Operation of homogeneous azeotropic distillation column sequences

A self-optimizing control concept is provided for sequences of three homogeneous azeotropic distillation columns with two recycles separating a ternary feed of a 020 mixture that has two simple distillation regions that are separated by a residue curve boundary. Studying a case with reduced complexity (columns of infinite length operated at infinite reflux), one of the two recycles (the entrainer recycle) is identified as the key manipulated variable for ensuring optimal operation of the sequence. Different setups of the scheme have different optimal feed regions. The theoretical results are illustrated with rigorous simulations of a 020 mixture: methanol (which acts as an entrainer), 2-propanol, and water. Dynamic simulations confirm that the process operates robustly by using the (L/D)Q(R) scheme for the individual columns and one additional control loop that controls the methanol (entrainer) composition of the distillate by manipulation of the overall feed of one column. Dynamic simulations also confirm that column profiles can cross residue curve boundaries, distillation boundaries, and even azeotropes if the feed composition is changed such that a column profile is only feasible in the other distillation region.