Modelling of mass transfer in packing materials with cellular automata

Engelbrecht, Alma Margaretha (2008-12)

Thesis (MScEng (Process Engineering))--Stellenbosch University, 2008.


The general objective for this thesis is to assess the ability of cellular automata to model relatively complex processes or phenomena, in particular thermodynamic scenarios. The mass transfer in packing materials of distillation columns was selected as an example due to the sufficient level of complexity in the distillation process, and its importance in a wide range of applications. A literature survey on cellular automata that summarizes the information currently available in formal publications and the internet is included to provide a general overview on the basic theoretical principles and the application of cellular automata models in the process engineering industry. The literature study was also used to identify potential requirements for the new research project. The study objective includes the construction of a cellular automata model that is able to represent transition of solutes from the fluid on the micro-surfaces of packing materials to the by-passing vapour stream, as well as the steady-state equilibrium between evaporation and condensation. Iterated model parameters sufficient for the realistic modelling of mass transfer as a result of thermodynamic driving forces, are required to meet this objective. The model behaviour was compared and the parameters subsequently adjusted according to the behaviour that is theoretically expected from the system being simulated. Qualitative (although sometimes in a quantitative format) rather than quantitative observations and comparisons were made seeing that the model has not yet been calibrated. The model that has been developed to date is not able to simulate the individual effects of chemical and thermodynamic properties although a realistic simulation of the cumulative effect exerted by these factors, or change thereof, on a system has been achieved. The accuracy of the results that have been obtained by using iterated parameters cannot be guaranteed for scenarios that deviate too much from the systems that have already been modelled successfully. The trade-off between the ability of the model to incorporate the effect of polarization, its ability to represent separation, in particular the condensation of hydrophilic substances, for strong hydrophilic packing materials and its ability to incorporate a large number of species limits the range of scenarios that can be successfully modelled. The model is able to represent the effect of a declining driving force (difference between the component vapour pressure of the gas phase and that of the liquid phase) that is typical of a system which is allowed to reach equilibrium after an initial disturbance. The model is also able to represent an additional driving force for separation caused by the effect of intermolecular forces. The model also displays the potential ability to represent the effect of different surface structures of the packing material on the extent of separation achieved at steady state as well as the rate at which such steady state conditions have been achieved. The model must be correctly scaled to minimize inaccurate results. Although several adjustments are needed to eliminate some limitations, the model has proven itself worthy of further development due to its capability to represent the basic characteristics of mass transfer in packing materials.

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