Hybrid modelling membrane processes

Kislik, V., Eyal, A. (2000). Aqueous hybrid liquid membrane process for metal separation. Part I. A model for transport kinetics and its experimental verification. J. Membr. Sci., 169, 119-32. 

Several authors have already developed methodologies for the simulation of hybrid distillation-pervaporation processes. Short-cut methods were developed by Moganti et al. [95] and Stephan et al. [96]. Due to simplifications such as the use of constant relative volatility, one-phase sidestreams, perfect mixing on feed and permeate sides of the membrane, and simple membrane transport models, the results obtained should only be considered qualitative in nature. Verhoef et al. [97] used a quantitative approach for simulation, based on simplified calculations in Aspen Plus/Excel VBA. Hommerich and Rautenbach [98] describe the design and optimization of combined pervaporation-distillation processes, incorporating a user-written routine for pervaporation into the Aspen Plus simulation software. This is an improvement over most approaches with respect to accuracy, although the membrane model itself is still quite... 

In the present work a new developed heat integrated hybrid pervaporation distillation process is modeled and experimental studies are carried out to analyze the effect of the heat integration in the process. With the results of the experiments, the model is validated and a comparison between industrial scale non heat integrated and heat integrated processes is done. As a result, three main advantages are presented in the approach a) reduction of the necessary external energy supply into the process, b) improvement in the pervaporation separation performance and c) reduction in the necessary membrane surface. 

For the model validation and the analysis of the heat integration in the hybrid pervaporation distillation process, a laboratory plant has been built at the TU -Berlin and prepared for the connection with the distillation column (see fig. 3). With this plant experiments with a flat PVA-based (Polyvinylalcohol from GKSS) hydrophilic membrane have been done. A heat exchanger has been built within the pervaporation module. The temperature in the heat exchanger has been necessary to avoid the temperature drop between feed and retentate streams in the pervaporation process. In the process a 2-Propanol/ Water mixture has been separated. The concentration of 2-Propanol in the feed is between 80 and 90 % in weight and the temperature range in the experiments was between 70 and 90°C. The feed flow is turbulent and the system fully insulated to avoid heat looses. The pressure in the permeate side has been kept at 30 mbar and the feed pressure at 1.5 bar. 

The model equations employing biokinetics of Groot et al. and PV data of Gudernatsch et al. were solved using an iterative procedure in a computer program to optimize process parameters for minimum total cost. The hybrid PV + distillation process yields better utilization of the sugar in the feed because of decrease in inhibition. The results indicated that the raw material and membrane fixed cost contribute more than 80% of the direct production cost. Further, the direct production cost of EtOH cost for the hybrid process was found to be 12%-16% lower than the conventional process. 

The present chapter describes how various kinds of advanced models, based either on ANNs or on a hybrid approach, can be used to predict the behavior of some typical membrane processes and to implement efficient control systems. 

Exploitation of ANNs and hybrid modeling in membrane processes... 

In the present contribution, it has been shown that both ANNs and hybrid models definitely represent powerful computational tools, offering very reliable predictions of the actual behavior of membrane systems. The results obtained demonstrate, in particular, that the proper combination of a theoretical model with a straightforward neural model is able to widen the applicability of pure neural models beyond the training range, thus paving the way for the exploitation of the HNM for process optimization purposes and for the implementation of efficient on-line controllers operating on different kinds of membrane processes. 

Curdo, S., Calabro, V., lorio, G., Reduction and control of flux decline in cross-flow membrane processes modeled by artifidal neural networks and hybrid systems. Desalination, 2009,236(1-3), 234-243. 

For the study of the process, a set of partial differential model equations for a flat sheet pervaporation membrane with an integrated heat exchanger (see fig.2) has been developed. The temperature dependence of the permeability coefficient is defined like an Arrhenius function [S. Sommer, 2003] and our new developed model of the pervaporation process is based on the model proposed by [Wijmans and Baker, 1993] (see equation 1). With this model the effect of the heat integration can be studied under different operating conditions and module geometry and material using a turbulent flow in the feed. The model has been developed in gPROMS and coupled with the model of the distillation column described by [J.-U Repke, 2006], for the study of the whole hybrid system pervaporation distillation. 

B. Schlichter, V. Mavrov and H. Chmiel, Study of a hybrid process combination ozonation and membrane filtration — filtration of model solution, Desalination, 156 (2003) 257-265. 

J. F. De Jong, H. J. van Gerner, M. van Sint Annaland and J. A. M. Kuipers, Development of a Novel Hybrid Discrete Particle - Immersed Boundary Model for Fluidized Bed Membrane Reactors, Seventh International Conference on CFD in the Minerals and Process Industries. CSIRO, Melbourne, 2009. 

For a fundamental understanding of the hybrid process it is necessary to describe the interactions between two different unit operations with appropriate models. Making basic parameter studies the equilibrium stage model for distillation and a short-cut model for membrane separation is sufficient. The models are well established and the model parameters are quite accessible. This combination gives an first survey on the influence of structural and operational parameters on the concentration profiles in the column and on the maximum amount of water, which can be removed. 

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