Gas industries worldwide had found that vast quantities of natural gas available in many parts of the world. Some of this gas was associated with oil production and was simply flared considering as a waste product. Unfortunately the supplies of gas were very far away from the large energy markets. In some parts of the world major supply of the gas by pipeline were in use up to several thousand kilometres. However, piped gas transport and pipelines have number of drawbacks. It is technologically difficult to Lay pipelines at depth of more than about hundred metres below sea level and under water pipelines are expensive and sometimes at risk. Political conditions in the areas between producer countries and the consumption countries may not be favourable to investments. The problem was therefore to derive a best, safest and economic method of transporting large quantities of natural gas by sea. The solution was seen to be liquefying the gas by refrigeration down to cryogenic temperature. At -161°C natural gas condenses to a liquid at atmospheric pressure and only occupies 1/600th of its original volume. This volumetric advantage, without the high pressure containment problems, was most attractive for shipping economics.
In October 1964 the worlds first LNG importation project was inaugurated with the delivery of first cargo of the LNG to Canvey methane terminal in the United kingdom from the liquefaction terminal at ARZew in Algeria. The liquefaction plants and loading facilities are permanently tied to a particular gas field, the LNG ships, which represent the huge investment in an LNG scheme, can be moved from one supply system to another supply system. In the past forty years as the number of gas liquefaction plants around the world has increased this extra flexibility has been very beneficial. Similarly, the destination of LNG, once it is loaded can be changed in accordance to its demand. LNG once liquefied can be stored at atmospheric pressure and can be used as a gas reserve to be drawn up to cope with demand fluctuations.
The development of LNG as a massive energy source has continued during the past forty years with technological advances in purification and liquefaction process, LNG ship construction, loading and unloading systems and construction of Cryogenic storage tanks using new materials ,improved insulation and new methods .
LIQUEFIED NATURAL GAS TECHNOLOGY:
The 1/600th solution involves
Natural gas is a mixture of hydrocarbon gases consisting predominantly of methane. Inert gases such as carbon dioxide, nitrogen and traces of mercury, sulphur and occasionally water vapour and helium may be present.
At atmospheric pressure, LNG, depending on its composition will boil at a temperature ranging -157 C to -166 C. At an absolute pressure of 13.8 nar(200 lbsF in2), LNG can be stored at temperatures up to -118 C. The presence of nitrogen in LNG depresses the boiling point by about 2.5 C per volume percent of nitrogen.
The density of LNG is determined by its composition, pressure and temperature. Nitrogen increases the density by 5 Kgm 3 per volume percent.
Properties at atmospheric pressure
Critical Temperature
Critical Pressure
Boiling point
Liqud Density
Latent Heat
C
F
KGM
Lbft -3
kjkg
Btu lb
C
F
Bar
1
2
3
4
5
6
7
8
9
10
methane
-162
-260
424
26.5
509
219
-82
-116
46.0
Ethane
-89
-128
541
33.8
488
210
32
90
48.8
Propane
-42
-44
580
36.2
426
183
97
206
42.5
Iso-Butane
-12
10
599
37.4
365
157
134
273
37.5
n-pentane
36
97
620
38.7
358
154
197
387
33.6
n-butane
-1
30
599
37.4
386
166
152
305
38.1
Gas purification: Gas piped to a liquefaction plant from fields around hundred miles away is usually get minimum purification necessary at the wall head. The feed gas may therefore contain water, hydrogen sulphide, carbon dioxide, higher hydro carbons, and other impurities. The line must be checked regularly to prevent blockage and irregularities due to two-phase flow.
The first stage treatment will therefore include traps and facilities to collect liquids. Depending on ambient temperature, water content and pressure drop, methanol or glycol can be injected to prevent hydrate formation. In this case glycol or methanol recovery involving fractionation will be required on the aqueous layer in the gas or liquid separator . The second stage is gas sweetening by removing H 2S and CO2.
After sweetening the gas is generally saturated with the water which has to be removed before liquefaction. Drying can be carried out by
Simple refrigeration
Glycol dehydration
Solid desiccant adsorption
Gas liquefaction: - to liquefy a boiling gas it has to be cooled below its dew point . To cool a gas ,heat energy has to be removed from the compressed gas either by means of cooling water, or by means of an evaporating refrigerant. If the temperature of heat removal is lower than ambient.
The classical cascade cycle: - the cycle uses three separate refrigerants propane, methane and ethylene . In three compression refrigeration cycles operating at successively lower temperatures cycles each rejecting heat to the next warmer cycles that is cascading on to it. Operating conditions for cascade process are there fore largely defined once the type and the number of refrigerants have been selected.
Modified cascade cycles (mixed refrigerants cycle):- in these cycles refrigerant circulation is restricted to a single stream and by operating in multiple stages heat exchange at different pressures the mixed refrigerant produced by using the heavier hydro carbons from the natural gas itself can be pumped as a single fluid.
Pre-cooled mixed Refrigerant cycles (C3= MR cycle): this process was developed later and has been used in all the most recent LNG plants. It combines the simplicity of a mixed refrigerant cycle with the efficiency of cascade cycle and is now used at more than half of the base load LNG plant. It consists of mixed liquefaction refrigerant cycle and a another separate cycle for pre cooling the natural gas feed and the liquefaction refrigerant.
Liquefaction process technology selection is the most important part of developing an LNG production facility. considering that all liquefaction process technologies are dealing with the same thermodynamic limits, the technologies that best fit known . Generally, LNG is produced at locations where the gas is plenty and low cost, the equipment used plays a major part in the over all efficiency ,reliability, operability and cost of the plant .The liquefaction section typically contains 30-40% of the capital cost of the total plant, several licensed liquefaction process available with varying degrees of application and experience.
The C3-MR Process, licensed by AIR PRODUCTSis the most popular and widely used in the LNG industry for the last 30 years.
Ap-X tm process licensed by AIR-PROUCTS is still a non-proven technology but being used currently in the Qatar projects with large LNG train capacities, 7.8 MTPA.
The Dual Mixed Refrigerant Process (DMR Process) licensed by shell. Till date no LNG plants are plants are operated using this process yet, although at the moment shell is promoting this process for their LNG projects and in fact, this technology has been selected for an under constructing Sakhalin I LNG plant
The Mixed Fluid Cascade process (MFC Process) licensed by linde-statoil. Linde is present building for Statoil which is going to be the the first European Lng plant, in northern Norway, producing 4.2 MTPA of LNG in one single train and it is also the first commercial application of the MFC process.
The BHP nitrogen cycle is being considered for smaller LNG trains especially for FPSO applications,similar cycles have been used for small capacity peak shaving applications.
Shell has developed the latter option of the DMR process as the PMR process (Parallel Mixed Refrigerant process) . This process has not been selected yet.
PROPANE PRE-COOLED MIXED REFRIGERANT (C3-MR) PROCESS
the APCI Propane pre-cooled MR process is widely used process in the worlds base load LNG production. It uses a propane refrigeration cycle in cascade with a mixed refrigerant cycle.
The MR composition is normally comprises a mixture of nitrogen, ethane, methane and propane. The high pressure MR is first cooled with cooling water or air and then with propane. The MR is partially condensed on leaving the last propane exchanger and is separated into gas partially condensed on leaving the last propane exchanger and is then separated into liquid(heavily MR) and gas (light MR) streams. The heavy and light MR streams are then fed separately into the cryogenic exchanger, which is a spool wound coil type exchanger.
The liquid or heavy MR stream is sub cooled in the lower section or warm tube bundle of the cryogenic exchanger. It leaves the top of the warm tube bundle and is expanded and cooled to about -1100 C, prior to re-entry into the cryogenic exchanger, this time on the shell side. Here expanded heavy MR is combined with light MR liquids and vapours descending from the upper section, or cold tube bundle, of the exchanger. This mixture provides the first step of cooling the feed gas in cryogenic exchanger.
The light MR stream form the high pressure separator is passes through the lower warm bundle and then through the upper cold tube bundle of the cryogenic exchanger where it is cooled and completely condensed , the light MR is then expanded, before being cooled and partially vaporised at a temperature of about - 1700C. It is then reintroduced to the top of the cryogenic exchanger on the shell side. This mixture provides the final cooling step and liquefaction for the feed gas streamCONCO/PHILLIPS "OPTIMISED" CASCADE PROCESS
This process is the modern version of Phillips petroleum cascade process and was selected for Atlantic LNG project in Trinidad and Darwin LNG in Australia.
The optimised Conoco/Phillips process uses an open methane refrigeration circuit or feed flash system. Condensed product from the ethylene evaporators enters the open refrigeration cycle where it is combined with the partially vaporised methane refrigerant, to produce recycle gas for the methane refrigeration circuit and LNG product. plant fuel is with drawn from down stream of the methane refrigerant compressor there fore eliminating the need for a separate fuel gas compressor
APCI AP-X TM PROCESS
In 2001, Air products patented AP-X TMLNG process was introduced to the LNG industry to increase the production capacity of the single LNG train from 5 - 8 million metric tons per year without adding propane or MR compressor casings in parallel service. The MT and propane compressor casings can also remain within reasonable extensions of proven vendor frame sizes, and the dimensions of the main cryogenic heat exchanger do not need to increase beyond the size currently being manufactured. It also retains the major strength of the previously existing MCR cycles, namely the flexibility to maintain high efficiency through changes in feed composition and daily/seasonal temperature variations.
Shell has developed the parallel MR (PMR) process to meet the current challenge of the industry for
larger train sizes.
With a single pre-cooling cycle & two PMR cycles, the capacity can be increased up to 8MMTPA.
The process can either use MR or an C3 in pre-cooling. Proven refrigerant cycles can be used and the
design can be applied, without change in technology. The capacity can be increased further
with larger drivers.
Gas receipt and natural gas treating is followed by single propane pre-cooling cycle. After pre-
cooling, the flow is distributed over two parallel strings, each having a scrub column for NGL
extraction and an MR cycle for liquefaction and sub-cooling of the natural gas.
The scrub column overheads are cooled by the MR to create reflux and ensure the required
extraction level. Each liquefaction cycle has its own MR circuit, driven by a gas turbine. The sub
cooled liquid from the two liquefaction cycles is combined in an end flash system, where fuel gas is
flashed off and LNG is sent to storage at atmospheric pressure.
