Thermal cracking disadvantages

More recently a new approach employing BCI3 as reactant (Scheme 1) has been proposed as a substitute for the industrial thermal cracking process [27]. In most cases, quantitative conversion to the product isocyanates was achieved under mild conditions but with the disadvantage of a high BCI3 consumption. 

Partial oxidation is achieved at reactor conditions ranging from 1350 to 1600°C and pressures of up to 15 MPa (150 atm). This process is attractive because it allows utilization of hydrocarbon feeds that could not be handled in the more conventional vapor-phase processes, such as steam reforming. Particular disadvantages of the process (besides the need to furnish pure oxygen for POX reaetor injection) include cost and the inevitability of soot formation either via thermal cracking of the feedstock or through the Boudouard reaction (CO disproportionation). 

Data on the thermal conductivity for a variety of foams used at cryogenic temperatures have been presented by Kropschot Cryogenic Technology, R. W. Vance, ed., Wiloy, New York, 1963, p. 239). Of aJl the foams, polyurethane and polystiyene have received the widest use at low temperatures. The major disadvantage of foams is that they tend to crack upon repeated thermal cychng and lose their insulation value. 

The advantage of sol-gel technology is the ability to produce a highly pure y-alumina and zirconia membrane at medium temperatures, about 700 °C, with a uniform pore size distribution in a thin film. However, the membrane is sensitive to heat treatment, resulting in cracking on the film layer. A successful crack-free product was produced, but it needed special care and time for suitable heat curing. Only y-alumina membrane have the disadvantage of poor chemical and thermal stability. 

The major disadvantage of foams is not their relatively high thermal conductivity compared with that of other insulations, but rather their poor thermal behavior. When applied to cryogenic systems, they tend to crack on repeated cycling and lose their insulation value. 

The main disadvantage is that web temperatures of 275-325° F are required to cure the film. These temperatures affect the paper properties adversely, and therefore higher-quality, more-expensive paper must be used. Even so, blisterii or cracking in the folder may be a problem. Moreover, the solvent emission, although reduced significantly, is not eliminated, and, as with the ultraviolet light-cured inks, paper printed with these thermally-catalyzed inks cannot be recycled in the currently used processes (8, 9). 

Disadvantage. As any distillative process causes higher temperatures, aspects like thermal instability of terpenes are neglected. This results in radically or thermally induced transformation and crack processes of terpenes, forming precursors which subsequently lead to quick aroma destruction and a negative impact on the aroma profile of the concentrated oils. 

A disadvantage of Portland cement coatings is the sensitivity to damage by mechanical or thermal shock. Open tanks are easily repaired, however, by troweling fresh cement into cracked areas. There is evidence that small cracks in cold-water piping are automatically plugged with a protective reaction product of rust combining with alkaline products leached from the cement. In sulfate-rich waters, Portland cement may be attacked, but cement compositions are now available with improved resistance to such waters. 

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