WO1994015159A1

WO1994015159A1 - Dryer

Info

Publication number: WO1994015159A1
Application number: PCT/GB1993/002580
Authority: WO
Inventors: George David Halley; Charles Matthew Malcolm Thwaites; Malcolm Talbott
Original assignee: W.L. Gore & Associates (Uk) Limited
Priority date: 1992-12-18
Filing date: 1993-12-17
Publication date: 1994-07-07
Also published as: GB9226394D0; AU5706494A; CN1093798A; ZA939485B

Abstract

An apparatus is provided for drying materials, such as natural products including crops, cereals, vegetables, timber etc., which includes a sealed chamber (2) into which the materials are placed which retains a volume of gas. Water-vapour exchange means are provided either in the wall (30) of the chamber unit or in a separate unit (50), which comprise a selectively permeable membrane which is permeable to water-vapour but impermeable to gas. A heater (20) heats the gas inside the chamber, which absorbs water vapour from the material. The water vapour is then selectively transported through the membrane, whilst the heated gas is retained in the chamber. The drying process is energy-efficient.

Description

DRYER

Technical Field

The present invention relates to an apparatus and method for drying materials in a manner which is efficient in terms of energy usage. Though the apparatus has been designed for drying perishable natural products (especially cellulosic products) such as agricultural products (e.g. crops, cereals or vegetables) and timber products, the apparatus has wide application for the drying of many types of materials where water is to be removed.

Prior Art

Conventionally, cereal crops, such as wheat, barley or other grains, are sold with a water content of around 15% by weight. Produce which is harvested at a higher water content requires to be dried down to this level before it may be sold. Also, high levels of water encourage spoiling of the crop by fungal growths. Typically, wet cereal is placed in a silo with perforated sides, through which air is forced by means of a blower. Drying the cereal in this way is time consuming and relatively costly in terms of energy.

In tropical climates, it is common to dry perishable natural products, such as coffee beans, cocoa beans and other crops, and fish by laying the product out to dry in the sun. Unfortunately, these climates also tend to have high rain fall, so it becomes necessary to cover the drying crop when it starts to rain. If the cover is left on for long periods, there is a danger that moisture will accumulate under the cover and result in fungal growth and deterioration of the product. Also removal and replacement of the cover is labour intensive and can of itself result in physical damage to the product.

There are many requirements in other technologies such as manufacturing and process technologies where there is a need to remove water in an energy efficient manner. Usually, drying is brought about by circulating hot air whose reduced relative humidity (due to its higher temperature) encourages drying of the product. However, the used warm moisture laden air is then generally vented to atmosphere so that its heat content is lost. The water vapour may be extracted from the air, for example by condensing on a cold refrigerated surface, but this also results in substantial energy losses and substantial capital costs are involved.

Our earlier European patent application EP 0525842 (published 3rd February, 1993) discloses the use of a water-vapour-permeable gas-impermeable membrane for the production of a storage vessel for storing grain or other natural products. The membrane is impermeable to oxygen gas, which allows a reduced oxygen atmosphere (e.g. 3 to 7%) to be established within the vessel as oxygen is used up by insect pests and moulds within the grain, and by metabolism of the grain itself. The insects are unable to survive at such low levels of oxygen. Thus the grain is protected against insect pests without the use of chemical pesticides. The water-vapour-permeable nature of the membrane allows the grain to breath and prevents the build-up of condensation and possible mould growth within the stored product.

Summary of the Invention

It has now been found that if a heater is provided to heat the gas in contact with the stored material, the water-vapour may be transported efficiently through the membrane whilst the hot gas and its energy content is retained inside the vessel. This is an energy-efficient way of drying.

Thus, the present invention provides an apparatus for drying a material which comprises a chamber for containing the material, and substantially retaining a volume of gas therein; water-vapour exchange means forming part of the chamber which is substantially permeable to water vapour and substantially impermeable to said gas; heating means for supplying heat to the interior of the chamber for drying said materials.

The apparatus allows water vapour in said gas volume to selectively pass through said membrane whilst the gas is substantially retained within the chamber. The water-vapour exchange means usually comprises a membrane which is substantially water-vapour permeable and substantially impermeable to gas.

Another aspect of the invention provides a corresponding method of drying.

The chamber may be of any suitable size or shape to define a substantially closed volume. Some leakage of gas from the chamber may, of course, be permitted but to that extent the efficiency of the apparatus is reduced. The chamber may be provided with inlets and outlets as required, particularly an inlet for introducing the material to be dried and an outlet for removal of the dried material. In this case, suitable means may be provided within the chamber for conveying the material through the chamber in a manner such that contact with the gas is optimised for good drying. Suitable conveyors, such as perforated belt conveyors, perforated cones etc. are well known in the art. Alternatively, the chamber may be used on a batch basis, whereby the chamber is opened and the material to be dried are introduced, for example on trays. The chamber is then closed, the material dried and then removed.

Suitable means may be provided for recirculating the gas within the chamber, so as to improve evaporation of water from the material to be dried and transfer of water vapour laden gas to the selectively permeable membrane. Suitable fans are well known in the art. Recirculation may alternatively be carried out via external ducts which nevertheless form part of the substantially closed volume defined by the chamber.

Heating means are provided for supplying heat to the interior of the chamber to assist drying of the material. The use of the heating means is necessary in order to achieve rapid drying and to provide elevated water vapour gradients across the membrane between the interior of the chamber and the exterior thereof. The heating means enables positive drying to ,be carried out down to a much lower water-content than can be achieved without the positive application of heat. Thus, whilst the arrangement disclosed in our earlier patent specification EP 0525842 prevents the build-up of liquid condensation and provides some degree of drying, the present invention represents a substantial improvement.

The heating means may be any suitable means for providing a heated surface, for example a heat exchanger surface heated by means of hot combustion gas, or electrical heaters. In a particularly preferred embodiment for use in sunny climates, the chamber is provided with a blackened solar absorptive surface; or with a transparent portion in the chamber walls and the corresponding solar absorptive surface within the chamber, such as to allow for solar heating. For example, dark-coloured coffee beans may be heated and dried in this way. Also, the introduction of solar radiation including ultraviolet radiation may be useful in providing the right colour to ripening fruit. Soda glass may be used in order to provide suitable ultraviolet transmission properties.

The material to be dried may in principal be any solid material (or conceivably even a liquid, where water is to be removed) and is generally in particulate form. Powders such as milk powder may also be dried.As well as natural products, the material could be a mineral, a plastics material, a pharmaceutical powder, a catalyst powder or other material capable of heated gas drying.

The water vapour exchange means comprises a selectively permeable membrane which allows water vapour to pass through, such that water vapour is selectively exchanged between the heated gas inside the chamber and the outside ambient atmosphere, dependent on the water vapour pressure differential across the membrane.

The selectively permeable membrane may be any suitable membrane known in the art. A particularly suitable membrane preferably includes porous expanded polytetrafluoroethylene (PTFE) , which may be produced as described in US patent 3,953,566. Such membranes allow the passage of water vapour and do have gas permeability also. A limited gas permeability compared to the water vapour permeability characteristics can be tolerated in practice. However, membranes which are substantially gas-impermeable may be produced by coating the PTFE membrane with a water-vapour-permeable coating such as described in US patent 4,194,041. The PTFE membrane may be supported on a backing material such as a woven or non-woven natural or artificial textile material known in the art to be suitable for the purpose, in order to provide adequate mechanical strength.

Other water-vapour-permeable gas-impermeable materials known in the art may also be used, such as polyurethanes and polyesters, polyolefins, polyacrylates and mixtures thereof.

Such membranes are generally impermeable to liquid water, which protects the chamber from the ingress of liquid water and is weatherproof.

In a preferred construction, the membrane is formed of a laminate comprising porous expanded PTFE having a water-vapour-permeable coating thereon, and a further layer of porous expanded PTFE adhered by means of an adhesive over the coating. The coating layer is gas-impermeable but water-vapour-permeable. The adhesive layer is preferably formed of a breathable compound which is water-vapour-permeable and gas-impermeable, and which is applied as described for example in our US patent 4,532,316.

The membrane usually has a water-vapour-permeability (i.e. water vapour transmission rate) of at least 1500, for example 1500 to 35000 (preferably 3000 to 10,000)g/m²/day; and will be chosen according to the dampness of the material to be dried and the surface to volume ratio of the chamber. The aggregate gas leakage rate of the overall apparatus is preferably less than 100% of the total enclosed gas volume per day, preferably less than 50% per day, preferably less than 20% per day and most preferably less than 10% per day in accordance with the energy losses which may be tolerated. The permeability constant for oxygen is typically 3xl0~⁸ to 3xl0"⁶ MS"¹; and the permeability constant for nitrogen is generally less than for oxygen. Generally, the ratio of gas permeability (measured as oxygen permeability) to water-vapour-permeability of the membrane will be less than 1X10"² and preferably in the region 1X10~⁴ to 1X10"⁶, particularly 5X10^-4 to 2X10~⁵. Methods for measuring these parameters are described herein. Usually there is a relationship between water-vapour-permeability and gas-permeability in particular membrane materials so that where a high water-vapour-permeability is needed for quick drying, a correspondingly higher gas permeability has to be tolerated.

The term "permeable" (and correspondingly "impermeable") is used herein to describe the property of the membrane to transport (or not transport) a particular species, such as gas or water-vapour, through the membrane. The term describes the overall effect of mass transport, and in no way implies any particular scientific mechanism by which this occurs.

Because the apparatus is essentially sealed with respect to loss of gas from therein, the present invention has the additional benefit that where the material to be dried includes volatile components (e.g. in the case of coffee beans, cocoa beans, paprika, onions etc.) these volatiles are generally retained within the chamber and are not lost. For example, conventional drying of coffee often results in the loss of volatile aroma components, and these have to be replaced in the coffee before sale.

It is also possible to maintain a reduced oxygen (or substantially oxygen - free atmosphere) within the chamber, by allowing the oxygen in the air to be used up by processes, such as fungal or insect growth, occurring unaided within the chamber as disclosed in our EP0525842. Reduced oxygen content (e.g. less than 2%) can also be achieved by providing an inert gas atmosphere (e.g. of nitrogen, carbon dioxide, argon) within the chamber. Thus, the apparatus may be used for drying materials which are susceptible to oxidative degradation (e.g. peanuts) which may tend to affect the colour or flavour (by production of free fatty acids) of the materials being dried. Thus, the apparatus may be used for drying materials such as flowers, fruits (apricots, raisins, pears, peaches etc.), spices (cardamom, cinnamon etc.) and herbs (parsley, sage, thyme, rosemary etc.).

Other materials such as tea and cocoa may require the presence of oxygen during drying. However, in general the substantially gas-sealed nature of the apparatus allows a chosen gaseous atmosphere to be maintained within the chamber, whilst allowing egress of water-vapour. Description of Preferred Embodiments

Embodiments of the present invention are now described by way of example only in conjunction with the drawings wherein;

Figure 1 is a schematic sectional elevation of a drying apparatus for particulate material comprising a single conveyor;

Figure 2 is a schematic elevation of a dryer employing triple conveyors;

Figure 3 is a schematic cross sectional elevation of a dryer using a vertical feed system onto mesh cones;

Figure 4 is a schematic elevation of a dryer having a separate water-vapour exchanger;

Figure 5 is a perspective view of a grain dryer;

Figure 6 is a schematic cross-section of the grain dryer;

Figure 7 is a cross-section of a preferred laminate for use in the invention; and

Figure 8 is a psychrometric chart demonstrating the principles of the invention.

Figure 1 shows a drying apparatus according to a first embodiment which comprises a chamber 2 provided with an inlet 4 for wet grain in the form of a hopper and a rotary inlet valve 6 for introducing wet grain in a controlled manner into the chamber. An outlet 8 regulated by a rotary valve 10 is provided for discharging dried grain from the chamber. The inlet and outlet valves allow grain to be introduced and discharged whilst limiting loss of hot air from within the chamber.

The wet grain is introduced onto the upstream end of a conveyor 14 formed of a perforate mesh material through which the heated air may pass. The mesh conveyor is supported on rollers 12, 16 at either end and slowly transports the grain through the drier towards the discharge.

Air is circulated within the chamber by means of fan 18 which blows air through,a heat exchanger 20 into a manifold 22 provided with a series of warm air outlets 24 beneath the mesh conveyor. Warm air is thus passed through the mesh conveyor and through the wet grain and thence to ducting 26 (forming part of the chamber) wherein the cooled air is recirculated to the fan 18.

The chamber includes as part of its wall structure a cover 30 which includes a selectively permeable membrane material as shown in Figure 7. The sides of the chamber also include panels of this selectively permeable membrane material.

Figure 2 shows a second embodiment which is similar to the first embodiment except that a triple conveyor arrangement is employed. Analogous parts are labelled with the same reference numbers.

In this case, air from fan 18 is passed through heat exchanger 20 and then passes over the grain travelling on conveyors 32, 34 and 36. The conveyors may be formed of a mesh material or may be i perforate. The cooled gas is then recirculated through duct 26 to the fan.

In the embodiments of Figures 1 and 2, the warm air passes over or through the wet grain and picks up water vapour. The water vapour-laden air then passes across the selectively permeable membrane 30 which retains the air within the chamber but allows the passage through the membrane of water vapour such that it is lost from the chamber. When equilibrium is reached, the amount of water being dried from the grain equals the rate of water loss through the membrane 30. However, there is substantially no loss of heated air from within the chamber so that energy losses are minimised. The chamber can, of course, be insulated in conventional manner to minimise heat loss by conduction.

Figure 3 shows a third embodiment wherein the wet grain is transferred by vertical motion under gravity over a series of vibrating cones formed of a mesh. Analogous parts are marked with the same reference numerals as before.

The chamber 2 is formed of light weight aluminium tubing and the sides and top of the chamber are formed of the selectively permeable membrane, whilst the bottom is solid. No heater is provided, but the selectively permeable membrane is black on the outside to maximise absorption of solar radiation and thus provide the required heat input for removing the water from the wet product. The product is introduced through the hopper as before and regulated by means of rotary inlet valve 6. The wet grain falls onto vibrating mesh cones 40, 42 over which is passes as it moves downwardly before the dried produce is discharged from the lower end of the chamber. The mesh cones comprise a pair of upwardly facing cones 42 and a pair of downwardly facing cones 40, whereby the produce trickles down a zig zag path through the chamber.

As before, air is circulated within the chamber by means of a fan 18 and recirculating duct 26.

Figure 4 shows a drying apparatus wherein the membrane for removal of water vapour is provided in a separate water-vapour exchange device 50.

The drying apparatus comprises a chamber 52 having a suspended perforate floor 54. Above the floor, the chamber is filled with grain 56 to be dried, leaving a headspace 58. The chamber has an inlet 60 for introducing heated air beneath the perforate floor, and an outlet 62 from the headspace for removing moisture-laden air.

The moist air exits via outlet 62 and passes through duct 64 to the water-vapour exchange device 50. The exchange device comprises a housing 64 having an apertured top-plate 66 spaced below a top of the housing and an apertured bottom-plate 68 spaced above the bottom of the housing.

A series of tubes 70 formed of gas-impermeable water-vapour-permeable membrane are sealed to and extend between respective pairs of apertures in the top and bottom plates. A closed path for gas from the chamber is defined by the membrane tubes.

Ambient atmospheric air is passed through the housing from inlet 72 to outlet 74 by a fan (not shown) as indicated by the arrows. If required the ambient air may be initially dehumified by a dehumdifier (not shown) in order to enhance the water-vapour pressure differential across the membrane.

Dried air from the water-vapour exchanger is passed through duct 76 to a circulation fan 18 and then through a heater 20 before being returned to the chamber 52.

The benefit of this arrangement is that the membrane is protected from the weather by being situated in the water-vapour exchanger. Also, the construction of the chamber 52 may be varied, and is not constrained by the requirement to provide a large surface area of membrane in its walls.

Figures 5 and 6 show a further embodiment of the invention wherein the water-vapour-permeable membrane is zipped onto an impervious groundsheet.

Walls of the drying apparatus are formed from sections 90 of zinc coated steel bent into a right angle and braced if necessary, so as to present an upstanding portion 92 and a horizontal portion 94. Typically each portion is approximately one metre square. An impervious rubber groundsheet 96 of Hypalon (trademark of DuPont) forms the base and sides of the apparatus, and is provided with pockets 98 along its sides and closed end which fit over the upper ends of the steel sections. A pair of zip halves 100, 102 are provided along the upper edges 104, 108 of the rubber groundsheet.

A water-vapour-permeable membrane 110 having corresponding zip halves 101, 103 is zipped onto the top of the upper edges of the groundsheet so as to form an enclosed chamber. The zips form gas-tight seals.

The zip half 103 is stitched to membrane 110 and the stitching seam is sealed by the application of porous expanded PTFE tape. The zip half 102 is bonded to the groundsheet sidewall. A thin rubber or fabric strip (not shown) is stitched and seam-taped to the underside of membrane 110 to protect the zip. Two zips are provided, each of which starts in the middle of end edge 108 and extends up a respective side 104 of the chamber.

The front end 112 of the chamber is open and the side walls reduce in height towards the front to bring the front of the membrane down to ground level. Similarly the front of the groundsheet extends beyond the front of the sidewalls to lie beneath the front of the membrane. The open end of the enclosure is sealed by rolling up the front of the membrane and groundsheet together and applying weights thereon.

The chamber is filled with grain (not shown) prior to attachment of the membrane 110. Grain can be removed by opening the front end of the enclosure and resealing.

Fans and supply ducts are provided to assist drying of the grain. Air from within the chamber is recirculated to avoid introducing fresh air with a higher oxygen concentration. As shown in Figure 6, recirculation is accomplished by means of fan 18 which causes air to be withdrawn through outlet duct 114 and to re-enter through inlet duct 112, both ducts being located in the rear wall. The air is heated in heater 20 before re-entering the chamber. For initial drying of wet grain, a membrane 110 may be employed which has high water-vapour-permeability (and consequently somewhat higher gas permeability) . Once the grain has been subject to an initial drying to avoid mould growth, the membrane may be substituted with a further membrane of lower water-vapour-permeability and enhanced gas impermeability in order to allow reduction of oxygen concentration to the desired degree (typically 5-7%) .

Figure 7 shows a membrane suitable for use in the present invention. The membrane is in the form of a flexible laminate of two layers 80, 82 of expanded porous PTFE, such as sold under the Gore-Tex trademark by W.L. Gore & Associates, Inc., one layer coated with a continuous water-vapour-permeable polyurethane coating 83 such as described in US patent 4,194,041. The two PTFE membranes are adhered together by means of a sub layer 84 of adhesive applied between the coating and the second PTFE membrane. The adhesive may be applied as a continuous layer (in which case it must be water-vapour-permeable) or as a series of adhesive dots at spaced locations. The adhesive is preferably produced as described in US patent 4,532,316. The layers of expanded

PTFE are also impermeable to liquid water which protects the grain from rain. The laminate has a water vapour transmission rate of 4,000 g/m²/day, a resistance to water vapour of 351 s "¹ and a resistance to oxygen of 3.34xl0⁷ sm^-1; measured as described herein. The ratio of water vapour resistance to oxygen resistance was about 1.05 x 1₀-5.

In order to give physical protection to the membrane, a face fabric which is resistant to ultraviolet light (not shown) is adhered to one side of the membrane by a pattern of adhesive dots (e.g. the adhesive designated TP3 available from W.L. Gore & Associates, Inc.). A knitted nylon liner is adhered in the same way to the other side of the membrane.

Figure 8 is a phsychrometric chart which (without wishing to be limited by any particular scientific theory) illustrates our understanding of the way in which the invention operates to dry materials such as grain.

The psychrometric chart shows the relationship between relative humidity, air temperature and absolute humidity (the quantity of water vapour in a unit volume or mass of air e.g. grams per cubic metre or grams of water vapour per kilogram of air) . The grain and air conditions for a typical example are:

Starting moisture content 19%

Desired moisture content 14%

Ambient air temperature 8"C

Ambient relative humidity 80%

It should be understood that materials having a particular water content will have a particular water-vapour pressure in the air corresponding thereto (at a given temperature) . For example the equilibrium water-vapour pressures for grain are typically at 14^βC: Water content 19wt% = 92% relative humidity

" 14wt% = 65% " " These figures depend on the particular material. Let us consider the points 1, 2 and 3 on Figure 8. Points 1-2 - Air entering the fan from a headspace above the grain is blown through the heater where the air temperature is typically raised 5-10°C, and the relative humidity is reduced from about 88 to 65%. This air continues along the ducts and enters the grain through perforations in the pipes, (or a grain floor etc.). Points 2-3 - The air travels through the grain and as it does so it starts to heat and dry the grain next to the pipe. Because the relative humidity of the air corresponds to grain moisture content of about 14% the grain will not dry below this level. It emerges from the grain cooler and wetter. Points 3-1 - When the air travels along the head space on its way back to the fan, water vapour is transmitted across the membrane to the outside atmosphere. At the same time some heat is also lost.

The membrane transmits water vapour using a mechanism that operates in favour of drying grain as long as the concentration of water vapour inside the chamber is greater than that outside. Since the air inside the headspace is maintained at 5-10°C higher than ambient, water vapour will be transmitted through the membrane even when it is wet on the outside (See point 4 on the chart which represents the ambient air). At 14°C the relative humidity would have to drop to 55% or lower for water to re-enter the chamber under the ambient conditions listed above. This corresponds to a moisture content of about 13% and air circulation would be stopped at this moisture content since drying is complete. Lack of air circulation severely impedes the transmittance of water vapour so the possibility of rewetting the grain is obviated. The water vapour concentration difference between inside and outside determines the rate of drying - the larger the difference the faster the rate of drying. So for a constant temperature outside the drying rate will be faster when the outside ambient relative humidity is lower.

It should also be noted that the permeability of the membrane to water-vapour is itself a function of relative humidity. In order to maximise transport of water-vapour across the membrane a high relative humidity inside the chamber should be aimed for. The permeability of the membrane to oxygen is also a function of relative humidity, though to a lesser extent.

The fan and heater are carefully sized to make sure that exactly the correct amount of air at the right temperature and relative humidity is delivered to the grain. Operation of the Drier

When the chamber is filled with damp grain the relative humidity in the headspace corresponds to the moisture content of the grain. For 20% grain the relative humidity will be about 89%. The fan and heater are switched on and the drying process starts straight away. In the example above when the air passes from the pipe through the grain, it will only pick up moisture from grain which has an equilibrium relative humidity higher than the air. By monitoring the relative humidity in the headspace and setting the fan and heater controls to stop at the equilibrium relative humidity of the desired moisture content of dry grain, drying is stopped. For 14% grain the equilibrium relative humidity is about 63%. So if the air emerging from the grain is at about 65% the grain must be dry. Grain may typically be dried in six weeks. This could not be achieved without the use of a heater. Maintaining an Oxygen Deficient Atmosphere

When damp grain is kept in the chamber either prior or during drying, the grain itself and the fungi associated with it will consume oxygen and the level of oxygen will decrease from the normal 21% in air;. The presence of insects will have the same effect. Depending on the difference in oxygen concentrations between inside and outside the chamber, a very small amount of oxygen from the outside may re-enter the chamber. If the consumption of oxygen is relatively high the amount that can enter the chamber is not enough to raise the concentration significantly and a low level of oxygen is maintained in the chamber. When levels of oxygen are low, aerobic consumption is limited for both fungi and insects. Thus if there is a high biological or insect activity in the chamber, low oxygen levels can be maintained which in turn limits the aerobic consumption of grain.

Measurement of Water Vapour Transmission Rate (WVTR)

A description of the test employed to measure water vapour transmission rate (WVTR) is given below.

In the procedure, approximately, 70ml. of a solution consisting of 35 parts by weight of potassium acetate and 15 parts by weight of distilled water was placed into a 133ml polypropylene cup, having an inside diameter of 6.5cm at its mouth. An expanded polytetrafluoroethylene (PTFE) membrane having a minimum WVTR of approximately 85,000 g/m²/24 hrs. as tested by the method described in U.S. Patent 4,862,730 to Crosby and available from W.L. Gore & Associates, Inc. of Newark, Delaware, was heat sealed to the lip of the cup to create a taut, leakproof, microporous barrier containing the solution.

A similar expanded PTFE membrane was mounted to the surface of a water bath. The water bath assembly was controlled at 23"C ± 0.2°C, utilizing a temperature controlled room and a water circulating bath.

The sample of membrane to be tested was allowed to condition at a temperature of 23°C and a relative humidity of 65% prior to performing the test procedure. Samples were placed so the microporous polymeric membrane to be tested was in contact with the expanded polytetrafluoroethylene membrane mounted to the surface of the water bath and allowed to equilibrate for at least 15 minutes prior to the introduction of the cup assembly.

The cup assembly was weighed to the nearest 1/lOOOg. and was placed in an inverted manner onto the center of the test sample.

Water transport was provided by the driving force between the water in the water bath and the saturated salt solution providing water flux by diffusion in that direction. The sample was tested for 20 minutes and the cup assembly was then removed, weighed again within 1/lOOOg. The WVTR of the sample was calculated from the weight gain of the cup assembly and was expressed in grams of water per square meter of sample surface area per 24 hours.

Measurement of Oxygen Permeability

Oxygen permeability (and oxygen resistance) was measured by the test method given below.

The permeability was measured using a method based on the ASTM standard test designation F738-85. A stainless steel cell was divided into an upper and a lower chamber by the material under test. 100% nitrogen was passed through the lower chamber and 100% oxygen was passed through the upper chamber. The flow of gas was kept constant through both chambers using a mass flow controller. The concentration of oxygen was measured by gas chromatography. The permeability (P) is given by the following formula:-

P = CxF_/A where C = concentration of oxygen in the lower chamber F = Flow through the lower chamber A = Area of material under test Because of the nature of the membrane it could not be completely sealed in the permeability apparatus. A 'leak test' was therefore carried out by passing 100% nitrogen through both upper and lower chambers of the permeability cell. The concentration of oxygen leaking into the lower chamber was measured and an apparent permeability was calculated. A value for the oxygen permeability of the membrane corrected for leakage into the lower chamber could then be calculated by subtracting this apparent permeability from the measured permeability.

The permeability constant (k) for the laminate was as follows:- k = 2.99 x 10^-8 m s^-1 (a resistance of 3.34 x 10⁷sm^_1) where k = permeability/concentration gradient (g m-³)

Because the concentration in the lower chamber is so small compared to the concentration in the upper chamber the span of the concentration gradient can be taken as the concentration in the upper chamber.

Ratio of Water Vapour Permeability Constant to Oxygen Permeability Constant

The permeability constant for water vapour transmission can be calculated from the W.V.T.R. for a typical membrane (d = day) .

W.V.T.R. = 4000 gm~² d^_1

= 0.0463 gm-² s^-1 (/86400)

This is the permeability used in the equation below. The permeability constant, k, is

Perm = k x DELTAP or k = Perm gm^~ s^_1 / gm^-3

DELTAP where DELTAP is the difference between the water vapour concentrations inside (100%) and outside the fabric (20%) . DELTAP is the difference in absolute humidities at 100% and 20% relative humidity (rh) where absolute humidity, ABSHUM, is

ABSHUM = (E X 2170) / (273.15 + T) gm^-3 where E = Water vapour pressure kPa T = Temperature °C

The water vapour pressure, E, at a given rh is the saturated water vapour pressure, Es, multiplied by rh.

Es = exp(16.6536 - 4030.183/(T+235) ) kPA (6) For the W.V.T.R. test at 23°C and 100% and 20% rh's,

Es = 2.8087 kPa

DELTAP = (1.0 - 0.2) x Es x 2170 / (273.15 + 23) gm^-3 = 16.46 gm^-3 The permeability constant for water vapour is

k = 0.0463 gm-²s^-1/ ^n~2s-¹ 16.46

= 2.8 x 10^-3 rns^-1 The ratio of the permeability constants for the membrane is

k H20 = 2.8 x IP"³ k 02 2.99 x 10^-8

= 1 x 10⁵

Thus, this typical membrane is 100,000 times more permeable to water vapour than oxygen.

Claims

1. Apparatus for drying a material which comprises;

- a chamber for containing the material, and substantially retaining a volume of gas therein;

- water-vapour exchange means forming part of the chamber which is substantially permeable to water vapour and substantially impermeable to said gas;

- heating means for supplying heat to the interior of the chamber for drying said material.

2. Apparatus according to claim 1 or 2 wherein the chamber has a closable inlet and a closable outlet for passing said material to be dried through the chamber.

3. Apparatus according to claim 2 which further comprises conveyor means within the chamber for continuously passing said material through the chamber.

4. Apparatus according to any preceding claim which further comprises recirculation means for circulating the gas within the chamber.

5. Apparatus according to any preceding claim wherein the chamber has a wall means, and the water-vapour exchange means forms part of the wall means of the chamber for containing said material to be dried.

6. Apparatus according to any of claims 1 to 4 wherein the chamber has two units; the water-vapour exchange means is formed as the first unit of the chamber and is connected to the second unit of the chamber which contains the material, by connecting ducts adapted to pass water-vapour containing gas from around the material to be dried to the first unit and return reduced water-vapour content gas to said second unit.

7. Apparatus according to claim 6 wherein the water-vapour exchange means comprises at least one water-vapour permeable tube.

8. Apparatus according to any preceding claim which further comprises dehumidification means.

9. Apparatus according to claim 8 wherein the dehumidification means is adapted to reduce the water-vapour pressure of ambient air outside the water vapour exchange means.

10. Apparatus according to any preceding claim wherein the means for supplying heat comprises a heater for heating the gas.

11. Apparatus according to any of claims 1 to 9 wherein the means for supplying heat comprises a solar absorptive surface in thermal contact with the gas volume in the chamber.

12. Apparatus according to any preceding claim wherein said gas in said retained gas volume has a composition which comprises less than 2% by weight of oxygen.

13. Apparatus according to any preceding claim wherein said water-vapour exchange means comprises a membrane.

14. Apparatus according to claim 13 wherein the membrane comprises porous polytetrafluoroethylene.

15. Apparatus according to claim 14 wherein the membrane further comprises a substantially water-vapour permeable and substantially gas-impermeable coating applied on said porous polytetrafluoroethylene.

16. Apparatus according to claim 15 wherein a further layer of porous expanded polytetrafluoroethylene is adhered over said coating by means of a a water-vapour permeable adhesive layer.

17. Apparatus according to any of claims 13 to 16 wherein the membrane has a water-vapour-permeability of at least 1500 g/m /day.

18. Apparatus according to any of claims 13-17 wherein the ratio of gas permeability to water-vapour-permeability of the membrane is less than lxlO- .

19. Apparatus according to any of claims 13 to 18 wherein the membrane is impermeable to liquid water.

20. Apparatus according to any preceding claim wherein the gas leakage rate of the chamber is less than 100% of the total gas volume per day.

21. Apparatus according to claim 20 wherein the gas leakage rate is less than 50%.

22. Apparatus according to claim 20 wherein the gas leakage rate is less than 30%.

23. Apparatus according to claim 20 wherein the gas leakage rate is less than 10%.

24. A method of drying material which comprises:

- providing a chamber containing the material to be dried and substantially retaining a volume of gas therein;

- heating the gas;

- passing the heated gas into contact with the material and increasing the water-vapour content of the gas; and - bringing the increased water-vapour content gas into contact with a water-vapour exchange means which is substantially permeable to water-vapour and substantially impermeable to said gas, such that water-vapour passes selectively through the water-vapour exchange means.

25. A method according to claim 24 wherein the material to be dried contains volatile components, and wherein the water-vapour exchange means substantially retains said volatile components within the chamber.

26. A method according to claim 18 and 19 wherein the material to be dried is selected from the group consisting of coffee, cocoa, onions, flowers, fruits, spices and herbs.

27. A method according to claim 24 or 25 wherein the material to be dried is grain.

28. A method according to any of claims 24 to 27 wherein the oxygen content of said gas volume is less than 2% by weight.

29. A method according to any of claims 24 to 28 wherein an inert gas is introduced into said gas volume.