Page 300 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

302 Chapter 11 Distillation

The actual number of trays may also be obtained by introducing an overall efficiency defined as
(11.6)
0
h ¼ðNo. of equilibrium stagesÞ=ðNo. of actual traysÞ
The number of actual trays, calculated (separately for the enriching
Overall tray efficiency
section and the stripping section ¼ (Number of equilibrium stages in the
section)/ðh Þ.
0
In general, most practical separation problems involve 4 to 45 trays.
h is a complex function of the geometry and design of contacting trays, flow rates and flow paths
0
of the vapour/gas and liquid streams as well as their composition and properties. Tray efficiency in-
creases with
• decrease in liquid viscosity
• decrease in relative volatility for distillation/increase in gas solubility (lower K-value) for
absorption/stripping
• increase in flow path until it causes channelling
• reduction in fractional area of holes
• liquid level on tray, that is directly decided by the weir height. This is particularly true for
absorption of hydrogen sulphide/carbon dioxide with amines, caustic scrubbers, etc.
• higher pressure since this increases the tray temperature (bubble temperature) and lowers the
viscosity as well as relative volatility of tray liquid

Tray efficiency data in industrial columns are determined from data collected during test runs under
total reflux condition. However, the efficiency measured under total reflux can differ significantly from
that at design reflux ratio. Designers use empirical correlations to estimate efficiency.
Typical conservative values of h used in designing of different columns are shown in Table 11.5.It
0
may be noted that values of h for absorbers and strippers are typically low ( 50%).
0
Minimum reflux and total reflux condition
The limits of design of distillation column for a specific separation target are operations under
minimum reflux and total reflux conditions.
Under total reflux condition, neither any distillate nor a bottom product stream is drawn from the
column. The entire liquid from the condenser is refluxed back to the column and the entire liquid from
the bottom tray is vaporised in the reboiler and sent as vapour reflux (vapour reboil). Accordingly, both
the rectifying and stripping operating lines coincide with the 45 degrees line, their respective ends
being at ðx D ; x D Þ and ðx B ; x B Þ. As both distillate and bottom product flow rates are zero, the feed rate to
the column is also zero. This explains the lack of influence of feed condition and neither the feed
composition nor the feed line influences the staircase construction. With the operating lines located as
far as possible from the equilibrium curve under this condition, a minimum number of stages (N min ) are
required for achieving the desired top and bottom tray compositions.
A column is brought to total reflux only for test such as measurement of tray efficiency. Operation
at total reflux is sometimes used as an alternative to temporarily stopping production from the plant for
a short time as this is usually more convenient and requires less time and energy (in the form of reboiler
heat and coolant) as compared to shutdown and reestablishing operating conditions.
One may note that a total condenser is a practical device, whereas a total reboiler is not. This is
because complete vapourization at steady state will deposit any nonvaporized impurity present and
foul the heat transfer surface. In addition, the high heat transfer resistance of vapour films on the heat