(619f) 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)