The importance of liquefying natural gas is due to the economics of transporting
its bulk liquid form which only occupies 1/600th volume compared to the gaseous
form. LNG has a strong market demand worldwide as the source of fuel to generate
electricity and the demand is increasing every year. The high demand of LNG
fuel is because it emits less harmful gases into atmosphere compare to other
fossil fuels as discussed by Shukri and Wheeler (2004),
Pillarella et al. (2005) and James
(2004). Therefore, in order to meet this demand, LNG producers look for
better options to maximize their production by optimizing their current plant.
However, they need to consider several other factors for this option such that
they shouldnt bare extra expenditure for increasing their plants
capacity. An LNG projects represents a chain of capital-intensive investments,
consisting five links-field development, pipeline to on-shore, liquefaction
facility, tanker transportation and the receipt/regasification terminal.
The liquefaction unit process has been accounting for up to 50% of total project
cost of a liquefaction plant as shown in Fig. 1 which reviewed
by Finn et al. (2000). The train growth on the
global scale is shown on Fig. 2. The Evolution of LNG technologies
worldwide applied natural gas liquefaction technologies is shown in Fig.
3 and b.
||Typical breakdown of liquefaction plant capital costs
|| LNG train size growth
||Worldwide Natural Gas Liquefaction Technologies liquefaction
capacity within (a) 1964-2000 and (b) 2001-2012
One of the criterions for the selection of liquefaction process is the capacity
requirements. Designing a large plant and running it far below the capacity
rates is a waste of investments and potentially could result in greater maintenance
issues. Economy of scale means maximising profit based on fixed capital investment
as reported by Charles et al. (2005). Hence in
order to fully take advantage of their economy of scale, production must be
maintained near capacity. Since production level is based on what the market
will support, if the demand goes down, so would production. Likewise, if production
of LNG is greater than demand, the sale price will weaken and production rates
must be decreased which leads to further waste of capital investments as discussed
by Martin and Fischer (2003). Therefore, the selection
of process technology is not only concerned with capacity and stability but
also with maximising profit based on market demand.
Refrigeration for liquefaction: The refrigeration and liquefaction section
Is the key element of an LNG plant where it typically accounts for 30-40% of
the capital cost of the overall plant. Liquefaction of natural gas involves
the transfer of energy from hot stream of natural gas to cold stream of the
refrigerant via LNG heat exchangers. During this process, the phase of natural
gas changes from vapour to liquid. The basic principle of using refrigerant
to liquefy the gas to cryogenic temperature of approximately (-160°C) is
to match the cooling/heating curves of the process gas and refrigerant as closely
|| Natural gas refrigerant cooling curve
Results in a more efficient thermodynamic process requiring less power per
unit of LNG produced. After the liquefaction process takes place, LNG is pumped
into cryogenic storage tanks, until a tanker is available to transport LNG to
the market as reported by Doug (2002) and Bosma
and Nagelvoort (2009). These tanks are typically double-walled, with an
outer wall of reinforced concrete lined with carbon steel and an inner wall
of nickel steel as mentioned by Bosma and Nagelvoort (2009).
There is insulation between the two walls to prevent ambient air from warming
the LNG. After an empty tanker docks at the berth, LNG is loaded into the tanker
through insulated pipes that are attached to the tanker by rigid loading arms
as discussed by DOE/EIA-0484 (2009), APX
Energy Viewpoints (2005) and Ross et al. (2008).
Once the tanker is filled, the pipes are disconnected, the loading arm will
swing away from the ship and the tanker is ready to sail. Figure
4 shows an example of a typical temperature-heat diagram or cooling curve
for the cooling of natural gas using both pure and mixed refrigerants as reported
by Pillarella et al. (2005) and Eaton
et al. (2004).
The closer the line depicting the refrigerants is to the curve of the natural
gas, the more efficient is the cycle. Reducing the amount of work done on the
refrigerant can increase the efficiency of the heat exchange in the natural
gas liquefaction process. Higher efficiency is indicated by the closeness of
space between the refrigerant and the natural gas curves as reported by Rivera
et al. (2008) and Shuhaimi and Razik (2008).
APCI propane pre-cooled mixed refrigerant process (C3MR): In this technology,
there are two main refrigerant cycles. The precooling cycle which uses a pure
component propane and the liquefaction and sub-cooling cycle useing a Mixed
Refrigerant (MR) made up of nitrogen, methane, ethane and propane. The precooling
cycle uses propane at three or four pressure levels and can cool the process
gas down to (-40°C). It is also used to cool and partially liquefy the MR.
The cooling is achieved in kettle-type exchangers with propane refrigerant boiling
and evaporating in a pool on the shell side and with the process streams flowing
in immersed tube passes. Figure 5, shows C3MR process flow
sheet. A centrifugal compressor with side streams recovers the evaporated C3
streams and compresses the vapour to 15-25 bar to be condensed against water
or air and recycled to the propane kettles. In the MR cycle, the partially liquefied
refrigerant is separated into vapour and liquid streams. The refrigerant is
used to liquefy and sub-cool the process stream from typically -35°C to
the temperature range -150 to -160°C. This is carried out in a proprietary
spiral wound exchanger commonly known as the Main Cryogenic Heat Exchanger (MCHE)
as addressed by Finn et al. (2000). The MCHE
consists of two or three tube bundles arranged in a vertical shell with the
process gas and refrigerants entering the tubes at the bottom which then flow
upward under pressure.
The process gas passes through all the bundles to emerge liquefied at the top.
The liquid MR stream is extracted after the warm or middle bundle and is flashed
across a Joule-Thomson (JT) valve or a hydraulic expander onto the shell side.
It flows downwards and evaporates, providing the bulk of cooling for the lower
bundles. The vapour MR stream passes to the top cold bundle, liquefied and sub-cooled.
It is then flashed across a JT valve into the shell side over the top of the
cold bundle. It flows downwards to provide the cooling duty for the top bundle
and, after mixing with liquid MR, part of the duty for lower bundles. The overall
vaporised MR stream from the bottom of the MCHE is recovered and compressed
by the MR compressor to 45-48 bara. It is cooled and partially liquefied first
by water or air and then by the propane refrigerant, and recycled to the MCHE
as mentioned by Finn et al. (2000).
Dual mixed refrigerant process (DMR): The dual mixed refrigerant process
or DMR is very similar to the APCI liquefaction process. Figure
6 shows the DMR process flow sheet as discussed by Ross
et al. (2008). It is designed to overcome the inherent limitations
of using a single component refrigerant in pre-cooling in the C3MR design; the
additional degree of freedom resulting from the use of two mixed refrigerant
cycles allows full utilization of power in a design with two mechanically driven
compressors. Furthermore it allows keeping the compressors at their best efficiency
point over a very wide range of ambient temperature variations and changes in
feed gas composition.
|| C3MR process flow sheet
|| DMR process flow sheet
The natural gas stream is cooled via two stages. The first stage cools natural
gas to -50°C while the second column cools natural gas to LNG at -160°C.
The composition of the pre-coolant cycle is 50/50 of ethane/propane on molar basis and the coolant composition of the cooling cycle is similar to the composition of APCI. In this process the heat exchanger tower is divided into two sections and this concept allows the choosing of load on each refrigeration cycle through controlling the two compressors work before each column.
There are many aspects, that have been addressed to have influence on the efficiency of liquefaction process and affecting the quantity of LNG produced.
Tube side design pressure: The tube side design pressure in MCHE is increased from 76 to 83 barg using a feed compressor. Higher tube side design pressure allows for higher operating pressures which results in higher LNG production and lower specific power.
||Effect of feed pressure on production and specific power
|| Effect of end flash quantity on LNG production
Figure 7 shows the percentage increase in production and
reduction in specific power as the feed pressure is increased as reported by
Pillarella et al. (2005). It includes the feed
compressor power as well as the propane and mixed refrigerant compressor powers.
However, this has to take into consideration of the cost of the required inlet
plant equipment or the thermal efficiency options the pressure present for the
plant design and also the impact on upstream plant facilities supplying the
gas as discussed by Pillarella et al. (2005).
End flash quantity: The quantity of end flash usually corresponds to
the plant fuel gas consumption. If it is possible to increase the quantity (fuel
gas export to other plants and recycle etc) the cold end temperature of the
main exchange line will increase and the efficiency of the plant, thus the quantity
of LNG produced will also increase. However, the quantity of fuel gas cannot
be decreased below a certain quantity due to the nitrogen content of the feed
gas. Figure 8 shows the Effect of end flash quantity on LNG
production. The power requirement of the fuel gas compressor for the more generated
fuel gas also has to be considered. Nitrogen can have significant effect on
thermal efficiency, resulting in higher energy consumption and lower thermal
efficiency as this gas is cooled to LNG temperature. Hence, the rejection composition
of nitrogen should be evaluated by considering the impact on the size of equipment
(gas compression) and the piping system (relief valve, vessels and pipe) as
discussed by Martin and Fischer (2003).
||Effect on LNG production of condenser temperature approach
||Effect of using propane or mixed refrigerant on the size of
Temperature approach on the main condenser: A large condenser on the
first refrigerant cycle must evacuate the heat produced by the refrigeration
compressors. As the outlet of the compressor is at bubble point, to modify the
outlet temperature of this condenser will change the discharge pressure of the
Figure 9 shows the effect on LNG production of condenser temperature approach. The power of the compressor and overall efficiency will be affected. The closer is the temperature approach, the larger the LNG production. The size of the condenser depends upon whether the refrigerant of the first cycle is a pure component or a mixed refrigerant.
With a pure component, the condensation is done at a fixed temperature (the
dew point temperature is the same as the bubble point temperature) whereas with
a mixed refrigerant, the temperature varies linearly between the dew point temperature
and the bubble point temperature. Either the condenser will be much smaller
with a mixed refrigerant in the first cycle, or inversely with the same condenser
size, the LNG production will be increased.
|| Comparison between C3MR and DMR
||Effect of compressor polytropic efficiency on LNG production
|| Effect of LPG recovery on LNG production
Figure 10 shows effect of using propane or mixed refrigerant
on the size of the condenser as discussed by Martin and Fischer
Compressor efficiency: The LNG production can vary tremendously depending upon the compressor efficiency considered. The increase in compressor polytropic efficiency will also increase the LNG production as shown on Fig. 11.
LPG recovery: The recovery of LPG from the gas can make the project
economically sound. However, it will increase the power of LNG liquefaction.
Hence, with the increase of LPG recovery, it will lead to the decrease in LNG
production as shown on Fig. 12 as discussed by Martins
and Fischer (2003).
Liquefaction technologies: The significance of Dual Mixed Refrigerant (DMR) is from the modification of propane Pre-cooled Mixed refrigerant (C3MR) to maximize the utilization of power available from compressor drivers while maintaining efficient and efficient refrigerant compressor operation over a wide temperature range. Comparison of both technologies is shown on Table 1.
The selection of liquefaction technologies for an LNG plant depends on the overall economics, the required size train, the volume of natural gas reserves, market demand and equipment reliability. For many years, the propane pre-cooled Mixed Refrigerant (C3MR) process has remained the dominant liquefaction cycle in the LNG industry. This is due to the versatility of this cycle that makes it well-suited to accommodate this ever changing industry. Efficient integration of NGL/LPG recovery with the liquefaction process plays a key role in achieving lower heating value LNG requirements for a variety feed conditions. Though the mixed refrigerant processes rank the highest in equipment cost, it has the lowest operating cost. It consists of one big heat exchanger tower, massive compressors and propane chillers which raise the equipment cost and reduces the operating cost. The operating cost in mixed refrigerant process is lower due to two reasons. Firstly, having large heat exchanger tower which leads to reduction in ambient heat loss since all the heat exchanging process takes place in the tower rather than in separate heat exchangers. Secondly, less work is required due to having one large compressor with a heat exchanging tower which is more efficient than having many compressors running for each loop. The more compressors for each loop results in more frictional and head losses, hence greater compression work.
Process technology for liquefaction of natural gas is undergoing continuous improvement to meet with the increasing demand of LNG. This has demanded LNG plants to be volume flexible to match with market demand. Thus process evaluation techniques are required to identify process improvements which can be applied on existing plants to lower the operation cost and expand its capacity to increase LNG production. Selection of the optimum process cycle is indeed crucial to ensure the capacity of the plant will be fully utilized and the plant is run in a cost effective manner. The unique characteristic of an LNG plant is to be operating it at an optimum thermal efficiency without neglecting the economics merits and environmental impacts.
The authors thanks Universiti Tecknologi PETRONAS for support in research work.