May. 13, 2024
Sintered metal filters offer high efficiency in particulate removal, with capabilities for backwashing and long service life.
These filters are suitable for high-temperature applications and various industrial uses, including chemical and power generation sectors.
The design and selection of sintered metal filters depend on their particulate holding capacity and the characteristics of the particles being filtered.
They are advantageous for processes requiring high filtration efficiency, durability, and resistance to corrosive environments.
Filtration technology utilizing sintered metal media provides excellent performance for separation of particulate matter from either liquid or gas process streams (i.e., liquid/solids and gas/solid separation) in numerous industrial liquid and gas filtration applications. Sintered metal filter media, fabricated from either metal fibers or metal powders into filtration elements, are widely used in the chemical process, petrochemical, and power generation industries. Applications require particulate removal to protect downstream equipment, for product separation, or to meet environmental regulations.
Sintered metal media provide a positive barrier to downstream processes. Sintered metal media have demonstrated high particle efficiency removal, reliable filtration performance, effective backwash capability, and long on-stream service. These filters can provide particulate capture efficiencies of 99.9% or better using either surface or depth media. Operating temperature can be as high as 1000°C, depending on the selection of metal alloy. Along with the filtration efficiency consideration, equally important criteria include corrosion resistance, mechanical strength at service temperature, cake release (blowback cleanability), and long on-stream service life. These issues are critical to achieving successful, cost-effective operations.
The life of such filter media (filter operating life) will depend on its particulate holding capacity and corresponding pressure drop. This accumulating cake can be periodically removed using a blowback cycle. The effectiveness of the blowback cycle and filter pressure drop recovery is a critical function of the properties of the accumulating particles in the cake and the filter media. Depth filtration media configured in a polishing filter may be utilized in those applications with light particle loading.
In addition to providing superior filtration in a single pass, clean-in-place backwashable media reduces operator exposure to process materials and volatile emissions. While applications include high temperature and corrosive environments, any pressure-driven filtration process with high operating costs has the potential for improvement using sintered metal filtration technology.
This paper will discuss filter-operating parameters of sintered porous metal media and filtration system design criteria for optimizing performance in a number of chemical process streams.
The 21st century brings many economic and environmental challenges to the chemical industry. Major drivers for change include market globalization, demand for improved environmental performance, profitability, productivity, and changing workforce requirements. Future competitive advantage in the chemical processing industry will come from patented technology and technical know-how. New economical high-yield and high-quality processes will characterize much of the industry’s production capacity with improved environmental impact and energy efficiency.
A high percentage of the chemical industry’s products and processes involve solids (particulate) handling. Filtration technology offers a means of reducing solids through mechanical separation via patented filter design and unique systems operation. Filtration can improve product purity, increase throughput capacity, eliminate effluent contamination (minimizing or preventing air and water pollution), and provide protection to valuable equipment downstream of the filter. Advances in filtration technology include the development of continuous processes to replace old batch process technology. Cost savings include less hazardous waste for disposal and labor savings from new technology. Fully automated filter systems can be integrated into plant process controls.
Solids reduction includes the removal of suspended solids from process effluent waste streams and cleaning solvents. The liquid product recovered is valuable for recycling to another chemical feed stream. Waste minimization includes the reduction of hazardous solids materials for recovery or recycling and solids reduction of non-hazardous materials to landfill. Filtration can reduce wastewater feed stream BOD (Biological Oxygen Demand), COD (Chemical Oxygen Demand), TSS (Total Suspended Solids), and TOC (Total Organic Carbon). These are the main parameters for which current emissions are measured with regard to local and international standards.
Knowledge of filtration fundamentals is essential to ensure appropriate design of filter media and the optimum selection of appropriate media and filter design for each filtration application. Two main filtration modes can be considered, i.e., depth filtration and surface filtration. In the case of depth filtration, the particles are captured inside the media; while in surface filtration they are retained, as the term explains, at the surface where subsequently a cake of particles is formed.
Surface filtration is primarily a straining (sieving) mechanism where particles larger than the pore size of the filter media are separated at the upstream surface of the filter; their size prevents them from entering or passing through the pore openings. Subsequent particles accumulate as a cake that increases in thickness as more particle-laden fluid is forced into the filter medium. The cake, due to its potentially finer pore structure, may aid in the separation of finer particles than can be achieved by the filter media. However, the cake must exhibit sufficient porosity to permit continued flow through it as filtration proceeds. Processes can be run under constant flow/increasing pressure or constant pressure/decreasing flow. Because most surface filters are not perfectly smooth or have perfectly uniform pore structure, some depth filtration can take place that will affect the life of the filter.
Depth filtration is mainly used in applications where small particle levels have to be separated such as in the protection of downstream equipment against fouling or erosion, protection of catalysts from poisoning, and in product purification. The particles penetrate into the media and are subsequently captured within its multiple layer structure. This multiple layer structure prevents premature blocking of the media and increases the capacity to hold dirt and on-stream lifetime. Because the particles are captured within the depth of the media, off-line cleaning will be required. This off-line cleaning can be accomplished with solvents, ultrasonic vibration, pyrolysis, steam cleaning, or water back flushing. In addition, the media may be pleated, a configuration that minimizes housing size and cost.
Understanding the ability of a filter to remove particles from a gas stream passing through it is key to successful filter design and operation. For fluids with low levels of particulate contamination, filtration by capturing the particles within the depth of a porous media is key to achieving high levels of particle efficiency. The structure of sintered metal provides a tortuous path in which particles are captured. Particle capture continues as a cake of deposited particles is formed on the media surface; however, particles are now captured on previously deposited particles. The life of such filters will depend on its dirt holding capacity and corresponding pressure drop. For fluids with high particle load, the operative filtration mechanism becomes cake filtration. A particle cake is developed over the filter element, which becomes the filtration layer and causes additional pressure drop. The pressure drop increases as the particle loading increases. Once a terminal pressure is reached during the filtration cycle, the filter element is blown back with clean gas and/or washed to dislodge the filter cake. If the pore size in the filter media is chosen correctly, the pressure drop of the media can be recovered to the initial pressure drop. However, if particles become lodged within the porous media during forward flow, and progressively load the media, the pressure drop may not be completely recovered after the cleaning cycle.
Filtration rates are influenced by the properties of the feed particle concentration, viscosity, and temperature. The filter operating mode can be constant pressure, constant flow rate, or both with pressure rising and flow rate dropping while filtering. Filtration cycle will be constrained if solids are fast blinding and allowable pressure has been reached, or for cake filtration, if the volume for cake buildup has been filled, even if the allowable pressure drop has not been reached. Permeability is expressed as flow rate against pressure drop. Permeability is influenced by filter type, fluid temperature, and solids loading.
Sintered Powder Metal Media
Sintered metal media are manufactured by pressing metal powder into porous sheet or tubes, followed by high temperature sintering. A scanning electron photomicrograph of a typical sintered powder metal media is shown in Figure 1. The combination of powder size, pressing, and sintering operation defines the pore size and distribution, strength, and permeability of the porous element. Pore size of sintered metal media is determined using ASTM E-128. The media grade designation is equivalent to the mean flow pore, or average pore size of the filter. Sintered metal media are offered in grades 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 40, and 100. The filtration rating in liquid for media grades 0.2 to 20 is between 1.4 and 35 μm absolute. The filtration rating in gas ranges from 0.1 to 100 μm absolute.
Filter cartridges fabricated from sheet or tubes have an all-welded construction. The filter media is designed and engineered with a stable porous matrix, precise bubble point specifications, close thickness tolerances, and uniformity of permeability, which assure reliable filtration performance, effective backwash cleaning, and long on-stream service life.
Metal fiber filter media consist of very thin (1.5 to 80 μm) metal filaments uniformly laid to form a three-dimensional non-woven structure sintered at the contact points. A scanning electron photomicrograph of a typical sintered metal filter media is shown in Figure 2. These media are explicitly designed for either surface or depth filters. Either single or multi-layered construction is utilized with each layer comprised of potentially different diameter fibers to achieve optimal performance, e.g., pressure drop, filtration efficiency, particle loading capacity, and media strength. The multi-layered material has a graduated design, so the dirt holding capacity is much higher and consequently the life expectancy is longer. The final filter rating is determined by the weight per used layer, the fiber composition of the layer, and the combination of several layers. The availability of a high porous structure (up to 85%) offers very high permeability and hence a low-pressure drop.
The properties of metal fiber filters, fabricated from various metal alloys, for gas filtration applications allow the use in extreme conditions: high temperature, high pressure, and corrosive atmospheres. The primary benefits of sintered metal filters are: strength and fracture toughness, high pressure and temperature capabilities, high thermal shock resistance, corrosion resistance, cleanability, all-welded assembly, and long service life.
Fiber metal media have a higher porosity than powder metal media, thereby resulting in lower pressure drop. For high temperature or corrosive applications, Bekaert has developed fibers in other alloys besides AISI 316L. Inconel® 601 and Fecralloy® are used for high temperatures (up to 560°C and 1000°C respectively) whereas Alloy HR can withstand temperatures up to 600°C and wet corrosive environments.
The inherent toughness of the metal filters provides for continuous, back pulsed operation for extended periods. For high-temperature applications, additional criteria such as creep-fatigue interactions, and high-temperature corrosion mechanisms need to be addressed. Filters with semi-permanent media are cost-effective, since such units lend themselves to minimal downtime, closed and automatic operation with minimal operator intervention, and infrequent maintenance.
The proper selection of filter media with appropriate pore size, strength, and corrosion resistance enables long-term filter operation with high-efficiency particle retention. The filtration rating in liquid is between 2 and 35 μm absolute. The filtration rating in gas ranges from 0.1 to 10 μm absolute.
The filter design for liquid/solids separation is selected which produces the required filtrate, minimizes backwash or blowdown and maximizes throughput. Three types of filter configurations are described as follows:
1.) Outside-in filtration
Traditional liquid/solids barrier separation occurs on the outer perimeter of a closed-end tubular filter element (LSP). A gas-assisted pneumatic hydro-pulse backwash has proven to be the most effective cleaning method for sintered porous metal filters.
2.) Inside-out filtration
Liquid/solid barrier separation occurs on the inside of a closed-end tubular filter element (LSI). LSI backwash modes include: a.) Full shell slurry backwash, b.) Empty shell slurry backwash, c.) Empty shell and empty element wet cake backwash and d.) Empty housing wet cake discharge.
3.) Inside-out Multimode filtration:
Liquid/solids (barrier or crossflow) separation occurs on the inside of open-ended tubular filter element (LSM and LSX). Elements are sealed within two tube sheets, thereby allowing for either top or bottom feed inlet. The LSM filter, with a feed recirculation feature, has proven itself in several continuous loop reactor systems. The downward velocity controls the cake thickness of the catalyst; with the lower the velocity resulting in a thicker cake. Filter backwash modes are similar to LSI backwash modes and also include a bump-and-settle type backwash that allows the concentration of solids without draining the filter element or housing. Continuous loop reactor systems may not require backwashing.
Scalability of the filtration systems allows for accommodating high flow rates and increased solids capacity. Filtration units are suitable for batch or continuous processes. Single housing filter systems are recommended where flow rates allow and flow can be stopped for a few minutes prior to backwash, or if offline periods can be tolerated for maintenance. Two filter dual systems are recommended where continuous flow is required and short periods offline can be tolerated for maintenance. Three filter systems are recommended for continuous operation even during maintenance periods.
A valid method of evaluating filter performance is through bench scale and pilot testing. Filter testing typically begins with a simple disc feasibility test to qualify media and obtain critical filtration characteristics. Successful feasibility studies usually progress to more involved testing of pilot equipment. Pilot testing helps develop successful commercial separation practices. While bench scale tests produce a reliable indication of filter performance, data obtained in pilot scale testing on a process line will show filter operating parameters with normal process variations. Development programs require direct access to suitable equipment over an extended period. Pilot testing of sintered metal backwashable filters can provide the following information:
In addition to verifying filter performance, pilot testing provides the opportunity for the operating engineer to learn to use the equipment and conduct experiments that optimize filter operation for their particular process. Pilot test trials address significant technical questions and problems prior to full-scale commercialization. The outcome of pilot plant operations verify:
Feasibility Case Study: Catalyst Solids Removal
A typical approach for feasibility testing and media selection is illustrated in the following test case. The objective was to evaluate the filtering characteristics of a new catalyst to support an existing LSI commercial filter installation. Filtration studies were conducted with a 70-mm disc test filter using both Grades 5 and 10 media to compare filter performance. Catalyst particle size distribution (PSD) was measured using a Horiba LA-910 Laser Scattering Particle Size Distribution Analyzer. The size range (based on volume %) was 0.51 to 60 μm with a mean size of 13.4 μm. SEM microscopy at 2000- X magnification verified particle size distribution. Catalyst slurry was filtered once through at a constant rate using Grades 5 and 10 media housed in the 70-mm disc filter housing. A particle size distribution comparison of feed and filtrates (Grade 5) sample is shown. Test results indicate that filtration using Grade 5 media resulted in a lower rate of rise pressure than Grade 10 media. Filtrate turbidity samples were similar. Filtrate from the Grade 5 media measured 2.9 NTU, while filtrate from Grade 10 media measured 2.3 NTU. The 1/8-inch thick filter cake backwashed effectively from the Grade 5 media surface. Some catalysts remained in the porous structure of the Grade 10 media, indicating that the catalyst had blocked some of the surface pores.
Test results indicate that Grade 5 media is better suited for filtration of new catalyst samples using the HyPulse LSI filter configuration. Pilot testing at the commercial facility verified results of the feasibility study and resulted in the purchase of replacement cartridges for an existing filter vessel.
Laboratory disc tests conducted in April 1992 indicate the suitability of sintered metal filter for catalyst recovery application. Bench scale pilot filter tests were conducted at the customer’s lab facility to verify filter performance and filtrate quality. In November 1992 pilot testing with continuous catalyst filtration using a 2% slurry demonstrated consistent flux rates of 0.2 gpm/ft2. A comparison of filter performance from disc testing through pilot testing is listed. Axial velocity through the filter controlled cake thickness. The rate or velocity through the filter was optimized in bench scale testing. Optimal filter performance indicated that the filter could operate at pressures < 10 PSI without backwashing. Tests were conducted over about 1500 hours with no significant change in operating performance. The project gained approval to move to the final stage.
The objective of the pilot test development program was to convert the isomerization process from batch to continuous. The first commercial plant was scheduled for operation in 1994. The process was started up in July 1994 in accordance with parameters established during the pilot testing. System dynamics experienced during start-up and initial operation exhibited performance similar to the pilot test studies. The filter operated successfully to recover and recycle precious metal catalysts after solvent washing and
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