Thermal treatment, including regenerative thermal oxidation, of volatile organic compounds (VOCs) and other air pollutants works by a simple reaction of the harmful air pollutants with oxygen and heat. In this environment, the VOCs are converted to CO2, water vapor (H2O), and usable heat. These harmless byproducts are released to atmosphere or use an energy recovery technique to further lower the operational costs. Thermal oxidizers have gained acceptance as viable pollution control alternatives due to ever-rising fuel costs and environmental agencies' efforts to clean up the lowest emitters of VOCs.
Three case histories demonstrate how oxidizers reduced emissions and how they could help your process.
Many industries utilize silicone as an ingredient in their manufactured products. Silicone enhances the attributes of the product or helps to improve production characteristics. Whatever the reason may be, users of solvent-borne silicones face troubling issues when it comes to the selection of the right air pollution control system.
Among the air pollution control choices available to consider -- catalytic, recuperative thermal, and regenerative thermal oxidation -- all are susceptible to operational problems. The basis for these problems stems from the oxidation process itself. During operation of the production source, silicones are driven off along with the VOCs. Because the air pollution control equipment is installed to destroy VOCs, any system also must be designed to process the silicone vapors. During the oxidation process, silicone will convert to silicon dioxide (SiO2). This SiO2 is the culprit for failing or underperforming air pollution control equipment. The SiO2 is a talcum-powder-like substance that clings to metal surfaces, robbing any system of efficiency and putting at risk the oxidation system's ability to effectively destroy VOCs.
One silicone coater worked with Catalytic Products International, Lake Zurich, Ill., to design a thermal recuperative oxidizer capable of working efficiently through the varying ranges of operations and deal with the problems of SiO2. A thermal recuperative oxidizer works by combining the harmful air pollutants with oxygen and heat. In this controlled environment, the VOCs are converted to carbon dioxide and water vapor. These harmless byproducts are released over the shell-and-tube heat exchanger for energy recovery.
The application called for a pollution control system to destroy VOCs such as ketones, alcohols, acetates and glycols, among other such organic vapor emissions. Depending on the line speed and the products width, these solvents could range from 10 percent LEL to more than 25 percent LEL. Most of the time, the coating line operates at 15 percent to 17 percent LEL.
When designing the thermal rate efficiency of the oxidizer system, it is important to rate the maximum efficiency of the primary heat exchanger to the solvent load thought to be encountered near 80 percent of the time. This allows the oxidizer to use little natural gas during normal operations. It also is important to design a system that will be capable of working through the higher solvent-loading conditions. A hot-gas bypass is a standard recommendation on all thermal oxidizers. The hot-gas bypass becomes the safety valve for the system and allows higher LELs to be safely and effectively managed, without any need for operator involvement.
In this application, the customer expected that 2 to 5 lb/hr of silicone products such as silicone dioxide (an inorganic), hexamethylcyclotetrasiloxane and octamethyl-cyclotetrasiloxane (both organic) could be found in the exhaust stream. For this reason, CPI engineered the Quadrant SR-15,000 oxidizer to withstand the rigors of a considerable amount of particulate buildup.
Features of the oxidizer include specially designed silicone transitions that force the SiO2 through the inside of the tubes to allow thorough, efficient cleaning. Through the use of a special cleaning device, the system is fully restored to its original thermal efficiency. Also, the system's controls automatically track the oxidizer's thermal rate efficiency and warn the processor when cleaning should be scheduled.
This system replaced two square-box thermal oxidizers that incorporated expansion joints and poorly planned clean-out access. Beyond the short life of the primary heat exchangers due to the frequent cycling, these systems could not be effectively cleaned of the SiO2.
The modern pharmaceutical manufacturing plant can be considered among the world's more complex manufacturing environments. These facilities, which are constantly evolving their products, demand the highest quality control procedures, and use challenging VOCs in their process systems. In order for modern pharmaceutical plants to be EPA compliant, specialized systems must be incorporated into the manufacturing operations.
Faced with meeting the pharmaceutical industry maximum achievable control technology (MACT) standards, one Fortune 50 pharmaceutical manufacturer called on CPI. The application required venting more than 30 different reactors, batch processes, storage tanks and other sources of VOC emissions to a single control device. The challenge was to design a system that was able to process the ultra-high LEL (high concentration of VOCs) and deal with corrosive acids produced in the process off-gas and the post-combustion gases. The pharmaceutical processor has several sources that use hydrochloric acid and other corrosive compounds in the manufacturing processes. These acids are mixed with VOCs and hazardous air pollutants (HAPs). Because it is not economical to separate the acids from the off-gas prior to venting to the control device, CPI designed a unit that can tolerate the corrosive nature of the off-gas.
Adding to the challenge were many more sources that use halogenated VOCs in the process. A halogenated VOC is an organic hydrocarbon that contains chlorine (Cl). In the combustion process a normal organic hydrocarbon will be converted to:
HxCx + O2 ‹ H2O + CO2
The conversion of halogenated elements is changed to:
HxCxCl + O2 ‹ H2O + CO2 + HCl
The post-combustion gases contain some amount of hydrochloric acid. To meet the needs of local regulations and the industry MACT, the HCl must be removed from the exhaust stream prior to atmospheric release.
Recognizing that high BTU vent streams require a specialized approach to operate safely and economically, CPI supplied the Quadrant NRV thermal oxidizer that processes high-BTU off-gases directly through the burner system. The vent stream is treated as a fuel gas, so higher LEL concentrations can be processed without requiring excess dilution air.
For the pharmaceutical processor, the oxidizer was outfitted with a host of special alloys to address corrosive resistance, structural stability and high temperatures. And, to meet the high VOC removal rates for halogenated VOCs, the thermal oxidizer operates at 1,500°F (816°C). Because a scrubber cannot directly accept air temperatures greater than 300°F (149°C), a quencher is used to cool the gases after the combustion chamber before being introduced to the scrubber. The thermal oxidizer and quencher/scrubber combination allows more than 99.9 percent removal of all VOCs and HAPs entering the system with more than 99.9 percent removal of HCl gases. The system incorporates a control system that precisely monitors and controls the entire operation for safety, economy and regulatory verification.
Many chemical and petroleum processing facilities must meet national emission standards for hazardous air pollutants (NESHAPs). One specific NESHAP is 40 CFR part 63, subpart FFFF, which includes facilities that are major sources of HAP emissions and process units that:
- Produce organic chemicals that are not subject to other MACT standards.
- Process, use or produce organic HAP or hydrogen halide and halogen HAP.
- Batch process vent from hazardous organic (HON) process units.
Once qualifying in the NESHAP, a facility must group the emission. If it is determined that the emission is a Group 1 status, the emission must be vented to an appropriate air pollution control device. Following this, performance testing and establishment of operating limits and motoring will be made part of the process.
One Oklahoma petroleum processor worked with CPI to integrate a custom solution into their storage tank and transfer operations. The application required a special design that would meet a constant 98 percent or more destruction of the VOCs and HAPs across a range of solvent loadings.
CPI analyzed the application and made recommendations that addressed capital costs, space limitations, operating and maintenance costs, and assurance to meet stated MON Standards.
To meet the goals of this application, CPI installed a Triton-15.95 regenerative thermal oxidizer. The emissions are generated from storage tanks and transfer operations, and in some instances, high amounts of VOC emissions will be vented from cleaning operations. The oxidizer was designed to operate economically during a majority of the times when the VOC loads are very low. Complicating this application are the emissions generated when tank cars are loaded with the petroleum products and when the cars are cleaned with a mixture of solvents cleaning agents. The air volumes under normal operations are typically about 7,000 scfm. The system has a dual redundancy via dilution air and hot gas bypass.
Once installed, the system exceeded the NESHAP requirements by providing 98.5 percent destruction of all VOCs. The system's hot gas bypass and a dilution air system allow solvent loadings of more than 20 percent LEL to be safely processed.
This article was contributed by Catalytic Products International, Lake Zurich, Ill. Catalytic Products International designs and manufactures custom air pollution control systems such as catalytic oxidizers, concentrators, particulate control, energy conservation systems and special services. For more information, call (847) 438-0334 or visit www.cpilink.com.