Feature

Recent Developments in Membrane Technology

Posted on 08 February, 2010 | Tags: Advanced Processes

Membranes have been widely used for various applications such as drinking, wastewater treatment, process and ultrapure water. The articles focuses on  some of the critical considerations and highlights two state-of-the-art technologies viz. Electrodeionization (EDI) and Liquid Degasification using Membrane contractors having hollow fibre membranes.

-Pinakin J. Sampat & Mukesh C. Khagram

Ultrapure water has been the major requirement for Pharmaceutical industries as either purified water (PW) or water for injection (WFI), semiconductor complex, power stations, small laboratories and other process industries.
About a decade ago, ultrapure water was produced by the following methods:

• Ion exchange demineralisation + mixed bed polisher.
• Reverse Osmosis (Single or Double pass) + mixed bed polisher.

While conductivity upto 0.057 mS/cm was achieved, the water quality did not conform to the specifications with respect to TOC and bacterial levels generated from ion exchange systems.

Electrodeionization for Ultrapure Water Application
Research and development of EDI dates back to the year 1985 as a curiosity and it was only in the year 2000 that a major breakthrough was achieved. By the year 2002 EDI become a commercial viable technology and a well established process throughout the world.
The EDI process replaces the conventional DI mixed resin bed system to produce consistent ultrapure water quality.  

• EDI is a process to obtain ultrapure water.
• Produce water of consistent quality from 1-18.2 mΏ-cm resistivity or 1-0.057 µS/cm conductivity.
• Conforms to USP grade water as "purified water" (PW) and "water for injection" (WFI).
• Replaces conventional DI mixed resin bed system.
• Continuous process, no shutdown required.
• No regeneration required.
• Compact size.
• Low energy.

The RO feed to the EDI module has three separate streams:

• Product stream (90% water recovery)
• Concentrate stream (typically 10%)
• Electrolyte stream (10 lph, anolyte + catholyte to drain)
  The electrodeionisation process uses a combination of ion-selective membranes and ion-exchange resins sandwiched between two electrodes (anode (+) and cathode (-)) under a DC voltage potential to remove ions from RO pre-treated water.

Ion-selective membranes operate using the same principle and materials as ion-exchange resins, and they are used to transport specific ions away from their counter ions. Anion-selective membrane are permeable to anions but not to cations; cation-selective membranes are permeable to cations but not to anions. The membranes are not water-permeable.
By spacing alternating layers of anion- and cation- selective membranes within a plate-and-frame module, a "stack" of parallel purifying and concentrating compartments are created. The ion-selective membranes are fixed to an inert polymer frame, which is filled with mixed ion-exchange resins to form the purifying chambers. The screens between the purifying chambers form the concentrating chambers.
This basic repeating element of the EDI is called a "cell-pair". The "stack" of cell-pair is positioned between the two electrodes, which supply the DC potential to the module. Under the influence of the applied DC voltage potential, ions are transported across the membranes from the purifying chambers into the concentrating chambers, it becomes free of ions. This stream is the pure water product stream.

Example
Feed Conductivity :  5 mS/cm
Carbon dioxide : 3.5 ppm
Silica : 0.5 ppm
Hence,
FCE  =  5.0 + 2.79 (CO2) + 1.94 (SiO2) 
         =  5.0 + 2.79 (3.5) + 1.94 (0.5) 
         =  5.0 + 9.76 + 0.97
         =  15.73 mS/cm
FCE is in the working range (<33 mS/cm) but outside optimum range (<9 mS/cm).
Hence, it is important to reduce the Carbon dioxide and Silica content entering into the EDI unit.

The carbon dioxide can be reduced by caustic dosing to raise the pH level in the pre-treatment to RO appropriately so that RO permeate pH of 7 to 7.5 can be achieved for optimum EDI performance.

Alternatively, carbon dioxide can be removed by degasification method using Liqui-Cel Membrane Contractors having hollow fibre membrane in the post RO permeate.
It is in the best interest of the Designer to consider optimum working range in the EDI to obtain consistent good quality water and longer life.

Final Treated Water Quality from EDI
The final water quality from EDI system with varies between 1 - 18.2 megohm.cm resistivity for various applications. The final water quality will depend upon several factors:

1 Performance of RO and its pre- treatment.
2  Removal of all constituents such as hardness, iron, particulate matter, carbon dioxide, organics, and oxidising agents.
3 Proper instrumentation and controls.
4 Pressure drop across system.
5 Proper operation and maintenance of RO and EDI system.
6 Pure DC power supply.

Applications
EDI produces treated water quality conforming to the following applications:

• Pharmaceutical industries
• Automobile industries
• Power stations
• Boiler feed water
• Process industries
• Laboratories for process Water

Power Consumption
A typical EDI module operating at 2 m3/hr will require 300 VDC and draws w amps.
The Power Consumption can be calculated:

DC Power (Watts) = Voltage (VDC) x    Amps. (Current)
 = 300 VDC   x  2.0
 = 600 W
There is some power loss at the power supply during the conversion power AC to DC.  Assuming power supply efficiency of typical 85% (15% losses).

  DC Power
AC Power = 0.85
  600 W x 1 KW
 = 0.85        1000 W
 = 0.7 KW
Energy KWH = AC Power
  (KW) x 24 Hrs.
 = 0.7 KW x 24 Hrs.
 = 16.8 KWH.

Training
Training is one of the most important aspects for the successful operation of EDI. Training should cover pre-treatment, RO system design (Single pass or Double pass) EDI process, control philosophy, proper system monitoring instruments and controls. Daily log - data sheet have to be monitored and corrective action taken for system failure. Preventive maintenance, cleaning and sanitizing the system will enhance the life of EDI modules.

Liqui-cel® Membrane Contactors For Degasification Of Water
Dissolved oxygen, nitrogen and carbon dioxide are mainly present in water.  These gases should be carefully monitored and controlled as they can affect the products and process in which the water is used.

Example

• Dissolved oxygen can corrode the metal forming an oxide layer on the surface of finished products and piping material.
• Presence of Carbon dioxide in water interferes with the process and hampers its efficiency. 

Removal of dissolved gases is routinely accomplished by conventional treatments schemes such as degasification, deaeration and stripping with chemicals.

Comparison with Existing Technology
Conventional Vacuum Towers/Forced Draft Columns are used for the removal of oxygen and carbon dioxide in water.  These are typically tall columns filled with packing or trays and are used to bring a liquid phase in contact with the gases phase for the performance of removing dissolved gases from liquid.  The liquid runs from the top of the columns down around the packing.  The packing creates a large surface area for the gas phase to contact the liquid phase.
Membrane contactors perform the same task:  `However, they bring the two phases into contact at the pore without needing to disperse one phase into the other.  Membrane contactors have been in use over 15 years and have become a viable alternative for dissolved gas removal from water.

What Are Membrane Contactors
Membrane contactors are typically shell-and-tube device containing micro porous hydrophobic hollow-fibres. The material of construction of hollow-fibre is polypropylene having internal diameter: 200-220mm, outer diameter: 300 mm and having an average pore size of 0.03 mm. Water is passed on one side of the membrane and a gas is passed on the other.  Since the membrane is manufactured using a hydrophobic material and the pores are small, water does not easily pass through the pores.  The membrane essentially acts as a support between the gas and liquid phases and allows them to come in direct contact at the pore.  Gases, however, freely pass through the pore on a molecular level.

Principles Of Liquid Degasification
First, it is important to understand how and why gas is dissolved into water.  Whenever gas comes into contact with liquid it will dissolve into a liquid.  The amount of gas that dissolves into the liquid is proportional to the partial pressure of the gas in contact with liquid.  This relationship is governed by Henry's law given in Eq-1

Pi = HXi ...  Eq. 1

Where,
Pi = pressure of gas component i
H = Henry's Law coefficient, a
  function of water      temperature
Xi = concentration of dissolved    solute at equilibrium

Next, Dalton's law states that the total pressure of a gas mixture is equal to the sum of the partial pressures of the individual gases in the gas mixture (Equation 2):

P total = P1 + P2 + P3 + ... Eq. 2

The partial pressure component of each gas can be rewritten as in Equation 3:

P total = P total y1 + P total y2 + .... Eq. 3

Where yn = the mole fraction of the component

These relationships tell us that the pressure that a gas will exert in a fixed volume is dependent on the total pressure of the gas mixture and on the individual concentration of the gas in the mixture. This is an important relationship that is used to create the driving force to remove or dissolve gases into water. From these two relationships we can determine a particular gas equilibrium concentration in the aqueous gas phase if we know the total gas pressure and the concentration of that component in the gas phase.

Gas Transfer Mechanism
Under 1 atm and 25oC water will contain approximately 8.5 parts per million (ppm) of dissolved oxygen, 14.5 ppm of dissolved nitrogen, some trace amount of gases found in the atmosphere. If the partial pressure of the gas in contact with the water is reduced, the amount of gas dissolved in water will be reduced correspondingly.
The partial pressure of gas can be lowered in two ways. The total pressure of the gas phase can be lowered, or the concentration of the gases in the gas phase can be altered. To lower the total pressure of gas, a vacuum can be applied to the gas side of the membrane. To alter the concentration of the gases in contact with water, a strip gas that contains little or none of the gas being removed from the water can be introduced into the gas side of the membrane. This creates a driving force to remove gases from the liquid phase to the gas phase.

Gas Stripping Techniques
By changing the partial pressure of the gas we can either remove from or dissolve gas into water

• By lowering the partial pressure, the gas will be removed from the water.
• By increasing the partial pressure, the gas will dissolve into the water
  There are three operating modes for removal of dissolved gases from an aqueous stream:

1. Sweep gas mode is a process by which gas in lumenside (tube-side) of the membrane contactors flows counter-current to the water flow (shell-side). Sweep gas is the most economical technique for removing carbon dioxide from the liquid stream
2. Vacuum mode is a process in which a vacuum is applied to the lumenside (tube-side) of the fibre in the membrane contactor.  The vacuum method is recommended for total gas control and bulk gas removal.
3. Sweep-Assisted Vacuum (COMBO mode) is a method in which a sweep gas is applied to one lumenside (tube-side) port of the contactor, while the other is connected to a vacuum source.  This operating mode is the most efficient way to achieve low levels of dissolved oxygen or carbon dioxide.

Flow Capacities Of Membrane Contactors
Lab scale membrane contactors can typical handle upto 2500 ml/min.  Standard sizes are available in 2.5", 4" & 6", which can operate from 0.1 m3/hr to 11 m3/hr.  Industrial sizes are available in 10" & 14" and which can handle flow rate from 10 m3/hr to 90 m3/hr

System Design
Over the last few years, membrane contactors have become viable alternatives for dissolved-gas removal from water.  The membrane design has been improved to overcome hydrodynamic pressure-drop resistances, allowing individual contactors to handle large flow rates.  Because of the contactors' low pressure drop, they can be placed in series to increase their oxygen removal efficiency.
The membrane contactors allow for some unique design considerations.  Because of their nondispersed immobilized gas liquid interface, membranes can be operated over a wide range of flow rates without affecting the effective contact area for mass transfer.  The interface is unaffected even when the contactor is exposed to sudden changes in flow.  Because of the uniform membrane packing structure of the device, the sweep gas consumption or vacuum pump size can be minimized.  The modular design allows for systems to be built with a low profile, and can be tailored for each application.

Conclusion
Meeting the water specifications of various process industries is a challenging task.  The unique properties of membrane contactors allow them to be installed in almost any location in the high purity water system.  Membrane contactors can delivery 1 ppb of dissolved oxygen and less than 1 ppm carbon dioxide which offer the system designer improvements to the conventional high purity water treatment system design.  These improvements include redundancy, system stability and versatility.
Electropure EDI technology is offered by SnowPure LLC, USA world leader in high technology water, EDI since 1979. They excel in design, manufacture and marketing of proprietary electrodeionization technology. Liqui-cel membrane contactors are manufactured by Membrana - Charlotte (a division of Celgard, Inc), USA. The company has manufacturing facilities in Carolina, USA and Wuppertal, Germany.
In India, Evergreen Technologies Pvt. Ltd., Mumbai are technical support and sales representative for both Snowpure, LLC, USA for their Electropure EDI modules and for Membrana-Charlotte, USA for their Liqui-Cel Membrane Contactors and can provide design training, technical support and maintenance services related to these products.

Pinakin J. Sampat is Technical Advisor & Consultant at Evergreen Technologies Pvt. Ltd., Mumbai.

 

Mukesh C Khagram is the Director of Evergreen Technologies Pvt. Ltd., Mumbai.