Application
To control the air to acid gas ratio to the SRU, the ABC system has to be developed for assuring the correct H2S content in the gas to the SUPERCLAUS reactor stage. The ABC system can also be applied to control the H2S/SO2 ratio in a conventional Claus plant.
Description To cope with the flow rate variations, both the conventional control system and the Advanced Control System (ABC) use a feed-forward (ratio) control scheme. The conventional control system controls the air through the main air valve in ratio to the acid gas flow. It has the disadvantage that the main air valve is big and therefore slow. A step change in acid gas flow or composition does not result in a step change in the main air flow, and leads to an incorrect ratio during the (long) response time of the main air valve. The ABC system reacts faster to changes in the acid gas flow rate and provides superior process control and system performance.
The (total) flow controller (FC) manipulates the small (and thus quick) trim air valve, which operates parallel to the big main air valve. To prevent system upsets during a large flow rate disturbance, a trim valve position controller re-adjusts the trim air valve position to neutral by manipulating the main air valve.
The second type is a disturbance in the composition of the sour gas. A composition change of the sour gas results in a change in air demand. Both the conventional control scheme and ABC control schemes use a feedback system to compensate for acid gas composition changes. The conventional control scheme, however, is not able to react precisely to changes in flow rate and gas composition at the same time. The ABC system automatically adjusts the design air to acid gas ratio. A change in acid gas flow rate now results (through the adapted ratio) in the correct air demand, without any further re-adjustment from the quality controller.
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