OP-II-3

OP-I-20CFD INVESTIGATION OF MIXING AND SHEAR IN LAB AND PILOTSCALE STIRRED TANK REACTORS DURING HOTWASHRobin G. JordiSasol Technology Process Development R & D, Sasolburg, South Africa,robin.jordi@sasol.comThis paper presents computational fluid dynamic models of the hotwash cycle ofgas-liquid continuously stirred tank reactors (CSTR) utilised in studies of ahomogeneously catalysed oligomerization reaction. This reaction produces depositsof an unwanted polymeric by-product causing fouling of the internal surfaces andrequiring the reactor to be operated in campaigns alternating with a hotwash cycle toremove such deposits.The purpose of the study was to determine and compare shear rates within theCSTR and on polymer surfaces, evaluating the impact of scale to assist in scale-upto commercial scale units, and to predict the distribution of solvent in the pilot scalevessel which was difficult to assess independently.Extensive piloting in an agitated bubble column was conducted in a pilot scaleunit consisted of a baffled 316 litre jacketed high pressure vessel equipped with portsfor base sparging of reactant gases, continuous liquid recirculation and gaseous andliquid product draws. During the hotwash cycle the reactor was ~60 % solvent filledand agitated with a five stage agitator assembly consisting of five downward pumpingaxial flow impellers.Initial assessments of hotwash performance and required duration were madeusing a 40 % solvent filled 1 litre unbaffled CSTR with a six-blade downwardpumping axial impeller in which a 37 mm x 17 mm cylindrical sample of the polymericdeposit had been affixed to the wall.An unstructured tetrahedral and hexahedral mesh of ~3.5 million elementssuitable for a steady state multiple reference frame (MRF) analysis [3][5] was createdfor the laboratory scale unit and a high quality unstructured hexahedral mesh of ~4.6million cells suitable for a transient sliding mesh (SM) analysis [2][3] was fashionedfrom a dimensionally accurate CAD geometry for the analysis of the pilot scale unit.In both cases boundary layer cells were included to capture near wall viscous effects.74