(33a) Kinetic Modelling of N-Dodecane Dehydrogenation on Pt/Al2O3 Catalyst Based on Lhhw Mechanism

1. Introduction

Dehydrogenation of higher paraffins (C₁₀-C₁₄) to the corresponding mono olefins on Pt-Sn/Alumina catalyst is an industrially important process and a basic step in the manufacture of bio-degradable detergents. The reaction is carried out in an up-flow fixed bed reactor at higher temperature (733-773K) and low pressure (1-1.7bar) environment, with hydrogen as diluent. The main reaction is dehydrogenation to corresponding mono olefins, and some secondary dehydrogenation reactions may occur. According to these secondary reactions, some di-olefins and aromatics are formed. All these products would crack to light olefins. Isomerization reaction may also be possible which is ignorable. The kinetic studies help to develop the reaction mechanism, establish reaction network, design of commercial reactors and evaluate catalyst efficiency and attempt improvements thereof.

Very limited studies have been done on kinetics of higher paraffins dehydrogenation. These available literatures consider the kinetic of higher paraffins dehydrogenation such as n-decane and n-dodecane on promoted Pt/Al₂O₃ catalysts.  

2. Experimental

The Experimental set-up is a fully automated system supplied by Vinci-Technologies,
France. The schematic diagram of the experimental set-up used is shown in Fig. 1. Major sections in the unit are: (1) Hydrogen, Nitrogen and Air flow control modules (2) Liquid pumping system (3) Water pumping system (4) Pre-heater and mixer for hydrogen, n-dodecane and water (5) Reactor (6) Condenser and gas-liquid separator (7) PID controller system (8) Adequate flow meters to account material balance (9) Gas Counter. The reactor is an up-flow of homogenized hydrogen and n-dodecane vapor fixed bed reactor. The inner diameter of the reactor tube is 1.9cm. Space above and below the catalyst in the reactor was filled with inert SiC (particle size 250μm) to ensure proper distribution of fluid flow. The total bed height which is heated measures to 60cm. The bed temperature indication is carried out by 4 thermocouples which were inserted in 4 heating zones. One thermocouple was also inserted in the center of the pre-heater.

The catalyst bed dilution with inert particles of the same size allowed uniform dispersion of the catalyst particles. Accurate mass flow controllers measured amounts of hydrogen in to and out of system. Feed to reactor and product from reactor are measured accurately up to second decimal by electronic balances.

Fig. 1. Schematic diagram of the experimental set-up

3. Kinetic Analysis

Dodecene is the desired product in n-dodecane dehydrogenation. The liquid product of the reaction is about 99% and the rest is gas. All the products of n-dodecane dehydrogenation such as, dodecene, dodecadiene, etc. and n-dodecane itself can crack to form light paraffins. In this study, isomerization was ignored.

The reaction scheme corresponding to the three major dehydrogenation reactions have step for adsorption, surface reaction and desorption as each step could be the rate-determinig step. Therefore there could be three different rate equations.

4. Results

Figures 2 and 3 show the effect of W/F and H2/HC ratio on conversion and Selectivity of mono olefins and other products at three different temperatures. At constant W/F paraffin conversion increased with increase in reaction temperature. The selectivity of mono olefins and dienes decreased with increase in temperature. The paraffins conversion decreased with increase in H2/HC ratio. The decreased H2/HC ratio at a constant pressure lowers the hydrogen partial pressure, which favor higher conversion.

Fig. 2 Effect of W/F on Conversion

Fig. 3 Effect of W/F on Selectivity

Fig. 4 Effect of  H2/HC on Conversion and Olefin Selectivity

5. Conclusion

Kinetics of n-dodecane dehydrogenation on promoted industrial Pt-Sn/Al₂O₃ catalyst was investigated. The results support the stepwise mechanism of the reaction. Three sets of rate models based on LHHW mechanism for all the three dehydrogenation reactions were derived and subjected to parameter estimation and model discrimination. The kinetic parameters were estimated and determined using Genetic algorithm optimization method. The kinetic parameters of the selected model are statistically significant and follow thermodynamic laws.

6. Acknowledgments

This work was supported by Iran University of Science and Technology and Iran Polymer and Petrochemical Institute (project no. 85045, Petrochemical Department).

7. References

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