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Using on-line analysis to optimise the Claus Sulfur Recovery Process

The reduction of emissions from stacks is becoming an increasingly important environmental requirement. By increasing the efficiency of the sulfur recovery process we can increase the yield of Sulfur and also provide environmental benefits in the reduction of sulfur emissions as well as lowering the required energy costs associated with the process.

The Claus sulfur recovery process is commonly utilized to remove sulfuric compounds from fossil fuels and its use can be found in the sulfur recovery portion of most refineries. Automatic, continuous and accurate monitoring of H2S and SO2 are critical factors in the optimisation of the Claus process.

To achieve an increase in the efficiency of the recovery process it is essential to provide continuous on-line monitoring of the key components in the process. The main reactions of interest during the Claus Sulfur recovery process are:

Reaction Furnace: 3H2S + 3/2O2 → SO2 + H2O + 2H2S
Catalytic Converter: 2H2S + SO2 → 2H2O + 3Sv

For efficient conversion we need to control the stoichiometry in the sulfur kiln and require a H2S:SO2 ratio of 2:1. As such the process is optimized when the air demand = 0. The air demand can be used as a feedback control parameter and is defined as 2[SO2] – [H2S]. The Tail gas contains a number of sulfuric compounds including H2S, SO2, COS, CS2 as well as sulfur vapour. For increased efficiencies and to be able to obtain a very accurate reading of both H2S and SO2 it is essential that we are able to measure all components separately. This is most important for the H2S measurement as the COS and CS2 present in the gas stream interfere with determination of H2S and their concentrations must be taken into account. Even small concentrations of COS and CS2 can cause measurement errors of 25%.


Spectral Absorption for H2S


Spectral Absorption for SO2


Spectral Absorption for COS and CS2

The ideal analyser would provide for:

Applied Analytics
TLG-837

The Applied Analytics TLG-837 on-line analyser meets the above criteria and its design incorporates the following principles:

Spectrometer: The use of a solid state diode array spectrophotometer for detection which measures the complete spectrum from 190nm to 1100nm with 1nm resolution allows for a very wide concentration dynamic range and accurate measurements.

The Detector: A UV/VIS 1024 diode array detector. The photodiode array has a broad spectral response and high sensitivity in the ultraviolet range. The low dark current and large charge saturation enables signals to be obtained with a high signal to noise ratio. The detector’s aperture match the fiber optics numerical aperture, thus, maximizing the light throughput.

The Light Source: Either a very long life pulsed xenon lamp or a low noise Deuterium lamp. The light is focused onto the tip of a fiber optics via collection optics and not direct couple for maximum light throughput.

Absorbance Spectra: The analyzer must measure a complete spectrum of the process and analyzes the data via a calibration method to give very accurate and reliable concentration readings. In addition the provision for Standard gases to be introduced at any time to test the accuracy and reproducibility of the analyzer must be included. In continuous operation the complete spectrum should be displayed, this allows the operator to check the system and the validity of the actual reading - As opposed to filter based units where the operator has no access to the actual detection only to the final numbers.

Cold-finger probe

Sampling & the In-situ Probe: The tail gas stream , depending on the source, contain various amounts of sulfur. In order for the analyser to be applicable to any condition a demister probe was designed. The measurements are performed in-situ while the sulfur is reduced significantly by this ‘cold-finger’ probe, so no coating or plugging which is usually associated with sulfur streams is seen.

The probe is constructed from three concentric tubes. The outer one is 1.5" in diameter and is the outside diameter of the probe. The inner one is basically a "cold finger" that is designed to remove most of the sulfur vapor in a controlled manner. A tube leads to the bottom of the cold finger and brings air from the top of the probe in order to cool the sample . The head of the probe contains an air driven aspirator that provides the motive force for the sample to travel through the probe, past the cold finger where the sulfur is removed and into the integrated flow cell and back into the process through the waste tube.

Cold-finger probe
installed on site

Software:

Wash and blank cycle: The software allows the user to program the wash and blank cycle frequency. The duration depends on the amount of sulfur in the gas stream.

Temperature and pressure corrections: The temperature and pressure at the measurement point are monitored and the concentrations are corrected for via a user defined formula.

Control signal: A user defined formula based on the H2S and SO2 concentration such as H2S-2*SO2.

Display: Concentration is displayed in various screens including Bar and time chart, also displayed are the current spectra, intensity and absorbance.

Similar systems can also be configured to measure other parts of the refining process. Benzene, Toluene and Xylene (BTX) measurements in the alkylation unit of the plant are also possible.

For further information or enquiries on the analysis of your process please contact us.