GB1562208A - Separation of carbon dioxide from methane-containing hydrocarbonfeeds - Google Patents

Description

(54) SEPARATION OF CARBON DIOXIDE FROM METHANE-CONTAINING HYDROCARBON FEEDS (71) We, EXXON RESEARCH AND ENGINEERING COMPANY, a Corporation duly organised and existing under the laws of the State of Delaware.

United States of America, of Linden, New Jersey, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a process for the separation of carbon dioxide from a hydrocarbon feed containing methane, carbon dioxide and, optionally, hydrogen.

Gaseous hydrocarbons, particularly those produced in industrial operations, are characterized generally as admixtures of hydrocarbons in varying concentration, inclusive of nonhydrocarbon components.

Many include acid gas components which must be removed. Carbon dioxide and other acid gas components such as H2S, COS and SO2 often occur in admixture with hydrocarbons, notably methane, as in natural gas or synthetic natural gas, and must be separated from the hydrocarbon gas prior to its commerical use, e.g., as a fuel. A process of outstanding importance, in this regard, requires the separation of carbon dioxide and other acid gas components from a mixture of methane and synthesis gas (an admixture of carbon monoxide and hydrogen). The separation of carbon dioxide from such mixtures is quite burdensome, particularly since it is often contained within a gaseous mixture in concentrations ranging as high as thirty mole percent, or greater.Removal of the carbon dioxide by scrubbing with alkaline solutions, e.g., aqueous amine solutions, is usually prohibitive when the concentration of the carbon dioxide exceeds about two or three mole percent.

The separation of components of different boiling points by distillation usually provides advantages, but the separation of carbon dioxide from liquefied hydrocarbon streams is quite burdensome because carbon dioxide crystallizes.

solidifies or "ices up" over a wide range of temperature and pressure conditions, which ranges often overlap or correspond to those required for most effective separation. The formation of a solid phase in a distillation column for obvious reasons is generally viewed as intolerable.

An acute disadvantage in prior art processes employing only a single distillation column for the separation of carbon dioxide from gaseous hydrocarbon streams, notably methane streams, is that distillations conducted at economically feasible conditions leave significant amounts of the carbon dioxide present, and consequently cannot be used when it becomes necessary to remove greater amounts of the carbon dioxide. For example, a cryogenic separation process utilizing a single distillation column has been described which suggests that the removal of carbon dioxide from methane containing streams to provide methane which contains about 10 mole percent CO2 is possible by operation at 730750 psia at temperatures no lower than about -100"F..

but that further reduction of the CO2 content below the 10 mole percent level would not be practicable.

A process is also known for effecting the separation of carbon dioxide from a predominantly methane stream requiring the use of two distillation columns, each operated under different sets of conditions dependent on the concentration of carbon dioxide, (a) as ranging below 8 mole percent or (b) as ranging above 8 mole percent. In each instance the first and second distillation columns, respectively, of the two different types of operation are maintained under the same operating conditions, the respective operations differing only in that the feed is introduced at different locations.

Where the carbon dioxide is present in the lower concentrations, the feed is directly introduced into the first column of the series, and where the carbon dioxide is present in the higher concentrations, the feed is directly introduced into the second column of the series.

In each type of operation characterizing this known process, the first columns are operated at or below the critical pressure of methane and such that the feed to a respective column contains a carbon dioxide concentration below that which, on cooling at the operating pressure of the column. would produce a solid carbon dioxide phase. Effluents from the top of the second columns contain substantially the same concentration of carbon dioxide as the feeds to said first columns. The operating pressure applied to said second columns is maintained above a critical pressure defined as that at which a solid carbon dioxide phase will exist. and above which pressure a solid carbon dioxide phase will not coexist with a vapor. Whereas this process has provided certain advantages over previous processes, it nonetheless possesses acute disadvantages.A notable disadvantage is that two operating columns are required to effect the separation of carbon dioxide from a predominantly methane stream.

Moreover, the operation becomes particularly complex when it is required to treat methane streams of varying carbon dioxide concentration ranging above and below 8 mole percent carbon dioxide.

It is accordingly the primary objective of this invention to obviate, or mitigate, these and other prior art deficiencies, particularly by providing a new and improved distillation process for the separation in a single column of acid gas components from hydrocarbon streams.

According to the present invention, there is provided a process for the separation of carbon dioxide from a gaseous carbon dioxide-containing, methane-containing and, optionally, hydrogen-containing, hydrocarbon feed, which process comprises: forming a feed stream from said hydrocarbon feed such that the feed stream contains from 10 to 40 mole percent hydrogen and from 30 to 85 mole percent methane; introducing said feed stream at a temperature below -45"F. as a wholly gaseous feed into a single distillation column; maintaining within said column above the point of introduction of the gaseous feed, (i) a total pressure ranging from 710 psia to 1070 psia and (ii) a total partial pressure of carbon dioxide plus methane sufficient to avoid solids formation: condensing overhead vapor to produce a liquid; recycling at least a portion of the liquid as reflux: and recovering a gaseous product of reduced carbon dioxide content from the upper portion of said distillation zone.

The hydrogen may be present in the hydrocarbon feed ab initio, or it may be added in such amount that the feed stream contains the said 10 to 40 mole percent.

By distilling, or fractionating a feed stream comprising a hydrocarbon or hydrocarbon mixture of such character, in a single distillation column, at sufficiently high pressure and low temperature in the presence of sufficient hydrogen, solid carbon dioxide formation is prevented such that greater than 90 mole percent, suitably from 95 to about 99 mole percent, and higher, removal of the carbon dioxide originally present in the feed stream can be effected. In the practice of the process of the present invention CO2 reduction is possible such that residual carbon dioxide ranges below about 10 mole percent, even from about 5 to about 1 mole percent, and less.

In its preferred aspects, the present process makes it feasible to effect almost complete separation of carbon dioxide and other acid gas components from a methane containing feed gas such as natural gas, synthetic natural gas, or synthesis gas by distillation, or fractionation, in a single column. Preferably, pressures range from 1025 psia to 1070 psia. The feed gas, prior to or at the time of introduction into the distillation, or fractionation column, is cooled to below -45"F, and preferably to temperatures ranging from below -45"F. to -70"F., and more preferably from 500 F.

to F.

The distillation is carried out in a single column in conventional vapor-liquid contacting apparatus. These and other features of the present process will be illustrated, and consequently better understood, by reference to the attached drawings, the following description, illustrations and example which makes reference to the drawings.

In the drawings: Figure 1 depicts distillation apparatus in schematic form, and an arrangement of the apparatus and associated apparatus components adapted to carry out the present process.

Figure 2 depicts a diagram representative of the interrelationship between temperature and partial pressure of carbon dioxide plus methane (CO2+CH4) wherein solids phase formation can occur, which region is avoided in operation of the column.

Figure 3 depicts a diagram representative of upper stage temperature-composition profiles of a multi-component mixture containing methane, carbon dioxide, and hydrogen as exists in the upper stages of a distillation column.

Referring to Figure 1. there is shown a fractionating column 10 of the vapor-liquid contact type constituted generally of an outer metal shell within which is provided a plurality of vertically separated bubble cap trays (not shown). A gaseous feed, which in this example contains methane and hydrogen (in amounts in aforesaid mole percent ranges) together with carbon dioxide and carbon monoxide, after precooling by passage through a heat exchanger 12, is introduced via line 11 into about the middle of Column 10. Above this point of introduction the column 10 is operated at the aforesaid total and partial pressure conditions.Overhead vapors consisting primarily of methane and synthesis gas (or hydrogen alone if the feed is one not containing carbon monoxide) since the primarv function of the upper trays of the column is to reduce the quantity of carbon dioxide and other acid components leaving the top of the column, are removed via line 13. The vapors are passed through a condenser 14, which can be internal or external, but is illustrated for convenience as an external condenser. The uncondensed gas, principally methane and synthesis gas, is withdrawn from the top of accumulator 15 via line 16 and stored, and liquid is withdrawn from the bottom of accumulator 15 and reintroduced via line 17 into the top of the column 10 as reflux.The required liquid (condensed methane): distillate (gaseous methane) reflux ratio employed is related to the number of trays employed in the column, the relative amounts of carbon dioxide, methane and hydrogen present in the feed, and to the carbon dioxide level desired in the distillate.

It is set to achieve the required separation while avoiding solids formation. Suitably, the molar ratio of liquid: distillate used as reflux is at least 1.25:1, preferably at least 1.3:1. Liquid bottoms, which consist predominantly of carbon dioxide and other acid gas components, since the function of the lower part of the distillation column 10 is to reduce the quantity of methane and hydrogen (and carbon monoxide, if originally present) components in the acid gas stream leaving the bottom of the column, are removed via line 18 after passage of a portion thereof through a reboiler type heat exchanger 19. The proper heat exchange relationships are provided by a conventional refrigeration system (not shown), refrigerant being circulated via line 20 through heat exchanger 14.Heat exchange with a portion of the bottoms product is provided by passage of a portion of the bottom product via lines 8, 9 through heat exchanger 19, shown in heat exchangerelationship with a material contained in line 21. The remaining portion of the bottoms product is sent to storage or further processing via line 18.

In its preferred aspects, the fractionation is conducted at the highest total pressure possible with the stated range 710 to 1070 psia which will allow adequate separation between methane and carbon dioxide. The range of satisfactory operating conditions will vary to some extent dependent upon the specific composition of the feed gas of interest. For the separation of carbon dioxide from admixtures of methane (CH4) and synthesis gas (H2+CO) at molar ratios of CH4:(H2+CO) of 1:1 to 5:1 as conducted in a preferred embodiment of this invention, the upper portion of the column is maintained at a pressure greater than about 1025 up to but not exceeding 1070 psia, the critical pressure of carbon dioxide. At such pressure, even with reflux temperatures well below -1000F., the formation of solid carbon dioxide will not occur.Both gas and liquid phases will be present in the column at these pressures, which are well above 673 psia, the critical pressure of pure methane.

A feature of this invention is that the carbon dioxide can be reduced to very low levels within a hydrogen-containing hydrocarbon product by selection of the temperature and rate of reflux liquid, as desired.

Referring to Figure 2, there is graphically described an essential relationship between temperature, in "F., and the partial pressure of carbon dioxide and methane (CO2+CH4), expressed in pounds per square inch absolute, if solid formation is to be avoided in systems which contain methane and hydrogen (in the aforesaid mole percentage ranges) and CO2. It will be observed that, in order to avoid the formation of solid, operation of the column at temperatures ranging from about l70C F. to about 84a F., as shown on the x-axis, requires higher and higher partial pressures of carbon dioxide and methane, as shown on the y-axis, ranging from about 200 psia to about 710 psia at the higher temperature.

Thereafter, up to about -70"F., the partial pressure that is required declines. The relationship expressed in the graph which is required to avoid the solid formation region is tabulated for convenience as follows: Partial Pressure of (CO2+CH4), Temperature, "F. psia -170 > 200 -150 > 280 -130 > 420 -110 > 550 -90 > 700 -84 > 710 -70 > 75 In sharp contrast to prior art single column distillation processes for effecting such separations, which remove only about 90 percent of the carbon dioxide originally present, it has been found feasible to reduce carbon dioxide to a level of 1 mole percent, or less, in the admixture of carbon dioxide and methane, or methane in admixture with other hydrocarbons and hydrogen, e.g., methane and synthesis gas, particularly in a single column utilizing generally 20 to 30 theoretical distillation trays. This is conveniently illustrated by reference to Figure 3. This figure presents a diagram representative of upper stage (in this case the trays 21 to 24) temperature-composition profiles of a multicomponent composition containing methane, carbon dioxide, and hydrogen wherein 1025 psia total pressure is maintained on the column, and the column is operated by introducing the feed at a temperature of -50.8"F., while employing a liquid:distillate molar ratio of 1.35 in the overhead.The data graphically illustrated in Figure 3 are taken from a computer simulated run conducted on a gaseous feed having the following composition: Moles Hydrogen 24.1 Nitrogen 0.5 Carbon Monoxide 6.6 Methane 39.4 Carbon Dioxide 27.8 Ethane 0.4 Hydrogen Sulfide 1.1 The overhead vapor and bottoms liquid streams are 71.1 moles and 28.9 moles, respectively. The mole fractions of the components in the two streams are: Vapor Liquid Overhead Bottoms Hydrogen 0.339 0.000 Nitrogen 0.007 0.000 Carbon Monoxide 0.093 0.000 Methane 0.550 0.010 Carbon Dioxide 0.010 0.937 Ethane 0.000 0.013 Hydrogen Sulfide 0.000 0.040 In Figure 3, the temperature in Fahrenheit degrees is read on the y-axis, and the mole fraction of carbon dioxide in the binary fraction is read on the x-axis.The liquidus curve is representative of that region below and to the right of which curve a solid phase is formed, and above and to the left of which no solid phase is formed.

The left-most curve on the scale is representative of the vapor mole fraction, the intermediate curve is representative of the liquid mole fraction, and horizontal lines drawn therebetween are representative of theoretical trays of temperature below -700F., these ranging in number from 21 through 24+. These data show that it is possible to remove to a level of about 1 mole percent carbon dioxide present in the vapor phase mixture by use of less than 25 theoretical trays. It is particularly significant that the mole fraction of carbon dioxide in the liquid phase, at any given set of conditions, does not exceed the mole fraction of carbon dioxide given by the liquidus curve at corresponding conditions.

WHAT WE CLAIM IS: 1. A process for the separation of carbon dioxide from a gaseous carbon dioxidecontaining, methane-containing . and, optionally, hydrogen-containing, hydrocarbon feed, which process comprises: forming a feed stream from said hydrocarbon feed such that the feed stream contains from 10 to 40 mole percent hydrogen and from 30 to 85 mole percent methane; introducing said feed stream at a temperature below -45"F. as a wholly gaseous feed into a single distillation column; maintaining within said column, above the point of introduction of the gaseous feed, (i) a total pressure ranging from 710 psia to 1070 psia and (ii) a total partial pressure of carbon dioxide plus methane sufficient to avoid solids formation; condensing overhead vapor to produce a liquid; recycling at least a portion of the liquid as reflux; and recovering a gaseous product of reduced carbon dioxide content from the upper portion of said distillation zone.

2. A process according to claim 1, wherein the feed stream contains from 20 to 35 mole percent hydrogen, and from 50 to 80 mole percent methane.

3. A process according to claim 1 or claim 2, wherein said total pressure is from 1025 psia to 1070 psia.

4. A process according to any one of claim 1 to 3, wherein the temperature in the column above the point of introduction of the gaseous feed is from -170"F to --700F.

5. A process according to any one of claims 1 to 4, wherein the temperature of the gaseous feed introduced into the distillation column ranges from -450F to -70"F.

6. A process according to any one of claims 1 to 5, wherein the number of theoretical stages or trays of the distillation zone ranges from 20 to 30.

7. A process according to any one of claims 1 to 6, wherein the molar ratio of liquid: distillate used as reflux is at least 1.25:1.

8. A process according to claim 7, wherein the molar ratio of liquid: distillate used as reflux is at least 1.3:1.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (15)

**WARNING** start of CLMS field may overlap end of DESC **. In sharp contrast to prior art single column distillation processes for effecting such separations, which remove only about 90 percent of the carbon dioxide originally present, it has been found feasible to reduce carbon dioxide to a level of 1 mole percent, or less, in the admixture of carbon dioxide and methane, or methane in admixture with other hydrocarbons and hydrogen, e.g., methane and synthesis gas, particularly in a single column utilizing generally 20 to 30 theoretical distillation trays. This is conveniently illustrated by reference to Figure 3.This figure presents a diagram representative of upper stage (in this case the trays 21 to 24) temperature-composition profiles of a multicomponent composition containing methane, carbon dioxide, and hydrogen wherein 1025 psia total pressure is maintained on the column, and the column is operated by introducing the feed at a temperature of -50.8"F., while employing a liquid:distillate molar ratio of 1.35 in the overhead. The data graphically illustrated in Figure 3 are taken from a computer simulated run conducted on a gaseous feed having the following composition: Moles Hydrogen 24.1 Nitrogen 0.5 Carbon Monoxide 6.6 Methane 39.4 Carbon Dioxide 27.8 Ethane 0.4 Hydrogen Sulfide 1.1 The overhead vapor and bottoms liquid streams are 71.1 moles and 28.9 moles, respectively.The mole fractions of the components in the two streams are: Vapor Liquid Overhead Bottoms Hydrogen 0.339 0.000 Nitrogen 0.007 0.000 Carbon Monoxide 0.093 0.000 Methane 0.550 0.010 Carbon Dioxide 0.010 0.937 Ethane 0.000 0.013 Hydrogen Sulfide 0.000 0.040 In Figure 3, the temperature in Fahrenheit degrees is read on the y-axis, and the mole fraction of carbon dioxide in the binary fraction is read on the x-axis. The liquidus curve is representative of that region below and to the right of which curve a solid phase is formed, and above and to the left of which no solid phase is formed. The left-most curve on the scale is representative of the vapor mole fraction, the intermediate curve is representative of the liquid mole fraction, and horizontal lines drawn therebetween are representative of theoretical trays of temperature below -700F., these ranging in number from 21 through 24+. These data show that it is possible to remove to a level of about 1 mole percent carbon dioxide present in the vapor phase mixture by use of less than 25 theoretical trays. It is particularly significant that the mole fraction of carbon dioxide in the liquid phase, at any given set of conditions, does not exceed the mole fraction of carbon dioxide given by the liquidus curve at corresponding conditions. WHAT WE CLAIM IS:

1. A process for the separation of carbon dioxide from a gaseous carbon dioxidecontaining, methane-containing . and, optionally, hydrogen-containing, hydrocarbon feed, which process comprises: forming a feed stream from said hydrocarbon feed such that the feed stream contains from 10 to 40 mole percent hydrogen and from 30 to 85 mole percent methane; introducing said feed stream at a temperature below -45"F. as a wholly gaseous feed into a single distillation column; maintaining within said column, above the point of introduction of the gaseous feed, (i) a total pressure ranging from 710 psia to 1070 psia and (ii) a total partial pressure of carbon dioxide plus methane sufficient to avoid solids formation; condensing overhead vapor to produce a liquid; recycling at least a portion of the liquid as reflux; and recovering a gaseous product of reduced carbon dioxide content from the upper portion of said distillation zone.

2. A process according to claim 1, wherein the feed stream contains from 20 to 35 mole percent hydrogen, and from 50 to 80 mole percent methane.

3. A process according to claim 1 or claim 2, wherein said total pressure is from 1025 psia to 1070 psia.

4. A process according to any one of claim 1 to 3, wherein the temperature in the column above the point of introduction of the gaseous feed is from -170"F to --700F.

5. A process according to any one of claims 1 to 4, wherein the temperature of the gaseous feed introduced into the distillation column ranges from -450F to -70"F.

6. A process according to any one of claims 1 to 5, wherein the number of theoretical stages or trays of the distillation zone ranges from 20 to 30.

7. A process according to any one of claims 1 to 6, wherein the molar ratio of liquid: distillate used as reflux is at least 1.25:1.

8. A process according to claim 7, wherein the molar ratio of liquid: distillate used as reflux is at least 1.3:1.

9. A process according to any one of

claims 1 to 8, wherein hydrogen is added to the hydrocarbon feed, originally deficient in hydrogen or synthesis gas, to provide a feed stream containing the said hydrogen concentration.

10. A process according to any one of claims 1 to 9, wherein the feed stream comprises a gas which contains CH4, H2 and CO, the molar ratio of CH4:(H2+CO) ranging from 1:1 to 5:1.

11. A process according to any one of claims I to 10, wherein the feed stream further comprises one or both of nitrogen and hydrogen sulfide.

12. A process according to any one of claims 1 to 9, in which the gaseous feed stream introduced in the distillation column contains only methane, CO2 and H2, and wherein the temperature-pressure relationships between said carbon dioxide and said methane are maintained sufficient to avoid solid formation in accordance with the following: Partial Pressure of (CO2+CH4).

Temperature, OF psia -170 200 -150 280 -130 420 -110 550 -90 700 -84 710 -70 75

13. A process according to claim 1 and substantially as herein described.

14. A process according to claim 1 and substantially as herein described with reference to the accompanying drawings.

15. The gaseous product of reduced carbon dioxide content prepared by a process according to any preceding claim.