Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 239 of 331

recovery and media regeneration. The nature of an RTOs heat recovery process requires it to have at least two beds of appropriate heat recovery media. In many applications, the additional step of purging a bed before reversing the flow through it from inlet to exhaust is nec- essary to maintain very high destruction efficiencies. This purge step creates the requirement for an additional (or odd number) chamber making the RTO more complicated and more expensive than a recuperative oxidizer. RTO systems can utilize more than two beds (operating in parallel) in order to be capable of handling larger air volumes. The primary advantage of an RTO is lower operating costs due to high heat recovery and low fuel consumption. Depending on the mass of media included in an RTO, heat recoveries of up to 95% are common. Because of their capability for high heat recovery, RTOs are often operated in an "auto-thermal" or self-sustaining mode, where the heat content of the VOCs being oxidized is enough to sustain the combustion chamber temperature at setpoint, requiring no external fuel input. RTOs are a well-proven technology, but are being called on to become more efficient than ever, to reduce operating costs to even lower levels than have tra- ditionally been seen. That challenge has been met by developing improvements in heat transfer media, alternative oxidation technology and fuel usage opti- mization techniques. • Heat Transfer Media: Traditionally, the heat transfer beds of an RTO are composed of ceramic saddles, randomly packed into an insulated chamber. The airflow through the saddles is forced to make many changes in direction and velocity. Due to the turbulent nature of the airflow, the pressure drop across the bed increases with the square of the airflow. Dürr's investigations into the fundamental principles of RTO operation led to the development and application of a structured heat transfer media. These investigations indicated that a heat transfer media having straight airflow passages of constant cross-section offer significantly improved performance over traditional saddles by provid- ing more laminar airflow characteristics. The improved performance can be seen in a lower pressure drop across the packed beds of an RTO. Structured packing is a ceramic monolithic block, composed of silica alumna ceramic. Each block is approximately 12" tall, 6" wide and 6" long, and has hundreds of parallel passages, each approximately 1/8" square, extending from top to bottom. It's physical and performance characteristics allow for a higher airflow velocity through a packed bed, resulting in a more compact RTO which is attractive to land-locked plants that may not have the normal space required for an RTO. This higher bed velocity also allows for a unique solution to plants that have existing RTO equipment that may require additional airstream treat- ment capacity. Increased flow in a traditional saddle packed bed re- quires an exponential increase in pressure drop and motor horsepower, quickly overloading existing handling capacity. Replacement of an ex- isting saddle bed with ceramic monolith can not only reduce the pres- sure drop for existing capacity, but also provide almost a 40% increase 238

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