This column is based on “Vinyl Acetate from Acetic Acid and Ethylene — Cost Analysis,” a report published by Intratec. It can be found at: www.intratec.us/analysis/vinyl-acetate-production-cost.
Vinyl acetate (vinyl acetate monomer; VAM) is an important vinyl ester, primarily used in the production of polymers and copolymers for coatings, paints and sealants, as well as for binders and in textile and paper processing.
The following paragraphs describe a typical vapor-phase oxidation process for VAM production from acetic acid and ethylene. Figure 1 presents a simplified flow diagram of the process.
Reaction. Make-up and recycled acetic acid pass through a vaporizer, along with fresh and recycled ethylene. The feed stream, containing excess of ethylene over acetic acid, is mixed with oxygen, preheated and fed to multi-tube reactors. The reaction occurs over palladium and gold catalysts. Heat is removed by evaporative cooling on the reactors’ shell. At the end of this stage, 8–10 wt.% of ethylene and 15–35 wt.% of acetic acid are converted to VAM. Water, CO2 and small quantities of ethyl acetate, ethylidene diacetate, and glycol acetates are the main byproducts.
VAM separation. The reactor effluent is cooled and fed to the pre-dehydration column, where a crude VAM stream is withdrawn from the bottom and stored. The overhead stream is separated into an organic phase, which is recycled to the column, and an aqueous phase, which is directed to a decanter downstream. Uncondensed gases are washed by acetic acid, and the solution formed is routed to the crude VAM tank, while gases from the scrubber are recycled to the reaction. A portion of these gases is washed with water to remove residual acetic acid, which is also directed to the crude VAM tank. After the water wash, the gases are directed to an absorption column, for CO2 removal by a potassium carbonate solution.
VAM purification. In the azeotropic column, a VAM-water mixture is distilled from the column overhead and fed to a decanter, along with the aqueous phase separated in the pre-dehydration step. Here, an organic phase containing VAM is separated and directed to the light-ends column, while an aqueous phase is routed to the wastewater column. Ethyl acetate is withdrawn as a side stream and discharged, and acetic acid from the column bottom is recycled to the vaporizer.
The purpose of the light-ends column is to strip off acetaldehyde and other volatile materials from the crude vinyl acetate, while the wastewater column separates residual VAM from wastewater. Finally, residual acetic acid and heavy-ends are removed in the pure VAM column, yielding a vinyl acetate product with 99.9 wt.% from the overheads.
VAM production pathways
VAM commercial production is mostly based on the reaction of ethylene with acetic acid and oxygen, but other pathways are possible (Figure 2).
The total operating cost (raw materials, utilities, fixed costs and depreciation costs) estimated to produce vinyl acetate was about $1,100 per ton of VAM in the third quarter of 2013. The analysis was based on a plant with capacity to produce 350,000 metric tons per year of VAM.
Edited by Scott Jenkins
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