Page 288 - Chemical process engineering design and economics

Separator Design

Chapter 1 discussed two major types of separation processes, component and
phase separation. In component separation, the components are separated from a
single phase by mass transfer. An example is gas absorption where one or more
components are removed from a gas by dissolving in a solvent. In phase separa-
tion, two or more phases can be separated because a force acting on one phase
differs from a force acting on another phase or because one of the phases impacts
on a solid barrier. The forces are usually gravity, centrifugal, and electromotive.
Examples are removal of a solid from a liquid by impaction (filtration), gravity
(settling), centrifugal force, and the attraction of charged particles in an electro-
static precipitator. One exception to these mechanisms is drying by evaporating
unbonded water from a solid. In this case, separation of a liquid from a solid oc-
curs by mass transfer. For example, the water mixed with sand can be removed by
evaporating the water. Because many component separations require contacting
two phases, like liquid-liquid extraction, component separation is frequently fol-
lowed by phase separation. Phase separators can be classified according to the
phases in contact: liquid-gas, liquid-liquid, liquid-solid, solid-gas, and solid-solid.
Some of the more common phase separators will be discussed.
In addition to discussing phase separators, it is also appropriate to consider
the application of accumulators in processes. Accumulators or surge vessels are
necessary to reduce fluctuations in flow rate, pressure and composition and
thereby improve process control. Although accumulators are not phase separators,
they are discussed here because they are sometimes contained in the same vessel
as a phase separator. For example, in a gas-liquid separator, the volume of liquid
at the bottom of the separator is determined by the need to dampen fluctuations in
flow rate.
267

Copyright © 2003 by Taylor & Francis Group LLC