The raw materials used in the solvent crystallization process are usually filtered (solid-liquid separation), washed and vacuum dried after crystallization. The common vacuum dryer has a double cone dryer and a filter washing dryer (three in one). Solid-liquid separation process is different, material moisture content is different, moisture content (dry base) is usually 20% – 200%. The moisture content (dry base) of finished APIs after drying is generally less than 5% (except crystalline water). Solvents carried by wet materials are usually flammable, explosive, toxic and harmful hazardous chemicals, which are removed by vacuum pumps during drying. There are wet and dry vacuum pumps. Due to the limitation of using wet vacuum pump, the traditional wet vacuum drying system not only can not fully recover this part of the solvent, but also produces a large amount of waste water and exhaust gas. This not only creates cost pressure for chemical pharmaceutical enterprises, but also brings challenges to health, safety and environmental protection. The use of dry vacuum condensation system can realize the full recovery of solvents and zero discharge of wastewater and waste gas.
Characteristics of Diorganic Solvents and Condensation Ways
Organic solvents such as methanol, acetone, ethanol or other complex solvents are commonly used in the production of APIs. Their common characteristics are low boiling point and high saturated vapor pressure. For the drying process chain, there are usually two ways of condensation before and after the vacuum pump, also known as intake side condensation and exhaust side condensation, which need to cool the medium below the boiling point corresponding to the working pressure. The vapor pressure curves of several common organic solvents (see Figure 1). From the curve, it can be seen that the boiling point of the three solvents is between – 90 ~30 ~Pa in the limit pressure range (0.1~100 Pa) of drying process. Deep cooling or cold trap condensation must be used. At atmospheric pressure (<1.013*105Pa), the boiling points of the three solvents are between 56 ~78 ~C, and can be condensed only with circulating water. Comparing the two methods, the condensation recovery condition at the exhaust side is the simplest and the most economical.
Three-Wet Vacuum System
The liquid ring pump or Roots vacuum pump group are commonly used in wet vacuum system. Sealed coolant of liquid ring pump will dissolve or absorb part of organic solvent in operation. It is divided into waste gas and high concentration organic wastewater (or waste liquid) in gas-liquid separator. Both parts need to be treated. According to the operation principle of wet vacuum pump and the characteristics of organic solvents, there are many kinds of wet drying vacuum systems (see figure 2). The characteristics of each system are as follows:
3(1) Solvent type for pre-pump condensation
A condenser or a cold trap is added before the pump to recover some solvents. Advantages: Some solvents are recovered and the efficiency of vacuum system is improved. Disadvantage: It needs to be equipped with cryogenic or cold trap, which has high investment and operation cost.
3(2) Wastewater Discharge Type
There is no treatment before and after the pump, and the wastewater is directly discharged from the sewage treatment device or out-of-commission treatment. Disadvantage: sewage treatment
Large load, large water consumption and high cost.
3(3) Wastewater Recycling Type
Sealing fluid is cooled and recycled, and saturated waste water is treated regularly. Advantages: Low water consumption. Disadvantage: Cavitation and efficiency reduction will occur in liquid ring pump, and the cost of waste liquid treatment is high.
(4) After the pump, the solvent type is recovered to distill the waste liquid or recover the solvent by membrane separation, and the waste liquid is cooled and recycled. Advantages: Recovery of most solvents. Disadvantages: Recovery system covers a large area, has high energy consumption (especially distillation recovery), high investment and operation costs.
From the above analysis, we can see that although a variety of wet vacuum systems have been designed to recover solvents and reduce pollution emissions, the effect is not good. This is determined by the characteristics of the solvent and the wet vacuum pump itself. Only by using the dry vacuum pump system and the matching condensation recovery system can this problem be solved thoroughly.
Brief Introduction of Four-Dry Vacuum Pump
Dry vacuum pump is named for its working chamber, which does not require sealed coolant, as opposed to liquid ring pump. Dry vacuum pump can condense the exhaust gas at the exhaust side because it has no sealing fluid and the exhaust pressure is higher than atmospheric pressure.
Dry vacuum pumps include piston pump, Roots pump, claw pump and twin screw pump. Dry screw vacuum pump has the advantages of high efficiency, long life cycle and low cost, and has gradually become the most widely used dry vacuum pump.
The rotor of dry screw vacuum pump is a pair of screw with small clearance and no contact with each other. After changing speed through synchronous gear, the gas moves to the outlet. Its advantages are:
(1) It can operate at full speed under atmospheric pressure and extreme vacuum;
(2) The single stage can achieve a higher vacuum and the limit pressure can be less than 10 Pa.
(3) There is no oil or water contact process gas in the working chamber of the pump;
(4) The rotors are not in contact with each other, which reduces wear; the rotors can be coated with fluorine, nickel and other anti-corrosion coatings;
(5) The gas passage in the pump is short and can be exhausted quickly.
(6) It can be operated independently or as a front-stage pump.
Dry screw vacuum pump has two kinds: equal pitch and variable pitch, also known as external compression screw and internal compression screw. Compared with the equal pitch screw, the variable pitch screw pump has lower exhaust temperature and higher efficiency.
Design of Five-Dry Vacuum Condensation System
Dry screw vacuum pump and exhaust side condensation have been applied in pharmaceutical industry in recent years. Dry screw vacuum pump, vacuum pipeline and exhaust side condensation device can be defined as dry vacuum condensation system (see figure 4). Influenced by design, type selection and special process, there are some problems such as system mismatch, insufficient vacuum and unstable operation. How to optimize the design and configuration of the system is described with engineering cases.
The application of dry vacuum condensation system can realize the exhaust side condensation of organic solvents in the drying process, thus realizing the total recovery of organic solvents and zero discharge of wastewater and waste gas. Through the system optimization design, the system matches well, the maximum condensation load of the system is significantly reduced, and occupies less land. Dry vacuum condensation system can be used not only in new projects, but also in reforming the existing water ring vacuum system.