Meeting the food-processing industry's new MACT requirements — Part 1
MACT standards for eliminating VOCs from food-processing-plant stacks are a looming challenge for the industry. Part one of this article covers some prerequisites.
By Isaac Ray
When cooking food, some fatty components escape from the process as vapor in the form of volatile organic compounds (VOC). These emissions from the cooking process may contain dioxins and furans not unlike those from a chemical plant. That is why the U.S. government's Maximum Achievable Control Technology (MACT) standards for the food-processing industry, beginning in 2001, are as tough as those that apply to the chemical industry.
Blue Smoke
Cooking oils and fats are organics. Heated in the cooking process, some of the organic compounds vaporize into high-molecular-weight VOCs that manifest as visible blue smoke and invisible odor.
Smoke basically comprises tiny solid or liquid particles of VOCs of less than one micron suspended in the gaseous discharge. The opacity of the smoke from the stack—measured according to the U.S. Environmental Protection Agency's Method 9—can range from 0 (nil) to 100% (impenetrable).
The opacity of smoke actually relates to the quantity, rather than the weight, of particles present in the gas. For a given particle size and stack diameter, percentage opacity is a function of the grain loading or weight of particles, measured as grains per cubic foot (1 lb = 7000 gr).
To reduce an exhaust-stack discharge with opacity of 20% to the EPA-mandated "zero opacity," requires an air-pollution-control system with a removal efficiency greater than 95% for particles of less than one-micron size.
Odors
The odors associated with food-processing-plant emissions usually are caused by VOCs with molecular weight of less than 200. The odor molecules attach themselves to the particles of smoke and can be carried great distances from their point of origin.
However, where there is no smoke there is no odor, since once air-pollution-control equipment has abated the smoke, odor molecules have no vehicle to carry them. For this reason, installation of smoke-abatement equipment mostly solves the odor problem, too.
However, some food-preparation processes—cooking onions, for example—have an odor potential not entirely related to the smoke. If odor from low-molecular-weight, non-condensable organics continues to cause a problem, relatively inexpensive equipment is available to address it specifically.
Abatement Strategies
Selection of an air-pollution-control system should involve several orderly steps:
- evaluation of the existing exhaust system;
- consideration of exhaust-stream precooling; and
- consideration, comparison, and selection of optimum control equipment.
Existing-system Evaluation
To ensure that the existing system will properly complement any new control equipment requires evaluating the performance of the existing exhaust system and, if necessary, improving it. Major considerations include:
- keeping smoke out of the work area by maintaining an exhaust flow of 150 cfm for every square foot of hood area by keeping the pressure in the hood suitably negative above the cooking equipment; (the exhaust-gas volume also should be adequate to absorb moisture evaporated from the cooking process);
- paying particular attention that the exhaust-system ductwork (that leads from the cooking equipment to the cooling section of the pollution control system) be of all-steel construction and welded and tested for gas and liquid tightness (with the welded seams located at the top of the duct); and
- preventing the possibility of condensation of VOCs in the exhaust system and subsequent dripping of condensate back into the kettle or oven—to preserve product quality and FDA requirements and maintain pollution control.
Also, to keep the droplets moving and their temperature high until they enter the cooling section of the control system, the gas velocity through the duct should be sufficient to the purpose and the ductwork should be as short as possible. In addition, insulating the entire run of exhaust-system ductwork is usually a wise procedure.
Exhaust-stream Precooling
For the air-pollution-control system to capture VOCs, the vapors must be condensed. Cooling the air stream is therefore an important first step in the capture process. The efficiency of the cooling/condensation process depends on the boiling temperatures of the VOCs, their concentration in the exhaust gas, and operating temperature of the control equipment.
In food-processing applications, most VOCs condense at the saturation temperature of the exhaust stream, which usually ranges between 110ºF and 160ºF. As a rule of thumb, the amount of condensed VOC doubles for every 30ºF of temperature drop from the initial condensation point in the cooling section of the control equipment.
Occasionally—as when the exhaust contains substantial amounts of low-molecular-weight organics—reducing the gas temperature to 100ºF or less is needed to achieve reasonable efficiency of condensation.
However, excessive cooling is costly. The operating temperature of the cooling section therefore should be selected to ensure effective condensation at minimal capital and operating costs.
Cooling Equipment
Cooling can be accomplished indirectly with shell-and-tube heat exchangers or directly by injecting cooling liquid in the pre-scrubbing section of the control system.
The appropriate selection takes into consideration the composition of the exhaust, quantity of solids, and—if applicable—the processing plant's previous results with similar systems.
Tubed heat exchangers rarely are the optimum selection, because the tubes tend to clog, and cleaning them is difficult. If the exhaust contains relatively small amounts of easily removed organic oil, the heat-recovery potential of a tubed exchanger can effect meaningful savings. However, such savings can quickly be eroded by greater-than-expected maintenance costs.
Therefore an evaporative cooling system (cooling water sprayed into the exhaust gas) normally is the system of choice. It provides simple, effective cooling with minimum maintenance, for most food-processing applications. On the other hand, evaporative cooling produces water droplets contaminated by oil, adding to total inlet load on the treatment system.
About the author: Isaac Ray, PhD, is a principal with Croll-Reynolds Clean Air Technologies, Westfield, NJ.