6. Extraction Liquid Solid

The term extraction is derived from the Latin word extrahere. Ex specifies

the direction, i. e., out of, and trahere describes the action, namely drawing or

removing. Extraction is defined as the process of removing a substance or several

substances from another substance.

The process is extremely important in a wide range of technical applications, for

instance biotechnology, the pharmaceutical and food industries as well as

environmental protection. Extraction is a separating process which has the

advantage of low energy consumption.

In numerous areas of application, extraction is the more efficient, more selective

and less expensive alternative compared with competing separating methods

such as distillation, evaporation and membrane technology. Extraction has

become established in conjunction with the following process conditions:

1. Minor boiling point differences of the components to be separated or aceotropic

separation, e. g., separation of isomers, aromatic substances or aliphates.

2. Heat-sensitive or unstable substances, e. g., antibiotics.

3. Non-volatile substances, recovery and purification of catalysts or heavy metals.

4. Mixtures with inorganic components which would result in encrustation of

evaporator surfaces in conjunction with the thermal separating process.

5. Separation of low mass contents of a component which is not readily volatile.

Solid-Liquid extraction now a day is one of the mostly used methods which

accompany with other methods as globalism becomes wider and wider in all

corners of the world.

Solid-liquid extraction allows soluble components to be removed from solids

using a solvent. Applications of this unit operation include obtaining oil from oil

seeds or leaching of metal salts from ores.

An everyday example is the preparation of coffee. Here, water (solvent) is used

to remove the coffee flavors (transition component) from the coffee powder

(extraction material, consisting of solid carrier phase and transition component).

Ideally, this results in drinkable coffee (solvent with dissolved flavours), with

the completely depleted coffee grounds (solid carrier phase) remaining in the

coffee filter.

In reality, the solid carrier phase will still contain some transition component

after completion of the extraction. In addition, some of the solvent will still be

adsorptively bonded to the solid carrier phase.

To achieve the fastest and most complete solid extraction possible, the solvent

must be provided with large exchange surfaces and short diffusion paths. This

can be done by pulverizing the solid to be extracted.

However, an excessively small grain size can cause agglutination and make it

more difficult for the solvent to permeate.

In the simplest form of this unit operation,

the extraction material and the solvent are

mixed well. The solvent and the dissolved

transition component are then removed and

regenerated.

The extraction material can also take the

form of a fixed bed with the solvent flowing

through it. In a further form of the

application, the extraction material is led

through the solvent.

Figure 1. Displacement curves for each extraction method reveal that combined liquid

(ether) and solid-phase extraction (SPE) method improves E2 assay sensitivity at lower

concentrations (i.e.,

7. Measurement of Relative Diffusion for two phases flow

Diffusion is Movement of a fluid from an area of higher concentration to an

area of lower concentration. Diffusion is a result of the kinetic properties of

particles of matter. The particles will mix until they are evenly distributed.

Or Diffusion is the process in which molecules move from a higher

concentration to a lower concentration. This process happens at random.

For example: When a test tube containing Hydrogen Sulfide (H2S) leaved

for a while, the H2S will slowly diffuse into the air of a lab until equilibrium

is reached.

Diffusion has following properties:

Spontaneous movement of particles from an area of high concentration

to an area of low concentration.

Does not require energy (exergonic).

Occurs via random kinetic movement.

Net diffusion stops when concentration on both sides equal (if crossing a

membrane) or when there is a uniform distribution of particles.

Diffusion can be divided into two general types:

1. Absolute Diffusion:

Absolute diffusion or single particle diffusion is used to describe the average

movement of a particle with time, and can for instance be studied in this form:

2. Relative Diffusion

The term relative diffusion is used to describe relative motion of pairs of

particles viewed as a diffusion process. Richardson explained the importance

of using relative, rather than absolute, motion in turbulence studies. His main

intention was to separate the turbulent variations in the velocity field from the

average velocity field.

We can look at relative diffusion in a form similar to that used for absolute diffusion

Two phase flow in porous media is governed by capillary and viscous

forces, and their relative magnitude governs the two phase distribution and

flow regimes. Two phase flow is designated as a drainage process if the

invading fluid is non-wetting and an imbibition process otherwise. Liquid

water transport in a hydrophobic GDL is thus essentially a drainage process.

Lenormand proposed a phase diagram, illustrated in Fig.2, to describe

displacement of a wetting phase by a non-wetting phase in the absence of

buoyancy forces. They found that immiscible displacement is governed by

capillary number, Ca, and viscosity ratio, M, defined as

Where subscripts nw and wet stand for the non-wetting and wetting

phase, respectively, u is the velocity of non-wetting phase and is the surface tension.

Fig.2 Schematic

representation

of phase

diagram

showing various

flow regimes

and

characteristic

distributions of

non-wetting

phase for these

regimes.

In brief, the relative diffusion can be also represented by figures or curves

to be more illustrated, hence there are some curves were derived from the

experiments and according to their specific mathematical calculations has

been reported. Each of them is different from another which means that the

diffusion of each compound differs from another one, according to their

nature.

8. Measurement of Relative Diffusion for steady state fluid flow

In chemistry, a steady state is a situation in which all state variables are

constant in spite of ongoing processes that strive to change them. For an

entire system to be at steady state, i.e. for all state variables of a system to

be constant, there must be a flow through the system (compare mass

balance). A simple example of such a system is the case of a bathtub with

the tap running but with the drain unplugged: after a certain time, the water

flows in and out at the same rate, so the water level (the state variable

Volume) stabilizes and the system is in a steady state. The term steady state

is also used to describe a situation where some, but not all, of the state

variables of a system are constant. For such a steady state to develop, the

system does not have to be a flow system.

Diffusion is process which is NOT due to the action of a force, but a

result of the random movements of atoms (statistical problem)

- Consider diffusion of solute atoms (b) in solid state solution (AB) in X-axis between two parallel atomic planes (separated by x).

- If there is no changes with time in CB at these planes such diffusion condition is called steady-state diffusion.

Above equation called Fick`s first law which

J flux of atoms, atoms/(m2 s): the number of particles which pass through a unit area in a unit of time;

D diffusivity or diffusion coefficient, m2/s dC/dx concentration gradient, atoms/m4

Diffusivity D depends on:

1. Diffusion mechanism

2. Temperature of diffusion

3. Type of crystal structure (bcc > fcc)

4. Crystal imperfections

5. Concentration of diffusing species

9. Measurement of Relative Diffusion for non-steady state fluid

flow

In practice the concentration of solute atoms at any point in the material

changes with time non-steady-state diffusion.

For non-steady-state condition, diffusion coefficient, D - NOT dependent

on time:

The rate of compositional change is equal to the diffusivity times the rate

of the change of the concentration gradient.

Change in concentration in 2 semi-

infinite rods of Cu and Ni caused by

diffusion, from G. Gottstein

Physical Foundations of Material Science

With specific initial or boundary conditions this partial differential

equations can be solved to give the concentration as function of spatial

position and time c(x, y, z, t).

Let us consider two rods with different concentrations C1 and C2 which are

joined at x=0 and both are so long that mathematically they can be considered

as infinitely long.

The concentration profile

at t = 0 is discontinuous at x = 0:

10. Separation of GasLiquid using sound wave, determine the surface between Gas-Liquid

The presence of free gases in liquids can markedly alter the results as well

as complicate analysis regarding the prediction of water hammer pressure.

Gases may be present either in the dissolved or the entrained state, or both, in

cooling water system of fossil fuel and nuclear power stations, in sewage

pumping lines, or crude oil lines. The effect of compressibility of any free gas

on wave propagation speed, and on the resulting pressure changes must be

considered in any transient analysis for which even the smallest amount of gas

may be present. If the pressure changes during the transient lower the pressure

to, or near to, the saturation vapor pressure of the liquid, large quantities of gas

dissolved in the liquid may come out of the solution and considerably alter the

wave propagation speed.

The possibility of gas-mixture separation in the field of a traveling sound

wave was first investigated by P. Passau. The basic mechanism producing the

spatial separation of particles of different mass was assumed by P. Passau to be

barodiffusion, and the reason for the appearance of a time-averaged pressure

gradient in the traveling wave was considered to be the attenuation of sound

by dissipative volume processes. The true nature of the separation of the 1:1

mixture of CO2 and H2, has not been entirely clear: the process took hours to

settle, and the magnitude and sign of the effect did not fit the theoretical ideas.

The separation of mixtures by barodiffusion is conveniently described in terms

of an individual particle when the sound is excited in a light gas containing a

small heavy-particle impurity. Friction forces exerted on a test particle by

vibrating light molecules makes the heavy-particle concentration distributed in

space in accordance with the Boltzmann law

In which M is the mass of the test molecule and fi is the velocity of light

particles in the acoustic wave (the bar represents time-averaging over one

period of the acoustic waves).

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A mathematical model determining the propagation of sound waves in the

two-fraction mixture of a liquid with polydisperse vapor-gas and gas bubbles

with account of phase transformations is presented. The system of integro-

differential equations governing the disturbed flow of the two-phase mixture

is written, the dispersion equation is derived, and the equilibrium speed of

sound is determined.

The equilibrium speed of sound is shown to decrease with increase in the vapor

concentration. The theoretical predictions are compared with the available

experimental data on the phase velocity in the water with vapor bubbles and in

the mixture of Freon with vapor bubbles.

In the presence of any gas in liquid there is a region which separate the particles

of gas from the liquid, called Interface.

If two homogeneous bulk phases meet there is a region of finite thickness

where the properties changed. That region is called interface.

At a molecular level the thickness of the interfacial region is not zero, and it is

significant!

The properties of interfacial region can be important for colloid systems,

especially for dispersions, where the surface to volume ratio is not negligible.

The attractive forces acting on molecules at the surface are anisotropic, the net

force is oriented toward the liquid phase.

As a consequence, liquids tend to reduce their surface. Energy is required to

increase the surface to overcome the attraction.

The gas-liquid interface plays a crucial role in the transport of non-aqueous

phase liquids (NAPLs), colloids, bacteria, and other contaminants in

unsaturated porous media since these components often tend to adsorb to

interfacial surfaces. In two-fluid systems (e.g., solid, liquid, and gas phases),

the maximum degree of gas-liquid interfacial area available for a particular

porous medium occurs at, or near, residual liquid saturation.

Standard analytical methods are available for measuring the surface area of a

porous medium based on molecular adsorption and the resulting physical and

chemical responses. However, these measurements include the surface area of

internal micro-pores and other small defects in the particle grains and can

yield surface areas that may be much larger than those pertinent to fluid flow

and contaminant transport.