Page 383 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

• For distillation, remove the product with the highest heat of vaporization first, if possible. This
reduces the heating/cooling duties of subsequent units.
• Do not recombine separated streams. (This may seem obvious, but it is often disobeyed.)
• Do the easy separations first.
• Do not waste raw materials, and do not overpurify streams based on their uses.
• Remove hazardous or corrosive materials first.
• Use the less expensive, cruder separation technique first (e.g., liquid-liquid extraction before
chromatography).

As with all sets of heuristics, these can be mutually contradictory. However, in the initial topology of the
separation section, the main goal is to follow as many of these heuristics as possible.

Beyond these general guidelines, beware of azeotropes and multiple phases in equilibrium, especially
when water and organics are present. Special techniques are available to deal with these problems, some
of which are discussed later. On the other hand, if a single-stage flash will do the separation, do not use a
column with reflux.

For the separation section, other important questions to be considered include the following.
1. What are the product specifications for all products? Product specifications are developed to
satisfy customers who will use these products in their own processes. The most common
specification is a minimum concentration of the main constituent, such as 99.5 wt%. Maximum

impurity levels for specific contaminants may also be specified, as well as requirements for
specific physical properties such as color, odor, and specific gravity. A single separation
technique may not be sufficient to meet all the required product specifications, as demonstrated
in Example 12.1.

Example 12.1

In the production of benzene via the hydrodealkylation of toluene, it is necessary to produce a benzene
product stream that contains >99.5 wt% benzene that is water white in color (i.e., absolutely clear). If the
feed toluene to the process contains a small amount of color, determine a preliminary separation scheme
to produce the desired benzene product.

As a guide, we can look at the toluene hydrodealkylation process shown in Figure 1.5. Because the
volatilities of toluene and benzene are significantly different, the main purification step (the separation of
benzene from toluene) can be accomplished using distillation, which is consistent with Figure 1.5.

However, it has been found that the compound causing the discoloration of the toluene is equally soluble
in benzene and toluene, causing the benzene product to be discolored. It is further found by laboratory
testing that the benzene product can be decolorized by passing it through a bed of activated carbon. Thus,
a second separation step, consisting of an activated carbon adsorber, will be added to the process to
decolorize the benzene product.
2. Are any of the products heat sensitive? If any of the products or by-products are heat sensitive
(i.e., they decompose, deactivate, or polymerize at elevated temperatures), the conditions used
in the separations section may have to be adjusted, as in Example 12.2.

Example 12.2