and Environmental Protection
http://ago.helion.pl ISSN 1733-4381, Vol. 5 (2007), p-99-106
Catalytic carriers for selective oxidation of organic compounds
Krajewski W.*, Najzarek Z.**
*Instytut Inżynierii Chemicznej PAN - Gliwice ** Politechnika Opolska, Zakład Chemii
Streszczenie
Nośniki katalizatorów utleniania lotnych związków organicznych
W pracy zajmowano się katalizatorami osadzonymi na różnych nośnikach. Wykonano badania cieplno – przepływowe w rurze o średnicy 26 mm i wysokości 600mm dla kilkunastu nośników. Podano korelacje na współczynniki oporu hydraulicznego Fanninga oraz liczby Nusselta w zależności od liczb Reynoldsa. Badano przepływ powietrza. Przedstawione typy nośników obejmują dwa standardowe nośniki ceramiczne stosowane w reaktorach przemysłowych, cztery złoża utworzone z metalowych pierścieni Rashiga usypanych lub ułożonych, dwa nośniki strukturalne oraz cztery inne typy nośników. Porównania tych nośników dokonano przy pomocy kryterium entropowego.
Abstract
In the paper problems of catalytic agents mounted on different carriers have been presented. Described experiments were based on the stand equipped with tube (diameter 26 mm, height 600 mm) were carried out for several carriers. Dependence of Fanning hydraulic resistance coefficient and Nusselt number has been given. Air flow was examined. Used carriers are of two different ceramic types commonly used in industrial reactors, four beds created using metal Rashig rings, two structural carriers and four other types of carriers. Comparison of those carriers was done using entropy criteria.
1. Introduction
Industrial processes of selective oxidation of organic compounds generate usually large amounts of the reaction heat. The oxidation is performed most often with multi-tubular reactors, in gas-phase at a temperature of a few hundreds of Celsius degrees [1, 2]. Therefore, proper selection of the catalyst carriers in order to minimize resistance of flow through the catalytic bed, while the reaction heat is removed intensively, has become a considerable problem. Such a selection of an optimal carrier from among a few or over a dozen structures of either very similar nor very different properties is not an easy and unequivocal choosing as yet.
Recently, preparation of thin-layer catalytic systems on the carrier surface was elaborated [3]. The elaboration technique may extent substantially the application possibilities of advanced carriers and relevant catalysts.
2. Experimental set-up
In this work, heat-flow studies of the carrier characteristic were carried out on over a dozen structures with a measuring tube of 26 mm diameter and 0.6 m height.
Table 2.1. The heat-flow characteristic of the carriers
No Description of carrier a m2/m3 ε m3/m3 Friction factor correlation Nusselt number correlation 1 Double helical surface made of wire (double
wire strand) =0.56
386 0,937 f=6,194 Re-0,104 Nu=0,148 Re0,715
2 Helical tape i=13, e=19, h=24, t=0.5 27,6 0,994 f=207,82 Re-0,625 Nu=0,148 Re0,688
3 Spiral fin =24.3 (round a pipe e=10) h=7.5mm, t=0.5mm
258 0,803 f=1116,9 Re-0,460 Nu=0,0538 Re0,930
4 Dumped metal Raschig rings
5.1×5×0.25mm, randomly packed 905 0,892 f=190,4 Re-0,285 Nu=0,0604 Re0,927
5 Single helical surface made of wire (single wire strand) i/e=7.8/24.4, t=0.05
408 0,946 f=87,53 Re-0,184 Nu=0,0252 Re1,017
6 Metal rosettes (20 leafs=26, h=22, t=0.05) with a drop-like core (=11.8, h=19.1)
804 0,859 f=0,613 Re0,088 Nu=0,172 Re0,730
7 Dumped metal Raschig rings
5.1×5×0.25mm randomly packed in annular space round a core =13.8
898* 0,893* f =430,99
Re-0,339
Nu=0,0177 Re1,086
8 Metal Raschig rings 5.1×5×0.25mm stacked
in annular space round a core 14.8.
1033* 0,877* f=40,26 Re-0,309 Nu=0,537 Re0,559
9 Rosettes (20 leafs=26, h=10, t=0.05)
alternately with U-shaped rings type B (ie11.8/22.4, h=2.2, t=0.3)
918 0,956 f=62,34 Re-0,216 Nu=0,156 Re0,820
10 Metal rosettes (20 leafs=26, h=10, t=0.05) and U-shaped rings type A (i/e=14.1/24.3, h=2.0, t=0,3)
903 0,959 f=22,55Re-0,129 Nu=0,0338 Re0,986
11 Dumped metal rings 4.5×5×0.05mm,
randomly packed
1504 0,925 f=209,5 Re-0,298 Nu=0,018 Re1,058
12 Ceramic Raschig rings with rounded edges
×7×1.5mm, randomly packed 750 0,537 f=116,67 Re-0,272 Nu=0,0933 Re0,864
13 Metal rosettes (20 leafs=26, h=10, t=0.05) 917 0,997 f=0,965 Re0,08 Nu=0,285 Re0,688
14 Ceramic half-rings ×6×2 mm, randomly packed
690 0,546 f=155,47 Re-0,262 Nu=0,0217 Re1,048
*
Archiwum Gospodarki Odpadami i Ochrony Środowiska, vol. 5(2007) 101 φ - dimension; φe, φi – external, internal dimension; t – thickness of the carrier foil; h –
height (helical/screw unit); a – carrier specific surface; ε – void volume of the carrier Correlations for Fanning friction factor and Nusselt number vs. Reynolds number are shown in Table 2.1. Air was used as the process medium. The Reynolds number was defined on the tube diameter. Table 2.1 characterizes: two standard ceramic carriers of industrial reactor applications, four carrier beds stacked or randomly packed, two structural carriers, and four carriers of other kinds. Comparison between those carriers was performed by evaluation criteria based on the process entropy generation.
3. Generated entropy formulation and discussion
The rate of entropy generation is given by Shah and Seculic [4] as the equation (3.1).
St c m T D L q T c D π ρ fL m 2 S S S p . 2 m 2 m p 5 2 2 . . . . 4 ) / ( 3 " ' 2 + = + = (3.1)
The first term in the equation (3.1) describes the entropy generation due to flow resistance and the second-due to heat transfer.
According to the work by Ranasinghe et al. [5] one can determine disadvantage as regards the process economy because of the first-term entropy generation, and advantage– because of the second-term entropy generation controlling. Therefore, that approach allows for the possibility of the carrier selection with respect the process economy.
The entropy generation on the carriers by flow resistance, heat transfer and their addition are presented vs. Reynolds number in Figures 3.1a, 3.1b and 3.2, respectively. The smallest entropy generation was observed for small values of Reynolds number while the carriers of low efficiency of heat transfer were used. More informative was the ration of exchanged heat to generated entropy.
Figure 3.3 shows the ratio of Nusselt number to the generated entropy vs. Reynolds number. Fig. 3.4 presents relationships between the dimensionless term j/(s/(m·Cp)) and Reynolds number. It can be seen that the best effects have been attained at Reynolds number under 5000 for the carrier no. 9 composed of rosettes (height 20 mm) packed alternately with the B-type U-shaped rings. Within the Reynolds number range of 5000-8000 the best results were attained with the carrier no. 10 composed of the rosette with U-shaped ring type A, while within Re range of 8000-10000 the best performance showed the carrier no. 6 combined with the rosette and a drop-like core. Those carriers showed the best performance at the Colburn factor of 0.036; 0.0292; 0.0144, respectively, as it is showed in Fig. 3.5.
The presented method makes it possible to exact estimation of advantageous features of the catalytic carriers with respect to Reynolds number within conditions of relevant application. The studies show that the carrier performances may be evaluate with respect to the economic calculations concurrently with the literature data [5].
cp specific heat [J/(kg·K)], D - tube diameter [m], F - Fanning friction factor, j - Colburn
factor, L - ube length [m], .
m - mass flow [kg/s], Nu - Nusselt number , Re - Reynolds
number , q - heat flux [W]
0 0,02 0,04 0,06 0,08 0,1 0,12 0,14 0,16 1000 3000 5000 7000 9000 Re S ', W /K 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 0,005 0,01 0,015 0,02 0,025 0,03 0,035 0,04 1000 3000 5000 7000 9000 Re S ", W /K
Fig. 3.1. Rate of entropy generation for the carriers vs. Re number: the generation by hydraulic flow resistance
the generation by heat transfer
Inlet air temperature 325 K, outlet pressure 0.1 MPa, tube (wall) temperature 414 K, length 0.6 m, tube diameter 26 mm. The carriers are presented in Table 2.1.
a)
Archiwum Gospodarki Odpadami i Ochrony Środowiska, vol. 5(2007) 103 0 0,02 0,04 0,06 0,08 0,1 0,12 0,14 0,16 0,18 1000 3000 5000 7000 9000 Re S , W /K
Fig. 3.2. Rate of the total entropy generation on the carriers (presented in Table 2.1 and Fig. 3.1a). 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 1000 3000 5000 7000 9000 Re N u /S , K /W
Fig. 3.3. The ratio Nu /
.
0 0,5 1 1,5 2 2,5 1000 3000 5000 7000 9000 Re j/ (S /( m *c p ))
Fig. 3.4. Variation of the dimensionless term j/(
.
S
/( .m cp)) vs. Reynolds number for the
carriers (see Table 2.1, Fig. 3.1a).
0 0,5 1 1,5 2 2,5 0 0,01 0,02 0,03 0,04 0,05 j j/ (S /( m *c p ))
Fig. 3.5. The dimensionless term j/(
.
S
/( .m cp)) as a function of Colburn factor for the
Archiwum Gospodarki Odpadami i Ochrony Środowiska, vol. 5(2007) 105 .
S - entropy generation rate [W/K] St - Stanton number
Tm - gas mean temperature [K]
- gas density [kg/m3
] References
[1] M. S. Ray, M. G. Sneeby: Chemical Engineering Design Project. A Case Study Approach. Production of Phthalic Anhydride, 2nd ed., Gordon and Breach Sc. Publ., Amsterdam 1998
[2] W. Krajewski, M. Galantowicz, Stud. Surf. Sci. Catal., 1999, 126, 447 [3] W. Krajewski, Z. Najzarek, Petroleum & Coal, 2004, 42(2), 26
[4] R. K. Shah, D. P. Seculic, Fundamentals of Heat Exchangers Design, Wiley, New Jersey 2003
[5] J. Ranasinghe, S. Aceves-Saborio, G. M. Reistad, in Second Low Aspects of Thermal Design, HTD, ASME, N. Y., 1984
