Precipitation in Carbon Dioxide Capture Processes
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Climate change as a result of the greenhouse effect has become a global issue from both technical and political perspectives. Carbon dioxide (CO2), produced from fossil-fuelled power plants, gas sweetening processes and transportation, is a major contributor to the greenhouse effect. Absorption processes are considered as one of the most feasible CO2 capture technologies, but the operating costs related to energy consumption in these processes are an obstacle to industrial application widely. Researchers have been exploring new technologies and solvents to overcome the problem of high energy consumption. The use of precipitating systems is one of the options and this thesis is motivated by a need to understand the different aspects involved when introducing a crystallization process. In literature the concept is mainly analyzed from a thermodynamic and process design point of view. However the feasibility of any crystallization process depends on the development of a supersaturation with an analysis of the accompanying kinetics of nucleation and growth. The coupling of the absorption kinetics with solid phase formation kinetics will determine the precipitation sequence of different solid products, the crystal size distribution, polymorphism, and particle shape. The resulting yield and quality of the crystalline particle population will affect solid handling strategies relevant for absorber design, crystallizer design, fouling of heat exchangers, solid-liquid separation, dissolution rates and energy consumption. Three different precipitating systems were chosen for the study, varying with respect to the location of crystallization in the absorber/desorber swing operation. Potassium carbonate and amino acid salts were chosen as examples of systems that will precipitate as a result of absorption since the solubility of the absorption products are lower than the reactants. Piperazine (Pz) was chosen as an example of the opposite, in which case solid formation by crystallization might occur as a consequence of desorbing CO2. The crystallized solid phases from different concentrations of aqueous piperazine solutions were determined by powder-XRD. The products of solutions with lower concentrations than 44.3 wt% were found to be Pz6H2O. Crystallization experiments performed between the hexahydrate composition (44.3 wt%) and 65 wt% resulted in a mixture of piperazine hexahydrate and hemihydrate, while primary products from solutions above 75wt% were anhydrous piperazine, consistent with the phase diagram. A eutectic point was found at a temperature of 32.9°C for an initial concentration of 60 wt% piperazine. Due to the regeneration of piperazine when CO2 is stripped from the system, piperazine might crystallize when the lean solution is cooled prior to a new absorption cycle. It was found that the metastable zone widths (MZW) of the piperazine-H2O system were substantial even at lower cooling rates. However, the eutectic composition exhibits a smaller MZW than the other concentrations, which is believed to be caused by metastable precursor crystals. The crystallization at different piperazine concentrations (and different loadings) may in some cases lead to complete solidification. This is the case for the hexahydrate composition at a relatively high temperature of 43.0 ºC and for higher concentrations, at the eutectic temperature of 32.9 °C. This can be of high importance for safe operation of CO2-capture plants in case of unintended shut-downs where the process equipment is cooled below these respective temperatures. The effects of precipitation in three amino acid salt solutions, 5m’ (mol/kg solution) potassium sarcosine (KSar), 5m’ potassium L-Alanine (KL-Ala) and 5m’ sodium L-Alanine (NaL-Ala), were studied to evaluate the CO2 absorption efficiency. It was found that the 5m’ KSar solution exhibited the highest initial CO2 capture rate. Although the occurrence of precipitation lead to an upward shift on the absorption rate and CO2 capacity, it would take too long to obtain these crystals and the absorption rate is then too low. Thus, the KSar solution was not considered as a good candidate for a precipitating CO2 capture system unless an additional driving force is applied to generate supersaturation downstream the absorber. The NaL-Ala solution exhibited a lower initial CO2 capture efficiency and together with no effect of precipitation on the absorption rate this system was inferior to the others. The KLAla solution has a substantial CO2 capture rate and precipitated twice during absorption, first the amino acid and then potassium bicarbonate. The first precipitation provides a positive effect on its performance in terms of absorption rate, making the KL-Ala system a promising solvent for a CO2 capture process with precipitation. For amino acids that have high solubility, a crystallizer after the absorber should be considered in order to force out the solid product in a more efficient manner. The growth kinetics of potassium bicarbonate was studied in seeded batch experiments, at conditions relevant for post-combustion CO2 capture. Potassium bicarbonate seeds with favorable shape and size distribution were produced and the crystal growth rate was determined. A growth order of two indicated that the crystal growth is controlled by surface integration. The corresponding growth rate constants were 497±36, 590±66a and 767±51 μm/s at 10.00 °C, 20.00 °C and 30.00 °C, respectively. The high crystal growth rate of KHCO3 at all the investigated temperatures will assure fast solute concentration change of the solution, thereby achieving the benefits from precipitation fast. Additionally, the fast growth rate will promote secondary nucleation, which is more robust than primary nucleation for production of particles with more favorable characteristics. This makes the system a good candidate for cooling crystallization downstream the absorber. With the growth model it is possible to evaluate the effect of the different proposed promoters and catalysts that are necessary in order to increase the absorption rate in this solvent, since the promoters may affect the crystallization kinetics of the system and affect the particle size and shape of the crystals. A general discussion of the benefits and possible risks of CO2 capture systems with precipitation were discussed based on the three precipitating solvent systems. The possible risks associated with the presence of solids and the selection of equipment used for solid liquid separation and mass and heat transfer were described. The effect of precipitation on CO2 capture performances was discussed in relation to net CO2 capacity, absorption rate and energy consumption. The change of the absorption rate as a result of precipitation is attributed to its combined effects on the Henry’s law constant, the chemical reaction kinetics and the physical liquid-side mass transfer coefficient. Regarding the CO2 capacity, in some systems, such as potassium sarcosine, the occurrence of precipitation could lower the steepness of the gas-liquid equilibrium curve, thereby leading to a higher CO2 loading capacity. Moreover, the allowance of precipitation could lead to reduced energy penalty by decreasing the stripping heat and lowering circulating and pumping load in some systems. Regarding solvent regeneration, however, the formed solids require additional energy for dissolution. Therefore, the energy penalty required to dissolve solids have to be taken into account, and the heat of solution becomes an important parameter for the selection of solvent candidates. The results from the analysis of the particle size for both potassium bicarbonate and the amino acids illustrated that solid-liquid separation is hardly going to be the most difficult aspect of running these processes. The ratio of absorption rate and solid solubility and the accompanying supersaturation generation rate will not reach levels that favor excessive primary nucleation and hence a problematic fines fraction. However, should a smaller particle size be required to improve dissolution efficiency, then the ratio of the absorption rate to the solid solubility might be increased by either adding promoters or introducing an additional driving force to lower the solubility and thus increase the supersaturation. Precipitating CO2 capture is a promising concept but in order to advance it, it is essential to focus on the crystallization kinetics and how it is influenced by the absorption kinetics. Further work should focus on proper absorption rate measurements during precipitation and how promoters in combination with low cost additional driving forces for efficient precipitation can be incorporated.