Pressure-jump relaxation apparatus

Figure 4.5. Block diagram of pressure-jump relaxation apparatus with digitizing interface. [From Krizan and Strehlow (1974), with permission.]...

The apparatus s step change from ambient to desired reaction conditions eliminates transport effects between catalyst surface and gas phase reactants. Using catalytic reactors that are already used in industry enables easy transfer from the shock tube to a ffow reactor for practical performance evaluation and scale up. Moreover, it has capability to conduct temperature- and pressure-jump relaxation experiments, making this technique useful in studying reactions that operate near equilibrium. Currently there is no known experimental, gas-solid chemical kinetic method that can achieve this. 

Knoche, W., and Wiese, G. (1974). An improved apparatus for pressure-jump relaxation measurements. Chem. Instrum. (N.Y.) 5, 91-98. 

A pressure perturbation results in the shifting of the equilibrium the return of the system to the original equilibrium state (i.e., the relaxation) is related to the rates of all elementary reaction steps. The relaxation time constant associated with the relaxation can be used to evaluate the mechanism of the reaction. During the shift in equilibrium (due to pressure-jump and relaxation) the composition of the solution changes and this change can be monitored, for example by conductivity. A description of the pressure-jump apparatus with conductivity detection and the method of data evaluation is given by Hayes and Leckie (1986). 

Evaluation and Determination of Relaxation Times and Rate Constants in Single-Step Systems 70 Pressure-Jump (p-Jump) Relaxation 71 Historical Perspective 71 Pressure-Jump Apparatus 72... 

A sudden pressure release or application of pressure can be employed to cause the pressure jump. Ljunggren and Lamm (1958) described the first pressure-jump apparatus, which consisted of a sample cell connected to a nitrogen tank. With this apparatus, a pressure increase to 15.2 MPa could be obtained in 50 ms by quickly opening the valve. Chemical relaxation was monitored conductometrically. 

Using a pressure-jump apparatus the dissolution time for sodium dodecyl sulphate micelles has been measured in solutions which also contain polyvinyl pyrrolidone. The relaxation time associated with the association/dissolution of the micelles depends on the amount of polymer present and also its molecular weight. In addition the activation energy of the dissociation/ association process decreases in the presence of the polymer. These data suggest that the micelles are incorporated along the polymer chain and a simple mechanism for the dissolution of micelles on the polymer is proposed. 

These studies were undertaken using a pressure-jump apparatus over the time range 10-10 sec. incorporating a rapid data capture and analysis system. The polymer, polyvinylpyrrolidone, PVP, (Aldrich) was used without further purification. Sodium dodecyl sulphate, SDS, (Henkel) was chosen as the surfactant system, since a considerable wealth of equilibrium data were already available involving this surfactant. SDS was purified by repeated recrystallisation from two solvents until the relaxation time at the CMC agreed with literature values. 

Apparatus. (a)Pressure-jump apparatus The Pressure-jump apparatus used is a modification of that described by Knoche and Wiese. A pressure jump of 100 atm was realized within 80 ysec. An advantage of this apparatus is that chemical relaxation induced by pressure jump can be observed simultaneously by both changes in electric conductivity and optical absorbance. (b)Electric field pulse apparatus The electric field pulse apparatus used has already been reported . The electric field... 

The first p-jump apparatus was developed by Ljunggren and Lamm (1958). A conductivity cell was filled with the solution of interest and then placed in an autoclave connected to a 15-MPa N tank. The stopcock was rapidly opened to create a rapid pressure increase. With this method, one could obtain a pressure change of 15 MPa in 0.05 s. Ljunggren and Lamm (1958) followed the relaxation time conductometrically. 

In the work of Zhang and Sparks (1989), a Dia-log p-jump apparatus and conductivity detector were employed (Fig. 3-4). A digitizer is triggered upon the pressure release and membrane burst and the changes in conductivity of the suspension are monitored. The signals are digitized and then sent to a microcomputer. The results of the relaxation can be read from the computer print-out and displayed on an oscilloscope. 

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