GB1596500A - Removal of impurities from gases - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THE REMOVAL OF IMPURITIES FROM GASES (71) We, BRITISH GAS CORPORATION, of 59 Bryanston Street, London, W1A 2AZ, a British Body Corporate, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to the removal of impurity gases from fluids such as gas mixtures.

More particularly, the invention relates to improvements in thermal efficiency of such gas removal processes and to overall improvements in the thermal efficiency of the manufacture of synthesis gases and fuel gases which include one or more stages for the removal of impurity gases from the product.

In the production of fuel gases, such as Substitute Natural Gas by the catalytic steam reforming of hydrocarbons, it has been demonstrated that the biggest heat loss in the overall process occurs at the stage where carbon dioxide is removed from the gas. This is illustrated by reference to Table 1, which shows the heat losses occuring during a substitute natural gas manufacturing process for each 100 therms of feed and fuel employed in the process, employing a feedstock with a carbon/hydrogen ratio of 6.0.

TABLE I C/H Therms used Therms lost Therms lost Therms lost weight Therms in COo as stack in Gas in Gas ratio as SNG removal losses Drying Cooling 6.0 94.03 3.80 1.60 0.07 0.50 It will be seen that the greatest heat loss occurs during the CO2 removal process. This loss will be acute with heavier feedstocks, because of their higher carbon/hydrogen ratios.

Carbon dioxide is usually removed from the product gas by contacting the product gas with a solution containing an absorbent for the carbon dioxide. The rich liquor containing absorbed carbon dioxide is then subjected to a CO2 removal step to regenerate lean liquor by, for example, stripping, heating and/or by pressure reduction to boil or flash the absorbed gas. Conventional absorbents for removing carbon dioxide include hot aqueous solutions of carbonates and bicarbonates or organic absorbents such as monoethanolamine and diethanolamine. The CO2 rich liquors are usually regenerated by steam stripping. The steam for such stripping process is usually low grade steam, ie having a temperature of less than 200"C. However, it has been noted that in the production of substitute natural gas (SNG), for example by catalytic steam reforming of hydrocarbons, there is usually insufficient low grade waste heat available from the main gasification process streams to provide the heat required for the regeneration of the absorbent solution. This deficiency is accentuated if the overall steam to feedstock ratio is low. Thus, any heat requirement for the CO2 removal stage, which cannot be met by low grade process waste heat increases the requirement for direct-fired steam raising and, thus, the overall process efficiency is reduced. Several techniques have been proposed for reducing the net heat requirement for the CO2 removal stage and these proposals include modifications to the regenerator and/or heat recovery from the lean liquor or regenerator overhead streams.

In the known processes for carbon dioxide removal, pressure is reduced between the absorber and the regenerator stages. During this pressure reduction stage steam flashing occurs with a concomitant large heat loss from the liquor. Furthermore, owing to the kinetic limitations on the release of carbon dioxide from the rich liquor, little carbon dioxide is stripped when the flashing occurs. The net result is that a large heat loss occurs and the rich liquor still has to be subjected to a substantially complete regeneration stage and that a substantial quantity of sensible heat has to be supplied from external sources to make up the heat loss.

The present invention proposes a means whereby sensible heat may be recovered from the gas laden rich liquor by means of indirect exchange and to supply this heat to the regenerator where it can more usefully contribute to the stripping (regeneration) process.

As a result, heat loss in the pressure reducing flashing stage is reduced and there is a corresponding reduction in the net heat requirement on the overall gasification process.

In accordance with the present invention there is provided a process for removing impurity gases from fluids which process comprises contacting said fluid with a lean aqueous absorbent solution at a temperature in excess of 70"C thereby to absorb said gases into said absorbent solution to form a rich liquor, regenerating said lean absorbent solution by subjecting the rich liquor to steam, stripping and condensing the stripping steam after contact with the absorbent solution, wherein at least a part of the steam for said stripping is provided by contacting said condensed steam, or other feed water in indirect heat exchange relationship with said rich absorbent liquor.

The process of the present invention is particularly applicable to the removal of acid gases such as carbon dioxide from fuel gases such as methane containing gases to make substitute natural gas. However, the process of the present invention may be employed for the removal of any impurity gas from a fluid by an absorption-desorption process using steam as the regenerating medium.

The heat required for raising a part of the stripping steam is provided by direct heat exchange of the condensed stripping means or feed water from another source with hot rich liquor in a suitable boiler. The stripping steam thus raised may be at a lower pressure than the regenerator and can be compressed by any conventional means, such as a compressor, fan or turbine. However, it is preferred to employ a steam ejector since this is of low cost and provides a convenient means for adding any make up steam which may be required.

The lower the working pressure of the boiler the more heat can be recovered from the rich liquor. Table 2 illustrates the variation of the amount of steam produced in the boiler and the ejector driving steam requirements for various boiler pressures with a 5"C boiler approach temperature and using a steam ejector having an isentropic efficiency of 20% and dnven by saturated steam at a pressure of 100 psig.

TABLE2 BOILER CONDITIONS HEAT FLOWS EJECTOR TEMPER- DRIVING PRESSURE ATURE LOP STEAM STEAM FLOW DRIVING LP STEAM TOTAL (ata) ("C) FLOW REQUIRED STEAM tonne/h 1.00 100 19.8 20.2 21.5 21.2 42.7 1.09 102 15.4 11.3 12.0 16.5 28.5 1.17 104 11.0 5.5 5.8 11.8 17.6 1.25 106 6.6 1.8 2.0 7.0 9.0 1.36 108 2.2 0 0 2.3 2.3 The operating conditions for the absorption and regeneration systems may be those conventionally employed using the known absorbents. An example of such a commercially known process for removing carbon dioxide is the Benfield Process which employs a hot aqueous solutions of about 30 wt % potassium carbonate activated by diethanolamine at about 3 wt %.

The invention will be illustrated by reference to the accompanying drawing which is a schematic drawing of a CO2 removal facility commonly employed in a SNG manufacturing process.

Fluid, such as a synthesis gas containing acid gas impurities, is fed to the bottom of an absorption tower 1 through line 2. The fluid rises up the tower, leaving via line 3 and within the tower contacts an aqueous absorbent solution flowing counter-currently downwards.

The absorbent solution is fed to the tower under pressure through line 4 via pump 6. After contacting the fluid, the rich liquor, ie absorbent solution containing absorbed impurity gases, is drained from the bottom of the tower through line 8 and is passed to a boiler 10 where the rich liquor is subjected to an indirect heat exchange with an aqueous condensate.

The cooled rich liquor is then passed via a pressure reducer 9 to the top of a regenerator tower 11 where it is subjected to steam stripping to desorb the absorbed gases. The stripping is effected by passing the rich liquor downwards through the tower countercurrently against an upwardly rising stream of steam. The steam for the stripping is derived partly from the condensate boiled in the boiler 10 by indirect heat exchange with the rich liquor and this steam is fed to the bottom of regenerator tower 11 via line 12. A part of the stripping steam may be supplied from an external source to the base of the regenerator via line 5. To enable a water balance to be maintained in the system, a portion of the stripping steam may also be obtained by indirect heat exchange between the regenerated solution and steam or hot process fluids bed through line 18 in heat exchanger 7. The steam stream from the boiler 10 is compressed by motive steam fed through ejector 13. The desorbed or lean liquor is returned to the absorption tower via line 4. The overheads from the top of the regenerator twoer 11 are removed through line 14. These overheads consist of steam and the desorbed gases. The overheads are first subjected to cooling, for example using an air cooler 15 to condense a part of the steam and the condensed steam is collected in a knock-out pot 16. The gaseous components and the residual steam from the cooled overheads are then vented and the aqueous condensate is removed from the knock-out pot 16 and returned for re-use as the stripping agent to the bottom of the regenerator via boiler 10, steam ejector 13 and line 12. A part of the aqueous condensate, from pot 16, may alternatively be fed to the bottom of the regenerator lower via line 17.

WHAT WE CLAIM IS: 1. A process for removing impurity gases from fluids which process comprises contacting said fluid with a lean aqueous absorbent solution at a temperature in excess of 70"C thereby to absorb said gases into said absorbent solution to form a rich liquor, regenerating said absorbent solution by subjecting the rich liquor to steam stripping and condensing the stripping steam after contact with the absorbent solution, wherein at least a part of the steam for said stripping is provided by contacting said condensed steam, or other feed water, in indirect heat exchange relationship with said rich liquor.

2. A process as claimed in Claim 1, wherein the impurity gas is carbon dioxide.

3. A process as claimed in Claim 2 wherein the absorbent solution comprises an aqueous solution of potassium carbonate.

4. A process as claimed in any of the preceding Claims, wherein the fluid is a methane containing gas.

5. A process for removing impurity gases from fluids according to Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. tower 11 where it is subjected to steam stripping to desorb the absorbed gases. The stripping is effected by passing the rich liquor downwards through the tower countercurrently against an upwardly rising stream of steam. The steam for the stripping is derived partly from the condensate boiled in the boiler 10 by indirect heat exchange with the rich liquor and this steam is fed to the bottom of regenerator tower 11 via line 12. A part of the stripping steam may be supplied from an external source to the base of the regenerator via line 5. To enable a water balance to be maintained in the system, a portion of the stripping steam may also be obtained by indirect heat exchange between the regenerated solution and steam or hot process fluids bed through line 18 in heat exchanger 7. The steam stream from the boiler 10 is compressed by motive steam fed through ejector 13. The desorbed or lean liquor is returned to the absorption tower via line 4. The overheads from the top of the regenerator twoer 11 are removed through line 14. These overheads consist of steam and the desorbed gases. The overheads are first subjected to cooling, for example using an air cooler 15 to condense a part of the steam and the condensed steam is collected in a knock-out pot 16. The gaseous components and the residual steam from the cooled overheads are then vented and the aqueous condensate is removed from the knock-out pot 16 and returned for re-use as the stripping agent to the bottom of the regenerator via boiler 10, steam ejector 13 and line 12. A part of the aqueous condensate, from pot 16, may alternatively be fed to the bottom of the regenerator lower via line 17. WHAT WE CLAIM IS:

1. A process for removing impurity gases from fluids which process comprises contacting said fluid with a lean aqueous absorbent solution at a temperature in excess of 70"C thereby to absorb said gases into said absorbent solution to form a rich liquor, regenerating said absorbent solution by subjecting the rich liquor to steam stripping and condensing the stripping steam after contact with the absorbent solution, wherein at least a part of the steam for said stripping is provided by contacting said condensed steam, or other feed water, in indirect heat exchange relationship with said rich liquor.

2. A process as claimed in Claim 1, wherein the impurity gas is carbon dioxide.

3. A process as claimed in Claim 2 wherein the absorbent solution comprises an aqueous solution of potassium carbonate.

4. A process as claimed in any of the preceding Claims, wherein the fluid is a methane containing gas.

5. A process for removing impurity gases from fluids according to Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.