Modelling of mass transfer in packing materials with cellular automata

[摘要] The general objective for this thesis is to assess the ability of cellular automata tomodel relatively complex processes or phenomena, in particular thermodynamicscenarios. The mass transfer in packing materials of distillation columns was selectedas an example due to the sufficient level of complexity in the distillation process, andits importance in a wide range of applications.A literature survey on cellular automata that summarizes the information currentlyavailable in formal publications and the internet is included to provide a generaloverview on the basic theoretical principles and the application of cellular automatamodels in the process engineering industry. The literature study was also used toidentify potential requirements for the new research project.The study objective includes the construction of a cellular automata model that is ableto represent transition of solutes from the fluid on the micro-surfaces of packingmaterials to the by-passing vapour stream, as well as the steady-state equilibriumbetween evaporation and condensation. Iterated model parameters sufficient for therealistic modelling of mass transfer as a result of thermodynamic driving forces, arerequired to meet this objective. The model behaviour was compared and theparameters subsequently adjusted according to the behaviour that is theoreticallyexpected from the system being simulated. Qualitative (although sometimes in aquantitative format) rather than quantitative observations and comparisons weremade seeing that the model has not yet been calibrated.The model that has been developed to date is not able to simulate the individualeffects of chemical and thermodynamic properties although a realistic simulation ofthe cumulative effect exerted by these factors, or change thereof, on a system hasbeen achieved. The accuracy of the results that have been obtained by using iteratedparameters cannot be guaranteed for scenarios that deviate too much from thesystems 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 hydrophilicsubstances, for strong hydrophilic packing materials and its ability to incorporate alarge number of species limits the range of scenarios that can be successfullymodelled.The model is able to represent the effect of a declining driving force (differencebetween the component vapour pressure of the gas phase and that of the liquidphase) that is typical of a system which is allowed to reach equilibrium after an initialdisturbance. The model is also able to represent an additional driving force forseparation caused by the effect of intermolecular forces.The model also displays the potential ability to represent the effect of different surfacestructures of the packing material on the extent of separation achieved at steadystate 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 modelhas proven itself worthy of further development due to its capability to represent thebasic characteristics of mass transfer in packing materials.

[发布机构] Stellenbosch University