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

AIChE Spring Meeting and Global Congress on Process Safety
April 4, 2012 - 12:00am

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

The first step was to establish whether NOx is present in the natural gas feed via dry calorimetry. Although the majority of the results were below the detection limit, the presence of NOx in the natural gas feed could not be ruled out, so a NOx 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 NOx 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 NOx hydrocarbon gum and the frequency of methanol wash will be adjusted based on the analytical results of NOx downstream of the guard bed. As Guidance, a calculated maximum amount of 3lb NOx accumulation will be allowed before a methanol wash must be carried out.

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