JPH04330902A - Method for evaporating water solution containing water soluble solute having saturation solubility in water inversely proportional to temperature and equipment therefor - Google Patents

Abstract

PURPOSE:To reduce crystallization of a solute as scale on the surface of each horizontal heating tube of a heater when evaporating a water solution containing a water soluble solute having saturated solubility in water inversely proportional to temperature to the state of crystallization of the solute by a horizontal tube falling film evaporator. CONSTITUTION:A water solution after heated and evaporated on the outside surface of horizontal heating tubes 11c in an evaporator 10 at reduced pressure (under barometric pressure) is introduced into a flash evaporator 16 maintained at reduced pressure lower than that in the evaporator 10 where it is flashed. Then, after the water soluble solute is crystallized and the crystals are separated and removed, the water solution is sprayed over the outside surface of the horizontal heating tubes 11c in the evaporator 10.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is directed to water-soluble materials whose saturation solubility in water increases in proportion to temperature, such as potassium sulfate (K2 SO4), copper sulfate (CuSO4), or sodium chloride (NaCl). The present invention relates to an evaporation method for heating and evaporating water in an aqueous solution containing a solute, and an apparatus used in the method.

0002

[Prior Art] Conventionally, heating evaporation of aqueous solutions containing water-soluble solutes has been carried out as shown in Figures 7 and 6 (i) on page 405 of the revised 5th edition of "Chemical Engineering Handbook" published by Maruzen Co., Ltd. and edited by the Society of Chemical Engineers. A horizontal tube descending film type evaporator constructed as described and shown in FIG. 3 is used.

That is, in this horizontal tube falling film type evaporator, a heater 2 equipped with a large number of horizontal heating tubes 2a is provided in the upper part of an evaporator 1, and a condenser 3 is installed inside the evaporator 1. While reducing the pressure to below atmospheric pressure using a vacuum pump 4, the aqueous solution pumped out from the bottom of the evaporator 1 by a pump 5 was provided at the top of the evaporator 1 via a circulation pipe 6. The mixture is sent to a spray nozzle 7, and sprayed from the spray nozzle 7 onto the outer surface of each horizontal heating pipe 2a in the heater 2,
The aqueous solution is made to flow down the outer surface of each horizontal heating tube 2a in the form of a liquid film, and the aqueous solution is heated and evaporated during the flow down.

In the horizontal tube descending film type evaporator shown in FIG. 3, a part of the steam generated in the evaporator 1 is sucked and compressed by a steam ejector 8 driven by high-pressure steam from a boiler or the like. Afterwards, it is configured in a so-called self-compression type so that it is supplied into each horizontal heating tube 2a in the heater 2 and used as a heating source.

[0005]

[Problems to be Solved by the Invention] The above-mentioned horizontal tube descending film type evaporator heats and evaporates the aqueous solution in a state where the aqueous solution is in the form of a liquid film, thereby reducing the boiling point increase due to the depth of the liquid. In addition, since the overall heat transfer coefficient is high, it is illustrated in Figures 7 and 6 (k) or (l) on page 405 of the revised 5th edition of "Chemical Engineering Handbook" published by Maruzen Co., Ltd. and edited by the Society of Chemical Engineers. Compared to forced circulation type evaporators, this has advantages such as not only significantly higher thermal efficiency but also the ability to miniaturize the device.

However, on the other hand, the horizontal tube descending film type evaporator can produce an aqueous solution containing a water-soluble solute, such as potassium sulfate, copper sulfate, or sodium chloride, whose saturation solubility in water increases in proportion to temperature. When the solute evaporates to the point where it precipitates as crystals, the following problems arise. In other words, in the horizontal tube descending film type evaporator, an aqueous solution with approximately saturated solubility is poured down the outer surface of the horizontal heating tube in the heater in the form of a liquid film, and the temperature is approximately equal to the vapor temperature in the evaporator. Because it heats and evaporates while maintaining the same temperature,
Due to the heating and evaporation on the outer surface of the horizontal heating tube, the aqueous solution becomes supersaturated beyond its saturated solubility at that temperature, and as a result, crystals of the solute in the aqueous solution are formed on the outer surface of the horizontal heating tube. Since it precipitates and forms scale, it is necessary to stop the evaporation operation and remove the scale from each horizontal heating tube frequently.

[0007] According to the present invention, crystals of the solute are precipitated from an aqueous solution containing a water-soluble solute whose saturation solubility in water is proportional to temperature using the horizontal tube descending film type evaporator which has high thermal efficiency and can be made compact as described above. A technical problem is to provide a method that can reliably reduce the formation of scale on the surface of each horizontal heating tube in a heater when evaporating to a state of evaporation, and an apparatus for use in the method. It is.

[0008]

[Means for Solving the Problems] In order to achieve this technical problem, the method of the present invention is to prepare an aqueous solution containing a water-soluble solute, such as potassium sulfate or copper sulfate, whose saturation solubility in water is proportional to temperature. In an evaporation method in which a horizontal heating tube in an evaporator maintained at a pressure below atmospheric pressure is heated and evaporated by spraying the outer surface of the horizontal heating tube so as to flow down in a liquid film state, evaporation in the evaporator is performed. The resulting aqueous solution is introduced into a flash evaporator maintained at a reduced pressure lower than that of the evaporator for flash evaporation, and then crystals of the water-soluble solute are precipitated and the crystals are separated and removed. I decided to spray it on the horizontal heating tube inside the can.

Furthermore, the apparatus of the present invention includes an evaporator maintained at a reduced pressure below atmospheric pressure, a plurality of horizontal heating tubes provided in the evaporator, and a water-resistant evaporator such as potassium sulfate or copper sulfate. a dispersing means for dispersing an aqueous solution containing a water-soluble solute whose saturation solubility is proportional to temperature in the form of a liquid film over the outer surface of each of the horizontal heating tubes; and circulating the aqueous solution at the bottom of the evaporator to the dispersing means. In an evaporator comprising a supply circulation path, a flash evaporator maintained in a reduced pressure state lower than that of the evaporator in the circulation path, and a crystal precipitator for precipitating and separating crystals of water-soluble solutes in the aqueous solution. The flash evaporator was provided on the upstream side and the crystal precipitation tank was provided on the downstream side.

0010

[Effect] In this way, the aqueous solution coming out of the evaporator will be
The aqueous solution enters a flash evaporator maintained at a reduced pressure lower than that of the evaporator, undergoes flash evaporation, and its temperature decreases, causing the aqueous solution to move into the supersaturated region above the saturated solubility curve. As a result, crystals of the solute in the aqueous solution precipitate in the crystal precipitation tank, and the concentration of the aqueous solution approaches the saturated solubility.

[0011] Next, after the crystals are removed, the aqueous solution is sprayed in the form of a liquid film onto the outer surface of the horizontal heating tube in the evaporator via the spraying means, but the temperature of the aqueous solution is is lower than the vapor temperature in the evaporator due to flash evaporation in the flash evaporator, so the temperature of the aqueous solution flowing down the outer surface of the horizontal heating tube in a liquid film state is lower than that in the evaporator. It is heated until it becomes equal to the steam temperature at , and then it is heated and evaporated.

That is, the aqueous solution flowing down the outer surface of the horizontal heating tube in the form of a liquid film is heated until its temperature becomes equal to the steam temperature in the evaporator, and by this heating, the aqueous solution is once saturated. The state will be unsaturated below the solubility curve, and heating and evaporation will start from this unsaturated state, so the heating and evaporation on the outer surface of the horizontal heating tube can be compared to the saturated solubility curve. This can be done in the lower unsaturated region, or the extent to which it crosses the saturated solubility curve into the supersaturated region can be significantly reduced.

[0013]

Therefore, according to the present invention, an aqueous solution containing a water-soluble solute whose saturation solubility in water is proportional to temperature,
As mentioned above, the horizontal tube descending film evaporator, which has high thermal efficiency and can be miniaturized, ensures that scale does not form on the surface of each horizontal heating tube in the heater when the solute is evaporated to the point where crystals are precipitated. Since the number of times the evaporation operation must be stopped to remove scale from each horizontal heating tube can be reduced, the evaporation efficiency can be significantly improved.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Below, the drawings will be described in which an embodiment of the present invention is applied to evaporating an aqueous potassium sulfate solution in a self-vapor compression type horizontal tube descending film evaporator. In the figure, reference numeral 10 indicates a closed type evaporator, and the evaporator 1
A heater 11 consisting of a pair of left and right headers 11a, 11b and a large number of horizontal heating tubes 11c connecting both the headers 11a, 11b is installed at the upper part of the interior of 0. Each of the horizontal heating pipes 11c
A spraying nozzle 12 is provided to spray a liquid film onto the outer surface of the container.

The steam generated in the evaporator 11 is sucked and extracted by a steam ejector 14 driven by high-pressure steam supplied from a steam supply pipe 13 from a boiler (not shown).
After being compressed, the heater 11 is passed through the steam duct 15.
is supplied into one header 12a. code 16
is a closed type flash evaporator, and code 17 is pipe line 1.
8 each shows a condenser cooled by supplied cooling water,
A steam outlet 19 from the flash evaporator 16 is connected to the condenser 17 via a steam duct 20, and a vacuum generating device such as a vacuum pump 21 is connected to the condenser 17. discharge pump 22 is connected to the other header 11b of the heater 11.
A condensed water extraction line 23 from is connected thereto.

Further, the vapor duct 20 from the flash evaporator 16 or the condenser 17 includes the heater 11.
A non-condensable gas extraction line 24 from the other header 11b is connected, while a part of the steam in the steam duct 20 is transferred to an auxiliary pipe driven by high-pressure steam supplied from the steam supply line 13. After being suctioned and compressed by the steam ejector 25, it is supplied into one header 11a of the heater 11 through the steam duct 26, and with these configurations, the steam inside the evaporator 10 is heated to a temperature of 70°C.
The flash evaporator 1 is kept under reduced pressure at ℃.
5 is maintained at a reduced pressure state with a steam temperature of 65°C.

Then, the potassium sulfate aqueous solution at the bottom of the evaporator 10 is led into the flash evaporator 16 through the first circulation pipe 27 and flash-evaporated, and the potassium sulfate aqueous solution after flash-evaporation is After guiding it into the crystal precipitation tank 29 via the second circulation pipe 28, it is extracted as a supernatant liquid from the upper part of this crystal precipitation tank 29, and this is transferred to the circulation pump 30 and the third circulation pipe 3.
1, a spray nozzle 12 in the evaporator 10
On the other hand, an aqueous potassium sulfate solution is supplied from a pipe line 32 to the middle of the third circulation pipe line 31.

In this device, the amount of water evaporated in the evaporator 10 is set to 1,000 kg/h, and the amount of potassium sulfate aqueous solution circulated is set to 50,000 kg/h, which is 50 times the amount of evaporation.
g/h, and assuming that the boiling point rise of the potassium sulfate aqueous solution is, for example, 5°C, the evaporator 1
The temperature of the potassium sulfate aqueous solution flowing out from the bottom of 0 is 75
The potassium sulfate aqueous solution at 75°C is led into the flash evaporator 16 via the first circulation line 27, where it is flash evaporated until the temperature drops to 70°C.

FIG. 2 shows the concentration of the aqueous potassium sulfate solution flowing out from the bottom of the evaporator 10 in the unsaturated region below the saturated solubility curve of potassium sulfate in water. At point A, that is, 16.94 wt%, the flash evaporator 1
6, water flash evaporates by 449 kg/h, and as the temperature drops from 75°C to 70°C, the concentration decreases to 1.
By increasing the concentration to 7.09 wt%, this aqueous potassium sulfate solution moves from point A to point B in the supersaturated region above the saturated solubility curve, becomes highly supersaturated, and flows through the second circulation pipe 28. The crystal precipitation tank 29
Since the potassium sulfate in the potassium sulfate aqueous solution precipitates as crystals in the crystal precipitation tank 29, it precipitates at the bottom of the crystal precipitation tank 29.
To separate.

Due to the precipitation of the crystals, the concentration of the potassium sulfate aqueous solution flowing out from the crystal precipitation tank 29 decreases to approximately 16.6 wt% of saturation solubility when the temperature is 70° C., and shifts from the point B to the point C. do. Next, the potassium sulfate aqueous solution in this state is sent to the spray nozzle 12 in the upper part of the evaporator 10 via the circulation pump 30 and the third circulation pipe 31, and then from this spray nozzle 12 to each horizontal heating pipe. It is sprayed so as to form a liquid film on the outer surface of 11c.

At this time, the temperature of the aqueous potassium sulfate solution sprayed from the spray nozzle 12 onto the outer surface of each horizontal heating tube 11c is 70°C, while the steam temperature inside the evaporator 10 is 70°C. The heating and evaporation of the potassium sulfate aqueous solution does not start at the same time as the dispersion to each horizontal heating pipe 11c, but starts after the temperature has been heated to 75°C, and the heating and evaporation of the potassium sulfate aqueous solution starts after the temperature is heated to 75°C. By heating, the potassium sulfate aqueous solution moves from the state of point C to point D in the unsaturated region below the saturated solubility curve, and once becomes a considerably low unsaturated state, and this unsaturated Heating and evaporation starts from this state, and as a result, 1000 kg/h of water evaporates, resulting in the above-mentioned A.
Repeat the process of returning to the point.

[0022] That is, the potassium sulfate aqueous solution flowing down in the form of a liquid film on the outer surface of each horizontal heating tube 11c is heated until its temperature reaches 70°C to 75°C, and by this heating, the aqueous solution is once heated. , becomes a low unsaturated state below the saturated solubility curve, and by starting heating and evaporation from this unsaturated state, the heating and evaporation on the outer surface of each horizontal heating tube 11c becomes saturated. Since it can be carried out in the unsaturated region below the solubility curve, it is possible to significantly prevent the potassium sulfate crystals in the potassium sulfate aqueous solution from adhering and precipitating as scale on the outer surface of each horizontal heating tube 11c. This can be reduced to

Furthermore, even when the concentration of the potassium sulfate aqueous solution flowing out from the storage can 10 is brought close to the saturated solubility at 75° C., the concentration of the potassium sulfate aqueous solution flowing out from the storage can 10 is close to the saturated solubility at 75° C. , as mentioned above, once
This reduces the degree to which the aqueous potassium sulfate solution exceeds the saturated solubility curve and enters the supersaturated region during heating and evaporation on the outer surface of each horizontal heating tube 11a. Therefore, it is possible to reliably reduce the scale generated on the outer surface of each horizontal heating tube 11c.

The crystals precipitated at the bottom of the crystal precipitation tank 29 are sent to a centrifugal separator 34 by a slurry pump 33, separated into liquid by the centrifugal separator 34, and then taken out. Separated from crystals in tank 35
The potassium sulfate aqueous solution accumulated in the pump 36 is returned to the third circulation pipe 31 via a pipe 37 with a pump 36. This slurry pump 33 has a bypass for facilitating the extraction of crystals by returning a part of the liquid discharged from the slurry pump 33 to the suction part of the slurry pump 33 in the crystal precipitation tank 29. A conduit 38 is provided.

[0025] Although the above example shows the case where the present invention is applied to the evaporation of an aqueous solution of potassium sulfate, it goes without saying that the present invention is not limited to this and can also be applied to the evaporation of an aqueous solution of copper sulfate or an aqueous sodium chloride solution. When the slope of the saturation solubility curve in water is small, it is necessary to increase the amount of circulation of the aqueous solution. It goes without saying that the present invention can also be applied to a multiple effect evaporator.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing a saturation solubility curve of potassium sulfate in water.

FIG. 3 is a diagram showing a conventional horizontal tube descending film type evaporator.

[Explanation of symbols]

10 Evaporator 11 Heater 11c Horizontal heating tube 12 Spraying nozzle 16 Flash evaporator 29
Crystal precipitation tank 27, 28, 31
Circulation pipe 30 Circulation pump

Claims (2)

[Claims] Claim 1: An aqueous solution containing a water-soluble solute, such as potassium sulfate or copper sulfate, whose saturation solubility in water is proportional to temperature, is heated to a horizontal heating tube in an evaporator kept below atmospheric pressure. In an evaporation method that heats and evaporates by spraying the outer surface of a heating tube in the form of a liquid film, a flash in which the aqueous solution evaporated in the evaporator is maintained at a reduced pressure lower than that of the evaporator. The water-soluble solute is introduced into an evaporator for flash evaporation, and then crystals of a water-soluble solute are precipitated, separated and removed, and then sprayed onto a horizontal heating pipe in the evaporator. A method of evaporating aqueous solutions containing water-soluble solutes whose saturation solubility is proportional to temperature. Claim 2: An evaporator maintained at a reduced pressure below atmospheric pressure, a plurality of horizontal heating tubes installed in the evaporator, and an evaporator whose saturated solubility in water is proportional to temperature, such as potassium sulfate or copper sulfate. a dispersion means for dispersing an aqueous solution containing a water-soluble solute to the outer surface of each of the horizontal heating tubes in the form of a liquid film; and a circulation path for circulating and supplying the aqueous solution at the bottom of the evaporator to the dispersion means. In the evaporator, a flash evaporator maintained at a reduced pressure lower than that of the evaporator and a crystal precipitation tank for precipitating and separating crystals of a water-soluble solute in the aqueous solution are provided in the circulation path. An evaporation device for an aqueous solution containing a water-soluble solute whose saturated solubility in water is proportional to temperature, characterized in that an evaporator is provided on the upstream side and a crystal precipitation tank is provided on the downstream side.