Andrzej WILK, Lucyna WIĘCŁAW-SOLNY, Aleksander KRÓTKI, Dariusz ŚPIEWAK ? Centre for Process Research, Institute for Chemical Processing of Coal (IChPW), Zabrze, Poland
Please cite as: CHEMIK 2013, 67, 5, 399?406
The amine absorption processes are widely used in the industry to purify refinery gases, process gases or natural gas. Recently, amine absorption has also been considered for application in CO2 capture from exhaust fumes created as by-product of electric power generation from fossil fuels. Amine absorption is based on a reversible reaction between weak bases, e.g. ethanolamine, and acidic gases, including i.a. carbon dioxide. CO2-containing gas (e.g. exhaust fumes) is fed into the absorption column, where it comes in counter flow contact with unsaturated sorption solution. Carbon dioxide is absorbed by the solution, while the gas leaving the column is purified. The solution with absorbed CO2 is pre-heated with a solution regenerated in the heat exchanger, and then directed to the regenerator. In the regenerator, in the supplied heat, the amine-CO2 compound decomposes, eliminating carbon dioxide from the solution. Thus, a stream of virtually pure CO2 is obtained. The regenerated sorbent solution is redirected, via heat exchanger, back to the absorption unit. A simplified flow sheet of the process is provided in Figure 1.
One major disadvantage of amine absorption processes is the high energy consumption, arising from high energy levels required to regenerate the sorbent. There is a number of ways to reduce the energy consumption: by appropriate configuration of the technological process line , integration of CO2 capturing unit with the power plant system , choice of optimal process parameters or optimal sorbent  with the best possible kinetic and equilibrium parameters, and the lowest absorption heat and specific heat.
Commercially available technologies usually use amines such as: monoethanolamine (MEA) ? Econamine FG or Kerr-McGee/ ABB Lummus Crest process , diethanolamine (DEA) ? SNEA DEA process , N-methyldiethanolamine (MDEA) BASF aMDEA [11, 14], and amines with steric hindrance, e.g. 2-amino-2- methylpropan-1-ol (AMP) ? KM-CDR Process . Among those amines, each has a different, individual set of properties. Primary amines, such as MEA, are characterised by high absorption rate, but at the same time by high desorption heat levels. Furthermore, primary amines are less resistant to thermal and oxidative degradation than tertiary amines or amines with steric hindrance [10, 12]. Secondary amines in most cases are characterised by slightly lower absorption kinetics, but are more resilient to degradation and produce lower desorption heat. Tertiary amines are the slowest to absorb CO2, produce the lowest desorption heat and feature high resilience to degradation . The final group, i.e. amines with steric hindrance, combine the advantages of primary and tertiary amines. Due to their structure, they are characterised with relatively high absorption rate, with lower susceptibility to degradation and lower desorption heat that primary amines .
Individual amine groups can be combined to obtain solutions with the best characteristics, both in terms of kinetics and absorption capacity. Aside from mixing amines, good results can be obtained also by using amine blends with physical CO2 sorbents, e.g. with propylene carbonate . These are some of the advantages of using such compounds:
? the composed solution has a lower vapour pressure than both pure physical sorbent and aqueous amine solution
? requires less energy to regenerate the sorbent than amine/H2O compounds
? increases CO2 solubility in the absorbing solution as compared to pure physical sorbent and aqueous amine solution.
During the CO2 absorption process in alkanolamine solutions a number of chemical reaction occurs. We can distinguish 5 main reactions :
Those reactions can be reduced to two fundamental reactions that can be used to describe the CO2 absorption process in any amine type:
The first of the above reactions is characteristic for primary and secondary amines. The second reaction occurs for all types of amines ? in primary and secondary amines only to a minor extent, while in tertiary amines and amines with steric hindrance, where the structure of the molecule prevents the formation of stable carbamate, it is the main reaction [7, 13].
Equations (6) and (7) also provide way of explaining the phenomenon of CO2 absorption process activation in tertiary amine solutions by using small additions of quick-reacting amines, such as monoethylamine or piperazine.
In the case of tertiary amine solution, the reaction limiting the absorption rate is the slow reaction of dissociation of dissolved carbon dioxide (3). However, if the solution contains both tertiary and primary amines, then it is highly probable that one molecule of the tertiary amine and one of the primary amine will participate in the reaction with the carbon dioxide molecule. Due to its structure, tertiary amines undergo protonation, while primary amines form carbamate anions. The formed carbamate may undergo hydrolysis, thus creating a bicarbonate anion and a primary amine molecule is released, which can then again react with carbon dioxide. Thus, in the process of bonding carbon dioxide by tertiary amines, the free stage of CO2 dissociation is omitted. The diagram of reactions occurring in the solutions of activated tertiary amines is provided in Figure 2.
The following reagents were used for laboratory analysis: ethanolamine for synthesis, ?99%, MERCK; N-methyldiethanolamine, 99+%, ACROS ORGANICS; piperazine, ?99%, MERCK; 2-amine-2-methyl-1-propanol, ~95%, MERCK; N-methyl-2-pyrrolidone, ?99.5%, MERCK and carbon dioxide 4,5, Linde Gaz Polska.
Testing apparatus and experiment procedure
The analysis involved testing the impact of the activator quantity on the absorption capacity and CO2 absorption rate in tertiary amine solutions. Also tested was the impact of the amount of organic liquid in the primary amine / amine with steric hindrance / activator / organic liquid / water compound (as specified in previous tests) [5, 6] on the equilibrium and kinetic parameters of the solution. All tests were conducted in specially designed laboratory station for analysing equilibriums and kinetics of CO2 absorption in amine blends (Photo 1).
The laboratory station included a 2.5 dm3 thermostated glass reactor, equipped with cryostat to maintain temperature during the test, a cylindrical separatory funnel for liquid sample of the sorbent, and the system for measuring and automatic recording of pressure in the apparatus. The vacuum gauge enabled filling the apparatus with carbon dioxide or removing gas from the apparatus using a vacuum pump.
The tests performed with the laboratory station included establishing the isotherms of CO2 absorption in amine solutions in order to obtain data on the equilibrium absorption capacities, as well as testing the CO2 absorption rate.
For the purpose of equilibrium tests, samples of the absorbing solution were introduced into the apparatus several times. After instilling each sample the researchers waited for the equilibrium pressure in the system to stabilise. On the basis of the obtained points, the curve of correlation between the CO2 equilibrium pressure and the amount of carbon dioxide absorbed in 1 dm3 of solution was drawn, or per mol of amine groups. The tests of absorption kinetics included a single instillation of a solution sample into the system and recording the pressure fluctuations for approx. 10 minutes. Thus, it was possible to determine the correlation between the amount of absorbed CO2and the process duration. All tests were performed in temperatures of 20?60°C and for partial pressures of CO2 in the range of 0?90 kPa for each test. The stirrer rotated at approx. 750 rpm. For the set rotation speed the general absorption rate was conditioned by the chemical reaction rate, and the absorption process ran in the kinetic area instead of the diffusion course of the reaction.
The correctness of obtained results was verified by comparing the data from previous tests for aqueous solutions of N-methyldiethanolamine with other literature data and CO2 absorption models in tertiary amines .
Analysis of results
As part of the analysis of the tertiary amine/activator/H2O compounds, tests were performed on the following solutions: 30% MDEA; 50% MDEA; 30% MDEA + 2 (4, 6, 8, 10, 12 and 20%) PZ.
The comparison of the obtained data indicated that, as anticipated, the use of activator in the carbon dioxide absorption in tertiary amine solutions (MDEA) has a beneficial effect both on the process kinetics and the absorbing equilibrium.
In the case of absorbing equilibriums, a close correlation is visible between the amount of activator and the solution?s absorption capacity. The larger the activator addition, the higher the equilibrium absorption capacity for the given carbon dioxide partial pressure. As indicated by the data (Figs. 3 and 4), the correlation becomes much more prominent in higher process temperatures. In the carbon dioxide absorption rate the impact of the amount of activator does not appear to be as prominent as in absorption equilibriums. The increase of CO2 absorption rate is visible for a 2% addition of activator: increasing the concentration to 4% results in a considerable increase of the absorption rate, however, further rise of activator concentration does not significantly increase the values of kinetic parameters. Both the solutions containing 4% and 12% w/w of activator have similar absorption rates. Furthermore, due to limited solubility of piperazin in water-amine blends, it is beneficial to apply as low activator amounts as possible, since with piperazin concentration in the sorbing solution of approx. 20% w/w, the activator undergoes partial crystallisation already at 30°C. The results of the absorption kinetics tests in MDEA/PZ/ H2O are presented on the charts below (Figs. 5 and 6).
The analysis of the impact of the volume of organic liquid and water in the solution was performed on the compounds: primary amine/amine with steric hindrance/activator/organic liquid/water. The performed analysis included tests of equilibriums and kinetics of absorption for solutions with the following mass concentrations of water/organic liquid: 63/0, 53/10, 43/20, 33/30, 23/40, 13/50. As shown by the equilibrium data provided in Figure 7, the volume of organic liquid has a minor impact on the degree of carbonisation of the solution and thus on the absorption capacity. Increasing the amount of the organic liquid improves the absorption capacity of the solution to a small extent.
Unlike the equilibrium data, the data obtained from the tests of absorption kinetics indicate a major impact of the volume of organic liquid and water on carbon dioxide absorption rate. Using a minimum amount of water (ok. 13%) considerably increases the CO2 absorption rate, which is higher in this case even than with the widely used 30% monoethylamine solution. This arises from the fact that the absorption reaction is directed at carbamate formation due to limited amount of water and thus inability to form a bicarbonate. The correlation between the carbon dioxide absorption rate and the concentration of the organic liquid in the solution is provided in Figure 8.
The obtained results indicate a considerable impact of the appropriate solution composition on the equilibrium parameters and, more importantly, on CO2 absorption rate. Based on the solution developed in previous analyses, it was possible to obtain, by appropriate selection of concentration of organic liquid and water, a sorbent characterised by kinetic parameters that are comparable or ? for some concentrations ? even better than the ones in one of the most widely used solution, i.e. 30% MEA. Furthermore, using an organic liquid reduces the vapour pressure of the solution and thus its losses, and reduces the amount of energy consumed in the regeneration process due to its specific heat, which is several times lower than water?s. The analysis of the impact of the activator amount indicated that for MDEA/PZ compounds a considerable increase of the absorption rate can be obtained already with small activator amounts, i.e. 4?6% w/w. The kinetic data obtained from the analysis are only a set of comparative data used to choose the best among the tested CO2 sorbents. To apply the kinetic data e.g. in the design of CO2 capture systems, it is necessary to perform further study to comprehensively assess the kinetics of the absorption which, aside from the chemical reaction, includes also other partial phenomena, e.g. diffusion [17, 18].
The results presented in this paper were obtained through analysis co-financed by the National Centre for Research and Development under agreement SP/E/1/67484/10- Strategic Research Programme ? Advanced Technologies for Energy Generation: Developing a technology for high efficient zero emission coal blocks integrated with CO2 capture from exhaust gases, and under agreement SP/E/2/66420/10 ? Strategic Research Programme ? Advanced Technologies for Energy Generation: Developing a technology of oxyfuel combustion for pulverized fuel and fluidized-bed furnaces integrated with CO2 capture system.
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Andrzej WILK ? M.Sc., graduated from the Faculty of Chemistry of the Silesian University of Technology in Gliwice, majoring in chemical technology (2010). He is also a graduate of postgraduate studies at the Faculty of Power and Environmental Engineering of the Silesian University of Technology in Gliwice (2012), majoring in waste management. He is currently employed as engineer in the Institute for Chemical Processing of Coal in Zabrze. Specialisation: chemical technology and environmental protection. e-mail: firstname.lastname@example.org; phone: +48 517 178 510
Lucyna WIĘCŁAW-SOLNY ? Ph.D., Eng. graduated from the Faculty of Chemistry of the Silesian University of Technology (1998). She defended her doctoral thesis ?Obtaining catalytic coatings on metallic surfaces? in 2004. She specialises in chemical and process engineering. She is currently employed in the Institute for Chemical Processing of Coal in Zabrze as the deputy head of the Centre for Process Research.
Aleksander KRÓTKI ? M.Sc., graduated from the Faculty of Chemistry of the Silesian University of Technology in Gliwice (2010). She is currently employed in the Institute for Chemical Processing of Coal in Zabrze. Specialisation ? chemical industry and environmental protection apparatus.
Dariusz ŚPIEWAK ? M.Sc., graduated from the Faculty of Chemistry of the Silesian University of Technology in Gliwice (2011). He is currently employed in the Institute for Chemical Processing of Coal in Zabrze. Specialisation ? chemical and process engineering.