Field of the invention

The invention relates to a method for thermal concentration of fluid in a liquid state by using mechanical vapour recompression as defined in the preamble of the independent claim 1.

The invention relates also to a plant for thermal concentration of fluid in a liquid state by using a mechanical vapour recompression as defined in the preamble of the independent claim 6.

Background Art MVR evaporators use mechanical vapour recompression to recompress all the vapour from the vapour separator to be reused as heating steam either in the same stage or in previous stages.

In the 1980's high-pressure fan was developed and introduced for single-stage evaporators wherein its pressure difference was barely adequate. Fan evaporator plants are sensitive for air introduction with a product or a fluid or through leaks and for contamination of heat transfer surfaces because of their relatively low pressure increase. That is why extra attention has been brought into air removal of MVR evaporator's feedstock and the removal of inert gases. That has been done by using pre-heaters that can be coupled to inert removals and thereby diminish steam load going to a condensing apparatus.

A plant for concentrating tomato juice is known from WO 2005/027664 which discloses a concentration process for tomato juice wherein the juice is circulated in different sectors in order to concentrate the juice.

Objective of the invention The object of the invention is to improve the process for thermal concentration of a fluid so that the capacity of the MVR evaporator is more effectively controlled, the steam usage is optimized and the heat transfer surfaces are used optimally.

Short description of the invention

The method for thermal concentration of a fluid in a liquid state by using mechanical vapour recompression, comprising an evaporator for evaporating said fluid to be concentrated so that said fluid loses liquid in the form of vapour and fluid concentrate is produced, a pre-heater for pre-heating said fluid to be concentrated before said fluid is fed into the evaporator, a separation chamber in fluid communication with the evaporator for separating fluid concentrate from vaporized components of said fluid, a compressor in fluid communication with the separation chamber and in fluid communication with the evaporator for compressing said vaporized components of said fluid before being introduced back from the separation chamber to the evaporator, one or several outlets for discharging from the evaporator non-condensable gases of said fluid that are developed in the evaporator and an ejector or a compression device for compressing non-condensable gases of said fluid into higher pressure, wherein said non-condensable gases of said fluid exiting the ejector or compression device are fed through the pre-heater for using thermal energy of the compressed gases for pre-heating of fluid to be concentrated, is characterized by the definitions of the independent claim 1.

Preferred embodiments of the method are defined in the dependent claims 2 to 5.

The plant for thermal concentration of fluid in a liquid state by using a mechanical vapour recompression, comprising an evaporator for evaporating fluid to be concentrated so that said fluid loses liquid in the form of vapour and for producing fluid concentrate, a pre-heater for pre-heating said fluid to be concentrated before said fluid to be concentrated is fed into the evaporator, a separation chamber in fluid communication with the evaporator to separate fluid concentrate from vaporized components of said fluid, a compressor in fluid communication with the separation chamber and in fluid communication with the evaporator for compressing said vaporized components of said fluid before being introduced back from the separation chamber to the evaporator, one or several outlets for discharging non-condensable gases of said fluid that are developed in the evaporator from the evaporator, feeding means for feeding said non-condensable gases from the evaporator to the pre-heater for using thermal energy of the compressed gases for pre-heating fluid to be concentrated, and an ejector or a compression device arranged in said feeding means for compressing non-condensable gases of said fluid into higher pressure, is characterized by the definitions of the independent claim 6.

Preferred embodiment of the plant is defined in the dependent claim 7. A plant for thermal concentration of a fluid consists usually of different equipments in order to run the process. One of the equipments and the key equipment is an evaporator where the actual concentration of a fluid takes place. An evaporator is a heat exchanger wherein the fluid is heated and altered to a gaseous state. The evaporator is provided with a vertical tube bundle wherein a falling film of the fluid occupies the internal surfaces of the tubes and the heating fluid strikes the external surfaces of the tubes. While the fluid moves inside the tubes it heats up and loses water in the form of steam and becomes concentrated. The evaporator is usually heated by a compressor which aspirates the steam from the evaporator, in precise usually from the separation chamber of the evaporator, and compresses it to diminish the volume of the steam in order to increase the pressure of the steam. The compressed steam is returned to the central part of the evaporator which contains the tube bundle. Normally make-up steam can be added on the way before entering to the evaporator, if extra steam is needed. With this invention live steam is added during the removal of non-condensed gases from the evaporator.

When the process for thermal concentration of a fluid is started, normally at atmospheric pressure, the inert gas removal valve or modulating control device is opened and ejectors or inert blowers are set to work in order to replace the air with vapour to allow evaporation to commence. The plant can at the same time be in a hot water circulation while the air is aspirated. After air removal the inert removal valve or modulating control device is choked until the pressure in the pre -heater rises. The pressure rise is a sign for non-condensed gases, i.e. gases that have not changed into a fluid, collecting in the pre-heater. Then the valve is crack opened until the pressure in the pre-heater corresponds to the actual air content, so that minimum amount of non- condensed gases is flown to the evaporator.

With non-condensed gases an optional larger amount of steam can also be pulled out from the evaporator. The only limiting factor is the intensity of the feed fluid expansion into the evaporator and possible heat sensitivity of the feed fluid. With this invention a pre-heater which originally was used to pre-heat a fluid to a random temperature can now be used to pasteurize the fluid and at the same time make the process more energy efficient. The start-up sequence from atmospheric pressure commences by starting the vacuum pumps and running them until a reasonable vacuum is achieved, normally in the bracket 150-200 mbar abs. To evacuate the plant most effectively the inert removal control valve is fully opened during the vacuum pull. After reaching the level of vacuum proven optimum for start-up, heating of the plant is commenced along with starting the vapour compression fan and ramping up the speed. Evacuation of air- steam vapour mixture from the evaporator continues along with heating and fan acceleration. Evaporation commences when reaching a low enough level of residual air, a corresponding temperature level and a sufficient fan speed to create a temperature difference for heat transfer in the evaporator. In the prior art evaporator normally one degree Celsius sub-cooling of the condensing steam is considered reasonable to operate the plant. With this sub-cooling every kilogram of air or other non-condensable gases to be evacuated from the plant will bring about 15 kilograms of steam along with itself to the condenser. The energy corresponding to the evacuated air and/or vapour mixture must be replaced in order to maintain evaporation. In the invention the heat is recovered from the air and/or steam mixture before venting the evaporator to the condenser. Due to this feature a substantially larger amount of non-condensable gases mixture can be removed, in fact it could be even tenfold or more compared to the prior art.

In order for solvent to evaporate the inert gases, air and high boilers must be removed to a certain degree. The heat transfer in the evaporator is depending upon the residual non-condensable content. Every kilogram of non-condensable gas is accompanied by 15 kilogram of water vapour, if the condensing steam is allowed to sub cool by one degree Celsius. The effective temperature for heat transfer is reduced correspondingly and the fan power is increased by the ratio (dTF+l)/dTF, where dTF is the difference in condensing temperature over the heat exchanging surface. The reduction of steam loss with non-condensable gas has been reduced by controlling the release to the condenser. In this system loss of steam is compensated by adding make- up by e.g. electric heating. In the method and apparatus of the invention the heat is recovered before releasing non-condensable gases to the condenser. Therefore the amount can be chosen freely according to which flow is available to be heated. If the condensation is done in a distillation bottom product reboiler the flow can be so substantial, that the sub-cooling of condensing steam in the evaporator is almost negligible. For example in a cheese whey concentration the steam consumption can sink from 270 kg/h to 70 kg/h with a simultaneous decrease in fan power from 230 to 180 kW. In another example the temperature can be boosted by 3 ⁰ C by the recovery of condensable gases in a pre-heater. The plant capacity can be increased by 40% with the same fans due to the increased specific weight of the steam at the higher temperature. Simultaneously to the capacity increase the specific energy consumption of the fans can be decreased by 8 kWh/t water evaporation from 39 kWh/t to 31 kWh/t.

List of figures

In the following the invention will be described in more detail by referring to the figures, in which

Fig 1 shows a flow sheet of the prior art for warm feed fluids,

Fig 2 shows a flow sheet of the prior art for cold feed fluids,

Fig 3 shows a flow sheet of the preferred embodiment of the invention, and

Fig 4 shows a flow sheet of another embodiment of the invention. Detailed description of the invention

The figure 1 shows an example of a flow sheet of the prior art for a warm feed fluid wherein the fluid to be concentrated enters to a preliminary preparation P and then moves to the evaporator 1 in which the fluid is to be concentrated. The falling film concentration comprises the evaporator 1 provided with vertical tube bundle in which plurality of tubes are arranged in a known manner. The bottom zone of the evaporator 1 is known as a separation chamber 2 in which the fluid concentrate is separated from its vapours exiting to successive operations, for example to a second evaporator or a finisher or a further process like a crystallizer 3. The condensed vapours are discharged with removing means 4, for removing condensed vapours. The steam from the concentration passes through the separation chamber 2 to a compressor 5 which aspirates the steam and compresses it. After compression the steam is introduced back to the evaporator 1. Some times extra steam is needed and therefore make-up steam 6 is provided. Non-condensed gases are removed from the evaporator 1 through one or several outlets 7 and entered to a condensing apparatus 8 controlling the evaporation pressure. Non-condensable gases are pumped out through pump 9, which is for example a vacuum pump.

The figure 2 shows an example of a flow sheet of the prior art for a cold feed fluid wherein the fluid to be concentrated enters to a preliminary preparation P and then passes through a pre-heater 10 where the fluid warms-up and then moves to the evaporator 1 in which the fluid is to be concentrated. In the separation chamber 2 the fluid concentrate is separated from its vapours exiting to successive operations, for example to a second evaporator or a finisher or a further process like a crystallizer 3. The condensed vapours are discharged with removing means 4, for removing condensed vapours. The steam from the concentration passes through the separation chamber 2 to a compressor 5 which aspirates the steam and compresses it. After compression the steam is introduced back to the evaporator 1. Some times extra steam is needed and therefore make-up steam 6 is provided. Non-condensed gases, i.e. inert gases which are not condensed in the evaporator 1 , are removed from the evaporator 1 through one or several outlets 7 and entered to the pre-heater 10 wherein they are moved to the condensing apparatus 8 controlling the evaporation pressure. Non-condensable gases are pumped out through pump 9. The figure 3 shows an example of a flow sheet of a preferred embodiment of the invention wherein the fluid to be concentrated enters to a preliminary preparation P and then passes through a pre-heater 10 before moving to the evaporator 1 in which the fluid is to be concentrated. If the fluid needs to be pasteurized this can be done in the pre-heater 10 by rising the fluid temperature to 71 - 74 ⁰ C for 15-30 seconds. The falling film concentration comprises the evaporator 1 as described earlier in connection with the prior art flow sheet in the figure 1. The bottom zone of the evaporator 1 is known as a separation chamber 2 in which the fluid concentrate is separated from its vapours and exiting to successive operations for example to a second evaporator or a finisher or a further process like a crystallizer 3. The condensed vapours are discharged with removing means 4, for removing condensed vapours. The removing means 4 can be for example a discharge valve or a pump. As described earlier the steam from the concentration passes through the separation chamber 2 to a compressor 5 which aspirates the steam and compresses it. After compression the steam is introduced back to the evaporator 1.

Non-condensed gases are removed from the evaporator 1 through ejectors 12 or compression device wherein gases are compressed into higher pressure. The compression device can be a conventional steam ejector or other similar device. Non- condensed gases can also be compressed with a blower or a compressor. Live steam 11 is introduced as motive steam to be accelerated in the ejector 12 in order to aspirate non-condensed gases and help them to be compressed into higher pressure.

The compressed gas and live steam mixture is then introduced to the pre- heater 10 wherein the condensable part is condensed. The fluid heated in the pre- heater 10 expands into the evaporator 1 causing the fluid's velocity to increase. The removal of non-condensed gases from the pre-heater 10 is controlled by a modulating control device 13. In other words discharge of said non-condensable gases of said fluid from the evaporator 1 through the pre-heater 10 is controlled by a modulating control device 13. The modulating control device 13 is totally opened during the startup of the evaporator 1 when the process starts and the ejectors 12 are activated. The process can be in a hot water circulation while the air is aspirated. The modulating control device 13 is choked until the pressure in the pre-heater 10 rises and then the modulating control device 13 is opened so that minimum amount of the non- condensed gases are able to flow to the condensing apparatus 8. Non-condensable gases are pumped out through pump 9, which is typically a liquid ring or rotary vane vacuum pump. With the modulating control device 13 the pressure in the pre-heater 10 can be controlled effectively and the control range is 0-100%.

In a preferred embodiment of the invention the modulating control device 13 is used to control the temperature of the fluid feed in the pre-heater 10 before the fluid moves to the evaporator 1. Another benefit of the invention is that the cleaning of the process or the plant can be done at atmospheric pressure because with the help of the invention the air is removed effectively and vacuum is quickly arranged.

The figure 4 shows another example of a flow sheet of another embodiment of the invention wherein the fluid to be concentrated enters to the preliminary preparation P and then passes through a pre -heater 10 before moving to the evaporator 1 in which the fluid is to be concentrated. The falling film concentration comprises the evaporator 1 as described earlier in connection with the prior art flow sheet in the figure 1. The bottom zone of the evaporator 1 is known as a separation chamber 2 in which the fluid concentrate is separated from its vapours. The condensed vapours are discharged with removing means 4, for removing condensed vapours. The removing means 4 can be for example a valve or a pump. The fluid concentrate exits the separation chamber 2 to successive operation through outlet 14 and enters to another evaporator 15. The evaporator 15 comprises also a separation chamber 16 in which the fluid concentrate is separated from its vapours. The condensed vapours are discharged to the condensing apparatus 8 or to the separation chamber 2. The fluid concentrate is discharged through discharge means 17 to further processing.

A compressor 5 aspirates the steam from the first separation chamber 2 and the second separation chamber 16 and compresses it. After compression the steam is introduced back to the evaporator 1 and also to the other evaporator 15.

From the evaporator 15 non-condensed gases are removed through ejectors 18 or compression device wherein gases are compressed into higher pressure. As described earlier the compression device can be a conventional steam ejector or other similar device. Non-condensed gases can also be compressed with a blower or a compressor. The compressed gases are then introduced to the pre -heater 10 wherein the condensable part is condensed. The procedure is similar as with the first evaporator 1 where the non-condensed gases are removed also through ejectors 12 or compression device and compressed and then introduced to the pre -heater 10.

Live steam 11 or 19 is introduced in the ejector 12 or 18 as motive steam in order to aspirate non-condensed gases and help them to be compressed into higher pressure.

The removal of non-condensed gases from the pre-heater 10 is controlled by a modulating control device 13. In other words a modulating control device 13 controls the discharge of non-condensable gases of fluid from the evaporator 1 or 15 through the pre-heater 10. As described earlier the modulating control device 13 is totally opened when the process starts and the ejectors 12 are activated. The process can be in a hot water circulation while the air is aspirated. The modulating control device 13 is choked until the pressure in the pre-heater 10 rises and then the modulating control device 13 is opened so that minimum amount of the non-condensed gases are able to flow to the condensing apparatus 8. Non-condensable gases are pumped out through pump 9, which is typically a liquid ring or rotary vane vacuum pump. The other evaporator can also be pre- or post-evaporator or there can be several evaporators in series while having the common steam system. The pre-heater can be a pre-evaporator or a finisher.

In a preferred embodiment of the invention the modulating control device controls the discharge of non-condensable gases from several evaporators connected in series or in parallel. The fluid to be concentrated can be any fluid that contains some solid matter, for example milk.

It is apparent to a person skilled in the art that as technology advanced, the basic idea of the invention can be implemented in various ways. The invention and its embodiments are therefore not restricted to the above examples, but they may vary within the scope of the claims.