(633b) Activity Based Adsorption Isotherms for Adsorption From Mixed Solvents

Activity based adsorption
isotherms for adsorption from mixed solvents

C. Wegmann^a, P.J.A.M.
Kerkhof^a

^aEindhoven Univeristy
of Technology, Faculty of Chemical Engineering and Chemistry, P.O. Box 513,
5600 MB Eindhoven, The Netherlands

Adsorption is a very efficient
technology for the removal of dissolved organic components from aqueous
streams. High loadings on the sorbent can be achieved even at low
concentrations in the feed and the residual concentrations in the water are
low. For the removal of dissolved acrylonitrile from an industrial process
water stream ?Solvent Swing Adsorption? will be applied. Solvent Swing
Adsorption is a hybrid technology which combines adsorption with extraction and
overcomes one of the biggest drawbacks of adsorption which is the recovery of
the adsorbed compound. Acrylonitrile is a very toxic component and a valuable
raw material. It must be removed from the aqueous stream and be reused in the
production process for ecological and economical reasons.

Solvent Swing Adsorption consists
of two steps: In the first step the water stream containing dissolved
acrylonitrile is fed to an adsorption column where the acrylonitrile is
adsorbed. Just before the acrylonitrile concentration in the water outflow
starts to increase, the desorption step starts. In this second step a solvent
with a high affinity for acrylonitrile is fed to the column. Where the solvent
meets sorbent particles loaded with acrylonitrile, the acrylonitrile desorbs
and very high concentrations in the solvent result. As soon as the
acrylonitrile is removed from the column, the next adsorption step starts. It
is crucial to minimize the amount of solvent which is used per cycle and
maximize the acrylonitrile concentration in the solvent since the desorbed
acrylonitrile will be separated from the solvent by distillation. The
acrylonitrile will go back to the industrial production process and the solvent
will be reused in the next desorption step.

Fast kinetics and favorable
isotherms are prerequisites for the application of Solvent Swing Adsorption. Dowex
Optipore L-493 was selected as adsorbent material for the Solvent Swing
Adsorption process because the absence of functional groups on its internal
surface ensures complete reversibility of the adsorption. The intraparticle
diffusion coefficient of acrylonitrile in the Dowex Optipore particles (diameter:
0.8 mm) was experimentally determined and it was found to be larger than 4.6×10^-10
m²/s at 20ºC which is very high compared to other adsorbent
materials (e.g. activated carbon). This allows running adsorption columns at a high
superficial velocity of the feed which is desirable for industrial processes.

Solvents with a high affinity for
acrylonitrile were found by combining computer aided molecular design with lab
experiments. Acetone was eventually selected as desorption solvent for the
adsorption-desorption process because of its high affinity for acrylonitrile,
its miscibility with water, its low boiling point, and its good environment,
health and safety properties.

In order to determine the
adsorption equilibrium of acrylonitrile onto Dowex Optipore, solutions of
100-50,000 ppm acrylonitrile in water/acetone mixtures where mixed with the
adsorbent and shaken in flasks during 8 hours at 25ºC. It was shown in
preliminary experiments that the adsorption equilibrium was reached after less
than 1 hour. The acrylonitrile concentration in the liquid was measured and the
adsorbed amount was calculated using a mass balance. Fig.1 (dots) shows the
results of these batch experiments. It is clear that the adsorbed amount is
strongly dependent on the acetone fraction in the solvent which is desirable
for the process.

Figure 1:
Concentration based adsorption isotherms of acrylonitrile onto Dowex Optipore
L-493 for solvents with different acetone volume fractions. Experimental (dots)
and calculated by use of activity based isotherms as explained below (lines).

A correct description of the
adsorption isotherms is required for the design and the dimensioning of the
Solvent Swing Adsorption column. The description of the equilibrium with only
one equation which is applicable for all solvent compositions is preferable.
Since the adsorbed amount of acrylonitrile onto Dowex Optipore depends on the
water/acetone ratio of the solvent, the equilibrium cannot be described with a
conventional isotherm based on the acrylonitrile concentration in the solution.
Therefore, instead of describing the adsorbed amount as a function of the
equilibrium concentration, the loading was described as a function of the
acrylonitrile activity in the solution. For this purpose the activity of
acrylonitrile in the solution was calculated by use of UNIFAC. The calculation
is based on the functional groups of which the three molecules present in the
liquid mixture consist. The UNIFAC model uses group specifications and
group-group interaction parameters which can be found in literature^1,2.
For the range of solution compositions which can appear in the bulk solution of
the adsorber, the activity of acrylonitrile was described as a function of the
mol fractions of acetone and acrylonitrile. Fig. 2 shows the activity of
acrylonitrile estimated by use of UNIFAC. The low activity at high acetone
concentrations leads to small adsorbed amounts for acetone rich solvents.

Figure 2: Acrylonitrile
activity as a function of acetone and acrylonitrile mol fractions

For each experimentally
determined data point of the isotherms shown in Fig. 1, the activity of
acrylonitrile in the solution was found by use of Fig. 2. The activity based
isotherms are shown in Fig. 3. It can be seen that for water/acetone solvents
with 10-100v% acetone the adsorbed amount of acrylonitrile at a certain
activity is always the same no matter which is the water/acetone ratio of the
solution. The acrylonitrile adsorption for all solution compositions can
therefore be described by use of one common activity based Freundlich isotherm
which is given by Eq. (1) and shown in Fig. 3. The equation was fitted to the
data points given in Fig. 3 by adjusting the parameters k_F
and n.

Figure 3:
Activity based adsorption isotherms of acrylonitrile

onto Dowex Optipore L-493 for solvents with different acetone volume fractions

Eq. (1)

q          equilibrium
adsorbed amount of acrylonitrile, mg/g

k_F         Freundlich
parameter, mg/g (k_F = 725.4 mg/g)

n          Freundlich
parameter, 1 (n = 0.783)

a          acrylonitrile
activity in the solution, mol/mol_tot

The Freundlich isotherm in Fig. 3
can be converted into ten different concentration based isotherms which are
shown in Fig. 1 (lines). In order to convert the single activity based isotherm
into ten conventional concentration based isotherms, for each acetone fraction
the activity of AN is converted into a concentration by use of Fig. 2. It can
be seen that the model describes with good accuracy the whole range between
10v% and 100v% acetone.

These findings indicate that at
solvent compositions of 10-100v% acetone in water the acrylonitrile loading is
mainly controlled by its non-ideal behavior in the solution and not by competition
with adsorption of acetone and water molecules in the adsorbed phase.

A fast intraparticle mass transfer of acrylonitrile in the
sorbent particle and a high affinity of the solvent for acrylonitrile are
requirements for an efficient Solvent Swing Adsorption process. By selecting
Dowex Optipore L-493 as an adsorbent and acetone as a solvent these
requirements are fulfilled. The activity based acrylonitrile isotherm together
with the UNIFAC-based calculation of the acrylonitrile activity in the solution
enables us to describe the acrylonitrile concentration in the adsorber as a
function of time and position in the column. By use of these relations the
dimensions of the Solvent Swing Adsorption column and the operating parameter can
be optimized in order to minimize the acetone stream and therefore the energy
consumption of the separation of acrylonitrile from water.

References:

¹ B.E. Poling, J.M. Prausnitz, J.P. O'Connell, The
Properties of Gases and Liquids, fifth ed., McGraw-Hill, Singapore, 2001.

² R. Wittig, J. Lohmann, J. Gmehling, Vapor-Liquid
Equilibria by UNIFAC Group Contribution. 6. Revision and Extension, Ind. Eng. Chem. Res. 42 (2003) 183-188.