Abstract:
Acidic gases, such as H 2  S, COS, CO 2 , and HCN are removed from a mixture of light hydrocarbons by first separating the mixture into a gas portion and a liquid portion. The gas portion is compressed, water washed and scrubbed with an absorbing solution prior to separating at least one hydrocarbon from other hydrocarbons in the mixture. The liquid portion is stripped of H 2  S. The stripped vapors containing the stripped H 2  S may be recycled to the gas compression step.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for removing acidic gases such as H 2  S, COS, CO 2  and HCN from mixtures of lower boiling hydrocarbons. 
     2. Description of the Prior Art 
     Mixtures of light hydrocarbons, such as mixtures commonly known as light ends, which usually result from the thermal decomposition of high temperature petroleum refining operations are often recovered and separated into two or more hydrocarbon streams in a light ends treating operation, such as deethanizer and debutanizer towers. The acidic gases contained in these light ends cause corrosion of the light ends treating equipment. 
     Prior art technology typically consisted of compressing the gas portion and pumping the liquid portion of these light hydrocarbons and feeding them to a light ends fractionation system. Any acidic gases contained in the light ends were carried through the light ends fractionation towers and eventually removed from the separated products. In a typical light ends sequence, consisting of an absorber deethanizer tower followed by a dubutanizer, a portion of the acid gases would go to the overhead product from the deethanizer and be scrubbed out by a downstream scrubber using an amine solution or other absorbent. The remainder of the acid gases would typically concentrate on the overhead product of the debutanizer, usually a C 3  /C 4  LPG stream, and be removed by liquid/liquid treating with an amine and/or caustic solution. 
     The presence of corrosive agents in the light ends towers in such schemes as described above has been known to cause severe corrosion of the fractionating vessels and associated equipment. Also, the presence of HCN, which reacts irreversibly with the amine to form degradation products, has resulted in large amine losses. 
     The present invention provides a process which relieves both of these conditions by (1) placing the acid gas removal system upstream of the light ends towers and (2) placing a water scrubber to remove the bulk of the HCN upstream of the amine tower. 
     A method has now been found whereby the acidic gases are removed from the light ends prior to processing the light ends in light ends recovery equipment. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided, in a process for separating at least one hydrocarbon from other hydrocarbons in a mixture comprising light hydrocarbons and acidic gases, the improvement which comprises removing said acidic gases from said mixture prior to said hydrocarbon separation step by the steps which comprise: 
     A. SEPARATING SAID MIXTURE INTO A GASEOUS PORTION AND A LIQUID PORTION; 
     B. SUBJECTING SAID GASEOUS PORTION TO COMPRESSION, WATER WASHING AND SCRUBBING WITH AN AQUEOUS ABSORBING SOLUTION; AND 
     C. SUBJECTING THE LIQUID PORTION TO STRIPPING TO REMOVE AT LEAST A PORTION OF THE HYDROGEN SULFIDE THEREFROM AND TO PRODUCE A HYDROGEN SULFIDE-CONTAINING EFFLUENT. If desired the hydrogen sulfide-containing effluent may be recycled to the gas compression step. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic flow plan of one embodiment of the invention. 
     FIG. 2 is a schematic flow plan of another embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a mixture of light hydrocarbons containing acidic gas contaminants, such as H 2  S, CO 2  and HCN, is passed via line 10 into drum 12. Suitable mixtures of hydrocarbons include mixtures having hydrocarbon components boiling (at atmospheric pressure) in the range between Cl, and 400°F. The major source of light hydrocarbons is the thermal decomposition of high temperature petroleum refining operation. The mixture of light hydrocarbons may be, for example, the overhead vapor of a coker products fractionator or the overhead vapor of a catalytic cracking products fractionator, typical compositions of which are tabulated below. 
     
         ______________________________________    Coker Fractionator                   Catalytic CrackingStream:  Overhead       Fractionator OverheadConstituent    Mole %         Mole %______________________________________H.sub.2  10.58          2.20C.sub.1  27.76          11.78C.sub.2  15.68          9.58C.sub.3  10.05          14.22C.sub.4  5.35           12.69 C.sub.5 +    13.76          41.49 H.sub.2 O    7.22           4.31 H.sub.2 S    4.12           1.78CO       1.50           -- CO.sub.2    0.50           -- HCN     0.01           --Inerts   3.47           1.95    100.00         100.00______________________________________ 
    
     In drum 12, the mixture is separated into a liquid portion removed by line 14 and a gaseous portion removed by line 16. The gas portion is passed to a gas compressor 18, removed via line 20 and passed through aftercooler 22, then through line 24 into aftercooler drum 26 from where it is passed via line 28 into a conventional water washing stage in tower 30. Foul water is removed from water washing tower 30 via line 32. The water washed portion is removed via line 34 and sent to tower 36 in which the water washed portion is subjected to a conventional scrubbing stage with an absorbing aqueous solution, such as an amine solution for removal of carbon dioxide and hydrogen sulfide. Typical prior art amine scrubbing processes are described in U.S. Pat No. 3,144,301, and in U.S. Pat. No. 3,851,041, the teachings of which are hereby incorporated by reference. 
     The scrubbed gas is removed from the absorption tower 36 by line 38. The absorbent-rich solution is removed via line 40. The absorbent-lean solution enters the absorption tower via line 41. The scrubbed gas removed via line 38, which is a gas containing a negligible amount of acidic contaminants, is suitable as feed to a light ends recovery operation such as a deethanizing stage. 
     Returning to drum 12, the liquid portion is passed via line 14 to a reboiled stripping tower 42. Hydrogen sulfide and lighter components are removed from tower 42 by line 44 and, if desired, recycled to gas compressor 18 via line 46, intercooler 48, line 50, interstage drum 52 and line 54. The stripped liquid portion is removed from stripper 42 via line 56. The stripped liquid containing a negligible amount of acidic components is suitable as feed to a light ends recovery process, such as a deethanizing stage. 
     The following example is presented to illustrate the invention. 
     EXAMPLE 
     As illustrative example, the stream compositions and conditions of a specific embodiment of the scheme shown in FIG. 2 are given in Table II. Referring to FIG. 2, a vacuum pipestill overhead gas (stream 101), a coker naphtha hydrofiner stripper overhead (stream 102) and a coker fractionator vapor distillate (stream 103) are combined and passed to a coker fractionator distillate drum (201). The overhead gas from the coker fractionator distillate drum is compressed in a three-stage centrifugal gas compressor (202, 205, 208). In Table I are listed various stages indicated in FIG. 2 and their corresponding numerals. ##TBL2## 
     
                                           TABLE II__________________________________________________________________________Stream No.    101  102  103    104    105    106         107  108         Coker                                 1st         Naphtha                               Stage         N/F  Coker  Compressor                            Coker  1st         K.O. Compressor    Vacuum         Stripper              Fractionator                     1st    Fractionator                                   Stage       Drum 2ndStream   P/S  Overhead              Vapor  Stage  Liquid Intercooler Water                                                    StageName     Overhead         Vapor              Dist.  Inlet  Distillate                                   Feed        Cond.                                                    Inlet__________________________________________________________________________Conditions    Vapor         Vapor              Vapor  Vapor  LIquid Vapor Liquid                                               Liquid                                                    VaporPressure, Psig    2     75  2      1       55     30    30    25   25Temperature, °F    110  135  110    110    110    183   183   100  100Composition,Moles/Hr.H.sub.2  --   7.1  293.9  301.0  --     301.3 --    --   301.3H.sub.2 S    8.2  11.0 114.3  133.5  0.3    141.2 0.2   --   139.6C.sub.1  --   5.3  770.7  776.0  0.2    781.0 0.2   --   780.2C.sub.2 =    --   --   147.7  147.7  0.2    151.1 0.1   --   150.5C.sub.2  37.7 --   287.1  324.8  0.4    334.4 0.3   --   332.7C.sub.3 =    --   --   151.6  151.6  0.6    164.3 0.4   --   161.9C.sub.3  --   --   126.5  126.5  0.5    138.4 0.4   --   136.1iC.sub.4 --   --   6.7    6.7    0.1    7.5   --    --   7.1C.sub.4 =    --   1.5  93.7   95.2   1.3    105.5 0.6   --   100.0nC.sub.4 --   0.8  39.0   39.8   0.5    43.4  0.3   --   40.8C.sub.4 ==    --   --   7.2    7.2    0.1    7.6   0.1   --   6.9C.sub.5 +    --   2.0  232.3  234.3  149.8  316.2 72.8  --   137.3H.sub.2 O    3.5  --   200.6  204.1  --     204.1 --    143.0                                                    61.1CO       --   --   41.8   41.8   --     41.9  --    --   41.9N.sub.2  9.0  --   96.5   105.5  --     105.6 --    --   105.6CO.sub.2 --   --   14.3   14.3   --     14.6  --    --   14.6__________________________________________________________________________Stream No.   109    110       111    112  113    114    115  116   1st Stage               Com- 2nd Stage          3rd Stage   K.O. Drum          2nd Stage 2nd Stage                           pressor                                K.O. Drum                                       3rd Stage                                              Vapor                                                   K.O. DrumStream  Hydrocarbon          Intercooler                    K.O. Drum                           3rd Stage                                Hydrocarbon                                       K.O. Drum                                              Water                                                   HydrocarbonName    Condensate          Feed      Water Cond.                           Inlet                                Condensate                                       Water Cond.                                              Scrubber                                                   Condensate__________________________________________________________________________Conditions   Liquid Vapor               Liquid                    Liquid Vapor                                Liquid Liquid Vapor                                                   LiquidPressure,Psig    25     84   84   80     80   80    230    230  230Temperature, °F   100    170  170  100    100  100    100    100  100Composition,Moles/Hr.H.sub.2 --     301.2               0.1  --     301.1                                0.2    --     301.0                                                   0.1H.sub.2 S   1.8    139.5               1.9  --     135.4                                6.0    --     133.8                                                   1.6C.sub.1 1.0    779.4               1.8  --     177.5                                3.7    --     776.2                                                   1.3C.sub.2 =   0.7    150.2               1.0  --     148.7                                2.5    --     147.9                                                   0.8C.sub.2 2.0    332.0               2.7  --     327.5                                7.2    --     325.2                                                   2.3C.sub.3 =   2.8    161.6               3.1  --     155.0                                9.7    --     152.1                                                   2.9C.sub.3 2.7    135.9               2.9  --     129.5                                9.3    --     126.7                                                   2.8iC.sub.4   0.4    7.2  0.3  --     6.4  1.1    --     6.1  0.3C.sub.4 =   6.1    101.0               5.1  --     87.6 18.5   --     82.6 5.0nC.sub.4   2.9    41.3 2.4  --     35.0 8.7    --     32.7 2.3C.sub.4 =   0.8    7.1  0.6  --     5.5  2.2    --     5.0  0.5C.sub.5 +   251.7  185.0               204.0                    --     74.4 314.6  --     49.8 24.6H.sub.2 O   --     61.1 --   36.9   24.2 --     15.2   9.0  --CO      --     41.9 --   --     41.8 0.1    --     41.8 --N.sub.2 --     105.6               --   --     105.5                                0.1    --     105.5                                                   --CO.sub.2   --     14.6 --   --     14.4 0.2    --     14.3 0.1__________________________________________________________________________Stream No.   117      118  119  120       121   122   123    124                 Distillate                      Distillate   Distillate            Distillate                 Stripper                      Stripper  Distillate                                      MEA.sup.1Stream  Stripper Stripper                 Reboiler                      Reboiler  Stripper                                      Scrubber                                            Lean   RichName    Feed     Overhead                 Feed Return    Bottoms                                      Overhead                                            MEA    MEA__________________________________________________________________________Conditions   Vapor       Liquid            Vapor                 Liquid                      Vapor                           Liquid                                Liquid                                      Vapor Liquid LiquidPressure, Psig   50  50    50   52   52   52   52   226   250    230Temperature,°F   98  98   123  212  272  272  272   120   120    146Composition,Moles/Hr.H.sub.2 0.3 --   0.3  --   --   --   --    300.9 --     --H.sub.2 S   0.8 6.8  7.6  --   --   --   --    --     23.0  156.8C.sub.1 2.6 2.4  5.0  --   --   --   --    776.2 --     --C.sub.2 =   0.8 2.5  3.3  --   --   --   --    147.9 --     --C.sub.2 1.7 7.8  9.5  --   --   --   --    325.2 --     --C.sub.3 =   0.9 11.7 12.5 1.1  1.0  0.1  0.1   152.1 --     --C.sub.3 0.8 11.3 11.8 2.0  1.7  0.3  0.3   126.7 --     --iC.sub.4   --  1.4  0.7  3.3  2.6  0.7  0.7   6.1   --     --C.sub.4 =   0.5 23.0 9.6  62.4 48.5 15.9 13.9  82.6  --     --nC.sub.4   0.2 10.8 3.4  31.6 24.0 7.6  7.6   32.8  --     --C.sub.4 =   --  2.7  0.4  8.0  5.7  2.3  2.3   5.0   --     --C.sub.5 +   0.3 338.9            4.9  472.7                      138.4                           334.3                                334.3 49.8  --     --H.sub.2 O   --  --   --   --   --   --   --    14.8  6244.0 6238.2CO      0.1 --   0.1  --   --   --   --    41.8  --     --N.sub.2 0.1 --   0.1  --   --   --   --    105.5 --     --CO.sub.2   0.1 0.2  0.3  --   --   --   --    --    --      14.3MEA                                               460.0 460.0__________________________________________________________________________ .sup.1 MEA -- monoethanolamine