134
the SMART SM™ process based on the Classic SM™ by ABBLummus/UOP
[146]. In this process, ethylbenzene is dehydrogenated in the presence of molecular
oxygen resulting in the formation of water and styrene. The heat required for the
dehydrogenation reaction is supplied by the oxidative dehydrogenation reaction.
This modification results in the reduction of the superheated steam requirements
and eliminates the need for the costly interstage reheater and also results in EB
conversion of higher than 80% with 99.85 wt% purity of styrene monomer.
Auto-oxidation of Ethylbenzene, Followed by Dehydration (SMPO Process)
The process developed and used by Royal Dutch SHELL is another major styrene
production route and accounts for 15% of the styrene produced globally. It is an
altogether different approach to styrene [151]. Propylene epoxide is coproduced
along with styrene, starting from ethylbenzene [154]. The process comprises autooxidation of ethylbenzene to ethylbenzene hydroperoxide (90% selectivity) in the
presence of air over a catalyst mixture consisting of zinc and copper oxides. The
conversion of EB (13%) is kept low to minimize the formation of byproducts. The
oxygen transfer between the ethylbenzene hydroperoxide and propene then leads to
the formation of the corresponding 1-phenylethanol with >70% selectivity and
propene- epoxide with 70–85% selectivity in the presence of a metallic catalyst
along with acetophenone (5–7% selectivity). The metallic catalysts used are molybdenum in liquid-phase (ARCO) [147] or heterogeneous titanium catalyst (SHELL)
[155]. The acetophenone co-produced in the oxygen transfer process is also hydrogenated in the liquid phase to 1-phenylethanol to improve the yields. A selectivity
of 92% for 1-phenylethanol is obtained at acetophenone conversion of 90%.
Subsequently, styrene is formed by gas-phase dehydration of 1-phenylethanol over
highly selective titania/silica catalyst at 573 K, after which styrene monomer purity
of 99.7 wt% is typically obtained in the process. The ratio of styrene to propylene
epoxide produced in this process is approximately 2–2.5. The description of the
flow scheme of a typical ethylbenzene auto-oxidation plant containing the following
process steps and units is given below [155], and the flow scheme is given in Fig. 12.
The plant consists of the auto-oxidation reactor, where the EB oxidation takes
place in the presence of air or oxygen to produce ethylbenzene hydroperoxide, and
the second reactor, an epoxidation reactor, where the transfer of oxygen from the
obtained hydroperoxide to propene takes place to produce 1-phenyl ethanol and
propylene oxide. Both reactors are maintained at a temperature of 373–403 K and
pressure in the range of 20–50 bar. The effluent of the second reactor is sent to the
first distillation column where the unreacted propylene is separated from the propylene epoxide/1-phenylethanol mixture, and this effluent from the first distillation
column is subsequently fed to the second distillation column for the recovery of
propylene epoxide. The last step is the conversion of 1-phenylethanol to styrene in
the dehydration reactor. The SMPO is a “cleaner” and much less energy-intensive
process as compared to the conventional dehydrogenation route (less by-products).
However, the serious drawback of this combined reaction is that capacity of styrene
S. M. Pai et al.
the SMART SM™ process based on the Classic SM™ by ABBLummus/UOP
[146]. In this process, ethylbenzene is dehydrogenated in the presence of molecular
oxygen resulting in the formation of water and styrene. The heat required for the
dehydrogenation reaction is supplied by the oxidative dehydrogenation reaction.
This modification results in the reduction of the superheated steam requirements
and eliminates the need for the costly interstage reheater and also results in EB
conversion of higher than 80% with 99.85 wt% purity of styrene monomer.
Auto-oxidation of Ethylbenzene, Followed by Dehydration (SMPO Process)
The process developed and used by Royal Dutch SHELL is another major styrene
production route and accounts for 15% of the styrene produced globally. It is an
altogether different approach to styrene [151]. Propylene epoxide is coproduced
along with styrene, starting from ethylbenzene [154]. The process comprises autooxidation of ethylbenzene to ethylbenzene hydroperoxide (90% selectivity) in the
presence of air over a catalyst mixture consisting of zinc and copper oxides. The
conversion of EB (13%) is kept low to minimize the formation of byproducts. The
oxygen transfer between the ethylbenzene hydroperoxide and propene then leads to
the formation of the corresponding 1-phenylethanol with >70% selectivity and
propene- epoxide with 70–85% selectivity in the presence of a metallic catalyst
along with acetophenone (5–7% selectivity). The metallic catalysts used are molybdenum in liquid-phase (ARCO) [147] or heterogeneous titanium catalyst (SHELL)
[155]. The acetophenone co-produced in the oxygen transfer process is also hydrogenated in the liquid phase to 1-phenylethanol to improve the yields. A selectivity
of 92% for 1-phenylethanol is obtained at acetophenone conversion of 90%.
Subsequently, styrene is formed by gas-phase dehydration of 1-phenylethanol over
highly selective titania/silica catalyst at 573 K, after which styrene monomer purity
of 99.7 wt% is typically obtained in the process. The ratio of styrene to propylene
epoxide produced in this process is approximately 2–2.5. The description of the
flow scheme of a typical ethylbenzene auto-oxidation plant containing the following
process steps and units is given below [155], and the flow scheme is given in Fig. 12.
The plant consists of the auto-oxidation reactor, where the EB oxidation takes
place in the presence of air or oxygen to produce ethylbenzene hydroperoxide, and
the second reactor, an epoxidation reactor, where the transfer of oxygen from the
obtained hydroperoxide to propene takes place to produce 1-phenyl ethanol and
propylene oxide. Both reactors are maintained at a temperature of 373–403 K and
pressure in the range of 20–50 bar. The effluent of the second reactor is sent to the
first distillation column where the unreacted propylene is separated from the propylene epoxide/1-phenylethanol mixture, and this effluent from the first distillation
column is subsequently fed to the second distillation column for the recovery of
propylene epoxide. The last step is the conversion of 1-phenylethanol to styrene in
the dehydration reactor. The SMPO is a “cleaner” and much less energy-intensive
process as compared to the conventional dehydrogenation route (less by-products).
However, the serious drawback of this combined reaction is that capacity of styrene
S. M. Pai et al.