Recovery of biomass wastes by hydrolysis in sub-critical water

Resources, Conservation and Recycling 55 (2011) 409–416

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Resources, Conservation and Recycling journal homepage: www.elsevier.com/locate/resconrec

Review

Recovery of biomass wastes by hydrolysis in sub-critical water Guangyong Zhu, Xian Zhu ∗ , Qi Fan, Xueliang Wan Dept. Chem. Eng., Shanghai University, No. 333 Nanchen Road, Shanghai 200444, PR China

a r t i c l e

i n f o

Article history: Received 9 October 2010 Received in revised form 23 December 2010 Accepted 26 December 2010 Keywords: Amino acid Biofuel Sub-critical water Hydrolysis Biomass waste

a b s t r a c t The recovery of waste substances is important not only for the prevention of environmental issues, but also for the rational utilization of natural resources. Hydrolysis reaction in sub-critical water is a promising method for the treatment of organic wastes and has been attracting worldwide attention. In this paper, sub-critical water hydrolysis was employed as a method for producing amino acids, reducing sugars, biooil and gas fuels from biomass wastes. The current statuses of these useful chemicals production from biomass wastes by hydrolysis in sub-critical water were reviewed. The review indicates that sub-critical water hydrolysis can be an efficient process for recovering useful chemicals from biomass wastes. This method is renewable, sustainable, efficient, and safe for the environment. © 2010 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4. 5.

6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gas fuel production from biomass wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bio-oil fuels production from biomass wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reducing sugar production from biomass wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Amino acid production from biomass wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Reactor system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Amino acid production from alternative materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Reaction mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. The main influence factors on amino acid yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Biomass, consisting of plant material and animal products, is vastly available and considered as a valuable resource. The annual growth of plant-derived biomass is estimated to be 118 × 109 tons per year on a dry matter basis. The lignocellulosic biomass materials are abundant, cheap and renewable. The composition of lignocellulosic material (LCM) is as follows: 50% cellulose, 25% hemicellulose, and 20% lignin (Bobleter, 1994). Due to the increasing price and undesirable environmental effects of fossil fuels, production of energy from renewable resources has gained much attention in recent years. Being non-edible portion of the plant, lignocellulosic

∗ Corresponding author. Tel.: +86 21 66137727; fax: +86 21 66137727. E-mail addresses: [email protected], [email protected] (X. Zhu). 0921-3449/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.resconrec.2010.12.012

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biomass materials are attracting growing attention as sustainable and renewable energy sources. Because these resources are carbon neutral, they do not cause additional increase in the carbon dioxide level in the earth’s atmosphere. The release of carbon dioxide from bioenergy/biofuels is balanced by the carbon dioxide consumed in biomass growth. Therefore, using biofuels instead of fossil fuels reduces greenhouse gas emissions. Lignocellulosic materials contain cellulose and hemicellulose that are bound together by lignin. Cellulose and hemicellulose are both polymers built up by long chains of sugar units, which after pretreatment and hydrolysis, can be converted into intermediate products which can be transformed into biofuels or other industrially important products (Öztürk et al., 2010). Usually, animal biomass wastes such as hog hair, feather, shrimp shells and so on are proteinaceous. Amino acids can be produced from these protein-rich biomass wastes. Amino acids are the basic

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“building blocks” that combine to form proteins. In every species, proteins are constructed from the same set of twenty amino acids. Besides forming proteins, enzymes and other body tissue, they are also found throughout the body participating in a wide variety of chemical reactions, and are vital in basic energy production cycles, energy transfer and muscle activity. Amino acids play an important role in the physiology of all life-forms. However, not all of the 20 amino acids that are involved in the structure of natural proteins (so-called proteinogenic amino acids) can be metabolically synthesized by all creatures. Whereas plants can produce all of the 20 proteinogenic amino acids, some of them (so-called essential amino acids) cannot be produced by the human and animal organisms. These amino acids therefore have to be digested in sufficient amounts. Amino acids are also important for medical, cosmetic, animal husbandry and other industrial applications. Besides the direct chemical synthesis, the fermentation by microorganisms, the production by means of protein hydrolyzate extraction, and enzymatic methods are established techniques for the production of amino acids. However, if chemical synthesis is used, an enantiomer separation with subsequent racemization is needed. In conventional methods, the raw materials are split by acid, alkaline, or enzymatic hydrolysis, whereas the addition of further chemicals is necessary for the two former methods. The use of immobilized enzymes in different reactor types leads to incomplete hydrolyses, even at residence times of 8–20 h (Lasch et al., 1987). In the late 70s of last century, spurred by the first oil crisis, environmental concerns, and the investigation of supercritical fluids, people started to pay attention to the application of sub-critical and supercritical water. Sub-critical water refers to water above its normal boiling temperature (373 K), but below its critical temperature, which is kept in liquid state by applying pressure. Suband super-critical water have been gaining increasing attention as environmentally friendly solvent and attractive reaction medium for a variety of applications. It is cheap, non-toxic, non-flammable, non-explosive, and offers essential advantages compared to other substances, particularly in the field of “green chemistry”(Rogalinski et al., 2008). Its distinctly different behavior compared to water at ambient conditions is due to the dramatic changes in physical properties, namely dielectric strength and ionic product, which in turn can easily be altered by changing temperature and pressure (Alenezi et al., 2009). The ionic product of sub-critical water is as much as three orders of magnitude higher than under ambient conditions. Under these conditions, there is a high H3 O+ and OH− ion concentration. As such, some acid-catalyzed organic reactions can be carried out without acid addition. Hence, acid hydrolyzed cleavages of peptide bonds can pass off without the addition of catalysts. However, the ionic product decreases greatly above the critical point. This fact makes sub-critical water an ideal reaction medium for the hydrolysis of organic compounds (Zhu et al., 2010a) and for the recycling of different organic wastes, such as municipal solid wastes, refractory pollutants, sludge, different polymers (Chen et al., 2010; Esteban et al., 2008; Li et al., 2009; Zhang et al., 2010). In this paper, sub-critical water hydrolysis was employed as a method for producing reducing sugars, bio-oil and gas fuels from lignocellulosic biomass wastes and for producing amino acids from protein-rich biomass wastes. The current statuses of these useful chemicals production from biomass wastes by hydrolysis in subcritical water are reviewed. The prospects of this technology are offered.

2. Gas fuel production from biomass wastes The inter-related problems of energy and environment are among the biggest challenges facing the world today, in partic-

ular energy sustainability and carbon emissions from the fossil fuels. Hydrogen has been projected as one of the few long-term sustainable clean energy carriers, emitting only water vapor as a byproduct during the combustion or oxidation process (Tanksale et al., 2010). However, hydrogen is a gas that cannot be directly available in nature and, thus, must be produced from other substances. Most industrial processes for hydrogen production use reforming techniques, which require hydrocarbons, stemming from oil industry. Thus, hydrogen produced in that way cannot any more be considered as a ‘clean gas’, especially because of its bonds with oil production, which is limited by carbon dioxide formation and by geopolitical aspects (Calzavara et al., 2005). With the increasing interest in renewable energy sources and utilization of various wastes and by-products, production of hydrogen from biomass has been studied all over the world in the last several years. Converting plant biomass wastes into hydrogen can be accomplished in sub-critical and supercritical water. The subcritical and supercritical water gasification of wet biomass such as food waste is more advantageous than conventional gasification because it does not require a drying step. Kruse and Gawlik (2003) studied the degradation of biomass wastes in the ranges of 330–410 ◦ C and 30–50 MPa. The reaction time was 15 min. Gas fuels could be produced from biomass wastes. They also found that an increase in the temperature leads to an increase in the gasification efficiency. Above the critical temperature of water, a drastic increase in the yields of H2 and CH4 and a decrease of CO are found. As minor products, hydrocarbons such as ethane, ethene, propane, propene, isobutane, methylpropene, and butene were identified and determined by GC. The amount of major compounds H2 and CO2 as well as most of the minor compounds decreases with pressure. Only the yields of propene, ethene, and C4 compounds increase with the augmentation of pressure. Sodium hydroxide is a promoter of hydrogen gas production during the hydrothermal gasification of glucose and other biomass samples. Without sodium hydroxide, glucose decomposed to produce mainly carbon dioxide, water, char and tar. In the presence of sodium hydroxide, however, glucose initially decomposed to form mostly alkylated and hydroxylated carbonyl compounds, whose further decomposition yielded hydrogen gas (Onwudili and Williams, 2009). The alkali additive, NaOH, can increase the carbon–carbon decomposition of glucose and real food processing waste such as molasses and rice bran. The trend of gas production from glucose sub-critical water gasification was similar to that from real-world biomass waste molasses and rice bran in the presence of NaOH. Low temperature was sufficient to cause the decomposition of some intermediate reaction products leading to increased formation of H2 . With increasing reaction temperature and reaction time, the concentration of hydrogen and CH4 gases increased. Additionally, increasing feed concentration of glucose, molasses and rice bran led to lower hydrogen production but higher concentrations of other gases such as CO, CO2 , CH4 and C2 –C4 compounds. The major effect of NaOH addition was the inhibition of tar and char formation and promotion of CO-producing intermediates for H2 production by the water–gas shift reaction (Muangrat et al., 2010a). The production of hydrogen via sub-critical water gasification of glucose and glutamic acid as model compounds has been investigated by Muangrat et al. (2010b). The influence of NaOH, Ni/Al2 O3 and Ni/SiO2 catalysts and the combination of these catalysts in relation to hydrogen production was examined. Increasing concentration of NaOH enhanced the important decomposition reactions of the model compounds and promoted the water gas shift reaction leading to higher H2 , lower CO2 production and a CO-lean gas product. Also, the amount of oil/tar and char decreased in the presence of NaOH additive. The NaOH could promote glucose decomposition to water soluble products and CO formation.

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Chareonlimkun et al. (2010) studied reactions of lignocellulose biomass wastes in sub-critical water in the presence of heterogeneous acid catalysts. The presence of TiO2 and SO4 –ZrO2 promoted the hydrolysis and dehydration of xylose, glucose, cellulose, and lignocellulose to furfural and HMF whereas ZrO2 strongly promoted the isomerization reaction (Chareonlimkun et al., 2010). Lignin is one of the major fractions of woody biomass, which is a polymer of aromatic compounds, such as coniferyl alcohol, sinapyl alcohol, and coumaryl alcohol, and constitutes up to 30% of the weight and 40% of the energy of biomass. Hydrogen and methane can be produced from lignin in supercritical water over several supported metal catalysts using a batch reactor. The order of catalysts for the lignin gasification is as following: Ru/C > Rh/C > Pt/C > Pd/C > Ni/C. The order of hydrogen production is as following: Pd/C > Ru/C > Pt/C > Rh/C > Ni/C. Titania and activated carbon are stable supports of noble metal catalysts for lignin gasification in supercritical water (Yamaguchi et al., 2009).

3. Bio-oil fuels production from biomass wastes Sub-critical water hydrolysis is a promising liquefaction process, which can be used to convert a broad range of biomass wastes to liquid fuels. In the past, cattle manure has been considered a cheap fertilizer. Due to the intensification of livestock practice, it is now considered a waste product that causes environmental pollution. In 2007, the global cattle manure production reached over 23 million tons per day. Yin et al. (2010) studied the sub-critical hydrothermal liquefaction of cattle manure in the presence of NaOH. The maximum bio-oil yield of 48.78 wt.% was obtained at 310 ◦ C, a residence time of 15 min and the cattle manure to water mass ratio of 0.25. The heating value of bio-oil was 35.53 MJ/kg on average. The bio-oil contained function groups of poly alcohols, carboxylic acids, phenol derivatives and alpha, beta-unsaturated ketones. In the case of swine manure, an estimated 5.3 million tons of swine manure is produced annually in the U.S. Swine manure was hydrothermally liquefied in a 1 L batch reactor over a temperature range of 260–340 ◦ C, a holding time of 0–90 min by Xiu et al. (2010). Swine manure cannot be liquefied completely at a temperature of 260 ◦ C. Increasing the temperature from 260 ◦ C to 340 ◦ C was found to increase the oil yield from 14.9% to 24.2%. The maximum oil yield of 24.2% was obtained under the following conditions: reaction temperature of 340 ◦ C, retention time of 15 min. Oil palm is the most important agriculture crop in Malaysia. Decomposition of oil palm fruit press fiber waste to various liquid products in sub-critical water was investigated by Mazaheri et al. (2010). When the reaction was carried out in the absence of catalyst, the conversion of solid to liquid products increased from 54.9% at 210 ◦ C to 75.8% at 330 ◦ C. Simultaneously, the liquid yield increased from 28.8% to 39.1%. The liquid products were sub-categorized to bio-oil (benzene soluble, diethylether soluble, acetone soluble) and water soluble. When 10% ZnCl2 was added, the conversion increased slightly but gaseous products increased significantly. However, when 10% Na2 CO3 and 10% NaOH were added independently, the solid conversion increased to almost 90%. In the presence of catalyst, the liquid products were mainly bio-oil compounds. Although solid conversion increased at higher reaction temperature, the liquid yield did not increase at higher temperature. Qian et al. (2007) studied the direct liquefaction of silver birch (a widely used woody material in China) at temperatures ranging from 280 ◦ C to 420 ◦ C in the liquid water with sodium carbonate as the catalyst. The highest heavy oil yield of 53.3% was obtained at 380 ◦ C. The heavy oil was analyzed by FTIR and GC/MS. The analytical results show that the heavy oils are complex compounds

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containing hydrocarbon, aldehyde, ketone, hydroxybenzene and ester. Kruse and Gawlik (2003) investigated biomass waste conversion in sub- and supercritical water at 330–410 ◦ C. The number of compounds formed during biomass reaction in near-critical and supercritical water is huge. Key compounds identified of bio-oil are phenols (phenol and cresols), furfurals, acids (acetic acid, formic acid, lactic acids, and levulinic acid), and aldehydes (acetic aldehyde and formic aldehyde). Sawdust can be converted to bio-oil by hydrolysis in subcritical water. The biomass to water ratio has an important effect on product distribution and composition of oil products. Catalyst, K2 CO3 can increase the yield of bio-oil (Karagöz et al., 2006) Microalgae are an especially promising feedstock for advanced biofuels because of their higher photosynthetic efficiency, faster growth rate, and higher area-specific yield relative to terrestrial biomass. Microalgae can be cultivated in saline/brackish water and on non-arable land so there is no competition with conventional crop land. Further, microalgae have the ability to accumulate large amounts of lipids. Conventional thermochemical methods for fuel production from biomass (e.g., gasification, fast pyrolysis) require a dry feedstock or they will suffer a large energy penalty from vaporizing the moisture content. Many potential feedstocks, and especially microalgae, however, have very high moisture contents. Therefore, aqueous-phase processing of such biomass feedstocks is attractive from an energy perspective. Subcritical water hydrolysis technology can be employed as a method for conversion of wet biomass into biofuels. Duan and Savage (2010) produced crude biooils from the microalga by subcritical water hydrolysis at 350 ◦ C in the presence of six different heterogeneous catalysts. The heating value of the crude oil produced is about 38 MJ/kg.

4. Reducing sugar production from biomass wastes Reducing sugar, as a biomass energy precursor, can be further transformed to fuel alcohol in a fermentation process by means of micro-organisms or yeast. Cellobiose is a suitable model compound to study the hydrolysis of glycosidic bond and other reactions since cellobiose is the repeating unit of cellulose. The decomposition of cellobiose in supercritical water with and without sulfates has been performed by Kim et al. (2004) using a continuous reaction apparatus. For the reactions in pure supercritical water the conversions of cellobiose increase from 36.7% to 59.3% while the selectivities of glucose decrease from 19.9% to 12.8% by changing residence time between 0.05 and 0.15 s at 400 ◦ C and 30 MPa. Introducing CuSO4 into supercritical water and keeping the concentrations between 6.4 × 10−5 and 3.2 × 10−4 mol/l, they found that the conversions of cellobiose increase from 49.1% to 96.7% and the selectivities of glucose vary between 25.3% and 62.5% with residence time. The glucose selectivities of CuSO4 are three or four times greater than those of pure supercritical water. It is evident that high glucose yields may be obtained in a very short reaction time, e.g., 0.05–0.15 s, by employing extremely small amount of metal sulfates. Ma et al. (2010) investigated cellulose hydrolysis in sub-critical water to produce glucose. The maximum glucose yield is 14.3% at 280 ◦ C and 60 s without catalyst. Catalyst ZnCl2 , FeCl3 , CuCl2 or AlCl3 can accelerate the cellulose hydrolysis and the glucose decomposition simultaneously. With 0.01% catalyst AlCl3 , the maximum glucose yield is 46.05% at 260 ◦ C and 120 s. The ratio of charge and ionic radius, e/r, has been used as an approximate measure for the electron-withdrawing ability, or in other word, Lewis acidity of metal cation. Since Al3+ has a relatively higher value of e/r, AlCl3 is a strong Lewis acid. The acidic function of alumina is supposed to promote the hydrolysis of the cellulose into glucose.

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Fig. 1. Experimental setup of the continuous-flow hydrolysis reactor.

Lü and Saka (2010) produced reducing sugars from Japanese beech by sub-critical water hydrolysis. After the treatments, the monosaccharides, oligosaccharides and decomposition products in water-soluble portion were determined. The results demonstrated that the production of total saccharides increased with the temperature. The maximum yield of total saccharides was achieved at 250 ◦ C when treated by semi-flow sub-critical water. Corn stalks and wheat straw are two important lignocellulosic biomass wastes. Fermentable hexose can be produced from the two biomass wastes by hydrolysis in sub-critical and supercritical water. The highest yield of fermentable hexoses from corn stalks (27.4% of raw material) was obtained at 280 ◦ C, 27 s, and from wheat straw (6.7% of raw material) at 280 ◦ C, 54 s. Compared with the conventional technologies including acid treatment and enzymatic hydrolysis, the combined supercritical/sub-critical technology demonstrates obvious advantages, such as much higher reaction rate, not requiring additional catalyst, and no inhibitory reaction of intermediates (Zhao et al., 2009). Wood from sugi is one of the most common softwood in Japan. Saccharides can be produced from sugi wood powder quickly and effectively by treatment in sub-critical water. High yield of total saccharides was obtained at 310–320 ◦ C and 25 MPa. It appears that the decomposition of hemicellulose started (Matsunaga et al., 2008).

5. Amino acid production from biomass wastes Amino acids can be produced from protein-rich biomass wastes by hydrolysis in sub-critical water. The reactor systems, various biomass wastes, reaction mechanism, and effects of different factors (temperature, time and carbon dioxide) on yield of amino acid are discussed as follows.

5.1. Reactor system Biomass liquefaction in sub-critical water is a novel process. As characteristic reactors for sub-critical water liquefaction, continuous-flow apparatus and batch reactor are introduced. The continuous-flow apparatus is depicted in Fig. 1. This reactor was used for amino acid production from bovine serum albumin (BAS) by Rogalinski et al. (2005). The piping and the reactor consisted of non-corroding and heat-resistant Cr–Nistainless steel 1.4404 with an inner diameter of 3.05 mm (outer diameter 6.35 mm). The feed suspension was pressurized to an overpressure of 0.2 MPa with nitrogen in order to increase the pump’s inlet pressure and to prevent the feed from contact with air oxygen. The BSA solution was delivered to the preheater by the feed pump and heated up to a temperature below 453 K. In previous experiments this temperature was found to be low enough to avoid the start of the hydrolysis reaction. Pure water was pumped from the water reservoir by another pump and was heated. The feed solution was mixed with the hot water flow at the mixing point. Thus, the reaction mixture was rapidly heated to the desired reaction temperature. Immediately after leaving the reactor, the medium was cooled down in order to obtain a well-defined reaction time. For this purpose, the suspension was passed through a pipe-in-pipe heat exchanger. After quenching, the medium was depressurized so that samples could be taken. For monitoring the experimental conditions, temperatures in the preheaters as well as before and after the reactor were measured by a temperature control unit. The system pressure was adjusted by an overflow valve. The schematic diagram of the batch reactor apparatus is shown in Fig. 2. This reactor was used for amino acid production from bean dregs by Zhu et al. (2010b,c). The 300 ml batch reactor was made of (316) stainless steel. The experimental system includes water tank, high pressure metering pump, feeding vessel, nitrogen or carbon

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Fig. 2. Experimental setup of the batch hydrolysis reactor: (1) metering pump; (2) nitrogen gas cylinder; (3) heating coil; (4) reactor; (5) emptying; (6) sampling device; (7) safety valve for pressure limitation; (8) valve; T1, T2, temperature controller; P1, Piezometer for pressure indication.

dioxide bottle, feeding funnel, pressure reactor, sampling device, collector, etc. After emptying apparatus and checking no leakage, setting thermostat, putting deionized water quantified into vessel, shut valves and heat. When the temperature and pressure of vessel get to the condition of experiment, inject the feeding suspension (preheated to 90 ◦ C) into vessel by high pressure metering pump. It is very important that the feeding suspension injected into vessel is a small enough quantity when compared with the deionized water in vessel. Thus, the feeding suspension can be rapidly heated to the desired reaction temperature. Sample and analyze hydrolysate. Immediately after leaving the vessel, the sampling device was cooled down in order to obtain a well-defined reaction time. 5.2. Amino acid production from alternative materials Rice (Oyaza sativa) is one of the most staple diets for human especially in Asian countries. About 610 million metric tons of rice are produced annually in recent years. This huge amount of production results in commensurate amount of rice by-products. One of the major by-products is rice bran which accounts for 8% of milled rice. Sub-critical water can be effectively used to hydrolyze deoiled rice bran to produce useful protein and amino acids. The highest yield of amino acids was 8.0 ± 1.6 mg/g of dry bran, and were obtained at 200 ◦ C at hydrolysis time of 20 min. The amount of protein and amino acids produced are higher than those obtained by conventional alkali hydrolysis. The yields generally increased with increased temperature and hydrolysis time. However, thermal degradation of the product was observed when hydrolysis was carried out at higher temperature for extended period of time (Sereewatthanawut et al., 2008). The most common method for the production of rice protein is by alkali hydrolysis followed by acid precipitation. This method is simple because the agents required for the process is easily available. However, as a result of the degradation at high pH conditions, the protein yield is generally low. High pH conditions could also lead to undesirable results including molecular cross-linking and rearrangements resulting in decrease in nutritive value and formation of toxic compounds such as lysinoalanine. Furthermore, the remaining alkali needs to be washed thoroughly from the product, leading to generation of a large amount of wastewater. Alternatively, enzymatic process has been studied. Although the process produces no toxic chemicals, it takes a long time and the high cost of enzymes makes the process uneconomical.

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Spent Brewer’s yeast, the by-product from the brewing industry, is being produced in large amount annually from main beer manufacturers due to increased volume of beer production. It is generally sold primarily as inexpensive animal feed after inactivation by heat, and much of this by-product is considered as industrial organic waste that causes a great deal of concerns. Such wastes are generally incinerated or put into landfill, in which case, the remaining proteins and amino acids, and other useful substances were not recovered. Baker’s yeast cells, used as a model for spent Brewer’s yeast waste, was hydrolyzed in sub-critical water for amino acid production by Lamoolphak et al. (2006). The reaction was carried out in a closed batch reactor at various temperatures between 100 and 250 ◦ C. The reaction products were separated into water-soluble and solid residue. The results demonstrated that the amount of yeast residue decreased with increasing hydrolysis temperature. After 20 min reaction in water at 250 ◦ C, 78% of yeast was decomposed. The highest yield of protein and amino acids were 0.16 and 0.063 mg/mg of dry yeast, respectively. In the silk industry, tons of silk wastes are produced annually from damaged cocoons or from the cocoons that are difficult to unreel. Like silk, major components of silk waste are fibroin and sericin proteins. Non-catalytic hydrothermal decomposition of sericin and fibroin from silk waste into useful protein and amino acids was examined in a closed batch reactor at various temperatures, reaction times, and silk to water ratios to examine their effects on protein and amino acid yields (Lamoolphak et al., 2008). For the decomposition of sericin, the highest protein yield was found to be 0.466 mg protein/mg raw silk, obtained after 10 min hydrothermal reaction of silk waste at 1:100 silk to water ratio at 120 ◦ C. The highest amino acid yield was found to be 0.203 mg amino acids/mg raw silk, obtained after 60 min of hydrothermal reaction of silk waste at 1:20 silk to water ratio at 160 ◦ C. For the hydrothermal decomposition of fibroin, the highest protein yield was 0.455 mg protein/mg silk fibroin (1:100, 220 ◦ C, 10 min) and that of amino acids was 0.755 mg amino acids/mg silk fibroin (1:50, 220 ◦ C, 60 min) (Lamoolphak et al., 2008). Hog hair is one of the most important solid wastes unsuitable for human consumption produced in slaughterhouses. Hog hair constitutes up to 1% of total hog weight, and in Spain, more than 34,000 ton/year are generated as a by-product of the pig. This excess of material is discarded and may become an environmental problem because it is difficult to degrade and the enormous volumes of waste cannot be easily assimilated by natural processes, and therefore require special treatment. A recycling method using subcritical water hydrolysis to convert hog hair from slaughterhouses into amino acids was studied by Esteban et al. (2010). The influence of the reaction parameters such as temperature, time of reaction and initial substrate concentration were investigated in a batch reactor. The quality and quantity of amino acids in hydrolysates were determined and 17 kinds of amino acids were obtained. Under the tested conditions, the highest amino acid yield (325 mg/g protein) was reached at an initial substrate concentration of 10 g/l, a temperature of 250 ◦ C and a reaction time of 60 min. Simple amino acids (alanine and glycine) were mainly obtained at maximum yield of 85 and 55 mg amino acid/g protein, respectively. Sub-critical water hydrolysis was confirmed as an effective and practical process to recover amino acids from hog hair waste (Esteban et al., 2010). China is the largest fishing nation in the world. The country had the largest market for aquatic products, but the utilization rate of aquatic products was less than 30%. About 40–45% of the fish mass is waste material such as bones, bony parts, entrails, and waste meats. These wastes contain a lot of proteins and bio-active matter. Amino acids production from fish waste in sub-critical water with the addition of carbon dioxide has been studied by Zhu et al. (2009). The effects of reaction temperature, time, CO2 and pressure

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on the yield of amino acids were investigated, respectively. The experiment results show that the optimum hydrolysis conditions for amino acid production from fish waste are as follows: temperature 513 K, amount of CO2 5.0 MPa, amount of N2 3.0 MPa and reaction time 18 min. Under this condition, the yield of amino acids can reach 31.53%. The chemical reaction kinetics experiment results indicate that the total velocity constants of fish waste hydrolysis in sub-critical water are 0.036, 0.071, 0.14, 0.33 at 493, 513, 533, 553 K, respectively with the amount of CO2 of 4.9 MPa and pressure (adding N2 ) of 3.0 MPa. The reaction kinetics order is 1.28, active energy is 82.75 kJ/mol and the pre-exponential factor is 1.97 × 107 . 17 kinds of amino acids were obtained in hydrolysate. It is found that the nitrogen and carbon dioxide atmosphere should be used for leucine, isoleucine and histidine production while the air atmosphere might be used for other amino acids (Cheng et al., 2007; Zhu et al., 2008c). Fish meat was easily liquefied by hydrolysis under sub-critical conditions without Oxidants. Amino acids such as cystine, alanine, glycine, and leucine were produced in the temperature range 513–623 K with a maximum yield at 543 K. Amounts of cystine, alanine, glycine, and leucine produced in 5 min at 543 K (5.51 MPa) were 0.024, 0.013, 0.009, and 0.004 kg/kg of dry meat, respectively (Yoshida et al., 1999). Sub- and super-critical water was applied to the hydrolysis of waste fish entrails for the production of amino acids. Two main consecutive reactions were observed in the treatment of fish wastes: hydrolysis of proteins to amino acids and decomposition of amino acids to other products such as organic acids. In this regard, under sub- and super-critical conditions, proper control of reaction parameters such as temperature and time is necessary in order to efficiently obtain high yield of amino acids. The study on the effect of temperature showed that the maximum yield of total amino acids (137 mg/g dry fish) from waste fish entrails was obtained at sub-critical condition (T = 523 K, P = 4 MPa) at reaction time of 60 min using the batch reactor. The amino acids obtained in this study were mainly alanine and glycine (65 and 28 mg/g dry fish, respectively). Under supercritical conditions (e.g., T = 653 K, P = 45 MPa), the yield of amino acids decreases because of higher decomposition compared to production rate of amino acids at high temperatures and pressures. From the study on the effect of reaction time, it was observed that production is favored over decomposition of amino acids at reaction time less than 60 min. It is possible that equilibrium was reached inside the batch reactor after 60 min, giving way for amino acid to decompose. This could be the reason for the decrease in the yield of amino acid after 60 min. The result suggests that proper control of reaction time is necessary in order to get high yield of amino acids (Kang et al., 2001). In a local seafood processing company in Japan, 7–10 tons of shrimp are processed daily. About 30% of this amount, containing mostly shells, is discarded as wastes. Shrimp shells normally contain about 17% chitin and 42% proteins; recovery of these useful products or their derivatives is significant from an industrial and ecological viewpoint (Quitain et al., 2001). The possibility of amino acids and glucosamine production from the treatment of shrimp shells in sub-critical water was investigated by Quitain et al. (2001). Under the tested conditions, the highest amount of amino acids (70 mg/g of dry shrimp shell) from hydrolysis of proteins was obtained at a reaction temperature of 523 K in 60 min. The amount of simple amino acids such as glycine and alanine increased with increasing temperature up to 523 K and decreased thereafter. The solid waste generated from poultry process such as skin, feather, internal organs, blood, bones, and residual meat is one of the most important biomass solid wastes in the world. In order to improve the utilization of bird wastes, the hydrolysis technology optimization and reaction kinetics for amino acid production in sub-critical water have been conducted by Zhu et al.

(2010d). The effects of reaction temperature, reaction time and H2 SO4 concentration in reactant system on amino acid yields were investigated using the orthogonal test and the optimum hydrolysis technology conditions were obtained. The experimental results show that the best hydrolysis technology is: reaction temperature 533 K, reaction time 28 min, H2 SO4 concentration in reactant system 0.02 wt.%. Under this condition, the amino acid yield reaches 11.49%. The results of chemical reaction kinetics experiment indicate that the total velocity constants of chicken intestine hydrolysis in sub-critical water with H2 SO4 concentration in reactant system 0.02 wt.% are 1.52 × 10−2 , 3.35 × 10−2 , 11.58 × 10−2 , and 14.01 × 10−2 at 473 K, 503 K, 533 K, and 553 K, respectively. The reaction kinetics order is 1.52, activation energy is 64.44 kJ/mol and the pre-exponential factor is 1.91 × 105 . This method for amino acid production by hydrolyzing poultry wastes in sub-critical water is simple, efficient and friendly to environment (Zhu et al., 2010f). Soybean is one of the most staple human diets especially in Asian countries. China is rich in soybean and has a long history of soybean cultivation. Soybean is the main oil-bearing crop and an important food resource. Because soybean has comprehensive and rich nutrients, the soybean processing industries are in the ascendant. More than 80,000 tons wastes of bean dregs, the main by-product of soybean processing industry, are produced annually in recent years in China. Bean dregs have a high protein content which is about 20 wt% of bean dregs. Zhu et al. (2010b,c) investigated the possibility of amino production from bean dregs by hydrolysis in sub-critical water. The results show that a variety of amino acids are produced. The concentrations of arginine, lysine and alanine were relatively high in the hydrolysate. Temperature and time have a great influence on the hydrolysis reaction. The effects of reaction temperature and time on concentration of different amino acids vary. The best hydrolysis conditions i.e. reaction temperature 200 ◦ C, reaction time 20 min, were obtained. Under this condition, the total amino acid yield reaches 52.9%. Based on the results, this method could become an efficient method for bean dregs liquefaction and producing valuable amino acid. Feather is one of the most important solid wastes and contains more than 80% proteins. Zhu et al. (2008b) studied the hydrolysis of feather in sub-critical water to produce amino acids. 16 kinds of amino acids were obtained. The optimum hydrolysis technology conditions were determined by the orthogonal test. The research results show that the best hydrolysis technology is: reaction pressure 5.0 ± 0.5 MPa, temperature 250 ◦ C, reaction time 30 min. Under this condition, the amino acid yield reaches 6.54%. This method is renewable, sustainable, efficient, and safe for environment. 5.3. Reaction mechanism Sub-critical water has a high reactivity. The reactions are commonly summarized as “hydrolysis reactions”. These are reactions in which a compound is split by water according to the formal reaction: A − B + H − OH → A − H + B − OH

(1)

Since CO2 , dissolved in water, increases the availability of protons, the addition of CO2 to liquid water catalyses the hydrolysis reactions. Proteins are important bio-polymers. The reaction of sub-critical water can be of interest for producing oligomers and amino acids. In the hydrolysis reaction for proteins, first a proton is attached to the nitrogen atom of the peptide bonding. This leads to a splitting of the bonding, forming a carbo-cation and an amino group. In the next step, a hydroxide ion, from a dissociated water molecule, attaches to the carbon-cation, forming a carboxy group. The hydrolysis reac-

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tion mechanism described above can be shown as Eq. (2). (Brunner, 2009).

H R C N R+H O

H R C N R O H

technology for converting biomass waste to valuable chemicals. This method may provide a practical and economical solution for

O H H + H2O R C + N R R C + N R+ H H O OH H

(2)

amino acid and bio-fuel production and the disposal of biomass wastes.

5.4. The main influence factors on amino acid yield It was observed that the production of amino acids depends primarily on reaction temperature and residence time (Kang et al., 2001; Rogalinski et al., 2005; Zhu et al., 2010b). Two main consecutive reactions were observed in the treatment of biomass wastes: hydrolysis of proteins to amino acids and decomposition of amino acids to other products. In this regard, proper control of reaction parameters such as temperature and time is necessary in order to efficiently obtain high yield of amino acids. In general, the amino acid yield tends to increase as the temperature increases and attain a maximum value at a certain temperature. With further temperature increasing, the amino acid yield decreases. The decrease in amino acid yield may be due to the rapid decomposition rate compared to production of amino acids by hydrolysis of proteins (Kang et al., 2001). The effect of reaction time on amino acid yield is different for various biomass wastes, but the trends are similar. The amino acid yield increases with extension of reaction time, and then decreases with continued extension of reaction time when reaction time is extended to a certain value (Kang et al., 2001; Rogalinski et al., 2005; Zhu et al., 2008a, 2010b). The hydrolysis reaction is catalyzed by acids. Besides the idea of using pure water as reaction medium, the addition of carbon dioxide leads to an acid hydrolyzed catalysis due to the formation of carbonic acid according to Eq. (3) and the connected decrease in pH of the system water/CO2 . CO2 + H2 O  H2 CO3  HCO3 − +H  CO3 2− +2H+

415

(3)

Bovine serum albumin (BSA) was chosen as a model substance for protein hydrolysis. It can be seen that the addition of CO2 had a remarkable effect on the amino acid production: the amino acid amount without CO2 was 36.6 (mg amino acid/g BSA), whereas the addition of CO2 led to an amount that was four times higher (150.3 [mg amino acid/g BSA]). The addition of carbon dioxide had a positive effect on the amino acid production (Rogalinski et al., 2005). 6. Conclusion Waste substances are actually secondary resources in wrong place. Effective recovery of them is not only important for environmental protection, but also for rational utilization of natural resources. Sub-critical water have been widely investigated and developed as technologies for converting lignocellulosic biomass wastes to fuels and for converting protein-rich biomass wastes to amino acids. One advantage of these technologies is that they do not require a drying process for biomass wastes. The drying process, which is required in the conventional process, has a high energy cost. Sub-critical water results in a faster reaction under high temperature and high-pressure conditions. The reaction rate can be easily and continuously controlled by adjusting the temperature and pressure of the system. The other advantages of sub-critical water hydrolysis in comparison to conventional techniques are the harmlessness of the solvent water as well as the short residence times in the range of minutes. The addition of CO2 had a positive effect on the amino acid production and CO2 can be easily separated from the reaction mixture by depressurizing it to ambient conditions. Sub-critical water hydrolysis may become an important

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