Measuring water vapor in hydrocarbons

A method comprising:

• dehydrating a first sample of an olefin gas mixture containing an unknown concentration of water vapor and a varying concentration of one or more olefins to reduce the water vapor concentration;
• recording a first absorption spectrum of the first sample at a chosen wavelength;
• recording a second absorption spectrum of a second sample of the olefin gas mixture, the second absorption spectrum being obtained in parallel or sequentially with the first absorption spectrum, the first absorption spectrum and the second absorption spectrum being recorded in parallel in first and second sample cells with substantially identical optical path lengths;
• generating a differential absorption spectrum from the first absorption spectrum and the second absorption spectrum;
• and
• analyzing the differential spectrum to determine the concentration of water vapor in the olefin gas mixture.

An apparatus comprising:

• a modulated laser light source that emits light comprising a chosen wavelength;
• a sample cell;
• a dehydrator that reduces water vapor in a first sample of an olefin gas mixture, the olefin gas mixture containing a varying concentration of one or more olefins and, prior to entering the dehydrator, an unknown concentration of water vapor;
• one or more valves for alternately and sequentially providing the first sample and a second sample of the olefin gas mixture to the sample cell, the second sample containing the unknown water vapor concentration of the olefin gas mixture;
• a photodetector positioned to quantify light passing through the sample cell;
• and
• a microprocessor that records a first absorption spectrum from the photodetector using a harmonic spectroscopy method when the sample cell contains the first sample, records a second absorption spectrum from the photodetector using the harmonic spectroscopy method when the sample cell contains the second sample, calculates a differential absorption spectrum from the first and second absorption spectra, and calculates the concentration of water vapor in the olefin gas mixture based on the differential absorption spectrum.

An apparatus comprising:

• a laser light source that emits light comprising a chosen wavelength;
• a dehydrator to reduce water vapor in a first sample of an olefin gas mixture, the olefin gas mixture containing an unknown concentration of water vapor and a varying concentration of one or more olefins;
• a first sample cell for containing the first sample;
• a second sample cell for containing a second sample of the olefin gas mixture, wherein the second sample cell has a substantially identical path length to the first sample cell;
• a gas flow divider that directs the first sample to the first sample cell and the second sample to the second sample cell for parallel analysis;
• optical components for splitting the beam between the first sample cell and the second sample cell;
• a first photodetector positioned to quantify light passing through the first sample cell;
• a second photodetector positioned to quantify light passing through the second sample cell;
• and
• a microprocessor that records a first absorption spectrum from the first photodetector, records a second absorption spectrum from the second photodetector, calculates a differential absorption spectrum from the first and second absorption spectra, and calculates the concentration of water vapor in the olefin gas mixture based on the differential absorption spectrum.

An apparatus comprising:

• means for generating a beam of light at a wavelength where water molecules and other components of an olefin gas mixture have different absorbances, the olefin gas mixture containing an unknown water vapor concentration and varying concentrations of two or more olefins;
• means for reducing water vapor in a first sample of the olefin gas mixture;
• means for illuminating the first sample of the gas with the beam and recording a first absorbance spectrum;
• means for illuminating a second sample of the olefin gas mixture with the beam and recording a second absorbance spectrum, the second sample being illuminated in parallel or sequentially with the first sample;
• processing means for generating and analyzing a differential absorbance spectrum from the first absorbance spectrum and the second absorbance spectrum to determine the concentration of water vapor in the olefin gas mixture.