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Abstract： The concept of integrated forest biorefineries （IFBRs） has gained significant interest. The prehydrolysis kraft （PHK） dissolving pulp production process is a suitable example of IFBR concept for the production of dissolving pulp and utilization of dissolved hemicelluloses， acetic acid， and lignin in the prehydrolysis liquor （PHL）. This review paper highlights recent progress related to the recovery and utilization of dissolved organics （e.g.， hemicelluloses， acetic acid， and lignin） in the PHL of the PHK dissolving pulp production process. Integrated multi-step recovery and separation processes have been developed for this purpose to accommodate the complex nature of the PHL. Potential products， including xylan-based compounds， acetic acid， and lignin， are also discussed in detail.
Keywords： PHK； prehydrolysis； lignocellulosic material； biorefinery
1 Introduction
Increasing demand for renewable resources， such as biomass as an alternative to fossil fuels for the production of liquid fuels and electricity has led to the development of integrated forest biorefineries （IFBRs）. With the addition of biorefining units， existing pulp and paper mills can be upgraded to IFBRs with great economic potential for the production of biofuels， chemicals， and biomaterials， while maintaining production of traditional products like pulp， paper， and other wood products[1].
Dissolving pulp mills are particularly well-suited for this type of upgrade， and the resultant IFBR is energetically favorable without the need for fossil fuels due to the established infrastructure of pulp mills， well-trained workforce， strong community support， and abundant wood （especially fast-growing hardwood like poplar and eucalyptus） fiber supplies[2].
Lignocellulosic biomass consists of cellulose， hemicelluloses， lignin， and small amounts of extractives. By upgrading to IFBRs， several value-added products can be produced along with dissolving pulp. It was proposed that hemicelluloses can be used as raw materials for xylitol， furfural， acetic acid， and ethanol production， or as strength additives for papermaking[3]. The lignin of woody materials has a variety of industrial applications， e.g.， a fuel source， dispersing agent， emulsion stabilizer. It can also be used as a raw material for the production of value-added products such as phenols， carbon fibers， binder resins， soil additives， polyurethanes， and plastics/polymers[4-6].
Dissolving pulp has very low contents of hemicelluloses， lignin， and other extracted compounds. These impurities are extracted from the cellulose matrix by hydrolysis， pulping， and bleaching processes. Dissolving pulp is mainly produced by acid sulfite （AS） and prehydrolysis kraft （PHK） processes. Both processes are from paper grade pulping with an emphasis on hemicelluloses removal， but major differences exist between the two methods. The main difference between AS and PHK processes is that the removal of lignin and hemicelluloses occurs at different stages. In the AS pulping process， the lignin and hemicelluloses are removed from the wood chips during the same pulping process； whereas in the PHK process， they are removed in two separate steps： hemicelluloses in the prehydrolysis step and lignin mainly in the kraft pulping step[7]. Another significant difference between the AS and PHK processes is the acidity/alkalinity of the cooking liquor， which results in differences in the chemical changes of the carbohydrates chains and the subsequent pulp properties. In the AS process， the glycosidic linkages in carbohydrates chains are randomly cleaved under acidic conditions， while in the PHK process， stepwise peeling is the dominant mechanism[8-9]. Therefore， AS pulp exhibits higher viscosity， higher low-molecular-weight fraction content， and broader molecular weight distribution compared to the PHK pulp. However， the PHK pulp has a higher a-cellulose content and lower reactivity[10-11].
The AS process was once considered the most common dissolving pulp process， benefits of this technique include high removal rates of lignin and hemicelluloses in the cooking process[12]. However， the predominant process of producing dissolving pulp shifted from AS to PHK at the beginning of 21st century， because of the difficulty of chemical recovery in the AS process， and stricter environmental regulations[13-14].
PHK is performed in a combined process of both acidic （liquid or water/steam prehydrolysis） and alkaline （kraft cooking） conditions， as depicted in Fig.1[15-16]. Hemicelluloses are extracted and removed from the wood chips during the prehydrolysis stage， the kraft cooking stage， and multi-stage bleaching process to reach the desired purity of dissolving pulp[9， 17].
Acidic prehydrolysis prior to alkaline cooking to selectively degrade hemicelluloses is necessary because alkaline pulping processes are not capable of selectively removing long-chain hemicelluloses. Prehydrolysis can be conducted using autohydrolysis （water/steam prehydrolysis process） and liquid hydrolysis with addition of acid （liquid prehydrolysis and concentrated acid hydrolysis） at the laboratory scale. However， prehydrolysis at the industrial scale is performed only via hot water or steam due to economic considerations[9，18-20]. In a liquid hydrolysis operation， pH adjustments can easily be made， and the process features flexibility of acid usage， lower temperature， and shorter time requirements. However， liquid hydrolysis generates large volumes of useless （at present） hydrolysate， which is associated with severe environmental and economic problems[9]. Hot water/steam hydrolysis is an autocatalytic process （or autohydrolysis）， in which the cleavage of acetyl and uronic acid substituents produces acetic and other organic acids. The organic acids derived from the hemicelluloses decrease the pH value of the liquor to 3～4， providing the acidity needed to catalyze depolymerization and removal of hemicelluloses from the wood chips[21-23].
Water prehydrolysis is conducted using hot water at elevated temperature （140～170℃） and pressure. However， the hot water prehydrolysis process is unpopular because of operational and technical problems[19]. Steam hydrolysis is a well-established method that has been used to hydrolyze/remove hemicelluloses prior to kraft pulping in commercial dissolving pulp production processes， such as the Visbatch and the continuous PHK dissolving pulp process of Shandong Sun Paper[9，19].
The PHK dissolving pulp process represents a promising platform for the IFBR concept. Prehydrolysis liquor （PHL）， which is the aqueous stream separated in the PHK-based dissolving pulp production process， contains hemicelluloses， lignin related products， and degradation products. Hemicelluloses （5wt%～6wt%） and acetic acid （approximately 1wt%） in the PHL can be recovered as valuable products instead of being directly combusted[24]. The hexose- and pentose-based carbohydrates that are decomposition products from hemicelluloses represent valuable feedstocks for further conversion to value-added products such as ethanol， furfural， and xylitol[25]. In addition， the acetic acid generated from hemicelluloses can be translated into extra profits for the industry[26-27]. The main reactions associated with the above-mentioned processes are listed in Fig.2.
2 Separation processes
The relatively low concentration of dissolved materials in the PHL hinders its practical application in various downstream conversion processes. The dissolved materials in the PHL should first be separated from the liquor. The following section will cover the various separation techniques that can be applied to the PHL， which are mostly converted from wastewater treatment or chemical engineering processes in various other industries.
2.1 Acidification
Lignin is only sparingly soluble in aqueous solutions at low pH value， making acidification a simple and effective method to recover lignin from the effluents of a PHK process. Generally， inorganic acids （e.g.， HCl and H2SO4） and flue gas containing CO2 and SO2 are used to recover lignin[28-29]. For kraft pulping liquor， over 90% of the total lignin can be recovered after the liquor pH value is decreased to approximately 2[29]. Liu et al acidified the PHL to a pH value of 2 using sulfuric acid and found that 47% of the total lignin in the PHL was precipitated[30]. The lower recovery rate of lignin from the PHL compared to that from the pulping liquor was possibly caused by the lower concentration of lignin and its higher solubility in the PHL. In addition， due to the complex composition of the PHL， subsequent separation processes should be introduced to recover other components. 2.2 Adsorption
Adsorption is typically performed using materials with highly porous structure and high specific surface area， such as activated carbon （CA）. The adsorption materials can be used to separate/concentrate dissolved organics in the PHL. In this system， the adsorbent is mixed with the PHL to adsorb lignocellulosic materials under various conditions. The absorbent is usually inert， thus can be recycled. The surface area， pore size， and functional groups of the adsorbent determine its performance towards various PHL components. The main advantage of this system is its flexibility for selectivity towards the adsorption of a desired lignocellulosic material contained in the PHL. When the AC was used for adsorbing lignocellulosic materials in the PHL， the final adsorption of hemicelluloses， lignin， and furfural were 400～700， 200～300， and 100 mg/g， respectively， indicating differential selectivity of the AC for the lignocellulosic materials in the PHL[31].
2.3 Coagulation/flocculation
Coagulation/flocculation methods have been applied in wastewater treatment systems for decades. The components of the PHL are generally anionic since they originate from hydroxyl/carboxylic groups associated with their backbones. By introducing cationic polymers such as chitosan[32]， poly（diallyldimethylammonium chloride）[33]， and polyethylene oxide[34] to the PHL system， these groups can associate with the cationic groups to form flocs， which can be subsequently removed by filtering. Saeed et al used cationic chitosan to recover furfural from industrially produced PHL， showing good selectivity of separation （over 50% of the furfural was removed）， whereas the recovery of monomeric sugars and acetic acid was rather limited[32].
2.4 Ion exchange resin treatment
Resin containing functional groups is a useful medium for the separation of specific components in the PHL. At the laboratory scale， a weak base anionic resin with tertiary amine functionality was applied for the selective removal of acetic acid. Shen et al used ion exchange resin treatment to recover acetic acid from a lignin eliminated PHL and showed that approximately 70% acetic acid could be removed. In addition， the resin could be regenerated and reused by the addition of sulfuric acid to desorb acetic acid attached to the resin[25].
2.5 Membrane filtration
Membrane filtration is an efficient and cost-effective technique that can be used to separate different sized compounds by molecular weight cut-offs. Membrane filtration （ultrafiltration， nanofiltration， and reverse osmosis） have been widely investigated to concentrate low-concentration solutes and has been shown to be suitable for the concentration and recovery of organic components in the PHL. Ahsan et al used nanofiltration and reverse osmosis to recover/concentrate hemicellulosic sugars and acetic acid from the PHL. The sugars were concentrated from 48 g/L to 227 g/L by nanofiltration and the acetic acid concentration increased from 10 g/L to 50 g/L after reverse osmosis[35]. However， since the membrane separation is solely based on molecular weight， the separation selectivity of organic components with similar molecular weights is low. In addition， fouling presents a technical challenge for practical industrial application. 2.6 Solvent extraction
Various solvents that can selectively separate the organic components in the PHL are used for the recovery of dissolved organics. For example， Yang et al demonstrated an efficient reactive extraction approach using triisooctylamine （TIOA） diluted with decanol to recover acetic acid[27]. According the literature， the recovery of acetic acid could reach as high as 66.6% using solvent extraction techniques. However， a large amount of organic solvent is generally required， and solvent regeneration is quite energy intensive， impairing the commercial potential of this process.
2.7 Integrated multi-step separation process
Efficient isolation of the complex organic components in the PHL usually requires an integrated multi-step separation process. Several studies have focused on the recovery/purification of furfural from PHL during the PHK process[24，36-37]. Shen et al reported a novel process to recover/concentrate acetic acid by combining AC adsorption， ion exchange resin treatment， and membrane concentration （Fig.3）[25]. Their results indicated that the removal of lignin in the PHL was achieved in the AC adsorption step， and the ion exchange resin treatment isolated the acetic acid. Finally， membrane filtration was used to obtain a concentrated hemicellulosic sugar liquor.
Overall， acidification is effective for precipitating lignin from kraft pulping liquor but limits the amount of lignin that can be isolated from the PHL. Adsorption techniques using AC， cationic polymers， and ion exchange resins are able to selectively remove components with high affinity towards the absorbents. However， desorption should be considered for the recovery of organic substances and regeneration of the absorbents. The membrane process can concentrate the components of the PHL but cannot separate components with similar molecular weight （e.g. low molecular weight lignin and xylooligosaccharides）. Although the solvent extraction process is suitable for separating some organic components in the PHL， the scaling-up of this technique is complicated by the need for large quantities of organic solvents. Integrated multi-step separation processes are effective for isolating different components in the PHL with high yields and purity. The feasibility of integrated processes highly depends on the specific end product values. Therefore， the development of more commercially-relevant combinatory technologies for the efficient recovery of dissolved organics in the PHL is urgently required. 3 Products
Hydrolysis is crucial for the kraft-based dissolving pulp production process. In this stage， a majority of hemicelluloses and a part of lignin components are dissolved from the wood chips， which facilitates the production of the dissolving pulp. The resulting PHL contains hemicelluloses， which can be utilized in the production of various value-added products as part of an IFBR strategy. Therefore， various value-added products will be produced after the separation process， which can be classified into three categories including xylan-based products， acetic acid， and lignin. The three main product streams will be discussed in the following section.
3.1 Xylan-based products
3.1.1 Xylose and xylan
Hemicellulose degraded sugars， especially xylose monomers and oligomers， are major components of the PHL. Xylose can be further converted to xylitol， and furfural. The xylooligosaccharide （XOS） can be directly used as a food additive for anti-obesity diets and in prebiotic foods. The annual production capacity of XOS in China is approximately 5000 tons which accounted for 20% for the global market based on the data from 2015[38]. The market demand for xylitol is increasing with the growing consciousness of preventive health care. The most common commercial method for the production of XOS is combined chemical-enzymatic hydrolysis of corn cob， cotton seed hull， and bagasse.
In the PHK process， the first order pseudo homogeneous kinetic model with an Arrhenius-type dependence on temperature is a commonly used model to predict the hydrolytic reactions of the hemicellulose fraction of lignocellulosic materials[39]. The hydrolysis of hemicelluloses by hydronium ion-catalyzed reaction is a multi-step process. Hemicelluloses are firstly hydrolyzed to form both high and low molecular weight XOS， and the produced oligomers are further hydrolyzed to xylose monomers. Chen et al determined the optimized conditions for this process （160℃， 60 min； 180℃， 20 min； 200℃， 5 min） with XOS yields up to 13.5% （w/w） of the initial dry biomass by autohydrolysis of Miscanthus x giganteus. The highest recovery rate of 47.9% （w/w） of the total XOS can be obtained by adding 10% AC and performing ethanol extraction[40].
3.1.2 Xylitol
Xylitol is a pentahydroxyl sugar alcohol which is naturally present in very small quantities in fruits and vegetables. Xylitol is commonly used as a pharmaceutical additive to prevent dental caries and as a food sweetener to replace of sugar. The annual production of xylitol in China exceeds 0.1 million tons， which is greater than 2/3 of the global annual production of xylitol[41]. Also， it is expected to increase by 7%～8% for the global xylitol production. The current commercial production process for xylitol is hydrogenation at 80～140℃ and 50 atm， which is an energy intensive process. Several laboratory scale trials focused on the biological conversion of xylose to xylitol by fermentation with various microbial and yeast strains. Biological conversion is preferable due to its advantage of mild reaction conditions. Due to the large amount of xylose in the PHL， the hydrolysates are considered potential sources for fermentation. However， the xylose concentration in the PHL is low for direct fermentation. At the industrial scale， xylose is usually concentrated by triple effect evaporation prior to its conversion to xylitol[42]. Alternatively， reverse osmosis can be used to concentrate the feed xylose. Reverse osmosis is considered to be preferable over evaporation for concentrating sugars as it prevents scorification and requires less energy[42]. In addition， the hydrolysates are always accompanied by lignin degradation products， furfural， and acetic acid which are inhibitors of microbial fermentation[43-45]. Therefore， purification must be integrated into the recovery process of xylose for subsequent use in fermentation. Various microbial and yeast strains such as D. hansenii， Candida magnoliae， C. guillermondii， and C. tropicalis have been previously used in the fermentation process[46-49].
3.1.3 Furfural
Furfural is an important chemical intermediate derived from the acid hydrolysis of pentoses （xylose and arabinose）. Furfural is considered to be a key green chemical produced from the lignocellulosic feedstock of a biorefinery. Furfural and its derivatives represent strategic platform chemicals due to their large number of applications， and are likely to experience growing demand in various fields. Furfural is a feedstock for the synthesis of resins， plastics， and stabilizers due to its beneficial properties such as corrosion resistance， physical strength， and thermosetting properties[50].
The annual production capacity of furfural reached 0.5 million tons in China in 2015. Furfural is commercially produced from agricultural lignocellulosic materials under acidic conditions involving acid hydrolysis and dehydration reactions. Furfural has been demonstrated to be a potential product from C-5 sugars present in industrial PHL. Mineral acid （H2SO4 or HCl） hydrolysis processes using corn cob is known to be toxic， corrosive to equipment， and the generated liquid waste is difficult to treat[42]. A high yield （80%） of furfural was obtained using hemicelluloses from lignocellulosic materials by applying solid acid catalysts （H-mordenite） with g-valerolactone （GVL） as the solvent[51]. 3.1.4 Other compounds
Xylan-based PHL can also serve as a potential raw material for the production of other value-added chemicals and materials. Bustos et al proposed a method of utilizing the hemicelluloses in the hydrolysis liquor to produce lactic acid by simultaneous saccharification and fermentation with cellulases and L. rhamnosus[52-53]. Hemicelluloses from the PHL can be used as papermaking additive as a substitute for cationic starch to achieve strength-enhancement or for retention purpose. Their use resulted in high brightness after the semi-bleaching process （83%ISO）， and a charge density of 0.24 mmol/g under optimized conditions using a cascade treatment including acidification， ethanol precipitation， and cationization[54]. In addition， the hemicelluloses precipitated from the PHL can be used to increase the basis weight of high-yield pulp （HYP） paper products after bleaching （D0EpD1） at a dosage as high as 50 mg/g without affecting mechanical properties[55]. The hemicelluloses， derivatives， and polyphenols from the PHL of eucalyptus wood can be used as bio-reductant for the reduction and stabilization of Ag+ to form Ag nanoparticles[56]. As previously reported， freshly collected PHL was filtered using a 0.22 mm membrane to remove impurities and then diluted by a factor of 10 before being used as bio-reductant. With a dosage of 10 mL of prepared bio-reductant in 50 mL of 1.0 mg AgNO3 solution， Ag nanoparticles were obtained at a purity of 82.1wt%， demonstrating the feasibility of using xylan-based PHL as an effective bio-reductant[53].
3.2 Acetic acid
Acetic acid is an important potential product in the PHL of the kraft-based hardwood dissolving pulp， accounting for approximately 1wt% of the total PHL[21]. Acetic acid in the PHL is generated from acetyl groups of the hemicelluloses[57]， and can be used as a feedstock to produce terephthalic acid （PTA）， vinyl acetate， acetate ester， and acetic anhydride. Therefore， the recovery of acetic acid from the PHL instead of production through methanol carbonylation using non-renewable feedstock fits well into the IFBR concept[58].
Acetic acid is a significant component in the PHL[36， 59]， but it is known to be inhibitory during sugar fermentation， so the removal of acetic acid from the PHL is critical for further sugar valorization[60-61]. Recently， several studies have reported the separation of acetic acid from the PHL stream[25-27，35]. Ahsan et al investigated the feasibility of recovering and concentrating acetic acid from the PHL of a PHK dissolving pulp process using a combination of AC adsorption， nanofiltration， and reverse osmosis[35]. It was reported that 68% of the acetic acid in the PHL could be recovered and a final concentration of 50 g/L was achieved. Simultaneously， the sugars in the PHL were concentrated and recovered at 227 g/L during the developed process. Yang et al demonstrated an efficient reactive extraction approach using triisooctylamine （TIOA） diluted with decanol to recover acetic acid at a rate of 66.6% in 10 min without affecting the hemicelluloses in the PHL[27]. In addition， Shen et al reported a novel process to recover/concentrate acetic acid by combining AC adsorption （1 g AC to 30 g PHL）， ion exchange resin treatment （10 g PHL to 1 g resin）， and membrane filtration. The ion exchange resin treatment exhibited good selectivity for acetic acid recovery， with the recovery rate of acetic acid reaching as high as 70%[25]. 3.3 Lignin
Lignin is another main component of lignocellulosic biomass， besides cellulose and hemicelluloses， and should be essentially eliminated by the PHK process to produce dissolving pulp. In the PHK process， lignin is isolated from the wood chips in two sections， the prehydrolysis and kraft pulping sections. In the prehydrolysis section， lignin with higher contents of hydrophilic functional groups is removed from the wood chips， and in the following kraft pulping process， the majority of the lignin in the wood chips is eliminated.
Traditionally， lignin dissolved in the hydrolysis liquor and kraft pulping liquor is burned as a fuel source in the chemical recovery of dissolving pulp production. However， the low concentrations of the dissolved lignocellulosic materials in the PHL render this process less economically attractive. From a biorefinery perspective， lignin should be recovered and further applied to produce value-added products during the commercial operation of PHK processes.
Efforts have been made to find alternative methods of recovering dissolved lignin from the PHL and kraft pulping black liquor. For the lignin obtained from the kraft pulping process， acid precipitation is feasible for lignin recovery and purification since most impurities are soluble and remain in the supernatant[62]. However， disposal of the generated salt solutions should be considered if this purification process is adopted in a pulping mill.
For the recovery of lignin from the PHL， since the dissolved lignin is more hydrophilic and its concentration is lower， only approximately 50% of the total lignin can be separated via adjusting pH value to 2.0[30]. The remaining lignin in the acidified PHL can be further removed by adsorption and flocculation. Fig.4 shows a process flow diagram for separating lignin from the PHL. Stream 1 represents the possibility of using polymers （e.g.， poly ethylene oxide （PEO） and poly aluminum chloride （PAC）） to form complexes with lignin， facilitating lignin flocculation and precipitation. The PEO/PAC/lignin precipitates can be used as wet end additives （retention aids） for papermaking. Stream 2 depicts the possibility for precipitating lignin via solvent extraction using ethyl acetate （EC） or ether. The precipitated lignin via acidification and solvent extraction is very pure， and can be further utilized in different processes for the production of various value-added products. However， the solvent should first be recovered from the PHL， which can then be used in downstream biorefinery processes. Recently， due to the popularization of alkali-tolerant ceramic membranes， ultrafiltration has become a widely accepted method for separating lignin from the black liquor of kraft pulping[63]. It is a pressure-driven technique that purifies lignin from extracts by retaining lignin macromolecules and allowing inorganic salts added during the pulping process to pass through. Similarly， ultrafiltration can be adopted for separating lignin dissolved in the PHL. The advantage of ultrafiltration is that pH value and temperature adjustments are not required， and the molecular weight of the products can be controlled. However， since the molecular weight of lignin is similar to those of other dissolved organics in the PHL， the selectivity of ultrafiltration for removing lignin is low， and the process is costly and slow.
With increasing demand for cost-effective biorefinery processes， the development of lignin valorization， in particular for high-value products， has become an important research focus[64]. The native structure of lignin facilitates a wide range of applications as a phenol substitute for synthetic resins， dispersants for dyes and pesticides， and a binder for feeds and fertilizers. Moreover， other value-added products such as carbon fibers[65] as well as biomedical and smart materials[6]， are being developed. Recently， a number of reviews focusing on the applications of lignin and lignin-derived chemicals in a wide variety of fields have been published[4-6， 66-69]， reflecting the significant attention this topic is generating. Generally， the major research areas with respect to lignin applications could be classified into five categories： blocks/additives for polymers[5， 68]， adsorbents for wastewater treatment [70]， depolymerized fine chemicals/liquid fuels[4]， precursors for carbon materials[71]， and biomedical applications[69].
Although tremendous effort has been devoted to the development of lignin applications， the practical utilization of lignin remains unsatisfactory in the pulping industry because of the structural heterogeneity of lignin[72]. The heterogeneity primarily results from the free radical polymerization of three different monomers in the biosynthesis of lignin and the complex reactions that occur during its isolation[72]. In the PHK process， the reactions of lignin are also complicated since lignin depolymerization and repolymerization can occur during both the prehydrolysis and kraft pulping sections. The broad molecular weight distributions of lignin have a significant impact on the functional group contents and reactivity characteristics of lignin and， to a large extent， on the performance of lignin for subsequent applications. Therefore， in-depth study regarding the possible effects of lignin heterogeneity on its applications is highly desirable. Another limitation for lignin application is the lack of processes available to convert lignin into a single material in high yields[62]. The current market for lignin chemicals is rather limited and lignin should be used in considerable quantities so the biorefinery can be successfully implemented. 4 Summary
The PHK dissolving pulp production process has emerged as an important commercial process for the production of dissolving pulp， which has undergone significant growth in the past 10 years. Technological developments for recovering and utilizing dissolved organics in the PHL fit well with the IFBR concept. The potential value-added products produced from PHL， such as xylooligosaccharides， xylitol， acetic acid， and furfural， can provide additional revenue streams for PHK dissolving pulp mills， enhancing their competitiveness. The technologies discussed in this review represent a good starting point for future commercial exploitation of the dissolved organics in the PHL. Further research is required， which may include： ① case studies of specific mills targeting particular products， ② pilot/mill trials， and ③ detailed process design and economic analyses.
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Keywords： PHK； prehydrolysis； lignocellulosic material； biorefinery
1 Introduction
Increasing demand for renewable resources， such as biomass as an alternative to fossil fuels for the production of liquid fuels and electricity has led to the development of integrated forest biorefineries （IFBRs）. With the addition of biorefining units， existing pulp and paper mills can be upgraded to IFBRs with great economic potential for the production of biofuels， chemicals， and biomaterials， while maintaining production of traditional products like pulp， paper， and other wood products[1].
Dissolving pulp mills are particularly well-suited for this type of upgrade， and the resultant IFBR is energetically favorable without the need for fossil fuels due to the established infrastructure of pulp mills， well-trained workforce， strong community support， and abundant wood （especially fast-growing hardwood like poplar and eucalyptus） fiber supplies[2].
Lignocellulosic biomass consists of cellulose， hemicelluloses， lignin， and small amounts of extractives. By upgrading to IFBRs， several value-added products can be produced along with dissolving pulp. It was proposed that hemicelluloses can be used as raw materials for xylitol， furfural， acetic acid， and ethanol production， or as strength additives for papermaking[3]. The lignin of woody materials has a variety of industrial applications， e.g.， a fuel source， dispersing agent， emulsion stabilizer. It can also be used as a raw material for the production of value-added products such as phenols， carbon fibers， binder resins， soil additives， polyurethanes， and plastics/polymers[4-6].
Dissolving pulp has very low contents of hemicelluloses， lignin， and other extracted compounds. These impurities are extracted from the cellulose matrix by hydrolysis， pulping， and bleaching processes. Dissolving pulp is mainly produced by acid sulfite （AS） and prehydrolysis kraft （PHK） processes. Both processes are from paper grade pulping with an emphasis on hemicelluloses removal， but major differences exist between the two methods. The main difference between AS and PHK processes is that the removal of lignin and hemicelluloses occurs at different stages. In the AS pulping process， the lignin and hemicelluloses are removed from the wood chips during the same pulping process； whereas in the PHK process， they are removed in two separate steps： hemicelluloses in the prehydrolysis step and lignin mainly in the kraft pulping step[7]. Another significant difference between the AS and PHK processes is the acidity/alkalinity of the cooking liquor， which results in differences in the chemical changes of the carbohydrates chains and the subsequent pulp properties. In the AS process， the glycosidic linkages in carbohydrates chains are randomly cleaved under acidic conditions， while in the PHK process， stepwise peeling is the dominant mechanism[8-9]. Therefore， AS pulp exhibits higher viscosity， higher low-molecular-weight fraction content， and broader molecular weight distribution compared to the PHK pulp. However， the PHK pulp has a higher a-cellulose content and lower reactivity[10-11].
The AS process was once considered the most common dissolving pulp process， benefits of this technique include high removal rates of lignin and hemicelluloses in the cooking process[12]. However， the predominant process of producing dissolving pulp shifted from AS to PHK at the beginning of 21st century， because of the difficulty of chemical recovery in the AS process， and stricter environmental regulations[13-14].
PHK is performed in a combined process of both acidic （liquid or water/steam prehydrolysis） and alkaline （kraft cooking） conditions， as depicted in Fig.1[15-16]. Hemicelluloses are extracted and removed from the wood chips during the prehydrolysis stage， the kraft cooking stage， and multi-stage bleaching process to reach the desired purity of dissolving pulp[9， 17].
Acidic prehydrolysis prior to alkaline cooking to selectively degrade hemicelluloses is necessary because alkaline pulping processes are not capable of selectively removing long-chain hemicelluloses. Prehydrolysis can be conducted using autohydrolysis （water/steam prehydrolysis process） and liquid hydrolysis with addition of acid （liquid prehydrolysis and concentrated acid hydrolysis） at the laboratory scale. However， prehydrolysis at the industrial scale is performed only via hot water or steam due to economic considerations[9，18-20]. In a liquid hydrolysis operation， pH adjustments can easily be made， and the process features flexibility of acid usage， lower temperature， and shorter time requirements. However， liquid hydrolysis generates large volumes of useless （at present） hydrolysate， which is associated with severe environmental and economic problems[9]. Hot water/steam hydrolysis is an autocatalytic process （or autohydrolysis）， in which the cleavage of acetyl and uronic acid substituents produces acetic and other organic acids. The organic acids derived from the hemicelluloses decrease the pH value of the liquor to 3～4， providing the acidity needed to catalyze depolymerization and removal of hemicelluloses from the wood chips[21-23].
Water prehydrolysis is conducted using hot water at elevated temperature （140～170℃） and pressure. However， the hot water prehydrolysis process is unpopular because of operational and technical problems[19]. Steam hydrolysis is a well-established method that has been used to hydrolyze/remove hemicelluloses prior to kraft pulping in commercial dissolving pulp production processes， such as the Visbatch and the continuous PHK dissolving pulp process of Shandong Sun Paper[9，19].
The PHK dissolving pulp process represents a promising platform for the IFBR concept. Prehydrolysis liquor （PHL）， which is the aqueous stream separated in the PHK-based dissolving pulp production process， contains hemicelluloses， lignin related products， and degradation products. Hemicelluloses （5wt%～6wt%） and acetic acid （approximately 1wt%） in the PHL can be recovered as valuable products instead of being directly combusted[24]. The hexose- and pentose-based carbohydrates that are decomposition products from hemicelluloses represent valuable feedstocks for further conversion to value-added products such as ethanol， furfural， and xylitol[25]. In addition， the acetic acid generated from hemicelluloses can be translated into extra profits for the industry[26-27]. The main reactions associated with the above-mentioned processes are listed in Fig.2.
2 Separation processes
The relatively low concentration of dissolved materials in the PHL hinders its practical application in various downstream conversion processes. The dissolved materials in the PHL should first be separated from the liquor. The following section will cover the various separation techniques that can be applied to the PHL， which are mostly converted from wastewater treatment or chemical engineering processes in various other industries.
2.1 Acidification
Lignin is only sparingly soluble in aqueous solutions at low pH value， making acidification a simple and effective method to recover lignin from the effluents of a PHK process. Generally， inorganic acids （e.g.， HCl and H2SO4） and flue gas containing CO2 and SO2 are used to recover lignin[28-29]. For kraft pulping liquor， over 90% of the total lignin can be recovered after the liquor pH value is decreased to approximately 2[29]. Liu et al acidified the PHL to a pH value of 2 using sulfuric acid and found that 47% of the total lignin in the PHL was precipitated[30]. The lower recovery rate of lignin from the PHL compared to that from the pulping liquor was possibly caused by the lower concentration of lignin and its higher solubility in the PHL. In addition， due to the complex composition of the PHL， subsequent separation processes should be introduced to recover other components. 2.2 Adsorption
Adsorption is typically performed using materials with highly porous structure and high specific surface area， such as activated carbon （CA）. The adsorption materials can be used to separate/concentrate dissolved organics in the PHL. In this system， the adsorbent is mixed with the PHL to adsorb lignocellulosic materials under various conditions. The absorbent is usually inert， thus can be recycled. The surface area， pore size， and functional groups of the adsorbent determine its performance towards various PHL components. The main advantage of this system is its flexibility for selectivity towards the adsorption of a desired lignocellulosic material contained in the PHL. When the AC was used for adsorbing lignocellulosic materials in the PHL， the final adsorption of hemicelluloses， lignin， and furfural were 400～700， 200～300， and 100 mg/g， respectively， indicating differential selectivity of the AC for the lignocellulosic materials in the PHL[31].
2.3 Coagulation/flocculation
Coagulation/flocculation methods have been applied in wastewater treatment systems for decades. The components of the PHL are generally anionic since they originate from hydroxyl/carboxylic groups associated with their backbones. By introducing cationic polymers such as chitosan[32]， poly（diallyldimethylammonium chloride）[33]， and polyethylene oxide[34] to the PHL system， these groups can associate with the cationic groups to form flocs， which can be subsequently removed by filtering. Saeed et al used cationic chitosan to recover furfural from industrially produced PHL， showing good selectivity of separation （over 50% of the furfural was removed）， whereas the recovery of monomeric sugars and acetic acid was rather limited[32].
2.4 Ion exchange resin treatment
Resin containing functional groups is a useful medium for the separation of specific components in the PHL. At the laboratory scale， a weak base anionic resin with tertiary amine functionality was applied for the selective removal of acetic acid. Shen et al used ion exchange resin treatment to recover acetic acid from a lignin eliminated PHL and showed that approximately 70% acetic acid could be removed. In addition， the resin could be regenerated and reused by the addition of sulfuric acid to desorb acetic acid attached to the resin[25].
2.5 Membrane filtration
Membrane filtration is an efficient and cost-effective technique that can be used to separate different sized compounds by molecular weight cut-offs. Membrane filtration （ultrafiltration， nanofiltration， and reverse osmosis） have been widely investigated to concentrate low-concentration solutes and has been shown to be suitable for the concentration and recovery of organic components in the PHL. Ahsan et al used nanofiltration and reverse osmosis to recover/concentrate hemicellulosic sugars and acetic acid from the PHL. The sugars were concentrated from 48 g/L to 227 g/L by nanofiltration and the acetic acid concentration increased from 10 g/L to 50 g/L after reverse osmosis[35]. However， since the membrane separation is solely based on molecular weight， the separation selectivity of organic components with similar molecular weights is low. In addition， fouling presents a technical challenge for practical industrial application. 2.6 Solvent extraction
Various solvents that can selectively separate the organic components in the PHL are used for the recovery of dissolved organics. For example， Yang et al demonstrated an efficient reactive extraction approach using triisooctylamine （TIOA） diluted with decanol to recover acetic acid[27]. According the literature， the recovery of acetic acid could reach as high as 66.6% using solvent extraction techniques. However， a large amount of organic solvent is generally required， and solvent regeneration is quite energy intensive， impairing the commercial potential of this process.
2.7 Integrated multi-step separation process
Efficient isolation of the complex organic components in the PHL usually requires an integrated multi-step separation process. Several studies have focused on the recovery/purification of furfural from PHL during the PHK process[24，36-37]. Shen et al reported a novel process to recover/concentrate acetic acid by combining AC adsorption， ion exchange resin treatment， and membrane concentration （Fig.3）[25]. Their results indicated that the removal of lignin in the PHL was achieved in the AC adsorption step， and the ion exchange resin treatment isolated the acetic acid. Finally， membrane filtration was used to obtain a concentrated hemicellulosic sugar liquor.
Overall， acidification is effective for precipitating lignin from kraft pulping liquor but limits the amount of lignin that can be isolated from the PHL. Adsorption techniques using AC， cationic polymers， and ion exchange resins are able to selectively remove components with high affinity towards the absorbents. However， desorption should be considered for the recovery of organic substances and regeneration of the absorbents. The membrane process can concentrate the components of the PHL but cannot separate components with similar molecular weight （e.g. low molecular weight lignin and xylooligosaccharides）. Although the solvent extraction process is suitable for separating some organic components in the PHL， the scaling-up of this technique is complicated by the need for large quantities of organic solvents. Integrated multi-step separation processes are effective for isolating different components in the PHL with high yields and purity. The feasibility of integrated processes highly depends on the specific end product values. Therefore， the development of more commercially-relevant combinatory technologies for the efficient recovery of dissolved organics in the PHL is urgently required. 3 Products
Hydrolysis is crucial for the kraft-based dissolving pulp production process. In this stage， a majority of hemicelluloses and a part of lignin components are dissolved from the wood chips， which facilitates the production of the dissolving pulp. The resulting PHL contains hemicelluloses， which can be utilized in the production of various value-added products as part of an IFBR strategy. Therefore， various value-added products will be produced after the separation process， which can be classified into three categories including xylan-based products， acetic acid， and lignin. The three main product streams will be discussed in the following section.
3.1 Xylan-based products
3.1.1 Xylose and xylan
Hemicellulose degraded sugars， especially xylose monomers and oligomers， are major components of the PHL. Xylose can be further converted to xylitol， and furfural. The xylooligosaccharide （XOS） can be directly used as a food additive for anti-obesity diets and in prebiotic foods. The annual production capacity of XOS in China is approximately 5000 tons which accounted for 20% for the global market based on the data from 2015[38]. The market demand for xylitol is increasing with the growing consciousness of preventive health care. The most common commercial method for the production of XOS is combined chemical-enzymatic hydrolysis of corn cob， cotton seed hull， and bagasse.
In the PHK process， the first order pseudo homogeneous kinetic model with an Arrhenius-type dependence on temperature is a commonly used model to predict the hydrolytic reactions of the hemicellulose fraction of lignocellulosic materials[39]. The hydrolysis of hemicelluloses by hydronium ion-catalyzed reaction is a multi-step process. Hemicelluloses are firstly hydrolyzed to form both high and low molecular weight XOS， and the produced oligomers are further hydrolyzed to xylose monomers. Chen et al determined the optimized conditions for this process （160℃， 60 min； 180℃， 20 min； 200℃， 5 min） with XOS yields up to 13.5% （w/w） of the initial dry biomass by autohydrolysis of Miscanthus x giganteus. The highest recovery rate of 47.9% （w/w） of the total XOS can be obtained by adding 10% AC and performing ethanol extraction[40].
3.1.2 Xylitol
Xylitol is a pentahydroxyl sugar alcohol which is naturally present in very small quantities in fruits and vegetables. Xylitol is commonly used as a pharmaceutical additive to prevent dental caries and as a food sweetener to replace of sugar. The annual production of xylitol in China exceeds 0.1 million tons， which is greater than 2/3 of the global annual production of xylitol[41]. Also， it is expected to increase by 7%～8% for the global xylitol production. The current commercial production process for xylitol is hydrogenation at 80～140℃ and 50 atm， which is an energy intensive process. Several laboratory scale trials focused on the biological conversion of xylose to xylitol by fermentation with various microbial and yeast strains. Biological conversion is preferable due to its advantage of mild reaction conditions. Due to the large amount of xylose in the PHL， the hydrolysates are considered potential sources for fermentation. However， the xylose concentration in the PHL is low for direct fermentation. At the industrial scale， xylose is usually concentrated by triple effect evaporation prior to its conversion to xylitol[42]. Alternatively， reverse osmosis can be used to concentrate the feed xylose. Reverse osmosis is considered to be preferable over evaporation for concentrating sugars as it prevents scorification and requires less energy[42]. In addition， the hydrolysates are always accompanied by lignin degradation products， furfural， and acetic acid which are inhibitors of microbial fermentation[43-45]. Therefore， purification must be integrated into the recovery process of xylose for subsequent use in fermentation. Various microbial and yeast strains such as D. hansenii， Candida magnoliae， C. guillermondii， and C. tropicalis have been previously used in the fermentation process[46-49].
3.1.3 Furfural
Furfural is an important chemical intermediate derived from the acid hydrolysis of pentoses （xylose and arabinose）. Furfural is considered to be a key green chemical produced from the lignocellulosic feedstock of a biorefinery. Furfural and its derivatives represent strategic platform chemicals due to their large number of applications， and are likely to experience growing demand in various fields. Furfural is a feedstock for the synthesis of resins， plastics， and stabilizers due to its beneficial properties such as corrosion resistance， physical strength， and thermosetting properties[50].
The annual production capacity of furfural reached 0.5 million tons in China in 2015. Furfural is commercially produced from agricultural lignocellulosic materials under acidic conditions involving acid hydrolysis and dehydration reactions. Furfural has been demonstrated to be a potential product from C-5 sugars present in industrial PHL. Mineral acid （H2SO4 or HCl） hydrolysis processes using corn cob is known to be toxic， corrosive to equipment， and the generated liquid waste is difficult to treat[42]. A high yield （80%） of furfural was obtained using hemicelluloses from lignocellulosic materials by applying solid acid catalysts （H-mordenite） with g-valerolactone （GVL） as the solvent[51]. 3.1.4 Other compounds
Xylan-based PHL can also serve as a potential raw material for the production of other value-added chemicals and materials. Bustos et al proposed a method of utilizing the hemicelluloses in the hydrolysis liquor to produce lactic acid by simultaneous saccharification and fermentation with cellulases and L. rhamnosus[52-53]. Hemicelluloses from the PHL can be used as papermaking additive as a substitute for cationic starch to achieve strength-enhancement or for retention purpose. Their use resulted in high brightness after the semi-bleaching process （83%ISO）， and a charge density of 0.24 mmol/g under optimized conditions using a cascade treatment including acidification， ethanol precipitation， and cationization[54]. In addition， the hemicelluloses precipitated from the PHL can be used to increase the basis weight of high-yield pulp （HYP） paper products after bleaching （D0EpD1） at a dosage as high as 50 mg/g without affecting mechanical properties[55]. The hemicelluloses， derivatives， and polyphenols from the PHL of eucalyptus wood can be used as bio-reductant for the reduction and stabilization of Ag+ to form Ag nanoparticles[56]. As previously reported， freshly collected PHL was filtered using a 0.22 mm membrane to remove impurities and then diluted by a factor of 10 before being used as bio-reductant. With a dosage of 10 mL of prepared bio-reductant in 50 mL of 1.0 mg AgNO3 solution， Ag nanoparticles were obtained at a purity of 82.1wt%， demonstrating the feasibility of using xylan-based PHL as an effective bio-reductant[53].
3.2 Acetic acid
Acetic acid is an important potential product in the PHL of the kraft-based hardwood dissolving pulp， accounting for approximately 1wt% of the total PHL[21]. Acetic acid in the PHL is generated from acetyl groups of the hemicelluloses[57]， and can be used as a feedstock to produce terephthalic acid （PTA）， vinyl acetate， acetate ester， and acetic anhydride. Therefore， the recovery of acetic acid from the PHL instead of production through methanol carbonylation using non-renewable feedstock fits well into the IFBR concept[58].
Acetic acid is a significant component in the PHL[36， 59]， but it is known to be inhibitory during sugar fermentation， so the removal of acetic acid from the PHL is critical for further sugar valorization[60-61]. Recently， several studies have reported the separation of acetic acid from the PHL stream[25-27，35]. Ahsan et al investigated the feasibility of recovering and concentrating acetic acid from the PHL of a PHK dissolving pulp process using a combination of AC adsorption， nanofiltration， and reverse osmosis[35]. It was reported that 68% of the acetic acid in the PHL could be recovered and a final concentration of 50 g/L was achieved. Simultaneously， the sugars in the PHL were concentrated and recovered at 227 g/L during the developed process. Yang et al demonstrated an efficient reactive extraction approach using triisooctylamine （TIOA） diluted with decanol to recover acetic acid at a rate of 66.6% in 10 min without affecting the hemicelluloses in the PHL[27]. In addition， Shen et al reported a novel process to recover/concentrate acetic acid by combining AC adsorption （1 g AC to 30 g PHL）， ion exchange resin treatment （10 g PHL to 1 g resin）， and membrane filtration. The ion exchange resin treatment exhibited good selectivity for acetic acid recovery， with the recovery rate of acetic acid reaching as high as 70%[25]. 3.3 Lignin
Lignin is another main component of lignocellulosic biomass， besides cellulose and hemicelluloses， and should be essentially eliminated by the PHK process to produce dissolving pulp. In the PHK process， lignin is isolated from the wood chips in two sections， the prehydrolysis and kraft pulping sections. In the prehydrolysis section， lignin with higher contents of hydrophilic functional groups is removed from the wood chips， and in the following kraft pulping process， the majority of the lignin in the wood chips is eliminated.
Traditionally， lignin dissolved in the hydrolysis liquor and kraft pulping liquor is burned as a fuel source in the chemical recovery of dissolving pulp production. However， the low concentrations of the dissolved lignocellulosic materials in the PHL render this process less economically attractive. From a biorefinery perspective， lignin should be recovered and further applied to produce value-added products during the commercial operation of PHK processes.
Efforts have been made to find alternative methods of recovering dissolved lignin from the PHL and kraft pulping black liquor. For the lignin obtained from the kraft pulping process， acid precipitation is feasible for lignin recovery and purification since most impurities are soluble and remain in the supernatant[62]. However， disposal of the generated salt solutions should be considered if this purification process is adopted in a pulping mill.
For the recovery of lignin from the PHL， since the dissolved lignin is more hydrophilic and its concentration is lower， only approximately 50% of the total lignin can be separated via adjusting pH value to 2.0[30]. The remaining lignin in the acidified PHL can be further removed by adsorption and flocculation. Fig.4 shows a process flow diagram for separating lignin from the PHL. Stream 1 represents the possibility of using polymers （e.g.， poly ethylene oxide （PEO） and poly aluminum chloride （PAC）） to form complexes with lignin， facilitating lignin flocculation and precipitation. The PEO/PAC/lignin precipitates can be used as wet end additives （retention aids） for papermaking. Stream 2 depicts the possibility for precipitating lignin via solvent extraction using ethyl acetate （EC） or ether. The precipitated lignin via acidification and solvent extraction is very pure， and can be further utilized in different processes for the production of various value-added products. However， the solvent should first be recovered from the PHL， which can then be used in downstream biorefinery processes. Recently， due to the popularization of alkali-tolerant ceramic membranes， ultrafiltration has become a widely accepted method for separating lignin from the black liquor of kraft pulping[63]. It is a pressure-driven technique that purifies lignin from extracts by retaining lignin macromolecules and allowing inorganic salts added during the pulping process to pass through. Similarly， ultrafiltration can be adopted for separating lignin dissolved in the PHL. The advantage of ultrafiltration is that pH value and temperature adjustments are not required， and the molecular weight of the products can be controlled. However， since the molecular weight of lignin is similar to those of other dissolved organics in the PHL， the selectivity of ultrafiltration for removing lignin is low， and the process is costly and slow.
With increasing demand for cost-effective biorefinery processes， the development of lignin valorization， in particular for high-value products， has become an important research focus[64]. The native structure of lignin facilitates a wide range of applications as a phenol substitute for synthetic resins， dispersants for dyes and pesticides， and a binder for feeds and fertilizers. Moreover， other value-added products such as carbon fibers[65] as well as biomedical and smart materials[6]， are being developed. Recently， a number of reviews focusing on the applications of lignin and lignin-derived chemicals in a wide variety of fields have been published[4-6， 66-69]， reflecting the significant attention this topic is generating. Generally， the major research areas with respect to lignin applications could be classified into five categories： blocks/additives for polymers[5， 68]， adsorbents for wastewater treatment [70]， depolymerized fine chemicals/liquid fuels[4]， precursors for carbon materials[71]， and biomedical applications[69].
Although tremendous effort has been devoted to the development of lignin applications， the practical utilization of lignin remains unsatisfactory in the pulping industry because of the structural heterogeneity of lignin[72]. The heterogeneity primarily results from the free radical polymerization of three different monomers in the biosynthesis of lignin and the complex reactions that occur during its isolation[72]. In the PHK process， the reactions of lignin are also complicated since lignin depolymerization and repolymerization can occur during both the prehydrolysis and kraft pulping sections. The broad molecular weight distributions of lignin have a significant impact on the functional group contents and reactivity characteristics of lignin and， to a large extent， on the performance of lignin for subsequent applications. Therefore， in-depth study regarding the possible effects of lignin heterogeneity on its applications is highly desirable. Another limitation for lignin application is the lack of processes available to convert lignin into a single material in high yields[62]. The current market for lignin chemicals is rather limited and lignin should be used in considerable quantities so the biorefinery can be successfully implemented. 4 Summary
The PHK dissolving pulp production process has emerged as an important commercial process for the production of dissolving pulp， which has undergone significant growth in the past 10 years. Technological developments for recovering and utilizing dissolved organics in the PHL fit well with the IFBR concept. The potential value-added products produced from PHL， such as xylooligosaccharides， xylitol， acetic acid， and furfural， can provide additional revenue streams for PHK dissolving pulp mills， enhancing their competitiveness. The technologies discussed in this review represent a good starting point for future commercial exploitation of the dissolved organics in the PHL. Further research is required， which may include： ① case studies of specific mills targeting particular products， ② pilot/mill trials， and ③ detailed process design and economic analyses.
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