Abstract:
A gas under pressue is sent to a first compartment of a separation module having permeation membrane, the first modified gas mixture which exits from the first compartment being introduced into the enclosure to constitute therein a controlled atmosphere which is removed and circulated in counter-current in the second compartment of the module towards the discharge orifice of the latter so as to substantially recombine the two gaseous modified mixtues before discharging same thereby increasing the performances of the separation module.

Description:
BACKGROUND OF INVENTION 
     1. a) Field of the Invention 
     The present invention concerns a process for preparing a controlled atmosphere inside an enclosure and an enclosure for carrying out this process. 
     More particularly, the invention concerns a process for providing, within an enclosure, a controlled atmosphere, of the type comprising the steps of supplying a gaseous mixture under pressure to a first compartment of a membrane type separation module, circulating through the enclosure a first one of the modified gaseous mixtures from the module to constitute the controlled atmosphere, and recombining the controlled atmosphere withdrawn from the enclosure with the second of the modified mixtures emanating from the module before rejection into the surrounding atmosphere. 
     2. a) Description of Prior Art 
     Such a process is described in the document EP-A-0.358.359, recombination being carried out in the duct for the evacuation of the residual gas from the adsorbent or membrane type separation module. 
     OBJECT OF INVENTION 
     It is an object of the present invention to propose an improved process enabling to improve the operation and the yield of the membrane type separation module, for example by reducing the useful surface of the membrane and the flow of gaseous mixture under pressure, thereby reducing congestion in the module and its manufacturing cost, and enabling to more easily integrate same in or on the enclosures. 
     SUMMARY OF THE INVENTION 
     For this purpose, according to a characteristic of the invention, the recombination is carried out in the second compartment of the module, typically in the low pressure compartment, the first modified mixture, which is introduced into the enclosure, being expanded at a pressure which is slightly higher than the pressure within the low pressure compartment of the module, the controlled atmosphere, from the enclosure, being circulated in the low pressure compartment in counter-current with the gaseous flow from the high pressure compartment of the module. 
     It is also an object of the present invention to provide an enclosure structure which is associated with a membrane type separation module in a compact arrangement and of low manufacturing and assembling cost. 
     For this purpose, according to another characteristic of the invention, such an enclosure, of the type comprising at least one interior volume connected, by means of a first duct, to a first compartment of a membrane type separation module which can be connected to a source of a gaseous mixture under pressure, and, by means of a second duct, to an orifice for the discharge into ambient atmosphere communicating with a second compartment of the module, is characterized in that the second duct consists at least partially of the second compartment of the module. 
     The process and the enclosure according to the invention are useful for protection against oxidation or some contamination of raw materials, for example food products, semi-finished products, during their manufacture, or finished products, for a period of time which varies before transformation or marketing, for example, in the secondary electronic industry. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     Other characteristics and advantages of the present invention will appear from the description which follows of embodiments, given by way of illustration, but without limitations, with reference to the annexed drawing, in which: 
     FIG. 1 is a schematic view in cross-section of a first embodiment of the invention; and 
     FIG. 2 is a fragmentary partial cross-section view of another embodiment of the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the description which follows and in the drawing, identical or similar elements are referred to by the same reference numerals. 
     FIG. 1 illustrates a portion of a substantially impervious enclosure 1, defining at least an interior volume 2, associated with a module for separation by permeation 3, comprising at least one permeation membrane 4 separating an upstream or high pressure compartment 5 from a downstream or low pressure compartment 6. Module 3 includes, at a first end, a terminal 7 for connection to a source of gaseous mixture under pressure, typically a network for supplying industrial compressed air at a pressure between 2.5 and 6×10 5  Pa, opening in the high pressure compartment 5 and, at the other end, a duct 8 in communication with the high pressure compartment 5 and extending inside the volume 2. Duct 8 is provided with a throat 9 to reduce the pressure of the modified gaseous mixture from the high pressure compartment 5 of module 3 and which is introduced into the volume 2 at a slightly higher pressure than that of the low pressure compartment 6. The latter communicates, on the side of the first end of the module 3, with a duct 10 for discharge into the surrounding atmosphere and, in the vicinity of the second end of the module 3, with a duct portion 11 in communication with the upper part of volume 2 and provided with a trap valve 12. 
     With this arrangement, the first modified gaseous mixture, typically a mixture for providing inertness and which is poor in oxygen and consists essentially of nitrogen, which is introduced into the volume 2 of enclosure 1 to constitute therein a controlled atmosphere, escapes from the latter, slightly modified in its composition by the products which are present in the enclosure, through duct 11 and high pressure chamber 6 in counter-current to the gaseous flow which circulates in the high pressure chamber 5. The introduction of the mixture to provide inertness and which is poor in oxygen, into low pressure chamber 6 lowers the partial pressure of oxygen in the latter, which thus enables to substantially improve the operation of the membrane. By way of example for a membrane whose oxygen/nitrogen selectivity is 3;4, and which is made of polyimide, flushing of low pressure compartment 6 with the total gas for providing inertness condition which is produced and has circulated through volume 3 enables, for a same production of this gas for providing inertness, at the same purity of about 99%, to reduce the required membrane surface by about 25% and the necessary flow of compressed air by about 20%. The gaseous mixture which is withdrawn by discharge duct 10 has a composition which is essentially identical to that (air) introduced into connection 7, with an oxygen content of about 21%, which thus removes the risk which could be due, without recombination, to an over-oxygenation of the area surrounding the enclosure, for example a fire, or a rejection of too much gas for providing inertness, for example anoxia. 
     With such an arrangement, the separation module 3 enables on the other hand a recycling of the vapors or scents which are released by the products which are stored in enclosure 1. On the other hand, if enclosure 1 is maintained at a temperature (hot or cold) which is different from room temperature, module 3 permits a function of heat exchange between the injected gaseous mixture and the mixture which is extracted from the enclosure, which enables to reduce the energy consumption of the storage assembly. 
     In practice, the interior volume 2 of enclosure 1 is kept at a low pressure and the fact that the first modified gaseous mixture, typically one for providing inertness, issued from the high pressure compartment of the separation module, is utilized at a pressure only slightly above atmosphere pressure enables to substantially simplify the construction of the membrane type module. 
     FIG. 2 illustrates an embodiment wherein the first modified gaseous mixture is expanded by the membrane itself, which, for example, is made of a plurality of bundles of hollow fibers 4, for example of polyimide, each bundle of hollow fibers being inserted, through one of its ends, into an orifice 13 formed in a collector tube 14, for example of PVC or reinforced polyethylene, the imperviouness at the level of the orifices being obtained by the controlled application of epoxide resin (by dipping, brushing or injection under pressure via tube 14). Tube 14 and the bundles of fibers 4 are placed in a casing 15 mounted in the interior volume 2 of enclosure 1. Two partitions 16 and 17 of casing 15 which are parallel to one another respectively define a chamber 6 receiving the tube 14 and the bundles of fibers 4 and constituting the low pressure compartment of the module, and a chamber or a series of ducts 11 defining a communication between the upper part of the interior volume 2 and the bottom of chamber 6 via a pivoting flap constituting a trap valve 12. Tube 14 is placed in the upper part of chamber 6 and fibers 4 normally extend past a bottom 18 of casing 15 which defines with the bottom of enclosure 1, a duct 8 providing communication between the exit ends of fibers 4, ending in duct 4, and volume 2. The collector tube 14 is connected at an end of the enclosure to a connection 7 to a source of air under pressure, its other end being sealed. The upper partition of chamber 6 is provided with one or more openings 10 defining a discharge orifice towards the surrounding atmosphere of the permeation module. 
     According to a variant, in order to optimize the gaseous separation with the bundle of fibers 4, the partitions defining chamber 6 are arranged so that the transverse cross-section of the latter progressively decreases from top to bottom, i.e. from connector tube 14 to the lower narrowing duct receiving the free ends of the fibers. 
     It will be understood that according to embodiments, the length and the diameter of the hollow fibers 4, which interiorly define the high pressure compartment of the module, determine the composition of the first gaseous mixture produced for a given supply pressure, as well as the reduced pressure for delivering this first gaseous mixture, which enables to remove the sealed compartment under pressure on the production side of the first gaseous mixture which is normally found in traditional hollow fiber cartridges.