January 2025
Process Optimization
A novel process and machinery unit for the highly efficient liquefaction of renewable natural gases
The authors’ company has developed a gas expansion-based liquefaction processa using a proprietary machine called a compander. This article describes the newly developed process, including ease of operation and permitting, as well as high quality thanks to in-house design, manufacturing, assembly and testing of key components.
The authors’ company has developed a gas expansion-based liquefaction processa using a proprietary machine called a compander. Compared to conventional gas expander processes, the compander-based process offers 10%–15% higher process efficiency and requires less equipment. The highly efficient, sustainable and competitive solution is ideal for liquefying renewable natural gases (RNGs). To develop this technology, the authors’ company leveraged its comprehensive in-house expertise in both process and turbomachinery design and optimization.
This article describes the newly developed process. Highlights of the process include ease of operation and permitting, as well as high quality thanks to in-house design, manufacturing, assembly and testing of key components.
Background. The market for RNGs—such as biogas, landfill gas and e-methane—is expected to grow rapidly due to their potential as an energy source with a low-, zero- or even negative-carbon dioxide (CO2) balance. Regarding biogas, the International Energy Agency’s (IEA’s) Net-Zero Scenario forecasts a worldwide growth rate of 32% between 2023 and 2028.1 A key factor driving the rapid expansion of RNG is the seamless substitution of fossil-based liquefied natural gas (LNG) with liquid RNG through the utilization of existing infrastructure. However, to fully compete with fossil-based LNG in markets like heavy-duty land transport and ship bunkering, RNG must be liquefied.
The ability to liquefy gas is an essential part of the value chain in the gas market. In fact, storage and transportation would hardly be economically viable in many market segments without liquefaction. For LNG production rates of approximately 200 tpd–1,000 tpd, the nitrogen dual-expander cycle is the widely established liquefaction process. For RNG from biogas plants and other sources, small production capacities and low product gas pressures are the norm. There are many approaches to adapt and improve the standard nitrogen dual-expander configuration for lower outputs. However, these configurations often fail to integrate compact, highly efficient turbomachinery and require two or three separate rotating units.
This prompted the authors’ company to develop a more suitable liquefaction technology for use in a broad range of RNG applications.
Integrated team approach. The technology was developed according to a novel concept. Traditionally, in a first step, the liquefaction technology owner designs and optimizes the process, considering all pertinent project parameters. Then, the liquefaction technology owner approaches one or more rotating equipment original equipment manufacturers (OEMs) to inquire about refrigerant cycle machinery that can achieve the preselected process parameters as closely as possible. This process usually requires many iterations and compromises to arrive at the best possible and buildable design. Yet, even at the end of the exercise, it is often unclear whether an optimal process configuration and design parameters have been determined for the selected machine.
For this project, the authors’ company took a different approach and leveraged unique in-house expertise from the very beginning of the development process. The contribution from turbomachinery experts was especially valuable. Their experience gained from more than 70 projects involving similar machines for air separation and syngas plants can be beneficially employed for the development of natural gas processing and liquefaction technologies.
Best-in-class efficiency. In essence, the team's idea was to introduce a third pinion—with a compressor and an expander wheel—to an integrally geared machine, a compander. In total, the complete machine comprises three pinions with six wheels—three each for compression and expansion. For higher liquefaction capacities utilizing larger machinery frame sizes, four pinions are possible, which further increases energy efficiency by distributing compression work across multiple wheels.
The gas expansion process is designed so that all compander wheels operate at maximum efficiency. The integrated compression for low-pressure feed gas within the compander substantially improves overall process efficiency. Put simply, the feed gas pressure is optimized for minimum energy requirements in the subsequent liquefaction step. The third expansion wheel, driven by an open-loop feed gas precooling cycle, further improves energy efficiency and relieves the nitrogen expansion refrigeration cycle (FIG. 1). The pressure ratio in the nitrogen cycle is considerably reduced, so two compression stages are sufficient. In the case of low-pressure feed gas, as is often the case for RNGs, the newly developed concept provides overall energy savings of 10%–15% compared to conventional gas expander-based technologies.
FIG. 1. Newly developed process concept with an ideal fit to machinery with six wheels on three pinons and a matching plate-fin heat exchanger.
The system topology was another area for improvement: this involved developing specific compander frame sizes for different liquefaction capacity ranges, complemented by a standardized layout and sizes of the main cryogenic heat exchanger (plate-fin type), process gas coolers, piping and valves. This approach minimizes overall system investment costs.
Even though the innovative design is comprised of standardized, pre-engineered components, a sufficiently high degree of flexibility is still maintained. That is because the individual expansion and compression wheels are tailormade and can be easily adapted to project-specific feed-gas conditions—i. e., composition, pressure and temperature.
Depending on the quality of the feed gas, an upstream adsorptive heavy hydrocarbon removal unitb can be added. This helps maintain a high degree of standardization for the downstream liquefaction unit—as described above—regardless of the feed gas type and pressure (renewable methane or certified pipeline gas).
A compete product partner. Developing innovative technology is one thing, but seamlessly implementing it is quite another. The close collaboration among all experts in the authors’ company proved highly effective, resulting in an in-house design and the manufacturing, assembly and testing of key components, such as the compander and the plate-fin heat exchanger. In addition, this approach enabled the full control of the supply chain, beginning with process optimization and extending all the way through to the delivery of key components from a single source.
The compander skid (FIG. 2) is completely packaged with aftercoolers, an oil system, control valves, silencers, safety valves, a programmable logic controller (PLC) and seal gas. Liquefaction capacities range from 50 tpd–1,650 tpd LNG in a single-train configuration, covered by five available frame sizes.
FIG. 2. The compander is a modularized machinery string with standardized frame sizes that allow for efficient project execution.
Takeaway. The authors’ company’s gas expansion process represents a significant breakthrough, enabling the liquefaction of RNGs with 10%–15% higher process efficiency compared to traditional nitrogen expansion solutions. The result is significantly reduced operational expenditures and increased sustainability. The pure nitrogen refrigeration cycle, with hassle-free refrigerant makeup or import, ensures straightforward permitting and operation of the technology.
The integration of several compression and expansion services, such as feed gas compression within nitrogen booster-expanders, is achieved using only one integrally geared machine known as a compander. This approach keeps the technology’s equipment count to a minimum, resulting in an attractive capital expenditure and requiring minimum plot space.
The solution meets the highest quality standards thanks to in-house design and manufacturing, assembly and testing of key components in the authors’ company’s own workshops. Moreover, full control over the supply chain, from process design to delivery of key components, by one company allows for shorter delivery times.
NOTES
a LICOM™
b Linde Engineering’s HISORP® TS
LITERATURE CITED
1 IEA, “Renewable 2023: Analysis and forecast to 2028,” January 2024, online: https://iea.blob.core.windows.net/assets/96d66a8b-d502-476b-ba94-54ffda84cf72/Renewables_2023.pdf
The Authors
Andreas Bub joined Linde in 2008 as a Process Engineer for natural gas plants. In this role, he led several NGL, LNG, NRU and helium projects worldwide. Since 2023, Bub has led the Process Design Group for natural gas plants for the product line Sustainable Hydrocarbon Solutions. He earned an MS degree in chemical engineering from the Technical University of Karlsruhe.
Thilo Schiewe joined Linde in 1997 and has held various management positions in business development and sales, product management and cost estimation in the product line LNG and natural gas processing plants. Later, Dr. Schiewe headed the business unit for LNG plants at Linde Engineering’s subsidiary in Hangzhou, China. Since early 2023, he has held responsibility for the cost estimation group of the newly established product line Sustainable Hydrocarbon Solutions and drives new product developments, with a special focus on liquefaction of RNG. Dr. Schiewe earned a PhD in chemical engineering from the University Erlangen-Nuremberg in Germany.
Henry Howard is a Corporate Fellow and a member of the cryogenic process R&D group within Linde Engineering. In his 35 yr at Linde, he has focused on the development of new cycles and equipment configurations in support of atmospheric, CO2 and natural gas processing. His expertise also includes the advancement of distillation technology through field testing and process model reconciliation. He is the inventor of more than 75 granted U.S. patents and has authored several papers related to the integration and optimization of equipment within cryogenic processes. Howard earned BS and MS degrees in chemical engineering from Rensselaer Polytechnic Institute and the University of Michigan (Ann Arbor), respectively.
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