Dual Scrubber Guards Against POCl₃ Hazards In Chemical Unloading

Solutia (formerly Monsanto Chemical Group) has never had a serious spill of phosphorus oxychloride (POCl₃) in 30 years, but the company's safety guidelines stress preparation to meet the worst-case condition. For this reason, the Bridgeport, NJ, operation decided that all POCl₃ unloading and storage, previously done in the open, must take place in an enclosed area provided with emergency scrubbing equipment. This scrubbing system comprises a novel combination of a venturi jet ejector and a packed tower.

The main purpose of the new POCl₃ unloading building, which opened in the mid-90s, is to avoid or contain any spill problems. An ancillary benefit of isolating the unloading process in this manner is to detect and remove trace quantities of POCl₃ fumes that occasionally are generated during unloading.

The Bridgeport facility is a moderately sized plant producing specialty chemicals and employing approximately 150. POCl₃ is used in the manufacture of plasticizers, hydraulic fluids and lubricants, and as a raw material in other chemical products.

Automated unloading

POCl₃ reacts violently with water or water vapor to form hydrochloric acid gas. To protect the prefabricated walls of the enclosure, a factory-applied coating was specified. The building is outfitted with a dike and tank arrangement to contain liquid spills. As soon as a tank truck enters the building, the outer doors are manually closed. The truck passes over the dike to the isolation section where it is prepared for unloading. Unloading does not occur until the scrubber is operating.

As soon as unloading hoses are in place, all personnel leave the area, inner and outer doors are closed, and unloading proceeds automatically, monitored remotely from the control room. With this arrangement, even if a spill occurred while a rainstorm was going on outside, there would be little chance of hazardous mixing of POCl₃ and water.

Characteristics of the gas to be handled affected the selection of the fume-handling equipment. Wet quenching with water was determined to be the best choice.

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 (see figure). The combination provides the best solution in terms of capability to treat a large emergency release (a feature of the ejector scrubber) and provide extremely high overall removal efficiencies (a feature of the packed tower).

An additional advantage of this approach is the function of the ejector scrubber 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 to push gas through the packed tower. As a result, no fan is needed by the system.

The supplier of the ejector scrubber was Croll-Reynolds Co. (Westfield, NJ), which provided its Jet Venturi Fume Scrubber. Croll-Reynolds was also the supplier of the 30-ft tall packed tower. Both are on a common 2,500-gal sump tank.

Whole-building scrubbing

An unusual feature of this installation is that the entire interior of the unloading building can be continuously scrubbed. 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, is vented to the scrubber. Slight negative pressure ensures that leakage is always into the building, never out.

The scrubber system handles certain routine duties. It scrubs vapors and fumes. After each tank truck is unloaded, the building air is evacuated and scrubbed. Before any inside maintenance is performed, the venting system is again used to scrub vapors. And of course, the system is always ready to handle emergency situations.

HCl monitors inside the building detect gas releases. When and if this occurs, unloaded stops, and operators are automatically notified. The scrubber system is in operation around the clock.

The Solutia first-stage scrubber, packed tower and some of the ancillary equipment are made of corrosion-resistant FRP vinyl ester resin. The scrubbing solvent is water, which in the process of scrubbing eventually becomes a dilute (maximun 3 wt%) HCl solution.

The system is designed to operate with a gas inlet rate of 3,000 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 Croll-Reynolds Jet Venturi works on the principle of an ejector, entraining and scrubbing large volumes of gas without baffles or moving parts. Motivating fluid enters the scrubber through a nozzle, creating a draft which draws gases and vapors into the moving stream where they are absorbed.

The basic design of the packed tower is counterflow. Tailgas from the Jet Venturi enters the bottom of the column and works its way 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.

About the author

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