Devolatilization diffusion controlled

The objective is to reduce volatiles to below 50-100-ppm levels. In most devolatilization equipment, the solution is exposed to a vacuum, the level of which sets the thermodynamic upper limit of separation. The vacuum is generally high enough to superheat the solution and foam it. Foaming is essentially a boiling mechanism. In this case, the mechanism involves a series of steps creation of a vapor phase by nucleation, bubble growth, bubble coalescence and breakup, and bubble rupture. At a very low concentration of volatiles, foaming may not take place, and removal of volatiles would proceed via a diffusion-controlled mechanism to a liquid-vapor macroscopic interface enhanced by laminar flow-induced repeated surface renewals, which can also cause entrapment of vapor bubbles. 

Two types ofDV actions occur (1) volatile components diffuse to the plastic-vapor interface (called diffusional mass transport), and (2) volatile components evaporate at the interface and are carried away (called conveaive mass transport). If (1) is less than (2), the process is diffusion-controlled. This condition represents most of the plastic devolatilization processes because plastics diffusion contents are usually low. 

Predictions from these models suggest that the devolatilization rate is independent of particle size. This conclusion should not, however, be applied to typical fluidized beds because the results are based on devolatilization of 0.2 mm particles while much larger coal particles are fed to commercial fluidized beds. From data on devolatilization of various coal types in the absence of combustion. La Nauze concluded that the devolatilization time is proportional to d [9]. He ascribed this to an internal diffusion controlled evolution mechanism. 

In devolatilization, one or more volatile components are extracted from the polymer. The polymer can be either in the solid state or in the molten state. Two processes occur in the devolatilization process. First, the volatile components diffuse to the polymer-vapor interface then the volatile components evaporate at the interface and are carried away. Thus, the first part of the process is a diffusional mass transport and the second part a convective mass transport. If the diffusional mass flow rate is less than the convective mass flow rate, the process is diffusion-controlled. In polymer-volatile systems, the diffusion constants are generally very low, and, therefore, in many polymer devolatilization processes the process is diffusion-controlled. 

Theoretical description of devolatilization of particulate polymer can generally be achieved with a relatively high degree of accuracy. In most cases, the process will be diffusion controlled. The diffusion coefficients in solid polymers are very low, ranging from about 10" mys to 10 mys. The temperature in the polymeric particle can usually be taken as constant since the thermal diffusivity (a 10" mys) is many orders of magnitude higher than the diffusion coefficient. 

Devolatilization. Following polymerization of many polymers, removal of residual monomer or solvents is necessary. Ultimately this removal becomes controlled by diffusion from the polymer no matter what type of process is employed (27). Many processes use a steam-stripping operation for this purpose, and others employ devolatilization in a vented extruder. Removal of residual vinyl chloride monomer from poly(vinyl chloride) was an issue of much concern following the discovery that this monomer is a carcinogen. 

Co being the initial interface volatile concentration. In the devolatilization of polymer solutions and polymer melts, the diffusion of the volatiles through the polymer is usually the rate-controlling part of the process [4]. Therefore, the volatile concentration at the interface, Cj, is in equilibrium with the concentration of the volatile in the gas phase. The equilibrium concentration is related to the partial pressure of the volatile in the gas phase, i, by means of Henry s law, Eq. (4), where H, the Henry s law constant, depends on the temperature, pressure, and nature of the volatile. 

In the third regime, where the viscosity of the melt is very high and the degree of superheat has been reduced due to depletion of volatiles and temperature drop, bubbles are hardly formed and existing bubbles grow very slowly. Under these circumstances, the rate of volatile loss is very slow, as it is controlled by the molecular diffusion at the melt/vapor interface. Therefore, for a very low level of volatiles, flash devolatilizers are not efficient. 

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