An investigation into the viscosity of C-massecuite using a pipeline viscometer.

Abstract

Sugar is recovered by three stages of evaporative crystallisation, with each stage producing a two-phase mixture of sugar crystal and mother liquor commonly referred to as massecuite. The composition of each massecuite changes as increased amounts of sucrose crystallises out of solution, increasing the concentration of residual non-sugars and organic salts with each evaporative stage and resulting in C-massecuite possessing the lowest purity and highest viscosity. Tongaat Hulett maintains an interest in the viscosity of C-massecuite from a process and equipment design perspective as viscosity is a critical physical property in the selection of pumps and design of piping networks, evaporative and cooling crystallisers, crystalliser drives and reheaters in the C-station of a sugar factory. In the absence of a well-established correlation, viscosity data published by Andre Rouillard in 1984 is widely-used at Tongaat Hulett, however, this chart and other widely published data resulted from experimentation with a rotating viscometer that is believed to be unsuitable for this application. The rotating viscometer results in displacement of sugar crystals, interfering with the accuracy of the measurements. Whilst the rotational viscometer was accepted by the International Commission for Uniform Methods of Sugar Analysis as the standard technique for measurement of molasses viscosity, no standard technique is available for massecuite viscosity measurement. An investigation into alternative methods of viscosity measurement rendered the pipeline viscometer as best suited to this product as the method of measurement is not affected by the heterogeneous nature of the massecuite. The aim of this study is thus to design, construct and validate a pipeline viscometer which is to be used, together with non-Newtonian theory, to investigate the viscosity of C-massecuite. The pipeline viscometer was successfully constructed, validated and used, together with the power law model, to describe the viscosity of C-massecuite in terms of two rheological parameters; the flow behaviour index and consistency. The results of this study indicate that the average flow behaviour index of C-massecuite is 0.85. An empirical correlation for C-massecuite consistency as a function of temperature, dissolved solids concentration and crystal content was proposed with a regression coefficient of 0.7672 as well as additional equations to guide the estimation of C-massecuite viscosity. The massecuite consistency, assumed to be equivalent to apparent viscosity at a shear rate of 1s-1, was compared with the C-massecuite viscosity data currently used. A more rapid increase in massecuite viscosity with a reduction in temperature was found, however, the experimental data was found to fall within the recommended range for C-massecuite viscosity currently used. It is with confidence that the power law model can thus be used with a flow behaviour index of 0.85 and a consistency as predicted by the empirical correlation and guiding equations to yield an apparent viscosity for C-massecuite.