CA1107190A

CA1107190A - Hydrocarbon recovery

Info

Publication number: CA1107190A
Application number: CA341,332A
Authority: CA
Inventors: Donald D. Livingstone
Original assignee: Polysar Ltd
Current assignee: Polysar Ltd
Priority date: 1979-12-06
Filing date: 1979-12-06
Publication date: 1981-08-18
Also published as: AU539274B2; GB2066936B; BE886491A; AU6504280A; FR2471568B1; GB2066936A; FR2471568A1

Abstract

"HYDROCARBON RECOVERY"
Abstract of the Disclosure A process is provided for the recovery of hydro-carbons from a gas stream normally supplied to a flarestack for combustion therein, which process does not interfere with the safe operation of the flare gas system. The process yields a liquid hydrocarbon stream and a gaseous hydrocarbon stream and provides a means for conserving hydrocarbons.

Description

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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 .
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 :~ :

~ 3 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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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed, are defined as follows:

1. A process for the recovery of C4 hydrocarbons from a flare gas system of a C4 hydrocarbon processing plant in which at least a portion of the flare gas is removed from the flare system and supplied to the suction of a compressor, the suction pressure being from about 0.015 to about 0.05 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 of from about 2° to about 15°C whereby liquefiable hydrocarbons are liquefied, the liquefied hydrocarbons are separated for return to the C4 hydrocarbon processing plant and the non-liquefied hydrocarbons are either returned to the flare gas system or used as fuel gas.

2. The process of Claim 1 wherein the flare gas is compressed in two stages of compression with an intermediate cooling stage,

3. The process of Claim 2 wherein the compressed gas is finally cooled in two stages of cooling.

4. The process of Claim 3 wherein the gas is compressed to a final pressure of from 6.5 to 8 kg/cm2 and finally cooled to a temperature of from 2° to 10°C, preferably from 2° to 7.5°C.

5. The process of Claim 1 wkerein the flare gas comprises a major proportion of a mixture of methane, C3 hydrocarbons and C4 hydrocarbons.

6. The process of Claim 5 wherein the flare gas comprises from 70 to 85 weight per cent of C4 hydrocarbons.

7. The process of Claim 5 wherein the liquefiable hydrocarbons comprise a major proportion of C4 hydrocarbons and a minor proportion, up to about 7.5 weight per cent, of C3 hydrocarbons.

8. The process of Claim 5 wherein the non-liquefiable hydrocarbons comprise a major proportion of a mixture of methane and C4 hydrocarbons and a minor proportion, up to about is weight per cent, of C3 hydrocarbons.

9. The process of Claim 8 wherein the non-liquefiable hydrocarbons comprise from 15 to 60 weight per cent of methane, 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.

10. The process of Claim 1 wherein the weight ratio of liquefiable hydrocarbons to non-liquefiable hydrocarbons is from about 2:1 up to about 4:1.