Autoclave, internally heated measurements

For pVTx measurements in highly concentrated electrolyte solutions, Franck s group used a variable volume piezometer designed by Hilbert (1979) and shown schematically in Figure 2.6. The sample is filled in a pure nickel bellows to prevent corrosion and to permit elastic volume changes. This cell is located inside a stainless-steel autoclave immersed in an internally heated, argon-filled vessel. 

Despite the importance of the magnetic properties of expanded fluid mercury, experimental difficulties related to the high critical pressure have prohibited measurements of the static susceptibility except close to r j. As discussed in Sec. 3.2.2, the Faraday method is not readily adapted to use with internally heated autoclaves and the high critical pressure prevents the use of sealed sample cells. Thus measurement of the static, uniform susceptibility of mercury under conditions close to the critical point remains an open challenge to experimentalists. 

An exception to the use of internally heated autoclaves is the measurement of magnetic susceptibilities by the Faraday balance technique described in Sec. 7.3.1. 

Fig. 7.1. Internally heated autoclave for equation-of-state and electrical conductivity measurements on fluids at high temperatures and pressures (Gotzlaff, 1987 Hensel, 1987).

A stainless steel autoclave filled with 80 mg (0.1 mmol) of [(-)-Diop]PtCl(SnCl,) is evacuated and cooled to -11 °C. 30 mL (1.77 g, 0.31 mol) of l-butene (measured in a Schlenk tube at — 60 C) and 25 mL of ethylbenzene are introduced by suction. The autoclave is pressurized with 19.95 bar of CO and heated at 100 X in an oil bath. After 1 h the reaction is started by introducing an equivalent amount of H2 and by connecting the autoclave to a reservoir to maintain the pressure in the reaction vessel constant at 81.13 bar. After a gas uptake of 12.5 L (corresponding to an olefin conversion of 70%) the reaction is stopped by cooling, first with water and then with ice to 0 X. The solution is analyzed by GC using a Carbowax column at 100 C for aldehyde analysis and adimethylsulfolanecolumnat r.t. for olefin analysis. The aldehyde yield (63%) is determined by using the solvent as internal standard. 

Wang et al. [26] were the first to develop a power compensation calorimeter. They assumed that heat losses are constant throughout the reaction in order to link the measured and the generated heat. This can be achieved by maintaining the outside of the autoclave at a constant temperature that is slightly lower than that of the reaction. This temperature differential is maintained by the use of a smaller internal heater whose power can be externally controlled. If an exotherm occurs, then the system automatically reduces the power supplied to the internal heater to compensate and to maintain a constant temperature. The opposite happens when an endotherm occurs. 

Quantitative IR spectroscopy has turned out to be very usefid for measuring the decay of peroxide concentration or, alternatively, the inaease in the concentration of products from decomposition. Two types of discontinuous procedures have been used at lower reaction temperatures, the peroxide solution is contained in an internal cell (see Figure 3), which is filled and assembled in a glove box under an argon atmosphere. The internal cell is positioned into the optical high-pressure cell and pressurized with n-heptane acting as the pressure-transmitting medium. The assembly is heated to the reaction temperature and the collection of IR spectra is started. In the case that decomposition rate becomes too fast and a major fraction of the peroxide is decomposed before reaaion conditions of constant T and p are reached, the peroxide solution is dirertly fed into a preheated autoclave. The solution is then quickly pressurized and the collection of IR spectra is started. 

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