Adsorbing tracers

As mentioned in Section 11.3, fluidized-bed reactors are difficult to scale. One approach is to build a cold-flow model of the process. This is a unit in which the solids are fluidized to simulate the proposed plant, but at ambient temperature and with plain air as the fluidizing gas. The objective is to determine the gas and solid flow patterns. Experiments using both adsorbed and nonadsorbed tracers can be used in this determination. The nonadsorbed tracer determines the gas-phase residence time using the methods of Chapter 15. The adsorbed tracer also measures time spent on the solid surface, from which the contact time distribution can be estimated. See Section 15.4.2. 

The retardation term can also be expressed as the ratio of the breakthrough time of an adsorbed pollutant relative to the elution time of a non-adsorbed tracer. In addition, parameter R can be used to estimate the number of pore volumes of flow required to achieve breakthrough, assuming that breakthrough of a non-adsorbed tracer would occur at one pore volume of flow. 

A - gas reactant A a - adsorbing tracer app - apparent value e - at gas-liquid equilibrium gl - gas-liquid... 

In principle, by matching the theoretical moments with the corresponding experimental moments, the adsorption and diffusion parameters can be extracted. To facilitate the reliability of the parameter extracted, we can make use of an inert tracer as well as an adsorbing tracer, and carry out experiments for both of them at different flow rates. 

At the same flow rate, the first moment of the adsorbing tracer is given by eq.( 14.3-3). Taking the difference between the first moment for adsorbing tracer and that for the inert at the same flow rate, we get... 

The flood history is summarized in Table 3. First baseline pressure drops were determined across the 1.8 m core during simultaneous injection of brine (without surfactant) and gas at a fixed frontal advance rate and varying gas fractional flows. A frontal advance rate of 4.0 m/day was selected to ensure a flow rate higher than the critical rate for effective foam formation and propagation. A non-adsorbing tracer, tritiated water, was added to the brine so that the breakthrough of the tracer could be compared with the breakthrough of the gas. 

One obvious deficiency of the D-A equation is that it does not approach the Henry type equation at lower concentrations. According to chromatographic measurement using helium gas as a carrier (Chihara, Suzuki and Kawazoe, 1978), the adsorption equilibrium coefficient of the Henry type equation, which was assumed to hold at an extreme of C=0, could be determined for MSC SA. The results are shown in Fig. 3.14. In this experiment, since helium gas exists in large excess compared with adsorbable tracer gas, the coadsorption effect of helium may not have been negligible, making it possible to assume the Henry type isotherm for the tracer gases. 

The chromatographic experiment, based on introducing a pulse of adsorbable tracer of concentration Co and duration time r into the entrance of the bed, is illustrated in Fig. 6.1. For quantitative analysis of the effluent peak Ce(r), there are several alternative techniques to determine model parameters by comparing the mathematical solution of the fundamental equations and the experimental results. These are 1)... 

This inert moment can be evaluated from measurable properties of the bed and the velocity. Subtracting Eq. (6-25) from the counterpart expression for the adsorbable tracer (Eqs. (6-16) and (6-18)) gives... 

Even when the isotherm relation is nonlinear, chromatographic technique involves very small concentration change and the isotherm relation governing a small pulse may be considered to be locally linear (Fig. 6.7). Then, by utilizing the local slope of the isotherm as an apparent equilibrium constant, the theory developed for linear isotherm systems can be applied to the analysis of the behavior of the small concentration pulse introduced to the column kept at this point of the equilibrium relation. With this in mind, chromatographic measurement was made by introducing a small adsorbable tracer into the carrier streams which contain the same adsorbable components of different concentration levels. The detection of concentration perturbation at the outlet of a column is sometimes accompanied by fluctuation of a base... 

However, since E (s) is just a time delay it is clear that Ei2(t) = E (t + Bq). Thus, the adsorbing tracer curve, for the tracer with K equal to that of the reactant, can be used directly, by substituting t + for time t, to obtain the RTD for the react-... 

The boundary conditions are the same in both cases. This shows that the transform of the response for the adsorbing tracer, Ea( )> obtained from the transfer function for the non-... 

After we have eliminated the possibilities of the differences on microscopic (particle) scale we can explore the possibility of flow or contacting maldistribution by tracer methods using nonadsorbing and adsorbing tracers. 

In the above discussion the behavior of nonlinear reaction schemes was not considered. Conventional approaches to reactor modeling based on ideal reactor models are suggested and tracer studies with nonadsorbing tracers should be conducted to determine whether the assumptions of ideal flow patterns are satisfied. Adsorbing tracers can be used to assess catalyst contacting through the use of the central volume principle. 

Schwartz et. al. (128), Colombo et. al. (129) and Mills and Dudukovic (48) have shown that nonvolatile nonadsorbing and linearly adsorbing tracers can be used to determine the liquid-solid contacting efficiency. By the central volume principle the first moment of the normalized tracer impulse response for a nonvolatile tracer is ... 

Nauman and Collinge (159,160) expanded upon the concept of contact time distributions introduced by Orcutt et. al. (157) and showed how to obtain it by use of an adsorbing and a nonadsorbing tracer or of a pair of adsorbing tracers with different adsorption rate constants. The tracers must adsorb linearly and reversibly. 

The models for catalyst effectiveness in trickle bed reactors developed in this paper require explicit measurements or predictions of external contacting, ncE pore fill-up, rii In laboratory conditions this can be accomplished by tracer techniques (22, ). Fractional pore fill up may be determined by the difference in first moments of the impulse response tracer tests performed on two beds of same particle size and shape when one bed consists of porous the other of nonporous particles. Fractional pore fill-up can also be assessed from the measured volumetrically static holdup. External contacting is measured by adsorbable tracer tests on beds of nonporous particles ( ). In industrial conditions ncE hj would have to be evaluated from correlations. Unfortunately at present the existing correlations for ncE unsatisfactory since they were developed for fixed bed adsorbers with larger packing and correlations for m ate nonexistent but may be developed in the future. 

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