Modified percolation process in dilute-acid hydrolysis of biphasic hemicellulose

Applied Biochemistry and Biotechnology - To improve the performance of a percolation reactor in dilute-acid pretreatment of biomass containing biphasic hemicellulose, a reactor simulation study was...     

Two-stage dilute-acid pretreatment of softwoods

Whole treechips obtained from softwood forest thinnings were pretreated via single-and two-stage dilute-sulfuric acid pretreatment. Whole-tree chips were impregnated with dilute sulfuric acid and steam treated in a 4-L steam explosion reactor. In single-stage pretreatment, wood chips were treated using a wide range of severity. In two-stage pretreatment, the first stage was carried out at low severity tomaximize hemicellulose recovery. Solubilized sugars were recovered from the first-stage prehydrolysate by washing with water. In the second stage, water-insoluble solids from first-stage prehydrolysate were impregnated with dilute sulfuric acid, then steam treated at more severe conditions to hydrolyze a portion of the remaining cellulose to glucose and to improve the enzyme digestibility. The total sugar yields obtained after enzymatic hydrolysis of two-stage dilute acid-pretreated samples were compared with sugar yields from single-stage pretreatment. The overall sugar yield from two-stage dilute-acid pretreatment was approx 10% higher, and the net enzyme requirement was reduced by about 50%. Simultaneous saccharification and fermentation using an adapted Saccharomyces cerevisiae yeast strain further improved cellulose conversion yield and lowered the enzyme requirement.     

Dilute-acid pretreatment of corn stover using a high-solids percolation reactor

Pretreatment of corn stover by dilute sulfuric acid was investigated using a laboratory percolation (flowthrough) reactor operated under high-solids conditions. The effects of reaction conditions and operating parameters on the performance of the percolation reactor were investigated seeking the optimal range in which acceptable levels of yield and sugar concentration could be attained. It was demonstrated that 70–75% recovery of xylose and 6 to 7% (w/w) xylose concentration were attainable. The high sugar concentration was obtained as a result of dense packing of dry corn stover and the low liquid throughput. Xylose was mostly unreacted, rather than decomposed. The cellulose and the unreacted xylan of treated corn stover were both effectively hydrolyzed by a “cellulase” enzyme preparation that also exhibits some activity on xylan. The xylose yield was affected significantly by the flow rate under the same reaction time and conditions. This behavior appears to be related to sugar decomposition, mass transfer resistance, and the fact that acid is neutralized by the buffering components of the biomass.     

A process economic approach to develop a dilute-acid cellulose hydrolysis process to produce ethanol from biomass

Successful deployment of a bioethanol process depends on the integration of technologies that can be economically commercialized. Pretreatment and fermentation operations of the traditional enzymatic bioethanol-production process constitute the largest portion of the capital and operating costs. Cost reduction in these areas, through improved reactions and reduced capital, will improve the economic feasibility of a large-scale plant. A technoeconomic model was developed using the ASPEN Plus^TN modeling software package. This model in cluded a two-stage pretreatment operation with a co-current first stage and countercurrent second stage, a lignin adsorption unit, and a cofermentation unit. Data from kinetic modeling of the pretreatment reactions, verified by bench-scale experiments, were used to create the ASPEN Plus base model. Results from the initial pretreatment and fermentation yields of the two-stage system correlated well to the performance targets established by the model. The ASPEN Plus model determined mass and energy-balance information, which was supplied, to an economic module to determine the required selling price of the ethanol. Several pretreatment process variables such as glucose yield, liquid: solid ratio, additional pretreatment stages, and lignin adsorption were varied to determine which parameters had the greatest effect on the process economics. Optimized values for these key variables became target values for the bench-scale research, either to achieve oridentify as potential obstacles in the future commercialization process. Results from this modeling and experimentation sequence have led to the design of an advanced two-stage engineering-scale reactor for a dilute-acid hydrolysis process.     

Optimization of dilute-acid pretreatment of corn stover using a high-solids percolation reactor

We have previously demonstrated that pretreatment of corn stover with dilute sulfuric acid can achieve high digestibility and efficient recovery of hemicellulose sugars with high yield and concentration. Further improvement of this process was sought in this work. A modification was made in the operation of the percolation reactor that the reactor is preheated under atmospheric pressure to remove moisture that causes autohydrolysis. This eliminated sugar decomposition during the preheating stage and led to a considerable improvement in overall sugar yield. In addition, liquid throughput was minimized to the extent that only one reactor void volume of liquid was collected. This was done to attain a high xylose concentration in the hydrolyzate. The optimum reaction and operating conditions were identified wherein near quantitative enzymatic digestibilities are obtained with enzyme loading of 15 FPU/g glucan. With a reduced enzyme loading of 5 FPU/g glucan, the enzymatic digestibility was decreased, but still reached a level of 92%. Decomposition of carbohydrates was extremely low as indicated by the measured glucan and xylan mass closures (recovered sugar plus unreacted) which were 98% and 94%, respectively. The data obtained in this work indicate that the digestibility is related to the extent of xylan removal.     

Xylan hydrolysis in zinc chloride solution

Xylan is the major component of hemicellulose, which consists of up to one-third of the lignocellulosic biomass. When the zinc chloride solution was used as a pretreatment agent to facilitate cellulose hydrolysis, hemicellulose was hydrolyzed during the pretreatment stage. In this study, xylan was used as a model to study the hydrolysis of hemicellulose in zinc chloride solution. The degradation of xylose that is released from xylan was reduced by the formation of zinc-xylose complex. The xylose yield was >90% (w/w) at 70°C. The yield and rate of hydrolysis were a function of temperature and the concentration of zinc chloride. The ratio of zinc chloride can be decreased from 9 to 1.3 (w/w). At this ratio, 76% of xylose yield was obtained. When wheat straw was pretreated with a concentrated zinc chloride solution, the hemicellulose hydrolysate contained only xylose and trace amounts of arabinose and oligosaccharides. With this approach, the hemicellulose hydrolysate can be separated from cellulose residue, which would be hydrolyzed subsequently to glucose by acid or enzymes to produce glucose. This production scheme provided a method to produce glucose and xylose in different streams, which can be fermented in separated fermenters.     

Enzymatic hydrolysis of high-moisture corn fiber pretreated by afex and recovery and recycling of the enzyme complex

Corn fiber is a grain-processing residue containing significant amounts of cellulose, hemicellulose, and starch, which is collected in facilities where fuel ethanol is currently manufactured. Preliminary research has shown that corn fiber (30% moisture dry weight basis [dwb]) responds well to ammonia-fiber explosion (AFEX) pretreatment. However, an important AFEX pretreatment variable that has not been adequately explored for corn fiber is sample moisture. In the present investigation, we determined the best AFEX operating conditions for pretreatment of corn fiber at high moisture content (150% moisture dwb). The optimized AFEX treatment conditions are defined in terms of the moisture content, particle size, ammonia to biomass ratio, temperature, and residence time using the response of the pretreated biomass to enzymatic hydrolysis as an indicator. Approximate optimal-pretreatment conditions for unground corn fiber containing 150% (dwb) moisture were found to be: temperature, 90?C; ammonia: dry corn fiber mass ratio, 1:1; and residence time 30 min (average reactor pressure under these conditions was 200 pounds per square inch [psig]). Enzymatic hydrolysis of the treated corn fiber was performed with three different enzyme combinations. More than 80% of the theoretical sugar yield was obtained during enzymatic hydrolysis using the best enzyme combination after pretreatment of corn fiber under the optimized conditions previously described. A simple process for enzyme recovery and reuse to hydrolyze multiple portions of AFEX-treated corn fiber by one portion of enzyme preparation is demonstrated. Using this process, five batches of fresh substrate (at a concentration of 5% w/v) were successfully hydrolyzed by repeated recovery and reuse of one portion of enzyme preparation, with the addition of a small portion of fresh enzyme in each subsequent recycling step.     

Xylulokinase activity in various yeasts includingSaccharomyces cerevisiae containing the cloned xylulokinase gene

d-Xylose is a major constituent of hemicellulose, which makes up 20–30% of renewable biomass in nature.d-Xylose can be fermented by most yeasts, includingSaccharomyces cerevisiae, by a two-stage process. In this process, xylose is first converted to xylulose in vitro by the enzyme xylose (glucose) isomerase, and the latter sugar is then fermented by yeast to ethanol. With the availability of an inexpensive source of xylose isomerase produced by recombinantE. coli, this process of fermenting xylose to ethanol can become quite effective. In this paper, we report that yeast xylose and xylulose fermentation can be further improved by cloning and overexpression of the xylulokinase gene. For instance, the level of xylulokinase activity in S.cerevisiae can be increased 230fold by cloning its xylulokinase gene on a high copy-number plasmid, coupled with fusion of the gene with an effective promoter. The resulting genetically-engineered yeasts can ferment xylose and xylulose more than twice as fast as the parent yeast.     

Effects of operating parameters on countercurrent extraction of hemicellulosic sugars from pretreated softwood

In a previous study using a continuous countercurrent screw extractor for two-stage dilute-acid hydrolysis, which was focused on the effects of liquid-to-insoluble solids (L/IS) ratio, we demonstrated that by using low volumes of wash water soluble sugars can be recovered from first-stage pretreated softwood at high yields and also at high sugar concentrations. In this study, we investigated the effects of important operating parameters other than the L/IS ratio, such as the feed rates of water and pretreated biomass and the extractor inclined angle, on the performance of the extractor using first-stage pretreated softwood. As biomass and water feed rates increased at the same L/IS ratio, the recovery yield of soluble sugars decreased, probably owing to a reduced solids residence time in the extractor, which is related to the solid/liquid contact time. The sugar recovery yield was higher at a higher extractor inclined angle. This may be attributed to the effects of increased back mixing and a longer residence time for solids at a higher extractor angle. Countercurrent extraction was also carried out with other pretreated biomass having smaller particle sizes and poor drainage rates. The countercurrent screw extractor was found to be unsuitable for these fine materials due to the slow liquid drainage rate and filter-clogging problems. In a test for stability of soluble sugars in first-stage softwood hydrolysate, irrespective of the storage temperature and storage form, the sugar concentration slowly decreased with storage time. However, storage in slurry form showed higher sugar stability compared with that in liquor form at the same conditions.

     

Pretreatment of corn stover by low-liquid ammonia recycle percolation process

A pretreatment method using aqueous ammonia was investigated with the intent of minimizing the liquid throughput. This process uses a flow-through packed column reactor (or percolation reactor). In comparison to the ammonia recycle percolation (ARP) process developed previously in our laboratory, this process significantly reduces the liquid throughput to one reactor void volume in packed bed (2.0–4.7 mL of liquid/g of corn stover) and, thus, is termed low-liquid ARP (LLARP). In addition to attaining short residence time and reduced energy input, this process achieves 59–70% of lignin removal and 48–57% of xylan retention. With optimum operation of the LLARP to corn stover, enzymatic digestibilities of 95, 90 and 86% were achieved with 60, 15, and 7.5 filter paper units/g of glucan, respectively. In the simultaneous saccharification and fermentation test of the LLARP samples using Saccharomyces cerevisiae (NREL-D₅A), an ethanol yield of 84% of the theoretical maximum was achieved with 6% (w/v) glucan loading. In the simultaneous saccharification and cofermentation (SSCF) test using recombinant Escherichia coli (KO11), both the glucan and xylan in the solid were effectively utilized, giving an overall ethanol yield of 109% of the theoretical maximum based on glucan, a clear indication that the xylan content was converted into ethanol. The xylooligomers existing in the LLARP effluent were not effectively hydrolyzed by cellulase enzyme, achieving only 60% of digestibility. SSCF of the treated corn stover was severely hampered when the substrate was supplemented with the LLARP effluent, giving only 56% the overall yield of ethanol. The effluent appears to significantly inhibit cellulase and microbial activities.     

Evaluation of different biomass materials as feedstock for fermentable sugar production

Saline crops and autoclaved municipal organic solid wastes were evaluated for their potential to be used as feedstock for fermentable sugar production through dilute acid pretreatment and enzymatic hydrolysis. The saline crops included two woods, athel (Tamarix aphylla L) and eucalyptus (Eucalyptus camaldulensis), and two grasses, Jose tall wheatgrass (Agropyron elongatum), and creeping wild rye (Leymus triticoides). Each of the biomass materials was first treated with dilute sulfuric acid under selected conditions (acid concentration =1.4% (w/w), temperature =165 degrees C, and time =8 min) and then treated with the enzymes (cellulases and beta-glucosidase). The chemical composition (cellulose, hemicellulose, and lignin contents) of each biomass material and the yield of total and different types of sugars after the acid and enzyme treatment were determined. The results showed that among the saline crops evaluated, the two grasses (creeping wild rye and Jose tall wheatgrass) had the highest glucose yield (87% of total cellulose hydrolyzed) and fastest reaction rate during the enzyme treatment. The autoclaved municipal organic solid wastes showed reasonable glucose yield (64%). Of the two wood species evaluated, Athel has higher glucose yield (60% conversion of cellulose) than eucalyptus (38% conversion of cellulose).     

Dilute-acid hydrolysis of sugarcane bagasse at varying conditions

Sugarcane bagasse, a byproduct of the cane sugar industry, is an abundant source of hemicellulose that could be hydrolyzed to yield a fermentation feedstock for the production of fuel ethanol and chemicals. The effects of sulfuric acid concentration, temperature, time, and dry matter concentration on hemicellulose hydrolysis were studied with a 20-L batch hydrolysis reactor using a statistical experimental design. Even at less severe conditions considerable amounts (>29%) of the hemicellulose fraction could be extracted. The percentage of soluble oligosaccharides becomes very low in experiments with high yields in monosaccharides, which indicates that the cellulose fraction is only slightly affected. For the sugar yields, acid concentration appears to be the most important parameter, while for the formation of sugar degradation products, temperature shows the highest impact. It could be demonstrated that the dry matter concentration in the reaction slurry has a negative effect on the xylose yield that can be compensated by higher concentrations of sulfuric acid owing to a positive interaction between acid concentration and dry matter contents.     

Substrate Dependency and Effect of Xylanase Supplementation on Enzymatic Hydrolysis of Ammonia-Treated Biomass

Pretreatment based on aqueous ammonia was investigated under two different modes of operation: soaking in aqueous ammonia and ammonia recycle percolation. These processes were applied to three different feedstocks with varied composition: corn stover, high lignin (HL), and low lignin (LL) hybrid poplars. One of the important features of ammonia-based pretreatment is that most of the hemicellulose is retained after treatment, which simplifies the overall bioconversion process and enhances the conversion efficiency. The pretreatment processes were optimized for these feedstocks, taking carbohydrate retention as well as sugar yield in consideration. The data indicate that hybrid poplar is more difficult to treat than corn stover, thus, requires more severe conditions. On the other hand, hybrid poplar has a beneficial property that it retains most of the hemicellulose after pretreatment. To enhance the digestibility of ammonia-treated poplars, xylanase was supplemented during enzymatic hydrolysis. Because of high retention of hemicellulose in treated hybrid poplar, xylanase supplementation significantly improved xylan as well as glucan digestibility. Of the three feedstocks, best results and highest improvement by xylanase addition was observed with LL hybrid poplar, showing 90% of overall sugar yield.     

Biomass valorisation by staged degasification: A new pyrolysis-based thermochemical conversion option to produce value-added chemicals from lignocellulosic biomass

Pyrolysis of lignocellulosic biomass leads to an array of useful solid, liquid and gaseous products. Staged degasification is a pyrolysis-based conversion route to generate value-added chemicals from biomass. Because of different thermal stabilities of the main biomass constituents hemicellulose, cellulose and lignin, different temperatures may be applied for a step-wise degradation into valuable chemicals. Staged degasification experiments were conducted with deciduous (beech, poplar), coniferous (spruce) and herbaceous (straw) biomass. Thermogravimetry was used to estimate appropriate temperatures for a two-stage degradation process that was subsequently evaluated on bench-scale by moving bed and bubbling fluidised bed pyrolysis experiments. Degasification in two consecutive stages at 250–300 °C and 350–400 °C leads to mixtures of degradation products that originate from the whole biomass. The mixtures that were generated at 250–300 °C, predominantly contain hemicellulose degradation products, while the composition of the mixtures that were obtained at 350–400 °C, is more representative for cellulose. Lignin-derived fragments are found in both mixtures. Yields up to 5 wt% of the dry feedstock are obtained for chemicals like acetic acid, furfural, acetol and levoglucosan. Certain groups of thermal degradation products like C₂–C₄ oxygenates and phenols are formed in yields up to 3 wt%. Highest yields have been obtained for beech wood. Staged degasification is a promising pyrolysis-based route to valorise lignocellulosic biomass. Clear opportunities exist to increase product yields and selectivities by optimisation of reactor conditions, application of catalysts and specific biomass pretreatments like demineralisation and pre-hydrolysis.     

生物质半纤维素稀酸水解反应*

半纤维素是木质纤维素类生物质中第二大组分,半纤维素的高效、低成本转化是实现木质纤维素类生物质转化工艺实用化的一个技术关键。稀酸水解技术被广泛应用于水解生物质半纤维素,其对半纤维素糖的转化率高,得到的糖可进一步发酵生产燃料乙醇等。半纤维素还可直接水解制低聚糖等功能性食品和糠醛等化工产品。本文综述了半纤维素稀酸水解反应的研究进展。介绍了半纤维素的基本结构特征,解析了稀酸催化半纤维素水解的反应机理及反应网络,评述了半纤维素水解过程中反应条件等对目标产物的影响,并总结了半纤维素稀酸水解动力学模型。在此基础上,对今后半纤维素稀酸水解反应的研究方向与水解产物的利用进行了展望。     

Enhanced Saccharification of Biologically Pretreated Wheat Straw for Ethanol Production

The biological pretreatment of lignocellulosic biomass with white-rot fungi for the production of bioethanol is an alternative to the most used physico-chemical processes. After biological treatment, a solid composed of cellulose, hemicellulose, and lignin—this latter is with a composition lower than that found in the initial substrate—is obtained. On the contrary, after applying physico-chemical methods, most of the hemicellulose fraction is solubilized, while cellulose and lignin fractions remain in the solid. The optimization of the combination of cellulases and hemicellulases required to saccharify wheat straw pretreated with the white-rot fungus Irpex lacteus was carried out in this work. The application of the optimal dosage made possible the increase of the sugar yield from 33 to 54 %, and at the same time the reduction of the quantity of enzymatic mixture in 40 %, with respect to the initial dosage. The application of a pre-hydrolysis step with xylanases was also studied.     

Two-Stage Fractionation of Corn Stover Using Aqueous Ammonia and Hot Water

Hot water and aqueous ammonia fractionation of corn stover were used to separate hemicellulose and lignin and improve enzymatic digestibility of cellulose. A two-stage approach was used: The first stage was designed to recover soluble lignin using aqueous ammonia at low temperature, while the second stage was designed to recover xylan using hot water at high temperature. Specifically, the first stage employed a batch reaction using 15 wt.% ammonia at 60 °C, in a 1:10 solid:liquid ratio for 8 h, while the second stage employed a percolation reaction using hot water, 190–210 °C, at a 20 ml/min flow rate for 10 min. After fractionation, the remaining solids were nearly pure cellulose. The two-stage fractionation process achieved 68% lignin purity with 47% lignin recovery in the first stage, and 78% xylan purity, with 65% xylan recovery in the second stage. Two-stage treatment enhanced the enzymatic hydrolysis of remaining cellulose to 96% with 15 FPU/g of glucan using commercial cellulase enzymes. Enzyme hydrolyses were nearly completed within 12–24 h with the remaining solids fraction.     

两段式固定床反应器中焦油脱除的实验研究

通过两段式固定床反应器实验,研究了热裂解、部分氧化和炭层转化三种方法对焦油脱除的效果,并研究了生物质种类、反应温度、停留时间、生物质焦的粒径及种类等因素对热解焦油的脱除和转化规律。结果表明,随着温度的升高,三种脱除方法中焦油生成量下降,且降幅逐渐减小,实验过程中无论采取何种方法,都难以将焦油完全脱除;部分氧化和炭层转化对焦油的脱除效果都较相同温度条件下的热裂解要好,且在焦油脱除效果上,炭层转化>部分氧化>热裂解;联合部分氧化和炭层转化可达最高的焦油脱除效率,三种生物质热解焦油经1 000 ℃联合脱除后产量分别为,稻秆0.43%、玉米秆0.61%和杉木屑1.15%,转化率分别达到98.28%、97.23%和96.29%;相同实验条件下稻秆的热解焦油最容易脱除,这与其物料中含氧量较高有关;生物质焦种类对焦油的脱除效果影响较小。     

Alkali (NaOH) Pretreatment of Switchgrass by Radio Frequency-based Dielectric Heating

Radio-frequency (RF)-based dielectric heating was used in the alkali (NaOH) pretreatment of switchgrass to enhance its enzymatic digestibility. Due to the unique features of RF heating (i.e., volumetric heat transfer, deep heat penetration of the samples, etc.), switchgrass could be treated on a large scale, high solid content, and uniform temperature profile. At 20% solid content, RF-assisted alkali pretreatment (at 0.1 g NaOH/g biomass loading and 90°C) resulted in a higher xylose yield than the conventional heating pretreatment. The enzymatic hydrolysis of RF-treated solids led to a higher glucose yield than the corresponding value obtained from conventional heating treatment. When the solid content exceeded 25%, conventional heating could not handle this high-solid sample due to the loss of fluidity, poor mixing, and heating transfer of the samples. As a result, there was a significantly lower sugar yield, but the sugar yield of the RF-based pretreatment process was still maintained at high levels. Furthermore, the optimal particle size and alkali loading in the RF pretreatment was determined as 0.25–0.50 mm and 0.25 g NaOH/g biomass, respectively. At alkali loading of 0.20–0.25 g NaOH/g biomass, heating temperature of 90^oC, and solid content of 20%, the glucose, xylose, and total sugar yield from the combined RF pretreatment and the enzymatic hydrolysis were 25.3, 21.2, and 46.5 g/g biomass, respectively.     

Fractionation of Cynara cardunculus (cardoon) biomass by dilute-acid pretreatment

Cynara cardunculus L. (cardoon) is a Mediterranean perennial herb offering good potential as substrate for sustainable production of bioethanol. In this work the first approach to the study of dilute-acid pretreatment of cardoon biomass for biological conversion was made. The influence of temperature (160-200 degrees C), acid concentration (0-0.2% [w/w]), and solid concentration (5-10% [w/v]) in the formation of free sugars and sugar decomposition products in the prehydrolyzate was studied using a response surface methodology. Results show a negative interaction effect between acid concentration and temperature in xylose recovery yield in prehydrolyzate, whereas dry matter concentration does not exert a significant effect. Xylose recovery yield reaches a maximum of about 80% of the content in dry untreated raw material at 180 degrees C and 0.1 or 0.2% acid addition. At these conditions the ratio of monomers found in prehydrolyzate in relation to total sugar yield for xylose is close to 100%. Furfural concentration, the major furan determined in the prehydrolyzate, increases as pretreatment severity rises. Maximum furfural yield of 4.2 g/100 g dry untreated raw material was found at 200 degrees C and 0.2% acid concentration. The yield of furfural at the conditions in which maximum xylose recovery is attained is substantially lower, less than 2 g/100 g dry untreated raw material. This fact supports the idea of using moderate temperatures in dilute-acid processes, which at the same time provides reasonably high sugar recovery yield and avoids high inhibitory products formation.