Dynamics and control of multistage membrane plants
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The controllability of multistage membrane separation processes is examined, and methods of improving closed-loop performance are assessed in this thesis. Membrane systems exhibit non-linear time-variant behaviour which poses special challenges. Extensive analysis of dynamic process characteristics was undertaken to develop an understanding of inherent system behaviour. In this work two case studies are examined, based on membrane separations performed in the New Zealand dairy industry, one manufacturing a retentate and the other a permeate product. Both produce a foodstuff, and are subject to operating constraints specified in the interests of product safety. The initial part of this thesis reviews membrane separations, and develops a general framework for modelling the dynamic behaviour of multistage membrane processes. Specific models are developed for each case study, which exhibit characteristics representative of industrial membrane separations. These models are analysed extensively in this dissertation, and also used as the basis for open- and closed-loop process simulation. In order to improve closed-loop process performance it was first necessary to develop an understanding of dynamic process behaviour. Qualitative analysis of structural system models showed the inherent characteristics of the two case studies to be similar, with inverse response and oscillatory characteristics feasible within both separations. It was found that multistage membrane processes have widespread disturbance propagation, due to concentration-dependent permeate fluxes and constant volume plant design. Numerical controllability assessment confirmed the presence of inverse response and oscillatory behaviour within both case studies. Oscillatory eigenvalues were not present in flowsheets with few stages, showing that flowsheet design has a significant impact on dynamic process behaviour. Analysis also showed that the use of diafiltration injection hastens the onset of oscillatory behaviour in systems with few stages. This illustrates the significant effect diafiltration injection has on dynamic process characteristics. The conclusions drawn in this thesis are valid for both retentate and permeate product separations. Interaction between different control variable pairings was quantified using the relative gain array. This analysis tool indicated that high levels of interaction were present within both case study systems for certain input-output variable pairings. The preferred variable pairings for the retentate product case study are consistent with industrial practice in New Zealand. No preferred variable pairings were identified for the permeate product case study, and instead it was concluded that directly controlling the permeate stream properties is generally undesirable. For a multistage membrane plant producing a permeate product, it is best to control the concentration and purity of the retentate stream, selecting setpoints corresponding to the desired permeate stream properties. This analysis contributes to a fundamental understanding of multistage process behaviour, and is applicable to any liquid-phase pressure-driven membrane separation. Closed-loop simulation was used to examine achievable process performance. In both case studies, the retentate stream was controlled to achieve a composition corresponding to the desired retentate purity or permeate yield, using the preferred input-output variable pairings identified by the relative gain analysis. It was concluded that the performance of the multi-loop PID strategy is limited by the inherent characteristics of a membrane process. The regular addition of new separation stages also degrades the quality of control that can be achieved, and causes the dynamic process characteristics to change significantly over time. Analysis showed retentate composition control to be difficult in membrane plants, due to highly variable process dynamics and occasional inverse response behaviour. It was concluded that conventional diafiltration injection strategies limit the achievable closed-loop performance of multistage plants with variable membrane area. The closed-loop simulations successfully identified limitations on closed-loop performance, and highlighted the few options for the mitigation of these constraints. The final part of this thesis presents an innovative multivariable controller which attempts to avoid the identified limitations on process performance associated with conventional diafiltration injection strategies. This strategy attempts to maintain the total solids concentration and purity of each fractionation stage somewhere on a specified reference trajectory, by simultaneously manipulating all variables in the input set. Diafiltration flow rate inputs are directly manipulated, rather than maintained as proportions of the permeate flows. A real-time model is used to supply estimates of current process conditions, and predictions of the process response to chosen input combinations. The multivariable controller exhibits superior ability to reject measured system disturbances, such as those caused by the introduction of new separation stages. Most significantly, closed-loop simulation demonstrates the ability to maintain the desired retentate purity or permeate yield for the duration of production despite changes in process behaviour during this time. This controller strategy could easily be applied to any other multistage membrane plant using diafiltration injection to achieve enhanced separation.