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.
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.
Recuperative and regenerative are terms used to describe the heat exchangers in oxidation systems. This terminology is consistent with both the thermal and catalytic systems. Recuperative heat-recovery systems typically use a metal shell and tube or metal plate-type heat exchanger to recover energy used in the oxidation process. Regenerative type heat-recovery systems typically use ceramic media to collect and store energy. The ceramic media is contained in multiple towers or canisters, which are interconnected, by ducting and a valve system.
The valve system in regenerative designs directs the incoming exhaust stream between the various canisters of ceramic. By switching from one tower of ceramic to another (called 'cycling'), one ceramic bed will release its energy while the ceramic bed in the other tower absorbs energy. For both types of heat recovery systems, the recovered energy is used to preheat the process exhaust as it enters the oxidation system.
Oxidation systems using recuperative-type heat exchangers typically recover 40-80% of the oxidation process energy. Most system designs fall into the range of 60-70% recovery.
Several factors impact the successful use of metal recuperative heat exchangers. These factors include process-exhaust temperatures, system operating-temperature requirements, temperature stratification within the unit (which relates to flow turndown), type and concentration of the VOCs treated, and the process operating cycle. All these factors affect the efficiency and life of the unit. Temperature limitations of the metals in heat exchangers along with stresses induced by changing process conditions can severely reduce unit life.
Regenerative heat-recovery systems are capable of recovering up to 97% of the energy used in the oxidation process. Most operate in an energy-recovery range of 85-95%. The ceramic media used in these systems are typically capable of continuous operating temperatures of 1,800-1,900 deg F.
High-temperature capabilities, along with the use of hot-gas bypass systems allow today's regenerative systems to operate effectively over a wide range of airflows with VOC concentrations of nearly zero to 25% of the LFL (lower flammability limit for the VOC).
Energy recovered in the oxidation process comes from the energy generated from the burning of process solvents (an exothermic reaction) plus the energy introduced through an auxiliary burner (used in most systems). Increased energy available from the process solvents reduces the auxiliary fuel required to support the oxidation process.
Regenerative thermal, regenerative catalytic and recuperative catalytic systems can be designed to operate using only the energy available from the process solvents. This operating condition is referred to as 'self-sustaining.' In a self-sustaining mode, the oxidation system's burners are shut down and energy from the process solvents entirely supports the oxidation process. The self-sustaining condition is the most desirable operating condition for any oxidation system. Only through careful optimization of process-air requirements can this be done.
Secondary & tertiary heat recovery
In some oxidizer applications, the self-sustaining condition is easily achieved, and excess energy exists from the process solvents. Once the oxidation-process energy demand is satisfied, the excess energy could be used to reduce the energy demand of the process or other areas in the plant.
Secondary and even tertiary energy-recovery systems are becoming more popular as energy costs continue to rise. One example would be operations where the energy is put back directly into the process by returning oxidizer exhaust to the dryer or oven.
Energy recovery through the use of air-to-air heat exchangers or thermal-fluid systems such as hot oil or water glycol exchangers is also an option. Some facilities have hot/chilled water or steam demands that can be supplied through a waste-heat boiler or adsorption chiller fired by the oxidation system exhaust. In northern climates, facilities use this excess energy as make-up air for heating the building.
When looking at the potential for secondary and tertiary heat-recovery systems, it's important to confirm that the process and pollution-control system's energy demands are met first. Only when the process and oxidation-system energy demands are satisfied is it appropriate to investigate alternative energy recovery projects.
Optimal process design
In summary, optimal process design can only be achieved through understanding multiple equipment variables and the interrelationships between drying and oxidation systems. Dryer design clearly affects pollution-control design and operation. Pollution-control design can have effects on dryer-operating costs and facility-energy management. Optimal solutions balance the competing needs and keep the total process in mind.
Fred Horn, pollution-control product specialist for MEGTEC Systems, DePere, WI, holds an associate degree in instrumentation electronics from Northeast Wisconsin Technical College. He has 18 years of industry experience and has worked for MEGTEC Systems since 1990, providing technical sales support to the web printing and coating industries requiring VOC emission-control equipment.