Start-up procedure for a Fischer-Tropsch process

Abstract

The present invention generally relates to a Fischer-Tropsch process, in particular a Fischer-Tropsch process for converting a feed comprising a mixture of hydrogen and carbon monoxide gases, preferably in the form of a synthesis gas mixture, to hydrocarbons by contacting a cobalt-containing Fischer-Tropsch synthesis catalyst with a mixture of hydrogen and carbon monoxide in a reactor at a pressure of 4.0 MPa absolute or greater, wherein the process is initiated by a start-up procedure comprising the steps of: i) providing a feed comprising a mixture of hydrogen and carbon monoxide gases, preferably in the form of a synthesis gas mixture, to a reactor containing a cobalt-containing Fischer-Tropsch synthesis catalyst, wherein the pressure inside the reactor is 3.5 MPa absolute or below; and ii) maintaining the feed to the reactor, removing a product stream comprising hydrocarbons and maintaining the pressure inside the reactor at 3.5 MPa absolute or below for at least 15 hours, preferably for at least 50 hours.

Claims

1. A Fischer-Tropsch process for converting a feed comprising a mixture of hydrogen and carbon monoxide gases to hydrocarbons, the method comprising initiating the Fischer-Tropsch process by a start-up procedure comprising the steps of: i) providing a feed comprising a mixture of hydrogen and carbon monoxide gases to a reactor containing a cobalt-containing Fischer-Tropsch synthesis catalyst, wherein the pressure inside the reactor is 3.5 MPa absolute or below; and ii) maintaining the feed to the reactor, removing a product stream comprising hydrocarbons and maintaining the pressure inside the reactor at 3.5 MPa absolute or below for at least 15 hours; and conducting the Fischer-Tropsch process by contacting the cobalt-containing Fischer-Tropsch synthesis catalyst with the mixture of hydrogen and carbon monoxide in a reactor at a pressure of 4.0 MPa absolute or greater, and at a temperature less than a temperature in the reactor during initiating by the start-up procedure.

2. A process according to claim 1, wherein the pressure during steps i) and ii) of the start-up procedure is 3.3 MPa absolute or below, and/or wherein the pressure during steps i) and ii) of the start-up procedure is greater than 0.2 MPa.

3. A process according to claim 1, wherein the pressure in the reactor for the Fischer-Tropsch process conducted after completion of the start-up procedure is from 0.5 MPa to 3.5 MPa higher than the pressure in step (i) and/or step (ii).

4. A process according to claim 1, wherein the pressure in the reactor in step ii) of the start-up procedure is maintained for 100 to 600 hours.

5. A process according to claim 1, wherein the pressure and/or temperature in the reactor during step i) of the start-up procedure is different from the pressure and/or temperature in the reactor during step ii) of the start-up procedure.

6. A process according to claim 1, wherein the pressure in the reactor during the Fischer-Tropsch process conducted after completion of the start-up procedure is less than 10.0 MPa absolute.

7. A process according to claim 1, wherein the start-up procedure and/or the subsequent Fischer-Tropsch process following the start-up procedure is conducted at a temperature in the range of from 100 to 400 C.

8. A process according to claim 1, wherein the start-up procedure occurs in the same reactor as where the Fischer-Tropsch process is conducted after completion of the start-up procedure.

9. A process according to claim 1, wherein the cobalt-containing Fischer-Tropsch synthesis catalyst is a supported catalyst.

10. A process according claim 9, wherein the support material comprises a material selected from any of silica, alumina, silica/alumina, ceria, titania, gallia, zirconia, magnesia, zinc oxide and mixtures thereof.

11. A process according to claim 1, wherein the cobalt-containing Fischer-Tropsch synthesis catalyst comprises one or more promoters, dispersion aids, strength aids and/or binders.

12. A process according to claim 1, wherein during the Fischer-Tropsch process conducted after completion of the start-up procedure, the C.sub.5+ selectivity is at least 70%, and/or wherein the C.sub.5+ productivity is at least 90 g/L.Math.h, and/or wherein the CH.sub.4 selectivity is less than 15%.

13. A process according to claim 1, wherein during the Fischer-Tropsch process conducted after completion of the start-up procedure, the CO conversion is greater than 60%.

14. A process according to claim 1, wherein the process increases the selectivity of a Fischer-Tropsch process for the production of C.sub.5+ hydrocarbons or conversion in a Fischer-Tropsch process as compared to the Fischer-Tropsch process performed without initiating by the start-up procedure.

15. A process according to claim 1, wherein the process increases the selectivity of a Fischer-Tropsch process for the production of C.sub.5+ hydrocarbons and the conversion in a Fischer-Tropsch process as compared to the Fischer-Tropsch process performed without initiating by the start-up procedure.

16. A process according to claim 1, wherein the mixture of hydrogen and carbon monoxide gases is in the form of a synthesis gas mixture.

17. A process according to claim 1, wherein the Fischer-Tropsch catalyst is at least partially reduced prior to the start-up procedure.

18. A Fischer-Tropsch process for converting a feed comprising a mixture of hydrogen and carbon monoxide gases to hydrocarbons, the method comprising initiating the Fischer-Tropsch process by a start-up procedure comprising the steps of: i) providing a feed comprising a mixture of hydrogen and carbon monoxide gases to a reactor containing an at least partially reduced cobalt-containing Fischer-Tropsch synthesis catalyst, wherein the pressure inside the reactor is 3.5 MPa absolute or below; and ii) maintaining the feed to the reactor, removing a product stream comprising hydrocarbons and maintaining the pressure inside the reactor at 3.5 MPa absolute or below for at least 15 hours; and conducting the Fischer-Tropsch process by contacting the cobalt-containing Fischer-Tropsch synthesis catalyst with the mixture of hydrogen and carbon monoxide in a reactor at a pressure of 4.0 MPa absolute or greater.

Description

(1) The invention will now be further described by reference to the following Examples which are illustrative only. In the Examples, CO conversion is defined as moles of CO used/moles of CO fed100 and carbon selectivity as moles of CO attributed to a particular product/moles of CO converted100. Unless otherwise stated, temperatures referred to in the Examples are applied temperatures and not catalyst/bed temperatures. Unless otherwise stated, pressures referred to in the Examples are absolute pressures.

Example 1

(2) Catalyst Preparation

(3) The catalyst was prepared by the impregnation of a zinc oxide support with a sufficient quantity of an aqueous cobalt nitrate hexahydrate solution to achieve a cobalt loading of 10.5 wt % (this is 10.5 wt. % cobalt atoms compared to the total mass of catalyst which has been calcined but not yet reduced). The impregnated powder was extruded, dried and calcined.

Example 2 (Invention)

(4) Start-Up Procedure at 3.3 MPa, Operation at 4.3 MPa

(5) 10 ml of the catalyst prepared in Example 1 was charged into a microreactor and a reduction was completed under hydrogen in the microreactor to form a catalyst that is at least partially reduced (10 h, 240 C., 50% H.sub.2/N.sub.2, 0.7 MPa). The gaseous supply was switched to a mixture of hydrogen and carbon monoxide (H.sub.2/CO=1.8) further comprising 18% nitrogen by volume, which was introduced into the reactor at a gas hourly space velocity (GHSV) of 1250 h.sup.1 and at an applied temperature of 150 C. The temperature was then increased incrementally from 150 C. to 160 C. at a rate of 60 C./h, from 160 to 180 at a rate of 10 C./h, from 180 to 190 at a rate of 5 C./h before a final increase at 1 C./h to reach 60 to 65% CO conversion. This final temperature was then maintained for approximately 300 hours at a pressure of 3.3 MPa. The pressure in the reactor was then increased to 4.3 MPa and the applied temperature was varied to achieve a CO conversion level of 60 to 65% (210 C.) and maintained at this temperature throughout the Fischer-Tropsch synthesis.

Example 3 (Comparative)

(6) Constant Operation at 4.3 MPa

(7) The procedure of Example 2 was followed, except that a temperature of 216 C. and a pressure of 4.3 MPa were maintained from start-up and throughout the Fischer-Tropsch synthesis.

Example 4 (Invention)

(8) Start-Up Procedure at 3.3 MPa, Operation at 4.3 MPa

(9) The procedure of Example 2 was followed, except that the applied temperature was 229 C. during the start-up procedure and 219 C. during the subsequent Fischer-Tropsch synthesis.

Example 5 (Comparative)

(10) Constant Operation at 4.3 MPa

(11) The procedure of Example 3 was followed, except that the applied temperature was 219 C. throughout.

Example 6 (Invention)

(12) Start-Up Procedure at 3.3 MPa, Operation at 4.3 MPa

(13) The procedure of Example 2 was followed, except that the temperature was 212 C. during the start-up procedure and 213 C. during the Fischer-Tropsch synthesis.

Example 7 (Invention)

(14) Start-Up Procedure at 3.3 MPa, Operation at 4.3 MPa

(15) The process of Example 2 was monitored at 1560 hours on-stream at an applied temperature of 210 C.

Example 8 (Comparative)

(16) Constant Operation at 4.3 MPa

(17) The process of Example 3 was monitored at 1560 hours on-stream at an applied temperature of 214 C.

Example 9 (Comparative)

(18) Constant operation at 4.3 MPa then drop to 3.3 MPa

(19) The process of Example 8, running at 4.3 MPa was adjusted to drop the pressure from 4.3 MPa to 3.3 MPa. Data from Example 9 relates to the time period at 3.3 MPa.

Example 10 (Comparative)

(20) Constant Operation at 3.3 MPa

(21) The procedure of Example 3 was followed, except that the temperature was 213 C. and the pressure was 3.3 MPa throughout.

Example 11 (Invention)

(22) Start-Up Procedure at 1.3 MPa, Operation at 4.3 MPa

(23) The procedure of Example 2 was followed, except that the pressure was 1.3 MPa, the temperature was 220 C. and the GHVS was 800 h.sup.1 during the start-up procedure and the temperature was 208 C. during the Fischer-Tropsch synthesis.

Example 12 (Invention)

(24) Start-Up Procedure at 2.3 MPa, Operation at 4.3 MPa

(25) The procedure of Example 2 was followed, except that the pressure was 2.3 MPa and the temperature was 220 C. during the start-up procedure and the temperature was 205 C. during the Fischer-Tropsch synthesis.

(26) CO conversion, CH.sub.4 selectivity, and C.sub.5+ selectivity data were compiled and results for the above Examples are provided in Table 1 below. Exit gasses were sampled by on-line mass spectrometry and analysed. The C.sub.5+ selectivity is determined by difference from the C.sub.1-C.sub.4 components in the gas phase. The CH.sub.4 selectivity is determined by difference from the C.sub.2+ components in the gas phase. Values for CO conversion, CH.sub.4 selectivity, and C.sub.5+ selectivity are average values obtained at steady state over 200 to 700 hours on stream.

(27) TABLE-US-00001 TABLE 1 CO CH.sub.4 C.sub.5+ Applied Pressure Conversion Selectivity Selectivity Temp. Procedure (MPa) (%) (%) (%) ( C.) Example 2 3.3 (st) 59.28 14.74 76.74 213 4.3 65.44 11.31 81.56 210 Example 3 4.3 65.69 15.3 74.05 216 (comparative) Example 4 3.3 (st) 63.06 20.25 61.92 229 4.3 65.58 14.3 73.74 219 Example 5 4.3 64.6 18.6 69.1 219 (comparative) Example 6 3.3 (st) 63.4 15.6 74.1 212 4.3 63.2 13.0 77.5 213 Example 7 4.3 65.1 11.4 81.1 210 Example 8 4.3 61.4 13.0 76.9 214 (comparative) Example 9 3.3 46.7 16.0 71.9 214 (comparative) Example 10 3.3 65.1 19.9 69.4 213 (comparative) Example 11 1.3 (st) 56.4 28.5 52.1 220 4.3 63 12.6 77.8 208 Example 12 2.3 (st) 64.2 24.5 60.5 220 4.3 62.2 12.0 79.3 205 (st) = start-up phase. Rows in bold are post-start-up Fischer-Tropsch synthesis according to the invention

(28) The results in Table 1 show that significant improvements can be obtained by use of a start-up procedure at a pressure lower than 3.5 MPa followed by operation at a pressure of 4.0 MPa or greater. For example, at comparable levels of CO conversion, the processes using the start-up procedure of the present invention advantageously show increased C.sub.5+ selectivity and decreased methane selectivity. For example, a comparison of Examples 2 and 3 shows that where the present start-up procedure is not followed (i.e. Example 3), in order to achieve the same level of CO conversion, a higher reaction temperature is necessary, which leads to higher CH.sub.4 selectivity and lower C.sub.5+ selectivity. As previously mentioned, this reduction in CH.sub.4 selectivity and the enhanced C.sub.5+ selectivity is vital to improving Fischer-Tropsch commercialisation. Comparison of Examples 7 and 8 also shows that this advantageous selectivity is maintained over a longer time period, with the improvement still observed after 1560 hours on stream.

(29) Furthermore, comparison of Examples 4 and 5 shows that, using the same raised temperature during the Fischer-Tropsch synthesis, where the present start-up procedure was used (Example 4) all parameters show an improvement over the constant pressure process (Example 5). Example 6 also demonstrates improved results at an intermediate temperature between that of Examples 2 and 4.

(30) Example 9 shows that moving from a constant 4.3 MPa process to 3.3 MPa gives a large reduction in conversion and inferior C.sub.5+ selectivity, confirming that the start-up procedure is vital rather than the process pressure per se. Similarly, Example 10 shows that simply starting the process at 3.3 MPa and maintaining this pressure leads to lower C.sub.5+ selectivity and higher CH.sub.4 selectivity.

(31) Examples 11 and 12 show that the start-up procedure may equally be carried out at the lower pressures of 1.3 MPa and 2.3 MPa respectively, showing improved selectivity comparable with the start-up procedure at 3.3 MPa.

(32) Comparison of Example 10 with Examples 3 and 5 shows that, although higher methane selectivity is observed for the process at 3.3 MPa, catalyst activity overall is higher, requiring a lower temperature to reach the same level of conversion. This indicates that the catalyst undergoes better final activation at 3.3 MPa than at 4.3 MPa, which is taken advantage of in the present invention by subsequently raising the pressure after the initial lower pressure start-up procedure.