Minor Equipment Debottlenecking

An example of the value of relatively minor debottlenecking was studied for another LSR Stabilizer. This Stabilizer was in the same service as the one discussed previously. Although the refinery configuration was slightly different, the same products and desired separations applied. The Stabilizer under discussion was undersized for the service as the upstream equipment had been debottlenecked allowing higher charge rates. The undersized stabilizer led to high C₄- concentrations in the bottoms and high C₅+ losses in the overhead.

Case I shows that typical operation allowing almost 14 LV% of C₅+ material in the overhead product, the lowest attainable C₄- concentration in the bottoms was 3.9 LV%. This high C₄- content resulted in high RVP’s of 12 and greater. The high C₄- concentration in the Stabilized LSR was causing high RVP’s and significant RVP blending problems. A study was initiated to reduce the RVP with only minor equipment modifications desired.

Since the bottom portion of the tower was limiting, the study concentrated on increasing the feed preheat temperature to the tower. The tower had a feed/bottoms exchanger, but the feed still entered the column significantly subcooled. The subcooling of the feed contributed to the bottom tray flooding limit. The subcooled feed had to be heated up to its bubble point temperature by using the heat supplied by the reboiler. Using the reboiler heat increased the traffic in the loaded, bottom section of the tower. Alternatively, additional feed preheat could be added externally to the tower which would not increase the tower loadings. The higher preheat temperature could be achieved easily with another small feed/bottom exchanger.

Cases II and III illustrate the significant debottlenecking effect of additional feed preheat. Raising the feed preheat to 200° F cut the amount of butanes in the bottoms by more than half. The RVP dropped accordingly from 12.1 in Case I to 11.1 in Case II. The graph of bottoms RVP vs. feed preheat temperature illustrates how much higher feed preheat temperatures reduce the bottoms' RVP. While the column separation was still not good even at the higher preheat levels, the bottom RVP reduction was significant.

 

Additional feed preheat is not a panacea for all column debottlenecking. Preheat usually increases capacity in columns only with subcooled feeds that are reboiler limited or limited by tray flooding below the feed. Additional feed preheat also reduces the energy efficiency of the tower. For a given amount of feed and reboiler duty, a better column separation is achieved if more duty is applied to the reboiler. However, if additional preheat is available through feed/bottoms exchange, then the energy debit may become moot if the bottom heat was to be rejected against a utility. As with any debottlenecking effort, all of the ancillary equipment must be checked. Additional feed preheat increases the loads upon all of the equipment above the feed tray. Finally, there may be other debottlenecking options such as high capacity trays or packing which may be more suitable depending on the individual circumstance.

 

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