Use global parameters

While operational parameters are associated with process units, global parameters are free variables not associated with any process unit. This means they can be used in calculations and can be constrained, as required.

Global parameters can be of two types:

• Time-Averaged: the value of the parameter is averaged across the whole planning period.

• Multi-Period: you can specify an Opening Value of the parameter to use in your calculations, and then the parameter value is time-averaged over the planning period. For more information see Multi-Period Global Parameters below in this topic.

Add a global parameter

1. Add the global parameter

   Click Add > Add Global Parameter from the Manage group of the Home ribbon tab in the Constraints and Alerts page.

   

2. Enter the parameter details

   Enter the parameter details, including its name, unit of measure, default value and valid range. The valid range and default values are important during simulation and optimization, as they are used to help define ranges and starting points respectively.

   

   Tip: You can later view and edit the default value in the Default column of the Constraints and Alerts page.

3. Include in any calculations

   Once added, the global parameter can be accessed in any calculation by adding it as a formula variable.

   

4. Define necessary constraints

   You can constrain the global parameter, or calculations based on it, from the Constraints and Alerts page.

   

Multi-period global parameters

Global parameters are useful for calculations over multiple periods of time. Consider the following example.

A plant contains an FCC which uses a catalyst, and you need to know the cumulative use of the catalyst over a number of periods. To do this, you want to determine the amount of catalyst used in a each period, and add that to the sum of the catalyst usage from the preceding periods.

You need two global parameters:

5. The amount of catalyst consumed in the period (that is, the catalyst consumption per day multiplied by the period duration in days).

6. The cumulative catalyst usage (that is, the cumulative catalyst usage from the preceding period plus the catalyst usage in this period).

Imagine you calculate the catalyst usage in the current period as 0.0005 tonnes of catalyst per tonne of FCC feed. Since your FCC feed is calculated in t/d, the catalyst usage in the period is:

FCC Feed × 0.0005 × number of days

Note that this could be calculated as a utility consumption and included in the FCC process model.

The cumulative catalyst usage so far is the closing amount for cumulative catalyst usage from the preceding period plus the usage in this period. Normally this value for the closing amount is transferred between periods by automatically setting the opening value for the parameter equal to the closing value of the preceding case, in the same way as the opening inventory of crude in a tank is set equal to the closing inventory for the same tank in the preceding period.

Let us look at this month-by-month.

7. Month 1

   In the first month there is no preceding case, so you enter a manual value of 0.

   

   Note: In the first case of a case stack, the value of a multi-period global parameter must be overridden and set as a fixed value. If you do not set the value for the global parameter, it is not possible to optimize the case.

   Note that time-averaged parameters cannot have opening and closing values, so you do not need to set values for these.

   

8. Month 2

   In the second month, the multi-period parameter is switched so the opening value is set equal to the closing value of the parameter in the preceding case.

   

   

9. Month 3

   Month 3 again updates the opening value for the cumulative usage with the closing value from the preceding case (in this example month 2).