Optimal Safe Start-up of a Glycolether Process

Althougln start-up and shutdown only account for a small percentage of the total operating time of an industrial chemical plant, still up to 4 6% of the accidents occur during the start-up or shutdown. Furthermore, a large part of the costs of a start-up account for the extra personnel that is needed on site. Therefore, it is logical to optimise both start-up time and safety. For this optimisation, dynamic models that simulate start-up behaviour are needed. The basis for this graduation assignment is a dynamic model of a glycolether process that has been developed in the equation-oriented simulation environment gPROMS. This process has a typical process layout, which consists of a reaction and recovery section with recycle. The recycle is used for heat integration with the feed. During start-up the system must be heated, because the reaction can only start at a high temperature. In this work the reboiler provides the heat and through the recycle the reactor is heated, making the use of a start-up heater unnecessary. Starting up the system in an integrated way and the presence of discontinuities provide a numerically challenging problem. Discontinuities are present, because the distillation column model incorporates equations for liquid weeping and vapour flowing through a downcomer; flows that are typically present during the first phase of a start-up. General guidelines for start-up were used to generate a basic start-up procedure, which is used for comparison with the optimised procedure. The optimisation objectives are a minimisation of start-up time and a maximisation of safety during start-up of the integrated process. Safety includes a maximum breakthrough of reactants to the separation section, a maximum temperature in the reactor and a maximum temperature change everywhere in the process. A Pareto curve is used to visualise the trade-off between these objectives. Optimisation strategies for the reactor comprise choosing the optimal feed temperature (and thereby the optimal recycle temperature), choosing the optimal moment and quantity of reactant and catalyst addition and choosing the moment that the cooling water is started. Strategies for the distillation column involve establishing an optimal reboiler duty strategy and choosing the initial amount and composition of material in the reboiler and condenser. It has been found that ramping PO feed as well as the cooling water flow have a large influence on the safety objective. It has also been found that the total feed flow should be low for safety and for start-up time, because a large excess of MeOH absorbs so much heat from the process that the reaction can not start-up gradually. The temperature of the feed should be high to minimise start-up time, but low for a maximisation of safety. The Pareto curve obtained from the optimisations in gPROMS shows a trade-off between the objectives time and safety. It also shows that not much extra safety is gained if the start-up time is extended very long. However, which operating point on the curve is preferred depends on specific preferences.Process Systems EngineeringDelftChemTechApplied Science