Many of the process streams in a refinery contain unsaturated hydrocarbons, sulfur compounds, nitrogen compounds and other materials (metals) that interfere with some of the refining processes. Hydrotreating is a catalytic process that makes use of by-product hydrogen from the catalytic reforming process, to saturate hydrocarbons (change a carbon-to carbon double bond to a carbon-to carbon single bond) by adding a hydrogen atom to each of the two carbon atoms. In doing so, the process removes sulfur, nitrogen and metals from a wide variety of process streams. In general, each of the major streams coming off the atmospheric tower will have a hydrotreating unit.