INFLUENCE OF CHEMICAL ADDITIVES ON GAS HYDRATE FORMATION

第54卷增刊学报Vol 54 SuppL.2003年12月Journal of Chemical Industry and Engineering (China)December 2003INFLUENCE OF CHEMICAL ADDITIVES ONGAS HYDRATE FORMATIONTANG Cuiping and FAN Shuanshi(Center for Gas Hydrate Research, Guangzhou Institute of Energy ConueChinese academy of science, Guangzhou 510070, Guangdong, ChinaAbstract One surfactant as sodium dodecyl sulfate(SDS)and one synthesized sample as gas hydratenhibitor are introduced in this paper Through experiments we prove sodium dodecyl sulfate can acceleratethe formation rate of gas hydrate and the synthesized sample can inhibit the formation and growthKeywords natural gas hydrate, chemical additive, kinetic inhibitorINTRODUCTIONenhances safety. But there are still some problemsto prevent the industrialization of this storageNatural gas hydrates are crystalline inclusion method, next are some reasons: the formation ofcompounds formed by water and small hydrocarbon gas hydrate is extremely slow: when hydratemolecules. Through hydrogen bonding, water particles form, much free water is trapped in themolecules form a lattice structure like a cage. The interstices of particles, therefore, free waterhydrate lattices contain large vacancies or cavities, between particles will occupy much of thewhich are at least partially occupied by small gas expensive storage tank; the slurry separation andnolecules. The force between water and gas packing of hydrate particles formed in anolecules is van der waals [1.2]mechanically-stirred reactor seems difficultNatural gas hydrates have a great advantageBy agitation of the water, the water-gasfor natural gas storage but are formed slowly at interfacial area is increased and continuallylow temperature and high pressure To meet therenewed to improve hydrate formation rate [4demand, researchers are trying different methods Although satisfactorily high hydrate formationto change the formation conditions or formationrates can be achieved with a stirred system to serverates of natural gas hydrates.in an industrial-size process, inherent problemsGas hydrates used as storage have anmake it impractical. Energy costs from stirringincreasing advantage. 180 vol-gas/vol-hydrateincrease as slurry thickens In fact, thickeningcould be stored 3. Some positive factors are asof a stirredstem maylimit the hydfollows: safety of storing natural gas in hydrates mass in the water to as low as 5% [5, at whichis unmatched, for gas will be essentially encased intime filtering or clarification would be necessaryice; gas can be stored in the hydrates at relatively The separation of hydrates from the slurry requireslow pressure; the slow release of gas fromadditional labor as does packing the hydrates in ahydrates in the events of storage-tank rupturestorage vesseL. Problems with mechanical stirringital and maintenance costsCorresponding Author: FAN Shuanshi. ProfeWheth中国煤化工 red system,frehydrate@ms,giecac.cnFoundation item: supported by the National Natural ScienwaterCNMHGed between solidFoundation of China (No. 50176051)and the Statehydrate particles [6). Englezos found only 1.4%-Program for Basic Research of China(N. 2000026306)14% of the water after hydrate formation in an·38化工学December 2003experimental cell was bound in the hydrate inhibitors(KI). KI can be more effective at a lessstructure while most of the water was trapped concentration which will be less than 5% by massbetween solid particles [7Jand more economicMany researchers are trying to solve these1 EXPERIMENTproblems. In America Zhong and Rogers foundMicellar solutions could increase gas hydrate 1. 1 Experimental apparatus and procedureformation rate and alter formation mechanism forThe apparatus schematic is shown in fig. 1ethane and natural gas hydrates in 2000[3. In The apparatus consists of a high-pressureAustralia nimalan gnanendran and Robert Amin reservoir, a stainless steel reactor with a volume ofdiscussed hydrate promotion capabilities of 141 cm, a temperature-controlled water bathhydrotrope para-toluene sulfonic acid(P-TSA) in (rating 268 15--368. 15 K; accuracy: +0. 05 K;20038. In China Sun Zhigao proved an anionic model: SDC-6)manufactured by Ningbo Tianhengsurfactant, a nonionic surfactant, their mixtures Instrument Factory, and a data acquisition system.and cyclopentane could improve the hydrateaBas checkformation of a synthetic natural gas in a quiescentsystem and that the effect of an anionic surfactanton natural gas storage in hydrates was morepronounced compared to the effect of a nonionsurfactant(9. In this paper the surfactant will be used.However, the formation of gas hydrate shouldbe prohibited at the transportation of oil or naturalgas. There is an increasing tendency to exploitcylinder cylinderhydrocarbon reserves at increasing depths in theoceans with increasing distances for pipelineFig 1 Schematic of experimentalboth horizontal and verticalapparatus for hydrate formationdirection, and it is easy for water-containingPressure sensor is used to measure pressure ofhydrocarbon fluid to form hydrates in those the system from Shanghai Tianmu Instrrumentconditions. Once hydrate particles gather, it will Factory. A model Do8-80BM/ZM mass gasbecome a problem in transportation lines [1. n. So we flowmeter from Beijing Jiangzhong Machineryalso need some inhibitor hinder formation of hydrates. Factory is used to measure the gas added to theFormation of gas hydrates can be eliminated or reactor during hydrate formation and rTD tohindered by several methods. Traditionally record the temperature of the systemhermodynamic hydrate inhibitors such asThe gas mixture contains 91 98%(vol)methanol, glycol or electrolyte were used to methane, 5.02%(vol)ethane and 3%(vol)change the equilibrium condition of hydrate, so propane supplied by Foshan Kody Gas Chemicalthat hydrates could not be formed in the condition Industry Co., Ltd. In all test the de-ionized waterwhich hydrate should have been formed. The is prepared in our labeffectiveness of these inhibitors is well known, but 1. 2 Experimental procedurelarge concentrations are needed, which sometimesBefore an experimental run, we follow theimpact project economics. Some additives suchfolle中国煤化工 clean thee reactor bythanol are harmful to the environment. SinceCNMHGthen charge with1990, many researchers have worked for replacing quanthe thermodynamic hydrate inhibitors with kinetic Secondly connect the reactor to the system, turIVoL 54 Suppl.Journal of Chemical Industry and Engineering ( China)39on the temperature-controlled water bath (water inthe bath containing some glycol), vacuumize theT in bath: 273.65 Ksystem when the temperature of the bath reacheshe set point. Close thecontrol valve connected onhe reactor whevacuumizing of the system, open the control valveconnecting the reactor, then close it right now,9少少以快分妇repeat this procedure for about five times. Thenminwe consider the air is got rid of in the system.Next we can turn on the pressure regulator andsupply the system with proper pressure gas. In alltests the pressure near to the reactor is recorded byT in bath: 273. Kthe data acquisition system.2 RESULTS AND DISCUSSION62The surfactant using in the tests is sodiumdodecyl sulfate. Fig. 2 shows the relation of0183367550733971100t/minpressure to time with or without the surfactant.We choose that point as o point when the pressurereaches the set point in the beginning, theFig 2 Diagram of pressure to time with (a)oremperature of the bath keeps the constant atthout(b) the surfactant for hydrate formation273. 65 K, the liquid added to the reactor is 30 ml or not drop if the kinetic inhibitor is added into thed additives’ concentration Is300mg·kghreactor, but this method need very sensitiveThere is magnetic stirrer in the reactor. From theflowrate meter. In our experiment we use pressurefig 2, we can get some information: the pressure direction. When the system is stable, we close thedrops rapidly and falls down from 6. 56 MPa topressure regulator connected on the gas cylinder4. 40 MPa after 55 min in the test with sds, butand effective inhibitors can hinder the hydratethe pressure drops slowly and falls down for about formation or hydrate particles agglomeration. So0.2 MPa after 55 min in the test only with DI there is not pressure drop, or there is not evidentwater,even after 1200 min the pressure drops is Pressure drop. Of course the pressure will dropabout 0.5 MPa. In the end we open the reactor and with the hydrate formation after long time.find that all of the liquid with the surfactant hasIn fig 3 we choose that point as o point whenbecome solid and agglomerated together tightly.the pressure reaches the set point in the beginningWhile hydrate lump is dry except on the top after the temperature of the bath keeps the constant at55 min. After 1500 min the liquid without a 273. 65 K, the liquid added to the reactor is 30 mlsurfactant has also become hydrate lump, but the and inhibitors'concentration is 1% by mass.lump is loose, soft and watery, and there is still Inhibitors synthesized by myself. There issome water unreacted in the reactor. So we can get magnetic stirrer in the reactor. From the fig 3, weSDS accelerate the hydrate formation rate andcaninformation: first there is aincrease gas storage ability in some time.SSO中国煤化工 pressure drop isMany methods can be used to test the effect of rapidCNMHG, we can only see adirection. The curve of flowrate can drop smoothly the a beginning curve in the figure. Althoughkinetic inhibitors. Usually we use flowrateverticalginning pressures arerent, but we sti·40·化工December 2003know the pressure drops fast in the reactionThe synthesis sample can effectively inhibitithout the synthesis sample and that pressure hydrate formation and hydrate agglomeration. Indrop reaches 0. 5 MPa after 450 min, however the the reaction without the synthesis sample thepressure drop is only 0. 21 MPa after 450 min in the pressure drop reached 0.5 MPa after 450 minreaction with the synthesis sample. Both reactors however the pressure drop is only 0. 21 MPa afteropen after 1200 min, there is hydrate lump 450 min with the synthesis sample.formation in the reactor without the synthesisChemical additives can influence the state ofsample and the lump is loose and soft, but with the gas hydrate.synthesis sample there is no hydrateReferencesformation and hydrate particles are flowing slurry.o the synthesis sample can inhibit hydrate1 Liao Jian, Mei Donghai, Yang Jitao, Guo Tianmin. Study onformation and hydrate agglomeration.Phase Equilibrium for gas hydrate formation, Natural GasIndustry, 1998692 Rogers R E. Natural Gas Hydrate Storage Project. FinalT in bath: 273.65 K6日3 Zhong Y, Rogers R E. Surfactant effects on gas hydrateformation. Chemical Engineering Science, 2000, 55the synthesized sample4175-4187Narita H, Uchida T. Studies on formation/ dissociation rates ofmethane hydrates, In: Second international symposium on gashydrates, Toulouse, 1996. 191-1975 Vysniauskas A, Bishnoi P R. a kinetic study of methanedrate formation. Chemical Engineering Science, 1983, 38:1061-19726 Rogers R E. Natural gas hydrates storage project. IConceptual design and economic study. Final report°器禺浔子§西員器器器Department of Energy, 19997 Englezos P. Nucleation and growth of gas hydrate crystals inrelation to kinetic inhibition In, second internationaFig 3 Relation of pressure to time with or withoutsymposium on gas hydrates, Toulouse, 1996. 147-153inhibitors for hydrate formation8 Nimalan gnanendran, Robert Amin. The effect of hydrotropeson gas hydrate formation. Journal of Petroleum Science and3 CONCLUSIONSEngineering, 2003, 40: 37-46Sun Zhigao, Wang Ruzhu, et al. NaturalThe surfactant SDS can accelerate the hydratehydrates with the presence of promoters, Energy Conversionand management, 2003, 44: 2733--2742formation rate and increase gas storage ability.10 Ugur Karaaslan, Mahmut Parlaktuna PEO-A New HydrateThe pressure drops for about 0. 2 MPa after 55 minInhibitor Polymer. Energy &Fuels,2002, 16:1387--1391in the test with SDS and the pressure falls dowr11 Malcolm A Kelland Thor M Svartaas, et al. A New Class ofKinetic Hydrate Inhibitor. In: Annals New York Academy offor about 0.2 MPa after 55 min in the test with DISciences, 281--293water, even after 1200 min the pressure drop isabout 0. 5 MPa中国煤化工CNMHG