(222e) Dynamic Separation of Ethylene/Ethane By Ion-Exchanged ETS-4 Materials

Ethylene is the cornerstone feedstock in the petrochemical industry, particularly in the production of polyethylene polymers. The global production of ethylene reached 218 Mt in 2022, twice the amount in 2000, signifying its substantial increasing demand. The major concern in the production of polymer-grade ethylene (>99.9% purity) is the separation of ethylene from ethane since the conventional distillation process for separating ethylene/ethane mixture is energy-intensive and expensive. In this regard, finding an efficient alternative method to attain polymer-grade ethylene is imperative to fulfill the sustainable development intention [1].

The size-selective kinetic approach offers a promising avenue for the separation of light hydrocarbons. To synthesize a highly selective adsorbent, fine-tuning the pore sizes of the appropriate adsorbent is crucial [2]. Microporous Engelhard Titanosilicate-4 (ETS-4) is a small pore member of the titanosilicate family. ETS-4 has the pore size of 0.3–0.5 nm, which is in the range of the kinetic diameters of ethylene (4.163 Ã…) and ethane (4.443 Ã…), providing the selective separation of these molecules based on their size difference [3]. ETS-4 is composed of octahedral TiO₆ and tetrahedral SiO₄ units [4] resulting in the negative framework charge of −2. To counterbalance the negative framework charge, exchangeable cations such as Na⁺ or K⁺ are used [5]. The type of cation affects both the adsorption properties and the thermal stability of ETS-4. Studies indicate that ion-exchanging of Na⁺ with divalent Sr²⁺ and Ba²⁺ improves thermal stability and influences the adsorptive properties of ETS-4 adsorbent [4, 6]. The pore size of ETS-4 can be adjusted via two approaches of ion-exchange and thermal treatment [3]. Grande et al. [7] conducted a comprehensive investigation into the impact of activation temperature on the ethylene/ethane kinetic selectivity of Ca-ETS-4. Their findings revealed that the activation temperature not only alters the pore size but also influences the adsorption capacity of the adsorbent. In a study conducted by Anson et al., [3] ETS-4 titanosilicate molecular sieves were synthesized and ion-exchanged with various cations. Ethylene/ethane selectivity isotherms measurements demonstrated significant selectivity of these materials with Ca/H-ETS-4, Sr-ETS-4, and Ba-ETS-4 exhibiting a selectivity of 10.3, 11.5, and 7.92, respectively.

Despite the promising performance of ETS-4 materials in separating ethylene from ethane, there is still a considerable gap in thoroughly evaluating their effectiveness for this application. Therefore, we investigated the separation performance of different ion-exchanged ETS-4 materials for ethylene/ethane. Single-component adsorption isotherms of ethylene and ethane for different ion-exchanged ETS-4s were assessed using a volumetric device. The best performing samples were scrutinized in detail using dynamic column breakthrough (DCB) experiment. Breakthrough experiments were performed at various experimental conditions such as separation temperature (30-90 °C), feed concentration (10/90, 50/50, 90/10 %), and ETS-4 activation temperature (250-400 °C) to estimate the ETS-4 performance for process development. The results show that depending on the cation, the ethylene/ethane separation performance of ETS-4 varies significantly. An increase in temperature from 30 °C to 70 °C improved the ethylene/ethane separation. This could be due to the increased mobility of gas molecules at higher temperatures, that overcome the kinetic hinderance.

Reference

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