Wurster coating interview questions and answers : In this article we will discuss the frequently asked Wurster coating interview questions and answers
Wurster coating interview questions and answers
Wurster coating interview questions and answers
following are the frequently asked Wurster coating interview questions and answers
What is Wurster coating?
Wurster coating is a coating technique for powder coating or pellet coating using bottom-spray where the nozzle is spraying from the bottom of fluid bed coater.
What is the principle of wurster coating process?
The principle of the wurster coating process is to suspend particles in a fuid-like state using air introduces at the bottom of the bed. Depending on the position of the spraying nozzle, the coating solution is sprayed directly on the particles. the Air that act as a momentum carrier improves the coating and drying. hence, the coating layer of the particles is built up during wetting and drying together with particle motion.
What are the applications of Wurster coating ?
The bottom-spray Wurster method is very popular in the pharmaceutical industry. following are the applications of Wurster coating :
- Drug coating (API layering)
- Seal coating
- Polymer coating for modify or control drug release formulation
- Pellet (Multiple Unit Particulate Systems) coating.
Fluid bed processor are used of Top Spray Granulation, Drying, Spherical Granulation (Rotor Granulation) and Wurster Coating (Bottom Spray Coating).
What are the process variables for wurster coating?
The Wurster process has number of variables like:
- Inlet air flow (CFM)
- Atomization air pressure,
- Spray rate
- Product temperature,
- Inlet air temperature,
- Inlet %RH
- Absolute humidity
- Dew point
- Batch size
- Spray liquid viscosity and concentration
- Spray assembly setting,
- Air distributor plate type
- Column height
What is down bed in Wurster coating Process?
The region of outside of the partition is referred to as the down bed.
What is up bed in Wurster coating Process?
The region of inside of the partition is referred to as the up bed.
Who invented Wurster coating ?
In 1959, Dr. Dale Wurster introduced an air suspension technique now known as the Wurster coating process.
How to determine the batch size in Fluid Bed process?
Volume and bulk density – Volume and Bulk Density are the two important factors in determining the batch size.
Which one is better cartridge filters and bag filters ?
Cartridge Filters are much better at product retention, rated for up to 2 microns compared to Bag Filters, rated at 20 microns.
How to avoid agglomeration in Wurster coating ?
By optimizing process parameters like inlet air temperature, product temperature, spray rate, absolute humidity and atomization air pressure allows the evaporation of solvent to take place at a rate that is sufficiently slow for adequate spreading of spray droplets and coalescence of polymer particles, and fast enough to avoid agglomeration.
What is recommended product temperature for EUDRAGIT Wurster coating ?
In methacrylic acid (EUDRAGIT ) based coating, at higher temperature spray gun choked frequently due to film formation of low glass transition coating dispersion. So it recommended that run the methacrylic acid EUDRAGIT based coating below 30°C product temperature.
What is recommended product temperature for aqueous ethyl cellulose Wurster coating ?
In aqueous ethyl cellulose based coating required minimum 45°C product temperature due to high glass transition temperature of ethyl cellulose polymer.
How to select Air distribution plate in Wurster coating ?
Air distribution plate has to be selected to get consistent fluidization at minimum attrition. Therefore, when deal with the smaller particle, use plate with lesser opening area to create the resistance at the Plat to have better distribution of the air.
| Equipment | Pellet size in micron | Plate combination |
| 6” Wurster | < 500 Micron | A |
| 500 to 1200 Micron | B | |
| 1200 to 1500 Micron | C | |
| > 1500 Micron and Tablet | D | |
| For commercial models | < 300 Micron | A-I |
| 300 to 800 Micron | B-I | |
| 800 to 1200 Micron | B-H | |
| 1200 to 1500 Micro | C-H | |
| > 1500 Micron and Tablets | C-G |
Wurster coating interview questions and answers
Which type of Filter bags used in Wurster coating ?
Bonet type Filter bags with pore size 200 micron is generally used in Wurster coating.
A Filter bags are used to stop the loss of material and allow air to flow through. If the porosity is greater than ideal then it is likely that the amount of lost material is excessive. When the porosity of the material is less than the optimal level it will cause the filter to clog and the processing will cease and this will impact the yield. The filter bag is chosen according to the size of the particle in the material and prior experiences.
Why Dew point is important variable in Wurster coating ?
The air’s humidity can fluctuate between seasons. The fluctuation in the dew point of the air inlet alters the efficiency of evaporation of the inlet air. The lower humidity of the air that is inlet to increase the capacity to dry air, even at low temperatures however, it may create static charge that is too high in the product.
The excessive absolute humidity could result in a decrease in the air temperature, below the dew point. This causes the condensing of water on the surfaces of the product or machine. It is not advised to have high humidity on an aqueous-soluble substrate during the beginning.
What is Column height in Wurster coating ?
A proper adjustment of the gap between the partitions allows for proper flow of the substrate through the spray zone, and then up in the separation column. The height of the column can change depending on the properties of the particles such as shape, size flow and bulk density.
In the case of column the height is too high. the slow and slugged movement of particles in the column causes an the formation of agglomerates, and a lack of pressure differential to pull particles into the column.
If the gap is too small the pellets will not draw into the coating material and column is lost and there is a risk of over-wetting. Increase the column’s height so that the maximum amount of pellets are into the column.
The suggested Column size for 6″ Wurster is 15-25mm, and for 18″ Wurster is 40-50 millimeters.
How to select Batch size in Wurster coating ?
Keep the batch size within the recommended occupancy. Working volume of the batch at initial and final stage should be in 20 to 100% for nonfunctional coating and 20 to 80% for functional coating limit.
How to calculate spray rate for scale up batches for Wurster coating ?
Following equation used to calculate the spray rate in scale up batches for Wurster coating.
S2 = S1 x V2 / V1
Where, V1 =Air flow at lab batch
V2 =Air low for scale up model
S1 =Spray rate in Lab batch
S2 =Spray rate in scale up model
What are the challenges and limitations of Wurster (spray) coating of small particles (<100 μm particles) ?
Wurster (spray) coating of small particles in a fluid bed dryer (Fine particles with diameters of <100μm particles) can have severe agglomeration in a Wurster fluidized bed.
The principle of wurster coating process
FAQ – Wurster coating interview questions and answers
What is the height of a wurster column?
column height is used for appropriate fluidization in wurster coating. Generally column height is set in range of 25 mm – 35 mm.
What is difference between FBD and FBP?
Fluidized bed dryer (FBD) is used for only drying and, Fluid Bed Processor (FBP) is used for Top granulation, Wurster coating and Drying.
How do I reduce drying time in FBD?
by increasing the Inlet air flow (CFM) may improve more mass and heat transfer and resulting minimizing drying time in FBD.
What is material of construction (MOC) finger bag?
Finger bag is made up of Antistatics-statin, Nylon polymer, and Cotton polymer.