Page 378 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

value (i.e., maximum selling price) to the customer.

Similarly, there may be more than one chemical pathway to the product. Pathways of greatest interest to
the chemical engineer are not necessarily those of greatest interest to the chemist. The abilities to use
impure feed materials and to avoid the production of by-products reduce costs but may not be of interest
to a chemist. The costs of small-lot, high-purity laboratory reagents may not even qualitatively correlate
to those of multiple tank-car, industrial-grade raw materials. Isothermal operation of small laboratory
reactors is common but essentially impossible to achieve on a large scale. It is more economical per unit
volume to maintain high pressures on the plant scale than it is in the lab. Simple batch operations are
common in laboratory work, but, at plant scale, sophisticated optimization of scheduling, ramp rates,
cycle sequencing, and choice of operating mode (batch, semibatch, continuous) is vital. Thus, the
chemical process design engineer must be in touch with the chemist to make sure that expensive
constraints or conditions suggested by laboratory studies are truly needed.

12.1.2 Reaction Kinetics Data

Before reactor design can begin, the kinetics of the main reaction must be known. However, a knowledge
of the kinetics of unwanted side reactions is also crucial to the development of PFD structure or
topology (number and position of recycle streams; types, numbers, and locations of separators; batch or

continuous operating modes; sterilization operations needed for aseptic operation). Knowledge of
detailed reaction pathways, elementary reactions, and unstable reaction intermediates is not required.
Rather, the chemical process design engineer needs to know the rate of reaction (main and by-product
reactions) as a function of temperature, pressure, and composition. The greater the range of these
independent variables, the better the design can be.

For some common homogeneous reactions, kinetics are available [1,2,3]. However, most commercial
reactions involve catalysts. The competitive advantage of the company is often the result of a unique
catalyst. Thus, kinetics data for catalyzed reactions are not as readily available in open literature but
should be available within the company files or must be obtained from experiments. One source of
kinetics data for catalytic reactions is the patent literature. The goal of someone writing a patent
application, however, is to present as little data as possible about the invention while obtaining the
broadest possible protection. This is why patent information is often cryptic. However, this information is
often sufficient to develop a base-case PFD. The key data to obtain from the patent are the inlet
composition, temperature, pressure, outlet composition, and space time. If the data are for varying
compositions, one can develop crude kinetics rate expressions. If the data are for more than one
temperature, an activation energy can be determined. These data reduction procedures are described in

undergraduate textbooks on reaction engineering [4,5].

Without kinetics data, a preliminary PFD and cost analysis can still be done [6]. In this type of analysis,
the differing process configurations and costs for different assumed reaction rates provide estimates of the

value of a potential catalyst. If doubling the reaction rate reduces the cost of manufacture by $1 million
per year, for example, the value of catalysis research to increase the reaction rate (all other things being
equal) is clear. As a guideline, the economic breakpoint is often a catalyst productivity to desired product
of ~0.10 kg product per kg catalyst per hour [7]. Another guide is that activation energies are usually
between 40 and 200 kJ/mol.