US2794334A - Propane recovery - Google Patents

Description

` June 4, 1957 R. v. PEASLE ETL PROPANE RECOVERY Filed Oct. 9, 1953 2 Sheets-Sheet l E v. mg. M 2./ MPA I Nu T I .Y A m MM w 00 QN Ru Q/ Y Du m95. m Ks mor V n n l; u

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, June 4, 1957 R. v. PEASLEE ETAL 2,794,334

PROPANE RECOVERY Filed oct. 9, 1953' 2 sheets-sheet 2 SETTL/NG T/ME1 MINUTES E. SOLUB/L/TY GAL. N BUTANE /N /00 GALS. 0E REFR/GERANT l l l v 0 0 20 40 /00 ME THANOL, VOL. PERCENT IN WATER INVENTORS. ROBERT l/. PEASLEE JOHN F. HAINES United States PROPANE RECOVERY Robert V. Peaslee, Arcadia, and lohn F. Haines, Pasadena, Calif., assignors to C. F. Braun & Co., Alhambra, Calif., a corporation of California Application October 9, 1953, Serial No. 335,07@ Claims. (Cl. 62-175.5)

This invention is directed to the separation of a rich hydrocarbon gas into a liquid fraction and a gaseous fraction by a process of countercurrent contact of the gas with a liquid refrigerant. The liquid fraction generally consists of propane and higher boiling hydrocarbons and is conventionally referred to as LPG (liquetied petroleum gas). The invention is also directed to a refrigerant liquid for such process.

Propane and heavier hydrocarbons are now recovered from natural gas either by absorption or refrigeration, the latter process involving chilling the gas to a temperature at which a major portion of propane and heavier hydrocarbons are liquefied while ethane and lighter hydrocarbons remain in a gaseous state. lf the natural gas contains water, as is frequently the case, it becomes necessary to dry the gas before refrigeration to avoid the formation of ice and hydrocarbon hydrates at the low temperatures employed. Such preliminary drying represents not only an appreciable cost in capital equipment and processing, but is only partially satisfactory in that a temperature no lower than about 35 F. is economically practical in the subsequent refrigeration operation.

It has been proposed to accomplish rich gas refrigeration for LPG recovery by countercurrently contacting the gas with a chilled brine (U. S. Patent 2,198,142, issued to Henry N. Wade, April 23, 1940). This proposedprocess is alleged to eliminate need for preliminary drying because any water in the gas is absorbed in the brine refrigerant before ice or hydrate formation takes place. However, use of brine as a direct contag refrigerant in this manner is unsatisfactory in several respects. Brine is inherently limited to use at tem eratures above about 25 F. since below this temperature its viscosity is so great as to prevent the operation of the process. As a consequence, LPG recovery with brine is relatively inefficient. It is not possible with brine to achieve a degree of refrigeration as low as that obtainable with the more conventional refrigeration process. In addition, brine is highly corrosive and in such a direct contact process is diicult to recover because of dilution. Concentration of diluted brine by heat is impractical because of comparatively high heat load requirements. There is no known commercial use of this proposed process.

This invention is directed to a liquid refrigerant for recovery of LPG from natural gas by countercnrrent direct contact, and which refrigerant avoids and overcomes all of the objectionable features of the heretofore proposed brine refrigerant. The invention contemplates a refrigerant and water in which the alcohol-is present in an amount of not less than 60% by volume and not more than 90% by volume. The invention also contemplates the method of recovering LPG from hydrocarbon gas, which cornprises countercurrently contacting a hydrocarbon gas with a refrigerated soution of the above described composition, removing a gaseous fraction from the liquid fraction separating liquefied hydrocarbons from the solution, rechilling the separated solution and recontacting the rechilled solution with additional feed gas.

comprising a solution of methyl alcohol n 2,794,304 Patented June 4, 1957 The composition of the refrigerant solution is critical. Solubility of hydrocarbons in the solution, settling time, i. e. the rate of separation of solution from liquefied hydrocarbons, and solution viscosity are all factors of paramount importance. Solubility of hydrocarbons in the refrigerant increases markedly at alcohol concentrations in excess of about or 90%. Solution viscosity at the low temperatures employed as well as settling time increase markedly at alcohol concentrations below about 60% to 70%. Settling time at the lower range of alcohol concentrations can be improved by addition of 4a commercially available treating or dernulsifying agent, such as the mono or di-ammonium salts of petroeum sulfonic acids, olyl hydroxy stearic acid, oleic hydrogen sulfate, and benzene sulfo stearic acid. Settling agents of this type may be employed in amounts of from about 5 to about 50 parts per million parts of refrigerant. However, such an agent will not improve an unduly high viscosity which is encountered at lower alcohol concentrations. For these reasonsl the process is operative with an alcohol-Water solution in which the alcohol is present in not less than 60% and not more than 90% by volume. Preferably, the solution has an alcohol content of between 70% and 85% since in this concentration range the various factors mentioned above are the optimum values.

The invention will be clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

Fig. l is a diagram of a portion of a recovery plant for carrying out the process of the invention; and

Fig. 2 is a graph showing the relationship of solubility and settling time as a function of refrigerant composition.

The recovery plant shown diagrammatically in Fig. 1 comprises a contactor 10, which may be a conventional bubble tower. Feed gas is introduced to the contactor through a feed gas inlet line 11 opening into the lower portion of the tower and dry gas is removed through an overhead line 12. Settled refrigerant is withdrawn from the tower through a bottoms line i3 and is introduced to a surge 'tank 14. A return line l5 is connected from the tank 14 to the contactor at a point intermediate the bottom of the contactor and the feed gas inlet line. Liquid hydrocarbon is withdrawn through a bleed line 16 from the lower portion of the tower and is pumped into a secondary settling tank 17 by means of a pump 18 located in the line 16.

The refrigerant surge tank 14 is connected by a line 20 through a pump 21 to a heat exchange system including rst, second, third and fourth stage exchangers 22, 23, 245, 25 and 4from the heat exchange system through an inlet line 26 to the upper portion of the contactor.

Dry gas taken overhead from the contactor is fed successively through the exchangers 24 and 22.

A secondary refrigerant such as liqueed ethane is passed successively through exchangers 25 and 23 by means of a line 30.

Liqueed hydrocarbons are removed from the secondary settling tank 17 through an overhead line 34 and,

settling tank 17 through a line 35 and is introduced toA an alcohol recovery tower 36 where alcohol concentray tion is accomplished by conventional fractionation.

Alcohol and Water are taken overhead through line 38 and returned to the refrigerant surge tank 14 (together with any required refrigerant make up) through a makel up line 39. Water is removed from the lower portionof the recovery tower 36 through a line 40.

A typical operation as follows: Feed gas of the illustrated recovery plant is is introduced at the lower portion" of contactor through the feed gas inlet line, the gas Y being, for example, at a temperature of approximately 90 F. and at a pressure of approximately 400 p. s. i. g. Refrigerant is introduced to the top of the contactor The critical nature of refrigerant composition is' made apparent from the graph of Fig. 2 in which hydrocarbon solubility and settling time are plotted as functions of the volume percent methanol in the water-alcohol refrigthrough the line 26, the required temperature of the re- 5 erant. Dotted line curves A and B represent plots of frigerant being a function of the desired recovery eiisolubility against alcohol concentration at temperatures ciency. In this particular exampleV the refrigerant enterof 5 F. and 60 F. respectively. Solid line curves ing the contactor is at a temperature of approximately C, D, E, F and G are plots of settling time as a function 70 F; The downowing refrigerant solution mixes of alcohol concentration at temperatures ranging from countercurrently with the upowing gas, liquefying the IO 60 F. for curve C to 5 for curve G. Tt becomes liqueable portions thereof and accomplishing in effect immediately' apparent from Fig. 2 that pure methyl alcoa low temperature fractionation, at the same time abhol is totally unsatisfactory as a refrigerant, and it is sorbing water from the gas, equally apparent that an alcohol-water solution contain- With the described refrigerant no objectionable hying less than about 60% by volume of alcohol is equally drates or ice are formed even at these very low temperaunsatisfactory as a refrigerant. As previously mentioned, tures. In the lower portion of the contactor the liquid even if settling time as represented by curves C through components separate into a hydrocarbon phase and an G is reduced by means of a settling agent, the viscosity alcohol-water phase as indicated. The alcohol-water of a refrigerant containing more than about 40% by phase is withdrawn from the bottom of the tower through Volume of water at the temperatures contemplated sl the bottoms line into the surge tank 14. Any additional such as to render the refrigerant totally unacceptable. hydrocarbons separating from the alcohol-water phase We claim: in the surge tank are returned to the settled hydrocarbon l. The process of separating a hydrocarbon gas mixphase in the contactor through the line 15. Under the ture into a liquid fraction and a gaseous fraction which conditions mentioned, the refrigerant withdrawn from comprises introducing the gas mixture intoa region of the lower portion of the contactor is at a temperature of a separating column spaced from the ends of the column, approximately 50 F. removing gaseous hydrocarbons from the top of the Refrigerant is withdrawn from the surge tank by the column, removing liquefied hydrocarbons from the botpump 21 and is passed successively through the heat tom of the column, introducing a refrigerant into au exchangers 22, 23, 24, and 25 to lower its temperature upper part of the column and ata temperature below to approximately 70 F. as indicated. A refrigerant 30 that of the liquid and gaseous hydrocarbons leaving the medium such as ethane is introduced to the exchanger column, the refrigerant comprising a solution of methyl 25 through the line 30 at a temperature of approximately alcohol and water in which the alcohol is present in 80 F. and from the exchanger 25 flows through exfrom about 60% to about 90% by volume, the refrigerant changer 23 at a temperature of approximately 30 F. eing added in sufficient volume to liquefy said liquid The dry gas removed from the top of the contactor is at fraction as the refrigerant flows downwardly in the cola temperature of approximately 60 F. and passes umn countercurrently to the unliqueed hydrocarbons through the exchanger 24 at approximately this temowing upwardly in the column. perature. The dry gas flows from the exchanger 24 2. The process of separatingahydrocarbon gas mixture through the exchanger 22 at a temperature of approxiinto a liquid fraction and a gaseous fraction which commately 30 F. wherein the recycle refrigerant under- 40 prises introducing the gas mixture-into a region of a goes its rst stage refrigeration, the temperature of the separating column spaced from the ends of the column, refrigerant being reduced from about F. to about removing gaseous hydrocarbons from the top of the 24 F. in the first stage exchanger 22. In the second column, removing liquefied hydrocarbons from the botstage exchanger 23 the refrigerant temperature is dropped tom of the column, introducing a refrigerant into an'upper to about 20 F. by heat exchange with the secondary 45 part of the column and at a temperature below about refrigerant. In the third stage exchanger 24 the refrigminus 5 F. and below the temperature of the liquid and erant temperature is further reduced to about 33 F. gaseous hydrocarbons leaving the column, the refrigerant by heat exchange with the dry gas, and in the fourth comprising a solution of methyl alcohol and water in stage exchanger 25 it is reduced to approximately 70 which the alcohol is present in from about 60% to about F. by heat exchange with the secondary refrigerant. 50 90% by volume, the refrigerant being added in sufficient The settled hydrocarbon phase in the contactor is volume to liquefy said liquid fraction as the refrigerant withdrawn from it through the hydrocarbon bleed line by ows downwardly in the column countercurrently to the means of the pump 18 and introduced into the secondary unliquefied hydrocarbons flowing upwardly in the column. settling tank 17. Any further refrigerant settling from 3. The process of separating arhydrocarbon, gas mixture the hydrocarbon phase in the tank 17 is Withdrawn there- 55 into a liquid fraction and a gaseous fraction which comfrom through the line 35 and introduced into the alcohol prises introducing the gas mixture into a region of a seprecovery tower 36, the function of which has previously arating column spaced from the ends of the column, rebeen described. The hydrocarbon phase from the secmoving gaseous hydrocarbons from the top of the column, ondary settling tank 17 is withdrawn through line' 34. removing liquefied hydrocarbons from the bottom of the A typical hydrocarbon material balance-at the processcolumn, introducing a refrigerant into an upper part of ing conditions of the above described example is set the column and at a temperature below that of the liquid forth in the following table: and gaseous hydrocarbons leaving the coltunn, the refrig- Feed Cont Deethn. Deethn. Deethn. Deethn. Dry gas Fuel Gas Ovrhd Ovrhd. Product Feed Reflux to Comp.

c1 73.14 2.40 02m 10. 53 0.21 1.97 C3..- 9. 64 s. 14 s. 43 104.- 1. 96 1. 9s 1. 9e No4. 2. 2. 70 2. 70 105.. 0. 0. 75 0. 75 Not 0. 5s 0. 53 o. 53 o6 0.65 0. 65 0. 65 Mols/ Mol Feed 100.00 80. 4.45 14. 94 19.39 MMsCF./24 Hr 130 104.7 5.8 G. P. M. at 60 1L 410 490 erant comprising a solution of methyl alcohol and water in which the alcohol is present in from about 60% to about 90% by volume, the refrigerant being added in suicient volume to liquefy said liquid fraction as the refrigerant ows downwardly in the column countercurrently to the unliqueed hydrocarbons flowing upwardly in the column, separating refrigerant from the liquefied hydrocarbons, rechilling the separated refrigerant, and returning the rechilled refrigerant to the top of the column.

4. The process of recovering LPG and higher boiling constituents from a wet hydrocarbon gas mixture which comprises introducing the gas mixture into a region of a separating column spaced from the ends of the column, introducing a refrigerant into an upper part of the column at a temperature between about minus F. and minus 70 F. and below the temperature of the liquid and gaseous hydrocarbons leaving the column, the refrigerant comprising a solution of methyl alcohol and water in which the alcohol is present in from about 60% to about 90% by Volume, the refrigerant being added in suicient volume to liquefy the LPG and higher boiling hydrocarbons as the refrigerant flows downwardly in the column countercurrently to the unliqueed hydrocarbons flowing upwardly in the column, removing dry gas from an upper part of the column, separating the liquefied hydrocarbons from refrigerant in a quiescent zone at the bottom of the column, removing refrigerant, heat exchanging the removed refrigerant with dry gas and thereafter with a 6 second refrigerant to bring it to the temperature at which it is introduced to the column, reintroducing the refrigerant to the column, and removing liquefied hydrocarbons from the quiescent zone.

5. The process of separating a hydrocarbon gas mixture into a liquid fraction and a gaseous fraction which comprises introducing the gas mixture into a region of a separating column spaced from the ends of the column, removing gaseous hydrocarbons from the top of the column and liquefied hydrocarbons from the bottom of the column, introducing a refrigerant into an upper part of the column and at a temperature between about minus 5F. and minus 70 F. and below that of the liquid and gaseous hydrocarbons leaving the column, the refrigerant comprising a solution of methyl alcohol and water in which the alcohol is present in from about to about by volume, the refrigerant being added in sucient volume to liquefy said liquid fraction as the refrigerant ows downwardly in the column countercurrently to the unliquefred hydrocarbons owing upwardly in the column.

References Cited in the le of this patent UNITED STATES PATENTS 1,724,513 Pollitzer Aug. 13, 1929 2,198,142 Wade Apr. 23, 1940 2,214,678 Raigorodsky Sept. 10, 1940 2,295,809 Schuftan Sept. 15, 1942 2,522,640 Ruhemann Sept. 19, 1950

Claims (1)

1. THE PROCESS OF SEPARATING A HYDROCARBON GAS MIXTURE INTO A LIQUID FRACTION AND A GASEOUS FRACTION WHICH COMPRISES INTRODUCING THE GAS MIXTURE INTO A REGION OF A SEPARATING COLUMN SPACED FROM THE ENDS OF THE COLUMN, REMOVING GASEOUS HYDROCARBONS FROM THE TOP OF THE COLUMN, REMOVING LIQUEFIED HYDROCARBONS FROM THE BOTTOM OF THE COLUMN, INTRODUCING A REFRIGERANT INTO AN UPPER PART OF THE COLUMN AND AT A TEMPERATURE BELOW THAT OF THE LIQUID AND GASEOUS HYDROCARBONS LEAVING THE COLUMN, REFRIGERANT COPMPRISING A SOLUTION OF METHYL ALCOHOL AND WATER IN WHICH THE ALCOHOL IS PRESENT IN FROM ABOUT 60% TO ABOUT 90% BY VOLUME, THE REFRIGERANT BEING ADDED IN SUFFICIENT VOLUME TO LIQUEFY SAID LIQUID

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