PTS: Combines Dense Phase Transfer with Flat Filtration: “The Power Behind the Powder”
Both dense phase transfer and flat filtration are combined within the PTS. Thanks to their respective advantages even the most complex powder transfer problems can be solved. DEC Group encourage you to consider moving powders in the same way you move liquids.
The problems generally encountered with pneumatic transfer systems when attempting to transfer fine, sticky or wet powders are; plugged transfer hoses, blocked filters and difficulties in cleaning the air/powder separator usually some kind of filter arrangement.
In order to understand the advantages of the PTS, there are three main types of pneumatic transfer that we must consider:
1. Dilute phase transfer: powder is transferred at a high velocity (>25 m/s) and remains in suspension in the gas flow. The loss of pressure in the hose is proportional to the velocity of the gas.
A standard dilute phase system may use a rotary valve to feed powder into the transfer hose where suction or overpressure is then used to convey the powder. The motive force is obtained by means of a fan (low underpressure and high velocity) and usually a cyclone separator and/or large surface area filter is installed at the point of loading.
2. Semi-dilute phase transfer: when the gas velocity lies in a certain area (approximately 10-20 m/s) powder begins to settle in the lower part of the transfer hose, whilst it remains in suspension in the upper part.
This system utilises suction and incorporates a separator and filtration system installed on the receiving vessel. Suction is generated by a small fan or Venturi (providing medium suction velocity).
3. Dense phase transfer: when the velocity of the transfer gas is further reduced, the powder begins to form plugs in the hose. The loss of pressure is therefore proportional to the amount of powder in the hoses.
This type of system transfers powder by means of strong vacuum to induce powder transfer.
Not all powders are suitable for all three transfer methods. Semi-dilute or dilute phase systems are generally only suitable for transferring powders with good flow characteristics and a large particle size (> 300 microns). However, dense phase systems are suitable for the complete spectrum of powders. The DEC Group PTS being one such system is capable of transferring wet, sticky and explosive powders.
As the powder has to be separated from the transfer air, filtration is an unavoidable element of all pneumatic transfer systems. Many different types of filters (cartridge filters, filter bags, membranes, etc.) made of diverse materials operate according to different filtration methods (depth filtration or surface filtration).
Depth filtration: generally the filter medium consists of a fine mesh of fibres. Particles are stopped totally at random when they hit one of these fibres. This form of filtration has the advantage of being economical, but also the disadvantage that it is very difficult to clean the filter with a reverse pulse because the particles remain trapped in the fibres.
Surface filtration: the filter medium consists of one or several superimposed layers, the top one being fine, homogenous and with a clearly defined porosity (generally < 1 micron). The particles are caught on the surface and normally cannot penetrate the filter medium. The filter is easy to clean with a reverse pulse.
The two main filtration problems encountered in pneumatic transfer systems are the efficiency of the filtration and soiled filters. The efficiency of the filter is not only defined by the theoretical filtration degree (porosity) and the particle characteristics of the powder, but also, by the configuration of the pneumatic transfer system.
Depending on the kinetic energy of the particles (specific weight and velocity), their angle of impact and their (crystalline) shape, diminished filtration efficiency can be observed.
The most common problem is the soiling of filters. With time every filter has a tendency to become clogged and needs to be regenerated by different cleaning methods (mechanical, reverse gas flow).
In order to prevent clogging too rapidly, often filter systems are oversized. In addition to cost, they are housed in a voluminous air/powder separator. In some instances the volume of the filter can amount to 90% of the separator. The diameter of separators can be as great as 400-500 mm. In addition to being cumbersome to install and taking up floorspace, bridging and plugging regularly occur in the outlet cone of the separator when powders have poor flow characteristics.
The PTS (patented) offers an elegant solution to these problems. Consisting of a straight cylindrical chamber with a single flat filter membrane (porosity 1 micron) located in its upper part. By alternating pressure with vacuum, powder is sucked directly out of a container (sack, drum, Big-Bag, silo, process equipment, etc…) into the PTS chamber. Pressure is then used to empty the chamber, cleaning the filter at the same time.
Powder is transferred in dense phase because the velocity of the gas is limited by the surface area of the filter. Powder is transferred in plugs at reduced speed (<2-3 m/s). On entry into the chamber the powder is immediately separated from the transfer gas due to the cyclonic inlet on the upper part of the chamber. Thus the filter is protected against the direct impact of the particles and a thorough separation is guaranteed.
Cleaning is achieved by a pressure shock on the membrane. By instantaneously alternating between full vacuum and pressure (difference in pressure approx. 2 bar) the fine film of powder is removed from the surface of the entire filter membrane. The reverse gas flow which removes the remaining dust particles completes the cleaning cycle.
The following are advantages of flat filtration compared to traditional filtration:
– Quick and easy to install and change the membrane
– Low cost of membrane
– Possible to clean in place without changing the filter
– Choice between many different materials
Dense phase transfer simplifies and greatly improves the efficiency of installations:
– Powders can be sucked from many different types of container without the need for a complex feeding system or a significant need for fluidisation.
– Powders with poor flow characteristics or with low permeability (wet powders, sticky powders, etc.) can be transferred.
– No risk of damaging the particles (attrition) during transfer because there is very little friction between particles (powder is transferred in plugs at low velocity < 1-2 m/s).
– Powders with a Minimum Ignition Energy (MIE < 1 mJ) can be transferred in air without danger of explosion. Due to reduced speed and a high concentration of powder in the hoses, electrostatic charges are insignificant.
– Hygroscopic powders or powders that oxidize when in contact with air can be transferred. A reaction between the powder and the air is practically impossible. The active contact surface between the powder and the air is very limited (the powder particles are not in suspension in the gas flow) and the duration of the contact is only a few seconds.
– Powders can be transferred without their characteristics being modified. For example a mixture of powders can be transferred without any risk of being separated, as it is practically impossible to generate different particle velocities.
The DEC Group PTS (Powder Transfer System) combines dense phase transfer with flat filtration; a totally new approach to powder handling. Thanks to its compact build and robust construction it can be installed directly on the process equipment to be charged (reactor, dryer, etc.).
The system functions not only as a pneumatic transfer system but also as a security airlock which eliminates the air from the powder, even when the powder is transferred with ambient air. Consequently inert equipment or equipment under pressure can be loaded without introducing oxygen or allowing vapours to escape.
The PTS is unique in its design which conforms to the strict requirements of the process industries such as; enabling the transfer of many different products, a high level of containment, fast and easy cleaning and the exclusion of many dangers associated with traditional transfer methods.
From the introduction of raw materials and intermediate products during the different phases of production processes to the handling of the final product, DEC Group technologies offer the perfect solution for contamination free emptying and loading of drums, production equipment and vessels of all kinds.
For more information visit our web-site or contact Chris Broadbent, Director of DEC-UK at:
Come and visit Stand F63 at the PPMA exhibition at NEC Birmingham, 27th – 29th September 2011