Problem
A major chemical manufacturer required a pollution control
system to destroy the halogenated volatile organic compounds (VOCs)
emitted during the formulation of Pure Terephthalic Acid (PTA).
During the production process, methyl bromide is converted to
hydrogen bromide. The pollution control system had to be designed
to withstand the corrosiveness of the hydrogen bromide and varied
organic loadings which alter the temperature of the system.
Action
After thorough supplier evaluation, the plant selected and
contracted Anguil Environmental Systems, Inc. to solve their VOC
emission problem. Because of the high loadings of CO and the
presence of methyl bromide, a Catalytic Recuperative Oxidation
System was selected, utilizing a specialized catalyst specifically
designed for PTA plant exhausts by the Johnson Matthey Catalytic
Systems Division. Anguil's success with catalytic oxidation of
halogenated compounds and experience integrating control devices
into existing processes were crucial factors in the plant's
decision.
Solution
An Anguil catalytic oxidation system designed for 90,000 SCFM
was used to process the airflow. The presence of methyl bromide in
the emission stream presented additional design considerations. The
catalyst oxidizes the methyl bromide to hydrogen bromide (HBr). If
hydrogen bromide drops below its dew point it becomes corrosive to
the equipment. The equipment downstream of the catalyst must be
engineered to avoid any "cool spots" where the hydrogen bromide can
condense.
The heat exchanger is one potential "cool spot." Without proper
design, the heat exchanger could lower the temperature of the air
exiting the catalytic oxidizer to below the dew point of hydrogen
bromide. To avoid the potential condensation of hydrogen bromide
and subsequent corrosion to the system, Anguil incorporated a steam
preheater on the incoming process stream before the catalytic
oxidizer. The exhaust from the PTA plant is heated with a 316L
stainless steel plate and frame steam preheater. The preheated
process air then enters the tube side of the shell and tube heat
exchanger. The increased temperature of the process air entering
the tube side of the heat exchanger prevents the condensation of
HBr on the shell side of the heat exchanger, a critical issue in
the overall design of the system.
The process exhaust from the PTA plant varies in organic
loading; therefore, the catalytic oxidizer design must accommodate
these varying levels with minimal use of auxiliary fuel. This is
accomplished by utilizing a bypass on the 316L stainless steel
shell and tube heat exchanger. Under low organic loading
conditions, the heat exchanger bypass is closed so that the full
effectiveness of the heat exchanger is available to preheat the
incoming stream. At high organic loadings the outlet temperature of
the catalyst is raised, resulting in a high preheat temperature
from the heat exchanger. This high preheat temperature could cause
overheating and shutdown of the system. The "hot side" heat
exchanger bypass supplied by Anguil controls the preheat
temperature from the heat exchanger, preventing any
high-temperature conditions.
The selection of the proper catalyst that would oxidize both the
carbon monoxide and the methyl bromide at low temperatures was
critical to the project. Historically, halogenated compounds, which
include chlorine, bromine, iodine and fluorine, have had damaging
effects on both noble metal and base metal oxidation catalysts.
Several significant advances have occurred in recent years in
catalyst technology, resulting in the development of catalysts
suitable for the airstream under consideration. A
platinum/palladium-based catalyst deposited on a ceramic substrate
was utilized in this design. Anguil's proprietary catalyst rack
design, including specialty gasketing, eliminated the risk of gas
bypassing the catalyst which would result in incomplete
destruction.
This Anguil Catalytic Recuperative Oxidation
System is currently operating and achieving the regulatory
compliance demanded of the PTA plant. With the success of this
system, the customer purchased an identical 90,000 SCFM unit and
additional systems were installed for PTA manufacturers in the
Middle East and Southeastern United States.