Page 116 - Design of Simple and Robust Process Plants

4.2 The Methodology of Process Synthesis 101
load streams are also identifiable at this stage. With regard to the ethyl benzene pro-
cess, we need to know where impurities in the ethylene (e.g., methane, ethane, and
propylene) leave the process. The impurities in the benzene (e.g., water and toluene)
leave the process in the product and in a separate water outlet. An alternative might
be pretreatment at the supplier versus in the process plant. It is not always known
beforehand in which stream these impurities will leave the process, but the overall
balance should focus on their removal.

4.2.2.2 Selection of separations
The selection of separation techniques (Douglas, 1988; Smith, 1995; Biegler et al.,
1997; Seader and Henley, 1998; Seider et al., 1999) must be deduced from the physi-
cal properties, but also take into consideration the safety aspects. Thus, a list must
be prepared of available physical and safety properties for the components involved,
and their potential separations. If reliable information is missing, then estimation
techniques may be used for the initial selection, but this needs to be confirmed later
experimentally. Separations can be defined as either heterogeneous or homogeneous.
Heterogeneous separations These are governed by three basic rules:

1. Separation of different phases of a heterogeneous mixture is easier (lower
cost) than for a homogeneous mixture.
2. Separation of the phases of a heterogeneous mixture should be carried out
before homogeneous separation.
3. Larger particles/droplets are a preferable choice.
The constraints on phase separation provide a reasonable driving force (particle/
droplet size and density difference) for separation, the phase separations for consid-
eration being:
. Vapor±liquid
. Liquid±liquid
. Solid±vapor
. Solid±liquid
. Solid±solid
A number of different techniques are available for these varying phase separations.
The selection of technique is often highly self-directing for different concentrations
and particle/droplet sizes and distribution. In cases where the particle size forma-
tion is both understood and controllable ± for example known crystallization
kinetics, grinding, and/or cutting ± the direction is to choose particles of larger size.

Homogeneous separations Homogeneous separations can only be performed by
the addition or creation of another phase; examples include distillation, extraction,
and adsorption. Homogeneous separations can be further subdivided into gas and
liquid separations.