Production of para-xylene by the methylation of benzene and/or toluene (Exxonmobil)

PATENT
Production of para-xylene by the methylation of benzene and/or toluene (Exxonmobil)
Publication number US8399727 B2
Application number US 12/899,193
Publication date Mar 19, 2013
Also published as
CN102686541A, US20110092756, WO2011049765A2, WO2011049765A3
Inventors
James R. Lattner
Original Assignee
Exxonmobil Chemical Patents Inc.
Abstract
In a process for the production of para-xylene, an aromatic feedstock comprising toluene and/or benzene is reacted with methanol under alkylation conditions in a reactor in the presence of a fluidized bed of solid catalyst particles to produce a vapor phase effluent comprising para-xylene, water, unreacted toluene and/or benzene and solid catalyst fines. The vapor phase effluent is contacted with a liquid hydrocarbon quench stream under conditions to condense a minor portion of the vapor phase effluent and produce a condensate which contains at least some of the catalyst fines and which is substantially free of an aqueous phase. The condensate containing said catalyst fines is then separated from the remainder of the vapor phase effluent.
FIELD
This invention relates to a process for producing para-xylene by the selective methylation of benzene and/or toluene.
BACKGROUND
Para-xylene is an important starting material for manufacturing terephthalic acid, which is itself a valuable intermediate in the production of synthetic polyester fibers, films, and resins. These polyester materials have many practical, well known uses, such as in fabrics, carpets, and apparel.
One known route for the manufacture of para-xylene is by the methylation of benzene and/or toluene. For example, U.S. Pat. No. 6,504,072 discloses a process for the selective production of para-xylene which comprises reacting toluene with methanol under alkylation conditions in the presence of a catalyst comprising a porous crystalline material having a Diffusion Parameter for 2,2 dimethylbutane of about 0.1-15 sec−1 when measured at a temperature of 120° C. and a 2,2 dimethylbutane pressure of 60 torr (8 kPa) wherein said porous crystalline material has undergone prior treatment with steam at a temperature of at least 950° C. to adjust the Diffusion Parameter of said material to about 0.1-15 sec−1. The reaction can be carried out in a fixed, moving, or fluid catalyst bed.
In addition, U.S. Pat. No. 6,642,426 discloses a process for alkylating an aromatic hydrocarbon reactant, especially toluene, with an alkylating reagent comprising methanol to produce an alkylated aromatic product, comprising: introducing the aromatic hydrocarbon reactant into a reactor system at a first location, wherein the reactor system includes a fluidized bed reaction zone comprising a temperature of 500 to 700° C. and an operating bed density of about 300 to 600 kg/m3, for producing the alkylated aromatic product; introducing a plurality of streams of said alkylating reactant directly into said fluidized bed reaction zone at positions spaced apart in the direction of flow of the aromatic hydrocarbon reactant, at least one of said streams being introduced at a second location downstream from the first location; and recovering the alkylate aromatic product, produced by reaction of the aromatic reactant and the alkylating reagent, from the reactor system.
The reaction of toluene and methanol, particularly using the highly steamed catalyst described in the '072 patent is highly selective to the production of para-xylene. However, in addition to para-xylene the reaction product contains water, as a necessary by-product of the substitution of a hydrogen group of the benzene ring by the methyl group of the methanol. Other side reactions generate small quantities of oxygenated organic species, many of which are organic acids, such as formic acid, acetic acid and alkyl phenols (such as, methyl, dimethyl and ethyl phenols). Thus the effluent from a toluene methylation reaction includes product xylene, unreacted toluene, light gas products, heavier aromatic species and an aqueous vapor phase. Where the reaction is conducted in a fluidized bed, such as disclosed in the '426 patent, the effluent will also contain catalysts fines which are not recovered by the catalyst separation system of the reactor. These catalysts fines mostly end up in the aqueous phase which, by virtue of the presence of the organic acids, has a low pH. Such an environment is conducive to partial dissolution of the catalyst fines rendering them “sticky” and difficult to separate from the water/oil mixture. This solids separation problem is accentuated by the fact that the solids are in low concentration requiring the processing of large volumes of liquid in the solids recovery step (such as, filtration).
There is therefore a need for an effective process for recovering catalyst fines from the reaction effluent of a fluid bed methylation process that avoids the processing of a low pH aqueous phase having a very dilute catalyst solids concentration. According to the invention, this is provided by contacting the reaction effluent vapor with a liquid hydrocarbon quench stream so as to condense a controlled, minor part of the effluent vapor and produce a condensate which contains at least some of the catalyst fines and which is substantially free of an aqueous phase. After concentration of the solids in the condensate, the catalyst fines can be recycled back to the methylation process preferably using a flush stream containing methanol and/or the aromatic reagent of the process, namely benzene or toluene.
SUMMARY
In one aspect, the invention resides in a process for the production of para-xylene, the process comprising:
(a) reacting an aromatic feedstock comprising toluene and/or benzene with methanol under alkylation conditions in a reactor in the presence of a fluidized bed of solid catalyst particles to produce a vapor phase effluent comprising para-xylene, water, unreacted toluene and/or benzene and solid catalyst fines;
(b) contacting the vapor phase effluent with a liquid hydrocarbon quench stream under conditions to condense a minor portion of the vapor phase effluent and produce a condensate which contains at least some of the catalyst fines and which is substantially free of an aqueous phase; and
(c) separating the condensate containing said catalyst fines from the remainder of the vapor phase effluent.
Conveniently, said liquid hydrocarbon quench stream is contacted with said vapor phase effluent in countercurrent flow or alternatively in cocurrent flow.
Conveniently, said contacting condenses less than 10 weight %, such as less than 5 weight %, for example less than 2 weight %, of said vapor phase effluent.
Conveniently, said liquid hydrocarbon quench stream contains less than 1 volume % free water.
In one embodiment, said liquid hydrocarbon quench stream comprises at least part of the condensate separated in (c) and conveniently the process further includes:
(d) cooling the condensate containing said catalyst fines; and
(e) recycling the cooled condensate to the contacting (b).
Preferably, the process still further includes:
(f) removing at least part of said catalyst fines from said condensate prior to said recycling (e).
(g) returning at least part of the catalyst fines removed from said condensate to said reacting (a).
Conveniently, the catalyst fines are returned to said reacting (a) by flushing with a liquid flush stream comprising said aromatic feedstock and/or methanol. Typically, the liquid flush stream contains less than 5 volume %, such as less than 2 volume %, for example 1 volume %, free water.
In one embodiment, the process further comprises:
(h) continuously removing part of said solid catalyst particles from said reactor and feeding said removed solid catalyst particles to a regenerator;
(i) contacting the catalyst particles in the regenerator with an oxygen-containing gas to remove coke thereform and produce a flue gas effluent containing catalyst fines;
(j) continuously returning part of the solid catalyst particles in the regenerator to the reactor; and
(k) purging catalyst fines from said flue gas effluent to control the level of catalyst fines in the reactor and the regenerator.
In one embodiment, the solid catalyst particles comprise a porous crystalline material, typically having a Diffusion Parameter for 2,2 dimethylbutane of about 0.1-15 sec−1 when measured at a temperature of 120° C. and a 2,2 dimethylbutane pressure of 60 torr (8 kPa).
Conveniently, the porous crystalline material comprises an aluminosilicate zeolite, such as ZSM-5 or ZSM-11.
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