Feed Gas (Acid Gas) H2S Analyzer

Application Summary

Analysis Point Analyte Typ. Range Suitable Analyzer
Acid Feed Gas to Sulfur Recovery Unit H2S (hydrogen sulfide) 0-100% OMA H2S Analyzer
NH3 (ammonia) 0-50%
CO2 (carbon dioxide) 0-50% MicroSpec MCP-200 Infrared Analyzer


H2S is an extremely dangerous chemical which occurs naturally in fossil fuels and is removed through refining processes. The sulfur recovery unit (SRU) of a refinery converts the H2S to elemental sulfur which can be stored or sold off. A critical point in this process is the furnace, which combusts H2S in order to sustain a 2:1 stoichiometric ratio of H2S:SO2 in the subsequent catalytic reaction. The amount of available oxygen in the furnace is carefully controlled to ensure this ratio for the next step. The efficiency of the entire SRU therefore hinges on the ability to continuously measure the required amount of oxygen (i.e. “air demand”) in the furnace.

While tail gas analysis measures the H2S:SO2 ratio after the furnace to calculate air demand correction, a feed forward analysis system measures the H2S concentration in the acid feed gas before the furnace, to preemptively adjust air demand based on the real-time feed gas H2S level. While tail gas analysis provides the most accurate air demand calculation, this measurement occurs after the furnace. Feed forward analysis allows for air demand control with no process lag by immediately detecting sudden changes in acid feed gas composition, and preventing any losses in SRU efficiency.

The OMA system continuously measures 0-100% H2S concentration in the acid feed gas to provide real-time feed forward control. This system provides the perfect complement to the TLG-837 Tail Gas Analyzer for total SRU air control.

System Benefits: Sulfur Recovery Unit Feed Forward

  • Continuously measures H2S level in acid feed gas stream using UV-Vis spectrophotometer
  • Totally solid state build with no moving parts — modern design for low maintenance
  • Additional software benches for up to 4 more chemical analytes (e.g. NH3)
  • Ultra-safe fiber optic design with no sample gas inside analyzer unit — world’s safest solution for this application

Sulfur Recovery Unit Schematic

In the diagram below, the feed forward analysis point is identified by AT3.

Sulfur Recovery Unit Feed Forward Analysis--Flow-Diagram-SRU

H2S Absorbance Curve

Any single photodiode measurement is vulnerable to noise, signal saturation, or unexpected interference. This susceptibility to error makes a lone photodiode data point an unreliable indicator of one chemical’s absorbance.

As accepted in the lab community for decades, the best way to neutralize this type of error is to use collateral data in the form of ‘confirmation wavelengths,’ i.e. many data points at many wavelengths instead of a single wavelength:

Sulfur Recovery Unit Feed Forward Analysis--spectra

In the figures above, each diamond represents a single photodiode and data point. After being calibrated on a full spectrum of pure H2S, the OMA knows the absorbance-concentration correlation for each measurement wavelength; the system averages the modeled concentration value from each wavelength to completely eradicate the effect of noise at any single photodiode. The OMA visualizes the H2S absorbance curve in this manner and knows the expected relation of each data point to the others in terms of the curve’s structure.

This curve analysis is critical at the high level H2S analysis involved with acid feed gas streams. At very high concentrations, the absorbance will be very high, and a photodiode at a high-absorbance wavelength (e.g. 215 nm) may not register any light at all. Fortunately, the low-absorbance wavelengths (e.g. 240 nm) will be used to validate the curve and reject error.

PRO-500 Steam Heated Acid Gas Probe

The PRO-500 for acid gas is heated with steam above 120 °C to prevent high dew-point vapors in the process sample from condensing inside the sample probe. The probe is precision-machined in NACE compliant Stainless Steel 316L and has best-in-class Kalrez O-rings internally for process handling. Internal crescent steam caps maximize heat transfer from steam to the probe body, eliminating the threat of liquid dropout, plugging, or freezing in the sample probe.

With a particulate filter and membrane filter inside the probe, the PRO-500 preconditions the sample effectively before the process enters the sample conditioning system and undergoes sample analysis.

PRO-500 Steam Heated Acid Gas Probe Demonstration

PRO-500 Flow Diagram

PRO-500 Steam Heated Acid Gas Probe Flow Diagram

Example Acid Feed Gas Sample Conditioner

The system pictured below was built to monitor 0-100% H2S in SRU acid feed gas. Key features for this high level H2S application include:

  • SS316 fittings & Kalrez gaskets for extreme corrosion resistance
  • Heating to 80°C

Sulfur Recovery Unit Feed Forward Analysis--sample conditioning system-sourgas

The specifications below represent performance of the OMA-300 Process Analyzer with integrated MicroSpec modules in the feed forward SRU application.

For technical details about the OMA-300 Process Analyzer, see the data sheet:
DS-001A: OMA-300 Process Analyzer

For technical details about the MicroSpec MCP-200 IR Modular Analyzer, see the data sheet:
DS-003A: MicroSpec MCP-200 Infrared Analyzer

All performance specifications are subject to the assumption that the sample conditioning system and unit installation are approved by Applied Analytics. For any other arrangement, please inquire directly with Sales.

Application Data
Performance Specifications
Custom measurement ranges available; example ranges below.
Accuracy OMA-300(UV-Vis) H2S 0-100%: ±1% full scale
NH3 0-50%: ±1% full scale
MCP-200(NDIR) CO2 0-50%: ±2% full scale
*Whichever is larger

Note: Subject to modifications. Specified product characteristics and technical data do not serve as guarantee declarations.

Further Reading