Fischer-tropsch synthesis

Abstract

A Fischer-Tropsch synthesis process (10) includes feeding gaseous reactants (20) including at least CO, H.sub.2 and CO.sub.2into a reactor (14) holding an iron-based catalyst. The H.sub.2 and CO are fed in a H.sub.2:CO molar ratio of at least 2:1 and the CO.sub.2 and CO are fed in a CO.sub.2:CO molar ratio of at least 0.5:1. The reactor (14) is controlled at an operating temperature in the range from about 260 C. to about 300 C. A liquid product (22) and a gaseous product (24) including hydrocarbons, CO, H.sub.2, water and CO.sub.2 are withdrawn from the reactor (14).

Claims

1. A Fischer-Tropsch synthesis process, the process including feeding gaseous reactants including at least CO, H.sub.2 and CO.sub.2 into a reactor holding an iron-based catalyst, the H.sub.2 and CO being fed in a H.sub.2:CO molar ratio of at least 2:1 and the CO.sub.2 and CO being fed in a CO.sub.2:CO molar ratio of at least 0.5:1; controlling the reactor at an operating temperature in the range from about 260 C. to about 300 C.; and withdrawing a liquid product and a gaseous product including hydrocarbons, CO, H.sub.2, water and CO.sub.2 from the reactor; wherein the approach to water gas shift equilibrium in the gaseous product withdrawn from the reactor, according to Equation 5, is less than 0.9 $\begin{array}{cc}\mathrm{Approach}\mathrm{to}\mathrm{WGS}\mathrm{equilibrium}=\frac{\frac{P\left(H_2\right).Math.P\left({\mathrm{CO}}_2\right)}{P\left(\mathrm{CO}\right).Math.P\left(H_{2}O\right)}}{0.0102\exp \frac{\left(4730\right)}{T}}& 5\end{array}$ where T is the operating temperature of the reactor in kelvin and P is the partial pressure of the gases CO, CO.sub.2, H.sub.2 and water vapour in the gaseous product.

2. A Fischer-Tropsch synthesis process according to claim 1, wherein the H.sub.2 and CO is being fed in a H.sub.2:CO molar ratio of at least about 2.5:1.

3. A Fischer-Tropsch synthesis process according to claim 1, wherein the CO.sub.2 and CO are fed in a CO.sub.2:CO molar ratio of at least about 1:1.

4. A Fischer-Tropsch synthesis process according to claim 1, wherein the CO.sub.2 and CO are fed in a CO.sub.2:CO molar ratio of no more than about 4.

5. A Fischer-Tropsch synthesis process according to claim 4, wherein the CO.sub.2 and CO are fed in a CO.sub.2:CO molar ratio of no more than about 2:1.

6. A Fischer-Tropsch synthesis process according to claim 1, wherein the reactor is controlled at an operating temperature in the range from about 265 C. to about 285 C.

7. A Fischer-Tropsch synthesis process according to claim 1, wherein the approach to water gas shift equilibrium, according to Equation 5, is less than about 0.8.

8. A Fischer-Tropsch synthesis process according to claim 1, wherein the approach to water gas shift equilibrium, according to Equation 5, is at least about 0.2.

9. A Fischer-Trospch synthesis process according to claim 1, wherein at least 10 mass % of hydrocarbons in the liquid product and gaseous product withdrawn from the reactor have a carbon number of at least 19.

10. A Fischer-Tropsch synthesis process according to claim 9, wherein at least 20 mass % of the hydrocarbons in the liquid and gaseous products withdrawn from the reactor have a carbon number of at least 19.

11. A Fischer-Tropsch synthesis process according to claim 1, wherein at least a portion of the gaseous product withdrawn from the reactor is cooled to produce a condensed liquid phase including water which is separated from uncondensed gaseous product, and wherein at least a portion of the uncondensed gaseous product is recycled to the reactor as a recycle gas.

12. A Fischer-Tropsch synthesis process according to claim 11, wherein the volumetric flow rate of the recycle gas is at least about 50% of the volumetric flow rate of the gaseous reactants fed to the reactor.

13. A Fischer-Tropsch synthesis process according to claim 12, wherein the volumetric flow rate of the recycle gas is less than about 83% of the volumetric flow rate of the gaseous reactants fed to the reactor.

14. A Fischer-Tropsch synthesis process according to claim 1, wherein the per pass conversion of CO in the reactor is at least about 10%.

15. A Fischer-Tropsch synthesis process according to claim 1, wherein the per pass conversion of CO in the reactor is no more than about 60%, and/or wherein the CO.sub.2 selectivity is less than about 25%.

16. A Fischer-Tropsch synthesis process according to claim 1, wherein the H.sub.2 and CO is being fed in a H.sub.2:CO molar ratio of at least about 3:1.

17. A Fischer-Tropsch synthesis process according to claim 1, wherein the CO.sub.2 and CO are fed in a CO.sub.2:CO molar ratio of less than about 3.

18. A Fischer-Tropsch synthesis process according to claim 1, wherein the reactor is controlled at an operating temperature in the range from about 265 to about 275 C.

19. A Fischer-Tropsch synthesis process according to claim 1, wherein the per pass conversion of CO in the reactor is at least about 15%.

20. A Fischer-Tropsch synthesis process according to claim 1, wherein the per pass conversion of CO in the reactor is no more than about 50%, and/or wherein the CO.sub.2 selectivity is less than about 20%.

Description

(1) The invention will now be described in more detail with reference to the accompanying diagrammatic drawings and the examples.

(2) In the drawings,

(3) FIG. 1 shows a Fischer-Tropsch synthesis process in accordance with the invention; and

(4) FIG. 2 shows a graph of the water gas shift reaction equilibrium constant (K.sub.WGS) as a function of temperature.

(5) FIG. 3 shows a graph of CO.sub.2 selectivity as a function of the H.sub.2:CO molar ratio and CO.sub.2:CO molar ratio, according to Example 13.

(6) Referring to FIG. 1 of the drawings, a Fischer-Tropsch synthesis process in accordance with the invention is generally indicated by reference numeral 10. The process 10 broadly includes a synthesis gas generation stage 12, a Fischer-Tropsch synthesis stage 14 and a cooling and separation stage 16.

(7) A feed line 18 for a carbonaceous or hydrocarbonaceous material leads to the synthesis gas generation stage 12. A synthesis gas feed line 20 leads from the synthesis gas generation stage 12 to the Fischer-Tropsch synthesis stage 14.

(8) The Fischer-Tropsch synthesis stage 14 is provided with a liquid product withdrawal line 22 and a gaseous product withdrawal line 24. The gaseous product withdrawal line 24 leads to the cooling and separation stage 16, which is provided with a condensed liquid phase withdrawal line 26 and a recycle gas line 28. The recycle gas line 28 leads from the cooling and separation stage 16 to the synthesis gas feed line 20. A purge line 30 branches off from the recycle gas line 28.

(9) In the process 10, a hydrocarbonaceous material (natural gas in the embodiment illustrated in FIG. 1), is fed along the feed line 18 to the synthesis gas generation stage 12. In the synthesis gas generation stage 12, the methane in the natural gas is reformed to provide synthesis gas which includes at least H.sub.2 and CO, with the synthesis gas then being withdrawn by means of the synthesis gas feed line 20 for feeding to the Fischer-Tropsch synthesis stage 14 as gaseous reactants. The synthesis gas generation stage 12 may be any synthesis gas generation stage operable to produce a synthesis gas suitable for Fischer-Tropsch synthesis. The synthesis gas generation stage 12 may employ combinations of more than one synthesis gas generation technology. For example, the synthesis gas generation stage 12 may be supplied with different carbonaceous and/or hydrocarbonaceous feedstocks. Thus, for example, a combination of coal gasification and reforming of a Fischer-Tropsch tail gas may be employed in the synthesis gas generation stage 12, or combinations of different natural gas reforming technologies such as partial oxidation reforming, auto-thermal reforming and steam reforming may be employed in the synthesis gas generation stage 12. Reforming of a hydrocarbonaceous material such as natural gas is preferred over gasification of a carbonaceous material such as coal.

(10) The synthesis gas withdrawn from the synthesis gas generation stage 12 may optionally be subjected to one or more gas treatment steps, e.g. cleaning steps (not shown), where known Fischer-Tropsch catalyst poisons (e.g. H.sub.2S, COS and/or NH.sub.3), or other components, are removed from the synthesis gas prior to the synthesis gas being fed to the Fischer-Tropsch synthesis stage 14. The synthesis gas may also be subjected to one or more steps (not shown) to adjust the composition thereof, e.g. water gas shift and/or CO.sub.2 removal.

(11) The operation of a suitable synthesis gas generation stage 12 and the use of optional synthesis gas treatment steps and optional composition adjustment steps upstream of a Fischer-Tropsch synthesis stage are well-known to those skilled in the art and these process features are thus not described in any detail. Similarly, the operation of a Fischer-Tropsch synthesis stage such as the Fischer-Tropsch synthesis stage 14 is well-known to those skilled in the art and is also not described in any detail.

(12) The synthesis gas in the synthesis gas feed line 20 is mixed with recycle gas from the recycle gas line 28 before the resultant gas stream is fed to the Fischer-Tropsch synthesis stage 14 as gaseous reactants. The recycle gas is rich in CO.sub.2 and typically also includes unreacted H.sub.2 meaning that the use of the recycle gas increases the CO.sub.2:CO molar ratio and typically also the H.sub.2:CO molar ratio of the gaseous reactants being fed into the Fischer-Tropsch synthesis stage 14. The desired H.sub.2:CO molar ratio and the CO.sub.2:CO molar ratio of the gaseous reactants can also be achieved by adjusting the composition of the synthesis gas from the synthesis gas generation step, e.g. in a composition adjustment step, such as water gas shift step or CO.sub.2 removal. As will be appreciated, instead of feeding the recycle gas into the synthesis gas feed line 20 as shown in FIG. 1, the recycle gas can be fed directly to the Fischer-Tropsch synthesis stage 14 without first combining the recycle gas with the synthesis gas from the synthesis gas generation stage 12, as indicated by the broken line 32 in FIG. 1.

(13) In any event, irrespective of the method used to recycle CO.sub.2 to the Fischer-Tropsch synthesis stage 14, the flow rate of recycled CO.sub.2 (and typically also recycled H.sub.2) fed to the Fischer-Tropsch synthesis stage 14 in the form of recycle gas is such that the combined feed of gaseous reactants fed to the Fischer-Tropsch synthesis stage 14 has a H.sub.2:CO molar ratio of at least 2:1, e.g. about 3:1. The CO2 and CO in the gaseous reactants are present in a CO.sub.2:CO molar ratio of at least 0.5:1, but no more than 4, preferably less than 2:1, e.g. about 1:1.

(14) The Fischer-Tropsch synthesis stage 14 includes at least one Fischer-Tropsch synthesis reactor holding an iron-based catalyst. In the embodiment of the invention illustrated, the reactor of the Fischer-Tropsch synthesis stage 14 is a slurry bubble column reactor. In the reactor, a portion of the gaseous reactants fed to the reactor is converted in the presence of the iron-based Fischer-Tropsch catalyst to hydrocarbons via the well-known Fischer-Tropsch synthesis, which is a highly exothermic process. The reactor is controlled at an operating temperature in the range from 260 C. to 300 C., e.g. about 270 C. Typically, the reactor operating temperature is controlled by controlling the amount of heat removed from the reactor by means of a steam generating cooling circuit (not shown), as is well-known to those skilled in the art.

(15) The reactor is typically operated at a pressure in the range of from about 10 bar to about 50 bar, e.g. about 25 bar.

(16) In the reactor of the Fischer-Tropsch synthesis stage 14 and at the operating temperature and pressure of the reactor, hydrocarbons produced by the Fischer-Tropsch synthesis include gaseous hydrocarbons as well as liquid hydrocarbons that extend well into the wax range. In the temperature range of about 260 C. to about 300 C., the Fischer-Tropsch hydrocarbons produced in the reactor in the presence of the iron-based catalyst include economically significant amounts of valuable olefins in the C.sub.6 to C.sub.14 range.

(17) As is known to those skilled in the art, water is a byproduct produced by the Fischer-Tropsch reaction. As is also known to those skilled in the art, iron-based catalysts have an inherent propensity for the WGS reaction. Consequently, a portion of the CO fed to the Fischer-Tropsch synthesis stage 14 as part of the gaseous reactants reacts with the water and is converted to CO.sub.2 according to the WGS reaction of Equation 1. The reactor thus produces, in addition to hydrocarbons (which are in liquid and gaseous form at the operating conditions of the reactor), water, CO.sub.2 and H.sub.2.

(18) It is preferred that the per pass conversion (molar conversion) of CO in the Fischer-Tropsch synthesis stage 14 should be more than 10% but preferably less than about 60%. On the one hand, a too low per pass CO conversion is undesirable since a very high gas recycle rate to the Fischer-Tropsch synthesis stage 14 will be required in order to achieve an acceptable overall conversion of CO (i.e. to prevent excessive carbon loss), which results in an increase in the associated costs, e.g. the cost of recompressing the recycle gas. On the other hand, a too high per pass conversion of CO, which is associated with a high partial pressure of water present in the reactor, will be detrimental since the iron-based catalyst could be damaged by a high water partial pressure in the reactor.

(19) As will be appreciated, as a result of the restricted per pass CO conversion, some of the CO (and thus also some of the H.sub.2, which in any event is present in the reactor in stoichiometric excess) fed to the Fischer-Tropsch synthesis stage 14 will remain unreacted.

(20) The process 10 includes withdrawing gaseous product, which thus includes CO and H.sub.2, CO.sub.2 water (water vapour) and gaseous hydrocarbons formed by the Fischer-Tropsch synthesis, from the Fischer-Tropsch synthesis stage 14 by means of the gas withdrawal line 24.

(21) Fischer-Tropsch liquid product is withdrawn from the Fischer-Tropsch synthesis stage 14 by means of the liquid product withdrawal line 22. Withdrawal of liquid product, i.e. liquid phase from the slurry bed of the slurry bed reactor of the Fischer-Tropsch synthesis stage 14 also serves to maintain the slurry bed at a desired level inside the reactor. The liquid product withdrawn by means of the liquid product withdrawal line 22 is typically fed to downstream units for further processing into final products.

(22) It is to be noted however that in alternative embodiments of the invention (not shown) it is possible for the liquid and gaseous product to leave the Fischer-Tropsch synthesis stage 14 as a combined stream which is then fed to a separation stage where the liquid product and a gaseous product are separated.

(23) The gaseous product withdrawn from the Fischer-Tropsch synthesis stage 14 by means of the gas withdrawal line 24 is fed to the cooling and separation stage 16 where the gaseous product is cooled to produce a condensed liquid phase including water and other components that condense at the conditions in the cooling and separation stage 16. The condensed liquid phase is separated in the cooling and separation stage 16 from uncondensed gaseous product and withdrawn by means of the condensed component withdrawal line 26 for further processing which typically includes separation of reaction water from valuable hydrocarbons.

(24) The uncondensed gaseous product which includes H.sub.2, CO, and CO.sub.2 is withdrawn from the cooling and separation stage 16 by means of the recycle gas line 28 and recycled as recycle gas to the Fischer-Tropsch synthesis stage 14 as hereinbefore described. A portion of the uncondensed gaseous product from the cooling and separation stage 16 may be purged by means of the purge line 30 to control the accumulation of CO.sub.2 and inerts, e.g. nitrogen, in the process 10.

(25) It will be appreciated that the invention is not limited to a process such as the process 10 where a single synthesis gas feed stream from a synthesis gas generation stage is combined with a single recycle gas stream to form a single total feed stream which is introduced into a Fischer-Tropsch reactor. The invention can, for example, equally well be applied in a process where one or more fresh synthesis gas feed streams are introduced separately from a CO.sub.2-containing recycle gas stream into a Fischer-Tropsch reactor. Under such circumstances, the total gaseous reactants feed composition and flow rate fed into the reactor is simply the composition and flow rate of a stream that would have formed had all fresh synthesis gas streams and recycle gas streams been combined prior to entry into the reactor. Similarly, the gas recycle ratio can be defined as the volumetric flow rate of recycle gas divided by the volumetric flow rate of gaseous reactants, which is the sum of all the fresh synthesis gas streams and the recycle gas stream(s), irrespective of whether said streams are physically combined prior to entering the reactor.

(26) At the conditions described for the process 10, i.e. a relatively low Fischer-Tropsch reactor operating temperature and relatively low CO.sub.2 feed concentration (i.e. a relatively low CO.sub.2:CO molar ratio) to the Fischer-Tropsch reactor, it would be expected that the forward reaction of the water gas shift reaction would be favoured, resulting in a large production of CO.sub.2 in the Fischer-Tropsch synthesis stage 14. Surprisingly, it has however been found for the process of the invention that this is not the case. Instead, at the described operating conditions, the CO.sub.2 production in the Fischer-Tropsch synthesis stage 14 is reduced even though the conditions (i.e. partial pressures of CO, H.sub.2O, CO.sub.2 and H.sub.2) at the point where gaseous products are withdrawn from the Fischer-Tropsch reactor are still far from the water gas shift equilibrium at the operating temperature of the Fischer-Tropsch reactor.

(27) FIG. 2 illustrates the operating regime or envelope for the process according to some embodiments of the invention. Operating conditions for the Fischer-Tropsch synthesis stage 14, e.g. reactor feed stream composition, tail gas recycle rate, reactor temperature and pressure, catalyst loading and residence time of reactants are selected such that the reactor or reactors is/are operating at a temperature between about 260 C. and about 300 C., at an equilibrium constant for the WGS reaction (K.sub.WGS) according to Equation 2, falling between the graphs for 0.2 K.sub.WGS and 0.9 K.sub.WGS, preferably between the graphs for 0.2 K.sub.WGS and 0.8 K.sub.WGS, more preferably between the graphs for 0.2 K.sub.WGS and 0.6 K.sub.WGS

(28) Advantageously, by operating at the conditions described herein it is thus not necessary to achieve water gas shift equilibrium in order to suppress CO.sub.2 formation during Fischer-Tropsch synthesis using an iron-based catalyst. This in turn negates the need for a large gas recycle rate as taught in the prior art when carrying out Fischer-Tropsch synthesis at a temperature below 300 C. Further, as illustrated in the following examples, a higher H.sub.2:CO molar ratio by itself is not sufficient to lower the CO.sub.2 selectivity, but it is necessary simultaneously to increase the amount of CO.sub.2 being fed to the Fischer-Tropsch synthesis stage in order to observe a considerable reduction in the CO.sub.2 selectivity. Similarly, an increase in the CO.sub.2:CO molar ratio alone is not sufficient to reduce CO.sub.2 production in the Fischer-Tropsch synthesis stage 14. Thus, also the H.sub.2:CO molar ratio needs to be raised simultaneously with an increase in the CO.sub.2:CO molar ratio.

(29) At a reactor temperature in the operating temperature range for the process of the invention, it would be expected that the suppression of CO.sub.2 formation in the Fischer-Tropsch synthesis stage 14 could be achieved by the continued increasing of the CO.sub.2 content of the feed to the Fischer-Tropsch synthesis stage 14 to the extent that water gas shift equilibrium is reached. However, such a suggestion would lead to an uneconomical process, particularly in a process where the CO.sub.2 is provided by recycle of tail gas derived from the Fischer-Tropsch synthesis stage 14, as in the process 10, as this would mean an economically unfeasible high recycle rate to achieve water gas shift equilibrium. Advantageously, the process 10 of the invention, as illustrated, provides an alternative approach to reducing the CO.sub.2 selectivity in a Fischer-Tropsch synthesis process when employing a shifting Fischer-Tropsch catalyst such as an iron-based catalyst. Surprisingly, by operating the Fischer-Tropsch synthesis stage or reactor at a temperature between about 260 C. and about 300 C., at the selected ranges for the H.sub.2:CO and CO.sub.2:CO molar ratios, the inventors have found that it is possible to reduce the CO.sub.2 selectivity below 25%, without requiring operation of the Fischer-Tropsch synthesis stage at the water gas shift equilibrium.

(30) In addition to the suppression of the overall CO.sub.2 production, the Fischer-Tropsch product obtained from the Fischer-Tropsch synthesis process of the invention advantageously extends into the wax range and contains economically significant amounts of primarily linear -olefins. Moreover, the ability to operate at a higher temperature than the typical LTFT synthesis process leads to a favourable reaction rate for the Fischer-Tropsch synthesis reaction.

EXAMPLES

(31) A number of experimental examples are provided. In the examples provided, the terms example and experiment have been used interchangeably.

Example 1

(32) A Fischer-Tropsch synthesis experiment was performed in a once-through laboratory scale slurry reactor.

(33) Activation Method

(34) A shifting iron-based Fischer-Tropsch catalyst promoted with copper and potassium, having a composition on a mass basis of 100Fe/5Cu/20SiO.sub.2/5K.sub.2O was added to a heavy wax fraction inside a slurry reactor. The slurry reactor was closed and pressurised with Argon to 15 bar. The temperature of the reactor was increased to 255 C., and at that temperature, synthesis gas having an H.sub.2/CO molar ratio of 1.5 was passed through the reactor at a GHSV of 12000 ml(N)/g.sub.catalyst/hr and maintained for 16 hours.

(35) Fisher Tropsch Synthesis

(36) Once the iron-based catalyst had been activated using the above method, the reactor temperature was increased to 270 C. The reactor used was equipped with a mechanical stirrer and the stirring rate was sufficiently high to ensure that the reactor operated like a continuous stirred tank reactor, i.e. void of any significant temperature or concentration gradients. Synthesis gas, having a H.sub.2/CO molar ratio of 1.6, but containing no CO.sub.2 was fed to the reactor and outlet gas from the reactor was measured with a gas chromatograph. Argon was co-fed with the synthesis gas as an internal standard to interpret the gas chromatograph data quantitatively. The experiment was performed at a reactor temperature of 270 C. The results of the experiment are presented in Table 1. The CO.sub.2 selectivity was found to be very high (around 27%), as would be expected under these operating conditions.

Example 2

(37) Example 2 was carried out following the same method as that of Example 1 and employed the same inlet H.sub.2/CO molar ratio of 1.6, but in this case a substantial amount of CO.sub.2 was added to the feed to the extent that the feed CO.sub.2/CO molar ratio was 1.1. By suitably adjusting the reactor pressure and/or the flow rate of the Argon internal standard, it was ensured that the partial pressures of CO and H.sub.2 in the reactor were kept similar to that of Example 1. The co-fed CO.sub.2 hardly affected the CO.sub.2 selectivity, as would be expected from prior art teachings, seeing that the conditions at the reactor outlet were still very far from thermodynamic equilibrium. The results of the experiment are presented in Table 1.

Example 3

(38) Example 3 was carried out following the same method as that of Example 1, but was performed with an H.sub.2/CO molar ratio of 4.6, and with no CO.sub.2 in the reactor feed. The CO.sub.2 selectivity was again high, at about 19%, again according to expectations from the prior art teachings. The results of the experiment are presented in Table 1.

Example 4

(39) Example 4 was performed following the same method as that of Example 2, but with a similar inlet H.sub.2/CO molar ratio as that of Example 3. However, CO.sub.2 was added to the feed to obtain an inlet CO.sub.2/CO molar ratio of 0.7 in accordance with the present invention. Again the reactor pressure and/or Argon flow rate was adjusted to achieve the CO and H.sub.2 partial pressures in the reactor feed as for Example 3. The CO.sub.2 selectivity (around 10%) was substantially lower than in the case of Example 3. This means that the net CO.sub.2 formation rate was suppressed substantially by the addition of CO.sub.2 to the feed, despite the fact that the reactor outlet was still very far from thermodynamic equilibrium. The results of the experiment are presented in Table 1.

Example 5

(40) Example 5 was also performed following the same method as that of Example 2. However, an even larger amount of CO.sub.2 was added to the feed obtaining a CO.sub.2/CO molar ratio of 2.3 which suppressed CO.sub.2 formation to the point where the process had a net CO.sub.2 selectivity close to that of a non-shifting Fischer-Tropsch process. This is despite the fact that conditions in the outlet were still very far from thermodynamic equilibrium. The results of the experiment are presented in Table 1.

Examples 6 to 12

(41) Examples 6 to 12 were performed following the same method as that of Example 1, but sometimes with CO.sub.2 being fed to the reactor. However, the H.sub.2/CO and the CO.sub.2/CO molar ratios were varied in a range of from 2 to 3 and from 0 to 1 respectively. The results show that a significant reduction in the CO.sub.2 selectivity is achieved by raising the H.sub.2/CO molar ratio above 2 and the CO.sub.2/CO molar ratio above 0.5. The results also show that this reduction in CO.sub.2 production is achieved even though WGS equilibrium has not been reached. The closest approach to WGS equilibrium, 55.4%, is obtained in the case of the highest CO.sub.2/CO molar ratio of 2.3 and the highest H.sub.2/CO molar ratio, and at such conditions a CO.sub.2 selectivity of 2.8% is achieved, which is very close to that typically seen for cobalt-based catalyst (non-shifting catalyst). The results of the experiments are presented in Table 1.

(42) TABLE-US-00001 TABLE 1 Experimental results for Examples 1-12 Experiment number 1 2 3 4 5 6 7 8 9 10 11 12 H.sub.2/CO inlet (molar ratio) 1.6 1.6 4.6 4.5 4.1 2.0 2.0 2.5 2.5 3.0 3.0 2.0 CO.sub.2/CO inlet (molar ratio) 0.0 1.1 0.0 0.7 2.3 0.0 1.0 0.0 1.0 0.0 1.0 0.5 H.sub.2/CO outlet (molar ratio) 1.9 1.8 8.1 7.1 5.6 3.1 2.8 4.1 3.8 5.4 4.8 2.6 CO.sub.2/CO outlet (molar 0.3 2.3 0.2 1.5 3.9 0.4 2.5 0.3 2.5 0.3 2.4 1.2 ratio) Conversion %: H.sub.2 39.6 39.3 19.1 17.8 18.6 37.1 37.6 31.5 31.5 26.4 26.6 33.0 Conversion %: CO 51.3 47.5 53.9 47.9 40.2 59.3 55.1 59.1 54.5 59.0 53.8 49.5 Conversion %: CO + H.sub.2 44.2 42.5 25.3 23.3 22.8 44.5 43.5 39.5 38.1 34.5 33.4 38.5 CO.sub.2 selectivity (molar %) 26.8 26.7 18.7 10.1 2.8 26.0 20.5 23.9 19.7 23.6 17.0 24.8 CH.sub.4 selectivity (mass %) 2.6 2.9 5.7 5.5 4.3 2.6 2.8 4.7 5.0 7.1 6.8 5.6 Equilibrium constant 61.9 61.9 61.9 61.9 61.9 61.9 61.9 61.9 61.9 61.9 61.9 61.9 P(H.sub.2)P(CO.sub.2)/ 1.1 10.1 2.4 14.6 34.3 1.7 9.4 1.9 12.7 2.4 14.7 6.4 P(CO)P(H.sub.2O) Approach to Equilibrium 1.8 16.3 3.9 23.5 55.4 2.7 15.1 3.1 20.6 3.9 23.7 10.4 (%) Reactor outlet partial pressures (bar) H.sub.2 8.7 8.7 14.4 14.4 13.9 9.6 9.6 12.2 12.3 13.7 13.7 11.2 CO 4.4 4.7 1.8 2.0 2.5 3.1 3.5 3.0 3.3 2.5 2.9 4.3 H.sub.2O 2.2 2.0 1.3 1.5 1.6 2.2 2.5 2.2 2.4 1.9 2.2 2.1 CO.sub.2 1.3 11.1 0.4 3.1 9.6 1.2 8.6 1.0 8.0 0.9 6.7 5.2

Example 13

(43) Fischer-Tropsch synthesis experiments were performed in the same reactor and under the same reactor conditions as those of Example 1 and at the same temperature of 270 C. However, the Fischer-Tropsch synthesis was performed in the presence of a shifting iron-based Fischer-Tropsch catalyst promoted with potassium, having a composition on a mass basis of 100Fe/4.4Si/1.2K. The catalyst is a Low Alpha catalyst obtained from CAER (University of Kentucky). The iron-based catalyst was activated following the same activation method of Example 1. The experiments were initially carried out in the absence of CO.sub.2 in the feed to the reactor, and the H.sub.2/CO molar ratio was varied from 1.5 to 3. Then CO.sub.2 was added to the feed at a CO.sub.2:CO molar ratio of 1, and the H.sub.2:CO molar ratio was varied according to the initial experiment. The results of the experiments are presented in FIG. 3. As it is apparent, while increasing the H.sub.2:CO ratio does have an effect in decreasing the CO.sub.2 selectivity, it is only at the addition of CO.sub.2 at a CO.sub.2:CO molar ratio of above 0.5 in the feed that a significant reduction in the CO.sub.2 selectivity is seen. At these conditions the WGS equilibrium is not reached. The approach to WGS equilibrium in the case of the absence of CO.sub.2 in the feed into to the reactor was reported to be in the range of from about 9 to about 15%. In the case of CO.sub.2 being fed together with the synthesis gas into the reactor, the approach to WGS equilibrium was reported to be in the range of from about 23 to about 37%, which is significantly low.

Examples 14 & 15

(44) Examples 14 and 15 were performed following the same method as that of Example 1, with CO.sub.2 being fed to the reactor in both instances. The CO, H.sub.2 and CO.sub.2 were fed into the reactor at H.sub.2/CO and the CO.sub.2/CO molar ratios of 4.4 and 1.6 respectively (Example 14), and 4.2 and 2.3 respectively (Example 15). In each instance, the selectivities of hydrocarbons with a carbon number of at least 5 (C5+), carbon number of at least 10 (C10+), and a carbon number of at least 19 (C19+) were measured using a gas chromatograph. The selectivities are provided in Table 2 below, and expressed as a fraction (in percentage) of the total hydrocarbons product, i.e. excluding the CO.sub.2 in the product.

(45) TABLE-US-00002 TABLE 2 Experimental results for Examples 14 & 15 Experiment number 14 15 H.sub.2/CO inlet (molar ratio) 4.4 4.2 CO.sub.2/CO inlet (molar ratio) 1.6 2.3 H.sub.2/CO outlet (molar ratio) 6.0 5.2 C5+ Hydrocarbon Selectivity (mass %) 59.5 61.8 C10+ Hydrocarbon Selectivity (mass %) 33 29.5 C19+ Hydrocarbon Selectivity (mass %) 24 24.2

Example 4A and 5A

(46) The results of Experiments 4 and 5 were used to construct design cases for implementing the invention as a commercial Fischer-Tropsch synthesis reactor operated under recycle. The results are presented in Table 2 for purposes of illustration. With fresh feed synthesis gas compositions that can realistically be achieved on a commercial scale, and operating the reactors under recycle, the inlet and outlet reactor conditions of the experimental runs could be matched. As can be seen from Table 2, high overall conversions can be achieved. This shows that the invention can be implemented as a feasible commercial process at industrial scale, contrary to expectations from the prior art. The process designs based on Experiments 4 and 5 can of course be further improved with optimisation approaches known to those skilled in the art, e.g. by developing reliable Fischer-Tropsch process models that can predict reactor performance over a range of conditions so that a Fischer-Tropsch synthesis stage operated in accordance with the invention can be optimally integrated with a synthesis gas generation stage.

(47) TABLE-US-00003 TABLE 3 Reactor design information based on the results of Examples 4 and 5 Experiment number 4A 5A Fresh Feed composition (mol %) CO 30.2 29.8 H.sub.2 67.8 63.9 CO.sub.2 2.0 6.3 Fresh feed H.sub.2/CO molar ratio 2.2 2.1 Overall CO conversion (%) 89.7 93.9 Overall H.sub.2 conversion (%) 67.7 84.1 Overall synthesis gas conversion 74.5 87.2 (%) Recycle ratio 2.7 4.5