Modeling adsorption of cane sugar colorant in packed-bed ion exchangers

Abstract

The removal of cane sugar solution colorant by packed-bed ion exchangers was modeled using a linear driving force (LDF) adsorption model. Adsorption of colorant is of interest to the developers of the White Sugar Mill (WSM) process as it is a complex subject. The problem is that color is an indiscrete mixture of many components making it difficult to measure and even more challenging to model. Colorant formation was investigated using gel permeation chromatography (GPC) with the objective of developing a method to define pseudo-components representative of cane sugar solution colorants. WSM is a process for producing white sugar directly from sugarcane in the raw sugar mill by using ultrafiltration and continuous ion exchange technology. The ion exchange resins employed were a strong acid cation (SAC) resin in the hydrogen form, a weak base anion (WBA) resin in the hydroxide form and a decolorizing resin in the chloride form. Decolorization using the three resins was then analyzed using the GPC pseudo-component technique. Batch testing of the resin allowed the development of equilibrium isotherms that could be substituted into a standard LDF model. Column testing was then performed to investigate the dynamics of adsorption of colorant in packed-beds. Linear isotherms were measured for each of the three resins, indicating that the colorant is dilute. Results indicated that a plug-flow model with a constant linear isotherm was sufficient in all cases except the SAC resin. The SAC adsorption parameter decreased sharply as the pH increased, causing colorant to be desorbed from the resin. This situation must be avoided if optimal decolorization is to be achieved. The adsorption models can be utilized in the design of a WSM process to optimize the decolorizing capacity of the resins.