Submitted to: Journal of Food Preservation
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/12/2006
Publication Date: 2/1/2007
Citation: Eggleston, G., Monge, A. 2007. How time between cleanings affects performance and sucrose losses in Robert's evaporators. Journal of Food Preservation. 31:52-72. Interpretive Summary: Currently, sugarcane factory staff do not take into account losses of sucrose across evaporators for the improvement of evaporator performance. A large factory study was undertaken to measure the effect of evaporator scale build-up or fouling on sucrose losses. A wide range of 0.21-1.42% total sucrose losses were measured across the evaporator station. For the first time, it was shown that scaling had a dramatic effect on increasing sucrose losses, because it reduced flow rates through the station. This, in turn, increased retention times and temperatures in evaporators which increased the breakdown of sucrose. The quality of the cane juice also had a major impact on sucrose losses. The economic cost of sucrose losses is also described. Factory staff will ultimately benefit from better equations to predict sucrose losses, and will be able to improve the economical optimization of when to clean evaporators to remove scale
Technical Abstract: Tubular heat exchangers in Robert's-type evaporators in the sugarcane industry cannot last the grinding season without periodic cleanings because of scale build-up. Scaling reduces heat transfer but its effect on sucrose losses was unknown. Therefore, a comprehensive study was conducted at a U.S. factory, to determine the effects of time between evaporator cleanings on overall evaporator performance and sucrose losses. The factory had two pre-evaporators in parallel, and three sets of triple-effect evaporators in parallel. Each body was usually subjected to a nine day cleaning cycle, and two cleaning cycles were studied: one in early and one in midseason. Retention times (Rts) were 11.4 and 9.5 min in the two pre-evaporators, respectively, and increased from 10.0 to 21.8 min across the triple-effect evaporators. Gas chromatography was used to determine sucrose losses as delta %glucose/%sucrose ratios. A wide range of 0.21-1.42% total sucrose losses to acid inversion were measured across the evaporator 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, but these decreased in midseason, and were more a function of temperature, heating surface, Brix, scaling, and pH than Rt. Scaling had a dramatic effect on increasing sucrose losses, because scale causes increases in Rts due to decreased heat transfer coefficients and flow rates, and rises in the temperature of the heating juice to partially compensate for the decreased rate of heat transfer in scaled tubes. For both pre-evaporators, as the time between clean-outs increased from 1 to 8 days, the amount of sucrose inversion increased. In general, sucrose inversion occurred in the first and second evaporator bodies only when scale had built up, i.e., >6 days after the last clean-out, and became worse until the next clean. In early season, average lbs raw sugar lost/ton cane across the evaporation station was 3.45 lbs, and up to 44,160 lbs raw sugar was lost/day which is equivalent to a loss of US$ 9,274/day. In comparison, in midseason when juice processing quality is the lowest, 17,792 lbs raw sugar were lost/day, equivalent to a loss of US$ 3,736/day.