AbstractsEngineering

Deposition of amorphous silica (SiO2) and calcium oxalate (CaOx) on the calandria tubes of juice evaporators cause serious processing problems in Australian cane sugar mills. The removal of these deposits by mechanical and chemical means is a timeconsuming and costly experience. The cost of downtime and chemical cleaning can be several million dollars per year for the Australian sugar industry. The interactions between CaOx and SiO2 have not been investigated previously because conventional studies only address fouling by individual components. The present work evaluates their interactions using two experimental approaches: batch tests for assessing kinetic and thermodynamic behaviour, and fouling-loop experiments for examining composite fouling behaviour under different operating conditions. The above two approaches were employed both in the absence and in the presence of sugar to elucidate the effect of sugar on composite fouling mechanisms and to determine the controlling species responsible for composite fouling. The composite fouling experiments were performed in a novel closed-loop circulation system simulating the effect of feed composition of successive stages of evaporation cycle in a single run. In addition, the fouling-loop system was operated in a constant composition mode to study the effects of thermal hydraulic conditions on composite fouling. The combined information obtained from both the batch and fouling-loop tests in this study offer a unique insight into the mechanisms of composite fouling of CaOx and SiO2. Some of the highlights of the obtained results are as follows: �� Identification of a complex interactive process in calcium oxalate monohydrate �� silica (COM-SiO2) systems by investigation of the kinetics and thermodynamics of COM-SiO2 coprecipitation in water and sugar solutions, and an understanding of the mechanisms of these interactions; �� Development of a novel fouling-loop system, which is simple, efficient and cost effective for the study of the effect of juice composition on scale formation in various stages of juice evaporation; �� Elucidation of composite fouling mechanisms, e.g., a feed composition dependent fouling mechanism is proposed; �� Isolation and verification of the existence of certain species in composite deposits, which is known to be thermodynamically unstable. In other words, it is established that calcium oxalate trihydrate is stable under certain conditions; �� Evaluation of the role of thermal hydraulic operating parameters in determining the characteristics of subcooled flow boiling heat transfer and in determining the strength of the composite deposit; �� Development and validation of an empirical model to predict the subcooled flow boiling heat transfer coefficients in water and sugar solutions; ���� Development of an analytical model incorporating the effects of operating parameters for COM and SiO2 composite fouling in sugar solutions. This model predicted the experimental data better than available models. Results of this work…