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What conditions or principles usually dictate whether an impure feed should be separated prior to entering a process? Give three examples that violate these principles.

Question

What conditions or principles usually dictate whether an impure feed should be separated prior to entering a process? Give three examples that violate these principles.

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Step 1

Feed stream entering the process do not contain pure chemicals. There is scope for purification. The question of whether this purification step should be performed can be answered only by a detailed economic analysis.

Following are some conditions where purification is needed.

  1. If the impurities foul or poison the catalyst, then purify the feed. For example, one of the most common catalyst poisons is sulfur especially cobalt, nickel, platinum. In such case the impurities must be separated before entering a process.
  2. If the impurities react to form difficult-to-separate or hazardous products, then purify the feed.
  3. If the feed contains any harmful or unuseful components, then purify the feed.
  4. If the impurity is present in large quantities, then purify the feed. It is obvious because significant additional work and heating/cooling duties are required to process large amount of impurity.
Step 2

The following are the examples where separation of impurities can be avoided.

  1. If the impurities are not present in large quantities (say <10-20%) and these impurities do not react to form by-products. Example the hydrogen feed to toluene HDA process contains a small amount of methane (5 mol%). Because methane does not react and is present in small quantity, it is not worth considering separating it from hydrogen.
  2. If separation of impurities is difficult. For example, if an impurity forms an azeotrope with the feed or feed is a gas at the feed conditions, then do not separate them prior to feeding to the process. Again, methan...

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Chemical Engineering

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