Gas Scrubbing System

Natural and synthetic gases containing acidic components such as CO.sub.2, H.sub.2 S, COS, and the like, have been treated on a commercial scale with aqueous alkanolamine solutions to remove the acidic components. The severe corrosion which results from the presence of these components, particularly in the rich absorbent solution regeneration section, has necessitated keeping the concentration of the amine low, i.e., 10-30 percent generally and the loading, i.e. the amount of H.sub.2 S/CO.sub.2, in the absorbent solution, low. In light of the recent energy shortage and necessity to conserve energy it would be advantageous to more heavily load the gas treating solutions with the acidic components. It would be a further advantage to employ more concentrated absorbent solutions. However to accomplish these objects it is necessary to prevent any increase in corrosion and it would be more advantageous to reduce the corrosion to less than that experienced today at the lower loadings and solution concentrations.

A brief review of the prior art discloses that numerous patents have taught various polyamines as corrosion inhibitors for various fluids:

For example, Dickson et al. in U.S. Pat. No. 3,262,791, which is representative of many patents in the corrosion inhibitor field issued to Petrolite Corporation, describes polyalkylenimines of molecular weight above 800 as useful in preventing corrosion of iron, steel and ferrous alloys when using slushing oils. The patent also suggests these polyalkylenimines as capable of imparting corrosion resistance to brines, weak inorganic acids, organic acids, CO.sub.2, H.sub.2 S, etc.

Ting Sin Go in U.S. Pat. No. 3,819,328, also a Petrolite patent, discloses alkylene polyamines, such as ethylene diamine, to control acid corrosion in distillation columns, such as occurs in petroleum distillation columns. Preferably the alkylene polyamine is used in conjunction with a corrosion inhibitor, e.g., a film-forming corrosion inhibitor, i.e., morpholine, at regulated pH's of about 2. The corrosion of columns which have water condensers and which water has a high pH due to acidic materials such as H.sub.2 S, HCN, CO.sub.2, HCl, etc. is taught to be controlled.

Ulrich et al. in U.S. Pat. No. 2,143,393 discloses absorbing acid gases in an aqueous solution of a polymerized alkylenimine.

Johnson et al. in U.S. Pat. No. 3,137,654 teach using glycines as a corrosion inhibitor in an alkanolamine solution used to absorb CO.sub.2 from industrial gases.

Yeakey et al. in U.S. Pat. No. 3,829,494 teach using a water-soluble trialkanolamine in an aqueous 2-(2-aminoethoxy)ethanol gas scrubbing medium.

Singh in U.S. Pat. No. 3,535,260 teaches a similar result for monoethanolamine gas treating solutions.

The art is rife with disclosures of nitrogen containing compounds useful as corrosion inhibitors, but that only a few are commercially acceptable. It is evident to one skilled in the art operating a commercial installation that the old limitations on concentration of absorbent, loading of the absorbent solution, etc., while improved to some extent with these few commercially accepted inhibitors are lacking in ability to carry without excessive corrosion the demands for more efficient operation, higher absorbent concentrations and higher loading, brought on by the energy shortage.

Is is therefore an object of the present invention to provide an inhibitor for aqueous amine gas absorbing systems which will enable higher concentrations of the amine absorbent to be used as well as a higher loading of the acid gases in the absorbent with reduced corrosion.

These and other advantages of the present invention will be apparent to those skilled in the art from the following disclosure and claims.

In accordance with the present invention, a gas scrubbing solution, e.g., an aqueous alkanolamine solution, particularly, diethanolamine and N-methyldiethanolamine solutions, is rendered less corrosive toward metals, particularly ferrous metal and their alloys by incorporating into the solution from about 10 to about 3000 parts of an inhibitor composition consisting essentially of (1) from 10 to 2000 ppm of a compound having the formula ##STR2## wherein n is an integer from 1 to 3, m is an integer from 2 to an integer sufficient to yield a molecular weight of about 800; each R represents an independently selected member from the group consisting of --H, --C.sub.n' H.sub.2n' OH; --C.sub.n' H.sub.2n'+1 ; C.sub.n' H.sub.2n' N(R.sub.3)R.sub.4 ; wherein n' is an integer from 1 to 2 and wherein R.sub.1 -R.sub.3 and R.sub.2 -R.sub.4 may be joined together to form cyclic amines when n' is 2; (2) copper or a copper ion yielding compound in from 0 to 1000 ppm; and (3) sulfur or a sulfur atom yielding compound in from 0 to 1000 ppm.

Amine compounds falling within the scope of the formula set out above are for example, polyethylenimine (polyethylene polyamines) having molecular weights from about 60 to about 800 which may be mixtures of several amines, for example a commercial polyalkylene polyamine identified as EA-10 (or E-100) containing 8-9 weight percent tetraethylenepentamine, 30-35 weight percent pentaethylenehexamine, 55 weight percent hexaethyleneheptamine and the remainder branched and cyclic isomers, polyethylenimines, PEI 3 (molecular weight 300.+-.50), PEI 6 (molecular weight about 600), EA-17 tris(aminoethyl)amine (about 90 percent branched), EA-19 pentaethylenehexamine (MW about 232), EA-15 (a mixture of 35-45 percent diethylenetriamine, 10-15 percent triethylenediamine, 5-15 percent piperazine), EA-25 (reaction product of triethylenetetramine divinylbenzene), EA-26 (a divinylbenzene-ethylvinylbenzene-diethylenetriamine mixture), EA-11 (aminoethylpiperazine-diethylenetriamine mixture), 1,2-polypropylenimine, tetraethylenepentamine, triethylenetetramine, diethylenetriamine, ethylenediamine, dihydroxyethyl ethylenediamine, and the like.

Copper metal or a copper ion yielding compound such as CuCO.sub.3, or the like may be employed. (Get scope from earlier case.)

Sulfur or a sulfur ion yielding compound may be present and is present in those compositions containing H.sub.2 S, COS or other sulfur containing constituents.

The inhibitors are most effective in diethanolamine solutions where the H.sub.2 S to CO.sub.2 ratio is 10/1 to 1/10, respectively. The inhibitors reduce corrosion compared to present day inhibitors in systems containing up to 1/40 H.sub.2 S/CO.sub.2, respectively. The absorbents DEA and MDEA are effectively provided with less corrosion than present day inhibitor systems with H.sub.2 S only but the results are not as dramatic as when CO.sub.2 is present also.

The alkanolamines which have been employed as absorbents for the acidic gas are the mono C.sub.2-3 alkanolamines, such as monoethanolamine, monisopropanolamine, the dialkanol C.sub.2-3 amines, such as diethanolamine and their N-alkyl substituted derivatives, i.e., methyl diethanolamine. The dialkanolamines show marked improvement in corrosion protection when the inhibitors of the present invention are employed.

The laboratory data were obtained from tests run in a modified Sparkler Filter at 40 psig, at a temperature at 125.degree. C for about 17 hours. All 1020 mild steel test specimens (which were acid etched in 5N HCl, soap washed, water and acetone rinsed and weighted) were placed in 4 oz bottles containing the prospective inhibitor and 50% diethanolamine (DEA) -50 percent water, saturated with H.sub.2 S/CO.sub.2 at various gas ratios. The total liquid volume in each bottle at the start of the experiment was 120 mls. At the end of the experiment, the mild steel specimens were again etched in acid (containing HCl inhibitor) washed with soap and water and acetone rinsed and weighed. The corrosion rate and percent inhibition were calculated using the before and after weighing of the mild steel specimens according to the following equations:

where W = weight loss in mg; D = density of specimen in gm/cc; A = area in square inches; and T = exposure in hours. ##EQU1##

Each of the following tables tabulate the experiments conducted and the results obtained in a single series of tests. The corrosion rate for the uninhibited, blank tests was run for each series to enable a direct comparison to be made with the other tests run in that series.

Observation of data from 6 tests shows compounds PEI 6, CuCo.sub.3 + 10 cc MEA, E-100, and E-100 + CuCO.sub.3 + S.degree. as very good protectors of mild steel specimen in 50 percent aqueous DEA solutions and an H.sub.2 S/CO.sub.2 environment. Passivation was observed from 84 percent to 96 percent inhibition. E-100 offers protection at concentrations as low as 1 ppm with 74 percent inhibition; and with the addition of CuCO.sub.3 + S.degree. an additive effect is seen with passivation around 96 percent to 97 percent.

In the following tables, data is presented demonstrating the results achieved with various compounds falling within the scope of the invention as well as results obtained with closely related compounds and commercially available compositions. Abbreviations used hereinafter are set forth here to facilitate comprehension of the data.

The data in each of the following tables was obtained using the indicated compound(s) in the manner set forth in Example 1.

A 50 percent solution of N-methyl diethanolamine (N-MDEA) was saturated with H.sub.2 S/CO.sub.2 gases at a ratio of 9/1. Prospective inhibitors for N-MDEA were tested in the same manner used to test compounds for diethanolamine (DEA) inhibition. The corrosion rate and percent inhibition were determined by the same method used for DEA.

The data, listed in Table VII, shows that those compounds which were effective in preventing corrosion in DEA, were just as effective in inhibiting corrosion in N-methyl DEA. At 100 ppm, the materials offered protection around 93 percent and the polyamine, CuCO.sub.3 and S.degree. synergistic system also showed excellent protection of mild steel.