liquefinLNG新工艺介绍

Pierre-Yves Martin – Axens Jérôme Pigourier – Axens Pierre Boutelant – Axens

Introduction

The continuous expansion of LNG trade for now more than three decades has been achieved thanks to the permanent search for cost reduction, mainly using the size effect .

To pursue this expansion at the same sustained rate of 6 percent per year, some operators are now seriously considering trains with capacities of 6, 7 or even 8 MTPA. In order to reach such capacities, with always higher efficiency and without adding complexity in the process, it is necessary to depart from the traditional scheme.

IFP and Axens have developed the Liquefin process with the aim of producing an LNG cheaper than with any other process, at good conditions of reliability and safety, and more friendly to environment.

The process uses simple and reliable technologies, easy to operate and able to cope with isolated or harsh climate regions. With Liquefin, very high capacities can be reached with a simple scheme and standard compressors.

Beyond an initial success of curiosity, most of the Majors have now closely reviewed the process in conjunction with engineering contractors. They have also thoroughly compared Liquefin with its competitors.

These Majors consider Liquefin as a potential alternative for their future developments: one Front End Engineering Design is now completed and confirm all the expectations in term of efficiency and cost attractiveness.

Axens, is a 100 % subsidiary of IFP is in charge of marketing worldwide IFP licenses, including Liquefin.

Process description

The Liquefin process operates according to the basic flow scheme presented in Figure 1. The pre-refrigeration of the gas is achieved by using a mixed refrigerant instead of propane, vaporized at three different pressure to follow closely the LNG cooling curve (see figure 3). In this process, the pre-refrigeration cycle is operated at a much lower temperature than in a conventional dual-cycle process: the temperature is decreased down to a range of -50°C to -80°C (-60°F to -110°F). At these temperatures, the cryogenic mixed refrigerant can be completely condensed, no phase separation is necessary and moreover the quantity of cryogenic refrigerant is substantially reduced. The overall required power is decreased, as a good part of the energy necessary to condense the cryogenic mixed refrigerant is shifted from the cryogenic cycle to the pre-refrigeration cycle. This shifting of energy leads to a better repartition of the necessary heat exchange area: the same number of cores in parallel can be used all along between the ambient and the cryogenic temperature.

A significant advantage of this scheme is the possibility to use directly the full power provided by the selected drivers whatever they are, without any transfer of power from one cycle to the other. For example, Liquefin can adapt to a half/half power balance between the two cycles for two identical gas turbines, but also to a one-third/two third power balance in case of three identical gas turbines.

Liquefin has all the positive features of the cascade process with a much better efficiency and a smaller number of rotating equipment. One can summarize the advantages of Liquefin process as follows:

¾ No integrated cascade: As the light mixed refrigerant of the second cycle is fully

condensed, the two mixed refrigerants can be used in a similar way to the pure refrigerants used in the cascade process.

¾ A balanced power: the process is easily adjustable to get the same power for

each cycle. With two identical gas turbines, it avoids the difficulty, encountered with the C3/MR cycle, of having to transmit power somehow from the pre-cooling cycle to the cryogenic cycle.

¾ A compact heat-exchange line: the Liquefin process has also been defined to make the best use of plate-fin heat-exchangers. A single heat exchange line is used to cool gas from ambient temperature down to cryogenic temperatures. The process has been conceived to make the exchange line simple and compact.

Figure 1 - Liquefin general scheme

Heat Exchange Line

All the heat exchange between the natural gas and the two mixed refrigerants (and between the two mixed refrigerants) is done in a single exchange line, made of plate-fin heat exchangers inside a limited number cold boxes (see Figure 2)

From / To Compressorstop viewCold boxabout 10 mCore (PFHE)40°Cfront viewUpper coreabout 15 m- 60°CLower core-160°C about 25 m

Figure 2 – Liquefin typical cold box arrangement

¾ IFP/Axens have qualified Chart, Nordon Cryogenie and Kobe Steel. and have worked

closely with these manufacturers to derive a main heat exchange line with similar architecture of plate-fin heat exchangers inside the cold boxes. Thus, the cold boxes and their internals can now be purchased from any of the three vendors without impacting the plant design or the P&I diagrams

¾ This exchange line is modular: each cold box contains several parallel lines of two cores

in series. The number of cores and cold boxes depends upon the capacity of the unit and the site conditions.

¾ With the modular concept, all limitations in size that exist for spiral-wound exchangers are

removed.

The main exchange line arrangement being at the heart of the liquefaction technology, a significant effort has been made to ensure its optimal operation: dedicated teams in IFP and Axens have thoroughly studied fluid distribution and mechanical/thermal stress issues for the cores, the cold boxes and the connecting manifolds in close co-operation with the three qualified manufacturers. What was demonstrated confirmed what vendors knew already: PFHE are now extremely robust and reliable equipment.

Lower CAPEX:

With Liquefin, the liquefaction plant CAPEX will be reduced for several reasons:

• Liquefin is more efficient

Comparing like for like, the process will produce about 15% more LNG with the same gas turbines than other established liquefaction processes. This efficiency improvement is related both to the use of mixed refrigerant for the pre-cooling and to the use of plate-fin heat exchangers.

¾ The low temperature difference all along the cores between hot and cold side (see Figure 3) brings an improvement of the exergy efficiency and hence of the power required per ton of LNG (or for the same drivers, the quantity of LNG produced is bigger).

Enthalpy Curves - Pre-refrigeration CoreTemperature (°C)Temperature (°C)0501001502002506040200Enthalpy curves - cryogenic core0-20-40-60-80-100-120-140-160050100150-20-40-60-80Enthalpy (MW)Enthalpy (MW)

Figure 3 – Liquefin main exchange line enthalpy curves

¾ The cryogenic mixed refrigerant enters the cryogenic section in a fully liquid state

so that no energy of this cycle is wasted in condensing the mixed refrigerant. The quantity of cryogenic refrigerant is much lower than in the C3/MR process (roughly 1 mole of cryogenic mixed refrigerant for 1 mole of natural gas), thus the overall efficiency is improved.

¾ The pressure drop is very low on both hot and cold side of the plate-fin heat

exchangers and this brings an additional efficiency advantage for Liquefin.

¾ As Liquefin is not submitted to the manufacturing limits of main heat exchangers,

the efficiency is as good for very large capacities as it is for smaller ones.

¾ Liquid turbines, which are now proven and widely used in LNG plants, brings a higher increase of capacity with Liquefin because the total stream of the cryogenic mixed refrigerant passes through the turbine.

• the equipment is less expensive

¾ For the same capacities, compressors from an earlier generation, i.e; less

expensive can be used. On figure 4 are shown the operating area of many existing propane compressors. The large units now under construction (around 4.5 MTPA) are in the blue circle, very close to the mechanical limits of the wheels. Liquefin cases at slightly higher capacity (4.8 MTPA) are in the red area, away from sonic hazard. BiggestpropaneCompressors(45MTPA)1.251.20(Mu1.151.10)num1.05ber1.000.95Mac0.90h0.85Peri0.80phe0.75ral0.700.650.600.550.500.450.400.020.040.060.080.100.120.140.16FlowCoefficient (?1)1.000.950.900.85)inlet0.80(numb0.75er0.70relative0.65ma0.60ch0.55Tip0.500.450.400.350.300.020.040.060.08(4.5 MTPA)0.10?1)0.120.140.16FlowCoefficient (LiquefinMR1 compressors(4.8MTPA)Figure 4 - Propane or MR1 compressor: Mach number vs. flow coefficient

¾ A single PFHE exchange line replaces the spiral-wound heat exchangers and its associated large propane kettle chillers. The spiral wound exchanger alone costs twice the complete Liquefin exchange line. Thus the saving on this exchange line is very significant, specially when considering installed cost.

¾ The air-condenser size is reduced, first because of the higher efficiency, and secondly because of the use of a mixed refrigerant instead of propane (see figure

5). The mixed refrigerant condenses in a range of temperature instead of a constant temperature as a pure component. Therefore, the LMDT is increased by 35%. The area of the air (or water) condenser is decreased by 35 % for the same duty. Or on the reverse, it is possible to decrease the condensing temperature with the same condensing area, thus increasing the LNG production.

60555045°C40

SamecondensingtemperatureMR CondensationPropaneCondensationLarger LMDT (+35%),353025

Air or WaterSame Condensing Duty20

0

50

100

150

200DUTY

250

300

350

figure 5: Mixed refrigerant 1 condensing vs propane condensing

¾ Due to the compact exchange line, and to reduced air-cooler size, the plot area is

reduced. This means large savings on long runs of pipe work and cryogenic insulation, and on the civil and metal work.

¾ The multi-sourcing of all equipment including the main heat exchange line is another factor of cost reduction

• there is no hidden cost

Axens has no commercial interest with any vendor, including PFHE vendors, and has no arrangement with any engineering contractor. The cost of licence fee is clearly indicated in the budget.

Studies carried out by third parties (Majors oil & gas companies and their Contractors) shows that with Liquefin, the cost of the liquefaction unit itself can be decreased around 15%. (see figure 6). Overall, including utilities, pre-treatment, storage, etc, the difference is still around 7% for the whole plant. But taking into account the increased capacity for the same gas turbines, the cost per ton of LNG is lower by up to about 23 % when compared with competing process.

1009080706050403020100

Main exchange lineRefrigeration compressors anddriversTowers, drums, pumps, liquidturbines, misc.Air coolersC3/MR

Liquefin (capacity

+15%)

figure 6: cost of Liquefin compared to competing process

Towards higher capacities

By the economy of scale, the cost per ton of LNG decreases when the capacity increases. This has driven the continuous increase of capacity of liquefaction trains from less of 1 MTPA some 30 years ago to about 4 MTPA nowadays. (see figure 7)

4.5Train capacity MTPA43.532.521.510.501960

1965

1970

197519801985

1990

1995200020052010

Start-up date

figure 7 – The continuous increase of train capacity

Some of the present projects are around 4.5 MTPA. Already at that flow, C3/MR process requires a large amount of flash gas, positive for the efficiency but negative on an economic standpoint. Liquefin has no bottleneck at this capacity:

¾ With Liquefin, the exchange line being modular, no technical limitation to the size of the spiral wound exchanger has to be taken into account: an increased capacity is reached with more cold boxes in parallel.

¾ A severe limitation is the size of the axial compressor for the second mixed refrigerant. As Liquefin balances the power between the two cycles, the quantity of the second mixed refrigerant is decreased by about 30% compared to a C3/MR for the same capacity. With the same existing axial compressor, it is thus possible to build a plant with a capacity increased by 30% compared to actual C3/MR plants, all conditions being equal.

¾ A 6 MTPA train with 2 Frame 7 is feasible with Liquefin, whereas a C3/MR in the basic version with 2 cycles is limited below 5 MTPA.

A new liquefaction scheme: C3/MR/nitrogen cycle has been put on the market recently. This means that a third Frame 7 is used in the nitrogen cycle to reach capacities in the range of 7+ MTPA. Three cycles in series imply added complexity and a reduced availability compared to the two cycle scheme, although a change of composition of the mixed refrigerant allows possibly working at reduced capacity without the nitrogen cycle. Of course, a full shutdown must be done if either the C3 or MR driver or compressor has a problem.

With the same three Frame 7, the Liquefin scheme will be much simpler, as shown on figure 8 One Frame 7 drives the pre-refrigeration compressors, and the two others run in parallel on the liquefaction cycle. This allows reaching very large capacities ( 8+ MTPA), still with existing, proven compressors. The availability is slightly improved compared to the base scheme, since previously a failure of the single liquefaction compressor meant a complete shutdown, whereas with this scheme, if one of the liquefaction compressor is down, it is possible to operate at reduced capacity (minimum 50%, more by changing the pre-refrigeration temperature and the mixed refrigerant compositions). Only a failure of the MR1 compressor or driver will stop completely the unit.

CWDry naturalgas40°CFrame 7-20°CScrubberCWCWFrame 7CWCondensates-40°CMain heat exchangeline-150°CCWFrame 7LNG

Figure 8: Liquefin scheme with 3 Frame 7

Liquefin is a very flexible process, and offers more than one possibility to reach large and highly competitive capacities:

¾ Either by using larger gas turbines. The Frame 9 has very recently been qualified for mechanical drive. With Liquefin, this would allow capacities of 7 to 8 MTPA with only two main drivers (base scheme of figure 1). Although the volumetric flow-rates are of course seriously increased, a choice compressors can be found for this case, because the speed of the Frame 9 is lower than the Frame 7 speed (3000 rpm instead of 3600).

¾ Or by using very large gas turbines (combined cycle) to produce electricity, and using large electrical motors (up to 70 MW) in parallel on each cycle. (see figure 9). There is hardly any technical limit on the capacity, as we are in a completely modular scheme. This scheme has many advantages: available proven compressors, moderate size equipment, excellent efficiency (very low fuel gas consumption as combined cycles can reach 50-60% efficiency), and pretty good availability: no complete shutdown in case of failure of any driver/compressor.

NaturalGasMMMMR1 -1MR1 -2

LNGMR2 -1

MMR2 - 2

Figure 9: Modular arrangement of Liquefin (4 identical drivers)

Conclusion

In the continuous race for higher capacities and cheaper LNG, Liquefin is a real breakthrough.

The best present arrangement of proven equipment to produce the maximum quantity of LNG out of a given set of drivers at the lowest cost in the market.

Liquefin is particularly well adapted to the range of 4 to 8 MTPA per train, with many open options for designing and erecting a plant fully responding to the client’s needs. With Liquefin, the capacity can be chosen considering mainly the economics and the marketing possibilities, without being bothered by technical hindrances.