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by assuming that equilibrium is reached. The stream compositions can then be calculated
from the phase equilibrium data for the components. This approximation can often be
made for single-stage gas-liquid and liquid-liquid separators, such as quench towers,
partial condensers and decanters. It is particularly useful if one component is essentially
non-condensable and can be used as a tie substance (see Section 2.11). Some examples
of the use of this process constraint are given in Examples 4.2 and 4.4.
3. Fixed stream compositions
If the composition (or flow-rate) of one stream is fixed by “internal” or “external”
constraints, this may fix the composition and flows of other process streams. In Chapter 1,
the relationship between the process variables, the design variables and design equations
was discussed. If sufficient design variables are fixed by external constraints, or by the
designer, then the other stream flows round a unit will be uniquely determined. For
example, if the composition of one product stream from a distillation column is fixed
by a product specification, or if an azeotrope is formed, then the other stream compo-
sition can be calculated directly from the feed compositions; see Section 2.10. The feed
composition would be fixed by the outlet composition of the preceding unit.

4. Combined heat and material balances

It is often possible to make a material balance round a unit independently of the heat
balance. The process temperatures may be set by other process considerations, and the
energy balance can then be made separately to determine the energy requirements to
maintain the specified temperatures. For other processes the energy input will determine
the process stream flows and compositions, and the two balances must be made simulta-
neously; for instance, in flash distillation or partial condensation; see also Example 4.1.

Example 4.1

An example illustrating the calculation of stream composition from reaction equilibria,
and also an example of a combined heat and material balance.
In the production of hydrogen by the steam reforming of hydrocarbons, the classic
water-gas reaction is used to convert CO in the gases leaving the reforming furnace to
hydrogen, in a shift converter.

CO(g) C H 2 O(g) ! CO 2 (g) C H 2 (g) H Ž 41,197 kJ/kmol
298
In this example the exit gas stream composition from a converter will be determined for
a given inlet gas composition and steam ratio; by assuming that in the outlet stream the
gases reach chemical equilibrium. In practice the reaction is carried out over a catalyst,
and the assumption that the outlet composition approaches the equilibrium composition
is valid. Equilibrium constants for the reaction are readily available in the literature.
A typical gases composition obtained by steam reforming methane is:

CO 2 8.5, CO 11.0, H 2 76.5 mol per cent dry gas
Ž
If this is fed to a shift converter at 500 K, with a steam ratio of 3 mol H 2 Oto1 molCO,
estimate the outlet composition and temperature.