Problem

A chemical processing facility uses methanol as a raw material. The methanol is held in a 500,000 gallon tank. This tank is refilled with methanol on a daily basis. On average, 30,000 gallons of methanol is added to the tank daily over a 2-3 hour period. During filling, the head space in the tank is vented, resulting in approximately 4000 cubic feet of air emitted daily from the tank. This air is saturated with methanol vapor. Assuming this tank is filled every day of the year, this results in an emissions rate of 15.1 tons/year. It is necessary to remove 95% of the methanol from the tank vent stream before it can be vented to the atmosphere.

Solution

The tank filling process results in a gas stream with a very high methanol concentration. To control the methanol emissions using a biofilter, it is necessary to reduce the concentration and deliver the methanol vapors to the biofilter in a continuous manner. The method proposed to control the delivery of methanol vapor to the biofilter is a two stage process.

While the methanol storage tank is filling, the tank vent stream is diverted through a scrubber designed to remove greater than 95% percent of the methanol from the air. This dampens the "spike" loading to the biofilter. The effluent air from the scrubber is then vented to the biofilter.

A second packed tower is used as an air stripper before the biofilter. The water from the first column containing the methanol is gradually bled into the air stripper holding tank. The methanol is stripped from the water using ambient air and fed to the biofilter. This provides a more even and constant methanol loading to the biofilter.

This system allows gas streams containing high VOC concentrations to be treated with a smaller, less expensive biofilter by moderating and leveling out the VOC loading to the biofilter. The air stripper also acts as the pre-treatment system to humidify the gas stream before it enters the biofilter. It can also be used to control the temperature of the gas stream.

Preliminary analysis of this system indicates that it may be an economical solution to controlling cyclic, high concentration methanol emissions from storage facilities.

Equipment, Capital and Operating Costs

The system above can be provided as a skid mounted unit, ready for operation with customer connections. The foot print of this system would be approximately 20' x 40'. The capital cost for this system, F.O.B. Longview, TX is $98,000.00

The operating costs for this system would be approximately $0.25/hr. This includes electrical cost to run all pumps and the blower to pull air through the system.