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

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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 liquefac- tion 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 pro- cess 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 HYDROCARBON PROCESSING JULY 2010 13027.indd 1 7/30/10 3:55 PM SPECIALREPORT LIQUEFIED NATURAL GAS DEVELOPMENTS 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 HYDROCARBON PROCESSING JULY 2010 13027.indd 2 7/30/10 3:55 PM LIQUEFIED NATURAL GAS DEVELOPMENTS SPECIALREPORT 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.
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