Onlinetuningofasteadystatecrudedistillationunitmodelforreal time applications DhavalJ.Dave,MurtuzaZ.Dabhiya,S.V.K.Satyadev,SaibalGanguly 1 , DeokiN.Saraf* Process Control Laboratory, Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208016, India Abstract The steady state simulators, used for on-line performance prediction and for on-line optimization in crude distillation units are often sensitive to small variations in the feed composition, which is specified in terms of a True Boiling Point (TBP) vs volume percentdistilledcurve.TheexactfeedTBPisoftennotavailableduringtheplantoperation.Alsostratificationofrawcrudeoilinto layers in the large tank farm sections cause severe operating problems in terms of the stability of the column. If feed TBP can be predicted online, necessary feedforward action can considerably reduce the operating problems. A model has been developed for backcalculationoffeedTBPusingmeasuredplantparameters.Aheatbalanceisperformedaroundanenvelopeencompassingthe rectifying section of the fractionator and is followed by the calculation of Equilibrium Flash Vaporization (EFV) temperatures at six different locations of the column which are correlated with corresponding feed TBP temperatures. The second part of model tuning consists of calculating model parameters in the form of point efficiencies so as to minimize the discrepancy between the simulator predicted and measured column parameters which arises out of modelling approximations such as assumption of phase equilibriaateachstageanduseofimperfectthermodynamicscorrelations.Thesimulatorresults,aftertuning,werefoundtomatch theplantmeasurementswithintwopercentinallthecasesinvestigated.Thesimulatoroutputwasusedtopredictvariousproduct properties using a Property Prediction package and these were also found to match well with those of laboratory measurements. Both the backcalculation of feed TBP and the efficiency tuning need to be implemented on-line for inferential control and super- visory optimization. # 2003ElsevierScienceLtd.Allrightsreserved. Keywords: Onlinetuning;Crudedistillation;CrudeTBPbackcalculation;Distillationmodeltuning 1. Introduction Crude distillation unit (CDU) is at the heart of any petroleum refinery and one of the most complicated operationsinthefieldofseparationprocesses.Thepro- ducts from CDU are basically mixtures of various hydrocarbon compounds and it is not possible to char- acterize them in terms of individual components. Moreover, it is often sufficient to characterize these products in terms of certain properties such as Reid vaporpressureforvolatileproducts,flashpointforlight distillates, pour point for heavier fractions, etc. These product properties are usually measured off-line at intervals of 8–24 h as these measurements are cumber- someandtimeconsuming.Thestringentqualitycontrol requirement in a highly competitive market, makes it essential that all the requisite product properties are monitored online. Since the product properties are gen- erally not available online the column operation is con- ventionally controlled using ‘‘cut points’’ [1]. These cut pointsdefinetherangeofboilingpointsinagivenpro- duct. However, the product properties can vary sig- nificantly within the same cut point interval if the feed compositionchanges.Cutpointcontrolcanneverresult inacceptableuniformityofproductqualityifthenature of crude (paraffinic, naphthenic, etc.) changes in a refinery. A tighter product quality control is possible onlyiftheproductpropertiescanbemadeavailableon- line.Alsoon-lineeconomicoptimizationrequiresthata profit function be maximized keeping product quality within acceptable limits. Here again one needs to know the product quality in terms of the properties before economic optimization can proceed [2]. 0959-1524/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII:S0959-1524(02)00026-4 Journal of Process Control 13 (2003) 267–282 www.elsevier.com/locate/jprocont * Corresponding author. Tel.: +91-512-597-827; fax: +91-512- 590-104. E-mail address: dnsaraf@iitk.ac.in (D.N. Saraf). 1 Presently Associate Professor at Indian Institute of Technology, Kharagpur, India.