Catalyst Deactivation During Pyrolysis Gasoline Hydrogenation

Catalyst Deactivation during Pyrolysis Gasoline Hydrogenation

 

Javed Ali and S. David Jackson*

Centre for Catalysis Research, WestCHEM, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, Scotland, UK

• david.jackson@glasgow.ac.uk

  Introduction

  Pyrolysis Gasoline (PyGas) is a by-product of high temperature naphtha, with high contents of aromatics such as toluene, styrene and benzene, and unsaturated aliphatic hydrocarbons such as olefins and dienes [1].  It is desirable to stabilize olefins and dienes as these highly active species which can form gums and to reduce the aromatic content to meet strengthened fuel regulation [2].  To achieve this PyGas is hydrogenated over either nickel or palladium catalysts.  In this paper we report on the carbon deposition and catalyst deactivation associated with PyGas hydrogenation over Ni/alumina. Experimental

 

Hydrogenation of the PyGas over a commercial Ni/Al₂O₃ catalyst was investigated in a high-pressure micro reactor using the operating conditions: T= 140-200 ^oC, P_T= 10-20 bar, WHSV 4-8. A synthetic PyGas mixture containing 1-pentene, cyclopentene, 1-octene, n-heptane, n-decane, toluene and styrene was used. Coke deposits were analysed by in-situ temperature program oxidation (TPO); the process was monitored by online mass spectrometer. Both fresh and regenerated catalysts were characterized by powder XRD, BET and TGA-MS. Results/Discussion

 

Hydrogenation of the PyGas over was investigated over a range reaction temperature (T= 140-200 ^oC).  The deposition of coke is highly dependent upon the reaction conditions and especially on the reaction temperature. The increase in reaction temperature during PyGas hydrogenation not only increased the amount of coke deposition but also produced a more condensed hydrogen deficient type coke. The TPO results of catalysts used in the reactions preformed at 140^oC and 200^oC are compared below in Figure 1.

 

These results show that a higher amount of coke deposition was observed with an increase in the reaction temperature. The evolution of CO_2, CO and H₂O also start at comparatively higher temperatures suggesting hard type coke formation at the higher reaction temperature. A considerable increase was also observed in the evolution of aromatic species (styrene and benzene) and hydrogen in the TPO with the increase in reaction temperature, as shown in Figure 2. However, the evolution of other species i.e. cyclopentene, 1-pentene, pentane, toluene, methylcyclohexane, ethylbenzene, 1-octene, ethylcyclohexane and octane were found to be similar in both TPOs.

 

Figure 1. Comparison of in-situ TPO of Ni/Al₂O₃ at reaction temperature 140^oC and 200^oC [WHSV_PyGas = 4 h^-1, P_H2 = 20 barg]

 

Figure 2.  Comparison of aromatic species and H₂ evolution during in-situ TPO of Ni/Al₂O₃ at reaction temperature 140^oC and 200^oC [WHSV_PyGas = 4 h^-1, P_H2 = 20 barg]

 

The significant increase in the amounts of aromatic species (styrene and benzene) evolved in the TPO suggests that the formation of condensed polyaromatic (hard type) coke increased with a higher reaction temperature.

A distinct difference was also noted in the mode of H₂ evolution in the TPO with an increase in reaction temperature. Evolution of H₂ was observed with the evolution of CO₂ /CO in the TPO of the catalyst used in the reaction temperature at 140^oC. However, the evolution of H₂ in the TPO of the catalyst used at 200^oC showed a stepwise decrease until finally no hydrogen was evolved while the evolution of CO₂/CO was still observed, as shown in Figures 4.25-26. This indicates the presence of various types of carbonaceous residues on the catalyst used at the higher reaction temperature.

These results illustrate that the combustion of hydrogen rich coke (soft coke) occurred at lower temperatures and produced CO₂/CO and H₂O. Subsequently, the combustion of hydrogen deficient type coke took place and mainly produced CO₂/CO and H₂. Finally the evolution of CO₂/CO with no evolution of H₂O or H₂ at higher temperature shows the combustion of hard coke took place during TPO of the catalyst used at 200^oC. As the temperature is raised the amount of oxygen required to combust the deposit increased. 

The carbon balance of each species was determined at each reaction temperature and is shown in Figure 3.

Figure 3.  Carbon balance of PyGas components [T = 140-200^oC, P_H2 = 20 barg, WHSV_PyGas = 4 h^-1]

 

Styrene was observed to be the main precursor of coke formation, however the olefins also contribute a reasonable amount to coke deposition.  The carbon balance of styrene significantly decreased with an increase in the reaction temperature, which suggests a higher amount of styrene polymerisation to coke deposition at the higher reaction temperature.

These results illustrate that an increase in the reaction temperature of PyGas hydrogenation not only increases the amount of coke deposition on the catalyst but also the nature of coke changes to a condensed polyaromatic type with a higher C/H ratio.

The results of in-situ TPO indicate that residue on catalyst is mostly aromatic and hydrogenated aromatic components.

  References.

1.  Z. Zhou, Z. Cheng,, D.Yang, X. Zhou, W. Yuan, J.Chem.Eng.Data, 51, 972-976, (2006)

2.  P.Castano, B.Pawelec, J.L.G.Fierro, J.M.Arandes, J.Bilbao, Fuel, 86, 2262-74, (2007)