Page 340 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

342 Chapter 11 Distillation

Total volume of the still ¼ charge volume þ vapour space, typically 30% of total volume.
Foaming systems are provided with additional volume usually based on experience.
Dimensions: In case of stills with internal heating, the length and diameter of the still are selected
such that the heating element remains conveniently submerged at end of operation. This requires a
prior estimate of the heating surface and the heating element (coil or tubes) assembly size. For systems
with external heater, a circulating pump with adequate head needs to be located appropriately so that
the NPSH available is sufficient. In case of single-stage separation, the vessel must also accommodate
a demister pad typically of SS wire mesh around 100 mm thick to minimise entrainment. The nozzles
are sized based on the same considerations mentioned in Section 11.6.3.
Fractionation column
Internals e packing/trays: Selection is based on the considerations in Chapter 14, Sections 14.2 &
14.4.
Dimensions: Column height has to be sufficient to accommodate packed section, packing support
grid and packing restrainer grid (top) or the desired number of trays (usually sieve tray without
downcomer) as the case may be.
Column diameter is decided from flooding considerations which is a function of flow rates and
transport properties of vapour and liquid. The standard design procedure and correlations for packed
column flooding and the details of tray sizing are mentioned in Chapter 14.
The vapour and liquid flow rates corresponding to which sizing is done and checked depend on the
duration of distillation operation. The sizing is checked for

i) initial phase where vapour density is minimum
ii) end phase where the temperature is maximum
iii) maximum reflux ratio
Condenser and Heater (internal to the still or external) e These are designed as heat exchange
equipment. The external heater may be a shell and tube heat exchanger whose details are provided in
Chapter 4.
Estimation of heat duty for the condenser and the still heater
In either scheme of fixed reflux ratio or fixed top product composition, the total heat supply to the
still (Q still ) needs to be decided based on the batch time available. The duty should include an initial
load to bring the feed charge and the associated hardware to its bubble point (T bubF ), (sensible heat)
that should also include the estimated heat loss from the still and the column (Q 1 ) and heat required (Q)
during production run, i.e., boil up from (T bubF ) to bubble point of bottoms product (T bubB ).
If facility is kept to use a coolant after stopping of the heat input to the still and cool the final holdup
(B) of the still from T bubB to T B , the design of the heat transfer element should also consider this load
Q 2 as well in its sizing.

Q still ¼ Q 1 þQ (11.61)
where
(11.62)
Q 1 ¼ FC pF ðT bubF T F Þþ mC p ðT bubF T F Þ
In the above equation the second term is associated with hardware heating and heat losses are
considered negligible.