Multi-step hydrodesulfurization process

Details Multi-step hydrodesulfurization process Multi-step hydrodesulfurization process

1997-05-23

1999-10-19

Griffin, Walter D.

Mineral oils: processes and products

Refining

Sulfur removal

208210, 208211, 208212, 208216R, 208217, 208222, C10G 1702, C10G 4504

Patent

active

059683473

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to a process for hydrotreatment of a hydrocarbon feedstock.


BACKGROUND TO THE INVENTION

Crude oils, their straight-run and cracked fractions and other petroleum products contain sulphur in varying amounts, depending upon the source of the crude oil and any subsequent treatment that it may have undergone. Besides elemental sulphur, numerous sulphur compounds have been identified in crude oil including hydrogen sulphide (H.sub.2 S), C.sub.1 to C.sub.5 primary alkyl mercaptans, C.sub.3 to C.sub.8 secondary alkyl mercaptans, C.sub.4 to C.sub.6 tertiary alkyl mercaptans, cyclic mercaptans (such as cyclopentane thiol, cyclohexane thiol and cis-2-methylcyclopentane thiol), open chain sulphides of the formula R--S--R' where R and R' represent C.sub.1 to C.sub.4 alkyl groups, mono-, bi- and tri-cyclic sulphides, thiophene, alkyl substituted thiophenes, condensed thiophenes (such as benzo(b)thiophene, isothionaphthene, dibenzothiophene, and benzo(b)naphtho(2,1-d)thiophene), thienothiophenes, alkyl cycloalkyl sulphides, alkyl aryl sulphides, 1-thiaindans, aromatic thiols (such as thiophenol), and cyclic thiols such as cyclohexane thiol.
Most of the sulphur compounds that have been positively identified as components of crude oil boil below about 200.degree. C. Many other sulphur compounds of high molecular weight and high boiling point remain unidentified in crude oil.
For a variety of reasons it is necessary to treat crude oil and petroleum fractions derived therefrom to reduce or remove the sulphur components present therein. Otherwise subsequent processing may be hindered, for example because the sulphur components may adversely affect the performance of a catalyst. If the hydrocarbon fraction is intended for fuel use, then burning of the fuel will result in any sulphur components present therein being converted to sulphur oxides which are environmentally damaging. Moreover the level of particulates in the combustion products of liquid fuels is linked to the sulphur content of the fuel.
For these reasons it is necessary to remove as far as possible the sulphur content from hydrocarbon fractions derived from crude oil, such as gasoline fractions, diesel fuel, gas oils and the like. Typically such sulphur removal is carried out by a process known generally as hydrodesulphurisation. In such a process the hydrocarbon fraction is admixed with hydrogen and passed over a hydrodesulphurisation catalyst under appropriate temperature and pressure conditions. In such a process the aim is to If rupture the carbon-sulphur bonds present in the feedstock and to saturate with hydrogen the resulting free valencies or olefinic double bonds formed in such a cleavage step. In this process the aim is to convert as much as possible of the organic sulphur content to hydrocarbons and to H.sub.2 S.
Generally the cyclic sulphur-containing compounds are harder to hydrogenate than the open chain compounds and, within the class of cyclic sulphur-containing compounds, the greater the number of rings that are present the greater is the difficulty in cleaving the carbon-sulphur bonds. The presence of alkyl or other substituent groups on the basic ring system can further reduce the reactivity of the organic sulphur compounds towards hydrodesulphurisation.
The specification for Swedish Class I diesel fuel, which is required for use in urban areas, stipulates a maximum aromatics content of 5 volume % and a maximum sulphur content of 10 ppm, while the corresponding specification for Class II diesel fuel limits the aromatics to 20 volume % and sulphur to a maximum of 50 ppm; Class III diesel fuel has a 25 volume % limit on aromatics and a maximum of 500 ppm of sulphur.
When a hydrocarbon feedstock is treated with hydrogen in the presence of a suitable catalyst with the aim of effecting hydrodesulphurisation, other reactions may also occur. Hence hydrotreatment is often used as a more general term to embrace not only the hydrodesulphurisation reactions but also the other reactions including hydrocracki

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