Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2000-10-02
2004-04-27
Richter, Johann (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C560S205000, C562S598000
Reexamination Certificate
active
06727383
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for preparing acrylic acid that involves using oligomeric acrylic acid or a mixture comprising acrylic acid and oligomeric acrylic acid to cool a gaseous reaction mixture which comprises acrylic acid and is obtained in the gas-phase oxidation to prepare acrylic acid, and that produces a gaseous mixture comprising acrylic acid. It also relates to a process for the preparation of acrylates. Moreover, it relates very generally to the use of oligomeric acrylic acid or of a mixture comprising acrylic acid and oligomeric acrylic acid to cool a gaseous reaction mixture obtained in the gas-phase oxidation to prepare acrylic acid.
2. Description of the Background
Because of its highly reactive double bond and the acid function, acrylic acid is a valuable monomer for preparing addition polymers, for example, for aqueous polymer dispersions suitable as adhesives.
One route to acrylic acid is the gas-phase oxidation of propylene and/or acrolein with oxygen or gases comprising oxygen in the presence of catalysts at elevated temperature, preferably with dilution of the reactants with inert gases and/or steam owing to the high heat of reaction.
Catalysts employed in this oxidation are generally multicomponent oxide-type systems based, for example, on oxides of molybdenum, of chromium, of vanadium or of tellurium.
However, this process leads not to pure acrylic acid but rather to a gas mixture which in addition to acrylic acid comprises secondary components—mainly unreacted acrolein and/or propylene, steam, oxides of carbon, nitrogen, oxygen, acetic acid, formaldehyde, benzaldehyde, fufurals, and maleic anhydride—from which the acrylic acid must subsequently be separated.
Isolation of the acrylic acid from the gaseous reaction mixture is generally carried out by countercurrent absorption using, for example, a high-boiling solvent or solvent mixture and a plurality of subsequent distillative processing steps, as is described, for example, in DE-A 21 36 396 and DE-A 43 08 087. In EP-B 0 009 545, U.S. Pat. No. 5,154,800, DE-A 34 29 391 and DE-A 21 21 123 absorption takes place first with water/aqueous acrylic acid in countercurrent and is followed by extractive or azeotropic distillation.
Disadvantages associated with these processes are that in general they are technically complex and energy-intensive and that for the absorption and, where appropriate, extraction an additional organic solvent/solvent mixture is required which has to be separated off again in a separate distillation step and possibly purified before being used again.
A further disadvantage of these processes is that the acetic acid obtained alongside acrylic acid in the oxidation of propylene (purity of the acetic acid: from 0.5 to 10% by weight relative to the amount of acrylic acid) has to be separated off in a complex distillation stage. Because of the small differences in boiling point and the great tendency of acrylic acid to polymerize, this is a difficult task, as can be seen, inter alia, from U.S. Pat. No. 3,844,903.
In view of the known fact that acrylic compounds have a high tendency to undergo addition polymerization, processes operating with multistage distillative workup are disadvantageous in very general terms since they heighten the polymerization tendency of the acrylic acid.
SUMMARY OF THE INVENTION
It is now the object of the present invention to provide a simple process for obtaining acrylic acid which first requires no additional solvent/absorbent or extractant and second is energetically favorable in its implementation.
We have found that this object is achieved by the present invention, which provides a process for preparing acrylic acid comprising the following stage A:
A: cooling a gaseous reaction mixture which comprises acrylic acid and is obtained in the gas-phase oxidation to prepare acrylic acid, using oligomeric acrylic acid or a mixture of acrylic acid and oligomeric acrylic acid, to give a gaseous mixture comprising acrylic acid and a quench bottom product which comprises oligomeric acrylic acid.
DETAILED DESCRIPTION OF THE INVENTION
The term “gaseous reaction mixture which comprises acrylic acid” embraces, for the purposes of the invention, all reaction mixtures obtained in the gas-phase oxidation to prepare acrylic acid.
If propylene/acrolein are used as starting materials to prepare acrylic acid, the gaseous reaction mixture concerned is obtained with a temperature of approximately 200 to 300° C. from the gas-phase oxidation and comprises from about 1 to about 30% by weight of acrylic acid with the following byproducts: unreacted propylene (from about 0.05 to about 1% by weight), acrolein (from about 0.001 to about 2% by weight), propane (from about 0.01 to about 2% by weight), steam (from about 1 to about 30% by weight), oxides of carbon (from about 0.05 to about 15% by weight), nitrogen (from 0 to about 90% by weight), oxygen (from about 0.05 to about 10% by weight), acetic acid (from about 0.05 to about 2% by weight), propionic acid (from about 0.01 to about 2% by weight), aldehydes (from about 0.05 to about 3% by weight), and maleic anhydride (from about 0.01 to about 0.5% by weight).
This gaseous reaction mixture is cooled by means of oligomeric acrylic acid or a mixture comprising acrylic acid and oligomeric acrylic acid, generally to a temperature from approximately 100 to approximately 190° C., preferably from approximately 120 to approximately 180° C. and, in particular, from approximately 130 to approximately 160° C., to give a further gaseous mixture comprising acrylic acid.
As the cooling apparatus it is possible to employ any prior art apparatus known for this purpose, with preference being given to the use of Venturi scrubbers or spray coolers (quench), especially the latter.
The term “oligomeric acrylic acid” used in accordance with the invention here comprises the product formed by addition of the carboxyl group onto the olefinic double bond by Michael addition, having the following formula I:
CH
2
═CHCO
2
—(CH
2
CH
2
CO
2
)
n
—H n=1−10
Oligomeric acrylic acid is always formed during the preparation of acrylic acid and is influenced by the temperature and the residence time but cannot be influenced or prevented by inhibitors.
The resultant gaseous mixture comprising acrylic acid is preferably separated in a stage B to give a crude acrylic acid, a low-boiling fraction, and a bottom product which comprises oligomeric acrylic acid. In particular, the separation of stage B is carried out in a distillation column and this is done preferably such that the acrylic acid is obtained by way of a sidestream takeoff of a distillation column.
The procedure here is generally as follows:
The gaseous mixture from stage A, comprising acrylic acid, is passed into the lower part of a distillation column, in which the gaseous constituents and the low boilers, especially aldehydes, acetic acid and water, are separated off via the top of the column.
The acrylic acid is drawn off as crude acrylic acid in the lower third of the distillation column via a sidestream takeoff.
The high boilers, principally oligomeric acrylic acid, are obtained in the bottom (liquid phase) of the distillation column.
The columns which can be employed for the process of the invention are not subject to any particular restriction. In principle, suitable columns are all those having internals which provide for effective separation.
Suitable column internals are all customary internals, especially trays, and random and/or structured packings. Of the trays, preference is given to bubble-cap trays, sieve trays, valve trays and/or dual-flow trays. The column comprises at least one cooling apparatus. Suitable such apparatus comprises all those heat transfer devices or heat exchangers where the heat liberated during the condensation is dissipated indirectly (externally). All customary apparatuses can be employed for this purpose, with preference being given to tube bundle heat exchangers, plate heat exchang
Machhammer Otto
Nestler Gerhard
Schröder Jürgen
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Puttlitz Karl J
Richter Johann
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