Pharmaceuticals Interview Q&A – Production Basics
1. What is the main objective of industrial pharmacy?
To design, develop, manufacture, and evaluate pharmaceutical dosage forms and drug delivery systems ensuring safety, efficacy, and quality.
2. Name the three main editors of the third edition of the book.
Leon Lachman, Herbert A. Lieberman, and Joseph L. Kanig.
3. What is mixing in pharmaceutical context?
A process that tends to result in a randomization of dissimilar particles within a system.
4. Differentiate between Newtonian and non-Newtonian fluids.
Newtonian fluids have a constant viscosity independent of shear rate; non-Newtonian fluids have apparent viscosity that changes with shear stress.
5. What are the four categories of mixing mechanisms for fluids?
Bulk transport, turbulent flow, laminar flow, and molecular diffusion.
6. What is turbulent mixing?
Mixing due to random velocity fluctuations in fluid flow, characterized by eddies of various sizes.
7. Define scale of segregation and intensity of segregation.
Scale of segregation is the average size of "lumps" in a mixture; intensity of segregation measures variation in composition among portions of the mixture.
8. What are the common impeller types used in batch mixing?
Propellers, turbines, and paddles.
9. What is the role of baffles in mixing tanks?
To reduce swirl, prevent vortex formation, and improve mixing efficiency by redirecting flow.
10. What is continuous mixing?
A process that provides uninterrupted supply of freshly mixed material, often using pipes or chambers with recirculation.
11. What is a dimensionless group in mixing correlation?
A unitless number representing ratios between pertinent variables (e.g., Reynolds number, Froude number, Power number).
12. Define Reynolds number for mixing.
Re = (velocity × length × density) / viscosity. It indicates the ratio of inertial forces to viscous forces.
13. What are the three mechanisms of solids mixing?
Convective mixing, shear mixing, and diffusive mixing.
14. Why is segregation a problem in powder mixing?
Because differences in particle size, density, shape, or surface properties can cause particles to separate during or after mixing.
15. How can electrostatic charging during mixing be minimized?
By increasing humidity above 40%, adding surfactants, or using conductive surfaces.
16. What is the "degree of mixing" and how is it measured?
A statistical measure of uniformity; often assessed by standard deviation or variance of sample compositions.
17. What is a random mixture in statistical terms?
A mixture where the number of particles of a given component in samples varies according to chance, following a binomial distribution.
18. What is milling?
The mechanical process of reducing the particle size of solids (comminution).
19. What are the pharmaceutical applications of milling?
Increased dissolution rate, improved extraction, enhanced drying, better mixing, uniform coloring, and smooth texture in semisolids.
20. How is particle size expressed in milling?
In terms of mesh size (openings per linear inch) or micrometers (μm).
21. What is specific surface area?
Surface area per unit weight; increases with size reduction.
22. What is Stokes' diameter?
The diameter of a sphere that settles at the same rate as the particle in a fluid.
23. What is a size-frequency distribution curve?
A plot showing the percentage frequency of particles of different sizes in a sample.
24. What is the median diameter?
The diameter below which 50% of the particles lie.
25. What is geometric mean diameter?
The nth root of the product of the diameters of n particles.
26. What is the normal (Gaussian) distribution in particle size?
A symmetric bell-shaped curve of size frequency.
27. What is the log-normal distribution?
A distribution where the logarithm of particle sizes follows a normal distribution.
28. Name three methods for particle size measurement.
Microscopy, sieving, and sedimentation.
29. What is the lower limit of microscopy for particle size?
About 0.4 μm with visible light; can be extended to 0.1 μm with UV light.
30. What is sieving?
A method using a series of screens with defined openings to separate particles by size.
31. What is the Andreasen pipet method?
A sedimentation technique where samples are withdrawn at intervals from a suspension to determine particle size distribution using Stokes' law.
32. What is the Griffith theory of cracking?
A theory stating that solids contain flaws and cracks that concentrate stress, leading to fracture.
33. What is Kick's law of comminution?
Energy required for size reduction is proportional to the reduction ratio (D1/D2).
34. What is Rittinger's law?
Energy required is proportional to the new surface area created.
35. What is Bond's law?
Energy required is inversely proportional to the square root of the product diameter.
36. What is work index in milling?
The energy (kWh/ton) required to reduce material from infinite size to 100 μm.
37. What is milling limit?
The practical limit of fineness achievable due to material properties and milling conditions.
38. What is first-order milling kinetics?
The rate of disappearance of particles of a given size is proportional to the mass present.
39. Name four common types of mills in pharma.
Hammer mill, ball mill, fluid-energy mill, and colloid mill.
40. What is a hammer mill?
An impact mill using high-speed rotating hammers to fracture particles against a screen.
41. How does a hammer mill control particle size?
By adjusting rotor speed, hammer design, and screen opening size.
42. What is a ball mill?
A rotating cylinder partially filled with grinding media (balls or pebbles) that impacts and attrites material.
43. What is critical speed in a ball mill?
The speed at which centrifugal force equals gravitational force on the balls, causing them to centrifuge.
44. What is a fluid-energy (micronizer) mill?
A mill where particles are suspended in high-velocity air or steam and reduced by interparticle attrition.
45. What is a colloid mill?
A high-shear mill with a rotor-stator clearance used for emulsification and dispersion of liquids.
46. What is wet milling vs. dry milling?
Wet milling uses liquid medium, reduces dust, and often gives finer particles; dry milling is for moisture-sensitive materials.
47. What factors influence mill selection?
Material hardness, friability, moisture, heat sensitivity, desired particle size, and capacity.
48. What is drying in pharmaceutical context?
Removal of liquid (usually water) from a material by applying heat, resulting in mass transfer to an unsaturated vapor phase.
49. What are non-thermal drying methods?
Expression (squeezing), adsorption (desiccants), absorption (sulfuric acid column), and extraction with solvent.
50. Why is drying important in tablet manufacturing?
To produce stable granules, reduce chemical reactivity, prevent microbial growth, and improve flow and compaction.
51. What is psychrometry?
The study of air-water vapor mixtures, particularly humidity measurement and control.
52. Define absolute humidity.
Mass of water vapor per unit mass of dry air.
53. What is relative humidity?
Ratio of partial pressure of water vapor in air to vapor pressure of pure water at same temperature, expressed as %.
54. What is dew point?
Temperature at which air becomes saturated with water vapor and condensation begins.
55. What is wet-bulb temperature?
Temperature reached by an evaporating surface when heat gain equals heat loss by evaporation.
56. How is humidity measured with a sling psychrometer?
By whirling dry-bulb and wet-bulb thermometers and using psychrometric charts to find humidity.
57. What is the driving force for drying?
Vapor pressure difference between wet material and surrounding air.
58. What is constant rate period in drying?
Period when evaporation rate from surface is constant because surface is saturated.
59. What is critical moisture content?
Moisture content at which constant rate period ends and falling rate period begins.
60. What are falling rate periods?
First falling rate: surface becomes unsaturated. Second falling rate: moisture diffusion from interior controls drying.
61. What is equilibrium moisture content (EMC)?
Moisture content at which material neither gains nor loses moisture under given humidity and temperature.
62. What is water activity (aw)?
Ratio of vapor pressure of water in material to vapor pressure of pure water at same temperature.
63. How does water activity affect stability?
Lower aw reduces chemical reaction rates and microbial growth.
64. What are static-bed dryers? Give an example.
Dryers where solids remain stationary; e.g., tray dryer, truck dryer.
65. What is a tray dryer?
A cabinet with multiple trays where heated air circulates over material.
66. What is a truck dryer?
A tray dryer where trays are mounted on wheeled trucks for easy loading/unloading.
67. What is a tunnel dryer?
A continuous version of truck dryer where trucks move through a heated tunnel.
68. What is a turbo-tray dryer?
A continuous moving-bed dryer with rotating annular trays and central fans.
69. What is fluidized-bed drying?
Drying where hot gas suspends particles, providing excellent gas-solid contact and rapid drying.
70. What are advantages of fluidized-bed dryers over tray dryers?
Faster drying, better thermal efficiency, uniform temperature, and shorter handling time.
71. What is spray drying?
Drying of liquid feeds by atomization into hot gas, producing fine powder.
72. What types of atomizers are used in spray dryers?
Pneumatic (two-fluid) nozzles, pressure nozzles, and spinning disc atomizers.
73. What is spray congealing?
Solidification of molten droplets in cooled air to form solid particles.
74. What is vacuum drying?
Drying under reduced pressure to lower boiling point, suitable for heat-sensitive materials.
75. What is freeze drying (lyophilization)?
Removal of water by sublimation from frozen material under vacuum, preserving heat-sensitive structure.
76. What is microwave drying?
Drying using electromagnetic waves to generate heat internally within the material.
77. What is dielectric drying?
Similar to microwave drying but uses lower frequency RF waves for volumetric heating.
78. What is drum drying?
A method where slurry is spread on a heated rotating drum, dried, and scraped off.
79. What is pneumatic drying?
Drying where particles are entrained and conveyed in high-velocity hot gas (flash drying).
80. What is case hardening?
Formation of hard, impermeable surface layer during drying, trapping interior moisture.
81. What is loss on drying (LOD)?
Weight loss expressed as percentage of initial wet weight after drying under defined conditions.
82. What is moisture content (MC)?
Weight of water expressed as percentage of dry weight of material.
83. Why is MC more useful than LOD for dryer design?
MC directly indicates amount of water to be removed per unit dry product.
84. What is adiabatic saturation temperature?
Temperature reached by air-water system in adiabatic saturation; approximated by wet-bulb temperature for air-water.
85. How does air temperature affect drying rate?
Higher temperature increases humidity differential and heat transfer, speeding drying.
86. What is the role of air velocity in drying?
Higher velocity increases mass transfer coefficient and removes saturated air from surface.
87. What is a cyclone separator in spray drying?
A device using centrifugal force to separate dried powder from exhaust gas.
88. What is chamber product vs. cyclone product?
Chamber product is coarser and collected at chamber bottom; cyclone product is finer from cyclone separator.
89. What is the purpose of bag filters in dryers?
To capture fine dust from exhaust air, preventing pollution and product loss.
90. What safety precautions are needed in fluidized-bed drying?
Static grounding to prevent explosions, dust control, and temperature monitoring.
