Solvent-laden air

Solvent-laden air (SLA), 70 95 Solvent mixtures flash point of, 23 116 viscosity of, 23 99 Solvent Orange 107, 60, 63, 7 colorant for plastics, 7 374t Solvent polarity/hydrophobicity probing, polymethine dyes in, 20 517 Solvent polarity/temperature, in initiating systems, 74 268 Solvent power, 23 89 Solvent preparation, for Grignard reactions, 72 823 Solvent properties... 

When handling solvent-laden air, as is common in activated charcoal recovery plants, it is normal to operate with a flammable solvent content in the range of 25-40% of LEL. If information on the flash point of a mixture is not available the great majority of solvents have an LEL of 10,000 ppm (1%) with a few In the range of 7000-10,000 ppm. The flash point of straight run hydrocarbon solvents (e.g. white spirit) can be estimated from their initial boiling point (IBP)... 

For the eventual disposal of used solvent, whether in liquid or vapour form, the preferred method is usually burning. This may involve using the solvent as a fuel, possibly in a cement or lime kiln in which it may be used as a replacement for a more conventional, and more costly fuel. Alternatively, to reduce VOCs to a very low level in solvent-laden air, combustion in an incinerator, with or without added fuel, may be used. 

H can also be used to calculate the composition of solvent-laden air in contact with water at levels appropriate to TLV calculations thus... 

Every effort must be made to maintain a constant vapor level to avoid pumping action resulting in solvent-laden air being expelled from the tank. Sufficient heat input capacity should be provided so that the top of the work will be covered with vapors as quickly as possible for efficient vapor condensation and degreasing. 

For acetate spinning, it is necessary to recover 3 kg of acetone for each kg of fiber produced. The solvent-laden air exiting from the cabinet is passed to adsorption beds of activated... 

Many of the potential scrubbing liquids become viscous at low temperatures and do not spread well on the column packings which are generally used for absorption. Plate columns can be used but they have a higher pressure drop for the same duty, involving more fan power to move the solvent-laden air (SLA) through the system. 

To prevent an excessive temperature increase across the bed due to the heat of adsorption, inlet solvent concentrations are usually limited to about 50 g/m. In most systems the solvent-laden air stream is directed upwards through a fixed carbon bed. As soon as the maximum permissible breakthrough concentration is attained in the discharge clean air stream, the loaded adsorber is switched to regeneration. To reverse the adsorption of the solvent, the equilibrium must be reversed by increasing the temperature and decreasing the solvent concentration by purging. 

Adsorptive solvent recovery with steam desorption and condensation units with gravity separator and a stripper have become a standard practice in modem production plants. The solvents-laden air (toluene, xylene) is collected from emission points, e.g., rotogravure printing machines, drying ducts by means of a blower and passed through the recovery plant. 

Solvent-laden air is exhausted at the three rotogravure printing presses by several fans operating in parallel and is routed in an upward flow through four adsorbers packed with Supersorbon activated carbon. The solvent contained in the air is adsorbed on the activated carbon bed. Adsorption continues until breakthrough, when the full retentive capacity of the adsorbent for solvent vapors is used up. 

In the packaging printing industry solvents like ethyl acetate, ethanol, ketones, tetrahydrofuran (THF), hexane and toluene are used in printing. The solvent laden air generally contains between 2 and 15 g/m of solvent and, depending on the season, 5-18 g/m of water. 

A number of other systems are based on the use of polymers for adsorption of solvents, but perhaps of particular note is another process from the Dow Chemical Company [21]. Sorbathane is the trade name for the resin which has been specifically developed for the recovery of chlorinated solvents such as perchloroethylene and trichloroethylene. Units which use this resin are usually two-tank systems which sequentially adsorb and desorb. Adsorption is achieved by passage of the solvent-laden air through the resin which is characterised by a high surface area, small pore size, a swellable polymer matrix and fast adsorption kinetics. Desorption of the solvent occurs when the resin is heated to 80-90°C and the application of a vacuum of less than lOOmbar. The novelty and advantage of using this system is that adsorption and desorption of the stabilisers, required for these solvents, also occurs and therefore the need for restabilisation, as necessary following activated carbon recovery, is eliminated. 

In some cases, it is feasible to recover the solvent used in coatings. Solvent recovery is desirable, but feasibility is limited by low solvent concentration in the air stream, needed to stay below the lower explosive limits. VOC emissions can also be minimized by incineration. The effluent solvent-laden air stream is heated in the presence of a catalyst to a temperature high enough to bum the solvent. As with solvent recovery, this approach is feasible only when solvent concentrations are relatively high. Incineration has been found to be particularly applicable in coil coating. Most of the solvent is released in the baking oven part of the effluent air from the baking oven is recirculated back into the oven. The amoimt of such recirculation is limited so that the solvent content does not approach the lower explosive limit. The balance of the effluent air is fed to the gas burners that heat... 

These solvents are not required in the final tape product. Accordingly, they are typically evaporated off the tape during the drying process. Many companies collect the solvent laden air and route it to some form of solvent recovery. Here the solvents are recovered using a distillation process. This is an environmental friendlyprocess and typicaUyyields solvents ofbetter quality than those originally purchased... 

The solvent laden air gathered in the process is then sent to solvent recovery. This is a relatively simple process whereby the air is passed through a vessel containing activated charcoal. The charcoal absorbs the evaporated solvent and the air is passed into the atmosphere. The charcoal is then flushed with steam and the solvents recovered in a cracking tower as part of a distillation process. 

There are, generally, three sources of solvent-laden air within and around any type of vapor degreaser. They are ... 

Consequently, this subchapter provides brief but essential guidance for the engineering department of a site doing cleaning work for them to constmct their own "package" system for treatment of solvent-laden air by activated carbon. The necessary engineering (and chemistry) is straightforward — just not well known. 

Naturally, adsorbent capadty will be high for the initial discharge of solvent-laden air from the deaning chamber. Solvent contained in subsequent discharges of air won t be as wdl adsorbed on activated carbon — because the concentration has been reduced (Figure 4.17). 

Solvent-laden air generally enters the adsorber (pumped by the air amplifier) through a port in the top of the tank . A diffuser spreads the volume of fiow across the entire area of the adsorber bed. 

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