Configure modes of operation for your process units

A mode is a state of operation for the process unit with associated input feeds, process model, and outputs.

The Modes tab of the Process Units page allows you to configure the modes of the process unit selected in the Process Unit drop-down menu.

The Modes pane lists the available modes for the process unit. You can limit the number of active modes used during optimization by selecting Mode Limit.

Example: Say you have n available modes in your process unit. If you limit to just one active mode, the optimizer finds the best solution using only one of your available modes at a time. If you limit to n active modes, the optimizer finds the best solution using each mode individually, or a combination of all available modes.

You can add, edit, delete and move the different modes using the Modes group of the Home ribbon tab:

Moving the order of your modes has no effect on the optimization solution.

Mode connectivity

The Mode Connectivity grid allows you to define the connectivity between upstream process units and operating modes on the selected process unit. If an upstream mode can feed a particular operating mode on this unit, select the check box in the intersection between the upstream unit mode column and the current unit mode row. The Mode Connectivity grid and stream routings interact to control the flow of material around the plant in different operational modes.

See the following two figures for an example.

With this connectivity, when the Vac Res Pool is in Anode Coke Mode, it can only feed the Anode Code Mode of this process unit, and when the Vac Res Pool is in Default mode, it can only feed the Default mode of this process unit.

With this connectivity, when the Vac Res Pool is in Default mode, it can only feed the Default mode of this process unit. However, when the Vac Res Pool is in Anode Coke Mode, it can feed either the Anode Coke Mode or Default Mode of this process unit. The ratio with which these modes are fed is determined by the optimizer, subject to any other operating constraints. For example, if the Default operating mode for this process unit had a maximum capacity constraint of 0, that is, when in this mode it cannot receive any feed, then all feed to the unit would have to be routed to the Anode Coke mode.

Control the proportion of time a unit operates in a particular mode for

There are two ways to control the proportion of time a unit is in a mode for:

1. Constrain the flow

2. Time-slicing

Constrain the flow

To control the amount of a period the unit can operate in a mode, you can constrain the flow of the mode. You can do this using a calculation (as shown step-by-step below).

To calculate the proportion of the period a unit is operating in a mode for, use this formula:

Proportion in mode A = flow per day in mode A / flow per day in all modes

3. Open the list of constraints for the unit

   Open the Process Units page and click on the Constraints and Alerts tab. Select the correct unit from the Process Unit list.

   

4. Add a calculation

   Click Add in the Manage group of the Home ribbon tab. The Create New Calculation dialog box opens.

   

5. Add a flow variable

   Click the button to add a variable for the calculation formula. Choose Add Flow Variable.

   

6. Configure the variable

   Define the variable as the total flow of the unit feed, in all modes.

   

7. Add a second variable

   Select Add multiple variables and click the Add button. The first variable is now added to the Calculation Editor, and you can define the second one.

   From the Mode list choose the mode for which you want to constrain the amount of operation time. Leave all other settings unchanged.

   Click Add to add the variable, then Close to return to the Calculation Editor.

   

8. Write the formula

   Click in the Enter Formula pane and add the formula defining the calculation. The formula is the flow in the mode you have chosen divided by the flow in all modes.

   As you type, the auto-complete feature shows you a list of matching variable names.

   

9. Complete the calculation

   Enter a name for the calculation, and if necessary choose a Calculation types from the Type list.

   Click OK to close the Calculation Editor.

   

10. Add constraints

The new calculation is now listed in the Constraints and Alerts tab. Add constraints and alert thresholds as needed. In the image below, 0.20 has been entered in the Min column, meaning that the CDU1 is forced to run at least 20% of the time in Low Metal mode.

Time-slicing

Along with constraining the flow, time-slicing is another way to model the proportion of time a unit is operating in a particular mode for.

Within a single period, the results of optimization represent a time-average. It is not possible to know what is occurring at any instant in time, it is only possible to know what occurs on average.

This is true even when a unit has modes. The modes represent different operating states which are separate from each other (and so can be thought of as separate in time), but within these modes the flow (and other properties) is still the average of the mode. However, when a unit has modes, the flows represent the average over the whole period, even when the mode is not in use.

Example: A distillate hydrotreater may have an instantaneous flow rate maximum of 20 kbbl/d in a particular mode. Within the optimization, if the unit were active for 50% of the time and running at 20 kbbl/d the flow would be reported as 10 kbbl/d (20 * 0.5).

Therefore, within a mode to limit the active flow, it is necessary to know both the time average flow and the amount of time the mode is active for. This is commonly achieved through time-slicing.

The active flow rate in a unit is given by the equation:

Active Flow = Modal Flow / Mode Time Slice Proportion

For a unit that is always active (so its total time is 1) then:

Σ Mode Time Slice Proportion = 1.

So, for a unit with two modes (A and B):

Mode A Time Slice Proportion + Mode B Time Slice Proportion = 1.

Therefore, to model the active flow in a unit, you need to introduce a variable to model the proportion of time the mode is active. This can be done through global parameters. For each mode in the unit, add a global parameter (between 0 and 1), and then define a calculation so the sum of mode parameters equals 1.

For each modal flow constraint, rewrite the constraint equation as above.

For example, if the hydrotreater has two modes A and B (where B represents a high sulfur mode with a maximum capacity of 20 kbbl/d), then create an equation:

High Sulfur Flow / Proportion Time High Sulfur Mode <= 20.

As the proportion of time may be zero, it is not recommended to leave the calculation in this form (as the denominator may be 0 and then the equation becomes indeterminate). Instead, the equation should be re-written in a linear form:

High Sulfur Flow - 20 * Proportion Time High Sulfur Mode <= 0.