(309f) Recovery of Nicotinic Acid From Aqueous Solution Using Reactive Extraction with Tri-n-Octyl Phosphine Oxide (TOPO) in Kerosene

Abstract

Niacin, also known as nicotinic acid or vitamin B3, is a
water-soluble vitamin whose derivatives such as NADH (reduced form of NAD) play
essential roles in energy metabolism in the living cell. Nicotinic acid
(3-pyridine carboxylic acid) widely used in food, pharmaceutical and
biochemical industries is an important chemical, mainly obtained by chemical
synthesis, using 3-picoline or 2-methyl-5-ethyl-pyridine as starting-materials,
at high temperature and pressure. Besides the technical aspects, other
parameters such as desired quality, physical and chemical properties of the final
product, and the ecological problems complicate the chemical synthesis methods.
Due to these reasons, the chemical synthesis route for nicotinic acid
production will become unattractive in the future. In recent years, the
application of enzymes to organic chemical processing has attracted the
attention of researchers. Nitrilases enzymes are gaining popularity as
biocatalysts for the mild and selective hydrolysis of nitriles. The production
of nicotinic acid and nicotinamide can be intensified by enzymatic conversion
of 3-cyanopyridine or biosynthesis (Kumar and Babu, 2009). Very recently amidase-catalyzed production of nicotinic acid in batch and
continuous stirred membrane reactors has been studied by Cantarella et al (2008). Amidase enzyme, operated under mild conditions is
suitable for the synthesis of labile organic molecules and it is stable up to
50 °C. This fermentation process, because of various impurities and very
low concentration of product in the fermentation broth, requires an economic
separation method to compete with the synthetic process.

Many separation
processes such as liquid extraction, ultra filtration, electro-dialysis, direct
distillation, liquid surfactant membrane extraction, anion exchange,
precipitation and adsorption in chemical industries have been employed to
recover the organic acids from aqueous solution. Among various available
alternate processes for simultaneous removal of the product, extraction is
often the most suitable one. So a reactive extraction method has been proposed
to be an effective primary separation step for the recovery of bio-products from
a dilute fermentation process (Kumar et al, 2008).

Organophosphorus
compounds and long-chain aliphatic amines are effective extractants for the
separation of carboxylic acids from dilute aqueous solution (Kertes and King,
1986). Phosphorus-bonded, oxygen-containing extractants have a phosphoryl group
and a stronger Lewis basicity than those of carbon-bonded, oxygen-containing
extractants. Phosphorus-bonded, oxygen-containing extractants can only
co-extract small amounts of water, and show low solubilities in water. When
organophosphorus extractants are used, the solvation has a higher specificity. The
distribution of nicotinic acid between water and Alamine 300 (tri-n-octylamine)
dissolved in polar and non-polar diluents, is studied at 298 K using a phase
ratio of 1:1 (v/v) by Senol (2002). The comparative study of the reactive
extraction of nicotinic acid with Amberlite LA-2 (lauryl-trialkyl-methylamine)
and di-(2-ethylhexyl)-phosphoric acid (D2EHPA) has been presented by Cascaval
et al, 2007. Compared to D2EHPA, the use of Amberlite LA-2 allows the
possibility to reach higher extraction efficiency, the extraction degree being
supplementarily increased by increasing the solvent polarity. Kumar et al
(2008) studied reactive extraction of nicotinic acid with TBP and TOPO at a
fixed initial acid concentration to intensify the recovery from fermentation
broth.

The aim of the
present work is to study the reactive extraction of nicotinic acid (3-pyridine
carboxylic acid) from aqueous solutions using tri-n-octyl phosphine oxide
(TOPO) dissolved in kerosene to provide the extraction equilibrium data for
intensification of nicotinic acid production via enzymatic route.  The effects
of initial acid concentration and composition of extractant (TOPO) are also observed.
An equilibrium model based on mass action law is presented and used to
determine the equilibrium extraction constant (K_E) and the
number of extractant molecules per acid molecule (n) with graphical
method as well as an optimization procedure. Population based search algorithm,
differential evolution is used as optimization algorithm.

The extraction
equilibrium experiments are carried out at constant temperature (298 K) with
equal volumes (12 cm³ of each phase) of the aqueous and organic
solutions shaken at 100 rpm for 8 hours in conical flasks of 100 mL on a
temperature controlled reciprocal shaker bath. After attaining equilibrium, the
phases are brought into contact with each other for separation. The initial
concentration of nicotinic acid in aqueous solutions is varied between 0.02 - 0.12
kmol/m³. Tri-n-octylphosphine oxide (TOPO) concentration in organic
phase is kept in the range of 0.10 ? 0.60 kmol/m³. The concentration
of acid in the aqueous phase is determined using an UV spectrophotometer
(Systronics, 119 model, 262 nm). The acid concentration in the organic phase is
calculated by mass balance. The initial and equilibrium pH values of aqueous
solutions are measured using a digital pH-meter of Arm-Field Instruments (PCT
40, Basic Process Module) which varied in the range of (2.45 to 2.92) and (2.91
to 3.74) respectively.

The extraction
process is analyzed by means of the degree of extraction and distribution
coefficient. The distribution coefficient, K_D, is calculated
using Eq. 1.

                                                                                                     (1)

where,  is
the total (analytical) concentration of nicotinic acid in organic phase and  is
the total (analytical) concentration (dissociated and un-dissociated) in
aqueous phase at equilibrium.

The degree of
extraction is defined as the ratio of acid concentration in the extracted phase
to the initial acid concentration in aqueous solution by assuming no change in
volume at equilibrium as given by Eq. 2.

                                                                                          (2)

Tri-n-octyl
phosphine oxide (TOPO) is used as extractant to study the extraction equilibria
of nicotinic acid because of its excellent chemical stability, higher basicity
and low solubility in water. TOPO (as shown in Figure 1) contains a phosphoryl
group (>P=O) which serves as a stronger Lewis base for its high polarity. The
isotherms for nicotinic acid are determined from five aqueous solution
concentrations, four concentrations of TOPO dissolved in kerosene. For a higher
range of TOPO concentration, there is a linear relationship between acid
concentration in the two phases, and slightly nonlinear relationship for lower concentrations
of TOPO. The distribution coefficients (K_D) and degree of
extraction (E) are found to increase in the range of 0.38 to 1.44 and
27.5 to 59% respectively with an increase in TOPO concentration (0.10 to 0.60 kmol/m³)
in kerosene at fixed acid concentration of 0.02 kmol/m³. The distribution
coefficients (K_D) and degree of extraction (E) decreased
in the range of 1.34 to 1.44 and 57 to 59% respectively when the concentration
of acid is increased from (0.02 to 0.12 kmol/m³) at fixed TOPO
concentration of 0.60 kmol/m³. Different concentrations of extractant
(TOPO) have been used to derive the effect of initial acid concentration on
extraction efficiency.  The number of TOPO molecules in the acid:TOPO complex and
the extraction equilibrium constants are estimated through proposed
mathematical model (Eq. 3).

                             (3)

Due to
apparition of n under logarithm, an optimization route for estimation of
n and K_E is applied. If >>,
the initial extractant concentration can also be
used to determine n and K_E in the Eq. (3).

A plot of
equation (3) by taking, on y-axis
and  on x-axis
yields the straight line with a slope of n and intercept of log K_E.
A Population based search algorithm, differential evolution (DE), which is
simple and robust and has proven successful record (Babu, 2004), is also
employed to solve the model equation (3) for estimation of extraction
equilibrium constants (K_E) and the number of reacting
extractant molecules (n). An objective function based on least square
error between experimental data and predicted value of  has
been minimized. Chemical modeling approach is used for the determination of the
equilibrium extraction constant (K_E) and the number of
extractant reacting molecules (n), the estimated values of K_E
and n depend on the applied method. More exact values of K_E
and n have been found when optimization procedure (differential
evolution algorithm) is used to solve the model equations compare to graphical
method (with the assumption, >>).
Since the loading ratio was less than 0.5 in all the cases, no overloading was
obtained and only 1:1 complexes of acid and TOPO were formed using graphical
method and differential evolution algorithm. Maximum equilibrium extraction
constant was found to be 2.4 m³/kmol.

References

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