Particle circulation time

Based upon these observations, Rowe (1977) derived an expression for the average particle circulation time t around a bed in terms of excess gas velocity and bed height at minimum fluidization... 

Xu, M., and Turton, R. (1997), A new data processing technique for noisy signals Application to measuring particle circulation times in a draft tube equipped fluidized bed. Powder Technol., 92,111-117. 

Polymersomes size is crucial for the design of drug delivery systems. Size governs the fate of particles both in vitro and in vivo. Cellular internalisation can drop by three orders of magnitude, going from 100 to 400 nm [70], In vivo the size determines the particles circulation times, their ability to reach specific target, the... 

When comparing different impeller types, an entirely different phenomenon is important. In terms of circulation time, the phenomena shown in Figs. 18-18 and 18-19 stiU apply with the different impellers shown in Fig. 18-5. When it comes to blending another factor enters the picture. When particles A and B meet each other as a result of shear rates, there has to be sufficient shear stress to cause A and B to blend, react, or otherwise participate in the process. 

Solid-Liquid Mass Transfer There is potentially a major effect of both shear rate and circulation time in these processes. The sohds can either be fragile or rugged. We are looking at the slip velocity of the particle and also whether we can break up agglomerates of particles which may enhance the mass transfer. When the particles become small enough, they tend to follow the flow pattern, so the slip velocity necessary to affect the mass transfer becomes less and less available. 

Emulsions Almost eveiy shear rate parameter affects liquid-liquid emulsion formation. Some of the efrecds are dependent upon whether the emulsion is both dispersing and coalescing in the tank, or whether there are sufficient stabilizers present to maintain the smallest droplet size produced for long periods of time. Blend time and the standard deviation of circulation times affect the length of time it takes for a particle to be exposed to the various levels of shear work and thus the time it takes to achieve the ultimate small paiTicle size desired. 

However, for small scale equipment, Cheng (1993) and Xu (1993) measured the circulation times of a single tagged particle in a 150 mm diameter fluidized bed coater with a draft tube and found that typical circulation times were between 2-10 seconds. Robinson and Waldie (1978) made similar measurements in a 6" diameter conventional spouted bed and found mean circulation times in the range of 3-6 seconds. 

If we consider different particles, we can see from Eq. (11) that the variation in the total amount of material deposited on each particle is a function of the number of times the particle passes through the spray zone, TV and the amount of material that it receives in each pass, x. Since both x and N vary, we expect that Xlolal will vary between particles in the same batch. Mann (1974) has shown that for a batch coating process in which the operating time is greater than about 20 times the average circulation time,... 

PEG is hydrophilic and is widely used in biological research because it protects surfaces from interacting with cells or proteins. Thus, coated particles may result in increased blood circulation time. For their preparation, 10-mg magnetite particles were dispersed in 1.0 mU of deoxygenated water by sonication for 30 min. The aqueous dispersion of MNPs was dissolved in the aqueous cores of reverse micelles... 

New avenues could come from reacting small chemical entities with the remaining amines at the surface of the lipoplexes, to obtain completely neutralized DNA-loaded particles (57). Gain in circulation time was significant by this so-called postgrafting method. 

Although this expression may not accurately predict circulation time, and in any case particles do not follow a simple predetermined circuit around the bed, it serves to illustrate the significance of the excess gas velocity in determining particle mixing rates. The excess gas flow rate, proportional to the excess gas velocity, is essentially the bubble flow rate. A greater bubble flow generates more bubbles and therefore... 

The well-mixed drier has fhe disfinct disadvanfage of nof being able to dry particles to very low moisture contents because of fhe wide particle residence time distribution. In other words, some particles leave the bed immediately, having had no opportunity to dry and effectively bypass the bed some circulate within the bed for a long... 

A.T. Harris, R.B. Thorpe, J.F. Davidson, Stochastic modelling of the particle residence time distribution in circulating fluidised bed risers, Chem. Eng. Sci. 57 (2002) 4779-4796. 

In the emulsion phase/packet model, it is perceived that the resistance to heat transfer lies in a relatively thick emulsion layer adjacent to the heating surface. This approach employs an analogy between a fluidized bed and a liquid medium, which considers the emulsion phase/packets to be the continuous phase. Differences in the various emulsion phase models primarily depend on the way the packet is defined. The presence of the maxima in the h-U curve is attributed to the simultaneous effect of an increase in the frequency of packet replacement and an increase in the fraction of time for which the heat transfer surface is covered by bubbles/voids. This unsteady-state model reaches its limit when the particle thermal time constant is smaller than the particle contact time determined by the replacement rate for small particles. In this case, the heat transfer process can be approximated by a steady-state process. Mickley and Fairbanks (1955) treated the packet as a continuum phase and first recognized the significant role of particle heat transfer since the volumetric heat capacity of the particle is 1,000-fold that of the gas at atmospheric conditions. The transient heat conduction equations are solved for a packet of emulsion swept up to the wall by bubble-induced circulation. The model of Mickley and Fairbanks (1955) is introduced in the following discussion. 

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