A Novel Selective Flue Gas SO2 Removal with an ...

216 Int. J Sci. Emerging Tech

Vol‐6 No 1, July 2013

A Novel Selective Flue Gas SO2 Removal with an Amine Absorbent H.Mehrara*1, M.R.Shishesaz 1, B.Rouzbehani 1 1HSE Department of Abadan Institute of Technology Petroleum University of Technology North Bowardeh, Abadan, Iran *1

[email protected] [email protected] [email protected] Abstract— Wet flue gas desulfurization (WFGD) is the most widespread one among all SO2 removal methods. A novel desulfurization method is presented throughout this paper in which an amine based absorber is used to reduce SO2 concentration, from 8000ppm to less than 200 ppm. In this study, the selectivity of the new desulfurization method with consideration of operational parameters such as: SO2 concentration, column pressure, temperature of absorption process and final released SO2 emission are investigated in a steady-state condition. According to the experiments, sulfur dioxide which was in range of 4000 ppm to 8000 ppm in gas mixture. and was reduced to less than 200 ppm and 1000 ppm respectively was reduced from 4000-8000ppm to an amount of less than 200 ppm in the outlet of the pilot. During the scrubbing process, SO2 absorption was performed in atmospheric pressure and a temperature between 52 to 73 degrees Celsius. influences of PH, gas to liquid ratio (G/L) and the stripper temperature at certain condition are investigated. Keywords— Selective, desulfurization, amine, absorption, pilot plant.

1. Introduction As sulphur dioxide is a major atmospheric pollutant generating acid deposits, its abatement in flue gases is of significant industrial concern [1,2]. Combustion of fossil fuels in power plants, incinerators, boilers, roasting of sulfide ore in metallurgy and sulfuric acid industry are the major sources of sulfur oxides. Large part of sulfur oxides is also added in air due to volcanic eruption. cause of global environmental problems such as air pollution and acid rain [3,4]. Due to presence of moisture in atmospheric air these oxides react with moisture and forms sulforic acide The oxides react in the atmosphere forming sulfuric acid, which leads to acid rain. Because of the lowering of the permissible emission limits, there has been a rising interest in new treatment processes for absorbing gaseous SO2, using various reactants [5]. __________________________________________________________________________ International Journal of Science & Emerging Technologies IJSET, E‐ISSN: 2048 ‐ 8688 Copyright © ExcelingTech, Pub, UK (http://excelingtech.co.uk/)

Many processes have been developed, untill now. Among them limestone, calcium hydroxide and magnesium hydroxide slurries, sodium hydroxide solutions, and some organic solvents are used as absorbent in those processes [6-8].The most wellknown flue gas desulfurization processes are mainly based on scrubbing with limestone slurries of the flue gas. These are known as non-regenerative processes, [1, 9-10]. Typically, the byproduct is either discarded in a landfill or converted into gypsum for use in wallboard and cement manufacturing. Disposal in a landfill requires a large initial capital investment as well as significant resources to maintain the landfill throughout the life of the plant [11]. There are also regenerative SO2 removal processes like Wellmann-Lord and the citrate processes [9, 12] In these processes, SO2 is absorbed by aqueous solution, then recovered from the SO2 rich solution either as SO2 by steam stripping or as elemental sulfur by reacting to H2S[12-14]. The selectivity of this process with high performance separate this process from others, which can have lots of usage in industries. In this process Sulfur, in the form of sulfur dioxide (SO2 ) is recovered, which can be used in bleaching as a feed chemical, hydrosulfite manufacture, for pH adjustment and residual peroxide destruction. The recovery and recycling of sulfur are being aided by the commercial introduction of regenerable selective SO2 scrubbing from gas streams. This new process can reduce emissions to air while providing a new tool to solve the recycling puzzle [15].

2. Theoretical considerations In water solution, dissolved SO2 undergoes reversible hydration and ionization to produce bisulphite and sulfite according to the following equations. SO2(g)↔SO2(aq) SO2(aq)+H2O(L)↔H2SO3(L) H2SO3(L)↔HSO3-+H+↔SO2+2H+

(1) (2) (3)



Adding the amine, to the water increases the quantity of SO2 dissolved. According to equation (4), the buffer drives the above equilibria to the right by reacting with the hydrogen ions to form ammonium salts. The overall reaction, indicates that as the concentration of SO2 in the feed gas increases, the equilibrium moves to the right, i.e. the quantity of SO2 dissolved in the rich solvent increases. Thus, the scrubbing of more concentrated gas streams requires a less than the proportional increase in solvent circulation rate. Since the gas volume, and therefore the gas side equipment, remains constant, a relatively small total cost increase is caused by an increase in feed SO2 concentration. R1R2R3N + H+

R1R2R3NH+

solutions were prepared with distilled water. The CO2 and N2 gases were commercial grades with a purity of 99.99%, and the SO2 gas was used with a high purity of 99.9%. All these components are mixed in a collector designed for this purpose. Figure 1 shows a scheme of the pilot plant employed in the experiments. The absorber and regenerator, which were made of 316 stainless steel with an internal diameter of 15 cm, and a height of 1.5 (m) are used.

(4)

3. Experiment The tests were done in an experimental pilot plant. The construction of the plant was designed specifically to improve absorption. The pilot plant is made up of a packed column containing metal grid packing. The plant is specially designed for the experiments and uses an amine absorber with an innovative formulation. In the plant, the absorption tower, with a diameter of 15cm and a height of 150cm was designed, and the experiments were done using the gas to liquid ratio ranging from 375 (V/V) to 500(V/V). A valve is supplied in the path between deception tower and the storage tank, by opening which; desorption tower is filled with solvent. Two 2000 (w) elements controlled by a thermo set are used in the desorption tower which heats up the solvent to 120 °C. This cause the pressure increase up to 0.8 (bar). At the top of this tower, a thermocouple is placed to measure the temperature and set the top temperature and pressure at a favorable point. A pump is placed below the supply tank, stripper and absorption tower controlled by a thermo controller. A cooler is designed after stripper tower which leads the solvent enters the sell & tube heat exchanger to reduce reciprocating absorber temperature to about 60-65 degrees Celsius before entering the absorption tower. Before pump 2 sprays solvent in the absorption tower on packing, flow is adjusted by an adjustable rotameter. The flow of solvent in the inlet of the absorption column can be set from 100 to 400 milliliters per minute. After entrance of the absorber, it is then sent to the desorption column via a 750 (w) pump. Then the absorber flow is preheated in the mentioned shell and tube heat exchanger prior to entering the desorption tower. Feed gas is produced by using three gas cylinders, N2, CO2 and SO2 and a boiler to produce steam. Aqueous

Figure 1. The schemic of SO2 scrubbing pilot plant.

4. Results and discussions As SO2 is an acidic gas, the increase of pH of absorption solution is favorable to the absorption. [16]. Aquas amine compounds have base properties so its pH affects the absorption process significantly. Figure 2 indicates that at a certain operational condition, desorption temperature of 120 ºC, a G/L ratio of 375 and SO2 concentration of 6200 ppm. The PH value of the absorption agent is adjusted by changing the proportion between sulfuric acid and amine. The desulfurization efficiency increases asymptotically with the increase in PH. In other words By variation of PH. It is observed that the SO2 concentration has strongly changed in the outlet of absorption tower. As it is shown in Figure 2 by increasing absorbent PH, SO2 concentration decreases quickly in the outlet of absorption tower. The results indicate that the desulfurization efficiency increases with an increase of the pH value. In low PH value a decrease in the number of active groups for absorption are seen, while a pH value that is too high leads to an increase of volatile loss, so a suitable PH value must be found.



90 85 80 75 η%

70 65 60 55 50 45 40 2

2.5

3

3.5

4

4.5

5

5.5

6

PH

Figure 2. Effect of PH on desulfurization in the average SO2 concentration of 6100 ppm, at 120 °C and G/L ratio of 375. 70 60

η%

50

At constant liquid flow rate, an increase in the inlet gas flow rate will decrease the gas film thickness, and gas-liquid mass transfer area and coefficient increase a little, which is favorable to absorb but on the other hand, an increase in the inlet gas rate decreases the gas-liquid contact time, which is harmful to the absorption and effects dominantly on efficiency decrease. When the SO2 inlet concentration was increased from 2400 ppm to 9000 ppm, the SO2 absorption efficiency decreased from 94 % to 55%. The experimental results shown in Figure 4 indicate that the efficiency of absorption into amine solution decrease with an increasing in SO2 concentration of the inlet gas. It is obvious that by increasing the SO2 concentration the capability of solvent to absorb SO2 decrease and this is due to absorbent saturation at higher concentration [17]. When the SO2 concentration exceeds 7000 ppm, the buffer capacity declines, while too low of an SO2 concentration in the solution would result in more steam consumption in desorption [18].

40

90

30

80

20

70 60 η%

10 0 300

340

380

420

460

500

540

580

G/L ratio

20 10 0 95

100

100

105

110

115

120

125

Desorption temperature

95 90

Figure 5. Absorption variation vs. temperature Variation in SO2 concentration of 6100 ppm, at 110 °C and PH of 4.

85 80 η%

40 30

Figure 3. Absorption variation vs. Variation of G/L in SO2 concentration of 6100 ppm, at 110 °C and PH of 4.

75 70 65 60 55 50 2000

50

4000

6000

8000

10000

SO2 inlet

Figure 4. Absorption variation vs. Variation of SO2 concentration, at 110 °C, G/L of 375 and PH of 4. It can be seen from Figure. 3, that the increase of gas to liquid ratio, cause the efficiency decreases quickly.

Figure 5 shows that at 110 °C, the best performance is achieved. At 100 °C, the bound between SO2 and amine compound in absorbent cannot completely be recovered so by increasing the temperature to 110 °C better absorption capacity is reached. Experiment shows that by increasing temperature to 120 °C absorption highly decrease from 79% to 63%. By increasing the temperature from 100 °C to 120 °C the desorption pressure increase from 0.3 bar to 0.85 bar, which don’t permit the absorbent to desorb the dissolved SO2.

219


Analysis of the Safety and Economy. The solvent in the novel method has little toxicity, and absorption, and high absorption and operation is at atmospheric pressure and temprature is below 110 ºC. Furthermore, sulfuric acid is added to decreases the volatility of amine and increase PH. Therefore, the damage of the absorbent on air can be nearly ignored. One shoude consider the corrosion of equipmentat at this pH. favorable PH for better stripping is obtained to be around 5, which has no danger but equipment corrosion prevention should be considered [18].

3500 3000 2500 SO2 ppm

Int. J Sci. Emerging Tech

2000 1500 Inlet SO2

1000

outlet SO2

Selectivity investigation.

500

Time

Figure 7. SO2 variation in the Five hour plot of the flue gas desulfurization

13 12.5

Table 1. operating condition for 5 hour testing. Desorption Tower bottom P (bar)

solvent concentration (gr/lit)

8

108

0.8

0.85

0.05

60

8

105

0.75

0.8

0.05

2800

61

8

104

0.75

0.8

0.05

2470

59

8

107

0.8

0.85

0.05

2400

62

8

110

0.8

0.85

0.05

3300

57

8

110

0.8

0.85

0.05

2480

58

8

116

0.85

0.9

0.05

2536

55

8

110

0.8

0.85

0.05

2587

57

8

106

0.75

0.8

0.05

2394

59

8

110

0.8

0.85

0.05

2426

61

8

109

0.8

0.85

0.05

2242

60

8

107

0.8

0.85

0.05

2250

59

8

105

0.75

0.8

0.05

2220

52

8

110

0.8

0.85

0.05

Outlet CO2

10

5:05

4:50

4:15

3:50

3:30

3:10

2:00

1:25

1:00

0:45

0:30

0:20

0:00

9.5 4:35

% CO2

Inlet SO2

Inlet CO2

10.5

desorbtion Top T

61

2600

11

Absorption tower P (kpa)

2430

11.5

Absorption tower T (° C )

Desorption Tower top P (bar)

12

Time

Figure 6. CO2 variation in the five hour plot of the

flue gas desulfurization Figure 7 shows that the SO2 concentration in the outlet of the desorption tower is approximately constant on the average of 270 ppm, which shows the performance of about 90%. Operating condition is shown in Table 1. According to this table any parameter my affect absorption and desorption are presented. As can be noted from this table performance for the absorbent with the concentration of 0.05% (V/V) is good enough to arrange the experiment in a categorize the experiment as a selective high efficient desulfurization process. 6.

Conclusion

This paper has presented a new selective regenerative flue gas desulfurization process with high performance. In this study, four parameters were investigated, which involved SO2 concentration in

5:05

4:50

4:35

4:15

3:50

3:30

3:10

2:00

1:25

1:00

0:45

0:30

0 0:20

The selectivity of absorption to SO2 is presented in figure 6 and figure 7 respectively. Clearly, the inlet and outlet stream had little difference in CO2 concentration.

0:00

5.



flue gas, desorption temperature, PH of absorption and liquid to the gas ratio. Results showed that an increase in SO2 concentration from 2400 to 7800 decreases the performance of absorption from 94% to about 60 %, and desulfurization efficiency increases very smoothly with increase in G/L value for more absorbent and more contact area in the scrubber, but by variation of PH, it is observed that the SO2 concentration has strongly changed in the outlet of the absorption tower. It was shown that by increasing PH the absorbent shows better performance and tends to dissolve more SO2. The five-hour test also confirmed the solvent selectivity by which the absorbent showed very little CO2 interest in comparison to SO2. This is good enough to categorize the experiment in the experiment as a selective high efficient desulfurization process.

[7] Norberto Fueyo, Alfredo Tomás Antonio Gómez, "Detailed modelling of a flue-gas desulfurisation plant," Computers & Chemical Engineering, vol. 31, pp. 1419–1431, 2007.

7.

[10] Qin Lib, Fang Lia Yuan Wua, "Desulfurization in the gas-continuous impinging stream gas– liquid reactor," Chemical Engineering Science, vol. 62, pp. 1814 – 1824, 2007.

REFRENCES

[1] Jacques Vanderschuren, Diane Thomas Sandrine Colle, "Pilot-scale validation of the kinetics of SO2 absorption into sulphuric acid solutions containing hydrogen peroxide," Chemical Engineering and Processing, vol. 43, pp. 1397–1402, 2004. [2] Takao Kaneko, Tsutomu Tashimo, Tadashi Yoshida, Kunio Kato Xiaoxun Ma, "Use of limestone for SO2 removal from flue gas in the semidry FGD process with a powder-particle spouted bed," Chemical Engineering Science, vol. 55, pp. 4643-4652, 2000. [3] A. Savas Koparal, Ulker Bakır Ogutveren U mran Tezcan Un, Separation and Purification Technology , vol. 53, pp. 57–63, 2007. [4] Bal Raj Deshwal, InWon Kimb, Hyung Keun Leea Hyun Hee Parka, "Absorption of SO2 from flue gas using PVDF hollow fiber membranes in a gas–liquid contactor," Journal of Membrane Science, vol. 319, pp. 29–37, 2008. [5] Sandrine Colle, Jacques Vanderschuren Diane Thomas, "Kinetics of SO2 absorption into fairly concentrated sulphuric acid solutions containing hydrogen peroxide," Chemical Engineering and Processing, vol. 42, pp. 487-494, 2003. [6] Weiguo Pan a, Qiang Jin b, Wenhuan Wanga, Yu Li Binlin Dou a, "Prediction of SO2 removal efficiency for wet Flue Gas Desulfurization," Energy Conversion and Management, vol. 50, pp. 2547–2553, 2009.

[8] F. Vidal , P. Ollero , L. Salvador , and V. Cortés F. J. Gutiérrez Ortiz, "Pilot-Plant Technical Assessment of Wet Flue Gas Desulfurization Using Limestone," Industrial & Engineering Chemistry Research, vol. 45, pp. 1466–1477, 2006. [9] S. Kaytakoglu L. Akyalcın, "Flue gas desulfurization by citrate process and optimization of working parameters," Chemical Engineering and Processing, vol. 49, pp. 199– 204, 2010.

[11] Qin Zhong, Xuyou Fan, Qianqiao Chen, and Haibo Sun Yong Jia, "Modeling of ammoniabased wet flue gas desulfurization in the spray scrubber," Korean Journals of Chemical engineering, vol. 28, pp. 1058-1064, 2011. [12] Richard B. Nielsen Arthur L. Kohl,.: Elsevier Inc., 1997, ch. 7, pp. 466-669. [13] Y. Feng V. Ramanathan, "Air pollution, greenhouse gases and climate change," Atmospheric Environment - Fifty Years of Endeavour, vol. 43, pp. 37–50, 2009. [14] N.Karatepe, "A comparison of flue gas desulfurization processes," Energy Source, vol. 22, pp. 197–206, 2000. [15] Devin Shaw, "Cansolv CO2 Capture: The Value of Integration," Energy Procedia, vol. 1, pp. 237–246, 2009. [16] LIU Youzhi and GU Meiduo JIANG Xiuping, "Absorption of Sulphur Dioxide with Sodium Citrate Buffer Solution in a Rotating Packed Bed," Chinese Journal of Chemical Engineering, vol. 19, pp. 687-692, 2011. [17] Honglei Ding, Zhen Dua, Zuliang Wub, Mengxiang Fang, Zhongyang Luo, Kefa Cen Xiang Gao a, "Gas–liquid absorption reaction between (NH4)2SO3 solution and SO2 for ammonia-based wet flue gas desulfurization," Applied Energy 87, vol. 87, pp. 2647–2651, 2010.


[18] Chang-cheng Zhou, and Cheng Chen Zhi-gang Tang, "Studies on Flue Gas Desulfurization by Chemical Absorption Using an Ethylenediamine-Phosphoric Acid Solution," Industrial & Engineering Chemistry Research, vol. 43, pp. 6714-6722, 2004.
