Refinery planning models employ swing-cut representation (simplified model) of crude distillation units (CDUs), since rigorous CDU models are too large and too nonlinear for use in multiperiod planning models. Underlying assumption (“equidistance assumption”) in swing cut or fractionation index models is that the crude true boiling point (TBP) curve lies in the middle between the end point of the lighter cut and the initial boiling point of the heavier cut.
This work presents a single model of a crude distillation unit (preflash, atmospheric, and vacuum towers) suitable for planning, scheduling, and RTO. The model (“hybrid model”) combines first principles mass and energy balances with via partial least squares model of product TBP curves base on the feed TBP curve and operating conditions (flows, pumparound heat duties, furnace coil outlet temperatures). This enables prediction of crude distillation on par with a rigorous distillation model, with 0.5% RMSE over a wide range of conditions. Associated properties (e.g. gravity, sulfur) are computed for each product based on its distillation curve and corresponding property distribution in the feed.
We compare production planning results based on the swing-cut models and on the hybrid CDU model. For a given crude mix, refinery plan is optimized by selecting the best operating modes for the downstream process units and the best blend recipes. CDU is modelled either via swing-cut multi-mode unit or via hybrid model which can compute the best operating conditions for a given crude mix. We present the case studies for the swing-cut model with the bias terms added to the product yields and compare it to the plans based on the hybrid model of CDU.
Our case studies show that inaccurate CDU models can lead to huge reductions in refinery profits.
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