Chem Eng - Separations Flashcards
What are the (2) types of polymer/ceramic membrane?
Symmetric porous membranes:
Symmetric porous membranes are those characterised by little or no variation
along the thickness of the membrane through on a local scale (<10 um d,ave)
Asymmetric membranes:
Asymmetric membranes are those containing a minimum of two bands with differing porous networks or a combination that includes a minimum of one porous and one compact band.
• Asymmetric membranes of a special type are those with a porous structure continuously varying along the membrane thickness.
• Asymmetric membranes are in high demand as they may optimise separation ability, permeability, and mechanical strength. For an example a selective layer with a 100um thickness is fully adequate for a separation performed by 10 um pores.
What’s a hydrophobic/philic membrane?
A material that uses hydrophobicity to act as a membrane.
An appropriate chemical treatment (Plasma) or physico-chemical treatment of a membrane may render the surface of a hydrophilic or hydrophobic or activated molecular sieving effect.
What’s phase inversion?
Phase inversion or phase separation is a chemical phenomenon exploited in the fabrication of artificial membranes. It is performed by removing the solvent from a liquid-polymer solution, leaving a porous, solid membrane.
What materials are used to produce hollow fibre or flat sheet membranes?
High performance polymer materials such as polyethersulfone (PESF), polyvinylidendifluoride (PVDF) and modified polyethylene.
- These polymer materials exhibit excellent chemical, thermal and mechanical stabilities.
- As long as polymer could be dissolved in a solvent, hollow fibres membrane or flat sheet membranes could be made by phase inversion technique.
Polyamide is a typical polymer for membrane production.
The membrane may be either in the form of a flat sheet or a hollow fiber. In general, hollow fibers are preferred as they achieve a higher effective membrane area within a given module volume.
How are hollow fibre membranes manufactured?
1) After preparing a homogeneous dope (polymer dissolved in a solvent), the spinning dope was transferred to a stainless steel container, the feed vessel was pressurized to 5 – 6 bar using nitrogen during the spinning process.
2) A triple orifice spinneret (lace die) with external layer(dout/din,3.2/2.8.mm), internal layer (dout/din, 2.0/1.4 mm), and bore diameters 0.4 mm (small bore) and 0.8mm (standard bore) is used to obtain double or single-layer hollow fibres.
3) A distilled water/solvent mixture can be used as the internal coagulant and tap water as the external coagulant for all spinning runs.
4) Finally, in forming the hollow fibre the precursor was passed through an initial water bath to complete the phase inversion process.
5) The hollow fibre was then washed thoroughly in a second water bath. Automated fibre guide rollers avoided any mechanical dragging of the fibre.
6) Hollow fibres were collected at the end in a large water container to remove NMP. Water was changed twice over three days. Fibres were then removed from the water bath and allowed to dry under atmospheric conditions.
What are the advantages of Alumina Triple Layer Fibres with Quadruple Orifice Spinneret?
Pore size of the fibres are being controlled by alumina particle size in the formulation.
Important Points: strong fibres (mechanical strength), compact defect free, fine pore fibres with dense separative layer suitable for gas separation.
What are the main problems that arise with using membranes?
Selectivity
Productivity/cost
Operational reliability
What materials may gas separation membranes be?
Ceramic or polymeric
What are some advantages of using membranes?
Separation technology enables processors to simultaneously concentrate, fractionate, and purify their products.
Large volumes can be treated with remarkable energy efficiency since the technology does not require a phase change to effect water removal, nor do the processes require a steam source or ancillary equipment such as heat generators, evaporators, or condensers.
How is permeation rate important to membranes?
Gas separation with membrane technology- involves the separation of individual components on the basis of the difference in their rates of permeation through a thin porous membrane barrier.
The rate of permeation for each component is determined by the characteristics of the gas mixture, the characteristics of the membrane, and the partial pressure differential of the gaseous component across the membrane.
Since separation is based on a difference in the rates of permeation rather than on an absolute barrier to one component, the recovered component that flows through the membrane (the permeate) is never 100% pure.
However, that relatively high product purities and high recoveries are possible with membrane systems (at increased cost) by the use of multiple stages and recycle systems or when used in combination with other technologies.
What are the advantages of gas purification and separation by membrane permeation?
- Low capital investment
- Ease of operation; Process can be operated unattended
- Good weight and space efficiency
- Ease of scale up. However, there is little economy of scale
- Minimal associated hardware
- No moving parts
- Ease of installation
- Flexibility & Reliability
- Minimal utility requirements
- Low environmental impact
- Ease of incorporation of new membrane developments. Many users can install the next generation of membranes into existing equipment at the scheduled membrane replacement time.
What is PSA?
Pressure swing absorption
A technique used to separate some gas species from a mixture of gases (typically air) under pressure according to the species’ molecular characteristics and affinity for an adsorbent material.
What are some issues with polymeric membranes?
Plasticisation Compaction Aging Competitive sorption Fouling
Cannot withstand high temperatures and aggressive chemical environments.
Heavy hydrocarbons (in certain industries) can be a problem, particularly in hollow fibre modules.
Many polymers can be swollen or plasticised when exposed to hydrocarbons or CO2 with high partial pressure, even in low concentrations.
What are some key challenges in gas separation?
Fouling is a critical factor in ultrafiltration, microfiltration and in gas separation field therefore dominates the entire membrane operation.
In gas separation, fouling is usually not a problem and only minimal pretreatment of the feed stream is required. On the other hand, in a typical membrane gas- separation process, it is only necessary to develop one defect per square meter of membrane to essentially destroy the efficiency of the process.
The ability to make, and maintain, defect-free membranes is, therefore, a key issue in gas separation.
Another factor that leads to operational unreliability is poor membrane stability. In facilitated-transport membranes, instability is such a problem that the process has never become commercial.
Membrane instability has also proved to be a major problem area in reverse osmosis, gas separation and pervaporation.
What is membrane selectivity?
The membrane selectivity is a measure of the ability of the membrane to separate 2 gases A and B. It is the ratio of the permeabilities of A and B.
The selectivity is given by the ratio of permeability coefficients: (also called ideal separation factor):
a(AB ideal) = P(A) / P(B)
What are some of examples of membrane preparation techniques?
Membranes can be prepared from a wide range including both organic and inorganic materials.
Polymer membranes could be made from either rubbery or glassy polymers. Generally, membranes made from glassy polymers are porous and rubbery polymers tends to produce dense membranes.
Techniques employed using organic polymers include:
- stretching
- track-etching
- phase inversion
- sintering
Techniques employed using inorganic materials include (inorganic membranes e.g. alumina and silica):
- sintering
- sol/gelprocess
- anodicoxidation
What are the 2 inorganic membrane categories?
(i) dense inorganic membranes (mostly Pd and Pd-alloy) (Pd materials useful for H2 separation)
(ii) porous inorganic membranes
What are typical materials used for inorganic membranes?
- alumina (Al2O3)
- zirconia (ZrO2)
- titania (TiO2)
- silica & carbon molecular sieves
- zeolite
- stainless steel,
- porous glass
Generally ceramic membranes comprise a highly controlled surface membrane layer that is formed on the inner (feed-side) surface of a more open support
layer.
All of these materials are FDA-listed for suitability in pharmaceutical processes.
Ceramic membranes exhibit near zero non-specific adsorption of biological materials.
These ceramic membranes can separate gas mixtures based on Knudsen diffusion, which is inversely proportional to the square root of molecular mass leads to a low separation factor.
What provides the driving force for a membrane?
The driving force of the molecules transport is given by concentration,
pressure, electrical or chemical gradient across the membrane.
What are the main membrane module configurations?
- Hollow fibre (tube-in-shell module)
- Flat membranes (plate and frame)
- Spiral-wound membranes
What are the properties of the geometry of hollow fibre membrane modules?
The membranes are in the shape of very small diameter hollow fibres. The inside diameter of the fibres is in the range of 100 to 500μm range and the outside 200 -1000 μm with length up to 3 to 5m
long.
The hollow fiber configuration consists of thousands of hollow fibers packaged in bundles mounted in a pressure vessel resembling a shell and tube heat exchanger.
Thousands of fine tubes are bound together at each end into tube sheet that is surrounded by a metal shell having a diameter of 0.1 to 0.3 m, so that membrane area per unit volume could be up to 30,000 m2 m-3.
For high pressure applications, the fiber diameter is usually in the order of 100 um ID and 150-200um OD. The feed gas is introduced on the shell side because hollow fibers are much stronger under compression than expansion.
The faster permeating gases migrate into the fiber bore and exit via the open end of the bundle.
For low pressure applications the fibers have a diameter greater than 400 um and the feed gas enters the bore side while the permeate exits via the shell side. This configuration reduces pressure drop on the feed side.
What is potting?
When hollow fibres are sealed together using a resin-like material.
The end is then cut so that the hollow fibre tubes are open/exposed to gas, but the resin in between the fibres is completely gas-tight.
What occurs in one-side and both-side potting?
One side potting: If the feed gas mixture is fed to the outside surface of the tube bundle, then the bundle can be bent over in two in a U shape and the both ends of the bundle can be potted in a single, common tube, the permeate being collected from the lumen.
Both side potting: If the feed gas mixture is fed to the fiber lumen, then a separate tube sheet must be formed around each end of the bundle.
▪ To form a tube sheet, some form of resinous or other plastic or glue-like material must be caused to flow between the individual fibers (usually by slightly heating the resin) and fill all the inter-fiber interstices, so that no gaps are left between the fibers to allow a flow leak.
▪ A wide variety of thermoplastic and thermosetting materials are used for this purpose and most common ones are epoxy resins, polyurethane resins, silicone resin.
▪ First the fiber ends are clogged with a quick-setting resin to prevent the casting resin from filling the fiber to the height of the tube sheet casting.
▪ Pouring the liquid resinous casting material into a mold in which the ends of the fiber bundle have been previously placed and then allowing the mixture to harden. After the tube sheet is formed, the fiber ends are cut open to unclog the pre-sealed fibers.
Why is a membrane support and feed spacer required in spiral-wound membranes?
The material between the membranes (permeate channel spacer) supports them against the operating pressure and defines the permeate flow channel. A net-like spacer sheet that has two functions:
I) It keeps adjacent membranes apart to form a feed channel.
2) It promotes turbulence of the feed gas mixture as it passes through the module, thus reducing concentration polarisation.
What are the advantages of inorganic membranes?
Ease of use and high flux
Wide chemical and pH compatibility [more resistant to chemical attack]
Excellent thermal stability
Sanitisable and sterilisable
Generally element burst pressures are relatively high (> 50 bar)
Able to withstand high frequency back pulsing cycles
Need 100% bubble point integrity testing if using for gas separation
How is stretching used to produce membranes?
Stretching may be applied to semi-crystalline polymers, and the arrangement of crystallites appears to control crazing of the amorphous section.
Materials are often stretched and annealed (heat treated)
What is track etching?
A method of membrane production.
Ion etching - heavy, high-energy ions are accelerated towards the membrane / polymer film. This produces tracks on the film surface.
Chemical etching - a chemical solution is used and the damaged zone of a latent track is removed and transformed into a hollow channel.
What is phase inversion?
A method of membrane production.
It is performed by removing the solvent from a liquid-polymer solution, leaving a porous, solid membrane.
(Polymer is often cast and fed into a precipitation bath.)
What is a sol-gel process?
the sol–gel process is a method for producing solid materials from small molecules. The method is used for the fabrication of metal oxides, especially the oxides of silicon (Si) and titanium (Ti). The process involves conversion of monomers into a colloidal solution (sol) that acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers.
A “sol” (a colloidal solution) is formed that then gradually evolves towards the formation of a gel-like diphasic system containing both a liquid phase and solid phase.
Why is the sol-gel technique useful (for producing inorganic membranes)?
A ceramic can be obtained at relatively low temperatures
The particle size which controls the pore diameter is small and homogeneous,
The cut-off range is very narrow.
What are the two main types of membrane materials?
Natural polymers (wool, rubber, cellulose)
Synthetic polymers (PVDF, PES, PI)
What is the glass transition temperature?
The point at which a material alters state – going from a glass-like rigid solid to a more flexible, rubbery compound.
What are industrial gas separation membranes dependent on?
Material
Structure and thickness
Configuration
Module and system design
What is membrane permeability and what does it depend on?
The rate at which any component permeates through a membrane.
This depends on the thermodynamic factor (looking at partitioning of species between feed and membrane phases) and the kinetic factor (e.g. diffusion in a dense membrane or surface diffusion in a microporous membrane).
What is membrane selectivity?
The ability of a membrane to accomplish a given separation.
What are the various transport mechanisms occurring across membranes?
Soluble (Transport in dense/non porous materials - dominant factor is solubility)
Knudsen flow in narrow pores
Viscous flow (Poiseuille flow) in wide pores (not suitable for gas separation)
Surface diffusion along pore wall.
‘Slip’ flow
Sieving action - larger particles will not fit through membrane pores
