Vacuum conveying or pneumatic transfer is often regarded as a necessary evil in processing. Why spend money on a system which does not alter the process or add value?
The truth about vacuum conveying is quite the opposite, and a well-designed system can enhance a process, increasing throughput and efficiency, eliminating the risk of operator injury and exposure, and ensuring product containment.
The typical integrated pharmaceutical granulation suite is perhaps the best example of these principles being optimally applied.
The raw material is first loaded into a high-shear mixer / granulator. The dried granule is then passed through a second mill to remove agglomerates formed in the drying process, and then into bins or IBCs for onward movement to tabletting etc.
The movement of the wet mass from high-shear mixer, through the wet mill and into the dryer bowl is most commonly achieved under gravity, but it is the initial loading and final unloading where vacuum transfer is relevant.
The process of loading the dry ingredients for a large production batch into a mixer/granulator by hand involves an operator handling drums of weighed product up podium steps and tipping it into the opened lid. This not only exposes the operator to the product but can often lead to injuries, minor batch losses and airborne dust.
Once dried, the dryer bowl can be removed. It is tipped into the dry mill, then fed into the bin below under gravity; a very labour intensive and time consuming process. The dispensing height of the dry mill dictates the use of a relatively small destination bin beneath it – and this then often needs to be transferred into a larger vessel / IBC for onward movement, introducing another step and increasing batch losses and operator exposure.
The solution at both stages is simple – transfer the product under vacuum. The operator can tip the bag / drum of product directly into a sack-tip station at floor level, or can plunge a vacuum lance (akin to a domestic vacuum cleaner nozzle) into the bag or drum, eliminating the need to climb the podium steps. For more hazardous products, there are other means of unloading the supply containers without breaking the seal. The vacuum transfer system pulls the raw materials from this station or lance up to a hopper over the high-shear granulator, and drops it into the bowl in a contained manner.
Once dried, instead of separating the bowl from the dryer body and exposing the product, a vacuum conveying system is connected to the discharge valve just above the plate. The bed is fluidised and the vacuum turned on. Dried product is raised as a fluid bed, and flows out of the discharge port, drawn by the vacuum. Again, the unloading of large batches can be achieved in minutes, far quicker than by manual unloading.
The vacuum system may also be sized to incorporate a dry mill (to remove agglomerates formed in the drying process) as part of the transfer. This vacuum swept milling is again faster than milling under gravity and is done so in a totally contained manner. The milled product is drawn into a transfer hopper and deposited into the destination vessel (drum, IBC etc.).
As with any piece of equipment, there will be exceptions, but in the majority of cases it will be of huge benefit to the process. If lateral movement is required between operations for dry material, then vacuum conveying should always be a serious consideration. Dry product (powder, granule, flake, chips, even tablets) can be transferred safely under vacuum, without the risk of injury, in a contained manner, eliminating airborne dust and operator exposure. Add to this the fact that the inclusion of a pneumatic conveying system can speed up product transfer considerably, increasing efficiency and reducing batch process times – and you would have to ask why would you not include such a beneficial piece of equipment?