Process Description:
A venting system for capturing explosive solvent vapors emitted from process equipment in two buildings feeds 1500 cfm of air contaminated with a maximum of 70 lb/hr of vinylidene chloride (below 100oF), to an Enders double tank, explosion proof regenerative carbon adsorption system with automatic controls for switching tanks from adsorption mode to regenerative mode.
An Enders' Model 1400 DAES carbon adsorption system containing 1400 pounds of activated carbon in each stainless steel tank was supplied. The control system automatically regenerates each carbon bed once it becomes less than 70% saturated with solvent using adjustable timers to trigger the regeneration cycle.
The selected carbon (after 3 months on stream) will adsorb about 7% of it's weight of vinytlidene chloride solvent before it becomes saturated. Consequently a 1400 pound carbon bed will never become saturated if regeneration occurs every hour. On the other hand, if only 10 lb/hr of solvent (instead 70 lb/hr) is fed then it will take 7 hours before regeneration is required. To compensate for this lower solvent loading and save steam, the automatic regeneration cycle can be triggered every 7 hours by setting the timers accordingly. If, over the 7 hour period however more than 98 pounds of solvent enters the bed (because of surges or spills) then a risk of solvent bleed through occurs.
An option, not selected here because steam cost was not a consideration, is a bleed through detector which monitors solvent concentration in the treated exiting air stream. This detector is used to trigger the regeneration cycle on increasing solvent concentration and will minimize the amount steam used for regeneration. Typically about 2 to 3 pounds of steam is required per pound of solvent recovered if regeneration occurs at 70% of bed saturation otherwise steam usage increases.
During the regenerative cycle low pressure steam is used to release vinylidene chloride vapor from the activated carbon bed with both steam and solvent vapor flowing to a condenser for recovery. Unfortunately small amounts of hydrochloric acid vapors form which requires a corrosion resistant material where both solvent and steam contact metal. Consequently the client selected tanks be fabricated from ¼" thick T-316L stainless steel plate (and stress relived at 1100oF minimum to better resist chloride cracking) over Hastelloy C because of cost. This system has been operating well for the last 15 years.
Enders Regenerative Carbon Adsorption System for Recovery of Organic Solvent from Air Stream