Introduction

KINETICS OF ABSORPTION OF CO

₂

INTO AQUEOUS

essentially be kept constant at the desired level. Another significant advantage of this contactor is that, for most of the gas liquid systems the gas phase resistance to mass transfer for absorption of dilute solute (diluted by an inert gas) can be made negligible by increasing the gas flow rate with little effect on the hydrodynamics of the liquid film. Early studies on gas absorption with simultaneous chemical reaction in laminar falling film for short contact times have been reported by Nysing and Kramers [1] and Roberts and Danckwerts [2] for the absorption of CO2 in carbonate-bicarbonate buffer. Emmert and Pigford [3] estimated the enhancement factor due to chemical reaction and Hatta number for chemical absorption of CO2 into MEA in a falling liquid film absorber. Sharma [4, 5] presented the different values of rate constants for the reaction between CO2 and various amines using laminar jet and falling film absorbers. Sada et al. [6, 7] and Hikita et al. [8] measured the absorption rates of CO2 into aqueous solutions of MEA using laminar jet and wetted wall column absorbers. Gas absorption with first-order homogeneous chemical reaction in a laminar falling liquid film has been studied by Stepanek and Achwal [9] for the cases of zero and finite gas phase resistance.

Alvarez-Fuster et al. [10] and Blanc and Demarais [11] studied the kinetics of absorption of dilute CO2 into aqueous MEA and DEA and aqueous DEA solutions respectively using wetted wall column under conditions of negligible gas phase resistance. Yih and Shen [12]

studied the kinetics of the reaction between CO2 with AMP using a laboratory wetted wall column. Saha [13] studied the absorption of CO2 into aqueous AMP as well as aqueous MEA using a wetted wall contactor. Mshewa [14] used a wetted wall column to study the CO2

absorption/desorption with aqueous mixtures of MDEA and DEA. Saha et al. [15] studied the kinetics of CO₂ – AMP using a laboratory wetted wall contactor. Pacheco [16] studied the rates of mass transfer of CO₂ in mixtures of MDEA and DGA^® using a laboratory wetted wall column. Mandal et al. [17] studied the rates of absorption of CO₂ into mixtures of MDEA/MEA and AMP/MEA using a wetted wall column. Liao and Li [18] investigated the kinetics of absorption of CO₂ into aqueous blends of MEA and MDEA using wetted wall column. Yoon et al. [19, 20] studied the kinetics of absorption of CO₂ into aqueous solutions of AEPD and AMPD using a wetted wall column. Sun et al. [21] investigated the kinetics of absorption of CO2 into aqueous blends of PZ and AMP using wetted wall column.

It is evident from the review of literature presented in Chapter 1 (Section 1.3.1) that sterically hindered amines have become commercially attractive solvents. Recently many studies have been made on identifying new sterically hindered amines to reduce the total capital and operating cost in CO₂ absorption process. Baek and Yoon [22] proposed AMPD, which is a primary sterically hindered amine, as a potential CO₂ absorbent in the solubility study. Yoon et al. [19, 20] studied the reaction kinetics of CO2 with aqueous solutions of AMPD and AEPD using zwitterionic mechanism.

2-Piperidineethanol (2-PE) is a secondary sterically hindered amine having a naphthenic ring attached to amino group which is sterically hindered by a hydroxyl group (Figure 4.1). 2- Amino-2-hydroxymethyl-1,3-propanediol (AHPD) is a primary sterically hindered amine in which the amino group is connected to a tertiary carbon atom and the amino group is sterically hindered by three hydroxyl methyl groups connected to that tertiary carbon atom (Figure 4.1). There is very little information available regarding the absorption of CO2 in aqueous solution of 2-PE. Shen et al. [23] studied the kinetics of absorption of CO2 into aqueous solution of 2-PE at 303 K within the amine concentration range of 0.2-1.0 kmol m^-3

using a wetted-wall column. They found a second order forward rate constant of 195 m³ kmol^-1 s^-1 at 303 K. They considered the carbamate ion to be unstable, which readily

hydrolyzed to form bicarbonate ion. Xu et al. [24] explored the kinetics of absorption of CO2

into aqueous solutions of 2-PE at temperature range of 283-313 K within the amine concentration range 0.25-2.5 kmol m^-3 using a stirrer cell absorber. The authors did not consider the hydrolysis of carbamate ion in their reaction mechanism. They reported a much higher second order forward rate constant of 1468 m³ kmol^-1 s^-1 than that found by Shen et al.

[23].

Park et al. [25] measured the solubility of CO2 in aqueous solution of AHPD which showed a better CO2 loading capacity of AHPD compared to that of other amines like MEA, AEPD and AMPD at high CO2 partial pressure. Grøenvald et al. [26] described the reaction of CO2 with AHPD as a reaction of CO2 with alcohol and not with amine forming only monoalkyl carbonate in their study of reaction of CO2 with highly basic aqueous solution of AHPD. This reaction is however in general not expected to play a significant role in industrial CO2 absorption processes as the pH of the system is usually not high enough [27]. Further, the

reaction of CO₂ withAHPD was confirmed as a reaction of CO₂ with amine from the NMR spectroscopic analysis carried out by Park et al. [28]. Their investigation also confirmed the hydrolysis of unstable carbamate to form bicarbonate, as they detected very small amount of carbamate anion for a wide range of CO₂ partial pressures. So, from the above discussion and the literature reports it is clear that AHPD can be a potential solvent having good CO₂ absorption capacity and regeneration characteristics. But unless there is any information on the reaction kinetics of a solvent with CO2 it is not possible to understand the mechanism and the rational design of the gas treating unit. To the best of our knowledge, the kinetics of the reaction of CO2 with aqueous solution of AHPD is not reported so far.

Recent interest and developments in the bulk removal of CO₂ involve the addition of an activator to other suitable alkanolamines. The reason for the use of such blends is related to the relatively high rate of reaction of CO2 with the activator with the advantages of the other alkanolamines in the blends concerning regeneration and stoichiometric loading capacity, which leads to higher rates of absorption in the absorber column and a low heat of regeneration in the stripper section. PZ is such an activator which was first used in the activated MDEA technology of BASF and it is reported that PZ is more effective than the conventional rate accelerators [29]. Since then, several studies have reported on the characteristics and performance of piperazine activated MDEA blends [30–35]. The rate constant was reported an order of magnitude higher than primary amines such as monoethanolamine (MEA) or Diglycolamine (DGA) [35]. Following the performance of PZ we here propose 2-(1-piperazinyl)-ethylamine (PZEA) as a new activator towards the absorption of CO2.

In this work, new results on the kinetics of absorption of CO2 into aqueous solutions of sterically hindered 2-PE and AHPD, aqueous solutions of PZEA and aqueous blends MDEA with PZEA using a model laboratory wetted wall column are presented. Kinetic rate parameters were determined from the measured CO2 absorption rate. A parametric sensitivity analysis was investigated in order to study the effect of kinetic parameter on calculated CO2

absorption rates.