SIZE SEPARATION: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future pharmaceutical scientists and manufacturing professionals!
Size separation is a process used to separate particles of different sizes from a mixture. It divides materials into two or more size fractions using sieves or screens. Common names for this process include sieving, sifting, segregation, or classification. The separation depends on size, shape, and density of particles. Interestingly, large particles rise on shaking—this phenomenon is called the Brazil-nut effect.
As a pharmacy educator with years of experience teaching pharmaceutical engineering, I have observed that students often underestimate the importance of size separation in drug manufacturing. Let me tell you: Size separation is critical for ensuring uniform particle size, consistent drug release, and product quality.
In this comprehensive guide, I will walk you through the fundamentals of size separation—its definition, objectives, classification of powders as per IP, the cyclone separator, and sieves including their types, standards, and applications. By the end of this article, you will have a thorough understanding of how size separation works and its importance in pharmaceutical manufacturing. Let us begin!
Dpharmguru’s exam insights:
Size separation is a frequently tested topic in pharmacy exams. Remember: Size separation is also called sieving, sifting, or classification. The separation depends on particle size, shape, and density. Cyclone separator works on centrifugal force. Sieves are classified by number of meshes per 2.54 cm. Powder classification as per IP includes coarse, moderately coarse, moderately fine, fine, and very fine. These are classic exam questions!
INTRODUCTION TO SIZE SEPARATION
Definition
Size separation is a pharmaceutical process in which particles of different sizes are separated and classified into groups based on their size.
Objectives / Applications
- To get powder or granule fractions of narrow size range
- To control particle size and prevent variation in materials
- To classify materials for specific applications
- To improve performance, stability, and uniformity of dosage forms
- To ensure uniform mixing and maintain quality standards of finished products
CLASSIFICATION OF POWDERS (AS PER INDIAN PHARMACOPOEIA – IP)
| Type of Powder | Description (as per IP) |
|---|---|
| Coarse powder | All particles pass through sieve No. 10 (1.7 mm) and not more than 40% through No. 44 (355 µm) |
| Moderately coarse | All pass through No. 22 (710 µm), not more than 40% through No. 60 (250 µm) |
| Moderately fine | All pass through No. 44 (355 µm), not more than 40% through No. 85 (180 µm) |
| Fine powder | All particles pass through No. 85 (180 µm) |
| Very fine powder | All particles pass through No. 120 (125 µm) |
Dpharmguru’s exam insights:
Powder classification as per IP is frequently tested. Remember: Coarse powder passes through No. 10 (1.7 mm). Fine powder passes through No. 85 (180 µm). Very fine powder passes through No. 120 (125 µm). These are classic exam questions!
CYCLONE SEPARATOR
Principle
A cyclone separator works on the principle of centrifugal force. When a gas or liquid containing solid particles enters tangentially, it creates a vortex (spiral flow). The heavy solid particles move outward to the wall and fall due to gravity, while clean air/gas exits from the top.
Construction
A cyclone separator consists of:
- Cylindrical body (barrel) with conical base
- Tangential inlet for dusty air or fluid
- Gas outlet at the top (vortex finder)
- Solid outlet at the bottom
It has no moving parts and is available in various sizes (1–2 cm to 5 m diameter).
Working
- Mixture of solid and air (or gas) enters tangentially at high speed
- Rotary (spiral) motion develops inside
- Centrifugal force pushes heavy particles to walls
- Solids fall down to bottom and are collected
- Clean air/gas exits from the top outlet
Efficiency Depends On
- Particle size and density
- Air velocity
- Cyclone design
- Smoothness of inner wall
Applications
- To separate solids from liquids or gases
- Used in spray drying and fluid bed drying
- For separation based on size, density, or porosity
- Removes fine dust before it reaches central extraction systems in tablet manufacturing
- Used to produce clean, particle-free air in air-handling systems
Dpharmguru’s exam insights:
Cyclone separator is frequently tested. Remember: It works on the principle of centrifugal force. It has no moving parts. Efficiency depends on particle size, density, air velocity, and design. It is used to separate solids from gases or liquids. A common exam question is: “What is the principle of a cyclone separator?” (Answer: Centrifugal force).
SIEVES
Definition
A sieve (or sifter) is a device with small openings used to separate or classify particles based on size.
Construction
- Usually made of stainless steel, brass, or bronze (non-reactive, corrosion-resistant)
- May also be made from nylon or silk cloth for fine powders
- Sieves may have square, circular, or diamond-shaped openings
Types of Sieves
- Wire sieve – woven metal mesh
- Perforated sieve – metal plate with punched holes
- Vibrating sieve – vibrates to separate particles efficiently
- Inclined or circular motion sieve – uses rotating motion for flow and separation
- Static sieve – fixed slope, no moving parts, used for coarse materials
- Horizontal low-head sieve – accurate particle sizing but lower capacity
- Banana sieve (multi-slope) – variable slope for faster and efficient sieving
- Rotary scrubbers – cylindrical drums used to clean and separate coarse particles
Applications of Sieves
- Used for particle size analysis (dry or wet materials)
- Used to separate coarse and fine powders
- Ensures product quality in pharmaceutical production
- Used to prevent foreign body contamination
Standards of Sieves (as per I.P.)
| Term | Meaning |
|---|---|
| Sieve number | Number of meshes per 2.54 cm length |
| Nominal aperture | Size of openings (in mm or µm) |
| Nominal wire diameter | Thickness of wire used |
| Approx. percent sieving area | Percentage of open area for sieving (usually 35–40%) |
| Aperture tolerance | Allowed variation in hole size during manufacture |
Common I.P. Sieve Numbers and Apertures
| Sieve No. | Aperture (mm) | Wire Dia. (mm) | Approx. Area (%) | Aperture Tolerance (%) |
|---|---|---|---|---|
| 6 | 3.812 | 1.422 | 37 | 3.2 |
| 10 | 1.676 | 0.864 | 20.5 | 3.3 |
| 22 | 0.699 | 0.457 | 26 | 4.0 |
| 44 | 0.353 | 0.224 | 34.5 | 4.8 |
| 60 | 0.251 | 0.173 | 37 | 5.2 |
| 85 | 0.178 | 0.122 | 40 | 5.9 |
| 100 | 0.152 | 0.102 | 42 | 6.2 |
| 200 | 0.076 | 0.051 | 47 | 8.2 |
International Sieve Standards
| Country | Standard Code | Type of Sieve |
|---|---|---|
| Great Britain | BS 410 | Woven wire |
| USA | ASTM E11 | Woven wire |
| Germany | DIN 4188 / 4187 | Woven wire / Perforated plate |
| France | AFNOR NFX 11-501 | Woven wire |
| International | ISO R565:1972(E) | Woven wire, Perforated plate |
Dpharmguru’s exam insights:
Sieve standards are frequently tested. Remember: Sieve number means number of meshes per 2.54 cm length. IP sieves include No. 6 (3.812 mm) to No. 200 (0.076 mm). International standards include BS 410 (UK), ASTM E11 (USA), and DIN 4188 (Germany). These are classic exam questions!
COMPARISON: TYPES OF SIZE SEPARATION EQUIPMENT
| Feature | Cyclone Separator | Sieve |
|---|---|---|
| Principle | Centrifugal force | Size-based separation |
| Moving Parts | No moving parts | May be static or vibrating |
| Material | Metal body | Stainless steel, brass, nylon |
| Main Use | Separate solids from gases/liquids | Classify powders by size |
| Application | Spray drying, dust removal | Particle size analysis, powder separation |
FREQUENTLY ASKED QUESTIONS (FAQs)
1. What is size separation?
Size separation is a pharmaceutical process in which particles of different sizes are separated and classified into groups based on their size. It is also called sieving, sifting, segregation, or classification.
2. What is the principle of a cyclone separator?
A cyclone separator works on the principle of centrifugal force. When a gas or liquid containing solid particles enters tangentially, it creates a vortex. Heavy particles move to the walls and fall down, while clean air/gas exits from the top.
3. What are the types of sieves used in pharmaceutical processing?
Common types include wire sieve, perforated sieve, vibrating sieve, inclined/circular motion sieve, static sieve, horizontal low-head sieve, banana sieve (multi-slope), and rotary scrubbers.
4. What is the difference between coarse powder and fine powder as per IP?
Coarse powder passes through sieve No. 10 (1.7 mm) with not more than 40% through No. 44 (355 µm). Fine powder passes through No. 85 (180 µm). Very fine powder passes through No. 120 (125 µm).
5. What are the international sieve standards?
International sieve standards include BS 410 (Great Britain), ASTM E11 (USA), DIN 4188/4187 (Germany), AFNOR NFX 11-501 (France), and ISO R565:1972(E) (International).
6. What is the Brazil-nut effect?
The Brazil-nut effect is a phenomenon where large particles rise to the top when a mixture of particles is shaken, while smaller particles settle at the bottom. This is due to differences in particle size and density.
SUMMARY
Size separation is a fundamental unit operation in pharmaceutical manufacturing. This guide covered:
- Definition: Size separation divides particles into groups based on size
- Objectives: Control particle size, improve uniformity, and ensure product quality
- Powder Classification: As per IP – Coarse, Moderately Coarse, Moderately Fine, Fine, Very Fine
- Cyclone Separator: Works on centrifugal force; no moving parts; used for solid-gas/liquid separation
- Sieves: Types include wire, perforated, vibrating, and banana sieves; used for powder classification
- Sieve Standards: IP sieves from No. 6 to No. 200; international standards include BS 410, ASTM E11, DIN 4188
As I always tell my students: “Size separation is the key to uniformity—it ensures that every tablet, capsule, and powder contains the right amount of active ingredient.”
REFERENCES AND FURTHER READING
- Pharmacy Council of India (PCI). (2022). Pharmaceutics Syllabus. New Delhi: PCI.
- Indian Pharmacopoeia Commission (IPC). (2018). Indian Pharmacopoeia (8th ed.). Ghaziabad: IPC.
- Aulton, M. E., & Taylor, K. M. G. (2018). Aulton’s Pharmaceutics: The Design and Manufacture of Medicines (5th ed.). Elsevier.
- Subrahmanyam, C. V. S. (2019). A Textbook of Pharmaceutics. Vallabh Prakashan.
- Lachman, L., Lieberman, H. A., & Kanig, J. L. (2018). The Theory and Practice of Industrial Pharmacy (4th ed.). CBS Publishers.
- United States Pharmacopoeia (USP). (2020). USP-NF. Rockville: USPC.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns. Pharmaceutical regulations and guidelines may vary by region—always refer to your local regulatory authorities for specific requirements.
written by:
Dr. N. Sujith Kumar
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