3 Engineering Example 1
3.1 The ideal gas law and Redlich-Kwong equation
Introduction
In Chemical Engineering it is often necessary to be able to equate the pressure, volume and temperature of a gas. One relevant equation is the ideal gas law
(1)
where is pressure, is volume, is the number of moles of gas, is temperature and is the ideal gas constant ( , when all quantities are in S.I. units). The ideal gas law has been in use since 1834, although its special cases at constant temperature (Boyle’s Law, 1662) and constant pressure (Charles’ Law, 1787) had been in use many decades previously.
While the ideal gas law is adequate in many circumstances, it has been superseded by many other laws where, in general, simplicity is weighed against accuracy. One such law is the Redlich-Kwong equation
(2)
where, in addition to the variables in the ideal gas law, the extra parameters and are dependent upon the particular gas under consideration.
Clearly, in both equations the temperature, pressure and volume will be positive. Additionally, the Redlich-Kwong equation is only valid for values of volume greater than the parameter - in practice however, this is not a limitation, since the gas would condense to a liquid before this point was reached.
Problem in words
Show that for both Equations (1) and (2)
-
for constant temperature, the pressure decreases as the volume increases
(Note : in the Redlich-Kwong equation, assume that is large.)
- for constant volume, the pressure increases as the temperature increases.
Mathematical statement of problem
For both Equations (1) and (2), and for the allowed ranges of the variables, show that
- for = constant
- for = constant
Assume that is sufficiently large so that terms in may be neglected when compared to terms in .
Mathematical analysis
-
Ideal gas law
This can be rearranged asso that
-
at constant temperature
-
for constant volume
-
at constant temperature
-
Redlich-Kwong equation
so that
-
at constant temperature
which, for large , can be approximated by
-
for constant volume
-
at constant temperature
Interpretation
In practice, the restriction on is not severe, and regions in which does not apply are those in which the gas is close to liquefying and, therefore, the entire Redlich-Kwong equation no longer applies.
Exercises
-
For the following functions find
- For the functions of Exercise 1 (a) to (d) find
-
For the following functions find
and
-
- .
-
(a) 0 0 0 (b) 2 2 0 (c) 6 1 (d) 120 15 -