Exergy Analysis of a GTL Process Based on Low-Temperature Slurry F-T Reactor Technology with a Cobalt Catalyst Carmine L. Iandoli and Signe Kjelstrup* Department of Chemistry, Norwegian UniVersity of Science and Technology (NTNU), Trondheim, Norway ReceiVed December 19, 2006. ReVised Manuscript ReceiVed March 30, 2007 Interest in the gas-to-liquid process (GTL) using Fischer-Tropsch reactors (F-T) has increased in recent years because of its potential to replace ordinary petroleum with sulfur-free fuels. However, the efficiency of the process is still low compared with current routes; a large amount of the initial exergy of the gas is used to convert it into liquid fuel. In the present study, we analyze the overall thermodynamic efficiency of a GTL process and point out the main sources of entropy production. Next, we use exergy analysis to establish the impact of catalyst selectivity and of thermal losses on the process efficiency. In particular, we report the effect of selectivity losses in the F-T reactor and reformer, and the impact of high-temperature heat integration across the reforming unit. 1. Introduction The Fischer Tropsch (F-T) synthesis was originally devel- oped in Germany in the 1920s by Franz Fischer and Hans Tropsch; their aim was to use a mixture of CO and H 2 (referred to as synthesis gas, syngas) to produce hydrocarbons, chemicals, and liquid fuels. The production of syngas was achieved by coal gasification, and 10 small-scale plants (production < 1 million ton/year) were built before World War II in a push for petroleum independence. After World War II, oil was available, and commercial applications of F-T synthesis were less attractive. New plants based on F-T synthesis were built after 1955 in South Africa to overcome the oil shortage caused by the embargo imposed by the Arab Emirates (in 1973) and United Nations (in 1981). The process feedstock was still coal (coal- to-liquid, CTL). In the 1990s, natural gas was discovered in South Africa offshore, and a process using gas as feedstock was implemented. The process, known as gas-to-liquid (GTL), was based on two steps: first, steam reforming of natural gas into syngas and, then, Fischer-Tropsch synthesis of syngas into synthetic liquid fuels. A first plant was built in Mossel Bay (1992). Details about the historical development of F-T technology were reported by Freerks. 1 The history of F-T synthesis shows that, although the technology has been available since the 1920s, its practical use has been pushed by a lack of crude oil (high oil prices). Recently (2004-2006), the massive growth of GNP in China and India has caused crude oil prices to rise continuously, and F-T synthesis (including both coal and biomass as feedstock, BTL) has become once more an appealing technology. Recent interest in F-T technology especially in Europe and South America is driven by a focus on the gasification of biomass into fuel 2 (BTL). Moreover, European oil companies are also interested in GTL as a business opportunity outside Europe. New CTL plants are planned in China and the U.S.A. The Shell GTL plant in Malaysia has been in operation since 1993, and a new plant owned by Qatar Petroleum and Sasol will start production in Qatar in early 2007. GTL is seen as the main alternative to liquefied natural gas for monetizing so-called stranded gas worldwide. The GTL diesel fuel has near zero sulfur content and aromatic components and a very high cetane number. For this reason, GTL diesel fuels are able to reduce exhaust emissions from a variety of diesel engines. 3 The production of synthetic fuels implies CO 2 emissions, therefore, and a well-to-wheel analysis shows that the overall CO 2 emissions are still higher than those from the conventional oil-refining route. The overall energy balance and CO 2 emission sources are understood, but the relative impact of different technology elements of the GTL process have not been studied. How will high-temperature heat integration (HTHI) in the syngas unit affect the energy efficiency of the GTL plant, and how does it compare to improvements in the selectivity of the F-T synthesis? An exergy analysis can provide better understanding of the fundamental reason for the efficiency losses in a GTL plant. Only one exergy analysis is known to us, based on BTL, 2 but a number of life cycle analyses focusing on first law efficiency have been published by the oil companies. Our aim is therefore to study what the initial exergy of the natural gas exactly is used for, and where and why exergy is lost in the process. On the basis of process simulations of a promising GTL concept available today using Pro/II (Aspen/SimSci), the exergy balance and exergy losses of the process have been calculated. The relative impact of the following parameters has been analyzed: • improvement in F-T catalyst selectivity • once-through operation of the F-T reactor • high-temperature heat integration across the autothermal reformer (ATR) (HTHI, no metal dusting) • high-temperature heat integration: gas-heated reforming (GHR) • and syngas production via ideal low- and high-temperature catalytic partial oxidation (LTCPO and HTCPO). * Author to whom correspondence should be addressed. E-mail: signe.kjelstrup@chem.ntnu.no. (1) Freerks, R. AIChE Spring National Meeting, New Orleans, LA, 2003. (2) Prins, M. J.; Ptasinski, K. J.; Janssen, F. J. J. G. Fuel Process. Technol. 2005, 86, 375-389. BATCH: ef4a40 USER: eah29 DIV: @xyv04/data1/CLS_pj/GRP_ef/JOB_i04/DIV_ef060646y DATE: May 29, 2007 10.1021/ef060646y CCC: $37.00 © xxxx American Chemical Society PAGE EST: 7.9 Published on Web 00/00/0000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90