An apparatus for concentrating a solution by heating or crystallization from a solution. Generally, the solution is a dilute aqueous solution containing a non-volatile solid solute. The solution is heated to boiling to vaporize part of the solvent, thereby increasing the concentration of the solute in the solution. For example, the industrial preparation of caustic soda, salt and sugar is treated with an evaporator. In addition, the evaporator can also be used for purification of solvents, such as fresh water. The heat source of the evaporator is mainly saturated steam, but also flue gas, fire and solar energy.
The type evaporator is divided into three types according to the operating pressure: normal pressure, pressurization and decompression. According to the movement condition of the solution in the evaporator: 1 cycle type: the boiling solution passes through the heating surface multiple times in the heating chamber, such as central circulation tube type, hanging basket type, external heat type, column type and forced circulation type, etc. . 2 Single-pass type: The boiling solution passes through the heating surface once in the heating chamber, and does not circulate, that is, the concentrated liquid is discharged, such as a rising film type, a falling film type, a rising-falling film type, a stirring film type, and a centrifugal film type. 3 direct contact type: the heating medium is in direct contact with the solution for heat transfer, such as immersion combustion evaporator.
The central circulation tube evaporator consists of a heating chamber, an evaporation chamber and a separation chamber (Fig. 1). The heating chamber is actually a heat exchanger, and the solution in the tube is heated by steam between the tubes (heated steam, also called primary steam or fresh steam) to reach boiling and rise into the evaporation chamber. The solution pressure is lowered and the solvent is vaporized in a large amount to form steam (referred to as secondary steam). The unvaporized solution enters the large-diameter central circulation pipe. Due to the density difference between the vapor-liquid two-phase flow in the surrounding small-diameter heating pipe, a directional circulating flow is formed, which is transferred to the surrounding heating pipe. It is heated to evaporate and the concentrate is discharged from the bottom discharge valve. The evaporation chamber has a certain height (called a separation space), so that large droplets in the steam naturally fall, and preliminary separation is performed, and the separation space is generally about 800 mm. The initially separated secondary vapor enters the separation chamber to further separate the fine droplets, reducing mechanical entrainment losses. During the evaporation process, it should be noted that: 1 In the heating chamber, the boiling point of the solution increases with the concentration of the solution, and varies with the height of the position. The higher the liquid column above the position, the more static The higher the pressure, the higher the boiling point. Thus, the effective temperature difference between the heated steam and the solution is smaller. 2 In order to maintain a high heat transfer coefficient, the wall of the solution side should be clean, free of dirt and regular cleaning; the heating steam space outside the tube, especially at certain dead angles, should be provided with non-condensable gas emissions. Mouth, so as to avoid the accumulation of gas and affect heat transfer. This type of evaporator is also known as a standard evaporator. The utility model has the advantages of improved circulation of the solution, improved heat transfer efficiency, reduced scale formation, simple structure and convenient manufacture. This type of evaporator is commonly used in the sugar industry.
Suspension basket evaporator The heating chamber of the evaporator is a separate basket-type member that can be hung (or supported) on the evaporator wall, hence the name "suspension basket". The basket type evaporator is also a commonly used circulation type evaporator. The heating chamber has only a vertical tube bundle without a center tube, and the annular cross-sectional space between the heating chamber and the wall is used as a circulation loop of the solution. Since the cross-sectional area of ​​the annulus flow channel is large, the circulation of the solution is good, and the heat transfer efficiency is high; and one end of the heating chamber can be freely expanded to avoid the temperature difference stress between the tube and the tube sheet. Because of the ease of replacement of the heating chamber, basket-type evaporators are commonly used in the caustic soda industry.
External heat evaporator This evaporator heating chamber is placed outside the evaporation chamber. This not only reduces the height of the entire evaporator, but also facilitates cleaning and replacement. The heating tube bundle is long, generally more than 5 meters, and the circulation tube is not heated, and the tube is all liquid phase, so the circulation speed is also large, the heat transfer efficiency is high, and the crystal grains are not easy to scale. However, the upper portion of the heating tube bundle is easily worn and clogged.
The Levin evaporator is based on the principle of the Soviet Union PE in the 1950s. He lowered the heating chamber and placed a support section and a steady flow section between the heating chamber and the evaporation chamber so that it could be operated at a high liquid level (Fig. 2). In this way, a considerable liquid static pressure is established in the heating chamber to increase the boiling point of the solution, so that the solution is heated only in the heating chamber without heating (controlling the temperature is about 2 to 3 ° C below the boiling point), and thus will not There is grain precipitation. When the solution ascends, the pressure is lowered and the boiling point is lowered through the support section and the steady flow section to the evaporation chamber, that is, the boiling evaporation and the crystal grains are precipitated, thereby avoiding scaling on the heating pipe wall. Since the solution circulation speed is as high as 2 m/sec or more, a steady flow section must be provided at the outlet above the heating chamber to disperse the liquid flow to avoid "water hammer". The Levin evaporator is mainly used to evaporate a solution in which crystal grains are precipitated.
Forced Circulating Evaporator This is an evaporator that uses a pump to force a solution to circulate (Figure 3). Most of the circulation pumps are external, but they are also built-in. In the heating chamber outside the circulation pump, the solution flows from bottom to top; in the heating chamber built into the circulation pump, the solution flows from top to bottom and then passes through the annulus between the heating chamber and the wall. Up, through the guide diaphragm behind the pump, introduce a circulation pump and circulate downward. Below the heated tube bundle, there is also a guide baffle to make the flow uniform and reduce drag. The purpose of using forced circulation is to: 1 enhance heat transfer. For evaporation under small temperature difference conditions, low-energy steam with a boiling point of 3 to 5 °C is also available; and the evaporation temperature difference of natural circulation is generally above 7 to 10 °C. 2 reduce scaling. The solution is at a high speed of about 3 m/sec, in which small crystal grains are difficult to deposit, and the solution does not actually boil in the heating tube, so the small crystal grains do not grow.
Riser Evaporator This evaporator heating chamber has a plurality of bundles of vertical tubes (Fig. 4). The commonly used heating tube has a diameter of 25 to 50 mm, and the ratio of the length of the tube to the diameter of the tube is about 100 to 150. After the solution is preheated, it is introduced in the lower part of the heating chamber. During the process of ascending in the tube, the steam between the tubes is heated and rapidly boiled to form a large amount of bubbles. The bubble rises with the solution and forms a high-speed flow on the tube wall. Liquid film. In order to form a film effectively, the rate of rising steam should be maintained above a certain value. For example, a suitable outlet steam speed under normal pressure is generally 20 to 50 m/sec, and the steam velocity under reduced pressure will be higher. The vapor stream with a large amount of liquid foam enters the evaporation chamber, and the separated concentrated liquid is mixed with the liquid droplets separated from the separation chamber, and is discharged as a product from the lower end. The rising film evaporator is characterized by: high heat transfer coefficient, high heat transfer efficiency, short residence time on the heating surface, suitable for evaporation of heat-sensitive solution with moderate viscosity and concentration, foaming tendency; A concentrated solution which will crystallize in the boiling zone is not suitable.
Falling Film Evaporator The upper end of the evaporator heating tube is equipped with a liquid distributor with a spiral groove (Fig. 5). The solution is added from the upper part, and is scattered by the groove of the distributor in the spiral wall to form a liquid film, which flows down by gravity and is evaporated by heat. Since the pressure in the evaporation chamber is lower than the heating chamber, the steam in the heating tube is also sucked downward by the evaporation chamber (in the same direction as the liquid film), and is accelerated. The residence time of the solution in the heating tube is similar to that of the rising film type, and the use of this type of evaporator is similar to that of the rising film type.
Ascending and falling film evaporators This type of evaporator assembles the ascending and falling films in a single casing. The preheated solution is first raised by the rising film heating chamber, and then lowered by the falling film heating chamber, and then separated from the secondary steam in the separation chamber to obtain a concentrated liquid. This type of evaporator is mostly used in applications where the viscosity of the solution changes greatly, the amount of water evaporation in the solution is not large, and the height of the plant is limited.
Stirred Membrane Evaporator This type of evaporator has a mechanical agitation wiper. The mechanical stirring membrane sweeping device is a rotor composed of a rotating shaft and a blade. When working, the blade can apply the solution on the heat transfer surface in a film shape. Since the rotor is rotated, the film is frequently renewed to enhance heat transfer. The blades have many forms, such as flat plates, spirals and cylindrical shapes; the position of the blades on the rotor can be fixed or floating; the radial clearance between the rotor and the casing is also fixed. Reconcile the self-balancing zero gap to suit the needs of different viscosity solutions. The liquid is added in the tangential direction from the upper part of the evaporator. Under the gravity and the rotating blade scraping belt, the solution forms a film that rotates downward on the inner wall of the casing, and is continuously evaporated by heat to obtain a concentrated liquid at the bottom. Stirred membrane evaporators are primarily used to treat heat sensitive, high viscosity solutions that have a tendency to foam and scale.
Centrifugal Membrane Evaporator This is a high-efficiency evaporator that combines "centrifugal separation" and "thin film evaporation" (Figure 6). It utilizes the centrifugal force of the high-speed drum to form a very thin, continuous, high-speed liquid film layer on the heat transfer surface, thus having a high heat transfer coefficient, capable of rapid evaporation and efficient separation of secondary steam. Its basic components are the outer casing, the drum and the transmission. The drum is a rotary shell, and the core of the centrifugal membrane evaporator is equipped with a conical disc, which functions as heat transfer and separation. The conical dish is hollow, and the heated steam is taken inside. A plurality of conical discs are stacked in the drum, and the space between the discs is taken away. When the drum rotates at a high speed, the solution is attached to and distributed on the lower surface of the conical disc by the action of centrifugal force as a liquid film, where it is vaporized by heat and vapor-liquid separation is performed under the action of centrifugal force. At the same time, in the cavity of the conical disc, a condensation of steam and a centrifugal separation of the condensate are performed. Therefore, on both the inner and outer sides of the cone plate under the conical disc, there is phase change heat transfer, and their heat transfer coefficient is large, so the heat transfer coefficient is also large. Due to the high-speed rotation of the drum, the liquid film on the heat transfer surface is extremely thin, only 0.05 to 0.1 mm; the film flows at a high speed, and the residence time of the solution on the heat transfer surface is extremely short, usually not more than 1 second. Therefore, this type of evaporator is suitable for the treatment of highly heat sensitive solutions, especially for the purification and concentration of foods and drugs.
Immersion Combustion Evaporator This is a direct contact evaporator where the heat transfer medium is in direct contact with the treated solution for heat transfer (Figure 7). After the fuel (such as gas or oil) is mixed with air, it is burned in a combustion chamber immersed in the solution, and the generated high-temperature flame and flue gas are directly injected into the treated solution through the nozzle at the lower part of the combustion chamber, and the bubble is upwardly passed through the liquid. The layer is such that some of the solvent is rapidly vaporized. The generated steam is discharged from the upper outlet pipe of the evaporator together with the combustion gas. The evaporator has a simple structure and no fixed heat transfer surface, and is particularly suitable for evaporation of crystallization, scale and corrosive solution, and has good heat transfer effect and high heat utilization rate; but for a solution that is not allowed to be contaminated by smoke gas. It is not appropriate to use this type of evaporator. It has been widely used in the evaporation of spent acid, ammonium sulfate solution, arsenic acid, hydrochloric acid or clay mud and treatment.
Measures to save heating steam The evaporation device needs to consume a large amount of heating steam during operation. In order to save heating steam, a multi-effect evaporation device and a steam recompression evaporator can be used.
The multi-effect evaporation device connects several evaporators in series to form a multi-stage evaporation device system. The secondary steam from the evaporation chamber of the first-stage evaporator enters the heating chamber of the latter-stage evaporator and serves as a primary steam. . In such a system, each stage of the evaporator, which constitutes an "effect", such as a three-effect evaporator, is a three-stage evaporator system in which three evaporators are connected in series. In such a system, the evaporation pressure of each stage is Unequal, from front to back, the pressure is getting lower and lower (the vacuum is getting higher and higher), and accordingly the evaporation temperature is getting lower and lower. Since the secondary steam generated by each effect (except the final effect) is used as the next effective heating steam, this improves the utilization of steam. According to experience, the amount of fresh steam (kg) required to evaporate 1 kg of water is: single effect 1.1, two effects 0.57, three effects 0.4, four effects 0.3, five effects 0.27. In fact, the multi-effect evaporation device does not use too much effect, which is limited by the effective total temperature difference. For an electrolyte solution such as an aqueous caustic soda, it is usually two to three effects due to a large increase in boiling point; for a non-electrolyte solution such as an aqueous solution of sugar, the effect may be four to six effects.
The steam recompression evaporator passes the secondary steam from the evaporation chamber through a mechanical compressor (such as a turbo compressor) or a jet pump to increase the pressure and the corresponding saturation temperature before entering the heating chamber of the evaporator as a Steam is used to increase steam utilization. It is suitable for the concentration of solutions with low boiling point lift.
Development trend Since the evaporator itself is essentially a heat exchanger, with the development of the heat exchanger, the research on the evaporator has made some progress, mainly: 1 Some new heating chamber structures have emerged, such as A plate evaporator for treating various materials sensitive to high temperature; experimentally studied a plate-fin evaporator; proposed an evaporator with inner and outer fin heating tubes; developed a special application for concentrated syrup Shaft tube evaporators, etc. 2 Enhance the heat transfer performance of the original heat exchange area. Research and trial production of a copper fiber-like surface covering on the copper tube of the evaporator to form a high heat flow tube evaporator. 3 In order to solve the special conditions of corrosion resistance and high temperature, a circulating evaporator made of borosilicate glass with high SiO2 content appeared in the hydrochloric acid production device; in order to withstand high temperature, an evaporation element of metal-ceramic material was proposed. It can be used in vacuum equipment during metallization. 4 In terms of energy saving and new energy use, not only the steam recompression evaporator proposed based on the principle of heat pump has been popularized, but also a high efficiency solar evaporator appeared in the 1970s. 5 In the suppression and prevention of fouling in the evaporator, many scholars have done some research work, proposed to add seed crystals in the solution, add polyphosphate mixture and ultrasonic treatment.
The type evaporator is divided into three types according to the operating pressure: normal pressure, pressurization and decompression. According to the movement condition of the solution in the evaporator: 1 cycle type: the boiling solution passes through the heating surface multiple times in the heating chamber, such as central circulation tube type, hanging basket type, external heat type, column type and forced circulation type, etc. . 2 Single-pass type: The boiling solution passes through the heating surface once in the heating chamber, and does not circulate, that is, the concentrated liquid is discharged, such as a rising film type, a falling film type, a rising-falling film type, a stirring film type, and a centrifugal film type. 3 direct contact type: the heating medium is in direct contact with the solution for heat transfer, such as immersion combustion evaporator.
The central circulation tube evaporator consists of a heating chamber, an evaporation chamber and a separation chamber (Fig. 1). The heating chamber is actually a heat exchanger, and the solution in the tube is heated by steam between the tubes (heated steam, also called primary steam or fresh steam) to reach boiling and rise into the evaporation chamber. The solution pressure is lowered and the solvent is vaporized in a large amount to form steam (referred to as secondary steam). The unvaporized solution enters the large-diameter central circulation pipe. Due to the density difference between the vapor-liquid two-phase flow in the surrounding small-diameter heating pipe, a directional circulating flow is formed, which is transferred to the surrounding heating pipe. It is heated to evaporate and the concentrate is discharged from the bottom discharge valve. The evaporation chamber has a certain height (called a separation space), so that large droplets in the steam naturally fall, and preliminary separation is performed, and the separation space is generally about 800 mm. The initially separated secondary vapor enters the separation chamber to further separate the fine droplets, reducing mechanical entrainment losses. During the evaporation process, it should be noted that: 1 In the heating chamber, the boiling point of the solution increases with the concentration of the solution, and varies with the height of the position. The higher the liquid column above the position, the more static The higher the pressure, the higher the boiling point. Thus, the effective temperature difference between the heated steam and the solution is smaller. 2 In order to maintain a high heat transfer coefficient, the wall of the solution side should be clean, free of dirt and regular cleaning; the heating steam space outside the tube, especially at certain dead angles, should be provided with non-condensable gas emissions. Mouth, so as to avoid the accumulation of gas and affect heat transfer. This type of evaporator is also known as a standard evaporator. The utility model has the advantages of improved circulation of the solution, improved heat transfer efficiency, reduced scale formation, simple structure and convenient manufacture. This type of evaporator is commonly used in the sugar industry.
Suspension basket evaporator The heating chamber of the evaporator is a separate basket-type member that can be hung (or supported) on the evaporator wall, hence the name "suspension basket". The basket type evaporator is also a commonly used circulation type evaporator. The heating chamber has only a vertical tube bundle without a center tube, and the annular cross-sectional space between the heating chamber and the wall is used as a circulation loop of the solution. Since the cross-sectional area of ​​the annulus flow channel is large, the circulation of the solution is good, and the heat transfer efficiency is high; and one end of the heating chamber can be freely expanded to avoid the temperature difference stress between the tube and the tube sheet. Because of the ease of replacement of the heating chamber, basket-type evaporators are commonly used in the caustic soda industry.
External heat evaporator This evaporator heating chamber is placed outside the evaporation chamber. This not only reduces the height of the entire evaporator, but also facilitates cleaning and replacement. The heating tube bundle is long, generally more than 5 meters, and the circulation tube is not heated, and the tube is all liquid phase, so the circulation speed is also large, the heat transfer efficiency is high, and the crystal grains are not easy to scale. However, the upper portion of the heating tube bundle is easily worn and clogged.
The Levin evaporator is based on the principle of the Soviet Union PE in the 1950s. He lowered the heating chamber and placed a support section and a steady flow section between the heating chamber and the evaporation chamber so that it could be operated at a high liquid level (Fig. 2). In this way, a considerable liquid static pressure is established in the heating chamber to increase the boiling point of the solution, so that the solution is heated only in the heating chamber without heating (controlling the temperature is about 2 to 3 ° C below the boiling point), and thus will not There is grain precipitation. When the solution ascends, the pressure is lowered and the boiling point is lowered through the support section and the steady flow section to the evaporation chamber, that is, the boiling evaporation and the crystal grains are precipitated, thereby avoiding scaling on the heating pipe wall. Since the solution circulation speed is as high as 2 m/sec or more, a steady flow section must be provided at the outlet above the heating chamber to disperse the liquid flow to avoid "water hammer". The Levin evaporator is mainly used to evaporate a solution in which crystal grains are precipitated.
Forced Circulating Evaporator This is an evaporator that uses a pump to force a solution to circulate (Figure 3). Most of the circulation pumps are external, but they are also built-in. In the heating chamber outside the circulation pump, the solution flows from bottom to top; in the heating chamber built into the circulation pump, the solution flows from top to bottom and then passes through the annulus between the heating chamber and the wall. Up, through the guide diaphragm behind the pump, introduce a circulation pump and circulate downward. Below the heated tube bundle, there is also a guide baffle to make the flow uniform and reduce drag. The purpose of using forced circulation is to: 1 enhance heat transfer. For evaporation under small temperature difference conditions, low-energy steam with a boiling point of 3 to 5 °C is also available; and the evaporation temperature difference of natural circulation is generally above 7 to 10 °C. 2 reduce scaling. The solution is at a high speed of about 3 m/sec, in which small crystal grains are difficult to deposit, and the solution does not actually boil in the heating tube, so the small crystal grains do not grow.
Riser Evaporator This evaporator heating chamber has a plurality of bundles of vertical tubes (Fig. 4). The commonly used heating tube has a diameter of 25 to 50 mm, and the ratio of the length of the tube to the diameter of the tube is about 100 to 150. After the solution is preheated, it is introduced in the lower part of the heating chamber. During the process of ascending in the tube, the steam between the tubes is heated and rapidly boiled to form a large amount of bubbles. The bubble rises with the solution and forms a high-speed flow on the tube wall. Liquid film. In order to form a film effectively, the rate of rising steam should be maintained above a certain value. For example, a suitable outlet steam speed under normal pressure is generally 20 to 50 m/sec, and the steam velocity under reduced pressure will be higher. The vapor stream with a large amount of liquid foam enters the evaporation chamber, and the separated concentrated liquid is mixed with the liquid droplets separated from the separation chamber, and is discharged as a product from the lower end. The rising film evaporator is characterized by: high heat transfer coefficient, high heat transfer efficiency, short residence time on the heating surface, suitable for evaporation of heat-sensitive solution with moderate viscosity and concentration, foaming tendency; A concentrated solution which will crystallize in the boiling zone is not suitable.
Falling Film Evaporator The upper end of the evaporator heating tube is equipped with a liquid distributor with a spiral groove (Fig. 5). The solution is added from the upper part, and is scattered by the groove of the distributor in the spiral wall to form a liquid film, which flows down by gravity and is evaporated by heat. Since the pressure in the evaporation chamber is lower than the heating chamber, the steam in the heating tube is also sucked downward by the evaporation chamber (in the same direction as the liquid film), and is accelerated. The residence time of the solution in the heating tube is similar to that of the rising film type, and the use of this type of evaporator is similar to that of the rising film type.
Ascending and falling film evaporators This type of evaporator assembles the ascending and falling films in a single casing. The preheated solution is first raised by the rising film heating chamber, and then lowered by the falling film heating chamber, and then separated from the secondary steam in the separation chamber to obtain a concentrated liquid. This type of evaporator is mostly used in applications where the viscosity of the solution changes greatly, the amount of water evaporation in the solution is not large, and the height of the plant is limited.
Stirred Membrane Evaporator This type of evaporator has a mechanical agitation wiper. The mechanical stirring membrane sweeping device is a rotor composed of a rotating shaft and a blade. When working, the blade can apply the solution on the heat transfer surface in a film shape. Since the rotor is rotated, the film is frequently renewed to enhance heat transfer. The blades have many forms, such as flat plates, spirals and cylindrical shapes; the position of the blades on the rotor can be fixed or floating; the radial clearance between the rotor and the casing is also fixed. Reconcile the self-balancing zero gap to suit the needs of different viscosity solutions. The liquid is added in the tangential direction from the upper part of the evaporator. Under the gravity and the rotating blade scraping belt, the solution forms a film that rotates downward on the inner wall of the casing, and is continuously evaporated by heat to obtain a concentrated liquid at the bottom. Stirred membrane evaporators are primarily used to treat heat sensitive, high viscosity solutions that have a tendency to foam and scale.
Centrifugal Membrane Evaporator This is a high-efficiency evaporator that combines "centrifugal separation" and "thin film evaporation" (Figure 6). It utilizes the centrifugal force of the high-speed drum to form a very thin, continuous, high-speed liquid film layer on the heat transfer surface, thus having a high heat transfer coefficient, capable of rapid evaporation and efficient separation of secondary steam. Its basic components are the outer casing, the drum and the transmission. The drum is a rotary shell, and the core of the centrifugal membrane evaporator is equipped with a conical disc, which functions as heat transfer and separation. The conical dish is hollow, and the heated steam is taken inside. A plurality of conical discs are stacked in the drum, and the space between the discs is taken away. When the drum rotates at a high speed, the solution is attached to and distributed on the lower surface of the conical disc by the action of centrifugal force as a liquid film, where it is vaporized by heat and vapor-liquid separation is performed under the action of centrifugal force. At the same time, in the cavity of the conical disc, a condensation of steam and a centrifugal separation of the condensate are performed. Therefore, on both the inner and outer sides of the cone plate under the conical disc, there is phase change heat transfer, and their heat transfer coefficient is large, so the heat transfer coefficient is also large. Due to the high-speed rotation of the drum, the liquid film on the heat transfer surface is extremely thin, only 0.05 to 0.1 mm; the film flows at a high speed, and the residence time of the solution on the heat transfer surface is extremely short, usually not more than 1 second. Therefore, this type of evaporator is suitable for the treatment of highly heat sensitive solutions, especially for the purification and concentration of foods and drugs.
Immersion Combustion Evaporator This is a direct contact evaporator where the heat transfer medium is in direct contact with the treated solution for heat transfer (Figure 7). After the fuel (such as gas or oil) is mixed with air, it is burned in a combustion chamber immersed in the solution, and the generated high-temperature flame and flue gas are directly injected into the treated solution through the nozzle at the lower part of the combustion chamber, and the bubble is upwardly passed through the liquid. The layer is such that some of the solvent is rapidly vaporized. The generated steam is discharged from the upper outlet pipe of the evaporator together with the combustion gas. The evaporator has a simple structure and no fixed heat transfer surface, and is particularly suitable for evaporation of crystallization, scale and corrosive solution, and has good heat transfer effect and high heat utilization rate; but for a solution that is not allowed to be contaminated by smoke gas. It is not appropriate to use this type of evaporator. It has been widely used in the evaporation of spent acid, ammonium sulfate solution, arsenic acid, hydrochloric acid or clay mud and treatment.
Measures to save heating steam The evaporation device needs to consume a large amount of heating steam during operation. In order to save heating steam, a multi-effect evaporation device and a steam recompression evaporator can be used.
The multi-effect evaporation device connects several evaporators in series to form a multi-stage evaporation device system. The secondary steam from the evaporation chamber of the first-stage evaporator enters the heating chamber of the latter-stage evaporator and serves as a primary steam. . In such a system, each stage of the evaporator, which constitutes an "effect", such as a three-effect evaporator, is a three-stage evaporator system in which three evaporators are connected in series. In such a system, the evaporation pressure of each stage is Unequal, from front to back, the pressure is getting lower and lower (the vacuum is getting higher and higher), and accordingly the evaporation temperature is getting lower and lower. Since the secondary steam generated by each effect (except the final effect) is used as the next effective heating steam, this improves the utilization of steam. According to experience, the amount of fresh steam (kg) required to evaporate 1 kg of water is: single effect 1.1, two effects 0.57, three effects 0.4, four effects 0.3, five effects 0.27. In fact, the multi-effect evaporation device does not use too much effect, which is limited by the effective total temperature difference. For an electrolyte solution such as an aqueous caustic soda, it is usually two to three effects due to a large increase in boiling point; for a non-electrolyte solution such as an aqueous solution of sugar, the effect may be four to six effects.
The steam recompression evaporator passes the secondary steam from the evaporation chamber through a mechanical compressor (such as a turbo compressor) or a jet pump to increase the pressure and the corresponding saturation temperature before entering the heating chamber of the evaporator as a Steam is used to increase steam utilization. It is suitable for the concentration of solutions with low boiling point lift.
Development trend Since the evaporator itself is essentially a heat exchanger, with the development of the heat exchanger, the research on the evaporator has made some progress, mainly: 1 Some new heating chamber structures have emerged, such as A plate evaporator for treating various materials sensitive to high temperature; experimentally studied a plate-fin evaporator; proposed an evaporator with inner and outer fin heating tubes; developed a special application for concentrated syrup Shaft tube evaporators, etc. 2 Enhance the heat transfer performance of the original heat exchange area. Research and trial production of a copper fiber-like surface covering on the copper tube of the evaporator to form a high heat flow tube evaporator. 3 In order to solve the special conditions of corrosion resistance and high temperature, a circulating evaporator made of borosilicate glass with high SiO2 content appeared in the hydrochloric acid production device; in order to withstand high temperature, an evaporation element of metal-ceramic material was proposed. It can be used in vacuum equipment during metallization. 4 In terms of energy saving and new energy use, not only the steam recompression evaporator proposed based on the principle of heat pump has been popularized, but also a high efficiency solar evaporator appeared in the 1970s. 5 In the suppression and prevention of fouling in the evaporator, many scholars have done some research work, proposed to add seed crystals in the solution, add polyphosphate mixture and ultrasonic treatment.
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