INFLUENCE OF VISCOSITY ON PRODUCT DISTRIBUTION OF FAST COMPETITIVE CHEMICAL-REACTIONS

The objective of this study was to determine the influence of viscosit y on micromixing in turbulent flow. It was first necessary to find a s uitable viscosity-raising additive. HEC (hydroxyethyl cellulose) prove d to be better than previously studied additives [sorbitol and carboxy methylcellulose (CMC)]. In concentrations up to 1 wt-%, HEC solutions are almost Newtonian with viscosities independent of pH over the range 2 to 10. HEC had no effect on the reaction rate constants and the spe ctrophotometric analysis of the fast, competing reactions used - the d iazo coupling between 1-naphthol and diazotized sulphanilic acid. The viscosity can then be raised by around an order of magnitude by adding less than 1 wt-% HEC to this reaction system. Diazo couplings were co nducted in a 20 l semi-batch tank reactor stirred by a Rushton turbine at two viscosity levels (0.9 and 6.2 mPa s). Long feed times ensured that micromixing was controlling. More bisazo dye was formed in the mo re viscous solution, all other conditions being unchanged, indicating more intense segregation and slower micromixing. This was also shown b y visualising the extent of neutralisation zones, with more spreading and slower micromixing being observed in viscous solution. Higher turb ine speeds reduced this spreading. One feed point near and one far fro m the turbine were employed: the strong inhomogeneity of the turbulenc e led to smaller amounts of bisazo dye when the feed was added to the turbine suction, irrespective of the viscosity. All results agreed wit h the trends predicted by the engulfment model of micromixing. Its sim plest form assigns an average energy dissipation rate to the reaction zone: the values obtained are of similar magnitude to those measured b y physical techniques and were related to the spreading of the reactio n zone.