Problem
A major chemical manufacturer required a pollution control
system with minimal operational cost to destroy the Hazardous Air
Pollutant (HAP) by-products of a formaldehyde production process. A
key issue was system reliability; the plant operates 24 hours a day
and only shuts down once per year for plant-wide preventive
maintenance.
Action
After thorough technical evaluation, the company selected Anguil
Environmental Systems, Inc. to solve their HAPs emission problem
and put them in compliance. Anguil installed a catalytic oxidation
system that meets all applicable regulatory requirements. The
system uses a 65% effective shell and tube heat exchanger that
allows self-sufficient operation under normal process loadings.
Solution
In the formaldehyde production process, methanol reacts with air
in the presence of catalyst to produce formaldehyde. Process yields
are normally quite high, with over 95% - 98% of methanol ending up
as formaldehyde. Some ancillary compounds are generated in the
oxidation process due to catalyst inefficiencies. The emission
by-products that require control are carbon monoxide, dimethyl
ether, methanol and unscrubbed formaldehyde. Both formaldehyde and
methanol are classified as Hazardous Air Pollutants (HAPs) and
require stringent emission reduction.
Anguil's extensive experience with formaldehyde manufacturers
helped the company in examining possible pollution control
solutions. Due to the relatively high loadings to be fed to the
system (an exothermic/heat release of 300° F/149° C was expected
during oxidation), the customer's familiarity with catalytic
technology and the easily oxidizable nature of the by-product
compounds, a catalytic oxidation system was selected as the proper
solution. The effort then focused on the proper design of a
catalytic unit to meet all of the customer's operational goals.
A precious metal catalyst was selected as the core of the
catalytic system due to its ability to provide 98% DRE of the
formaldehyde, methanol, dimethyl ether, and carbon monoxide at a
catalyst bed inlet temperature as low as 500° F/260° C. The
catalyst is deposited on a monolithic, honeycomb, stainless steel
substrate. The catalyst design minimizes the pressure drop through
the system by reducing the upstream pressure required to overcome
the back pressure of the oxidizer. By achieving a low pressure
drop, the system needs less energy to operate and has significantly
lower operating costs.
Another cost-effective design feature was the integral 65%
effective shell and tube heat exchanger that was selected to
preheat the incoming process stream prior to the catalyst bed. The
exhaust air from the oxidizer is hot enough to allow operation of
the system with no additional heat input during normal plant
conditions. The 65% efficiency was determined taking into account
the inlet temperature to the oxidizer (outlet from the formaldehyde
reactor) of 81° F/27° C, the required oxidation temperature of 500°
F/260° C, and the expected 300° F/149° C exotherm from the
oxidation of the HAPs. The outlet temperature of the catalyst bed
is approximately 800° F/427° C.
Oxidizer warm-up following a plant shut-down was also a crucial
design consideration. Neither natural gas nor propane is readily
available at many of the company's formaldehyde plants. For this
reason an electric heating system was selected to bring the
oxidizer up to the 500° F/260° C operational temperature prior to
the processing of the plant off-gases. A small pre-heat fan,
capable of approximately 1/10th of the total system flow, was
designed to utilize fresh air in bringing the system up to initial
operating conditions. Once to temperature, the oxidizer process
inlet valve is incrementally opened to allow the exothermal
reaction to drive the oxidizer operation. As sufficient HAP loading
enters the system the electric preheat elements shut off, enabling
"self-sufficient" oxidizer operation.
The system is operating and providing well over the 98+%
destruction efficiency required. The success of this system
encouraged this company to purchase over ten other systems for
formaldehyde plants around the world. Anguil's experience,
innovative design and quality manufacturing resulted in another
satisfied Anguil customer and a long-term partner in air pollution
control.