Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention is direeted to a process or
the recovery of hydrocarbons from a waste gas stream which
is no~nally burned in a flarestack.
In the processing and recovery of hydrocarbons all
S plants are equipped with flare systems whereby the hydro~
carb~n can be burned. Such flare systems usually handle the
hydrocarbons as gaseous compon nts and are used to dispose
of waste materials, mixtures of materials obtained from
various locations throughout the plant and to dispose of
ma~erials during emergencies or upsets in pla~t operations.
The flare systems must operate at essentially atmospheric
pressure and wi~h no means capable of generating a back
pressure in the system. It has long been known that burning
of the materials in the flare system may be wasteful in
terms of loss of the heating v~lue t~ereof but little could
be done abou~ such waste because of the extreme variability
; of flow rates and frequently of composition of the gas stream
: to the flare and the concurrent need ~o have the flare system
operate without interruption as the qafety outlet for the
;: 20 plant.~ .
: I have now discovered a proc~ss ~or the recover~ of
hydrocarbons from the 1are gas system of a hydrocarbon
proc~ssing plant so that at least a portion o the hydro-
carbons may be returned to the hydrocarbon processing plant
:25 and~ a further portion of the hydrocarbons may be used as
fuel gas or retuxned to the flare gas system.
According to my invention, there is p~ovided a
pro ess for the recovery of C~ hydrocarbons from a flare gas
system of a C4~hydrocarbon processing plant, in which at
30~: least~a~portion~of the ~flare gas is removed from the flare :
gas system and supplied to the suction of a compressor, the
suction pr~ssure being from about O.OlS to about O.OS kg~cm2,
the gas is compressed to a pressure of from about 5.5 to
about 8.5 kg/cm and cooled to a temperature o from about
2 to about 15C whereby liqueflable hydrocarbons are lique-
fied, the liquefied hydrocarbons are separated for return to
the C4 hydrocar~on processing plant and the non-liquefied
hydrocarbons are either returned to the flare gas system or
used a~ fuel gas.
A ~ypical C4 hydrocarbon stream from a refinery
operation will comprise a mixture of the bu~anes, butenes
and isobutylene, butadienes, small amounts of the acetylenes
and small amounts of ~arious C2, C3 and C5 hydrocarbons. In
a typical C4 hydrocarbon processing plant, the isobutylene
and butadienes are separated as pure materials from the C~ ¦
~: stream or use as monomers in subsequent polymerization or
for use in a variety of chemical reactions and the remaining
hydrocarbons may be returned to the refinery operation.
Depending on the uses to be made of the C4 hydrocarbons, one
or more of the butenes may be separated out as a pure .
: material. In all of these operations, small quantities of
wa~te material are continuously generated, such as by col- ,~
lection of material from leaks, by purgin~ operations or as
residual materials from various steps o~ the separation
stages. These small quantities of waste material continuously
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generated can build, over a twenty four hour period, to be
quite sizeable quanti~ies and are absolutely ~navoidable in
a commercial operation. Also during such separati.on
: operakions, process upse~s or equipment malfunctions may .
; 30 occur~which, for safety reasons, re~uire one or more streams
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to be partially or completely vented to the flare gas system.
Reference is made to the Figures where Figure 1 is a
schematic diagram of a conventional flare gas system, and
Figure 2 is a schematic diagram of a flare gas system modi-
fied to incorporate an embodiment of the present invention.
With reference ~o Figure 1, flarestack l is supplied
wi~h the com~ustible gas by line 2. Seal drum 3 is main-
tained fill~d with water 4 to a predetermined level and in-
let line 5 ends below the wa~er level so that ~here is a
predetermin~d back pressu.re equi~alent to the height o water
covering line 5, this height of water usually being from
; about 15 cm to about 35 cm thereby causing a back pressure
of about 0.02 to about 0.06 kg/cm2. Inlet line 5 is
connected to line 6 which in t~rn collects the gas from the
liquid knock out drum 7. The various sources o~ hydrocarbons
are collected in line 8 and thereby fed to the liquid knock
out drum. Many minor variations are known of such systems
but they all incorporate as essential components a liquid
knock out drum to ensure that only gas i5 supplied to the
flare6ta~k and a seal dru~ to main~ain slight positive
pressure in the 1are lines.
In Figure 2 there îs shown a modified flarestack
sys~em with an embodi~ent of the present invention incor-
porated. The various supplies of hydrocarbon are collected
~25 and fed to line 17 whichsupplies them to the liquid knock
out drum 16.: The gas from drum 16 is passed by line 15 to
seal drum 12 in which the inlet pipe 14 terminates below the
water level 13. T~e gas flows by 7ine 11 to the flarestack
10. Attached to line 15 is line 18 whereby all or a por~ion
of ~he. gas flowing in l m e 15 may be removed. Line 18 is ; :
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connected to a further liquid knock out drum 19 and the gas
from this knock out drum is fed by line 20 to the first stage
of a compressor 22. Liquid knock out drum 19 is not essen-
tial but serves to protect the compressor from receiving any
liquid: any liquid collected in knock out drum 19 is removed
through line 21 by pump 31 and is fed through line 32 into
line 30. The first s~age of compression compresses the gas
to a pressure of about 1.5 to about 3.5 kg/cm2, preferably
about 2 to 3 kg/cm2, and the gas is ~hen passed through heat
10 exrhanger 23 to liquid knock out drum 24. Th~ heat exchanger
23 will be supplied with cold water to cool the compressed
gas to about lS to about 20C. The gas from liquid knock
. out drum 24 is fed by line 25 ~o a second stage of a
: compressor 27, the outlet pressure of which is from about 5.5
to about 8.5 kg/cm2, preferably from about 6.5 to about
8 kg/cm2. Liquid from knock out drum 24 is recycled by line
26 back to knock out drum 19. The compressed gas then passes
through line 28 to heat exchanger 29, which is preferably a
: water cooled exchanger, and then by line 30 to heat exchanger
33, which is preferably a liquid ammonia cooled exchanger and
by line 34 into sep~ration ~es~el 35. The gas/liquid mixture
: in line 34 will be at a temperature of rom about 2 to about
15C, most preferably from about 2 to about 7..5C. Separ-
ation vessel 35 acts to separate the liquefiable hydrocarbons
25. from the non-liquefiable hydrocarbons, the gaseous non-
liquefiable hydrocarbons being removed by line 37 and
lique~ied hydrocarbons being removed by line 36. The gaseous
hydrocarbons in line 37 may be directed to the flarestack
: system or may be directed to a co~bustion facility for use
as fuel gas (not shown). The liquefied hydrocarbons are
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returned to the C~ hydrocarbon~ processin~ plant (not shown)
where isobutylene, butadiene and optionally one or more of
the butenes may be recovered.
In a preferred process of my invention, the hydro-
carbon stream ~s co~pressed to a final pressure of 6.5 to
8 kg/cm2 and cooled preferably to 2~ to lO~C, most preferably
2~ to 7.5C. The compressing and eooling of the hydroearbon
stream may be achieved wi~h a single stage of compression
and cooling but preferably is achieved with two stage com-
pression with an intermediate stage of cooling and a sub- -
sequent two stages of cooling. The liquefiable hydrocarbons
preferably comprise a ma~or proportion of C4 hydrocarbons
and a minar proportion, ~Ip to about 7.5 weight per cent, of
C3 hydrocarbons. ThP non~ uefiable hydrocarbons preferably
comprise a ~ajcr proportion of a mixture of ~ethane and C~
hydrocarbons and a mi~or proportion, up to about 15 weight
per cent, of C3 hydrocarbons. Most preferably, the non-
li~ueflable hydrocarbons comprise from 15 to 60 weight per
cent of methane and from 30 to 60 weight per cent of C4
hydrocarbons,~ up to 15 weight per cent of C3 hydrocarbons
- and up to 5 weight per cent of other hydrocarbons. The 1are
gas stream preferably comprises a major proportion of a
mixture of methane, C3 hydrocarbons and C~ hydrocarbons and
most preerable comprises rom 70 to 85 weight per cent of
C4 hydrocarbons. Preferab~y the weight ratio of liquefiable
hydrocarbons ~to non-liquefiable hydrocarbons is from about
2:1 up to about 4:1. Ho~ever, the process of the present
invention will work at essentially any such ratio providing
that the equipment i~s appropriately sized.
: 30 AB an example o the process o the invention, a
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flare gas hydrocarbon recovery system was installed according
to Figure 2, in which compressors 22 and 27 are driven by a
single drive ~echanism and heat exchangers 23 and 30 are
water cooled and heat exchanger 33 is liquid ammonia cooled.
S The flarestack stream flow rate ranged fxom about 4,000 to
about 20,000 kg per day, the liquefiable hydrocarbons re-
covered ranged from about 2,500 to abou~ 15,000 kg per day
and the non-liquefiable hydrocarbons tran~ferred to a boiler
system for use as fuel gas ranged from about 1,250 to about
S,000 kg per day. With regard ~o the liquefied hydrocarbons
recovered, the butadiene-1:3 content ranged from about 15 to
about 60 weight per ~ent and the isobutylene content ranged
from about 15 to about 60 weight per cent of the total
liquefied hydrocarbons. With regard to the non-liquefiable
hydrorarbons recovered, the methane content ranged from about
15 to about 55 weight per cent, the C4 hydrocarbon content
ranged from about 30 to about 60 weight per cent, and the C3
hydrocarbon eontent ranged from about 1 to ab~ut 15 weigh~
per cent of the total non-liqueiable hydrocarbons. The
process operated satis~actorily and was brought into
operation and shut o~f without affecting the safety aspects
of the flarestack operation.
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