PROCESS-CONTROL FOR ENERGY INTEGRATED DISTILLATION SCHEMES

Two energy integrated distillation schemes far the separation of three different ternary mixtures of four typical feed compositions have bee n rigorously investigated and compared to the best conventional two-co lumn distillation scheme. The two energy integrated schemes are: the h eat integrated two column system and the fully thermally coupled disti llation column (FTCDC), also known as Petlyuk column. The comparison o f the energy savings and the total costs of the investigated schemes s how that the heat integrated two column scheme is economically always. better than the conventional scheme. The FTCDC shows considerable ene rgy savings in several cases, however, it can be competitive with the heat integrated two column system only in those rare cases where the c oncentration of the middle component is high, the split between the fi rst 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 controllab ility studies for the two energy integrated distillation schemes. Degr ees of freedom analysis and steady-state multivariable control structu re synthesis,tools show that both investigated schemes can be controll ed by conventional decentralised control structures, however, in the c ase of the heat integrated two column scheme heuristics and also the s teady-state control indices show that the interaction among the contro l loops is less than in the case of the FTCDC. The dynamic simulations of the two energy integrated schemes without and with the most promis ing control structures support the steady-state analysis. They also de monstrate that in the case of the control of the FTCDC due to the sign ificant interaction among the control loops detuning of the controller s is necessary. Higher overshoots, longer settling time and more slugg ish behaviour are expected compared to the heat integrated two column system. (C) 1998 Published by Elsevier Science Ltd. All rights reserve d.