Para hydrogen conversion rate measurement

The catalyzed ortho-para hydrogen conversion rate may be measured in either a flow, or a static, reactor. The former is the more convenient, the latter is generally used for obtaining absolute rates. Both methods have been used to study the extrinsic field effect, but most of the data have been obtained by the flow method. 

The catalytic work on the zeolites has been carried out using the pulse microreactor technique (4) on the following reactions cracking of cumene, isomerization of 1-butene to 2-butene, polymerization of ethylene, equilibration of hydrogen-deuterium gas, and the ortho-para hydrogen conversion. These reactions were studied as a function of replacement of sodium by ammonium ion and subsequent heat treatment of the material (3). Furthermore, in some cases a surface titration of the catalytic sites was used to determine not only the number of sites but also the activity per site. Measurements at different temperatures permitted the determination of the absolute rate at each temperature with subsequent calculation of the activation energy and the entropy factor. For cumene cracking, the number of active sites was found to be equal to the number of sodium ions replaced in the catalyst synthesis by ammonium ions up to about 50% replacement. This proved that the active sites were either Bronsted or Lewis acid sites or both. Physical defects such as strains in the crystals were thus eliminated and the... 

When the rate is measured for a catalyst pellet and for small particles, and the diffusivity is also measured or predicted, it is possible to obtain both an experimental and a calculated result for rj. For example, for a first-order reaction Eq. (11-67) gives directly. Then the rate measured for the small particles can be used in Eq. (11-66) to obtain k. Provided is known, d) can be evaluated from Eq. (11-50) for a spherical pellet or from Eq. (11-56) for a fiat plate of.catalyst. Then 7caic is obtained from the proper curve in Fig. 11-7. Comparison of the experimental and calculated values is an overall measure of the accuracy of the rate data, effective diffusivity, and the assumption that the intrinsic rate of reaction (or catalyst activity) is the same for the pellet and the small particles. Example 11-8 illustrates the calculations and results for a flat-plate pellet of NiO catalyst, on an alumina carrier, used for the ortho-para-hydrogen conversion. 

Specific conversion rates are calculated in the usual way for a flow reactor k = (F/S) ln[(Ceq - C0)/(Ce<, - Cx)], where F is the flow rate (mol s 1), S the total catalyst surface (m2), C, the ortho-para equilibrium ratio at the reactor temperature, C0 the ratio for hydrogen entering the reactor and Cx the ratio for hydrogen leaving the reactor. For different samples of the same catalyst the zero field conversion reproducibility is seldom better than by a factor of 5, but the fractional change AkH = (kH - k0)/ko may often be reproduced to 5%. In some cases a change of 0.5% is measurable. (kH is the specific rate in a field H, k0 that in zero or negligible field). 

Example 12-1 Wakao et al. studied the conversion of ortho hydrogen to para hydrogen in a fixed-bed tubular-flow reactor (0.50 in. ID) at isothermal conditions of — 196°C (liquid nitrogen temperature). The feed contained a mole fraction P-H2 of fc, = 0.250., The equilibrium value at — 196°C is = 0.5026. The catalyst is Ni on AI2O3 and has a surface area of 155 m /g. The mole fraction p-Hj in the exit stream from the reactor was measured for different flow rates and pressures and for three sizes of catalyst granular particles of equivalent spherical diameter, 0.127 mm, granular particles 0.505 mm, and nominal f x -in. cylindrical pellets. The flow rate, pressure, and composition measurements are given in Table 12-1. 

A fourth assay measures the rate of interconversion of ortho and para hydrogen. Since any type of rupture of the hydrogen-hydrogen bond that permits random recombination will be detected by this assay, it was suggested that this is the most specific measurement of hydrogenase. However, the finding that a D2O medium substitutes the exchange reaction (HD formation) for the ortho-para conversion indicates that... 

In considering the properties of the solid surface and its influence on the chemistry of the reactants, I should like to recall to your attention papers by Harrison and McDowell (9) which merit, I believe, a measure of careful consideration. The authors were principally concerned with a detailed and quantitative examination of the phenomenon published in 1941 by Turkevich and Selwood. These authors had found that a mixture of zinc oxide and a,a-diphenyl- 3-picrylhydrazyl was much more powerfully a converter of para- to orthohydrogen than would be concluded on the basis of the mixture law and their separate activities in the conversion process. This phenomenon can be rationalized on the basis of concepts developed by Wigner. The more recent paper of Harrison and McDowell demonstrates, however, that, whereas neither the hydrazyl nor zinc oxide has any marked ability to produce the hydrogen-deuterium exchange reaction at 77° K, the reaction proceeds on the mixture at a rapid and reproducible rate, 2.4 times faster than the parahydrogen conversion on the mixture at the same temperature ( ) and 81 times faster than it would have occurred on the zinc oxide constituent. 

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