1. technology and computing

ACID EXCHANGE RESINS DEACTIVATION IN THE ESTERIFICATION OF
FREE FATTY ACIDS
Riccardo Tesser(°), Martino Di Serio(°), Luca Casale(°), Lucio Sannino(+), Marianna
Ledda(+), Elio Santacesaria(°)
(°)
University of Naples Federico II, Department of Chemistry, via Cintia 80126 Naples,
Italy
(+)
ASER S.r.l. Co, via F.Icace n.1, 84131, Salerno, Italy
Abstract: In this work the deactivation of an exchange resin, used as catalyst for
promoting the esterification of fatty acids for producing biodiesel, has been studied. At
this purpose, a dynamic mathematical model, suitable to describe the performances of a
tubular reactor, containing the catalysts mixed with stainless steel springs as inert diluent,
and its evolution with time due to the catalyst deactivation has been developed. The
catalyst deactivation has been shown to depend mainly on the poisoning effect of iron
that was present as impurity in the fatty acids used as feedstock.
Keywords: Esterification; Ion-exchange resins; Deactivation, Free Fatty Acids, Biodiesel
1. INTRODUCTION
The worldwide interest towards biofuels has recently significantly grown as a direct result of the renewed need of
facing the global warming effect by reducing the greenhouse gases emissions that are related to the wide use of
fossil fuels. With this respect, biodiesel represents a valuable alternative to petroleum-derived fuels due to both its
renewable nature and its substantially null net carbon dioxide emission. This biofuel is conventionally produced
through batch or continuous transesterification of refined vegetable oils with methanol by using homogeneous
alkaline catalysts like sodium or potassium hydroxides or methoxides (Ma and Hanna, 1999; Fukuda et al., 2001).
Glycerol is the by-product of this reaction in a ratio of 10% by weight of the oil. The mentioned technology,
however, is only compatible with highly refined oils which content of free fatty acids (FFAs) don’t exceed the
threshold value of about 0.1% by weight. As a matter of fact, FFAs in the presence of an alkaline catalyst, give
place to soaps forming stable emulsions between biodiesel and glycerol characterized by a long settling time for a
complete separation of the two liquid phases. The main limitation for a wider biodiesel market diffusion is,
therefore, represented by the relatively high raw material cost: the steps of production, transportation, storage and
refining of vegetal oils affect for more than 85% of the total biodiesel cost (Van Gerpen, 2007) making biodiesel by
the conventional production technology significantly more expensive than diesel oil from petroleum.
A possible solution to this drawback could consist in the development of new technologies able to employ waste
raw materials like fried oils or waste oils from various sources that cannot be treated in the conventional process for
their high content in free fatty acids. This perspective is very interesting and discloses the way toward the
development of innovative biodiesel production processes like that, as example, based on supercritical methanol
(Kusdiana and Saka, 2001), or the two-stage process (Lacaze-Dufaure and Mouloungui, 2000; Berrios et al., 2007) .
In this last, the oil acidity is reduced below the acceptable limit by an esterification pre-treatment with methanol
producing methylesters (biodiesel) and water while, in the subsequent step, the traditional transesterification can be
performed producing biodiesel and glycerol being the FFAs almost totally converted in the first step. The
esterification reaction of acid oils or animal fats can then be used both as biodiesel direct production and as pretreatment step in the framework of a conventional transesterification process.
(*)
To whom correspondence must be addressed ([email protected]).
The esterification processes for FFAs abatement are generally based on homogenous acid catalyzed reaction
(Lacaze-Dufaure and Mouloungui, 2000; Berrios et al., 2007) or by ionic-exchange acid resins as heterogeneous
catalysts. These resins are subjected to a remarkable swelling phenomena (Flory, 1953; Mazzotti et al., 1997; Lode
et al., 2004) when are contacted with polar solvents like methanol or water. The high liquid volume retained and the
selectivity towards the adsorption of polar substances can results in a significant alteration of liquid composition and
of the kinetics of the reaction occurring inside the resin particles.
A paper published by Tesser et al. (2005) reported the esterification reaction kinetics of oleic acid with methanol in
the presence of triglycerides, catalyzed by an acid exchange resin in a batch reactor. Furthermore, Santacesaria et al.
(2007a; 2007b) have shown that the esterification reaction, performed in a continuous packed bed tubular reactor
(PBR), is possible, but for obtaining high conversions long residence times and consequently low volumetric flow
rates are required resulting in very low Reynolds numbers at which the external fluid-to-solid mass transfer
resistance becomes significant in comparison with the intrinsic kinetics.
In this work, the esterification with methanol of a mixture of free fatty acids has been studied in the presence of an
acid exchange resin catalyst of the type Resindion Relite CFS in a fixed bed tubular reactor operated for long timeon-stream. The experimental runs have been shown a progressive deactivation of the resin mainly due to the
presence of iron dissolved in the feeding oil. The experimental data have been interpreted with a mathematical
model based on both a new more suitable kinetic approach and a reasonable deactivation mechanism.
2. EXPERIMENTAL SECTION
2.1 Reagents.
Reagents used in the experimental runs are: Relite CFS of Resindion, which properties are summarized in Tab. 1,
methanol of high purity (99.8 %) and a mixture of free fatty acids containing the equivalent of 95% of oleic acid.
The amount of iron and other metals dissolved in this mixture is reported in Tab. 2.
Table 1. Characteristics of the resins RELITE CFS.
matrix
porous copolymer styrene-DVB
functional groups
Sulfonics
acidity
3.6 meq/g
particles mean diameter
0.7 mm
particles size range
0.3-1.18 mm
total exchange capacity
2.0 eq/L
maximum operating temperature
140 °C
0.840 g/cm3
bulk density
Table 2: Metals content in the fatty acids mixture (ppm)
Na
K
Ca
Cd
Cr
Fe
Mg
133.5
3.0
26.7
0.1
0.4
455.2
1.3
2.2 Experimental apparatus.
The experimental runs have been performed in a stainless steel (AISI 304) tubular reactor with length 50 cm and an
internal diameter of 8.2 cm. The catalytic bed contained 1 kg of exchange resin and 1.8 Kg of stainless steel springs
having the scope of compensating the swelling effect avoiding an excessive pressure drop along the catalytic bed
(Siano et al., 2006) . Methanol and fatty acids mixture were mixed in a weight ratio 0.72:1 in a reservoir, gently
mixed, preheated at 35°C. This solution was fed to the reactor with a constant flow rate. The reactor was kept at
100°C and 6 bars to maintain methanol in liquid phase. The reactor was operated continuously 12 hours per day and
samples were withdrawn at different times and at different heights of the reactor (see Fig. 1) measuring, in each
sample, the residual acidity by titration with KOH 0.1 N in ethanol.
Fig. 1. 1. Feeding reservoir containing free fatty acids and methanol. 2.Feeding pump. 3. Packed bed tubular
reactor. 4.Thermostatted heating system. 5-8. Valves for withdrawing samples of the reaction mixtures at different
reactors heights (5=inlet; 6,7 = intermediate heights; 8= outlet)
3. RESULTS AND DISCUSSION
From the values of acidity determined by titration, it is possible to evaluate the experimental conversion profile
along the reactor by using the relation (1):
xhexp
Ac0 Ach
Ac0
(1)
where Ac0 is the acidity at the reactor inlet, while, Ach is the acidity determined at the height h.
The activity loss of the catalyst can be expressed as in (2):
Activityh
where
xhexp
xhexp
t
0
xhuc is the conversion of the uncatalysed reaction, while, xhexp
xhuc
xhuc
t
0
(2)
is the one of the catalysed reaction in the
absence of deactivation (corresponding to 0 time), both related to the different heights of the catalytic bed. The
activity data are reported in Fig.2.
Mathematical model for interpreting the experimental runs. The catalyst deactivation profiles of Fig. 2 have
interpreted by developing a mathematical model able to simulate the dynamic behaviour of the tubular reactor
considered as a sequence of 50 different cells operating as dynamic CSTRs in series.
1.0
0.9
0.8
Activityh (-)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Time/Time of reaction (h/h)
Fig. 2. □ Experimental data at intermediate height (6) ∆ Experimental data at intermediate height (7) ○
Experimental data at reactor outlet (8). ── Simulation at intermediate height (6) ─ ─ Simulation at intermediate
height (7) ─ ∙ ─ Simulation at reactor outlet (8).
This model consider also the physical partition of the reagents and products between the internal and the external
part of the catalytic particles, the chemical adsorption on the catalytic sites and the effect of the external mass
transfer. The mass balance of the j-th cell is given by the following system of differential equations:
VLC
dCib, j
dt
Q Cib, j
1
Q Cib, j +
i
rncb , j VLC
rncr , j VrigC
C
rcat , j Wcat
60
(3)
b
where Q is the volumetric flow rate of oil and methanol, Ci , j is the i-th component concentration in the liquid bulk
(i = A – acid, M – methanol, E – ester, W – water, T – triglycerides) of the j-th cell,
i
is the stoichiometric
coefficient of the reaction: “+1” for the products (W, E) , “-1” for the reagents (A, M), “0” for the inerts (T). The
apex r, b, means respectively the resin and the bulk, while apex C means that the related value corresponds to a
single cell. Therefore,
C
are respectively the bulk liquid volume, the swelling volume of the resin and
VLC , VrigC , Wcat
b
r
the mass the catalyst all referred to a single reaction cell. ruc , j ed ruc , j are the uncatalysed reaction rates
respectively in the liquid bulk and in the resin of the j-th cell. These reaction rates are calculated with the relations
(4) and (5):
rucb , j
kuc C Ab , j
2
rucr , j
kuc C Ar , j
2
CMb , j
(4)
CMr , j
(5)
rcat , j is the catalysed reaction rate that can be calculated with relation (6):
j
kcat H A C Ar , j
rcat , j
1
where
j
H E CEr , j CWr , j
k
cat
CMr , j
H A C Ar , j
H E CEr , j
HW CWr , j
CMr , j
CMr , j
CMr , j
(6)
is the deactivation parameter variable from 0 and 1 according to the kinetics reported below.
Kinetic constants kn are dependent on the temperature according to the Arrhenius equation (7):
kn
where,
knrif exp
E A, n
R
1
T rif
1
T
(7)
knrif is the kinetic constant of the n-th reaction at the reference temperature Trif of 373.16K.
The Hi parameters, the ionic exchange constants of the i-th component with methanol, have been considered
independent of the temperature in the examined range. The values of all the parameters used in the simulations are
reported in Tab. 3.
Table 3: Kinetic and ion-exchange parameter
Uncatalyzed reaction
Catalyzed reaction
k
ref
cat
12.11 mL g -1cat min -1
kucref = 65.78 mL2 mol-2 min -1
E A,cat
12.79 kcal mol -1
16.28 kcal mol-1
k refcat
5.77 mL g -1cat min -1
E A,uc
E A,
8.02 kcal mol -1
cat
HA
0.46
HW
2.40
HE
0.15
The mass transfer limitation has been evaluated by assuming a pseudo-steady state condition for the amount of fatty
acid diffusing and reacting. According to this condition we can write:
b
i, j
k S aS C
s
i, j
C
i
r
nc , j
rcat , j Ccat + r
VrigC
V
C
(mol min-1 mL-1)
(8)
where, ks, the mass transfer coefficient, has been roughly assumed equal to the one of oleic acid for all the
components and estimated by the Olive correlation (Seguin et al., 1996). For the estimation of the mass transfer
properties the composition at the reactor inlet has been considered. as is the specific surface area calculated as the
geometric surface area external to the resin particles referred to the reactor volume; CSi,j is the i-th component
concentration on the surface of the resin in the j-th cell; CCat is the catalyst concentration defined as mass of catalyst
per volume of the reactor; VC is the volume of a single cell.
The concentrations of each component in the liquid bulk and on the surface of the resin are linked by the partition
equilibrium and can be calculated by relations (9).
K i Cis, j
Mwi
K z C zs, j
z
Cir, j
where Ki (or Kz) are the partition constants, while
i
ie
(9)
Mwi are respectively the density and the molecular weight of
the i-th component. An index of the affinity of the i-th component for the resin is given by the effective partition
constant of relation (10):
Ki i
Mwi
Kieff
(10)
All the estimated partition parameters are reported in Tab. 4.
Table 4. Partition constants.
Component
K i (mL mol-1)
Kieff (-)
Water
1
0.0542
Methanol
0.503
0.0109
Acid
1
0.00293
Ester
0.503
0.00135
Kinetics of catalyst deactivation. The catalyst deactivation parameter j depend on the mechanism of catalyst
poisoning and related kinetics. By analysing the poisoned catalyst it is possible to observe that poisoned resin retain
large amounts of the iron that is present in the fatty acids mixture fed to the reactor. For this reason we have
attributed to iron the main effect of catalyst deactivation. The effect of poisoning can be interpreted as the result of a
reaction of the type:
Feq+
kd
Sitefree
Siteoccupied
(11)
a power law kinetics of the type reported in relation (12) can be assumed as a first approximation:
rd
(mmol min-1 gcat-1)
kd CFe CS
(12)
where, rd is the rate of poisoning, kd is the deactivation constant, CFe is the iron concentration in the flowing reagent
(mmol mL-1), Cs is the concentration of active sites per gram of resin (meq H+ gcat-1) and is the number of active
sites poisoned per mole of adsorbed iron. The best value for resulted equal 2.
Deactivation kinetics is taken into account in the dynamic model of the tubular reactor by considering the mass
balance related to the poison in each j-th cell, as reported in the following differential equation:
VLC
dCFe, j
dt
Q CFe, j
1
where the rate of poison disappearing from the liquid bulk,
Q CFe, j
rd , j WcatC 60
rd , j , is given by:
(13)
kd CFe, j CS0
rd , j
where
j
(mmol min-1 gcat-1)
(14)
CS0 is the initial concentration of the acid sites of the resin. The deactivation factor
j
corresponds to the
ratio between the un-poisoned residual and the initial acid sites for each cell. In order to evaluate
j
, the mass
balance on the un-poisoned acid sites must be solved, see equation (15);
d
rd , j
j
dt
C
0
S
60 (h-1)
(15)
The initial conditions for solving the deactivation model are:
CFe, j (t
j
(t
0) 0.00394 mmol mL-1
(16)
0) 1
The described dynamic model of the tubular reactor with deactivation can be solved by a numerical approach, using
the Rosenbrock algorithm for the solution of the differential equation. In the meantime kd has been determined by
mathematical regression analysis.
The acidity profiles for different reactor heights, reported in Fig. 2, have been determined by equation (17) and
reported in the plot as a function of time reported as dimensionless value, that is as a ratio between the effective time
and the total time-on-stream.
Ac j %
C Ab , j PM A
C Ab , j PM A CEb , j PM E
CTb, j PM T
100
(17)
In fig. 2, are reported both experimental data and the reaults of simulations by assuming a deactivation constant kd
equal to 0.844 gcat mL min-1 meqH+ -2. By observing Fig. 2 it is possible to observe a good agreement for the profiles
related to the intermediate heights (6) and (7). The profile at reactor outlet (8) is not adequately simulated. This is
probably due to the release of chromium from the springs used as catalyst diluent. Chromium acts as additional
poison in the last part of the reactor. This phenomenon has been experimentally observed but not simulated because
dependent on the particular device employed.
4. CONCLUSION
This study has shown that the deactivation of exchange resins used as catalysts in the esterification of fatty acids is
mainly due to the ionic exchange of the proton with iron or other metals that can be present in the feedstock
probably as a consequence of the corrosive action of free fatty acids during the storage. A dynamic model has been
developed for describing the performances of tubular reactors containing exchange resins more or less diluted with
an inert material. The model can foresee the life of the catalyst if the amount of iron dissolved in the feedstock is
known.
5. AKNOWLEDGMENTS
Thanks are due to ASER srl Co. for the financial support
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