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1、 某某學(xué)校 畢業(yè)設(shè)計（論文）外文文獻(xiàn)翻譯 （本科學(xué)生用） 題 目：為了未來的發(fā)展，液化天然氣工藝處理過程中應(yīng)該注意的問題 學(xué) 生 姓 名： 學(xué)號： 學(xué) 部 （系）:城市建設(shè)工程學(xué)部 專 業(yè) 年 級:級建筑環(huán)境與設(shè)備工程班 指 導(dǎo) 教 師： 年月日 LNG PROCESS SELECTION CONSIDERATIONS FOR FUTURE DEVELOPMENTS John B. Stone Senior LNG Consultant Dawn L. Rymer Se

2、nior Engineering Specialist Eric D. Nelson Machinery and Processing Technology Supervisor Robert D. Denton Senior Process Consultant ExxonMobil Upstream Research Company Houston, Texas, USA ABSTRACT The history of the LNG industry has been dominated by the constant search for economies of sc

3、ale culminating in the current Qatar mega-trains undergoing final construction, commissioning,start-up and operations. While these large trains are appropriate for the large Qatar gas resources, future, smaller resource developments will necessitate different process selection strategies. The actual

4、 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 mixed refrigerant (DMR) process that has th

5、e 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

6、 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 search for economies of scale in the LNG industry. However, theapplication of these very large trains for general LNG a

7、pplications 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

8、 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 reso

9、urces 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 these large tra

10、ins, 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 selecting a process that gives the maximum flexibility for utilizing compressors, heat exchangers, and drivers with multiple competing vendors

11、. Another desirable feature is using refrigerant as a utility to allow for facilitated expansion if there is a possibility that several resources can be staged for expansion trains. PROCESS COMPARISON LNG process selection has often been highly influenced by the specific power consumption, i.e., r

12、efrigerant compression power divided by the train capacity. This is certainly an important parameter, since refrigerant compressors are the largest single cost and energy consumption components in an LNG train. Conventional wisdom would be that lower specific power consumption would result in lower

13、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 processes against the number of cycles employed based on consistent conditions. SMR -

14、 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 DMR-SWHE - Dual Mixed Refrigerant with single pressure levels and SWHEs DMR-BAHX - Dual Mixed Refrigeran

15、t with multiple pressure levels and BAHXs TMR - Triple Mixed Refrigerant Figure 1 - Process Specific Power Comparison In general, mixed refrigerant processes are more efficient than pure component processes and additional cycles improve efficiency. However, both of these efficiency improvements c

16、ome at the expense of increased 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 efficienc

17、y can overwhelm some of the differences shown. Considerations for the generation and distribution of electric power for motor driven LNG processes can further complicate the comparison. The LNG industry is changing in a number of areas that can also impact the selection of the best liquefaction pro

18、cess. 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

19、simpler than LNG production and process selection is generally not an important consideration. All these factors point to the need for more compact, lighter mechanical designs. Another important future consideration is the increasing need to reduce greenhouse gas emissions. Aeroderivative gas turbi

20、ne drivers are an obvious choice for higher thermal efficiency or modular application but are not available in sizes as large as industrial gas turbines. Consequently, a process suitable for large 95 MW industrial gas turbines may not be well suited for a 35 MW aeroderivative gas turbine. Combined-c

21、ycle power generation is another option for achieving increased thermal efficiency and can be adapted to any of these processes, 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 limite

22、d aeroderivative gas turbine choices for very large (>100MW) power generators. The value of thermal efficiency can also become a more important process selection criterion when the feed gas to the LNG plant is relatively expensive or supply is limited. An efficient process can allow for a reduced c

23、ost development plan through a lower gas rate, or extend the gas production plateau from the reservoir to make a more profitable project.  IMPACT OF EQUIPMENT COSTS Our process research comparing liquefaction processes has demonstrated that the primary difference in the costs for the different l

24、iquefaction processes is the choice of equipment utilized. Process licensors tailor their process to make it capital and thermally efficient given the owners preferences and constraints. However, they do not always have control over the cost (both equipment and installation) in the final analysis.

25、Gas Turbines Gas turbine costs exhibit a reasonably high economy of scale. Large industrial gas turbines are the least expensive, but their cost advantage is lost in a modular or offshore environment due to their large weight and space requirements. Therefore, aeroderivative based designs will be m

26、ore attractive. However, once the drivers are selected, then a process that is flexible in allowing a shift in refrigerant power loads to maximize the utilization of the available turbine power would be the best process. A multiple mixed refrigerant process, without the fixed atmospheric boiling tem

27、peratures characteristic of pure refrigerants, has the flexibility to allow such shifting. An alternative to mechanical-drive gas turbines would be electric motor drives with very large power generators for economy of scale. In this case, gas turbine costs would be lower because of standard designs,

28、 multiple manufacturers, and possibly greater economies of scale, but there would be additional costs for motors, spare turbine generators and power distribution which can reduce the overall efficiency in a simple cycle configuration. This efficiency loss can be overcome with combined cycle, but in

29、simple or combined cycle the net result is usually a higher capital cost. The implementation of an all-electric drive configuration is even more difficult at reduced economies of scale where the use of larger lower cost turbines becomes problematic due to difficulties managing the dynamic response t

30、o electrical load changes spread across fewer units. In the end though, the choice of an all-electric drive configuration is condensed to a trade off between a higher capital cost and the increased plant availability that electric motors can achieve. Compressors Compressors exhibit a very high eco

31、nomy of scale. Refrigerant compression costs are primarily a function of the number of compressor cases needed. Consequently, it is important to minimize the number of compressor cases. Likewise it is important to limit the required rotor diameter of the centrifugal compressor wheels to stay within

32、 the capabilities of multiple vendors. This requires limiting the volumetric flow rate feeding these compressors through reduced refrigerant circulation or higher refrigerant suction pressure. Again the dual mixed refrigerant process allows the process designer the flexibility to optimize the compre

33、ssor inlet suction volumetric rate to maximize throughput within the design capability of at least four suppliers. Heat Exchangers Cryogenic heat exchanger costs are primarily related to the surface area supplied. There will always be a tradeoff between exchanger area and compressor power to reach

34、 a minimum overall cost. Spiral Wound Heat Exchangers (SWHEs) are the standard cryogenic heat transfer equipment for the base load LNG industry. SWHEs have an excellent service record in LNG service; however, they are expensive, have long delivery times, and are limited to two manufacturers. Anothe

35、r option is to use brazed aluminum heat exchangers (BAHXs), which have a lower cost per unit area than SWHEs, and can be aggregated easily into blocks of surface area to meet large heat transfer requirements effectively. BAHXs also easily accommodate side-streams which allow refrigerant systems with

36、 multiple pressure-levels to be readily incorporated. BAHXs have been demonstrated in LNG service in cascade processes and smaller mixed refrigerant processes. BAHXs are built in small units (cores) typically manifolded together and insulated in a cold box. A typical design would require about 30 co

37、res to provide the exchanger area needed for a 3 MTPA LNG train. These exchangers are available from five manufacturers. Having multiple vendors ensures not only competitive prices, but also flexibility in acquiring the exchangers in time to meet the project schedule. PROCESS SELECTION What would

38、an ideal liquefaction process look like? It would be a DMR process such as shown in Figure 2 below for low specific power consumption and flexibility to optimize compressor design. Including multiple levels of cooling in the warm mixed refrigerant circuit allows more flexibility to meet compressors

39、volumetric limitations. ExxonMobil has synthesized these traits with known liquefaction processes, adding our own proprietary optimizations resulting in this configuration. Figure 2 - ExxonMobil DMR-BAHX Process Schematic It would utilize BAHX exchangers to provide: ? Multiple

40、 manufacturers for cost and schedule benefits, ? Economic scale up over a wide range of throughputs, ? Ease of modularization The BAHX exchangers would be protected from operational and design problems associated with multi-phase maldistribution by effecting refrigerant separation at each pressur

41、e level of the warm refrigerant and feeding only liquids to the BAHX cores while bypassing the vapor back to the compression system. It would utilize gas-turbine-driven centrifugal compressors large enough to capture the economy of scale available but small enough to ensure that multiple compressor

42、 vendors are capability of supplying the sizes needed. The results of our LNG process research applying these principles to a potential LNG development are shown in Figure 3. By using BAHXs and a dual mixed refrigerant process to match the best fit of compressors and drivers available from multipl

43、e vendors, the resulting process will have a lower specific power requirement, and could have a lower capital cost than traditional technologies. The DMR process with brazed aluminum heat exchangers shows a unit cost advantage across a broad range of plant capacities and optimizes the trade-offs of

44、efficiency versus cost for a wide size range (3-6 MTPA) of plants . EFFICIENT EXPANSION LNG plants have long benefited for profitable expansion trains, typically provided from the same large resource. While the number of discovered large fields available for multi-train development is shrinking,

45、 there is still the potential for economical expansion from nearby smaller resources. In many cases these other fields cannot be aggregated into one large project for a variety of reasons: difficulty aligning several commercial interests, waiting on reduced development costs for more difficult resou

46、rces, or near-field discoveries identified after the LNG project is underway. For all of these reasons it is desirable to have an easily expandable LNGplant. Treating refrigerant as a utility is a way to maximize the expandability and reliability of a multtrain facility. In this configuration all o

47、f the refrigerants that serve the same process function are combined into a single header and delivered as required to the LNG liquefaction sections. The refrigerant as a utility concept can be done with any liquefaction process, but is most suited for dual mixed refrigerants where the refrigerant r

48、eturn pressures can be higher resulting in smaller piping for distribution of refrigerant across the LNG plant. Figure 4 shows one such configuration Treating refrigerant as a utility has several benefits: ? The trains do not necessarily need to be the same size, leading to cus

49、tomizable expansion to match commercial needs. ? All the refrigerants can be re-tuned to match changes in feed gas composition to machinery limits as new gas supplies are brought on-line. ? Any spare capacity identified by testing after start-up can be designed for and utilized during expansion.

50、 ? A mixture of gas turbine, steam turbine, and motor drivers can be used giving more flexibility to the driver selection and energy utilization. ? In the event of driver failures, the liquefaction train may be able to turn-down instead of shut-down. ? During planned driver maintenance the other

51、 drivers can be run at their maximum rates and potentially take advantage of seasonal swings. ? A driver and hence refrigerant supply can be easily spared across the whole plant, increasing plant availability. ? Various cold streams, such as LNG-loading vapors, can be effectively integrated into t

52、he process scheme to allow the impact of flow fluctuations in these streams to be evenlyspread across all trains for operational stability. With these advantages, a refrigerant as a utility concept could be beneficial to provide options for any project with uncertainty in its expansion possibilitie

53、s. CONCLUSION In conclusion, a dual mixed refrigerant process with brazed aluminum heat exchangers that treats refrigerant as a utility has the scalability, flexibility and expandability required for the next generation of LNG projects. The system incorporates the guiding principle that capital co

54、sts can be minimized by ensuring that there will be multiple vendors or contractors that can supply the equipment or services for the duties required. The design appropriately balances economies of scale against the expense of sole source purchase and results in a more readily scalable configuration

55、. The refrigerant as a utility concept allows for effectively dealing with uncertain expansion plans while providing operational and design flexibility. ExxonMobil can incorporate these process characteristics with these key success factors to ensure a successful project: ? Demonstrated Mega-projec

56、t management and execution expertise ? Close working relationships with equipment vendors ? Proven start-up and commissioning experience ? Rigorous technology qualification process ? Value-driven process and vendor selection procedure 為了未來的發(fā)展，液化天然氣工藝處理過程中

57、應(yīng)該注意的問題 John B.Stone 高級液化天然氣顧問 Dawn L.Rymer 高級工程專家 Eric D.Nelson 機(jī)械和加工技術(shù)主管 Robert D.Denton 高級工藝工程顧問 ?？松梨谏嫌尾块T研究公司 美國，德克薩斯州，休斯頓 摘要 液化天然氣產(chǎn)業(yè)的歷史已經(jīng)被經(jīng)濟(jì)規(guī)模的擴(kuò)張所控制著，并且在目前卡塔爾超級列車經(jīng)歷最后的建設(shè)、調(diào)試、啟動和操作中達(dá)到發(fā)展的高峰。雖然這些大型的列車適合于卡塔爾大型的燃?xì)赓Y源，但是未來更節(jié)能的發(fā)展要求使不同的處理工藝方法成為必要性。現(xiàn)行的液化天然氣處理工藝僅是影響最佳選擇的諸多因素之一。設(shè)備的選擇，尤其是低溫?zé)峤粨Q器和制

58、冷壓縮機(jī)的選擇能夠克服在效率方面的小差異。埃克森美孚國際公司已經(jīng)在發(fā)展一種雙極混合制冷工藝，這一工藝具有能夠提供新的工程建設(shè)要求的潛力，同時也能使設(shè)備供應(yīng)商的數(shù)量達(dá)到最大值，從而允許更廣泛的競爭，并使成本處于可控的范圍之內(nèi)。這一工藝也能靈活地適應(yīng)大范圍的供給、價格和產(chǎn)品銷售要求。 背景 在卡塔爾，每年780萬噸載重的火車標(biāo)志著目前在液化天然氣工業(yè)方面經(jīng)濟(jì)規(guī)模探索的最高峰。然而，這些大型LNG火車的普偏應(yīng)用是受限制的。生產(chǎn)此數(shù)量的液化天然氣需要1500MSCFD的原料氣。關(guān)于大型液化天然氣火車的討論中常常被人們忽略的是：維持這樣的火車工作超過25年需要約 370 GCM的資源，這幾乎和425

59、 GCM的印度阿倫場一樣大，這個產(chǎn)量是這個地區(qū)液化天然氣廠的極限。對于新的液化天然氣發(fā)展，擁有一個750 GCM的真正世界級的資源是必需的。即使資源能夠支持如此龐大的火車，龐大的氣體處理和備用火車也需要確保：在非液化天然氣設(shè)施中，它也不會失去作用?？紤]到將來有限的氣體資源能夠支持這些大型火車，將需要找到新的方法以更小的生產(chǎn)來維持成本的優(yōu)勢，做到這一點的一種方法就是選擇一個過程，以提高工程的執(zhí)行力，這個過程提高最大的靈活性去利用壓縮機(jī)，熱交換器，并且和許多競爭的供應(yīng)商一起控制。如果有一種可能性，即一些能源可以應(yīng)用到火車上，那么另一個可取的特點就是使用制冷劑作為一種實用工具，來允許其作用的擴(kuò)展。

60、 工藝比較 液化天然氣的工藝過程往往受到具體功率（即火車做功除以壓縮機(jī)做功）的高度影響，這顯然是一個重要的參數(shù)，因為制冷壓縮機(jī)在一輛液化天然氣火車上是最大的成本和最大的能源消耗體。傳統(tǒng)的觀點認(rèn)為：較低的功率消耗將會導(dǎo)致較低的制冷劑壓縮成本和較低額外生產(chǎn)液化天然氣的原料氣。實際上它的描述很復(fù)雜，針對基于一定循環(huán)次數(shù)出現(xiàn)的各種各樣的液化過程，圖1繪制了具體的功率消耗過程。 在一般情況下，混合制冷劑工藝比單一制冷劑工藝更有效，并且額外的周期能提高工作效率，然而，工藝過程的復(fù)雜性都提高了工作效率。 造成如上圖表過程復(fù)雜的另一個因素是：它僅考慮了一個過程的比較，而不是一個制冷壓縮機(jī)或

61、驅(qū)動程序的比較。壓縮機(jī)功率的不同、一個高速傳動裝置的需求、或者是驅(qū)動器的效率可以掩蓋一些差異?？紤]液化天然氣摩托車中電能的產(chǎn)生和分配可以進(jìn)一步使比較復(fù)雜化。 液化天然氣行業(yè)正在改變，在一些領(lǐng)域，也可以影響最好的液化過程。然而“棒內(nèi)置”的液化天然氣廠仍然傳統(tǒng)，模塊化的液化天然氣設(shè)施對于近海地區(qū)的應(yīng)用或者是勞動成本高且生產(chǎn)率低的地方更具吸引力。模塊化結(jié)構(gòu)通常適用于海上石油加工，然而石油加工過程比液化天然氣的生產(chǎn)過程簡單很多，工藝的選擇一般不是重要的考慮因素。所有些因素都指向需要更緊湊、更輕的機(jī)械設(shè)計。 未來另一個重要的考慮因素是對減少溫室氣體排放量不斷增長的要求，對于更高的熱效率或模塊化的應(yīng)用

62、模式，航改燃?xì)廨啓C(jī)驅(qū)動是顯而易見的選擇，因此，適合95兆瓦的大型工業(yè)燃?xì)鉁u輪機(jī)的過程未必適合35兆瓦的航改燃?xì)廨啓C(jī)。聯(lián)合循環(huán)發(fā)電機(jī)是實現(xiàn)增加熱效率的另一選擇，可適應(yīng)任何這些過程，但由于電動機(jī)、發(fā)電機(jī)和配電設(shè)施，以及選擇發(fā)電機(jī)（>100MW）受到限制的航改燃?xì)廨啓C(jī)的額外重量，使它不適合模塊化或境外申請。 當(dāng)液化天然氣廠的原料氣相對昂貴或者供應(yīng)商有限的時候，熱效率值也可以成為一個更重要的過程選擇準(zhǔn)則。一個有效率的進(jìn)程可以通過較低的氣量或者是從氣田中擴(kuò)大天然氣生產(chǎn)平臺來考慮降低成本，以此使工程更有利可圖。 設(shè)備成本的影響 我們所做的比較液化工藝的研究已經(jīng)證明：在不同的液化工藝的成本差異中最主要

63、的不同處是對利用設(shè)備的選擇。調(diào)整自己的過程，使其資本和熱效率的過程中協(xié)議業(yè)主的編好和約束，然而，他們總不能在最后的分析中控制成本（包括設(shè)備及安裝）。 燃?xì)鉁u輪機(jī) 燃?xì)鉁u輪機(jī)成本表現(xiàn)出相當(dāng)高的經(jīng)濟(jì)性，大型的工業(yè)燃?xì)鉁u輪機(jī)是最便宜的，但由于重量和體積大的原因，其成本優(yōu)勢在模塊化或者是近海環(huán)境內(nèi)未能體現(xiàn)，因此，航改的設(shè)計將是更具吸引力。然而，一旦驅(qū)動被選中，那么靈活地改變制冷負(fù)荷，從而最大限度地利用現(xiàn)有的渦輪動力，這將是最好的過程。沒有單一制冷劑的固定沸點溫度的特點，一個多元混合制冷的過程能夠靈活地允許這樣的轉(zhuǎn)變。 機(jī)械式驅(qū)動燃?xì)廨啓C(jī)將是具有非常好的經(jīng)濟(jì)性的電動傳動裝置，在這種情況下，因為標(biāo)準(zhǔn)

64、設(shè)計、多個廠家及有可能的更大經(jīng)濟(jì)性，燃?xì)鉁u輪機(jī)的成本將會降低，但對于發(fā)動機(jī)、備用發(fā)電機(jī)和配電會有額外的費用，這些因素能在一個簡單的周期配置中減少整體效率。聯(lián)合循環(huán)可以克服效率損失，但是單一循環(huán)通常有較高的成本。全電動驅(qū)動器配置的實施，更是難以減少經(jīng)濟(jì)性，由于在較少的單位電力負(fù)荷變化的動態(tài)響應(yīng)中管理困難，更低成本渦輪機(jī)的使用成了問題。最后，一個全電氣化的驅(qū)動配置被認(rèn)為是較高的成本和提高工廠的可用性之間的一個折中的選擇。 壓縮機(jī) 壓縮機(jī)表現(xiàn)出非常高的經(jīng)濟(jì)性，制冷劑壓縮成本主要是所需壓縮機(jī)數(shù)量的函數(shù)，因此，最重要的是要減少壓縮機(jī)的數(shù)量。同樣重要的是要限制所需的轉(zhuǎn)子離心壓縮機(jī)車輪直徑，這就要通過減

65、少制冷劑循環(huán)量來限制體積流量或者是更高的制冷劑吸入壓力供給這些壓縮機(jī)。利用二次雙混合制冷劑工藝使流程設(shè)計變得靈活，在至少四家供應(yīng)商的能力范圍內(nèi)，以優(yōu)化壓縮機(jī)的進(jìn)氣口吸氣容積來最大限度地提高生產(chǎn)量。 這將會利用釬焊鋁熱交換器來提供： l 多個廠家的成本和進(jìn)度的利益 l 經(jīng)濟(jì)規(guī)模較大的吞吐量 l 易于模塊化 釬焊鋁熱交換器在每個壓力水平下影響制冷劑的分離，從操作和設(shè)計相關(guān)的問題中得到保護(hù)，只有液體輸送到釬焊鋁熱交換器的核心部位，而繞過蒸汽回到壓縮系統(tǒng)中。 它會利用燃?xì)廨啓C(jī)驅(qū)動離心壓縮機(jī)達(dá)到足以獲得經(jīng)濟(jì)性，但它必須確保多個壓縮機(jī)供應(yīng)商的供應(yīng)。 我們的液化天然氣流程研究將這些原則應(yīng)用

66、到一個潛在的液化天然氣發(fā)展中，其結(jié)果顯示在圖3中。通過使用釬焊鋁熱交換器和雙混合制冷劑，使壓縮機(jī)和驅(qū)動器達(dá)到最佳匹配，由此產(chǎn)生的過程將會有一個更低的功耗要求，并且有一個比傳統(tǒng)技術(shù)更低的資本成本，釬焊鋁熱交換器的DMR過程表明一個單位耗資有優(yōu)勢。 高效擴(kuò)增 LNG廠使擴(kuò)張的火車長期受益，通常從同樣大的資源中得到提供，雖然可用于多級列車發(fā)展的已發(fā)現(xiàn)的大油田的數(shù)量正在減少，但附近的小資源對經(jīng)濟(jì)的擴(kuò)張仍是有潛力的。在許多情況下，這些其它領(lǐng)域不能聚合成一個大工程的各種原因有：一些商業(yè)利益的調(diào)整、為了更困難的資源而等待降低開發(fā)成本、或者是附近LNG工程資源正在進(jìn)行中。對于所有這些原因，它需要一個有易于擴(kuò)張的液化天然氣工廠。 作為一個實用的制冷劑，它是一種以最大限度來提高擴(kuò)展性和可靠性的途徑，在此配置中所有服務(wù)過程中的制冷劑合并成一個單一的頭，并交付給LNG液化環(huán)節(jié)。作為通用的制冷劑可以用于任何液化過程，但最適合雙混合制冷劑，制冷劑的回饋壓力可以更高，從而導(dǎo)致較小的管道分布橫跨制冷劑液化天然氣廠，圖4顯示了一個這樣的的配置。 處理的制冷劑作為一種實用工具有幾個好處： l 列車不一