helkewt
Post on 07-Jan-2016
20 Views
Preview:
DESCRIPTION
TRANSCRIPT
-
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).
-
Above graph is ( Sulzer ) which is a major player in the field of gas/liquid and liquid/liquid separation technology, offering a full range of innovative products and related services.
Our commitment to development of technology, combined with application knowhow and
consistent fabrication standards ensures that a well-engineered solution is available for most
separation problems.
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.
top related