Membrane technology

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Membrane technology encompasses the scientific processes used in the construction and application of membranes. Membranes are used to facilitate the transport or rejection of substances between mediums, and the mechanical separation of gas and liquid streams. In the simplest case, filtration is achieved when the pores of the membrane are smaller than the diameter of the undesired substance, such as a harmful microorganism. Membrane technology is commonly used in industries such as water treatment, chemical and metal processing, pharmaceuticals, biotechnology, the food industry, as well as the removal of environmental pollutants.

After membrane construction, there is a need to characterize the prepared membrane to know more about its parameters, like pore size, function group, material properties, etc., which are difficult to determine in advance. In this process, instruments such as the

, and Liquid–Liquid Displacement Porosimetry are utilized.

Introduction

Membrane technology covers all

Typically

Membrane Overview

Membrane separation processes operate without heating and therefore use less energy than conventional thermal separation processes such as

can be very effective in removing colloids and macromolecules from wastewater. This is needed if wastewater is discharged into sensitive waters especially those designated for contact water sports and recreation.

About half of the market is in medical applications such as artificial kidneys to remove toxic substances by hemodialysis and as artificial lung for bubble-free supply of oxygen in the blood.

The importance of membrane technology is growing in the field of environmental protection (

.

Mass transfer

Two basic models can be distinguished for mass transfer through the membrane:

• the solution-diffusion model and
• the hydrodynamic model.

In real membranes, these two transport mechanisms certainly occur side by side, especially during ultra-filtration.

Solution-diffusion model

In the solution-diffusion model, transport occurs only by

). Concentration polarization is, in principle, reversible by cleaning the membrane which results in the initial flux being almost totally restored. Using a tangential flow to the membrane (cross-flow filtration) can also minimize concentration polarization.

Hydrodynamic model

Transport through pores – in the simplest case – is done

.

Membrane operations

According to the driving force of the operation, it is possible to distinguish:

• Pressure-driven operations
  • microfiltration
  • ultrafiltration
  • nanofiltration
  • reverse osmosis
  • gas separation
• Concentration driven operations
  • dialysis
  • pervaporation
  • forward osmosis
  • artificial lung
• Operations in an electric potential gradient
  • electrodialysis
  • membrane electrolysis e.g. chloralkaline process
  • electrode ionization
  • electro filtration
  • fuel cell
• Operations in a temperature gradient
  • membrane distillation

Membrane shapes and flow geometries

There are two main flow configurations of membrane processes: cross-flow (or tangential flow) and dead-end filtrations. In cross-flow filtration the feed flow is

.

Flat plates are usually constructed as circular thin flat membrane surfaces to be used in dead-end geometry modules. Spiral wounds are constructed from similar flat membranes but in the form of a "pocket" containing two membrane sheets separated by a highly porous support plate.[6] Several such pockets are then wound around a tube to create a tangential flow geometry and to reduce membrane fouling. Hollow fiber modules consist of an assembly of self-supporting fibers with dense skin separation layers, and a more open matrix helping to withstand pressure gradients and maintain structural integrity.^[6] The hollow fiber modules can contain up to 10,000 fibers ranging from 200 to 2500 μm in diameter; The main advantage of hollow fiber modules is the very large surface area within an enclosed volume, increasing the efficiency of the separation process.

• 
  Hollow fiber membrane module
• 
  Separation of air into oxygen and nitrogen through a membrane

The Disc tube module uses a cross-flow geometry and consists of a pressure tube and hydraulic discs, which are held by a central tension rod, and membrane cushions that lie between two discs.^[7]

Membrane performance and governing equations

The selection of synthetic membranes for a targeted separation process is usually based on few requirements. Membranes have to provide enough mass transfer area to process large amounts of feed stream. The selected membrane has to have high selectivity (rejection) properties for certain particles; it has to resist fouling and to have high mechanical stability. It also needs to be reproducible and to have low manufacturing costs. The main modeling equation for the dead-end filtration at constant pressure drop is represented by Darcy's law:^[6]

${\displaystyle {\frac {dV_{p}}{dt}}=Q={\frac {\Delta p}{\mu }}\ A\left({\frac {1}{R_{m}+R}}\right)}$

where V_p and Q are the volume of the permeate and its volumetric

${\displaystyle S={\frac {C_{p}}{C_{f}}}}$

where C_f and C_p are the solute concentrations in feed and permeate respectively. Hydraulic permeability is defined as the inverse of resistance and is represented by the equation:[6]

${\displaystyle L_{p}={\frac {J}{\Delta p}}}$

where J is the permeate flux which is the volumetric flow rate per unit of membrane area. The solute sieving coefficient and hydraulic permeability allow the quick assessment of the synthetic membrane performance.

Membrane separation processes

Membrane separation processes have a very important role in the separation industry. Nevertheless, they were not considered technically important until the mid-1970s. Membrane separation processes differ based on separation mechanisms and size of the separated particles. The widely used membrane processes include microfiltration, ultrafiltration, nanofiltration, reverse osmosis, electrolysis, dialysis, electrodialysis, gas separation, vapor permeation, pervaporation, membrane distillation, and membrane contactors.^[8] All processes except for pervaporation involve no phase change. All processes except electrodialysis are pressure driven. Microfiltration and ultrafiltration is widely used in food and beverage processing (beer microfiltration, apple juice ultrafiltration), biotechnological applications and pharmaceutical industry (antibiotic production, protein purification), water purification and wastewater treatment, the microelectronics industry, and others. Nanofiltration and reverse osmosis membranes are mainly used for water purification purposes. Dense membranes are utilized for gas separations (removal of CO₂ from natural gas, separating N₂ from air, organic vapor removal from air or a nitrogen stream) and sometimes in membrane distillation. The later process helps in the separation of azeotropic compositions reducing the costs of distillation processes.

Pore size and selectivity

The pore sizes of technical membranes are specified differently depending on the manufacturer. One common distinction is by nominal pore size. It describes the maximum pore size distribution

of a globular molecule that is retained to 90% by the membrane. The cut-off, depending on the method, can by converted to so-called D₉₀, which is then expressed in a metric unit. In practice the MWCO of the membrane should be at least 20% lower than the molecular weight of the molecule that is to be separated.

Using track etched mica membranes^[10] Beck and Schultz^[11] demonstrated that hindered diffusion of molecules in pores can be described by the Rankin^[12] equation.

Filter membranes are divided into four classes according to pore size:

Pore size	Molecular mass	Process	Filtration	Removal of
> 10		"Classic" filter
> 0.1 μm	> 5000 kDa	microfiltration	< 2 bar	larger bacteria, yeast, particles
100-2 nm	5-5000 kDa	ultrafiltration	1-10 bar	bacteria, macromolecules, proteins, larger viruses
2-1 nm	0.1-5 kDa	nanofiltration	3-20 bar	viruses, 2- valent ions[13]
< 1 nm	< 100 Da	reverse osmosis	10-80 bar	salts, small organic molecules

The form and shape of the membrane pores are highly dependent on the manufacturing process and are often difficult to specify. Therefore, for characterization, test filtrations are carried out and the pore diameter refers to the diameter of the smallest particles which could not pass through the membrane.

The rejection can be determined in various ways and provides an indirect measurement of the pore size. One possibility is the filtration of macromolecules (often

, the so-called "bacteria challenge test", can also provide information about the pore size.

Nominal pore size	micro-organism	ATCC root number
0.1 μm	Acholeplasma laidlawii	23206
0.3 μm	Bacillus subtilis spores	82
0.5 μm	Pseudomonas diminuta	19146
0.45 μm	Serratia marcescens	14756
0.65 μm	Lactobacillus brevis

To determine the pore diameter,

holes) is assumed. Such methods are used for membranes whose pore geometry does not match the ideal, and we get "nominal" pore diameter, which characterizes the membrane, but does not necessarily reflect its actual filtration behavior and selectivity.

The selectivity is highly dependent on the separation process, the composition of the membrane and its electrochemical properties in addition to the pore size. With high selectivity, isotopes can be enriched (uranium enrichment) in nuclear engineering or industrial gases like nitrogen can be recovered (gas separation). Ideally, even racemics can be enriched with a suitable membrane.

When choosing membranes selectivity has priority over a high permeability, as low flows can easily be offset by increasing the filter surface with a modular structure. In gas phase filtration different deposition mechanisms are operative, so that particles having sizes below the pore size of the membrane can be retained as well.

Membrane Classification

Bio-Membrane is classified in two categories, synthetic membrane and natural membrane. synthetic membranes further classified in organic and inorganic membranes. Organic membrane sub classified polymeric membranes and inorganic membrane sub classified ceramic polymers.^[14]

Synthesis of Biomass Membrane

The composite biomass membrane

Fabrication of pure biomass based membrane

A biomass-based membrane is a membrane made from organic materials such as plant fibers.

Equipment and instruments used in the process

List of instruments used in membrane synthesis procedures:

• Centrifuge
• Casting Machine
• Plane casting glass
• Magnetic Stirrer
• Glass ware:
  measuring cylinders, flask
  etc.
• Oven
• Mortar and pestle

Membrane Characterization

After casting and synthesis of membrane there is need to characterize the prepared membrane to know more details about membrane parameters, like pore size, functional groups, wettability, surface charge, etc. It is important to know membrane properties so we are able to remove and treat a particulate pollutant, which causes pollution in the environment.^[18] For characterization following different instruments are used:

• Scanning Electron Microscope (SEM)
• Transmission electron Microscope (TEM)
• Fourier Transform Infrared Spectroscopy
  (FTIR)
• Atomic force microscopy
• Contact angle meter
• Zeta potential (streaming potential)
• X-ray Diffraction
  (XRD)
• Liquid–Liquid Displacement Porosimetry (LLDP)

Biomass Membrane Applications

Water treatment

Water treatment is any process that improves the quality of water to make it more acceptable for a specific end-use. Membranes can be used to remove particulates from water by either size exclusion or charge separation.^[19] In size exclusion, the pores in the membrane are sized such that only particles smaller than the pores can pass through. The pores in the membrane are sized such that only water molecules can pass through, leaving dissolved contaminants behind.^[20]

Gas separation

Utilization of membranes in gas separation, like carbon dioxide (CO₂), Nitrogen oxides (NO
_x),  Sulphur oxides (SO
_x), harmful gasses can be removed to protect the environment.^[21] Biomass Membrane gas separation more effective then commercial membrane.^[22]

Hemodialysis

Membrane application in hemodialysis is a process of using a semipermeable membrane to remove waste products and excess fluids from the blood.^[23]

See also

• Particle deposition
• Synthetic membrane

Notes

References

• Osada, Y., Nakagawa, T., Membrane Science and Technology, New York: Marcel Dekker, Inc,1992.
• Zeman, Leos J., Zydney, Andrew L., Microfiltration and Ultrafitration, Principles and Applications., New York: Marcel Dekker, Inc,1996.
• Mulder M., Basic Principles of Membrane Technology, Kluwer Academic Publishers, Netherlands, 1996.
• Jornitz, Maik W., Sterile Filtration, Springer, Germany, 2006
• Van Reis R., Zydney A. Bioprocess membrane technology. J Mem Sci. 297(2007): 16-50.
• Templin T., Johnston D., Singh V., Tumbleson M.E., Belyea R.L. Rausch K.D. Membrane separation of solids from corn processing streams. Biores Tech. 97(2006): 1536-1545.
• Ripperger S., Schulz G. Microporous membranes in biotechnical applications. Bioprocess Eng. 1(1986): 43-49.
• Thomas Melin, Robert Rautenbach, Membranverfahren, Springer, Germany, 2007,
  ISBN 3-540-00071-2
  .
• Munir Cheryan, Handbuch Ultrafiltration, Behr, 1990,
  ISBN 3-925673-87-3
  .
• Eberhard Staude, Membranen und Membranprozesse, VCH, 1992,
  ISBN 3-527-28041-3
  .