Control Strategy Used in the Dynamic Simulation

In order to dynamically show the influence of adding a small flowrate of the m-xylene component into feed stream F3 to the overall column performance, the Aspen Plus simulation was converted to a dynamic simulation using Aspen Dynamics. The tray sizing option in Aspen Plus is utilized to calculate the column diameter to be 1.58 m and the hay spacing is 0.6096 m. Other equipment sizing recommended by Luyben is used. The volume of the reboiler is sized to give 10 min holdup with 50% liquid level. The decanter is sized to be bigger to allow for two liquid phases to separate. A holdup time of 20 min is used in the dynamic simulation. A pressure-driven simulation in Aspen Dynamics is used with the top pressure of the azeotropic column controlled at 1.1 atm to allow for some pressure drop in the condenser and decanter so that the decanter can be operated at atmospheric pressure. The pressure drop inside the column is automatically calculated in Aspen Dynamics. 

The determination of the temperature control point inside the column is from a combination of closed-loop sensitivity analysis and verification by open-loop sensitivity analysis. For the closed-loop sensitivity analysis, closed-loop simulation runs are made in which both the top and bottom product purities are held at their specifications for different feed compositions. This is to mimic ideal (although not practical) situation with two online composition measurements and two composition control loops setting aqueous reflux flow and reboiler duty. The tray temperature with the least amount of variation for changes in feed composition is selected as the temperature control point. The specific feed composition changes are feed F3 water molar composition +10% changes while keeping the total molar flowrate of F3 the same by adjusting the acetic acid molar composition. 

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