Theoretical and Experimental Studies on Startup Strategies for a Heat-integrated Distillation Column System

Due to their higher efficiency of energy utilisation heat-integrated column systems have been widely used in chemical industry. However, the heat-integration leads to difficulties in startup of such columns, i.e. a long startup time and thus considerable costs will be resulted. In this work, a study consisting of modelling, simulation, optimisation and experimental verification is made for developing optimal operation strategies for heat-integrated columns to reduce the startup time. A pilot twopressure column system with bubble-cap trays is considered. As a result, more than 35% of the startup time can be reduced in comparison to conventional startup procedure. Heuristics for startup operation of such processes are suggested. INTRODUCTION Process intensification has been recently extensively studied and applied in chemical industry with the aim of reducing the costs of equipment and operation. An example of such intensification is the adoption of heat-integrated distillation column systems. Heat integration of two columns is based on the idea of utilising the overhead vapour of one column to provide the latent heat for boiling up the other column. Thus the energy consumption can be reduced while accomplishing the same separation task. However, the heat-integration leads to difficulties in startup operation due to strong coupling of the two columns. A long time period is usually needed for startup of the process. A long startup time costs considerable amount of energy and off-product. Thus optimal strategies are desired from industry practice to shorten the startup period. In spite of its importance, very few previous work has been done on startup optimisation for distillation processes. Conventionally, the values of control variables corresponding to the steady state values are set to the columns for startup and one just waits for columns running to the desired steady state (so called direct setting strategy). Empirical startup strategies like total or zero reflux flow and maximum reboiler duty for single columns have been proposed [1, 2, 3] to improve the startup performance. Since a column startup is influenced by many factors such as column structure, type of trays and packings, components in the mixture to be separated as well as top and bottom specifications, these empirical strategies are suitable only for some specific cases. Therefore, systematic approaches concerning these influential factors are needed in order to solve general startup problems for distillation column systems. This calls for methodologies of modelling, simulation and optimisation. In addition, due to their higher complexity, very few studies have been made on startup of heat-integrated columns [4]. The essential difficulty in modelling column startup lies in the fact that it is a complex dynamic process. In most startup models for distillation columns, the "three-phasemodel" (discontinuous, semicontinuous and continuous phase) proposed by Ruiz et al. [5] has been used. The discontinuous phase is the time period from an empty cold column to the beginning state of equilibrium. Hangos et al. [6] studied this discontinuous phase with a simplified non-equilibrium model. Wang et al. [7] proposed a model considering the tray-by-tray state transfer from non-equilibrium to equilibrium that takes place at the boiling temperature at the operating pressure. Compared with the other two phases in which the column is in the vapor-liquid equilibrium state, this discontinuous phase is much shorter. In addition, hydrodynamic properties on trays and packings in the column play an important role in modelling startup processes and thus should be considered. The model equations result in a large-scale nonlinear differential algebraic equation (DAE) system. Based on an established model, simulation can be made by solving the DAE system to study the startup behaviour [7, 8, 9]. For simulation an operating policy during startup has to be defined a priori. This means it may be neither optimal (in the sense of minimising the startup time) nor feasible (in the sense of holding the process constraints, e.g. the product specifications at the desired steady state). Thus, a mathematical optimisation has to be employed to search for an optimal as well as feasible operating policy. Optimisation approaches to solving large-scale problems have been proposed in the previous studies [10, 11, 12]. The basic idea of these approaches is to discretize the dynamic system into a large nonlinear programming (NLP) problem so that it can be solved by an NLP solver like sequential quadratic programming (SQP). These dynamic optimisation approaches can be used to solve startup optimisation problems for distillation columns. In this work, a systematic study on startup strategies for heat-integrated distillation columns is made. A pilot two-pressure heat-integrated column system is considered. The process is modelled with a detailed dynamic model consisting of the MESH equations and hydraulic relations. The model is validated by experimental studies on the pilot plant. Simulation and optimisation are carried out for a preliminary study to gain a rough insight into the dynamic behaviour and to develop the tendency for optimal operating policies with the aim of reducing the startup time. The optimisation approach used is a sequential method with a capability of dealing with large-scale problems. Extensive experiments are conducted to test these results and to adapt them to different column configurations. As a result, significant reduction of the startup time can be achieved by implementing the optimal operating policy. Based on the results of simulation, optimisation and experimental study, some heuristic rules for optimal startup for heat-integrated column systems are suggested. PLANT AND MODEL DESCRIPTION As shown in Fig. 1, the plant considered is a pilot two-pressure column system consisting of a high pressure and a low pressure column both with a diameter of 100 mm. The columns have a central down-comer with 28 and 20 bubble-cap trays, respectively. The overhead vapour from the high pressure column (HP) is introduced as the heating medium to the reboiler of the low pressure column (LP). The plant is so constructed that it is possible to operate the process in downstream, upstream and parallel arrangements. Fig. 1 (right) shows the flow-sheet of the plant with the parallel arrangement. The plant is equipped with temperature, pressure, level and flow rate measurements and electrical valves for flow control. All input/output signals are treated by a process control system. For the experimental study, startup of the plant to a steady state for the separation of a methanol-water mixture is considered. The reboiler duty of HP and reflux flow rate of both HP and LP are the manipulated variables to be optimised for the startup procedure.