Research & Reviews: Journal of Chemistry

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1 Research & Revews: Journal of Chemstry Hydrate Rsk Evaluaton durng Transport and Processng of Natural Gas Mxtures contanng Ethane and Methane Kvamme B* and Sapate A Department of Physcs and Technology, Unversty of Bergen, Norway Research Artcle Receved date: 22/07/20 Accepted date: 23/09/20 Publshed date: 2/09/20 *For Correspondence Bjørn Kvamme, Department of Physcs and Technology, Unversty of Bergen, Norway; Tel: E-mal: bjorn.kvamme@ub.no Keywords: Natural gas hydrates, Ppelne transport, Rsk analyss, Rust ABSTRACT Transport of hydrocarbon on the seafloor of the North Sea nvolves condtons of methane hydrate formaton for most of the transport condtons from delvery to the recevng end. Hydrate stablty regons are further extended through addtonal content of ethane. The crtcal queston s therefore whether water wll drop out from the gas and how t wll drop out. Water can obvously condense out as lqud water, as has been the usual bass for hydrate rsk evaluaton schemes. Ppelnes are rusty even before they are placed out on the seafloor and the queston s f the water wll beneft from droppng out onto these rusty surfaces at lower concentratons than dew-pont concentratons for the same system at local temperatures and pressures. In ths work we have used state of the art theoretcal models to estmate maxmum water content before condensaton, and smlar for adsorpton on Hematte (rust). It s found that the maxmum content of water that would be permtted for dew-pont s more than 8 tmes than what would be permtted f adsorpton on rust was the crtera for water drop-out. These ratos do not change sgnfcantly by addng ethane but the absolute tolerance lmt for water mole-fracton s reduced. It s therefore recommended that dehydraton unts should be dmensoned accordng to estmated maxmum water content before adsorpton on rusty surfaces. Hematte s a domnant form of rust but t s stll recommended that smlar analyss s conducted for Magnette and Iron oxde. INTRODUCTION Clathrate hydrates formed from hydrocarbons and water commonly called gas hydrates (GH), are ncluson compounds formed by the combnaton of hydrogen bonded water molecules formng cavtes, and sutably szed molecules fllng these cavtes. Hydrates are formed when consttuents come n contact wth each other under favorable hydrate formng condtons of low temperature and hgh pressure. Due to the huge amounts of methane trapped n ce-lke form as hydrates, GHs are recognzed as fuel of the future. Most of the natural gas hydrate s trapped under sealng formatons (normally clay or shale) n sedments, or permafrost wth varyng addtonal sealng n addton to frozen layers above the hydrate zones. The orgn of the methane n natural gas hydrates s, n most natural occurrences, from bogenc degradaton of organc materal n the upper few hundreds of meters of the crust. These hydrates are typcally very rch n methane, n contrast to hydrocarbons released from thermogenc degradaton of organc materal deep down n the crust. Dfferent orgn of hydrocarbons offshore Norway s reflected n the composton of natural gas beng produced. Gas from Troll s very rch n methane and the lmted amount of ethane and heaver hydrocarbons has not been consdered as valuable enough to justfy a costler (nvestment and operaton) separaton plant for very low temperature separaton. The Troll separaton plant therefore s a smple so-called dew-pont separaton plant, n whch the lowest temperature s -22 C at 9 bar. Ths s n contrast to gas from Kvtebjørn. Ths gas s separated on the same locaton (Kollsnes outsde Bergen n Norway) but s a separate plant. The hgher content of ethane and heaver justfes a more complex process plant whch goes down to -70 C. For the Troll plant the fnal separator s one of the crtcal ponts n terms of

2 possble hydrate and ce formaton due to the low temperature and hgh concentraton of ethane n the lqud outlet from the fnal separator. Snce content of propane and hgher s very low we mght expect a methane domnated structure I hydrate n the gas phase and structure I domnated by ethane n the lqud phase out from ths separator. Any lmted content of propane wll lead to some structure II hydrate but lkely small amounts n the Troll separaton system. In addton to methane (CH ), GHs contan other lght hydrocarbons lke ethane (C 2 H ), propane (C 3 H 8 ), butane (C H 0 ) etc. and non-hydrocarbons such as carbon doxde (CO 2 ), Hydrogen sulfde (H 2 S), Ntrogen (N 2 ) etc []. These guest molecules enter hydrate as supportng stablzaton whle some of them are unable to make hydrate as pure components at these condtons, lke for nstance N 2. The composton of natural gas vares from feld to feld and regon to regon. Although more hydrate structures had been dscovered durng the latest two decades there are three structures whch are the most common and well known; structure I, structure II and structure H. Unt cell of si hydrate s made up of water molecules and contans two small and sx large cavtes, small cage Pentagonal dodecahedron ( 2 ) whle large cage s a Tetrakadecahedron ( 2 ). A cubc unt cell has dmenson of about 2.0 Å at 0 C. Unt cell s the smallest symmetrcal unt crystal whch s repeated n all volumetrc dmensons nto macro crystals. One-unt cell of structure II consst 3 water molecules and has 8 large and small cavtes. Large cage s Hexakadecahedron ( 2 ) arranged n terms of faces, made up of -sded cavty wth 2 pentagonal faces and four hexagonal faces. The cubc unt cell dmenson s 7.3 Å at 0 C. In all of the three structures, the small cage s a Pentagonal dodecahedron ( 2 ) snce t has the shape of 2 pentagonal faces []. Structure H s rarer and we could not fnd any references to fndngs of structure H n ndustral hydrate plugs formed durng processng or transport. Formaton of these varous hydrate structures depends on the avalable space n cavtes compared to sze of molecules enterng these. The actual stablzaton depends on short range nteractons, so-called van der Waal type nteractons, and n some cases also coulombc nteractons between partal charges n guest molecules and host molecules n the lattce wthout any chemcal bondng. An example Hydrogen sulfde, whch has a dpole moment whch s strong enough to have sgnfcant coulombc attractons towards water n the lattce but weak enough to ensure that the water lattce do not collapse [2]. Generally small guest molecules such as methane, Ethane can form both structure I and structure II hydrate, but the sze of the large cavty s very benefcal for ethane. Pure components of the two, as well as mxtures of these wll predomnantly form structure I hydrate. Propane (C 3 H 8 ) s too large to ft nto the large cavty of structure I and only forms structure II hydrate through occupaton of the large cavty n that structure. The small cavty of that structure s very smlar to the small cavty n structure I and s well stablzed by occupaton of methane. Mxtures of methane and propane wll therefore form structure II hydrates ntally but when all of propane s extracted from the gas over to hydrate the remanng methane wll form structure I hydrate. Long term transportaton of natural gas to reasonably close markets s normally accomplshed by ppelnes, as for example wth most of the ppelnes transportng natural gas from felds offshore Norway and also ncludng ppelne transport to Germany for further dstrbuton. Shppng of natural gas as Lquefed Natural gas (LNG) system or Compressed Natural Gas (CNG) system s normally costler and dedcated to destnatons far away. LNG from Snøhvt offshore the north of Norway, as one example, was targeted towards the USA market. After USA started large scale producton of gas from shale ths market dsappeared. Most of the condtons durng separaton of natural gas n a plant lke for nstance the Troll plant s nsde the hydrate formaton regon of temperature and pressure f free water s avalable, or f water can drop out from the gas as separate phase(s). Transport of natural gas from Norway to the contnent nvolves hgh pressures and low temperatures as well. Typcal delvery pressure for transport s bar. Pressures on recevng ends vary, dependng on desred transport velocty on the recevng end and further nfrastructure for processng, petrochemstry or transport dstrbuton. The mnmum pressure range used n ths work corresponds to 0 bar. Ths temperature s somewhat random but based on transport lnes to further dstrbuton and/or further processng. Smlar analyss for lower mnmum pressures n the hydrate formaton regon s of course trval along the lnes descrbed n ths paper. Temperature outsde the ppelnes on the seafloor s rarely above C and also well wthn hydrate formng condtons f a separate water phase s avalable or can drop out from water dssolved n the gas. Gven these ranges of condtons then methane hydrate wll form f water drops out from the gas. Although methane s the prmary component of natural gas t may, as dscussed above, contan other lght hydrocarbons and non-hydrocarbons. Ntrogen has lmted stablzaton effect on the hydrate cavtes and represents rather a dluton of the natural gas, whch alters the hydrate formaton to hgher pressures and/or lower temperatures. Hydrogen sulfde, on the other hand, stablzes the hydrate very effcently so even lmted amounts of hydrogen sulfde shfts the hydrate formng condtons to lower pressures and/or hgher temperatures. Normally water s followng the hydrocarbon stream from the producng well. Water s separated from the hydrocarbon stream by a three phase separator. The mnmum amount of water followng the hydrocarbons further nto the processng wll be saturaton concentraton before the water phase was removed. Typcally, some addtonal water s dstrbuted nto the hydrocarbon stream n the form of nano to mcro droplets due to hydrodynamcs. Dependng on the local condtons of temperature and pressure, gas composton and mole-fracton of water n the gas, water can drop out and form separate water phases. Water can condense out or adsorb on the sold surfaces (typcally rusty ppelnes) as two examples examned n ths study. And f hydrate forms the adsorpton of water from gas on hydrate crystals yet another possblty. Theoretcally hydrate can also form drectly from dssolved water n the gas although the low concentraton of water makes t rather unlkely. In partcular n vew of the assocated released heat, heat whch needs to be transported away through a heat nsulator (non-polar gas).

3 Hydrate formng from a separate water phase and gas components can eventually form substantal ce-lke plugs that can severely obstruct the flow and n worst case lead to total blockng and damage to ppelne or equpment. The safest soluton s to remove water from the gas down to a level of mole-fracton, whch does not represent a rsk n formaton of a separate water phase, whether that means a lqud water phase or water adsorbng on rust. Estmaton of upper lmt of water content that can be permtted n a natural gas stream durng processng or transportng for gven ranges of temperature and pressure s a crtcal ssue n the evaluaton of how much water that needs to be removed. The most common way to dry the gas s through absorpton n glycol, whch can be utlzed down to around -2 C. Other systems for water removal, mostly adsorpton technology, are needed for lower temperature separaton. In ths work we prmarly focus on two dfferent routes that can lead to hydrate formaton from a gas mxture. One route nvolves the condensng of water and subsequent hydrate formaton from the free water phase and hydrate formers from the gas phase. A second route goes through adsorpton of water from the gas on rust, and then formaton of hydrate from free water and hydrate formers from the gas phase. Hydrate cannot form drectly on the rusty surface because of ncompatblty between the dstrbuton of partal charges of hydrogens and oxygens n the hydrate lattce, and atom charges n the rust surface. But the rust wll serve as a catalyst for kckng out water from the gas and then hydrate can form slghtly outsde of the frst two or three water layers (roughly nm). As mentoned above hydrate formaton drectly from water dssolved n gas s not lkely due to lmtatons n mass- and heat-transport. Ths route s stll evaluated as a reference to see f t thermodynamcally feasble. If the water/hydrocarbon system were totally unaffected by surface stress from flow, then a hydrate flm would form rapdly on the water/hydrocarbon nterface and rapdly block further transport of hydrate former and waters through the hydrate flm (very low dffusvty coeffcents). In that case hydrate would grow from the hydrate formers dssolved n water, and hydrate could grow from water dssolved n gas, whch then would beneft from nucleaton on the hydrate surface. But n a flowng stuaton wth turbulent shear forces ths s not a realstc stuaton. Yet another mportant dfference between a flowng stuaton and a statonary constant volume, constant mass experment n laboratory s that supply of new mass s contnuous. As such there s not the lmtng stuaton that all water s consumed and hydrate formaton stops. Rust formed from ron and oxygen under mpact of water s a mxture of several dfferent ron oxdes lke Magnette (Fe 3 O ), Hematte (Fe 2 O 3 ) and ron oxde (FeO). Magnette often forms very early whle n the long run Hematte appears to be one of the most thermodynamcally stable forms of ordnary rust. By ordnary rust we then mean varous ron oxde forms as a result of ron exposed to water and oxygen. Impurtes of components lke Carbon doxde and Hydrogen sulfde can lead to conversons over to ron carbonates and varous ron/sulfur components. HYDRATE FORMING ROUTES Water wll always follow the hydrocarbon stream enterng an offshore or onshore processng plant, from a producton well. Amount of water vary, from a separate water phase beng produced along wth the hydrocarbons, and downwards to water dssolved n hydrocarbon gas and lqud phases. Gas processng nvolves several unt operatons that can lead to thermodynamc condtons favorable for hydrate formaton. Turbnes leadng to gas coolng and low temperature flash tanks are two typcal examples. In the North Sea alone there s 7800 km of rusty ppelnes transportng ol and gas. The range of operatng pressures n terms of delvery pressure from separaton plant and pressure on recevng plant, or connecton pont vares, as dscussed above. Temperatures on the seafloor are typcally n the range of - to C. A pressure range of 0 to 20 bar covers large portons of the North Sea transport condtons. Hydrate wll therefore form durng ppelne transport f free water phases can be formed from water n the hydrocarbon system. Remans of water wll always preval after hydrocarbon processng and the queston s how hgh concentraton of water that can be tolerated wthout rsk of water droppng out from the hydrocarbon soluton. Hydrates formed n ppelnes could be removed usng varous technologes but the smplest possble way to reduce rsk of hydrate formaton s to evaluate the upper lmt of water content that can be permtted n natural gas stream durng transport. The prmary focus of ths work s to compare hydrate rsk evaluaton as based on two dfferent schemes. As mentoned above, estmaton of water dew pont pressures for local temperatures s the bass for the more classcal ndustral approach use n most ndustry. Wth water dew-pont mole-fracton as the dependent varable rather than temperature or pressure the lmt of water content before water condensaton can be calculated for each local temperature and pressure durng the transport. The resultng maxmum water content that can be tolerated s the bass for desgn of water removal systems. Yet another possblty s that water adsorbs on rusty ppelne surfaces and subsequently forms hydrates heterogeneously from water layers slghtly outsde [3]. The frst step controllng hydrate formaton s to evaluate at what temperature-pressure condtons n a partcular system are conducve to gas hydrate formaton. However, the estmaton of maxmum amount of water that can be tolerated n gas durng ts ppelne transport wthout a rsk of hydrate formaton s a very complex ssue due to competng phase transton mechansms and dfferent routes to hydrate formaton, where both knetcs and thermodynamcs plays vtal role. The dynamc stuaton becomes even complex due to the fact that hydrates formed n transport ppelnes are unable to reach thermodynamc equlbrum due to the lmtatons of Gbbs phase rule. In non-equlbrum stuaton, chemcal potentals of dfferent hydrate formng components are dfferent across all the phase boundares.

4 Our study s based on free energy analyss snce hydrate formed from dfferent phases wll have dfferent free energes because of dfferent chemcal potental of guest molecules. The bref descrpton of hydrate phase transton scenaro can be seen n []. In equlbrum stuaton the classcal way to solve for equlbrum s to solve smultaneously the condtons for equlbrum, mass-conservaton, and energy-conservaton. In a non-equlbrum stuaton, the equlbrum condtons wll be replaced by the combned st and 2 nd law of thermodynamcs through some schemes for mnmzng free energy locally under constrants of massand energy-conservaton. In case of hydrate formaton and dssocaton, each phase transton s modeled as pseudo reactons wth correspondng changes n free energes as drvng force for phase transton tself and dynamcally coupled to mass transport & heat transport []. Impact of competng phase transtons needs to couplngs to knetcs of mass and heat transport. Free energy changes assocated wth all phase transtons can be calculated by usng equaton. ( ) ( ) G = δ X µ µ + X µ µ H, H, P H, H, P W W W gas gas gas Where x=composton, H=hydrate phase, =phase transton scenaro, µ=chemcal potental, p=lqud, gas, adsorbed phases, =+ for hydrate formaton and - for hydrate dssocaton. Varous possble hydrate formaton routes, as well as routes that can lead to hydrate dssocaton durng transport of natural gas through ppelnes have been dscussed by Kvamme [] on the bass of changes n free energy assocated wth the varous phase transtons. Note n partcular that for a non-equlbrum stuaton chemcal potentals for guest molecules n varous phases are not the same. Ths mples that the chemcal potentals for the guest molecules n the hydrate wll be dfferent as well, as can be seen from a Taylor expanson from an equlbrum pont [-8]. The hydrate phase transton tself, whether t s formaton or dssocaton, s a nano-scale process whch s knetcally controlled by what happens on a thn nterface-typcally 3- water layers (roughly -. nm). The mplctly coupled mass transport s therefore a molecular scale dffuson transport. The support of mass from a larger surroundng couples ths dffuson to hydrodynamcs of the flow stuaton. But the thermodynamc drvng force for the phase transton tself s of course also mplctly coupled to the assocated heat transport needed. For hydrate formaton then the formaton enthalpy needs to be transported away. And for dssocaton the heat must be suppled by heat transport from the surroundngs. The absolute value of heat that needs to be transported s consstently gven by Equaton and standard thermodynamc relatonshps..e.: G RT T (2) = RT H PN, 2 Ths can be evaluated numercally and analytcally based on the models for chemcal potentals ncorporated, as dscussed later. The actual heat transport dynamcs, as mplctly coupled to the phase transton thermodynamcs (Equaton ) s ndvdual for each phase transton. Hydrate formaton on the nterface of water adsorbed on rust wll have fast heat transport through lqud water and sold rust. Smlarly, for hydrate formed from dssolved and/or adsorbed hydrate formers n ths layer. For these stuatons heat transfer may be two to three orders of magntudes faster than mass transport [9] and as such no rate lmtng process. THERMODYNAMICS As dscussed above the system n consderaton cannot reach equlbrum. If a quas-equlbrum approach s utlzed n order to evaluate thermodynamc benefts of varous routes of hydrate formaton or dssocaton, then the classcal equlbrum equatons apples: T (I) =T (II) =T (III).. =T Thermal Equlbrum (3) P (I) =P (II) =P (III)..=P Newton s law, Mechancal Equlbrum () μ (I) =μ (II) =μ (III)..=μ Chemcal Equlbrum () In above equatons superscrpt (I), (II), (III) and further are phase ndex for each of the co-exstng phases beng consdered. Resdual thermodynamc through the use of an equaton of state SRK (Soave-Redlch-Kwong) [0] s utlzed for all components n all phases, ncludng hydrate, lqud water and ce. Ths s accomplshed usng molecular dynamcs smulaton results for water n varous phases (empty hydrates, lqud water, and ce) []. Flud Thermodynamcs The chemcal potental of component n gas phase s calculated by: dea lgas µ (T,P, y) µ (T,P, y) = RT ln φ (T,P, y) where ф s fugacty coeffcent for component n gven phase and y wth arrow denote mole fracton vector of the gas. lm (ф ).0.for deal gas () () 7

5 Also note that the deal gas chemcal potental ncludes the trval mxng term due to deal gas mxng of gases at constant pressure. Aqueous Thermodynamcs Estmaton of chemcal potental of pure water are based on samplngs from molecular smulatons []. Symmetrc excess: Chemcal potental of component n water phase s gven by, deallqud µ (T,P, x) µ (T,P, x) = RT ln γ(t,p, x) Where γ.0 when x.0 Note that the deal lqud term contans the trval deal mxng term as well as the pure lqud value. Asymmetrc access: Components lke methane, ethane and hgher hydrocarbons have lmted solublty n water and a reference state of nfnte dluton s approprate. µ (T,P, x) µ (T,P, x) = RT ln x γ (T,P, x) lm (γ ).0 when x 0 On the bass of these values t appears thermodynamcally possble for hydrate to form from soluton n water, also n accordance wth earler studes [2,3]. Nevertheless, the lmted amount of methane and ethane dssolved nto water does not make ths route to hydrate a very mportant one n gas transport. It does not mean that t cannot play a role at some stage durng massve hydrate growth. But t s lkely not a key route n a prmary hydrate rsk evaluaton. In Equaton 8, superscrpt ndcates chemcal potental of component n water at nfnte dluton, γ ndcates actvty coeffcent of component n aqueous phase based on the same reference state. R s the unversal gas constant. Solublty of methane and ethane are both ndvdually so low that they may almost be consdered as ndependent. As such Equaton 9 below may be utlzed wth Equaton 8, and smlar for ethane based on solublty models may be mportant for stuatons of hydrate growth durng shut-down and absence of sgnfcant hydrodynamc shear forces. µ,j(t,p, x) µ,j(t,p) + RT ln x,jγ,j(t,p, x) (9) In Equaton 9, subscrpt and j refers to dfferent phases and components respectvely. Hydrate Thermodynamcs For chemcal potental of water n hydrate the usual Langmur type adsorpton model s utlzed but n the form derved by Kvamme & Tanaka ], whch also accounts for lattce movements and correspondng mpacts of varous guest molecules: ( ) H 0, H µ W = µ W RTv ln + k= k h k (0),2 Where superscrpt H refers to hydrate phase, µ H W refers to chemcal potental of water n hydrate, µ0,h W denotes chemcal potental of water n empty clathrate structure, v k ndcates fracton of cavty type k per water molecule, h k s cavty partton functon of component k n cavty type j. The canoncal partton functon s gven by: H nc ( gk ) h e β µ k = () Where β s the nverse of gas constant tmes temperature, g nc refers to mpact on hydrate water from ncluson of the k guest molecules n the cavty k. EQUILIBRIUM THERMODYNAMICS OF HYDRATE At equlbrum chemcal potental of component n hydrate has to be equal to ts chemcal potental n the phase t has been extracted from []. In our study, water wll totally domnate the dew pont and hydrate formaton from dropped out lqud water can be obtaned by followng Equaton 2. However, the chemcal potental of all gas components of hydrate are estmated by usng Equaton when dssolved n gaseous phase. 0,H Purewater µ + =µ + γ W RTv = k ln( h k),h2o (T,P) RTln x k,2,h2o,h2o (T,P, x) (2) Based on the model descrbed by [], the chemcal potental of empty hydrate structure s estmated and has been verfed to have predctve capabltes, whch makes any emprcal formulatons redundant and may be unphyscal snce chemcal potental s a fundamental property. Throughout ths study we approxmate the rght hand sde of (2) by pure water. Ths wll mply a lmted shft to the chemcal potental of lqud water. As an example the correcton at 0 bar and 27 K wll be kj/mole and slghtly hgher for 200 bar and 20 bar but stll not dramatc for the purpose of ths study. (7) (8) 8

6 Below Equaton 3 proves benefcal for the calculaton of free energy change related to a hydrate phase transton g H : H n H H H p G = δ x ( µ µ ) (3) = In above equaton, H and P s hydrate and parent phase of molecule respectvely. The relaton between the fllng fracton, the mole fractons & cavty partton functon s gven by Equaton : H xk hk θ k = = v x h k ( T ) + k Where x T = total mole fracton of all guests n the hydrate. θ k = Fllng fracton of component n cavty type k. x k H = Mole fracton of component n cavty type k. MODEL VERIFICATION We dd not make any attempts to ft nteracton parameters n order to reproduce expermental data and prorty on keepng the statstcal mechancal model [] free of adjustable parameters n all terms, ncludng also empty hydrate chemcal potentals and chemcal potentals for ce and lqud water. Any other users of the methods and type of analyss descrbed through ths work may adjust ther ndvdual models at ther own dscreton. Nevertheless, t s stll mportant to examne the qualtatve agreements between estmates from our model systems [], and representatve expermental data. In below Fgures 3, compared the estmated hydrate equlbrum curve wth expermental values from lterature [-] resp. for smple ethane hydrate equlbrum. Even wthout utlzng any emprc data fttng, we consdered the devatons qute acceptable for further llustraton of maxmum water content permtted for dfferent scenaros leadng to hydrate formaton. In partcular, predcted values are good enough for the range of relevant temperatures (27 K-280 K). () 0 9 Pressure (Bar) Fgure. Comparson of estmated and expermental data for ethane hydrate equlbrum for a system contanng pure ethane. Sold lne (-) s estmates whle astersk (*) are expermental data from lterature []. 0 9 Pressure (Bar) Fgure 2. Comparson of estmated and expermental data for ethane hydrate equlbrum for pure ethane. Sold lne (-) s estmates whle astersk (*) are expermental data from lterature [7]. Verfcaton of Hydrate Equlbrum Estmates for Bnary Mxtures Estmated hydrate equlbrum data for bnary mxture of methane and ethane are compared wth expermental data from lterature [] n Fgures and below. The devatons between predctons and expermental data are acceptable for a qualtatve analyss and llustraton of the 9

7 dfference between the classcal hydrate evaluaton approach and approaches that also accounts for other routes toward hydrate formaton. Others that want to use our approach to evaluate hydrate rsk usng ther own mathematcal models and tuned codes. 2 0 Pressure (Bar) Fgure 3. Comparson of estmated and expermental data for ethane hydrate equlbrum for pure ethane. Sold lne (-) s estmates whle astersk (*) are expermental data from lterature [8]. 8 Pressure (Bar) Fgure. Estmated and expermental hydrate equlbrum curve, for gas mxture of 0. % methane and 0.% ethane. Sold lne (-) s estmates whle astersk (*) are expermental data from lterature [7] Pressure (Bar) Fgure. Estmated and expermental hydrate equlbrum curve, for gas mxture of 0.9 % methane and 0.0 % ethane. Sold lne (-) s estmates whle astersk (*) are expermental data from lterature [7]. RESULTS AND DISCUSSION Water avalable for hydrate formaton n transport ppelne can exst n three dfferent states. Water dssolved n gas s nevtably. Water can be present as nano to mcro droplets and water can also condense out as lqud water droplet. Yes, a thrd opton s that water can adsorb on rust (n our study hematte). Drect hydrate formaton from water dssolved n gas and hydrate formers s thermodynamcally feasble but mass transport s lmtaton. Here we have consdered two approxmatons for water, frst water beng dropped out as lqud droplets at specfc temperature pressure condtons and forms hydrate, second alteratve nclude water beng adsorbed on ppelne surfaces resultng n subsequent heterogeneous hydrate formaton [3]. Fgures llustrate the estmated upper lmt of water content n a system contanng varyng concentraton of ethane and methane gas mxture wth small amount of mpurtes ncludng Carbon doxde, Ntrogen and propane, before droppng out as lqud water and before adsorpton on hematte. In our work, we have chosen the temperature range wthn 27 K to 280 K snce the actual condtons at the North seafloor resembles and pressure range between mnmum pressure ranges of 0 bar to hgher pressure 20 bar. Based on absolute thermodynamcs we have nvestgated the dfferent routes to hydrate formaton wth deal gas as a reference state. Ths makes comparson between dfferent hydrate formng pathways more clear and consstent n free energy changes and relevant enthalpy changes as well

8 x0 8 - Mole - fracton of water before water drop out Fgure. Maxmum water content before lqud drop out for mole fracton of gas contanng pure ethane. Curves from bottom to top correspond to pressure 0 bar, 90 bar, 30 bar, 70 bar, 20 bar and 20 bar respectvely.. x0- Mole - fracton of water before adsorpton on hematte Fgure 7. Maxmum water content before adsorpton on hematte for mole fracton of gas contanng pure ethane. Curves from bottom to top correspond to pressure 0 bar, 90 bar, 30 bar, 70 bar, 20 bar and 20 bar respectvely. 7 x0- Mole - fracton of water before lqud drop out Fgure 8. Maxmum water content before lqud drop out for mole fracton of gas contanng 0. % ethane and 0. % methane. Curves from bottom to top correspond to pressure 0 bar, 90 bar, 30 bar, 70 bar, 20 bar and 20 bar respectvely. Fgure 9. Maxmum water content before adsorpton on hematte for mole fracton of gas contanng 0. % ethane and 0. % methane. Curves from bottom to top correspond to pressure 0 bar, 90 bar, 30 bar, 70 bar, 20 bar and 20 bar respectvely. In order to estmate the amount of water dropped out, a smple flash calculaton can be appled. Under sutable T, P hydrate stablty condtons, deposted water can form hydrate. Even though ths s a trval route to hydrate formaton, water wll prefer to drop out as adsorbed more quckly on rust [7] then hydrate forms from a number of addtonal pathways ncludng ether from adsorbed water and hydrate formers n gas phase or from both adsorbed water and hydrate formers. 7

9 x0 -. Mole - fracton of water before lqud drop out Fgure 0. Maxmum water content before lqud drop out for mole fracton of gas contanng 0.0 % ethane and 0.9 % methane. Curves from bottom to top correspond to pressure 0 bar, 90 bar, 30 bar, 70 bar, 20 bar and 20 bar respectvely. 3 x0- Mole - fracton of water before adsorpton on hematte Fgure. Maxmum water content before adsorpton on hematte for mole fracton of 0.0 % ethane and 0.9 % methane. Curves from bottom to top correspond to pressure 0 bar, 90 bar, 30 bar, 70 bar, 20 bar and 20 bar respectvely. Table. Table for a system contanng gas mxture of ethane + methane showng when wll be the dew pont and adsorpton pont occur and at what pont wll the hydrate be formed at a partcular temperature, pressure condtons. T Mn =27.0 (K) T max =280 (K) gas gas Y C2H +Y CH (Molefracton) (Bar) At pressure Dew Pont AdsorptonPont Dew Pont AdsorptonPont Y Y max H20 max H20 Ymax H20 Y max H 2 0 Pure Ethane % Ethane + % Methane % Ethane + 9 % Methane From above Table, n a system contanng pure ethane at temperature 27 K and pressure 0 bar, the estmated water permtted based on dew-pont as bass for water drop-out (Fgure ) s 9 tmes hgher than a lmt based on water drop-out as adsorbed on Hematte (Fgure 7). Smlarly comparng the same system at qute hgher pressure 20 bar, t s observed that the mole-fracton of water before lqud drop out (Fgure ) s 9.2 tmes hgher than the mole-fracton of water before adsorpton on hematte (Fgure 7). Now n a system contanng % ethane + % methane at temperature 280 K and pressure 90 bar, the estmated water tolerance based on dew-pont as bass for water drop-out (Fgure 8) s 8 tmes hgher than a lmt based on water drop-out as adsorbed on Hematte (Fgure 9). Our results clearly suggest that water wll prefer to drop out faster as adsorbed on sold surfaces ndcatng that lqud water drop-out as adsorbed phase onto rust s a domnatng route to hydrate formaton durng ppelne transport of natural gas. 72

10 It s therefore recommended that smlar analyss s conducted for models systems of Iron oxde and Magnette. A more rgorous dynamc analyss can be accomplshed through model systems usng Phase Feld Theory [8] wth sold surface models extracted from Molecular Dynamcs smulatons [9,20]. CONCLUSIONS Local condtons of temperature and pressure durng transport of methane n ppelnes on the seafloor of the North Sea nvolve condtons of methane hydrate formaton. Addton of ethane ncreases the ranges of hydrate stablty. Accordng to Gbbs phase rule and the combned st and 2 nd law of thermodynamcs hydrates formed durng transport of gas stream contanng domnated by methane but wth ethane as addtonal component are unable to establsh equlbrum. As a consequence, the local free energy mnmum wll determne phase dstrbutons and assocated concentratons under constrants of mass and heat transport nvolved n the phase transton. In ths work we have examned the lmts of water mole-fractons n methane/ethane mxtures that can be tolerated before the water drops out as ether lqud water droplets (dew-pont) or adsorbs on rust (as modeled by Hematte). For pure methane the tolerance lmts as estmated from dew-pont has been estmated to roughly 8 tmes hgher than correspondng lmt for adsorpton on Hematte (rust) at 280 K. Smlar ratos for 27 K are roughly 9 tmes. These ratos do not change much by addng ethane, but the tolerance levels for water are reduced compared to the pure methane system. Accordng to the estmates presented through ths work, hydrate rsk evaluaton should be based on predctons of maxmum water content before adsorpton on rusty surfaces and dehydraton unts should be dmensoned accordngly. REFERENCES. Sloan ED, Koh CA. Clathrate Hydrates of Natural Gases. 3 rd ed. CRC press, USA; Kvamme B and Førrsdahl OK. Polar guest-molecules n natural gas hydrates. Effects of polarty and guest-guest-nteractons on the Langmur-constants. Flud phase equlb. 993;83: Kvela PH, et al. Phase feld theory modelng of methane fluxes from exposed natural gas hydrate reservors. Int J Greenhouse Gas Control. 202;29: Kvamme B, et al. Hydrate Formaton durng Transport of Natural Gas Contanng Water and Impurtes. J Chem Eng Data 20;: Jema K, et al. Theoretcal studes of CO 2 hydrates formaton and dssocaton n cold aqufers usng RetrasoCodeBrght smulator. WSEAS Trans. on Heat Mass trans. 20;9:0.. Kvamme B, et al. Can hydrate form n carbon doxde from dssolved water? Phys Chem Chem Phys. 203;: Kvamme B, et al. Investgatons of the Chemcal Potentals of Dssolved Water and H 2 S n CO 2 Streams Usng Molecular Dynamcs Smulatons and the Gbbs-Duhem Relaton. J Chem Eng Data. 20;0: Kvamme B, et al. Consequences of CO 2 solublty for hydrate formaton from carbon doxde contanng water and other mpurtes. Phys Chem Chem Phys. 20;: Svandal A. Modelng hydrate phase transtons usng mean-feld approaches. Dssertaton for the degree phlosophae doctor (PhD) at the Unversty of Bergen, Soave G. Equlbrum Constants From a Modfed Redlch-Kwong Equaton of State. Chem Eng Sc. 972;27: Kvamme B and Tanaka H. Thermodynamc Stablty of Hydrates for Ethane, Ethylene, and Carbon Doxde. J Phys Chem 99;99: Kvamme B. Knetcs of Hydrate Formaton From Nucleaton Theory. Int J Offshore Polar Eng. 2002;2: Kvamme B. Droplets of dry ce and cold lqud CO2 for self transport to large depths. Int J Offshore Polar Eng. 2003;3:39-.. Holder GD and Grgorou GC. Hydrate dssocaton pressures of (methane + ethane + water) exstence of a locus of mnmum pressures. J Chem Thermo. 980;2: Deaton W-M and Frost EM Jr. Gas Hydrates and Ther Relaton to the Operaton of Natural Gas Ppe Lnes. US Bureau of Mnes Monograph Roberts OL, Brownscombe ER. Ol and Gas J. 90;37: Kvamme B, et al. Adsorpton of water and carbon doxde on hematte and consequences for possble hydrate formaton. Phys Chem Chem Phys. 202;: Kvamme B, et al. Hydrate phase transton knetcs from Phase Feld Theory wth mplct hydrodynamcs and heat transport. Int J Greenhouse Gas Control 20;29:

11 9. Kvamme B, et al. Molecular dynamcs studes of water deposton on hematte surfaces. AIP Conf Proc. 202;0: Kuznetsova T, et al. Water-wettng surfaces as hydrate promoters durng transport of carbon doxde wth mpurtes. Phys Chem Chem Phys. 20;7: