Measurement and modeling of pressure drop of HEPA filters clogged with ultrafine particles

A PM2.5 pollution-level adaptive air filtration system based on elastic filters for reducing energy consumption

Exacerbated by human activities and natural events, air pollution poses severe health risks, requiring effective control measures to ensure healthy living environments. Traditional filtration systems that employ high-efficiency particulate air (HEPA) filters are capable of effectively removing particulate matter (PM) in indoor environments. However, these systems often work without considering the fluctuations in air pollution levels, leading to high energy consumption. This study proposed a novel PM2.5 pollution-level adaptive air filtration system that combined elastic thermoplastic polyurethane (TPU) filters and an Internet of Things (IoT) system. The developed system can effectively adjust its filtration performance (i.e., pressure drop and PM2.5 filtration efficiency) in response to real-time air quality conditions by mechanically altering the structures of TPU filters. Furthermore, while operating in varied pollution conditions, the proposed system demonstrated remarkable reductions in pressure drop without notably compromising the pollution control capability. Finally, the energy consumption of the pollution-level adaptive air filtration system was estimated when applied in mechanical ventilation systems in different cities (Hong Kong, Beijing, and Xi'an) with various pollution conditions. The results revealed that, compared to a traditional fixed system, the annual energy consumption could be reduced by up to ∼26.4 % in Hong Kong.

Improving the Dust Removal Efficiency of a Natural Gas Filter: An Experimental and Numerical Simulation Study

This study aims to evaluate the dust removal efficiency and working conditions of a filter separator through a pressure drop under various operating conditions. Typical horizontal filter separators in natural gas stations were taken as the research objects, and the computational fluid dynamics method was first attempted to investigate the static and dynamic characteristics of the pressure drop and the dust removal efficiency under different operating times and pressures. Then, the simulated results were compared with those obtained from online dust detection. At a constant standard flow rate, the detected pressure drop deviated from the fitted optimal quadratic curve with an increase in the operation duration of the filter separator, and the dust removal efficiency also tended to decline. The declining trend was particularly faster at lower operating pressures caused by the fast air flow, which leads to more coalesced particles flowing out and increases the dust concentration downstream. A higher initial pressure drop of the filter separator was also maintained at a low operating pressure. The dust removal efficiency rapidly decreased at a higher throughput load, and the decreasing rate became moderate at a lower input load. An optimum operating throughput of the filter was obtained when the input load varied in the range of 100 × 104-270 × 104 Nm3/d. Good agreement was achieved between the simulated and experimental dust removal efficiency, and the relative errors are within ±20%. Both methods applied in this work were verified to have high accuracy and reliability through the actual on-site amount of dust captured.

Pressure Drop Dynamics during Filtration of Mixture Aerosol Containing Water, Oil, and Soot Particles on Nonwoven Filters

The pressure drop dynamics during the filtration of three-component mixture aerosols are investigated and compared with two and single-component aerosols. The main area of interest is the effect of the addition of a small quantity of liquid (oil) and solid (soot) particles during the filtration of aerosol containing water mist. In addition, calculations of the change in filter mass during oil aerosol filtration have been carried out and compared with the experimental results. The new, improved filtration efficiency model takes into account a better coefficient fitting in the filtration mechanism equations. The limitations in the change in fibre diameter and packing density resulting from the filter loading have been implemented in the model. Additionally, the calculation model employs the fibre size distribution representation via multiple average fibre diameters. The changes in fibre diameter are dependent on each fibre’s calculated filtration efficiency. The improved filtration model has been utilised to predict the mass change of the filters during the filtration of pure and mixture aerosols. The pressure drop calculation model based on changes in filter mass has been formulated. The model is then utilised to calculate pressure drop changes resulting from the filtration of the oil aerosol and water and oil mixture aerosol.

Self-Powered Fine Dust Filtration Using Triboelectrification-Induced Electric Field

Particulate matter (PM) in the environment can adversely affect the health of living things. However, high removal efficiency and low-pressure loss are crucial design challenges for any air filtration system. To circumvent the challenge, here, we demonstrate a novel triboelectric (TE) air filtration system that is based on a rotation-type triboelectric nanogenerator (TENG) and a filter comprising two sets of plates: primary and secondary, that are placed in the airflow channel. When the TENG charges the two plate sets with opposite charges, the flowing air particles are charged at the primary plates and are collected, due to an electric field, at the secondary plates. The TE filter has demonstrated a PM_2.5 removal efficiency of ~ 99.97% for the fine dust particles, and it remains stable even after several washing cycles. The pressure loss is almost two orders less than the high-efficiency particulate air filter. Since the airflow itself can drive the TENG, the TE filter can potentially be integrated with any air conditioning system for fine dust filtration in offices, automobiles, etc.

Experimental Detection of Particle Structures Detachment from a Stretchable Single Fiber during Multiple Consecutive Stretching Cycles

The deposited particulate material within a fibrous filter affects the pressure drop which develops through three different stages. The implementation of a time-adjustable matrix is intended to cause detachment of particle structures from fibers within the upstream layers at low flow velocities. The deposited particle structures are transported further within the filter and clear up void space for an extension of filter service life. As in previous studies observed fiber stretching initiate cracks and following detachment of particle structures with a simultaneously applied airflow. For complete detachment of the particle structure, five consecutive stretching cycles are performed in this study. The elongation velocity, the flow velocity and the particle loading are varied. Using an image analysis technique and a laser-light measurement technique simultaneously, the cumulative fraction of detached particle structures and the size of detached particle structures are determined. A high initial particle loading on the fiber induces early detachment of larger structures from the fiber. The size of detached structures is increased by the increase of the elongation velocity. The mean value remains almost constant whether the elongation velocity or superficial velocity are increased. For small initial structures on the fiber, a decrease in superficial velocity causes detachment of larger particle fragments.

Cooperative physical separation of oil and suspended solids from methanol-to-olefin wastewater: A pilot study

Methanol-to-olefin (MTO) is an important non-petroleum chemical process for the preparation of light olefins. However, the MTO process consumes copious amounts of water and produces large amounts of untreated effluent. Therefore, the realization of efficient wastewater treatment and recycling is key to the green low-carbon development of MTO. Here, a cooperative process combining swirl regenerating micro-channel separation (SRMS) and combined fibrous coalescence (CFC) technologies was proposed to separate high contents of oil and suspended matter in MTO wastewater. Using a pilot device with a treatment capacity of 1 m3/h, the average oil content in MTO wastewater decreased from 750 mg/L to <30 mg/L, while the average content of suspended matter decreased from 108 mg/L to <15 mg/L. Compared with a commercial MTO wastewater treatment process (olefin production capacity of 0.6 million tons per annum), the proposed method could reduce wastewater discharges and costs by 57% and US$ 0.23 million per annum respectively. Equipment costs and operational energy consumption were also reduced by 30% and >95% respectively. The combined process may provide the basis for the green and sustainable treatment of MTO wastewater and its recycling.

Ultralow Resistance Two-Stage Electrostatically Assisted Air Filtration by Polydopamine Coated PET Coarse Filter

Airborne particulate matters (PM) pose serious health threats to the population, and efficient filtration is needed for indoor and vehicular environments. However, there is an intrinsic conflict between filtration efficiency, air resistance, and service life. In this study, a two-stage electrostatically assisted air (EAA) filtration device is designed and the efficiency-air resistance-filter life envelope is significantly improved by a thin coating of polydopamine (PDA) on the polyethylene terephthalate (PET) coarse filter by in situ dopamine polymerization. The 8 mm thick EAA PDA-140@PET filter has a high filtration efficiency of 99.48% for 0.3 µm particles, low air resistance of 9.5 Pa at a filtration velocity of 0.4 m s^-1 , and steady performance up to 30 d. Compared with the bare PET filter, the penetration rate for 0.3 µm particles is lowered by 20×. The coated PDA is of submicron thickness, 10^-3  × the gap distance between filter fibers, so low air resistance could be maintained. The filter shows steadily high filtration efficiency and an acceptable increase of air resistance and holds nearly as many particles as its own weight in a 30 day long-term test. The working mechanism of the EAA coarse filter is investigated, and the materials design criteria are proposed.

Quantification and prediction of water uptake by soot deposited on ventilation filters during fire events

Soot samples from different fuels were produced in small and pilot combustion test benches at various O₂ concentrations, and were then characterized in terms of primary particle diameter, specific surface area and oxygen content/speciation. Water sorption measurements were then carried out for soot compacted into pellet form and in powder form, using both a gravimetric microbalance and a manometric analyser. Water adsorption isotherms are all found to be Type V, and reveal the central role of the specific surface area and the oxygen content of soot. A single parametrization of the second Dubinin-Serpinsky model gives a proper fit for all isotherms. To the best of our knowledge, this is the first study to provide physico-chemical parameters and water sorption results for fire soot. This enables a better description of the soot cake formed on filters during a fire, in particular in industrial confined facilities as simulated in this study. Humidity can be then explicitly considered in the same way as other parameters influencing the aeraulic resistance of soot cakes. These results can be used to improve predictions of the consequences of fires on the containment of toxic materials within industrial facilities.

Evolution of pressure drop across electrospun nanofiber filters clogged by solid particles and its influence on indoor particulate air pollution control

Because of the relatively low pressure drop and high particle removal efficiency, nanofiber filter media can be potentially used for indoor particulate air pollution control. However, the influence of particle loading on the long-term performance of nanofiber filters in indoor particle control has not been well studied. This study first experimentally investigated the relationship between the pressure drop and solid particle loading mass for 42 nanofiber filter samples under various face velocities. The results show that the air resistance coefficient increased with the solid particle loading mass for the nanofiber filter media. Furthermore, the air resistance coefficient was positively associated with the face velocity, as a higher air velocity tended to make the particle cake tighter with higher resistance. Based on the experimental data, a semi-empirical equation was developed for predicting the pressure drop under different particle loading masses and face velocities. The developed semi-empirical model was then used to assess the long-term performance of an air cleaner equipped with nanofiber filter media in indoor PM_2.5 control. The case study demonstrated that an air cleaner equipped with nanofiber filter media could effectively control indoor PM_2.5, but the lifetime of the nanofiber filter was shorter than that of traditional HEPA filters.

Facile Functionalization via Plasma-Enhanced Chemical Vapor Deposition for the Effective Filtration of Oily Aerosol

With the growing concern about the health impacts associated with airborne particles, there is a pressing need to design an effective filter device. The objective of this study is to investigate the effect of plasma-based surface modifications on static charges of electrospun filter media and their resulting filtration performance. Polystyrene (PS) electrospun web (ES) had inherent static charges of ~3.7 kV due to its electric field-driven process, displaying effective filtration performance. When oxygen species were created on the surface by the oxygen plasma process, static charges of electret media decreased, deteriorating the filter performance. When the web surface was fluorinated by the plasma-enhanced chemical vapor deposition (PECVD), the filtration efficiency against oily aerosol significantly increased due to the combined effect of decreased wettability and strong static charges (~-3.9 kV). Solid particles on the charged media formed dendrites as particles were attracted to other layers of particles, building up a pressure drop. The PECVD process is suggested as a facile functionalization method for effective filter design, particularly for capturing oily aerosol.

Skin layer in cyclic loading-cleaning of a nanofiber filter in filtering nano-aerosols

•

Cyclic loading/cleaning nanofiber filter with nano-aerosols.

•

Thin skin layer upstream of filter controls loading behavior, permeability drops to 10–20% of initial value.

•

Capillary model for aerosol deposit, pore bridging, cake model match 6-cycle loading tests.

•

Transit from depth-to-surface-filtration – tests and modelling.

Nano-aerosols from viruses to virgin pollutant particulates from combustion, 100 nm or smaller, are harmful to our health as they penetrate readily into our body causing various diseases. Nanofiber filter can capture effectively these nano-aerosols. However, over time the pressure drop increases dramatically and cleaning of the filter by backpulse/backblow is essential for filter reuse. The cyclic loading-and-cleaning of a nanofiber filter has been investigated for the first time experimentally and theoretically.

The “skin” layer, a thin region upstream of the nanofiber filter, plays a pivoting role in controlling the pressure drop excursion of the filter. We model the skin layer to be made up of numerous fine capillaries and examine how continuous aerosols deposited in the capillaries affect rapid rise in pressure drop followed by bridging of aerosols across the capillary openings leading to more bridging and ultimately formation of cake on top of the bridges and filter surface. We have been able to describe the deposition of aerosols in the capillary pores for depth filtration, the deposition of aerosols in the cake (surface filtration), and the intermediate bridging regime between these two. We can depict the complete pressure drop excursion including the S-shaped curve behavior from depth filtration transiting to surface filtration for a filter with severe skin effect. Our prediction matches extremely well with the 6 cycles of loading/cleaning on a 280-nm nanofiber filter subject to challenging nano-aerosols, 50–400 nm. During cyclic loading and cleaning, the porosity and permeability in the skin layer for our experiment drop to 68% and 11–21% of their original values, respectively, and the effective pore diameter also drops from 1.2 to 0.6 μm.

Effect of long-range electrostatic repulsion on pore clogging during microfiltration

We perform computer simulations based on adhesive contact mechanics to demonstrate the clogging process of charged microparticles at the single-pore level. The effect of long-range Coulomb repulsion on clogging is characterized in terms of bulk permeability, the number of penetrating particles, and particle capture efficiency. Results indicate that the repulsion among particles delays or even totally prevents the formation of clogs. A clogging phase diagram, in the form of the driving pressure and a proposed charge parameter κ_{q}, is constructed to quantify the clogging-nonclogging transition. In addition, a critical state, where the capture efficiency of particles decreases to its minimum, is identified as a clogging-nonclogging criterion for repulsive particles. The distributions of the local volume fraction show that the structure of clogs is mainly determined by short-range adhesion. With relatively strong adhesion, a loose clog will be formed and it is easier for particles to penetrate. Finally, a schematic representation of the clogging process, considering both long-range repulsion and adhesion, is proposed to show the relationship between the clogging results and the interparticle interactions.