EP1968727A1

EP1968727A1 - Filter to filter equalization

Info

Publication number: EP1968727A1
Application number: EP06828769A
Authority: EP
Inventors: Martin Elsøe NIELSEN
Original assignee: Aker Cool Sorption AS
Current assignee: Cool Sorption AS
Priority date: 2005-12-30
Filing date: 2006-12-29
Publication date: 2008-09-17
Also published as: WO2007076867A1

Abstract

The invention relates to an apparatus and a method for regenerating a filter comprising at least two filter units, and a plant where the apparatus for regener- ating the filter is combined e.g. with an absorber or a compressor unit to remove pollutants such as volatile organic compounds from a gas stream. An apparatus according to the invention comprises at least two filter units (I, II, III) each receiving polluted gas through a first connection (11 +13,12+14, 31 , 32, 33) and each discharging cleaned gas through a second connection (15, 16, 28) and at least one third connection (20, 34, 35, 36) connects the at least two filters (I, II, III) in such a way that gas can flow from one filter to the other when a device (19, 5, 6, 29) allows for passage of a gas stream. The first and second connections each are provided with devices (1 , 2, 3, 4, 23, 27) which allow for passage of a gas stream or block the gas stream, each filter (I, II, III) is working both at a higher pressure p1 and at a lower pressure p2, and the third connection or tertiary connections combine the entrance or discharge of one filter to the entrance of another filter.

Description

Filter To Filter Equalization

Technical field

The invention relates to an apparatus and a method for regenerating a filter comprising at least two filter units, and a plant where the apparatus for regenerating the filter is combined e.g. with an absorber or a compressor unit to remove pollutants such as volatile organic compounds from a gas stream.

Background of the invention DK 172.772 B1 relates to a method for recovery of volatile organic compounds (VOC) from a gas stream. According to this known method at least two adsorbing filter units are used to adsorb the volatile compounds, and while a first filter is adsorbing VOC from the VOC containing gas stream, a second filter is being regenerated by applying a vacuum to the filter. A filter unit according to this inven- tion is preferably based on activated carbon, silica gel or zeolite.

A filter unit being in adsorption mode at a higher pressure p1 is regenerated at a lower pressure p2 by closing of the outlet of cleaned gas and the admission of VOC and thereafter reducing the pressure in the filter to the required lower pres- sure p2 by opening a conduct to a vacuum pump. The vacuum pump lowers the pressure and transports the gas from the filter unit under regeneration to an absorption column.

This process has the disadvantage that a lot of energy is used when the vacuum pump has to reduce the pressure from p1 to p2 and transport the gas stream to the absorption column. Also the vapor pressure of VOC in the first part of the gas stream, i.e. the part of the gas stream which is removed at a pressure higher than p2, is below saturation which means that this part of the gas stream is saturated with VOC inside the absorption column and thereby the content of VOC in the gas stream is actually increased in the absorption column.

US patent no. 4.256.469 relates to a method for adsorption of a component from a gaseous mixture e.g. air which gaseous mixture exhibit differing adsorbability on carbon molecular sieves. According to this method the repressurisation is conducted in at least two steps/stages, as it is recognized that having slower repressurisation step allows closer approximation to equilibrium which especially is a problem when using a carbon molecular sieve.

In this document it is taught (column 4, lines 4-29) that the pressure increase in the first pressure build up stage is preferably effectuated by connecting a de- sorbed, optionally evacuated adsorber, with a loaded adsorber which is under pressure. It is stated as advantageous to conduct this pressure equalization via the outlet ends of both adsorbers, so that a residual charge of more readily adsorbable components, i.e. nitrogen, present in the proximity of the outlet end of the low-pressure, desorbed adsorber will be pushed toward the inlet side. It is stated as even more advantageous for the purpose of pressure equalization to connect both the inlet ends and the outlet ends of a pressurized adsorber and a desorbed adsorber with each other, since in this way the gas from the inlet of the pressurized adsorber which is richer in the more adsorbable component passes to the inlet of the low-pressure adsorber; and the gas from the outlet of the pressurized adsorber depleted of the more readily adsorbable components passes into the outlet end of the low-pressure adsorber, so that at this outlet end the load of the adsorbent with more readily adsorbable components is even more greatly reduced than in case the process variant described above.

Description of invention

The object of the invention is to provide a filter with two or more filter units with similar ability to remove pollutants as the known filters but acquiring reduced cost of installation and operation compared to the known filters. The reduced cost of installation is obtained as smaller filter units having less adsorbent are required and smaller inlet pipes will be required for the filter units. Also the operation costs are reduced as the vacuum pump will have to work less; both because the initial pressure of the filter unit to be regenerated will be lower and because less gas has to be transferred through the vacuum pump. Another object of the invention is to provide an optimized filter comprising at least to filter units where recurrently one unit operates at a high pressure and the other operates at a low pressure. According to the present invention a filter is a unit where a mixture of components such as volatile organic components are removed from a carrier, e.g. air or nitrogen gas. This object is obtained by equalizing the pressure from either the outlet end or the inlet end of the high pressure filter to the inlet end of the low pressure filter. Normally the pressure will be equalized from the inlet end of the high pressure filter to the inlet end of the low pressure filter.

Equalization to the inlet of the second filter (the just regenerated filter at low pressure) reduces the build-up of low boiling components of the mixture (light ends) in the filters. This probably results from the amount of low boiling components being transferred from one filter to the other is low as the low boiling components at the end of an adsorption period primarily are found at the top of the first filter.

In one embodiment the equalization step in the second filter is started with letting in atmospheric air through the outlet end (purging), and according to this embodiment atmospheric air can be let in until the pressure in the second filter has increased with around 100% e.g. from around 100 mbar (abs) to around 200 mbar (abs).

The present invention concerns an apparatus comprising - at least two filter units each receiving polluted gas through a first connection and each discharging cleaned gas through a second connection

- where the first and second connections each are provided with devices which allow for passage of a gas stream or block the gas stream,

- and where each filter unit is both working at a higher pressure p1 and at a lower pressure p2. The apparatus is characterized in that at least one third connection provided with at least one device which allows for passage of a gas stream or blocks the gas stream, connects the at least two filter units in such a way that gas can flow from one filter unit to the inlet end of another when the device allows for passage of a gas stream.

Adsorption is normally performed at a higher pressure p1 which is around at- mospheric pressure (1013 mbar (abs)), preferably between 900 - 1100 mbar (abs). Regeneration / desorption is performed at a lower pressure p2 which is normally beyond 900 mbar (abs), preferably p2 < 500 mbar (abs), preferably p2 < 200, more preferred 35 mbar (abs) < p2 < 130 mbar (abs) and most preferred 80 mbar (abs) < p2 < 100 mbar (abs).

In one embodiment the apparatus comprises at least three filter units (I, II, III) which will allow for continuous adsorption although a filter unit is being regenerated. In this preferred embodiment the apparatus comprises three tertiary connections connecting respectively filter unit I and II, filter unit I and III and filter unit Il and III.

An embodiment could also have 4 or 5 filter units as the optimum number of filter units depending upon whether it will be an advantage to have more and smaller filter units compared to fewer larger filter units. This is a balance between installation costs and operation/maintenance costs.

In another embodiment the third connection or tertiary connections combine the entrance of one filter unit to the entrance of another filter unit.

"Entrance" is here understood to be the end of the filter unit where the gas stream to be treated normally enters, and the third connection might exit into an inlet pipe leading to the filter unit or exit directly into the filter unit. Alternatively the third connection could connect the exit end of the first filter unit with the inlet end of the second filter unit.

In one embodiment the at least two filter units of the apparatus comprise a filter material which can adsorb volatile organic compounds (VOC) e.g. activated carbon, zeolites or silica gel. The advantages of using activated carbon are high working capacity, low price. The advantages of using zeolites and silica gel are that these materials are unable to combust and can e.g. upgrade biogas to natural gas pipeline quality by separation of CO₂.

The invention also comprises a method where polluting compounds are adsorbed in a first filter unit at a higher pressure p1 and simultaneously a second filter unit is regenerated at a lower pressure p2; a) when the first filter unit is saturated with polluting substance to a certain degree, the inflow and the outflow from both the first and the second filter units are blocked, b) then a connection allowing gas to flow directly between the first filter unit and the inlet end of the second filter unit is opened thereby allowing gas to flow from the first filter unit to the second filter unit equalizing the pressure in the two filter units; c) after equalizing the pressure the connection between the first filter unit and the second filter unit is closed and the first filter unit is subjected to an active reduction of pressure e.g. by a vacuum pump in order to regenerate the first filter unit.

The invention also comprises a method where polluting compounds are adsorbed in the first filter unit at a higher pressure p1 , simultaneously a second filter unit is regenerated at the same time at a lower pressure and a third filter unit is either adsorbing polluting compounds at a higher pressure p1 or being pre- pared for adsorbing polluting compounds; a) when the first filter unit is saturated with polluting substance to a certain degree, inflow to and outflow from the third filter unit is opened and inflow to and outflow from both the first and the second filter units are blocked, b) then a connection allowing gas to flow between the first filter unit and the inlet end of the second filter unit is opened thereby allowing gas to flow from the first filter unit to the second filter unit equalizing the pressure while polluted gas passes through the third filter unit; 2

6 c) after equalizing the pressure the connection between the first filter unit and the second filter unit is closed and the first filter unit is subjected to an active reduction of pressure e.g. by a vacuum pump in order to regenerate the first filter unit.

In an embodiment of the invention the steps a) to c) are repeated each time a filter needs to be regenerated. Both/all the filter units will at some time be the "first" filter, the "first" filter is a filter unit which is just facing regeneration.

In another embodiment of the invention the pressure difference Δp = p1-p2 between the first and the second filter is between 1100 mbar and 500 mbar, preferable Δp > 700 mbar.

Especially the filter of the invention is directed for use in connection with a Vapor Recovery Unit (VRU). A VRU controls and reduces the emission of VOC to the atmosphere caused by loading of volatile organic liquids from one storage or transport unit to another e.g. loading of gasoline into road tankers or loading of crude oil into ship. The VRU will have a positive influence on the following factors:

A) Environmental protection The air pollution will be reduced in the area of the volatile organic liquid handling facility.

B) Safety

The risk of fire or explosion due to the inflammable vapors will be reduced.

C) Health The personnel at the volatile organic loading facility will be protected against inhalation of unhealthy vapors.

D) Economy

Returning recovered product back to storage allows the installation to be profitable in the long term. Without recovery valuable volatile organic compounds are released into to the atmosphere.

Description of the drawings W

7

The invention is explained in greater detail below with reference to the accompanying drawings wherein preferred embodiments of the invention are shown.

Fig. 1 shows an embodiment of the filter comprising two filter units;

Fig. 2 shows the embodiment of figure 1 together with an absorber for re- moving VOC from the regenerated filters;

Fig. 3 shows an embodiment of the filter comprising three filter units.

The filter of figure 1 comprises two filter units I, Il which in this embodiment have a common inlet conduit 10 through which a gas stream containing pollutants such as VOC is fed. The inlet conduit 10 is split up in separate inlet conduits 11 , 12 for each filter unit I, II. In this embodiment the separate inlet conduits 11 , 12, respectively, are provided with inlet valves 1 and 2 and enter filter conduit 13 leading to filter unit I and filter conduit 14 leading to filter unit Il respectively. Gas streams pass in both directions through the filter conduits 13 and 14 in this embodiment. When a filter unit is in adsorbing mode, the ingoing gas stream passes from the entrance of the separate inlet conduit 11 or 12 to the filter unit I or II. When a filter unit is being regenerated, the gas stream passes from the filter unit I or Il through the filter conduit 13 or 14 then through a vacuum conduit 17 or 18, where access is controlled by suction valves 5 or 6, to a common vacuum inlet for a vacuum pump 9. The vacuum pump 9 transports the gas stream to a unit where the polluted gas will be recovered e.g. to an absorber where the pollutant is removed from the gas by absorption. On the exit side of the filter units I and Il the outlet conduits 15 and 16 in this embodiment lead the gas streams to a common outlet to which there will be access when outlet valves 3 or 4 are open. This embodiment of the filter also comprises an equalizing conduit 20 provided with a valve 19 which^' equalizing conduit connects the filter conduit 13 of filter unit I with the filter conduit 14 of filter unit II.

When a first filter unit, e.g. filter unit I, is in adsorption mode the inlet valve 1 and the outlet valve 3 are open and the suction valve 5 is closed. Concurrently a second filter unit e.g. filter unit Il is being regenerated which means that inlet valve 2 and outlet valve 4 are closed and suction valve 6 is open (step 1 and 2). This step stops after a given time has passed or when the concentration of pollutant in the outlet conduit 15 exceeds an upper limit, usually regeneration will be stopped before adsorption stops. Normally purge air will be added to the second filter unit during regeneration in order to increase desorption of the pollutants. After having ended the adsorption step in the first filter unit and the vacuum regeneration of the second filter unit the pressures in first and second filter units will be equalized. As the filter unit changes to equalizing mode the inlet valve 1 , the outlet valve 3 of the first filter unit and the suction valve 6 of the second filter unit are closed whereafter the equalizing valve 19 is opened (step 3).

After opening the access between the two filter units thereby equalizing the pressure, the pressure has been reduced in the first filter unit which was in adsorbing mode and increased the pressure in the second filter unit which unit was being regenerated. After equalizing the pressure between the two filter units the equalizing valve 19 is closed and a vent valve 22 of the second filter unit is opened in order to further increase the pressure of the regenerated filter to atmospheric pressure.

Before the second filter unit i.e. filter unit Il enters adsorption mode the vent valve 22 is closed and then the inlet valve 2 and the outlet valve 4 is opened allowing polluted gas to flow through the filter unit II. At the same time the suction valve 5 of the first filter i.e. filter unit I is opened and regeneration of the filter unit I starts. Again the adsorption/regeneration step stops after a given time has passed or when the concentration of pollutant into the outlet conduit 16 exceeds an upper limit. When the adsorption/regeneration step ends, the suction valve 5 of filter unit I is closed and the inlet valve 2 and the outlet valve 4 of filter unit Il are closed whereafter the equalizing valve 19 is opened. After equalizing the pressure between the two filter units the equalizing valve 19 is closed and a vent valve 21 of the first filter unit is opened in order to equalize the pressure of the regenerated filter with atmospheric pressure.

In table 1 is given an overview of the continuous repetition of adsorption/regeneration/equalizing cycles for a filter with two filter units: Table 1

VO

o = valve open ; c = valve closed

Table 1 also shows an example of the approximate pressures appearing in the filter at the end of each step.

Fig. 2 shows a plant for recovery of volatile organic compounds where an embodiment of the invention has been employed.

In this plant a mixture of hydrocarbons and air collected during loading or filling operations is, by method of displacement or blower, passed through a bed of activated carbon pellets. The hydrocarbons are adsorbed onto the activated carbon (known as adsorption) and the purified gas is released to the atmosphere. From adsorption the carbon pellets become saturated and require regeneration. Regeneration is attained by applying a vacuum to the bed of pellets. At least two filter units are required to allow continuous vapor recovery. While one filter unit is adsorbing hydrocarbons the other is regenerated by vacuum.

The regeneration of the activated carbon is achieved by applying a vacuum e.g. by using a liquid ring pump 9 where the sealing liquid for the pump can be a mixture of mono ethylene glycol and water but other vacuum pumps such as a rotary vane vacuum pump or a screw vacuum pump can also be used in the process. To lower the partial pressure in the activated carbon (AC) bed in the regeneration phase, a small amount of purge air is lead into the bed by opening the valve 7, 8.

The hydrocarbonrich gas is passed on to the packed column 41 in which the bulk of hydrocarbon is absorbed in a stream of absorbent. The absorbent is preferably a partial stream of the product being loaded. The absorbent and recovered product collected in the lower part 42 of the combined separator and absorber 40 is returned to the product stream by a pump 43. Non-condensed hydrocarbons and air leave the top of the absorber column through a connection 45 and is routed to a filter unit in the adsorption mode through the connection 10.

During loading or filling operations the liquid surface of the product being loaded will come into contact with air or inert gasses in the tanks and will thus evaporate. The degree of saturation is depending on several factors, one of the more critical of these is temperature. At 0 ⁰C e.g. air saturated with gasoline vapor will contain approximately 1 I gasoline/m³ compared to approximately 2 l/m³ at 23 ⁰C.

The degree of evaporation also depends on: Vapor pressure of the product being loaded, the degree of saturation in the compartments caused by previous products being contained, temperature on hull and the way the filling nozzle is fitted to the tank being filled.

Taking cognizance of the considerations given above the vapor flow generated during filling operations always tends to be slightly higher than the volumetric filling rate of liquid loaded. The vapor flow divided by the product flow is defined as the vapor growth factor.

The vapor flow consisting of hydrocarbon and air or inert gas is led through a vapor header, manifold or line, to the VRU where it enters a filter unit in adsorption mode (I or II).

The vapors entering the carbon bed normally contain up to 1400 g hydrocar- bons /m³. In the standard VRU system this concentration is reduced to less than 10 g hydrocarbons /m³ in the outlet 15, 16, 28 from the filter units of the VRU.

When the VRU is in operation, one filter unit is in adsorption mode while at least one other filter unit is being regenerated (desorption phase). The beds are automatically switched between adsorption and desorption e.g. according to a fixed time cycle of approximately 15 minutes. The process of switching between adsorption and desorption can also be in economy mode, in economy mode there can be several hours between a switch between adsorption and desorption. When a vacuum regenerated carbon bed is loaded with a mixture of air and hydrocarbon, a temperature rise occurs within the bed. An increase of 20 - 40 Kelvin is normal.

During adsorption the bed can be divided into three zones namely the Inlet Zone, Adsorption Zone and the Outlet Zone. The inlet zone is on the bottom of the bed where the carbon is saturated with hydrocarbons and equilibrium has been achieved. The higher the concentration of hydrocarbon vapor, the more hydrocarbons will be adsorbed. The second zone, the adsorption zone, is also called the Mass Transfer Zone (MTZ). The concentration of hydrocarbons in the MTZ decreases as the distance from the Inlet Zone increases. The outlet zone is in the top of the bed. In this zone the adsorbent is free from adsorption. Should the MTZ reach the top of the bed, a "break through" will occur where a high concentration of hydrocarbons will leave at the outlet from the VRU.

Under normal circumstances an adsorption cycle of around 15 minutes will ensure that a shift to a clean bed is carried out well before a break through of the MTZ can take place. Under normal circumstances a fixed cycle time will be at least 5 minutes as is necessary to have operation time beside the time it takes time to change valves position during operation.

During the adsorption phase in an activated carbon bed there is a potential risk of a "hot-spot" developing in the bed. The risk of forming a hot spot is increased if compounds like ketones, aldehydes or organic acids are passed through the VRU.

In order to detect a possible hot spot the VRU can be equipped with two independent safety measures. One system will indicate a rise of temperature in the top and in the bottom of the beds by means of temperature transmitters. The second system monitors the CO concentration in the outlet by means of a gas detector. Even a small hot spot will develop CO which is detected by the CO monitor. If a hot spot is detected by a safety system, the VRU is shut down. Automatic valves at the inlet and outlet of the VRU isolate the adsorption bed. This will shut the oxygen supply to the adsorption bed off, preventing the hot spot from developing any further.

In order to remove a hot spot and cool the bed, the bed first needs to be purged with nitrogen. The heat can be removed by purging with nitrogen for a certain period of time.

When a adsorbent bed of a filter unit has been saturated with hydrocarbons, it needs to be regenerated to restore its adsorption capacity. As adsorption is a reversible process, it is possible to desorb the hydrocarbons by changing the adsorption equilibrium. Desorption is achieved by lowering the pressure in the adsorbent vessel hereby changing the adsorbate partial pressure.

In practice the bed is evacuated to 105 mbar (abs) by means of the vacuum pump 9. The vacuum can e.g. be generated by a vacuum pump that runs continuously. In order to prevent overloading of the pump 9 and the separa- tor/absorber 40, the flow of gas is restricted by a not shown control valve placed in the suction line inlet to the vacuum pump. The suction pressure of the pump can initially be kept constant at 300 mbar (abs) by means of a pressure controller. Once 300 mbar is reached in the bed being subjected to desorption, the control valve is opened fully. The pressure then decreases further and at ap- proximately 105 mbar (abs) the valve 7 or 8 on top of the bed is opened to purge the bed with air. This ensures the complete removal of hydrocarbons (complete desorption). Depending on the load of hydrocarbons on the adsorbent, the time for reaching the 105 mbar (abs) will vary. A timer function in a PLC (Programmable Logic Control) will ensure that the purge air valve is open during a minimum time period, e.g. the last 4 minutes, of the desorption sequence. When the desorption sequence is finished the valve 5 or 6 in the vacuum conduit 17 or 18 is closed and the bed pressure is equalized by 21 or 22.

Once the bed has been equalized it is ready for switching to adsorption mode. The purge and equalizing valves are closed and vapor inlet and outlet valves are opened. As a safety feature the bed pressure is checked at the end of the equalizing phase to ensure that there is no vacuum in the bed. Should there be a vacuum, the VRU will automatically shut down and an alarm is raised.

PLC programs can control the adsorption / desorption of the filter units in a cycle. The cycle consists of a sequence of steps and these steps can be divided into two half-periods. In the first half period one bed is in adsorption mode while the other undergoes regeneration, in the second half-period the beds have switched.

The hydrocarbons desorbed in the filter units enter the absorber column 40 from the bottom and rise towards the outlet in the top. The gas containing hydrocarbons is scrubbed by a counter current flow of absorbent entering through connection 46 running through the absorption packing down the column. The effi- ciency of the absorption is dependent of the absorber liquid vapor pressure, temperature of the absorbent, pressure in the column and the ratio in which gas and liquid are mixed.

The "fresh" absorbent is pumped by pump 44 through connection 46 to the ab- sorber 40. After being used in the absorber, the absorbent, plus the recovered product, is collected in the absorbent section 42. The absorbent containing the recovered hydrocarbons is returned to the product line by a pump 43.

Fig. 3 illustrates an embodiment of the invention where the filter comprises three filter units I, Il and III. The three filter units I₁ II and III have vapor inlet valves 1 , 2 and 23 and vapor outlet valves 3, 4 and 27. When the vapor inlet valves are open, the polluted gas passes through the inlet conduits 31 , 32 or 33, and when the vapor outlet valves are open clean gas can pass through the outlet conduits 15, 16 or 28. The filter units are provided with vents or equalizing valves 21 , 22 and 25 for equalizing the pressure inside the filter units with atmospheric pressure and with purge air valves 7, 8 and 26 for letting in purge air during regeneration. The filter units are also provided with suction/balancing valves 5, 6 and 29 which valves are used both when regenerating a filter unit and when balancing the pressure between the filter units. When the suction/balancing valves are used during regeneration they are open at the same time as the common suction valve 30.

In table 2 is given an overview of the continuous repetition of the cycles of adsorption, regeneration and equalizing for a filter with three filter units.

Table 2

r-

Step description Step in ^as

Unit/Register at Stop ^as σ\ ^as

III -desoφtion phase 1 (suction) 1 a a O O X X X O O X X X X X X X O O III -desorption phase 2 (purge) 2 a a O O X X X O O X X X X X O X O O III - balancing phase 3 a X X X X O O O X X X X X X X O X III - equalisation phase 1 4 a X X X X X O O X X X X X X O X X

I -desorption phase 1 (suction) 5 a a X X X X O O O X X X O O X X X O I -desorption phase 2 (purge) 6 a a X X O X ^iil ^Eve^quazo^{r ng} vaⁱ O O O X X X O O X X X O I -balancing phase 7 a X X X X O ^bl ^Stnaanuconc X X X X O O O X X X X

I - euqalisation phase 1 8 a X X X O X X X X X X O O X X X X

II -desorption phase 1 (suction) 9 a a O O X X X X X X X O O O X X X O II -desoφtion phase 2 (purge) 10 a a O O X X X X X O X O O O X X X O II - balancing phase 11 a O O X X X X X X X O X X X X O X

II - euqalisation phase 1 12 a O O X X X X X X O X X X X X X X x : Valves closed ; o : Valves open ; a : Bed in adsoφtion

^f B^{d il} ^Poe^errvu^rge ^! va ^{fdll i} B ^E;eo^rev^qa ^■ n^g vau ^{il bli} ^Stv n^g vaeccoⁱaanu

^il C^t on vavme n ^sucom Step 1 : The filter units I and Il are in adsorption mode where the inlet and outlet valves 1 ,3,2,4 are open allowing polluted gas to pass through the filter units. The filter unit III is in regeneration mode and therefore the suction/balancing valve 29 is opened as well as the common suction valve 30 is opened. Step 2: The filter units are in same mode as in step 1 but purge air is let into the filter unit III by opening the purge air valve 26.

Step 3: Purge air valve 26 has been closed and filter unit III has finished regeneration and enters balancing mode as suction valve 30 is closed while the suction/balancing valve 29 is kept open. Filter unit I has finished adsorption mode by closing the inlet and outlet valves 1 and 3 and has entered balancing mode by opening the suction/balancing valve 5. Filter unit Il stays in adsorption mode by keeping the inlet and outlet valves 2 and 4 open and suction/balancing valve 6 closed. Step 4: Suction/balancing valves 5 and 29 for filter unit I and III are closed and equalizing valve 25 for filter unit III is opened in order to equalize the pressure in filter unit III to atmospheric pressure.

Step 5: The filter units Il and III are in adsorption mode, and the inlet and outlet valves 2, 4, 23, 27 are open allowing polluted gas to pass through the filter units. The filter unit I is in regeneration mode and the inlet and outlet valves 1 and 3 are closed, the suction/balancing valve 5 as well as the common suction valve 30 are opened.

Step 6: The filter units are in same mode as in step 5 but purge air is let into the filter unit I by opening the purge air valve 7. Step 7: Purge air valve 7 has been closed and filter unit I has finished regen- eration and enters balancing mode as suction valve 30 is closed while the suction/balancing valve 5 is kept open. Filter unit III stays in adsorption mode by keeping the inlet and outlet valves 23 and 27 open and suction/balancing valve 29 closed. Filter unit Il enters balancing mode by keeping the suction/balancing valve 6 open while the suction/balancing valve 30 is closed. Step 8: Suction/balancing valves 5 and 6 for filter unit I and Il are closed and equalizing valve 21 for filter unit I is opened in order to equalize the pressure in filter unit I to atmospheric pressure. Step 9: The filter units I and III are in adsorption mode, and the inlet and outlet valves 1 , 3, 23, 27 are open allowing polluted gas to pass through the filter units. The filter unit Il is in regeneration mode and therefore the inlet and outlet valves 2 and 4 have been closed and the suction/balancing valve 6 is opened as well as the common suction valve 30 is opened.

Step 10: The filter units are in same mode as in step 9 but purge air is let into the filter unit Il by opening the purge air valve 8.

Step 11: The purge air valve 8 is closed and the filter unit III has left adsorption mode by closing the inlet and outlet valves 23 and 27 and has entered balanc- ing mode by opening suction/balancing valve 29 and closing common suction valve 30. Filter unit I stays in adsorption mode by keeping inlet and outlet valves 1 and 3 open and suction/balancing valve 5 closed. Filter unit Il enters balancing mode by keeping the suction/balancing valve 6 open while the common suction valve 30 is closed. Step 12: Suction/balancing valves 6 and 29 for filter unit Il and III are closed and equalizing valve 22 for filter unit Il is opened in order to equalize the pressure in filter unit Il to atmospheric pressure.

Claims

1. Apparatus comprising

- at least two filter units (I, ll.lll) each receiving polluted gas through a first connection (11 +13,12+14, 31 , 32, 33) and each discharging cleaned gas through a second connection (15, 16, 28) and

- at least one third connection (20, 34, 35, 36) provided with at least one device (19, 5, 6, 29) which allows for passage of a gas stream or blocks passage of a gas stream, connects the at least two filters (I, II, III) in such a way that gas can flow from one filter to the other when the device (19, 5, 6, 29) allows for passage of a gas stream,

- where the first and second connections each are provided with devices (1 , 2, 3, 4, 23, 27) which allow for passage of a gas stream or block the gas stream, and

- where each filter (I, II, III) is working both at a higher pressure p1 and at a lower pressure p2, characterized in that the third connection or tertiary connections combine the entrance or discharge of one filter to the entrance of another filter.

2. Apparatus according to claim 1 , characterized in that the apparatus com- prises at least three filters (I₁ II, III).

3. Apparatus according to claim 2, characterized in that the apparatus comprises four or five filters.

4. Apparatus according to claim 2, characterized in that it comprises three tertiary connections (34, 35, 36) connecting filter I and II, filter I and III and filter Il and III, respectively.

5. Apparatus according to claim 1-4, characterized in that the third connection or tertiary connections combine the entrance of one filter to the entrance of another filter.

6. Apparatus according to claim 1 , characterized in that the at least two filters (I, II, III) comprise a filter material which can adsorb volatile organic compounds (VOC) e.g. activated carbon, zeolites or silica gel.

7. Method where polluting compounds are adsorbed in a first filter at a higher pressure and a second filter is regenerated at the same time at a lower pressure; a) when the first filter is saturated with polluting substance to a certain degree, the inflow and the outflow from both the first and the second filter are blocked, b) then a connection (20) allowing gas to flow between the first filter and the inlet end of the second filter is opened thereby allowing gas to flow from the first filter to the second filter in order to equalise the pressure; c) after equalising the pressure the connection between the first filter and the second filter is closed and the first filter is subjected to an active reduction of pressure e.g. by a vacuum pump in order to regenerate the first filter.

8. Method where polluting compounds are adsorbed in the first filter at a higher pressure, a second filter is regenerated at the same time at a lower pressure and a third filter is either adsorbing polluting compounds at a higher pressure or being prepared for adsorbing polluting compounds; a) when the first filter is saturated with polluting substance to a certain degree, inflow to and outflow from both the first and the second filter are blocked and inflow to and outflow from the third filter is opened, b) then a connection allowing gas to flow between the first filter and the inlet end of the second filter is opened thereby allowing gas to flow from the first filter to the second filter in order to equalise the pressure while polluted gas flows into the third filter; c) after equalising the pressure the connection between the first filter and the second filter is closed and the first filter is subjected to an active reduction of pressure e.g. by a vacuum pump in order to regenerate the first filter.

9. Method for regeneration of a first filter according to claim 7 or 8, characterized in that the steps a) to c) are repeated each time a filter needs to be regenerated.

10. Method according to claim 7, 8 or 9, characterized in that the pressure difference Δp between the first and the second filter is between 1100 mbar and 500 mbar, preferably Δp > 700 mbar.