Recovering/concentrating of hemicellulosic sugars and acetic acid by nanofiltration and reverse osmosis from prehydrolysis liquor of kraft based hardwood dissolving pulp process
Introduction
The integration of forest biorefinery concept into existing pulp mills has received a lot of attention due to global marketing competition, environmental concern and biofuel/biochemcials/bio-materials demand. The fast growing dissolving pulp mill is not exceptional. In addition to the dissolving pulp as main product, these pulp mills have the potential to produce bio-materials, bio-fuels/energy. In the current practice the prehydrolysis liquor and pulping waste liquor are burnt. In case of prehydrolysis liquor (PHL) it is not economical as the heating value of hemicelluloses is rather low (van Heiningen, 2006). On the other hand, hemicelluloses can be converted to biochemicals such as furfural, xylitol and ethanol.
The PHL from the Kraft based hardwood dissolving pulp process contains around 10 g/L acetic acid (Shen et al., 2012) and it is an important commodity chemical and a key raw material for the production of wide range of products like terephthalic acid, vinyl acetate, acetic anhydride, esters, monochloroacetic acid and so on. Acetic acid in PHL is from acetyl groups (around 5% in hardwood), which are mainly bounded to hemicelluloses and hydrolyzed during the prehydrolysis process (Li et al., 2010). Separation of acetic acid from the sugar rich PHL is very crucial as its presence inhibits fermentation process. An effective recovery of acetic acid can lead to another commodity chemical, while it can help utilization of hemicellulosic sugars in the downstream processing. Our previous study was done on recovery of acetic acid by adsorption onto resin and by reactive extraction process using trioctyl amine in octanol as a diluent. The resin adsorption process showed lower. The reactive extraction showed 63–65% recovery of acetic acid of TPHL at 1:1 amine to acetic acid molar ratio (Ahsan et al., 2012). In the back extraction process 83–90% of acetic acid was regenerated without hampering the extraction efficiency of the recycled organic phase (Ahsan et al., 2013). In spite of better recovery octanol and trioctyl amine were expensive and would be difficult to handle at an industrial scale.
In the present study we focused on consecutive nanofiltration (NF) and reverse osmosis (RO) concepts. Generally, membranes are industrially used for concentration and purification. They are relatively easy to scale up and can be operated with low energy consumption. They do not require any additional organic chemicals and are not involved with phase change. To reduce the load of fouling compounds like lignin, the PHL was first treated by using activated carbon (AC) adsorption. The removal of lignin can enhance the subsequent NF and RO as the fouling is decreased.
The main objective of this study was to assess the feasibility of the proposed concept for recovering and concentrating of sugars and acetic acid by combination of AC treatment, NF and RO.
Section snippets
Materials
The Pre-hydrolysis liquor (PHL) sample was collected from a dissolving pulp mill located in Eastern Canada. The mill used a mixture of maple (70% wt.), poplar (20% wt.), and birch (10% wt.) as raw materials. Wood based powdered activated carbon (CR325W-Ultra) was purchased from Carbon Resources.
Activated carbon (AC) treatment
The PHL and powdered activated carbon (AC) were mixed at the ratio of 20:1 in a flask and shaken at the speed of 150 rpm at room temperature for 5 h. Then the treated PHL (TPHL) was collected after filtration
Proposed concept for recovering and concentration of hemicellulosic sugars and acetic acid by activated carbon and multistage membrane process
PHL contains lignin, sugars, acetic acid, and therefore various separation units would be required. Starting with lignin it was already established that activated carbon is a good adsorbent for removing polyphenolic compounds (Liu et al., 2011, Fatehi et al., 2013). Wood based powder activated carbon is the best choice in this case for its higher surface area and functional groups (Fatehi et al., 2013). Subsequently the activated carbon can be regenerated and reused though our research did not
Conclusion
The combined concept of adsorption and multistage NF and RO were showed as a promising process to separate/concentrate and acetic acid. The total sugar concentration after the NF increased to 227 g/L from 48 g/L under the conditions of VRF 5, pH 4.3, and 500 psi. In addition, the RO stage increased the acetic acid concentration to about 50 g/L from 10 g/L under the conditions of pH 4.3 and 500 psi. Multiple stages of RO can increase the overall acetic acid recovery up to about 70% under the
Acknowledgements
The authors gratefully acknowledge the Grant of NSERC and Atlantic Innovation Fund (AIF) for financing this project.
References (14)
- et al.
Adsorption of lignocelluloses of model pre-hydrolysis liquor on activated carbon
Bioresour. Technol.
(2013) - et al.
Removal of inhibitors from pre-hydrolysis liquor of kraft-based dissolving pulp production process using adsorption and flocculation processes
Bioresour. Technol.
(2012) - et al.
A combined process of activated carbon adsorption, ion exchange resin treatment and membrane concentration for recovery of dissolved organics in pre-hydrolysis liquor of the kraft-based dissolving pulp production process
Bioresour. Technol.
(2013) - et al.
Separation of acetic acid from monosaccharides by NF and RO membranes: performance comparison
J. Membr. Sci.
(2013) - et al.
Simultaneous acetic acid separation and monosaccharide concentration by reverse osmosis
Bioresour. Technol.
(2013) - et al.
Recovery of acetic acid from pre-hydrolysis liquor of the kraft-based dissolving pulp production process by reactive extraction with tri-octyl amine(TOA) and octanol
J. For.
(2012) - et al.
Recovery of acetic acid from the prehydrolysis liquor of kraft based dissolving pulp production process: sodium hydroxide back extraction from the trioctylamine/octanol system
Ind. Eng. Chem. Res.
(2013)