Simulation Of Startup Operation For Batch Distillation Starting From A Cold State

Due to its high flexibility the batch distillation is widely used in chemical industry. A property of batch distillation is that the batch column will be frequently started up from a cold empty state. The initial charge in the reboiler, heating style and reflux operation may be variable from batch to batch. The performance of startup from these initial conditions will impact the whole batch operation. Therefore, the study of startup behaviors of batch distillation is important to industrial practice. Modeling of batch distillation has been mostly based on equilibrium model in previous studies. It leads to a large-scale differential algebraic equations (DAEs) system. Due to its dynamic nature, initialization of the system has been a problem. A model to express startup behaviors of batch columns starting from an empty cold state is proposed to handle these issues.For each tray, we can get its dynamic feature through observation and analysis of the phenomena on the tray. At the very beginning we define the trays in the column are at the state of empty (EM). Then the tray is transited from EM to the state of liquid accumulation (LA), due to the condensation of the rising vapor on the tray. At the same time the condensed liquid on this tray is heated by the rising vapor from the reboiler. When the temperature increases at its bubble point, the state of the tray switches to vapor-liquid equilibrium (VLB). The sequence of the transitions of a tray is from EM to LA and VLB. When the vapor arrives in the total condenser, the holdup of the trays will be increased significantly after the reflux valve opened. The startup is ended when the liquid flow reaches the lowest tray of the column, which means that all trays are in VLE and have an enough liquid holdup. The startup features are common to general batch tray columns. The fact that the temperature rises tray by tray from bottom to top and the column is filled with the liquid flow essentially from top to bottom during startup is well-known in industry practice.A unified equation system can describe the switching from a non-equilibrium phase in which only mass and energy transfer are taking place to an equilibrium phase inwhich the vapor-liquid equilibrium is held. The switching point between these two phases is decided by the relationship of bubble point temperature at the operating pressure. The liquid holdup of each tray is related to the geometry of the trays, different weir height is used to describe the holdup change during startup period. The model for startup operation appears discontinuous between nonequilibrium and equilibrium phase. The structure discontinuous mode of the model related with the working state of the column during startup operation. But to a unified equation system, the structure of differential equations must be set to the same in all different phases. So trivial equations are introduced to realize this aim. In the numeric method, three types of variables are divided, company with three different ways to set variables' initial value. The software package gPROMS is used for solving the equations system. It is unique amongst commercial simulators in its facilities for describing hybrid processes with discontinuities.Two cases are used to validate the correctness of the model. The first process considered is a well-known batch distillation given by Nad and Spiegel (1987). The second is a pilot batch column in lab. It can be seen that the model can describe the startup process with a high accuracy by the comparison of the simulation results and experiment data. The dissertation is concluded with a summary and prospect of future researches.