Emphasis on air management within converting-process dryers and ovens is critical to the design of pollution-control systems for treating exhausts containing VOCs (volatile organic compounds). Efficient air management allows the pollution-control system to be optimally sized to meet the needs of the process. Good process control further defines the emission stream by avoiding the need for large safety factors that increase control equipment size (and cost) and reduce efficiency.
Oxidation systems are used as 'end-of-pipe' pollution-control solutions for emissions from many solvent-based processes. Today's regulations demand very high VOC-control efficiencies. For many coating applications, overall emissions-control requirements of 95% are standard. In other areas, overall VOC control of 98% is required by law.
To determine overall control efficiency for a pollution-control system, you must first examine the 'capture' (or collection) of VOCs at the process. To achieve high capture of the VOC, most systems today include rooms around the coating equipment. These coater 'rooms' are known as permanent total enclosures (or PTEs). They contain (or capture) 100% of the emitted process solvents allowing them to be ducted directly to the pollution-control system. The exhaust from these enclosures also is sometimes used as makeup air to the oven system. The decision as to how the PTE exhaust is handled is a function of process conditions and oven design.
Once the capture system has been designed, the next consideration in defining the oxidation system is to quantify the concentration of VOC and other contaminants in the exhaust stream. Using the design parameters for the oven system, VOC types in the process can be identified, the minimum and maximum solvent concentrations defined, and the exhaust volumes and temperatures determined. Any particulate that may be generated by the coating equipment or process chemistry should also be identified. These are all critical parameters used in the selection of the oxidation system.
The operating schedule for the process is another important consideration. The variability in uptime on a process can have a tremendous impact on the efficiency and life expectancy of pollution controls. Processes that generate high-solvent concentrations and are frequently online and off line can put extreme stress on equipment components.
Running schedules also impact oxidation system selection. Different oxidation equipment may be selected for processes that run 8 hr/day, five days a week versus a process that runs 24-7.
Choices available
Oxidation systems are available in a wide variety of designs and configurations. It's important to remember that the end result expected from all of these systems is a 98% or higher destruction of process VOCs. The design variations available in the marketplace exist to address differences in process conditions, operating schedules, installation space requirements and operating costs.
Various oxidizer types used to control coating-line VOC exhaust include direct thermal systems, recuperative thermal systems, regenerative thermal systems, recuperative catalytic systems and regenerative catalytic systems. Descriptions of each type follow:
Direct-thermal oxidizers operate at temperatures from 1,200 to 1,800 deg F. These units are capable of treating a wide variety of VOCs and are available for use on exhaust streams ranging from flows of 200 to 20,000 cfm (ft3/min) or higher. Direct-thermal oxidizers do not include heat recovery and for that reason are costly to operate. These units are generally applied to low-flow processes that have minimal operating times or in operations where a heat recovery device can be used to recover the energy to the process.
By Fred HornCourtesy of MEGTEC Systems Originally published 2003
How to Optimize Oxidation Systems for Efficient Operation
