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Mass transfer in a ‘droplets column’ in presence of solid particles
Chemical Engineering and Processing 40 (2001) 167– 174 www.elsevier.com/locate/cep
Mass transfer in a ‘droplets column’ in presence of solid particles N. Muller, B. Benadda *, M. Otterbein LAEPSI, INSA, 20 A6. A. Einstein, 69621 Villeurbanne, France Received 17 March 1999; received in revised form 14 June 1999; accepted 31 May 2000
Abstract This work studies the mass transfer efficiency of a new process: the ‘droplets column’. The special feature of this apparatus is the use of high gas throughputs (up to 14 m/s) comparable to smoke from a household waste incinerator. The advantage, as the results show, is the ability to carry out acid gas scrubbing and dust removal simultaneously at a low energy cost. An hydrodynamic study has already been carry out on the droplets column. It begins by measuring pressure drops and liquid hold-ups with three different methods for given operating points. The tendency of the curve remains identical, i.e. a decrease in hold-ups versus liquid flow rate and gas velocity. Considering the errors made on the different methods, the manometric method is more accurate than the other. Therefore, this method can be validated. The study is followed by the liquid residence time determination. All these different results enable to modelize the liquid flow in the column by comparison with well-known models. In this way, the droplets column is assimilated to a sequence of four continuous flow tank reactors. The results concerning hydrodynamic study have been published in N. Muller, B. Benadda, M. Otterbein, Chem. Eng. Tech. 7 (1997) 469– 74. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Mass transfer efficiency; Droplets column; Solid particles
1. Introduction Due to the stricter regulations and standards for gas effluents, industry has had to find more efficient methods of air pollution control. This involves the installation of more or less expensive depollution processes. The most frequently used processes depend on the nature of the pollutant. To treat particles, often dry mechanical processes are used: sedimentation, cyclone, filter or electrofilter. For gases, in particular acid gases, and heavy metals, different depollution techniques, can be used, i.e. dry, semi-wet or wet processes. At the present time, few devices carry out the simultaneous treatment of acid gases and dust. The droplets column is a gas – liquid contactor derived from classical bubbles columns. In such columns, increasing the gas velocity up to UG of 14 m/s, transforms progressively the type of dispersion
* Corresponding author. Tel.: + 33-4-72438185; fax: +33-472438717. E-mail address: [email protected] (B. Benadda).
from gas bubbles in a liquid into liquid droplets in a gas: this is the operating range of the droplets column [1,2]. This apparatus enables simultaneous gas scrubbing through absorption and dust removal in the same reaction volume: all the gas pollutants are transferred to the liquid phase and then may undergo an appropriate treatment. The advantage of this process is the use of high gas throughputs which, for the same depollution efficiency as other gas –liquid reactors and for the same diameter, can treat higher gas flow rates. These flow rates are comparable to those of smoke from household waste incinerators. For this reason, our work studied the efficiency of the process as regards the type of pollutants found in this smoke (HCl, SO2, dust). In this article we will first study the determination of the volumetric transfer coefficient on the liquid side using the oxygen probe method to evaluate the absorption potential of the process. The second part focuses on the determination of process efficiency to absorb both gas pollutants and dusts.
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2. Description of the pilot The droplet column operates with a co-current gas – liquid upflow. Reaction takes place throughout the column which is made of transparent PVC (d = 0.15 m, height H= 3.2 m). It is described in Fig. 1. The high gas velocities are obtained using a centrifugal high pressure fan. The gas enters at the base of the column through a perforated plate with a single large central hole. At the column outlet the gas escapes into the atmosphere. The liquid flows in a closed circuit. It is injected into the column by a cross-tube distributor. Each tube has eight holes and each hole serves an identical surface of the column section. The optimum number and diameter of holes was chosen to ensure the highest possible liquid velocity and to take into account possible clogging of
the holes if the liquid transported solid particles. In this way the liquid is transported and dispersed by the gas velocity all along the column. This also allows recuperation of at least a fraction of the potential energy of the recycled liquid and takes into account possible clogging if the liquid contains solid particles. Separation of water and air is carried out by a direction-changing separator. The separated liquid is collected in a storage tank with an overflow maintaining operation at constant volume. Thus a fixed load can be applied to the diaphragm device and a large range of flow rates can be covered. The liquid flow rate is precisely measured by a flow meter situated downstream. Pressure taps are situated all along the column connected to manometric tubes in order to evaluate pressure drops and hold-ups.
Fig. 1. The droplets column.
N. Muller et al. / Chemical Engineering and Processing 40 (2001) 167–174 Table 1 Type of dust particles used for the study Dust particles
Grain size d50 (mm)
Large ashes — thermal 150 power plant Ashes — household waste 90 incinerator Ashes — chemical plant 50 Ash fines — thermal 40 power plant Talc 9.3 Quartz 1.5 a
Solubility (g/l)a
1 35 12 1.8 0.6 0
At 20°C in water.
169
The minimum oxygen concentration [O2]t 0of the solution must be as low as possible. For this, a quantity of sulphites in great excess is added in order to make all the dissolved oxygen react and thereby approach zero concentration. The [O2]t concentrations can be read from the graph obtained at given times. By tracing ln[([O2]*− [O2]t 0)/([O2]*− [O2]t )] versus time (t− t0), the slope of the straight line gives us readings of kLaL. This coefficient was determined for different operating points in the presence or absence of particles, in order to measure the absorbing potential of the column. Our experiments are carried out between 17 and 20°C. However, all the results were expressed at 20°C taking into account the following correction formula [5]: (kLaL)20°C =
(kLaL)T 1.024(T − 20)
This formula is only valid for + 5°CB TB + 35°C. The study concerning the influence of liquid flow rate and gas velocity on kLaL has already been published [3]. We have shown the increase of the volumetric transfer coefficient on the liquid side with the liquid flow rate whereas the gas velocity does not interfere. Therefore, in this paper, we will only present here the influence of particles and their solubility on this parameter.
Fig. 2. Influence of particle size on kLaL.
3. Determination of the volumetric transfer coefficient on the liquid side The volumetric mass transfer coefficient on the liquid side is determined by a physical absorption method: the oxygen probe method which studies reoxygenation of water. First, deoxygenating is carried out using sodium sulphites in the presence of cobalt sulphate. The reoxygenation of water is measured by an oxygen probe connected to a recorder. This method allows us to determine the volumetric mass transfer coefficient on the liquid side kLaL expressed according to the total volume of liquid present in the column. The equation is given in the form ln[([O2]*− [O2]t 0)/([O2]* −[O2]t )] kLaL = (t − t0) The concentration in saturated oxygen [O2]* depends on the temperature of the solution and the atmospheric pressure. The solubility tables corresponding to the regulatory standard NF T 90 032 [4] allow the determination of this value.
3.1. Influence of the presence of solid particles on the coefficient kLaL For this study, we only take into account the influence of the presence of particles on kLaL, without considering the quantity of particles captured. Table 1 presents the two main characteristics we have chosen for the study of the influence of particles on the volumetric transfer coefficient on the liquid side.
3.1.1. Influence of particle grain size on the coefficient kLaL Two categories of dust were chosen mainly due to their grain size: talc with a grain size from 0 to 50 mm such that 50% of the dust has a diameter greater than 5 mm, micronized quartz with a grain size from 0 to 15 mm such that 90% of the dust has a diameter lower than 6.4 mm. The injection rate of dust is maintained constant in both cases at 750 g/h. It can be noted as in Fig. 2 that kLaL increases with the liquid flow rate in the same way, whether dust is present or not. These results allow to predict that this type of reactor used on multiple pollutants (acid gases and dusts) will give higher depollution efficiencies of acid gases in the
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presence of fine particles (diameters B 5 mm) than in the presence of larger particles (\5 mm). This conclusion is valid only if the two kind of particles used are wettable in the same way in our work conditions.
3.1.2. Influence of the quantity of dust on kLaL For this study we used ashes from a thermal power plant which we injected at two different rates. As before, we measured the volumetric transfer coefficient on the liquid side at different operating points. As shown in Fig. 3, the volumetric transfer coefficient decreases when the dust flow rate increases. It would therefore seem that while part of the water traps the dust, the other part remains free for gas transfer. If we suppose that the exchange coefficient kL, which depends on the hydrodynamics of the column, is not influenced by the dust, then it must be the interfacial area which decreases. The higher the liquid flow rate the more the difference between the values of kLaL should decrease in
Fig. 3. Influence of the quantity of particles in water on kLaL.
Fig. 4. Influence of dust solubility on kLaL.
absence and presence of dust. In fact, the fraction of water which traps dust remains identical as it does not depend on the quantity of dust, whereas the fraction of water which carries out gas transfer increases. This progression does not appear clearly in the curves however (they should in fact show a greater tendency to converge for higher liquid flow rates). This is only valid in the case where we suppose that all the injected dust is captured by the circulating water.
3.1.3. Influence of dust solubility on kLaL The dusts were chosen according to their solubility. Dusts of similar grain size were considered in order to be able to compare the results without running the risk that the values of the volumetric transfer coefficient on the liquid side be affected by the grain size. In the series of assays, fine ashes from a thermal power plant as well as ashes from incineration of chemical products were used. Thermal power plant ashes are less soluble than ashes from chemical product incineration. Their grain size is similar. The dust flow rate used in this series of assays is 3 kg/h. There is a small difference between the values of kLaL obtained in the presence of soluble dust and insoluble dust. In both cases the values are lower than those obtained in the absence of dust (Fig. 4). The difference found shows that there is a repercussion of dust solubility on the exchange capacity between the liquid and gas phases. The values of kLaL are higher in the case of insoluble dust (ashes from incineration of chemical products), which means that as the solubility increases the possible transfer between the liquid and gas phases decreases. Dust of very high solubility would greatly decrease the kLaL. 3.1.4. Conclusions on the 6olumetric transfer coefficient on the liquid side Knowledge of the volumetric transfer coefficient on the liquid side is just as important as knowledge of the interfacial area. The results obtained on the droplet column are of great interest. We have shown that an increase in liquid flow rate gives rise to an increase in the volumetric transfer coefficient on the liquid side whereas the coefficient is little affected by an increasing gas velocity [3]. The original feature of this work resides in the presence of solid particles in the measurement of the volumetric transfer coefficient on the liquid side. Therefore, we have been able to demonstrate the influence of the grain size, the quantity and solubility of the particles on the volumetric transfer coefficient on the liquid side. These different parameters affect kLaL. It can be thought that if we consider the droplets column as a gas –liquid –solid reactor, the acid gas absorption po-
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Table 2 Results of depollution efficiency for HCl QL (m3/h)
CI (g/N m3)
CF×103 (g/N m3)
E (%)
Regulatory standard (mg/N m3)a
0.8 1.0 1.2 1.3
1.515 1.515 1.515 1.515
14.4 7.6 4.5 3.0
99.1 99.5 99.7 99.8
50 50 50 50
a
The gas velocity places us in the context of furnaces with a capacity greater than 3 t/h, i.e. under the strictest regulatory conditions.
Table 3 Results of depollution efficiency for SO2
Without addition of NaOH
With NaOHb
a b
QL (m3/h)
CI (g/N m3)
CF×103 (g/N m3)
E (%)
Regulatory standard (mg/N m3)a
0.8 1.0 1.2 0.8 1.0 1.2
0.531 0.531 0.398 0.531 0.531 0.531
292.1 265.6 185.9 79.7 53.1 26.6
45.0 50.0 53.0 85.0 90.0 95.0
300 300 300 300 300 300
The gas velocity places us in the context of furnaces with a capacity greater than 3 t/h, i.e. under the strictest regulatory conditions. [NaOH]=1 g/l, pH =8.
tential for example, will be less efficient in the presence of solid particles. These results are to be compared to efficiency measurements of the process during air pollution control of standard smoke fumes.
4. Absorption of gas effluents The air entering the droplets column is artificially polluted with hydrogen chloride and sulphur dioxide. Regarding the quantities injected, we based our assays on values found in household waste incinerators, i.e. 1500 mg/N m3 for HCl and 300 mg/N m3 for SO2. The air is analysed at the column input and output.
4.1. Absorption of HCl All the presented assays were carried out with an air flow rate of 795 m3/h, i.e. a velocity of 12 m/s. Table 2 presents the main results in the case of HCl for different given operating points. From these results, it can be seen that the droplets column is very efficient for the treatment of gases loaded with HCl whatever the liquid flow rate used. The quantities of HCl found at the output are much lower than the French regulatory standard. The results obtained are coherent with those from the study of the volumetric transfer coefficient on the liquid side as regards the liquid flow rate. The coefficient kLaL is higher when the liquid flow rate increases.
4.2. Absorption of SO2 As far as SO2 is concerned, the results are presented in Table 3. The quantities injected are voluntarily higher than those found in household waste incinerators, taking into account that the quantities from these plants (300 mg/N m3) already meet the national standards. The assays were also carried out at 12 m/s, which are the conditions found in furnaces of a capacity higher than 3 t/h. The depollution efficiency of SO2 increases with increasing liquid flow rate. As the quantity of water present in the column is greater for the same quantity of gas pollutant, the gas –liquid mass transfer is consequently higher. These results, which give only relatively average efficiencies are nevertheless satisfactory. They are comparable to those obtained in certain industrial processes operating with pulverised water and without addition of a base [6]. The depollution efficiencies are clearly improved in the presence of a base, even for very low flow rates. In fact, the reactions take place in the solution: Absorption of SO2 in the solution: SO2 + H2O H+ + HSO3− HSO3− + 1/2O2 H+ + SO24 − Followed hydroxide:
by
the
neutralisation
2NaOH + H2SO4 Na2SO4 + 2H2O
with
sodium
N. Muller et al. / Chemical Engineering and Processing 40 (2001) 167–174
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Table 4 Influence of dust on acid gas capture
HCl
SO2
a
Qp (kg/h)
CI (g/N m3)
CF×103 (g/N m3)
E (%)
0 2.7 8.6 0 2.0 9.4
1.515 1.515 1.212 0.531 0.531 0.531
7.6 7.6 7.6 53.1 53.1 53.1
99.5 99.5 99.3 90.0 90.0 90.0
Regulatory standard (mg/N m3)a 50 50 50 300 300 300
The gas velocity places us in the context of furnaces with a capacity greater than 3 t/h, i.e. under the strictest regulatory conditions.
The liquid side resistance is so largely decrease by the kinetic of neutralisation chemical reaction. Here again, the results confirm those obtained from the study on absorption potential.
4.3. Influence of the presence of particles on acid gas capture A known quantity of ashes from a thermal power plant is injected in the air flow together with HCl and SO2. The quantity of gas pollutants is measured at the input and output without taking into account the quantities of dust at the output. For a given operating point, different dust flow rates were used (Table 4). Whatever the flow rate of dust injected, the presence of particles does not affect the efficiency of acid gas capture. These results are different from those obtained from the study concerning the influence of dust on kLaL as we had observed a decrease in this coefficient as the quantity of dust injected increased. We can suppose that, given the high solubility in water of the gases used (HCl and to a lesser extent SO2) as against that of oxygen, the quantity of particles is not sufficient to perturb mass transfer. In the same way, similar assays were carried out testing the influence of grain size or solubility of the particles used on the efficiency of acid gas absorption, but no variation was demonstrated. Taking into account these first results, the pollutant contents of HCl and SO2 can be highly reduced in a process such as the droplets column. The quantities collected at the output for these two acid gases are always much lower than the regulatory standards and are only slightly modified by the presence of dusts.
5. Dust removal To test the efficiency of the column for dust removal, we have first to simulate an industrial smoke consisting of a vector gas, in our case air, transporting solid particles. For this, and as described previously, the dusts were injected into the input gas flow.
Measurement of the quantity of dust at the output was carried out using a probe placed in the air conduit in such a position so as to respect the isokinetics so that the sample would be representative. Different powders were used. The choice was made according to their grain size and ease of manipulation. silica (0–7.5 mm), talc (0–40 mm), cement powder (0–360 mm), ashes from a thermal power plant (0–400 mm).
5.1. Results The first assays for dust capture were carried out at the following operating point: QL = 0.8 m3/h UG = 10.4 m/s Table 5 summarises the results for the above-mentioned dusts. These first results already demonstrate the efficiency of the droplets column as a dust remover. However, they do not allow us to establish a precise relationship between the grain size range and efficiency of dust removal. Only the assays on silica, with a grain size between 0.9 and 7.5 mm, tend to show that for particles of a few microns, efficiency could be better. However, Table 5 Efficiency of the process for different particles Dusts
Qp (kg/h)
CI (g/N m3)
CF×103 (g/N m3)
E (%)
Ashes
0.7 4.4 1.05 14.9 15.6 1.7 5.9 0.21 0.16 0.2
0.86 0.56 1.60 22.5 23.7 2.60 9.10 0.32 0.24 0.30
5.6 4.5 9.7 143.2 128.7 9.2 31.2 26.5 10.9 1.0
99.4 99.9 99.4 99.1 99.5 99.6 99.7 91.8 95.6 99.6
Cement
Talc Silica
N. Muller et al. / Chemical Engineering and Processing 40 (2001) 167–174 Table 6 Efficiency according to liquid flow rate with cement QL (m3/h)
P (W) for 1000 m3/h of gas
CI (g/N m3)
CF×103 (g/N m3)
E (%)
0.4 0.8 1.1
270 360 440
14.1 12.2 13.5
146 92 58
99.0 99.2 99.6
as this powder is very light, the injection system does not allow injection of large quantities. These results are therefore not sufficiently significant. We can not affirm that the droplets column is not efficient for this type of particle. It would be necessary to decrease the liquid drop size or increase the impact velocity for considering the collection of dust of any particular size [7]. It can also be noted that, given the velocity used in the assays for dust removal, the regulatory standard for incineration facilities of urban and assimilated residues, places us in the context of installations with a capacity of 1 – 3 t/h, which corresponds to a total quantity of dust at the output of 100 mg/N m3. Only the assays with cement and relatively high initial concentrations of dust have given output concentrations higher than the regulatory standard. For each category of dust, there probably exists a maximum quantity which can be injected while still satisfying the regulatory standards. Further assays must be carried out in this direction. It appears that dust removal is little affected by dust concentration in the gas to be treated. Further assays with higher initial dust concentrations in the air must be carried out until saturation of the circulating water is reached. However, in the industrial application of this process, water is partially regenerated by removing the sludge and adding clean water in the storage tank, the concentration of dust in water, maintained constant, would avoid the problem of saturation.
5.2. Influence of liquid flow rate on dust remo6al To study the influence of the liquid flow rate on dust
173
removal efficiency, cement was injected at an initial concentration of 13 g/N m3, at three operating points as shown in Table 6. Therefore, to treat this smoke (CI = 13 g/N m3 of cement) and to reach the threshold value at the output of 100 mg of dust per N m3 of depolluted air, a water flow rate of 1.1 m3/h is necessary. If the input gas velocity were increased to 12 m/s, the dust content would correspond to the regulatory standard for household waste incinerator plants with a capacity greater than 3 t/h, i.e. 30 mg of dust per N m3 of depolluted air. It would therefore probably be necessary to increase the water flow rate in order to meet this standard. It can be supposed that for each type of dust which can be treated with the droplets column, we must define experimentally an adequate liquid flow rate to allow efficient treatment without wasting energy.
6. Conclusion concerning gas treatment The droplets column is a depollution system for smoke with a twofold action, as efficient for gas pollutants (HCl or SO2 in the presence of NaOH) as for dust removal. In general, the results of gas effluents are very promising as the quantities of pollutants found at the output are much lower than the regulatory standard. The efficiency of the process for dust removal, using low liquid flow rates (0.8 –1.1 m3/h) in a closed circuit and for smokes which are not too heavily loaded with dust (3–13 g/N m3) attains: 99.9% for relatively coarse grained dusts (0–40 and 0–400 mm) 96.0% for finer dusts (0–7.5 mm). Its efficiency with fine particles seems better than that of most other dust removal systems as shown in Table 7. The process is remarkable for its simplicity: increasing the liquid flow rate improves efficiency. This simplicity allows us to suppose a less cumbersome and less costly installation than for other processes.
Table 7 Comparison of droplets column with other separators Deduster
Cyclone Ven turi scrubber Electrofilter Bag filter
Efficiencies of deduster (%) 30 mm
20 mm
10 mm
5 mm
99 99.9 99.9 99.99
90 99 99 98–99
50–60 90–95 80–90 90
30–40 80–90 50–60 70
1 mm B10 50–70 10–20 30–40
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The gas velocities used are high (up to 14 m/s with this pilot). For equal diameter, this allows to treat higher gas flow rates. As far as energy consumption is concerned, the droplets column appears quite comparable to other dust removal systems. It approaches the electrofilter, the most advantageous system at the moment (for 1000 m3/h of gas treated, the energy cost is 400 W, whereas it is 500 –1000 W for the electrofilter [8]). We will be cautious in our comparisons with other processes because we should consider the complete depollution system. Here, we have only studied the transfer of pollution from air to water without considering the treatment in the aqueous phase.
d E H kL P QL Qp t t0 T UG
diameter of the column (m) depollution efficiency (%) column height (m) partial transfer coefficient on the liquid side (m/s) energy cost (W) liquid flow rate (m3/h) dust flow rate (kg/h) time (s) initial time (s) temperature (K) gas velocity (m/s)
References Appendix A. Nomenclature
aL C or [..] C *A
CI CF
interfacial area per unit volume of liquid (m2/m3) concentration (mol/m3) concentration of solute A in thermodynamic equilibrium with the gas containing species A at pressure PA (mol/m3) quantity of pollutant injected at input (g/N m3) quantity of pollutants at output (g/N m3)
.
[1] R. Botton, B. Benadda, R. Bressat, M. Otterbein, Can. J. Chem. Eng. 75 (1997) 527– 534. [2] B. Benadda, R. Bressat, K. Kafoufi, M. Otterbein, N. Muller, Actes de la Deuxie`me Confe´rence Maghre´bine de Ge´nie des Proce´de´s, Gabe`s et Djerba. Tome 1, 1996, pp. 487– 490. [3] N. Muller, B. Benadda, M. Otterbein, Chem. Eng. Tech. 7 (1997) 469– 474. [4] AFNOR NF T 90-032 Essais des eaux. Table de solubilite´ de l’oxyge`ne dans l’eau, 75-04, pp. 1 – 6. [5] J.K Bewter, W.R Nicholas, L.B Polkowski, Water Res. 4 (2) (1970) 115– 123. [6] C. Dimitrov, La Technique Moderne, Novembre– De´cembre, 1987, pp. 41 – 44. [7] W. Muhlrad, Techniques de l’Inge´nieur, J2-II, 1979, A 5700, pp. 14 – 21. [8] M. Otterbein, Actes du Colloque: Traitement des Fume´es, ATEE, Lyon, France, Avril 1993, pp. 51 – 57.
Mass transfer in a ‘droplets column’ in presence of solid particles N. Muller, B. Benadda *, M. Otterbein LAEPSI, INSA, 20 A6. A. Einstein, 69621 Villeurbanne, France Received 17 March 1999; received in revised form 14 June 1999; accepted 31 May 2000
Abstract This work studies the mass transfer efficiency of a new process: the ‘droplets column’. The special feature of this apparatus is the use of high gas throughputs (up to 14 m/s) comparable to smoke from a household waste incinerator. The advantage, as the results show, is the ability to carry out acid gas scrubbing and dust removal simultaneously at a low energy cost. An hydrodynamic study has already been carry out on the droplets column. It begins by measuring pressure drops and liquid hold-ups with three different methods for given operating points. The tendency of the curve remains identical, i.e. a decrease in hold-ups versus liquid flow rate and gas velocity. Considering the errors made on the different methods, the manometric method is more accurate than the other. Therefore, this method can be validated. The study is followed by the liquid residence time determination. All these different results enable to modelize the liquid flow in the column by comparison with well-known models. In this way, the droplets column is assimilated to a sequence of four continuous flow tank reactors. The results concerning hydrodynamic study have been published in N. Muller, B. Benadda, M. Otterbein, Chem. Eng. Tech. 7 (1997) 469– 74. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Mass transfer efficiency; Droplets column; Solid particles
1. Introduction Due to the stricter regulations and standards for gas effluents, industry has had to find more efficient methods of air pollution control. This involves the installation of more or less expensive depollution processes. The most frequently used processes depend on the nature of the pollutant. To treat particles, often dry mechanical processes are used: sedimentation, cyclone, filter or electrofilter. For gases, in particular acid gases, and heavy metals, different depollution techniques, can be used, i.e. dry, semi-wet or wet processes. At the present time, few devices carry out the simultaneous treatment of acid gases and dust. The droplets column is a gas – liquid contactor derived from classical bubbles columns. In such columns, increasing the gas velocity up to UG of 14 m/s, transforms progressively the type of dispersion
* Corresponding author. Tel.: + 33-4-72438185; fax: +33-472438717. E-mail address: [email protected] (B. Benadda).
from gas bubbles in a liquid into liquid droplets in a gas: this is the operating range of the droplets column [1,2]. This apparatus enables simultaneous gas scrubbing through absorption and dust removal in the same reaction volume: all the gas pollutants are transferred to the liquid phase and then may undergo an appropriate treatment. The advantage of this process is the use of high gas throughputs which, for the same depollution efficiency as other gas –liquid reactors and for the same diameter, can treat higher gas flow rates. These flow rates are comparable to those of smoke from household waste incinerators. For this reason, our work studied the efficiency of the process as regards the type of pollutants found in this smoke (HCl, SO2, dust). In this article we will first study the determination of the volumetric transfer coefficient on the liquid side using the oxygen probe method to evaluate the absorption potential of the process. The second part focuses on the determination of process efficiency to absorb both gas pollutants and dusts.
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N. Muller et al. / Chemical Engineering and Processing 40 (2001) 167–174
2. Description of the pilot The droplet column operates with a co-current gas – liquid upflow. Reaction takes place throughout the column which is made of transparent PVC (d = 0.15 m, height H= 3.2 m). It is described in Fig. 1. The high gas velocities are obtained using a centrifugal high pressure fan. The gas enters at the base of the column through a perforated plate with a single large central hole. At the column outlet the gas escapes into the atmosphere. The liquid flows in a closed circuit. It is injected into the column by a cross-tube distributor. Each tube has eight holes and each hole serves an identical surface of the column section. The optimum number and diameter of holes was chosen to ensure the highest possible liquid velocity and to take into account possible clogging of
the holes if the liquid transported solid particles. In this way the liquid is transported and dispersed by the gas velocity all along the column. This also allows recuperation of at least a fraction of the potential energy of the recycled liquid and takes into account possible clogging if the liquid contains solid particles. Separation of water and air is carried out by a direction-changing separator. The separated liquid is collected in a storage tank with an overflow maintaining operation at constant volume. Thus a fixed load can be applied to the diaphragm device and a large range of flow rates can be covered. The liquid flow rate is precisely measured by a flow meter situated downstream. Pressure taps are situated all along the column connected to manometric tubes in order to evaluate pressure drops and hold-ups.
Fig. 1. The droplets column.
N. Muller et al. / Chemical Engineering and Processing 40 (2001) 167–174 Table 1 Type of dust particles used for the study Dust particles
Grain size d50 (mm)
Large ashes — thermal 150 power plant Ashes — household waste 90 incinerator Ashes — chemical plant 50 Ash fines — thermal 40 power plant Talc 9.3 Quartz 1.5 a
Solubility (g/l)a
1 35 12 1.8 0.6 0
At 20°C in water.
169
The minimum oxygen concentration [O2]t 0of the solution must be as low as possible. For this, a quantity of sulphites in great excess is added in order to make all the dissolved oxygen react and thereby approach zero concentration. The [O2]t concentrations can be read from the graph obtained at given times. By tracing ln[([O2]*− [O2]t 0)/([O2]*− [O2]t )] versus time (t− t0), the slope of the straight line gives us readings of kLaL. This coefficient was determined for different operating points in the presence or absence of particles, in order to measure the absorbing potential of the column. Our experiments are carried out between 17 and 20°C. However, all the results were expressed at 20°C taking into account the following correction formula [5]: (kLaL)20°C =
(kLaL)T 1.024(T − 20)
This formula is only valid for + 5°CB TB + 35°C. The study concerning the influence of liquid flow rate and gas velocity on kLaL has already been published [3]. We have shown the increase of the volumetric transfer coefficient on the liquid side with the liquid flow rate whereas the gas velocity does not interfere. Therefore, in this paper, we will only present here the influence of particles and their solubility on this parameter.
Fig. 2. Influence of particle size on kLaL.
3. Determination of the volumetric transfer coefficient on the liquid side The volumetric mass transfer coefficient on the liquid side is determined by a physical absorption method: the oxygen probe method which studies reoxygenation of water. First, deoxygenating is carried out using sodium sulphites in the presence of cobalt sulphate. The reoxygenation of water is measured by an oxygen probe connected to a recorder. This method allows us to determine the volumetric mass transfer coefficient on the liquid side kLaL expressed according to the total volume of liquid present in the column. The equation is given in the form ln[([O2]*− [O2]t 0)/([O2]* −[O2]t )] kLaL = (t − t0) The concentration in saturated oxygen [O2]* depends on the temperature of the solution and the atmospheric pressure. The solubility tables corresponding to the regulatory standard NF T 90 032 [4] allow the determination of this value.
3.1. Influence of the presence of solid particles on the coefficient kLaL For this study, we only take into account the influence of the presence of particles on kLaL, without considering the quantity of particles captured. Table 1 presents the two main characteristics we have chosen for the study of the influence of particles on the volumetric transfer coefficient on the liquid side.
3.1.1. Influence of particle grain size on the coefficient kLaL Two categories of dust were chosen mainly due to their grain size: talc with a grain size from 0 to 50 mm such that 50% of the dust has a diameter greater than 5 mm, micronized quartz with a grain size from 0 to 15 mm such that 90% of the dust has a diameter lower than 6.4 mm. The injection rate of dust is maintained constant in both cases at 750 g/h. It can be noted as in Fig. 2 that kLaL increases with the liquid flow rate in the same way, whether dust is present or not. These results allow to predict that this type of reactor used on multiple pollutants (acid gases and dusts) will give higher depollution efficiencies of acid gases in the
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presence of fine particles (diameters B 5 mm) than in the presence of larger particles (\5 mm). This conclusion is valid only if the two kind of particles used are wettable in the same way in our work conditions.
3.1.2. Influence of the quantity of dust on kLaL For this study we used ashes from a thermal power plant which we injected at two different rates. As before, we measured the volumetric transfer coefficient on the liquid side at different operating points. As shown in Fig. 3, the volumetric transfer coefficient decreases when the dust flow rate increases. It would therefore seem that while part of the water traps the dust, the other part remains free for gas transfer. If we suppose that the exchange coefficient kL, which depends on the hydrodynamics of the column, is not influenced by the dust, then it must be the interfacial area which decreases. The higher the liquid flow rate the more the difference between the values of kLaL should decrease in
Fig. 3. Influence of the quantity of particles in water on kLaL.
Fig. 4. Influence of dust solubility on kLaL.
absence and presence of dust. In fact, the fraction of water which traps dust remains identical as it does not depend on the quantity of dust, whereas the fraction of water which carries out gas transfer increases. This progression does not appear clearly in the curves however (they should in fact show a greater tendency to converge for higher liquid flow rates). This is only valid in the case where we suppose that all the injected dust is captured by the circulating water.
3.1.3. Influence of dust solubility on kLaL The dusts were chosen according to their solubility. Dusts of similar grain size were considered in order to be able to compare the results without running the risk that the values of the volumetric transfer coefficient on the liquid side be affected by the grain size. In the series of assays, fine ashes from a thermal power plant as well as ashes from incineration of chemical products were used. Thermal power plant ashes are less soluble than ashes from chemical product incineration. Their grain size is similar. The dust flow rate used in this series of assays is 3 kg/h. There is a small difference between the values of kLaL obtained in the presence of soluble dust and insoluble dust. In both cases the values are lower than those obtained in the absence of dust (Fig. 4). The difference found shows that there is a repercussion of dust solubility on the exchange capacity between the liquid and gas phases. The values of kLaL are higher in the case of insoluble dust (ashes from incineration of chemical products), which means that as the solubility increases the possible transfer between the liquid and gas phases decreases. Dust of very high solubility would greatly decrease the kLaL. 3.1.4. Conclusions on the 6olumetric transfer coefficient on the liquid side Knowledge of the volumetric transfer coefficient on the liquid side is just as important as knowledge of the interfacial area. The results obtained on the droplet column are of great interest. We have shown that an increase in liquid flow rate gives rise to an increase in the volumetric transfer coefficient on the liquid side whereas the coefficient is little affected by an increasing gas velocity [3]. The original feature of this work resides in the presence of solid particles in the measurement of the volumetric transfer coefficient on the liquid side. Therefore, we have been able to demonstrate the influence of the grain size, the quantity and solubility of the particles on the volumetric transfer coefficient on the liquid side. These different parameters affect kLaL. It can be thought that if we consider the droplets column as a gas –liquid –solid reactor, the acid gas absorption po-
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Table 2 Results of depollution efficiency for HCl QL (m3/h)
CI (g/N m3)
CF×103 (g/N m3)
E (%)
Regulatory standard (mg/N m3)a
0.8 1.0 1.2 1.3
1.515 1.515 1.515 1.515
14.4 7.6 4.5 3.0
99.1 99.5 99.7 99.8
50 50 50 50
a
The gas velocity places us in the context of furnaces with a capacity greater than 3 t/h, i.e. under the strictest regulatory conditions.
Table 3 Results of depollution efficiency for SO2
Without addition of NaOH
With NaOHb
a b
QL (m3/h)
CI (g/N m3)
CF×103 (g/N m3)
E (%)
Regulatory standard (mg/N m3)a
0.8 1.0 1.2 0.8 1.0 1.2
0.531 0.531 0.398 0.531 0.531 0.531
292.1 265.6 185.9 79.7 53.1 26.6
45.0 50.0 53.0 85.0 90.0 95.0
300 300 300 300 300 300
The gas velocity places us in the context of furnaces with a capacity greater than 3 t/h, i.e. under the strictest regulatory conditions. [NaOH]=1 g/l, pH =8.
tential for example, will be less efficient in the presence of solid particles. These results are to be compared to efficiency measurements of the process during air pollution control of standard smoke fumes.
4. Absorption of gas effluents The air entering the droplets column is artificially polluted with hydrogen chloride and sulphur dioxide. Regarding the quantities injected, we based our assays on values found in household waste incinerators, i.e. 1500 mg/N m3 for HCl and 300 mg/N m3 for SO2. The air is analysed at the column input and output.
4.1. Absorption of HCl All the presented assays were carried out with an air flow rate of 795 m3/h, i.e. a velocity of 12 m/s. Table 2 presents the main results in the case of HCl for different given operating points. From these results, it can be seen that the droplets column is very efficient for the treatment of gases loaded with HCl whatever the liquid flow rate used. The quantities of HCl found at the output are much lower than the French regulatory standard. The results obtained are coherent with those from the study of the volumetric transfer coefficient on the liquid side as regards the liquid flow rate. The coefficient kLaL is higher when the liquid flow rate increases.
4.2. Absorption of SO2 As far as SO2 is concerned, the results are presented in Table 3. The quantities injected are voluntarily higher than those found in household waste incinerators, taking into account that the quantities from these plants (300 mg/N m3) already meet the national standards. The assays were also carried out at 12 m/s, which are the conditions found in furnaces of a capacity higher than 3 t/h. The depollution efficiency of SO2 increases with increasing liquid flow rate. As the quantity of water present in the column is greater for the same quantity of gas pollutant, the gas –liquid mass transfer is consequently higher. These results, which give only relatively average efficiencies are nevertheless satisfactory. They are comparable to those obtained in certain industrial processes operating with pulverised water and without addition of a base [6]. The depollution efficiencies are clearly improved in the presence of a base, even for very low flow rates. In fact, the reactions take place in the solution: Absorption of SO2 in the solution: SO2 + H2O H+ + HSO3− HSO3− + 1/2O2 H+ + SO24 − Followed hydroxide:
by
the
neutralisation
2NaOH + H2SO4 Na2SO4 + 2H2O
with
sodium
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Table 4 Influence of dust on acid gas capture
HCl
SO2
a
Qp (kg/h)
CI (g/N m3)
CF×103 (g/N m3)
E (%)
0 2.7 8.6 0 2.0 9.4
1.515 1.515 1.212 0.531 0.531 0.531
7.6 7.6 7.6 53.1 53.1 53.1
99.5 99.5 99.3 90.0 90.0 90.0
Regulatory standard (mg/N m3)a 50 50 50 300 300 300
The gas velocity places us in the context of furnaces with a capacity greater than 3 t/h, i.e. under the strictest regulatory conditions.
The liquid side resistance is so largely decrease by the kinetic of neutralisation chemical reaction. Here again, the results confirm those obtained from the study on absorption potential.
4.3. Influence of the presence of particles on acid gas capture A known quantity of ashes from a thermal power plant is injected in the air flow together with HCl and SO2. The quantity of gas pollutants is measured at the input and output without taking into account the quantities of dust at the output. For a given operating point, different dust flow rates were used (Table 4). Whatever the flow rate of dust injected, the presence of particles does not affect the efficiency of acid gas capture. These results are different from those obtained from the study concerning the influence of dust on kLaL as we had observed a decrease in this coefficient as the quantity of dust injected increased. We can suppose that, given the high solubility in water of the gases used (HCl and to a lesser extent SO2) as against that of oxygen, the quantity of particles is not sufficient to perturb mass transfer. In the same way, similar assays were carried out testing the influence of grain size or solubility of the particles used on the efficiency of acid gas absorption, but no variation was demonstrated. Taking into account these first results, the pollutant contents of HCl and SO2 can be highly reduced in a process such as the droplets column. The quantities collected at the output for these two acid gases are always much lower than the regulatory standards and are only slightly modified by the presence of dusts.
5. Dust removal To test the efficiency of the column for dust removal, we have first to simulate an industrial smoke consisting of a vector gas, in our case air, transporting solid particles. For this, and as described previously, the dusts were injected into the input gas flow.
Measurement of the quantity of dust at the output was carried out using a probe placed in the air conduit in such a position so as to respect the isokinetics so that the sample would be representative. Different powders were used. The choice was made according to their grain size and ease of manipulation. silica (0–7.5 mm), talc (0–40 mm), cement powder (0–360 mm), ashes from a thermal power plant (0–400 mm).
5.1. Results The first assays for dust capture were carried out at the following operating point: QL = 0.8 m3/h UG = 10.4 m/s Table 5 summarises the results for the above-mentioned dusts. These first results already demonstrate the efficiency of the droplets column as a dust remover. However, they do not allow us to establish a precise relationship between the grain size range and efficiency of dust removal. Only the assays on silica, with a grain size between 0.9 and 7.5 mm, tend to show that for particles of a few microns, efficiency could be better. However, Table 5 Efficiency of the process for different particles Dusts
Qp (kg/h)
CI (g/N m3)
CF×103 (g/N m3)
E (%)
Ashes
0.7 4.4 1.05 14.9 15.6 1.7 5.9 0.21 0.16 0.2
0.86 0.56 1.60 22.5 23.7 2.60 9.10 0.32 0.24 0.30
5.6 4.5 9.7 143.2 128.7 9.2 31.2 26.5 10.9 1.0
99.4 99.9 99.4 99.1 99.5 99.6 99.7 91.8 95.6 99.6
Cement
Talc Silica
N. Muller et al. / Chemical Engineering and Processing 40 (2001) 167–174 Table 6 Efficiency according to liquid flow rate with cement QL (m3/h)
P (W) for 1000 m3/h of gas
CI (g/N m3)
CF×103 (g/N m3)
E (%)
0.4 0.8 1.1
270 360 440
14.1 12.2 13.5
146 92 58
99.0 99.2 99.6
as this powder is very light, the injection system does not allow injection of large quantities. These results are therefore not sufficiently significant. We can not affirm that the droplets column is not efficient for this type of particle. It would be necessary to decrease the liquid drop size or increase the impact velocity for considering the collection of dust of any particular size [7]. It can also be noted that, given the velocity used in the assays for dust removal, the regulatory standard for incineration facilities of urban and assimilated residues, places us in the context of installations with a capacity of 1 – 3 t/h, which corresponds to a total quantity of dust at the output of 100 mg/N m3. Only the assays with cement and relatively high initial concentrations of dust have given output concentrations higher than the regulatory standard. For each category of dust, there probably exists a maximum quantity which can be injected while still satisfying the regulatory standards. Further assays must be carried out in this direction. It appears that dust removal is little affected by dust concentration in the gas to be treated. Further assays with higher initial dust concentrations in the air must be carried out until saturation of the circulating water is reached. However, in the industrial application of this process, water is partially regenerated by removing the sludge and adding clean water in the storage tank, the concentration of dust in water, maintained constant, would avoid the problem of saturation.
5.2. Influence of liquid flow rate on dust remo6al To study the influence of the liquid flow rate on dust
173
removal efficiency, cement was injected at an initial concentration of 13 g/N m3, at three operating points as shown in Table 6. Therefore, to treat this smoke (CI = 13 g/N m3 of cement) and to reach the threshold value at the output of 100 mg of dust per N m3 of depolluted air, a water flow rate of 1.1 m3/h is necessary. If the input gas velocity were increased to 12 m/s, the dust content would correspond to the regulatory standard for household waste incinerator plants with a capacity greater than 3 t/h, i.e. 30 mg of dust per N m3 of depolluted air. It would therefore probably be necessary to increase the water flow rate in order to meet this standard. It can be supposed that for each type of dust which can be treated with the droplets column, we must define experimentally an adequate liquid flow rate to allow efficient treatment without wasting energy.
6. Conclusion concerning gas treatment The droplets column is a depollution system for smoke with a twofold action, as efficient for gas pollutants (HCl or SO2 in the presence of NaOH) as for dust removal. In general, the results of gas effluents are very promising as the quantities of pollutants found at the output are much lower than the regulatory standard. The efficiency of the process for dust removal, using low liquid flow rates (0.8 –1.1 m3/h) in a closed circuit and for smokes which are not too heavily loaded with dust (3–13 g/N m3) attains: 99.9% for relatively coarse grained dusts (0–40 and 0–400 mm) 96.0% for finer dusts (0–7.5 mm). Its efficiency with fine particles seems better than that of most other dust removal systems as shown in Table 7. The process is remarkable for its simplicity: increasing the liquid flow rate improves efficiency. This simplicity allows us to suppose a less cumbersome and less costly installation than for other processes.
Table 7 Comparison of droplets column with other separators Deduster
Cyclone Ven turi scrubber Electrofilter Bag filter
Efficiencies of deduster (%) 30 mm
20 mm
10 mm
5 mm
99 99.9 99.9 99.99
90 99 99 98–99
50–60 90–95 80–90 90
30–40 80–90 50–60 70
1 mm B10 50–70 10–20 30–40
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The gas velocities used are high (up to 14 m/s with this pilot). For equal diameter, this allows to treat higher gas flow rates. As far as energy consumption is concerned, the droplets column appears quite comparable to other dust removal systems. It approaches the electrofilter, the most advantageous system at the moment (for 1000 m3/h of gas treated, the energy cost is 400 W, whereas it is 500 –1000 W for the electrofilter [8]). We will be cautious in our comparisons with other processes because we should consider the complete depollution system. Here, we have only studied the transfer of pollution from air to water without considering the treatment in the aqueous phase.
d E H kL P QL Qp t t0 T UG
diameter of the column (m) depollution efficiency (%) column height (m) partial transfer coefficient on the liquid side (m/s) energy cost (W) liquid flow rate (m3/h) dust flow rate (kg/h) time (s) initial time (s) temperature (K) gas velocity (m/s)
References Appendix A. Nomenclature
aL C or [..] C *A
CI CF
interfacial area per unit volume of liquid (m2/m3) concentration (mol/m3) concentration of solute A in thermodynamic equilibrium with the gas containing species A at pressure PA (mol/m3) quantity of pollutant injected at input (g/N m3) quantity of pollutants at output (g/N m3)
.
[1] R. Botton, B. Benadda, R. Bressat, M. Otterbein, Can. J. Chem. Eng. 75 (1997) 527– 534. [2] B. Benadda, R. Bressat, K. Kafoufi, M. Otterbein, N. Muller, Actes de la Deuxie`me Confe´rence Maghre´bine de Ge´nie des Proce´de´s, Gabe`s et Djerba. Tome 1, 1996, pp. 487– 490. [3] N. Muller, B. Benadda, M. Otterbein, Chem. Eng. Tech. 7 (1997) 469– 474. [4] AFNOR NF T 90-032 Essais des eaux. Table de solubilite´ de l’oxyge`ne dans l’eau, 75-04, pp. 1 – 6. [5] J.K Bewter, W.R Nicholas, L.B Polkowski, Water Res. 4 (2) (1970) 115– 123. [6] C. Dimitrov, La Technique Moderne, Novembre– De´cembre, 1987, pp. 41 – 44. [7] W. Muhlrad, Techniques de l’Inge´nieur, J2-II, 1979, A 5700, pp. 14 – 21. [8] M. Otterbein, Actes du Colloque: Traitement des Fume´es, ATEE, Lyon, France, Avril 1993, pp. 51 – 57.