Spent solvent is a common byproduct of the lithographic printing process, typically from inks and cleaning solvents. Spent solvent can be recycled through a distillation process. A distillation unit (also known as a "solvent still") is a piece of equipment which essentially boils dirty solvent, captures the "clean" vapor and directs it to a condensation zone, which condenses the vapor to a liquid allowing it to deposit into a separate collection vessel. When boiled, the solvent becomes a vapor and leaves the solids (called still bottoms) behind.
Distillation units can be purchased or leased for on-site distillation and solvent reuse. This reclaimed solvent is often used in cleaning operations and saves the printer the cost of buying virgin solvent while reducing the volume of hazardous waste generated and the associated disposal costs. Additionally, service providers can distill solvent off-site. A printer may have the reclaimed solvent returned from the off-site distiller or choose to sell the material.
Distillation units come in varying sizes, from small bench top units to large scale, high volume (5,000 gal) units. Some use direct heat to evaporate the dirty solvent, while others use a combination of heat and vacuum. The vacuum allows the unit to operate more quickly and efficiently at lower temperatures.
Reclaimed solvent may need further treatment before it can be reused, particularly if added to inks as thinning solvent. Depending upon the specific blend of solvents used for cleaning, additional make-up solvent may also have to be added prior to the solvent being reused to clean critical press and other equipment components.
The still bottoms, or sludge generated from separating dirt and other contaminants from the solvent solution, are usually classified as hazardous waste due to low flashpoints (<140°F). As hazardous wastes, still bottoms need to be handled and disposed of properly. Any spent cleaning solvents that contain "listed" chemicals will also cause the still bottom to be classified as hazardous waste. To learn more about listed wastes, see the Hazardous Waste Section.
A solvent still is a source of VOC emissions. In most states, an air emission permit is required to operate a solvent still. It is advisable to contact the state/local air permit agency to determine if a permit is necessary and if so, what type of permit is required.
Depending upon the facility’s hazardous waste generator classification (e.g., exempt quantity generator, small quantity generator, large quantity generator) and state hazardous waste regulations, a hazardous waste permit may be necessary. It is advisable to contact the state/local agency to determine if a permit is necessary and if so, what type of permit is required.
Health & Safety
Due to the fire and explosion risk associated with operating a solvent distillation unit, explosion proof wiring may be required. The local or state fire protection authority (local fire department or Local Emergency Planning Committee) may require that the building or room in which the distillation unit is installed be constructed according to fire and explosion protection standards.
Operating a distillation unit requires strict use of personal protective equipment. For smaller volumes of solvent, gloves and goggles are required. For larger volumes, gloves, goggles, and a splash apron are required.