oa South African Journal of Chemical Engineering - Using a Turbine Expander to Recover Exothermic Reaction Heat - a Case Study on a Phthalic Anhydride Process
Phthalic anhydride (PA) is produced from alkyl-substituted- or multinuclear aromatic compounds such as o-xylene or aphthalene by partial oxidation. With reaction enthalpies as high as -1 793 kJ/mol naphthalene reacted, there are many opportunities for energy recovery making these systems attractive for process integration research.<p>The production of PA from naphthalene is used to investigate two energy recovery schemes. The first is a Rankine cycle that uses steam, raised during the cooling of the reactor, to produce power. The second also includes a turbine expander at the reactor exit. It expands hot reactor product gases to deliver additional power and reduces the pressure of the reactor product before it enters the separation section.<p>Simulation results showed that, under certain operating conditions, the steam cycle alone could not supply the feed compression power requirement. However, when a turbine expander is included in the process, all of the power requirements can be fulfilled and as much as 2 651 MJ/ton PA can be exported for use elsewhere.<p>Unlike power generation from fossil fuels, no carbon dioxide is emitted to the atmosphere when an expansion turbine is used. Savings in CO<sub>2</sub> emissions between 0.46 and 0.75 ton CO<sub>2</sub>/ton PA are estimated for this unit.<p> An exergy analysis showed that the thermodynamic loss of the process with the second energy recovery scheme is 4.5% lower than that of the first. The reason being that power can be recovered when expansion takes place in a turbine instead of an isenthalpic expansion valve.
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