ARTICLE pubs.acs.org/IECR
Heat- and Mass-Transfer Kinetics of Carbon Dioxide Capture Using Sorbent-Loaded Hollow Fibers Matthew D. Determan, Dhruv C. Hoysall, and Srinivas Garimella* Sustainable Thermal Systems Laboratory, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ABSTRACT: Sorbent-loaded hollow fibers operating in a rapid temperature-swing adsorption cycle are a unique platform for the capture of CO2 from power plants. They are ideally suited for heat recovery strategies that will reduce the operating costs of capture facilities. Accurate estimates of the fiber-level heat- and mass-transfer kinetics are critical for the design and implementation of these systems. A detailed coupled heat- and mass-transfer model of the adsorption process in sorbent-loaded fibers is developed here. The effects of varying fiber geometry on the heat- and mass-transfer kinetics are presented. The rapid diffusion and adsorption in the fiber and the direct cooling of the fibers will enable efficient capture of CO2, as well as substantial recovery of the sensible heat capacity of the beds, thus reducing energy costs of the thermal-swing adsorption process.
1. INTRODUCTION Growing concerns about the long-term implications of increased CO2 levels in the atmosphere for the global climate have led to significant interest in reducing anthropogenic emissions of CO2. The major sources of such emissions are power generation facilities, with emissions from coal burning plants accounting for approximately 40% of worldwide CO2 emissions.1 Although the increased use of nonemitting renewable sources such as solar and wind will offset some emissions, the use of fossil fuels will continue to be the major source of primary energy for electricity production for the near future. Production of low- or zero-CO2-emission electrical power from these fossil fuels can be aided by some form of carbon capture system. Precombustion capture systems such as thee integrated gasification combined cycle (IGCC) will require purpose-built power generation facilities and cannot address the emissions of the already installed fleet of power plants. Postcombustion capture systems, however, will be able to be retrofitted to existing plants, thus providing a pathway to transition existing facilities into a low- or zero-emission future. Postcombustion capture requires that relatively low-concentration CO2 be removed from large volumes of flue gas. Methods under development or pilot-plant operation include aqueous monoethanolamine (MEA) absorption, gas separation membranes, pressure-/vacuum-swing adsorption, temperature-swing adsorption, and chilled ammonia stripping. All of these methods require that additional primary energy be consumed to satisfy the energy demands of the capture and compression facilities. In MEA systems, CO2 from the flue gas is absorbed into an aqueous solution of monoethanolamine. The solution is then pumped to a stripping column, and the CO2 is removed by heating the solution. The optimal operating conditions for minimal energy consumption require concentrations of MEA that can lead to severe corrosion effects and increased rates of solvent degradation.2 Furthermore, carryover of MEA with the cleaned flue gas increases with higher concentrations, requiring extra gas-scrubbing equipment, as well as makeup solvent. r 2011 American Chemical Society
Although membrane separation systems for CO2 capture from flue gas require some degree of compression of the entire flue gas stream to provide sufficient driving force for the separation, work continues on developing membranes with CO2/N2 selectivity high enough to achieve economical separation.3 The main energy input for these systems is high-grade electrical power, unlike the main input for absorption or adsorption systems, which is typically low-grade (low-temperature) thermal energy. The chilled ammonia process is similar to MEA processes. Flue gas is contacted with chilled aqueous ammonia (210 °C), and the CO2 is absorbed into the ammonia solution. Depending on the loading of CO2 in the solution, the operating temperature, and the pressure of the system, solid phases of ammonium carbonate and bicarbonate can exist in the absorber. The CO2 is then removed from the solution in a stripping column.4 The claimed advantage of the chilled ammonia process over the MEA process is a much lower heat of reaction.5 A gas-cleaning system must still be used to limit emissions of ammonia into the ambient, and there are significant energy costs associated with maintaining the absorber at subambient temperatures and chilling the flue gas stream. The refrigeration-plant compressor energy input is offset somewhat by the higher-pressure CO2 product obtained, and thus lower CO2 compressor energy input, but it is still a net power draw.6 In adsorption processes, CO2 is removed from the flue gas stream when it adsorbs onto a solid sorbent. The sorbent is then regenerated in a pressure-swing adsorption (PSA) or vacuumswing adsorption (VSA)710 process by lowering the pressure or in a thermal-swing adsorption (TSA)11 process by increasing the temperature of the sorbent. As the CO2 desorbs from the sorbent, it is concentrated and captured. The PSA cycle requires compressors to increase the feed pressure of the system during Received: September 8, 2011 Accepted: November 3, 2011 Revised: November 1, 2011 Published: November 03, 2011 495
dx.doi.org/10.1021/ie201380r | Ind. Eng. Chem. Res. 2012, 51, 495–502
Industrial & Engineering Chemistry Research
ARTICLE
thermal recovery techniques.15 An illustration of the proposed heat recovery technique is shown in Figure 2. As the flue gas enters module 1, the cooling fluid enters the fiber bed in a coflow orientation with respect to the flue gas. As the cooling fluid removes the heat of adsorption in the mass-transfer zone of the module, it also removes the sensible heat in the module, thus combining the adsorption and cooling steps. This creates a “thermal wave” in the coupling fluid leaving module 1. This warm coupling fluid is then reheated with a trim heater to the desorbing temperature, Tmax, and flows into module 2, where it supplies the energy to heat the fibers and desorb CO2 from that module. Purge gas might be introduced at some time during the desorption step to efficiently remove and capture the CO2 from module 2. Once the temperature of the coupling fluid leaving module 1 drops below a threshold, the process and the coupling fluid flow directions are reversed, and the thermal wave leaving module 2 is used to heat module 1. In this way, a substantial portion of the sensible heat from one module is used to heat the other module. A trim cooler is used to return the coupling fluid to the heat rejection temperature of Tmin before it is reused as coolant. While only a two-module system is shown here for simplicity, this method could be extended to multimodule systems with the use of thermal reservoirs, thus eliminating the need for a one-to-one correspondence of coupling fluid flow rates in the modules. The requirement for successful implementation of this heat recovery technique is a steep temperature gradient in the coupling fluid along the axial length of the module. To achieve this steep axial temperature gradient, the fibers must have extremely fast radial transport of both mass and thermal energy. Although similar techniques for heat recovery have been used in adsorption heat pumps,1621 those systems have not had to account for multicomponent mixtures and mass-transfer resistances in the gas phase and the associated dispersion of the masstransfer zone. Therefore, a critical element for successful use of this carbon capture platform is the design and modeling of the hollow fibers and the modules in which they will be assembled to achieve high sensible heat recovery during adsorption of CO2 from flue gas while also producing high-purity CO2 during the desorption stage at high capture efficiencies. In this work, the heat- and mass-transfer kinetics of a sorbent-loaded hollow fiber with an internal barrier layer used in rapid temperature-swing adsorption were investigated. The influences of fiber geometry and internal and external heat- and mass-transfer resistances were also analyzed.
Figure 1. Schematic of a sorbent-loaded hollow fiber.
the adsorption step; regeneration of the sorbent happens at nearambient pressures. The VSA cycle requires a vacuum pump to lower the pressure of the beds to regenerate the sorbent. Both the PSA and VSA cycles use high-grade electricity to affect the required pressure differentials. The TSA cycle uses low-grade thermal energy to drive the process. Typically, in a TSA application, a heated inert gas would be passed through the adsorbent bed to desorb the CO2, thus diluting it in the heating gas. In a system where the adsorbed component is a trace contaminant in the product stream, this is not an issue, but in CO2 capture processes where the required purity of CO2 is quite high, TSA systems have not been seen as suitable capture platforms. Some researchers12,13 have proposed using direct electrical heating of the bed to avoid dilution in the heating gas, but this process would require the use of high-grade electricity. Merel et al.11 evaluated an indirect heating method for use in a TSA system, avoiding the regeneration of the system by a hot gas purge. They reported high product purities (98%) and capture efficiency (83%), with a volumetric productivity of 30 kgCO2/m3 3 h. Recently, Lively et al.14 demonstrated the ability to produce hollow fibers with very high mass fractions of embedded sorbent material and proposed their use for the capture of CO2 from the flue gas of a large coal-fired power plant in a rapid temperatureswing adsorption (RTSA) system. The adsorbent particles are small (∼3-μm) crystals of zeolite that are suspended in an opencell polymer matrix. These fibers have high void fractions, which allow the CO2 to diffuse quickly into the fiber to reach the sorbent material. As shown in Figure 1, the inner bore of the fibers can be sealed to produce a barrier layer between the active sorbent material and a coupling fluid that can be used to either heat or cool the sorbent material. This local thermal access to the sorbent material creates the possibility for extremely fast cycling between fully adsorbed and desorbed states. Furthermore, the low heat- and mass-transfer resistance in the radial direction would allow the establishment of steep temperature and concentration fronts in a module made of such fibers. Any postcombustion CO2 capture platform is extremely energy-intensive because of the low partial pressure of the CO2 that is to be separated from the flue gas. The parasitic load of the capture facility on the power plant must be minimized to make any such system feasible for widespread deployment beyond the pilot-plant scale. In a capture platform working on an RTSA cycle with hollow-fiber sorbents, the driving energy can be low-grade thermal input (
