Page 333 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.8 Batch distillation 335

fractionation and needs to be avoided and a steady regulated heating rate in the still is achieved by the
flow control of the circulating heating oil or by pressure control of the heating steam supply.
Batch operation
Batch distillation is an unsteady state operation, i.e., the distillate and still compositions contin-
uously vary with time. The process operates through time cycles of
i. loading of the feed charge in the batch still
ii. heating up/start-up phase e when the process absorbs energy without any distillate being
withdrawn. (Duration: t 1 )
iii. production run e this is the period during which the distillate is withdrawn. (Duration: t 2 )
iv. cooling down the still and preparing the equipment for the next batch. (Duration: t 3 )
Operation without fractionating stages and reflux e single-stage batch distillation
Following the initial inspection of the readiness of equipment the charge is loaded into the still. The
coolant flow in the condenser, which is cooling water in most cases, is started. If the process needs to
be connected to a vacuum system, the same is done and vacuum pulling is done right at the cold state.
If vacuum is pulled later when the still is already heated up, sudden vaporisation and foaming lead to
carryover of liquid with the vapour.
Production run starts as the first drop of liquid falls into the accumulator vessel. During production
run the distillate cut is collected. In case of more than one cut, separate accumulators are used. The still
pressure, if required to be kept at slightly above atmospheric pressure, is regulated by controlled
bleeding of noncondensable gases from the accumulator vessel top. In small set ups there may be a
water seal through which this gas may be bled, limiting the accumulator gauge pressure to the seal
liquid dip head. Such a seal system should also include arrangement to prevent seal-liquid suck-back in
case of vacuum creation in the equipment during cooling down or even otherwise. This is usually
ensured by a liquid trap installed in the line upstream of the seal. The cut specification control is done
by (i) periodic sampling of the liquid from the accumulator and its analysis, (ii) ascertaining the yield
of the cut against previous instructions based on assessment of feed composition. The vapour tem-
perature leaving the still is the dew point corresponding to the instantaneous composition of the
vapour. This temperature is monitored to control the components going to a particular cut of distillate.
Operation of multistage column
The operational procedures of start-up, production and shutdown phases may differ slightly for
different column configurations but in general, the operation follows the following sequence.
Start-up phase
In practice, an empty conventional multistage batch column is started up in the following
sequential steps:
1. The still is charged with the feed to be processed and heat is applied to bring the material to its
boiling (bubble) point temperature.
2. Depending on the heat input, a part of the charge vaporises and the vapour travels upward through
the column internals. Part of the vapour condenses in contact with the cold column and its
internals that get heated in turn. The condensate travels down the column to the still as reflux. As
the tower and internals get hotter, the condensing amount reduces and after a while the vapour
reaches the condenser.