Elsevier

Bioresource Technology

Volume 155, March 2014, Pages 111-115
Bioresource Technology

Recovering/concentrating of hemicellulosic sugars and acetic acid by nanofiltration and reverse osmosis from prehydrolysis liquor of kraft based hardwood dissolving pulp process

https://doi.org/10.1016/j.biortech.2013.12.096Get rights and content

Highlights

  • A concept was proposed to recover sugar and acetic acid from the PHL.

  • Activated carbon adsorption was applied to remove lignin.

  • NF and reverse RO were followed to recover and concentrate sugars and acetic acid.

  • Pressure and pH effect were also observed to optimize the process.

Abstract

This work investigated the feasibility of recovering and concentrating sugars and acetic acid (HAc) from prehydrolysis liquor (PHL) of the kraft-based dissolving pulp process prior to fermentation of hemicellulosic sugars, by the combination of activated carbon adsorption, nanofiltration (NF) and reverse osmosis (RO) processes. To reduce the fouling PHL was subjected to adsorption on activated carbon, then the treated PHL (TPHL) passed through a nanofiltration (NF DK) membrane to retain the sugars, and the permeate of acetic acid rich solution was passed through a reverse osmosis membrane (RO SG). It was found that for NF process sugars were concentrated from 48 to 227 g/L at a volume reduction factor (VRF) of 5 while 80 to 90% of acetic acid was permeated. For the reverse osmosis process, 68% of acetic acid retention was achieved at pH 4.3 and 500 psi pressure and the HAc concentration increased from 10 to 50 g/L.

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.

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