METHOD FOR OBTAINING HYDROCARBONS FROM LOWER ALCOHOLS

The invention relates to a method for the production of energy from lower alcohols, particularly methanol, wherein such alcohols are obtained from synthesis gas by means of a process comprising the gasification-pyrolysis of wet crushed coal. According to the method of the invention, the aforementioned alcohols are subject to a catalytic dehydration using a zeolite catalyst as an acid catalyst, giving rise to olefins which in turn, using the same catalyst as a molecular sieve, give rise to highly branched paraffins and cyclic and aromatic compounds, by means of hydrogenation, alkylation and isomerisation, using the hydrogen present in the above-mentioned synthesis gas.

Thus, the object of the present invention is to provide a procedure for the production of energy from lower alcohols, particularly methanol, wherein such alcohols are derived from a synthesis gas obtained through a process involving the gasification-pyrolysis of coal, which allows the conversion of such alcohols into olefins, after their dehydration with zeolites, such olefins being in turn transformed into highly branched paraffins and cyclical and aromatic compounds.

To that effect, the Methanol obtained from the synthesis gas, as it has been previously explained, is passed trough a zeolitic catalyst at a temperature ranging from 340° C. to 375° C., and this temperature is reached through heating with circulating water obtained from the gasification-pyrolysis device.

The catalytic properties of zeolites is the direct consequence of their high surface area and types of active centres. The dehydration, or elimination of water from an alcohol molecule leads to the formation of alkenes or olefins. This elimination reaction requires an acid catalyst, which is used to protonate the hydroxyl group and convert it into a proper leaving group through the formation of a carbonyl ion and, consequently, the reactivity depends on the easiness to form such ion. In some cases, a protonated alcohol may be attacked by another alcohol molecule. This reaction occurs when the dehydration takes place in primary non impaired alcohols, and the result of the process is the formation of water and one ether.

Generally speaking, the acidity of a zeolite depends on the Al atoms present at its crystalline network. However, not all the acid centres of zeolite show the same degree of activity and, therefore, not all of them are able to catalyse these dehydrogenation reactions. For that purpose, the zeolitic catalyst used in the process according to the invention may be optionally activated, preferably with ammonium or nitric acid. For instance, the values of the conversion percentages for 1-pentanol reveal that the activated zeolite is an excellent catalyst for the dehydration of linear alcohols, since they show a high degree of activity in the conversion of 1-pentanol, without a specific order of catalyst activity, and the conversion ranges between 99 and 100%. However, it changes in the case of the conversion of Isopropyl alcohol (branched alcohol). The most probable explanation for this behaviour is reflected on the CIC, since CIC decreases when zeolite is activated with acid and, therefore, the number of cations present at the structure and available for their exchange gradually decreases. This implies that, since there are less exchangeable cations, the space between them must be larger, and the final result is the presence of larger pores, close to the meso-region. The opposite procedure (activation with ammonium nitrate) entails a greater CIC (a larger number of exchangeable ions) and a greater number of cations is incorporated into the structure of zeolite, and the pores of the structure are smaller, located in the micro- region, which entails a limitation for this catalyst prepared to be used in reactions involving a voluminous substrate. The products obtained in the different reactions were 1-pentene and isopropene, respectively.

On the other hand, the alkenes obtained in these alcohol dehydration reactions are converted into the relevant alkanes through hydrogenation, wherein the required hydrogen is derived from the synthesis gas obtained during the gasification-pyrolysis process, and in turn, these alkanes are converted into other branched alkanes with a higher octane rating by means of isomerization and reforming processes, through the relevant hydrogenation, isomerization and reforming processes carried out within the relevant reactors.

Thus, as it has been previously mentioned, and according to the method of the invention, the methanol obtained from the synthesis gas derived from a coal gasification-pyrolysis process is passed through an optionally activated zeolitic catalyst, at a temperature ranging from 340° C. and 375° C., such temperature being reached by heating it with the circulating steam obtained from the gasification-pyrolysis device.

At this point the dehydration of the alcohol occurs through the active acid centres of the zeolite catalyst, and the result will be a mixture of olefins which, in turn, are partly converted into paraffins by the same catalyst, by means of an alkylation process, and the catalyst operates as a molecular sieve which allows to separate the molecules obtained on the basis of their pore size.

The mixture which now comprises different molecular species is passed through a fractional distillation column to separate them, resulting in alkane fractions of a linear and branched type, as well as aromatic compounds and, to a lesser extent, residual alkenes.

The method according to the invention allows to eliminate the main disadvantage of the reaction of Methanol with hydrocarbons, namely, the deactivation of the zeolitic catalysts through the formation of deactivating coke inside the pores of the catalyst, since the processing temperature and the recirculation of the residual hydrogen and water towards the gasification-pyrolysis reaction do not allow the deposition of C, neither on the surface nor on the pores of the catalyst.