hydrate formation of (ch4 + c2h4) and (ch4 + c3h6) gas mixtures

Fluid Phase Equilibria 191 (2001) 41–47

Hydrate formation of (CH4 + C2H4) and(CH4 + C3H6) gas mixtures

C.-F. Ma, G.-J. Chen, F. Wang, C.-Y. Sun, T.-M. Guo∗High Pressure Fluid Phase Behavior and Property Research Laboratory,

University of Petroleum, Beijing 102200, PR China

Received 4 June 2001; accepted 9 August 2001

Abstract

Hydrate formation conditions of (CH4 + C2H4) and (CH4 + C3H6) binary gas mixtures in the presence of purewater were measured in a sapphire cell using the “pressure search” method. The experimental temperature-rangewas 273.7–287.2 K, and pressure-range was 0.53–6.6 MPa. Ethylene content in the gas mixtures varied from 7.13to 100 mol%, and the propylene content varied from 0.66 to 71.96 mol%. The Chen–Guo hydrate model has beensuccessfully applied to represent the measured data. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Hydrate formation; Data; Gas mixture; Methane; Alkene

1. Introduction

(Methane+ alkene) and (methane+ hydrogen) mixtures occur widely in the petroleum refining andpetrochemical processes. Recently, a promising method for separating hydrogen from methane throughhydrate formation/dissociation was developed in our laboratory [1]. For exploring the feasibility of ex-tending this new technology to (methane+ alkene) gas mixtures, basic hydrate formation data are re-quired. Some earlier work related to the initial hydrate formation conditions of (methane+ethylene) and(methane+propylene) mixtures are available [2,3], but they are not sufficient for our project. In addition,the experimental data were presented in figures, which are inconvenient for practical applications. Inthis work, we have systematically measured the hydrate formation data of (methane+ ethylene) and(methane+ propylene) gas mixtures.

Five synthetic (methane+ ethylene) and four (methane+ propylene) binary gas mixtures have beenprepared. The hydrate formation conditions of those gas mixtures and pure ethylene were measured byusing the “pressure search” method [4]. The experimental temperature-range was 273.7–287.2 K, andpressure-range was 0.53–6.6 MPa. The ethylene and propylene contents in the mixtures varied from 7.13

∗ Corresponding author. Tel.:+86-10-6234-0132; fax:+86-10-6234-0132.E-mail address: [email protected] (T.-M. Guo).

0378-3812/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0378-3812(01)00610-0

42 C.-F. Ma et al. / Fluid Phase Equilibria 191 (2001) 41–47

to 100 mol%, and 0.66 to 71.96 mol%, respectively. The recently developed Chen–Guo hydrate model[5] was used to represent the measured data.

2. Experimental

2.1. Apparatus

The experimental apparatus used in this work had been described in detail in the previous paperspublished by this laboratory [6–8]. The apparatus consists of a cylindrical transparent sapphire cell(2.54 cm in diameter, effective volume 60 cm3) installed in an air–bath and equipped with a magneticstirrer for accelerating the equilibrium process. The formation/dissociation of the hydrate crystals in thesolution can be observed directly through the transparent cell wall. The accuracy of temperature andpressure measurement is±0.2 K and±0.025 MPa, respectively.

2.2. Materials and preparation of samples

Analytical grade methane (99.99%), ethylene (99.95%) and propylene (99.95%) supplied by BeifenGas Industry Corporation were used in preparing the synthetic gas mixtures. The compositions of gasmixtures were analyzed by a Hewlett-Packard gas chromatograph (HP 6890).

2.3. Experimental procedure

Firstly, the transparent cell was washed by distilled water and then rinsed three times with deionizedwater. After the cell was thoroughly cleaned,∼10 cm3 deionized water was added into the cell. The vaporspace of the cell was purged with the gas mixture under study. A gas sample was collected and analyzedto ensure the absence of air. The air-bath temperature was then adjusted to the chosen temperature. Oncethe temperature was stabilized, the following “pressure search” method [4] was applied to determine thehydrate formation conditions.

The pressure in the cell was raised to∼1 MPa higher than the estimated equilibrium pressure (usingan in-house software) via the floating piston. When a trace of hydrate crystal was observed, the pressurewas reduced gradually to allow the hydrate crystals decompose slowly. When all the hydrate crystalsdisappeared, the pressure of the system was raised again by a small pressure-step of 0.05 MPa until thehydrate crystal appears again (clinging to the cell wall or floating on the water surface). Maintain thesystem temperature and pressure for 6 h, if the hydrate crystals disappeared during this period, the pressureof the system was raised slightly until a trace of hydrate crystals appeared again. When the hydrate crystalsare kept in the cell after 6 h, the system pressure is taken as the equilibrium hydrate formation pressureat the given temperature. The above procedure was repeated for a series of temperatures.

2.4. Experimental results

Following the above procedure, the initial hydrate formation data (in the presence of pure water)of pure ethylene, five (methane+ ethylene) and four (methane+ propylene) gas mixtures have beenmeasured.

C.-F. Ma et al. / Fluid Phase Equilibria 191 (2001) 41–47 43

Fig. 1. The comparison of the hydrate formation data measured for pure ethylene: (�) this work; (�) data of Snell et al. [2].

The comparison of the measured hydrate formation data of pure ethylene with those data reported bySnell et al. [2] is depicted in Fig. 1. From Fig. 1, it can be seen that the agreement between the two datasets is excellent.

The compositions of the five (methane+ethylene) and four (methane+propylene) gas mixtures studiedand the corresponding hydrate formation data measured are presented in Tables 1 and 2.

Table 1Hydrate formation conditions of (methane+ ethylene) gas mixtures

Composition of gas mixture (mol%) T (K) P (MPa)

100% C2H4 273.7 0.665275.2 0.739277.2 0.920278.2 1.010279.2 1.100281.2 1.439283.2 1.838285.2 2.345286.2 2.830287.2 3.210

5.60% CH4 + 94.40% C2H4 273.7 0.712278.2 1.178281.2 1.592283.2 1.956286.2 2.916

34.09% CH4 + 65.91% C2H4 273.7 0.784278.2 1.292281.2 1.755283.2 2.220286.2 3.115


Table 1 (Continued )


64.28% CH4 + 35.72% C2H4 273.7 1.146278.2 1.875281.2 2.406283.2 3.120

85.69% CH4 + 14.31% C2H4 273.7 1.800278.2 2.714281.2 3.758283.2 4.640

92.87% CH4 + 7.13% C2H4 273.7 2.230278.2 3.448281.2 4.720283.2 6.002

Table 2Hydrate formation conditions of (methane+ propylene) gas mixtures


28.04% CH4 + 71.96% C3H6 273.7 0.529a

278.2 1.081a

281.2 1.515a

283.2 1.963a

92.40% CH4 + 7.60% C3H6 273.7 1.081278.2 1.765281.2 2.501283.2 3.161

96.60% CH4 + 3.40% C3H6 273.7 1.421278.2 2.381281.2 3.287283.2 4.121

99.34% CH4 + 0.66% C3H6 273.7 2.531278.2 3.681281.2 5.179283.2 6.585

a Four-phase equilibrium (V–Lw–LHC−H).

3. Data processing

Chen–Guo hydrate model [5] was used to represent the experimental data measured in this work. Therequired parameters for (methane+ ethylene) system are available in [5]. In our calculation, (methane+ethylene) gas mixtures were presumed to form structure I hydrates in the full composition range. Asthe anomalous increase of deviation from experimental data was not observed (which might occur for


Fig. 2. Hydrate formation conditions of (methane+ ethylene) gas mixtures: (�) pure ethylene; (�) M1; (�) M2; (�) M3; (�)M4; (�) M5; (+) pure methane [10]. Calculated by Chen–Guo hydrate model [5].

(methane+ethane) system due to the hydrate structure change from I to II at higher methane concentration(>62 mol%) [9]), we believe that the structure transition will not be happened in the hydrate formationfor (methane+ ethylene) system. Certainly, this presumption should be further confirmed by using morereliable experimental methods such as Raman spectra and NMR technique [9]. The calculation resultsfor (methane+ ethylene) system are illustrated in Fig. 2. The overall average absolute deviation (AAD)of the calculated hydrate formation pressures is 2.48%.

For the (methane+ propylene) system, as the hydrate-vapor–water-rich liquid equilibrium of(propylene+ water) system exists over a very narrow temperature range 273.15–274.50 K, the purepropylene hydrate formation data are not sufficient for correlating its parameters (A′, B′ andC′) to beused in the Chen–Guo hydrate model [5]. Thus, propylene’s parameters were determined indirectly fromthe hydrate formation data of (methane+propylene) system measured in this work. As propylene cannotform structure I hydrate, structure II was assumed in the regression. The regressedA′, B′, C′ values forpropylene are listed in Table 3 along with the binary cross parameterAij value for (methane+propylene)system. The calculated results are illustrated in Fig. 3. The overall AAD of the calculated hydrate for-mation pressures for (methane+ propylene) system is 3.36%. The hydrate formation pressure data of

Table 3Parameter values used in the Chen–Guo hydrate model

Gas species A′ (Pa) B′ (K) C′ (K) Aija

Ethyleneb (structure I) 4.8418× 1016 −5597.59 51.80 0Propylene (structure II) 2.4256× 1031 −18421 −42.35 506

a Binary cross parameterAij for (methane+ ethylene) and (methane+ propylene) systems.b Parameter values adopted from Chen and Guo [5].


Fig. 3. Hydrate formation conditions of (methane+ propylene) gas mixtures: (�) M1; (�) M2; (�) M3; (�) M4; (�) purepropylene [3]; (+) pure methane [10]. Calculated by Chen–Guo hydrate model [5].

pure propylene have been predicted also and compared with literature data [3] as shown in Fig. 3. Themaximum absolute deviation between predicted values and literature data is less than 3%.

4. Conclusions

The hydrate formation data of five synthetic (methane+ ethylene), four (methane+ propylene) gasmixtures and pure ethylene gas have been measured in the temperature range of 273.7–287.2 K andpressure range of 0.53–6.6 MPa using the “pressure search” method. The uncertainty of temperature andpressure measurement is±0.2 K and±0.04 MPa, respectively. The reported data are valuable for testingexisting hydrate models/software.

Chen–Guo hydrate model was used to represent the experimental data measured in this work andsatisfactory calculation results were obtained.

Acknowledgements

The financial support received from the National Natural Science Foundation of China (No. 29806009)and the China National Petroleum and Natural Gas Corporation is gratefully acknowledged.

References

[1] C.-F. Ma, Separation of Gas Mixtures using Hydrate Technology, Ph.D. Thesis, University of Petroleum, 2001.[2] L.E. Snell, F.D. Otto, D.B. Robinson, AIChE J. 7 (1961) 482–485.[3] F.D. Otto, D.B. Robinson, AIChE J. 6 (1960) 602–605.


[4] D.-H. Mei, J. Liao, J.-T. Yang, T.-M. Guo, Ind. Eng. Chem. Res. 35 (1996) 4342–4347.[5] G.-J. Chen, T.-M. Guo, Chem. Eng. J. 71 (1998) 145–151.[6] S.S. Fan, T.M. Guo, J. Chem. Eng. Data 44 (1999) 829–832.[7] D.-H. Mei, J. Liao, J.-T. Yang, T.-M. Guo, J. Chem. Eng. Data 43 (1998) 178–182.[8] S.-X. Zhang, G.-J. Chen, C.-F. Ma, L.-Y. Yang, T.-M. Guo, J. Chem. Eng. Data 45 (2000) 908–911.[9] S. Subramanian, R.A. Kini, S.F. Dec, E.D. Sloan Jr., Chem. Eng. Sci. 55 (2000) 1981–1999.

[10] E.D. Sloan, Clathrate Hydrates of Natural Gases, 2nd Edition, Marcel Dekker, New York, 1998.

hydrate formation of (ch4 + c2h4) and (ch4 + c3h6) gas mixtures

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