Thermal Treatment Systems for Volatile Organic Control

The following is a brief summary of the various thermal treatment systems available to the process engineer in treating volatile, hazardous organic wastes in an air stream. The purpose is to discuss the subject of oxidation.

Oxidation, either catalytic or thermal, is a well developed technology for VOC control. Although oxidation is not the only treatment available for controlling VOC's, it is advantageous in the fact that the pollutants are destroyed, as opposed to being captured. Other approaches to the control of VOC's are condensation, adsorption, absorption and biofiltration. Oxidation units can destroy nearly 100 percent of the VOC and toxic emissions targeted by the Clean Air Act Amendments (CAAA) of 1990. In fact, depending on the type of system, destruction efficiencies of over 99.99% can be attained. According to some sources, industry will spend more than $200 million on fume incinerators this year, which represents about 40 percent of all VOC control equipment purchases.

The decades-old technology operates on a simple premise: Sufficiently heating a VOC consisting of carbon and hydrogen, in the presence of oxygen, will reduce the VOC to harmless carbon dioxide and water.

In this general reaction, the heat of formation of the products, typically, is less than that of the reactants, thus yielding a heat release. To initiate this reaction, a certain amount of energy is required. If the amount of energy released in the formation of the products is greater than the amount required to initiate the reaction, the reaction will sustain itself without further input of energy. When a catalyst is used, the reaction remains the same except the energy required to activate the reaction is lowered. Typical catalyst used are either precious metal-based or metal oxide-based.

Sometimes the organic compounds contain additional elements such as nitrogen, chlorine or sulfur. The addition of these elements creates complications with additional products of combustion. When the chlorine is present, the products of the general reaction will contain HCl and Cl2.

The amount of Cl2 will decrease with increasing combustion chamber temperature. At times it is more desirable to have the chlorine in the form of HCL due to the ease which it can be scrubbed. Both of these compounds are undesirable in the exhaust gas stream, with the need for an acid gas scrubber usually required. The presence of the HCl is also cause for downstream corrosion and necessitates careful selection of materials of construction.

If sulfur is present in the reactants, the percentage of SO3 decreases with increasing oxidation temperature. In practice though, SO3 is generally seen. Similarly, an acid gas scrubber is typically required downstream of the oxidizer.

As with any incineration process, the critical elements of successful thermal treatment are time, temperature and turbulence. This means the waste gas must be kept at incineration temperature for an adequate amount of time, generally 0.75 to 1 second. To destroy 98 - 99 percent of non-chlorinated VOCs, a thermal process will heat VOCs to about 1500°F, whereas a catalytic system will heat the pollutants to around 700°F.

Factors influencing the temperature of an individual application include the chemical makeup of the VOC, and oxygen availability. For some chlorinated hydrocarbons, the thermal incineration temperature can be in the 1800°F range. The following table lists the destruction temperatures at one and two second residence times for several compounds for thermal incineration. The following data verifies that the chlorinated compounds generally require higher temperatures for adequate oxidation. When a catalyst is used, it is best to have actual performance data due to the difficulty in generating a model.

It should be noted that the data as listed in the above table was generated using kinetic theory, which assumes ideal mixing, which is not the case. Less than ideal mixing forces the design engineer to place a safety factor into the design oxidation temperature. As a measure of mixing, turbulent flow ensures adequate contact between process air and VOCs. Turbulence may be achieved in a variety of ways, such as through the use of fluidized beds, stoneware, internal baffles, etc.

Applications for oxidizers include treating fumes from chemical/hydrocarbon processing operations, printing, paint finishing, coil coating, soil vapor extraction and pharmaceuticals manufacturing. Oxidation units are often utilized with a plant's coating, drying and other processes.

High energy demands make heat recovery a must when treating high-flow, low-concentration air streams. Aside from whether they use an open flame or catalyst to oxidize VOCs, what often sets oxidizer types apart is how they recover this heat. The oxidizer's approach to heat recovery usually is the limiting factor to the system's process applicability. An oxidizer which is highly efficient in capturing heat will not be feasible for use with a gas stream which has a high amount of available energy when oxidized. In a similar fashion, a gas stream which has very little available heat when combusted will necessitate a highly efficient heat recovery device if the oxidizer is going to be cost effective.

The two primary approaches to heat recovery are recuperative and regenerative. The recuperative exchanger transfers heat through a surface (a heat exchanger tube, for instance) from one fluid to another as long as a temperature gradient exists between the two fluids. In a regenerative system, a hot fluid transfers heat to the surface over a period of time, after which the colder fluid is then passed over the same surface, absorbing heat from the surface.

Depending on the type of oxidation system and recovery design, as much as 95 percent of the thermal energy can be recovered to be reused in the oxidizer, in other industrial processes, or to provide building heat, etc. A brief description of several different types of oxidizers follows.

Afterburner
The typical afterburner consists of a burner, a burner train, a combustion blower and, if necessary, a process fan. Usually, this scheme is not used unless the concentration of the organic pollutant is elevated high enough to yield available energy to heat the products of combustion up to the chosen oxidation temperature. If the gas stream does not contain enough energy, then a burner is supplied to provide the necessary heat. Often, this type of system proves to be uneconomical from an operating cost basis, due to the high gas usage when oxidizing low concentrations of VOC's in the gas stream.

Recuperative
In a recuperative unit, the basic operation of the afterburner is retained except that much of the waste heat is captured. The fact that the system can capture this heat allows for it to operate very economically down into the 25% LEL range. In this type of system, a metallic tube or plate-type heat exchanger is built into the exhaust end of the combustion chamber of the oxidation system. Typically, a plate exchanger is used when the exhaust gas stream does not contain elevated amounts of particulate and the maximum amount of heat recovery is desired. It is not unusual for a plate type heat exchanger to be capable of achieving 70% heat recovery, while a tubular exchanger will recover 40% to 50% of the available heat. The heat recovery of a heat exchanger is defined as the amount of heat transferred divided by the maximum amount heat that could be transferred.

Recuperative systems generally are smaller and lighter in weight than other oxidation systems, allowing for skid mounted installations.

Recuperative systems work best with relatively high and stable VOC concentrations. Otherwise, auxiliary fuel is needed for air streams with little fume energy, or for cyclical applications. With recuperative systems, high nitrogen oxides (NOx) production might be a concern because typical designs allow for direct flame contact. Also of concern is the possibility of autoignition in the tube or plate section of the heat exchanger where the gas stream temperature passes through the autoignition point of the organic compounds it contains.

Due to the use of metallic heat exchangers, recuperative systems are typically limited to operating temperatures below 1400°F, restricting the process engineer to resort to increasing residence times and higher turbulence. When chlorinated compounds are encountered, acid resistance refractories are needed. This causes the cost of the systems to increase.

Catalytic
Catalytic oxidation systems are another option for low VOC concentrations. These units are similar in design to recuperative units, but oxidize solvents with precious metal or metal-oxide based catalysts, instead of open flames. Operating at about half the temperature of thermal oxidizers, catalytic units have small footprints and relatively low operating costs.

Catalytic beds may be fixed, in which the catalyst is not allowed to move; or fluidized, with pellatized catalytic elements in a turbulent environment to allow greater contact between VOCs and the catalyst. Disadvantages of the catalysts are their susceptibility to poisoning, masking, or high temperature deactivation. Poisoning of the catalyst consists of the reaction of the catalyst metal with some component of the gas stream (lead for example), while masking is a process whereby the catalyst is coated with some inorganic material (sodium, for example). A catalyst which has been masked can usually be cleaned and returned to service, while poisoned catalyst must be replaced. Elevated temperature also has a negative effect on catalysts. The thermal aging process of a precious metal-based catalyst usually involves the migration of the well-dispersed, low concentration precious metal from many activation sites on the surface of the catalyst, to fewer, high concentration activation sites. In a metal oxide-based catalyst, the mechanism usually involves the breakdown of the crystalline structure of the metal oxide. In general, precious metal catalysts have better elevated temperature aging resistance than metal oxide catalysts.

Fixed or monolithic beds have more surface area for a given pressure drop than do the pellatized or fluidized bed catalyst systems, requiring less energy consumption.

A catalyst in a properly designed system may be effective for several years, but still might become coated and less effective through contact with compounds such as silicone, heavy hydrocarbons or particulate. Of particular note, very few catalysts can remain active in a chlorinated environment. For this reason, catalytic systems work best with relatively clean air streams with well-defined contents. Process upsets also must be scrupulously avoided to prevent damage to temperature-sensitive catalysts.

Breakthroughs in catalyst technology, however, continue to make catalysts more resistant to deactivation. A fluidized system using chromia-alumina catalyst beads, for instance, is successfully treating chlorinated compounds in air streams from stripped groundwater at an Air Force base in California. Trichloroethylene concentrations of (1 to 2 ppm) are being destroyed up to 97.5% efficient.

Regenerative
A regenerative system provides extremely high thermal energy recovery. This process uses a ceramic heat exchange bed to preheat process air to within 5 percent of the oxidation temperature. The ceramic bed typically consists of either structured or random packing. Historically, random packing has been used, with 1 inch porcelain "saddles" being the most common media, but recently, structured packing in the form of honeycomb ceramic monolith has been utilized in many applications. The main advantage of the monolith media is its lower pressure drop derived from its greater void fraction and its inherent laminar flow characteristic. Saddle-based systems typically are lower in cost due to the mature market for ceramic saddles and their lower cost to manufacture.

Initially, the incoming process gas passes through a ceramic heat recovery bed before entering the combustion chamber. It is during this step in the process that the gas is preheated to within 5% of the combustion chamber temperature. After the process stream exits the ceramic bed, the already hot gases are further heated to the desired combustion chamber temperature. These gases are then sent through another heat exchange bed, where energy is absorbed and stored to heat the next cycle of contaminated air. Up to 95 percent of heat energy can be recovered with this multiple-bed approach. Low VOC concentrations can be processed in a self-sustaining mode without burning extra fuel.

Regenerative systems are usually large and capital intensive, but recent trends and a competitive marketplace are reducing the size and the cost. The typical bed velocity is between 200 and 250 fpm on a standard basis with random packing (gas volume referenced to 70 deg F. and 14.696 psia), or 300 to 350 fpm when structured packing is used. The typical pressure drop across a system is about 22 in WC for a random packed unit and 15 in WC for a monolith based unit.

Preventative maintenance and downtime is typically more extensive when hydraulic or electrically driven valves are used, versus mechanically driven valves. The potential drawbacks of the large and capital intensive systems are offset by fuel savings, the ability to operate at high temperatures (2000°F), and high destruction efficiencies. Furthermore, with this process, a highly pre-heated waste stream's exposure to high temperature flames is minimized, thus reducing concerns about NOx generation.

Regenerative systems are recommended for air flows typically above 10,000 scfm and solvent concentrations below about 10 percent LEL. For air streams with compounds that might build up in the heat recovery beds, regenerative systems can incorporate a "bake-out" feature to elevate bed temperatures and burn off condensable organic compounds. Due to dependence on the sealing characteristic of the diversion valves, destruction efficiencies above 99%, although possible, is not typical.

Regenerative Catalytic
A more recent addition to the oxidation technologies available to the process engineer is the regenerative catalytic oxidizer. This device is very similar in operation to an RTO, but with a layer of catalyst in the combustion chamber. Both precious metal and metal oxide-based catalysts are presently in use. This technology has only recently been developed, with long term success or failure still to be determined.

The net advantage of this type of unit is an additional reduction in operating costs by approximately 30%. This savings comes from the decreased combustion chamber temperature and the reduction in pressure drop across the system.

Conclusion
Companies considering VOC control methods should be mindful of the versatility of oxidation systems. Energy recovery, in turn, should be a major consideration when selecting a specific oxidation unit. The following example points out some of the more basic calculations used when designing a thermal oxidation unit.

The basis for this calculation is taken from the original design specification for the Ott/Story/Cordova design specification. The example examines the effect of changing the heat exchange media from in the RTO, to a 50 cell, honeycomb monolith media.

References

• Theodore, L., Reynolds, J., DuPont, R., Hazardous Waste Incineration Calculations, Wiley-Interscience, New York, 1991.
• Lee, K.C., Morgan, N., Hansen, J., Whipple, G., Revised Model For The Prediction of the Time Temperature Requirements For Thermal Destruction of Dilute Organic Vapors And Its Usage For Predicting Compound Destructibility, Union Carbide Corporation, South Charleston, West Virginia, 1982.