Fibre Chemistry, Vol. 41, No. 5, 2009
USE OF MODIFIED POLYMER FIBRE MATERIALS AS FILTER ELEMENTS FOR TREATMENT OF MULTIPHASE MEDIA A. G. Kravtsov,* S. V. Zotov,** V. A. Gol’dade,** and K. V. Ovchinnikov**
UDC 677.494
Fibre materials manufactured with modern, environmentally clean, solution-free, meltblown technology are convenient and economical polymer filters. The method of manufacturing fibre materials for such filters can be introduced in polymer processing plants interested in implementing progressive technology and expanding the line of polymer fibres and commercial articles manufactured from them.
Filtration of multiphase media — liquids or gases — is the unavoidable companion of many modern manufacturing processes. With respect to structural organization, polymer fibre materials have long been considered optimum filtration systems, so that they are used in the most varied sectors [1-3]. A number of promising versions of fibre filter elements based on melt blowing technology — aerodynamic spinning of fibres from melts of large-tonnage manufactured polyolefins — has now been developed [4, 5]. The developments in creating polymer fibre filters in Belarus are gradually approaching industrial introduction [6]. Liquids and gases are filtered directly on porous fibre filters under the effect of many mutually interrelated factors; the fundamental ones are the flow rate of the filtered dispersion medium, the concentration of disperse particles of contaminants in it, the shape and size of pores and channels in the porous fibre filter, and the intensity of interactions in the “contaminant particle—filter membrane” system. The presence of electric or magnetic fields of determined strength in the system, whose simultaneous effect on the contaminant particles can significantly increase the filtration efficiency, is especially important. In industrial separators, the fields are usually regenerated with powerful voltage sources. However, some varieties of porous polymer fibre filter materials have electrical and/or magnetic activity, so that using them as feed for filtration systems is expedient and economically justified [1-3]. For this reason, the search for progressive methods of modifying polymer fibre materials becomes pressing. There are two groups of such methods — incorporating active additives in the fibres and treating the fibres in electric or magnetic fields. Many studies have shown that it is useful to combine several methods of treating the fibres [4, 5, 7]. Electrical treatment of the spun hot melt-blown fibres in a corona discharge field of varying strength is usually used to give filter materials electrical activity, and filling the fibres with magnets (usually barium or strontium ferrites) followed by treatment in pulsed magnetic fields is used to impart magnetic activity [1]. At the same time, it has repeatedly been noted [5, 7] that the electret and magnetic properties become interconnected in such fibres. For this reason, we can hypothesize that composite fibre materials can exhibit combined — adsorption, electrical and magnetic — activity with respect to filtered multiphase media in contact with them [8]. We demonstrated the possibilities of some varieties of polymer fibre filter elements obtained with melt blowing technology in treatment of multiphase media in real industrial conditions. The basic problems of the study included selecting model filterable media, fabricating experimental samples of modified fibre filter elements by the melt blowing method, and experimentally investigating filtration of multiphase media (liquids and gases). We investigated modified polymer fibre materials, including sources of electric and magnetic fields. Melt blowing technology — high-temperature aerodynamic spinning of fibres from polyolefin melts (high-pressure polyethylene — HPPE, *Gomel’ Branch, National Academy of Sciences of Belarus; **V. A. Belyi Institute of Mechanics of Metal-Containing Polymer Systems, National Academy of Sciences of Belarus, Gomel’. Translated from Khimicheskie Volokna, No. 5, pp. 4449, September-October, 2009. 328
0015-0541/09/4105-0328 © 2009 Springer Science+Business Media, Inc.
TABLE 1. Results of Tests of Electret FE in Filtration of Recycled Water Containing SF Change in opti cal density (opacity) of detergent medium i n comparison to initial value, %
Test type
5 nC/cm
2
8 nC/cm
2
Change in mass of FE due to accumulati on of contaminant s, g 2
2
5 nC/ cm
8 nC/cm
Preliminary low-s peed fi ltration of water to remove undiss olved SF particl es
–(15-20)
–(15-20)
+(0-0.1)
+(0-0.1)
Filtration of water from washin g prim ary glass container
–(13-17)
–(13-17)
+(1.5-2.5)
+(1.5-2.5)
Repeated filtrat ion of the same volume of water from washing ret urnable container
–(5-10)
–(12-15)
+(0.5-1.0)
+(1.0-2.0)
______________
Testing conditions: 800-1200 mg/liter concentration of SF, 20 liters of washing liquid, 250 g weight of FE.
14
U
15 h
13 16
11
III 12 3
5
4
II
8 7 9
6
I
10
2 1
Fig. 1. Diagram of pneumatic extrusion unit: 1) motor; 2) channel for cooling hopper zone; 3) channel for output of melt into shaping head; 4) cylinder (body) heaters; 5) cylinder; 6) “worm”; 7) filling funnel; 8) hopper; 9) thrust bearing; 10) reducing gear; 11) injector; 12) compressed air feed pipe; 13) gas-polymer stream; 14) shaping mandrel. “Worm” zones: I) feed (loading); II) compression (plasticization); III) extrusion (batching). Electrification zone: 15) high-voltage electrode; 16) ground electrode. polypropylene — PP) was the method of manufacturing the porous-fibre systems. A diagram of the manufacturing process is shown in Fig. 1. The fibres were modified by incorporating particles of carbon or magnetic barium ferrite in their structure and electrifying the fibre materials in a coronal discharge field of negative polarity and magnetizing them in a pulsed magnetic field simultaneously or immediately after spinning the fibres. Several versions of the experimental samples of filter elements made of melt-blown HDPE and PP fibres 20-50 μm in diameter with a packing density of 0.2-0.3 were prepared in this way. In many cases, the fibre materials passing through the filtration cycle were reprocessed again by the melt blowing method. FILTRATION OF RECYCLED WATER CONTAINING SURFACTANTS We prepared 50 cylindrical filter elements (FE) consisting of cohesively bound HDPE fibres with an electret charge with effective surface density of 5-8 nC/cm2 containing no ultradisperse modifiers. Secondary fibre FE with the same electret charge parameters were prepared from the FE passing the test with this method [9]. The structure of the fibre material is presented in Fig. 2. The experimental studies included preliminary filtration to remove undissolved particles of water (20-liter sample per filter element) containing an anion-active surfactant (SF) in the form of a foaming detergent and indicated for washing glass containers (original and recycled) and repeated filtration of the same volume of water from washing the glass container in the conditions of a bottle washer shop. The efficiency of filtration was evaluated with the following criteria: — change in the optical density, i.e., opacity, of the detergent medium before and after filtration, determined by photocolorimetry; — accumulation of contaminants in the filter element, determined by weighing; 329
a
b
Fig. 2.
a
b
Fig. 3.
Fig. 2. Fibre structure of FE for original (a) and carbon-filled (b) filtration of recycled water. Scale given in μm. Fig. 3. Fibre structure of FE for electret (a) and carbon-filled (b) filtration of wastewater. Scale given in μm. retention of dissolved SF in the treated washing liquid, determined with the initial and final concentration of active anions using a Spectroquant (Germany) analytical complex. It was found that low-speed filtration of the washing liquid with an electret (charge density of 5 nC/cm2) filter element allowed effectively trapping undissolved solid or unemulsified particles. Regardless of the initial ratio of SF and water in the washing liquid (the concentration of active anions varied within the limits of 800-1200 mg/liter), the optical density (opacity) of the liquid increased by 15-20% as a result of one cycle of preliminary low-speed filtration (Table 1), although no significant increase in the weight of the FE was found. After washing the original glass container, this characteristic almost did not worsen. However, after washing the recycled container, the optical density additionally increased 5-10% and did not change after repeated filtration. Using electret filter elements with a higher charge density (8 nC/cm2) did not markedly improve the results of filtering the original liquid. However, such FE were effective in treating the liquid undergoing washing cycle of the recycled glass container, decreasing its opacity by 5-7% relative to the value measured immediately after washing. Accumulation of contaminants in the fibre filter was investigated by weighing the FE before and after the filtration cycle, and in the second case, the filter was dried and blown with hot air to remove liquid residues. Weight gain of one FE of 1.5-2.5 g after filtration of the initial washing liquid and by 0.5-1.0 g after filtration of the used liquid were found for filter elements with a charge density of 5 nC/cm2. At the same time, these indexes were 1.5-2.5 and 1.0-2.0 g for FE with a charge density of 8 nC/cm2. The concentration of active anions in washing liquid filtered with both types of FE decreased insignificantly, by 4-5%, for the initial liquid. The concentration also decreased by 10-12% after washing the original container and by 25-35% after the recycled container washing cycle. We can hypothesize that this is not due to adsorption of SF on the FE fibres, but to deactivation of the active constituent of the detergents due to contact with contaminants. We also found that the secondary FE demonstrated almost the same characteristics of filtration of recycled water containing detergents. —
FILTRATION OF SEWAGE We made 30 cylindrical filter elements from HDPE and PP fibres of the following varieties for filtration of sewage: 1 — carrying electret charge of up to 10 nC/cm2; — 2 — containing channel black particles in the amount of 0.5-1.5 wt. % injected during aerodynamic spinning; 2 — 3 — containing channel black particles and additional carrying electret charge of up to 10 nC/cm introduced in induced electrification of viscofluid fibres in corona discharge. Secondary fibre materials with the same electret discharge parameters were made from the FE passing the test. The structure of the fibre material is shown in Fig. 3. The experimental studies included investigating the effectiveness of filtering sewage and the possibility of using these filter elements for finishing of the water used in manufacturing several products in the dairy industry (yogurt, fruitmilk mixtures, powdered beverages). —
330
5
1 a
a 0.8
2
3
1
2
3
C, Mg/liter
C, Mg/liter
4
1
0.6 2 0.4 3
0.2
0 0
5
10
15
0
20 τ, days
0
5
10
15
20
τ, days
1
5 b
0.8
2
3
1
2
3
1 0 0
5
10
15
20
τ, days
C, Mg/liter
4 C, Mg/liter
1
b 1
0.6 0.4
2
0.2
3
0
0
Fig. 4.
5
10
15
20
τ, days
Fig. 5.
Fig. 4. Concentration of suspended particles 5-20 μm in size in analyzed medium as a function of duration of work of HPPE (PP) fibre FE: a) primary; b) secondary; 1) carrying electret charge (10 nC/cm2) introduced in induced electrification of spun fibres in corona discharge; 2) containing particles of channel black (0.5-1.5%) injected in aerodynamic spinning of the fibres; 3) carrying electric charge (10 nC/cm2) and containing particles of channel black (0.5-1.5%). Fig. 5. Concentration of organic contaminants in the analyzed medium as a function of time of use of HPPE (PP) fibre FE: a) primary; b) secondary; 1) carrying electret charge (10 nC/cm2) introduced in induced electrification of spun fibres in corona charge; 2) containing channel black particles (0.5-1.5%) injected in aerodynamic spinning of fibres; 3) carrying electric charge (10 nC/cm2) and containing channel black particles (0.5-1.5%). In filtration of sewage, all of the listed filter elements were satisfactorily effective (Figs. 4 and 5, indexes for HPPE and PP almost coincide). Using fibre filters No. 3 seems most attractive and confirms the expediency of combining different kinds of activity in one FE (in the given case, adsorption and electrical). The secondary FE exhibit similar filtration characteristics with slightly faster “saturation” of the filter. This is probably due to the decrease in the fibre diameter by 1.5-2 times, which usually takes place in repeated processing with the melt blowing method. In finishing of water used for manufacturing some dairy products, we found that using filter elements containing carbon particles does not totally ensure the required water quality. However, when the treated water passes through a system with an electret fibre filter placed after such a filter, the required indexes can be attained while respecting the low-speed filtration conditions. The photocolorimetric studies revealed almost no change in the optical absorption coefficient of the water treated with this method. The organoleptic indexes of the water satisfy the standards established for food products. FILTRATION OF POLLUTED AIR For the tests in conditions of the ventilation systems of paint and varnish plants, we fabricated 10 m2 of HPPE and PP filter cloth containing barium ferrite particles in the amount of 5 wt. %. The average fibre diameter was 50 μm (Fig. 6). The spun filter cloth was successively magnetized (to 0.4 mT) and electrified (to 7 nC/cm2). The experimental studies assumed placement of the samples of filter elements in a special chamber in which the conditions in the air of a plant shop in application of paint coatings on aviation engineering elements were simulated in the free volume and a study of the change in the penetration coefficient of oil aerosol and the air drag of the fibre filter as a result of its absorption of contaminants from the air of the working zone (determined on a plant laboratory filtration unit). 331
TABLE 2. Change in Indexes of Fibre Composite Filter Element in Conditions fo the Work Place in a Paint and Varnish Plant Exposure t ime, h
Penetration coefficient, %
Air drag, Pa
0*
0.30
50
1
0.29
56
2
0.27
62
3
0.25
69
4
0.22
77
5
0.18
89
6
0.14
101
7
0.10
124
0.06
145
8 ______________
*Before loading FE sample into chamber. a
b
Fig. 6. Fibre structure of FE for primary (a) and secondary (b) treatment of sewage. Scale given in μm. In addition to the previously obtained data [10], we found that the composite fibrous filter element was an effective absorbent for contaminants suspended in the air of the work zone and in the liquid or solid state (Table 2). The penetration coefficient of the test aerosol through such a filter element after 1 testing cycle (1 work day) decreased by 5 times and the air drag increased by almost 200% for the same time. This illustrates the effect of clogging the pores and capillaries in the bulk of the FE by an important amount of adsorbed pollutants. The pollutants are trapped by the filter and retained due to the combined effect of the electric and magnetic fields and the sieve effect realized in the “pollutant particle—fibre” system. We also found that magnetization of the FE after regeneration with the melt blowing method almost did not change. The electret charge density decreased insignificantly, which could indicate stabilizatin of the electret state in the magnetfilled fibres due to the structural changes that take place in secondary formation of fibre FE. CONCLUSION The results of the studies and technical tests of the filter elements led to the following conclusions. 1. The electret charge improves adsorption of disperse contaminants of different natures by fibrous FE, while only weakly affecting the ionic composition of the detergent. For this reason, electret fibre filters containing no additives can be successfully used for filtration of water used for washing original and recycled glass containers. We can hypothesize that the higher chemical inertness and dimensional stability of such FE allows using them to replace paper and cardboard filters, including in treatment of wine stock and other food products.
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2. Polyolefin fibre FE carrying an electret charge and/or containing adsorbent particles can be used to increase the efficiency of filtration of sewage from industrial enterprises. The primary data suggest that water used for fabricating some dairy products can be finished, in successive placement of adsorption-active and electret FE in the filtration system and in observing the conditions of low-speed filtration. 3. Polyolefin fibre FE, which are simultaneously sources of electric and magnetic fields, can effectively absorb contaminants which are liquid and solid particles suspended in air (finely disperse wastes from paint and varnish plants). 4. In all cases investigated, secondary melt-blown FE exhibited efficient filtration compared to the efficiency of primary fibre FE and similar characteristics of the change in it. This indicates the possibility of regenerating fibre FE by melt blowing while preserving the combined activity of the fibres with respect to treated multiphase media. These results confirm the hypothesis concerning the necessity of comprehensive modification of fibres (addition of additives and treatment in physical fields) to increase the efficiency of filtration of complex multiphase media, including those used in the food industry. The previously established [5, 11] partial preservation of the electrical activity of electret fibres in liquids (up to 70% effective surface charge density for 1 month) was confirmed, which could be explained by the reversible character of the reaction of the electret charge with both the liquid and the disperse contaminants it contained. The characteristics of the combined effect of charge, magnetic field, and modifiers added to FE require further investigation using industrial equipment. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
L. S. Pinchuk (ed.), Melt-blown Polymer Fibre Materials [in Russian], IMMS NANB, Gomel’ (2000). A. G. Kravtsov, V. A. Gol’dade, and S. V. Zotov, in: Polymer Electret Filter Materials for Protection of the Respiratory Organs [in Russian], L. S. Pinchuk (ed.), IMMS NANB, Gomel’ (2003). A. G. Kravtsov, S. A. Marchenko, and S. V. Zotov, Polymer Fibre Filters for Overcoming the Environmental Consequences of Emergency Conditions GTTU im. P. O. Sukhogo, Gomel’ (2008). S. V. Zotov, Fibre Electret Filter Materials Made of Polyolefins for Individual Protection of the Respiratory Organs, Candidate Dissertation 05.02.01, Minsk (2006). A. G. Kravtsov, Electric and Magnetic Fields in Polymer Fibre Filter Elements for Fine Treatment of Multiphase Media, Doctoral Dissertation 04.01.07, Gomel’ (2007). A. G. Kravtsov and S. V. Zotov, “Polymer materials for solving environmental problems,” Nauka Innovatsii, 68, No. 10, 70-72 (2008). A. G. Kravtsov, Izv. NAN Belarusi, Ser. Fiz.-Tekhn. Nauk, No. 1, 23-26 (2003). A. G. Kravtsov and S. V. Zotov, in: Proceedings of the International Conference Polikomtrib-2009, June 22-25, 2009, Gomel’ (Belarus’) [in Russian], IMMS NANB, Gomel’ (2009), pp. 119-120. A. G. Kravtsov, Materialovedenie, No. 4, 47-52 (2001). A. G. Kravtsov, L. S. Pinchuk, and V. A. Gol’dade, Khim. Volokna, No. 6, 42-46 (2000). A. G. Kravtsov, S. V. Zotov, et al., Plast. Massy, No. 6, 15-19 (2004).
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