It’s amazing how some problems keep repeating themselves. That’s where the RCE experience and expertise comes into play for our clients. Recently a gas plant was operating with high inlet rates and occasionally (at least weekly) the demethanizer would flood and cause major problems. The RCE engineer on site noticed the Cold Separator liquid level control valve position was swinging quite a bit. This leads to a cycle of high / low liquid feed rates to the column. It was during these high liquid feed rates that the flooding started and propagated up the column. The tuning on the Cold Separator liquid level controller was modified so that the feed rate to the column was much more steady. Operations believes this added 3-5 percent more capacity with higher recoveries before column flooding.
We’ve seen this many times before where a feed to a column is controlled on level from an upstream vessel. While the vessel level hardly moves, the control valve swings greatly causing very unsteady conditions within the column. The upstream vessel must be treated as a “surge” vessel and the level allowed to swing slightly while keeping the control valve output and the column feed more steady. This leads to the best overall performance.