How to calculate relative volatility for multicomponent?
Answers
Multicomponent Distillation
need more than one distillation tower, for
n
components, n-1 fractionators are required
Specification Limitations
The following are established at the beginning
1. Temperature, pressure, composition, and rate of
the feed.
2. Pressure of the distillation
3. Feed to be introduced on that tray which will
result in the least total number of trays
(optimum feed-tray location)
4. Heat loses (even if assumed to be zero)
Under these circumstances only three additional items of
the following list need to be specified, any other items
must be calculated.
1. Total number of trays.
2. Reflux ratio
3. Reboil ratio (ratio of vapor produced by the
reboiler to residue withdrawn)
4. Concentration of one component in one product
(a maximum of two may be chosen)
5. Ratio of flow of one component in the distillate
to the flow of the same component in the
residue, or split of the component (a maximum
of two may be chosen)
6. Ratio of total distillate to total residue
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Key components
The feed components are listed in order of their relative
volatility. The more volatile components are called
light, the less volatile are heavy. The components
separated in a tower are called light key (more volatile)
and heavy key (less volatile). Any components lighter
than the light key are called light components, while
those heavier than the heavy key are called heavy
components.
In hydrocarbon systems, because of nonidealities, the
equilibrium data are often represented by
y
A = KA
x
A, y
B= K
B
x
B, y
C = K
C
x
C, y
D = K
D
x
D
where KA is the vapor-liquid equilibrium constant or
distribution coefficient for component A. K is a function
of temperature and pressure, but is assumed to be
independent of composition, which is sufficiently
accurate for most engineering calculations. For an ideal
system, K
A = P
A*/P
T = vapor pressure/total pressure.
The relative volatility in multicomponent distillation is
always computed with respect to the heavy key
j
j
hk
je je
hke hke
K
K
y
x
y x
/
/ (1)
The value of
is greater than 1 for any component
lighter than the heavy key and less than 1 for any heavier
component