Hydrocarbon Processing - August 2021 - 51

Process Optimization
It affects the performance of the downstream units and the
whole economy of the OCM process, considering that the
quality of the methane, oxygen and the accompanying inert
components are crucial factors in determining the operation
and the economy of the OCM process.
The following parameters are the bases for analyzing the
impact of the feed specifications on the performance process.
Methane-to-oxygen ratio-It has been proved that, in
terms of methane conversion and ethylene selectivity and
yield, using lower methane-to-oxygen ratios is preferred to
target the highest ethylene yield with safe operations, i.e., securing
a safe operation of OCM reactors while reducing the
methane-to-oxygen ratio below 2 is essential. Under such
conditions of low methane-to-oxygen ratio, the potential
thermal integration with the dehydrogenation step of ethane
is manifested.
Methane feed conditioning-Injecting an inert component,
such as nitrogen or steam, for diluting purposes is usually
recommended to provide flexibility in controlling the thermal
performance of the OCM, which is considered a strongly
exothermic reaction. This will result in a better distribution of
oxygen along the catalytic bed, a relatively higher selectivity
and an improved ethylene yield.
Ethane byproduct and reactant-Considering the shared
products and complementary exothermic/endothermic thermal
nature of the OCM process with ethane cracking systems,
sharing the downstream units with an ethane cracking unit is
well justified. Therefore, the heat integrated reactor, combining
the exothermic OCM and endothermic ethane cracking
reaction systems, and shared unit operations are utilized. In
this integrated process, the increase of the C2
yield and ethylene-to-ethane
ratio by heat management of the OCM reactor
have been realized by establishing a proper temperature profile
along the bed and intensifying the gas phase dehydrogenation
reaction for maximizing ethylene yield.
Formation of ethane and COx
products. Produced methyl
radicals are coupled in the gas phase of the ethane production
step, undergoing homogeneous reactions. Ethane can be converted
into ethylene by oxidative dehydrogenation of C2
and ethyl radical (C2
H6
oxygen or in the gas phase.
The formation of COx
pathways:
C2
H6 -------- C2
CH3* + O2
CH3* + O2
- ------- CH3
H5) * by their interaction with surface
can occur by any of the following
H4 ------- CO, CO2
O- ------- CO, CO2
--------- CH3 O2* ------------ CO, CO2
The formation of total oxidation products CO and CO2
proceed mainly through the oxidation of CH3
occurs parallel to the formation of C2
H6
CO2
removal section. CO2
.
is generated as an undesired
and
in terms of
byproduct of the OCM reaction and should be separated
right after the reactor. Due to their corrosive nature, CO2
water are the first components to be separated in the OCM
downstream section. Suitable amine solutions are selected to
attain the highest removal performances of CO2
lowest regeneration energy consumption, and ethylene losses
in the absorber.
radicals. This
ABBAS EZZAT works as a Petrochemical Professor at
Pharos University and a Distinguished Scientist at the
Materials Science Department within the Institute of
Graduate Studies and Research at Alexandria University.
He is also a local consultant for the Egyptian petroleum
and petrochemical sectors and a Senior Associate Consultant
for Channoil Consulting Ltd. in London. Prior to joining
academia, he occupied several top management positions in the Egyptian
petroleum and petrochemical industries. Dr. Ezzat holds an MSc degree in
chemical engineering from Washington University and a PhD in petrochemical
applications from Alexandria University. He completed his postgraduate
studies in petroleum processing technologies from the School of Chemical
Engineering at Oklahoma State University.
Hydrocarbon Processing | AUGUST 2021 51
Takeaway. This technology is envisaged to meet challenges
with tangible economic advantages, driven by feedstock availability
and the extreme pricing dynamics of the energy industry.
Based on cost forecasts for natural gas, the gap between the
ethylene price obtained with conventional technologies and
the one obtained with OCM technology is expected to progressively
become smaller, forecasting OCM to be competitive
with traditional technologies in the near future.
Ethylene purification section. Ethylene purification is an
energy intensive process. The separation occurs in three columns:
the demethanizer, C2
-splitter and C3
+ recovery. The
optimum design of this section has a significant impact on the
economy of the whole process.
Demethanizer. The methane conversion in the OCM process
is unavoidably low (i.e., < 50%). Consequently, a significant
amount of the remaining unreacted methane should be
separated from the rest of the gaseous species. Conventionally,
this is performed via energy-intensive cryogenic distillation,
which stands as a major source of energy deficiency in the
OCM process.
Given the high concentration of methane and other light
, CO, etc.) in the feed stream to be separated in the
gases (H2
demethanizer, a high pressure is required to efficiently fulfill
the condensation task. Normally, liquid methane has been used
as a cryogenic refrigeration utility, which is required for partial
condensation upstream (CH4
, H2, N2, CO) in the condenser.
The first cascade in the refrigeration cycle is methane; two
other cascades operating with ethylene and propylene are used
for efficiently closing the refrigeration cycle and internally cooling
the media. This enables establishing a condenser temperature
of about -112°C under 33.5 bar operating pressure in an
8.5-m diameter distillation column. The separation performance
of this system is designed in a way to secure traces of methane in
the C2
-rich stream and minimize the ethylene loss to < 0.3%.
Ethylene/ethane splitter. A similar approach was applied
in the C2
splitter for separating ethane from ethylene under 20
bar pressure, utilizing propylene refrigerant for establishing a
condenser temperature of about −29°C using a distillate-tofeed
ratio of 0.65 to secure ethylene product purity of 99.5%.
Heat integration. The optimum utilization of the heat
sources and sinks in the OCM process are key factors to improve
the economy and energy performance of the process,
utilizing the significant generated OCM reaction heat in the
EDH reactor section.
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