M.Sc Thesis

M.Sc StudentRing Shiran
SubjectCharacterization of Donnan Dialysis Separations of Applied
DepartmentDepartment of Chemical Engineering
Supervisors PROFESSOR EMERITUS David Hasson
Full Thesis textFull thesis text - English Version


Donnan dialysis is a process that utilizes ion exchange membranes (IEMs) to achieve separation or concentration of ionic species. Available Donnan dialysis studies are of two types: investigation of process mechanisms based on elaborate models and practical investigations reporting empirical results. The objectives of the present research were to develop simplified engineering models for practical characterization of Donnan dialysis processes and to verify their applicability by experimental data.

The steps involved in a Donnan separation process are convective mass transfer of ions through feed and receiver solution boundary layers and diffusion of the ions through the IEM. Simplified kinetic models describing the transport mechanism of target ions from feed solution to receiver solution were derived. The models enabled characterization of the separation process by an overall transport coefficient combining mass transfer resistance and membrane diffusion resistance.

Discrimination of the controlling mechanism was based on the effect of the solutions Reynolds number on the overall transport coefficient. Mass transfer control was evident when variation of the overall transport coefficient with Reynolds numbers conformed to the form of standard correlations. Diffusion control was evident when variation of Reynolds number had no effect on the overall coefficient. Mass transfer control also shifted to membrane diffusion control by increase of solutions concentrations.

Experimental work was conducted in a batch mode dialyzer with a 100 mmol/L NaCl receiver solution. The volume of the feed and receiver solutions was 2 L each and the membrane area was 0.02 m2. Flow rates of the feed and receiver solutions were in the range of 0.5-13 L/min corresponding to Reynolds numbers of 288-7475.  Feed solution concentrations were in the range 0.05-4.20 mmol/L .

Model verification was based on results derived from the following experimental systems:

A.    Single divalent component system: Separation of SO42-.

Experimental data clearly indicated that transport was mass transfer controlled when feed concentrations were low (0.63-4.90 mmol/L) and membrane diffusion controlled when feed concentration was high (38mmol/L). The mass transfer coefficients derived from the low concentration data were shown to conform to the standard correlations. 

B.     Single monovalent component systems: Separation of NO3- or H2PO4- or HCO3- at dilute feed concentrations of 0.05-4.20 mmol/L.

The experimental data of the NO3- and HCO3- systems were found to be mass transfer controlled and the derived mass transfer coefficients conformed to the standard correlations. However, the data for the H2PO4- system indicated that membrane diffusion control was not negligible. This result can be ascribed to the relatively high magnitude of the H2PO4- ionic radius. 

C.    Dual monovalent component systems: Simultaneous separation of NO3- HCO3-or NO3- H2PO4-.

The experimental data of the NO3- HCO3- were found to be mass transfer controlled. However, the data of the NO3- H2PO4- system indicated mass transfer control for the NO3- and as in case B, non negligible resistance for the H2PO4-.

In conclusion, experimental data lend support to the simple engineering models for analyzing Donnan dialysis processes.

Part of the above research was published in JMS paper, reproduced here in Appendix C.