Introduction to Flow of Fluids, Size Reduction & Size Separation
Unit 1 covers fundamental engineering principles governing the movement of liquids in pharmaceutical plants (fluid flow) and the operations that control particle size – critically important for drug dissolution rate, content uniformity, and bioavailability.
Syllabus & Topics
- 1Flow of Fluids: Viscosity, density, and their effect on flow.
- 2Types of Flow: Laminar (Newtonian, streamlined, Re<2100) and Turbulent (Re>4000).
- 3Types of Manometers: Simple (U-tube), Differential, Inclined manometers – principle and applications.
- 4Reynolds Number: Re = ρvD/µ. Significance: predicts flow regime. Re < 2100 = laminar; 2100-4000 = transitional; > 4000 = turbulent.
- 5Bernoulli’s Theorem: In an ideal fluid, P/ρg + v²/2g + h = constant (total energy per unit weight is constant). Applications: Venturimeter, Pitot tube, flow nozzles.
- 6Energy Losses: Friction losses in pipes (Darcy-Weisbach equation), minor losses (bends, valves, fittings).
- 7Flow Measuring Devices: (1) Orifice meter – constriction plate, permanent pressure drop, cheapest. (2) Venturimeter – constriction section with recovery, less pressure drop, expensive. (3) Pitot tube – measures velocity at a point, used for gas flow. (4) Rotameter (Variable Area Flow Meter) – float in a tapered tube, suitable for liquids and gases.
- 8Size Reduction: Objectives (increase surface area, improve dissolution, improve content uniformity, prepare powder mixtures).
- 9Mechanisms of Size Reduction: Impact, Attrition/Abrasion, Cutting, Compression.
- 10Laws of Size Reduction: Kick’s Law (coarse grinding, P ∝ log(Df/Dp)), Rittinger’s Law (fine grinding, P ∝ 1/Dp – 1/Df), Bond’s Law (intermediate – geometric mean).
- 11Factors Affecting Size Reduction: Feed size, hardness, moisture content, heat sensitivity, abrasiveness.
- 12Size Reduction Equipment:
- 13Hammer Mill: Rotating hammers crush material; produces coarse to fine powder; suitable for hard, dry materials (spices, drugs); M&D: simple, low cost / over-heating possible.
- 14Ball Mill: Steel balls in rotating cylinder; impact and attrition; batch or continuous; for soft/hard materials; suitable for wet and dry milling; produces very fine powder.
- 15Fluid Energy Mill (Jet Mill): High-velocity gas jets cause particle-particle collisions; produces ultra-fine (<10 µm) particles; no mechanical parts; contamination-free; suitable for heat-sensitive materials.
- 16Edge Runner Mill (Muller mill): Heavy rollers on a circular track; crushing and attrition; for tough, fibrous materials.
- 17End Runner Mill (Pestle-mortar type): Rotating pestle in bowl; mixing and size reduction.
- 18Size Separation: Objectives (ensure uniform particle size for consistent formulation), mechanisms (vibration, gravity, centrifugal force).
- 19Official Standards: IP/BP/USP sieve numbers and mesh sizes.
- 20Size Separation Equipment: (1) Sieve Shaker (Rotap machine) – vibrating sieves for size analysis. (2) Cyclone Separator – centrifugal separation of fine particles from air; continuous operation. (3) Air Separator – elutriator type; separates by particle size/weight. (4) Bag Filter – collects fine powder from air streams; used with mills. (5) Elutriation Tank – wet separation by upward liquid flow.
Learning Objectives
Frequently Asked Questions (FAQs)
Q1. What is Reynolds Number and Why is it Important?
Reynolds Number (Re) = ρvD/µ (density × velocity × diameter / dynamic viscosity). It is a dimensionless number used to predict the flow regime of fluids. If Re < 2100, the flow is laminar (parallel streamlines); between 2100–4000, the flow is transitional; and if Re > 4000, the flow is turbulent (chaotic). In sterile fluid manufacturing lines, laminar flow is preferred to minimize shear stress on sensitive biological products.
Q2. What are the Advantages of a Fluid Energy Mill Over a Ball Mill?
Fluid Energy Mill (Jet Mill): (1) Produces ultra-fine particles (1–10 µm), which are difficult to achieve with ball mills. (2) No moving mechanical parts, so minimal contamination. (3) Suitable for heat-sensitive drugs since air acts as the grinding medium with minimal heat generation. (4) Sterile operation possible. Disadvantages: High energy consumption and high cost.
Q3. What are the Different Mechanisms of Size Reduction?
(1) Impact: Particle struck by rapidly moving hammer or ball; suitable for hard, brittle materials. (2) Attrition/Abrasion: Rubbing of particles against rough surfaces; produces fine material from soft substances. (3) Cutting: Shearing action with blades; used for fibrous materials. (4) Compression: Squeezing between two surfaces; suitable for hard and tough materials. Most mills use a combination of these mechanisms.
Q4. Explain How a Cyclone Separator Works.
Feed air containing particles enters tangentially into a conical chamber. The rotating air generates centrifugal force, pushing larger and heavier particles toward the outer wall and downward into the collection hopper. Clean air with very fine particles exits through the central vortex tube at the top. The system has no moving parts, allows continuous operation, and requires low maintenance.
Q5. What is the Difference Between Kick’s and Rittinger’s Laws?
Kick’s Law states that the energy required for size reduction is proportional to the logarithmic ratio of initial to final particle size and is most applicable for coarse grinding. Rittinger’s Law states that energy is proportional to the new surface area created and is best suited for fine grinding. Bond’s Law provides an intermediate approach and is most commonly applied in practice. All three laws are theoretical approximations, and actual energy requirements are determined experimentally.