Article pubs.acs.org/IECR
Mass Transfer Performance for Low SO2 Absorption into Aqueous N,N′‑Bis(2-hydroxypropyl)piperazine Solution in a θ‑Ring Packed Column Fengyu Wei,*,† Yuan He,†,‡ Pan Xue,† Yunjin Yao,† Chengwu Shi,† and Peng Cui† †
School of Chemical Engineering, Hefei University of Technology, Hefei 230009, People’s Republic of China Hefei Huaqing Metal Surface Treatment Company, Ltd., Hefei 230088, People’s Republic of China
‡
ABSTRACT: The mass transfer performance of SO2 absorption in an aqueous solution of a newly proposed organic amine absorbent, N,N′-bis(2-hydroxypropyl)piperazine (HPP), was studied by the dynamic absorption method in a θ-ring packed column. The absorption performance was interpreted in terms of the overall mass transfer coefficient, KGa. The experimental results showed that the KGa value increased with an increase in the liquid flow rate, concentration, and initial pH of HPP absorbent. However, it decreased with an increase in temperature, SO2 partial pressure, and gas flow rate. An empirical correlation for the overall mass transfer coefficient is subsequently developed as a function of the process parameters. The obtained KGa from empirical correlation is found to be in relatively good agreement with the experimental results with a mean relative deviation of 6.6%, which could be used for engineering design.
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INTRODUCTION A global issue that is getting more and more attention is the air pollution caused by SO2 from oil- and coal-burning fuel gas.1 Many techniques for removing SO2 have been put forward. Some conventional procedures, such as ammonia scrubbing and limestone scrubbing, have some intrinsic shortcomings, involving a larger water requirement, high capital and operating costs, secondary pollution, and poor quality of byproduct.2,3 The interest in flue gas desulfurization (FGD) with organic amine aqueous solution stems from the fact that it has high desulfurization efficiency, high utilization of SO2 recovery, excellent selective for SO2, and no recontamination.4−7 Recently, the commonly used organic amines are ethylenediamine7 and alkanolamines.8 Ethylenediamine has higher efficiency for desulfurization, but it has relatively high vapor pressure, which can bring about significant solvent loss and vaporization.9 Alkanolamines, such as monoethanolamine (MEA), have good chemical activity, but there are some disadvantages, such as thermal and oxidation degradation, both of which can generate toxic byproducts,9−11 which also result in low SO2 absorption capacity and low desulfurization efficiency.8 Ravary et al.12 have developed a diamine piperazine, which is the most attractive absorbent. It can overcome the drawbacks of ethylenediamine and alkanolamines, such as high vapor pressure and low desulfurization efficiency. Thus, a new diamine piperazine desulfurization agent called N,N′-bis(2hydroxypropyl)piperazine (HPP) has been proposed by our group.13 The molecular structure of HPP is presented in Figure 1. Knowledge about the mass transfer performance of SO2 in HPP−sulfate aqueous solution is very important for its potential application in SO2 removal. In previous works, we have studied the kinetics of SO2 with HPP−sulfate aqueous solution and found HPP has excellent desulfurization efficiency, high regeneration ability, high chemical stability, and low cost as compared with other reported amine absorbents.14 © 2014 American Chemical Society
Figure 1. Molecular structure of HPP.
Because the mass transfer coefficient directly affects the chemical reaction and the design of the packed column, it is one of the most important parameters for gas−liquid countercurrent flow in a packed column.15 The differential method is considered to be a convenient and effective way for determining the mass transfer coefficient in a packed column. Many researchers make use of this way to determine the volumetric overall mass transfer coefficient (KGa). For example, Aroonwilas et al.16,17 studied the KGa value of CO2 absorption into NaOH, AMP, and MEA aqueous solutions in a packed column. The KGa value of CO2 with 4-diethylamino-2-butanol solvent was researched by Sema et al.18 The KGa value of CO2 with DEAB/MEA aqueous was investigated by Maneeintr et al.19 In our laboratory, we have studied the KGa value of SO2 with one kind of piperazine diamine.20 To our knowledge, although there is a lot of work published on the use of amines to absorb SO2, there have been few studies concerning the mass transfer performance for SO2 in organic amine absorbent, which is a critical parameter for designing the absorption column. The aim of this study is to investigate the mass transfer performance of a low concentration of SO2 into HPP aqueous solution in a θ-ring packed tower. The effects of different operating variables, such as initial pH and concentration of absorbent, absorption temperature, Received: Revised: Accepted: Published: 4462
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ratio of gas to liquid, and SO2 partial pressure in the inlet gas, on KGa are also investigated. Moreover, the present work is also an attempt to propose a new empirical KGa correlation that is suitable for a SO2−HPP system.
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DETERMINATION OF THE VOLUMETRIC OVERALL MASS TRANSFER COEFFICIENT (KGa) On the basis of the two-film theory, the mass flux of component A at a steady state can be expressed as21,22 NA = K GP(yA − yA∗ )
(1)
where NA is the mass flux of A, KG is the overall mass transfer coefficient for the gas phase, P is the system pressure, yA represents the mole fraction of component A in the gas bulk, and y∗A is the equilibrium mole fraction of component A. Considering an element of packed column with height dz, the material balance equation can be given as ⎛ y ⎞ NAadz = Gd⎜⎜ A ⎟⎟ ⎝ 1 − yA ⎠
Figure 2. Schematic diagram of gas absorption apparatus: 1, SO2 cylinder; 2, N2 cylinder; 3, valve; 4, gas flow meter; 5, buffer tank; 6, mouth of gas phase sampling; 7, liquid storage bottle; 8, constant temperature water bath; 9, peristaltic pump; 10, liquid flow meter; 11, pack tower; 12, tail gas absorption bottle.
(2)
where a and G are the effective interfacial area per unit volume of packing and the inert gas molar flow rate of nitrogen, respectively. According to eqs 1 and 2 ⎛ y ⎞ K GaP(yA − yA*)dz = Gd⎜⎜ A ⎟⎟ ⎝ 1 − yA ⎠
Experimental Procedure. The desired concentration of absorption solution was prepared by mixing HPP with deionized water. Its pH was regulated to a desired value by sulfuric acid and measured with a pH meter (model pH-3C, INESA Scientific Instrument Co., Ltd.). When the pH of the absorbent exceeds 8.2, the regeneration rate of the absorbent is slow, and the absorption rate is not high when the pH is below 3.8.14 Therefore, the initial pH was in the range of 4.0−7.9. At the start of each experiment, the N2 and SO2 were set to the desired rates through the flow meters, mixed, and introduced into the bottom of the column and flowed upward. To verify the concentration of SO2 in the gas stream, the SO2 flue gas analyzer was used. Simultaneously, the prepared solution was pumped to the top of the column from the liquid storage bottle at the desired flow rate. This brought both liquid and gas phases into contact countercurrently, and SO2 was transferred from the gas phase into the liquid phase. The treated gas left from the top of the packed column, and the rich solution with SO2 from bottom of the column was collected in a liquid storage bottle. To get credible experimental results, the experiment was maintaned at steady-state conditions. After 10−15 min, the SO2 concentration along the column was measured and recorded. All experiments were implemented under atmospheric conditions, and the details of the operational parameters are summarized in Table 1. To ensure the accuracy of the experiment, each experiment was carried out three times.
(3)
When the partial pressure of SO2 in the gas phase is lower, yA/(1 − yA) can be replaced by yA. For an instantaneous chemical reaction, y∗A is very low and can usually be negligible.16 Thus, from eq 3, KGa can be defined as
K Ga =
⎛ G ⎞ d ln yA ⎜ ⎟ ⎝ P ⎠ dz
(4)
The measured SO2 concentration (yA) can be converted into ln yA, then, the gradient d ln yA/dz can be obtained by plotting ln yA against the height of the column (z).
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EXPERIMENTAL SECTION Chemicals. HPP was synthesized in our laboratory with a purity of ≥99%. Nitrogen and SO2 (with a purity of ≥99%) were supplied by Hefei Zhongyi Chemical Products Co. Ltd., China. Sulfuric acid (analytical reagent) was purchased from Traditional Chinese Medicine Group Chemical Reagent Co. Ltd. Experimental Apparatus. Figure 2 shows the schematic diagram of the experimental setup. The packed column was made of glass. Its height and inside diameter were 650 and 40 mm, respectively. The column was filled with θ-ring (316 stainless steel) with height and outer diameter of 4 mm. The total height of the packing section was approximately 400 mm. To determine d ln yA/dz, the gas concentration along the column must be sampled. A SO2 flue gas analyzer, model Testo 300 (Beijing Orient Hengyun Science and Technology Development Co. Ltd., China), was used to measure the SO2 concentration in the gas stream. The gas and liquid flow meters were used to control the gas and liquid flow rates, respectively. The temperature of the liquid inlet was controlled by the constant-temperature water bath. To pump the solution to the liquid inlet of the column, a peristaltic pump (TP10-20) was used.
Table 1. Experimental Conditions of the Packed Column
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parameter
conditions
pressure SO2 partial pressure (kPa) gas flow rate(kmol·m−2·h−1) absorbent concentration (mol·L−1) liquid flow rate (m3·h−1·m−2) temperature (K) initial pH of absorbent
atmospheric 0.38−0.47 9.30−12.08 0.005−0.025 4.41−10.59 298.15−338.15 4.0−7.9
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Figure 3. Relationship of ln yA with z: (a) G = 9.33 kmol·m−2·h−1; (b) G = 10.76 kmol·m−2·h−1; (c) G = 11.48 kmol·m−2·h−1; (d) G = 12.2 kmol· m−2·h−1) (T = 328.15 K, pA = 0.4661 kPa, initial pH = 4.40, CN = 0.01 mol·L−1, L = 5.293 m3·h−1·m−2).
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RESULTS AND DISCUSSION
To verify the feasibility of eq 4, the correlation of ln yA with z is determined from the results of several runs at different gas flow rates. As shown in Figure 3, the correlation between ln yA and z meets the linear relationship. This means that it is feasible to determine the overall mass transfer coefficient with eq 4. Effect of Gas Flow Rate on KGa. The effect of changing the gas flow rate on KGa under different liquid flow rates is presented in Figure 4. It shows that an increase in the gas flow rate leads to a lower KGa. The effect of gas flow rate on KGa is very complicated for a chemical absorption under a different absorption system. Many researchers16,19,23 have put forward that the gas flow rate has no significant effect on KGa for CO2 absorption with aqueous solutions of AMP, DEAB-MEA, and DETA. Our results corresponded well with those of Luo et al.,24 who reported that KGa decreased with the gas flow rate for CO2 absorption into mixtures of potassium carbonate and piperazine. According to the two-film theory, increasing gas flow rate should consequently increase KGa. However, Strigle et al.25 brought forward that controlling the absorption process is not the gas-side mass transfer but the liquid-side mass transfer, and the residence time of the gas−liquid contacting the packed column decreases with the increase of the gas flow rate, which is not conducive to the reaction of SO2 with HPP−sulfuric acid
Figure 4. Effect of gas flow rate on KGa (T = 328.15 K, pA = 0.4661 kPa, initial pH = 4.40, CN = 0.01 mol·L−1).
aqueous solution when the SO2 partial pressure of inlet gas is stable. This is unfavorable to the mass transfer. Effect of Liquid Flow Rate on KGa. The liquid flow rate has a significant effect on KGa in SO2−HPP absorption. It is 4464
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clear that KGa increases with the liquid flow rate as shown in Figure 5. This is ascribed to the increase of the liquid-side mass
with the temperature. Therefore, a high liquid feed temperature would discourage SO2 absorption into aqueous HPP solution. Dey et al.26 found that KGa decreased with the temperature when the CO2 loadings are above 0.35 mol/mol, but Fu et al.23 found that the KGa values of CO2 into both MEA and DETA increased when the liquid feed temperaturewas in the range of 303−323 K. Effect of SO2 Partial Pressure pA on KGa. The effect of SO2 partial pressure pA on KGa in the packed column is presented in Figure 7 at different temperatures. It is shown that
Figure 5. Effect of liquid flow rate on KGa (T = 328.15 K, pA = 0.4661 kPa, initial pH = 4.40, CN = 0.01 mol·L−1).
transfer coefficient (kL) with the liquid flow rate. In addition, the wetting degree of the packing, the droplet velocity, and the update speed of the liquid film rise with the liquid flow rate. Furthermore, the gas−liquid contact area increases with the liquid flow rate. The results were confirmed by many papers.16,20,23 Effect of Temperature on KGa. The absorption performance is obviously affected by the temperature. The relationship between KGa and temperature is shown in Figure 6. It is found
Figure 7. Effect of SO2 partial pressure on KGa (CN = 0.01 mol·L−1, initial pH = 4.36, G = 10.73 kmol·m−2·h−1, L = 5.293 m3·m−2·h−1).
KGa decreases with increasing partial pressure of SO2. According to the chemical reaction rate and two-film theory, the reaction rate of SO2 with HPP increases with SO2 partial pressure and the increased SO2 partial pressure allows more SO2 molecules to travel from gas bulk to the gas−liquid interface, which is beneficial for reducing the mass transfer resistance of the gas phase. However, the concentration of SO2 in the liquid phase increases with the partial pressure of SO2, which directly results in the increase of the mass transfer resistance of the liquid phase. KGa values denote mass transfer rate per unit driving force, and the increase of the driving force with the SO2 partial pressure results in a decrease of KGa values.26 These indicate that the addition of SO2 partial pressure is not helpful to KGa. This is similar to many experimental results reported in the literature.21,26 Effect of Absorbent Concentration on KGa. The effect of the absorbent concentration on KGa in the packed column is an important issue because it has an effect on the design.19 It is evident that the KGa values increase with the absorbent concentration from Figure 8. The free active amine HPP molecules per unit volume and the chemical reaction enhancement factor (E) increase with the solution concentration, which is conducive to fast absorption of SO2 into the amine solution. Moreover, an increase in the chemical reaction enhancement factor E results in a lower mass transfer resistance in the liquid phase, which is favorable to KGa. Besides, the rate of SO2 from the gas−liquid interface toward the liquid phase increases with the absorbent concentration, which results in the increase of absorption driving force of gas phase. These results are in accordance with the conclusions reported by Aroonwilas et al.,16 Kuntz et al.,17 and Maneeintr et al.19
Figure 6. Effect of absorption temperature on KGa (CN = 0.01 mol· L−1, initial pH = 4.48, pA = 0.456 kPa, G = 10.73 kmol·m−2·h−1).
that the KGa value decreases with the absorption temperature. Such behavior is attributable to the reaction rate constant and the diffusion coefficient increasing with the temperature according to the principle of molecular dynamics, which is beneficial to improve KGa. However, the reaction of SO2 with HPP−sulfuric acid aqueous solution is exothermic reaction, which gives rise to KGa decreasing with the absorption temperature. On the other hand, SO2 solubility decreases 4465
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absorption operation. Some shortcut correlations to estimate KGa for the different systems and the packings have been proposed by many researchers. Kohl and Riesenfeld27 developed a correlation to predict KGa of an MEA−CO2 system in random packing as ⎛ L ⎞2/3 K Ga = F ⎜⎜ ⎟⎟ [5.7(αeq − α)C exp(0.0067T − 3.4PCO2) ⎝ μL ⎠ + 1]
(5)
where F represents the packing correction factor; L represents the liquid mass flow rate; μL represents the liquid viscosity; PCO2 represents the CO2 partial pressure over the solution; αeq is the CO2 loading of solution in equilibrium with PCO2; α represents the CO2 loading of solution; C is the concentration of the amine solution; and T is the absorption temperature. It should be pointed out that this correlation might not be applicable with other amine systems23,28 and structured packing,29 especially the SO2 absorption with amine. KGa is affected by the packing, the gas−liquid flow, and the absorbent.30 By combining our previous work20 and eq 5, an empirical correlation of KGa for SO2 absorption into the HPP− sulfuric acid aqueous solution was developed on the basis of the experimental data. It is expressed as
Figure 8. Effect of absorbent concentration on KGa (initial pH = 4.0, pA = 0.456 kPa, G = 10.73 kmol·m−2·h−1, L = 5.293 m3·m−2·h−1).
Effect of Absorbent Initial pH on KGa. The initial pH of the absorbent solution is vital in a gas separation process, which refers to the sum of free active amine HPP molecules, and subsequently affects the sum of SO2 dissolved in solution by physical and chemical absorption. Figure 9 presents the
⎛ G ⎞α1 K Ga = K1⎜ ⎟ [Q (pH)α2 C Nα3 e α4PA + α5/ T + 1] ⎝L⎠
(6)
where G/L is the ratio of gas to liquid; α1, α2, α3, α4, and α5 are the coefficients for the respective parameters in eq 6; and K1 and Q are the empirical constants. Furthermore, eq 6 can be rewritten as follows: K Ga G α1 L
( )
= K1Q (pH)α2 C Nα3 e α4PA + α5/ T + K1 (7)
The coefficients in eq 7 can be obtained by linear regression based on the single-factor experiment, giving α1 = −0.7809, α2 = 0.8604, α3 = 0.3910, α4 = −6.079, and α5 = 1453. and the confidence level is 98%. Thus, eq 7 can be expressed as follows: K Ga G −0.7809 L
Figure 9. Effect of absorbent initial pH on KGa (G = 10.73 kmol·m−2· h−1, pA = 0.456 kPa, T = 328.15 K, CN = 0.01 mol·L−1).
( )
= K1Q (pH)0.8604 C N0.3910 e−6.079PA + 1453/ T + K1 (8)
relationship between KGa and the absorbent initial pH at different liquid flow rates. It is shown that an increase in the absorbent initial pH results in the increase of KGa. The possible reason for this behavior is that a higher absorbent initial pH gives rise to the following: (1) The effective amine involved in the reaction increases with the absorbent initial pH, and this is helpful to improve the driving force of liquid phase. (2) The driving force of the gas phase is increased, which results from the increase in the reaction rate of SO2 with HPP−sulfuric acid aqueous solution. Aside from the effects of pH on mass transfer, it has a great influence on the desorption and recycle use of absorbent. To optimize the pH, multiple factors should be taken into account. Empirical Correlation of KGa. An accurate correlation for the calculation of KGa is very useful for the design of the packed column and predicting the effects of different parameters on the
To confirm the relationship between the term KGa/(G/ L)−0.7809 and (pH)0.8604C0.3910 e−6.079PA + (1453/T), twice linear N regression is used to find K1Q and K1, which are 0.9966 and −0.27, respectively, at a confidence level of 95%. Therefore, eq 8 can be replaced by the following final correlation: ⎛ G ⎞−0.7809 K Ga = 0.27⎜ ⎟ [3.691(pH)0.8604 C N0.3910 ⎝L⎠ e−6.079PA + 1453/ T − 1]
(9)
In eq 9, 0.27, −0.7809, and 3.691 represent the packing characteristic, the gas−liquid flow characteristic, and the absorbent characteristic, respectively. The mean relative deviation (MRE) between the experimental data (KGa, exptl) and predicted value (KGa, calcd) is calculated by the equation31 4466
dx.doi.org/10.1021/ie404000w | Ind. Eng. Chem. Res. 2014, 53, 4462−4468
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1 N
N
⎛ K Ga , exptl −K Ga , calcd ⎞ ⎟ K Ga , exptl ⎝ ⎠
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Wuhu Bengbu Joint Innovation Project (No. 2013AKKG0388) is gratefully acknowledged.
∑ ABS⎜ 1
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(10)
where N represents the runs of experiment. From Figure 10, it is found that the calculated KGa values from eq 9 are in
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REFERENCES
(1) Xu, Y. Improvements in the operation of SO2 scrubbers in China’s coal power plants. Environ. Sci. Technol. 2011, 45, 380−385. (2) Wang, C.; Cui, G.; Luo, X.; Xu, Y.; Li, H.; Dai, S. Highly efficient and reversible SO2 capture by tunable azole-based ionic liquids through multiple-site chemical absorption. J. Am. Chem. Soc. 2011, 133, 11916−11919. (3) Sun, Z.; Zhao, Y.; Gao, H.; Hu, G. Removal of SO2 from flue gas by sodium humate solution. Energy Fuels 2010, 24, 1013−1019. (4) Maneeintr, K.; Idem, R. O.; Tontiwachwuthikul, P.; Wee, A. G. H. Synthesis, solubilities, and cyclic capacities of amino alcohols for CO2 capture from flue gas streams. Energy Procedia 2009, 1, 1327− 1334. (5) Zhang, J.; Zhang, P.; Han, F.; Chen, G.; Zhang, L.; Wei, X. Hydrogen bonding and interaction in the absorption processes of sulfur dioxide in ethylene glycol + water binary desulfurization system. Ind. Eng. Chem. Res. 2009, 48, 1287−1291. (6) He, Z.; Liu, J.; Li, L.; Lan, D.; Zhang, J. Absorption properties and spectroscopic studies of dilute sulfur dioxide in aqueous glycerol solutions. Ind. Eng. Chem. Res. 2012, 51, 13882−13890. (7) Tang, Z. G.; Zhou, C. C.; Chen, C. Studies on flue gas desulfurization by chemical absorption using an ethylenediamine− phosphoric acid solution. Ind. Eng. Chem. Res. 2004, 43, 6714−6722. (8) Choi, W. J.; Min, B. M.; Shon, B. H.; Seo, J. B.; Oh, K. J. Characteristics of absorption/regeneration of CO2−SO2 binary systems into aqueous AMP + ammonia solutions. J. Ind. Eng. Chem. 2009, 15, 635−640. (9) Long, X. L.; Xin, Z. L.; Chen, M. B.; Li, W.; Xiao, W. D.; Yuan, W. K. Kinetics for the simultaneous removal of NO and SO2 with cobalt ethylenediamine solution. Sep. Purif. Technol. 2008, 58, 328− 334. (10) Vevelstad, S. J.; Eide-Haugmo, I.; da Silva, E. F.; Svendsen, H. F. Degradation of MEA; a theoretical study. Energy Procedia 2011, 4, 1608−1615. (11) Goff, G. S.; Rochelle, G. T. Monoethanolamine degradation: O2 mass transfer effects under CO2 capture conditions. Ind. Eng. Chem. Res. 2004, 43, 6400−6408. (12) Ravary, P. M.; Sarlis, J. N.; Parisi, P. J.; Hakka, L. E. Low energy regenerable SO2 scrubbing process. U.S. Patent 7,214,358, May 8, 2007. (13) Shi, C.; Liu, Q.; Cui, P.; Wei, F. The regeneration of wet flue gas desulfurization and preparation methods. Chinese Patent 200910117195.7, July 20, 2011.
Figure 10. Comparison between KGa calculated from proposed correlation and experimental data.
relatively good agreement with the experimental data with the mean relative deviation of 6.6% calculated by eq 10. However, it is not known whether this new correlation developed for the SO2−HPP system is applicable to other SO2 absorption systems. Therefore, the application of this correlation for other SO2−amine systems should be further explored.
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CONCLUSIONS In the present work, the mass transfer performances of SO2 absorption into an aqueous solution of HPP were comprehensively studied in a laboratory-scale absorption column packed with a θ-ring. The absorption performance was evaluated in terms of the overall mass transfer coefficient KGa. It was found that the mass transfer performance of SO2 absorption into HPP−sulfuric acid aqueous solution is effected by many process parameters. KGa increases with liquid flow rate, initial pH, and concentration of absorbent. However, it decreases with inert gas flow rate, absorption temperature, and SO2 partial pressure. A mass transfer correlation for KGa that involved the effect of the main operating variables is successfully proposed for the HPP−SO2 absorption system. In addition, the KGa value calculated by the empirical correlation is found to be in good agreement with the experimental data, and the mean relative deviation is 6.6%.
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NOMENCLATURE α = specific area of contactor (m2·m−3) CN = concentration of the absorbent (mol·L−1) E = enhancement factor (dimensionless) F = packing correction factor G = inert molar flow rate of nitrogen (kmol·m−2·h−1) L = liquid flow rate (m3·m−2·h−1) KGa = volumetric overall mass transfer coefficient (kmol· m−3·h−1·kPa−1) NA = mass flux of A into reactant B (kmol·m−2·h−1) pA = SO2 partial pressure (kPa) P = total system pressure (kPa) T = absorption temperature (K) yA = mole fraction of component A in the gas bulk (mol· mol−1) y∗A = mole fraction of component A in equilibrium with the bulk liquid (mol·mol−1) z = height of the column (m)
AUTHOR INFORMATION
Corresponding Author
*(F.W.) Phone: +86 551 62901548. Fax: +86 551 62901450. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the Ministry of Anhui Provincial Science and Technology (No. 08010202124) and Hefei 4467
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