DISSOLUTION KINETICS OF CA-MALEATE CRYSTALS - EVALUATION FOR BIOTRANSFORMATION REACTOR DESIGN

In order to develop a bioreactor for solid to solid conversions the bi ocatalytic conversion of solid Ca-maleate to solid Ca-D-malate is stud ied. The dissolution of Ca-maleate is the first step in this process a nd is described here. A kinetic model, based on the interfacial-barrie r theory and the diffusion-layer theory, was developed which describes the increase in Ca-maleate concentration due to dissolution with the help of the time-dependent parameters. According to the model two proc esses contribute to the dissolution of Ca-maleate . H2O crystals: (1) the dissolution land dissociation) reaction of Ca-maleate at the solid -liquid interface, characterized by a time-independent reaction rate c oefficient, and (2) the transport of Ca2+ and maleate(2-) across a bou ndary liquid film, characterized by a time-dependent mass-transfer rat e coefficient. In addition, the surface of a crystal and the driving f orce are time;dependent variables. Since Ca-maleate . H2O crystals are not uniform, a crystal-size distribution was also used in the model. The effects of stirring speed, temperature, pH, and initial Ca2+ conce ntration on the dissolution rate of Ca-maleate . H2O crystals were det ermined experimentally in order to evaluate the model. The model fitte d the data well (R-2 > 0.97). In order to determine whether the overal l dissolution process was reaction or transport controlled, a method b ased on overall reaction and transport rates (per unit of driving forc e) was developed. This showed that the dissolution of Ca-maleate was r eaction controlled. Temperature influenced the reaction rate coefficie nt the most; it ranged from 5.7 x 10(-6) m s(-1) at 10 degrees C to 67 x 10(-6) m s(-1) at 60 degrees C. The reaction rate coefficient was a lso influenced by the pH and the initial Ca2+ concentration, but, as e xpected, hardly by the stirring speed. Simplifying the model by omitti ng the time-dependent mass-transfer rate coefficient and by assuming u niform crystals, resulted in only slightly worse fits of the data with R-2 being at most 0.004 smaller. (C) 1988 Society of Chemical Industr y.