Page 24 - Chemical process engineering design and economics

Processes and Process Engineering 11

operations. The nine basic process operations will be discussed separately. More
than one process operation can occur in a single piece-of-equipment, which is
called a process unit.
Conversion of material from one form to another is a task of the chemical
engineer. Table 1.3 lists a number of ways conversion can be accomplished, de-
pending on what form of energy is supplied to the reactor. The most common form
of energy is heat to carry out a reaction thermochemically.
Rarely do the reaction products have an acceptable degree of purity. Thus,
separators are necessary process units. Together, conversion and separation con-
stitute the heart of chemical engineering. In turn, separations consist of two parts,
component and phase. In component separations, the components in a single
phase are separated, usually by the introduction of a second phase. Molecules of
different substances can be separated because their chemical potential in one phase
differs from their chemical potential in a second phase. Thus, separation occurs by
mass transfer, whereas phases separate because a force acting on one phase differs
from a force acting on the other phase. Usually, it is a gravitational force. Exam-
ples are sedimentation and clarification, where a solid settles by the gravitational
force acting on the solid. Generally, phase separation follows component separa-
tion. For example, in distillation vapor and liquid phases mix on a tray where
component separation occurs, but droplets and possibly foam form. Then, the va-
por is separated from the liquid drops and foam, by allowing sufficient tray spac-
ing and time, for small drops to coalesce into large drops and the foam to collapse.
The large drops and collapsing foam then settle on the fray by gravity.
Mixing, the reverse of component and phase separation also occurs fre-
quently in processes. This operation requires energy to mix the two phases. For
example, in liquid-liquid extraction, one of the liquid phases must be dispersed
into small drops by mixing to enhance mass transfer and increase the rate of com-
ponent separation. Thus, extractors must contain a method for dispersing one of
the phases.
Material is transferred from one process operation to another by compres-
sion, pumping or conveying; depending on whether a gas, liquid or a solid is trans-
ferred. This operation also requires energy to overcome factional losses.
Many of the process operations listed in Table 1.3 require an energy input.
Energy must be supplied to the process streams to separate components and to
obtain favorable operating temperatures and pressures. For example, it may be
necessary to compress a mixture of gases to achieve a reasonable chemical con-
version. This work is potentially recoverable by expanding the reacted gases
through a turbine when the system pressure is eventually reduced downstream of
the reactor. Similarly, a high-pressure liquid stream could be expanded through a
hydraulic turbine to recover energy. Heat transfer and expansion of a gas or liquid
through a turbine are energy transfer operations. In addition to elevating the gas
pressure to obtain favorable reaction conditions, gases are also transferred from a
previous process unit to the reactor. This material transfer operation requires work
to overcome frictional losses. Both the material and energy transfer operations are

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