Performance of Pelletized Biosorbents for Gas Dehydration

Abstract

Moisture removal from vapors/gases, such as natural gas, ethanol, and butanol, is critical in energy, chemical, and other industries. Crude natural gas, as an example, contains moisture of different levels and lowering its humidity is essential to ensure compliance with standards. Otherwise, the existing water vapor causes numerous processing issues, such as low combustion efficiency, corrosion in pipes and equipment, forming hydrates, and blocking pipelines. Sorption, including absorption and adsorption, is one of the most feasible and effective techniques to selectively remove water vapor from gaseous mixtures and achieve fuel-grade products. Although various sorbents/adsorbents have been employed in sorption, cost-effective agriculture by-products, such as oat hull (OH), canola meal (CM), and their composites, are efficient and effective materials for developing biosorbents. These agricultural biomaterials mainly consist of cellulose, hemicellulose, lignin, and protein. The components contain polar functional groups such as carbonyl, hydroxyl, and amide groups that enhance the surface hydrophilicity of biosorbents and perform the dehydration process. However, most of the current research has used biosorbent particles with random size and shape, for industrial applications, it is crucial to pelletize the biosorbents to control their size, shape, water uptake, and related qualities. In addition, the characteristics of the pellets need to be investigated. Hence, the pelletization process was implemented to enhance biomaterial quality and characteristics of the pellets in moisture was investigated in this work. The first phase of this work investigated the impacts of biomass composition, as well as die temperature and compressive force during the pelletization on biosorbent pellet quality. In this regard, raw oat hull and canola meal and their 50%-50% composition were pelletized under three levels of compressive force (2000, 3000, and 4000 N) at 75, 85, and 95 °C die temperature. Results showed that increasing die temperature and compressive force enhanced the pellet strength and density. Canola meal was superior in water uptake, while oat hull was important in strengthening pellets because of their material composition and structure. Moreover, sodium lignosulfonate (LSNa) as an additional binder improved pellet quality. By statistical optimization of the Response Surface Methodology (RSM), four pelletized biosorbents (18.6% OH-81.4% CM-0% LSNa; 100% CM-4% LSNa; 54.5% OH- 45.5% CM-4% LSNa; 91.9% OH-8.1% CM-4% LSNa) were developed, which were promising for industrial usage. The optimized biosorbents possessed relatively high density (up to 1214 kg/m3) and tensile strength (up to 1.30 MPa), and their water uptake capacity was in the range of 0.32 – 0.38 g water/g dry biosorbent, which is comparable to or higher than those of commercial adsorbents. Therefore, the introduced biosorbents could be effective alternatives to the conventional materials for gas dehydration. The second phase studied sorption kinetics, mechanisms, and structural analysis of the biosorbents. In the case of 100% OH pellet, sorption on active sites was the predominant mechanism initially, followed by intraparticle diffusion until equilibrium. However, sorption on the 100% CM pellets was delayed at the initial stage of the process, owing to the external mass transfer resistance; then, intraparticle diffusion controlled the process until the end. Adding LSNa strengthened the pellet structure and enhanced water uptake capacity though it slightly increased the mass transfer resistance and reduced the sorption rate at the beginning of the moisture uptake process. The third phase focused on sorption equilibrium and thermodynamics, as well as sorption-desorption and reusability of biosorbent pellets. The water uptake process was exothermic and physisorption, mainly due to the hydrogen bonds formed between water molecules and polar functional groups on the surface. Furthermore, the sorption process was multilayer at 25 °C and 101.33 kPa. However, primarily monolayer sorption was achieved at higher temperatures (35 and 45 °C) under atmospheric pressure. Regarding the reusability of biosorbents, the pellets were stable over 50 sorption-desorption cycles inside the adsorption column; hence, the biosorbents demonstrated promising potential for industrial applications.