Page 118 - Design of Simple and Robust Process Plants

4.2 The Methodology of Process Synthesis 103
The standard separation technique for homogeneous liquid systems is by simple
distillation (separation on two key components). There are several reasons why dis-
tillation/evaporative separations are a first choice:
. Distillation is a well-known and proven technique which can easily be de-
signed based on vapor liquid equilibrium data. These data are widely avail-
able from data banks, and prediction techniques of these data also gained
confidence for preliminary designs at synthesis stage 1.
. Distillation separations can be performed very sharp on the key components,
and generally does not require an additional unit for purification.
. Distillation is carried out in one unit; other separation techniques need more
units to achieve separation.
The disadvantages of distillation are its limitations for the separation of tempera-
ture-sensitive materials, and its high capital and operational costs.
Extractive distillation is often applied when the relative volatilities are low (<1.1)
and conventional distillation would result in over 100 theoretical trays with very
high reflux ratios over 10. The disadvantage is the much higher investment cost, as
an additional tower is required for separation to be effected.
When extraction is compared with distillation, an extraction unit always requires
purification of the extract, and of the raffinate from the solvent. Similar arguments
are valid for extractive distillation, and adsorption. For extraction, an additional point
to overcome is that tests are required to size the height of a theoretical separation
stage and to judge its tendency to emulsify due to the presence of minor impurities.
When comparing crystallization with distillation, the design of a crystallizer
requires crystallization kinetics to be applied that are seldom available to good-sized
crystals, and the crystals formed often need to be freed from the mother liquor.
When the final product is needed in crystalline form (e.g., sugar), there is no alter-
native technique available.
Membrane separations have a limited application area due to several reasons: the
competitiveness of the membrane with the feed streams; fouling characteristics;
temperature sensitivity; and cost of the modules. Despite this, membranes are being
increasingly applied in the field of water purification.
Reaction is often used as a last resort if all other techniques fail to perform, or are
highly priced. An exception must be made for ion exchange, which is in widespread
use but is already facing competition from membrane separations in water purifica-
tion systems. Low-purity product streams which can be sent to a downstream plant
where separation takes place in the reactor by selective removal of the target prod-
uct. This is an example of process integration.
Pre-selection guidelines are:
. For separation of main components with relative volatility >1.1, distillation/
evaporative separation should be used.
. For separation of main components with relative volatility <1.1, extractive distil-
lation/extraction/reaction should be considered. Examples include butadiene
and benzene extractive distillations, and hydrogenation in olefin processes.