Vinyl acetate-methanol pressure swing distillation
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This post was last edited by defeil on 2018-5-21 at 10:22. Vinyl acetate and methanol are an azeotrope; through ASPEN analysis, it was determined that this system is pressure-sensitive and can be separated using pressure-swapped distillation. Therefore, a simulation model was developed with a pressure of 40 kPa for the low-pressure column and 200 kPa for the high-pressure column. Since it is an azeotropic system, separation was attempted by continuously adjusting the operating parameters of each column, but no matter how these parameters were adjusted, it was not possible to achieve a vinyl acetate content of 99.6% in the effluent from the high-pressure column. The simulation file is attached.
Your process does not include a section for removing ALD; since ALD is a light component, it cannot be removed from the bottom of the tower, which leads to an increasing accumulation of this substance. Therefore, you should add an additional tower for removing light components, or vent gas from the top of the high-pressure tower (this approach may result in significant waste)
Sensitive to pressure? What is the basis for this judgment? Thank you!
There are a few points LZ should pay attention to: 1. Agree with the view of zhxbkofkyo above – first remove the light components completely； 2. In this system, the VLE lines for methanol and vinyl acetate are very close to each other, which makes separation difficult. This can also be seen from their respective boiling points and azeotrope temperatures; usually the difference in boiling points is less than 15°C, requiring either a large number of theoretical plates or a high reflux ratio, resulting in high energy consumption ； 3. Based on your phase diagram analysis, this system cannot be considered a pressure-sensitive system; it is only moderately sensitive to pressure. Moreover, the difference in azeotropic composition between 40 kPa and 200 kPa is less than 0.1. It is therefore impossible for the tops of the two columns to be exactly at the azeotrope temperature – both will shift inward, resulting in an even smaller difference in composition or even none at all. This leads to a very large circulation volume. Suggestion: To implement variable-pressure distillation, increase the pressure in the high-pressure column appropriately to enhance the difference in azeotrope compositions. Replace the low-pressure column with a column at atmospheric pressure to avoid the use of vacuum pumps; meanwhile, condensed water at 32°C–42°C can be used at the column top ； Theoretically, even if it is achieved, the energy consumption will still be high.
Vinyl acetate polymerizes slowly on its own, and the polymerization accelerates when heated.
Yes, vinyl acetate will undergo self-polymerization. Is this self-polymerization carried out in the gas phase or in the liquid phase? What is the typical temperature for such self-polymerization? Is it possible to add a polymerization inhibitor to the high-pressure column during pressure swing distillation? Currently, in the practical operation of industrial extractive distillation, a polymerization inhibitor is added during distillation
When the pressure in the high-pressure tower is increased, the azeotrope temperature rises; for example, when the pressure is raised to 500 KPA, the azeotrope temperature can reach around 100°C. At this temperature, vinyl acetate tends to polymerize on its own, and it is unclear what the effect of adding a polymerization inhibitor at this temperature would be
Then I don’t recommend using pressure swing distillation; it has no future. This system is not pressure-sensitive; the PSD cycle ratio is high, resulting in high energy consumption.
Currently, the energy consumption associated with industrial production of extractive distillation is also high; a large amount of extraction water must be used, and subsequently the methanol solution is distilled to obtain 99.9% pure methanol. This process generates a significant amount of wastewater. For 25 tons of vinyl acetate-methanol solution, 16 tons of extraction water are required, and the steam consumption for the distillation of methanol from water in a single column is approximately 60 tons. In total, this entire separation process requires around 70 tons of steam. If variable-pressure distillation is used, with a pressure of 500 kPa in the high-pressure tower, simulation calculations show that approximately 36 tons of steam are required for the separation of vinyl acetate from methanol in order to purify vinyl acetate, while about 26 tons of steam are needed to distill methanol to 99.9% purity. In total, around 62 tons of steam are required, and this approach allows for a reduction of about 16 tons of wastewater.
At 500 kPa, the azeotrope temperature is 108°C; what is the bottom temperature of the atmospheric pressure column? It should be less than 108. The steam from the top of the pressurized tower can be used to heat the atmospheric pressure tower, which allows for the saving of over 20 tons of steam. If the temperature difference is not sufficient, the pressure in the atmospheric pressure tower can be reduced slightly
Self-polymerization occurs in the liquid phase, and it accelerates in a boiling state. Please check the book \"Polyvinyl Alcohol Production Process\"; it contains information on this topic. The inhibitor is dissolved in vinyl acetate; its solubility is very low, and the amount added is also small, entering the tower along with the reflux.