Precipitation of Piperazine in Aqueous Piperazine Solutions with and without CO 2 Loadings Xiaoguang Ma, † Inna Kim, ‡ Ralf Beck, † Hanna Knuutila, ‡ and Jens-Petter Andreassen* ,† † Department of Chemical Engineering, NTNU, N-7491 Trondheim, Norway ‡ SINTEF Materials and Chemistry, N-7465 Trondheim, Norway ABSTRACT: The crystallization of piperazine in water as well as in systems loaded with CO 2 has been studied for piperazine concentrations of 30-70 wt %, representing conditions relevant for CO 2 capture. The use of a LabMax reactor system equipped with probes for in situ focused beam reflectance measurement (FBRM) and particle vision measurement (PVM) made it possible to determine solid-liquid transitions, crystal habit, and chord length distributions in these highly concentrated systems without disturbing the solid-liquid-gas equilibrium during crystallization. As shown by powder X-ray diffraction analysis, three phases including piperazine hemihydrate, piperazine hexahydrate, and anhydrous crystals were precipitated from the aqueous piperazine solutions at different concentrations and temperatures, as also supported by findings from FBRM and PVM. It was found that the metastable zone widths of the piperazine-H 2 O system were substantial even at the lower cooling rates, which could allow for a higher tolerance with respect to cooling prior to a new carbon dioxide absorption cycle. However, the eutectic composition exhibits a smaller metastable zone width than the other concentrations, which is believed to be caused by the precursor needle- shaped crystals, assisting the precipitation of the final product. 1. INTRODUCTION Amine-based absorption/stripping systems utilized for removal of CO 2 from coal-fired power plants have been widely studied. 1 Except for liquid based processes, such as established monoethanolamine (MEA) absorbents, precipitating systems used for CO 2 capture have attracted increasing attention in recent years. In such postcombustion CO 2 -capture processes, absorption and stripping are performed in columns by performing a temperature swing. Carbon dioxide is captured in the absorber at a temperature of 30-50 °C (MEA). The “rich” solvent is then pumped to a stripper, where CO 2 is desorbed at the temperature of around 120 °C, thereby regenerating the solvent. The “lean” solvent is sent back to the absorber for a new cycle of CO 2 capture. In systems with the potential for precipitation, like K 2 CO 3 /KHCO 3 and (NH 4 ) 2 CO 3 /NH 4 HCO 3 (the chilled ammonia process), the selective removal of reaction precipitates (bicarbonates) from the reaction mixture will essentially shift the equilibrium toward the product side, thereby increasing the CO 2 absorption capacity. An additional benefit of such processes is offered by the fact that only concentrated slurry needs to be sent to the stripper, thereby reducing both the recycling load and sensible heat requirements. Another promising candidate for CO 2 capture is the cyclic ethyleneamine piperazine (Pz) which comprises two secondary amine groups resulting in high reactivity with CO 2 . It has been shown to be an efficient promoter to enhance the CO 2 mass transfer rate in systems such as MDEA/Pz 2 and K 2 CO 3 /Pz. 3,4 Due to the relatively low solubility of Pz, the concentration is normally between 0.5 and 2.5 m to avoid precipitation of piperazine-based compounds. 3 In the study of Freeman et al., 5 however, concentrated aqueous piperazine (more than 8 m) was shown to be a promising solvent for CO 2 capture by itself. In the piperazine-H 2 O-CO 2 system, the following reactions may take place: 3 ↔ CO (g) CO (aq) 2 2 (1) + ↔ + − + CO (aq) 2H O HCO HO 2 2 3 3 (2) + ↔ + − − + HCO HO CO HO 3 2 3 2 3 (3) ↔ + + − 2HO HO OH 2 3 (4) + ↔ + + + PZH HO PZ HO 2 3 (5) + + ↔ + − + PZ CO HO PZCOO HO 2 2 3 (6) + ↔ + + − + − HO H PZCOO HO PZCOO 2 3 (7) + + ↔ + − − − + PZCOO CO HO OOCPZCOO HO 2 2 3 (8) Equations 5 to 8 show how piperazine reacts into hydrogenated piperazine and carbamates during the CO 2 absorption process. Since the resulting carbamates exhibit higher solubility than piperazine itself, this system behaves differently from the carbonate systems. Precipitation will not happen as a result of CO 2 absorption, but rather in the lean solvent, if the concentration of piperazine is sufficiently high. During the desorption of carbon dioxide, the concentration of piperazine will increase as CO 2 is stripped off, and crystallization of piperazine will eventually occur if the concentration of piperazine is sufficiently high and also at Received: April 27, 2012 Revised: June 21, 2012 Accepted: August 27, 2012 Published: August 28, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 12126 dx.doi.org/10.1021/ie301101q | Ind. Eng. Chem. Res. 2012, 51, 12126-12134