Hydrate phase equilibrium of CH4+N2+CO2 gas mixtures and cage

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Hydrate phase equilibrium of CH4+N2+CO2 gas mixtures and cage occupancy behaviors Youhong Sun, Sheng-Li Li, Guo-Biao Zhang, Wei Guo, and You-Hai Zhu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01093 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017

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Industrial & Engineering Chemistry Research

Hydrate phase equilibrium of CH4+N2+CO2 gas mixtures and cage occupancy behaviors You-Hong Sun1, Sheng-Li Li1∗, Guo-Biao Zhang1, Wei Guo1, You-Hai Zhu2

1 Key Laboratory of Drilling and Exploitation Technology in Complex Conditions of Ministry of Land and Resources, College of Construction Engineering, Jilin University, Changchun, 130026, China 2 The Key Laboratory of Unconventional Petroleum Geology, Oil and Gas Survey, China Geological Survey, Beijing, 100029, China

Corresponding information: Corresponding author: Sheng-li Li E-mail: [email protected] (S. L. Li). Tel.: +86 18946798682 Fax: +86 431 88502678. Postal Address：Ximinzhu Street, Changchun City, 130026, China.

Authors’ email addresses and affiliations: Youhong Sun: [email protected], Jilin University, China; Shengli Li: [email protected], Jilin University, China; Guobiao Zhang: [email protected], Jilin University, China; Wei Guo: [email protected], Jilin University, China; Youhai Zhu: [email protected], Oil and Gas Survey, China Geological Survey, China.

∗

To whom correspondence should be addressed. Fax: +86 431 88502678. E-mail: [email protected] (S. L. Li).

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ABSTRACT Methane, nitrogen and carbon dioxide co-exist after the injection of CO2/N2 mixture into CH4 hydrate. For a better understanding of CH4 recovery and CO2 sequestration by gas exchange involving N2, the hydrate phase equilibrium of CH4/N2/CO2 ternary gas mixtures in pure water system were determined in a sapphire cell using the pressure-search method under isothermal conditions. The hydrate equilibrium data were analyzed with respect to the concentration of CH4 and the ratio of N2/CO2. The cage occupancies of CH4, N2 and CO2 were calculated by Chen-Guo hydrate model and CSMHYD program. The result revealed the cage-specific guest distributions and preferential partitioning of guest molecules in cages were involved in the formation of mixed gas hydrates. The enthalpy of the ternary hydrates was calculated using the Clausius−Clapeyron equation. It was deduced that the enthalpy of the mixed hydrates was changed with the occupancy of CO2 molecules in large cavities.

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1. INTRODUCTION Gas hydrates are ice-like crystalline compounds composed of a framework of hydrogen-bonded water molecules encaging small guest molecules. Natural gas hydrates, mainly containing methane as guest, are found worldwide in continental margins as well as in and below permafrost regions where a low temperature and high pressure condition is proper for the formation of gas hydrate. Gas hydrates generally crystallize into structure I (sI), structure II (sII), and structure H (sH).1 Pure CH4 hydrate and CO2 hydrate are usually found to be sI, with a unit cell organized into 2 small cages and 6 large cages. N2 is known to form sII as N2 molecules occupy and stabilize the small cage of structure II.2 Considerable efforts have been made to develop efficient methods to extract methane from hydrate-bearing sediments. It has been investigated that the injection of CO2 into CH4 hydrate reserves could result in both CO2 sequestration and CH4 exploitation simultaneously.3-13 The hydrate equilibrium of CH4-CO2 gas mixture formed after CO2 injection has been extensively explored to confirm the feasibility of methane recovery by CO2 swapping method.8, 14-17 More recently, Park et al.18 proposed that the injection of CO2 + N2 gas mixture was more efficient than pure CO2 for recovering CH4 from hydrates; a significant increase of CH4 recovery efficiency up to 85% was observed. Following reports on the CH4 recovery by the exchange of CO2 + N2 mixture appeared later.19-25 The inclusion of N2 in the gas exchange process between CO2 and CH4 in hydrates could offer an additional control on hydrates equilibria.26-28 To analyze and understand this complex thermodynamic phenomenon, the complete hydrate phase behavior of the CH4-N2-CO2 mixture must be investigated to predict the exchange conditions and understand the mechanism occurring in the replacement in a gas hydrate reservoir. Lee et al.28 studied the hydrate phase equilibrium behavior of the CH4-N2-CO2 mixture with specific compositions according to the landfill gas characteristics to evaluate whether the landfill gas can be stored and transported in the form of hydrate. Kakati et al.26 investigated the phase equilibrium of CH4 + CO2 + N2 mixture hydrates in synthetic seawater for the handling, processing, and transportation of these gases as hydrates. However, these studies cannot provide effective information of the ternary hydrates equilibrium behaviors in the replacement process. In addition, the evolution of CO2 and N2 cage occupancies during the exchange remains unclear. Although Lee et al.28 presumed that the addition of N2 molecules considerably promoted the gas exchange as CH4 molecules in the small cages and 3



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large cages are preferentially replaced by the smaller N2 and the larger CO2 molecules, respectively. The competitive occupancy behaviors of the multi-guests CH4, N2 and CO2 in hydrate cavities are not yet well understood. Further work is needed to investigate these gas species occupancies in small or large cages in order to establish the details of this exchange process. The dissociation enthalpy of gas hydrates has been regarded as a critical factor to study the heat flow feature in the exploitation of natural gas hydrates

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, which is significant in the

replacement of CH4 by CO2-N2, because the generation, diffusion and flow of latent heat always accompany the formation, dissociation, and replacement processes of hydrates. Lee et al.22 measured the dissociation enthalpy of CH4 hydrate before and after the replacement by flue gas (CO2-N2) with specific compositions. Kakati et al.26calculated the dissociation enthalpy of CH4-CO2-N2 hydrates to study the dissociation behaviors. However, there are large differences between the results of the two works. Dissociation enthalpy data of CH4–N2-CO2 ternary hydrates are still needed with respect to the hydrate replacement process. In this study, the hydrate formation conditions of the CH4-N2-CO2 ternary gas mixtures with compositions similar to that of the gases coexisting in replacement recovery were measured in pure water over a wide temperature and pressure range. In addition, the cage occupancy behaviors of guest molecules were studied based on a thermodynamic model, which were helpful for a better understanding of gas exchange in CH4-N2-CO2 hydrates. Furthermore, the enthalpies of dissociation for the ternary gas hydrates were calculated with the application of Clausius–Clapeyron equation.

2. EXPERIMENTAL SECTION 2.1. Experimental apparatus A sapphire cell was used to observe the hydrate phase equilibrium behavior of the ternary gas mixture system considered in this work. The schematic diagram of the apparatus is shown in Figure 1. The cell consists of a cylindrical transparent sapphire tube (25.4mm in diameter and effective volume of 51mL) sealed by stainless steel flanges. Then the formation and dissociation of hydrates in the cell can be observed and recorded directly through the transparent sapphire tube. A magnetic stirrer was equipped to accelerate the hydrate equilibrium process. The whole apparatus was placed in an air bath. To maintain the compositions of gas mixtures during the experiment, the sapphire cell was connected with an equilibrium cell (maximum volume of 2 L) with a piston containing feed gas. The pressure in the sapphire cell can be controlled by a hand pump connected to the equilibrium 4


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cell. The uncertainties of temperature and pressure measurements in the system are 0.1 K and 0.02MPa, respectively.

Figure 1. Schematic diagram of the experimental apparatus. 2.2. Materials Analytical grade methane (99.99%), carbon dioxide (99.99%), and nitrogen (99.99%) were supplied by Beifang Special Gas Industry Corporation, which were used for the preparation of the (CH4 +N2 + CO2) ternary gas mixtures. 2.3. Experimental procedure The experimental procedure for hydrate equilibrium was similar to that in Chen’s works.29-32 First, the sapphire cell was washed more than three times with distilled water and a small quantity (2 to 4 ml) of deionized water was added into the cell. Then the whole system was vacuumized and purged four to five times with the gas mixture prepared to expel the air completely. And the air bath temperature was adjusted to the chosen temperature. After the temperature was stabilized, the sapphire cell was charged with gas mixture from the equilibrium cell. To shorten the induction time for the hydrate formation from pure water, a pretreatment process was operated as follows. The piston of the equilibrium cell was driven upward slowly with a hand pump(Figure 1). Thus the pressure in the cell was raised to a value much higher than the hydrate equilibrium pressure estimated and maintained for hydrate formation. After the formation of abundant hydrate, the pressure in the cell was decreased slowly by withdrawing the piston to ensure the thoroughly dissociation of the hydrate. The pretreatment procedure was repeated enough times before each 5



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experimental run. After the pretreatment, a ‘pressure-search’ method under isothermal conditions was adopted to measure hydrate phase equilibrium conditions of the ternary gas mixture as described below. 29-32 After the stabilization of the system temperature, the system pressure was increased to a value that was ~1MPa higher than the hydrate equilibrium pressure by compressing the gas mixture in the equilibrium cell with the piston. When an amount of hydrate crystal formed and existed in a long time range, the system pressure was decreased slowly to make the hydrate dissociate slowly. When the hydrate crystals disappeared completely, the system pressure was increased again with a small step of 0.05MPa until the hydrate crystal appeared in the aqueous phase again. If the hydrate crystal was observed after 6 h, the temperature and pressure in the cell can be determined as a hydrate equilibrium condition. If the hydrate crystal disappeared after 6 h, the system pressure was increased in steps of 0.05 MPa for the appearance of hydrate crystal and then maintained for another 6 h observation. To eliminate the change of gas composition resulting from the dissolution of CO2 in water, the gas phase was replaced three times with the gas feed in the equilibrium cell or the water in the cell was renewed before each experimental run. To obtain the hydrate equilibrium line, the above procedure of searching pressure was repeated under a series of assigned temperatures. In this manner, the hydrate formation conditions of (CH4 + N2 + CO2) gas mixtures with different compositions in pure water were measured.

3. CALCULATION OF GUEST OCCUPANCY IN CAGES Chen and Guo

33, 34

proposed a two-step mechanism for hydrate formation. According to their

theory, basic hydrate structure forms in the first step by chemical reaction between gas and water and then gases are adsorbed into the linked cavities in the second step. Therefore, two kinds of equilibrium, chemical reaction equilibrium and the physical adsorption equilibrium, exist in hydrate formation. For the first step, basic hydrate i is formed by a quasi-chemical reaction:

H 2 O + ∑ λ2,i G i → ∑ G i, λ2,i H 2 O i

(1)

i

The chemical equilibrium in the reaction is constrained by:

∑xµ i

i

B ,i

= µW + ∑ λ2,i µ g ,i

(2)

i

where µ B ,i is the chemical potential of basic hydrate i, µW and µ g ,i are the chemical potential 6


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of water and gas in basic hydrate i, respectively, and λ2,i is the mole ratio of gas molecule i to water molecule (large cavities) in basic hydrate. In the second step, the adsorption of gas molecules into linked cavities lowers the chemical potential of the basic hydrate i, µ B ,i , which could be expressed as:

µ B ,i = µ B0 ,i + λ1,i RT ln(1 − ∑ θ j )

(3)

j

where θ j is the occupancy of gas species j in linked cavities, µ B0 ,i represents the chemical potential of basic hydrate without adsorption ( ∑ θ j ), and λ1,i is the number of linked cavities per water molecule in the basic hydrate i. 33, 34. The adsorption in the second step complies with Langmuir theory, and θ can be calculated by:

θj =

Cj f j

(4)

1 + ∑ Ck f k k

where f j is the fugacity of gas j in hydrate phase, and C j denotes the Langmuir constant that is a function of both temperature and species of gas molecule j. In Chen-Guo model, a simple correlation of C j is applied as follows:

 Yj C j = X j exp  T −Z j 

  

(5)

The value of constants X j , Y j and Z j could be referred to Chen and Guo model33, 34. The chemical potential of gas species i in the fluid phase, µ g , could be calculated by:

µ g ,i = µ g0,i (T ) + RT ln fi

(6)

where µ g0,i denotes the chemical potential of gas species i at the ideal state. The establishment of hydrate equilibrium means that the two kinds of equilibrium of the chemical reaction equilibrium and the adsorption equilibrium are reached at the same time. The chemical potential of basic hydrate µ B ,i in the chemical equilibrium equals to that in the adsorption equilibrium. Combining eqs 2, 3 and 6, we obtain:

µ B0 ,i + λ1,i RT ln(1 − ∑ θ j ) = µW + λ2,i  µ g0 ,i (T ) + RT ln fi  j

7


(7)


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Eq 7 is rearranged as α

f i = xi f i 0 (1 − ∑ θ j )

(8)

j k

∑x

i

= 1.0

(9)

i =1

where xi is the occupancy of gas component i in large cavity, α =λ1 / λ2 , (for sI hydrate, α = 1/3; for sII hydrate, α = 2). f 0 is a function of T , P , aW , and the properties of guest molecules in basic hydrate. Therefore, f i 0 could be expressed as:

f i 0 = xi f iT0 (T ) f ( P) f (aW )

(10)

The effect of T on f i 0 is can be calculated by the following equations:

 Bi'  f i (T ) = A exp  '   T − Ci  0

' i

 D ( 273.15 − T )  '  Bi'  f i (T ) = exp   × Ai exp  '  T  T − Ci    0

T ≥ 273.15 K

T < 273.15 K

(11a)

(11b)

Because f i 0 is a function of species of guest molecules i in basic hydrate, Ai' , Bi' , and Ci' in eq 11 are distinct for different guest molecules, which could be fitted by experimental data. The constant D in eq 11b equals to 22.5 for sI hydrate, and 49.5 for sII hydrate, respectively. The effect of P on f i 0 can be calculated by:  βP f ( P ) = exp    T 

(12)

where β = ∆V (λ2 R ) . β equals to 4.242 K/MPa for sI hydrate and 10.224 K/MPa for sII hydrate, respectively. The effect of aW on f i 0 can be calculated by 33, 34:

f 0 (aW ) = aW−1/ λ 2 =( fW / fW0 )−1/ λ 2

(13)

As the interactions between the guest molecules in the linked cavities and in the basic cavities should be taken into account, Eq 11 for evaluating f iT0 (T ) can be corrected by:

8


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 −∑ Aijθ j  j f (T ) = exp  T   0 iT

  Bi'   ' A × exp  '   i  T − Ci   

(14)

where Aij are the binary interaction parameters for the interaction between guest components i and j. The Aij values for typical binary pairs can be referred to Chen and Guo.33, 34 In Chen-Guo model,

the interactions between guests are only considered for sII hydrate. The constants X j , Y j and Z j in Eq. (5) and the constants Ai' , Bi' , and Ci' in Eq. (14) for gas species of CH4, N2 and CO2 in this work are listed in Tables 1 and 2, respectively. The calculation procedure for predicting the hydrate formation conditions of the ternary gas mixture and CH4, N2 and CO2 molecules occupancies both in the large cavities (51262, L) and the small cavities (512, S) of hydrates are summarized in a flow diagram that is shown in Figure 2. Input T, yi

assign hydrate structure

Input initial

f

Calculate

θ

Calculate

P values

Calculate

f iT0 (T )

by Eq 14

Calculate

f 0 ( P)

by Eq 12

by EOS

by Eq 4

Calculate

f 0 (aW )

Calculate

Adjust

P

by secant method

NO

f − f

0

α

(1 − θ )

fi 0

by Eqs 13

by Eq 10

Hydrate phase equilibrium of CH4+N2+CO2 gas mixtures and cage

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