Natural Gas Hydrate Dissociation by Presence of Ethylene GlycolShuanshi Fan, Yuzhen Zhang, Genlin Tian,* Deqing Liang, and Dongliang Li

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

ReceiVed July 19, 2005. ReVised Manuscript ReceiVed October 11, 2005

In the current article, natural gas hydrate dissociation experiments have been conducted on a self-designedapparatus, which consists of a high-pressure reaction vessel, visualization system, chiller with circulation system,gas supplier, and data acquisition system. Gas hydrate was synthesized in the reaction vessel, and then thedissociation experiment was performed by addition of ethylene glycol (EG) as an inhibitor. The results showthat dissociation rate depends on the concentration and flow rate of EG because it reduced the dissociationheat, and less energy is required for dissociation at higher ethylene concentration. Thus, EG concentration andflow rate can be optimized in practical utilization.

Introduction

Gas hydrate is considered an important clean energy resource.More and more countries pay special attention to its researchand development.1 Now in China, energy consumption mainlydepends on coal, with a high percentage of 67%.2 China is thebiggest coal-consuming country, with a great amount ofdischarge of CO2. However, using oil and gas to replace coaland improve energy consumption is limited by resource reserves.Thus, looking for a new energy source is very important inChina. It is well-known that natural gas hydrate (NGH) is apotential energy resource with great amounts around the world.3

A primary survey and a series of investigations show theexistence of NGH reservoirs, which could be the clean energyof the future.4,5 To improve energy consumption and solveenergy demand problems, it is necessary and important toinvestigate the possible ways to develop NGH.

Many potential methods, such as heat exchange, depres-surization, and injection of thermoinhibitors, have been em-ployed to exploit NGH with respect to different conditions.6

For instance, in ocean reservoir development, injection ofthermoinhibitors is most feasible because of its easy operation.Although ethylene glycol (EG) is believed to be an inhibitor toprevent hydrate formation, we still lack detailed information of

experimental data, and in particular, the mechanism of dissocia-tion of NGH by ethylene glycol is not well understood yet.Another reason to study the influence of EG is because it canalso be used as an inhibitor to prevent hydrate formation duringnatural gas transportation and production.7 EG is widely usedin oil and gas industries to prevent hydrate formation in theproduction tubing and transportation pipes.8,9 Its concentrationand amount need to be improved with thermodynamic study.

Here we report the study of EG as an inhibitor for thedissociation of NGH by self-made setup. Using differentconcentrations and flow rates of injected EG, we attempted toobtain the NGH dissociation behaviors under various conditions.

Materials and EquipmentDistilled water is used in all the experiments. Gas mixture with

a composition of C3H8 4.96%, C2H6 4.03%, and CH4 90.01% isobtained from Foshan Huawen Gas Factory, China. Ethylene glycol(CR) is from Guangzhou Chemical Reagent Factory, China.

Experiment apparatus is designed and set up in our laboratory.It can be used for both hydrate formation and dissociation test.The schematic diagram is shown in Figure 1. The apparatus mainlyconsists of a gas supplier, a high-pressure transparent reactionvessel, a video camera, a temperature control system, a vacuumpump, and a data acquisition system.

Experimental ProceduresAfter the whole system is tested, experiments can be conducted

as following with several main steps: Vacuum the reaction vesselfor more than 30 min to completely remove the inner air, then setup chiller’s temperature at 0°C to reduce the temperature of thevessel, supply the gas with a small pressure gradient to 3.5-5 MPa,observe the reaction directly and through pressure change, and injectethylene glycol to observe the dissociation at different rates.

Results and DiscussionTo conduct dissociation experiments, NGH was synthesized

first by mixing gas and water at 0°C in the presence of a verysmall amount of SDS as an inducer. The reaction time is aslong as 7 days. The NGH sample is a white snowlike crystal,which was observed from the transparent reactor directly.

* Corresponding author. E-mail: tiangl@ms.giec.ac.cn.(1) Fan, S. S.; Chen, Y.; Liang, D. Overview on the development of

NGH. Mod. Chem. Eng.2003, 23, 1-5.(2) Guo, Y. T. Analysis and prediction of middle and long term coal

supply in China.Chin. Coal2004, 10.(3) Lee, S. Y.; Holder, G. D. Methane hydrates potential as a future

energy source.Fuel Process. Technol.2001, 71, 181-186.(4) Shipley, T. H.; Houston, M. H.; Buffler, R. T.; Shaub, F. J.; McMillen,

K. J.; Ladd, J. W.; Worzel, J. L. Seismic evidence for widespread possiblegas hydrate horizons on continental slopes and rises.Am. Assoc. Pet. Geol.Bull. 1979, 63, 2204-2213.

(5) Iseux, J. C. Gas Hydrates: Occurrence, Production and Economics.SPE Production Operations Symposium, Oklahoma City, OK, April 7-9,1991; Paper 21682.

(6) (a) Islam, M. R. A new recovery technique for gas production fromAlaskan gas hydrate. SPE 22924. (b) Staykova, D. K.; Kuhs, W. F.;Salamatin, A. N.; Hansen, T. Formation of Porous Gas Hydrates from IcePowders: Diffraction Experiments and Multistage Model.J. Phys. Chem.B 2003, 107, 10299-10311. (c) Sun, Z. G.; Ma, R. S.; Wang, R. Z.; Guo,K. H.; Fan, S. S. Experimental Studying of Additives Effects on Gas Storagein Hydrates.Energy Fuels2003, 17, 1180-1185. (d) Circone, S.; Stern, L.A.; Kirby S. H. The Role of Water in Gas Hydrate Dissociation.J. Phys.Chem. B2004, 108, 5747-5755. (e) Moon, C. M.; Taylor, P. C.; Rodger,P. M. Molecular Dynamics Study of Gas Hydrate Formation.J. Am. Chem.Soc.2003, 125, 4706-4707.

(7) Sloan, E. D. Natural Gas Hydrates.J. Pet. Technol.1991, 11, 1414-1417.

(8) Yousif, M. H. Effect of Underinhibition with Methanol and EthyleneGlycol on the Hydrate-Control Process.Soc. Pet. Eng., Prod. Facil.1998,13, 184-189.

(9) Tang, C. P.; Fan, S. S. Advancement on the research of new typesof NGH inhibitors.Chem. Eng. Pet. Natural Gas2004, 33, 157-159.

324 Energy & Fuels2006,20, 324-326

10.1021/ef0502204 CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 11/08/2005

For the dissociation test, water or EG solution is injected intothe vessel. Figure 2 shows the pressure changes during dis-sociation when different EG solutions are injected. Beforeinjection of EG solution, the free gas was removed to reducethe pressure for safety reason, which causes the pressure dropat the very beginning.

It can be seen that the pressure gradient is greater at the be-ginning, and then the slope decreases. This means that the dis-sociation rate at the beginning is faster because the amount ofNGH is more and there is no free water from NGH to dilutethe solution. The dissociation rates are nearly constant at thevery beginning and then reduce to nearly zero when the reactionis nearly finished.

Although dissociation is a dynamic process, it can also beenseen from Figure 2 that, with increase in ethylene glycolconcentration, dissociation rates also increase. This is becauseEG can reduce the dissociation heat. For example, when EGconcentrations are 10, 20, and 30%, dissociation heats are 73.35,65.16, and 60.65 KJ‚mol-1, repectively.10 The reduction ofdissociation potential energy makes NGH be “solved” quickly.It is known that other substances such as EG can also changephase transition conditions. These make the dissociation rateincrease with EG concentration.

To know the influence of EG on dissociation rate, averagedissociation rates have been drawn in Figure 3. It can be seen

clearly that with increasing EG concentration, the dissociationrate increases greatly.

To show the NGH system pressure behaviors upon the in-hibitor flow rate, we select the EG solution with a concentrationof 20%. Figure 4 shows the system pressure performance whenEG is injected at different flow rates. The system pressure wasdecreased to 0.8 MPa before the valve was closed and EG sol-ution was injected. Among the all of the tested experiments,the dissociation rate is fast at the beginning stage. This is becausethe lowest system pressure and only a little H2O from dissocia-tion exist. Thus, the injected EG solution is not diluted. But atthis stage the driving force for dissociation is the biggest in thewhole dissociation process. When dissociation continued, moreand more gas formed and resulted inincreasing pressure. Fur-thermore, the EG concentration decreases at dissociation inter-face because of the dilution by dissociated H2O. Therefore, itcan be seen that there is a plateau in the curve for every test. Itshould be noted that increasing the EG concentration offers asteeper curve, meaning that the shorter dissociation times areneeded in this case. But at the ending stage when all NGH isdissociated, the pressures are nearly all the same in the reactionvessel. Calculated from the data, the dissociation rates are dem-

(10) Fan, S. S.Technology of NGH storage and transportation; ChemicalIndustrial Press: Beijing, 2005.

Figure 1. NGH experiment system.

Figure 2. Dissociation curves of NGH by injection of ethylenesolutions.

Figure 3. Dissociation rate to ethylene glycol concentration.

NGH Dissociation by Presence of Ethylene Glycol Energy & Fuels, Vol. 20, No. 1, 2006325

onstrated to be nearly linear to the flow rate. They are 0.056,0.075, and 0.085 mol/min at the flow rate of 80, 100, and 120mL/min, respectively. The above result also shows that at sucha flow rate there is a fine contact for the solution and hydrate.Because dissociation rate depends on the flow rate for practicalutilization, the dissociation rate should be optimized in practicalutilization. From these experiments, it can be seen that ethyleneglycol can help hydrate dissociation a lot. Not only importantfor hydrate development, dissociation is also important to pre-vent natural gas from forming hydrate during production andtransportation.

With injection of EG, dissociation heat and energy can alsobe reduced. Figure 5a indicates the dependence of heat flowduring hydrate dissociation on heating and injection of EG at293 K. It can be seen that heat flow from heater grows at first,corresponding to the NGH with lowest temperature. It is thebig temperature gap between NGH and environmental water.However, the absorbed heat was consumed by the dissociation.Since the pressure increases and temperature goes down due tothe fast dissociation of NGH, the dissociation rate becomes slowand heat flow reduces. One can easily see that there is a suddenchange at 30 min, which indicates that NGH stops thedissociation at this point.

The total heat input by circulating water and EG varies withtime (Figure 5b). The heat flow increases very fast whendissociation starts and reaches an almost constant value after 3min. At this stage, the heat flow is mainly contributed to thedissociation. After 8 min, the heat flow starts to decreasesuddenly, meaning that the NGH stops dissociation. At the endthere is still 500 W heat left in the system.

From calculation, it is known that total energy used byheating is:

Total energy used by injection of EG is:

The energy cost by heating is more than 3 times as much asthat by injection of EG. It has been mentioned previously thatEG can reduce the dissociation heat, and therefore less energyand heat are needed. By using ethylene glycol as an inhibitoror production media, we can reduce both time and energy.

In the experiment, the amount of NGH for heating was 4.3mol and for injection of EG it was 5.1 mol, separately. Thetheoretic dissociation heat efficiency results are:

It can be seen that with injection of ethylene glycol, heatefficiency is nearly 3.5 as high as that by heating, meaning thatheat energy can be used more efficiently.

ConclusionsEG can accelerate the dissociation of natural gas hydrate,

mainly because it can reduce the dissociation heat. Dissociationrate can be promoted by increasing both EG concentration andflow rate. Therefore, it can be optimized in practical utilization.Injection of EG not only can increase dissociation rate, but alsocan save energy by increase in heat efficiency. This study isimportant not only for NGH development, but also for naturalgas transportation and production.

EF0502204

Figure 4. Relationship of dissociation rate with injection rate.

Qh ) heat flow× time ) 1500× 2400) 3600 kJ

Qi ) heat flow× time ) 2250× 480) 1080 kJ

Figure 5. (a) Heat flow by heating. (b) Heat flow by injection ofethylene glycol.

Qth ) 4.3× 74.12) 318.7 kJ

Eh ) Qth/Qh ) 318.7/3600) 10.3%

Qti ) 5.1× 74.12) 378.0 kJ

Ei ) Qti/Qi ) 378.0/1080) 35%

326 Energy & Fuels, Vol. 20, No. 1, 2006 Fan et al.