GB2092019A - Gas scrubbing apparatus - Google Patents

Abstract

Gas scrubbing apparatus is free-standing and comprises means (e.g. including a fan 5) for causing gas to pass in through an inlet 2 in the apparatus, over an oxidant 3 which can absorb/adsorb and oxidise ethylene, and out through an outlet 6 in the apparatus. Wheels 7 may be provided. The apparatus is suitable for use in controlling the atmosphere under which vegetable produce, and particularly fruits such as bananas, is stored. <IMAGE>

Description

SPECIFICATION Gas scrubbing apparatus The present invention relates to gas scrubbing apparatus which can have particular utility in modifying/controlling the atmosphere under which fresh vegetable products are stored.

Fresh vegetable products such as fruit, vegetables or flowers, deteriorate on storage. The deterioration of such products is associated with the presence of ethylene and other hydrocarbons which may function as ethylene precursors as well as, possibly, other gaseous materials in the atmosphere surrounding the product. Such products evolve ethylene during the process of ripening and maturation.

The presence of ethylene can promote the deterioration of ripe fruit. In particular, the process can be autocatalytic, with the result that the presence of ethylene accelerates the production of more ethylene and the deterioration of the product. Certain fungi evolve ethylene, and bruising shock and stress can also cause the evolution of ethylene, and these factors therefore accelerate deterioration by the mechanism described.

The need to store fresh vegetable products is the result of two main factors. Firstly, it is unecomomic to transport large consignments of vegetable products from their source to their destination by air. It is therefore desirable that the fruit may be stored satisfactorily during transport by road or sea. Secondly, consumers often wish to obtain fresh vegetable products out of their local season.

Vegetable products are usually now stored, not least for the purposes of transport, in refrigerated containers under an atmosphere of high, e.g. 90-95% , relative humidity.

Odours and other gases (such as carbon dioxide and including ethylene) are conventionally removed from such containers by venting.

This procedure has the disadvantage that the uncontaminated air which is introduced to replace that lost in venting has to be humidified and cooled.

By way of example, bananas are often stored in a sealed room at a constant temperature of about 1 3on. When desired, ethylene may be injected into the room for a given period, the atmosphere then exhausted and the room again sealed. The intention of this procedure is to allow the ethylene to contact the fruit for only a predetermined period of time. However, it is observed that ethylene may be absorbed by, for example, corrugated cardboard boxes, and only desorbed after the atmosphere has been exhausted from the room. On desorption, the respiration rate is increased, contrary to the desired effect, and this in turn leads to increasingly fast ripening.

U.S. Patent Specification No. 3,049,399 discloses the use of an inorganic activated water-adsorbing substrate inpregnated with a permanganate, for the purpose of de-odorising air in enclosed spaces. It is proposed that the composition may be loosely packed in a filtertype bed, or that the air may be impinged on the solid material. It is also proposed that the composition may be used to deodorise spaces in which foods are stored, but no exemplification of any such system is given, nor is there any indication of which foods might most beneficially be deodorised.

Gas scrubbing apparatus according to the present invention is free-standing and comprises means for causing gas to pass in through an inlet in the apparatus, over an oxidant which can absorb/adsorb and oxidise ethylene, and out through an outlet in the apparatus.

Apparatus of the invention may be of simple construction. It may comprise, for example, a container having inlet and outlet apertures, a simple fan, and the oxidant contained within a permeable enclosure positioned in the gas passage path. The enclosure may comprise a substantially rigid housing which is permeable to the gas; alternatively, the oxidant may be provided supported on, or bonded into, a permeable blanket or similar material. The enclosure or blanket forms a filter element in which, if desired, a number of permeable sachets containing pellets or granules of the oxidant, may be used, although it will usually be preferred to provide a plurality of parallel pockets substantially filled with pellets of the oxidant. Alternatively, a bed of pellets may be used, oriented horizontally or vertically.

Sachets or parallel pockets may be provided by bonding, e.g. by heat-sealing or sewing, together two facing sheets of a suitable material. The material may be a fabric, in which case it may have the desired degree of permeability, or of a polymer film which may be punctured before or after bonding to provide the desired permeability. A biaxially-orientated polyolefin material is particularly suitable for use in sachets of the type described. This material is tear-resistant, allows puncturing to provide clean circular holes for permeability and is transparent, thereby allowing inspection of the oxidant if that is one which changes colour during use.

Fabrics for the preparation of pockets of the type described may be woven or non-woven.

A 100% polyester spun-bonded fabric has proved to be particularly satisfactory in that its properties allow the complete filling of pockets following insertion of a given amount of oxidant and sealing. When full, such pockets retain their tubular shape, and thus maximum surface area, well.

Pockets of the type described provide one or a plurality of flexible cylindrical tubes.

Alternatively, the oxidant may be mounted in rigid, permeable housing. Rigid individual tubes may be formed of, for example, a polymer or plastics mesh filled with pellets of the oxidant. The tubes are advantageously caused to rest parallel and in contact with adjacent tubes in a frame. The frame may be, for example, a rectangular doublewalled frame. The tubes then span the frame between the walls and preferably, the space between the walls is such that the axes of adjacent tubes are offset from one another in a staggered relationship. This allows a high degree of contact between the oxidant and the atmosphere passing through the frame.

It is very desirable that the oxidant should be packed in a material which is non-moisture-absorbent. This ensures that, when used in an atmosphere of high relative humidity, the material remains self-supporting. Further criteria for the packaging are that it should not degrade, react with or adsorb the oxidant or any other reduced form thereof.

The oxidant must oxidise, and preferably also adsorb/absorb ethylene, under the prevailing conditions. These may comprise a storage temperature which is ambient, for instance 10 to 30 , but which is often lower, for example the temperature conventionally prevailing in refrigerated containers, ships' holds, cold stores etc.

The relative humidity may be at least 85, e.g. 90-95, %. The conditions may be controlled, modified and/or hypobaric.

The preferred oxidant for use in the invention, which can satisfy all the conditions described immediately above, comprises a substantially non-friable absorbent support impregnated with a suitable oxidising agent. The oxidising agent is preferably in substantially dry form. The oxidant is suitably provided in pellet, preferably spherical, form.

The oxidant may be made by spraying an aqueous or other solution of an oxidising agent onto a absorbent support material. This material is generally in particulate form, most preferably in pellet form, during the spraying and the pellets or other particles are preferably in agitation during the spraying. The agitation may, for instance, be a tumbling movement. Thus, the solution may be sprayed onto the particles while they are tumbled in a rotating drum. In order to obtain optimum distribution of the solution, and thus of the oxidising agent, within the pellets or other particles, tumbling or other agitation of them preferably continues after the spraying has been terminated, and thereafter they are dried to constant weight, again generally by tumbling or agitating them, for instance, in the same rotating drum.Drying may be conducted by contact with air at ambient or an elevated temperature.

The oxidant may then be put into whatever form is desired for its final use. For instance, it may be packed in permeable containers, for instance sachets that may be formed of porous plastics film, or it may be bonded into a blanket or other filter, for instance by filling the particulate material into pockets of a blanket or other support. The oxidant product is preferably then sealed in a container that is substantially non-translucent and substantially impermeable to the atmosphere and stored in this until it is removed for use.

The preferred oxidant is potassium permanganate, for reasons of availability, cost and safety. Other oxidants which may be used include other alkali and other metal manganates, potassium or other alkali metal chlorates or chromates and a wide variety of other peracid salts and other oxidising agents. The oxidising agent is generally an inorganic compound and will usually be soluble in water, in which event any solution which is sprayed onto a particulate support is normally an aqueous solution. Depending upon the particular oxidant which is used, the solution may be neutral, acid or alkaline. The preferred solution is made simply by dissolving potassium permanganate in water (which may be deionised).

The absorbent support should be one that can carry a large amount of the oxidant and remain highly absorbent to ethylene. The preferred support is activated alumina. However, other supports could be used, including for instance activated clays, aluminosilicas, bauxite, zeolites, dried silica gel and so forth, provided that they are non-friable. The support preferably has a crush strength of at least 5, more preferably at least 8 and most preferably at least 10, kg; this value can be measured using a compressometer and is a measurement of the load required to break the material between flat parallel plates.

The oxidant preferably contains from 2.5 to 50, usually 5 to 20% , of the oxidising agent, by weight. Preferably the oxidant has a surface area of at least 100, for instance 1 50 to 500, and most preferably 300 to 350, m2/g.

It may have a micropore volume of, for instance, 20 to 100, e.g. 40, cm3/100 g.

When it is in the form of pellets, these may be substantially spherical balls having a diameter of, for instance, 2 to 1 5 mm, preferably 2 to 5 mm. The oxidant preferably has a moisture content of 5 to 25, e.g. 12, % by weight.

Oxidant pellets for use in the invention have been prepared by the following process (which is given by way of example): 0.23 kg potassium permanganate were dissolved in 3.5 kg deionised water, the solution being heated to ensure that the permanganate dissolved completely. 4.5 kg of activated alumina pellets were placed in a rotating drum and the permanganate solution was sprayed onto the tumbling pellets over a period. After all the solution had been sprayed, the drum was allowed to continue to rotate for an addtional period to ensure uniformity of impregnation. The impregnated pellets were then dried to remove moisture until there was no apparent change in weight of the pellets between weighings at intervals of time.

The free-standing nature of the apparatus of the invention allows it to be moved readily between, say, fruit storage rooms, e.g. in a ship's hold. The apparatus can be used to control the level of ethylene in the atmosphere under which vegetable products are stored. In particular, it can be used to avoid the problems associated with the desorption of ethylene from containers in storage volumes into which ethylene has been injected and the atmosphere subsequentially exhausted, as described above.

Use of apparatus of the invention has the advantage that satisfactory control is achieved without the need for modication of existing storage atmosphere control mechanisms.

Thus, the novel apparatus avoids the need for modification of existing atmosphere pumping systems. Despite the fact that apparatus of the invention is not associated with the total- exhaustion of the storage atmosphere, it can provide satisfactory treatment of all the atmosphere in a storage volume which may be very large indeed, in comparison with the volume of the apparatus which might be thought to be satisfactory only for the treatment of the immediately surrounding atmosphere.

Use of apparatus of the invention does not require any external control of the storage atmosphere. There is thus no need, for example, for extra pumping or cooling systems of the type associated with complete removal of storage atmosphere for the removal of ethylene.

It is an advantage of the use of potassium permanganate in the oxidant that the activity of the oxidant can be readily checked. Breakage of a single pellet of the oxidant will reveal whether the purple colour of potassium permanganate remains, in which case the oxidant is still suitable for use, or has disappeared, in which case the oxidant needs replacing.

Although the apparatus of the invention has been described for use with the control of ethylene levels in the atmosphere, oxidants which can have the given effect on ethylene can also be used to remove undesirable gases in factories, sewage odours, ammonia from copying machines and sulphur dioxide in paper mills. If desired, the oxidant may be associated with a carbon dioxide-removing material such as soda. In the apparatus, different materials may be provided in separate filter beds.

The invention will now be described by way of example with reference to the accompanying drawing which is a cut-away perspective view of apparatus embodying the present invention. The drawing shows a container 1 having an inlet 2 in the form of a grille, a particulate oxidant material 3 contained within two rigid, permeable sheets 4, a fan 5 and an outlet 6. Although inessential, the apparatus is shown, for convenience of use, mounted on wheels 7.

Claims (7)

1. Gas scrubbing apparatus which is freestanding and comprises means for passing gas in through an inlet in the apparatus, over an oxidant which can absorb/adsorb and oxidise ethylene, and out through an outlet in the apparatus.

2. Apparatus according to claim 1, in which the oxidant is in particulate form and has a crush strength of at least 5 kg.

3. Apparatus according to claim 1 or claim 2, in which the oxidant is in the form of substantially spherical pellets.

4. Apparatus according to any preceding claim, in which the oxidant comprises an abosorbent support impregnated with an oxidising agent which can oxidise ethylene.

5. Apparatus according to claim 4, in which the support comprises alumina.

6. Apparatus according to claim 4 or claim 5, in which the oxidising agent is potassium permanganate.

7. Apparatus according to claim 1, substantially as described herein with reference to the accompanying drawing.