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.

Why commercial sulfuric acid having 98 to 99% concentration?

We are taking data of sulfuric acid vapor pressure at different concentration & temperature.

We take this data from Perry’s chemical engineering handbook, 7th edition.

Temperature	98	98.5	99	99.5
0	1.17E-08	7.68E-09	4.79E-09	3.13E-09
10	3.91E-08	2.61E-08	1.66E-08	1.13E-08
20	1.21E-07	8.12E-08	5.28E-08	3.73E-08
30	3.44E-07	2.34E-07	1.55E-07	1.14E-07
40	9.14E-07	6.30E-07	4.25E-07	3.23E-07
50	2.28E-06	1.59E-06	1.09E-06	8.61E-07
60	5.38E-06	3.79E-06	2.64E-06	2.16E-06
70	1.20E-05	8.56E-06	6.05E-06	5.14E-06
80	2.57E-05	1.84E-05	1.32E-05	1.17E-05
90	5.24E-05	3.90E-05	2.77E-05	2.53E-05
100	1.03E-04	7.51E-05	5.55E-05	5.27E-05
110	1.94E-04	1.43E-04	1.07E-04	1.06E-04
120	3.54E-04	2.63E-04	2.01E-04	2.06E-04

In the sulfuric acid plant, a commercial grade of sulfuric acid is 98.5 to 99.0% concentration.

In DCDA (Double Contact double absorption) plant, produce sulfuric acid has higher concertation than older methods & also has low SO2 emissions. That’s why DCDA process is widely acceptable in present.

But,

Why is a commercial grade of sulfuric acid having 98.5 to 99.0% concentration?

In DCDA process, SO3 (sulfur trioxide) gas having maximum absorption in sulfuric acid at concentration between 98.5% to 99.0%.

From “Kirk-Othmer Encyclopedia of Chemical Technology, 5th Edition”

Looking at the DCDA process, where SO3 gas is absorbed in Sulfuric acid twice. During the absorption, the most controlled parameter is the concentration of sulfuric acid. From data of vapor pressure of sulfuric acid, it is found that it has the lowest vapor pressure at 98.0% to 99.5% concentration.

Sulfuric acid vapor pressure profile

The graphical representation of sulfuric acid vapor pressure at operating temperature from 0 to 120 deg C.

A graph represents sulfuric acid vapor pressure increase with temperature at respectively fixed concentrations. Vapor pressure is minimum for 99.0 & 99.5 % concentration.

During absorption operation, SO3 gas absorbed in sulfuric acid & that change the sulfuric acid concentration in the tower. We will plot the difference in vapor pressure during a change of concentration.

Here is the plot of Change in vapor pressure with the difference in concentration for 98.0 to 98.5, 98.5 to 99.0, 99.0 to 99.5% concentration.

Sulfuric acid vapor pressure profile

Between this concentration changes vapor pressure decrease.

Vapor pressure profile of 99.0 to 99.5 represent increase in vapor pressure after 100 C. So, we need to take account of concentration range where vapor pressure decreases with concentration increase & also having a minimum vapor pressure. So, here we can take concentration range of 98.5 to 99.0%, So, if we take concentrated 98.5% sulfuric acid in absorption towers & during absorption its percentage will increase, but that will decrease the vapor pressure & that does not affect the process.

We also need to observe in the field for the concentration range where the stack is minimum.

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.

Disposing sulfuric acid

sulfuric acid sludge

Sulphuric acid is most produced chemical in the world by volume. It has desirable chemical properties like

• Reaction with metals & non-metals 
• Strong oxidizing agent 
• Reaction with salt 
• Good dehydrating agent 

This property & it’s the least price make it commonly used chemical in industries like

• Fertilizers 
• Pharmaceuticals 
• Automobile batteries 
• Water treatment 
• Sulfonation agents 
• Cellulose fibers 
• Steel manufacturing 
• Coloring agents 
• Amino acid intermediates 
• Regeneration of ion exchange resins 

In this industries, sulfuric acid is used from primary to intermediate to after treatment processes.


This processes in the industry generate wastewater containing sulfuric acid sludge.

Sulfuric also used for household purposes in

• Drain cleaners 
• Car batteries 
• Fertilizers 

This stuffs sometimes creates problems for disposing of sulfuric acid

Click here to get how it could be a problem for disposing of sulfuric acid in your house.

Despite looking in every sector of industries, We will take a look at the sulfuric acid plant where it is produced & how can we dispose of it without any hazard.


In the sulfuric acid plant, acid spill or leakages are countable problems.

Because of its corrosive nature, leakages are common problems in the sulfuric acid plant.

During shut down of the sulfuric acid plant, cleaning of equipment generates wastewater containing acid mist.

Sulfuric acid & its mist both have toxic effects on the human body & environment.

Refer to its MSDS for more information under section 11 & 12.

Due to its toxic effects, It’s disposal should be fast as possible.

For disposal, it should be neutralized first.

For neutralization Water and calcium oxide (or hydroxide) can be used.

Sulfuric acid reacts with water is exothermic reaction & requires large quantities to neutralize compared to calcium oxide (or hydroxide).

The acid-base reaction can help us to solve the problem of acid neutralization.

Sulfuric acid forms a slightly soluble salt or precipitates with calcium oxide or hydroxide, the least expensive and most readily available base.

For household purposes we can use baking soda (NaHCO3) is also a base for acid neutralization.



Click here to get an idea of How much quantity of base needs to neutralize the acid.

After neutralization, it is safe for disposal.

After disposal to a safe location, there are also various techniques developed by company or scientist to recover the sulfuric acid from its sludge.


See patents published on a method for recovering sulfuric acid.

http://bit.ly/2IphiBV

http://bit.ly/2pavdDf



Recovery or recycling has a positive impact on the environment. we want to know how you think of recovery of sulfuric acid from its waste.


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