Industrial applications of the divided-wall column for the separation of ternary mixtures have increased in recent years with about 40 columns reported to be in service. The divided-wall column is a practical way to implement the topology of the Petlyuk column that features two columns (a prefractionator into which the feed is introduced and a main column from which a sidestream product is withdrawn) with interconnected vapor and liquid streams arising from a single reboiler and a single condenser. Many papers discuss the steady-state design issues and propose heuristic and rigorous design optimization methods. The dynamic control of the divided-wall column has been explored in a relatively small number of papers. Control is more difficult than with a conventional two-column separation sequence because there is more interaction among controlled and manipulated variables since the four sections of the column are coupled. The vapor split is fixed at the design stage and cannot be changed during operation, but the liquid split can be manipulated to achieve some control objective. A number of control structures and algorithms have been proposed, but the reported results present a somewhat confusing picture. Different authors draw conflicting and inconsistent conclusions. Most papers control the purities of the three product streams using reflux flow rate, sidestream flow rate, and vapor boilup. This paper proposes a new control structure that controls these purities and also minimizes energy consumption. This is achieved implicitly by controlling a composition of the heaviest component in the prefractionator. Disturbances in feed flow rate and feed composition are used to demonstrate the effectiveness of the proposed control structure. A comparison of the dynamic controllability of the dividedwall column with a conventional configuration is also provided.

The control of a divided-wall column is more difficult than the control of a conventional two-column separation sequence for the separation of ternary mixtures because there is more interaction among control loops. In a previous paper, a control structure using four composition loops was shown to provide effective control of the purities of the three product streams and also achieve minimum energy consumption for both feed flow rate and feed composition disturbances. The numerical example studied the separation of benzene, toluene, and o-xylene. The four manipulated variables were reflux flow rate (R), side-stream flow rate (S), reboiler heat input (Q R), and liquid split (βL) at the top of the wall. In this paper we explore the use of temperatures to avoid expensive and high-maintenance composition analyzers. Two types of temperature control structures are studied. In the first, three temperatures located in the main column and one temperature on the prefractionator side of the wall are used to adjust the four manipulated variables. Feed flow rate disturbances are well handled with this structure, but product purities start to deviate significantly from their desired values for feed composition changes greater than about 10%. In the second control structure, four differential temperature control loops are used. Performance is improved and disturbances of 20% in feed composition are well handled with only small deviations in product purities. This structure also handles large changes in column operating pressure.

The divided-wall column is an energy-saving promising technology for the separation of ternary mixtures, but control is difficult because of its complexity structure and interactions of control loops. Although a few control methods have been reported, the general control method for all ternary mixtures is still pending because of the difference of relative volatilities. In this work, we explore a new control structure by control the remixing of the intermediate at the top trays in the prefractionator section for the separation of benzene, toluene, and o-xylene. It is found that avoiding the remixing of toluene helps the divided-wall column work very close to the optimal conditions. The remixing of toluene is studied at steady state first. Then, a remixing control structure with one remixing control loop and three composition control loops is presented. Minimized energy consumption is achieved by avoiding the remixing of toluene at the top of the prefractionator. Disturbances in feed flow rate and feed compositions are used to demonstrate the effectiveness of the proposed control structure. Remixing profile analysis of intermediate compounds gives an easy and new way to design and control the divided-wall column at energy saving conditions.

The effect of the solvent deasphalting (SDA) process of vacuum residue (VR) blending with coal tar (CT) was studied, and the experimental conditions were optimized. The results revealed that the SDA process was improved by blending with coal tar. The yield of deasphalted oil (DAO) from VR blended with 10% CT was 2.02 wt % higher than that obtained from VR alone. The contents of nickel and vanadium decreased by 5%, and the sulfur content also decreased, whereas the contents of carbon residue and nitrogen remained unchanged. Components such as alkanes and low-condensation aromatics were easily extracted into DAO, leading to an increase of its yield and quality. The viscosity and colloid stability of VR declined after being blended with CT. Furthermore, the structures of VR and CT were characterized, and their structural models were proposed. Thus, their solubility parameters were calculated. According to dissolved balance theory, the mechanism of how the blended CT improved the SDA process was analyzed based on the test results of the surface tension of the extraction and the analysis of the average molecular structure parameters of DAO.