isotherm test rev2

8/13/2019 Isotherm Test Rev2

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MEASURING

ADSORPTIVE

CAPACITY OFPOWDERED

ACTIVATED

CARBON


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Introduction.

Norit Americas Inc. produces over 150 different varieties of

activated carbon. This diverse product line provides manyoptions for carbon adsorption. Isotherm testing can aid in the

process of choosing the best carbon for your purification needs.

Pore volumeava ilab le to

both ads orbatesand solvent.

Pore volumeava ilab le onlyto solvent andsmaller adsor-

ba te molecules.

Porosityava ilab le only

to solvent.Po re s izetoo small

for adsorptionof impuritiesfrom liquids.

Electron microscope photo of stea m a ctivated ca rbon.


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A pow de red ca rbons a bility to remo ve impurities

from a liquid is evaluated using laboratory scale

ba tch trea tment tests or Iso therms. Data from

these laboratory adsorption tests may be used to

ca lculate the c a rbo ns ca pac ity for impurity remova l,

or its a ds orptive ca pac ity. These la bo ratory sca letests are also useful to identify the best performing

ca rbon type and to de fine the relative ec onomics of

powdered carbon treatment.

The sa me laboratory proce dures ca n also b e used

as an initial screening of granular carbons, provided

they are ground to a fine powder before testing.

Normally, granular carbon is used in columns or

tanks, with the process liquid flowing continuously

through the bed of granular carbon. As a result, alaboratory batch adsorption test at equilibrium will

not give a cc urate d ata for des ign of a granular

ca rbon a ds orption sys tem. P ilot co lumn tests are

required to accomplish this and define process

ec onomics whe n granular ca rbon is us ed. The pro-

cedure for running pilot column tests is described

in brochure number NA00-4 a nd c a n be ob tained

by calling NoritAmericas Inc.


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In a batch treatment process using powdered acti-

vated carbon, adsorption of impurities from a liquid

is es sentially a n eq uilibrium pheno meno n. The firs t

step of the process involves migration or diffusion

of the impurities into the porous cavities within the

ca rbon particles. Once inside the pore structure,the impurity molecules are attracted to the internal

pore s urfac es by w ea k electrostatic forces known

as van d er Waa ls forces . This physica l ads orp-

tion of impurity mo lec ules onto the internal surfac e

of the pore structure is the most common type of

ad so rption a nd is reversible. P hysica l ads orption

ca n be a n adva ntage in some separation sc enarios

in which the adsorbed material is the desired

product. After a ds orption, a cha nge in proc es sing

co nditions ma y ca use the produc t to des orb fromthe carbon. Gold recovery and antibiotic production

are two examples of this.

In some cases, the adsorbed material may interact

with ac tive sites on the carbon pore surfac es a nd

form chemica l bonds with the surfac e. This proce ss

is called chemisorption and is considered irreversible.

P ow dered a ctivated ca rbon us ually w orks well in

applications involving removal of trace contami-

nants from liquids. Evaluating the economics of the

proce ss req uires a nsw ering two q uestions:

will carbon a ds orb the impurity to a n ac cepta ble

level and wha t is the c os t? Most organic chemical

impurities are ad sorbed on c arbon to so me extent.

To d etermine how well a c tivated ca rbon w ill purify

your process liquid, you must determine the carbons

a ds orptive c apa city. This is done in the lab oratory

by c onducting a n ad sorption isotherm test.

S tanda rd laboratory analytica l tests such a s Iodine

Numbe r, P henol Value, or S urfac e Area ca n be

misleading a s a predictor of ca rbon performanc e.

These s urroga te tests do not neces sa rily relate

to a ca rbons ab ility to a ds orb the s pec ific impurities

in your process stream. For example, the Iodine

Number tes t proced ure mea sures removal of iodine

from a standa rd s olution. Ca rbons with the highes t

Iod ine Number value are freq uently not the be stchoice for color removal in food processing or for

tas te and odo r remova l from pota ble wa ter. What

must be done to select the best carbon for your

application is to conduct an adsorption isotherm

test on your specific proc ess stream.

There are seve ral important fa cto rs that m ust b e

cons idered w hen evaluating ad sorptive c apa city.

In mos t proce ss streams, there a re numerous types

of impurities present a t d ifferent c oncentration levels.All of these impurities are competing with each other

in the ad so rption proc ess. S ince d ifferent impurity

types are adsorbed at different rates and in different

amounts, it is very important that laboratory tests

be cond ucted using the a ctual proc ess liq uid. An

accurate measurement of a carbons adsorptive

capacity also requires duplication of the actual plant

processing conditions in the laboratory. Physical

and chemical properties such as pH, temperature,

impurity concentration, and viscosity can affect

ad so rption a nd must be duplica ted in your lab ora-

tory isotherm testing. Additionally, the physical size

distribution of the ca rbon s amples tes ted must b e

consistent because carbon particle size affects the

rate of a ds orption.


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Measuring Adsorptive Capacity.

Laboratory tests for measuring adsorptive capacity are

designed to be rapid screening methods for determining the

performance of activated carbon. For batch treatment using

powdered carbon, results from laboratory isotherm tests con-

ducted at equilibrium will correlate directly with full scale plant

process performance. The laboratory tests can accurately

measure adsorptive capacity of different types of activated carbon

and identify the carbon type with the best cost performance.

To achieve accurate results, the adsorption measurements

must be made under equilibrium conditions. Therefore, Step 1

in your laboratory testing is to determine the minimum contact

time required to establish equilibrium conditions.


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STEP 1: Determine the Test Conditions.

An ac tivated c arbon sa mple must reac h ad so rption

equilibrium to measure its total adsorptive capacity.

Norma lly, a co ntac t time o f one ho ur is sufficient.

How ever, a pplica tions involving visc ous liq uids , low

temperatures, or impurities that are difficult to adsorbmight require long er con ta ct times . To determine

the optimum conta ct time, you should e xpose a

series of liq uid s amples to the s ame ca rbon treat-

ment dosage for different time periods using test

conditions that match the plant process (impurity

type, concentration, pH, temperature, etc.).

The lab procedure commonly used is as follows:

1. Lab oratory tests must be run on sa mples of theac tual proce ss liq uid a nd a ctual plant proces sing

cond itions must b e duplica ted a s c lose ly as poss ible.

2. A predetermined w eight of c arbon is a dd ed to

ea ch of five bo ttles . The am ount of ca rbon used is

typically 0.1% to 0.5%, by weight, for treating

proces s liq uids , w hich usua lly ha ve relatively high

concentrations of impurity present. For potable

wa ter or wa stew ater with less than 5 ppm of impu-

rity, the ca rbon d os ag e used may b e in the rang e

of 5 to 200 ppm. The ca rbon d osa ge used in these

co ntact time tests s hould b e near the sma llest

dosage you expect to use in the adsorptive capacity

tests, which will be conducted later.

3. Trans fer 200 mL (or 200 grams ) of the proces s

liquid to ea ch of the bo ttles and sea l the bottle.

4. Agitate the s amples a t the sa me proces s temper-

ature a nd tes t cond itions fo r different time period s.

Contact times of 10, 20, 30, 60, and 120 minutes

may be used.

5. After mixing the carbon and liquid for the specified

contact time, immediately filter each sample to

sepa rate the proce ss liq uid from the ca rbon.

Analyze the filtrate to determine the impurity

rema ining in the s olution. You c an a nalyze the

residual impurities by any number of methods,depending on the impurity of interest. Commonly

used ana lytica l methods include spe ctrophotom etry,

titration, chromatography, or gravimetric methods.

6. P repa re a grap h w ith the impurity removed (or

remaining) on the Y-axis vs. the carbon contact

time on the X-axis. The tes t res ults w ill produc e a

plot a s sho wn in Figure 1. There is usua lly a sha rp

break in the curve at the optimum contact time.

Co ntact times in exces s of this value ha ve a verylimited effect on removal of additional impurity or

increases in ca rbon a ds orptive ca pac ity.

When evaluating several different carbon samples,

you will find it necessary to determine the contact

time for eac h ca rbon type. This is p a rticula rly true

when comparing samples with different particle sizes.

CONTACT TIME (min)

IMPURITYRE

MOVED(%)

Figure 1

CONTACT TIME TEST RESULTS


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Step 2: Obtain Your Adsorption Data.

Once the optimum contact time is determined, the

next step is to measure the total adsorptive capacity

of the ac tivate d c a rbon. This mus t be do ne using

the ac tual proce ss liquid and running the tes ts at

actual process conditions that exist in the full-scaleplant. You s hould also e sta blish a trea tment goa l

for impurity removal to provide a ba sis for selection

of the rang e of ca rbon do sa ges to use. This ma y

be a n arbitrary number to obta in an a cc eptab le

product quality level or it may be a number estab-

lished by a regulation governing the process.

Impurity removal levels may range from 50% to

over 99%, depending on the nature of the impurity

to be removed.

The following method may be used to measure

the adsorptive capacity of a carbon sample.

1. Different amounts of carbon are added to six

bo ttles . At leas t one of the bottles should c ontain

sufficient carbon to remove impurities to a co ncen-

tration level that is below the desired treatment

goa l. Ca rbon dosa ges may range from a low of

0.05% to 5% when trea ting proc es s liq uids .

Dosa ges for wa ter and wa stewa ter treatment

are usually significantly lower.

2. Add 200 mL (or 200 grams) of the test liquid to

ea ch bo ttle, and se a l the b ottle. Tes t liq uid (200

mL) is also a dd ed to a bottle c ontaining no c arbon,

to ac t as a control sample.

3. Agitate the samples under identica l process condi-tions for the optimum contac t time de termined ea rlier.

4. Immed ia tely filter the s a mples to remove the

carbon from the solution.

5. Analyze the s amples us ing the b est method

for determining the residual impurity level you are

trying to reac h.

If at lea st one of the ca rbon trea ted s amples meetsthe impurity removal goal, you are ready to interpret

the test results. If the target level is not reached,

you may need to repea t the test w ith higher ca rbon

dos ag es. In the event you are still unable to meet

your treatment goal with higher carbon dosages,

different process c onditions or techniq ues ma y

be req uired .


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Step 3: Interpret Your Results.

Carbon Dosage Method

There a re tw o b as ic a pproac hes to interpreting the

tes t res ults for ca rbon ad so rptive ca pa city. The

first is to directly plot the amount of impurity

rema ining vs. the ca rbon dos a ge . Ide a lly, this w illproduce a plot a s s how n in Figure 2. This linea r

sc ale Isotherm P lot ca n be used to determine the

req uired ca rbon dos ag e. Loca te on the curve the

targeted impurity remaining level, and draw a

vertical line down to the X-axis to identify the

req uired c arbon dos ag e. You may also ca lculate

the ad so rptive capa city or c arbon loa ding

at this c arbon dosa ge a s follows :

Adsorptive Wt. Impurity AdsorbedCapacity Carbon Weight

This simplified iso therm technique ma y b e us ed to

co mpare the performanc e of d ifferent ca rbon s am-

ples and to identify the mos t c ost-effective c arbon.

You c ould a lso use it to de termine ho w impurity

removal and ad so rptive ca pac ity are influenced by

changes in process conditions, such as pH, tem-

perature, impurity concentration, or impurity type.

Freundlich Adsorption Equation Method

Sometimes the lab isotherm data does not produce

a plot that is easy to interpret, as indicated in

Figure 2. The curve may b e very fla t at high levels

of impurity remo va l. This ma kes it difficult to d eter-

mine the ca rbon usa ge rate ac curately. In suchca ses , a different approach is used to evaluate the

data based on the Freundlich Adsorption Equation.

This e q uation des cribe s ma thema tica lly the rela-

tionship between the amount of impurity adsorbed

a t equilibrium and the impurity concentra tion. It is

usually written as follows:

X/M = KC1/n

X = Amount o f impurity ad sorbe d a t eq uilibriumM = Weight of ca rbon used

C = Conc entrat ion of impurity rema ining in liquid

K &n = Co nstants spec ific to tes t conditions a nd

the carbon type used

When expressed in logarithmic form, this empirical

eq uation be come s a straight line eq uation w ith a

s lope of 1/n and Y-axis intercept of log K. A loga -

rithmic s ca le plot o f X/M on the Y-axis a nd C on the

X-axis usua lly res ults in a s traight line. S uch plots

a re referred to a s Freundlich Ads orption Iso therms.

CARBON DOSAGE, g/200mL

IMPURITYRE

MAINING,mg/L

Figure 2

ADSORPTION ISOTHERM

=


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An a ds orption isotherm plotted o n a log a rithmic

sc ale often makes it ea sier to de termine c arbon

loading at the desired level of impurity removal.

Figure 3 is an example of a Freundlich Isotherm plot.

Adsorption isotherms provide useful information forestimating performance in a full scale process

strea m. First, they help de termine if it is po ss ible to

reach a desired purity level with activated carbon

treatm ent. This is e spec ially impo rtant w hen multi-

ple impurities are present and one or more impuri-

ties are poorly ad sorbed . S eco nd, isotherms allow

ca lcula tion of c a rbon load ing (X/M) a t eq uilibrium,

which has a major impact on process economics.

Note that Figure 3 a nd the Freundlich eq uationshow that ca rbon loading d epends upon the eq ui-

librium c onc entrat ion of impurity rema ining in the

process liquid. Carbon loading will be higher at

higher impurity concentrations.

Ads orption isotherms ca n also be used to predict

the rela tive performance of d ifferent types of c ar-

bo n. The po sition a nd slope of the iso therm lines

reveal how well one carbon performs relative to

another carbon. A higher isotherm line means that

carbon has better adsorptive capacity than one

with a low er isothe rm line. When the iso therm line

is nearly horizontal, it means the carbon has good

adsorption of impurity throughout a wide range of

impurity concentration. A nearly vertical isotherm

line shows poor adsorptive properties at lower

impurity co nce ntra tions .

The follow ing s a mple prob lem a nd c a lcula tion w ill

illustrate how you may make effective use of the

Freundlich Ads orption Iso therm to d etermine a ca r-

bons ad sorptive capa city. It also s hows how to

co mpa re the rela tive performa nce of two different

types of powdered activated carbon.

Sample Calculation.

A process liquid conta ins 400 ppm (mg/L) of a n

impurity that must be reduced by 95%. Labo ratoryadsorption tests were conducted with two types of

carbon (A and B) to determine if the desired impuri-

ty remo val co uld b e a chieved. The purifica tion g oa l

is less tha n 20 mg /L of impurity rema ining a fter

carbon treatment. A tabulation of the test results

is g iven in Ta ble 1.

The tes t d a ta from Ta ble 1 a re plotted on a log arith-

mic scale in Figure 4, which shows the adsorption

isotherms for Carbon A and Carbon B. Both carbons

a chieved the d es ired level of purifica tion (les s than

20 mg /L of impurity rema ining). You c a n ea s ily s ee

IMPURITY REMAINING, mg/L

CARBONLOADING,mg/g

Figure 3

FREUNDLICH ADSORPTION ISOTHERM

IMPURITY REMAINING, mg/L

X/M,mg/g

Carbon ACarbon B

Figure 4

ADSORPTION ISOTHERM


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that Carbon A is the best performing carbon,

because at all impurity concentrations the amount

of impurity adsorbed per unit weight of carbon

(X/M) is higher tha n the corres pond ing va lue fo r

Carbon B. Carbon usage rates are calculated for

each carbon as illustrated below.

1. Determine ca rbon loa ding (X/M) direc tly from

the isotherm plots at the desired final impurity level

(20 mg/L).

Carbon A X/M = 74 mg/g

Carbon B X/M = 48 mg/g

2. Ca lculate impurity remova l. This is the s a me for

bo th ca rbons : 400 mg/L 20 mg/L = 380 mg/L

3. Ca rbon usa ge rate is determined b y dividing the

a mount of impurity remove d (380 mg /L) by the c a r-

bon loa ding (X/M).

Carbon A Usage = 380/74 = 5.1 g carbon/L

Carbon B Usage = 380/48 = 7.9 g carbon/L

From the calculated results, it is apparent that

a pproxima tely 55% more C arbo n B is req uired to

ac hieve the sa me trea tment results a s obta ined

with Ca rbon A. Finally, the unit prices of ea ch c arbon

type ca n be us ed to determine the most c ost-effec-

tive activated carbon for this purification application.

Conclusion.

Lab oratory a ds orption tes ting is the only w ay you

ca n determine how effective an a ctivated ca rbon

will be in your purification process. Accurate, unbi-

as ed tes ting on the ac tual proce ss liq uid a t plant

proce ss cond itions will provide da ta that you c an

use confidently. This type o f tes ting is the only

effective w ay to co mpare ca rbons for a particular

application.

NoritAmerica s Inc. produces the mos t diverse

produc t line of a ctivated ca rbo n in the wo rld. With

over 80 years of technological expertise, we are

one of the most experienced manufacturers of pow-

dered, granular, and extruded a ctivated c arbons.

NoritAmericas Inc. has the capabilities and the

resources to recommend the right carbon for your

purifica tion need s.

CarbonDosage

(g/200mL)

00.08

0.160.32

0.64

1.95

CarbonDosage

(g/L) (M)

0

0.4

0.81.6

3.2

9.75

ImpurityRemoved

(mg/L)(X)

0

152

236308

366

390

Impurity

Removed / Unit

Weight of Carbon(mg/g) (X/M)

0380

295

193114

40

Concentration

of ImpurityRemaining

(mg/L) (C)

400

248

16492

34

10

Carbon

Dosage

(g/200mL)

0

0.080.16

0.32

0.643

Carbon

Dosage(g/L) (M)

00.4

0.8

1.63.2

15

Impurity

Removed

(mg/L) (X)

0

120200

280

340391

ImpurityRemoved / Unit

Weight of Carbon(mg/g) (X/M)

0

300250

175106

26

Concentration

of ImpurityRemaining

(mg/L) (C)

400

280

200120

60

9

CARBON

A

CARBON

B

Table 1

ISOTHERM DATA


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2001 NORIT NA00-3 2000

Norit Nederland BV

Amers foo rt, The Netherla nds

Telephone : 31334648911

Telefa x: 31334617429

Norit (U.K.) Ltd.

Glasgow, S cotlandTelephone: 441416418841

Telefa x: 441416410742

Norit (France) S.a.r.l.

P aris, Franc e

Telephone : 33145910808

Telefa x: 33148673603

Norit Italia S.p.A

Ravenna, Italy

Telephone : 39544451514

Telefa x: 39544451283

Norit Deutschland G.m.b.H.

Dsseldorf, G ermany

Telephone: 49211906020

Telefa x: 49211161115

N.V. Norit Belgium S.A.

Brussels, BelgiumTelephone : 3226750645

Telefa x: 3226751119

Norit (Japan) Co. Ltd.

Mina to-Ku, Tokyo, J a pa n


Telefa x: 81352952860

Norit Singapore Pte. Ltd.

S ingapo re, S inga pore


Telefa x: 657353166

Norit Americas Inc.

3200 West University Avenue

Ma rsha ll, TX 75670

800-641-9245

Telephone: 903-923-1000

Telefax: 903-923-1003

www.norit-americas.com

e-mail: [email protected]

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