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and practically it has better sense from the water flow dynamics point of view. This model is suitablefor a number of processes (river flows, flow of chemically active substances in porous media, etc.).2.3.3. Combination of basic modelsThe models reviewed above are obviously not able to represent all possible tracer experiments.It is therefore necessary to have a set of rules for combining these models, in order to accommodateany shape of RTD or impulse response function. Basically, models can be associated in three ways:parallel, series and with recycling (Fig. 16 a, b, c).(a) Parallel combinationSystem 1 System 2Flow rate Q(b) Combination in seriesFlow rate (1+α)QFlow rate QNode ASystem 1Flow rate QSystem 2Flow rate αQ(c) Models with recycling (part of flow rate (αQ) is recirculating)FIG.16. Combination of basic models.2.3.4. Optimization procedure - Curve fitting methodResidence time distribution (RTD) model is a time function with some parameters. Modelingmeans to match the RTD model to the experimental RTD curve. The evaluation of the modelparameters is performed by means of the optimization (curve fitting) of the experimental RTD E exp (t)with the model (or theoretical RTD) E m (t,p i ), where p i are the model parameters (which represent theprocess parameters).Fitting the model RTD function with the experimental RTD curve is performed by the leastsquare curve fitting method.The quality of the fit is judged by choosing the model parameters to minimize the sum of thesquares of the differences between the experimental data and model.19
The values of the model parameters corresponding to the minimum value of the squares of thedifferences are chosen as the best.There are various commercial and homemade RTD software packages for modelingexperimental RTD curve.The experience has shown that:• tracer work is first of all an experimental one and all of efforts should be dedicated to the qualityand reliability of the measurements,• experimental data may be interpreted at different level by tracer specialist or in collaboration withchemical engineering specialist in complex cases ,• tracer experimental data remain of prime importance and the results of the experiments are notlinked to the output of particular software, but different software facilitates extraction ofinformation and interpretation, in particular for complex process analysis.2.4. RESIDENCE TIME DISTRIBUTION FOR TROUBLESHOOTINGRTD and MRT are two important parameters of continuously operating industrial processsystems. They are analyzed either to identify the cause of malfunctioning or to characterize the degreeof mixing in the system. There are a number of reasons for imperfect mixing, i.e. presence of dead (orstagnant) volume, occurrence of channelling, split in parallel or preferential flows, bypass or shortcircuit exit flows and holding-up. These symptoms are reflected in the experimental response curves.Experience has shown that two common phenomena may happening in any kind of reactors areshort circuiting stream and dead volume (Fig. 17). A short circuiting stream is characterized with ashort residence time within the tank. Dead volumes are stagnant or scaled zones that are not active inthe process. Dead zones reduce the effective (active) volume, as a consequence, the process efficiencyis lower that expected.FIG.17. Illustrations of a porous media with channeling, and of a tank reactor with short circuitingstream (bypass) and with dead zone.20
Page 1 and 2: Radiotracer Applications inWastewat
Page 3 and 4: The following States are Members of
Page 5 and 6: TRAINING COURSE SERIES No. 49RADIOT
Page 7 and 8: EDITORIAL NOTEThe use of particular
Page 9 and 10: PRACTICAL RECOMMENDATIONS .........
Page 11 and 12: The success of radiotracer applicat
Page 13 and 14: skimmed out. Flotation, as currentl
Page 16 and 17: 1.3.3. Primary clarifiersA clarifie
Page 18 and 19: esidence times of phases, mixing ch
Page 20 and 21: FIG.8. Secondary clarifier: periphe
Page 22 and 23: processes that occur in natural env
Page 24 and 25: The RTD and MRT are parameters that
Page 26 and 27: (b) Perfect mixers in series modelA
Page 30 and 31: The experimental RTD curve gives ma
Page 32 and 33: FIG.18. Experimental and CFD calcul
Page 34 and 35: Optical tracers can be divided in t
Page 36 and 37: Table 1b. Liquid tracers.- Field of
Page 38 and 39: TABLE 2. SOME WATER RADIOTRACERSIso
Page 40 and 41: The detection efficiency (ϕ) is de
Page 42 and 43: Since on-line radiotracer technique
Page 44 and 45: source. The dose received is direct
Page 46 and 47: 4. CASE STUDIESWastewater from indu
Page 48 and 49: FIG.26. Scheme of equalizer-clarifi
Page 50 and 51: FIG.29. Exp. RTD of S2 and S3 jet m
Page 52 and 53: FIG.33. Experimental RTD curves mea
Page 54 and 55: 1. Radiotracer testsRadiotracer Br-
Page 56 and 57: 4.00E-033.50E-03ExperimentalModel3.
Page 58 and 59: 3. Conclusions• The aeration tank
Page 60 and 61: FIG.50. Experimental response curve
Page 62 and 63: 4.8. DIAGNOSIS OF A CYLINDRICAL TWO
Page 64 and 65: FIG.56. RTD simulation for experime
Page 66 and 67: The radiotracer was chosen to trace
Page 68 and 69: (a) D1 - D3 (b) D1 - D4(c) D1 - D6
Page 70 and 71: (d) ConclusionsThe diagnosis of dig
Page 72 and 73: • average gamma energy of 400 keV
Page 74 and 75: To estimate the MRT of the mud insi
Page 76 and 77: FIG.71. Experimental RTD curve and
Page 78 and 79:
FIG.75. Biological aeration tank.FI
Page 80 and 81:
4.12.2. Radiotracer experiment(a) L
Page 82 and 83:
FIG.80. The experimental RTD curve
Page 84 and 85:
FIG.84. Experimental RTD curve of l
Page 86 and 87:
FIG.88. Characteristic curves of se
Page 88 and 89:
tracer background level detection.
Page 90 and 91:
As seen from Fig. 94, the tanks-in-
Page 92 and 93:
FIG.99. Experimental RTD curves wit
Page 94:
The experimental RTD curves obtaine
Page 98:
BIBLIOGRAPHYBORROTO J.I., DOMINGUEZ
Page 102:
CONTRIBUTORS TO DRAFTING AND REVIEW
Page 106:
I S S N 1 0 1 8 - 5 5 1 8
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