Process control for energy integrated distillation schemes

Abstract

Two energy integrated distillation schemes for the separation of three different ternary mixtures of four typical feed compositions have been rigorously investigated and compared to the best conventional two-column distillation scheme. The two energy integrated schemes are: the heat integrated two column system and the fully thermally coupled distillation column (FTCDC), also known as Petlyuk column. The comparison of the energy savings and the total costs of the investigated schemes show that the heat integrated two column scheme is economically always better than the conventional scheme. The FTCDC shows considerable energy savings in several cases, however, it can be competitive with the heat integrated two column system only in those rare cases where the concentration of the middle component is high, the split between the first and second components is harder than the split between the second and third components, and the separation is not too sharp (Annakou and Mizsey, 1996). The rigorous economic study is supported by controllability studies for the two energy integrated distillation schemes. Degrees of freedom analysis and steady-state multivariable control structure synthesis tools show that both investigated schemes can be controlled by conventional decentralised control structures, however, in the case of the heat integrated two column scheme heuristics and also the steady-state control indices show that the interaction among the control loops is less than in the case of the FTCDC. The dynamic simulations of the two energy integrated schemes without and with the most promising control structures support the steady-state analysis. They also demonstrate that in the case of the control of the FTCDC due to the significant interaction among the control loops detuning of the controllers is necessary. Higher overshoots, longer settling time and more sluggish behaviour are expected compared to the heat integrated two column system.