Initialisation with Custom Constraints
In this example (see the previous section) a constraint is used to define a state block.
State blocks have an initialisation routine, sb.initialize(), which is used to figure out good guesses for all the variables in the state block.
More complex state blocks have other variables and other constraints (e.g temperature might be a variable, with a constraint linking temperature and pressure and enthalpy) and so they need to be solved for. Even in the simple helmholtz case, part of the initialization involves leaving all state variables (flow_mol, pressure, and enth_mol) fixed so that the unit operation can solve with those values - even if they aren't the correct values it's a good guess.
The problem arises when we add our custom constraints (e.g our flow_mass constraint.) As all the state variables have been fixed by the initialisation, flow_mol is already fixed (to some guessed values), and so the flow_mass constraint over-defines the system. This leads to a degrees of freedom error when solving.
Our solution is a wrapper method for the state block, creating our own custom initialisation routine that doesn't fix flow_mol if a flow_mass constraint is defined. This is done as part of the property_packages repository.
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