- Open Access
Investigating the mechanism of nanofiltration separation of glucosamine hydrochloride and N-acetyl glucosamine
© The Author(s) 2016
- Received: 3 March 2016
- Accepted: 20 June 2016
- Published: 27 June 2016
Glucosamine hydrochloride (GAH) and N-acetyl glucosamine (NAG) are chitin derivatives. Owing to their excellent biological activity, they have long been used as pharmaceuticals and nutraceuticals. However, both of them exist simultaneously in chitin hydrolyzate or fermentation production. The aim of this study is to identify the feasibility of separating GAH and NAG by nanofiltration on the basis of appropriate adjustments of physical conditions.
One commercial spiral nanofiltration membrane (QY-5-NF-1812) was used. Experiments were carried out in full recycle mode and the membrane separation performance was investigated at various mass ratios (mass ratios of GAH to NAG were from 1:14 to 1:2), pressures (4–22 bar), temperatures (15–35 °C), and electrolytes (NaCl, MgSO4, and MgCl2). The influence of temperature on molecular characteristics that play an important role in the separation process was also studied.
Owing to the steric-hindrance effect, electrostatic effect, and different solute permeability, the GAH separation factor increased with increasing GAH concentration. Furthermore, upon temperature increasing, the permeability difference between GAH and NAG decreased, thus decreasing the GAH separation factor. Simultaneously, with increasing temperature, the polarities and calculated molecular diameters for both GAH and NAG increased evidently. The calculated reflection coefficients for both GAH and NAG can be well fitted by the steric-hindrance pore (SHP) model, suggesting that steric-hindrance effect played an important role on the separation process. Furthermore, owing to Donnan repulsion and solute diffusion effects, three electrolytes had noticeable effects on nanofiltration separation efficiency.
The nanofiltration separation efficiency of GAH and NAG was significantly affected by their physical properties in this system, and the mechanisms for GAH and NAG separation were elucidated. The current study could provide a certain basis for the nanofiltration separation of GAH and NAG on an industrial scale.
- Concentration Polarization
- Separation Performance
- Transmembrane Pressure
- Similar Molecular Weight
- Molecular Diameter
As chitin derivatives, glucosamine hydrochloride (GAH) and N-acetyl glucosamine (NAG) are widespread (Chen et al. 2012). Given that GAH and NAG have significant biological activity and can be used as ligands in coordination chemistry (Tao et al. 2014), both of them have long been used as pharmaceuticals and nutraceuticals to treat osteoarthritis and maintain cartilage and joint health (Zhu et al. 2015). GAH is usually produced by the HCl hydrolysis of chitin or fermentation (Zhu et al. 2005a; Chen et al. 2012) and is acidic in aqueous solutions. NAG can be prepared by GAH acetylization or glucose fermentation (Chen et al. 2012; Zhu et al. 2015) and is neutral in aqueous solutions. Both of them exist simultaneously in chitin hydrolyzate or in fermentation production (Deng et al. 2005; Chen et al. 2012). However, since both of them have similar molecular weights (GAH with molecular weight of 215.5 g/mol and NAG with molecular weight of 221.0 g/mol) and physical properties, it is difficult to separate them from their mixture solutions. Although some monosaccharides can be separated by chromatography (Brereton and Green 2012), the cost of this technique is relatively higher with poor selectivity of an appropriate stationary. Most columns face a number of problems such as column stability, lifetime, and separation reproducibility (Ghfar et al. 2015). Nanofiltration (NF) technology is a good approach for separation due to their advantages, including lower energy consumption, sustainable processing and relatively easy scale-up over other filtration procedures (Kolfschoten et al. 2011; Aroon et al. 2010).
Currently, NF membranes have been applied to many aspects, including the separation of multi-component solution in soybean molasses (Zhao et al. 2013), recycling of phosphoric acid from sewage sludge (Niewersch et al. 2010), recycling wastewater in the dairy industry (Chen et al. 2016), dye removal from aqueous and organic solutions (Kebria et al. 2015), and removal of fermentation inhibitors from wood extracts (Xie and Liu 2015). NF membranes have received increasing attention because of saccharides separation (Dong et al. 2014; Moreno-Vilet et al. 2014).
On the basis of previous research, NF performance can be affected by many factors during separation. Qin et al. (2014) reported that the increase of salt concentrations resulted in rejections for both salt and trisulfonic acid decrease. Sharma et al. (2003) indicated that with increasing temperature, pure water permeability increased because of the increase in polymeric membrane pore size and cutoff size (Desal-5 DL). Wang et al. (2002) demonstrated that the negatively charged membrane showed different rejections for different types of electrolytes with the order of R(MgSO4) > R(K2SO4) > R(MgCl2) > R(KCl) > R(NaCl) as a result of the Donnan and steric-hindrance effects. Sjoman et al. (2007) indicated that different mass ratios significantly influenced the rejections for both xylose and glucose through the steric-hindrance effect. Furthermore, saccharides, such as pectate oligosaccharides (which are acidic in aqueous solutions), carry an electric charge that affect their separation by NF membranes (Iwasaki and Matsubara 2000).
However, the possibilities of NF separations for monosaccharides with similar molecular weights have seldom been studied, and the transport mechanisms for monosaccharides are not fully understood. From the traditional point of view, membrane filtration would require a tenfold difference in molar mass or threefold difference in hydrodynamic radius for separation of components from each other (Sjoman et al. 2007). Simultaneously, many mathematical models have been proposed to describe and predict the process of NF. Generally, the filtration mechanisms involve mainly steric hindrance (Bowen et al. 1997), Donnan exclusion (Schaep et al. 2001), and dielectric exclusion effects (Yaroshchuk 2001). Nevertheless, the NF separation mechanisms for mixed monosaccharides with similar molecular weights were seldom studied and many of them focused on the study of neutral monosaccharides, such as xylose and glucose (Sjoman et al. 2007). Different from neutral monosaccharides, GAH is one kind of the cationic monosaccharides and the molecular weights of GAH and NAG are similar.
Thus, the aim of this study is to identify the feasibility of separating GAH and NAG using NF by regulating and controlling physical conditions, including different mass ratios, pressures, temperatures, and three types of electrolytes. Thereafter, the separation performance is evaluated by a series of models to lay the foundation for large-scale industrial utilization of monosaccharide purification with the same or similar molecular weights. Furthermore, varying molecular characteristics such as the changing molecular diameters and polarities caused by temperature are also investigated to obtain an insight into the processing of glucosamine fractions separation.
A series of models used for describing the membrane separation performance and molecular characteristics are explored as follows:
Concentration polarization model
Irreversible thermodynamic model
Calculation of the molecular diameter
The research in this study found that the pressure, feed concentration and ion strength hardly have influence on the calculated molecular diameter but the temperature has some influence on the calculated molecular diameter.
Simultaneously, molecular polarity can be obtained from HyperChem.
Steric-hindrance pore model
Max. Pressure (bar)
Pure water permeance (L m−2 h−1 bar−1)a
MgSO4 retention (%)b
Na2SO4 retention (%)b
NaCl retention (%)b
CaCl2 retention (%)b
Chemicals and reagents
GAH and NAG were of analytical grade and purchased from Shandong Aokang Biotechnology Co. Ltd. (Shandong, China) and Zhejiang Aoxing Biotechnology Co. Ltd. (Zhejiang, China), respectively. Several analytical grade salts, namely, MgSO4, Na2SO4, NaCl, MgCl2, CaCl2, NaOH, and EDTA (ethylene diamine tetraacetic acid), were supplied by Lingfeng Chemical Reagent Co. Ltd. (Shanghai, China). The deionized water (conductivity ≤10 μS cm−1) used for experiments and cleaning was supplied by Shanghai Huazhen Co. Ltd. (Shanghai, China).
The conductivity was measured by an electric conductivity meter (Shanghai Jingke, DDS-307, China), and the viscosity was measured by Ubbelohde Viscometer. The solution pH was measured by the FE20 pH meter (Mettler Toledo, Shanghai, China). The GAH and NAG contents of the samples were analyzed by the HPLC method (Agilent 1200, Agilent, USA) equipped with a high-performance sugar column (Sugarpak-I column, Waters, USA) and an RI detector.
Influence of different mass ratios on separation performance
Influence of different mass ratios on feed volume flux
Estimated pore radius (r p ), osmotic pressure difference (Δπ) and effective thickness (Δx/A k )
Different mass ratios
R p (nm)
Δx/A k (μm)
Rejection difference between single solution and mixed solution
Figure 4b illustrates that compared with single solution, the NAG rejection in the mixed solution decreases; this result is consistent with that of Luo and Wan (2011). One reason is a partial dehydration of solute because the “salting-out” effect (also called the Hofmeister effect [Kunz et al. 2004)] causes a decrease of solute hydrodynamic radius, thus inducing a decrease in solute retention. Another reason is that, at a higher GAH concentration, the membrane becomes more compacted than that with a lower GAH concentration (Bargeman et al. 2014). Correspondingly, the double electric layer becomes compressed, leading to the thinner double electric layer and the increase of the channel in the membrane pore. Therefore, NAG rejection decreases in the mixed solution.
Influence of different mass ratios on GAH separation factor
The above results can be attributed to the following explanations: both the MWCO and molecular diameter [obtained from Eq. (9)] of GAH are smaller than NAG (at 25 °C, the molecular diameters for GAH and NAG are 0.61 and 0.67 nm, respectively). Therefore, the GAH rejection is smaller than NAG rejection because of the steric-hindrance effect (Sjoman et al. 2007). Furthermore, the positive charge of GAH can interact with the negative charge at the membrane surface because of electrostatic attraction, which reduces the electric quantity on membrane surface and diminishes the dielectric effect of the membrane (Qin et al. 2014). This result promotes that GAH penetrates the membrane. Different from GAH, neutral NAG cannot interact with the membrane, under the effect of steric hindrance, so GAH is less rejected than NAG. Furthermore, the permeability [obtained from Eq. (8)] of GAH and NAG is apparently different. As shown in Fig. 5c, GAH permeability is larger than NAG permeability at every mass ratio, and the permeability difference value is larger at higher GAH concentrations. Therefore, given the influence of permeability, MWCO, molecular diameter, and electrostatic effect, the rejection of GAH is less than NAG rejection, and the separation performance presents different results with different mass ratios. On the basis of the above results, the mass ratio of 1:2 (GAH:NAG) has been chosen in the next experiments.
Influence of different temperatures on separation performance
Influence of different temperatures on rejections for GAH and NAG and feed volume flux
Influence of different temperatures on molecular characteristics
The temperature influences not only the NF membrane but also the molecular characteristics (Jian et al. 2015; Paul and Paul 2015; Xing et al. 2013). Therefore, temperature plays an important part in the NF process. Figure 8a, b illustrates the effects of temperature on the molecular polarities and calculated diameters. Figure 8a shows that the GAH dipole moment increases from 3.54 to 3.74 Debye with increasing temperature from 15–35 °C. Compared with GAH, the dipole moment of NAG increases rapidly and the maximum dipole moment is up to 5.94 Debye when the temperature reaches 35 °C. The results provide a good explanation on why both GAH and NAG retentions decrease with increasing operating temperatures (Fig. 6a, b).
Influence of different temperatures on the GAH separation factor
Figure 10c shows that temperature influences both GAH and NAG solute permeability significantly. Compared with the solute reflection coefficient, solute permeability, which also represents the retention performance of membrane for GAH and NAG, shows rising tendency with increasing temperature. For GAH, the solute permeability increases rapidly from 15–20 °C and then increases slowly when temperature increases from 20–30 °C. Solute permeability of NAG increases slowly from 15 to 30 °C and then increases rapidly when temperature increases from 30 to 35 °C. Therefore, the permeability difference value between GAH and NAG decreases with increasing temperature. That is why the GAH separation factor decreases with increasing temperature. This result indicates that high temperatures have an adverse effect on the separation for the two mixed solutions.
Influence of different electrolytes on separation performance
Influence of different electrolytes on rejections for GAH and NAG
Diffusion coefficients of three electrolytes at 25 °C (Schaep et al. 1998) and permeability of NAG influenced by three electrolytes
Diffusion coefficient (10−9 m2 s−1)
Permeability of NAG (L m−2 h−1)
Influence of different electrolytes on GAH separation factor
NF membrane separation performance was significantly affected by physical conditions in this system. When the mass ratio of GAH:NAG was 1:2, the maximum membrane flux was up to 42.3 L m−2 h−1. Under this condition, the permeability difference obtained from the irreversible thermodynamic model was the largest. Therefore, the GAH separation factor under this condition was up to 1.22. When the temperature was 35 °C, the permeability of GAH and NAG was 9.6 and 8.7 L m−2 h−1, respectively, and the permeability difference was the minimum. Therefore, lower operation was not good for GAH and NAG separation. Simultaneously, the calculated molecular diameter and increasing temperature showed good exponential relationship, providing supplement for the separation process. After adding salts, by the analysis using the irreversible thermodynamic model, electrostatic repulsion was the essential influencing factor for GAH rejection, and solute diffusion was an important transport mechanism for NAG rejection. With NaCl addition, the GAH separation factor was up to 1.13. The explored mechanisms could be used to understand the process of NF separation for monosaccharides with similar molecular weights and provide a certain basis for large-scale separation of chitin derivatives in the future.
molecular weight cut-offs
- C p :
solute concentrations in the permeate (g L−1)
- C b :
solute concentrations in the bulk feed (g L−1)
- C m :
solute concentration on the membrane surface (g L−1)
- d c :
calculated molecular diameter (nm)
- J v :
permeat volume flux (L m−2 h−1)
- L p :
pure water permeability (L m−2 h−1 bar−1)
- k :
mass transfer coefficient (L m−2 h−1)
- ΔP :
transmembrane pressure (bar)
- P s :
solute permeability (L m−2 h−1)
- r p :
mean pore radius (nm)
- r s :
solute radius (nm)
- R :
- R o :
- Δπ :
osmotic pressure difference (bar)
- Δx/A k :
the ratio of the effective membrane thickness to membrane porosity (m)
- μ :
solute viscosity (mPa s)
- σ :
the reflection coefficient
This paper is the result of joint efforts. Prof. LZ designed the whole experimental plan and confirmed the main objective of this paper. Prof. JZ developed the statistical methods for experimental data. SZ was responsible for the optimization of the nanofiltration technology and the partial investigation of the mechanism in the process. YQ was responsible for the quantification of total sugars. Prof. LJ and Prof. LF helped us complete the paper writing and correcting some grammatical errors in details. All authors read and approved the final manuscript.
This work is financially supported by the National Natural Science Foundation of China (NO.31371725). It is also supported by the National High Technology Research & Development Program of China (863 Program) (No. SS2014AA021202).
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
- Aroon MA, Ismail AF, Mastuura T (2010) Performance studies of mixed matrix membrane for gas separation: a review. Sep Purif Technol 75:229–242View ArticleGoogle Scholar
- Bargeman G, Westerink JB, Miguez OG, Wessling M (2014) The effect of NaCl and glucose concentration on retentions for nanofiltration membranes processing concentrated solutions. Sep Purif Technol 134:46–57View ArticleGoogle Scholar
- Ben Amar N, Saidani H, Palmeri J, Deratani A (2009) Effect of temperature on the rejection of neutral and charged solutes by Desal 5 DK nanofiltration membrane. Desalination 246(1–3):294–303View ArticleGoogle Scholar
- Bowen WR, Mohammad AW, Hilal N (1997) Characterization of nanofiltration membranes for predictive purposes—use of salts, uncharged solutes and atomic force microscopy. J Membr Sci 126:91–105View ArticleGoogle Scholar
- Brereton KR, Green DB (2012) Isolation of saccharides in dairy and soy products by solid-phase extraction coupled with analysis by ligand-exchange chromatography. Talanta 100:384–390View ArticleGoogle Scholar
- Bui AV, Nguyen MH (2004) Prediction of viscosity of glucose and calcium chloride solutions. J Food Eng 62(4):345–349View ArticleGoogle Scholar
- Chen X, Liu L, Li JH, Du GC, Chen J (2012) Improved glucosamine and N-acetylglucosamine production by an engineered Escherichia coli via step-wise regulation of dissolved oxygen level. Bioresour Technol 110:534–538View ArticleGoogle Scholar
- Chen ZW, Luo JQ, Chen XR, Hang XF, Shen F, Wan YH (2016) Fully recycling dairy wastewater by an integrated isoelectric precipitation-nanofiltration-anaerobic fermentation process. Chem Eng J 283:476–485View ArticleGoogle Scholar
- Deng MD, Severson DK, Grund AD, Wassink SL, Burlingame RP, Berry A, Running JA, Kunesh CA, Song LS, Jerrell TA, Rosson RA (2005) Metabolic engineering of Escherichia coli for industrial production of glucosamine and N-acetylglucosamine. Metab Eng 7:201–214View ArticleGoogle Scholar
- Dong HZ, Wang YS, Zhou JC, Xia QM, Jiang LH, Fan LQ, Zhao LM (2014) Purification of DP 6 to 8 chitooligosaccharides by nanofiltration from the prepared chitooligosaccharides syrup. Bioresour Bioprocess 1:1–12View ArticleGoogle Scholar
- Freger V, Arnot TC, Howell JA (2000) Separation of concentrated organic/inorganic salt mixtures by nanofiltration. J Membr Sci 178(1–2):185–193View ArticleGoogle Scholar
- Ghfar AA, Wabaidur SM, Yacine Badjah Hadj Ahmed A, Alothman ZA, Khan MR, Al-Shaalan NH (2015) Simultaneous determination of monosaccharides and oligosaccharides in dates using liquid chromatography-electrospray ionization mass spectrometry. Food Chem 176:487–492View ArticleGoogle Scholar
- Iwasaki K, Matsubara Y (2000) Purification of pectate oligosaccharides showing root-growth-promoting activity in lettuce using ultrafiltration and nanofiltration membranes. J Biosci Bioeng 89(5):495–497View ArticleGoogle Scholar
- Jian WJ, Siu KC, Wu JY (2015) Effects of pH and temperature on colloidal properties and molecular characteristics of Konjac glucomannan. Carbohydr Polym 134:285–292View ArticleGoogle Scholar
- Kebria MRS, Jahanshahi M, Rahimpour A (2015) SiO2 modified polyethyleneimine-based nanofiltration membranes for dye removal from aqueous and organic solutions. Desalination 367:255–264View ArticleGoogle Scholar
- Kedem O, Katchalsky A (1958) Thermodynamic analysis of the permeability of biological membranes to non-electrolytes. Biochim Biophys Acta 27:229–246View ArticleGoogle Scholar
- Kolfschoten RC, Janssen AEM, Boom RM (2011) Mass diffusion-based Separation of sugars in a microfluidic contactor with nanofiltration membranes. J Sep Sci 34:1338–1346View ArticleGoogle Scholar
- Koter S (2006) Determination of the parameters of the Spiegler–Kedem–Katchalsky model for nanofiltration of single electrolyte solutions. Desalination 198(1–3):335–345View ArticleGoogle Scholar
- Kunz W, Henle J, Ninham BW (2004) ‘Zur Lehre von der Wirkung der Salze’ (about the science of the effect of salts): Franz Hofmeister’s historical papers. Curr Opin Colloid Interface Sci 9(1–2):19–37View ArticleGoogle Scholar
- Luo JQ, Wan YH (2011) Effect of highly concentrated salt on retention of organic solutes by nanofiltration polymeric membranes. J Membr Sci 372(1–2):145–153View ArticleGoogle Scholar
- Luo JQ, Wan YH (2013) Effects of pH and salt on nanofiltration-a critical review. J Membr Sci 438:18–28View ArticleGoogle Scholar
- Mehiguene K, Garba Y, Taha S, Gondrexon N, Dorange G (1999) Influence of operating conditions on the retention of copper and cadmium in aqueous solutions by nanofiltration: experimental results and modelling. Sep Purif Technol 15(2):181–187View ArticleGoogle Scholar
- Moreno-Vilet L, Bonnin-Paris J, Bostyn S, Ruiz-Cabrera MA, Moscosa-Santillan M (2014) Assessment of sugars separation from a model carbohydrates solution by nanofiltration using a design of experiments (DoE) methodology. Sep Purif Technol 131:84–93View ArticleGoogle Scholar
- Murthy ZVP, Chaudhari LB (2009) Rejection behavior of nickel ions from synthetic wastewater containing Na2SO4, NiSO4, MgCl2 and CaCl2 salts by nanofiltration and characterization of the membrane. Desalination 247(1–3):610–622View ArticleGoogle Scholar
- Nakao SI, Kimura S (1982) Models of membrane transport phenomena and their applications for ultrafiltration data. J Chem Eng Jpn 15:200–205View ArticleGoogle Scholar
- Niewersch C, Meier K, Wintgens T, Melin T (2010) Selectivity of polyamide nanofiltration membranes for cations and phosphoric acid. Desalination 250(3):1021–1024View ArticleGoogle Scholar
- Paul S, Paul S (2015) Influence of temperature on the solvation of N-methylacetamide in aqueous trehalose solution: a molecular dynamics simulation study. J Mol Liq 211:986–999View ArticleGoogle Scholar
- Qin JX, Dai XG, Zhou Y, Zhang L, Chen HL, Gao CJ (2014) Desalting and recovering naphthalenesulfonic acid from wastewater with concentrated bivalent salt by nanofiltration process. J Membr Sci 468:242–249View ArticleGoogle Scholar
- Schaep J, Van der Bruggen B, Vandecasteele C, Wilms D (1998) Influence of ion size and charge in nanofiltration. Sep Purif Technol 14:155–162View ArticleGoogle Scholar
- Schaep J, Vandecasteele C, Mohammad AW, Bowen WR (2001) Modelling the retention of ionic components for different nanofiltration membranes. Sep Purif Technol 22–23:169–179View ArticleGoogle Scholar
- Sharma RR, Agrawal R, Chellam S (2003) Temperature effects on sieving characteristics of thin-film composite nanofiltration membranes: pore size distributions and transport parameters. J Membr Sci 223(1–2):69–87View ArticleGoogle Scholar
- Sjoman E, Manttari M, Nystrom M, Koivikko H, Heikkila H (2007) Separation of xylose from glucose by nanofiltration from concentrated monosaccharide solutions. J Membr Sci 292(1–2):106–115View ArticleGoogle Scholar
- Spiegler KS, Kedem O (1966) Thermodynamics of hyperfiltration (reverse osmosis): criteria for efficient membranes. Desalination 1:311–326View ArticleGoogle Scholar
- Tao CZ, Zhang ZT, Wu JW, Li RH, Cao ZL (2014) Synthesis of unnatural N-glycosyl alpha-amino acids via Petasis reaction. Chin Chem Lett 25(4):532–534View ArticleGoogle Scholar
- Van der Bruggen B, Schaep J, Wilms D, Vandecasteele C (1999) Influence of molecular size, polarity and charge on the retention of organic molecules by nanofiltration. J Membr Sci 156(1):29–41View ArticleGoogle Scholar
- Wang KY, Chung TS (2005) The characterization of flat composite nanofiltration membranes and their applications in the separation of Cephalexin. J Membr Sci 247(1–2):37–50View ArticleGoogle Scholar
- Wang XL, Wang WN, Wang DX (2002) Experimental investigation on separation performance of nanofiltration membranes for inorganic electrolyte solutions. Desalination 145(1–3):115–122View ArticleGoogle Scholar
- Xie YP, Liu SJ (2015) Purification and concentration of paulownia hot water wood extracts with nanofiltration. Sep Purif Technol 156:848–855View ArticleGoogle Scholar
- Xing PY, Sun T, Li SY, Hao AY, Su J, Hou YH (2013) An instant-formative heat-set organogel induced by small organic molecules at a high temperature. Colloids Surf A Physicochem Eng Asp 421:44–50View ArticleGoogle Scholar
- Yaroshchuk AE (2001) Non-steric mechanisms of nanofiltration: superposition of Donnan and dielectric exclusion. Sep Purif Technol 22–23:143–158View ArticleGoogle Scholar
- Zhao LM, Zhao HF, Nguyen P, Li AR, Jiang LH, Xia QM, Rong Y, Qiu YJ, Zhou JC (2013) Separation performance of multi-components solution by membrane technology in continual diafiltration mode. Desalination 322:113–120View ArticleGoogle Scholar
- Zhu XL, Cai JB, Yang J, Su QD (2005a) Determination of glucosamine in impure chitin samples by high-performance liquid chromatography. Carbohydr Res 340:1732–1738View ArticleGoogle Scholar
- Zhu ZZ, Hao ZL, Shen ZS, Chen J (2005b) Modified modeling of the effect of pH and viscosity on the mass transfer in hydrophobic hollow fiber membrane contactors. J Membr Sci 250(1–2):269–276View ArticleGoogle Scholar
- Zhu YQ, Liu YF, Li JH, Shin HD, Du GC, Liu L, Chen J (2015) An optimal glucose feeding strategy integrated with step-wise regulation of the dissolved oxygen level improves N-acetylglucosamine production in recombinant Bacillus subtilis. Bioresour Technol 177:387–392View ArticleGoogle Scholar