Modeling of hydrate deposition in loading and offloading flowlines of marine CNG systems

Transcription

1 Internatonal Journal of Sustanable and Green Energ 214; 3(6-1): 1-6 Publshed onlne Februar 11, 215 ( do: /j.jrse.s Modelng of hdrate deposton n loadng and offloadng flowlnes of marne CNG sstems Esam I. Jassm Department of Mechancal Engneerng, Prnce Mohammad Bn Fahd Unverst, Al-Khobar, Saud Araba, Emal address: ejassm@pmu.edu.sa To cte ths artcle: Esam I. Jassm. Modelng of Hdrate Deposton n Loadng and Offloadng Flowlnes of Marne CNG Sstems. Internatonal Journal of Sustanable and Green Energ. Specal Issue: newable Energ and Its Envronmental Impacton. Vol. 3, No. 6-1, 214, pp do: /j.jrse.s Abstract: The man am of ths paper s to demonstrate the predcton of the model capablt of predctng the nucleaton process, the growth rate, and the deposton potental of hdrate partcles n gas flowlnes. The prmar objectve of the research s to predct the rsk hazards nvolved n the marne transportaton of compressed natural gas. However the proposed model can be equall used for other applcatons ncludng producton and transportaton of natural gas n an hgh pressure flowlne. The proposed model emplos the followng three man components to approach the problem: computatonal flud dnamcs (CFD) technque s used to confgure the flow feld; the nucleaton model s developed and ncorporated n the smulaton to predct the ncpent hdrate partcles sze and growth rate; and the deposton of the gas/partcle flow s proposed usng the concept of the partcle deposton veloct. These components are ntegrated n comprehended model to locate the hdrate deposton n natural gas flowlnes. The present research s prepared to foresee the hdrate deposton locaton that could occur n a real applcaton n Compressed Natural Gas loadng and offloadng. A ppelne wth 12 m length and dfferent szes carred a natural gas s taken n the stud. The locaton of hdrate deposton formed as a result of restrcton s determned based on the procedure mentoned earler and the effect of water content and downstream pressure s studed. The crtcal flow speed that prevents hdrate to accumulate n the certan ppe length s also addressed. Kewords: Hdrate deposton, Natural gas, CFD, Multphase flow 1. Introducton In Compressed Natural gas flow, the formaton of hdrate s lkel common, partcularl n loadng and offloadng operaton. The ppe that connects the offshore termnal wth the custom CNG shp merges n the sea water and carres hgh pressure gas. Snce the sea temperature s relatvel low, the hdrate could form and accumulate somewhere n the ppe resultng a partal blockage whch eventuall could plug the ppe and completel nterrupt the flow (Jassm et al., 21). The transmsson sstem must operate n a safe, effcent, and relable manner throughout the desgn lfe. Falure to do so has sgnfcant economc consequences, partcularl for offshore gas producton and transportaton sstem. The avodance or remedaton of hdrate problems s the ke aspect of flow assurance that enables the desgn engneer to optmze the producton sstem and to develop safe and costeffect operatng strateges for the range of expected condtons, ncludng start-up, shutdown, and turndown scenaros. Annuall, an operatng expense greater than $5 mllon s devoted to hdrate preventon, almost half of that s devoted for hdrate nhbton (Sloan, 23). In addton, offshore operatons spend approxmatel $1,, per mle for nsulaton of subsea ppelnes to prevent hdrates. The am of the research s to ntegrate a comprehensve model n order to dentf the locatons where hdrate accumulaton would most lkel occur, to stud the sze dstrbuton of partcles, to model the partcle-wall nteracton, to smulate the deposton process based on forces balance, and to stud the nfluence of some parameters on the locaton of the deposton such as flow condtons. The comprehensve models capable to predct partcle deposton and accumulaton n fluds have been successfull mplemented (Chen et al, 1997; Joseph, et al, 21; Kvasnak et al, 1993; Legendre, 25; Tan and Ahmad, 27). However, based on our extensve lterature surve and best knowledge, models specfcall developed to predct the most probable locaton for hdrate deposton under

2 2 Esam I. Jassm: Modelng of Hdrate Deposton n Loadng and Offloadng Flowlnes of Marne CNG Sstems condtons where natural gas flows through restrctons n ppelne sstems are stll unavalable and need to be developed. The proposed model conssts of the followng components:. The computatonal flud dnamc (CFD) technque to confgure the flow feld;. A new correlaton for hdrate growth and dstrbuton based on the satsfacton of the Law of Mass acton to predct the ncpent hdrate partcles sze and growth rate;. The ncluson of the concept of partcle deposton veloct to track the partcle moton n the turbulent regme; and v. A novel approach to descrbe the partcle behavour near wall regon. 2. Summar of the Model The flowchart presented n Fg (2.1) summarzes the procedure of the partcle mgraton and process of deposton n turbulent flow. Further detals can be found n (Jassm, 28). As partcles travel n the full turbulent regon, deposton veloct s evaluated (dependng on ther szes) and used to determne collecton factor and the number of partcles at the wall at each tme step. In the sublaer regon, the partcle sze s used to relate the phenomena of the deposton to the proper model of deposton process, whch s ether the balance of the forces experenced b the partcle(cherukat et al, 1994; Jassm et al, 21; Fan and Ahmad, 1993; Kvasnak et al, 1993; L and Ahmad, 1993; Shams and Ahmad, 2; Tan and Ahmad, 27; Wang and Lev, 23) or the probablt of bouncng (Gondret et al, 22; Jassm et al, 21; Legendre et al, 26; Legendre et al, 25; Wang and Lev, 23). 3. Case Stud Fgure (2.1). Flowchart of deposton model The followng nformaton pertanng to a near real applcaton was used for ths stud to predct the deposton of the hdrate partcle n a flexble loadng lne to a CNG shp: Ppe length: L=12 m; Gas water content = 16, 64, 16 mg/m 3 of wet gas; Lne pressures: P = 345 and 69 kpa ; Ppe dameters: D ppe = 25.4, 5.8, 11.6, 152.4, 23.2, and 254 mm (1, 2, 4, 6, 8, 1 nches) Flow rate was calculated as follows: 2 Q πu gd ppe 4 = where U g 15 = (m/s); ρ g These are the lmts of eroson veloct predcton based on API 14E gudelnes used n the stud to predct the flow rates n the flow lne. Usng the dagram of water content n hdrocarbon gas (Engneerng Data Book GPSA), the temperature of the gas could be predcted. The ppelne was assumed to be n thermal balance wth the chamber envronment Calculatng the Dstance of the Deposton Accordng to the proposed model, the procedure for calculatng the dstance traveled b a sngle partcle before beng deposted on the wall could be summarzed as follows: The deposton veloct s frst determned usng the model of Wells and Fredlander (Wells and Chamberlan, 1967; Crowe, 26). The tme requred for the partcle to reach the sublaer

3 Internatonal Journal of Sustanable and Green Energ 214; 3(6-1): regon can be found from: t = (3.1) Where,, the dstance between the local poston of the partcle and the wall, s measured from the tube wall. The travelng dstance n the turbulent regon, assumng the ntal veloct of the partcle s equal to the flud veloct, becomes: X u( ). 1 = ; where, Vd V d U u( ) u = * 2.5ln R (3.2) In the sublaer regon, the partcles smaller than the thckness of the boundar laer could mgrate further as a result of external forces. Hence, the total dstance from the ntal poston would be: X t X + = 1 (3.3) X δ For partcles larger than the sublaer thckness, the bouncng dstance (X B ), the dstance taken b the partcle to settle as a result of rebound, s added to the dstance traveled n the turbulent regon The Deposton Dstance X X + = 1 (3.4) t X B Fgure (3.1) shows the dstance from the spot of partcle formaton to the locaton of deposton as a functon of nolds number. The graph concludes that the deposton dstance ncreases wth ncreasng n nolds number. The trend s almost lnear. It s mportant to note here that the dstance of partcle deposton, called the crtcal dstance, s defned as the dstance between the spot of hdrate formaton to the locaton where the partcles settle on the wall. The mnmum sze of the partcle that depost n a crtcal dstances, s called the crtcal sze. Hence all szes equal or larger than the crtcal sze wll depost n the same dstance whle smaller partcles wll be settled somewhere further ahead n the lne. Fgure (3.2) llustrates such crtcal szes wth respect to nolds number. The fgure shows that each value of nolds number s desgnated wth a certan crtcal sze of hdrate. The larger the magntude of nolds number s, the smaller s the partcle crtcal sze Effect of Pressure Pressure of the ppelne could change the dstance of the deposton and the smallest partcle sze deposted (crtcal sze). Fgure (3.3) llustrates the crtcal sze of hdrate partcles as a functon of nolds number for dfferent flowlne pressures. For each partcular nolds number, the mnmum sze of hdrate deposted on the wall ncreases when the pressure nsde the ppelne decreases. Deposton Dstance (m) Fgure (3.1). Deposton dstance versus flow nolds number dp cr (µm) Pressure (ps) Fgure (3.2). Crtcal partcle sze depost as a functon of nolds number dp cr (µm) Pressure (ps) Fgure (3.3). Influence of ppelne pressure on the crtcal sze of hdrate The deposton dstance s also nfluenced b the pressure varaton nsde the ppe. Fgure (3.4) shows that the deposton dstance ncreases as the pressure nsde the ppe

4 4 Esam I. Jassm: Modelng of Hdrate Deposton n Loadng and Offloadng Flowlnes of Marne CNG Sstems decrease. Such dscrepanc becomes more pronounced for hgher nolds number. Deposton Dstance (m) Pressure 1 ps 5 ps dstance ( x) as a functon of nolds number for dfferent flow pressure and water contents. It could be concluded that the dfference n the deposton dstance ncreases wth the ncrease n nolds number. Fgure (3.7) was developed to show the sgnfcance of ths trend. The graph represents the dstance the partcle moves n the sublaer regon as a fracton of total deposton dstance the. From the fgure, t can be seen the dstance fracton decreases as the nolds number ncreases. That means that the partcle-wall effect becomes sgnfcant at relatvel low nolds number despte that the dfference n the deposton dstance s hgher for hgh nolds number lb/mmscfd; 1ps 1lb/mmscfd; 1ps 4lb/mmscfd; 5ps 1lb/mmscfd; 5ps 8 12 Fgure (3.4). Influence of ppelne pressure on the deposton dstance 3.4. Effect of Water Content Another factor studed n ths sample case was the amount of natural gas water content. Three values were chosen to present the effect of the water content on the deposton dstance. Fgure (3.5) demonstrates such effect. It can be observed that there are sgnfcant dfferences n the deposton dstance as a result of water contents n natural gas flow, specfcall at hgh nolds number. Ths trend comes from the fact that wth ncrease n water content, the fugact of water n gas phases ncreases and the drvng force for mass transfer ncreases too. Hence, the rate of hdrate growth ncreases. Deposton Dstance (m) lb/mmscfd; 1 ps 1 lb/mmscfd; 1 ps 1 lb/mmscfd; 1 ps Fgure (3.5). Effect of water content on the deposton dstance 3.5. The Near Wall Effect As the model consders the partcle behavour nsde the sublaer regon, t would be approprate to llustrate the nfluence of such behavour b comparng the deposton dstance wth and wthout partcle-wall nteracton. Fgure (3.6) shows the dfference n the deposton x (cm) Fgure (3.6). the ncrement n the deposton dstance resulted b wall effect % extended dstance lb/mmscfd; 1ps 1lb/mmscfd; 1ps 4lb/mmscfd; 5ps 1lb/mmscfd; 5ps Fgure (3.7). the sgnfcance of near wall effect as a functon of nolds number 3.6. The Crtcal Speed Ths secton presents the speed requred b the natural gas to flow so that no accumulaton ma occur wthn the ppelne. In other words, all the sold partcles wll move outsde the ppelne and no deposton occurs on the ppe wall. Table (1) shows such veloctes for the followng constrants: P= 69 Kpa (1 ps), Water content= 16 mg/m 3 (1 lb/mmscfd). It s shown that small ppe sze needs more gas speed to force the partcles outsde the lne. Ths s

5 Internatonal Journal of Sustanable and Green Energ 214; 3(6-1): because the dstance to the wall s smaller and less tme s requred to get the partcles travel towards the wall. Table (1). Mnmum flow veloct requred to prevent accumulaton Lne sze (ID) Veloct (m/s) Lc (m) Concluson A new approach s developed to locate the hdrate deposton n a natural gas ppelne. The concept of the partcle deposton veloct s ntroduced to help to predct the trajector of the partcle moton n the turbulent regon. The model presented n ths research proposes a new approach to track the partcle moton merged n the sublaer regon usng the forces actng on the partcle. For partcles wth szes larger than the sublaer thckness, the model ntroduces the nfluence of the bouncng concept to explan the near wall effects. The man conclusons of the research can be summarzed as follows: 1 The contnuum equatons should be corrected when the moton of submcron partcles s addressed. Snce ver tn partcles behave as flud partcles, Brownan effect s taken nto account b ncludng the slp correcton factor n the contnuum equatons. 2 The stud showed that the dstance of deposton decreases as the partcle sze ncreases. However, the analss has ntroduced a certan sze of partcle n whch further partcle growth has no effect on the dstance of deposton. Such sze was called deposton crtcal sze. 3 Deposton dstance ncreases wth the decreasng of ppelne pressure. 4 Water content n the natural gas has sgnfcant nfluence on the deposton dstance partcularl at hgh nolds number. The partcles depost faster when the content of water n the natural gas ncreases. 5 Increasng n nolds number reduces the effect of the dstance that occurs as a result of near wall effect,.e. the dstance traveled b the partcle n the boundar laer regon. 6 More speed requred b the smaller ppe sze to prevent hdrate accumulaton. Nomenclature D ppe d p E e L m p Ppe dameter Partcle dameter Partcle Energ Coeffcent of resttuton Ppe length Mass of Partcle P Q R r t T U g u * V d V c V X ρ ferences Pressure Flowrate Ppe Radus Partcle Radus nolds Number tme Temperature Flud Veloct frcton veloct Deposton Veloct Crtcal Veloct Partcle Veloct deposton dstance dstance perpendcular to the wall denst [1] Chen Q., and Ahmad G., Deposton of partcles n a turbulent ppe flow, Journal of Aerosol Scence, 28, , [2] Cherukat P., McLaughln J.B., The Inertal Lft on a Rgd Sphere n a Lnear Shear Flow Feld Near a Flat Wall, Journal of Flud Mechancs, 263, 1 18, [3] Cleaver J.W., and Yates B., A sublaer model for the deposton of partcles from a turbulent flow, Chem.Engng Sc. 3, , [4] Crowe C.T., Multphase Flow Handbook, Talor & Francs Group, 26. [5] Engneerng Data Book/CD-ROM SET, 12 th Edton, Gas Processors Supplers Assocaton, Tulsa- Oklahoma, 24. [6] Fan F-G., and Ahmad G., A sublaer model for turbulent deposton of partcles n vertcal ducts wth smooth and rough surfaces, Journal of Aerosol Scence, 24, 45-64, [7] Gondret P., Lance M., Pett L., Bouncng Moton of Sphercal Partcles n Fluds, Phscs of Fluds, 14(2), , 22. [8] Jassm E.I., Locatng Hdrate Deposton n Multphase Compressed Natural Gas Flow Lnes Usng Computatonal Flud Dnamcs Technques, PhD Thess, Memoral Unverst of Newfoundland, St. Johns, 28. [9] Jassm E.I., Abednzadegan Abd M., and Muzchka Y., A new approach to nvestgate hdrate deposton n gasdomnated flowlnes, Journal of Natural Gas Scence and Engneerng, Vol.2, Issue 4, Sept.21, Pages [1] Joseph G.G., Zent R., Hunt M.L., Rosenwnkel A.M., Partcle-Wall Collson n a Vscous Flud, Journal of Flud Mechancs, 433, , 21. [11] Kvasnak W., Ahmad G., Baer R., Ganes M., Expermental Investgaton of Dust Partcle Deposton n a Turbulent Channel Flow, Journal of Aerosol Scence, 24 (6), , [12] Legendre D., Zent R., Danel C., Guraud P., A Note on the Modelng of the Bouncng of Sphercal Drops or Sold Spheres on a Wall n Vscous Flud, Chemcal Engneerng Scence, 61, , 26.

6 6 Esam I. Jassm: Modelng of Hdrate Deposton n Loadng and Offloadng Flowlnes of Marne CNG Sstems [13] Legendre D., Danel C., Guraud P., Expermental Stud of a Drop Bouncng on a Wall n a Lqud, Phscs of Fluds, 17(1), 1-13, 25. [14] L A., and Ahmad G., Deposton of aerosols on surfaces n a turbulent channel flow, Internatonal Journal of Engneerng Scence, 31, , [15] Shams M., Ahmad G., Rahmzadah,H., A sublaer model for deposton of nano- and mcro-partcles n turbulent flows, Chemcal Engneerng Scence, 55, , 2. [16] Sloan E.D, Fundamental prncples and applcatons of Natural Gas Hdrates, Nature Publcaton Group, 426, , Nov. 23. [17] Tan, L., and Ahmad G., Partcle deposton n turbulent duct flow-comparsons of dfferent model predctons, Journal of Aerosol Scence, 38, , 27. [18] Wang J., and Lev E.K., Partcle Motons and Dstrbutons n Turbulent Boundar Laer of Ar-Partcle Flow Past a Vertcal Flat Plate, Expermental Thermal and Flud Scence, 27, , 23. [19] Wells A.C., and Chamberlan A.C., Transport of Small Partcles to Vertcal Surfaces, Brt. J. Appl. Phs., 18, 1793, 1967.