Page 381 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

especially important if these by-products are formed in large quantities and are to be purified
for sale. For high-selectivity reactions, it may be more economical to dispose of by-products as
waste or to burn them (if they have high heating values), which simplifies the separation
section. However, for environmental reasons, great emphasis is placed on producing either
salable by-products or none at all.
7. What is the approximate single-pass conversion? The final single-pass conversion is
determined from detailed parametric optimizations (Chapter 14); however, the range of feasible
single-pass conversions affects the structure of the separations section. If extremely high single-
pass conversions are possible (e.g., greater than ~98%), it may not be economical to separate
and recycle the small amounts of unreacted feed materials. In this case, the feed materials

become the impurities in the product, up to the allowable concentration.
8. For gas-phase oxidations, should the reactor feed be outside the explosive limits? For
example, there are many reactions that involve the partial oxidation of hydrocarbons (see
acrylic acid production in Appendix B and phthalic anhydride production in Appendix C [on
the CD]). Air or oxygen is fed to a reactor along with hydrocarbons at high temperature. The
potential for explosion from rapid, uncontrolled oxidation (ignition) is possible whenever the
mixture is within its explosive limits. (Note that the explosive limits widen significantly with
increase in temperature.) An inherently safe design would require operation outside these
limits. Often, steam is added both as a diluent and to provide thermal ballast for highly
exothermic reactions—for example, in the acrylic acid reactor (Figure B.9.1).

12.3 Separator Section

After the reactor section, the separator section should be studied. The composition of the separator feed is
that of the reactor effluent, and the goal of the separator section is to produce a product of acceptable

purity, a recycle stream of unreacted feed materials, and a stream or streams of by-products. The ideal
separator used in the GBFD represents a process target, but it generally represents a process of infinite
cost. Therefore, one step is to “de-tune” the separation to a reasonable level. However, before doing that,
one must decide what the by-product streams will be. There may be salable by-products, in which case a
purity specification is required from the marketing department. For many organic chemical plants, one by-
product stream is a mixture of combustible gases or liquids that are then used as fuel. There may also be a
waste stream (often a dilute aqueous stream) to be treated downstream; however, this is an increasingly
less desirable process feature.

Prior to enactment of current environmental regulations, it was generally thought to be less expensive to
treat waste streams with so-called end-of-pipe operations. That is, one produced, concentrated, and
disposed of the waste in an acceptable manner. As regulations evolved, the strategy of pollution
prevention or green engineering has led to both better environmental performance and reduced costs.
More details are given in Chapter 25, but the overall strategy is to minimize wastes at their source or to
turn them into salable products.

The separation section then generally accepts one stream from the pre-separation unit and produces
product, by-product, and (sometimes) waste streams. In the development of the PFD, one must consider
the most inclusive or flexible topology so that choices can be made in the optimization step. Thus, each
type of stream should be included in the base case.