Solvent recovery in vacuum drying process

The practical parameters give theoretical and empirical data and provide insights on the improvement and development of peripheral equipment for vacuum drying equipment. Vacuum drying and vacuum distillation have become a mature technology. However, there are still many practical scientific and technical problems. To be solved for powder materials, under vacuum, the heat and mass transfer mechanism in the porous powder body, between the powder body and between the powder body and the outside (drying room), including under vacuum conditions such as thermal conductivity, moisture vapor transmission coefficient, The determination of transmission parameters such as channel factors, the most economical and practical operating pressure cycle, the selection of operating temperature cycle parameters, and the degree of influence on the social environment are all areas of object nanotechnology that we must study in depth, especially considering ultra-fine The unique surface characteristics of the powder may affect the heat and mass transfer process. Outside of the powder material under vacuum conditions, the temperature distribution field inside the drying room space environment, the pressure field, the moisture distribution of the component velocity field, and the liquid-vapor phase transition process. The research is also very important, such as the ability to capture moisture in the condenser, gas conduction capacity, and vacuum Or the actual pumping speed relationship of the vacuum pump system, such as the actual drying process, generally achieves two purposes, ie, drying and extraction (distillation). For the former, the useful material is the material, and the latter, if the solvent is more expensive, needs to be recycled, that is, to obtain the solvent extraction. process. The recovery of the solvent is to condense the evaporated solvent gas into liquid, including the evaporation process and the condensation process. In view of the above problems, technically, it is expressed how to perform the vacuum system and the condensation system and parameters thereof in the vacuum drying process. An excellent match This article gives a brief introduction to the design and parameter selection of the condensing system and the vacuum system that are matched in this process (discussing the dry evaporation chamber separately) 1 General vacuum drying system components General vacuum drying system components The relevant parameters are set as follows.

Dryer: temperature Ta vaporized gas absolute partial pressure Pa condenser temperature K condenser internal gas absolute pressure Pb vacuum pump: inlet temperature Tc absolute pressure Pc block diagram is as follows: For a specific material at temperature T, when its saturated vapor partial pressure is Pv In the condenser, when Pv=Pb, the condensation speed=evaporation speed W/m2.K disadvantage is the large amount of cooling water consumed, the surface of the water pipe, in this case, the dynamic balance process, at this time there is continuous material condensation, and often In contact with air, it is easy to grow moss plants. Corrosion of cooling pipes has continuous secondary evaporation of materials. The critical state is shown below. The larger the material into the evaporation of the non-condensable gas out ° ° liquid I; condensing refrigerant out will cause changes in other indicators, to break this balance and establish a new balance, the condensation rate and evaporation rate will change very large Pc, Tb , Ta is the main parameter that determines this balance and change. The material is continuously evaporated in the desiccator and condensed in the condenser to achieve solvent volatilization. At the same time, the solvent recovery Pc is determined by the vacuum system. The boundary is determined by the refrigerant in the condenser. Ta is determined by the evaporator heating temperature required for the material.

To ensure that the material during the operation is not deteriorated, not oxidized, but also able to evaporate, select the parameter Ta, select the appropriate Pa for rapid evaporation, and then determine the size and form of the vacuum system according to the refrigerant system K. Theoretically, the refrigerant system Tb The lower the vacuum pump system Pc is, the higher the dryer heating temperature limit is and the higher the overall system efficiency is. However, in the actual industrial production process operation, this idealized state does not exist, and it is also uneconomical. In order to realize a simple theoretical index, it will inevitably increase equipment costs and increase operating costs.

Therefore, according to the process parameters, choosing the right equipment, including the dryer condenser, vacuum system, etc., is the key to the desiccant process and whether the recycling process can be industrialized. 2 Selection and design of condensers bookmark2 2.1 Selection of type in the drying system The condenser is a vacuum container, so to meet the strength requirements of the external pressure container, the leakage of the cylinder and each joint should meet the requirements of vacuum sealing. The condenser equipped with most of the drying system is the shell-and-tube condenser ( Generally water-cooled. There are three types of commonly used water-cooled condensers: Vertical shell-and-tube condensers Horizontal shell-and-tube condensers and spiral-plate condensers. 21.1 Vertical shell and tube condensers Cooling water can be evenly distributed to each The root pipe spirals down along the inner wall of the pipe to increase the heat transfer effect of the pipe. Its vertical installation, small footprint, direct passage of cooling water through the top to remove the scale and rust in the tube is convenient, can also be used for cooling water with poor water quality 2.1.2 horizontal shell and tube condenser construction and vertical condenser Similarly, the cylinder is placed horizontally. It has a compact structure and a good heat transfer effect. The cooling water flows through the heat transfer tubes along the various processes, increasing the flow rate of the cooling water in the tube, and has a large heat transfer coefficient. 900 ~ but less water consumption. The main drawback is that the cooling pipes are prone to corrosion, especially when the cooling water flow rate is large. In addition, it is also difficult to remove the dirt from the cooling water pipe. Therefore, the horizontal shell-and-tube condenser is mostly operated in a dirt-like state for a long time, which affects its heat transfer effect.

It is a new type of condenser. The cooling water makes a spiral movement at a higher speed, so the heat release coefficient is higher. According to the data, the experimental results show that the heat transfer coefficient of the spiral plate condenser is about 50% higher than the heat transfer coefficient of the horizontal shell-and-tube condenser, and the volume is smaller and the weight is lighter. Its main drawback is that the water side of the condenser is heavily rusted, and cleaning is difficult. At the same time, the flow resistance of the water is larger. At the same flow rate, it is about 4 to 5 times higher than that of the horizontal shell-and-tube condenser. At present, the condenser still belongs to the trial stage. In view of the advantages and disadvantages of the above three types of condensers, users can choose according to their own conditions, the purpose is to achieve the best cooling effect. 2.2 Design of the condenser 2.21 Determination of the heat transfer area Since many kinds of parameters, such as the type of temperature medium, the flow velocity and the structure of the medium, affect the heat transfer area, the determination of the heat transfer area is the most important. However, the calculation method in the traditional sense is very complicated, and the error will be very large in the calculation. If the area is large, the effect will be better, but the cost will be relatively high; if the heat transfer area is small, the cooling requirement cannot be met. Therefore, the heat exchange area of ​​the condenser equipped with a general vacuum drying system is selected according to the selected drying. The volume of the device is selected from 2 to 5 times. The following formula can also be used to roughly calculate the heat transfer area of ​​the A-condenser, the heat load of the m2Q condenser, and the heat transfer coefficient of the kW type condenser is relatively low. 900, lishingHouK—The heat transfer coefficient of the condenser, W/mCnki.net is not only the vacuum system but also the condensing system. The following results for the vacuum system's oil emulsification A result in contamination of the work ring. ,Maintenance and maintenance frequency bookmark3 ATm one by one average heat transfer temperature difference, KATmax one or two medium heat exchange at the inlet or outlet larger temperature difference, K ATmin- one or two medium heat exchange at the inlet or outlet is smaller The temperature difference, K Note: The heat load of the condenser can be calculated according to the set value of each parameter of the system and the characteristics of the material to be dried, refer to the relevant chemical manual 22.2 Determination of heat transfer temperature difference The inlet and outlet temperature difference of the horizontal shell-and-tube condenser AT = f6C; vertical shell and tube condenser inlet and outlet temperature difference AT = 2 ~ 4C. The above value is only the value under normal circumstances, heat transfer temperature difference is with the pressure change in the dryer and the pressure is high , Temperature difference 2 2.3 Determination of the medium flow rate In the shell-and-tube condenser, the water speed is generally 1.5 ~ 25m / s (take smaller copper pipe, take a larger number of steel pipe), brine generally take 1CkPa gas, generally taken Kh20m/s; 50) XltfkPa gas, generally take 3 ~ 5m / s, in order to reduce the fluid resistance 2.4 The determination of several major structural parameters Shell and tube condenser tubes are generally arranged in a regular triangle, tube center distance and diameter ratio Generally s/d is more than 1.25, but the edge of the tube hole The distance should not be less than 4mm, the distance between the edge of the outermost tube hole and the shell surface is not less than 3~5mm. The tube sheet and the heat transfer tube can be welded and expanded. When the welding method is adopted, the length of the tube tube protruding from the heat transfer tube is recommended. Value: When the pipe diameter 25mm, take.

The condenser used in the drying system is mostly single-tube or double-tube.

According to the heat transfer area, the number of tubes, the length of the tube, etc., the shape of the condenser can be determined. The design of other aspects of the condenser can refer to the relevant chemical equipment manual.

3 Selection of vacuum system The vacuum system is the power for drying. The greatest role of the vacuum system in the drying process is to form a vacuum and reduce the temperature required for the evaporation of the material solvent so that the required heating temperature can be reduced to achieve the purpose of drying. Theoretically speaking, the gas discharged from the vacuum system is non-condensable gas. The solvent evaporated in the dryer is completely condensed in the condenser. However, the actual situation is not so. The condensation process is an unbalanced dynamic process. There is always a partial introduction of some solvent vapors into the vacuum system, such as the choice of factors that affect the overall system vacuum (pressure).

The vacuum system excludes the source of the gas as non-condensable gas in the V1-solvent, LV2 gas leaking into the system one by one from the outside, LV3- gas that evaporates secondarily, LV4-water vapor that the condenser does not condense The volume of L water vaporized in a vacuum can be calculated by the following formula: (M/) RTV M-mass of a gas, molar mass of kg gas, kg/mol T-gas temperature, KP-gas residual pressure, Pa The solvent quickly reaches the evaporation point, must ensure that the vacuum system has a sufficient effective pumping rate, the formula is as follows C 1PP Kq-a correction factor, the value is generally between 1 and 4, related to the pressure P at the termination of pumping equipment , The pressure is large, the value is large, the suction time is 1-1, the pressure at which a device starts pumping, PaP-the ultimate pressure of a vacuum device, PaP-the pressure after pumping at t, Pa chooses the vacuum pump system The comprehensive effective pumping speed and limit pressure are indicated by t. When selecting the limit pressure, the lower the better, the test proves that the saturation pressure corresponding to the Tb of the condenser must be lower than the operating pressure of the vacuum system when Pb/Pc At 0.55 the condenser capture rate can reach a maximum When the operating pressure is reduced, the trapping rate of the condenser will not be much longer due to the change of the gas flow state anyway; on the other hand, the degree of vacuum is too high, which is beneficial to evaporation, and at the same time the secondary evaporation Addition, the recovery of the solvent is disadvantageous. The conventional vacuum pump system generally uses a steam jet pump, and its supporting equipment is complicated and energy-consuming, and gradually eliminated and then developed to use a reciprocating vacuum pump, a rotary vane vacuum pump, etc. The sealing medium used in the vacuum pump is a vacuum pump oil, which is easy to mix with the material vapor and has a high operating cost.

The water ring vacuum pump and Roots water ring unit developed in recent years can be used not only for general gas extraction, but also for suctioning gas containing moisture or a small amount of dust. Compared with general mechanical vacuum pumps, they are not afraid of oil pollution and are not afraid of it. Water vapor and dust, compared with the general water ring vacuum pump, has the characteristics of high vacuum degree and high pumping speed under high vacuum degree. At the same time, it can change the configuration method in the drying and distillation industry according to the vacuum degree of the process requirements. Has a wide range of applications, and achieved good results. The vacuum degree of this unit can be arbitrarily configured from 1330 to 2500 L/s. 4 The integrated configuration principle of the condensing system and the vacuum system selects or determines the recovery rate according to the recovery value of the solvent, and generally takes 50% to 95%, of course, when the system The main effect is distillation, solute is waste, recovery is of course the most important, the recovery rate must be high, can mainly consider the condensing system if the system and process mainly for drying, evaporants are water or other low-value substances, can not be recycled, however, At the same time, the temperature requirements of the vacuum pump system for the entire system must be considered. The necessary condensing measures must be taken to protect the vacuum pump system in order to increase the pumping efficiency.

In particular, there are environmental pollution of volatile substances, and more should be done to recover measures to protect our living space no longer suffer from pollution. This is the social factor that we must prioritize in the configuration of vacuum systems and condensing systems. 5 Concluding remarks The above is about vacuum drying. In the process, a brief introduction of the condensing system and the vacuum system required when the solvent needs to be recovered. Vacuum drying technology will surely make greater progress with the development of related industries, because drying involves the quality, properties, and different physical states of different types of products before and after drying. At present, the condensing system and the vacuum system have been used together in industries using vacuum drying such as pharmaceuticals and chemicals. With the development of society and advancement in science and technology, the integrated support of dryer condensers and vacuum pump systems in vacuum drying equipment will be highly valued by various industries considering both economic and social factors.

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