Tail Gas Analyzer

The analyzer
The analyzer’s design8 adheres to the principle of no moving parts and no sample lines in sulfur recovery applications. The same diode array analyzer is utilized here, but an in-situ probe is used in place of the typical flow cell.
Demister “cold finger” Probe
A patent-pending demister (“cold finger”) probe is used for in-situ measurements. The setup requires very low maintenance, benefiting from the innovative in-situ probe. The probe is designed to draw a continuous sample into its body, and remove the sulfur vapor. This is done by a “cold finger” which condenses the sulfur out of the gas in a controlled manner, and an aspirator, which draws the sample into the sample chamber, and returns it to the process line.
The probe is constructed from three concentric tubes. The outer tube, called the “sample chamber”, is 1.5" in diameter. This tube passes down through a ball valve into the process line, where its angled tip helps draw the tail gas into the chamber. The cold finger tube located inside the sample chamber is kept much cooler than the process gas. This causes most of the sulfur to condense and drip back into the process, creating a sample stream that is free of sulfur vapor. Cooled air is constantly fed to the bottom of the cold finger through a smaller internal tube and is exhausted out the opposite side. These three tubes are welded to the first section of the probe’s head, a 1” thick disk made of 316 stainless steel. Above this is a second disk, which contains an air-driven aspirator. This provides a vacuum, which draws the process gas into the sample chamber, past the cold finger, and up through an integrated flow cell at the top of the probe head. The aspirator then pulls the sample down through a waste tube, where it is exhausted back into the process.
All air/gas connections in the head are ¼” Swagelok tube fittings. These accommodate the aspirator air in, cold finger air in and out, and calibration gas in. The integrated flow cell has fiber optic connections on each side. The system periodically washes and zeros itself. Calibration gases can be introduced into the probe at any time for data verifications, although recalibration or spanning is not required, since it is a solid-state analyzer.


Tail gas analyzer - UV absorbance spectra
The concentrations of up to five components are measured: H2S and SO2 for process control, COS and CS2 for catalyst efficiency, and sulfur vapor for signal compensation. Each one of these components has unique UV absorbance spectra. Figures 3 and 7 show the absorbance spectra of H2S and SO2 at different concentration levels. Figure 8 shows the total signal of a mix of 1% H2S and 1% SO2 and the individual signals. The absorbance spectra of the individual components are superimposed to give the total absorbance spectrum of the process sample (see Figure 9). To de-convolute the signals of each of the components, a full-spectrum, multi-component algorithm is used