Evaporation in food industry
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Evaporation is the partial removal of water from liquid food by boiling. For instance, liquid products can be concentrated from 5% dry solids to 72%, or even higher, depending on the viscosity of the concentrates. Evaporation is used to pre-concentrate food, to increase the solid content of food, to change the colour of food and to reduce the water content of a liquid product almost completely, e.g. as in edible oil drying (BAT in the Food, Drink and Milk Industries, June 2005).
2. FIELD OF APPLICATION
Evaporation is used in many food, drink and milk applications. For example, it is used to process milk, starch, derivatives, coffee, fruit juices, vegetables pastes and concentrates, seasonings, sauces, sugar and edible oil (BAT in the Food, Drink and Milk Industries, June 2005).
3. DESCRIPTION OF TECHNIQUES, METHODS AND EQUIPMENT
- General description of evaporators: (Operations in Food Processing - the Web Edition, R. L. EARLE, 1983)
- The typical evaporator is made up of three functional sections: the heat exchanger, the evaporating section, where the liquid boils and evaporates, and the separator in which the vapour leaves the liquid and passes off to the condenser or to other equipment. In many evaporators, all three sections are contained in a single vertical cylinder. Steam, vapour or exhaust gases from other drying operations, are usually used as the heating medium.
- Principal of evaporation: (BAT in the Food, Drink and Milk Industries, June 2005)
- The latent heat of condensation is transferred to the liquid food to raise its temperature to boiling point, to evaporate the water. The vapour is then removed from the surface of the boiling liquid.
- Temperature rates: (BAT in the Food, Drink and Milk Industries, June 2005)
- Since food products are heat sensitive, it’s necessary to work at low temperatures. This is achieved by boiling the liquid part under vacuum. Evaporation normally occurs in the range of 50-100oC, although it can be as high as 130oC in the sugar industry.
- Typical total solids concentration for various types of evaporators: (BAT in the Food, Drink and Milk Industries, June 2005)
- The level of total solids in the outlet depends on the composition of the product to be concentrated.
BAT for Food, Drink and Milk Industries, 2005
- Types of evaporation equipment:
- In its simplest form, evaporation is carried out by boiling off water to the air, using immersed electric heaters. However, in practice the most common used equipment is multistage shell and tube evaporators, or plate evaporators (BAT in the Food, Drink and Milk Industries, June 2005).
- Below there is a description of different types of evaporation equipment (Operations in Food Processing - the Web Edition, R. L. EARLE, 1983):
- Open pans:
- The most elementary form of evaporator consists of an open pan in which the liquid is boiled. Heat can be supplied through a steam jacket or through coils, and scrapers or paddles may be fitted to provide agitation. Such evaporators are simple and low in capital cost, but they are expensive in their running cost as heat economy is poor.
- Plate evaporators:
- The plate heat exchanger can be adapted for use as an evaporator. The space between the plates can be increased and appropriate passages provided so that the much larger volume of the vapours, when compared with the liquid, can be accommodated. Plate evaporators can provide good heat transfer and also ease of cleaning.
- Horizontal-tube Evaporators:
- The horizontal-tube evaporator is a development of the open pan, in which the pan is closed in, generally in a vertical cylinder. The heating tubes are arranged in a horizontal bundle immersed in the liquid at the bottom of the cylinder. Liquid circulation is rather poor in this type of evaporator.
- Vertical-tube Evaporators:
- By using vertical, rather than horizontal tubes, the natural circulation of the heated liquid can be made to give good heat transfer. The standard evaporator, shown in Figure 1, is an example of this type. Recirculation of the liquid is through a large “downcomer” so that the liquors rise through the vertical tubes about 5-8 cm diameter, boil in the space just above the upper tube plate and recirculate through the downcomers. The hydrostatic head reduces boiling on the lower tubes, which are covered by the circulating liquid. The length to diameter ratio of the tubes is of the order of 15:1. The basket evaporator shown in Figure 1(a) is a variant of the calandria evaporator in which the steam chest is contained in a basket suspended in the lower part of the evaporator, and recirculation occurs through the annular space round the basket.
Literature: Unit Operations in Food Processing - the Web Edition, R. L. EARLE, 1983, Published by NZIFST (Inc.), http://www.nzifst.org.nz/unitoperations
- (a) Long tube evaporators:
- Tall slender vertical tubes may be used for evaporators as shown in Figure 1(b). The tubes, which may have a length to diameter ratio of the order of 100:1, pass vertically upward inside the steam chest. The liquid may either pass down through the tubes, called a falling-film evaporator, or be carried up by the evaporating liquor in which case it is called a climbing-film evaporator. ::Evaporation occurs on the walls of the tubes. Because circulation rates are high and the surface films are thin, good conditions are obtained for the concentration of heat sensitive liquids due to high heat transfer rates and short heating times. Generally, the liquid is not recirculated, and if sufficient evaporation does not occur in one pass, the liquid is fed to another pass.
- In the climbing-film evaporator, as the liquid boils on the inside of the tube slugs of vapour form and this vapour carries up the remaining liquid which continues to boil. Tube diameters are of the order of 2.5 to 5 cm, contact times may be as low as 5-10 sec. Overall heat- transfer coefficients may be up to five times as great as from a heated surface immersed in a boiling liquid.
- In the falling-film evaporator, the tube diameters are rather greater, about 8 cm, and these are specifically suitable for viscous liquids.
LITERATURE: BAT in the Food, Drink and Milk Industries, June 2005
- (b) Forced circulation Evaporators:
- The heat transfer coefficients from condensing steam are high, so that the major resistance to heat flow in an evaporator is usually in the liquid film. Tubes are generally made of metals with a high thermal conductivity, though scale formation may occur on the tubes which reduce the tube conductance.
- Increasing the liquid-film coefficients:
- The liquid-film coefficients can be increased by improving the circulation of the liquid and by increasing its velocity of flow across the heating surfaces. Pumps, or impellers, can be fitted in the liquid circuit to help with this. Using pump circulation, the heat exchange surface can be divorced from the boiling and separating sections of the evaporator, as shown in Figure 1(c). :::Alternatively, impeller blades may be inserted into flow passages such as the downcomer of a calandria-type evaporator. Forced circulation is used particularly with viscous liquids: it may also be worth consideration for expensive heat exchange surfaces when these are required because of corrosion or hygiene requirements. In this case it pays to obtain the greatest possible heat flow through each square metre of heat exchange surface.
- Various scraped surface and agitated film evaporators :
- Under the heading of forced-circulation evaporators are various scraped surface and agitated film evaporators. In one type the material to be evaporated passes down over the interior walls of a heated cylinder and it is scraped by rotating scraper blades to maintain a thin film, high heat transfer and a short and controlled residence time exposed to heat.
- Evaporation for Heat-sensitive Liquids:
- In evaporators which have large volumes into which incoming feed is mixed, the retention time of a given food particle may be considerable. The average retention time can be obtained simply, by dividing the volume of the evaporator by the feed rate, but a substantial proportion of the liquor remains for much longer than this. Thus with heat sensitive materials a proportion may deteriorate and lead to general lowering of product quality. This difficulty is overcome in modern high flow-rate evaporators; in which there is a low hold up volume and in which little or no mixing occurs. Examples are long-tube evaporators with climbing or falling films, plate evaporators, centrifugal evaporators, and the various scraped-plate thin-film evaporators.
- Food products with volatile flavour constituents: (Operations in Food Processing - the Web Edition, R. L. EARLE, 1983)
- Many food products with volatile flavour constituents retain more of these if they are evaporated under conditions favouring short contact times with the hot surfaces.
- Low viscocity solutions:
- This can be achieved by climbing-film and falling-film evaporators, either tubular or plate types.
- Inceased viscocity solutions:
- As the viscosity increases, for example at higher concentrations, mechanical transport across heated surfaces is used to advantage. Methods include mechanically scraped surfaces, and the flow of the solutions over heated spinning surfaces. Under such conditions residence times can be fractions of a minute and when combined with a working vacuum as low as can reasonably be maintained, volatiles retention can be maximized.
- Highly viscous food products: (BAT in the Food, Drink and Milk Industries, June 2005)
- Centritherm evaporators, wiped film evaporators (WFE), thin film evaporators and vacuum pans are specially designed for the evaporation of highly viscous products.
4. COMPETITIVE TECHNOLOGIES AND ENERGY SAVING POTENTIALS
- a) Changes in the process
- Multiple-effect evaporators: (BAT in the Food, Drink and Milk Industries, June 2005)
- Multiple-effect evaporators are used when evaporation requires sufficient energy, e.g. in sugar beet processing, starch production and the evaporation of milk and whey. These evaporations use fresh steam or exhaust gases from other operations, and so recover or re-use energy, to boil off water vapour from the liquid in the first effect. The evaporated water still has sufficient energy to be the heat source for the next effect and so on. A vacuum is applied to a multiple-effect series to allow the water to boil off. The liquid being processed is passed from one evaporator body through the others so it’s subjected to multiple stages of evaporation. In this way, one unit of steam injected in the first evaporator can remove three to six units of water from the liquid.
- Vapour recompression: (BAT in the Food, Drink and Milk Industries, June 2005)
- Additional energy can be saved by recompressing the vapour using a thermal vapour recompressor (TVR) or a mechanical vapour recompressor (MVR).
- Mechanical vapour recompression (MVR):
- The evaporated vapour is compressed by a mechanical compressor and then re-used as a heat source. The latent heat is higher than the power input of the compressor and a large COP is available. With MVR all the vapour is compressed so a high degree of heat recovery is achieved. The system is driven by electricity but needs a steam heated “finisher” to attain higher temperatures. Two types of compressors are in operation, i.e. a fan and a high speed turbine. In practice, the fan is the most widely used compressor type as it has better energy efficiency. The principle of MVR is shown below:
Figure 3: MVR evaporator principle LITERATURE: BAT in the Food, Drink and Milk Industries, June 2005
- Thermal vapour recompression (TVR):
- TVR makes use of steam injection compressors to compress the vapour. Steam injection compressors may have fixed or variable injection nozzles. The thermal energy needed for compression is live steam from a boiler. The live steam passes through the injection nozzle and is throttled to the pressure level of the receiving vapour. Vapour is entrained as a result of the difference in speed. Vapour and live steam are mixed in the mixing chamber. Changing the flow aperture in diffuser determines the pressure at which the mixed steam leaves the steam injection compressor. TVR consumes more energy than MVR.
- However, both cases can lead to steam contamination, making it unsuitable for return to the boilers, and therefore, increasing the waste water load. In the final stage, the vapour may be condensed by cooling with cooling water. Some of the vapours can be drown off the evaporators to be used as heat sources for other process requirements. The condensate may be of such quality that it may even be fed to other processes as process water.
- Regular cleaning: (BAT in the Food, Drink and Milk Industries, June 2005)
- During processing, product compounds gradually deposit on the heat-exchange surfaces. This can affect the efficiency of the heat-exchange and lead to heat losses in the system. These deposits may be inorganic or organic, depending on the product. The installation must, therefore, be cleaned at regular intervals to prevent too high a heat loss occurring with a subsequent loss of product quality.
- Membrane processes:
- Reverse osmosis filtration (RO) membranes are permeable to water but not minerals and are therefore used for dewatering, concentration of whey or skimmed milk, polishing evaporator condensate and in water treatment, e.g. softening and salt removal (BAT in the Food, Drink and Milk Industries, June 2005).
- There have been several demonstration projects using membranes in the fruits and vegetables industry. At Golden Town Apple Products in Canada, a combination of ultra-filtration and reverse osmosis was used for apple juice concentration (CADDET 1996). In this process, the juice is heated to about 60°C and afterwards passed through the reverse osmosis membrane and the ultrafiltration membrane. The system has a maximum capacity of 3,000 l/hr for feedstock, 1,500 l/hr for final concentrate and 1,500 l/hr for water removed by reverse osmosis. It is most economical for small systems that need to remove no more than 2040 to 4080 kg of water an hour. The energy savings are estimated to be 66% compared to an evaporation process, while the volume of the equipment is reduced by 50% as are the transportation costs. The payback period of the combined system is about 2.5 years (CADDET 1996) (Northwest Food Processors Association:
- b) Changes in the energy distribution system
No information is available.
- c) Changes in the heat supply system
No information is available.
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