Influence of pretreatment conditions on composition of liquid hydrolysate and subsequent enzymatic saccharification of remaining solids

In the present work, the effect of dilute acid pretreatment was studied on the composition of liquid hydrolysate obtained after pretreatment by employing different combinations of process variables (temperature, time and acid concentration). The effect of pretreatment was also studied on subsequent enzymatic saccharification of remaining solids to obtain maximum yield of sugars. The efficiency of pretreatment was measured in terms of high‐xylose and low‐glucose yields, which was found most suitable at pretreatment conditions of 120°C, 120 min and 2% (v/v) acid concentration. With increased severity of pretreatment, xylose yield decreased with concomitant increase in glucose yield. The decrease in xylose yield was attributed to conversion into degradation products such as 5‐hydroxylmethyl furfural (HMF) and acetic acid. The percentage of enzymatic saccharification increased with increased pretreatment severity. Saccharification of biomass pretreated at 180°C, 7 min and 0.5% (v/v) acid concentration produced the maximum glucose yield of saccharification of 352 g/kg dry matter, compared to just 97 g glucose/kg dry matter in the case of untreated biomass. The same pretreatment conditions resulted in maximum total sugar yield of pretreatment and saccharification of 459 g/kg dry matter, which was more than 67% of the total potential sugars in biomass. © 2012 Canadian Society for Chemical Engineering     

Low-Temperature Dilute Acid Hydrolysis of Oil Palm Frond

Oil palm frond (OPF) fiber, a lignocellulosic waste from the palm oil industry, contains high cellulose and hemicellulose content, thus it is a potential feedstock for simple sugars production. This paper describes the two-stage hydrolysis process focusing on the use of low-temperature dilute acid hydrolysis to convert the hemicellulose in OPF fiber to simple sugars (xylose, arabinose, and glucose). The objective of the present study was to evaluate the effect of operating conditions of dilute sulfuric acid hydrolysis undertaken in a 1 L self-built batch reactor on xylose production from OPF fiber. The reaction conditions were temperatures (100–140°C), acid concentrations (2–6%), and reaction times (30–240 min). The mass ratio of solid/liquid was kept at 1:30. Analysis of the three main sugars glucose, xylose, and arabinose were determined using high-pressure liquid chromatography. The optimum reaction temperature, reaction time, and acid concentration were found to be 120°C, 120 min, and 2% acid, respectively. Based on the potential amount of xylose (10.8 mg/mL), 94% conversion (10.15 mg/mL) was obtained under the optimum conditions with small amount of furfural (0.016 mg/mL). To enhance the effectiveness of dilute acid hydrolysis, the hydrolysis of OPF fiber was also performed using ultrasonic-pretreated OPF fiber. The effects of ultrasonic parameters power (40–80%) and ultrasonication times (20–60 min) were determined on sugar yields under optimum hydrolysis conditions (2% acid sulfuric, 120°C and 120 min). However, the use of ultrasonication was found to have detrimental effect on the yield of simple sugars due to the 10-fold increase in the formation of furfural.     

有机酸对甜高粱渣亚临界水解产物的影响

半纤维素的高效转化是提高甜高粱渣原料全组分利用的关键技术之一。采用亚临界水热预处理方法,并将强度因子R₀引入研究过程,考察了不同温度（160~200℃）和反应时间（10~60 min）对甜高粱渣水解反应的影响。在这基础上,进一步考察了多种有机酸（乳酸、醋酸及乳酸+醋酸）对亚临界水解效果的影响。实验表明,当强度因子lgR₀=3.96（180℃,40 min）时,采用不外加酸的亚临界水热预处理工艺得到的最大木糖浓度为4.79 g·L^-1;有机酸的加入可强化水解反应,提高木糖浓度;与单一乳酸或醋酸处理方法相比,加入同浓度的乳酸+醋酸既可以促进半纤维素水解,又可以抑制副产物生成;在温度180℃,时间40 min,乳酸+醋酸（乳酸：醋酸=6：4）的浓度1%（质量）的条件下,木糖浓度为7.92 g·L^-1。     

A kinetic study of xylose recovery from a hemicellulose-rich biomass for xylitol fermentative production

Hemicellulose in the complex structure of lignocellulosic substances is mainly composed of xylan which is a polymer based on monosaccharide xylose. Using acidic or enzymatic hydrolysis, hemicellulose can be depolymerized into its constituent monomer. The kinetics of hemicellulose depolymerization and decomposition in oat hull was investigated under moderate pressures with catalyst (H₂SO₄) concentration up to 0.55?N and temperatures of up to 130?°C for a total residence time of 150?min. Different trends of recovery or generation and kinetic mechanisms obtained for the components in the hydrolysate which could be described by different kinetic models, that is, a single-phase kinetic mechanism with product decomposition (two-step sequential reaction) could describe xylose generation. However, generation of arabinose, furfural, and acetic acid followed a single-phase mechanism with no decomposition (one-step reaction). Generation of glucose in the hydrolysate followed a biphasic mechanism due to the fast- and slow-releasing fractions into the liquid phase which was apparently with no decomposition. A pentose recovery of almost 80% was achieved under optimal conditions. Parameters of xylitol bioproduction indicated that a xylitol/xylose conversion yield of 0.80?g/g is achievable from the concentrated hydrolysate with no complementary treatment proving its low toxicity compared to other hemicellulose resources.     

Design of process parameters for the production of xylose from wood sawdust

Meranti wood sawdust (MWS) is a cheap and widely available lignocellulosic biomass, which can be a potential source of xylose. This xylose can be an economic raw material for the production of a wide variety of specialty chemicals, mainly xylitol. It is particularly important to establish rapid hydrolysis conditions, which can yield xylose-rich hydrolysate that do not require further treatment. The aim of this research was to study the effect of residence time, temperature, acid concentration, and liquid to solid ratio (LSR) on the formation of xylose and byproducts. Batch hydrolysis was performed using different levels of residence time (10–120 min), temperature (105–130 °C), H₂SO₄ concentration (2–12%), and LSR (8–20 g/g). One-factor-at-a-time (OFAT) method was followed to select the optimum level of parameters. The residence time, temperature, and acid concentration were found to be the major factors affecting xylose production with the effective level of 60 min, 125 °C, and 4%, respectively. In these conditions, the xylose concentration was 17.9 g/l, corresponding to a yield of above 86% of the potential concentration.     

Delignification of intact biomass and cellulosic coproduct of acid‐catalyzed hydrolysis

The kinetics of acid‐catalyzed hemicellulose removal and also alkaline delignification of oat hull biomass were investigated. All three operational parameters namely, catalyst concentration (0.10–0.55 N H₂SO₄), temperature (110–130°C), and residence time (up to 150 min) affected the efficiency of hemicellulose removal, with 100% of hemicellulose removed by appropriate selection of process parameters. Analysis of delignification kinetics (in the temperature range of 30–100°C) indicated that it can be expressed very well by a two‐phase model for the crude biomass and also for the hemicellulose‐prehydrolyzed material. The application of acid‐catalyzed prehydrolysis improved the capacity of lignin dissolution especially at lower temperatures (30 and 65°C) and accelerated the dissolution of lignin. This acceleration of delignification by prehydrolysis was possible at all levels of temperature in the bulk phase; however, results were more significant at the lower temperatures in the terminal phase. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1783–1791, 2015     

Fractionation of barley straw with dilute sulfuric acid for improving hemicellulose recovery

Dilute acid fractionation of barley straw improves dissolving hemicellulose fraction of the straw, while leaving the cellulose more reactive and accessible to enzyme as a strategy of pretreatment. To characterize the fractionation process, the effects of the acid concentration, temperature and reaction time on the hemicellulose removal as well as on the formation of by-products (furfural, 5-hydroxymethylfurfural and acetic acid) were investigated. The optimum fractionation conditions of barley straw were 1% (w/v) concentration of sulfuric acid, 158 °C of reaction temperature and 15 min of reaction time. Under the optimum conditions, 87% of xylan was hydrolyzed and recovered in liquid hydrolyzate, which was 7% higher than that of the predicted yield. The hydrolyzate contained glucose 2.44 g/L, arabinose 1.70 g/L, xylose 13.41 g/L, acetic acid 1.55 g/L, levulinic acid 0.03 g/L, 5-HMF 0.03 g/L and furfural 0.75 g/L.     

Modeling and optimization of dilute nitric acid hydrolysis on corn stover

BACKGROUND: Because of its high cost, nitric acid has not been widely employed as the catalyst for hydrolysis of lignocellulosic biomass to obtain fermentable sugars. However, recently more and more research results have reported that nitric acid was more effective than other acids for the hydrolysis of lignocellulose. Therefore, it is necessary to find an optimum condition for nitric acid pretreatment and a means of reducing the cost. RESULTS: In this work, low concentrations of nitric acid and short reaction times were considered to optimize the pretreatment process. The kinetic parameters of models to predict the concentrations of xylose, glucose, arabinose, acetic acid and furfural in the hydrolysates were obtained. Applying the kinetic models, the optimum conditions were: 150 °C, 0.6% HNO₃ and 1 min, which yielded a solution containing up to 22.01 g L^?1 xylose, 1.91 g L^?1 glucose, 2.90 g L^?1 arabinose, 2.42 g L^?1 acetic acid and 0.21 g L^?1 furfural, which were consistent with the predicted values. The influence of temperature was also studied using the Arrhenius equation. CONCLUSIONS: A combination of experimental data and model analysis suggested that 96% xylose yield can be achieved by using low concentration nitric acid for a short reaction time, which could greatly reduce the pretreatment cost. Therefore, dilute nitric acid could be considered a good choice for the hydrolysis of corn stover. Copyright © 2010 Society of Chemical Industry     

EFFECT OF PRETREATMENT CONDITIONS ON STRUCTURAL CHARACTERISTICS OF WHEAT STRAW

Structural characteristics of lignocellulosic biomass such as surface area, pore volume, crystallinity, hemicellulose, and lignin content significantly affect the yield of fermentable sugars for bioethanol production. In the present work, the effect of dilute acid pretreatment was studied on structural characteristics of wheat straw, using different combinations of process variables (temperature, time, and acid concentration). Pretreated wheat straw (PWS) exhibited higher available surface area and pore volume along with low hemicellulose and lignin content. Crystallinity index (CrI) of biomass at different pretreatment conditions showed an increased trend followed by sharp decrease at high temperature (190°C) conditions. Maximum increase in surface area (7.1 m²/g compared to 4.0 m²/g for untreated wheat straw) was obtained at pretreatment conditions of 180°C temperature, 0.5% (v/v) acid, and 7 min time. SEM imaging of biomass revealed that pore breaking, compression of pores, and partial pore blocking in the case of high temperature (190°C) pretreatment conditions may be the reason behind decreased surface area of biomass. FT-IR analysis showed almost complete hemicellulose removal and acid-soluble lignin removal after dilute acid pretreatment but insufficient removal of acid insoluble lignin.

[Supplementary material is available for this article. Go to the publisher's online edition of Chemical Engineering Communications for the following free supplemental resource: figure showing XRD pattern of biomass with respect to different pretreatment conditions.]     

高温液态水中甜高粱渣半纤维素水解及其机理

为了回收甜高粱渣中的半纤维素衍生糖并了解其水解机理,在自行设计的Flowthrough反应器中对甜高粱渣进行了高温液态水水解,分别考察了不同反应温度和反应液流量下水解液中产物的生成情况。研究表明,相对葡萄糖和阿拉伯糖而言,木糖的生成受反应温度和反应液流量影响更大。温度高于195 ℃时糖降解加剧,总木糖浓度不断降低;低流量（5 ml?min-1）条件下生成的木糖不能被及时排出而进一步降解。通过产物分析可知,甜高粱渣半纤维素中含有典型的O-乙酰基-4-O-甲基葡萄糖醛酸基阿拉伯糖基木聚糖结构,木糖、葡萄糖、阿拉伯糖、各种低聚糖、乙酸和葡萄糖醛酸等是半纤维素水解的直接产物,糠醛和5-羟甲基糠醛等是糖类的降解产物, 甲酸等小分子酸是它们的进一步降解产物。     

Applicability of sub- and supercritical water hydrolysis of woody biomass to produce monomeric sugars for cellulosic bioethanol fermentation

In this study two woody biomasses, poplar and pitch pine wood, were treated with sub- and supercritical water (SCW) at temperature of 325–425 °C, at pressure of 220 ± 10 atm and residence time of 60 s, respectively, to develop a time saving and efficient conversion process for the production of fermentable sugars from woody biomasses using supercritical water system. Cellulose/hemicellulose was easily hydrolyzed during SCW treatment into monomeric sugars with the total yield of 7.3% and 8.2% based on the oven dried weight of poplar and pitch pine, respectively. Total yield of the monomeric sugars was increased about threefolds to 23.0% and 25.1% in the presence of 0.05% of hydrochloric acid. Model experiment confirmed that glucose and xylose were readily converted into low molecular weight compounds during SCW hydrolysis. According to GC/MS analysis main compounds converted from glucose and xylose by SCW were identified to 5-hydroxymethyl furfural and 4-oxo-5-methoxy-2-penten-5-olide, respectively.     

Pretreatment of lignocellulosic biomass by autohydrolysis and aqueous ammonia percolation

A two-stage biomass pretreatment process-a combination of autohydrolysis and aqueous ammonia percolation-was experimentally studied as a method to remove and recover hemicellulose from lignocellulosic biomass. Hemicellulose was completely separated from the biomass after 1 hr of autohydrolysis at 200‡C. As reaction temperature and/or time of autohydrolysis was increased in the range of 170-200‡C and 1–2.5 hr, respectively, the amount of hemicellulose solubilization was increased ; however, more sugars were decomposed. Most of the extracted hemicellulose was recovered as xylose oligomer. Hemicellulose was found to inhibit the enzymatic hydrolysis of cellulose. When the biomass was consecutively pretreated with pure water at 180‡C for 30 min and with 10 wt% ammonia solution at 180‡C for 30 min, about 62% of the hemicellulose was extracted. The enzymatic digestibility of the pretreated biomass was as high as 95 %.     

Design of a continuous process for the alkaline treatment of xylose into lactic acid

A compact process based on a gas/liquid contactor was designed for the continuous alkaline oxidation of xylose into lactic acid. An experimental design using Doehlert's method was used to study the effect of: i) the NaOH concentration, ii) the xylose concentration, iii) the reaction temperature, and iv) the residence time, on the yield of lactic acid. Results indicate that the conversion of xylose at concentrations between 5-15% (w/w) is complete and nearly instantaneous at both high temperatures (over 200°C) and high sodium hydroxide concentrations ((> 10% w/w). Yields of lactic acid reaching 50% (with a theoretical yield of 60% (w/w)) were obtained under the most favorable experimental condition suggested by the model developed.     

Effects of dilute acid pretreatment on enzyme saccharification of wheat stubble

BACKGROUND: Prehydrolysis of wheat stubble using moderate temperatures and dilute acid strength is an effective means for solubilizing hemicellulose fractions and improving cellulose hydrolysis. Variation in prehydrolysis parameters (temperature, time, and acid strength) and enzymatic saccharification conditions were examined for conversion of wheat stubble into fermentable sugars. RESULTS: Elevating temperature and acid strength maximized sugar release in prehydrolysate liquors. The optimal conditions of 2.0% H₂SO₄/60 min/121 °C effectively solubilized 79% of the available hemicellulose. Production of inhibitory hydrolysis and degradation products such as acetic acid and levulinic acid, were detected at levels of 3.4 g L^?1 and 0.64 g L^?1, respectively. Sugar yields in prehydrolysate and saccharified liquors were found to increase with treatment severity. Temperature had the greatest impact on sugar release, followed by acid concentration and time. Optimizing prehydrolysis conditions at 1.0% H₂SO₄/90 min/121 °C, produced a 3.2‐fold improvement in cellulose hydrolysis with recoveries approaching 82%. The addition of β‐glucosidase and xylanase to the cellulase preparations assisted monomeric sugar release. CONCLUSION: Although treatment conditions for hemicellulose and cellulose hydrolysis differ, the study's findings suggest a good degree of overlap and process flexibility which should permit high recovery of pentose and hexose sugars. Copyright © 2011 Society of Chemical Industry     

Production of xylose,furfural, fermentable sugars and ethanol from agricultural residues

With the developing shortage of petroleum, reliance on biomass as a source of chemicals and fuels will increase. In the present work, bagasse and rice husk were subjected to dilute acid (H₂SO₄) hydrolysis using pressurised water to obtain furfural and fermentable sugars. Various process conditions such as particle size, solid-liquid ratio, acid concentration, reaction time and temperature have been studied to optimise yields of furfural, xylose and other fermentable sugars. The use of particle sizes smaller than 495 μm did not further increase the yield of reducing sugars. A solid-liquid ratio of 1:15 was found to be the most suitable for production of reducing sugars. Hydrolysis using 0.4% H₂SO₄ at 453 K resulted in selective yields (g per 100 g of dried agricultural residues) of xylose from bagasse (22.5%) and rice husk (21.5%). A maximum yield of furfural was obtained using 0.4% H₂SO₄ at 473 K from bagasse (11.5%) and rice husk (10.9%). It was also found that hydrolysis using 1% H₂SO₄ at 493 K resulted in maximum yields of total reducing sugar from bagasse (53.5%) and rice husk (50%). The reducing sugars obtained were fermented to ethanol after removal of furfural. The effect of furfural on the fermentation of sugars to ethanol was also studied. Based on these studies, an integrated two-step process for the production of furfural and fermentable sugars could be envisaged. In the first step, using 0.4% H₂SO₄ at 473 K, furfural could be obtained, while in the second step, the use of 1% H₂SO₄ at 493 K should result in the production of fermentable sugars.     

Pretreatment of <Emphasis Type="Italic">Helianthus tuberosus</Emphasis> residue by flow-through process for production of fermentable sugar

The pretreatment of Helianthus tuberosus residue was studied for fermentable sugar production. The pretreatment was performed by varying the temperature, type of chemical solution, and concentration. Two different catalytic pretreatments using sulfuric acid and aqueous ammonia were operated and compared in a flow-through column reactor system. The flow-through process was required to increase the sugar production yield of biomass. To selectively remove the lignin of biomass and achieve fractionation of hemicellulose in the liquid phase to produce pentose, the flow-through process could be controlled by the pretreatment conditions. Furthermore, the remaining solid underwent enzymatic hydrolysis for hexose production. The mass balances of biomass pretreated with aqueous ammonia and sulfuric acid solution were compared in terms of production of fermentable sugars. The glucose recovery compared to the initial biomass was 71.2% in the pretreatment using aqueous ammonia at 170 °C, and pretreatment using sulfuric acid solution at 150 °C was 52.3%.     

Kinetic studies of hemicellulose hydrolysis of corn stover at atmospheric pressure

The object of this work was to study the xylose production by hydrolysis of corn stover with diluted sulfuric acid at 100 °C. Several concentrations of H₂SO₄ (2%, 4% and 6% w/w) and reaction time (0–300 min) were evaluated. Kinetic parameters of mathematical models for predicting the concentrations of xylose, glucose and furfural in the hydrolysates were found. Optimal conditions for hydrolysis were 5.5% H₂SO₄ at 100 °C for 60 min; under these conditions, 86.7% of xylose yield and 2.82 g/g selectivity were attained, leading to liquors containing up to 18.73 g/l xylose, 6.64 g/l glucose and 0.63 g/l furfural. The models could be successfully used to predict the concentrations of xylose, glucose and furfural within 0–300 min under experimental acid concentrations. Furthermore, the hydrolysis process of corn stover using dilute acid could be conceived as the first stage of an integrated strategy for corn stover utilization.     

Optimization of formic acid hydrolysis of corn cob in xylose production

Dilute acid pretreatment of lignocellulosic material is one of the significant steps in a biorefinery. We used response surface methodology to determine the important factors of formic acid concentration (2%–6% wt%), treatment time (30–150 min), reaction temperature (120–160 °C), and liquid to solid ratio (3–11 mL/g) on dilute acid hydrolysis of corn cob to produce xylose. A xylose yield of 81.6% and selectivity of 15.1 g/g were achieved under the optimal conditions (5% acid concentration, 150 min, 135 °C, and 7 mL/g liquid to solid ratio). The addition of trivalent salts (FeCl₃, Fe(NO₃)3, and Fe₂(SO₄)₃) to the reaction system enhanced the xylose yield but decreased selectivity. The FeCl₃ concentration over 0.75 mol/L had a negative effect on xylose production.     

草酸水解甜高粱秸秆渣的Saeman动力学模型

利用草酸作为催化剂水解甜高粱秸秆渣制备木糖,测定了不同温度下的木糖收率和副产物糠醛产量;依据半纤维素水解的Saeman模型,计算得到了木聚糖水解和木糖降解的动力学数据,其活化能分别为5.89×104,1.38×104J/mol。分析结果表明:木聚糖水解反应速度快,但是生成的木糖容易发生降解;模型最优化反应条件为125℃和77min,实验得到的木糖收率为52.11%。草酸作为一种有机酸,能够用于催化半纤维素水解制备木糖,副产物糠醛的产率较低。     

Enzymatic Hydrolysis of Corn Stover after Pretreatment with Dilute Sulfuric Acid

Although many previous studies have been carried on the enzymatic hydrolysis of corn stover after pretreatment with dilute sulfuric acid, this paper emphasizes the use of different conditions to attain the highest yields of two sugars, xylose and glucose, from both stages. The pretreatment was performed at a range of sulfuric acid concentrations of 2, 4 and 6 % at 80, 100 and 120 °C. Up to 77 % xylose yield was obtained while the glucose yield was only 8.4 %. The corresponding solid phase was hydrolyzed by cellulase and the influences of five factors and their interactions on enzyme hydrolysis were evaluated by response surface methodology based on one‐factor‐at‐a‐time experiments. The optimal levels for each variable to obtain the highest reducing sugar yield were as follows: enzyme concentration of 22 FPU/g substrate, substrate concentration of 77 g/L, temperature of 49 °C, pH 4.8 and reaction time of 38 h. A reducing sugar yield of 42.11 g/100 g substrate was achieved, which was consistent with the predicted value of 42.13 g/100 g substrate. Compared with the one‐factor‐at‐a‐time experiments, there was a 9.4 % increase in reducing sugar yield when the enzyme concentration was decreased to 3 FPU/g substrate, the substrate concentration increased to 17 g/L and the reaction time dropped to 22 h. The total sugar released from the two stages was 62.81 g/100 g substrate.