Problem
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 Anguil 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.