Molar Heat Capacity (C p ) of Aqueous Cyclic Amine Solutions from (298.15 to 353.15) K Saeed Poozesh, Aravind V. Rayer, and Amr Henni* Acid Gas Removal Laboratory, Faculty of Engineering and Applied Science, University of Regina, Saskatchewan, Canada S4S 0A2 ABSTRACT: Molar heat capacities at 11 different temperatures in the range (298.15 to 353.15) K are reported for aqueous 1,4-dimethyl piperazine (1,4-DMPZ), 1-(2-hydroxyethyl) piperazine (1,2-HEPZ), 1-methyl piperazine (1-MPZ), 3-morpholinopropyl amine (3-MOPA), and 4-(2-hydroxyethyl) morpholine (4,2-HEMO) solutions. Molar heat capacities of MDEA and MEA were also measured and compared with published data resulting in a 1 % experimental uncertainty. Molar excess heat capacities were correlated as a function of mole fraction and temperature employing the Redlich- Kister relation. Excess partial molar quantities were calculated and reported. Group additivity analysis was performed using the molar heat capacities for all the above cyclic amines, and data were fitted with less than 0.5 AAD%. Among the five amines studied, 3-MOPA had the highest values of molar heat capacity and 1-MPZ had the lowest. The values of molar heat capacity of amines were dominated by -CH 2 and -OH group contributions. The contributions of -CH 2 , -N, -OH, -O, and -NH 2 groups increased with increasing temperature, and the contributions of -NH and -CH 3 groups decreased with increasing temperature for these cyclic amines. 1. INTRODUCTION Global warming of the Earth’s atmosphere has been mainly attributed to the contribution of one gas, CO 2 . Therefore, the issue of acid gas removal became increasingly significant in the treatment of natural and industrial gases. For the last few decades, alkanolamine solutions have received a lot of attention by the research community because of their industrial importance in natural gas processing, synthetic ammonia plants, fossil-fuel-fired power plants, steel production, and chemical and petrochemical industries. Acid impurities in gas streams include carbon dioxide (CO 2 ), hydrogen sulfide (H 2 S), and sulfur dioxide (SO 2 ). 1 Monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA) have been widely used in natural gas sweetening. Although there has been development in the absorption technique and the availability of solvents for acid gas removal, certain issues need to be addressed like the associated heat duty of regeneration per CO 2 loading, corrosion, solvent degradation, and the high cost of the absorption/ stripping columns. To address these issues, there is a need to investigate more reliable solvents that possess good physical and chemical properties such as high CO 2 loading capacities to reduce solvent circulation rates, low vapor pressures to minimize solvent loss, low to moderate viscosities for high mass transfer, thermal and chemical stabilities to reduce solvent reclaimer costs, and high kinetic rates of absorption to minimize the size of absorption/stripping columns. Piperazine has recently been found to be a very efficient additive to conventional solvents such as MEA or DEA because of its high acid gas loading capacity along with its high reaction rate. Because of its cyclic and diamine characteristics, the reac- tivity and solubility of piperazine is higher than other secondary amines. The reaction kinetics, mass transfer, and solubility of carbon dioxide into a single or mixed aqueous solution of piper- azine have been studied by various researchers. 2-8 Kadiwala et al. 8 concluded that piperazine (at high pressures) has a capacity for absorbing CO 2 in the ratio of three molecules of CO 2 per one molecule of piperazine, which is three times higher than industry “standard” monoethanolamine (MEA). Research is therefore continuing to find the best piperazine derivative. In this paper, five amines with structures like piperazine (piperazine derivatives) have been selected, and their molar heat capacities were measured. The molecular structures of the studied cyclic amines are given in Table 1. Molar heat capacity is a fundamental thermodynamic property and the knowledge of heat capacities at various temperatures is necessary for the calculation of thermodynamic properties such as enthalpy (H), entropy (S), and Gibbs energy (G). These data are required for the design of absorbers, regenerators, condensers, heat exchangers, and reboilers used in gas plants. To the best of our knowledge, no published data could be found for the molar heat capacity (C p ) of aqueous 1,4-dimethyl piperazine (1,4-DMPZ), 1-(2-hydroxyethyl) piperazine (1,2- HEPZ), 1-methyl piperazine (1-MPZ), 3-morpholinopropyl amine (3-MOPA), and 4-(2-hydroxyethyl) morpholine (4,2- HEMO) solutions. The objective of this study is to determine experimentally the molar heat capacities of these cyclic amines at eleven different temperatures in the range (303.15 to 353.15) K. Experimental C p values were used to calculate the molar excess Received: February 14, 2013 Accepted: June 6, 2013 Published: June 27, 2013 Article pubs.acs.org/jced © 2013 American Chemical Society 1989 dx.doi.org/10.1021/je400178k | J. Chem. Eng. Data 2013, 58, 1989-2000