Abstract:
The hydrolysis of carbon oxysulfide in a gas or liquid stream is catalyzed by morpholines and piperazines at a temperature in the range from about 50° about 90° C. The process can be used in the treatment of refinery gases, coal gasification streams, and other such gases as well as liquid hydrocarbons which contain COS and other acidic contaminants.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of Ser. No. 936,675 filed Aug. 24, 1978 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method for hydrolyzing carbon oxysulfide, more particularly to a method whereby the hydrolysis is catalyzed by morpholines and piperazines. 
     Carbon oxysulfide or carbonyl sulfide (COS) is often present in minor amounts in petroleum refinery gases and it is found in larger amounts in coal gasification product streams. This toxic contaminant poses a special problem in sour gas purification processes since it is neither readily separated nor easily hydrolyzed by known gas treating solvents under ordinary conditions. It is an undesirable contaminant in a Claus plant feed stream, since it is not readily converted to sulfur, and it is necessary to remove it from fuel gases and the like for reasons of corrosion and air pollution. Carbon oxysulfide reacts irreversibly with ethanolamine but this is not usually a practical method for removing it from a gas stream because the solvent cannot be regenerated. The use of sulfolane, diisopropanolamine, methyldiethanolamine, and mixtures thereof are known to be useful to hydrolyze COS but the present invention gives a higher percent hydrolysis. 
     It is known from Belgium Pat. No. 750,177 that aqueous morpholine solution reacts with COS to form a precipitate which is recovered and subsequently treated to heat and pressure to recover morpholine. The present invention eliminates the need for the formation of a precipitate. 
     U.S. Pat. No. 4,100,256 describes a process for COS hydrolysis using an aqueous solution of piperazinone compounds however the present invention is more effective in that a higher degree of hydrolysis is obtained. For example, a 50% solution of morpholine gave 83% hydrolysis at 100° C. and under the same conditions 1,4-dimethyl piperazinone gave 15% hydrolysis. 
     SUMMARY OF THE INVENTION 
     It has now been found that carbon oxysulfide is rapidly and efficiently hydrolyzed to H 2  S and CO 2  when a gas or liquid stream containing it is contacted with an aqueous solution of one or more of the N-heterocyclic compound having the formula: ##STR1## wherein Z is an oxygen atom or ##STR2## each R is independently a hydrogen atom or an alkyl group of 1-2 carbon atoms and R&#39; is the 2-amino ethyl group when Z is ##STR3## and R&#39; is a hydrogen atom when Z is an oxygen atom. 
     The hydrolysis can be carried out effectively using an N-heterocyclic compound or mixtures thereof in a solution containing an amount sufficient to hydrolyze the COS or at least about 25% by weight of the N-heterocyclic compounds, but preferably the hydrolysis process is operated with a solution containing about 25% to about 75% by weight of the N-hetercyclic compounds. The process temperature can range from about 50° to about 90° C. with the preferred range being 80°-90° C. The process of this invention is useful hydrolyzing COS in gas streams such as natural gases, synthetic gases, refinery gases, cracked gases and the like. The invention is also applicable to liquid hydrocarbon streams containing COS. 
     This invention is particularly useful to treat gas streams containing H 2  S, and CO 2 , in addition to COS since H 2  S and CO 2  do not inhibit the COS hydrolysis reaction. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The N-heterocyclic compounds useful in this invention generally comprise morpholine, and piperazine, and their alkylated derivatives which are water soluble. 
     Examples of useful morpholines are morpholine, 3,3-dimethylmorpholine, 2,6-dimethylmorpholine, 2,3,6-triethylmorpholine, 2-ethylmorpholine, 2,3,5,6-tetramethylmorpholine and the like. 
     Examples of useful piperazines are piperazine, N-methyl piperazine, N-ethyl piperazine, 2,6-dimethyl piperazine, N-aminoethyl piperazine, 1,3-dimethyl piperazine and the like. 
     These morpholines and piperazines (hereinafter N-heterocyclic compounds) are hygroscopic and, since most gas streams of the kind treated by this process normally contain more or less water vapor, in practice there is almost always some water present as the process continues even when the N-heterocyclic compound solution is initially essentially anhydrous. 
     The aqueous N-heterocyclic solution can also include a substantial proportion of one or more known gas-treating solvents; for example, an alkanolamine or sulfolane. Lower alkanolamines such as ethanolamine, diethanolamine, N-methyldiethanolamine, triethanolamine, isopropanolamine, and diisopropanolamine are common examples of that class. Such an additional solvent can comprise up to about 50 percent by weight of the solution although preferably it is limited to about 30 percent of the total. The hydrolysis of COS in the presence of the N-heterocyclic compounds is apparently a catalytic effect and this effect is facilitated by the increased solubility of COS in the solution provided by an additional solvent such as sulfolane or an alkanolamine. Any significant proportion of N-heterocyclic compound in the solution will promote the hydrolysis to some extent, but a more practical rate of hydrolysis is obtained when the N-heterocyclic compound comprises at least about 25 percent of the gas-treating solution. The range of N-heterocyclic compound in the aqueous solution is 25-75% by weight with 40-60% by weight being the preferred range. 
     The hydrolysis process can be run at any convenient pressure and atmospheric or superatmospheric pressure is preferred. Contact time can vary widely from a few seconds to a few minutes, i.e., five minutes or more depending upon the temperature, concentration and composition of the aqueous N-heterocyclic solution, and the amount of COS present in the gas being treated. 
     If desired, desorption of H 2  S and CO 2  formed by hydrolysis is efficiently accomplished by merely heating the aqueous N-heterocyclic medium to a temperature of about 85° C. or above, preferably to 100°-150° C. Moderately reduced pressure will accelerate the desorption to some extent although this is usually not necessary. 
    
    
     EXAMPLES 1-5 
     The apparatus used in these examples utilized a 20-tray 1 inch diameter Oldershaw column as the gas-liquid contactor. A liquid solution was continuously pumped to the top of the column in a closed type circuit. Natural gas was introduced at the bottom of the column and vented. The liquid was preheated before entering the column. The liquid reservoir contained about 3000 gms total. The natural gas was modified by adding CO 2 , H 2  S, and COS in the amounts set forth below. 
     The following data of Table I was obtained using a 50 wt.% aqueous solution of morpholine. 
     EXAMPLES 6 AND 7 
     Following the procedure set forth in Examples 1-5, the data of Table II was obtained using a 25 wt.% aqueous solution of morpholine. 
     CONTROL A 
     Following the above procedure the data in Table III was obtained using a 90 wt.% aqueous solution of morpholine. 
     The above system worked well for hydrolysis but the entire apparatus quickly plugged up with morpholine salts of CO 2  and H 2  S which could not be held in solution because of the small amount of water present. 
     EXAMPLES 8-18 
     Following the procedure set forth in Examples 1-5 the data of Table IV was collected utilizing aqueous solutions of various piperazine compounds. 
     EXAMPLES 19-46 
     The apparatus used in these examples utilized an 8 foot tower 3 inches in diameter packed with 3/8&#34; ceramic saddles. Natural gas was introduced at the bottom of the column. All data collected was with 50 wt.% aqueous morpholine which was preheated and introduced at the top of the column. The natural gas was modified with CO 2 , H 2  S and COS as in Examples 1-5. The data in Table V shows the temperature effects. 
     
                                           TABLE I__________________________________________________________________________ExampleLq. Rate     Lq. Temp.           Gas Rate                Mole %&#39;s In Mole % Out                                   HydrolysisNo.  cc/min.     °C.           l./min.                CO.sub.2                    H.sub.2 S                        COS COS    %__________________________________________________________________________1    18   75    1.0   6.9618                    9.5888                        0.1191                            0.0029 97.562    18   75    2.0   7.9729                    7.3969                        0.1101                            0.0000 1003    10   70    2.0  12.4502                    1.0906                        0.0587                            0.0071 87.904     5   70    2.0  11.4585                    1.2252                        0.0844                            0.0106 87.445    10   70    2.0  11.7270                    0.8692                        0.1152                            0.0110 90.45__________________________________________________________________________ 
    
     
                                           TABLE II__________________________________________________________________________ExampleLq. Rate     Lq. Temp.           Gas Rate                Mole %&#39;s In Mole % Out                                   HydrolysisNo.  cc/min.     °C.           l./min.                CO.sub.2                    H.sub.2 S                        COS COS    %__________________________________________________________________________6    10   70    2.2  9.3980                    0.0000                        0.1228                            0.0224 81.767    10   70    2.2  9.1092                    0.0000                        0.1261                            0.0622 50.67__________________________________________________________________________ 
    
     
                                           TABLE III__________________________________________________________________________Ref. Lq. Rate     Lq. Temp.           Gas Rate                Mole %&#39;s In Mole % Out                                   HydrolysisNo.  cc/min.     °C.           l./min.                CO.sub.2                    H.sub.2 S                        COS COS    %__________________________________________________________________________ControlA    10   25    2.0  9.4470                    0.0000                        0.1508                            0.0004 99.73__________________________________________________________________________ 
    
     
                                           TABLE IV__________________________________________________________________________Example       Lq. Rate              Lq. Temp.                    Gas Rate                         Mole %&#39;s In Mole % Out                                            HydrolysisNo.  Compound Wt. %         cc/min.              °C.                    l./min.                         CO.sub.2                             H.sub.2                                 COS COS    %__________________________________________________________________________8    Piperazine 22         18   70    1.4  10.7644                             1.3790                                 0.1332                                     0.0451 66.19    Piperazine 22         10   75    1.4  10.7644                             1.3790                                 0.1332                                     0.0492 63.110   Piperazine 22         10   75    0.8   9.0714                             0.6961                                 0.1028                                     0.0362 64.811   Piperazine 22         10   95    1.4  10.7644                             1.3790                                 0.1332                                     0.0435 67.312   Piperazine 32         10   75    2.0  10.8371                             0.0000                                 0.0939                                     0.0218 76.813   Piperazine 50         18   70    2.5   9.2360                             0.0000                                 0.1006                                     0.0190 81.114   Piperazine 50         18   70    2.5  14.2290                             0.0000                                 0.1473                                     0.0154 89.515   Piperazine 50         18   70    2.5  12.0865                             0.0000                                 0.1342                                     0.0190 85.816   Piperazine 50         18   70    2.5  11.1082                             0.0000                                 0.1339                                     0.0202 84.917   Aminoethyl-piperazine 50         18   70    2.3   5.9389                             0.0000                                 0.1129                                     0.0204 81.918   Aminoethyl-piperazine 50         10   70    2.3   8.6509                             0.0000                                 0.1241                                     0.0510 58.9__________________________________________________________________________ 
    
     
                                           TABLE V__________________________________________________________________________ExampleGas Rate     Lq. Rate          Mole %&#39;s In Mole % Out                             HydrolysisNo.  SCFM GPH  CO.sub.2               H.sub.2                  COS COS    %__________________________________________________________________________     Hydrolysis at 60° C.19   2.1  7.68 44.582               1.327                  0.0500                      0.0165 60.020   2.1  7.68 45.227               1.855                  0.0270                      0.0165 38.921   2.1  7.68 23.590               0.912                  0.0090                      0.0039 58.3     Hydrolysis at 70° C.22   2.5  8.04  3.740               0.710                  0.5971                      0.0416 93.023   2.5  8.04  8.701               0.236                  0.3690                      0.0780 78.924   2.4  7.02 10.657               0.987                  0.7050                      0.1015 85.625   2.4  7.08 11.562               1.163                  1.385                      0.1500 89.226   2.4  6.96 17.486               1.251                  0.687                      0.1160 83.1     Hydrolysis at 80° C.27   1.96 7.08 2.1110               0.584                  0.1570                      0.0175 88.828   1.96 6.96 2.7620               0.270                  0.2660                      0.0235 91.229   1.96 7.08 6.852               0.367                  0.1990                      0.0290 85.430   2.03 6.96 3.614               0.452                  0.2390                      0.0255 89.331   2.03 7.08 3.437               1.090                  0.3890                      0.0230 94.132   2.03 7.08 3.012               0.369                  0.2280                      0.0195 91.433   2.03 7.08 2.256               0.325                  0.1920                      0.0175 90.934   1.96 7.08 2.107               0.266                  0.1960                      0.0195 90.035   1.96 7.20 2.232               0.279                  0.1870                      0.0210 88.836   1.96 6.84 2.612               0.391                  0.2210                      0.0255 83.5     Hydrolysis at 90° C.37   1.58 7.08 2.529               0.405                  0.4590                      0.0310 93.238   1.58 7.08 2.152               0.439                  0.5160                      0.0133 97.439   1.26 6.96 3.794               1.836                  0.0190                      0.0020 89.540   1.58 6.96 3.070               1.115                  0.8110                      0.0345 95.841   1.58 6.96 2.887               1.087                  0.896                      0.0305 96.642   1.58 6.96 2.560               1.070                  0.704                      0.0245 96.543   1.26 7.08 3.463               1.853                  0.0140                      0.0010 92.944   1.26 7.08 3.282               1.721                  0.0110                      0.0010 90.945   1.26 7.14 3.359               1.779                  0.0100                      0.0000 10046   1.26 7.14 3.519               1.759                  0.0100                      0.0010 90.0__________________________________________________________________________ 
    
     EXAMPLES 47-53 
     Following the procedure set forth in Examples 19-46, a 50 weight percent aqueous morpholine solution was used at 90° C., a liquid rate of 11-12 gallons per hour (gph) to contact a modified natural gas stream flowing at 1.1 to 1.5 standard cubic feet per minute (SCFM) to give a residence time of 54-67 seconds. The averages of several runs are set forth in Table VI. 
     
                       TABLE VI______________________________________Example Mole %   Mole %   Average %No.     COS IN   COS Out  Hydrolysis                             Notes______________________________________47      0.05-0.10            0.0042   94.76   Av. of 9 runs48      0.10-0.15            0.0072   94.01   Av. of 13 runs49      0.15-0.20            0.0113   93.03   Av. of 10 runs50      0.10-0.25            0.0143   93.48   Av. of 8 runs51      0.25-0.40            0.0206   93.72   Av. of 4 runs52      0.40-0.75            0.0390   92.14   Av. of 7 runs53      0.75-1.25            0.1240   87.90   Single run______________________________________ 
    
     EXAMPLES 54-57 
     In a manner similar to Examples 47-53 a 50% aqueous solution of morpholine at 90° C., a liquid rate of 12-19 gph, a gas rate of 1.3-1.5 SCFM was used to give a residence time of 50-60 seconds. The averages of several runs are set forth in Table VII. 
     
                       TABLE VII______________________________________Example Mole %   Mole %   Average %No.     COS IN   COS Out  Hydrolysis                             Notes______________________________________54      0.05-0.10            .0052    94.6    Av. of 2 runs55      0.10-0.15            .0090    93.1    Av. of 9 runs56      0.15-0.20            .0115    93.3    Av. of 8 runs57      0.20-0.25            .0119    94.6    Av. of 7 runs______________________________________ 
    
     EXAMPLES 58-62 
     Using the same hydrolysis temperature and solution as in Examples 47-53 with a gas rate of 1.0-1.3 SCFM, a liquid circulation rate of 25 gph and a gas residence time of 60-75 seconds, the run averages of Table VIII were obtained. 
     
                       TABLE VIII______________________________________Example Mole %   Mole %   Average %No.     COS IN   COS Out  Hydrolysis                             Notes______________________________________58      0.05-0.10            0.0030   97.0    Single run59      0.10-0.15            0.0065   95.6    Av. of 2 runs60      0.15-0.20            0.0062   96.5    Av. of 13 runs61      0.20-0.25            0.0097   96.1    Av. of 9 runs62      0.25-0.35            0.0086   97.0    Av. of 6 runs______________________________________ 
    
     EXAMPLES 63-68 
     Using the equipment set forth in Examples 19-46, the tower was flooded almost to the top leaving a small air space. It was found that under these conditions, the flow rate could be reduced to 8 or 9 gallons per hour and the percent hydrolysis could be increased. In these examples, a 50% aqueous morpholine solution was used at a temperature of 90° C. The inlet gas flow rate was 1.5 SCFM and the residence time was 50-53 seconds. The results are set forth in Table IX. 
     
                       TABLE IX______________________________________Example Mole %   Mole %   Average %No.     COS IN   COS Out  Hydrolysis                             Notes______________________________________63      0.05-0.10            0.0060   91.4    Single run64      0.10-0.15            0.0043   96.9    Av. of 5 runs65      0.15-0.20            0.0060   96.4    Single run66      0.20-0.25            0.0044   98.1    Av. of 3 runs67      0.25-0.35            0.0043   98.5    Av. of 7 runs68      0.35-0.76            0.0033   99.3    Av. of 5 runs______________________________________ 
    
     While the examples illustrate the use of a single compound in aqueous solution, similar results are obtained when one or more of these compounds are used.