On the optimization of the start-up plan for liquid chlorine purification 【I’ve just graduated and have been working for a year; I don’t understand many things yet】
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Let’s get straight to the point. The purification of liquid chlorine, which contains CO2, O2, N2, and other small amounts of non-condensable gases, is carried out using a pressurized packed tower (at 11 bar) and a vertical thermosyphon reboiler (heated with hot water at around 58°C). After driving again following the last emergency stop, as the liquid level in the tower bottom increased (the normal level is 50), the temperature of the tower bottom also rose. The set temperature should have been 40°C, but it rose to 41.5°C before dropping back to around 40°C. So, what are some ways to carefully control the temperature so that it doesn’t exceed 40°C? I have been working for just one year since graduating from chemical engineering, and I haven’t encountered many issues yet. I hope those with more experience will be kind enough to offer their guidance. Thank you!
Single-point temperature control (that is, direct control of the reboiler heat input based on the temperature at the bottom of the tower) is very unstable, as heat transfer is a process that is generally lagging; therefore, temperatures experience certain fluctuations after startup or shutdown； Cascade control can be employed: the temperature at the bottom of the tower is primarily controlled by the heat source in the reboiler, with an additional secondary circuit that uses the flow rate of the heat source to further regulate it
You also mentioned that there might be some non-condensable gases inside. Therefore, during the initial startup phase of the system, fluctuations in tower pressure may occur due to the amount of non-condensable gases present, which in turn can affect the temperature at the bottom of the tower. Hence, during the startup phase, it is necessary to carry out a full cycle for this tower in order to eliminate the problem related to non-condensable gases. Additionally, if similar issues persist even after that, it might be necessary to check whether there have been any changes in the parameters of your hot water system (such as flow rate and temperature). The above are some points shared for your reference
Thank you for your reply. Do you mean that the bottom of the tower temperature is controlled in cascade with the hot water valve? So, what exactly is it about controlling the flow rate of the heat source to regulate the heat source itself? Today I went to the control room to check the records. The temperature of the tower bottom increased in two stages: from 15°C to 31°C, after which it remained stable for about 20 minutes. During that time, the hot water valve was opened wider, but the temperature stayed at 31°C. When the hot water flow was approaching its maximum value of 23 t/h, the temperature started to rise again from 31°C; by the time it reached 41°C, the hot water flow had dropped back to around 14.5 t/h. In simple terms, what happens in this situation is that the temperature at the bottom of the tower first rises, then stays constant or even drops slightly; as a result, the hot water valve is opened wider, and eventually the temperature at the bottom of the tower rises sharply, exceeding the set value (there’s a similar post at https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=1376699&page=1). So at 31°C, how should the hot water valve be adjusted to allow the temperature to rise gradually? Is there a way to calculate it? Thank you.
Thank you for the answer! Do you mean by full circulation complete reflux at the top of the tower with no product taken out from the bottom of the tower?
In the initial operating state, in order to achieve discharge, the entire system must first reach stability (in terms of temperature and pressure); otherwise it may affect the downstream processes. In your example, since the feed is continuous (the feed rate is controlled by the liquid level), the heat source at the bottom of the tower is increased gradually to increase the amount of vaporization. Once a liquid level appears in the buffer tank at the top of the tower, the pump can be activated to bring the entire system to a stable state. At that point, discharge can then take place from the bottom of the tower according to the operational procedures