Adsorption and membrane filtration for the separation and valorization of hemicellulose from organosolv beechwood hydrolyzates

The sustainable use and conversion of biomass for the production of energy, fuels, and chemicals can help to counteract the declining availability of fossil resources and increasing climate change. In biorefineries, woody raw materials are fractionated into their three main constituent’s cellulose, hemicellulose, and lignin. The dissolved hemicellulose and its degradation products usually end up in the wood hydrolyzate. Due to relatively low concentrations and inhomogeneous composition of this process stream, it is usually not recovered. However, if the hemicellulose and its derived sugars can be separated and purified, they could be used to produce high-value-added products and raise up the competitiveness and sustainability of existing and future biorefineries. Within the scope of this work, the separation and valorization of hemicellulose from beechwood hydrolyzates by adsorption and membrane filtration were experimentally investigated and techno-economically assessed. At first, an adsorption process was developed to separate residual lignin from hemicellulose out of beechwood hydrolyzate. Four polymeric resins and one zeolite were compared in batch experiments, and competitive adsorption isotherms were modeled. The most efficient resin SP700 was studied in more detail in column tests. The adsorption of lignin and hemicellulose from beechwood hydrolyzate could be fitted best to the extended Freundlich isotherm. In a continuous adsorption process, 80 % of lignin was removed with 99.5 % hemicellulose recovery. Adsorbed lignin could be efficiently desorbed with a 50 wt.% ethanol solution. Membrane filtration was developed, on the one hand, for the recovery and concentration of hemicellulose from beechwood hydrolyzate by ultrafiltration, and on the other hand, for the separation of xylose from hydrothermally pretreated beechwood hydrolyzate by nanofiltration. The ultrafiltration was optimized in terms of high permeate flux and hemicellulose retention as well as low lignin retention as a function of transmembrane pressure, temperature, and pH. In addition, the effect of a prior adsorption step on ultrafiltration of beechwood hydrolyzate was studied. For the process design the statistical approach of response surface methodology and Pareto optimization was used. Optimum process parameters using the polymeric membrane UA60 were found to be a transmembrane pressure of 0.98 MPa, a temperature of 55 °C, and a pH of 2.5, resulting in a permeate flux of 49 L/(m²h), hemicellulose retention of 85 %, and lignin retention of 41 %. Adsorption previous to ultrafiltration enhanced the permeate flux by 174 % and reduced the hemicellulose and lignin retention by 12 and 54 %, respectively. Hence, higher losses of hemicellulose, but simultaneously a purer concentrate and a higher throughput. For nanofiltration, the influence of hydrothermal pretreatment of beechwood hydrolyzate on the separation of xylose from fermentation inhibitors as well as the overall process performance was investigated. At first, a hydrothermal process was developed, to convert the remaining oligomeric hemicellulose into xylose. Then, the nanofiltration process using untreated and hydrothermally pretreated beechwood hydrolyzate was assessed by a performance and fouling analysis. The average permeate flux increased by up to 33 % during filtration of the pretreated solution. It was found that this is due to reduced concentration polarization and membrane fouling as a result of macromolecular sugar degradation. Thus, due to higher xylose concertation and lower inhibitor retentions a cleaner retentate stream could be obtained. To design an energetically favorable process appropriate parameters were identified. Using the polymeric membrane Alfa Laval NF, a volume reduction of 80 % was achieved with an average permeate flux of 22.5 L/(m²h) and retentions for xylose, furans, and acetic acid of 95, 31, and 4 %, respectively. The experimental findings were used to assess two industrial scale purification cascades. Purification cascade 1 consists of adsorption and ultrafiltration and aimed a purified hemicellulose stream. Purification cascade 2 consists of hydrothermal treatment and nanofiltration, with the aim to achieve a purified xylose stream. Both purification cascades were simulated in Aspen Plus® to calculate mass and energy balances and conduct a techno-economic assessment. In purification cascade 1, 80 % of the lignin was removed by adsorption, and 7.6 t/h of a purified hemicellulose solution with a concentration of 200 g/L was obtained using ultrafiltration. In purification cascade 2, almost the entire oligomeric hemicellulose was hydrothermally converted to xylose and purified by nanofiltration to 8.9 t/h of a xylose solution with a concentration of 200 g/L. The energy efficiency of the cascades was 24 and 56 %, respectively. Furthermore, the estimation of specific production costs showed that hemicellulose could be recovered from beechwood hydrolyzate at 135.1 EUR/t and xylose at 71.4 EUR/t.

Author dc:creator

• Nitzsche, Roy

Advisors dc:contributor.advisor

• Kraume, Matthias
• Gröngröft, Arne

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Identifier URI

https://doi.org/10.14279/depositonce-18217

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/19420