Method for the purification of natural gas having a high contents of acidic gases
This invention relates to a method for the purification of natural gas having a high content of acidic gases. 
     The natrual gas often contains considerable amounts of acidic gases, such as, essentially, CO 2  and H 2  S. 
     The purification of natural gas from such acidic gases is necessary to permit the utilization of the natural gas and, at any rate, in order to reduce the volume of the useful gas to be shipped, thus reducing the shipping costs. 
     A composition of natural gas rich in acidic gases is the following: 
     --CH 4  26.46% by volume 
     --C 2  H 6  0.44% by volume 
     --C n  H 2n+2  0.27% by volume 
     --N 2  0.49% by volume 
     --CO 2  71.80% by volume 
     --H 2  S 2  0.54% by volume 
     Other composition may obviously occur according to the natural deposit from which the natural gas is extracted. 
     In view of the fact that natural gas rich in acidic gases is often found in deposits which are offshore and far from the coast, it can easily be understood that the purification made necessary by the presence of such acidic gases requires that technical and ecological problems be solved, which are not negligible and are connected with the necessity of carrying out the purification operations on platforms so as to reduce the cost of the sealine to convey the natural gas to the coast: such a cost would grow in an excessive manner if one would convey the natural gas as such, due to the very high content of inerts. 
     The discharge in the atmosphere of enormous quantities of acidic gases could also create serious air pollution problems. 
     A method has now been found for the purification of natural gas having a high content of acidic gases, by which method the acidic gases in question are removed in a simple and cheap way while preventing pollution, the purification operation being susceptible of being easily carried out directly on an offshore platform. 
     The subject of the present invention is a method for the purification of natural gas, which essentially comprises the following steps: 
     (1) Subjecting the natural gas to desulfurization by causing the sulfurous compounds (from 0.1% to 5% by vol.), especially H 2  S, contained in said gas, to be absorbed by a selective solvent for them, more particularly and preferably dimethyl-ether-dipolyglycol or propylene carbonate; 
     (2) REMOVING MAJOR FRACTION OF THE CO 2 , up to 20-30% of the CO 2  content of the desulfurized gas, by distillation at low temperatures (cryogenic stripping), the low temperatures which are required being obtained by evaporation at low pressure of the liquid CO 2  separated by such a distillation; 
     (3) REGENERATING THE SELECTIVE SOLVENT BY STRIPPING IT OF THE SULFUROUS COMPOUNDS; 
     (4) UTILIZING THE REGENERATED SELECTIVE SOLVENT TO ABSORB THE RESIDUAL CO 2  still contained in the natural gas emerging from the cryogenic stripping stage, and 
     (5) recycling the CO 2  -containing selective solvent from stage 4 above to the absorption stage, 1, of the sulfurous compounds. 
     The temperatures as adopted in the desulfurization stage lie within the range from 20° C to 100° C whereas the pressure is generally in the range between 100 and 200 kilograms/sq. cm. absolute. 
     Low-temperature distillation is carried out at a columnhead temperature below -35° C and under a pressure in the range between 60 and 80 kgs./sq.cm absolute. 
     The regeneration of the selective solvent is generally carried out in a usual stripping column at a temperature in the range between 100° and 140° C and under a pressure of from 1 to 2 kgs/sq.cm absolute. 
     The final absorption of CO 2  (stage 4) is carried out at a temperature in the range between 3° C and 20° C and under a pressure of from 60 to 80 kgs.sq.cm. absolute. 
     The selective solvent which is recycled to stage 1 has a content of CO 2  equal to 9 to 16% of the total CO 2  contained in the natural gas (in the gaseous stage) at the start of the treatment according to the invention. 
     The invention will now be described in more detail with reference to the diagram shown in the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a schematic representation of apparatus adapted for use in carrying out the process which we have invented. 
    
    
     The natural gas rich in acidic gases is available under a pressure of 150 kgs/sq.cm and at room temperature and is fed, through 1, to the desulfurization column 2. 
     The natural gas has the composition as reported in the introductory part of this specification. 
     The absorption of H 2  S is carried out by using, as the solvent, dimethyl ether polyglycol fed via 3. 
     The natural gas is desulfurized down to 5 parts per million (ppm) of residual H 2  S, whereas the CO 2  is absorbed only partially to a residual content of 69% by volume, approximately. 
     The solvent, and the substances dissolved thereby, is discharged through 4 and is subjected to a flashing operation under a pressure of about 75 to 80 kgs/sq.cm. A gaseous phase, 6, is thus separated in the collector 5, said phase being essentially rich in methane which, as compressed by the compressor 7, is recycled via the main 8 to the desulfurization column 2. 
     The liquid phase emerging from 5 is sent, upon preheating by the exhausted solvent at 9, through the duct 10 to the stripping column 11 wherein, at the head; there is obtained H 2  S at 12, which is then sent to burnout (conversion into SO 2  and H 2  O) whereas, at the tail, the solvent 13 is obtained, which is sent for the absorption of the residual CO 2  in the absorption column 15, after having supplied the heat which is necessary for the cryogenic separation of the CO 2  at 14 and after having been cooled at 16 down to about 5° C. 
     The desulfurized gas which emerges from the column 2 is sent to the separation column 18 (pressure : 70 kgs/sq.cm) after having been cooled at 19 by the cold effluent which emerges from the top and the bottom of the column 18 aforesaid. 
     Prior to entering the column 18 and after having cooled as aforesaid, the gas 17 is caused isoenthalpically to be expanded to a pressure of about 70 kgs/sq.cm, thus separating in the separator 21 a liquid phase which, through the line 22, is sent to the low portion of the column 18, and a gaseous phase, which, after a further cooling at 23 by evaporating CO 2  (that which forms also a liquid phase) is conveyed to the intermediate portion of the column 18. 
     The liquid reflux to the head of the column 18 is obtained by condensing a portion of the vapors with evaporating CO 2 , whereas liquid CO 2  is obtained as a bottom product and, by evaporation, supplies the refrigeration which is necessary for the refluxing and the condensation at 23 and at 19 as well. 
     The natural gas 20, which still contains CO 2  (20-30% by vol.) is sent to the absorption column 15 wherein the CO 2  is absorbed by the solvent 13 fed to the column 15 in counterflow relationship relative to the gas fed in the neighborhood of the column bottom. The purified gas is discharged through the duct 24 and is sent to the point of utilization through sealines or pipes. The CO 2  -containing solvent is discharged from the bottom of the column 15, and via 3, is recycled to the column 2. Solvent make-up can obviously be effected at any point of the installation.