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Wednesday, September 25, 2013

Stoichiometry Problem - Material Balance, 16

Category: Chemical Engineering Math, Algebra

"Published in Suisun City, California, USA"

Fresh juice contains 15% solids and 85% water by weight and is to be concentrated to contain 40% solids by weight. In single-evaporation system, it is found that volatile constituents of juice escape with water leaving the concentrated juice with a flat taste. In order to overcome this problem, part of the fresh juice bypasses the evaporator. The operation is shown in the figure below

Photo by Math Principles in Everyday Life

 Calculate:
(a) The fraction of juice that bypasses the evaporator.
(b) The amount of concentrated juice produced (containing 40% solids) per 100 kg of fresh juice fed to the process.

Solution:

The given word problem is about evaporation of fresh juice to concentrated juice with bypassed fresh juice to be added to concentrated juice which involves the principles of Stoichiometry. The total amount of a substance in the reactants or incoming ingredients must be equal to the total amount of a substance in the final products. In short, the Law of Conservation of Mass must be followed all the time. To illustrate the given problem, it is better to label further the flow diagram as follows

Photo by Math Principles in Everyday Life

Basis: 100 kg of Fresh Juice

Let x = be the amount of Fresh Juice fed to evaporator
      y = be the amount of Fresh Juice bypasses the evaporator
      z = be the amount of juice leaving the evaporator
      p = be the amount of Concentrated Juice obtained

Overall Material Balance before Evaporator:


Overall Material Balance after Evaporator:


Material Balance of Solids over Evaporator:



Material Balance of Solids after Evaporator:


but


then the above equation becomes






Substitute the values of x and y to the first equation, we have





The value of x will be equal to




The value of y will be equal to




The value of p will be equal to




Therefore,





The amount of Concentrated Juice (40% solids) produced per 100 kg of Fresh Juice is 37.498 kg.


Tuesday, September 24, 2013

Stoichiometry Problem - Material Balance, 15

Category: Chemical Engineering Math, Algebra

"Published in Suisun City, California, USA"

A waste acid from a nitrating process contains 23% HNO3, 57% H2SO4, and 20% water by weight. This acid is to be concentrated to contain 27% HNO3, 60% H2SO4 by the addition of concentrated H2SO4 containing 93% H2SO4 and concentrated HNO3 containing 90% HNO3. Calculate the amounts in kg of waste and concentrated acids that must be combined to obtain 1,000 kg of desired mixture.

Solution:

The given word problem is about the mixing of waste and concentrated acids which involves the principles of Stoichiometry. The total amount of a substance in the reactants or incoming ingredients must be equal to the total amount of a substance in the final products. In short, the Law of Conservation of Mass must be followed all the time. Since all incoming substances are acids, then there's no chemical reactions involved in the mixture. To illustrate the given problem, it is better to draw the flow diagram as follows

Photo by Math Principles in Everyday Life

Basis: 1,000 kg of Desired Mixture

Let x = be the amount of Waste Acid
      y = be the amount of Concentrated Sulfuric Acid
      z = be the amount of Concentrated Nitric Acid

Overall Material Balance of Mixer:


Material Balance of H2SO4:





Material Balance of HNO3:



 
    

Substitute the values of y and z to the first equation, we have





Substitute the value of x to the second equation, we have



 

Substitute the value of x to the third equation, we have
 
 



Therefore,

Amount of Waste Acid = 418.618 kg
Amount of Conc. Sulfuric Acid = 388.548 kg
Amount of Conc. Nitric Acid = 192.834 kg