CA2215539A1

CA2215539A1 - Catalyst and method for catalytic off-gas cleaning in the dmt process

Info

Publication number: CA2215539A1
Application number: CA002215539A
Authority: CA
Inventors: Dietrich Maschmeyer; Ulrich Neutzler; Reinhard Sigg
Original assignee: Huels AG
Current assignee: Huels AG
Priority date: 1996-09-17
Filing date: 1997-09-15
Publication date: 1998-03-17
Also published as: HUP9701546A2; HU9701546D0; DE19637792A1; MX9706920A; KR19980024648A; BG101555A; TR199700969A2; HUP9701546A3; EP0829295A2; CZ287997A3; PL321885A1; JPH10137586A; CN1176847A; BG62558B1; TR199700969A3; EP0829295A3

Abstract

Disclosed are a catalyst for cleaning a pressurized off-gas obtained after the oxidation of para-xylene with air in the liquid phase in the preparation of dimethyl terephthalate, which comprises:
a) at least one oxide of titanium, and b) at least one elements form subgroup VIII of the Periodic Table of the Elements in the metal or oxide form, a process for preparing the catalyst by applying the component b) to a shaped article which comprises at least the component a), individually or in a mixture by dip impregnation or spray impregnation, and a method of cleaning a pressurized off-gas obtained after the oxidation of para-xylene, in which the pressurized oxygen-containing off-gas from the oxidation is led first via a single-stage or multistage condensation, a single-stage or multistage absorption and subsequently, with or without the supply of oxygen, via a catalytic afterburner operated under pressure employing the catalyst.

Description

CA 0221~39 1997-09-1~

Catalyst and method for catalytic off-qas cle~n;nq in the DMT process The invention relates to a catalyst for cleaning a pressurized off-gas obtained after the oxidation of para-xylene (p-X) with air in the liquid phase in the preparation of dimethyl terephthalate (DMT), to a process for preparing such a catalyst, and to a method of cleaning the pressurized off-gas obtained after the oxidation of para-xylene (p-X) where the pressurized, oxygen-containing off-gas from the oxidation is guided first of all via a single-stage or multistage condensation, a single-stage or multistage absorption and subsequently, with or without the supply of oxygen, via a catalytic afterburner which is operated under pressure.
It is known that the current Witten DMT process essentially comprises the steps of:
(1) oxidation of para-xylene (p-X) and methyl para-toluate (p-TE), generally with downstream off-gas cleaning, (2) esterification with methanol of the reaction products from the oxidation, (3) separation of the resulting so-called crude ester (DMT, crude)into a p-TE-rich fraction, which is customarily recycled to the oxidation, a crude DMT
fraction, which generally contains more than 85~ by weight of DMT, and a high-boiling residue fraction, workup thereof, when required, for example by a downstream methanolysis or thermolysis, and subsequent recovery of the catalyst, and o.z. 5104 CA 0221~39 1997-09-1~

(4) purification of the crude DMT fraction, for example by washing, recrystallization and distillation ("Terephthalsauredimethylester", Ullmann Vol. 22, 4th edition, pp. 529-533; EP 0 464 046 B1; DE-A 40 26 733). It is also known that terephthalic acid of appropriate quality can be prepared from DMT, i.e. from particularly DMT-rich fractions or ultrapure DMT, by specific hydrolysis.
A mixture of para-xylene (p-X) and methyl para-toluate (p-TE or p-T ester) is oxidized generally with atmospheric oxygen in the presence of a heavy metal catalyst (DE-C 20 10 137) at a temperature from about 140 to 180~C and at a pressure of from about 4 bar to 8 bar absolute in the liquid phase. The oxidation stage produces a reaction mixture which predominantly contains monomethyl terephthalate (MMT), p-toluic acid (p-TA) and terephthalic acid (TA) dissolved or suspended in p-TE and is esterified with methanol, at a temperature from about 250 to 280~C and at a pressure of from 20 to 25 bar absolute. In addition, the oxidation produces an off-gas which depending on the pressure and temperature is substantially saturated with aliphatic and also aromatic compounds. Thus the off-gas contains not only target products but also reaction by-products, including the low-boiling compounds acetaldehyde, formaldehyde and the corresponding methyl acetals, dimethyl ether, acetic acid and formic acid and their methyl esters. Apart from these organic constit-uents the off-gas from the oxidation essentially comprises atmospheric nitrogen, a residual oxygen content of from about 0.5 to about 4~ by weight, CO2 at from about 1 to 3~ by weight, and from about 0.3 to 2.0~ by weight of CO. In the O.Z. 5104 CA 0221~39 1997-09-1~

Witten DMT process the off-gas is usually first of all subjected to multistage cooling, in the course of which the relatively high-boiling and middle-boiling target products gradually condense out. The target products that have remained in the off-gas, predominately methanol and p-X, are subsequently removed in all but traces from the off-gas in a multistage absorption procedure, the target products that have accumulated in the absorbents being recycled to the process.
In numerous countries there are statutory regulations requiring the removal of organic carbon compounds and CO from off-gases and expelled air.
EP 0 544 729 Bl discloses a process for cleaning an oxidation off-gas originating from atmospheric oxidation of xylene and under a pressure of from 5 to 50 bar, wherein at least the xylene is first of all substantially removed in an off-gas scrubber under pressure by absorption with an ester, for example methyl para-toluate (p-TE) or methyl benzoate (BME) or an ester mixture, for example of p-TE and BME. It is also possible for the absorption procedure to be preceded by a condensation stage. In addition, it is intended that oxidiz-able substances still present in the off-gas after the absorptive cleaning should be incinerated under pressure and that the pressurized off-gas should be utilized in an expansion turbine for producing energy. The incinerators required for such incineration of off-gases under pressure are complex and expensive both to procure and to operate.
Furthermore, appropriate pressure incineration chambers are now almost impossible to obtain on the market.
In this context EP 0 544 726 Bl mentions the O.Z. 5104 CA 022l~39 l997-09-l~

possibility of subjecting the oxidizable constituents in the off-gas to catalytic incineration under pressure, where the off-gas to be incinerated, following the absorption scrubber, is generally in a state in which it is saturated with steam.
The VDI [German Engineers Association] guideline 3476, "Katalytische Verfahren der Abgasreinigung" [Catalytic methods of waste-gas purification], VDI-Handbuch Reinhaltung der Luft, Volume 6 (June 1990), describes inter alia the removal of CO, hydrocarbons and NOx from car exhaust gases using Pt/Rh/Pd on ceramic supports at temperatures from 300 to 950~C. Support materials employed for catalysts for off-gas cleaning include metals in the form of shaped sheets (expanded mesh), weaves, nets, moldings of metal oxide, such as A1203, SiO2, TiO2, ZrO2, MgO, and also natural and synthetic minerals, such as pumice, mullite, cordierite, steatite or zeolites. Mention is also made here, quite generally, for the removal of CO and vapors of organic compounds from industrial waste gases, of noble metal catalysts or metal oxide catalysts on ceramic supports, supported catalysts of high surface area, or unsupported catalysts.
EP 0 664 148 Al provides a process for cleaning a pressurized off-gas by catalytic afterburner at an operating temperature of between 250 and 800~C and at a pressure of between 2 and 20 bar, with the use in particular of a catalyst comprising platinum and/or palladium on ~-aluminum oxide.
Experiments show that under the operating conditions in an off-gas from the oxidation of para-xylene in the preparation of DMT such a catalyst is deactivated after only a short operating period and that the required reduction in pollutants O. Z . 5104 CA 0221~39 1997-09-1 is thus no longer achieved.
A major object of the invention, therefore, is to provide a catalyst system which satisfies the requirements for the cleaning of the off-gas produced in the oxidation in the course of the preparation of DMT . An additional'concern of the present invention is to tie such an off-gas cleaning measure into the DMT process from an economic standpoint as well. The invention also applies to the cleaning of an off-gas from the oxidation of other alkyl aryls, for example o-xylene, m-xylene, toluene, and xylene and toluene derivatives.
It has now been found that a catalyst which comprises at least one oxide of titanium and at least one element from subgroup VIII of the Periodic Table of the Elements in the metal or oxide form is outstandingly suitable for the method of cleaning a pressurized off-gas originating from the oxidation of para-xylene (p-X) with air in the liquid phase in the preparation of dimethyl terephthalate (DMT), since even after a comparatively long operating period no notable deactivation was found, the required off-gas values were achieved, and thus the catalyst is also notable for an excellent economic service life.
The present invention therefore provides a catalyst for cleaning a pressurized off-gas obtained after the oxidation of para-xylene (p-X) or a mixture of para-xylene with methyl para-toluate (p-TE) with air in the liquid phase in the preparation of dimethyl terephthalate (DMT), wherein the catalyst comprises:
a) at least one oxide of titanium, and b) at least one element from subgroup VIII of the O.Z. 5104 CA 0221~39 1997-09-1~

Periodic Table of the Elements in the metal or oxide form.
The novel catalyst preferably contains, based in each case on the weight of the catalyst:
the component a) in an amount of from 50 to 99~ by weight, more preferably 80 to 99~ by weight, and the component b) in an amount of from 0.01 to 5~ by weight, where the component b) is calculated in total and as metal.
The titanium oxide or a precursor compound for component a) preferably originates from the so-called sulfate process, so that the novel catalyst preferably contains as additional component c), a sulfate. The component c) is possibly present, in particular, in an amount of from 0.1 to 10~ by weight, based on the weight of the catalyst.
The sulfate as the component c) can be present in the novel catalyst as such or else, for example, as hydrogen sulfate or oxide sulfate or water-containing oxide sulfate of a corresponding metal or in the form of sulfuric acid which is adhering to or has formed an adduct with a metal oxide compound. The sulfate can also be present in two or more of the abovementioned forms alongside one another.
In the novel catalyst, the component a) is preferably present as titanium dioxide, for example as rutile.
More preferably, however, in the novel catalyst, the component a) is predominantly in the anatase form. It is also possible, however, for titanium oxide with an oxygen deficit or water-containing oxide, oxide hydroxide or hydroxide or sulfate to be present, including hydrogen sulfate, for example oxide O.Z. 5104 CA 0221~39 1997-09-1~

sulfate or water-containing oxide sulfate of titanium, or titanium oxide with sulfuric acid in adduct form or adhering.
It is also possible for a proportion of the component b) to be present in the novel catalyst in a sulfatic form.
Preferably, the sulfate c) is present in the form of sulfate of barium, tungsten, vanadium, zirconium or a mixture thereof.
As additional components, the novel catalyst may optionally include an oxide of tungsten, an oxide of vanadium, an oxide of zirconium, or a corresponding phosphate, including a hydrogenphosphate, an oxide of silicon or a silicate.
The structure given to the novel catalyst is not critical and, may be any practical form such as spherical, strand-like, tubular, annular or else like a saddle or honeycomb. The novel catalyst preferably in general has a geometric surface area of from around 200 to 2,000 m2/m3 and is preferably a strand extrudate, for example with a diameter of 4 mm. The novel catalyst may also have a honeycomb structure, especially such a structure in which the hydraulic diameter (4a/U=dh where a = inside channel cross-sectional area, U = circumference of the inside channel cross-sectional area and dh = hydraulic diameter) of the individual channel cross-sections of the flow-traversed honeycomb channels is in the range from 1.5 mm to 6.5 mm.
The present invention also provides a process for preparing the above-mentioned catalyst, which comprises applying the component b) to a shaped support which is formed essentially of the component a), individually or in a mixture by means of a dip impregnation or spray impregnation.

O.Z. 5104 CA 0221~39 1997-09-1~

A process for preparing a shaped support based on titanium oxide, also referred to as a molding, is disclosed for example, in DE-C 26 58 569. The novel catalyst is generally prepared by using a molding based on titanium oxide which may be obtained inter alia by the preparation and extrusion of a moldable composition containing titanium oxide and subsequent drying and calcining of the molding, and preferably has a BET surface area of between 10 and 200 m2/g and a pore value of between 0.1 and 0.6 cm3/g. To improve the mechanical properties, the molding may be reinforced by means of glass fibers, for example.
In general, the component b) of the novel catalyst is applied to the support by impregnation. To this effect, the molding is generally brought into contact with a solution which contains the component b), preferably in the form of a dissolved salt. In the novel process, the molding may be impregnated using a solution prepared preferably by employing a nitrate of the component b). In preparing the solution, it may be necessary to adjust the pH of the solution in an appropriate manner, for example by adding an organic or inor-ganic acid, for example nitric acid, or an alkali, or to add a complexing agent or stabilizer, for example for stabilizing a noble metal sol. The impregnation can be effected by spraying the molding one or more times with the solution or by dipping the molding one or more times into the solution.
The impregnated molding is dried, preferably with the ingress of air, in a temperature range of from 30 up to 650~C and is subsequently calcined, i.e. subjected to a thermal aftertreatment.

O.Z. 5104 CA 0221~39 1997-09-1~

When a nitrate is employed, a particular advantage of the novel preparation process is that the component b) can be fixed on the molding by a simple thermal treatment of the impregnated molding in the metal and/or oxide form and without further residues, for example halides. In this way it is also possible to save costly and time-consuming operations which may be necessary, for example, in the case of wash coating, or a reduction step with hydrogen in the gas phase.
In the novel catalyst, the component b) is prefer-ably present predominately in the region of a surface of the catalyst molding; in other words, preference is given here to a so-called shell impregnation. As the component b), the novel catalyst preferably comprises at least one of platinum, palladium and rhodium. Particular preference is given to the novel catalyst having a platinum content in the range from 0.05 to 0.5~ by weight, especially from 0.1 to 0.2~ by weight of Pt, based on the weight of the catalyst.
The present invention provides, furthermore, a method of cleaning a pressurized off-gas obtained after the oxidation of para-xylene (p-X) or a mixture of para-xylene and methyl p-toluate (p-TE) with air in the liquid phase in the preparation of dimethyl terephthalate (DMT), where the pressurized, oxygen-containing off-gas from the oxidation is led first of all via a single-stage or multistage conden-sation, a single-stage or multistage absorption and subsequently, with or without the supply of oxygen, via a catalytic afterburner which is operated under pressure, using the catalyst as defined previously.
In general, in the novel off-gas cleaning method, O.Z. 5104 CA 0221~39 1997-09-1 the target products present in the oxidation off-gas from air oxidation of p-X (these products are essentially p-X, p-TE, DMT, methyl benzoate (BME) and methanol) are predominately removed from the off-gas in a condensation unit, also in an optional downstream scrubber unit or absorption unit, and are recycled appropriately to an earlier stage of the process.
The condensation is in general operated at a temperature in the range from 15 to 80~C and at a pressure of from 3 to 20 bar absolute.
The pressurized gas originating from the condensation is preferably heated in a countercurrent heat exchanger, for example from about 25~C to around 120~C at a pressure of from 3 to 20 bar absolute, and then is run into a scrubber unit (i.e., absorption unit). The absorption unit may consist of a plurality of scrubbing stages; for example, off-gas scrubbing with BME or an ester mixture can be carried out first of all.
Figure 1 shows a preferred embodiment of the novel method for cleaning a pressurized off-gas according to the present invention.
Downstream of such an absorption unit, the off-gas (110) may preferably be further saturated with wastewater (100) obtained in the process, the so-called process water generally containing organic constituents. In general this is done using a so-called saturator (AS). The process water is preferably circulated via a pump (P1) and a heat exchanger (W1) which is heated with low-pressure steam, and is fed to the top of the saturator (AS). In this procedure the off-gas generally becomes saturated with water and with the O.Z. 5104 CA 0221~39 1997-09-1~

essentially volatile organic constituents present in the original process water. In order to avoid clogging of the saturator (AS) as a result of the accumulation of solids and of high-boiling constituents which are generally obtained in the bottom of the saturator, it is possible to draw off a small amount of the wastewater (120) from the bottom section of the saturator (AS) and to recycle it to the process at an appropriate point, for example.
The off-gas leaving the absorption unit, preferably the saturator (AS), normally comprises by-products obtained in the DMT process, such as CO and also low-boiling compounds such as acetaldehyde, formaldehyde, methyl acetate, dimethyl ether, acetic acid and formic acid and their methyl esters.
In order to dispose of these by-products in the most environmentally friendly way, the pressurized off-gas may then be passed to a catalytic afterburner.
In the catalytic afterburner (KNV) the novel catalyst is employed here. The reactor of the catalytic afterburner is in general designed as a fixed-bed reactor and is able to accommodate bulk catalysts and monolithic catalysts. It is appropriate to employ honeycomb-shaped monoliths, which are generally notable for a very low pressure loss, which in the case of the novel off-gas cleaning method has a positive effect in connection, in particular, with energy recovery by an off-gas turbine.
In the novel method the catalytic afterburner is preferably operated at a pressure of between 2 and 20 bar absolute, particularly preferably at from 5 to 10 bar absolute, and at a working temperature of between 160 and O.Z. 5104 CA 022l~39 l997-09-l~

650~C, particularly preferably between 200 and 550~C. With preference, the catalytic afterburner in the novel method is operated with a space velocity (GHSV) in the range from 1000 h-1 to 50,000 h-1, particularly preferably in the range from 5000 h-1 to 80,000 h-1 (GHSV = V(s.t.p.)/Vcat [m3 (s.t.p./
m3 ~ h], where V(s.t.p.) = volume flow under standard conditions [m3 (s.t.p.)/h] and Vcat = catalyst volume [m3]) For the first run-up of the reactor (KNV), process air (141) is in general first of all heated via an electri-cally operated heat exchanger (W4) and passed via the reactor (KNV) until the latter has reached its intended operating temperature, which ensures the activation of the catalyst.
The reactor can then be loaded with off-gas. Suitably, the catalytic afterburner then runs with substantial thermal autonomy. In the case of the novel method, the off-gas upstream of the catalyst usually contains water as steam in an amount of from 0.04 to 2 . 8 kg/m3 (s.t.p.), in particular from 0.1 to 0.4 kg of water per3 m (s.t.p.) of off-gas.
Before it enters the catalyst region, the off-gas stream to be cleaned (150) is preferably preheated (160,161, 170) in countercurrent with the cleaned off-gas (clean gas) (180) coming from the reactor by means of an appropriate arrangement of corresponding heat exchangers (W2, W3 ) .
The oxidation in the DMT process is generally set such that the oxygen required for catalytic combustion is already present in the off-gas before the off-gas enters the reactor. The oxygen content in the off-gas (130) may, if required, be raised by supplying compressed air (140). In this way the organic compounds and CO present in the off-gas O.Z. 5104 CA 022l~39 l997-09-l~

are generally converted almost completely to carbon dioxide and water over the novel catalyst. The oxygen content in the clean gas (180, 190) is preferably adjusted to a level of from 0.5 to 2 % by volume.
In order to recover the compression energy, it is possible to depressurize the clean gas (190) by way of an off-gas turbine (TU) to produce mechanical or electrical energy.
For energy recovery by means of an expansion turbine, use is generally made of those clean gas streams which are under a pressure of, preferably, more than 3 bar absolute. Since the off-gas is then generally in accordance with requirements, it can be led off (210) via a stack. For this purpose the temperature may be controlled, possibly by supplying unheated off-gas, such that the clean gas leaving the off-gas turbine has a temperature of about 125~C.
In the case of the novel method, it is also possible to divert at least some of the clean gas stream upstream of the turbine, to dry it appropriately and then cool it (W5) to a temperature lower than 40~C, and to use it again (200) appropriately as inert gas, for example for blanketing in the process.
In addition to the good economic catalyst service life obtained, the particular advantages of the novel method also consist in the simultaneous utilization of the waste water obtained, in the incineration of all of the by-products which are obtained in the oxidation and which pass into the off-gas, and the minimization of the CO content in the clean gas to a value which in many cases can no longer be detected with measuring instruments used in everyday operation.

O. Z . 5104 CA 0221~39 1997-09-1 - 13a -The invention is illustrated in more detail by the following examples:
Example~
Example 1:
- Preparing a catalyst for cleaning the off-gas from the oxidation in the Witten DMT process -2000 g of a commercial, TiO2-based catalyst support in strand form (type H9050 from Huls AG prepared by a process described in DE-C 2658 569) were charged to a rotating drum and heated to 110~C by means of a stream of hot gas which was guided onto the support. When the temperature reached, 50Oml of an aqueous Pt nitrate solution where w(Pt) = 5.2 g/l were sprayed on at 110~C over a period of about 20 minutes. In the course of this procedure the solvent evaporated and the metal was deposited on the support in a thin marginal zone. The material was then removed and calcined at 450~C in a stream of air for 4 hours.
Example 2:
- Cleaning the off-gas from the oxidation in the Witten DMT
process -Figure 1 shows a preferred embodiment of the novelmethod of cleaning pressurized off-gas from the oxidation in the Witten DMT process. In this connection, Table la + b sets out material streams, their composition and the respective operating conditions; cf. key. The amounts of off-gas and wastewater and their compositions are typical of a DMT/PTA
plant with a capacity of 240 kt/a.
The off-gas which is obtained downstream of the condensation and absorption unit in the DMT/PTA plant and has o.z. 5104 CA 0221~39 1997-09-1 - 13b -been substantially freed from useful products enters the bottom section of the saturator (AS) as a material stream (110) at a rate of 70,768 kg/h and at a temperature of 120~C.
The wastewater (20,000 kg/h) (100) originating from the DMT/PTA plant is fed in at the top section of the saturator.
The saturator is fitted either with valve trays or with structured packing. By means of a circulation pump (P1) the wastewater is circulated (121) via a heat exchanger (W1) which is heated with low-pressure steam. In order to avoid accumulation of solids in the saturator with the possibly attendant functional disruptions to the plant, a small part of the bottom product from the scrubber is removed (120) and recycled to the DMT/PTA process at an appropriate point.
For the run-up of the reactor (KNV), process air (141) is first of all passed via the electrically heated heat exchanger (W4) into the catalytic afterburner, until a temper-ature is reached which ensures the commencement of the reaction.
The scrubbed off-gas (130) which is saturated with water and with the organic constituents of the wastewater, with a temperature of 121.5~C and a pressure of 7.1 bar, is heated by the countercurrent heat exchangers to 250~C (150, 160, 170) and is passed into the reactor of the catalytic afterburner.
The entry temperature of the off-gas before the reactor can be adjusted both in the course of run-up and during normal operation by means of the material stream (161).
In addition, atmospheric oxygen required for combustion can be supplied by way of the material stream (140).

O.Z. 5104 CA 022l~39 l997-09-l~

The reactor (KNV) is charged with the catalyst type H 5922 from Huls AG - cf. Example 1 - and is operated with a space velocity in the region of around 30,000 [m (s.t.p.)/m h].
Combustion is complete and requires only relatively small excesses of oxygen.
As a result of the heat of reaction of the exother-mic combustion process, the cleaned off-gas (clean gas) leaves the reactor with a temperature of 401~C (180) and is guided onto the jacket side of the heat exchangers (W2, W3) for heating of the off-gas, in the course of which the clean gas is cooled to a temperature of 277~C (190) and is then let down (210) to atmospheric pressure by way of the off-gas turbine (TU) coupled to the air compressor, and is led off into the atmosphere by way of the stack.
In order to obtain inert gas it is possible to cool the clean gas (190), in part under pressure, and to use it (200) in the DMT/PTA plant for blanketing.

O . Z . 5104 CA 02215539 1997-09-lS

- 14 - o. z . 5104 Key to Fiqure 1, Table 1 and the mater;al streams Figure 1 shows a preferred embodiment of the novel method of cleaning pressurized off-gases from the oxidation in the Witten DMT process.

Material streams in Figure 1 and Tables 1a and b:

5 100: process water 110: off-gas from the condensation and absorption unit 1 2û: bottom product, recycled to the process 121: saturator circuit 130 off-gas from the saturator 10 140: - compressed air 141: compressed air for running up the reactor 142: air intake 150: off-gas, enriched with oxygen 160: flow of off~as to the neat exchangers W 2 + 3 15 161: flow of off-gas for regulating the reaction temperature 170: off-gas heated by countercurrent heat exchanger, upstream of the reactor 180: clean gas downstream of the reactor 190: clean gas downstream of the countercurrent heat exchanger 20 200: substream of c!ean gas, cooled and under pressure, recycled to the process as inert gas 210: residual, depressurized stream of clean gas upstream of the stack Plant comp~nents in Figure 1:

AS : saturator 25 P1 : pump W1 : heat exchanger, operated with steam W2+3: c~untercurrent heat exchangers KNV : reactor for catalytic afterburning W4 : heat exchanger, electrically operated 30 W5 : heat exchanger TU : expansion turbine Key to Tables 1a and b:

In Tables 1a and b the material streams, their compositions and the respective operating conditions are set out in relation to Figure 1.

CA 0221~39 1997-09-1~

- 15 - o . z . 5 104 Key to abbreviations:

p-X : para-xylene p-TA : para-toluic acid p-TE : methyl paratoluate (pT ester) BME : methyl benzoate HM-BME : methyl hydroxymethylbenzoate MM-BME : methyl methoxymethylbenzoate DMT : dimethyl terephthalate DMT, crude = crude ester (DMT crude ester stream after the l 0 esterification) Crude DMT : dimethyl terephthalate fraction after the crude ester distillation DMT-ultrapure : ultrapure dimethyl terephthalate (highly pure DMT
intermediate or end product) DMO : dimethylorthophthalic acid DMI : dimethylisophthalic acid DMP : dimethyl phthalate = isomer mixture of DMT, DMO
and DMI
MMT : monomethyl terephthalate (terephthalic acid 2 0 monomethyl ester) TA : terephthalic acid MTA : medium-purity terephthalic acid PTA : high-purity terephthalic acid PTA-p : very pure, i.e. ultrapure, terephthalic acid (contents 2 5 of MMT and p-TA together of < 50 ppm by weight) TAS : terephthalaldehyde acid (4-CBA) TAE: : terephthalaldehyde acid methyl ester Formaldehyde-DMA: formaldehyde dimethyl acetal Acetaldehyde-DMA: acetaldehyde dimethyl acetal CA 022l5539 l997-09-l5 o u~ O tq ~ ~

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Table 1b Malerlal slream- 100 110 120 130 140 150 1.70 180 190 210 Amounls Ikg/hl 20000 7076B s68 90200 54 90254 90254 90254 90254 90254 Amounls [m lhl 16229 7.97 16242 21648 28382 23109 106052 T. - ~ e l-Cl35 110 105 1215 110 122 250 401 277 123 Pressure Ibarl 7.1 7.1 7.1 7.1 7.5 71 7 7 6 9 1.1 Cr , - -~ r r1% by wl.l 1~h by wl.ll% by wl.ll% by wll 1% by wt IlUh by wt I seo 150 lUh by w1.1 see 180 1% by wt.l D
W~ter 95.47 0.23 94.01 20 75 20 74 21.33 21.33 1 Melhanol 0.01 0 03 0.02 0 03 0 03 _ p-TE traces Iraces traces Iraces X
p-xyleno 0 01 0 1 ~
BME traces 0.04 0 01 0.03 0 03 O
Acellc acld2.60 0 00 4 56 0 55 o 55 Fommlc acld1.20 0 00 1.36 0 26 0 26 r. j~- 0.02 0.15 015 Ac~ I I jd~0.66 Melhyl acotato 0 01 0 02 0 02 0.02 ~ Elhyl acetato Iraces Iraces Iraces Iracoa w C ~ 'Dorl~h by wt.ll% by wt.l1% by wl.l 1% bY ~ 11% by v~ 11~h bY wt l see 1501% by w~.l see 180 1% by wt.

Dlmethyl ether lraces Iraces Iraces traces Methvl torm. ~ 0 05 0 01 0 02 0 02 0 02 o ~ ~ U~ o o a~

ao ~ ~ U~ o o U7 ~ ~ ~ 0 'D ~
- _ g ~ ('~ O

O (D
~r ~

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5 ~ r N ~ b ~o o o -- o o ~
_ g g ,~ o oO 9 9 ,, s . o 1- 5 '~ Z 0 8 8

Claims

1. A catalyst for cleaning a pressurized off-gas obtained after the oxidation of para-xylene (p-X) or a mixture of para-xylene with methyl para-toluate (p-TE) with air in the liquid phase in the preparation of dimethyl terephthalate (DMT), wherein the catalyst a shaped article comprising:
a) at least one oxide of titanium, and b) at least one element from subgroup VIII of the Periodic Table of the Elements in the metal or oxide form.

2. A catalyst as claimed in claim 1, which the component b) comprises at least one member selected from the group consisting of platinum, palladium and rhodium.

3. A catalyst as claimed in claim 1 or 2, which comprises:
the component a) in an amount of from 50 to 99% by weight, and the component b) in an amount of from 0.01 to 5% by weight, wherein the component b) is calculated in total and as metal, based in each case on the weight of the catalyst.

4. A catalyst as claimed in any one of claims 1 to 3, which also comprises c) a sulfate.

5. A catalyst as claimed in claim 4, which comprises as the sulfate c), a sulfate of barium, tungsten, vanadium, zirconium or a mixture thereof in a total amount of from 0.1 to 10.0% by weight, based on the weight of the catalyst.

6. A catalyst as claimed in any one of claims 1 to 5, which comprises at least one member selected from the group further consisting of an oxide or phosphate of tungsten, vanadium, zirconium or silicon and a silicate.

7. A catalyst as claimed in any one of claims 1 to 6, which comprises, as the component a), titanium oxide which is predominantly in the anatase form.

8. A catalyst as claimed in any one of claims 1 to 7, which has a honeycomb structure.

9. A catalyst as claimed in any one of claims 1 to 8, wherein the component b) is permanently present in the region of a surface of the catalyst.

10. A process for preparing a catalyst as claimed in any one of claims 1 to 9, which comprises applying the component b) individually or in a mixture by dip impregnation and spray impregnation, to a shaped support which is formed essentially of the component a).

11. The process as claimed in claim 10, wherein the shaped support has a BET surface area of between 10 and 200 m2/g and a pore volume of between 0.1 and 0.6 cm3/g.

12. The process as claimed in claim 10 or 11, wherein the shaped support is impregnated using a solution containing a nitrate of the component b).

13. The process as claimed in any one of claims 10 to 12, wherein the shaped support, after impregnation is subjected to thermal aftertreatment with ingress of air.

14. A method of cleaning a pressurized oxygen-containing off-gas obtained after the oxidation of para-xylene (p-X) or a mixture of para-xylene (p-X) with methyl para-toluate (p-TE) with air in the liquid phase in the preparation of dimethyl terephthalate (DMT), which comprises passing the off-gas through:
[A] a single-stage or multi-stage condensation step;
[B] a single-stage or multi-stage absorption step; and [C] subsequently, with or without a supply of oxygen, a catalytic afterburner which is operated under pressure using the catalyst as defined in any one of claims 1 to 9 or as prepared by the process of any one of claims 10 to 13.

15. The method as claimed in claim 14, wherein the catalytic afterburner is operated in the temperature range between 160 and 650°C and at a pressure of from 2 to 20 bar absolute.

16. The method as claimed in claim 14 or 15, wherein the catalytic afterburner is operated with a space velocity (GHSV) in the range from 1000 h-1 to 50,000 h-1.

17. The method as claimed in any one of claims 14 to 16, wherein after the last stage of the absorption step [B] and before the afterburner [C], water containing organic constituents is passed via a saturator to the off-gas stream.

18. The method as claimed in any one of claims 14 to 17, wherein the off-gas upstream of the catalytic afterburner [C]
contains water in the form of steam in an amount of from 0.04 to 2.8 kg/m3 (s.t.p.).

19. The method as claimed in any one claims 14 to 18, wherein the catalytically cleaned off-gas is depressurized by way of a turbine in order to produce mechanical or electrical energy.

20. The method as claimed in claim 19, wherein the catalytically cleaned off-gas, which is under a pressure of more than 3 bar absolute, is depressurized in an expansion turbine in order to produce mechanical or electrical energy.

21. The method as claimed in any one of claims 14 to 20, wherein the catalytically cleaned off-gas is reused as an inert gas.

22. A method of cleaning a pressurized oxygen containing off-gas obtained after the oxidation of para-xylene (p-X) or a mixture of para-xylene (p-X) with methyl para-toluate (p-TE) with air in the liquid phase in the process for the preparation of dimethyl terephthalate (DMT), the off-gas containing (i) target products that include para-xylene, methyl para-toluate, dimethyl terephthalate, methyl benzoate and methanol, (ii) low-boiling by-products that include acetaldehyde, formaldehyde, acetaldehyde methyl acetal, formaldehyde methyl acetal, dimethyl ether, acetic acid, formic acid, acetic acid methyl ester and formic acid, methyl ester and (iii) atmospheric nitrogen, residual oxygen, CO2 and CO, which comprises passing the off-gas through:
[A] a single-stage or multi-stage condensation step at a temperature of 15 to 80°C at a pressure of 3 to 20 bar absolute, to condense out predominantly the target products from the off-gas;
[B] a single-stage or multi-stage absorption benzoate or an ester mixture at a temperature of 25 to 120°C at a pressure of 3 to 20 bar absolute, to remove predominantly methanol and p-xylene;
[C] a saturation step by saturating the off-gas in a saturator with wastewater obtained in the process and containing volatile organic constituents, whereby the off-gas is saturated with the wastewater and contains CO and the low-boiling by-products; and [D] with or without a supply of oxygen, a catalytic afterburner which is operated at a pressure of 2 to 20 bar absolute at a working temperature of 160 to 650°C using the catalyst as defined in any one of claims 1 to 9 or as prepared by the process of any one of claims 10 to 13.