Control System Design for Energy Efficient On-Target Product Purity Operation of a High-Purity Petlyuk Column

Abstract

Energy efficient, on-target product purity operation of a high-purity three product benzene−toluene−xylene ternary Petlyuk column is studied. The basic regulatory control system consists of four temperature inferential control loops with a fixed prefractionator vapor-to-fresh feed ratio. An economic control system on top of the regulatory layer adjusts these five set points. It consists of three product purity controllers that adjust three temperature set points along with a reboiler duty reduction controller that adjusts the remaining two free set points in the regulatory layer. The latter makes these adjustments to prevent the downward curvature in the prefractionator and main column middle section temperature profiles from being too large. Closed loop results for large feed composition changes show significant energy savings (up to 15%) are realized via temperature profile curvature control compared to constant set point column operation. The case study highlights the need for innovative control strategies for realizing the sustainability benefit of the integrated complex Petlyuk column during actual operation. ■ INTRODUCTIONIn the process industry, distillation remains the most preferred and widely used unit operation for separating liquid mixtures into constituent pure (pseudo) components. 1 The basic idea is to utilize the difference in the volatility of the mixture components to purify it by repeated flashing. This is accomplished via countercurrent vapor−liquid contact on the trays of a simple distillation column with the reboiler providing the vapor stream into the bottom and the condenser providing refluxed liquid to the top of the column. The process is then naturally energy intensive with the reboiler heat driving the separation so that distillation alone can contribute up to 53% of plant energy costs. 2 Thus, innovations toward energy efficient distillation configurations for a given separation task have traditionally been of interest to the process industry. The volatility in energy prices in recent years has renewed interest in the synthesis, design, operation, and control of complex column configurations that can be significantly more energy efficient than a conventional light-out-first (direct sequence) or heavy-out-first (indirect sequence) train of simple distillation columns.In pioneering work, Petlyuk et al. 3 suggested a complex configuration consisting of a prefractionator followed by a main column with a side draw for separating a ternary ideal mixture into its constituent pure components (Figure 1a). Compared to a conventional two-column direct or indirect sequence, the prefractionator in the Petlyuk configuration mitigates remixing of the middle boiler, which distributes itself between the prefractionator top and bottom products. This reduces the inherent process irreversibility, leading to potentially significant energy savings. Literature reports (see e.g. Triantafyllou and Smith 4 ) indicate impressive energy savings up to 40% for a Petlyuk configuration over a conventional two-column...

Optimal Operation and Control of Four-Product Dividing-Wall (Kaibel) Distillation Column

“…Install a partition or dividing wall in the middle third of the distillation column e.g., 33-66% of height of distillation column.…”

Dynamic Performance of a Complex Distillation System to Separate a Five‐Component Hydrocarbon Mixture

“…Thus, innovations toward energy‐efficient distillation configurations for a given separation task traditionally have been of interest to the process industry. The volatility in energy prices in recent years has renewed interest in the synthesis, design, operation, and control of complex column configurations that can be significantly more energy‐efficient than a conventional light‐out‐first (direct sequence) or heavy‐out‐first (indirect sequence) train of simple distillation columns .…”

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

“…This design has been adapted from a recent article from our group. 13 For more realistic simulations, we consistently use pressure driven dynamic simulations, in which the internal column pressure profile changes with operating conditions. Theoretically, the DWC has four steady-state dofs corresponding to the liquid split (β L ) to the prefractionator, the reflux rate (R), the side-draw rate (S) and the reboiler duty (Q R ).…”

“…Figure shows the base-case design and operating conditions, including temperature and composition profiles, of the DWC for the equimolar fresh feed. This design has been adapted from a recent article from our group . For more realistic simulations, we consistently use pressure driven dynamic simulations, in which the internal column pressure profile changes with operating conditions.…”

“…This design has been adapted from a recent article from our group. 13 For more realistic simulations, we consistently use pressure driven dynamic simulations, in which the internal column pressure profile changes with operating conditions.…”