Enhanced filtration performance by adding nanofiber layer on bag filter media and controlling surface roughness
- 주제(키워드) Air filtration , Electrospinning , Filtration mechanism , PVA nanofiber , Ultrafine particle , Filter quality factor , Bag filter , Polyamide nanofiber , Thermal stability , Bag house system , Contact angle , Surface free energy , Kinetic energy
- 발행기관 고려대학교 대학원
- 지도교수 이기봉
- 발행년도 2022
- 학위수여년월 2022. 2
- 학위구분 박사
- 학과 대학원 화공생명공학과
- 원문페이지 229 p
- UCI I804:11009-000000257746
- DOI 10.23186/korea.000000257746.11009.0001241
- 본문언어 영어
초록/요약
Particulate matter having a particle diameter smaller than 2.5 μm is harmful to the human body as it causes various diseases such as cardiovascular and respiratory when introduced into the human body. In general, the bag house system has the advantages of high particle collection efficiency and easy operation, regardless of the work process. Therefore, it is used to remove particulate matter present in the flue gas generated in most industrial processes. The bag filter is a very crucial component that determines the filtration and particle removal performance of the bag filter. Surface filtration, which is one of the filtration methods of bag filters, is advantageous as the pressure drop increases initially and then gradually decreases as the operation progresses because the pores inside the filter are maintained under the influence of the surface filtration layer. In addition, water and oil repellency properties are imparted to the filter surface through various treatments, such as coating, lamination, and plasma, to simultaneously improve the particle removal performance and filter life. Accordingly, this dissertation aims to develop a high-performance bag filter with water repellency and oil repellency by coating a nanofiber layer on the bag filter surface via surface treatment. It also addresses the existing challenges pertaining to the practical application of the bag filter by the following measures: (i) Preparation, coating, and performance analysis of the nanofiber layer, (ii) providing the nanofiber layer with stability toward the thermal, mechanical, and chemical environment, and (iii) presenting the coating with water and oil repellency properties via surface treatment of the filter medium. First, the purpose of the preparation, coating, and performance verification of the nanofiber layer coated on the surface of the filter medium. In this study, a nanofiber layer was prepared using an electrospinning method and coated on the surface of the filter medium. The polyvinyl alcohol (PVA) used to manufacture the nanofiber in this study was selected because it can be facilely electrospun with PVA, and the diameter and morphology of the fiber can be controlled relatively easily. As the coated PVA nanofiber layer also acts as a filter, the effects of diffusion and interception, which are the main filtration mechanisms for ultrafine particles, were confirmed. To clearly observe only the effect of the fiber diameter on the filter’s filtration performance, a PVA nanofiber filter was prepared with the same physical properties, such as the fiber layer thickness, packing density, and fiber weight per unit area that affect the filtration performance, through the control of the electrospinning process variables. Both the diffusion and interception mechanisms are significantly affected by the fiber diameter constituting the filter. It was confirmed that as the diameter of the fiber decreased, the particle collection efficiency for diffusion and interception increased. Based on the filtration theory, the effect of the fiber diameter was theoretically evaluated by comparing the experimental and theoretical values of the filtration performance. In addition, the filter quality factor, which indicates the performance of the filter, tended to increase as the fiber diameter decreased. In a previous study of PVA nanofibers, a nanofiber layer was successfully manufactured, and it showed an excellent filtration performance. However, as the temperature of the flue gas passing through the bag filter used in the actual process was approximately 200 °C, the PVA nanofiber layer was not suitable for the practical applications. Therefore, a commercially available bag filter was coated with heat-resistant meta-aramid by electrospinning, and the properties and filtration performance of the nanofiber layer were evaluated. In this study, meta-aramid nanofibers were prepared by appropriately setting the electrospinning process parameters, such as the polymer solution concentration, solution feed rate, electric field strength, and tip-to-collector distance, similar to the conditions of the aforementioned study. The fiber diameter, one of the most important physical properties determining the filter performance, increased as the polymer solution concentration and tip-to-collector distance increased, whereas it decreased as the electric field strength increased. The filter quality factor of the filters with different fiber diameters increased as the fiber diameter decreased and agreed well with the theoretical calculation results. In addition, the meta-aramid nanofiber filters showed excellent thermal stability in thermogravimetric analysis (TGA), and the mechanical properties that could withstand the exhaust pressure of the dust filter were confirmed through tensile strength tests. Therefore, the possibility of applying the nanofiber layer coating to the surface of the medium- and high-temperature bag filters used in the actual process was demonstrated. The filter media installed in the bag filter should simultaneously possess a high particle collection efficiency and high filter cleaning efficiency. Although both characteristics were considered in the studies mentioned above, a deeper focus was on the high particle collection efficiency due to the fiber diameter. To retain all these characteristics, there is a method of surface-treating a filter medium with a high particle collection efficiency. The increased contact angle and surface roughness acquired through surface treatment have a close relationship with each other, and as a result, a low surface free energy is maintained. Sandblasting is used for the surface treatment, which is one of the simplest methods of applying a physical force through particles to the filter surface. Among the various process parameters of sandblasting, the optimized surface treatment conditions were determined by analyzing the contact angle between water and oil, field emission scanning electron microscope (FE-SEM) image of the filter surface, and the pressure drop measurement. Under the optimized conditions, the surface-treated filter had a superior water-repellent property with a contact angle of 150° or more, and the surface free energy calculated based on the theoretical formula had a very small value of approximately 1.5 mJ/m2. In addition, compared to the filter medium used as the surface treatment support, there was no significant change in the pressure drop, and it was determined that the physical structure of the filter did not change. Meanwhile, the kinetic energy of the sandblasted particles was less than 8.0 J/m2, and this value was within a range that did not damage the filter surface.
more초록/요약
2.5um보다 작은 입자 직경을 가지는 입자상 물질은 인체 유입되게 되면 심혈관계, 호흡기질환 등 같은 다양한 질병들을 유발하는 원인으로 인체에 유해하다. 일반적으로 여과집진장치는 높은 집진효율과 작업공정에 관계없이 운전이 용이하다는 장점을 가지고 있기 때문에, 대부분의 산업공정에서 발생하는 배가스에 함유된 입자상 물질을 제거하기 위해 사용되어진다. 여과백은 여과집진장치의 여과 및 탈진성능을 결정짓는 매우 중요한 부품이다. 여과백의 여과방식 중 하나인 표면여과는 표면여과층의 영향으로 필터 내부의 기공을 유지하기 때문에 점차 운전시간이 지날수록 압력손실의 상승이 낮아진다는 장점을 가진다. 게다가, 필터 표면에 coating, laminated, plasma 같은 다양한 처리를 통해 발수성 및 발유성의 성질을 부여하여 탈진성능을 향상시키고 동시에 필터 수명의 증대 또한 기대할 수 있다. 이에 따라 본 학위 논문에서는 여과백 표면에 나노섬유층을 코팅하고 표면처리를 통해 발수성과 발유성을 가지는 고성능 여과백의 개발을 목표로 하며, 여과백의 실제 공정 적용에 있어 반드시 해결되어야 할 다음의 문제점들을 다룬다. (i) 나노섬유층에 대한 제조, 코팅 및 성능, (ii) 나노섬유층의 열적, 기계적 및 화학적 환경에 대한 안정성, (iii) 여과재 표면 처리에 의한 발수성 및 발유성 부여. 가장 먼저, 챕터 3에서는 여과재 표면에 코팅되는 나노섬유층의 제조, 코팅 및 성능 확인을 목표로 하였다. 본 연구에서 나노섬유층은 전기방사법을 이용하여 제조되었으며, 여과재 표면에 코팅되어졌다. 나노섬유 제조에 사용된 고분자는 PVA로 손쉽게 전기방사가 가능하며, 섬유의 직경 및 morphology를 비교적 쉽게 제어할 수 있다는 점에 선택하였다. 코팅되는 PVA 나노섬유층 또한 하나의 필터로써 작용되기 때문에 초미세입자의 대한 주된 여과 메커니즘인 확산과 차단의 영향을 확인하였다. 필터 여과 성능에서 섬유 직경에 대한 영향만을 명확하게 보기 위해서 전기방사 공정변수들의 제어를 통해 여과 성능에 영향을 미치는 섬유층 두께, packing density, 단위면적당 섬유무게 같은 물리적인 특성들을 동일하게 가지는 PVA 나노섬유 필터를 제조하였다. 확산과 차단, 2가지 메커니즘 모두 필터를 구성하는 섬유 직경에 아주 큰 영향을 받으며, 섬유 직경이 얇아질수록 확산과 차단에 대한 입자 포집효율 또한 증가하는 것을 확인하였다. 여과 이론을 기반으로 여과성능에 대한 실험값과 이론값의 비교를 통해 섬유 직경에 대한 영향을 이론적으로 풀어내었다. 또한, 필터의 성능을 나타내는 필터성능지수는 섬유 직경이 감소함에 따라 증가하는 경향을 보였다. 앞서 진행한 PVA 나노섬유 연구에서 나노섬유층이 성공적으로 제조되고 여과성능 또한 우수하다는 것을 증명하였다. 하지만, 실제 공정에 사용되는 여과백을 통과하는 flue gas의 온도는 200도 부근으로 PVA로 제조된 나노섬유층의 경우, 실제 공정 적용에 적합하지 않았다. 따라서, 챕터 4에서는 상업적으로 이용가능한 여과백에 내열성을 가지는 meta-aramid를 전기방사하여 코팅시키고 나노섬유층에 대한 특성 및 여과 성능을 평가하였다. 이 연구에서 전기방사 공정변수는 PVA 나노섬유 필터 연구의 조건과 유사하게 고분자 용액 농도, 용액 feed rate, electric field, 및 tip-to-collector distance로 선택하여 meta-aramid 나노섬유를 제조하였다. 필터 성능을 결정하는 가장 중요한 물리적 특성 중 하나인 섬유 직경은 고분자 용액 농도와 tip-to-collector distance가 증가함에 따라 증가하는 경향을 보였으며, electric field가 증가할수록 감소하였다. 각기 다른 섬유직경을 가지는 필터들의 필터성능지수는 섬유 직경이 감소할수록 증가하였으며, 이론식을 기반으로 한 결과와 잘 일치하는 경향을 보였다. 또한, meta-aramid 나노섬유 필터들은 TGA 분석을 통해 훌륭한 열적 안정성을 보여주었으며, 인장강도 test를 통해 여과집진장치의 탈진 압력에도 견딜 수 있는 기계적 특성을 확인하였다. 따라서, 실제 공정에 사용되는 중고온용 여과백 표면에 나노섬유층 코팅 적용 가능성을 증명하였다. 여과집진장치에 장착되는 여과재는 높은 입자 집진효율과 높은 필터 cleaning 효율을 동시에 가져야만 한다. 위에 언급된 연구들에서는 이 두가지를 모두 고려하여 진행하였지만, 보다 깊은 초점은 섬유직경에 기인한 높은 입자 집진효율이었다. 이러한 특성들을 모두 보유하기 위해서는 높은 입자 집진효율을 가지는 여과재에 표면 처리를 하는 방법이 있다. 표면처리를 통해 증가하는 접촉각과 표면 거칠기는 서로 밀접한 관계를 가지고 있으며, 이 결과에 따라 낮은 표면 자유 에너지를 보유하게 된다. 표면처리에는 샌드블라스팅을 사용하였으며, 이 방법은 필터 표면에 입자들을 통해 물리적인 힘을 가하는 간단한 방법 중 하나이다. 샌드블라스팅의 다양한 공정 변수들 중 최적화된 표면처리 조건은 물과 오일에 대한 접촉각, 필터 표면에 대한 FE-SEM 이미지, 및 압력손실 측정으로 결정되어졌다. 최적화 조건에서 표면처리된 필터의 물에 대한 접촉각은 150도 이상으로 초발수 특성을 보유했으며, 이론식을 기반으로 계산된 표면 자유 에너지는 1.5 mJ/m2에 근접할 정도로 매우 작은 값을 가졌다. 또한, 표면처리 지지체로 사용된 여과재와 비교하였을 때, 압력손실의 큰 변화가 없는 것으로 보아, 필터 자체의 물리적인 구조가 변하지 않은 것으로 판단되었다. 이 때의 샌드블라스팅 되는 입자들이 가지는 운동에너지는 8.0 J/m2보다 작았으며, 이 값은 필터 표면에 손상을 주지 않는 범위였다.
more목차
Abstract i
국문 요약 vi
Contents x
List of figures xv
List of tables xxi
1. Introduction 1
1.1. Research objectives 7
2. Theoretical background 11
2.1. Characterization of fine particles 11
2.1.1. Definition of fine particle 11
2.1.2. Generation of fine particle 12
2.1.3. Hazards of fine particle 13
2.1.4. Classification of fine particle 14
2.2. Filter for air purification 19
2.2.1. Definition of filter 19
2.2.2. Mechanical filter 20
2.2.3. Electrostatic filter 21
2.3. Hot gas control device 24
2.3.1. Bag house system 29
2.4. Filtration mechanism and theory 33
2.4.1. Types of filtration mechanism 33
2.4.1.1 Surface filtration 33
2.4.1.2 Depth filtration 34
2.4.2. Filtration mechanism of a fibrous filter with nanofiber 37
2.4.2.1 Diffusion 42
2.4.2.2 Interception 44
2.4.2.3 Dielectrophoretic 45
2.4.3. Filter performance evaluation 47
2.4.3.1 Collecton efficiency 47
2.4.3.2 Pressure drop 48
2.4.3.3 Filter quality factor 49
2.4.4. Prediction of theoretical single fiber collection efficiency 50
2.5. Filter surface treatment technique 51
2.5.1. Electrospinning 51
2.5.2. Sandblasting 55
3. Comparison of experimental and theoretical filtration performance using air filters with identical physical properties 58
3.1. Introduction 58
3.2. Experitmental method 63
3.2.1. Preparation of PVA solution used for electrospinning 63
3.2.2. PVA nanofiber filters produced by electrospinning 64
3.2.3. Evaluation of the characteristics for the fabricated filters 69
3.3. Results and discussion 72
3.3.1. Effects of electrospinning parameters on PVA nanofiber morphology 72
3.3.2. Filtration thoery calculations for fibrous filters 81
3.3.3. Effects of the fiber diamter on the filtration performance of filters with similar physical properties 83
3.4. Conclusions 92
4. Fabrication of meta-aramid nanofibers filter by electrospinning 94
4.1. Introduction 94
4.2. Filtration thoery of air filters 97
4.3. Experimental method 99
4.3.1. Fabrication of the spinning solution using electrospinning 99
4.3.2. Production of meta-aramid nanofiber filters through electrospinning process 100
4.3.3. Filter characterization 104
4.3.4. Measurement of filtration performance 106
4.4. Results and discussion 108
4.4.1. Effects of electrospinng conditions on nanofber morphology 108
4.4.2. Effects of sintering on nanofiber morphology 114
4.4.3. Filter characteristics 116
4.4.4. Filtration characteristics 123
4.5. Conclusion 130
5. Adjustment of the surface free energy by sandblasting filter media 131
5.1. Introduction 131
5.2. Experimental method 134
5.2.1. Test filters 134
5.2.2. Surface treatment of PTFE composite filtets by sandblating 135
5.2.3. Characterization of the surface-treated filters 139
5.2.4. Wenzel’s theroy of superhydrophobic 140
5.2.5. Calculation of the particle kinetic energy 141
5.3. Results and discussion 142
5.3.1. Effects of sand-blasting process paramters of filter surface morphology 144
5.3.2. Effects of surface treatment on the liquid-drop contact angle with the filter surface 149
5.3.3. Calculation of the kinetic energy according to the surface treatment conditions and relationship with the pressure drop 154
5.3.4. Effects of surface treatment on the filter surface roughness and filter surface free energy 160
5.4. Conclusion 168
6. Overall conclusion 169
References 174
Nomenclatures 199
