es, the new controlled

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indicate that achieved

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I and by so ~amic ,t for the

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ulled Turbine teering, '. Lld., U.K. rolled Cryogenic Id Pumps,

.E.: "Power n ic Proceedings lwer echnology

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jnics: An Y.A. and McGraw-Hill

T he growth in LNG trade has continued at a significant level over the last decade. The market for

LNG, however, is tightening up and thus potential investors are looking for lower capital and operating costs for new

, facilities with increasing intensity. This has led to greater interest in alternative technologies for pre-treatment, liquefaction

1 and storage. The Base Load LNG industry has up to

recently been dominated by one process technology, the Air Products & Chemicals Inc.(APCI) precooled Mixed Refrigerant Process (known as MCR). This process currently has an unrivalled track record of over 50 plants in operation, using wound coil cryogenic heat exchangers. For many years Technip, in combination with L'Air Liquide and others also offered a technically sound and commercially attractive wound coil heat exchanger Mixed Refrigerant technology, the Tealarc Process, but did not succeed in penetrating the base load LNG plant market. Recently, the situation has changed with the established technology being aggressively challenged by processes using plate fin heat exchangers (PFHEs). These include the Phillips Optimised Cascade for the Atlantic LNG project at Point Fortin in Trinidad. Also, the Pritchard - PRlCO mixed refrigerant process has been operating at Skikda for many years and modern designs offer a further alternative process for the Base Load LNG market.

In addition, the Peak Shave LNG market is seeing a revival through a significant potential for replacement of older, less efficient plants operating at peak shaving

, facilities around the world. It has been

Liquefaction

b y Terry Lavin, Linnhof f M a r c h L im i ted ; Harry lsalski a n d Philip Hindley, Tractebel I n d u s t r y Eng inee r i ng S.A.; a n d David Linnett, Consu l t an t i n

C ryogen i cs & Gas Processing (UK).

This article is an updated, edited version o f a paper presented a1 the Conference organized by IIR Limited : "Market Opportunities for LNG Projecls", London, October

14-15, 1997.

satisfied by a much larger number of suppliers of technology including APCI, Pritchard, Linde, L'Air Liquide, Gaz de France, Snamprogetti, CBI, BOC-and. Costain. The Peak Shave LNG market \ has probably seen the widest range of liquefaction technologies applied, including a variety of mixed refrigerant processes comprising one to five stages of refrigeration, classic cascade refrigeration processes, inert gas and natural gas

Terry Lavin is a Principal Consultant at Linnhoff March. His speciallies include cryogenics, gas processing and distillation. I n previous positions w i th the BOC Group and Brit ish Gas, he was responsible for design and commissioning of several LNG peak shave plants. Recent wo rk includes detailed thermodynamic analysis of LNG

I base load designs.

Harry lsalski has over 2 7 years experience in the conceptual design, delailed engineering and commissioning of a l l types of gas processing, cryogenic and LNG facilities. This includes al l design, engineering and startup of t w o LNG peak shave plants and process optimisation studies of mid-sized and base load LNG plants. He is Technical Director at Traclebel lndustry Engineering, OK Branch Office.

David Linnett has been an independent consullant in cryogenics and gas processing since 1987 and was previously Process Engineering Manager w i t h BOC Process Plants. He has over 30 years experience in LNG plant technology, including major assignments i n many LNG projects of a l l sizes from peak shave l o base load.

Philip Hindley graduated w i t h a n honours degree in Chemical Engineering from Loughborough University, and has 4 years o fgas processinq desiqn experience. He is a Process Enqineer w i t h Tractebel - - . lndustry Engineering SA.

expansion cycles. Some of these processes are also suitable for mid-sized and base load applications.

As the range of potential technology increases, the operating companies have a much more difficult choice to make when evaluating the various schemes and making the optimum choice of process route for a specific application. This article describes various methods of process review and the perceptions that influence process selection. It discusses simple efficiency benchmarking and recommends a more structured way forward which includes Exergy Analysis as a technique that gives a clearer picture in any comparison methodology.

Criteria for Process Evaluation Natural gas liquefaction consumes a large amount of refrigeration energy to bring the gas from elevated pressure and ambient temperature to a liquid at near atmospheric pressure and minus 162'C. The way the refrigeration is applied has a dramatic effect on the capital and operating costs of the liquefaction plant. Currently, common methods of evaluating LNG liquefaction processes use:

specific power, kwhftonne of LNG specific capital cost. $ or Sftonne of

annual capacity . past experience in these plants The above methods are useful as an

initial assessment, but with the number of process options available today and the pressure on establishing low life cycle costs, a more rigorous approach is

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EU market 0 Energy liberalisation is set to radically change the shape of European electricity and gas markets, as major EU I

power companies and new entrants fight j for a slice of a USS125 billion market, according to a new report from Reuters Business Insight (RBI).The passing of ' ! the European Commission's electricity 1 1 directive in February 1997 means thaf'f'r' ' by 1999, at least 60% of the EU ., electricity market will be OF

competition. 3

I 30

necessary. Other qualitative aspects used for comparing processes include:

process complexity number and type o f machines driving

the process type of heat exchangers used (wound

tube or plate fin) - what ambient cooling methods should be adopted

what design margins should be applied I n any refrigeration process, the

refrigerant compressor dominates the capital and operating costs. When the plants are large, the economies of scale help in increasing the machinery efficiency considerably. For the larger refrigerant compressors, polytropic efficiencies over 86% can now be reached. This means that for a given power input, more refrigeration can be provided and more LNG can be made.

If gas is cheap, the view could be taken that process efficiency should not be a critical issue. This may hold true for the gas turbine drivers in the liquefaction plants because it may be that the more efficient driver may be more expensive to buy. When coupled up together, the compressor and driver is a high proportion of the LNG plant liquefier capital cost. A lower cost, more robust gas turbine driver may be the most cost effective solution even though it may have a lower efficiency in terms of conversion of natural gas into compressor shaft energy.

However, the process efficiency will always matter once a given compressor and driver has been chosen. This is because the designer has chosen a machine for a given specific cost per kW of power supplied to the drive shaft. Once that energy is at the shaft, it must be put to the best use possible in order to maximise the production of LNG. This will have the effect of driving down the specific cost of the plant per tonne of LNG. Therefore, the design of the liquefaction section itself needs very careful attention. In selecting a liquefaction process, the designer has many parameters that he can change to optimise the process, including:

type of cycle; mixed refrigerant, pure

component, expander, cascade, pre- cooled or a combination of these

which components to use in the refrigerant - number and temperatures of stages of refrigeration

pressures at the suction and discharge of the main compressor

ambient cooling media - pressure drops in equipment Given previous experience, the choices

can be narrowed down considerably. However, i t is wise to analyse the options. particularly when the nature of the LNG market, and the locations and sizes of plants are changing significantly. A more rigorous approach for the highly competitive environment of the 1990's is warranted.

Thermodynamic Approaches The simplest thermodynamic approach is to consider the law of conservation of available energy. This has often been used in natural gas processing to determine a given process' yield of heating value. Putting this simply, the ratio of calories out to calories in (the caloric ratio) gives a benchmark of how much energy has been lost in the process through compression power and fuel gas to generate heat and steam. In a base load LNG plant, this figure is often used as a yardstick for a given plant performance. When applying this to an LNG plant, values in the range of 89% - 94% can be found. This always looks a reasonably high value and can give false comfort to management. Plant improvements may not present an attractive improvement to an already high number.

A second benchmark sometimes used to compare process routes is an overall entropy based efficiency (E) term, E =

Wm/Wa, where Wm is the minimum work required to liquefy the gas, and Wa is the actual work input to the process. A similar ratio was used by Bronfenbrenner (1). The use of this equation is a good starting point for process comparison because it is quick and needs few calculations; as long as the comparison is on the same basis, it provides the user

Table 1 - Overall Comparison o f LNG Processes (Cold Section Only)

Process Route Approx. Caloric Cold Section Specific Ratio x 100% Thermodynamic Power

Efficiency (%) kW/kg LNG

I-Stage MRC 93.0 5 5 - 6 1 0.34 - 0.36

5-Stage MRC 93.7 60 - 65 0.30 - 0.32

5-Stage MRC 93.9 63 - 66 0.29 - 0.3 1

(fully sub-cooled LNG) C3 Precooled MCR 93.4 60 - 63 0.32 - 0.34

3 turbine N2 cycle 90.5 35 - 45 0.45 - 0.55

cade, pre- f these e in the

IS of stages of

and discharge

nent the choices

derably. the options.

of the LNG d sizes of ntly. A more rhly he 1990's is

:es : approach is vation of ten been lg to ield of heating e ratio of 5 caloric row much rocess 3nd fuel gas In a base often used as >erformance. 3 plant, 34% can be asonably comfort to

ments may rovement to

?times used an overall

term. E =

inimum work i d Wa is the ess. A nfenbrenner is a good

nparison ; few omparison is s the user

n Cycle wi

Ftgue 1: Slngle Stage MLxed Refriserent Cycly

FEED GAS

MAIN EXCHANGER

CONDENSER

LNG

CYCLE COMPRESSOR

NITROGEN CYCLE

HEAVY HYDR

EXPANDER 1 EXPANDER 2 EXPANDER 3 NZ COMPRESSOR

with an initial guide to the efficiency of each route. However, it can only look at the overall picture; i t does not identify where the major inefficiencies are. This can be illustrated by comparing several LNG liquefaction processes as shown in Table 1. It is important to note that this article compares only the cryogenic units of the LNG plant.

The above two methods would most likely be used by an assessor of process routes during preliminary appraisal of the economics of the project and for initial comparison between potential suppliers of technology. The designer, however, needs something more accurate to help h im search for the optimum route for liquefaction. It is imperative that the designer can identify the location of the major inefficiencies and see what effect design changes have on the system.

Using the principles of Conservation of Energy (First Law), one can use heat exchanger composite curves to some good effect. However, it does not always show the most effective route, as illustrated in a paper by Lavin & lsalski (2), which compared the composite heat load curves for the cryogenic sections of the single stage and a 5-stage mixed refrigerant cycle for the liquefaction of natural gas. When comparing the heating and cooling curves, at first sight, the 5- stage process looked inefficient when compared to the single stage process because the temperature differences were larger, implying greater losses. Therefore, a cursory view may lead to the conclusion that the single stage process is more efficient. The true picture is very different in that the 5-stage cycle consumes some 20% less power to liquefy the same amount of LNG. The reason is that the flow of refrigerant in the 5-stage process is much lower at the cold end than at the warm end, making the irreversible losses smaller at the cold end. Also, the final stage refrigerant only comprises the lightest components, nitrogen and methane. which are the most effective at the cold end. In the single stage process. the flow and composition of refrigerant are the same at the warm end as at the cold end. This greatly increases the heat load and reduces both efficiency and operating flexibility.

This proves that it is necessary to go into more detailed analysis to illustrate the merits of liquefaction processes. Exergy Analysis provides an excellent tool to achieve this.

Exergy Analysis Exergy or "availability" can be defined as the maximum amount of reversible work that can result from the interaction of a closed system with its surroundings. Exergy Analysis is a well-known technique for analysing irreversible losses in a separation process. By combining exergy

analysis with flowsheet optimisation, one can effectively compare various processes and identify the largest loss locations and look for ways to minimise them. The concepts have been adapted during recent work to allow easy interaction with HYSYS, which is being used for process optimisation. Exergy analysis has been used to provide data for this article by applying it to the process simulations developed for the following processes:

a single stage mixed refrigerant process - Figure 1

a five stage mixed refrigerant process - Figure 2

a three-turbine nitrogen gas compression/expansion cycle - Figure 3 Note: in practice, the small

flows of the returning heavy hydrocarbons stream would be re-heated in the main exchangers. This is not shown on the diagram.

These are compared in Table 2. Overall irreversible losses are tabulated as well as individual item losses.

I Comparison basis for Exergy Analysis of Process Schemes

Table 2 - Energy Analysis Comparison for 3 Process Routes

Plant Items I-Stage MRC 5-Stage MRC 3-Turblne Cycle Heat Exchangers 72 78* 51

Valves 26 15 16 Turbines 0 0 33** Mixers 2 7 negligible

Total 100 100 100 Total Losses 1.58 1.16 1.9 with turbine brake

(kW/kg mole LNG) 6.0 without brake

'As an example -exchangers broken down into 5 sections yield the following loss breakdown from warm end to cold end: 31; 18; 13; 14; 2% or a total of 78% '*This example quoted for turbine inclusive of brake recovery

Heat Exchanger Design Some of the recent advances in the manufacture of brazed aluminium plate fin heat exchangers (PFHEs) have dispelled worries about their use in base load LNG plants. This has been manifested in the award of the Trinidad project to a process that uses these extensively. Moreover. there are many smaller LNG plants that have operated using PFHEs for many decades. They are ideal for use in complex arrangements where small temperature differences are required. Their design naturally allows excellent heat integration of multiple heating and cooling streams giving excellent process efficiencies. The air separation industry has used PFHEs as standard for many years and has maximised these benefits, continuing to expand the frontiers of what is a mature technology. Large multi-exchanger assemblies approaching the size needed for a base load LNG plants are already in successful operation in both the air separation and LNG industries.

In using PHFEs in the liquefaction process, their multi-stream capability makes them more difficult to assess. Rigorous analysis using exergy can make it much easier to appreciate comparisons between several process options. As is shown in Table 2, exergy analysis can highlight losses in any process item including heat exchangers, valves, and mixing devices as well as identifying pressure drop losses.

This analysis can lead to process enhancements, having identified its effect relative to the whole process. For all the processes, we can see from Table 2 there

F or this comparison, a simulation was constructed for each of the schemes so that it was able to adjust compositions of refrigerants in order to get to a workable solution.

Exergy analysis was then carried out to identify the areas where the losses were greatest and adjustments were made to the process to gain a better solution. The Table uses data from simulations on the same basis, optimised to the same level of accuracy to make the comparison meaningful. The calculations assume process streams are cooled to 33°C in compressor aftercoolers, etc. Each of these has assumed a 50 barg and 40'C clean natural gas feed containing about 5% by volume of ethane and about 4% of propane and heavier. The same compressor isentropic efficiency was used for all cases. The minimum exchanger temperature approaches have also been normalised to about 2.5"C pinch. The expansion turbines have been assumed to achieve 85% isentropic efficiency. The mixed refrigerant processes have used a mixture of nitrogen, methane, ethane, propane, butanes and pentanes. If ethylene were used instead of ethane in the mixed refrigerant process options, a 5 - 10 % lower power would be expected.

The nitrogen cycle is assumed to use pure nitrogen as the circulating fluid. The expansion turbines take the bulk of the exergy loss as expected. However, the majority will be recovered in the compression section through the brakes linked to the expander. There are high losses in the warm end heat exchangers, which could easily be reduced by precooling the nitrogen with say, propane, yielding a relatively efficient process, albeit with five machines. Figure 3 shows a nitrogen

cycle with three turbines in parallel, which was only one of the typical starting options considered. Exergy analysis showed that a more efficient configuration is two turbines in series with intermediate reheat, plus a third turbine in parallel. These options have all been considered and used in appropriate applications for many years : however, improved design techniques, including detailed exergy analysis, now enable much better comparison of cycle options. Cycle selection and optimisation can then be closely tailored to specific applications.

are significant losses in the heat exchangers. It is easy to see where the losses are in the 5stage process. However, for a single stage process the analysis does not show where in that exchanger the loss is the greatest. Therefore, it is necessary that the exchanger is broken down into several segments, to allow segment by segment analysis of the exchanger.

Conclusions The varieties of LNG processes available today require careful selection to ensure that the correct process is chosen for a given application. This selection will depend on the plant size and location. However, all but the very small plants will benefit from a rigorous analysis to help make the right choice.

Condensing mixed refrigerant cycles show a considerably higher overall efficiency than gas expansion cycles, even when these use three turbines. Multi- stage mixed refrigerant processes are better than single stage ones from the efficiency point of view.

Exergy analysis has shown that the heat exchangers have the largest irreversible losses in the process and is used to identify where they are. The use of exergy analysis as part of the optimisation process, together with major advances in the design and manufacture of PFHEs. enables high efficiency processes to be tailored to meet the specific requirements of individual applications.

References I . Bron/enbrenner,J.C., "The Air Products Propane Precooled/Mixed Refrigeranl LNG Process", The LNG J o ~ ~ r n a l , Nou/Dec, 1996. 2. Lauin, J.T. & Isalski, W.H.. "lmprouing the E//iciency o/ Liquefaclion Processes lhrough the Applicalion of Exergy Analysis", IIR Conlerence on LNG. London, 15th Ocl 1997.

Acknowledgements Our lhanks are due to [he management of Traclebel Industry Engineering S.A. and Linnhoff March L ld for permission to publish this arlicle.