Q: Process Airflow measurement indicated by the machine, are those shown as standard cfm, std m3/hr etc?
A: Each manufacturer of equipment would specify the units. F-V provides units based on the customer’s requirements. And do we need to correct for inlet temperature in order to get the actual volume? Yes, if the temperature is different than the reference standard temperature selected for the air flow station. If yes, how to convert? The first step is to know the volumetric air flow at the flow meter under flow conditions (temperature, pressure, and density of the air). An on-line calculator can assist with this, such as can be found at www.dwyer-inst.com/flowcalc/. Note that the initial calculation can be done with dry or moist air. In all processes the air has some moisture. In addition, air flow stations manufacturers may use a correction factor for their flow meters. Once the air flow is known at the flow conditions, the actual volumetric flow at the inlet to the fluid bed can be found utilizing the ideal gas law (reference slide 29 of the presentation).
Q: How does different type of filter affect scale-up? Can you give a more descriptive or specific example?
A: A switch from bag to cartridge filters, or vice versa, can change the height of the fluid bed above the Wurster partition and therefore change the rate of drying before the product returns in the down bed. In addition, the differences in media may allow for differences in the size of particles being retained.
Q: What in-process controls are available to monitor coating in real time?
A: There are particle size analyzers that can be placed on systems to measure average film thickness (provided the beginning spherical core size is known); R&D studies have used NIR to measure coatings as well; however, models must be built for each product to apply it for controlling a given process. Some of the coating manufacturers have “computer tools” to calculate how much coating is required to provide a given thickness, but these tools are proprietary; one needs to contact the coating manufacturer for assistance.
Q: Can you clarify what is the particle/air velocity ratio? e.g. particle number /velocity? particle size/velocity?
A: If I said “ratio” that was a mistake on my part. Ideally, you want the particle velocity through the spray zone to be the same in both the small and large systems. The particle velocity can be assumed to the same as the air velocity exiting the top of the Wurster partition.
Q: Can tablets be coated in a Wurster process?
A: Yes; however, the tablets must be robust enough to withstand the vigorous movement that occurs. My personal recommendation would be to use a pan coater for coating tablets since it generally has a more gentle tablet movement as compared to a Wurster process.
Q: How aqueous coating be used for moisture sensitive actives?
A: Since drying is occurring as fast as the coating is being applied to the product, the small amount of moisture and time it is in contact with the active may not be detrimental; however, that will be product dependent.
Q: What is lowest RH% that can be achieved inside coater without causing spray-drying?
A: While RH% of the incoming air has a bearing on how fast drying can occur, other factors are equally or more important with regards to spray drying of droplets; such as air temperature, air flow, percent moisture of solution being sprayed, and droplet size.
Q: Does Spray gun Nozzle size matter?
A: Yes, as it will have a governing factor on droplet size. In lab scale equipment mostly 1.0 mm size is used whereas large scale equipment has 1.5 mm. A good rule to use as a starting point is to use 1000 times the atomization air flow as the solution flow. Increase this ratio if smaller droplets are desired.
Q: Please explain advantages of bottom spray and why not go for top spray standard fbd for spraying particles and tablets.
A: Bottom spray is co-current with the product flow and therefore leads to more uniform coating application.
Q: Is it necessary to start spraying at a lower spray rate and then ramp up to target to avoid overwetting?
A: While not required to commence spraying at a slower rate, it is a good place to start developing a process. Once you know how the process/product responds with no agglomeration, solution application can be started at faster rates.
Q: Can you please again elaborate the required evaporation rate for spray rate?
A: In order to prevent over-wetting, the evaporation rate must equal the moisture addition rate. The moisture addition rate can be determined by multiplying the spray rate by the percent solvent in the solution/suspension.
Q: How is the relationship between coating weight gain at small scale and larger scale provide similar drug release?
A: Provided the same size cores are used at both scales, final moisture content is the same at both scales, and the coating has the same characteristics, the release should be similar.
Q: Is there a similar system to coat particles smaller than 1 micron?
A: Not that I am aware of.
Q: Can you comment on how to evaluate substrate strength and its possible impact on coating film quality and consistency, especially for ER products?
A: Friability testing is one way; another way is to put uncoated product in the machine and operate it under expected processing conditions to see how it withstands the vigorous product movement encountered during Wurster processing. If the product attrits, and the small particles are not allowed to leave the processing chamber, these particles can become entrapped in the coating layer and/or on the surface of the coating. This, in turn, will cause a faster release of the active.
Q: You mention the maximum relative size of the sprayed droplets of 1:10 to the size of the coated particle, while this is the maximum, where would you estimate the optimum?
A: As small as possible without resulting in spray drying of the droplet.
Q: Is there any relationship between particle size and spray gun pressure or solution volume?
A: If particle size is what is meant, I am not aware of any correlation. However, if droplet size is what is of concern, then yes, there is a relationship between atomization air velocity (volume or velocity is the result of pressure), solution velocity (related to flow), spray gun orifice, and solution viscosity, etc. The best way to determine the correlation is to conduct spray droplet tests for a given solution using a laser diffraction device to measure the droplet size.
Q: Should the relative humidity of inlet air be under control?
A: Yes, for consistent control of a process; however, if the humidity can not be controlled, one must be able to adjust inlet air temperatures or spray rates to control the process. Note, I did not include process air volume as it really is fixed for a given Wurster process to provide proper product flow.
Q: Can particles 10 to 100 microns be coated using Wurster coating?
A: Yes, provided the particles have a narrow particle size distribution. If the distribution is too wide, the particles can agglomerate at a higher percentage.
Q: How to avoid the doublets/ triplets cause due to static charge during process?
A: One way is to increase the “moisture” in the system. This can be done in a couple of ways:
1) by spraying as soon as the product begins moving or spraying faster (but this may not be sustainable for the duration of the process) and
2) by increasing the humidity level of the incoming process air (this usually means that a slower spray rate will be required to sustain the process).
Q: How can we get the mass flow rate of process air and atomization air?
A: If you know the volumetric flow and the density of the air (function of humidity and temperature) you can calculate the mass flow rate … where mass flow = volumetric flow (volume/time) x density (mass/volume).
