Note: Descriptions are shown in the official language in which they were submitted.
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.
- 2 -
-
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
- 4 -
:
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
-- 5 --
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.
- 6 -
lq~
- 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
-- 8 --
~ 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
3 -170 200
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 "
. .
- 10 -
.~ , . . .' .
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.
- 11 -
