A grain size distribution model for non-catalytic gassolid reactions

Heesink, Albertus B.M.; Prins, Wolter; Swaaij, W.P.M. van

doi:10.1016/0923-0467(93)80004-g

Cited by 44 publications

(43 citation statements)

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“…Further, when a pellet core is encased in a shell of stronger material, there is a built-in case for adding an additional resistance to the SCM, thus making this model a prime candidate for describing the sulfidation reaction. It is noteworthy, however, that the grain model has been successfully used for the sulfidation of precalcined limestone with different microstructures (Heesink et al 29 ). The micrograin-grain model has also been successfully used in the sulfidation of calcium oxide (Dam-Johanson et al 30 ).…”

Section: Overview Of Gas-solid Reaction Modelsmentioning

confidence: 99%

A Plausible Model for the Sulfidation of a Calcium-Based Core-in-Shell Sorbent

Hasler

Doraiswamy

Wheelock

2003

Ind. Eng. Chem. Res.

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A promising reusable sorbent for desulfurizing hot coal gas is being developed in the form of core-in-shell pellets which consist of a highly reactive CaO core encased in a porous protective shell made largely of an inert material. The spherical pellets are made by pelletizing plaster of Paris and then applying a coating of alumina (80%) and limestone (20%) particles. Subsequent high temperature treatment converts the cores to CaO and sinters the coating material to form the porous shells. The pellets absorb and react with H2S at high temperatures (e.g., 800−900 °C) to form CaS and are regenerated by applying a cyclic oxidation/reduction method. The rate of sulfidation does not appear to be controlled by chemical reaction but, instead, seems to be controlled by one or more of the following diffusion resistances: external gas film, porous shell, micropores between CaO grains in the core, and product layer surrounding CaO within each grain. After considering various models, a brief review of which is included in the paper, a semiempirical model was chosen which represents the process well over a limited range of conversion (approximately 0−85%). This is not a serious limitation for the model because above this range the rate of conversion becomes too slow for most applications. The model assumes that within this range the rate of conversion is controlled by the resistance to diffusion offered by the porous outside shell together with the resistance to diffusion presented by a porous layer of reacted material. The model is in effect an extension of the well-known shrinking, unreacted core model for gas−solid reactions. As more basic diffusion data become available, a more rigorous model, such as a modified grain model, is expected to be developed. Disciplines Chemical Engineering CommentsReprinted with permission from Ind. & Eng. Chem. Research, 42, 2644Research, 42, -2653Research, 42, (2003 A promising reusable sorbent for desulfurizing hot coal gas is being developed in the form of core-in-shell pellets which consist of a highly reactive CaO core encased in a porous protective shell made largely of an inert material. The spherical pellets are made by pelletizing plaster of Paris and then applying a coating of alumina (80%) and limestone (20%) particles. Subsequent high temperature treatment converts the cores to CaO and sinters the coating material to form the porous shells. The pellets absorb and react with H 2 S at high temperatures (e.g., 800-900°C ) to form CaS and are regenerated by applying a cyclic oxidation/reduction method. The rate of sulfidation does not appear to be controlled by chemical reaction but, instead, seems to be controlled by one or more of the following diffusion resistances: external gas film, porous shell, micropores between CaO grains in the core, and product layer surrounding CaO within each grain. After considering various models, a brief review of which is included in the paper, a semiempirical model was chosen which represents the process well over a limited range of conv...

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Section: Overview Of Gas-solid Reaction Modelsmentioning

confidence: 99%

A Plausible Model for the Sulfidation of a Calcium-Based Core-in-Shell Sorbent

Hasler

Doraiswamy

Wheelock

2003

Ind. Eng. Chem. Res.

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“…In a particularly useful grain model, proposed by Heesink et al, 29 the original model is made much more acceptable by accounting for grain size distribution, change in grain size with conversion, and pore blocking. Instead of using a continuous distribution of grain size, the grains are divided into groups and the conversion calculated for each group using the SCM.…”

Section: Gas-solid Reaction Modelsmentioning

confidence: 99%

Engineering a New Material for Hot Gas Cleanup

Wheelock¹,

Doraiswamy²,

Constant³

2003

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The overall purpose of this project was to develop a superior, regenerable, calcium-based sorbent for desulfurizing hot coal gas with the sorbent being in the form of small pellets made with a layered structure such that each pellet consists of a highly reactive lime core enclosed within a porous protective shell of strong but relatively inert material. The sorbent can be very useful for hot gas cleanup in advanced power generation systems where problems have been encountered with presently available materials.An economical method of preparing the desired material was demonstrated with a laboratory-scale revolving drum pelletizer. Core-in-shell pellets were produced by first pelletizing powdered limestone or other calcium-bearing material to make the pellet cores, and then the cores were coated with a mixture of powdered alumina and limestone to make the shells.The core-in-shell pellets were subsequently calcined at 1373 K (1100°C) to sinter the shell material and convert CaCO 3 to CaO. The resulting product was shown to be highly reactive and a very good sorbent for H 2 S at temperatures in the range of 1113 to 1193 K (840 to 920°C) which corresponds well with the outlet temperatures of some coal gasifiers. The product was also shown to be both strong and attrition resistant, and that it can be regenerated by a cyclic oxidation and reduction process.A preliminary evaluation of the material showed that while it was capable of withstanding repeated sulfidation and regeneration, the reactivity of the sorbent tended to decline iv with usage due to CaO sintering. Also it was found that the compressive strength of the shell material depends on the relative proportions of alumina and limestone as well as their particle size distributions. Therefore, an extensive study of formulation and preparation conditions was conducted to improve the performance of both the core and shell materials. It was subsequently determined that MgO tends to stabilize the high-temperature reactivity of CaO. Therefore, a sorbent prepared from dolomite withstands the effects of repeated sulfidation and regeneration better than one prepared from limestone. It was also determined that both the compressive strength and attrition resistance of core-in-shell pellets depend on shell thickness and that the compressive strength can be improved by reducing both the particle size and amount of limestone in the shell preparation mixture.A semiempirical model was also found which seems to adequately represent the absorption process. This model can be used for analyzing and predicting sorbent performance, and, therefore, it can provide guidance for any additional development which may be required.

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“…………………………………………………………………………………………….……………………………………………… 195 Figure 9.24. Schematic representation of grain size distribution (GSD) model for a partially reacted particle: M denotes the macro-pores between clusters, and m represents micro-pores within the clusters, between grains (Heesink, 1993). ……………………………………………………………………………….……………… 196 List of Tables Table 2.1 Structural and physical properties of iron oxides (Monsen, 1992).…”

Section: Statement Of Parts Of the Thesis Submitted To Qualify For Thmentioning

confidence: 99%

“…Figure 9.24. Schematic representation of grain size distribution (GSD) model for a partially reacted particle: M denotes the macro-pores between clusters, and m represents micro-pores within the clusters, between grains (Heesink, 1993).…”

Section: Proposed Micromodels To Describe the Phenomenamentioning

confidence: 99%

The high temperature decomposition of hematite under reactive gas atmospheres: for use in chemical looping combustion

Simmonds¹

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Chemical looping combustion (CLC) is an alternative method of combustion, which enables the efficient recovery of heat and process gas whilst also producing a high CO 2 concentration product gas potentially suitable for carbon capture and storage (CCS). This is a potentially important option for greenhouse gas reduction in future hydrocarbon combustion technology. Fundamental to the process are oxygen carriers; metal oxides that transfer oxygen from the air reactor to the fuel reactor where CO 2 and steam are released. The coupled gaseous reduction and oxidation of iron oxides hematite (Fe 2 O 3 ) and magnetite (Fe 3 O 4 ) has been identified as a promising option for low cost oxygen carrier material. The aim of the present study is to understand the microstructural basis of key properties of oxygen carrier/metal oxide materials, such as reactivity and strength, and the how these may be influenced by the process conditions. This has been performed by studying the structural changes taking place in single stage reduction and oxidation of hematite and magnetite samples, and examination of cross sections at the reaction interface.Investigations have been undertaken using a range of reduction conditions of interest to CLC at 800-1100°C and CO:CO 2 ratios of 0.01:1 to 0.33:1. The samples were reduced for relatively short times, commensurate with the residence times in fluidised bed reactors used in the CLC process.Examination using optical and scanning electron microscopy techniques have revealed that different reaction product morphologies are formed on reduction of hematite depending on the reaction temperature and CO:CO 2 ratio. These include a porous magnetite and dense platelet or "lath" type morphologies. The critical conditions for the formation of these product structures have been identified in the present study. Porous magnetite formation is important to the application of CLC as the high surface area is associated with high reactivity. The porous magnetite structures are shown to form gas pores from instabilities created at the initially dense material through reaction with the gas.The oxidation of magnetite is illustrated to occur through the formation of dense hematite layers on the particle surface. This dense hematite forms through lath type shear transformations or solid state diffusion of iron or oxygen through the product layer.Cyclic reduction and oxidation experiments have been undertaken to simulate the conditions experienced by the hematite oxygen carrier in the CLC process. It has been shown that the initial reduction cycle can have a major effect on the subsequent reactivity of the oxide carrier. The observations also indicate that the effective life of the hematite oxygen carrier is dependent on reduction gas composition, temperature and particle size.The present systematic study of reduction and oxidation reactions on natural hematite has revealed a number of phenomena previously not reported in the literature and has contributed to more fully understanding the changes taking place i...

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A grain size distribution model for non-catalytic gassolid reactions

Cited by 44 publications

References 23 publications

A Plausible Model for the Sulfidation of a Calcium-Based Core-in-Shell Sorbent

A Plausible Model for the Sulfidation of a Calcium-Based Core-in-Shell Sorbent

Engineering a New Material for Hot Gas Cleanup

The high temperature decomposition of hematite under reactive gas atmospheres: for use in chemical looping combustion

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