Difficulty in applying additional balances

Data reconciliation, as applied to process plant meters, is the procedure of optimally adjusting measured data so that the adjusted values obey mass balance and other constraints, while staying as close to the actual measured values as possible to retain a maximum degree of accuracy.

Data reconciliation is well known in the process industries and often practiced in petrochemical plants, refineries, platforms, mining operations, oil sand separation plants and many other types of process industry plants. Many such plants have introduced data reconciliation software to achieve this goal but usually they reconcile data with regard only to mass balance constraints. Additional types of balance constraints, such as component and enthalpy balances are seldom applied. Restricting the balance constraints to use only mass balance results in low accuracy of the reconciled data. This is especially true in plants with a lower degree of redundancy of flow measurements.

We can improve the accuracy of reconciliation using component data from online analyzers and laboratory component analysis results, as well as process data such as temperature and pressure, so why, then, do plants continue to balance only the overall mass? Some data reconciliation software have the capabilities to reconcile using component and enthalpy balances along with mass balance but this feature is not likely to be used because of the following limitations.

• The addition of component or enthalpy balance constraints to the mass balance constraints make the data reconciliation calculation into a nonlinear optimization problem, which is harder to solve than linear optimization. The solution time may be so high as to make the problem intractable.

• It is very difficult to configure a full component or enthalpy balance in complex data reconciliation models.

• With traditional approaches to adding component or enthalpy balance, one does not gain the ability to add additional constraints from engineering performance equations. Below, we shall see an approach that offers this.

Additional constraints

The new approach that we present in this guide addresses the limitations above by building an additional balance model, then adding the linear constraints from the additional model to the solution of the existing balance model (mass balance). Because these constraints come from an additional model, we refer to this approach as using “additional” constraints; that is, additional to the overall mass balance constraint. This new approach permits users to specify many kinds of additional linear constraints between the additional balance model and the mass balance model, including component and enthalpy balances.