Jim Stone is senior sales manager for Anguil Environmental Systems, which specializes in exhaust air purification; jim.stone@anguil.com ; tel: (414) 365-6400 ext 586.  

Jim Stone

	As is the case with many industries, thermal and catalytic oxidizers are widely applied air-pollution control devices used throughout the pharmaceutical industry for the destruction of hydrocarbons. Many pharmaceutical processes, however, also generate emissions that contain halogenated hydrocarbons such as chlorine. Because of their stability and persistence in nature, most regulatory agencies require further treatment or removal for these halocarbons at the plant level.

	When treated by thermal oxidation, halogenated compounds present additional challenges. In particular, they form corrosive compounds in the oxidizer and in treated exhaust air. For example, methylene chloride can easily be converted to carbon dioxide and water vapor in a thermal oxidizer, at rates of 99% and greater, but the reaction results in the formation of acid gas. This condition presents a few important considerations in oxidizer design.

	Regenerative thermal oxidation (RTO) has become a common solution to treat volatile organic compounds (VOC) and hazardous air pollutants (HAP) present in the emissions of many pharma production processes. RTOs are flexible control devices, able to handle a wide variety of VOCs and HAPs in a wide range of concentrations. The key feature of an RTO is its ability to recover nearly all the energy required to heat up and combust the pollutants-up to 97% efficiency. In air streams with low pollutant concentrations, this feature results in a very low auxiliary fuel requirement in comparison with other types of thermal or catalytic oxidizers.

	This high level of heat recovery results in an RTO outlet temperature only slightly higher than the inlet; although the combustion chamber temperature where the pollutant conversion occurs is approximately 1550 degrees Fahrenheit or higher, the outlet temperature is typically only approximately 70 degrees above the inlet temperature. This condition, when combined with water vapor and acid gases in the outlet exhaust, can result in aggressive corrosion of carbon steel in improperly designed RTO support structures. Further, if the acid dewpoint is reached in the oxidizer, any formed acids could condense and cause further corrosion.

	Strategies to protect RTOs against corrosion in these applications include:

Upgrade materials of construction in key RTO subsections. 

Heat exchanger support areas, air diverter valves, and connection ducts are all crucial to ongoing RTO operation and therefore should be constructed of corrosion-resistant alloys.

 The outer RTO shell, while not required to be upgraded to alloy construction, could see acid gas vapors, and corrosion resistant coating over carbon steel should be considered.

By artificially raising the inlet temperature to the RTO, usually with a stand-alone heat source or recirculation of clean RTO exhaust gas to the inlet, the process air stream stays above the acid gas dew point.

All RTOs come with a process exhaust fan used to advance the polluted air through the oxidizer and out the clean air stack. Although an induced draft fan arrangement sees higher temperatures than a forced draft option, the system remains under negative pressure, and any leaks that may develop in ductwork or expansion joints over time would have air drawn through them into the system, as opposed to acid gas leaking out.

	Material selection for RTO construction in these applications is crucial. Designers should review specific defenses, such as avoiding pitting and crevice corrosion from chlorides and minimizing chloride stress-corrosion cracking. Several commercially available alloys are specific solutions to these challenges and have proven themselves over decades of operation.

	Pharmaceutical process emissions are not typical industrial applications, but strategies exist to address them successfully. The key is to address potential fail points in the typical industrial design, particularly in the materials of construction. Time spent wisely at the design stage will ensure uptime and long life of the emission control equipment downstream.

	Jim Stone is senior sales manager for Anguil Environmental Systems, which specializes in exhaust air purification; 

Articles

Regenerative thermal oxidation (RTO) has become a common solution to treat volatile organic compounds (VOC) and hazardous air pollutants (HAP) present in the emissions of many pharma production processes. Regenerative thermal oxidation devices used to treat halocarbons should be designed to prevent corrosion. Anguil Environmental Systems explains best practices for addressing potential fail points in the materials of construction when designing process emission handling systems. 
