adsorption & properties of activated carbon

7/29/2019 adsorption & properties of activated carbon

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INTRODUCTION

The problem of protecting the environment from pollution and contamination by various types of

discharges is now in the focus of attention all over the world. At present hundreds of millions of tonsof diverse substances acting as a source of harm to the health of people, plant life, and useful

microorganisms are discharged to the aquosphere and in number of these places the concentration ofharmful impurities is already impermissible and their level exceeds the maximum permissible

concentration [65].So the government and the private enterprises are together and/or separately seeking the best

technological process in order to control the pollution problems by using a clean technology ordeveloping more efficient waste treatment facility [26].

There are three identified types of pollution, namely 'air, water, and solid pollution. In everycommunity, the three types of pollution are present in different proportions ' according to the type of

community and industrial activities practiced it.

The control of water pollution is one of today STO ajpr area of scienU gains an important position inmost pollution conferences, and research due to the constant fear of water depletion as a result of the

huge increase in population in the last decades.

Consequently, the reuse and recycle of purified wastewater became the most ideal solution [16].One of the most important leading sources of man-made pollutants are industrial waste, which enter

the water as complex mixtures whose specific chemical compositions are frequently not known. The

industrial wastewater is essentially the water supply of the community after it has been fouled by avariety of uses. If untreated wastewater is allowed to accumulate, the decomposition of the organicmaterials it contains can lead to the production of large quantities of malodorous gases [34]. A major

step towards effective pollution control is a complete chemical and biological analysis of suspectwater.

In addition, untreated industrial wastewater usually contains pathogenic, or disease causingmicroorganisms that dwell in the human intentional track or that may be present in certain industrial

waste [34]. Wastewater also contains nutrients which can stimulate the growth of aquatic plants, and itmay contain toxic compounds. For these reasons, the immediate and nuisance free removal of

wastewater from its sources of generation, followed by treatment and disposal, is not only desirablebut also necessary in an industrial society.

MAIN WASTEWATER CONTAMINANTS

Depending on the nature of the industry and the projected uses of the waters of the receiving stream,

various, 'waste constituents may have to be removed before discharge. These may be summarized asfollows:

Soluble Organics

They cause depletion of dissolved oxygen since most receiving waters require maintenance of

minimum dissolved oxygen, the quantity of soluble organics is correspondingly restricted to thecapacity of the receiving waters for assimilation or by specified effluent limitations [18].

Suspended Solids

Deposition of solids in quiescent stretches of a stream will impair the normal aquatic life of the stream.

Sludge blankets containing organic solids will undergo progressive decomposition resulting in oxygendepletion and the production of noxious gase [I8].

Heavy metals. Cyanide and Toxic Organics

The presence of heavy metals in the wastewater is very dangerous to the human been because it is

toxic, and also it is non degradable materials [19] .Color and Turbidity

These present aesthetic problems even though they may not be particularly deleteriol for most wateruses. In some industries, such as pulp and paper, economic method are not presently available for color

removal [18].

Oil and Floating Materials

These produce unsightly conditions and in most cases are restricted by regulating [18].

Volatile Materials.

Hydrogen sulfide and other volatile organics will create air pollution problems and ^ usually restrictedby regulations [18].

Nitrogen and Phosphorous

When effluents are discharged to lakes, ponds and other recreational areas, 1 presence of nitrogen andphosphorous is particularly undesirable since it enhances eutrophication and stimulates undesirable

algae growth [18].


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Adsorption Is Key to Removing Contaminants

When filtering impurities out of drinking water, mechanical filtration typically is used. But what

happens if there are particles in water that are dissolved or are too small and cannot be removed bymechanical filtration? Fortunately, there is a process called adsorption, which can remove very tiny

particles or dissolved contaminants from water such as lead, PCBs, some pesticides, viruses andasbestos fibers.

What is adsorption?

Adsorption is a physical process in which dissolved molecules or small particles (the adsorbate) areattracted and become attached to the surface of something larger (the asdorbent). The attraction is

similar to that of a magnet on a refrigerator, but on an atomic or molecular scale. Energy differencesand electrical attractive forces, known as van der Waals forces, cause molecules of the adsorbate to

physically fasten and stick onto the adsorbent.

Common in nature, the laboratory and industry, adsorption often occurs between solids and liquids orgases. It is responsible for the transport of nutrients into the soil, assisting in plant and animal growth,

the chemical separation of proteins and enzymes, and industrial processes like air purification, sugar

refining and desalting of sea water.Adsorption should not be confused with the completely different process of absorption, in which

liquids and particles penetrate into another substance, such as a sponge that soaks in liquids.

The amount of material adsorbed depends on a number of factors including the degree of attraction, thesurface area exposed to mobile particles, the concentration of the contaminants, and the pH andtemperature of the liquid. Typically, the strongest adsorbents are microporous or finely divided solids

(clays, charcoal, powdered metals) and liquids (fine droplets like aerosols and sprays( .

The Role of Activated Carbon

Throughout history, people have used carbon (charcoal) as an effective adsorbent, in such processes as

water treatment, sugar purification and color removal from liquids. In water treatment systems, animproved form of carbon, called "activated carbon," is the adsorbent most commonly used to attract

and hold dissolved contaminants.Activated carbon is made from carbon-based materials like coal or wood that is first heated without

oxygen to produce charcoal and tar or pitch, which bubbles out. The charred material is then heatedwith steam or carbon dioxide to above 1000 Celsius, which further erodes and corrodes it to remove

everything but the carbon. The result is an airy, delicate structure that is nearly pure carbon and full ofholes. It is then crushed to a powder and mixed with binders to form granules of desired size ranges fordifferent filter media.

Chemicals are sometimes added during activation of the carbon to produce different surface chemical

natures that adsorb different contaminants. For example, acids produce carbon with maximum capacityfor adsorbing heavy metals.

Activated carbon's enormous surface area is a critical factor in its effectiveness to adsorb variouscontaminants. The surface area typically is about 1,000 square meters per gram. As an example, a piece

of carbon the size of a pea has an area the size of half a football field. The structure and distribution ofpores in activated carbon are key factors for adsorption because they determine the size of molecules

that can be adsorbed. Adsorption can only occur when molecules enter activated carbon's pores. As aresult, the size and porosity of activated carbon determines the rate at which contaminants are

adsorbed.

Depending on the desired results, activated carbon may be used in powdered or granular form.Granular activated carbon is commonly used in water treatment facilities where the water is passedthrough a granular carbon bed to remove tastes, colors, odors, and dissolved organics. Powdered

carbon also is used in treatment facilities at various points for its quicker rate in removing variouscontaminants. Powdered carbon is the preferred choice in point-of-use water filtration systems because

it is faster and is a better mechanical filter than granular activated carbon. It also takes up a minimumof space given its large surface area-to-volume ratio.

Filter Solutions

Filters with activated carbon are available in a variety of types and sizes, depending on use in the home

or commercial operations. Their effectiveness at adsorption depends mainly on how long the water is incontact with the activated carbon. The longer that water is in contact with the activated carbon, the

more materials can be adsorbed.

Point-of-use systems include faucet-mounted, countertop, pour-through and under-the-sink filters. Thefaucet-mounted is the most common filter, which can be easily and quickly installed by attaching it


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directly to a faucet outlet or on the counter attached with flexible tubing. Under-the- sink activatedcarbon systems come in two types, ones that filter all cold water passing through the faucet and others

that attach to the cold water line and go to a separate faucet, in order to prolong the life of the filter.Whole-house units filter all water serving a household. These systems are key in removing

contaminants that can be absorbed through the skin during bathing or showering, or from inhalation.For activated carbon filters to be most effective, cartridges need to be replaced periodically. The life of

the cartridge varies with the amount of water passing through the filter and the amount of impurities orcontaminants present in the water. Expected minimum capacities are expected to be declared on

product labels.

Effect on Water Quality

As activated carbon adsorbs dissolved molecules and sub-micron particles, the effect is the reduction of

contaminants resulting in more aesthetically pleasing and healthy drinking water. Adsorption removes

disinfectant chlorine both as "free chlorine" and a special "combined" form (mixture of chlorine andammonia) that is often used in municipal water treatment. The taste and odor of disinfectant chlorine is

the most common complaint and the most common reason people buy filters.

Adsorption also can remove many kinds of pesticides and other synthetic organic chemicals, includingchlorinated hydrocarbons, gasoline, industrial solvents, and disinfection by-products. Adsorption also

can remove heavy metals like lead and cadmium that get in water from corrosion of plumbing

materials.Everpure precoat filters are highly effective at removing dissolved and sub-micron contaminants. Theycontain Micro-Pure, a proprietary powdered media mix containing mostly powdered activated carbon.

Finer than talcum powder, it is very efficient at removing dissolved chemicals and suspended particlesbecause there is less space between carbon particles for contaminants to travel. Everpure precoat filters

are small and convenient for foodservice and home use.In an Everpure precoat filter, flowing water deposits Micro-Pure onto a septum, which is a supported

fabric where the powdered media mix forms a firm layer known as the precoat cake. The cake capturesdissolved chemicals through adsorption and particles through mechanical filtration. The treated water

then passes through the septum and to the outlet. Everpure precoat filters remove 99.9 percent ofparticles one-half micron (1/50,000 of an inch) and larger. The filters also remove 99.9 percent of the

protozoan cysts of Cryptosporidium and Giardia, which cause waterborne illnesses.Everpure systems with Micro-Pure have achieved NSF International's highest ratings under Standard

42 for Aesthetic Effects and Standard 53 for Health Effects. NSF International is an independenttesting agency that sets product standards and certifies the performance of point-of-use drinking watersystems.

For more than 60 years, Everpure has been a leading manufacturer of water filtration and treatment

systems for foodservice and residential use and offers a full line of systems to meet all water qualityneeds.

Properties of Activated Carbon

Activated carbon is the generic term used to describe a family of carbonaceous adsorbents with ahighly crystalline form and extensively developed internal pore structure. A wide variety of activated

carbon products is available exhibiting markedly different characteristics depending upon the rawmaterial and activation technique used in their production. In selecting an activated carbon, it is

important to have a clear understanding of both the adsorptive and physical characteristics of the

material in order to optimise the performance capabilities.

Adsorptive Characteristics

Surface area (BET N2) - measurement of the extent of pore surface developed within the matrix of theactivated carbon by nitrogen (N2) adsorption. Used as a primary indicator of the activity level, based

on the assumption that the greater the surface area, the higher the number of adsorptive sites available.Pore size distribution - determination of the pore size distribution of an activated carbon is an

extremely useful way of understanding the performance characteristics of the material.The International Union of Pure and Applied Chemistry (IUPAC) defines the pore size distribution as:

Micropores r < 1nm

Mesopores r 1-25nmMacropores r > 25nm

The macropores are used as the entrance to the activated carbon, the mesopores for transportation and

the micropores for adsorption. View of pore structure - scanning electron microscope


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Iodine number - measurement of the porosity of an activated carbon by adsorption of iodine fromsolution.

Carbon tetrachloride activity - measurement of the porosity of an activated carbon by adsorption of

saturated carbon tetrachloride vapour.

Physical Characteristics

Hardness - an important factor in system design, filter life and product handling. There are largedifferences in the hardness of activated carbons, depending on the raw material and activity level.Bulk Density - should be carefully considered when filling fixed volumes as it can have considerable

commercial implications. The backwashed and drained density will show a lower value due to thewater film between the particles of activated carbon.

Particle size distribution - the finer the particle size of an activated carbon, the better the access to thesurface area and the faster the rate of adsorption kinetics.

In vapour phase systems this needs to be considered against pressure drop, which will affect energycost. Careful consideration of particle size distribution can provide significant operating benefits.

The important properties of activated carbon relevant to specific applications are considered further inthe text.

Adsorptive materials

A large variety of different materials are included in this group of removal techniques. The basicprinciple is that the radio nuclides are retained on a fixed matrix (adsorbent), which may be a batch of

i.e. activated aluminium oxide, barium sulphate crystals, manganese oxide, activated carbon or a salt of

phosphoric acid, Ca3(PO4)2, CaHPO4. Sometimes inorganic ion exchangers such as sodium titanate,mica and zeolite are considered as mineral adsorbents, too.

Usually, these materials cannot be regenerated. They can be applied in the same way as ion exchangers

(see Chapter 4.5), POE, POU or pour-through.Depending on the volume of the adsorbent bed, different amounts of water may be treated with them.

Table IV summarises the reductions of different radio nuclides obtained by these materials. (Theinformation is based on the literature and on laboratory experiments within WP 5 and WP 6.)

Table IV. Summary of the removal efficiencies and distribution coefficients (KD) values for radio

nuclides attained by various adsorbents.

Removal efficiency (%)

Adsorbent U 226Ra 210Pb 210Po

Al2O3, activated (1) good(3) 6080

BaSO4 (2) >99 MnO2 90 8090

GAC fair(3) fair 3595 5090CaHPO4 7080 >90 >90 good

KD (ml/g)

Sodium titanate 7 600 2 030 000 MnO2 650 5 400 000

Zeolite A 82 106 000

Na-4-mica 35 300 146 000

no information

(1) easy to regenerate(2) cannot be regenerated

(3) poor capacity

It seems that removal of radium can easily be carried out with different adsorbents. The best materialsare barium sulphate and manganese oxide, which cannot be regenerated. It is important to recognise

that radium generates radon, which is dissolved in the water passing through the bed.The best application for these materials is thereby pour-through units where no high concentration of

radon can be formed in the treated water and the gamma radiation levels remain low.Lead can best be removed by adsorption on MnO2 or CaHPO4. As was the case for radium, pour-

through units are most suitable.


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Adsorption

Adsorption, the binding of molecules or particles to a surface, must be distinguished from absorption,

the filling of pores in a solid. The binding to the surface is usually weak and reversible. Just aboutanything including the fluid that dissolves or suspends the material of interest is bound, but compounds

with color and those that have taste or odor tend to bind strongly. Compounds that containchromogenic groups (atomic arrangements that vibrate at frequencies in the visible spectrum) very

often are strongly adsorbed on activated carbon. Decolorization can be wonderfully efficient byadsorption and with negligible loss of other materials.

The most common industrial adsorbents are activated carbon, silica gel, and alumina, because theypresent enormous surface areas per unit weight. Activated carbon is produced by roasting organic

material to decompose it to granules of carbon - coconut shell, wood, and bone are common sources.Silica gel is a matrix of hydrated silicon dioxide. Alumina is mined or precipitated aluminum oxide and

hydroxide. Although activated carbon is a magnificent material for adsorption, its black color persists

and adds a grey tinge if even trace amounts are left after treatment; however filter materials with finepores remove carbon quite well.

A surface already heavily contaminated by adsorbates is not likely to have much capacity for additional

binding. Freshly prepared activated carbon has a clean surface. Charcoal made from roasting wooddiffers from activated carbon in that its surface is contaminated by other products, but further heating

will drive off these compounds to produce a surface with high adsorptive capacity. Although the

carbon atoms and linked carbons are most important for adsorption, the mineral structure contributes toshape and to mechanical strength. Spent activated carbon is regenerated by roasting, but the thermalexpansion and contraction eventually disintegrate the structure so some carbon is lost or oxidized.

Temperature effects on adsorption are profound, and measurements are usually at a constanttemperature. Graphs of the data are called isotherms. Most steps using adsorbents have little variation

in temperature.

Study Guide

Students at R.P.I. will be tested on the following:

Terms and definitionsDerivation of the Langmuir equation

Sketching an adsorption isotherm given the data for a laboratory testDesign of a batch adsorption process

Batch adsorption when carbon is added in portions for several stages (not covered in hypertext yet)Concepts of designing column adsorption but no calculations

Applications

There are a few applications for finely powdered carbon as an adsorbent. However, a large amount of

fine carbon is used in water treatment. When a process fluid will be filtered anyway, it may make senseto add powdered carbon to take advantage of its great surface. Retaining the carbon in packed columns

is less costly than collecting it by filtration after batchwise addition. Powdered carbon is not used incolumns because the large head losses for passing fluid through such a fine structure cannot be

tolerated.Granular carbon, roughly the size of a pea, provides sufficient surface without excessive resistance to

flow in a column. It has been used as a"roughing step"for recovery of fermentation products. The

surface is much more than the outer area of the particles because there is a vast network of pores. The

concentration at which the column can be considered exhausted depends on engineering and costfactors. A column must be regenerated in time to take its place at the head of the series of columns-

there is no point in continuing to adsorb with a column that is so loaded that it accomplishes little.

Freundlich Equation

The most common shape of the graph of amount adsorbed per unit weight of adsorbent versus the

concentration in the fluid in equilibrium is:

Typical Adsorption Isotherm

This graph very closely resembles that for microbial specific growth rate coefficient versus substrateconcentration. These data often fit nicely the empirical equation proposed by Freundlich:

q = Kf Cn

where: Kf and n are coefficientsq = weight adsorbed per unit wt of adsorbent
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C = concentration in fluidTaking logs and rearranging:

log q = log Kf + n log CThe coefficients Kfand n can be estimated from slopes and by substituting values from a line fitted to a

graph of log q versus log C. With a personal computer handy, the method of least squares can be usedto get a statistical fit, however outlying data points are not as obvious as with a graph.

Langmuir equation

Langmuir derived a relationship for q and C based on some quite reasonable assumptions. These are: a

uniform surface, a single layer of adsorbed material, and constant temperature. The rate of attachmentto the surface should be proportional to a driving force times an area. The driving force is the

concentration in the fluid, and the area is the amount of bare surface. If the fraction of covered surface

is , the rate per unit of surface is:

rate going on = k1 C ( 1 - )

The evaporation from the surface is proportional to the amount of surface covered:

rate leaving = k2

where k1 and k2 are rate coefficientsC = concentration in the fluid

= fraction of the surface covered

At equilibrium, the two rates are equal, and we find that:

By dividing the numerator and denominator by k1, and making use of the fact that q will be

proportional to , the useful form of the equation is:

where qm = q for a complete monolayerKa = a coefficient

Taking reciprocals and rearranging: ( details of math for sleepy people)

A plot of versus should indicate a straight line of slope and an intercept of . The

graph shows data points and lines fitted to both Freundlich and Langmuir equations.

Equations Fitted to Data
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Try this yourself. The equations are handled separately. The idea is to imagine a data set and to observe

that either equation can usually fit pretty well if you fiddle with the coefficients.

B.E.T. Equation

A different equation is more likely to describe adsorption where theadsorbate exceeds a monolayer.

The Brunauer-Emmett-Teller (BET)equation is:

where C = concentration at which all layers are filled

K = a coefficient

Its assumptions are:

Adsorbed molecules stay put

Enthalpy of adsorption is the same for any layer

Energy of adsorption is the same for layers other than the first

A new layer can start before another is finished.

It is important that many unusual adsorption isotherms are fitted well bythe BET equation. This is to beexpected when there are three coefficients to manipulate. The maximum loading, Qm, just multipiles to

move the entire curve up and down. The coefficient, Kb, has a major effect on shape. The concentration

at which all sites are saturated (maybe several layers) can be adjusted to get a portion of the isotherm.In other words, you can look at just part of the curve. As C approaches Cs, the denomenator of theequation becomes small, and the curve shoots up.

Plotting the B.E.T. Equation with Various Coefficients

Constructing the Isotherm

The first thing to do is to set up an analysis for the material that will be adsorbed.You need some sort

of measurement procedure. An inexpensive and common method uses a colorimeter orspectrophotometer. Light is passed through a sample; the higher the concentration, the less the light

that escapes. It would be possible to weigh out material each time to make up solutions of knownconcentration, but it is quicker and easier to weigh out just once to prepare the solution of highest

concentration. The other solutions are made by pipetting from the strongest solution into volumetric f


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lasks and diluting up to the mark. For the very lowest concentrations, it may be more accurate topipette from one of the intermediate concentrations rather than have to measure a very small amount of

the most concentrated solution.Here are some typical numbers for the solutions of known concentration:

1) We were trying for 0.1 g, but the amount of powdered dye that we weighed in a dish on theanalytical balance was 0.183 g. This was placed in a 100 ml volumetric flask, and water was added to

the mark.0.183 g per 100 ml = 1.83 g per liter

2) 5 ml from #1 was added to another 100 ml flask and diluted to the mark. There are several ways tocalculate;

5 to 100 is 1 to 201.83 / 20 = 0.0915 g/L

or

0.005 L (5 ml) times 1.83 g/L = 0.00915 g in 100 ml = 0.0915 g/L3) 1 ml from # 1 was added to Flask #3 and diluted to the 100 ml mark.

0.001 L times 1.83 g/L = 0.00183 g in 100 ml = 0.0183 g/L

4) 10 ml from Flask #2 was added to Flask #4 and diluted to 100 ml.0.01 L times 0.0915 g/L = 0.000915 g in 100 ml = 0.00915 g/L

This is not enough known solutions for a good calibration curve; eight or ten would be better.

Batch Adsorption

Design of a batch adsorption process starts with plotting the isotherm. This is part of this hypertext

package. This is a typical plot:

Note what is plotted. This is an equilibrium graph of loading ( milligrams of material adsorbed per

gram of carbon ) versus concentration of the solution in equilibrium with that carbon.

Batch Adsorption (continued)

Now we resort to a mass balance. In very simple terms, we know how much carbon and how muchsolution we add to this batch process at the start. What ever disappears from the solution can only end

up adsorbed on the carbon. Let's establish some notation for this:

V is the volume of solution with initial concentration Co.B is the weight of carbon with initial loading qo. This loading is zero for fresh carbon.


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C1 is the concentration of the final solution, and q1 is the final loading of the carbon.The mass balance equation is: V ( Co - C1 ) = B ( q1 - qo )

We know Co and qo. The design problem is either to estimate the final concentration and final loadingwhen we are given values for V and B or to calculate what V and B should be to achieve a given

removal of material. The latter is the most common situation, so let's do the calculation. Each large stubline on the abscissa marks 5 more concentration units. We mark a point for the start; in this case the

feed solution is 25 mg per liter while the carbon is fresh with zero loading. The point is on the abscissa( X-axis) at 25 units on the scale.

The final point must lie on the equilibrium line at the desired final concentration. In this case, we havedecided that 5 mg per liter is our target. We move up from 5 units (first big stub line) on the abscissa

until we intersect the isotherm line. We can read the loading by moving horizontally to the ordinate (Y-axis). This is about 15 mg/g. Now we know everything in the equation but V and B. Probably we do

know V because we have so much solution to treat per day or per batch. Then we merely plug into the

equation to find B, the amount of carbon needed.

The orange line is known as the operating line. Even if we were not at equilibrium, the material thatleaves the solution is on the carbon, and this operating line satisfies the mass balance. If we know V

and B, we know the slope of the line. We can extend from the starting point and draw a line with this

slope. When it intersects the equilibrium isotherm line, we have the values to solve the design problemfor the situation where we are given the startingconcentration and loading and must estimate what agiven amount of carbon will do.

Column Adsorption

Design of a column for adsorption starts with laboratory testing to establish the breakthru curve. At

time intervals, the effluent from a column is sampled. Time zero is when the solution is applied to thecolumn.

At first, the adsorbent (usually activated carbon) is fresh with all its adsorption sites. Essentially noneof the material to be removed escapes from the column. As time passes, some of the adsorption sites

are used up, and concentration in the effluent rises.The shape of the graph may vary considerably for different situations. Usually there is a long time

before the effluent concentration rises sharply and then levels off. If all the sites were occupied, wewould expect the inlet concentration and the outlet concentrations to become the same.


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Note the lines in blue. The breakthrough concentration is determined by the process specifications.This is the allowable concentration. If a pollutant is being removed, the breakthrough concentration

might be the government regulation for what your plant can discharge. For a commercial product

where the column is removing color, the breaktrhough concentration is determined by yourspecification for product quality. The point is that breakthrough concentration is not some fundamental

number but depends on how you decide to operate your process.

The exhaustion concentration also depends on process considerations. If there are not enough workers

on the night shift, the column may have to be removed in time for regeneration by the day shift so that

it can ready in time the next day to become the t rail column. Note that the column could still adsorbsome more material. However, you get little benefit by running for a longer time. You pay for labor,for electricity, for plant costs, etc. When the benefits are not worth the costs, the column is considered

exhausted.

Column Adsorption, continued

A practical way to design an adsorption column is to experiment with a laboratory column. If it isroughly the same length as the length of a full size column and of sufficient diameter to minimize wall

effects, scale up is merely a matter of increasing the area to match the volume to be treated. Even if thedimensions of the production column are undecided, operating the lab column until breakthrough

shows how much solution has been passed through so much carbon. This leads to a simple calculationof capacity of the carbon. A major source of error is the effect of flow rate because this determines

contact time, and the approach to equilibrium takes time.

We have seen that columns in series are operated past the breakthrough point to the exhaustion point. Ifthe graph is steep, there will be little error in neglecting the volume that passes after breakthrough.

There is a method to account for this volume that will be presented here. Note that a rate constant is

needed for the main equation. It is more trouble to measure this rate than to do experiments with alaboratory column for the scale up already mentioned. Nevertheless, following the logic of the

mathematical method improves understanding of countercurrent adsorption.

Another weakness in theory

The shape of the breakthrough curve relates to the adsorption isotherm. The time for the adsorptionzone to become defined and to move to the end of the column is:

t = Ve / Qt
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where t = time as exhaustion zone exits

Ve = volume applied to column, volume liquid/area

Qt = flow rate to column, volume/time-areaThe time for an established adsorption zone to move a distance corresponding to its own thickness is:

t = ( Ve - Vb ) / Qt

where t = transit time

Vb = volume, starts at conc. for breakthroughThe amount adsorbed in the adsorption zone is the integral from Vb to Ve of ( Co - C ) dV

We next need to construct the shape of the adsorption zone from the adsorption isotherm. This meansthat equilibrium is assumed.

As is customary in the analysis of staged separations, we consider a differential element in the column.The liquid and the solid adsorbent are assumed to have velocities entering the element even though the

solid is actually motionless. A material balance for the entire column is:

Qt ( Co - 0 ) = Bt ( q - 0 )

A material balance for the section of the column containing the adsorption zone just as it is exiting is:Qt ( C - 0 ) = Bt ( q - 0 )

These material balances define an operating line shown in the figure.

Each point on the operating line represents compositions of liquid and solid in contact at some point inthe column. For the differential element:

Qt dC = K ( C - C* ) dZwhere K = a rate coefficient

Z = distance

C* = equilibrium solute concentrationThe equation for the column is:


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where C = concentration deemed the breakthrough

C = concentration at the other end of the element

The quantity ( C - C* ) is the horizontal distance between the operating line and the isotherm line. Itcan be integrated graphically by taking the area under the function as in the following figure :

There are two integrations, one from the top of the column to the end and one for the adsorption zone.

Column Adsorption, overlooked factor

Design using theory is based on a column packed with adsorbent and operated from start to exhaustion.

Real columns are usually lead-and-trail. A design should take into consideration that the lead column

has reached the breakthrough point and is delivering that concentration to the trail column to produce afinal effluent to meet specifications. At the start of the analysis there is a discontinuity in the adsorbent

from that in the lead column that has partial loading from contact with the solution to adsorbent in the

trail column that starts out freshly regenerated with little or no loading depending on how well it wasregenerated.


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Removal of Oil from Wastewater UsingAdsorption Technique

I. Adsorbents.a- Low-cost Adsorbents.

b- Activated Carbon.

II. Adsorbate.

III.Adsorption.

IV. Removal.

V. Wastewater.

I. Adsorbent:

a- Low-cost Adsorbents:-

- Clays.

- Cement dust.

- Polymers.

b- Activated Carbon.

II. Adsorbate:

Oil

III. Adsorption.

IV. Removal.

V. Wastewater.

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adsorption & properties of activated carbon