Role of Nonlinear Effects in Benzene–Toluene–Xylene Dividing Wall Column Control System Design

Abstract

Steady-state bifurcation analysis is applied to a ternary benzene−toluene−xylene (BTX) dividing wall column (DWC) to synthesize robust temperature inferential controlled variables (CVs). The coupling between the prefractionator and the main column causes steady-state multiplicity in conventional tray temperature CV loops. Using physical insights into the DWC behavior, differential tray temperature across the side-draw and double differential temperature control across the prefractionator rectification section are shown to significantly mitigate the steady-state multiplicity. Control system variants (with and without prefractionator liquid split manipulation) with alternative temperature-based CVs exhibit nonlinear dynamic phenomena of seeking an infeasible temperature/purity set point and input multiplicity induced "wrong" control action leading to a low purity steady-state transition under closed loop operation. These nonlinear dynamic phenomena correlate with the bifurcation analysis results. The recommended control structure with robust temperature inferential CVs is shown to effectively reject very large feed composition changes, including for on-target product purity operation via temperature inferential CV update. The work brings out the importance of robust CV design for effective control of the highly coupled and nonlinear ternary DWC process. ■ INTRODUCTIONThe ternary dividing wall column (DWC) emulating the Petlyuk sequence 1 in a single column shell has been known to be significantly more energy efficient and cheaper than the conventional light/heavy-out-first distillation sequence. For a ternary mixture of components A (light), B (intermediate), and C (heavy), the feed side of the dividing wall (see Figure 1) acts as the prefractionator (TS1 and TS2) and does the easy AC split. The tray sections above and below the side-draw (TS4 and TS5) prevent, respectively, light A and heavy C leakage in the side-draw. The main column rectifying (TS3) and stripping (TS6) sections prevent B leakage up the top and down the bottoms, respectively. With the prefractionator performing the easy AC split, the intermediate boiler B distributes evenly in the prefractionator top and bottom mitigating its remixing and thus inherent thermodynamic irreversibility, compared to a conventional two-column sequence. Consequently, significant energy saving in the range of 20−40% can be achieved. 2−5 Despite its high energy efficiency, reports of industrial DWC applications have only recently appeared in the literature, BASF being the industry pioneer. 6 More recently, extractive DWC distillation 7 and multiple DWCs 8 have been reported in the literature. The reluctance on part of the industry to adopt DWC technology is due to the control system design being significantly more complex compared to conventional simple distillation. Specifically, the highly nonlinear interaction between the prefractionator and the main column makes tight quality control on the intermediate boiler (B) side-draw stream around a stringent target (e.g., >99%...

Numerical and Experimental Research on a Kaibel Divided-Wall Column: Design and Steady-State and Dynamic Operation

“…Although simple temperature control structure is widely used due to its great industrial prospect, it may lead to a steady-state deviation for the control of DWCs. More advanced temperature control structures were proposed and studied for the dynamic control of DWCs (e.g., asymmetrical temperature control, double temperature difference control, and pressure-compensated temperature control structures), ,, and the application of these advanced temperature control schemes to handle the industrial control of the KDWC need to be further studied in the future due to more complex issues, like pressure disturbance in the column, impurity sources, and higher control requirement.…”

Inferential Self-Optimizing Control of a Reactor–Separator–Recycle Process

“…Direct rectifying tray temperature control gave unacceptably loose regulation of x C R , which is what necessitated direct x C R control. In the literature, tray temperature combinations have been used for tighter inferential composition control. − One of the most commonly applied combinations is the difference between tray temperatures (Δ T ) . The possibility of using Δ T as a CV in lieu of direct x C R control is explored.…”

Configuring Effectively Double Temperature Difference Control Schemes for Distillation Columns

“…4,5 Gupta and Kaistha also indicated the reasonability and effectiveness of employing simplified DTDC schemes to deal with the nonlinear effect of a benzene−toluene−xylene DWDC. 6 These unexpected outcomes have reminded us of the drawbacks of the conventional method adopted for the synthesis and design of the DTDC schemes and stimulated us to trace the problems that caused such a deficiency. As a result, a more effective method that permits tight inferential control of various distillation columns is likely to be derived, and this is the major purpose of the current study.…”

“…Owing to the involvement of more temperature measurements, double temperature difference control (DTDC) schemes outperform generally temperature control schemes and temperature difference control schemes in the inferential control of various distillation columns. − Not only are the static offsets in the controlled product qualities reduced, but also the capability of rejecting feed composition disturbances is enhanced. Contrary to this common sense, we found recently in our studies about the operation of dividing-wall distillation columns (DWDCs) that some simplified DTDC schemes, which involved relatively a reduced set of temperature measurements, could still give quite similar or even better performance than the DTDC schemes. , Gupta and Kaistha also indicated the reasonability and effectiveness of employing simplified DTDC schemes to deal with the nonlinear effect of a benzene–toluene–xylene DWDC . These unexpected outcomes have reminded us of the drawbacks of the conventional method adopted for the synthesis and design of the DTDC schemes and stimulated us to trace the problems that caused such a deficiency.…”

“…Thus, the deficiency of the DTDC schemes seems to be not closely related to the identification of sensitive stages. For the determination of reference stages, not much attention has been given so far and they have frequently been determined via insightful operation analysis. ,, Based on SVD analysis Ling and Luyben suggested locating their positions that yield the highest sensitivities between the temperature differences (TDs) thus formed with the given sensitive stages and the manipulated variables adopted . By following exactly the same principle, Wu et al determined the upper and lower reference stages and proposed consequently a DTDC scheme for the operation of DWDCs .…”