Para-xylene separation process comprising pretreatment by select

Details Para-xylene separation process comprising pretreatment by select Para-xylene separation process comprising pretreatment by select

1998-01-27

1999-12-21

Griffin, Walter D.

Chemistry of hydrocarbon compounds

Purification, separation, or recovery

By plural serial diverse separations

585319, 585254, 585479, 585258, 585253, 585260, C07C 700

Patent

active

060051566

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention concerns a novel sequence of catalytic processes, purification processes and separation processes for the production of high purity aromatic compounds such as benzene, ortho-xylene and para-xylene.
It also concerns a novel process for the treatment of hydrocarbon cuts which are rich in aromatic compounds, more particularly a process for the elimination of diolefins and the reduction and possible elimination of olefins from mixtures of aromatic compound-rich hydrocarbons using a hydrogenation reaction by means of particular catalysts.
More particularly, it concerns the production of very pure para-xylene, in particular for the synthesis of terephthalic acid which is mainly used in the textile industry.
Mixtures of aromatic compound-rich hydrocarbons are used in petrochemical plants which produce benzene, toluene, ethylbenzene, meta-xylene, ortho-xylene, para-xylene, ethylbenzene and styrene after various separation and purification treatments.
The sources of aromatic compound-rich cuts containing different concentrations of diolefins and olefins are generally: distillation processes for crude hydrocarbons such as crude oil, condensed natural gas, coal; thermal processes such as steam cracking of naphtha; processes dedicated to the production of aromatics from light aliphatic cuts (C.sub.3 -C.sub.5, more particularly C.sub.3 and C.sub.3 /C.sub.4), C.sub.6 and C.sub.6 /C.sub.7 aliphatics, and heavy naphtha cuts (>C.sub.6 for various catalytic reforming processes); and processes for transforming aromatic products, such as processes for trans-alkylation and isomerisation of ortho- and meta-xylenes to para-xylene.
Catalytic reforming is the major process for producing mixtures which are rich in aromatic compounds. At first, catalytic reforming was carried out in two types of facilities, depending on whether it was to be used for refining or petrochemistry. Later on, this distinction, linked to the severity of the operating conditions, has become blurred.
Nowadays, in order to satisfy increased energy constraints, the industry is again seeking more specific processes. They thus use, for refining, catalytic reforming units which operate at high severity but which have greater operational stability and improved spirit yields, and for petrochemistry aromatic production (benzene, toluene and xylenes) is optimised by using a reactor which operates at low pressure.
The use of reformers operating at high severity is accompanied by an increase in the concentration of olefins and diolefins in the reformates. The bromine number of a stabilised reformate can reach a maximum value of over 7000 mg of bromine per 100 g of product for units operating at very low pressure. The presence of these olefins and diolefins is particularly prejudicial to aromatic separation processes. As an example, olefins and diolefins tend to polymerise in the solvents used for extraction. Purification treatments using natural silico aluminates, usually activated (for example attapulgite, bentonites and montmorillonites activated by treatment in the presence of acids) are currently used. Those purification materials are generally termed activated clays.
Purification treatments using activated clay have a number of problems, among them: a short lifetime (generally 4 to 6 months, sometimes as low as one month for feeds containing high concentrations of olefins, >0.6% by weight), low catalytic activity (hourly space velocities in the range 0.5 to 3 volumes of feed/volume of catalyst/hour), and great difficulty in purifying feeds containing more than 1.5% by weight of olefins and diolefins, which limits the operating severity of the reformer. Purification of the feed is accompanied by the production of high molecular weight compounds due to alkylation of the aromatics. These products must then be separated out. They cannot be regenerated economically and are thus eliminated in the waste with their toxic residual aromatic products.
Conventional schemes for the production of aromatic compounds contai

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