The latter usually equals the number of liquid levels with no steady-state effect, including
most buffer tank levels. However, note that some liquid levels do have a steady-state effect,
such as the level in a non-equilibrium liquid phase reactor, and levels associated with adjustable heat transfer areas. Also, we should not include in Ny0 any liquid holdups that are
left uncontrolled, such as internal stage holdups in distillation columns.
We o find Ny0 is nonzero for most chemical processes, whereas we often have Nm0 = 0. A simple example where Nm0 is non-zero is a heat exchanger with bypass on both sides,
(i.e. Nm = 2 ). However, at steady-state Nss = 1 since there is really only one operational
degree of freedom, namely the heat transfer rate Q r (which at steady-state may be achieved
by many combinations of the two bypasses), so we have Nm0 = 1
The optimization is generally subject to several constraints. First, there are generally
upper and lower limits on all manipulated variables (e.g. fully open or closed valve). In
addition, there are constraints on many dependent variables; due to safety (e.g. maximum
pressure or temperature), equipment limitations (maximum throughput) or product specifications. Some of these constraints will be active at the optimum. The number of “free”
unconstrained variables “for steady-state optimization”, Nss , free , is then equal to