Operation Under Reduced Surface Pressure

A variety of swirl motions are known to occur in a bath agitated by gas injection when the bath surface is exposed to the atmosphere, as described in Sect. 5.2.1.4 [18,23, 29-37]. In particular, two types of swirl motions typically occur in a circular cylindrical bath agitated by single-nozzle bottom gas injection, as schematically illustrated in Fig. 5.6 [29, 30] One is observed over an aspect ratio, H /D, from approximately 0.2-1.0. The other appears for H /D 2. No swirl motion occurs when the aspect ratio falls in the range of 1.0-2.0. The former swirl motion is caused by bath surface oscillations due to quasi-periodic generation and subsequent arrival of bubbles at the bath surface. It resembles the rotary sloshing of a water bath contained in a circular cylindrical vessel [16,17,38]. The latter is caused by the Coanda effect [26], which appears when a bubbling jet approaches the side wall of the vessel [29,30,39]. 

The mixing time in a cylindrical water bath with aspect ratio between H /D 0.2-1.0 is significantly shortened when the bath is accompanied by the first kind of swirl motion [35, 37]. This motion may thus be beneficial for shortening the mixing time in real refining processes agitated by bottom gas injection. Most of these processes are operated under reduced pressure on the bath surface. However, measurements of the characteristics of the swirl motion and the relationship between the mixing time and the swirl motion in model experiments have been carried out solely under atmospheric pressure on the bath surface. 

It is quite difficult and dangerous to carry out systematic experiments to study swirl motion using real processes under reduced pressures on the bath surface. Thus model experiments are conducted using water models under two different 

101 kPa. The starting time obtained under atmospheric pressure on the bath smface decreases with an increase in the normal gas flow rate. This is because the energy supplied by gas into the water bath increases with 0gN. A similar trend is observed for the two reduced pressures on the bath surface. In addition, 7s becomes shorter as the pressure on the bath surface, Ps, decreases. 

The relationship between the volumetric gas flow rate at the nozzle tip 2g and Ps can be expressed by, 

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The permeability can be reduced by a physical vapor deposition (PVD) process and chemical vapor deposition (CVD). PVD processes operate under reduced pressure and include evaporation and sputtering, in the absence of chemical reactions in the gas phase and at the substrate surface. An overview of PVD processes show that CVD processes utilizes volatile precursors that are decomposed by means of heat, photons, or plasma. Plasma enhanced CVD (PECVD), is applicable for thermally sensitive substrates, such as polymers. It has become the most widely used process for the deposition of silicone coatings. 

We do not discuss equilibrium because the molecular distillation is a nonequilibrium process. Molecular distillation belongs to the class of processes that uses the technique of separation under high vacuum, operation at reduced temperatures, and low exposition of the material at the operating temperature. It is a process in which vapor molecules escape from the evaporator in the direction of the condenser, where condensation occurs. Then, it is necessary that the vapor molecules generated find a free path between the evaporator and the condenser, the pressure be low, and the condenser be separated from the evaporator by a smaller distance than the mean free path of the evaporating molecules. In these conditions, theoretically, the return of the molecules of the vapor phase to the liquid phase should not occur, and the evaporation rate should only be governed by the rate of molecules that escape from the liquid surface therefore, phase equilibrium does not exist. 

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