(28d) Systematic Field Tests Turn Loss into Gain in High-Capacitytrays Part 1: Excursion

This paper originates from a systematic troubleshooting effort that successfully diagnosed two complex independent problems which severely limited the capacity of an atmospheric crude distillation tower. The crude tower had been retrayed (by others) with modern high-capacity trays, but upon start-up achieved less than 80% of its previous capacity.

The troubleshooting identified two bottlenecks: premature flooding on some of the heavy naphtha-diesel fractionation trays, and a phenomenon which we termed ?excursion,? in which sudden temperature reductions around the diesel draw, accompanied by distillate yield loss, occurred upon rate increase.

This paper focuses on the excursion bottleneck. Systematic field tests showed conclusively that the excursions occurred when the diesel draw rate exceeded about 2000 tons/day (t/d). This was lower than the diesel flow rate that could be drawn prior to the retray. The new bottleneck was diagnosed to result from a seemingly benign modification to the downcomer that transported diesel to its draw pan. This modification halved the degassing time in the downcomer, leading to aerated liquid reaching the diesel draw pan. The excursions occurred when the diesel draw nozzle and rundown line reached their self venting flow limit. The excursion reflected the switch from self-venting flow to choked flow in the diesel rundown line.

Based on this diagnosis, some of the diesel was drawn out of an available kerosene draw nozzle further up in the tower. This unloaded the diesel draw, keeping it in the self-venting mode. This mode of operation also helped the premature flooding, which was the second bottleneck in the tower (this will be described in another paper). As a result, drawing kerosene permitted the tower to recover almost all the capacity loss without any hardware modification. The longer term solution was to replace the diesel draw by a seal-welded chimney tray and to increase the sizes of the diesel draw nozzle and rundown line. These modifications completely eliminated the excursions, permitting higher-than-design capacities to be achieved.