Gas-liquid phase instability
Thread Content
Reasons for the instability of the gas-liquid phase in the xylene tower, and corresponding treatment methods, solution
Premise: Due to the high liquid level and low temperature in the reflux tank, when efforts were made to reduce the liquid level in the reflux tank and lower the temperature, the liquid level at the bottom of the tower was not monitored properly, which led to instability in the tower, resulting in unstable temperatures, as well as unstable liquid levels in both the tower and the reflux tank
Premise: Due to the high level of liquid in the reflux tank as well as high temperatures, efforts were made to reduce the liquid level in the reflux tank and lower the temperature. However, attention was not paid to the liquid level at the bottom of the tower, which led to instability in the tower; this in turn caused instability in temperatures, as well as instability in the liquid levels of both the tower and the reflux tank. The measure taken at that time was to increase the reflux flow in order to control the low temperatures. But low temperatures are relevant to the heat exchange process at the bottom of the tower used for separating heptane oil, and controlling the temperature of the xylene tower would result in substandard quality of the heptane oil. What should be the correct way to handle this situation?
Adjusting the reflux disrupts the original dynamic equilibrium, which indeed leads to a series of changes and instabilities; it takes time to establish a new dynamic equilibrium.
To establish a new balance, where is the easiest place to start? I began by considering the temperature at the top of the tower, but it’s not easy to do so because one has to take into account the temperature in the deheptanization tower as well
First, stabilize the bottom temperature, followed by steam, reflux, and feed. It needs to be done gradually; it is necessary to determine an approximate ratio between the processing volume and the amount of steam based on past data, making fine adjustments. Achieving stability is a gradual process – the more randomly adjustments are made, the less stable the situation becomes
Upon re-analysis, it was found that the reason was instability caused by significant adjustments to the reflux flow rate; the pressure at the top of the tower was ignored, which led to a drop in that pressure. When making adjustments, the pressure at the top of the tower should be increased first, followed by controlling the reflux flow rate, and only then can the temperature be reduced. While lowering the temperature, the feed rate should be well controlled. Then the top temperature can return to its original stable value.