|VX Cycle block diagram. Source: Expansion Energy. Click to enlarge.|
Dresser-Rand—among the largest suppliers of rotating equipment solutions to the worldwide oil, gas, petrochemical, and process industries—has entered into a definitive agreement with Expansion Energy LLC under which Dresser-Rand is granted a worldwide exclusive (for capacities up to 100,000 gallons per day) license to Expansion Energy’s proprietary VX Cycle technology for the small-scale production of Liquefied Natural Gas (LNG).
Dresser-Rand believes that the patented VX Cycle is the first technology to provide a cost-effective small-scale LNG production process with capacities as low as 1,500 gallons per day—smaller than any other LNG production system commercially available today. The VX Cycle yields approximately 85% LNG from every unit of natural gas that enters the plant, with 15% of the gas used as fuel for the prime mover that converts the NG to LNG, according to Expansion Energy. (A 90/10 ratio of product to fuel is possible with certain optimizations.)
In our on-going evaluation of technologies to complete the value chain for the burgeoning shale markets, as well as solutions for addressing global natural gas flaring and the rapidly expanding markets for LNG vehicle fueling, Expansion Energy’s LNG production process met all of the requirements we identified and embodies technology that we can bring to the market quickly. We already have several clients anxious to take multiple units to fuel their fleet operations. We presently believe that the market for this technology will be in the range of $100 to $200 million within just the next 2 to 3 years, and will continue to grow robustly beyond that.—Brad Dickson, Dresser-Rand’s Vice President and Chief Marketing Officer
The VX Cycle uses standard CNG equipment to produce LNG. The low-pressure stream natural gas is separated into a fuel stream for the prime mover (engine or turbine), and a product stream (85%) to be liquefied (or compressed). CO2 and water are removed in a multi-vessel molecular sieve, which requires periodic regeneration. The regeneration gas is sent to the prime mover for use as fuel. The cleaned gas is then sent to a multi-stage CNG compressor, such as used at existing CNG stations.
The feed gas is compressed, in stages, from 60 psia to approximately 400 psia. That choice is an essential feature of the invention, according to Expansion Energy, because up to 3,500 psia is routinely provided by most CNG compressors. Operating a CNG compressor at lower pressures reduces its workload and the heat of compression.
The CNG compressor is both the feed gas compressor and the recycle compressor. This is possible because the VX Cycle is an “all methane” cycle, in which the working fluid (refrigerant) and the feed stream are both methane.
The VX Cycle uses an integrated absorption chiller to counteract the heat of compression and to pre-cool the CNG immediately after it exits the compressor’s after-cooler.
To achieve -250 °F LNG at 65 psia, significantly more refrigeration is needed than can be provided by the front-end chiller. Two sources of refrigeration are at work near the main heat exchanger.
The first is a throttle valve. The pre-cooled CNG at +/- 400 psia is sent through the main heat exchanger where it is cooled to -170° F by the other streams within the exchanger. That combination of approximately 400 psia and -170° F allows for “plate fin” heat exchangers rather than the more-expensive coil wound units.
A portion of the -170° F stream, at +/- 400 psia, is sent through the throttle valve, which yields approximately -254° F vapor and liquid at a pressure of only 19 psia. That cold vapor + liquid stream is used to sub-cool the portion of the stream that is still at -170° F and 400 psia, cooling it to -251° F and still at +/- 400 psia. The sub-cooled product is dropped in pressure to 65 psia; forming LNG at -250° F, which can be sent to the storage tank, without any “flash” (vapor) formation.
The low-pressure stream that cooled the main product stream in the sub-cooler is sent back toward the beginning of the process as part of the recycle stream.
Prior to the return trip through the main heat exchanger, the recycle stream is mixed with the recycle stream from a compressor-loaded cryogenic methane turbo-expander—the second source of refrigeration. The turbo expander is needed because JT (Joule Thompson) refrigeration is not efficient enough.
The mobile, skid-mounted equipment configuration for this process technology opens up a variety of applications in markets currently underserved or not served at all. Upstream applications include: the monetization of flared gas or associated gas to increase revenues for oil companies and reduce their environmental impact; the production of stranded natural gas fields which are not close to existing pipeline infrastructure; and on-site fuel supply for drilling rigs converted to run on LNG.
Downstream applications include the production of vehicle-grade LNG, allowing LNG to compete effectively with diesel fuel on a cost-per-energy-content (BTU) basis
The VX Cycle enables the distributed production of LNG with small-scale plants, as the technology can utilize natural gas from high- or low-pressure pipelines or distribution lines, or from stranded wells. As such, the VX Cycle eliminates the need for the costly trucking of LNG long distances from large, centralized plants to LNG fueling depots, as is the practice today. Instead, the VX Cycle produces LNG right at the fueling station or at the wellhead. The VX Cycle technology can also be used to upgrade existing CNG stations to produce LNG and/or a colder, denser CNG product with a higher BTU density versus standard CNG.
Under the agreement, Dresser-Rand will design, package and sell equipment embodying the VX Cycle production technology including Dresser-Rand reciprocating compressors and Guascor engine-generator sets, and associated control systems. Dresser-Rand advises that the technology is scalable to the tens of thousands of gallons per day, producing either LNG or CCNG (Cold Compressed Natural Gas) and will be designed in numerous skid-mounted, mobile trailer-mounted and/or stationary modular configurations to suit its clients’ requirements.