Gas-Phase RTD

Commonly, hehum is used as a tracer gas in air or nitrogen, in the usual method for measuring RTD with a step change or pulse in the input gas stream. The tracer gas must exhibit the following properties  

The method is described in Section 4-4.5. It is important to ensure that the dead spaces (gas volumes between the liquid surface and the exit gas concentration detector) are deconvoluted from the measured response. Gas-phase RTD has been measured by Hanhart et al. (1963) and Gal-or and Resnick (1966) and is often in between the ideal limits of plug flow and perfectly backmixed. 

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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... 

Many studies report gas phase RTD data using both radioactive argon and krypton (Martin et al. [83], Viitanen, [99], Patience and Chaouki, [98], and Contractor et al. [28]). Relatively small quantities of gas are required to obtain good signal-to-noise ratios from radioactive sources, which minimizes injection volumes required and perturbations of the gas flow. Radioactivity measurements are made in-situ (non-intrusive detection), on-line, and, in small diameter units, represent a radial average. 

RTD, gas phase Nearly plug flow Backmixed Backmixed Narrow... 

Recycling to monomers, fuel oils or other valuable chemicals from the waste polymers has been attractive and sometimes the system has been commercially operated [1-4]. It has been understood that, in the thermal decomposition of polymers, the residence time distribution (RTD) of the vapor phase in the reactor has been one of the major factors in determining the products distribution and yield, since the products are usually generated as a vapor phase at a high temperature. The RTD of the vapor phase becomes more important in fluidized bed reactors where the residence time of the vapor phase is usually very short. The residence time of the vapor or gas phase has been controlled by generating a swirling flow motion in the reactor [5-8]. 

Shetty et al. (1992) studied gas-phase backmixing for the air-water system in bubble-column reactors by measuring RTDs of pulse-injected helium tracer. 

In monolithic catalyst carriers with wider channels, the hquid forms a film on the channel walls, whereas in the core of the channel a continuous gas phase exists. As shown by Lebens [10], countercurrent gas-liquid operation is now possible, and shows certain advantages over the countercurrent trickle bed operation. Typical channel diameters are 3-5 mm, and the geometric surface areas are between 550 and 1000 m2 m 3. Below the flooding point, almost no hydrodynamic interaction between the gas and hquid can be observed for example, the RTD is the same for both co-current and countercurrent operation. Apart from some surface waves, the film flow is completely laminar. 

Each experiment was repeated at least four times, and good reproducibility was found between the different experiments. Air was used as medium for the gas phase, but it transpired that gas flow has almost no influence on the liquid RTD. 

The determination of the residence time of the gas phase r by identification of the experimental RTD curves, allows us to obtain the total gas hold-up [4] by the equation ... 

As for the RTD study in the gas phase, we have used the method of two injections. The response of the conductimetric probe to the input and output injections, with a Dirac tracer impulse, is shown in figure 6. In order to model the liquid flow, several theoretical models were tested. The liquid flow corresponded to plug flow with axial dispersion as shown in figure 7. 

As for the gas phase, the liquid hold-up was determined from identification of RTD curves. A series of experiments allowed us to study the variation in liquid hold-up according to liquid and gas flow rates in the pressure range 0.1 to 1.3 MPa. 

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. 

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. 

To this author s knowledge, no data are currently available on the RTD in the gas phase for cocurrent gas-liquid upflow through a packed column For unpacked bubble-columns with large length-to-diameter ratios, the gas phase is usually assumed to be in plug flow. The same should be true for the bubble-flow regime in a packed bubble-column. 

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. 

De Waal and Van Mameren14 studied the RTD in the gas phase for an air-water system in a 30.48-cm-diameter, 3.05-m-tall countercurrent flow trickle-... 

P13-9x Consider again the nonideal reaetor eharaeterized by the RTD data in Examples 13-land 13-2. The irreversible gas-phase nonelementary reaetion... 

As suggested, RTD measurements should be combined with other techniques to best quantify riser gas-phase hydrodynamics. Injection and detection methods are critical to interpreting the data. Iso-kinetic injection at different radii may help deconvolute inlet boundary conditions and flow structure. Multiple detectors along the riser length also are preferred. However, combining radial gas sampling, as practiced with steady state tracers, with radioactive impulse experiments could provide sufficient data to completely characterize riser gas-phase hydrodynamics. 

RTD models have been developed to characterize mixing in both gas and liquid phases. Actually, mixing in the gas phase has received very little attention because its influence on the behaviour of trickle flow reactors is rather weak. 

In the design of optimal catalytic gas-Hquid reactors, hydrodynamics deserves special attention. Different flow regimes have been observed in co- and countercurrent operation. Segmented flow (often referred to as Taylor flow) with the gas bubbles having a diameter close to the tube diameter appeared to be the most advantageous as far as mass transfer and residence time distribution (RTD) is concerned. Many reviews on three-phase monolithic processes have been pubhshed [37-40]. 

Contact Time Distribution Models. To overcome this difficulty and still use the information given by the RTD, models were proposed which assumed that faster gas stayed mainly in the bubble phase, the slower in the emulsion. Gilliland and Knudsen (1971) used this approach and proposed that the effective rate constant depends on the length of stay of the element of gas in the bed, thus... 

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