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Methanol Azeotrope with Acetone

  • 1.  Methanol Azeotrope with Acetone

    Posted 25 days ago
    Hi All

    As per the figure below, the azeotrope forms at around a 78.8% acetone in acetone-methanol mixture at atmospheric pressure. However, how much acetone in initial mixture, is considered to be practically sufficient to form azeotrope in a distillation column?

    Thanks
    Musallam

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    Musallam Al-Awaid PE,CSP
    Process Optimization Team Leader
    Salalah Methanol Company
    Salalah
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  • 2.  RE: Methanol Azeotrope with Acetone

    Posted 24 days ago
    The answer is the feed composition of acetone or methanol has no effect on the formation of an azeotrope in a distillation column. The first thing you need to realize is that the separation is between acetone and the azeotrope or methanol and the azeotrope. You have to think of the azeotrope as a separate chemical compound. The composition of the azeotrope is 78.8% acetone, 21.2% methanol. If a mixture at the azeotropic composition is fed to the column, no separation occurs. Let's consider a feed mixture consisting of <78.8% acetone -- say 50% acetone.  For 100 moles/hr of feed, you have 50 moles/hr of acetone and 50 moles/hr of methanol.

    In an azeotropic column, you can obtain very high purities without a large number of trays or a large reflux ratio. That's because the azeotrope boils at a much lower temperature than the components that make it up. When designing an azeotropic column for this case, you would either separate the acetone or the methanol from the azeotrope. This produces the azeotrope as the overhead product and either high purity acetone or methanol as the bottom product.

    In our example, we have 50 moles/hr of acetone in the feed. If that is removed via the azeotrope, it will require 13.45 moles/hr of methanol being removed in the overhead product. That leaves 36.55 moles/hr of methanol in the high purity bottom product. The result is the column has an overhead product rate of 63.45 moles/hr with a composition of 78.8% acetone and 21.2% methanol and a bottoms product rate of 36.55 moles/hr with a composition of 99%+ of methanol. What happens as the composition of acetone in the feed decreases is that you produce less and less of the overhead product. The only limit is how small an overhead product rate is acceptable.

    What happens if the acetone concentration in the feed is >78.8%. Let's look at a feed that consists of 85% acetone. For a feed of 100 moles/hr, you have 85 moles/hr of acetone and 15 moles/hr of methanol. To remove all of the methanol via the azeotrope will require 55.75 moles/hr of acetone. That leaves 29.25 moles/hr of acetone in the high purity bottom product. The result is a column that has an overhead product rate of 70.75 moles/hr with a composition of 78.8% acetone and 21.2% methanol and a bottoms product rate of 29.25 moles/hr with a composition of 99%+ of acetone. Similar to the previous case, an increase in acetone concentration in the feed results in decreases in the overhead product rate.

    In summary, if the acetone concentration in the feed is <78.8%, a column that produces the azeotrope as the overhead product and a high purity methanol stream as the bottom product is required for separation. If the acetone in the feed is >78.8% a column that produces the azeotrope as the overhead product and a high purity acetone stream as the bottom product is required for separation. The azeotrope will always form. What varies is the quantity of the azeotropic product.

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    John Braccili
    Wallingford, PA
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  • 3.  RE: Methanol Azeotrope with Acetone

    Posted 24 days ago
    Thanks alot John

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    Musallam Al-Awaid PE,CSP
    Process Optimization Team Leader
    Salalah Methanol Company
    Salalah
    ------------------------------