Operation control issues of distillation columns
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Hello everyone! I’m using a packed tower and operating at atmospheric pressure; I’ve conducted experiments for some time and have encountered many difficulties. Please help analyze them. 1. How to control the evaporation rate: Due to the excessive amount of gas at the top of the tower (with the heat exchange area fixed and the cooling water volume at its maximum), a large amount of gas is lost as it cannot be cooled sufficiently. My equipment can only control the temperature of the reboiler; it cannot adjust voltage or current. If the temperature of the reboiler is set (because only at such a temperature are the products at the top and bottom of the tower of acceptable quality), then isn’t the amount of evaporation fixed? 2. How to determine the amount of fluid accumulated inside the tower? 3. How is the pressure difference of the tower measured, and can its measurement be maintained throughout the testing process? How to achieve it? 4. How to test for leaks in a tower? 5. When conducting experiments, I mainly control the temperatures of the reboiler and the top of the tower. I’ve seen in other reports that factors such as the recovery rate at the top of the tower are mentioned as affecting the separation efficiency, but the temperatures of the reboiler and the top of the tower are not discussed. Question: How can the recovery rate be controlled? In my opinion, since the raw materials are known and the separation requirements are fixed, the recovery rate should also be constant. The key is to control the temperature well (of course, under the condition that other factors such as the reflux ratio meet the requirements). What are everyone’s thoughts? It seems like distillation is a simple task, but I keep failing to carry out the experiments properly; please give me more guidance! Thank you! This post was last edited by huazig on 2008-2-25 16:56.]
It may be because the experimental equipment is small, making control relatively difficult. For distillation operations, reference can be made to large-scale distillation processes.
1. With a constant composition, constant column pressure, and constant reboiler temperature, the evaporation rate should be constant. As you said, the temperature at the bottom of the tower remains constant and the product quality is satisfactory; however, insufficient heat exchange in the condenser at the top of the tower results in losses of product there. The problem lies in the condenser. Since the heat exchange area is fixed and the water flow rate is set at its maximum, it might be advisable to adjust the temperature difference further in order to lower the temperature of the condensate inlet. 2. Liquid accumulation in the tower – I’m not sure if you mean the amount of liquid accumulated on the tray or within the tower bottom ； 3. The simplest way should be to use a U-tube, right? ； 4. For testing the tower for leaks, soap water is sufficient ； 5. There are only a few parameters in total; by fixing two or three of them, the others are also determined. That parameter serves as the control indicator, and processes are prioritized based on their importance and ease of control. . That’s what I think; I’m not sure if it’s correct.
Regarding your first question, since none of your parameters can be changed, this is basically the only option. Personally, I think the condenser is the part that deserves to be modified the most; adding another condenser or lowering the temperature of the cooling water can increase the amount of condensation, which in turn increases the flow rate of the liquid being extracted or returned (it depends on which parameter you adjust)； Regarding the amount of evaporation, if you increase the feed rate and keep the temperature constant, the evaporation amount will also increase. I didn’t understand whether your distillation process is batch or continuous. The pressures at the top and bottom of the third tower are actually related to the temperature; therefore, temperature is also very important ； For the fourth leak test, full reflux can be used to conduct the test and then check whether the pressure remains stable. For the fifth recovery rate, in fact, if you conduct a simple material balance calculation, you will be able to determine where there is a reduction as a result of increasing the amount recovered. For example, if you maintain the feed rate and the bottom product discharge from the tower while reducing the reflux ratio, the yield can also increase. The above are all my immature opinions; please don’t take them seriously. Let’s keep communicating more.
Thank you. May I ask how to control the tower pressure at a constant level? I’ve never measured the tower pressure at all, and I don’t know if it’s constant or not. To test for leaks, does one have to fill the tower with nitrogen?
I’m operating continuously. Regarding the fourth point you mentioned: how does one operate it? Thank you