Dynamic-Simulation-LNG-Processes-eop.pdf

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

LIQUEFIED NATURAL GAS DEVELOPMENTS

SPECIALREPORT

Dynamic simulation of liquefied natural gas processes Here’ss how to improve the process design and operation of your facility Here’ G. STEPHENSON, Honeywell Process Solutions, London, Ontario, Canada; and L. WANG, Honeywell Process Solutions, Calgary, Alberta, Canada

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multi-tube, spirally-wound, cryogenic heat exchanger, the main heat exchanger (MHE) is the principal piece of heat-transfer equipment in mixed-refrigerant liquefaction cycles for producing liquefied natural gas (LNG).  An MHE unit operation operation model called the spirally-wound tube-bundle module was developed as an integral component of the dynamic simulation capabilities for a process modeling package. The model prep redicts the axial temperature, vapor fraction and pressure profiles for each tube stream and shell stream and axial and radial temperature profiles for the tube walls, shell  wall and insulation. ins ulation. The Th e spirally-wound spiral ly-wound tube bundle module, together with other key unit operation modules, can be deployed in dynamic process models, for many applications, such as evaluating and optimizing equipment design, controllability and operating procedures during the detailed design phase; training process operators before commissioning and throughout the lifetime of plant operations; as well as engineering studies for troubleshooting and debottlenecking with challenging situations in plant operations. Mixed-refrigerant natural gas liquefaction.  LNG production processes involve removing acid gases, helium,  water, dust and heavy hydro hydrocarbo carbons, ns, as  wel l as co cool oling ing the co cond ndens ens ati on and natural gas to approximately ( –162°C) through one of several commonly used liquefaction cycles. In the propane pre-cooled, mixedrefrigerant cycle, a classical propane liquefaction cycle precools both the feed

and the mixed refrigerant. 1 Precooling is followed by a mixed refrigerant liquefaction cycle that provides low-temperature refrigeration. Several advantages can be realized with this system. 2 It allows more LNG production when driver size is limited, substantially reduces the size of the cryogenic exchangers, permits some exchangers to be manufactured in steel, and reduces the number of high-pressure refrigerant separators. The propane system also provides fixed temperature levels for feed drying as well as recovery of components from the feed for export or use as Propane compressor

makeup refrigerants. Finally, the low suction temperatures (about –35°C) reduce compressor inlet flow volumes.  As ill illus ustra trated ted in Fig. 1, the mix mixededrefrigerant liquefaction cycle cools the high-pressure mixed refrigerant and natural gas feed in a common cryogenic heat exchanger,, the MHE, against the low-presexchanger sure refrigerant returning to the compressor suction. The mixed refrigerant from the compressor discharge is partially liquefied against propane and then separated in the high-pressure (HP) separator. In this instance, the MHE has two spirally-wound LNG storage

Feed LNG

Drier

Fuel

MR compressors

~

HP separator Fractionation FIG. 1

Propane precooled, mixed-refrigerant liquefaction process.1

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LIQUEFIED NATURAL GAS DEVELOPMENTS

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Dehydration

Liquefied natural gas plant

 AG  Acid gas recovery

N2 removal and fuel gas compressor

FG

Liquefaction

LNG

HP FG

HP NG Refrigeration

Condensate stabilization

NGL Refrigerant preparation FIG. 2

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

tube bundles. The liquid from the HP sepa- scrub column is re-introduced into the are kept constant for all layers. For the rator passes through the first (warm) bundle main heat exchanger at the bottom of the large exchangers used in LNG plants, the of the MHE, where it is sub-cooled. It is middle bundle where it is cooled further. tube diameter ranges from 3 ⁄8 in to 3 ⁄4 in then flashed into the shell at the warm bun-  Also, the natural gas pressure is reduced and the tubes are applied to the mandrel dle top, joining with the refrigerant from through a Joule-Thomson valve before final  with a winding ang le of approximately the top (cold) bundle to provide refrigera- cooling against the low-pressure refriger- 10°. The tubes are connected to tubesheets tion. Vapor from the HP separator passes ant in the top bundle. Product purity is at each end of the heat exchanger and each through both bundles where it is partially adjusted using liquefied petroleum gas, layer contains tubes from all the differcondensed. It is then flashed into the shell  which is cooled and at least partially con- ent streams so the shell-side duty is unito provide refrigeration for the top bundle. densed in the bottom and middle bundles form. The heat exchanger operates in  As the mixed refrigerant progresses down prior to being mixed with the natural gas total counter-flow, with evaporating fluid the shell toward the compressor suction, at the bottom of the top bundle as it enters flowing downwards on the shell side and the liquid becomes heavier in composition the bottom bundle of the MHE. high-pressure, condensing fluid flowing and boils at higher temperatures, providupwards on the tube side. ing evaporative cooling at a continuum of Main heat exchanger. A multi-tube, For the multi-bundle exchangers used temperatures. The last amount of liquid is spirally-wound heat exchanger is made in natural gas liquefaction processes, the vaporized in the bottom bundle and the up of tubes that are spirally wound on a bundles are housed within a single shell. resulting mixed refrigerant vapor is super- mandrel, as thread or cable is wound on a  Additionally, there is a reservoir for each heated before reaching the compressor. spool.4 As shown in Fig. 3, a layer of tubes bundle within the mandrel to collect and  Alternatively, the MHE can have three is wound (left to right) on the mandrel and redistribute the liquid phase of the refrigertube bundles rather than the two bundle spacers (bars, wire, etc.) are attached to ant over the annular rings within the shell configurations, as illustrated in Fig. 2, that them. This is followed by a second layer of the tube bundle. shows a high-level flowsheet for dynamic of tubes wound in the opposite direction simulation of an LNG plant. With the (right to left) and then a third layer (left M o d e l i n g t h e m a i n h e a t three-bundle configuration, the bottom to right again), each layer complete with exchanger. It is evident from the process bundle serves as the condensing heat its own set of spacers. This procedure is description that the basic unit operation exchanger for the fractionation (scrub) repeated until the required number of tubes required to model the MHE is a spirallycolumn, rather than using the precool- has been wound onto the mandrel.  wound shell-and-tube heat-exchanger buners for this purpose. Vapor (almost pure The longitudinal distance between the dle having multiple tube streams and a sinnatural gas) from the reflux drum of the tubes in a layer and the tube inclination gle shell stream. Although numerous papers HYDROCARBON PROCESSING JULY 2010

LIQUEFIED NATURAL GAS DEVELOPMENTS have been published and/or presented at conferences that discuss modeling of LNG processes on a qualitative basis, there are few publications that discuss these modeling processes, in particular modeling the main heat exchanger, on a quantitative basis.  A simplified model of a spirally-wound tube bundle will not predict the expected dynamic process behavior over the range of operation for which dynamic simulation is required. For example, a simplified model  will not accurately predict startup dynamics, when, during initial startup, volumetric capacitance influences the refrigerant charging procedures and compressor suction conditions are influenced by the refrigerant supply as a function of the exchanger duty. Simplified modeling of heat exchangers also produces irrational temperature profiles  with crossovers at segment boundaries and between individual shell-and-tube streams. Consequently, a first-principles mathematical model for a tube bundle of a

FIG. 3

FIG. 4

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spirally-wound heat exchanger, employing the shell stream, and an axially and radirigorous physical property calculations and ally distributed model for the heat flow thermodynamic flashes, was developed as a through the tube walls and the shell wall dynamic unit operation of a process model- and insulation. To predict phase change in ing package. This unit operation, called the the tube streams and the shell stream, the spirally-wound tube-bundle module, when model for the material flows incorporates an used in a flowsheet with the standard unit isobaric-isenthalpic (PH) flash at each grid operations of process modeling, reflects point. The solution of a spatially distribthe behavior of natural gas liquefaction uted model incorporating flash calculations processes with the fidelity, reliability and for a multiple-tube stream countercurrent robustness necessary to yield meaningful flow configuration is very challenging from results over the range of process operations a computational perspective—stability, typical of dynamic simulation studies and robustness and speed. Solution stability is simulation-based training of process opera- addressed by employing the equations-oritors. The spirally-wound tube-bundle mod- ented solution architecture that solves all the ule predicts: modeling equations for the unit operation • Exit flow, temperature, pressure, simultaneously. Solution robustness and vapor fraction and composition for each of calculation speed are addressed by replacing the outlet streams the highly nonlinear PH flash equations by • Phase change within each of the tube first-order Taylor series expansions whose streams and the shell stream coefficients are updated by exception as the • Tube and shell wall temperatures solution moves through the operating space • Intermediate temperatures along the and by employing a multilayer grid for the heat exchanger process streams, calculating some quantities • Thermal profiles in the shell wall and on a course grid and projecting values for insulation. these quantities onto the finer solution grid. Fig. 4 shows the standard views of the The model formulation and solution spirally-wound tube-bundle module of the methodology employed in the spirallyprocess modeling package, illustrating a  wound tube-bundl e unit op eration is great detail of what is captured in the model. proven technology, having been successfully In large-scale, real-time and faster-than- deployed in dynamic simulation models of real-time dynamic simulations typical of more than 10 LNG plants. 3 dynamic studies and simulation-based operator training, fidelity and calculation The power of dynamic simulation. speed are always competing objectives. The key value of dynamic simulation is Simplifying assumptions, such as using a the improved process understanding it representative tube winding for each tube provides. 6 After all, plant operations are stream and lumping the shell-side annular by nature dynamic. Realistic dynamic rings into a single shell stream, were made models can be used to enhance the design  when formulating the mathematical model of the control system, improve basic so as to balance these objectives. plant operation, and train both operaThe model formulation incorporates tors and engineers. Spirally-wound heat exchanger an axially distributed model for the matewith four streams.5 rial flows in the multiple tube streams and Plant life cycle—early stages.  In the design phase, dynamic simulation models can help identify operability and control issues and influence the design accordingly. They serve as valuable tools for designing, testing and tuning control strategies prior to startup. They can also be used for reconciling trade-offs between optimized steadystate design (targeted at minimizing capital expenditures and operating utility costs) and dynamic operability. In addition, such models often assist in the development of operating procedures. However, using dynamic models for training plant operators before commissioning is, by far, the most well-known application of dynamic simulation.7 With a good understanding of the production process and knowledge Standard views of the spirally-wound tube-bundle module of the process modeling of the control procedures applicable to norpackage. HYDROCARBON PROCESSING JULY 2010

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LIQUEFIED NATURAL GAS DEVELOPMENTS

mal and abnormal operations, well-trained operators ensure productive plant operations from day one. Throughout the lifetime of a plant. Once a plant is in operation, it can benefit from dynamic simulation models for improved operation on a daily basis. The dynamic models allow process engineers and plant operators to perform  what- if studies; tes t out the imp act of potential changes in feed stocks, operating conditions, control strategies or operating schemes and troubleshoot difficulties encountered during plant operation. It reduces the risk of disruption and, hence, improves the efficiency and reliability of process operation. In parallel, the dynamic models used in precommissioning operator training can be updated to as-built and used for continuous training.8 Analysis has shown that approximately 90% of plant incidents are preventable and that the majority of incidents—by some estimates the vast majority—result from the actions or inactions of people. Because people will always play an integral role in plant operations, continuous training of plant personnel is crucial to achieving safe, reliable and efficient operation. Dynamic simulation has the power to create significant value throughout the life cycle of a project, from initial investigation of the processing concepts right through to plant operation. Although this value is described here in broad terms without specific reference to LNG projects, it can certainly be realized in LNG projects, as shown by the following case study. Case study—Ras Laffan LNG— Train 3. A precommissioning dynamic simulation study (DSS) was undertaken for Train 3 of the Ras Laffan LNG facility to confirm operational readiness of key plant assets.3 The dynamic model encompassed the liquefaction process (feed dryers, feed pre-coolers, scrub column and main cryogenic heat exchanger) and the refrigeration process (closed-loop mixed-refrigerant and propane compression system). The DSS was conducted during the front-end engineering design (FEED) and detailed design stages of the project. During FEED, the objective of the DSS was to confirm whether the project specifications and plant design basis were suitable for equipment selection, and whether the control strategies met operability and assetprotection requirements. During this study phase, a simplified control implementation  was necessarily employed because the con-

trol system configuration was not available and after initial startup, troubleshooting at this early stage of the project. Eighteen operating problems and validating prosimulations were performed to predict and posed changes to plant operations before analyze the response of the process and the implementation. Addition of the spirallycontrol system to upsets imposed in the pro-  wound tube bundle module to the propane and mixed-refrigerant compressor sys- cess modeling package enables this value tems, including tripping anti-surge valves, to be realized for mixed refrigerant LNG tripping the gas turbine and loss of cooling facilities. This is proven dynamic simulato condensers. As is typical of such studies, tion technology, having been deployed in model validation included a complete (vir- numerous dynamic simulation studies and tual) startup of the liquefaction and refrig- operator training systems. HP eration systems, optimizing the sequence of operations and establishing reasonable LITERATURE CITED guidelines for initial refrigerant charging. 1 Edwards, T. J., C. F. Harris, Y. N. Liu and C. During detailed design, the objective of L. Newton, “Analysis of Process Efficiency for the DSS was to confirm operational readiBaseload LNG Production,” Cryogenic Processes ness of all actual plant assets prior to conand Equipment, Fifth Intersociety Cryogenics struction and commissioning. The dynamic 2 Symposium, ASME, New Orleans, 1984. Lom, W. L., “Liquefied Natural Gas,”  Applied model was updated with the configuration Science Publications , 1979. data for the selected equipment; its scope 3 Henderson, P., H. Schindler and A. Pekediz,  was extended to include the nitrogen rejec“Dynamic Simulation Studies Help Ensure Safety by Conforming Operational Readiness of LNG tion compressor and the LNG and mixed Plant Assets,” AIChE Spring Conference, New refrigerant turbines; and the simplified Orleans, 2004. control implementation was replaced with 4 Crawford, D. B. and G. P. Eschenbrenner, “Heat the actual control system, emergency shutTransfer Equipment for LNG Projects,” Chemical down logic, gas turbine startup sequences Engineering Progress , Vol. 68(9), p. 62, 1972. 5 Fredheim, A. and P. Fuchs, “Thermal Design of and compressor anti-surge control. EvaluLNG Heat Exchangers,” Proceedings for the ation of the automation system was critical European Applied Research Conference on to Ras Laffan because its configuration was Natural Gas, Trondheim, Norway, p. 567, 1990. new and unique. The simulations performed 6 Svrcek, W. Y., D. P. Mahoney and B. R. Yong, “A Real-Time Approach to Process Control,” John during the initial phase of the DSS were  Wiley and Sons, Ltd., Chichester, England, repeated and supplemented by six additional 2000. simulations using the updated and extended 7 Tang, A. K. C. and G. Stephenson, “LNG dynamic model. Plant Operator Training,” Petroleum Technology Generally, the DSS showed that the Quarterly , Autumn, 1997. 8 Stephenson, G., P. Henderson and control strategies were sufficient to protect H. Schindler, “Profit More from Process the equipment and personnel during upset Simulation,” Chemical Processing, August, 2009. situations and that the new and unique automation system was effective. A significant finding from an operability perGrant Stephenson is an engispective was sensitivity of the compressors neering fellow of Honeywell Automato overload during upset conditions with tion Control Solutions. In his current high flow rates. However, possibly the role, Mr. Stephenson serves as the global simulation architect for Hongreatest single result of the DSS was the eywell Process Solutions. Based in London, Ontario, confidence it provided in readiness for safe Canada, he has worked in the field of process simulaoperation through realistic simulation of tion for more than 35 years and has held positions with the many operating scenarios investigated. DuPont, Atomic Energy of Canada, the University of Following the conclusion of the DSS, the Western Ontario’s Systems Analysis Control and Design focus of the dynamic model shifted from Activity (SACDA), and Honeywell. Mr. Stephenson is the of the Shadow Plant dynamic simulator and engineering to operation. Operating pro- originator is a pioneer of the hybrid solution architecture and its cedures were prepared and then validated application to large-scale dynamic simulation. He has an against the dynamic model, and process MS degree in applied mathematics. operators were trained on process fundamentals and process operation during norLaurie Wang  is a senior prodmal operation and abnormal situations. Conclusion. Dynamic simulation has the power to create significant value throughout the life cycle of an LNG project, testing and refining the design, virtually commissioning the control system prior to startup, training operations personnel both before

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 hand s-on experience with process simulation and specializes in chemical engineering thermodynamics. Ms. Wang has also worked at the National Research Council o f Canada as a research scientist.

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