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Transcript of 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
Telephone: 81352952850
Telefa x: 81352952860
Norit Singapore Pte. Ltd.
S ingapo re, S inga pore
Telephone: 657353066
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]