1 Korean J. Chem. Eng., 34(7), 1-14 (2017) DOI: 10.1007/s11814-017-0186-y INVITED REVIEW PAPER pISSN: 0256-1115 eISSN: 1975-7220 INVITED REVIEW PAPER † To whom correspondence should be addressed. E-mail: rosiah@ukm.edu.my Copyright by The Korean Institute of Chemical Engineers. Evaluation of the operating parameters for the separation of xylitol from a mixed sugar solution by using a polyethersulfone nanofiltration membrane Khalefa Atya Faneer * , ** , *** , Rosiah Rohani * , ** ,† , Abdul Wahab Mohammad * , ** , and Muneer Mohammed Ba-Abbad * , ** , **** *Department of Chemical and Process Engineering, Universiti Kebangsaan Malaysia, UKM Bangi, 43600 Selangor, Malaysia **Centre for Sustainable Process Technology (CESPRO), Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia ***Department of Environment Engineering, High Institute for Comprehensive Professions, Bent Baya, Wadi Al-Ajal, Libya ****Department of Chemical Engineering, Faculty of Engineering and Petroleum, Hadhramout University of Science & Technology, Mukalla, Hadhramout, Yemen (Received 2 April 2017 • accepted 2 July 2017) Abstract-Nanofiltration (NF) membranes may offer a good route for the recovery of xylitol due to the difference in the size of its particles compared to the other sugars. We evaluated the ability of an in-house polyethersulfone (PES) NF membrane to separate xylitol from a simulated broth solution containing xylose and arabinose. Initially, a Box-Behnken design was utilized to optimize the factors that were significantly involved in the recovery of xylitol, such as the con- centration of the components, the composition of the solution, and the pressure. The results obtained from the analy- sis of the experimental response revealed that the fabricated PES membrane was able to retain 92% of the xylitol and remove 50% of the arabinose, with the purity of the xylitol being enhanced accordingly. The results of fouling showed a good membrane performance for long-term filtration. The concentration polarization was dominated by the mem- brane pores and the charge. It could be concluded that nanofiltration has a high potential to recover xylitol from its corresponding sugars. Keywords: PES Membrane, Box-Behnken, Xylitol Solution, Fouling, Concentration Polarization INTRODUCTION Xylitol is widely used in the food, pharmaceutical and dental fields as it has multiple properties, such as sweetness with low calorific content, is anticarcinogenic, has tooth hardening and remineral- ization properties, and also prevents otitis media [1]. Xylitol comes mainly from the fermentation of xylose by catalytic hydrogenation or through bioprocesses using microorganisms such as bacteria, fungi or yeast [2]. Xylose and arabinose are also common mono- saccharides that are found in the fermentation process. The pres- ence of these sugars in fermentation broths is believed to inhibit the production of high purity xylitol. The main challenge in puri- fying xylitol from these sugars is the similarity in the physical and chemical properties of xylitol and these sugars, especially at high concentrations, which limit the membrane selectivity [3]. It is known that when low concentrations of sugars are present in a broth, the membrane selectivity between xylitol and the sugars is increased, while at high concentrations of sugars, the membrane may lose this selectivity. However, at high sugar concentrations the membrane is still able to separate xylitol and its sugars from other impurities present in the broth such as yeast extract and proteins. The utilization of other separation techniques, such as adsorption, could cause xylitol to leach out from the broth, while it would be difficult to perform the separation due to the presence of impuri- ties such as proteins, that would reduce the purity of the xylitol and push up the separation cost further [4]. Previous studies have shown that separation using nanofiltration (NF) membranes is more effective at purifying xylitol in terms of saving energy and low maintenance cost [5,6]. Several variables affect the recovery of xylitol from its media, including (i) the com- position of the solution, namely xylitol, xylose and arabinose; (ii) the concentration of xylitol and sugars in the solution, where the concentration varies based on the type of biomass used, the type of microorganism, the pH, and the temperature; (iii) the pressure, where a high pressure will force the xylitol to pass through the mem- brane, while a low pressure will push the sugars through the mem- brane, while enabling the xylitol to be retained; and (iv) the effect of temperature is always omitted since the process is usually con- ducted at room temperature [3]. On this basis, three variables were included in this study (composition, concentration and pressure), while the temperature effect was excluded since the experimental work was carried out at 25 o C. The screening process for the xyli- tol recovery through the use of single-factor experiments was con- ducted and reported in previous works [7,8] to confirm the range of compound concentrations. The concentration ranges were based on literature studies [9,10] to determine the composition and con- centration of the solution. The interactions between the factors or the process parameters are a major concern to many engineers and