CA1041004A

CA1041004A - Separation of carbon dioxide and other acid gas components from hydrocarbon feeds

Info

Publication number: CA1041004A
Application number: CA262,558A
Authority: CA
Inventors: James M. Eakman; Harry A. Marshall
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1975-10-03
Filing date: 1976-10-01
Publication date: 1978-10-24
Also published as: JPS5246002A; ZA765623B; GB1562208A; AU506246B2; NL7610896A; NO763357L; AU1827476A

Abstract

ABSTRACT OF THE DISCLOSURE
A process for the separation of carbon dioxide and other acid gas components in a single column by high pressure, low temperature distillation from a gaseous feed stream comprised of one or more hydrocarbons, including particularly methane, and hydrogen. The separation is effected in a single column usually having from about 20 to about 30 theoretical distillation stages, and the feed can be one constituted ab initio within the desired ranges of composition, or modified by addition of components just prior to or at the time of distillation.

Description

1 Gaseous hydrocarbons, particularly those produced

2 in industrial operations, are characterized generally as

3 admixtures of hydrocarbons in varying concentration,

4 incl~sive of nonhydrocarbon components. Many include acid gas components which must be removed. Carbon dioxide and - 6 other acid gas components such as H2S, COS and S02 often 7 occur in admixture with hydrocarbons, notably methane, as 8 in natural gas or synthetic natural gas, and must be 9 separated from the hydrocarbon gas prior to its commercial use, e.g., as a fuel. A process of o~tstanding importance, 11 in this regard, requires the separation of carbon dioxide 12 and other acid gas components from a mixture of methane and 13 synthesis gas (an admixture of carbon monoxide and hydrogen).
14 The separation of carbon dioxide from such mixtures is quite burdensome, particularly since it is often contained within 16 a gaseous mixture in concentrations ranging as high as 17 thirty mole percent, or greater. Removal of the carbon 18 dioxide by scrubbing with alkaline solutions, e.g., aqueous 19 amine solutions, is usually prohibitive when the concentra-tion of the carbon dioxide exceeds about two or three mole 21 percent.
22 The separation of components of different boiling 23 points by distillation usually provides advantages, but the 24 separation of carbon dioxide from liquefied hydrocarbon streams is quite burdensome because carbon dioxide crystal-26 lizes, solidifies or "ices up" over a wide range of tempera-27 ture and pressure conditions, which ranges often overlap or 28 correspond to those required for most effective separation.
29 The formation of a solid phase in a distillation column for obvious reasons is generally viewed as intolerable.

-1~4 ~0 ~

1 An acute disadvantage in prior art processes 2 employing only a single distillation column for the 3 separation ~f carbon dioxide from gaseous hydrocarbon 4 streams, notably methane streams, is that distillations conducted at economically feasible conditions leaves 6 significant amounts of the carbon dloxide present, and 7 consequently cannot be used when it becomes necessary to 8 remove greater amounts of the carbon dioxide. For example, 9 a cryogenic separation process utilizing a single distilla-10 tlon column has been described which suggests that the ~-11 removal of carbon dioxide from methane containing streams 12 to provide methane which contains about 10 mole percent C02 13 is possible by operation at 730~750 psia at temperatures 14 no lower than about -100F., but that further reduct;on of the CO2 content below the 10 mole percent level would 16 require operation close to the solid C02-vapor region, and 17 close to the critical pressure loci.
18 A process is also known for effecting the separa-19 tion of carbon dioxide from a predominantly methane stream requiring the use of two distillation columns, each 21 operated under different sets of conditions dependent on the ~ -22 concentration of carbon dioxide, (a) as ranging below 8 mole 23 percent or (b) as ranging above 8 mole percent. In each ~-24 instance the first and second distillation columns, respectively, of the two different types of operation are 26 maintained under the same operating conditionsg the respec-27 tive operations differing only in that the feed is intro-28 duced at different locations. Where the carbon dioxide is 29 present in the lower concentrations, the feed is directly introduced into the first column of the series, and where 0~
1 the carbon dioxide is present in the higher concentrations, 2 the feed i9 directly introduced into the second column of 3 the series.
4 In each type of operation characterizing this known process, the first columns are operated at or below 6 the critical temperature of methane such that ~eeds to a 7 respective column provide a carbon dioxide concentration 8 below that which, on cooling at the operating pressure of 9 the column, would produce a solid carbon dioxide phase.
Effluents from the top of the second columns contain 11 substantially the same concentration of carbon dioxide 12 as the feeds to said first columns. The operating pressure 13 applied to said second columns is maintained above a 14 critical pressure defined as that at which the carbon dioxide phase will exist, and above which pressure a solid 16 carbon dioxide phase will not coexist with a vaporO Where-17 as this process has provided certain advantages over previous 18 processes, it nonetheless possesses acute disadvantages.
19 A notable disadvantage is that two operating columns are required to effect the separation of carbon dioxide from 21 a predominantly methane stre~m. Moreover3 the operation 22 becomes particularly complex when it is required to treat 23 methane streams of varying carbon dioxide concentration 24 ranging above and below 8 mole percent carbon dioxide.
It is accordingly the primary objective of this 26 invention to obviate these and other prior art deficiencies, 27 particularly by providing a new and improved distillation 28 process for the separation in a single column of acid gas 29 components from hydrocarbon streams.
A particular object of this invention is to :

1 provide a process wherein carbon dioxide can be separated 2 from methane gas streams by distillation, particularly one 3 requiring use only of a single distillation column.
4 A specific object of this invention is to provide a process requiring only a single distillation column for 6 the more effective separation of carbon dioxide from gaseous 7 methane streams, notably gaseous streams wherein methane 8 is contained in admixture with carbon monoxide and hydrogen.
9 These objects and others are achieved in accor-dance with the present inventionJ characterized as a process 11 for the separation in a single distillation column of carbon 12 dioxide and other acid gas components by distillation from 13 a gaseous hydrocarbon or mixture of hydrocarbons, inclusive 14 of methane and hydrogen. The hydrogen is present in the feed stream ab initio, or added to the distillation zone 16 to provide a hydrogen concentration ranging from about lO
17 to about 40 mole percent, preferably from about 20 to 18 about 35 mole percent, within the feed stream introduced 19 into the distillation zoneO Preferably9 the predominant hydrocarbon within the gaseous feed stream is methane, More 21 preferablyg the feed streams contain rom about 30 to about 22 85 mole percent, and preferably from abcut 50 to about 80 23 mole percent methane. By distilling3 or fractionating a 24 feed stream comprising a hydrocarbon or hydrocarbon mixture of such character, in a single distillation column, at 26 sufficiently high pressure and low temperature in the 27 presence of sufficient hydrogen, solid carbon dioxide 28 formation is prevented such that greater than 90 mole 29 percent, suitably from about 95 to about 99 mole percent, and higher, removal of the carbon dioxide originally present 1 in the feed stream can be effectedO Suitably, pursuant to 2 the practice of this process, the residual carbon dioxide 3 ranges below about 10 mole percent, preferably from about 5 4 to about 1 mole percent, and less.
In its preferred aspects, the present process 6 makes it feasible to effect almost complete separation of 7 carbon dioxide and other acid gas components from a methane 8 containing feed gas such as natural gas, synthetic natural 9 gas, or synthesis gas by distillation, or fractionation, in a single column at total pressures no greater than about 1070 lL psia (pounds per square inch absolute), the critical pressure 12 of carbon dioxide. Preferably, pressures range above 710 13 psia to 1070 psia, and more preferably from about 1025 psia 14 to 1070 psia The feed gas, prior to or at the time ofin~o-duction into the distillation, or fractionation column, is 16 cooled below -45F., preferably to temperatures ranging from 17 about -45F. to about -70F~, and more preferably from about 18 -50F. to about -55F.
19 The distillation is carried out in a single column in conventional vapor-liquid contacting apparatus.
21 These and other features of the present process will be 22 illustrated, and consequently better understood, by refer-23 ence to the attached drawings, the following description, 24 illustrations and example which makes reference to the drawings.
26 In the drawings:
27 Figure 1 depicts distillation apparatus in schema-28 tic form, and an arrangement of the apparatus and associated 29 apparatus components adapted to carry out the present pro-cess.

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- 1 Figure 2 depicts a diagram representative of the 2 interrelationship between temperature and partial pressure ; 3 of carbon dioxide plus methane (C02 + CH4) wherein solids 4 phase formation can occur~ which region is avoided in operation of the column.
6 Figure 3 depicts a diagram representative of 7 upper stage temperature-composition profiles of a multi-8 component mixture containing methane, carbon dioxide, and 9 hydrogen as exists in the upper stages of a distillation column.
11 Referring to Figure 1, there is shown a fractionat-12 ing column 10 of the vapor-liquid contact type constituted 13 generally of an outer metal shell within which is provided 14 a plurality of vertically separated bubble cap trays (not shown). A gaseous feed, after precooling by passage 16 through a heat exchanger 12, is introduced via line 11 17 into about the middle of Column lOo Overhead vapors 18 consisting primarily of methane and synthesis gas or 19 hydrogen, since the primary function of the upper stages of the column is to reduce the quantity of carbon dioxide i 21 and other acid components leaving the top of the column, 22 are removed via line 13. The vapors are passed through a 23 condenser 14, which can be internal or external, but is 24 illustrated for convenience as an external condenser. The ; 25 uncondensed gas, principally methane and synthesis gas, is - 26 withdrawn from the top of accumulator 15 via line 16 and 27 stored, and liquid is withdrawn from the bottom of 28 accumulator 15 and reintroduced via line 17 into the top 29 of the column 10 as refluxO The required liquid: distillate reflux ratio employed is related to the number of trays 1 employed in the column, the relative amounts of carbon 2 dioxide, methane and hydrogen present in the feed, and to 3 the carbon dioxide level desired in the distillate. It is 4 set to achieve the required separation while avoiding solids formation. Suitably, the molar ratio of liquid:distillate 6 used as reflux ranges about 1.25:1 and greater, preferably 7 about 1.3:1 and greater. Liquid bottoms, which consist 8 predominantly of carbon dioxide and other acid gas 9 components~ since the function of the lower part of the distillation column 10 is to reduce the quantity of methane 11 and syn gas components in the acid gas stream leaving the 12 bottom of the column, are removed via line 18 after passage 13 of a portion thereof through a reboiler type heat exchanger 14 19. The proper heat exchange relationships are provided by a conventional refrigeration system (not shown), refrigerant 16 being circulated via line 20 through heat exchanger 14. Heat 17 exchange with a portion of the bottoms product is provided 18 by passage of a portion of the bottom product via lines 8, 19 9 through heat exchanger 19, shown in heat exchange-relation-ship with a material contained in line 21~ The remaining 21 portion of the bottoms product is sent to storage or further 22 processing via line 18.
23 In its preferred aspects~ the fractionation is 24 conducted at the highest total pressure below mixture critical which will allow adequate phase separation for 26 the high purity carbon dioxide in the lower part of the 27 column and the reboiler. The range of satisfactory 28 operating conditions will vary to some extent dependent 29 upon the specific composition of the feed gas of interest.
For the separation of carbon dioxide from admixtures of ~ 5~ ~
': :
1 methane (CH4) and synthesis gas (H2 + C0) at molar ratios 2 of CH4:(H2+C0) of about 1:1 to about 5:1 as conducted in a 3 preferred embodiment of this invention, the upper stages of 4 the column are maintained at a pressure greater than about S 1025 up to but not exceeding 1070 psia, the critical 6 pressure of carbon dioxide. At such pressure, even with 7 reflux temperatures well below -100F., the formation of 8 solid carbon dioxide will not occur. Both gas and liquid 9 phases will be present in the column at these pressures, which are well above 673 psia, the critical pressure of pure 11 methane. A feature of this invention is that the carbon 12 dioxide can be reduced to very low levels within a hydrogen 13 containing hydrocarbon product by selection of the tempera-14 ture and rate of reflux liquid, as desiredO
Referring to Figure 2, there is graphically 16 described an essential relationship between temperature, 17 in F., and the partial pressure of carbon dioxide and 18 methane (C02 + CH4)~ expressed in pounds per square 19 inch absolute, if solid formation is to be avoided in such ; 20 systems. It will be observed that, in order to avoid the 21 formation of solid, operation of the column at temperatures 22 ranging from about -170F~ to about -84Fo~ as shown on 23 the x-axis, requires higher and higher partial pressures of 24 carbon dioxide and methane, flS shown on the y-axis, ranging from about 200 psia to about 710 psia at the higher 26 temperature. Thereafter, up to about -70Fo ~ the partial 27 pressure that is required declines. The relationship 28 expressed in the graph which is required to avoid the solid 29 formation re~ion is tabulated for convenience as follows:

_ 9 _ ., . ~ '.

l(~A1~4 1 Partial Pressure of 2 Temperature, F. (C02 + CH/~ psia 4 -150 ~280 -130 ~420 6 -110 >550 7 -90 ~700 8 -84 ~710 9 -70 ~75 In sharp contrast to prior art single column 11 distillation processes for effecting such separations, 12 which remove only about 90 mole percent of the carbon 13 dioxide, it has been found feasible to remove carbon 14 dioxide to a level of 1 mole percent, or less, in the admixture of carbon dioxide and methane, or methane in 16 admixture with other hydrocarbons and hydrogen, e.g., 17 methane and synthesis gas, particularly in a sin~le column 18 utilizing generally 20 to 30 theoretical distillation 19 stages. This is conveniently illustrated by reference to Figure 3. This figure presents a diagram representative 21 of upper stage temperature-composition profiles of a 22 multicomponent composition containing methane, carbon 23 dioxide, and hydrogen wherein 1025 psia total pressure is 24 maintained on the column, and the column is operated by introducing the feed at a temperature of -50.8F., while 26 employing a liquid:distillate molar ratio of 1.35 in the 27 overhead. The data graphically illustrated in Figure 3 28 are taken from a computer simulated run conducted as 29 follows:
Hytrogen 24.1 Moles 31 Nitrogen 0.5 "
32 Carbon Monoxide 6.6 "
33 Methane 39.4 "
34 Carbon Dioxide 27.8 "
Ethane 0.4 "
36 Hydrogen Sulfide 1.1 "

. .

.~ , . . .' .

1 The overhead vapor and bottoms liquid streams are 71.1 2 moles and 28.9 moles, respectivelyO The mole fractions of 3 the components in ~he two streams are:
4 Vapor Liquid 5Overhead Bottoms o Hydrogen 0.339 0.000 7 Nitrogen 0~007 0.000 8 Carbon Monoxide 0~093 0.000 9 Methane 0.550 0.010 Carbon Dioxide 0~010 0.937 11 Ethane 0.000 0.013 12 Hydrogen Sulfide 0.000 0.040 13In Figure 3, the temperature in Fahrenheit degrees 14 is read on the y-axis, and the mole fraction of carbon dioxide in the binary fraction is read on the x-axis. The 16 liquidus curve is representative of that region below and 17 to the right of which curve a solid phase is formed, and 18 above and to the left of which no solid phase is formed.
19 The left-most curve on the scale is representative of the vapor mole fraction, the intermediate curve is represen-21 tative of the liquid mole fraction, and horizontal lines 22 drawn therebetween are representative of theoretical stages 23 of temperature below -70F., these ranging in number from 24 21 through 24+. These data show that it is possible to remove to a level of about 1 mole percent carbon dioxide 26 present in the vapor phase mixture by use of less than 25 27 theoretical trays. It is particularly significant that the 28 mole fraction of carbon dioxide in the liquid phase, at 29 any given set of conditions, does not exceed the mole fraction of carbon dioxide given by the liquidus curve at 31 corresponding conditions.
32 It is apparent that various modifications can be 33 made in the process without departing the spirit and scope 34 of the present invention.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a process for the separation of carbon dioxide from a gaseous carbon dioxide containing hydro-carbon feed, inclusive of methane, the improvement com-prising forming a feed stream from said hydrocarbon feed, inclusive of about 10 to about 40 mole percent hydrogen, introducing said feed stream at temperatures ranging below -45°F. into a single distillation column, maintaining within the upper portion of said column total pressures ranging above about 710 psia to about 1070 psia, partial pressures of carbon dioxide and methane sufficient to avoid solids formation, while condensing overhead vapor to produce a liquid, recycling the liquid as reflux, and recovering a product containing less than 10 mole percent carbon dioxide from the upper portion of said distillation zone.

2. The process of Claim 1 wherein the feed stream contains from about 10 to about 40 mole percent hydrogen, and from about 30 to about 85 mole percent methane.

3. The process of Claim 2 wherein the feed stream contains from about 20 to about 35 mole percent hydrogen, and from about 50 to about 80 mole percent methane.

4. The process of Claim 1 wherein the upper portion of the distillation column is operated at total pressures ranging from about 1025 psia to 1070 psia, and temperatures range from about -170°F. to about -70°F.

5. The process of Claim 4 wherein the tempera-ture of the feed introduced into the upper portion of the distillation column ranges from about -45°F. to about -70°F.

6. The process of Claim 4 wherein the number of theoretical stages of the distillation zone ranges from about 20 to about 30, and the level of the carbon dioxide in the product ranges from about 5 to about 1 mole percent, and lower.

7. The process of Claim 6 wherein the molar ratio of liquid:distillate used as reflux ranges about 1.25:1, and greater.

8. The process of Claim 7 wherein the molar ratio of liquid:distillate used as reflux ranges about 1.3:1, and greater.

9. The process of Claim 1 wherein hydrogen is added to the hydrocarbon feed, originally deficient in hydrogen or synthesis gas, to provide a feed stream con-taining the required hydrogen.

10. The process of Claim 1 wherein the feed stream comprises a synthesis gas which contains CH4, H2 and CO, the molar ratio of CH4:(H2+CO) ranging from about 1:1 to about 5:1.

11. In a process for the separation of carbon dioxide from a gaseous carbon dioxide containing methane feed, the improvement comprising forming a feed stream from said methane feed, inclusive of about 10 to about 40 mole percent hydrogen, introducing said feed stream at temperatures ranging below about -45°F. into a single distillation column, maintaining within the upper portion of said column total pressures ranging above about 710 psia to about 1070 psia, and temperature-pressure relationships sufficient to avoid solid formation in accordance with the following:

condensing the overhead vapor to produce a liquid, recycling the liquid as reflux, and recovering a product from which about 95 to about 99 mole percent, and higher, of the carbon dioxide has been removed.

12. The process of Claim 11 wherein the feed contains from about 20 to about 35 mole percent hydrogen, and from about 50 to about 80 mole percent methane.

13. The process of Claim 11 wherein the molar ratio of liquid:distillate used as reflux ranges about 1.25:1, and greater.

14. The process of Claim 13 wherein the molar ratio of liquid:distillate used as reflux ranges about 1.3:1, and greater.

15. The process of Claim 11 wherein the feed stream comprises a synthesis gas which contains CH4, H2 and CO, and the molar ratio of CH4:(H2+CO) ranges from about 1:1 to about 5:1.