Development of optimal simulated moving bed processes for high-purity production of highly valuable rare sugars

Abstract

In this study, continuous chromatographic separation processes for the economical production of two highly valuable rare sugars, xylobiose (X2) and fucose, were developed. First, we developed highly-economical simulated moving bed (SMB) processes for the production of high-purity X2, which has recently drawn a lot of attention because of its outstanding prebiotic function. In such processes, xylooligosaccharides (XOS) or xylose (X1) was adopted as the starting material. In the first place, the task of developing an optimal SMB process for continuous-mode recovery of X2 from XOS with high purity and high yield was carried out. This work began by obtaining the intrinsic parameters of XOS components through a single column experiment. The determined intrinsic parameters were used in the optimal design of the SMB process. In the optimization stage, a relevant SMB optimization tool based on NSGA-II-JG and the detailed column model was utilized. The results from the optimization process were validated from both computer simulations and a lab-scale SMB experiment, which proved that the developed SMB process in this study could successfully recover X2 from XOS with high purity and high yield.

Next, the task of developing a highly efficient X2-production process from X1 using β-xylosidase reaction and SMB purification was carried out. This task began by preparing the Bacillus pumilus IPO β-xylosidase, which was utilized as the enzyme for the X1→X2 reaction in this study. The use of such enzyme in the X1→X2 reaction was found to give much higher X2 reaction yield and reaction efficiency, compared to those reported in the literature. As a next step, we developed an efficient SMB process that could recover X2 from the β-xylosidase reaction output with nearly 100% (w/w) purity and 92% (w/w) yield. The developed SMB process in this study could also recover the unreacted X1 almost completely. If the recovered X1 from the SMB process is reused as the reactant of the upstream process, the overall reaction yield of X2 can increase further, and reach almost 100% ultimately. Thus, the results of this study will enable a highly economical and environmentally-friendly production of high-purity X2.

Additionally, a comprehensive optimization of the SMB process for separation between X2 and X1 resulting from the aforementioned b-xylosidase reaction was accomplished by using an SMB optimization tool based on standing-wave-design method. Through such optimization task, the productivity of the X2-separation SMB could be maximized while meeting the constraints on X2 purity, X2 product concentration, and pressure drop. The results from such optimization work revealed that the maximum productivity of the X2-separation SMB could be attained by choosing the particle size such that the effect of the X2-concentration limiting factor could be balanced with that of pressure-drop limiting factor.

Following the above work, we developed efficient SMB processes for economical production of fucose from marine biomasses, which included macroalgae (or seaweed) and microalgae. The first fucose-production method was to use the seaweed of Fucus vesiculosus. This work began by the acid hydrolysis of F.vesiculosus. After the hydrolysis, a series of pretreatment operations consisting of an activated-carbon treatment, an electrodialysis, and an ion-exchange processing were carried out for decolorization and deionization of the hydrolysate. The resultant output from the pretreatment step was then loaded into the final-stage SMB purification process, which was optimized to recover fucose from the pretreated hydrolysate. Such SMB optimization work was facilitated by using the prepared optimization tool based on a genetic-algorithm and a column-model. The SMB experiments were then carried out on the basis of the optimization results and the self-assembled SMB unit. The SMB experimental results revealed that the developed SMB process in this study could recover fucose from the pretreated F.vesiculosus hydrolysate with a very high purity and high recovery in a continuous mode.

Finally, a highly efficient SMB process for continuous-mode recovery of fucose from the residue remaining after the extraction of lipid from microalgae for biodiesel production was developed. As a first step of this work, the screening of an effective adsorbent for the fucose-separation process was accomplished through a series of preliminary pulse tests. For the selected adsorbent, multiple frontal experiments were carried out for the purpose of estimating the intrinsic parameters of each component in the defatted-microalgae hydrolysate. Using the resultant parameters, we performed the optimal design of the fucose-separation SMB of interest. The designed process was then validated with detailed model simulation first, and then with a series of the relevant SMB experiments. The experimental results verified that the developed SMB process in this study was highly effective in continuous recovery of fucose from the defatted-microalgae hydrolysate with high purity and high yield.