GB2243090A

GB2243090A - Hydrocarbon vapour recovery

Info

Publication number: GB2243090A
Application number: GB9006027A
Authority: GB
Inventors: Georg Rettig
Original assignee: IT MCGILL Ltd
Current assignee: IT MCGILL Ltd
Priority date: 1990-03-16
Filing date: 1990-03-16
Publication date: 1991-10-23
Also published as: GB9006027D0

Abstract

Hydrocarbon vapour contamination is removed from an air mixture, especially from air vented from a tank previously containing liquid hydrocarbon, by reducing the hydrocarbon content of the contaminated air mixture (10) to less than 10 grams per cubic meter at STP in a first stage (12, 16) and then supplying the output (26) from the first stage to a diesel engine (28), which has a separate fuel supply (29). The output from the first stage forms at least a part of the air supply to the engine. The first stage is advantageously an adsorption process or plant in accordance with GB-A-1564464. The output (37) from the engine may be used to drive pump (48) which produces a vacuum in bed (12) or (16) during desorption of hydrocarbon therefrom, or to drive pump (47) which is used to withdraw recovered hydrocarbon liquid from absorber (70), or to generate electricity. <IMAGE>

Description

Hydrocarbon Vapour Recovery This invention relates to a process for the recovery of hydrocarbon components in an air/hydrocarbon vapour mixture.

Such recovery is desirable in situations where tanks and containers are refilled with liquid hydrocarbon, for example petroleum spirit, and the air vented from the container in the filling process is laden with hydrocarbon vapour. Apart from the waste of hydrocarbon if this air was vented directly to atmosphere, many countries have laws proscribing such venting because of its pollution of the environment.

There are numerous methods currently available to recover hydrocarbons from vented air. One such method is to cool the vapour so that the hydrocarbons condense. Another is to compress the vapour, again, so that the hydrocarbons condense. Another involves the use of semi-permeable membranes to concentrate the hydrocarbons in the mixture. And a fourth involves the adsorption of the hydrocarbons onto an adsorbing medium. Each of these methods is variously successful, but all of them leave a residual component of the hydrocarbons in the final air stream. However, venting of even minimal amounts of hydrocarbon is in some countries proscribed, and so it is necessary in some instances to render the vented gas almost entirely free of hydrocarbon.

This last stage has usually been achieved by means of catalytic burning of the remaining hydrocarbons which, although it is effective, suffers from the drawback that operators have a natural fear of high temperatures in the locality of large volumes of petroleum spirit.

Also it is rather wasteful of the potential energy in the residual hydrocarbons.

So one alternative which has been suggested in EP-A-0222158 is to employ the product of a first stage of vapour recovery in a gas engine whose own output is employed to provide the motive power for the first stage. In a refrigeration process, for example, the gas engine would drive the refrigerant compressor. In a compression process, it would drive the compressor.

In practice, however, such a process is fraught with its own problems.

The main problem is one of control. The vented air from tanks being refilled is never of constant hydrocarbon composition. Thus it is necessary at some point to employ a gasometer to store the gas and mix it, so that it becomes of constant composition. The gasometer is nowadays usually in the form of a bladder tank having a membrane which contains the hydrocarbon mixture.

The gasometer may be disposed before the first stage of hydrocarbon recovery, so that that stage need not be so large as to cope with the likely peak load of hydrocarbon contamination in the air mixture.

Or it may be disposed on the output of the first stage, so as to ensure that the input to the gas motor is constant, particularly if the first stage is such that a consistent output from it cannot be guaranteed, even if its own input is relatively constant. But in the latter case (which is preferred), it is necessary to ensure that the first stage is capable of handling anticipated peak loads. In either event, however, a bladder tank is necessary and this, of itself, presents problems. Firstly they are large and expensive, and secondly they leak. Thirdly, at some point in the process there is bound to exist a volume of hydrocarbon-containing-air, which must be processed, and which is at such a concentration to render it explosive. This is not a condition which operators like.

However, beyond all this, is the necessity to provide a gas engine which must be considerably modified to cope with running on the output from the first stage. These are not insignificant problems. For instance, depending on the process employed in the first stage, the output from it is likely to be variable, hence the desirability of a bladder tank after the first stage. However, all the presently known systems are efficient at removing heavier hydrocarbons and less efficient at removing lighter ones. Consequently, if the gas engine is a petrol or diesel engine, the output will need to have its octane value enriched in order for the engine to run smoothly. In any event, sophisticated control mechanisms need to be employed.

Thus it is an object of the present invention to provide a process for the recovery of hydrocarbon vapour from contaminated air which does not suffer from, or at least mitigates the effects of, the aforementioned problems.

In accordance with the present invention, there is provided a process for removing hydrocarbon contamination from an air mixture, comprising reducing the hydrocarbon content of the contaminated air mixture to less than 10 grams per cubic meter at STP in a first stage and supplying the output from said first stage to a heat engine, arranged with a separate fuel supply, and in which said output from said first stage forms at least a part of the air supply to said engine.

STP as used herein means standard temperature and pressure, that is to say, 20C and 105 Newtons per square metre.

Thus the object of the invention is achieved by so reducing the hydrocarbon content at the output of the first stage that, when the output is employed as the air supply for the gas engine, the hydrocarbons left in the output are insufficient materially to affect the performance of the engine. Consequently, these hydrocarbons can be ignored for all practical purposes, except that they will be burnt in the engine, thus achieving the primary purpose of the whole system, ie the removal of substantially all hydrocarbon from the ultimate exhaust of the process.

Moreover, much of the peripheral requirements associated with the process described in EP-0222158, can be dispensed with. For example, the bladder tank, and the sophisticated control mechanisms for the heat engine, are not needed with the present invention. Furthermore, the engine can be chosen, not on the basis of the fuel on which it must be capable of running, but on the basis of whatever task or purpose it is desired to employ the engine.

The engine may comprise a single diesel engine, for example, or indeed a plurality of smaller engines.

The invention finds particular application when the first stage is an adsorption process, because this process is particularly efficient at reducing hydrocarbon content in a single step. GB-A-1564464 describes such a system, wherein contaminated air is fed to one of a pair of adsorption beds in which hydrocarbons are adsorbed onto activated carbon forming the beds and substantially hydrocarbon free air is vented to atmosphere. In the present invention, such air is not vented to atmosphere but is instead passed to an engine, for example a diesel engine, which is supplied with its own fuel, as the air intake of such engine.

In GB-A-1564464, when a bed approaches saturation, the feed of contaminated air is switched to the second bed, and the first bed is regenerated by vacuum stripping. For this purpose a vacuum pump is provided and the gaseous output from that is fed to an absorber where recovered hydrocarbon is absorbed by liquid absorbent.

It may be desirable to employ some damping means on the output of the absorber, or indeed on the output from the adsorbers, to control the extra plugs of vapour input to an adsorber when it is in adsorption mode and when the other adsorber is initially being regenerated. This damping means may simply take the form of an expansion vessel disposed in the line feeding the engine or in the line returning from the absorber.

The engine of the present invention can be employed to operate said vacuum pump, although it is a feature of the adsorption system described in said patent, that the energy requirements for the vacuum pump are likely to be less than the energy provided by the engine, given the size of engine which would be required in order to cope with the anticipated air volume issuing from the adsorber.

Consequently it is anticipated that the engine might be employed as an electrical generator for connection to the grid, and the vacuum pump be driven by its own motor, for example an electric motor.

Where the engine comprises a plurality of engines, one may be employed to drive the vacuum pump, while the others are employed elsewhere. One advantage with a plurality of smaller engines is that this introduces flexibility and also more security in the event that one should break down, since one engine could be held in reserve and employed as a back-up.

It should also be appreciated that the present invention is not less energy efficient than the system described in EP-A-0222158, merely because the present invention requires a separate fuel supply for its engine. In fact, it is equally efficient, because it requires the first stage to recover more of the hydrocarbon from the contaminated air; hydrocarbon which otherwise would be burnt in the engine. The advantage with the present invention is that, a standard engine and/or generator setup can be employed without any modification beyond a simple ducting arrangement to direct output from the first stage to the air intake of the engine. It is also desirable to provide a separate air intake for the engine if the first stage is shut down for any reason.

While the invention finds particular application with an adsorption system, it is equally applicable to other systems if these can be arranged to produce an output of less than 10 grams per cubic metre at STP. With such other systems, whose energy requirements are larger (particularly to achieve the reduction in hydrocarbon content required by the first stage of the present invention), the energy available from the engine might usefully be employed to drive the first stage, as indeed suggested in EP-A-0222158. However, in that event, no energy is available for use elsewhere or to be sold to the electricity generating authorities, for example.

Furthermore, as mentioned above, another advantage with the present invention is that, providing the first stage is adapted to cope with peak loads of hydrocarbon contamination in the feed air mixture, no bladder tank or other gasometer is required, so that this major cost and inconvenience can be avoided. Also, with a system according to the present invention, the danger of having to process large volumes of gas in an explosive mixture is avoided.

The invention is further described hereinafter, by way of example only, with reference to the accompanying drawing, in which a system according to the present invention is schematically illustrated.

In the drawing, line 10 is connected from a source of light hydrocarbon contaminated air, such as that expelled from a gasoline storage tank, to the lower portion of adsorbent vessel 12. Line 14 is connected between line 10 and the lower portion of a second adsorbent vessel 16. Valve 18 is located in line 10 between adsorbent vessel 12 and the juncture of lines 10 and 14. Valve 20 is located in line 14. The adsorbent vessels are of standard design for holding solid adsorbent material, typically activated carbon, capable of selectively adsorbing the hydrocarbon components from an air-hydrocarbon mixture introduced therein. Lines 22 and 30 are connected from the upper portions of adsorbers 12 and 16 respectively to check valves 24 and 32 respectively. Check valves 24 and 32 are connected to line 26.

Line 26 supplies the air intake of a diesel engine 28, which is supplied with fuel from a source 29 thereof. The engine 28 is of sufficient size, and is arranged to run at sufficient speed, that its demand for air is substantially the same as, or exceeds, the rate of flow in line 26. In the event that the engines demand does exceed that supplied in line 26, for instance especially when the plant is inoperative, it can obtain further air via check valve 33 from a fresh air source 35. The exhaust from the engine 28, which will be free of hydrocarbons, is vented to atmosphere.

The engine 28 needs no modification from a standard, widely available, set; except in respect of the air-intake arrangements which must be made. The efficiency of the adsorption system described herein is such that the output in line 26 can be arranged to contain less than 5 grams per cubic metre of air at STP, and so the fuel value of this air is negligible compared with the 60-80 grams per cubic metre needed to run the engine. Consequently no modification is required to the control systems of the engine.

What use is made of the output 37 of the engine 28 is somewhat arbitrary. It may be employed to generate electricity, for sale to the local power generating authority, for instance, or it may be employed to drive pumps 48 and 76, described further below. However, it is a feature of the adsorption system described herein, that its own energy requirements are fairly small, and unlikely to employ the whole output of the engine 28 when it is processing the full flow of air supplied in line 26.

Referring again to the adsorption process, line 34 is connected to line 10 between valve 18 and adsorber 12 and to valve 38. Line 36 is connected to line 14 between valve 20 and adsorber 16, and to valve 40. Valves 38 and 40 are connected to line 46 via lines 42 and 44 respectively. Line 14 is connected to the suction port of a vacuum pump 48, and the pump discharge line 50 is connected between the pump 48 and a separator 52.

The hydrocarbon contaminated air is introduced to the process at a pressure slightly above atmospheric, and at the temperature of the mixture source, which is normally ambient, through line 10 to the lower portion of the adsorber bed 12 by opening valve 18 and closing valves 20 and 38. As the air-hydrocarbon mixture flows through the adsorbent packed in the vessel, substantially all of the hydrocarbon is adsorbed therefrom, and clean air is expelled from the top of the adsorber through line 22 and check valve 24. From check valve 24, the air passes through line 26. As is apparent, check valve 32 prevents effluent air from adsorber 12 from entering the top of the adsorbent bed 16.

Prior to reaching the saturation point of the adsorbent material in adsorbent vessel 12, valves 18 and 40 are closed, and valve 20 is open to route the air-hydrocarbon mixture from line 10 through line 14 to adsorber 16 for use during the regeneration of the adsorbent in adsorber 12.

Pump 48 is a liquid ring vacuum pump capable of producing a near vacuum in either adsorbent bed during regeneration. The use of a liquid ring pump Is advocated to minimise risks of explosion. Valve 38 is opened, and the vacuum produced by pump 48 in the adsorbent vessel desorbs the hydrocarbon from the adsorbent to produce a rich air-hydrocarbon mixture comprised of approximately 85 to 90% hydrocarbon by volume. As this rich air-hydrocarbon mixture comes in direct contact with pumping liquid used by the pump, cooling of the air-hydrocarbon mixture will occur, and a portion of the heavier hydrocarbon components therein will condense.

Effluent from pump 48 is admitted to separator 52 via line 50.

Separator 52 is a vessel operated slightly above atmospheric pressure and designed to separate the vapour and liquid components of the pump effluent and to further separate this immiscible liquid used by the liquid ring pump from any recovered hydrocarbon liquid condensed by the inherent cooling action of said pump. The liquid components of the pump effluent are separated by means of a weir 54 located in the bottom portion of the separator 52 over which the lighter hydrocarbon liquid may flow. The heavier pumping liquid, typically water, trapped by the weir, is withdrawn from the bottom of the separator 52 by line 56 and is returned to the pump 48 in lines 56,60 via a cooler 58.

Recovered liquid hydrocarbon overflowing the weir 54 is withdrawn from separator 52 through line 62 connected between the lower portion of separator 52 and line 74 for use as a liquid absorbent or return to a fuel supply.

The vapour phase of the pump effluent is withdrawn from the separator 52 through line 64 connected between the upper portion of the separator and the bottom portion of absorber 70. Absorber 70 is a conventional absorber operated near ambient temperature and slightly above atmospheric pressure and may be of either tray or packed tower design. In absorber 70, the vapour from separator 52 comes in direct counter current contact with hydrocarbon liquid introduced at the upper portion of the absorber via line 72.

The hydrocarbon components in the vapour are substantially absorbed.

Recovered hydrocarbon liquid is withdrawn from the bottom of the absorber through line 74 by pump 76. The recovered hydrocarbon liquid is discharged from the pump through line 78, and either a portion of said recovered hydrocarbon liquid forms the supply of absorbent in line 72, via line 80 and cooler 82, or else it is passed to a store and a separate, or the same, store 73 supplies absorber 70.

Vapour from the top of the absorber, comprising approximately 20 to 30% hydrocarbon by volume, passes by line 84 from the top of the absorber to line 10 for recycling through the adsorption bed 16. It is readily observed that this embodiment permits the continuous removal of hydrocarbon from hydrocarbon contaminated air as a result of operating two adsorption beds in reciprocal modes. After the desired level of reactivation is attained in adsorption bed 12 said bed may again be placed in the adsorbent mode, and adsorption bed 16 may be regenerated at that time.

While the invention has been described with reference to specific elements and combinations of elements, it is envisaged that each element may be combined with any other or any combination of other elements. It is not intended to limit the invention to the particular combinations of elements suggested. Furthermore, the foregoing description is not intended to suggest that any element mentioned is indispensable to the invention, or that alternatives may not be employed. What is defined as the invention should not be construed as limiting the extent of the disclosure of this specification.

Claims

CLAIMS 1. A process for removing hydrocarbon contamination from an air mixture, comprising reducing the hydrocarbon content of the contaminated air mixture to less than 10 grams per cubic meter at STP in a first stage and supplying the output from said first stage to a heat engine, arranged with a separate fuel supply, and in which said output from said first stage forms at least a part of the air supply to said engine. 2. A process as claimed in claim 1, wherein said first stage is an adsorption process comprising a pair of adsorption beds operated in turn to adsorb the hydrocarbons in the air mixture, one bed being regenerated by vacuum desorption while the other bed is in an adsorption mode. 3. A process as claimed in claim 2, wherein gaseous effluent of said vacuum desorption is fed to an absorber supplied with liquid hydrocarbon absorbent and which absorbs most of the hydrocarbons in said gaseous effluent, vapour issuing from said absorber being recycled as part of the input to said other adsorption bed. 4. A process as claimed in claim 2 or 3, wherein said vacuum desorption is performed by a vacuum pump driven by energy produced by said engine. 5. A process as claimed in any preceding claim, wherein said engine is a diesel engine. 6. A process as claimed in claim 4 and 5, wherein said diesel engine is part of a standard generator set, whose electrical output is directly or indirectly employed to drive said vacuum pump. 7. A process as claimed in any preceding claim, wherein said engine comprises a plurality of engines. 8. A process as claimed in claim 7, and in any of claims 4 to 6, wherein said vacuum pump is driven by one of said plurality of engines, the remaining engine or engines being employed elsewhere. 9. A process as claimed in any preceding claim, wherein said first stage reduces the hydrocarbon content of the contaminated air mixture to less than 5 grams per cubic meter at STP. 10. A process substantially as hereinbefore described with reference to the accompanying drawing. AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS.

1. A process for removing hydrocarbon contamination from an air mixture, comprising reducing the hydrocarbon content of the contaminated air mixture to less than 10 grams per cubic meter at STP in a first stage and supplying the output from said first stage to a heat engine, arranged with a separate fuel supply, and in which said output from said first stage forms at least a part of the air supply to said engine.

2. A process as claimed in claim 1, wherein said first stage is an adsorption process comprising a pair of adsorption beds operated in turn to adsorb the hydrocarbons in the air mixture, one bed being regenerated by vacuum desorption while the other bed is in an adsorption mode.

3. A process as claimed in claim 2, wherein gaseous effluent of said vacuum desorption is fed to an absorber supplied with liquid hydrocarbon absorbent and which absorbs most of the hydrocarbons in said gaseous effluent, vapour issuing from said absorber being recycled as part of the input to said other adsorption bed.

4. A process as claimed in claim 2 or 3, wherein said vacuum desorption is performed by a vacuum pump driven by energy produced by said engine.

5. A process as claimed in any preceding claim, wherein said engine is a diesel engine.

6. A process as claimed in claim 4 and 5, wherein said diesel engine is part of a standard generator set, whose electrical output is directly or indirectly employed to drive said vacuum pump.

7. A process as claimed in any preceding claim, wherein said engine comprises a plurality of engines.

8. A process as claimed in claim 7, and in any of claims 4 to 6, wherein said vacuum pump is driven by one of said plurality of engines, the remaining engine or engines being employed elsewhere.

9. A process as claimed in any preceding claim, wherein said first stage reduces the hydrocarbon content of the contaminated air mixture to less than 5 grams per cubic meter at STP.

10. A process substantially as hereinbefore described with reference to the accompanying drawing.

11. Apparatus for removing hydrocarbon contamination from a hydrocarbon/air mixture, comprising a first stage adapted to reduce the hydrocarbon content of the contaminated air mixture to less than 10 grams per cubic meter at STP, a heat engine having a separate fuel supply, and means for supplying the air output from said first stage to said engine as at least a part of the air supply thereto.

12. Apparatus as claimed in claim 11, wherein said first stage is an adsorption process comprising a pair of adsorption beds adapted to be operated in turn to adsorb the hydrocarbons in the air mixture, one bed being regenerated by vacuum desorption while the other bed is in an adsorption mode.

13. Apparatus as claimed in claim 12, comprising means for feeding gaseous effluent of said vacuum desorption to an absorber supplied with liquid hydrocarbon absorbent for absorbing most of the hydrocarbons in said gaseous effluent, and means for recycling vapour issuing from said absorber as part of the input to said other adsorption bed.

14. Apparatus as claimed in claim 12 or 13, comprising a vacuum pump adapted to be driven by energy produced by said engine for performing said vacuum desorption.

15. Apparatus as claimed in any one of claims 11 to 14, wherein said engine is a diesel engine.

16. Apparatus as claimed in claim 14 and 15, wherein said diesel engine is part of a standard generator set, whose electrical output is directly or indirectly employed to drive said vacuum pump.

17. Apparatus as claimed in any one of claims 11 to 16, wherein said engine comprises a plurality of engines.

18. Apparatus as claimed in claim 17, and in any one of claims 14 to 16, wherein said vacuum pump is adapted to be driven by one of said plurality of engines, the remaining engine or engines being employed elsewhere.

19. Apparatus as claimed in any one of claims 11 to 18, wherein said first stage is adapted to reduce the hydrocarbon content of the contaminated air mixture to less than 5 grams per cubic meter at STP.

20. Apparatus substantially as hereinbefore described with reference to the accompanying drawing.