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.
How do pneumatic conveyors fit into new applications and processes involving continuous operations?
Pneumatic receivers that operate under a dilute phase vacuum transfer principle are often used as refill devices, particularly for those loss-in-weight (LIW) feeders, which are critical to continuous operations. The mode of refill of bulk material for a LIW feeder that is feeding into a continuous pharmaceutical process can be almost as critical as choosing the right feeder technology.
In a refill operation, the pneumatic system utilizes vacuum to draw the required material into a vacuum receiver mounted over the LIW feeder on its own support structure (Figure 2). The receiver is filled to a predetermined level and then holds this material charge until the feeder below requests a refill. The level of fill in the receiver is determined by level sensors; when a refill request is received from the feeder below, the discharge valve opens and the receiver contents are discharged into the feeder hopper. At the same time, a gas pulse is sent through the filter mounted in the vacuum receiver, in order to release any material that may have settled on the filter. The filter material can vary, including options on laminated membrane-type materials, for quick release and easy clean properties.
After discharging the material into the feeder hopper below, the valve is closed again and then the receiver vacuum cycle immediately begins in order for the pneumatic receiver to be instantly ready for the next refill request. The use of pneumatic receivers as refill devices allows for an uninterrupted source of refill from bags, drums, IBC's or supersacks.
The discharge valve at the outlet of the pneumatic receiver is also critical to the refill process. The flow cutoff action of the selected valve must be quick and secure, since a slow tapering off of the refill flow needlessly lengthens refill time. Any leakage in the refill valve can cause an unavoidable, measurable weight disturbance, but will always result in a flow error in the positive direction. Butterfly valves are usually the discharge valve of choice in the pharmaceutical industry because of their easy clean design, and their availability in high containment options.
In all cases it is imperative that the overall sequencing of the product pickup process and relative sizing of both the feeder hopper and pneumatic receiver be designed in such a way as to allow for quick changeover of these pickup points without interruption of the process. For example, in cases with an IBC docking station equipped with a pneumatic pickup and a split butterfly valve, the time for changeover to a new bin can often be as much as 15 minutes. In this case, the timing of the refill sequencing and the overall feeder hopper volume must always be able to provide continuous material flow for the process, even when there is a lack of material delivery during changeover of these pick up components. Continuous system designs ensure that the overall material delivery and process feed components are tightly integrated in any continuous process, as the quality of the process is entirely dependent upon this coordinated integration.
What options and design improvements are available for cleaning pneumatic systems?
Stainless steel filter media, retractable spray balls integrated into the product pickup or vacuum receiver hoppers and specialised quick clean hopper designs are just two of the available options for pneumatic systems. For example, more recent pneumatic receivers include easy access clamp designs on the filter receiver body as well as swing away filter receiver heads. These swing away filter heads aid not only in accessibility for cleaning, but also in ease of filter removal. In addition, the use of retractable spray balls eliminates the worry of possible contamination traps that exist with internal static spray ball assemblies. With the retractable design, the spray ball is introduced into the receiver body only during the cleaning cycle, by means of a water pressure-activated spring. This design allows the end-user to wet the internals of the pneumatic system, without exposure to the operators, and create a "mud condition" with the product. This mud condition then prevents any airborne particulates and resultant dust exposure when the system is opened.
Finally, as stated earlier, filter media is available in a variety of materials of construction, depending on the cleaning requirements, material release properties, and filtration efficiencies that are required.
How are conveying systems engineered to handle potent compounds that must be contained?
In addition to the containment of dust by introducing the wash in place design on the pickup points and filter receivers, the actual product source docking stations and discharge points can be equipped with split butterfly valves for added containment.
The use of product delivery via glove box designs can also be easily integrated to include vacuum pickup hoppers for complete contained conveyance of the active powder to the vacuum receiver.
As an added component, the use of bag in bag out secondary inline filters are often used after the vacuum receiver, for the airflow which exits the receiver prior to the vacuum pump. This added filtration unit ensures no particulates bypass to the pump, especially in the case of a broken or poorly installed internal vacuum filter, thus ensuring containment of the active powder throughout the process.