RTD measurement

Previously, the RTD funetions for flow systems were deseribed. In praetiee, the RTD is determined experimentally by injeeting a traeer at the inlet of the system and monitoring the response at the outlet information about the system is derived from these results. In a tubular reaetor, the RTD measurements are used to determine how elosely the plug flow assumption is obeyed. 

Rhodes et al. (1991) conducted RTD measurements to study longitudinal solids mixing in a circulating fluidized-bed riser by pulse-injection of a sodium chloride tracer. 

A set of typical data calibrated is shown in Fig. 3.7. It demonstrates that there is good regularity and reproducibility of the data, suggesting the calibrated curves can be used as the input signals to the impinging stream device for RTD measurement to yield sufficiently accurate results. 

The bridge circuit is used whenever extremely accurate resistance measurements are required (such as RTD measurements). 

Among the many mathematical models of fluidized bed reactors found in the literature the model of Werther (J ) has the advantage that the scale-dependent influence of the bed hydrodynamics on the reaction behaviour is taken into account. This model has been tested with industrial type gas distributors by means of RTD-measurements (3)and conversion measurements (4), respectively. In the latter investigation (4) a simple heterogeneous catalytic reaction i.e. the catalytic decomposition of ozone has been used. In the present paper the same modelling approach is applied to complex reaction systems. The reaction system chosen as an example of a complex fluid bed reaction is the synthesis of maleic anhydride (Figure 1). 

The linear drop and exponential recovery shape of these transformations also appear in power-compensated DSC traces, but for different reasons. The temperature measuring device (RTD) measures its own temperature, which is influenced by all substances in the chamber, the housing, the sample crucible, as well as the melting sample. The device adds power to the sample side as needed to compensate for the cooling effect on the chamber due to sample melting. This energy requirement increases lineaxly since the setpoint sample temperature increases linearly. When melting is over, the need for extra heat flow to the sample chamber side drops exponentially as the chamber temperature quickly catches up to the setpoint. 

The mixing time is measured by the schlieren or chemical decolorization method. For short mixing times, consecutive dye reaction can be used. For viscous liquids, the decoloration or thermal methods are recommended. The RTD measurement requires a good choice of tracer. 

Computation of axial dispersion coefficients for solids was provided by Patience et al. (1991), in the following simple one-dimensional model using the data obtained from RTD measurements of a radioactive solid tracer in a 82.8 mm i.d. by 5 m high circulating fluidized bed, that is,... 

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. 

The Otake-Kunugita type of correlation for the Peclet number has also been used by Co and Bibaud.10 Bennett and Goodridge2 and Michell and Furzer.35 Co and Bibaud10 examined trickle-flow air-water systems in 12.7- and 27.94-cm i.d. columns. The columns were packed to 304.8 cm with either 1.27- or 2.54-cm Raschig rings for the RTD measurement. The data were correlated by the following relation ... 

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 ... 

The RTD measurement is a dynamic measurement meaning that the volume in the column measured with the experiments (Vflow-through) Can be determined. Static volume measurements can also be done to determine the column hold up, Fhoid up-The average residence time, Ty, based on the column hold up can be determined by ry ... 

In Figure 8 and 9 two examples are shown for the continuous measurements and determination of process parameters such as temperature, excess air, state of mixedness and mean residence time. The measurement of temperature, excess air, and helium concentration takes place at the outlet of the SCC (SCCn i), The CO emissions and the excess air are measured separately in the chimney of the biomass furnace. This measurement equipment works simultaneously with the RTD measurement, but with a higher time resolution. 

The Helium injection occurs via secondary air flow to determine the state of mixedness in the SCC (see Figure 4), Square waves with a period of 30 s are used to excite the system during 5 minutes. Therefore 10 step responses in both directions result, such that every 15 seconds new measurement data of the state of mixedness are provided. Each step response is evaluated individually as shown in Figure 7 and thus the model parameters are determined off-line for every 15 sec. This corresponds to the time resolution of the RTD measurement system. 

The total flow rate is controlled by a commercial mass flow controller (MFC), which contains an internal servo mechanism that links a mechanical valve to a resistance thermal device (RTD). The RTD measures mass flow (rather than gas velocity) by the change of electrical resistance in a sensing wire heated by an adjacent hot wire. Because this measurement is affected by the specific heat of the gas, the MFC must be calibrated for each individual gas. The desired MFC flow is set by applying a voltage to the MFC that corresponds to the voltage generated by the RTD at that flow. A comparator in the MFC opens or closes the internal valve to balance the RTD and the applied voltage. 

Pruvost J, Legrand J, Legentilhomme P, MuUer-Peuga A Lagrangian trajectory model for turbulent swirling flow in an annular cell. Comparison with RTD measurements, Chem Eng Sci 57(7) 1205-1215, 2002b. 

In Fig. 6.34a, two curves are plotted that show the time course of the discontinuous process kinetics and the residence time distribution in a continuous reactor of arbitrary RTD. Normalized values are used for ease of comparison = final concentration at time fend — total concentration of the pulse used to initiate the RTD measurement). 

Yianatos J, Contreras F, Diaz F. (2010a) Gas holdup and RTD measurement in an industrial flotation cell. Miner. Eng., 23 125-130. 

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. 

An estimate of the Pe number can be obtained from an RTD measurement, that is, variance of residence times in the experiment. Empirical correlation giving the Pe number as a function of liquid Re (and possibly other dimensionless numbers) is also... 

Many types of sensors and transducers have particular signal conditioning requirements. For example, thermocouples require cold-junction compensation for the thermoelectric voltages created where the thermocouple wires are connected to the data acquisition equipment. Resistive temperature devices (RTDs) require an accurate current excitation source to convert their small changes in electrical resistance into measurable changes in voltage. To avoid errors caused by the resistance in the lead wires, RTDs are often used in a 4-wire configuration. The 4-wire RTD measurement avoids lead resistance errors because two additional leads carry current to the RTD device, so that current does not flow in the sense, or... 

Radioactive Na2C03 RTD measurement in CFBs of various sizes Helmrich et al. (1986)... 

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