Evaporation is a process by which a liquid is brought to its boiling point by external heating thereby transforming the water into vapour, which escapes from the surface of the liquid. The rate of evaporation depends primarily:
The evaporation of water from milk requires special attention because of heat sensitivity. Therefore the evaporation has to be carried out under vacuum to:
The water contents of the most important liquid dairy products, their concentrates for spray drying and dried powders are given in Table 2.1. From these figures it is obvious that a substantial part of the water is removed by vacuum evaporation and only a fraction by spray drying and possibly fluid bed after-drying.
|Product||Water content %|
Beside these three main dairy products, others are also processed by evaporation and spray drying, like buttermilk, acid whey, permeates from ultrafiltration, mixtures of dairy liquid products with other components to produce special formulations e.g. milk formulas and icecream mixes. Therefore, whenever the expression 'milk' is used in the following text, it has to be considered as a general designation which may mean any of the other products mentioned.
There are two main reasons for using evaporation prior to spray drying:
Consequently, removing as much water as possible by evaporation improves the overall heat economy of the process.
Having these reasons in mind, the basic principles for the design of an evaporator are:
The main components of an evaporation plant are:
As the milk to be evaporated has a temperature of 5-10°C it has to be heated to the boiling temperature of the first effect in order to enable evaporation. The milk is therefore first passed through a vapour cooler/preheater, placed between the last effect’s separator and the condenser, thereby saving cooling water as well. From the vapour cooler the milk is passed through the preheating section of the last effect and then backwards to the first effect, before it enters either the pasteurization system or directly into the boiling section of the first effect. The preheating system can technically be carried out in different ways:
The spiral tubes are placed inside the heating room in the calandria surrounding the falling-film tubes, thus being heated by vapour. The system is simple, but does not offer the possibility of inspection for deposits or leakage. In modern evaporators they are not used any longer. See Fig.2.1.
The straight-tube preheaters are placed vertically outside the evaporator and like the spiral tubes heated by vapour from the corresponding calandria. The vapour connection is at the top of the calandria, so that uncondensable gasses can easily be extracted. See Fig. 2.2. This ensures an optimum utilization of the heating surface of the evaporation tubes. With this system inspection and manual cleaning are possible, if in rare cases it should prove necessary. The heat transfer surface in the preheater is arranged in groups of parallel tubes with small diameter resulting in a large surface. Each group of tubes is connected by normal dairy fittings at the end. Due to the parallel flow, the holding time is very short. The viscosity of the final concentrate is therefore lower in evaporators equipped with straight-tube preheaters.
The large surface of the preheaters and the temperature level prevailing during operation (5- 65°C) offers, however, optimal growth conditions for mesophilling and thermophilie bacteria. After 14-16 h of operation a bio-film is formed on the inner surface of the preheaters, where they can form spores. Unless special attention is paid, one cannot expect a 20 hour production without increase of mesophile and thermophile bacteria and their spores during the last 4-5 hours of a 20 hour production.
The table below indicates typical growth temperatures and inactivation temperatures/time of spore forming bacteria, their vegetative cells and spores.
|Spore forming Bacteria||Growth temperatures (°C)||Usual inactivation in milk by heat|
|B.Stearothermophilus||30-45||55-60||60-70||12 s 85°C||8-15 m 121°C|
|B. Cereus||5-20||30-37||45-48||10 s 72°C||0.5 m 121°C|
|B. Coagulans||15-25||35-50||55-60||20 s 72°C||3-5 m 121°C|
|B. Licheniformis||15||30-45||50-55||20 s 72°C||3-5 m 121°C|
|B. Subtilis||6-20||30-40||45-55||20 s 72°C||3-5 m 121°C|
|C. Botulinum||3||25-40||48||20 s 72°C||3-4 m 121°C|
|C. Perfringens||8-20||45||50||20 s 72°C||1-4 m 121°C|
|C. Tyrobutiricum||20 s 72°C||1-4 m 121°C|
Spore forming bacteria are bacteria which under adverse growth conditions, such as too high or too low temperature or lack of nutrition, transform themselves into a dormant state - they sporulate and become extremely heat resistant. When growth conditions become favourable again, they re-vegetate and develop.
It has been found that the development of spore forming bacteria in evaporators takes place in the preheaters, as that is the only place where bio-films are formed.
To ensure production of powder during a 20 hour operation without problems the following type of preheaters can be used:
By using a direct contact regenerative preheater of similar design as the direct contact regenerative flash chambers (see section 2.2.2.), the heating from 5°C to 40°C and from 40°C to 70°C can be done in fractions of a second without heat surfaces where biofilms can be formed. The milk is pumped to the inlet of the direct contact preheater(s), where vapour from one of the calandrias is introduced by means of live steam through a small thermo-compressor. See Fig. 2.3.
By applying this technology it is possible to operate the plant for 20 hours or more without growth of mesophile and/or thermophile bacteria and their spores at reduced steam consumption.
By installing duplex preheaters, see fig 2.4., it is possible to have a continuous run of 20 hours, as the preheaters are cleaned before the critical level has been reached. Additional costs for cleaning and effluent treatment must be taken into account. Further, the investment is higher, but the actual direct production costs and time are not affected.
As mentioned, the spore forming bacteria only develop in biofilms in the preheaters. Therefore, an obvious solution would be to by-pass the preheaters, where temperatures are between 5 and 70°C. This will, however, result in increased overall steam consumption, as direct steam injection is necessary to bring up the temperature from 5°C to the pasteurization temperature, and further the water from dilution of the condensing steam has to be evaporated again.
If for some reason, one does not want to use the above described method, but still wants to operate the plant for 20 hours without problems with spore forming bacteria, the following measures can be implemented:
If the evaporator is cleaned after 10 hours, the problem is solved, but approx. 10% of effective production time is lost, and further there are expenses for cleaning agents and waste disposal.
By heating the milk to 140°C in 4 sec. after the preheaters, the problem is solved, however, the dead cells are still traceable, and it will not be possible to make powders with “tailor-made” functional properties.
Further, there will be additional steam consumption, and the maximal running time is depending on the milk quality.
The indirect heaters are working as ordinary heat exchangers, either the plate, straight-tube or spiral-tube type. If temperatures up to 110°C are wanted, it is recommended to have two heaters, where one is in operation while the other one is being cleaned.
The advantage of the indirect heating is that the product will not be mixed with the condensating steam and neither will the product be diluted. The disadvantage is that it takes a long time for the product to be heated in the interval from 80°C to 110°C resulting in a concentrate with high viscosity. This is because the whey proteins, when unfolded, will react with each other and the k-casein. For improved efficiencies one or more regeneration systems can be incorporated.
The direct pasteurization is done in two different ways, either by direct steam injection, where the live steam is mixed into the milk using a Tangential Swirl Heater (TSH), see photo. It offers a controlled and short residence time with no mechanical impact, even at temperatures >120°C. It can operate 20 hours or more without intermediate cleaning. Alternatively, milk is sprayed into a steam atmosphere (infusion) at a sufficient pressure. The steam must be of good quality, i.e. for use in products for human consumption. Culinary steam boilers, where milk condensate is heated up in an indirect coil-type heater by means of live steam, can be used. The advantage of direct pasteurization is the short time it takes to reach the desired temperature.
The direct heating will further have a less pronounced effect on the denaturation of the whey proteins at the same pasteurization temperature/time.
|Whey protein denaturation||Thiamin loss|
|Direct system 35%||0.5 - 0.8%|
|Indirect system 65%||1.4 - 4.4%|
As for the indirect preheating, regenerative flash chambers are used, if high pasteurizing temperatures are needed. The temperature of the milk will drop due to the evaporation, and the vapours are used for preheating prior to the pasteurizer. The regenerative flash chamber can be either indirect as shown in Fig. 2.8., or direct contact as shown in Fig. 2.9. The direct contact regenerative system is preferable for its short residence time and there is no heat contact surface, where deposits can develop.
The pasteurization temperature will of course have a direct influence on the total steam consumption, which will increase by increasing the temperature. For the same pasteurization temperature the direct pasteurization will result in higher steam consumption compared with indirect pasteurizing due to the need of evaporation of the extra water formed by the condensation. However, the additional steam used is - after flashing off - used as heating medium in the subsequent calandrias and some of the applied energy is reused.
The holding is practically always done in horizontally placed holding tubes, with specific length and diameter to give the desired holding time. There are usually several tubes of the same length but with various diameters, the combination of which enables the holding time to be varied. For instance four tubes corresponding to holding times 0.5, 1, 2 and 4 minutes allow the holding time to be varied from 0.5 to 7.5 minutes in half minute intervals.
It is very important that the product to be evaporated is distributed evenly into all the tubes in the calandria to get a good coverage. The distribution system is therefore given special attention when designing an evaporator. In principle there are two different systems:
In the dynamic distribution system, the necessary kinetic energy for distribution is obtained by a pressure drop of the product over a full-cone nozzle. As the product is superheated in relation to the pressure inside the tubes, flash vapour will instantaneously be formed. The mixture of product and vapour is sprayed into the inlet of the tubes thus being covered by product. This system is very inflexible as to capacity variations and not used in dairy evaporators designed for various milk products with different solids content.
In the static distribution system the incoming superheated product is first separated in flash vapour and product. The product enters a distributor plate placed inside an open cone, as the product enters the calandria. The cone is placed above a distributor bowl with a number of holes. Here a certain level of product is maintained. The product flows through the holes in the plate by gravity. Each hole is placed just above the area between the tubes. Thus the product flows onto the tube plate and then over the edge down along the surface of each tube. The flash vapour also enters the tubes and pushes the product against the inner surface of the tubes giving it its initial velocity. See Fig. 2.10.
This distribution system is much more flexible with respect to capacity, as an increase in the level in the distributor bowl - as a result of increased capacity - will make the product flow through the holes at a higher velocity, thus maintaining the level. During CIP of the evaporator and especially the pasteurizing equipment, some jelly lumps of milk protein deposits may cause blocking of the holes in the distributor plate. To avoid this, a self-cleaning hydro cyclone may be installed in the product line between the discharge from the flash vessel of the regenerative pasteurizer and the inlet to the first calandria. See Fig. 2.11.
The liquid to be evaporated is evenly distributed on the inner surface of the tubes. The liquid will flow downwards forming a thin film, from which the boiling/evaporation will take place because of the heat applied by the steam. The steam will condense and flow downwards on the outer surface of the tube. A number of tubes are built together side by side. At each end the tubes are fixed to tube plates, and finally the tube bundle is enclosed by a jacket, see Fig. 2.12. The steam is introduced through the jacket. The space between the tubes is thus forming the heating section. The inner side of the tubes is called the boiling section. Together they form the so-called calandria. The concentrated liquid and the vapour leave the calandria at the bottom part, from where the main proportion of the concentrated liquid is discharged. The remaining part enters the subsequent separator tangentially together with the vapour. The separated concentrate is discharged (usually by means of the same pump as for the major part of the concentrate from the calandria), and the vapour leaves the separator from the top. The heating steam, which condenses on the outer surface of the tubes, is collected as condensate at the bottom part of the heating section, from where it is discharged by means of a pump.
In order to understand the heat and mass transfer, the basis for the evaporation, it is necessary to define various specific quantities.
Feed (A) means a liquid product supplied to the evaporator to be evaporated (B) and concentrate (C) is the resulting product. And thus:
The evaporation ratio (e) is a measure for the evaporation intensity and can be defined either as the ratio between the amount of feed and concentrate or the ratio between the total solids (TS) percentage in the concentrate and in the feed.
If the concentrations or the evaporation ratio are known the quantities A, B or C can be calculated, if one of them is known.
|Given quantity||To be found||Formula|
|Quantity to be treated A||B||B = A * (e - 1) / e|
|C||C = A / e|
|Evaporated quantity B||A||A = B * e / (e - 1)|
|C||C = B * 1 / (e - 1)|
|Concentrate quantity C||A||A = C * e|
|B||B = C * (e - 1)|
A: feed in kg/h
B: evaporation in kg/h
C: concentrate in kg/h
e: evaporation ratio
Since milk, due to the protein content, is a heat-sensitive product, evaporation (i.e. boiling) at 100°C will result in denaturation of these proteins to such an extent that the final product is considered unfit for consumption. The boiling section is therefore operated under vacuum, which means that the boiling/evaporation takes place at a lower temperature than that corresponding to the normal atmospheric pressure. The vacuum is created by a vacuum pump prior to start-up of the evaporator and is maintained by condensing the vapour by means of cooling water. A vacuum pump or similar is used to evacuate incondensable gases from the milk.
At 100°C the evaporation enthalpy of water is 539 Kcal/kg and at 60°C it is 564 Kcal/kg. As the milk has to be heated from e.g. 6°C to the boiling point, and as energy, approx. 20 Kcal/ kg, is required for maintaining a vacuum corresponding to a boiling point of 60°C, we get the following energy consumption figures, provided we estimate the heat loss to be 2%. Corresponding to about 1.1 kg of steam/kg of evaporated water.
|Net energy consumption||Kcal/kg||633||638|
|Heat loss, approx.||Kcal/kg||15||15|
|Total energy consumption||Kcal/kg||648||653|
The role of the separator is to separate vapour from the evaporated liquid. Milk evaporators are working exclusively with centrifugal type separators.
As the vapours generated from the evaporation are used as heating media in the “next” calandria, any product must be separated, since it would otherwise contaminate the condensate and further represent a loss.
The majority of the concentrate is discharged from the bottom of the calandria below the tube bundle. Due to the high vapour velocity some of the concentrate will be carried along with the vapour as small droplets. The separation is done in a separator with tangential vapour inlet; see Fig. 2.13., connected to the calandria below the tubes.
Special care is taken to design the separator to avoid product carry-over at lowest possible pressure drop, as a drop in the pressure is equal to drop in heating enthalpy in the following calandria with an all-over drop in the efficiency as a result.
One way of saving energy is by using a thermo-compressor which will increase the temperature/pressure level of the vapour, i.e. compress the vapour from a lower pressure to a higher pressure by using steam of a higher pressure than that of the vapour. Thermo-compressors operate at very high steam flow velocities and have no moving parts. The construction is simple, the dimensions small, and the costs low. See Fig. 2.15.
In the live steam nozzle (1) the pressure of the inflowing steam is converted into velocity. A jet is therefore created which draws in part of the vapour from the separator connected to the calandria. In the diffuser (2) a fast flowing mixture of live steam and vapours is formed, the speed of which is converted into pressure (temperature increase) by deceleration. This mixture can now be used as heating steam for the evaporator. In Fig. 2.16 a flow sheet of a three-effect evaporator with thermo-compressor is shown.
The best efficiency in the thermo-compressor, i.e. the best suction rate, and thereby a good economy, is obtained when the temperature difference (pressure difference) between the boiling section and the heating section is low.
Thermo-compressors must be adapted to the operating conditions. But these conditions can vary, be it, for example, that the heat resistance of the heating surfaces increases during operation due to deposits on the heating tubes. The suction rate will then decrease considerably. In evaporators that have to serve various capacities a number of thermo compressors with different characteristics are used. Further, a thermo-compressor, which has been designed for a higher live steam pressure, can draw a larger amount of vapour from the separator than one built for a lower pressure. For simplification we will in the following use an efficiency of 1:2, but new designed thermo-compressors will under certain conditions operate with an efficiency of 1:3.
By adding a thermo-compressor we have then in a three-effect evaporator by means of 1 kg live steam evaporated 5 kg of water, i.e. the saving of steam is as great as that obtained by addition of two effects in multi-effect evaporation. Dividing a given total Δt between the first and last effect in multi-effect evaporators requires an enormous heating surface and consequently an expensive installation.
As an alternative to the thermo-compressor, the mechanical vapour compressor has during the last fifteen years found extensive use in evaporators in the dairy industry. The applied energy for the compressor is usually electricity, but also diesel and gas motors are used. Other processes may require steam at low pressure, and the compressor may be driven by a steam turbine acting as a reducing valve. All determined by local price policy for energy. As a rule of thumb, however, an MVR solution is profitable, if the price/kW < price/kg steam x 3. However, the decision as to which type of compressor to use, is nowadays influenced by the end product quality - the milk powder - and in the MVR evaporator there is a very short residence time, resulting in low viscosity of the concentrate.
The mechanical vapour compressor is a fast revolving high pressure fan (~3000 rpm) capable of operating under vacuum. At low boiling temperatures the volume of the vapours is enormous. Consequently, there is a limit as to the lowest temperature levels used in practice. As the energy applied to the compressor is utilized most efficiently by low compression ratios, the obtained temperature/pressure increase is limited. Therefore, a large heat transfer surface is required tending to increase the capital costs of the equipment.
As it is essential to operate an MVR unit at a low overall temperature difference between the vapour evolved from the product and the heating medium as a result of the compression, it is a must that the boiling point elevation of the product is kept at a minimum, as this would otherwise even further minimize the temperature difference available for the evaporation. This, too, limits the maximum concentrations aimed at in evaporators of this kind. Fig 2.17., illustrates a one-effect MVR evaporator. The incoming cold milk is first preheated by concentrate then by condensate from the heating section of the calandria followed by a final pasteurization by means of live steam. The vapour is compressed in the MVR unit and used as heating medium, as it releases the latent heat by condensation.
A vacuum pump, together with a small amount of cooling water, maintains the desired vacuum in the system.
As it can be seen no energy leaves the plant in form of warm condensate, and only a minor part via the cooling water (depending upon the pasteurization temperature desired). The MVR evaporator is in this context very often used as pre-condenser of milk products for transport purposes, where the required solids content is in the range of 30-35% and thus the boiling point elevation is limited. With the concentrate leaving the plant at low temperature, this kind of installation is a strong competitor to hyperfiltration.
The vapour is by the MVR fan sucked from the separator and the compressed vapour is desuperheated by spraying water into the outlet of the compressor. The compressed vapour is condensed on the heat exchanger surface in the subsequent calandria, where it is discharged as condensate. Simultaneously, water is evaporated from the milk and separated as vapour in the separator.
The MVR evaporator offers much better capacity flexibility / turn-down capability, as only the RPM on the fan needs to be adjusted.
Usually, the MVR evaporator is combined with a TVR unit, if solids contents suited for a spray drying plant are aimed at, see Fig. 2.18. The steam consumption per kg evaporated water is of course less than in a multi-effect evaporator, but if the MVR unit is driven by an electric motor, the electrical energy consumption will be bigger. As only very little cooling water is required, this combination offers a very attractive solution, however, a higher investment should be anticipated. Under special energy price conditions it is advantageous to replace the TVR unit with an additional MVR unit to compress the vapour over the last effect, see Fig. 2.19. It is therefore recommendable that each case be studied carefully taking local conditions such as steam, electricity and cooling water prices into consideration.
|5-effect TVR||7-effect TVR||1-effect MVR /
|PRODUCT||Skim milk||Skim milk||Skim milk|
|Solids in/out, %||9/50||9/50||9/50|
|Pasteurization temp., °C||90||90||90|
|Holding time, sec.||30||30||30|
|Steam consumption, kg/h||1,610||1,190||375|
|Steam pressure, bar||10||10||10|
|Condensate temp., °C||54||51||22|
- MVR, kW
- Motors, kW
|Cooling water cons., m3/h||32||3.5||2 *|
|Cooling water temp in/out, °C||28/35||28/35||12/50|
|Power cons. cool. tower, kW||10||2.5||-|
|Residence time, min.||12||18||6|
In multi-effect evaporators - be it a TVR or MVR plant or combinations hereof - any “subsequent” calandria – operated at a lower boiling temperature - is used as condenser for the ”warmer” vapour coming from the separator of the previous calandria. Water is used as cooling medium in a condenser to condense the vapour from the last calandrias separator either indirectly (shell and tube surface condenser) or directly (spray mixing condenser). Surface condensers are more expensive and need 10-15% more water. The type of condenser has no effect on the performance of the evaporator. In plants processing products containing volatile acids, surface condensers are preferred to avoid contamination of cooling water by acid.
In a mixing condenser numerous nozzles and plates are installed in order to get a good mixing of the vapour and the cooling water, see Fig. 2.20. The water and condensed vapour are removed at the bottom. As there will be the same vacuum in the mixing condenser as in the last effect, the pump to remove the water and condensate should be capable of discharging from this vacuum.
Another solution is to place the mixing condenser barometrically high, i.e. about 11 meter above the pump. The water will run into a well from where it is pumped away, either to a cooling tower or to a natural water reservoir.
The advantage of the mixing condenser is low investment costs and lower cooling water consumption. The disadvantage is that condensate is mixed with the cooling water which may have the effect that the cooling tower is contaminated. Since there is an open connection between the product in the last effect and the, possibly contaminated, cooling water they represent a bacteriological hazard and thus should be avoided.
The surface condenser is working and built according to the same principle as an ordinary straight tube heat exchanger. The advantage of a surface condenser is that cooling water and vapour condensate remain separate. As only the vapour condensate has to be pumped out of the vacuum, it has never been considered to place it barometrically as is the case for the mixing condenser. Surface condensers should always be used in plants where acid products such as acid whey are evaporated in order to separate acid vapour condensate from the cooling water.
The vacuum in the last effect of the evaporator is a function of the power of the vacuum equipment and the amount of cooling water and the temperature to maintain the vacuum once created. The vacuum in the first and intermediate effects is created by the subsequent calandria acting as a condenser for the vapours from the previous effect. Any change in the evaporation rate in one effect, due to fouling for example (decrease of K factor), therefore means that less vapour is condensed. This results in increased boiling temperature in the previous effect, the Δt decreases and so does the overall evaporation capacity. Each effect is connected to the condenser to ensure the de-aeration of incondensable air and gas.
Saturated steam which is used as heating steam contains also considerable amount of air and other non-condensable gases. So does the product to be concentrated. It amounts usually to about 0.5% and increases especially in multi-effect evaporators up to 1%. The noncondensable gases reduce the heat transfer coefficient considerably. Therefore it is important to provide effective degassing of the calandrias. The heating steam may contain some milk solids creating deposits on the steam side of the tubes, due to incomplete separation of entrained droplets from vapour in the separator.
This also reduces the heat transfer. To create and maintain (due to the incondensable gases and leaks) the vacuum in the evaporator, two types of pumps are used:
Vacuum pumps such as the water-ring pump are used. Normally two units are installed; they are both used for quick start-up of the plant, while only one is used during production to maintain the vacuum. Only stainless steel material should be considered, as bronze - even it is cheaper - has a very short lifetime, especially if the plant has to process whey, due to corrosion.
The steam jet vacuum unit is in principle designed like the thermo-compressor discussed earlier. This system has a low maintenance cost, but the additional steam requirement should be taken into consideration.
Very often the required concentrate temperature is lower than the one obtained from the last effect. The concentrate can naturally be passed over a cooling surface, such as a plate heat exchanger, but as the viscosity is high at this stage, it is not recommended. Instead, flash coolers are used. The system is simple and consists only of a vacuum chamber (vacuum created by steam jet vacuum units) into which the concentrate is sprayed. See Fig. 2.21. Depending upon the vacuum the concentrate will flash and due to the evaporation a cooling will take place simultaneously resulting in a slight increase of the solids content.
The flash cooler is mainly used for whey, where it is especially advantageous, as the cooling takes place instantaneously, thereby avoiding problems with crystallization of the lactose, which would create blockages between the plates.
All falling-film evaporators have transport pumps for passing the milk concentrate from one effect to another. The amount of pumps depends on the number of effects, and whether the effects are split or not. As the pumps work under vacuum efficient sealing is necessary to avoid any air leaking making it difficult to maintain the vacuum. This sealing is done with water. Each pump requires about 100- 200 l/h of sealing water of which normally ½-1 l/h enters the milk flow. The sealing water system may be designed, so that each pump is furnished with a small funnel to see if there is any excessive waste of sealing water and - which is more important - if a pump is suddenly using more water than normal, which means that the sealing ring is wearing out.
Many factories are placed near lakes, rivers or other natural water reservoirs, and the amount and temperature of cooling water are therefore no problem, provided the increased temperature in the return water is not causing any environmental problems.
However, not all factories have got access to unlimited water supply, and the situation where the cooling water requirement cannot be covered may arise. The problem could be solved by recycling the water, but hot water is not a good cooling medium, so the vacuum in the evaporator would soon disappear. By installing a cooling tower, see photo, this problem is overcome. In the cooling tower the water is cooled (how much depends on local conditions for ambient temperature and wet-bulb temperature) by evaporation, as the water is distributed over a big surface, and a fan ensures the necessary air turbulence. The flow of water goes from the cooling tower to the condenser from where it is pumped back to the cooling tower.
Due to the evaporation of water in the cooling tower water has naturally to be added, but the amount is low. When a mixing condenser is used the extra water requirement is practically nil, as the condensed vapours are mixed with the water. It is recommended at certain intervals to renew the recycled water completely to avoid excessive bacteria and algae growth.
Saturated steam is used exclusively as the heating medium for evaporation of milk. The essential aspect to consider when designing a milk evaporator is to estimate the heating surface. Generally it must be large enough to secure the required heat transfer but not excessively large to keep still good coverage of the over-all tube surface by the evaporating liquid. It is calculated by following equation:
The amount of heat required for evaporation Q is calculated from the required duty, i.e. the amount of water to be evaporated W (kg/h) and the specific heat of evaporation I under given conditions (vacuum and temperature):
The heat transfer coefficient is the most critical factor. The numerical values of K are influenced by a number of external factors as well as the properties of the evaporating liquid at any stage of the process (i.e. density, viscosity, surface tension, temperature, boiling point elevation, heat conductivity, specific heat) properties of the heating steam, tube wall material, surface treatment and cleanliness, velocity of the film flow, thickness of the film etc.
The numerical value of the heat transfer coefficient varies between 3000 and 100 W/m²/°C for low viscous liquids and clean surfaces to high viscous liquids and dirty surfaces respectively. Therefore, under continuous operating conditions in a multi-effect evaporator the heat transfer coefficient decreases from stage to stage due to rising viscosity and formation of deposits (mainly calcium phosphates and precipitated proteins) on the heating surfaces. The heat transfer coefficient for skim milk is about 2500 W/m²/°C in the first effect and drops down to below 1000 in the last effect. For whole milk values are about 15% lower.
Burnt deposits in the tubes occur especially if the tube surfaces are not completely covered due to a low average flow of liquid per tube or to poor distribution.
The increased demand for big multi-effect evaporators requiring bigger heating surface in order to obtain better specific consumption figures, can be met by using more tubes. This would, however, mean that less liquid is getting into each of the tubes, and the produced film becomes too thin. At high solids contents the viscosity will increase, the film will not flow any more, and there will be a risk of burnt deposits. This will result in a concentrate with small jelly lumps, very often discoloured and found in the powder as “scorched particles”, as they will not dissolve when the powder is reconstituted. In extreme cases the tubes block completely and manual cleaning is necessary.
The designer therefore operates with the so-called coverage coefficient defined as:
The coverage problem was some years ago overcome by recirculating part of the feed from the outlet of the calandria to the inlet of same, thus increasing the amount of liquid sufficient to cover all the tubes.
From a technical point of view this is the ideal solution, as it is cheap and simple, but from a product point of view it should not be tolerated, as it means that part of the product is exposed to the high temperature for a long uncontrollable time. This means that the final concentrate will get increased viscosity and possibly protein denaturation, both resulting in a powder with an inferior solubility.
In modern falling-film evaporators, the so-called “singlepass” evaporators, the problem is solved by dividing the effects with low coverage coefficient in two or more separate calandrias with the same boiling temperature and often one combined separator. See Fig. 2.22.
Another method is to split the calandria by dividing it into two or more sections in a “multi-flow” evaporator. The product is pumped to one section, from the outlet of which it is pumped direct to the next section, and so forth. Having passed through the last section it is pumped to the next effect. This system is almost as cheap as the recirculation, but has the advantage of the divided calandria and no circulation is necessary.
The trend today is to manufacture the calandria with longer tubes in order to obtain more heating surface per tube and to combine it with calandrias with two or more “splits” maintaining the coverage coefficient at a safe level. About forty years ago the evaporators were equipped with 3-4 m tubes and operated with a temperature difference of about 15°C, whereas evaporators 15 years ago had tubes with a length of up to 14 m and a temperature difference down to 2°C. Today most new evaporators have tube lengths up to 16 m. The advantage is that less product passes are needed to obtain sufficient coverage, fewer pumps, and reduced residence time. The disadvantage is that there will be an increased pressure drop of the vapour over the longer tubes, and that is on the account of overall evaporation capacity. Tube length up to 18 m has been tried, but the speed of the vapour becomes so high when it leaves the boiling tubes that the concentrate gets “atomized” and ends up in the heating side of the subsequent calandria and is then discharged as condensate with high BOD level.
When designing an evaporator/spray dryer the main product is therefore always selected, and the evaporator calandrias are designed, so that optimal coverage coefficients are ensured, also for the other products.
As mentioned above, the vapour generated from the evaporation, contains all the applied energy (less heat loss). The applied energy can thus be reused if the vapour condenses in a subsequent calandria operated at a lower product boiling pressure. The energy applied to the system can therefore be reduced to 50% if a second calandria is installed and 33% if a third calandria is used and so forth. But the vapour needs to be separated from the evaporated product before reused.
A very important factor for evaporator design is the selection of the boiling temperature throughout the whole evaporator profile. The principle of multi-effect evaporation requires a temperature cascade of steam temperatures and boiling temperatures from stage to stage. Most common is the so-called feed-forward system, in which each subsequent evaporator stage has lower values of both heating and boiling temperature, than the previous stage. Milk is a heat sensitive liquid and thus the maximum permissible boiling temperature in the first effect has an upper limit. Usually this is 66 to 68°C. It is somewhat higher for whole milk than for skim milk. Due to increasing concentration during evaporation the viscosity of the concentrate rises as well. This increase is further supported by the temperature drop and therefore there is also a limit for minimum permissible temperature. Therefore the available working temperature range is about 25-30°C which means that the temperature drop between the individual stages, which depends on number of stages, is in practice 10 to 3°C. The evaporation capacity of an evaporator is:
Thus the capacity of an evaporator can be increased by more surface or higher boiling temperature in the first effect. It is not recommended to use higher temperature than 66- 68°C, as discussed above. The thermo-compressor is incorporated between the separator and the shell of the first effect (mono-thermal compression), the separator of the second and the shell of the first (bi-thermal compression), or between the separator of the third and the shell of the first effect (tri-thermal compression). The influence on the steam economy and the investment costs is significant. However, one major drawback in multi-effect evaporators is the long residence time, where the product is exposed to heat. Although it is at low temperature, it will have a negative effect on the viscosity of the concentrate.
The temperature obtained from the last preheater is in multi-effect evaporators lower than the boiling temperature in the first effect. Additional preheating is therefore necessary to obtain the minimum required 2-3°C above the boiling temperature of the first effect. A separate preheater heated by live steam, usually via a thermo-compressor, is then used. However, some products may require higher temperatures, but the primary purpose of the heat treatment in an evaporator, apart from bacteriological requirements, is not ”pasteurization”, but obtaining a tool to get functional properties in the final powder. The reasons for the heat treatment are:
A pasteurization directly before the evaporation will naturally influence the bacteria count in the final powder, and the higher the temperature and the longer the holding the more efficient the killing.
The heat treatment applied should under any circumstances meet or exceed legal requirements.
Skim milk powder is often produced according to a fixed degree of denaturation of the whey proteins and is classified according to the whey protein nitrogen index (mg WPNI/g powder) which expresses the content of undenaturated whey proteins. Different temperature and time combinations have an influence on the index as shown in Fig. 2.23., as well as % denaturation of -lactoglobulin in milk in Fig. 2.24.
This type of powder is used for reconstitution for making evaporated, sterilized milk, especially in the Far East. After reconstitution to 25-27% TS the product has to be sterilized using temperatures of 120°C or higher during 20 min. The heat stability of the recombined product is controlled by the pasteurization temperature/time combination prior to the evaporation and drying. A direct contact heating system gives a better result.
|Pasteurization||Temperature Interval °C|
|Indirect °C||From 60 to 80|
|Direct °C||From 80 to 110 *|
|Direct °C||From 110 to 125|
|Holding time in min.||2-4|
When producing whole milk powder one problem is the shelf-life, as the fat easily becomes oxidized, if the powder is not packed using an inert gas. As a lot of powder is shipped in bags, it is not possible to protect the powder effectively, and antioxidants are in most cases not permitted.
By pasteurizing (direct) the milk prior to the evaporation to 90-95°C and keeping the temperature for ½-1 min., some natural antioxidants will be formed, as -SH groups, originating from the amino acids cystine, cysteine and methionine. They are liberated and will act as antioxidants. Higher pasteurization temperatures will form more -SH groups, but they will react with casein and not be found in free form. See Fig. 2.25. The free -SH groups will at the same time give the milk a cooked flavour, which, however, is liked by many consumers.
To produce instant whole milk powder with good reconstitution properties in cold water and at the same time with a good “coffee stability” - that is no coagulation should take place when the powder is added to hot coffee as a “whitener”. It is recommended to use a temperature/ time combination to achieve a WPNI of > 3.5 mg/g, which corresponds to approx. 45% denaturation of -lactoglobulin, see further Fig. 2.24.
For further and a more elaborate reading please see chapter 10. Achieving product properties.
The pasteurization can be carried out in different ways, either:
The concentrate leaving the last effect of the evaporator is liquid. The concentrate may however have different viscosity depending upon the composition, heat sensitivity of the proteins, pre-treatment, temperature and solids content.
Whole milk concentrates are generally less viscous than skim milk concentrates, and as a general rule the viscosity should not exceed 60 and 100 cP, respectively, if the atomization should be optimal. Higher viscosities can of course be handled in the dryer, but not without losing capacity (bad atomization - big droplets) and an inferior product will be the result.
The composition will influence the viscosity, especially on the protein (P) content in relation to the lactose (L) content. When the ratio P:L is high the concentrate will get a high viscosity. This is especially a problem with jersey cows during the whole year, but other breeds tend to give problems during the beginning and/or the end of the lactation period. The ratio P:L can be adjusted by adding lactose. As a general rule it can be concluded that a higher fat and lactose content will give lower viscosity. Higher protein content will give higher viscosity.
When milk is exposed to a high heat treatment, especially in indirect pasteurizing systems, prior to the evaporation, the viscosity of the concentrate will be higher.
The concentrate temperature will naturally have a direct influence on the viscosity and higher temperature means lower viscosity.
The solids content of the concentrate will have a very significant influence on the viscosity, and the higher the concentration the higher the viscosity.
However, the above only states the direct influence of some parameters on the viscosity. One of the main influences on the viscosity is the time, i.e. the viscosity is a function of time, also known as age-thickening. This means that the viscosity will increase if the concentrate is left for some time. The increase is depending on composition, mainly proteins binding to each other, temperature and concentration. The age-thickening is only partly reversible by agitation.
A temperature increase will naturally result in a viscosity drop; but as the age-thickening is more pronounced at higher temperatures, the viscosity will soon increase to the same level and further on as the time passes. See Fig. 2.26.
The age-thickening will also be influenced by the solids content and will be more pronounced the higher the solids content in the concentrate. See Fig. 2.27. The composition will have same influence on the age-thickening as on the viscosity. If the concentrate should be kept for some length of time, or transported over long distances before further processing, the concentration and temperature should be low. The low temperature will at the same time limit bacterial growth.
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Handbook of Milk Powder Manufacture
The Handbook of Milk Powder Manufacture is a valuable reference book for dairy processing engineers wanting to take a deeper look at the complex world of milk powder processing.