Methanol (CH3OH) reacts to form formaldehyde (HCHO) either by decomposition to formaldehyde and hydrogen (H2) or by oxidation to form formaldehyde and water:
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CH3OH → HCHO + H2
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(1)
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CH3OH + 0.5O2 → HCHO + H2O
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(2)
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A mixture containing 98 mol% methanol and 2 mol% of an inert contaminant is available as a feed to a formaldehyde plant. The fresh feed to a process consists of 1000 kmol/h of this mixture along with 200 kmol/h of oxygen. All of the oxygen is consumed in the reactor. The reactor output is sent to a separation unit, where formaldehyde, water and hydrogen are removed, and methanol and the contaminant are sent back to the reactor. To control the contaminant level in the reactor, a purge stream is taken off the recycle stream. The recycle stream that directly goes back to the reactor contains 4400 lbmol/h of methanol.
If the overall fractional conversion of methanol to products is 80%, determine (a) degree of freedom, (b) the single pass conversion of methanol in the reactor, (c) production rate of formaldehyde, (d) the recycle: purge ratio.