NOx Risk Mitigation for the Co-Feeding of Natural Gas to the Sasol CTL Complex

Natural gas has been a co-feedstock to the reformers in the Sasol CTL complex since 2004. A more beneficial way is to re-route natural gas to the cryogenic separation unit and thus only reform the separated methane in the natural gas feed.

However, introducing NO_x into the cryogenic separation unit where conjugated diolefins from the tail gas of the CTL complex are present creates the risk of explosive NO_x hydrocarbon gum formation. The steps taken on the project to quantify and mitigate the NO_x safety risks are described below.

The first step was to establish whether NO_x is present in the natural gas feed via dry calorimetry. Although the majority of the results were below the detection limit, the presence of NO_x in the natural gas feed could not be ruled out, so a NO_x guard bed was added to the design. A novel adsorption technology was chosen versus the more conventional catalytic process to act as a NO₂ guard bed. In order to monitor the effectiveness of the guard bed several cold liquid streams downstream of the guard bed will be analyzed for NO₂. From theoretical calculations it was established that the separated liquids will contain up to ten times higher concentrations of NO₂ compared to the gaseous inlet stream. Detection of NO₂ in the liquid streams will thus indicate a high level of NO₂ in the feed if the removal system fails.

NO will fractionate with methane, hence it will not come into contact with conjugated diolefins. In addition a procedure of periodical washing the system with methanol has been proposed to remove NO_x hydrocarbon gum if it is formed. The procedure has been developed in such a way that the impact on production will be minimal. The methanol wash sample will be analyzed for any indications of NO_x hydrocarbon gum and the frequency of methanol wash will be adjusted based on the analytical results of NO_x downstream of the guard bed. As Guidance, a calculated maximum amount of 3lb NO_x accumulation will be allowed before a methanol wash must be carried out.