(666f) Biosorptive Dehydration of Alcohol/Water Azeotropes Using Compound Starch-Based Adsorbent

Water removal represents a serious problem in most
monohydric alcohol production because most monohydric alcohols form azeotropes with water at atmospheric pressure, except for
methanol. Various
techniques have been developed to break the azeotropic
point, such as salting-out method, azeotropic
distillation, extractive distillation, reactive distillation, adsorptive
distillation and pervapration. Among the methods, the
adsorptive option is most attractive, especially the biosorption,
which has distinct advantages over conventional methods as it is environmentally
friendly, highly selective, easily available, easy to operate, cost effective and
reusable in repeated cycles in the treatment of aqueous organics. In this work,
a specially formulated compound starch-based adsorbent (CSA), which consists of
corn, sweet potato and foaming agent, was developed, and biosorptive
dehydrations of ethanol, isopropanol and tert-butyl alcohol
(TBA) using CSA were investigated, respectively.

In the first step, a systematic study of the biosorptive process was performed. The net retention time
and separation factor of alcohol and water were obtained using inverse gas
chromatography (IGC), the results indicated that the net retention time of
water on CSA is significantly longer than that of alcohol which attested the
feasibility of this adsorptive separation purpose and low temperature could be propitious
to the process which was consistent with the traditional theory of adsorption.

Then, through a batch adsorption experiment, the
optimum adsorption condition was determined under different bed depths, bed
temperatures and kettle temperatures by single-factor method and orthogonal
design method, which indicated that the optimization condition for ethanol was
at Vapor superficial velocity of 0.14m/s,
bed temperature of 81°æ and feed
concentration of 92.5wt%, isopropanol at kettle temperature of 87°æ, bed temperature of 88°æ and bed depth of 26 cm and TBA, kettle temperature of 102°æ,bed
temperature of 84°æ and bed
height of 30cm.
And take TBA for instance, the optimal regeneration condition was obtained under
different air flow rates, air temperature and regeneration time by treating CSA
with hot air by single-factor method and orthogonal design method and the
results showed that the optimal regeneration condition was at air flow rate of 60L/h, air temperature of 120°æ
and regeneration time of 40min. In order to characterize the service life of
CSA, five cycles experiments of adsorption-regeneration were carried out at the
optimum adsorption and regeneration condition, the results showed that there is
practically no change in the regeneration efficiency of CSA after the cyclic
process adsorption¨Cregeneration. It is due to the properly chosen regeneration
conditions assuring complete desorption of the water adsorbed without changes
in the CSA structure. This unchanged adsorption capacity after a number of
cycles demonstrates reusability of the adsorbent.

Finally, field emission scanning electron microscopy
(FESEM) and mercury porosimetry (MP) were applied to
investigate the change of surface morphology and microstructure of CSA before
and after the adsorption, as well as after regeneration, as shown in Fig.1 and
Fig.2. The FESEM images show that the dry CSA is composed of wizened, irregular
particles with many void spaces between them. After an adsorption experimental
run, the particles become enlarged, the void spaces become smaller, and some
particles adhered to each other. There is a significant change in the surface morphology
of the CSA before and after adsorption, which indicated that the CSA was
saturated with water. However, the particles of CSA are wizened as the nature
adsorbent after a regeneration experimental run, which indicated that nine
tenths of water was desorbed form CSA. The MP data indicated that the total
pore volume, total pore area, average pore diameter and porosity of the CSA
before adsorption are
larger than those parameters after adsorption. In other words, a large numbers
of water molecules were adsorbed on the surface of the pores in the adsorption
process, which resulted in decreases of the above parameters after adsorption.
However, all the above parameters of the CSA just slightly lower than the
natural adsorbent after regeneration experimental run, which demonstrated that
almost all the adsorbed water molecular was desorbed from the surface of the pores. The pore size
distribution (PSD) curves show that below 5000 nm, the behavior of the two
curves of the CSA before and after adsorption is very similar. Before
adsorption, the curve exhibits two peaks at 6000 and 17000 nm. However, after
adsorption, the peak at 6000 nm was no longer present, and the peak at 17000 nm
became significantly smaller, which indicates that the CSA is mainly composed of
macropores that play a significant role in water
adsorption. Numerous macropores in the CSA were
saturated with water molecules, which resulted in the disappearance of macropores of certain sizes and a reduction in total pore
volume. After regeneration, the two peaks at 6000 and 17000 nm were reappeared,
the PSD curve of the CSA was almost identical to the curve of before
adsorption, which further manifested that the spent adsorbent can be
regenerated completely and restored to the natural structure.

Thermodynamic discussions are subsequently done with
parameters DH_s and DG_s for the adsorption
determined respectively. The adsorption enthalpy of water for ethanol/water azeotrope was -31.6kJ/mol, for
isopropanol/water azeotrope was -55.7kJ/mol and tert-butyl alcohol/water azeotrope, -38.3 kJ/mol, which
indicated that the biosorption was physisorption in nature and exothermic. And the adsorption
enthalpy of water significantly higher than the corresponding value of alcohol,
which indicated that the CSA shows stronger preferential sorption of water over
alcohol. The negative values of DGs indicate
the spontaneous of the adsorption process, and the larger negative values for
water at lower temperatures indicate that adsorption of water is favored at
lower temperatures. Alternatively, the larger negative values of DGs for water compared to those
values for alcohol further confirm the preferential adsorption of water.

On the basis of the above conclusions, a more
energy-efficient hybrid program that combines normal distillation with this
crucial biosorptive pathway to break the azeotropic point was developed for the production of
anhydrous alcohol, and the simulation results showed that the hybrid option was
definitely reduced the energy and material requirements by a large margin
compared to the prevailing azeotropic or extractive
distillation schemes. These results indicated that the starch-based adsorbent
can be considered as an effective and low-cost biosorbent
to dehydrate alcohol.

Fig.1 FESEM
images of the CSA before (A) and after adsorption (B) as

well as
after regeneration (C).

Fig.2. Pore size
distributions (PSD) of the CSA before and after adsorption as well as after
regeneration.

Fig.3. Diagram of
the coupling process of biomass-based fixed-bed adsorption