PROCESS FOR REFORMING HYDROCARBONS

Abstract

A process for the production of synthesis gas by the use of autothermal reforming in which tail gas from downstream Fischer-Tropsh synthesis is hydrogenated and then added to the autothermal reforming stage.

Claims

1. Process for the production of liquid hydrocarbons from a hydrocarbon feedstock comprising: a) passing said hydrocarbon feedstock through an ATR, CPO or POx, and withdrawing a stream of hot effluent synthesis gas from the ATR, CPO or PDX, b) converting the synthesis gas into liquid hydrocarbons via Fischer-Tropsch synthesis, c) passing tail gas from the Fischer-Tropsch synthesis stage through a hydrogenation stage to produce a hydrogenated tail gas containing less than 1 mol olefins, and d) adding the hydrogenated tail gas directly to said ATR, CPO or Pox; e) optionally recovering the liquid hydrocarbons produced.

2. Process according to claim 1, wherein said hydrocarbon feedstock is a gas that has passed through at least one adiabatic pre-reforming stage.

3. Process according to claim 1, wherein said hydrocarbon feedstock is a gas that has passed through at least one steam reforming stage.

4. Process according to claim 1, wherein said hydrocarbon feedstock is a gas mixture resulting from dividing a raw hydrocarbon feed gas into two streams, passing the first stream through at least one steam reforming stage to form a primary reformed gas, using the second stream as a by-pass stream to said steam reforming stage, and subsequently combining said primary reformed gas with the by-pass stream to form said hydrocarbon feedstock.

5. Process according to claim 1, comprising dividing a raw hydrocarbon feed gas into two streams, by which one of the streams becomes said hydrocarbon feedstock, and passing the other stream through at least one steam reforming stage to form a reformed gas.

6. Process according to claim 3, wherein the steam reforming stage is heat exchange reforming, and where at least a portion of the hot effluent synthesis gas from the ATR, CPO, or POx is used as heating medium in said heat exchange reforming.

7. Process according to claim 5, wherein said hot effluent synthesis gas is combined with said reformed gas before, during or after said hot effluent synthesis gas has delivered heat to the heat exchange reforming.

8. Process according to claim 6, comprising adding a stream comprising steam to said hot effluent synthesis gas, said reformed gas, or the combined stream of hot effluent synthesis gas and reformed gas.

9. Process according to claim 4, wherein the, at least one adiabatic pre-reforming stage is conducted prior to dividing said raw hydrocarbon feed.

10. Process according to claim 1, comprising mixing the hydrogenated tail gas with the hydrocarbon feedstock prior to conducting reforming in the ATR, CPO or POx.

11. Process according to claim 1, comprising adding the hydrogenated tail gas to the ATR, CPO or POx as a separate stream.

12. Process according to claim 4, comprising mixing the hydrogenated tail gas with said by-pass stream prior to conducting reforming in the ATR, CPO or POx.

13. Process according to claim 4, comprising mixing the hydrogenated tail gas with said primary reformed gas.

14. Process according to claim 1, wherein the hydrogenated tail gas of step c) contains below 0.5 mol %.

15. Process according to claim 1, wherein the hydrogenated tail gas of step c) contains less than 0.2 mole %.

16. Process according to claim 1, wherein the hydrogenated tail gas of step c) contains less than 0.1 mole %.

16. Process according to claim 1, wherein the hydrogenated tail gas of step c) contains less than 0.1 mole %.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] The invention is further illustrated by reference to the accompanying figures.

[0057] FIG. 1 shows a schematic view of the invention when using a stand-alone autothermal reformer yet including a pre-reformer.

[0058] FIG. 2 shows heat exchange reforming and autothermal reforming in series with hydrogenated tail gas addition to the primary reformed gas.

[0059] FIG. 3 shows a process with by-pass of the primary reforming stage, with addition of hydrogenated tail gas to the by-pass stream, or to the combined stream of primary reformed gas and by-pass stream.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0060] The accompanying FIG. 1 shows a general schematic view of an embodiment for the production of synthesis gas for Fischer-Tropsch synthesis using a stand-alone autothermal reformer. Clean (free of sulphur and other poisons to reforming catalysts) hydrocarbon feed gas 100 such as natural gas or other hydrocarbon containing gas source is mixed with process steam 110, optionally partly via saturator/humidifier. The mixture is preheated and pre-reformed adiabatically in pre-reformer 300 in order to convert any higher hydrocarbons into H.sub.2, CO, CO.sub.2 and CH.sub.4. This resulting hydrocarbon feedstock mixture 120 is fed to the autothermal reformer 400 together with oxygen 130, protection steam 140 and hydrogenated tail gas 150. From the autothermal reformer 400 at hot effluent of synthesis gas 160 is withdrawn and further processed to form the synthesis gas feed to the downstream Fischer-Tropsch section 500. Liquid hydrocarbons 170 are produced and tail gas recycle stream 180 is passed through hydrogenating stage 600 prior to entering the autothermal reformer 400. Notably, hydrogenated tail gas is added directly from the hydrogenator to the autothermal reformer 400.

[0061] In FIG. 2, a mixture of the raw hydrocarbon feed gas and steam 10 is passed to heat-exchange reformer 25 where it is catalytically steam reformed and thereafter leaves the heat-exchange reformer as stream 30. The primary reformed gas stream 30 is mixed with hydrogenated tail gas 65 from Fischer-Tropsch section 150 forming the ATR feed stream 70. The mixed stream 70 is fed to an autothermal reformer 75 with oxidant 80 and protection steam (not shown) also being supplied. The primary reformed gas is partially combusted and brought towards equilibrium over reforming catalyst in the autothermal reformer 75. The hot effluent synthesis gas 110 from the autothermal reformer is passed through the heat exchange reformer 25. The synthesis gas is cooled by heat exchange with the gas undergoing reforming over the catalyst in the heat-exchange reformer 25. The thus cooled synthesis gas leaves the heat exchange reformer as stream 120 and is further processed to form the synthesis gas feed to the Fischer-Tropsch section 150 downstream. Liquid hydrocarbon products 140 are withdrawn together with a tail gas recycle stream 60. The tail gas recycle stream 60 passes through hydrogenator 160 to form hydrogenated tail gas stream 65 before being combined with primary reformed gas 30. Notably, hydrogenated tail gas is added directly from the hydrogenator to the autothermal reformer 400.

[0062] In FIG. 3, a mixture of raw hydrocarbon feed gas (10) is divided into two streams 20 and 40. The first stream 20 is fed to the heat-exchange reformer 25 where it is catalytically steam reformed and thereafter leaves the heat-exchange reformer as primary reformed gas 30. The second stream 40 is preheated in a heat exchanger 45 and bypasses the heat exchange reformer. The primary reformed gas 30 is mixed with the preheated second stream 50. Hydrogenated tail gas 65 is added to this mixed stream or to the preheated second stream 50 thus forming the ATR feed stream 70. The ATR feed stream is fed to the autothermal reformer 75 to which oxidant 80 and protection steam (not shown) are also supplied. The ATR feed stream is partially combusted and brought towards equilibrium over reforming catalyst in the autothermal reformer 75. The hot effluent synthesis gas 110 is passed through the heat exchange reformer 25. The mixture stream is cooled by heat exchange with the gas undergoing reforming over the catalyst in the heat-exchange reformer 25. The thus cooled synthesis gas leaves the heat exchange reformer as stream 120 and is further processed to form the synthesis gas feed to the Fischer-Tropsch section 150 downstream. Liquid hydrocarbon products 140 are withdrawn together with a tail gas recycle stream 60. The tail gas recycle stream 60 passes through hydrogenator 160 to form hydrogenated tail gas stream 65 which is then combined with primary reformed gas 30 or by-pass stream 50. Alternatively, the hydrogenated tail gas 65 may also be added to the primary reformed stream 30. Notably, hydrogenated tail gas is added directly from the hydrogenator to the autothermal reformer 400.

EXAMPLE

[0063] Two tests were made in the same experimental setup: An 800 mm long sample of Inconel 690 was placed in a reactor. The reactor was placed in an oven with three heating zones. The temperature of the Inconel 690 sample varied with the position in the oven. The sample temperatures were 200 to 640 C. The sample was exposed to a continuous flow of gas with the composition given in Table 1 as Test 1. The flow rate was 100 Nl/h. The pressure was 29 barg. The conditions were kept for 626 hours. The sample was examined after the test using stereo microscope and scanning electron microscope. The sample was attacked by metal dusting corrosion.

[0064] The second test was made analogous to the first test, with the exceptions that the gas composition used was as given in Table 1 as Test 2 and the conditions were kept for 672 hours. Examination of the sample after the test showed that the sample was not attacked by metal dusting corrosion.

TABLE-US-00001 TABLE 1 Gas compositions (mole %) Component Test 1 Test 2 Hydrogen 12.1 12.1 Water 22.6 22.6 Carbon 6.9 6.9 monoxide Carbon 7.8 7.8 dioxide Ethylene 0.14 0 Ethane 0 0.14 Methane 49.8 49.8 Propane 0.45 0.45 1-Butene 0.21 0 Butane 0 0.21

[0065] The two gas compositions in the two tests are identical with the exception that the gas in test 1 contains the olefins (alkenes), whereas the gas in test 2 contains the corresponding alkanes. Metal dusting attack occurs in Test 1 but not in Test 2, which is of longer duration.

[0066] The presence of alkenes makes a gas more aggressive with respect to metal dusting corrosion. Thus, the use of a hydrogenated tail gas conveys the reduction or elimination of metal dusting compared to a situation where tail gas is used without being hydrogenated.