6) Surface Cooling Crystallization
In general, continuous surface cooling crystallization is not preferred in modern processes. Because, supersaturation is created on the surface of a heat exchanger. Incrustations accumulated on the surface of heat exchanger decrease the production output proportionally.
Surface cooling crystallization can be preferred for batch processes as the incrustations are dissolved in the beginning of each batch.
Fig. 1. Principle of Multiple
Effect Evaporation
Fig. 2. Principle of Mechanical
Vapor Recompression (MVR)
Fig. 3. Principle of Thermal
Vapor Recompression (TVR)
3) Thermal Vapor Recompression (TVR)
Evaporated water vapor of the evaporator/crystallizer is reused in the main heater of the evaporator by increasing the pressure with a steam jet ejector. Approximately 50% of the evaporated water vapor from the first effect can be reused with TVR. Remaining 50% goes to main heater of the second effect evaporator or a condenser. So, by adding this simple steam ejector to the system, you increase the number of effects almost by one. If the cost of steam ejector and additional evaporator body is compared, it is advantageous to use it in evaporation / crystallization processes. Main advantages of TVR are as follows;
- Low investment cost
- Simple operation
- No moving parts, so no maintenance.
2) Mechanical Vapor Recompression (MVR)
If electrical power is available with low cost, then this technology can be advised. Evaporated water vapor of the crystallizer is reused in the main heater of the system by increasing the pressure of vapor with a radial turbo compressor or industrial blowers connected in series. Main advanteges of MVR are as follows;
- Since all evaporated vapor is reused by recompression, almost no steam is consumed. Only at start-up, small amount of steam is required.
- No cooling water is required.
- Low structural costs due to the compact design of the plant
- Easy operation and start-up
1) Multiple Effect Evaporation / Crystallization
If low pressure steam is available with low cost, then this technology can be advised. It is the classical process to produce crystal product or concentrate the feed solution. If the vapor from one evaporator is passed into the steam chest of the second evaporator, saving of energy can be achieved. It is known as multiple-effect evaporation. As many as six effects have been used in practice. Increasing the number of effects will decrease the live steam consumption. On the other hand, investment cost will increase. Optimum design depends on the relation between operation costs and investment costs. Preheating of the feed solution plays an important role in order to decrease specific steam consumption.