State and federal regulations require that a range of industries control organic vapors from their processes. Applications exist in food processing, finishing, ethanol/biodiesel fuels, plastics/rubber, packaging/printing, pharmaceuticals, refining, electronics, pulp/paper/engineered wood and other industries. This article covers the major issues involved in design and specification of vapor control systems.
Controls for volatile organic compounds — also referred to variously as VOCs, air toxics, organic vapors and SVOCs — generally are required to provide greater than 95 percent removal. In addition to old limits such as 100 tons/year of VOCs per facility, some compounds are subject to more stringent limits of 10 tons/year, depending on the severity of air pollution in a non-attainment area and the emitted compound.
In addition to production emissions, fugitive emissions such as those from tank storage, valve stem leaks and pump shaft seals also are regulated.
Sources of and Controls for VOC Emissions
Many processes generate and vent gases containing VOCs. Table 1 lists a range of sources and typical emission controls.
Major VOC controls are thermal destruction, adsorption via activated carbon (and/or zeolites), condensation and biofilters. The following data is required to choose the right type:
• Process vent flow rate.
• Concentration of the VOCs.
• Chemical properties (especially sulfur and chlorine, which form acid gases) and dust.
• Cycle (continuous; batch).
• Potential for use of waste heat.
The majority of industrial VOC controls are oxidation based. Other controls such as condensers are used for small flow/high concentration (for example, 50 percent by volume VOC). Adsorbers are used for small-volume applications at low concentration (for example, laboratory vents) and for applications where recovered solvents may be returned to the process.
Figure 1 shows, in general, where different types of control equipment fit based on process flow and VOC concentration. Choosing the wrong oxidizer means higher fuel use, higher emissions and potential for overtemperature shutdowns.
Thermal Destruction of VOCs
Combustion safety is a major aspect of design for VOC treatment systems. A major consideration is whether to design and operate above or below 25 percent lower explosion limit (LEL). Systems running below that value are covered by basic controls per NFPA codes. Higher concentrations require more robust and sophisticated controls such as seal pots and flame arrestors as well as safety precautions such as VOC monitors. The converse of the LEL is the upper explosion limit (UEL), and mixtures between the LEL and UEL will burn when ignited.
Table 2 summarizes LEL and UELs for typical VOCs. These values are at standard conditions with VOC mixed in air. Preheating the VOC mixture or enriching it with oxygen expands the range between LEL and UEL.
Explosions can occur when VOC concentration changes suddenly and a combustible mixture enters the oxidizer. The mixture can flash back through ductwork and cause equipment damage and injury. Good design of the process and controls includes consideration of probable failure modes, in particular, loss of electrical power. The system should fail-safe and should not be able to restart until VOC concentrations are again in the design range. NFPA and other codes offer guidance on this subject, and equipment vendors such as those selling flame arrestors (figure 2) are an additional resource.
Thermal systems combust the VOCs in the vent stream by addition of thermal energy from VOCs and/or auxiliary fuel. It is critical that the choice of oxidizer be made to match the VOC fuel value to prevent overheating or overtemperature shut down, as well as excessive use of auxiliary fuel for too low a concentration fume. Table 3 lists temperature rise versus LEL values for selected VOCs.
Flares. Flares are used primarily to dispose of combustible gases during a process upset condition while heat may be recovered from regular emissions of such combustible waste gases. Flares are used for waste gases that are above the UEL, and boost fuel can be added in marginal cases to provide a burnable mixture. Air or steam may be injected to promote mixing with air and reduce cracking and soot formation and smoking. Figure 3 shows an elevated enclosed flare, which has the bonus of not requiring stack testing because EPA considers it to have 98 percent organic destruction efficiency.
Direct Thermal Oxidizers.Oxidizers use the VOC, boosted by auxiliary fuel as required, to raise the gas stream to the temperature required for good destruction. Most VOCs will be removed to desirable levels at exit gas temperatures of greater than 1500°F (815°C).
Theoretical flame temperatures of 2200°F (~1200°C) are necessary to sustain combustion in a flame. This is attainable with mixtures of air and VOC that are greater than 120 BTU/ft3 when fed to high-intensity burners. Auxiliary fuel is required for mixtures below this level, with much of the destruction occurring outside the flame using a half to two seconds residence time, augmented by good mixing of air and fume. When odor is the prime problem (sewage treatment plants, coffee roaster, etc.), oxidizer temperatures of 1200 to 1400°F (649 to 760°C) are normal. Direct thermal oxidizers are of particular use for dirty streams (figure 4) that contain dust, which would foul regenerative thermal oxidizers (RTOs) and foul and erode recuperative heat exchangers.
Regenerative Thermal Systems. Regenerative thermal oxidizers (RTOs) offer higher fuel efficiencies than either the catalytic or direct thermal oxidizers, but they are much larger and more expensive to purchase (figure 5). They employ beds of heat-absorbing ceramic materials. The VOC-air mixture passes through a ceramic bed (heated in a previous cycle) and is raised to a temperature usually exceeding 1200°F. It then passes into an oxidization chamber and is mixed with the products of combustion from a small auxiliary fuel burner, raising the mixture to about 1500°F. The hot effluent from this chamber passes through a second ceramic bed on its way to the stack, preheating the ceramic material to a high temperature. When the ceramic material has been fully heated by exhaust gases, pairs of valves in the VOC inlet and regenerator chamber exhaust gas lines reverse position, starting another cycle.
For odor control when using an RTO, a three-chamber type or one with a “puff chamber” is a good idea to prevent a half-second or so emission of untreated fume when the switched bed valves change over.
Regenerative units typically are designed for 95 percent heat recovery. At extra cost, a hot gas bypass can be added to stretch the normal 3 to 5 percent of LEL limit to 20 to 25 percent of LEL.
Catalytic Recuperative Oxidizers. In these oxidizers, the catalyst is like those in an automobile exhaust. It increases the rate of the reaction and allows it to occur at lower temperatures, thereby using less auxiliary fuel. Catalytic oxidizers are used for low (less than 25 percent LEL) concentrations of VOCs. A rule of thumb for catalytic oxidizer design is that 1 percent of LEL will produce a 25°F (13.8°C) temperature rise across the catalyst. Care must be exercised in catalytic oxidizer application to prevent overheating, poisoning and fouling.
Exhaust and Makeup Air Design
The design of ductwork for high-volume/low-concentration VOC streams is critical to proper control of emissions, worker health and plant safety. For tank farm vents, elements such as vertical slip-fits (which allow air/fume in but prevent liquid entry due to overfilling), conservation vents, trapped overflow piping and controls to shut down the vent system if overfilling of tanks occurs must be part of the design.
Proper design of hoods/pickup points for the VOC vapor is required to minimize fume flow and maximize capture for manufacturing operations. Makeup air must be supplied properly in terms of volume, velocity and point of delivery. For permanent total enclosures (PTE)3 such as those used for flexographic printing), good design provides a slight negative pressure (typically 0.01 to 0.1” w.c.; EPA’s minimum is 0.007” w.c., corresponding to 200 ft/min average velocity though natural draft openings) to prevent fugitive emissions. One benefit of PTE design is that no “capture test” is required, and capture is assumed to be 100 percent. Poor design with “short-circuiting” of makeup and exhaust air can cause high VOC concentrations in stagnant areas and low concentrations in areas with high air velocity. The principles of good HVAC design1,4,5 must be employed to do a proper job.
In summary, the economical and safe control of VOCs — and doing so while burning a minimum of auxiliary fuel — entails:
• Having good data on fume flow and concentration at the start of the project.
• Being aware of constituents such as chlorine and sulfur that may create acid gases.
• Being aware of dust levels that may cause fouling as well as poisoning of catalysts.
When that data is in hand, the right VOC control system can be chosen and proper design of ductwork and hoods can be executed to complete the design.