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.
0 Comments: