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CALGARY, Alberta, Canada, – Galvanic Applied Sciences, Inc. reveals an innovative new control strategy for sulfur recovery units: DOUBLE AIR DEMAND (DAD).

DAD is designed to overcome the limitations of traditional SRU control strategies. Tail gas analysis is inherently insufficient due to long process lag time. Feed forward analysis is prone to interferences and becomes expensive when monitoring multiple acid gas streams.

DAD incorporates two measurements, one at the usual location at the end of the catalytic stage (tail gas), with an additional analyzer located at the outlet of the first condenser. External programmed logic synchronizes the DAD output signals with the acid gas and air flow ratio control. The benefit of using two analyzers is that if one fails or is down for scheduled maintenance, the plant is still controlled automatically.

“The DAD control strategy results in a more accurate and faster response to changes in feed gas composition,” comments Parisa Akhshi, senior portfolio manager. “This minimizes SO2 emissions and optimizes sulfur recovery.”

SRU upsets are easily handled using DAD thanks to the rapid response time. For example, selective oxidation tail gas treating units (TGTU) are bypassed if H2S exceeds limits. Regenerative TGTUs are bypassed if increased SO2 levels are detected from reduction reactor breakthroughs. This leads to plugging, corrosion, and damage throughout the TGTU. Recovering from sour water stripper excursions is even more complicated and requires extensive cleaning and repair activities. Excessive downtime forces reduced upstream capacity in hydrocarbon processing units.

Implementing DAD requires an air demand analyzer to handle the difficult upstream conditions present at the outlet of the first condenser. Sulfur vapor is five times higher and the sample temperature is significantly greater compared to the tail gas. In order to successfully operate at this location, robust sample handling is critical. The following factors greatly contribute to the success of an installation after the first condenser. 

  • Close-Coupled Mounting: Minimizes sample transport time to prevent plugging associated with extractive installations.
  • In-Line Condensing: Sulfur handling within the process avoids plugging issues associated with an externally condensed sulfur method.
  • Sample Probe Cooling: Instantly adjusting the cooling medium according to changing conditions to maintain a consistent, and efficient sulfur knockout temperature.
  • Electric Oven: Ensures the oven temperature remains above sulfur dew point without the need for medium or high-pressure steam.
  • Back-Purge: Clears out the sample path and vent path for rapid recovery from any excursion to avoid downtime.

There is only one model of tail gas analyzers that meets these criteria: Brimstone™. “DAD is only possible thanks to the robust sample system of our Brimstone™ tail gas analyzer,” comments platform manager, Dennis Hnidan. “The analyzer’s sealed fiber gland and purged enclosure also helps protect against corrosion from high sulfur vapor.”

Brimstone™ performs a broadband UV analysis covering 530 pixels from 200 to 400 nm. This ensures effective interference compensation when measuring H2S, SO2, COS, and CS2. Monitoring COS and CS2 provides operators with trending for catalyst condition and conversion efficiency.

“Galvanic has been supporting the process gas industry for over 40 years. The DAD control strategy provides measurement certainty by offering the best response time to SRU operators,” comments David Haydt, director of research & development.


Keywords: DAD, Double Air Demand, Air Demand, Tail Gas, SRU, Sulfur Recovery Unit, Brimstone

Applications: Refinery, Natural Gas Processing, Sulfur Recovery