Process for the synthesis of a methanol/dimethyl ether...

Details Process for the synthesis of a methanol/dimethyl ether... Process for the synthesis of a methanol/dimethyl ether...

2000-01-25

2001-02-20

Kumar, Shailendra (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Treatment of feed or recycle stream

C518S700000, C518S707000, C518S713000

Reexamination Certificate

active

06191175

ABSTRACT:

The present invention relates to a process for the synthesis of methanol and dimethyl ether (DME) from an essentially stoichiometrically balanced synthesis gas comprising H
2
/CO/CO
2
.
A product mixture with a high DME to methanol ratio is preferred as a product richer in DME in most cases represents a higher product value.
The product with the ultimate DME to methanol ratio obtained is a pure DME product, which is at present mainly produced at a high cost by dehydration of methanol by use of a dehydration catalyst in a fixed bed reactor, and rectification of the product to recover a DME product with high purity as required by the aerosol industry.
In many aspects a DME/methanol raw product mixture rich in DME is sufficient and thus preferred to a pure DME product if obtained at a lower cost than by methanol dehydration. Several methods are described in the literature where DME is produced directly in combination with methanol by a combined synthesis from synthesis gas by use of a catalyst active in both the synthesis of methanol from synthesis gas and methanol dehydration (DD Patent No. 291,937, EP Patent Nos. 164,156 and EP 409,086, GB Patent Nos. GB 2,093,365, GB 2,097,383 and GB 2,099,327, U.S. Pat. Nos. 4,417,000, 5,254,596, 4,177,167, 4,375,424 and 4,098,809, DE Patent Nos. 3,220,547, DE 3,201,155, DE 3,118,620, DE 2,757,788 and DE 2,362,944 and DK Patent Nos. 6031/87 and DK 2169/89).
Suitable catalysts for the use in the synthesis gas conversion stage include conventional employed methanol catalysts such as copper and/or zinc and/or chromium based catalysts and methanol dehydration catalysts, which usually comprise alumina or alumina silicates as active compounds arranged in a physical mixture or layered beds as cited in WO 96/23755.
The combined synthesis of methanol and DME from synthesis gas is conducted according to the following reaction schemes (all equilibrium reaction steps being exothermic, meaning heat is evolved, when displaced to the right hand side):
CO
2
+3H
2
←→CH
3
OH+H
2
O  (1)
2CH
3
OH←→CH
3
—O—CH
3
+H
2
O  (2)
CO+H
2
O←→CO
2
+H
2
  (3)
Reaction schemes 1 and 3 are catalysed by catalysts active in methanol formation from synthesis gas, whereas reaction scheme 2 is catalysed by catalysts active in methanol dehydration. Combined catalysts, i.e. active in both methanol formation from synthesis gas and methanol dehydration thus catalyse all three reactions. The formation of the combined methanol (MeOH) and DME product is limited by chemical equilibrium. The equilibrium conversion of synthesis gas to the combined product increases with increasing pressure and decreasing reactor exit temperature.
Typical synthesis conditions are temperatures ranging from 200° C. to 310° C. and pressures in the range of 40-120 kg/cm
2
.
Normally, unconverted synthesis gas is separated from the combined product downstream the synthesis reactor and recycled by means of a recycle compressor in order to obtain a higher overall conversion of the synthesis gas. The degree of separation of product from the recycled synthesis gas determines the equilibrium conversion per pass of synthesis gas. Methanol is substantially removed at moderate pressures by simple condensation at a temperature of the reactor effluent obtained at a low cost e.g. cooling by cooling water, whereas an efficient separation of DME from the synthesis gas requires either washing, cooling at lower temperature at substantially higher cost than obtained by cooling water, or condensation at high pressures or combinations thereof. Consequently, increasing the DME content in product results is increasing expenses to recover the DME/methanol product from the synthesis.
Increased deactivation of the catalyst sites active in methanol formation is observed at conditions with a high partial pressure of water, which makes restrictions for the application of catalysts with a methanol synthesis activity as to operating conditions.
The reactivity of the dehydration function of the combined catalyst increases more steeply with reaction temperature than does the methanol function, and at the same time the equilibrium conversion of dehydration is less sensitive to the temperature.
The methanol synthesis function is prone to deactivation at high temperature (e.g. more than 310° C.), whereas the dehydration function is far more resistant.
Likewise the composition of the synthesis gas decides for the obtainable conversion. Normally, when high conversions are desired a synthesis gas composition with a so-called module
M
=
(
n
H
2
-
n
CO
2
)
(
n
CO
+
n
CO
2
)
About 2 is desired, as the components active in the reaction schemes are then stoichiometrically balanced. Compositions having modules of about the value 2 are essentially balanced, like typically module values between 1.8 and 2.2 are representing compositions considered essentially balanced.
Several compositions of synthesis gas meet the criterion. The higher the content of CO
2
in the synthesis gas, the lower is the equilibrium conversion.
The combined synthesis can be conducted in one or more fixed bed reactors loaded with the combined catalyst, it be cooled reactors, whereby reaction heat is removed from the reaction bed, or adiabatic type reactors typically placed in series with intercooling in a number providing for an appropriate conversion per pass. At high production capacities, it is found that adiabatic reactors are preferred to cooled reactors due to a favourable scale of economy. Typically, for obtaining a high conversion, three adiabatic reactors with two intercoolers in between are used.
It has now been found that a novel combination of process steps provides for an improved synthesis than the direct synthesis described above based on an essentially stoichiometrically balanced gas; the novel combination of process steps comprising
Mixing make-up synthesis gas with unconverted recycle synthesis gas;
Heating the admixed synthesis gas to a predetermined methanol reactor inlet temperature; Optionally splitting a part of the preheated admixed synthesis gas;
Converting the remaining admixed preheated synthesis gas in a cooled reactor loaded with catalyst active in the synthesis of methanol from synthesis gas forming a cooled reactor effluent comprising methanol, water and unconverted synthesis gas;
optionally adding the split stream to the cooled reactor effluent;
passing the cooled reactor effluent to one or more beds of catalyst optionally comprising methanol synthesis function and/or combined catalyst function converting the synthesis gas further to DME, and a dehydration function converting the methanol further to DME, forming a DME reactor effluent;
cooling the DME reactor effluent;
separating the cooled DME reactor effluent into a stream mainly comprising unconverted synthesis gas, secondly comprising inert and reduced amounts of methanol and DME, and a stream mainly comprising the combined methanol and DME product and water;
splitting a purge stream from the said stream mainly comprising unconverted synthesis gas; and
passing the remaining stream mainly comprising unconverted gas to a compressor raising the pressure of the stream to at least the pressure of the make-up synthesis gas providing a stream of unconverted recycle synthesis gas.
The catalysts active in methanol formation from synthesis gas may be selected from conventional employed methanol catalysts such as copper and/or zinc and/or chromium based catalysts, and the catalyst active in methanol dehydration may be selected from conventional employed methanol dehydration catalysts such as alumina or alumina silicates.
The advantages obtained when applying the technical features of the invention as presented above are that the combined synthesis step is split into at least two optimized steps:
Firstly, a pure methanol conversion, which accounts for the major conversion of synthesis gas, at conditions where the water from the dehydration of the methanol is eliminated, thus serving for a lower deactivation rate of the metha

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