HOW TIME BETWEEN CLEANINGS AFFECTS PERFORMANCE AND SUCROSE LOSSES IN ROBERT'S EVAPORATORS

摘要

In U.S. sugarcane factories, multiple-body evaporator stations mostly consist of Robert's-type calandria (rising film) evaporators that are simple, robust and easy to operate, but are susceptible to scaling and require periodic cleanings. Scaling reduces heat transfer, but its effect on sucrose losses is unknown. Therefore, a comprehensive study was conducted at a U.S. factory to determine the effects of time between evaporator cleanings on overall sucrose losses and evaporator performance. The factory operated two Robert's pre-evaporators in parallel, and three sets of triple-body Robert's evaporators in parallel. Each body was usually subjected to a 9-day cleaning cycle, and two cleaning cycles were studied in early and midseason, respectively. Gas chro-matography was used to determine sucrose losses as Δ% glucose/% sucrose ratios. A wide range of 0.21-1.42% total sucrose losses to acid hydrolysis were measured across the station, which were affected by seasonal changes in clarified juice quality. Losses in the pre-evaporators were up to 98% of the total station losses in early season but decreased in midseason, and were more a function of temperature, heating surface, °Brix (% dissolved solids), scaling and pH than retention time (R_t). Scaling had a dramatic effect on increasing sucrose losses, because it causes increases in R,s as a result of decreased heat transfer coefficients and flow rates, and rises in the heating juice temperature to partially compensate for the reduced heat transfer in scaled tubes. For both pre-evaporators, as time between cleanings increased from 1 to 8 days, the amount of sucrose hydrolysis increased. In general, hydrolysis occurred in the first and second evaporator bodies only when scale had built up, i.e., > 6 days after the last cleaning, and became worse until the next cleaning. The contribution of sucrose losses to economic losses is described.