Sulfuric acid absorptive capacity of SO3 gas

Temperature & concentration disturbance changes in sulfuric acid vapor pressure. A schematic diagram is shown for absorption of SO3 gas into sulfuric acid.

Sulfuric acid plant: Absorption tower

At a constant flow rate of sulfuric acid & SO3 gas, a controlled parameter of absorption process is temperature & concentration of sulfuric acid.

Now, look at the rate constant equation of absorption process.

H2SO4 → SO3 + H2O

Equilibrium Rate constant Kp = (P SO3 X P H2O) / P H2SO4

From this equation, we can understand

Kp > 1, No further absorption of SO3 gas

Kp < 1, Absorption can take place

Now, because the constant flow rate of acid & gas, temperature & concentration of acid change the value of equilibrium rate constant Kp. We can say that Kp indicates the absorptive capacity of sulfuric acid for SO3 gas.

At higher concentration & temperature Kp will increase, decrease the absorptive capacity of sulfuric acid leads to stack gas emission in the sulfuric acid plant.

During absorption operation, the sulfuric acid temperature in tower bottom outlet leads to change its properties than the top area of the tower. SO3 gas is first getting in contact with tower bottom portion of acid. So acid temperature in tower bottom portion is a significant factor of SO3 absorption.

Controlling acid tower outlet temperature will give us significant benefits in operation & metallurgical aspects of the tower.

Calculating low melting liquid sulfur trioxide

Liquid sulfur trioxide

Sulfur trioxide having monomeric and polymeric forms in liquid and gas state respectively.

It is produced by the heating of oleum followed by condensation process.

In this process produced liquid SO3 having traces of sulfuric acid.

Sulfuric acid enters into sulfur trioxide during heating of oleum.

Heating at higher temperature than oleum boiling point causes sulfuric acid enters into liquid sulfur trioxide.

A lab result of liquid sulfur trioxide gives us pure SO3 % content in the sample. Other than SO3 content remaining is sulfuric acid as residue.

This sulfuric acid molecule provides additional bonding between SO3 molecules & make it low melting SO3.

Sulfur trioxide molecules come together with additional bonding caused by sulfuric acid molecules.

We will calculate how much amount of sulfuric acid forms low melting of SO3?

For 1 ton of SO3 content it lab result is 99.51% of pure SO3 & remaining as acid content.

Pure SO3 content = 995.1 kg = 12.44 Kmol

1 mol = total 6.023E+23 molecules

1 kmol = total 6.023E+26 molecules

Total SO3 molecules = (12.44 kmol) ✖ (6.023E+23) = 7.49E+27 molecules

Considering S3O9 forms of Liquid SO3

Total S3O9 molecules = (7.49E+27 / 3) = 2.50E+27 molecules
 
SO3 hydrogen bond acceptor count = 3


H2SO4 content in SO3 sample = 0.49 % of total SO3 = 4.9 kg = 0.05 kmol



1 mol = total 6.023E+23 molecules

1 kmol = total 6.023E+26 molecules

 
Total H2SO4 molecules = (0.05 kmol) ✖ (6.023E+23) = 3.01E+25 molecules

Total additional bonds created with sulfuric acid content

= 3.01E+25

 
A typical bonding between S3O9 form create 1 poly molecule shown below.

S3O9-------S3O9-------S3O9-------S3O9



Total poly molecules will be created = (3.01E+25) / 3 = 1.00E+25 poly molecules



1 poly molecule has 4 S3O9 molecule

So, total S3O9 molecules take participate in poly molecule will be

= (1.00E+25) ✖ 4

= 4.02E+25 S3O9 molecules take participate in creating poly molecules

= (4.02E+25) ✖ 3

= 1.20E+26 SO3 molecules take participate in creating poly molecules

= (1.20E+26) / (6.023E+26)

= 0.20 kmol of SO3

= 0.20 ✖ 80

= 16 kg of SO3


So, Total 16 kg of SO3 out of total 1 ton of liquid So3 having 99.51 % of pure SO3 will be of low melting.

This change can’t be observed in continuous process.

Temperature cycle & residence time could be effect on formation of low melting So3.

This is the simple calculation based on hypothetical concept of bonding between molecules.


This is all hypothetical calculation. It may be not meet with the real experimental data, but this calculation or fundamentals give us an opportunity to think further on this topic.