Pneumatic Vacuum Conveying - Dilute and Dense Phase

An Easy-Clean, Efficient, and Safe Mode of Transfer for Difficult or Harmful Materials Used in Chemicals, Plastics, Food and Pharmaceutical Processes

Bulk material transfer by pneumatic vacuum conveying is one of the more popular means of transport for a wide variety of chemical, plastics, pharmaceutical, neutraceutical and food materials. The use of vacuum in either dilute or dense phase ensures that the powder or granulate remains within the process, as opposed to leaks or losses that often result in positive pressure transfer systems.

The use of either dilute phase vacuum or dense phase vacuum can be applied to a wide variety of operations, including the loading of blenders, feeders, sifters and mills. In the plastics industry vacuum conveying is the preferred means of transport for additives and minor ingredients. Vacuum conveying is used in chemical processes to transport catalysts such as potasium chloride, magnesium sulfate and potasium sulfate. In pharmaceutical operations you will also see vacuum conveying used with capsule fillers, tablet presses and even finished tablet and capsule handling without damage to the finished product.

graphic of dilute phase flow
Dilute Phase Normal Material Loading

Dilute Phase Principle of Operation

Most vacuum transfer operations operate via the dilute phase principle of powder transfer. Typical dilute phase conveying operations involve materials where segregation in the conveying line is not a concern. Comparative velocities in a 76 mm  [3 in] pipe for dilute phase can range from 914 m/min up to 1,829 m/min [3,000 ft/min up to 6,000 ft/min].

In most applications the gas is air; however, in the pharmaceutical industry, nitrogen is widely used due to its inert properties as well as the natural purity of the gas. Inert gas is also required in explosive environments.

Conveying and Dispensing of Powders

Vacuum transfer systems can easily be adapted for pickup of material from a variety of sources.

These sources can include open drums (for nonhazardous materials) whereby the pickup mode is made by means of a pick up wand or Aerolock rotary valve. In cases where higher containment is required, specialty glovebox design pick up bins can be incorporated into the vacuum line in order to protect both the operator and the product.

graphic of dense phase flow
Dense Phase Flow (slugs)

Dense Phase Principle of Operation

By definition, dense phase means a higher product to gas ratio, or a smaller amount of gas is used to move a large quantity of product. The less the gas requirement, the less the power consumed by the exhauster or vacuum pump.

Typically material is picked up from the outlet of a specialty hopper, which creates a pressure differential and allows slugs of product to form. In addition, the hopper also includes a type of makeup air inlet, which aids in the forming of the slugs as they travel and pulse through the conveying line. The combination of the relatively low air velocity and an expanded line size result in a type of “siphon-like” effect for conveying to the vacuum receiver, with less resultant attrition and segregation.

Conveying of Pre-blended or Fragile Materials via Dense Phase

The lower gas velocity used with dense phase conveying will lower product degration. This gentle action also reduces the segregation issues often experienced with the more aggressive dilute phase operation.

It should be noted, however, that there are limitations to dense phase conveying. These limitations include conveying distances in excess of 3.6 m (12 ft) vertical and 30 m (100 ft) horizontal, and conveying materials which are cohesive, hygroscopic, or so coarse in particle size that they will not readily form slugs.

Conveying of Speicalty Products, Food, Capsules and Tablets

Vacuum conveying systems are not just for transporting powders and granules. In the plastics industry vacuum conveyors are employed in component conveying. Vacuum conveying is used in hot melt glue systems. The food industry uses vacuum conveying to move all kinds of food products such as bread crumbs, grated cheese, frozen vegitables and more.

The conveying of filled capsules or tablets requires special modifications to the pneumatic receiver in order to avoid any damage to the product. Due to the individual particle density of filled capsules or tablets, and their overall higher mass, a lower overall velocity of the individual tablet results.

In addition, the receiver is often lined with a soft food grade rubber liner to avoid any product degradation which may occur after impact to a standard stainless steel receiver.

picture of Coperion K-Tron's Pharma Feeders
Coperion K-Tron Pharma Feeders are ideal for feeding even difficult powders

Material Handling Automation for Continuous Processing in the Pharmaceutical Industry

As the pharmaceutical industry undergoes a change in manufacturing from batch to continuous operations, Coperion K-Tron continues to be the supplier of choice for feeding and material handling equipment.

Regardless of the exact continuous process utilized, the method of feed and refill to the process is critical in maintaining process and product quality/efficiency. Highly accurate Coperion K-Tron loss-in-weight feeders with clean and quick-reacting Coperion K-Tron receivers for automatic feeder refill are the perfect combination to ensure consistent continuous operations.

Continuous processes which are currently utilizing Coperion K-Tron equipment include: hot melt extrusion and extrusion granulation, wet granulation, drying, direct compression, blending/mixing, and coating.