The Chemzcal Engmeerzng Journal, 44 (1990) B41 - B46 B41 Experimental Determination of the Mass Transfer Boundary Layer around a Spherical Biocatalyst Particle C M HOOIJMANS, S G M GERAATS, J J M POTTERS and K Ch A M LUYBEN Department of Bzochemzcal Engmeerrng, Delft Unzverszty of Technology, Delft (The Netherlands) (Recewed 27 June, 1989, m fmal form 5 January, 1990) ABSTRACT An oxygen mzcrosensor was used to deter- mzne the thzckness of the boundary layer outszde a spherzcal bzocatalyst partzcle zn lzquzd flow Dzfferent partzcle dzumeters and fluzd velocztzes were used The results were compared wzth correlatzons avazlable zn the lzterature uszng the superfzczal lzquzd veloczty The measured thzckness of the boundary layer appeared to be smaller than the thzck- ness calculated wzth the lzterature correla- tzons The reason for thzs dzsagreement could be that the local lzquzd veloczty exzstzng zn the measurzng equzpment was hzgher than the superfzczal veloczty used zn the calculatzons No experzmental dzstznctzon could be made between the results obtazned wzth dzfferent partzcle d zame ters The d zame ter dependence zs small at hzgh Re numbers, whzle at low Re numbers the measurzng error was too large due to vortzces However, zt zs clear that care must be taken when uszng correlatzons from the lzterature to predict external mass transfer reszs tance An lmmoblhzed biocatalyst 1s constructed by the lmmoblllzatlon of enzymes or cells m a gel or other carrier material. Due to this unmoblllzatlon a concentration profile of the substrate consumed inside the particle will result. Apart from the effective dlffuslon coefficient and the mtrmslc kinetics, the external mass transfer resistance will influence the exact profile m the particle. This reas- tance 1s the result of a boundary layer around the particle with a thickness dependent on the velocity or corresponding Re number of the liquid flowing around the particle. If there 1s no flow, transport 1s only by dlffuslon 2 LITERATURE RELATIONS Various relations can be found m the literature to calculate the thickness of the boundary layer for different condltlons They all assume the mass transfer resistance to be located m a stagnant boundary layer with a certain thickness through which mass transfer takes place (film theory). The mass transfer coefficient 1s then defined by the followmg relation 1 INTRODUCTION This paper describes boundary layer mea- surements performed with an oxygen mlcro- sensor around a spherical biocatalyst particle made of gel contammg an oxygen consummg enzyme. The sun was to see d the approprl- ate literature relations descrlbmg mass trans- fer for flow around a spherical particle could be used for the sltuatlon of a fixed sphere m a flow chamber. This flow chamber 1s part of a piece of equipment that 1s being used for measuring internal oxygen profiles m blo- catalysts [l] and will be presented later m the paper. ShD k,=; = _ 4 (1) If mass transfer 1s only by dlffuslon the thickness of the assumed boundary layer 1s equal to the radius of the particle (the Sh number 1s 2 for this situation). When there 1s liquid flow, the thickness of the boundary layer will decrease. The relations descrlbmg the mass transfer for flow around a sphere can have an emplrlcal or a theoretical back- ground. A theoretical analysis, checked by experl- ment, 1s that of Brian and Hales [2]. The equation resulting from their analysis 1s well 0300-9467/90/$3 50 @ Elsevler Sequola/Prmted m The Netherlands