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Pressure Swing Adsorption Separation of H2S/CO2/ CH4 Gas Mixtures with Molecular Sieves 4A, 5A, and 13X

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Pressure Swing Adsorption Separation of H2S/CO2/ CH4 Gas Mixtures with Molecular Sieves 4A, 5A, and 13X Separation Science and Technology ISSN: 0149-6395 (Print) 1520-5754 (Online) Journal homepage: http://www.tandfonline.com/loi/lsst20 Pressure swing adsorption separation of H2S/CO2/ CH4 gas mixtures with molecular sieves 4A, 5A, and 13X Howell H. Heck, Merilyn L. Hall, Rudy dos Santos & Manolis M. Tomadakis To cite this article: Howell H. Heck, Merilyn L. Hall, Rudy dos Santos & Manolis M. Tomadakis (2018) Pressure swing adsorption separation of H2S/CO2/CH4 gas mixtures with molecular sieves 4A, 5A, and 13X, Separation Science and Technology, 53:10, 1490-1497, DOI: 10.1080/01496395.2017.1417315 To link to this article: https://doi.org/10.1080/01496395.2017.1417315 Published online: 26 Dec 2017. Submit your article to this journal Article views: 101 View Crossmark data Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=lsst20 SEPARATION SCIENCE AND TECHNOLOGY 2018, VOL. 53, NO. 10, 1490–1497 https://doi.org/10.1080/01496395.2017.1417315 Pressure swing adsorption separation of H2S/CO2/CH4 gas mixtures with molecular sieves 4A, 5A, and 13X Howell H. Hecka, Merilyn L. Hallb, Rudy dos Santosb, and Manolis M. Tomadakisb aDepartment of Civil Engineering, College of Engineering, Florida Institute of Technology, Melbourne, FL, USA; bDepartment of Chemical Engineering, College of Engineering, Florida Institute of Technology, Melbourne, FL, USA ABSTRACT ARTICLE HISTORY Pressure swing adsorption experiments were carried out for the separation of equimolar mixtures Received 18 October 2016 of carbon dioxide and methane containing small amounts of hydrogen sulfide, utilizing 4A, 5A, Accepted 11 December 2017 and 13X molecular sieves. High-purity methane of zero or nearly zero hydrogen sulfide concen- KEYWORDS tration was produced in the adsorption stage with 13X and 5A sieves, at high product recovery H2S; CO2;CH4; molecular rates; high-purity carbon dioxide was obtained with the same sieves in the desorption stage. sieves; pressure swing Zeolite 4A was found capable of raising considerably the hydrogen sulfide concentration in the adsorption accumulated desorption product (vs. the adsorption feed) at high recovery rates too. Adsorption selectivity values derived from the experimental results for all three gas pairs were in line with some theoretical predictions and experimental data of the literature. Introduction different gases including biogas, using a molecular mod- eling approach. The results indicated that the 13X and 5A The separation of ternary gas mixtures of H S, CO ,and 2 2 zeolites were the top candidates for H SandCO removal CH has been the subject of a number of experimental and 2 2 4 from H S/CO /CH biogas systems, based on selectivity, theoretical investigations, mostly using various types of 2 2 4 – whereas 4A was also ranked among the best four adsor- membranes as the separation agent.[1 4] Adsorption has bents for this separation. The model employed pressures also been studied experimentally and theoretically as a up to 4 MPa and a temperature of 303 K, with a biogas method for separating these gases, often from biogas or consisting of 70% CH ,29.8%CO, and 0.2% H S. natural gas, using activated carbons or carbon molecular 4 2 2 – The removal of H S from its ternary mixtures with sieves,[5 8] mesoporous silica,[9] multilayer graphene 2 CH and CO has also been studied by Cosoli et al.,[14] nanostructures,[10] or dolomite suspensions.[11] Asmall 4 2 who used grand canonical Monte Carlo (GCMC) simu- number of literature studies focused on pressure swing lations to model biogas adsorption on different zeolites, adsorption (PSA), some of them using 4A, 5A, and – namely, FAU NaX, FAU NaY, LTA, and MFI. The 13X[12] or similar zeolites,[13 15] and others utilizing differ- model used a biogas with partial pressures of H S ran- ent sieves such as Si-CHA,[16] naturally occurring zeolites,- 2 ging from 0.01 to 1 kPa at 1 atm and 298 K. The results [17,18] or a mixture of activated carbon and 13X zeolite.[19] indicated that zeolite FAU NaY exhibits the highest Other studies investigated the separation of H S, CH , and/ 2 4 selectivities for H S over CH and CO (ranging from or CO from natural gas by means of both activated carbon 2 4 2 2 300 to 1000 for H S/CH and 10 to 100 for H S/CO ), and membranes,[20,21] or gas hydrate crystallization.[22] 2 4 2 2 whereas MFI exhibits the lowest values of these pairwise Literature reviews of the employed membrane separation- selectivities (ranging from 0.10 to 0.20 for H S/CH and [23,24] and biogas purification[25] processes have also been 2 4 0.25 to 1.8 for H S/CO ). Cosoli et al.[15] coupled the reported. 2 2 GCMC with molecular dynamics simulations to further To our knowledge, only one journal article reported on investigate the adsorption of low-pressure H S and its the separation of ternary mixtures of H S, CO ,andCH 2 2 2 4 selective adsorption behavior toward CO and CH for using molecular sieves 4A, 5A, and 13X: Peng and Cao[12] 2 4 each of the previously mentioned four sieves. Zeolite carried out a theoretical investigation of 18 types of por- FAU NaY was once again found to be the best option ous materials, including carbons, zeolites, and metalor- for H2S removal from the simulated biogas mixtures. ganic frameworks, for the removal of H2SandCO2 from CONTACT Manolis M. Tomadakis [email protected] College of Engineering, Florida Institute of Technology, 150 W. University Blvd., Melbourne, 32901 Florida, United States. Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsst. © 2018 Howell H. Heck, Merilyn L. Hall, Rudy dos Santos, and Manolis M. Tomadakis SEPARATION SCIENCE AND TECHNOLOGY 1491 Gholampour and Yeganegi[26] created a constricted slit Table 1. Molecular sieve properties.[27]. pore model for nanoporous carbon (NPC) and applied it Sieve type in a molecular simulation study of the separation of 4A 5A 13X Characteristic pore size (Å) 4 5 10 binary and ternary mixtures of CH4,CO2, and H2S. Pellet size (in.) 1/16 1/16 1/8 Adsorption selectivities were also calculated for both Bed porosity (dim/less) 0.37 0.38 0.41 binary and ternary mixtures of the gases and compared Bulk density (lb/ft3) 41 43.4 44 Heat capacity (Btu/lb°F) 0.23 0.23 0.23 to literature values for the corresponding selectivities of zeolites 13X, ZIF-8, NaX, LTA, NaY, MFI, and Si-CHA. This study is the first experimental investigation of composition, and adsorption takes place. The column is the separation of ternary mixtures of H2S, CH4, and then blown down to atmospheric pressure, and the bed is CO2 by PSA using molecular sieves 4A, 5A, or 13X. purged with nitrogen for desorption to occur. Throughout Equimolar mixtures of CH4 and CO2 are utilized, con- the adsorption and desorption steps, temperatures and flow – taining 0.1 0.7% H2S, undergoing adsorption at pres- rates are recorded at regular intervals, and gas samples are sures of 50–90 psig. In addition to the measured drawnfromthesamplingportsontheinletandoutlet adsorption and desorption product compositions and streams with pressure-tight valved syringes and are ana- recovery values, pairwise adsorption selectivities are lyzed by the gas chromatograph. Figure 1 illustrates the four also derived and compared to theoretical predictions steps of the PSA process, highlighting the main products and experimental data of the literature. obtained in the adsorption and desorption stages. A total of nine PSA experiments were carried out: five with molecular sieve 13X, two with 4A, and two Experiments with 5A. Table 2 presents the type of molecular sieve, The PSA apparatus consists of an adsorption column, a column pressure, feed flow rate, and H2S feed concen- reciprocating compressor, and a Gow-Mac 600 gas tration employed in each adsorption run. As shown in chromatograph. The adsorption bed is 2 feet long and the table, the adsorption feed pressure was maintained 3 inches in internal diameter, made of stainless steel, at 70 psig for seven of the nine runs, while the other and fitted with thermocouples, pressure gages, rota- two experiments were performed at a lower (50 psig) or meters, and sampling ports on all inlet and outlet higher (90 psig) pressure, to investigate the effect of lines. The feed gases are 99.5% chemically pure H2S, pressure on process performance. Similarly, the H2S 99.8% industrial grade CO2, and 99% commercial grade feed concentration was assigned values of 0.1%, 0.5%, CH4; ultrahigh purity 99.998% nitrogen is used as the and 0.7% in various runs, so that the effect of this purge gas. The solid adsorbents are molecular sieves 4A parameter on process performance is also examined. by UOP and 5A and 13X by W. R. Grace; Table 1 presents some characteristic sieve properties. Results and discussion First, the column is pressurized to the desired adsorption pressure, using methane. Next, the mixture of H2S, CO2, Figure 2 presents the CO2 breakthrough curves for experi- and CH4 is supplied to the bed at the predetermined ments A1, A3, A5, and A8. Three of the four curves are Figure 1. PSA process steps highlighting main adsorption and desorption products in bold. 1492 H. H. HECK ET AL. Table 2. Adsorption process parameters. breakthrough times of all gases (tH2S, tCO2, tCH4) for Adsorption feed each run; as indicated in the table, breakthrough times Pressure Flow rate H2S content were in some cases too small to be detected in time Sieve Run# (psig) (gmol/min) (mol %) with the manual syringe sampling system.
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