Page 379 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

12.1.3 Physical Property Data

In addition to kinetics data, physical property data are required for determining material and energy
balances, as well as for sizing of heat exchangers, pumps and compressors, and separation units. These
data are, in general, easier to obtain and, when necessary, easier than kinetics data to estimate.

For the material and energy balances, pure-component heat capacity and density data are needed. These
are among the most widely measured data and are available on process simulators for more than a
thousand substances. (See Chapter 13 for details of process simulators.) There are also reasonably
accurate group-contribution techniques for use when no data are available [8]. The enthalpies of mixtures
require an accurate equation of state for gases and nonionic liquids. The equations of state available on
process simulators are accurate enough for these systems. However, additional heat of solution data are
needed for electrolyte solutions, and these data may not be as readily available. For these systems, care
should be taken to use accurate experimental data, because estimation techniques are not as well defined.

The design of heat exchangers and the determination of pressure drops across units require thermal
conductivity and viscosity data. These data are usually available (often in the databanks of process
simulators) and, if unavailable, can be estimated by group-contribution techniques [8].

The most crucial and least available physical property data are for phase equilibrium. Most separators
are based on equilibrium stages; thus, these data are usually needed for a process design. For vapor-
liquid equilibrium, such as for distillation, either (1) a single equation of state for both phases or (2) a
combination of vapor-phase equation of state, pure-component vapor pressure, and liquid-state activity

coefficient model is required. The choice of thermodynamics package for process simulators is explained
in Section 13.4. The key experimentally determined mixture parameters for either equations of state or
activity-coefficient models are called BIPs (binary interaction parameters), and they have great effect
on the design of separation units. A poor estimation of them (e.g., assuming them to be zero!) can lead to
severely flawed designs. The solubilities of noncondensables in the liquid phase are also essential but
difficult to estimate.

12.2 Reactor Section

For a process with a reactor, often the synthesis of the PFD begins with the reactor section of the GBFD.
(See Chapter 20.) A base-case reactor configuration is chosen according to the procedures described in
reaction engineering textbooks. This configuration (e.g., plug flow, CSTR, batch, semibatch, adiabatic,
isothermal) is used at some base conditions (temperature, pressure, feed composition) and some
preliminary base specification (e.g., 60% conversion) to calculate the outlet composition, pressure, and
temperature. The goal at this stage is to develop a feasible PFD for the process. Optimization of the PFD
can begin only after a suitable base case is developed. If there are obvious choices that improve the

process (such as using a fluidized bed instead of a packed bed reactor, or batch operation instead of
continuous), these choices are made at this stage; however, these choices should be revisited later.

To enable later optimization, the general effects of varying the feed conditions should be investigated at

this point by using the trend prediction approach of Chapter 17. A list of possible reactor configurations