Case study

Selectively building a component or enthalpy balance in this way can yield a great improvement of the accuracy of the reconciled mass quantities.

The mass balance model

This figure shows the mass balance model for an ammonia plant.

Figure: The mass balance model for the ammonia plant

Performing only a mass balance does not guarantee consistency with the component data from on-line analyzers or laboratory samples. At the same time, given that there are many unmeasured streams in such a plant and given that the chemical reactions are very complex, it would be impossible to perform a component balance for every stream and reactor in the plant when calculating the mass balance.

From the major mass balance streams, we were able to abstract the nitrogen component balance and represent it as a separate model. We also imposed additional constraints to establish the relationship between the mass balance model and the nitrogen component balance model.

The nitrogen component balance model

This figure shows the nitrogen component balance model for an ammonia plant.

Figure: The nitrogen component balance model for the ammonia plant

A major goal of data reconciliation in the ammonia plant is to accurately determine the production of ammonia.

Nitrogen component information for various streams is available from laboratory samples or online analyzers. Though the mass balance model was quite detailed, it was initially unable to provide insight into the balance of the nitrogen content. For instance, in the case of the ammonia plant, the nitrogen content of the process air being fed in should balance with the nitrogen content in the produced ammonia, with due consideration for venting.

Though balancing the nitrogen content along with the mass balance greatly increases the accuracy of the reconciled ammonia production, adding a nitrogen balance at every point in the mass balance model would be far too complex an undertaking. For that reason, the overall nitrogen balance model was built by extracting the streams related to nitrogen such as process air, ammonia production, recycled hydrogen streams, etc.

The following shows how the nitrogen balance was configured separately from the mass balance using a subset of the plant’s streams with constraints to establish the relationship between them.

The nitrogen balance constraint

This figure shows the nitrogen balance constraint between the mass balance and nitrogen component balance.

The constraint object named “AMMONIA_NITROGEN_CONSTR” defines the relationship of the nitrogen stream coming from the ammonia plant to the total mass of the material coming from the ammonia plant in the mass balance model. That is, “AMMONIA_NITROGEN_CONSTR” defines the relationship between the mass of mass of stream “S16”and the calculated nitrogen component thereof, which appears as stream “S16_N”. That relationship is defined using the following equation.

This constraint equation is easily configured in the equation editor of the smart constraint object.

We can also impose additional constraints without actually creating an additional model graphically. Simply imposing a constraint through the use of a suitable equation implicitly creates an additional model, which does not have to be represented graphically.

Enthalpy balance constraint

Here, we see an enthalpy balance constraint between the mass balance and enthalpy around the heat exchanger, enforced without graphically creating another model.

Figure: An enthalpy balance constraint between the mass balance and enthalpy around the heat exchanger, enforced without graphically creating another model

The mass balance constraint around the heat exchanger can be expressed by the following two equations, which are both enforced.

There are no meters on “S9” and “S12” streams, which impedes our ability to obtain accurate mass flow quantities around the heat exchanger using only a mass balance constraint.

To improve our estimation, we can take into account the fact that enthalpy around the heat exchanged must also balance. We represent his using a constraint object named “E4_Enthapy_Balance” around the heat exchanger, using the following equation: