Überkritisches Wasser als Lösungsmittel

Process Effluent Recycling in the Supercritical Water Gasification of Dry Biomass

The influence of process water recycling during the Supercritical Water Gasification (SCWG) of dry biomasses was investigated. Dry biomass has to be diluted with water to a dry matter content of approximately 10 wt.% to gasify it in the process of supercritical water gasification. The treatment of wastewater in the SCWG process is cost intensive due to organic contaminants; therefore, the recycling of the process effluent is attractive. Salt separation is needed to avoid accumulation of salts in the effluents, since salts enhance corrosion rates and might cause blocking of the flow when the effluent is recycled. The grass Reed Canary Grass and grapevines were gasified. The recycling of the process effluent did not influence the composition of the product gas. In both cases the carbon efficiency decreased by 4% when wastewater was used to dilute the biomass. An increase in organic carbon and potassium in the reactor effluent was observed after gasification of the biomass with recycled process effluent. The addition of potassium hydroxide to the feed as a homogenous catalyst needs to be closely monitored and adjusted according to the potassium content of the reactor effluent. Insufficient salt separation proved to be an issue regarding formation of solid deposits in the reaction system.

Supercritical Water is not Hydrogen Bonded

Thinking about water is inextricably linked to hydrogen bonds, which are highly directional in character and determine the unique structure of water, in particular its tetrahedral H-bond network. Here, we assess if this common connotation also holds for supercritical water. We employ extensive ab initio molecular dynamics simulations to systematically monitor the evolution of the H-bond network mode of water from room temperature, where it is the hallmark of its fluctuating three-dimensional network structure, to supercritical conditions. Our simulations reveal that the oscillation period required for H-bond vibrations to occur exceeds the lifetime of H-bonds in supercritical water by far. Instead, the corresponding low-frequency intermolecular vibrations of water pairs as seen in supercritical water are found to be well represented by isotropic van-der-Waals interactions only. Based on these findings, we conclude that water in its supercritical phase is not a H-bonded fluid.

Governing chemistry of cellulose hydrolysis in supercritical water

At extremely low reaction times (0.02 s), cellulose was hydrolyzed in supercritical water (T=400 °C and P=25 MPa) to obtain a sugar yield higher than 95 wt%, whereas the 5-hydroxymethylfurfural (5-HMF) yield was lower than 0.01 wt %. If the reaction time was increased to 1 s, the main product was glycolaldehyde (60 wt%). Independently of the reaction time, the yield of 5-HMF was always lower than 0.01 wt%. To evaluate the reaction mechanism of biomass hydrolysis in pressurized water, several parameters (temperature, pressure, reaction time, and reaction medium) were studied for different biomasses (cellulose, glucose, fructose, and wheat bran). It was found that the H(+) and OH(-) ion concentration in the reaction medium as a result of water dissociation is the determining factor in the selectivity. The reaction of glucose isomerization to fructose and the further dehydration to 5-HMF are highly dependent on the ion concentration. By an increase in the pOH/pH value, these reactions were minimized to allow control of 5-HMF production. Under these conditions, the retroaldol condensation pathway was enhanced, instead of the isomerization/dehydration pathway.

Ecofriendly porphyrin synthesis by using water under microwave irradiation

Water, under microwave irradiation and at a temperature of 473 K, reaches pressures above 16 bar, being capable to act as catalyst, without the use of organic solvents and oxidants, for meso-substituted porphyrin synthesis. Sustainability of the reaction is proved by E Factor=35 and EcoScale value of 50.5, the highest so far obtained for porphyrin synthesis. Methodology's wide versatility is clearly demonstrated by the good yields obtained for both aryl and alkyl substituted porphyrins. These reaction conditions represent a huge development, not only by using very high concentrations, minimizing organic solvent usage, but also by eradicating toxic expensive solvents and oxidants.

Homogeneous catalysis of valeronitrile hydrolysis under supercritical conditions

Supercritical nitrile hydrolysis can be used for both, amide and acid production as well as waste water treatment, as the hydrolysis products show good biodegradability. The conventional process at ambient conditions requires large amounts of mineral acid or base. Approaches that use supercritical water as a green solvent without a catalyst have been investigated over recent years. Findings for valeronitrile hydrolysis presented recently showed promising reaction rates and valeric acid yields. In an attempt to further maximize product yield and to better understand the impact of the pH, reactions in dilute sulfuric acid (0.01 mol L(-1)) were performed in a continuous high-pressure laboratory-scale apparatus at 400-500 °C, 30 MPa, and a maximum residence time of 100 s. Results from both reaction media were compared with regard to productivity and sustainability.

Subcritical water as reaction environment: fundamentals of hydrothermal biomass transformation

Subcritical water, that is, water above the boiling and below critical point, is a unique and sustainable reaction medium. Based on its solvent properties, in combination with the often considerable intrinsic water content of natural biomass, it is often considered as a potential solvent for biomass processing. Current knowledge on biomass transformation in subcritical water is, however, still rather scattered without providing a consistent picture. Concentrating on fundamental physical and chemical aspects, this review summarizes the current state of knowledge of hydrothermal biomass conversion in subcritical water. After briefly introducing subcritical water as a reaction medium, its advantages for biomass processing compared to other thermal processes are highlighted. Subsequently, the physical-chemical properties of subcritical water are discussed in the light of their impact on the occurring chemical reactions. The influence of major operational parameters, including temperature, pressure, and reactant concentration on hydrothermal biomass transformation processes are illustrated for selected carbohydrates. Major emphasis is put on the nature of the carbohydrate monomers, since the conversion of the respective polymers is analogous with the additional prior step of hydrolytic depolymerization.

Valeronitrile hydrolysis in supercritical water

Nitriles act as important intermediates for the chemical industry and are accessible on a large scale through hydrocyanation or ammonoxidation. Nitrile hydrolysis yields amides and acids used in various applications. The conventional nitrile hydrolysis process relies on stoichiometric amounts of mineral acid or base which inherently deliver great amounts of waste brine. Improving this process towards green chemistry would require reaction conditions which can provide technically significant results without the use of catalysts. Under these conditions, the hydrolysis of valeronitrile in pure supercritical water was investigated. The experiments were performed in a continuous high pressure laboratory-scale apparatus at a temperature between 400 and 500 degrees C, 30 MPa pressure and maximal residence time of 100 s. Nitrile conversion and valeric acid selectivity greater than 90% were achieved.