Microfiltration is widely used in fine filtration operations, with dead-end filtration and cross-flow filtration. The microfiltration filter element or the microfiltration membrane is easy to be polluted by impurities in the water and causes fouling, resulting in flux attenuation. The flux can be expressed by Darcy’s law and attenuation model. In this paper, two industrial titanium rod sintered filter elements ( Ø60 × 960 mm and Ø60 × 960 mm) of different specifications are selected, and tap water (1.0 NTU) is used for constant pressure dead-end filtration. The amount tends to be the same, about 0.435 m 3 ·m −2 ·h −1, which has nothing to do with the filtration accuracy of the filter element but only depends on the characteristics of the filter cake and the filter membrane. Through the analysis of the two models, it is found that the two filtration flux models are not universal and difficult to be applied in engineering.
Microporous filtration uses the pore size of the microporous membrane to trap particles, bacteria, and suspended substances that are larger than the pore size of the microporous membrane in the filtrate under the pressure difference, so as to achieve the purpose of removing the particulates and clear solution in the filtrate. Generally, the pore size of the microporous membrane used in the microfiltration process is in the range of 0.05 - 10 μm [
The microfiltration process has two operation modes: terminal microfiltration and cross-flow microfiltration, as shown in
Cross-flow microfiltration is based on the sieving mechanism of membrane surface filtration. The operation mode is to use a pump to send the filtrate into a pipe or thin-layer flow channel with a porous membrane wall. The filtrate flows along the tangential direction of the membrane surface. Under the driving force, permeate is cross-flowed through the membrane. As shown in
There are many microporous filtration models, of which two typical mathematical models have been widely recognized by scholars, namely the filter cake filter flux model and the microporous filter flux attenuation model. The core expressions are shown in
| Conventional filter cake filtration flux model | Microporous filtration flux attenuation model |
|---|---|
| Filter flux: J = 1 A × d V d t = Δ p μ ( R m + R c ) (1) Membrane resistance: R m = k m ( 1 − ε m ) 2 S m l m ε m 3 (2) Cake resistance: R c = k c ( 1 − ε c ) 2 S c l c ε c 3 (3) | Filter flux: J = 1 A × d V d t = β ( α t + β ) 2 × 1 A (4) Model parameters: α = 4 π h m n p d p 2 × N A ρ s ( 1 − ε ) (5) Model parameters: β = h m N × 128 μ π d p 2 A × Δ p TMP (6) |
| J—Filtration flux (also called membrane filtration rate), m3·m−2·h−1; A—Filter area, m2; V—Permeate filtrate volume, m3; t—Filter time, h; Δp—The pressure difference applied to the membrane and the filter cake, kPa; μ—Viscosity of the suspension fluid, Pa·s; Rm, Rc—Respectively the resistance of the membrane layer and the filter cake layer, N; km, kc—Respectively filter membrane constant (km = 2) and filter cake constant (kc = 5); εm, εc—Respectively, membrane porosity and filter cake porosity, dimensionless, Sm, Sc—Respectively, the specific surface area of the filter membrane and the specific surface area of the filter cake, m3·m−2; lm, lc—Respectively the thickness of the filter membrane and the thickness of the filter cake, m. | J—Filtration flux (also called membrane filtration rate), m3·m−2·h−1; A—Filter area, m2; V—Permeate filtrate volume, m3; t—Filter time, h; α, β—Model parameters; μ—Viscosity of the suspension fluid, Pa·s; hm—Circulation depth, m; np—Number of membrane holes; dp—Porous outer diameter before filtration, m; N—Concentration of suspended solids entering the porous, kg·m−3; ε—The porosity of the pore surface layer structure is dimensionless, ΔpTMP—The pressure difference applied to the membrane and the filter cake, kPa; ρs—Solid particle density, kg·m−3. |
The filtration flux test device is shown in
The titanium rod filter element is provided by Shijiazhuang Gold Titanium filter element, and its specifications are Ø60 × 960 mm and Ø60 × 960 mm, and the filtration area of a single rod is 0.181 m2. When the flow rate is greater than 0.2 m3/h, use an electromagnetic flowmeter to read, and when the flow rate is less than 0.2 m3/h, use an electronic scale and a stopwatch to measure. The turbidity of tap water is 1.0 NTU, and the turbidity is 0 NTU after filtering by the filter element. The flux attenuation trend is as follows shown in
The mathematical average of the flux in the test interval is obtained by the Origin scatter integral, namely:
J ¯ = 1 Δ t ∫ t 1 t 2 J d t
Know from
1) As the filtration progresses, the filtration resistance continues to increase, the filter element continues to be contaminated, and the flux continues to decrease. The flux decreases rapidly at the beginning of the filtration, and gradually slows down after the filter cake is formed;
2) Large pores have a large flux, the filtration resistance increases faster, the fouling speed is fast, and the flux decays fast. The small pores have a small flux, the filtration resistance increases relatively slowly, and the fouling speed also slower;
3) Whether it is a 1 μm or 3 μm filter element, the flux basically tends to be the same after 2 h. Therefore, it can be inferred that the ability to intercept suspended solids after the curve enters the flat section is basically independent of the micropore diameter, and the backwash cycle should not exceed 2 h.
In this article, tap water is filtered with micropores. Tap water contains both organic matter and minerals, as well as microorganisms and other impurities. During filtration, a gel layer is formed near the surface of the membrane. This gel particle has a certain degree of elasticity. The particles deform greatly and enter the microporous pore channel under pressure, which not only thickens the filter cake layer on the surface of the filter membrane, but also passes through the adsorption bridging and salting out in the gap after entering the pore channel of the filter membrane, which affects the filtration The factors are complicated. Both the filter membrane resistance Rm and the filter cake resistance Rc are no longer constants, and it is difficult to use a reliable formula for effective expression. Therefore, the filter cake filtration flux model is rarely used in engineering design.
The pre-expression of the flux decay model is:
t V = α t + β (7)
Rearrange and differentiate the formula (7) to obtain the formula (4) in
Obviously, the relationship between t/V ~ t is not a straight line, that is, α and β are not constants as stated in the model, but a variable that changes with the
filtering time. Not only that, the pore size of the filter element is different, and the shape of the attenuation curve is also different, so the model does not have universality and versatility.
α 1 = 1.39468 × 10 − 7 t 2 − 4.0926 × 10 − 5 t + 0.00555 , β 1 = 0.04228 , R 2 = 0.9990 (Ø60 × 960 mm)
α 2 = 3.36011 × 10 − 8 t 2 − 6.88277 × 10 − 6 t + 0.00101 , β 2 = 0.01768 , R 2 = 0.9974 (Ø60 × 960 mm)
Substituting the obtained model parameters into Equation (4):
J 1 = 0.04228 [ ( 1.39468 × 10 − 7 t 2 − 4.0926 × 10 − 5 t + 0.00555 ) t + 0.04228 ] 2 × 1 0.181 (8) (Ø60 × 960 mm)
J 2 = 0.01768 [ ( 3.36011 × 10 − 8 t 2 − 6.88277 × 10 − 6 t + 0.00101 ) t + 0.01768 ] 2 × 1 0.181 (9) (Ø60 × 960 mm)
Two kinds of filter elements need two different model parameters to express. Obviously, this has lost the meaning of the model, so the method of experimental determination is still used in engineering.
Titanium rod sintered filter element is made of industrial pure titanium powder (purity > 99.7%) as the main raw material, sintered at high temperature, and the thickness of the microporous film is generally 90 - 170 μm. Its uniform structure, high porosity, low filtration resistance, good permeability, high filtration precision (up to 0.1 μm), acid and alkali corrosion resistance, high temperature resistance, etc., can be widely used in the separation and purification of solid-liquid and solid-gas. It is especially suitable for pre-filtration of ultrafiltration, nanofiltration and reverse osmosis.
Membrane fouling is usually caused by the formation of an adhesion layer on the membrane surface and the blockage of the membrane channels [
The pollution of the titanium rod filter element can be divided into two types: reversible pollution and irreversible pollution:
1) Reversible pollution. That is, after the filter element is contaminated, the flux can be restored after physical methods such as air washing, water washing, and ultrasonic cleaning. Air washing and water washing are usually performed online, while ultrasonic cleaning needs to be performed offline [
2) Irreversible pollution. That is, the filter element cannot be recovered by physical methods after being contaminated, and must be treated with chemical cleaning or even calcination. Chemical cleaning can usually be done online, while the calcination process must be done offline.
The pollution of seawater treatment by the titanium rod sintered filter element mainly comes from the following three aspects:
1) Suspended solids in seawater and tiny flocs and particles produced during seawater treatment;
2) Microorganisms and their dead bodies in seawater;
3) Inorganic salt in seawater crystallizes and scales on the filter element, causing filter hole blockage.
The removal of the filter element mainly comes from physical and chemical methods.
Physical cleaning: mainly air blowing, water washing and ultrasonic cleaning.
Chemical cleaning: first strong acid washing, then strong alkali washing and then water washing, sometimes it is necessary to add strong oxidants. A lot of practice has proved that the cleaning sequence cannot be reversed, otherwise the cleaning effect will be worse [
Microfiltration has two operation modes, namely dead-end filtration (terminal filtration) and cross-flow filtration (dynamic filtration). The former has a rapid and continuous decline in flux and a short operation period, while the latter has a flux that drops to a certain extent and stabilizes in a certain range, can implement continuous filtration operation for a long time to extend the backwash cycle of the filter element.
There are two typical mathematical models for filter element filtration, namely the filter cake filtration flux model and the flux attenuation model. Although the flux attenuation model is more effective in expressing the actual situation of microfiltration, its versatility is still poor and it has not been engineered.
The dead-end filtration test of the titanium rod sintered filter element shows that whether the filter element with different precision adopts constant pressure filtration or variable pressure filtration, the flux tends to be the same after a period of time. In most cases, the backwash time of the filter element should not exceed 2 h, otherwise, the filtration flux is very small and seriously affects the production capacity. Constant flow filtration (or boosted filtration) is complicated to control, which is not good for backwashing filter elements and is not suitable for industrialization.
The pollution of the microporous filter element mainly comes from the surface adhesion of organic polymers, microorganisms, and inorganic salts, pore bridging, and pore blockage, which requires regular online backwashing and regular offline cleaning.
The authors declare no conflicts of interest regarding the publication of this paper.
Xiong, R.L., Liu, P., Zhang, Y.Y., Nan, Y., Chen, J.D., Chen, X.J., Ma, S.Z., Lin, G.Q. and Zhang, Q. (2021) Analysis of Pollution and Blockage of Titanium Rod Sintered Microporous Filter Element. Open Journal of Applied Sciences, 11, 614-622. https://doi.org/10.4236/ojapps.2021.115044