Evaluation and optimization of VPSA processes with nanostructured zeolite NaX for post-combustion CO₂ capture

Highlights

• Nanostructured zeolite saves 30% energy over the microsized zeolite for CO₂ capture.

• Optimization of a 2-bed 6-step VPSA process for performance and energy consumption.

• Enhanced mass transfer rate, large capacity for CO₂ and high CO₂/N₂ IAST selectivity.

• Binder ratio and sintering temperature effect on nanostructured zeolite NaX pellets.

Abstract

We present a comprehensive experimental and simulation study on a low-cost and efficient CO₂ capture technique using nanostructured zeolite NaX in a vacuum pressure swing adsorption (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. A series of nanostructured zeolite NaX pellets are prepared with different ratios of binder material and at different sintering temperatures. The results from mechanical properties, adsorption isotherms, kinetics and breakthrough experiments demonstrate 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. Process optimization studies employing the above adsorbents were performed to minimize the energy consumption of the process for a specified product purity and recovery rate. 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.

Introduction

Global warming resulted from greenhouse gas 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 [1].

Traditionally, CO₂ capture can be achieved by absorption with amine commonly used in large-scale carbon removal from the flue gas. The process is efficient but 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 [2]. Two generic cycle concepts known as temperature swing adsorption (TSA) and pressure swing adsorption (PSA) are employed to carry out the essential adsorption and desorption steps in conjunction with a variety of other complimentary steps [3]. PSA is an industrial unit operation for separating gas mixtures where the preferred gas component is adsorbed at a high pressure and then desorbed at an atmospheric pressure repeatedly. PSA has been widely applied for carbon capture due to its simplicity, high efficiency and low energy requirement [4], [5]. 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 more efficient than PSA processes when dealing with flue gas due to the low partial pressure of CO₂ and high partial pressure of N₂ in the flue gas stream [6].

Many conventional adsorbents have been used for N₂/CO₂ separation, such as zeolites, activated carbons and modified mesoporous silica [7], [8], [9]. 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 with other CO₂ capture methods like absorption and membrane separation. 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 [10]. An ideal adsorbent for CO₂ capture should 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 [11]. New research has found that nanostructured zeolite NaX is particularly promising as CO₂ capture materials. For example, Pham et al. [12] found the decrease in the particle size of zeolite crystals from microscale to nanoscale leads to a significant increase in specific surface area, thus providing more active sites for adsorption of CO₂. Jiang et al. [13] have reported that the synthesized T-type zeolite showed higher adsorption capacities than micro-level T-type zeolite for the separation of CO₂/N₂. Eskandari et al. [14] pointed that reduction of the particle size resulted in increasing the adsorption capacity for carbon dioxide as well as the adsorption selectivity on the X type zeolite particles.

Although progress has been made on the development of nanostructured zeolite as CO₂ capture adsorbent material, most work only reported the basic properties of the material such like adsorption isotherms, porosity, density and surface areas. More work is required to analyze the novel adsorbent through lab scale breakthrough experiments as well as process modeling to determine the feasibility of the material for practical applications. In addition, 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.

A large number of simulation and experimental studies has been reported for post-combustion CO₂ capture from flue gas mixture by employing the VPSA technique [15], [16], [17], [18], [19], [20], [2], [21]. 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 [22]. 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.

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. 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 objective is to minimize the energy consumption of the VPSA process for the specified CO₂ purity and recovery target by altering the operating conditions.