Unstable fraction

Control of color darkening and sludge formation when cycle oil, cracked gas oil, or some other unstable fraction is blended into fuel. These fractions contain unstable olefins and aromatics in high percentages. 

The process based on cationic polymerization of 1,3,5-trioxane employs a different principle for stabilization of polymer. Trioxane is copolymerized with a few percent of 1,3-dioxolane (or ethylene oxide). The sequence of —OCH2— units is then separated from time to time by —OCH2CH2— units. The product of copolymerization is subsequently heated to eliminate the terminal units (unstable fraction). Depropagation proceeds until the stable —CH2CH2OH group is reached ... 

When cross-linked crystals of thermolysin were applied in peptide synthesis in ethyl acetate, they were stable for several hundred hours at amazingly low enzyme consiunption, whereas a soluble enzyme preparation became inactive within a short period of time. Again it is worthwhile to consider the quality of the soluble enzyme preparation. When soluble thermolysin was stored in mixed aqueous-organic solutions, it lost about 50% of its activity within the first day of incubation only to be then quite stable for the next 15 days. It is possible that the initial inactivation was caused by an unstable fraction of thermolysin and that crystals of thermolysin no longer contained this unstable fraction [118]. Productivity comparable to that of crystals was achieved with thermolysin adsorbed on Amberlite XAD-7 resin which was employed in continuous plug flow reactors with tert-amyl alcohol as solvent [119]. 

From the practical point of view a disadvantage of this process is longer reaction times and the advantage is easier temperature control. The presence of polyEO leads also to a decrease of the molecular weight of the product and decreases the content of the thermally unstable fraction, as shown in Fig. 7.5. 

The low content of the unstable fraction (defined as the fraction of polymer, which is thermally degraded under the standard conditions of measurement) in copolymers of TXN with small amounts (2-5 %) of DXL is an evidence that this condition is fulfilled, i.e. that the ethylene oxide units are uniformly distributed in the chain. 

Table 7.17. The content of the unstable fraction in the l,3,5-Trioxane-l,3-Dioxolane and 1,3,5-Tri-oxane-l,3-Dioxepane copolymers 138)...

In contrast to Kucera s data Fejgin et al.138) found that the content of the unstable fraction does not change significantly when the proportion of DXL units increases above 2.5 mol %, Typical results are shown in Table 7.17. Similar results were obtained for the radiation-induced polymerization copolymer prepared from a 95 5 mixture of TXN and DXL still contained 12% of the unstable fraction. 

These discrepancies may be due, at least in part, to the different methods of copolymerization and different chances of copolymers to reach the statistical distribution, and/or to the different determination methods of the unstable fraction content. 

Comonomer [Comonomer] PBF3] %mol %mol Yield %wt Unstable fraction %wt Comonomer units in copolymer %wt fo] dig 1... 

The main limitation to thermal conversion is that the products can be unstable. Thermal cracking at low pressure gives olefins, particularly in the naphtha fraction such olefins yield an unstable product that tends to form gum as well as heavier products that form sediments (5). 

The flask is then heated in an oil bath maintained at a temperature of 190-210° and the mixture subjected to steam distillation. The sulfonate groups are hydrolyzed in this process and the bromophenol passes over as a heavy, colorless or pale yellow oil. In about one hour the distillate is clear. The product is extracted with ether, the ether is removed by distillation from the steam bath, and the residue is distilled at atmospheric pressure (Note 3). The fraction boiling at 194-200° represents practically pure o-bromophenol. The yield is 70-75 g. (40-44 per cent of the theoretical amount) (Note 4). t -Bromophenol is a colorless liquid with a very characteristic odor. It is rather unstable and decomposes on standing, becoming brown or red in color. 

M, extremely unstable conditions B, moderately unstable C, slightly unstable D, neutral conditions E. slightly stable conditions F, moderately stable conditions Applicable to heavy overcast, day or night The degree of cloudiness is defined as that fraction of the sky above the local apparent horizon which is covered by clouds. 

The fractional conversions in terms of both the mass balance and heat balance equations were calculated at effluent temperatures of 300, 325, 350, 375, 400, 425, 450, and 475 K, respectively. A Microsoft Excel Spreadsheet (Example6-ll.xls) was used to calculate the fractional conversions at varying temperature. Table 6-7 gives the results of the spreadsheet calculation and Eigure 6-24 shows profiles of the conversions at varying effluent temperature. The figure shows that die steady state values are (X, T) = (0.02,300), (0.5,362), and (0.95,410). The middle point is unstable and die last point is die most desirable because of die high conversion. 

All RBMK reactors have positive void coefficients which means that increasing the boiling rate increases the steam fraction in the core which increases reactivity causing more steam void which causes more reactivity and so on. Competing factors provide stability, but startup, shutdown and maneuvering below about 600 MWt are unstable, hence, there is a rule prohibiting extended operation below 700 Mwt. 

Sotolon (4,5-dimethyl-3-hydroxy-2(5H)-furanone) and solerone (4-acetyl- y-butirrolactone) were claimed to be responsible for some aroma characteristic of flor sherries wines. These compounds are present only as traces, and are chemically unstable. A system of two gas chromatographs coupled with a four-port switching valve was used to quantitate these components without previous fractionation. The first chromatograph was equipped with an on-column injector, in order to avoid thermal degradation of sotolon in the heated injector, a DB-5 column and an FID. The second chromatograph was equipped with an on-column injector, a DB-1701 column and an FID. The method allowed quantification of solerone and sotolon at concentrations as low as a few ppb (29). 

H- and 3//-Azepines are generally unstable in aqueous acid solution and the few examples of simple azepinium salts, namely perchlorates,77 bromides,105 picrates35201 and a solitary iodide,105 have been prepared under nonaqueous conditions. The fractional crystallization of oxalate salts has been used for the separation of mixtures of 4- and 6-substituted 3f/-azepines,66 and 3,6-di-tm-butyl- and 2,5-di-tert-butyl-3//-azepine, on treatment with tetrafluoroboric acid in acetonitrile, are converted quantitatively into their crystalline tetrafluoroboratc salts.70... 

These two parameters describe the change in fraction unconverted with a percentage change in kt or in c0. The first sensitivity is also the slope of the curves in Fig. 28. The values of these sensitivities are given in Table IX. In a piston flow reactor where the conversion level is c/c0 = 0.1, the value of Stt is —0.23 for the first-order kinetics, —0.90 for the zero-order kinetics, and —4.95 for the negative first-order kinetics. In the stirred tank reactor, the value of the sensitivities Skt is —0.09 for the first-order kinetics, — 0.90 for the zero-order kinetics, and +0.11 for the negative first-order kinetics. A positive sensitivity means that as kt is increased, the fraction unconverted also increases, clearly an unstable situation. 

---