Constrain your case

The Constraints and Alerts page shows any constraints that have been applied to the current case, and any alerts associated with these constraints.

Alerts provide warning and error ranges, without being incorporated in any optimization problem. Alert messages are shown in the Diagnostics window, and the affected process unit is highlighted on the flowsheet. You define the ranges for warning and error alerts under the Warning and Error column bands. An amber or red disc appears in the Alert column if a warning or error alert is raised during optimization.

Constraints limit the value for a variable or calculation in the solution. You can define Minimum, Maximum and Fixed constraints. You can constrain the following:

• The amount of particular components in final product blends.

• Calculations. Note that calculations can only be applied for the current active plant. If calculations that span plants are required in a multi-plant model, these must be created within the multi-plant environment.

• Global parameters, which can be used across the supply chain model in any formula, for any calculation.

• Operating parameters (that is, parameters for particular process units).

• The split ratios from splitters and manifolds in the flowsheet.

• Capacity (a constraint for the amount of material handled by an asset).

• Property constraints limit the property values for materials in the plant. For final products, the property constraints are defined using Specifications for the particular product grade. However, for internal streams, it may also be necessary to apply a constraint, and these may be added using the Constraints and Alerts page. For example, you may want to limit the acidity of a process unit feed to less that 0.5 mgKOH/g. You could apply this as a constraint on the crude feed.

• Purchases and sales to control the amount of materials bought and sold. Note the you must set these from the Purchases and Sales pages, rather than the Constraints and Alerts page.

To add, edit or delete constraints, use the buttons in the Manage group of the Home ribbon tab:

The Process Units > Constraints and Alerts tab shows the constraints applied to the process model. The constraints shown are a subset of the constraints included in the main Constraints and Alerts page. The Process Model column shows the process model the constraint applies to.

Tip: You can copy and paste values into the Constraints and Alerts page, including formulae. When pasting formulae, take care to use the exact formula names already present in the calculation and take care to ensure the formula evaluates correctly.

Constraint solutions

After optimization, the Constraints and Alerts page shows the solution results for each valid constraint in the Solution column.

If a result cannot be calculated (because the calculation is invalid, but inactive) then no solution value is displayed.

If the constraint is inactive in the current optimization (its Active state is disabled in the Constraints and Alerts page) then the solution value is calculated via simulation, but the constraint may be violated.

Marginal values

When a constraint is at a limit, a marginal value is shown for the constraint. This is the benefit (or cost) of violating the constraint by one unit (that is, breaking the constraint and allowing an extra unit of the constraint).

For example, imagine your CDU is running at its maximum capacity. The marginal value shows the extra profit from increasing the maximum capacity. So, if the maximum capacity were 100 kbbl/d, and the marginal value were 4399 $/UoM, then if you were able to increase the capacity of the CDU to 101 kbbl/d, you would make an extra $4399.

When properties are constrained, the marginal value is for a change in the property value at the current flow rate. Therefore, to calculate the marginal value due only to the property, you need to divide by the flow rate.

Example: Imagine your FCC feed sulfur is constrained to 1%, that is, the feed to the FCC cannot have more than 1% (by wgt) sulfur. After optimization, this has a reported marginal value of $19,000, so increasing the sulfur constraint from 1% to 2% should increase the profit by $19,000. This value is for the current FCC flow rate, which you calculate as 16,000. Therefore, the marginal value due to sulfur alone is: 19,000 / 16,000 = $ 1.19 per percent.

Infeasibility breakers

Although constraints will try to be met during optimization, if your constraints result in the overall problem being infeasible, you can invoke Infeasibility Breakers and break one or more constraints to make the problem feasible.

For this to happen, infeasibility breakers must be enabled for the optimization (see: Infeasibility Breaker Settings), and you must select the check box in the Infeasibility Breakers column for each constraint you want to break.

If a constraint cannot be met during optimization, and infeasibility breakers are disabled for that constraint, the optimization will fail.

Carefully review constraints and alerts to ensure that the displayed values are valid. Where a constraint exists on a stream with zero flow, the constraint value does not have any meaning (as the pipe has zero flow, the properties for the material in the pipe cannot be determined. However, a property result is still returned for the pipe generated via simulation).

For example, in the image above, the underlined value is generated for the sulfur content of the High Acid stream. The constraint is not violated and the solution is valid. The high acid stream has zero flow, and therefore its true property values cannot be determined. A value is returned via simulation (see: Post-Optimization Simulation) to indicate typical values for this stream, which can be useful for model analysis and debugging (for example, the unit of measure of the sulfur is incorrectly set in this example).

Lock your constraint solutions

Solution values can be locked during subsequent runs. This means that a solution value will not change in any subsequent run, even if other input to the run is changed.

Example: The flow of naphtha from a crude could be locked, and in subsequent runs it would not be able to change. Therefore, if a fixed swing cut point was changed and the model re-run, the crude blend would have to change in order to match the fixed flow rate.

Locking a solution value acts in a similar way to fixing the value. The advantage over fixing a value is that the lock can be clearly seen in the user interface, so it is simpler to distinguish true fixed values from those which are temporarily locked.

To lock one or more solution values, select the corresponding rows in the table and do either of the following:

• Click Lock Solution in the Behaviour group of the Home ribbon tab.

• Right-click on the table and choose Locking > Lock from the context menu.

To unlock one or more solution values, select them and either click Unlock Solution in the ribbon or choose Locking > Unlock from the context menu.