Abstract:
A novel class of solvents is described that have improved selectivity for H 2  S as compared to CO 2 . The solvents are based on adding suitable second partially miscible solvent to known solvents. In specific, it is shown that adding 20% of dodecane to NMP (normal methyl pyrolidinone) increases the selectivity of the solvent for H 2  S as compared to CO 2  by 50%. This leads to important process improvements and reduction in the cost of the removal process.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     Physical solvents such as methanol, NMP (normal methyl pyrolidinone) [U.S. Pat. No. 3,103,411 and U.S. Pat. No. 4,208,382]selexol, propylene carbonate [U.S. Pat. No. 2,926,751]and others are widely used for the removal of H 2  S from gases such as natural gas and syngas from coal gasification. In such cases the solvent is contacted counter currently with the gas and the solvent is then heated and later stripped to remove one of the dissolved gases, H 2  S [U.S. Pat. No. 4,198,388]. In many of those applications, CO 2  is also present and it is important to minimize the CO 2  that is released together with the H 2  S in the stripping of the solvent. This is due to the fact that high concentration of CO 2  would make complete conversion of the H 2  S in a chemical process more difficult. This complexes the design of the extraction column, and there are a number of commercial designs that have solved that problem [Kohl and Riesenfeld, 1985, Newman, 1985]. The details of these designs are not relevant to our inventions. It is enough to know that satisfactory design exists. 
     One important and well known feature of these designs is that they require a solvent which is highly selective for H 2  S as compared to CO 2 . This is especially important if the concentration of CO 2  is much higher than that of H 2  S. 
     It is obvious to those skilled in the art that any improvement in selectivity has great value in making the cost of separating and extending the range of relative conditions over which the method is commercially viable. The field of the present invention is the description of a method which simplifies the search for improved solvents as well as specific examples of solvent mixtures with greatly enhanced selectivity properties. 
     Partially Miscible Solvent Mixtures With A Critical Point of Miscibility. 
     Let us consider a binary mixture of solvents which have a critical point of miscibility. The meaning of such a critical point can be explored by a phase diagram (see FIG. 1). 
     This figure describes the concentration of component A and B as a function of temperature. The region marked &#34;one phase&#34; represents conditions of complete miscibility of components A and B, whereas regions marked &#34;two phase&#34; correspond to separation into two phases. When the phase diagram shows a maximum (FIG. 1a), it is called an upper critical solution temperature (UCST), whereas a minimum (FIG. 1b) corresponds to a lower critical solution temperature (LCST). 
     Liquids posses also a vapor-liquid critical point. Above the critical temperature, there is only one fluid phase independent of pressure. In the last twenty years, it was found that near this critical point there are some very interesting thermodynamic properties, [Paulaitis, et al., 1983]. Those unique properties have resulted in the development of separation processes named supercritical extraction. However, that field should not be confused with the present art. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a process for selectively removing H 2  S from a gas stream containing both H 2  S and CO 2  which comprises: 
     contacting said gas stream with a solvent comprising a mixture of N-methylpyrrolidone (NMP) and dodecane, said mixture having a critical point of miscibility, to selectively dissolve H 2  S in said solvent thereby providing a gas stream of reduced H 2  S content and a solvent containing dissolved H 2  S. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1(a) is a phase diagram of an immiscible binary system with an upper critical solution temperature. 
     FIG. 1(b) is a phase diagram of an immiscible binary system with a lower critical solution temperature. 
     FIG. 2 is a phase diagram of an acetonitrile/water system. 
     FIG. 3 is a phase diagram of an NMP/dodecane system. 
     FIG. 4 is a phase diagram of an NMP/methyl-cyclohexane system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Description of the Solvents Proposed 
     Ludmer et al. [1987]have previously shown that a class of solvent mixtures in the vicinity of their critical point of miscibility obtain an increased solubility for some solids. Contrary however to our previous publication, we have found here that the maximum effect is not in the neighborhood of the critical point. In fact it is often more pronounced in a region where two liquid phases exist. It was detected in some systems to have a maximum effect close to the phase transition and not necessarily at the critical composition. 
     Let us now look as to what available evidence and theory predict for the solubility of a gas in a solvent mixture as a function of composition. If one uses, for example, a mixture of two solvents, then solubility of a gas at low gas pressure can be expressed as follows. If H is the Henry constant defined by 
     
         H.sub.1 -p.sub.i /S.sub.i                                  (1) 
    
     where p i  is the partial pressure of the gas compound in equilibrium with the solution and x i  the molar fraction of the dissolved gas in the mixed solution, then the solubility of the gas in an ideal mixture and low gas concentration can be computed [O&#39;Connell, 1971]by Krischevsky [1937]equation 
     
         lnH.sub.3,m -S.sub.1 lnH.sub.3,1 +X.sub.2 lnH.sub.3,2      (2) 
    
     Where 3 denote for the dissolved gas, 1 and 2 for the solvents and m for the mixture. H 3 ,1 and H 3 ,2 are the Henry constants in the pure solvents. Equation 2 is derived by computing the thermodynamic potential in an ideal mixture of two solvents with no interactions. Obviously, pariially miscible solvents have strong interactions, which can be indicated by either positive (higher Henry Constant) or negative (decreased Henry&#39;s Constant) deviations compared to Eq. 2. However, the solubility of different gases in the same CST forming mixture can result in different deviation in magnitude, and in some solvent gas systems, also in the deviation sign. These phenomena can result in increased selectivity when compared to the pure constituents of the mixture. 
     In tables 1 to 4, we present some typical gas solubility results for selected mixtures of partially miscible solvents both above and below the critical point of miscibility, and as a reference, table 5 summarizes the solubility of the gases in the pure solvents. The example solvents being three solvent pairs NMP-dodecane, NMP-methyl-cyclohexane and acetonitrile-water. In FIGS. 2 to 4 we show the diagrams of the above solvent pairs. 
     We found strong effects, as far as selectivity is concerned, in all of the above mentioned solvent mixtures. However, only in the NMP-dodecane and the acetonitrile-water mixtures is the effect useful for our purposes. In the NMP-methyl-cyclohexane case the nonlinear effect favors CO 2  over H 2  S. At present, we have no method that would predict the direction of the effect in a specific solvent mixture All what we have established is that the effects are surprisingly large and that such effects have never been reported in the literature. As one can notice from tables 3 and 4, the maximum selectivity is not close to the CST but rather in the two phase region. The total solubility in these cases is reduced but still acceptable. The increase in selectivity is technically and economically of some importance. 
     We do not claim that all mixtures of partially miscible solvents with a critical point of solubility will have these effects or that adding a second solvent that forms a critical mixture will always increase selectivity. What we claim is that addition of such a second solvent can in some systems lead to drastic improvements in selectivity as illustrated by the above examples. However, it should be pointed out that some solvent systems may favor CO 2  over H 2  S. The references cited above are cited in greater detail below. 
     Description of the Proposed Process 
     As said before, removal of acid compounds such as H 2  S and CO 2  from gas streams by physical solvents is common commercial practice, and proven process equipments and flow-sheets are available. 
     In our process modification, the type of equipment used remains the same. What our proposed process modification does, is to increase the selectivity of the gas removal by improving the selectivity of the solvent. As an example we provide data for removing H 2  S in the presence of CO 2 . That could in principal be achieved in two ways. One is to find a better, more selective solvent. A solvent mixture of the type described here could serve this purpose and we will give an example later. A second way is to take the primary solvent that is used in the process and add to it a second solvent that forms with it a solution with a critical point of miscibility. An example of such a type is NMP (normal methyl pyrolidinone) which is used in the Purisol process. If 8% (17% by volume) dodecane is added to it then at a temperature of about 35° C., the selectivity is increased by 60%. 
     The solubility and selectivity for this example are given in tables 1 to 4. We note that the mixture with 8% dodecane and at 35° C. is not at the critical composition or temperature which are 50% dodecane, 50% NMP at temperature of 78° C., but in the NMP rich phase. However, similar effects could also be observed in the one phase region. We argue that it is the nature of such mixtures to promote non-linear effects over a wide range of concentrations and our claim is over the whole region where such effects persist. 
     A second example is the case where acetonitrile is the primary solvent. It is not commonly used in gas purification, but has excellent properties. Here addition of 30% (by volume) water at 5° C. leads to a greatly improved solvent with higher selectivity for H 2  S compared to CO 2 , as can be seen from the data in tables 3 and 4. 
     Solvent mixtures with a critical point are also formed if more than one additional solvent is added. Even solid compounds can be added to change the properties of the solvent mixture as well as 
     the composition and temperature of the critical point. Such cases do not substantially modify the basic concept of our invention and are therefore part of the improved process method claimed. 
     There is a large class of such solvent mixtures both binary and ternary that are potential candidates for improved gas absorption and the above solvent systems are considered as examples of a practical realization of our model concept. 
     
         ______________________________________Reference Cited______________________________________UNITED SATE PATENTS2,926,751    Kohl et al.   Mar. 1, 19603,103,411    Werner        Sep. 10, 19634,198,388    Laslo ey al.  Apr. 15, 19804,208,382    Blume et al.  Jun. 17, 1980OTHER PUBLICATIONSKohl, A. and Riesenfeld, F., &#34;Gas Purification&#34;,Fourth ed., Gulf Publication Company, 1985.Krischevsky, I., R., Zh. Fiz. Khim., Vol. 9, 41, 1937.Ludmer, Z., Shinnar, R. and Yakhot, V., &#34;Solubilityin Binary Mixtures at the Immiscibility Critical Point&#34;,AIChE J., Vol. 33, 1776-1780, 1987.Newman A. S. (editor), &#34;Acid and Sour Gas TreatingProcesses&#34;, Gulf Publication Company, 1985.O&#39;Connell, J. P., &#34;Molecular Thermodynamics ofGases in Mixed Solvents&#34;, AIChE J., Vol. 17,658-663, 1971.Paulaitis, M. E., et al., &#34;Supercritical Fluid Extraction&#34;,Rev. Chem. Eng., Vol. 1, 179,______________________________________ 
    
     
                                           TABLE 1__________________________________________________________________________Solubility of carbon dioxide and hydrogen sulfide (at 1 atmpartial pressure) in the one phase region.                  Solubility Ratio                  Mixture/                        Mixture/Solvents   Solute        Temp.     Good  Poor  Mixture/System  (gas)        °C.             Solub..sup.1                  Solvent                        Solvent                              Raoult&#39;s.sup.2__________________________________________________________________________Acetonitrile 10   4.40 0.464 3.451 1.973(good solv.) +        15   3.87 0.444 3.686 2.026Water   CO.sub.2        20   3.54 0.426 3.933 2.082(poor solv.) 25   3.21 0.401 4.280 2.169(0.609).sup.3        15   16.30                  0.534 5.552 2.876        20   13.85                  0.519 5.368 2.778   H.sub.2 S        45   7.81 0.462 5.049 2.543        60   5.23 0.400 4.144 2.117NMP     CO.sub.2        78   0.84 0.467 1.024 0.832(good solv.)Dodecane   H.sub.2 S        78   11.58                  0.739 5.514 2.305(poor solv.)(0.297).sup.3NMP          18   4.40 0.894 2.018 1.401(good solv.) 20   4.20 0.878 1.974 1.372        30   3.50 0.854 1.938 1.338Methyl- CO.sub.2        40   2.92 0.859 1.879 1.352Cyclohexane  50   2.39 0.885 1.833 1.317(0.412).sup.3        60   1.90 0.805 1.753 1.228        70   1.42 0.703 1.632 1.107   H.sub.2 S        10   53.55                  0.729 3.688 2.031        20   45.08                  0.717 3.840 2.009        30   33.52                  0.638 3.520 1.774        40   24.58                  0.598 3.208 1.598        50   18.52                  0.593 3.046 1.515        60   13.99                  0.593 3.937 1.461        70   11.00                  0.564 3.013 1.430__________________________________________________________________________ .sup.1 Volume of the gas at standard condition over solutefree solvent volume. .sup.2 Ideal mixture (eq. 2). .sup.3 Volume fraction of the good solvent 
    
     
                                           TABLE 2__________________________________________________________________________Solubility of carbon dioxide and hydrogen sulfide in critical mixtures(at 1 atm partial pressure) in the two phase region.                  Mixture/                       Mixture/Solvents      Temp.    Good Poor  Mixture/System  Phase °C.             Solub..sup.1                  Solvent                       Solvent                             Raoult&#39;s.sup.2__________________________________________________________________________Aetonitrile   ACN   -5  11.84                  1.024                       6.072 2.349(good solv.) +   rich   0  8.76 0.826                       5.078 2.246Water          3  7.25 0.706                       4.560 2.287(poor solv.)   5  5.48 0.545                       3.651 2.148+CO.sub.2 (gas)   Water -5  3.21 0.277                       1.644 1.367   rich   0  3.44 0.324                       1.992 1.555          3  3.92 0.382                       2.465 1.790          5  5.33 0.530                       3.553 2.141   Average         -5  8.00 0.692                       4.103 2.548          0  6.40 0.604                       3.710 2.974          3  5.78 0.563                       3.635 2.189          5  5.41 0.538                       3.607 2.147+H.sub.2 S (gas)   ACN rich          5  28.90                  0.858                       7.298 2.555   Water rich          5  8.95 0.266                       2.260 1.731   Average          5  21.44                  0.636                       5.414 3.017NMP     NMP   20  3.12 0.652                       2.940 0.735(good solv.) +   rich  35  2.09 0.572                       2.173 0.635Dodecane      45  1.53 0.553                       1.776 0.628(poor solv.)  65  1.18 0.538                       1.426 0.658+CO.sub.2 (gas)   Dodecane         20  1.06 0.222                       1.000 0.855   rich  35  0.97 0.265                       1.007 0.876         45  0.89 0.321                       1.031 0.909         65  0.83 0.379                       1.005 0.863   Average         20  1.49 0.312                       1.406 0.738         35  1.20 0.329                       1.250 0.710         45  1.02 0.369                       1.186 0.732         65  0.91 0.414                       1.096 0.743+H.sub.2 S (gas)   NMP   20  55.60                  0.885                       8.633 1.226   rich  35  44.50                  0.931                       8.768 1.340         50  32.24                  0.856                       7.694 1.187         65  19.94                  0.924                       6.453 1.388   Dodecane         20  9.02 0.144                       1.400 1.153   rich  35  9.49 0.198                       1.870 1.490         50  10.03                  0.266                       2.394 1.896         65  11.27                  0.522                       3.647 2.621   Average         20  22.94                  0.365                       3.562 1.451         35  20.20                  0.422                       3.981 1.597         50  17.96                  0.422                       3.981 1.721         65  14.08                  0.653                       4.558 2.003NMP     NMP   10  5.56 1.010                       2.269 1.304(good solv.) +   rich  15  5.03 0.978                       2.206 1.315Methyl- MCH   10  3.67 0.667                       1.498 1.380cyclohexane   rich  15  3.50 0.680                       1.534 1.300(poor solv.)   Average         10  5.10 0.927                       2.080 1.451+CO.sub.2 (gas)         15  4.66 0.906                       2.042 1.419__________________________________________________________________________ .sup.1 Volume of the gas at standard condition over solutefree solvent volume. .sup.2 Ideal mixture (eq. 2) 
    
     
                                           TABLE 3__________________________________________________________________________Selectivity of hydrogen sulfide in respect to carbon dioxide in criticalmixtures (at 1 atm partial pressure) in the one phase region.                       Selectivity Ratio           Volume      Mixture/                             MixtureSolvents    Temp.           Fraction    Good  PoorSystem      °C.           (good solv.)                   Selc..sup.1                       Solvent                             Solvent__________________________________________________________________________Acetonitrile       15  0.609   4.212                       1.203 1.506(good solvent) +       20          3.912                       1.219 1.365Water (poor solvent)NMP (good solvent) +       78  0.297   13.786                       1.582 5.383Dodecane(poor solvent)NMP (good solvent) +       20  0.412   10.733                       0.817 1.946Methyl-cyclohexane       30          9.577                       0.748 1.816(poor solvent)       50          7.749                       0.670 1.662       60          7.363                       0.736 1.676       70          7.746                       0.802 1.846__________________________________________________________________________ .sup.1 Ratio- mole H.sub.2 S to mole CO.sub.2 absorbed 
    
     
                                           TABLE 4__________________________________________________________________________Selectivity of hydrogen sulfide in respect to carbon dioxide in criticalmixtures (at 1 atm partial pressure) in the two phase region.                       Selectivity Ratio           Volume      Mixture/                             Mixture/Solvents    Temp.           Fraction    Good  PoorSystems Phase       °C.           (good solv.)                   Selec..sup.1                       Solvent                             Solvent__________________________________________________________________________Acetonitrile   ACN  5  0.70    5.277                       1.578 2.00(good solvent) +   richWater   Water        5  0.45    1.679                       0.502 0.636(poor solvent)   richNMP     NMP 20  0.85    17.84                       1.358 2.937(good solv.) +   rich       35  0.83    21.33                       1.628 4.040Dodecane   Dodec.       20  0.06    8.51                       0.648 1.401(poor solv.)   rich       35  0.07    9.81                       0.748 1.858__________________________________________________________________________ .sup.1 Ratio  mole H.sub.2 S to mole CO.sub.2 absorbed. 
    
     
                       TABLE 5______________________________________Solubility of carbon dioxide and hydrogen sulfide(at 1 atm and 20° C.) in pure solvents.Solvent         Solute (gas)                      Solubility.sup.1______________________________________Acetonitrile    CO.sub.2   8.31           H.sub.2 S  26.69NMP             CO.sup.2   4.78           H.sub.2 S  62.82Methyl-cyclohexane           CO.sub.2   2.13           H.sub.2 S  11.74Dodecane        CO.sub.2   1.06           H.sub.2 S  6.44Water           CO.sub.2   0.89           H.sub.2 S  2.58______________________________________ .sup.1 Volume of the gas at standard condition over solutefree solvent volume.