Modes of operation

At any one point in time, a plant is in a particular mode of operation. A mode of operation represents all the operating parameters of the plant process units, and the stream routings of all materials.

Some plants may operate in a constant state and therefore have a single mode of operation. Other plants may have to adjust their operations at different points in order to meet operational or product requirements. These plants have several modes of operation.

In AVEVA Unified Supply Chain, the mode of operation of the plant is defined by setting the individual modes of operation of all the assets in the plant. Note that many assets do not operate in a modal sense and process material in the same way, no matter what mode the other units are operating in.

Each mode for a process unit may have a different process model and different stream routings. That is, the sources which feed the unit in that mode can be different, and the destinations where the unit's products go can be different.

These are typical examples you can model using modes:

• Segregated feed. Some refineries segregate crudes into low- and high-sulfur types. These may then be processed separately in the refinery, so the two crude types are never refined at the same time.

• Operating conditions. Due to seasonal changes diesel may have different product specifications. In order to make diesel which meets these specifications it may be necessary to run the refinery differently depending on the season.

• Product crude dependence. Some products, such as lubricating oil, can only be made from certain crude oils. When manufacturing these products, the refinery must be run in a mode where these crudes are segregated from all other crudes.

• Stream routing. A refinery is usually configured to allow a complex interchange of streams between units. However, at any one time some of these units will be inactive, and the routing of streams will have to be changed accordingly by using a different mode.

Modes and constraints

An important part of defining the optimization problem in AVEVA Unified Supply Chain is defining the constraints in the plant, that is, the limits for particular variables. Many of these constraints refer to flows in the plant, that is, the amount of material passing through the pipe at a particular point in time. By default, any applied constraint refers to the sum of the flows of all the modes in which that pipe can operate.

Modes and optimization

A mode of operation represents how the plant operates at a single point in time, while optimization aims to find the best way of operating the plant over a period of time. Therefore the final solution given by AVEVA Unified Supply Chain will often result in the plant being operated in several different modes. For example, the refinery might have a summer and winter operating mode with different cut points in the CDUs. The result of the optimization might be to run the summer mode for 75% of the period and the winter mode for 25% of the period.

To ensure that modes are run during optimization, or to limit their operations, it is necessary to set constraints for particular operating modes, rather than the default sum of all modes. By setting a constraint on a particular mode it is possible to control the degree to which that mode is used in the solution.

Modes and simulation

In contrast to optimization, simulation represents the plant running at a particular point in time. That is, during simulation the plant is operating in one single mode of operation and all of the process unit settings and routings must be valid for this mode of operation. Therefore, any applied constraints must also be valid for the mode of operation.

Mode connectivity examples

Anode grade coke production

When manufacturing anode grade coke, only some crude grades are suitable for use. These must be kept segregated from the other crude types all the way to the unit which produces the anode grade coke. The vacuum residue feeding the coker would need to be segregated, and when fed with suitable feed the product of the coker can then be passed to the calciner to manufacture anode grade coke. Therefore each unit between the calciner and crude feed pool needs to have segregated modes to handle the different feedstock types.

In the default mode, material from Crude Pool 1 passes through the process units which are all in their default modes. As material from Crude Pool 1 is not suitable for anode coke production, the coke produced from this feed can only be sold as green coke. Therefore this material, once it reaches the splitter, is routed to a pool for sales.

In the anode coke mode, material from the anode coke feed pool passes through the process units which are all in their anode coke modes. As the material from the anode coke feed pool is suitable for calcining, the coke produced from this feed is directed to the calciner, where it is further processed to make anode grade coke.

At any one time, the refinery is operating in only one of these modes, and the two types of feed and their products are always separate. The solid coke products of the two modes are never mixed and are sold separately. During optimization, the ratio in which to run the two modes is decided based on the economics and constraints of the problem.

On the flowsheet, the process units are directly connected. The calciner receives feed from the coker, which in turn receives feed from upstream units. Crude feed is segregated into two groups, a standard group and a group of approved crudes which can feed the calciner. As the calciner needs to be able to 'see' these two groups, it is necessary to add modes on every process unit between the calciner and the feed units.

The calciner itself only has one operational mode. The Modes tab for the calciner process model shows the upstream units which the calciner can receive feed from, in this case the green coke split. The calciner can only receive green coke originating from crudes which have been approved for calcined coke production. Therefore, to ensure these crudes are also segregated in the splitter, the splitter itself must have an anode coke and default mode.

The mode configuration for the calciner is then set up to allow the calciner to only receive feed from the Green Coke Split when the splitter is in its Anode Coke Mode.

The Green Coke Split needs two modes to segregate residue from crudes approved for anode coke production. These are added on the Modes tab for the splitter. For the residue to be segregated, any process unit the residue passes through must also have these two modes present. Therefore these have also been added to the coker which feeds the Green Coke Split.

When the Green Coke Split is operating in Anode Coke mode, it can receive feed from the coker also operating in Anode Coke mode. This is configured by selecting the check box in the intersection between the source unit mode column and receiving unit mode row.

When the Green Coke Split is operating in its Default mode, it can receive feed from the coker operating in both modes. This is configured by selecting both check boxes for the receiving unit row.

Once material reaches the Green Coke Split, it can be directed to either the green coke sales or calciner. Some of the material from the Anode Coke mode of the Green Coke Split may be directed to the calciner, depending on economics or constraints in the model. When the Green Coke Split is in Default mode, it can only direct material to the sales pool.

To force green coke to be directed to the calciner, a number of possibilities exist. If the Green Coke Split were connected to another process unit, this could have its mode connectivity defined to not receive material from the Green Coke Split in Anode Coke Mode. Thus when the Green Coke Split was in Anode Coke mode, it would only be possible to pass material to the calciner.

In this example there is no downstream unit of the Green Coke Split, so this approach is not possible. Instead it is possible to turn off the output from a splitter by setting its availability to zero.

On the Splitting tab of the Stream Routing page it is possible to enter limits on the amount of material that can flow out of a particular pipe in a splitter. This can be done on a modal basis to prevent routing through a pipe originating in a particular mode. In the above example, the Green Coke Split has been configured so that when it is Anode Coke Mode the availability of the pipe to the Green Coke sales pool is zero. Thus when the Green Coke Split is in its Anode Coke mode the material can only go to the calciner.

The crude mixer is the source of the actual segregations. It can receive crude from two different feedstock groups, one approved for anode coke production and the other for use in default operations. Unlike all the other connections between units, which are essentially single pipes operating in different modes, for the crude mixer there are two physical feed pipes, one for each mode.

The crudes in a feedstock group are shown on the Feedstocks page. A single crude can be in several feedstock groups if required.

For an operational mode to be used, it must be economically advantageous, or there must be an operational constraint which forces the mode to be employed at least some of the time. In this example, a constraint has been placed on the CDU1 so that it has a minimum capacity to operate in Anode Coke mode. This ensures the crude unit sometimes operates in anode coke mode.

Operating FCCs with or without atmospheric residue feed

A refinery may have an FCC that can be fed with atmospheric residue. Typically, the process model associated with the FCC will be different when the FCC has long residue feed, and when it does not have long residue feed. Each of these operating states is a different mode.

In the flowsheet, the atmospheric residue is split; in the LR to FCC mode the atmospheric residue can be routed to the vacuum tower or the FCC feed pool, in the No LR to FCC mode the atmospheric residue can only be routed to the vacuum tower.

The Atm Res Splitter therefore has two modes of operation, one of which allows the material to be split between the vacuum tower and FCC pool, and the other which diverts all atmospheric residue to the FCC. The split ratio is defined by the availability on the Stream Routing page.

When the Atm Res Splitter is in Atm Res to FCC Feed Pool mode, 100% of the atmospheric residue can go to the FCC Feed Pool, or the HVU. When the Atm Res Splitter is in No Atm Res to FCC Feed Pool mode, no atmospheric residue can go to the FCC Feed Pool.

The FCC Feed Pool must also carry these modes, and therefore must have matching LR to FCC Unit and No LR to FCC Unit modes. When the Atm Res Splitter is in Atm Res to FCC Feed Pool mode, the FCC Feed Pool is configured to only allow LR to FCC Unit mode. That is, if the Atm Res Splitter is delivering atmospheric residue to the FCC then the FCC Feed Pool must be in LR to FCC mode. This configuration is necessary to allow the FCC to correctly obtain feed.

The FCC has two modes of operation, Bottoms to FCC and No Bottoms to FCC. In the Assign Models to Modes section the Bottoms to FCC mode is assigned to use the CCU LR process unit model; the No Bottoms to FCC mode is assigned to use the CCU VGO Feed process unit model.

The Mode Connectivity section allows the modes to be linked to the FCC Feed Pool. When the FCC is in Bottoms to FCC mode, then the FCC Feed Pool must also be in LR to FCC Unit mode (and this implies that the Atm Res Splitter must be in Atm Res to FCC Feed Pool mode).

Selectively operating a kerosene hydrotreater based on the input crude types

A refinery may choose to operate a kerosene hydrotreater at a reduced capacity, or not at all, if the kerosene stream has a low enough sulfur content to still be suitable for jet fuel.

In the above flowsheet, the kerosene hydrotreater is preceded by a splitter. This splitter can divert straight run kerosene to the hydrotreater or to the kerosene pool.

The splitter can operate in two modes, one of which diverts kerosene for hydrotreating, and the other which does not allow hydrotreating. These modes are linked to modes in the CDU. When the CDU is in its Standard Cuts - Low Kero S mode, the splitter can be in its Kero Treating or No Kero Treating mode. When the CDU is in its Max Kero - Kero Treat / Deep mode, the splitter can only be in its hydrotreating mode.

The flow through the splitter is controlled by the availability of the unit.

When the splitter is in its Kero Treating mode, it can divert incoming feed to the kerosene hydrotreater, or straight to the kerosene blending pool. This is because the availability of these outputs is 100%, that is, they are always available.

When the splitter is in its No Kero Treating mode, it can only divert the kerosene straight to the kerosene blending pool. This is because the availability of the hydrotreater feed pipe is set to 0%.

The CDU has two operational modes, a Standard Cuts - Lower Kero S mode and a Max Kero - Kero Treat / Deep mode. Each of these modes is associated with a different process model in the Assign Models to Modes section. The difference between the models is the cut points set for the fractions.

The CDU is connected to the Crude Feed Mixer, which is operates in two modes. When the CDU is in Standard Cuts - Low Kero S mode, the mixer must be in High and Low S Mix mode. When the CDU is in Max Kero - Kero Treat / Deep mode then the Crude Feed Mixer can operate in either mode.

The mixer modes determine which feeds can be sent to the CDU. When the mixer is in High and Low S Mix mode, crudes from the High S Crude Feeder and Low S Crude Feeder can both be sent to the CDU. When the mixer is in the Low S Only mode, only crudes from the Low S Crude Feeder can be sent to the CDU.

So crudes in the Feedstock Group from the High S Crude Feeder can only go to the Crude Feeder when the Crude Feeder is operating in High and Low S Mix mode. When the Crude Feeder is in this mode the CDU can only be in Max Kero - Kero Treat / Deep mode. When the CDU is in this mode it uses the CDU (Max Kero / Deep) process model, and the Kero Splitter must be in Kero Treating mode. When the Kero Splitter is in this mode the splitter can divert feed to the kerosene pool, or the kerosene hydrotreater.

Crudes in the Feedstock Group from the Low S Crude Feeder can be mixed with the High S Crudes when the Crude Feed Mixer is operating in High and Low S Mix mode. In this situation the modes of the other units are the same as those described above.

However if the Crude Feed Mixer is in Low S Only mode then the Low S Crude Feeder crudes are the only crudes that can be processed. In this situation the CDU can operate in either its Standard Cuts - Low Kero S mode or its Max Kero - Kero Treat / Deep mode.