Reactive Extraction of Nicotinic Acid Using Tri-n-Octylphosphine Oxide (TOPO) Dissolved in a Binary Diluent Mixture

Reactive Extraction of Nicotinic Acid using Tri-n-Octylphosphine Oxide (TOPO) Dissolved in a Binary Diluent Mixture

Sushil Kumar^1*  Dipaloy Datta¹ and B V Babu²

¹Department of Chemical Engineering

Birla Institute of Technology and Science (BITS), PILANI – 333031 (Rajasthan), INDIA

*Corresponding Author:

E-mail: sushilk2006@gmail.com; skumar@bits-pilani.ac.in 

Phone : +91-1596-245073 Ext 215; Fax: +91-1596-244183

Homepage: http://universe.bits-pilani.ac.in/pilani/skumar/profile

Abstract

Nicotinic acid (pyridine-3-carboxilic acid) is a water-soluble vitamin and precursor to coenzymes (NADH, NAD⁺, NADP⁺, and NADPH) which serves an important role in the redox reactions taking place inside the human living cells for the metabolism activity. Niacin helps in both DNA repair and formation of steroid hormones in the adrenal gland. A deficiency of niacin can cause pellagra, a serious disease that has paralyzed mankind for centuries. Also, a mild deficiency slows down the metabolism of the body, causing decreased tolerance to cold (Kumar and Babu, 2009). The production of organic acids by biochemical fermentation route is comparatively a clean and a green technology. The biosynthesis process produces nicotinic acid at a lower rate and also the concentration of the acid in the fermentation broth is found to be very low. Therefore, to make the fermentation route efficient and effective, there is a need to develop novel fermentation processes and efficient separation techniques. The reactive extraction with higher distribution coefficient is proposed to be an efficient and eco-friendly primary separation process (Kertes and King, 1986; Kumar and Babu, 2008).

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 (Kertes and King, 1986). These extractants are dissolved in organic diluents (ketones, alcohols, hydrocarbons) to provide appropriate physical properties (density, viscosity, etc.,) to the extractant-diluent system. The diluents are categorized in two groups based on their activity: (i) inactive (inert) diluents, and (ii) active diluents (modifiers). The presence of polar functional groups in the modifiers enables them to act as better solvation medium for the acid-extractant complex by the formation of hydrogen bond. Also, a modifier enhances the extracting power of organophosphoric extractant as compared to an inert diluent in the extraction of organic acids (Mariya et al., 2005; Yankov et al., 2004).

The present work is aimed to intensify the recovery of nicotinic acid using reactive extraction with tri-octyl phosphine oxide (TOPO) in a diluent mixture consisting of an active and an inactive diluent [MIBK + kerosene (1:1 v/v)]. The aqueous solutions of nicotinic acid are prepared in the concentration range 0.02 to 0.12 mol/L using distilled water. Organic solutions are prepared by varying the concentration of TOPO (0.1 and 0.5 mol/L) in the mixture of MIBK + kerosene (1:1 v/v). MIBK is used as the active diluent (modifier). Kerosene is used as inactive diluent to control density and viscosity of the organic phase. The extraction equilibrium experiments are carried out with equal volumes (20 ml) of the aqueous and organic solutions in conical flasks of 100 ml and shaken at 100 rpm for 8 hours in a temperature controlled reciprocal shaker bath (HS 250, Remi Labs, India) at constant temperature (298 K). After attaining equilibrium, the mixture of aqueous and organic phases is kept for separation in separating funnel (125 ml) for 2 hrs at 298 K. After separation of both phases, the aqueous phase acid concentration is analyzed by titration using NaOH solution of 0.01 N with phenolphthalein as an indicator and also by UV-Vis Spectrophotometer (Evolution 201, Merck, India at 262 nm). The acid concentration in the organic phase is calculated by mass balance. The equilibrium pH values of aqueous solution are measured by a digital pH-meter (PCT 40, ArmField Instruments, UK). The experimental data are analyzed by calculating distribution coefficient (K_D = C_org/C_aq), degrees of extraction [E = K_D / (1 + K_D)] and loading ratios (Z = C_org/ ).

To determine the physical extraction parameters (partition coefficient = P and dimerization constant = D), the following equation is fitted linearly in the Origin 8.0 (software package).

                                                                                           (1)

The values of of nicotinic acid with diluent mixture are found to be in the range of 0.074 to 0.149 which are less than one.  The values of P and D are obtained as 0.0598 and 115.66, respectively. The use of non-polar solvents such as kerosene in the extraction of carboxylic acid at higher initial acid concentration may also lead to the formation of a stable emulsion and dimer in the organic phase. Therefore, a modifier (active diluent) is generally added with the inert diluent to avoid the formation of a stable emulsion and dimer in the organic phase.

The equilibrium chemical extraction experiments for the recovery of nicotinic acid is carried out using TOPO dissolved in MIBK + kerosene (1:1 vv). The extraction degree decreases with an increase in acid concentration in the aqueous phase. This may be due to the lower amount of TOPO used in the initial organic phase, and TOPO concentration is found to be a limiting parameter for the extraction. Generally, for a high initial acid concentration, the distribution coefficient (K_D) may decrease with an increase in the acid concentration in aqueous solution using TOPO with diluent mixture. The distribution coefficients (K_D) and degree of extraction (E) are found to increase with an increase in TOPO concentration (0.10 to 0.50 mol/L). TOPO/diluent system favors the formation of ‘not overloaded' complexes of polar acid-TOPO structures with the Z factors restricted mainly between 0.032 and 0.684. With TOPO (0.365 mol/L) dissolved in MIBK + kerosene (1:1 v/v), the maximum value of K_D is to be 4.168 for an acid concentration of 0.02 mol/L.

The experimental results are also interpreted in terms of the distribution coefficient of acid by chemical extraction ( ) with extractants (TBP and TOA) dissolved in diluent mixtures and given by Eq. (2).

                                                                                             (2)

where ν is the volume fraction of diluent mixture.

The overall distribution coefficient ( ) by physical and chemical extraction is obtained from the following equation.

                                                                                            (3)

The Z values less than 0.5 indicate a formation of (1:1) acid-TOPO complex in the organic phase. Therefore, with assumption of (1:1) acid-extractant complexes in the organic phase, the following model equations of reactive extraction mechanism are developed incorporating the effect of physical extraction.

                       (4)

The m is the loading of acid in organic phase by diluent mixture.

                                                                                                                 (5)

The plots of  versus [HC] yield a straight line with a slope representing the corresponding K_E value of the reactive extraction. The equilibrium complexation constants (K_E) predicted by the Eq. (4) are presented with the coefficient of determination (R²) and standard deviation (SD). The model predicted values of K_E are showing good correlation with R² > 0.98 and maximum value of SD = 0.092. The highest value of K_E is found for lower acid and extractant concentration which shows a faster mass transfer of solute into the organic phase.

References

(1)   Kertes, A. S.; King, C. Extraction Chemistry of Fermentation Product Carboxylic Acids. Biotechnol. Bioeng. 1986, 28, 269-282.

(2)   Kumar, S.; Wasewar, K.L.; Babu, B.V. (2008) Intensification of Nicotinic Acid Separation using Organophosphorous Solvating Extractants: Reactive Extraction. Chem. Eng. Technol. 31, 1584-1590.

(3)   Kumar, S., Babu, B.V. (2008) Process intensification for separation of carboxylic acids from fermentation broths using reactive extraction, J. Fut. Eng. Technol. 3, 19.

(4)   Kumar, S.; Babu B. V. Process Intensification of Nicotinic Acid Production via Enzymatic Conversion using Reactive Extraction. Chem. Biochem. Eng. Q. 2009, 23, 367-376.

(5)   Mariya, M., Albet, J., Molinier, J., Kyuchoukov, G. Specific Influence of the Modifier (1-Decanol) on the Extraction of Tartaric Acid by Different Extractants. Ind. Eng. Chem. Res. 2005, 44, 6534-6538.

Figure 1. Physical equilibria for extraction of nicotinic acid at 298 K

 

Figure 2. Equilibrium complexation constant (KE) determination for extraction of nicotinic acid at 298 K with TOPO in MIBK + kerosene (1:1 v/v)