A chemical manufacturer required a pollution control system with minimal operational cost to destroy the hazardous air pollutant (HAP) byproducts of a formaldehyde production process. One of the company's key issues was system reliability. The plant operates 24 hours a day, shutting down only once a year for plant-wide preventive maintenance.
To solve the HAPs emission problem and put the plant in compliance with regulatory requirements, Anguil Environmental Systems Inc., Milwaukee, installed a catalytic oxidation system using a 65-percent-effective shell-and-tube heat exchanger that allows self-sufficient operation under normal process loadings.
In the formaldehyde production process, methanol reacts with air in the presence of a catalyst to produce formaldehyde. Process yields normally are quite high, with more than 95 percent to 98 percent of the methanol ending up as formaldehyde. Still, some ancillary compounds are generated in the oxidation process due to catalyst inefficiencies. The emission byproducts that require control are carbon monoxide, dimethyl ether, methanol and unscrubbed formaldehyde. Both formaldehyde and methanol are classified as HAPs and require stringent emission reduction. Anguil's experience with other formaldehyde manufacturers helped it perform the critical evaluation process and devise pollution control solutions.
Due to the relatively high loadings 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 byproduct compounds, a catalytic oxidation system was selected as best solution. Anguil then focused on the proper design of a catalytic unit to meet all the customer's operational goals.
Anguil chose a precious-metal catalyst as the core of the catalytic system because of its ability to provide 98 percent destruction and removal efficiency 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, says the company.
Another design feature was the integral 65-percent-effective shell-and-tube heat exchanger that preheats the incoming process stream prior to the catalyst bed. The exhaust air from the oxidizer is hot enough to allow system operation with no additional heat input during normal plant conditions. The 65 percent efficiency was determined by 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 shutdown was also a crucial design consideration. Neither natural gas nor propane is readily available at many of the company's formaldehyde plants. Therefore, an electric heating system was selected to bring the oxidizer up to the 500°F operational temperature prior to the processing of the plant's off-gases. A small preheat fan, capable of approximately one-tenth 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.
During the months of operation since installation, the system has provided well over the 98-percent destruction efficiency required, and the success encouraged the customer to purchase more than 10 other Anguil systems for its formaldehyde plants around the world.