BE659695A - - Google Patents

Description

       

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  "Improvements to the preservation of fruits and similar products"
The present invention relates to improvements in the preservation of fruits and similar products, either at a fixed position or during transport.



     It is known that the ripening of fruits in an enclosure can be delayed by reducing the oxygen concentration of the atmosphere in contact with the fruits, and by limiting the increase in its concentration of carbon dioxide. It is also known that the rate of ripening of the fruits can be accelerated by the action of certain gases such as ethylene.

     Putting this teaching case into practice, French patent 699.360 of July 25
1930 introduced a method of storing, transporting and processing fruits and the like, with adjustment of the composition of the gas atmosphere or contact with the fruit; according to this known process, a material selectively permeable to the fruit is used. certain gases to code the concentration of gases in the atmosphere

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 sparkling.

   The apparatus for carrying out this procedure comprises a chamber, one part of which is formed of a body impermeable to gases and the other part of material selectively permeable to certain gases whose concentration inside the chamber. must be adjusted; the apparatus may also include means for taking gas inside the chamber (s) as well as a `` diffusion device to which the gas thus withdrawn is supplied and in which it s' flows into contact with the selectively permeable materials to be introduced into the chamber if desired. change by diffusion the composition of the atmosphere of said chamber.

   As a material selectively permeable to different *! gas, said patent cites among others the rubber whose membranes used can be protected by perforated lids or other means of protection. The apparatus described in said patent may include means for fixing the ratio between the internal and external pressures.



   The inventors' studies made it possible to develop the knowledge described in the above patent; these studies made it possible to determine with precision the conditions which must be met by conditioning the atmosphere of enclosures containing products endowed with both respiratory and photosynthetic activity, such as various plant organs such as fruit and / that of closed enclosures, such as greenhouses, intended for the cultivation of plants.

   As described in the aforementioned patent, the packaging of plant products

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   . improved or not by various accessories more particularly intended for the absorption or elimination of carbon dioxide emitted by said plants such as fruits, entails various drawbacks. It is in fact a matter of exercising close monitoring of the composition of the atmosphere of the conservation chamber so that the scrubber or absorber is implemented as soon as the CO 2 content reaches a certain limit. Such a device also entails fairly high equipment costs and is not convenient to use for warehouses whose candidates can vary between wide limits.

   In all cases, on the other hand, where carbon gas is thus absorbed - except, however, if a water scrubber is used - the odorous products given off by the fruits accumulate, in the storage chamber and can cause physiological diseases such as scalding.



   It is also known that a small quantity of ethylene comes from the fruits in storage and no washer or absorber provided exclusively for carbon dioxide of the type of those which have been used up to now. , cannot at the same time remove the ethylene formed. This can present drawbacks if the storage is carried out at a temperature close to ambient; because we know that ethylene is detrimental to the good preservation of certain fruits above about 7.



   It has been observed during the new research carried out that the desired results, avoiding the above drawbacks and improving the procedure described in the aforementioned patent, could be improved 'and adapted to numerous applications, when some:

   

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 special conditions are met, in particular by using exchangers made of synthetic material. It is then possible, by virtue of the selective properties obtained and chosen, to obtain the desired oxygen and carbon dioxide levels in the storage atmosphere. res, while removing odorous substances and ethylene, which has the effect of facilitating storage in a controlled atmosphere close to room temperature.



   The purpose of the following description is to recall and specify the conditions for setting the cover of the exchanger made of synthetic material solen the invention, in order to determine the properties. We first know that the gas mixture contained in the storage enclosure must include known partial pressures. These can be found in specialized works on the problem of conservation in “controlled atmospheres”, namely partial pressures. x of oxygen and z of carbon dioxide x and z being for example expressed in atmospheres.
To maintain the partial pressures of oxygen and carbon dioxide at said known and predetermined values,

   the exchanger must allow the introduction into the enclosure of a suitable quantity of oxygen and at the same time the suitable elimination of carbon dioxide. In fact, on account of the respiration of the plant organs, oxygen tends to become scarce on contact, while carbon dioxide tends, on the contrary, to accumulate.

   We denote by R and R 'respectively the quantities of oxygen and carbon dioxide exchanged per unit of time by the total mass of the fruit .. treated, at the temperature considered and in the gas mixture .. of oxygen and carbon dioxide at the respective partial pressures x and z, in order to determine the permeability characteristics, for diffusion, of the exchanger making it possible to ensure the constancy of the composition of oxygen and carbon dioxide in the enclosure of -

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 storage. Firstly, the total pressure prevailing inside the storage enclosure must be equal to the pressure which prevails outside said enclosure, that is to say at atmospheric pressure.

   Consequently, there is a relation (1) between the partial pressures of the gases contained in the storage enclosure, If we denote by y the partial pressure of nitrogen, we have substantially: x + y + z = 1 (1) x, y and z being expressed in atmospheres.



   In most cases, y is greater than the partial pressure of nitrogen in air (0.79 atm).



   Under these conditions, the nitrogen contained in the storage enclosure tends to escape by diffusion through the exchanger. To maintain the pressure of an atmosphere in the enclosure, it is therefore necessary to provide a continuous inlet of 'air.



   Let U be the air inlet flow. In order for the oxygen and carbon dioxide composition of the storage atmosphere to remain constant, the diffusive exchanges through the walls of the exchanger must satisfy the following equalities: Incoming O2 flow rate = R - 1 U 5
Outgoing CO2 flow = R '
Outgoing N2 flow = 4 U
5
In the present description, we denote by #x, # y and # z respectively the variations of the partial pressures! of oxygen,% of nitrogen and z of carbon dioxide, after the mixture of composition (x, y, z) has passed through the exchanger. The variations in the volumetric flow Q at the outlet of the exchanger can be neglected.

   We can then write the equalities (2), (3) and (4) below:
Q. # z = R '(2) Q. # x = R - 1/5 U (3)
Q. # y = 4/5 u (4)

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The importance of variations in the tensions (or concentrations) of carbon dioxide, oxygen, and nitrogen, which are respectively
 EMI6.1
 depends in the respective importance of the diffuse exchanges which install you through the walls of the exchanger and the flow rate of circulation of the gas mixture in the exchanger.

   The exchanges by diffusion R ', R - 1/5 U and 4/5 U are generally low (of the order of a fraction of per liter hour or of a few liters at most /). It is therefore easy to choose a value of Q large enough to be able to consider # x, # y and # z as negligible.

   Under these conditions, if we denote by P (O2), P (CO2) and P (N) the respective total permeabilities of the exchanger with respect to oxygen, carbon dioxide and nitrogen , we easily obtain the following relations 1
 EMI6.2
 
The permeabilities defined in relations (5), (6), and (7) will be calculated in units as the flow rates R, R 'and U, that is to say in gas volumes,

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 per unit of time and per atmosphere, since x, y and a are expressed in atmospheres.

   These permeabilities for one. exchanger made up of a diffusing wall with a total surface area S and a thickness to can still be reduced to the unit of surface and thickness.



   The conditions, expressed by the equalities (5), (6) and (7) above make it possible to establish a relationship between the unit permeability characteristics of the synthetic material, namely PU (02) 3, Pu (CO2) and Pu (N2) and certain biological data, which are, on the one hand, the “gas climate” corresponding to the values of the partial pressures and z (atm) and, on the other hand, the respiratory quotient of products R '/ R or its inverse R / R'.



   R R '
Indeed, we draw the equalities (1), (5) (6) and (7) by eliminating y and U the following expression (8):
 EMI7.1
 
Now the total permeabilities of the exchanger are, if S is the surface of the diffusing wall and has its thickness! P (O2) = S / a Pu (O2); P (CO2) = S / a Pu (CO2) and P (N2) = S / a Pu (N2).



   Relation (8) then becomes:
 EMI7.2
 
Consequently, the synthetic materials which are suitable for producing the exchanger according to the invention have oxygen permeabilities.

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 Pu (O2), carbon dioxide Pu (CO2) and 1 nitrogen Pu (N2) such that, under the planned working conditions, the equality (8) is appreciably satisfactory,
By way of illustration of the above, we have chosen the particular case of the preservation of various varieties of apples or pears which is improved in a known manner by storage in an atmosphere containing 2% oxygen and 5% carbon dioxide.

   In this atmosphere, these varieties of apples and pears have a respiratory quotient a little higher than unity and we will take the case where R '/ R = 1.3 (or R / R' = 0.77).



   Knowing the respective values of x (se = 0.02) and z (z = 0.05), we find z / 0.21-x = 0.26.



  0.21-x
Relation (9) shows that, in this particular case, the synthetic materials suitable for constituting the heat exchanger according to the invention are suitable for which:
 EMI8.1
 
The choice of the practically usable synthetic material therefore depends on the value of the expression
 EMI8.2
 characteristic 1 * a 4PU (02) + PU (N2) 3pos PU (C02) + pu (N2) of the material considered.

   Those skilled in the art will obviously be able to choose from among the materials available to them, those whose value r is closest to 0.26.

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   This is how we are led in the example considered: - to eliminate the polypropylene for which r = 0.39 - to eliminate the polyvinyl chloride for which r = 0.17 - but to retain the polyethylene for which r = 0.26.



   The invention also proposes another method for determining the synthetic material than that which has just been described, in the case where it is not known - or if it is difficult to practically determine - the respiratory quotient of plant organs to be treated. The invention then provides a more practical means for enabling those skilled in the art to exercise their choice.



  The Application has in fact found that the permeabilities of the plastics suitable for the invention must satisfy the double inequality (10), below, which was established taking into account the range of possible variations of the quotient. respiratory R '/ R in the case of good conservation of the fruits in the envisaged gas mixture.
 EMI9.1
 



  By denoting by # the dimensionless relation
 EMI9.2
 4 Pu (02) + Pu (N2) of a synthetic material, 4 Pu-rCO2) + Pu {N2r- the invention provides that the plastics capable of being used for the manufacture of a sample. geur, for obtaining and maintaining in contact with products, such as fruits, an atmosphere containing x% oxygen and z% carbon dioxide are those whose

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   the characteristic responds appreciably to the double condition -% ion (11)
 EMI10.1
   'By applying the latter method of determining the plastics which can be used in the sense of the invention, to the treatment of the fruits with an atmosphere of 2% oxygen and 5% carbon dioxide,

   the double inequality (11) becomes 0.18 # # 0.26
For propylene # = 0.30
For P.V.C # = 0.17
For polyethylene, on the other hand, it is verified that 1 0.18 ## = 0.20 <0.26
For a material based on silicones (dimethylpolysiloxane) we also check
0.18 <# = 0.19 <0.26
The diffusion of the respiratory gases and of the nitrogen through the exchanger in accordance with the process of the invention can therefore be determined as a function of the needs and it is in general advantageous, for the prolonged conservation of the fruits,

  to choose materials having in addition the following general characteristics: -
Permeability to water vapor: P (H2O) very low "to ethylene:: P (C2H4) high" to, fragrant essences: P (essences) very high.

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   By means of a suitable choice of the synthetic material constituting the exchanger, it is possible to ensure permanently in an enclosure where fruits, such as apples are stored, an appropriate composition of the en-. storage enclosure; for example, the values of X, y, z defined above can be maintained as follows: x = 0.02 atm y = 0.93 atm z = 0.05 atm
When the appropriate synthetic material has been chosen, the surface area to be given to the diffusion membrane should be determined, obviously involving the mass of products to be treated.

   For this purpose, one can appeal, for example, to the above-mentioned equality (6) which is written; s
P (CO2) = R '/ z (6) in which R' represents the quantity of carbon dioxide released by the total mass of products stored in the chosen unit of time. This quantity R ′ is a common data item in terms of conservation of products.

   If it is a question of a homogeneous membrane and a regime of diffusion of gases through matter, to the exclusion of perforations, the relation (6) will be written, by making intervene Pu (CO2), the permeability reduced to the unit of surface (S) and thickness (a) of membrane, (6) P (CO2) = Pu (CO2) x S / a = R '/ z The dimensional characteristics of the membrane or the diffusion plate can. then be determined

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 thanks to relation (12),
 EMI12.1
 in which 8 and a respectively represent the surface area and the thickness of the exchanger to be used.



   In the foregoing account, it was assumed that the flow rate Q of the gas mixture which circulates in the exchanger was large compared to the values of the respective partial pressures of oxygen, carbon dioxide and nitrogen in the exchanger. mixed. In this way, the values of the variations of said partial pressures (or of the concentrations) could be validly considered as negligible. However, it is possible, when the mass of the organs to be treated is large or when it is desired to use a low flow rate Q (for example to avoid too much overpressure of the atmosphere in the exchanger, or to decrease the cost of the circulation pump ....) that the values # x, # y and # z are no longer negligible.

   The permeabilities P (C02) P (02) -and P (N) of the exchanger are then defined by the equalities:
 EMI12.2
 

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If we set in general = P e-P / Q the relations 5 ', 6', 7 'become
 EMI13.1
 which are the generalization of expressions (5). (6) and (7) and where @ designates the effective permeability of the exchanger, which depends on Q, while P, obviously independent of this latter value, is the real (or intrinsic) permeability of the exchanger.



   To define the materials that can be used to make 1 exchanger, we are therefore theoretically led to use the double inequality
 EMI13.2
 which is the generalization of condition (la), the latter being in principle valid only for.Q infinite.



   But the terms e-P / Q ,, in fact are little different from 1, the smallest value of the ratio - P / Q being greater than - 1/2. Indeed, the calculation shows that the minimum value of Q in the expressions (5 '), (6') and (7 ') is fixed by that of the constituents (CO2, 0 or N2) which diffuses the most rapidly through the walls of the exchanger. Assuming, as is often the case, that it is carbon dioxide, we demonstrate that:
Q minimum. R 'e / z, e = base of the logarithms neperions.

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 is
 EMI14.1
 Given that P (CO2) greater than z P (CQ) we see, finally that ....... Q "2- '- is greater than -1 - when it reaches its minimum value.



   So it is fitting to write that
 EMI14.2
 -P / Q> - 1/2
The preceding considerations finally lead to admit that the double inequality (10) defines the material to be used in a sufficiently suitable manner whatever the value of the flow rate Q of the gas mixture.
 EMI14.3
 To determine the dimensional characteristics of the exchanger in the most general case, it is
 EMI14.4
 It is therefore nice to solve the system of equations (13 and (14) below:

     
 EMI14.5
 
 EMI14.6
 P (C02) a (S / a) Pu (CO2) (14)
To calculate the overall permeability of the exchanger P (C02), we write the relation (13) in logarithmic form, to obtain the relation (15) t
 EMI14.7
 Loge P (C02) - Loge RVx "-f1Q2J .. (15)
We can then determine the graphical solution of the system by intersection of the, function
 EMI14.8
 , 1.

   Loge P (CO2) and from the line y, 2 a P (CO2) + Loge R '/ z After having determined P (C02) 8 using equality (15) we can calculate the dimensional characteristics
 EMI14.9
 

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 EMI15.1
 z, as an example, the area of an exchanger consisting of a membrane woven of synthetic fibers, for example of "nylon", and coated with a silicone rubber (dimethylpolysiloxane), was calculated below, to obtain ob- hold and maintain in contact with a ton of apples (va-
 EMI15.2
 riety "Golden ï3, ic. under an atmosphere at atmospheric pressure and whose composition is as follows! a a oeo4 ato x" 1,0, 'r atm y = 0,

  93 atm
The respiration of the fruits in this mixture
 EMI15.3
 provides, for one tonne, a value of 1 0 # 04 liter of CO2 Per minute at 15.



   In the example considered, we had a
 EMI15.4
 sample of the aforementioned coated fabric, of which the thickness 1 was fixed * Per unit area and at 15, 8 S 'Pu (02) * 3.3 1 / / d Pu 2 = 20 1 / d / dm' Pu (N2 ) - 1.7 1 / / dm2 We note first that the ratio
 EMI15.5
 
 EMI15.6
 is well between 2Î - x- and 'o'21 * x "" because .1 the double inequality (10) is written: 1.1 0.5 <1 m 0.18 <. 0.22.

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   To define the surface of the exchanger, we solve equation (15) which is written in the example considered with Q = 5 1 + min
 EMI16.1
 
We can graphically solve this' SAlite by the intersection of the two curves 11 == Loge P (CO2) .. y2 = 1/5 P (CO2)
We thus find, in the particular case considered: P (CO2) = 1.3 1 / min / ton or 1872 1 / d / ton We thus obtain:
 EMI16.2
 P (C02> 1872 93.6 a 0 # 9 z ton.



  8 W F u Co 2 "2 * g3 '6' 0.9 m ton.



   The exchanger will therefore be made of a coated fabric with an area equal to 0.9 m2 per tonne of fruit stored.



   It is important to emphasize that in the temperature range 0 -15 (or more) where the process of the invention is carried out, the quantity P (CO2), as well as the other permeability characteristics which come into play. , P (02) and P (N2), increase in substantially the same way, as a function of temperature, as the respiratory activity R '(or R). Under these conditions, the di @ en-

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 sions (surface, thickness) defined for a temperature included in this range are suitable for all other thermal conditions in the same range.

   This observation amounts to saying in practice that the exchanger according to the invention is perfectly suitable for the storage temperature of the products, a temperature which is preferably between 0 and 15, and that the exchanger thus calculated retains sensiblemt for all other thermal conditions in the preferred range 00-15 & the same efficiency. The inefficiency of the same exchanger can be modified by adjusting the magnitude of the flow of fluid which passes through it over the extent of its free surface or even by the conditions of its external ventilation.



   It may however be useful to have an exchanger the efficiency of which can be varied at will in a simple and gradual manner. The invention, according to one variant, provides very simple means of achieving this: it suffices, in fact, to heat or cool the diffusing membrane.



   Indeed, we know that the transfer of gas by diffusion through plastics, such as polyethylene increases with temperature; it is therefore possible either to increase the efficiency of the exchanger by supplying heat or reducing it by supplying cold.



   The reheating of the diffusion membrane (or plate) can be carried out by any means, for example by means of external natural hot air blown through the exchanger, or of an air stream, heated by the condenser of the group. refrigeration intended to cool the enclosure of 'food storage. Resistors can also be provided

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 , electrical tances incorporated into the diffusion membrane (or applied to this membrane).



   The cooling of the diffu membrane. Zion can be obtained variously and easily by means of external natural cold air blown into the exchanger. One can also, inter alia, artificially cool the membrane, in particular by means of cells with "Peltier effect" -disposed on the membrane, etc * ,,,
Thus, the invention makes it possible to adapt the operation of the exchanger to the particular operating conditions provided for and avoids having to carry out an extensive calculation of the exchanger at the start.



   According to an advantageous characteristic of the inventive one, the heating of the exchanger therefore offers great interest in the case of plastics, the permeability of which increases very quickly with temperature.



   In fact, it considerably reduces the area of the exchanger.



     By way of illustration, it was previously determined that, in order to store 10 tonnes of apples in a gas mixture comprising 3% oxygen and 4% carbon dioxide, an exchanger is required, for example consisting of a membrane. of silicone-based elastomer, having a surface area of 9 m2.



   This area has been calculated for an exchanger operating at the same temperature as the mass of the fruits stored, which, to fix ideas, will be assumed equal to 5 C. If the storage is maintained at 5 ° C, but if the heat exchanger temperature is raised to 30 C, the over-

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 EMI19.1
 diffusing face will only deviate from about 3 M2. oe which therefore represents an imposing economy of matter, - synthetic consi11'tuti1J'I 'dfj the ech8ngeur <In a general way we can considerably improve 1 (' 1 1 "ono ....

   The temperature of the exchanger according to the invention had been varied in the temperature of said exchanger during the storage of the products to ensure greater flexibility in the process of adjusting the composition of the atmosphere. storage; either by keeping the fruit at
 EMI19.2
 temperature lower than the exchanger to reduce the en- .. 'oombronenb doe eel'mi-oi We have seen that, according to the general form of the process:

        
 EMI19.3
 Belon invented it was possible to form selectively permeable diffusing membranes based on oiliconesg by 6xepL membranes woven from synthetic fibers such as nylon, coated with uaeilioone *
The exchanger can be located either outside or in whole or in part inside the enclosure containing! products.A preferred embodiment of the invention. results from the following,

   it has been found that the selective membranes based on silicones are much more advantageous than all the others because their gas permeability remains practically independent of the temperature
 EMI19.4
 within a wide temperature range commensurate with the use of the devices provided; we were able to 'by putting this realization to the point, specify the influence of the nature of the support of the exchange membrane on
 EMI19.5
 the gas poreability of said membrane and study the influence of condensations and the relative humidity of the air on the permeability of the silicone elastomer membranes.

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   As regards the action of temperature, it has been found, by examining the above teachings that
 EMI20.1
 the roupiratoîre istonsity p of the products to be treated (a'eet- towdire "1.absorption of.02 or the estonia of C02 at the tompdra- 'tearie according to the law' hereafter! 2: log ro + kt oû ro is the absorption intensity of 02 or that of the C02 emission at 010 where1 ç is a characteristic eoefficient of the proda # .tdaeuaib, emnt equal to 004 for pears and pcames laws and 0'4 t is the It follows that if t increases, it is necessary to increase the exchange surface 8 or the flow rate Q of the gas mixture to be conditioned, and
 EMI20.2
 'that a3. jg diminua ', it is necessary to decrease a or Q.

   We saw above on this subject that the flow of gas diffusion through the exchanger is directly proportional to S and that
 EMI20.3
 the real peraad, bi3itéa x of the exchanger are linked to the start
 EMI20.4
 bit Q by the relations s pu (002) 00 <(COg) - pr.020 '? (0z),; . X (02) to P02- '0
 EMI20.5
 where Po 'and peo are the total permeability of the eohan-
2 CO2 generator for O2 and CO2 respectively.

   On the other hand ,

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 the first method of carrying out the process provides the means of calculating the S / a ratio of the surface of the exchanger to its thickness as a function of the unit permeabilities and of the unit quantities of oxygen and CO2 exchanged in the unit of time by the total mass of the products (fruit or other) treated * The inventors then observed that a support made of fine and tight mesh fabric decreases the permeability of the exchange membrane when comparing two membranes with the same surface S and of the same thickness a of silicone elastomer, one imaginary, produced without support, the other, real,

  in which the threads constituting the fabric occupy an area, it has been found that the permeability is decreased by the fabric in the ratio. s / S port. For example, an imaginary silicone elastomer membrane 40 microns thick would have a carbon dioxide permeability of 60 liters / 24 hours per 1 dm2. On the other hand, the corresponding real membrane, made from 1 dm2 of a gas% type fabric with a thickness of 23/100 mm, with 20 warp threads and 8 weft threads per cm, keeps a free area equal to only 0.60 dm2, so that its permeability is reduced to 35 liters / 24 hours.



   The tests carried out on silicone elastomer membranes have made it possible to establish the weak influence of the relative humidity of the gases brought into contact with the .silicones- elastomer exchange membranes. During measurements carried out on a membrane between 70% and 100% relative humidity, values which correspond to frequent fluctuations in humidity during the storage or treatment of plant organs, no modification of the humidity. permeability could not be detected in this interval.

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   Tests have also shown that the condensations deposited on the silicone-based exchange membranes have no effect on their gas permeability, provided that the membranes are not mounted horizontally and that the support by its structure or nature does not retain not the water condensed on the exchange surface.

   Starting from these: raits, a variant of the invention consists in that the support of the silicone elastomer membrane consists of a non-hygroscopic fabric, such as nylon, whose free surface ratio is to the total surface is all the greater as .the desired permeability is greater and that said support with its membrane or the silicone membrane alone is mounted in a position preventing the stagnation of water in contact with them, for example in a non-horizontal position, preferably vertical.



   The improved exchange membranes according to the invention can form part of equipment mounted inside or outside a room, or constitute part of the vertical wall (or s) of this room.



   When the room or enclosure can be considered to have an atmosphere of stable and homogeneous composition throughout its volume, without the need for artificial mixing, the permeabilities to O2,
CO2 and N2 are provided by the aforementioned equalities (5), (6) and (7).

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 EMI23.1
 When the atmosphere of the room or enclosure must be homogenized by mixing, it is necessary to know the flow Q, of air at the contact of the membrane.
 EMI23.2
 debangouseî if this flow Q is weak,

   relative to the exchanges to be carried out through the membrane, a tangential displacement of the air on the membranes is ensured and the dgalitdac5l) t (6 ') and (71) oî-desausb are then applied
The improved membranes according to the invention can be used directly to constitute all or
 EMI23.3
 part of the walls of packaging, especially fruit, a further feature of the invention is that, for large capacity packaging and contrary to certain indications of the prior art, the sealing; strictness of the envelope is not necessary:

   '
 EMI23.4
 in fact, the appearance of a vacuum in the packaging, which would be liable to deteriorate the walls or the fruits, is avoided by some defects of etanoheity; the membrane then functions as an exchanger incorporated in the wall of the packaging. an enclosure whose atmosphere does not need to be stirred and the permeability of the window is calculated;

       'by relations 5-6-7 recalled above * These faults
 EMI23.5
 Voluntary etanoheity are carried out in a judicious manner according to the results of the established constructions, and can be optionally obtained by means of suitable holes.



   The invention is described in detail below, without being limited in any way, with reference to the accompanying drawings for which $
 EMI23.6
 F3..1 represents schematically in longitudinal section a first device for the implementation of the p2aed of the invention in which the exchanger is located
 EMI23.7
 outside the enclosure of 8O 'Icase

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Fig. 2 is a diagram (seen in plan) of a variant of implementation of the similar to the invention with two exchangers placed inside the storage enclosure;

     
Fig. 3 shows a device for preserving fruit with a changer according to the invention;
Fig. 4 is a detail view of an example of an element of the exchanger according to the invention during manufacture; Fig. 5 is a diagram showing over time the variations in the composition of the gas mixture (O2 and CO2) in the storage enclosure of the device of Fig. 3;
Fig. 6 schematically shows in axial section another type of heat exchanger capable of being used according to the invention; Fig. 7 is a diagram showing the variations in the composition of the hollow mixture in the storage chamber over time, the latter having or not having passed through the exchanger of FIG, 6.



   In the installation for the conservation of fruits shown in FIG. 1, the fruit mass 1 is placed in the storage enclosure 2. The exchanger 43 according to the invention is placed outside the enclosure 2, and the gaseous atmosphere prevailing in the enclosure runs through the closed circuit installation thanks to pump 5.in pipes 6 and 7 while being continuously circulated by fan 4.

   In addition, there is provided in the storage chamber an opening connected to a pressure equalizer 9, which allows the entry of a little air.

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 in the enclosure when the pressure therein tends to drop below the atmosphere and which, in the example shown, is constituted by a tube 9a, one end of which opens out to the air in the ambient atmosphere and the other end is immersed in a water tank 9b hermetically connected to the opening 8 of the storage enclosure. - The gas flow circulating in the installation has been shown by arrows.

   When, after a few days, a stable operating speed of the system is obtained, there enters in 8 a flow / U of air intended to balance the internal and external pressures. The atmosphere prevailing in the storage chamber, circulated by the fan 4- is drawn in by the pump 5 through the pipe 6.



  Before reaching exchanger 3, the original gas mixture was enriched in CO2 and depleted in O2 by the respiration of the fruits. By passing over the exchanger, according to the invention, the gas mixture gives up to the ambient atmosphere per unit of area and time a quantity of nitrogen equal to 4/5 U, a quantity of carbon dioxide equal to R ', as well as ethylene and fragrant essences, while a quantity of oxygen (R-1/5 U) is introduced by diffusion through the exchanger 3 into the gas mixture.



  This mixture sucked by the pump 5, in line 7, constitutes a gaseous mixture suitably enriched in O2 and depleted in CO2 which is recycled into the storage chamber 2. It is understood, from the above, that the process of the invention is advantageously continuous.



   In the embodiment shown in FIG. 2. the storage enclosure 12, plan view, con-

 <Desc / Clms Page number 26>

   holds a mass of fruit 11. According to the invention, two exchangers 13a andn13b made of synthetic material are provided which pass through the storage enclosure respectively via openings 18a, 20a and 18b, 20b. The internal and external pressure balance is ensured at 18c by an air inlet valve, for example by a device similar to that shown in the embodiment of FIG. 1.



   The operation of such an installation is substantially the same as that of the device in FIG. 1. To remove the excess nitrogen and carbon dioxide formed by respiration to the outside, as well as ethylene and odorous essences, In this case, provision is made for the exchangers 13a and 13 to be swept by air, which, introduced at 18a and 1811, respectively, entrains the aforementioned gases and exits at 20a and 20b. The pure air introduced in 18a and 18b therefore exits in 20a and 20b enriched in CO2, C2H4 and fragrant essences and depleted in oxygen.



   It is shown in elevation in FIG. 3 an installation practically used for the preservation of fruits according to the invention. The mass 30 of fruit is placed in boxes or superimposed trays 31; protective boards 31a and 31b have been placed at the upper and lower part of the elements 31, the lower board 31b rests on the ground by means of bars
31c, of wood for example, which can be used for handling and allowing the isolation of the batch of fruit.



   The fruits are surrounded by a 'gas-tight' enclosure 32, advantageously of weldable synthetic material, such as polyethylene or the like. This enclosure 32 may or may not be cooled. In the example shown

 <Desc / Clms Page number 27>

 felt, it has the form of a simple tarpaulin completely surrounding the batch of fruits 30. One can easily achieve the sealing of such an enclosure by first placing a horizontal sheet of weldable plastic material 32b on plate 31b, after which we place the fruits 30.

   The sheet 32b overflows along the sides of the board 31b, which allows the flaps 32a of the sheet to be welded to the edges of the sheet 32b, by welds 32c. The tarpaulin 32 is provided, on two opposite sides, with two openings 38a 38b. A tubing 36 starts from the opening; Si. and leads the gases to the exchanger 33. The opening 38b receives the gases coming from the exchanger 33 through the pipe 37 possibly provided with a cooler 42. Different measuring and control devices are mounted on the pipe 36 or 37, in particular a water pressure gauge 40, a device 41 for taking gas samples. A sealed pump 35 mounted on a frame 35a ensures the circulation of the gas flow.

   A fan 34, placed in front of the exchanger 33, creates an air circulation to drive the gases diffused out of the exchanger. Finally, a pressure balancer, in the space a water valve 39, ensures the necessary inflow of air into the storage atmosphere to maintain the balance of the external and internal pressures. The gas mixture contained in the fruit chamber is drawn in by the sealed pump 35 and delivered into the exchanger 33. The mixture then returns directly to the fruit chamber. The pump 35 can also be placed upstream of the exchanger on the pipe 36.



   For example, a tarpaulin formed from a sheet of polyvinyl chloride 300 microns thick was used. 250 kg of "Jonathan" apples were stored in 15 crates stacked in three rows per side.

 <Desc / Clms Page number 28>

   ralleles each comprising 5 superimposed boxes @ The dimensions of the tarpaulin were approximately 60 x 120 x 135 cm, which corresponded substantially to an interior volume, of the order of 1 m. This storage enclosure was placed in an isothermal chamber at 20 and was connected to a changer described in detail! below,
We implemented,

  as a synthetic material for the constitution of the exchanger, a thin sheet in the form of a flat sheath, which has been pleated and held by an adhesive tape.



   As can be seen in FIG. 4, this operation is carried out without difficulty by threading in the sheath 43, warning of a roller 44 and inflated with air coming from a small compressor at 45, a light tube 46, made of glass for example, with a diameter smaller than that of the flat sheath 43. It is then easy to fold the plastic film around the tube
46 in the transverse direction, an adhesive strip 47 is then glued along two opposite generatrices. Tube 46 is then withdrawn, which provides a central channel which allows the gases to flow without appreciable pressure drop.



   During this work, in cases where a good seal is desired, all necessary precautions must be taken so as not to perforate the sheath 43 by friction on rough surfaces or by impacts.



   The two ends of the sheath 43 are then welded after the establishment of two inlet and outlet pipes for connection to the circuit.

 <Desc / Clms Page number 29>

 



   In the aforementioned particular case of the treatment of heads, polyethylene in the form of thin films of two different thicknesses (50 / u and 80 / u) was used as the material constituting the changer. For such a material, the respective permeabilities to oxygen and carbon dioxide are, with a pressure difference par. tielle of 1 atmosphere and at 15::
 EMI29.1
 P (02) t 30 cm3fdm'flr h for 30M of thickness 19 cz3f Dmf 2. h for 9 t of thickness P (C02) s 150 cm3 / dm2 / 24 h for SO J1 of thickness 95 cm "J / dm2 / 24 h for 80 g1 of thickness.



   The surface of the exchanger was determined so as to allow obtaining a gas mixture very poor in oxygen (approximate composition of the atmosphere
 EMI29.2
 storage sphere s 2 02 $ 5% C02 and 93% N2) at the temperature of the isothermal storage room (200).



     A 150 m long sheath was made.
 EMI29.3
 form formed "of '" C sections with 50 m of polyethylene sheet 80 microns thick
 EMI29.4
 (surface d) 6changej 12 m2) - 1000 m of polyethylene sheet 50 microns thick
 EMI29.5
 (exchange surface! t 24 m, ..



  Thanks to the pleating described above, the apparent length of the exchanger can be reduced considerably. In fact, a pleated exchanger aetre corresponds to approximately six meters of flat duct. Congestion
 EMI29.6
 total of the interchange is therefore about 25 meters. The exchanger thus pleated was then arranged as a serpentine and

 <Desc / Clms Page number 30>

 kept it in a rigid frame, the volume of which was about 0.15 m3.



   As an indication, with such an exchanger, the continuous circulation of the gases is ensured by the pump 35 with a flow rate of 3 liters / min, the suction depression then being 2 cm of water. The exchanger introduces a pressure drop of 0.5 cm of water.



   The evolution of the percentages of oxygen and carbon dioxide in the gas storage atmosphere was determined from day to day, and the results obtained were recorded in the diagram in FIG.
FIG. 5 remains relating to the conservation of apples in the installation of FIG. 3, as has been said above. We can observe, over time, two successive stages of evolution of the atmosphere in the storage enclosure.



   In a first or transitional stage, which lasts only a few days, the O2 percentage decreases sharply, while the CO2 percentage first increases, then gradually decreases before becoming substantially stationary. This stage is accompanied, as is normal, by large introductions of air through the water valve 39, since the fluctuations of the internal pressure are high.



   The second stage, which is the so-called preservation stage in the treatment described in FIG. 5, corresponds to a stabilization of the composition of the gas mixture and has been shown for the first 12 days. The mixture then has a composition

 <Desc / Clms Page number 31>

 substantially constant, namely:
O2 1 2.5%
CO2: 6.5%
N2: 91%
At the same time, the air intakes have become extremely small, thus visibly materializing the restoration of permanent operation.
This stabilization of the composition of the atmosphere lasts several weeks or several months depending on the temperature, the species, the variety and the initial stage of the fruits.



   The practical example considered shows that the process of the invention, with which a synthetic material heat exchanger is used, makes it possible to obtain and maintain at a substantially constant value a determined composition of the storage atmosphere. , in particular a very low oxygen content (2% to 3%) and a low carbon dioxide content (5% to 6%).



   Such a result has been obtained with a pleated exchanger made from a thin sheet of polyethylene. It should also be noted that such an exchanger can be manufactured from a single sheet of polyethylene, of thickness 50 for example.



   Each particular case of storage treatment can be solved by the method of the invention by means of a suitable choice of the characteristics of the exchanger (exchange surface), if a given synthetic material is provided which meets the conditions of permeability mentioned above. We can therefore understand that the

 <Desc / Clms Page number 32>

 Practical applications of the process of the invention can be numerous and are not limited to particular types of synthetic material. Thus, a material endowed with improved permeability properties may give rise to the use of less bulky exchangers with a smaller surface area.



     By way of illustration of these latter indications, a membrane made up of a fabric coated with a layer of elastomer (silicones) having a high permeability was used for the manufacture of the exchanger. gas. The elastomer used was dimethylpolysiloxane. Such a pure compound can be used in the form of a tube or in a thin layer on a support of a textile fabric. FIG. 6 shows schematically an advantageous embodiment of an exchanger composed of a small number of elements made watertight by gluing, welding, etc. It has in fact been observed that the membrane coated with silicones was extremely permeable to odors and, moreover, very permeable to ethylene.



   The exchanger described in FIG. 6 is then suitable for particular. well to the needs of the invention. The exchanger 50 comprises a series of elements 51 consisting of a membrane (cloth) coated with silicones (dimethylpolysiloxane), the contiguous edges 52 of which have been secured and made watertight by bonding the elastomer layer. Two tubes 54, 55 are tightly sealed to each end member of the exchanger 50. The gas mixture to be controlled is swept along 56 into the interior of the exchanger through the tubes 54, 55.

 <Desc / Clms Page number 33>

 



   The permeabilities to carbon dioxide and oxygen are much higher than that of polyethylene, as shown in the following table
 EMI33.1
 
<tb> Permeability <SEP> to <SEP> 15 <SEP> for <SEP> Ratio
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> CO2 <SEP> expressed <SEP> in <SEP> P (CO2) / P (O2)
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> cm3 / 24h / dm2 / 1 <SEP> atm.
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>



  Dimethylpolysiloxa-
<tb>
<tb>
<tb>
<tb>
<tb> ne <SEP> on <SEP> fabric <SEP> (en-
<tb>
<tb>
<tb>
<tb> duction <SEP> from <SEP> 50 <SEP> to <SEP> 60
<tb>
<tb>
<tb>
<tb>
<tb> microns <SEP> approximately) <SEP> 20,000 <SEP>. <SEP> 6 <SEP> - <SEP> 7
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> Polyethylene
<tb>
<tb>
<tb>
<tb>
<tb> (50 <SEP> microns) <SEP> 150 <SEP> 4.5
<tb>
 
The inefficiency of a changer made from dimthylpolysiloxane is therefore greater than the pleated changer based on polyethylene. The exchange surface can therefore be considerably reduced and it has for example been found that a fabric covered with silicones, having a surface area of 0.9 m2, could be suitable for storing 1 ton of fruit.



   FIG. 7 illustrates the results obtained by using such an exchanger for the preservation of approximately 1 ton of “Golden Delicious” apples at 12; the com .. position of the atmosphere in contact with the fruit was maintained at the desired levels of O2 (3%) and CO2 (5%).



   In the diagram in Figure 7, curves (a) and (c) respectively represent the CO2 and O2 contents of the gas atmosphere of the storage enclosure, while curves (b) and (d) respectively represent -. ment these same contents after passage of the gas mixture, at a flow rate of 5 liters / min, through the exchanger described above with reference to FIG. 6.

 <Desc / Clms Page number 34>

 



   The invention generally relates to any use of synthetic material for preserving fruit; the choice of the synthetic material and the corresponding calculation of the characteristics of the exchanger must be considered as being within the reach of a person skilled in the art and do not depart from the scope of the present invention,
The invention applies not only to premises or packaging for fruits and the like, but also makes it possible to ensure any gas conditioning of greenhouses according to special crops.

   She offers:. : also an industrial interest for the construction of manholes on small packaging or on the linings of polyethylene cases or bags, of panels thermally insulated or not for the packaging commonly called "containers", of panels for uninsulated watertight removable chambers - refrigerated or not - possibly accommodating in traditional cold rooms,. panels on insulated, removable cold rooms or on traditional airtight rooms, internal or external heat exchangers for airtight cold rooms of all types, etc ...

   It will be recalled, however, that the invention can also easily be used to regulate the composition of the atmosphere of enclosures containing green organs, whole plants, etc., which, by photosynthesis, absorb carbon dioxide and reject oxygen. Thus, the method of the invention is suitable for conditioning the atmosphere of greenhouses or shelters for growing plants.


    

Claims (1)

EMI35.1 -BEVBtDICATIONS - * '' 1. Method for controlling the composition of the atmosphere in contact with products exchanging oxygen and carbon dioxide with the ambient medium, and in particular for the preservation of products which absorb oxygen and release gas. carbon dioxide such as fruits, said process making use of selective membranes and / being characterized in that one brings into contact, in a closed circuit and, preferably, continuously, the atmosphere prevailing in the enclosure where said products are stored and which is normally isolated from the ambient atmosphere, with at least one exchanger made of synthetic material providing essentially by diffusion, the introduction of oxygen in the circuit and the elimination of carbonic gas towards the ambient atmosphere, the pressure. in the storage enclosure. being kept substantially equal to that of the ambient atmosphere, for example thanks to the controlled introduction into the closed circuit of a gas, such as air * 2. Method according to claim 1 characterized in that the storage is carried out as appropriate at a temperature substantially between 0 and 15. 3. Method according to one of the preceding claims, characterized in that the ethylene and the odorous essences originating from the stored products also diffuse towards the ambient atmosphere through the exchanger made of synthetic material. <Desc / Clms Page number 36> 4. Method according to at least one of the preceding claims characterized in that the sum of the pressures Partial $ of oxygen, carbon dioxide and nitrogen inside the storage enclosure is equal to atmospheric pressure. 5. Method according to at least one of the preceding claims characterized in that the respective unit permeabilities of the synthetic material constituting the exchanger to the various gases mainly involved, namely to carbon dioxide Pu (CO2), to Pu oxygen (O2) and Pu nitrogen (N2) substantially satisfy relationship (5) EMI36.1 in which R / R 'denotes the inverse of the respiratory quotient of the products, z and x the respective partial pressures of carbon dioxide and oxygen. 6. Method according to at least one of the preceding claims characterized in that said permeabilities substantially satisfy the inequality doubled! EMI36.2 7. Method settling at least one of the preceding claims, characterized in that the efficiency of an exchanger calculated for certain operating conditions is increased by heating, for example by means of blown hot air or resistors. electrics incorporated into the diffusion membrane by increasing the gas circulation flow rate in the exchanger, by increasing the <Desc / Clms Page number 37> surface or external ventilation of this exchanger, 8. Process according to at least one of the preceding claims, characterized in that the efficiency is reduced. of the exchanger by cooling, for example by means of blown cold air or cells working by the "Peltier effect", and deposited on the diffusion membrane. 9. Method according to at least one of the preceding claims, characterized in that at least one exchanger.si- fait outside the storage enclosure, is traversed. in a closed circuit by the atmosphere of the enclosure. 10. Method according to at least one of the preceding claims characterized in that at least one exchanger, located inside the storage enclosure, is swept by air which releases the substances into the ambient atmosphere. diffused through the exchanger. 11. Method according to at least one of the preceding claims, characterized in that the exchanger is made of polyethylene. 12. Method according to at least one of the preceding claims, characterized in that 3 exchanger is based on silicon (, for example on dimethylpolysiloxane). 13. Method according to at least one of the preceding claims, characterized in that in order to determine the dimensional characteristics of an exchanger the material of which satisfies equality (5), or, more generally, the double inequality (8 ), we first calculate the value P (CO2) knowing the composition of the gas atmosphere to be controlled and the flow rate of the gas mixture passing through the exchanger, after which the ratio - of the surface of the exchanger to the thickness of the material is determined <Desc / Clms Page number 38> which constitutes it * 14. Process according to at least one of the preceding claims, characterized in that the support of the silicone elastomer membrane consists of a non-hygroscopic fabric, such as nylon, the ratio of the free area to the total area of which is the greater the greater the desired permeability, and that said support with its membrane or the silicone membrane alone is mounted in a position preventing the stagnation of water in contact with them, for example in a non-horizontal position, preferably vertical. @ 3. Device for implementing the method of claims 1 to 14, characterized in that it comprises a sealed storage enclosure, where the products to be preserved are arranged, having an inlet pipe and an outlet pipe, with at least one exchanger made of synthetic material connected to said enclosure by pipes constituting a closed circuit with the inlet and outlet pipes of said enclosure, while means are provided in said circuit for measuring the pressure of the storage atmosphere, means for circulating the gas flow, and means for balancing the pressure of the storage enclosure and that of the ambient atmosphere. 16. Device according to claim 15, characterized in that @ a water valve in communication with the atmosphere. ambient phère is placed on the path of the gas flow in the circuit, after the exchanger and before the inlet pipe of the storage enclosure, said valve ensuring the introduction of air into the circuit to balance the internal pressures and external. <Desc / Clms Page number 39> 17. Device according to at least one of claims 15 and 16 characterized in that the sealed storage enclosure is a tarpaulin. weldable plastic. 18. Device according to at least one of claims 15 to 17 characterized in that the exchanger is traversed on its face in contact with the ambient atmosphere by a current of air. 19. Device according to at least one of claims 15 to 18 characterized in that the exchanger is made from a thin sheath of a synthetic material such as polyethylene, which is kept pleated in the. direction of its longitudinal axis. 20. Device according to at least one of claims 15 to 19 characterized in that the exchange = is a fabric, covered with a thin layer of silicones, and comprising a plurality of elements made waterproof by gluing. .