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CHE1002 - Process Calculations

Dr. S. K. Behera School of Chemical Engineering Vellore Institute of Technology, Vellore, TN, India – 632 014.

Module 3 Material balance without chemical reactions

Mixing/Blending

This operation is carried out in the chemical industry to obtain a product of the desired quality by mixing weak and

concentrated streams. Example: Preparation of a desired mixed acid

Overall Material balance: Feed stream 1 + Feed stream 2 + Feed stream 3 = Desired product stream

If the feed stream 1 contains components A, B and C; feed stream 2 contains A and C; feed stream 3 contains components B and C; product stream contains A, B and C; then:

Material balance of A: A in Feed stream 1 + A in Feed stream 2 = A in desired product stream

Material balance of B: B in Feed stream 1 + B in Feed stream 3 = B in desired product stream

Problem -1 Waste acid from a nitrating process contains 25% HNO3, 55% H2SO4 and 20% H2O by weight. This is to be concentrated to get fortified acid containing 27% HNO3, 60% H2SO4 and 13% H2O. This is done by adding concentrated H2SO4 of strength 93% H2SO4 and concentrated HNO3 of strength 90% HNO3 in suitable quantities to the waste acid. If 1500 kg of fortified acid is to be produced, calculate the kg of various solutions mixed.

‘x’- kg of waste acid, ‘y’– kg of aqueous H2SO4 & ‘z’ – kg of aqueous HNO3

Overall Material balance:

Material balance for H2SO4:

Material balance for HNO3:

Solving these three equation, we get

Problem - 2 It is required to make 1000 kg mixed acid containing 60% H2SO4, 32% HNO3 and 8% water by blending: (i) Spent acid containing 11.3% HNO3, 44.4% H2SO4 and 44.3% H2O, (ii) Aqueous 90% HNO3 and (iii) Aqueous 98% H2SO4 All percentages are by weight. Calculate the quantities of each of three acids required for blending.

‘x’ – kg of waste acid ‘y’ – kg of aqueous H2SO4 ‘z’ – kg of aqueous HNO3

Overall Material balance:

Material balance for H2SO4:

Material balance for HNO3:

Solving these three equation, we get

Practice Problem 1000 kg of mixed acid of composition 40% H2SO4, 45% HNO3 and 15% H2O is to be produced by strengthening waste acid of composition 30% H2SO4, 36% HNO3 and 34% H2O by weight. Concentrated sulfuric acid of strength 95% and concentrated nitric acid containing 80% are

available for this purpose. How many kilograms of spent acid and concentrated acids are to be mixed together?

Practice Problem It is required to make 1000 kg mixed acid containing 60% H2SO4, 32% HNO3 and 8% water by blending (i) spent acid containing 10% HNO3, 50% H2SO4, 40% H2O (ii) aqueous 90% HNO3 (iii) aqueous 98% H2SO4. All percentages are by weight. Calculate the quantities of each

of three acids required for blending.