Silent oil-free vacuum pump involves a large number of vacuum separation processes in the production process of pharmaceutical and chemical industry, such as vacuum coarse recovery or low vacuum recovery of organic solvents, high vacuum distillation of high boiling point heat sensitive materials, vacuum filtration to remove solid impurities, true air drying of powdered intermediates or products, etc. vacuum acquisition equipment vacuum pump is used. The reasonable selection and configuration of vacuum pump is of great significance to the optimization of vacuum separation process. With the promotion and implementation of GMP, ISO14001 environmental management system and OHSAS18001 occupational health and safety management system, efficiency, energy conservation, consumption reduction, safety and environmental protection have become the main theme in the production of pharmaceutical and chemical industry. Accordingly, the selection and configuration of equipment in the field of pharmaceutical and chemical industry are also facing new and higher requirements. This paper focuses on the practical application of oil-free vacuum pump in clean production of pharmaceutical and chemical industry.
1、 Comparison with other commonly used vacuum pumps
In the past, we used a large number of water ring pumps, water flushing pumps, slide valve pumps, etc. as the front-stage vacuum pumps. Last year, after investigation, we adopted WLW type vertical oil-free vacuum pump. After use, we believe that the pump has obvious advantages in its principle and performance, especially suitable for the requirements of pharmaceutical and chemical cleaner production.
Water flushing pump is a vacuum generated by pumping gas when the water in the pool is sprayed to the variable diameter nozzle through the pump at high speed. Its advantages are: ⑴ simple structure, low equipment cost, convenient operation and maintenance; ⑵ corrosive occasions are easy to solve as long as the materials are suitable. The basic disadvantages are: ⑴ due to the limitation of steam pressure of water itself, the limit vacuum degree is not high; ⑵ the limit vacuum degree is related to the water temperature. In order to maintain the vacuum degree, it is necessary to continuously add new water to cool the circulating water of the pool. At the same time, the pool also needs to have a certain volume and occupy a large area; (3) water may be poured back into the vacuum system; (4) in the pool, the organics sucked in by vacuum float on the surface of the pool, so its overflow water should be treated as waste water. The amount of waste water is large, and the volatile tail gas volatilizes from the water, causing serious waste gas pollution; (5) there is a lot of noise when there is no flow impact. To sum up, the water flushing pump does not meet the requirements of cleaner production, and we have phased it out.
Water ring pump is to put an appropriate amount of water into the pump body. When the eccentric impeller rotates at high speed, a crescent shaped water ring space is formed, and this space is divided into several small cavity spaces equal to the blades by the impeller. With the continuous change of the volume of the small cavity, the gas is constantly sucked in and discharged, so as to obtain a vacuum. The advantages of water ring pump are: ⑴ few moving parts, reliable operation, simple operation and wide use; ⑵ light structure and small land occupation; (3) the water ring has the function of supplementary condensation; (4) corrosive occasions are easy to solve. The disadvantages of water ring pump are: ⑴ similar to water flushing pump, due to the limitation of steam pressure, the limit vacuum is not high; (2) the water discharged from the water ring pump is used for wastewater treatment, and the amount of wastewater is large; ⑶ the energy utilization rate of the water ring pump is very poor, and the heat generated by the pump during operation is serious. If the water is recycled, it is necessary to use frozen brine to take away the heat generated by the pump; (4) water may be poured back into the vacuum system;
The structural principle of liquid ring pump is similar to that of water ring pump. The difference is that the circulating medium water is replaced by vacuum pump oil and other liquids with high boiling point. Its advantages are: (1) high limit vacuum; ⑵ the amount of waste water is small, and the cooling water can be recycled; (3) the low boiling point solvent can be recovered again through the exhaust gas condensation system. Its disadvantages are: ⑴ the liquid medium is easy to be polluted by the pumped materials, and there is the possibility of coking; ⑵ pump oil has the possibility of backflow; (3) low energy utilization rate
For slide valve pump, due to the reciprocating rotation of the outer circle of the slide valve attached to the cylinder wall driven by the eccentric wheel, the volume in the pump cavity is periodically expanded and reduced, so as to absorb and exhaust and obtain vacuum. The advantages of slide valve pump are: ⑴ high limit vacuum; ⑵ the amount of waste water is small, and the cooling water can be recycled. Its disadvantages are: ⑴ it has great limitations in use, and is not suitable for gases with high oxygen content, corrosive to ferrous metals, and chemical reaction to vacuum oil, nor for pumping from one container to another container as a delivery pump; ⑵ pump oil has the possibility of backflow;
The oil vacuum pump is the piston reciprocating in the cylinder to form the expansion and compression of the gas in the cylinder, and the suction and exhaust action is formed through the axially arranged intake and exhaust valves to obtain vacuum. The advantages of oil-free vacuum pump are: ⑴ high limit vacuum; ⑵ clean vacuum can be obtained without oil lubrication; (3) there is no waste water, and the cooling water can be recycled; (4) high energy utilization rate, obvious energy saving (50% of water ring pump), water saving and floor area saving; (5) due to the elimination of shear wave bad vibration, the vibration is small, and the noise is low after reasonable configuration; (6) stainless steel, titanium and other materials can be used in acid-base corrosive occasions; (7) the pure solvent can be recovered twice through the exhaust gas condensation system; (8) in case of sudden power failure or shutdown, the pump does not need to worry about the backflow of pump fluid, but also has a self sealing function, so that the vacuum system is not directly connected with the atmosphere, so as to ensure the safety of the vacuum system, which cannot be achieved by the above other forms of vacuum pump. For example, in the running high vacuum distillation system, the sudden entry of cold air will cause coking, combustion, explosion and other dangers of materials in the tower. Reliable safety devices are crucial. Its disadvantages are: ⑴ there are many moving parts, because the processing accuracy of the parts of the manufacturer we choose is not high, and the manufacturing quality is rough, resulting in a relatively high maintenance rate; ⑵ due to some deficiencies in the product structure design of the manufacturer, there are still many areas worthy of improvement. For example, the type of sealing surface, the cooling structure of the cylinder, the elimination of liquid accumulation in the pump cavity, the muffler, etc. are not very reasonable. As users, we hope to have more scientific research machinery and manufacturers to further study and improve this type of vacuum pump, because its excellent performance is just what modern chemical cleaner production needs. It has great promotion value.
2、 Application of silent oil-free vacuum pump in vacuum recovery
The configuration of silent oil-free vacuum pump and other related equipment in the system is directly related to whether the performance of oil-free vacuum pump can be effectively played. The above figure is a schematic diagram of a material decompression and recovery process applied in actual production. In the figure, the material vapor is basically condensed into liquid in the condenser, but under vacuum conditions, the movement speed of gas molecules is accelerated, the gas density is reduced, the condensation temperature of materials is reduced, and the total heat transfer coefficient of the condenser is doubled, so the material condensation (primary condensation) under vacuum is often not very sufficient, especially for low boiling point solvents. Part of the gas that has not been fully condensed is liquefied with the change of pressure in the vacuum pump, and the other part is condensed under normal pressure in the condenser on the exhaust side (secondary condensation). Since the oil-free vacuum pump has no vacuum medium (vacuum pump oil, water, etc.), the gas passing through the oil-free vacuum pump is not polluted by the vacuum medium. Therefore, the recovered solvent is clean. In this way, the condensing part under vacuum is converted to condensing under normal pressure, which greatly improves the recovery rate of solvent materials, especially for materials that are difficult to condense under vacuum. Through this process configuration, the discharged tail gas basically contains no organic waste gas. In addition, after the pump exhausts the gas and passes through the shell side of the tubular condenser, the noise is also greatly reduced.
