Case study: MDI production plant

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Courtesy of Anguil Environmental Systems, Inc.

Overview

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 airstream represented a unique set of constituents and variables which required an intense application engineering process. One factor was the effluent contained no oxygen; another consideration was the primary chlorinated constituent, chlorobenzene, has a high heat of combustion and therefore generates a great deal of heat when combusted. The system also had to be designed to neutralize the hydrochloric acid (HCl) that is produced during oxidation and to resist corrosion from this acid gas and the halogenated compounds.

Action

After thorough technical evaluation, the plant selected the Anguil Environmental Systems solution for their VOC and HAP emission issues. Due to the required 99.9% destruction efficiency and the presence of extremely high levels of chlorobenzene, a Thermal Recuperative Oxidizer with a high temperature Scrubber was selected. Anguil's previous success with oxidation of halogenated compounds and their experience with the chemical industry's engineering and hazardous operations review standards were crucial factors in the plant's decision.

Solution

As previously noted, the process stream from MDI manufacturing contains high levels of chlorobenzene. The customer's environmental policy dictated that greater than 99.9% of the chlorobenzene and other VOCs and HAPs had to be controlled and treated. Several of Anguil's competitors offered technologies that used the process gas as a direct feed source for the burner to oxidize the contaminants. Anguil's solution was a 6,000 SCFM Chlorinated Thermal Oxidizer with a high temperature Scrubber. This approach offered both lower capital and operating costs and guaranteed the required destruction efficiency.

The airstream that Anguil needed to treat was coming from a phosgene scrubber. The influent temperature to Anguil's system was approximately 100 degrees F with humidity levels reaching nearly 100% saturation. The process stream was unique because of it's low flow and the high concentration of chlorobenzene coupled with the absence of oxygen. The air flow range was 220 - 378 SCFM and the chlorobenzene concentration could peak as high as 655 lbs/hr. Anguil's 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 exotherm (heat release) associated with the chlorobenzene.

An important design development was the addition of dilution air to the process stream. Dilution air was necessary for three reasons. First, the incoming process air lacked oxygen, therefore 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, Anguil was able to cool the high temperature chlorobenzene exotherm as it occurred, thus protecting the integrity of the system. Finally, the dilution air allowed Anguil 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, Anguil protected the system by using a Heresite™ mastic coating on all of the internally exposed surfaces. In addition to the oxidizer coating, a high temperature scrubber with hastelloy C crossover pipe and water-jacketted nickel alloy quench was used. A byproduct of treating halogenated airstreams and the associated acid gases is a brine water discharge from the scrubber. The high saturation temperature of this high temperature scrubber led Anguil to supply a liquid to liquid heat exchanger to lower the blow-down water temperature prior to discharge.

With over sixty chlorinated systems currently in use, Anguil has proven experience resolving halogenated compounds issues. Anguil's experience with chemical plants was another important factor in this project. As with every project, Anguil organized a detailed HazOp meeting with the customer's process engineers, maintenance technicians and the consultant to launch the project. Anguil's ability to provide complete engineering services and support for every aspect of the project guaranteed the successful integration with the process and the interface with the existing Distributed Control System (DCS); a key for nearly every chemical plant.

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