Method And Device For Separating A Mixture Of Hydrogen, Methane And Carbon Monoxide By Cryogenic Distillation

Abstract

A method and to a device for separating a mixture of hydrogen, methane and carbon monoxide by cryogenic distillation is provided. In particular, the present invention relates to a method for producing a mixture of H2/CO with a low CH4 content, possibly combined with the production of CH4 in liquid form.

Description

The present invention relates to a method and to a device for separating a mixture of hydrogen, methane and carbon monoxide by cryogenic distillation. In particular, it relates to a method for producing a mixture of H₂/CO with a low CH₄ content, possibly combined with the production of CH₄ in liquid form.

Units for the production of carbon monoxide and hydrogen can be split into two parts:

• • generation of the syngas (mixture containing H₂, CO, CH₄, CO₂, Ar and N₂ essentially). Among the various industrial routes used to produce syngas, steam reforming is the most important. The design of this unit, which comprises a furnace, is based on the required productions of CO and hydrogen. However, units based on the gasification of coal may present numerous advantages with regard to operating costs and appear to be undergoing increasing development particularly in countries such as China. The design of such units, which comprise a reactor for gasifying the coal with oxygen, is based on the required productions of CO and hydrogen.
  • purification of the syngas. This features:
    • a liquid-solvent scrubbing unit for removing most of the acidic gases contained in the syngas
    • a purification unit purifying on a bed of adsorbents
    • a cryogenic separation unit known as a cold box for producing CO and/or hydrogen and/or a mixture of carbon monoxide and of hydrogen known as oxygas, with a defined H₂/CO ratio.

In general, the syngas contains a high-pressure (between 15 and 60 bar) mixture containing the following compounds: H₂, CO, CH₄, N₂, Ar.

There are two known main families of method for separating cryogenically a mixture containing hydrogen, carbon monoxide and methane, these being methane scrubbing and partial condensation.

One of the advantages of methane scrubbing is that it allows the production of hydrogen under pressure with good purity, the CO content of which may range from 0.5 mol % to a few ppm. However, with this type of method using methane scrubbing, the residual CH₄ content cannot generally be got below 1 mol %.

In some instances where it is desirable to produce hydrogen or an H₂/CO mixture containing less than 1 mol % of CH₄ (particularly for the production of MeOH), the only alternative is a method employing partial condensation in which the syngas is cooled down to a temperature of the order of −186° C. in order to reach a thermodynamic equilibrium that allows the CH₄ content in the H₂/CO gaseous mixture to be lowered below 1 mol %.

Reaching this temperature level is costly in terms of energy in the cycle compressor.

The idea postulated in this proposed invention would enable the separation energy (and therefore the energy to be supplied to the cycle compressor) to be reduced appreciably.

U.S. Pat. No. 4,488,890 and U.S. Pat. No. 6,098,424 propose methods with a liquid carbon monoxide scrubbing column in which method substantially all the cold energy is produced by a carbon monoxide cycle.

In the context of the production of a mixture of hydrogen and of CO containing a low CH₄ content (generally of below 1 mol %), the idea is to cool the syngas to a temperature close to −167° C. (therefore 20° C. warmer than in a partial condensation scheme), the vapor phase being treated in a CO scrubbing column into which liquid CO is injected at the top of the column.

One subject of the invention is a method for separating a mixture of hydrogen, carbon monoxide and methane by cryogenic distillation, in which method the mixture is cooled in an exchange line and at least some is sent to a liquid CO scrubbing column, a gas is tapped off at the top of the scrubbing column, the bottom liquor from the scrubbing column is sent, possibly after purification, to a CO/CH₄ separation column, a CO-rich liquid is drawn off at the top of the CO/CH₄ separation column, at least partially pressurized and at least some is sent to the top of the scrubbing column and a methane-rich liquid is drawn off at the bottom of the CO/CH₄ separation column by way of end product, at least some of the separation energy being provided by a closed cycle using nitrogen, methane, oxygen, argon, helium or hydrogen as the cycle fluid.

In other optional aspects:

• • the gas tapped off at the top of the scrubbing column is a mixture of H₂ and of CO containing less than 1 mol % of CH₄;
  • the closed cycle is used to condense the head liquors of the CO/CH₄ column;
  • the closed cycle is used for reboiling the bottom liquors of the CO/CH₄ column and/or of a stripping column;
  • the closed cycle provides at least some of the energy used to liquefy this CH₄;
  • two liquids of the closed cycle vaporize at least two different pressures in the exchange line;
  • at least some of the carbon monoxide-rich liquid drawn off from the CO/CH₄ column is pressurized using a pump and at least some of the pumped liquid is sent to the scrubbing column;
  • at least two of the temperatures in the following list differ at most by 5° C.:
    • the inlet temperature at which the mixture enters the scrubbing column
    • the temperature of the CO-rich liquid from the CO/CH₄ column
    • the temperature of the supercooled liquid methane.

Another aspect of the invention provides a device for separating a mixture of hydrogen, carbon monoxide and methane by cryogenic distillation, the device comprising a liquid CO scrubbing column, a CO/CH₄ separation column, an exchange line in which the mixture is cooled and means for sending at least some of the cooled mixture to the scrubbing column, means for tapping off a gas at the top of the scrubbing column, means for sending the bottom liquor from the scrubbing column, possibly after purification, to the CO/CH₄ separation column, means for drawing off a CO-rich liquid from the CO/CH₄ separation column, means for pressurizing at least some of the drawn-off liquid, and means for sending at least some of the pressurized liquid to the top of the scrubbing column and means for drawing off a methane-rich liquid at the bottom of the CO/CH₄ separation column by way of end product, and a closed cycle using nitrogen, methane, oxygen, argon, helium or hydrogen as the cycle fluid to provide at least some of the separation energy.

According to other optional aspects, the device comprises:

• • means for drawing off CH₄ in liquid form by way of end product;
  • a closed cycle providing at least some of the energy used to liquefy this CH₄;
  • a stripping column for purifying the bottom liquor of the scrubbing column upstream of the CO/CH₄ separation column;
  • a pump connected to the top of the CO/CH₄ column and to the top of the scrubbing column.

The invention will be described in greater detail with reference to the figures which show devices according to the invention.

In FIG. 1, the syngas 1 available at high pressure (generally between 15 and 60 bar) is cooled in the main exchanger 3 and partially condensed in the exchange line down to a temperature of the order of −167° C. The vapor phase is sent to the bottom of a scrubbing column 5 where it is scrubbed with liquid CO 51 injected at the top of the column 5. This makes it possible to lower the CH₄ content in the vapor 7 produced at the top of the scrubbing column 5 to under 1 mol % so that it can be treated after reheating in the exchange line in an MeOH unit for example.

The liquid phase 11 at the bottom of the liquid CO scrubbing column 5 is very rich in CH₄ and also contains CO and dissolved hydrogen. This liquid 11 is sent to the top of a stripping column 13, having a bottom reboiler 15, to separate out the hydrogen and reduce its content in the bottom liquors of the flash column 17 so as to reduce the amount of hydrogen that cannot be condensed during the separation of CO and CH₄ in the column 33.

The top gases 21 from the stripping column are heated up in the exchange line 3 and act as fuel.

The bottom liquors 17 from the stripping column 13 are supercooled by the exchanger 19 then sent to a CO/CH₄ separation column in two parts. Some 27 is expanded in the valve 31 and sent to the upper part of the column 33. The remainder 23 is expanded in the valve 29 then heated by the heating 25 and then sent to the lower part of the column 33. The CO is produced in liquid form 47 at the top and sent to pumps 49 which raise its pressure up to the pressure level of the CO scrubbing column 5. There is therefore an internal liquid-CO loop through at least one cryogenic pump 49 and one valve 53 between the top of the CO/CH₄ column 33 and the top of the CO scrubbing column 5.

Possibly, some of the liquid CO 55 may be sent through the valve 57 at the head gases 7 of the scrubbing column 5 to form a mixed flow 9. This makes it possible to adjust the CO/H₂ ratio of the gas.

The CH₄ 39 is produced at the bottom of the CO/CH₄ column 33 in liquid form. The CO/CH₄ column has a bottom reboiler 37 and a top condenser 35.

One of the options with this scheme is to produce, to supplement this H₂/CO mixture 7 with a low CH₄ content, pure methane containing small traces of CO so that it can be marketed in the form of LNG 45.

This liquid CH₄ 39 which has left the bottom of the CO/CH₄ column will be supercooled in the exchange line 41 before being sent for storage in order to limit the production of so-called “boil off” vaporized liquid. A valve 43 can be used to short-circuit the line 41.

Possibly, head gases 59 from the CO/CO₄ column 33 are compressed in a compressor 61 to form the flow 63, condensed in the exchange line and sent to the top of the scrubbing column 5 in place of or in addition to the pumped flow from the pump 49.

The separation energy is provided by an external closed cycle. This cycle will also make it possible to supply energy for liquefying this CH₄ 39.

The gas used for the cycle may be chosen from the list N₂, CH₄, O₂, Ar, He, H₂, etc. The gas 65 is used to reboil the CO/CH₄ column and then forms the liquid 67 which is split into two. Some 71 passes through the valve 73 and is sent to the top condenser 35. The flow vaporized in the condenser is sent by way of flow 81, 83 to the series compressor 85, 87, 89. The flow 91 compressed in the compressor 89 is split into two portions 93, 95 which are compressed in two compressors 97, 99 in parallel. The compressed flows 95, 101 are combined to form a flow 103 which is split into two. Some 105 is partially cooled in the exchange line 3 before being split into two. A fraction 109 is expanded to an intermediate temperature in the turbine 111 and the expanded flow 113 is sent back to the flow 81 at an intermediate temperature level in the exchange line 3. The other fraction is sent to the turbine 115 at a temperature level lower than the temperature to which 109 is cooled in the exchange line 3 and combines with the flow 81 upstream of the exchange line 3. The flow 107 is completely cooled in the exchange line 3 and sent by way of flow 65 to reboil the CO/CH₄ column.

Vaporizing the flows 77, 81 in the exchanger 3 at two different pressures makes it possible to optimize heat exchange.

In FIG. 2, the syngas 1 available at high pressure (generally between 15 and 60 bar) contains 15 mol % of methane. It is split into two, some 1A being cooled in the main exchanger 3 and the remainder 1B bypassing the main exchanger before being mixed back in with the flow 1A and sent to the bottom reboiler 37 of the CO/CH₄ column 33 and the ringed flow 3. The ringed flow 4 cooled in the bottom reboiler is sent back at an intermediate temperature of the main exchanger 3 and partially condensed in the exchange line to a temperature of the order of −167° C. It is sent to the bottom of the scrubbing column 5 where it is scrubbed with liquid CO 51 injected at the top of the column 5. That makes it possible to lower the CH₄ content in the vapor 7 produced at the top of the scrubbing column 5 to under 1 mol % so that it can be treated after heating in the exchange line, in an MeOH unit for example.

The liquid phase 11 at the bottom of the liquid CO scrubbing column 5 is very rich in CH₄ and also contains CO and dissolved hydrogen. This liquid 11 is sent to the top of a stripping column 13, having a bottom reboiler 15, to separate out the hydrogen and reduce its content in the bottom liquors of the flash column 17 so as to reduce the amount of hydrogen that cannot be condensed during the separation of CO and CH₄ in the column 33.

The head gases 21 from the stripping column are heated up in the exchange line 3 and act as fuel.

The bottom liquors 17 from the stripping column 13 are supercooled by the exchanger 19 then sent to a CO/CH₄ separation column in two parts. Some 27 is expanded in the valve 31 and sent to the upper part of the column 33. The remainder 23 is expanded in the valve 29 then heated by the heating 25 and then sent to the lower part of the column 33. The CO is produced in liquid form 47 at the top and sent to pumps 49 which raise its pressure up to the pressure level of the CO scrubbing column 5. There is therefore an internal liquid-CO loop through at least one cryogenic pump 49 and one valve 53 between the top of the CO/CH₄ column 33 and the top of the CO scrubbing column 5.

Possibly, some of the liquid CO 55 may be sent through the valve 55 to the head gases 7 of the scrubbing column 5 to form a mixed flow 9. This makes it possible to adjust the CO/H₂ ratio of the gas.

The CH₄ 39 is produced at the bottom of the CO/CH₄ column 33 in liquid form. The CO/CH₄ column has a bottom reboiler 37 and a top condenser 35.

One of the options with this scheme is to produce, to supplement this H₂/CO mixture 7 with a low CH₄ content, pure methane containing small traces of CO so that it can be marketed in the form of LNG 45.

This liquid CH₄ 39 which has left the bottom of the CO/CH₄ column will be supercooled in the exchange line 41 before being sent for storage in order to limit the production of so-called “boil off” vaporized liquid. A valve 43 can be used to short-circuit the line 41.

Possibly, head gases 59 from the CO/CO₄ column 33 are compressed in a compressor 61 to form the flow 63, condensed in the exchange line 3 and sent to the top of the scrubbing column 5 in place of or in addition to the pumped flow from the pump 49.

The separation energy is provided by an external closed cycle. This cycle will also make it possible to supply energy for liquefying this CH₄ 39.

The gas used for the cycle may be chosen from the list N₂, CH₄, O₂, Ar, He, H₂.

The reboiling of the stripping column 13 is performed by a flow of cycle gas 169. The cooled flow 171 is expanded in a valve 173 and sent to the top condenser 35 of the CO/CH₄ column 33 by way of flow 177. The flow 175 is split to form the flows 177 and 179. The flow 177 cools the condenser 35. The flow 179 is sent through the valve 181 to the exchanger 3 where it is heated up. The flow 180 heated up in the reboiler 35 is mixed with the flows 167 and 194 to become the flow 183. This flow 183, once it has been heated up slightly, combines with the flow 179. The combined flow 185 at 10 bar is sent to the series cycle compressors 85, 87 and then in part the compressor 89. Some 169 of the flow compressed at 89 is sent at 39 bar for reboiling of the column 13 and the remainder 191 is compressed in the compressor 197 to 50 bar to form the flow 201. The flow 201 is split into two to form the flow 203 which proceeds through the valve 205 to the turbine 211 to become the expanded flow 167. The flow 202 passes right through the exchanger 3 and is split into three. The flow 190 is sent to the turbine 211 also, the flow 174 is mixed with the flow 171, and the flow 186 is heated up in the exchanger 3 before being combined with the flow 192 from the compressor 87 to form a combined flow 189. The flow 189 is sent to the compressor 199, is partially cooled in the exchanger 3 and is expanded in the turbine 215 to form the expanded flow 194.

The compressor 197 is coupled to the turbine 211 and the compressor 199 is coupled to the turbine 215.

Vaporizing the flows 179, 186 in the exchanger 3 at two different pressures makes it possible to optimize heat exchange.

Throughout this document, the term “top of the column” comprises positions ranging from the top of the column in the strictest sense to a position at most 10 theoretical plates below this position.

Claims

1-13. (canceled)

14. A method for separating a mixture of hydrogen, carbon monoxide and methane by cryogenic distillation, comprising: a) cooling the mixture in an exchange line and sending at least a portion of said cooled mixture to a liquid CO scrubbing column,b) removing a gas at the top of the scrubbing column,c) removing the bottom liquor from the scrubbing column, and sending said bottom liquor to a CO/CH4 separation column,d) removing a CO-rich liquid at the top of the CO/CH4 separation column, at least partially pressurizing said CO-rich liquid, and at sending at least a portion of said pressurized CO-rich liquid to the top of the scrubbing column, ande) removing a methane-rich liquid at the bottom of the CO/CH4 separation column by way of end product, at least some of the separation energy being provided by a closed cycle using a cycle fluid selected from the group consisting of nitrogen, methane, oxygen, argon, helium and hydrogen.

15. The method of claim 14, wherein step c) further comprises removing the bottom liquor from the scrubbing column, purifying said bottom liquor, and sending said bottom liquor to a CO/CH4 separation column.

16. The method of claim 14, in which the gas removed from the top of the scrubbing column comprises a mixture of H2 and of CO, containing less than 1 mol % of CH4.

17. The method of claim 14, wherein the closed cycle is used to condense the head liquors of the CO/CH4 column.

18. The method of claim 14, wherein the closed cycle is used for reboiling the bottom liquors of the CO/CH4 column and/or of a stripping column.

19. The method of claim 14, wherein the closed cycle provides at least some of the energy used to liquefy this CH4.

20. The method of claim 14, wherein, at least two liquids of the closed cycle vaporize at least two different pressures in the exchange line.

21. The method of claim 14, wherein at least two of the temperatures selected from the group consisting of the inlet temperature at which the mixture enters the scrubbing column; the temperature of the CO-rich liquid from the CO/CH4 column; and the temperature of the supercooled liquid methane; differ at most by 5° C.:

22. The method of claim 14, wherein the cycle fluid is methane.

23. The method of claim 14, wherein the cycle fluid is nitrogen.

24. A device for separating a mixture of hydrogen, carbon monoxide and methane by cryogenic distillation, the device comprising; a) a liquid CO scrubbing column,b) a CO/CH4 separation column,c) an exchange line in which the mixture is cooled,d) means for sending at least some of the cooled mixture to the scrubbing column,e) means for tapping off a gas at the top of the scrubbing column,f) means for sending the bottom liquor from the scrubbing column to the CO/CH4 separation column,g) means for drawing off a CO-rich liquid from the CO/CH4 separation column,h) means for pressurizing at least some of the drawn-off liquid,i) means for sending at least some of the pressurized liquid to the top of the scrubbing column,j) means for drawing off a methane-rich liquid at the bottom of the CO/CH4 separation column by way of end product,k) a closed cycle using a cycle fluid selected from the group consisting of nitrogen, methane, oxygen, argon, helium and hydrogen; to provide at least some of the separation energy.

25. The device of claim 24 further comprising a pump connected to the top of the CO/CH4 column and to the top of the scrubbing column.

26. The device of claim 24, wherein the closed cycle provides at least some of the energy used to liquefy this CH4.

27. The device of claim 24, further comprising a stripping column for purifying the bottom liquor of the scrubbing column upstream of the CO/CH4 separation column.