T h e primary cracking of pure hydrocarbons both with and without catalysts has been studied in terms of the distribution by carbon number of the cracked fragments to allow arriving a t a mechanism of molecular disintegration. The secondary reactions of the cracked fragments have been followed by analyses of the product fractions to allow a further definition of the nature of the cracking system. On the basis of this work, cracking systems are assigned to two fundamental classes; each class is described by a set of characteristic reactions covering both the primary cracking and the secendary reactions. Correspondingly, two types of reaction mechanisms are proposed, one a free radical (thermal type) mechanism based on the Rice-Kossialroff theory of cracking, the other a carbonium ion (acid-activated type) mechanism RIOR work on the catalytic cracking of pure hydrocarbons P has led to a general characterization of the rates of cracking and product distributions of the principal classes of petroleum hydrocarbons (10-13). I n addition, a number of secondary reactions of olefins have been investigated and the effects of structural isomerism on the rates of cracking of several types of hydrocarbons were examined (9, 54). Consistent mechanisms of reaction are now proposed, based on the primary hypothesis that any hydrocarbon reacting over this type of catalyst is transformed into a carbonium ion (33, which then cracks or undergoes secondary reactions according to definite rules. This hypothesis is directly coupled with the requirement that the acidic oxide type of cracking catalyst must make available reactive positive hydrogen ions (protons) capable of producing carbonium ions on contact with the hydrocarbon feed. A similar type of approach was proposed independently by Thomas (52).The properties of carbonium ions, which are postulated to represent the reactive form of the hydrocarbon in conventional catalytic cracking, also determine the mechanism of reaction and the type of product in many other acid-catalyzed hydrocarbon reactions, such a8 the isomerization, polymerization, parafKn alkylation, and hydrogen transfer reactions of olefins, the isomerization of paraffins, and the alkylation of aromatics. Funda-derived from the work of Whitmore and others on the properties of carbonium ion systems. Cracking catalysts are available for either type of reaction mechanism; those which accelerate free radical type reactions are nonacidic, and those which accelerate carbonium ion type reactions are acidic. Commercial acid-treated clay and synthetic silica-alumina cracking catalysts belong to the latter class. Activated carbon, a highly active, nonacidic catalyst, gives a unique product distribution which is explained as a quenched free radical type of cracking. Activated pure alumina has weakly acidic properties and produces moderate catalysis of both types of reaction mechanism, the primary cracking corresponding to a free radical mechanism and the secondary reactions of product olefins following a carbonium ion mechanism. me...

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The barrier properties of ZrO2 to inward migration of deuterium have been investigated with a view to understanding the hydriding mechanisms of a Zr-2.5% Nb alloy used in CANDU nuclear reactors fuel channels. Thin film oxide specimens, grown in steam to ∼1 μm thickness, have been heated to 350 °C and exposed to deuterium gas at pressures ranging from 6×10−3 Pa to 101 kPa (1 atm) and times from 10 to 810 min. Some irreversible uptake can be measured for all exposures using secondary ion mass spectrometry. At low exposures, the shape of the deuterium concentration profile is can be fitted to a Fickian relationship. During longer exposures, the rate of deuterium ingress is sharply curtailed, presumably due to passivated outer oxide surface. Reactions between D2O vapor and the thin film oxide in the 10−3 Pa pressure region and above show a sharply higher uptake of deuterium than in the equivalent pressure of D2 gas. This is ascribed to a more efficient decomposition of D2O on the ZrO2 surface compared to D2.

Figitre 12. Storage and Finishing Cellar, Five-Barrel P l a n t i Storage and Finishing Cellar. Thc storage cellar, which ib held at 32" t o 34" F. by brine-cooled air diffmers, is somewhat larger than the fermenter room. Figure 12, a picture of the storage area, shows most of the equipment in this cellar. There are twelve tanks; most of these are glass-lined, but mveral have a baked phenolic lining. Each holds one brew.This area is also used for filtration, carbonation, and othei Enishing operations. There is a stainled8 steel diatomaceous earth filter, with 5 square feet of filter area, and a small pulp filter and pulp cake press. Transfers are made through 1.5-inch brewers' hose, using either a rotary pump or a cent,rifugal carbonating pump Package filling is performed with a one-arm racker for kegs or a small hand-operated bottle filler Bottled beer is pasteurized in normal fashion.