Page 22 - Gas Purification 5E

12 Gas Puri$cation

Figure 1-4. Typical structured packing (Intalox). Courtesy of Norton Chemical Process
Products Company

Design Approach

The design of countercurrent absorbers normally involves the following steps: (1) selec-
tion of contactor, including type of trays or packing, based on process requirements and
expected service conditions; (2) calculation of heat and material balances; (3) estimation of
required column height (number of trays or height of packing) based on mass transfer analy-
sis; (4) calculation of required column diameter and tray or packing parameters based on gas
and liquid flow rates and hydraulic considerations; and (5) mechanical design of the hard-
ware. The steps are not necessarily performed in the above order and may be combined or
reiterated in the design procedure. In the design of spray contactors, steps 3 and 4 are
replaced by design calculations that define the configuration and operating parameters of the
liquid breakup and separation equipment. For cocurrent contactors selecting and sizing the
mixing elements are the principal design tasks.
The key data required for the design of absorbers are the physical, thermal, and transport
properties of the gases and liquids involved; vaporniquid equilibrium data; and, if chemical
reactions are involved, reaction rate data. Configuration data on the trays or packing are, of
course, also required. Appropriate data are included, when available, for processes described
in subsequent chapters.
The design of absorbers (and strippers) typically involves a computer-assisted, tray-by-
tray, heat- and material-balance calculation to determine the required number of equilibrium
stages, which are then related to the required number of actual trays by an estimated tray
efficiency. More recently, a non-equilibrium stage model has been developed for computer
application which considers actual trays (or sections of packing) and performs a heat and
material balance for each phase on each actual tray, based on mass and heat transfer rates on
that tray.
To facilitate the use of computers in the design of absorbers, Kessler and Wankat (1988)
have converted a number of commonly used correlations to equation form. These include