02.11 Reference States


We can streamline process calculations by defining a common reference state and computing values of enthalpy for all streams. A convenient path for tabulating properties relative to a reference state is illustrated in Figure 2.6c. It is very similar to the common calculation of DH illustrated in section 2.10. We define the common reference state to be the ideal gas at 25C (298K). Then (1) compute the change in ideal gas properties to the temperature of the stream. (2) Use Eqn. 2.47 to check Psat/Tsat in case a liquid may be forming (3) if liquid, use Eqn. 2.45 to compute the change from the ideal gas to the saturated liquid (4) if PLiq >> Psat, compute ΔH = VLΔP. This process is easy to automate using a spreadsheet and you can quickly tabulate all the stream enthalpies of interest, as illustrate using sample calculations for DME (uakron, 17min). Remember to push the pause button as soon as you read the problem statement and see if you can perform the calculation on your own. Then use the screencast to catch any mistakes you might have made. The procedure for a single component can be extended to multiple components to provide a spreadsheet utility for quickly performing energy balance calculations for an entire process (uakron, 7min) Note that an entire process may involve mixtures or reactions. The extension to mixtures is presented in Section 3.5.

Comprehension Questions:
1. Tabulate the stream enthalpies of methanol relative to the ideal gas reference state at (298K,1bar) at: (a) 300K,1bar (b) 350K,1bar.
2. Compute ΔH for going between these states (a) and (b) and compare to the similar problem in Section 2.10.