In this study, the conditions for the absorption of CO2 into different single and blended amine solutions were selected in such a way as to ensure that absorption occurred in the fast pseudo first-order reaction regime which requires fulfilling the condition stated by Eq. (2.59).
The rate of absorption of CO2 was calculated by Eq. (2.68) for which the condition given by Eq. (2.58) also should be satisfied. So, the overall condition for the reaction regime and the rate of absorption is,
[ ]
0
0
*
*
2 [A ] [B ]
1 B
3 [A ]
m n
A mn
B
L A
D k D
m
k Z D
< + (2.85)
In the laminar jet absorber a jet of liquid and in the case of a wetted wall column a film of liquid moves continuously through the gas, to which it is exposed for a known length of time.
The contact time of interest in these absorbers range from a few seconds to 10-3 s or lower. In the laminar jet apparatus a jet of liquid enters the gas space through a circular hole, and leaves through a slightly larger hole. We can conveniently vary the concentration of A as well as of B. An important advantage offered by the jet apparatus is that the contact time is uniquely determined by the jet length and diameter and the liquid flow rate, and is independent of the viscosity and the density of the liquid. The contact time in the jet apparatus can be varied over a wide range (0.001 – 0.1 s).
In the wetted wall column, the liquid flows in the form of a film under the influence of gravity down a surface which is usually a vertical tube or rod. The concentration of both A and B can be varied easily. When an inert gas is used to vary partial pressure of A, due care should be taken either to eliminate the gas phase resistance to mass transfer or evaluate it by adopting an appropriate method. The contact time in a wetted wall column can be varied in the range 0.1 – 2 s by changing the absorption length or the liquid-flow rate or both.
In the stirred cell, a cross shaped stirrer with vertical flat blades just skims the surface of the liquid. This arrangement gives higher values of kL than are obtainable when the blades are completely immersed. The gas-liquid interfacial area is known and is equal to the area of the flat liquid surface minus the area occupied by the stirrer blades. The concentration of A in the stirred cell can be varied by changing the total pressure of the system. In some cases, however, it may be possible by using an inert gas to change the partial pressure of A and hence the concentration of A. The range of concentration of B should be such that the viscosity of the liquid does not change significantly. The speed of agitation should be varied under such conditions that no vortex is formed (normal stirrer speed range, 20-150 rpm). The volume of the liquid per unit transfer area can also be conveniently varied here, at least by a factor of 2 without significantly affecting the value of kL. However, in many cases the gas- side resistance, in case of using an inert diluent for the solute gas, cannot be eliminated conveniently in a stirred cell.
In the mechanically agitated contactor the gas is usually introduced at the bottom of the contactor either through a single tube or through a sparger. It is desirable to use a small contactor with a capacity in the range 1–6×10-3 m3. The stirrer is a disk turbine with four or six straight blades. The stirrer speed is varied in the range 400 – 2800 rpm. In this contactor the gas phase and the liquid phase both are essentially back mixed above the critical stirring speed. It should be noted that the mechanically agitated contactor is usually associated with high values of kL and a.
The stirred contactor is in effect a stirred vessel with an undisturbed flat gas-liquid interface and with provisions for independently stirring the gas and the liquid phases. The main advantage of this type of apparatus is that at lower speed of agitation it can be used as a stirred cell and at higher speed of agitation it can be used as a mechanically agitated contactor.
Further, when it is used as a stirred cell, the gas-side mass transfer coefficient can be independently varied over a wide range, and hence it is much easier to eliminate the gas-side resistance when one is concerned with finding the order of the reaction with respect to the solute gas by varying its partial pressure by dilution with an inert gas. However, the main drawback of this contactor is that kL values are low and these cannot match the values of kL
encountered in those contactors where a gas is dispersed in a pool of liquid.
Recently, membrane contactors are getting considerable interests due to their advantages in operation over the conventional gas-liquid contactors. The microporous membrane used in this process acts as a fixed interface between the gas and the liquid phase without dispersing one phase into another. The operational flexibility is due to the absence of interpenetration of the phases in the contactor and hence the liquid and gas phase flow rates can be manipulated independently of each other, without any consequences like flooding, entrainment and weeping, as encountered in column type contactors. The modularity of membrane modules makes the design simple and easy to be scaled up linearly.
In the present work for the experimental study, wetted wall column is used as model laboratory contactor for absorption measurements of CO2 into single and blended amines.
Also, a performance analysis of different single and blended aqueous alkanolamine solvents towards absorption of CO2 has beencarried out through simulation of models developed to
describe the operation in membrane contactors. Both the wetted wall and membrane contactors are discussed further in details in Chapters 4 and 5, respectively.