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by Alexander ROZOVSKY, Dr. Sc. (Chem.), head of laboratory of kinetics, RAS Institute of Petrochemical Synthesis (named after A. Topchiev)
The problem of producing motor fuels from alternative raws, such as natural or by-product gas, coal, etc., has long been attracting the attention of specialists working in this field. Since the available oil resources will be exhausted sooner or later, research labs in various countries have been focusing on ways and means of producing synthetic motor fuel substitutes. In recent time these efforts have been stimulated by the introduction of stringent norms on the levels of car exhausts. The challenging task of producing new synthetic fuel (or additives) which could ensure "clean", or at least low-pollution exhausts, was, and still is just as important as it was hard to translate into reality.
Methane as a chemical raw can be the basis for the production of most of the organic compounds which are produced from oil, including large-scale production of fuels for transport and energy generation. Unfortunately, at the present state of the art the methods of processing of this raw are not enough for the realization of its full potential since the products obtained turn out to be more expensive than their natural petroleum analogues. Research and development of the past few years, however, are likely not only to close this gap, but yield products less expensive than their "natural" analogues.
Natural gas as a chemical compound is a sufficiently inert one. That is why at the first stage of its processing it is turned into a more reactive synthetic gas, or what we call synthes-gas (mixture of carbon oxides and hydrogen) which is then turned into motor fuel by catalytic methods. There are various methods of synthes-gas production, including steam or carbon dioxide conversion and oxidation with air or pure oxygen. The alternative ways of further processing of synthetic gas include the Fischer-Tropsch (indirect hydrogenation) process and methanol synthesis. The former yields a kind of an equivalent of oil - a mixture of carbohydrates which require further processing. The latter provides the basis for commercial production (world output approaching 30 mln t) which is quite common now. Its main drawback is "unfavourable" thermodynamics which interferes with the production of the required compounds of high enough concentrations. This makes it necessary to recycle the gas mixture through the reactor many times over which requires large amounts of electricity and pushes up the cost of gasoline thus obtained from methanol.
Experts from our Institute, working in conjunction with their colleagues from other agencies, have implemented a number of projects which make it possible to boost the efficiency and bring down the costs of the reprocessing of natural gas and other hydrocarbon raws into more "effective" energy carriers. With regard to the first stage of synthetic gas production one of our experts, Yuly Kolbanovsky, Dr. Sc. (Chem.) suggested technical solutions based on the combustion of natural gas in modified diesel or compression engines operating in an unusual regime. The idea was put into practice in 1998 at an industrial unit with the capacity of 10,000 m 3 of synthetic gas per hour.
The two obvious advantages of the proposed new process make it especially attractive for remote regions. First, it can use as the raw low-pressure natural gas, including that emanating from wells not fit for operation in the usual manner. Apart from that the initial raw can be oxydized by ordinary air and the engine can be used simultaneously for both-the chemical reaction and its main function of electricity generation. It should be noted, however, that using air results in a high level of nitrogen in the synthetic gas (50-60 percent) which causes problems with its subsequent processing.
Of considerable interest in this respect is an apparatus developed by Valentin Kubikov, Cand. Sc. (Tech.) in conjunction with his colleagues. This apparatus for the oxydation of natural gas by oxygen-generator of synthetic gas takes into account the experience of jet engines design. The output of this reactor, per unit of its volume (with the installation itself being of a moderate size) is tens and even hundreds of times greater than that of its available industrial analogues. One of its drawbacks, however, consists in the fact that using oxygen in the process involves considerable investments into its generation. On the other hand, the synthetic gas produced, as different from the one in the previous version, does not contain any ballast nitrogen which, naturally, has a positive role to play at the stage of production and especially isolation of the end product - gasoline of dimethyl ether.
And it should be stressed at this point that all of the aforesaid processes are based on the high-temperature oxydation of methane, bringing up the mixture to a composition close to an equivalent one. This not only sharply reduces the working volume of the apparatuses, but produces a "negative feature": the synthetic gas composition becomes a parameter which is hard to control. The most practically feasible appears to be the Н 2 /СО ratio of 1.5-1.6. And although composition corrections are possible, they "spoil" the economic indexes involved.
Our Institute researchers have also studied in detail the processes which occur at the second stage - Fischer-Tropsch synthesis and synthesis of methanol. This led, among other things, to a revision of the commonly accepted notions of the mechanism and regularities of the latter, up to the components of chemical reactions. As has been demonstrated at our lab by Galina Lin, Cand. Sc. (Chem.) and her coworkers the "textbook" reaction СО+2Н 2 =СН 3 ОН does not really take place and the methanol synthesis occurs as a result of the transformation СO 2 +ЗН 2 =СН 3 ОН+Н 2 О. On the basis of that we have worked out what we call the new physico-chemical basis of the process as such and then suggested a technology for obtaining the required product which makes it possible to double the output per unit of the reactor volume.
But still and all, as applies the general scheme of reprocessing of the natural (by- product) gas, the methanol synthesis remains a "weak spot" due to the above thermodynamic restrictions. More preferable, therefore, is the synthesis of dimethyl ether which practically rules out these limitations.
Indeed, in this case methanol is formed in the first place and in accordance with the aforesaid reaction and then it turns into dimethyl ether: 2СН 3 ОН=СН 3 ОСН 3 +Н 2 О. If these reactions occur simultaneously, methanol is constantly evacuated from the system and does not
accumulate in any considerable amounts. In this way it is possible to bypass the undesirable thermodynamic limitations...
As was revealed by the subsequent studies, dimethyl ether (DME) is a fine raw for the synthesis of gasoline superior to methanol. As a result an alternative technique emerged for turning synthetic gas into gasoline wherein the two stages boast higher efficiency than the traditional version. Finally, it was discovered quite recently that DME is a very promising diesel fuel and an excellent alternative to liquid gas for gas-turbine units. In view of the above DME has moved into a prominent position with the scale of its potential uses probably comparable with such common fuels as gasoline and diesel oil.
In ordinary conditions DME is a gas (boiling point is 24.9 0 C) which is easily liquefied under pressure (5 atm at 20 0 C, 8 atm at 38 0 C). What is more, it is non-toxic and can be used in aerosol packages. It rapidly disintegrates in the air and can therefore be used as a replacement of freons which are causing environmental concerns. By its physical properties DME is similar to the traditional propane-butane mixtures. And although its energy content is 1.5 times (per unit of mass) less than that of the conventional diesel fuel, its other parameters are obviously superior. Thus one of its main parameters - cetane rating - is 55 to 60 as compared with 40-55 for the ordinary diesel fuel, and the flash points are respectively 235 and 250 0 C. The presence of oxygen atoms in DME provides for its smokeless combustion and for easy cold starts of engines. The noise level is also reduced, although the main asset of DME as diesel fuel are its low levels of environmental pollution. Its low levels of nitric oxide makes it possible to meet the most stringent international ecological standards of EURO-3 and ULEV without exhaust purification.
In the opinion of experts both at home and abroad the switching of automobile transport to the new fuel should not run into any serious obstacles. Serious problems arise only in connection with the need of developing a corresponding infrastructure because the one we have today (for propane-butane mixtures) can serve the purpose only in part.
At the present time DME is obtained by means of methanol dehydration on aluminum oxide and other catalysts at the level of commercial production of about 150,000 tons/year. But quite recently two companies - Mobil (USA) and Haldor Topsoe (Denmark-started direct synthesis of DME from synthetic gas. Later on a similar innovation (liquid-phase process) was introduced by the NKK of Japan and also at our own Institute with the participation of experts from other agencies (gas-phase process). In the latter version the process occurs at a pressure of 5-10 MPa and is highly effective which is best demonstrated by a comparison with the synthesis of methanol involving similar technology. For instance, while in the first case 60-80 percent of carbon oxides are transformed in the catalytic reactor, in the second case this is only 15-20 percent. Respectively, there is a sharp increase in the output per unit of volume of the reactor which serves to improve all of the technico-economic parameters. The process is so effective, it can be used with a "lean" synthes-gas obtained by the oxydation of natural gas by air and containing 50-60 percent of nitrogen and only 10-15 percent of CO 2 .
As a result, direct synthesis of DME from synthes-gas turns out according to different estimates 5-20 percent more economic than the synthesis of equal amounts of methanol.
The high efficiency of the direct DME synthesis from synthes-gas is accompanied by the release of large amounts of heat and this necessitated a careful reappraisal of the engineering solutions involved. Our Institute researcher Ivan Kubikov and his team have designed and built a special apparatus which provides for an intensive heat removal from the reaction zone.
And what we call the next stage of gasoline production has also been implemented now. A research team headed by G. Lin of our staff, working in conjunction with a team from the RAS Institute of Organic Chemistry headed by E. Mortikov, Dr. Sc. (Chem.) have synthesized high-octane gasoline directly from synthes-gas through DME. And the important thing is that this motor fuel boasts better ecological parameters than ordinary gasoline. With the octane number of 92-93 it contains practically no harmful admixtures (benzene, durol, isodurol) and its low content of non-saturated carbohydrates (1 percent) provides for its good stability.
The findings of our researchers have provided the basis for an industrial pilot unit which is now being prepared for operation.
Summing up the above, it should be pointed out that the obtained results exceed the framework of the main problem under consideration. And this primarily applies to the direct synthesis of DME from synthes-gas. An important feature of this process consists in the fact that it provides for using synthes-gas with a broad range of its compositions; the H 2 CO ratio, which should exceed 2 in the synthesis of methanol, can vary within broad range, including 1:1, in the DME synthesis. Synthes-gas, which can be obtained from coal, timber wastes and such like sources of carbon, has just that or similar composition. And that means that moving ahead along this road it should be possible to launch the production of motor fuels from different raw materials sources, including renewable ones.
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