Note: Descriptions are shown in the official language in which they were submitted.
1~70483
The invention relates to a process for purifying an
acid gas containing gaseous hydrocarbon stream and to gaseous
hydrocarbon obtained by such process.
Acid gases, such as hydrogen sulphide, carbon dioxide
and carbonyl sulphide are often present in gaseous hydrocarbon
streams. It i8 usually necessary to remove these acid gases 80
as to purify the hydrocarbon stream or recover the acid gas or
both. Representative of the hydrocarbon streams that often re-
quire acid gas removal include streams such as natural gas, flue
gas, hydrogen, and cracked gas produced by pyrolysis cracking.
Another ga~eous hydrocarbon stream containing undesirable acid
gases is the ethylene recycle stream in an ethylene oxide process.
The usual method employed to remove these acid gases from the
gaseous hydrocarbon stream is to contact the stream with an acid
gas absorbent, and then regenerate the ab~orbent by stripping
the acid gas from the fat absorbent. Typical acid gas absor-
bents include aqueous solutions of alkali metal carbonates~
such as potassium carbonate. Other typical acid gas absorbents
are alkanol amines, such as dl-isopropanol-amine, alkyl Plkanol
amines, and alkali metal salts of amino acids. Other selective
~olvents commonl4Y employed include water, methyl alcohol,
acetone, and propylene carbonate. A specific absorbent to
remo~e acid gases i8 for example gamma-butyrolactone.
Common to most of the acid gas remo~al processes is
that during the scrubbing of the gQseous hydrocarbon stream,
~mall ~mount~ of hydrocarbon are dis~olved and/or entrained iD
the fat absorbent stream and are ultimately lost with the acid
gas stream. It ~ould be highly desirable if recovery of
the hydrocarbons could be efficiently accomplished both to
enhance the economics of the acid gas removal process and to
substantially pre~ent release of these hydrocarbons to the
enYironment. Moreover, when recovery of the acid gas iB
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desired, the hydrocarbon impurities may be detrimentnl in ~ome
applications.
One process to remove the trace hydrocarbons found in the
carbon dioxide vent gas stream of an ethylene oxide process is taught
in Canadiun Patent No. 1,035,780. This process requires the use of a
fla~h ~e~sel and an intermediate stripping zone to remove the trace hydro-
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carbons present in the carbon dioxide-fat absorbate stre~m. While
this process reduces the a~ount of hydrocarbon vented with the
carbon dioxide stream, it also results in an increased cspital
cost for the two additional vessels.
The invention relates to a process for purifying an acid gas-
containing, gaseous hydrocarbon stream, which comprises
(a) contacting the acid gas-containing, gaseous hydrocarbon
stream with an acid gas-absorbing liquid in a scrubbing zone,
thereby producing (i~ a hydrocarbon-rich overhead stream and
(ii) a hydrocarbon-containing, acid gas-rich absorbate stream
which is routed to the top of a stripping zone, and
(b) stripping the hydrocarbon-containing, acid gas-rich absorbate
stream in said stripping zone, thereby producing (i) an acid
gas-lean, hydrocarbon-rich overhead stream, (ii) a hydrocarbon-
lean, acid gas-rich stream taken at a point intermediate
between the point at which the hydrocarbon-containing, acid gas-
rich absorbate stream enters the stripping zone and the bottom
of the stripping zone, and (iii) a lean absorbate bottoms stream
which is recycled to said scrubbing zone as said acid gas-
absorbing liquid.
The present invention is particularly useful in removing trace
quantities of hydrocarbon from the carbon dioxide vent gas stream in
an ethylene oxide process, wherein the ethylene recycle stream is
scrubbed with an absorbent such as aqueous potassium carbonate.
A modification of the process comprises recycling a major part
of the lean absorbate bottoms stream to the said scrubbing zone as
said acid gas-absorbing liquid and a minor part of the lean absorbate
bottoms stream being recycled to the top of the stripping zone.
Another modification comprises cooling a minor portion of the
said hydrocarbon-containing, acid gas-rich absorbate stream and
routing the cooled portion of the said hydrocarbon-containing acid
gas-rich absorbate stream to the top of the stripping zone and
routing the major portion of the hydrocarbon-containing, acid gas-
rich absorbate stream to said stripping zone at a point intermediate
between the top of the stripping zone and the point at which a
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hydrocarbon-lcan, acid gas-rich product of said stripping zone
leaves said stripping zone.
The accompanying three figures diagrammatically illustrate a
preferred embodiment of the present invention. Shown therein is
a scrubbing zone, a stripping zone and their intérconnections.
Pieces of equipment such as pumps, compressors, surge vessels,
accumulators and the like which are not essential for an under-
standing of the invention have been omitted for purposes of
clarity.
In particular, Figure 1 reveals a process employing a stripper
with a sidedraw to remove hydrocarbon-free acid gas. Figure 2
reveals an alternative embodiment of the process shown in Figure 1,
wherein a portion of the fat absorbate stream from the scrubbing zone
is routed through a cooler before entering the stripping zone.
Figure 3 also reveals an alternative embodiment of the process
shown in Figure 1, wherein a portion of the 'ean acid gas absorbent
stream is routed directly to the top of the stripping zone.
With reference to the attached Figure t, a gaseous, acid gas-
containing hydrocarbon stream, such as natural gas, enters the
scrubbing zone 10 along line 11 where it is contacted with a lean
ab~orbent, such as potassium carbonate, which enters the scrubbing
zone along line 21. The particular design of the scrubbing zone is
well known in the art, and includes both packed columns and columns
with fractionation trays. Absorption conditions vary depending upon
the particular absorbent employed and on the particular acid gas to
be removed. Typical temperatures of from 10C to 360C and pressures
from 1 to 70 atmospheres are used. When the invention is employed
in the removal of carbon dioxide from the recycle ethylene stream
in an ethylene oxide process employing potassium carbonate as the
absorbent, scrubbing zone temperatures are from 38C to 122C and
pressure~ are from 10 to 25 atmospheres. The amount of acid gas to
be removed from the gaseous hydrocarbon stream also varies widely.
Typical acid gas concentrations in the hydrocarbon stream vary from
about O.t vslume percent to about 25 volume percent. From the
scrubbing zone, the overhead stream containing reduced quantities of
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acid gas is withdrawn via line 12. The bottom stream from the
scrubbing zone is the fat absorbate (acid gas-rich absorbate stream)
and exits the scrubbing zone via conduit 13 before entering the top
of the stripping zone 20. The acid gas-fat absorbate stream exiting
the scrubbing zone along line 13 also contains a small amount of
hydrocarbon.
In the stripping zone 20, the acid gas and any entrained hydro-
carbon are removed from the fat absorbate. The stripping zone 20
is typically a distillation column containing packing or a
plurality of fractionation trays. The pressure in the stripping
zone is typically from 1 to 4.5 atmospheres and the stripping action
is usually provided by steam introduced along line 22 or alternatively
by heating coils or a reboiler. When the invention is being employed
in the removal of carbon dioxide from the recycle ethylene stream
in an ethylene oxide process, the stripping zone pressure is from
1 to 2 atmospheres. An acid gas-rich, hydrocarbon-lean stream 23
is withdrawn from the stripping zone as a sidedraw stream so as to
reduce the hydrocarbon contamination in the acid gas stream. The
acid gas sidedraw stream is located at a point on the stripping
zone intermediate between the point at which the acid gas-rich,
fat absorbate stream enters the stripping zone along line 13 (line
t4 in Figure 2) and the bottom of the stripping zone. Typically,
the sidedraw is located such that there are a suitable number of
trays or feet of packing above the sidedraw location to provide
sufficient stripping of the hydrocarbons from the absorbate. The
overhead Prom the stripping zone comprises only a fraction of the
amount of acid gas removed along line 23 and most of the hydrocarbons
that would otherwise have been vented with the acid gas stream.
Typically, this overhead stream 24 is recycled via line 26 to the
scrubbing zone 10. Alternatively, this overhead stream is routed
to a separate service along lines 24 and 25. The bottoms stream from
the stripping zone is the lean acid gas absorbent, and is routed to
the scrubbing zone along line 21.
The advantage of withdrawing the acid gas as a sidedraw stream
rather than as the top product stream in the stripping zone is that
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the hydrocarbons entrained and/or dissolved along with the acid
gas in the scrubbing zone are more volatile than the acid gas
and are accordingly found in ereater concentrations above the
point at which the fat absorbate enters the stripping zone.
Therefore, the acid gas stream, which is withdrawn below the
lowest point at which the fat absorbate stream enters the
stripping zone, contains lesser quantities of hydrocarbons than
would be the case if the acid gas were withdrawn as the overhead
stream from the stripping zone.
Figure 2 reveals a preferred embodiment of the present in-
vention, wherein a portion of the fat absorbate stream withdrawn
as the bottoms stream from the sru~bing zone 10 is first routed
through a cooler before entering the top of the stripping zone 20.
The main portion of this stream enters the stripping zone at a
point intermediate between the top ~ the stripping zone and the
point at which the acid gas sidedraw stream leaves the stripping
col~ . Referring to Figure 2, the major portion of the acid gas-
rich, fat absorbate containing small quantities of entrained and/or
dissolved hydrocarbon is routed from the scrubbing zone 10 via
conduits 13 and 14 to the stripping zone 20. A portion of the fat
absorbate stream, typically between 1% and 30% by weight is routed
via conduit 30 through a cooler 31 wherein the temperature of the
fat absorbate stream is reduced to a temperature between ~3C and
40C before entering the top of the stripping zone 20 along line 32.
By splitting the flow of the fat absorbate stream to the stripping
; zone wherein the cooled portion enters the top of the stripping zone
and the uncooled fat absorbate ~tream enters the stripping column
at a point intermediate between the top of the column and the point
at which the acid gas sidedraw stream leaves the column, the acid
gas content of the stripper overhead stream exiting the stripper
along line 24 is reduced. The cooled absorbate from line 32 achieves
this reduction in acid gas content of overhead stream 24 as a
result of a more favourable equilibrium. A lower temperature
generally increases the capacity of the absorbent for acid gases.
For volatile absorbents, the lower temperature also reduces the
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loss of absorbent in stream 2~. The acid gas-rich, hydrocarbon-
lean stream is withdrawn from the stripping zone along line 23
a~ a sidedraw stream. Stripping action in the stripping zone is
provided by steam via conduit 22 or alternatively by reboil heat
or the like. The lean absorbate stream exits the stripper 20 as
the bottoms stream 21 and is routed to the scrubbing zone 10. It
is important to withdraw only as much heat in the cooler 3t as
is required to reduce the acid gas content of the stripper over-
head stream and the hydrocarbon content of the strippe~r sidedraw
stream to acceptable levels, since whatever heat that is with-
drawn in the cooler 31 must be supplied by the steam entering the
stripper along line 22 or by other reboil heat, if the fraction-
ation efficiency of the stripping zone is to be maintained.
Figure 3 reveals an alternative embodiment of the present in-
vention wherein a portion of the lean absorbate stream withdrawn
as the bottoms stream from the stripping zone 20 is recycled to
the top of the stripping zone and the fat absorbate stream enters
the column at a point intermediate between the top of the column
and the point at which the acid gas sidedraw stream leaves the
column. Referring to Figure 3, the acid gas-rich, fat absorbate
stream containing small amounts of entrained and/or dissolved
hydrocarbon is routed from the scrubbing zone tO via conduit 13
to the stripping zone 20. Stripping action is provided in the
stripping zone 20 by steam introduced along line 22 or alternatively
by heating coils or a reboiler. Betw~en about 1% and about 20%, prefer-
ably between about 1% and about 10% of the lean absorbate stream
taken as the bottoms stream from the stripping zone 20 along line
21 is routed back to the stripping zone along lines 40 and 42,
the remainder being routed to the scrubbing zone 10 along line 27.
The lean absorbate stream being routed directly to the top of the
stripping zone reduces the acid gas content of the stripper overhead.
An acid gas-rich, hydrocarbon-lean stream 23 is withdrawn from the
stripping zone as a sidedraw stream and a hydrocarbon-rich, acid
gas-lean Rtream 24 is withdrawn as the overhead product. Cooling of
stream 40 in cooler 41 prior to its entry to the top of stripping
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zone 20 along line 42 is a possible alternative. Doing so would
reduce the loss of volatile solvent components into stream 24 if
such are present in the absorbate.
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