MNTAP Fact Sheet: Selecting a Still for On Site Recycling

Minnesota Technical Assistance Program
1313 5th Street SE Suite 207
Minneapolis, Minnesota 55414-4504
(612) 627-4646
(800) 247-0015 (Minnesota only)
FAX (612) 627-4769

The Minnesota Office of Waste Management's Mn TAP program is supported with a grant to the School of Public Health, Division of Environmental and Occupational Health, at the University of Minnesota.

Background Information

Many solvents can be reprocessed and then reused (recycled). Distillation is the most common method of solvent recycling, although filtration can be used to remove solids and nonmixable liquids. Recycling can be done at your facility or off site. For information about off-site recycling services, request MnTAP's publication, "Solvent Recovery and Fuel Blending," by calling 612/627-4646 or 800/247-0015.

On-site distillation is generally viable for recovering large volumes of solvent. For small volumes of solvent, a careful evaluation of the feasibility of small-scale on-site distillation should be made before purchasing distillation equipment. This fact sheet provides information that companies should consider before purchasing a small still.

Feasibility Considerations

Before buying a still, each company must evaluate the basic feasibility of solvent recycling. Suppliers of distillation equipment are good sources of information related to economic and safety considerations. Other sources of information are companies in your industry that already distill solvents for reuse. The names of these companies may be available from equipment vendors and members of trade associations.

Before you purchase a still, answer the following questions:

• Can the distilled solvent be used again for the same process at your company?

In many cases, solvent blends are used for cleaning and as components in paint, ink or adhesive formulas. If the solvents in the blends have a wide range of boiling points, distillation will often yield a solvent that is different from the virgin blend. In addition, if some components are missing, or are present at too low of a concentration, the recycled blend may be unsuitable for the original use unless the solvent strength is rebuilt. In other cases, contaminants that are not sufficiently removed can cause problems. For example, the breakdown of chlorinated degreasing solvents creates acids, and residual acid catalyzes the formation of more acid, which can result in the corrosion of both metal products and process equipment.

• If the distilled solvent cannot be used in the original process, is there another process in which the solvent can be used?

Uses where requirements are less stringent than the original use are good candidates for using distilled solvent.

• Should various types of waste solvents be stored separately?

If various solvents are used for different purposes, and they are accumulated together in a single waste drum, simple batch distillation will not separate the solvents into their original form. Such a mixture may be useless for many purposes. Make sure that any procedure necessary to prevent the cross-mixing of different solvents can and will be followed by all employees. Also consider reducing the number of different solvents used, if possible.

Economic and Operational Considerations

• Will the equipment supplier distill a sample of your waste solvent so that you can evaluate the quality and quantity of both the distilled product and the still bottoms?

Distill enough waste so that the recycled solvent can be tested in your facility.

• What is the percentage of solids or nonvolatile liquid contaminants in your waste?

Many stills claim a high solvent recovery percentage; however, if only half your waste is solvent, a high recovery rate may still be inadequate.

• What is the physical nature of the still bottoms or residue produced by the still? Is the still designed to handle this type of residue?

Solids and liquids that gel or polymerize can require significant amounts of labor to remove them from the still. High solids can also coat and insulate the still's heating surfaces. This causes the distillation to take more time and may cause hot spots and chemical decomposition. Waste of this type may require that you use still liners for smaller units, or a still with an automated surface scraper (scraped-surface still) for larger applications.

• Should waste solvent be distilled under vacuum?

Since vacuum distillation lowers the temperature at which liquids boil, it has the following advantages:


1. allows the distillation of some materials that are not safe to distill at atmospheric pressure;
   
2. speeds up most distillations; and
   
3. protects some materials from breaking down under high temperatures. However, vacuum distillation can add significantly to the purchase price and operating costs of the still.
• How will the still bottoms be managed, and what are the disposal costs?

Most still bottoms must be handled as hazardous waste. The disposal costs for pumpable liquid wastes will generally be less per pound than for solids or semi-solids. However, if the volume can be reduced sufficiently, the disposal of solids may be economically feasible. In some machining operations, oily still-bottoms can be reused as a lubricant.

NOTE: regulatory agencies generally require evidence that reused still bottoms are providing a valuable function and are not being disposed of in an unregulated manner.

• What are the costs of operating the still?

These costs include:


1. labor to operate the still (i.e., load, monitor, unload and clean),
   
2. electricity,
   
3. condenser water (including sewer or treatment charges), and
   still liners, if needed.
• Will any additional equipment, special utilities or plant modifications be needed for the still to operate? At what cost? What is required to install the still? Is it easy to operate or will it require extra labor or supervision?

These indirect costs should be factored into the total cost of purchasing a still.

• How much maintenance will the still require? Will the solvent eventually deteriorate any of the still components?

Stainless steel and teflon fittings and gaskets are required for some solvents, but they add cost. Check to see if the chemicals are compatible with the equipment you are considering.

• How much operational support will be available from the manufacturer or distributor? How much might be needed? Will the manufacturer/distributor train your operators? Are written instructions clear? Will the manufacturer/distributor make an operational guarantee and back its equipment?

Many of these questions can only be answered by other companies using the equipment for similar purposes.

Safety Considerations

• Is there a safe location for the still?

When choosing a location, consider fire code requirements, sources of ignition, ventilation needs, and the possibility of spills. Check with your fire marshal and insurance company to ensure that all their requirements are met.

• Can the electrical installation of the distillation equipment be easily inspected and approved by the local electrical inspector or the Minnesota State Board of Electricity?

Most industrial equipment, whether it is a hard-wired installation or simply plugged into an electrical outlet, must be inspected. Approval of the entire piece of equipment by an independent testing laboratory, such as Underwriter's Laboratories or Factory Mutual, allows for the easiest inspection approval. Using approved components is the next best situation. Consider requiring a performance guarantee for the installation and successful inspection from the electrical inspector and fire marshal.

• Are the following minimum safety features present on the machine that you are considering?
  
1. Automatic shut down for high pot temperature to prevent the still bottoms from continuing to cook after all solvent has boiled off.
   
2. Automatic shut down for high condenser-water temperature to prevent solvent vapors from being lost due to malfunctions that cause low or no coolant flow.
   
3. A relief valve for pressure buildup.
   
4. Controls that are explosion-proof, intrinsically safe or installed in a remote location if flammable solvents are processed. Equipment installed in a hazardous location (as defined by the fire code) should be approved by an independent testing laboratory to verify compliance with the National Electrical Code.
   
5. Interlock features that prevent opening of the still until it has cooled to a certain temperature.
• How are the solvent vapors condensed?

Water is generally the simplest and least costly cooling medium. Air cooling can make sense in some cases, such as for very small units, but electric fan motors are generally not explosion-proof and their use should be carefully evaluated. Refrigerated chillers are expensive.

• Does your solvent contain nitrocellulose?

Nitrocellulose is explosive when dry. Special precautions are required to distill materials containing nitrocellulose. Automotive lacquers, and flexographic and gravure inks can contain nitrocellulose. Contact MnTAP for more information about products that contain nitrocellulose.

Regulatory Overview

• Recycling is a waste treatment process that is exempt from hazardous waste regulations. However, most solvents are hazardous waste after they are used in industrial applications. This is because the solvents themselves have toxic or flammable properties that make them hazardous, or the solvents become contaminated during use making the waste solvents hazardous. Therefore, from the time the solvent leaves the industrial process as a hazardous waste to the time recycling begins, companies need to comply with hazardous waste storage, labeling, transportation and inspection requirements for the waste solvents and solvent containers.
• Still bottoms are generally hazardous. If a listed solvent (Minnesota Rules Chapter 7045.0135) is distilled, then the still bottoms are by definition hazardous waste. If the solvent is a characteristic waste (has hazardous properties but is not specifically named in the rules), then still bottoms will be hazardous only if they exhibit significant hazardous characteristics of their own. This generally needs to be determined by laboratory analysis.
• Generator size implications. Recycling should decrease waste volumes and solvent purchases. Consequently, recycling can reduce regulatory requirements for companies if their lower waste volumes allow them to be reclassified to a smaller generator size class. Minnesota hazardous waste rules state that recycled materials will only be counted as wastes the first time they are generated. For example, when a tank is emptied and the solvent is distilled and then returned to the tank, the annual waste volume is the solvent capacity of the tank -- counted one time -- plus the volume of all still bottoms generated over one year.
• Closed-loop recycling exists when the recycling process is integrated into the manufacturing process and contaminants are removed as they accumulate. In a closed-loop system, wastes (other than still bottoms) are never removed from the manufacturing process and, in the regulatory sense, are never generated. Closed-loop recycling is difficult to accomplish on a small scale, but it is preferred over other recycling methods when it is possible.

For More Information

The considerations listed above are some of the more important points to evaluate before purchasing a still. A list of small still manufacturers, as well as a list of off-site solvent recovery and fuel blending services, is available from MnTAP. To request copies of these publications, call MnTAP at 612/627-4646 or 800/247-0015 from greater Minnesota.