Reaction kinetic data and calculation of effectiveness factor are showed in [12].
A Dynamic Model For Decoking Process Of Acetylene Hydrogenation Reactor With Two Configuration
O. Dehghani, A. Bolhasani*, S. Karamiyan
Research and Development Department, JAM Petrochemical Complex, Assaloyeh, Iran
Abstract
Selective hydrogenation is used to reduce acetylene
concentration less than 1 ppm due to poisons the catalysts
in polymerization plants. the catalysts have to regenerate
due to green oil formation during hydrogenation. close
monitoring of the two regeneration cycles in An olefine
plant in JAM petrochemical complex, have revealed
complications that caused a dramatic reduction in catalyst
lifetime and also disrupted the temperature profile in the
reactor overtime. instead of conventional configuration
after validation of the dynamic model a new configuration
was proposed, simulated and constracted. As a result, the
regeneration period decreases significantly by 28 h.
Results and discussion
In order to solve these equations, the following boundary
conditions have been applied:
Introduction
Series reactors are used due to high flexibility in control of
reactor temperature and product conversion. After catalyst
deactivation, it is essential to regenerate the coked catalysts
completely [1]. Yajun et al. proposed the general formula for
green oil as: CnH(1.8–1.9)n (14 < n < 17) [2]. Van Deemter
presented a model for coke burning process [4]. Gonzalez et
al. examined the effects of temperature changes on reaction
] In 1988, Westerterp
p et
kinetics and chemical diffusion [[5].
al. introduced a model to investigate heat and mass transfer
impacts simultaneously considering internal mass and heat
transfer and using Chilton‐Colburn equation as well as Lewis
number [6]. In 2010, catalyst regeneration and coke burning
processes in a fixed bed reactor were simulated by Zhang for
a Cr2O3/Al2O3 catalyst in a propane dehydrogenation
process [9]. the catalyst surfaces is only consists of carbon
[7]. For this reason, complete reaction of pure carbon with
oxygen was only considered in these studies. However, in
1967 Massoth showed that hydrogen exists in the coke
structure [3]. Santamaria‐Ramiro et al. showed that coke
formation method (parallel or series) and coke distribution
along the reactor did not affect the temperature profile
considerably [8].
In the present research, a model for coke burning process
was developed. Moreover, the model results were validated
with industrial data from domestic plant for typical fixed bed
series reactors. Subsequently, a novel configuration is
suggested based on the model predictions and so the
pipelines were changed in order to implement the proposed
method to the reactors. It is interesting that a model
predictions and plant data from reconfiguration beds are in
good agreements.
The required physical specifications in the model are taken
from a typical catalyst used in 10th olefine plant of JAM
petrochemical complex [12].
Model validation
Model validation Experimental data are collected from 10th olefine plant in
JAM petrochemical complex during regenaration. The
temperature profiles which are plant data and model's
output for both guard and lead bed are compared to gather
in fig. 1 and fig. 2. Some peaks could not be simulated with
the model because they are heat front points. In these
points, no combustion reaction occurs and they only
detected heat fronts of previous combustion zones [12].
Conclusions
A study on two two‐year‐old regeneration cycles in this
domestic Petrochemical Company, uncovered several
problems associated with the configuration of reactors and
pipelines which could lead to diminished catalysts lifetimes.
lifetimes
Therefore, a new configuration was suggested to overcome
these complications in regenerative process and improve
catalysts efficiency. In this situation, each bed is regenerated
individually which should be consistent with the
manufacturer’s time frame. By doing this, the amounts of
nitrogen and steam consumptions are reduced noticeably
relative to the actual amounts observed in the plant and
sudden temperature increases before decoking are
prevented. in fig. 5 old and new pipe line are illustrated.
References
Kinetics of reaction
The coke can be formulated as CHn, with n varying between
0.4 and 1.3 [3]. The typical TPO tests of deactivated catalysts
in 10th olefine plant of JAM petrochemical complex showed
that the best quantity for n is 0.5. In the presence of catalysts
and above 400 _C the coke reacts completely with oxygen
[10]. So:
The rate of reaction is first order kinetic according to both oxygen and coke [13]. So: Fig. 2. lead bed's temperature profile(old pipe line)
Fig. 1. guard bed (old pipe line)
Fig. 1. Guard bed's temperature profile(old pipe line)
Fig. 2. lead bed
(old pipe line)
Reaction kinetic data and calculation of effectiveness factor
are showed in [12].
Mathematical modelling
A plug flow was assumed in the reactor. Regeneration is an
adiabatic process. Because of high thermal conductivity, there
is no temperature gradiant with in the catalyst. Radial
concentration and temperature distribution, effect of external
mass transfer and axial dispersion of mass and heat are
negligible. Coke distribution is uniform. Due to the high
superficial velocity, heat conduction through the bed is
negligible in comparison with heat convection. the pressure
drop across the bed is very low. So mass and energy balance
are:
A mixture of air and steam was injected stepwise during
decoking. First of all, a small amount of air was injected and
so the reactor temperature increased rapidly owing to the
coke combustion. After that, the air injection stopped until
the bed temperature fell off to its original value and again
oxygen injection started.
After the regeneration cycles it was observed the SOR
temperature had to be higher than the vendor’s suggestion
and the life cycle of catalysts decreases noticeably. In other
words the distance between SOR and EOR temperatures
words,
decreased and the catalysts deactivated sooner than
expected. this means that they did not regenerate properly.
Interestingly, coke deposition on platinum and palladium
might lead to loss of ethylene selectivity and increased
ethane production. If only 0.2 mol% ethylene of 10th olefine
in JAM complex converts to ethane in 1 day, we may lose
$28,800 per day which is approximately $10 million per year.
A novel configuration was proposed to regenerate each bed
individually. In fig. 3 and fig. 4 the reactor temperature and
model outputs for both beds during air injection stage after
pipeline reconfiguration are compared. In this stage, air was
injected continuously to avoid sudden upsurges in the bed
temperature which could be harmful for catalysts. As shown
in these figures, not only the coke burning process for the
guard bed lasted less than the lead bed, but also the lead
bed maximum temperature was significantly higher than the
guard bed one. These events are owing to the coke load
accumulated in the reactors [12].
Fig. 1. lead bed's temperature profile(new pipe line)
Fig. 3. guard bed (newpipe line)
Fig. 2. lead bed's temperature profile(new pipe line)
Fig. 4. lead bed
(newpipe line)
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