Pervaporation Of Liquid Organic Mixtures

Izák P.: Pervaporation of liquid organic mixtures Supervisor: doc. M. Šípek This PhD thesis deals with a description of transport parameters during pervaporation of binary liquid mixture through non-porous polymer membrane. The main task of this work is the application of the model that would be able to describe the transport of the penetrant in the membrane. The influence of the C-number of aliphatic alcohols on transport parameters is also under study. It was necessary to determine experimentally the concentration dependent density of the membrane, pure component vapor sorption isotherms, the dependence of pure component diffusion coefficients on their relative pressure and the dependence of the solubility of liquid binary mixture in a membrane. All dependencies were mathematically described and than introduced into Maxwell-Stefan equations. Literature survey shows that these equations represent the most practical model for the description of pervaporation process. A modified Maxwell-Stefan model takes into accounts the non-ideal multicomponent solubility effect, concentration dependent diffusion coefficients of all permeating components, concentration dependent density of the membrane and diffusion coupling. The model was experimentally verified by using the liquid binary mixtures (methanol, ethanol, propan-1-ol, butan-1-ol, pentan-1-ol, hexan-1-ol with toluene) and the low-density polyethylene membrane at 25°C. To describe concentration dependence of the membrane density we used a device, which is able to measure the extension of the polymer membrane during swelling continuously, while the membrane is immersed in liquid. This device was developed at the Department of Physical Chemistry of the Institute of ChemicalTechnology in Prague. The sorption apparatus with quartz spring balance was also constructed for suitable measuring of vapor sorption isotherms and vapor sorption kinetics of pure solvents. All measured sorption isotherms increased exponentially with increasing relative pressure of the solvent. Our results show that the solubility of toluene in PE membrane is in one order of magnitude higher than the solubility of aliphatic alcohols. From our vapor sorption measurements we observed an increase of mass fraction of the solvent in the membrane with increasing C-number. The diffusion coefficients of all aliphatic alcohols increase exponentially with the increase of their relative pressure. Swelling of the polyethylene membrane, during

which the solvent extends polyethylene chains and therefore the penetrants can pass through the network more easily, could cause this increase. The values of diffusion coefficient obtained for toluene pass maximum in the dependence on relative pressure. Forming of clusters at higher relative pressure probably causes the decrease of the diffusion coefficient of toluene with its relative pressure. In order to establish the composition of the sorbed phase in the polyethylene membrane, which differs from that of the bulk equilibrium solution, two different experiments

were

performed

and

compared:

interferometry

and

permittivity

measurements in all measured systems in PE membrane. Both used methods show a negligible solubility of aliphatic alcohols in comparison with toluene. Much higher solubility of toluene and its diffusion coefficient (bigger than the one of aliphatic alcohols) are responsible for good separation of these binary compounds by PE membrane. Pervaporation data for all six systems were measured in order to calculate coupled diffusion coefficient and to compare the model with the experiment. The data are described by the experimental dependencies of the weight fraction of toluene in permeate, partial fluxes of permeates, enrichment factor and separation factor on the weight fraction of toluene in retentate. The azeotropic mixtures were separated by pervaporation in the whole concentration range. The dependencies of pervaporation fluxes and weight fraction of toluene in permeate on concentration of toluene in the retentate are compared with the calculations based on the 1st Fick’s law with constant diffusion coefficient and a modified Maxwell-Stefan model. As the toluene concentration in retentate was becoming higher, the pervaporation flux through the polyethylene membrane was increasing exponentially. We did not find any influence of the C-number in the alcohol molecule on pervaporation flux through the membrane. The separation process in all measured binary mixtures is most successful in cases, where there is a small amount of toluene in alcohol. The mixture with the highest separation factor, i.e. in which the separation is most efficient, is hexan-1-ol with toluene. The experimental error in all experimental data does not exceed 5% of the total value.

It turns out that the strong non-ideal solubility and diffusivity behavior of the measured liquid binary mixture components in polyethylene membrane and diffusive coupling effects play the dominant roles in the pervaporation process. Therefore the results based on the 1st Fick’s law with constant diffusion coefficient are completely unsatisfactory. An essential improvement of the model description can be achieved by introduction of the concentration dependence of all diffusion coefficients, the density of the membrane and diffusion coupling using Maxwell-Stefan model with one additional adjustable parameter. These diffusion coupling coefficients were calculated using Newton iterative method. Our results show that this model is able to describe the performance of a pervaporation membrane for given separation problems very well.