Aromatisation of paraffins.

The present invention relates to a process for the production of aromatic hydrocarbons from a paraffinic hydrocarbon feedstock containing ethane and at least one C₃-C₆ paraffinic hydrocarbon.

Our prior applications describe processes for converting ethane into aromatics (EP 0050021) and for converting higher paraffinic hydrocarbons into aromatics (GB 1561590) both using a gallium loaded zeolite catalyst.

The present invention describes a method of sequentially converting two types of feedstock in a single reactor system in a specific manner.

Accordingly, the present invention is a process for the conversion of C₂-C₆ paraffinic hydrocarbons to aromatic hydrocarbons over a gallium loaded zeolite at elevated temperature characterised in that the process comprises the steps of: <ul id="ul0001" list-style="none"><li>(a) bringing a feedstock comprising predominantly ethane into contact with a freshly prepared or regenerated gallium loaded zeolite catalyst at a temperature from 550° to 650°C over a period from 10 to 50 hours;</li><li>(b) bringing a paraffinic feedstock comprising predominantly C₃-C₆ hydrocarbons into contact with the residual catalyst from step (a) after contact with ethane at a temperature from 470 to 580°C over a period from 40 to 200 hours;</li><li>(c) regenerating the residual catalyst from step (b) after contact with the C₃-C₆ hydrocarbon; and</li><li>(d) recycling the regenerated catalyst from step (c) to step (a).</li></ul>

The C₃-C₆ paraffinic hydrocarbon feedstock is preferably propane.or a butane.

Steps (a) and (b) are both suitably carried out at a pressure in the range of 1 to 20 bar absolute using a weight hourly space velocity (WHSV) of 0.3 to 8.

The gallium loaded zeolite used in the process is suitably that which contains from 0.1 to 10% by weight of gallium, preferably from 0.2 to 5% by weight of gallium.

The zeolite on to which gallium is loaded to form the catalysts used in the present invention is an aluminosilicate which preferably has a high silica to alumina ratio, i.e. greater than 5:1. Methods of preparing such zeolites are claimed and described in our published European Patent Application Nos. 0024930 and 0030811. Zeolites of the MFI type are most preferred.

The gallium can be loaded on the zeolite from a gallium compound by ion-exchange or impregnation by well known techniques of the art, e.g. as described in our published European Patent Application No. 0050021.

The gallium loaded zeolite may be activated prior to reaction with ethane using the process described for instance in our published European Patent Application No. 0119027.

The sequential reaction is suitably carried out in a pressure swing reactor system as shown in the accompanying schematic drawing. Alternatively a moving bed system may be used whereby fresh catalyst is fed to the first reactor and it gradually deactivates due to deposition of carbon as it passes through a series of reactors, ethane is fed to the first reactor and the C₃-C6 hydrocarbons to any of the subsequent reactors.

Upon reaction with ethane in step (a), the gallium loaded catalyst is deactivated to some extent. This is due to the deposition of carbon in a concentration of e.g. 3 to 10% w/w on the catalyst in step (a). However, in spite of the deactivation of the original catalyst in step (a), the residual catalyst surprisingly retains sufficient activity to allow conversions of the higher C₃-C₆ paraffin hydrocarbons into aromatics at relatively lower temperatures in step (b). Moreover, in step (b) the selectivities to liquid hydrocarbons are substantially the same as those observed with a fresh catalyst.

The catalyst deactivated in step (b) may be regenerated using conventional methods, e.g. by burning off the deactivating carbon deposited thereon using air diluted with an inert gas e.g. nitrogen at elevated temperature.

The process of the present invention is further illustrated schematically in the accompanying drawing and in the following Example.

Example

The gallium loaded zeolite was prepared by a method similar to that described below. The zeolite had a silica to alumina ratio of 33:1.

A sample of an MFI-type zeolite containing ammonium cations (zeolite prepared using ammonia as template according to the general process of our published EP 0030811-A) was impregnated with 20 ml of an 0.04M aqueous solution of Ga (NO₃)₃. This solution was just sufficient to wet the zeolite. The aqueous solvent was then removed by drying at 130°C at subatmospheric pressure.

The gallium impregnated zeolite was then bound in an inert silica matrix and sieved to give catalyst particles which passed through a standard 12 mesh sieve but were retained by a 30 mesh sieve.

This procedure gave a final catalyst containing 0.8% Ga.

Thereafter, the catalyst was modified by steam and hydrogen treatment using a procedure similar to that described in Ep-A-0119027, Example 1(1) (b).

Ethane was passed over the catalyst at 625°C and 4.5 bar absolute with a WHSV of 1. Over a period of 24 hours an average of 37X of the ethane was converted and a 20% wt yield of aromatic liquid was obtained. The catalyst then contained 5.3% wt carbon. A mixture of propane (75% wt) and butane was then passed over the catalyst at 550°C and 6 bar absolute with a WHSV of 2. Over 52 hours an average of 90% of the butane and 56X of the propane was converted. A 36% yield of substantially aromatic liquid was obtained.