Emissions Equipment

Q. I am looking for advice on emission equipment to meet 10 pounds per day requirement for releasing isopropyl alcohol. Our current process emits about 30 pounds per day of isopropyl alcohol into the air. L.K.

A. Your question would make a nice size environmental consulting project so with the limited amount of space, let me quickly describe the issues and possible technologies.  Isopropyl alcohol (IPA) is regulated as a smog forming volatile organic compound (VOC). 

First, let’s estimate the minimum exhaust rate for this process assuming that your process operates eight hours per day, IPA has a vapor density of 27.63 pounds per cu ft at standard temperature and pressure, IPA has a lower explosion limit of 2.5% and the maximum allowable IPA concentration in the exhaust is 25% of lower explosion limit (LEL) for safety reasons.  Using this data, we have the following calculations:

• 30lbs IPA/(8 hrs × 60 min/hr) = 0.063 lbs IPA/min
• 0.063lbs IPA/min × 27.63 cf/lb = 1.74 cf of IPA vapor/min
• 1.74 cf/min/(2.5% × 25%) = 278 cf/min

If we round up for safety to 300 cfm of exhaust, we still have quite a small volume to deal with.  Furthermore, we need a technology that needs to have a removal rate of somewhat more than 67% ((30 - 10 lbs/day/30 lbs/day) × 100%).
 Second, let’s identify potential technologies for IPA removal.

By passing the IPA laden air past a refrigeration coil, one can condense out the IPA but so will water vapor condense out.  To recover the IPA, you would likely have to go through a second distillation or use membrane filtration to separate the IPA from water.  The capital and operating costs for this technology are very significant.

Scrubbing out the IPA with water can work, but it would take a significant amount of water, and you would have to get permission to discharge this scrubber water to the sewer where the IPA can revaporize into the sewer and air.  Your local air pollution control agency would likely not allow this technology unless you can otherwise remove the IPA from the scrubber water.

Activated carbon can also be used to remove IPA from air.  Assuming that the activated carbon can absorb 25% of its weight in IPA, you would need to use 80 pounds per day ((30-10 lbs/day)/25%) of activated carbon. Activated carbon canisters come in sizes as small as 55-gal drums weighing about 140 pounds and providing 1.75 operating days of capacity to thousands of pounds. An exchangeable canister containing 1,000 pounds of activated carbon can provide you 12.5 days of operation.  While activated carbon systems have fairly low capital costs, in your situation, it will have a very high operating costs to regenerate the activated carbon after it is spent.

The three most feasible technologies to meet your needs are thermal oxidation, biofiltration and biotrickling filter.  Over the last ten or so years, these technologies have been successfully miniaturized to meet the needs of thousands of bioremediation projects, such as leaking underground storage tanks, spills, abandoned factories, Superfund hazardous waste site, brownfield cleanup needed for development and so forth.  These technologies can economically treat small air flows that you need.
Thermal oxidation can be accomplished directly by incineration at 1,200–1,800°F or by catalytic oxidation at 300–800°F. The obvious advantage of catalytic oxidation is its significantly reduced operating energy costs, however, the catalyst’s initial and periodic replacement costs can be significant.  Most suppliers provide both types of technology and can quickly assist you to determine which makes the most sense for your application. These technologies can easily provide +98% removal.
Biofiltration and biological scrubber technologies use IPA as a food source for bacteria to convert the IPA into water, carbon dioxide and more “bugs” and can easily provide +80% removal of IPA. With biofiltration, the air and IPA vapors are blown through a fixed moist media bed, like a controlled compost pile.  As the IPA vapors pass through this media, the IPA is absorbed into the liquid film so that it is available as food for the bacteria in the bed. While the incoming air has to be humidified in order to keep the media moist, its energy consumption is quite low and the media bed can last for years. Because the bacteria are fixed to a media, which contains stored food, this process is better able to withstand long shutdowns of your process. 

Biological scrubbers basically combines biological activity and air scrubbing.  In this technology, the recirculating scrubbing fluid contains a very large amount of bacteria.  As the IPA vapors enters the scrubber and flows through its media, the IPA is absorbed into the scrubbing fluid and the bacteria in the fluid use the IPA as its food source, releasing carbon dioxide and water and creating more “bugs.”  To keep the “bugs” healthy, a solution of nutrients (nitrogen, phosphorus, etc) is metered into the scrubber fluid. Periodically, the extra “bugs” can be discharged to the sewer. Since your process can have long shutdowns, you will need to periodically “feed” these bugs during non-operating times to keep them alive and ready to work when your process starts its operation.

For a listing of air pollution control equipment suppliers, check out PRODUCT FINISHING’S DIRECTORY AND TECHNOLOGY GUIDE under Pollution Control Systems Engineering, Air or online at www.pfonline.com/suppliers.