Cane and beet sugar milling and refining

Massecuite solids content and mother liquor concentration are commonly monitored parameters in the vacuum pan as they are used to control the crystallization process and therefore have an effect on the end product quality, the sugar crystals. Massecuite solids content is the mixture of crystals and mother liquor resulting from the crystallization process and mother liquor concentration is the liquid phase in the massecuite during crystallization. Massecuite solids content, or total sugar content, is typically determined using a microwave meter or nuclear density probe whereas mother liquor concentration (dissolved sugar) is measured using a refractometer. The measurement scale is usually Brix.

Online monitoring of batch cooling crystallization

Batch cooling crystallization is a widely practiced production process in several industries. In the sugar industry, it is mainly used as the first step to produce massecuite (footing magma) to seed batch or continuous evaporating crystallizers and to increase the final exhaustion of sugar from the basic raw material (beet or cane). In the pharmaceutical industry, it is used to produce the
final product directly. Cooling crystallization is carried out in different types of equipment, the typical ones having cooling coils or jacketed vessels with built-in stirrers.

Crystal growth is a favorite subject of crystallization research. Due to the complexity of the process, there are several more or less different approaches to its study. Most of the theoretical studies rely on mass, energy, and population balance equations, kinetic correlations, and model formulations sometimes based on questionable assumptions and simplifications. Combined methods (theoretical plus small scale laboratory experiments) can provide useful information on the effect of one or the other parameter on crystal growth, but they are usually restricted to the study of a single (or only a few) crystals in well-controlled environments.
The problems of crystal growth in industrial-scale crystallization, when there are billions of crystals in the massecuite are rarely studied, and if so, the validation of the findings is rare or is completely missing. Production of good quality sugar however should be based on an in-depth knowledge of the processes which characterize mass production. The major difficulty of the study of crystal growth in large populations was the long-standing lack of appropriate instruments needed to monitor on-line the governing parameters of crystal growth.
With the introduction of the SeedMaster Instruments on-line data acquisition of the most important parameters of crystallization, including supersaturation, the crystal content, and mother liquor purity became reality in quite a few mills and refineries of the world.

It is generally acknowledged that supersaturation is the most important parameter in sugar crystallization. It has a key role in determining product quality and yield, cost of production, profitability, and survival of the manufacturer. Besides supersaturation, the other important parameters are crystal content, mother liquor purity (or purity drop), and massecuite solids content (“Brix”). 

Sugar crystallization is a process of key importance. Currently, a large percentage of crystallized sugar is produced under manual control using instruments that arguably do not provide actionable information, and apparently resemble methods used several decades ago. This kind of approach is completely unfit to mass-produce quality sugar at a competitive cost. 

In Part 1 of this paper some of the most important details of sugar crystallization, like product quality, cost of production, crystal growth, pan circulation, and seeding were discussed. The role and importance of supersaturation in all of these details were duly emphasized. See ISJ (2008), 110 (1315): 403-413. In Part 2 the emphasis is placed on real-life case studies and on some new tools which can be used to shed light on the inner workings of a crystallizer in real-time and use the information for advanced control of crystallization. 

There is a growing realization of the importance of supersaturation in sugar crystallization. It has a profound effect on product quality and on the cost of production as well, parameters that determine the chances of survival of plants all over the world.