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Numerical investigation of the outward growth of ZnS in the removal of H2S in a packed bed of ZnO
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.ORCID iD: 0000-0002-1881-2686
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.ORCID iD: 0000-0001-7995-3151
2017 (English)In: Chemical Engineering Science, ISSN 0009-2509, E-ISSN 1873-4405, Vol. 158, p. 328-339Article in journal (Refereed) Published
Abstract [en]

Sulfidation of zinc oxide is a viable option for the removal of hydrogen sulfide from raw syngas and biogas. Recent experiments showed that sulfidation of nanoscopic zinc oxide particles leads to an outward growth and the formation of voids inside the particles. In this work, we derive a micro-scale model to describe this phenomenon. The model accounts for nucleation and growth of voids inside the particles, diffusion of Zn and O through the product layer, and deposition of the solid product at the particle surface as a result of the reaction between ZnO and H2S. The model is thus opposite to the well known shrinking core model where an inward growth of the product layer is assumed. To explore the effect of the outward growth on the dynamics of a packed bed adsorber the micro-scale model is combined with a macro-scale model that accounts for intra-pellet diffusion and convection along the packed bed. In the limit of fast nucleation and growth of voids inside the zinc oxide particles, the micro-scale model shows a qualitatively similar conversion profile to the shrinking core model, while when nucleation controls an inflection point in conversion profile is found. On the macro-scale, the outward growth can cause the clogging of pores inside the pellets which prevents the pellets from reaching full conversion. This leads to shorter breakthrough times of the packed bed due to the sealing of unreacted zinc oxide. Our results thus provide a possible explanation of the incomplete conversion of zinc oxide in packed beds.

Place, publisher, year, edition, pages
Elsevier, 2017. Vol. 158, p. 328-339
Keywords [en]
Gas-solid reaction, Syngas cleaning, H2S removal, ZnO sulfidation, Outward growth, Hollow particles
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-199459DOI: 10.1016/j.ces.2016.10.038ISI: 000389068900033Scopus ID: 2-s2.0-84993996109OAI: oai:DiVA.org:kth-199459DiVA, id: diva2:1067942
Note

QC 20170123

Available from: 2017-01-23 Created: 2017-01-09 Last updated: 2019-10-29Bibliographically approved
In thesis
1. Biomass to Biofuel: Syngas Cleaning and Biomass Feedstock
Open this publication in new window or tab >>Biomass to Biofuel: Syngas Cleaning and Biomass Feedstock
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis builds around the idea of a biofuel production process that is comprised of biomass production, biomass gasification, gas cleaning, and fuel production. In this work, we specifically looked into H2S removal as a part of cleaning the producer gas and flocculation of microalgae which is involved in the harvesting of microalgae after biomass production. One of the impurities to remove from the producer gas is hydrogen sulfide which can be removed by using a packed bed of zinc oxide. Despite the regular use, it was only recently shown that during reaction with H2S, nano-size particles of ZnO exhibit void formation and outward growth. In this work, a micro-scale model was introduced to describe the void formation and outward growth. On the macro-scale, the simulations captured pore clogging of pellets due to the outward growth. The pore clogging prevents the full conversion of pellets and consequently leads to shorter breakthrough times of beds. The second problem investigated here deals with the flocculation of microalgae. Microalgae is produced in relatively low concentrations in the incubator liquid medium and during the harvesting, the concentration is increased to an acceptable level. The harvesting process includes a flocculation followed by a filtration or centrifuge unit. During flocculation, microalgae are stimulated to aggregate and form clusters. The experiments showed that the mean size of clusters formed during flocculation increases with time to a maximum and then starts decreasing, resulting in an overshoot in the mean size profile. The size of clusters influence the efficiency of the afterward filtration or centrifuge, thus it is of interest to carefully track the size evolution of clusters, making the studying of overshoot a crucial research topic. In this work, the possible mechanisms behind this overshoot were investigated.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2017. p. 63
Keywords
Gas-Solid Reaction; ZnO Sul_dation; Outward Growth; Hollow Particles; Algae Flocculation; PBEs; Fractal Clusters; Restructuring of Clusters; Deposition of Clusters; Primary Particle Aggregation.
National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:kth:diva-204570 (URN)
Presentation
2017-03-31, LOGOS, Teknikringen 42, plan 6, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20170330

Available from: 2017-03-30 Created: 2017-03-28 Last updated: 2017-03-30Bibliographically approved
2. Modeling in Biomass Harvesting, Biomass Pyrolysis and Producer Gas Cleaning
Open this publication in new window or tab >>Modeling in Biomass Harvesting, Biomass Pyrolysis and Producer Gas Cleaning
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biomass is a viable alternative in order to mitigate the environmental effectscaused by the aggressive use of fossil feedstock during the last century.This thesis builds around the idea of a biofuel production process that iscomprised of biomass production, biomass gasication, gas cleaning andfuel production. Biomass production includes microalgae synthesis followedby harvesting to supply microalgae to the gasication process. In the gasi-cation process, the prepared microalgae is dried, pyrolyzed and gasiedto obtain a producer gas containing syngas and impurities. The producergas is cleaned from the impurities and fed to a fuel production unit, e.g. amethanation process.This thesis investigates three distinct aspects related to this processscheme, namely the occulation of microalgae as part of harvesting, biomasspyrolysis as a part of gasication, and sulfur removal from the producer gasas a part of gas cleaning.The investigation of occulation focuses on secondary phenomena thataccompany the aggregation and breakup of the suspended particles, namelyrestructuring, decay of oc strength and settling. For the study of theconsidered phenomena, a population balance model is developed.Slow pyrolysis of biomass is studied on both the reactor scale and thepellet scale. A model for a rotary drum reactor, using principles of isoconversionalanalysis, is developed for the study of dierent biomass feedstock.The proposed model allows for deriving a preliminary reactor design withminimal experimental input data. A one-dimensional nite volume schemeis developed for the investigation of pyrolysis on the pellet scale. The proposedscheme accounts for convective and diusive heat and mass transfer,and is tested against analytical solutions and commercial software packages.Sulfur removal by metal oxides in a packed bed is studied on both thesystem level and the process level. Criteria for the selection of metal oxidesand the design of packed bed units are derived. A detailed analysis isundertaken to study the reaction of H2S with ZnO in a packed bed, wherethe nano-particles of ZnO experience void formation and outward growth.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. p. 80
Series
TRITA-CBH-FOU ; 2019:65
Keywords
PBE modeling of flocculation; Pyrolysis in Rotary Kilns; Pellet Pyrolysis; Gas Cleaning; Desulfurization.
National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:kth:diva-263085 (URN)978-91-7873-367-5 (ISBN)
Public defence
2019-11-28, Kollegiesalen, Brinellv. 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 2019-10-29

Available from: 2019-10-29 Created: 2019-10-29 Last updated: 2019-11-06Bibliographically approved
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