(398i) Thermokinetic Properties and Mass Transfer of CO2 Absorption in Aqueous Benzylamine Solvents for CO2 Capture

In March 2017, atmospheric Carbon dioxide (CO₂) concentration level of 407 ppmv was recorded by the monitoring system of Mauna Loa Observatory, Hawaii. This is expected to rise in future largely due to massive fossil fuel firing as major source of industrial power plants application to meet the global energy demand. Global warming due to Greenhouse effect is one of the most concerned environmental effects due to accumulation of acid gases in troposphere. Reduction of heating value of natural gas, corrosion in pipes and process equipments, and catalyst poisoning in ammonia synthesis are some of the adverse effects due to the potential amount of CO₂presence in respective industrial gas streams. Mankind are at the verge of a great threat of future disaster. Scientists, environmentalist and engineers are very concerned about the possible solutions of this major environmental threat.

Amine based CO₂ absorption is the most mature technology to reduce greenhouse gas emissions by coal fired power plants. Chemical solvents remain the best solution to absorb CO₂ because of low partial pressure of CO₂ in post combustion exhaust gases from industrial operations. Aqueous alkanolamine absorbents such as monoethanolamine (MEA) and diethanolamine (DEA) are widely used for post combustion CO₂ capture. Several drawbacks are associated with conventional amines such as large energy demand to regenerate CO₂, high reaction enthalpy, low absorption capacity and high oxidative and thermal degradation. At high concentration, viscosity of MEA is found to be high which resisting the liquid phase mass transfer and high corrosion rate is also related to concentrated MEA. However, unlike secondary amines (like piperizine), carcinogenic nitrosamines are not formed directly with primary amines in the presence of NO_xand therefore primary amines are commercially more acceptable for PCC applications.

Recently, Benzylamine (BZA) has been identified as a potential absorbent for CO₂ removal with higher cyclic capacity similar to that of a tertiary or sterically hindered amine solution. BZA, a primary amine functional group attached to a benzyl group, is water soluble in all proportions. Aqueous BZA is readily biodegradable and its volatility is comparable to MEA. The corrosion rate of BZA is also lower than MEA. Vapour-liquid equilibrium data and kinetics parameters are the key requirements to design absorption and regeneration column. Density, viscosity and physical solubility data are needed to find kinetic parameters and thus the determination of required solvent circulation rate, and the size of the absorption column to achieve a specified CO₂ removal rate. Solubility of CO₂ in new solvents has been reported in recent years to develop new solvent system for acid gas removal. Density, viscosity and CO₂ solubility data in aqueous BZA solvent are very rare in open literature. There is no direct measurement data of kinetics of BZA available in open literature. In our recent publication, the CO₂ solubility in BZA with ENRTL modelling have been studied in detail to present BZA as potential solvent for CO₂ removal.

In this work, kinetics of CO₂ absorption in aqueous BZA has been studied in the temperature range of 303.15-333.15K. The reactive absorption rate was determined in stirred cell by pressure drop method in a reaction calorimeter system. Molar concentrations of BZA studied in this work are 2.78M, 1.87M, 0.94M and 0.47M. Thermokinetic properties have been measured which are used in kinetic measurement of CO₂ absorption in aqueous BZA solution. Pure component density and viscosity data of BZA are experimentally measured and presented by empirical model equation. The densities and viscosities of binary mixtures composed of benzylamine and water are measured over experimental temperature and molar concentration range. Experimental density and viscosity data are correlated using a Redlich-Kister type equation in terms of mole fraction and Grunberg-Nissan model with prediction error of 0.02% and 5.05% respectively. For kinetic measurement of BZA-CO₂ reaction, low CO₂ partial pressure was maintained to satisfy the criterion for a pseudo-first-order reaction condition. In pseudo-first-order reaction regime, second order rate constant for (BZA+H₂O+CO₂) system is evaluated using Zwitterions mechanism and correlated using Arrhenius power law expression. Reaction order has been found unity for both amine and CO₂. Hatta number and instantaneous enhancement factor are calculated and found to be satisfied the range of pseudo-first-order reaction regime condition. It can be seen that the enhanced mass transfer coefficient in liquid phase, k_LE, varies with the concentration of BZA solutions and temperature. It manifests that the CO₂ absorption rate increases with increasing BZA concentration and temperature. It can be obtained from the Arrhenius power law model that the reaction activation energy (E_a) of the reaction of aqueous BZA+CO₂ is 23.74kJ/mole, indicating lower energy barrier for the reaction of aqueous BZA solution with CO₂.

References:

Mukherjee, S., Bandyopadhyay, S. S., Samanta, A. N. 2017. Vapor-Liquid Equilibrium (VLE) of CO₂in Aqueous Solutions of Benzylamine: New Data and Modeling Using ENRTL-Equation,. Int. J. Greenh. Gas Control, 56, 12-21.

Gilles Richner, 2013. Promoting CO₂absorption in aqueous amines with benzylamine. Energy Procedia 37, 423 – 430.

Gilles Richner, Graeme Puxty, Amanda Carnal, William Conway, Marcel Maeder, Pauline Pearson, 2015. Thermo kinetic properties and performance evaluation of benzylamine-based solvents for CO₂capture. Chemical Engineering Journal 264, 230–240.

Ma’mun, S., & Svendsen, H. F., 2009. Solubility of N₂O in aqueous monoethanolamine and 2-(2-Aminoethyl-amino)ethanol solutions from 298 to 343 K. Energy Procedia, 1(1), 837–843.

Mondal, B.K., Bandyopadhyay, S.S., Samanta, A.N., 2017. Kinetics of CO₂ absorption in aqueous hexamethylenediamine. Int. J. Greenh. Gas Control 56, 116–125.