(231ag) A Model for Vitamin E Recovery from By-Product of Edible Oil Refining By Adsorption/Desorption of Anion-Exchange Resin

Introduction

Vitamin E (V_EH) has high antioxidant and anti-aging activities and widely used in food and pharmaceutical fields¹⁾_. The deodorized distillate, a by-product of edible oil refining, is used as a raw material for V_EH recovery. Its typical composition is free fatty acid (FFA) 44wt%, triglyceride 14wt%, sterol 11wt%, and V_EH 1-5wt% and main component FFA has separation characteristics similar to V_EH. In the current industrial V_EH recovery, a multi-stage molecular distillation at high temperature, 100-250°C, is mainly utilized. Its recovery yield is low, about 50%, because of V_EH thermal degradation and many compounds having similar boiling point remained as impurities, 65%²⁾. Thus, further purification is necessary, this causes high production cost.

We have proposed a novel low-temperature process by adsorption/desorption using ion-exchanges resins³⁾. This process consists of esterification of FFA using cation-exchange resin catalyst to convert into fatty acid ester without adsorption activity and adsorption/desorption of V_EH using anion-exchange resin at 50°C. Under specific condition, V_EH was completely recovered without thermal degradation and only FFA remained as an impurity, 22%. However, the recovery amount of V_EH after desorption step greatly changed with feedstock supply volumes during adsorption step. This was because V_EH and unreacted FFA competitively adsorbed on the anion exchange resin, and FFA with higher acidity was re-exchanged with the adsorbed V_EH to elute it from the resin. Thus, it is important to determine the optimum feedstock supply volume during the adsorption step for giving maximum V_EH recovery amount.

In this study, a model for competitive adsorption of V_EH and FFA on the anion exchange resin was constructed by considering mass transfer into the resin and adsorption by the ion-exchange reactions. The competitive adsorption behavior of V_EH and FFA was simulated by the model in the column reactor. The V_EH recovery amount during desorption step was predicted by the model.

Experiments

Diaion PA306S (porous-type, average particle diameter: 0.32 mm, ion-exchange capacity: 0.94 mmol/g wet basis) was used as most active anion-exchange resin. The model esterified deodorizer distilllate was V_EH (δ-tocopherol) and FFA (oleic acid) dissolved in ethanol. A glass column (i.d. 1.1cm, length 10cm) was packed with the resin (6.7 g, wet basis) and its temperature was kept constant at 50°C. The model feedstock was supplied to the bottom of the column. After the adsorption, ethanol was fed to the column to wash out the feedstock and then desorption eluent, the acetic acid in ethanol, was also fed to the top of the column. Under all operations, the flow rate was 1.0 cm³/min. The feedstock supply volume in the adsorption step was changed in the range of 40-250 cm³. The concentrations of V_EH and FFA in the effluent from the column were determined using an HPLC system.

Construction of a model

The following assumptions are set for construction of competitive adsorption model. A linear partition equilibrium for concentrations between the bulk liquid phase and the resin liquid phase was established. Three ion-exchange reactions, i)V_EH adsorption onto the resin by ion-exchange with hydroxyl group, ii)FFA adsorption onto the resin with hydroxyl group, iii)V_EH escape from the resin by re-exchange with FFA, occurred on the resin solid surface. The model equations in the column reactor were derived from mass balance of V_EH, FFA and hydroxyl group considering mass transfer by the convection and dispersion in the bulk liquid phase, partition between the bulk liquid phase and the resin liquid phase and reaction on the resin solid surface. There are unknown constants such as the partition coefficients of each component, kinetic constants and equilibrium adsorption constants of each ion-exchange reaction, and resin porosity. The model constants were estimated by fitting with the separate experimental results of competitive adsorption in batchwise system. The dispersion coefficient is calculated by the empirical equation⁴⁾for the fixed-bed reactor.

Simulation by the kinetic model

The model well described the concentration profiles of V_EH and FFA in the effluent experimentally obtained during the adsorption step with the feedstock supply volume of 250 cm³. The overshooting of V_EH concentration in the effluent from the column was observed since the adsorbed V_EH was flowed out by the re-exchange with FFA. On the other hand, the breakthrough of FFA was delayed compared with that of V_EH since the re-exchange as well as the adsorption of FFA occurred.

The model can also simulate the concentration profiles of each component along the axis of the column reactor at each feedstock supply volume condition. By integrating the concentration profiles from inlet to outlet of the column reactor, the adsorbed amount of each component on the resin at each condition was calculated under various feedstock supply volumes. The adsorbed amount calculated by the model simulation were compared with the recovery amounts experimentally obtained during the desorption step. Under any condition, the calculated adsorbed amounts were in good agreement with the experimental recovery amount. This meant that the adsorbed V_EH on the resin was perfectly recovered in the desorption step. The V_EH recovery amount increased with feedstock supply volumes in the adsorption step and it became a maximum at 71 cm³ of feedstock supply volume. Then, V_EH adsorbed amount became the almost zero at the supply volume of more than 150 cm³. The model well predicts the optimum feedstock supply volumes for giving a maximum V_EH recovery amount.

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