Fischer-tropsch process

Details Fischer-tropsch process Fischer-tropsch process

2003-11-03

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

Reexamination Certificate

active

06822005

ABSTRACT:

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.
BACKGROUND OF THE INVENTION
In the Fischer-Tropsch synthesis reaction a gaseous mixture of carbon monoxide and hydrogen is reacted in the presence of a catalyst to give a hydrocarbon mixture having a relatively broad molecular weight distribution. This product is predominantly straight chain hydrocarbons which typically have a chain length of more than 2 carbon atoms, for example, 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.
GB 728543 relates to a process for the production of hydrocarbons by the reaction of synthesis gas in the presence of a catalyst which may be suspended in finely divided form within the hydrocarbon oil (contact oil). A mechanically moved stream of contact oil circulating after the separation of the gas, and the synthesis gas is introduced into the reaction chamber below a cooling arrangement disposed therein, suitably by means of one or a series of nozzles. Cooling of the contact oil or mixture of contact oil and gas in the reaction chamber is effected in a number of stages in such manner that the mixture of synthesis gas and contact oil successively flows through cooling stages at increasing temperature. Owing to the fact that the individual cooling stages have a temperature increasing from the bottom upwards, the reaction can be retarded in places where the concentration of carbon monoxide and hydrogen is highest, namely in the lower part of the reaction tower, by the application of low temperatures. In accordance with the reduction of the concentration of the reaction substances, the temperature is then increased in the higher zones of the reaction tower, so that the complete reaction between the carbon monoxide and the hydrogen, corresponding substantially to equilibrium, is obtained in the neighborhood of the top of the reaction tower. Thus, GB 728,543 relates to a plug flow reactor vessel where the reaction conditions vary in the individual cooling stages.
SUMMARY OF THE INVENTION
We have recently found that a Fischer-Tropsch process may be operated by contacting synthesis gas with a suspension of catalyst in a liquid medium in a system comprising at least one high shear mixing zone and a reactor vessel. The suspension is passed through the high shear mixing zone(s) where synthesis gas is mixed with the suspension under conditions of high shear. The shearing forces exerted on the suspension in the high shear mixing zone(s) are sufficiently high that the synthesis gas is broken down into gas bubbles and/or irregularly shaped gas voids. Suspension having gas bubbles and/or irregularly shaped gas voids dispersed therein is then discharged from the high shear mixing zone(s) into the reactor vessel where the majority of the conversion of synthesis gas to liquid hydrocarbon products takes place. The suspension present in the reactor vessel is under such highly turbulent motion that any irregularly shaped gas voids dispersed therein are constantly coalescing and fragmenting over a rapid time frame, for example, over a period of up to 500 ms. The transient nature of these irregularly shaped gas voids results in improved heat transfer and mass transfer of gas into the liquid phase of the suspension when compared with a conventional slurry bubble column reactor. This process is described in WO 0138269 (PCT patent application number GB 0004444) which is herein incorporated by reference.
It has now been found that the catalytic conversion of synthesis gas to hydrocarbon products is improved when the kinetic energy dissipation rate in the high shear mixing zone(s) is at least 0.5 kW/m
3
relative to the total volume of suspension present in the system.
The present invention therefore relates to a process for the conversion of synthesis gas to hydrocarbons by contacting the synthesis gas at an elevated temperature and pressure with a suspension comprising a solid particulate Fischer-Tropsch catalyst suspended in a liquid medium, which contacting takes place in a system comprising at least one high shear mixing zone and a reactor vessel wherein the volume of suspension present in the high shear mixing zone(s) is substantially less than the volume of suspension present in the reactor vessel, suspension is mixed with synthesis gas in the high shear mixing zone(s), the resulting mixture of suspension and synthesis gas is discharged from the high shear mixing zone(s) into the reactor vessel and wherein kinetic energy is dissipated to the suspension present in the high shear mixing zone(s) at a rate of at least 0.5 kW/m
3
relative to the total volume of suspension present in the system.
For avoidance of doubt, the conversion of synthesis gas to hydrocarbons is initiated in the high shear mixing zone(s) although the majority of the conversion generally occurs in the reactor vessel.
Without wishing to be bound by any theory it is believed that when kinetic energy is dissipated to the suspension present in the high shear mixing zone(s) at a rate of at least 0.5 kW/m
3
relative to the total volume of suspension present in the system, the rate of mass transfer of synthesis gas to the suspension is enhanced. Suitably, in the process of the present invention, the volumetric mass transfer rate is in the range 2 to 10,000, preferably, 25 to 1000, more preferably 5 to 100 kg-moles/h of carbon monoxide transferred per m
3
of suspension. Suitably, in the process of the present invention, the mass transfer rate is in the range 5×10
−3
to 5×10
−6
kg-moles carbon monoxide transferred per m
2
of bubble and/or irregularly shaped void surface area per hour.
Preferably, the kinetic energy dissipation rate in the high shear mixing zone is in the range of from 0.5 to 25 kW/m
3
, relative to the total volume of suspension present in the system, more preferably from 0.5 to 10 kW/m
3
, most preferably from 0.5 to 5 kW/m
3
, and in particular, from 0.5 to 2.5 kW/m
3
.
The high shear mixing zone(s) may be part of the system inside or outside the reactor vessel. Preferably, the high shear mixing zone(s) project through the walls of the reactor vessel such that the high shear mixing zone(s) discharges its contents into the reactor vessel. Suitably, the system comprises 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. The plurality of high shear mixing zones may discharge their contents into or may be located within a single reactor vessel which has an advantage of significantly reducing the size of a commercial Fischer-Tropsch plant. It is envisaged that a plurality of the reactor vessels may be connected in series, preferably, 2 to 4, for example, 2 or 3, in which case the high shear mix

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