By the end of this chapter, you will be able to:
Size reduction is a unit operation in which materials are reduced to coarse particles or fine powder before formulation into suitable dosage forms. Other terms used for size reduction include comminution, grinding, milling, and pulverizing.
Harder the material, more difficult to reduce its size.
Soft but tough material creates problem in size reduction and its toughness is reduced by decreasing temperature.
Gum and resinous substances cause problems in size reduction.
<5% moisture suitable for dry grinding and >50% for wet grinding.
Drying is the process of removal of small amounts of water or any liquids from material by application of heat.
Psychrometry is the science of studying the thermodynamic properties of moist air and the use of these properties to analyze conditions and processes involving moist air.
The Dry Bulb, Wet Bulb and Dew Point temperatures are important to determine the state of humid air. The knowledge of only two of these values is enough to determine the state—including the content of water vapor and the sensible and latent energy (enthalpy).
The dry bulb temperature, usually referred to as air temperature, is the air property that is most commonly used. When people refer to the temperature of the air, they are normally referring to its dry bulb temperature.
The dry bulb temperature refers basically to the ambient air temperature. It is called “Dry Bulb” because the air temperature is indicated by a thermometer not affected by the moisture of the air. It can be measured using a normal thermometer freely exposed to the air but shielded from radiation and moisture.
The Wet Bulb temperature is the temperature of adiabatic saturation. This is the temperature indicated by a moistened thermometer bulb exposed to the air flow.
Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer and the cooling effect is indicated by a “wet bulb temperature” lower than the “dry bulb temperature” in the air. The wet bulb temperature is always lower than the dry bulb temperature but will be identical with 100% relative humidity.
The dew point is the temperature at which water vapor starts to condense out of the air (the temperature at which air becomes completely saturated). Above this temperature, the moisture will stay in the air.
If moisture condensates on a cold bottle taken from the refrigerator, the dew-point temperature of the air is above the temperature in the refrigerator.
Humidity ratio w (kg/kg) of a given moist air sample is defined as the ratio of the mass of water vapor (mw) to the mass of dry air (ma) contained in the sample.
m = mw/ma ……… 1.
When the dry air and water vapor occupy the same volume and temperature, by applying the characteristic equation of state for perfect gas, Eqn. 1. becomes:
W = 0.622 Pw/Pat – Pw ……… 2.
Where:
Pw = partial pressure of water vapor in moist air
Pat = atmospheric pressure of moist air
Relative humidity (RH) is defined as the ratio of the mole fraction of the water vapor (Xw) in a given moist air sample to the mole fraction of water vapor in an air sample of saturated moist air (Xws) at the same temperature and pressure.
RH = Xw/Xws …….. 3.
By definition, the mole fraction of the water vapor (xw) is the ratio of the number of moles of water vapor in a given moist air sample to the total number of dry air and water vapor.
Xw = Nw/Nw+ Na ……… 4.
When the dry air and water vapor occupy the same volume and temperature, by applying the characteristic equation of state for perfect gas, Eqn. 3. becomes:
RH = Pw/Pws ……… (5)
Where Pw = partial pressure of water vapor in moist air
Pws = partial pressure of water vapor in saturated moist air
Relative humidity is usually expressed as percentage (%).
It is the number of pounds of water per pound of dry solid at any given temperature and humidity. This E.M.C. is low for non-porous solids and zero for sand, china clay and higher for fibrous and colloidal organic substances.
It is present as liquid in solids which exert vapor pressure less than of pure liquid at same temperature. The substance containing bound water is called Hydroscopic.
It is amount of water removed from wet solid under given condition. Free moisture content = Total pound of water of dry solid – E.M.C.
It exerts its full vapor pressure and held in voids of solid. Bound and Unbound water depend on property of material itself while E.M.C. depend on particular conditions.
Crystallization is the (natural or artificial) process of formation of solid crystals from a uniform solution. It is also a chemical solid-liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs.
The crystallization process consists of two major events: nucleation and crystal growth.
It is the step where the solute molecules dispersed in the solvent start to gather into clusters, on the nanometer scale (elevating solute concentration in a small region), that becomes stable under the current operating conditions. These stable clusters constitute the nuclei. Nucleation can occur spontaneously or be induced artificially by any foreign surface. These two cases are referred to as homogenous and heterogeneous nucleation respectively.
It is the subsequent growth of the nuclei that succeed in achieving the critical cluster size. It occurs through four stages:
Nucleation and growth continue to occur simultaneously while the supersaturation exists. Super saturation is the driving force of the crystallization; hence the rate of nucleation and growth is driven by the existing super saturation in the solution.
It is well defined curve for any defined condition of heterogeneous nucleation can be established in super saturation zone which is parallel to solubility curve.
It is applicable when solubility depends on temperature like inorganic salt, organic substance. At higher temperature, solution is saturated and at lower temperature solution is supersaturated.
It is applicable to those whose solubility independent to temperature like NaCl.
A. Salting out: It is applied when solubility of substance is very high so super saturation is difficult by method 1. and 2.
B. Precipitation
C. pH change
Evaporation is simply vaporization from surface of liquid. It means the removal of liquid from solution by boiling the liquor in suitable vessel and withdrawing vapor, leaving concentrate liquid residue. Heat supply is latent heat of vaporization.
Greater the surface exposed to evaporation higher will be the rate of evaporation like in film evaporator.
Higher the temperature, higher will be evaporation.
It breaks scum or layer and increase rate of evaporation.
It is a unit operation in which two or more than two components in separately or roughly mixed. So each particle lies as nearly as possible.
It is irreversible mixing and formed from gases and miscible liquid by diffusion process. E.g., Sugar in water.
It requires energy for mixing and difficult to prepare. E.g., Two immiscible liquids are mixed to form emulsion.
It is neither mixing nor de-mixing means no tendency to mix spontaneously or to segregate when mixed.
It requires localized mixing and general movement.
It is a neutral mixture. Three mechanisms may be involved:
E.g., Ribbon mixer gives only convective mixing while Barrel mixer gives diffusion mixing.
The dilatants plastic or materials are difficult to mix than Newtonian liquids.
Filtration is the process of separation of solids from fluid by passing the same through porous medium that retain the solids, but allows the fluid to pass through.
It is similar to sieving, means the particles of larger size cannot pass through the smaller pore size of filter medium.
Solids move with streamline flow and strike the filter medium.
Particle becomes entangled in mass of fiber due to small size of particle than pore size.
Solids are retaining due to attractive forces between particles and filter medium.
Centrifugation is a unit operation employed for separating the constituents present in dispersion with aid of centrifugal force.
Heat flow from high region temperature to lower region temperature. According to principle of thermodynamic, whenever physical or chemical transformation occurs, heat flows into or leaves the system.
When heat flow in body is achieved by transfer of momentum of individual atoms or molecule without mixing. This mechanism is based on Fourier’s law.
Fourier’s law: It states that the rate of heat flow through a uniform material is proportional to the area and temperature drop and inversely proportional to length of path of flow.
A. Forced convection
When mixing of fluid is achieved by use of agitator or stirrer or pumping the fluid for recirculation, such process in heat transfer is called forced convection.
In force convection, the stagnant films (film or surface coefficients) are of great importance in determining rate of heat transfer.
Film coefficient is the quantity of heat flowing through unit area of film for unit drop in temperature.
Factor affecting film coefficients: Thermal conductivity of the liquid, Specific heat of the film, Density of the liquid, Turbulence of the liquid, Thickness of the film
B. Natural convection
Mixing of fluid is accomplished by the currents set up, when body of fluid is heated. Such process is known as natural convection. Fluid circulation caused by change in the density due to temperature difference in the fluid which depends on geometry of the system and shape of vessel in which the fluid is enclosed. This natural convection is observed when extracts are evaporated in open pans.
Radiation is a process in which heat flows through space by means of electromagnetic waves.
Thermal radiation: Heat transfer by radiation is known as thermal radiation.
Stephen-Boltzmann law: It gives the total amount of radiation emitted by black body.
q = bAT4
Where: q = energy radiated per second, A = area of radiating surface, T = absolute temperature of radiating surface, b = constant