The Faces of Catalytic Oxidation

Oxidation technology "has arrived." Improved catalyst design coupled with increased operational experience now provides a rational basis for appropriate oxidizer selection. "Off-the-shelf" systems no longer are necessarily reasonable long-term solutions for controlling solvent emissions from converting operations.

Selecting the optimum oxidation technology and heat-recovery system to meet current and future process needs is one of the most important choices facing users of solvent-based coatings. Many companies that are new to air-pollution-control requirements should consult with knowledgeable vendors who have the catalytic and thermal expertise to present alternatives for evaluation and selection. However, the companies also should be aware that selecting a thermal or catalytic approach to emission control involves recognizing the disadvantages and advantages of each in relation to the application at hand and future operational needs.

Catalyst poisoning is the main deterrent to using catalytic oxidation. Fast-acting poisons such as bismuth, arsenic, antimony, mercury, chlorine, fluorine, bromine, and phosphorous must be avoided. Heavy metals such as zinc, lead, and tin also will poison a catalyst, reducing life span. Typical fouling agents to be avoided are ceramic dusts, iron oxides, carbon and oils. However, where such fouling occurs, the catalyst can be cleaned, if required, by compressed air or steam.

The presence of sulfur or vanadium generally precludes using a catalytic system when firing heavy oil (although both catalytic and thermal systems operate with natural gas, propane, butane, and light fuel oils). A catalytic oxidizer, unlike a thermal oxidizer, may not tolerate silica. Still other agents' being released into the process exhaust stream may foul or poison catalytic systems, making them less tolerant than thermal systems of short-term process upsets.

There are other drawbacks to the use of the catalytic oxidizer. As the catalyst ages, its ability to oxidize hydrocarbons decreases. In contrast, emissions' reduction of a thermal system remains constant over time--if the equipment is properly maintained. Therefore, the overall economic analyses for each system should factor in the cost to replace the catalyst (generally after five years).

The fuel saving of the catalytic oxidizer, however, can offset these previously mentioned operational limitations. The catalytic system operates at 550°F to 650°F compared with the 1,400°F to 1,500°F temperature for thermal systems. This is assuming a slightly higher capital cost and an equal-efficiency heat exchanger. This saving, of course, will be diluted by the eventual cost of the catalyst reconditioning.

Oxidation with no heat recovery imposes enormous fuel costs. A primary heat exchanger that preheats the effluent prior to its entering the burner can achieve acceptable fuel costs. Beyond that, economics may justify including secondary heat exchanges. In fact, secondary recovery, combined with the gas solvents' burning to help "power" the reactor, can produce more heat than required by the oxidizer burner and should be seriously considered.

The options available for secondary heat recovery include air-to-gas heat exchange for process or plant heating or production of hot water, steam, hot oil, or electricity through cogeneration systems. In many cases, a minimal operation of 2,000 hours per year and a process exhaust volume of 2,000 scfm can be a justified candidate for heat recovery. These options must be evaluated for each need. A further option is to use a portion of the clean oxidizer exhaust gases for process heat. This approach carries a very attractive advantage in that no heat exchangers--only ducting, mixing boxes, fans and controls--are required. Thermal systems, with their higher temperature head availability, typically use this direct recirculation design more frequently. This concept can be incorporated following primary heat recovery and provides air at approximately 900°F to the process.

Overall, today's economics justify maximum heat-recovery options. Indeed, primary and secondary heat recovery and proper equipment selection are vital to successfully implementing oxidizer systems. Solvent-based-coatings' users therefore should use a knowledgeable company with a proven track record in emission-code compliance and heat-recovery design. The company should also provide responsive nationwide customer service for troubleshooting and training both during and after the warranty period.

Look for companies with capability in both thermal and catalytic oxidizer systems. This allows them to determine the best selection for specific operating and processing needs by optimizing for an individual application and economic requirement. Such equipment companies consider all process operation factors in the selection of thermal versus catalytic design, primary heat-exchanger efficiency, and secondary energy needs. Also, they use state-of-the-art catalyst support structure, such as the metallic honeycomb monolith. This structure is more resistant than a nonmetallic substrate to mechanical shock, high temperatures, frequent start-ups/shutdowns, and possible damage caused by cleaning and handling the unit. Plus, such companies may offer special financing for converters' requiring oxidation systems but having short-term cash flow problems.

Air-pollution-control equipment may often appear as being a financially burden, with large capital expenditures and operating costs. However, careful equipment choices generally can provide minimal impact on overall plant fuel costs. In some cases, through optimum efficiency selection of the equipment, deploying pollution control can actually save the converter money.