Environmental Engineering Reference
In-Depth Information
more volatile components in the feed vaporize upon entering the column, rise to the top
and are collected as distillate ( D ). The less volatile components remain in the liquid phase,
move to the bottom of the column and are usually termed bottoms ( B ). The temperature
at the top of the column is just above the boiling point of the most volatile component and
that at the bottom is just below that of the least volatile. If the feed stream contains more
than two substances to be separated, multiple product streams can be removed down the
length of a column. Often, as will be discussed later, a portion of the distillate stream is
sent back to the column as reflux.
If the relative volatility of the components to be separated is quite low (close to one),
a mass-separating agent (MSA) can be added to alter the phase equilibrium. If the MSA
is relatively non-volatile and exists at the bottom of the column, the process is called
extractive distillation. Azeotropic distillation occurs when the MSA forms an azeotrope
with one or more components in the feed mixture such that the separation is limited to the
azeotropic composition.
A fundamental requirement of distillation, as well as all other separations unit opera-
tions, is that intimate contact must occur between the phases at each stage in a cascade. In
continuous distillation, this means intimate contact between the vapor and liquid phases
in each stage. Typical equipment to achieve this requirement is a sieve tray.
A sieve tray, Figure 4.2, consists of a circular horizontal tray, A, with a downpipe, B,
which acts as a weir. When the level of liquid sitting on top of each tray becomes too high,
it spills over the weir to the next lower tray. The downpipe, C, from the next higher tray
reaches nearly to tray A, allowing overflow liquid to travel down to A. It is designed such
that the overflow liquid is injected below the surface of the liquid on A. The weirs insure
column operation by maintaining a constant liquid level on each sieve tray, regardless of
the liquid flowrate through the system.
Across the surface of each sieve tray there are small holes, typically 0.25 to 0.50 inches
(6.35 to 13 mm) in diameter. Vapor from the next lower tray flows upward through the
C
B
A
Vapor
Figure 4.2 Sieve tray.
 
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