WO1997000718A1

WO1997000718A1 - Vapour recovery process and unit

Info

Publication number: WO1997000718A1
Application number: PCT/GB1996/001495
Authority: WO
Inventors: Christopher Jerome Lunt
Original assignee: Vistair Engineering Limited; Christopher Jerome Lunt
Priority date: 1995-06-23
Filing date: 1996-06-21
Publication date: 1997-01-09
Also published as: GB9512864D0

Abstract

A process, and a unit (10) to give effect to the process, is provided for the recovery of hydrocarbon from a contaminated mixture (12) of air and hydrocarbon vapour. The contaminated mixture is passed through an inlet (20) to a first solid adsorbent bed (24). The bed is capable of selectively adsorbing hydrocarbon components from the contaminated mixture to leave substantially pure air (32). Hydrocarbons are then desorbed from the first bed when the bed becomes saturated. This is achieved by lowering the pressure of the bed by vacuum pumping (50) so as to produce a rich air/hydrocarbon vapour mixture. The rich mixture is fed to an absorber (54, 70) in which most of the hydrocarbons in the rich mixture are absorbed in a liquid absorbent. Overhead gas (76) of the absorber is recycled to a second adsorbent bed (26) through a venturi ejector orifice (16, 78) in said inlet.

Description

Vapour Recovery Process and Unit

This invention relates to a process for the recovery of hydrocarbon from a mixture of air or other gas (eg a process gas such as nitrogen) contaminated with hydrocarbon vapour. This situation particularly arises when a tank is filled with liquid petroleum and the air previously in the tank is vented. The present invention also relates to a unit for such recovery.

GB-A- 1564464 describes a process in which a contaminated mixture of air and hydrocarbon vapour is passed through a first solid adsorbent bed operated at a pressure above atmospheric, and capable of selectively adsorbing hydrocarbon components from the mixture to leave substanially pure air, which is vented to atmosphere. When the bed becomes substantially saturated with hydrocarbons, the hydrocarbons are desorbed from the bed by lowering the pressure of the bed by vacuum pumping. This produces a rich air/hydrocarbon vapour mixture, which is then passed to an absorber in which most ofthe hydrocarbons in the rich mixture are absorbed in a liquid absorbent. The absorber overhead gas is recycled to a second adsorbent bed.

Switching arrangements between the two beds switch the supply of contaminated mixture from the saturated bed to a previously stripped bed, and initiates stripping ofthe saturated bed. A problem with this arrangement is that very soon after each switching operation, the pressure at the inlet to the new bed tends to rise as the recycle from the absorber starts to come through as a result of commencement of stripping ofthe saturated bed. This rise in pressure is felt as a back pressure in the tank being filled. In certain circumstances, this could cause leakage and pollution. For example, some older rail car tanks and bulk storage tanks cannot be sealed effectively and/or cannot withstand higher pressures, so that restrictions on the flow of air being vented from the tank as the tank is filled must be minimised to keep the pressure in the tank within prescribed limits.

One known solution to this problem is to make the inlet piping to each adsorber larger than would otherwise be necessary, so that the pressure rise is reduced. Pressure can also be reduced by arranging for the adsorbent beds to have a larger diameter. This latter measure, however, reduces adsorption efficiency and increases emissions of hydrocarbon from the adsorbers.

Another known solution is to install an induced draft fan at the outlet ofthe adsorber. This is usually arranged to operate continuously with appropriately controlled valving to maintain a constant inlet pressure or flow. Again, however, not only does this introduce complications regarding control, it may also reduce working pressures periodically in the adsorber and hence reduce adsorption efficiency. Moreover, it is necessary to provide additional pipework to manifold the adsorber outlets together, so that only one fan is required, and also precautions may need to be taken to minimise any spark hazzard the fan may represent.

A further problem with vapour recovery units ofthe type to which the present invention relates is that, on each regeneration of each adsorber, a mass of air (equal to that contained, substantially at atmospheric pressure, in the volume ofthe adsorber, less the volume occupied by the adsorbent, plus any purge air permitted to enter the adsorber towards the end of its regeneration), is drawn out ofthe adsorber, only for this mass of air subsequently to be recycled after passing through the absorber. On its return, it is saturated with hydrocarbon from the absorber. If this recycling of hydrocarbon could be eliminated, or at least reduced, the whole process would be more efficicient. It is already known to employ a pressure sustaining valve on the outlet ofthe absorber, which mitigates this effect by pressurising the air in the absorber and so reducing the amount of hydrocarbon which it can carry. However, this has the penalty that the high pressure is applied throughout the absorber and so is felt at the output ofthe vacuum pump attempting to evacuate the adsorber bed being regenerated, so reducing the efficiency ofthe pump and the amount of hydrocarbon it can strip from the bed.

It is an object ofthe present invention, therefore, to eliminate the foregoing problems, or at least to mitigate their effects. In accordance with this invention there is provided a process for the recovery of hydrocarbon from a contaminated mixture of gas and hydrocarbon vapour, wherein the contaminated mixture is passed through an inlet to a first solid adsorbent bed capable of selectively adsorbing hydrocarbon components from the contaminated mixture to leave substanially pure gas; when the bed becomes substantially saturated with hydrocarbons, isolating the first bed and switching the contaminated mixture to an inlet to a second solid adsorbent bed; desorbing hydrocarbons from the first bed by lowering the pressure ofthe bed by vacuum pumping so as to produce a rich gas/hydrocarbon vapour mixture; passing the rich mixture to an absorber in which most ofthe hydrocarbons in the rich mixture are absorbed in a liquid absorbent; recycling overhead gas ofthe absorber to the inlet of said second bed, characterised in that the overhead gas is introduced through a venturi ejector in said second inlet.

This process provides several advantages. Firstly, the introduction of recycled gas/hydrocarbon mixture through a venturi ejector has the effect of reducing pressure in the inlet, rather than raising it, because inlet contaminated mixture is drawn through the venturi by rapidly ejected recycled mixture. Thus the problem of inlet back pressure is eliminated, or at least reduced. Thus both enlarged inlet piping and enlarged diameter adsorbent beds are rendered unnecessary so that reduced efficiency adsorption is avoided. Indeed, temporarily increased adsorbent pressure enhances adsoφtion efficiency. By the same token, the use of induced draft fans can be avoided.

Secondly, the ejector has the effect of restricting flow from the absorber and so increases the pressure ofthe absorber overhead gas. Thus the mass of gas which is recycled can contain less hydrocarbon. Hence less is recycled so that overall efficiency is improved.

Thirdly, the pressure is only increased at the time when high pressure is needed; that is to say, when the gas is stripped from the bed and is. being recycled saturated with hydrocarbon. Once the output from the vacuum pump drops, there is less flow from the absorber for the ejector to restrict so that the pressure upstream of the ejector can progressively drop, assisting the efficiency ofthe vacuum pump by reducing its discharge pressure.

The present invention is also particularly effective in situations where there is additional pressure on the tank being filled, for example where a vapour recovery unit is sited a long way from the tank being filled. In this case the venturi ejector serves to assist flow by drawing contaminated mixture from the tank. However, because the venturi would serve to restrict flow from the tank being filled once the recycle pressure drops below a threshold value, it is in any event preferable, and particularly where the recovery unit is remote, to install a bypass ofthe venturi ejector which is opened when said threshold pressure is reached. From another aspect, the present invention permits the vapour recovery unit to be sited more remotely from the tank being filled because the greater pressure drop inherent with larger distances can be accommodated satisfactorily.

5 The present invention also provides a vapour recovery unit comprising first and second solid adsorbent beds, each with an inlet and an outlet; a supply pipe for the supply of contaminated mixture of air and hydrocarbon vapour to the inlets of said beds; a vacuum pump, having an inlet and discharge; a regeneration pipe from said o beds to the pump inlet; an absorber connected to said discharge, the absorber having an entrance for the supply of liquid absorbent, a liquid exit for the discharge of liquid from the absorber and a gas exit for the discharge of gas from the absorber, the gas exit being connected with said supply pipe; and control means to control 5 supply of contaminated mixture to one bed while the other is connected to said pump and to switch the beds periodically, characterised in that a venturi ejector is disposed in said supply pipe and said gas exit connects to said supply pipe through said ejector.

Preferably, a by-pass line in said supply pipe by-passes said venturi 0 ejector and, when the pressure in said gas-exit drops below a threshold value, a valve in said by-pass line is arranged to open. Said control means may open said by-pass valve on a timed basis. The present invention is further described hereinafter, by way of example, with reference to the accompanying drawing, which is a process flow diagram of a vapour recovery unit according to the present invention.

In the drawing, a vapour recovery unit 10 comprises a supply pipe 12 which branches to a by-pass valve 14 and a venturi 16 before entering a supply manifold 18, controlled by inlet valves 20,22, of two adsorbent beds 24,26. The supply pipe 12 is connected to a filling point of a hydrocarbon storage tank (not shown) so that air vented from the tank as it is filled with liquid hydrocarbon is vented to supply pipe 12.

Each bed 24,26 comprises solid hydrocarbon adsorbent, such as activated carbon, so that hydrocarbon vapour (contaminating air vented from the tank being filled and being fed to the beds 24,26 through supply pipe 12) is selectively adsorbed as the contaminated air filters up through the beds. Each bed 24,26 has an outlet 28,30 which each immediately branch, first to a valve 32,34 connected to atmosphere, and second to a check valve 36,38, both of which are connected to a purge valve 40. The purge valve 40 leads to a dry air inlet 42.

The supply manifold 18 further comprises two valves 44,46, each in a branch from a respective bed 24,26, and each supplying vacuum line 48 leading to the inlet of a vacuum pump 50. Pump 50 is employed to desorb hydrocarbons from the beds 24,26 when they become saturated and it discharges through line 52 to a separator 54 having a weir 56. Pump 50 is here ofthe liquid ring variety, and pump coolant, such as water, is separated from liquid hydrocarbon stripped from the beds by weir 56 and recycled to the pump 50 through line 58 and via a heat exchanger 60. Liquid hydrocarbon floating on the pump coolant over-spills weir 56 and collects in sump 62, from where it is drawn off through line 64 to a storage tank 66.

Hydrocarbon vapour discharged from the pump 50 passes through a gauze 68,which traps liquid droplets and allows gas/vapour to pass through to an absorber column 70. Cool liquid absorbent for hydrocarbon, which absorbent conveniently is itself hydrocarbon, is flooded through the column, in contra-flow to the vapour rising through the column, from an inlet 72 at the end of absorbent supply line 74. The line 74 comes, via heat exchanger 60, from the storage tank 66. Liquid absorbent, and hydrocarbon absorbed by the absorbent collect in sump 62. Finally, an absorber overhead gas exits the absorber 70 through recycle line 76 which terminates in an ejector 78 into venturi 16.

The vapour recovery unit 10 is controlled, by means not shown, to operate the following process. Only one ofthe valves 20,22 is permitted to open at any one time and say, for the sake of example, that valve 20 is opened initially. Thus contaminated vapour is fed into the adsorber bed 24 and substantially pure air is vented through valve 32, which is also open,

5 to atmosphere. After a time, bed 24 becomes saturated with hydrocarbon and so valve 20 closes and valve 22 opens simultaneously so that there is no interruption to the flow in supply 12. Thus bed 26 commences adsoφtion. At the same time, valve 34 is opened and valve 32 closed. When valves 20 and 32 of bed 24 are o closed, valve 44 is opened so that the vaccuum pump 50 begins to evacuate bed 24. At first, substantially pure air is pulled from the bed 24, but as the pressure therein drops, so hydrocarbon vapourises from the carbon in the bed to produce a very rich air/hydrocarbon mixture. After a time, the process of desoφtion is assisted by a 5 purge of air as the valve 40 is opened and non-return valve 36 permits entry ofthe purge flow to bed 24.

Hydrocarbon desorbed from bed 24 mixes with pump coolant and is discharged from the pump 50 through line 52 to the separator 54. A proportion ofthe hydrocarbon discharged from the pump is liquid, 0 and this floats on the coolant and over-spills weir 56 where it collects in sump 62. However, hydrocarbon vapour enters the bottom of absorber column 70 where it meets in contra-flow liquid hydrocarbon trickling down. Most ofthe hydrocarbon is absorbed in this process. Nevertheless, there is a residue of gas (air) which is 5 substantially saturated with hydrocarbon vapour, and which exits the cloumn to line 76. This contaminated air is recycled to the supply line 12 where it joins the flow presently going to bed 26. Bed 26 adsorbs recycled hydrocarbon, venting substanially pure air to atmosphere through valve 34, and the above described process

5 continues in a continuous cycle.

The effect and consequences of ejector 78 and venturi 16 are many:- i) The primary effect is to pressurise the gas in recycle line 76. This reduces the volume ofthe air being recycled. Consequently less hydrocarbon is transported and thus recycled in that reduced o volume of air and so the whole process is more efficient. ii) Since recycled gas is ejected under pressure into the venturi 16, it reduces the pressure in the supply line 12 and boosts the pressure in manifold 18, thereby assisting flow therein. Consequently the unit 10 can be employed to service filling points 5 located more remotely from the unit 10 (ie presenting a large pressure drop with a consequent lack of flow to the unit). Alternatively, it can service filling points having tanks whose filling pressure cannot be permitted to rise above a low maximum, and having a similarly consequential effect on the flow to the unit. That 0 is to say, the effect in this respect ofthe ejector 78 and venturi 16 is to reduce back pressure in the supply line 12. iii) Line 76 is only under pressure while there is flow in it and while there is a need to pressurise the mass of air extracted from the bed being stripped so that such mass of air transports less 5 hydrocarbon. However, once most ofthe air and its hydrocarbon content has passed through ejector 78, the pressure in line 76 begins to drop. The primary result of this is that the back pressure which will have developed in the column 70, separator 54 and pump discharge line 52 is dissipated so that the efficiency ofthe pump 50 is increased because the pressure on its discharge side is reduced. Thus more effective stripping ofthe relevant adsorber bed can be achieved.

As the pressure in line 76 drops, so also will the entraining and pressure reducing effect ofthe ejector 78 in the supply line 12. Its effect is steadily lost, and instead the venturi 16 merely becomes a restriction in the line 12 inhibiting flow and increasing back pressure in the line 12. Thus in situations where this back pressure cannot be tolerated, the by-pass valve 14 is opened until the pressure in pipe 76 again rises due to switching ofthe beds 24,26. Control of by-pass valve 14 can be effected by monitoring ofthe pressure in the manifold 18 or upstream ofthe ejector 78, but it is most conveniently controlled simply on a timed basis, since the pressure regime in the unit is largely predictable. Similarly, many other control functions in the recovery unit are also most conveniently determined in this way.

After stripping of a bed 24,26 is completed (eg on a timed basis) valve 44,46 closes and the bed is at a low pressure. The stripped bed must be returned to atmospheric pressure before the supply line 12 can be reconnected. This repressurisation is achieved by opening valve 40, air entering the bed via check valve 36,38. During this time, however, no flow is available to the vacuum pump 50 and without flow it will cause cavitation and may suffer damage. Consequently, a flow is provided by opening recycle valve 82 in line 80 so that vapour recirculates through the absorber until the next bed is to be stripped, whereupon valve 82 closes again.

Because the ejector and venturi reduce the back pressure in the supply line 12, the necessity for an induced draft fan in the outlets 28,30 ofthe beds 24,26 is in most cases removed, with a consequent reduction in pipe manifolding, fan control requirements, adsorber bed pressure loss and even spark hazzard. Furthermore, there is even the opportunity to reduce inlet/outlet piping diameters and valve sizes, and even adsorber bed diameters because ofthe increase in differential pressures in the unit 10 providing a consequential, if marginal, improvement in adsoφtion efficiency and reduced hydrocarbon emission levels.

Claims

1. A process for the recovery of hydrocarbon from a contaminated mixture of gas and hydrocarbon vapour, wherein the contaminated mixture is passed through an inlet (20) to a first solid adsorbent bed (24) capable of selectively adsorbing hydrocarbon components from the contaminated mixture to leave substanially pure gas; when the bed becomes substantially saturated with hydrocarbons, isolating the first bed and switching the contaminated mixture to an inlet (22) to a second solid adsorbent bed (26); desorbing hydrocarbons from the first bed by lowering the pressure ofthe bed by vacuum pumping so as to produce a rich gas/hydrocarbon vapour mixture; passing the rich mixture to an absorber (70) in which most ofthe hydrocarbons in the rich mixture are absorbed in a liquid absorbent; recycling (76) overhead gas ofthe absorber to the inlet of said second bed, characterised in that the overhead gas is introduced through a venturi ejector (16,78) in said second inlet.

2. A process as claimed in claim 1, characterised in that, when the pressure upstream (76) ofthe venturi ejector drops below a threshold value, flow to said inlet by-passes (14) the venturi ejector.

3. A process as claimed in claim 2, characterised in that, said threshold value is predetermined on a timed basis.

4. A vapour recovery unit (10) comprising first and second solid adsorbent beds (24,26), each with an inlet (20,22) and an outlet (28,30); a supply pipe (12) for the supply of contaminated mixture of air and hydrocarbon vapour to the inlets of said beds; a vacuum

5 pump (50), having an inlet (48) and discharge (52); a regeneration pipe (44,46) from said beds to the pump inlet; an absorber (70) connected to said discharge, the absorber having an entrance (72) for the supply of liquid absorbent, a liquid exit (64) for the discharge of liquid from the absorber and a gas-exit (76) for the discharge of gas o from the absorber, the gas-exit being connected with said supply pipe; and control means to control supply of contaminated mixture to one bed while the other is connected to said pump and to switch the beds periodically, characterised in that a venturi ejector (16,78) is disposed in said supply pipe and said gas-exit connects to said 5 supply pipe through said ejector.

5. A unit as claimed in claim 4, characterised in that, a by-pass line (14) in said supply pipe by-passes said venturi ejector and, when the pressure in said gas-exit drops below a threshold value, a valve in said by-pass line is arranged to open.

0 6. A unit as claimed in claim 5, characterised in that, said control means opens said by-pass valve on a timed basis.