Low-temperature control at the top of the deethanizer tower

The question is difficult; no one is participating in the discussion! The main problem is that the temperature at the top of the deethanizer fluctuates greatly, often remaining above zero, which results in propane being present in the dry gas. The reasons for large temperature fluctuations are likely the precision and set range of the temperature control components, delays, or insufficient capacity of the ammonia refrigeration evaporator, resulting in slow response? It is recommended to try adjusting it first in terms of control!

Control has been maintained throughout. Part of the reason is the blockage of the conical filter in the bottom extraction line, which causes fluctuations in the bottom temperature of the tower. Additionally, the issues related to ammonia refrigeration control are likely the main cause. I would like to consult experts with experience in ammonia refrigeration control in order to learn from their expertise

Generally speaking, ammonia refrigeration is controlled via the throttle valve; but based on your previous response, the temperature fluctuations are caused by a clogged conical filter in the product outlet line at the bottom of the tower, so it’s necessary to resolve that clogging issue first

Blockage is part of the reason. I think the main reason is likely that the ammonia refrigeration load is insufficient. When the amount of liquid ammonia supplied is large, liquid forms and frost appears at the compressor inlet, the temperature at the top of the tower fails to drop, and the vaporization rate of liquid ammonia is low. When the amount of ammonia supplied is low, the top temperature also cannot be reduced. Is this situation caused by the limited heat exchange area of the tower top evaporator, which results in the overall ammonia refrigeration system not being able to meet the required load?

It seems there’s no issue with the size of the compressor; so I’ll ask the design team whether it’s possible to increase the heat exchange area or to use a structure with higher heat exchange efficiency – that should be feasible

But the size of the evaporator is fixed; to increase the heat exchange area, one has to use an evaporator with a greater height. The specific reason is not yet known; plans are to suspend operations for cleaning up first and then reassess the situation

Thank you all for your enthusiastic participation. After stopping operations for investigation, the reasons for the fluctuations in the top temperature of the tower were identified as follows: First, the product extraction point at the bottom of the tower was at the same level as the bottom of the tower, and there was no filter, which allowed a large amount of rust to enter the extraction line; this led to blockages at the inlet of the reboiler and uneven heat distribution ； II. The compressor does not operate smoothly, and the separation of ammonia from oil is incomplete, resulting in oil remaining at the bottom of the evaporator and a reduced heat exchange efficiency ； III. The position of the liquid ammonia supply control valve is too far forward; as a result, the pressure drops after throttling, leading to significant temperature loss. This in turn creates high resistance in the supply of liquid ammonia to the evaporator, and causes delay in regulation ； IV. The filter at the compressor’s suction inlet is clogged, resulting in poor gas flow back in the evaporator. After maintenance and rectification, the ammonia system can meet the process requirements. The above are the several factors I have identified; I hope everyone can offer their feedback, and I also hope this may be of some reference to colleagues facing similar issues.