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Vapor adsorption system
A typical vapor recovery unit is comprised of two adsorption
vessels filled with activated carbon, a
vacuum system used to strip hydrocarbon vapor from the carbon
bed during the carbon
regeneration process, and a recovery device which turns the
regenerated vapors into a liquid
product.
Adsorption-Absorption System
Its most common application is the control of hydrocarbon vapor
emissions at terminals handling
petroleum fuel products, e.g. gasoline bulk terminals.
In this configuration, the system is equipped with two identical
adsorber vessels filled with activated
carbon. One adsorber vessel is on-stream in the adsorption mode
while the other is off-stream in the
regeneration mode. Switching valves are provided to
automatically alternate the adsorbers between
adsorption and regeneration so that one adsorber is always on
stream to assure uninterrupted vapor
processing capability.
The inlet hydrocarbon vapor-air mixture to be processed flows up
through the on-stream adsorber
vessel. In the on-stream adsorber, the activated carbon adsorbs
the hydrocarbon vapor and allows
clean air to vent from the bed with only minimal hydrocarbon
content. The second adsorber is
offline being regenerated. Carbon bed regeneration is
accomplished with a combination of high
vacuum and purge air stripping which removes previously adsorbed
hydrocarbon vapor from the
carbon and restores the carbons ability to adsorb vapor during
the next cycle.
This design utilizes a liquid ring vacuum pump as the source of
vacuum for regeneration. The vacuum
pump extracts concentrated hydrocarbon vapor from the carbon bed
and discharges into a three
phase separator which separates the vacuum pump seal fluid,
hydrocarbon concentrate, and non-
condensed hydrocarbon/air vapor.
The seal fluid is pumped from the separator through a seal fluid
cooler to remove the heat of
compression from the seal fluid, and returned to the liquid ring
pump. In some applications, e.g.
chloro-hydrocarbon vapor recovery, a dry type vacuum pump is
substituted for the standard liquid
ring pump to avoid incompatibility of the vapor with the seal
fluid required by the liquid ring pump.
Non-condensed hydrocarbon vapor plus hydrocarbon condensate flow
from the separator to an
absorber column which functions as the recovery device. In the
absorber the hydrocarbon vapor
flows up through the absorber packing where it is liquefied and
subsequently recovered by
absorption into a liquid hydrocarbon absorbent.
A circulating liquid hydrocarbon absorbent, supplied from
storage, serves the dual purposes ofabsorbing the recovered
hydrocarbon vapor and providing cooling for the vacuum pump seal
fluid.
The absorbent is normally the hydrocarbon liquid which was the
original source of the vapor
generation. For example, in gasoline vapor control applications,
gasoline product from a storage
tank is used as the absorbent. The recovered product is simply
returned along with the circulating
gasoline back to the product storage tank. A lean absorbent
supply pump and a rich absorbent
return pump are provided to circulate the required absorbent. A
small stream of air and residual
vapor exits the top of the absorber column and is recycled to
the on-stream carbon bed where the
residual hydrocarbon vapor is re-absorbed.
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New technology
Legislation is being prepared world-wide or is already in place
to limit gaseous hydrocarbon
emissions to atmosphere. A large portion of these emissions are
formed by volatile hydrocarbon
compounds derived from the manufacturing, transportation, and
application of most liquid
petroleum products.
It is a well-known fact that hydrocarbon vapors emitted to the
atmosphere, and in the presence of
ultra violet light from the sun, for low-level atmospheric
ozone. Low level ozone has been identified
as a major precursor to the formation of SMOG. These and other
hydrocarbon components such as
Benzene, Toluene and Xylene are known as carcinogenic (cancer
causing to humans).
In an effort to minimize air pollution and the incidence of
cancer, many organizations around the
world have established guidelines for maximum emission levels of
various hydrocarbon and chemical
components. These are known globally as the USEPA, the German
TA-Luft, and the EC Directive for
petroleum terminals and many similar national regulations.
To help reduce air pollution from the manufacture and handling
of chemical and petrochemical
products vapor recovery system was developed.
