Multi-effect evaporation process is a combination of multiple evaporators after the evaporation operation process. Multi-effect evaporation requirements after the effect of the operating pressure and the boiling point of the solution are lower than the former effect, the introduction of the former effect of the secondary steam as the latter effect of the heating medium, that is, after the effect of the heating chamber to become the former effect of the condenser of the secondary steam, only the first effect of the need to consume raw steam. General multi-effect evaporation of the last effect or after a few effects are always operated under vacuum, due to the effects (except for the last effect) of the secondary steam as the next effect of the heating steam, so improve the utilization of raw steam, that is, the economy. It should be emphasized that the evaporation is proportional to the amount of heat transfer, multi-effect evaporation does not improve the evaporation, but only saves the heating steam, the cost of equipment investment increases. In the same operating conditions, the production capacity of multi-effect evaporator is not greater than the heat transfer area and one of the effect of single-effect evaporator production capacity. Particular attention should be paid to the view that the production capacity of the multi-effect evaporator is several times that of the single-effect evaporator is wrong.
According to the evaporator to add raw materials, can be divided into parallel flow feeding, countercurrent feeding and flat flow feeding three kinds of evaporation process. The following three effects as an example are introduced:
1. Parallel flow evaporation process
Parallel flow of three-effect evaporation process, the solution and the flow of the heating steam is the same, are from the first effect of the beginning of the order of the flow to the end of the third effect. Among them, there are two kinds of heating steam, the first effect is raw steam, that is, the steam generated by other steam generator, the second and third effect of steam is the secondary steam, the first effect of evaporation of steam is the second effect of evaporation of heating steam, the second effect of evaporation of secondary steam is the third effect of evaporation of the heating steam. The raw material liquid enters the first effect to be concentrated and discharged from the bottom, and enters the second effect and the third effect in turn, and is continuously concentrated in the second and third effects. The finished liquid is discharged from the bottom of the third effect.
Advantages of the parallel-flow charging method are the use of the pressure difference between the effect of transporting the liquid material; because of the former effect of the temperature and pressure is higher than the effect can not set up a preheater; less auxiliary equipment, compact process, temperature loss is small; easy to operate, stable process, equipment maintenance is small. The disadvantage is: after the effect of temperature reduction, the solution viscosity increases by effect, reducing the heat transfer coefficient, the need for a larger heat transfer area.
2. Countercurrent charging process
In the countercurrent charging process, liquid and steam to the opposite direction. Liquid from the end effect to join the evaporation and concentration, with a pump to send the concentrated liquid into the previous effect until the end of the effect, the completion of the liquid; raw steam from the first effect to join the exothermic condensation into a liquid, resulting in the second steam into the second effect, in the liquid heated to condense into a liquid, the second effect of the second steam into the third effect of the liquid heating of the raw material, the release of heat and condensation into a liquid discharged.
Countercurrent charging process, as the concentration of concentrated liquid increases and the temperature increases by effect, so the viscosity of the effect of the difference is small, the heat transfer coefficient is roughly the same; the completion of the liquid discharge temperature is higher, can be further flash concentration under reduced pressure. The disadvantages are: more auxiliary equipment, the need for pumps to transport raw material liquid; because of the effect of feeding below the boiling point, it must be set up preheater. Energy consumption is also a disadvantage. Countercurrent charging process is mainly used in the concentration of viscous liquids.
3. Plain stream charging process
In the plain stream evaporation process, the raw material liquid was added to the evaporator in each effect, the completion of the liquid from each effect respectively, the steam flow is from the first effect into the raw steam, the secondary steam into the second effect and release heat condensed into liquid, the second effect of the second steam into the third effect, the release of heat in the third effect condensed into liquid and discharged. This method is mainly used for viscosity, easy to crystallize the occasion, can also be used for two or more different liquids at the same time evaporation process.
Multi-effect evaporation process only in the first effect of the use of raw steam, so saving the need for raw steam, the effective use of the heat in the second steam, reduce production costs, improve economic efficiency. In actual production, but also according to the specific circumstances, the above basic process combination, designed to adapt to the production needs of the multi-effect process.
The principle of three-effect evaporation is: from one effect into the steam is used three times, so as to improve the heating efficiency of the steam is also thermal economy, the essence of multi-effect evaporation is to reduce the consumption of steam