Best Practices for Vacuum Conveying of Pharmaceutical Powders

Automated material handling can eliminate risks associated with operators manually carrying and pouring bags of powder into feeder hoppers. Pharmaceutical Technology spoke with David Kennedy, business development manager at equipment supplier VAC-U-MAX, about best practices in the use of vacuum conveying for transferring powders in a pharmaceutical manufacturing facility.

Considering automation

PharmTech: What are some of the situations where an automated vacuum conveying system could be used instead of manual transfer of powders? 

Kennedy (VAC-U-MAX): The first factor to consider is the type of process that the vacuum conveyor is feeding. For example, a high-speed packaging machine, such as an auger-filler, will be well-served by an automated conveying system so that the filler never runs out of material. Automated transfer (i.e., a permanent installation) also reduces the ladder-climbing and lifting that accompanies a manual operation. This benefit would also apply to tablet press loaders or capsule fillers.

Even small-rate applications, however, can still involve repetitive motions that will fatigue the operator or put them at risk for musculo-skeletal disorders. Loading V-blenders or double-cone blenders, feeders, and mixers can be a labor-intensive operation that can be improved by using a bag dump station with vacuum conveyor or direct-charge blender loading to transfer powders from ground level up to a mezzanine instead of the intensive manual lifting and climbing activities. Some low-rate applications can still be ‘automated’ with an operator who manages a suction wand to draw powders from drums to a vacuum conveyor that lifts the powder to a process that is 10 or 20 ft. in the air. Other applications include high-rate gel-cap inspection systems that are typical fed by a specialized vacuum conveyor.

Benefits of automated systems include reduced fugitive dust (i.e., escaping out of the process), improved product quality due to the enclosed system, improved ergonomics for workers, and increased throughput over manual operations, which is particularly important in times of qualified labor shortages. 

In addition, automated systems aid in compliance with United States (US) Occupational Safety and Health Administration (OSHA) safety regulations for preventing falls by eliminating the need to climb ladders or stairs with containers of raw materials. Some features of vacuum conveying systems, such as mobile lift frames or column lifts, can bring the conveying equipment down to ground level for maintenance or sanitation to eliminate the need for performing those duties as higher elevations. Vacuum conveying systems also keep walking/working surfaces clean of debris. For example, it not unusual to see a pallet of bags forklifted up to a mixer mezzanine for manual dumping into a mixer. As each bag is emptied, product can fall on the walking surface. A properly-designed vacuum conveying system leaves the bag-dumping operation at ground level.

Specifying a system

PharmTech:What are the factors in sizing a system? 

Kennedy (VAC-U-MAX): Several factors to consider include:

• Bulk density (weight of powder in a common volume, lb/ft³or g/cc³)

• Convey rate (expressed in lbs or ft³/hour)

• Duty factor (one small batch per minute, or one large batch per hour)

• Conveying distance (longer distance requires more horsepower)

• Number of elbows in the conveying route (more elbows converts to longer convey distance)

• Particle size and characteristics (e.g., fine particles need more filter area in the conveyor, sticky powders need good techniques for discharge from the conveyor, abrasive materials need special consideration)

• Combustibility (requires explosion protection techniques)

• Available headroom over the process being filled

• Equipment construction (materials, sanitation and polish requirements, chemical compatibility)

• Ingredient accuracy (batch weighing and/or loss-in-weight feeding).

PharmTech: Do you need to characterize the powder when setting up the system? 

Kennedy (VAC-U-MAX):Materials must be characterized for particle shape, size, percentage of fines, flowability, discharge friendliness, aeration or de-aeration characteristics, friability (breakage factors), moisture absorption, sensitivity to temperature, and so on. After characterization, even when you think you know everything about the powder, a responsible supplier will run a test in their lab to prove out all of their hypotheses.

PharmTech:What precautions are needed to mitigate explosive risks? 

Kennedy (VAC-U-MAX): First the customer must have the material tested for explosivity by a qualified lab. There are several types of explosion protection techniques standardized by the US National Fire Protection Association (NFPA) that have advantages and trade-offs for a customer, which include: 

• Explosion venting (allows the explosion to take place and directs it out a certain vented direction, but the process vessel must be located near an exterior wall, requires equipment rebuild, and has higher downtime)

• Chemical suppression (prevents the thermal event from occurring by injecting dry chemical powder into the vessel before deflagration; process vessel can be located anywhere inside or outside the plant, has less equipment damage and reduced downtime)

• Explosion venting with flameless venting (also allows explosion to take place, but flameless vent allows process equipment location anywhere within the plant, but still have internal equipment damage and rebuild time)

• Reduced oxygen concentration (uses an inert gas, such as nitrogen or argon, as the conveying medium instead of plant air that contains oxygen, which is a fuel for combustion; more costly than ambient plant air; more controls and monitoring for oxygen concentration levels; respiratory concerns for plant workers; no special requirements to relocate the equipment; no explosions results in no downtime)

• Explosion containment (build the equipment to withstand the pressure and contain the energy of the explosion inside the vessel, high up-front costs, equipment rebuilding and re-certification). 

• Handling of combustible powders requires equipment bonding and grounding per NFPA standards, and all electrical components must comply with the hazardous area designations (class, division, and groups) (1,2). To further improve explosion protection, all equipment should be manufactured in anti-sparking stainless steel. All controls need to be classified for the area.   

PharmTech:What are the considerations for vacuum conveying in a GMP environment? In a cleanroom?

Kennedy (VAC-U-MAX): Considerations include equipment cleanability and sanitary manufacturing techniques, washdown-resistant materials of construction and electrical enclosures, and FDA-approved soft materials (e.g., gaskets and filter media).

Best practices for poorly flowing or potent powders

PharmTech:What are some best practices for transferring powders that don’t flow easily? 

Kennedy (VAC-U-MAX):Some powders may have improved flow characteristics with the use of bin vibrators, bin activators (i.e., live-bottom bins), or pulse-air devices in the hopper section. Other materials require hopper designs with offset cones (e.g., straight on one side and a 60–70 degree slope on the other) or flared-tube hoppers (e.g., straight-sided tube hoppers and full-opening valves) that will handle materials with high fat or moisture content. It is also critical to understand the process conditions for the equipment being fed by the conveyor: are moisture or vapors generated when the process is filled or is there a high-temperature condition in the vessel being filled?

PharmTech:What are the considerations for containment for highly potent powders?

Kennedy (VAC-U-MAX):Use vacuum technology for conveying because if it develops a leak, it will only leak inward, not outward. Dust-tight designs on the powder supply end and the discharge end are crucial. Maintenance that can be performed without entering the equipment itself (i.e., confined spaces) is desirable. Some high-risk or high-potency applications will call for bag-in, bag-out filter changing designs to reduce operator exposure.

References

1. NFPA, NFPA 77 Recommended Practice on Static Electricity (2019).
2. NFPA, NFPA 70E Standard for Electrical Safety in the Workplace (2018).

Article Details

Pharmaceutical Technology
Supplement: Solid Dosage Drug Development and Manufacturing
April 2019
Pages: s18–s19

Citation

When referring to this article, please cite it as J. Markarian, “Best Practices for Vacuum Conveying of Pharmaceutical Powders," Solid Dosage Drug Development and Manufacturing Supplement (April 2019).