Phase Equilibria of CO2 Hydrate Formation in Glucoamylase Aqueous

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Phase Equilibria of CO2 Hydrate Formation in Glucoamylase Aqueous Solutions Jing Bai,*,† Lu Zhang,† Jing Li,§ Teng-bo Liang,† Chun Chang,† Shu-qi Fang,*,† Xiu-li Han,† and Deqing Liang‡ †

School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, Henan, China Guomao Engineering Design Institute, Academy of State Administration of Grain, State Administration of Grain, Beijing 100037, China ‡ Key Laboratory of Renewable Energy and Natural Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China §

ABSTRACT: In this research, the effects of water-soluble glucoamylase at four concentrations of 2.79, 5.58, 13.95, and 27.9 wt % on CO2 hydrate formation phase equilibrium have been investigated. Traditional isochoric method was employed in 300 mL autoclave to provide these phase equilibrium data. The results are also compared with the hydrate formation in pure water and tetra-n-butyl ammonium bromide (TBAB) solution. The concentration of glucoamylase for CO2 hydrate formation has optimum value. When the concentration of glucoamylase was 5.58 wt %, it has positive effect on lowering CO2 hydrate formation requirements. Compared with TBAB, the promotion effect of glucoamylase is a little bit milder. When the concentrations of glucoamylase were 27.9 and 13.95 wt %, the glucoamylase has inhibiting effect on hydrate formation. Compared with methanol, the inhibiting effect of glucoamylase system is a little bit milder. 2.5 MPa and 273.75 K. Sakamoto et al.12 investigated the thermodynamic stability on the hydrogen-TBAF semiclathrate and found that the TBAF hydrate was more stable than TBAB hydrate. G Chen et al.13 proposed a double-process hydrate initiation mechanism and indicated that the new model was superior to the traditional van der Waals−Platteeuw14 typed models. In recent years, some researchers have focused their attention on organic macromolecule. Daimaru et al.15 used a series of surfactants with sodium sulfonic acid groups in common but with different carbon chain lengths (C4, C12, and C18) as hydrate promoter and investigated that for a given weight concentration, the surfactant with different chain length showed different acceleration. Fakharian et al.16 used the potato starch as methane hydrate promoter and presented that the potato starch could enhance the hydrate formation rate and storage capacity. Chen et al.17 indicated that lecithin did not significantly affect the hydrate thermodynamic equilibrium conditions, but it could improve the hydrate formation rate. Wang et al.18 presented that lignosulfonate, which could be used as promoter for the formation of methane hydrates with a high capacity up to 170 v/v and a high formation rate.

1. INTRODUCTION The usage of fossil energy leads to the concentration of CO2 increasing sharply in the atmosphere.1 The volume fraction of CO2 in the atmosphere will be more than 5.5 × 10−4 if no measures are taken.2 Sequestration of CO2 is a near-term goal for reduction of emissions. Many ways have been developed to reduce CO2 emissions, such as adsorption, absorption, membrane separation, and so on.3 Gas hydrate as a medium for CO 2 containing gas mixture separation has been investigated recently.4,5 Extensive research has been presented in literature. Seo6 was the first to report the hydrates formation process, which can be used to capture CO2 from flue gas. Englezos et al.7 presented incipient equilibrium experimental data for hydrate formation in pure water and solutions of NaCl, KCl, and CaCl2. Fan et al.8 reported the and equilibrium experimental data for hydrate formation in TBAB and found that TBAB could be used as promote for hydrate formation. Lee et al.9 indicated equilibrium experimental data for hydrate formation in different concentrations of tetrahydrofuran (THF) and found that 1.0 mol % THF was the optimum concentration for CO2 capture among the tested concentrations. Linga et al.10,11 also investigated a medium-pressure CO2 capture process based on hydrate crystallization in the presence of THF. The thermodynamic data was used to develop a new flow diagram of the proposed process involving three hydrate stages coupled with a membrane-based gas separation process, which operated at © XXXX American Chemical Society

Received: August 31, 2015 Accepted: December 24, 2015

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DOI: 10.1021/acs.jced.5b00733 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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vacuumed, an amount of 100 mL prepared solution was introduced into the evacuated reactor with barometric pressure and gravity. The mixer was started to operate at a rate of 1000 rpm. After the CO2 was charged into the reaction cell until the given pressure, the system temperature was slowly lowered to form hydrates. During the experiment, the temperature and pressure were recorded by data logger. When the system pressure and temperature were constant, it means the formation process is over. After that, the temperature in the equilibrium cell was increased at a very slow rate 0.05 K/h in order to obtain the thermodynamic equilibrium condition for CO2 hydrate dissociation. After the hydrate was completely dissociated, the pressure−temperature diagram was obtained to analyze. The phase equilibrium point can be determined from the measured P−T curve where a sharp slope change during the final heating process was observed. 2.4. Reliability of Equipment and Method. Before the experiment, the reliability of the experiment apparatus and procedures were checked by four experiments of CO2 hydrate formation in pure water. The data were presented in Table 1 and plotted in Figure 2, as is contrast with Fan,22 Adisasmito,23 and Li.24

Glucoamylases can be found among a wide range of different organisms such as fungi, yeasts, and eubacteria.19 It is an important enzyme and have been used in a wide variety of applications that require the hydrolysis of starch (such as glucose production and other monosaccharides from starch). Glucoamylases are extensively used to produce high fructose corn sweeteners and ethanol production, which comprise over 50% of the sweetener market in the United States.20,21 In this work, glucoamylase is being used as additive to investigate the hydrate formation process. Glucoamylase as organic macromolecule has never been studied on its effect on CO2 hydrate formation conditions. This work first provides the effect of different concentrations of glucoamylase on the CO2 hydrate thermodynamic equilibrium. The findings would be an alternative for the selection of chemicals on lowering gas hydrate formation conditions.

2. EXPERIMENTAL SECTION 2.1. Materials. The CO2 (>99.999% purity) used in this experiment was supplied by Inha Tech (Beijing) Technology Co., Ltd., China. Distilled water made by our own laboratory was used to prepare all solutions. Additives used in the experiments were: tetra-n-butyl ammonium bromide (>99% purity) from Tianjin Guangfu Fine Chemical Research Institute with chemical formula of (C16H36BrN), glucoamylase from Novozymes with 27.9 wt % and the enzyme activity is 80 000 u/ml. 2.2. Apparatus. The schematic flow diagram for this experimental system is shown in Figure 1. It mainly consists of

Table 1. Equilibrium Data for CO2 Hydrate in Pure Water T/K

P/MPa

277.6 278.5 280.2 281.6

2.07 2.34 2.93 3.61

Figure 1. Schematic diagram of the experimental apparatus. 1, Gas cylinder; 2, vacuum pump; 3, magnetic stirrers; 4, programmable controller cooling bath; 5, pressure gauge; 6, liquid storage vessel; 7, pressure transducer; 8, temperature sensor; 9, data logger; 10, computer.

a stainless steel autoclave (6.3 cm in diameter, effective volume 300 cm3) equipped with a magnetic stirrer (Haian Scientific Research Apparatus Co., Ltd., China). The reactor is designed to be operated at pressure up to 20 MPa. The temperature of the reactor is controlled by circulating the coolant from a thermostat with a stability of ±0.05 K inside the jacket around the cell (Ningbo Tianheng Instrument Factory THD-2010). The temperature of the reactor is monitored by Pt(100) resistance thermometers (Westzh WZ-PT100) with an accuracy of 0.1 K, which is placed in the top of the reactor. A pressure transducer (Westzh CyB-20S) with an accuracy of 0.01 MPa is being used to measure the pressure inside the reactor. The pressures and temperatures of the reactor are recorded by data logger (Agilent 34970A). 2.3. Procedure. This research employs the isochoric method to measure the equilibrium temperatures and pressures. The reaction cell and the liquid storage vessel need to be rinsed three times with the prepared solutions. Put the reaction cell in the cooling bath after postdrying it. After the reactor was

Figure 2. Phase equilibrium conditions for CO2 + water: ●, this work; ○, ref 18; △, ref 19; × , ref 20.

The sum of square residuals (SQR), as a statistical criteria of data reproducibility and calculated by eq 1 SQR = (Pexp − Pcal)2

(1)

where Pexp was the experimental equilibrium pressure and Pcal was the pressure calculated by the empirical third-order polynomial correlation mentioned above. SQR could be used to explain the deviations of the experiment system. The results in the present research were plotted in Figure 3. In this replication experiment, the maximal SQR value was 0.038 at 277.6 K. The absolute average deviation was within 3% B

DOI: 10.1021/acs.jced.5b00733 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 3. SQR value of the experimental equilibrium pressures for CO2 hydrate in pure water from those calculated by the empirical correlation investigated on the basis of the experimental data obtained in the present reaesrch: ●, this work; ○, ref 18; △, ref 19; × , ref 20.

Figure 4. Phase equilibrium conditions for CO2 in glucoamylase solutions: *, 27.9 wt %; ■, 13.95 wt %; □, 5.58 wt %; ▰, 2.79 wt %; ☆, pure water.

experiment results were compared with CO2 hydrate in pure water. When the concentration of glucoamylase was 5.58 wt %, the phase equilibrium curve was located under pure water hydrate phase equilibrium curve, which means it has positive effect on hydrate formation. When the concentration of solutions was too high (i.e., 27.9 and 13.95 wt %), the p−T data in phase equilibrium curve were presented to illustrate the inhibition effect of glucoamylase. When the concentration of solutions was too low (i.e., 2.79 wt %), the effect was almost the same as pure water. It is also the critical concentration of glucoamylase solution which appeared positive effect or inhibition effect on CO2 hydrate formation. The reason may be that the amount of additive was too little to influence the hydrate formation, the hydrate was mainly formed by pure water. Figure 4 illustrates that when the concentration of glucoamylase was 5.58 wt %, glucoamylase may has positive effect on hydrate formation. On the basis of the above experiments, plenty of work had been done to confirm phase equilibrium when the concentration of glucoamylase was 5.58 wt %, and the data was plotted in Figure 5. The data in pure

between the data obtained in this replication experiment and the data in the literature. Meanwhile, on the basis of the average difference division principle, there are only two facilities which existence errors. Therefore, the instrumental uncertainty is only 7.5 × 10−3. It indicated that the experiment equipment and experiment methods are reliable.

3. RESULTS AND DISCUSSION 3.1. Phase Equilibrium. The phase equilibrium data of CO2 hydrate at glucoamylase solutions were investigated. Because of complex composition of glucoamylase, it still remains unclearly about it is composition. So the mass fraction was used to quantify it. The mass fraction of enzyme stoste is 27.9 wt % with the enzyme activity is 80 000u/ml. Aqueous solutions with concentrations of 2.79, 5.58, 13.95, and 27.9 wt % of glucoamylase were used. The results were given in Table 2 and plotted in Figure 4. As shown in Figure 4, there is an optimum concentration of glucoamylase for hydrate formation process. All of the Table 2. Equilibrium Data for CO2 Hydrate in Glucoamylase Solutions with Five Different Concentrations m/wt %

T

P

27.9

274.71 278.38 281.29 277.15 278.15 279.25 280.35 281.49 283.25 284.81 286.32 276.9 279.7 280.7 282.5

1.82 2.96 4.64 2.61 2.89 3.25 3.74 2.41 2.85 3.69 4.34 1.78 2.52 3.11 4.58

13.95

5.58

2.79

Figure 5. Phase equilibrium conditions for CO2: ■, pure water; ○, glucoamylase (5.58 wt %); △, TBAB(5 wt %). C

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water22−24 and TBAB24 were also presented to illustrate the promotion effect of glucoamylase. Although the data had some deviations, it can be indicated that when the concentration of glucoamylase was 5.58 wt % it had positive effect on hydrate formation phase equilibrium. But compared with TBAB, the promotion effect was a little bit milder. As illustrated in Figure 4, when the concentration of glucoamylase were 27.9 and 13.95 wt %, glucoamylase may have inhibiting effect on hydrate formation. In order to confirm it, plenty of work had been done when the concentration of glucoamylase was 27.9 wt %, and the data were plotted in Figure 6. The data in pure water22−24 and methanol25 were also

residues. When amino acids are incorporated into polypeptides, hydrogen bonds can be formed. In gas hydrate formation, gas molecules (guests) are trapped into water cavities (host), which are composed of hydrogen-bonded water molecules. Hydrogen bonds in the polypeptides could be used in hydrate formation. And some functional groups in the long chains may also have some positive effect on hydrate formation. 2. Takamichi et al.15 has discussed the effect of surfactant carbon chain length on hydrate formation. He found that for a given weight concentration, the surfactant with different chain length showed different accelerations. As illustrated in Figure 7, the glucoamylase may have some long-chain molecules. 3. Yousif27 have discussed about the inhibition effect of methanol and ethylene glycol on the hydrate-control process. He found that although methanol and ethylene glycol are effective hydrate inhibitors when added in sufficient quantities, they actually have positive effect on hydrate formation when added at low concentrations to the water. As illustrated in Figure 7, enzyme is made up of amino acids and includes one or more hydroxide radical.26 In this research, it appeared to have an inhibition effect on hydrate formation when the concentrations of glucoamylase were 27.9 and 13.95 wt %, and it appeared to have a positive effect on hydrate formation when concentration of glucoamylase was 5.58 wt %. There were similar phenomenon among glucoamylase, methanol, and ethylene glycol. The mechanism of the positive effect and inhibition effect on gas hydrate formation may be the same as methanol and ethylene glycol.

Figure 6. Phase equilibrium conditions for CO2: ■, pure water; ○, glucoamylase (27.9 wt %); △, methanol (10 wt %).

4. CONCLUSIONS The effect of water-soluble glucoamylase at four concentrations of 2.79, 5.58, 13.95, and 27.9 wt % on CO2 hydrate formation data were obtained. When the concentration of glucoamylase solution was 5.58 wt %, it has positive effect on lowering CO2 hydrate formation requirements. Compared with TBAB, the promotion effect was a little bit milder. When the concentrations of glucoamylase were 13.95 and 27.9 wt %, it appeared inhibition effect for hydrate formation. The reason for glucoamylase could influence the hydrate formation is not clear. The mechanism of the positive effect and inhibition effect for gas hydrate formation needs further detailed investigations.

presented to illustrate the inhibiting effect of glucoamylase. As shown in Figure 6, the inhibiting effects on hydrate formation phase equilibrium are found for both glucoamylase and methanol systems. However, compared with methanol, the inhibiting effect of glucoamylase system was a little bit milder. 3.2. Discussion. 1. As shown in Figure 7, the enzyme is made up of amino acids, and including much long chains of amino acid

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AUTHOR INFORMATION

Corresponding Authors

*Jing Bai. Fax: +86-371-67780093. E-mail: [email protected]. *Shu-qi Fang. Fax: +86-371-67780093. E-mail: fangsq@zzu. edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Fund of China (NSFC-U1404519), the National Natural Science Fund of China (NSFC-21176227).

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REFERENCES

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Figure 7. Structure models of the active site of glucoamylase26 D

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DOI: 10.1021/acs.jced.5b00733 J. Chem. Eng. Data XXXX, XXX, XXX−XXX