Offshore Natural Gas Liquefaction Research Articles

On the sustainability of small-scale expansion-based natural gas liquefaction: Supersonic separator, Joule-Thomson, and turbo-expander

Expansion-based liquefaction technologies are prime candidates for small scale offshore natural gas liquefaction, aiding in reducing NG chain logistical costs, due to their compactness. This study assesses the sustainability and profitability of three expansion-based small-capacity natural gas liquefaction technologies: the rigid-geometry supersonic separator, the Joule-Thomson valve and the turbo-expander. The rigid-geometry supersonic separator was simulated for liquefaction of nearly pure methane via a thermodynamically rigorous steady-state modeling. Supersonic separator inlet conditions were optimized via a response-surface predictor of its critical flowrate in terms of temperature and pressure. Such optimum inlet conditions were found to be T = -15.79°C, P = 80 bar for a maximum 2.5 Mach. With these conditions a multi-criteria analysis was conducted combining sustainability metrics to heuristic criteria into a single one-dimensional index, the Sustainability Degree. Turbo-expander was unveiled as the most sustainable process as it has the highest SD = 11.39 due to its lowest power intensity (0.78 kWh/kgLNG), highest net present value (15.89 MMUSD) and minimal carbon emissions. Single unit supersonic separator has a medium performance (SD = 3.5), weighed down by the large natural gas recycle needed, while Joule-Thomson is the worst process due to the largest power consumption (1.66 kWh/kgLNG) and the lowest net present value (4.4 MMUSD).

Single-Solution-Based Vortex Search Strategy for Optimal Design of Offshore and Onshore Natural Gas Liquefaction Processes

Propane-Precooled Mixed Refrigerant (C3MR) and Single Mixed Refrigerant (SMR) processes are considered as optimal choices for onshore and offshore natural gas liquefaction, respectively. However, from thermodynamics point of view, these processes are still far away from their maximum achievable energy efficiency due to nonoptimal execution of the design variables. Therefore, Liquefied Natural Gas (LNG) production is considered as one of the energy-intensive cryogenic industries. In this context, this study examines a single-solution-based Vortex Search (VS) approach to find the optimal design variables corresponding to minimal energy consumption for LNG processes, i.e., C3MR and SMR. The LNG processes are simulated using Aspen Hysys and then linked with VS algorithm, which is coded in MATLAB. The results indicated that the SMR process is a potential process for offshore sites that can liquefy natural gas with 16.1% less energy consumption compared with the published base case. Whereas, for onshore LNG production, the energy consumption for the C3MR process is reduced up to 27.8% when compared with the previously published base case. The optimal designs of the SMR and C3MR processes are also found via distinctive well-established optimization approaches (i.e., genetic algorithm and particle swarm optimization) and their performance is compared with that of the VS methodology. The authors believe this work will greatly help the process engineers overcome the challenges relating to the energy efficiency of LNG industry, as well as other mixed refrigerant-based cryogenic processes.

Experimental and numerical investigation of two-phase flow outside tube bundle of liquefied natural gas spiral wound heat exchangers under offshore conditions

In order to efficiently use spiral wound heat exchangers (SWHEs) in the field of offshore natural gas liquefaction, the heat transfer mechanism in two-phase flow outside tube bundle should be known. In the study, the heat transfer characteristics of two-phase flow were numerically investigated based on the experimental results under offshore conditions using propane as medium. A comparison between the heat transfer of the proposed model and that of experimental data was analyzed. The validation results showed that the trends of simulation results were basically identical to those of the experimental ones, the deviation was generally within ±10%. It obtained that liquid film acted as a key role during the heat transfer procedure for two-phase flow outside tube bundle under offshore conditions, and increases of sloshing frequency would improve the velocity of liquid films escaping from tube surface, which resulted in an adverse effect on heat transfer. Furthermore, heave had the weakest effect on heat transfer procedure as well as the influence of roll was the strongest for the two-phase flow outside tube bundle. The results obtained by the numerical model could be the basis for design of liquefied natural gas spiral wound heat exchangers (LNG-SWHEs) installed on buoyant platforms.

Krill-Herd-Based Investigation for Energy Saving Opportunities in Offshore Liquefied Natural Gas Processes

For offshore natural gas liquefaction operation, the single mixed refrigerant process is considered as one of the best and most suitable processes. However, multivariable nonlinear thermodynamic interactions among operating conditions and flow rates of mixed refrigerant ingredients lead to energy losses, which ultimately contributes to high energy consumption for liquefied natural gas production. The present work investigates the energy saving opportunities in the single mixed refrigerant liquefaction process through “krill-herd” strategy which is based on the biological flocking (herding) behavior of individual krill. To find the energy saving opportunities, the krill-herd approach effectively reduced the exergy losses of the compression units and cryogenic heat exchanger up to 18.6 and 41.1%, respectively, as compared to the published liquefaction process. The figure of merit was found as 27.0% in the krill-herd-optimized single mixed refrigerant process, whereas it was 22.2% in the base case.

Distribution characteristics of gas-liquid mixture in plate-fin heat exchangers under sloshing conditions

Gas-liquid two-phase maldistribution is an important problem for plate-fin heat exchangers (PFHEs), which has hampered the application of PFHEs in offshore natural gas liquefaction. In order to efficiently use PFHEs in the field of offshore natural gas exploration, it is necessary to study the two-phase distribution performance of PFHEs under sloshing conditions. In this paper, the experimental bench of gas-liquid distribution was built and the distribution characteristics of distributor in PFHEs referred to as liquid-injected seal was investigated under sloshing conditions using air and water as mediums. The effects of vapor quality (0.2–0.4) and mass flow rate (647–1310 kg/h) under sloshing conditions were analyzed. The results showed that by increasing vapor quality, the gas distribution performance in the distributor changed more significantly than the liquid one and the liquid distribution performance at low vapor qualities cases strongly depended on sloshing conditions. It was also proved that the gas-liquid distribution performance of liquid-injected seal enhanced with the increase of mass flow rate, and its sloshing resistance was increased as well.

Framework for work‐heat exchange network synthesis (WHENS)

Work and heat are the two predominant forms of energy in the process industry. Considerable savings can be achieved by synergizing the work and heat requirements of process streams. A generalized framework for integrating heat and work simultaneously is proposed based on a mixed‐integer nonlinear programing model for work‐heat exchange network synthesis. Starting with a set of streams with known flows, temperatures, and pressures, a network of single‐shaft‐turbine‐compressors with motors/generators, valves, heat exchangers, and utility heaters/coolers is synthesized for minimized total annualized cost. In contrast to existing works, (1) streams are not preclassified as hot/cold or high/low pressure, (2) pressure changes are allowed for streams with no net pressure change, (3) liquid‐vapor phase changes are allowed, and (4) phase‐based property correlations are used. Successful application of our approach to C3 splitting yields a nonintuitive configuration. Another application of an offshore natural gas liquefaction process is also studied. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2472–2485, 2018

Sensibility Analysis of Pre-cooling Cold Box Pipeline Blockage in DMR Liquefaction Process

As the key equipment of onshore and offshore natural gas liquefaction (LNG) plants, the performance of cold box influences operation reliability of the whole system. Due to the fact that the temperature of pre-cooling cold box is much lower, its pipelines can be blocked by the hydrates. In this paper, a dynamic model of dual mixed refrigerant (DMR) liquefaction process is established. The variations of gas-phase and liquid-phase plugging ratios are selected as disturbances to test the dynamic responses of the DMR liquefaction process. In addition, an experimental device of DMR liquefaction process is constructed to verify the accuracy of dynamic model. The results show that the process parameters from dynamic model agree well with the experimental data. When the plugging ratio of liquid-phase pipeline is more than 21.75% or plugging ratio of gas-phase pipeline is more than 29%, control structures of DMR liquefaction process fail.

Design and optimization of offshore natural gas liquefaction processes adopting PLNG (pressurized liquefied natural gas) technology

Space limitation and energy consumption are two significant considerations for offshore natural gas liquefaction. This study designs three pressurized liquefied natural gas (PLNG) processes without CO2 pretreatment facility for natural gas containing less than 0.5% CO2, in which natural gas is converted into LNG at an elevated pressure (2 MPa), associated with a raised temperature (165.7 K). The three proposed processes, the cascade, the single mixed refrigerant (SMR) and the single expander PLNG processes, are simulated with Aspen HYSYS and optimized by the genetic algorithm (GA) to achieve the minimum specific power consumption. The cascade, the SMR and the single expander PLNG processes show an energy saving of 46%, 50% and 63%, respectively, compared to representative conventional liquefied natural gas (LNG) process. Moreover, the cascade, the SMR and the single expander PLNG processes present a smaller heat transfer area than representative conventional LNG process by 42%, 25% and 4%, respectively. In conclusion, the proposed PLNG processes can achieve both low energy cost and small footprint, providing a promising option for offshore natural gas liquefaction.

A decomposition methodology for dynamic modeling of cold box in offshore natural gas liquefaction process

Natural gas liquefaction process using mixed and/or cascade refrigerant is popular in onshore LNG (liquefied natural gas) plant. Similar attempt has been adopted for FLNG (floating LNG) but still needed for the improvement of the process to enhance its efficiency as well as reliability. The dynamic modeling of cold box which is a core equipment in LNG/FLNG plant enabling to liquefy natural gas is crucial in order to develop or improve a liquefaction process concerning operability and controllability. A decomposition methodology for dynamic modeling of cold box in the case of lack of internal design data at early design stage is presented. The proposed methodology is validated through the industrial application of offshore natural gas liquefaction process and expected to be extensively applied to the various process designs which require dynamic simulation of cold box unit.

Design and analysis of multi-stage expander processes for liquefying natural gas

Multi-stage expander refrigeration cycles were proposed and analyzed in order to develop an efficient natural gas liquefaction process. The proposed dual and cascade expander processes have high efficiency and the potential for larger liquefaction capacity and are suitable for small-scale and offshore natural gas liquefaction systems. While refrigeration cycles of conventional expander processes use pure nitrogen or methane as a refrigerant, the proposed refrigeration cycles use one or more mixtures as refrigerants. Since mixed refrigerants are used, the efficiency of the proposed multi-stage expander processes becomes higher than that of conventional expander processes. However, the proposed liquefaction processes are different from the single mixed refrigerant (SMR) and dual mixed refrigerant (DMR) processes. The proposed processes use mixed refrigerants as a form of gas, while the SMR and DMR processes use mixed refrigerants as a form of gas, liquid- or two-phase flow. Thus, expanders can be employed instead of Joule-Thomson (J-T) valves for refrigerant expansion. Expanders generate useful work, which is supplied to the compressor, while the high-pressure refrigerant is expanded in expanders to reduce its temperature. Various expander refrigeration cycles are presented to confirm their feasibility and estimate the performance of the proposed process. The specific work, composite curves and exergy analysis data are investigated to evaluate the performance of the proposed processes. A lower specific work was achieved to 1,590 kJ/kg in the dual expander process, and 1,460 kJ/kg in the cascade expander process. In addition, the results of exergy analysis revealed that cycle compressors with associated after-coolers and companders are main contributors to total exergy losses in proposed expander processes.

Plant-wide control for the economic operation of modified single mixed refrigerant process for an offshore natural gas liquefaction plant

The marine operation of floating liquefied natural gas (FLNG) demands process compactness, flexibility, simplicity of operation, safety, and higher efficiency. The modified single mixed refrigerant (MSMR) process satisfies the FLNG process requirements and is accepted as a suitable technology for FLNG operation. The aim of this study was to develop a plant-wide control structure or strategy that can sustain the economic efficiency of the MSMR process. The NGL recovery and liquefaction units were integrated in the MSMR process to provide a compact plant structure with an efficient operation. Steady-state optimality analysis was intensively conducted in a rigorous dynamic simulation environment to determine the correct variable to sustain the economic efficiency of MSMR process. The results showed that the flow rate ratio of heavy and light mixed refrigerant (HK/LK ratio) is a promising self-optimizing controlled variable. Controlling this variable can sustain the MSMR optimality, even when the process is operated under off-design operating conditions or in the presence of disturbances. Based on the control structure tests, the control configuration with the HK/LK ratio loop showed excellent performance, maintaining the process stability against a range of disturbances. The proposed approach can also be applied to any cryogenic liquefaction technology for determining a possible optimizing controlled variable.

Synthesis of heat exchanger networks at subambient conditions with compression and expansion of process streams

AbstractThis article presents an optimization formulation for the synthesis of heat exchanger networks where pressure levels of process streams can be adjusted to improve heat integration. Especially important at subambient conditions, this allows for the interconversion of work, temperature, and pressure‐based exergy and leads to reduced usage of expensive cold utility. Furthermore, stream temperatures and pressures are tuned for close tracking of the composite curves yielding increased exergy efficiency. The formulation is showcased on a simple example and applied to a case study drawn from the design of an offshore natural gas liquefaction process. Aided by the optimization, it is demonstrated how the process can extract exergy from liquid nitrogen and carbon dioxide streams to support the liquefaction of a natural gas stream without additional utilities. This process is part of a liquefied energy chain, which, supplies natural gas for power generation while facilitating carbon dioxide sequestration. © 2010 American Institute of Chemical Engineers AIChE J, 2011

Offshore Natural Gas Liquefaction Process and Development Issues

Summary Liquefied natural gas (LNG) has yet to be deployed in the development of offshore fields in spite of several detailed studies completed and offshore technology development demonstrating its technical feasibility. The perceived risks associated with deploying unproven technology in a high construction cost and volatrile gas price environment have so far inhibited offshore liquefaction projects. The potential deployment of such technologies is of paramount importance considering the massive volumes of natural gas currently deemed as "stranded" and the exploitation of which is compelling not only because of the inherent economic benefit but also because of the otherwise adverse impact on oil production. It is conceivable that deep water offshore locations may contain quantities of natural gas rivalling those of onshore locations. Such a statement cannot even be confirmed because drilling for offshore natural gas reservoirs, expected to be found considerably deeper than oil reservoirs, has been unattractive exactly because of the absence of coherent exploitation strategies. If anything, the mere presence of large natural gas deposits even in the form of solution gas in oil is now often considered as largely undesirable because of the cost of just handling non-monetized natural gas. This paper discusses potential offshore LNG processes and reviews natural gas liquefaction cycles in the context of compactness, ease of operation, process safety, and efficiency. Particular attention is paid to the lower-efficiency turboexpander processes for plant capacities up to 3 million tonnes per annum (MTPA, approximately 0.43 Bcf/d). These cycles offer several advantages over the alternative optimized cascade and mixed refrigerant (MR) liquefiers for offshore applications.