Reprinted from HYDROCARBON ENGINEERING FEBRUARY 2004

Technology selection starts at an early stage in the life ofa baseload LNG project and is typically addressed at thefeasibility study and pre-FEED definition stages.

Process routes must be chosen for the process, utilities andoffsite units of the plant, which include proprietary and non-proprietary technologies. This also applies to the upstreampart of the chain, which supplies the gas to the plant.

Potential options must be identified and evaluation criteriaestablished. The selection could be between alternative pro-cessing technologies for the operating units, the type of majorequipment, or utilities schemes.

This article presents an overview of the LNG process andan introduction to the main processes available for the lique-faction section of a baseload LNG plant. It also discussessome selection issues relating to the main technologies thataffect LNG plant configuration.

The LNG processAn example of a LNG plant overall flow scheme, and the mainprocess units and supporting utilities, is shown in Figure 1. Theprocess and utility requirement depend, amongst other things,on the site conditions, feed gas quality and product specifica-tion.

In a typical scheme the feed gas is delivered at high pres-sure (for example, up to 90 bara) from upstream gas fields viatrunk lines and any associated condensate will be removed.The gas is metered and its pressure controlled to the designoperating pressure of the plant.

The gas is first pre-treated to remove any impurities thatinterfere with processing or are undesirable in the final prod-ucts. These include acid gases and sulphur compounds (forexample, CO2, H2S and mercaptans), water and mercury.

The dry sweet gas is then cooled by refrigerant streams toseparate heavier hydrocarbons. The remaining gas is madeup mainly of methane and contains less than 0.1 mol% of pen-tane and heavier hydrocarbons. It is further cooled in the cryo-genic section to approximately -160 ˚C and is completely liq-uefied. The resulting LNG is stored in atmospheric tanks readyfor export by ship.

The heavier hydrocarbons separated during cooling arefractionated to recover ethane, propane and butane. Ethane isnormally reinjected into the gas stream to be liquefied. Thepropane and butane can either be reinjected or exported asLPG products. The remaining hydrocarbons (pentane andheavier components) are exported as a gasoline product.

The utilities required to support the processing unitsinclude fuel gas (derived from the process streams) to gener-ate electric power, cooling medium (water or air), heatingmedium (steam or hot oil system), and other services such asinstrument air and nitrogen.

Liquefaction technologyThe refrigeration and liquefaction section is the key element ofthe LNG plant. There are several licensed processes availablewith varying degrees of application and experience. There areothers proposed or under development but are not consideredhere.

The basic principles for cooling and liquefying the gasusing refrigerants involve matching as closely as possible thecooling/heating curves of the process gas and the refrigerant.This results in a more efficient thermodynamic process requir-ing less power per unit of LNG produced. This applies to all liq-uefaction processes. Typical cooling curves are shown inFigure 2.

However, the way this is achieved and the equipment usedplay a major part in the overall efficiency, operability, reliabilityand cost of the plant. The liquefaction section typicallyaccounts for 30 - 40% of the capital cost of the overall plant.

Key equipment items include the compressors used to cir-culate the refrigerants, the compressor drivers and the heatexchangers used to cool and liquefy the gas and exchange

Dr Tariq Shukri, Foster Wheeler, UK, discusses available LNG technologies and the important criteria for selection.

Figure 1. LNG block flow diagram.

Figure 2. Typical natural gas/refrigerant coolingcurves.

LNG technology

selection

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heat between refrigerants. For recent baseoad LNG plants thisequipment is among the biggest of its type and at the leadingedge of technology.

The natural gas, being a mixture of compounds, liquefiesover a wide temperature range. Heat curves can be matchedby minimising the temperature difference between the coolingprocess gas and refrigerant streams. This is achieved by usingmore than one refrigerant to cover the temperature range andusing the refrigerant at different pressure levels to further splitthe temperature ranges to closely matching ones. The processgas side is normally operated at high pressure (for example,40 - 55 bara) to reduce equipment size and provide more effi-cient refrigeration.

The composition of the refrigerant gives an added control

parameter as it can be made either from pure ormixed components. With a mixed refrigerant thecomposition can be adjusted to suit the processconditions.

The heat exchangers used, for example, thespiral/coil wound heat exchangers (CWHE) orthe plate fin heat exchangers (PFHE), havevery large surface areas and a large number ofpasses, enabling close temperatureapproaches.

The main available liquefaction processesare described below. The MCRTM process willbe described in greatest detail. Many of the prin-ciples apply to other processes. The main dif-ferences will be highlighted.

APCI propane pre-cooledmixed refrigerant process(MCR™) This process accounts for a very significant pro-portion of the world’s baseload LNG productioncapacity. Train capacities of up to 4.7 million tpy

were built or are under construction. It is illustrated in Figure 3as part of an overall LNG plant flow scheme.

There are two main refrigerant cycles. The precoolingcycle uses a pure component, propane. The liquefactionand sub-cooling cycle uses a mixed refrigerant (MR)made up of nitrogen, methane, ethane and propane.

The precooling cycle uses propane at three or four pres-sure levels and can cool the process gas down to -40 ˚C. It isalso used to cool and partially liquefy the MR. The cooling isachieved in kettle-type exchangers with propane refrigerantboiling and evaporating in a pool on the shell side, and with theprocess streams flowing in immersed tube passes.

A centrifugal compressor with side streams recoversthe evaporated C3 streams and compresses the vapour to15 - 25 bara to be condensed against water or air andrecycled to the propane kettles.

In the MR cycle, the partially liquefied refrigerant is sepa-rated into vapour and liquid streams that are used to liquefyand sub-cool the process stream from typically -35 ˚C tobetween -150 ˚C - -160 ˚C. This is carried out in a proprietaryspiral wound exchanger, the main cryogenic heat exchanger(MCHE).

The MCHE consists of two or three tube bundlesarranged in a vertical shell, with the process gas andrefrigerants entering the tubes at the bottom which thenflow upward under pressure.

The process gas passes through all the bundles to emergeliquefied at the top. The liquid MR stream is extracted after thewarm or middle bundle and is flashed across a Joule Thomsonvalve or hydraulic expander onto the shell side. It flows down-wards and evaporates, providing the bulk of cooling for thelower bundles. The vapour MR stream passes to the top (coldbundle) and is liquefied and sub-cooled, and is flashed acrossa JT valve into the shell side over the top of the cold bundle. Itflows downwards to provide the cooling duty for the top bun-dle and, after mixing with liquid MR, part of the duty for thelower bundles.

The overall vaporised MR stream from the bottom of theMCHE is recovered and compressed by the MR compressorto 45 - 48 bara. It is cooled and partially liquefied first by wateror air and then by the propane refrigerant, and recycled to theMCHE. In earlier plants all stages of the MR compressionwere normally centrifugal, however, in some recent plantsaxial compressors have been used for the LP stage and cen-trifugal for the HP stage. Recent plants use Frame 6 and/or

Reprinted from HYDROCARBON ENGINEERING FEBRUARY 2004

Figure 3. APCI propane precooled mixed refrigerant process (typical).

Figure 4. Phillips optimised cascade process.

Figure 5. Black & Veatch PRICO process.

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Frame 7 gas turbine drivers. Earlier plants used steam turbinedrivers.

A recent modification of the process, which is being con-sidered for large LNG capacity plants (> 6 million tpy), is theAPX-process, which adds a third refrigerant cycle (nitrogenexpander) to conduct LNG subcooling duties outside theMCHE.

Phillips optimised cascade processThis process, a modified version of a process used in an ear-lier plant in Alaska during the 1960s, was used for the AtlanticLNG plant in Trinidad and for a baseload plant under con-struction in Egypt. Train capacities of up to 3.3 million tpy havebeen constructed with larger trains in development. Thisprocess is illustrated in Figure 4.

Refrigeration and liquefaction of the process gas is

achieved in a cascade process using three pure componentrefrigerants; propane, ethylene and methane, each at two orthree pressure levels.

This is carried out in a series of brazed aluminium PFHEsarranged in vertical cold boxes. Precooling could be carriedout in a core-in-kettle type exchanger.

The refrigerants are circulated using centrifugal compres-sors. Each refrigerant has parallel compression trains. Frame 5 gas turbine drivers were used.

Black & Veatch PRICO® processThis is a single mixed refrigerant process used on an earlierbaseload plant in Algeria. Train capacity has been uprated to1.3 tpy per train. It is illustrated in Figure 5.

The mixed refrigerant is made up of nitrogen, methane,ethane, propane and iso-pentane. The cooling and liquefac-tion is carried out at several pressure levels, in PFHEs in coldboxes. The refrigerant is compressed and circulated using asingle compression train. In the Algerian plant axial compres-sors driven by steam turbines were used.

Statoil/Linde mixed fluid cascadeprocess (MFCP)In this process three mixed refrigerants are used to provide thecooling and liquefaction duty. It has been selected for theSnøhvit LNG project (Ekofisk, Norway) which is underdesign/construction. This is a single train 4 million tpy LNGplant. The process is illustrated in Figure 6.

Pre-cooling is carried out in PFHE by the first mixed refrig-erant, and the liquefaction and subcooling are carried out in aspiral wound heat exchanger (SWHE) by the other two refrig-erants. The SWHE is a proprietary exchanger made by Linde.It may also be used for the pre-cooling stage. The refrigerantsare made up of components selected from methane, ethane,propane and nitrogen.

The three refrigerant compression systems can have sep-arate drivers or integrated to have two strings of compression.Frame 6 and Frame 7 gas turbine drivers have been proposedfor large LNG trains (> 4 million tpy). A novel feature of theSnøhvit project is that all motor drivers will be used for themain refrigerant compressors, with sizes up to 60 MW.

The SWHE itself is being installed with other liquefactionprocesses, in new and expansion projects or as areplacement for old cryogenic exchangers.

Axens Liquefin™ processThis is a two-mixed refrigerant process, which isbeing proposed for some new LNG base load pro-jects of train sizes up to 6 million tpy. It is illustratedin Figure 7.

Detailed studies have been made including inputfrom main equipment vendors. All cooling and lique-faction is conducted in PFHE arranged in coldboxes. The refrigerants are made up of componentsfrom methane, ethane, propane, butane and nitro-gen. The first mixed refrigerant is used at three dif-ferent pressure levels to precool the process gasand precool and liquefy the second mixed refriger-ant. The second mixed refrigerant is used to liquefyand subcool the process gas.

Using a mixed refrigerant for the precoolingstage allows a lower temperature to be achieved(for example, -60 ˚C) depending on refrigerant com-position.

The PFHEs are non-proprietary and can besupplied by independent vendors. Two large dri-vers can drive the refrigerant compression sys-tems. Frame 7 gas turbines are being proposed

Reprinted from HYDROCARBON ENGINEERING FEBRUARY 2004

Table 1. Some technology selection parametersTechnology selection items Pros ConsSpiral wound exchanger Flexible operation Proprietary/more expensivePFHE Competitive vendors Require careful design to

available. Lower pressure ensure good 2-phase flowdrop and temperature distribution in multiple differences exchanger configurations

Axial compressors High efficiency Suitable only at high flow rates.

Large gas turbines Proven, efficient and cost Less reliable/stricteffective maintenance cycle/

more complicated control/fixed speed

Large motor drivers Efficient, flexible & more Untried in LNG at speedsavailable needed/require large

power plant.Mixed refrigerant process Simpler compression system. More complex operation.

Adjusting composition allows process matching

Pure component cascade Potential higher availability More equipment andprocess with parallel compression complicated compression

systemAir cooling (compared to Lower cooling system Less efficient processsea water cooling) CAPEX /higher operating costsFluid medium heating Eliminates the need for Higher reboiler costs(compared to steam) steam generation & water

treatmentLarger train capacity Lower specific costs Some equipment/

(CAPEX per tonne LNG) processes may requirefurther development

Figure 6. Statoil/Linde mixed fluid cascade process(MFCP).

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for the large LNG trains.

Shell double mixed refrigerantprocess (DMR)This is a dual mixed refrigerant process, which is being appliedin the Sakhalin Island project with a capacity of 4.8 million tpy per train.

Process configuration is similar to the propane pre-cooledmixed refrigerant process, with the precooling conducted by amixed refrigerant (made up mainly of ethane and propane)rather than pure propane. Another main difference is that theprecooling is carried out in SWHEs rather than kettles. Theprecooling and liquefaction SWHEs will be supplied by Linde.

The refrigerant compressors are driven by two Frame 7gas turbines. An axial compressor is also used as part of thecold refrigerant compression stages.

Other processesThe above processes are used in current LNG plants or areapplied in LNG projects in progress. There are otherprocesses developed or in development for baseload LNGapplications, which can be or are being considered in feasibil-ity studies or for future projects but are not discussed here.

The trend is to extend the capability of existing processesand develop new processes to support large LNG capacitiesof over 5 million tpy per train. Larger train capacities result inlower specific costs.

Process selectionTechnology selection of process and equipment will be basedon technical and economic considerations. Foster Wheelerhas carried out selection studies as part of major LNG projectsand proposals during the various phases of feasibility, FEEDand detailed engineering. In addition to an extensive in-houseLNG database, contacts are made with the liquefaction licen-sors and main equipment vendors to obtain data and developdesigns to enable valid comparisons and optimum selections.

Depending on the stage of project development, sufficientprocess details must be developed to define main equipmentand operating parameters to evaluate options using relevantcriteria.

Technical considerations include process and equipmentexperience, reliability, process efficiency, site conditions andenvironmental impact. Economic issues include capital cost,operating cost and lifecycle costing. All of these aspects willneed to be evaluated to arrive at the optimum solution.

Technical risks associated with a process relate to thetrack record of the process in operation, and any develop-ments required for the project for example, capacity increase.

Process efficiency, for example, energy required to

produce LNG, is not solely related to the thermodynamic effi-ciency of the liquefaction process but also to the efficiency ofthe main equipment such as the main refrigerant compressorsand drivers.

Site conditions may favour one type of process over another.For example, with very cold ambient temperatures multi-mixedrefrigerant processes may offer the optimum solution.

Process requirements and configuration will have an influ-ence on selection. A requirement for greater LPG recoverymay suit processes with lower precooling temperatures.

Wider feed gas range will require better process adapt-ability and may favour mixed refrigerant processes with theadded flexibility of changing refrigerant composition.

Refrigerants made up from components that can be pro-duced in the process (in the fractionation unit) will obviate theneed for external supply to make up refrigerant losses.

Compressors and driversThe rotating equipment selection is affected by the character-istics of the process, such as composition and flow rate of therefrigerant and head required. Some will fit available framesand casings while others will require some development.

The choice of drivers, compressors and driver arrange-ments, and their fit with the process and power generation iscritical to the selection process.

The larger the drivers and compressors the more efficientand cost effective they are likely to be. However, if somemachinery is limited by available designs, smaller provenequipment may be installed in parallel trains, offsettingincreased costs by higher availability.

The choice of drivers for the main compressors is not lim-ited to gas and steam turbines. Studies carried out by FosterWheeler have shown that the use of large electric motor dri-vers is a feasible option to support high capacity baseloadLNG plants.

The selection of cooling system will have an impact oncompressor design, as it dictates compressor interstage anddischarge conditions.

Often the selection of process and drivers, particularly forexpansion projects, is dictated by the desire to stay with famil-iar designs and configurations and to standardise sparing, etc.

EquipmentAll the main processes are licensed processes, and some alsouse proprietary equipment. The main spiral wound heatexchangers used by APCI and Linde are both proprietary. Tubeleakage problems experienced previously with some designs ofspiral wound exchangers have been addressed several yearsago and minimised. The PFHEs used by some processes arenon-proprietary and can be offered by different vendors.Installations on large capacity plants comprising multiple paral-lel exchangers require more careful design for two phase(vapour and liquid) flow conditions within the unit. Some otherconsiderations for equipment selection are given in Table 1.

Other selectionsAnother important area is deciding the heating and coolingmedia types as they directly impact process and equipment.Cooling medium is normally a choice between air and water ina direct or indirect system. For the heating medium, steam orhot oil systems can be considered. For example a selection ofair for cooling, oil for heating and gas turbine drivers eliminatesthe need for a steam generation system including water treat-ment, and a cooling water system which may include a costlyseawater intake.

The above criteria are typical of the main issues that mustbe considered when selecting the technology for an LNGplant.

Reprinted from HYDROCARBON ENGINEERING FEBRUARY 2004

Figure 7. Axens Liquefin process.

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