Fortschritte bei der Modellierung von Festbettreaktoren

Abstract

Forschungsarbeiten iiber die Modellierung von Festbettreaktoren haben in den letzten Jahren zu einem besseren Verstandnis der mit der Modellierung verbundenen Probleme und zu einer praxisnaheren Modellierung gefuhrt. Es ist heute moglich, mit Standardroutineprogrammen, selbst fur sehr komplexe Reaktionssysteme, Festbettreaktoren sowohl zu analysieren als auch auszulegen. Messungen der axialen und radialen Konzentrations-und Temperaturverteilung in Festbettreaktoren haben den EinfluB der Schuttschichtstruktur gekliirt . Das Problem der Mehrkomponentendiffusion bei komplexen Reaktionen in porosen Katalysatoren ist quantitativ losbar und 1aBt sich in die Reaktormodellierung einbeziehen. Die begrenzte Verfugbarkeit von Transportparametern in Schiittschichten zwingt allerdings zu einer deutlichen Einschrankung bei der praktischen Anwendung denkbarer Modelle.Progress in the modeling of fixed bed reactors. Recent work on the modeling of catalytic fixed bed reactors have led to more realistic mathematical models and to a better understanding of problems connected with modeling. Nowadays, standard routine computer programs are available for use in the design (and analysis) of catalytic fixed bed reactors for very complex reaction systems. The influence of bed structure on transport parameters was illustrated by experimental determination of axial and radial concentration and temperature distribution. Moreover the problem of multicomponent diffusion coupled with complex reactions in porous catalysts has been solved and can be included in the reactor model. However, the practical application of possible reactor models is limited by the availability of transport parameters for fixed beds.

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

“…In order to further support the statement that the apparent dependence of 2off and ~w on chemical reaction is highly significant, we note that Hofmann (1979b) reports that the number ~ should roughly equal 0.24).3. However, he finds that values of 2elf and ~w are higher for exothermic reactions, not lower as predicted by us.…”

“…Thus ~ varies in the axial direction of the tube, or, if we calculate 2eff and ct w by fitting the radial temperature profile measured at one specific location in the axis of the tube, it depends on the velocity what the local value of ~ is in that particular place. Since we do not know where Hofmann (1979b) measured his radial temperature profiles we cannot give exact data for the values to be expected for ~ in his experiments. In Appendix C an order of magnitude is estimated for the number ~.…”

“…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). Since a 10% error in those correlations will induce a much larger error in the value calculated for ( [see eqs (57) and (59)I, a considerable spread in the values for ( must be expected.…”

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

“…In order to further support the statement that the apparent dependence of 2off and ~w on chemical reaction is highly significant, we note that Hofmann (1979b) reports that the number ~ should roughly equal 0.24).3. However, he finds that values of 2elf and ~w are higher for exothermic reactions, not lower as predicted by us.…”

“…Thus ~ varies in the axial direction of the tube, or, if we calculate 2eff and ct w by fitting the radial temperature profile measured at one specific location in the axis of the tube, it depends on the velocity what the local value of ~ is in that particular place. Since we do not know where Hofmann (1979b) measured his radial temperature profiles we cannot give exact data for the values to be expected for ~ in his experiments. In Appendix C an order of magnitude is estimated for the number ~.…”

“…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). Since a 10% error in those correlations will induce a much larger error in the value calculated for ( [see eqs (57) and (59)I, a considerable spread in the values for ( must be expected.…”

Modeling of heat transfer in wall-cooled tubular reactors

“…Nowadays most cooled tubular reactors are not designed on the basis of kinetic data and model calculations, but experiments are carried out using single tubes in pilot scale reactors at practical conditions. Previous studies of heat transport phenomena in wall-cooled tubular reactors have shown a discrepancy between the e!ective radial conductivities of the catalyst bed measured with and without reaction (Hall et al 1949;Ho!man et al 1979;Schwedock et al 1989;Schouten et al 1994). Schwedock et al, 1989) found that the radial heat conductivity was about 50% higher in the presence of reaction than when no reaction occurs.…”

Simulation of thermal conversion of solid fuel by the discrete particle method

“…A lot of studies rely on the radial effective heat conductivity λ eff and the heat transfer coefficient at the wall α w in packed beds to describe heat transfer as employed by Zehner [15], Hennecke and Schlün-der [16], Zehner and Schlünder [17], Lerou and Froment [18], Bauer [19], Dixon and Cresswell [20], Hofmann [21], and Wellauer et al [22]. A more detailed review may be found in the articles of Westerterp et al [23] and Tsotsas and Martin [24].…”

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

“…In order to further support the statement that the apparent dependence of 2off and ~w on chemical reaction is highly significant, we note that Hofmann (1979b) reports that the number ~ should roughly equal 0.24).3. However, he finds that values of 2elf and ~w are higher for exothermic reactions, not lower as predicted by us.…”

“…Thus ~ varies in the axial direction of the tube, or, if we calculate 2eff and ct w by fitting the radial temperature profile measured at one specific location in the axis of the tube, it depends on the velocity what the local value of ~ is in that particular place. Since we do not know where Hofmann (1979b) measured his radial temperature profiles we cannot give exact data for the values to be expected for ~ in his experiments. In Appendix C an order of magnitude is estimated for the number ~.…”

“…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). Since a 10% error in those correlations will induce a much larger error in the value calculated for ( [see eqs (57) and (59)I, a considerable spread in the values for ( must be expected.…”

Modeling of heat transfer in wall-cooled tubular reactors

“…Nowadays most cooled tubular reactors are not designed on the basis of kinetic data and model calculations, but experiments are carried out using single tubes in pilot scale reactors at practical conditions. Previous studies of heat transport phenomena in wall-cooled tubular reactors have shown a discrepancy between the e!ective radial conductivities of the catalyst bed measured with and without reaction (Hall et al 1949;Ho!man et al 1979;Schwedock et al 1989;Schouten et al 1994). Schwedock et al, 1989) found that the radial heat conductivity was about 50% higher in the presence of reaction than when no reaction occurs.…”

Simulation of thermal conversion of solid fuel by the discrete particle method

“…A lot of studies rely on the radial effective heat conductivity λ eff and the heat transfer coefficient at the wall α w in packed beds to describe heat transfer as employed by Zehner [15], Hennecke and Schlün-der [16], Zehner and Schlünder [17], Lerou and Froment [18], Bauer [19], Dixon and Cresswell [20], Hofmann [21], and Wellauer et al [22]. A more detailed review may be found in the articles of Westerterp et al [23] and Tsotsas and Martin [24].…”

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

“…In order to further support the statement that the apparent dependence of 2off and ~w on chemical reaction is highly significant, we note that Hofmann (1979b) reports that the number ~ should roughly equal 0.24).3. However, he finds that values of 2elf and ~w are higher for exothermic reactions, not lower as predicted by us.…”

“…Thus ~ varies in the axial direction of the tube, or, if we calculate 2eff and ct w by fitting the radial temperature profile measured at one specific location in the axis of the tube, it depends on the velocity what the local value of ~ is in that particular place. Since we do not know where Hofmann (1979b) measured his radial temperature profiles we cannot give exact data for the values to be expected for ~ in his experiments. In Appendix C an order of magnitude is estimated for the number ~.…”

“…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). Since a 10% error in those correlations will induce a much larger error in the value calculated for ( [see eqs (57) and (59)I, a considerable spread in the values for ( must be expected.…”

Modeling of heat transfer in wall-cooled tubular reactors

“…Nowadays most cooled tubular reactors are not designed on the basis of kinetic data and model calculations, but experiments are carried out using single tubes in pilot scale reactors at practical conditions. Previous studies of heat transport phenomena in wall-cooled tubular reactors have shown a discrepancy between the e!ective radial conductivities of the catalyst bed measured with and without reaction (Hall et al 1949;Ho!man et al 1979;Schwedock et al 1989;Schouten et al 1994). Schwedock et al, 1989) found that the radial heat conductivity was about 50% higher in the presence of reaction than when no reaction occurs.…”

Simulation of thermal conversion of solid fuel by the discrete particle method

“…A lot of studies rely on the radial effective heat conductivity λ eff and the heat transfer coefficient at the wall α w in packed beds to describe heat transfer as employed by Zehner [15], Hennecke and Schlün-der [16], Zehner and Schlünder [17], Lerou and Froment [18], Bauer [19], Dixon and Cresswell [20], Hofmann [21], and Wellauer et al [22]. A more detailed review may be found in the articles of Westerterp et al [23] and Tsotsas and Martin [24].…”