Competitive pressures continuously motivate process heating equipment users to examine heat processes for opportunities to increase quality, increase productivity and decrease costs.  Effective heat management or control can reduce operating costs.

Figure 1. In a direct thermal oxidizer, a burner fires into the exhaust airstream, heating it to the combustion temperature. All of the heat is exhausted to atmosphere.

Convection dryers coupled to thermal oxidizers are a fixture in many energy-intensive processes such as those associated with the manufacture of products utilizing water or VOC-based solvents.  Both dryers and oxidizers heat, circulate and exhaust large volumes of air.  Without proper design, large amounts of costly, usable energy can be carried out the exhaust stack.
For both types of equipment, the goal of process heat management is to minimize the volume and temperature of the exhaust stream.  Heat recovery is the process of utilizing the heat that is generated but not consumed by a process. This heat may be directed back to the process as primary heat recovery; utilized by a related or connected process as secondary recovery; or provided to an unrelated process as tertiary heat recovery.

An oxidizer is an air pollution control device that operates by heating a VOC-laden airstream to its combustion temperature, then converting the solvents to carbon dioxide and water.  Typically, the combustion chamber operates in the range of 1,400 to 1,600oF (760 to 871oC) to achieve adequate volatile organic compound (VOC) destruction.  In a direct thermal oxidizer, a burner fires into the exhaust airstream, heating it to the combustion temperature (figure 1).  The clean, hot airstream is exhausted to atmosphere.  In this case, all of the energy put into the heating of the airstream -- as well as the heat released in the VOC combustion process -- is exhausted out the stack as waste heat. Equipment of this design is suitable for intermittent, low flow applications where the capital cost of heat recovery is large compared to savings in the operating cost. This situation is rarely encountered in converting processes.

Primary Heat Recovery in the Oxidizer

Figure 2. A recuperative thermal oxidizer utilizes an air-to-air heat exchanger to preheat the incoming airstream.

Most oxidizer designs incorporate a primary heat recovery system to preheat the incoming airstream.  A heat exchanger is used to extract heat from the high temperature airstream exiting the combustion chamber and transfer it to the cooler airstream entering the combustion chamber. Depending on the type of heat exchanger employed in the design, an oxidizer is referred to as recuperative or regenerative.

A recuperative oxidizer utilizes an air-to-air heat exchanger to preheat the incoming airstream (figure 2).  Typical thermal efficiency of a recuperative heat exchanger is 40 percent to 70 percent.

Figure 3. A regenerative thermal oxidizer uses multiple beds of ceramic heat exchange media. At regular intervals, a switching valve reverses the airflow through the media beds. The beds cycle between absorbing heat from the process (left) and releasing heat to the process (right).

A regenerative oxidizer utilizes multiple beds of ceramic heat exchange media (figure 3). One bed absorbs heat from the outgoing airstream while another bed releases heat to the incoming airstream.  At regular intervals, a switching valve reverses the airflow through the media beds. The beds cycle between absorbing heat from the process and releasing heat to the process.  Typical thermal efficiency of a regenerative heat exchanger is 80 to 95 percent. High efficiency heat recovery combined with the exothermic combustion reaction creates the opportunity for the oxidizer to operate without the need for additional fuel. In general, the higher the solvent concentration in the airstream, the lower the heat exchanger efficiency required to maintain this “self-sustaining” operation.  Regenerative oxidizers reach this operating condition at solvent concentrations as low as 5 percent LEL.

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