Abstract
Membrane contactor technology has been demonstrated in a range of liquid/liquid along with gas/liquid software in wastewater therapy, fermentation, drugs, chiral separations, and semiconductor creation, material ion removal, and protein removal.
These are functionally similar to continuous contact mass transfer devices like forced draft aerators or vacuum towers.
Even some kind of long-term human missions in space require a continuous and self-sufficient supply of fresh water for consumption, hygiene and maintenance. In these type of missions the sources of wastewater is hygiene wastewater, urine and humidity condensates.
To satisfy these concerns and other requirements there should be one equipment or technology or process required. Similarly the process is needed to be cost effective, efficient and lightweight. There is one equipment or process called membrane contactors which usually satisfy almost all above needs can be acquired.
This Article will describe about this equipment or process used for this purpose and also explains which are the techniques, procedure and applications in different fields.
Introduction
Membrane contactors are devices that allow a gaseous phase and a liquid phase to come into direct contact with each other, for the purpose of mass transfer between the phases, without dispersing one phase into the other.
Membrane contactors are manufactured with hydrophobic hollow-fiber microporous membranes. The hollow fiber wall is very thin (25-100 micron) and highly porous. Hydrophobic nature of membrane will hold the water, and the force required for water to enter into membrane pores is calculated through Young-Lapace equation.
Membrane contactors can be made from following materials
- Polypropylene
- Polyethylene
- PVDF
- PTFE
- PFA
- ABS
- Polyvinyl chloride
- FRP
- Stainless steel
Young-Lapace Equation
Young-Lapace equation explains capillary pressure difference sustained across the interface between two static fluids, such as water and air.
δP1 = σ(1/R1 + 1/R2)
Where
δP1 - pressure drop
σ - Surface tension
By watchful control of the pressure difference relating to the fluids, one of the fluids is usually immobilized inside pores of the membrane in order that the fluid/fluid interface is found at the particular mouth of each one pore.
This approach offers quite a few important benefits over typical dispersed phase contactors, including absence of emulsions, zero flooding from high stream rates, unloading from low stream rates, density distinction between body fluids required.
Indeed, membrane contactors typically offer thirty times additional area than what's achievable in gas absorbers in addition to 500 times what's obtainable in liquid/liquid removal columns, leading to remarkably reduced height of a transfer unit (HTU) values.
Membrane contactor’s primary function is recovery of potable water from wastewater and secondary functions include oxygen reconstitution and humidity control systems.
Procedure
Fig. [1] Membrane contactors working procedure diagram
If hollow fiber membranes are pressurized from the outside by a liquid and membrane prevents the liquid from entering from pores. The pressure utilized for this purpose is called critical entry pressure. Critical entry pressure is dependent on the water surface tension, membrane pore size and the contact angle of water on membrane surface. The membrane prevents any water flow across the hollow fiber wall as long as the water pressure is less than this critical pressure. The pores remain dry and allow all volatile species dissolved in water to pass unhindered through the pores if a proper driving force is applied and maintained.
Steps
- First reverse osmosis (RO) of wastewater
- Second Direct osmosis process
Direct/ Forward osmosis process
In Direct/ Forward osmosis process two membrane contactors are used for the pretreatment of two streams of wastewater before treatment with RO. The stream of hygiene wastewater mixed with humidity condensate then followed by mixing with urine and treated with a unique dual membranes process direct osmosis and osmotic distillation. Then RO subsystem is used to produce two liquid streams.
It is a novel and prospective water treatment process that emerges as a result of water scarcity and energy crisis. In the FO process, a solution of considerably high concentration is utilized to generate a hydrostatic osmotic pressure gradient across a semi-permeable membrane to extract freshwater from feed solution such as seawater or brine, which is on the other side of the membrane.
Osmotic Distillation
Osmotic distillation (OD) is an evaporative membrane contactor process. OD technique can be used to extract selectively the water from aqueous solutions under atmospheric pressure and at room temperature, thus avoiding thermal degradation of the solutions. This process involves the contact of two streams of liquid with a hydrophobic microporous membrane. If the operating pressure is kept below the capillary penetration pressure of liquid into the pores, the membrane cannot be wetted by the solutions.
The difference in solute concentrations, and consequently in water activity of both solutions, generates, at the vapor–liquid interface, a vapor pressure difference causing a vapor transfer from the dilute solution towards the stripping solution.
Applications
Removal of dissolved oxygen from microelectronics industry, food industry, beverage industry
Removal of carbon dioxide from water in Electrodeionization technology
Removal of dissolved nitrogen from water from blanketed storage tanks
Removal of volatile organic compounds from water
Humidification of air and other gases
Removal of ammonia content from wastewater
Removal of dissolved carbon dioxide from water
Carbon dioxide (CO2) is one of major impurities in water. Generally CO2 present in water as inorganic form of carbon. There are several ways to remove CO2 from water includes
By Anion exchange columns
By lowering water ph
By deaeration
Reference
[1] http://www.unr.edu/cee/homepages/amyec/PDFs/Membrane%20contactor%20processes%20for%20wastewater%20reclamation%20in%20space%20II.%20Combined%20direct%20osmosis.pdf
[2] http://inside.mines.edu/~tcath/publications/CathPub/2_Membrane_contactor_processes_for_wastewater_reclamation_in_space_I.pdf
Image Reference
Fig. [1] Membrane contactors working procedure diagram © From
http://www.unr.edu/cee/homepages/amyec/PDFs/Membrane%20contactor%20processes%20for%20wastewater%20reclamation%20in%20space%20II.%20Combined%20direct%20osmosis.pdf
To contact the author mail: articles@worldofchemicals.com
© WOC Article