Hydrogen cyanide (HCN) is an important precursor in the production of organic po
ID: 525860 • Letter: H
Question
Hydrogen cyanide (HCN) is an important precursor in the production of organic polymers. e.g. methyl methacrylate (used to produce Plexiglas). One method of producing hydrogen cyanide is via the catalytic reaction of nitrogen and acetylene. The reaction is given by: N_2 (g) + C_2 H-2 (g) rightarrow 2 HCN (g) Due to thermodynamic constraints the single pass conversion is low (15%), and therefore a recycle stream is required. The gas stream exiting the reactor is cooled in a condenser to produce a product stream and a recycle stream. A schematic of the reactor is shown below If it is desired to produce 15,000 kg/hr of HCN, determine: The number of moles per hour and the percentage composition of the material leaving the reactor (point X). The recycle to fresh feed ratio.Explanation / Answer
Flow rate of HCN= 15000 kg/hr, molar flow rate of HCN= mass flow rate/ molar mass = 15000/54 kg moles/hr= 278 moles/hr
From the reaction given N2+C2H2----->2HCN
2 mole of HCN requires 1 mole of N2 and 1 mole of C2H2
Writing overall balance, molar flow rate of each of N2 and HCN= 278 kgmoles/hr
Total flow rate of fresh feed= 278+278= 556 kg moles/hr
writing balance after reactor
X= R+278, R= X-278
Feed entering the Reactor = 278+278+R
= (R+556)
Percentage conversion is 15%. 2 moles of N2 and C2H2 together give 2 mole of HCN
(R+556)*0.15= 278, R= 1297 moles/hr
Unreacted , X= 1297+278= 1575 moles/hr
R= 1575-278= 1297 moles/hr, Ratio of recycle to feed= 1297/278= 4.66
1 mole of N2 and 1 mole of C2H2 react,
Composition of N2 and C2H2 remain at 0.5 :0.5 molar ratio.
At X , moles of HCN= 278 kgmoles/hr, Rest is R at 1297 moles/hr at 0.5 : 0.5 ratio
Hence molar flow rate ( kgmoles/hr) : HCN= 278 , N2 and C2H2= 1297*0.5= 648.5
Composition at X : HCN= 278/1575= 0.18, N2 = C2H2= 648.5/1575= 0.42
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