Feed Gas (Acid Gas) H₂S Analyzer

Introduction

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.

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:

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 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

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.

Installation and Product Photos

Further Reading