Scale Formation in Sugar Juice Heat Exchangers

Abstract

To understand fouling in sugar juice heat exchangers, it is first necessary to look at the sugar manufacturing process as a whole (Figure 8.1), where raw sugar is produced from sugarcane. The first step in the process is the harvesting and transport of sugarcane to the mills, where the cane is shredded and then crushed on the milling train to squeeze out the juice. The leftover plant material is called bagasse, and it is used to fire the boilers to create process steam and electricity. After the cane is crushed, the juice is heated to ~76°C, incubated to remove starch and then heated to 100°C, and clarified to remove soluble and insoluble impurities, such as plant material, dirt, and scale-forming ions. During clarification, calcium hydroxide (as milk of lime or lime saccharate) is added to raise the pH (from ~5.4 to ~7.6-7.8) and to form calcium phosphate with the phosphate already present in the juice. If the phosphate levels in the juice are low and the juice is poorly clarifying, phosphoric acid can be added. Calcium phosphate is used to remove both soluble and insoluble impurities, and it forms flocs in the process. A sodium polyacralyate copolymer is added to aid the flocculation of the calcium phosphate flocs. Once the juice is clarified, it is drawn from the top of the clarifier and sent to the evaporators, where the clarified juice is concentrated from 12 to 65 wt% sucrose to form syrup. Within each evaporator unit, the juice is boiled in stainless steel tubes heated by circulating steam in a multieffect arrangement, and approximately 40% of the tube height is submerged in sugar juice during processing. The juice typically passes through five such evaporator units before being transferred to crystallization pans. The evaporators are where the majority of the fouling occurs.