Catalytic Oxidation is one of the many options you have when looking at air pollution control systems at your facility. Catalysts are used to reduce the oxidation temperature, resulting in lower operating costs through the reduction of fuel needed to heat an air pollution control system.
As we discussed in our recent blog comparing thermal and catalytic oxidizers, these pollution control abatement systems are designed for specific industries in which a catalyst is employed to reduce the amount of energy needed to oxidize a pollutant. By using a catalytic oxidizer, companies can reduce operating costs and minimize maintenance costs during operation.
Most notably used in automotive paint booth exhaust and parts manufacturing, metal decorating, printing, coating, laminating, converting, engineered wood manufacturing, Bakery and Food Processing, Soil Vapor extraction, and pharmaceutical applications, catalytic oxidizers have specific advantages in cost and operation that we discussed in a recent blog.
However, when looking at catalytic oxidizers, there are many considerations that go into such a product’s construction and operation. Today, we explore some of the basics in catalytic oxidation, providing a brief look at the options available when you are looking at this type of equipment.
Common Metals used as Catalysts for Oxidation
Depending on the exhaust streams, different catalysts are used in the oxidation process, but are generally metals used both as catalysts and support for catalysts.
Non-Noble Metal Oxides
Metal oxides are occasionally used as a catalyst. Cheaper and more resistant to poisoning than precious metal catalysts, they are often less durable and efficient than noble metals. Non-noble metal based catalysts can be either supported or unsupported metal oxides, and are readily available at a low price compared to noble metals.
The most commonly used metal-oxide catalysts include copper oxide, manganese dioxide, iron oxide, nickel oxide, chromium oxide, and cobalt oxide, used alone or with the support of clay or Aluminosilicate.
- Chromium: Chromium oxide catalysts are a group of very active catalysts, particularly for the removal of halogenated VOCs. Chromium oxide catalyst was effective in the removal of carbon tetrachloride, chloromethane, trichloroethylene, ethyl chloride, chlorobenzene, and perchloroethylene. However, high toxicity of chromium causes serious catalyst disposal problems.
- Cobalt: Co3O4 is one of the most active low-cost metal oxides, which has been used to treat Acetylene, Propylene, 1,2-Dichloroethane, Ethyl acetate, and propane.
- Nickel: NiO is another active metal oxide used for various catalytic applications including the oxidation of VOCs.
- Manganese: Manganese oxide is among low cost active catalysts for the oxidation of VOCs, and has been used as a catalyst for the destruction of many VOCs, including n-hexane, benzene, ethanol, toluene, acetone, propane, and NOx
- Titanium: Titanium is a relatively low cost, readily available, and chemically stable catalyst suitable for the removal of a range of VOCs.
- Vanadium: Vanadium-based catalysts originally designed for the removal of nitrogen oxides have proven to be active in the destruction of various polychlorinated pollutants.
- Cerium: Cerium is the most abundant among the rare earth elements. Cerium-based catalysts have unique properties due to their abundant oxygen vacancies associated with strong interactions with other metals, oxygen storage capacity. However, these are better suited for non-chlorinated VOCs.
- Copper: Cupric oxide is also a highly active catalyst for the deep oxidation of CO, methane, methanol, ethanol, and acetaldehyde, and is also used to catalyze the oxidation of methanol to methyl formate and propylene to acrolein.
Mixed Oxide Catalysts
Also, certain combinations of these oxides can provide a synergistic effect that improves performance.
For more information on the temperatures and efficiencies, read Catalytic oxidation of Volatile Organic Compounds—a Review from ResearchGate.
More commonly used than base metal catalysts, precious metals are attractive as catalysts due to their high efficiency for the removal of VOCs at low temperatures. Often supported by a ceramic or metallic material to prevent poisoning or sintering, there are many options available and performance is dependent on method of preparation, precursor type, particle size, metal loading, concentration of the VOCs, reactor type, and the overall gas flow rate.
Several types of supports with good thermal stability and high surface area have been used in conjugation with noble metals depending on exhaust stream, which include the following: alumina (Al2O3), zirconia (ZrO2), CeO2, SiO2, titania (TiO2),SnO2, CuO, Fe2O3, La2O3, MgO, montmorillonite, zeolites, and carbon based materials.
- Platinum: Platinum, either alone or in combination with other noble metals, is a common catalyst for air pollution control. In the supported form it is more active than the best of the base metal catalysts, it is stable up to at least 750°C and is resistant to poisoning by most elements except lead and phosphorus.
- Gold: Long considered a poor catalyst, gold combined with the right support can be effective as a catalyst for the destruction of ethyl acetate, xylene, toluene, benzene, and propane, as discussed in this Atmospheric Environment article.
- Palladium: Palladium catalysts have a higher thermal and hydrothermal resistance compared to other noble metal catalysts (Huang et al., 2008b). Palladium has been found effective as a catalyst for o-Xylene, Toluene, Xylene, and more.
Additionally, a blend of these metals with proper support could be used to provide a synergistic effect (source). While these three metals are popular, this will be analyzed and discussed in much more detail during the technical engineering meetings.
When designing a catalytic oxidizer, another design consideration is the method of contact between a catalyst and pollutant. Again, depending on the industry, concentration, and exhaust stream makeup, catalysts may take the following shape.
The most widespread method of applying a catalyst to an exhaust stream is through the use of a catalyst monolith—a porous solid block of the catalyst to be applied.
In this design, the monolith contains parallel, non-intersecting channels aligned in the direction of the gas flow, providing minimal attrition during startup and shutdown and a low pressure drop, as well as a high surface area for maximum efficiency.
Alternatively, catalytic oxidizers may use a bed of catalyst particles. In this method, catalyst particles are supported in a tube or positioned in trays through which gas passes. The tray-type/pelletized arrangement is popular in certain industries, especially those in which phosphorous or silicon compounds are present. Additionally, catalyst particulates come in different shapes, including spheres, granular pellets, extrudates, and precious metal saddles, each offering their own advantages for specific applications.
A third option available is a fluid-bed catalytic incinerator, which provides high mass-transfer rates, efficient heat transfer, the ability to handle waste gasses with traditionally higher heating values, and better particulate matter (PM) tolerance.
Catalytic Oxidizers from The CMM Group
For the right application, catalytic oxidizers provide many advantages: Lower fuel requirements, lower operating temperatures, minimized risk of fire, smaller footprint, and less need for insulation, all providing lower ongoing costs that usually offset the higher initial costs. At The CMM Group, we have been in the business of helping organizations like yours to select, install, and operate the best possible air pollution control system for their needs, and can advise you on what works best for your facility based on concentration, pollutant makeup, and more. We invite you to learn more about our products and to download our VOC Abatement Guide for more information.