Troubles with the gas separation unit?
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Hello, dear colleagues. Our company has a plant with an annual production capacity of 40,000 tons. We have a problem that requires your assistance in solving: the pressure in the propylene distillation tower in our plant keeps fluctuating back and forth, which in turn causes fluctuations in the pressure in the reflux tank as well as in other temperatures. In severe cases, this even leads to interruptions in the cold reflux at the top of the tower. We hope you can take the time to help us identify the reasons for this issue and suggest appropriate solutions. Thank you so much! ! !
The main cause of pressure fluctuations in our system is significant variations in the composition of the materials, specifically large changes in the C2 content within the raw materials. This can be determined directly from the pressure changes in the raw material buffer tank (the pressure tends to be higher during hot summer weather). When there is an excess of C2, it can be handled by releasing gas through lines connected to the raw material buffer tank; however, such releases have a considerable impact on the yield of propylene. We usually release this gas in the reflux tanks of the propane/ethane removal towers, and only as a last resort do we release it from the raw material buffer tank. Another cause of pressure fluctuations is unstable operation – the heat supply at the bottom of the tower varies greatly, as does the amount of reflux. Distillation operations require very stable conditions; it takes time to establish equilibrium within the tower. Operations with large fluctuations are not suitable for distillation towers. This is related to the skill level of the operators, and management can set limits on how much certain parameters can vary within a certain time period
The main focus is on controlling the steady return flow temperature to ensure an adequate amount of return flow
I believe the objective reason is that changes in the C2 component in the raw materials have the greatest impact on operations; the rest are related to operational aspects, which can be overcome by maintaining stable operations.
It’s likely that the control scheme for tower pressure is inadequate, and it fails to maintain stable pressure. Is it choke control or thermal bypass control? Your device is too small; is the control valve too large? What is the opening degree of the pressure control valve? The position of the control valve should ideally be between 40% and 70%; if it is set to less than 20%, fluctuations are likely to occur.
Additionally, the PID parameters need to be properly tuned.
The content of the C2 component in the raw material is unstable; this issue needs to be addressed at the catalytic absorption stabilization unit by ensuring proper stabilization. Additionally, there should be no significant fluctuations in the temperature of the propylene tower, which will help keep the pressure stable within the range of 1.7–1.8 MPa. The purity of the propylene should be 99.5%
First, ensure the quality of the liquefied gas is stable, guaranteeing that it is free of C2. Distillation operations require careful, steady control, with gradual adjustments made to maintain stable reflux temperatures and temperatures at the bottom of the tower. The pressure control valve is now operated manually to stabilize the pressure.
First, ensure the quality of the liquefied gas is stable by guaranteeing that it contains no C2 compounds. Next, maintain a constant level of reflux and reflux temperature; for now, ignore the pressure issue as it will stabilize over time. Do not adjust all parameters every time there is a fluctuation, as there is always a certain delay in the effects of distillation adjustments
First, it is necessary to understand the proportion of each component in the liquefied gas being processed； Adjust the control scheme appropriately according to the properties of the raw materials ； Especially the understanding of the C2 content ； Its content directly affects the stability of the entire system as well as the quality of the product. Then, follow the approach of fine tuning, subtle tuning, and micro-tuning to ensure smooth operation!
Thank you all for your help. I would like to add a few more points: 1) The C2 content in the raw materials remains relatively stable, generally not exceeding 0.5%; 2) The heat supply valve at the bottom of the tower is always operated manually to ensure uniform heat distribution. I initially suspect that the fluctuation in the liquid level of the reflux tank is caused by the fact that the vapor at the top of the tower doesn’t cool down properly during its ascent; as a result, not all of it turns into liquid and enters the reflux tank, so some of it remains in the cooler. This is what leads to fluctuations in the liquid level of the reflux tank. Is there any way to solve this problem?