4.2.1 Materials
All the chemicals used in the present work were procured from Sigma-Aldrich (USA). The specifications of all the chemicals are given in Table 4.1. CO2 gas having purity (> 99.9 %) used for solubility study was procured from Linde India Ltd. Deionized water is used to prepare all the solvent used in the present work.
4.2.2 Experimental methodology
4.2.2.1 Equilibrium solubility measurement
Equilibrium solubility of CO2 in aqueous amine system was measured in the stainless steel (SS) stirred equilibrium cell (600 × 10-6 m3) connected to a (1200 × 10-6 m3) SS buffer cell. The schematic of the experimental set up developed in this work is shown in Fig. 4.1.
The buffer cell is fitted with a pressure transmitter (0-500 psia, Honeywell STA74L, USA) and has an accuracy of ±0.065% of the full scale. The equilibrium cell is fitted with a pressure transmitter (0-100 psia, Rosemount 2051T, Emerson Process Management, USA) and has an accuracy of ±0.065% of the full scale. Temperature sensor (Pt 100, Julabo, FRG) with an accuracy of ± 0.1 K are mounted at the respective cell to measure the temperature. A magnetic stirrer (5MLH, Remi Instruments Ltd., India) was used for liquid phase stirring which is placed under the equilibrium cell. The equilibrium and buffer cell assembly is submerged in a thermostated water bath. The temperature of the water bath is controlled by water circulating temperature controller (F32 HL, Julabo, FRG) with an accuracy of ± 0.1 K. The tubing and needle valves used for regulating the gas flow and solvent inlet to the system are of Swagelok make.
For each experimental run, the equilibrium cell and the buffer cell were allowed to reach the thermal equilibrium with the external temperature controller to attain the experimental temperature condition. Both the cells were evacuated simultaneously by applying high vacuum using a vacuum pump (IV-50, INDVAC, India). The vacuum pump was then kept disconnected.
After the evacuation, the buffer cell was made to fill with pure CO2 from the external CO2
cylinder till it attains a pressure of about 300 psia. The equilibrium cell was then fed with a known amount of aqueous amine solution (50 × 10-6 m3) from the solvent injection port with the
help of an attached burette, and the port was fully closed by operating the needle valve. The amine solution was again degassed to remove any dissolved gases in the liquid by applying an instant vacuum and it was kept under this condition for about 20 min so that the liquid inside the cell exists under its own vapor pressure.
The vapor pressure corresponding to the experimental temperature inside the equilibrium cell ( Pv) and the initial pressure in the buffer cell(Pb1) were measured using the pressure transducers mounted on each cell. A certain amount of CO2 gas from the buffer cell was then transferred to the equilibrium cell by operating the needle valve between them. The magnetic stirrer kept under
Fig. 4.1 Experimental arrangement for CO2 solubility measurement: 1- CO2 Cylinder, 2- Buffer Cell, 3- Equilibrium Cell, 4- Magnetic stirrer, 5- Vacuum Pump, 6- Temperature sensor, 7- Pressure Transducer, 8- Temperature sensor for water bath, 9- Inlet provision for liquid solvent, 10- Water bath, 11- Temperature controller
the equilibrium cell was kept on for liquid phase stirring. As CO2 gas is absorbed by the solution inside the equilibrium cell, the total pressure of the cell decreases. Vapor and liquid phase equilibrium of CO2 gas was attained when there was no change of total pressure in the equilibrium cell noted at least for 1 hr. At this condition total pressure in the equilibrium cell,
)
(PT and the final pressure in the buffer cell, (Pb2) were recorded simultaneously. The equilibrium partial pressure of CO2 at the experimental temperature was calculated by taking the difference between total pressure and amine vapor pressure( )
2
v T
CO P P
P . The liquid phase CO2 loading
CO2
was calculated using equations (4.1-4.3) as reported in the literature [16].This procedure was repeated to obtain liquid phase CO2 loading at different higher CO2 partial pressures and at different temperatures.
The moles of CO2,
CO2
n transferred from the buffer cell to the equilibrium cell is calculated as [15]:
2 2 1 1
2 Z
P Z P RT
nCO Vb b b (4.1)
Where,Vbis the volume of the buffer cell, Rand Trepresents the gas constant and the system temperature, Z1 and Z2 are the compressibility factors corresponding to the initial pressure (Pb1) and final pressure (Pb2) of the buffer cell, respectively. At high CO2 pressure, the gas phase may behave non-ideal and compressibility factor (Z) is required to calculate using an equation of state (EOS). In the present work, Peng-Robinson equation of state is used to calculate ‘Z’ and used in Eq. (4.1). The moles of CO2 remaining in the gas phase (ngCO2) after the attainment of phase equilibrium is calculated by Eq. (4.2).
RT Z
P n V
CO CO g gCO
2 2 2
(4.2)
Where, Vg and ZCO2 represent the gas phase volume and the compressibility factor of CO2 in the equilibrium cell, respectively. The gas phase volume was calculated by subtracting the liquid
volume from the volume of the equilibrium cell. Equilibrium CO2 loading (αCO2) expressed in moles of CO2 per moles of total amine corresponding to a CO2 partial pressure can be determined using a mass balance of CO2 in liquid and vapor phase. The expression is given by:
am CO g CO
CO n
n
n 2 2
2
(4.3)
Where nam represents the total moles of amine in the liquid phase. The sample calculation corresponding to the calculation of CO2 loading has been presented in Appendix II.
4.2.2.2 Standard uncertainty in the solubility measurement
The standard uncertainty associated with the estimation of equilibrium CO2 loading data can be determined by error propagation theory [16-17]. According to this theory, the standard uncertainty u(y) corresponding to the measured output variables can be expressed as a function of the standard uncertainties associated with each input variableu(xi). The functional relation can be represented as:
n
i
i i
x x u y f
u
1
2
2( ) ( ) (4.4)
The standard uncertainty in the measurement of CO2 loading can be considered as a function of uncertainty in temperature, reactor volume, pressure and composition of the inlet solution. The uncertainties associated with each measured variable as well as the methodology for uncertainty calculation is reported in Appendix III.