Fischer-tropsch synthesis process

Details Fischer-tropsch synthesis process Fischer-tropsch synthesis process

2003-06-17

2004-12-28

Parsa, J. (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

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

C518S700000, C518S710000

Reexamination Certificate

active

06835756

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 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 which employ particulate fluidised beds in slurry bubble column reactors are described in, for example, U.S. Pat. Nos. 5,348,982; 5,157,054; 5,252,613; 5,866,621; 5,811,468; and 5,382,748. Slurry bubble column reactors operate by suspending catalytic particles in a liquid and feeding gas phase reactants into the bottom of the reactor through a gas distributor which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalytic system, they can be converted to both liquid and gaseous products. If gaseous products are formed, they enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered by passing the slurry through a filter which separates the liquid from the catalytic solids. A principal advantage of slurry reactors over fixed bed reactors is that the pressure of a circulating/agitated slurry phase greatly increases the transfer rate of heat to cooling surfaces built into the reactor. A distinct advantage of bubble columns over mechanically stirred reactors is that the required mixing is effected by the action of rising bubbles, a process significantly more efficient in energy than mechanical stirring.
U.S. Pat. No. 5,252,613 described a method and means for improving catalyst particle distribution and mixing in slurry bubble column, the catalyst being 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, said improved catalyst distribution and mixing being obtained by introducing a secondary stream of gas into the slurry bubble column by use of a secondary gas introduction means located within the column at a location above the gas distribution means at the bottom of the slurry bubble column. The secondary gas stream may comprise a portion of the reactive feed gas or recycle gas or it may be separately added inert gas, or condensed light hydrocarbons or process end products which vaporize under the conditions present at the location of introduction.
SUMMARY OF THE INVENTION
It has now been found that at least a portion of the heat of reaction can be efficiently removed from a slurry by vaporising a low boiling solvent in a slurry reactor, withdrawing a gaseous stream comprising unconverted synthesis gas and vaporised low boiling solvent from the slurry reactor, cooling the gaseous stream to a temperature sufficient to form a two phase mixture of gas and condensed liquid and recycling the condensed liquid and gas either separately or together to the slurry reactor. Evaporation of the low boiling solvent in the slurry reactor and cooling of the gaseous recycle stream results in the removal of at least a portion of the heat of reaction. Evaporation of the low boiling solvent in the slurry reactor also assists in maintaining the catalyst particles suspended in the slurry.
Accordingly, the present invention relates to a process for the conversion of synthesis gas to liquid hydrocarbon products comprising:
a) contacting, in a slurry reactor, synthesis gas at an elevated temperature and pressure with a suspension of catalyst in a liquid medium,
b) introducing a low boiling solvent into the slurry reactor
c) vaporising at least a portion of the low boiling solvent in the slurry reactor,
d) withdrawing from the slurry reactor, a gaseous stream comprising unreacted synthesis gas and vaporised low boiling solvent,
e) cooling at least a portion of the gaseous stream to a temperature at which liquid condenses out so as to form a two phase mixture of gas and condensed liquid, and
f) recycling at least a portion of the gas and at least a portion of the condensed liquid to the slurry reactor.
The process of the present invention is advantageous in that it can reduce or eliminate altogether the need for removal of heat of reaction from the slurry reactor by heat exchange of the slurry with a heat transfer material which may, for example, be circulating on the shell side of a shell and tube reactor when the Fischer Tropsch reaction takes place in the tubes, or through the tubes when the reaction takes place on the shell side. Without wishing to be bound by any theory, it is believed that vaporisation of the low boiling solvent in the slurry reactor and cooling of at least a portion of the withdrawn gaseous stream to below a temperature at which liquid condenses out, removes at least some of the exothermic heat of reaction thereby allowing more control over the product selectivities and minimising the production of gaseous by-products, for example, methane.
The slurry reactor may be any reactor suitable for carrying out highly exothermic, three phase, catalytic reactions. Suitably, the slurry reactor is a “slurry bubble column” as described in, for example, U.S. Pat. Nos. 5,348,982; 5,157,054; 5,252,613; 5,866,621; 5,811,468; and 5,382,748 which are herein incorporated by reference.
Suitably, the ratio of hydrogen to carbon monoxide in the synthesis gas is in the range of from 20:1 to 0.1:1, especially 5:1 to 1:1 by volume, typically 2:1 by volume. The synthesis gas may contain additional components such as nitrogen, water, carbon dioxide and lower hydrocarbons such as unconverted methane.
Preferably, the liquid hydrocarbon products comprise a mixture of hydrocarbons having chain lengths 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. For avoidance of doubt by “liquid hydrocarbon products” is meant hydrocarbons which are liquid under the process conditions.
Low boiling solvent is defined herein as a solvent having a boiling point, at standard pressure, in the range of from 30 to 280° C., preferably from 30 to 210° C. Preferably, the low boiling solvent is selected from the group consisting of aliphatic hydrocarbons having from 5 to 10 carbon atoms, alcohols (preferably, alcohols having from 1 to 4 carbon atoms, in particular, methanol), and water. In order to simplify the process, it is preferred that the low boiling solvent is a low boiling liquid hydrocarbon product or mixtures thereof, such as hydrocarbon products having from 5 to 10 carbon atoms, in particular, pentanes, hexanes, or hexenes.
The liquid medium may comprise a low boiling solvent and/or a high boiling solvent. By high boiling solvent is meant a solvent having a boiling point, at standard pressure of greater than 280° C. In order to simplify product recovery, it is preferred that the high boiling solvent is a high boiling liquid hydrocarbon product.
For practical reasons the slurry reactor is generally not totally filled with suspension during the process of the present invention so that above a certain level of suspension a

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