Environmental Protection

Meeting MACT Head-On

A new air toxics control system helps manufacturers achieve EPA standards by handling VOC and HAP emissions

The DuPont Front Royal plant has been the leading name in the automotive refinishing industry. DuPont Performance Coatings, formed from DuPont Automotive finishes and DuPont's acquisition of Herberts, is the world's largest supplier of original equipment manufacturer (OEM) and aftermarket coatings and the world's third largest coatings company, overall. DuPont has continually reduced solid waste generation and air emissions over the past five years. Since 2000, DuPont has reduced solid waste by 30 percent and air emissions by more than 50 percent.

The DuPont Front Royal site is continuing its commitment to environmental excellence by installing an additional abatement system to further reduce its air emissions 50 percent.

Like with many other manufacturing facilities, such as semiconductor fabricators, miscellaneous organic chemical manufacturing, miscellaneous coating manufacturing, reinforced plastic composites, painting processes, and others that utilize volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) in their products, low concentration, high volume (LCHV) room ventilation exhaust becomes a major economic issue in meeting their maximum achievable control technology MACT requirements under the Clean Air Act.

The low-concentration, high-volume (LCHV) VOC / HAP process exhaust streams substantially increase the capital and operating costs associated with most air emission control equipment. As most plant manager have already realized, the cost of natural gas has gone through the roof, and their low-efficiency thermal oxidation equipment is burning up their production profits faster than they can produce them. Natural-gas costs have nearly doubled in the past year, and it is expected to continue to increase in the years to come.

To meet MACT requirements and minimize operating costs, DuPont selected a hybrid concentrator and rotary-valve regenerative thermal oxidizer (RL RTO) system. The hybrid system virtually eliminates the need for additional auxiliary natural gas. The energy produced from the combustion of the high-concentration, low-volume (HCLV) of VOCs / HAPs provides nearly all of the energy to self-sustain the operation of the RL RTO system.

The system provides control for several areas, including manufacturing building exhaust with concentrations less than 75 PPMv (parts per million), solvent storage tank vents with high polar vapor component (Methanol), manufacturing process vents with VOC / HAP, resin-manufacturing process vents with a wide vary of components, and solvent-recovery process vents. All of the various areas have different process flows, solvent types, solvent concentrations, and operating schedules that required a specialized control approach in order to minimize operating costs and maximize control performance to meet MACT requirements.

The overall system control efficiency is critical for a site to meet synthetic emission minor status and maintain its right to operate. Therefore, to achieve the required reduction in emissions, all of the factors from the sources must be addressed to ensure the plant stays below MACT limits. The combined overall destruction removal efficiency (DRE) must include equipment uptime as well as capture and destruction efficiency (see Table 1).

Table 1

Capture efficiency
Concentrator wheel removal efficiency 97.0%
RTO Destruction efficiency 99.0%
System Uptime
(Not Counting Planned Down Time)
99.0%
Overall System
(Capture X Destruction X Uptime)
95.1%

This overall control efficiency must meet MACT requirements, which are based on standards for each listed source category according to a prescribed schedule and are based on the best demonstrated control technology or practices within the regulated industry. Typically these sources are based on the Title III requirements (see sidebar).

The Hybrid System
The system includes additional features to allow unexpected downtime of an individual concentrator module and still maintain the full process flow through the remaining system. This, along with other redundant and/or high-reliability designs, ensures that the system will maintain the high efficiency uptime requirements necessary for successful compliance.

The basic system operates in the following fashion. The LCHV process streams are directed to four two-stage pre-filter systems to remove any potential dust and dirt prior to the adsorber units. These units include monitoring and easy access to clean and replace filters as needed without disrupting the overall process operation. After the filters, the process then enters into the zeolite rotating concentrator disc, where zeolite media adsorbs and removes the incoming VOCs / HAPs, before the clean air is discharged to the atmosphere through an induced-draft, low-horsepower fan to a common stack.
A slip stream results from the LCHV process flow after the filtration is used to cool each concentrator zeolite disc after the higher temperature de-sorption process. This stream is then directed to an air-to-air heat exchanger where it is preheated by the hotter exhaust from the RL RRTO unit before it is returned to the concentrator disc to strip the disk of collected VOCs / HAPs. The air stream, with its concentrated VOCs / HAPs, is then sent to the rotary valve RL RTO unit for thermal oxidation into carbon dioxide and water.

The honeycomb zeolite structure of the adsorbent media disc yields a very low pressure drop, which minimizes electricity needed for concentrator exhaust fan power. In addition, it provides heat transfer to minimize the overall fuel costs. The fuel costs are also kept extremely low by the reduced size of the thermal oxidizer -- only 5 to 20 percent of the process flow is actually sent to the oxidizer (but this includes nearly all of the VOC / HAPs), which in most cases will self-sustain in the RL RTO unit, requiring little or no auxiliary fuel.

The concentrated stream, along with the other non-compatible process streams, are drawn into the inlet side of the RL RTO rotary valve and directed up through five of the 12 heat recovery chambers, which provide 95 percent thermal energy recovery. The HCLV stream is preheated in the ceramic heat-recovery bed to the auto-ignition temperature of the solvents. The spontaneous combustion in the heat recovery beds, approximately 3 percent of the lower explosive level (LEL) or greater, will provide all the energy to complete the oxidation process. This allows the RL RTO system to self-sustain or maintain the purification temperature to complete the oxidation process with little or no auxiliary fuel. The hot, purified clean-air stream is drawn down through another set of five heat recovery chambers where the energy is recovered and stored for preheating the next cycle of concentrated solvent air as it enters the RL RTO.

The rotary valve sequences each heat-recovery chamber from inlet to flushing, which provides the ability to achieve more than 99 percent destruction-removal efficiency. The RL RTO unit includes a variable energy system that provides additional energy to the air-to-air heat exchanger system for desorption on the concentrators. In addition, it can provide the periodic high-temperature energy for regeneration of the concentrator disc. The HCLV or non-compatible concentrator solvents go directly to the inlet of the RL RTO unit and are processed with the concentrated stream from the disc.

The system includes:

  • A completely negative pressure (induced-draft) system to eliminate the potential of fugitive leakage or emissions;
  • A separate control room for the complete system control;
  • Four skid-mounted modular concentrator units that allows individual module shutdown while maintaining full process flow through the rest of the system;
  • Four skid-mounted modular integral two-stage filtration system that also allows individual module shutdown while maintaining full process flow through the rest of the system;
  • Design provisions for additional future concentrator modules to provide increased capacity and/or removal efficiency;
  • An online full-flow, bake-out feature with rotary valve regenerative thermal oxidizer (RL RTO) unit;
  • An integral RL RTO dual-fuel, nature-gas, and #2 Oil combustion system is included, which eliminates the need for a separate inline concentrator heater;
  • A system design that eliminates the potential nitrogen oxide (NOx)and concentrator media degradation from a direct fired nature gas and # 2 oil inline heater operation; and
  • A single process exhaust stack to facilitate a single continuous emission monitoring (CEM) location
DuPont selected a hybrid concentrator and rotary-valve regenerative thermal oxidizer (RL RTO) system to meet their MACT requirements and minimize their overall operating costs.
This type of hybrid system with thermally efficient RL RTO units, increased reliability and up-time design, and short installation designs with shop-assembled, skid-mounted modular units, make it possible for industry to remain in regulatory compliance while continuing to turn a profit.

Title III: Air Toxics
The list of source categories under this section of the Clean Air Act must include: (1) major sources emitting 10 tons per year of any one, or 25 tons per year of any combination of those pollutants; and, (2) area sources (smaller sources, such as dry cleaners).

This article originally appeared in the 09/01/2006 issue of Environmental Protection.

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