INVESTIGATING REASONS FOR FILTER BAG FAILURE AND DEVELOPING A METHOD TO IMPROVE ITS LIFE SPAN

Transcription

1 INVESTIGATING REASONS FOR FILTER BAG FAILURE AND DEVELOPING A METHOD TO IMPROVE ITS LIFE SPAN Dr. İbrahim ÇAKMANUS FFT FAN FİLTER SYSTEM CO. LTD. ABSTRACT This paper reports a study on the reasons for filter bag failure at steel plants and ways to overcome such problems. Clean and used filter bags were subjected to evaluate filtration, tensile and bursting strengths and air permeability properties. This study is based on the sources given in References section. The structural changes in all bags (clean, used and chemically treated) were analyzed. One of the major reasons for the filter bag failure was attributed to poor acidic resistance of the bags. SEM images of the bags showed perceptible degradation of the fibers after acidic treatments and also clogging of the filter bags as the dust particles were passing through the bags. In order to improve the life of the bags, these bags were coated with a mixture of acrylic binder (7% concentration) and nanoclay (2% by weight). Furthermore, filter fabrics were also produced from the spunlaced process and coated with the binder and nanoclay coatings. The results showed that filters bags, as well as developed samples, showed improved resistance without any failure behavior. KEYWORDS Air filtration, Bag filter, Failure analysis, Filter media, Nanoclay coating, Nonwovens 1. Introduction Air filtration plays an important role in improving air quality and cleaner environment. The demand for better air quality has increased greatly due to new regulations, advent of new scientific knowledge and a change in health consciousness [1-2]. Majority of the filters used in the manufacturing sectors, such as steel, power, cement and mining are expensive. A number of factors affect the life span of a filter in actual operating conditions. Premature failure of the filter bags not only increases the pollution level, but also increases the manufacturing cost. Filters used in the steel plants mainly capture the fly-ash which is acidic in nature. The same is true about the filters used in the power plants, which need to filter hot flue, mainly consist of NO2, SO2, CO2 and CO. In both the cases, ability of the filter to operate and withstand acidic conditions (acid dew point) will determine its service life. Traditionally, filter bags used in the above sectors are coated with the polyurethane or Polytetrafluoroethylene (PTFE) membranes in order to improve various functional properties. Some of the problems associated with these coatings are their durability and price sensitivity. Another problem with the coating on the filters is the associated increase in the pressure drop characteristics of the filters. Furthermore, the widely used polyacrylonitrile (PAN) filter bags in power plants are susceptible to shrinkage. Shrinkage in the PAN filter bags can be thermal, chemical or physical in nature (or a combination of these). The first two factors, thermal (heat induced shrinkage) and chemical (acid hydrolysis predominately by sulfuric acid - H2SO4 and oxidation) are dominant in deciding the life span of the filters. Filter shrinkage affects the durability and performance of the bags in actual operating condition and eventually lead to filter bag failure. The aim of this paper is to study the possible reasons for filter bag failure and means to tackle such problems by a coating of nanoclay-binder.

2 2. Materials and Methods 2.1. Filter Bags Detailed information about the tests which are explained briefly here could be obtained from the given references. Clean and used filter bags were obtained from the steel plants. Used filter bags were 4 months old. These bags were made up of glass fibers. One side of these tubular bags was woven fabric and the other side was the needle-punched nonwoven (Figure 1). Both the woven and nonwoven components were joined by chemical bonding. The woven fabric remains exposed to the dust filtering side and the nonwoven to clean side. The filtering gases are passing from the woven side to the nonwoven side. The area weights of the clean filters bags were around 1200 g/m2. On the other hand, very few of the consumers in Turkey have knowledge about grammage of the bags, permeability values, real dust load, particle dimension distribution, physical and chemical properties of the fluids, energy consumption of the filter systems. However, it is very important to make extensive examinations, to be in line with the latest developments and accumulation of knowledge in the fields like these which are very high cost. Figure 1: Actual photographs of the filter bags: a) clean bag, b) used bag Spunlaced Filter Fabric Preperation PAN fiber was used for sample preparation and Fleissner Aquajet was used for producing the samples. Spunlaced filter fabrics were prepared by using manifold pressures of 120 bar. Two filter fabrics were prepared, one using 2.2 dtex, 50 mm PAN fibers (100%). Other fabric was produced from a 92:8 blend (by weight) of 2.2 dtex, 50 mm PAN fibers and 1.7 dtex, 50 mm PAN fibers (PAN blend). The spunlacing processing parameters were: manifold pressures (bar) - 10, 120, 120; output speed (m/min) - 4; drying temperature C and 2 layer structure. Thickness of 100% PAN sample was 3.36 mm and PAN blend was 3.45 mm, respectively. The area weights of the fabrics were kept constant at 450 g/m2. Filter fabrics were sprayed with a coating which was a mixture of acrylic binder (Acronal 32d, 7% concentrations) and nanoclay (nanomer 1.3 PS obtained from Sigma Aldrich, 2% by weight). Nanomer 1.3 PS was used as it is without modifications as the clay was already functionalized. A mixture of the nanoclay and binder was stirred for 24 hours before spaying on to filter samples with the help of a spray gun. The coated samples were dried in an oven at 90 C for 2 hours. The clean filter bags were also coated with the same mixture of acrylic binder (7% concentration) and nanoclay (2% by weight) Properties Evaluation Clean and used filter bags were subjected to various performance tests such as tensile strength and elongation, bursting strength, air permeability, ph and acid dew point. The structural changes in all the bags (clean, used and chemically treated) were analyzed under the SEM. All samples were conditioned for 24 hours in a standard testing atmosphere maintained at 20 ± 2 C and 60± 5% relative humidity prior to testing Tensile Properties Tensile properties were measured on an Instron tensile tester according to ISO standard [3]. Strip

3 tests were conducted which used the following testing parameters: fabric width 50 mm, gauge length: 120 mm, traverse or stretching speed 100 mm/min Bursting Strength Bursting strength were measured on a bursting strength tester according to ISO standard [4]. A test specimen was clamped over an expansive diaphragm by means of a circular clamping ring. Increasing compressed air pressure was applied to the underside of the diaphragm, causing inflation of the diaphragm and the fabric. The pressure was increased gradually until the test specimen burst. A test area of 1.0 cm2 was used Air Permeability The air permeability test was carried out on WIRA air permeability tester according to EN ISO 9237:2011 standard [5]. The rate of airflow passing perpendicularly through a known fabric surface area was adjusted to obtain a prescribed air pressure difference between two surfaces of the fabrics. From this rate of airflow, the air permeability of fabric was determined. The air permeability was expressed in ml/s/cm². Test surface diameter was 40 mm and pressure head maintained for testing was 98 Pa ph Value The ph values were measured according to ISO 3071:2005 standard [6]. The method involves the electrometric measurement of the ph value of the aqueous extract of textiles at room temperature by means of a glass electrode. Test specimens of 2 g were taken from the samples and agitated in a stoppered flask containing 100 ml of deionized water (ph 5 and 7.5) for 2 hours. The ph meter used for the subsequent measurements of the aqueous extract is calibrated using standard buffer solutions Acid Dew Point Acid dew point was measured according to the methods discussed in the references [7] and [8]. Clean filter bag samples were subjected to 10% H2SO4 at 90 C for 24 hours. The weights of the samples before and after acidic treatment and linking acid dew point with other properties like tensile and bursting strengths. Nanoclay and binder coated spunlaced filter samples and one of the nanoclay coated filter bag sample was also tested for acid dew point properties Micro Structure of the Filter Bags In house SEM, FEM quanta 200 was used to examine the micro-structure of the filter bags. For the SEM analysis, longitudinal view and cross-section of the bags were observed under different magnifications. SEM analysis was done on the clean, used and chemically treated bags as well as on the spunlaced samples. 3. Results and Discussion 3.1. Tensile Properties The percentage changes in the tensile properties of the clean, used and chemically treated bags are shown in Table 1. There was 58% drop in the bag strength when comparisons were made between clean and used bags. There was an 87% drop in the bag strength when comparisons were made between used and chemically treated bags. These values indicate the poor resistance of the glass fibers to the acidic conditions as well as poor durability of the glass fibers. There was also 33% reduction in the elongation value and 625% increase in this value while comparing between clean and used bags and used and chemically treated bags. These values suggest that fibers were weak after acidic treatment, which resulted in such a high increase in the elongation values. Sample code Clean bags (C) Used bags (U) Chemically treated bags (H2SO4) (CT) Table 1: Percentage changes in the tensile properties of various bags. Average maximum force (N) Average extension (%) strength between C and U (C-U/C*100) strength between C and CT (C-CT/C*100) elongation between C and U (C-U/C*100) elongation between C and CT (C-CT/C*100)

4 Above physical test results were further substantiated by capturing images of the various bags with the help of SEM images. These images throw a light on the changes in the fiber micro-structure after acid treatment. SEM images of the clean and used bags are shown in Figure 2a and 2b. The nonwoven side was used for the comparison purpose. There are no signs of fiber damage in a clean bag (Figure 2a). The used bag was covered with dust particles; therefore, it is difficult to see the fiber damage (Figure 2b). The SEM image of the H2SO4 treated bag was clearly showing the signs of fiber damage. These images confirm the finding that fibers were damaged by the acid. Therefore, a significant drop in the strength of the bag as compared to the original bags was observed. Figure 2 a): SEM image of the clean bag (nonwoven side), magnification 500X. Figure 2 b): SEM image of the used bag (nonwoven side), magnification 500X.

5 Signs of fiber damage by acid Figure 2 c): SEM image of the chemically treated bag (nonwoven side), magnification 1000X. Signs of fiber damage by the action of H2SO4 were clearly visible Bursting Strength The bursting strength values of the clean, used and chemically treated bags are shown in Table 2. A 37% drop in the bursting strength between the clean and used bags was observed. A 77% drop in the bursting strength between used and chemically treated bags was observed. These values indicate the poor resistance of the glass fibers to the acidic conditions as well as their poor durability. Table 2: Percentage changes in the bursting strength of various bags. Sample type Bursting strength bursting strength between C and U bursting strength between C and CT (C-CT/C*100) (Kg/cm 2 ) (C-U/C*100) Clean bags (C) Used bags (U) Chemically treated bags (H2SO4) (CT) Air Permeability The air permeability values of various bags are shown in Table 3. A 84% drop in the air permeability values between clean and used bags and 17% increase between clean and chemically treated bags were observed. The reason for such a drop was due to bag surfaces were covered with the dust particles, thereby preventing the passage of airflow and decreasing the air permeability. Lower values of air permeability indicate lower filtration efficiency of the bags and increasing the pressure drop characteristics of the bags. SEM images of the clean and used bags also confirm the blocking of pores within the fabrics by the dust particles and particles were passing right through the entire cross-section of the used bags. The increase in air permeability after acid condition was due to the slackness in the fiber network structure which allows easy passage of airflow.

6 Sample type Table 3: Air permeability values of clean and used bags. Air permeability (ml/s/cm²/98 pa) air permeability between C and U (C-U/C*100) air permeability between C and CT (C-CT/C*100) Clean bags (C) Used bags (U) Chemically treated bags (H2SO4) (CT) Dust particles Figure 3: SEM image of the used bag cross-section (woven), magnification 100X ph Values The ph values of the clean and used bags were 6.4 and 2.7, respectively. From almost neutral values of clean bags, bags became acidic after usage. The possible reason for this could be due to high acidic content in the filtering gas. There should be systems such as desulphurization in place to reduce the acidic content of the emission before it passes through the filter bags in order to improve the life span of the bags Acid Dew Point Earlier discussion regarding tensile strength and other properties indicated that there were losses in most of the properties after treating the clean bags with H2SO4, because of the fiber damage. The weight loss in the clean and H2SO4 treated clean bags after acid dew point test is shown in Table 4. There was a 3% loss in the weight of the clean bags when treated with 10% H2SO4 at 90 C for 24 hours. If this loss continues in this manner for a prolonged period of time, then the bags will last just over one month, as an acid increasingly damages the fibers, which ultimately damaging the filter bags and resulting in the bag failure. Some of the SEM images captured on the woven side of the H2SO4 treated clean bag also reconfirming the fiber damage (Figure 4). Therefore, there should be some sort of controlling system (such as desulphurization) in place in order to reduce the acidic content of the emission before it passes through the filter bags so as to improve the life span of the bags. Also, filtering gas temperature should be more than the acid dew point temperature of the incoming flue gas in order to avoid the formation of H2SO4.

7 Table 4: Weight loss in the clean and H2SO4 treated clean bag after acid dew point test. Sample type Clean bags (C) Chemically treated bags (H2SO4) (CT) Weight (gm) weight after acidic treatment (C-CT/C*100) Fiber damage by H2SO4 Figure 4: SEM image of the H2SO4 treated clean bag (woven side), magnification 1000X Performance of bags and spunlaced filter fabrics after acid dew point test Acid dew point results showed that there were negligible reduction in the sample weight after acid treatment in case of the nanoclay and binder materials as shown in Table 5. As a result of nanoclay coating, it prevents the chemical degradation and thereby, prevents weight loss in the samples. Since it was a spray coating on the surfaces in order to maintain other performance properties, there was some weight reduction where there was no coating applied to the samples (Figure 6).

8 Table 5: Weight losses in various bags and spunlaced filter sample after acid dew point test. Sample code Weight before acidic treatment (gm) X Weight after acidic treatment (gm) Y weight after acidic treatment (X-Y/X*100) Clean bag (C) Nanoclay and binder coated bag (CB) Nanoclay and binder coated spunlaced fabric1- PAN 100% (CBSL1) (a) (b)

9 Figure 5: Spunlaced CBSL2 coated with a mixture of 9% nanoclay and binder, a) view at a magnification of 150X and b) closure view of the sample at 1000X 3.7. Conclusions A significant reduction in tensile and bursting strengths after acidic treatment with H2SO4 was observed in clean bags. Poor acidic resistance of the clean bags was one of the main reasons for the early failure of the filter bags. SEM images of the bags showed fiber degradation after acidic treatments. Furthermore, clogging of the filter bags with dust particles were observed for the used bags. The nanoclay and binder coated filter bags and filter fabrics developed from spunlacing process showed resistance to acidic degradation. References 1. Patanaik A. and Anandjiwala R. D. Hydroentanglement nonwoven filters for air filtration and its performance evaluation, Journal of Applied Polymer Science, Vol. 117, No. 3, , Patanaik A. Anandjiwala R. D. and Boguslavsky L. Development of High Efficiency Particulate Absorbing Filter Materials, Journal of Applied Polymer Science, Vol. 114, No. 1, , ISO :2013, Textiles - Tensile properties of fabrics - Part 1: Determination of maximum force and elongation at maximum force using the strip method. 4. ISO :1999, Textiles - Bursting properties of fabrics- Part 1: Hydraulic method for determination of bursting strength and bursting distension. 5. ISO 9237:2011, Textiles - Determination of the permeability of fabrics to air. 6. ISO 3071:2005, Textiles - Determination of ph of aqueous extract. 7. Renaud C. M. and Greenwood M. E. Effect of glass fibers and environments on long-term durability of GFRP composites, 9 th EFUC meeting, Wroclaw, Poland, Nov Acid induced stress corrosion cracking, Smithers Rappa and Smithers Pira Ltd