Dynamic Simulation of Liquefied Natural Gas Processes Here’S How to Improve the Process Design and Operation of Your Facility

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

LIQUEFIED NATURAL GAS DEVELOPMENTS SPECIALREPORT

Dynamic simulation of liquefied natural gas processes Here’s 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

multi-tube, spirally-wound, cryo- and the mixed refrigerant.1 Precooling is makeup refrigerants. Finally, the low suc- genic heat exchanger, the main followed by a mixed refrigerant liquefaction temperatures (about –35°C) reduce A heat exchanger (MHE) is the tion cycle that provides low-temperature compressor inlet flow volumes. principal piece of heat-transfer equipment refrigeration. Several advantages can be As illustrated in Fig. 1, the mixed- in mixed-refrigerant liquefaction cycles for realized with this system.2 It allows more refrigerant liquefaction cycle cools the producing liquefied natural gas (LNG). LNG production when driver size is high-pressure mixed refrigerant and natu- An MHE unit operation model called the limited, substantially reduces the size of ral gas feed in a common cryogenic heat spirally-wound tube-bundle module was the cryogenic exchangers, permits some exchanger, the MHE, against the low-pres- developed as an integral component of exchangers to be manufactured in steel, sure refrigerant returning to the compres- the dynamic simulation capabilities for a and reduces the number of high-pressure sor suction. The mixed refrigerant from process modeling package. The model pre- refrigerant separators. The propane system the compressor discharge is partially lique- dicts the axial temperature, vapor fraction also provides fixed temperature levels for fied against propane and then separated in and pressure profiles for each tube stream feed drying as well as recovery of compo- the high-pressure (HP) separator. In this and shell stream and axial and radial tem- nents from the feed for export or use as instance, the MHE has two spirally-wound perature profiles for the tube walls, shell wall and insulation. The spirally-wound tube bundle module, together with other Propane LNG compressor storage key unit operation modules, can be deployed in dynamic process models, for many applications, such as evaluating and Feed optimizing equipment design, control- lability and operating procedures during LNG the detailed design phase; training process operators before commissioning and throughout the lifetime of plant opera- Drier tions; as well as engineering studies for troubleshooting and debottlenecking with challenging situations in plant operations.

Mixed-refrigerant natural gas liquefaction. LNG production pro- Fuel cesses involve removing acid gases, helium, water, dust and heavy hydrocarbons, as well as cooling the condensation and natural gas to approximately (~ –162°C) MR compressors through one of several commonly used liquefaction cycles. HP separator In the propane pre-cooled, mixed- Fractionation refrigerant cycle, a classical propane liq- FIG. 1 Propane precooled, mixed-refrigerant liquefaction process.1 uefaction cycle precools both the feed

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Dehydration N2 removal and FG Liqueﬁed natural fuel gas compressor gas plant AG

Acid gas recovery 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 angle 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 differ- condensed. It is then flashed into the shell which is cooled and at least partially con- ent streams so the shell-side duty is uni- to 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, provid- upwards 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 refriger- tube 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 Modeling the main heat 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 spirally- column, rather than using the precool- has been wound onto the mandrel. wound shell-and-tube heat-exchanger bun- ers for this purpose. Vapor (almost pure The longitudinal distance between the dle having multiple tube streams and a sin- natural gas) from the reflux drum of the tubes in a layer and the tube inclination gle shell stream. Although numerous papers

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have been published and/or presented at spirally-wound heat exchanger, employing the shell stream, and an axially and radi- conferences that discuss modeling of LNG rigorous physical property calculations and ally distributed model for the heat flow processes on a qualitative basis, there are few thermodynamic flashes, was developed as a through the tube walls and the shell wall publications that discuss these modeling dynamic unit operation of a process model- and insulation. To predict phase change in processes, in particular modeling the main ing package. This unit operation, called the the tube streams and the shell stream, the heat exchanger, on a quantitative basis. spirally-wound tube-bundle module, when model for the material flows incorporates an A simplified model of a spirally-wound used in a flowsheet with the standard unit isobaric-isenthalpic (PH) flash at each grid tube bundle will not predict the expected operations of process modeling, reflects point. The solution of a spatially distrib- dynamic process behavior over the range of the behavior of natural gas liquefaction uted model incorporating flash calculations operation for which dynamic simulation is processes with the fidelity, reliability and for a multiple-tube stream countercurrent required. For example, a simplified model robustness necessary to yield meaningful flow configuration is very challenging from will not accurately predict startup dynam- results over the range of process operations a computational perspective —stability, ics, when, during initial startup, volumetric typical of dynamic simulation studies and robustness and speed. Solution stability is capacitance influences the refrigerant charg- simulation-based training of process opera- addressed by employing the equations-ori- ing procedures and compressor suction tors. The spirally-wound tube-bundle mod- ented solution architecture that solves all the conditions are influenced by the refrigerant ule predicts: modeling equations for the unit operation supply as a function of the exchanger duty. • Exit flow, temperature, pressure, simultaneously. Solution robustness and Simplified modeling of heat exchangers also vapor fraction and composition for each of calculation speed are addressed by replacing produces irrational temperature profiles the outlet streams the highly nonlinear PH flash equations by with crossovers at segment boundaries and • Phase change within each of the tube first-order Taylor series expansions whose between individual shell-and-tube streams. streams and the shell stream coefficients are updated by exception as the Consequently, a first-principles math- • Tube and shell wall temperatures solution moves through the operating space ematical model for a tube bundle of a • 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 spirally- process modeling package, illustrating a wound tube-bundle unit operation 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 opera- The model formulation incorporates tors and engineers. FIG. 3 Spirally-wound heat exchanger an axially distributed model for the mate- with 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 recon- ciling trade-offs between optimized steady- state 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 FIG. 4 Standard views of the spirally-wound tube-bundle module of the process modeling package. of the control procedures applicable to nor-

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mal and abnormal operations, well-trained trol system configuration was not available and after initial startup, troubleshooting operators ensure productive plant opera- at this early stage of the project. Eighteen operating problems and validating pro- tions from day one. simulations were performed to predict and posed changes to plant operations before analyze the response of the process and the implementation. Addition of the spirally- Throughout the lifetime of a control system to upsets imposed in the pro- wound tube bundle module to the pro- plant. Once a plant is in operation, it pane and mixed-refrigerant compressor sys- cess modeling package enables this value can benefit from dynamic simulation tems, including tripping anti-surge valves, to be realized for mixed refrigerant LNG models for improved operation on a daily tripping the gas turbine and loss of cooling facilities. This is proven dynamic simula- basis. The dynamic models allow process to condensers. As is typical of such studies, tion technology, having been deployed in engineers and plant operators to perform model validation included a complete (vir- numerous dynamic simulation studies and what-if studies; test out the impact of tual) startup of the liquefaction and refrig- operator training systems. HP potential changes in feed stocks, operating eration systems, optimizing the sequence conditions, control strategies or operat- of operations and establishing reasonable LITERATURE CITED ing schemes and troubleshoot difficulties guidelines for initial refrigerant charging. 1 Edwards, T. J., C. F. Harris, Y. N. Liu and C. encountered during plant operation. It During detailed design, the objective of L. Newton, “Analysis of Process Efficiency for reduces the risk of disruption and, hence, the DSS was to confirm operational readi- Baseload LNG Production,” Cryogenic Processes improves the efficiency and reliability of ness of all actual plant assets prior to con- and Equipment, Fifth Intersociety Cryogenics struction and commissioning. The dynamic Symposium, ASME, New Orleans, 1984. process operation. 2 model was updated with the configuration Lom, W. L., “Liquefied Natural Gas,” Applied In parallel, the dynamic models used in Science Publications, 1979. precommissioning operator training can be data for the selected equipment; its scope 3 Henderson, P., H. Schindler and A. Pekediz, updated to as-built and used for continuous was extended to include the nitrogen rejec- “Dynamic Simulation Studies Help Ensure Safety training.8 Analysis has shown that approxi- tion compressor and the LNG and mixed by Conforming Operational Readiness of LNG mately 90% of plant incidents are prevent- refrigerant turbines; and the simplified Plant Assets,” AIChE Spring Conference, New Orleans, 2004. able and that the majority of incidents—by control implementation was replaced with 4 Crawford, D. B. and G. P. Eschenbrenner, “Heat some estimates the vast majority—result the actual control system, emergency shut- Transfer Equipment for LNG Projects,” Chemical from the actions or inactions of people. down logic, gas turbine startup sequences Engineering Progress, Vol. 68(9), p. 62, 1972. Because people will always play an integral and compressor anti-surge control. Evalu- 5 Fredheim, A. and P. Fuchs, “Thermal Design of ation of the automation system was critical LNG Heat Exchangers,” Proceedings for the role in plant operations, continuous train- European Applied Research Conference on ing of plant personnel is crucial to achieving to Ras Laffan because its configuration was Natural Gas, Trondheim, Norway, p. 567, 1990. safe, reliable and efficient operation. new and unique. The simulations performed 6 Svrcek, W. Y., D. P. Mahoney and B. R. Yong, “A Dynamic simulation has the power to during the initial phase of the DSS were Real-Time Approach to Process Control,” John create significant value throughout the life repeated and supplemented by six additional Wiley and Sons, Ltd., Chichester, England, 2000. cycle of a project, from initial investigation simulations using the updated and extended 7 Tang, A. K. C. and G. Stephenson, “LNG of the processing concepts right through dynamic model. Plant Operator Training,” Petroleum Technology to plant operation. Although this value Generally, the DSS showed that the Quarterly, Autumn, 1997. is described here in broad terms without control strategies were sufficient to protect 8 Stephenson, G., P. Henderson and the equipment and personnel during upset H. Schindler, “Profit More from Process specific reference to LNG projects, it can Simulation,” Chemical Processing, August, 2009. certainly be realized in LNG projects, as situations and that the new and unique shown by the following case study. automation system was effective. A significant finding from an operability perspective was sensitivity of the compressors Grant Stephenson is an engi- Case study—Ras Laffan LNG— neering fellow of Honeywell Automa- Train 3. A precommissioning dynamic to overload during upset conditions with tion Control Solutions. In his current simulation study (DSS) was undertaken for high flow rates. However, possibly the role, Mr. Stephenson serves as the Train 3 of the Ras Laffan LNG facility to greatest single result of the DSS was the global simulation architect for Hon- confidence it provided in readiness for safe eywell Process Solutions. Based in London, Ontario, confirm operational readiness of key plant Canada, he has worked in the field of process simula- 3 assets. The dynamic model encompassed operation through realistic simulation of tion for more than 35 years and has held positions with the liquefaction process (feed dryers, feed the many operating scenarios investigated. DuPont, Atomic Energy of Canada, the University of pre-coolers, scrub column and main cryo- Following the conclusion of the DSS, the Western Ontario’s Systems Analysis Control and Design genic heat exchanger) and the refrigeration focus of the dynamic model shifted from Activity (SACDA), and Honeywell. Mr. Stephenson is the engineering to operation. Operating pro- originator of the Shadow Plant dynamic simulator and process (closed-loop mixed-refrigerant and is a pioneer of the hybrid solution architecture and its propane compression system). cedures were prepared and then validated application to large-scale dynamic simulation. He has an The DSS was conducted during the against the dynamic model, and process MS degree in applied mathematics. front-end engineering design (FEED) and operators were trained on process funda- detailed design stages of the project. Dur- mentals and process operation during nor- ing FEED, the objective of the DSS was mal operation and abnormal situations. Laurie Wang is a senior product manager with Honeywell and is to confirm whether the project specifica- responsible for the UniSim Design tions and plant design basis were suitable Conclusion. Dynamic simulation has the Suite products. She is a registered for equipment selection, and whether the power to create significant value through- professional engineer with a PhD control strategies met operability and asset- out the life cycle of an LNG project, testing from the University of Ottawa. She has hands-on expe- and refining the design, virtually commis- rience with process simulation and specializes in chemi- protection requirements. During this study cal engineering thermodynamics. Ms. Wang has also phase, a simplified control implementation sioning the control system prior to startup, worked at the National Research Council of Canada as was necessarily employed because the con- training operations personnel both before a research scientist.

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