Note: Descriptions are shown in the official language in which they were submitted.
'7~
BACKGROllND OF TEIE INVENTION
. . .
Field of the Invention
. . . _
This invention relates to the recovery of
condensable components from a hydrocarbon gas mixture
particulary condensable hydrocarbons, especially ethane and
propane.
Priox Art
~a,n~- processes have been tauyht for the separation
Oæ gases, e.g. the separ~Ltion of nitrogen from methane, the
sepaxation of methane, ethane, propane, et cetra from
hydrocarbon gas streams. Some of these processes even
utilize subcooling means. For example, U.S. 3,559,418 to
Ho~frqan discloses th~e subcooling of the methane product
prior to E~offman, U.S. 3,568,458 and U.S. 3,589,137 disclose
. .
the subcooling of the liquid hydrocarbon, the hydrocarbon
obtained after fractionation, so it can be used to condensè
nitrogen out of the hydrocarbon gas~ In turn, nitrogen and
~ydrocarbon vapors subcool the li~uid hydrocarbon. Tracy et
.
al in U.S. 3,791,157 in teaching the separation of nitrogen
from hydrocarbon gases uses the liquid phase from 'che second ~
` separation to subco~1 the feed gas, i.e. the contaminated ;
-~ naturai gasO Thùs, the liquid phase is actually heated in
.~ .
the subcooler. Another use of a subcooler is disclosed by
Eakin et al in IJ.S. 2,823,523.
. - .
Previously the prior art would increase recovery of
a condensable component by lowering the temperature of the
vapor in order to condense more of the desired h~drocarbon. -
However, the lower the pressure to which the vapor is reduced~
the more recompression is required of the final residue gas
prior to storage or transportation. Additionally, the
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ability to do this is some-times limi-ted by the amount of
carbon dioxide the hydrocarbon gas contains because ag the
temperature is lowered it will tend to freeze creating plugging
problems.
None of the prior ar1 teaches the subcooling of a
pressurized, condensed saturated liquid produced in hydrocarbon
recovery processes in order to increase the recovery of the
desired component from the hydrocarbon gas mixture. By sub-
cooling the saturated liquid, it was found that more of the
component could be recovered than what is present in the
liquid streamts~ that make up the liquid stream which i5
processed for.th.e recovery of the desired component.
~ lthough the.prior art teaches the subcooling of
refrigerants, it does not teach the use of subcooling to
reduce energy requirements of processes recovering condensable
components from hydrocarbon gases. Thus, in processes
utilizing vapor expanders, this technique permits a higher
expander outlet pres.sure for a given recoYery of a condensable
component. Ener~y sayings are realized throug~ the reduction ;.
of recompress~on horsepower due to a hlgher expander outlet
pressure. An added benefit can be the aYoidance of problems
caused by the freezin~ of carbon dioxide~
SUMMARY QF THE`INVENTION
Enhanced and!or more efficient recovery of
condensable co~ponents from relative~y high pressure hydrocarbon
gas mixture can be obtained in processes for the recovery of
.
condensable components ~hich utilize low temperatures to ~:
partially conaense the gas into saturated ~apor and saturated
liquid phases by subcooling the saturated liquid prior to
the liquid being depressured for further processing.
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I'he tw~ phases can be separated with-the saturated liquid phase being
subcooled and depressurcd separately from the depressuring of the
vapor stream. Thereafter, -the two streams (each of which is usually
tw~-phased) may be recombined. The co~densation of the desired
canponent obtained, and, thus, the recovery of the c~nponent, will be
greater than the ~nount of condensable ccmponent the separate liquid
phases contained prior to their reccmbination. Plternatively, for a
given recovery of condensable ccmponents frcm hylrocarbon gas, the low
pressure section of the process can be run at higher pressures. This
results in energy savings due to a reluction in the amount of
recompression of the residue gas needed prior to its storage or
transportation.
m is technique is applicable to a nu~ber of processes which
partially condense a hydrocarbon gas mixture to create a saturated
liquid phase in order to remove or recover a condensable component
from the hydrocarbon gas. Examples of such processes includes straight
refrigeration processes, Jc~le-Thcmpson valve expansion processes and -
processes utilizing one or more expansion enginesl For example, a
process to which this subcooling is beneficial is the ccmmon technique
for the recovery of condensable camponents from hydrocarbon gas
which involves the cooling of pressuriz~d hydracarbon gas to partially
condense it. mis tWD phase muxture is then introduced into a separator
~referred to as a chiller separator) to effect a separation between
the vapor and liquid phases. The vapor is fed into an expander where
it is let down to a lower pressure, thereby doing w~rk, cooling and~
usually partially condensing the vapor, Thereafter, the liquid phase
fram the chiller separator is recombined with the partially condensed
vapor fram the
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expander~ The mixture of vapor ancl liquid are again separated
in a low pressure separ~-tor -to a vapor phase and liquid phase.
The liquid from this separa-tor is processed, e.g., in a frac-
tionation tower, to remove undesirable components. The vapor
can be used to satisfy a portion o~ the process refrigeration
demand, after which it is recompressed and leav~s the process
as a residue gas.
Thus, the invention of the present process entails
the subcooling of the condensed liquid phase of the hydrocarbon
- 10 gas mixture. It is preferred that the liquid phase be sub-
cooled so -that when it is depressured it is essentially the
same temperature as the temperature of the vapor phase prior
to recombining it with the vapor phase or prior to subjecting
it to a separation process to obtain the various components of
~; the hydrocarbon gas. This improvement is particularly appli-
cable in the recovery of hydrocarbon components, e.g. ethane
,,
or propane, from a hydrocarbon gas mixture.
Broadly speaklng and in summary of the above, the
.-: - : i
present invention may be seen to provide a process for the
:- ~
20 recovery of a condensable component from a pressurized gas '
... .
mixture wherein the pressurized gas mixture is coole~ to
selectively condense at least a portion of the condensable
` component to produoe a saturated vapor phase and a saturated
liquid phase whlch contains the deslrable component, the im-
.. , ~
~; provement comprising: (a)separating the saturated vapor phase
and the saturated liquid phase into separate streams; (b) sub-
cooling and then depressuring the saturated liquid stream into
- a vapor and a liquid phase; (c) depressuring the saturated vapor
~ stream into a vapor and a liquid phase;(d) recombining the two
." g
.. - . . . ~ : . .
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phase, depressured, subcooled liquid s-tream and the two phase,
depressured vapor stream; (e) separating the recombined stream
into a vapor stream and a liquid s-tream; and (E) processing
; the liquid stream of step (e) .for the recovery of the desired
condensable component.
BRIEF DESCRIPTION OF THE DR WINGS
: FIG. l is a schematic representation of the process
showing the recombining of the vapor and liquid phases from the
;` chiller separator;
FIG. 2 is a schematic flow sheet of the process
-; showing separate processing of the vapor and liquid phases of
the chiller separator; and ~
FIG. 3 is a schematic representation of Example 3. .
~:. DESCRIPTION OF T~IE PREFERRED EMBODIMENTS
This invention is useful in any process for the
recovery or removal o~ condensable components from a gas mixture
which is pressurized and cooled to condense at least a portion
~ of
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the desired component into a saturated liquid phase. The gas
mixture can be inorganic gases, organic gases or a mixture of
inorganic and organic gases. The invention is particularly
useful in any process for the recovery of removal of
condensable components, e.g., methane, ethane, propane,
butane, from hydrocarbon gas m~tures wherein some of said
component is contained in a conclensed saturated liquid phase
of the hydrocarbon gas mixture and the process is conducted
under conditions which cause some vaporization of the liquid
phase. The invention is especially useful in the recovery of
condensable hydrocarbon, e.g., ethane, propane, from hydro-
carbon gas. In processes for the recovery of condensable
components which utilize low temperatures to treat relatively
high pressure hydrocarbon feed gas containing methane and
condensable components, it was found by subcooling the
essentially saturated liquid produced from a relatively high
pressure separator prior to the liquid being depressured for
further processing that enhanced recovery of the desired
~: .
~-~ hydrocarbon components and/or energy savings could be obtained.
Generally, in a process for the recovery of
- condensable components from a hydrocarbon feed gas, the
hydrocarbon feed gas having a pressure of from about 2,760
-.,:
(400 p.s.i.a.) to about 13,790 kilopascals ~2,000 p.s.i.a.) is
cooled to a temperature of from about 10C to about-100C,
. :
preferably from about -35 to about -75C in order to partially
condense the hydrocarbon feed gas. The hydrocarbon sought to
be recovered will be more concentrated in the liquid phase.
This two phase mixture is introduced into a separator, often
call~d a chiller separator, in order to effect a separation
between the vapo~ and the l~uid phases. The chiller separator
, -6
bm:
7 ~
is opera-te~ at a tempera-ture of from about 10C to about -100C,
preferably from about -35 to about -75C and at a pressure from about
2,760 (400 p.s.i.a.) to about 13,790 kilopascals (2,000 p.s.i.a.),
and preferably fram about 4,137 kilopascals (600 p.s.i.a.) to about
6,895 kilopascals (1000 p.s.i.a.). The vapor is fed into one or more
expansion engines, referred to as an ~pander or expanders, where as
a result of the work it is doing, is lowered to a pressure of from
about 515 (75 p.s.i.a.) to about 2,760 kilopascals (400 p.s.i.a.)
and preferably from about 690 (100 p.sOi.a.) to about 2,070 kilopascals
(300 p.s.i.a.). This causes the vapor to be caoled and usually
become partially condensed. The liquid frcm the chiller separator is
subcooled. It is preferred the saturated liquid be cooled so that its
temperature after depressurizing will be about as cold as the
temperatwre of the expander effluent. However, more or less subcooling
of the saturated liquid stream prior to being depressurized will be
beneficial.
In any process for the recovery of condensable ccmponents,
it is kncwn by those skilled in the art that the parameters can be varied
in accordance with physical properties of the desired ccmponent sought
.
to be recovered. It is also kncwn by those in the æ t that a parameter
or parameters can be varied in the process and this usually results in
other parameters being varied elsewhere in the process. These variances
are within the kncwn chemical and physical properties of the hydrocarbon
gas and ccmponents being recovered. The condensed liquid phase is
sukcooled to maximize the recovery of the desired ccmponent or the
maximize energy savings where the amount of recovery remains constant
; or used to provide more moderate recovery parameters. Thus~ it will be
obvious to one skilled in the art what the desire amount of
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subcoolincJ will be for a given process, since it will involve
a balancing of the econom~ics of a process, closeness o~
temperature approach in heat exchangers, amount and type of
component being recovered, et cetera.
The refrigeration for the subcooler can be supplied
by other streams produced in the process, e.g., vapors from
the low pressure separator, the net overhead vapors from a
demethanizer, by a deme~hanizer feed, or it can be supplied
by an external source.
After the liquid from the chiller separator has been
subcooled, it can be recombined with the partially condensed
-`- vapor which are thereafter subjected to a low pressure
- separation process~ Alternatively, the partially condensed
vapor and the subcooled liquid can be subjected to individual
low pressure separation processes. The invention can also be
used in processes ~hich use a straight refrigeration process
to recoYer desirable components from hydrocarbon gas. The
subcooling ~ill benefit the recovery of the condensable `~
hydrocarbon in either of those latter two processes; however,
- 20 greater benefit is seen in a process ~hich recombines the
depressured subcooled liquid with an expander effluent~
The lo~ pressure separatorCs~ are operated at a
pressure of from about 515 ~75 p~s.i.a.) to about 2,760
kilopascals ~4qO p.s.i.a.~ and preferably from about 6~0
- (100 p.s.i.a.¦ to about 2,070 kilopascals t300 p.s.i.a.)
: .
~-~ The invention can also be illustrated by refexence ;
`~ to the figures which are illustratiYe of an ethane recoYery
process. In Fiyure 1 a hydrocarbon gas feed stream is cooled
in heat exchanger 1 such that a portion of i-t is condensed
to produce a two phase stream consisting of a vapor and a
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liquid. The two phase stream is introduced to a high pressure
separator vessel 2 called a chiller separa-tor which separates
-the vapor and liquid in-to separate s-treams 3 and 5, respectively.
The vapor is then passed to an expander 4 where it is expanded
to a lower pressure in order to perform work. As a result of
this expansion the Yapor is cooled and usually partially
condensed. The liqu~d from the chiller separator 5 is flowed
through a su~cooler 6. The subcooler 6 is cooled by a
refrigerant stream 7. The subcooled chiller separator liquid
is. then passed through control ~alve 8 which controls the
liquid leyel in the separator 2 and also depressures the liquid
to the same pressure as the vapor from expander 4. Thereafter,
the chiller separator liquid 9, which is usually a t~o phase
stream., is reco~bined with the partially condensed vapor from
ex~ander 4. Th~s ~ixed stream 10 is flo~ed into a low pressure
separator 11. The lo~ pressure separator 11 separates the
h~drocarb.on mixture into a vapor stream 12 and a liquid stream
~3. Th.e ~apo.r stream 12 can be recycled to the process as
a xefr~gerant, for example, in a heat exc~anger 1 or in
-.
.. 20 subcooler 6. T~ereafter, t~e ~apor is~ recompressed ready
. fo~ sto~a~e qr tr~ns.porta~i~on for suBsequent use. The
.. condensed hxdrocarb.on l~qu~d in stream l3 is subjRcted to a
urther ~ecover~ process, e.g., fract~onat~on, ~n order to
- xeco.vex the eth.ane,.
. Figure 2 depicts an alternative embodiment of
. Figure 1 wherein the subcooled chiller separator liquid 9 and
the effluent are not recombined, but rather are subjecked
to separate low pressures separations ln low pressure
separators llA and llB respectively. Again, the vapor
produced from these separators 12A and 12B can be recycled to ..
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refrigeration steps of th~ process and ultima-tely recompressed
for storage or -transportation. The liquids 13A and 13B are
processed for the recovery of -the desired condensed component.
The use of individual separators for these two streams allows
them to be ope,rated at different pressures which may be
advanta~eous with certain gas compositions, supply pressures
or recover~ levels.
The invention is illust~ated by the comparaltive
examples presented below. These examples are meant to be
illustrative of the invention but not limiting thereof.
'EXAMPLES
.
;~ Example 1
~, 31,689 pound moles per hour of hydrocarbon gas at a
pressure o~ 5,654 kilopascals (820 p.s.i.a.~ was cooled to
-64C to condense a portion of the feed gas. The two phase
~',, hydrocarbon gas was then introduced to a chiller separator
; .
- haying a temperature o~ -64C and a pressure of 5,516
kilo~ascals ~800 p.s.i.a.~. 'The stream ~as separated into
~, a, va,por stxeam and a liquid st~eam. 'The Vapor stream was
$ent ~rough an expander-~here'it dld work and was reduced
to a ~ressuxe of 1,0,34 ki~lop~sca~s (150 p.s.i.a.l~and a
tempeXature of -116.5G. ,~he'liquid phase ~rom the chiller
~''' sepa,rato~ wa,s sent throug~ a control valve which reduced t~le
;' l,iqu;~d?s pressure to 1,034 k~lopasca~s C150 p.s.~.a.1; the
,~ 'tempera-ture of the liquid was -110,5C. The depressured
chiller liquid stream was recombined with the expander
~'' effluent, resulting in a mixed stream havin~ a temperature
,' of -112.5C and a pressure of 1,034 kilopascals (150 p.sOi.a.). ~ '
, Thereafter, the stream was again separated in a low pressure
,, 30 separa-tor to vapor and liquid streams. The liquid stream was
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processed :Eor the recovery of ethane. Data relating to the
composition of the s-treams and the amount of ethane recovered
is given below in Tables 1 and 2.
Example 2
Example 2 was run exact:Ly the same as Example 1 except
that the chiller separator liquid was subcooled such that when
it was depressured to the expancler exhaust pressure its
temperature was -116.5C prior to being recombined with the
~ expander exhaust. The resultant stream from the combining
- 10 of the expander exhaust and depressured subcooled chiller
liquid was at -116.5C and 1,034 kilopascals (150 p.s.i.a.).
The comparative results to Example 1 are given below
`- in Tables 1 and 2.
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Example 3
This example is described with reference to
Figure 3. 34,586 pound moles per hour of hydrocarbon gas
1 at a pressure of 5,654 kilopascals (820 p.s.i,a.) was
cooled in chiller train 2. In this example an intermediate
separation of a small condensed amount of the inlet gas
stream is made in separator 3 at -36.5C. This small
intermediate separation could he done at other temperatures
or not done at all without significantly affecting the value
of the chiller liquid subcooler, After the intermediate
separation the vapor was further cooled and partially
condensed down to a temperature of -64C, This two phase
hydrocarbon stream was then introduced into a chiller -
separator 4 having a temperature of -64C and a pressure of
~- 5,516 kilopascals (800 p,s.i.a,), In the separator 4~ the
~ ~ .
stream was separated into a vapor stream 5 and a liquid
stream 6. The vapor stream 5 was sent through an expander ~;
7 where it did work and was reduced to a pressure 1,034
kilopascals (150 p,s,i.a.~ and a temperature of ~116.5C,
The liquid phase 6 from the separator 4 was sent through
a control valve 8 which reduced the li~uid's pressure to
1,034 kilopascals ~150 p.s.i.a,); the temperature of the
liquid was -110C. The two phase li~uid stream 9 was
combined with the expander 7 effluent stream 10 to form
stream 11. Stream 11 had a temperature of -112,5C and a
pressure of 1,034 kilopascals (150 p,s.i,a,l, Thereafter, -
stream 11 was separated in a low pressure separator 12 to a
vapor stream 13 and a liquid stream 14, For refrigeration
recovery the vapor stream 13 flowed through a demethanizer
reflux condenser 15 and thence as stream 16 to the process
.. ..
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~ - : . . : :: . . :
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as coolant for the initial chiller train 2 along with stream 17.
Thereafter, those vapor streams 17 (undesirable light ends frcn the
fractionator) an~ 16 were sent through compression 18 to compress
the vapors prior to transportation. The recompression service i5
performed using a brake-ccm~ressor dr:iven by the power developc~d by
the expander plus a separate recompreC3sor. The liquid strean 14 from
the low pressure separator 12 is directed through the demethanizer
reflux condenser 15 and is recycled back through the process as a
refrigerant 19, for example, in the chiller train 2. This liquid stream
~: 10 19 is then sent to a fractionation tGwer 20 having a reboiler 21 to
recover the ethane in stre~m 22.
The data concerning the . ccnpositions of ~hese various
.. streams stated in po md moles per hour is given in l'able 3 c~n~ the
-: energies required for this process are given in Table 5.
TA~IE 3
(No Subcooler Us0d - Exam~le 31
Stream No.
Component 1 6 10 13 19 17 ?2
Nz 1110 349 738 1055 55 55
CO2 138 78 47 38 100 75 25
C1 27565 1253313608 20594 6971 6871 100
~ C2 4~79 2706 919 267 4212 190 4022
j C3 1124 581 81 3 1121 1121
i-C4 52 19 1 T 52 52
. n-C4 73 22 1 T 73 73
:i i-C5 17 3 T 17 17
n-C5 14 2 T 14 14
n~C614 1 T 14 14
(T denotes trace amount, less than 0,5 pound moles per hour)
~ -15-
- '. .. ' :, '
,
lV'~J1.rJ~ 7
~btal pouu~l
moles~. 3458616294 15385 21957 12629 7191 5438
~lec~ar
weight 19.5019.89 17.69 16.84 24.11 16.80 33.79
Pressure
kilopascals5,6545,5161,034 1,034 1,434 1,379 1,482
(p.s.i.a.)(820)(800) (150) (150) (208) (200) (215)
T~nperature
C 32 -64 -116.5 -112.5 -41.5 -101 -15
10Mole Percent
Vaporized 100 0 81.6 100 80 100 0
Example 4
The same hydrocarbon gas as in Example 3 is subjected to the
`` same type of process with the exception that the liquid produced from the
dhiller separator 4 is subcool~d in a subcooler prior to going through
control valve 8. Additionally, because the same amount of ethane is
sought to be recovered, the low pressure separator 12 is operated at a
-~ higher pressure in Example 4 than in Example 3, So stream 11 has a
pressure of 1,310 kilopascals (190 p.s.i.a.) and a temperature o~ -110.5C,
The bene~icial results of such an operation are given below in ~ables
4 an~ 5.
. .. ~ .
Table 4
- (Subcooler Used - Example 4)
~,~ Component 1 _ 6 _ 10 13 19 17 22 _
N2 1110 349 738 1015 95 95
: CO2 138 78 47 26 112 87 25
Cl 27565 1253313608 17199 10366 10265 101
; C2 4479 2706919 183 4296 271 4025
..
3 1124 581 81 2 1121 1122
ic4 52 19 1 T 52 52
n-C4 73 22 1 T 73 73
C5 17 3 T 17 17
n-c5 14 2 T 14 14
~, n-C6 14 1 T 14 14
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Ibt~l pou~
moleY/hr. 34586 16294 15395 18425 16160 10718 5443
Mblecular
~ei~ht 19.50 19.89 17.69 16.88 22.47 16.73 33.79
Pressure
kilopascals 5654 5516 1310 1310 1434 1379 1482
(p.s.i.a.) (820) (800) (190) (190) (208) (200) (215)
Temperature
C 32 -64 -110.5 -110.5 -48.5 -101.5 -15
Mole Percent
Vaporize~ 100 0 81.9 100 as loo o
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