SusChemE 2015
International Conference on Sustainable Chemistry & Engineering
October 8-9, 2015, Hotel Lalit, Mumbai
Kinetics of Extraction of Nitric Acid into Binary Mixture of Tri-n-butyl Phosphate and
Normal Paraffin Hydrocarbon
Elizabeth Augustine1, Remya Murali1, N. Desigan1, N.K. Pandey1*, Kamachi Mudali1 and J.B. Joshi2
1
Reprocessing Group, Indira Gandhi Centre for Atomic Research, Kalpakkam
2
Institute of Chemical Technology, Matunga, Mumbai and HBNI, Mumbai
*nkpandey@igcar.gov.in
1. Introduction:
Kinetics of solvent extraction is of paramount importance to all industrial applications and is largely determined by the
interfacial chemistry. The commercial extraction equipments in operation are designed based on the number of equilibrium
stages required to effect a given separation. Design from equilibrium isotherms automatically neglects kinetic effects. Hence, for
the rational design and operation of extraction equipment, one should know the mass transfer characteristics, intrinsic kinetics
and the extent of axial mixing. The present work is concerned with the identification of reaction location and extraction regime,
estimation of the overall rate of extraction and determination of intrinsic rate constant and mass transfer coefficient.
2. Material and Methods:
Nitric acid supplied by M/s. Fischer Chemicals Ltd (AR grade, Min. Assay: 69–71%) was used as aqueous phase in the
experiments. Tributyl phosphate (supplied by M/s. Spectrochem Pvt Ltd, Mumbai) diluted with normal paraffin hydrocarbon
was used as the extractant (organic phase). The constant interfacial area stirred cell (Lewis cell) used for kinetic studies (as
shown in figure (1)) was a glass vessel with a jacket for maintaining constant temperature [1]. Experiments were conducted by
varying different parameters like aqueous phase concentration of HNO3 (1-4M), organic phase concentration (0.3652-1.466
kmol.m-3), impeller speeds (110 to 150 rpm), the ratio of aqueous to organic phase (0.6-1.2) and temperature (15-45oC).
3. Results and Discussion:
The complexation of nitric acid by TBP can be described on the basis of the following reaction equilibria:
H   NO3  TBP  TBP  HNO3
(1)
3.1 Identification of the reaction location
The relationship between the extraction rate and the interfacial area is an important criterion to decide whether the chemical
reactions that control the rate of extraction occur in the bulk phase or at the interface. The low aqueous phase solubility (10-3M)
and surface activity of TBP supported the possibility of the occurrence of reaction between nitric acid and TBP at the interface.
From the experimental results, initial rate of nitric acid extraction was found to be directly dependent on the specific interfacial
area (as shown in Fig. 2) which is the characteristic of an interfacial reaction.
3.2 Identification of the reaction regime
For establishing the intrinsic kinetics of the extraction of nitric acid, the theory of mass transfer with chemical reaction has been
used. There are primarily two regimes of extraction – the kinetic regime and the diffusion regime [2]. In a kinetic regime system
the kinetics of solvent extraction can be described in terms of chemical reactions occurring in the bulk phases or at the interface.
In diffusion controlled extraction process, kinetic parameters are unimportant and the extraction rate directly depends on
interfacial area and turbulence. When both chemical reactions and film diffusion processes occur at rates that are comparable,
the solvent extraction kinetics is said to take place in a mixed diffusion-kinetic regime. The extraction rate expression for the
mixed diffusion-kinetic regime is given by equation (2)
1
SusChemE 2015
International Conference on Sustainable Chemistry & Engineering
October 8-9, 2015, Hotel Lalit, Mumbai

d[B] aq
dt



Va  Va


 
[B] aq , t  0 
[B] aq  K e [A ] o 
Vo  Vo

S
S 




RB 
Va
Va

1 
 1  K e [A ] o


 


k
k Laq
k Lo 

 b 

(2)
The experimental data on nitric acid concentration vs time were fitted into model equations of three regimes using a nonlinear
regression technique to obtain mass transfer coefficient and rate constant. Figure 3 shows the comparison between experimental
and calculated values of aqueous phase nitric acid concentration.The impeller speed dependency on the rate was checked to
estimate the contribution of diffusion mass transfer to the overall rate of extraction. A further criterion that enables a distinction
between a diffusion-controlled and a kinetic regime is the experimental determination of the activation energy from the
temperature effect of the extraction process. The activation energy for the intrinsic rate constant value was found to be 26.04
kJ/mol which assured the fact that the overall extraction rate is controlled by both the intrinsic kinetics and mass transfer of the
species involved.On the basis of present study, a correlation is also developed for the estimation of individual mass transfer
coefficients.
-4
1
x 10
120rpm(exp)
120rpm(calc)
130rpm(exp)
130rpm(calc)
140rpm(exp)
140rpm(calc)
150rpm(exp)
150rpm(calc)
0.98
[HNO3]aq/[HNO3]aq,t=0
RB,t=0.a (kmol.m-3.s-1)
0.96
3
2
0.94
0.92
0.9
0.88
0.86
0.84
1
12
Figure 1: Lewis cell set up
14
16
18
20
22
24
Interfacial area (m2/m3)
26
28
Figure 2: Effect of interfacial area on
initial extraction rate of nitric acid
0.82
0
1000
2000
3000 4000
Time (s)
5000
6000
7000
Figure 3: Experimental and calculated
concentration profiles of nitric acid
4. Conclusions:
The extraction of nitric acid by TBP was found to be an interfacial reaction. Dependence of extraction rate on stirring speed,
temperature, specific interfacial area, and the concentrations of reactants have been studied using constant area stirred cell. The
overall extraction rate was found to be controlled by both the intrinsic kinetics and the mass transfer of the species involved, for
which a comprehensive model has been developed.
References
1. Doraiswami L.K., Sharma M.M. Heterogeneous Reactions: Analysis, Examples and Reactor Design. Volume II, John Wiley
and Sons, 1984.
2. Danesi P.R. Solvent Extraction Kinetics, Solvent Extraction Principles and Practice, Second Edition, Revised and Expanded,
Chapter-5, Edited by Jan Rydberg, Michael Cox, Claude Musikas and Gregory R. Choppin, 2004
2