CARBON DIOXIDE RECOVERY USING AN ABSORPTION COLUMN IN COMBINATION WITH OSMOTIC FILTERS

Abstract

A process for the removal of CO.sub.2 from flue gases using a co-current flow absorption column followed by one or more serial osmotic filters. The permeate is heated to strip out the CO.sub.2. Both sides of the filters are fed back to the column input.

Claims

1. A process for the recovery of carbon dioxide from flue gases comprising a gravity fed co-current absorption column charged with an aqueous solution of salt and amine to absorb carbon dioxide from flue gases applied co-currently to the input of the column and fed by gravity through the column; and said column is followed by one or more osmotic filters from which the retentate is cycled back to the column input while the permeates are heated to 50 C. to 120 C. to strip out the carbon dioxide wherein the column operates at a temperature between about 15 C. and 50 C.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0006] FIG. 1 is a schematic drawing of a system for carry out the process of the present invention.

DETAILED DESCRIPTION

[0007] The present invention can best be appreciated by reviewing the relevant chemistry. Chemical reactions that occur are unique to the process. They indicate the principal advantages, namely, the efficiency of removing carbon dioxide from a gas stream and the ease with which the solvent can thereafter can be filtered and regenerated.

[0008] In the first step of the process when the gas stream is contacted with the aqueous solution the following chemical reactions take place:

NH.sub.3+CO.sub.2+H.sub.2O.fwdarw.NH.sub.4HCO.sub.3 1.

NH.sub.4HCO.sub.3+NaCl.fwdarw.NaHCO.sub.3+NH.sub.4Cl 2.

[0009] In the first equation ammonia reacts rapidly with carbon dioxide and water to provide ammonia bicarbonate. The second equation shows the rearrangement of ammonia bicarbonate with salt to give sodium bicarbonate and ammonia chloride. By combining these two reactions the following expression is obtained for the overall reaction in the absorption step.

NH.sub.3+CO.sub.2+H.sub.2O+NaCl.fwdarw.NaHCO.sub.3+NH.sub.4Cl 3.

[0010] The regeneration of the aqueous solution is shown by the following two equations.

2 NaHCO.sub.3.fwdarw.Na.sub.2CO.sub.3+H.sub.2O+CO.sub.24.

Na.sub.2CO.sub.3+NH.sub.4Cl>NaHCO.sub.3+NaCl+NH.sub.3 5.

[0011] The decomposition of sodium bicarbonate to give sodium carbonate and release carbon dioxide is indicated by equation 4. This reaction occurs under the application of heat. Finally, sodium carbonate, strongly alkaline in solution, reacts with ammonium chloride to form sodium bicarbonate, salt, and ammonia. When equations 4 and 5 are combined, the following equation is obtained showing the regeneration process.

NaHCO.sub.3+NH.sub.4Cl.fwdarw.->NH.sub.3+NaCl+H.sub.2O+CO.sub.26.

[0012] As one can see from this expression, the resulting solution containing ammonia and salt is identical to the solution used in scrubbing carbon dioxide shown in equation 3. Thus, the regenerated solution can be recycled to the absorption step.

[0013] The conditions of the process are important. The absorption step is conducted at atmospheric pressure or close to this level. The temperature for absorption is in the range of about 15 C. to approximately 50 C. Below this range, sodium bicarbonate begins to precipitate from solution causing fouling of the equipment. Above this range, carbon dioxide is released.

[0014] This regenerative step uses relatively mild conditions. The pressure of the process remains the same as in step one. The temperature is raised to a point in the range of 50 C. to 120 C., but a level below boiling is generally sufficient for the complete evolution of carbon dioxide.

[0015] Because of the limited solubility of sodium bicarbonate, sodium chloride in the scrubbing solution can be replaced by potassium chloride. The resulting chemistry is essentially the same.

[0016] Also, an amine can be substituted for ammonia in the process. For example, n-propylamine has a boiling point of 49 C. and is miscible in water. Recourse to the common solvent monoethanolamine is also possible.

[0017] FIG. 1 represents a co-current flow system that minimizes the loss of ammonia. The apparatus comprises a modified gravity fed absorption column 10 fed with the flue gas containing carbon dioxide at 12 and the solvent comprises an aqueous solution of sodium chloride and ammonia at input 14. The flows are co-current downwardly by gravity through the absorption column 10 with vent air exiting at 11 while the carbon dioxide charged solvent exits on line 16 and enters the first of three osmotic filters 18, 20 and 22 each of which has an osmotic filter membrane 40a, 40b and 40c respectively. The retentate sides of the filters 18, 20 and 22 are recycled by way of lines 24, 26 and 28 respectively to line 30 which runs back to the input 14 of the absorption column. The permeate sides of the filters 18, 20 pass to the next filter and exit from the final filter in series on line 32 and are heated at 34 to a temperature approximately 50 C. to 120 C. and flow by way of line 30 back to line 38 where the carbon dioxide is stripped out on line 36 while the solvent returns to the absorption column 10 by way of line 38. The column 10 operates at a temperature of about 15 C. to 50 C.

[0018] In summary, the process of the present invention is directed toward the recovery of carbon dioxide from flue gases using a gravity fed co-current absorption column charged with an aqueous solution of salt and amine to absorb carbon dioxide from the input flue gas. The column is followed by one or more osmotic filters from which the retentate is cycled back to column input while the permeates are heated to 50 C. to 120 C. to strip out the carbon dioxide. The column recycles the solvent and operates at a temperature between about 15 C. and 50 C.

EXAMPLE

[0019] Half a cup of ammonia solution used by consumers as an all-purpose cleaner was added to half cup of sparkling carbonated water. After complete mixing, 1 teaspoons of salt were added to the solution and stirred vigorously. No bubbles appeared. Next, the solution was heated to temperatures describe above. Considerable foaming occurred shortly before the solution began to boil and released carbon dioxide.