Pneumatic conveying via vacuum has been used in the pharmaceutical industry for some time now. What are the newest trends in conveying of pharmaceutical
powders and blends?
It is true that powder transfer via vacuum has been in use in the pharmaceutical industry for many years. Obviously, vacuum, (usually via dilute phase) has been the transfer mode of choice because of its inherent ability to keep the powder contained within the
system due to the suction created by the vacuum. This is in contrast to positive pressure conveying, which is often used in
other industries for high volume and long distances. Positive pressure systems have the disadvantage of possible outward leakage,
particularly in cases of improperly aligned or incorrectly installed pipework.
Newer trends in conveying involve dense phase vacuum transfer for pre-blends and granulations, specialty designs for containment
and cleaning, and the increased use of specially designed pneumatic receivers for refill operations for continuous pharmaceutical
processes, such as continuous granulation, mixing/blending, and extrusion.
What are the differences between dense phase and dilute phase conveying?
Dilute phase conveying is typically used with materials where segregation or attrition in the conveying line is not a concern.
Comparative gas/air velocities in a 3 inch pipe for dilute phase can range from 15.2 up to 35.6 m/sec (3000 to 7000 ft/min).
In dense phase operations (Figure 1), a reduced gas velocity range of 0.4 to 8.6 m/sec (80 to 1700 ft/min) is utilised. In most applications the gas is air,
however, in the pharmaceutical industry nitrogen is also widely used because of its inert nature, as well as the natural purity
of the gas.
By definition, dense phase means a higher product to gas ratio, in other words a smaller amount of gas is used to move a larger
quantity of product. The less gas required for transfer, the lower the power consumption of the exhauster or vacuum pump.
Typically material is picked up from the outlet of a specialty hopper, which minimises the amount of air entrained in the
material, and allows the slugs of product to form. In addition, the hopper also includes a type of make-up air inlet, which
aids in forming the slugs as the material enters the conveying line. The combination of a relatively low air velocity and
an expanded line size result in a "siphon-like" effect, transporting the material to the vacuum receiver.
The lower gas velocity used in dense phase conveying results in a much gentler action, reducing wear on the conveyed powder
or granulate. This gentle action also reduces the segregation issues often experienced with the more aggressive dilute phase
operation, making it ideal for the conveying of powder blends to tabletting machines or roller compactors. It should be noted,
however, that there are limitations to the application of dense phase vacuum conveying; for example, conveying distances may
not exceed 3.7 m (12 ft) vertical and 4.6 m (15 ft) horizontal. Also, dense phase conveying is not appropriate for conveying
materials that are cohesive, hygroscopic or so coarse in particle size that they will not readily form slugs. While dilute
phase vacuum conveying is fairly forgiving, dense phase is very dependent on the material characteristics; therefore it is
highly advisable to perform full-scale tests when modeling a dense phase vacuum system.