Dual Scrubber Helps Isolate POCl₃ Hazards

Solutia (Bridgeport, NJ, formerly Monsanto Chemical Group) didn't want to spoil its record of never having a serious spill of phosphorus oxychloride (POCl₃). Therefore, it decided that all POCl₃ unloading and storage, previously done in the open, would take place in a sealed building.

The unloading building's main purpose is to avoid or contain any spill problem. An ancillary benefit is detecting and removing trace quantities of POCl₃ fumes that sometimes occasion unloading.

For this purpose that building has an emergency scrubbing equipment including a venturi jet ejector and a packed tower.

POCl₃ reacts violently with water or water vapor to form hydrochloric acid gas. Therefore the first-stage scrubber, packed tower and some of the ancillary equipment comprise corrosion-resistant FRP vinyl ester resin.

Automated unloading

As soon as a tank truck enters the building, the outer doors are manually closed. The truck passes over a spill-containment dike to the isolation section for unloading. Unloading does not occur until the scrubber is operating. The scrubber system operates around the clock.

All personnel leave the area, closing the inner and outer doors, as soon as unloading hoses are in place. Unloading proceeds automatically, monitored remotely from the control room. The arrangement leaves little chance of POCl₃ mixing with water were a spill to occur even during a raging rainstorm.

HCl detectors inside the building monitor for gas releases. Any release stops the unloading stops, and operators are notified.

After each tank truck is unloaded, the building air is evacuated and scrubbed. The venting system also is used to scrub vapors in advance of any inside maintenance.

A 16-in. main-duct header inside the building pulls in air and collects trace amounts of fumes that occasionally escape in normal operation. The entire system, including hoses, vents to the scrubber. A slight negative pressure ensures that leakage always is into the building.

The scrubber

Solutia engineers examined a variety of wet-scrubbing arrangements, and opted for a two-stage system consisting of a first-stage ejector scrubber and a second-stage, countercurrent packed tower.

The combination provides maximum capability to treat (via the ejector scrubber) a large emergency release as well as high overall removal efficiency (a feature of the packed tower).

The ejector scrubber offers the additional advantage of functioning as a "gas mover." Using the scrubbing liquid as the motivating fluid, the ejector creates a suction vacuum to pull fumes from the unloading building and push gas through the packed tower. Therefore no fan is needed.

The system design calls for a gas inlet rate of 3000 acfm. Fumes and any airborne particles are knocked down into the fluid stream for collection and routing to the plant's waste-treatment system. The two-stage system is designed to provide 99.9% removal of POCl₃ and HCl.

The venturi entrains and scrubs large volumes of gas without baffles or moving parts.

Tail gas from the venturi enters the bottom of the column and flows upward through the packed bed designed to provide a high surface area and intimate contacting. The scrubbing liquid is distributed through a spray nozzle over the surface of the bed and flows downward by gravity. This allows for optimum mass transfer of vapor from the airstream into the scrubbing liquid.

Croll-Reynolds Co. (Westfield, NJ) supplied the ejector scrubber (a Jet Venturi Fume Scrubber) and the 30-ft-tall packed tower. Both are on a common 2500-gal sump tank.

About the author:Satish Shah is a process safety engineer and engineering specialist at Solutia, Inc., Bridgeport, NJ.

The previous case study was adapted from one that appeared on Chemical Online, a companion VerticalNet site to Pollution Online.