Model-based Optimisation of Mixed Refrigerant LNG Processes

Natural gas liquefaction processes are energy and cost intensive. This thesis pursues the optimisation of propane precooled mixed refrigerant (C3MR) processes considering variations in upstream gas well conditions, in order to maximise gas well life. Four objective functions were selected for the design optimisation of the C3MR and dual mixed refrigerant (DMR) processes: 1) total shaft work (W), 2) total capital investment, 3) total annualised cost, and 4) total capital cost of both compressors and main cryogenic heat exchanger (MCHE). Optimisation results show that objective function 4 is more suitable than other objective functions for reducing both W and UA (MCHE design parameter). This leads to 15% reduction in specific power for C3MR and 27% for DMR, while achieving lower UA values relative to baseline. The operation optimisation of the C3MR process and its split propane version (C3MR-SP) was performed using four objective functions: 1) total shaft work, 2-3) two different exergy efficiency expressions, and 4) operating expenditure (OPEX). Objective function 3 results in the lowest specific shaft work 1469 MJ/tonne-LNG. For C3MR-SP, however, the lowest specific shaft work is found to be under objective function 1. A comparison of optimisation results across literature studies is impractical due to dissimilar process conditions, feed gas conditions, product quality, and equipment size. A sensitivity analysis highlights the effect of feed gas conditions on performance of the C3MR. For instance, as LNG production decreases from 3 MTPA to 2.4 MTPA over time, the specific OPEX increases from $128/tonne-LNG to$154/tonne-LNG. A subsequent study was conducted focusing on energy benefits of two configurations: integrating natural gas liquids (NGL) recovery unit with C3MR. An integrated NGL recovery within C3MR shows a 0.74% increase in energy consumption as methane concentration of the feed gas decreases, however a frontend NGL recovery unit only has a 0.18% decrease

Framework for Embedding Process Simulator in GAMS via Kriging Surrogate Model Applied to C3MR Natural Gas Liquefaction Optimization

Rigorous black-box simulations are useful to describe complex systems. However, it cannot be directly integrated into mathematical programming models in some algebraic modeling environments because of the lack of symbolic formulation. In the present paper, a framework is proposed to embed the Aspen HYSYS process simulator in GAMS using kriging surrogate models to replace the simulator-dependent, black-box objective, and constraints functions. The approach is applied to the energy-efficient C3MR natural gas liquefaction process simulation optimization using multi-start nonlinear programming and the local solver CONOPT in GAMS. Results were compared with two other meta-heuristic approaches, Particle Swarm Optimization (PSO) and Genetic Algorithm (GA), and with the literature. In a small simulation evaluation budget of 20 times the number of decision variables, the proposed optimization approach resulted in 0.2538 kW of compression work per kg of natural gas and surpassed those of the PSO and GA and the previous literature from 2.45 to 15.3 %.The authors acknowledge the National Council for Scientific and Technological Development – CNPq (Brazil), processes 148184/2019-7, 440047/2019-6, 311807/2018-6, 428650/2018-0, and Coordination for the Improvement of Higher Education Personnel – CAPES (Brazil) for the financial support

OPTIMIZATION OF OPERATING PARAMETERS IN LNG AP-X PROCESS

Natural gas (NG) has been known as the cleanest fossil fuel since it releases low level of harmful products when being burnt. Natural gas can be transported either in pipelines or in liquefied natural gas (LNG) carriers. In LNG carriers, LNG is liquefied to the temperature of -162 degree Celsius at atmospheric pressure so that its volume can be reduced up to 600 times. There are a lot of techniques available for liquefying natural gas. The most potential technique developed by APCI is AP-X process. This is an improvement from C3MR process by using nitrogen in the subcooling loop at the end of the process. It is very beneficial to know the optimum refrigerant flow rate for the purpose of saving energy consumed in the process. Moreover, the operating refrigerant flow rate also is optimized with subject to the compensation with the compressor load and the energy efficiency. HYSYS software is utilized to model the nitrogen loop of AP-X process. LNG flow rate, compressor load and heat duties exchanged are taken from HYSYS model. In this study, the optimum pure nitrogen flow rate was found to be at around 2500 kg/h. Besides, the flow rate for 5% methane mixed refrigerant is 2375 kg/hr, so that the process is most beneficial in term of revenue as well as energy efficiency. The optimum capacity of LNG plant using AP-X process is found at 9.1 MTPA, according to around 13.5% increase in train capacity compared with the current operating train capacity in Qatar

Process Efficiency Optimisation of Cascade LNG Process

The aimed of this thesis is to optimise the Cascade LNG process efficiency of 5 MTPA production capacity. The cascade process was modelled and simulated in Aspen HYSYS version 7.2 using Peng Robinson equation of state. The optimisation of cascade process was carried out from operation and design perspectives. It focused on two main cycles which are propane and ethylene refrigeration cycles as they are the main energy consumers of this process

The developing role of gas in decarbonizing China's energy system:system analysis of technical, economic and environmental improvements of LNG and low carbon gas supply chains and infrastructure

The gas system in China is essential for an energy transition towards a low-carbon future. The optimization of gas supply chains and gas infrastructure deployment is crucial for energy-saving, cost-saving, and GHG emissions mitigation. In the short term, natural gas can act as a transition fuel by replacing oil and coal in especially power and transport sectors. In the long term, gradually replacing natural gas with low-carbon gas can secure the role of the gas infrastructure in a low carbon energy system. As China is undergoing an energy transition from a coal dominated energy system to a low-carbon energy system, the main objective of this thesis is to investigate how gaseous energy carriers and the NG infrastructure can be used in the most efficient way for a low-carbon energy system in China towards 2050. In this thesis, the potential role of LNG in the short term and low-carbon gases in the long term for a low-carbon energy system with infrastructure deployment pathway in China are investigated by assessing the energy efficiency, GHG emissions, and costs of the supply chains

Comprehensive review of current natural gas liquefaction processes on technical and economic performance

This paper provides a quantitative technical and economic overview of the status of natural-gas liquefaction (LNG) processes. Data is based on industrial practices in technical reports and optimization results in academic literature, which are harmonized to primary energy input and production cost. The LNG processes reviewed are classified into three categories: onshore large-scale, onshore small-scale and offshore. These categories each have a different optimization focus in academic literature. Besides minimizing energy consumption, the focus is also on: coproduction for large-scale; simplicity and ease of operation for small-scale; and low space requirement, safety and insensitivity to motion for offshore. The review on academic literature also indicated that optimization for lowest energy consumption may not lead to the lowest production cost. The review on technical reports shows that the mixed-refrigerant process dominates the LNG industry, but has competitions from the cascade process in large-scale applications and from the expander-based process in small-scale and offshore applications. This study also found that there is a potential improvement in adopting new optimization algorithms for efficiently solving complex optimization problems. The technical performance overview shows that the primary energy input for large-scale processes (0.031–0.102 GJ/GJ LNG) is lower than for small-scale processes (0.049–0.362 GJ/GJ LNG). However, the primary energy input for identical processes do not necessarily decrease with increasing capacity and the performance of major equipment shows low correlation with scale. The economic performance overview shows specific capital costs varying significantly from 124 to 2255 $/TPA LNG. The variation could be, among others, caused by the different complexities of the facility and different local circumstances. Production cost, excluding feed costs, varies between 0.69 and 4.10$/GJ LNG, with capital costs being the dominant contributor. The feed cost itself could be 1.51–4.01 $/GJ LNG, depending on the location. Lastly, the quantitative harmonization results on technical and economic performance in this study can function as a baseline for the purpose of comparison

Multi-objective simulation optimization via kriging surrogate models applied to natural gas liquefaction process design

A surrogate-based multi-objective optimization framework is employed in the design of natural gas liquefaction processes using reliable, black-box process simulation. The conflicting objectives are minimizing both power consumption and heat exchanger area utilization. The Pareto solutions of the single-mixed refrigerant (SMR) and propane-precooled mixed refrigerant (C3MR) processes are compared to determine the suitability of each process in terms of energy consumption and heat exchanger area. Kriging models and the ɛ-constraint methodology are used to sequentially provide simple surrogate optimization subproblems, whose minimizers are promising feasible and non-dominated solutions to the original black-box problem. The surrogate-based ɛ-constrained optimization subproblems are solved in GAMS using CONOPT. The Pareto Fronts achieved with the surrogate-based framework dominate the results from the NSGA-II, a well-established meta-heuristics of multi-objective optimization. The objective functions of non-dominated solutions go as low as 1045 and 980.3 kJ/kg-LNG and specific UA values of 212.2 and 266.9 kJ/(°C kg-LNG) for SMR and C3MR, respectively. The trade-off solutions that present the minimum sum of relative objectives are analyzed as well as the dominance of C3MR over SMR at low power consumption values and conversely at low heat exchanger area utilization.The authors LFS, CBBC, and MASSR acknowledge the National Council for Scientific and Technological Development–CNPq (Brazil), processes 200305/2020-4, 148184/2019-7, 440047/2019-6, 311807/2018-6, 428650/2018-0, 307958/2021-3 and Coordination for the Improvement of Higher Education Personnel–CAPES (Brazil) for the financial support. The author JAC acknowledges financial support from the “Generalitat Valenciana” under project PROMETEO 2020/064 and the Ministerio de Ciencia e Innovación , under project PID2021-124139NB-C21

Lean Feed 조건에서의 NGL/LNG 통합공정에 대한 최적 설계와 경제성평가에 관한 연구

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 조선해양공학과, 2019. 2. 임영섭.최근 호주와 미국을 비롯한 국가들의 비 전통가스전 개발이 증가함에 따라 lean 가스필드에 대한 수요가 점차 증가하고 있으며 따라서 조성이 lean 한 조건하에서의 NGL 회수공정 및 LNG 공정에 대한 성능 및 경제성 재평가가 필요할 것으로 예상된다. 본 논문은 다양한 NGL 회수 공정과 액화공정 그리고 NGL/LNG 통합 공정에 대하여 HHV 스펙을 고려한 공정최적화 및 경제성평가를 진행하고 비교 분석하였다. NGL회수공정은 네 가지 서로 다른 공정들에 대하여 다양한 lean가스조성 하에서 공정성능 및 경제성평가를 진행하였다. 그 중 ISS 와 IPSI 공정은 전통 feed 조건하에서의 대표적인 공정인 반면에 HHC separator 와 scrub column 은 장치수를 최소화하여 단순한 공정도를 가진 공정들로 feed 조성이 lean 할 경우 강점을 가질 것으로 예상되는 공정이다. 공정성능평가 결과 비교적 많은 장치를 사용하여 복잡한 공정도를 가지고 있는 IPSI 공정이 가장 좋은 분리효율을 가지고 있으므로 다른 공정들에 비해 가장 적은 재료비를 사용하였다. 그러나 상대적으로 높은 자본투자가 IPSI 공정의 전체적인 경제성에 더 큰 영향을 주었다. 가장 단순한 공정인HHC separator 공정은 다른 공정들 대비 가장 적은 자본투자비를 보였지만 상대적으로 좋지 않은 분리효율로 인하여 가장 많은 재료비를 사용하였다. 자본투자비용 및 운전비용 등 전반적인 경제성을 고려 했을 경우 ISS 공정이 feed GPM 값이 2.5로 근접할 때 가장 좋은 경제성을 보였고 scrub column 공정은 feed 조성이 일정하게 lean 할 경우 다른 공정들 대비 가장 좋은 경제성을 보였다. 이는 전반적인 경제성측면에서 봤을 때 조성이 일정하게 lean 할 경우에는 상대적으로 복잡한 ISS 공정이나 IPSI 공정보다도 scrub column 공정을 NGL 회수 공정으로 사용하는 것이 더 유리함을 보여 준다. 해상용 천연가스 액화공정의 경우 제한된 공간 및 안전성 등 원인으로 인하여 일반적으로 육상보다 더 복잡한 선정기준을 가지고 있다. 예를 들어 육상에서 최대 점유율을 보이고 있는 C3MR 액화공정은 상대적으로 많은 공간 필요 및 공정안전성에 영향을 주는 많은 양의 프로판 성분을 필요로 하고 있으며 이로 인하여 해양 플랫폼에는 실제 사용 된 경우가 없다. 본 논문은 해상에서 사용 가능한 하나의 N2 엑스펜다 공정, 두 가지 종류의 단일혼합냉매 공정 (SMR) 및 세가지 타입의 듀얼혼합냉매 공정 (DMR) 등 총 6가지 타입의 공정들에 대해 공정성능 및 경제성평가를 진행하였다. N2 엑스펜다 공정은 비가연성인 질소를 단일냉매로 사용하므로 플랜트 운전측면 및 안전성 측면에서 강점을 가지고 있지만 다른 공정들 대비 가장 좋지 않은 효율을 보이는 것으로 알려져 있다. DMR 공정은 두 개의 혼합냉매를 사용하므로 N2 엑스펜다 및 SMR 공정보다 더 좋은 효율을 보이고 트레인당 용량도 가장 크므로 상대적으로 큰 용량의 액화공정 선정에 사용가능하다. 반면에 상대적으로 많은 장치를 사용하므로 복잡한 공정도를 가지고 있고 따라서 가장 큰 투자비를 필요로 한다. SMR 공정은 N2 엑스펜도 와 DMR 공정의 사이의 공정성능을 보인다. 성능 및 경제성평가 결과 N2 엑스펜더 공정이 다른 공정들 대비 가장 낮은 효율과 수익성을 보였다. SMR 2 공정은 가장 적은 투자비와 payout time 을 보였고 DMR base 공정이 다른 공정들 대비 높은 액화효율을 보여주었고 따라서 가장 높은 수익성과 가장 적은 운전비용을 필요로 하였다. SMR 2 공정은 투자비측면에서 가장 좋은 경제성을 보였고 DMR base 공정은 상대적으로 복잡한 공정도를 가지고 있어 높은 초기 투자비를 보이지만 높은 액화효율을 가지고 있어 운전비용 측면에서 다른 공정들 대비 강점을 가지고 있으므로 해상용 액화공정 선정 시 프로젝트 상황에 따라 SMR 2 공정 또는 DMR base 공정을 액화공정 후보로 고려할 수 있다. 통합공정은 본 논문에서 제안 한 두 개의 최대한 장치수를 간소화 한 NGL/LNG 통합공정과 전통적인 통합공정을 feed 가 lean 한 조성 하에서 LNG HHV 스펙을 고려하여 성능 및 경제성을 비교 분석하였다. 액화공정 파트는 SMR 2 공정과 DMR base 공정을 각각 사용하였고 유전자 알고리즘을 공정 글로벌 최적화에 적용하여 공정 최적운전조건을 도출하였다. 공정 최적화 결과 제안 한 Case 1 공정이 전통공정에 대비 액화효율은 조금 낮았지만 NGL회수 공정부분 장치를 최대한 줄임으로 인하여 상당히 낮은 자본투자비를 보였다. 또한 전반적인 경제성 측면에서도 Case 1 공정이 가장 좋은 성능을 보여주었다. 액화공정에 SMR 를 사용한 Case 2 공정의 경우에는 비교 분석한 세 개의 공정 중에서 가장 낮은 자본투자비를 보였으나 운전비 측면에서 다른 공정들 대비 비교적 큰 차이를 보여 전반적인 경제성 측면에서는 좋지 않았다. 하지만 플랜트 운영기간이 15개월보다 짧을 경우에는 가장 좋은 수익성을 보여주었다. 그러므로 제안된 Case 2 공정은 플랜트 운전기간이 짧은 peak shaving 플랜트 또는 feed reservoir 수명이 짧은 해양플랜트에 적용하면 전체 투자비를 줄일 수 있을 뿐만 아니라 전체 수익성 측면에서 유리하여 NGL/LNG 통합 공정 선정 시 하나의 좋은 공정옵션으로 고려할 수 있음을 보여주었다.In recent years, the demand for lean gas fields has increased due to the development of unconventional gas reservoirs in Australia and U.S. Therefore, the re-evaluation of natural gas liquid (NGL) recovery and NGL/LNG integrated processes under lean feed conditions are required. This dissertation performs process optimization and economic evaluation for the various representative NGL recovery, natural gas liquefaction and NGL/LNG integrated processes considering the liquefied natural gas (LNG) higher heating value (HHV) specification. Four different NGL recovery process schemes were evaluated under various lean feed conditions. The ISS and IPSI (A company name who owns it) processes are the representative processes in the conventional NGL recovery feed conditions. On the other hand, heavy hydrocarbons (HHC) separator and scrub column schemes are the simplified processes which may have advantages for lean feeds. The results indicate that IPSI process requires lowest raw material cost due to high process efficiency. However, its high total capital cost offset its overall economic performance. The HHC separator scheme shows the lowest total capital cost because of a simplified configuration, but requires the highest raw material cost among the processes due to the poor separation efficiency compared with the other processes. ISS scheme shows best economic performance when the feed GMP value reaches 2.5. The scrub column scheme shows the best overall economic performance among the process schemes in the wide range of lean feed conditions. The results demonstrate that scrub column scheme can be seen as a good candidate of the NGL recovery processes for economically when the feed is in the considerably lean conditions. An offshore platform has limited deck area different from onshore liquefaction plants. So the selection criteria for a liquefaction process is different compared to the onshore liquefaction processes. In this study, six types of liquefaction processes that applicable for offshore units were selected and analyzed both the process efficiency and economic performance. The six types of processes are a dual N2 expander, two single mixed refrigerant (SMR) and three kinds of dual mixed refrigerant (DMR) processes. The N2 expander process uses nonflammable pure nitrogen as the refrigerant that has advantages of safety and relatively simple operation. However, the liquefaction efficiency is the lowest one compared with the other processes. The DMR process includes two mixed refrigerant cycles that owns the highest liquefaction efficiency and per train capacity than the N2 expander and SMR processes. However, it has more complex process configuration than the other processes that normally used in the large scale liquefaction plants. On the other hand, the process performance of a SMR process is between the N2 expander and DMR processes. The results present that dual N2 expander process has the lowest process efficiency and net profit among the compared processes. The SMR process 2 shows the lowest capital expenditure and payout time. The DMR base case scheme indicates the highest profitability and lowest operating cost among the processes because of the highest liquefaction efficiency. The results show that both SMR process 2 and DMR base processes have advantages in terms of some aspects compared with the other processes. The proposed two simple integrated NGL/LNG processes and a conventional LNG and NGL coproduction process were also investigated with consideration of LNG HHV specification under the lean feed condition. The SMR process 2 and DMR base cycles were selected for the liquefaction part and the genetic algorithm (GA) method was used for the process optimization. The results show that the proposed integrated process, case 1, has overall economic advantages compared to the conventional base case scheme. The capital cost reduced remarkably by simplifying NGL recovery part, and only a little loss of liquefaction efficiency (less than 1%). The proposed process case 2, which adopts SMR process 2 as the liquefaction process, shows the lowest total capital cost and best profitability when a plant operating time is less than a certain period. Therefore, it could be a good process option when a plant reservoir lifetime is relatively short such as some peak shaving plants and special offshore applications in terms of economic aspect.Abstract i Contents iv List of Figure vi List of Tables viii CHAPTER 1. Introduction 1 1.1. Research motivation 1 1.2. Research objectives 3 1.3. Outline of the thesis 3 CHAPTER 2. Economic evaluation of the conventional NGL recovery and liquefaction processes 5 2.1. Evaluation of the representative NGL recovery processes considering LNG HHV specification 5 2.1.1. Overview 5 2.1.2. Process selection and description 9 2.1.3. Design criteria and specifications 19 2.1.4. Economic evaluation 23 2.1.5. Profitability analysis 28 2.1.6. Total annualized cost (TAC) optimization 28 2.1.7. Results and discussion 41 2.1.8. Summary 48 2.2. Optimization and economic analysis of natural gas liquefaction processes for offshore units 49 2.2.1. Overview 49 2.2.2. Process description 51 2.2.3. Results and discussion 65 2.3. Evaluation of a representative NGL/LNG integrated process considering LNG HHV specification 81 2.3.1. Overview 81 2.3.2. A conventional integrated process description 84 CHAPTER 3. The proposed integrated processes under the lean feed conditions 87 3.1 Overview 87 3.2 The proposed integrated process description 88 3.2.1. The proposed integrated process with DMR cycle 88 3.2.2. The proposed integrated process with SMR cycle 91 3.3 Feed compositions and specification 93 3.4 Liquefaction efficiency analysis 96 3.5 Economic evaluation 99 3.6 Results and discussion 99 3.7 Summary 109 CHAPTER 4. Concluding Remarks 111 4.1. Conclusions 111 4.2. Future works 113 References 114 Nomenclature 123 Abstract in Korean (국문초록) 125Docto

A case study: Application of energy and exergy analysis for enhancing the process efficiency of a three stage propane pre-cooling cycle of the cascade LNG process

The propane pre-cooling cycle has been widely used in most LNG plants as the first cooling cycle in the natural gas liquefaction process. As LNG plants consume high amounts of energy, enhancements in the process design and plant operation will minimize the overall energy consumption of the plant. The aim of this study is to enhance the process efficiency of a three stage propane pre-cooling cycle of the Cascade LNG process for the large-scale LNG train by determining the optimal operating conditions of the propane evaporator that will minimize the overall energy consumption. Energy and exergy analysis methods are adopted to evaluate the process efficiency of the propane pre-cooling cycle. Six case studies were presented to determine the optimal operating conditions of the propane evaporator that gives maximum energy reduction. The propane pre-cooling cycle is modelled and simulated using Aspen HYSYS with detailed thermodynamic information obtained to calculate the exergy loss. The results of the energy and exergy analysis indicate that Case 6 gives the highest coefficient of performance (COP) and the maximum exergy efficiency compared to the baseline case, which are 15.51% and 18.76% respectively. The results indicate that by reducing the cooling duty at the intermediate stages of propane evaporator about 13.5% energy saving can be achieved compared to the baseline case

An Analysis of the Energy Consumption and Environmental Impact on the Emissions of Carbon Dioxide and Methane of the Offshore Natural Gas Liquefaction Process in Facilities with Utilizing Dual Mixed Refrigerant (DMR) Process Technology

The objective of this thesis is to evaluate the effects of the dual mixed refrigerant (DMR) process on energy consumption and greenhouse gas (GHG) emissions in terms of carbon dioxide (CO2) and methane (CH4) in an offshore liquid natural gas (LNG) facility based on the reliable data from the Gulf of Mexico in the United States. After comparing energy consumption and GHG (carbon dioxide and methane) emissions for a number of liquefaction technologies: including mixed fluid cascade (MFC), dual mixed refrigerant (DMR), single mixed refrigerant (SMR), mixed refrigerant with propane precooling (C3MR), and the Conoco Phillips Optimized Cascade processes, the evaluation showed that dual mixed refrigerant (DMR) process features both high energy consumption and high GHG (carbon dioxide and methane) emission in regards to the liquefaction process. With this result in mind, more technical enhancement is needed to update the DMR process to decrease energy consumption, and more research is needed to support the reduction of negative environmental impacts in the future. The results of this thesis will benefit research on the liquefaction process regarding energy consumption and environmental impacts