Theoretical prediction of effective heat transfer parameters in packed beds

Dixon, Anthony G.; Cresswell, David L.

doi:10.1002/aic.690250413

Cited by 368 publications

(158 citation statements)

References 23 publications

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“…According to the dispersion model proposed by Levenspiel [9] and taking into account the operating conditions listed in Table 1, the Peclet number (Pe=uL/D e ) can be expected to be higher than 3000, and therefore a plug flow pattern can be assumed in the Cu oxidation stage. For the conditions employed in this work, a gas-solid heat transfer coefficient (h fs ) of about 300 W m -2 K -1 was obtained [11]. This value implies a very fast gas-solid heat exchange with no significant difference between the temperatures of the gas and solid at any point in the reactor.…”

Section: Mathematical Model Descriptionmentioning

confidence: 73%

Modeling of Cu oxidation in adiabatic fixed-bed reactor with N2 recycling in a Ca/Cu chemical loop

Fernández

Abanades

Murillo

2013

Chemical Engineering Journal

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A dynamic model has been constructed to describe the Cu oxidation reaction within a largescale Cu/CuO chemical looping process in adiabatic fixed-bed reactors. Careful control of the temperature is required during Cu oxidation because of its high reaction enthalpy. The recycling of a large amount of nitrogen, previously cooled down, and its mixture with air for Cu oxidation regulates the temperature in the reaction front, ensuring that undesirable hot spots that would lead to the irreversible loss of Cu activity are avoided. Since the gas/solid heat exchange front advances faster than the reaction front, the bed is eventually left at a low temperature. An additional stage was added to allow a gas/solid heat exchange between the hot recycled gas and the oxidized bed. This ensures that the fixed-bed is ready for the next reaction step that involves the reduction of CuO by a fuel gas. A sensitivity analysis of the main operating parameters confirms the theoretical viability of this operation. Cu oxidation is favoured at high pressure and therefore fast reaction rates were achieved, even with low contents of oxygen in the feed (around 3-4%). The recirculation of more than 80% of the exit gas from the oxidation reactor and its subsequent cooling down to around 423 K keep the maximum temperature down to within reasonable values (1173 K). Although this work was focused on the boundary conditions 2 of the Ca/Cu looping process for hydrogen production and/or power generation, some of the trends observed may be considered valid for other CLC systems that use similar N 2 recycles.

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Section: Mathematical Model Descriptionmentioning

confidence: 73%

Modeling of Cu oxidation in adiabatic fixed-bed reactor with N2 recycling in a Ca/Cu chemical loop

Fernández

Abanades

Murillo

2013

Chemical Engineering Journal

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“…4, where ~-eff and ~w are plotted vs the Peclet number for the case where there is no chemical reaction (for definition of Pe see Notation). The straight lines were calculated from the correlations given by Zehner (1973) and Dixon and Cresswell (1979); the dots indicate the values measured by Hofmann (1979b). Notice that there is a 25-80% deviation between the literature correlations and the values measured by Hofmann (1979b).…”

Section: R(t~)(-ah)r 2 R(t)(-ah)rmentioning

confidence: 99%

“…Several studies have been conducted on the effective heat conductivity (2~ff) and the heat transfer coefficient at the wall (~w) inside packed beds [-see, for example, Damk6hler (1937), Zehner (1973), Hennecke and Schliinder (1973), Lerou and Froment (1977), Schlfinder (1966Schlfinder ( , 1978, Bauer (1977), Dixon and Cresswell (1979) and Hofmann (1979a, b)]. To this purpose models were developed for heat transfer inside packed beds.…”

Section: Introductionmentioning

confidence: 99%

Do the effective heat conductivity and the heat transfer coefficient at the wall inside a packed bed depend on a chemical reaction? Weaknesses and applicability of current models

Wijngaarden

Westerterp

1989

Chemical Engineering Science

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Al~tract--Many studies have been conducted on the effective heat conductivity (2eft) and the heat transfer coef~cient at the wall (~w) inside packed beds. It has been mentioned that the values of )~ff and ~w are changed when a chemical reaction occurs in the packed bed. We give an explanation for such a phenomenon. The properties 2of f and ew are lumped parameters which usually are determined by both the measured temperature profiles and the model used to calculate the temperature profiles from 2off and ew. If either the experimental data are wrong or the model is erroneous the error will manifest itself in the values of ~.~ff arid ew-At least a part of the change in the values of 2~rf and ew due to a chemical reaction is caused by the fact that a homogeneous model with catalyst and gas having the same temperature is chosen, whereas a heterogeneous model with cata!yst and gas having different temperatures should be used. If no reaction occurs the catalyst and gas will have the same temperature and the homogeneous model yields a good description. Hence, when fitting temperature profiles with this model the correct values of 2 m and ew are found. If reaction does occur the catalyst and the gas will have different temperatures because the heat of reaction must be transferred from the catalyst to the gas. If, despite this fact, a homogeneous model is used to calculate the temperature profiles, an error is introduced which is reflected in the values of 2~ and ~. As a consequence we create an apparent dependence of ).,ff and ew on the reaction rate. We derive criteria to determine which model must be used. We discuss results presented in the literature on the dependence of 2¢f e and e~ on the chemical reaction. The explanation is both qualitative and quantitative. INTRODUCTIONSeveral studies have been conducted on the effective heat conductivity (2~ff) and the heat transfer coefficient at the wall (~w) inside packed beds [-see, for example, Damk6hler (1937), Zehner (1973, Hennecke and Schliinder (1973), Lerou and Froment (1977), Schlfinder (1966, 1978, Bauer (1977), Dixon and Cresswell (1979) and Hofmann (1979a, b)]. To this purpose models were developed for heat transfer inside packed beds. With these models the temperature profiles inside packed beds were fitted by varying 2elf and ~w. Thus the values of ~eff and ~w obtained in the literature are best-fit values. Several authors reported that 2eff and ~ are dependent on a chemical reaction occurring inside the packed bed [-see for example, Hofmann (1979b) and Chao et al. (1973)]. It is hardly likely that this dependence can be explained by experimental errors. It could also be possible that )let f and. c~w are affected by chemical reaction in some physico-chemical way. However, to our knowledge there is no indication whatsoever that can support this vision. The most probable explanation is that the model we use to fit 2ef¢ and ce w yields wrong results when chemical reaction occurs. Since 2ef f and ~ are fit parameters, an error in the model used will manifest ...

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“…Also, with a column of a particular bed height, the heat transfer surface area to reactor volume ratio (i.e., specific surface area, excluding the headers) varies inversely as the radius. Therefore, when columns with large cross-sectional areas are used, the available heat transfer area could become a limiting factor [16][17][18]. Where this is the case, multi-tube packed-column reactors are commonly used [19][20][21][22].…”

Section: Introductionmentioning

confidence: 99%

Cuboid Packed-Beds as Chemical Reactors?

Ghosh

2018

Processes

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Columns are widely used as packed-bed or fixed-bed reactors in the chemical process industry. Packed columns are also used for carrying out chemical separation techniques such as adsorption, distillation, extraction and chromatography. A combination of the variability in flow path lengths, and the variability of velocity along these flow paths results in significant broadening in solute residence time distribution within columns, particularly in those having low bed height to diameter ratios. Therefore, wide packed-column reactors operate at low efficiencies. Also, for a column of a particular bed height, the ratio of heat transfer surface area to reactor volume varies inversely as the radius. Therefore, with wide columns, the available heat transfer area could become a limiting factor. In recent papers, box-shaped or cuboid packed-bed devices have been proposed as efficient alternatives to packed columns for carrying out chromatographic separations. In this paper, the use of cuboid packed-beds as reactors for carrying out chemical and biochemical reactions has been proposed. This proposition is primarily supported in terms of advantages resulting from superior system hydraulics and narrower residence time distributions. Other potential advantages, such as better heat transfer attributes, are speculated based on geometric considerations.

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Theoretical prediction of effective heat transfer parameters in packed beds

Cited by 368 publications

References 23 publications

Modeling of Cu oxidation in adiabatic fixed-bed reactor with N2 recycling in a Ca/Cu chemical loop

Modeling of Cu oxidation in adiabatic fixed-bed reactor with N2 recycling in a Ca/Cu chemical loop

Do the effective heat conductivity and the heat transfer coefficient at the wall inside a packed bed depend on a chemical reaction? Weaknesses and applicability of current models

Cuboid Packed-Beds as Chemical Reactors?

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