How to scale chemical species


This scaling methods are meant to be used with the ‘gradient-based’ scaling functionality built into ipopt. It is not advised to use ‘user-scaling’ with the chemical modules, as this tends to have poor convergence.

In How to scale chemical reactions, we discussed at length how you need to apply scaling factors to variables and constraints associated with all the different types of reactions your system may have. Here, we will add to that discussion how the variables for the chemical species in the system should also be scaled.


The scaling factor constants used in this demonstration are the best found for these types of problems based on numerical testing of a variety of problems. These values can and may need to change for your particular problem.

Scaling Chemical Species Example


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.

For this example, our GenericParameterBlock (named thermo_params) is using the “state_definition” of FTPx. This may change what variables the scaling is applied to. See How to setup simple chemistry for more information on “state_definition” options.

# Try adding scaling for species
min = 1e-6
for i in model.fs.unit.control_volume.properties_out[0.0].mole_frac_phase_comp:
    # i[0] = phase, i[1] = species
    if model.fs.unit.inlet.mole_frac_comp[0, i[1]].value > min:
        scale = model.fs.unit.inlet.mole_frac_comp[0, i[1]].value
        scale = min
    iscale.set_scaling_factor(model.fs.unit.control_volume.properties_out[0.0].mole_frac_comp[i[1]], 10/scale)
    iscale.set_scaling_factor(model.fs.unit.control_volume.properties_out[0.0].mole_frac_phase_comp[i], 10/scale)
    iscale.set_scaling_factor(model.fs.unit.control_volume.properties_out[0.0].flow_mol_phase_comp[i], 10/scale)
        model.fs.unit.control_volume.properties_out[0.0].component_flow_balances[i[1]], 10/scale)
    iscale.constraint_scaling_transform(model.fs.unit.control_volume.material_balances[0.0,i[1]], 10/scale)