Controlled thermal closure

Figure 4.23 Nozzles with controlled thermal closure...

CannedFoods, Principles of Thermal Process Control, Acidification, and Container Closure Evaluation, 5th ed.. The Food Processors Institute, Washington, D.C., 1988. D. M. Considine and G. D. Considine, eds.. Foods and Food Production Fnyclopedia, Van Nostrand Reinhold Co., New York, 1982. 

Varying the steam-to-condensate interface level to control the reboiler duty will promote steam leaks in the channel head-to-shell flanged closure. This is caused by the thermal cycling and stresses that result from constantly varying the level of condensate in the channel head. However, when low-pressure steam (<60 psig) is used, this becomes a minor problem, which may be safely ignored. 

The Nazarov cyclization is an example of a 47r-electrocyclic closure of a pentadienylic cation. The evidence in support of this idea is primarily stereochemical. The basic tenets of the theory of electrocyclic reactions make very clear predictions about the relative configuration of the substituents on the newly formed bond of the five-membered ring. Because the formation of a cyclopentenone often destroys one of the newly created centers, special substrates must be constructed to aUow this relationship to be preserved. Prior to the enunciation of the theory of conservation of orbital symmetry, Deno and Sorensen had observed the facile thermal cyclization of pentadienylic cations and subsequent rearrangements of the resulting cyclopentenyl cations. Unfortunately, these secondary rearrangements thwarted early attempts to verify the stereochemical predictions of orbital symmetry control. Subsequent studies with Ae pentamethyl derivative were successful. - The most convincing evidence for a pericyclic mechanism came from Woodward, Lehr and Kurland, who documented the complementary rotatory pathways for the thermal (conrotatory) and photochemical (disrotatoiy) cyclizations, precisely as predicted by the conservation of orbital symmetry (Scheme 5). 

The formation of the C(l)-C(4) a bond via the participation of two 7T electrons of the HOMO requires that the 2p,r orbitals in atoms C(l) and C(4) undergo rotation in the same sense conrotatory mode) so that positive (in-phase) overlap occurs (bonding effect). If they rotate in opposite directions (disrotatory mode) then there will be negative overlap and an anti-bonding effect. Accordingly, the conrotatory closure of (a) in a thermally controlled reaction leads to the cis isomer of dimethylcyclobutene (b) and not to (c). 

Gavin, A. and Weddig, L.M. 1995. Canned Foods Principles of Thermal Process Control, Acidification and Container Closure Fvaluation, 6th edition. Washington, DC The Pood Processors Institute. 

Exposure of the 8-bromo-2,2 -dimethyl-6-nitro-l -phenyl-(2H-[l]benzospiro-pyran-2,2 -indoline) to light causes it to isomerize into the coloured form. In a solvent it rapidly converts to the bleached form and exhibits first-order kinetics.The process is thermally activated and will require volume for the relative rotation of the two parts of the molecule to effect ring closure. However, if the same process is carried out with the dye in a polymer matrix the process now depends on there free volume available for the rotation of the molecule required for ring closure to occur. Studies of the kinetics (Figure 7.15) indicate that simple kinetics is no longer followed and that a more complex analysis is required. Good fits of the data can be obtained if it is assumed that the process is split into two processes. The initial fast decay is essentially the same as that seen in a liquid and indicates that the ring closure process is only subject to thermal control. This implies that these molecules have sufficient free volume available to execute the process. 

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