Distillation

Principle of distillation

Distillation defined as the technique of preferential separation of the more volatile components from the less volatile ones in a feed solution by partial vaporization of the feed followed by condensation (Binay K.Dutta). Component inside the distillation column is in two phases where it is governed by the vapor-liquid equilibrium relationship. Large degree of separation can be achieved when there is a multistage contact between the vapor and the liquid phases.

The principles of distillation column start when feed enters at a suitable point of the tower. Exchange of mass normally takes place between the liquid and the vapor phase. The more volatile components move from the liquid to the vapor phase and the less volatile move from the vapor to the liquid phase. The heavy liquid flows down as the bottom product then reboiler normally heated by steam will partially vaporize the liquid. The light vapor is withdrawn as the top product by entering the condenser and the rest is fed back into the distillation column as reflux which flows down the trays and packing.

Driving force of distillation column

Driving force can be generated by various techniques related to different chemical or physical properties. In distillation the driving force is the difference in composition between the vapor phase and liquid phase which is caused by a difference of properties such as vapor pressure.The present components from an ideal mixture in both phases and the vapor or liquid equilibrium can be described with Raoult’s law (Zuric, Feb 6, 2014). Raoult’s law states that the vapor pressure of a solution is equal to the sum of the vapor pressures of each volatile component if it were pure multiplied by the mole fraction of that component in the solution. Raoult’s law only works for ideal solutions (Binay K.Dutta). Expression of Raoult’s law is written below for two components. Normally both components can be known as A and B to give the equilibrium partial pressure in a binary liquid mixture.

If x is the mole fraction of A in the binary solution, that of B is (1-x).

PA* = xPAv and pB*= (1-x)PBvThe total pressure: P=PA* + PB* = xPAv + (1-x) PBvPA* and PB* are the equilibrium partial pressure of A and B in the vapor, PAV and PBV are the vapor pressure of A and B at the given temperature. The mole fraction of A in the vapour is given by

yA* =pA* /P = (xPAv)/P

This equation can be used to calculate the vapor-liquid equilibrium data (x-y*) for an ideal binary mixture. In distillation, as the driving force approach zero, separation of the component from the mixture becomes difficult while compare when the driving force approach maximum, the energy necessary to maintain the two phase system is minimum.

The driving force as defined by Gani and Bek-Pedersen given by:

Fij = yi – xi = xi?ij1+xi(?ij-1) – xi

In the above equation, xi and yi are the composition of component I in two co-existing phases, Fij is the driving force for component i, ?ij is the relative separability factor for component I with respect to property j.?ij = f(T, P, composition, ?) where ? indicates external factors such as resistance to mass transfer and heat transfer.

According to Bek-Pedersan the driving force diagram can be use to obtain near optimal sequence of distillation trains. These diagram has been modified and extend to allow the presence of azeotropes in the multicomponent mixture to generate hybrid separation schemes and to allow for scaling in the distillation column design when extreme condition for the feed mixture or composition are specified.

Drying

Principle of drying

Drying is the process where liquid is removed from the solid material (Oldrich Hole?ek). In mass transport point of view, drying is understood as a diffusion process. Drying process can be divided into three parts which is first warm-up period. During this period material starts to be heated from initial temperature tp to the wet bulb temperature. Evaporation start to take place from the moisture surface. Mass and heat transfer are non-zero. Water was decrease in its concentration. Warm up period usually happened in short time and sometimes can be non-catchable. Second part is first drying period or known as (constant-rate period of drying). At this point, temperature of the material is equal to the wet thermometer temperature and stays constant. First drying period can get the moisture gradient inside the material. Lastly is the second drying period where water is transported inside material to the surface (internal diffusion). The drying rate starts to decrease. The temperature of the material is rising and causes decreasing of the mass and heat transfer driving forces. The forces controlling the vapor diffusion determine the final rate of drying and these are largely independent of the conditions outside the material.

Driving Forces of drying process

In drying process the partial pressure differential is the driving force which means that is the difference between the material’s internal moisture vapor pressure and the partial pressure of the moisture in the surrounding drying gas. Mass and heat transport proceed together during drying. This can be seen based on the process determining overall process rate (the slowest process).

The mass transfer driving force (which is equal to the concentration gradient) can be used for description of drying. The drying rate expressed as an intensity of the mass flow can be defined as

?A = d2mAdAd?Drying rate normally can be described by gas-phase mass transport equation in form

?A= kY(YAW-YA)

Where kY is the mass transfer coefficient based on the driving force defined by relative mass fraction of the moisture in air, YA represent the average moisture in air and YAW is the moisture content on the boundary between air and saturated material. Another possible definition of the drying rate should be using partial pressure of the vapors in air

?A = kp (pAW-pA)

Freeze drying is one of the general methods of drying where water is sublimed from frozen material. During a steady state freeze drying process the heat flux can be defined as the conduction of heat transfer. Where the formula is shown below:

q =h(Te-Ts) = k?L (Ts-Tf)

q = heat flux (j/s)

Te = external temperature

Ts = surface temperature of dry material (C)

Tf = temperature of sublimation on ice front(C).

k = thermal conductivity

?L = thickness of dry layer

However during steady state freeze drying process, the mass flux can be defined as following:

Na = D’RT?L (pfw –psw) = kg (psw-pew)

Na = mass flux in kg.mol/s.m2

kg = external mass transfer coefficient kg.mol/s.m2.atm

psw = partial vapor pressure on the surface of dried later in atm pew = external partial vapor pressure in atm pfw = partial vapor pressure at ice front

D’=diffusivity in dry layer m2/s

References

Design and Synthesis of Distillation System Using a Driving Force Based Approach, Erik Bek- Pedersen and Rafiqul Gani.

Binay K.Dutta, Principle of Mass Transfer and Separation Process, 2009

Andrzej Gorak. Distillation Fundamental and Principles, 2014

Drying, Oldrich Holecek

Drying Basics, John J. Walsh