A METHOD OF REMOVING HYDROCARBONS FROM THE AIR

Abstract: A method serves to strip a gas mixture of air and hydrocarbons and/or petrol vapours of its content of hydrocarbons. At least one diesel engine (40) is provided for this purpose, to which the gas mixture, oil and supplementary air are fed to an extent which, together with the air content of the gas mixture, is correct for simultaneously combusting both the hydrocarbons and the oil during the working stroke. The method is used to special advantage as the second step after the process and the system which are described in the applicant's European Patent 0 079 364. Hereby, emitted petrol vapours from e.g. a tank installation may be recovered to a considerable extent, and the residual petrol vapours are utilized for driving the recovery process. Further, optimum cleaning of the air is achieved.

Description: A method of removing hydrocarbons from the air The invention concerns a method of stripping a gas mixture of air and hydrocarbons and/or petrol vapour of its con¬ tent of hydrocarbons. Many different processes emit considerable amounts of hydrocarbons, and this takes place not least from tank installations in the form of petrol vapour. This emission is very detrimental to the environment and moreover constitutes a considerable loss of expensive fuels, and accordingly it has long been endeavoured to solve this problem. Thus, EP 0 298 288 discloses a method and a system where the content of petrol vapour in a mixture of petrol vapour and air is first reduced under cooling in a substance exchanger, following which the residual vapour is combust- ed in a petrol engine. However, because of the high vol¬ tage system for igniting the engine the use of this pro¬ cess involves safety hazards in precisely the areas where the inflammable and, under certain circumstances, explo¬ sive mixtures of air and hydrocarbons are handled, and to this should be added that the process is difficult to con¬ trol and therefore frequently proceeds uneconomically, and that the resulting combustion products themselves moreover contain considerable amounts of unco busted hydrocarbons. The applicant's European Patent 0 079 364 discloses a method and a system fo the recovery of petrol from a mix¬ ture of petrol vapour and air. A very high degree of recovery can be obtained by means of this method, but it leaves petrol vapours which must be vented to the atmos- phere or be removed in another manner. The object of the ivention is therefore to provide a method of the type stated in the opening paragraph which can strip contaminated air of a content of hydrocarbons and/or petrol vapours better than before with simultaneous utilization of these. This is achieved by providing at least one diesel engine to which the gas mixture, oil as well as supplementary air are added to an extent which, together with the air con- tent of a gas mixture, is correct for simultaneously com¬ busting both the hydrocarbons and the oil during the working stroke. Since a diesel engine does not work with electric ignition, a considerable improvement in safety is obtained, and it is moreover possible to save a catalyst since the combustion products from a diesel engine do not contain nearly as large amounts of uncombusted hydrocarbons as the combustion products from a petrol engine. The output power of the diesel engine may e.g. be used for driving an electric generator, and it is advantageous in this connection that the number of revolutions and output moment of the diesel engine are kept constant. This can take place according to the invention in that the oil supply is varied automatically in response to the hydro¬ carbon content of the gas mixture. In some cases the hydrocarbon content of the contaminated air is rather small, and according to the invention, the stripping process can then be regulated by using several diesel engines, there being coupled a number such that none or just a single engine needs supplementary air supply. In particular when working with petrol vapours it may be advantageous to insert a gas mixture buffer in front of the diesel engine. This can then always run with optimum efficiency, since it starts only at a predetermined maximum filling of the buffer and stops at another predetermined minimum filling degree. The method of the invention can be used to special ad¬ vantage in connection with a system of the type described in the applicant's EP Patent 0 079 364. The residual petrol vapours from this process are then removed by com- bustion in the diesel engine, and simultaneously the recovery process fully or partly receives the energy emitted by the diesel engine during this. This results in the greatest possible recovery of the expensive petrol at the least possible cost, since the energy for the recovery is obtained partly form the combustion of the residual petrol vapours, partly from the combustion of the oil which is cheaper to use than petrol. For the recovery process is used a cooling system which can be driven either directly from the output shaft of the diesel engine via a transmission, or indirectly by means of the current from an electric generator which is con¬ nected with the output shaft. The exhaust gases from a diesel engine are very hot and can therefore be used for driving a vapour generator, which can in turn supply vapour for distillation of the petrol in the distillation column of the recovery system. The invention will be explained more fully below with reference to the drawing, in which fig. 1 is a block diagram showing a first embodiment of the method. fig. 2 is a block diagram showing a second embodiment of the method with a gas mixture buffer inserted in front of the diesel engine, fig. 3 is a block diagram showing a third embodiment of the method with several serially connected diesel engines, fig. 4 is a block diagram showing a fourth embodiment of the method, which in this case comprises a recovery pro¬ cess. In the block diagram in fig. 1 the diesel engine is indi¬ cated by the reference numeral 40. The output shaft 41 of the engine is coupled to an electric generator 42, which supplies the electric wires 43 with electric current during operation. Instead of the electric generator, the output shaft 41 may be coupled to another power consuming machine, e.g. the compressor in a cooling system. The engine is moreover provided with an exhaust pipe 44 lead¬ ing the very hot exhaust gases from the combustion process through a vapour generator 45, which, via a water conduit, receives water which, via a heat exchanger (not shown) in the vapour generator, is converted to vapour that is supplied to a service place (not shown) via a vapour con¬ duit 47. The cooled exhaust gases are vented to the atmos- phere, and in this connection it is not necessary to use a catalyst since the hydrocarbon content of the exhaust gases is very low. The shown engine is water cooled in this case, and the coolant is conveyed into the cooling jacket of the engine through a coolant conduit 48 and out again through a return coolant conduit 49. For example at a filling station, the hot coolant may be used for heating service water via a heat exchanger or be used directly as hot service water. The oil for driving the engine is supplied through an oil conduit 50, and the air for the combustion is added through an air supply conduit 51. This conduit is connected with a gas mixture conduit 52, which supplies the gas mixture which is to be stripped and which may consist of a mixture of air and hydrocarbons in the form of e.g. petrol vapour or of petrol vapour alone. The gas mixture from the gas mixture conduit 52 and the sup- plementary air from the air supply conduit 51 then flow together via a common conduit 53 into the diesel engine 40 during its suction stroke. The hydrocarbons are combusted during the working stroke together with the added oil and consume the oxygen in the supplementary air as well as the oxygen in the air present in the gas mixture. To obtain optimum combustion, the total amount of air must correspond to the amount of air which can be deduced from the combustion equation. The pressure and the velocity of the gas mixture as well as its content of hydrocarbons and air must therefore be measured by means of probes which apply signals about the measured quantities to a control, which adjusts the sup¬ plementary air to the correct quantity via a valve or a throttle in response to the signals and the injected amount of oil. This regulation mechanism, which is not shown in the figure, is known per se and will therefore not be described in greater detail here. When, as shown, the diesel engine is to drive an electric generator, this is usually to be driven with a constant number of revolutions and moment. However, the amount and the composition of the gas mixture are not constant, but can often fluctuate greatly. To compensate for this, the engine must be regulated by other means, and this is obtained particularly advantageously by means of a diesel engine which can be controlled exclusively by the variable supply of oil and thus does not need any form of throttle or lambda control. As mentioned, the gas mixture will often flow rather irre- gularly to the diesel engine, which, however, works best with an even supply of the gas mixture. With a view to obtaining this, a gas mixture buffer 54 capable of collecting a suitable amount of gas mixture is therefore inserted in the gas mixture conduit 52, as shown in fig. 2. Then, the diesel engine is started only when a pre¬ determined maximum filling degree is achieved in the buffer 54, and runs until the filling degree has dropped to a predetermined minimum size. The diesel engine can hereby be driven optimally, because it is now not necessary constantly to stop and start the engine again. As will be seen, a constant maximum speed faster than the flame front can thereby be maintained in the suction, thereby eliminating the risk of back flow through the common conduit 53. Fig. 3 shows an embodiment of the method with several diesel engines which are connected in series by means of a connecting conduit 55. This embodiment is useful in particular when gas mixtures containing a considerable constituent of air, i.e. relatively deluted mixtures, are to be handled. When the stripping process is to be initiated, the first engine is then initially started. If this engine requires no supplementary air in addition to the air already present in the gas mixture, the next engine in the series is started, and this continues until an engine requiring addition of supplementary air for working optimally is reached. The engines are coupled successively, a valve 56, which receives a control signal via an electric control wire 57, being opened for each individual engine. Finally, fig. 4 shows a particularly advantageous embodi¬ ment of the method, which is worked in cooperation with method and the system described in the applicant's EP Patent 0 079 364. The total process sequence will be described briefly below, the details of which being described more fully in said European patent. The gas mixture to be cleaned is passed to the system via a conduit 11 after having first passed various non-return valves, etc. The gas mixture is first stripped off by far the greatest part of its water content in the heat exchanger 1, from which the mixture is conducted to an absorbtion means 2 which also receives cooled petroleum distillate via a controllable valve 15. In the absorbtion means 2 the petrol vapour is absorbed in the petroleum distillate, which is passed to a buffer tank 3, while the cleaned cold air is discharged into a conduit 52 via the heat exchanger 1, so that the cold air provides the cooling effect required to condense water. The petrol- containing petroleum distillate is conducted from the buffer tank 3 to a stripping means, in the shown system a distillation column 5 where the petroleum distillate is stripped of petrol by heating and is then passed to a storage tank 7, in which the petroleum distillate is cooled before being recircled to the absorbtion means 2. The heat exchanger 4 reduces the power consumption of the system since the cold petroleum distillate from the buffer tank 3 is preheated before being transferred to the dis- tillation column 5, while the hot petroleum distillate from the distillation column 5 is precooled before being transferred to the storage tank 7. It is observed that the heat exchanger 4 provides optimum savings in energy, because it can be dimensioned to a predetermined flow volume, which is substantially constant under all operating conditions owing to the presence of the buffer tank 3. The petrol vapours given off by the distillation are transferred to a washing chamber 8, e.g. a packed column, through which liquid petrol absorbing the petrol vapours flows via conduits 9, 10. Alternatively, the vapours may be condensed or compressed in a manner known per se. The numerals 13 and 14 represent a cooling system and a heat exchanger, respectively, for cooling the petroleum distillate in the storage tank. By means of this process it is possible essentially to recover the expensive petrol, but the cold air flowing out into the conduit 52 from the heat exchanger 1 will still have a content of petrol vapours unacceptably high under certain conditions. The air thus still contaminated to a certain degree is therefore conducted via the conduit 53 into the diesel engine 40, in which the residual petrol vapours are combusted as described before, thereby cleaning the air optimally and utilizating the petrol vapours for generating various forms of energy. This energy is now used advantageously in the preceding recovery process. The electric current, transmitted by the generator 42 into the electric wires 43, is thus caused to drive an engine 58 which drives the cooling system 13, this, however, may also be driven directly by the output shaft 41 of the diesel engine 40 via a suitable transmis¬ sion. Correspondingly, the vapour, generated in the vapour generator 45 and fed through the vapour conduit 47, is used for heating the petroleum distillate in the dis- tillation column 5 via a heating coil 59. Of course, many other arrangements are conceivable than those described above and shown in fig. 4 for utilization of the energy developed by the diesel engine during its combustion of the petrol vapours, but in all cases their energy content is utilized fully instead of being wasted like before while cleaning the air optimally. Finally, if desired, a catalyst and optionally also a soot filter may be inserted in the exhaust pipe of the engine with a view to restricting the pollution from the exhaust gases additionally and keeping it within the prescribed threshold values.

Claims: P a t e n t C l a i m s: 1. A method of stripping a gas mixture of air hydro- carbons and/or petrol vapour of its content of hydro¬ carbons, c h a r a c t e r i z e d by providing at least one diesel engine to which the gas mixture, oil and supplementary air are supplied to an extent which, together with the air content of the gas mixture, is cor- rect for simultaneously combusting both the hydrocarbons and the oil during the working stroke. 2. A method according to claim 1, c h a r a c t e r ¬ i z e d by varying the oil supply automatically in response to the hydrocarbon content of the gas mixture such that the number of revolutions and output moment of the diesel engine are kept constant. 3. A method according to claim 1, c h a r a c t e r - i z e d by providing several diesel engines, and coupling such a number of these during the stripping of the gas mixture that none or just a single engine needs addition of supplementary air. 4. A method according to claims 1, 2 or 3, c h a r a c ¬ t e r i z e d by providing a gas mixture buffer, and starting and stopping the stripping process in response to this filling degree. 5. A method according to one or more of claims 1-4 using a method and a system of the type described in the applicant's European Patent 0 079 364 for recovery of petrol from a mixture of petrol vapours and air, c h a r a c t e r i z e d by removing residual petrol vapours from this process by combustion in the diesel engine, and fully or partly supplying the energy emitted by the diesel engine during said process to the recovery process. 6. A method according to claim 5, c h a r a c t e r - i z e d in that the output shaft of the diesel engine drives a generator supplying current to the cooling system of the recovery system. 7. A method according to claim 5, c h a r a c t e r - i z e d in that the output shaft of the diesel engine operates the cooling system of the recovery system via a transmission. 8. A method according to claims 5, 6 or 7, c h a r a c t e r i z e d by using the exhaust gases from the diesel engine for driving a vapour generator supplying vapour for distillation of the petrol in the distillation column of the recovery system.

Inventors: JACOBSEN ANKER JARL, JACOBSEN, ANKER, JARL
Applicants: JACOBSEN ANKER JARL, JACOBSEN, ANKER, JARL
Application year: 1991
Main classification: F23G7
IPC codes: B01D53, F02B1, F02B3