Jul-2014

Wastewater stripper fouling cured

Sulzer was contacted by two separate refiners who noted a continued fouling problem and short run lengths on their wastewater strippers. The configuration of the units was very similar, with both strippers processing wastewater from the desalters, coker blowdown, and various waste sumps from around the refinery.

Rodney Alario
Sulzer Chemtech

Article Summary

Refiner A was operating a retrofitted column, formerly utilised as a coker main fractionator. It wanted to install new internals to fit within the existing tray support rings to avoid welding on a stress-relieved vessel. The refiner targeted an increase in run length from eight weeks to six months.

Refiner B had noted its fouling problem becoming worse with opportunity crudes being processed. They were batching flow to the stripper, with a surge tank upstream. Refiner B was controlling stripping steam injection to maintain a middle tray temperature target. It had planned to replace the trays in kind, but wanted to investigate other options before the next outage for cleaning.

The strategy
Benzene stripper columns, built so that refineries can meet the US National Emissions Standards for Hazardous Air Pollutants (NESHAP), operate with several unique conditions – low vapour rates, high liquid loads, and a high tendency toward fouling make designing well-balanced, effective internals difficult.

Benzene steam stripper columns tend to have very low vapour rates because the refiner seeks to minimise energy consumption and steam usage. The vapour load on the top trays is primarily stripped hydrocarbon, with much of the steam having condensed on the lower trays. The low vapour rates are coupled with high liquid rates, which can make a good, balanced tray design tricky.

Without the appropriate stripping steam, the 
top vapour loads can be non-existent. Sulzer 
starts with a simple heat balance in the column. A significant amount of latent heat from the steam is transferred to sensible heat in the water and unable to be used to strip the benzene. Steam injection must be sufficient to where enough steam remains in the vapour phase or no tray design will be successful in this application.
An anti-fouling tray design should be considered. There are several features that Sulzer implements in many services to reduce fouling accumulation like a deck design to reduce stagnation zones, high performance downcomers to enhance liquid disengagement, and large fixed valves like the Sulzer V-Grid LVG trays.

The solutions
Presented with an anti-fouling tray option or a less efficient shed deck option, Refiner A chose to equip their column with V-Grid LVG trays and channel baffles in order to create an active area with adequate sweep of liquid and adequate valve density to maintain efficiency.

For Refiner B, Sulzer recommended an alternate control scheme to maintain a stripping steam to feed rate ratio, ensuring that the system had adequate duty to strip out benzene. Refiner B’s stripper was equipped with antifouling trays, featuring sloped downcomers to keep a higher velocity in order to sweep solids from the exit.

For more information: TowerTech.CTUS@sulzer.com