Impacts of superheated steam on juice degradation and evaporator performance

Process steam used in juice evaporation is typically supplied from a turbine exhaust or from a pressure let-down station and so is usually superheated. Superheat raises the temperature difference between the heating surface and the temperature of the bulk juice in the first evaporator. There is anecdotal evidence that large amounts of superheat can reduce the heat- transfer coefficients of the first evaporator as the superheated vapour can blanket the heating surface, which impedes condensation, and this in turn raises the calandria pressures and increases the scaling rates of the evaporators. It is generally considered ideal that the amount of superheat is small (e.g. 5°C or lower) so that most of the heat transfer in the steam space occurs from saturated vapour. This study investigated the impact of high surface temperature on juice quality using a specially designed laboratory evaporator rig. Juice samples were boiled and analysed for sucrose loss, pH and colour formation under a range of operating conditions. Results were also compared against empirical correlations and kinetic models in order to allow predictions on the impact of surface temperature on sucrose loss. The results showed higher surface temperatures had limited impact on sucrose loss and colour formation reactions which was attributed to effective mixing within the vessel that allowed the bulk juice temperature to be adequately maintained. Juice pH was very influential on sucrose losses but had limited impact on colour formation. The heat transfer and operational impacts on evaporation performance due to superheated steam are also discussed and modelling showed minimal overall impact to the evaporator station occurs for a reduction of 10% in heat transfer performance (used to simulate impact of superheated steam) in the first effect. Key words Sucrose degradation, evaporators, superheated steam, temperature differential, juice quality, colour