PROCESS FOR CLEANING CARBON DIOXIDE-CONTAINING PROCESS GASES FROM THE PREPARATION OF VINYL ACETATE

Abstract

The invention provides processes for cleaning carbon dioxide-containing process gases from the preparation of vinyl acetate after reaction of ethylene with acetic acid and oxygen in heterogeneously catalyzed, continuous gas phase processes, characterized in that carbon dioxide-containing process gases, for removal of carbon dioxide, are contacted with one or more scrubbing solutions, and one or more scrubbing solutions comprise one or more oxides of metals (metal oxides) selected from the group comprising vanadium, niobium, tantalum, chromium, molybdenum, manganese and arsenic.

Claims

1. A process for cleaning carbon dioxide-containing process gases from the preparation of vinyl acetate comprising the steps of: reacting ethylene with acetic acid and oxygen in a heterogeneously catalyzed continuous gas-phase process to form a carbon dioxide-containing process gas; and contacting the carbon dioxide-containing process gas with one or more alkaline scrubbing solutions comprising: I) one or more oxides of metals form the group comprising KVO.sub.3,NaVO.sub.3, KV.sub.2O.sub.5,. Na.sub.4V.sub.2O.sub.7 and K.sub.4V.sub.2O.sub.7 and also II) one or more oxides of metals of the group comprising KBO.sub.2, Na.sub.2B.sub.2O.sub.4, K.sub.2B.sub.2O.sub.4, Na.sub.2B.sub.2O.sub.7 and K.sub.2B.sub.2O.sub.7.

2. (canceled)

3. The process of claim 1, wherein the one or more scrubbing solutions are situated in a CO absorber having a sump, said process further comprising the steps of: feeding the carbon dioxide-containing process gas to the CO.sub.2 absorber in the region of the sump thereof; passing the carbon dioxide-containing process gas through the CO.sub.2 absorber whereby carbon dioxide is removed from the process gas and is taken up by the scrubbing solution; and withdrawing the cleaned process gas from the CO.sub.2 absorber and feeding it back to the reaction forming vinyl acetate.

4. The process of claim 1, wherein the one or more scrubbing solutions contain 0.5 to 20% by weight of oxides of metals, based on the total weight of the scrubbing solution.

5. (canceled)

6. (canceled)

7. The process of claim 1, wherein the one or more scrubbing solutions contain 0.5 to 15% by weight of oxides of metals of group I and 1 to 10% by weight of oxides of metals of group II, in each case based on the total weight of the scrubbing solution.

8. The process of claim 1, wherein the scrubbing solutions have a weight ratio of the oxides of metals of group I to the oxides of metals of group II of 1:2 to 4:1.

9. (canceled)

10. The process of claim 1, wherein the one or more scrubbing solutions contain alkaline earth metal hydroxides, alkali metal hydroxides, alkaline earth metal carbonates or alkali metal carbonates.

11. The process of claim 3, wherein the cleaned process gas withdrawn from the CO.sub.2 absorber contains 1.6% by weight of carbon dioxide, 70 to 95% by weight of ethylene, and further amounts of inerts selected from the group consisting of ethane, methane, nitrogen, and argon, wherein % by weight is based on the total weight of the cleaned process gas withdrawn from the CO.sub.2 absorber.

Description

GENERAL PROCESS DESCRIPTION

[0040] In a plant according to FIG. 1, ethylene-containing circuit gas is charged with acetic acid (AcOH) in the acetic acid saturator 1 (AcOH saturator), thereafter oxygen (O.sub.2) is added and fed to the tubular reactor 3 via a steam heated line 2. The circuit gas mixture leaving the reactor, which mixture substantially contained ethylene, vinyl acetate, acetic acid, carbon dioxide, oxygen and also inerts, was fed via line 4 to the predewatering column 5. In the predewatering column 5, the mixture was separated, wherein the sump product substantially containing VAM, acetic acid and water (H.sub.2O) was fed to the crude vinyl acetate container 21 via line 19, and, after transfer via line 22 to the azeotrope column 23 was separated into a VAM fraction and an acetic acid fraction each of which were further worked up in process steps that are not shown here.

[0041] The overhead product of the predewatering column 5 was withdrawn and freed from gaseous VAM in the downstream circuit gas scrubber 6 by means of scrubbing with acetic acid. The gas mixture (circuit gas) was compressed with the circuit gas compressor 7 to a pressure about 3 bar higher. The majority of the circuit gas was recirculated via 18 to the acetic acid saturator 1.

[0042] A fraction of 12 or 18% by volume of the circuit gas, as stated in the table for the respective (comparative) example, was branched off on the pressure side from the circuit gas compressor 7 and transferred via line 8 to the water scrubber 9 and there, to remove further vinyl acetate, was treated with acetic acid and then water. The bottoms product comprising acetic acid, water and vinyl acetate was introduced via line 20 directly into the predewatering column 5.

[0043] The overhead product of the water scrubber 9 was passed via line 10 in the region of the sump at a rate of 10 tonnes per hour into the CO.sub.2 absorption column 11. The CO.sub.2 absorption column 11 was operated at a circulation rate of 90 tonnes per hour or 45 tonnes per hour of a 25% strength by weight aqueous potassium carbonate scrubbing solution that optionally contained 6% by weight of potassium vanadate and 4% by weight of potassium borate as additives, as specified in more detail in the table. Otherwise, the CO.sub.2 absorption column 11 and the CO.sub.2 desorption column 13 were operated in a conventional manner.

[0044] The sump of the CO.sub.2 absorption column 11 contained, as usual, potassium hydrogen carbonate and was passed via line 12 to the top of the CO.sub.2 desorption column 13. The sump of the CO.sub.2 desorption column 13 contained potassium carbonate and was introduced via line 14 in the region of the top into the CO.sub.2 absorption column 11. The mass flow rate transferred from the CO.sub.2 absorption column 11 into the desorption column 13 corresponded to the mass flow rates transferred from the desorption column 13 to the CO.sub.2-absorption column 11. The scrubbing solution introduced into the CO.sub.2-desorption column 13 was heated by means of heating steam and thereby freed from carbon dioxide. The heating steam rates used in the respective (comparative) example are stated in the table. Via line 15, CO.sub.2 was removed from the desorption column 13.

[0045] The overhead product of the CO.sub.2-absorption column 11 was passed via the line 1616 into the ejector 17, there admixed with ethylene feed and finally introduced via the acetic acid saturator 1 into the reactor 3.

[0046] Comparative example 1 (CEx.1) and example 2 (Ex.2) make it clear that the addition according to the invention of potassium vanadate and potassium borate considerably increases the efficiency of the circuit gas scrubbing in the CO.sub.2-absorption column 11. As a result, the circulation rates of the potassium carbonate solution scrubbing solution in the CO.sub.2-absorption column 11 and the required heating steam rate in the CO.sub.2-desorption column 13 were able to be halved, and moreover with separation of carbon dioxide retained, as the CO.sub.2 concentration in the circuit gas before entry into the acetic acid saturator 1 indicates in the table. As a result, the energy requirement in the process according to the invention can be reduced in comparison with the conventional process.

TABLE-US-00001 TABLE CEx. 1 Ex. 2 Ex. 3 Fraction of the circuit gas 12 12 18 transferred into the water scrubber 9 [% by volume] Circulation rate of the potassium 90 45 90 carbonate scrubbing solution in the CO.sub.2-absorption column 11 [t/h] Additions to the scrubbing solution: Potassium vanadate KVO.sub.3 + + Potassium borate KBO.sub.2 + + Heating steam rate introduced into 4.5 2.3 4.5 the CO.sub.2-desorption column 13 [t/h] CO.sub.2 concentration of the process 20 20 20 gas that was fed to the water scrubber 9 via line 8 [% by volume] CO.sub.2 concentration in the circuit 18.3 18.3 17.5 gas before entry into the acetic acid saturator 1 [% by volume]

[0047] Comparison of example 3 (Ex.3) and comparative example 1 shows that the capacity of the scrubbing solution according to the invention, in comparison with a conventional scrubbing solution, is considerably increased, in such a manner that with otherwise identical mode of operation to that in the prior art, 50% more circuit gas can be introduced via the water scrubber 9 into the CO.sub.2-absorption column 11 and freed from carbon dioxide, without additional expenditure in terms of apparatus and energy being incurred thereby. As a result, finally, the ethylene content of the circuit gas can be increased, for example, before entry into the acetic acid saturator 1, and thereby also on entry into the reactor.

[0048] This specifically means that, in example 3, by introducing a larger fraction of the circuit gas into the water scrubber 9 and thereby also into the CO.sub.2-absorption column 11, in the cleaning that is performed there, the carbon dioxide fraction was decreased from 19% by weight to below 3% by weight of carbon dioxide and in total 50% more carbon dioxide were removed than in the comparative example 1. Finally, the consequence was that the carbon dioxide content in the circuit gas was decreased by cleaning from 18.3% by weight in the comparative example 1 to 17.5% by weight in example 3. The circuit gas volume no longer demanded by carbon dioxide was made up by ethylene on the ejector 17 in such a manner that the ethylene content before entry into the acetic acid saturator 1 was increased by 0.8% by weight.