Case study: Formaldehyde production

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Courtesy of Anguil Environmental Systems, Inc.

Overiew

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.

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