LNG PROCESS SELECTION CONSIDERATIONS FOR FUTURE DEVELOPMENTS John B. Stone Senior LNG Consultant Dawn L. Rymer Senior Engineering Specialist Eric D. Nelson Machinery and Processing Technology Supervisor Robert D. Denton Senior Process Consultant ExxonMobil Upstream Rearch Company Houston, Texas, USA 九记牛腩ABSTRACT The history of the LNG industry has been dominated by the constant arch for economies of scale culminating in the current Qatar mega-trains undergoing final construction, commissioning,start-up and operations. While the large trains are appropriate for the large Qatar gas resources, future, smaller resource developments will necessitate different process lection strategies. The actual LNG process is only one of many factors affecting the optimal choice. The choice of equipment, especially cryogenic heat exchangers and refrigerant compressors, can overwhelm small differences in process efficiencies. ExxonMobil has been developing a dual 一个深夜的记忆mixed refrigerant (DMR) process that has the potential of offering the scalability and expandability required to meet the needs of new project developments, while also maximizing the number of equipment vendors to allow broader competition and keep costs under control. The process will also have the flexibility to accommodate a wide range of feed compositions, rates, and product sales requirements. BACKGROUND The startup of the 7.8 million tonnes per year (MTPA) trains in Qatar mark the most recent pinnacle in the arch for economies of scale in the LNG industry. However, theapplication of the very large trains for general LNG applications is very limited. To produce this amount of LNG requires ~42 MSCMD (1500 MSCFD) of feed gas. What is often overlooked in the discussion of large LNG trains is that a resource of about 370 GCM (13 TCF) is needed to support the operation of one such train over a 25-year life. This is nearly as large as the Arun field in Indonesia 425 GCM (15 TCF), which was the backbone of the LNG plant development in that region. For new LNG developments that are often built with a minimum of two identical trains, a truly world-class resource class of 750 GCM (26 TCF) would be required. Even for resources capable of supporting such large trains, very large gas treating and preparation trains with a minimum of parallel equipment are also needed to ensure that economies of scale are not lost in the non-LNG facilities. Given the limited supply of gas resources capable of supporting the 童趣的古诗large trains, future projects will need to find ways to maintain some cost advantages at smaller capacities. One way to do this is to improve the project execution by lecting a process that gives the maximum flexibility for utilizing compressors, heat exchangers, and drivers with multiple competing vendors. Another desirable feature is using refrigerant as a utility to allow for facilitated expansion if there is a possibility that veral resources can be staged for expansion trains. PROCESS COMPARISON LNG process lection has often been highly influenced by the specific power consumption, i.e., refrigerant compression power divided by the train capacity. This is certainly an important parameter, since refrigerant compressors are the largest single cost and energy consumption 招商银行待遇components in an LNG train. Conventional wisdom would be that lower specific power consumption would result in lower refrigerant compression costs and additional LNG production from a fixed feed gas rate. In actuality it is a more complicated picture. Figure 1 plots the specific power consumptions for a variety of liquefaction process against the number of cycles employed bad on consistent conditions. |
SMR - Single Mixed Refrigerant C3MR - Propane pre-cooled Mixed Refrigerant C3MRN2 - Propane pre-cooled Mixed Refrigerant plus Nitrogen expander cycle Cascade - Pure propane, ethylene, and methane 极致追击姓史DMR-SWHE - Dual Mixed Refrigerant with single pressure levels and SWHEs DMR-BAHX - Dual Mixed Refrigerant with multiple pressure levels and BAHXs TMR - Triple Mixed Refrigerant Figure 1 - Process Specific Power Comparison In general, mixed refrigerant process are more efficient than pure component process and additional cycles improve efficiency. However, both of the efficiency improvements come at the expen of incread process complexity. Another factor that complicates the picture above is that it only considers a process comparison and not a refrigerant compressor or driver comparison. Differences in compressor efficiency, the need for a speed-increasing gear, or driver efficiency can overwhelm some of the differences shown. Considerations for the generation and distribution of electric power for motor driven LNG process can further complicate the comparison. The LNG industry is changing in a number of areas that can also impact the lection of the best liquefaction process. While stick-built LNG plants are still the norm, modularization of LNG facilities are more attractive for offshore applications or where labor costs are very high and/or productivity is low. Modular construction is routinely applied for offshore oil processing. However, oil processing is much simpler than LNG production and process lection is generally not an important consideration. All the factors point to the need for more compact, lighter mechanical designs. Another important future consideration is the increasing need to reduce greenhou gas emissions. Aeroderivative gas turbine drivers are an obvious choice for higher thermal efficiency or modular application but are not available in sizes as large as industrial gas turbines. Conquently, a process suitable for large 95 MW industrial gas turbines may not be well suited for a 35 MW aeroderivative gas turbine. Combined-cycle power generation is another option for achieving incread thermal efficiency and can be adapted to any of the process, but is not well suited for modular construction or for offshore application due to the additional weight of motors, generators and distribution equipment as well as limited aeroderivative gas turbine choices for very large (>100MW) power generators. The value of thermal efficiency can also become a more important process lection criterion when the feed gas to the LNG plant is relatively expensive or supply is limited. An efficient process can allow for a reduced cost development plan through a lower gas rate, or extend the gas production plateau from the rervoir to make a more profitable project. |
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