Mineral oils: processes and products – Chemical conversion of hydrocarbons – Cracking
Reexamination Certificate
2000-11-17
2003-04-08
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Cracking
C208S109000, C208S110000, C208S111010, C208S950000
Reexamination Certificate
active
06544407
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the preparation of middle distillates starting from prevalently paraffinic charges.
More specifically, the present invention relates to a process for the production of middle distillates in a single reactive step comprising a hydrocracking reaction, starting from charges prevalently consisting of mixtures of n-paraffins in which a significant fraction has a boiling point higher than that of middle distillates.
2. Description of the Background
Mixtures of hydrocarbons comprising a relevant fraction with a high boiling point are normally obtained as distillation residue in refining processes of fuels of petroleum origin. More recently, mixtures of hydrocarbons with a high boiling point have also been obtained from certain degradation processes and recycling of polymeric materials. The production of hydrocarbon mixtures, essentially consisting of n-paraffins, in which a significant fraction has a boiling point exceeding 370° C., by means of direct synthesis from mixtures of hydrogen and carbon monoxide (synthesis gas) in so-called Fischer-Tropsch processes, from the name of the inventors of the first synthesis of this type in the thirties', is also well known.
The above mixtures are normally in solid or semi-solid form (waxes) at room temperature and in no case can be used as fuels or lubricants in this form. However, they represent a raw material which is potentially very significant as an energy source and for other uses, and are therefore subjected to various kinds of degradative and/or regradative treatment to improve their characteristics and allow them to be used as fuels. In particular, due also to the present tendency in the fields of automobile and air transport, in which there is an increasing demand for Jet Fuel and high quality gasolines for Diesel cycle engines, the necessity is strongly felt for obtaining, with the least possible number of steps and maximum yield, mixtures with typical properties of so-called “middle distillates”.
The term middle distillates usually refers to a mixture of hydrocarbons with a range of boiling points corresponding to those of “kerosene” and “gas oil” fractions obtained during the atmospheric distillation of petroleum. In this distillation, the boiling point range which defines the “middle distillate” generally ranges from 150 to 370° C. The middle distillate cut consists in turn of: 1) one or more kerosene fractions with a boiling range generally between 150 and 250° C.; 2) one or more gas oil fractions with a boiling range generally between 180 and 370° C.
It is known that hydrocarbon mixtures suitable for producing the above fuel cuts, with a good yield, after fractionation, can be obtained by subjecting a high-boiling mixture of hydrocarbons, normally having a distillation range exceeding 350° C., to a degradative thermal process in the presence of hydrogen. These processes, more commonly defined as “hydrocracking”, are normally carried out in the presence of a bifunctional catalyst, containing a metal with a hydrogenating activity supported on an inorganic solid usually comprising an oxide or a mixture of oxides with acid characteristics.
Hydrocracking catalysts typically comprise metals of groups 6 to 10 of the periodic table of elements (in the form approved by IUPAC and published by “CRC Press Inc.” in 1989, to which reference is continually made hereafter), especially nickel, cobalt, molybdenum , tungsten or noble metals such as palladium or platinum. Whereas the former are more suitable for processing hydrocarbon mixtures with relatively high sulfur contents, the noble metals are more active but are poisoned by the sulfur and require a feeding essentially without its presence.
Carriers normally used for the purpose are various types of zeolites (&bgr;, Y), X-Al
2
O
3
(wherein X can be Cl or F), silico-aluminas, the latter amorphous or with varying degrees of crystallinity or mixtures of crystalline zeolites and amorphous oxides. A very detailed description of the various catalysts, specific characteristics and different hydrocracking processes based on these, is provided, among the many available in literature, in the publication of J. Scherzer and A. J. Gruia “Hydrocracking Science and Technology”, Marcel Dekker, Inc. Publisher (1996).
The availability of high-boiling mixtures or waxes, produced directly, for example, by means of synthesis processes of the Fischer-Tropsch type, although greatly desired (absence of polycondensed aromatic compounds, asphaltenes, sulfur and nitrogen), requires however a particular selection of catalysts and process conditions which makes this alternative possible at costs competitive with the traditional sources of liquid mineral fuels.
In fact, owing to the particular growth mechanism of the hydrocarbon chains during the Fischer-Tropsch reaction, it is not achievable in practice the synthesis of a product with a narrow chain-length range. Regardless of the type of catalyst and operating conditions, the Fischer-Tropsch reaction produces a mixture of products characterized by an extremely wide molecular weight distribution, ranging from methane to normal-paraffin waxes, even containing more than 100 carbon atoms. An appropriate choice of synthesis catalyst and operating conditions allows the type of product to be varied in terms of the relative content of paraffin, olefin and oxygenated compounds in the mixture, and average hydrocarbon chain length. It is consequently possible to obtain different mixtures of hydrocarbons with a composition more or less approaching heavy products, but always with a relatively wide distribution. For example, in the case of the products obtained with catalytic systems of the most recent generation based on cobalt, which tend to produce long-chain hydrocarbons, generally only 40-60% of the hydrocarbon fraction having at least 7 carbon atoms (abbreviation C
7
+
) consists of a middle distillate, whereas the complement to 100% consists of heavier products.
A critical element in the hydrocracking process of products coming from the Fischer-Tropsch synthesis is the reactivity of the molecules with an increase in the paraffinic chain length. To obtain high selectivities to middle distillates, using a “full range” charge (i.e. C
5
+
), it is necessary for the reactivity towards cracking the components of the naphtha, kerosene and gas oil fractions to be much lower than that of the fraction having a boiling point higher than 370° C. (370+° C. fraction). Otherwise, during the hydrocracking reaction, a fraction consisting of middle distillates is converted to gas (C
1
-C
4
) and naphtha (C
5
-C
9
) with a consequent decrease in the selectivity to middle distillates. Another consequence which derives from not respecting the above condition is the considerable increase in the kerosene/gas oil ratio, during the reaction, even at relatively low conversion degrees of the heavy fraction (370+° C.). This is a particularly negative aspect if the yields to gas oil are to be maximized.
In the processes of the known art, there is generally a considerable increase in the kerosene/gas oil ratio and a consistent decrease in the selectivity to middle distillates at high conversion degrees of the 370+° C. fraction. In order to limit these drawbacks, when C
5
+
or 150+° C. charges are processed, it is normal to operate with conversions of the heavy fraction (360+° C.) ranging from 35 to 60%. Alternatively, the hydrocracking reaction is only carried out on 260+° C. or 370+° C. cuts obtained by means of a preliminary fractionation step.
Another critical aspect of hydrocracking processes of Fischer-Tropsch synthesis products is the isomerization degree of the products which, with the same range of boiling points, strongly influences their properties at low temperatures. In this respect, to obtain products which satisfy the specifications for use as fuels for car transport (Diesel) and air transport (Jet Fuel), the percentage of iso-paraff
Calemma Vincenzo
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Pavoni Silvia
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