Key Concepts

Determine the allowable level of contaminants in your area.

The choice of oil/water separator depends on the composition of the contaminant.

Disposal of filters must be done according to local regulations.

Sections: Condensate generation Separator types Biodegradable fluids Installation and maintenance More Info:

Since the 1970 Clean Water Act, state and local legislatures developed their own initiatives, and the proper disposal of wastewater generated by manufacturing facilities has been a focal point of environmental regulations. This concern exemplifies the operating and environmental challenges confronting plant engineers.

The disposal of air compressor lubricant carryover in condensate is a concern, as is the variety of regulations across the country. For example, allowable concentrations of contaminants in wastewater can range from 10 ppm to 100 ppm. It is important to note that such standards usually apply to total plant effluent, not only that from a compressor.

Since local, state, and federal regulations regarding contaminant disposal vary substantially, plant engineers need to determine the specific requirements that apply to their location. This information is readily available from local waste treatment authorities.

Condensate generation

During the process of compressing air, atmospheric air along with water vapor and airborne contaminants, are drawn into the compressor intake. Additionally, the compression chamber of most compressors requires oil for lubrication, sealing, and cooling.

Once compressed, the air flows into an after-cooler. As it cools, most water and hydrocarbon vapors condense. Additional condensation takes place as the air is further cooled in piping and in air dryers. The condensed moisture must be removed to prevent damage to downstream components and processes.

Drain valves installed on moisture separators. coalescing filters, receivers, dryers, and drip legs remove this condensate. The condensate passes through automatic drain valves and is piped to oil/water separators to remove the oil from the condensate prior to discharge to a drain.

If not treated, condensate can be collected into drums or storage tanks. The drums or tanks are then taken to an approved disposal facility. Transportation, storage, and disposal costs can exceed $500 for a single 55-gallon drum. A typical 25 hp compressor can generate approximately 20 gallons of condensate in one day. Eleven 55-gallon drums of condensate are produced in one month. That comes to $5,500 a month — a significant disposal cost.

Since condensate is approximately 99% water and 1% oil, oil/water separators have been developed to reduce or eliminate the amount of oil in the condensate (Fig. 1).

This ratio will vary with local climate conditions. In dry areas or in cold ambient conditions, less water will condense from compressed air. Oil carryover will remain the same. This results in a higher percentage of oil in the condensate.

Separator types

The challenge of ensuring proper condensate separation is daunting because all coolants and lubricants form solutions with water. Condensate can look clean and still contain hundreds of parts per million of contaminant.

Chemical absorption separators are filled with a chemical media developed to attract oil while repelling water. Depressurized oily condensate drains into the chemical absorption separator where the oil bonds to the media. Clean water then flows to the drain (Fig. 2).

The life of the chemical separator depends on the quantity of oil in the condensate. Oil concentration in the condensate can be as low as 40 ppm but can reach much higher values. Actual oil concentration is dependent on the type and condition of the compressor, the type of oil used, and the amount of water vapor in the ambient air (Fig. 3).

Since absorption capacity is approximately 50% of the media bed weight, a 15-gallon absorption separator will capture 7 to 8 gallons of contaminants.

Gravity separation is accomplished by flowing the condensate into a settling tank. Oil is skimmed off the top and water is pumped out from the bottom of the tank. The water removed still contains oil. The amount depends on the demulsibility of the oil.

Gravitational separation devices are simple and will separate free oils that have migrated to the top of the settling tank. Gravitational oil/water separators are not effective on oils that have emulsified in the water since the oil will not naturally separate from the water. No matter how long an oil and water emulsion stands, some of the oil, an excess amount by federal standards, will remain in the water, regardless of what type of oil was used.

Mechanical separation is done by a pressure drop through a coalescer, a tortuous pathway, and oleophobic attraction. The pressure drop across the coalescer causes some of the oil to drop out of phase and separate.

Pores create a tortuous path through the coalescer and oil droplets adhere to it. To enhance the oil’s collecting and forming droplets and draining off the coalescer element an oleophobic (oil resistant) filter media is used. The combination of these methods makes coalescers 99% effective in separating oil from water (Fig. 4).

The actual efficiency of the coalescer depends on the type of oil being separated. Some synthetic lubricants will not be removed by a coalescing element. Elements foul over time and require replacement, based on pressure drop.

Activated charcoal adsorbs oil and most synthetic lubricants. The condensate is passed through a charcoal chamber. The charcoal chamber fouls over time and requires constant monitoring and scheduled replacement. As the filter element reaches the end of its operating life, contaminant carryover may climb above acceptable limits.

Currently, the only filter material that complies with the limits established by federal regulations is activated carbon, which can remove contaminants down to 10 ppm.

Filtration system operators must also address another critical question: How to properly dispose of used filter elements. Often, filtration systems with carbon elements are used in conjunction with other separation processes such as gravity or membranes. When used in tandem, filtration elements typically last longer.

Vaporization oil/water separators utilize an external heat source, such as electric or steam heaters, to boil off water. The oil remaining is then drained into a container for proper disposal. Vaporization separators efficiently separate almost all oils. Attention must be given to the construction of the oil/water separator since the oily waste can be corrosive. Since nothing is sent to the drain, there is no conflict with any water pollution regulations.

The main drawback with this approach is energy costs. Separating a gallon of condensate requires about 2.3 kW of energy. In many parts of the United States, this equates to an energy cost of 12 cents per gallon.

Semi-permeable membranes are similar to filtration systems. This method uses different membranes to screen out various contaminants from the condensate prior to disposal.

The primary drawback with this method is the rate at which membranes foul with contaminants. Some membrane designs require back flushing frequently. Over time, this maintenance procedure can cause premature membrane failure due to frequent flow reversals and high differential pressures.

A membrane designed for one base stock lubricant may not work well with a different oil. Depending on the individual situation, a plant engineer may require several different membranes if lubricants vary in each compressor.

Membrane separation efficiency is affected by the concentration of contaminant in the condensate. If the condensate contains unusually high levels of contaminants, the discharge from the membranes will contain relatively higher levels as well.

Biodegradable fluids

A cost-effective, environmentally acceptable alternative to condensate separation systems may be using biodegradable compressor lubricants. When approved by waste treatment authorities, these products require no separation from condensate.

The use of biodegradable lubricants enables plant engineers to send low levels of compressor condensate directly to a sanitary sewer without pretreatment. With approval, biodegradable lubricants would eliminate the costs of separation systems as well as concerns regarding long-term performance.

While there is no universally accepted definition of biodegradability, plants should use compressor lubricants that have passed the EPA’s test method 796.3100.

Although some compressor lubricants are considered biodegradable, they can still be toxic and difficult to treat by waste water facilities. Great care must be taken when disposing of these fluids.

Local codes, local climate conditions, the amount of compressed air used, and the amount of waste water produced by a plant must all be taken into account before condensate containing any compressor lubricant is drained into a sanitary sewer.

Installation and maintenance

Installation of an oil/water separator is simple. The oily condensate from each drain valve is individually piped to a depressurization chamber to reduce pressure to atmospheric pressure.

The clean water is then piped to an approved drain. The separated oil is piped separately to a collection vessel for proper disposal.

Oil/water separators are normally installed indoors to prevent freeze-up of the water during cold weather operation. If outdoor installation cannot be avoided, contact the manufacturer for installation requirements.

Maintenance of oil/water separators varies by design and manufacturer, however the following guidelines apply:

Drain collected oil as necessary. Properly dispose of all collected oil as required by law.

Clean the settling tank or boiling tank with water to remove particulate build-up at least once a year. Dispose of the water properly.

Change the activated carbon absorber before it is fully saturated with oil.

Replace the coalescer element based on pressure drop.

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Plant Engineering extends its appreciation to Atlas Copco Compressors Inc., CompAir, Ingersoll-Rand, Kaeser Compressors, Inc., and ultrafilter, inc. for their assistance in the preparation of this article.

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