Particles contact time

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. 

Kaneko, 2000) obtained for three values of spring stiffness constant (spread over four orders of magnitude) are shown in Fig. 12.7. It can be seen that if the objective is to understand the macroscopic behavior of the fluidized bed, low values of spring stiffness can be used for faster simulations. It must, however, be remembered that when such artificially low values of spring stiffness constant are used, the predicted values of contact time between solid particles are not realistic. When the objective is to understand local particle to particle heat or mass transfer, it is important to make accurate predictions of particle contact times. For such cases, it is necessary to use realistic values of spring stiffness constant at the expense of increased computational resources. 

The importance of the intraparticle heat transfer resistance is evident for particles with relatively short contact time in the bed or for particles with large Biot numbers. Thus, for a shallow spouted bed, the overall heat transfer rate and thermal efficiency are controlled by the intraparticle temperature gradient. This gradient effect is most likely to be important when particles enter the lowest part of the spout and come in contact with the gas at high temperature, while it is negligible when the particles are slowly flowing through the annulus. Thus, in the annulus, unlike the spout, thermal equilibrium between gas and particles can usually be achieved even in a shallow bed, where the particle contact time is relatively short. 

Ryu et al. (2003) have reported the formation of dibenzofnran from phenol and benzene in combnstion gas exhanst systems. Chlorination at 200-400°C (392-752°F) over the particle beds containing CuCh produced chlorinated dibenzofurans. The authors have detected 2,3,7,8 congeners of both chlorinated dibenzofurans and -dibenzodioxins in the product mixtures the formation of which depended on the gas-particle contact time. 

Influence of Ambient Temperature and Particle Contact Time on Adhesion. 

In the extraction of citms juices it is desirable to have as gende an extraction pressure as possible. There should be minimal contact time between juice and pulp to reduce the amount of bitter substances expressed from the peel into the juice. The amount of suspended soHds in citms juice is controlled in a subsequent separation in a finisher. A screw action is used to force the juice through a perforated screen and separate the larger pulp particles from the juice. The oil level in the juice is adjusted by vaporizing under a vacuum (10). The separated pulp is washed and finished several times to produce a solution which is then either added back to the juice to increase juice yield, or concentrated to produce pulp wash soHds, also called water extract of orange soHds, which can be used as a cloudy beverage base. 

Fluid coking (Fig. 4) is a continuous process that uses the fluidized soflds technique to convert atmospheric and vacuum residua to more valuable products (12,13). The residuum is converted to coke and overhead products by being sprayed into a fluidized bed of hot, fine coke particles, which permits the coking reactions to be conducted at higher temperatures and shorter contact times than they can be in delayed coking. Moreover, these conditions result in decreased yields of coke greater quantities of more valuable Hquid product are recovered in the fluid coking process. 

Limiting flow rates are hsted in Table 23-16. The residence times of the combined fluids are figured for 50 atm (735 psi), 400°C (752°F), and a fraction free volume between particles of 0.4. In a 20-m (66-ft) depth, accordingly, the contact times range from 6.9 to 960 s in commercial units. In pilot units the packing depth is reduced to make the contact times about the same. 

There are data showing that at the same contact time, but different linear velocities, there is no difference in the performance of a carbon system. It is obvious then that the effect of linear velocity on the diffusion through the film around the particle and the ratio of the magnitude of the film diffusion to the pore diffusion are the factors that determine the effects, if any, that occur. Therefore, the linear velocity cannot be ignored completely when evaluating a system. Systems at the higher linear velocity (LV) treat more liquid per volume of carbon at low-concentration levels and the mass-transfer zone (MTZ) is shorter. 

Anderson (A2) has derived a formula relating the bubble-radius probability density function (B3) to the contact-time density function on the assumption that the bubble-rise velocity is independent of position. Bankoff (B3) has developed bubble-radius distribution functions that relate the contacttime density function to the radial and axial positions of bubbles as obtained from resistivity-probe measurements. Soo (S10) has recently considered a particle-size distribution function for solid particles in a free stream ... 

Sorption of plutonium (l.fixlO-11 M) and americium (2xl0-9 M) in artificial groundwater (salt concentration 300 mg/liter total carbonate 120 mg/liter Ref. 59) on some geologic minerals, quartz, biotite, o apatite, o attapulgite, montmorillonite. Dashed lines indicate the range for major minerals in igneous rocks. Experimental conditions room temperature, particle size 0.04-0.06 mm, solid/liquid ratio 6-10 g/1, aerated system, contact time 6 days. 

A well-defined bed of particles does not exist in the fast-fluidization regime. Instead, the particles are distributed more or less uniformly throughout the reactor. The two-phase model does not apply. Typically, the cracking reactor is described with a pseudohomogeneous, axial dispersion model. The maximum contact time in such a reactor is quite limited because of the low catalyst densities and high gas velocities that prevail in a fast-fluidized or transport-line reactor. Thus, the reaction must be fast, or low conversions must be acceptable. Also, the catalyst must be quite robust to minimize particle attrition. 

Pneumatic dryers, also called flash dryers, are similar in their operating principle to spray dryers. The product to be dried is dispersed into an upward-flowing stream of hot gas by a suitable feeder. The equipment acts as a pneumatic conveyor and dryer. Contact times are short, and this limits the size of particle that can be dried. Pneumatic dryers are suitable for materials that are too fine to be dried in a fluidised bed dryer but which are heat sensitive and must be dried rapidly. The thermal efficiency of this type is generally low. 

Kukkonen, J., Landrum, P.F. (1998) Effect of particle-xenobiotic contact time on bioavailability of sediment-associated benzoin) pyrene to benthic amphipod, Diporeia spp. Aqua. Toxicol. 42, 229-242. 

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