Sulphur Recovery Process
This refinery case study concerned the recovery of sulphur from acid gases coming from an Amine Regeneration Column using a EuroClaus (catalytic reduction of SO2) and a SuperClaus process (selective oxidation) as shown alongside. The control objective was to keep the H2S concentration in the tail gas to 0.9% volume at the input to the SuperClaus stage in order to maximise the efficiency of this process. The ultimate objective was to minimise the amount of H2S finally entering the atmosphere to comply with emissions regulations.
Aimed to maintain a desired ratio between air (required for combustion and supplied by blowers) and acid gas. By measuring the acid gas flow rate, the expected air demand was calculated for complete combustion and the air flow rate controlled using a double valve arrangement. The set point of H2S was adjusted from lab analysis to compensate for possible instrumentation errors.
The accompanying figure shows a comparative performance under stable load changed, the time lag between air entering, the flow rate changing and the air/gas ratio changing, resulted in large peaks in H2S under PID control. This required the operator to make any changes in load gradual, waiting at least 10 minutes between incremental changes in the gas flow rate to moderate the acid gas peaks.
Under ADEX control there is no peaks problem and load changes are not restricted.
