Experimental Investigation on Cyclopentane–Methane Hydrate

Dec 5, 2016 - †Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, ‡Guangdong Key Laboratory of New and Renewable Energy Rese...
1 downloads 10 Views 640KB Size
Subscriber access provided by GAZI UNIV

Article

Experimental Investigation on CyclopentaneMethane Hydrate Formation Kinetics in Brine Xueru Zang, Qiunan Lv, Xiao-Sen Li, and Gang Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02435 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 10, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 18

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

Energy & Fuels

1

Experimental Investigation on Cyclopentane-Methane

2

Hydrate Formation Kinetics in Brine

3

Xueru Zang†,‡,§,∥, Qiunan Lv†,‡,§, Xiaosen Li*,†,‡,§, Gang Li†,‡,§



4



5

of Sciences, Guangzhou 510640, PR China

6



7

Academy of Sciences, Guangzhou 510640, PR China

8

§

9

PR China

10 11

Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy

Guangdong Key Laboratory of New and Renewable Energy Research and Development, Chinese

Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640,



Nano Science and Technology Institute, University of Science and Technology of China, Suzhou

215123, PR China

12 13

ABSTRACT: Based on the hot brine in situ seafloor prepared for marine NGHs

14

exploitation, formation kinetic behavior of cyclopentane (CP)- methane hydrate was studied

15

at various depths of ocean water. The effects of the temperature, pressure, gas-liquid ratio and

16

ratio of CP/ liquid phase on gas uptake were investigated to reveal the affecting factors of the

17

hydrate rapid formation. The experimental results indicated that the driving force played an

18

important role in the hydrate formation. In addition, the CP/liquid phase ratio of 5% was

19

beneficial to gas uptake. When the conditions of driving force and the CP/liquid phase ratio

20

were very favorable for hydrate formation, the gas uptakes slightly changes with the

21

gas-liquid ratio. On the contrary, a smaller gas-liquid ratio was conductive to gas uptake.

22 23

1. INTRODUCTION

24

Gas hydrates are solid crystals formed from water and small gas molecules at low

25

temperature and high pressure. Water molecules are linked through hydrogen bonding and

26

they create cavities (host lattice) that can enclose a large variety of molecules (guests). There

27

is no chemical bonding between the host water molecules and the enclosed guest molecules.1,2

28

The characteristics of gas hydrates lie in that their empty hydrate lattice is just like an efficient

29

gas storage, which is evidenced by the fact that 1 m3 methane hydrates can be decomposed to

ACS Paragon Plus Environment

Energy & Fuels

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

30

160−180 m3 of methane gas. Hydrates have been detected in marine and permafrost region

31

throughout the world.3 Natural gas hydrates (NGHs) have the potential to meet the global

32

energy needs in the future. Therefore, the NGHs have been attracting considerable attention in

33

development of methods for producing natural gas from hydrates.

34

However, exploiting NGHs in safety and high efficiency is the key issue.4 The major

35

methods of gas production from NGHs reservoirs are depressurization, thermal stimulation

36

and chemical injection.5, 6 Each of these methods has its own advantages and disadvantages.

37

The depressurization method is demonstrated to be a high energy-efficient when the NGH

38

deposits are in conjunction with free-gas zone.7-9 However, it will not likely provide a steady,

39

reliable gas production. The faster gas production rate is seen in initial stage, the local

40

temperature will continuously drop while NGHs dissociate, and the dissociation of NGHs

41

might stop once the local temperature drops to the equilibrium temperature at a certain

42

pressure.10 The thermal injection method could effectively avoid the problems that emerged in

43

the depressurization process11-14, such as the low rate of gas production, blocking caused by

44

secondary hydrate formation or ice formation.15 However, the technical difficulty of thermal

45

stimulation method is that the heat would diffuse before it reaches the decomposition zone.

46

The heat loss leads to low efficiency of NGHs exploitation system.16 Among the three

47

methods, the chemical injection is considered to be a highly effective method for hydrate

48

dissociation,17 but it has the defects of large reagent consumption and high cost. And

49

large-scale use of chemicals will cause environmental pollution.18 In addition, Ohgaki et al19

50

proposed CH4-CO2 replacement in NGHs. This method has the problem that replacement rate

51

is extremely low and the CO2 hydrate will cover on the surface of NGHs, which leads to heat

52

and mass transfer barriers during the decomposition.

53

In this paper, we developed a novel technique to prepare hot brine in situ seafloor for

54

marine NGHs exploitation based on hydrate technology.20 The basic idea of this method is

55

described as follows: an apparatus is installed into situ seafloor, the apparatus is extended

56

from the surface of the sea to the seafloor, by adding hydrate formation agent into the

57

apparatus, hydrate forms rapidly at the conditions (pressure and temperature) in the seafloor.

58

Due to a lot heat released in the process of hydrate formation, the unreacted brine is

59

concentrated and heated to hot brine. Then, the hot brine prepared is expelled from the

ACS Paragon Plus Environment

Page 2 of 18

Page 3 of 18

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

Energy & Fuels

60

apparatus and injected into NGH deposits for exploitation. The solid hydrates float up to the

61

surface by its own buoyancy. Then it decomposes into water and the hydrate formation agent

62

at the temperature and pressure of sea surface. The hydrate formation agent can be recycled

63

due to its insoluble in water. This method combines thermal stimulation and chemical

64

injection and avoids heat loss during transiting the heat carrying medium from the ocean

65

surface to the NGH regions in conventional thermal stimulating methods. It is very important

66

to develop an economical, efficient technology of hydrate-forming by utilizing appropriate

67

hydrate formation agent. The appropriate hydrate formation agent must meet the following

68

requirements: non-toxic or low toxic and insoluble with seawater, the density of hydrate

69

formation agent is lower than that of brine so as to facilitate its recovery and recycling. Yan et

70

al.21 proved that CP is an effective hydrate formation additive that can accelerate gas hydrate

71

formation, and the hydrate formation enthalpy of CP - methane is as high as 130.25 kJ·mol−1

72

at 285 K. Therefore, in this study, CP is considered as appropriate hydrate formation promoter.

73

Recent years, there are many researchers studied the hydrate formation and dissociation

74

kinetics in fresh water and NaCl solution or seawater.11,22-27 The investigation into gas hydrate

75

formation of methane in a large-size bubble column reactor was carried out in the presence of

76

CP11. Kerkar et al.22 investigated the influence of pressure on methane hydrate formation and

77

dissociation kinetics in seawater. The kinetics of the CP–methane binary hydrate growth in

78

fresh water and brine solutions were studied by Cai.23,

79

formation rate in salt water was 2–3 times smaller than that in fresh water at small subcooling

80

degree (∆T