METHOD AND APPARATUS FOR REGULATING AN ATMOSPHERE

Abstract: A method and an apparatus for regulating an atmosphere in an essentially closed space are described. The atmosphere contains a condensable substance, inert gas and oxygen, and the method comprises the following steps: withdrawing said atmosphere from the space and passing the same, forming a process gas flow to a concentrator unit wherein said condensable substance is separated from the rest of said process gas flow; returning at least a part of said process gas flow to the space; and bringing said condensable substance from said concentrator unit to a condensation unit and condensing said condensable substance in said condensation unit.

Description: METHOD AND APPARATUS FOR REGULATING AN ATMOSPHERE FIELD OF INVENTION The present invention relates generally to a method and an apparatus for regulating the atmosphere m an essentially closed space. BACKGROUND It is previously known to inert spaces, into which objects are brought and continuously treateα. These spaces often contain condensable suostances, such as volatile organic compounαs (VOC), e.g. solvents, and hydrocarbons. The object of the inerting process is to regulate the atmosphere, e.g. to keep the level of 0 2 on a level at which the atmosphere is non-explosive. Another reason to keep the 0; low is to maintain the quality of the treated objects because a high 0 2 level may have a detrimental effect on the final result of the treatment. A preferred inert gas supplied to tne space is e.g. nitrogen. In this kind of mertion, a large flow of nitrogen to the inerted space is often needed which leads to large costs for nitrogen. Because there is solvent vapour m the space the outgoing process flow from the space will contain not only nitrogen and oxy- gen but also e.g. solvent vapour as well. This means additional costs for the solvent and also for environmental influence. Also, m many countries stricter environmental requirements have required investment m purifying equipment m order to keep the amount of dis- charged VOC low and thereby to continue with the operation. The European patent publication EP-0 094 172 discloses a method and an apparatus for recovering solvent vapour from an oven chamber or driver wherein a material balance is maintained with respect to the chamber atmos- phere. The atmosphere is withdrawn from the oven at a substantially constant flow rate and the uncondensed gas stream is returned to the oven at a rate that depends on sensed changes in the solvent vapour concentration. Thereoy the combined rates at which solvent vapour is formed m saiα oven and at which the uncondensed gas stream is returned to the oven mass balance the rate at which the oven atmosphere is withdrawn from the oven. A problem with the described method and apparatus is that that the requirements for low discharges of VOC are not fulfilled m a cost-efficient way. OBJECT OF THE INVENTION An object of the present invention is to provide a etnod and an apparatus for regulating the atmosphere m an essentially closed space whereby the above mentioned drawbacks of prior art are avoided or at least mitigated and which are cost-efficient and limits tne amount of discharged VOC. SUMMARY OF THE INVENTION The invention is based on the realisation, that the outgoing process gas flow can be purified m a cost- efficient way by means of a combination of a concentrator and a condensation plant. According to a first aspect of the invention there is provided a method for regulating an atmosphere in an essentially closed space, said atmosphere containing a condensable substance, inert gas and oxygen, comprising the following steps: a) withdrawing said atmosphere from the space and passing the same, forming a process gas flow to a concentrator unit wherein said condensable substance is separated from the rest of said process gas flow; b) returning at least a part of said process gas flow to the space; and c) bringing said condensable substance from said concentrator unit to a condensation unit and condensing said condensable substance in said condensation unit. According to a second aspect of the invention there is provided an apparatus for regulating an atmosphere m an essentially closed space, said atmosphere comprising condensable substance, said apparatus comprising a source of inert gas connected to said space, and a condensation unit for condensation of said condensable substance, characterised m that the apparatus further comprises a concentrator unit provided between said space and said condensation unit, said concentrator unit increasing the level of condensable substance m the flow leaving the concentrator unit to the condensation unit compared to that of the flow entering the concentrator unit from the space. Further preferred embodiments are defined m the dependent claims. BRIEF DESCRIPTION OF DRAWINGS The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 is a block diagram showing an apparatus according to the invention for maintaining a desired atmosphere m an essentially closed space; FIG. 2 is a diagram showing a concentrator shown m FIG. 1 m more detail; and FIG. 3 is a block diagram of an alternative embodiment of an apparatus according to the invention for maintaining a desired atmosphere m an essentially closed space . DETAILED DESCRIPTION OF THE INVENTION In the following, a first embodiment of an apparatus and a process according to the invention for treatment of objects in an essentially closed space will be described witn reference to Figs. 1 and 2. The objects (not shown) that are to be treated are moved into and through an essentially closed space 2. The space 2 is well known m the art and comprises openings for the objects to be treated, m some cases some type of gas curtain or other device m order to minimise the amount of oxygen entering the space through the openings etc. The space 2 is connected to a concentrator 5 by means of an outgoing conduit 3 and an ingoing conduit 4. An outlet valve 6 is provided m the ingoing conduit 4, the function of which valve will be described later. The concentrator 5 is m turn connected to a cryo condensation plant (CCP) 9 through an outgoing conduit 7 and an ingoing conduit 8. Finally, there is provided a source of liquid nitrogen (LIN source) 10 connected to the CCP 9 through a con- duit 11. The LIN source 10 is also connected to the space 2 through an evaporator 12 and conduits 13 and 14. The conduit 13 also connects to the CCP 9. The operation of the above-described apparatus will be described m the following. At start-up, the 0 2 level m the space 2 exceeds a desired level, e.g. 3%. In order to lower the Oc level pure nitrogen is conducted to the space 2 from the LIN source 10 through the evaporator 12 and conduits 13 and 14. Thus, the function of the evaporator 12 is to avoid having nitrogen m liquid form enter the space 2, which would lead to damage to the space and objects therein. Nitrogen is supplied from the LIN source 10 to the space 2 until the 0_ level therein is lowered to the desired level. The objects to be treated, such as glass bottles or video tapes, are then brought into and through the space 2 through openings (not shown) provided therefor. Nitrogen is constantly supplied to the space 2 m order to keep the oxygen level on the de- sired level, because new oxygen constantly leaks into the space 2. In order to maintain the pressure m the space 2, a flow of process gas comprising nitrogen, solvent vapour and oxygen will then be conducted by means of a fan (not shown) from the space 2 through the outgoing conduit 3 and into the concentrator 5. The fan can be provided m either conduit 3 or conduit 4. The concentrator 5 will now be described m detail with reference to FIG. 2. The process gas flow enters the concentrator 5 through the conduit 3 and is brought to one of two purifying beds 20a, 20b. There is provided a valve 21a-d m each inlet and outlet of the beds, by which valves the bed to be used for purifying the process gas flow is selected. The bed not used for purifying the process gas flow is then shut off by means of the valves 21a-d. If e.g. bed 20a is to be used, valves 21a and 21b are open whereas valves 21c and 21d are closed. The beds 20a, 20b comprise some suitable purifying material, such as zeolite or active carbon. When the process gas flow is led through the beds, the solvent vapour is separated from the process gas and is retained in tne purifying beds. The purified process gas flow then leaves the concentrator via the conduit 4. The beds 20a, 20b are also connected to the conduits 7 and 8 through valves 22a, 22b and 22c, 22d, respec- tively. These connections are used when the beds are to be cleaned, i.e., when they are saturated with solvent. This cleaning process will now be described. The conduits 7 and 8, together with the concentrator 5 and the CCP 9, form an essentially closed system for nitrogen. A fan (not shown) is provided m the conduit 8 for the circulation of the nitrogen. There is also provided a heater 23 m the conduit 8 for heating the nitrogen that is about to enter the concentrator 5, see FIG. 2. The nitrogen entering the concentrator from conduit 8 is brought to the one of the beds 20a, 20b that is saturated with solvent and therefore presently not used for purifying the process gas flow from the space 2. The selection of bed 20a, 20b is effected by means of valves 22a-d. Thus, when bed 20a is used for purifying the process gas flow, the nitrogen from conduit 8 is brought to the saturated bed 20b and vice versa. Thus, when valve 21a is open then valve 22a must be closed and vice versa, i.e., the two valves 21a, 22a must not be open at the same time. However, it is possible for the two valves 21a, 22a to be closed at the same time, e.g. when the bed 20a has been cleaned but the other bed 20b is still functioning for purifying the process gas flow. The same applies for the other valve pairs 21b, 22b etc. When the heated nitrogen enters the bed 20a or 20b from conduit 8, the solvent m the bed is evaporated and brought with the heated nitrogen flow leaving the concentrator 5 through the conduit 7 to the CCP 9. In that way the selected bed is cleaned and can thereafter be used to purify the incoming process gas from the space 2. The function of the CCP 9 will now be described. It works according to the principles of cryo condensation, which will now be explained. In a cryo condensation plant for condensing e.g. solvent vapour, the effi- ciency depends on the concentration of solvent m the process gas. At higher solvent concentrations, a larger part of the cooling effect can be used for condensation of solvent. At lower concentration a larger part of the cooling effect will be used for cooling nitrogen, and this leads to a less efficient use of the cooling capacity. In other words, in order to condense the same amount of solvent a much larger amount of LIN would be needed for cooling m the case no concentrator was used. Thus, the incoming nitrogen-solvent flow from conduit 7 is cooled to a very low temperature, whereby a major part of the solvent is condensed. The cooling is effected by means of liquid nitrogen brought from the LIN source 10 via the conduit 11. The CCP 9 then works as a heat exchanger, wherein the liquid nitrogen from the LIN source is evaporated thereby cooling the nitrogen- solvent flow coming from the concentrator 5. The condensed solvent is collected and can thereafter be returned to the merted space 2 and reused m the process by means of some suitable piping means, shown as arrow 26 m the figures. Thus, this reuse of solvent provides an inexpensive and effective means for lowering tne costs for solvent. Due to the concentrator 5, the cooling process m the CCP 9 is a very effective one because the nitrogen- solvent flow from the concentrator 5 contains a relatively large portion of solvent. The evaporated pure nitrogen from the LIN source 10, after being used for cooling, is then carried to the space 2 as a part of the nitrogen added through conduit 13. In normal operation, when limited amounts of oxygen is leaking into the space 2, the nitrogen used for the cryo condensation and the amount of nitrogen needed for mertmg the space 2 is essentially balanced, i.e., all nitrogen used for cooling the CCP goes directly onward to the merted space 2 and small amounts of or no nitrogen has to be used m the CCP that can not be used m the merted space 2. This means that there is no additional cost for the cryo condensation process the CCP 9. In effect, addition to working as a con- denser, the CCP 9 also functions as an evaporator, further increasing the cost-efficiency of the inventive method and apparatus. A second embodiment of the invention will now be de- scribed with reference to FIG. 3. The only change from the embodiment described with reference to Figs. 1 and 2 lies m the CCP 9. In the first embodiment, this functioned as a heat exchanger with two separate systems, viz. the system connected to the concentrator 5 with the function to condense the solvent and the system connected to the LIN source 10 with the function to cool the nitrogen carrying the solvent, respectively. In this second embodiment, the CCP works a different way. As seen m FIG. 3, the CCP 9 is still connected to the LIN source 10 and the liquid nitrogen supplied through conduit 11 is used for cooling. However, instead of being carried to the space 2, the then evaporated nitrogen used for cooling is supplied to the concentrator 5 through the conduit 8. This nitrogen is heated by heater 23, see FIG. 2, before being brought to one of the beds 20a, 20b to be cleaned, just as in the first embodiment. In this second embodiment, however, the fan (not shown) used for circulating the nitrogen m con- duit 8 can be omitted, as the pressure from the LIN source 10 is sufficient for forcing the nitrogen through the system. After leaving the concentrator 5, the nitrogen used for cleaning the beds is carried through conduit 7 to the CCP 9, wherein the solvent is condensed. Thereafter, the nitrogen, now with only a minor part of solvent, is lead to the space 2 for ertion. Thus, the system CCP 9, concentrator 5 and conduits 7, 8 no longer forms a closed system for nitrogen. This second embodiment provides several advantages. Firstly, the fan m conduit 8 can be omitted, thus saving costs. Secondly, the nitrogen supplied through this conduit 8 is pure, i.e., it contains essentially no oxygen as it comes essentially directly from the LIN source 10. This eliminates the hazards connected to the neater 23 as already small amounts of oxygen in contact therewith can cause an explosion. The self-regulatmg system of mert g nitrogen coming from the CCP 9 and the evaporator 12 will now be described with reference to FIG. 1. As is seen m that figure, there are provided two pressure controllers 28, 29 m conduit 13 and the conduit 14 leaving the evaporator 12, respectively. The set pressure value of controller 28 is slightly higher than that of controller 29, e.g. 200 mbar and 190 mbar, respectively. The nitrogen conduit 13 then comes primarily from the CCP 9 and only m case there is not sufficient nitrogen supply therefrom, i.e., the pressure drops m conduit 13 before pressure controller 28, nitrogen will be supplied from the evaporator 12 and conduit 14. In that way it is ensured, that the nitrogen used for cooling the CCP 9 also will be used for mertmg the space 2 and that nitrogen coming from the LIN source 10 through the evaporator 12 will be used only m case the nitrogen used for cooling is not sufficient for mertmg the space 2. Thus, this provides a very cost-efficient way of reducing the amount of LIN used. The function of the valve 6 will now be described with reference to FIG. 1. A fundamental feature of the process is that oxygen leaks into the space and an in- erting gas, e.g. nitrogen, must therefore be supplied to the space 2. Oxygen and nitrogen together with solvent vapour is carried as a process gas flow from the space 2 to the concentrator 5. The nitrogen together with oxygen is then returned to the space 2 through conduit 4. In case all the nitrogen-oxygen mixture is returned, this would cause an excess pressure in the space 2. Therefore, some of this nitrogen-oxygen mixture is bled off through valve 6 and into the surrounding environment. This has been made feasible due to the fact that this nitrogen-oxygen mixture is essentially free of solvent vapour and thus does not constitute an environmental hazard. By using the method and the apparatus according to the invention, the consumption of LIN can be substantially reduced. It has been experimentally shown, that the LIN consumption can be lowered by a factor 8-10. The number of times the nitrogen can be reused depends on how much oxygen is leaking into the space 2. In addition, the condensed solvent can be brought back to the process, whereby the amount of added solvent can be reduced. The LIN used for reducing the level of oxygen can be used not only for that but also to condense the solvent after the concentrator. Often when using solely cryogen condensation the requirements on VOC discharge can not be fulfilled. With the method and the apparatus according to the invention, wherein also a concentrator is used, the VOC discharge can be kept on a low level complying with the requirements also countries with very strict rules regarding VOC discharge. In view of the foregoing description it will be evident to a person skilled m the art that various modifica- tions may be made within the scope of the claims. For example, although two cleaning beds have been shown, any number of beds can be used. If only one bed is used, the process is run intermittently, i.e., the process is run until the bed is saturated with solvent and then the mertmg process is shut down and the process cleaning the bed is initiated. Also, the inventive method and apparatus are not limited to VOC, such as solvents, but is also applicable to other types of condensable substances, such as hydrocarbons.

Claims: CLAIMS 1. A method for regulating an atmosphere in an essentially closed space, said atmosphere containing a condensable substance, inert gas and oxygen, comprising the following steps: a) withdrawing said atmosphere from the space and passing the same, forming a process gas flow to a concentrator unit wherein said condensable substance is separated from the rest of said process gas flow; b) returning at least a part of said process gas flow to the space; and c) bringing said condensable substance from said concentrator unit to a condensation unit and condensing said condensable substance in said condensation unit . 2. A method according to claim 1, charac- teri s ed i n that said condensing in step c) comprises cryo condensing. 3. A method according to claim 1 or 2, characteri sed i n that step a) comprises bringing the process gas flow to a cleaning bed in said concen- trator unit and in that step c) comprises evaporating said condensable substance and bringing said condensable substance from said cleaning bed to said condensation unit by means of heated inert gas. 4. A method according to any of claims 1-3, characteri sed in that step c) comprises bringing cool inert gas to the condensation unit for effecting said condensing. 5. A method according to claim 4, characteri sed in the additional step d) of bringing said inert gas used for effecting said condensing to the space for inerting the atmosphere therein. 6. A method according to claim 4, characterised in the additional step d) of bringing said inert gas used for effecting said condensing to the concentrator unit for evaporating said condensable substance and bringing said condensable substance from said cleaning bed to said condensation unit. 7. An apparatus for regulating an atmosphere in an essentially closed space (2), said atmosphere com- prising condensable substance, said apparatus comprising a source of inert gas (10) connected to said space (2), and a condensation unit (9) for condensation of said condensable substance, characteri sed in that the apparatus further comprises a concentrator unit (5) provided between said space (2) and said condensation unit (9), said concentrator unit increasing the level of condensable substance in the flow leaving the concentrator unit to the condensation unit (9) compared to that of the flow entering the concentrator unit from the space (2) . 8. An apparatus according to claim 7, characteri sed in that said concentrator unit (5) comprises at least one cleaning bed (20a, 20b) con- nected to said space (2) and said condensation unit (9) . 9. An apparatus according to claim 8, characteri sed in that said at least one cleaning bed (20a, 20b) comprises zeolite.

Inventors: JOHANSSON SVEN-AAKE, JOHANSSON, SVEN-AAKE
Applicants: AGA AB, JOHANSSON SVEN AAKE, JOHANSSON, SVEN-AAKE
Application year: 1999
Main classification: F26B
IPC codes: F26B21, F26B25