Recovery of Solvent and Other Vapors

The escape of solvent vapors into work rooms can create health, fire, and explosion hazards. Apart from this, the amount of solvent thus vaporized can represent a considerable portion of the operating cost, and recovery becomes desirable for economic reasons.1 14,15,16,17 

The discussion to be provided here will be limited to an outline of the general principles involved and certain precautions that must be observed. One important precaution involves fire and explosion hazards. Many solvent vapors, when mixed with air in certain proportions, are explosive, the degree of explosiveness depending upon the proportion of vapor to air. No explosion will occur when the vapor concentration is below a critical amount which varies from 0.5-10% according to the chemical nature of the vapor. As the vapor concentration increases above this lower critical point, the 

There are practical difficulties in working with vapor concentrations in the upper nonexplosive zone, because there is always a possibility that the concentration may be diluted to a dangerous point through leakage of air. In the relatively few cases where it is necessary to use high vapor concentrations, it becomes desirable to employ a noninflammable solvent, or else, provide an inert atmosphere. The latter involves installation of completely enclosed equipment to prevent access of air. 

Practical considerations usually require that the vapor concentration is kept in the lower safe zone. Such concentrations generally are too low to permit practical recovery of the solvent by condensation or by scrubbing with a high-boiling-point liquid. As a result, the general trend has been to install the active-carbon process which maintains a high efficiency even when the vapor concentration is low. 

Although the concentration of vapor must be kept well below the danger zone, it should not be reduced to a needlessly low point as this involves handling unnecessarily large volumes of air. The vapor concentration is maintained at the optimum point by proper design of the hoods at which the vapor-laden air is collected. Devices are available which automatically maintain the vapor concentration at the proper point. 

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