Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2000-09-26
2001-05-15
Richter, Johann (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
Reexamination Certificate
active
06232507
ABSTRACT:
A number of processes for preparing formaldehyde from methanol are known (see, for example, Ullmann's Encyclopedia of Industrial Chemistry). Processes which are carried out industrially are predominantly the oxidation
CH
3
OH+½O
2
→CH
2
O+H
2
O
over catalysts comprising iron oxide and molybdenum oxide at from 300° C. to 450° C. (Formox process) and the oxidative dehydrogenation (silver catalyst process) described by:
CH
3
OH→CH
2
O+H
2
H
2
+½O
2
→H
2
O
at from 600° C. to 720° C. In both processes, the formaldehyde is initially obtained as an aqueous solution. Particularly when used for preparing formaldehyde polymers and oligomers, the formaldehyde obtained in this way has to be subjected to costly removal of water. A further disadvantage is the formation of corrosive formic acid which has an adverse effect on the polymerization as by-product.
The dehydrogenation of methanol enables these disadvantages to be avoided and, in contrast to the abovementioned processes, enables virtually water-free formaldehyde to be obtained directly:
In order to obtain an ecologically and economically interesting industrial process for the dehydrogenation of methanol, the following prerequisites have to be met: the strongly endothermic reaction has to be carried out at high temperatures in order to be able to achieve high conversions. 35 Competing secondary reactions have to be suppressed so as to achieve sufficient selectivity for formaldehyde (uncatalyzed, the selectivity to formaldehyde is less than 10% at conversions of over 90%). Residence times have to be short or the cooling of the reaction products has to be rapid in order to minimize the decomposition of the formaldehyde which is not thermodynamically stable under the reaction conditions:
CH
2
O→CO+H
2
.
Various processes for carrying out this reaction have been proposed; thus, for example, DE-A-37 19 055 describes a process for preparing formaldehyde from methanol by dehydrogenation in the presence of a catalyst at elevated temperature. The reaction is carried out at a temperature of 300° C. to 800° C. in the presence of a catalyst comprising at least one sodium compound.
J. Sauer and G. Emig (Chem. Eng. Technol. 1995, 18, 284-291) were able to liberate a catalytically active species which they presumed to be sodium from a catalyst comprising NaAlO
2
and LiAlO
2
by means of a reducing gas mixture (87% of N
2
+13% of H
2
). This species is able to catalyze the dehydrogenation of methanol introduced downstream in the same reactor, i.e. not coming into contact with the catalyst bed, to produce formaldehyde. When nonreducing gases were used, only a slight catalytic activity was observed.
According to J. Sauer and G. Emig and also results from more recent studies (see, for example, M. Bender et al., paper at the 30th annual conference of German catalyst chemists, Mar. 21-23, 1997), sodium atoms and NaO molecules were identified as species emitted into the gas phase and their catalytic activity for the dehydrogenation of methanol in the gas phase was described. In the known processes, the starting material methanol is always diluted with nitrogen and/or nitrogen/hydrogen mixtures for the reaction.
Various publications, for example EP-A 0 130 068, EP-A 0 261 867 and DE-A 25 25 174 propose using the gas mixture formed in the reaction as fuel after separating off the formaldehyde.
Although the known processes already give good results, there is still plenty of room for improvements from engineering and economic points of view.
It is therefore an object of the invention to provide an improved and more economical process.
In the search for a solution to achieve this object, the following was found:
A great improvement in engineering and economic terms, particularly in respect of energy, can be achieved if the product gas mixture formed in addition to the formaldehyde is used for diluting the methanol starting material.
It is also advantageous to generate a catalytically active species from a primary catalyst in a carrier gas stream at a temperature which is different from the dehydrogenation temperature. The regions for generating the catalytically active species and for carrying out the reaction are advantageously physically separated from one another. This enables different temperatures and residence times to be set for the carrier gas streams passed through these units.
The circulating gas stream is obtained by, after separating off the formaldehyde, recirculating at least part of the by-products of the dehydrogenation, primarily H
2
and CO, to the reactor by means of a suitable apparatus.
Heating suitable primary catalysts in the primary catalyst decomposition zone and passing gas over them at temperatures which are different from the dehydrogenation temperature results in one or more catalytically active species which are able to catalyze the dehydrogenation of methanol being generated or carried out by the gas. Such a fluid catalyst is transported to the reaction zone via the feed lines. Setting the temperatures separately and matching them to the respective conditions for liberation/vaporization of catalyst or generation of a catalytically active species on the one hand and for the reaction on the other hand makes it possible, in particular, to lower the reaction temperature. This reduces the decomposition of the unstable formaldehyde as a result of secondary reactions and increases the yield.
Although the homogeneously catalyzed, nonoxidative dehydrogenation of methanol by means of sodium species in the gas phase and the introduction of the catalytically active species by thermal decomposition of a primary catalyst or vaporization of various substances has been described in the literature, attention has not previously been paid to possible losses of catalyst on the way to the reaction zone.
It has surprisingly been found that the catalyst utilization and formation of deposits in the region in which the active catalyst species is generated depends strongly on the carrier gases used.
When using H
2
/CO mixtures or the circulating gas as carrier gas for the catalyst species, growing deposits comprising mainly carbon and additionally sodium, oxygen and hydrogen are formed in the feed line upstream of the actual reactor. This adversely affects the primary catalyst utilization, results in partial inactivation of the catalyst and limits the trouble-free operating time of the plant. This negative synergistic effect due to formation of a deposit and trapping of the catalyst impairs the economic operation of the plant.
Furthermore, it has surprisingly been found that it is possible to increase the primary catalyst utilization and to avoid the formation of deposits in the region of the catalyst feed line and the reactor inlet.
The invention accordingly provides a process for preparing formaldehyde from methanol by dehydrogenation in a reactor at temperatures in the range from 300 to 1000° C. in the presence of a catalyst which is introduced into the reactor with the aid of a carrier gas to give a product gas mixture, wherein the formaldehyde is separated from the product gas mixture and at least part of the remaining product gas mixture is recirculated to the reactor in a circulating gas stream and the carrier gas used is a carbon-free gas or gas mixture.
Particular embodiments are disclosed in the subclaims. One or more of these embodiments, either individually or in combination, can also achieve the object of the invention and the features of the embodiments can also be combined in any way.
The amount of catalyst-containing carrier gas stream as a proportion of the total gas stream is here from 1 to 50%, preferably from 5 to 40%.
The process of the invention makes it possible to obtain formaldehyde which is low in water in an ecologically and economically favorable way. The utilization of the hydrogen-rich by-products of the reaction, i.e. the product gas after separating off the formaldehyde, for diluting the methanol starting material for the dehydrogenation enab
Kaiser Thomas
Meister Christine
Schweers Elke
Connolly Bove & Lodge & Hutz LLP
Richter Johann
Ticona GmbH
Witherspoon Sikarl A.
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