Recuperative Thermal Oxidizer with HCL Scrubber to Treat Chemical Processing Emissions

The Challenge

A major chemical manufacturer required a pollution control system to destroy 99.9% of the halogenated volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) emitted during the MDI manufacturing process. MDI is a chemical commodity used as a lightweight insulating material for ice chests and coolers as well as for sound and collision insulation in automobile bumpers. The process exhaust stream represented a unique set of constituents and variables, which required a long and intensive application engineering process. One factor was the exhaust stream contained no oxygen; another consideration was the primary chlorinated constituent, chlorobenzene, generates a large exothermic reaction (heat release) when combusted. The system also had to be designed to neutralize the hydrochloric acid (HCL) produced during the oxidation and also resist corrosion from the acid gas and the halogenated compounds.

The Solution

After thorough technical evaluation, the plant selected a thermal recuperative oxidizer with HCL scrubber solution for their VOC and HAP emission issues with high levels of chlorobenzene. The decision was based upon previous success with oxidation of halogenated compounds and experience with the chemical industry’s engineering and hazardous operations and standards.

The process exhaust stream requiring treatment was coming from a phosgene scrubber. The influent temperature to the thermal recuperative oxidizer was approximately 100 degrees F with humidity levels reaching nearly 100% saturation. The process stream was unique because of it’s low air flow range between 220 – 378 SCFM and the high concentration of chlorobenzene, as high as 655 lbs/hr, coupled with the absence of oxygen.

The system was sized for 6000 SCFM (not 380 SCFM) in order to incorporate the dilution air necessary for combustion of the air stream while also controlling the exothermic reaction associated with the chlorobenzene. An important design development within the project was the addition of dilution air to the process stream. As the incoming process air lacked oxygen,  dilution air was necessary to provide sufficient oxygen for combustion of the chlorobenzene in the oxidation chamber; Second, by providing the proper amount of dilution air, the system was automatically controlled to cool the high temperature chlorobenzene exotherm as it occurred, thus protecting the integrity of the system; and the dilution air allowed the system to meet NFPA regulations that require the lower explosive limit (LEL) of the airstream to be below 50% LEL.

Due to the corrosive nature of the process stream, the system was protected by using a special coating on all of the internally exposed surfaces. A byproduct of treating halogenated airstreams and the associated acid gases is a brine water discharge from the scrubber. This required the system to be supplied with a liquid to liquid heat exchanger to lower the blow-down water temperature prior to discharge.

The Results

The final solution of a large Chlorinated Thermal Oxidizer with a high temperature Scrubber, which used the process gas as a direct feed source for the burner to oxidize the contaminants , offered both lower capital and operating costs and guaranteed the required destruction efficiency.