JPH0369274B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention This invention relates to a process for the efficient and continuous production of volatile fermentation products such as alcohols, ethers or ketones.

(Prior Art) Generally, methods for producing volatile substances by fermentation have the advantage of being able to be produced at room temperature and near normal pressure, but production takes a long time and the concentration of the product is low. Moreover, there is a drawback that the production rate decreases rapidly as the concentration of volatile substances increases.

Furthermore, since the concentration of the target fermentation product is low, there is a drawback that a large amount of energy is required for concentration and separation. On the other hand, at present, the fermentation heat in the fermenter is cooled and removed by various coolers such as shower coolers, flash coolers, or cooling coils, and is wasted.

Various methods and devices have been proposed to solve the above drawbacks.

For example, firstly, as described in U.S. Pat.
As described in No. 39-26041, there is a method of removing heat generated by fermentation production by circulating the fermentation liquor through a vacuum evaporative cooler.

Second, as an energy-saving ethanol fermentation production method, for example, a portion of the liquid extracted from the fermenter is passed through a heat exchanger, heated, and then introduced into a vacuum flash evaporator to extract ethanol-rich vapor into the gas phase, which is then purified under reduced pressure. A method in which the vapor at the top of the rectifying column is adiabatically compressed using a compressor and used as a heat source for the rectifying column and a heat source for the heat exchanger before the vacuum flash evaporator (ATPAL).
Law, Chemical.Age, Nov.21, (1980) p.11).

Third, methods for efficiently carrying out ethanol fermentation in a fermenter include, for example, Japanese Patent Application Laid-Open No. 57-2685 and Japanese Patent Publication No. 59-43156. In these methods, a part of the fermentation liquor in the fermenter is extracted and introduced into a vacuum flash evaporator, and the fermentation liquor is used to separate into ethanol-rich steam and ethanol-poor fermentation liquor. It will be sent back.

Furthermore, as a method similar to this,
A method described in Japanese Patent No. 120791 includes a method in which a part of the fermented liquid in the fermenter is extracted and supplied to a simple distillation unit corresponding to one or a small number of distillation stages for distillation.

(Problem to be solved by the invention) However, in the first example above, the US patent No.
In the method of No. 2440925, the extracted steam is cooled in a condenser, and the fermentation heat is discarded. Additionally, there are other problems, such as the need for power to compress the condenser to normal pressure during cooling, and the need for deep cooling to cool the condenser. Furthermore, the method of Japanese Patent Publication No. 39-26041 was intended to concentrate the fermentation liquor, and had the disadvantage that only the volatile fermentation products (and water) could be obtained in proportion to the calorific value of the fermentation reaction.

In the second ATPAL method mentioned above, the amount of heat for evaporating ethanol is transported in the form of sensible heat from the heat exchanger to the vacuum flash evaporator, so the amount of circulating fermentation liquid is large, and the required heat transfer area of the heat exchanger is also large. It has the following drawbacks.

Furthermore, in the case of the third example above, when the fermented liquor from the fermenter is separated in the vacuum flash evaporator, there is a considerable amount of ethanol in the fermented liquor at the outlet of the vacuum flash evaporator due to the relationship between the equilibrium solubility of ethanol in the gas phase and the liquid phase. of ethanol remains. When this fermentation liquid is returned to the fermenter, the ethanol concentration in the fermenter increases, and the fermentation rate inevitably decreases compared to the case where the fermentation liquid is not returned to the fermenter.

The problem of this decrease in fermentation speed was discussed in Japanese Patent Application Laid-Open No. 55-120791.
The method disclosed in the publication also has the same problem, but this method also has the disadvantage that the amount of circulation to the fermenter increases, resulting in poor energy efficiency.

(Means for Solving the Problems) As a result of various studies in order to overcome the drawbacks of the above-mentioned conventional methods, the present inventors have found that an evaporator with a built-in heat exchanger has the equivalent of 4 to 15 theoretical plates. By incorporating a built-in tray and introducing the fermented liquid from above after removing microorganisms to treat the fermented liquid, the recovery efficiency of volatile fermentation products is extremely high, and the volatile content remaining in the fermented liquid is It is possible to significantly reduce the concentration of volatile substances, prevent a decline in the activity of microorganisms, and increase the productivity of desired volatile substances.Furthermore, by effectively utilizing the fermentation heat, the required energy is saved and stable operating conditions are maintained. I found out what I can do. The present invention has been made based on this knowledge.

That is, in the fermentation production of a volatile fermentation product selected from alcohols, ethers, or ketones, the present invention uses a heat exchanger and a heat exchanger to remove microorganisms from the fermented liquor extracted from a fermenter. It is introduced into the upper part of the tray of an evaporator which has a built-in tray corresponding to the tray, evaporates the volatile fermentation product in the fermentation liquid, and adiabatically compresses the vapor of the volatile fermentation product from the evaporator. After raising the temperature and using it as a heating source for the heat exchanger built into the evaporator, the bottom liquid, which is the fermented liquid after being recovered and treated to reduce the concentration of volatile fermentation products to an extremely low concentration, is sent to the fermentation tank. The present invention provides a continuous method for producing characterized volatile fermentation products.

The invention will be described in detail below with reference to the drawings showing preferred embodiments of the invention.

FIG. 1 is a flow sheet showing the basic configuration of a fermentation apparatus used in the method of this invention. In the figure, 1 is a fermenter using immobilized microorganisms, and fermentation raw materials are supplied to the fermenter 1 through a line 9 and fermented by the microorganisms. This produces volatile substances and non-condensable gases. Take out the fermented liquid (mash) at any fermentation stage from the fermenter and transfer it to line 1.
1 to an evaporator 3 with built-in heat exchanger having trays 2 inside to generate vapor of the volatile fermentation product. In this case, a reduced pressure evaporation tank (flash tank) is preferably used as the evaporator 3. moreover,
This steam is extracted from line 12 and adiabatically compressed by compressor 4 to raise its temperature, and is used as a heating source for evaporator 3 with built-in heat exchanger 5 from line 13. Further, this evaporator 3 is provided with a shelf 2 to improve the recovery rate of volatile fermentation products and to reduce the concentration of volatile fermentation products in the fermentation liquor in the line 14 at the outlet of the evaporator 3. Therefore, this fermentation liquid is transferred to line 14,
When circulating to the fermenter 1 via 15 and 16, it is possible to prevent the concentration of volatile fermentation products in the fermenter 1 from increasing, thereby reducing fermentation inhibition by fermentation products and improving fermentation productivity. Furthermore, in order to compensate for the insufficient heat source in the evaporator 3, steam is added to the auxiliary heating section 5a of the heat exchanger built into the evaporator.
Hot water or the like may be supplied, or steam may be blown directly. Thereby, sufficient steam can be generated and the evaporator 3 can be operated stably regardless of fluctuations in fermentation.

Most of the non-condensable gas produced by fermentation is discharged through line 10. The fermentation products, volatile substances, are condensed and recovered via line 17.

On the other hand, the fermentation liquid after volatile substances evaporation, which is a fermentation product, is returned to the fermenter 1 as described above, but also,
A portion is discharged through line 18. If necessary, it is also possible to supply this fermentation liquid to the next fermenter.

FIG. 2 is a flow sheet of another embodiment of the method of the present invention, in which the evaporator 3 is a heating evaporation tank, and the volatile substances in the fermentation liquid are evaporated under reduced pressure in the downstream stage of the evaporator 3. A reduced-pressure evaporation tank (for example, a flash tank) 6 is provided that can lower the temperature of the fermented liquor (mash) to a temperature suitable for fermentation at the same time as recovery. Furthermore, a condenser 7 is provided to recover evaporated volatile substances. As a result, the evaporator 3 can be operated at a higher pressure, so the volume of gas to be evaporated is reduced, and the diameter of the evaporator 3 can be reduced. For this embodiment,
The purpose of the evaporator 3 is mainly to separate volatile substances that are fermentation products, and the purpose of the reduced pressure evaporator 6 is mainly to lower the temperature of the fermented liquor to an appropriate temperature. As the heat exchanger built into the evaporator 3, a falling thin film heat exchanger 8 is preferably used, which has less influence on raw material components that are unstable to heat in the fermentation liquid. In this figure, the same reference numerals as in FIG. 1 indicate the same things. In addition, 19 is a condenser 7 for steam from the reduced pressure evaporation tank 6.
20 is a line for returning the fermentation liquor taken from the vacuum evaporation tank 6 to the fermentation tank 1 via lines 15 and 16, and 21 and 22 are discharge lines for non-condensable gas and condensate from the condenser 7. be.

FIG. 3 is basically the same as the case of FIG. 2, but is a flow sheet when flocculent microorganisms or planktonic microorganisms are used as the microorganisms in the fermenter. Since the fermented liquid taken out from the fermenter 1 through the line 11 contains microorganisms, a separator 23 is provided to separate the microorganisms and return them to the fermenter 1. Specific examples of the separator 23 include a centrifugal separator, a sedimentation separator, a filter, and the like. The fermented liquid from which microorganisms have been removed is sent from line 27 to evaporator 3. The isolated microorganisms are in line 24,
Most of it is returned to the fermenter 1 through line 25, but if necessary, it can be taken out through line 26 and recycled. Other configurations are as follows:
Figure 1 or Figure 2 is the same as Figure 1 or Figure 2.
The same reference numerals as in the figure indicate the same thing.

In this invention, as the evaporator, evaporators such as a reduced pressure evaporation tank and a heated evaporation tank are used in appropriate combinations, and at least one of these evaporators is provided with a heat exchanger and a tray. In addition, when using a heating evaporation tank, as shown in Figure 2 or 3, a vacuum evaporation tank is connected to the subsequent stage, and the temperature of the fermented liquor after evaporation is lowered in the vacuum evaporation tank, and then the fermentation tank is heated. It is preferable to send it to

In the method of the present invention, in the embodiment shown in FIG. 1, a reduced pressure evaporation tank with a built-in heat exchanger is used as the evaporator 3 and is operated under reduced pressure, but the degree of reduced pressure depends on the type of fermentation reaction and the time. Although it varies depending on the fermentation temperature of the fermenter, it is usually 20 to 720 mmHg・
abs, preferably in the range of 20 to 200 mmHg·abs.

Further, in the case of the embodiment shown in FIG. 2 or 3, a heating evaporation tank is used as the evaporator 3, and the temperature is 75 to 105°C when the pressure is 0.8 to 1.2 atm, or the temperature is 95 to 105°C when the pressure is 1.2 to 10 atm. The most preferred evaporation pressure and temperature conditions are within the range of 180°C. In this case, the remaining volatile substances are then evaporated and recovered in the reduced pressure evaporation tank 6, and the temperature thereof is lowered. The degree of pressure reduction in the vacuum evaporator tank 6 at this time is related to the fermentation temperature, but is 20 to 720 mmHg・abs,
Preferably it is within the range of 20 to 200 mmHg·abs.

There is no particular restriction on the type of tray used in this invention, but it is one that reduces the concentration of volatile substances in the bottom liquid of a heating evaporation tank, a vacuum evaporation tank, etc., and this purpose is achieved. If so, it can be used without any particular restrictions. Therefore, the shelf includes a perforated plate, a bubble cap tray, etc., and a packed column type having equivalent performance.

In this invention, the number of shelves of the evaporator is 4 to 15.
A theoretical plate or, in the case of a packed column type, a packed bed having a packing height corresponding to the theoretical plate. If the number of plates is less than 4 theoretical plates, it is not easy to reduce the concentration of volatile substances sufficiently, and even if the number of plates exceeds 15 theoretical plates, the effect will not change, but the equipment cost will increase, and it will not be possible to scale up in the long term. The pressure loss inside the tower increases due to the adhesion of such substances, which may cause operational inconveniences. Volatile fermentation products in the fermentation liquid supplied to the evaporator are evaporated and removed to the gas phase while flowing down the tray or packed bed, but they pass through the heating section at the bottom of the evaporator and reach the bottom. When this is done, the concentration of volatile products in the fermentation liquid is preferably reduced to 0.1-1.0 wt%. This fermentation liquid is supplied in whole or in part to the original fermenter or another fermenter directly or, if a reduced pressure evaporator (flash tank) is installed, via the reduced pressure evaporator. In this way, the concentration of volatile products in the fermentation liquid supplied to the fermenter after completing the evaporation process in the evaporator is sufficiently low to reduce the effect of inhibition on the fermentation reaction in the destination fermenter.

In this invention, the fermented liquid from which volatile substances have been removed, which is a fermentation product, is sent to a fermenter, and this fermenter may be an original fermenter or another fermenter. Furthermore, two or more combinations of fermenters and evaporators may be assembled in series or in parallel.

In this invention, there are no particular restrictions on the type of heat exchanger built into the evaporator used, and a normal multi-tube type may be used, but it may An efficient heat exchanger that has little influence is also selected as appropriate. for example,
A structure such as a falling thin film type or a thin film type (horizontal tube type) is preferable.

In this invention, changes in the concentration of volatile substances that are fermentation products in the fermentation liquid in the fermenter cause changes in the concentration of volatile substances in the vapor evaporated from the evaporator, and the condensation of this vapor The temperature may change from the original design value. Here, if the temperature difference required for heat transfer in the heat exchanger becomes small, there is a possibility that heating of the fermented liquid and condensation of vapor containing volatile substances in the evaporator will not be performed sufficiently.

In order to compensate for the insufficient heating source, an auxiliary heating section is provided in the heat exchanger with a built-in evaporator, and/or steam is directly blown into the evaporator to supply an auxiliary heat source, thereby ensuring sufficient evaporation and heating of the evaporator. Stable operation is possible. Particularly when returning the fermented liquor to the fermenter, it is important to operate stably so that no volatile substances remain in the fermented liquor at the evaporator outlet in order to prevent a decrease in the fermentation rate in the destination fermenter.

The method of the present invention can be carried out in accordance with a conventional fermentation method except for using the above-mentioned evaporator. That is, when producing volatile substances such as alcohols, ethers, and ketones, immobilized microorganisms, flocculent microorganisms, or planktonic microorganisms are used as microorganisms to ferment raw materials, and the microorganisms are fermented at the desired fermentation stage. Separated from the fermentation liquid.

The microorganisms used in this invention include:
Various yeasts such as Saccharomyces cerevisiae, Saccharomyces uvaram, etc.
Alternatively, bacteria such as Zymomonas mobilis may be mentioned.

In this invention, the fermented liquor extracted from the fermenter and introduced into the evaporator as described above is one after removing microorganisms, but this means that it does not substantially contain microorganisms, and usually Microorganisms are separated by means of separation such as centrifugation, membrane separation, immobilization, or sedimentation. It is difficult to completely remove microorganisms using such separation means, and microorganisms may remain at a concentration of 5 g dry cell/or less. In addition, when immobilized microorganisms are used as described above, the extracted fermentation liquid does not need to be subjected to any separation means, but even in such a case, the fermentation liquid after removing the microorganisms according to the present invention may be used. Equivalent to.

When flocculent microorganisms or planktonic microorganisms are used as microorganisms, they are separated by separation means such as centrifugation, filtration, or sedimentation. The separated microorganisms are returned to the fermenter, and the fermented liquid after the microorganisms have been separated is supplied to an evaporation device with a built-in tray and heat exchanger, where it is heated or depressurized to release volatile substances, which are fermentation products. and non-condensable gases are evaporated and volatile substances are recovered. In this evaporation means, since microorganisms are not contained in the fermentation liquid during heating, the heating pressure and temperature conditions can be set arbitrarily.

(Effects of the Invention) Conventionally, the amount of evaporation of volatile substances in a fermentation liquid is determined by the relationship of vapor-liquid equilibrium, and the concentration of volatile substances remaining in the fermentation liquid is high. Therefore, since this fermentation liquid is supplied to the original fermenter or other fermenters,
The concentration of volatile products in the fermenter becomes high and has an inhibiting effect on fermentation, leading to a decrease in the productivity (efficiency) of the fermenter.

On the other hand, according to the method of the present invention, it is possible to increase the concentration of the fermentation raw material supplied to the original fermenter by returning it within a range that does not cause a decrease in the productivity of the fermenter. This in turn leads to a reduction in the amount of waste liquid from the entire fermentation process, making it possible to significantly reduce waste liquid treatment costs.

According to the method of the present invention, the required energy is saved by effectively utilizing the fermentation heat, and stable operating conditions can be maintained.

In this invention, the volatile fermentation-rich steam removed by evaporation in the evaporator with a built-in tray and heat exchanger is adiabatically compressed and raised in temperature and pressure, and then heated by the built-in heat of the evaporator. It is used as a heating source for the exchanger and is cooled and liquefied in its evaporator. That is, the fermentation heat is completely utilized for evaporation of the fermentation product. After adiabatic compression, the vapor rich in volatile fermentation products is
The temperature has increased by ~20℃. With this method, an additional heat source for evaporation in the evaporator is hardly required. In other words, volatile fermentation products can be evaporated and recovered using less heat than before. Conversely, it cools the steam rich in volatile substances, which is a fermentation product.
It does not require a cooling source such as cooling water to liquefy, so it has great advantages such as saving on the cooling system.

(Examples) Next, the method of the present invention will be explained in more detail with reference to Examples and Comparative Examples.

Example 1 Ethanol fermentation was carried out according to FIG. Immobilized yeast 400, which is made into a spherical gel by enveloping yeast in calcium alginate, is packed into fermenter 1, and the raw material with a total sugar concentration of 35.3wt% is passed through lines 9 and 16.
It was supplied at a rate of 114.4Kg/h. The fermentation temperature was 32°C.
Carbon dioxide gas generated during fermentation was discharged from line 10 at a rate of 14.4 kg/h. On the other hand, ethanol
Fermentation liquid containing 5.1wt% (yeast concentration 0.5g dry cell/
) was extracted at a rate of 299.5 kg/h and sent through a line 11 to an evaporator (flash tank) 3 with a height of 6 m and a diameter of 700 mm, which has 10 stages. In order to evaporate and separate ethanol from the fermentation liquid, steam was first supplied at 5 kg/h to the auxiliary heating section of the heat exchanger with a built-in evaporator. The ethanol-containing vapor evaporated from the evaporator 3 was adiabatically compressed by the compressor 4, returned to the evaporator 3, and used as a heating source, so the amount of evaporation gradually increased and became constant at 52.5 kg/h. At this time, the ethanol concentration was 28.5 wt%. The temperature inside the evaporator is 32℃ to 34℃, the pressure is 40mmHg・abs
was maintained and stable operation was possible.

Steam temperature at compressor 4 outlet is 60℃, pressure is 72mm
It becomes Hg/abs, which is led to evaporator 3 and condensed 35
It became an ethanol aqueous solution at ℃ and was recovered at a rate of 52.5 kg/h.

On the other hand, the fermented liquid at the outlet of evaporator 3 is extracted through line 14 at 247 kg/h, and is drawn out through line 15 and 16.
through line 18 at 199.5 kg/h and the remainder was discharged through line 18 at 47.5 kg/h. The ethanol concentration contained in this fermentation liquid is 0.16wt%
It became. In this way, the fermentation liquor with low ethanol concentration is returned to the fermenter, so the ethanol concentration in the fermenter is kept low, and the ethanol fermentation rate is 35g/
- Fermentation tank h was obtained and fermentation productivity was improved.

Comparative Example 1 Ethanol fermentation was carried out under the same operating conditions as in Example 1 using a device similar to that shown in FIG. Since this fermentation liquid is returned to the fermenter, the ethanol concentration at the exit of fermenter 1 changes from 5.1wt% in Example 1 to 6.3wt%, and compared to Example 1, the fermentation productivity is approximately 2.0wt%. It decreased by 40%. In order to produce the same amount of ethanol as in Example 1, the amount of immobilized yeast was required to be 700. Also,
The recovered ethanol concentration was 27.1 wt%, which was a lower value compared to Example 1. As described above, when there are no shelves, there is a drawback that fermentation productivity is reduced.

Comparative Example 2 Same as Example 1 except that the same device as Example 1 was used, the steam compressed by the compressor 4 was not used as the heating source for the evaporator 3, and only the steam was used as the heating source for the auxiliary heater 5a. Approximately 50 kg/h of steam was required to obtain a evaporation amount of 52.5 kg/h. The operating conditions of the fermenter 1 and the evaporator 3 were the same as in Example 1, and the inside of the evaporator was maintained at reduced pressure by a vacuum pump. As a result, when comparing the total amount of steam for auxiliary heating and the required power for the compressor, Example 1 consumed about 30% of the energy compared to Comparative Example 2.
Furthermore, Comparative Example 2 requires a new condenser and cooling utility to recover ethanol from the vapor at the inlet or outlet of the vacuum pump, which has the disadvantage of increasing both equipment cost and energy cost.

Example 2 Ethanol fermentation was carried out according to FIG.
The conditions of the fermenter 1 were the same as in Example 1. Molasses with a total sugar concentration of 35.3wt% is passed through lines 9 and 16.
It was supplied at a rate of 114.4Kg/h. Carbon dioxide gas generated during fermentation was discharged from line 10 at a rate of 14.4 kg/h. On the other hand, a fermentation liquid containing 5.2 wt% ethanol (yeast concentration was the same as in Example 1) was extracted at a rate of 299.7 kg/h and sent through line 11 to evaporator 3 having 10 stages. Steam was supplied at 3.5 kg/h to the auxiliary heating section of the heat exchanger with a built-in evaporator, and the generated steam was adiabatically compressed by the compressor 4 to recover heat. As a result, the amount of evaporation was 35.3Kg/h. Moreover, the ethanol concentration was 40.3wt%. The temperature inside the evaporator is 88℃ to 93℃, the pressure is 600℃
It was maintained at mmHg・abs. Under these operating conditions, the evaporator 3 had the same height as in Example 1, but the diameter was about 1/5. The temperature at the compressor 4 outlet is 120℃, and the pressure is
The ethanol aqueous solution at 95°C was recovered at a rate of 35.3 kg/h. The fermented liquid at the evaporator outlet is extracted through line 14 at a rate of 264.4 kg/h, and is pumped at 40 mmHg by a vacuum pump.
Reduced pressure evaporation tank (flash tank) leaning on abs 6
The solution was flashed under reduced pressure to evaporate the remaining ethanol and the temperature was lowered to 34°C. Flush steam is condenser 7
After cooling, it was recovered as an ethanol aqueous solution at a rate of 24.6 kg/h. Moreover, the ethanol concentration was 2.6 wt%.

On the other hand, the fermented liquid after evaporating from the flask is on line 20.
Fermenter 1 at 199.7Kg/h through line 15.
The remaining fermented liquor is returned to the tank through line 18.
It was discharged at a rate of 40.1Kg/h. Moreover, the ethanol concentration of the fermentation liquid was 0.5wt%.

[Brief explanation of drawings]

FIG. 1 is a flow sheet showing the basic configuration of a fermentation apparatus used in the method of the present invention, and FIGS. 2 and 3 are flow sheets of other embodiments, respectively. Explanation of symbols, 1... Fermentation tank, 2... Shelf, 3...
...Evaporator, 4...Compressor, 5...Heat exchanger, 6...
...Reduced pressure evaporation tank, 7... Condenser, 8... Falling film heat exchanger.

Claims (1)

[Claims] 1. In fermentation production of a volatile fermentation product selected from alcohol, ether or ketone, the fermented liquid extracted from the fermenter and after removing microorganisms is transferred to a heat exchanger and It is introduced into the upper part of the tray of an evaporator which has a built-in tray corresponding to 15 theoretical plates, and the volatile fermentation product in the fermentation liquid is evaporated, and the vapor of the volatile fermentation product from the evaporator is adiabatically compressed. The temperature is raised and used as a heating source for the heat exchanger built into the evaporator, and then the bottom liquid, which is the fermentation liquid after treatment to reduce the concentration of volatile fermentation products, is sent to the fermentation tank. A method for continuously producing a volatile fermentation product, characterized in that: 2 The built-in heat exchanger of the evaporator is divided into two or more parts, and as a heat source for each, one uses adiabatically compressed steam containing volatile fermentation products evaporated in the evaporator, and the other uses The method according to claim 1, wherein hot water and/or steam is used as an auxiliary heat source. 3. The method according to claim 1, wherein the evaporator comprises (a) a reduced pressure evaporation tank or (b) a combination of a heated evaporation tank and a reduced pressure evaporation tank connected to its subsequent stage. 4. The method according to claim 3, wherein the vacuum evaporator (a) or the heating evaporator (b) includes a built-in heat exchanger and trays. 5 The vacuum evaporator in (a) is 20~200mmHg・abs, 25~
The method according to claim 4, wherein the temperature is maintained at 65°C. 6 The heating evaporation tank in (b) is maintained at 0.8 to 1.2 atm, 75 to 105 °C or 1.2 to 10 atm, 95 to 180 °C, and the subsequent vacuum evaporation tank is maintained at 20 to 200 mmHg・abs, 25 to 65 °C. The method according to claim 4. 7. The method according to claim 1, wherein the heat exchanger built into the evaporator is of a falling film type. 8. The method according to claim 1, wherein the trays built into the evaporator are of a perforated plate type, a bubble cap tray type, or a packed column type.