Liquefaction Focuses on Flexibility

22 January 2008 (Last Updated January 22nd, 2008 18:30)

As we increasingly rely on LNG production the challenges of the liquefaction process become increasingly complex. The current crop of LNG cooling technologies provides the platform for innovation required to meet challenging environments head on.

Liquefaction Focuses on Flexibility

The rapid growth of the global market for liquefied natural gas (LNG), along with historically high prices for energy, have incentivised the industry to find and exploit new reserves. The potential return for capital-intensive LNG projects is attracting producers to harsher, more remote locations and technology development must keep pace.

Efforts to improve liquefaction technology have needed to yield quick results in a limited time frame by improving efficiency, lowering cost, raising capacity and becoming sufficiently flexible to suit a range of challenging environments, as global energy markets undergo rapid change.

"It's a sellers market now, which was not true five years ago," observes Royal Dutch Shell's gas technology manager, Rob Klein Nagelvoort.

"Most commercially available LNG technologies have very high thermal efficiency."

LNG liquefaction technology involves refrigeration cycles, using hydrocarbons and inert agents to absorb heat from natural gas, which is cooled through multiple expansion cycles, before LNG passes to gas turbine driven refrigerant compressors.

Adapting this basic process to suit different implementations has been a key focus, as technology developers target smaller or more complex fields and look towards offshore production.

Most commercially available LNG technologies have very high thermal efficiency, so versatility and lower cost of operation have been key priorities. Innovative approaches to refrigerants, aeroderivatives and design continue to emerge, delivering more compact, robust and adaptable systems.

INNOVATIVE TECHNOLOGIES

Among the most successful liquefaction technologies is ConocoPhillips's optimised cascade process. Designed around a 'two-train-in-one' concept to improve reliability, the process has been successfully proven over decades of plant operation in Kenai, Alaska and elsewhere.

The refrigerants in successively colder heat exchangers are in three loops of essentially pure propane, ethylene and methane, a factor that improves stability and ease of operation.

"We are the second most licensed process," notes Jim Rockwell, manager gas-to-liquids, ConocoPhillips LNG. "It is well proven, which very few processes can claim. All of our plants have met and exceeded design production, been on budget and on schedule."

ConocoPhillips was the first company to use replace steam turbines with gas-fired turbines to drive refrigeration compressors, and the first to use high-efficiency, high up-time aeroderivative turbines in an LNG plant, so reducing fuel consumption by around 25% compared to industry-standard Frame turbines.

The company currently has nine LNG trains in operation. After Kenai, the first license of the optimised cascade SM process was to Atlantic LNG in Trinidad in 1999, since when other projects have included Darwin LNG in Australia and Egyptian LNG. The latest in the series – Atlantic Train Four, where the optimised cascade process was deployed in 2006 - was then the world's largest operating LNG plant.

"Efforts to improve liquefaction technology have needed to yield quick results in a limited time frame."

"All plants have used at least two parallel compressors for refrigeration compression – our 'two-in-one' design – which improves reliability by allowing a plant to continue operating when a turbine or compressor is down. In addition, the thermal efficiency of the process remains high for varying conditions," remarks Rockwell.

The two-in-one design illustrates ConocoPhillips's drive for flexibility. Atlantic Four was the first time it had used three gas turbine-compressor strings on a single plant, to increase up-time and make its operation more versatile.

A PLATFORM FOR INNOVATION

ConocoPhillips has had to cope with the demanding environments like Alaska. Shell has had to tackle the likes of Sakhalin Island, Russia.

Land-based projects there are challenging enough, but in 2007 Sakhalin Energy began drilling Russia's first offshore gas wells. The Lunskoye-A platform, which will provide the main volume of gas for LNG production, is designed to operate year-round in a severe, rough seas and ice, and withstand significant seismic activity.

The commissioning of Russia's first LNG plant is also now under way. The plant at Aniva Bay will use Shell's new double-mixed refrigerant (DMR) process to improve operational performance in temperatures ranging from 30°C to -30°C.

"The climate changes a lot between summer and winter. The process must be efficient, maintaining the highest output through the whole temperature range. There are demands on water quality, as fisheries are a big source of income in the region. So, I based the design on air cooling," says Shell's Klein Nagelvoort.

The DMR process adds greater flexibility to the typical liquefaction process of pre-cooling and final cooling cycles. The conventional propane / mixed refrigerant (C3/MR) process uses a propane refrigerant for pre-cooling, whereas DMR uses a mixture of ethane and propane for pre-cooling. There are two cooling cycles in series, with a mix of refrigerants extracted from natural gas.

By varying the concentrations of these refrigerants, plant operators using DMR can respond to changing climatic conditions. Increasing the proportion of propane creates a heavier mix in summer, while adding ethane yields a lighter mix for winter. Traditional C3/MR pre-cooling cannot be adjusted this way.

The DMR process cools gas to -40°C in summer and -65°C in winter, feeding pre-cooled natural gas into the liquefaction process. It gives producers another option, alongside propane pre-cooling or single-mix refrigerants. DMR aims to maximise flexibility in harsh environments and, as Sakhalin is very remote, the system is compact by design.

"Among the most successful liquefaction technologies is ConocoPhillips's optimised cascade process."

"The process is very good for cold climates or places where the temperature changes a lot. We have a similar process for a plant in the Middle East, where the temperature ranges from 5°C to 50°C. Most of the other processes in our portfolio use propane pre-cooling, being very practical, but less flexible when temperature and feed quality change considerably." remarks Klein Nagelvoort.

"Around the equator in places like Brunei, Nigeria, or in Northern Australia, for example, temperatures vary less, so it makes sense to use propane first, then DMR," he adds.

The development of DMR for Sakhalin has provided valuable lessons for future cooling technology development, which Klein Nagelvoort believes will rest on a portfolio of solutions adaptable to different environments and capacity needs. Shell is, in fact, preparing to launch a new suite of LNG technology early in 2008, which will show its commitment to versatility.

Capacity variation is an increasingly important factor. Shell has the technology for a propane pre-cooled liquefaction process of 11 million tonnes a year, Sakhalin in currently seven million tonnes a year and growing, and it is building facilities in Qatar with capacity for eight million tonnes a year. The company recognises, however, that its solution must increasingly enable smaller operations.

"At the other end of the scale people are turning to the development of smaller gas reserves, with an LNG production capacity of two to three million tonnes per annum, which would use single-mix refrigerants," remarks Nagelvoort.

HEADING OFFSHORE

As the Sakhalin development shows, optimising offshore liquefaction capability will be crucial in the years ahead. For Shell, DMR is one stage in a series of innovations that will help it to address the challenges of the offshore environment.

"DMR was developed for Sakhalin, but we also have developed a variant for a floating LNG system. With refrigerants being lighter than propane there is less risk of a pool of liquid forming when a small leak would occur. Furthermore, the compact design of the system for Sakhalin makes the technology suitable for offshore applications," notes Klein Nagelvoort.

"The two-in-one compressor design illustrates ConocoPhillips's drive for flexibility."

ConocoPhillips also continues to build on its past innovations, and one of its key foci going forward is the further development of aeroderivatives, which it sees as appropriate for offshore installations.

It is also working to develop electric motors for refrigeration drivers and improving efficiency by capturing waste heat from aeroderivative turbine exhausts.

These companies are obviously not alone in developing more versatile, adaptable and cost-effective solutions for LNG cooling, but they are among those matching the speed of innovation that is needed to make production viable as the industry explores further and deeper for reserves.