Supercritical oxidations reactions

Reaction vessels for supercritical water oxidation must be highly corrosion resistant because of the aggressive nature of supercritical water and oxidation reaction products at extreme temperatures and pressures. Supercritical oxidation of PCBs and some chlorinated hydrocarbons can be difficult... 

One of the most studied technologies is supercritical fluid extraction with SC-CO2. The advantages of SC-CO2 include its low processing temperature, which minimizes thermal degradation the ease of separation with no solvent residue left in the final product and minimization of undesirable oxidation reactions. 

The process employs the supercritical fluid carbon dioxide as a solvent. When a compound (in this case carbon dioxide) is subjected to temperatures and pressures above its critical point (31°C, 7.4 MPa, respectively), it exhibits properties that differ from both the liquid and vapor phases. Polar bonding between molecules essentially stops. Some organic compounds that are normally insoluble become completely soluble (miscible in all proportions) in supercritical fluids. Supercritical carbon dioxide sustains combustion and oxidation reactions because it mixes well with oxygen and with nonpolar organic compounds. 

In the wet oxidation process, materials partially or completely dissolve into a homogeneous, condensed-phase mixture of oxygen and water, and chemical reactions between the material and oxygen take place in the bulk water phase. This condensed-phase makes wet oxidation an ideal process to transform materials which would otherwise be non-soluble in water to a harmless mixture of carbon dioxide and water. Since oxidation reactions are also exothermic, the high thermal mass of supercritical water makes this reaction medium better suited for thermal control, reactor stability, and heat dissipation. The purpose of this research was to establish a new method for selectively oxidizing waste hydrocarbons into new and reusable products. 

Partial wet oxidation or controlled wet oxidation is, in a sense, similar to that of catalytic oxidation. Catalytic oxidation provides a conventional catalyst in order to boost and control the oxidative reaction, whereas wet oxidation provides a favored atmosphere for the reaction to occur. More accurately, catalytic oxidation provides a surface upon which intimate contact between the reactants takes place compared to the thermodynamic (or fluid dynamic) contact provided by wet oxidation. In wet oxidation, it can be said that the supercritical water phase acts as the "catalyst for the reaction. 

Table 10.5 provides performance data regarding the SCWO process. Typical destruction efficiencies (DEs) for a number of compounds are also summarized in Table 10.5, which indicates that the DE could be affected by various parameters such as temperature, pressure, reaction time, oxidant type, and feed concentration. Feed concentrations can slightly increase the DE in supercritical oxidation processes. For SCWO, the oxidation rates appear to be first order and zero order with respect to the reactant and oxygen concentration, respectively. Depending upon reaction conditions and reactants involved, the rate of oxidation varies considerably. Pressure is another factor that can affect the oxidation rate in supercritical water. At a given temperature, pressure variations directly affect the properties of water, and in turn change the reactant concentrations. Furthermore, the properties of water are strong functions of temperature and pressure near its critical point. 

Holgate, R.H. and Tester, J.W., Oxidation of hydrogen and carbon monoxide in sub-and supercritical water reaction kinetics, pathways, and water-density effects. 2. Elementary reaction modeling, ]. Phys. Chem. Technol., 98, 810-822, 1994. 

In 2002, an interesting concept was proposed for coupling a C02-based supercritical extraction with air oxidation in order to remove and decompose pollutants from gases or liquids (134). An exemplary process scheme according to this preliminary concept is shown in Figure 5. Possible (future) environmental applications of such an integrated supercritical extraction-reaction system include treatment of liquid effluents, regeneration of catalysts and adsorption materials, and soil decontamination. 

In many cases, CO2 is seen as the most viable supercritical solvent. It is inexpensive and readily available (by-product of fermentation and combustion), non-toxic and non-flammable. It cannot be oxidised and therefore oxidation reactions using air or oxygen as the oxidant have been intensively investigated. In addition, it is inert to free-radical chemistry, in contrast to many conventional solvents. 

Another important bulk chemical that could be derived from glycerol is acrylic acid (Craciun et al., 2005 Shima and Takahashi, 2006 Dubois et al., 2006). Shima and Takahashi (2006) reported a complete process for acrylic acid production involving the steps of glycerol dehydration in a gas phase followed by the application of a gas phase oxidation reaction to a gaseous reaction product formed by the dehydration reaction. Dehydration of glycerol could lead to commercially viable production of acrolein, which is an important and versatile intermediate for the production of acrylic acid esters, superabsorber polymers or detergents (Ott et al., 2006). Sub- and supercritical water have been applied by Ott et al. (2006) as the reaction media for glycerol dehydration, but the conversion and acrolein selectivities that have been achieved so far are not satisfactory for an economical process. 

Holgate HR, Tester JW. Oxidation of hydrogen and carbon monoxide in sub-and supercritical water reaction kinetics pathways, and water-density effects ... 

On the other hand, it is interesting to use the supercritical C02 as an alternative solvent for various photochemical reactions, particularly for photo-initiated oxidation reactions, due to the non-ignitability of C02. Thus we are studying photo-induced oxidation reactions of ternary mixtures of hydrocarbon/02/C02. Partially oxidized products have been found to be produced in the case of C Hg, ethylene (C2H4) and cyclohexane (C6H,2) which have been studied so far. The present paper mainly reports the case of C Ii, in order to understand the behavior of typical olefinic hydrocarbons which are expected to trap some active species during the reaction. 

In order to explain the data of Aronowitz et al (12) and previous shock—tube and flame data, Westbrook and Dryer (12) proposed a detailed kinetic mechanism involving 26 chemical species and 84 elementary reactions. Calculations using tnis mechanism were able to accurately reproduce experimental results over a temperature range of 1000—2180 K, for fuel—air equivalence ratios between 0.05 and 3.0 and for pressures between 1 and 5 atmospheres. We have adapted this model to conditions in supercritical water and have used only the first 56 reversible reactions, omitting methyl radical recombinations and subsequent ethane oxidation reactions. These reactions were omitted since reactants in our system are extremely dilute and therefore methyl radical recombination rates, dependent on the methyl radical concentration squared, would be very low. This omission was justified for our model by computing concentrations of all species in the reaction system with the full model and computing all reaction rates. In addition, no ethane was detected in our reaction system and hence its inclusion in the reaction scheme is not warranted. We have made four major modifications to the rate constants for the elementary reactions as reported by Westbrook and Dryer (19) ... 

Catalytic supercritical water oxidation is an important class of solid-catalyzed reaction that utilizes advantageous solution properties of supercritical water (dielectric constant, electrolytic conductance, dissociation constant, hydrogen bonding) as well as the superior transport properties of the supercritical medium (viscosity, heat capacity, diffusion coefficient, and density). The most commonly encountered oxidation reaction carried out in supercritical water is the oxidation of alcohols, acetic acid, ammonia, benzene, benzoic acid, butanol, chlorophenol, dichlorobenzene, phenol, 2-propanol (catalyzed by metal oxide catalysts such as CuO/ZnO, Ti02, MnOz, KMn04, V2O5, and Cr203), 2,4-dichlorophenol, methyl ethyl ketone, and pyridine (catalyzed by supported noble metal catalysts such as supported platinum). ... 

A way to further minimize corrosion is by adding base to the feed or reactor, so dial acids formed during the oxidation reaction are immediately neutralized. However, one must then deal with the resulting salts. Whether formed during reaction or already contained in the feed, salts will quickly precipitate in supercritical water. As these salts tend to adhere to and accumulate on the reactor walls and other surfaces within the reactor, they can inhibit and ultimately block process flow unless they are removed or their accumulation is controlled. Nonsalt solids (e.g., metal oxides, grit), by contrast, have little tendency to stick to process surfaces but can be a problem with respect to erosion and system pressure control. Methods that have been developed to manage and/or minimize the impact of corrosion, salt precipitation/accumulation, and solids handling are discussed in Sections 6.5 and 6.6. 

Building on the results of these initial studies, Modell began adding oxygen in order to explore the effect of the supercritical water environment on oxidation reactions of organic species. In 1980, he started MODAR as the first company established to develop and commercialize the SCWO process. Many of the characteristic attributes that are commonly associated with SCWO were first discovered and described by personnel at MODAR, and they established an... 

H. R. Holgate and J. W. Tester, Oxidation of Hydrogen and Carbon Monoxide in Sub-and Supercritical Water Reaction Kinetics, Pathways, and Water Density Effects. 1. Experimental Results, J. Phys. Chem., 98, 800-809 (1994). 

---