Liquid-Phase RTD

The method is the same as described in Section 4-4.5. The probes must be mounted or shielded such that the gas bubbles do not interfere with the readings. 

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During an experiment in a multiphase system, the tracer should not be transferred from one phase to another phase. For example, a gaseous tracer used in a gas-liquid reactor should not be absorbed by liquid and a liquid tracer used to measure the liquid-phase RTD curve should not be volatile. Similarly, a solid tracer used to measure the RTD curve for the solid phase in a gas-liquid- solid slurry reactor should not dissolve in the liquid, etc. 

A review on earlier studies of liquid-phase axial dispersion in unstirred bubble-columns with no solids is given by Ostergaard.97 Van de Vusse138 has discussed the liquid-phase RTD in stirred slurry reactors. 

RTD, liquid phase Narrow Narrower than for entrained Wide Wide Narrow... 

In large-size mechanically-agitated multistage contactors, the speed of the agitator is kept at a relatively low level and horizontal baffles are provided. Here, RTD in the gas and liquid phases may show different behavior compared to a small-scale unit. 

The residence-time distribution in the liquid phase of a cocurrent-upflow fixed-bed column was measured at two different flow rates. The column diameter was 5.1 cm and the packing diameter was 0.38 cm. The bed void fraction was 0.354 and the mass flow rate was 50.4 g s l. The RTD data at two axial positions (which were 91 cm apart in Run 1 and 152 cm apart in Run 2) are summarized in Table 3-2. Using the method of moments, estimate the mean residence time and the Peclet number for these two runs. If one assumes that the backmixing characteristics are independent of the distance between two measuring points, what is the effect of gas flow rate on the mean residence time of liquid and the Peclet number Hovv does the measured and the predicted RTD at the downstream positions compare in both cases ... 

In general, an unsteady-state operation is obtained as a result of catalyst aging. The reactor is, of course, not very useful for gathering the kinetic data when the catalyst decays rapidly. Multiple taps can be employed to give multiple conversion and selectivity points for each experimental run. As shown in Chaps. 6 through 8, the RTD characteristics of the gas and liquid phases depend on the orientation of the gas and liquid flows (e.g., both cocurrent downwards, both cocurrent upwards, etc.). 

As discussed in Chap. 3, there are a large number of models proposed to evaluate macromixing in a trickle-bed reactor. A brief summary of the reported experimental studies on the measurements of RTD in a cocurrent-downflow trickle-bed reactor is given in Table 6-7. Some of these experimental studies are described in more detail in a review by Ostergaard.94 Here we briefly review some of the correlations for the axial dispersion in gas and liquid phases based on these experimental studies. 

The experimental studies have shown that, in gas-liquid trickle-bed reactors, significant axial mixing occurs in both gas and liquid phases. When the RTD data are correlated by the single-parameter axial dispersion model, the axial dispersion coefficient (or Peclet number) for the gas phase is dependent upon both the liquid and gas flow rates and the size and nature of the packings. The axial dispersion coefficient for the liquid phase is dependent upon the liquid flow rate, liquid properties, and the nature and size of the packings, but it is essentially independent of the gas flow rate. 

The determination of the gas or liquid holdup is largely made from RTD measurements. Some investigators have measured the RTD of the gas phase, while others have studied the liquid phase. It should be noted that, in principle, the holdup of only one phase is required because the holdup for the other phase can be calculated if the total voidage is known. 

Woodburn55 measured the gas-phase RTD for extremely high ratios of liquid rate/gas rate in a 29.2-cm i.d. and 97-cm tall column packed with 2.5-cm stoneware Raschig rings (e = 0.714). The data were obtained in the range... 

Significant literature on the axiaj dispersion in gas and liquid phases for countercurrent-flow packed-bed columns have been reported. Trickle- and bubble-flow regimes have been considered. Unlike the holdup, there is quite a discrepancy in the results of various investigators. Almost all the RTD data are correlated by a single-parameter axial dispersion model. A summary of the reported axial dispersion studies in countercurrent flow through a packed bed is given in Table 8-1. 

Michelsen and Ostergaard82 extended the study of Ostergaard and Michelsen104 to a wider range of flow rates and other system conditions. The data were obtained in a 15.2-cm-i.d. and 11-m-tall concurrent-upflow air-water-sol id system. The RTD of both the gas and liquid phases were measured by a radioactive tracer... 

Calculate the mean conversion in the reactor we have characterized by RTD measurements in Examples 13-1 and 13-2 for a first-order, liquid-phase, irreversible reaction in a completely segregated fluid ... 

P13-12b An RTD analysis was carried out on a liquid-phase reactor [Chem.Eng. /, 76 (1970)]. Analyze the following data ... 

Example 13-9 RTD and Complex Reactions Consider the following set of liquid-phase reactions ... 

The consequence of recycling is a high backmixing of the liquid phase and a large RTD corresponding to continuous stirred tank reactors. Compared to plug flow reactors (Bo > 100), the specific performance of recycle reactors is significantly... 

Joshi et al. (30) proposed reactor models based on the shrinking core mechanism. Since the particles take part in the reaction their role was evaluated based on the residence time distribution. For extremely fine pyrite particles, (< 100 ym), it has been shown (31) that the RTD of the solid and liquid phases can be asstimed to be identical and the RTD of the solid phase is given by the diffusion-sedimentation model. Various rate controlling steps that were considered are (1) gas-liquid mass transfer (2) liquid-solid mass transfer (3) ash diffusion (4) chemical reaction and, (5) intraparticle diffusional resistance (for particles encased in the coal matrix). 

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