Eliminating Emissions from an MDI Manufacturing Plant
Problem
A major chemical manufacturer required a pollution control system to destroy 99.9% of the halogenated VOCs and hazardous air pollutants (HAPs) emitted during the manufacture of MDI (methyl di-isocyanate, used in the production of polyester plastics.)
The effluent air stream contained chlorobenzene (the primary chlorinated constituent, which has a high heat of combustion) but no oxygen. It also contained hydrochloric acid (HCl).
The pollution-control system, in addition to controlling pollution, would have to resist corrosion from the acid gas and the halogenated compounds
Action
After thorough technical evaluation, the plant selected the Anguil Environmental Systems solution for its VOC and HAP emission issues based on Anguil's previous success with oxidation of halogenated compounds and experience with the chemical industry's engineering and hazardous operations review standards.
Anguil chose to use a thermal recuperative oxidizer with a high temperature scrubber. Anguil's
As with every project it undertakes, Anguil organized a detailed HazOp meeting with the customer's process engineers, maintenance technicians, and the consultant to launch the project.
Design
Several of Anguil's competitors had proposed direct feed of the process gas to the burner where the contaminants would oxidize. Anguil's solution was a 6000-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 air stream that Anguil had to treat came from a phosgene scrubber. The influent temperature to Anguil's system was approximately 100ºF with humidity levels approaching 100%. The process stream, with its low flow and the high concentration of chlorobenzene plus the absence of oxygen, was unusual. The airflow range was 220 to 378 scfm and the chlorobenzene concentration could peak as high as 655 lb/hr.
Anguil's oversized its system to incorporate the dilution air necessary for combustion of the air stream while controlling the heat released during oxidation of the chlorobenzene.
An important design development was adding dilution air to the process stream in order to:
- provide sufficient oxygen for combustion of the chlorobenzene in the oxidation chamber,
- cool the high temperature chlorobenzene exotherm as it occurred (thus protecting the integrity of the system), and
- meet NFPA regulations, which require the lower explosive limit (LEL) of the air stream to be below 50%.
Because of the corrosive nature of the process stream, Anguil protected the system by using Heresite mastic coating on all of the internally exposed surfaces. In addition to the oxidizer coating, the company chose a high-temperature scrubber with Hastelloy C crossover pipe and a water-jacketed nickel-alloy quench.
A byproduct of treating halogenated air streams and the associated acid gases is a brine water discharge from the scrubber. The high saturation temperature of this high temperature scrubber caused Anguil to supply a liquid-to-liquid heat exchanger to lower the blow-down water temperature prior to discharge.
Outcome
Anguil's ability to provide complete engineering services and support for every aspect of the project guaranteed the successful integration of its system with the existing distributed control system (DCS); a key for nearly every chemical plant.
Contact: Anguil Environmental Systems, Inc., 8855 N. 55th Street, Milwaukee, WI 53223. Tel: 414-365-6400; Fax 414-365-6410; E-mail: info@anguil.com.
Contact: Anguil Environmental Europe Ltd. Tel (UK): 44-01785-761910; Fax (UK): 44-01785-761911. Tel (Italy): 39-50-220046; Fax (Italy): 39-50-501801.
Contact: Anguil Asia, Eurox Co. Ltd. Tel (Taiwan): 8-862-705-2886; Fax (Taiwan): 8-862-709-0448.