Phase Equilibrium Data for the Hydrates of Synthesized Ternary CH4

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Phase Equilibrium Data for the Hydrates of Synthesized Ternary CH4/ CO2/N2 Biogas Mixtures Xiaoya Zang†,‡,§ and Deqing Liang*,†,‡,§ †

Guangzhou Institute of Energy Conversion, Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, China ‡ CAS Key Laboratory of Gas Hydrate, Guangzhou 510640, China § Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China ABSTRACT: To enhance the energy efficiency of biogas and reduce environmental pollution, impurities such as CO2 and N2 must be removed. In this context, hydrate-based gas separation (HBGS) is a novel technology that has been utilized in gas purification. As such, the hydrate phase equilibrium data of the CH4/CO2/N2 gas mixture are crucial for application of the HBGS process in biogas separation. This study presents our investigation into the phase equilibrium conditions for the hydrates of synthesized ternary CH4/CO2/N2 biogas mixtures. All experiments were conducted at 276.2−286.3 K and 2.59− 8.84 MPa using an isochoric method with different ternary CH4/CO2/N2 gas concentrations. The results obtained herein indicated that the hydrate phase equilibrium conditions shifted to lower pressures and higher temperatures with increasing CO2 contents. In addition, with a N2 content of about 10%, the hydrate phase equilibrium data of the ternary gas mixtures approached that of pure CH4.

1. INTRODUCTION As a replacement for conventional natural gas, biogas can be considered a potential clean energy source for current and future applications.1,2 Although CH4 is the main component of biogas, CO2 and N2 are often present as impurities.3,4 The presence of such impurities is disadvantageous, as in industrial applications, the calorific value of the fuels decrease, leading to a reduction in utilization efficiency. In addition, the emission of additional gas combustion products can increase environmental pollution. As such, the development of a suitable method for the removal of impurities such as CO2 and N2 is of particular importance. To date, the most common gas separation technologies employed include low temperature distillation, pressure swing adsorption separation, and membrane separation,5−9 However, these technologies have a number of disadvantages, such as the high operating cost of low temperature distillation, the poor separation efficiency of adsorption separation, and facile membrane blockage or damage in membrane separation technology.10−12 These drawbacks therefore hinder the industry application of these common gas separation technologies. In recent years, hydrate-based gas separation (HBGS) has received growing attention as an emerging and promising separation technique.13−15 Compared with traditional gas separation technologies, the advantages of HBGS, such as the use of moderate pressures and temperatures, its stable gas storage ability, and its low energy consumption, render it potentially practical for industry applications. As gas hydrates are nonstoichiometric crystalline solid structures, small guest © XXXX American Chemical Society

molecules (such as CH4, CO2, and N2) can be captured in the hydrogen-bonded cage structures that form in the presence of water at high pressures and low temperatures.16 In general, the natural gas hydrates found in sediments below the ocean and in permafrost zones are considered potential clean energy sources, and the special cage structure can be utilized in gas separation.17,18 Thus, HBGS technology is based on differences in the hydrate phase equilibrium conditions between the different gas components, as some components will form hydrates under a defined set of conditions, while others will remain in the gas phase. As such, this process has been extensively studied for CO2 capture from both fuel gas (CO2/ H2) and flue gas (CO2/N2).19−25 Recently, studies addressing the separation of CO2 from CH4/CO2 mixtures have also emerged.26−28 In terms of the biogas composition, the CH4 content range from 30 to 70%, the CO2 content can range from 25 to 50%, and biogas tends to have higher N2 contents (2− 17%) compared to natural gas.2 Therefore, binary gas mixtures of CH4/CO2 and CH4/N2 do not represent the character of typical biogas. On the basis of the application of the HBGS process in CO2 separation from binary gas mixtures, HBGS technology should also be suitable for CH4 purification from ternary gas mixtures containing CH4, CO2, and N2. However, only a few studies have been published within this area to date. For example, Kakati29 studied the formation of hydrates in the Received: September 15, 2017 Accepted: December 7, 2017

A

DOI: 10.1021/acs.jced.7b00823 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram of the experimental apparatus employed herein.

containing different component ratios were obtained from the Guangzhou Puyuan Gas Plant, China. The gas molar fractions were determined by gas chromatography (GC) and are listed in Table 1. Deionized water from an in-house water purification system was employed in all experiments.

CH4/CO2/N2 gas mixture followed by dissociation in various aqueous electrolyte concentrations, and provided ternary gas hydrate phase equilibrium data for a single given component (i.e., CH4/CO2/N2 = 89.89/5/5.11). In addition, Lee30 investigated the phase equilibrium behavior of landfill gas with lower CH4 contents (i.e., 41, 46, 52, and 55 mol %) and four different N2 contents (i.e., 5, 10, 20, and 30 mol %) between 273 and 282 K and at pressures < 6 MPa. The obtained results indicated that the CO2/CH4/N2 hydrates have the potential to be utilized in gas storage and transportation applications. However, the reported gas components do not represent the typical gas compositions of anaerobic digestion biogas (i.e., CH4, 53−70%; CO2, 30−50%). In addition, the N2 content of biogas generally does not exceed 17%, and the relatively low temperature and pressure ranges are not suitable for the industrial application of the HBGS process. However, under different conditions, the contents of the three gas components in biogas vary significantly, and so it is necessary to investigate the hydrate phase equilibrium conditions of the ternary mixed gas under a range of concentrations. At present, extremely limited phase equilibrium data are available for the ternary CH4/CO2/N2 gas hydrate. Although such data can be predicted by mathematical models, deviations remain between the calculated and experimental data. Thus, to test the phase equilibrium data of ternary gas mixtures over an extensive range of molar ratios and extended P−T conditions, and to ultimately obtain basic data for optimization of the design of hydrate separation processes and technologies, we herein report an experimental study aimed at determining the hydrate phase equilibrium data of ternary CH4/CO2/N2 gas mixtures. We expect that the obtained data will provide a theoretical basis and data foundation for application of the HBGS process in biogas separation.

Table 1. Compositions of the Purchased Ternary CH4/CO2/ N2 Gas Mixtures sample name

component

concentration/mol %

analytical method

Gas 1 Gas 2 Gas 3 Gas 4 Gas 5 Gas 6 water

CH4/CO2/N2 CH4/CO2/N2 CH4/CO2/N2 CH4/CO2/N2 CH4/CO2/N2 CH4/CO2/N2 water

69.91/25.17/4.92 60.08/35.02/4.90 49.95/45.03/5.02 70.05/19.98/9.97 60.01/29.97/10.02 49.05/40.02/10.93 deionized water

GCa GCa GCa GCa GCa GCa

a

GC = gas chromatography.

2.2. Procedures. After cleaning and drying the reactor cell, ∼250 mL distilled water was added, and the reactor was connected to the gas pipelines and placed in the water bath. Air was then evacuated from the reaction system using a vacuum pump, after which the ternary gas mixture was injected into the reaction cell at a constant pressure, and the reactor cell was disconnected from the pipeline. The water bath was then set at 273 K, and the mixture of ternary gas and deionized water was agitated for >10 h (100 R/min). After hydrate formation had reached completion, the temperature was increased to 300 K in a stepwise manner (see below) to dissociate the hydrates. Throughout the hydrate formation and dissociation processes, the temperature and pressure changes were recorded and monitored by an Agilent data acquisition unit (product model: 34970A). Thus, the dissociation process can be divided into three stages. In the first step, the temperature was increased at a rate of 1 K·h−1. Although hydrate dissociation did not take place in this stage, a pressure increase was recorded in the reactor cell, likely due to expansion of the gas phase volume. In the second stage, hydrate dissociation took place as the temperature was increased at a rate of 0.1 K·h−1 and held for ∼4−6 h at each increment to allow an equilibrium to be reached in the reactor cell. In this stage, the pressure increase could be attributed both to the gas phase volume expansion and to hydrate dissociation. Finally, in the third stage, hydrate dissociation was complete, and so the observed pressure

2. EXPERIMENTAL SECTION 2.1. Apparatus and Materials. The experimental apparatus employed herein (see Figure 1 for a schematic representation) was composed of a high pressure visual reaction cell, a mechanical stirring device, gas pipelines, a vacuum system, and a data acquisition system. Additional details regarding the setup can been found in the literature.31 The accuracies of thermometers and the pressure transducer employed were 0.1 K and 0.025 MPa, respectively. The ultrahigh purity synthesized ternary CH4/CO2/N2 gas mixtures B

DOI: 10.1021/acs.jced.7b00823 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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increase was due only to the gas phase volume expansion. As such, the slope corresponding to the pressure increment decreased accordingly in the third stage. The pressure− temperature inflection point between the second and third stages could be considered as the phase equilibrium point. Additional details regarding the experimental procedure can be found in our previous study.31

3. RESULTS AND DISCUSSION For the experiments carried out herein, verification of the experimental reliability is detailed in the literature,31 where we previously confirmed that our system is suitable for testing the mixture gas hydrate phase equilibrium data and obtain definite conclusions. Thus, the hydrate phase equilibrium data of six different biogas mixtures were determined, where the CH4 content ranged from ∼49.05 to 70.05%, the CO2 content ranged from ∼19.98 to 45.03%, and the N2 content ranged from ∼4.90 to 10.93% (see Table 1). The experimental results are reported in Table 2, which lists the three-phase equilibria

Figure 2. Hydrate phase equilibrium data for the ternary CH4/CO2/ N2 gas mixtures based on different component molar ratios. The solid lines represent the pure CH4 and CO2 gas hydrate phase equilibrium conditions predicted using the CSMHYD, ref 16.

Table 2. Three-Phase Equilibrium Conditions of the Gas Hydrates for Ternary CH4/CO2/ N2 Gas Mixtures Based on Different Component Molar Ratios P/MPa

T/K

P/MPa

Gas 1 3.38 4.32 5.42 5.98 6.54 8.84

277.3 279.8 281.9 283.1 283.5 286.0

3.36 4.00 4.76 6.02 8.36

277.0 280.4 282.5 283.8 284.4 285.0 286.3

4.06 5.00 5.82 6.95 8.02

276.2 279.7 282.5 284.5 285.5 286.2

3.46 4.58 5.80 6.82 8.40

276.2 278.0 279.7 281.6 284.7 Gas 5

Gas 3 2.59 3.73 5.11 6.04 7.11 7.94

T/K Gas 4

Gas 2 2.9 4.14 5.15 5.94 6.65 7.38 8.58

CO2 hydrates at an N2 content of ∼5%. Upon increasing the molar ratio of N2 to ∼10%, the hydrate phase equilibrium conditions of the ternary gas mixtures were similar to those of the pure CH4 hydrate, as shown in Figure 2. We also compared the experimental data obtained herein with previously reported literature data for similar systems.29,30 As shown in Figure 3, our results complemented the hydrate

278.5 280.6 282.0 283.7 285.0

Gas 6 277.1 279.8 282.2 283.7 285.5

data of the hydrates of the ternary gas mixtures based on different component molar ratios. The deviation between experimental value and predictive value of CSMHYD16 ranged from 0.5% to 11%. In addition, Figure 2 shows the corresponding plots of the hydrate equilibrium data for the different ternary CO2/CH4/N2 gas mixtures outlined in Table 2. As shown, the influence of the CH4 molar ratio on the hydrate equilibrium conditions increased upon increasing the temperature at a given N2 content. As the hydrate phase equilibrium pressure of CH4 is greater than that of the pure CO2 hydrate at a comparable temperature, a higher CH4 gas molar ratio results in a greater hydrate equilibrium pressure at a constant N2 concentration. Furthermore, the hydrate phase equilibrium conditions of the ternary gas mixtures were between those of the pure CH4 and

Figure 3. Comparison of the hydrate phase equilibrium data obtained herein with reported literature data. The solid lines represent the pure CH4 gas hydrate phase equilibrium conditions predicted using the CSMHYD.16

phase equilibrium data of the ternary gas mixtures with CH4 contents of 49.05−70.05%, and CO2 contents of 19.98−45.03% under extended pressure and temperature ranges. In addition, it was apparent that hydrate phase equilibrium conditions shifted to higher pressures and lower temperatures upon decreasing the CO2 content. Interestingly, at higher N2 concentrations than those examined herein (i.e., at 30 mol % N2), the hydrate phase equilibrium data of the ternary gas mixtures crossed the C

DOI: 10.1021/acs.jced.7b00823 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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pure CH4 hydrate curve between 274.2 and 281.9 K.30 Furthermore, our experimental results also indicated that the hydrate phase equilibrium conditions of the CH4/CO2/ N2(70.05/19.98/9.97) ternary gas mixtures crossed that of the pure CH4 hydrate at temperatures >281.6 K (as shown in Figure 2). Therefore, we conclude that the influence of N2 on the mixture gas hydrate phase equilibrium conditions is greater than that of CO2 with the same gas contents. To explain the hydrate phase equilibrium conditions more clearly, a hydrate phase equilibrium curve was constructed using the experimental data obtained herein. More specifically, the difference in hydrate equilibrium pressure (ΔP) between the ternary gas mixture and the pure CH4 was calculated and compared between 278 and 284 K, as shown in Figure 4, where

Figure 4. Differences in the phase equilibrium pressure between the ternary mixture gas hydrates and the pure CH4 hydrate.

the subscript n represents the gas number indicated in Table 1. Thus, a greater ΔP value indicated an increased deviation in the hydrate phase equilibrium from that of pure CH4, and the influence of CO2 gas component was enhanced at constant N2 concentrations. Compared with the N2 hydrate, the phase equilibrium condition of the pure CH4 hydrate is closer to that of pure CO2. In addition, as ΔP increased, the ternary gas mixture hydrate phase equilibrium pressure decreased at a comparable temperature, thereby indicating that hydrate formation from the CH4 and N2 gas components became more challenging. In contrast, hydrate formation from CH4 and N2 became easier at lower ΔP values. Furthermore, the negative value of ΔP6 at temperatures >281 K indicates that the hydrate phase equilibrium pressure of the ternary CH4/CO2/N2 (70.05/19.98/9.97) gas mixture is higher than that of the pure CH4 hydrate. Finally, we examined the variation in hydrate equilibrium pressure upon altering the CH4 and CO2 gas molar ratios (i.e., at a fixed N2 content), and the results are plotted in Figure 5 for N2 contents of 5% (Figure 5a) and 10% (Figure 5b). As shown, an increase in the CH4 molar ratio caused a shift in the phase equilibrium conditions for the ternary mixture gas hydrates to higher pressures at any given temperature. On the basis of these observations, we could conclude that both an increase in the CH4 and N2 concentrations and a decrease in the CO2 concentration caused a shift in the phase equilibrium conditions to lower temperatures and higher pressures.

Figure 5. Variation in the hydrate phase equilibrium pressure with CH4 content at a range of temperatures and at N2 contents of (a) ∼5% and (b) ∼10%.

4. CONCLUSIONS The hydrate phase equilibrium data of CH4/CO2/N2 gas mixtures are of significant importance in the context of biogas separation using the hydrate-based gas separation process, a technique that permits the removal of CO2 and N2 from biogas to enhance its energy efficiency and reduce environmental pollution. We therefore investigated the phase equilibrium conditions for the hydrates of ternary CH4/CO2/N2 gas mixtures containing a range of gas molar ratios. Our observations indicated that an increase in the N2 and CH4 concentrations shifted the hydrate phase equilibrium conditions to higher pressures and lower temperatures, that is, an increase in the CO2 content shifted these conditions to lower pressures and higher temperatures. In addition, the hydrate phase equilibrium conditions of the ternary gas mixtures were located between that of the pure CH4 and CO2 hydrates, crossing that of the pure CH4 hydrate at temperatures >281.6 K with CH4, CO2, and N2 contents of 70.05, 19.98, and 9.97%, respectively. Thus, as the contents of the three gas components in biogas vary significantly under different conditions, we expect that our D

DOI: 10.1021/acs.jced.7b00823 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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results will contribute to the collection of basic data for optimization of the design of hydrate separation processes and technologies.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 86-20-87057669. Fax: 86-20-87057669. E-mail: [email protected]. ORCID

Deqing Liang: 0000-0001-7534-4578 Funding

This work was supported by National Natural Science Foundation of China (NSFC: 51676197), National Key Research and Development Plan of China (2017YFC0307305), CAS Program (KGZD-EW-301). Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.jced.7b00823 J. Chem. Eng. Data XXXX, XXX, XXX−XXX