A realistic vapour phase heat transfer model for the weathering of LNG stored in large tanks

A new non-equilibrium model relevant to LNG weathering in large storage tanks under constant pressure has been developed. It treats the heat influx from the surroundings into the vapour and liquid phases separately and allows for heat transfer between the two phases. The main heat transfer mechanisms in the vapour phase are assumed to be advection, due to upward flow of evaporated LNG, and conduction. It has been observed that the vapour temperature increases monotonically as a function of the height, in agreement with recent experimental results. In all the simulations performed the vapour to liquid heat transfer was small, also in line with recent experimental findings, and is estimated to contribute less than 0.3% to boil-off gas rates. The results of this work indicate that the heat transfer by the advective upward flow dominates the energy transfer within the vapour, while the natural convection, in the body of the vapour, can be neglected. The initial liquid filling has a pronounced effect on all the relevant variables, leading to a decrease in vapour temperature and boil-off gas temperature and an increase in boil-off rates. A rule of thumb for estimating the boil-off gas temperature in industrial storage tanks is provided

Modelling the evaporation of cryogenic liquids in storage tanks

Cryogenic liquids are substances with a normal boiling point below -150°C. Recently, the interest in cryogenic liquids has skyrocketed because of their role in the energy transition, particularly for LNG and liquid hydrogen. Cryogenic liquids are stored in highly insulated tanks, which are nevertheless subject to heat ingress from the surroundings. The heat ingress drives thermal stratification, natural convection, pressure build-up and evaporation. Managing the evaporated cryogen, denominated boil-off gas (BOG), pose techno-economic, safety and environmental challenges. To facilitate the design and operation of cryogenic storage tanks, new models for cryogenic liquids evaporation have been developed. For isobaric storage, a 1-D model has been developed. The model includes wall heating, heat conduction and advection in the vapour phase. The model shows that advection dominates vapour heat transfer. A 2-D CFD model has been developed to validate the assumptions of the 1-D model. The CFD model validates the 1-D model assumption of one-dimensional advective flow. Additionally, the CFD model shows that thermal stratification dampens natural convection in the vapour. Analytical solutions of the 1-D model valid for the pseudo-steady state have been developed. The analytical solutions constitute an easy-to-use tool for practitioners to improve BOG management. For non-isobaric storage, a 1-D model that considers wall heating, heat conduction and wall boiling has been developed. The 1-D model demonstrates that wall boiling is relevant even for low heat fluxes. The 1-D model predictions were in good agreement with experimental pressure and vapour temperature profiles. The assumptions of the 1-D model have been validated by developing a new single-phase CFD model. A multiphase model has been developed to investigate interfacial transport phenomena. It shows that interfacial momentum transfer slightly enhances liquid heat transfer, and that vapour heating dominates pressure build-up at the beginning of the storage period.Open Acces

Analytical solutions for the isobaric evaporation of pure cryogens in storage tanks

New analytical solutions have been derived for the isobaric evaporation of a pure liquid cryogen. In particular, expressions have been provided for the liquid volume, evaporation rate, Boil-off-Gas (BOG) rate, vapour temperature and vapour to liquid heat transfer rate as a function of time. Both equilibrium and non-equilibrium scenarios have been considered. In the former, the vapour and liquid cryogen are assumed to be in thermal equilibrium, while in the latter the vapour is treated as superheated with respect to the liquid and acts as an additional heat source. The derived solutions for two scenarios were validated against the numerical results for the evaporation of liquid methane and of liquid nitrogen in small, medium sized and large storage tanks that are used in industry. For the equilibrium model, the analytical solutions are exact. For the non-equilibrium model, the analytical solutions are valid for the whole duration of evaporation, except for a short transient period at the beginning of the evaporation. For physical quantities of industrial interest, they provide accurate estimates of liquid volume, BOG rate and BOG temperature, with the maximum deviations not exceeding 1%, 2% and 4.5%, respectively. The vapour to liquid heat transfer rate is also well predicted to within a maximum deviation of 5%

Modelling the weathering process of stored liquefied natural gas (LNG)

Weathering occurs in stored liquefied natural gas (LNG) due to the removal of the boil-off gas (BOG) from the LNG container and results in the remaining LNG being richer in heavier components. A model has been developed to predict stored LNG weathering in containment tanks, typically used in regasification. The model integrates a vapour-liquid equilibrium model, and a realistic heat transfer model. It provides a number of advances on previously developed models: (i) heat ingress is calculated based on outside temperature and LNG composition, allowing for daily/seasonal variations; (ii) boil-off-ratio is not an input; (iii) LNG density is estimated using an experimentally based correlation. The model was validated using real industry data and the agreement obtained in predicting overall composition, density and amount vaporized was within industry requirements. Two modelling approaches have been developed: (i) assuming thermodynamic equilibrium between vapour and liquid; and (ii) assuming heat exchange between the two phases. Both models were run in a predictive mode to assess the BOG under different scenarios. One of the main results of this work is that the BOG generation is 25% less when considering the non-equilibrium approach, which will have a significant impact on industry where simple equilibrium models are used. In the initial stages of weathering nitrogen content of LNG has a marked effect on BOG generation. Even 0.5% mol of nitrogen leads to nearly 7% BOG decrease, making the initial BOG unmarketable. That is a result of preferential evaporation of nitrogen and increase in the direct differential molar latent heat. In the final stages of weathering the heavier hydrocarbons govern the BOG dynamics, which becomes a strong function of initial composition and the LNG remaining in the tank.Open Acces

A Study on the Optimization of BOG Handling for LNG-Fueled Ship under Various Bunkering Scenarios

Liquefied natural gas (LNG) as a marine fuel is considered as a realistic and feasible solution that complies with the stringent emissions regulation issued by International Maritime Organization (IMO). For LNG-fueled ships, the bunkering process of LNG and heavy fuel oil are completely different since the cryogenic liquid transfer generates a considerable amount of boil-off gas (BOG). In this study, the commercial software, Aspen HYSYS V10, for process design is used to investigate and analyze the optimization in the dynamic simulation on the BOG handling between the cargo tank of a bunking ship and bunker tank of a receiving ship (LNG fueled ship) under various bunkering scenarios. With respect to the modeling of the study, for the standard ship-to-ship (STS) and truck-to-ship (TTS) LNG bunkering methods, the diameter of the bunkering lines are set as 8 inch and 3 inch while that of the BOG return pipelines are set as 4 inch and 2 inch to satisfy the pressure of the receiving ship and BOG generation, respectively. The capacities of the cargo tank and fuel tank for bunkering and receiving ships are set as 4,538 m3 (70 m3) and 700 m3 (70 m3) for the STS and TTS LNG bunkering methods, respectively. The results indicated that the BOG amount with different LNG bunkering scenarios is variable. The BOG flow rate varies proportionally with the temperature difference, methane number and diameter of BOG/LNG pipe in case of temperature, methane number (MN) and pipe diameter disturbance and inversely with respect to the bunkering time limit after 20 min. in case of different bunkering time limits. Additionally, for the optimal BOG handling (STS bunkering method), it is necessary to control the bunkering time within 120 min. since additional BOG is generated when the capacity of the pump exceeds 100,000 kg/h. Meanwhile, when the diameter of the BOG line (DB) divide the diameter of the LNG line (DL) DB/DL = 0.5 is considered the best value both in the STS and TTS LNG bunkering methods, thus the tank pressure difference between bunkering and receiving ship may be reduced. It is believed that the results of the research could provide feasible assistance for STS and TTS LNG bunkering for the ports, and could give a specific guideline for the amount of the BOG generation and the standardized diameter of pipeline ratio.Contents ⅰ List of Tables ⅳ List of Figures ⅴ Abstract ⅷ Chapter 1 Introduction 1 1.1 Research Background 1 1.2 Dissertation Outline 5 Chapter 2 Overview of LNG Bunkering 7 2.1 LNG Bunkering Supply Chain 7 2.2 LNG Bunkering Method 9 2.3 LNG Tank and Bunkering Equipment 13 2.4 Literature Review 19 2.4.1 LNG Bunkering Technology and Safety 19 2.4.2 Current International Regulations, Standards, Class Rules and Guidelines 21 2.4.3 Challenges on BOG Handling 27 Chapter 3 Methodology & Process Design 30 3.1 Mathematical Model Classification 32 3.2 Dynamic Model of Main Bunkering Facilities and Equipments 38 3.2.1 LNG Tank Setup 38 3.2.2 Pipeline System Setup 45 3.2.3 Pump System Setup 49 3.3 Bunkering Process 52 3.4 Equation of State Selection & System Description 54 3.4.1 Equation of State Selection 54 3.4.2 System Description (STS & TTS Case) 56 3.5 Initial Conditions 57 3.6 Model Validation 60 Chapter 4 Results and Discussion 62 4.1 Disturbance of Temperature 62 4.2 Disturbance of Methane Number 69 4.3 Optimal BOG Generation in Different Bunkering Time Limits 74 4.4 Optimal BOG Generation in Different Pipe Diameter Ratio 86 Chapter 5 Conclusions 93 References 96Docto

Mathematical Modelling of LNG Dispersion Under Various Conditions

The global demand of liquefied natural gas (LNG) has risen rapidly in recent years. A new modelling method, direct CFD simulation method, was developed, due to the risks associated in handling, storage and transport of LNG. This method was shown to accurately model a LNG spill, pool formation and dispersion; and has been used to study the effect of (a) Impoundments, (b) Sea and air temperature and; (c) Sea and air stability

Expanding the Horizons of Manufacturing: Towards Wide Integration, Smart Systems and Tools

This research topic aims at enterprise-wide modeling and optimization (EWMO) through the development and application of integrated modeling, simulation and optimization methodologies, and computer-aided tools for reliable and sustainable improvement opportunities within the entire manufacturing network (raw materials, production plants, distribution, retailers, and customers) and its components. This integrated approach incorporates information from the local primary control and supervisory modules into the scheduling/planning formulation. That makes it possible to dynamically react to incidents that occur in the network components at the appropriate decision-making level, requiring fewer resources, emitting less waste, and allowing for better responsiveness in changing market requirements and operational variations, reducing cost, waste, energy consumption and environmental impact, and increasing the benefits. More recently, the exploitation of new technology integration, such as through semantic models in formal knowledge models, allows for the capture and utilization of domain knowledge, human knowledge, and expert knowledge toward comprehensive intelligent management. Otherwise, the development of advanced technologies and tools, such as cyber-physical systems, the Internet of Things, the Industrial Internet of Things, Artificial Intelligence, Big Data, Cloud Computing, Blockchain, etc., have captured the attention of manufacturing enterprises toward intelligent manufacturing systems

"Tool to assess the cost of hydrogen considering its supply chain"

Hydrogen is envisioned to become a fundamental energy vector within the decarbonization of the energy systems. Despite already being employed in several industries, its production comes almost completely from processes based on fossil fuels. The upcoming challenge towards a hydrogen economy includes the development of low- and zero-carbon processes, the creation of an adequate infrastructure, and the diffusion of new, hydrogen-based applications. Two key factors that will define the success of hydrogen are its sustainability and competitiveness with alternative solutions, e.g., electrification. This study therefore aims at assessing the economic feasibility of hydrogen supply chains, with a focus on their final use in Germany, Spain, and France. The different production methods for each stage (production, transmission and distribution, storage) are discussed and evaluated. Consequently, the entire supply chains are analyzed, comparing domestic production with hydrogen imports from favorable locations. The economic assessment is based on an indicator, the levelized cost of hydrogen, the LCOH. The study results in an Excel-based tool calculating the LCOH for different supply chains. Different scenarios are developed for each end-use country. In Germany, domestic production is compared with imports, also addressing the need for adequate storage. Blue hydrogen imports from close locations present the lowest LCOH, with values as low as 2.1 €/kg in 2030. This requires pipeline transmission and a monthly storage in depleted natural gas or oil reservoirs. Longer storage durations increase the supply security but also the related costs. In Spain, local, small-scale supply chains are evaluated in opposition to central, larger-scale alternatives. Both configurations are competitive with costs around 3.6 €/kg, suggesting that both supply pathways are feasible. This can spark competition between different players towards a hydrogen economy. In France, domestic hydrogen production via electrolysis is studied, considering different electricity sources, such as the power grid, electricity from nuclear plants and from renewable energy sources. Despite the high interest of France in pink hydrogen, renewables produce the cheapest product, at an LCOH of 4.4 €/kg for onshore wind. If this result is compared to the other two countries, French hydrogen is not competitive. However, the focus on solid oxide electrolysis and novel nuclear technologies might determine a decline in hydrogen costs.Vätgas är tänkt att bli en grundläggande energivektor i samband med avkolning av energisystemen. Trots att vätgas redan används i flera industrier kommer produktionen av vätgas nästan helt och hållet från processer som bygger på fossila bränslen. Den kommande utmaningen mot en vätgasekonomi inbegriper utveckling av processer med låga eller inga koldioxidutsläpp, skapande av en lämplig infrastruktur och spridning av nya vätgasbaserade tillämpningar. Två nyckelfaktorer som kommer att avgöra vätgasens framgång är dess hållbarhet och konkurrenskraft i förhållande till alternativa lösningar, t.ex. elektrifiering. Denna studie syftar därför till att bedöma den ekonomiska genomförbarheten av vätgasförsörjningskedjor, med fokus på slutanvändning i Tyskland, Spanien och Frankrike. De olika produktionsmetoderna för varje steg (produktion, överföring och distribution, lagring) diskuteras och utvärderas. Följaktligen analyseras hela försörjningskedjorna genom att jämföra inhemsk produktion med import av vätgas från gynnsamma platser. Den ekonomiska bedömningen baseras på en indikator, den genomsnittliga nuvärdesberäknade kostnaden för vätgas, LCOH. Studien resulterar i ett Excel-verktyg som beräknar LCOH för olika försörjningskedjor. Olika scenarier utvecklas för varje slutanvändarland: i Tyskland jämförs inhemsk produktion med import, där man också tar hänsyn till behovet av lämplig lagring. Import av blå väte från närliggande platser ger de lägsta LCOH-värdena, med värden så låga som 2.1 €/kg år 2030. Detta kräver överföring via rörledningar och en månatlig lagring i uttömda naturgas- eller oljereserver. Längre lagringstider ökar försörjningstryggheten men också de relaterade kostnaderna. I Spanien utvärderas lokala, småskaliga försörjningskedjor i motsats till centrala, storskaliga alternativ. Båda konfigurationer är konkurrenskraftiga med kostnader på omkring 3.6 €/kg, vilket tyder på att båda försörjningsvägarna är genomförbara. Detta kan utlösa konkurrens mellan olika aktörer i riktning mot en vätgasekonomi. I Frankrike studeras inhemsk vätgasproduktion via elektrolys med hänsyn till olika elkällor, t.ex. elnätet, el från kärnkraftverk och förnybara energikällor. Trots Frankrikes stora intresse för rosa vätgas är det förnybara energikällor som producerar den billigaste produkten, med en LCOH på 4.4 €/kg för landbaserad vindkraft. Om detta resultat jämförs med de andra två länderna är fransk vätgas inte konkurrenskraftig. Fokuseringen på SOEC-teknik och ny kärnkraftsteknik kan dock leda till att vätgaskostnaderna sjunke

Simulation of LNG rollover in storage tanks

LNG rollover is the sudden mixing of stratified LNG layers, which can generate significant amounts of boil-off gas. Such event is a severe safety concern; however, there are no reliable models at industrial scales available in the literature. In this research, we extend the definition of the hydrostatic stability ratio for binary mixtures to multi-component mixtures. Moreover, the fundamental issues associated with LNG rollover are reviewed, and a new model for simulating rollover is presented

Sustainability and risk management of LNG as a fuel for marine transportation

The use of liquefied natural gas (LNG) as an alternative ship fuel marks a fundamental step towards the reduction of emissions linked to maritime transportation of goods and passengers. Despite the positive safety record of the LNG shipping industry, natural gas is a hazardous substance and safety concerns for its use onboard passenger ships demand a thorough evaluation. This study aims at a comprehensive safety and sustainability assessment of marine LNG technologies, focusing on small-scale applications, seeking to fill the current knowledge gap in this field. An in-depth evaluation of the safety of existing technologies for LNG bunkering and onboard fuel gas supply systems is performed, providing key information about the credible accident scenarios and their expected consequences. The safety criticalities are identified based on the application of specifically developed models for the evaluation of the inherent safety performance of LNG bunkering and propulsion technologies. Another part of the work is dedicated to the development of a computational fluid dynamic (CFD) setup to model the behaviour of cryogenic tanks exposed to accidental hydrocarbon fires, overcoming the limitations of the previous modelling approaches, and providing precise data for further analysis of the tank structural integrity under extreme conditions. Furthermore, a preliminary CFD modelling of LNG fire scenario consequences occurring inside the fuel preparation room of gas-fuelled ships is carried out to evaluate the heat flux received by the ship structure. The obtained results represent a first step towards a wider approach aimed at enhancing the safety of the entire LNG supply for maritime propulsion. Furthermore, these results can make a valuable contribution to support the decision-making process for shipowners and port authorities in the design and safety assessment of such systems, both in port areas and onboard ships, also providing guidance for emergency responders