distributed and suspended in the slurry by the energy imparted from the synthesis gas rising from the gas distribution means at the bottom of the slurry bubble column as described in, for example, U.S. Pat. No. 5,252,613.
The Fischer-Tropsch process may also be operated by passing a stream of the liquid medium through a catalyst bed to support and disperse the catalyst, as described in U.S. Pat. No. 5,776,988. In this approach the catalyst is more uniformly dispersed throughout the liquid medium allowing improvements in the operability and productivity of the process to be obtained.
However, there remains the need for further improvements in the mode of operation of a Fischer-Tropsch process.
SUMMARY OF THE INVENTION
The present invention relates to a process for the conversion of gaseous reactants to liquid hydrocarbon products by contacting the gaseous reactants at an elevated temperature and pressure with a suspension comprising catalyst suspended in a liquid medium, in a system comprising a high shear mixing zone and a post mixing zone wherein the process comprises:
a) passing the suspension comprising catalyst suspended in the liquid medium through the high shear mixing zone where a gaseous reactant stream comprising the gaseous reactants is mixed with the suspension;
b) discharging a mixture comprising gaseous reactants and suspension from the high shear mixing zone into the post mixing zone;
c) converting at least a portion of the gaseous reactants to liquid hydrocarbon products in the post mixing zone to form a product suspension comprising catalyst suspended in the liquid medium and the liquid hydrocarbon products;
d) separating a gaseous stream comprising unconverted gaseous reactants from the product suspension;
e) recycling the separated gaseous stream to the high shear mixing zone; and
f) recycling at least a portion of the product suspension to the high shear mixing zone.
An advantage of the process of the present invention over conventional Fischer-Trospch processes is that enhanced mass transfer in the high shear mixing zone and the post mixing zone improves the contact between the gaseous reactants, liquid medium and solid catalyst and hence promotes the catalytic conversion of the gaseous reactants to liquid hydrocarbon products. For avoidance of doubt, the conversion of the gaseous reactants to liquid hydrocarbon products is initiated in the high shear mixing zone although the majority of the conversion generally occurs in the post mixing zone.
Preferably, the gaseous reactants comprise a mixture of carbon monoxide and hydrogen (synthesis gas). Preferably, the ratio of hydrogen to carbon monoxide in the synthesis gas is 2:1 by volume.
The synthesis gas may be prepared using any of the processes known in the art including partial oxidation of hydrocarbons, steam reforming, and autothermal reforming. A discussion of these synthesis gas production technologies is provided in “Hydrocarbon Processing” V78, N.4, 87-90, 92-93 (April 1999) and “Petrole et Techniques”, N. 415, 86-93 (July-August 1998). It is also envisaged that the synthesis gas may be obtained by catalytic partial oxidation of hydrocarbons in a microstructured reactor as exemplified in “IMRET 3: Proceedings of the Third International Conference on Microreaction Technology”, Editor W Ehrfeld, Springer Verlag, 1999, pages 187-196. Alternatively, the synthesis gas may be obtained by short contact time catalytic partial oxidation of hydrocarbonaceous feedstocks as described in EP 0303438. Preferably, the synthesis gas is obtained via a “Compact Reformer” process as described in “Hydrocarbon Engineering”, 2000, 5, (5), 6769; “Hydrocarbon Processing”, 79/9, 34 (September 2000); “Today's Refinery”, 15/8, 9 (August 2000); WO 99/02254; and WO 200023689.
Preferably, the liquid hydrocarbon products comprise a mixture of hydrocarbons having a chain length of greater than 5 carbon atoms. Suitably, the liquid hydrocarbon products comprise a mixture of hydrocarbons having chain lengths of from 5 to about 90 carbon atoms. Preferably, a major amount, for example, greater than 60% by weight, of the hydrocarbons have chain lengths of from 5 to 30 carbon atoms.
Suitably, the liquid medium comprises one or more of the liquid hydrocarbon products which has the advantage that there is no requirement to separate the liquid hydrocarbon products from the liquid medium.
The high shear mixing zone may be part of the system inside or partially outside the post mixing zone, for example, the high shear mixing zone may project through the walls of the post mixing zone such that the high shear mixing zone discharges its contents into the post mixing zone. The system may comprise a plurality of high shear mixing zones, preferably up to 250 high shear mixing zones, more preferably less than 100, most preferably less than 50, for example 10 to 50 high shear mixing zones. Preferably, the plurality of high shear mixing zones discharge into a single post mixing zone which has an advantage of significantly reducing the size of a commercial Fischer-Tropsch plant. Preferably, the plurality of high shear mixing zones may be spaced uniformly inside or partially outside the post mixing zone, for example, the high shear mixing zones may be spaced uniformly at or near the top of the post mixing zone. Preferably, the high shear mixing zones discharge the mixture of gaseous reactants and suspension in a downwards direction into the post mixing zone.
The high shear mixing zone(s) may comprise any device suitable for intensive mixing or dispersing of a gaseous stream in a suspension of solids in a liquid medium, for example, a rotor-stator device or an injector-mixing nozzle.
The injector-mixing nozzle(s) can advantageously be executed as venturi tubes (c.f. “Chemical Engineers' Handbook” by J. H. Perry, 3
rd
edition (1953), p. 1285, FIG. 6I), preferably an injector mixer (c.f. “Chemical Engineers' Handbook” by J H Perry, 3
rd
edition (1953), p 1203, FIG. 2 and “Chemical Engineers' Handbook” by R H Perry and C H Chilton 5
th
edition (1973) p 6-15, FIGS. 6-31) or most preferably as a liquid-jet ejector (c.f. “Unit Operations” by G G Brown et al, 4
th
edition (1953), p. 194, FIG. 210). Alternatively, the injector-mixing nozzle(s) may be executed as “gas blast” or “gas assist” nozzles where gas expansion is used to drive the nozzle (c.f. “Atomisation and Sprays” by Arthur H Lefebvre, Hemisphere Publishing Corporation, 1989). Where the injector-mixing nozzle(s) is executed as a “gas blast” or “gas assist” nozzle, the suspension of catalyst is fed to the nozzle at a sufficiently high pressure to allow the suspension to pass through the nozzle while the gaseous reactant stream is fed to the nozzle at a sufficiently high pressure to achieve high shear mixing within the nozzle.
Suitably, the gaseous reactant stream is fed to the high shear mixing zone at a pressure of at least

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