Vitrification line

The Windscale Vitrification Plant vitrifies high level (highly active) liquid waste arising from reprocessing operations at Sellafield. The plant operates two identical vitrification lines with a current combined throughput of 350 product containers per year. A third line is currently under construction and will commence operation in the year 2000. The key safety function of the plant is to convert mobile material into a solid immobile form which can be more easily managed, stored, and transported. 

NOx Cell. There are two nitrogen oxide (NOx) cells, one for each vitrification line each contains a single NOx absorber column which receives the off-gas from the vitrification cell process. Their function is to remove radioactive species from the off-gas stream and to absorb NOx compounds in the off-gas. 

Figure 2.29. Schematic TXT cure diagram for a thermosetting system. The gelation and vitrification lines divide the liquid, rubbery and glassy state. Tgo, and Tgoo are the glass transition temperatures of the uncured and fully-cured resin, respectively gei Tg is the temperature at which gelation coincides with vitrification.

In case of glassy amorphous polymers the melting line may be replaced by a vitrification line. This concept may be applied to various systems and table III - 4 summarises. some examples of this thennally induced phase separation (TIPS) process. 

Figure 4.4 Hypothetical effective phase diagram, showing the effects of vitrification.-lines...

The vitrification line generally curves down to lower temperatures from pure polymer as it becomes more highly plasticized see Figure 4.4 (8). Frequently the curvature is well represented by the Fox equation (see Section 8.8). 

Figure 4.13 shows three possible adiabatic trajectories in the CTT diagram. For the trajectory with the lowest ATad(< Tgoo — T0), the straight line is intercepted by the vitrification curve. But when the system vitrifies, the reaction practically ceases, and there is no more possibility of getting out from the glassy state because the only source of temperature increase is the chemical reaction therefore, full conversion cannot be attained, and a postcure step at T > Tgoo is necessary to complete the polymerization. 

In a manner similar to the extension of the full cure line into the glassy region, note that the gelation line is extended beyond the vitrification curve (Fig. 1). This implies that gelation can occur below e Tg once vitrification has occurred this could aflea the storage temperature of partially-reacted materials. 

Isoconversion curves, if shown, would approximately parallel the gelation line (as well as the full cure line) because gelation is considered to be an isoconversion state The extent of conversion after vitrification changes very slowly, but does not cease. Reference to a complete 11 1 diagram enables a time-temperature path of cure to be selected which will follow a desired viscosity-conversion path. 

Fig. 15. TTT cure diagram TJT vs. times to gelation and vitrification. Theoretical (solid lines) First-order kinetics using the following parameters T = —19 °C T, = 166 °C Ej/Em = 0.34 FJPft = 0-19 E, = 12.6 kcal/mole A = 4.5x 10 min" pgy, = 0.75 g,Tg = 49 °C. Experimental , pregel (TBA) , gelation (TBA) Q, vitrification (TBA) , diffusion control (infra spectroscopy) A, gelation (gel fraction). The system studied was Epon 828/PACM-20 (see Fig. 4 caption)...

Fig. 22. Extent of reaction at vitrification vs. reaction temperature for linear free-radical polymerization (styrene) for f = 0.5 and [II, = 0.10 mole/1. The solid line is for the results from the T,-mole-cular weight model [Eq. (21)] the dashed line is for the results from the free volume theory [Eq. (26)]. [Aronhime, M, T., Gillham, J. K. J. Coat. Tech. 56 (718), 35 (1984)]...

Fig. 6.19. Free energies of crystal, (solid line), cluster, (dashed line) and cluster in an ensemble, (dash-dotted line) at different temperatures, T = 0.6 (a) and T = 0.55 (b). Kinetic vitrification temperature is close to 0.6. Temperature, free energies and radius are scaled by Tm, Hm, and a, respectively...

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