RMP Lecture Notes

Multiple Step Processes

Most processes consist of a number of interconnected unit processes. Streams will split and combine. Determining a full set of flows and compositions will usually require writing balances around various groups of equipment.

Just as we can write material, component, and atom balances around the entire system, we can also write each type of balance around any subsystem of the plant. A subsystem doesn't have to be a piece of equipment; it might be a mixing point (pipe junction) or splitter.

Absorber/Stripper The figure shows an absorber-stripper system for gas cleanup. If we consider the process to have 4 components (gas, H2S, oil, air), we can write a bunch of different balances:

5 balances (total material, gas, H2S, oil, air) on the overall system
5 balances on the absorber
5 balances on the stripper
5 balances on the heat exchanger (although they don't really tell us much)

The trick -- as with all these problems -- is to write the balances in a well-chosen order, so that the unknowns are managed and the problem is kept as simple as possible.

References:

Felder, R.M. and R.W. Rousseau, Elementary Principles of Chemical Processes, 2nd Edition, John Wiley, 1986, pp. 107-113.
Felder, R.M. and R.W. Rousseau, Elementary Principles of Chemical Processes, 2005 3rd Edition, 2005, p. 104-10.
Himmelblau, D.M., Basic Principles and Calculations in Chemical Engineering, 3rd Edition, Prentice-Hall, 1974, pp. 195-198.

R.M. Price
Original: 6//94
Modified: 9/17/98; 1/5/2005