Page 79 - Applied Process Design For Chemical And Petrochemical Plants Volume II

68 Applied Process Design for Chemical and Petrochemical Plants

y7 0.02 (from graph) To properly handle the changing composition relation-
x,=-= = 0.004 ships it is almost essential to utilize some electronic com-
5.0 5.0
puter techniques if good accuracy is to be achieved. Even
(5.0/1.618 - 1) 0.004/0.0005 - 1) + 1- three component systems become tedious using desk size
In
(1/1.618) (5.0 - 1) electronic calculators without significant internal memo-
ry. Computers can be well programmed to handle the
complexities of trial and check for convergence to a pre-
set acceptable limit.
NB = 1.71 trays (theoretical) not including reboiler, but
including tray number 7, the one used as Techniques for convergence of the digital computer
reference. program are often the heart of an efficient multicompo-
Total trays = 7 (from diagram plus (1.71 - 1) = 7.7 theoretical, nent calculation. There are several techniques incorporat-
plus a reboiler or 8.7 including a reboiler. ed into many programs [27,76,112,135,139, 1681.

Tray efficiency is calculated as previously demonstrated Key Components
and will not be repeated, except that normally stripping
tray efficiencies run lower than rectification efficiencies. The two components in a feed mixture whose separa-
For ammonia-water stripping such as this example most tions will be specified.
over-all efficiencies run SO-SO%.
Note that if the problem of accurate graphical repre- 1. Adjacent keys: key components that are adjacent with
sentation occurs in the rectification end of the diagram, respect to their volatilities.
the corresponding relation to use to calculate the balance 2. Split keys: key components that are separated in
of the trays, assuming straight line operating and equilib volatilities by a non-key component, i.e., the system of
rium lines in the region is [59] : components contains one or more whose volatilities
Rectdjing section: fall between the volatilities of the designated keys.
3. Light key: the designation of the key component with
the highest volatility of the two key components.
4. Heavy key: the designation of the key component
with the lowest volatility of the two key components.
5. Example: component designations
where IS = equilibrium constant for the hmt volatile
component, K’ = y/x Relative Volatility
N, = number of plates above (but not including) Component al/h-7’F. and 550 psia Designation
reference plate n
y‘, x‘ = mol fractions least volatile component Hydrogen 11.7 Lighter than Key
Methane 3.7, a1 Light Kq, 1
Multicomponent Distillation Ethylene 1.0, ah Heavy Kq, h
Ethane 0.72 Heavier than Key
The basic background and understanding of binary dis- Propylene 0.23 Heavier than Key
tillation applies to a large measure in multicomponent Propane 0.19 Heavier than Key
problems. Reference should be made to Figure 8-1 for the
symbols. Hengstebeck [137] presents a simplified procedure for
Multicomponent distillations are more complicated reducing a multicomponent system to an equivalent bina-
than binary systems due primarily to the actual or poten- ry using the “key” components. From this the number of
tial involvement or interaction of one or more compo- stages or theoretical plates and reflux can be determined
nents of the multicomponent system on other compo- using conventional binary procedures and involving the
nents of the mixture. These interactions may be in the McCabe-Thiele method.
form of vapor-liquid equilibriums such as azeotrope for- Liddle [136] presents a shortcut technique for multi-
mation, or chemical reaction, etc., any of which may affect component calculations based on improving the Fenske
the activity relations, and hence deviations from ideal rela- and Gilliland correlations.
tionships. For example, some systems are known to have
two azeotrope combinations in the distillation column. Minimum Reflux Ratio-Infinite Plates
Sometimes these, one or all, can be “broken” or changed
in the vapor pressure relationships by addition of a third This is the smallest value of external reflux ratio (L/D)
chemical or hydrocarbon. which can be used to obtain a specified separation. This is