FR2544992A1 - Plant for drying a liquid product - Google Patents

Abstract

Plant for drying a liquid product comprising a drying column 10 in which the sprayed liquid product travels countercurrentwise to a hot air being external air heated in the secondary circuit 52 of a heat exchanger 50. This plant is characterised in that it comprises a scrubbing column 20 eliminating the solid matter remaining in the air leaving the drying column, a heat pump consisting of a compressor 31 heating the air leaving the scrubbing column, and a pressure reducer 32 lowering the temperature of the air before it exits into the atmosphere, the hot air travelling between the compressor and the pressure reducer gives up its heat energy to the secondary circuit 52 of the heat exchanger 50. This plant is employed chiefly in the food industry.

Description

Drying plant for a liquid product
The present invention relates to a drying installation of a liquid product comprising a drying column in which the liquid product sprayed circulates against the current of a hot air, being the outside air heated in the primary circuit of a heat exchanger .

 In many food factories and especially in dairy plants, some liquid products involve a drying treatment, preceded or not, of a concentration operation.

 The final operation bringing the product to a water content reduced to less than 10% (# to 5% in the case of powdered milk, for example) is very often carried out in vertical drying towers in which the product is introduced. liquid to be dried, concentrated or not, at the top by means of a spraying device for the purpose of forming a mist of liquid particles offering the maximum of heat exchange surface introduced into the tower at the lower part to circulate from bottom to top counter-current with the droplets of liquid in contact with which the air cools by absorbing in the form of vapor the water extracted with the liquid.

A dry pulverulent product is collected at the bottom and the upper part of the air cools while being loaded with moisture.

 In known installations, air is admitted into the drying column at a temperature of between 180 and 1900 ° C.

 At the outlet of this column, the air has a temperature of about 900 ° C. after being charged with a quantity of water of between 35 and 40 g / kg of dry air, which corresponds to a dew point temperature. relatively low between 30 and 400 C, thus making it difficult to recover the latent heat fraction contained in the air.

The air is therefore released into the atmosphere at a temperature of the order of 900C and loaded with milk powder up to 0.5 to 1.5% of the tonnage collected at the bottom of the column. This poses a number of problems related to pollution, either outside air or waste water.

 The present invention therefore aims to recover the solids contained in hot air leaving the drying column, and even to value them by marketing them in an optionally degraded form.

 In addition, in this prior installations, the air heating needs are provided most often by means of a heat transfer battery in which hot water or steam circulates from a boiler consuming a certain amount of fuel, or in some cases, by means of a gaseous fuel directly heating the air.

 The present invention therefore also aims to allow an operating economy obtained by the use of a heat pump that eliminates the use of a boiler, so a fuel, in favor of a single energy (for example electricity) which significantly improves the operating cost of the installation.

 It also has the added advantage of applying outside air only a heating treatment before it passes through the drying column. In fact, the exchange of heat in the column is done directly, and therefore the outside air must not undergo any other treatment which could alter its physical or chemical characteristics.

 The present invention is characterized in that it comprises, a washing column removing the remaining solids in the air leaving the drying column, a heat pump composed of a compressor heating the air coming out of the column. washing and a regulator lowering the temperature of the air before its exit into the atmosphere, the hot air circulating between the compressor and the regulator, yields its calories to the secondary circuit of the heat exchanger.

 According to an alternative embodiment, the hot air circulates between the compressor and the expander in the primary circuit of another air / water heat exchanger and yields its calories to the secondary circuit of the first air / water heat exchanger via a water circuit constituted by the secondary circuit of the second exchanger and the primary circuit of the first exchanger.

 According to another variant embodiment, the hot air circulates between the compressor and the expander in a washing column and transfers its calories to the secondary circuit of the air / water heat exchanger via a circulating heat transfer fluid against the current of hot air in the washing column and passing through the primary circuit of the exchanger.

 Advantageously, the air leaving the expander passes through a washing column, against the flow of a fluid.

 According to yet another embodiment, the hot air circulates between the compressor and the expander in the primary circuit of an air / air heat exchanger, and thus gives up its calories to the secondary circuit.

 Advantageously, the air exiting into the atmosphere is recycled to the primary circuit of the heat exchanger and, before this recycling, the water contained in this air is recovered in the primary circuit of the exchanger placed between the compressor and the expander or in the coolant circuit of the wash column, and removed by a pipe.

 Preferably, the outside air is preheated in the secondary circuit of a heat exchanger, the primary circuit being traversed by an auxiliary heat source.

 Hereinafter, by way of example and with reference to the figures of the accompanying drawings, a drying installation according to the invention is described.

 FIG. 1 represents the complete drying installation provided with a drying tower of a washing column and means of heating the air necessary for drying.

 Figure 2 shows a variant adaptable auxraoyems heating.

 Figure 3 shows another variant adaptable to the heating means, as well as other ancillary arrangements.

 In FIG. 1, the installation comprises a drying column 10 provided with a device (not shown) for spraying the liquid, the pulverized liquid being introduced into the drying column via the pipe 11 and recovered once dried by the cana 12 at the bottom of the column.

 The hot air required for drying is introduced at the bottom of the column through line 2.

 The hot air is obtained as follows: the outside air (arrow E) enters via a pipe 1 into a secondary circuit 52 of an air / water heat exchanger 50 to be heated to about 1800 ° C., and enters through the pipe 2 (arrow F) at the bottom of the drying column.

 The air leaving the drying column, at a temperature of about 900 ° C., enters via a pipe 3 into the bottom of a washing column 20 to remove the solids entrained during drying, which still represent approximately 0.5 125% of the dried product.

 The air flows upward against the current of an oil mist coming out of the diffuser 23, and the solids are then driven and discharged into the oil 24 located at the bottom of the washing column. This oil 24 circulates through a pump 21, and it can be used in food preparations for livestock feed.

 A sample of a certain percentage of the flow rate of the pump may be carried out bypassing the oil circuit to pass into a centrifuge for the purpose of separating a certain fraction of the solids. These materials are recovered as a paste containing a higher or lower percentage of edible oil.

After a while the renewal of the oil charge will be necessary but this oil, as well as the extract obtained thanks to the centrifuge, can be valorized as livestock feed.

 This washing is isothermal and the air which is treated at a temperature close to 900 ° C., but freed of its solids, is directed to the outlet of the washing column 20 and enters, via a pipe 4 (arrow G), into a centrifugal or axial compressor 31 which is intended to compress it to a corresponding pressure at a final temperature of 210 to 2300 C.

 The pressurized hot air enters via a pipe 5 into the primary circuit 41 of a finned tube air / water heat exchanger 40, which makes it possible to heat a pressurized 6d water circuit to around 2000C, of which the object is to transfer the heat released by the compressed air in the primary circuit 41 of the exchanger 40 to the air flowing in the secondary circuit 52 of the exchanger 50 of the same design. Circulation is ensured permanently by the operation of a water pump 61 and the circuit is completed by an expansion vessel 62 under a nitrogen or air sky, protected by a safety valve 63.

The cooled compressed air enters via a pipe 6, in a centripetal or axial expansion valve 32 mechanically connected to the compressor 31 by a shaft 71. The shaft 71 is driven by the expander 32 which recovers a portion of the compression energy and the additional energy is provided, for example, by an electric motor 70
The compressed air entering the regulator at approximately 55C will be discharged into the atmosphere through a pipe 7 (arrow S> at a variable temperature depending on the initial water content and the amount of water that has been absorbed in the drying column This temperature will however be close to the temperature of the outside air creating no disturbance or pollution.

 In FIG. 2, the installation has an alternative embodiment, and the drying and washing columns 20 are not shown, but they are, of course, connected respectively to the pipe 2 (arrow F) and to the pipe 4 (FIG. arrow G).

 This variant consists in replacing the heat exchanger 40 provided on the compressed air at the outlet of the compressor 31 (FIG. 1) by a second washing column 80 using a heat-transfer fluid flowing through a pump 81. air and the sprayed heat transfer fluid leaving the diffuser 84 flow against the current to obtain better heat exchange conditions and therefore a reduction in energy consumption of the heat pump. The final temperature at the outlet of the pipe 7 can thus be reduced by 10 or t5 degrees C with respect to the installation of FIG.

 The heat transfer fluid recovered at the bottom of the washing column thus circulates via the po # pe in the primary circuit 51 of the finned tube air heat exchanger 50 to heat the outside air passing through the secondary circuit. 52.

 In this variant, it is possible to add another washing column 90 to recover the greater part of the vaporized fraction of the heat transfer fluid u04, which could have been entrained in the expander in the form of steam. This involves removing Line 7 and replacing it with Line 91.

The air then enters the bottom of the column 90 and flows against the flow of a washing fluid driven by a pump 92 and leaving the diffuser 93. The air is then rejected externally by a pipe 94 (FIG. arrow S ').

 In FIG. 3, the installation has another variant embodiment and the drying and washing columns 20 are not shown, but they are, of course, connected respectively to the pipe 2 (arrow F) and to the pipe 4 (FIG. arrow G).

 This variant consists in replacing the heat exchangers 40 and 50 (FIG. 1), or the exchanger 50 and the washing column 80 (FIG. 2), by a single air / air heat exchanger 50 '. The air leaving the compressor 31 through the pipe 5 passes through the primary circuit 511 of the exchanger 50 'and then passes into the expander 32.

 The air coming from the outside through the pipe 1 passes through the secondary circuit 52 'to be heated and distributed in the drying column through the pipe 2.

 This figure also shows additional amenities.

 A first arrangement, adaptable to the three embodiments of the heating means, is to recycle the air exiting the expansion valve 32 (arrow S) by passing through a pipe 7 'connected to the pipe 1 (arrow E).

 This makes it possible to work in a closed circuit, thus in a pressure circuit that is favorable to drying.

 However, the water contained in the air must be eliminated before recycling it and this is done during a cooling phase, preferably at the primary circuit 51 'of the exchanger 50' (or 41.degree. the exchanger 40 Figure 1, or at the level of the pipe 85 (Figure 2) by a pipe 115 connected to a siphon 120 which discharges the waste water through a pipe 125.

 A second arrangement, independent and therefore adaptable to all embodiments, is to heat the fresh air, prior to the final heating obtained through the heat pump compression / expansion of air, by means of a heat source lost recoverable at a relatively low thermal level (for example in wash water before sewage), this may reduce the heat demand to be provided by the heat pump.

 This source of heat passes through the primary circuit 101 of a heat exchanger 100 and gives up its heat to the secondary circuit 102 through which the outside air coming from the pipe 103 (arrow E2) passes and which penetrates once heated in the pipe 1 of the air outside (arrow E), by a pipe 104.

 Without departing from the scope of the invention it is obvious that the described invention can be extended to other cases of drying of food products or not.

Claims (8)

1 / Apparatus for drying a liquid product comprising a drying column (10) in which the liquid product sprayed circulates against the current of a hot air being the outside air heated in the secondary circuit (52, 52 '). a heat exchanger (50, 50 '), characterized in that it comprises, a washing column (20) removing the remaining solids in the air leaving the drying column, a heat pump composed of a compressor (31) heating the air leaving the washing column and a pressure reducer (32) lowering the temperature of the air before its exit into the atmosphere, the hot air flowing between the compressor and the pressure reducer, gives way its calories to the secondary circuit (52, 52 ') of the heat exchanger (50, 50'). 2 / drying installation according to claim 1, characterized in that the hot air flows between the compressor and the expander, in the primary circuit (41) of an air / water heat exchanger (40), and gives up its calories to the secondary circuit (52) of the air / water heat exchanger (50) via a water circuit (60) constituted by the secondary circuit (42) of the exchanger (40) and the primary circuit (51) of the exchanger (50). 3 / drying plant according to claim 1, characterized in that the hot air circulates between the compressor and the expander, in a washing column (80) and sells its calories to the secondary circuit (52) of 11 heat exchanger air / water (50) via a heat transfer fluid circulating against the flow of hot air in the washing column and passing through the primary circuit (51) of the exchanger (50). 4 / A drying installation according to claim 3, characterized in that the air exiting the expansion valve passes through a washing column (90) against the flow of a fluid. 5 / drying plant according to claim 1, characterized in that the hot air circulates, between the compressor and the expander, in the primary circuit (51 ') of an air / air heat exchanger (50), and gives way thus his calories to the secondary circuit (52 ').  6 / drying plant according to one of claims 1 to 5, characterized in that the air exiting into the atmosphere (arrow S or S ') is recycled into the primary circuit (51, 51') of the exchanger of heat (50, 50 '). 7 / drying plant according to claim 6, characterized in that, before the recycling of air, the water contained therein is recovered in the primary circuit (41, 51 ') of the exchanger placed between the compressor and the expander or in the coolant circuit of the wash column (80), and removed by a pipe (115). 8 / Apparatus according to one of claims 1 to 7, characterized in that the outside air is preheated in the secondary circuit (102) of a heat exchanger 100, the primary circuit (101) being traversed by a source of heat annex.