Abstract
The inevitable decline of membrane performance in membrane separation processes can be optimized through a good understanding of the mass transfer phenomenon and the transport resistances involved in the operation. Thus, this chapter focused on the discussions of mass transfer mechanisms and models in membrane separation based on several types of driving forces. This includes the pressure from a mechanical operation, partial pressure, osmotic pressure, concentration, and also thermal gradients. The chapter elaborates on the transport resistances in membrane resulting from membrane fouling and concentration polarization. The author hopes that readers, especially engineers and technical operators, gain a deep understanding and comprehensive knowledge regarding the theories and are able to utilize the knowledge to optimize the membrane operation.
TopThe Driving Forces In Membrane Separation Processes For Water And Wastewater
In general, Figure 1 illustrates the membrane separation whereby the membrane separation process takes place as a result of the specific driving force that transport component across the membrane from one phase to the other. In membrane separation, a feed mixture comprising of two or more components is separated to an extent through a semipermeable membrane which acts as an interphase between two phases. The membrane separation process can be classified based on several factors including the driving force, operation mechanism, membrane structure, and phases involved. In essence, it focuses on a certain component that can be readily transferred across the membrane from other components (selectivity) and the transfer rate of that particular component (flux). These parameters, selectivity and flux largely determine the efficiency and performance of the membrane separation process respectively.
Figure 1. Schematic of the membrane separation process showing different types of driving forces
The operation of membrane separation is mainly governed by the specific driving force mechanism of mass transport used. The driving forces in membrane separation can be classified into pressure-driven (in reverse osmosis, ultrafiltration, and microfiltration processes), concentration gradient-driven (in forward osmosis, and dialysis), partial pressure-driven or thermal-driven (in pervaporation, and membrane distillation processes), and electrical potential-driven (in electrodialysis). More than one driving force may be involved in some membrane operations, such as concentration and pressure in gas separation.
This section will focus on the operations by different driving forces that are typically used in water and wastewater treatment, which are the pressure, and partial pressure (thermal) gradients.
Key Terms in this Chapter
Concentration Polarization: Phenomenon which occurs naturally due to the build-up of concentration gradient at the membrane-solution interface due to the membrane’s permselective property.
Membrane Fouling: Accumulation of solutes and/or other materials on the membrane surface and/or inside the membrane pores causing reduced in membrane flux.
Mass Transfer: In the context of the membrane, mass transfer is the net movement of mass from the feed side to the permeate side.
Transport Resistance: Obstacle to the transport of heat/mass across the membrane.
Convective Transport: The transport of heat/mass through the porous membrane which is induced by the bulk motion of fluid-driven by the applied driving force.
Modelling: In the mathematical context, it is the process of translating a phenomenon from qualitative explanation into the mathematical language which can describe the phenomenon quantitatively.
Membrane Separation: A process which utilizes a membrane that selectively separates components based on size exclusion or other selectivity mechanisms.
Diffusive Transport: Transport of fluid down the concentration gradient.