Simulation and experimental testing of reactive distillation column for ETBE synthesis

Abstract

Reactive distillation is a novel alternative to the sequential operation in reactor and distillation. It offers advantages in chemical reaction by shifting the chemical equilibrium and separation by overcoming distillation boundaries. Economic advantages also result from direct energy integration and reduction of equipment costs. Reactive distillation is accepted as a preferred technology for producing oxygenates, including ethyl tert-butyl ether (ETBE). The available reactive distillation column in School of Chemical Engineering, Curtin University of Technology was designed to produce ETBE. The mathematical models of the column written in Prom and SpeedUp Simulation Packages were significantly modified to study other aspects of reactive distillation phenomena, which were not previously considered by using steady state as well as dynamic simulations. The ETBE reactive distillation column exhibits multiplicity phenomena, which influence the transient condition during start up and shut down operations. The input multiplicity region that depends on the operating condition is significant during start up operation. The input multiplicity could increase or decrease the overall process performance depending on the chosen operating condition. Regarding output multiplicity, different start up strategy results in different output conditions. This result implies that the column could be sent to an inappropriate operating condition due to the lack of understanding of the output multiplicity. Three scenarios for shut down operation were considered in order to maintain high isobutylene conversion as well as ETBE purity. Since the multiplicity phenomena result from interaction between reaction and separation effects, the existence of mUltiplicity depends on the number of separation and reaction stages. Several columns were simulated to investigate the effects of different number of either separation or reaction stages on the multiplicity phenomena. It is speculated that input multiplicity always occurs in any reactive distillation column. This result implies that several reactive distillations maybe designed to satisfy the same process requirements. On the other hand, output multiplicity region could be magnified by increasing the number of reactive and separatio tages. Besides, output multiplicity could be found for low internal rates for high number of stages of the reactive distillation column. The control schemes for the ETBE reactive distillation were also compared with respect to their disturbance rejection capabilities. One-point control schemes may be used to implicitly control the two main objectives of this reactive distillation column. It was found that disturbance gains resulting from steady state analysis could be used to predict the dynamic changes of the system resulting from the same disturbances. Among the control schemes studied, the LB scheme is the most suitable scheme to reject load disturbances at one-point scheme for this reactive distillation. The LB scheme also has excellent capability to change the steady state conditions through changes in the set-point temperature. Although the LV control scheme also has capability to reject load disturbances, the small step changes in the set-point temperature could destabilise the system. This results from the limitation in the reboiler sump, which is dry for step increases and floods for step decreases in the setpoint temperature. It is recommended to use bigger reboiler sump in this reactive distillation. A detailed comparison for the LV and LB. control schemes were investigated by using proportional-integral (PI) controllers with various values for the gains and reset times. It was found that the same values cannot satisfy both positive and negative disturbances. This results from the high degree of non linearity and bi-directionality of the system. The Amberlyst-15 catalyst was first tested to determine the activity. The results show that the catalyst particles still have adequate activity to proceed the synthesis of ETBE. Experiments were then conducted under proposed batch operation to avoid any polymerisation of isobutylene. However, continuous operations could not be run due to pump failure. Other reasons for these difficulty and possible solutions to solve this problem are explained in the thesis. Efforts were also made to connect the controllers to a PC having SCAN 3000 through an interface converter, so that the rig can be used for cor.trol studies in future projects.