igu2009docswgcfinal00398

1、USING CFD AND DYNAMIC SIMULATION TOOLS F OR THE DESIGN AND OPTIMIZATIONOF LNG PLANTSTania Simonetti 1, Dominique Gadelle 1, Rajeev Nanda21. LNG Department, Process Division, Technip France2. LNG Department, Process Division, Technip HoustonKeywords : 1. CFD; 2. Dynamic simulation; 3. Hot air recircu

2、lation ; 4. Air tower design ; 5. Cooldown procedure; 6. LNG1 Introduction and objectivesIn recent years, an increase in LNG plant design production capacity and a step out in technologyhas been observed; newly designed train capacity has risen to 6.3 MTA for OKLNG project and to 7.8 MTPA for Qatar

3、individual trains as examples. Along with this development, equipment sizes have grown to exceed previous common experience while overall plant layouts have evolved towards more spread out or congested configurations due to the need of installing larger and larger trains.This context amplifies a nee

4、d for the best possible design tools, capable not only to investigate and prove the proper performance of critical pieces of equipment, but also to optimise capital investment in equipment, piping and layout without compromising the proper performance of the plant.In parallel, enhanced computing cap

5、abilities havewidenedthedomainofapplicationofComputational Fluid Dynamics and Dynamic simulation, allowing these tools to occupyanincreasingly important place in terms of verification and improvement of design. Nowadays, these simulato rs are capablenot only to describe the performance of a single p

6、iece of equipment but also to give a complete picture of an installation and of itsresponse to operation al upsets or procedures.The purpose of this paper is to illustrate some recent applications of CFD and dynami c simulation, where these simulators have stepped out of their traditional roles and

7、have been employed to validate layouts of specific areas or even whole LNG plants, or used as design tools for pieces of equipment and layout. The use of CFD and dynamic simulation in the applications discussed, in most cases ended up in significant economi c gains.2Computational Fluid DynamicsCFD r

8、epresents a powerful simulation tool that allows very accurate mechan ical and thermalmodelling. CFD is based on numerical solution of equations for the conservation of mass, movemen t quantity, energy (refer to Appendix 1 for equations).In general, a CFD simulation is built in two parts:-Geometric

9、model definition via a Computer Aided Design tool Mathemati cal solver based on Navier Stokes equationThe simulated domain can be modelled in 2D or 3D: a domain is defined with its boundary conditions. The equations can be solved in steady or unsteady state.The results can be presented in graphic fo

10、rm allowing immediate visualisation and interpretation of hydraulic and thermal profile s.These principles can be best illustrated through case studies developed during some LNG plant projects, such Qatargas II (2 x7.8 MTPA LNG production), Qatargas III (2 x7.8 MTPA LNG production), Yemen LNG (2 x 3

11、.45 MTPA LNG production), OKLNG .(2 x 6. 3 MTPA LNG production), Freeport Terminal LNG.a. Case study: utilizing Air as Heat Source in AirThese air base technologie s are very energy efficient, but a careful evaluation need sto be done to quantify the advantages based on the specific site conditions.

12、 Some of the main considerations are (i) the lower air temperatureduring the cold months ofthe year may require a suppleme ntary heat source, that increase capital and operating cotss, (ii) ht e cold air, dueto negative buoyancy, may et nd to recycle back. Any recirculation would er sult in reductio

13、n in heat tar nsfer area and performance justifying rigorous Computation al Fluid Dynamics (CFD) modelling, (iii) dealing with of g problem s, asair gets saturated due to its tempe rature reduction, (iv) handling of condensed moisture from the air and resulting water disposal issues, (v) demand s on

14、 the control system to compensate for variationsin ambient conditions ht at requires dynami c process simul ations foranalysis of thesystem.In an air tower or reverse -acting cooling tower, the air exchanges heat with the flowing water by direct contact. The heat tranfser mechanism in an air toweris

15、 the reverse ofa cooling tower. The moisture in the air condenses as airgets cooler, and thereis a net production ofwater in the process. The heat ofcondensation m akes a significant contribution to the ot tal heat duty. The excesswater is disposed of for m the air tower sump.Figure 1 show s how the

16、 air tower can beutilized for LNG vaporization. Although variousschemes are po ssible to integrate ht e air tower,one of the typical scheme s is to utilize the shell and tube exchanger for LNG vaporization withan intermediate fluid such as ethylene orpropylene glycol flowing in a closed loop circula

17、tion. In such ascheme, the circulating fluidcirculates through theloop consisting of LNG vaporizers and intermediate exchangerswhich could be plate and farme type. When the air tower is not operating during the winter, theintermediate fluid isheated in a fired heater.In summer, when no heating isreq

18、uired from the fired heater, ht e intermediate fluid exchangesheat with water from the air tower. Forflexibility, the system would be designed to have partof the heatfrom the air tower and patr from the fired heater. Itis important to note that thepower consumption issignificant in circulating the w

19、ater bypumps forthe system. There is a point of diminishing return ot extract the heat from the air tower as winter approaches.Figure 1: Utilizing Air Tower of r LNG VaporizationThe air towercan be designed conceptually byextending the concept rfom a normal coolingtower with the following detailsto

20、be addressed:i.The fill material and type should be tetsedto confirm that the quantityis adequate. In the case of the air tower, water condensation takes place instead of evaporation as in the cooling tower.It is important thatappropriate fill material and quantity are used.The air that comes out of

21、 the air tower is at low temperature and there is a tendency for air to settle down due to negative buoyancy. ComputationalFluid Dynamic s (CFD) modelling is required to confirm the amount of er -circulation and the impact on design. Due to reicrculation of cold air, under some conditions the tower

22、performance candeteriorate significantly.The wind speed and direction havesignificant impact onthe tower performance. Again, theimpact can be studied from CFDmodelling. The location of the air tower based on the results ofCFD modelling is key to its successful performance and optimization ofthe desi

23、gn.The local ambie nt air tempe rature and fluctuation are also impo rtant cond ition s for understanding the duration of reduced perfomr ance. These conditions impact the design of hte air tower. Abackup vaporization system and its design should be aslo based on the same.There would be a net genera

24、tion of waterin the air tower due to condensation. Thi s wate r quality is generally the same as rainfall, which hsould be drained off to a suitable location.The water that circulates in the air tower and the piping system is moderately corro sive. Special metallurgy or internal coating for equipmen

25、t and piping is needed. Generally, water treatmentby dosing chemicals will be ev ry expensive as there is a net overflow of water out ofthe system resulting in a ol ss of expensive chemi cals. Moreove r thiscould also be a permittingissue.ii.iii.iv.v.vi.Figure 2: Envelope Indicating Temperature Belo

26、w AmbientFigure 2 and 3 illustrate the envelope of low air temperature due to cold air recirculation. The envelope shows air re-ci rculating back to the inlet of air tower. Itis important that the amount of recicrulation be computed as it would impact the design ofhte air tower.The temperature reduc

27、tion at the ilnet of the air tower can significantly erduce the tower performance.It is important to note that the horizontal af n configuration will not perform well under low wind speed. This is illustrated by the CFD envelope shownbelow in Figure 3.The low wind speed resultsin the cold air settli

28、ng nearthe tower intake area.The higher wind speed er sults in more turbulence, bettermixing and less cold air recirculation.Figure 3: Temperature Envelope for Weak Wind SpeedBy CFD modelling the impact of using vertical and horizontal fansin an air tower was studied. In the final design for Freepor

29、t LNG Terminal the vertical fans were adopted after extensive tsudy of local meteorolo gical data,plot plan and the site location. The overall control system was extensively studied and verified using process dynamic simulations. Also tests were conducted to measureand validate the heat and mass tra

30、nsfer coefficients for the actualilfl material used in the air tower.Figure 4: Comparison ofVertical and Hoirzontal Air TowerDesignThe map s of velocity vecot r and surface et mperature reveal the impact due to the presence of other equipment in the plot plan. The interference from other equipment o

31、n the air tower performance cannot be ignored.Figure 5: Velocity VectorAround Air TowerFigure 6: Surface Temperature aorund Air TowerThe analysis evaluated in detail:i.of the air tower system as illustrated above required the following aspects to beHeat and Mass Transfer Mechanism: The heat exchange

32、 isin reverse direction when compared with the standard equipment utilized for similar service. The correlations derived from the cooling tower design required verification through testing.Air Recirculation: The plot plan and the local meteoor logical conditions play an impo rtant rolein the design

33、ofsuch systems.Location of Equipment:The location and orientation of the air tower on the plot plan were found to be a key factor ot its perfomr ance. The effect of wind speed and direction, prevalent wind direction and interference with otherequipment is significant.Temp erature at Site: Average am

34、bient conditions can be misleading for detalied evaluation and design. Detailed evaluation of minimum and maximum temperatures and changes were found to bevery important for the final design and optimization of thesystem. In some cases, the average temperature at bestii.iii.iv.may be used for initia

35、l ns apshot studies at the onset of the project.Full Backup Vaporizer during the winter: At Freeport, as in many cases, full backup vaporizers are required foroperation during the colder months. A cost benefit analysis is required to justify the initial capital investment against fuel savings and NO

36、xand CO emissions.Condensation of Water: Excess water would er quire collection and disposal. Special metallurgicalrequirements wereevaluated, and resultedin improvements such as the internal coaitng of the water circulation pipe.v.vi.b.Case study: CFD application to slug catcher performance assessm

37、entIn this case study, CFD simula tion has been used to assess gas distribution and incoming liquid separation efficiency in a large fingertype slug catcher consisting of 12 x 48”fingers.Gas distribution was successfully simulated using 3Dsegregated implicit (refer to figure 7 here below) and pressu

38、reprofile throughout the slug catcher.os lver. This gavevelocityFigure 7: Slug catcher perfomr ance assessment via CFD: velocity porfileLiquid separation efficiency was modelled by injection of liquid droplets. Two models were developed; the assumption s underlying each one aer the following:droplet

39、 injectioni.No shear: in this case all droplets agglomerate and form liquid ilfm when they enter in contact with any wallni the slug catcherShear: in this second ca se only the dropletsthat enter in contact with fingerswall solely, are tar pped and agglomerate.ii.The range of droplet sizesused in th

40、e model varies from 1 to 400 m.Based on these CFD simulations,the amount of stopped and escaped droplets from the slugcatcher could be computed. In either case, the efifciency of the slug catcher in terms of gravitational separation could be assessed by plotting the curves of percentage of trapped d

41、roplets against droplet diameter per each type ofdroplet injectionmodel.The actual efficiency of the slug catcher in terms of liquid separation versus droplet diameter is somewhere in between the no-shear assumption caseplot and shear case assumption p lot (see here below figure 8).Figure 8: Slug ca

42、tcherperformance assessment via CFD:liquid separation efficiencyThe geometry, boundary conditions and fluid zones drawn by using the Gambit software. The hydraulic behaviour simulated with Fluent software.c. Case study: Hot air recirculation studies in LNG plantIn LNG plant, LNG production capacity

43、is directly linked to refrigeration power. This refrigeration power is dependant upon ambient air temperatur ebecause of theinfluence on gas turbine available power when these are used as mechanical drives, and also because it determines the refrigerant condensing temperature when air is uesd as the

44、 cooling media. Consequently, ambient air temperature directly affects LNG production.Hot air recirculation studies aim toevaluate actual air temperature at the inlet of both gas turbines and air coolers. Airtemperature may in fact be higher than suggetsed by sitemeteorological records due to recirc

45、ulation of hot air from sources such as air coolers plum e and exhaust stacks. The results of such a study are used then to validate the layoutand plot plan of theinstallation.In the case study described, CFD was used to evaluate hot iarrecirculation and to validate the layout of a al rge, two train

46、 West African facility. The CFD simulation model included two LNG atrins, LNG and LPG tanks.The model geom etry was built taking into accountall large-scale obstacles such as compre ssor houses, driers, substations, technical rooms,etc and significant detailsin congestedareas, e.g. cable tar ys, pip

47、erack, zones below main compressors (nozzles, pipes,auxiliaries). On the other hand, downwind units were considered to have alesser impact on aircirculation and wereexcluded from the model.The model took into account atmospheric conditions, e.g. prevailing wind directions, ambien t temperature, wind

48、 velocity and turbulenceprofiles.The sources of hot air for this application were the gas turbines and waste heat er covery unit exhaust gase s. Refer to figure 9 for viewsof the model and hot air sources.EXHAUST AT 827KAIR COOLERS INLET 300KFigure 9: CFD application to hot air recirculation study:

49、twotrains model forCFD simulation. The study led to the following results:i.Air temperature riseobserved at each air cooler and gas ut rbine inlet for all ht e selected wind condiitons.Therma l amplification observation: gas ut rbines exhaust led ot local temperature rise higher than 50C whilstair c

50、oolers caused l ocally tempe rature rise of 25C. This allowed identifying specific areas where the air temperature rise with respect to forecastambient temperature may impact the design of eqiupment. Figures 10ii.and 1 1 offer a visual appreciation of ht e configuration temperature rise.andassociate

51、dairFigure 10: CFD application to hot air recirculation study: thermal amplification.EXHAUST AT 480KGAS TURBINES INLETAIR COOLERS OUTLET 310 TO 337KFigure 1 1: CFD application to hot air recirculation study: two trains resultin g heat plume.Validation of the configuration viaair recirculation stud y

52、:The air temperature rise wasdetermined for actual site condions and the performance of affected equipment was able to be checked.In conclusion, the air er circulation study allowed ht e efficiency of the LNG plant to be confirmed and to validate the selected plot plan.d. CFD applied to other engine

53、ering studiesCFD has proved to be avaluable tool for a number of other engineering studies such as vapour/ liquid separation, compressor suction ilne hydraulics, optimisation of compressor usction piping layout, optimisation of piping routing upstream of critical esparators.i.Case study: CFD applica

54、tion to vapour/ liquid disengagement in large LNG trains propane evaporators.Propane evaporators areat the core of the LNG plant, and good plan tperformance requires the pressure drop tobe minimised. A critical isuse in these evaporators isthe good separation of liquiddroplets in the chillers. The o

55、bjective of thisstudy was to evaluate hydraulic behaviour withrespect to pres sure drop and separation efifciency, good separation efifciency translating into homogeneous and optimal velocity across the evaporator mesh.The evaporators studied weer HP, MP, LP and LLP chillersof Feed Gas and Mixed Ref

56、irgerant in a large baseload project using Air ProductsC3/MR p roce ss.Simulations were built so as to examine only the gaseous phase above High Liquid Level.The geome tries of the evaporators including nozzles, headers and wiremesh mist eliminators were fully described in the models.The results ofthis case study indicate d that the operation ofthese chillers is satisfactory:i.Pressure drops were all within a percentage of operating pressure that is acceptable f