Methane the Promising Career of a Humble Molecule

Transcription

1 Journal of Natural Gas Chemistry 13(2004)71 78 Methane the Promising Career of a Humble Molecule Serge Kioes, Waldemar Liebner mg engineering-lurgi AG, Lurgiallee 5, D Frankfurt am Main, Germany [Manuscript received May 12, 2004] Abstract: Methane, CH 4, here represents natural gas (NG) of which it is the main constituent. Routes of chemical utilisation of NG as opposed to energy usage are discussed. A main step is the conversion of NG to synthesis gas, a mixture of CO and H 2. Simple molecules derived from synthesis gas, like methanol, can be further reacted to longer-chained hydrocarbons like propylene and other olefins and even to gasoline and diesel. Key words: CH 4, natural gas conversion, methanol, propylene, MTP, MtSynfuels 1. Methane and more Talking about natural gas and natural gas conversion, we essentially consider methane, the main component of all natural gases. This now as the simplest hydrocarbon indeed is a humble molecule: CH 4, one carbon atom and four hydrogen atoms. Chemically speaking this molecule is rather inert. It does not readily react, even with air as in combustion, where its ignition temperature is high. This unwillingness to undergo transformations is one of the hurdles on its way to higher ranks of chemicals like longer chain hydrocarbons. All routes of direct coupling (direct oxidation) as ingeniously described and developed so far have not surpassed the experimental stage and shall be left aside here with this honourable mention. Instead we shall concentrate on the technically feasible or better even, the technically successful. For all its stubborn inertia our humble molecule can be induced, if not to say forced, to change shape and appearance if we offer the right partners and conditions. These may be steam and oxygen as in Steam Methane Reforming or Autothermal Reforming. By these the CH 4 changes into CO and H 2, i.e. a mixture of carbon monoxide and hydrogen which is called synthesis gas or syngas for short. With syngas now starts the veritable career of our molecule: by a tendency to re-shape it becomes H 3 COH, also known as methanol. From this it transforms to H 3 COCH 3, dimethyl-ether and then further to longer chain hydrocarbons, the formerly elusive goal. We will see products like the valuable propylene and also transportation fuels like gasoline and diesel: an impressive career indeed for the small, humble CH 4! Fully acknowledging that many companies, institutions and individuals are active in the field of methane transformation or, as the title of this conference states in natural gas conversion, the presenter today will concentrate on what he knows best, his companies portfolio. What first comes to mind with natural gas conversion is GTL, mostly meant to be Fischer-Tropsch, the classic route from coal or natural gas to transportation fuels (synfuels). Lurgi on the other hand promotes methanol-based technologies for upgrading of natural gas to value-added products. These primarily would be DME (dimethyl ether), propylene, synfuels and gas-based petrochemicals. Corresponding author. dr waldemar liebner@lurgi.de

2 72 Serge Kioes et al./ Journal of Natural Gas Chemistry Vol. 13 No Figure 1. Flared natural gas (2002) [1] Since Lurgi introduced its new groundbreaking MegaMethanol R process for plants with a production of 5,000 tons of methanol per day and more, methanol will be available at a constant low price in the foreseeable future. This development has an enormous impact on downstream technologies for the conversion of methanol to more valuable products. The first derivative of methanol in this context is DME which has a high potential as alternative to conventional diesel fuel as feedgas for gas turbines in power generation and as supplement to LPG. The next step is the use of methanol as feedstock for the production of olefins which is one of the most promising new applications. Lurgi s new Methanolto-Propylene (MTP R ) process presents a simple, costeffective and highly selective technology. Both routes allow for the production of petrochemicals which then would be gas-based. Lurgi also proposes a methanolbased technology for production of synfuels which compares well with the FT-processes. 2. Natural gas conversion a solution for the 21 st century The total proven gas reserves amount to approx. 180 trillion cubic meters world-wide which translates into a gas reserve-to-production ratio, i.e. a gas reserve lifetime of 70 years. Furthermore, estimated additional gas reserves will cover a lifetime of 65 years more [1]. Compared with the reserve lifetime of 41 years for petroleum and 230 years for coal, there is no doubt that natural gas will be a key fuel component in the 21 st century. However, currently a considerable portion of this reserve is wasted yearly: a brief look at Figure 1 Flared Natural Gas explains the main incentive for engineers and environmentalists as well to come up with novel ideas for the utilisation of this gas. Existing technologies for natural gas conversion are shown in Figure 2: via the conversion to syngas, hydrogen and ammonia, Fischer-Tropsch products, methanol and DME are produced. Currently, the production of chemicals requires only around 5% of world gas consumption [2]. Figure 2. Uses of natural gas Both, economic and environmental benefits from the use of natural gas are driving and supporting the continuous innovation of gas-based technologies. Lurgi AG focuses on new routes from C 1 to valuable products by combining a chain of proprietary Lurgi technologies that are based on low cost natural gas supply and large scale methanol plants. These are new DME and synfuels technologies and an exciting new process for the highly selective conversion of methanol to propylene. Certainly, there is healthy competition already in these new fields: just to name single-step DME synthesis and Methanol to Olefins, MTO, which produces ethylene and propylene together. 3. Lurgi MegaMethanol R : basis for more valuable products The term MegaMethanol R refers to plants with

3 Journal of Natural Gas Chemistry Vol. 13 No a capacity of more than one million metric tons per year, the actual standard size being t/a (equivalent to 5000 t/d). To achieve such a large capacity in a single-train plant a special process design is required. For this reason Lurgi focused on the most efficient integration of syngas generation and methanol synthesis into the most economical and reliable technology for the new generation of future methanol plants [3]. The unique advantages of the Lurgi MegaMethanol R technology result in ex-gate methanol prices of about 65 /t or less and make this process ideally suited as part of Lurgi s route from C 1 to propylene and others. This year two such plants of 5000 t/d capacity will be started up: Atlas/Trinidad in summer and Zagros/Iran by year s end. Conceptual studies and engineering activities for MegaMethanol R plants with single-train capacities of up to 7500 t/d and more have been successfully finalised making these plant sizes ready for commercialisation. An environmental sidenote: 80 billion cubic meters of natural gas are flared or vented annually, see Figure 1 [1]. That amount would be sufficient to feed about 60 MegaMethanol R plants with a capacity of 102 million tons per year in total. 4. DME a valuable product from methanol Dimethyl Ether, DME, is industrially important as the starting material in the production of the methylating agent dimethyl sulphate and is used increasingly as an aerosol propellant. In the future DME can be an alternative to conventional diesel fuel or a feedgas for power generation in gas turbines. Both applications are based on large-scale production facilities in order to achieve an economic fuel price. Traditionally, DME was obtained as by-product of the high-pressure methanol synthesis. Since the lowpressure methanol synthesis was established, DME has been prepared from methanol by dehydration in the presence of suitable catalysts. The dehydration is carried out in a fixed-bed reactor. The product is cooled and distilled to yield pure DME. A modification of the methanol synthesis would allow for co-generation of DME within the methanol synthesis loop. This technical path comprises two disadvantages. While dehydrating methanol, the water vapour content increases, thus enhancing the water gas shift reaction. By converting CO into CO 2, the quality of the synthesis gas deteriorates. The kinetics of the reaction of CO 2 and H 2 is slower than the one of CO and H 2. As a result, the synthesis catalyst volume and the recycle loop capacity have to be increased. In addition, due to its low boiling point a cryogenic separation is required in order to separate DME from the synthesis recycle loop. As a result of these disadvantages of the cogeneration of methanol and DME Lurgi favours the concept of generating DME from methanol by dehydration. If a DME unit is added to the MegaMethanol R plant, the distillation of methanol is reduced from a three-tower system to one tower at considerable savings. Figure 3 shows the simple and inexpensive flowsheet for the dehydration of methanol. In this process all types and qualities of DME can be produced. The different specifications for fuel gas, power generation or pure DME can be achieved just by varying size and design of the DME distillation towers. Figure 3. DME production by methanol dehydration Without going into the details of the economic analysis which has been presented elsewhere it can be summarised that DME can be produced at a capacity of 5000 t/d from natural gas priced at US 0.5/MMBtu with a reasonable profit of 20%ROI at 93 US /t. Delivered cost of DME after trans-ocean transport from the production site, the stranded gas location, will be about 4 US /MMBtu. From all this it follows that DME, a traditional derivative of methanol, can be a promising alternative fuel for power generation, diesel, LPG or the manufacture of olefins when produced in large capacities. These uses of DME are promoted by a group of technology providers and contractors together with interested institutions in the International Dimethyl-Ether Association, IDA, and its Japanese Equivalent, JDA. See the website 5. Propylene an attractive product with high value Demand growth of propylene till 2002 was above

4 74 Serge Kioes et al./ Journal of Natural Gas Chemistry Vol. 13 No % and after that is projected at higher than 5%. Polypropylene is by far the largest and fastest growing of the propylene derivatives, and requires the major fraction of about 60% of the total propylene. The increasing substitution of other basic materials such as paper, steel and wood by PP will induce a further growth in the demand for PP and hence propylene. Other important propylene derivatives are acrylonitrile, oxo-alcohols, propylene oxide and cumene. The average growth rate for propylene itself is estimated very conservatively to be 4.5% per year for the next two decades. How to satisfy this demand for propylene? Currently, steam crackers and FCC units supply 66% and 32%, respectively of propylene fed to petrochemical processes. However, as FCC units primarily produce motor gasoline, and steam crackers mainly ethylene, propylene will always remain a by-product (e.g t/t of ethylene for steam crackers with ethane feedstock and t/t, respectively of motor gasoline and distillates production for FCC units). Current forecasts indicate an increasing gap of propylene production that has to be filled by other sources. Lurgi s new MTP process directly aims to fill that gap. 6. Lurgi s methanol to propylene (MTP R ) technology Lurgi s new MTP R process is based on an efficient combination of the most suitable reactor system and a very selective and stable zeolite-based catalyst. Since the process has been described in detail elsewhere [4], suffice it to say here that Lurgi has selected a fixedbed reactor system because of its many advantages over a fluidised-bed. The main points are the ease of scale-up of the fixed-bed reactor and the significantly lower investment cost. Furthermore, Süd-Chemie AG manufactures a very selective fixed-bed catalyst commercially which provides maximum propylene selectivity, has a low coking tendency, a very low propane yield and also limited by-product formation. This in turn leads to a simplified purification scheme that requires only a reduced cold box system as compared to on-spec ethylene/propylene separation. With Figure 4 a brief process description reads: methanol feed from the MegaMethanol R plant is sent to an adiabatic DME pre-reactor where methanol is converted to DME and water. The high-activity, high-selectivity catalyst used nearly achieves thermodynamic equilibrium. The methanol/water/dme stream is routed to the first MTP R reactor where also the steam is added. Methanol/DME are converted by more than 99%, with propylene as the predominant hydrocarbon product. Additional reaction proceeds in the second and third MTP reactors. Process conditions in the three MTP R reactors are chosen to guarantee similar reaction conditions and maximum overall propylene yield. The product mixture is then cooled and the product gas, organic liquid and water are separated. Figure 4. MTP R : simplified process flow diagram with production figures The product gas is compressed and traces of water, CO 2 and DME are removed by standard techniques. The cleaned gas is then further processed yielding chemical-grade propylene with a typical purity of more then 97%. Several olefin-containing streams are sent back to the main synthesis loop as an additional propylene source. To avoid accumulation of inert materials in the loop, a small purge is

5 Journal of Natural Gas Chemistry Vol. 13 No required for light-ends and the C 4 /C 5 cut. Gasoline is produced as a by-product. Water is recycled to steam generation for the process; the excess water resulting from the methanol conversion is purged. This process water can be used for irrigation after appropriate and inexpensive treatment. It even can be processed to potable water where needed. An overall mass balance is included in Figure 4 based on a combined MegaMethanol R / MTP R plant. For a feed rate of 5000 tons of methanol per day (1.667 million tons annually), approx tons of propylene are produced per year. By-products include fuel gas (used internally) and LPG as well as liquid gasoline and process water. Further integration and optimization of the total plant complex including syngas, methanol, propylene production and offsite facilities will again decrease the capital investment and production costs. The technological status of MTP R in the areas of process and catalyst can be summarized as follows: The basic process design data were derived from more than 9000 operating hours of a pilot plant at Lurgi s Research and Development Centre. Besides the optimization of reaction conditions also several simulated recycles have been analysed. Parallel to that Lurgi decided to build a larger-scale demonstration unit to test the new process in the framework of a world-scale methanol plant with continuous 24/7 operation using real methanol feedstock. After a cooperation agreement with Statoil ASA was signed in January 2001 the Demo Unit was assembled in Germany and then transported to the Statoil methanol plant at Tjeldbergodden (Norway) in November Later in 2002 Borealis joined the cooperation. The Demo Unit was started up in January 2002, and the plant has been operated almost continuously since then. As of March 2004, the Demo Unit completed the scheduled 8000 hours life-cycle test and an additional 3000 hours with a new batch for counterchecks. With that the main purpose of the test was achieved: to demonstrate that the catalyst lifetime meets the commercial target of 8000 hours on stream. Cycle lengths between regenerations have been longer than expected. Deactivation rates of the methanol conversion reaction decreased with operation time. Propylene selectivity and yields were in the expected range for this unit with only a partial recycle. Also, the high quality of the by-product gasoline and the polymerisation grade quality of the propylene were proven [5]. The catalyst development is completed and the supplier commercially manufactures the catalyst. 7. GTP economics Since propylene by itself is more an intermediate than an end product, an economics estimate was performed for a complete natural gas to polypropylene complex. In this case of integrating a MegaMethanol R and a MTP R plant we designate the resulting unit as Gas to Propylene, GTP R, as shown in Figure 5. Figure 5. Block flow diagram-pp complex Thus, the economic assessment included the GTP route with a polypropylene unit for the production of a more saleable, higher-value end product. To summarise the extensive economic study presented elsewhere, it can be said that again based on natural gas priced at US 0.5/MMBtu, competitively priced polypropylene with the side-product gasoline can be produced at ROIs of 19% to 23% or, stated differently, with IRRs before tax of 21% to 26%. 8. Fischer-Tropsch, FT, an old natural gas conversion process FT simply condenses the syngas derived from the humble molecule CH 4 into longer chains (-CH 2 -) n, i.e. hydrocarbons like gasoline (medium long chains), diesel (long chains) and waxes (very long chains). Historically, Lurgi was one of the developers of FT in the ties. FT in the form of (fixed bed) ARGE-synthesis was commercialised in 1952 in Sasolburg, RSA. All five original reactors are still in operation. A sixth one was started in 1987 as capacity extension. Modern FT reactor technology prefers slurry phase reactors, either tubular or fluidised bed. Lurgi has commercial experience in all these reactor designs. Also, Lurgi has designed all syngas production units of all currently operating industrial FT-plants: Sasol/Secunda, RSA, utilising coal gasification; Mossgas, RSA, -combined reforming of NG and SMDS Bintulu, Malaysia-partial oxidation of NG. The syngas production route which among others is used for MegaMethanol R is offered by Lurgi as MegaSyn R and is available for FT syntheses also.

6 76 Serge Kioes et al./ Journal of Natural Gas Chemistry Vol. 13 No A new route to transportation fuels: MtSynfuels R Given the economically highly attractive technologies of MegaMethanol R and MTP R as described above it nearly follows by itself to combine them with an industrially proven process for the conversion of olefins to diesel. A gas-based synfuels plant using this process, then named COD (derived from Conversion of low molecular weight Olefins to Diesel), was developed and built by Lurgi for Mossgas (today: PetroSA), RSA, in 1992 and is performing well since its start-up in Remarkably, the industrial design was based on a scale-up factor of 3600 over the preceding demonstration plant. This basically was possible through the use of fixed-bed catalysis (on zeolite basis) which lends itself to easy scale-up. Other important process features are semi-continuous operation and a 98% conversion of C 3 - and C 4 - olefins. The Lurgi route to synfuels, MtSynfuels R shown in Figure 6 is a combination of this type of process with MegaMethanol R and a simplified MTP R. Extensive engineering and estimating studies have been performed to prove the feasibility and economic viability of this new route. All studies show that MtSynfuels R compares well with FT processes. Investment costs are lower and efficiencies are better than for FT. MtSynfuels R produces on-spec gasoline and diesel at about 23 US /bbl which makes this route attractive at crude oil prices of 21 US /bbl already [6]. Figure 6. Gas refinery via methanol lurgi s MtSynfuels R Admittedly MtSynfuels R lacks full commercialisation, but so do most of the other FT processes discussed currently. In contrast to these, MtSynfuels R is proven in three of four steps with the demo unit for the third step (MTP R ) having confirmed the lab results by a hours test run. 10. From gas to petrochemicals the real career of the humble molecule It has been shown above that propylene produced via MTP R competes well with cracker-derived product. In more general terms it develops that the chain of Lurgi s technologies described here provides an alternative route to petrochemicals. Almost all steps are technically proven and the economic competitiveness mainly depends on the natural gas price. This again follows from market pressures and the need or willingness to monetise gas reserves. Accidentally, the technology chain described here also represents the career of the humble molecule : methane becomes a supplemental basis of the broad field of petrochemistry. Figure 7 shows how the conventional cracker route from crude oil through olefinic and aromatic intermediates to highly valued petrochemical products is complemented -and replaced possibly- by gas-tomethanol-and-others processes. There is even the possibility to use coal (or biomass!) as the primary feedstock for this methanol-to-petrochemicals route, an alternative seriously considered here in China which lacks large oil or gas reserves but has an abundance of coal. 11. Conclusions There are abundant natural gas reserves providing low cost feedstock for methanol production and aiming at better use of natural resources especially in the case of associated gases being flared. DME and propylene produced from methanol will increase the value of natural gas considerably and offer an exciting potential of growth and a high earnings level.

7 Journal of Natural Gas Chemistry Vol. 13 No Figure 7. Gas-based petrochemistry Lurgi s MegaMethanol R technology brings down the net methanol production cost below US 50 per ton, wherever low cost natural gas is available. This opens up a completely new field for downstream products like DME, propylene and synfuels. Based on simple fixed-bed reactor systems, conventional processing elements and operating conditions including commercially manufactured catalysts, Lurgi s MegaDME, MTP R and MtSynfuels R technologies provide attractive ways to monetise natural gas. Alert competition offers alternatives in several cases. Here, the markets will have the last say. Driven by the excellent market prospects and additional environmental aspects, Lurgi has developed its own technology chains starting from natural gas via methanol to DME or propylene and polypropylene, based on the combination of highly efficient concepts at low investment costs. In the next step these concepts lead to gas-based refineries and gasbased petrochemicals. Figure 8 summarises the gas to chemicals routes. With the exception of FT and MTO which are offered as licensed technologies, all others depicted here are Lurgi proprietary technologies a direct result of the high importance Lurgi always attached to gas and syngas conversion. MtPower depicts the utilisation of methanol and DME as energy carriers, made possible by the low production costs associated with the Mega-plants. Figure 8. Gas to chemicals processing routes The many routes our humble molecule CH 4 can wander, making a career of its own

8 78 Serge Kioes et al./ Journal of Natural Gas Chemistry Vol. 13 No Eventually, financial, strategic and political interests will determine the ultimate selection of any gas-to-value technology. The task of the engineering company is to provide as many attractive alternatives as possible to accommodate for all sorts of local conditions. With the technology portfolio described above Lurgi is up to this challenge. References [1] Cedigaz. The 2002 Natural Gas Year in Review, April 2003, [2] Quigley Th M, Fleisch Th H. Technologies for the Gas Economy, EFI Gas to Market Conference, San Francisco, October 11 13, 2000 [3] Streb S, Göhna H. MegaMethanol R paving the way for new down-stream industries, World Methanol Conference, Copenhagen (Denmark), November 8 10, 2000 [4] Rothaemel M, Holtmann H-D. MTP, Methanol To Propylene Lurgi s Way, DGMK-Conference Creating Value from Light Olefins Production and Conversion, Hamburg, October 10 12, 2001 [5] Koempel H, Liebner W. Gas to Liquids? Gas To Chemicals? Gas to Value!, ERTC Petrochemical Conference, Amsterdam, February 20 22, 2002 [6] Rothaemel M, Koempel H, Liebner W. Progress Report on MTP with focus on DME, AIChE Spring National Annual Meeting, New Orleans, April 25 29, 2004, Session: Olefins Production