- By Peter Anderson
- Feb 01, 2002
Demanding clean air standards and rising energy costs combine to make catalytic oxidation progressively more popular as the preferred choice for abatement of volatile organic compounds (VOCs). But how does the catalyst really work? What types of abatement scenarios favor this approach? How much fuel does catalytic oxidation really save? Here are some straightforward, useful answers.
How Does It Work?
VOCs are common pollutants found in the exhaust air of manufacturing plants in a variety of industries including pharmaceutical, semi-conductor, printing, painting, coating, refining, fiber and textile and other operations. Removal of these VOCs is appropriate in order to comply with clean air standards under the Clean Air Act aimed at reducing smog-creating and other toxic implications. Thermal oxidation provides an effective VOC abatement process, with a destruction removal efficiency (DRE) in a well-designed system often in excess of 99 percent.
Raising the temperature of the air stream to a point where sufficient energy is provided to overcome the activation energy is required for the oxidation reaction to proceed -- typically in excess of 1400 degrees Fahrenheit where the VOC molecules are actively broken apart and oxidized to carbon dioxide and water. In the presence of an effective catalyst (often a noble metal such as platinum or palladium), however, the activation energy barrier is reduced. The rate of reaction is accelerated and oxidation is accomplished at a lower system temperature -- typically starting at approximately 400 to 500 degrees Fahrenheit. Thermodynamically, the reaction will yield the same net change in enthalpy (heat content). The catalyst only affects the rate at which the reaction proceeds. Classically, the catalyst remains unchanged and may go on to repeat this feat indefinitely until compromised by other factors. Most importantly, the catalyst facilitates a faster reaction in smaller volume equipment and an attractive energy savings associated with operation at significantly lower temperatures than would otherwise be required.
Catalytic VOC oxidation obviously requires intimate contact between the VOCs, oxygen and the catalyst. Manufacturers typically employ a ceramic honeycomb structure for catalyst support and pass the heated reactants (air and VOCs) over its surface. A metallic foil-like substrate is also available that offers improved thermal resilience, lower weight, smaller footprint and silhouette. These are important features when rooftop or other installations require small size and low weight.
The parallel honeycomb channels may be hexagonal, square, triangular, etc. in cross-section and arranged in a density of several hundred cells per square-inch. In order to increase the available surface area and maximize contact with the catalyst, this substrate is normally wash-coated with a micro-porous refractory, such as aluminum oxide and then impregnated with platinum or the catalyst of choice. Depending on its crystal structure, the alumina may have a surface area of 100 to 200 square meters per gram (m2/gm.) This provides an effective structure for attaching the catalyst and attracts reactants via adsorption. Optimized honeycomb or cell design is critical in achieving maximum DRE while imposing minimum pressure drop (and therefore minimizing fan horsepower) on the air stream.
Environmental applications favoring catalytic oxidation typically have VOC concentrations from a few hundred to a few thousand parts per million (ppm.) In this concentration range, the kinetic steps in achieving VOC destruction are limited by the rate of bulk mass transfer of the reactants to and from the catalytically active surface. Although this might suggest that smaller is better when designing the honeycomb cells, it can be overdone and the reaction actually quenched. This is somewhat akin to a charcoal briquette that has just started to burn and can be accelerated by a small flow of air but extinguished by too much. Ideally, the honeycomb cells would be presented in a smaller size as air progresses through the catalyst bed and the concentration of reactants decreases -- a "graded cell" approach.
Catalyst life is theoretically unlimited, but in actual practice may often be reduced to five to eight years by inadvertent deactivation. Aging mechanisms include poisoning, masking and sintering. Noble metal catalysts like platinum are deactivated by exposure to certain chemical poisons, such as mercury, lead and cadmium, which form inactive alloys with the platinum. The catalyst sites can be covered up or masked by accumulation of inorganic scale, phosphorous (from lube oils), dirt, etc. Various forms of sintering can occur at high temperature, in which the catalyst particles tend to agglomerate into larger crystals with a resulting decrease in activity. The crystal structure of the aluminum oxide (wash coat) may also change at elevated temperatures with the occlusion (retention) of the catalyst particles and a reduction in activity. The alumina can be treated to resist sintering by removing fluxing agents like sodium oxide and incorporating refractory additives, such as lanthanum oxide. Sintering is further discouraged by the use of high temperature shut-off controls in catalytic equipment. Inevitably, however, some form of deactivation normally leads to catalyst aging and the need for occasional replacement.
When Should It Be Used?
The energy savings, smaller equipment size and other operating features associated with catalytic oxidation can be very attractive, but like all technologies it has its "best fit" and limitations. A careful assessment of many factors is needed to optimize the selection of a treatment scheme. For common environmentally motivated VOC abatement (air purification) applications, consider the following
Typical system capacities range from 100 to 10,000 cubic feet per minute (cfm.) At lower rates, the fixed components of a catalytic system tend to be pricey and harder to justify. At higher flows, particularly with lower VOC levels, concentrating devices such as zeolite rotors may extend the attractive capacity range for a catalytic system.
Catalytic oxidation has been chosen as the preferred technology for treating relatively low VOC concentrations (~10 ppmv.) At lower VOC levels (a few ppm or parts per billion (ppb)) it becomes progressively more difficult to justify oxidation verses other alternatives, such as carbon adsorption (if the VOCs are well adsorbed). An upper concentration range is limited by thermal considerations. In order to avoid an excessive temperature rise across the catalyst, systems are normally considered for streams having a heat content less than five to six (British thermal units per standard cubic feet (Btu/scf.)) The corresponding VOC concentration will depend on specific chemical composition. Typical levels are 2000 to 3000 ppmv, but higher concentrations can be accommodated in the inlet stream by introducing a controlled flow of dilution air. Efficient heat recovery can minimize fuel cost and preserve the attractive thermal advantage of the catalyst.
Exposure to chlorine and other halogens will inhibit traditional noble metal catalysts containing platinum or palladium and is therefore avoided by many system suppliers. Others use alternative catalysts, but these must operate at considerably lower space velocities than catalysts with non-halogenated VOCs and equipment tends to be much larger.
Lower Explosive Limit
It is generally recommended that the inlet be below 25 percent of the LEL for safety considerations.
Catalytic systems typically have a smaller "footprint" than a similarly tasked thermal system. This can be particularly important when space limitations demand rooftop installation.
How Much Does It Fuel Require?
Some supplemental fuel is required to achieve the threshold operating temperature for effective catalyst operation. Here is an easy-to-use graphical approximation of the fuel required for catalytic oxidation of various flow rates, compounds and concentrations. As an example, consider a 5000 scfm air stream containing 50 pounds per hour (lb/hr) of isopropyl alcohol (IPA).
The first step is to determine the heat content of the inlet air using Figure 3. Notice that separate correlations are provided for "Typical VOCs" (non-oxygenated) and those that already have some oxygen in their structure (like IPA). In this example, the IPA laden air will have a heat value of about two Btu/cf. This heat content will be liberated as the warm air stream passes over the catalyst and the IPA is oxidized. The supplemental fuel required for the system burner can be estimated from Figure 4, and is about 700 thousand British thermal units per hour (MBtu/hr) for our IPA example. Heat exchange efficiency may be restricted by the heat content of the inlet stream, since catalyst temperature and burner control can become difficult to maintain if excessive heat is recovered from "hot" (high VOC concentration) streams.
By contrast, oxidizing the IPA in this example using a thermal oxidizer without the benefit of a catalyst (operating at about 1500 degrees Fahrenheit) would require about 2300 MBtu/hr (using high efficiency heat exchange). At a fuel cost of $5/MMBtu, this earns catalytic oxidation a potential savings of nearly $70,000 a year.
The Final Answer
In summary, catalytic oxidation has become a common VOC abatement choice for a broad range of flow rates and concentrations. Well designed systems are smaller and lighter weight than their non-catalyzed thermal alternates. Operating at lower temperature, catalytic units suffer less corrosion, provide longer equipment life and require less fuel -- an attractive combination of features.
This article originally appeared in the 02/01/2002 issue of Environmental Protection.