Refinery Stock Balancing

Before the advent of linear programming (LP) models, process-planning studies were done by hand with desktop calculators and usually large printed or duplicated worksheets. Little optimization was possible via trial and error, as this would involve calculating a stock balance over and over again until a satisfactory answer was arrived at.

Refinery LP models now do stock balancing. Many LP packages are available that facilitate plant yield calculations and optimize product blending.

However, stock balancing must be done by hand at times. A refinery operation planner may take an LP-optimized stock balance and redo it by hand, taking into account the conditions in the refinery that cannot be conveniently incorporated into the LP model,1 for example, a critical pump-out of service, partially coked-up furnace, catalyst bed with high pressure drop or low activity, a delayed ship causing severe ullage constraints, or a change of specifications can upset the best-laid plans.

LP models are price driven and cannot handle nonlinear blending. LP models sometimes give complicated solutions to simple problems, which often need to be compromised for practical reasons. Also, LP solutions may require large number of changes to the model to realize small real benefits and tend to overoptimize, unless they are very sophisticated. For these reasons, they are not considered a good tool for producing a practical plan for the refinery operations.

Long-term process planning studies may also be done by hand when no LP model of the refinery in question is available; and putting together an LP model and testing it takes more time than a simple hand balance. Hand balancing is done on a personal computer (PC) with a spreadsheet program. The spreadsheet simulates a typical refinery flow diagram. Each box on the spreadsheet corresponds to a refinery unit. Each unit is represented by a performance equation that relates the output of the unit to change in the input or its operating conditions. The equations need not be linear.

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