(377d) Evaluation and Optimization of Vpsa Processes with Nanostructured Zeolite NaX for Post-Combustion CO2 Capture

Global warming resulted from greenhouse gases emission has been an issue of great concern. Concentration of greenhouse gases in the atmosphere has been continuously increasing over the last few decades due to the strong energy dependence of fossil fuels. Among the greenhouse gases, carbon dioxide is considered the main contributor of the global warming due to its huge emission amount. Thus, reducing anthropogenic CO₂ emission and controlling CO₂ concentration have become one of the most urgent global environmental issues. The carbon capture and storage technology (CCS) requires the reduction of carbon emissions primarily from large stationary point, such as coal-based plants. It is necessary to separate CO₂ from N₂ in post-combustion emissions before full utilization of CO₂.

Though CO₂ capture can be achieved by absorption with amine, the process is energy costly due to the high temperature requirement for regeneration. Adsorption stands out to be an attractive operation over amine capture due to its relatively low cost, flexible operating and more efficient regeneration. Vacuum pressure swing adsorption (VPSA) is an emerging technology that applies pressurized gas to the adsorption process as well as a vacuum during the desorption stage. VPSA is efficient when dealing with flue gas due to the low partial pressure of CO₂ and high partial pressure of N₂ in the flue gas stream.

The key challenge in CO₂ capture by adsorption technique is to find a suitable solid adsorbent material with good separation performance, as the adsorbent plays a critical role in the overall process performance. Many conventional adsorbents have been used for N₂/CO₂ separation, such as zeolites, activated carbons and modified mesoporous silica. However, the adsorption selectivity of CO₂ to N₂ and adsorption capacity of CO₂ are not high enough as current commercial adsorbents, which makes the adsorption process less competitive. An ideal adsorbent for CO₂ capture usually have a high CO₂ adsorption capacity, a large pore dimension that enables fast mass transfer, a decent CO₂ selectivity over N_2, and good stability.

In this work, we present a comprehensive experimental and simulation study on a low-cost and efficient CO₂ capture technique using nanostructured zeolite NaX in VPSA process. The nanostructured zeolite NaX adsorbent has a high CO₂ adsorption capacity, a relatively high adsorption and desorption rate, and a large CO₂ selectivity over N₂. Therefore, it is suitable for post combustion CO₂ capture from dry flue gas. Current work first presents a material study for a group of nanostructured zeolite NaX samples prepared with different binder ratios and sintering temperatures. The porosity properties, mechanical properties, adsorption isotherm, kinetics, IAST selectivity, and column dynamics for the group of nanostructured zeolite NaX samples are compared. The results demonstrated that nanostructured zeolite material prepared with a low ratio of binder that sintered at 500°C has the best separation performance for a CO₂/N₂ mixture. In addition, the results indicate that the nanostructured zeolite NaX samples lead to a better separation performance compared with the commercial microsized zeolite NaX. To apply the nanostructured zeolites for CO₂ capture by VPSA, the operating conditions of the VPSA process is desired to be preliminarily designed and optimized before running the process in an actual pilot plant, which is both costly and time-consuming.

It is clear that product purity, recovery, productivity and energy consumption are four important performance indicators of the VPSA process, and they varied as the operating conditions changed. In addition, the performance of the VPSA process depends on the CO₂ concentration of the flue gas. To obtain over 90% CO₂ purity with a recovery of 90% for CO₂ capture from a flue gas containing 15% of CO₂, a deep vacuum (<0.05 bar) is often required. However, the deep vacuum involves multistage pump units which will dramatically increase the capital cost and energy consumption, which becomes the major obstacle of applying the CO₂ capture process by adsorption to the power plants. There is a great need to employ an advanced adsorbent for the VPSA process for CO₂ capture, and to optimize the process to lower the energy consumption while meeting CO₂ purity and recovery target.

Simulation and optimization studies of a two-bed six-step VPSA process was then performed in gPROMs environment for post-combustion CO₂ capture from dry flue gas using the optimal nanostructured zeolite NaX and a commercial microsized zeolite NaX. The simulation model has been validated with both breakthrough experimental data and reported data from literature. The objective is to minimize the energy consumption of the VPSA process for the specified CO₂ purity and recovery target by altering the operating conditions. The decision variables include the feed pressure, blowdown pressure, evacuation pressure, feed flow rate, and length to diameter ratio of the adsorption bed. The effect of cycle time was investigated independently due to the need for synchronization of the multi-bed configuration. The optimization results indicate that the energy consumption of the process with nanostructured zeolite is about 30% lower while achieving a higher CO₂ purity and productivity compared with a process employing a commercial microsized zeolite. This is due to the more moderate conditions (e.g. higher blowdown pressure, higher evacuation pressure and lower feeding pressure) the process required to meet specified CO₂ purity and recovery target. In short, the optimization results have shown great potential for employing nanostructured zeolite NaX by VPSA process for CO₂ capture from the flue gas with low CO₂ concentrations, as a CO₂ purity of over 95% and a recovery of 90% was obtained with a practical vacuum level (>0.1 bar). Lower vacuum levels or multi-stage operations may be possible to achieve products with higher purity.