There are many areas throughout North and South America where there are pockets of “stranded gas,” and many communities that need that gas but have no pipeline infrastructure. That includes individuals and businesses like mines and manufacturers.
To respond to the needs of gas producers and their customers, Okra Energy has developed small-scale LNG plants that make it possible to get gas where it’s needed. They call it a “virtual natural gas pipeline,” and the core of it is modular liquefaction and regasification.
Okra’s chief technology officer, Don Gongaware, provided a comprehensive description of the various stages of the process, and just like regular LNG plants, their process uses many valves and actuators.
THE PLANT IN MODULES
The liquefaction process is divided into the following six major stages along with their related modules. You can see the process in a video at THIS LINK.
1. Reception, Metering and Drying
The first module receives the natural gas and consists of the equipment and components required to isolate the plant from the gas source, reduce pressure to the required plant operating pressure, and remove any entrained water or particles from the natural gas.
Water droplets are knocked in an inlet separator. Pressure is controlled downstream of the inlet separator. The gas passes through an inlet pressure control valve that will regulate the plant feed gas pressure at a constant control pressure. A pressure/temperature compensated flow transmitter is installed and configured to measure and record plant inlet gas flow.
2. Water and Carbon Dioxide Removal Module
The gas then flows to a temperature-swing adsorption (TSA) module for drying the gas and removing CO2. The TSA module consists of three beds, one heater, one air cooler and associated valves and instruments.
The TSA dries the gas before prior to removing flowing to a skid dedicated to the carbon dioxide removal skid. One of the substrate process beds is removing water and CO2 while another bed is either regenerating or in a wait state/on standby.
Regeneration is completed using the vent gas stream from the cold box consisting of methane, ethane and nitrogen from the LNG cold box. The final gas composition is based on operating parameters and the inlet gas composition.
The purified gas from the TSA module is routed to the dry gas dust filters to remove any impurities prior to entering the cryogenic portion of the plant. At this stage, the process gas is ready for pre-cooling and liquefaction.
3. Liquefaction Module
The liquefaction module consists of the equipment and components required to liquefy the natural gas. It also includes the equipment required to remove nitrogen, ethane and other trace particles.
The pretreated gas from the purification module first flows into a primary heat exchanger and is precooled to approximately -200oF.
The gas then flows through a Joule-Thomson (JT) valve. The cooled natural gas pressure is dropped across a JT valve to approximately 30 psi and -240oF. The precooled gas flows into a flash separation vessel to separate out most of the nitrogen from the natural gas.
The LNG composition is approximately 99% methane and 1% nitrogen and can change based on the inlet gas composition. The vent gas is used for regenerating the TSA beds and as fuel for an engine-generator.
4. Refrigeration Module
The refrigeration module consists of the equipment and components to provide the necessary energy to support the liquefaction of the LNG. The refrigeration module includes two independent trains. Each train can run from 15 to 100%, thus providing a high degree of flexibility.
The refrigeration is provided by a closed loop nitrogen cycle. Nitrogen is compressed to approximately 300 psi. The selected compressor provides a wide range of capacity by adjusting either the discharge control slide valve or the volume slide valve. The two slide valves help improve the efficiency of the compressor by compressing only the required nitrogen to reach the desired discharge pressure.
The compressed nitrogen flows into the compressor on the expander-compressor, which uses the energy given off by the expansion of the nitrogen from approximately 400 psi.
To minimize the loss of the nitrogen, the oil system on the expander-compressor is a pressurized oil tank. This allows the nitrogen pressure to settle out with minimal loss into the atmosphere.
5. LNG Storage and Truck Loading
This module includes three 16,000-gallon storage tanks with associated valves and instruments. The truck loading module includes all equipment and components to safely load an LNG transport trailer. It includes a transfer pump, truck scale, weigh printers, instruments, valves and associated piping.
6. Power and Controls Module
The power and controls module includes all equipment and components required to distribute power to each skid, plant component and facility equipment. Each skid/module has its own power distribution panel and controls panel designed for the maximum voltage required on the skid.
Okra supplies an uninterruptible power supply system (UPS) as backup power. The UPS is for the primary control system. This allows the operators to ensure all valves and equipment default to a safe state in the event of an emergency or system error and allow an orderly shutdown.
HOW CAN VALVE MANUFACTURERS HELP?
According to Gongaware, the system uses a variety of valve types to include manual, actuated on/off, or controlled. They use ball (full, v-port and reduced port), check valves, butterfly valves, globe, plug and pressure safety valves. However, Okra works to limit the number of valves throughout the process for several reasons. “First, is to minimize leak points,” he said. “Each valve has a minimum of two fittings and each fitting is a potential leak point.” The second reason is to reduce maintenance because each valve is another maintenance point.
We asked Gongaware how valve manufacturers could help in the development of more of these plants, and to make them more efficient.
“Cost is of course is a factor for these small modular plants. Our goal is to minimize CAPEX cost and minimize the life cycle cost,” said Gongaware. “More parts and components increase life-cycle costs, so we often have to use fewer valves. I also use socket-weld versus flanged, and minimize control valves and/or actuated valves because of the cost differences.” Valve manufacturers could help by working with the plants to decrease control valve costs, make the valves available directly and shortening delivery times. “We need to able to get valves faster than 4 to 12 weeks,” he said.
Just as modular nuclear plants can provide power to stranded communities, modular LNG plants could be an answer to the problem of stranded natural gas. Valve manufacturers have an opportunity to make this process even faster and more efficient.
Gongaware passed on this message: “To be competitive in a small-scale plant, a single dollar difference has a huge impact. To be competitive, valve suppliers need to work with smaller companies like us to help drive down cost and delivery time.”