Question

In: Chemistry

1000 mol/min of a mixture of 90 mol% air and 10 mol% HCl enters an absorber...

1000 mol/min of a mixture of 90 mol% air and 10 mol% HCl enters an absorber column and is contacted with a stream of pure water entering the column at the rate 300 moles/min. 90% of HCl in the feed gas is absorbed by water. The two exit streams are gas stream (air+HCl) and liquid stream (water+HCl), respectively.

1. Draw and label a flowchart.

2. Define a basis of calculation.

3. Carry out a degree of freedom analysis and determine flow rates in mole/min and compositions in mole fractions of the exit streams.

Solutions

Expert Solution

1. Flowchart

2. Basis of calculation

Purpose of the process: To prepare aqueous mixture of HCL + water.

Required calculations: Finding the mass flow rates of streams 1 and 2 and composition of stream 3 and 4.

Material Balance Equations: There are 4 independent material balance equations:

Total mass balance ? m1 + m2 = 1300 mol/min
air (g) + Hcl (g) balance ? (1000mol/min) m1
water balance ? (300mol/min) m2

3 . Since there is no chemical reaction, the generation and consumption terms are zero, and the balance equation for steady-state physical process will be:

Input = Output  

There are two unknowns - Input HCl(g) and output HCl(aq.) - and since there are two substances - HCl and H2O - in the input and output streams, two balances may be written to solve for them. The total mass balance and the water balance involve both unknowns, but the Hcl balance involves only one.

HCl Balance

mol% In = mol% out

1000 mol/min of mixture contains 10% HCL (g) = 1000 x 10 /100 = 100 mol/min HCl (g)

90% of HCl(g) absorbe in water stream moving in 300mol/min

therfore

the actual HCl (g) dissolve in water stream = 100 x 90 / 100 = 90 mol/min (in aqueous solution)

The concentration of HCl (g) remains in gas stream (Out stream)

HCl (g) In stream = HCl (aq) + HCl (g)

100 mol/ min = 90 mol/min + 10 mol/min

Degree of freedom analysis:

A problem with just the right amount of information is uniquely specified. A degree of freedom analysis is used to determine whether a process is uniquely specified.

To perform a degree of freedom analysis for a given system, we must know:

N(unknown) find the number of unknowns in the problem
+N(reaction) add the number of reactions in the system
-N(material) subtract the number of independent material balances

-N(other)and subtract the number of other relationships given in the problem statement
If Ndof equals zero, the problem is uniquely specified. If is Ndof greater than zero, the problem is underspecified. Finally, as you may have guessed, if Ndof is less than zero, the problem is overspecified.

For this examples N dot equals Zero.

queen_honey_blossom answered 2 years ago
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