DE68916026T2 - Manufacture of middle distillates. - Google Patents

Description

This invention relates to a process for producing middle distillates, in particular distillates with high paraffin content, e.g. jet fuel.

One group of products produced by petroleum refineries in large quantities are middle distillate products, which include materials such as diesel fuel, jet fuel and heating oil. Some of these products may be fuel products that are of relatively higher quality than others and deserve a premium value as fuel products. For example, certain jet fuels of high quality may be traded as premium products with a correspondingly higher price. With increasing demand for these fuels, there is a demand for increasing the pool for these fuel products and for this purpose, various synthetic and semi-synthetic processes have been considered to produce both gasoline and middle distillate products.

One recommended process is the so-called MOGDL process, in which light olefins, particularly olefinic streams from a catalytic cracking unit, are oligomerized to products in the gasoline boiling range and above, using an oligomerization catalyst in the form of a medium pore size zeolite. Processes of this type are described, for example, in U.S. Patent Nos. 3,760,024, 4,211,640, 4,227,992, 4,513,156 and 4,476,330. However, the products from these processes tend to an aromatic character. For this reason, the middle distillate products are generally unsuitable for use as jet fuel or diesel fuel, which require the aromatics content to be maintained at a low level.

Another proposed semi-synthetic process converts low quality paraffinic streams to more valuable products of generally aromatic character, particularly high octane gasoline. These processes are generally characterized as aromatization processes which convert the paraffins present in the feed to aromatic products, usually in the gasoline boiling range. The aromatic character of the products generally renders them unsatisfactory as jet fuel or diesel fuel, although they are good blending components for the gasoline pool. Examples of these processes can be found in U.S. Patent No. 3,729,409, which describes a process for upgrading a paraffinic reformate feedstock by contacting the feed with a ZSM-5 zeolite catalyst, optionally containing a metal component, at relatively high conversion temperatures, typically from 260 to 540°C (500 to 1000°F). US 3,951,781 similarly describes a process for upgrading a distillate gasoline in combination with a solvent extracted residue and a paraffinic material using the catalyst ZSM-5, possibly with a metal component. Other aromatization processes are described in US Patent Nos. 4,590,323, 4,304,657 and the article by Garwood and Chen, ACS 125(1), 84 (1980): "Octane Boosting Potential of Catalytic Processing of Reformate Over Shape Selective Zeolites" describes the conversion of n-octane to benzene. over the ZSM-5 catalyst, producing only trace amounts of C₂- in the product.

It has now been discovered that paraffinic middle distillate products can be produced by disproportionation of low boiling, low molecular weight paraffinic streams using medium pore size zeolite catalysts at relatively low to mild temperatures below 500°F (about 260°C).

The present invention therefore provides a process for producing a middle distillate fuel product typically boiling in the range of 330 to 645°F (about 165 to 345°C) by disproportionating a light C5 to C8 paraffinic stream in the presence of an acidic solid porous catalyst comprising a medium pore size zeolite, optionally with a hydrogenation component in the form of a metal, e.g. platinum. This reaction is normally carried out at moderately elevated temperatures below 500ºF (260ºC), preferably 200 to less than 260ºC (400 to 500ºF), and preferably at elevated pressure, usually 50 to 1000 psig (440 to 7000 kPa) and preferably from 400 to 1000 psig (2460 to 7000 kPa abs.). It has been found that the use of these conditions results in a product which is predominantly paraffinic in character and which is advantageous as a premium quality jet fuel or as other paraffinic distillates including diesel fuel. Accordingly, the present invention provides a way to convert low quality paraffinic streams, e.g. solvent raffinates or C5-C6 distillate gasolines, into premium quality distillate fuels.

The feed used in the process of the invention is a light, low boiling stream which is essentially paraffinic in character, i.e. it does not contain significant amounts of aromatics. Streams of this type may typically comprise C5-C6 distillate gasolines, low boiling solvent raffinates obtained from petrochemical processing, e.g. raffinates from solvent extraction processes such as UDEX extraction or sulfolane extraction, and other similar petroleum refinery streams. The UDEX and sulfolane processes are described in Modern Petroleum Technology, Hobson, G.D. (Ed.), Applied Science Publishers Ltd., Barking, 1973, pp. 409-410, to which reference is made for the description of these processes. Streams of this type may be characterized as C5-C8 streams, although minor amounts of C9+ paraffins may be present, typically in amounts of no more than about 1 wt.%. These streams generally have at least 60 wt.% in the C5-C7 boiling range and usually contain less than 5 wt.% aromatics, typically benzene and toluene. A particular advantage of the process of the invention, however, is that the aromatics content of the feed is significantly reduced during processing so that the final product has an almost entirely paraffinic character, making it very suitable as a jet fuel or diesel fuel.

This feed is contacted with a medium pore size zeolite under conditions of moderately elevated temperature and pressure. This zeolite catalyst may contain a metal hydrogenation component such as platinum or palladium, although this is not essential in this process. When this metal component is present, it is typically present in an amount up to about 1 wt.% based on the catalyst, usually from 0.1 to 0.5 wt.% based on the catalyst. The catalyst may also comprise a binder, e.g. alumina, silica or silica-alumina, to improve mechanical strength.

The essential component of the catalyst is a zeolite with medium pore size, e.g. ZSM-5, ZSM-11, ZSN-22, ZSM-23 or another zeolite with a constraint index of 1 to 12. These described zeolites are known materials. The constraint index can be determined by the method described in U.S. Patent No. 4,016,218, while the importance of the constraint index as an indication of zeolite structure is described in J. Catalysis 67, 218 - 222 (1981).

The process is carried out as above under conditions of moderately elevated temperature and pressure, with temperatures of from 400 to less than 500°F (200 to 260°C) being preferred. A pressure of at least 440 kPa (50 psig) should be used, and a pressure of from 2460 to 7000 kPa (400 to 1000 psig) is preferred. The space velocity is maintained at moderate values, typically from 0.5 to 2 WHSV, and in addition, no hydrogen is added to the feed. At these conditions a significant degree of paraffin disproportionation occurs, and typically C7 paraffins are disproportionated to lower molecular weight C4-C6 paraffins, with concomitant production of C8-C10 paraffins. This type of reaction differs from aromatization-type reactions associated with the above-mentioned known processes, which operate at relatively high temperatures and low pressure. Disproportionation to a product with a higher molecular weight occurs in the boiling range of the middle distillate, and a special feature of this process is that the product obtained by this disproportionation can contain a smaller amount of aromatics than the feed. Product recovery can take place by suitable fractionation of the effluent stream from the disproportionation process.

Claims (6)

1. A process for producing distillate range products by disproportionating a C5-C8 paraffinic feed stream boiling below the distillate boiling range in the presence of an acidic solid porous catalyst comprising a zeolite having a constraint index of 1-12 at a temperature of less than 260°C (500°F) and recovering a distillate range paraffinic product. 2. The process of claim 1 which is conducted at a pressure of 440 to 7000 kPa (50 to 1000 psig). 3. The process of claim 1 which is conducted at a pressure of 2460 to 7000 kPa (400 to 1000 psig). 4. A process according to claim 1, wherein the catalyst comprises a hydrogenation component in the form of platinum or palladium. 5. The process of claim 1 wherein the zeolite is ZSM-5. 6. The process of claim 1 which is conducted at a temperature of 200 to 260ºC (400 to 500ºF).