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

Babu, B.V. (2004) Process plant simulation, Oxford University Press, India.

Cantarella, M., Cantarella, L., Gallifuico, A., Intellini, R., Kaplan, O., Spera, A., Martínková, L. (2008) Enzyme Microb. Technol. 42, 222.

Cascaval, D., Galaction, A.I., Blaga, A.C., (2007) Camarut, M. Comparative Study on Reactive Extraction of Nicotinic Acid with Amberlite LA-2 and D2EHPA. Sep. Sci. Technol. 42, 1-13.

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

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.

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.

Kumar, S.; Babu B.V. (2009) Process Intensification of Nicotinic Acid Production via Enzymatic Conversion using Reactive Extraction. Chem. Biochem. Eng. Q. accepted (In press).

Senol, A. (2002) Extraction Equilibria of Nicotinic Acid using Alamine 300/Diluent and Conventional Solvent Systems. Turk. J. Chem. 26, 77.