Page 413 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

simulators allow the flowsheet topology to be input both graphically and by keyboard. Certain
conventions in the numbering of equipment and streams are used by the simulator to keep track of the
topology and connectivity of the streams. When using the graphical interface, the streams and equipment
are usually numbered sequentially in the order they are added. These can be altered by the user if
required. Care must be taken when connecting batch and continuous unit operations, because it is often
assumed that “continuous” units approach equilibrium instantaneously.

13.2.4 Select Feed Stream Properties

As discussed in Section 13.1, the sequential modular approach to simulation requires that all feed streams
be specified (composition, flowrate, vapor fraction, temperature, and pressure). In addition, estimates of
recycle streams should also be made. Although feed properties are usually well defined, some confusion
may exist regarding the number and type of variables that must be specified to define completely the feed
stream. In general, feed streams will contain n components and consist of one or two phases. For such
feeds, a total of n + 2 specifications completely defines the stream. This is a consequence of the phase

rule. Giving the flowrate (kmol/h, kg/s, etc.) of each component in the feed stream takes care of n of these
specifications. The remaining two specifications should also be independent. For example, if the stream
is one phase, then giving the temperature and pressure of the stream completely defines the feed.
Temperature and pressure also completely define a multicomponent stream having two phases. However,
if the feed is a single component and contains two phases, then temperature and pressure are not
independent. In this case, the vapor fraction and either the temperature or the pressure must be specified.
Vapor fraction can also be used to specify a two-phase multicomponent system, but if used, only
temperature or pressure can be used to specify completely the feed. To avoid confusion, it is
recommended that vapor fraction (υf) be specified only for saturated vapor (υf = 1), saturated liquid (υf =
0), and two-phase, single-component (0 < υf < 1) streams. All other streams should be specified using the
temperature and pressure.

Use the vapor fraction (vf) to define feed streams only for saturated vapor (vf = 1), saturated
liquid (vf = 0), and two-phase, single-component (0 < vf < 1) streams.

By giving the temperature, pressure, and vapor fraction for a feed, the stream is overspecified and errors
will result.

13.2.5 Select Equipment Parameters

It is worth pointing out that process simulators, with a few exceptions, are structured to solve process
material and energy balances, reaction kinetics, reaction equilibrium relationships, phase equilibrium
relationships, and equipment performance relationships for equipment in which sufficient process design
variables and batch operations scheduling have been specified. For example, consider the design of a
liquid-liquid extractor to remove 98% of a component in a feed stream using a given solvent. In general, a
process simulator will not be able to solve this design problem directly; that is, it cannot determine the
number of equilibrium stages required for this separation. However, if the problem is made into a
simulation problem, then it can be solved by a trial-and-error technique. Thus, by specifying the number