TLG-837 Sulfur Recovery Tail Gas Analyzer/Air Demand Analyzer main view

Tail Gas/Air Demand Analyzer

The world’s safest tail gas analyzer. TLG-837 continuously measures the concentrations of H2S, SO2, COS, and CS2 in the Claus process tail gas stream. Using the patented in situ DEMISTER sampling probe and a full-spectrum UV-Vis spectrophotometer, this system provides extremely fast, accurate response for tight process control.

  • Continuously measures concentrations of H2S and SO2 and outputs Air Demand signal (user-defined formula)
  • Patented DEMISTER sampling probe with internal sulfur vapor removal
  • Totally solid state with no moving parts, sample lines, or heat tracing — modern design for low maintenance
  • Ultra-safe fiber optic design — no toxic/explosive sample gas in analyzer enclosure
  • Superior Off-Ratio range ( 100:1 < H2S/SO2 ratio < 1:20 )

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Applications: TLG-837 Tail Gas Analyzer

The Claus Process

H2S is toxic at 10 ppm, entirely lethal at 800 ppm, highly corrosive to equipment, flammable when in excess of 4.3% by volume in air, and unpleasantly odorous at a threshold of less than 1 ppb.

Unfortunately, H2S occurs abundantly in the world’s fossil fuel reserves. The sulfur recovery unit (SRU) of a refinery is dedicated to processing the H2S stripped from the hydrocarbon fuel through a series of operations that convert it into water and harmless elemental sulfur, which can be sold and repurposed in fertilizer, gunpowder, and more.

The Claus process is the industry standard for treating the H2S-rich “sour” gas. In a furnace, H2S is combusted:

3H2S + 32O2 SO2 + H2O + 2H2S

A catalytic converter reacts the products of the combustion to create elemental sulfur in various crystalline forms:

2H2S + SO2 2H2O + 3XSX

As can be deduced from the second reaction above, the typical Claus reaction runs most efficiently when the stoichiometric ratio of H2S to SO2 is controlled at 2:1. The 1st reaction above demonstrates that this ratio is controlled by adjusting the amount of available oxygen.

Sulfur Recovery Unit Feed Forward Analysis--Flow-Diagram-SRU

Air Demand

As demonstrated above, the efficiency of sulfur recovery hinges on the ability to maintain a set H2S/SO2 ratio in the Claus reaction. This adjustment requires knowing the exact H2S/SO2 ratio in the tail gas at all times.

A tail gas analyzer measures H2S and SO2 in the stream and continuously outputs the “Air Demand” control signal, calculated by multiplying the expression (2[SO2] - H2S) by a scaling factor. Additionally, operators sometimes require online measurement of COS and CS2 due to side reactions in the reactor.

Related Applications:

Tail Gas Absorbance Spectra

To analyze tail gas, the TLG-837 detects the distinctive absorbance curve of each chemical analyte and mathematically isolates this structure from the total sample absorbance. In accordance with the Beer-Lambert Law, the TLG-837 correlates the height of each curve directly to the real-time concentration of its corresponding chemical.

Absorbance Curve: H2S and SO2

Absorbance (AU)
Wavelength (nm)

Sampling: TLG-837 Tail Gas Analyzer

The TLG-837 uses our patented in situ DEMISTER Probe for tail gas sampling. Learn more about this technology below.


The DEMISTER sampling probe was designed to be lightweight and compact, so it can easily be installed by a single technician. The probe is mounted on the process pipe via flange.

The actual interaction between the sample gas and the light signal occurs in the flow cell disk within the probe head, where fiber optic connections on opposite ends transmit light across the path length of the disk.

Sulfur Recovery Tail Gas Demister Probe Demonstration

Automatic Sulfur Vapor Removal

Tail gas contains elemental sulfur which is quick to condense and plug mechanical cavities or obstruct optical signals. The DEMISTER Probe removes sulfur from the rising sample as an internalized function within the probe body. Recycling the steam generated by the Claus process, the probe controls the temperature along its body at a level where all sulfur vapor in the rising sample condenses and drips back down to the process pipe.

Inside the probe, an internal ‘demister’ chamber (concentric to the probe body) is fed with low pressure steam. Since the LP steam is much cooler than the tail gas, this chamber has a cooling effect on the rising sample. Elemental sulfur has the lowest condensation point of all of the components in the tail gas. Due to the internal probe temperature maintained by the LP steam, all of the elemental sulfur in the rising sample is selectively removed by condensation while a high-integrity sample continues upward for analysis in the probe head.

The point of interaction between the light signal and the sample gas occurs in the flow cell disk inside the probe head. The flow cell disk has a built-in HP steam channel to heat the cell and ensure that any present sulfur remains gaseous—eliminating the possibility of condensation on the optical windows.

An aspirator creates a Venturi effect which pulls the sample up the probe body intake path, through the flow cell for analysis, and down the return line. The used sample is released back into the process pipe.

System Overview

Diagram: TLG-837 Sulfur Recovery Tail Gas Analyzer with Demister Probe

Utility Control Panel

TLG-837 Utility Control Panel

The Utility Control Panel (UCP) regulates utilities going to the DEMISTER sampling probe for the TLG-837. The UCP is optional; the customer can decide to purchase the UCP from Applied Analytics or build your own integration panel. View datasheet

  • Regulates LP steam pressure for demister chamber in probe body
  • Regulates HP steam pressure for flow cell steam tracing in probe head
  • Provides zero gas for Auto Zero sequence
  • Provides span gas in case Auto Span is desired
  • Controls aspirator flow rate
  • Provides steam failure blowback feature: in the event of faulty steam utilities, the flow cell disk is sealed from the sample and the cell is purged with nitrogen from the UCP

Ultra-Safe Design

TLG-837 Sulfur Recovery Tail Gas Analyzer safety diagram

The major safety flaw of other tail gas analyzers is that they bring the toxic sample fluid into the analyzer enclosure for analysis. Not only does this practice expose the system electronics to higher corrosion effects, it also poses a lethal threat: if there is any leak in the instrument — especially inside a shelter — the human operator is placed at enormous risk.

The key difference between the TLG-837 and other tail gas analyzers is the use of fiber optic cables: we bring the light to the sample instead of bringing the sample to the light. The toxic sample only needs to circulate through the probe, and never enters the analyzer electronics enclosure.

  • No danger of leaks inside the analyzer because the tail gas does not enter the analyzer enclosure
  • No need for a shelter — system designed for outdoor environment
  • Custom fiber length up to 6 meters allows for distance between analyzer and probe
  • User can safely perform service on the analyzer while process is running — no exposure to sample gas
  • Digital link (e.g. Modbus) provides additional process data during any upset conditions — personnel do not need to physically visit the analyzer during potentially dangerous situations
  • Full port 2” ball valve provides process seal, allowing isolation and removal of probe while process running
Analyzer Safety Demonstration-Tail Gas Analyzer/Air Demand Analyzer for Sulfur Recovery Plant

Specifications: TLG-837 Tail Gas Analyzer

Note: All performance specifications are subject to the assumption that the utility control panel and unit installation are approved by Applied Analytics. For any other arrangement, please inquire directly with Sales.

Measurement PrincipleDispersive ultraviolet-visible (UV-Vis) absorbance spectrophotometry
Detectornova II™ UV-Vis diode array spectrophotometer
Spectral Range200-800 nm
Light SourcePulsed xenon lamp (average 5 year lifespan)
Signal Transmission600 μm core 4 meter fiber optic cables
Other lengths available
Sample IntroductionIn situ DEMISTER Probe
Analyzer CalibrationCalibrated with certified calibration fluids; no re-calibration required after initial calibration; measurement normalized by Auto Zero
Reading VerificationSimple verification with samples
Human Machine InterfaceIndustrial controller with touch-screen LCD display running ECLIPSE™ Software
Data StorageSolid State Drive
Analyzer EnvironmentIndoor/Outdoor (no shelter required)
Ambient TemperatureStandard: 0 to 35 °C (32 to 95 °F)
With optional temperature control: -20 to 55 °C (-4 to 131 °F)
To avoid radiational heating, use of a sunshade is recommended for systems installed in direct sunlight.
Standard Outputs1x galvanically isolated 4-20mA analog output per measured analyte
5x digital relay outputs for indication and control
1x K type ungrounded thermocouple input
Optional OutputsModbus TCP/IP; RS-232; RS-485; Fieldbus; HART
Wetted MaterialsStainless Steel 316/316L, Kalrez
Other materials available
Analyzer Enclosurewall-mounted NEMA 4X stainless steel type 304 Enclosure
Other enclosures available
IP RatingIP66
Probe MaterialStainless Steel 316/316L
Other materials available
System DimensionsAnalyzer: 24” H x 20” W x 8” D (610mm H x 508mm W x 203mm D)
Probe (Average): 36” length x 12“ widest diameter (914mm x 305mm)
System WeightAnalyzer: 32 lbs. (15 kg)
Probe (Average): 29 lbs. (13 kg)
Accuracy/RepeatabilityAnalyteTypical RangeAccuracyRepeatability
H2S0-2% ± 1% full scale ± 0.4%
SO20-2% ± 1% full scale ± 0.4%
air demanduser-defined ± 1% full scale ± 0.4%
Off-Ratio Range100:1 H2S:SO2 20:1
Response Time1-5 seconds
Sensitivity ±0.1% full scale
Noise ±0.004 AU at 220 nm
Standard Design General Purpose
Available Options ATEX, IECEx, EAC, PESO, JPN
Please inquire with your sales representative for additional certifications (CSA, FM etc.).

FAQ: TLG-837 Tail Gas Analyzer

Modern refineries are required to remove sulfur containing compounds such as hydrogen sulfide (H2S) or sulfur dioxide (SO2) to meet emission and environmental regulations. This sulfur is removed from the process gas and recovered using the modified Claus process. In this process hydrogen sulfide is partially burned to form a mixture of H2S and SO2. The resulting mixture is then catalytically converted to elemental sulfur. Further recovery can be achieved using a tail gas treatment unit, or TGTU. These technologies have enabled the production of low sulfur fuels and prevent environmental hazards such as H2S and SO2 from being released into the atmosphere.

The TLG-837 uses dispersive ultraviolet spectroscopy to determine the concentration of H2S and SO2 in the tail gas. This feature enables the analysis of multiple components such H2S, SO2, CS2 or COS without the need for delicate filters or moving parts.

Measurements are made in-situ or “in-place” using our patented cold-finger DEMISITER Probe. Rather than the sample being drawn through a sample line to the process analyzer, analysis light is brought to the mixture instead. This setup enables a safer and more reliable process analyzer, as hazardous hydrogen sulfide does not need to be extracted and carried to the process analyzer. This design has the added benefit of eliminating the possibility of sample lines becoming clogged in the future.

The DEMISTER Probe utilizes low pressure process steam flowing through the probe’s internal demister chamber to lower the temperature of the process gas, causing elemental sulfur to precipitate prior to the composition measurement in the probe’s flow cell.

The TLG probe can be used in multiple ways including as an air demand analyzer and as a sulfur pit headspace analyzer. If you have an application that’s not listed here, please reach out to Applied Analytics for more information.

The TLG-837 requires access to steam, instrument air, and an electrical connection. The exact specifications are listed below:

  1. Electrical requirements: 85 to 264 VAC, 47 to 63 Hz
  2. Instrument air: 80 psig (-40 °C dew point)
  3. Steam pressure:
    1. 20 psig for DEMISTER chamber
    2. 55-100 psig for probe blowback function
    3. 55-100 psig for optional ball valve steam jacket

The TLG-837 will automatically zero at regular preset intervals with instrument air to ensure a repeatability of ±0.4%.

The TLG-837 typical measurement is between 0%-2% for both H2S and SO2 with an accuracy of ± 1% the full scale.

The standard fiber length is 4m.

The typical time is 1-5 seconds.

The analyzer’s dimensions 24” H x 20” W x 8” D (610mm H x 508mm W x 203mm D) and weighs approximately 32 lbs (15 kg). The tail gas probe’s dimensions are 36” L x 12” W (914mm x 305mm) and weights approximately 29 lbs (13kg).

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