Cloud phase

Aniline and mixed aniline point (DIN 51 775 modified). It is similar to the cloud point test except that the solvent is aniline, a very polar liquid. The aniline point is defined as the temperature at which a mixture of equal parts of aniline and the resin show the beginning of phase separation (i.e. the onset of clouding). Phase separation for aromatic resins occurs between I5°C and below zero for resins with intermediate aromaticity, it lies between 30 and 50°C and for non-aromatic resins, it is 50 to 100°C. Sometimes the mixed aniline point is used. It is similar to the aniline point except that the solvent is a mixture of one part of aniline and one part of w-heptane. The problem of both procedures is that precipitation of resins can be produced before the cloud is generated. 

Because of the inadequacies of the aforementioned models, a number of papers in the 1950s and 1960s developed alternative mathematical descriptions of fluidized beds that explicitly divided the reactor contents into two phases, a bubble phase and an emulsion or dense phase. The bubble or lean phase is presumed to be essentially free of solids so that little, if any, reaction occurs in this portion of the bed. Reaction takes place within the dense phase, where virtually all of the solid catalyst particles are found. This phase may also be referred to as a particulate phase, an interstitial phase, or an emulsion phase by various authors. Figure 12.19 is a schematic representation of two phase models of fluidized beds. Some models also define a cloud phase as the region of space surrounding the bubble that acts as a source and a sink for gas exchange with the bubble. 

Therefore, the area occupied by the bubble and cloud phase is... 

Figure 1. Schematic of two-phase and three-phase representations for fluidized beds operating in the bubble regime B, bubble phase C, cloud phase D, dense phase E, emulsion phase Two-phase models, a and b three-phase models, c...

The paper by Peters t (17) is welcome in that it attempts a new approach to the two phase flow distribution problem. Further details are given in another paper by the same authors (40). However, the authors fail to distinguish clearly between "visible" and invisible flow components in the bubble and cloud phases. At this time their approach must be regarded as a purely empirical method which appears to give a reasonable match with selected experimental data. 

Equations 14 and 16 along with 18 give the concentration profiles in the bubble and cloud phases. The constants and R appearing in these equations can be evaluated subject to initial conditions given by Equations 9 and 10 and can be written in matrix form as... 

Where, i = 1 for the bubble phase, i = 2 for the cloud phase, and i = 3 for the emulsion phase. Note from the term on the right-hand side of Eqn. (1) that a first order rate equation for particulate collection is assumed (10). The inlet gas corresponds to the zero compartment, thus. 

F. Volume ratio of cloud to bubble phases, 62/6]. The volume ratio of the cloud phase to the bubble phase may be estimated from the model of Murray (17)... 

Pressure drop of bed Specific converting power also net transport defined by Eq. (4-7) also dimensionless heat generated or rejected in Figs. 90-91 Flow rate of gas in gas cloud phase... 

The bed is assumed to consist of three phases, the bubble, cloud and emulsion, with the wake region considered to be a part of the cloud phase. 

For a single reaction a generic species mole balance for component A in the stagnant cloud phase gas is written as ... 

In the latter relation the volume of the thin cloud phase is ignored. 

Water vapor exists in the atmosphere in concentrations on the order of grams per m3 of air while its concentration in the aerosol phase is less than lmgm 3 of air. As a result, transport of water to and from the aerosol phase does not affect the ambient vapor pressure of water in the atmosphere. This is in contrast to the cloud phase, where a significant amount of water exists in the form of cloud droplets (see Chapter 17). Thus the ambient RH can be treated as a known constant in aerosol thermodynamic calculations. Considering the equilibrium... 

Let us first check where we are with respect to the Geldhart classification of Figure 8.4. The values of dp and (p, — pfj given as data land us well into the A region of Geldhart A particles, so the parameter correlations presented above should be valid. Further mq = 16M ,y, and 90 u f (shown subsequently), so that we are dealing with a fine-particle bed, fast bubbles, and a thin cloud phase. From equation (8-52),... 

Also, the picture is more complicated than depicted in Figure 12.10a. As the bubble rises, it carries with it a small amount of the solids as wake. Thus a rigorous model should really recognize four regions emulsion, bubble, cloud, and wake. In the K-L model, it is assumed that the wake solids are evenly distributed in the cloud phase. This simplifies the computations without seriously affecting the accuracy. 

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