The Kinetics of Activation of Industrial and Model Iron Catalysts for Ammonia Synthesis in Dried and Wet Atmosphere

Guni, L et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elscvier Science Publishers B.V. All rights reserved

THE KINETICS OF ACTIVATION OF INDUSTRIALAND MODEL IRON CATALYSTS FOR AMMONIA SYNTHESIS IN DRIED AND WET ATMOSPHERE A. Baranski, A. Kotarba, J. M.Lagan, A. Pattek-Janc& E. Pyrczak and A. Reuer

Jagiellonian University, Karasia 3, 30-060 Cracow, Poland

The oxide precursor of the catalyst is activated by a hydrogen reduction in situ in industrial reactors or in especially built installations [l]. The kinetic data are of primary importance for the technological design of the reduction process Kinetic equations for the reduction of industrial catalyst in dried gas phase were previously proposed by us [2,3]. Their significance was emphasized in the review articles [4,5]. Water evolved during the reduction significantly retards the reduction rate. The retarding effect is due to the alumina [6]. It implies a necessity of a study of industrial and model catalysts in dried and wet atmosphere.

.

1. ABBREVIATIONS

Mlcatl-42 is a typical label of a model catalyst. It reads: doubly promoted (by K and Al) catalyst containing 42 wt % of wustite. Letter M will end the label if wustite is lacking, i.e. the catalyst is based on magnetite only. Letter u at the beginning will denote an unpromoted catalyst. 2. EXPERIMENTAL

Model catalysts were prepared in industrial conditions. If necessary, they were preoxidized or prereduced in CO/CO, mixture at 1370K in order to remove wustite or magnetite. The alumina and K,O content amounts to 3-4 and to 1.5-2 w t % respectively. Reductions were carried out in a flow Mc Bain thermobalance [2]. The water content (ppm) in the 3H,/N, mixture amounts to ca. 100 (dry atmosphere), -2600, -6000 and -10000 (wet atmosphere). 3. THE KINETICS OF REDUCTION OF THE INDUBTRIAG CATALYSTS IN

THE DRY ATMOSPHERE

Two mixed-control equations: Seth-Ross (SR) (see [2]) and Spitzer (S) (see [3]) were previously used for the kinetic description of the reduction of the industrial iron catalysts

-

2024 in dry atmosphere. A complex linearized form of (SR) equation was replaced by the relation R=R(t) (where R was the reduction degree and t was time ) . The simple linear regression method was replaced by the direct numerical solution of the nonlinear equation. The parameters of the equation were calculated by a minimization procedure. This treatment proved that SR was valid also for the initial part of the kinetic curve. The discrimination procedure reveals that SR is a more adequate empirical equation than S. The detailed description of the outlined results will be presented in a separate paper.

Dry atmosphere (-100

ppm HzO)

Time/mCn

Wet atmosphere

A1catl-M

(-lo Oo0

ppm H2O)

Time/min

KM I

'oar 80

Time/min

Figure 1. The kinetic curves of the reduction of the iron catalysts.

2025 4. THE EBBECT OF WATER OU THE REDUCTIOU RATE

Two industrial catalysts (Danish KM I and Polish PS3INS) and Alaatl-Y were chosen for the study of the effect. The catalysts were reduced at varying temperatures and atmospheres. The kinetic curves are shown in Fig. 1. Some of them are averaged basing on several (2-10) kinetic runs. A spectacular decrease of the reduction rate can be observed in wet atmosphere for industrial catalysts (for T< 500OC) and for Alaatl-W (for T<550°). For the model catalyst the reduction rate at 5OOOC and below is undetectably small. The Seth-Ross equation and the parabolic equation R=k t" (kp and n are paramekers) were fitted to all kinetic data. The fitness of both equations is compared in Table 1 by means of a factor y defined as the ratio of residual standard deviations (y=u JuSR). The SR equaaon is undoubtedly better for the runs in dry atmosT [OCI phere. The same is true for the experiments performed at the highest temperature in Figure 2. The fitness of parabollic wet atmosphere. All equation to the kinetic data in dry the rate constants of and wet atmospheres. SR equation (k) for Alaatl-I4 at 6OOOC and industrial catalysts at 55OOC are of the same order of magnitude (. 02
relations valid at wet and dry atmosphere respectively. This is a starting point for studying the reduction in intermediate atmospheres typical of industrial conditions. Table 1 Comparison of the kinetic models. The values of the factor y

100

"20 content

6000

ppn

10000

'

3.9

1.03

11.1

1.4

3.7

5.6

1.5

0.98"

1.2

2.0

0.77" 0.74"

1.07

2.0

6.2

1.5

0.96'

1.60

1.25

-

unreasonably high values o f the r a t e constant o f SR equation

5. THE EFFECT OF POTASSIUM AND WUSTITE ON THE REDUCTION RATE

OF MODEL CATALYSTS.

Potassium and wustite accelerate reduction and make the initial part of the kinetic curve linear. It implies that the initial rate is a reasonable measure of reducibility. The ratio of the initial rates indicates that ucatl-33 is reduced faster than ucatl-M by a factor of ca 1.5. The same is true for ucatl-33 and Kcatl-28. Thus, 2% of K and 30% of wustite yield similar rate increase. The initial reduction rates of Kcatl-28 and KAlcatl-29 samples are close, independently on the water content in the gas phase. One can conclude that in the presence of potassium alumina does not affect significantly the reduction rate. The detailed results will be presented in a separate paper. 6. REFERENCES

1 D. Honti, The Nitrogen Industry, Akademiai Kiado, Budapest, 1976, p. 124-126. 2 A. Barahski, A. Bielahski and A. Pattek, J. Catal., 26 (1972) 286. 3 A. Barafiski, J.M. Lagan, A. Pattek, A. Reizer, L.J. Christiansen and H. Tops@e, Preparation of Catalysts 11, Elsevier, Amsterdam, 1979, p. 353. 4 A. Nielsen, Catal. Rev. Sci. Eng., 23 (1981) 17. 5 R. Schlagl, in J.R. Jennings (ed.), Catalytic Ammonia Synthesis, Plenum Publ. Co., 1991, chapter 2, p. 49. 6 A. Barahski, A . Reizer, A. Kotarba and E. Pyrczak, Appl. Catal., 40 (1988) 67. 7 A. Barahski, A. Kotarba and J.M. Lagan, Appl. Catal., 71 (1991) L1.