Chemical Kinetics in Supercritical Water
J.A. Manion, V. Anikeev, R.E. Huie, and W. Tsang
Objective: To develop an experimental apparatus to obtain transferrable information on the rates and mechanisms of a variety of chemical reactions in supercritical water (SCW).
Problem: The physical properties of water change rapidly near its critical point (374 °C, 221 bar). One result is that rates and mechanisms of chemical reactions can be strongly affected by small changes in process variables. This makes the supercritical environment extremely interesting for use in hazardous waste destruction, the oxidation of biomass and metabolic wastes, and the development of new chemical synthesis strategies. A detailed understanding of the chemical reactions of organic compounds in SCW remains lacking, however, in part due to significant experimental difficult. For instance, the limited solubility of most organics in sub-critical water causes mixing difficulties and can necessitate complex corrections for diffusion. A more serious problem is that many reactions are faster in hot, dense water than in the supercritical environment. Classic static and flow systems involve long heat-up and cool-down times during which the organic is in contact with hot water. This can lead to spurious or ambiguous results. Finally, a widely applicable analytical system is needed, suited to studies of reactions about which little is known. Traditional post-reaction analysis, however, is both time-consuming and difficult in SCW studies because of phase separation of gases and of polar and non-polar liquids.
Approach. We have designed and built an innovative reactor that overcomes many experimental deficiencies of previous approaches. A schematic of the reactor is shown at right. The 125 mL static reactor is rated to 773 K and 600 bar, sufficient for studies with water densities ranging from gas-like to those approaching liquid water. The reactor features precise temperature and pressure controls and a stirrer rated to 3000 rpm. Water is added into the reactor using a high-pressure syringe pump. To avoid studying "hot water" chemistry, the organic of interest is injected directly into the pre-existing SCW environment. An automated high-pressure valve and loop system allows withdrawal and storage of micro-scale samples (6
mL) without perturbing the reactor conditions. Separate analyses of light gases and heavier components are performed on the dual-column GC. Sequential analyses allow the time progression of the reaction to be easily followed in a single experiment, vastly improving the efficiency of data collection.Results and Future Plans. Systematic studies of the reversible dehydration of alcohols are underway. Results to date show that the rate of reaction is much faster in SCW than in the gas phase and is highly dependent on the density of the fluid. These observations suggest that ionic processes are important and occur even at relatively low fluid densities (0.2 g/cm3). It is interesting to note that the reaction in SCW is significantly slower than in hot water. This demonstrates the importance of direct sample injection to obtain the correct kinetic parameters. Studies are being extended to other reaction classes involving hydrolysis and molecular reactions. In conjunction with the experimental program, we are collaborating with NIST molecular modelers and computational chemists to develop better theoretical models of reactions in SCW.
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Last modified: 21 February 2000