Supercritical water pyrolysis

Hydrogen can be produced from biorenewable feedstocks via thermochemical conversion processes such as pyrolysis, gasification, steam gasification, steam reforming of bio-oils, and supercritical water gasification (SWG) of biomass. 

The flash vacuum pyrolysis of alkynes, arynes, and aryl radicals has been reviewed. A discussion of secondary reactions and rearrangements is included. The pyrolysis of cyclopentadienes has also been examined. The rates for the initial C—H bond fission and the decomposition of C-C5H5 have been calculated. A single-pulse shock study on the thermal decomposition of 1-pentyl radicals found alkene products that are formed by radical isomerization through 1,4- and 1,3-hydrogen migration to form 2- and 3-pentyl radicals. The pyrrolysis of f-butylbenzene in supercritical water was the subject of a report. ... 

Conversion of polymers and biomass to chemical intermediates and monomers by using subcritical and supercritical water as the reaction solvent is probable. Reactions of cellulose in supercritical water are rapid (< 50 ms) and proceed to 100% conversion with no char formation. This shows a remarkable increase in hydrolysis products and lower pyrolysis products when compared with reactions in subcritical water. There is a jump in the reaction rate of cellulose at the critical temperature of water. If the methods used for cellulose are applied to synthetic polymers, such as PET, nylon or others, high liquid yields can be achieved although the reactions require about 10 min for complete conversion. The reason is the heterogeneous nature of the reaction system (Arai, 1998). 

Hua et al. (1995) proposed a supercritical water region in addition to two reaction regions such as the gas phase in the center of a collapsing cavitation bubble and a thin shell of superheated liquid surrounding the vapor phase. Chemical transformations are initiated predominantly by pyrolysis at the bubble interface or in the gas phase and attack by hydroxyl radicals generated from the decomposition of water. Depending on its physical properties, a molecule can simultaneously or sequentially react in both the gas and interfacial liquid regions. 

The reaction of benzyl phenyl amine in supercritical water and supercritical methanol is shown to involve parallel pyrolysis and solvolysis reaction pathways. 

Supercritical water can be used to controllably depolymerize the rubber compounds. Tires decompose into high-molecular-weight olefins (MW 1000-10000), or oils (max. 90%). Roy [9] discussed vacuum pyrolysis at 2nd ISFR. 

Pyrolysis in supercritical water [67, 68] owing to the many special characteristics of supercritical water, waste plastics can be degraded efficiently in supercritical water, which has recently received great attention has been studied comprehensively. This technology can not only realize the recovery of valuable products from waste plastics, but also provide a solution to the ever-growing energy crisis and environmental pollution. No catalysts or reaction agents are needed here, so the cost is very low. 

The pyrolysis of mixed plastics containing PVC in supercritical water has also been demonstrated [96]. The temperature in the reactor increases from 200°C at the top to 1200°C at the bottom. HCl is generated in the first reaction zone. In the second zone, HCl continues to react with alkali metal and is removed, and residue and fuel gas which mainly consists of H2 and CHj are produced by reaction of plastic waste and supercritical water. In the third reaction zone, part of the residue produced was oxidized and CO and fuel gases were generated. 

Water used in this supercritical state behaves very differently from water under normal pressure and temperare." In such a supercritical state, the water can be expected to act as an acid or base, but by returning the system to ordinary conditions before pyrolysis occurs, glucose and its derivatives could be obtained in water from cellulose. Therefore, supercritical water treatment can be superior to enzymatic saccharification or oidinaiy acid hydrolysis mentioned above, for the chemical conversion of biomass to useful chemicals. 

The crude black, viscous pyrolysis oil product requires an upgrading step to make it suitable as a refinery feedstock. This is accomplished by high-pressure hydrogenation in a manner very similar to the upgrading step used for the coking products in tar sands processing. The influence of overall process conditions on the polycyclic aromatic compounds found in the product has been examined [62], and the supercritical water extract of the pyrolytic product has been characterized [63]. 

On the other hand, reactions of high molecular mass free radicals, e.g. occurring during pyrolysis, are slowed down by a so-called cage effect caused by solvent molecules at high pressure [5]. In some cases these effects may be reinforced by the special properties of water. A similar effect may be the reason why organometaUic complexes are able to exist and even act as catalysts at rather high temperatures in supercritical water. 

Sonochemical reactions in water are characterized by the simultaneous occurrence of supercritical water reactions, direct pyrolyses, and radical reactions especially at high solute concentrations. Volatile solutes such as CCI4 (13) and hydrogen sulfide (20) undergo direct pyrolysis reactions within the gas phase on the collapsing bubbles or within the hot interfrcial region as shown below ... 

S. Sawamoto, T. Adschiri, K. Arai, Pyrolysis of Hydrocarbons in Supercritical Water in K. Arai (Eds.), Tohoku University Press, Sendai,... 

Unlike polycondensation polymers, polymers of addition polymerization such as polyethylene and polypropylene when depolymerized in inert atmosphere (39) or in supercritical water (37) do not convert to just the monomer, but a homologous series of oligomers (alkanes and alkenes). Compared to pyrolysis in argon, for polyethylene, the portion of the lighter products increases in supercritical water depolymerizations conducted at 693 K and water densities of 0.13 and 0.42 g/cm. The 1-alkene to n-alkane ratio also increases in supercritical water and with density. These are shown in Figure 11. These results are attributed to the fact that in argon pyrolysis, the reaction proceed in the molten state of the polymer, whereas in supercritical water, some of degradation products... 

Kruse, A. and Ebert, K.H., (1996) Chemical reactions in supercritical water - 1. Pyrolysis of tert.-butylbenzene, Ber. Bunsenges. Phys. Chem. 100,80-83... 

Later studies showed that the mechanism of reactions, in particular ionic versus free-radical, could vary. Townsend [15] has studied the reaction of a series of coal model compounds (alkyl-aryl hydrocarbons and ethers) in supercritical water. For the hydrocarbons a free-radical pyrolysis route does not take advantage of the medium. However, for the ethers enhanced rates of reaction through a hydrolysis route occurs. As a result of different possible pathways, decomposition products of some organics in supercritical water have been shown by several workers to vary with solvent strength. In the absence of water, Pr(H20) = 0, pyrolysis is dominant and yields a variety of products including polycondensates. The main products of decomposition of neat methoxy... 

Decomposition products of some organics in supercritical water have been shown by several workers to vary with solvent strength. In the absence of water, pr(H2O) = 0, pyrolysis is dominant and yields a variety of products including polycondensates. The main products of decomposition of neat... 

Plots for yield of hydrogen from supercritical fluid (water) extraction, pyrolysis gasification (water/solid = 2) of beech wood at different temperatures... 

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