Editing Contact process (section)

== Double contact double absorption ==
The next step to the contact process is ''double contact double absorption'' (DCDA). In this process the product gases (SO<sub>2</sub>) and (SO<sub>3</sub>) are passed through absorption towers twice to achieve further absorption and conversion of SO<sub>2</sub> to SO<sub>3</sub> and production of higher grade sulfuric acid.

SO<sub>2</sub>-rich gases enter the catalytic converter, usually a tower with multiple catalyst beds, and are converted to SO<sub>3</sub>, achieving the first stage of conversion. The exit gases from this stage contain both SO<sub>2</sub> and SO<sub>3</sub> which are passed through intermediate absorption towers where sulfuric acid is trickled down packed columns and SO<sub>3</sub> reacts with water increasing the sulfuric acid concentration. Though SO<sub>2</sub> too passes through the tower it is unreactive and comes out of the absorption tower.

This stream of gas containing SO<sub>2</sub>, after necessary cooling is passed through the catalytic converter bed column again achieving up to 99.8% conversion of SO<sub>2</sub> to SO<sub>3</sub> and the gases are again passed through the final absorption column thus  achieving not only high conversion efficiency for SO<sub>2,</sub> but also enabling production of a higher concentration of sulfuric acid.

The industrial production of sulfuric acid involves proper control of temperatures and flow rates of the gases as both the conversion efficiency and absorption are dependent on these.