Reactions and Separations
ingle-stage distillation systems are widely
used in petrochemical, chemical and phar-
maceutical plants. This unit operation goes
by several names, but is often referred to
as a flash drum or evaporator. We will use
“evaporator” here, with the understanding that we are
not referring to a heat-transfer device, but, instead, to
the entire single-stage distillation system (an adiabat-
ic “evaporator” system has no source of heat input).
A review of the design and operation of this most
basic unit operation is available in various sources
and will not be covered here. However, we need to
ensure that the terminology of evaporators is under-
stood, since there are several design features that are
common to most such processes.
Evaporators typically employ heat to concentrate
solutions or to recover dissolved solids by precipitat-
ing them from saturated solutions. The scope of our
practical solutions will be primarily evaporators of
the former classification. A typical industrial evapo-
rator has tubular heating surfaces, a vessel to hold in-
ventory and disengage vapor from liquid, and a heat
exchanger to condense the lighter overheads product.
These units can operate at atmospheric or elevated
pressures, but are often run under vacuum to reduce
the system temperature. This unit operation can be
run continuously, semi-batchwise or fully batchwise.
In the pharmaceutical industry, it is our empirical ob-
servation that most evaporations are either semi- or
fully batch. While the product from the evaporation
step can either be the overhead or pot contents, the
solution to be separated is usually either wide-boiling
or has a non-ideal vapor/liquid equilibrium (such as
an azeotrope), which results in a more discreet sepa-
ration between mixture components. Figure 1 shows
a typical evaporator system.
A recent capacity initiative at Eli Lilly uncovered
several varied and creative methods to reduce cycle
time, increase throughput, and/or increase product
quality in our semi-continuous train of evaporators.
These were based on observations of a series of vacu-
um evaporator units that remove and exchange various
solvents from a temperature-sensitive, high-boiling
product stream (dissolved solute or slurry). These im-
provement methods were simplified and generalized to
illustrate select ways to boost capacity or reduce cycle
time in similar batch distillation operations. In most
cases, the practical solutions are applicable to both vac-
uum and atmospheric evaporators, and can sometimes
be extended to continuous units, as well.
With a basis established as to what an evaporator
train may look like, as well as the terminology to be
used, discussion can now move into capacity creation
and/or recovery. The question typically posed by man-
agement is: “How can we increase our capacity?” To
answer this, other questions need to be resolved:
1. By how much do we need/want to increase the