Design of an Inductance Measurement System for Determination of

Article pubs.acs.org/IECR

Design of an Inductance Measurement System for Determination of Particle Residence Time in a Dual Circulating Fluidized Bed Cold Flow Model Diana Carolina Guío-Pérez,*,† Tobias Pröll,‡ Johann Wassermann,§ and Hermann Hofbauer† †

Institute of Chemical Engineering and §Institute of Mechanics and Mechatronics, Vienna University of Technology, A-1060 Vienna, Austria ‡ Institute of Chemical and Energy Engineering, University of Natural Resources and Life Sciences, A-1180 Vienna, Austria ABSTRACT: A tracer impulse method is designed and implemented for determination of the residence time distribution (RTD) of particles in a cold flow model of a dual circulating fluidized bed chemical looping pilot plant. Ferromagnetic particles are used as a tracer. The method based on inductance measurements previously proved its suitability and convenience for determination of the concentration of ferromagnetic particles and the bed porosity. The current paper presents an improved system for the determination of inductance, based on an impedance circuit bridge. Injection and detection systems were designed and optimized for an extremely small amount of tracer, about 0.05% by mass. The system is entirely described, variables relevant for the attainment of an appropriate measurement are analyzed, and the first results of RTD experimentally determined are presented. Signals of high quality were obtained and successfully fitted to a simple model. The system shows a great capacity regarding the response time, quality of the signal, and suitability of results.

1.1. Tracer Measurements in Gas−Solid Systems. Tracer detection methods are the most common techniques for the study of the residence time distribution in any process. In gas−solid contact units, the residence time for each of the phases needs to be considered. In fluidized beds, gas residence time has been comparatively more extensively studied; this is primarily due to the facility of finding a tracer gas with a fluiddynamic behavior similar to the original fluidization medium, and to the ability to introduce such a tracer without perturbing the global performance of the unit. Results for diverse experimental units and operating conditions have been reported.4 The techniques for tracer injection and detection are numerous and diverse. On the side of the solids, tracer experiments are not always simple to perform. Even though many different methods have been tested and reported, the methods differ from one another not only in the nature of the materials used but also in the procedures used for injection and measurement of the tracer concentration. In the literature many different methods can be found, for example, salt tracers, fluorescent tracers, radioactive tracers, and ferromagnetic tracers.5−7 A unification of results is still difficult, since experimentation conditions differ widely from one study to another. The basic requirements and expected features of the RTD measurement system are well identified. Tracer particles should hold very similar fluid-dynamically-relevant properties in comparison with the bed material and should not involve any health risk. Injection and detection devices or methods should not imply any disruption of the internal flow pattern

1. INTRODUCTION The use of gas−solid reactors is widespread in numerous industries, petrochemicals (fluid catalytic cracking), metallurgy (roasting), combustion, gasification, ceramics, powder manufacturing, and lately even CO2 capture (carbonate looping,1 chemical looping combustion, and reforming2). More precisely, circulating beds are increasingly present in many applications and processes due to the advantages they have regarding mass and heat transfer characteristics and operational flexibility. The efficiency and extent of the thermal and chemical processes that take place in a fluidized reactor depend on the solid and gas mixing, the contact efficiency, and the contact time between the phases. These factors are particularly important in catalytic processes and in those where high reactivity is critical, or where adverse reactions are involved (poisoning, inhibition, or degradation, for instance). Given the characteristics of the reactions taking place and the desired product yields, the unit itself and the operating conditions must be set in order to achieve the necessary residence times. Investigations on residence time distribution (RTD) as well as on distribution of solids are essential for reactor design and scale-up, plant operation, and optimization of existing circulating fluidized beds (CFBs). However, the complexity of the flow structure in gas−solid reactors often makes this analysis difficult in both experimental and theoretical attempts. Diverse methods have been investigated and reported in the literature for the determination of residence time,3 but the optimization of such measurements still needs to be reached. The objective of the present investigation is, thus, the design of a nonintrusive method able to quantitatively detect a fluid-dynamically similar tracer material in a certain control volume of a fluidized bed cold flow model. © 2013 American Chemical Society

Received: Revised: Accepted: Published: 10732

January 18, 2013 June 26, 2013 July 10, 2013 July 10, 2013 dx.doi.org/10.1021/ie400211h | Ind. Eng. Chem. Res. 2013, 52, 10732−10740

Industrial & Engineering Chemistry Research

Article

characterized, and the fluid-dynamic suitability of the chosen ferromagnetic tracer particles was confirmed. The inductance sensor (coil) was installed around the pipe where particles were fluidized. The change in concentration of ferromagnetic material in the core of the coil was reflected on the measured inductance. The results evidence the reproducibility and the accuracy of the measurements. The proportionality between the measured parameter and the concentration (and also the net amount) of ferromagnetic material in the core of the coil was found to be approximately linear. On this basis, the bed density of a fluidized bed was successfully determined. Because the resonance frequency was the variable measured, the time resolution of the measurement could only be improved to the detriment of accuracy. Thus, in order for the signal from the coil to be used to determine the RTD, the signal would have to be processed in another way. An improved sensor is under examination in the present work. Here a slightly different version of the measurement method based on impedance (instead of frequency) is implemented for the residence time determination in a cold flow model. Thanks to the possibility of working at ambient conditions and with harmless materials, measuring the residence time in a cold model significantly simplifies the procedures, which would be otherwise costly, laborious, and time-consuming. Efforts can then be put into enhancing the quality of the measurements. Given the fluid-dynamic similarity between the hot unit and its corresponding cold flow model, the results obtained from the cold flow model in this respect are qualitatively valuable for improving industrial processes.

(particularly, pressure and inventory should not be affected). Injection of the particles should be possible under stable operating conditions and be the shortest possible (nearly Dirac delta pulse). The response of the detection method should be fast enough to capture the features of the RTD, which are usually very short; i.e., the response should perform a high time resolution. Removal of tracer particles from the bed should not entail long time periods or complicated procedures. Finally, calculation of the concentration should be possible without the need of intricate assumptions and the measurement should be repeatable and replicable within reasonable periods of time. 1.2. Magnetic Tracers. After investigating various tracer methods used to determine residence time distributions of solids in fluidized beds, ferromagnetic tracers were selected as a promising method entailing a number of advantages in contrast to other approaches:8 • Ferromagnetic particles do not require special handling, nor are they toxic. • The density and size can be modified to fit fluid-dynamic requirements by forming a composite with a polymer. • The magnetic properties of the material do not deteriorate with time or use, and temperature has only a slight influence as long as it is kept below certain limits. • The particles can be easily separated from the bed material by means of magnets. Some important research has already been done on magnetic tracer measurements. Avidan and Yerushalmi9 used a sensitive bridge circuit to study solids mixing; their method is considered disruptive insofar as the tracer material differs from the bed solids and demands separation of the materials after experimentation. This problem can be overcome if the magnetic tracer has fluid-dynamic properties similar to those of the bed material. The method will additionally enhance its precision if the tracer can be introduced without adverse effects on the original fluid-dynamic behavior. Such features are more easily realizable in a cold model thanks to its simplicity in comparison with a hot, reactive, and big unit. Later, Goldblatt10 proposed a sensor based on electromagnetic induction to determine trajectory and to measure the RTD of particles in a circulating fluidized bed. However, in order to eliminate measurement ambiguity at open boundaries (which was the case of Avidan and Yerushalmi), single tracer particles were introduced. This significantly increased the sensitivity requirements of the measurement and limited the success of the measurement to the use of large particles, which did not meet the fluid-dynamic properties required. Goldblatt also indicated two important factors to be considered when implementing inductance tests: (i) the generated magnetic field must not appreciably affect the fluid dynamics of particles or the trajectory of tracer particles; (ii) the signal due to the tracer must be greater than the undesired signals, or noise, due to mechanical vibrations for example. That is, tracer particles should be in a proportion large enough to provide a well-defined signal, but small enough not to produce effects on the normal flow of particles. 1.3. Previous Work. A previous investigation reported the fundamental principles for measuring ferromagnetic material concentration in the core of a simple coil.8 The inductance of the coil was measured using a simple LC resonator. The measurement was based on the resonance frequency of the resonator. The materials, tracer particles, and bed material were

2. EXPERIMENTAL SECTION 2.1. Cold Flow Model Configuration. The system chosen to implement the measurement is a cold flow model of a dual circulating fluidized bed (DCFB) designed for chemical looping.11 The system (Figure 1) consists of two interconnected circulating fluidized beds. In the main loop, particles are fluidized in the main reactor (air reactor, AR), separated in a cyclone, and fed into the secondary reactor (fuel reactor, FR) after passing through a seal (upper loop seal, ULS). The loop is completed by a seal located at the lower end of the reactors (lower loop seal, LLS), and the flow of particles in this loop is called the “global circulation rate”. The secondary loop includes the secondary reactor (FR) and a system for internal recirculation of particles, cyclone, and seal (internal loop seal, ILS); the particle flow in this section is called “internal circulation”. The cold flow model was designed according to Glicksman’s scaling criteria12 on a 3:1 scale of the pilot plant (hot unit). A complete description of the fluid-dynamic parameters for both hot and cold units and both air and fuel reactors is presented by Pröll et al.11 The secondary reactor of this system has been selected for the residence time measurements; this reactor is of special interest in the industrial process due to the particular flow conditions and the high conversion requirements.13,14 For the present work, the internal loop is not considered and the ILS was shut down; the control volume studied is indicated in Figure 1, for which there is only one input and one output. The configuration was designed to avoid invasive procedures or elements in both injection of tracer and sensing of concentration. For the injection, instead of using air pulse injection devices to add new particles in each measurement, a system was built to collect the tracer at a certain point in the 10733

dx.doi.org/10.1021/ie400211h | Ind. Eng. Chem. Res. 2013, 52, 10732−10740


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the measurement is made during stable operation. No additional process is needed to clean or regenerate the bed material. The magnet arrangement is able to separate the ferromagnetic particles from the solids stream even for very low concentrations (few grams of tracers). To sense the concentration of ferromagnetic particles in the stream, three coils identical in size and made of the same materials were wound directly on the sensing points. The inner diameter of the coils corresponds exactly to the outside diameter of the duct where they are installed. The duct did not need to be modified in any way for the installation of the coils. Each coil is connected to an independent signal acquisition system. The exact configuration of the sensing device is presented in section 2.3. The input signal was measured at the only solids input to the FR, namely at the ULS. The closeness between injection and the input measurement point guarantees a good signal quality. Since the measurement is done in the outlet duct of the loop seal, the residence time in the seal itself is not taken into consideration. Two points are taken for output signal measurement, namely one in each of the solids outlets of the FR, one in the LLS for the global loop, and one in the ILS for the internal loop. The loop seals were selected for the concentration measurements since they provide the densest zones available, a fact that enhances the resolution and accuracy of the measurement. A diluted bed would increase the sensitivity requirement for the measurement and would introduce uncertainties to the measured tracer concentrations. 2.2. Characterization of Materials. The fluid-dynamic similarity of bed material and trace particles was widely presented in ref 8, and the absence of agglomeration effects and similarity in shape were confirmed. Table 1 presents the most Table 1. Properties of the Bed Material (Bronze Particles) and the Tracer Material (Steel Particles) parameter

bronze

steel

units

8730 6.80 × 10−5 1 1.07 × 102 8.03 × 10−2

7579 7.20 × 10−5 1 1.11 × 102 8.28 × 10−2

kg·m−3 m

1.69 × 10−2

1.65 × 10−2

m·s−1

Figure 1. Cold flow model indicating the main parts and the locations of measuring coils and collecting magnets. The part surrounded by the dashed line indicates the control volume under study for residence time measurment. The grayed-out section is not considered in the present work.

particle density (ρP) Sauter mean particle diam (dP) particle sphericity (ϕ) Archimedes no. (Ar) Reynolds no. (min fluidization, Remf) min fluidization velocity (Umf)

unit in order to use the same particles for the next experiment. In this way operations otherwise needed are avoided, such as the separation of the tracer after each experiment or the correction of the measurement to a new zero (background tracer noise). The position of the collecting point was selected such that it serves directly as an injector; i.e., it is close enough to the input signal measuring point. The collection/injection system was placed at the lowest end of the AR cyclone body (along the whole perimeter) and consists of a number of strong magnets in a row that, when in the closed position, collect at the wall the ferromagnetic particles passing through the cyclone. The injection of particles is done by pneumatically removing the magnets from the wall; the particles fall immediately and flow together with the stream of bronze powder toward the ULS. With such a device a number of measurements can be done with the same inventory alternating periods of collecting and measuring. The operating conditions can be set before the injection is done, and therefore

relevant parameters for both materials, while Figure 2 shows their particle size distributions. The similarity between both types of particles is clear and enables the use of the chosen particles as a tracer. It can be said that the tracer behavior is representative for the bed. 2.3. Description of the Sensor and Associated Electronics. Figure 3 shows the principle measuring and control arrangement. It consists of a specially developed multichannel carrier frequency amplifier with a very high amplification factor. Because of this, a high sensitivity and a measuring signal with high quality are obtained with a very small percentage of ferromagnetic particles. For each measuring position, the flow of the ferromagnetic particles is detected with a coil surrounding the tube (this corresponds to Z1 in Figure 4) and used in a measuring bridge combination of three other passive elements: two resistors and one capacitor (Z2, Z3, Z4 in Figure 4). Using a capacitor instead of an additional inductor reduces the overall sensitivity against 10734



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In addition, a closed housing for the electronics is used to obtain a constant temperature (after a short warm-up time). Due to a synchronized support of all channels and coils with the same carrier frequency signal, a remarkable higher stability of the n-channel measuring results is obtained. The measuring signals are filtered with active high order low-pass filters and visualized on a PC screen. As mentioned above, for the synchronized injection of ferromagnetic particles, a controlled collector/injector was developed. It consists of an arrangement of permanent magnets mounted on a circular holder which is tightly pressed on the tube in the collecting position. For the injection the circular holder is opened with the aid of an electro-pneumatic valve. In the opened position only a very weak magnetic induction is active inside the tube. As a result, all the particles previously collected fall synchronously due to the flow inside the tube. With the aid of a program that controls the electro-pneumatic valve, the injection can be automatically initiated during the investigations whenever needed.

Figure 2. Particle size distribution of bed material (bronze, open symbols) and tracer particles (steel, solid markers).

3. CALCULATIONS 3.1. Residence Time Distribution. In principle, in an impulse-response experiment, a sufficiently small amount of a nonreactive tracer material is injected into the system under stable operating conditions at a certain location and its concentration is measured at some point downstream; the concentration is recorded against time (C(t) curve). The input signal should ideally approach a Dirac delta function; i.e., the injection should be instantaneous. If so, the output C(t) curve corresponds to the residence time distribution. In practice the input impulse diverges from a Dirac pulse, and such divergence must be considered. The residence time distribution is then the transfer function that relates the output and the input signals and can be calculated based on them.15−17 The function representing the distribution of the time that particles need to leave the unit is called the “exit age distribution” or “residence time distribution” and is denoted by E. Considering that the tracer leaving the unit at time t is equal to all the tracer entering t′ seconds earlier than t and staying for t′ in the unit, the E function would relate the input and output signals (by the so-called convolution integral) as follows:

Figure 3. Measuring and control arrangement of the measuring system.

Cout(t ) =

∫0

t

C in(t ′) E(t − t ′) dt ′

Cout = E∗C in

(1) (2)

where Cout(t) and Cin(t) are the normalized output and input curves. The mean residence time, τ, is defined as the first moment of the distribution curve if there are no dead or stagnant zones within the reactor: τ=

∫0

∞

t E(t ) dt

(3)

Additionally, the second moment of the distribution curve (variance, σ2) indicates the degree of dispersion around the mean and is given by

Figure 4. Multichannel carrier frequency amplifier for signal acquisition and conditioning.

σ2 =

electromagnetic interferences. All these components must have high stability to temperature variations so that they do not generate a remarkable offset drift in the output signal.

∫0

∞

(t − τ )2 E(t ) dt

(4)

There must be a clear and known correlation between the variable measured and the concentration of tracer in order to use the measurement for the calculations presented above. In a 10735



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previous work8 a nearly linear proportion was found for concentrations between 0 and 100% by weight. Additionally, tests were done in the unit used in the present work (with the impedance-based inductance measurement system), varying the concentration of ferromagnetic tracer and recording the strength of the background signal, which results from the changes in magnetic permeability with gas bubbles passing the core of the detection coil. The signal fluctuates only when the particles show a magnetic permeability significantly different from the gas, and the amplitude of the fluctuation is, thus, a measure for the concentration of ferromagnetic particles. The correlation between these two variables showed a nearly linear proportionality as well (Figure 5). On this basis, the signals are used as recorded in volts.

Figure 6. Raw residence time data typically obtained from the inductance measurement system. Fluidization conditions: 15 N m3·h−1 AR; 2 N m3·h−1 FR; 1 N m3·h−1 ULS; 1 N m3·h−1 LLS; total inventory 5 kg; circulation rate 55.60 kg·m−2·s−1; total steel mass 3 g (0.06 wt %).

Figure 5. Correlation between signal strength in volts and concentration of ferromagnetic particles.

Figure 7. Corrected and normalized data typically obtained from the inductance measurement system. Fluidization conditions: 15 N m3·h−1 AR; 2 N m3·h−1 FR; 1 N m3·h−1 ULS; 1 N m3·h−1 LLS; total inventory 5 kg; circulation rate 55.60 kg·m−2·s−1.

The signals obtained experimentally need to be modified in order to be used in the calculations. Only the period of time where the pulse and response appear is considered. After having extracted these data, the following two procedures are required: Background Correction. The values of each signal before and after the pulse (or response) are used as reference to level the signal to zero. The background correction was performed to additionally compensate for any possible drift of the signals. That is, the average values of the sections before and after the pulse (or response) are calculated and used to generate a linear background function, B(t). The corrected signal is numerically obtained by subtraction of B(t) for all values of t: C(t )corrected = C(t ) − B(t )

directly used to calculate the mean residence time and to find the E curve. 3.2. Data Fitting. In this work, the E curve that relates the experimental input and output signals is found on the basis of a previously proposed ideal system, which should approach the conditions expected to be found in the reactor for the corresponding experiment. In other words, a tentative E curve is proposed. This proposed transfer function is used to calculate a simulated output signal based on the experimental input signal and according to eq 1. The model consists of a combination of ideal reactor models for which the E functions are known. The simulated output is then compared with the experimental output, and the divergence of the simulated signal is minimized by means of a least-squares fitting routine; the parameters to be varied depend on the proposed model. This methodology is comparable to the one proposed by Michelsen,18 which was later evaluated by Boskovic and Loebbecke19 in terms of suitability and accuracy. The most important advantage of this alternative methodology in contrast to the usual deconvolution procedures20,21 is that the drastic increase of noise (due to the retransformation) is avoided and the RTD model is directly obtained. It is not necessary to implement noise filters, smoothing procedures,

(5)

Normalization of the Area under the Curve. In order to be used for calculation and modeling, the input and output curves have to be normalized. The normalized curve is obtained by numerical integration: C(t )norm =

C(t ) ∞

∫0 C(t ) dt

(6)

Figure 6 shows an example of the signals as they are recorded (raw data). The injection signal is also included in the plot, which indicates the position of the magnets: 0 V, open position, injection; 5 V, closed position, collecting. In Figure 7 the same data as in Figure 6 are presented after background correction and normalization procedures. This last form of the signals is 10736



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and additional steps to find the RTD model. The method is robust and reliable and allows modeling diverse features of the unit. However, comparatively longer computation time is required. The ideal reactor models considered are the plug flow and the ideally mixed stirred tank, for which the residence time distribution is well-known, e.g., from Levenspiel:16 plug flow: E(t ) = δ(t − τ )

(7)

ideally mixed stirred tank: E(t ) =

1 −t/ τ e τ

(8)

Figure 8. Graphic representation of the simulation results of a typical RTD obtained from the inductance measurement system. Fluidization conditions: 15 N m3·h−1 AR; 2 N m3·h−1 FR; 1 N m3·h−1 ULS; 1 N m3·h−1 LLS; total inventory 5 kg; circulation rate in terms of AR net solids flux 55.60 kg·m−2·s−1.

More complex features can be modeled by considering extended models, such as plug flow with dispersion and several stirred tanks in series. Additionally, combination and specific configurations could be assessed in order to obtain a good description of the experimental system. plug flow with dispersion: E(t ) =

⎡ (L − ut )2 ⎤ u3 exp⎢ − ⎥ 4π DL ⎣ 4DL /u ⎦

the methodology used for the modeling of the residence time shows a high potential due to the agreement reached and the simplicity in implementation. Different fluidization conditions will in fact require more specific models that may be tested in the same way. 4.2. RTD Dependence on the Solids Circulation Rate. The influence of the circulation rate variation on the residence time in the secondary reactor (fuel reactor) was investigated. Very low fluidization velocity (4 N m3·h−1) was kept in the fuel reactor to guarantee bubbling bed conditions. The total pressure difference in the fuel reactor was kept constant, and it is, therefore, assumed that the mass of bed material was constant for all experiments. The E curves calculated (according to section 3) for three different circulation rates are presented in Figure 9.

(9)

ideally mixed stirred tanks in series: ⎛ t ⎞N−1 1 E(t ) = ⎜ ⎟ e−t / ti̅ (N − 1)! ⎝ ti̅ ⎠

(10)

4. RESULTS AND DISCUSSION 4.1. Data Fitting: Initial Approach. The fitting of the results with a combination of ideal reactor models was investigated. Bubbling bed conditions in the secondary reactor were considered; under these conditions there is no circulation of particles in the internal loop and the dispersion effects are minimized. Therefore, an ideal configuration of one plug flow reactor plus one stirred tank reactor connected in series is used as the initial approach. It must be noted that, for the bubbling regime, there is a strong internal circulation of solids in the bed. The simple ideal models used in the current simulation do not explicitly consider this fact. No additional recirculation currents are built in the model, and the flow is considered unidirectional. This is an assumption made to keep the simplicity of the model in the first phase, and it will need to be modified in order to approach the model more closely to reality. The parameters optimized are the mean residence times of both plug flow and stirred tank reactors, τPFR and τSTR, respectively. The input signal (after background correction and normalization) is fed to the model to obtain the ideal output signal, and the mean residence times are varied to minimize the difference between the experimental and the simulated output signals. The E curve can then be plotted based on the parameters obtained from the simulation (Figure 8). A very accurate agreement of the simple RTD model is observed; the configuration proposed clearly describes the flow of the particles in the reactor well at the experimental conditions tested. On the basis of this result two important facts can be concluded that are of relevant importance for further studies. First, the accuracy of the signals is optimal; they can be used without need of further correction steps. Second,

Figure 9. Simulated residence time distribution curves (absolute time dependent) for solids in the fuel reactor under bubbling bed conditions for different global particle circulation rates. Fluidization conditions: 4 N m3·h−1 FR; 1 N m3·h−1 ULS; 1 N m3·h−1 LLS; 0 N m3·h−1 ILS; 10, 20, and 30 N m3·h−1 AR; total inventory 5 kg; circulation rate in terms of AR net solids flux.

From the results, it can be clearly seen that the mean residence time (τPFR + τSTR) decreases as the circulation rate increases, which corresponds to the expected effect: τ decreases inversely proportionally to the throughput of material for a constant mass inventory. Both fractions, plug flow and stirred tank, decrease, but a more significant change is seen in the fraction of residence time attributable to a stirred tank. Since 10737



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order to prevent the effect of the temperature; signal drift was in this way reduced to acceptable levels. Parasitic Impedances. Cable connections are kept as short as possible; long cables introduce parasitic inductances and capacitances that are inherently added to the circuit balance and can hardly be quantified. Furthermore, as capacitors, these long cables can be easily reached by undesired magnetic fields in the surroundings, which would substantially modify the measurement. Also for the same reason, any conducting cable is kept away from the coils. Electrostatic Charging (Triboelectric Effect). Because it is made of acrylic glass, the cold flow model undergoes electrostatic charging due to the friction caused by the fast and constant movement of the fine particles. A conductor is installed for this purpose all along the unit to continuously conduct this electric charge away (electrical grounding). The sensor coil and the connections are especially sensitive to electrostatic discharges; therefore, the sections near the coils needed to be carefully grounded. Peaks in the signal associated with such discharges were also reported by Goldblatt.10 Influence of Vibrations. Vibrations, mostly originating from the fluidization itself, modify the measurement. Vibrations cause fluctuations in the spacing between the turns of the coil, and particularly at the ends of the bundle, which results in variations of the coil’s inductance. Also, slugging of particles causes spacing between the duct and the coil to vary which generates random fluctuations in the coil’s parasitic capacitance.10 The coil and the ends of the bundle are fixed to the selected position to avoid relative movement of the coil and the duct and to avoid unnecessary peaks and noise in the signal. External sources of vibration are avoided; the injection system was, therefore, improved not to perform sudden movements. From an experimental point of view, it is important to mention some operational conditions that needed to be optimized before a proper RTD measure could be obtained. Determination of Optimal Tracer Concentration. In order to minimize distortion of the flow by injecting tracer particles, only a small amount of material should be used. The measurement system is in principle able to detect an impulse of about 0.5 g with good precision. However, the noise in the system can only be reduced to a certain extent; the nature of the flow, number of particles (even for such a small amount of material), and the high detection capacity creates a permanent noise. Since a high signal-to-noise ratio is desired to avoid uncertainties and further to favor calculations, very small amounts of tracer are not optimal. A range of 20 V was chosen as the maximum amplitude of the input signal (the sharpest of all signals), which corresponds approximately to 3 g of ferromagnetic particles. Determination of Optimal Collecting Time. Once the amount of particles to be used in the tracer impulse is fixed, the collecting time period is varied systematically while the input signal is recorded. Five minutes is estimated as sufficient for the complete collection of the 3 g of ferromagnetic particles. This time period guarantees negligible noise in the base signals and similar input signals for different experiments. Determination of Optimal Recording Frequency. Both measurement system and associated software are equipped to perform a high sampling rate (high time resolution). A frequency of 20 samples·s−1 was found to be satisfactory for generating a high quality signal but avoiding at the same time unnecessarily large data files.

the mass in the reactor is constant, it can be said that the mixing in the reactor is more extensive the lower the circulation rate is. In order to more precisely compare the E curves for the different mass flows, the nondimensional function curves, E(θ), are calculated and normalized (Figure 10). It is easily noticeable

Figure 10. Simulated residence time distribution curves (dimensionless time dependent, θ = t/τ) for solids in the fuel reactor under bubbling bed conditions for different global particle circulation rates. Fluidization conditions: 4 N m3·h−1 FR; 1 N m3·h−1 ULS; 1 N m3·h−1 LLS; 0 N m3·h−1 ILS; 10, 20, and 30 N m3·h−1 AR; total inventory 5 kg; circulation rate in terms of AR net solids flux.

that under the conditions tested here the dispersion in the reactor slightly decreases with increase of circulation rate. This result agrees with the expectations: a larger net flow through the reactor reduces the mixing effects, decreases in this way the dispersion, and creates a more plug-flow-like behavior in the fuel reactor. This can be also appreciated if observing the fraction of τPFR to τ in Figure 9, which changes from 32% at 79.95 kg·m−2·s−1 to 19% at 26.71 kg·m−2·s−1. 4.3. Attainment of a Suitable Measurement. Some of the aspects that usually require special attention in an impulse tracer measurement are omitted here given the advantageous properties of the method. These are signal f iltering or smoothing, when elimination of noise by means of mathematical methods or additional signal processing is needed, and data extrapolation, whenever recording time restrictions appear. Apart from these inherent advantages, this section of the paper is devoted to the discussion of some negative influences on the measurement that were identified and subdued before the experiments were performed. Thanks to the analysis and the actions taken, most of the possible uncertainties and discrepancies in the measurements were corrected. Some of these problems directly affected the field of the coils; others affected the electronics associated with the sensor. This discussion aims to clarify the method, give support to its suitability, and identify possible bias. Offset Adjustment (Bridge Circuit Balance). Given that the measurement is based on a bridge circuit, a balance of this bridge is needed before the experiment can start in order to obtain the maximum measuring range during the experiment (this because of the high filter amplification used). Temperature Dependence. The system showed significant sensibility to the changes in temperature. Although it was expected that the apparatus needed a certain time before the measurement could stabilize (preheating time), the influences were larger than estimated. A thermal isolation was built in 10738



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Determination of Optimal Total Measurement Time. After completion of the collecting time, the signal recording is started, and the base signals are recorded for 1 min before the impulse is injected and 2 min after the last significant change in signal is observed. The injection pulse itself is of about 5−10 s (amplitude of about 15 V). This provides the information needed for offset correction and identification of any abnormal change in the signal of the circulation of particles. By putting the magnets back in the collection position, the appearance of a second impulse (after the particles complete the circulation loop) is avoided.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the scientific advice of Prof. Martin Kozek (Vienna University of Technology), the efforts of engineer Manfred Neumann on the development of the RTD measurement device, the support of Simon Hinterhuber in the cold model operation, and financial support from the European Commission (Grant Agreement No. RFCP-CT-2012-00006, Project ACCLAIM).

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5. CONCLUSIONS A system for the determination of the residence time distribution in a cold flow model of a dual circulating fluidized bed was designed and built. The detection system measures inductance changes in a coil through changes in the concentration of ferromagnetic particles in its core. The system was optimized and is able to detect a pulse of only a few grams of tracer in 5 kg of total inventory (percentage by mass of about 0.05%) with enough precision and a high time resolution (up to 100 samples per second). There is no need for smoothing or further filtering before using data for calculation because the signals are of sufficient quality. The results obtained experimentally are realistic, reproducible, accurate, and appropriate for modeling calculations. The first results were already successfully fitted to a simple RTD model. Both the sensing instruments and the injection device are installed externally; they are nonintrusive by any means. The tracer is collected internally to be used in the next experiment; there is no need for addition or extraction of particles during the experiments. The same inventory can be used since the property of the particles on which the measurement is based is intrinsic and does not deteriorate with the time or particular experimentation conditions. Measures were taken in order to subdue the experimental factors affecting the measurement such as electrostatic charging, vibrations, parasite impedances, and temperature influence. In this way uncertainties and errors in the measurements were minimized. Effects of the magnetic field of injection and detection systems were not observed. The characteristics of the method as presented in this work show the possibility for its implementation not only in RTD determination but also for measurement of circulation rates and bed densities in similar units. For these purposes, the measurement principle can be kept identical; only the placement of the coils and the design of the experiment need to be reconsidered. Even though the method seems to be restricted to cold flow model determinations for high temperature processes such as gasification or chemical looping, it could potentially be directly applied in real units of any low to medium temperature fluidized or moving bed system based process.

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NOTATION Ar = Archimedes number B = background function Cin = concentration function of input signal Cout = concentration function of output signal C corrected = concentration function after background correction Cnorm = concentration function after normalization D = inner riser diameter [m] D/uL = vessel dispersion number dP = mean particle diameter [m] E = residence time distribution function N = number of tanks Remf = particle Reynolds number at minimum fluidization velocity t = time [s] ti̅ = mean residence time for tank i [s] u = flow velocity [m·s−1] U = superficial gas velocity [m·s−1] Umf = minimum fluidization velocity [m·s−1] Ut = terminal fluidization velocity [m·s−1] σ2 = variance of residence distribution curve δ = Dirac delta function ϕ = mean particle sphericity ηG = dynamic gas viscosity [Pa·s] ρG = gas density [kg·m−3] ρP = particle density [kg·m−3] τ = mean residence time [s] τPFR = mean residence time, plug flow reactor model [s] τSTR = mean residence time, stirred tank reactor model [s] θ = dimensionless time (t/τ) Z = impedance [ohm]

Abbreviations

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AR = air reactor CFB = circulating fluidized bed Ch = transmission channel DCFB = dual circulating fluidized bed FR = fuel reactor ILS = internal loop seal LLS = lower loop seal ULS = upper loop seal RTD = residence time distribution

REFERENCES

(1) Abanades, J. C.; Anthony, E. J.; Lu, D. Y.; Salvador, C.; Alvarez, D. Capture of CO2 from Combustion Gases in a Fluidized Bed of CaO. AIChE J. 2004, 50, 1614−1622. (2) Adanez, J.; Abad, A.; Garcia-Labiano, F.; Gayan, P.; de Diego, L. F. Progress in Chemical-Looping Combustion and Reforming Technologies. Prog. Energy Combust. Sci. 2012, 38, 215−282. (3) Harris, A. T.; Davidson, J. F.; Thorpe, R. B. A Novel Method for Measuring the Residence Time Distribution in Short Time Scale Particulate Systems. Chem. Eng. J. 2002, 89, 127−142.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 0043 (1) 58801166362. Fax: 0043 (1) 58801-16699. Notes

The authors declare no competing financial interest. 10739



Article

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Design of an Inductance Measurement System for Determination of

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