Selective Absorption of SO.sub.2 From Gases Containing the Same

Numerous patents and literature describe techniques for removing sulfur compounds from gas stream containing the same. By far the most common technique is that used to treat natural gas having one or more of the acid gases, H.sub.2 S, SO.sub.2, COS and CO.sub.2 with an aqueous liquid lean (with respect to the acid gases) absorbent to produce a rich absorbent stream and regenerate the rich absorbent stream by thermal treatment to produce a recycleable lean stream. Many compounds have been suggested and used as the absorbent, some to selectively remove H.sub.2 S or CO.sub.2 and others more general in nature to remove as much of each of the acid gases present as is possible. Now, with the renewed interest in coal fired boilers and the like coupled with the greater concern for the environment, there is a need to provide a low pressure, at or below atmospheric, low temperature selective process to remove sulfur dioxide from the flue gases emitted from such plants without removal of any major portion of the carbon dioxide. One process for removing the SO.sub.2 widely in use today is the old and well documented limestone scrubbing process. The disadvantage of this process is that there is produced a large volume of solid waste, calcium sulfite-sulfate, often contaminated with fly ash, which must be disposed of. In certain areas of the country, that is in those areas which have paper pulp operations being carried out, the waste is oftentimes usable, but such situations are not widespread.

Another system recently in the forefront is the use of potassium or sodium citrate as disclosed and taught in U.S. Pat. No. 4,366,134. While the absorbent is regenerated and recycled, the make-up costs can be high due to thermally stable salts being formed. In addition, it has been found to be necessary to employ stainless steel for the entire plant to prevent excessive corrosion of the metals.

It would be advantageous to have a process which selectively absorbs sulfur dioxide to the almost exclusion of the other acid gases, particularly carbon dioxide, which has low chemical make-up cost, reduced operating costs and permits economical construction of equipment to process low pressure, high volume, gas streams, such as flue gas, to reduce or eliminate the sulfur dioxide content of such gases.

In accordance with the present invention a gas stream containing sulfur dioxide and carbon dioxide which may also contain one or more of the other acid gases, for example H.sub.2 S, COS, commonly associated with hydrocarbon, natural or synthetic and/or combustion gases (flue gas) is contacted in an absorber with a lean aqueous absorbent solution containing from about 0.1 molar to the saturation content of at least one of a piperazinone or morpholinone compound having the general formula ##STR1## wherein X represents oxygen or .dbd.NR'; Y represents H.sub.2 or oxygen; and each of R and R' represent an independently selected lower alkyl moiety of 1 to 5 carbon atoms.

The rich absorbent, containing most of the SO.sub.2 and little of the CO.sub.2, is removed from the contactor and thermally regenerated to produce a lean absorbent solution for recycle to the absorber.

The absorber (contactor) is preferably operated at from 5.degree. to about 95.degree. C. under about atmospheric pressure conditions. Higher temperatures and pressures do not materially effect the process although equipment design may require modification to handle the higher temperatures and pressures.

The concentration of the sulfur dioxide in the gas streams may vary from about 10 ppm to about 45 percent by volume of the gas stream being treated.

The process for regeneration may be one of the conventional methods employed in conventional gas sweetening units as well as by steam stripping.

FIG. 1 represents a schematic diagram of the essential components of a process used to treat gases in accordance with the present invention.

An integrated absorber-stripper (contactor-regenerator), as illustrated in FIG. 1, was constructed by piping a ten tray Oldershaw column, 10, having a one inch internal diameter and 11/4 inch tray spacings in a manner to receive a lean absorbent solution at its upper end, 11, and a contaminated gas stream at its lower end, 12. The top, 13, and bottom, 14, were each independently piped to collect the treated gas at the top and the rich absorbent at the bottom, respectively. The rich absorbent was piped to a shell and tube cooler, 15, which passed the hot lean absorbent on the shell side and the cool rich absorbent on the tube side. The rich absorbent was then delivered to the upper end, 16, of a stripper, 17. The stripper, 17, was a two-foot one-inch internal diameter column packed with 1/4 inch Berl saddles. The sulfur dioxide exited the top, 18, with some water vapor and was sent to a condenser, 19, wherein the water vapor was condensed and the condensate and sulfur dioxide sent to a degasifier, 20, from which the sulfur dioxide was vented and the condensate returned via pump, 20A, to the top, 18, of the stripper, 17, as reflux. The liquid collecting in the bottom, 21, of the stripper, 17, was substantially lean absorbent, a part of which was passed through a reboiler, 22, and back into the stripper below the packed section. The remainder of the lean absorbent collecting in the bottom, 21, was piped to the cooler, 15, wherein it gave up most of its heat to the rich absorbent. The cool absorbent was drawn to the intake side of a pump, 23, passed through another cooler, 24, and then to the lean feed point of the absorber, 10.

The data collected from several runs is set forth in the table below. The alphabetic headings refer to like alphabetically numerated streams in FIG. 1.

A series of tests were run to screen the efficiency of various compounds known to absorb SO.sub.2 with respect to their absorbent characteristic for CO.sub.2. The equipment, a steel bomb filled with glass balls, was fitted with a valve at one end through which CO.sub.2 and absorbent could be added. The bomb was also fitted with a pressure sensing instrument. The bomb was pressurized to 760 mm Hg with CO.sub.2 and filled with a measured quantity of a 1 molar solution of a specific absorbent. The bomb was then left at ambient temperature ca 24.degree. C., or heated as indicated in the table below, to various temperatures and the pressure drop measured over a 10 minute period for each condition. The results were as follows:

Using the ten tray Oldershaw column described previously various compounds were tested for CO.sub.2 and SO.sub.2 absorption characteristics. A synthetic N.sub.2 /CO.sub.2 /SO.sub.2 gas mixture of the composition set forth in the following table was fed to the bottom of the column at 55.degree. C. and 4/5 liters/minute. The liquid flow at the top was about 10 cc/minute. The analysis of the gas in and out was obtained and weight percent CO.sub.2 and/or SO.sub.2 absorbed calculated. The results are set forth below.

This run established that NNDP will absorb in excess of one mole of SO.sub.2 per mole of NNDP.

These two runs establish that SO.sub.2 will be absorbed selectively vis-a-vis CO.sub.2 at temperatures above 50.degree. C., the normal water/gas wash temperature, at 5% concentration as well as 20% concentration.