Industrial & Engineering Chemistry Research, Vol.44, No.24, 9208-9220, 2005
Startup of homogeneous azeotropic distillation sequences with multiple steady states
The operability of an azeotropic sequence, which consists of an azeotropic distillation column with multiple steady states and an entrainer recovery column with the corresponding recycle stream, is analyzed by dynamic simulation during its startup. The goal is to study the effect of the recycle stream on the start-up performance. The well-known benzene-heptane-acetone (BHA) system is studied. Given open-loop (manual) start-up strategies, the way in which the system reaches different steady states by applying different start-up configurations and/or policies can be observed through the temperature profile evolutions. These are generated using different initial charges in one or both columns, and by applying different schedules for the evolution of the operative variables. Indeed, a partition of the start-up policy space yielding different solutions is achieved. Therefore, it is possible to identify a set of critical values for the supervision of the startup. Based on these results, very simple closed-loop start-up policies, using a simple PI controller, are analyzed. The study shows that the desired steady state can always be obtained by providing a minimum benzene flow rate, which has to be maintained during the startup. Thus, a methodology that uses dynamic simulation to search systematically suitable start-up procedures is presented. An adequate start-up operation mode (minimizing start-up time) is determined for the BHA system. Moreover, this start-up strategy is also applied to another well-known system with multiplicity: the methanol-methylbutyrate-toluene (MMT) case. In this example, the start-up time is also reduced to a minimum, in comparison with all the other policies explored.