Some recent fluidization developments in powder handling

March 1, 2015

Introducing a compressed gas into a bulk powder via a porous media allows the powder “to behave like a liquid,” making for ease of movement.

Powder fluidization applications are widespread in the pharmaceutical, food, beverage and construction industries and are used to move powders — including drugs, flour, plastics and cement — and to empty silos, road tankers and rail tankers.

Introducing a compressed gas into a bulk powder via a porous media allows the powder "to behave like a liquid," making for ease of movement.

In general, the smaller the powder particle size, the more cohesive it can become and the more difficult to move. There is, however, an extensive range of fluidization media to furnish practical solutions to most fluidization challenges.

The market demands that powder fluidization systems have compact, economic designs and be easy to clean with minimal maintenance.

The art of managing powders includes filtration systems operation, but porous materials are present in many applications other than filtration. Notably they are used in sound attenuation/silencers, vacuum table covers and porous supports in applications such as process chromatography. Sintered porous materials also exhibit strength and very uniform porosity critical in many of these applications.

Powder properties

However, where powders must be moved efficiently, properties can be evoked that offer particular benefits. In such applications, the powder is moved by making it "flow like a liquid," using air to fluidize the powder.

The materials involved are of high uniform porosity, minimizing energy required to make the powder flow. Unified porosity means that in fluidization there are no dead spots. Dead spots and dead zones hinder performance, causing poor flow or stagnation. For pharmaceuticals, stagnation can cause powder degradation.

For example, two materials for these applications are Sinterflo M® porous sintered mesh and Vyon® sintered porous plastic. Both are strong, free standing and can be fabricated into shapes as complex as those of the fluidizing cones used in silos, for example.

Formats of fabrication

The materials can be supplied in a variety of different formats, including flat sheet or fully fabricated aeration cones. Also available are filter elements in either a pleated or plain cylindrical form. Both mechanically and chemically strong, these materials allow for effective cleaning for reuse, thereby delivering cost-effective solutions for difficult applications.

Sinterflo® sintered porous metal materials are ideal for applications that involve aggressive chemical environments, high temperatures and high pressures.

Vyon® is a porous sintered material, manufactured from either high-density polyethylene (HDPE) or polypropylene (PP). This material can be supplied as a ready fabricated fluidizing cone liner or in flat sheet form, for use as a tank liner or in an end user secondary fabrication.

Sinterflo® sintered mesh can be fabricated into complete hopper bottoms, tubes and custom shapes. The material can be supplied as a flat-panel porous plate and unlimited size in butt-welded sheets. It is easily custom engineered for non-standard applications.

Sintered porous Sinterflo® mesh is made by weaving sintered metal meshes together to achieve a strong and rigid porous structure. It is particularly suitable for pharmaceutical applications as it can be steam sterilized.

It also has a uniform pore structure giving an even, total area fluidization. It is self-supporting due to its semi-rigid nature, thereby reducing the need for the external support structures that are required with canvas and felt media.

Selection, application

Sintered porous plastic Vyon® is made by sintering particles of the plastic materials together to form a porous structure. Typical materials used are polyethylene and polypropylene. Again, the material is rigid and strong and can be made to a specific pore size distribution.

Selection depends on application. For example, in the pharmaceutical and food industries, sintered porous stainless steel mesh is a preferred option because of the ability to steam sterilize. A broad range of media has been used to fluidize a wide variety of powders.

In industry and construction, media fluidize cement, gypsum, silica, resin bonded sand, sulphur, soda ash, pulverized fly ash, coal dust and glass beads. Furthermore, in chemicals and plastics, it can be applied to pigments, carbon black, titanium dioxide, anti-pest powders, calcium carbonate, detergents, PVC, EVA, polyethylene, epoxy and polyester paint powders.

These materials also bring other benefits. Traditionally, canvas is used as a fluidizing media, and is supported between two stainless steel frames. These materials are strong, single free-standing units. They don’t require support.

Filtration-recovered powders

Porvair has experience in the recovery of powder materials using filtration. Pulsed Jet cleaning is a method used in gas filtration for the in-situ removal of accumulated particulate from the surface of a filter element.

Historically, pulsed Jet cleaning was used in conjunction with bag-house filter systems, but fabric filters are unsuited to more stringent applications because of low-filtration efficiencies and relatively poor physical and chemical properties. This has been overcome by adapting the principle for use with metal fiber filters, to meet exacting requirements of powder-handling applications within many process industries.

Pharmaceutical companies are now among many investing in custom-designed pulsed jet blowback filters for recovering and containing valuable powders for a wide range of applications, such as drying, milling, blending, conveying and tableting in powder production and processing.

In such processes, the gas being withdrawn or expelled from the system is passed through one or more sintered stainless steel filter elements. Any of the powder that is carried to the filter surface is periodically dislodged by pulsed jet cleaning, allowing it to fall back into the system. Each pulsed jet cleaning cycle has a duration of less than half a second and creates only a minor instantaneous overpressure, which causes no disruption to the process.

Pulsed jet cleaning with metal fiber filters has found diverse applications across all aspects of pharmaceutical production. With continuing emphasis being placed on improving quality procedures and ever tightening legislation on airborne emissions, it is anticipated this will continue as engineers realize the full extent of the potential of this technology to help run their process plants safely, reliably and economically.

Porvair Filtration Group is an international leader in the development and supply of high performance, innovative materials and solutions for applications in filtration and separation. Its expertise in a wide range of media and the dedication of its design, manufacturing, sales, test and research teams ensure it is at the forefront of filtration technology. For nearly 50 years, Porvair has delivered world class performance to the most demanding of environments, including aerospace and defense, nuclear, energy, chemical process, industrial process, water treatment and life sciences markets.

With offices and manufacturing sites across the UK, Europe and US, Porvair prides itself on its continuous innovation and research to meet global demands.

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