On line product quality estimator based on True Dew Point curve for control of Crude Distillation Units

Operational excellence for Distillation Units and particularly for Crude Distillation Units is achieved by tight control of product quality which is usually estimated by inferential models. This article is focused on a novel approach leading to calculate an appropriate input to the inferential model strictly related to the characteristics of real feed to the column rectifying section where the separation of products occurs. The calculation simplicity based on usually available plant measurements is an additional feature of the proposed approach. This article introduces the definition of a novel curve characterizing hydrocarbon streams: the True Dew Point curve (TDP), whose name recalls the strong relationship with True Boiling Point curve (TBP). Inferential results obtained with the TDP curve have been performed adopting an approximation of the rectifying feed TDP curve with the assumption that any more accurate TDP curve description would give better results. Inferential validation has been carried out comparing the estimations with two different approaches: one based on model input derived from the crude feed TBP curve, assuming that is known and considering these results as benchmark, the other based on a pilot tray temperature (assuming available) as model input for each product. A general database has been built running rigorous simulation models of an atmospheric fractionator, with variations of most operating parameters affecting the distillation ASTM 95% quality chosen as product specification. This article suggests the way to solve problems deriving from high nonlinearity of feed TBP curves, showing the application of TDP methodology to a vacuum fractionator. An application to real laboratory and plant data of atmospheric fractionator is also shown.