United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-91/048 Dec. 1991
Project Summary
Evaluation of the MIDDAS
System for Designing GAC
Adsorbers
Walter J. Weber, Jr., Margaret C. Carter, Kevin P. Olmstead, and Lynn E. Katz
The micro-diaimster-depth-adsorptlon
system (MIDDAS) was evaluated for its
usefulness In determining equilibrium
parameters for adsorption In granular
activated carbon (GAC) systems. The
system employs a column configura-
tion for determining such parameters,
rather than the traditional completely
mixed batch reactor (CMBR) configura-
tion. The equilibrium results were em-
ployed in the homogeneous surface dif-
fusion model (HSDM) in conjunction
with the short bod adsorber (SBA) tech-
nique to determine rate parameters for
trichloroethylena (TCE) adsorption on
GAC in both single-solute and more
complex systems. The results of these
studies indicated that the equilibrium
capacity for TCE adsorption from dis-
tilled water was lower to a statistically
significantly extent when determined by
the MIDDAS technique than when de-
termined by the CMBR technique. The
rate parameter associated with trans-
port within the adsorption particle (Dg,
the surface diffusion coefficient in the
HSDM) was significantly dependent on
the selected isotherm capacity param-
eter, whereas the rate parameter asso-
ciated with external mass transfer (k(,
the film transfer coefficient) was not
significantly affacted by changes In the
Isotherm. These calibrated rate param-
eters were used in subsequent verifi-
cation studies with deep adsorption
beds; the results Indicated that rate
parameters determined using the
MIDDAS technique tended to provide
better predictions of deep bed behav-
ior than did those determined with the
CMBR methodology.
The MIDDAS methodology was also
employed In determining parameters for
both a bl-solute system Involving TCE
and p-dlchlorobenzene (DCB) and for a
system Involving TCE In a background
of naturally occurring organic matter.
In the bi-solute case, the equilibrium
interactions between the two adsor-
bates were evaluated using the ideal
adsorbed solution theory (IAST), which
generally described the data well.
Model predictions resulting from the
parameters obtained with the use of
these techniques were, however, not
as good, with the predictions for DCB
generally being superior to those for
TCE. In the study involving the pres-
ence of background organic matter,
TCE was added to Huron River water
before use in the MIDDAS system. The
predictions resulting from the param-
eters obtained in this case were poor,
indicating the possibility of a depen-
dence of adsorption capacity on adsor-
bent particle size or on other factors
not accounted for in the MIDDAS ap-
proach.
This Project Summary was developed
by ERA'S Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that Is fully documented In a separate
report of the same title (see Project
Report ordering Information at back).
Introduction and Background
GAC treatment technology has been
designated as the best available technol-
ogy for removal of synthetic organic chemi-
cals (SOCs) from contaminated water.
While GAC treatment has indeed proven
Printed on Recycled Paper
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to bo an excellent option for removal of a
broad range of SQCs commonly found in
raw water sources, difficulties still remain
with regard to design of technically and
economically feasible systems.
Fixed-bed (or column) adsorbers repre-
sent the most practical category of reactor
configuration for GAC treatment systems.
The objective of GAC system design in
such instances is to optimize empty bed
contact time, hydraulic surface loading,
and system configuration (i.e., series or
parallel column operation) to yield maxi-
mum utilization GAC adsorption capacity
white meeting specific treatment objec-
tives. Various approaches have been de-
veloped to aid in the design process; com-
puter-based mathematical models offer the
dual advantage of furnishing the design
engineer with important system informa-
tion, as well as providing an avenue for
Indopth investigation of factors affecting
the adsorption process.
The HSDM is a predictive model that
has been validated over an extensive
range of conditions. It incorporates math-
ematical descriptions of the major physi-
cochemlcal mechanisms recognized to
occur in fixed-bed systems: namely, axial
flow with dispersion, local equilibria at the
surface of the particle, mass transfer re-
sistance across a hydrodynamic boundary
layer surrounding the particle, and
Intraparticle diffusion along pore surfaces
within the particle. Input parameters to
the HSDM consist of phenomenological
rate and equilibrium coefficients. A vari-
ety of multi-parameter isotherm models
generally describe GAC equilibria well. To
calibrate equilibrium models, two or more
characteristic constants must be deter-
mined; in the case of the widely used
semi-empirical Freundfch model, the pa-
rameters are an equilibrium capacity pa-
rameter, Kp, and a parameter, n, which
relates to the magnitude of the driving
force for adsorption. Rate processes in
the HSDM, modeled as two resistances in
series, are characterized by an external
mass transfer coefficient, k(, and an
Intraparticle surface diffusion constant, Ds.
Accurate estimation of model param-
eters is critical to reliably predict perfor-
mance. Isotherm parameters such as KL
and n are traditionally obtained from CMBR
equilibrium data. Values for kf are typi-
cally determined from mass transfer cor-
relations derived from experimental data
for other systems (so-called "literature" cor-
relations), and values for Df are com-
monly obtained by fitting CMBR rate data
with the use of an appropriate mathemati-
cal model. A number of difficulties and
potential errors are associated with these
parameter estimation techniques. Mass
transfer correlations are usually developed
for systems substantially different from the
system to which they are applied. Fur-
thermore, the hydrodynamic and contami-
nant removal characteristics of CMBR rate
systems differ dramatically from those of
the fixed-bed reactors used in practical
GAC system designs. It is impossible to
predict which solutes or classes of solutes
may be most influenced by reactor con-
figuration from first principles. If capacity
measurements are in fact strongly depen-
dent on reactor configuration for a particu-
lar GAC application, then use of CMBR-
derived parameters to attempt simulation
of column configurations may lead to an
inaccurate description of adsorptive be-
havior of solution components.
The above discussion delineates one of
the major difficulties associated with the
use of mathematical models for GAC sys-
tem design: namely, accurate parameter
determination. The development of meth-
odologies to practically and economically
determine accurate input parameters has
been a major focus of research over the
past decade. This research has led to
development of a number of bench-scale
methodologies for determination of both
equilibrium and rate model coefficients.
The MIDDAS technique employs a combi-
nation of the SBA technique and a modi-
fied version of a high-pressure minicolumn
method. It was developed as a means to
provide greater accuracy in determining
model equilibrium parameters in a system
with the same hydrodynamic attributes as
the full-scale adsorber, while doing so in a
manageable time frame. The SBA tech-
nique was developed to provide greater
accuracy in estimating rate parameters.
The MIDDAS methodology allows simul-
taneous determination of external film
transfer and intraparticle surface diffusion
coefficients from the same set of experi-
mental data; this obviates the need for
literature correlations and CMBR-based
rate studies and eliminates potential error-
compounding by mutual compensation of
individual errors in these coefficients dur-
ing parameter search/regression analysis.
The application of this methodology to es-
timate mass transport parameters has
been demonstrated previously, and the
approach offers substantial promise as a
means for developing bench-scale infor-
mation that can be used to facilitate and
enhance full-scale system design proce-
dures.
The overall goal of this study was to
extend previous investigations of the ap-
plicability of the MIDDAS methodology to
a range of practical multiple-solute/back-
ground water supply conditions for se-
lected SOCs. A number of specific objec-
tives relating to this aim were investigated
in the studies described here, including:
1) comparison of MIDDAS and CMBR
methodologies for determination of equi-
librium parameters; 2) evaluation of the
SBA methodology for estimation of mass
transport parameters; and 3) evaluation of
ideal adsorbed solution theory (IAST) in
conjunction with the MIDDAS isotherm
technique as an approach to modeling
multisolute systems.
Materials and Methods
Materials
The adsorbent used in all experiments
was Filtrasorb 400* activated carbon
(Calgon Corp., Pittsburgh, PA). GAC par-
ticle size fractions were obtained by crush-
ing and sieving carbon samples obtained
from one lot of carbon. The resulting
fractions were washed in distilled, deion-
ized water to remove fines, dried over-
night at 105°C, and transferred to airtight
containers for storage. Carbon for imme-
diate use was dried again to remove any
moisture adsorbed in storage and stored
in a desiccator.
TCE was the primary target SOC em-
ployed in this work; DCB was employed
as a second target solute in bi-solute in-
vestigations. TCE and DCB were se-
lected because they have been designated
as U.S. Environmental Protection Agency
priority pollutants by the EPA, have been
identified in contaminated surface and
groundwaters, are relatively straightforward
to analyze, and represent a broad class of
compounds commonly found in the envi-
ronment.
Feed solutions of the target solutes were
prepared through direct injection of a high-
concentration stock solution of the target
solute dissolved in methanol into the back-
ground water. Experiments were con-
ducted at a temperature of 250±2°C. All
samples were extracted immediately into
hexane and analyzed for TCE or DCB by
packed or capillary column gas chroma-
tography with electron capture detection.
To obtain high purity water, distilled-
deionized water was processed through a
Milli-Q water system (Millipore). Huron
River water (HRW), collected from Argo
Park in Ann Arbor, was stored and refrig-
erated in 55-gal stainless-steel drums un-
til used. Approximately 100 mg/L sodium
azide was added to retard bacterial growth
* Mention of trade names or commercial products does
not constitute endorsement or recommendation for
use.
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and degradation of organic matter. Batch
reactor isotherm studies have shown that
sodium azide does not significantly affect
the adsorption equilibria of target com-
pounds. Before use, the HRW was fil-
tered through a 1.0 u. filter (Gelman Sci-
ences) to remove particulate matter. No
attempt was made to control pH.
Methods
Isotherm Parameters from CMBR
Data
Individual 250-mL borosilicate bottles
containing preweighed masses of 80/100
mesh activated carbon were filled with a
solution of the designated background
water spiked with either TCE or DCB.
Initial solution concentrations of the target
compound and carbon masses were esti-
mated to achieve equilibrium results that
spanned the concentration range of inter-
est. Filled bottles were sealed headspace-
free with the use of Teflon-lined septa and
allowed to equilibrate on a rotary tumbler
for 7 days. Rate studies with TCE and
DCB showed that an equilibration time of
7 days was sufficient to achieve equilib-
rium for the carbon size used in this study.
Filled, sealed bottles without carbon were
also tumbled simultaneously with the
sample bottles to assess volatility losses.
After equilibration, samples were taken
from the bottles containing carbon using a
gaslight syringe (Hamilton), filtered through
a prewashed glass fiber filter (Gelman Sci-
ences) in a stainless-steel filter holder
(Fischer Scientific) to remove any fines in
the sample, and analyzed for TCE or DCB.
Other studies indicated that TCE and DCB
losses onto the filters and filter holder
were negligible. The mass of solute on
the carbon at equilibrium, qo, was obtained
by a mass balance on the closed system:
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Tabifl.lsothormParamotersforTCEinMilli-QWaterwiththBUseofMIDDASandCMBRTechniques
Technique Kp* n
MIODAS
95% Conf. limits
CMBR
95% Conf. Limits
1.10
(0.846, 1.43)
1.94
(1.72, 2.20)
0.564
(0.511,0.616)
0.527
(0.498, 0.555)
* Based on q. In ng/mg and C, in jtg/L
7«W» 2. Effect of TCE Concentration on Rate Parameters
Ce(ng/L)
D,(x10locmz/sec)
95% Conf. Limits
k,(x 103 cm/sec)
95% Conf. Limits
104.6
398
1052
0.70
(0.26,2.8)
1.6
(0.7,3.4)
6.6
(2.2,250)
21
(6.7,30)
18
(10,23)
11
(8.3.14)
anticipated influent concentration as pos-
sible.
Large Column Verification
Studies
A set of large column adsorber data for
TCE in Milli-Q water on 30/40 particle size
GAC was obtained. The average influent
concentration was approximately 1000 u.g/
L. Rate parameters obtained using both
the CMBR-based and the MIDDAS-based
isotherm and the SBA technique are re-
ported in Table 3. Both parameter sets
ware used in the homogeneous surface
diffusion model to predict the breakthrough
profile for these systems. The predica-
tions along with experimental breakthrough
data are presented in Figure 3. Although
early time predictions fit the data reason-
able well, in the latter portions of the break-
through, the MIDDAS-derh/ed parameters
clearly demonstrate a better fit to the data
than do those based on the CMBR iso-
7>W«3. Ftito Parameters for Largo Column Data
therm. The CMBR-based predication
shows a breakthrough delayed from the
actual data, which is indicative of the larger
capacity given by the batch reactor iso-
therm.
Bl-solute Predictions
SBA data were obtained for the bi-sol-
ute system consisting of DCB and TCE in
Milli-Q water. To calibrate the bi-solute
version of the HSDM model, the SBA data
obtained from this experiment were used
in conjunction with the IAST model and
with both the single-solute MIDDAS and
CMBR isotherm parameters for TCE and
DCB. The calibrated bi-solute parameters
were used to predict deep bed perfor-
mance for a bisolute MIDDAS experiment.
Figure 4 presents one set of data with the
accompanying predictions. The data rep-
resent the experimental breakthrough pro-
files for influent concentrations of TCE
and DCB of 754 and 2,332 u.g/L, respec-
tively. As can be seen in Figure 4, the
GAC size isotherm
k(cm/secx 103)
(95% Conf. Limits)
p.(cm2/secx 1010)
Limits
30/40
30/40
MIDDAS
CMBR
3.9 (3.8,4.2)
4.0 (3.7,4.2)
8.7(7.3,11.0)
3.2(3.0,4.1)
prediction for the DCB breakthrough by
.the MIDDAS-based parameters is very
good, whereas that for the CMBFt-based
parameters overpredicts adsorption of
DCB. The model also nicely captures the
shape of the TCE breakthrough and over-
shoot for both sets of parameters, although
both predicted curves are displaced slightly
from the experimental data.
TCE In Huron River Water
The geometric mean regression method
was used to determine the Freundlich pa-
rameters for TCE in HRW. Both the
MIDDAS and CMBR experimental meth-
odologies were employed, with trends in
1C and n similar to those observed for the
Milli-Q water. SBA data were obtained for
TCE in HRW for the 30/40 particle sizes.
The data were used with the HSDM model
to determine rate parameters for both the
MIDDAS and CMBR isotherms. The re-
sults of the model calibrations are given in
Table 4.
Predictions were made of TCIE large
column data with the use of both MIDDAS
and CMBR isotherm and rate data (Figure
5). As can be seen, neither parameter
set predicts the data well, with both pre-
dictions greatly overpredicting removal of
TCE. Because even the MIDDAS method
cannot yield a good prediction, additional
factors not accounted for by the MIDDAS
approach are active in this system. A
rough approximation of the capacity of the
carbon bed used in the large column veri-
fication study indicates a value 25% lower
than would be predicted by the MIDDAS
isotherm conducted in HRW. Although
rate parameter determinations do indicate
some reduction in D. in the HRW from the
Milli-Q case, which can cause some re-
duction in apparent capacity because of a
slowing of the kinetics of adsorption, it is
unlikely that as large of a capacity reduc-
tion as was estimated could be precipi-
tated by only a moderate reduction in D,.
The major difference between the
MIDDAS and large column systems is par-
ticle size. The MIDDAS methodology,
which uses the smaller 80/100 particle
size to expedite determinations of equilib-
rium data, is predicated on different par-
ticle sizes having the same equilibrium
capacity. Although this assumption ap-
pears to hold for the studies conducted in
the Milli-Q water, additional factors that
violate this requirement are clearly mani-
fest in the HRW study. Further study is
necessary to determine if modification can
be made to the MIDDAS technique to
enable accurate description of large col-
umn breakthrough data when background
organic matter is present.
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Three-way
valve
Headspace
free cylinder
Teflon
plunger
Injection and
refill port
Magnetic stirrer
Divert flow line
to waste
Stainless steel
tubing (typ.)
Figure 1. Schematic of the MIDDAS system.
100
10-
10
100
o
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O
1.2-
1.0-
0.8-
0.6-1
0.4-
0.2-
0.0
a data
MIDDAS prediction
— GMBR prediction
—j—
20
40
—I—
60
—i—
80
100
Throughput (1000 bed volumes)
. Prediction of large column breakthrough data for TCE in Milli-Q water using MIDDAS-
and CMBR-defived Isotherm and rate parameters for 30/40 particle size.
o
5
1 -
,Q DOB Data
• TCE Data
MIDDAS DCB Pred.
MIDDAS TCE Pred.
— • CMBR DCB Pred.
-- CMBR TCE Pred.
20 40 60 80
Throughput (1000 Bed Volumes)
100
120
FJgur»4. Sample prediction of bi-solute data.
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1.0
o
Q
0.8-
0.6-
0.4-
0.2-
B .data
MIDDAS prediction
— CMBR prediction
0.0-9
20 40 60 80 100
Throughput (1000 bed volumes)
Figure 5. Prediction of large column breakthrough data for TCB in HRW using MIDDAS and CMBR-
derived parameters
Table 4. Rate Parameters for TCE in Huron River Water
k, (cm/sec x 103) D (cm2/sec x 1010)
GAG size isotherm (95% Conf. Limits) (95% Conf. Limits)
30/40
30/40
MIDDAS
CMBR
4.0 (3.7.4.3)
4.0 (3.2,4.4)
1.6(1.3,2.1)
0.80 (0.74,0.98)
Conclusions
The equilibrium capacity for TCE in I
Q water determined in the MIDDAS sys-
tem was statistically lower than that ob-
tained using the CMBR method, whereas
the n value for the MIDDAS tended to be
higher than that of the CMBR. The impor-
tance of these differences were confirmed
in large column verification studies for the
30/40 and 16/20 particle sizes; the predic-
tions made with the use of parameters
based on the MIDDAS isotherm yielded
predictions superior to those from CMBR-
derived parameters.
As with the adsorption from Milli-Q wa-
ter, the capacity for TCE in HRW in the
MIDDAS system was significantly de-
pressed from that in the CMBR determi-
nations. Unlike the Milli-Q water results,
however, neither MIDDAS- nor CMBR-
based parameters provided adequate pre-
dictions of a large column verification
study, with overpredictions of removal in
both cases. Although isotherm determi-
nations indicated a reduction in capacity
because of competitive substances in the
HRW as well as compounding reductions
because of the column configuration of
the MIDDAS protocol, an approximate es-
timate of the capacity of the carbon bed
used in the large column verification study
indicates a value lower still than was pre-
dicted by the MIDDAS isotherm in HRW.
Rate parameter determinations do indi-
cate a reduction in D, in the HRW from
the Milli-Q case; these determinations can
cause some reduction in apparent capac-
ity because of a slowing of the kinetics of
adsorption. It is unlikely, however, that as
large a capacity reduction as was esti-
mated could be precipitated by only a
moderate reduction in D,. One obvious
difference between the two systems is
GAG particle size. The smaller 80/100
particle size employed in the MIDDAS sys-
tem is to provide rapid attainment of equi-
librium parameters while still maintaining
the hydrodynamic attributes of a column
system. A major assumption in the devel-
opment of the MIDDAS protocol was that
equilibrium capacity is not particle size
dependent. Although no dependency of
equilibrium capacity on particle size was
manifest in the Milli-Q water studies, par-
ticle size apparently does affect the equi-
librium capacity of TCE in HRW. Thus,
although estimates in the MIDDAS sys-
tem do improve predictions somewhat over
the CMBR case, it is clear that achieving
similar hydrodynamics is not necessarily
enough to ensure estimate of accurate
isotherm parameters and that other fac-
tors, such as particle size and the pres-
ence of background organic matter, must
be addressed in further refinement of the
MIDDAS methodology.
In the bi-solute studies, reductions of
TCE adsorption capacity in the presence
of DCB were captured qualitatively by
IAST. Predictions for DCB breakthrough
were quite acceptable; the MIDDAS-de-
rived parameters achieved better predic-
tions of the data than did the CMBR coun-
terparts. Predictions of deep bed adsorber
data for TCE were, however, only moder-
ately satisfactory as best, comparable re-
sults were achieved by using both
MIDDAS- and CMBR-based parameters.
The research presented here has sought
to bring parameter estimation methods one
step closer to the reactors commonly used
in practice. The results have proved in-
conclusive, however. In some cases, the
capacity estimations of the MIDDAS
method were better than those of the
CMBR technique; the MIDDAS method
did not universally improve predictions of
both single- and bi-solute data over those
from CMBR-derived parameters. More-
over, although the SBA methodology itself
remains a viable and facile method of rate
parameter determination, the demonstrated
dependency of the SBA-D, values on ca-
pacity measurements makes such deter-
minations susceptible to propagation of
isotherm inaccuracies and hence brings
the problem full-circle. The nature of these
findings indicates that there are still many
research areas deserving of pursuit with
regard to applying the MIDDAS approach
to simulation and to design of fixed-bed
GAC reactors.
It is clear from this and others' works
that an a priori assessment of which com-
pounds will be most significantly affected
by reactor configuration is not possible at
•&U.S. GOVERNMENT PRINTING OFFICE: 1992 - 648-080/40117
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this Juncture. Further work Is required to
elucidate the exact mechanisms that bring
about the reductions seen in column based
systems. Factors that may play an impor-
tant role in determining the sensitivity of a
compound's capacity to reactor configura-
tion Include the presence of other
adsorbing species, molecular size, hydro-
phobicity, polarity, functional groups, and
carbon type.
Because the capacity of GAC for TCE
is dependent on particle size for adsorp-
tion from HRW, the MIDDAS methodology
may have to be modified. Further studies
are necessary to determine the mechanis-
tic causes of the observed effect.
The full report was submitted in fulfill-
ment of Interagency Agreement CR-
814135-01-0 by the University of Michi-
gan under the sponsorship of the U.S.
Environmental Protection Agency.
W. J. Weber, Jr., M.C. Carter, K.P. Olmstead, and L.E. Katz are with the University
of Michigan, Ann Arbor, Ml 48109-2125.
Thomas F. Spath is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of the MIDDAS System for Designing GAC
Adsorbers,"(OrderNo.PB91-234617/AS;Cost:$19.00,subjecttochange)willbe
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati, OH 45268
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