Measuring H₂S + Mercaptans in LPG

Introduction

H₂S is highly undesirable in liquefied petroleum gas (LPG) for several reasons:

• Storage. When storing produced LPGs for later distribution, H₂S causes corrosion in the expensive sphere tanks.
• Polymer yield. Sulfur compounds reduce the polymer yield in polyolefins like polypropylene.
• Catalysts. H₂S is known to poison catalysts in reactions that use LPGs as feedstock.
• Emissions. When burning propane or any other LPG, H₂S will oxidize to SO2, a particularly harmful emission.

Controlling H₂S and mercaptans in LPG requires online concentration measurement. Whether measuring the LPG entering the storage tank or analyzing the propylene feed to plastics production, the ability to respond quickly to changes in sulfur loading is critical for tight process control.

The OMA H₂S Analyzer uses a dispersive UV-Vis spectrophotometer to measure H₂S and mercaptans simultaneously in hydrocarbon streams with fast response and solid state reliability.

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. The OMA measures absorbance at each integer wavelength within the measurement wavelength range and produces analyte absorbance curves. This use of ‘confirmation wavelengths’ instead of simple peak absorbance measurement serves to eradicate the effect of noise at any single photodiode, allowing for a much more accurate analysis and the ability to deconvolute components using AAI’s multi components algorithm*.

*Provide sample matrix to AAI for verification.

Example Installation

The system below measures mercpatans in propane (0-50 ppm) in Peru:

The specifications below represent performance of the OMA-300 Process Analyzer in a typical LPG application.

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

Further Reading