Acid-catalyzed Steam Pretreatment Research Articles

Process scale-up of an efficient acid-catalyzed steam pretreatment of rice straw for xylitol production by C. Tropicalis MTCC 6192

The present study focuses on operational parameters for the efficient acid catalyzed rice straw pretreatment process for xylitol production. 75.77 % xylose yield was attained when the 24 h presoaked rice straw (≤10 mm or ≤ 15 mm) in 1.5 % (v/v) H2SO4 was pretreated in the same reactor at 121 °C for 30 min. Neutralization with barium hydroxide produced insoluble salt and noticeably reduced HMF and furfurals. Xylitol yield of 0.6 g/g of xylose, was achieved by fermenting rice straw hydrolysate medium with C. tropicalis MTCC 6192. This two-step process of production of xylitol from xylose rich hydrolysate is much simpler and produced minimal inhibitors including organic acids such as acetic acid. This process is modified for upscaling at optimized parameters and will simultaneously minimize the pollution problem caused by rice straw and is also promising for commercial scale.

A comparison of different pre-extraction methods followed by steam pretreatment of bamboo to improve the enzymatic digestibility and ethanol production

In this work, investigation of pre-extraction of bamboo under three different conditions (alkaline, neutral, and acidic) prior to acid catalyzed (AC) steam pretreatment on the conversion of polysaccharides into fermentable sugars and ethanol was reported. Pre-extraction process was demonstrated to significantly improve the enzymatic digestibility of bamboo and reduce the formation of inhibitory compounds for fermentation. Comparing to the pre-extraction of bamboo under neutral and acidic conditions, alkaline pre-extraction of bamboo yielded the highest monomeric sugar yields following enzymatic hydrolysis, which were approximately 86% of glucose and 82% of xylose (based on the initial sugar composition). The fermentation of sugars released during enzymatic hydrolysis with pentose-hexose fermenting Saccharomyces cerevisiae Lg8-1 strain achieved the final ethanol concentrations of 50.1, 46.3, and 49.0 g/L, corresponding to the ethanol yields of 70.5%, 68.9%, and 65.1% (based on theoretical ethanol yield of the initial bamboo) for the prep-extraction of bamboo under alkaline, neutral, and acidic conditions, respectively. The results indicate that the pre-extraction of extractives and ash followed by AC steam pretreatment is a promising approach to improve the bioconversion of bamboo biomass to biofuels.

Comparison of alkaline and acid-catalyzed steam pretreatments for ethanol production from tobacco stalk

In this study, tobacco stalk (TS), the solid residue from tobacco industry, was utilized as the raw material for the production of bioethanol through combining processes of alkaline pretreatment or acid-catalyzed (AC) steam pretreatment followed by enzymatic hydrolysis and fermentation. The results indicate that both the two pretreatment strategies could efficiently improve the enzymatic digestibility of TS. Compared to alkaline pretreatment, AC steam pretreatment solubilized more xylan into the liquid phase while preserving more lignin in the pretreated solid substrate. Following enzymatic saccharification of whole pretreated materials from two pretreatment methods at a modest enzyme loading of 15 mg protein/g glucan, the slurry from alkaline pretreatment achieved a slightly higher total monomeric sugar yields than that of AC steam pretreatment. The sugars in the obtained enzymatic hydrolysates from both the two pretreatment technologies showed good sugar-to-ethanol conversion during fermentation with Saccharomyces cerevisiae Lg8-1 strain. The overall mass balances showed that yields of ethanol were 2.75 and 2.69 kg per 10 kg of untreated TS for alkaline and AC steam pretreatments, respectively.

The effect of blending spruce and poplar on acid-catalyzed steam pretreatment and enzymatic hydrolysis

The aim of this work was to explore the possibility of mixed feedstocks utilization using acid-catalyzed steam pretreatment in the context of forest biomass. Three steam pretreatment conditions were applied to characterize the interactions between spruce (softwood) and poplar (hardwood) that were concurrently processed by SO2-catalyzed steam pretreatment and enzymatic hydrolysis. The effects of blending feedstocks were evaluated in terms of composition and enzymatic digestibility. The combined sugar yields after steam pretreatment and enzymatic hydrolysis ranged from 58% to 71%. No synergistic or antagonistic interactions were observed in the concurrent use of spruce and poplar—our linear interpolation model accurately predicted the overall sugar recovery after steam pretreatment and enzymatic hydrolysis for a 50:50 blend to within 3%, based on the results of the individual feedstocks.

Bioethanol Production from Musambi Peel by Acid Catalyzed Steam Pretreatment and Enzymatic Saccharification: Optimization of Delignification Using Taguchi Design

Citrus waste is an attractive feedstock for second generation bioethanol due to its richness in carbohydrates. For the production of bioethanol, removal of lignin through pretreatment to liberate more cellulose with minimal loss is essential. In the present work, the removal of lignin from musambi (Citrus limetta) peel by acid-catalyzed steam pretreatment and the characterization of musambi peel before and after pretreatment was studied. A Taguchi design was used to optimize solid loading, sulphuric acid concentration and time, for steam pretreatment with delignification as the response. The optimum conditions of process variables were 17% (w/v) for solid loading, 0.25% (v/v) for acid concentration with a time of exposure of 60 min with maximum delignification of 65%. The decrease in peak intensity in FTIR spectra and increase in the crystallinity index of pretreated peel indicated the reduction in pectin, hemicellulose, and lignin. The scanning electron micrographs of pretreated peel clearly showed the delignified microfibrils of cellulose. Enzymatic saccharification of pretreated musambi peel and the effect of peel loading, temperature, time, pH and the loading of cellulase, beta-glucosidase, and pectinase on reducing sugar were also investigated. The maximum reducing sugar obtained after the enzymatic hydrolysis was 386 mg/g for steam pretreated musambi peel at the optimum conditions whereas it was only 258 mg/g for the raw musambi peel. Thus, steam pretreatment of musambi peel resulted in better hydrolysis than untreated musambi peel. The yield of bioethanol production at the end of 48 h of fermentation was 85.97%.

Statistical optimization of acid catalyzed steam pretreatment of citrus peel waste for bioethanol production

Citrus waste is an attractive lignocellulosic biomass for the production of bioethanol due to the richness in carbohydrates and low lignin content. In this study, sweet lime peel was chosen as the lignocellu- losic biomass. To increase the cellulose for enzymatic hydrolysis, the statistical optimization of process parameters namely, solid loading, time of exposure and sulphuric acid concentration for pretreatment of sweet lime peel were accomplished by Taguchi orthogonal array design. The sweet lime peel was exposed to acid catalyzed steam pretreatment for solid loading [10%, 12%, 15% and 17% (w/v)], time of exposure [15 min, 30 min, 45 min and 60 min] and sulphuric acid concentration [0.25%, 0.5%, 0.75% and 1% (v/v)]. The cellulose content was found to be an optimum at 35% for 17% (w/v) solid loading and 0.25% (v/v) acid concentration and steam exposure for 60 min. With these optimized process parameters, enzymatic hy- drolysis of pretreated sweet lime peel was investigated at 50 °C for 48 h using in vitro isolated enzymes, viz., cellulase and pectinase from Aspergillus Niger with an activity of 1.7 FPU/ml and15 IU/ml respectively. 7.09 mg of reducing sugar/ml of hydrolysate was released in enzymatic hydrolysis which was estimated by DNS method. For the production of bioethanol, fermentation of hydrolysate was carried out at 30 °C for 72 h using baker’s yeast. The yield of ethanol was 18%. From this study, it is proved that citrus waste is a promising source for the production of bioethanol.

Removal of Water-Soluble Extractives Improves the Enzymatic Digestibility of Steam-Pretreated Softwood Barks

Softwood bark contains a large amounts of extractives—i.e., soluble lipophilic (such as resin acids) and hydrophilic components (phenolic compounds, stilbenes). The effects of the partial removal of water-soluble extractives before acid-catalyzed steam pretreatment on enzymatic digestibility were assessed for two softwood barks—Norway spruce and Scots pine. A simple hot water extraction step removed more than half of the water-soluble extractives from the barks, which improved the enzymatic digestibility of both steam-pretreated materials. This effect was more pronounced for the spruce than the pine bark, as evidenced by the 30 and 11% glucose yield improvement, respectively, in the enzymatic digestibility. Furthermore, analysis of the chemical composition showed that the acid-insoluble lignin content of the pretreated materials decreased when water-soluble extractives were removed prior to steam pretreatment. This can be explained by a decreased formation of water-insoluble “pseudo-lignin” from water-soluble bark phenolics during the acid-catalyzed pretreatment, which otherwise results in distorted lignin analysis and may also contribute to the impaired enzymatic digestibility of the barks. Thus, this study advocates the removal of extractives as the first step in the processing of bark or bark-rich materials in a sugar platform biorefinery.

Statistical optimization of acid catalyzed steam pretreatment of citrus peel waste for bioethanol production

Abstract Citrus waste is an attractive lignocellulosic biomass for the production of bioethanol due to the richness in carbohydrates and low lignin content. In this study, sweet lime peel was chosen as the lignocellulosic biomass. To increase the cellulose for enzymatic hydrolysis, the statistical optimization of process parameters namely, solid loading, time of exposure and sulphuric acid concentration for pretreatment of sweet lime peel were accomplished by Taguchi orthogonal array design. The sweet lime peel was exposed to acid catalyzed steam pretreatment for solid loading [10%, 12%, 15% and 17% (w/v)], time of exposure [15 min, 30 min, 45 min and 60 min] and sulphuric acid concentration [0.25%, 0.5%, 0.75% and 1% (v/v)]. The cellulose content was found to be an optimum at 35% for 17% (w/v) solid loading and 0.25% (v/v) acid concentration and steam exposure for 60 min. With these optimized process parameters, enzymatic hydrolysis of pretreated sweet lime peel was investigated at 50 °C for 48 h using in vitro isolated enzymes, viz., cellulase and pectinase from Aspergillus Niger with an activity of 1.7 FPU/ml and15 IU/ml respectively. 7.09 mg of reducing sugar/ml of hydrolysate was released in enzymatic hydrolysis which was estimated by DNS method. For the production of bioethanol, fermentation of hydrolysate was carried out at 30 °C for 72 h using baker's yeast. The yield of ethanol was 18%. From this study, it is proved that citrus waste is a promising source for the production of bioethanol.

The use of predictive models to optimize sugar recovery obtained after the steam pre‐treatment of softwoods

Acid catalyzed steam pre‐treatment is recognized as an effective method for defibrillating plant cell walls while providing good hemicellulose sugar recovery and a more readily accessible cellulosic component for subsequent enzymatic hydrolysis. However, much of the past work to try to optimize the overall sugar recovery after pre‐treatment and enzymatic hydrolysis has been limited to more qualitative comparisons which offer little insight into the steam pre‐treatment process itself. Better prediction of sugar recoveries from steam pre‐treated biomass will likely prove to be invaluable in helping us design more effective steam pre‐treatment reactors. The work discussed here attempted to determine which of three options – namely the severity factor Ro, the combined severity factor CS, and response surface methodology RSM – was best suited for the development of predictive empirical equations that would be used to optimize the acid catalyzed steam pre‐treatment of softwood chips of industrially relevant size and provide maximum soluble sugar recovery. It was apparent that the combined severity factor CS resulted in predictions that were slightly more accurate than those of the severity factor Ro, and that RSM and the severity factor Ro possessed similar predictive capability. A comparison of several RSM models that were used to evaluate the SO2 catalyzed steam pre‐treatment of softwood indicated that a hybrid design, when used in conjunction with a narrow process space, provided the most robust model. The applicability of an RSM model which was developed when pretreating radiata pine was assessed against pre‐treated lodgepole pine and was found to provide good predictability.