Mean residence times of particles

The mean residence times of the particles in various sub-spaces of the impinging stream device are determined by interpretation of the experimental data of 27 sets in total, and the conditions tested range as follows  

Mass flow rate ratio of particles to air mp/ma= 0.466-1.4 and Impinging distance S = 0.06-0.1 m. 

The results of Item (4) above are somewhat unfortunate, because the parameter t-im is the most relevant. The major reason for this may lie in the fact that the operating variables have contradictory influences on t m. For example, an increase in the 

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The total mean residence time of particles in the whole device is between 4.8 and 6.0 s and... 

Particle residence time distribution in a fluidized bed is more close to that of a stirred tank reactor (CSTR) or ideal backmix reactor than that of a plug flow reactor (Yagi and Kunii, 1961). In a perfect plug flow reactor, all particles have the same residence time, which is equivalent to the mean residence time of particles and can be calculated by... 

Mean residence time of particles Air volume rate Temperature of drying air... 

Operating principle. Particles of terminal velocity Vi > mq will tend to settle therefore design for Vi < mq of the smallest particle present in the feed stream. In other words, the settling time should be less than the mean residence time of the up-flowing fluid. 

The atmospheric reservoir (8) represents P contained on dust particles. Because the mean residence time of dust in the air is very short, the standing stock of P in the atmosphere is rela-... 

The piston flow case assumes that the particles spend the same time in the reactor, i, even though the fluid phase is well mixed. This case resembles the mass transfer situation of piston flow in contact with a CSTR as considered in Section 11.1.4. The particles leave the reactor with size Ro — kf i. None will survive if f > Ro/k". Note that i is the mean residence time of the solid particles, not that of the fluid phase. 

The turbulent fluctuation frequency can be estimated by means of turbulent measurements. Mockel 124] found that the wave number k = 2Tlft/u in the interesting dissipation range is k>ko with the limiting value ko= (0.1. ..0.2)qL-From this becomes the frequency to ft> (0.016...0.032)u/qL. An important measure should be the related number of turbulent fluctuation z/zp which occur during the residence time of particles ti=Vi/qp inside the fictive impeller volume Vj at one circulation. It follows to ... 

What became evident was that interactions between adsorbed particles can also exert an influence on their surface mobility and therefore the residence time at a particular site. The mean residence time of an isolated oxygen adatom at the Ru(0001) surface varies from 60 to 220 ms when a second oxygen adatom is located two lattice constants a0 apart from the first but only 13 ms when the... 

The holdup is calculated from the definition of mean residence time of solid particles ... 

A further consequence of the assumptions made, and of equation 22.2-11, is that the mean residence time of solid particles is independent of particle size ... 

Consider the fractional conversion ( /B) of solid (B) in a gas-solid reaction taking place in a reactor, with mean-residence time of solid fB, and time for complete reaction of solid particles t (R). The particles are not all of the same size. 

Bar and Tamir [63] studied the two-impinging stream dryer with two pairs of airfeeding tubes, as briefly shown in Fig. 6.7. The purpose of adding the lower two air streams is to increase the hold-up and the mean residence time of the particles in the dryer, and also aims to enhance the turbulence between phases in order to increase the drying intensity. However, the experiments did not show that the secondary air streams increased either the hold-up or the transfer coefficient. On the other hand, the induction of the two secondary air streams results in the greatly increased hydraulic resistance of the system. The pressure drop across the dryer, with two pairs of air-feeding tubes and with a volume treble that of the dryer shown in Fig. 6.6, is as high as 3800 Pa. 

Figure 5. Effect of mean residence time of the first reactor on the number of polymer particles formed (SF = 12.5 g/L H20 lF = 1.25 g/L H20 = 0.5...

This means that as long as a CSTR is used as the first stage reactor and all the recipe ingrediants are fed into the first stage reactor, one cannot have more than 57% of the number of particles produced in a batch reactor with the same recipe as in continuous operation. The validity of these expression is clear from the comparison between the experimental and theoretical values shown in Figure 5. From Figure 5, it is found that the optimum mean residence time of the first stage reactor is about 10 minutes under these reaction conditions. Equation(30) predicts 10.0 minutes, while experimental value is 10.4 minutes where the number of polymer particles is about 60% of that produced in a batch reactor. 

The experimental data also showed that when the superficial liquid velocity is zero or very small, the presence of partition plates gives the stepwise profile of solid particles. This avoids uniform distribution and gives large solid holdup. Continuous operation in this range, therefore, results in a higher mean residence time of the solid particles than that in the column without partition plates. 

There are four unknown parameters in the theoretical impulse response for porous particles, h(t) the pellet diffusion time, tdif (which contains the effective diffusion coefficient of the pair T-C, Dtc, td.fs R p/D.f( , R is the radius of the pellet equivalent sphere), the mean residence time of the carrier-gas in the interparticle space, tc (tc = v/L with the carrier gas linear interstitial velocity, v, and column length, L), Peclet number, Pe (Pe = L.v/E, with E the effective axial dispersion coefficient) and the adsorption parameter, 5q (see below). Because matching with four unknown parameters would give highly correlated parameters, it is better to determine some parameters independently,... 

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