Questions about circular evaporators
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Recently, I was learning about *processes in the workshop and came across multi-effect falling-film circular evaporators. I still have several questions regarding the purpose and significance of circular evaporators; there are quite a few of them, and my understanding might be a bit superficial. I would appreciate everyone’s guidance: 1. Why does some of the liquid after evaporation need to be recycled back to the same stage of evaporation, rather than going directly to the next stage? If the concentration required for this efficiency design is reached, should the cycle continue? 2. Taking the first effect heated by raw steam as an example, as the number of cycles increases, the concentration of the concentrate at the outlet of the gas-liquid separator rises as well, and its boiling point also increases. Is the evaporation process in such an evaporator still a steady-state process? Will the temperature of the steam required also increase? 3. Some liquids proceed to the next stage after evaporation, while others are recycled back to this stage; as a result, the number of times a liquid passes through the evaporator varies. How can it be ensured that the concentration of all the concentrated liquids resulting from evaporation meets the specified requirements?
Multi-effect evaporators have not been studied; here are my thoughts: 1. The purpose of multi-effect cyclic evaporation is to increase the circulation volume. The amount of heat required remains constant, and by increasing the circulation volume, the temperature at the outlet of the reboiler can be reduced. Therefore, it is necessary for most of the fluid to be returned to the same effect, with only a small portion being taken out. 2 is a steady-state process. 3. The part that returns is only used for the cycle and is no longer involved in mass transfer; therefore, the concentration of the extracted substance remains constant.
Hello, thank you for your reply. I know very little; I have two more questions regarding your reply. Why is it necessary to lower the outlet temperature of the reboiler? Isn’t the outlet temperature of the liquid mixture always the boiling point temperature at the corresponding pressure?
1. Some substances are heat-sensitive, so they must be handled at their thermal sensitivity temperature. 2. You have ignored the composition; increasing the circulation rate can raise the content of light components, which naturally lowers the outlet temperature.