Experiments, RTD

To run the residence time distribution experiments under conditions which would simulate the conditions occurring during chemical reaction, solutions of 15 weight percent and 30 percent polystyrene in benzene as well as pure benzene were used as the fluid medium. The polystyrene used in the RTD experiment was prepared in a batch reactor and had a number average degree of polymerization of 320 and a polydispersity index, DI, of 1.17. 

A curve showing the differential weight fraction versus chain length for the batch-prepared polystyrene is given in Figure 3. Figure 4 illustrates the exit age distributions obtained from the RTD experiments with benzene and with a 30 weight percent polymer solution. 

According to what has been stated above, good results have been obtained as a result of a spectrophotometric technique that entails a colored tracer. Two measuring probes are set up one at the inlet and the other at the outlet of the device. The acquisition time is set to 0.12 s. The operating protocol adopted during RTD experiments is as follows ... 

In a final RTD experiment, a sheet of dye was frozen as before and positioned in the feed channel perpendicular to the flight tip. The sheet positioned the dye evenly across the entire cross section. After the dye thawed, the extruder was operated at five rpm in extrusion mode. The experimental and numerical RTDs for this experiment are shown in Fig. 8.12, and they show the characteristic residence-time distribution for a single-screw extruder. The long peak indicates that most of the dye exits at one time. The shallow decay function indicates wall effects pulling the fluid back up the channel of the extruder, while the extended tail describes dye trapped in the Moffat eddies that greatly impede the down-channel movement of the dye at the flight corners. Moffat eddies will be discussed more next. Due to the physical limitations of the process, sampling was stopped before the tail had completely decreased to zero concentration. 

The actual residence time of a reactor is measured by employing residence time distribution (RTD) experiments utilizing tracing techniques. Furthermore, several correlation forms estimating the fluid holdup can be found in the related literature. 

The study of nonideal flow and liquid holdup can be done by residence time distribution (RTD) experiments (tracing techniques) or by use of correlations derived from literature. Dining this step, physical mechanisms that are sensitive to size are investigated separately from chemical (kinetic or equilibrium) studies (Trambouze, 1990). Here, the fixed bed is... 

The state of mixing in a given reactor can be evaluated by RTD experiments by means of inert tracers, by temperature measurements, by flow visualization and, finally, by studying in the reactor under consideration the kinetics of an otherwise well-known reaction (because its mechanism has been carefully elucidated from experiments carried out in an ideal reactor, the batch reactor being generally chosen as a reference for this purpose). From these experimental results, a reactor model may be deduced. Very often, in the laboratory but also even in industrial practice, the real reactor is not far from ideal or can be modelled successfully by simple combinations of ideal reactors this last approach is of frequent use in chemical reaction engineering. But... 

A well-known traditional approach adopted in chemical engineering to circumvent the intrinsic difficulties in obtaining the complete velocity distribution map is the characterization of nonideal flow patterns by means of residence time distribution (RTD) experiments where typically the response of apiece of process equipment is measured due to a disturbance of the inlet concentration of a tracer. From the measured response of the system (i.e., the concentration of the tracer measured in the outlet stream of the relevant piece of process equipment) the differential residence time distribution E(t) can be obtained where E(t)dt represents... 

In section 4.5, the following designations are made Qi, Qij and qij are the mass flow rates of the particles stream (kg particles/sec) and Cj is the concentration of the tracer particles (kg tracer particles/kg particles). In such systems, the tracer particles are, usually, those of the original ones. They are, however, made radioactive or are painted, in order to distinguish them from the original particles. The latter makes it possible to determine their concentration versus time in the RTD experiments [73, p.l76], thus their mean residence time tm in the system. tm =... 

RTD experiments showed that the fixed-bed almost behaves like a plug-flow reactor and the infrared cell like a continuous stirred tank reactor. This fixed-bed is described by the tanks-in-series model, using 9 tanks for the catalyst compartment. The two kinetic models (Equations 1-6) are able to describe the stop-effect experiments at 180 and 200°C, and the considerations made in this work are valid for both temperatures. However, for the sake of clarity, only model discrimination at 180°C will be presented here. In the experimental conditions used here, both models can be simplified the first adsorption step is considered as irreversible, and instantaneous equilibrium is assumed for the second one. With these hypothesis the total number of kinetic parameters is reduced from five (ki, Li, k2, k.2 and ks) to three (ki, K2 and ks), and the models can be expressed as follows ... 

At the moment of introduction of tracer into the tube, the tracer concentration in the film is in equilibrium with the tracer concentration in the slug, hence the initial condition for RTD experiments is different from that of the reacting mass transfer experiments. The high Sherwood number for the inlet transition region of the... 

A new consideration, namely the RTD of the less dense layer, must also be considered. Mechanical agitation in the top layer now becomes imperative otherwise, a short-circuit from entrance to exit will occur. The goal of any tracer RTD experiment should be to decide on the most appropriate baffles and impeller type, and to adjust the inlet and outlet ports to proper locations, so that a RTD is obtained with minimum short circuits and dead volumes. 

In the dispersed case, new complications will probably arise due to the need for settling zones (either internal or external). The settling zones can have at least two effects. First, they can act as noticeable dead volumes and this will show up in any RTD experiments. Secondly, and perhaps more worrisome, the stagnant settling zones might expose the intermediates to significantly different solute compositions. This might lead to accelerated deactivation of the system. 

In addition to the RTD experiments, the exact structure of the gaseous jets was obtained by means of a chemiliiminescent reaction of phosphorous vapor (21). It was observed that the gas mixture was approximately homogeneous. 

Residence time distribution (RTD) is a classical tool in the prediction of the comportment of a chemical reactor provided that the reaction kinetics and mass transfer characteristics of the system are known, the reactor performance can be calculated by combining kinetic and mass transfer models to an appropriate residence time distribution model. RTDs can be determined experimentally, as described in classical textbooks of chemical reaction engineering (e.g. Levenspiel 1999). RTD experiments are typically carried out as pulse or step-response experiments. The technique is principally elegant, but it requires the access to the real reactor system. In large-scale production, experimental RTD studies are not always possible or allowed. Furthermore, a predictive tool is needed, as the design of a new reactor is considered. 

RTD methods are based on the concept of age distribution functions and make use of the experimentally measured or calculated residence time distribution of fluid elements in a reactor vessel (Figure 12.3-1, C and D). A Lagrangian perspective is taken and the age of a fluid element is defined as the time elapsed since it entered the reactor. In what follows, steady state operation of a vessel fed with a volumetric flow rate F is considered. A residence time distribution (RTD) experiment can be performed with inert tracers, such that at an instant of time all fluid elements entering a reactor or process vessel are marked. The injection of an impulse of tracer into the vessel at time zero can be mathematically represented by means of the Dirac delta function or perfect unit impulse function ... 

The RTD experiment consists of measuring the time variations of concentration Cs(t) in the outlet section of the reactor. Experimentally, the products injected are often ions, and the measurement of concentration level is performed by measuring the electrical conductivity at the reactor s outlet. What is actually plotted is the normalized concentration ... 

Actually, the RTD models reported in this section evidence some questions that cannot be answered by RTD experiments but rather by a detailed analysis of the liquid flow hydrodynamics the scale of mixing (e.g. the number of stages in a stage-wise model)... 

Among RTD experiments, washout experiments are generally preferred since W(oo) = 0 will be known a priori but F(oo) = Co must usually be measured. The positive step experiment will also be subject to errors caused by changes in Co during the course of the experiment. However, the positive step change experiment requires a smaller amount of tracer since the experiment will be terminated before the outlet concentration fully reaches Co. Impulse response experiments that measure f(t) use still smaller amounts. 

Zacchi, P., Bastida, S.C., Jaeger, P., Cocero, M.J., and Eggers, R. (2008) Countercurrent de-acidification of vegetable oils using supercritical CO2, holdup and RTD experiments. Journal of Supercritical Fluids, 45, 238—244. 

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