08-06-2013, 04:53 PM
Ethanol distillation: the fundamentals
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Fundamentals of a distilling system
Certain fundamental principles are common to
all distilling systems. Modern distillation systems
are multi-stage, continuous, countercurrent,
vapor-liquid contacting systems that operate
within the physical laws that state that different
materials boil at different temperatures.
Represented in Figure 1 is a typical distillation
tower that could be employed to separate an
ideal mixture. Such a system would contain the
following elements:
a. a feed composed of the two components to
be separated,
b. a source of energy to drive the process (in
most cases, this energy source is steam, either
directly entering the base of the tower or
transferring its energy to the tower contents
through an indirect heat exchanger called a
reboiler),
Considerations in preliminary design
The engineer, given the assignment of designing
a distillation tower, is faced with a number of
fundamental considerations. These include:
a. What sort of contacting device should be
employed? (e.g. trays or packing). If trays are
chosen, what type will give the most intimate
contact of vapor and liquid?
b. How much vapor is needed? How much
liquid reflux is required? (What ratio of liquid:
vapor is required?)
c. How much steam (energy) will be required?
d. What are the general dimensions of the
distillation tower?
Distillation contactors
Trays are the most common contactor in use.
What are the functions expected of tray contactors
in the tower? Figure 5 depicts a single tray
contactor in a distillation tower and shows the
primary functions desired:
- Mixing rising vapor with a falling fluid.
- Allow for separation after mixing.
- Provide path for liquid to proceed down the
tower.
- Provide path for liquid to proceed up the
tower.
Figure 6 depicts a perforated tray contactor with
certain accoutrements required to control the
flow of liquid and vapor and to assure their
intimate contact. Another type of tray contacting
device, the disc-and-donut or baffle tray is shown
in Figure 7. The characteristics of this type of
contactor make it especially useful for distilling
materials such as dry-milled grain beer, which
would foul ordinary trays.
Energy analysis
In addition to the selection of the basic contacting
device, the energy requirement must be established.
This is accomplished by analyzing the
vapor/liquid equilibrium data from Figure 4, for
the liquid:vapor ratio to perform a continuous
series of steps within the limits of the equilibrium
curve. Table 1 demonstrates a simplified procedure
to calculate the approximate energy
requirement from the liquid:vapor ratio that will
be employed in the tower design. Repetition of
this type of calculation for different conditions
produces a design chart like that shown in Figure
8 for the ethanol-water system.