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.

How to maximize conversion: A practical approach

Chemical processes are designed to give maximum efficiency. Here, efficiency referred to as conversion of raw material into products.

If we can’t approach to total conversion, at least it’s possible to reach as close as possible to maximum conversion that can achieve.

In the chemical industry, people maintain & operate plant parameters based on their experience & monitoring the data.

But, how conversion increase in practical ?

Here, are some practical approaches to consider, when to think about the increase in conversion.

On a molecular scale, all matters on how much energy we are giving to the reaction.

A general practice can we make while increasing the energy rate we are providing, In the process we are providing reaction to the energy in the forms of (temperature, pressure, and flow), while changing & operating these forms or parameters like Temperature, pressure and flow as we can get to the maximum possible conversion we can achieve.

Providing extra space for reaction

Often for many chemical processes, we are using a catalyst. As we know catalyst decrease activation energy but also providing the extra space for a reaction where reactant molecules get accumulated in pores of catalyst & get reacted to form a product. This depends on the reaction & based on the size & shape of the catalyst.

Catalyst shape is selected for a particular reaction, with the observation of experimental trials using different shapes.

Removing resistance for the reaction

Reactant molecules should be contacted with each other for the maximum conversion. After 1st stage reaction when the reactant gets partially converted (25, 50 or more %).

To achieve maximum possible total conversion, we need to remove the unconverted reactant from the reaction mass. In 2nd stage reaction, product molecules will be the resistance for the unconverted reactant molecules to contact with each other.

Reactant surface area

When reactant having less surface area, it’s difficult to transferring energy to each molecule. Surface area is the important factor in some reaction. It could be a reaction rate controlling factor.

Increasing surface area affects the conversion & we can ensure the maximum energy is transferring to each reactant molecules.

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.


Type in the comments section below & you can also contribute to the blog.


Contact us if you willing to contribute to this website.