Page 384 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

It is known that acrylic acid starts to polymerize at 90°C when it is in a concentrated form. Acrylic acid
must be separated from acetic acid to produce the required purity product, and the volatilities of both
acids are significantly different. This points to distillation as the separation method. The normal boiling
points of acrylic acid and acetic acid are 140°C and 118°C, respectively. How should the separation be
accomplished to avoid degradation of the acrylic acid product?

The distillation column must be run under vacuum to avoid the problem of acrylic acid degradation. The
pressure should be set so that the bottom temperature of the column is less than 90°C. From Figure B.9.1
and Table B.9.1, we see that a column pressure of 0.16 bar at the bottom can accomplish the desired
separation without exceeding 90°C.
3. Are any of the products, by-products, or impurities hazardous? Because separation between
components is never perfect, small quantities of toxic or hazardous components may be present
in product, fuel, or waste streams. Additional purification or subsequent processing of these
streams may be required, depending on their end use.

12.3.2 Sequencing of Distillation Columns for Simple Distillation

Because distillation is still the prevalent separation operation in the chemical industry, it will now be
discussed in more detail. Simple distillation can be defined as distillation of components without the

presence of any thermodynamic anomalies. The most apparent thermodynamic anomaly in distillation
systems is an azeotrope. Azeotropic distillation is discussed in the next section. The remainder of this
section is for simple distillation.

As stated earlier, as a general guideline, a minimum of N–1 separators are needed to separate N
components, and this guideline also applies to distillation systems. Therefore, one distillation column is
required to purify both components from a two-component feed. This is the type of problem most often
studied in separation classes. To purify a three-component feed into three “pure” components, two
distillation columns are required. However, there are two possible sequences, and these are illustrated in
Figures 12.1(a) and 12.1(b). Ultimately, the choice of sequence depends upon the economics. However,
the results of the economic analysis often follow the guidelines in Table 12.2. For example, if the heavy
component (C) is water, it should be removed first due to its high heat of vaporization, so the sequence in
Figure 12.1(b) is likely to be more economical for such a situation. This is because the heating and
cooling duties in the second column are reduced significantly if the water is removed first. The sequence
in Figure 12.1(b) is also likely to be a better choice if component C is present in the largest amount, or if
component C is the only corrosive component. This is because in the former case, the second column will
be smaller, and in the latter case, the second column may not need the expensive materials of construction

needed in the first column.

Figure 12.1 Column Arrangements for Simple Distillation of Three-Component Feed