Heat exchangers have evolved to meet the market demands of LNG for improved productivity, efficiency, easier maintenance and energy savings
Heat exchangers have evolved to meet the market demands of LNG for improved productivity, efficiency, easier maintenance and energy savings
New process cycles, increased liquefaction train capacity, decreased LNG plant footprints and the introduction of floating LNG (FLNG) vessels have resulted in new cool(ing) solutions from heat exchangers manufacturers.
Used in the majority of LNG facilities for liquefying and subcooling natural gas, coil-wound heat exchangers (CWHEs) can contain hundreds of metres of helically wound tube bundles housed within an aluminium or stainless-steel pressure shell. The heat exchangers may consist of one, two, or three bundles, each made up of several tube circuits with the tube circuit areas being matched to the process requirements.
Accounting for about three-quarters of global LNG liquefaction capacity, CWHEs provide efficient liquefaction and have proved to be a stalwart in the LNG market because of their excellent heat transfer characteristics and flexibility.
One of the new wave of US LNG export terminals, the Golden Pass Products, LLC project has selected US-based Air Products to supply three of its MCR main cryogenic heat exchangers (MCHEs) for its US$10Bn Sabine, Texas, facility. Along with the MCHEs, Air Products will supply its propane pre-cooled mixed refrigerant AP-C3MR natural gas liquefaction technology and equipment for the three liquefaction trains that will have a total nameplate capacity of 15.6 mta of LNG.
The selection of AP-C3MR natural gas liquefaction technology by Golden Pass Products is hardly surprising. Indeed, more LNG is produced using Air Products’ mixed component refrigerant and liquefaction processes than any other processes in the world.
Last year, Air Products liquefaction processes accounted for 72% of total global liquefaction capacity, according to 2019 IGU World LNG Report. Representing 42% of the global liquefaction capacity was Air Products’ AP-C3MR, while AP-C3MR/SplitMR accounted for 18% and AP-X process 12%.
"As the heart of the liquefaction process, MCHEs have grown in size to meet the LNG market’s demands for increased productivity"
Air Products processes have also proved to be popular with new projects, with Yamal LNG and Cameron LNG both using the company’s AP-C3MR design and Cove Point LNG, Freeport LNG and Ichthys LNG all utilising AP-C3MR/SplitMR technology. By 2024, AP-C3MR/SplitMR will have a 20% share of the global liquefaction market.
As the heart of the liquefaction process for the facility, MCHEs have grown in size to meet the LNG market’s demands for increased productivity. Over the last 50 years, the diameter of CWHEs has swollen 40%, their volume trebled, and weight quadrupled, according to Air Products. The typical exchanger may be as large as 5 m in diameter, 55 m high and weigh 450 metric tonnes. And size does matter; the larger MCHEs can perform the role of multiple heat exchangers.
Air Products says the larger size of the individual heat exchanger tube bundles facilitates the design of large process trains. Providing economies of scale, the larger units simplify piping and control systems and, as a result, reduce installation, operation, and maintenance costs.
As a consequence of the increasing market demands for larger MCHEs, Air Products opened a new manufacturing facility in Port Manatee, Florida, in 2014. The facility can build MCHEs up to 60 m high and 6 m in diameter and has ready access to a deepwater port, allowing large MCHEs to be shipped worldwide.
Air Products had previously manufactured its heat exchangers in Wilkes-Barre, Pennsylvania, but closed the facility in 2017.
A joint venture of Qatar Petroleum (QP) and multi-national energy giant ExxonMobil, Golden Pass Products expects to begin commercial operations in 2024.
Germany’s Linde AG can manufacture CWHEs with heating surfaces of up to 40,000 m2, with bundle diameters of 5,500 mm and unit weights up to 250 metric tonnes. Linde has supplied CWHEs for Norway’s Snøhvit LNG terminal, Brunei LNG terminal, Russia’s Sakhalin Energy and Australia’s North West Shelf Venture and Pluto LNG.
A Linde coil-wound heat exchanger being installed at an LNG plant (image: Linde)
In 2014, Linde was awarded a contract to provide engineering and procurement services for a 2.1 mta Woodfibre LNG Project near Vancouver, British Columbia, Canada. The Woodfibre LNG will use Linde’s proprietary Limum technology, a multi-stage mixed refrigerant process.
As part of the contract, Linde is responsible for the extended basic engineering as well as the procurement and supply of critical equipment items, including the CWHE. Using an "all-electric-drive" based on hydro power, the plant will minimise its greenhouse gas emissions.
Brazed aluminium heat exchangers
US-based Chart Industries offers brazed aluminium heat exchangers (BAHXs) as a competing technology to CWHEs. It has manufactured more than 12,000 brazed aluminium heat exchangers for cryogenic applications since the 1950s. Chart BAHX can be supplied as single units, manifolded assemblies or integrated solutions comprising fully assembled cold boxes including separator drums, vessels, interconnecting pipe work, valves, instrumentation and flanged connections.
A BAHX unit is composed of primary and second heat transfer surfaces and side bars. The parting sheets, fins and side bars are fabricated using a vacuum brazing manufacturing process developed in the 1980s. The process has enabled manufacturers to develop the size, complexity and upper pressure limits of the plate fin units that are required in the LNG market.
A key feature of the BAHX is its compactness as compared with CHWE and shell-and-tube technology, says Chart. It says a BAHX has a heat transfer area density of 1,000 to 1,500 m2/m3, which is about six to 10 times greater than a CWHE and 20 times that of a shell-and-tube unit.
New and established US LNG export terminals could be employing Chart Industries’ BAHX technology if approved. Tellurian’s 27 mta Driftwood LNG, LNG Ltd’s 8.8 mta Magnolia LNG and Cheniere’s stage three expansion at Corpus Christi LNG are all on board in utilising BAHX units.
Chart has already secured some high-profile LNG projects in 2019, booking an order for 18 cold boxes and BAHX units for the 10 mta Calcasieu Pass LNG project in Calcasieu, Louisiana and Golar’s Gimi (Tortue) FLNG vessel to be deployed at the Greater Tortue Ahmeyim project offshore Senegal and Mauritania.
Designing CWHE for FLNGs
Because of ship motions, safety and space constraints, floating LNG vessels use different liquefaction processes and different CWHE designs. CWHE used onboard an FLNG vessel, for example, have to be engineered for both significant blast load, due to the compact layout, and for cyclic fatigue caused by ship motions due to wave action. As a result of these design criteria, stainless-steel pressure vessel shells were used as opposed to aluminium. While the shell was changed to a stainless-steel alloy, the internal components are still manufactured from aluminium.
The first FLNG vessel, PFLNG Satu uses the AP-N process, as will the PFLNG Dua under construction at South Korea’s Samsung Heavy Industries (SHI). PFLNG Dua is destined for the deepwater Rotan gas field 130 km off Sabah in the South China Sea. It is designed to operate for 20 years without the need to be drydocked and will have a production capacity of 1.5 mta of LNG.
Shell’s Prelude FLNG incorporates Shell’s own Dual Mixed Refrigerant (DMR) liquefaction process, but it does incorporate an Air Products cryogenic heat exchanger.
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