Dynamic Simulation LNG Processes

8/11/2019 Dynamic Simulation LNG Processes

1/4

LIQUEFIED NATURAL GAS DEVELOPMENTS SPECIALREPORT

Dynamic simulation of liquefiednatural gas processesHeres how to improve the process design and operation of your facility

G. STEPHENSON,Honeywell Process Solutions, London, Ontario, Canada;and L. WANG,Honeywell Process Solutions, Calgary, Alberta, Canada

Amulti-tube, spirally-wound, cryo-genic heat exchanger, the mainheat exchanger (MHE) is the

principal piece of heat-transfer equipmentin mixed-refrigerant liquefaction cycles forproducing liquefied natural gas (LNG).An MHE unit operation model called thespirally-wound tube-bundle module wasdeveloped as an integral component ofthe dynamic simulation capabilities for aprocess modeling package. The model pre-dicts the axial temperature, vapor fractionand pressure profiles for each tube streamand shell stream and axial and radial tem-perature profiles for the tube walls, shellwall and insulation. The spirally-woundtube bundle module, together with otherkey unit operation modules, can bedeployed in dynamic process models, formany applications, such as evaluating andoptimizing equipment design, control-lability and operating procedures duringthe detailed design phase; training pro-cess operators before commissioning andthroughout the lifetime of plant opera-

tions; as well as engineering studies fortroubleshooting and debottlenecking withchallenging situations in plant operations.

Mixed-refrigerant natural gasliquefaction. LNG production pro-cesses involve removing acid gases, helium,water, dust and heavy hydrocarbons, aswel l as cooling the condensation andnatural gas to approximately (~162C)through one of several commonly usedliquefaction cycles.

In the propane pre-cooled, mixed-

refrigerant cycle, a classical propane liq-uefaction cycle precools both the feed

and the mixed refrigerant.1Precooling isfollowed by a mixed refrigerant liquefac-tion cycle that provides low-temperaturerefrigeration. Several advantages can berealized with this system.2It allows moreLNG production when driver size islimited, substantially reduces the size ofthe cryogenic exchangers, permits someexchangers to be manufactured in steel,and reduces the number of high-pressurerefrigerant separators. The propane systemalso provides fixed temperature levels forfeed drying as well as recovery of compo-nents from the feed for export or use as

makeup refrigerants. Finally, the low suc-tion temperatures (about 35C) reducecompressor inlet flow volumes.

As illustrated in Fig. 1, the mixed-refrigerant liquefaction cycle cools thehigh-pressure mixed refrigerant and natu-ral gas feed in a common cryogenic heatexchanger, the MHE, against the low-pres-sure refrigerant returning to the compres-sor suction. The mixed refrigerant fromthe compressor discharge is partially lique-fied against propane and then separated inthe high-pressure (HP) separator. In thisinstance, the MHE has two spirally-wound

MR compressors

LNG

Feed

Drier

Propanecompressor

LNGstorage

Fractionation

HP separator

Fuel

Propane precooled, mixed-refrigerant liquefaction process.1FIG. 1

Originally appeared in:July 2010, pgs 37-44.Used with permission.

HYDROCARBON PROCESSING JULY 2010


2/4

LIQUEFIED NATURAL GAS DEVELOPMENTSSPECIALREPORT

tube bundles. The liquid from the HP sepa-rator passes through the first (warm) bundle

of the MHE, where it is sub-cooled. It isthen flashed into the shell at the warm bun-dle top, joining with the refrigerant fromthe top (cold) bundle to provide refrigera-tion. Vapor from the HP separator passesthrough both bundles where it is partiallycondensed. It is then flashed into the shellto provide refrigeration for the top bundle.As the mixed refrigerant progresses downthe shell toward the compressor suction,the liquid becomes heavier in compositionand boils at higher temperatures, provid-ing evaporative cooling at a continuum of

temperatures. The last amount of liquid isvaporized in the bottom bundle and theresulting mixed refrigerant vapor is super-heated before reaching the compressor.

Alternatively, the MHE can have threetube bundles rather than the two bundleconfigurations, as illustrated in Fig. 2, thatshows a high-level flowsheet for dynamicsimulation of an LNG plant. With thethree-bundle configuration, the bottombundle serves as the condensing heatexchanger for the fractionation (scrub)column, rather than using the precool-ers for this purpose. Vapor (almost purenatural gas) from the reflux drum of the

scrub column is re-introduced into themain heat exchanger at the bottom of the

middle bundle where it is cooled further.Also, the natural gas pressure is reducedthrough a Joule-Thomson valve before finalcooling against the low-pressure refriger-ant in the top bundle. Product purity isadjusted using liquefied petroleum gas,which is cooled and at least partially con-densed in the bottom and middle bundlesprior to being mixed with the natural gasat the bottom of the top bundle as it entersthe bottom bundle of the MHE.

Main heat exchanger.A multi-tube,

spirally-wound heat exchanger is madeup of tubes that are spirally wound on amandrel, as thread or cable is wound on aspool.4As shown in Fig. 3, a layer of tubesis wound (left to right) on the mandrel andspacers (bars, wire, etc.) are attached tothem. This is followed by a second layerof tubes wound in the opposite direction(right to left) and then a third layer (leftto right again), each layer complete withits own set of spacers. This procedure isrepeated until the required number of tubeshas been wound onto the mandrel.

The longitudinal distance between thetubes in a layer and the tube inclination

are kept constant for all layers. For thelarge exchangers used in LNG plants, the

tube diameter ranges from 38 in to 34 inand the tubes are applied to the mandrelwith a winding angle of approximately10. The tubes are connected to tubesheetsat each end of the heat exchanger and eachlayer contains tubes from all the differ-ent streams so the shell-side duty is uni-form. The heat exchanger operates intotal counter-flow, with evaporating fluidflowing downwards on the shell side andhigh-pressure, condensing fluid flowingupwards on the tube side.

For the multi-bundle exchangers used

in natural gas liquefaction processes, thebundles are housed within a single shell.Additionally, there is a reservoir for eachbundle within the mandrel to collect andredistribute the liquid phase of the refriger-ant over the annular rings within the shellof the tube bundle.

Mo d el ing the main heatexchanger.It is evident from the processdescription that the basic unit operationrequired to model the MHE is a spirally-wound shell-and-tube heat-exchanger bun-

dle having multiple tube streams and a sin-gle shell stream. Although numerous papers

HP NG

Condensatestabilization

Acid gasrecovery

Dehydration

Liquefaction

N2removal andfuel gas compressor

Liquefied naturalgas plant

LNG

FG

Refrigeration

Refrigerant preparation

HP FG

AG

NGL

Process flow diagram (flowsheet) for a dynamic simulation of an LNG plant.3FIG. 2



3/4

LIQUEFIED NATURAL GAS DEVELOPMENTS SPECIALREPORT

have been published and/or presented atconferences that discuss modeling of LNGprocesses on a qualitative basis, there are fewpublications that discuss these modelingprocesses, in particular modeling the mainheat exchanger, on a quantitative basis.

A simplified model of a spirally-woundtube bundle will not predict the expecteddynamic process behavior over the range ofoperation for which dynamic simulation isrequired. For example, a simplified modelwill not accurately predict startup dynam-ics, when, during initial startup, volumetriccapacitance influences the refrigerant charg-ing procedures and compressor suctionconditions are influenced by the refrigerantsupply as a function of the exchanger duty.Simplified modeling of heat exchangers alsoproduces irrational temperature profileswith crossovers at segment boundaries andbetween individual shell-and-tube streams.

Consequently, a first-principles math-ematical model for a tube bundle of a

spirally-wound heat exchanger, employingrigorous physical property calculations andthermodynamic flashes, was developed as adynamic unit operation of a process model-ing package. This unit operation, called thespirally-wound tube-bundle module, whenused in a flowsheet with the standard unitoperations of process modeling, reflectsthe behavior of natural gas liquefactionprocesses with the fidelity, reliability androbustness necessary to yield meaningfulresults over the range of process operationstypical of dynamic simulation studies andsimulation-based training of process opera-tors. The spirally-wound tube-bundle mod-ule predicts:

Exit flow, temperature, pressure,vapor fraction and composition for each ofthe outlet streams

Phase change within each of the tubestreams and the shell stream

Tube and shell wall temperatures Intermediate temperatures along the

heat exchanger Thermal profiles in the shell wall and

insulation.Fig. 4 shows the standard views of the

spirally-wound tube-bundle module of theprocess modeling package, illustrating agreat detail of what is captured in the model.

In large-scale, real-time and faster-than-real-time dynamic simulations typical ofdynamic studies and simulation-basedoperator training, fidelity and calculation

speed are always competing objectives.Simplifying assumptions, such as using arepresentative tube winding for each tubestream and lumping the shell-side annularrings into a single shell stream, were madewhen formulating the mathematical modelso as to balance these objectives.

The model formulation incorporatesan axially distributed model for the mate-rial flows in the multiple tube streams and

the shell stream, and an axially and radi-ally distributed model for the heat flowthrough the tube walls and the shell walland insulation. To predict phase change inthe tube streams and the shell stream, themodel for the material flows incorporates anisobaric-isenthalpic (PH) flash at each gridpoint. The solution of a spatially distrib-uted model incorporating flash calculationsfor a multiple-tube stream countercurrentflow configuration is very challenging froma computational perspectivestability,robustness and speed. Solution stability isaddressed by employing the equations-ori-ented solution architecture that solves all themodeling equations for the unit operationsimultaneously. Solution robustness andcalculation speed are addressed by replacingthe highly nonlinear PH flash equations byfirst-order Taylor series expansions whosecoefficients are updated by exception as the

solution moves through the operating spaceand by employing a multilayer grid for theprocess streams, calculating some quantitieson a course grid and projecting values forthese quantities onto the finer solution grid.

The model formulation and solutionmethodology employed in the spirally-wound tube-bundle unit operat ion isproven technology, having been successfullydeployed in dynamic simulation models ofmore than 10 LNG plants.3

The power of dynamic simulation.

The key value of dynamic simulation isthe improved process understanding itprovides.6After all, plant operations areby nature dynamic. Realistic dynamicmodels can be used to enhance the designof the control system, improve basicplant operation, and train both opera-tors and engineers.

Plant life cycleearly stages.Inthe design phase, dynamic simulation mod-els can help identify operability and controlissues and influence the design accordingly.

They serve as valuable tools for designing,testing and tuning control strategies priorto startup. They can also be used for recon-ciling trade-offs between optimized steady-state design (targeted at minimizing capitalexpenditures and operating utility costs)and dynamic operability. In addition, suchmodels often assist in the developmentof operating procedures. However, usingdynamic models for training plant opera-tors before commissioning is, by far, themost well-known application of dynamicsimulation.7With a good understandingof the production process and knowledgeof the control procedures applicable to nor-

Spirally-wound heat exchangerwith four streams.5

FIG. 3

Standard views of the spirally-wound tube-bundle module of the process modelingpackage.

FIG. 4



4/4

LIQUEFIED NATURAL GAS DEVELOPMENTSSPECIALREPORT

mal and abnormal operations, well-trainedoperators ensure productive plant opera-tions from day one.

Throughout the lifetime of aplant.Once a plant is in operation, itcan benefit from dynamic simulationmodels for improved operation on a dailybasis. The dynamic models allow processengineers and plant operators to performwhat- if studies; tes t out the impact ofpotential changes in feed stocks, operatingconditions, control strategies or operat-ing schemes and troubleshoot difficultiesencountered during plant operation. Itreduces the risk of disruption and, hence,improves the efficiency and reliability ofprocess operation.

In parallel, the dynamic models used inprecommissioning operator training can beupdated to as-built and used for continuous

training.8Analysis has shown that approxi-mately 90% of plant incidents are prevent-able and that the majority of incidentsbysome estimates the vast majorityresultfrom the actions or inactions of people.Because people will always play an integralrole in plant operations, continuous train-ing of plant personnel is crucial to achievingsafe, reliable and efficient operation.

Dynamic simulation has the power tocreate significant value throughout the lifecycle of a project, from initial investigationof the processing concepts right through

to plant operation. Although this valueis described here in broad terms withoutspecific reference to LNG projects, it cancertainly be realized in LNG projects, asshown by the following case study.

Case studyRas Laffan LNGTrain 3.A precommissioning dynamicsimulation study (DSS) was undertaken forTrain 3 of the Ras Laffan LNG facility toconfirm operational readiness of key plantassets.3The dynamic model encompassedthe liquefaction process (feed dryers, feed

pre-coolers, scrub column and main cryo-genic heat exchanger) and the refrigerationprocess (closed-loop mixed-refrigerant andpropane compression system).

The DSS was conducted during thefront-end engineering design (FEED) anddetailed design stages of the project. Dur-ing FEED, the objective of the DSS wasto confirm whether the project specifica-tions and plant design basis were suitablefor equipment selection, and whether thecontrol strategies met operability and asset-protection requirements. During this studyphase, a simplified control implementationwas necessarily employed because the con-

trol system configuration was not availableat this early stage of the project. Eighteensimulations were performed to predict andanalyze the response of the process and thecontrol system to upsets imposed in the pro-pane and mixed-refrigerant compressor sys-tems, including tripping anti-surge valves,tripping the gas turbine and loss of coolingto condensers. As is typical of such studies,model validation included a complete (vir-tual) startup of the liquefaction and refrig-eration systems, optimizing the sequenceof operations and establishing reasonableguidelines for initial refrigerant charging.

During detailed design, the objective ofthe DSS was to confirm operational readi-ness of all actual plant assets prior to con-struction and commissioning. The dynamicmodel was updated with the configurationdata for the selected equipment; its scopewas extended to include the nitrogen rejec-

tion compressor and the LNG and mixedrefrigerant turbines; and the simplifiedcontrol implementation was replaced withthe actual control system, emergency shut-down logic, gas turbine startup sequencesand compressor anti-surge control. Evalu-ation of the automation system was criticalto Ras Laffan because its configuration wasnew and unique. The simulations performedduring the initial phase of the DSS wererepeated and supplemented by six additionalsimulations using the updated and extendeddynamic model.

Generally, the DSS showed that thecontrol strategies were sufficient to protectthe equipment and personnel during upsetsituations and that the new and uniqueautomation system was effective. A sig-nificant finding from an operability per-spective was sensitivity of the compressorsto overload during upset conditions withhigh flow rates. However, possibly thegreatest single result of the DSS was theconfidence it provided in readiness for safeoperation through realistic simulation ofthe many operating scenarios investigated.

Following the conclusion of the DSS, thefocus of the dynamic model shifted fromengineering to operation. Operating pro-cedures were prepared and then validatedagainst the dynamic model, and processoperators were trained on process funda-mentals and process operation during nor-mal operation and abnormal situations.

Conclusion.Dynamic simulation has thepower to create significant value through-out the life cycle of an LNG project, testingand refining the design, virtually commis-sioning the control system prior to startup,training operations personnel both before

and after initial startup, troubleshootingoperating problems and validating pro-posed changes to plant operations beforeimplementation. Addition of the spirally-wound tube bundle module to the pro-cess modeling package enables this valueto be realized for mixed refrigerant LNGfacilities. This is proven dynamic simula-tion technology, having been deployed innumerous dynamic simulation studies andoperator training systems. HP

LITERATURE CITED

1 Edwards, T. J., C. F. Harris, Y. N. Liu and C.L. Newton, Analysis of Process Efficiency forBaseload LNG Production, Cryogenic Processesand Equipment, Fifth Intersociety CryogenicsSymposium, ASME, New Orleans, 1984.

2 Lom, W. L., Liquefied Natural Gas, AppliedScience Publications, 1979.

3 Henderson, P., H. Schindler and A. Pekediz,Dynamic Simulation Studies Help Ensure Safety

by Conforming Operational Readiness of LNGPlant Assets, AIChE Spring Conference, NewOrleans, 2004.

4 Crawford, D. B. and G. P. Eschenbrenner, HeatTransfer Equipment for LNG Projects, ChemicalEngineering Progress, Vol. 68(9), p. 62, 1972.

5 Fredheim, A. and P. Fuchs, Thermal Design ofLNG Heat Exchangers, Proceedings for theEuropean Applied Research Conference onNatural Gas, Trondheim, Norway, p. 567, 1990.

6 Svrcek, W. Y., D. P. Mahoney and B. R. Yong, AReal-Time Approach to Process Control, JohnWiley and Sons, Ltd., Chichester, England,2000.

7 Tang, A. K. C. and G. Stephenson, LNGPlant Operator Training, Petroleum Technology

Quarterly, Autumn, 1997. 8 Stephenson, G., P. Henderson andH. Schindler, Profit More from ProcessSimulation, Chemical Processing,August, 2009.

Grant Stephensonis an engi-neering fellow of Honeywell Automa-

tion Control Solutions. In his current

role, Mr. Stephenson serves as the

global simulation architect for Hon-

eywell Process Solutions. Based in London, Ontario,

Canada, he has worked in the field of process simula-

tion for more than 35 years and has held positions with

DuPont, Atomic Energy of Canada, the University of

Western Ontarios Systems Analysis Control and Design

Activity (SACDA), and Honeywell. Mr. Stephenson is the

originator of the Shadow Plant dynamic simulator and

is a pioneer of the hybrid solution architecture and its

application to large-scale dynamic simulation. He has an

MS degree in applied mathematics.

Laurie Wangis a senior prod-uct manager with Honeywell and is

responsible for the UniSim Design

Suite products. She is a registered

professional engineer with a PhD

from the University of Ottawa. She has hands-on expe-

rience with process simulation and specializes in chemi-

cal engineering thermodynamics. Ms. Wang has also

worked at the National Research Council of Canada as

a research scientist.

Article copyright 2010 by Gulf Publishing Company. All rights reserved. Printed in U.S.A.

Not to be distributed in electronic or printed form, or posted on a website, without express written permission of copyright holder.

Documents

Dynamic Simulation LNG Processes