Chemical Mass Transfer Mechanism and Characteristics of Flue Gas

Sep 11, 2017 - Flue gas desulfurization (FGD) technology with SO2 recyclable regeneration has been increasingly used for basic aluminum sulfate (alumi...
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Chemical Mass Transfer Mechanism and Characteristics of Flue Gas Desulfurization of Basic Aluminum Sulfate by Bubbles Zijing Zhang,*,† Linmao Lang,‡ Jianwen Wang,† Zhiqiang Zhang,† and Gao Wen† †

College of Energy and Power Engineering, Inner Mongolia University of Technology, Hohhot 010051, P. R. China Beijing Boqi Power Technology Co. Ltd., Beijing 100022, P. R. China

‡

ABSTRACT: Flue gas desulfurization (FGD) technology with SO2 recyclable regeneration has been increasingly used for basic aluminum sulfate (aluminum base); in this process, chemical reaction and physical mass transfer synergistically affect desulfurization. In this study, experiments and ANSYS numerical simulation were performed to determine the multicomponent continuity equation, gas-phase volume overall mass transfer coefficient (KGae), and chemical mass transfer complex factor (F) by analyzing the reaction kinetics and using two-film theory. The chemical reaction rate (r) was calculated using the consumption rates of SO2 and active Al2O3, and KGae was obtained using flue gas apparent velocity and SO2 mole fraction. The effect of different factors on the chemical mass transfer of aluminum base FGD was then determined. Results showed that both r and dynamic field of gas−liquid two-phase flow affected the mass transfer capacity and desulfurization effect of the aluminum base. The inlet SO2 concentration exhibited a greater influence on KGae and F than the concentration of the absorption solution. Chemical mass transfer originated at the phase interface, and the reaction mainly occurred in the liquid-phase boundary film. Mass transfer resistance was mainly concentrated in the gas-phase boundary film. The aeration rate affected bubble formation, diffusion morphology, and gas−liquid contact time. Increasing the number of small bubbles and preventing coalescence improved the desulfurization efficiency. Moreover, KGae increased linearly with the increasing aeration rate (0.05−0.15 m/s), and F remained stable at a high level (about 0.05). The mass transfer capacity was mainly controlled by aeration rate, and the comprehensive effect of aluminum base FGD was mainly controlled by liquid-phase mass transfer conditions caused by the chemical reactions. Under the condition with 12.5% absorption solution and 0.05 m/s aeration rate, KGae was only 2.346 × 10−6 kmol/(s·m3·kPa). Furthermore, KGae increased rapidly with increasing concentration of absorption solution, whereas F decreased rapidly to the minimum value of 0.03443 with increasing aeration rate (0.15−0.2 m/s) under the conditions of low absorption solution concentration. The mass transfer capacity and comprehensive effect of aluminum base desulfurization were controlled by both physical mass transfer conditions due to the aeration rate and the liquid-phase mass transfer conditions caused by chemical reactions.

1. INTRODUCTION Flue gas desulfurization (FGD) technology with SO2 recyclable regeneration is not only an effective way to achieve sustainable development for circular economy but also an effective means which can replace limestone gypsum to reduce CO2 emissions and ecological destruction. Basic aluminum sulfate (aluminum base) regeneration desulfurization technology has gained increasing attention because aluminum base absorbents exhibit advantages, such as nontoxicity, high absorption capacity, high absorption rate, low volatilization, and rare fouling in acidic environments. Gao et al.1−3 performed an in-depth study on desulfurization performance, desorption effect, and technological process. Then, it was found that the desulfurization capacity of aluminum base absorption solution could be restored by aluminum base which was regenerated by desulfurization product (Al2(SO4)3·Al2(SO4)3) under the thermal action. This research indicated that improved absorption rate, reduced desorption time, increased desorption depth, and decreased loss of aluminum are necessary for industrial application of aluminum base desulfurization. The researchers also determined the suitable conditions for the zero discharge of aluminum base desulfurization wastewater by flue gas spray evaporation technology in a 600 MW unit. Qiao et al.4,5 conducted small pilot tests on different absorption and desorption © XXXX American Chemical Society

devices to obtain the relationship of desulfurization efficiency to temperature, aeration volume, gas concentration, and aluminum base concentration. The regeneration performance of aluminum base solution and the mass transfer effect were analyzed by adding ethylene glycol to inhibit SO32− oxidation. These experimental studies focused on the effects of various factors on desulfurization performance and absorbent regeneration to evaluate the feasibility of the method for large-scale application. Despite the large amount of data accumulated, the depth of research on aluminum base desulfurization is insufficient, resulting in the lack of theoretical support for its largescale industrial application. The macro effect of aluminum base on absorbing SO2 is determined by both physical mass transfer conditions and chemical reactions. Thus, the mechanism and characteristics of chemical mass transfer are the basis for optimizing the desulfurization process and improving the desulfurization efficiency. Although the chemical mass transfer theory of aluminum base FGD has been rarely investigated, numerous reports are available on wet FGD. The equilibrium-level model and Received: May 24, 2017 Revised: September 5, 2017 Published: September 11, 2017 A

DOI: 10.1021/acs.energyfuels.7b01488 Energy Fuels XXXX, XXX, XXX−XXX

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mathematical analytical solution and thus restrict the comprehensive and reliable use of the model. To understand the mechanism of chemical mass transfer in aluminum base desulfurization, this paper investigated the use of aluminum base solution for the removal of SO2 in a single hole bubbling absorber. An analysis model was established by combining reaction kinetics and physical mass transfer theory. The model combined with numerical simulation was employed to determine characteristics, such as bubble formation in liquid phase, flow field distribution of bubble diffusion, mass transfer laws, and absorption efficiency. The results will provide a theoretical reference for the industrial application of aluminum base FGD by bubbles.

rate-level model of gas absorption were used to describe desulfurization;6,7 the latter has been widely used considering multicomponent mass transfer and chemical reactions. Sada8 studied the mechanism and characteristics of lye (liquid-phase containing dispersed solid particles) for SO2 absorption and established a single-reaction surface absorption rate model under the assumption that the diameter of solid particles is smaller than the thickness of the liquid film in the fast reaction. The desulfurization experiments in a stirred tank showed that the proposed model was unsuitable for high SO2 concentration (≥5%); however, the chemical reaction on the active solid particle dissolution-promoting effect was not considered. Manoj et al.9 used a similar method to establish a double-reaction surface model for studying the characteristics of Mg(OH)2 and Ca(OH)2-mixed slurry on SO2 absorption; the results are similar to those reported by Sada. Rochelle et al.10 investigated the mechanism of lime slurry on absorbing SO2 in a spray tower and concluded that appropriate additives can promote mass transfer. Hikita et al.11 evaluated the reaction mechanism and characteristics of Na2CO3 solution on SO2 absorption and CO2 desorption in a gas−liquid double stirred reactor to establish a SO2 absorption rate model based on two-film theory. The exact solution of the nonlinear differential equations was replaced by the approximate analytical solution of the model, and the results are consistent with the experimental data. Chang et al.12 used a modified two-film theory to simulate limestone slurry on SO2 absorption in a batch reactor and optimize some parameters. Brogren et al.13 used an infiltration model to establish a mathematical model of limestone slurry absorbing SO2 in spray tower and determined the effects of SO2 absorption, CO2 desorption, and SO32− oxidation on mass transfer; the multielement Gauss−Newton method was used to obtain the concentration of each component in the model. Juan et al.14 established a mathematical model of wet FGD based on the whole mixed-flow of a jet bubbling reactor; the model was used to calculate the concentration distribution of each component and change in pH in the liquid-film. Gang et al.15 compared the desulfurization reaction activity of CaCO3, MgO, and MgO/ MgSO4 in a laboratory-scale bubbling reactor and deduced the mass transfer and diffusion law of SO2 at the gas−liquid interface by combining experimental phenomena. Using microreactors with different structures, You et al.16 measured the average specific surface area in phase boundary and the liquidphase absorption mass transfer coefficient under the conditions of gas−liquid countercurrent contact by using chemical absorption method; the influence of reaction inlet structure size and fluid flow on the mass transfer performance was also determined. Yang et al.17 investigated the main influence of UV/H2O2 combined with Ca(OH)2 on the removal of NO and SO2 in a small ultraviolet bubble column. Considering that the gas−liquid mass transfer of the bubble was stronger than that of the spray, Qing et al.18 conducted a seawater desulfurization experiment on a self-made jet bubbling column; then, they estimated the mass transfer performance by using two-film theory combined with empirical formula to obtain the variation law of liquid-phase total mass transfer coefficient with exhaust gas flow and seawater temperature. On the whole, the current studies on chemical mass transfer mainly focus on experimental research of specific objects or simplified solution using models combined with empirical formula. The specificity of the theoretical model and the complexity of actual industrial conditions, particularly in terms of practicality and flexibility, complicate the calculation of the

2. EXPERIMENT AND NUMERICAL SIMULATION In this paper, experimental research was combined with numerical simulation. First, the reaction rate in aluminum base desulfurization and the mass transfer model suitable for the bubble experiment system were determined by analyzing the reaction kinetics and using two-film theory, respectively. Then the intrinsic link between chemical factors and flow field characteristics was established in simulated environment by chemical and physical parameters measured in the experiments. In this way, the numerical simulation of chemical mass transfer under different experimental conditions could be researched. The experimental system is shown in Figure 1.

Figure 1. Schematic diagram of the experimental apparatus.

The inlet SO2 concentration of the simulated flue gas was controlled by adjusting the inlet flow of SO2 and air. The outlet SO2 concentration was measured by testo350 flue gas analyzer, which has a standard range of 0−5000 ppm of SO2 and 5 times the range of expansion with a maximum measurable range of 0−25 000 ppm of SO2. Numerical simulation was conducted by ANSYS software embedded with user defined function (UDF) and considering the chemical mass transfer process. 2.1. Experimental Method. The desulfurization reaction of aluminum base is shown in eq 1: (1 − x)Al 2(SO4 )3 ·x Al 2O3 + 3xSO2 = (1 − x)Al 2(SO4 )3 ·x Al 2(SO3)3 2−

(1)

3+

The SO3 and Al in the solution were determined by iodometry and ethylenediamine tetraacetic acid (EDTA) complexometric titration, respectively.19,20 Then, the amount of aluminum in the absorption solution represented by the Al2O3 (g/L) could be obtained by converting the Al3+ concentration. Basicity (x) represents the fraction of free active Al2O3 in the total amount of aluminum in the solution. The aluminum base solution was prepared with 25 g/L Al2O3 and 25% x and used as the basic absorption solution, with a concentration of 100%. Different concentrations of the absorption solution were obtained by adding different volumes of deionized water. B

DOI: 10.1021/acs.energyfuels.7b01488 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Different concentrations of simulated flue gas were prepared using different volumes of pure SO2 with air. Hence, SO2 absorption experiments using variable concentrations and temperatures were conducted under different aeration rates. 2.2. Physical Model. Single-hole aluminum base desulfurization cylindrical absorber exhibited a height Z = 0.5 m, radius R = 0.04 m, and vent hole diameter = 0.002 m. The “Gambit” software was used to establish a physical model (Figure 2), and uniform hexahedral grids were divided into 250 thousands.

(4) Combining the absorption data of SO2 in the liquidphase, UDF with chemical mass transfer was prepared for numerical simulation. The boundary conditions were set as follows: (1) The initial state of the absorption solution was static. (2) The inlet surface of the simulated flue gas was speed conditions; the upper outlet surface of exhaust was pressure conditions, and the other surfaces were standard wall surface. 2.4. Mass Transfer UDF Accompanied by Chemical Reaction. The UDF was controlled by ANSYS’s own continuity equation (mass equation), momentum equation, and energy equation and closed by the ideal gas state equation. However, the multicomponent continuity equation with chemical reactions should be solved using experimental data because of different specific objects and reaction conditions. Particularly, the chemical reaction rate (r) and KGae were the keys to the solution. 2.4.1. Solving r in the Aluminum Base Desulfurization Process. The multicomponent continuity equation with chemical reaction described by Fick’s second law is shown in eq 2.24−27

Figure 2. Physical model of numerical simulation.

∂ρA

For the calculation domain, the center of the vent hole was used as the coordinate origin and divided into two parts to avoid unstable fluctuations of the liquid surface under high aeration rates. The lower part of 0.4 m was filled with the absorption solution, and the upper part of 0.1 m was supplied with simulated flue gas. The points were solved by controlling differential gap according to the reaction, quality, and energy coupling. 2.3. Mathematical Model. The Euler mixed model was used to study the characteristics of gas bubble formation and unsteady rising diffusion in the liquid phase. The volume of fluid (VOF) mixed model was used to determine the concentration distribution of the reactant and the mass transfer characteristics in the liquid phase. A multiphase flow model with chemical reaction was then established.21−23 For the absorption solution as the continuous phase, the concentration distribution of the reactant was solved by the simple algorithm with standard k−ε two-equation model in the Euler coordinate system. For simulated flue gas as discrete term, the stochastic orbit model was used to solve the flow field in the Lagrange coordinate system. For the governing equations of the system components, the following assumptions were considered: (1) In the gas−liquid two-phase flow, the gas phase included SO2 and air, and the liquid phase included aluminum base solution. Since the volume concentration of CO2 in air is only 0.03% and the solubility in aqueous solution is low, there is only a trace of CO2 (or H2CO3) in the absorption solution. In addition, active AL2O3 cannot react with H2CO3 (weak acid). As a result, the effect of CO2 on aluminum base desulfurization could be ignored. (2) Considering minimal changes in the temperature and concentration of the aluminum base solution during absorption, we ignored the changes in the density and viscosity of the reactant. (3) The flue gas entered the liquid phase in the form of bubbles; hence the simulated flue gas was regarded as a discrete term.

∂t

= DAB∇2 ρA − ∇(ρA u ⃗) + rA × MA

(2)

The effective reactants for aluminum base desulfurization were SO2 and free active Al2O3. Hence, the gas−liquid phase continuity equation could be expressed by the consumption of SO2 and active Al2O3, as shown in eqs 3 and 4, respectively. ∂ρG ∂t ∂ρL ∂t

= DGL ∇2 ρG − ∇(ρG uG⃗ ) − rSO2MSO2

(3)

= DGL ∇2 ρL − ∇(ρL uL⃗ ) − rAl 2O3MAl 2O3

(4)

2

where D is the diffusion coefficient (m /s), u is the apparent velocity (m/s), M is the molar mass (g/mol), ρ is the density (kg/m3), and t is the time (s). The r (mol/L·s) stands for mass exchange rate per unit volume due to the chemical reaction to takes a positive value with material generation, while a negative value with material consumption. The subscripts A, B, G, and L stand for material A, material B, gas phase, and liquid phase, respectively. SO2 consumption during aluminum base desulfurization could also be expressed as an increment of SO32−, and r can be solved dynamically by initial rate method.28 Figures 3 and 4 show that the reaction order (α) of SO2 was 1.01, whereas the order (β) of Al2O3 was 0.06. Under the conditions of 100% reactant concentration, the relationship between ln(r(SO32−)) and 1/T was obtained by fitting the experimental data of temperature change and absorption (Figure 5). Parameters such as the pre-exponential factor k0 (0.362 s−1), activation energy Ea (1.56 kJ/mol), and reaction rate constant k (0.191 s−1) at 20 °C were obtained using the Arrhenius formula. The r characterized by SO32− is shown in eq 5. Combining with eqs 1 and 5, the rSO2 and rAl2O2 in eqs 3 and 4 could be expressed as eqs 6 and 7, respectively, where c represents the molar concentration (mol/L).

C

rSO32− = 0.191 × c(SO2 )1.01 × c(Al 2O3)0.06

(5)

rSO2 = 0.191 × c(SO2 )1.01 × c(Al 2O3)0.06

(6)

DOI: 10.1021/acs.energyfuels.7b01488 Energy Fuels XXXX, XXX, XXX−XXX

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equation. It offers the possibility to study the chemical mass transfer performance. 2.4.2. Determination of KGae. According to two-film theory, the SO2 absorption rate (NSO2) in the bubbling desulfurization process could be expressed by eq 8, which is also associated with eqs 9 and 10. * ) NSO2 = K GP(ySO − ySO

(8)

1 1 H = + 0 KG kG IkL

(9)

2

2

* ) = PSO − PSO * P(ySO − ySO 2 2 2

(10)

2

KG in aluminum base SO2 absorption cannot be simply evaluated by chemical reaction theory or two-film theory because both r*SO2 and I were difficult to measure. The microprocess of the chemical reaction of aluminum base desulfurization remains unclear. Aroonwilas et al.29,30 reported that KG can be effectively determined by measuring changes in the concentration of gas components during the absorption experiment. With the bottom of the absorber as basis, the differential equation of mass transfer along the height Z could be expressed by eqs 11 and 12.

Figure 3. Reaction order fitting chart for the determination of SO2 at 20 °C.

NSO2aedZ = −GdYSO2 YSO2 =

(11)

ySO

2

1 − ySO

(12)

2

For medium-quick reaction, the gas-phase partial pressure balanced against SO2 concentration in the liquid phase was 0. Combining eqs 8 to 12, the KGae can be obtained by integrating the absorber (eq 13). K Gae =

Figure 4. Reaction order fitting chart for the determination of Al2O3 at 20 °C.

⎞ ⎛ uG ⎜ 1 1 ⎟ − ZR SO2T ⎜⎝ 1 − ySO ,in 1 − ySO ,out ⎟⎠ 2

2

(13)

3

where H is Henry’s law coefficient (kPa·m /kmol), kG is the gas-phase mass transfer coefficient (kmol/s·m2·kPa), kL is the liquid-phase mass transfer coefficient (m/s), k0L denotes liquid-phase mass transfer coefficient without chemical reactions (m/s), I is the enhancement factor due to the chemical reaction, KG is the gas-phase overall mass transfer coefficient (kmol/s·m2·kPa), ae is the effective interface area per unit volume absorption solution (m2/m3), KGae is the gas-phase volume overall mass transfer coefficient (kmol/s·m3·kPa), NSO2 is the absorption rate of SO2 (mol/s·m2), PSO2 is the gas-phase partial pressure of SO2 (Pa), P*SO2 is the gas-phase particle pressure which is balanced against SO2 concentration in the liquid phase (Pa), y is the mole (volume) fraction, G is the molar flow of gas (mol/s·m2), and RSO2 is the gas constant of SO2 which is taken for the universal gas constant of ideal gas (8.314 kJ/kmol·K). Thus, the mass transfer performance under various experimental conditions could be simulated by writing KGae UDF, and the relevant parameters are shown in Table 1.

Figure 5. Fitting chart of relationship between the reaction rate and temperature.

rAl 2O3 =

1 × 0.191 × c(SO2 )1.01 × c(Al 2O3)0.06 3 1.01

= 0.064 × c(SO2 )

0.06

× c(Al 2O3)

3. RESULTS AND DISCUSSION 3.1. Effect of Air Bubble on Mass Transfer in the Aluminum Base Desulfurization Process. In general, SO2 industrial emission range was 0−0.5% in coal-fired power plant flue gas and 1.5−8% in smelting flue gas. Meanwhile, the higher inlet SO2 concentration might make the difference of KGae

(7)

In this way, the intrinsic relationship between the physical and chemical parameters in the chemical mass transfer process was established by UDF which be embedded ANSYS continuity D

DOI: 10.1021/acs.energyfuels.7b01488 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Parameter Table of UDF parameter name aeration rate u SO2 concentration in flue gas CSO2 aluminum base absorption solution concentration CAl2O3 Ergun correction factor a Ergun correction factor b Ergun correction factor c Ergun correction factor d Ergun correction factor E1 Ergun correction factor E2 wetted perimeter Lp surface tension coefficient Sig acceleration of gravity g gas density ρG liquid density ρL gas-phase viscosity mu_G liquid-phase viscosity mu_L liquid-phase extension factor f_spread Henry coefficient H gas diffusion coefficient D atmospheric pressure P effective interface area per unit volume ae gas volume overall mass transfer coefficient KGae reaction rate constant k

unit

numerical value

m N/m m/s2 kg/m3 kg/m3 Pa·s Pa·s mm Pa·m3/mol m2/s kPa m2/m3 kmol/(s·m3·kPa) s−1

measurements calculated calculated 1.9 1.37 0.17 1.16 160 0.16 4.05 0.0728 9.81 1.225 998.2 1.72 × 10−5 1.1 × 10−3 7.4 73.21 2.8 × 10−7 100 0.005 calculated 0.191

m/s mol/L mol/L

source experimental experimental experimental selection selection selection selection selection selection selection selection selection selection selection selection selection selection selection selection selection selection eq 13 experimental

scheme scheme scheme

calculated value at 20 °C

that the bubble size minimally influenced the r of aluminum base desulfurization. The proportion of SO2 reaction consumption in big bubbles was small during the same reaction time, indicating a high concentration of SO2. Thus, the same amount of aeration volume can effectively improve the desulfurization efficiency by increasing the number of small bubbles and preventing coalescence in bubble rising. The mass transfer reaction of SO2 in the gas−liquid phase boundary film at the microlevel was further analyzed. The distribution cloud chart of aluminum base concentration for a single bubble in the liquid phase is shown in Figure 7.

more significant. In view of these considerations above, we selected the flue gas of inlet SO2 concentration 5% as the desulfurization object. Then, aluminum base desulfurization was simulated with the absorption solution concentration of 10% and aeration rate of 0.05 m/s. The effect of bubble size on SO2 concentration distribution in the height of 0.2−0.23 m interval in the absorber is shown in Figure 6. The release of bubbles was mainly affected by buoyancy, gravity, and orifice viscous force. The size and diffusion state of bubbles in the liquid-phase could be changed by modifying aeration speed. In addition, the coalescence of bubbles increased the resistance of bubbles rising, thereby decreasing the diffusion rate. As a result, the size and position distribution of bubbles in the liquid phase were not uniform. The large size and low surface tension of the bubble were good for gas−liquid mass transfer; on the basis of the SO2 concentration distribution in the bubbles, the SO2 concentration in the large bubble was high on the whole. This finding indicates

Figure 7. Aluminum base concentration distribution of a single bubble in the liquid phase.

The aluminum base concentration in the bubbles was lower, and the chemical reaction mainly occurred in the liquid-phase boundary film. This phenomenon was found to be related to the rapid reaction of aluminum base desulfurization obtained by analyzing the reaction kinetics. The aluminum base concentration in the absorption solution remained constant outside the liquid-phase boundary film. The aluminum base concentration distribution of the enlarged phase boundary film is shown in Figure 8; the high concentration of aluminum base also represented the low concentration of SO2.

Figure 6. Effect of the bubble size on SO2 concentration distribution in the height of 0.2−0.23 m interval in the absorber. E

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transfer resistance existed or the SO2 concentration reached the balance state in the gas−liquid phase. No concentration gradient was found in the gas−liquid phase body, besides the stagnant film. The liquid phase was not in direct contact with the gas phase, leading to homogeneous material composition and stable aluminum base concentration. In summary, the mass transfer resistance mainly occurred in the gas-phase boundary film. The chemical reaction rate affected mass transfer capability in bubble aluminum base desulfurization. 3.2. Effect of Aeration Rate and Absorption Solution Concentration on Mass Transfer. The aeration rate affected not only the bubble size but also the diffusion process and gas−liquid effective contact time. In general, at high aeration rates, the dispersion area of the bubbles increased during rising, thereby promoting the desulfurization reaction. However, at a limited absorber height, the residence time of SO2 in the liquid phase decreased. Thus, the desulfurization efficiency and KGae were affected by the combination of chemical reaction and physical mass transfer. F was defined as the chemical mass transfer complex factor, as shown in eq 14. Under the same inlet SO2 concentration, F increased with decreasing ySO2,out, and the stability of F could be used to determine the effect of different factors on the comprehensive desulfurization effect of the aluminum base. To further illustrate the effect of absorption solution concentration on the mass transfer at the same aeration rate, the relative increase rate of KGae (δ) was defined as shown in eq 15. Where KGae′ and KGae″ are the KGae at the absorption solution concentrations of 12.5% and 100%, respectively, (kmol/s·m3·kPa).

Figure 8. Aluminum base concentration distribution in the phase boundary film.

The concentration of aluminum base reduced stepwise in the phase boundary film. The aluminum base molecules were consumed constantly by SO2 in the absorption solution, thereby inducing SO2 to circulate from mass transfer to the reaction and mass transfer in the gas−liquid phase. The change in the aluminum base concentration in the gas− liquid phase boundary film was relatively gentle. According to two-film theory, the phase interface is a stable mass transfer reaction interface formed by contact between the reaction gas and absorption solution. Both sides of the interface exist as a layer of stagnant film, in which mass transfer resistance was determined by partial pressure or concentration of SO2. Through molecular diffusion, SO2 passes the stagnant film from the gas-phase main body to the liquid phase. In the entire mass transfer process, SO2 was absorbed by the absorption solution and formed a certain concentration gradient, and the concentration of the aluminum base decreased at the interface. When the reaction reached equilibrium stability, no interfacial mass

⎞ ⎛ 1 1 ⎟ F = ⎜⎜ − ⎟ 1 y 1 y − − ⎝ SO2 ,in SO2 ,out ⎠

(14)

Figure 9. Relationship of F and KGae with the aeration rate. T, 293 K; inlet SO2 concentration, 5%; and absorption solution concentration: (a) 12.5%, (b) 25%, (c) 50%, (d) 100%. F

DOI: 10.1021/acs.energyfuels.7b01488 Energy Fuels XXXX, XXX, XXX−XXX

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G

10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−5 10−6 10−6 10−6 10−5 × × × × × × × × × × × × × × × × 2.346 4.928 7.254 7.066 2.437 5.015 7.496 8.323 2.501 5.066 7.792 1.001 2.594 5.236 7.922 1.032 2.38 × 10−6 4.959 × 10−6 7.193 × 10−6 6.836 × 10−6 2.417 × 10−6 5.044 × 10−6 7.447 × 10−6 8.245 × 10−6 2.489 × 10−6 5.138 × 10−6 7.73 × 10−6 1.015 × 10−5 2.583 × 10−6 5.234 × 10−6 7.916 × 10−6 1.037 × 10−5 0.04638 0.04832 0.04673 0.03331 0.04711 0.04914 0.04837 0.04017 0.04850 0.05007 0.05022 0.04946 0.05035 0.05100 0.05142 0.05052 10.5 7.0 −6.6 −5.7 −6.9 7.8 −7.5 −3.0 −5.8 27.3a −16.6a 21.5a −8.8 −1.2 −3.3 11.8 0.686 0.459 0.548 1.788 0.512 0.375 0.392 1.194 0.387 0.326 0.201 0.384 0.208 0.161 0.117 0.236 0.621 0.429 0.587 1.896 0.550 0.348 0.424 1.231 0.411 0.256 0.241 0.316 0.228 0.163 0.121 0.211

Represents the larger difference rate. a

100

50

25

0.05 0.1 0.15 0.2 0.05 0.1 0.15 0.2 0.05 0.1 0.15 0.2 0.05 0.1 0.15 0.2

simulation (%) experiment (%)

ySO2,out

Table 2. Comparison between Experimental and Simulated Values

difference rate (%)

The experimental and simulation trends are consistent. The difference rate of outlet SO2 concentration (ySO2.out) was 10%. However, a large difference rate existed when the absorption solution concentration was 50%, which might be related to the comprehensive effect of chemical reaction and physical mass transfer. The difference rate of F and KGae was less than 1.4%, which is lower under the condition with 100% absorption solution concentration because of high reaction rate. When the absorption solution concentration was 12.5% and the aeration rate was 0.2 m/s, the difference rate reached the maximum value of 3.4%, which could be due to the effect of short contact reaction time on low-concentration absorption solution. Under the same aeration rate, KGae increased with increasing absorption solution concentration. When the aeration rate varied from 0.05 to 0.15 m/s, KGae increased linearly with increasing aeration rate. Meanwhile, the absorption solution concentration minimally affected mass transfer (δ: 4.7−10.5%) under the same aeration rate. The mass transfer capacity was mainly controlled by aeration rate, and the minimum value of KGae was only 2.346 × 10−6 kmol/(s·m3·kPa) when the absorption solution concentration was 12.5% and the aeration rate was 0.05 m/s. When the aeration rate varied from 0.15 to 0.2 m/s, KGae was affected by both aeration rate and absorption solution concentration. Moreover, the influence of absorption solution concentration on the mass transfer increased rapidly (maximum of δ up to 46%) under the same aeration rate. In this case, the mass transfer capacity was controlled by the combination of aeration rate and liquid-phase mass transfer conditions caused by chemical reaction. The maximum value of KGae reached 1.032 × 10−5 kmol/(s·m3·kPa) when the absorption solution concentration was 100% and the aeration rate was 0.2 m/s. Under the condition of high concentration of the absorption solution (50%, 100%), the increase in “I” reduced the mass transfer resistance of the liquid film. The improvement of the

0.04572 0.04802 0.04712 0.03443 0.04749 0.04887 0.04870 0.04055 0.04875 0.04936 0.05062 0.04878 0.05055 0.05102 0.05146 0.05027

−1.4 −0.6 0.8 3.4a 0.8 −0.6 0.7 0.9 0.5 −1.4 0.8 −1.4 0.4 0.0 0.1 −0.5

simulation [kmol/(s·m3·kPa)] experiment [kmol/(s·m3·kPa)] difference rate (%) experiment

Figure 10. Under different aeration rates, the effect of absorption solution concentration (from 12.5% increasing to 100%) on δ. T, 293 K; and inlet SO2 concentration, 5%.

12.5

F

KGae

When the inlet SO2 concentration was 5%, the effects of aeration rate and absorption solution concentration on mass transfer are shown in Figures 9 and 10, respectively. The comparison between experimental and simulated values is shown in Table 2.

simulation

(15)

aeration rate (m/s)

K Gae′

difference rate (%)

K Gae′′ − K Gae′

absorption solution concentration (%)

δ=

−1.4 −0.6 0.8 3.4a 0.8 −0.6 0.7 0.9 0.5 −1.4 0.8 −1.4 0.4 0.0 0.1 −0.5

Energy & Fuels

DOI: 10.1021/acs.energyfuels.7b01488 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels desulfurization efficiency maintained a high level of F (about 0.05). As such, aeration rate minimally influenced desulfurization efficiency, and the comprehensive effect of aluminum base desulfurization was mainly controlled by the liquid-phase mass transfer conditions caused by the chemical reaction. The average value of F was 0.04967 and 0.05103 under the condition of absorption solution concentrations of 50% and 100%, respectively. Under the condition of low concentration of absorption solution (12.5%, 25%), F showed different characteristics with an increasing aeration rate. When the aeration rate was less than 0.15 m/s, the F was kept around 0.048, with average values of 0.04744 and 0.04866 under the condition of absorption solution concentrations of 12.5% and 25%, respectively. As a result, the comprehensive effect of aluminum base desulfurization was mainly controlled by the liquid-phase mass transfer condition caused by the chemical reaction. When the aeration rate was more than 0.15 m/s, F decreased rapidly to 0.03443 with increasing aeration rate; this finding could be related to the decrease in gas−liquid contact time at high aeration rates. The average values of F were 0.04004 and 0.04507 under the condition of absorption solution concentrations of 12.5% and 25%, respectively. The comprehensive effect of aluminum base desulfurization was controlled by both physical mass transfer condition due to aeration rate and liquid-phase mass transfer condition caused by chemical reaction, which depended on the combined effect of the overall flow state and chemical reaction rate.31,32 These conclusions are consistent with those reported by Aroonwilas33 in their study using NaOH and AMP solution to absorb CO2 and with those presented by Qing34 in their study using ammonia−water to absorb CO2 in a packed column. 3.3. Effect of Inlet SO2 Concentration on Mass Transfer. Ignoring the effect of physical mass transfer, desulfurization experiments were carried out under the conditions of fixed aeration rate of 0.1 m/s, aluminum base concentration of 12.5% (25%, 50%, 100%), and inlet SO2 concentration of 0.5% (2%, 5%, 10%, 20%). With the mass transfer model (13), the relationship between KGae and inlet SO2 concentration under the different absorption solution concentrations is shown in Figure 11. The relationship between F and absorption solution concentration under different inlet SO2 concentrations is shown in Figure 12.

Figure 12. Relationship between F and absorption solution concentration under the different inlet SO2 concentrations. T, 293 K; and aeration rate, 0.1 m/s.

Under the same inlet SO2 concentration, KGae increased slightly with increasing the absorption solution concentration from 12.5% to 100%, with an average increase only of 5.92%. Under the same absorption solution concentration, KGae increased significantly with increasing inlet SO2 concentration, and the average increase reached up to 54 times. The KGae reached the maximum value of 2.538 × 10−5 kmol/(s·m3·kPa) when the inlet SO2 concentration was 20% and absorption solution concentration was 100%. Under the same inlet SO2 concentration, F increased slightly with increasing absorption solution concentration. Under the same absorption solution concentration, F increased significantly with increasing inlet SO2 concentration and was low (0.00418−0.051) in the range of low inlet SO2 concentration (0.5−5%). F reached the maximum value of 0.2473 when the inlet SO2 concentration was 20% and the absorption solution concentration was 100%. These results could be explained as follows. At a high partial pressure of SO2, the driving force of SO2 molecule diffusing into gas−liquid phase boundary film was also high. The larger the SO2 reaction order, the more favorable the chemical mass transfer will be. Overall, KGae and F increased with increasing reactant concentration, in which the inlet SO2 concentration had a great influence. The comprehensive effect of aluminum base desulfurization was mainly controlled by liquid-phase mass transfer condition caused by inlet SO2 concentration.

4. CONCLUSIONS The chemical mass transfer mechanism for aluminum base desulfurization was comprehensively examined by combining bubbling SO2 absorption experiments and numerical simulations under various conditions. The results showed that the proposed method is effective and feasible, and r and KGae are the key factors for the solution. The aeration rate affected bubble formation, diffusion morphology, and gas−liquid contact time. The bubble size minimally influenced the r of aluminum base desulfurization; however, increasing the number of small bubbles and preventing coalescence could improve the desulfurization efficiency under the same amount of aeration volume. The chemical reaction of bubbling aluminum base desulfurization mainly occurred in the liquid-phase boundary film. The mass transfer resistance was mainly concentrated in the gas-phase boundary film. Moreover, r affected the mass

Figure 11. Relationship between KGae and inlet SO2 concentration under the different absorption solution concentrations. T, 293 K; and aeration rate, 0.1 m/s. H

DOI: 10.1021/acs.energyfuels.7b01488 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

(5) Min, C.; Xian, H. D.; Fei, Q. H. Removal of SO2 from Flue Gas Using Basic Aluminum Sulfate Solution with the Byproduct Oxidation Inhibition by Ethylene Glycol. Energy Fuels 2016, 30, 1183−1191. (6) Shi, J. S. Study on gas-liquid absorption and desorption process of refuse leachate-flue gas desulfurization system. Degree Thesis of Sichuan University of China, 2004. (7) Seader, J. D. Rate-based approach for modeling staged separations. Chemical Engineering Progress. 1989, 85 (10), 41−49. (8) Sada, E. H. K.; Nishimu, H. Absorption of SO2 into aqueous double slurries containing limestone and Mg(OH)2. Chem. Eng. Sci. 1983, 29 (5), 60−63. (9) Dagaonkar, M. V.; Beenackers, A. A. C. M.; Pangarkar, V. G. Enhancement of gas-liquid mass transfer by small reactive particles at realistically high mass transfer coefficients: absorption of sulfur dioxide into aqueous slurries of Ca(OH)2 and Mg(OH)2 particles. Chem. Eng. J. 2001, 81 (6), 203−212. (10) Rochelle, G. T.; King, C. J. The effect of additives on mass transfer in CaCO3 or CaO slurry scrubbing of SO2 from waste gases. Ind. Eng. Chem. Fundam. 1977, 16 (1), 67−75. (11) Hikita, H.; Konishi, Y. The absorption of lean SO2 in aqueous solutions of Na2CO3 accompanied by the desorption of CO2. Chem. Eng. J. 1983, 27 (3), 167−176. (12) Chang, C. S.; Dempsey, J. H.; Borgwardt, R. H.; Toprac, A. J.; et al. Effect of limestone type and grid on SO2 scrubber performance. Environ. Prog. 1982, 1 (1), 59−65. (13) Brogren, C.; Karlsson, H. T. Modeling the absorption of SO2 in a spray scrubber using the penetration theory. Chem. Eng. Sci. 1997, 52 (18), 3085−3099. (14) Juan, W.; Qin, Z. Model for wet flue gas desulfurization in jet bubbling reactor. Journal of Nanjing University of Science and Technology 2000, 24 (4), 364−368. (15) Gang, Y.; Yong, L. M.; Li, D. W. Studies on the Reaction characteristics of magnesium oxide flue gas desulfurization. Chinese Journal of Environmental Engineering 2010, 4 (5), 1134−1138. (16) You, F. L.; Hong, Q. Y.; Hu, Z.; Xian, D. H. Studies on gasliquid mass transfer in impinging stream micro-reactors. Chemical Engineering 2012, 40 (3), 48−52. (17) Yang, X. L.; Jun, Z.; Zhu, L. W. Simultaneous desulfurization and denitrification by using combination of UV/H2O2 oxidation with Ca(OH)2 absorption. CIESC Journal 2012, 63 (10), 3277−3283. (18) Qing, F. Z.; Xiao, P. C. Desulfurization properties of seawater by jet bubbling reactor. CIESC Journal 2016, 67 (4), 1572−1579. (19) Hong, L. X. Inorganic and Analytical Chemistry Experiments; East China University of Science and Technology Press: Shanghai, China, 2009. (20) Hong, Z. P.; Xiao, L. Z.; Hui, L. Y. Analysis of water used in boiler and cooling system - Determination of sulfite (GB/T14426-93); China Standard Press: Beijing, China, 1993. (21) Ekambara, K.; Nandakumar, K.; Joshi, J. B. CFD Simulation of bubble column react or using population balance. Ind. Eng. Chem. Res. 2008, 47 (21), 8505−8516. (22) Hulme, I.; Clavelle, E.; van der Lee, L.; et al. CFD Modeling and Validation of Bubble properties for a Bubbling Fluidized Bed. Ind. Eng. Chem. Res. 2005, 44 (12), 4254−4266. (23) Sokolichin, A.; Eigenberger, G.; Lapin, A.; et al. Dynamic numerical simulation of gas-liquid two-Phase flows Euler/Euler versus Euler/Lagrange. Chem. Eng. Sci. 1997, 52 (4), 611−626. (24) Bird, R. B.; Steuart, W. E.; Lightfoot, E. N. (Book). Gan, C. D.; Shun, X. R.; Yan, F. S. (Translation). Transport phenomena; Chemical Industry Press: Beijing, China, 2004. (25) Danckwerts, P. V. Gas-liquid Reaction; McGraw-Hill, Inc.: New York, 1970. (26) Cheng, F. Z. Gas-Liquid Reaction and Reactor; Chemical Industry Press: Beijing, China, 1985. (27) Yang, X. L.; Jian, F. P.; Yong, L. Mass transfer-reaction kinetics for NO-removal by combination process of UV/H2O2 oxidation and CaO absorption. CIESC Journal 2013, 64 (3), 1062−1068.

transfer ability. In the low aeration rate range (0.05−0.15 m/s), absorption solution concentration minimally influenced mass transfer, and the mass transfer capacity was mainly controlled by aeration rate. In the high aeration rate range (0.15−0.2 m/s), the influence of the absorption solution concentration on mass transfer increased rapidly, and the mass transfer capacity was controlled by the aeration rate and liquid-phase mass transfer conditions caused by the chemical reactions. At high concentrations (50%, 100%) of the absorption solution, the stability of F was maintained at a high level (about 0.05), and the comprehensive effect of aluminum base desulfurization was mainly controlled by liquid-phase mass transfer conditions caused by the chemical reactions. At low concentrations (12.5%, 25%) of the absorption solution, F showed different characteristics with increasing aeration rate. When the aeration rate was less than 0.15 m/s, the comprehensive effect of aluminum base desulfurization was mainly controlled by liquid-phase mass transfer conditions caused by chemical reactions. When the aeration rate was more than 0.15 m/s, F rapidly decreased with increasing aeration rate, and the comprehensive effect of aluminum base desulfurization was controlled by both the physical mass transfer conditions due to aeration rate and liquid-phase mass transfer conditions caused by chemical reactions. Under the same aeration rate, KGae and F increased with increasing reactant concentration. The inlet SO2 concentration had a great influence on mass transfer capacity; thus, the comprehensive effect of aluminum base desulfurization was mainly controlled by liquid-phase mass transfer condition caused by the inlet SO2 concentration.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/fax: +86-0471-6576714. E-mail: nmgdlxyzzj@sina. com. ORCID

Zijing Zhang: 0000-0001-5692-9052 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (51566013 and 51468048), the Natural Science Foundation of Inner Mongolia (2014MS0513), the Science Research Project in Colleges and Universities of The Inner Mongolia Autonomous Region (NJZY14063), and the Inner Mongolia Doctoral Student Scientific Research and Innovation Project (B20141012802Z).

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REFERENCES

(1) Gao, W.; Xiao, L. Y. Study on Alkalinity Parameters of aluminum base wet flue gas desulfurization. Journal of Inner Mongolia University of Technology (Natural Science) 2011, 30 (3), 356−358. (2) Yi, G.; Zi, J. Z.; Gao, W.; et al. Experimental study on cyclic absorption properties of aluminum base regeneration. Thermal Power Generation 2014, 43 (2), 47−50. (3) Zi, J. Z.; Jian, W. W.; Yi, G.; et al. Flow field simulation of smoke and gas evaporation characteristics of desulfurization wastewater in coal-fired power plant. Journal of China Coal Society 2015, 40 (3), 678−683. (4) Qiao, Y. W.; Xian, H. D. Adsorption of sulfur dioxide by basic aluminum sulfate solution. Chinese Journal of Environmental Engineering 2013, 7 (12), 4940−4444. I

DOI: 10.1021/acs.energyfuels.7b01488 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels (28) Chen, J. Kinetic Analysis of Sulfitolysis and Thiosulfatolysis for Polythionates. Degree thesis of China University of Mining & Technology, 2014. (29) Aroonwilas, A.; Tontiwachwuthikul, P. High-efficiency structured packing for CO2 separation using 2-amino-2-methyl-1-propanol (AMP). Sep. Purif. Technol. 1997, 12 (1), 67−79. (30) Lin, F.; Hai, F. W. Mass transfer and reaction kinetics of SO2 absorption with cystine solution. Chemical Intermediates 2013, 2, 27− 30. (31) Pei, C. L.; Zhen, J.; Zhi, B. Z. Volumetric mass transfer coefficients of Potassium carbonate/Piperazine mixed absorbent for removal of CO2 in packed column. CIESC Journal 2005, 56 (1), 53− 57. (32) Kuntz, J.; Aroonwilas, A. Mass transfer efficiency of a spray column for CO2 capture by MEA. Energy Procedia 2009, 1 (1), 205− 209. (33) Aroonwilas, A.; Tontiwachwuthikul, P. Mass Transfer Coefficients and Correlation for CO2 Absorption into 2-Amino-2methyl-1-propanol(AMP) Using Structured Packing. Ind. Eng. Chem. Res. 1998, 37 (2), 569−575. (34) Qing, Z.; Yin, C. G.; Zhen, Q. N.; et al. Mass transfer performance of CO2 absorption into aqueous ammonia in a packed column. CIESC Journal 2011, 62 (S1), 146−150.

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DOI: 10.1021/acs.energyfuels.7b01488 Energy Fuels XXXX, XXX, XXX−XXX