Oct-2002

Operating an FCCU at multiple constraints

Integration of an FCCU operational model within a rigorous refinery-wide flow sheet, incorporating the use of software for process modelling, say the authors, enables refineries to increase profitability without the need for capital investment

Greg Tragitt and Gloria Chukman, KBC Advanced Technologies

Article Summary

The use of refinery process modelling has been demonstrated in the past. However, the benefits increase when the unit model is integrated within a detailed flowsheet package or a rigorous refinery-wide model. Use of software allows refineries to enhance profitability, typically without any capital investment. These benefits have been achieved at several refineries in conjunction with fluid catalytic cracking unit (FCCU) operations.

To maximise FCCU capacity and achieve economically optimum operation, it is desirable to operate at multiple unit constraints. There are often multiple unit constraints based on equipment capacities and operating limits. In general, FCCU profitability is greater when more constraints are met. If there is operating flexibility, operating variables can be manipulated with the model until those variables become constrained. Sometimes manipulation of a variable may remove another constraint or may cause another variable to become constrained.

Model utilisation may require optimisation of multiple operating variables to find an optimum combination of operational variables. In addition, detailed analysis with a model can disprove system constraints that are no longer valid.

FCCU yields, capacity and product properties can be greatly influenced by feedstock quality changes and operating condition changes.

Quantification of these changes is a very difficult task because the FCCU simultaneously maintains a heat and pressure balance. As one variable is manipulated, several conditions can change to allow the unit to achieve a new balance. Furthermore, changes to any major refinery conversion unit can be costly if not done properly.

FCCU operating conditions can greatly influence unit capacity. Use of a process model can predict the effect of reactor and regenerator operational changes on unit yields, and a separation system model can predict the effects on individual equipment. An engineer can determine or challenge FCC equipment constraints using a model. A simulation can then be used to examine operational changes within the fractionator and gas plant, which can often alleviate these constraints.

Modelling capabilities
The heart of FCCU modelling is simulation of the reactor unit. FCCU models have been available as stand-alone models for many years, and can also be integrated within a flow sheeting package or within third-party software to extend their usefulness.

KBC tunes its proprietary model using reconciled plant data to allow the model to match a specific FCCU. After the model is tuned, it can then be utilised to predict operating conditions, heat balance and product yields from information about the feed, the unit dimensions, and the operating conditions. The model also determines calculated parameters such as catalyst-to-oil ratio and conversion.

The effluent from this detailed FCCU prediction model is then fed to a simulation model that includes the main fractionator and gas plant in various levels of detail, depending upon the use of the simulation. For simple screening studies or refinery-wide models, the separation section can be modelled with simple yield separation models. For more detailed optimisation of operation, separation facilities can be modelled with tray-to-tray fractionation columns and rigorous models of compressors, pumps, heat exchangers, etc.

With the detailed simulation model, individual equipment items can be modelled to include such details as tray dimensions and compressor curves. The use of these models in different levels of detail can demonstrate the effects of debottlenecking affected equipment and can also quantify benefits of proposed changes to increase capacity.

While online systems and reactor-only systems are useful to direct the unit engineer towards optimum operation, the integration of these changes to downstream units are often not taken into account in more than a very simplistic manner. A flow diagram of the FCCU and associated separation equipment is shown in Figure 1.

 Most refinery-wide studies are performed with a linear program (LP) that does not have the capability to provide detailed step-out capability of the reactor or sufficient detail on the downstream separation facilities. A rigorous refinery-wide flowsheet is a very valuable tool in assessing overall refinery profitability as these variables are manipulated. This flowsheet maintains stream properties that allow products to be blended to actual specifications. The flowsheet provides overall refinery economics based on product qualities as well as product quantity.

Model creation and tuning
The first step in creating the model is to gather data for tuning or calibration. Operating data is gathered and reconciled with material and heat balances before the process models can be tuned. A single data set can be utilised to tune the model, but multiple data sets improve the calibration of FCCU process models. A very good heat and material balance is required to properly tune a model. With KBC’s model, reactor and regenerator dimensions are used as an integral part of tuning the process model and are independent of the unit’s manufacturer.

These dimensions are utilised in calculation and evaluation of such calculated parameters as riser residence time, cyclone velocities and regenerator superficial velocities. These calculated parameters are useful benchmarks for evaluation of equipment capacities and are often some of the key process indicators (KPI) of the unit. These parameters are also influenced by operating conditions. Once the process model is tuned it can be used to predict unit operation at different conditions or with different feed quality and feed rates.