Should VOCs/HAPs Be Recovered or Destroyed?
Solvent recovery is a feasible option for many larger printing and web coating operations, particularly when the VOC or HAP concentration is above 500 ppm, the process exhaust temperature is low (typically below 150 degrees F) and a single solvent or simple solvent blend is used. Recovery works best when the solvent blend is simple and the distillate will not retain water in the purified solvent. Retention of water may be detrimental in the opportunity to reuse the solvent for high quality printing. For higher temperatures and multiple solvent applications, the costs associated with applying the solvent recovery technology can be more expensive than destruction, but recovery can pay for itself in about 5 years. Historically small to medium size operations have not often used solvent recovery systems, nor is solvent recovery for all types of printers and web coaters. As an example, the complex solvent blend of flexographic printing inks, as well as those solvents that are aqua-phobic, may preclude the use of recovery in a flexographic printing application.
Gravure ink systems are frequently less complex, consisting of a limited number of basic solvents, making recovery systems more practical. All publication gravure printers use solvent recovery, each site saving many millions of dollars each year. Also, many large packaging printers, particularly those using expensive solvents to produce high quality materials, find recovery advantageous. If recovery is technically feasible, the key is to have the ability to reuse the recycled solvent in the printing and ink manufacturing process so as to gain the economic advantages. Compared to VOC/HAP destruction technologies, solvent recovery systems may be more complex and higher in capital costs. Nevertheless, there is a significant return on the investment.
Destruction technologies (i.e. catalytic and thermal oxidizers) can be used to control emissions from virtually all printing and web coating operations. These control technologies involve the destruction of VOCs or HAPs contained in the process exhaust fumes by subjecting them to elevated temperatures. The actual method employed to destroy these exhaust fumes, the operating temperatures required to assure a high level of VOC/HAP destruction and the operating efficiencies of the APCS itself are the primary differences between the various destructive air pollution control technologies.
A Basic Review of the Technologies
Solvent Recovery Systems are installed to remove VOCs/HAPs from the process exhaust fumes and return the solvent as a liquid for reuse. Recovery efficiencies of 98%+ can typically be guaranteed. The basic design concept of solvent recovery requires an inline adsorption and desorption process. Recovery begins when the solvent-laden air is passed through static activated-carbon beds, which are used to capture the solvent. The solvents are retained in the activated carbon pores and the clean air passes through and is discharged into the atmosphere. Multiple beds of activated carbon are used to provide continuous automatic operation. When a carbon bed has been saturated with solvent, it is automatically taken off-line for desorption or regeneration. These adsorption and desorption cycles are monitored by various analyzers and controlled by a computer. During the regeneration cycle, the carbon is heated in a counter-flow direction by steam or inert gas, which releases the solvent. The solvent-vapor and steam (or inert gas) mixture is directed to a condenser and subsequent separation into solvent and water occurs. The recovered solvent is clear and free of water and often is suitable for direct reuse.
Thermal Oxidizers are designed to completely destroy VOCs/HAPs from the process exhaust fumes. Destruction efficiencies of 99%+ can typically be guaranteed. The basic design concept of thermal oxidization is to promote a chemical reaction of the VOC/HAP with oxygen at elevated temperatures. This reaction destroys the pollutant in the air stream by converting it to CO2, H2O and heat. The rate of reaction is controlled by three-(3) interdependent and critical factors; time, temperature and turbulence.
In operation, the process exhaust fumes are preheated by use of an integrated heat exchanger and natural gas fired burner, are thoroughly mixed with oxygen (turbulence) and are held in the combustion chamber at elevated temperatures of 1,400F to 1,800F (temperature) for a residence time of between 0.3 and 1.0 seconds (time). A recuperative thermal oxidizer utilizes a multi-pass shell-and-tube type heat exchanger that is fabricated of heavy-duty stainless steel and includes the necessary thermal expansion joints. Thermal efficiencies range from 40% to 70%. Regenerative thermal oxidizers utilize ceramic media packed in multiple canisters as a high efficiency heat exchanger. Thermal efficiencies range from 85% to 97%. To maintain low external shell temperatures and minimize radiant heat loss, the combustion chamber is insulated with long-life ceramic fiber modules.
Catalytic Oxidizers are also designed to completely destroy VOCs/HAPs from the process exhaust fumes. Destruction efficiencies of 98%+ can typically be guaranteed. The basic design concept of catalytic oxidation is to utilize an industrial grade catalyst to promote the chemical reaction at lower temperatures, as compared to thermal oxidation. The VOC/HAP still must be mixed with oxygen and heated to an elevated temperature, thus destroying the pollutant in the air stream by converting it to CO2, H2O and heat. The rate of reaction is controlled by the temperature in the catalyst chamber and the amount of time that the pollutant spends within the catalyst itself. Because of lower operating temperatures, catalytic oxidation commonly requires less energy to operate.
In operation, the process exhaust fumes are preheated to 500F to 650F by use of an integrated heat exchanger and natural gas fired burner, are thoroughly mixed with oxygen and passed through the bed of industrial grade catalyst. A catalytic oxidizer incorporates a high efficiency counter-flow plate type heat exchanger that is fabricated of heavy-duty stainless steel. Thermal efficiencies range from 50% to 70%. The internal chambers of a catalytic oxidizer are manufactured entirely of heavy gauge stainless steel. Thermal expansion joints are incorporated where necessary and the internal chambers are covered with blanket insulation and then clad, typically with aluminum sheet. Most modern APCS utilize high efficiency heat exchangers, efficient natural gas fired burners, industrial grade blowers/fans, electric or pneumatic actuators and a programmable logic controller (PLC), all of which ensure safe and efficient long-term operation. The PLC provides primary electrical and temperature control of the oxidizer. A touch screen terminal provides the operator with a window-based interface for simple “hands-off” operation.