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As appeared in Tablets & Capsules April 2015 www.tabletscapsules.com

tablet coatingSelecting a cartridge dust collection system for tablet coating operations

David A. SteilCamfil Air Pollution Control

The dusts generated by tablet coating operations are likesnowflakes: No two are alike. The operations themselves alsovary widely. This article reviews the variables and standardsthat you must consider when selecting a dust collection systemfor tablet coating operations.

ecause of the complexities and the unique nature ofdifferent coatings and coating systems, it’s impossible toapply a cookie-cutter solution to every dust collectionchallenge. Rather, we must drill down to details duringthe planning phase, and tablet coating and dust collec-tion equipment suppliers must work together. That coop-

eration, coupled with a thorough, upfront examination ofthe coating system requirements, will lead you to the bestoptions in dust collection components.

Why is it so important to go through this process? Asa pharmaceutical manufacturer, you’re naturally con-cerned with employee safety, optimizing product quality,and minimizing downtime. At the same time, the dustcollection system must satisfy:

• The FDA, whose overriding concern is patientsafety;

• The National Fire Protection Association (NFPA),which sets standards for fire and explosion protection,especially important when the coating uses solvents;

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• The Occupational Safety and Health Admini stra tion,which assesses indoor air quality, combustible-dustexplosion risks, and other employee safety concerns;and

• The Environmental Protection Agency, which moni-tors the air exhausted outdoors.

Because tablet coating operations can involve toxic orpotent materials, as well as potential fire and explosionhazards, the risks must not be taken lightly. Review the fol-lowing questions and discussion for guidance in selecting adust collection system for your tablet coating operation.

What is the proper air-to-media ratio?The air-to-media or air-to-cloth ratio is defined as the

volume of airflow (cubic feet per minute) that flowsthrough the dust collector in relation to the squarefootage of cartridge filter media within the collector. Therecommended air-to-media ratio for cartridge filters usedin most tablet coating applications typically ranges from1.5 to 2.5 cubic feet of air per square foot of media, avery conservative ratio. The actual ratio will vary withthe particulars of each application and should be dis-cussed with the coating equipment supplier.

The single most important factor in determining air-to-media ratio is whether coating is done in batches orcontinuously. Continuous coating systems are associatedwith very heavy dust loading. They can also introducelarge amounts of moisture into the system because thecoating is sprayed over longer periods.

Other variables in the equation: How many layers ofcoating are applied to the tablets and at what intervals?How many spray guns or nozzles are used? How big is thecoating pan? Are solvents involved or are aqueous solu-tions used? Will the dust collector be adjacent to thecoater—where moisture will be a bigger concern—or willdust travel to a more remote collector? All these factorsshould be considered.

An air-to-media ratio that is too high may lead to fre-quent (excessive) pulse cleaning, shortening the servicelife of the filters and impeding dust collection efficiency,which will cost you more money over time. Worse still, ahigh air-to-media ratio may lead to blinded filters andthus unscheduled system shutdowns. These unexpectedshutdowns are very costly, and opting for a conservativeair-to-media ratio is a better approach. In the most de -manding or “worst-case” scenario—i.e., a large, continuouscoating operation that includes a wash-in-place/clean-in-place (WIP/CIP) system—even a 1.5-to-1 ratio might notbe conservative enough.

A conservative (low) air-to-media ratio reduces the riskof serious operational problems and will allow you tooperate longer between filter changes. Yes, the initialexpense of the dust collector will be higher, but the pay-back in operational and maintenance savings can be veryshort. Plus, the system will be much more reliable.

On average, airflows and pressures are higher in dustcollectors serving coating operations than in those servingdry processes, such as tablet manufacturing. As a result, thedust collectors are typically larger, especially when coatingis continuous, a case where the heavier dust loads and addi-tional moisture may require more frequent filter changes.

What is the best filter media?Tablet coating operations generate fine, potentially

hazardous hybrid dusts that require high-efficiency filtra-tion media, usually with some degree of moisture-resis-tance and/or other special properties. The type and dura-tion of the coating process will have an impact on mediaselection (Figure 1).

The two basic, most common media for coating opera-tions are nonwoven cellulosic blends and spun-bondedpolyesters or polyester-silicon blends. The cellulosic mediaare the most economical for dry dust collection at operat-ing temperatures to 160°F. Sometimes cellulosic media willincorporate a coating that enhances moisture resistance,

Figure 1Cartridge filter media commonly used in tablet coating dust collection systems

a. Conductive media for use where conveyeddusts generate electrostatic charges that requiredissipation, and/or for hybrid mixtures that

include solvents in the airstream

b. Synthetic spun-bonded media for high moisture environments

c. Nano-fiber-treated media for better performance and longer filter life

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but they do not, as a category, resist moisture as well aspolyesters. Both spun-bonded polyester media and poly-ester-silicon blends are lightweight and washable. In dryapplications, they can operate in temperatures of 180°F to265°F. They can recover from a moisture excursion, butthey are not intended for continuously wet applications.

Both cellulosic and polyester media can include a sur-face layer of nano-fibers to boost their efficiency andprovide other benefits. The main benefit: better surfaceloading of fine dust to prevent it from penetrating deeplyinto the base media. That translates into better dustrelease during pulse cleaning cycles and lower pressuredrop readings over the life of the filter, which extend fil-ter life and reduce energy consumption.

In batch coating processes, particularly those withsmall coaters and in less demanding applications, bothtypes of media are used, either as standard versions orwith a nano-fiber layer.

Both media can also include conductive elements thatdissipate electrostatic charges that can build up whendusts are conveyed. Typically, static dissipation isachieved by impregnating a cellulose filter with a carboncoating or a synthetic filter with an aluminized coating.Conductive media are often required when solvents arepresent in the airstream or when the application mustmeet requirements issued by the NFPA or ATEX, aEuropean directive that governs explosive environments.

If the coating operation uses a WIP/CIP system, aspun-bond polyester media with an oleophobic treatmentmight be warranted. This oil- and water-repellant finishallows filters to withstand moisture from the cleaning sys-tem. A bypass damper can also reduce or eliminate mois-ture in the airstream before it reaches the collector,allowing you to clean in place without shutting down thedust collector. For the most demanding high-moistureapplications, such as a very large continuous coating sys-tem with WIP/CIP, consider using heavy-duty, spun-bonded oleophobic media.

Filter service life varies a lot, but high-efficiency, long-life filters in a properly sized collector will last 3 to 6months on average. One year is probably the maximumservice life in a coating application. Many pharmaceuticalmanufacturers prefer to change filters more frequently,however, usually to coincide with other cleaning and main-tenance tasks at the end of a coating cycle. They don’t waitfor the filters to reach the maximum allowable restriction.

What kind of dust testing is needed?There are two types of dust testing: 1) explosibility

testing, which determines whether a dust is combustible;and 2) bench testing, which assesses the physical proper-ties of the dust.

Explosibility testing. These tests are required to deter-mine the degree of risk that a combustible dust could lead toan explosion. Conducted in accordance with ASTM meth-ods (as stipulated by the NPFA), the tests determine a dust’sKSt (the rate of pressure rise), Pmax (the pressure developedinside the collector), and MIE (minimum ignition energy).

Unless the dust is completely inert (KSt = 0), someform of explosion protection must be incorporated intothe dust collection system. Coating operations often gen-erate hybrid dusts and dust-solvent mixtures, and theconcentration of solvent can have a dramatic impact onKSt values. The NFPA Standard 68, “Explosion Pro tectionby Deflagration Venting,” states: “Where test data are notavailable for hybrid mixtures with gases that have com-bustion characteristics similar to those of propane (funda-mental burning velocity ≤1.3 times that of propane) andSt-1 and St-2 dusts, the design shall be permitted to bebased upon Pmax = 10 bar and KSt = 500 bar-m/s.” In thissituation, the dust collection equipment supplier will usea worst-case scenario and incorporate a very high level ofexplosion protection. To avoid over-designing the sys-tem, it is always preferable to test samples of the actualdust that the dust collector will capture.

Bench testing. Testing dust samples in a laboratory is along-established practice for making informed decisionsabout filter media and other dust collector components.It’s not always necessary but if there is anything at allunusual about the process and/or the dust—as is the casein coating applications—it’s a very useful practice.

Bench testing encompasses a series of tests that revealvaluable information about a dust’s physical characteris-tics. With a dust sample gathered from waste material, it’spossible to determine the dust’s particle size distribution,particle shape, moisture percentage by weight, the dust’sability to absorb moisture, and other characteristics. Somedust collection equipment suppliers offer bench testing,often at no charge, because it generates very useful infor-mation. You can also have it done at an independent lab.

Is contained dust collection required?Sometimes an inert coating is sprayed onto an active

tablet core; sometimes an active coating is applied to aninert core; in other cases, both coating and core are ac -tive. The specifics of the application determine whethercontainment equipment is needed to prevent hazardousdust from being released into the environment.

In operations where the coating machinery is fullycontained, and/or where cross-contamination of manu-facturing processes is a concern, a contained dust collec-tion system should be used. It will incorporate bag-in/bag-out (BIBO) technology for the filter cartridges toensure filters are replaced safely (photo, page 12). It willinclude dual-valve continuous liners at the system’s dis-charge to capture and contain the dust that releases fromthe cartridges and enters the hopper during pulse clean-ing. High-efficiency particulate air (HEPA) after-filters—sometimes called safety-monitoring filters—are also typi-cally specified for hazardous dust applications, and thesealso incorporate BIBO technology.

If the dust is deemed not to be highly hazardous, con-tainment may not be necessary, but HEPA after-filters areoften used as backup protection to the dust collector.HEPA filters put a final “polish” on the air after primaryfiltration in the dust collector, allowing it to be released

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directly outdoors or recirculated into the building.Which type of equipment to use depends on whether

the dust is potent, toxic, and/or allergenic. These proper-ties will, in turn, help determine the OccupationalExposure Limit (OEL) of the active pharmaceutical ingre-dient. The OEL is the maximum air concentration towhich a worker can be exposed over an 8-hour shift (40-hour work week) without suffering adverse health effects.This concentration is expressed as a time-weighted aver-age in micrograms per cubic meter of air. A risk-basedexposure evaluation should be performed to determinewhat level of filtration and containment is needed for aspecific application.

What are the negative-pressure requirements?In a typical coating operation, air cycles through the

coating pan at specific volumes, temperatures, humiditylevels, and other parameters, and all these parameters canchange throughout the process, creating additional dust-collection challenges. Typically, air moves through thesystem at negative static pressures that can reach 45inches water gauge. The coating pan must remain undernegative pressure (partial vacuum) throughout theprocess to maintain a controlled environment and ensuredust remains in the housing. Using a dust collector that isproperly sized (correct air-to-media ratio) and contained(if required) will create and maintain the proper negativepressure environment and enable you to operate the coat-ing system at peak performance without interruption.

It is also critical that you control the dust collector’sreverse-pulse cleaning system. Otherwise, you risk creat-ing positive pressure that can lead to a containmentbreach, improper operation of the coating equipmentand, ultimately, downtime while you correct the problem.

Where will the dust collector be located?Dust collectors for tablet coating applications are usu-

ally located indoors (photo), either in a maintenance or

mechanical area adjacent to the Good ManufacturingPractice (GMP) space, or within the GMP space itself. Iflocated within the GMP space, the dust collector mustmeet strict FDA requirements, just as all equipment in thatspace must, which may raise the costs of the collector.

Less often, usually due to space constraints or localrequirements, collectors are placed outdoors. If a com-bustible dust is involved, the collector will likely beequipped with an explosion vent, and you will need tocalculate the explosion vent area required and how toproperly direct the explosion to a safe location. Outsideinstallations typically have longer duct runs, and thus cor-rectly configuring them takes on added importance toensure consistent airflow from the coater to the collector.If incorrectly configured, the ductwork will lead toshorter filter life or blinding, which will cause the dustcollector to malfunction and the coating system to fail.

What is the best type of explosion-protection equipment?Explosion venting is the most basic and economical

form of explosion protection, but it may not be an optionif the collector is located within the GMP space and/ordeep inside the building. In fact, that is where most coat-ing operations and associated dust collectors are, and thatmakes venting an explosion to an outside wall difficult. Inthese situations, and/or if the dust is harmful or toxic(particularly if solvents are used), the only option may bea chemical suppression system, which is much morecostly than venting. See Figure 2.

A chemical suppression system detects an explosionhazard within milliseconds and releases a chemical agentthat extinguishes the flame before a deflagration canoccur, protecting the dust collector from explosion. Theinlet and/or outlet ductwork itself must also be protectedfrom explosion. If chemical suppression is used on thedust collector, chemical isolation is typically used for theductwork. Installed on the inlet and/or outlet ducting, itcreates a chemical barrier that suppresses and isolates theexplosion within the ducting and reduces the propaga-tion of flame. It also minimizes the pressure rise withinconnected process equipment.

Mechanical isolation is another option. In these sys-tems, passive dampers or fast-acting valves prevent adeflagration from reaching upstream equipment.

Determining which explosion-protection and duct-iso-lation methods are best for your coating operationrequires conducting a risk assessment. Because of theimportance and complexity of the task, hire an indepen-dent professional engineer and have him or her workwith your dust collector supplier. That’s the best way toensure compliance with the applicable standards and therequirements of your insurance carriers.

You can also conduct real-world destructive testing tohelp make decisions. The NFPA uses relatively conservativecalculations in developing its standards for explosion-protec-tion equipment, which is understandable. But the NFPA alsoallows you to use real-world destructive test data instead ofits textbook calculations, as long as the equipment supplier’s

A skid package facilitates transport and installation. It incorporates two high-efficiency cartridge dust collectors, contained HEPA housings, explosion-protection

devices, fans, controls, and other components.

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data proves that the design of the dust collection systemmeets a specific set of criteria for a given situation.

But few people use real-world destructive test data,probably because it is not required and can prolong theproject and add cost if your equipment supplier hasn’talready conducted similar testing. Still, many equipmentsuppliers recommend gathering the real-world databecause it can pay off very quickly by preventing youfrom paying for an overdesigned system.

The dust collector’s vessel strength is another factor thatcan only be verified by physical testing. A heavy-duty dustcollector—made with a thicker metal and bearing a higherpressure rating—will better withstand a dust explosion andmay allow you to specify a simpler and less costly explo-sion-protection system. Stronger vessels also increase theoptions for equipment placement because they allow longerexplosion vent ducting, potentially saving space inside yourfacility if you can locate the dust collector in a more remotelocation. Find out if your dust collector supplier can providereal-world test data to assist with this strategy.

Is air recirculation an option?If you operate in a region where it gets very hot or

cold, consider air recirculation downstream of the dustcollector. By recirculating heated or cooled air throughthe facility, you eliminate the cost of replacing that con-ditioned air, and the energy savings can be dramatic.Most dust collection equipment suppliers have softwarethat can estimate the savings based on system airflow, cli-mate, utility rates, and other factors.

When recirculating air downstream of the collector,adding a HEPA after-filter is recommended and, if you’refiltering hazardous or toxic dusts, it’s required. HEPA fil-ters provide backup protection and a final scrub of the airbefore it returns to the facility. Ductwork and transitionsections are usually required to connect a secondary filtermodule to the dust collection system. But in some appli-cations, an integral HEPA after-filter module mounts ontop of the collector to save floor space and eliminate theneed for transitional ducting, saving money.

For contained coating operations, safe-change contain-ment HEPA filter systems are available, and the designand selection of after-filter components should be basedon a risk assessment and the specifics of the application.

If the collector is handling an explosive dust, HEPA after-filters—as well as the collector and ductwork—must complywith NFPA standards for combustible dust handling. T&C

Figure 2Explosion-protection methods

a. Explosion vent b. Chemical suppression c. Chemical isolation d. Passive isolation valve (active mechanicalisolation valves also available)

Most dust collectors used with tablet coating operations are located indoors, as thisone is. It includes two high-efficiency filter cartridges and a chemical suppression

and isolation system.

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David A. Steil is pharmaceutical market manager at CamfilAir Pollution Control, 3505 South Airport Road, Jonesboro,AR 72401. Tel. 870 933 8048. Website: www.camfilapc.com/pharma. His responsibilities include sales and marketing to thepharmaceutical and nutraceutical industries in North Americaand Canada. Steil spent 12 years in the environment, health,and safety group at a major pharmaceutical company. He is amember of the International Society for Pharmaceutical Engi-neering and the American Industrial Hygiene Association.

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