Removal of CO2 from Biogas by Using Tert-Butyl Peroxy-2

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Removal of CO2 from Biogas by using TertButyl peroxy-2-ethylhexanoate and water Shuanshi Fan, Qi Li, Yanhong Wang, Xuemei Lang, and Jun Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04656 • Publication Date (Web): 11 Mar 2016 Downloaded from http://pubs.acs.org on March 12, 2016

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Industrial & Engineering Chemistry Research

Removal of CO2 From Biogas by using Tert-Butyl peroxy-2-ethylhexanoate and water

Shuanshi Fan*, Qi Li, Yanhong Wang, Xuemei Lang, Jun Chen

Key Lab of Enhanced Heat Transfer and Energy Conservation Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China * To whom correspondence should be addressed. Tel: + 86-20-22236581, Fax: +86-20-22236581, E-mail: [email protected]

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ABSTRACT. In this work, the effects of Tert-Butyl peroxy-2-ethylhexanoate (TBPO) and the operating conditions on the separation of CO2 from a CO2/CH4 (67.0 mol %) gas mixture based on hydrate formation have been studied through experiments. The results showed that CO2 separation was effectively achieved by adding TBPO to aqua-solutions. The content of methane in the residual gas hiked when the gas liquid volume ratio decreased. Methane concentration in residual gas phase was increased from 67.0 % to 89.6 %, obtained when the initial gas liquid volume ratio was 2.23 by one stage hydrate separation with 26.0 wt % TBPO at 287 K and 2MPa. The CH4 recovery was 0.91 and the CO2 separation factor was 17.01 by adding 5.0 wt % TBPO. By a hand pump to keep pressure constant for the residual gas in the vessel to form hydrates, CH4 concentration in residual gas phase increased to as high as 93.3 %. KEYWORDS. CO2, gas hydrate, biogas, carbon capture and sequestration, absorption

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1. INTRODUCTION Clathrate hydrates are non-stoichiometric, ice-like crystalline solids formed by a network of hydrogen-bonded of water molecules (called host molecules), which can encage various species (called guest molecules) such as methane, carbon dioxide, nitrogen, cyclopentane, and acetone in their cavities1. When the guest molecule is a gas, the clathrate hydrate is called a ‘‘gas hydrate’’. The three main hydrate structures2 are ‘‘structure I’’, ‘‘structure II’’, and ‘‘structure H’’, usually denoted as (sI), (sII), and (sH) respectively. Recently, applications of hydrate-based technologies have been proposed in many fields such as gas storage and transportation, refrigeration, desalination, and gas separation3. Clathrate hydratebased gas separation techniques are currently at its experimental research stage4,5. The theoretical footing these gas separation techniques2 build itself upon the principle that each gas has its certain hydrate formation pressure. The concentration of each component in the hydrate phase and that in the residual gas phase display different characteristics depends on the conditions of how a gas mixture forms hydrate with water. The component that forms hydrate more easily will be enriched in the hydrate phase. Based on this principle, it is considered possible to separate gas mixtures through forming hydrate. Numerous hydrate-based separations have already been studied with different gas mixtures containing carbon dioxide (CO2)6,7, methane (CH4)8, oxygen (O2)9, hydrogen(H2)10 and other gases11. This separation technology may be an interesting alternative that is economically competitive12,13 compared with conventional techniques such as pressure swing adsorption (PSA)14, chemical or physical absorption15,16, cryogenic separation17 and membrane separation18. Methane (CH4) and carbon dioxide (CO2) are commonly present in fuel gases such as biogas19, natural gas, production of methane with carbon dioxide injection from hydrates, shale gases. However, as one of the main greenhouse gases, CO2 is the main factor contributing to the greenhouse effect and global climate change. Besides, CO2 is also undesirable because it contributes to decreasing the heating value of such gases20. In order to get high purity methane, CO2 need to separate from biogas. Owing to the proximity of the CH4 and CO2 hydrate

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equilibrium curves21,22, it is fairly difficult to efficiently separate CO2 from a CH4-CO2 gas mixture using a hydrate-based process. However, recent results have shown that adding certain chemicals to the water, such as surfactants23,24, organic molecules or salts25,26, can significantly enhance the hydrate formation rate, modify the position of the equilibrium curves or allow the selective enclathration of one of the gases in the hydrate phase. Most of the existing additives reported in literatures27 applied for hydrate formation can be categorized into two classes, thermodynamic additives and kinetic additives. Tetrahydrofuran (THF)28, Cyclopentane

(CP)29,

Tetra-n-butylammonium bromide

(TBAB)30 and Tri-n-

butylphosphine oxide (TBPO)31,32 have been found to be good hydrate thermodynamics promoters that could effectively reduce the phase equilibrium conditions. Kinetic additives33,34 (generally surfactant molecules) could accelerate hydrate formation without changing the hydrate equilibrium conditions. Appropriate promoters shall be applied to the separation of gas mixture in order to boost efficiency. And the hydrate formation rate of CO2 was more quickly than CH4 under certain operating conditions. This work aims to investigate the effects of an organic compound on the separation of CO2 from a CO2 / CH4 (67.0 mol %) gas mixture based on hydrate formation. The main objective is to fill the knowledge gap regarding the effects of Tert-Butyl peroxy-2-ethylhexanoate (TBPO) on gas separation by forming hydrate and contribute experimental data to the study. The paper studies the influence of TBPO and the effects of the operating conditions on the hydrate formation in particular. 2. EXPERIMENTAL 2.1Materials Table 1 list the materials used in this work. CH4/CO2 gas mixture containing 67.0 mol % CH4 and 33.0 mol % CO2 was used in this work to simulate biogas. The additive used in this study was TBPO. Deionized water was used to prepare all solutions. The mole fractions of the mixture gas were 0.67 CH4 and 0.33 CO2 with an uncertainty of ± 0.001. The uncertainty in the mass fraction of TBPO is ± 0 .001.

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Table 1. Experimental Materials Used in This Work Materials

Purity ( mole fraction )

Supplier

CH4/CO2

0.67 and 0.33

Zhaoqing Gaonengda gas Co.

TBPO

0.98

Guangzhou Summit Chemical Co.

water

deionized self-made

2.2 Apparatus and methods The schematic of the experimental apparatus employed for gas separation was shown in Figure 1. The apparatus mainly consisted of a stainless-steel vessel of 22.6 cm in height and 7.5 cm in diameter, with 1000 ml effective volume. The maximum operating pressure of the vessel was 20 MPa. During the experiments, the contents in the vessel were agitated by a magnetic stirrer (Hai’an Scientific Research Apparatus Co., China). The temperatures of the gas and liquid phase in the vessel were measured by two resistance thermometers (Westzh WZ-PT100) within 0.1K in accuracy. The pressures inside the vessel were measured by a pressure transducer (Westzh CyB20S) with an accuracy ± 0.01 MPa. The temperatures and pressures of the vessel were recorded by a data logger (Agilent 34970A) at 20s intervals. The compositions of the gas phase were analyzed by gas chromatography (KeChuang GC9800: TCD detector, Porapak Q packed column) during the experimental process. The GC measurement accuracy is ± 0.001.

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Figure 1. Schematic diagram of the experimental apparatus.

Before starting the experiment, the reactor was cleaned using deionized water and dried. The solution prepared with a desired concentration was introduced into the evacuated vessel, and the reactor was purged with mixture gas of CH4/CO2 five times to ensure it was air-free. Then the vessel was cooled to setting temperature. When the solution temperature was kept constant, the mixture gas of CH4/CO2 was charged into the vessel until the initial operating pressure. Timer started counting from zero at the moment of opening the magnetic stirrer. The temperature and pressure were recorded and the gas sample was analyzed with GC during the experiment. When the system pressure was kept constant for more than one hour, it was considered to be the end of hydrate formation. However, in the further experiments of hydration, the pressure was recovered to the initial pressure by using hand pump, when the pressure had no obvious change. Cycling several times until the pressure had no obvious change, it was considered to be the end of reaction. The stirrer was stopped, the vent valve was opened, and the remaining gas was purged. Then, the vessel was warmed to room temperature to dissociate the hydrate completely. 2.3 Calculation methods Number of moles of gas consumed The molar number of gas that has been consumed during hydrate formation can be calculated as equation 1.

∆ = − = −

(1)

Where z is the compressibility factor calculated by the PR equation of state and subscripts 0 and t refer to time during the experiment process. Where P0 and Pt are defined as the pressure of the time zero and t respectively, V0 and Vt are defined as the volume of the gas phase at the time zero and t respectively. The volume of gas is assumed constant during the hydrate formation process for the reason that the phase transitions are neglected. CH4 recovery and CO2 separation factor

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The CH4 recovery of gas mixture in residual gas phase is calculated as equation 2. R =

(2)

!!" Where n is defined as the moles of CH4 in the gas phase at the end of the experiment, n is

defined as the moles of CH4 in the feed gas. The separation factor (denoted S.F.) is calculated as equation 3. S. F. =

× &'

&' ×

(3)

Where n )' is defined as the moles of CO2 and n is defined as the moles of CH4 in the hydrate

phase at the end of the experiment, n)' is defined as the moles of CO2 in the gas phase at the end of the experiment. 3. RESULTS AND DISCUSSION 3.1 Effects of experimental conditions 3.0

4.0

2.5

3.5

2.0

3.0 P / MPa

P / MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5

2.5

1.0

2.0

0.5

1.5 1.0

0.0 0.0

0.5

1.0

1.5

2.0

0.0

2.5

0.5

1.0

1.5

2.0

2.5

t/h

t/h

(a) (b) Figure 2. Pressure versus time in 26 wt % TBPO + 74 wt % water + CH4/CO2 systems with different initial pressures at 287K (a) and 290K (b).

The typical trends of the feed pressure during the hydrate formation process in 26 wt% TBPO +74 wt% water + CH4/CO2 systems with different initial pressures at 287 K and 290 K were shown in Figures 2. The results revealed that the system pressures decreased rapidly during the first 0.25 h

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and then were stable, which indicate that the hydrate formation had completed and the system reached equilibrium. It was noted that the pressure drop was both only 0.9 MPa at 287 K and 290 K when the feed pressure was 3.0 MPa. The trends of pressure change were similar to our previous experimental results35. The pressure rapidly reduced when TBAB and TBPO were added. Table 2 listed the composition analysis and separation efficiencies of 26 wt % TBPO + CH4/CO2 systems in different experimental conditions. The results showed that the temperature had little effect on gas uptake of each experiment at the same feed pressure. The gas uptake increased as pressure rose. However, the gas to hydrate ratio decreased when the pressure was higher at the same temperature. As seen from Table 2, the highest CH4 concentration in residual gas phase happened at 287 K and 2.0 MPa and the highest CO2 separation factor was happen at 287 K and 1.0 MPa. Low temperature and high pressure were not favorable for the separation of CO2 from CH4/CO2 gas mixture. The CH4 recovery was 0.86 highest at 290 K and 4.0 MPa, while the CH4 recovery was close to 0.8 at the other conditions. In this case, it can be inferred that 287 K and 1.02.0 MPa were appropriate for the separation of CO2 from CH4/CO2 gas mixture by adding 26 wt % TBPO. The separation effects were also similar to to our previous experimental results35. However, the experiment temperature was high and pressure was lower used in this work to separate CO2 and obtain high CH4 concentration. Table 2. Experimental results at different conditions for 26 wt % TBPO + CH4/CO2 systems.

T/K

Pfeed / MPa

Gas uptake / mol

Gas consumption ratio / %

CH4 concentration in residual gas phase / mol %

R

S.F.

285

1.0

0.060

32.0

80.22

0.81

6.30

287

1.0

0.065

36.3

82.16

0.78

6.58

287

2.0

0.128

35.6

83.06

0.80

5.48

287

3.0

0.190

34.2

80.20

0.79

5.68

290

2.0

0.128

34.3

79.81

0.78

5.35

290

3.0

0.201

35.5

81.35

0.78

5.51

290

4.0

0.228

29.1

81.59

0.86

3.10

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3.2 Effects of the initial gas liquid volume ratio The initial gas liquid volume ratio (Rv) is defined as: R * + ,,- n *.

Where n0 is the number of moles of feed mixed gas, VL is the volume of the liquid phase. 2.4 2.2 2.0 1.8 1.6

P / MPa

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1.4 1.2 1.0 0.8 0.6 0.4 0.0

0.5

1.0

1.5

2.0

2.5

t/h

Figure 3. Pressure versus time in 26 wt % TBPO + CH4/CO2 systems with different initial gas liquid volume ratios at 287K and 2.0 MPa: ■,CO2, Rv=13.60; ●, CO2, Rv=5.18; ▲, CO2, Rv=2.23.

Figure 3 showed the pressure curves during the hydrate formation process for 26 wt % TBPO + CH4/CO2 system with different gas liquid volume ratio at 287 K and 2.0 MPa. As shown in Figure 3, the pressures dropped quickly in 0.3 h and then reached balance in 0.62 h, 0.85 h and 0.62 h, respectively. The pressure drop was increased by reducing initial gas liquid volume ratios for the reason that more solution and less gas participated in the hydrate reaction. Figure 4 showed the curves of CO2 and CH4 consumptions during the hydrate formation process for 26 wt % TBPO + CH4/CO2 system with different gas liquid volume ratio at 287 K and 2.0 MPa. As shown in Figure 4, CO2 hydrate formation rate was higher than CH4 hydrate formation rate at the beginning of experiments, which had been proved by the other report36. In this case, the concentrations of CH4 in gas phase were improved. The gas consumptions of CO2 were significantly more than the gas consumptions of CH4 when the initial gas liquid volume ratio was 13.60. However, the gas consumptions of CO2 were only a little more than the gas consumptions of CH4 when the initial gas liquid volume ratio was 5.18 and the gas consumptions of CH4 were more than the gas

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consumptions of CO2 after 0.5 h when the initial gas liquid volume ratio was 2.23. It was indicated that low initial gas liquid volume ratios may lead to more CH4 to hydrate ratio, which may low the CH4 recovery.

0.08

Gas consumtion / mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.06

0.04

0.02

0.00 0.0

0.5

1.0

1.5

2.0

2.5

t/h

Figure 4. Gas consumptions versus time in 26 wt % TBPO + CH4/CO2 systems with different initial gas liquid volume ratios at 287K and 2.0 MPa: ■,CO2, Rv=13.60; ●, CO2, Rv=5.18; ▲, CO2, Rv=2.23; ▼, CH4, Rv=13.60; ◄, CH4, Rv=5.18; ►, CH4, Rv=2.23.

Table 3 listed the separation effect of CO2 from 26 wt % TBPO + CH4/CO2 system with different gas liquid volume ratio at 287 K and 2.0 MPa. The results show that the influence of the gas liquid volume ratio on separation was remarkable. The gas to hydrate ratio increased from 35.0% to 66.6% when the gas liquid volume ratio decreased. Nonetheless, the CH4 recovery decreased when the gas liquid volume ratio decreased. The CH4 concentration in residual gas phase was increased, which indicated that CO2 was preferentially incorporated into hydrate. The maximum concentration (89.65 mol%) was obtained with low gas liquid volume ratio of 2.23 for 26 wt % TBPO + CH4/CO2 system at 287K and 2 MPa. Meanwhile, the change of CO2 separation factor was consistent with CH4 concentration and the CO2 separation factors were increased from 5.89 to 10.62, when the gas liquid volume ratios were decreased from 13.60 to 2.23. It can be concluded from the above discussions that CH4 concentrations in residual gas phase and the CO2 separation factors were improved by decreased gas liquid volume ratio.

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Table 3. Effects of the gas liquid volume ratio on the separation for 26 wt % TBPO + CH4/CO2 system at 287K and 2MPa. Gas liquid volume ration

R

S.F.

35.0

CH4 concentration in residual gas phase / mol % 81.36

0.79

5.89

0.097

52.3

85.80

0.61

6.21

0.060

66.6

89.65

0.45

10.62

n0 / mol

Gas uptake / mol


13.60

0.364

0.128

5.18

0.185

2.23

0.090

3.3 Effect of TBPO water ratio

2.2 5 wt % 15 wt % 26 wt %

2.0 1.8 P / MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.6 1.4 1.2 1.0 0.0

0.5

1.0

1.5

2.0

t/h

Figure 5. Pressure versus time with different TBPO water ratio at 287K and 2.0 MPa: ▇, 5 wt % TBPO; ●, 15 wt % TBPO; ▲, 26 wt % TBPO.

Figure 5 showed the pressure changes with different TBPO water ratio at 287 K and 2.0 MPa. As shown in Figure 5, the pressures dropped quickly in 0.1 h and then reached balance in 0.25 h with 5 wt % TBPO. The pressure drop was 0.52 MPa, which was lower than 0.67 MPa by adding 26 wt % TBPO. All of the three concentrations, 15 wt % TBPO was the largest pressure dropper, the pressures dropped less than 0.1 h and then reached balance in 0.25 h. Table 4 listed the separation effects with different TBPO concentrations at 287 K and 2.0 MPa. The results show that the CH4 concentration in residual gas phase was close with different TBPO concentrations. However, CH4

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recovery and CO2 separation factor were remarkable changed by adding different TBPO concentrations. The CH4 recovery was 0.80 when TBPO concentration was 26.0 wt %, while the CH4 recovery increased to 0.91 when TBPO concentration was 5.0 wt %. The CO2 separation factor was 17.01 by adding 5 wt % TBPO, which was higher than 5.48 by adding 26 wt % TBPO. Due to lower TBPO concentrations, the gas to hydrate ratio was 0.265. Table 4. Effects of the different TBPO concentrations on the separation at 287K and 2MPa.

T/K

Pfeed / MPa

Gas uptake / mol


CH4 concentration in residual gas phase / mol %

R

S.F.

5.0

287

2.0

0.095

26.5

82.73

0.91

17.01

15.0

287

2.0

0.133

37.2

85.76

0.81

11.30

26.0

287

2.0

0.128

35.6

83.06

0.80

5.48

TBPO / wt%

3.4 Effect of recovering pressure 4.0

95

3.5

90

3.0 2.5 80 2.0 75

CH4 / vol %

85 P / MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.5 70

1.0 0.5

65 0.0

0.5

1.0

1.5

2.0

t/h

Figure 6. Evolution of the reactor pressure and CH4 concentration in residual gas phase versus time in 26 wt % TBPO aqueous solution at 287K.

Figure 6 showed the evolution of the reactor pressure and CH4 concentration in residual gas phase versus time in 26 wt % TBPO aqueous solution at 287K. The system pressure decreased rapidly in 0.75 h, and the CH4 concentration in residual gas phase was quickly increased from 67.0% to 89.52%. It was noted that the hydrate formation rate became low when the pressure drop to 0.89 MPa. That was because the concentration of TBPO in the solution was becoming gradually less

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during hydrate formation, and the low pressure was not enough to form hydrate. It was also noted that the residual gas continued to form hydrate when the pressure was improved by adding the same solution into vessel by using hand pump. Meanwhile, the CH4 concentration in residual gas phase was increased by recovering pressure. By recovering pressure to 2 MPa three times, the system pressure achieved equilibrium and the maximum CH4 concentration was reached to 93.3%. The results revealed that the CH4 concentration can be improved by recovering pressure. 4. CONCLUSIONS In this work, the effects of TBPO on the separation of CO2 from a CO2 / CH4 (67.0 mol %) gas mixture had been studied through experiments. The results showed that the content of methane in residual gas was higher when the gas liquid volume ratio was decrease. The highest CH4 concentration in residual gas phase was increased from 67.0 % to 89.6 %, obtained in 26 wt % TBPO aqueous solution with an initial gas liquid volume ratio of 2.23 by one stage hydrate separation at 287 K and 2 MPa. The CH4 recovery was 0.91 and he CO2 separation factor was 17.01 by adding 5.0 wt % TBPO. By recovering the pressure after the hydrate formation began using hand pump, the residual gas in the vessel continued to form hydrates, then the CH4 concentration in residual gas phase increased to the highest of 93.3%. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (51176051), the Fundamental Research Funds for the Central University (2013ZZ0032 and 2013ZM0036).

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with Tetra-N-Butylammonium Bromide Solution in the Presence of 1-Butyl-3-Methylimidazolium Tetrafluoroborate. Energy & Fuel., 2015, 29, 3143. (36) Malegaonkar, M. B.; Dholabhaii, P. D.; Bishnoi, P. R. Kinetics of Carbon Dioxide and Methane Hydrate Formation. Can. J. Chem. Eng. 1997, 75, 1090.

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For Table of Contents Only 4.0

95

3.5

90

3.0 2.5 80 2.0 75

CH4 / vol %

85 P / MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.5 70

1.0 0.5

65 0.0

0.5

1.0

1.5

2.0

t/h

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Page 18 of 18

Removal of CO2 from Biogas by Using Tert-Butyl Peroxy-2

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