engleski

Natural gas as a source of petrochemical products and ecologicaly clean liquid fuels

The natural gas world recoverable reserves are estimated at above 173 × 10^12 Nm3 with an annual production rate of about 2.53 × 10^12 Nm3, where is those of oil about 210 × 10^9 Nm3 and 4 × 10^9 Nm3, respectively. The most reserve of the fossil fuels is coal estimated to about 985 × 10^9 t. Furthermore, tremendous reserve of natural gas in the form of methane-hydrate of about 21 × 10^15 – 1700 × 10^15 Nm3 has been estimated, as well. Moreover, the rate of discovery of new natural gas reserve exceeds the increase in the rate of consumption, and it is anticipated that this trend will extend well in 21st century. Most of the natural gas is currently being used directly as a source of energy, and a part of it, mostly as pure methane is converted to synthetic gas, a mixture of carbon monoxide and hydrogen. From syngas a number of petrochemical products and highly ecologically acceptable liquid fuels, under the conditions of Fischer-Tropsch synthesis (FTS) can be produced. Two general processes producing synthetic gas: by catalytic steam reforming of methane or lower hydrocarbons and by partial oxidation of petroleum residues or coal (water gas). The capital cost of syngas plant based on methane is much lower than others, therefore is preferred process. The main petrochemical intermediate and products from syngas, besides hydrogen and carbon monoxide, are methanol, ammonia and their derivatives. Two other potential large scale petrochemical products from methane via methanol under development, are olefin hydrocarbons as ethylene and propylene by oxidative coupling reaction, so called methanol-to-olefin (MTO) process, and well as direct conversion of methane to aromatics. Conversion of syngas to a range of liquid hydrocarbon as a gasoline, diesel fuel and a vaxy product (gas-to-liquid, GTL) is a chain reaction of carbon monoxide and hydrogen under a FTS conditions. The catalyst, pressure, temperature and the ratio of reactants have significant influence upon the selectivity and product distribution. The catalyst is either iron or cobalt based and reaction is highly exothermic. Two classes of reactor are used, multi-tubular, fixed bed and fluidized bed reactors both ensure essentially isothermal conditions. Since synthetic gas is previously purified by the known end effective methods, all the FTS products are almost completely free of sulfur and nitrogen compounds, aromatics and also lead and rarely metals. Also, the obtained diesel fuel has a very high cetane number. Beside, FTS process the syngas could be converted to gasoline over methanol, so called MTG process using a special type of zeolite catalyst. The hydrocarbons obtained are in the gasoline range. In this paper, the review of current status and development predictions of FTS, GTL, MTG and MTO petrochemical processes and products have been presented. / Pričuve prirodnog plina u svijetu procijenjene su na 173 × 10^12 Nm3, a godišnja je proizvodnja 2, 53 × 10^12 Nm3, dok su naftne pričuve oko 210 × 10^9 Nm3, uz proizvodnju od oko 4, 0 × 10^9 Nm3. Novootkrivene količine prirodnog plina veće su od utrošenih količina, a utvrđene su i ogromne zalihe metan-hidrata, procijenjene na oko 21 × 10^15 – 1700 × 10^15 Nm3. Najveće pričuve fosilnih goriva, pričuve su ugljena od oko 985 × 10^9 t. Prirodni plin najviše se upotrebljava izravno kao energent, a manji dio, u obliku čistog metana, prevodi se u sintezni plin, smjesu ugljikovog monoksida i vodika, od kojega se zatim dobiva veliki broj petrokemijskih proizvoda kao i kapljevitih, ekološki čistih goriva, prvenstveno benzina i dieselskoga goriva. Ta goriva mogu se proizvoditi procesima temeljenim na Fischer-Tropschovoj sintezi (FTS), pretežito dieselska goriva (engl. gas-to-liquid, GTL), ili pretvorbom sinteznog plina prvo u metanol, a njegovom dehidratacijom u prisutnosti posebnih vrsta djelotvornih zeolitnih katalizatora izravno prevodi u benzinske ugljikovodike (engl. methanol-to-gasoline, MTG). Sintezni plin se proizvodi dvama temeljnim postupcima: parnim reformiranjem metana ili nižih ugljikovodika, i procesom djelomične oksidacije naftnih destilacijskih ostataka ili ugljena (vodeni plin). Cijena postrojenja proizvodnje sinteznog plina od prirodnog plina, značajno je niža od cijena drugih izvora, pa je taj proces i najzastupljeniji. Najznačajniji petrokemijski proizvodi dobiveni od sinteznog plina, osim CO i H2, su metanol i amonijak i njihovi brojni međuproizvodi i proizvodi. U posljednje vrijeme u razvitku su i procesi izravne pretvorbe metana u metanol kao i u olefinske i aromatske ugljikovodike (engl. methane-to-olefine, MTO). Fischer-Tropshova sinteza vrlo je egzotermna lančana reakcija smjese CO + 2 H2 uz nastajanje viših ugljikovodika. Iscrpak, selektivnost i raspodjela proizvoda, osim o procesnim uvjetima značajno ovise o katalitičkom sustavu, a najznačajniji su na temelju željeza ili kobalta. Dvije su izvedbe FTS procesa kojima se održavaju potrebni izotermni uvjeti: proces u cijevnom reaktoru i nepokretnom sloju katalizatora, i proces u vrtložnom katalitičkom sloju. Sintezni plin prije uporabe se obrađuje uklanjanjem svih nečistoća i zato FTS proizvodi ne sadrže sumporove i dušikove spjeve, kao ni aromatske ugljikovodike, olovne i spojeve drugih teških metala. Dobiveno dieselsko gorivo ima vrlo visoke vrijednosti cetanskoga broja. Procesom MTG, sintezni plin se prvo prevodi u metanol, a zatim u prisutnosti posebnih vrsta djelotvornih zeolitnih katalizatora, dehidratacijom prevodi u benzinske ugljikovodike. U ovom preglednom radu, opisani su petrokemijski proizvodi na temelju prirodnog plina, kao i GTL, MTG i MTO procesi proizvodnje ekološki čistih kapljevitih goriva, prvenstveno benzina i dieselskog goriva, te njihov predvidivi razvitak u 21. stoljeću.

natural gas; methane; Fischer-Tropsch synthesis; synthetic gasoline; synthetic diesel fuel

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