Interpass absorption process

IPA [Interpass absorption] Also called Double absorption, and Double catalysis. An improved version of the Contact process for making sulfuric acid, by which the efficiency of the conversion of sulfur to sulfuric acid is increased from 98 percent to over 99.5 percent. 

The contact process was invented by Phillips in England in 1831 but was not used commercially until many years later. Today 99% of all sulfuric acid is manufactured by this method. It was developed mainly because of the demand for stronger acid. All new contact plants use interpass absorption, also known as double absorption or double catalysis. This process will be described in detail in Fig. 2.3. 

FIGURE 9.1 I Schematic diagram of the converter section of a contact sulfuric acid plant employing an interpass absorption system for both better sulfur conversion and emission abatement. Product acid (or oleum) is cooled indirectly with process water prior to storage for sale. 

Apart from the benefits of the high pressure mixing of SO3 with water to produce sulfuric acid, the proposed cold process for the manufacture of sulfuric acid has also been conceived to avoid the complexity of requiring a sulfur furnace and the related heat recovery system, the multipass static converter, counter current heat exchangers, the interpass absorption tower (IPAT), drying tower (DT), final absorption tower (FAT), mist eliminators, acid coolers, and alkali scrubber. The resulting plant is, as a result, of much lower cost in equipment and land use. 

Minimise the escape of unreacted inputs from the plant by better process. A good example is the modified 3 + 2 DCDA process for the production of sulphuric acid wherein five catalyst beds are used instead of four. Three beds are used before the interpass absorption tower and two are after it. This results in the conversion of up to 99.85 % of SO2 to SO3. By providing separate acid circuit for the final absorption tower, the emission of SO2 in exit gases can also be brought down further as compared to the earlier design where aU acid towers had a common circulation tank. 

Absorption/distillation towers shall have packings with glazing/non-corrosive material to prevent contamination of process streams which shall be tested for compatibility with process fluids at operating conditions (e.g. the tower packing in any of the drying, interpass absorption or final absorption towers of the sulphuric acid plant shall not get affected by 98.5 % sulphuric acid at 120 °C). 

Manufacture of sulphuric acid by the modified 3 + 2 DCDA process (having three catalyst passes before interpass absorption tower and two passes after it) with caesium-promoted catalyst resulted in better conversion of SO2 to SO3 and negligible escape of SO2 from exit gases. An alkali scrubber for tail gases was provided for taking care of this. 

Following the primary converter, the process gas passes through an interpass absorption tower. This tower removes SO3 from the process gas to provide improved equilibrium conditions for further oxidation of SO2 to SO3 in a second converter. SO3 produced in the second converter is absorbed in the final absorber. 

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