A poorly designed dust collection system has the potential to do more harm than good to the tablet press and the overall operation.
When compressing tablets, the press’s airflow, static pressure, climate control, material handling, and compression force are critical to achieving the desired production rates and product quality. Pharmaceutical manufacturers must also keep in mind that fugitive dust from tableting operations can hinder reliable, consistent performance.
Tablet presses don’t generate large volumes of dust, but the dust can be hazardous when it contains active pharmaceutical ingredients and/or potent compounds. It can cross-contaminate other products and affect the health of operators and other workers in the facility. Therefore, it is critical for facilities to implement systems to safely collect and dispose of dust, preferably at its source.
A poorly designed dust collection system has the potential to do more harm than good to the tablet press and the overall operation. Consider the following points to ensure that your tablet presses and dust collectors are working in harmony for effective, consistent performance.
Regulatory Compliance for Dust Colors
The ideal dust collector for tablet compression applications is a high efficiency cartridge-style dust collector because it efficiently removes dust using a relatively small footprint. However, a dust collector’s productivity is not the only concern. A variety of government agencies monitor regulatory compliance to ensure the safety of employees, the environment and the public including:
- FDA – assesses the safety of all products delivered to patients
- NFPA – monitors your fire and explosion safety measures
- OSHA – evaluates the fugitive dust’s potential to explode, as well as indoor air quality, and employee health and safety
- EPA – measures exhaust system output to assess what is being released into the environment.
Required Level of Isolation and Containment for Fugitive Dust
When choosing the best dust collection system for your tableting facility, consider first the toxicological properties of the material being captured. This means understanding the dust’s potent, toxic or allergenic properties. This is a vital step in determining the Occupational Exposure Limit (OEL), or the amount of material determined to be the maximum air concentration to which each worker can be safely exposed to for an eight-hour shift, 40 hours per week, without suffering adverse health effects.
To determine proper control, complete a risk-based exposure evaluation including a dust hazard assessment. In most cases, the hazardous nature of pharmaceutical dust requires some level of containment since it can’t be released into the environment. Highly potent applications can require any or all of the following:
- HEPA secondary filter systems
- Bag-in/bag-out technology for the primary filter cartridges
- Continuous liner technology to capture dust released from cartridges during cartridge pulse cleaning.
The Required Air-to-Media Ratio
An often miscalculated, yet important calculation for selecting the proper dust collector design is the air-to-media (cloth) ratio. This ratio is defined as the cubic feet of air per minute to square feet of filter cloth/media.
For tablet press applications, the typical air-to-media ratio should be 2.5:1 to 3:1. Some equipment suppliers recommend a much higher ratio to use a smaller collector that takes up less floor space and provides a lower initial cost.
However, an overly high ratio can cause inconsistent airflow, leading to other problems that negatively impact dust collector performance. A more conservative ratio will enable the tablet press to perform reliably for up to one to two years without filter changes, and reduce maintenance costs. Reputable dust collector manufacturers have technical personnel who can help manufacturers determine the correct ratio for their specific application.
How Surrogate Testing Supports Risk Assessment
A surrogate test program can aid the risk assessment of fugitive dust. A surrogate (or substitute) compound simulates the active pharmaceutical ingredient (API) to safely evaluate the performance of the containment equipment.
This testing is especially beneficial for tablet presses working with an API with unknown toxicological properties. Surrogate testing can also be used during factory acceptance testing and/or site acceptance testing to confirm that equipment is performing as required.
Manufacturers that validate during the engineering process can reduce costs while selecting the best system for their specific project or application. At Camfil, we have performed surrogate testing on our equipment to demonstrate the potential performance of our containment systems.
Negative Pressure Ensures Effective Dust Capture
Tablet presses generally exhaust dust to the collector from a local pick-up point—typically where the material is fed into the die. Maintaining negative pressure in the turret enclosure ensures that the dust is captured inside the housing so there is no fugitive dust escaping into the workplace or outdoors. This controlled environment is necessary if you are using a contained tablet press system for a potent or hazardous API.
It is important to use the correct size dust collector with the proper air-to-media ratio, to create the necessary negative pressure environment inside the tablet press housing. To avoid creating positive pressure and a potential breech in containment, you need to have precise control over the dust collector’s reverse pulse cleaning system along with proper duct design.
Special Considerations for Combustible Dust
Applications that process combustible dusts require extra measures to protect against explosions that can occur in the dust collector. Most often, dust collectors are fitted with an explosion vent or a chemical suppression system.
The most cost-effective option is the explosion vent. However, the vent is not always possible if the collector is located indoors without access to an outside wall or without ability to vent through the ceiling. Chemical suppression with isolation is used when:
- The collector is located within the Good Manufacturing Practice (GMP) space
- The collector is located in a mechanical area that is processing a hazardous material that can’t be released directly into the atmosphere
- There is no direct access to an outside wall or ceiling location where the explosion vent ducting can protrude.
If an explosion vent is feasible, you will then determine what size components are required, the location where the dust collector will be placed and how much explosion vent ducting (if any) will be required. The inlet and outlet ductwork must also be protected against the possibility of an explosion as determined by a risk assessment.
Because combustible dust issues are very complex, and incidents can be devastating, it is important to use an independent professional engineer. That way you are certain that your facility is in compliance with NFPA, ATEX, and/or the requirements of your local fire marshal and insurance carriers.
Proper Placement of Dust Collectors
Dust collectors are typically placed either outdoors or indoors in a maintenance or mechanical area next to the GMP space. Their placement is determined by the required functionality, cost, available space, and convenience.
There are dust collector selections that do not require any type of additional explosion protection equipment other than isolation for the inlet ducting. Ensure that these types of collectors have been independently tested for the appropriate pressure resistance and have documentation to support the testing results.
Regardless of collector location, you must ensure consistent, stable airflow to your tablet press by properly installing the required ducting in accordance with the design parameters. Both the length and configuration of ducting can dramatically affect airflow.
Dust collectors placed in the GMP space are subject to the same tight FDA controls that govern all equipment in the processing area. As mentioned above, if the tableting application produces combustible dust, the system will need to have some type of device to protect from explosions or are properly pressure-resistant rated for the dust being collected.
Selecting the Correct Filter
As referred to earlier, tablet presses can handle hazardous materials in the form of fine, dry dust. Often, the dust collectors must use cartridges with high efficiency filtration media to process and contain this dust. For example, a filter media made of polyester-cellulose blend with a microfiber synthetic melt-blown laminated nanofiber surface layer can provide 99.99 – 99.999 percent efficiency for 0.5 micron and larger particles.
When dust collectors are placed indoors in climate-controlled environments, temperature and humidity may not factor into the filter media choice. But collectors located outdoors can pose a challenge, as they require filter media that can handle varying environmental conditions.
Another case for considering special filters is when the tablet press uses a wash-in-place (WIP) system. In this case, the filter media is exposed to moisture during the cleaning process. The following methods help to maintain dust collector performance in this situation:
- Use a high efficiency spun-bond polyester media that may include an oleophobic treatment
- Use a pleated filter style that is designed to promote optimum airflow and dust release characteristics through the system
- Use a bypass damper to eliminate the moisture before it reaches the collector
- Use a conservative air-to-media ratio to provide more filter surface area and extend filter life.
Selecting the correct filter ensures consistent airflow, extended filter life, reduced maintenance costs, and fewer production shutdowns.
About the Author:
David Steil is the pharmaceutical market manager for Camfil Air Pollution Control (APC), where he manages North American and Canadian sales and marketing to the pharmaceutical/nutraceutical industries. Steil is a member of the International Society for Pharmaceutical Engineering (ISPE) and the American Industrial Hygiene Association (AIHA).