In this work, the absorption of CO2 in aqueous solutions of sterically hindered and mixed amines is considered. Kinetics of the absorption of CO2 in a new activator such as 2-(1-piperazinyl)-ethylamine (PZEA) and in the mixtures of PZEA and N-methyldiethanolamine (MDEA) are also performed.
CO 2 ABSORPTION INTO ALKANOLAMINES: BASIC CHEMISTRY AND THEORY OF MASS TRANSFER
Results and discussion 83 .1 Results of physical solubility measurement 83
Results and discussion 156
Results and discussion 204
CONCLUSIONS AND FUTURE DIRECTIONS 248–252
Calculation of Uncertainty in the Experimental Measurements
Tabulated Representation of Physicochemical Properties (Supplementary Information of Chapter 3)
Derivations of model equations of membrane contactors (Supplementary Information of Chapter 5)
Dimensionless Forms of Model Equations of Membrane Contactors (Supplementary Information of Chapter 5)
188 Figure 4.21 Effect of errors in second-order rate constants on. calculated absorption rate of CO2 in aqueous solutions of AHPD. 234 Figure 5.7b Radial concentration profile of pure CO2 in the individual amine. solution at the liquid exit of the fiber in HFMC.
Introduction
Specifications for natural gas pipelines usually limit the CO2 content to 2-5%, and in the case of liquefied natural gas production CO2. CO2 can be used in industry for carbonated beverages, firefighting and solvent extraction as a supercritical fluid.
Carbon dioxide capture technologies
- Removal of CO 2 by absorption
- Physical absorption
- Mixed physical/chemical absorption
- Chemical absorption
Ammonia-based processes are rarely used because of the higher corrosiveness of the loaded solutions and the complicated flow scheme compared to alkanolamine-hot potassium carbonate-based processes. The advantage of aqueous alkanolamine solutions is the lower price compared to physical solvents.
- Primary and secondary amines for CO 2 absorption
- Sterically hindered amines for CO 2 absorption
- Tertiary amines for CO 2 absorption
- Blended amines for CO 2 absorption
They demonstrated that the absorption of CO2 on hindered amines can be explained in terms of the mechanism established for the reaction of CO2 with conventional alkanolamines. 98] studied the kinetics of CO2 absorption in mixed aqueous solutions of MDEA and PZ.
Importance and objectives of present work
In view of the extensive use of alkanolamines for the absorption of acid gases in industry and the significant energy requirements of acid gas treatment plants, there is considerable incentive for the development of more efficient and flexible methods for the separation of acid gases. Measurement of the required physicochemical properties (e.g. density, viscosity and surface tension of the amine solutions, diffusivity and physical solubility of CO2 in aqueous amine solutions) over a wide range of temperatures and amine concentrations and development of useful correlations for these properties.
Introduction
Absorption of CO2 in alkanolamine solutions involves chemical reaction of the CO2 with the amines. The effect of the chemical reaction in the liquid phase is generally to increase the liquid-film absorption coefficient over simple physical absorption case. In the case of very slow reactions, the dissolved molecules appear to migrate well into the body of the liquid before reaction occurs so that the overall mass transfer rate is not significantly increased by the occurrence of the chemical reaction.
The rate of absorption with chemical reaction in a contactor is partly determined by the hydrodynamic conditions (flow rate, geometry of packing or plate of absorption column, physical properties of liquid, etc.) and partly by the physico-chemical properties of the system (solubilities ) of CO2 in the absorbent, diffusivity of dissolved gases and reactants in solutions, kinetics of reactions occurring in solution, etc.). In this context, various theories or hydrodynamic models, which have been proposed for physical absorption, such as the film model and various surface renewal models, can be effectively used to predict the rate of absorption with chemical reaction.
- Zwitterionic mechanism
- Termolecular mechanism
- Base-catalyzed hydration mechanism
- Alcohol-group bonding of CO 2
- Molecules with multiple amine functionalities
In the limiting case where the contribution of amine to zwitterion deprotonation is much more significant than other bases such as H2O and OH–, the overall reaction is second order with respect to amine. The kinetics of the reaction of CO2 with monoethanolamine (MEA) has been widely studied and adequately described using zwitterionic mechanisms [4, 7, 8]. The kinetics of the reaction of CO2 with another secondary amine, diisopropanolamine (DIPA), in aqueous solutions was studied by Camacho et al.
The total rate of all CO2 reactions in an aqueous solution is given by the sum of the reaction rates given by Eq. The total rate of all CO2 reactions in an aqueous solution is therefore represented by the sum of the reaction rates given by Eq.
Theory of mass transfer with chemical reaction
- Mass transfer models
- Effect of chemical reaction on absorption
- Identification of different regimes of chemical absorption
If the liquid contains a constituent that reacts with the dissolving gas, both the rate of mass transfer and the capacity of the liquid for the gas are increased. The reactant B and the products of the reaction are assumed to be non-volatile. This ratio is equivalent to the ratio of the rate of chemical absorption to that of physical absorption called the 'Hatta number'.
The boundary condition given by Eq. 2.74) implies a constant reactant concentration, [Bi], in the immediate vicinity of the interface. By definition, the specific mass transfer rate is equal to the solute flux at the interface.
Laboratory gas-liquid contactors
In the laminar jet apparatus, a liquid jet enters the gas space through a circular hole and leaves through a slightly larger hole. 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 stirring cell, a cross-shaped stirrer with vertical flat blades just skims the surface of the liquid.
The concentration of A in the stirred cell can be changed by changing the total pressure of the system. In the mechanically stirred contactor, the gas is usually introduced at the bottom of the contactor, either through a single tube or through a sprinkler.
Notations
DEA 2
Introduction
These properties were also used to analyze the experimental data for CO2 adsorption on aqueous alkanolamines presented in Chapter 4. The solubility and diffusivity of CO2 in aqueous amine solutions cannot be measured directly since CO2 undergoes chemical reactions with these solvents. The similarity in mass, molecular structure and molecular interaction parameters between CO2 and N2O has led Clarke [2] to assume that the solubility and diffusivity ratios of CO2 and N2O in water and in aqueous solutions of organic solvents are similar within 5% or preferably at the same temperature.
Similarly, the diffusivity of CO2 in amine solutions can also be estimated using Eqs. The same assumption is made in this paper, except for the case of CO2 absorption in aqueous solutions of 2-PE.
Literature review
- Physical solubility and diffusivity
- Density and viscosity
The diffusion coefficients for 2-PE in solution are readily available in the literature and were calculated by Eq. Numerous solubility and diffusivity data for N2O in aqueous amine solutions were reported in the literature for the binary and ternary systems, such as (MEA + H2O. Using a diaphragm technique, the mutual diffusivities of MEA, DGA, DIPA and TEA in aqueous solution have been reported in the literature [39, 40].
Based on these results, the solubilities and diffusivities of CO2 in the aqueous amine solutions are estimated using the N2O analogy. Therefore, the density and viscosity measurements in this work have been performed in the temperature range 288 – 333 K to bridge the gap.
Experimental
- Materials
- Apparatus and procedure .1 Measurement of physical solubility
- Measurement of diffusivity
- Measurements of density and viscosity
A precise manometric device was used to maintain atmospheric pressure in the cell throughout each experimental run. The partial pressure of N2O in the experiments was corrected for the vapor pressure of the solution. The gas-liquid contact time in the column can be varied from 0.3 – 0.8 s by changing the absorption length but keeping the liquid flow rate constant.
The absorption temperature was controlled in the same way as already discussed in section 3.3.2.1. The experimental uncertainties in the measured density and viscosity were estimated to always be within ±0.1% and ±1%, respectively.
Results and discussion
- Results of physical solubility measurement
- Results of diffusivity measurement
- Results of density measurement
- Results of viscosity measurement
16], Versteeg and van Swaaij [4], and Li and Lai [22] were in good agreement with the solubility calculated from Equation (3.7), the solubility values reported by Al-Ghawas et al. The calculated solubilities from Eq. 3.9) are in good agreement with the experimental results of this study. The calculated solubilities from Eq. 3.10) are in good agreement with the experimental results of this study.
The calculated diffusivities from Eq. 3.13) is in good agreement with the experimental results of this study. The calculated solubilities from Eq. 3.14) is in good agreement with the experimental results of this study. A general set of temperature-dependent parameters has been developed using experimental data in the temperature range 288 – 333 K. The determined parameters are shown in table 3.24.
The calculated solubilities from Eq. 3.36) is in good agreement with the experimental results of this study.
Introduction
Saha [13] investigated the absorption of CO2 in aqueous AMP as well as aqueous MEA using a wetted wall contactor. 17] investigated the rates of absorption of CO2 in mixtures of MDEA/MEA and AMP/MEA using a wetted wall column. Liao and Li [18] investigated the kinetics of absorption of CO2 in aqueous mixtures of MEA and MDEA using wetted wall column.
19, 20] studied the kinetics of CO2 absorption in aqueous solutions of AEPD and AMPD using a wetted wall column. 21] investigated the kinetics of CO2 absorption in aqueous mixtures of PZ and AMP using a wetted wall column.
Theory
The contact time θ is calculated from Eq. 4.5) and can be changed by changing the flow rate VL or the length h of the liquid film.
Experimental
- Materials
- Apparatus
- Procedure
The length of the liquid film could be changed by an arrangement to slide the column up or down. Thermostatic water was circulated through the jacket of the glass envelope to maintain the gas phase temperature at the desired level. Thermostatic water was circulated through the annular tubes of the wetted wall column to control the temperature of the liquid film.
The amine solution, which was controlled at the temperature of the absorption, was taken to the overhead storage. The absorption temperature was controlled within approximately ±0.2 K using the circulation pump temperature controller.
Reaction mechanism
- Hydration of CO 2 in aqueous solutions
- Reactions of CO 2 with sterically hindered alkanolamines (2-PE and AHPD)
- Reactions of CO 2 with PZEA
- Reactions of CO 2 with blends of (PZEA + MDEA)
Following the mechanism first proposed by Caplow [42] and reintroduced by Danckwerts [43], the general consensus for the reaction of CO2 with primary or secondary amines is the formation of a zwitterion intermediate instead of a one-step carbamate formation. Assuming that the Brønsted relation is also applicable to PZEA, it seems reasonable to neglect the reaction of CO2 with the tertiary amine group, while the reaction of CO2 with the primary and secondary amine groups to form primary and secondary carbamates should be considered. Since in this study the conditions for CO2 absorption in PZEA solutions are in the fast pseudo-first-order reaction regime, the (interfacial) concentration of PZEA is not appreciably reduced by the reaction with CO2.
Based on the above considerations regarding the different reactions with CO2 in aqueous PZEA solutions, it seems justified to conclude that the overall absorption rate in the first pseudo first order kinetic regime is influenced by the reaction between CO2 and PZEA to form primary and secondary carbamates. Because the differences between the reaction rate constants considering and neglecting reaction of CO2 and OH ion were obtained less than 0.1.
Physicochemical properties
Results and discussion
- Specific rate of absorption
- Determination of overall and apparent reaction rate constants
- Determination of second order reaction rate constants
- Parametric sensitivity analysis
The same phenomenon about the contribution of the reaction of CO2 with OH ion was also reported for absorption of CO2 in aqueous solutions of other amines such as MDEA and AMPD [20, 56]. Thus, this indicates at higher temperatures, AHPD is more preferable solvent than 2-PE for the absorption of CO2. The second order rate constant of PZEA for any specific temperature obtained for the absorption of CO2 in (PZEA + H2O) and (PZEA + MDEA +H2O) systems are very close to each other (average deviation is 5.6%).
The parameters considered for the analyzes are the Henry's law constant for CO2, the diffusivity of CO2 in amine solutions and the second-order reaction rate constants for the absorption of CO2. C., “Chemical kinetics of the reaction of CO2 with amines in pseudo-mn-order conditions in polar and viscous organic solutions”, Chem.
