In-Situ Tail gas / Air Demand analyzer

"Cold Finger" in- situ probe     

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Introduction

The Claus sulfur recovery process  is commonly utilized to remove  sulfuric compounds from fossil fuels. Instantaneous and accurate monitoring  of H2S and SO2 in the process are  critical factors in the Claus process.

Applied Analytics, Inc. design adheres to the principle of no moving parts no moving sample in sulfur recovery applications. The TLG-837 uses solid state diode array spectrophotometer for detection, measuring complete spectrum from 190nm to 800nm with 1nm resolution. It can readily be used for absorbance measurements of up to 2AU . Allowing for a very wide concentration dynamic range and accurate measurements. The light source is a pulsed xenon with extremely long life time or a very low noise deuterium source. The TLG-837 uses fiber optics to transmit light to and from the detector allowing the electronics to be physically separated from the probe. A patent pending demister probe is where The light interacts with the sample . The new design is simple and requires very low maintenance, benefiting from the availability of the state of the art detectors and fast computers for instantaneous data analysis.

Claus Tail gas application

Tail gas is another important multi-component application. The Claus sulfur recovery process is commonly utilized for the removal of sulfur compounds from sour gas. The process is optimized when the correct stoichiometric ratio of H₂S to SO₂ is achieved. The efficiency of the recovery process depends on accurate measurements of the H₂S to SO₂ ratio. To obtain this accuracy, other stream components such as CS₂, COS, and sulfur vapor need to be taken into account.

Process Control

Typically, the H₂S to SO₂ ratio should be 2:1 for efficient conversion. The Feedback control parameter (air-demand) is equal to 2[SO₂]-[H₂S] and the process is optimized when the air-demand equals zero. Other similar processes require different ratios; the full-spectrum analyzer allows for wide concentration dynamic range and therefore can easily be applied to the control of other possible ratios. The control formula is a user-defined parameter. 

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.

Main Feature

• Diode array detection

• Solid state no moving parts

• Wide wavelength range

• High resolution

• Accurate concentration readings

• No maintenance

• Insitu detection cold finger demister probe

• Fast response

• no sample lines

• Steam, nitrogen and sample gasses can be easily introduced into the probe

• Fiber optics

• Low Noise / long life

• source

Specifications:

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View PDF sulfur recovery plant

 H2S analyzer