Paper title:  State of the Art and Recent Progress in Membrane Contactors

Journal: Chemical Engineering Research and Design

Authors: E. DRIOLI [1],[2], E. CURCIO [2] and G. DI PROFIO [1]
[1] Institute on Membrane Technology ITM-CNR c/o University of Calabria, Arcavacata di Rende, Italy
[2] University of Calabria, Department of Chemical Engineering and Materials, Arcavacata di Rende, Italy

Process intensification is a response to the demand for smaller equipment size, energy saving, increasing personal and environmental safety, and improved process control/automation, among other demands.

Membrane contactors are tools for inter-phase mass transfer.  Other than other membrane processes, the membrane does not act as a selective barrier but as a support for the fluid interface.  They have advantages, such as a large stable contact area, controlable inter-phase flow rates, modularity for an easy scale-up, and can promote mass transfer.  Disadvantages can be an additional resistance, a low break through pressure, and high membrane costs.

The paper further outlines the most important equations for membrane contactors:
- Breakthrough pressure
- Transmembrane flux
- Membrane resistance
- concentration profile at the interface (partition coefficient)
- Enhanced vapor pressure as a function of the curvature of the vapor liquid interface
- Enhanced transmembrane flux using the dusty gas model
- Heat flux
- Temperature polarization coefficient
- Enhanced free energy for crystal nucleation as a function of the fluid angle with the membrane (Because the hydrophobic membrane improves crystal formation and growth)

The authors present the following membrane contactor technologies and their applicability in process intensification:

- Supported liquid membranes: A SLM lies between two liquid phases and contains "carriers". These carriers selectively transport solute for which they have an affinity. This technology could improve performance of, e.g. hydrometallurgical facilities. SLMs are highly selectivebut also instable.  The authors refer to recent advances in membrane stability.

- Membrane absorbers and strippers: The authors focus on gas-liquid membrane contactors.  They can extract and immerse gases into liquids.

- Membrane distillation: After an introduction of the basic principles, and the four membrane distillation types, (theoretically) 100% rejection of non-volatiles, low temperature gradient, thus the possibility to use plastic materials as a constructon material, the possible contributions of MD to brine management are presented.  A combination of RO and MD was able to reach a recovery of 87.6%.  The authors also refer to other applications such as removal and recovery of HCl, H2SO4 and VOCs, and the use in the food industry to produce concentrates at reduced thermal stress for the ingredients.

- Osmotic distillation: A membrane contactor technology in which the water is removed by a hypertonic solution (draw solution).  Here there is virtually no thermal stress and thus this technology is suitable to the food industry.  Especially since the larger molecules of food flavour and fragrance are effectively retained by the membrane.

- Membrane crystallization:  Up to now MCr has it's field especially in the crystallization of chemicals, such as proteins, in the pharmaceutical, chemical, and biotechnological industry.  The quality of the produced crystals is very high and the process is well controlable.  But the quantities that can be processed are still too low for many applications.
- Membrane emulsification:  As opposed to traditional processes that use shear forces from moving equipment or pressure dispersing membrane emulsification uses permeation through a membrane to disrupt larger droplets.  Two configurations can be used. First, in cross-flow, the liquid to disperse (e.g. oil) is passing through a membrane and enters in a uniform bubble size into the continuous phase (e.g. water).  Second, in dead-end, the pre-mixed emulsion is passed through a membrane and the bubble size is unified.  This technology allows for a defined bubble size and protects labile compounds that could be destroyed by the large shear forces of conventional processes.
- Membrane contactors in phase transfer catalysis: In this process a catalyst separates two phases which contain either an reagent or a catalyst.  Especially the combination of membrane crystallization and phase transfer catalysis could be of interest.  Here the catalyst promotes a reaction which has solid crystalline product.

Contribution to the lab's work:
This paper reviews different membrane technologies that require low energy since they use low, to no heat gradient and low to no pressure gradient.  They could play a future role in brine management.  Having these different processes put together in one paper helps to think about potential innovative process combinations.

Contact: Michael.Hoyer@gmx.com