Lu Jiming

(Shandong Hongji Heat Exchange Technology Co., Ltd., Jinan 250022, China)

Abstract: Starting from the principle and technology of pervaporation technology, the characteristics of spiral threaded tube heat exchanger and its application in solvent dehydration system are expounded.

Key words: pervaporation; high efficiency heat exchanger; spiral threaded tube heat exchanger; dehydration; application

Dehydration or separation of organic matter by pervaporation technology is one of the most advanced liquid separation methods, and it is widely used in the recycling and reuse of solvents in pharmaceuticals, fine chemicals and other industries. The dehydration treatment of organic solvents by pervaporation technology is about 50% more energy efficient than the traditional distillation process. As a key equipment for the pervaporation unit, the efficiency of the heat exchanger is directly related to the separation effect and production efficiency. SECESPOL heat exchanger is a spiral threaded tube heat exchanger. It is one of the most advanced shell-and-tube heat exchangers in the world. It inherits the high temperature and pressure resistance of the tube heat exchanger, reliable operation and plate heat exchanger transmission. High thermal coefficient, small footprint, easy maintenance and many other advantages. In this paper, combined with the process principle of pervaporation technology, the application advantages of SECESPOL heat exchanger in this process point are analyzed.

1 Principle and technology of pervaporation technology

1.1 Principle of pervaporation technology

Pervaporation (Pervaporation, PV for short) is a novel membrane separation process that uses the difference in osmotic pressure difference of the components in the mixture to achieve separation of the mixture by the difference in dissolution and diffusion rates of the components in the membrane.

The pervaporation membrane separates the feed liquid and the permeate into two separate streams, the liquid side generally maintains a normal pressure, and the permeate side maintains a lower component partial pressure by vacuuming. The components of the feed liquid diffuse through the membrane and are vaporized into permeate vapor at the back side of the membrane, driven by the differential pressure of the components on both sides of the membrane. Due to the difference in physicochemical properties of the components in the feed liquid, there are differences in the thermodynamic properties (solubility) and kinetic properties (diffusion velocity) of the components in the membrane, so that the components in the feed liquid permeate through the membrane at different speeds and are easily permeable. The fraction of the component in the permeate vapor is increased and the concentration of the hardly permeable component in the feed liquid is increased.

1.2 Pervaporation technology

1.2.1 Pervaporation process

The membrane divides the membrane module into two chambers on the upstream side and the downstream side, the upstream side is a liquid phase chamber, the downstream side is a gas phase chamber, and the gas phase chamber is connected to a vacuum system. The aqueous liquid is heated to a certain temperature through a preheater and a heater, and then enters the liquid phase chamber. The membrane has a selective passage of water molecules in the liquid, and the water molecules are dissolved and adsorbed on the surface of the membrane, and the partial pressure difference of steam on both sides is Under the action, the diffusion is preferentially passed, and the outlet of the membrane module obtains a water-free product; the component permeating to the lower side through the polymer membrane vaporizes on the surface of the membrane because the partial pressure of steam is less than its saturated vapor pressure, and then enters the condenser and is condensed. Liquid permeation product, recyclable. The pervaporation process is shown in Figure 1.


1.2.2 Characteristics of pervaporation technology

Energy saving (low energy consumption, generally 1/2 to 2/3 energy saving than azeotropic distillation), clean (the process does not introduce other components, the product and environment will not be polluted), easy to enlarge, couple and integrate technology.

Pervaporation gasification is especially suitable for the separation of near-boiling and constant-boiling mixtures which are difficult to separate or cannot be separated by ordinary rectification, the removal of trace water in organic solvents and mixed solvents, the recovery of small amounts of organic matter in wastewater, and the separation of organic/organic matter. It has obvious economic and technical advantages, such as coupling with the reaction and continuously removing the reaction product.

2 Application of heat exchanger in membrane pervaporation process

Through the membrane pervaporation gasification process, it is known that the material before entering the liquid phase chamber needs to be preheated and heated to a certain temperature, and the material vaporized after passing through the membrane needs to be condensed and recovered. In a permeate gasification system, the material needs to pass through the permeable membrane multiple times, and the temperature of the material that does not pass through the permeable membrane will decrease, requiring heating through the heat exchanger again, and so on, until effective separation is achieved. Since the temperature of the permeate feed directly affects the permeation efficiency, the efficiency of the heat exchanger is directly related to the production efficiency of the permeate gasification system, so the choice of heat exchanger is very important.

2.1 Characteristics of SECESPOL heat exchanger

The SECESPOL heat exchanger, as shown in Figure 2, is a shell-and-tube spiral-threaded tube heat exchanger imported from Europe. The heat exchanger uses a unique spiral threaded tube as the heat transfer tube. The heat exchange efficiency is the traditional shell-and-tube type. More than three times the heat exchanger is the most suitable heat exchanger for the condensing system. Its characteristics:


(1) High efficiency. The unique spiral threaded tube is used, and the heat exchange tube is reversely wound. This structure greatly changes the turbulence effect of the fluid, and the fluid reheater forms a strong turbulence effect. The length of the heat exchange tube is 4-6 times of the length of the shell, the residence time of the fluid in the heat exchange tube is prolonged, the heat exchange is more sufficient, and the gaseous medium is sufficiently condensed. Under the best working conditions, the heat transfer coefficient can reach 14 000 W/(m·2 °C). Under normal working conditions, it is 5 to 7 times more efficient than conventional shell-and-tube heat exchangers.

(2) Good security. It is welded with high quality stainless steel, and the material has a uniform thermal expansion coefficient, which is not easy to leak. The overall welding is more secure than the gasket of the conventional heat exchanger and has a wider application range. The heat exchanger has a maximum temperature resistance of 400 °C.

(3) Small size. Under the same working conditions, the SECESPOL heat exchanger is only 1/10 of the volume of the traditional tube heat exchanger, and no special design platform is required, which greatly saves installation costs. It can be directly connected to the pipeline during installation, and it can be easily fixed.

(4) The tendency to scale is low. The application of CFD (Computer Fluid Dynamics) and FEM (Finite Element Technology) has increased the design flow rate up to 5.5 m/s, which is the main reason why the heat exchanger is not easily fouled. The unique heat exchange tube length is 4 to 6 times that of the shell, and the temperature gradient in the heat exchange process is low, which reduces the tendency to scale. With 100° connection, the heat exchangers all participate in heat exchange without leaving dead angles; the fluid automatically flushes the pipeline to reduce the tendency to scale.

(5) Long service life. Adopting the implementation of EU standards, using the Owen turbulent chattering frequency criterion, the minimum gap design of the heat exchange tube is used to effectively eliminate the turbulent chattering phenomenon of the heat exchanger and prolong the service life, and the design life is 40 years.

2.2 application examples

The following is an example of ethanol purification in a pharmaceutical factory in Shandong Province to illustrate the use of heat exchanger in pervaporation dehydration. The maximum treatment capacity of the pervaporation membrane unit for ethanol feedstock is 4 000 t/year, and the moisture content of the ethanol feedstock to be treated. Not more than 7%, the pH value is between 6 and 8. The customer requires that the moisture content of the treated product be ≯0.5%.

2.2.1 The heat exchanger is required for the three process points of the system.

(1) Preheater: Preheating of ethanol after purification by liquid phase requires osmosis of purified ethanol solution. The high temperature ethanol is about 88 ° C, the temperature after passing through the heat exchanger will be 45 ° C; the low temperature side ethanol is from normal temperature. Heat to 40 ° C. This process uses high-temperature ethanol to preheat the materials and make full use of the heat medium energy, which can effectively reduce energy consumption. A system with a treatment capacity of about 1.5 t/h requires a heat exchange area of ​​12 to 15 m2 for a conventional tube heat exchanger. After using a SECESPOL heat exchanger, only one heat exchanger with a heat exchange area of ​​2.1 m2 is required. Process requirements. The volume is only about 1/5 of the original heat exchanger.

(2) Heater: The preheated ethanol solution is heated to the temperature required for pervaporation by steam. The ethanol permeation gasification process needs to heat the material from 40 ° C to 90 ° C, and the heat medium is 0.25-0.3 kg saturated steam. . For a system with a processing capacity of about 1.5 t/h, a traditional tube-tube heat exchanger requires a heat exchange area of ​​12 to 15 m2, and a SECESPOL heat exchanger requires only one heat exchanger with a heat exchange area of ​​2.1 m2. Process requirements. The volume of the heat exchanger is only about 1/5 of the volume of the original heat exchanger. After one infiltration, the concentration of the ethanol solution is increased, and the temperature is slightly lowered. In order to ensure the effect of re-infiltration, it is required to be heated to 90 ° C by a heat exchanger, which requires 1.3 to 2 m 2 of the conventional tube heat exchanger. The SECESPOL heat exchanger is only 0.5 m2.

(3) Condenser: Ethanol water vapor passing through the permeable membrane needs to be condensed and recovered. The heat exchange efficiency of the heat exchanger directly affects the recovery rate of ethanol. Since the infiltration process requires high vacuum to maintain the pressure difference across the membrane, in the high vacuum State, if the heat exchanger is not efficient, it will inevitably result in material loss. In order to meet the condensation effect under high vacuum, traditional heat exchangers usually adopt a method of increasing the heat exchange area of ​​the heat exchanger, which not only increases equipment investment, but also increases the cost of infrastructure, installation and maintenance. In the ethanol purification process with a treatment capacity of 1.5 t/h, a heat exchanger of about 80 m2 is required at the process point. The SECESPOL heat exchanger requires only 21.6 m2 to meet the process requirements, and the heat exchanger volume is only 1/10 of the original heat exchanger volume.

2.2.2 Comparison of SECESPOL heat exchanger and tube heat exchanger

The pair using the SECESPOL heat exchanger and the tube heat exchanger is shown in Table 1. It can be seen from Table 1 that the heat exchange area required for the SECESPOL heat exchanger is only 1/4 of the original design heat exchange area, and the heat exchanger volume is only about 1/5 of the original heat exchanger volume.

After the SECESPOL heat exchanger, the osmosis device is more compact, which can save capital investment. More importantly, the SECESPOL heat exchanger can improve the solvent recovery rate in the condensation process. At the same time, due to the high heat exchange efficiency of SECESPOL, it can be The cold medium is raised to a higher temperature, the refrigerant consumption is reduced, and the operating cost of the equipment is reduced. The most critical heat exchanger efficiency can condense the solvent to a lower temperature, effectively reducing solvent loss and improving solvent recovery rate.

3 Conclusion

The use of pervaporation technology to dehydrate or separate organic matter, energy saving, environmental protection, easy operation, can replace traditional separation methods such as distillation, extraction, adsorption, etc., and can achieve separation requirements that are difficult or impossible to achieve in these conventional methods, in organic matter or various groups. It has obvious advantages in the removal of small or trace amounts of water in mixed organic matter, and has been widely used in the recycling and reuse of solvents in pharmaceutical, fine chemical and other industries. As pervaporation separation is widely used in pharmaceutical and chemical companies, SECESPOL heat exchangers will be widely used as high-efficiency, energy-saving and environmentally friendly products.


[references]

[1] Ma Xiaochi. New and efficient heat exchangers at home and abroad. Progress in Chemical Engineering, 2001

[2]SECESPOL heat exchanger design selection manual