Ultrahigh-temperature processing

D. S. Hsu, UltraHigh Temperature Processing andAseptic Packaging of Daig Products, Damana Tech., New York, 1979. 

Hsu, D. S., Ultrahigh Temperature Processing and Aseptic Packaging of Dairy Products Dairy and Food Technology Handbook, Damana Tech, 1970. 

Another continuous pasteurization process, known as ultrahigh temperature (UHT), employs a shorter time (2 s) and a higher temperature (minimum 138°C). The UHT process approaches aseptic processiag (Fig. 3). 

Other Continuous Processes. Various pasteurization heat treatments ate identified by names such as quick time, vacuum treatment (vacreator), modified tubular (Roswell), small-diameter tube (MaHotizer), and steam injection. The last three methods are ultrahigh temperature (UHT) processes (see Fig. 3). Higher treatment temperatures with shorter times, approaching two seconds, are preferred because the product has to be cooled quickly to prevent deleterious heat effects. 

In the surrounding world, physical objects, such as various solids, industrial materials, constructions, buildings, and natural substances, are subjected to continuous or periodical impact of a constant mechanical stress that may lead to their creep. For this reason, scientists have widely studied the problems of the creep resistance of metals at ultrahigh temperatures and polymeric and other materials under normal conditions, as well as slow creep processes occurring in some areas of earth crust. These studies have been particularly extensive in the last few decades. 

Also see MILK AND MILK PRODUCTS, Section headed "Processing Milk" PASTEURIZATION and ULTRAHIGH TEMPERATURE STERILIZATION.)... 

Polymer-derived ceramics (PDCs) represent a rather novel class of ceramics which can be synthesized via cross-linking and pyrolysis of suitable polymeric precursors. In the last decades, PDCs have been attaining increased attention due to their outstanding ultrahigh-temperature properties, such as stability with respect to decomposition and crystallization processes as well as resistance in oxidative and corrosive environments. Moreover, their creep resistance is excellent at temperatures far beyond 1000 °C. The properties of PDCs were shown to be strongly related to their microstructure (network topology) and phase composition, which are determined by the chemistry and molecular structure of the polymeric precursor used and by the conditions of the polymer-to-ceramic transformation. 

Fibers produced from pitch precursors can be manufactured by heat treating isotropic pitch at 400 to 450°C in an inert environment to transform it into a hquid crystalline state. The pitch is then spun into fibers and allowed to thermoset at 300°C for short periods of time. The fibers are subsequendy carbonized and graphitized at temperatures similar to those used in the manufacture of PAN-based fibers. The isotropic pitch precursor has not proved attractive to industry. However, a process based on anisotropic mesophase pitch (30), in which commercial pitch is spun and polymerized to form the mesophase, which is then melt spun, stabilized in air at about 300°C, carbonized at 1300°C, and graphitized at 3000°C, produces ultrahigh modulus (UHM) carbon fibers. In this process tension is not requited in the stabilization and graphitization stages. 

The dynamics of high-temperature CO adsorption and desorption over Pt-alumina was analyzed in detail using a transient mathematical model. The model combined the mechanism of CO adsorption and desorption (established from ultrahigh-vacuum studies over single-crystal or polycrystalline Pt surfaces) with extra- and intrapellet transport resistances. The numerical values of the parameters which characterize the surface processes were taken from the literature of clean surface studies ... 

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