GB2232093A - Hydrocarbon vapour recovery. - Google Patents

Abstract

A method and unit for hydrocarbon vapour recovery from air contaminated with such vapour involves an adsorption phase in which the contaminated air is passed through a first adsorption bed (A) venting substantially pure air to atmosphere (34), and a subsequent regeneration phase in which recovered hydrocarbon is desorbed from said bed by vacuum extraction (84), the extract being passed to an absorber column (90) supplied with liquid absorbent (95) which absorbs hydrocarbon vapour extracted from said bed. The vapour from the head (99) of absorber is recycled through a second adsorption bed (B) during the adsorption phase of the process for that bed. The increase in weight of each adsorption bed during each adsorption phase is monitored by computer means. This enables the amount of hydrocarbon recovered to be measured, and in addition provides means of controlling the sequential operation of the adsorption phases to improve their efficiency. The supply can be cut and/or an alarm raised if any bed should approach saturation. Moreover, if one bed is not nearly saturated and regeneration of the other bed is completed, the vacuum pump can be turned off, saving energy.

Description

Hydrocarbon Vapour Recovery This invention relates to hydrocarbon vapour recovery in a recovery unit of the type comprising one or more adsorbent beds through which is passed an air mixture contaminated with hydrocarbon vapour and from which it is desired to extract the hydrocarbons in order to recover them and to render the remaining air sufficiently pure to vent to atmosphere.

Such a unit is presently used where tanks of petroleum liquid are refilled after having been emptied of their petroleum content. During emptying, air is introduced to replace the liquid being drawn off and this air inevitably becomes laden with vapour. It is this vapour which is recovered by the recovery unit when the tank is subsequently refilled and the air is displaced.

The adsorbent beds generally comprise activated carbon or similar adsorbent material which is regenerated by vacuum extraction, the resultant extract being treated to an absorption process to recover the previously adsorbed hydrocarbons. Such a process is disclosed in United Kingdom patent number 1564464.

While the adsorption bed is being regenerated in the vacuum extraction phase, a further adsorption bed may be arranged to treat further contaminated air, so that the system is essentially continuous.

Because the process described above is very successful in recovering hydrocarbons from vapour, it would be useful to know precisely how much is recovered, if only because excise duty may well have been paid in respect of the hydrocarbon recovered by the process and it would be desirable to avoid, if possible, the necessity of having to pay further excise duty on the recovered hydrocarbon.

Two methods of estimating the quantity of hydrocarbon recovered are presently known, but neither is very satisfactory. In one case, the quantity of hydrocarbon recovered is determined simply by measuring the flow of hydrocarbon absorbent being fed to the absorber and measuring the flow of absorbent plus recovered hydrocarbon being output from the absorber, the difference between them representing the quantity of hydrocarbon recovered. This would be quite satisfactory if flow gauges were able to measure relatively high flow rates with an accuracy better than 0.01%. Unfortunately such flow gauges are not readily available, nor likely to be acceptable to the relevant authorities.

In another method, the concentration of hydrocarbon in the air fed into and vented from the hydrocarbon recovery unit is measured continuously and, knowing the volume of the tank being vented or measuring the volume of vapour being processed, the quantity of hydrocarbon being recovered can be approximated. However, this suffers two disadvantages.

Firstly the vapour analyser measuring the concentration of hydrocarbon is calibrated on a specific hydrocarbon, for example pentane or butane, while in fact the vapour is a wide mixture of different hydrocarbons. Secondly, where vapour volume is not directly measured but is estimated from the volume of the tank being filled, evaporation of the liquid during its transfer between tanks introduces an unknown variable which affects the accuracy of the method.

Moreover, this method could not be used where tank balancing was employed. Tank balancing is where vented air from the tank being filled is fed into the tank doing the filling. In this event there is always somme extra air but its precise volume is not known. Direct measurement of the volume of vapour being treated is difficult because the flow rates and pressure differentials are very small and do not admit of accurate measurement.

Thus it is one object of the present invention to provide a method of measuring the quantity of hydrocarbon recovered by the process.

A different concern with vapour recovery units of the type described above is the danger of hydrocarbon breakthrough in one or other of the adsorption beds should either become so saturated with hydrocarbon that it is no longer capable of holding the hydrocarbon being filtered through it. The cycle time between the adsorption and subsequent regeneration phases is arranged so that even with fairly concentrated hydrocarbon vapour at the adsorption bed input, the adsorption phase is short enough to ensure, in most cases, that breakthrough does not occur. However, the cycle time cannot be reduced beyond the time it takes to regenerate a bed. Consequently there is at least a theoretical possibility of hydrocarbon breakthrough.

This may in some cases lead merely to a temporary, if undesirable, venting of hydrocarbon vapour, but some jurisdictions are so particular about pollution that even the normal output from the adsorption beds is not sufficiently pure for venting to atmosphere. In these circumstances it is the present practice to pass the output of the adsorption beds through a catalytic burner which removes the last vestiges of hydrocarbon from the air. However, in these circumstances, hydrocarbon breakthrough from an adsorption bed represents a potentially dangerous hazard since the concentration of hydrocarbon in the vapours then entering the catalytic burners may approach their lower explosive limit.

Consequently it is a further object of the present invention to provide a method of ensuring as far as practical that hydrocarbon breakthrough does not occur.

Indeed, because the cycling period is presently set at a level which largely precludes the possibility of hydrocarbon breakthrough, it is found that this is such a short period that one or other of the adsorption beds is always being regenerated so that the vacuum pump is permanently in operation. This is despite the fact that with such short cycle times and the normal concentration of hydrocarbons in the contaminated air input to the adsorption beds, the beds are generally only employed to a fraction of their full potential and do not always require regeneration at the end of a cycle.

Consequently it is a further object of the present invention to provide a method whereby operation of the vacuum regeneration unit may be optimised.

Thus in accordance with the present invention, there is provided a method of hydrocarbon vapour recovery from air contaminated with hydrocarbon vapour, said method comprising the steps of: in an adsorption phase, passing the contaminated air through a first adsorption bed and venting substantially pure air to atmosphere; in a regeneration phase, desorbing recovered hydrocarbon from said beds by vacuum extraction, passing said extract through an absorber column supplied with liquid absorbent which absorbs the majority of the hydrocarbon extracted from the adsorption beds and passing the vapour from the head of the absorber through a second adsorption bed during the adsorption phase of the process for that bed, and monitoring the increase in weight of the adsorption beds during each adsorption phase.

In accordance with a second aspect of the present invention there is provided a hydrocarbon recovery unit suitable for putting said method into effect, said unit comprising a first adsorption bed the input of which is connected during an adsorption cycle thereof to a source of contaminated air from which it is desired to remove hydrocarbons and the output thereof venting substantially pure air to atmosphere, an adsorption bed regeneration unit, comprising a vacuum extraction unit arranged to desorb the bed during a regeneration cycle thereof, and an absorber connected to said vacuum extraction unit and being arranged to absorb hydrocarbon desorbed from said adsorption bed, residual air from the absorber being supplied to the input of a second adsorption bed during its adsorption cycle; each adsorption bed being mounted through weighing mechanisms enabling the increase in weight during each adsorption cycle to be measured.

Where it is said to vent substantially pure air to atmosphere it is of course understood that even this air will include traces of hydrocarbon which in some jurisdictions may be sufficient to require this air to be passed through a further treatment process, such as catalytic conversion, to remove this last vestige of contamination.

In the absorption phase of the process, the air and hydrocarbon mixture extracted by vacuum from the adsorption beds is contacted with an absorbent which is conveniently a liquid hydrocarbon of the type being recovered. While most of the hydrocarbon in the air extract is absorbed in the process, nevertheless the remaining air is left heavily laden with hydrocarbon vapour. This is recycled to the adsorption beds as a part of the input contaminated air mixture.

The first object of the present invention is thus solved in that the increase in weight of the adsorption bed in each adsorption phase can be employed to calculate the total amount of hydrocarbon recovered by the process. All that is required is to subtract from the measured increase in weight of the bed in each cycle an amount of hydrocarbon estimated to be recycled in each adsorption phase from the absorber.

An immediate advantage of this method is that it is a direct measurement of the hydrocarbon actually recovered and informs the user of the quantity of valuable product being saved. Moreover, the method uses presently available and easily understood technology and is thus more likely to be acceptable to the relevant authorities for the purpose of avoiding duplicate payment of excise duty on hydrocarbon product.

Although the weight of recovered hydrocarbon in a given adsorption phase may represent only as much as 0.2% of the total weight of the bed, nevertheless presently available load cells can give an accuracy of better than 0.05%.

Thus it is preferred that each adsorption bed is mounted through load cells which are connected to a computer programmed to sum the increases in weight during each adsorption cycle and to subtract the estimated weight of recycled hydrocarbon in each adsorption cycle.

Preferably each bed is mounted on legs and a load cell is disposed in each of said legs.

It is of course necessary to provide a means of estimating the quantity of hydrocarbon which is merely recycling through each adsorption phase so that this amount can be subtracted from the measured increase in weight during each adsorption phase. This might conveniently be achieved at the end of a working period of hydrocarbon recovery by not allowing introduction of contaminated air during one adsorption phase of one bed, so that the increase in weight of that bed is on this occasion entirely the result of recycling from the other adsorption bed during its regeneration cycle.

The second object of the present invention is also solved in that each adsorption bed has a finite capacity for adsorption, in terms of mass, before hydrocarbon breakthrough occurs. Since this mass is considerably more than the maximum loading at which it would normally be preferred to operate the adsorption beds, measurement of the increase in weight of the adsorption beds provides the desired alarm mechanism against hydrocarbon breakthrough. Should a particularly contaminated amount of air be input to the unit and saturation of the adsorption bed is approached before the allotted end of the adsorption cycle, the computer monitoring the increase in weight can easily be programmed to cut off input to the adsorption bed once a predetermined weight limit is reached.

The third object of the present invention is also solved in that, during an adsorption phase, should the contaminated air input be relatively clean and, as a result, the adsorption bed increases in weight by less than a predetermined minimum, the adsorption phase can be extended. This then allows the vacuum pump to be switched off temporarily after regeneration of the other bed has been completed. This may, on occasions, lead to considerable energy savings in operation of the vacuum pump.

Indeed it is quite feasible that instead of employing a time based cycling procedure, the increase in weight of each adsorption bed could be used as the trigger for switching between the adsorption and regeneration phases (as long, that is, as the cycle times are not thereby reduced, which would of course affect the proper regeneration of the beds). Allowing weight to determine cycle phasing would optimise the use of the adsorption beds. Moreover, the accuracy of the weight measurements would, on the whole, be improved because the weight at which each cycle is switched could be set at a level which would generally be greater than normally achieved with typical hydrocarbon concentration of the contaminated input air when a simple time based cycle is employed.

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

The hydrocarbon recovery unit 10 comprises two activated carbon adsorption beds A,B each having an input 12a,b and an output 14a,b respectively. The inputs and outputs are isolated from the beds by flexible couplings 16. Each bed is mounted through an array of load cells 18a,b disposed on each leg 19 supporting the beds A,B on a solid base 20. Electrical signals from the load cells are fed to a computer 22 which gives an output 24 in the form of a simple display or printout, or indeed process control commands. The cells 18 are arranged to detect changes in weight in the beds A,B and the computer makes calculations on the basis of these changes in weight to give meaningful results as explained below.

The outputs 14 from each bed are controlled by respective valves 30a,b. The inputs 12 are connected to a manifold 26 of valves 50,70a,b, which, together with the valves 30a,b, are sequentially operated by control means not shown to operate the recovery unit in a cyclic manner in two phases as further described hereinbelow.

In a first phase, one of the beds, say A, is in an adsorption phase while the other bed B is in a regeneration phase. In this mode, valves 30a and 70a are open while valve 50a is closed, enabling air contaminated with hydrocarbon vapour from a source 32 to pass at a gauge pressure of about 10-40 mbar through the bed A and be vented to atmosphere at 34 in a substantially pure condition, the hydrocarbons having been adsorbed in the process on the activated carbon contained in the bed A. Alternatively 34 may represent further processing as in a catalytic burner, for example.

In the meantime, bed B is undergoing regeneration. In this condition, valves 30b and 70b are closed and valve 50b is opened. Thus the vacuum generated by the pump 84 begins to evacuate bed B through valve 50b.

The air initially withdrawn from bed B is at this stage relatively pure and would collect hydrocarbons if passed through an absorber column 97 in an absorber 90 described further below. However a bypass is provided which is controlled by a valve 80. Thus initially valve 80 is opened, allowing the output from the pump 84 to be fed directly to the input of bed A via valve 70a, along with the supply to bed A from source 32.

However, once the pressure inside bed B begins to drop, the hydrocarbons adsorbed onto the activated carbon begin to be stripped off and there is a rapid increase in the concentration of hydrocarbons in the extracted air from bed B. At this time therefore valve 80 is closed so that the output from the liquid ring pump 84 must pass through the absorber 90.

A weir 92 separates the absorber 90 into two parts. In the first part 94, liquid from the output of pump 84 separates into two phases comprising relatively heavy pump sealant and lubricant, which may be water, and lighter recovered hydrocarbon liquid which floats on the water and eventually spills over the weir 92 into a hydrocarbon liquid reservoir 96. The pump liquid is returned to the pump 84 under the control of a drain valve 88.

Hydrocarbon liquid collecting in the reservoir 96 is periodically drawn off under the control of valve 98 and returned to a reservoir of the liquid hydrocarbon. The absorber 90 has an input 93 which is supplied from a source 95 with liquid absorbent for hydrocarbon which may itself be hydrocarbon liquid.

Hydrocarbon vapour fed from the pump 84, and which has not already condensed, now must pass up through the absorber column 97 where it contacts liquid filtering down from input 93. Here the liquid hydrocarbon absorbs a majority of the vapour. However the flow of absorbent liquid is arranged to be sufficient to maintain an absorber overhead vapour of constant composition. This is output from the absorber at 99 and where it mixes with the contaminated air from source 32. Thus the output from the absorber constitutes a part of the input to adsorption bed A via valve 70a.

After a period of time, the cycles are switched by the control means so that bed B, now largely desorbed of hydrocarbons, enters the adsorption phase and bed A, replete with adsorbed hydrocarbons, enters the regeneration phase. Since this is merely the mirror image of the condition described above no further description is deemed necessary here.

As mentioned above, the load cells 18 detect changes in weight of their respective adsorption beds A,B. It is therefore necessary to protect the beds from extraneous influences such as wind and rain etc and of course to isolate the beds from the rest of the recovery unit so that accurate weight measurement can be achieved.

As will be apparent from the foregoing description, not all the increase in weight of a given bed during an adsorption phase will be due to hydrocarbon recovered from the source 32. Some will be due to hydrocarbon recovered from a previous cycle which has not been entirely captured in the reservoir 96 but has been recycled from the absorber 90. Consequently an allowance has to be made against each load measured in any adsorption phase for this recycled hydrocarbon.

This can be achieved by isolating source 32, perhaps at the end of a working period of hydrocarbon recovery, and noting the increase in weight of the bed in the next adsorption phase, which increase will be due only to the regeneration of the other bed. This increase can then be taken to equal the amount of hydrocarbon recycled in each of the preceding adsorption phases and can be multiplied by the number of such adsorption phases which have taken place in the working period in question and subtracted from the total of all the weight increases which occurred in such working period.

Although the foregoing description assumes a fixed cycle time which has hitherto provided the most convenient and simplest form of control of the recovery unit, the present invention provides for the increase in weight of the beds to be used to control cycle phasing at a much finer level.

Even without this method of control, the weight increase can be used to trigger an alarm in the unlikely event that hydrocarbon breakthrough threatens a given bed at any time.

Thus the present invention provides that the output 24 from the computer 22 be employed to provide an alarm should either bed A or B increase in weight beyond a predetermined maximum during any adsorption phase.

Furthermore, the computer may be programmed to switch each bed between its adsorption phase and subsequent regeneration phase, so long as sufficient time has elapsed for regeneration of the other bed to be completed, when the weight of the adsorbing bed has increased by a predetermined amount.

With the increase in weight being used to control switching of cycles between the beds, two benefits ensue. Firstly, each bed can be allowed to continue in its adsorption phase until its weight increase reaches a predetermined limit. The time taken to reach this limit will vary in dependence upon the concentration of hydrocarbon in the contaminated source air and the volume thereof. On occasions this time will be much longer than generally allowed for switching, which hitherto has been calculated on a worst case basis. This renders the margin of error in the weight calculations much less because weight changes are larger. Secondly, since it takes only ten minutes or thereabouts to regenerate a bed, once a bed is regenerated, the pump 84 can be switched off if it is estimated that the other bed will take a significant time to reach the stage where it requires regeneration.Thus a significant energy saving can be realised.

As a check, it is possible to monitor the reduction in weight of a bed during its regeneration. This can serve two purposes. Firstly to provide a zero datum for subsequent weight increases in the next adsorption cycle of the relevant bed, and secondly to ensure that all the hydrocarbon adsorbed during the adsorption phase is in fact desorbed during regeneration. Any difference between the two measurements may be noted and if sufficiently large (whether alone or in aggregate with previously noted differences) to suggest a fault in the bed or its regeneration, an alarm can be raised.

This can be very important insofar as hydrocarbon breakthrough is concerned if less hydrocarbon is desorbed in successive regeneration phases than is adsorbed in previous adsorption phases. Without this check, a particular weight gain during one adsorption phase might not exceed the amount required to give a breakthrough alarm but nevertheless may result in the bed becoming saturated and allowing hydrocarbon breakthrough to occur.

It will of course be apparent to those skilled in the art that the use and monitoring of the load cells 18, and the implementation of the computer 22 to enable it to make the necessary calculations in order to give weight measurements, or to perform control operations (whether to provide an alarm in the event of possible hydrocarbon breakthrough, to control cycle times or to control any other process operations) are within the knowledge of those skilled in the art and require no further elucidation herein.

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 (19)

1. A method for the recovery of hydrocarbon vapour from air contaminated with hydrocarbon vapour, said method comprising the steps of: a) in an adsorption phase: passing the contaminated air from a source thereof through a first adsorption bed and venting substantially pure air to atmosphere; b) in a subsequent regeneration phase: desorbing recovered hydrocarbon from said bed by vacuum extraction; passing said extract through an absorber column supplied with liquid absorbent which absorbs hydrocarbon vapour extracted from said bed; and passing the vapour from the head of said absorber through a second adsorption bed during the adsorption phase of the process for that bed; and, c) monitoring the increase in weight of each adsorption bed during each adsorption phase.

2. A method as claimed in claim 1 in which the increase in weight of each adsorption bed is monitored by computer means which sums the noted increases in weight of each bed during each adsorption phase over a given working period, and subtracts from said sum an amount corresponding to the weight of hydrocarbon recycled from the absorber to each adsorption bed during each adsorption cycle.

3. A method as claimed in claim 2 in which said amount corresponding to the weight of hydrocarbon recycled is determined by disconnecting one adsorption bed from said source during one of its adsorption cycles, monitoring its increase in weight during said cycle, and multiplying the weight so monitored by the number of cycles employed during said working period.

4. A method as claimed in claim 1, 2 or 3 in which said first and second beds are sequentially switched between alternate adsorption and regeneration phases in an essentially continuous process, and in which said switching is at least in part controlled in response to said monitored increases in weight.

5. A method as claimed in claim 4 in which said vacuum extraction is effected by a vacuum pump which is turned off by said computer means when regeneration of one bed is completed and when the increase in weight of the other bed has not exceeded a predetermined minimum.

6. A method as claimed in claim 5 in which said vacuum pump which is turned on, or maintained on, by said computer means when regeneration of one bed is completed and when the increase in weight of the other bed has exceeded a predetermined desired level and regeneration of said other bed is required.

7. A method as claimed in claim 4, 5 or 6 in which said computer means prevents further input of contaminated air to an adsorption bed when a predetermined maximum increase in weight of said bed is exceeded.

8. A hydrocarbon recovery unit, said unit comprising: a first adsorption bed, the input of which is connected during an adsorption cycle thereof to a source of contaminated air from which it is desired to remove hydrocarbons, and the output thereof venting substantially pure air to atmosphere; an adsorption bed regeneration unit, comprising vacuum extraction means arranged to desorb the bed during a regeneration cycle thereof, and an absorber connected to said vacuum extraction unit and being arranged to absorb hydrocarbon desorbed from said adsorption bed; a second adsorption bed, adapted to be supplied, during its adsorption cycle, with residual air from the absorber; and, weighing mechanisms mounting each adsorption bed and enabling the increase in weight of each bed during each adsorption cycle to be measured.

9. A recovery unit as claimed in claim 8, further comprising computer means, which sums the noted increases in weight of each bed during each adsorption phase over a given working period, and subtracts from said sum an amount corresponding to the total weight of hydrocarbon recycled over said working period from the absorber through each adsorption bed during each adsorption cycle.

10. A recovery unit as claimed in claim 9, further comprising means to disconnect one adsorption bed from said source during one of its adsorption cycles, said computer means monitoring the increase in weight of said adsorption bed during said cycle, and determining said amount corresponding to the weight of hydrocarbon recycled by multiplying the increase in weight so monitored by the number of cycles employed during said working period.

11. A recovery unit as claimed in claim 8, 9 or 10, in which each adsorption bed is mounted through load cells.

12. A recovery unit as claimed in claim 11, in which each bed is mounted on legs and a load cell is disposed in each of said legs.

13. A recovery unit as claimed in any of claims 8 to 12, in which said computer means is adapted to provide an alarm if the increase in weight of an adsorption bed exceeds a predetermined maximum amount during any one adsorption cycle.

14. A recovery unit as claimed in any of claims 8 to 13, in which said computer means is arranged to monitor the decrease in weight of each adsorption bed during each regeneration cycle thereof.

15. A recovery unit as claimed in claims 13 and 14, in which said computer means is adapted to adjust said maximum amount during any subsequent adsorption cycle by an amount corresponding to any difference between the monitored increase and decrease in weight of an adsorption bed in any previous consecutive adsorption and regeneration phases.

16. A recovery unit as claimed in any of claims 13 to 15, in which said computer means is further adapted to disconnect an adsorption bed from said source once said predetermined maximum increase is exceeded.

17. A recovery unit as claimed in any of claims 8 to 16, in which said regeneration of each adsorption bed is effected by a normally operating vacuum pump and said computer means is further adapted to control, at least partially, switching between said adsorption and regeneration cycles of each adsorption bed, said computer means being arranged to turn off said vacuum pump when regeneration of one bed has been completed, and when a minimum predetermined increase in weight of the other adsorption bed has not been reached.

18. A recovery unit as claimed in claim 17, in which said computer means turns on, or maintains on, said vacuum pump when a predetermined desired increase in weight of one adsorption bed as been reached during its adsorption cycle, said desired increase being intermediate said minimum and said maximum increases, and switches said one bed to its regeneration cycle and said other bed to its adsorption cycle.

19. A method of recovering hydrocarbon from a contaminated air source, and a hydrocarbon recovery unit for performing such method, substantially as hereinbefore described with reference to the accompanying drawing.