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404 Lawrence K.Wang et al.

Equation (7) is used to obtain emission stream flow rate per total adsorbing system,
(Q ). The emission stream flow rate per adsorbing bed, Q' , is then obtain by divid-
e,a e,a
ing Q by the number of beds adsorbing (NA):
e,a
Q = Q (T + 460)/537 (7)
e,a e e
Q' = Q /NA (7a)
e,a e,a
where T is the emission stream temperature (ºF). Because of the trucking restrictions,
e
the diameter of the vessel (D ) and the vessel length (L ) are limited to 12 ft and 50 ft,
v v
respectively. After the diameter and length of the vessel are determined, the vessel surface
area is calculated using Eq. (8):

S =π D (L + D /2) (8)
v v v
In this handbook, the density of the HAP can be determined using

D = PM/RT (9)
HAP
where P is the system pressure (atm) (usually 1.0), M is the HAP molecular weight
3
(lb/lb-mol), R is the gas constant (0.7302 ft atm/lb-mol ºR), and T is the temperature (ºR).
Regeneration of carbon beds is commonly accomplished using steam, followed by
condensation. The quantity of steam required is dependent on the required removal
efficiency (outlet concentration) and how much material (adsorbate) is to be
removed (desorbed) from the carbon bed. The steam provides heat and carries media.
The steam raises the bed temperature to its regeneration temperature and provides
heat for the desorption process to occur. Approximately 60–70% of steam is required
to carry the desorbed VOCs. Complete desorption of the carbon is not usually accom-
plished because it is not cost-effective; acceptable working capacities of adsorption can
be achieved without utilizing large quantities of steam. A general rule of thumb for a
solvent recovery system requires 0.25–0.35 lb of steam per pound of carbon. This steam
usage ratio can be increased for applications where the outlet VOC concentrations need
to be fairly low.
In this handbook, a HAP outlet concentration of 70 ppmv can be achieved after
regeneration at the steam ratio of 0.30 lb of steam per pound of carbon. To achieve a
HAP outlet concentration of 10–12 ppmv, a steam ratio of 1.0 lb of steam per pound of

Table 3
Carbon Adsorber System Efficiency Variables
Steam requirement for regeneration, St (lb steam/lb carbon)
0.3 1.0
Outlet HAP concentration, HAP (ppmv) 70 10–12
o
a
Adsorption cycle time, θ (h) 2 2
ad
Regeneration cycle time, θ (h) 2 2
a
reg
a In some instances, cycle times may be considerably longer than the values given here. The values in
this table are approximate, not definitive.
Source: ref. 7.