Aug-2019

Best practice for refinery flowsheets

Refinery-wide flowsheets can demonstrate the true representation of changes in key operating variables and their associated impact on refinery operation.

Jitendra Chellani and Asma Al Sariyahi
KBC (A Yokogawa Company)

Article Summary

Due to the integrated nature of process units within a refinery, change in key operating variables has an impact on overall refinery operation and product blending as well. Refinery-wide flowsheets demonstrate the true representation of this effect as all process units are linked together. Non-linear kinetic or equilibrium models in the flowsheet represent the conversion units. Fractionation models capture the efficiency of separation between different cuts or components.

KBC (A Yokogawa Company) has developed and used refinery-wide flowsheets for more than 30 years. Developed in the 1980s, Petrofine was a FORTRAN based tool capable of refinery-wide flowsheeting. In 2004, KBC launched Petro-SIM with additional features to simulate refinery process units. These standalone models are combined to create a complex-wide flowsheet which includes all process units within the refinery.

Due to the integrated nature of process units within a refinery, change in key operating variables has an impact on overall refinery operation and product blending as well. Refinery-wide flowsheets demonstrate the true representation of this effect as all process units are linked together. Non-linear kinetic or equilibrium models in the flowsheet represent the conversion units. Fractionation models capture the efficiency of separation between different cuts or components.

KBC (A Yokogawa Company) has developed and used refinery-wide flowsheets for more than 30 years. Developed in the 1980s, Petrofine was a FORTRAN based tool capable of refinery-wide flowsheeting. In 2004, KBC launched Petro-SIM with additional features to simulate refinery process units. These standalone models are combined to create a complex-wide flowsheet which includes all process units within the refinery.

Petro-SIM is KBC’s process simulator used for rigorous modelling of the entire refinery and petrochemical complex, from crude to finished products. Since each unit, including conversion units, is modelled meticulously, the overall simulation suitably reflects the non-linearity of petroleum refining which enables sensitivity analysis over a wide range of operating variables and feedstocks.

The conversion units are based on comprehensive kinetic models that predict the unit yields and product qualities. The kinetic models are calibrated specifically to match the available plant data for a particular unit. This allows the simulation to be representative of the specific unit’s operation, independent of the  licensor.

Product separation is simulated using fractionation technology that represents current operation and heat balances. Heat-and-material balanced distillation models which use a section-by-section approach rather than simulating each tray are calibrated to plant data.

KBC and the company’s clients around the world have developed numerous refinery-wide flowsheets. Petro-SIM based flowsheets are being used for identification and evaluation of margin improvement opportunities which include optimisation of stream routings, blending strategies, molecular management, throughput maximisation, feedstock selection, and improvements in the unit operating conditions. Refinery-wide flowsheets have also been used for configuration studies for grassroots and revamp configurations.

Flowsheet development
Standalone models for the process units are the main building blocks for the refinery-wide flowsheet. Detailed kinetic and equilibrium based Petro-SIM models are calibrated using test run data. Unit configuration, operating parameters from historian and laboratory data are used to calibrate standalone models for the process units. The data are reconciled to close mass, sulphur, nitrogen, carbon, and hydrogen balance. A calibrated process model mimics the performance of the process unit. The models are valid over a wide range of operation as they are based on first principles and are non-linear in nature.

To understand overall refinery operation, a base month is selected. Base month operation provides an insight into marginal mechanisms in the refinery. The data used for standalone models are based on test run operating conditions and these test runs may have been conducted at a different time period. Due to this, it is essential to prepare a consistent basis for operating conditions of all process units.

The following guidelines are used to select the base month for the flowsheet:
• Crude blend for the month represents the typical crude blend used by the refinery
• Crude throughput should be close to the typical crude throughput of the refinery
• Most of the process units in the refinery should be operating at typical capacities and at normal operating conditions
• Most of the process units should operate continuously in stable conditions
• Changes in the inventory of intermediate streams should not be significant.   

One of the major challenges is with regard to inventory. Inventory changes are not simulated in the flowsheet as it represents steady state operation of the refinery. Inventory changes in feed and product are used to estimate the net feed processed and net products produced in the refinery. Inventory changes for intermediate streams affect throughputs of the process units and hence it is essential that the base month operation has minimum inventory changes for the intermediates.

Mass balance for different process units may be inconsistent for refineries which do not use data reconciliation tools, for instance, the throughput of a delayed coking unit measured by the feed meter may not be the same as the vacuum residue production measured in the vacuum distillation unit. Validating consistencies for a product which is routed to more than one process unit and blending is more challenging. Due to these issues, data for the base month also requires  reconciliation.

To build a refinery-wide flowsheet, all standalone models are combined. Crude blend for the crude distillation units (CDU) and throughputs are updated so that the flowsheet feed represents base month operation. Routing strategies used by the refinery are replicated in the flowsheet. During the flowsheet development phase, refinery strategies are used for routings rather than using optimum routings. Sensitivity analysis to validate optimum strategies is performed after completing the base case which represents ‘as-is’ refinery operation.

Understanding refinery stream routing strategy can be a difficult task if the refinery has multiple trains or two to three process units for a purpose, for instance more than one diesel hydrotreating unit. In this case, routings are fixed based on the feedback from planning engineers and base month data.

Petro-SIM’s blending unit operation allows flexibility of optimising refinery blends based on prices, product demand, product specifications, and other constraints. Marginal mechanisms used by the refinery must be reflected in the flowsheet product blenders. The flowsheet must hit key specifications for each blend, such as octane for gasoline, flash for diesel, viscosity for fuel oil, and so on. The marginal streams used by the refinery should be reflected in the flowsheet as well.

The refiner also needs to identify major changes which are planned in the near future including the revamp of a process unit or a significant shift in the crude basket. The refinery-wide flowsheet may be updated with these changes after which it represents the base case operation of the refinery.

The work process to develop the refinery-wide flowsheet is shown in Figure 1.