Supercritical Water Conversion Research Articles

Plastic Waste Recycling, Applications, and Future Prospects for a Sustainable Environment

Plastic waste accumulation has been recognized as one of the most critical challenges of modern societies worldwide. Traditional waste management practices include open burning, landfilling, and incineration, resulting in greenhouse gas emissions and economic loss. In contrast, emerging techniques for plastic waste management include microwave-assisted conversion, plasma-assisted conversion, supercritical water conversion, and photo reforming to obtain high-value products. Problems with poorly managed plastic waste are particularly serious in developing countries. This review article examines the emerging strategies and production of various high-value-added products from plastic waste. Additionally, the uses of plastic waste in different sectors, such as construction, fuel production, wastewater treatment, electrode materials, carbonaceous nanomaterials, and other high-value-added products are reviewed. It has been observed that there is a pressing need to utilize plastic waste for a circular economy and recycling for different value-added products. More specifically, there is limited knowledge on emerging plastic waste conversion mechanisms and efficiency. Therefore, this review will help to highlight the negative environmental impacts of plastic waste accumulation and the importance of modern techniques for waste management.

Molecular dynamic study on mechanisms of polyvinylidene fluoride decomposition by using supercritical water

As a thermoplastic fluoropolymer, polyvinylidene fluoride (PVDF) has been widely used as a binder in lithium batteries. However, PVDF is hard to be degraded naturally. Due to the unique properties of supercritical water (SCW), SCW conversion of polymers is considered as a clean and efficient conversion technology. The effects of different reaction parameters on the decomposition of PVDF in SCW were analyzed by the molecular dynamics (MD) method. The results show that higher reaction temperature, longer reaction time, lower raw material concentration, and the addition of oxidant are more conducive to the decomposition of PVDF in SCW. In addition, the molecular structure of products, product yield, and reaction rate of PVDF in SCW under different conditions were analyzed. The decomposition path, HO2 radical formation mechanism, and F element migration path of PVDF in SCW were also studied by using the MD visualization tool. Compared with SCWO, SCWG has a lower reaction rate and the products from SCWG has a longer molecular chain. The main path for HF generation is the direct interaction between the free F atom and H atom in the water molecules.

Structural Group Characteristics of Resins and Asphaltenes of High-Sulfur Natural Asphaltite and Products of Its Conversion in Supercritical Water

The structural group characteristics of resins and asphaltenes, isolated from high-sulfur natural asphaltite of the Ivanovskoe oilfield in Orenburg oblast and the liquid products of its conversion in a flow of supercritical water (SCW) at 400°C and 30 MPa with and without admixture of zinc or aluminum, have been investigated. It has been shown that as a result of SCW treatment, the molecular weight of the resins and asphaltenes decreases two- to fivefold in comparison with the components of the initial asphaltite. This is determined by a decrease in the number of structural blocks in the molecules which become more compact due to the decrease both in the number of rings, predominantly naphthenic ones, in the blocks and in the number of carbon atoms in the aliphatic substituents. The fraction of aromatic carbon atoms substantially increases, and the degree of substitution of the aromatic nucleus decreases in the resins and asphaltenes isolated from the conversion products. The differences in the structural chemical characteristics of the resins and asphaltenes manifest themselves to a greater extent in the case the metal admixtures.

Super-viscous oil conversion in supercritical water fluid

In this article, experimental research results of super-viscous oil conversion in supercritical water are shown. Reaction mixtures consisting of super-viscous oil, carbon substances and complex catalytic additive were influenced by the conversion. A catalytic additive is a mixture of dispersed nickel and copper salts of carbonic acid, aluminum oxide and coal. A content increase of paraffinonaphthenic hydrocarbon up to 76.2 wt % against 40.8 wt % of an initial oil is a main characteristic of supercritical conversion products. Light hydrocarbons formation is implemented by aromatic, resinous and asphaltene components destruction. Based on experimental methods of super-viscous oil conversion in a supercritical water, a principal technological conversion scheme is proposed.

A numerical study on gasification of a single char particle in supercritical water for hydrogen production

Supercritical water gasification technology provides a promising way for the efficient and clean utilization of coal. Char conversion in supercritical water is a complex coupled process of flow, heat transfer and reaction, and particle-scale study can provide important details for its in-depth understanding. A numerical work was conducted in this paper to investigate the influence of multiple factors on the gasification behavior of a single char particle in supercritical water. The results showed that the increase of temperature significantly benefited the char conversion and hydrogen production. Convection promoted the char gasification mainly by enhancing the heat transfer, and its improvement on effective mass transfer was quite limited. The existence of radiation is conducive to the char conversion, and cannot be ignored in some cases. The results derived from surface Damköhler number indicated that the gasification lied in kinetically controlled regime and tended to move towards diffusion controlled regime with larger inflow temperature, lower Reynolds number and larger particle size.

ХАРАКТЕРИСТИКА ТЕРМИЧЕСКОЙ УСТОЙЧИВОСТИ МАСЕЛ ПРИРОДНЫХ БИТУМОВ И НЕФТЕЙ

Актуальность исследования обусловлена тем, что термическая деструкция является одним из самых распространенных процессов переработки нефтяных остатков, тяжелых нефтей и природных битумов. Термические и термокаталитические процессы конверсии перечисленного углеводородного сырья сопровождаются образованием при термодеструкции смолисто-асфальтеновых веществ исходных объектов дополнительного количества дистиллятных фракций («вторичных» масел). Одновременно происходят процессы превращения масел исходного объекта и «вторичных» масел при температурах, соответствующих термической деструкции «слабых» и «прочных» связей в соединениях, входящих в их состав. При выборе оптимальных режимов термической и термокаталитической переработки тяжелого нефтяного сырья должны учитываться результаты оценки термической устойчивости содержащихся в них компонентов. Одним из методов изучения термической стабильности органического вещества осадочных пород является пиролитический анализ в варианте Rock-Eval. Сведения о возможностях этого метода для анализа масел весьма ограничены. В то же время из пирограммы масел можно получить информацию об их термической устойчивости и характеристических температурах деструкции содержащихся в них компонентов. А также получить информацию о различии этих параметров для масел, выделенных из нефтей и природных битумов, отобранных в различных нефтегазоносных провинциях и существенно отличающихся своим компонентным составом. Цель:сравнительный анализ с использованием метода Rock-Eval параметров термической устойчивости масел, выделенных из природных битумов и нефтей, отличающихся суммарным содержанием смолисто-асфальтеновых веществ, а также продуктов их лабораторной биодеградации и конверсии в сверхкритической воде. Объекты: масла, выделенные из 18 образцов нефтей и природных битумов, продуктов их лабораторной биодеградации и конверсии в сверхкритической воде. Методы: анализ элементного состава, ЯМР 1Н-спектроскопия, пиролитический анализ в варианте Rock-Eval. Результаты. Показано, что методом Rock-Eval можно получить информацию о термической устойчивости масел, выделенных из нефтей и природных битумов, отобранных в различных нефтегазоносных провинциях и отличающихся компонентным составом. Пирограммы масел содержат три пика, проявляющихся в различных интервалах температур. Пик S1 (изотерма 180 °С) соответствует процессу испарения бензиновой фракции (НК…180 °С). Пик S2a (180…350 °С) соответствует наложению процесса испарения «средних» фракций масел и деструкции «слабых» C–S, C–O связей в структурных фрагментах их компонентов. Пик S2b (350…550 °С) соответствует наложению испарения «высококипящих» компонентов масел и деструкции «прочных» C–O, C–С связей в их структурных фрагментах. Изученные образцы более чем на порядок (от 0,32 до 3,91) различаются отношением выхода летучих продуктов при 350…550 и 180…350 °С (S2b/S2a), а также отношением выхода бензиновой фракции масел – S1 и (S2b+S2a) (от 0,044 до 0,518). Изученные образцы масел также различаются температурой максимальной скорости выхода углеводородов в процессе пиролиза в области пика S2b (от 445 до 466 °С) и более существенно в области пика S2a (от 292 до 350 °С).

Transformation of lignin under uniform heating. I. Gasification in a flow of water vapor and supercritical water

The paper presents the research results on the lignin gasification in a flow of water vapor and supercritical water (SCW) at uniform heating (1.5 K/min) of the tubular reactor to 928 K. It was revealed that the highest yield of thiols and H2S occurs at T < 613 K; at that, the amount of sulfur in the thiols is by ≈ 3.5 times greater than in the H2S. The temperature dependence of the CO2 yield has a minimum within the range of 703–748 K that results from the superposition of two following processes: exhaustion of O-containing groups in lignin, and acceleration of the steam reforming and water gas shift reactions with increasing temperature. The latter is indicated by a sharp increase in the yield of H2 at T > 748 K. The maximum yield of phenols is observed at 658–703 K. It is shown that in consequence of SCW gasification of lignin, a carbonized microporous residue is formed. According to X-ray diffraction analysis, burkeite, Na6(CO3)(SO4)2 is the major mineral component of the lignin SCW conversion residue.

Effect of the Heating Rate on Palm Oil Mill Effluent (POME) Conversion in Supercritical Water.

Effect of the Heating Rate on Palm Oil Mill Effluent (POME) Conversion in Supercritical Water.

Mathematical model for coal conversion in supercritical water: reacting multiphase flow with conjugate heat transfer

Providing heat for supercritical water gasification (SCWG) of coal by coupling subsequent products oxidation in integrated supercritical water reactor (ISWR) provides an effective method for directional control of temperature field and avoids excessive hot spots caused by uniform heating. An exploratory numerical model incorporating particle-fluid flow dynamics, multispecies transport and thermal coupling between endothermic coal gasification and exothermic product oxidation was established to simulate the reacting multiphase flow process of coal conversion in a novel lab-scale ISWR. An eleven-lump kinetic model was proposed for the prediction of chemical reactions. And the thermal coupling relationship was described by conjugate heat transfer boundary conditions (BC). Detailed physical and chemical field distribution in ISWR were analyzed and influence factors were discussed. The results showed that oxidation of gas products as inner heat source could promote the gasification reaction with only slight or even little maximum temperature increase of the pressure-bearing wall. Coal feeding rate and oxygen supply method significantly affected the field distribution. The multi-injection compressed-air supply method provided a more uniform temperature field but would reduce heat transfer temperature difference. The carbon gasification efficiency (CGE) in the gasification zone could easily reach up to 97% under mild conditions (less than 650 °C).

Conjugated processes of black liquor mineral and organic components conversion in supercritical water

The conversion of black liquor (BL) in supercritical water (SCW) at a pressure of 30 MPa and different temperatures ranged from 400 to 750 °C was studied using a tubular reactor made of stainless steel. It follows from the mass spectrometric analysis that at 600 °C the gasification degree of BL organic matter is α = 65% wt., while when increasing the temperature to 750 °C, α > 100% due to water decomposition. It was revealed that the conversion of the main mineral components of BL (NaOH and Na2S) to Na2CO3 results from their interaction with the products of SCW gasification of BL organic matter. As was detected, solid products of SCW conversion of BL contain elemental sulfur. It is shown that regeneration of alkali is effectively realized by pumping SCW through the layer of Na2CO3 powder that results from removal of CO2, generated through reaction of Na2CO3 + H2O → 2NaOH + CO2.

Interaction of Phenol and Cyclohexanol with Supercritical Water

Transformation of phenol and cyclohexanol was studied in supercritical water (SCW) in a flow reactor at 300 bar, 500–750°С, and LHSV = 1.2–2.0 h–1. At 500–600°С the only products of phenol conversion are aromatic hydrocarbons—benzene, toluene, naphthalene and polycyclic aromatic hydrocarbons. Almost complete conversion of phenol is achieved at 750°С with selectivity to gaseous products (H2, CO2, CH4, C2H4, C2H6) not higher than 30%. A preliminary catalytic hydrogenation of phenol to cyclohexanol facilitates both overall reaction with SCW and the formation of gaseous products. At temperatures as low as 600°С gaseous products are formed from cyclohexanol, and its complete conversion is achieved at 700°С. In the latter case, the yield of the gaseous products reaches 70%, including 40% of C1–C2 hydr°Carbons. The differences in the rate of conversion in SCW and in selectivity to gaseous products between phenol and cyclohexanol at 500–750°С are associated with the stability of the aromatic ring.

Mechanism of glucose conversion in supercritical water by DFT study

Using density function theory (DFT) simulations with B3LYP/AGU-cc-pVDZ level of theory, six chemical reaction pathways of glucose decomposition in supercritical water were proposed to explore the formation mechanism of some main outcomes (levoglucosan, 5-hydroxymethylfurfural, hydroxylacetaldehyde, erythrose, glyceraldehyde and fructose). In addition, seven potential dehydration ways in glucose were investigated with and without the assistance of water molecule. All dehydration reactions are accelerated when water molecules take part in the reaction, because it can effectively lower the energy barrier of dehydration reaction. In supercritical water surrounding with assistant of water molecule, pathways 3 and 4 are preferred routes of glucose conversion to hydroxylacetaldehyde and erythrose with the lowest energy barrier of 127kJ/mol. While a higher energy barrier (163kJ/mol) is required to form glyceraldehyde and fructose in pathways 5 and 6. Levoglucosan and 5-hydroxymethylfurfural are hard to generate during this chemical processes because of their higher energy barrier without water participating in. It seems like that water molecule acts as a magic catalyst that can transfer hydrogen atom in dehydrations, keto-enol tautomerization and structure rearrangement, which reducing the distance of hydrogen atom moving, result in reducing these reactions energy barriers dramatically. The computational results open a window to produce hydroxylacetaldehyde and erythrose in theory. Moreover, it sheds some light on the various proportions of different products in conversion of glucose in supercritical water, as well as suggesting concrete reaction pathways to form these main products, contributing to the elaboration of the mechanism of glucose conversion and cellulose decomposition from a molecule level.

The products of heavy sulfur-rich oil conversion in a counter supercritical water flow and their desulfurization by ZnO nanoparticles

The conversion of heavy sulfur-rich oil (gross-formula CH1.59N0.01S0.027O0.03) under its continuous supply into a counter supercritical water (SCW) flow and the temperature gradient along the reactor axis (at the top −400°C and at the bottom −550°C) at 30MPa has been studied. The products dissolved in SCW were alternately discharged from the reactor into the samplers through two parallel pipelines, on one of which the cell, heated up to 400°C and loaded with ZnO nanoparticles, was mounted. Zinc oxide was synthesized during pumping SCW through a layer of zinc shavings at 400°C, 30MPa. The yields of volatile and liquid products and conversion residue have been found to be equal to 23.5, 62.8, and 11.9%, respectively, relative to the weight of oil supplied into the reactor. Upon SCW conversion the fraction of asphaltene in the liquid products was found to have decreased from 12 to 1wt% while the fraction of saturated hydrocarbons increased by 4–5 times as much, compared to crude oil. The composition of the liquid products is defined by means of gas–liquid chromatography, IR and 1H NMR spectroscopy. When reactants were pumped through the cell loaded with ZnO, zinc sulfide was formed as a result of the interaction of S-containing products with ZnO accompanied by near complete removal of H2S, decrease in the S/C atomic ratio in group components of the liquid products and insignificant increase in the O/C atomic ratio and fraction of resin in the liquid products.

Brown coal gasification in combustion in supercritical water

The yield and composition of conversion products are investigated in a layout that provides countersupply of reagents (brown coal, supercritical water (SCW), and O2) into a vertical tubular reactor and drain of reactants into replaceable collectors under isobaric conditions (30 MPa). The coal (gross formula CH0.96N0.01S0.002O0.31) incorporated into coal-water slurry (CWS) stabilized by starch addition (1 wt.%), was supplied through the top end of the reactor, while SCW and SCW/O2 fluids were supplied through the bottom end. Based on the results of elemental analysis of liquid products and solid residue of conversion, and mass spectrometric analysis of volatile products, we obtained gross reactions of brown coal conversion in SCW and SCW/O2 fluids. It was found that addition of O2 to SCW leads to autothermal conversion conditions and an increase in the contribution from heterogeneous reactions between carbon and water, which provides additional yield of H2 and CH4.

Lewis-acid catalyzed depolymerization of Protobind lignin in supercritical water and ethanol

The use of metal acetates, metal chlorides and metal triflates as Lewis acid catalysts for the depolymerization of soda lignin under supercritical conditions was investigated. The reactions were carried out at 400°C in water and ethanol. Lignin conversion in supercritical water led to formation of insoluble char and resulted in low yields of monomeric products. When the reaction was performed in supercritical ethanol, char formation was inhibited and higher yields of low molecular-weight organic products were obtained. The ethanol solvent was also converted in two ways. Firstly, the lignin depolymerization products were alkylated by ethanol. Secondly, ethanol was converted into a range of higher hydrocarbons including paraffins and olefins. Possible mechanisms of the lignin and ethanol conversion reactions are discussed.

Experimental study on Zhundong coal gasification in supercritical water with a quartz reactor: Reaction kinetics and pathway

China has fairly rich reserves of Zhundong coal but the utilization of it is limited due to the high alkali metal content which can cause great troubles in a traditional boiler. Supercritical water gasification is a novel thermochemical conversion technique to convert wet feedstock into hydrogen-rich gas product with near zero emission. Reaction kinetics of Zhundong coal gasification in supercritical water was investigated with a quartz reactor in this study. The effects of reaction temperature (650–850 °C) and coal concentration (5–15 wt%) on the gasification behavior were investigated in a wide range of residence time (0–30 min). Experimental results showed that the experimental method employed presents good performance in revealing the continuous change of gas formation with residence time. Carbon gasification efficiency can reach nearly 100% at 850 °C with the residence time of 15 min and the mole fraction of hydrogen was more than 50%. A simplified gasification reaction pathway was also claimed at the end. The results show a promising prospect of Zhundong coal conversion in supercritical water and provide some useful information for reaction kinetics model establishment of coal gasification in supercritical water.

Effect of temperature on bitumen conversion in a supercritical water flow

This article deals with a study of bitumen conversion (the gross-formula CH1.47N0.01S0.007) in a supercritical water (SCW) flow continuously supplied at the bottom of the vertically located tubular reactor. At the first stage, bitumen was continuously supplied from the top of the reactor into a counter-current SCW flow (400°C, 30MPa) for 60min. At the second stage (after ceasing the supply of bitumen into the reactor), SCW was pumped through the layer of bitumen residue at uniform (2.5°C/min) temperature increase from 400 to 700°C at 30MPa. The amount and composition of the liquid and volatile conversion products were measured. It is revealed that during bitumen supply into the reactor and subsequent pumping of SCW through the layer of bitumen residue in the temperature increasing mode from 400 to 500°C, the yields of liquid conversion products are equal to 26.9 and 45.4%, respectively, relative to the weight of bitumen supplied into the reactor. Oils are the major components of these liquid products. Participation of H2O molecules in redox reactions became evident due to the formation of CO and CO2 even at 400°C. A significant increase in the yields of H2, CH4, and CO2 are detected at T>600°C. Based on the sulfur balance, it can be stated that the degree of bitumen desulfurization at 400–700°C due to sulphur removal in form of H2S accounts for 21.6wt.% A solid carbonaceous bitumen residue, obtained after SCW conversion, is characterized by high specific surface (224m2/g).

Density functional theory study of glyceraldehyde conversion in supercritical water

Density functional theory calculations of glyceraldehyde conversion were performed to elucidate the relative reactivity between dehydration and retro-aldol reaction. In this study, structures and energies of the reaction system were determined by B3LYP/AUG-cc-pVDZ level of theory to explore the catalytic role of water molecule. The water-assisted retro-aldol reaction is the most favorable pathway among glyceraldehyde conversion. Water molecule participation in the reaction system lowered the threshold energies for both reaction pathways, suggesting that a water molecule acts as homogeneous catalyst. Comparing the threshold energy values between non-water and water-assisted mechanisms, the threshold energy difference in dehydration was found to be greater than that in the retro-aldol reaction. This large catalytic water influence on the dehydration means that the glyceraldehyde dehydration is more enhanced at fairly higher density conditions. The reaction analysis in terms of water molecule participation can explain the influence of supercritical water density upon the glucose conversion and the catalytic role of water molecule in molecular level.

Conversion of sulfur-rich asphaltite in supercritical water and effect of metal additives

The conversions of sulfur-rich asphaltite (the gross-formula CH1.23N0.017S0.037O0.01) in supercritical water (SCW) flow at 400°C, 30MPa without and with addition of aluminum and zinc shavings to asphaltite have been studied. At SCW conversion of asphaltite without addition of metals the yields of volatile and liquid products were found to be equal to 10.3 and 46.0%, respectively. The amount of oil in the liquid product was by 1.6 times higher than that in raw asphaltite. Hydrogen evolution during the oxidation of 〈Al〉 and 〈Zn〉 by supercritical water provided for the hydrogenation of asphaltite in situ. When 〈Al〉 and 〈Zn〉 were added, the portion of the insoluble conversion residue decreased from 44.5 up to 11.3 and 26.3%, respectively. The degree and efficiency of asphaltite hydrogenation with addition of 〈Al〉 were higher than the ones with addition of 〈Zn〉. The amount of O-containing substances in the products and the conversion residue was found to have increased as compared with raw asphaltite. At conversion without addition of metals, the bulk of oxygen was mainly concentrated in the conversion residue, while with addition of 〈Al〉 and 〈Zn〉 it was detected in the composition of CO and CO2. According to the GC–MS, IR and NMR 1H spectroscopy data, addition of metals to asphaltite resulted in decrease in the content of sulfoxides and carbonyl-containing substances and in increase in the content of polyaromatic substances in the liquid products. When 〈Al〉 was added to asphaltite, more than 70% of sulfur passed into H2S and when 〈Zn〉 was added, more than 60% of sulfur passed into ZnS.

Hydrogenation of bitumen in supercritical water flow and the effect of zinc addition

The composition of liquid and volatile products and residue of bitumen conversion in supercritical water (SCW) flow at 400°C and 30 MPa with and without addition of zinc savings to bitumen is investigated. The yield of liquid products upon bitumen SCW conversion reaches 47.3%. The data on oxygen and hydrogen balance in the initial bitumen, products, and conversion residue indicate the chemical participation of H2O molecules in the conversion. The yield of liquid products is increased up to 62.3% if zinc is added to the initial bitumen with a simultaneous increase of the H/C atomic ratio in the products. Hydrogenation is due to hydrogen evolution during zinc oxidation by supercritical water. Under these conditions, ZnO nanoparticles with a size of 50–100 nm are formed. The data of transmission electronic microscopy and X-ray diffraction analysis of oxidized zinc demonstrate the presence of about 0.5 wt % of ZnS and about 1.2 wt % of metal zinc clusters (up to 10-nm size) in the samples. According to the sulfur atoms’ balance accounting products and conversion residue, 12.6 wt % of bitumen sulfur participates in the formation of zinc sulfide.