Characterisation of steady state of crystallisation from melt in a continuous oscillatory baffled crystalliser

Abstract

This is the first time that temporal and spatial data of crystal sizes and solute concentration are obtained and analysed for different operating conditions, such that useful parameters (such as agglomeration/attrition rates) could be predicted. Carrying out steady state analyses is an unchartered territory with respect to palm oil (a typical fat material) crystallisation and some interesting but useful correlations and discoveries were made. Studies were undertaken in a batch OBC, investigating the effect of cooling profile and mixing on the kinetics parameter, e.g. MSZW and it was established that while cooling rate showed similar effects as it would be for a typical solution crystallisation in that higher cooling rate increased the crystallisation rate, mixing had a negligible effect; this is part of what distinguishes this process from a generic solution crystallisation. Generally, the MSZW for this palm oil system was between 17 – 22 °C. The effects of these parameters on filtration properties, such as filtration rate, yield, were also examined. Continuous crystallisation of palm oil was performed in a continuous oscillatory baffled crystalliser where steady states of solute concentration and crystal size along the column length were examined with respect to process conditions. Temporal steady state of size was achieved for all the conditions considered with a ≤ 10% change over a 30-min period while solute was continuously being consumed with time (based on the % transmittance data). However, the consumption reduced considerably when the flowrate was increased (effect short residence time), and when oscillation frequency was reduced (effect of high shear rates). Spatial size increase was observed between the measurements windows (Jn7 → Jn10) and this was attributed to agglomeration (as opposed to crystal growth) due to the minimal spatial change (1 – 4%) in the % transmittance (an indication of solute consumption) along this tube length. The rate of this agglomeration was averaged at 0.025 μm/s. Only the fast flowrate condition showed an opposite trend in that size decrease was observed from Jn7 to Jn10; another effect of short residence time. High mixing conditions had relatively more influence on the nucleation rate than the crystal size.