Spent solvent is a common by product of the flexographic printing process and its sources include 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 (dirt and grime) behind. The solid material left behind is called the still bottom.
Distillation units can be purchased or leased for on-site distillation and reuse of the solvent. There are also service providers that can either come on-site or will distill the solvent off-site. The printer may have the reclaimed solvent returned from the off-site distiller or choose to permit them to sell the material. Another option is to contract with a solvent recycler or supplier to take the spent solvent away and replace it with fresh solvent.
Distillation units come in varying sizes, from small bench top units to large scale, high volume (5,000 gal) capacity. 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.
Flexographic printers who use solvent for cleaning or solvent-based inks can recover solvents for reuse in the facility. This reclaimed solvent is often used in cleaning operations and saves the printer the cost of buying virgin solvent and reduces the volume of hazardous waste generated and the subsequent disposal costs.
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) and as such need to be properly handled in the facility and require proper disposal. Spent cleaning solvents that contain "listed" solvents (i.e., a special list of chemicals designated as hazardous by EPA), will also cause the still bottom to be classified as a hazardous waste.
The solvent still is a source of VOC emissions. In most states, an air emission permit to operate from the environmental regulatory agency(ies) is usually required. 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 would be required.
Depending upon the facilities 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 would be required.
Health & Safety
Due to the fire and explosion risk associated with operating a solvent distillation unit, explosion proof wiring may be required for installation. 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 by constructed according to fire and explosion protection standards for building structure.
Special attention is required for the operators of the units from a personal protective equipment standard perspective. For smaller volumes of solvent, gloves and goggles are required. For larger volumes, gloves, goggles, and a splash apron should be worn.