Choosing a heat recovery media for a regenerative thermal oxidizer (RTO) is not as simple as meets the eye. There are numerous misconceptions to overcome and many variables to consider in choosing a heat recovery media, especially in regard to thermal efficiency and VOC destruction efficiency.
Thermal efficiency and destruction efficiency do not necessarily go hand in hand. Paradoxically, in a typical two-chamber regenerative thermal oxidizer, higher thermal efficiency often means lower VOC destruction efficiency. Here is the paradox: stretching the cycle time to improve destruction efficiency lowers thermal efficiency. In addition, the stretch also generates larger temperature swings in the heat recovery media: The elongated cycle time depletes the media of stored heat.
That being said, how do these media types differ, and what does one need to know in order to choose?
Heat Recovery Media and Valve Cycle TimeCommon types of heat recovery media for regenerative thermal oxidizers include monolith, structured and random packed media. While monolith and structured media have a greater contact surface area to absorb heat, the thickness of the surface is only one-sixth the thickness of random packed media. So, while monolith and structured media may absorb heat quickly, the amount of heat the media is able to absorb and hold per unit area is smaller than what random packed media can hold.
During regenerative thermal oxidizer operation, one must remember that while one chamber is absorbing heat, its counter chamber is depleting heat. With limited heat storage on the exhaust cycle, the heat stored in the monolith or structured media may be quickly depleted to the incoming process air entering on the inlet cycle.
To achieve the reported thermal efficiency with monolith media, the cycle soak time - the length of time that the process fume enters a particular chamber before chambers are transferred - must be kept short (approximately every 3 minutes). Cycle times may need to be reduced further, to 1.5 minutes, when using high efficiency structured media.
Yet longer cycle times are desirable. Why? Most two-chamber regenerative thermal oxidizers have a generic slug of unburned VOC that is left unoxidized within the regenerative thermal oxidizer’s valve housing, inlet plenum and voids within the combustion chamber. Every time a chamber transfer takes place, there is an unoxidized amount of VOC, commonly referred to as the “peak” or “puff,” which is emitted into the atmosphere via the unit’s stack. Simply put, the more frequently regenerative chambers are transferred, the greater the amount of unoxidized VOC that enters the atmosphere.
Unfortunately, there is no foolproof method of eliminating the puff. However, longer recovery chamber cycle times and minimizing entrapment areas can reduce peak or puff emissions, thus increasing the overall removal efficiency of the regenerative thermal oxidizer.
Saddles Get a MakeoverBy the end of the 20th century, many regenerative thermal oxidizer manufacturers had switched from random packed saddle-type media to monolith and structured media. Why? At that time, monolith and structured media offered slightly higher thermal efficiencies, lower pressure drop and less particulate plugging over the old generation saddles. Since then, saddles have undergone a makeover. The new designs of random packed ceramic saddles combine improved shapes and configurations specifically designed for use in regenerative thermal oxidizers. In use since 2002 in both Europe and the United States, the modern saddle design has a pressure drop equivalent to that of structured/monolith media (on an equal velocity basis) and offers improved destruction efficiency.
In addition, the modern saddle media’s heat storage capacity allows cycle times to be maintained for up to 6 minutes without excessive media temperature or efficiency loss. In contrast, the high efficiency structured media requires chamber transfers as often as every 1.5 minutes.
Remember that thermal efficiency and destruction efficiency are not inclusive of each other. It is up to the oxidizer supplier, not the media supplier, to pick a media that will interface properly with the many other components in a regenerative thermal oxidizer. Final and correct design results are subject to the oxidizer manufacturer’s understanding of the relationship between the design parameters and the components.
No one media can be considered a panacea. Each application’s heat recovery media must be based upon its ability to interface with the specific industrial process, the type of regenerative thermal oxidizer, and its mandated VOC-destruction efficiency.