Question

Fresh orange juice is largely water and solids though much of the flavor comes from volatile organics present in small amounts. In producing concentrated orange juice the trick is to drive off the water but not to loose the organics that produce the taste. If fresh orange juice can be thought of as containing 12.0% solids with the balance being water (ignoring the small amount of organics), concentrated orange juice contains 42.0% solids. Initially a single evaporation process was used for the concentration, but the volatile constituents of the juice escaped with the water, leaving the concentrate with a flat taste. The current process overcomes this to a degree by bypassing the evaporator with a fraction of the fresh juice. The juice that enters the evaporator is concentrated to 58% solids, and the evaporator product stream is mixed with the bypassed fresh juice to achieve the desired final concentration. Calculate the amount of of product (42% concentrate) made and the fraction of the feed that bypasses the concentrator. Why not increase the bypass even more to get greater flavor in the concentrate?

Answer 1

Basis : 100 kg of fresh juice

Fresh juice   contains 12% solids. Solids in the fresh juice =12*100/100 =12 kgs

Final product contains 12 kg solids

But the final products contains 42% solids. Hence

P*0.42=12

Where P= final product

P= 12/0.42=28.6 kg

Water evaporated =100*0.88-28.6 ( since only evaporator removes water and by-pass does not remove water) =59.4

Let x =amount by-passed

At mixing point (M)

A*0.58+x*0.12=   12   (1) ( solids balance

Dividing by 0.58 gives

A +x*0.12/0.58= 12/0.58

A+x*0.21 =21 (1A)

A*0.42+x*0.88= 16.6 (2)

Dividing by 0.42 gives

A+x*2.1x= 39.52   (2A)

Eq.2A – Eq.1A givs

x*(2.1-0.21)= 18.52

x= 9.8 kg

Fraction by-passed = 9.8%

feed sent to evaporator = 100-9.8= 91.2 kg

when more stream is by-passed, the feed after evaporator containing more than 58% solids now does not flow.