Definition of membrane process
Membrane: A phase that acts as a fence to prevent the mass movement and allow one or several objects to pass through in an orderly manner.
Ultrafiltration (UF) is a membrane separation technology that uses the high-precision retention performance of synthetic polymer semipermeable membranes to separate solids and liquids or classify substances of different molecular weights by using pressure as the driving force.
Ultrafiltration is between nanofiltration and microfiltration, and its definition domain is about 5000-500000 molecular weight cut-off, and the approximate value of the corresponding pore size is 2 10-9-100 10-9 meters.
The process of separation of solutes by ultrafiltration membranes.
The separation process of solute by ultrafiltration membrane mainly includes:
1.adsorption on the surface of the membrane and in the micropores;
2.Stay in the hole and be removed (blocked);
3.Mechanical sieving (sieving) on the membrane surface.
The principle of the ultrafiltration process
Under the action of the pressure difference between the two sides of the membrane, the ultrafiltration membrane is used as the filter medium. Under a certain pressure, when the solution flows through the surface of the membrane, only water, inorganic salts and small molecules are allowed to pass through the membrane, and the suspended solids, colloids, proteins and microorganisms in the water are prevented from passing through, so as to achieve the purpose of purification, separation and concentration of the solution.
Basic terms
Water production: per unit time, the amount of liquid permeated through the membrane per unit membrane area is 2h.
Molecular weight cut-off (MWCO): The cut-off rate is9(0.95) when the molecular weight of the smallest substance retained by the membrane.
Rejection (%) is a quantitative measure of the membrane's ability to retain solutes under specific conditions.
Transmittance (%)Permeability of solutes on the membrane Rejection rate.
Hydrophilicity: A measure of the membrane's affinity with water.
Ultrafiltration concentrate: Water that is discharged during the ultrafiltration process without passing through the membrane.
Transmembrane pressure difference: The pressure difference between the membrane retentive side and the permeate side.
Concentration factor: The ratio of the initial volume (mass) of the feed solution to the final volume (mass) of the ultrafiltration concentrate.
Full Filtration (Dead End Filtration): The process of separation by adding a solvent to the concentrate. (continuous and intermittent).
Cross-flow filtration: refers to the filtration method in which part of the inlet water passes through the membrane element to form produced water, and the rest of the water forms concentrated water.
Concentration polarization: The accumulation of solutes trapped by the membrane on the membrane surface leads to an increase in solvent migration resistance or an increase in local osmotic pressure (both of which decrease the flux) and may alter the membrane's retention performance.
Flow ditch: The space in which fluid flows in the membrane module.
Membrane module: The membrane unit and the cylinder (pressure vessel) that houses the membrane unit are components.
According to the membrane material, ultrafiltration membrane can be divided into organic polymer materials and inorganic materials.
Organic polymer materials
1.Cellulose esters.
The main ones are CA, CTA, CA-CN
Advantages: good hydrophilicity, good porosity, convenient materials, low cost.
Disadvantages: poor acid and alkali resistance (pH4-6);Solvent resistance is also poor;Low operating temperature (30-35);Poor resistance to microbial degradation;
2.Polysulfones.
PS, SPS, PES, etc.
Advantages: excellent mechanical strength and high temperature resistance, chemical attack resistance;Wide range of operating temperatures;Wide pH range;Good chlorine resistance;
Disadvantages: The membrane is hydrophobic and easy to pollute.
3.Polyolefins.
pp、pan
It has good mechanical strength, heat resistance and chemical resistance, and is the most used membrane material at present.
4.Fluorine-containing materials.
pvdf、ptfe
Ultrafiltration membranes prepared from these materials are the best quality membranes, with excellent mechanical strength, high temperature resistance, and chemical attack resistance. It can be used in a wide range of operating temperatures, including strong acids, strong alkalis, and various organic solvents.
The disadvantage is that the material is highly hydrophobic.
Other polymer materials.
Inorganic materials
Mainly ceramic, glass and metal. The most prominent advantages are high temperature resistance, good resistance to organic solvents, not easy to age, strong renewable, and suitable for special separation.
Basic properties of ultrafiltration membrane polymers
The polymer must be compatible with the chosen film-forming method;
Polymers must have properties that meet the objectives of a specific application;
Polymers must be commercially available,**
It has good film-forming properties
Strong affinity for permeable components.
According to the shape of the membrane, it is divided into
Flat Plate Membranes (Plate Ultrafilters;Spiral ultrafilter).
Tubular membranes (internal pressure ultrafilters;External pressure ultrafilter).
Hollow fiber membranes (internal pressure ultrafilters;External pressure ultrafilter, curtain ultrafilter).
Characterization of the structure and properties of membranes
Morphological structure of the membrane
Ultrafiltration membranes prepared by asymmetric process (L—S) are asymmetrical. When making the film, the side that comes into contact with the air is 01～0.A dense layer of 25 microns, the lower part of which is 01～0.2 mm porous support layer. By adjusting the exchange rate between the precipitant and the solvent during the gel, it is possible to prepare membranes with finger pore structure and spongy pore structure. The finger pore structure has less resistance to the fluid and a higher permeability rate, but has poor resistance to compression, while the opposite is true for the cavernous pore.
Performance characterization of ultrafiltration membranes
Definitions
The characterization of membrane properties usually refers to the physicochemical and separation properties of the membrane. The physicochemical properties of the membrane mainly include the mechanical strength, chemical resistance, heat resistance temperature and applicable pH range of the membrane. The separation performance mainly refers to the water permeability rate and molecular weight cut-off and cut-off rate of the membrane.
Water permeability rate
Water permeability rate refers to the amount of water permeated by a membrane per unit area per unit time under a certain working pressure.
Determination of the permeability rate of pure water
Determination method: using a cup evaluation cell or a small ultrafilter, at 01～0.At 5MPa pressure, at 25 hours, the permeability of pure water per unit membrane area and unit time was measured.
Polyethylene glycol standards
It has good water solubility, narrow distribution of relative molecular weight, easy to obtain raw materials, and no or little pollution to most organic membranes, but large molecular weight polyethylene glycol is not easy to obtain.
Polyethylene glycol test method
1. Preparation of reagents.
1.1 Preparation of bismuth potassium iodide reagent.
Preparation of liquid a: accurately weigh 0800g of bismuth nitrate was put into a 50ml volumetric flask, 10ml of glacial acetic acid was added, and diluted to the scale with distilled water.
Preparation of solution B: accurately weigh 20000g potassium iodide into a 50ml brown volumetric flask and diluted to the scale with distilled water.
1.2 Acetic acid Preparation of sodium acetate buffer solution:
Take 02mol l sodium acetate solution 590ml and 02mol l glacial acetic acid solution 410ml into a 1000ml volumetric flask to prepare pH 48 acetic acid sodium acetate buffer solution.
1.3. Preparation of standard solutions.
The polyethylene glycol was dried at 60 for 4 hours, and the polyethylene glycol 1 was accurately weighed000g in a 1000ml volumetric flask and pipette separately. 0 in 100 ml of volumetric flasks, diluted to the scale with distilled water, prepared into a standard solution with a concentration of mg L.
1.4. The production of standard curves.
Take 5ml of each of the above polyethylene glycol standard solutions and add them to 10ml of volumetric flasks, add 1ml of D reagent and 1ml of acetic acid-sodium acetate buffer solution respectively, and dilute them to the scale with distilled water. After 15 min, the absorbance of the solution was measured at a wavelength of 510 nm, with distilled water as the blank. The test data was based on the polyethylene glycol concentration as the abscissa and the absorbance as the ordinate to plot the standard curve.
1.5. Determination of membranes.
One molecular weight polyethylene glycol was selected, and an aqueous solution of 100-200mg l was prepared as the inlet water for ultrafiltration, and the polyethylene glycol content of the influent water and the permeate water was detected in the standard curve after 30min of operation.
Ultrafiltration membrane separation experimental apparatus QY-HGGY43