Experimental

4.3.1 Materials

The reagents used for the absorption experiments were same those used for the measurement of physicochemical properties mentioned in Section 3.3.1 of Chapter 3.

Calibrated CO2 and N2 gas mixtures for the absorption experiment were obtained from Vadilal Gases, India.

4.3.2 Apparatus

Figure 4.2 is the schematic of the experimental set-up for absorption studies in the wetted wall column. The experimental facility shown in Figure 4.2 is complete with connections for two types of gases, CO₂ and gaseous mixtures of CO₂ and N₂, used for absorption measurements of CO₂. Photograph of the real experimental set-up is shown in Figure 4.3. The common sources of errors in wetted wall column are the entrance effect, appearance of ripples and the rigid film formation [36]. Appropriate measures, as suggested by Danckwerts [36] and described below, have been incorporated in the design of the wetted wall absorber used in this work to minimize the errors.

The column consisted of three concentric stainless steel tubes. The outer surface of the tube (2.81×10^-2 m o.d), provided the interfacial area for gas absorption. The liquid first flows up the central tube and across a liquid distributor located at the top of the column and then flows downwards through an annular gap, 1×10^-3 m wide and finally the liquid flows across a liquid distributor located at the top of column and moving downwards as a film on the outside of the outermost tube. The middle concentric tube formed the two-pass heat exchanger to keep the temperature of the liquid film at the desired level by circulation of thermostated water. The liquid, at the end of absorption length was collected in the liquid receiver located

at the bottom of the column. The outlet from the liquid receiver was adjusted so that a constant liquid level and a liquid seal were maintained in the receiver. An appropriately designed separator at the bottom of the column (with an annular gap of 1.5 mm between the separator and wetted wall surface) ensured separation of the gas and liquid beyond the absorption space with acceptable accuracy. The liquid level in the receiver was always kept about 15 mm below the top surface of the separator to minimize the error due to rigid film formation. The length of the liquid film could be altered by an arrangement to slide the column upward or down. The maximum length of the film exposed could be 200 mm by this arrangement. However, to minimize error due to end effects on the one hand and ripple formation on the other, the length of the film was varied between 50 and 100 mm only. A jacketed corning shroud made of glass with gas inlet at the top and three equally spaced gas outlets at the bottom made the enclosure for the gas space. Thermostated water was circulated through the jacket of the glass shroud to maintain the gas phase temperature at the desired level.

The liquid was fed to the contactor from an overhead storage located about 2 m higher than the absorption chamber. Precalibrated rotameters were used to control the liquid and gas flow rates. Thermostated water was circulated through the annular tubes of the wetted wall column to control the temperature of the liquid film. Calibrated platinum sensors (Pt-100, Julabo, FRG) along with temperature indicators (RW 2025G, Jeio Tech) were used to monitor gas phase and liquid phase temperatures.

4.3.3 Procedure

Before each run the absorption surface of the column was thoroughly cleaned with neutral EXTRAN (N) cleaning solution (Merck) and distilled water to ensure cleanness of outer surface of the column for getting continuous ripple free film. Circulation of the thermostated water through the jacket of the glass shroud, wetted wall column and through the bath, with the gas inlet coil immersed in it, was established to reach the desired temperature for absorption.

Pure CO2 or gas mixtures of CO2 and N2 were passed through the coil immersed in the controlled temperature bath and through the saturators immersed in the same bath. The gas,

saturated with water vapour at the temperature of absorption, was fed to the top of absorption space of the wetted wall column. The amine solution, thermostated at the temperature of the absorption, was taken in the overhead storage. The liquid was then fed to the contactor at the desired flow rate. The temperature of absorption was controlled within about ±0.2 K with the help of the circulator temperature controller. All experiments were performed under atmospheric pressure with initial CO2 loading of the solution equal to zero.

When the system reached a steady state condition with respect to the gas and liquid flow rates and the gas phase and liquid film temperature, a liquid sample was withdrawn from the outlet of the wetted-wall column into a vessel containing excess NaOH solution which converted free dissolved CO₂ and RNHCOO^- into the non-volatile ionic species,HCO₃⁻, and eventually toCO₃⁼. An excess amount of BaCl2 solutions was then added to the solution. The solution was shaken well to permit all absorbed (physically and chemically) CO2 to precipitate the carbonate as BaCO₃. The excess NaOH was titrated with HCl solution using phenolphthalein as the indicator. HCl was continued to be added to the solution using methyl orange as the indicator. The amount of HCl consumed after adding methyl orange indicator led to the CO₂ loading in terms of the moles of CO₂ per mole of amine. The titration procedure is the same as those described by Sun et al. [21]. Each reported or calculated value was obtained after averaging at least three titration measurements. Some typical calculations of specific absorption rate measurement are shown in Section IV.1.1 of Appendix IV. The uncertainties in the specific absorption rate measurements were always with in ±3.5%. The typical calculation of experimental uncertainty is shown in Appendix II.

The conditions for the absorption of CO₂ in amine solutions were selected in such a way as to ensure that absorption occurred in the fast pseudo first-order reaction regime which requires proper selection of CO₂ partial pressure to carry out the absorption experiments so that the following condition is maintained:

3<Ha E_∞ (4.7)

where, the Hatta number, Ha

ov CO2

L

Ha k D

= k (4.8)

and the enhancement factor in instantaneous reaction regime, E_∞, is

[ ] [

2

]

CO^Am₂

Am CO E D

Z D

∞ = (4.9)

When Eq. (4.7) is satisfied, the enhancement factor is equal to Hatta number (discussed in details in Section 2.3.3 of Chapter 2). The specific rate of mass transfer of CO₂ is given by [37]:

[ ]

[ ] ( )

2 2

2

2

2

CO CO

0 CO ov 2

CO ov

d CO dx CO

tanh

x

i

L

N D

D k

D k k

=

⎛ ⎞

= − ⎜ ⎟

⎝ ⎠

=

(4.10)

For Ha > 3, tanh Ha is near one; thus, the specific rate of mass transfer of CO2 becomes

[ ]

²

2 2 2

2

CO

CO 2 CO ov CO ov

CO

CO _i p

N D k D k

= = H (4.11)

The k_ov were calculated using Eq. (4.11) with the help of other known parameters,

CO2

p ,

CO2

D ,

CO2

H and the measured specific rate of absorption of CO₂,

CO2

N .