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Student Name: Clinton Oru
Student Number: 1705179
Module Title: Units Operations and separations process, 4ET 012
Assessor: Professor Chike F Oduoza
1)
a) Air Stripping
In Air stripping, liquid such as waste water is moved with air, so that the unwanted volatile
contaminants present in the liquid phase can be released and carried off by the gas. The movement
of the air would cause the volatiles to evaporate at a faster rate. Examples of these VOCs (Volatile
Organic Compounds) are benzene, ethyl benzene, toluene and xylene.
Air stripping simply works by using an air stripper to force air through contaminated water and
evaporate the volatile organic compounds. There are two types of air strippers, these include sieve
tray system and packed tower system.
A sieve tray system is the less common air stripper. See Figure 1 below.
(Fischer, 2015)
In a sieve tray system, contaminated water is pumped to the top of the tank, it flows over an inlet
weir, moving from one tray to another. The inlet weir acts like a sieve. Air is bubbled upwards
through holes in the trays which then creates a turbulence to prevent the water and air from having
contact. The VOCs (Volatile Organic Components) are transferred to the vapor phase for removal.
The treated water will then exit at the bottom after it goes pass the last tray (Rousseau, 1987).
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A packed tower system is the most popular air stripper because it is economical, efficient and
requires less pressure drop. Here, Contaminated water is pumped downwards through the column
via gravity. This is done through either a randomly or structured packed material like steel, plastic or
ceramic. At the Same time, air flows up through the column, this is a counter- current flow. As the
water and the air pass each other in a counter-current flow, the VOCs are then transferred from the
water phase into the air phase. The water phase leaves the bottom of the column having most of the
volatile organic components removed. The volatile organic components in the air phase exit from
the column top (Leibbert). See Figure 2 Below.
A factor that can influence air stripping is volatility. Volatility is the tendency of a compound to
evaporate under normal atmospheric conditions. The efficiency of air stripping is limited by the
volatility of the contaminant. When it is more volatile, it is more likely to become a gas and it is
extremely efficient at removing volatile organic compounds (VOCs).
Potential Applications of air stripping in the chemical industry
o
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Air stripping can be used to powdered activated carbon adsorption of geosmin and 2methylisoborneol to remove odour causing compounds known for odour in drinking water
(Jung, 2010)
Chemical industries also use air stripping to remove ammonia in the form NH3. The
ammonia content of a wastewater stream is lowered.
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b) Leaching
Leaching is a liquid-solid operation. It is the separation of a solute from solid mixture by dissolving it
in a liquid phase. In leaching, the liquid is very important as it facilitates the ability to extract a given
substance from a material.
During the leaching process, the solvent is transferred from the bulk solution to the surface of the
solid, where the solvent diffuses into the solid. The solute dissolves from the solid into the solvent.
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The solute diffuses through the mixture to the surface of the solid and it is transferred to the bulk
solution.
Leaching process can be either batch, semi batch, or continuous. It usually operates at high
temperatures to increase the solubility of the solute in the solvent. The Feed to a leaching system is
usually solid made of insoluble carrier material and a soluble compound. The feed must be prepared
by grinding or chopping. It is then mixed with a liquid solvent. The desired material dissolves and so
leaves when the liquid is drawn off as overflow.
Potential Applications of Leaching in the chemical industry
o
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Biological and food processing industries use leaching for the separation of sugar from sugar
beets with hot water.
In the metals processing industry, leaching is used to remove the metals from their ores,
which contains many undesirable constituents such as solute salts.
Gold is also leached from its ore using an aqueous sodium cyanide solution.
c) ion exchange chromatography
Ion exchange chromatography is the process of separating biomolecules based on their total charge.
In this case, the liquid phase is the crude sample containing charged molecules. When it passes
through the chromatographic column, molecules bind to oppositely charged sites in the stationary
phase (Healthcare, 2016).
The mechanism for this process is as follows; an impure protein sample is loaded into the ion
exchange chromatography column at a certain pH, charged proteins will bind to the oppositely
charged functional groups in the resin. A salt gradient is used to elute separated proteins. At low salt
concentrations, proteins having few charged groups are eluted and at higher salt concentrations,
proteins with several charged groups are eluted. The unwanted proteins and impurities are removed
by washing the column (Healthcare, 2016).
Potential Applications of ionic exchange chromatography in the chemical industry
o
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Ion exchange chromatography can be used for separation and Purification of blood
components.
Ion exchange chromatography can also be used during Fermentation as cation exchange
resins are used to monitor the fermentation process.
It can also be used for the analysis of production equipment cleaning solutions, waste
streams and container compatibility (Acikara, 2013).
d) Centrifugation
Centrifugation is a process of separating substances by using a centrifugal force provided by a
centrifuge. The centrifuge separates the particles based on their size, shape, density, viscosity and
rotor speed.
During this process, particles with a higher density will sink and lighter particles will float to the top.
The greater the difference in density, the faster they move. If there is no difference in density the
particles will stay steady and objects that are less dense are displaced and moved to the centre.
There are four type of centrifugation. These include; Density Gradient Centrifugation, Rate-Zonal
Density-Gradient Centrifugation, Isopynic Centrifugation and Differential Pelleting (differential
centrifugation).
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Potential Applications of centrifugation in the chemical industry
o
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The most common application centrifugation is to separate two miscible substances
Centrifugation can be use in removing fat from milk to produce skimmed milk
It can also be used in the clarification and stabilization of wine
And also to purify mammalian cells.
2)
a) Distillation is a physical process for the separation of liquid mixtures that is based on differences
in the boiling points of the constituent components.
The mechanisms of a distillation operation involve vaporization, condensation, and the collection of
the distillate. See figure 3 below for an example of a distillation unit with a single feed.
(Pati)
The liquid feed mixture is introduced into the feed tray. This feed tray divides the column into two
sections, enriching or rectification section and the stripping section. The feed flows down to the
column and it is collected in the reboiler. In order to generate vapour, heat is supplied to the
reboiler (the source of heat for this is usually steam). This vapour is brought back into the unit at the
bottom column and the liquid removed from the reboiler is called the bottoms product. The vapour
moves up the column to exit the top of the unit where a condenser cools it down, this condensed
liquid is stored as the reflux drum (holding vessel). The liquid that is recycled back to the top of the
column is known as the reflux meanwhile the condensed liquid removed from the system is the
distillate (Kister).
b) Industry scale chemical engineering processes which apply the principles of distillation
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Oil Refining; Crude oil can be separated into fractions by fractional distillation. This process takes
place in a fractionating column. See figure 4 below.
(Irwin, 2007)
The crude oil is heated to a high temperature of about 400 degrees Celsius inside a heater and it is
fed into the base of the fractionating column filled with many trays. Here, a large percentage of the
crude oil will turn into vapour. The vapour will then elevate in the column through various with
bubble caps which eases the collection of liquids that are formed as a result of condensation. As
vapour rises through the fractionating column, the vapour forces through the bubble caps and
Bubbles through a condensed liquid. The bottom of the tower has high temperatures as opposed to
the top which has low temperatures.
The vapour will condense into liquid when certain vapours reach a fraction in the column with a
temperature similar to the boiling point of the substance in the vapour. The substances with lower
boiling points will condense at a higher fraction in the column whereas substances with higher
boiling points will condense lower in the column. The accumulated liquid substances may pass
to condensers, which cool them further, and then go to storage tanks or for more chemical
processing (Irwin, 2007).
Another process that applies the principles of distillation is the purification of water. See figure 5
below.
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(Kamrin, 1990)
Contaminated water is heated to form steam. Inorganic compounds and large non-volatile organic
molecules do not evaporate with the water and are left behind. The steam then cools and condenses
to form purified water.
Distillation effectively removes inorganic compounds such as metals (lead), nitrate, and other
nuisance particles such as iron and hardness from a contaminated water supply. The boiling process
also kills microorganisms such as bacteria and some viruses. Distillation removes oxygen and some
trace metals from water (Kamrin, 1990).
c) Azeotropes are mixtures with the same vapour and liquid compositions. This makes them not
able to be separated by distillation. This is because both the components will boil at the same
temperature (Kiss, 2013).
A process that can be used to separate Azeotropes from other components is known as Azeotropic
distillation. The azeotropic distillation unit consists of a container to feed the azeotrope, decanter
and a steamer. This process can be used in the separation of ethanol with water from its aqueous
solution. Water has a boiling point of 100 degrees Celsius and ethanol has a boiling point of 78.3
degrees Celsius. Benzene is added to the azeotropic mixture and ethanol is separated from the
solution. With the presence of benzene, a new solution is formed with a low boiling point of 22.8%
ethanol and 53.9% benzene at 64.8 degrees Celsius. Pure water leaves as the overhead product and
pure ethanol leaves the column as bottoms product.
3)
a) Filtration: The basic principle in filtration is to separate larger molecular mass components from
bulk liquid with porous medium (Stanbury, 2016). The driving force can be pressure difference,
valence, concentration difference, pressure difference or affinity (Shuler, 2002). Filtration can be
used in four different ways;
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i.
ii.
iii.
iv.
Clarifying the cells from fermentation broth and conditioning the cells for mechanical or
chemical disruption,
Clarifying the products from homogenate of cellular debris after disruption process,
Clarifying extracellular product from the chromatography,
Concentration and dia-filtration of a clarified product for chromatography.
At industrial scale, during filtration, waste water that contains suspended matter is applied to the
top of the filter bed. As the water passes through the filter bed, the suspended matter in the
wastewater is removed by a variety of removing mechanisms. With passage of time, as material
accumulates within the interstices of the granular medium, the headless through the filter starts to
build up beyond the initial value. After some time, the operating head-loss reaches a predetermined
turbidity value, and the filter is back-washed to remove the material which is the suspended solids
that has accumulated within the granular filter bed. The flow is reversed through the filter to
accomplish the back-wash. Enough flow of wash water is applied until the granular filtering medium
is expanded causing particles of the filtering medium to abrade against each other (Metcalf & Eddy,
2003).
b) Methods of filtration include cake filtration, deep bed filtration and membrane filtration.
1. Cake Filtration;
In cake filtration, slurry (solid suspension) is passed through a porous medium. The solids in
the slurry are retained on the surface of the medium where they build up, forming an
increasing thicker cake. See figure 6 below for a typical cake filtration.
As the thickness of the cake increases, resistance to flow of filtrate increases and the rate of
filtration gradually decreases. If rate is maintained to be constant then pressure difference
driving force will increase.
This filtration method is a batch process. Hence, it can be expected that as filtration
proceeds the cake will build up and the pressure drop across the cake will increase. A filter
cake is formed by the substances that are retained on a filter. The filter cake grows in the
course of filtration and it becomes thicker. With increasing layer thickness the flow
resistance of the filter cake increases. After a certain time of use the filter cake has to be
back flushed from the Filter. Examples of Cake Filters include Filter presses, Belt filters
Vacuum filters (Rotary vacuum belt filters, Rotary vacuum precoat filters, Vacuum disk
filters).
Cake filtration in application
Cake filters are used to remove large amounts of solids from a slurry solution. They would
normally be seen in biotechnology in the primary clarification of fermentation batches and
in a variety of solids removal steps in the production of drugs via organic synthesis
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Cake filtration limitation
Properties of the filter cake affect the filtration rate, and it is desirable for the particle's size
to be as large as possible to prevent pore blockage by using a coagulant. Also, , filter cakes exhibit
compressive behaviour, namely, they become more compact as the cake compressive stress
increases (C. Tien, 2001).
2. Deep bed filtration; in deep bed filtration, particles are removed when passed through beds
of granular or fibrous filter material. Deep bed filtration differs from other kinds of filtration
in that the solid particles suspended in the fluid are generally smaller than the pores of the
filter medium, as shown in Figure 6 below (Ives, 1970).
(Ives, 1970)
As the suspension travels through the filter, the particles deposit at differing depths on the
filter grains which constitute the bed. In application, deep bed filtration usually is used to
treat raw water after the processes of coagulation, flocculation and sedimentation (Ives,
1970). Its limitations are based around the nature of and the conditions leading to the
retention of particles throughout a filter bed, the change of the filter media structure due to
deposition, and its effect on filter performance.
3. Membrane filtration; A membrane is a thin layer of semi-permeable material that separates
substances when a driving force is applied across the membrane (Wagner, 2009). The
membrane acts as a very specific filter that will let water flow through, while it catches
suspended solids and other substances. Examples of methods to allow substances to
penetrate the membrane are the applications of high pressure, the maintenance of a
concentration gradient on both sides of the membrane and the introduction of an electric
potential (Wagner, 2009). In application, this membrane processes are used for removal of
bacteria, microorganisms, particulates, and natural organic material, which can impart
colour, tastes, and odours to water and react with disinfectants to form disinfection byproducts. Some limitations of membrane filtration are; Membrane fouling, Production of
polluted water (from backwashing) and Membranes have to be replaced on a regular basis.
4)
a) Circumstances under which liquid-liquid extraction would be preferred to distillation
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Liquid-liquid extraction is a separation process used in a wide range of applications in the chemical
industry. Unlike distillation, which is based on boiling point differences, extraction separates
components based on their relative solubility. Liquid-liquid extraction is usually preferred over
distillation for separation applications that would not be cost-effective, or even possible, with
distillation. Other circumstance where Liquid-liquid extraction is preferred over distillation is when
distillation requires excessive heat which has to be avoided. Also azeotropes being formed make it
hard for distillation to be used because they cannot be separated by distillation. Sometimes, the
components that need to be separated are different in natures, some are volatile and others are
non-volatile, in this case, extraction is the preferred method for separation. Where solvent recovery
is easy and energy can be saved, extraction is also preferred over distillation. This is because as
chemical engineers, we economise and try to use the less expensive method possible.
b)
a) Logarithmic mean driving force for the column
0.004−0.0003
0.004
(𝐼𝑛0.0003)
= 0.00143
b) Height of Absorber
Gm(Y1 - Y2) = 𝐾𝐺 a (Y – Ye)lmZ
0.015(0.03 – 0.0003) = 0.04 x 0.00143Z
Z=(
0.000446
0.00000572
) = 7.8 m
c) Height of the transfer column
𝐻𝑂𝐺 =
𝐺𝑚
𝐾𝐺𝑎
-
0.015
0.04
= 0.375 m
d) Number of units required
𝑁𝑂𝐺 =
7.79
0.375
= 21
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References
Acikara, Ö. B. ( 2013). Ion-Exchange Chromatography and Its Applications.
C. Tien, S. K. (2001). Cake ltration analysis. National University of Singapore, .
Fischer, D. (2015). Part 2 - Sieve tray System Design. Ann Arbor, MI / San Leandro, CA: David Fischer.
Healthcare, G. (2016). Ion Exchange Chromatography. Sweden.
Irwin, D. (2007). Chemistry Contexts 1: Preliminary Course(2nd edition) 17.3 Separation of petroleum
(crude oil). Australia.
Ives, K. J. (1970). Rapid filtration. Water Research.
Jung, S.-M. P. (2010). Journal of Water Supply: Research and Technology-Aqua.
Kamrin, M. N. (1990). Distillation For Home Water Treatment," Cooperative Extension Service,.
Michigan State University.
Kiss, A. (2013). Distillation | Azeotropic Distillation. The Netherlands.
Kister, H. Z. (n.d.). Distillation Operation, Chapter 4. New York, Saint louis.
Leibbert, E. M. (n.d.). A COMPARISON OF PACKED-COLUMN AND LOW-PROFILE SIEVE TRAY AIR
STRIPPERS.
Metcalf & Eddy, T. G. (2003). Wastewater engineering: treatment and reuse.
Pati, K. (n.d.). Distillation Operations: Methods, Operational and Design Issues. India.
Rousseau, E. R. (1987). Absorption and Stripping, Handbook of Separation Process Technology. New
York.
Shuler, M. L. (2002). The ABCs of filtration and Bioprocessing. New York: Prentice Hall.
Stanbury, P. A. (2016). Principles of Fermentation Technology.
Wagner, J. (2009). Membrane Filtration Handbook Second Edition.