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Separation and Purification Technology 153 (2015) 162–169

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SO2 removal from simulated flue gas using various aqueous solutions: Absorption equilibria and operational data in a packed column Farhad Rahmani a,b, Dariush Mowla a,b,⇑, Ghloamreza Karimi a, Ali Golkhar a,b, Behnaz Rahmatmand a,b a b

School of Chemical and Petroleum Engineering, Shiraz University, P.O. Box 7194685315, Shiraz, Iran Environmental Research Center in Petroleum and Petrochemical Industries, Shiraz University, P.O. Box 7194685315, Shiraz, Iran

a r t i c l e

i n f o

Article history: Received 5 February 2014 Received in revised form 23 October 2014 Accepted 30 October 2014 Available online 11 November 2014 Keywords: SO2 removal Absorption SO2-aqueous solution equilibrium data Packed column

a b s t r a c t In this study, SO2 absorption performance of the novel amino acid salt aqueous solutions such as sodium lysinate and sodium glycinate from simulated flue gas were studied and compared with frequently utilized absorbents including ethylenediamine, 2-amino2-methyl1-propanol (AMP) and disodium hydrogen phosphate. SO2-aqueous solution equilibrium data of these absorbents at concentration of 0.1 M in a stirred batch reactor exhibited that amino acid salt aqueous solutions and especially sodium lysinate had higher SO2 solubility at 298 K. In order to ensure the SO2 removal efficiency at practical condition, the suggested absorbents were applied in a packed column. The experimental results imply better removal efficiency of amino acid salt solutions consistent with absorption equilibrium data. The effects of parameters such as temperature, SO2 inlet concentration and liquid–gas flow ratio on removal efficiency were investigated for 1 M concentration of sodium lysinate solution. SO2 removal efficiency enhanced with increasing gas–liquid flow ratio, but both temperature and SO2 inlet concentration had opposite effects. Ó 2015 Published by Elsevier B.V.

1. Introduction Undoubtedly, inordinate increase of SO2 emission has become the focus of the world, and especially industrial countries [1]. The majority of industrial SO2 emissions, which come from the burning of fossil fuels, are mainly led to air pollution, acid rain and urban smog [2]. It is well known that SO2 is not only harmful to human health and the ecosystems, but also support the reactions that create ozone depletion in the stratosphere. Therefore, it is considerably imperative to abate the SO2 emitted from the combustion of fossil fuels and other industrial processes. This requires collective actions and close cooperation between industries and researchers. Post-combustion capture has been recognized as a viable technology option to reduce SO2 emission due to simplicity and retrofit possibility on existing power plants. Among the wide variety of technological options proposed to capture SO2, chemical solvent absorption method is considered as a reliable and relatively competitive method for the mitigation of SO2 emission from fossil fuel power plants due to high removal efficiency and low cost. Other

⇑ Corresponding author at: School of Chemical and Petroleum Engineering, Shiraz University, P.O. Box 7194685315, Shiraz, Iran. Tel.: +98 711 6133764; fax: +98 711 6133700. E-mail address: [email protected] (D. Mowla). http://dx.doi.org/10.1016/j.seppur.2014.10.028 1383-5866/Ó 2015 Published by Elsevier B.V.

advantage of this process is that it can be applied to the existing power plants with relatively minor plant retrofitting aspects. The main challenge is to find the most optimal process for capturing SO2. Developing a novel solvent with desirable characteristics will promote chemical absorption. In general, the absorption capacity of SO2 is one of the main absorption characteristics of absorbents for SO2 removal. Besides, design of gas–liquid contactors, used in acid gas treating process, requires its information. The solvent capacity of an absorbed gas is a function of its partial pressure, process temperature and chemical kinetics. Numerous researches have been carried out to find the efficient absorbent with high removal efficiency, low cost and no byproducts. The processes of SO2 absorption by buffers is a regenerative processes of gas desulfurization and do not have the residues problems that can be found in non-regenerative processes such as SO2 absorption into lime slurries [3]. The use of buffers offers many advantages including: acceptable energy requirement, low total operating cost and negligible liquid effluents disposal. Among the inorganic solutions, sodium based buffer solutions especially sodium citrate and disodium hydrogen phosphate is generally considered as a fast, safe, green and approximately high solubility method for SO2 removal [4–10]. Of course precipitation of crystals of buffer salts may involuntarily occur in the regeneration process which is the main concern about buffer absorbent SO2 process. Due to increasing industries demand for resource recovery and recycling, recovery-type SO2 removal processes are becoming more

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preferable, among which amine-based SO2 removal is one of the most effective processes, because of the technical simplicity, economic advantage and lack of waste by-product [11,12]. Dimethylsulfoxide, pyridine, monoethanolamine, dimethylacetamide, methyldiethanolamine and ethylenediamine are the frequently utilized organic solvents used to separate SO2 from different flue gases [13]. In between, ethylenediamine has been found to be a good alternative for SO2 removal [14,15]. Recently, a new class of amines, sterically hindered amines is suggested as one of the attractive amine solution or as an additive for acid gas removal and especially CO2 [16]. 2-Amino2-methyl1-propanol (AMP) is a sterically hindered primary amine with relatively fast reaction kinetics, low regeneration energy, low metal corrosives and low biodegradation [17]. But few investigations have been performed on the measurement of equilibrium and operational data of SO2 absorption with AMP solution, so it might also be interesting for SO2 removal. Several difficulties of above mentioned absorbents such as low biodegradation, no environmental friendly, high vapor pressure and relatively low absorption capacity in low solvent concentrations, led to new type of absorbents with features close to the amines solution called amino acid salts were tested in SO2 removal. In previous researches on CO2 removal, amino acid salt solutions showed attractive performance for CO2 absorption because of their low environmental impact, with characteristics such as low volatility due to its ionic structure, low ecotoxicity, and high biodegradability [18]. Recently SO2 solubility of six water-soluble amino acids aqueous solutions was studied to remove SO2 from SO2–N2 system [19]. But no sufficient SO2 absorption equilibria and no operational data of amino acid salts solution has been reported. In order to improve the SO2 removal efficiency and reduce the cost, apart from choosing good absorbing liquid, it is very important to select effective equipment and proper operating conditions. Packed column are commonly used as a means of promoting efficient contact between gases and liquids. The mitigation of SO2 emission using packed column is a well-established technology being used widely in the chemical industrial due to its high efficiency, high capacity and low pressure drop. Efficient and flexible technologies, operating over a wide range of concentration levels and a wide range of volumetric flow rates and capable to remove greenhouse gases are needed. Numerous experimental works have been already carried out to investigate the performance of packed column in CO2 removal by different absorbents [20–23]. According to these studies, packed columns are still more popular due to maturity of the process compared to the rest. In recent years, several researchers have exerted packed columns for effective SO2 absorption with various aqueous absorbents and considered effects of operational parameters on SO2 removal efficiency [24,25]. This work evaluates and compares SO2-aqueous solution equilibrium data of sodium glycinate and sodium lysinate solutions with 2-amino2-methyl1-propanol (AMP) and ethylenediamine as various kinds of amine solutions and disodium hydrogen phosphate as an effective buffer solution using a batch stirred absorption apparatus. Next, the operational behavior of the used absorbents was studied further in a packed column. Also, the effects of various parameters on the SO2 removal efficiency including temperature, SO2 inlet concentration and gas–liquid flow ratio are investigated by using sodium lysinate solution. 2. Experimental section 2.1. Materials and preparation of absorbents The a-amino acid salts (sodium glycinate, sodium lysinate) and amines (ethylenediamine, AMP) purity >99% were obtained from

MERCK company and Disodium hydrogen phosphate purity >99% was from APPLICHEM company and their aqueous solutions were prepared with demonized water. SO2 with high purity (>99.9%) was supplied from commercial cylinders. 2.2. Process chemistry The use of buffers such as phosphate buffer absorbent significantly increases the capacity of water to absorb SO2. Utilization of an alkali metal phosphate absorbent such as Na2HPO4 in SO2 absorption is of interest. The following reactions may take place when SO2 is absorbed in aqueous solution of Na2HPO4:

SO2

ðgÞ

SO2

ðlÞ

$ SO2

ð1Þ

ðlÞ

þ Na2 HPO4

ðlÞ

þ H2 OðlÞ $ NaHSO3

ðlÞ

þ NaH2 PO4

ðlÞ

ð2Þ

The mechanism is that the basic mono-hydrogen-phosphate + ions (HPO2 4 ) react with the acid hydronium ions (H3O ), which are release when SO2 is transformed into bisulfate (HSO3); then the acid di-hydrogen-phosphate-ions (H2PO 4 ) are formed. The cations also are added in the form of sodium (Na+). Reaction (2) is instantaneous and reversible. The serious difficulties associated with alkali metal phosphate based SO2 process is that the buffer gets supersaturated and non-ideal aqueous solution; results in precipitation of crystals containing Na2HPO4(s) in the regeneration process [26,27]. So the overall reaction may be formulated:

NaHSO3

ðlÞ

þ NaH2 PO4

ðlÞ

$ SO2;

ðl gÞ

þ Na2 HPO4

ðlÞ

þ Na2 HPO4ðsÞ þ H2 Oðl gÞ

ð3Þ

In addition to reaction mechanism, molecular structure of absorbents has very effective role in their SO2 absorption capacity. Table 1 show the molecular structure and CAS number of used absorbents. In molecular structure of Na2HPO4, the oxygen atoms act as electron donors and thereby the dative bonds (O–S) are formed which increase the SO2 solubility [28–30]. Adding an amine into the water also enhance the amount of SO2 dissolved. In water solution, dissolved SO2 undergoes reversible hydration according to the following equations:

SO2 þ H2 O $ Hþ þ HSO3

ð4Þ

Table 1 Molecular structure and CAS number of used absorbents. Absorbent

CAS number

Disodium hydrogen phosphate

7558-79-4

AMP

124-68-5

Ethylenediamine

107-15-3

Sodium glycinate

6000-44-8

Sodium lysinate

56-87-1

Molecular structure

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The ethylenediamine leads the above equilibrium to the right by reacting with the hydrogen ions. Ethylenediamine molecule has two amino functional groups. One of the amine functionalities react with SO2 as given below:

NH2 —CH2 —CH2 —NH2 þ SO2 ðlÞ þ H2 O $ NHþ3 —CH2 —CH2 —NH3 HSO3 ð5Þ (NH+3–CH2–CH2–NH3HSO3) is the lean amine that removes SO2. Reactions (4) and (5) are reversible and regenerate the absorbent [12]. The chemical reactions between SO2 and AMP (RNH2) as a sterically hindered primary amine are based on the assumption that the SO2 first react with water and then combine with aqueous alkali solution. The reaction is as follows:

SO2 þ H2 O $ 2Hþ þ SO2 3

ð6Þ

H2 SO3 þ 2RNH2 $ ðRNH3 Þ2 SO3

ð7Þ

The overall reaction of amine-SO2 is as follows [31]:

SO2 þ 2RNH2 þ H2 O $ 2RNHþ3 þ SO2 3

ð8Þ

In molecular structure of ethylenediamine, two amino functional groups and for AMP, one amino functional group and one oxygen atom act as electron donors. Demyanovich compared SO2 solubility of eight different organic solvents and reported that the reduction sequence of solvents capacity was: N (amine)˜O (glycol ether)˜PO4˜O@S@O. Therefore it looks that the amines have higher tendency to form dative bonds (OAS and NAS) compared to alkali metal phosphate absorbent [32–35]. Alkanolamine is a good reference in determination of the reaction mechanism of amino acid with SO2. Generally, amino acid (RCHNH2COOH) in aqueous solution exists in the form of zwitterion (RCHNH+3COO). When they dissolve in water, the charged groups NH+3 and COO of amino acids act as breaker of the H bound network of water as shown below [36]:

RCHNHþ3 COO þ Hþ $ RCHNHþ3 COOH

ð9Þ

RCHNHþ3 COO þ OH $ RCHNH2 COO þ H2 O

ð10Þ

Therefore, the mechanism for SO2 absorption in the example of glycine as a simple amino acid may be illustrated in the reaction equations below: 1. Dissolution of SO2 in water

SO2 þ H2 O $ Hþ þ HSO3

ð11Þ +

2. Combination of carboxyl (from glycine) with H

NHþ3 CH2 COO þ Hþ $ NHþ3 CH2 COOH

ð12Þ

This reaction could advance the dissolution of SO2. The hydrogen sulfite ions also may react with NH+3CH2COOH. As illustrated before, Na+ has the role of cation. The detailed reaction mechanism for lysine will be discussed in Section 3.3. The presence of both identical amino functional groups and carboxylic acid group in amino acid salt molecular structure which is shown in Table 1, make it more attractive in SO2 removal [30–33]. Amino acids are important organic compounds which made of a side chain (R) group excluding two functional groups. These are classified based on (R) group containing with non-polar (R) group, uncharged polar (R) group, charged polar (R) group. Glycine is nonpolar amino Acid and has no charge on the (R) group. These amino acids have equal number of amino and carboxyl groups. In contrary, lysine is a polar amino acid with positive charge and high

solubility in water. The polar amino acids have more amino groups as compared to carboxyl groups making it basic. 2.3. SO2 solubility apparatus and procedure The schematic diagram of experimental apparatus used to measure the solubility of SO2 is presented in Fig. 1. It should be noted that the employed apparatus is a modification of the one applied by Derks et al. [37]. It is composed of a batch gas–liquid contactor and a gas vessel for storing the sulfur oxide with calibrated volumes, equipped with temperature and pressure indicators. The contactor, a 200 ml round-bottomed Pyrex flask which is magnetically stirred, has two spans, one for gas and liquid injection and other for pressure sensor installation. During the experiment, pressure of system was measured continuously via an electronic pressure sensor (GEFRAN), sent to the computer by means of a data logger and recorded by Lab View software. In a typical experiment, the apparatus is allowed to reach to the desire temperature with the help of the thermostatic bath. After that, a certain amount of SO2, supplied from gas vessel which is filled with pure SO2, was injected into the absorbent vessel and the initial pressure, Pin, is recorded. Then, a known volume of absorbent solution is transferred to the contactor. The stirrer is then switched on and the solution equilibrium was allowed to be established at the desired temperature. As the SO2 is absorbed, total pressure decreases. When the total pressure of the cell does not change (or remained constant), the equilibrium was assumed to be reached and the final pressure, Peq, is recorded. The partial pressure of SO2 was calculated from the difference between initial and final (equilibrium) contactor pressure and the absorption capacity of solution was obtained by applying ideal gas law. In this study, the SO2 absorption capacity was defined as mass ratio of the absorbed SO2 to the solution. To achieve different mixture of SO2 and air, another certain amount of SO2 was injected into the gas–liquid contactor. 2.4. SO2 removal measurements To ensure the effectiveness of the selected solvents at operational conditions, SO2 removal efficiency of absorbents were assessed in a bench scale packed absorption system. Fig. 1 shows the schematic diagram of experimental set-up including flue gas simulation and absorbents supply systems, a bench-scale scrubber and a flue gas analysis system (see Fig. 2). The liquid feed is pumped into the top of the column from a heat jacketed feed tank and distributed on the top of the bed by means of a nozzle. On the other hand, the simulated flue gas is made from SO2 supplied by a high pressure cylinder and air blown by a compressor, introduced to the bottom of the absorption column. The solvent and air rates were measured with separate glass rotameters prior to entering the tower. The SO2 is fed into the carrier gas, which is homogenized and heated through a cylindrical conduit containing packing material and electrical elements installed around it. The SO2 stream was also measured via a small rotameter before mixing with air and entering the tower. The absorption performance experiment was conducted in the packed column which gas mixture and liquid contact to each other counter-currently. The specifications of designed packed column are given in Table 2. The packing was packed in the absorber randomly. The absorption process was operated until steady state condition was achieved. Every experimental run was about 15–20 min to attain the absorption equilibrium state; and two to three experimental runs were regarded as an experimental series. Besides, a mass balance calculation at the end of each experiment was performed to confirm the validity of the run. The aeration oxidation (AO) method was applied to measure the SO2 concentrations at inlet and outlet air streams. To this aim, the

F. Rahmani et al. / Separation and Purification Technology 153 (2015) 162–169

165

Fig. 1. Schematic of the stirred gas–liquid contactor setup.

Fig. 2. Schematic diagram of the bench scale packed absorption system.

3. Results and discussions Table 2 Specifications of designed packed column. Column body material Column height (mm) Column diameter (mm) Packing material Support plate diameter (mm) Support plate thickness (mm) Support plate material

3.1. Absorption equilibria of dilute SO2 Glass, QVF 750 68 Berl Saddle 49 4 PP

sampling gas was entered to the sample container and absorbed by hydrogen peroxide solution. Then, produced sulfuric acid titration is done by standard sodium hydroxide solution in presence of indicator to calculate absorbed SO2 concentration using consumed volume of caustic soda.

3.1.1. Validation of apparatus performance and used procedure accuracy Before beginning the experiments, the reliability of experimental setup and procedure was tested by measuring SO2 solubility of water at 313 K and compared with previously reported data as shown in Fig. 3 [34]. Inspection of the figure reveals that the obtained results are compatible with previous data, implying the reliability of apparatus performance and the precision of the used procedure. 3.1.2. SO2 solubility in various aqueous solutions at 298.15 K The experimental values of the solubility of SO2 in 0.1 M concentration of studied absorbents at 298.15 K are shown in Fig. 4. It has been found that amino acid sodium salts and especially

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25 Rabe et.al This Work

300

SO2 Partial Pressure (Kpa)

SO2 Partial Pressure (Kpa)

350

250 200 150 100 50 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

SO2 Weight Fraction

0.1 M 0.5 M 1 M

20

15

10

5

0 0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

SO2 Weight Fraction

Fig. 3. SO2 solubility in water, T = 313 K.

Fig. 5. Solubility of SO2 into aqueous solution of sodium lysinate at different concentrations, T = 298 K.

sodium lysinate aqueous solution exhibits a greater capacity for absorption of SO2 over the pressure range studied. It may be due to the presence of both amino acid and carboxylic functional groups in their molecular structure. Also, comparison of SO2 solubility in various solutions reveals that ethylenediamine and AMP have greater SO2 loading compared to disodium hydrogen phosphate especially in higher partial pressure of SO2. Moreover, it is observed that the absorption capacity rises with the increasing of the SO2 partial pressure at a given temperature. This could be attributed to the increased diffusion driving force as a result of the increase in SO2 partial pressure, enhancing SO2 absorption solubility. Based on this screening study, the sodium lysinate aqueous solution seems to be promising solvent for SO2 capture from the view point of absorption capacity. However, it requires further study. 3.1.3. The effect of sodium lysinate concentration on the SO2 solubility at 298.15 K To more evaluate the absorption capacity of the sodium lysinate solution and also illustrate the effect of concentration, measurements with 0.5 and 1 M sodium lysinate solutions were also carried out. Fig. 5 displays a relationship between the SO2 absorption capacity and various concentrations of sodium lysinate solution. It is obvious that the SO2 solubility of sodium lysinate enhances with increase of solvent concentration because of promoting absorption capacity of solution. As can be seen, at constant

SO2 partial pressure of 12 KPa, with increasing sodium lysinate solution concentration from 0.1 to 1 M, the absorption capacity becomes about twice. It is worthy to note that by following the curve related to 0.1 M concentration, it is concluded that changes in absorption rate at lower partial pressures of SO2 is very faster than higher partial pressures, whereas for high sodium lysinate concentration such as 1 M, SO2 absorption rate dose not considerably decline with increasing of SO2 partial pressure. Thereby, at high SO2 partial pressures, it is not operationally beneficial to utilize the low concentration of sodium lysinate solution; however absorbent cost should be taken into account. 3.2. Evaluation of SO2 removal efficiency The obtained results from equilibrium data of previous section showed that sodium lysinate had better performance in view point of SO2 solubility. To introduce this absorbent as a proper alternative in desulfurization process and also to ensure the performance of sodium lysinate in practical conditions, the operational data of SO2 removal efficiency in a scrubber column is essential. To this aim, the experiments were carried out in a structured absorption column and specified operating conditions were applied to evaluate SO2 removal efficiency of suggested absorbent. The performance of the packed bed column was defined in term of SO2 removal efficiency using following equation:

SO2 Partial Pressure (Kpa)

25

20

Na2 HPO 4 Ethylenediamine AMP Sodium Glycinate Sodium Lysinate

Disodium hydrogen phosphate

15

AMP

10

Ethylendiamine

Sodium Glycinate 5 Sodium Lysinate 0 0.00

0.02

0.04

0.06

0.08

0.10

SO2 Weight Fraction Fig. 4. Solubility of SO2 into various aqueous solutions at 0.1 M concentration, T = 298 K.

0

20

40

60

80

SO2 Removal efficiency (%) Fig. 6. SO2 removal efficiency of various absorbents (0.1 M) in packed column at T = 313 K, SO2 inlet conc. = 1400 ppm, L/G = 0.01.

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ðC in  C out Þ  100 C in

where Cin is the SO2 inlet concentration; Cout is the SO2 outlet concentration and g is the removal efficiency.

glycinate solutions are superior to amine and buffer solutions. Physicochemical properties of sodium lysinate which were mentioned in Section 2.2 are the main reason of its predominance. Moreover, disodium hydrogen phosphate has lowest removal efficiency around 43%, as expected according to solubility data.

3.2.1. Comparison with other studied absorbents In order to compare the SO2-removal efficiency of sodium lysinate solution in the bench scale packed-bed absorber with that of other considered absorbents, the absorption tests were carried out for initial SO2 concentration of 1400 ppm, liquid to gas ratio of 0.01, absorbent concentration of 0.1 M and temperature of 313 K. As known, the removal performance of an absorbent and also effectiveness of an absorption system in toxic gas removal depends on the solubility of the gaseous contaminant. Fig. 6 reveals that the performances of SO2 removal using sodium lysinate and sodium

3.2.2. Influence of operating conditions The operating conditions that their effects were evaluated during the experiments were the volume percentage of SO2 at the scrubber inlet, the liquid to gas (L/G) ratio, and the operating temperature. The measured SO2 removal efficiencies changed in the range of 48.1–92.9%. The efficiency – as it was expected – decreased with an increase in SO2 inlet concentration, process temperature and the liquid to gas ratio. Effect of L/G ratio and initial SO2 concentration on the SO2 removal efficiency at various temperatures is illustrated in Fig. 7.

70 60

0.010 0.009

50

70

0.009 50 1800

0.008 1600

0.007 1400

1200

SO i 2 nlet co 50 60 70 80 90

0.006

0.010

60

0.008 1600

0.007 1400

1200

SO i 2 nlet co

G

1800

80

0.006 1000

G

iency (%)

80

Removal effic

90

90

0.005

nc.

L/

iency (%)

b

100

L/

ð13Þ

a

Removal effic

0.005

1000

nc.

50 60 70 80 90 100

c

70 60

0.010 0.009

50

1800

0.008 1600

SO2

0.007 1400

inlet

1200

0.006 1000

G

50 60 70 80 90

80

L/

y (%)

90

ienc Removal effic

gSO2 ¼

0.005

conc

.

Fig. 7. Effect of different parameters on the SO2 removal efficiency of sodium lysinate aqueous solution (1 M), a (T = 298 K), b (T = 313 K), c (T = 333 K).

F. Rahmani et al. / Separation and Purification Technology 153 (2015) 162–169

3.2.2.1. SO2 inlet concentration. Fig. 7 shows the removal efficiency of 1 M concentration of sodium lysinate when the feed concentration varies from 900 to 2000 ppm. As can be seen, by increasing the concentration of sulfur dioxide in the feed gas, the removal efficiency decreases. The possible reason is that the solvent becomes saturated at high SO2 concentrations. It is important point that, increasing SO2 inlet concentration leads to increase driving force and finally absorption rate, but it does not imply increase in SO2 removal efficiency [38]. On the contrary, decreased SO2 removal efficiency might be due to faster increase in the amount of SO2 than that needs to be absorbed [39], as well as this issue can be refer to Eq. (2). It is thought that there are enough reactant molecules to react with SO2 molecules at the low feed gas concentration. However, the number of reactant molecules is limited to react with the SO2 molecules fed at the high feed concentration, so that the removal efficiency decreases. 3.2.2.2. Liquid to gas (L/G) ratio. From the economic point of view, in the packed column liquid–gas volumetric flow ratio (L/G) is the effective parameter for evaluating absorption performance [40]. This factor affects on the absorption rate through the liquid film thickness and could have influence on mass transfer coefficient of gas phase. Increasing the liquid flow rate would increase the efficient wetting of packing material, and subsequently, effective gas– liquid interface for mass transfer, which is favorable to SO2 absorption [24]. As can be seen in Fig. 7, in certain condition of temperature and SO2 inlet concentration, removal efficiency increases with increasing L/G. However, greater L/G leads to higher power consumption and not economically advantageous. When L/G is increased from 0.007 to 0.008, the rate of SO2 removal increased from 72.9% to 83.7%, in SO2 concentration of 1400 ppm and temperature of 298 K. It is because of more solvent is in contact with gas. According to Fig. 7, when L/G is larger than 0.008, the SO2 removal increases relaxed. Regard to above discussions, the L/G in the range of 0.007–0.008 could be appropriate for this condition of packed column. 3.2.2.3. Effect of process temperature on SO2 removal. The effect of temperature on SO2 removal efficiency is depicted in Fig. 7. As is indicated, the blue1 color and orange color represent lower and higher removal efficiency, respectively. This figure reveals that SO2 removal efficiency slightly decreases with increasing temperature. For example, with respect to range of colors, the blue area in Fig. 7(a) which is plotted at 298 K is less than Fig. 7(b) at 313 K, also the orange area that show high removal efficiency is larger at 298 and 313 K compared to temperature of 333 K. To illustrate the effect of process temperature on SO2 removal efficiency more clearly, average inlet concentration of SO2 (1400 ppm) and optimum liquid to gas (L/G) ratio (0.008) were appointed. Fig. 8 explains the SO2 removal efficiency variation with increasing temperature in selected condition. In discussion the results, it should be noted, although increasing of temperature enhances the diffusivity of SO2, at the same time, it decreases the SO2 solubility and increases the rate of solvents evaporation. The results of this study also show that the negative effects of temperature on SO2 solubility and evaporation are slightly higher than its positive effect on diffusivity [2,19]. This is in consistence with the results obtained in most cases of gas dissolution into a liquid [41]. It is obvious from the results that low temperature is favorable for SO2 capture of absorbents. 3.3. Proposed SO2 reaction mechanism with lysine As mentioned earlier, presence of charged polar (R) group and more number of amino functional group in lysine, led to increasing 1 For interpretation of color in Fig. 7, the reader is referred to the web version of this article.

Removal Efficiency (%)

168

84 83 82 81 80 79 78 77 76 75 298 K

Fig. 8. SO2 removal efficiency conc. = 1400 ppm, L/G = 0.008.

313 K variation

333 K with

temperature,

SO2

inlet

SO2 solubility of this amino acid compared to glycine that was proved by experimental results. But details of SO2 absorption with lysine are presented by reaction mechanism. In Section 2.2, we explained that SO2 interacts with carboxyl group of glycine through two-step reactions. In the first step, the dissolution of SO2 in water occurs and hydrogen and bisulfite ions are generated which is common for SO2 interaction with lysine. But combination of hydrogen ions with COO is accomplished with suggested reaction below:

NH2 ðCH2 Þ4 CHðNHþ3 ÞCOO þ Hþ $ NH2 ðCH2 Þ4 CHðNHþ3 ÞCOOH ð14Þ + Moreover, the HSO 3 will reacts with NH2(CH2)4CH(NH3)COOH and forms hydrogen bond (dotted line in reaction (15)) according to following reaction:

ð15Þ

However, this reaction may be prevented because an electron withdrawing inductive effect of NH+3 on COOH [18].

4. Conclusions In this paper, SO2 solubility of two novel amino acid salt aqueous solutions including sodium glycinate and sodium lysinate salts were evaluated in detail and compared with the various types of used absorbents. The SO2-aqueous solution equilibrium data indicated that in low concentration (0.1 M) of absorbents, amino acid salt solutions and particularly sodium lysinate had the maximum SO2 solubility that mainly attributed to its physicochemical properties and high absorption capacity. The SO2 reaction mechanisms of frequently used absorbents were mentioned to interpret absorption performance according to previous works and at the end, SO2 absorption mechanism of sodium lysinate solution was proposed. Based on the experimental results, the amino acid solutions had good activity on SO2 removal with sodium lysinate solution exhibiting the highest removal efficiency in a packed column compatible to SO2 solubility data. So, in order to introduce sodium lysinate solution as an appropriate alternative for desulfurization process, various important operating factors affecting the removal efficiency including SO2 inlet concentration, process temperature and L/G were discussed. These experiments reveal that high temperature is not favorable for SO2 removal efficiency, also increasing SO2 inlet concentration led to decrease in removal efficiency. In contrast, removal efficiency could be realized by raising L/G, but according to economical issue the optimum ratio was recommended.

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