Dosage Form Design-1 Unit Operations PDF

Dosage form design-1  Unit Operations This lecture includes a brief account of:  Introduction to Unit Operations  Factors affecting the transfer of mass from a solid to a fluid.  Factors affecting the transfer of mass from a fluid to a fluid.  The effect of boundary layer.  The i...

Dosage form design-1  Unit Operations This lecture includes a brief account of:  Introduction to Unit Operations  Factors affecting the transfer of mass from a solid to a fluid.  Factors affecting the transfer of mass from a fluid to a fluid.  The effect of boundary layer.  The influence of mass transfer phenomena on the unit operations.  Unit Operations Introduction  Every industrial chemical process is based on Unit Operations (physical treatment) and Unit Process (chemical treatment) to produce economically a desired product from specific raw materials.  Unit Operations  The raw materials are treated through physical steps to make it suitable for chemical reaction.  So, knowledge of unit operations like ‘Mixing and agitation of liquid’ and’ heat flow’ is very much necessary.  The subject Unit Operations is based on fundamental laws and physicochemical principles.  Unit Operations  Unit Operations gives idea about science related to:  Specific physical operation;  Different equipment-its design, material of construction and operation; - Calculation of various physical parameters (mass flow, heat flow, mass balance, power and force etc.).  Unit Operations Examples of Unit Operations  Heat flow  Fluid flow  Mixing  Drying  Absorption  Evaporation  Adsorption  Distillation  Unit Operations  Condensation  Crystallization  Vaporization  Separation  Extraction  Sedimentation  Filtration  Crushing Mass transfer Solid/Fluid mass transfer  When considering a crystal of a soluble material immersed in a solvent in which it is dissolving.  A situation will exist where the crystal is surrounded by a stationary boundary layer of the solute, with the bulk of the fluid able to move. Solid/Fluid mass transfer  Such movement could be:  Natural convection: arising from temperature or density changes.  Forced convection: resulting from agitation. Solid/Fluid mass transfer  Hence, transport of the molecules of the dissolving solid will take place in two stages:  First, the molecules move through the boundary layer by molecular diffusion, with no mechanical mixing or movement, a process that is analogous to heat transfer by conduction. Solid/Fluid mass transfer  Second, once material has passed through the boundary layer, mass transfer take place by bulk movement of the solution, known as eddy diffusion, and analogous to heat transfer by convection. Solid/Fluid mass transfer  Since there is virtually no limit to the vigour of the movement of the bulk of the fluid, the controlling factor in the rate of solution of the crystal will be the molecular diffusion through the boundary layer.  Eddy diffusion will not be considered further.  In general molecular diffusion is the controlling process. Solid/Fluid mass transfer  Mass transfer by molecular diffusion can be represented by an equation similar to conduction heat transfer, in which: w=DA(C1 -C (2 θ/L Or W =DA(C1 -C 2 )/L w= weight of solute diffusing; W= weight of solute diffusing in unit time; D= diffusion coefficient; A= area; θ = time; C1 = concentration of solute at the interface; C2 = concentration of solute in bulk; L= film thickness. Solid/Fluid mass transfer  The term fluid includes gases and vapours as well as liquids.  The preceding discussion can refer equally to mass transfer from a solid to a gas.  As example, if a solid is drying in air, the vapour molecules must diffuse through the air boundary layer to the atmosphere.  The driving force in this case will be the partial vapor pressure gradient through the air boundary layer. Solid/Fluid mass transfer  A similar equation can be written for a vapour: W= DA(P1 -P 2 )/L Where P1 = partial pressure of vapour at the interface; P2 = partial pressure of vapour in the atmosphere. Fluid/Fluid mass transfer  An equivalent situation occurs when mass transfer takes place between two immiscible fluids.  This may be two liquids or a liquid and a gas or vapour.  In this case, there will be boundary layers of both fluids on each side of the interface, where the slope of the concentration gradients depends on the diffusion coefficients of the two materials. Influence on Unit Operations  Mass transfer theory can be applied to any operation in which material changes phase, whether it is:  Solid/liquid.  Solid/gas (vapour).  Liquid/liquid.  Liquid/gas (vapour). Influence on Unit Operations  The effect can be seen in simple operations, such as making a solution of a solid in liquid.  The rate of solution can be increased by: 1) Agitation:↓ the thickness of the boundary layers and disperses any local concentrations of solutions, so↑ concentration gradient. Influence on Unit Operations 2) Elevated temperature: ↑ the solubility of most materials, ↑ the diffusion coefficient and ↓ the viscosity of the liquid so ↓ boundary layer thickness. 3) Size reduction of the solid: ↑ the area over which diffusion can occur. Influence on Unit Operations  Solvent extraction of drugs and drying a solid in a stream of air are another examples.  The most complex case arises when a liquid of two components is required to reach equilibrium with a vapour consisting of the same two components. Influence on Unit Operations  This may occur in distillation, and can involve counter- current diffusion, that is, both components may diffuse in opposite directions to each other.  Distillation additionally includes simultaneous heat and mass transfer, which complicates the situation further. Design of Mass Transfer Equipment  Design of mass transfer equipment must take the following considerations: 1. Turbulent flow conditions. 2. Maximum concentration or partial pressure gradients. 3. Largest possible surface area. Thanks