U.S. patent number 3,633,371 [Application Number 04/719,032] was granted by the patent office on 1972-01-11 for gas separation.
This patent grant is currently assigned to Phillips Petroleum Company. Invention is credited to Joseph W. Davison.
United States Patent |
3,633,371 |
Davison |
January 11, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
GAS SEPARATION
Abstract
A method for recovering ethane and heavier from a normally
gaseous material by partially condensing the gas by cooling to a
temperature in the range of from about -70.degree. to about
-120.degree. F., removing uncondensed gas with an absorbent at a
temperature in the range of from about -70.degree. to about
-120.degree. F.
Inventors: |
Davison; Joseph W.
(Bartlesville, OK) |
Assignee: |
Phillips Petroleum Company
(N/A)
|
Family
ID: |
24888515 |
Appl.
No.: |
04/719,032 |
Filed: |
April 5, 1968 |
Current U.S.
Class: |
62/632; 95/172;
95/237 |
Current CPC
Class: |
F25J
3/0209 (20130101); C07C 7/005 (20130101); C07C
7/005 (20130101); C10G 5/04 (20130101); F25J
3/0247 (20130101); C07C 9/02 (20130101); C07C
7/11 (20130101); C07C 9/02 (20130101); F25J
3/0238 (20130101); C07C 7/11 (20130101); F25J
3/0233 (20130101); F25J 2205/04 (20130101); F25J
2205/50 (20130101) |
Current International
Class: |
C07C
7/11 (20060101); C07C 7/00 (20060101); F25J
3/06 (20060101); F25J 3/02 (20060101); C10G
5/00 (20060101); C10G 5/04 (20060101); F25j
003/05 (); F25j 003/00 () |
Field of
Search: |
;55/48,86,84
;62/17,23,24,27,28 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Yudkoff; Norman
Assistant Examiner: Purcell; Arthur F.
Claims
I claim:
1. A process comprising providing a feed gas containing at least 50
volume percent of a mixture of substantial amounts of each of
methane, ethane, and propane; cooling said feed gas to a
temperature in the range of from about -70.degree. to about
-120.degree. F., said temperature and suitable pressure being
sufficient to liquefy from about 5 to about 95 volume percent of
said gas and thereby produce an ethane and heavier rich liquid;
separating said ethane rich liquid from the remaining unliquefied
gas in a separation zone; separately removing from said separation
zone said unliquefied gas and said ethane rich liquid and
thereafter recovering ethane from said ethane rich liquid as a
product of the process and not returned thereto; passing at least a
portion of the unliquefied gas which was separated from said ethane
rich liquid to an absorbing zone which is maintained at a
temperature in the range of from about -70.degree. to about
-120.degree. F.; contacting said unliquefied gas in said absorbing
zone with a precooled absorbent which is liquid at the temperature
maintained in said absorbing zone and which consists essentially of
saturated hydrocarbons having from four to eight carbon atoms per
molecule, inclusive; separately removing from said absorbing zone
gas rich in methane and liquid absorbent rich in ethane and
thereafter separately recovering ethane from said liquid absorbent
rich in ethane as a product of the process and not returned
thereto.
2. The method according to claim 1 wherein said gas and liquid
separating step and said absorbing step are each carried out at
substantially the same pressure as the initial pressure of said
feed gas.
3. A method according to claim 1 wherein said gas and liquid
separating step is carried out at a substantially lower than the
initial pressure of said feed gas to cause flashing of a part of
the liquefied feed gas, and said absorbing step is carried out at
substantially the same pressure as said gas and liquid separation
step.
4. A method according to claim 1 wherein said feed gas consists
essentially of a finite amount up to 25 volume percent ethane, the
remainder being essentially methane and propane; and the absorbent
consists essentially of normal hexane.
Description
This invention relates to a new and improved method for recovering
ethane from a normally gaseous material.
Heretofore cooling gases to at least partially condense same has
been used as a type of a reverse distillation procedure.
According to this invention maximum recovery of ethane from a
normally gaseous material at minimum cost per volume of ethane
recovered is achieved by first cooling the gaseous material to a
temperature in the range of from about -70.degree. to about
-120.degree. F., said temperature being sufficient to liquefy a
portion of said gaseous material and thereby produce an ethane and
heavier rich liquid, separating the liquid from the remaining,
uncondensed gas, and then contacting at least a portion of the
remaining gas with an absorbent as hereinafter defined to remove
additional ethane from the gas, the absorbing step being carried
out at a temperature in the range of from about -70.degree. to
about -120.degree. F.
By this procedure separation of ethane by liquefaction thereof is
practiced to the point where maximum amounts of ethane as well as
heavier hydrocarbons are liquefied and past which point the
additional ethane liquefied would not be worth the cost of
obtaining the necessary lower temperatures, i.e., the cost of the
additional refrigeration necessary. After the cooling step the
nonliquefied gaseous portion can be treated with certain absorbents
while still in the temperature range of from about -70.degree. to
about -120.degree. F. and additional ethane thereby recovered from
the gas. In this manner maximum ethane recovery is achieved by
using the partial condensation technique only to the extent that
the amount of ethane recovered thereby justifies the refrigeration
cost and thereafter still more ethane is recovered by a less
expensive absorbing step. It is important to note that the
absorbing step is carried out in the temperature range of from
about -70.degree. to about -120.degree. F. since this procedure
insures maximum removal of ethane by the absorbent.
By the procedure of this invention, a maximum amount of ethane is
recovered from a gas in the most economic manner because the
refrigerating step is employed only to the extent that it remains
economical based on the amount of the ethane recovered thereby, and
thereafter additional ethane is recovered by using a less expensive
absorbing step. However, the absorbing step is carried out on the
already refrigerated gas thereby obtaining maximum benefit of (1)
the refrigeration step since the absorbent is, because of its
refrigerated condition, in a state of increased capacity for ethane
and also since the ethane is tending toward liquefaction, and (2)
the function of the absorbent.
The ethane produce of this invention can be dehydrogenated using
known procedures and the resulting ethylene employed as a monomer
in known polymerization procedures to produce polyethylene. The
polyethylene so produced is useful for making with known techniques
such as blow molding a wide variety of articles of commerce such as
bottles, e.g., baby bottles, wastepaper baskets, and the like.
Accordingly, it is an object of this invention to provide a new and
improved process for separating ethane from normally gaseous
materials.
It is another object of this invention to provide a new and
improved process for removing a maximum amount of ethane from a gas
at minimum cost by utilizing partial condensation techniques to the
extent that they remain economic and thereafter using an absorbing
step on the already refrigerated gases, thereby taking advantage of
the increased efficiency of the refrigeration and absorbing steps
at the same time in the absorbing step.
Other aspects, objects and the several advantages of this invention
will become apparent from a study of the disclosure, drawing and
appended claims.
The drawing shows a system which embodies this invention.
In the drawing, there is shown line 1 which carries the feed to the
system.
The feed can be any conventional ethane-containing gas such as
natural gas or a gasoline plant residue gas. Such gases are well
known in the art. Generally, the feed gas can contain at least 50
volume percent of methane, ethane, and propane, the remainder being
essentially butane and heavier saturated and unsaturated normally
gaseous hydrocarbons, nitrogen, carbon dioxide, helium, water, and
the like. The amount of water present should not be sufficient to
cause plugging of the apparatus by the formation of ice crystals.
The methane, ethane, and propane present in the feed gas are each
present in substantial amounts. For example, of a mixture of
methane, ethane, and propane present in the feed gas, at least 50
volume percent can be methane, the remainder being from about 5 to
about 25 volume percent each of ethane and propane.
The feed in line 1 passes to heat exchanger 2 which can be composed
of one or more conventional heat exchange units and/or one or more
conventional refrigeration units. The heat exchange system 2 is
designed to cool the feed gas to a temperature in the range of from
about -70.degree. to about -120.degree. F.
As shown in the drawing, recycle gas in line 3 can pass through the
system. This is advantageous because it conserves refrigeration
expenses by utilizing the already cooled gas after it is through
the absorbing treatment, thereby cooling the feed and also heating
the already treated gas before it is passed to a pipeline or other
disposition by way of line 4. It should be noted that the feed gas
can be cooled in heat exchange system 2 by a combination of cooling
steps such as by first being cooled by coming in heat exchange
relationship with already treated and refrigerated gas in line 3,
and thereafter being further cooled by other heat exchange
relationships and/or refrigeration processes which are not related
to nor dependent upon the use of the gas in line 3.
The cooling step should reduce the temperature of the feed gas into
the range set forth hereinabove, and should also liquefy at least a
portion of that feed gas, preferably from about 5 to about 95
volume percent of the feed gas being processed. This partial
liquefaction step in the temperature range recited will produce an
ethane and heavier rich liquid in contact with a methane rich gas.
This gas also contains recoverable amounts of ethane.
The partially condensed feed stream then passes by way of line 5 to
separator 6 wherein the uncondensed gas is allowed to separate and
collect at the top of the separator and the liquified portion of
the feed gas is allowed to collect at the bottom of the separator
and is removed by way of line 7.
The liquid removed by way of line 7 is rich in ethane and heavier
hydrocarbons. This liquid can be subjected to a conventional
demethanization distillation process to remove any methane present
in the liquid, thereby leaving a liquid consisting essentially of
ethane, propane, and any heavier components that happen to be in
the feed gas and that boil at temperatures higher than propane. The
demethanized liquid can then be subjected to another distillation
step wherein substantially pure ethane is recovered as an overhead
product.
The overhead product from separator 6 passes by way of line 8 and
line 9 into absorbing zone 10. It is important to maintain the
refrigerated status of the overhead gas passing into absorbing zone
10 in order to make the most advantage of the prior refrigeration
step when carrying out the absorbing step. If in the transfer of
the overhead gas from separator 6 to absorbing zone 10 the gas
happens to be heated to a temperature outside of the range of
-70.degree. to -120.degree. F. an additional cooling system 11 can
be employed to cool that gas so that the gas in the absorbing zone
10 is at a temperature in the range of from about -70.degree. to
about -120.degree. F.
The gas in absorbing zone 10 tends to pass upwardly and in so doing
is contacted countercurrently with downwardly passing absorbing
liquid from line 12.
The absorbent employed is precooled to substantially the same
temperature as the gas present in the absorbing zone, and therefore
is at a temperature in the range of from about -70.degree. to about
-120.degree. F. The absorbent employed is a liquid at the
temperature of the gas present in the absorbing zone and consists
essentially of saturated hydrocarbons having from four to eight
carbon atoms per molecule, inclusive, preferably hexanes such as
normal hexane. Mixtures of these hydrocarbons can be employed
also.
The downwardly passing absorbent very readily absorbs ethane from
the upwardly passing refrigerated gas because of the high capacity
of the absorbent for ethane at low temperatures. Thus the
contacting of the thus-refrigerated ethane with the liquid
absorbent maximizes the removal of ethane from the gas into the
absorbent.
The liquid absorbent rich in ethane is removed by way of line 13
for further treatment such as distillation to remove ethane
therefrom as an overhead product.
The combination of ethane removed by way of lines 7 and 13
constitutes the ethane product of this invention, and will
generally represent from about 5 to about 95 volume percent of the
ethane present in the feed gas in line 1. By following the
procedure of this invention an ethane extraction as deep as 70 to
80 volume percent of the ethane present in the feed gas can be
obtained at costs substantially less than using other techniques
such as absorption or partial condensation by themselves.
Separator 6 can be operated at a pressure substantially the same as
the pressure of the feed gas in line 1, taking into account
pressure drops caused by friction and the like in transporting the
feed gas through the refrigeration system 2 and into separator 6.
Similarly, absorbing zone 10 can also be operated at substantially
the same pressure as the pressure in separator 6 also taking into
account normal pressure drop occurring in lines 8 and 9, and
cooling system 11 if present.
Alternatively, separator 6 can be operated as a flash unit in that
the pressure maintained in that separator can be substantially less
than the pressure of the feed gas in line 1, thereby causing
partial volatilization of the liquid passing into separator 6 and
additional temperature reduction. This partial volatilization will
decrease the amount of methane carried by the liquid that passes
from the separator by way of line 7. Generally, the pressure
decrease in separator 6 below the pressure of the feed gas in line
1 is at least 10 p.s.i.g., preferably from about 10 to about 100
p.s.i.g. In this embodiment, the absorbing zone 10 is maintained at
a pressure substantially the same as reduced pressure in separator
6.
Apart of the overhead gas in line 8 and all of the overhead gas in
line 14 can be disposed of at will such as by reheating and passing
to a pipeline or other storage facility. However, for maximum
economics, all of the refrigerated, overhead gas in line 14 is
passed through lines 15, 16, and 3 into heat exchange relationship
with the incoming feed gas from line 1 as discussed hereinabove. In
addition, if desired, a portion of the refrigerated, overhead gas
in line 8 can be passed through line 17 into line 16 and thereby
through line 3 into the heat exchange system 2 to also serve to
cool the incoming feed gas from line 1. Normally, substantially all
of he overhead gas in line 8 is passed through line 9 into
absorbing zone 10. However, any portion or all of the overhead gas
in line 8 can be passed into line 17. Preferably, at least 5 volume
percent of the gas in line 8 passes into line 9 for treatment in
absorbing zone 10.
EXAMPLE
Two runs are made using the apparatus shown in the drawing and
using different feed compositions for each run. The two different
feeds are denoted herein as Feed A and Feed B, and the compositions
of these feeds are given in detail hereinbelow in table I. Feed A
contains about 3.4 mol percent ethane, the remainder being
essentially methane, propane and heavier hydrocarbons; while Feed B
contains about 12 mol percent ethane, the remainder being
essentially methane, propane, and heavier hydrocarbons. The
absorbent employed consists essentially of normal hexane.
Feed A is passed under pressure of 500 p.s.i.a. to cooling system
2, cooled to a temperature of about -90.degree. F., and thereafter
passed into separation zone 6. A portion of the overhead gas from
separation zone 6 is passed into absorption zone 10 and therein is
at a temperature of about -80.degree. F. The absorbent is passed
into absorption zone 10 also at a temperature of about -80.degree.
F. Feed B is under pressure of 500 p.s.i.a. is cooled in heat
exchange system 2 to a temperature of about -70.degree. F., and
then passed to separation zone 6. Part of the overhead gas from
separation zone 6 is passed to absorption zone 10. During this
passage the gas is cooled to about -80.degree. F. before being
passed into the absorption zone. The absorbent, again consisting
essentially of normal hexane, is precooled to a temperature of
about -80.degree. F. and is passed into the absorbing zone 10.
In both runs, the pressures in separation zone 6 and absorbing zone
10 are substantially the same as the initial 500 p.s.i.a. pressure
of the feed allowing of course for normal pressure drop in the
pipes due to friction and the like.
The results of the runs are as follows: ##SPC1##
Reasonable variations and modifications are possible within the
scope of this disclosure without departing from the spirit and
scope thereof.
* * * * *