How to scale energy balance for chemical process

In How to scale chemical reactions and How to scale chemical species we covered the most complex parts associated with scaling aqueous chemistry models in WaterTAP. The last piece to mention is that we also must scale the energy balances for the model.


IMPORTANT: Scaling of just the energy balances is insufficient for solving a chemistry module. User’s MUST also scale the chemical reactions in the system due to the dilute nature of aqueous chemistry. See How to scale chemical reactions. Additionally, user’s MUST also scale the chemical species. See also How to scale chemical species.

After you have set all scaling factors for reactions, species, energy balances, etc., you need to call the ‘calculate_scaling_factors’ function on the entire model before attempting to solve. See below:

# After all scaling factors and constraint transformations are complete,
# call the following function on the model.

Scaling the energy balances


The following example assumes that your unit model is named unit and your FlowsheetBlock is named fs. For information on setup of the basic chemistry modules, see How to setup simple chemistry.

The scaling of the energy balances can be done based solely on how the enthalpy flow term expressions are formulated in the model. This can be accomplished simply by doing the following:

# Setup default min and max for scaling
max = 1
min = 1
for phase in model.fs.unit.control_volume.properties_in[0.0].enth_mol_phase:
    val = abs(value(model.fs.unit.control_volume.properties_in[0.0].enth_mol_phase[phase].expr))
    if val >= max:
        max = val
    if val <= min:
        val = min
        model.fs.unit.control_volume.properties_in[0.0]._enthalpy_flow_term[phase], 10/val)
        model.fs.unit.control_volume.properties_out[0.0]._enthalpy_flow_term[phase], 10/val)

iscale.constraint_scaling_transform(model.fs.unit.control_volume.enthalpy_balances[0.0], 10/max)