Page 225 - Petroleum and Gas Field Processing

8.2.3 Vapor Recovery Methods
Ideally, it would be best to design a tank or a storage system to operate at
pressures high enough to suppress evaporation; hence minimizing eva-
poration losses. However, this is not generally economical; also, refiners
require crude oil to meet maximum vapor pressure specifications.
Various methods can be recommended to recover vapors generated in
storage tanks and from other sources such as liquified petroleum gas (LPG)
tankers. These usually involve one or a combination of the following
schemes implemented through what is referred to as the VRU:

Absorption: Usually carried out under pressure using a liquid
solvent of higher molecular weight than that of the vapors being
recovered. Vapors are then separated from the rich solvent, which
is recycled in the process as ‘‘lean solvent.’’
Condensation: Vapors can be totally or partially condensed by
compression and cooling, as shown in Figure 3.
Simple cooling: Cooling the vapors without compression may
condense the vapors, but it is not normally economical unless
refrigeration is applied.
Adsorption: Hydrocarbon vapors mixed with noncondensable
gases, such as air, can be adsorbed by molecular sieves, activated
charcoal, or silica gel. Heat or depressurization will remove the
adsorbed vapors from the solid bed. The vapors could then be
condensed for recovery.
The basic part of equipment operating the VRU is the vapor
regulator setup (see Figure 4). The basic functions of the regulator are the
following:
1. Release vapor from the storage tank battery when the normal
operating pressure within the system increases beyond a preset
value

Figure 4 Vapor regulator system connected to storage tanks.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.