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1.8.3 Recovery Credits and Disposal Costs During the desorption cycle, VOC is desorbed, condensed, and separated from the steam condensate. The recovered VOC can be re-used onsite (e.g., as a solvent or burned as a fuel), sold to recyclers, or sent to a disposal site. If the recovered VOC is sufficiently pure, it can be reused onsite or sold. As the example problem in Section 1.9 illustrates, if the quantity of recovered VOC is large enough, its value can offset the annual costs of the control device, resulting in a net annual credit. However, the current market price of the VOC and its purity impact the size of the credit. The greater the purity, the higher the value. However, if the recovered VOC contains impurities or is a mixture of compounds, further treatment, such as distillation, will be required. Purification and separation costs are beyond the scope of this chapter. Needless to say, the costs of these operations can be significant and should be included in estimates of the capital and operating costs. They may offset the revenues generated by the sale of the VOC. Where the facility is unable to re-use or sell the waste VOC collected, the facility may incur costs associated with its off-site disposal. Costs of disposal for wastes containing hazardous pollutants, such as benzene, can be high and alternative control systems, such as incinerators, may be more cost effective than a carbon adsorber. Recovery Credits: The following equation can be used to calculate the recovery credits: where RC = mvoc = θs = pvoc = E = recovery credit ($/yr) RC mvoc s pvoc E VOC inlet loading (lbs/h) system operating hours (h/yr) resale value of the recovered VOC ($/lb) adsorber VOC control efficiency (1.40a) By definition, the efficiency (E) is the difference between the inlet and outlet VOC mass loading, divided by the inlet loading. However, during an adsorption cycle the outlet VOC loading will increase from essentially zero at the start of the cycle to the breakthrough concentration at the end of the cycle. Because the efficiency is a function of time, it should be calculated via integration over the length of the adsorption cycle. However, this approach requires knowledge of the temporal variation of the outlet loading during the adsorption cycle. If this knowledge is not available, a conservative approximation of the efficiency may be made by setting the outlet loading equal to the breakthrough concentration. Disposal Costs: In situations where the collected VOC must be sent off-site for disposal, the following equation should be used to calculate the disposal costs: Disposal Cost mvoc s Dvoc E (1.40b) 1-33PDF Image | Carbon Adsorbers
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