FR2780491A1 - Closed circuit air drying procedure e.g. for foodstuffs - Google Patents

Abstract

The procedure consists of placing the product to be dried in a silo (9) inside a thermally-insulated drying chamber (8) and passing air through it in a closed circuit with programme-controlled temperature and hygrometry. The air picks up water vapour as it passes through the product and it is then passed through a heat exchanger (12) where the water vapour condenses. The resulting dry air is heated and fed back to the drying chamber.

Description

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  The present invention relates to a drying process by air treatment in a closed circuit and implementation device. It is an artificial drying process by dehydration of air in a closed circuit, with energy recovery and without

carbon dioxide emission.

  This drying can be applied to many bulk food products

  such as: pulses, aromatic or medicinal plants, chestnut cereals, etc.

  The present application has been experimented with drying nuts and this

for 9 seasons.

  Before developing this, it is advisable to compare a drying cycle by the current method (drying by a gas-heated air generator) to a cycle of

  air drying in a closed circuit.

  In a gas heated air generator drying, the air is taken outside, passes through the burner, the product to be dried, and is then released to the open air. Thus, the causes of energy losses are numerous: - generation of water vapor at the level of combustion, ie 1.55 times the weight of the gas (butane or propane), which is then sent to the nuts to be dried. This cause of

  loss is relatively small, around 3 to 5%.

  - outside air intake for one season, October, where the air is cool and humid (average RH of 80% and T = 10 to 12 C), hence the need to heat more to

lower the relative humidity.

  - the water, in the liquid state in the nuts can only be extracted through the evaporation phase, which absorbs energy (around 70 KWH per 100 liters of water

  evaporated) then returned to the atmosphere.

  - all of the energy generated at the burner is thus discharged into

  the atmosphere in a sensitive and latent form.

  - release of carbon dioxide into the atmosphere (3 times the weight of the gas burned).

  - the heated air, passing through the nuts, drops in temperature and becomes charged with humidity, thus introducing a drying front which is difficult to correct. In practice, the thickness of the nuts to be dried is reduced; some manufacturers have introduced a patch

mechanical to remedy this.

  In a drying cycle by air treatment in a closed circuit, the air circulates along a route for which thermal insulation has been ensured, whether in the

  dehydrator, or in the nut drying enclosure.

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  The advantages are as follows: - drying is no longer dependent on external climatic conditions and

  benefits from thermal insulation, thus reducing energy losses.

  - the heating is supplied by the sector via the compressor and the condenser.

  - there is no release of carbon dioxide into the atmosphere. - safety and regulation are easy to implement through temperature and hygrometry probes, because it suffices to cut electrical circuits so

  automatic or manual, depending on the operating mode chosen.

  - the noise level of the installation is low.

  - the drying front is reduced by the fact that we start to extract water at low temperatures (around 15 C). Thanks to thermal insulation, said front

  drying can be compensated by stopping the compressor while maintaining ventilation.

  The temperature and hygrometry of the drying chamber are then made perfectly homogeneous and this with respect to temperature and hygrometry thresholds

programmable.

  - last but not least, the energy of water evaporation is recovered in the

  condensation thus considerably reducing energy consumption.

  The invention relates to a set of dehydration of agro-products

  food. For the implementation of the method according to the invention, this assembly comprises two units:

- a drying enclosure 1.

- a dehydration device 2.

  The apparatus for implementation is illustrated by the single figure l (see annex). The two units of this set are joined by simple juxtaposition, a joint

  flexible ensuring sealing between said units.

  Air circulation takes place in a closed circuit between the two units 1, 2 and is ensured by fans 3,4,5. The air circulates in the sealed drying enclosure 1, so as to make it pass from top to bottom, the products to be dried. This same air circuit enters 6 in the dehydrator where it is dried and then heated before reintroduction in 7

in the drying cabinet.

  According to one embodiment of the invention, this device comprises: a drying enclosure, the outer wall 8 of which is thermally insulated; a silo 9, where the product to be dried is loaded, the section of which is octagonal and having a bottom made of perforated sheet metal 10 to ensure the circulation of air; a separation 11 intended to force the

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  passage of air over the product to be dried, while ensuring the separation between the air which enters

7 and the air coming out 6.

  According to another embodiment of the invention, the apparatus comprises: a dehydrator 2 o the air is dehumidified and then heated. The circuit of this air is thermally isolated throughout its course. The air leaving the drying chamber enters a heat exchanger 12 where it is cooled and then dehumidified by condensation of water vapor. Water resulting from

  this condensation is collected by gravity in a tank 13.

  Said air circuit then passes through a second heat exchanger 14 which heats it. A compressor 15 coordinates the connection between the exchangers 12 and 14 already mentioned. Part of the air circuit (not shown) can be diverted out of the exchanger

  thermal 12 in the event of further drying.

  According to another embodiment of the invention, the apparatus comprises a regulation 16 which ensures the signals, the security and the optimization of the drying cycle from the temperature sensors T 17 18 and of hygrometry 19. The setting the drying cycle is carried out by a separate order, depending on the most suitable choice. The control panel presents the system features which are: 1 Manual; - In this case the temperature and the humidity are indicated but none of the

  programmed values (T HG ATs) are not taken into account.

  -2 Automatic without regulation; The start of the cycle is done using the "START" button. The programmed values T HG and ATs are taken into account throughout the cycle but said cycle

Do not stop.

-3 Automatic with regulation;

  Same as 2 but when the values of T and HG are reached, the cycle stops.

  In the case of an automatic drying cycle operation, the following parameters are entered by programming:

  - T = temperature not to be exceeded.

  - HG hygrometry threshold where you want to stop drying.

  - ATs = time during which we stop extracting water to homogenize the

  temperature and humidity in the drying chamber.

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  These parameters can be changed during the drying cycle.

  As soon as the device is switched on, the temperature and humidity are signaled and all the fans are supplied, thus constituting a prerequisite for everything

operating mode.

  The invention relates to a process for drying agri-food products, this process being able to be implemented by the apparatus described above. The process will now be described in relation to FIG. 1. The dehydration process according to the invention consists in depositing the bulk material (nuts in our application) in the silo 9 located inside the drying enclosure 1. Air treatment in a closed circuit (materialized by arrows) gives hot and dry air at 7 (HG <40%) The energy of this incoming air remains constant during the crossing of the nuts, under latent (water vapor) and sensitive (temperature) form. On contact with this atmosphere, the water in the nuts evaporates, thereby increasing the latent energy and lowering the sensitive energy of the air flow. It is

  thus produces in 6 a less hot and more humid air flow.

  In contact with the heat exchanger 12, the air being optimally cooled, there is condensation of the water vapor coming from the nuts. Said water, resulting from condensation and thus recovering the energy of evaporation, flows by gravity into the tank

  of condensates 13. We obtain an air cooling materialized by AT1 20.

  Note that the amount of water collected is a precise indication

  as to the degree of drying of the product placed in the silo 9.

  The air circuit, cooled and dehydrated by passing through the heat exchanger 12, is then directed to another heat exchanger 14 where it recovers, in sensitive form, all the energy taken from 12 and to which the energy taken is added. to the sector through the compressor 15. This energy is materialized by AT2 21 and we

always have, AT2> to AT1.

  Thanks to the thermal insulation throughout the air circuit and to the energy taken from the sector, we obtain, by the additive effect of this, a slow rise in the

air temperature.

  Thus, throughout the drying cycle, we observe, gradually, a rise in temperature and a regression in humidity allowing therefore to push

  drying as required.

  When the slowly rising temperature reaches the programmed value of

  T, the regulation introduces a heating stop for a programmable time ATs.

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  During this time, the temperature and humidity of the drying chamber become

  homogeneous, thus correcting the drying front in drying chamber 1.

  It should be noted that this method of correcting the drying front does not entail any additional investment in the design of the device. Only one programming element will be taken into account. The parameters T HG and ATs being programmable and modifiable during the cycle, it is therefore possible to conduct a drying cycle very gradually and adapt it to

a wide range of products.

  When the programmed values of T and HG are reached, drying is

  considered finished and the "cycle in progress" indicator goes out.

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Claims (2)

  1 drying process by air treatment in a closed circuit consisting of the dehydration of the products to be dried, characterized in that it comprises the following stages: - depositing the bulk product in the silo 9 inside the thermally insulated drying enclosure. - implement the drying cycle consisting in circulating in a closed circuit and over the product to be dried, air whose temperature and humidity are controlled by programming. By passing through said product, the air is charged with water vapor, then is sent to a heat exchanger on which the water vapor condenses. The air thus dried and then heated is directed to the inlet of the drying chamber and through the product to be dehydrated. 2. Dehydration process according to claim 1, characterized by the following steps:   - program the following parameters in a first step.   T = maximum temperature to which the product must be subjected. HG = end of cycle hygrometry.   ATs = drying homogenization time.   - after performing a first cycle, program again, if necessary in a second step, the parameters T, HG and ATs in order to reach the final drying   gradually and well adapted to the product.   3. Device for implementing the method characterized in that it comprises a control panel for a drying line either: manual or - automatic without regulation or   - automatic with regulation, depending on the product to be dried.   4. Device according to claim 1 characterized in that it comprises: a drying enclosure 1 thermally insulated and juxtaposed with   dehydrator 2. Sealing between the two units is ensured by a flexible seal.   - a means for causing the circulation of air 3, from top to bottom, through of the product to be dried. 7 2780491   5. Device according to claim 1 characterized in that it comprises:   - a thermally insulated dehydrator, inside which the air to be treated circulates.   - fans 4 5 causing the flow of air leaving the enclosure   drying 6 towards the inlet 7 of said drying enclosure.   a heat exchanger 12 placed first in the air circuit and cooling it so as to cause the condensation of water vapor, which   then flows by gravity into a tank 13.   - a compressor, taking energy from the sector and ensuring through its   thermodynamic cycle optimized heat exchanges between the exchangers 12 14.   Said energy thus taken from the sector, is added to the air circuit via the exchanger 14   and constitutes the heating source.   - A heat exchanger 14 restores the energy taken from the heat exchanger 12 and adds the energy taken to the sector to heat the air before its reintroduction in the drying chamber 1.   -a slow rise in temperature thanks on the one hand to thermal insulation   throughout the air circuit and to the additive effect of the energy taken from the sector.   6. Device according to claims 1 2 3 characterized in that it comprises:   - a correction of the drying front, included in the programming of the drying homogenization time ATs, which thanks to the thermal insulation makes it possible to   maintain the energy in the enclosure practically constant during the time ATs.   7. Device according to claim 1 characterized in that it comprises: recovering the energy of the evaporation phase of the water from the product to be dried, in the condensation phase of said water vapor, thus reducing   considerably energy consumption.   8. Application of the method according to claim 1 or 2 to the drying of food products.