Residence time and distribution

Radioactive tracers [14] are a useful tool to measure unit parameters such as residence times and distribution of the catalyst and vapors in the reactor, stripper, or regenerator. Bypassing can be detected, slip factors calculated and dilute phase residence times are examples of useful calculations that can point the way to future modifications. This technology is also useful for detecting and analyzing equipment malfunctions. Plugged distributors, erratic standpipes, and main fractionator problems such as salt deposits or flooding can be detected with tracers. 

The total residence times and distribution coefficients of 7Be and 210Pb in different estuaries illustrate the importance of the concentration of dissolved and particulate constituents in controlling the dynamics of these radionulides (table 7.3). 

Also, the starting point of the reaction is affected by the screw speed. Due to thermal effects, the reaction will start at a later point in the extruder at increasing rotation rates. The shorter reaction length has also a negative effect on the conversion due to a loss of mixing and residence time. Nevertheless, the influence of this effect is small and the conversion of the reaction is mainly determined by the two competitive effects of mean residence time and distributive mixing. 

The materials residence time and distributive mixing can also be modified by the use of specially designed spindles with machined openings in the planetary spindles, which disrupt the forwarding action of the polymer blends in combination with the gap clearances within the dosing rings between PRE sections as well as the number and speed of the planetary central shaft. 

Glaser and Lichtenstein (G3) measured the liquid residence-time distribution for cocurrent downward flow of gas and liquid in columns of -in., 2-in., and 1-ft diameter packed with porous or nonporous -pg-in. or -in. cylindrical packings. The fluid media were an aqueous calcium chloride solution and air in one series of experiments and kerosene and hydrogen in another. Pulses of radioactive tracer (carbon-12, phosphorous-32, or rubi-dium-86) were injected outside the column, and the effluent concentration measured by Geiger counter. Axial dispersion was characterized by variability (defined as the standard deviation of residence time divided by the average residence time), and corrections for end effects were included in the analysis. The experiments indicate no effect of bed diameter upon variability. For a packed bed of porous particles, variability was found to consist of three components (1) Variability due to bulk flow through the bed... 

The general case of coupled heat-transfer and multicomponent mass-transfer in swarms of bubbles with residence-time and size distributions is treated in Section IV, L. In previous sections simplified cases of uncoupled mass transfer are considered starting from the most simplified models available for gas-liquid dispersions. 

L. Coupled Heat Transfer and Multicomponent Mass Transfer, with Residence-Time and Bubble-Size Distributions... 

In this section, a general formulation will be given for the effect of bubble residence-time and bubble-size distributions on simultaneous and thermodynamically coupled heat- and mass-transfer in a multicomponent gas-liquid dispersion consisting of a large number of spherical bubbles. Here one can... 

This result can be useful for design purposes when the diffusivities, partition coefficients, feed-stream conditions, dispersed-system volume, gas-phase holdup (or average residence time), and the size distribution are known. When the size distribution is not known, but the Sauter-mean radius of the population is known, (293) can be approximated by... 

Example 14.6 derives a rather remarkable result. Here is a way of gradually shutting down a CSTR while keeping a constant outlet composition. The derivation applies to an arbitrary SI a and can be extended to include multiple reactions and adiabatic reactions. It is been experimentally verified for a polymerization. It can be generalized to shut down a train of CSTRs in series. The reason it works is that the material in the tank always experiences the same mean residence time and residence time distribution as existed during the original steady state. Hence, it is called constant RTD control. It will cease to work in a real vessel when the liquid level drops below the agitator. 

From equation (12) it can be concluded that the required residence time 0 to achieve a certain value of s is inversely proportional to the average concentration of ozone present in tire reactor. The value of the required residence time and therefore the required volume V of the reactor is completely independent of the distribution of ozone over the reactor. This also holds for the conversion of component A and component B. 

In order to carry out comparable catalytic experiments in a high-throughput parallel mode, equal reaction conditions for each library member have to be ensured. This is especially true for reactant distribution, as variances in residence time and space velocity can have a dramatic impact on the... 

In the case of a tubular centrifuge with a free liquid surface at radius r, operating at steady state, the grade efficiency is related to the radius rd which divides the area available for fluid flow (in the axial direction) such that all particles of minimum size d introduced at radii r > rd with the feed, reach the centrifuge wall within the residence time (tR) of the fluid in the equipment. The remainder of the particles (introduced at r < rf) become distributed with increasing non-uniformity in the axial direction across the entire area for flow. rd is a function of the separating power, the residence time and the fluid/particle... 

The biomass is fed overbed through multiple feed chutes using air jets to help distribute the fuel over the surface of the bed. Variable-speed screw conveyors are usually used to meter the fuel feed rate and control steam output. Feedstocks such as bark and waste wood are chipped to a top size of 25 mm (1 in) to ensure complete combustion. The bed usually consists of sand around 1 m (3 ft) deep. This serves to retain the fuel in the furnace, extending its in-furnace residence time and increasing combustion efficiency. It also provides a heat sink to help maintain bed temperature during periods of fluctuating fuel moisture content. 

The number of inputs which are available for controlling crystallisation processes is limited. Possible Inputs for a continuous evaporative crystallisation process are, crystalliser temperature, residence time and rate of evaporation. These Inputs affect the crystal size distribution (CSD) through overall changes in the nucleatlon rate, the number of new crystals per unit time, and the growth rate, the increase in linear size per unit time, and therefore do not discriminate directly with respect to size. Moreover, it has been observed that, for a 970 litre continuous crystalliser, the effect of the residence time and the production rate is limited. Size classification, on the other hand, does allow direct manipulation of the CSD. 

Fig. l.(a) Histogram of residence times, and (b) limiting form of this histogram giving the exit age distribution, E(f). 

Dekker et al. [170] have also shown that the steady state experimental data of the extraction and the observed dynamic behavior of the extraction are in good agreement with the model predictions. This model offers the opportunity to predict the effect of changes, both in the process conditions (effect of residence time and mass transfer coefficient) and in the composition of the aqueous and reverse micellar phase (effect of inactivation rate constant and distribution coefficient) on the extraction efficiency. A shorter residence time in the extractors, in combination with an increase in mass transfer rate, will give improvement in the yield of active enzyme in the second aqueous phase and will further reduce the surfactant loss. They have suggested that the use of centrifugal separators or extractors might be valuable in this respect. 

A number of studies have documented that concentrations of some of the directly emitted species found in outdoor atmospheres can be quite high indoors if there are emission sources present such as combustion heaters, gas stoves, or tobacco smoke. In addition, there is evidence for chemistry analogous to that occurring outdoors taking place in indoor air environments, with modifications for different light intensities and wavelength distributions, shorter residence times, and different relative concentrations of reactants. In Chapter 15, we briefly summarize what is known about the chemical composition and chemistry of indoor atmospheres. 

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