Question

Methane reacts with chlorine to produce methyl chloride and hydrogen chloride. Once formed, the methyl chloride may undergo further chlorination to form methylene chloride, chloroform, and carbon tetrachloride. A methyl chloride production process consists of a reactor, a condenser, a distillation column, and an absorption column. A gas stream containing 80 mole% methane and the balance chlorine is fed to the reactor. In the reactor, a single-pass chlorine conversion of essentially 100% is attained, the mole ratio of methyl chloride to methylene chloride in the product is 5:1, and negligible amounts of chloroform and carbon tetrachloride are formed. The product stream flows to the condenser. Two streams emerge from the condenser: the liquid condensate, which contains essentially all of the methyl chloride and methylene chloride in the reactor effluent, and a gas containing the methane and hydrogen chloride. The condensate goes to the distillation column in which the two component species are separated. The gas leaving the condenser flows to the absorption column where it contacts an aqueous solution. The solution absorbs essentially all of the hydrogen chloride and none of the methane in the feed. The liquid leaving the absorber is pumped elsewhere in the plant for further processing, and the methane is recycled to join the fresh feed to the process (a mixture of methane and chlorine). The combined stream is the feed to the reactor.

Using the information provided do the following

Carefully draw and label the process flow diagram

Using a basis of 100 total moles of methane and chlorine calculate the composition and molar flowrate of all the streams.

Using scaling relations to answer part (c).....what molar flow rates and compositions of the fresh feed and recycle stream are required to achieve a methyl chloride production of 1000kg/h?

Answer 1

Balanced chemical reactions are

Methane and chlorine produces methyl chloride and hydrogen chloride

CH₄ + Cl₂ = CH₃Cl + HCl

Methane + chlorine = methyl chloride + hydrogen chloride

methyl chloride produces methylene chloride, chloroform, and carbon tetrachloride

CH₃Cl + Cl₂ = CH₂Cl₂ + HCl

methyl chloride + chlorine = methylene chloride + hydrogen chloride
CH₂Cl₂ + Cl₂ = CHCl₃ + HCl

methylene chloride + chlorine = chloroform + hydrogen chloride
CHCl₃ + Cl₂ = CCl₄ + HCl

chloroform + chlorine = carbon tetrachloride + hydrogen chloride

In the feed

Mole% of methane = 80%

Mol% of chlorine = 20%

Mol ratio of methane to chlorine = 80:20 = 4:1

mole ratio of methyl chloride to methylene chloride in the product = 5:1

Mass flow rate of methyl chloride = 1000 kg/h

Molar flow rate of methyl chloride

= mass/molecular weight

= (1000 kg/h) /(50.49 kg/kmol)

= 19.8 kmol/h

Process flow diagram

CH3Cl balance around condenser and distillation(Still) column

5 n6 = 19.8

n6 = 19.8/5 = 3.96 kmol/h

Overall Carbon balance

n1 = n6 + 19.8 = 3.96 + 19.8 = 23.76 kmol/h

Overall H balance

4 n1 = 3*19.8 + 2 n6 + n5

4*23.76 = 3*19.8 + 2*3.96 + n5

n5 = 27.72 kmol/h

Overall chlorine balance

n2 + n2 = 19.8 + 2 n6 + n5

2 n2 = 19.8 + 2*3.96 + n5 = 27.72 + 27.72

n2 = 27.72 kmol/h

Material balance at mixing point of recycled stream

n4 + n1 = 4 n2

n4 + 23.76 = 4*27.72

n4 = 87.12 kmol/h

Molar flow rate of fresh feed stream = n1 + n2

= 23.76 + 27.72 = 51.48 kmol/h

Mol% of CH4 = 23.76*100/51.48 = 46.15%

Mol% of Cl2 = 27.72*100/51.48 = 53.85%

Molar flow rate of recycled stream = n4 = 87.12 kmol/h

Molar flow of HCl = n5 = 27.72 kmol/h