August 19, 2002 - "New liquefaction process promises lower costs"
Oil & Gas Journal
Yet another LNG liquefaction process has recently become commercially available and claims to cut the cost of LNG production by 20%.
Liquefin, developed and commercialised by IFP-Axens, France, has been developed in cooperation with a major oil and gas company, says the company. The benefits of the new process, it says, were recently demonstrated in several studies conducted by engineering contractors and of several vendors for the main compression and the main exchange lines.
MR basis
In the process (Fig. 1), the pre-refrigeration of the gas is achieved by use of a mixed refrigerant instead of propane. In this process, the pre-refrigeration cycle operates at a much lower temperature than in a conventional dual-cycle process: The temperature is decreased to -50° to -80° C. (-60° - 110° F.).
At these temperatures, says Axens, the cryogenic mixed refrigerant can be completely condensed. No phase separation is necessary, and the quantity of cryogenic refrigerant is substantially reduced.
The molar ratio between the cryogenic mixed refrigerant and LNG can be, in some cases, lower than 1.
The overall required power is decreased because much of the energy necessary to condense the cryogenic mixed refrigerant is shifted from the cryogenic cycle to the pre-refrigeration cycle.
This leads to a better distribution of the required heat-exchange area, says the company: The same number of cores in parallel can be used between the ambient and the cryogenic exchange sections.
In the Liquefin process, both mixed refrigerants are used in the same way as pure components. The mixed refrigerant is condensed and vaporized at different pressure levels in each section, without any phase separation or fractionation. This way, the exchange line can be very simple and compact, says Axens.
A significant advantage of this new scheme, says the company, is the possibility to adjust the power balance between the two cycles. It is thus possible to use directly the full power provided by two identical gas turbines, with no transfer of power from one cycle to the other.
The process has the positive features of the cascade process with a much better efficiency, says Axens, and fewer pieces of rotating equipment. Advantages, according to the company, include:
∙ No integrated cascade. As the mixed refrigerant of the second cycle is fully condensed, the two mixed refrigerants can be used in a similar way as that with the pure refrigerants used in the cascade process.
∙ A balanced power. The process is easily adjustable to obtain the same power for each cycle. With two identical gas turbines, it avoids the difficulty, encountered with the C3-mixed refrigerant cycle, of having to transmit power somehow from the pre-cooling gas turbine to the cryogenic cycle.
∙ 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 to cryogenic temperatures. The process has been conceived to make the exchange line simple and compact.
Cold-box design
The plate fin heat exchanger (PFHE) arrangement is at the heart of the liquefaction technology.
Aluminium offers negligible resistance to heat transfer between fluids, says Axens. Moreover, the extended surface provided by the fins leads to very high heat-transfer efficiency, enabling a good exchange and a low-temperature differential. A very high surface density of up to 2,000 sq m/cu m can be attained, which is important in the reduction of footprint, weight, and therefore cost.
Unlike shell and tube exchangers, in which a single stream can be used on the shell side, plate-fin heat exchangers may handle a warm and cold streams at various pressures simultaneously.
The Liquefin process is especially adapted to the use of plate-fin heat exchangers: All fluids entering the main exchange line, except the outlets of the Joule-Thomson valves, are in a single phase—vapor or sub-cooled liquid. Moreover, a special arrangement at the outlet of the Joule-Thomson valves ensures the right amount of refrigerant in each parallel core and even distribution between the different channels of a core. A good distribution is one of the key parameters to ensure the full efficiency of the process.
For a capacity around 4.5 million tonnes/year, the total precooling and liquefaction heat exchange is deployed in four cold boxes, each with six parallel lines consisting of 2 PFHE cores in series. The total heat exchange of a large train is thus gathered in a 250 sq m (2,750 sq.ft) area, with a height no more than 15 m (50 ft).
The exchange line is the main novelty of this process, says Axens. In addition to PFHE manufacturers’ extensive experience of very similar equipment, advanced research and development studies were carried out on PFHE by IFP-Axens, related to thermal efficiency, fluids dynamic, and mechanical behavior. Sophisticated stress analysis on all parts of the cryogenic assembly have been performed in close co-operation with PFHE manufacturers, says Axens.
∙ Use of mixed refrigerant instead of propane for pre-refrigeration affords a closer approach of the heating-cooling enthalpy curves than with a pure component in the pre-refrigeration section.
∙ Use of a PFHE combined with this special process allows a very close fit of the warming and cooling enthalpy curves all along the liquefaction section, thus improving the thermodynamic efficiency. In the C3-MR case, even if the mixed refrigerant is optimized to reduce the pinch, the maximum size of the spiral wound exchanger forbids such a low LMTD for the high train capacities.
∙ The use of mixed refrigerant instead of propane for pre-refrigeration enables a reduction of the break point between the pre-refrigeration section and the liquefaction section from -30° C. down to -60° C, - 80°C depending on the feed composition. Thus, the cryogenic mixed refrigerant is completely liquefied in the pre-refrigeration section, resulting in a lower power requirement.
∙ The air-cooler size is lower for the same approach with a mixed refrigerant compared to a pure component (or stated in another way a higher capacity is possible with the same air-cooler size).
∙ The low pressure-drop on both sides of a PFHE gives an efficiency advantage to the Liquefin process. The high pressure-drop on tube side of a spiral wound exchanger leads to an efficiency loss by decreasing natural gas liquefaction pressure and by decreasing the mixed refrigerant pressure at the liquid turbine inlet.
Investment cost
Within the framework of front end engineering designs (FEEDs) and pre-FEEDs currently performed with majors and national oil and gas companies, detailed cost estimates of the liquefaction plant have been performed by several engineering contractors, says Axens. Their results show significant cost reduction.
The cost savings obtained come mainly from the heat exchange line. In the C3-MR process, in addition to the spiral wound exchanger, already much more expensive by itself than the whole Liquefin exchange line, says the company, the large number of C3 kettles leads to increased equipment cost, plot area, and piping cost.
The lower air-cooler size for Liquefin, due to the use of a mixed refrigerant instead of a pure component, increases further the cost difference as well as the plot area savings.
An estimation of the total investment cost was made for the entire liquefaction plant, says Axens. Even when including the pre-treatment, utilities, storage costs, the Liquefin process still gives a significantly lower cost for a much higher capacity.
Overall, the cost per tonne of LNG can be reduced by 20 %.
Through economy of scale, the cost per ton of LNG decreases with increasing capacity. This has driven the continuous increase of capacity of liquefaction trains from less of 1 million tonnes/year to more than 4 million tonnes/year, currently, says Axens. The main limitation, besides the maximum size gas turbine (Frame 7 at present) results from the large spiral-wound exchangers used in the conventional C3-MR units.
With Liquefin, says the company, the exchange line being modular, no such limitation exists. In addition, employing parallel, modular compression lines, if required, eliminates technological capacity limit. The notion of "train" becomes obsolete, says Axens.
The ongoing Liquefin development is at present oriented towards increasing the capacity with a given set of gas turbines (two Frame 7s). Several options can help to increase the capacity further. Axens estimates that through a combination of additional improvements, a capacity close to 6 million tonnes/year in one line with two Frame 7s can be reached, for a cost per tonne of LNG 10% lower than the currently low base case.
