Subcritical Water Conditions Research Articles

Two-step sub/supercritical water and ethanol processes for non- catalytic biodiesel production

The catalyst-free two-step process has been developed for biodiesel production using low-grade feedstocks. The first step consists of triglycerides hydrolysis under subcritical water conditions to generate and increase free fatty acid (FFA) content for ethyl ester production. In its subcritical state, water can be used as both a solvent and a reactant for the hydrolysis of triglycerides. The hydrolyzed product mixture is separated by decantation into the oil phase of FFA (upper layer) and a water phase that contains glycerol (lower layer). In the second step, the hydrolyzed products of free fatty acids were successfully esterified to their ethyl ester in supercritical ethanol conditions without any catalyst. Under the sub- and supercritical conditions of water and ethanol, the hydrolysis and the esterification reactions proceed quickly, with a conversion of greater than 98 % after 10−20 min. This two-step process for biodiesel production offers several advantages, such as milder reaction conditions and pollution reduction due to the use of water instead of organic solvents. Also, the glycerol is removed after the hydrolysis reaction so that the backward reaction between the glycerol and the ethyl ester disappears, and lead to the biodiesel yield and quality improvement. The aim of this study is making a comparison between our previous one-step process and the two-step reaction process to find the best pathway for designing and building an integrated reactor. Indeed, the two-step process is more applicable for low-grade feedstocks with a high amount of FFA and water.

Stability and Extraction of Vanillin and Coumarin under Subcritical Water Conditions.

In order to facilitate the development of the green subcritical water chromatography technique for vanillin and coumarin, the stability of the compounds under subcritical water conditions was investigated in this work. In addition, their extraction from natural products was also studied. The stability experiments were carried out by heating the mixtures of vanillin and water or coumarin and water at temperatures ranging from 100 °C to 250 °C, while subcritical water extractions (SBWE) of both analytes from vanilla beans and whole tonka beans were conducted at 100 °C to 200 °C. Analyte quantification for both stability and extraction studies was carried out by HPLC. After heating for 60 min, vanillin was found to be stable in water at temperatures up to 250 °C. While coumarin is also stable at lower temperatures such as 100 °C and 150 °C, it undergoes partial degradation after heating for 60 min at 200 °C and higher. The results of this stability study support green subcritical water chromatographic separation and extraction of vanillin and coumarin at temperatures up to 150 °C. The SBWE results revealed that the extraction efficiency of both analytes from vanilla beans and tonka beans is significantly improved with increasing temperature.

Green approach in food nanotechnology based on subcritical water: effects of thyme oil and saponin on characteristics of the prepared oil in water nanoemulsions.

Thyme oil in water nanomulsion was prepared under subcritical water conditions using water and saponin, as solvent and emulsifier, respectively. Gas chromatography revealed that there were 44 bioactive components in the extracted thyme essential oil which, thymol and carvacrol were two mains of them. Experiments were designed based on central composite design and effects of amounts of saponin and thyme essential oil were evaluated on particle size, polydispersity index (PDI) and zeta potential of the prepared nanoemulsions using response surface methodology. Obtained results revealed that more desirable thyme oil nanoemulsions with minimum particle size (184.51nm) and PDI (0.514), and maximum zeta potential (- 22.51mV) were prepared using 0.94g of saponin and 0.28mL of thyme essential oil. Furthermore, results indicated that prepared nanoemulsion using obtained optimum production conditions had relatively high antioxidant activity (24%) and high antibacterial and antifungal activities against Staphylococcus aureus and Penicillium digitatum.

Production of Tagatose from Galactose in a Batch-type Reactor Using a Phosphate Buffer under Subcritical Water Conditions

Isomerization of galactose to produce rare sugars, tagatose and talose, was performed in a Teflon screw-lid container, used as a batch reactor, at approximately 120 °C. Galactose was dissolved at 5% (w/w) in NaH2PO4-Na2HPO4 buffer at pH 6.0, 7.0 or 8.0. A high yield (approximately 13%) for the conversion of galactose to tagatose was achieved at pH 7.0, with slight changes in the solution color. The tagatose yield increased with increasing buffer concentration of 10 mmol/L; however, it was nearly constant at concentrations of >10 mmol/L. Conversely, little talose was produced at buffer concentrations of <10 mmol/L; however, its yield significantly increased at buffer concentrations of 10 mmol/L. When the galactose concentration was ≥5%, the tagatose yield decreased; however, the tagatose concentration increased with increasing galactose concentration; for example, the tagatose concentration was 1.71% (w/w) when the initial galactose concentration was 25% (w/w).

Green biorefinery - the ultra-high hydrolysis rate and behavior of Populus tomentosa hemicellulose autohydrolysis under moderate subcritical water conditions.

A high monosaccharide conversion rate of hemicellulose in a green solvent and under moderate reaction conditions for industrialization is one of the most important keys in a lignocellulosic biorefinery. The behavior of Populus tomentosa hemicellulose polysaccharides, crystallinity and the furfural formation in the autohydrolysis process under moderate subcritical water conditions (160–180 °C, 0.618–1.002 MPa) were studied. The results have shown that the hemicellulose was converted to corresponding monosaccharides at an ultra-high hydrolysis rate. Factor analysis indicates that the temperature is the most important factor affecting hemicellulose autohydrolysis. When the autohydrolysis temperature increased from 160 to 180 °C for 2 h, the hydrolysis rate of xylose, rhamnose, galactose, mannose, and glucose from hemicellulose increased from 70% to 91%, 71% to 100%, 82% to 95%, 42% to 58%, and 34% to 37%, respectively. Arabinose was completely dissolved in 30 min. The xylose, rhamnose, galactose, and arabinose from hemicellulose could be almost completely removed under the conditions. The hemicellulose removal rate obtained herein exceeded the values reported for most acid, alkali, ionic liquid, or deep eutectic solvent treatments. It is notable that almost all glucose in hemicellulose was dissolved and the glucose in cellulose was partially hydrolyzed. An analysis of the sugar composition and the crystallinity change in the process at 180 °C demonstrate that hydrolysis reaction started to shift from amorphous regions to crystalline regions, due to the partial hydrolysis of crystalline cellulose after 90 min at 180 °C. Overall, these results show that the moderate subcritical water autohydrolysis of hemicellulose in Populus tomentosa may be a potential bio-refinery process.

Combined Ultrasound/Microwave Chemocatalytic Method for Selective Conversion of Cellulose into Lactic Acid

Cellulose is the main component of lignocellulosic biomass. Its direct chemocatalytic conversion into lactic acid (LA), a powerful biobased chemical platform, represents an important, and more easily scalable alternative to the fermentative way. In this paper, we present the selective hydrothermal conversion of cellulose and simple sugars into LA, under mild reaction conditions in presence of ErCl3 grafted on the mesoporous silica (MCM-41) surface. High yields and selectivity were obtained for the conversion of sugars under microwave (MW) irradiation at a relatively low temperature (200 °C) and short reaction times (10 min) under microwave (MW) irradiation. Ultrasounds (US) pre-treatment was investigated to reduce the cellulose crystallinity, before the MW-assisted conversion, providing LA with a yield of 64% within 90 min at 220 °C below the subcritical water conditions with increased operational safety. We finally discuss the scalability of the process and the recyclability of the catalyst.

Preparation of Fe3O4@C composite nanoparticles with core-shell structure in subcritical water condition

Fe3O4@C composite nanoparticles with core-shell structure were prepared. The carbon shell was accomplished by the cracking and carbonization of oleic acid (OA) in the subcritical water condition. The cracking products absorbed on the surface of Fe3O4 nanoparticles were characterized by X-ray diffraction (XRD), differential scanning calorimetry-thermogravimetric (DSC-TG) curves, Raman and Fourier transform infrared (FTIR) spectra. The results proved the occurrence of the shell structure with a thickness of 10 nm and it was mainly composed of carbon materials with disordered and crystal structure, which could reach 30% of the mass of Fe3O4. The Reactive Force Field (ReaxFF) molecular dynamic was employed to simulate the cracking process of OA. It is found that the dissociation of CC bonds in OA could lead to the formation of hydrocarbon fragments, which further recombined to form graphite sheet and amorphous carbon, finally mixed and absorbed on the surface of Fe3O4 to form a carbonaceous shell.

Hydrothermal upgrading of heavy oil in the presence of water at sub-critical, near-critical and supercritical conditions

In this study, the hydrothermal upgrading (HTU) of high sulfur-content heavy oil was investigated at sub-critical (Sub-CW), near-critical (NCW) and supercritical water (SCW) conditions. Products obtained after HTU, including gases, liquid, and coke (if formed), were analyzed to understand the upgrading performance at different conditions. At Sub-CW (200, 250, and 300 °C), 250 °C is the optimum temperature where a viscosity reduction from 2073 to 1758 mPa s was achieved with a slight removal of sulfur (mainly sulfur) and the generation of a small amount of light and non-condensable hydrocarbons in gas phase (C1–C4, isoalkanes and alkenes, H2S, CO2 and H2, etc.). At NCW (350 °C) and SCW (400 °C), heavy oil was upgraded into light oil with a significant removal of heteroatoms, an increase of saturates content, a reduction of aromatics, resins and asphaltenes contents, and a high yield of light gaseous hydrocarbons (mainly methane). Simultaneously, each SARA fraction was also greatly ameliorated: the content of light alkanes with low molecular weight in saturates was increased, diaromatics content in aromatics was increased with a reduction of polyaromatics content, aromatics-type carbon atoms in resins was increased with a decrease in aliphatic hydrocarbons. Moreover, MALDI-TOF measurements of asphaltenes show that the molecular weights of asphaltenes were reduced. All these results indicated that HTU at Sub-CW can be used for heavy oil pre-treatment (in-situ or ex-situ upgrading) considering its main effect of viscosity reduction with a small removal of heteroatoms, while HTU at NCW and SWC has a great potential in in-situ and ex-situ upgrading and oil refining as it can convert heavy oil into light oil.

Barley starch behavior in the presence of rutin under subcritical water conditions

Starch-flavonoid complexation has relevance for functional food applications; therefore, this study was carried out to understand the behavior of barley starch and the flavonoid rutin, via treatment in an environmentally friendly media known as subcritical water. Barley starches with varying amylose contents of 37%, 22%, and 0% with rutin (4.90 mg), and without rutin were heat-treated at 80 °C and subcritical water-treated at 100, 120, 140, and 160 °C and 70 bar for 30 min. The highest rutin content (0.87 ± 0.06 mg/g modified starch, dry matter) was obtained for 0% amylose starch at 100 °C. Loss of apparent amylose was an indication of V-amylose inclusion complexation in 37% amylose starch at 100 °C (7.16%), and 22% amylose starch at 120 °C (6.67%). FTIR analysis showed that the rutin complexation in the starches was related to the short-range order of the treated starches. All treated starches expanded but 37% amylose starch with rutin had the highest expansion of 6.10 ± 0.12 mL/g (392% increase) at 160 °C from 1.24 ± 0.01 mL/g of native 37% amylose starch at room temperature (25 °C). Viscoelastic characterization showed that the treated starches were weak gels with 37% amylose, showing higher elastic component than 22% and 0% amylose starch gels. There was no significant difference in storage modulus G’ for 0% amylose starch gels. Yellowness index was high in samples of 0% amylose at 100 °C (6.96 ± 0.00), and 22% amylose at 80 °C (6.60 ± 0.06).

Preparation of Seasoning with Shrimp-like Flavor from the Aqueous Residue of Isada Krill under Subcritical Water Conditions

Preparation of Seasoning with Shrimp-like Flavor from the Aqueous Residue of Isada Krill under Subcritical Water Conditions

Lipids Extraction from Wet and Unbroken Microalgae &lt;i&gt;Chlorella vulgaris &lt;/i&gt;Using Subcritical Water

Lipids extraction from wet and unbroken microalgae (Chlorella vulgaris) using subcritical water with aid of co-solvents has been investigated. Lipids extraction from wet and unbroken microalgae has a crucial role in order to eliminate dewatering and drying steps. Subcritical water is able to extract lipids from feedstock with high water content. This work was conducted to study several factors affecting in subcritical water extraction (SWE) from wet and unbroken microalgae. In this study, effect of co-solvent types (without co-solvent, chloroform, methanol, ethanol, ethyl acetate, and n-hexane) under subcritical water (microalgae = 5g (dry weight), moisture content= 94.12%, T= 160°C, P = 80 bar, t= 30 min), extraction time (15 min, 30 min, 1 h, 3 h, and 5 h), and temperature (160o C, 180o C and 200o C) on yield of lipids were investigated orderly. Yield of lipids obtained without co-solvent (water polarity index = 10.2) and with co-solvents of methanol (PI=5.1), ethanol (PI=5.2), chloroform (PI=4.1), ethyl acetate (PI=4.4), and n-hexane (PI=0.1) were 38.73%, 26.47%, 26.12%, 51.93%, 53.40%, and 25.59%, respectively compared to the yield of lipids extracted using Bligh and Dyer method. Ethyl acetate is solvent with moderate PI, therefore can extract more lipids that is also have moderate polarity. Ethyl acetate shows a good performance to extract lipids from wet and unbroken microalgae because ethyl acetate can extract broader range of lipids including neutral and polar lipids. This study also found that increasing of extraction time and temperature to extract lipids in subcritical water condition can increase yield of lipids.

Amorphous Carbon Doping Nano-Magnesium Silicate Hydroxide with Significant Tribological Property

Nanocomposite lubricant additive consisting of Magnesium silicate hydroxide and amorphous carbon (MSH@C) was prepared. Amorphous carbon in the nanocomposite of MSH@C was obtained by hydrothermal decomposition of polytetrafluoroethylene (PTFE) under subcritical water condition. The MSH@C was characterized by Scanning electron microscope, X-ray diffractometer, Fourier transform infrared spectrometer, Raman spectrometer, and X-ray photoelectron spectrometer. The analysis results show that addition of PTFE in the process of synthesizing MSH can inhibit the tubular crystallization of MSH@C powder. Tribological experiments show that the doping of amorphous carbon materials can significantly enhance the friction reduction and wear resistance of nanocomposite powders as lubricant additive, and is superior to MSH and fully formulated commercial lubricant 5W30. The tribofilm on the worn balls including amorphous carbon originated from MSH@C nanoparticles accounts for the significant tribological properties.

Microwave-Assisted Subcritical Water Extraction of Steviol Glycosides From Stevia rebaudiana Leaves

A rapid method that combines microwave heating and subcritical water extraction to extract steviol glycosides from Stevia rebaudiana (Bertoni) or stevia leaves was developed and compared to conventional thermal solvent extraction methods. Within only 1 minute after subcritical water condition at 140°C being reached, the yields of major steviol glycoside, rebaudioside A and stevioside, and less polar steviol glucoside, rebaudioside C, from stevia leaves were comparable to conventional extraction method that used 70% ethanol under sonication for 45 minutes. This new hybrid extraction technique may provide a cost-effective and green approach for commercial production of high-purity steviol glycoside sweeteners and their “organic” versions without the use of highly expensive “organic” ethanol.

Reducing sugars production from cellulosic wastes by subcritical water hydrolysis in a continuous lab scale unit

Second generation biofuels are those produced from lignocellulosic biomass or crops that do not compete with food production. Cellulose and hemicellulose in plants can be transformed to building blocks, pentoses and hexoses, by hydrolysis. However, most processes require a physic-chemical pretreatment to remove lignin, which acts as a barrier for chemicals during hydrolysis. In fact, pretreatment for removal of lignin or lignin byproducts formed during hydrolysis can be considered as limiting steps for subsequent sugar transformation processes. Alternatively, industrial cellulosic residues, in which lignin and most of hemicellulose have been previously removed through chemical treatment, could be used as raw material for the production of reducing sugars without the drawbacks related to pretreatment and purification steps. Such is the case for diaper industry, which generates a polyacrylate-cellulose dust (POCEL), which cannot be recycled to the process. POCEL is comprised by around of 60 wt. % cellulose and 40 wt. % super absorbent polymer sodium polyacrylate. Due to the high content of cellulose in comparison to lignocellulosic biomass, in which the cellulose content usually oscillate between in the range 20 to 40 wt.%, POCEL could be used as a low cost raw material for glucose production, that can be subsequently fermented to produce bioethanol.In this work, experimental results of POCEL suspensions hydrolysis under subcritical water conditions in a continuous laboratory scale unit are presented. POCEL suspensions in water at 1.2 wt. % were prepared by adding a few drops of hydrochloric acid up to pH 2 to prevent sodium polyacrylate jellification with water. The hydrolysis unit in subcritical water at continuous laboratory scale consisted of a high pressure pump, preheater, a tubular reactor immersed in an electrical oven, heat exchanger and needle valve to regulate system pressure. Experiments were carried out in the temperature range 350 to 420 °C, pressures of 100 to 136 atm and suspension flowrates of up to 6.4 mL/min. Reactor effluent samples were characterized by means of dinitrosalicylic acid (DNS) colorimetric method. The highest concentration of reducing sugars was obtained at 390 °C and a water flowrate of 1.9 mL/min, which corresponds to a residence time of 126.7 s in the reactor. The estimated productivity of sugar production based on a cellulose content of 60 wt.% in POCEL was 85.4%, which indicates this kind of cellulosic waste could be used to obtain high reducing sugars concentration in hydrolysates through a continuous subcritical water process.

Solid Acid Catalysts from Biowaste and Their Potential for Biodiesel Production

Biodiesel production at commercial scale is viable option to substitute petrodiesel in Pakistan. Solid catalyst for biodiesel production has advantages over homogenous catalyst in terms of ease of separation, reusability, economic efficiency etc. This will focus on the production of carboneous material using green carbonization procedure from various biowaste, followed by sulfonation. Biowastes were hydrothermally carbonized under subcritical water conditions under endogenous pressure for more than 4 hours. Resulting carboneous materials were refluxed with sulfuric acid. Structural and morphological characterization, acid density and catalytic efficiency of such materials will be discussed in this study. The sulfonated carbon-based solid acid materials showed high acidity and good catalytic activities for the esterification of oleic acid with methanol.

Kinetics and capacity of phosphorus extraction from solid residues obtained from wet air oxidation of sewage sludge

Solid residues from thermal treatments of sewage sludge (SS) represent a valuable source of phosphorus (P) for the fertilizer production. This study aims at evaluating the P recovery potential from solid residues obtained from wet air oxidation of SS under subcritical water conditions (WAO residues). A series of P extraction experiments was performed by acidic and alkaline leaching at different liquid to solid ratios. Hot chemical extractions and P fractionations were also carried out to characterize the chemical composition of the WAO residues. The main objectives of this work were to determine the best operating conditions for P extraction, and to describe and understand the kinetics and the main mechanisms leading to P release. The results obtained in this study indicate that 1 M citric acid and 1 M HCl at the liquid to solid ratio of 10 L/kg can extract 61% and 65% of the total P content after 2 h of contact time at room temperature, thus giving P extraction capacities of 81 and 86 g P/kg WAO residues, respectively. The analysis of kinetic data indicates that P extraction with 1 M HCl is faster, but 1 M citric acid can give higher P extraction efficiencies at the equilibrium. The molar ratios of Ca to P of the leachates suggest that P extraction from WAO residues was primarily due to the dissolution of a mixture of various Ca-P complexes.

Hydrothermal Dehydration of Monosaccharides Promoted by Seawater: Fundamentals on the Catalytic Role of Inorganic Salts.

In biorefining, the conversion of carbohydrates under subcritical water conditions is a field of extensive studies. In particular, the hydrothermal decomposition of benchmark C6- and C5-monosaccharides, i.e., D-glucose and D-xylose, into furanics and/or organic acids is fully considered. Herein, we propose to establish the fundamentals of the decomposition of D-glucose and D-xylose under subcritical water conditions in the presence of specific salts (i.e., NaCl and KI) and in seawater. Our results demonstrated that the introduction of inorganic salts was found to modify sugars dehydration yields. Different NaCl concentrations from 0.21 to 1.63 mol L−1 promoted the conversion of D-xylose to 2-furfural (2-F) from 28 to 44% (molar yield). NaCl also improved 5-hydroxymethylfurfural (5-HMF) generation from D-glucose as well as rehydration of 5-HMF to levulinic and formic acid. KI favored other pathways toward formic acid production from D-glucose, reaching 20% in the upper concentration. Compared to a solution of equivalent NaCl concentration, seawater enhanced selectivity toward lactic acid which was raised by 10% for both monosaccharides, and sugars conversion, especially for D-glucose whose conversion was increased by 20%. 5-HMF molar yield around 30% were achieved from D-glucose in seawater at 211°C and 20 bars after 15 min.

Coupling of glycerol-APR and in situ hydrodeoxygenation of fatty acid to produce hydrocarbons

Glycerol valorization by coupling APR to yield hydrogen with hydrogenation of a model compound as tandem reactions is approached using Pt-Ni/Al2O3 as efficient catalyst. Oleic acid is used as a C18 fatty acid model compound for in situ hydrogenation reaction. The combination of the two catalytic transformations to a single tandem process appears particularly interesting since this protocol reduces the number of synthetic steps towards the target compound and avoids external hydrogen supply.Glycerol reforming is carried out in a catalytic fixed bed batch reactor under subcritical water condition between 175 and 275 °C (2–6.21 MPa as autogenous pressure). The coupled reactions were carried out using 1 wt% of catalyst, adding oleic acid to the mixture of water/glycerol and without any external hydrogen supply. In APR, maximum glycerol conversion of 80.3% was achieved using Pt-Ni/Al2O3 catalyst at 200 °C with H2-selectivity close to 80% and CO formation below to 3.7%. In the coupled reactions, no hydrogen is observed in the gas-off, which is associated to the hydrogen consumption by oleic acid at 275 °C; reaching a hydrogenation conversion value close to 91.3% and deoxygenation conversion of 32% giving a C17 paraffinic like-hydrocarbon.

Preparation, Characterization and Evaluation of Curcumin Nanodispersions Using Three Different Methods – Novel Subcritical Water Conditions, Spontaneous Emulsification and Solvent Displacement

Abstract Curcumin as a lipophilic bioactive compound can be incorporated into water-based formulations when it turns into curcumin nanodispersions. In fact, nanodispersion systems, increase curcumin bioavailability, solubility and stability, and furthermore increase curcumin uses in aqueous food and pharmaceutical formulations. Present study focuses on the preparation of curcumin nanodispersions under subcritical water conditions (temperature of 120 °C and pressure of 1.5 bar for 2 h) and using selected another two different methods namely, spontaneous emulsification and solvent displacement. Lecithin as carrier oil, Tween 80 as emulsifier and polyethylene glycol as co-surfactant, with a ratio of 1:8:1, were used in all the preparation techniques. Obtained results indicated that curcumin nanodispersions with smallest mean particle size (70 nm), polydispersity index (0.57), curcumin loss (5.5%) and turbidity (0.04 Nephelometric Turbidity Unit), and maximum loading ability (0.189 g/L), loading efficiency (94.5%) and conductivity (0.157 mS/cm) were obtained under subcritical water conditions. The results also exhibited that the prepared spherical curcumin nanoparticles in the water by this technique had desirable physical stability as their mean zeta potential value was (−12.6 mV). It also observed that, as compared to spontaneous emulsification and solvent displacement methods, the prepared curcumin nanodispersions via subcritical water method had highest anti-oxidant and antibacterial activities.

Novel hybrid process for the conversion of microcrystalline cellulose to value-added chemicals: part 3: detailed reaction pathway

In this study, a novel method of hydrothermal electrolysis of microcrystalline cellulose (MCC) under sub-critical water conditions (200 °C) was investigated by applying direct current at constant voltage with the presence of acid catalyst of 5 mM H2SO4. Direct current at constant voltage of 2.5 V, 4.0 V and 8.0 V was applied between cylindrical anode (titanium) and cathode (reactor wall). Hydrothermal electrolysis reactions were carried out in a batch reactor (450 mL-T316) for the reaction time of 240 min. Decomposition products of MCC were analyzed by GC–MS and the decomposition pathway of cellulose under applied voltage was postulated. Levoglucosan and levoglucosenone formations were detected as the first hydrolysis products of MCC and further hydrolysis yielded to formation of glucose and fructose. The major decomposition products of cellulose were detected as levulinic acid (LA), 5-HMF and furfural. Further reactions of LA such as electrochemical decarboxylation, dehydration, hydrogenation, resulted in the formation of 2-butanone, 2-butanone-3-hydroxy, gamma-valerolactone, respectively. Most dramatic results on the product distribution were obtained at applied 2.5 V voltage in which LVA and 5-HMF were selectively produced.