(161f) Tricky Liquid Distributors

Despite of continuing professional interest, liquid distributors selection and design are still challenging even for reputable vendors. Overlooking the basics often result in costly operational problems.

Case Study #1: An intermediate petrochemical product is manufactured using ammonia as the main feedstock. Unconverted ammonia is removed from the gaseous product by sulfuric acid absorption in the absorber column.

Absorber column has been revamped some years before with structured packing and new internals. After a period of reliable operation, absorber column performance suddenly droppedand unit throughput had to be reduced accordingly causing a substantial profit loss.

Field observation addressed liquid distributor as a problem cause, despite of dissent of tower internals vendor. A short shutdown was planned for column inspection and distributor repair. When the absorber column was openeda massive damage of packing was revealed. Surprisingly the column shell was also damaged as a consequence of pyrophorics self-ignition during the shutdown operations.

Investigation has shown that wrong distributor selection wasthe mainroot cause of the problem. New tower internals were provided on an emergency basis, including a new distributor of different type. Column shell was repaired in the vessel manufacturer’s shop. Unit was successfully put into operation after a three weeks’ repair period.

Case Study #2:Atmospheric distillation of crude oil is a core process of anyoil refinery. The technology is well known for many decades and equipment design had been standardized, apparently leaving no room for surprises.

After the startup of grass-root atmospheric distillation unita main fractionator column could not been operated at design conditions. When feed heater outlet temperature was increased to design condition,the columnwas shaking because of frequent hydraulic shocks in pumparound section. Hydraulic shocks could be prevented only at cost of valuable diesel fraction loss to bottom residue, keeping feed heater outlet temperature low.

Problem analysis addressed pumparound return distributor as a problem cause, which tower internals supplier could not believe. Column internals modifications were performed during a short shutdown. Unit was successfully started after the modifications and reached design capacity and product quality without any sign of hydraulic shocks.