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Ind. Eng. Chem. Res. 2007, 46, 6770-6776
Sulfur Dioxide Absorption in a Bubbling Reactor with Suspensions of Bayer Red Mud Elisabetta Fois, Antonio Lallai, and Giampaolo Mura* Dipartimento di Ingegneria Chimica e Materiali, UniVersita` di Cagliari, Piazza d’Armi 09123 Cagliari, Italy
In this work, we studied the SO2 absorption with suspensions of Bayer red mud (the solid residue of the Bayer process for alumina production from bauxite) using a laboratory-scale bubbling reactor with continuous feed of both gas and liquid phases. A few preliminary tests were carried out in order to calculate the physical mass transfer coefficients in both the gas and liquid sides. Then, the ability of red mud suspensions for sulfur dioxide absorption was studied at several different flow rates of both the liquid and gas streams. The absorption rate was measured for four different suspension concentrations. The absorption phenomena were modeled using the film theory and assuming two different fluid dynamics for the gas phase. The liquid-side mass transfer coefficient and the enhancement factor for chemical absorption were calculated from the experimental results using the model. Introduction Red mud is the solid residue obtained in the Bayer process for alumina production from bauxite. In this process, bauxite is reacted with a concentrated solution of caustic soda at high temperature and high pressure. At these conditions, most of the aluminum oxide converts into soluble sodium aluminate. The other compounds remain in the solid state, so they can be easily separated by filtration from the solution. The residue from the filtration is a concentrated suspension that can contain up to 60% w/w of dry solid of a deep red color and is called “Bayer red mud”. This solid has a highly variable composition depending on the composition of the original bauxite. As a rule, in a typical chemical composition, several metal oxides are present: the higher concentrations are those of iron and titanium oxide. However, the most important fraction of red mud is that of the compounds generated by the caustic reaction of the silicates initially present in bauxite. These compounds are referred to as desilication products (DSP). Most of these compounds belong to the tectosilicates group; sodalite is considered the most representative among those compounds.1 Such silicates are able to exchange Na+ ions with the solution that they are in contact with; therefore, the red mud suspensions usually exhibit a caustic pH in the range 10.5-11. A further fact to consider is that red mud is filtered from an aqueous phase that is a strongly caustic solution due to the presence of sodium hydroxide and sodium carbonate (1-6% w/w for the two compounds, expressed as Na2O) remaining from the caustic attack to bauxite. The chemical properties previously described make the red mud suspensions suitable to be used as sorbents in a flue gas desulfurization process, a property that is due to both the caustic solution surrounding the solid and the ability of the DSP to exchange Na+ ions with the solution. The first attempts to take advantage of the caustic properties of red mud for flue gas desulfurization date back to the 1970s when the Japanese firm Sumitomo constructed a demonstrative plant treating 220.000 Nm3/h of a flue gas containing 1260 ppm of SO2.1 Subsequent developments led to the construction in various parts of the world of a few industrial plants, usually based on the original * To whom correspondence should be addressed. Tel.: +39 070 675 5051. Fax +39 070 675 5067. E-mail:
[email protected].
flow sheet set up by Sumitomo. One of these plants was located in our region (Sardinia, Italy) in 1999. Despite the presence of these industrial applications, only a few papers regarding the experimental and theoretical study of desulfurization processes using red mud are present in existing literature. A few studies were produced regarding dry desulfurization,2 but only sporadic references are available on wet desulfurization, although several patents may be found.3 Since no theoretical work on wet desulfurization with Bayer red mud is available to be used as a reference, in this work we refer to the absorption process by means of limestone suspensions, which several papers have dealt with. The technical conditions of an industrial wet desulfurization process usually regard flue gas streams containing SO2 at a relatively low concentration. The reactions of sulfurous acid production and the consequential dissociation reactions should be considered reversible at these conditions. Moreover, the whole process can be well-modeled using the film theory.4,5 When the desulfurizing compound is present in a solid phase, the solid dissolution in the film of the gas-liquid transfer is often taken into account.6 On the other hand, equally good results can be obtained in many cases if the solid dissolution in the film is neglected.7 Albeit with some different model propositions (such as the assumption of a zero flux of charge in the interface), a similar approach has been adopted by Rochelle,8 obtaining negligible differences both in the mass flux and in the enhancement factor calculation. A more rigorous procedure coupling the mass transfer and the ionic reactions taking place in the stagnant film adhering to the gas liquid interface has been proposed by Lancia et al.9 Moreover, though the solid dissolution and the chemical reactions could take place either in series or in parallel, the two processes are supposed to be in series inside the laminar layer at the conditions of their experimental setup. Even if the two processes are substantially comparable, red mud desulfurization shows a few important differences in comparison with limestone desulfurization. The main difference is that the experimental tests on limestone desulfurization have been carried out at such limestone concentrations where it is totally dissolved in water10 or it forms a low concentrated suspension, just above the solubility limit.9 However, our experimental work used suspensions containing a very high solid fraction, in the order of that used in the industrial process. At
10.1021/ie0616904 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/01/2007
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Figure 1. Framework of the experimental apparatus.
these conditions, the ion exchange takes place in a mass of solid that is by far greater than the amount of solid that is present in the gas liquid film. So, we opted to neglect the contribution of the solid dissolution kinetics, supposing that the process is controlled only by the transport of sulfur dioxide from the gas bulk to the liquid bulk. Moreover, we also assume here that the mass transfer mechanism is similar to that governing the analogous process using limestone suspensions. Experimental Apparatus The absorption reactor is an unbaffled cylinder vessel of 0.08 m i.d. and 0.1 m height, made of Plexiglas. A magnetic stirrer was used to provide a thorough mixing of the liquid phase; the stirrer speed can vary in the range between 0 and 10 s-1. For all the experimental tests here reported, the speed of the magnetic stirrer was fixed at ∼6.6 s-1. This stirring speed derives from a few preliminary tests performed on this vessel to study the residence time distribution (RTD). The stimulusresponse tests were carried out using a KCl solution as the stimulus and measuring the conductivity of the stream leaving the reactor. The RTD was determined at different values of the gas and liquid flow rates as well as with different speeds of agitation, and the results obtained at the conditions used in this paper show that the liquid phase is well-mixed. The gas stream entering the reactor is a mixture of gases (N2 and SO2) premixed at a known certified concentration (4000 or 2000 ppmv of SO2) and stored in a cylinder. Streams at other SO2 concentrations were also used for a few tests by mixing the original stream with an additional N2 stream coming from a cylinder. Gas flow rates in the range 0.003-0.045 L/s were used during all the tests. The red mud samples used for this work were taken at the outlet of the filtration section in a bauxite refinery (Eurallumina SpA, Portoscuso, Italy). The red mud suspension is stored in a 4 L vessel that is continuously agitated in order to avoid sedimentation. The suspension is then fed to the reactor by means of a peristaltic pump with the suspension rate varying in the range 0.003-0.007 L/s. In any case, the experimental results obtained show that sulfur dioxide removal is barely
influenced by the suspension rate modification in the range here investigated. The red mud leaving the reactor is recirculated to the storage vessel. A nitrogen stream is supplied to the vessel, which is able to remove most of the absorbed sulfur dioxide, so the suspension pH has only a slight variation during each test. In any case, the pH of the recirculated stream is continuously measured in order to establish if the suspension should be renewed. In our procedure, the suspension is changed before it attains a pH value 0.3 points lower in comparison to that of the freshly prepared suspension The SO2 absorption rate was evaluated at a steady state by measuring the SO2 concentration in the outlet gas stream by means of a NDIR analyzer (Horiba PIR-2000) at a 2000 ppm or 6000 ppm full scale. Figure 1 shows the whole framework of the experimental apparatus. The absorption rate was evaluated at several red mud concentrations of the suspension in the range from 50 to 200 g/L. The suspension pH (10.5-11) was roughly equivalent to that of natural red mud. A few preliminary tests were first performed in order to evaluate the gas-side and the liquid-side mass transfer coefficients. This part of the research derives from the peculiarity of the system here studied, which makes it difficult to obtain the mass transfer coefficients from the literature.11 First, contrary to most of the other experimental systems used for mass transfer studies, our reactor has no baffle. Moreover, it is very difficult to find in the literature most of the physical parameters concerning the sorbent here used, which is a suspension of a solid mixture in an electrolytic solution. The experimental runs were performed by absorbing SO2 in 1 N NaOH or 1 N HCl aqueous solutions. In particular, the liquid-side resistance can be neglected if a NaOH aqueous solution is used to absorb SO2. On the other hand, the SO2(aq) dissociation can be neglected in the SO2 absorption by means of a HCl aqueous solution; therefore, it may be assumed that only physical absorption takes place. Each mass transfer coefficient (akg in the gas side and akl0 in the liquid film side) has been determined for several different values of the gas and liquid flow rates. The influence of the impeller speed was not studied in this work since all the absorption tests were carried out with red
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Figure 2. Experimental results obtained with a gas stream containing 4000 (full symbols) and 2000 (empty symbols) ppmv of SO2.
mud at the same value of speed. The results were expressed by the following relationships, where the three constants were calculated by means of a nonlinear regression procedure:
akl0 ) (5.56 × 10-7)uS2tL-1.9
(1)
akg ) (4.5 × 10-8)uS1.26tL-0.55
(2)
The values of akg and of akl0 were calculated with the assumption of a continuous stirred tank (CST) fluid dynamics for both the gas and liquid phases in the reactor. The values obtained with the correlations here reported are in good agreement with those shown by Lancia et al.10 in a similar work regarding SO2 absorption in limestone suspensions. A few tests with tap water as the liquid sorbent were carried out in order to verify the reliability of the experimental setup. The experimental results obtained were used to calculate the mass transport coefficients that were then compared with those reported by Chang and Rochelle.4 Good agreement was found between the two sets of data. A few experimental results obtained with our experimental setup are presented in Figure 2. These values have been obtained with two concentrations of the gas stream entering the reactor and at the same flow rate of the suspension. Other tests were carried out at different flow rates of the suspension, but the SO2 concentration measured in the gas outlet, at the same suspension concentration and gas flow rate, showed only negligible variations. Some other results were obtained with a reactor quite similar to the one used here, but where a higher residence time of the gas could be set. These results show that a removal efficiency that is comparable to that of the usual industrial systems may be obtained. The Model The absorption phenomena were studied using film theory to describe the liquid-side mass transfer. Then, the diffusion resistance in the liquid-phase side is assumed to be concentrated in a layer, whose thickness in general depends on the system fluid dynamics but is independent of the nature of the reactions taking place. The properties of the film surrounding each bubble, in particular, the concentration difference between the phases, vary during the time when the bubble is present in the reactor. Then, they also depend on the fluid dynamics of the two phases between which the mass transfer occurs. According to what is obtained by means of the RTD tests carried out on the liquid phase, it is possible to model gas absorption by considering a CST flow model for the liquid phase. The behavior of the gas phase may be different since each bubble follows in the reactor a journey that is often fully
independent of that of the other ones. The bubbles are, in this case, totally segregated, and the overall mass transfer with the bulk liquid depends on the macromixing. The general problem may become very complicated, so we can imagine reducing it to two extreme situations describing the bubbles’ fluid dynamics. The first is one where the bubbles are fully macromixed, while the other is that of a plug-flow fluid dynamics.12 The experimental results obtained with our absorption reactor were modeled assuming a CST flow model for both gas and liquid phases. The suspension pH is quite high at the beginning of each test. It remains almost constant during the tests as a result of the exchange with the solution of the Na+ ions present in the solid DSP. As was previously affirmed, as a consequence of the presence of a great excess of solid in the suspension, we assume that the ion exchange process is always in a condition near the equilibrium in the whole suspension. Therefore, the model neglects to take into account the kinetics of Na+ ions transport from the solid to the liquid bulk. The entire process is so described only by means of the following equilibrium equation:
Na+r.m. + H+ a Na+ + H+r.m. The value of the equilibrium constant was estimated beginning from the value of the natural pH of the mud (10.5-11); a value of Kr.m. ) 1.81 has been assumed in this paper. The whole process of sulfur dioxide transfer in a red mud suspension is composed of a diffusion operation and several chemical reactions. Diffusion could be considered to consist of two stages: the first takes place in the gas film toward the gasliquid interface and the second takes place in the liquid film toward the liquid bulk. The reactions accompanying the SO2 absorption are ionic, so they are much faster than the associated diffusive transfer in the liquid film. This condition is referred to as the instantaneous reaction regime. This leads us to assume that the reacting species are at thermodynamic equilibrium throughout the liquid. In an industrial process, the reactions occurring in the liquid film can be arranged in two sets. The first one contains the reaction of sulfurous acid formation and the subsequent dissociations. The second set includes the reactions deriving from carbon dioxide hydration. Only gas streams containing nitrogen and sulfur dioxide were used in the experimental setup used for this research work, so in the model we can neglect all the reactions concerning CO2 absorption and dissociation. Therefore, the following equations should be considered in order to describe the process.
SO2 + H2O a H2SO3 K1 ) 1.1225
[L/mol]
H2SO3 a H+ + HSO3- K2 ) 1.316 × 10-2
[mol/L]
HSO3- a H+ + SO3) K3 ) 7.0152 × 10-8
[mol/L]
H2O a H+ + OH- Kw ) 1 × 10-14
[mol2/L2]
The values of the respective equilibrium constants, calculated from the thermodynamic properties at 25 °C, are also reported for each equation; these values are similar to those reported by other researchers.10 This constant value has been used for our calculations since all the experimental tests were carried out at the room temperature of the laboratory in the range between 20 and 25 °C.
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The equilibrium constants of the previous reactions were defined as a function of the activity of the components present in solution by means of equations of this kind, Kj ) ∏i(ciγi)ν. The values of the activity coefficients γi for all the ionic species present in the solution were defined by the equation of Debye-Hu¨ckel assuming a very low concentration of all the ions.
log γi ) -
B‚zi2‚xF
(3)
1 + xF
In this equation, B is a constant, zi is the number of elementary charges of the single ion, and F is the total ionic strength defined by the equation F ) 0.5∑izi2ci. The activity coefficients of the ions present in the active solid phase (the desilication product of the red mud) were calculated by means of the Wilson equation for the binary system Na+H+. The symbol yi,r.m. in the following equation is the molar fraction of the ith component in the desilication product that is part of red mud. The values used for the binary constants are the same reported by Mehablia et al.13 2
log γi,r.m. ) 1 - log[
∑ j)1
2
yj,r.m.Λij] -
yk,r.m.Λki
∑2 k)1 yj,r.m.Λkj ∑ j)1
(4)
The equilibrium equations should be solved subjected to the electroneutrality condition. Specifically, for our system, the equation is the following
[H+] + [Na+] ) [OH-] + [HSO3-] + 2[SO3)] The following material balances were also taken into account. (1) Mass Balance for the Sulfur Compounds in the Liquid Phase. The value of [SO2]in is the theoretical concentration of sulfur dioxide absorbed in the liquid phase, before ion dissociation.
[SO2]in ) [SO2] + [H2SO3] + [HSO3-] + [SO3)] (2) Mass Balance of the Total Amount of Sodium Ions Present in the Whole System (Both Solution and Red Mud).
[Na+] + [Nar.m.+] ) [Na+]in + [Nar.m.+]in (3) Balance of the Positive Sites Present in the Desilication Product of Red Mud. Our assumption was that all the sites are initially occupied by the sodium ions that can then be exchanged with the H+ ions present in the solution.
[Hr.m.+] + [Nar.m.+] ) [Nar.m.+]in The following equations should be used to describe both the gas-side and liquid-side mass transport:
Gm(yin - yout) ) kGaPVl(yout - yo) Gm(yin - yout) )
Lm (c - cin) ) RVl ctot out
(5) (6)
The symbol y° corresponds to the molar fraction in the gas phase that would be in equilibrium with the bulk liquid-phase composition. The Henry’s equation has been used to describe this equilibrium, and the value of 82 500 (Pa L)/mol for Henry’s
constant at 25 °C has been assumed for SO2.9 The molar flow rate Lm is calculated as the sum of the molar liquid flow and the solid molar flow. A mean molecular weight is calculated for the solid assuming a typical composition of red mud.1 For this calculation, the desilication product is considered to be composed by sodalite only. The concentrations cout and cin are the sums of the concentrations of all the sulfur compounds present in the suspension. The overall mass transfer coefficient is defined by the following equation as a function of the mass transfer coefficients both in the gas and liquid phases.
1 H 1 ) + kG kl kg
(7)
The specific surface of the bubbles (a) present in eq 5 is defined by the following equation as a function of the gas holdup Hg and the bubble’s diameter
a)
6Hg db
For our calculations, the bubble diameter was considered to be uniform in the whole vessel. Its value was established by means of high-speed pictures of the vessel during the operation, and the value of the gas holdup was obtained with the same experimental tests, simply measuring the volume variation of the liquid phase in the reactor. The specific surface so obtained is in good agreement with that calculated with the same equation, but where the value of the gas holdup is computed by using the correlation proposed by Yoshida et al.14 This correlation includes the superficial velocity of the gas stream and the main mixing parameters of the reactor, such as the impeller speed and the specific power of the impeller. The set of equations 1-7 together with the chemical equilibrium, the mass balance, and the electroneutrality equations form a nonlinear system that was solved by using the Newton-Raphson method. Calculation is possible since the values of yin and yout are known from the experimental tests and the concentration cin is initially assumed. The concentrations of each sulfur compound in the suspension leaving the reactor are calculated from the solution of the nonlinear equation system described. Then, the value of kG can be easily calculated from eq 5. Finally, eq 7 allows us to obtain the value of kl. Results and Discussion The experimental results obtained for sulfur dioxide absorption with our experimental setup are plotted in Figure 2, where the SO2 concentration in the gas stream leaving the reactor is reported as a function of both the gas flow rate and the suspension concentration. The experimental runs show that high removal efficiencies can be obtained in the bubbling reactor operating with a value of the liquid-to-gas ratio quite similar to that used in industrial applications. The figures clearly show that, in the field here investigated, the SO2 removal efficiency increases when the gas flow rate is increasing, with all the other parameters being equal. Moreover, this efficiency assumes a higher value when a higher SO2 concentration of the gas stream is used. Similar results, even if less marked, were also obtained with the tests carried out with the same equipment using pure water as the sorbent. The SO2 removal efficiency assumes higher values for the higher concentrations of the red mud suspensions. It is interesting to point out that the removal efficiency obtained with the
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Figure 3. Enhancement factor as a function of the gas flow rate; SO2 gas concentration 4000 ppm (full symbols) and 2000 ppm (empty symbols).
Figure 4. Enhancement factor as a function of the liquid flow rate; SO2 gas concentration 4000 ppm (full symbols) and 2000 ppm (empty symbols).
low concentration suspensions (
