The unusual feature in this arrangement, is that the entire interior of the unloading building can be continuously scrubbed.

Solutia has never had a serious spill of phosphorous oxychloride (POCl3) in thirty years, but the company's safety guidelines stress preparation to meet the worst-case condition. For that 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 arrangements.

The main purpose of the new POCl3 unloading building, opened for business in March, 1994, is to avoid or contain any major spill problem, with the ancillary benefit of isolating the material, and detecting and removing the trace quantities of POCl3 fumes that occasionally escape during the unloading process.

The Bridgeport plant is a moderately sized facility, producing industrial specialty chemicals and employing approximately 150 people in all. POCl3, delivered here from the supplier's production facility, is used in the manufacture of plasticizers, hydraulic fluids and lubricants, and as a raw material in other chemical products.

Planning for Enclosure

POCl3 reacts violently with water or water vapor to form hydrochloric acid (HCL) gas, so it was an obvious candidate for enclosed unloading. The unloading building appears to be a conventional, prefabricated steel structure. Inside, however, a special applied-at-the-factory coating protects the walls from chemical attack.

The building is outfitted with a dike and tank arrangement to contain any possible liquid spill. As soon as a tank truck enters the building, the outer doors are manually closed. The truck then passes over the dike to the isolation section where it is prepared for unloading. Unloading does not take place unless 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.

Selecting a Scrubber

Characteristics of the gas to be handled affect the selection of pollution control equipment. POC13 fumes can react with water, and it was determined that to remove HCL fumes a wet quenching process was required.

After considering the many wet scrubbing methods and equipment available, it was determined that a two stage system consisting of a first stage ejector scrubber and second stage counter current packed tower was the preferred choice. This 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 characteristic of the counter current packed tower). An additional advantage of using 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 the gases from the unloading building and push the gases through the packed tower. As a result no fan is needed in the system.

Croll-Reynolds Co., Inc., of Westfield, NJ has designed and manufactured many similar systems for various applications during its more than 30 years in the air pollution control field. The ejector scrubber (Croll-Reynolds' Jet Venturi Fume Scrubber) was designed to mount along with the Croll-Reynolds' 30 ft. tall packed tower on a common 2500 gallon sump tank.

Whole-building Scrubbing

The unusual feature in this arrangement, is that the entire interior of the unloading building can be continuously scrubbed.

A 16-inch main duct header system inside the building pulls in air and collects the traces of fumes that very 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 any vapor or fumes generated by normal unloading procedures. 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 POC13 vapors. And, of course, the system is always ready in case of emergency vapor or liquid release inside the building - although this will, hopefully, never occur.

HCL monitors inside the building detect gas releases. When and if this occurs, unloading is immediately aborted, and operators notified, all automatically. The scrubber system is in operation around the clock.

Design Details

The Solutia POCl3 first stage Jet Venturi Fume Scrubber, second stage countercurrent packed tower scrubber, 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 water/hydrochloric acid mixture, a maximum of 3‡% HCL by weight.

The system is designed to operate with gas inlet rate of 3000 acfm. Fumes and any airborne particulates 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/HCL.

How the Scrubbing System Works

The Jet-Venturi fume scrubber 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 continuously scrubbed or absorbed.

Basic design of the packed-tower type scrubber is counterflow. Tailgas from the Jet Venturi enters the bottom of the column and works its way upward through a bed of packing designed to provide a high surface area and intimate contact. The scrubbing liquid is distributed through a spray nozzle over the surface of the packing and flows downward by gravity - countercurrent to the gas. This allows for optimum mass transfer of the toxic contaminant from the gas to the scrubbing liquid and a fine, "polishing" result.