Removable of polar contaminants from aromatic feedstocks

Details Removable of polar contaminants from aromatic feedstocks Removable of polar contaminants from aromatic feedstocks

2000-08-16

2003-09-09

Griffin, Walter D. (Department: 1764)

Chemistry of hydrocarbon compounds

Aromatic compound synthesis

By condensation of entire molecules or entire hydrocarbyl...

C585S467000, C585S820000, C585S823000, C585S824000

Reexamination Certificate

active

06617482

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a process for removing polar contaminants from aromatic feedstocks and, in particular, to an aromatic alkylation process employing pretreatment of the aromatic feed to remove nitrogen contaminants.
BACKGROUND OF THE INVENTION
In a typical aromatic alkylation process, an aromatic compound is reacted with an alkylating agent, such as an olefin, in the presence of acid catalyst. For example, benzene can be reacted with ethylene or propylene to produce ethylbenzene or cumene, both of which are important intermediates in the chemical industry. In the past, commercial aromatic alkylation processes normally used AlCl
3
or BF
3
as the acid catalyst, but more recently these materials have been replaced by molecular sieve catalysts. Thus, it is known from U.S. Pat. No. 4,891,458 to employ a zeolite beta catalyst in the alkylation of aromatic compounds with C
2
to C
4
olefins. In addition, it is known from U.S. Pat. No. 4,992,606 to employ MCM-22 in the alkylation of aromatic compounds with short chain (namely having 1-5 carbon atoms) alkylating agents.
Aromatics alkylation processes employing molecular sieve catalysts can be conducted in either the vapor phase or the liquid phase. However, in view of the improved selectivity and decreased capital and operating costs associated with liquid phase operation, most commercial alkylation processes now operate under at least partial liquid phase conditions. Unfortunately, one disadvantage of operating under liquid phase conditions is that the molecular sieve catalysts tend to be more sensitive to the presence of impurities in the feedstocks, particularly. polar compounds such as nitrogen compounds. Such impurities reduce the acid activity of the catalyst and hence decrease the cycle time between required regenerations of the catalyst.
The use of guard beds to remove trace contaminants from hydrocarbon feed streams is well known in the art. This is especially true for petrochemical and specialty chemical operations where product purity is critical. Normally, materials like bentonite clay, kaolin clay or special activated aluminas are used and are placed upstream of a catalyst-containing reaction vessel. The clay or alumina materials trap impurities in the feedstocks so that product purity specifications can be met and poisoning of the catalyst can be reduced. However, known clay and alumina guard beds have limited ability to reduce the basic nitrogen impurities in aromatic feedstreams to the low levels required for use in liquid phase alkylation processes. Moreover, clays are generally not susceptible to regeneration and hence must be discarded when their adsorption capacity is reached.
U.S. Pat. Nos. 5,744,686 and 5,942,650 describe processes for the removal of nitrogen compounds from an aromatic hydrocarbon stream by contacting the hydrocarbon stream with a selective adsorbent comprising a non-acidic molecular sieve having a silica to alumina molar ratio in excess of 100 and an average diameter less than 5.5 Angstroms. In both cases the selective adsorbent is a molecular sieve selected from the group consisting of pore-closed zeolite 4A, zeolite 4A, silicalite, F-silicalite, ZSM-5, and mixtures thereof.
PCT Publication No. WO 98/07673 discloses a process for preparing an alkylated benzene or mixture of alkylated benzenes in which the benzene feedstock is initially contacted with a solid acid in a pretreatment zone at a temperature greater than about 130° C. and less than about 300° C. The solid acid employed may be an aluminosilicate selected from ZSM-5, ZSM-11, ZSM-35, clinoptilolite, ferrierite, stilbite, EU-1, NU-87, mordenite, zeolite omega, zeolite beta, faujasites, gmelinite, ZSM-12, cancrinite, zeolite L, MCM-22, MCM-41, MCM-49, MCM-56 and MCM-58. The pretreated benzene is then contacted with an alkylating agent in an alkylation zone or with a transalkylating agent in a transalkylation zone in the presence of an alkylation/transalkylation catalyst which may be selected from mordenite, zeolite beta, ZSM-5, ZSM-12, zeolite Y, zeolite omega, EU-1, NU-87, zeolite L, MCM-22, SSZ-25, MCM-36, MCM-49, MCM-56, MCM-58, and a porous crystalline magnesium silicate.
In accordance with the present invention, it has now been found that molecular sieves having pores and/or surface cavities with a cross-sectional size greater than 5.6 Angstroms are more effective for removing nitrogen contaminants from aromatic feedstocks than the small pore materials described in U.S. Pat. Nos. 5,744,686 and 5,942,650. Moreover, it has been found that the removal can be effected at a temperature less than the 130° C. minimum taught in PCT Publication No. WO 98/07673 thereby reducing the formation of by-product hydrocarbon species and hence in potential yield losses, as well as reducing the construction and operating cost of the purification system.
SUMMARY OF THE INVENTION
In one aspect, the invention resides in a process for removing polar compounds from an aromatic feedstock containing polar compounds, said process comprising the steps of:
(a) contacting said feedstock in an adsorption zone with an adsorbent selective for the adsorption of said polar compounds, said adsorbent comprising a molecular sieve having pores and/or surface cavities with cross-sectional dimensions greater than 5.6 Angstroms and said adsorption zone being at a temperature of less than or equal to 130° C.; and
(b) withdrawing from said adsorption zone a treated feedstock which is substantially free of said polar compounds.
The term “substantially free of said polar compounds” is used herein to mean that the treated feedstock contains less than 0.1 ppm, and more preferably less than 0.03 ppm, of said polar compounds.
Preferably, the molecular sieve has pores with cross-sectional dimensions greater than 5.6 Angstroms. More preferably, the molecular sieve is selected from the group consisting of zeolite X, zeolite Y, Ultrastable Y (USY), ZSM-12, mordenite, zeolite beta, zeolite L, and zeolite omega.
Alternatively, the molecular sieve has surface cavities with a cross-sectional size greater than 5.6 Angstroms. More preferably, the molecular sieve is selected from the group consisting of MCM-22, MCM-49 and MCM-56.
Preferably, the molecular sieve has a silica to alumina molar ratio less than 100.
Preferably, said temperature is 20 to 125° C. and most preferably is 25 to 110° C.
In a further aspect, the invention comprises an aromatic alkylation process comprising the steps of:
(a) contacting an aromatic feedstock containing polar compounds in an adsorption zone with an adsorbent selective for the adsorption of said polar compounds, said adsorbent comprising a molecular sieve having pores and/or surface cavities with a cross-sectional dimensions greater than 5.6 Angstroms and said adsorption zone being at a temperature of less than or equal to 130°;
(b) withdrawing from said adsorption zone a treated feedstock which is substantially free of said polar compounds; and
(c) passing said treated feedstock to an alkylation zone containing an alkylation catalyst comprising a molecular sieve and contacting said treated feedstock with an alkylating agent in said alkylation zone under liquid phase alkylation conditions so as to produce an alkylated aromatic compound.
Preferably, the alkylation catalyst comprises a molecular sieve selected from the group consisting of zeolite beta, MCM-22, MCM-49 and MCM-56.
DETAILED DESCRIPTION OF THE INVENTION
This invention relates to a process for removing polar contaminants from an aromatic feedstock. More particularly, this invention relates to a liquid phase aromatics alkylation process which includes subjecting the aromatic feedstock to a pretreatement step for the selective removal of polar contaminants that poison aromatic alkylation catalysts. Such contaminants include nitrogen, sulfur, and oxygen containing compounds, particularly those that boil in the same ranges as benzene, toluene or xylenes. Especially problematic are basic nitrogen compounds, such as pyridine, quinoline, N-formyl-morpholine and N-methyl-pyr

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