Solvent recovery may be preferable to steam oxidation if the solvent can be reused, allowing companies to save significantly on the purchase of new solvents. While vapor oxidation can return some values where the energy comes from the solvent, usually the "chemical" value is significantly higher than the energy value.
There are two important methods for solvent recovery: activated carbon and direct condensation. Carbon methods are much more expensive. The condensation method is not always suitable, but the cost is much lower if the dryer and condenser for the coated web are designed as a system.
carbon adsorption
When carbon beds are installed to capture solvent vapors that exit the dryer with the air, two or more carbon beds are typically provided and periodically desorbed from one carbon bed while being adsorbed in the other.
Desorption is accomplished by heating the bed and purging with steam, usually with steam. The steam and steam are then condensed, and the steam condensate (water) needs to be mixed with a solvent. Typically about 3 to 4 or as much as 10 pounds of steam are used per pound of solvent recovered. If the recovered solvent is soluble in water (alcohols, ketones, etc.), the water needs to be separated at additional cost. Some vapors, such as methyl ethyl ketone, will be absorbed onto the carbon with substantial heat release and precautions need to be taken to prevent spontaneous combustion of the air-swept carbon bed of combustion air.
There are many carbon adsorption systems where the value of recovered solvent outweighs the cost of operation. However, usually the cost of recycling solvent is higher than the cost of purchasing new solvent. In this case, higher recycling costs are needed to avoid air pollution.
direct steam condensation
Direct steam condensation is more economical than using carbon beds for solvent recovery. This energy consumption is typically less than 10% of the energy used to run a traditional air-swept dryer, and the carbon bed, and the process does not add water to the solvent. Furthermore, no exhaust gas flows into the atmosphere in the preferred design. Vapor condensers are much smaller than required for carbon bed recovery systems, yet another advantage is that when two or more dryers are used with different solvents or solvent mixtures, separate vapor condensers avoid mixing of solvents and solvents The cost of separating them. Typically, a chiller type refrigeration system is required, but one system can serve all condensers.
However, there are limitations to the applicability of the process: process dryers need to be essentially "gas-tight" to allow the contained atmosphere to be recirculated numerous times with minimal exchange with the outside atmosphere. Indirect heaters, such as steam coils, are necessary. In addition, the level of oxygen contained in the atmosphere should be kept below the sustainable limit (10 to 13% O2) for combustion; otherwise, very cold condensers and relatively high recirculation rates are required to keep vapor levels safely at Vapor below LEL limit. A relatively small flow of inert (low oxygen content) gas is required to counteract the tendency of the wet web to drag air (21% O2) into the dryer, and it is necessary to maintain a readily available pressurized inert gas reservoir(a) to facilitate Quick air purge as well as start the dryer after it has been turned on for any reason, and (b) provide a safety mat for failsafe shutdown during a power outage.
Required sources of inert (low oxygen) gas include flue gas from gas-fired steam boilers and purchased liquid nitrogen or carbon dioxide. Where flue gas is used, the gas burner needs to be of the type that can maintain a low excess air to fuel ratio at various fuel firing rates. A compressor and pressurized storage tank can provide ready storage for last start-up and fail-safe shutdown, or a liquid nitrogen tank with vaporizer can be used.
In some important respects, the operation of an inert airtight dryer is inherently better than that of a traditional air-swept dryer. In an air-swept dryer, there is a transition zone between the flammable wet-wet interface and the non-flammable exhaust, and it is possible for a temporary excess solvent load to enter the dryer to create a large flammable mixture. In an inert dryer, there is no flammable interface and any temporary excess solvent loading will not form a flammable mixture. When the coating process and the wet web are stopped for any reason, no outside air enters to exchange with the air in the dryer, except that air may be sucked in to replace volumetric vapor condensation, or to compensate for the volumetric contraction of the contained gas as it cools . In the Wolverine system, the normal operating vapor concentration in the dryer is designed to prevent condensation due to the amount of vapor drawn into any unsafe air into the dryer. Typically, the coating process is stopped when the vapor is selected at the condenser temperature so that the vapor concentration is below the organic LEL level . This provides a double safety factor and safe O2 LEL levels and safe airborne organics LEL levels.
When the dryer is turned off overnight or on weekends, it is not necessary or desirable to purge contained atmosphere to outside atmosphere.
