Restrictive legislation has been accepted world wide requiring a persistent increase in emission control of toxic substances. To comply with these demanding regulations a reliable, accurate and continuous analytical technique needs to be implemented which can continuously verify the hydrogen sulfide levels in product gas. The process of removing the poisonous gas is costly and therefore it is of benefit to keep the levels just below regulatory values thus optimizing operational costs.

No Moving Parts              
The H2S-220 is a solid state analyzer adhering to the principle of no-moving parts in process applications.

Analytical technique:

The H2S-220 is a diode array UV fiber optics process spectrometer. It is designed to address on-line applications that require reliable, accurate, rugged and maintenance free monitoring of H2S concentration. Either a fiber optics in-situ probe or flow cell can be used.

Hydrogen sulfide has a unique UV absorbance spectra as shown below, in many cases mercaptans are also present. Therefore, the ability of the diode array to handle multi-components is clearly relevant in this case.

In contrast to a filter based instrument, which measures at a few discrete wavelengths, the diode array spectrometer processes the absorbance signal obtained from the complete spectrum by a multi-component method,  the measurement accuracy is greatly enhanced allowing for a higher quality and more reliable control signal.

Designed for versatility, fast-response, accuracy, and long-term durability in sulfur applications. The system consists of four major subunits: 1) a xenon light source, which sends light through fiber optic cables into the 2) flow cell, in which the unique characteristics of the sample define its interaction with the light; the light then enters a 3) holographic grating, which separates white light into its components such that the 4) diode array detector can measure the intensity of radiation at each wavelength. These measurements report on the absorption of light by the various components in the sample, and thus reveal the concentrations of the components.

The advantages of the H2S-220 over paper tape are economically, environmentally, and technologically significant. The spectrophotometric principles of the analyzer eliminate the need for costly, polluting consumables like lead acetate; the only part that would ever need to be replaced is the inexpensive and long-lasting xenon lamp. The analyzer is solid state (no moving parts), and since it connects to the sample through fiber optic cables, it can be installed far from any explosive or poisonous gases. 

While lead acetate tape gives slow, discrete measurements, the H2S-220 provides instantaneous and continuous measurements. Using the full high-resolution spectrum obtained by the diode array, the analyzer can easily switch between extreme wavelength ranges, allowing measurements of 0-10 ppm and 0-100% in the same instrument. Comparable range flexibility using paper tape might require dilutions, producing an additional sampling concern and possibly inviting inaccuracies.

The cross-interference of H2S, mercaptans (the functional S-H group), SO2, and other sulfur compounds is a concern which makes the paper tape method incompatible with the current technological demands of industry. AAI’s innovative multi-component analysis corrects for this interference, mathematically de-convoluting the interfering compounds and allowing simultaneous measurement of the total sulfur without the need for physical separation.

The beauty of the H2S-220, however, is its capacity for multi-component analysis. In the direct method, the analyzer mathematically derives the concentration of each sulfur compound from the full, high-resolution absorbance spectra it obtains from the original process stream. There is no need to mechanically consolidate all sulfur into one specific component, as by combustion in the standard method. The concentrations of sulfur based components like H2S, COS, and mercaptans are computed individually from the spectrum. The H2S-220 analyzer demonstrates the long-term benefits of incorporating AAI’s flagship technology into your process.

Sampling System

The sampling system comprises of   the valve mechanism necessary to take the sample gases from the inlet connections through the flow cell and back to the outlet connection. The valve mechanism is arranged such that either the sample gases flow through the flow cell or instrument air for blanking or span gas for system verification.  The pressure is continuously measured and the H2S concentrations are automatically corrected. All valves are software controlled.

Specifications

Feed Forwards Control in SRU

The efficiency of sulfur recovery, a procedure of utmost importance in many industrial processes, has gradually improved over the years. In maximizing this efficiency, control over oxygen input is paramount. Proper control in this respect requires accurate monitoring of H2S levels in acid gas; this in turn necessitates consideration of certain parameters such as time delay between measured signals from the beginning and the end of the process, or the extreme variation of feed gas concentrations throughout the process. It has been demonstrated that feed forward control in conjunction with tail gas control in sulfur recovery units drastically increases efficiency of recovery.

AAI's Feed Forwards Analyzer monitors the H2S concentration in feed gas or acid gas (H2S 0-100%) via a direct UV spectroscopic technique. The signals obtained in this method are used to control oxygen input to the SRU (Sulfur Recovery Unit) in question, as well as continuously analyzing and evaluating process effectiveness. AAI analyzers respond extremely rapidly to high fluctuations of H2S concentration while easily correcting for pressure and flow variations in the stream.

Some of the advantages of integrating an AAI analyzer include:

Safety of operation and analyzer components: H2S, particularly at high concentration, is extremely toxic and corrosive. AAIs FFA product line utilizes fiber optics to transmit light to and from the flow cell, allowing the electronics to be housed in a separate enclosure that is protected from the gas flow. AAI prioritizes the safety of our analyzers as well as the safety of those operating them.

No moving parts: AAIs solid state technology is a clear benefit in the corrosive environments it is designed for.

Fast response and auto-compensation: The full-spectrum diode array FFA analyzer responds rapidly to wide fluctuations in concentration. The instrument also automatically compensates for pressure variation by continuously checking measurement point pressure against calibration pressure.

Flexibility: If necessary, the system can incorporate infrared detection for monitoring concentrations of CH4 and CO2 all integrated and using the same controller and sampling system.

Hydrogen Sulfide monitoring in Sweet / Sales Gas

One of the most common applications is the direct monitoring of H2S 0-10 PPM +/- 0.5 PPM. This application is common in sweet / sales gas where the H2S levels in the sales gas are strictly regulated. The continuous monitoring of the H2S in the sales gas allows for cost affective and accurate control of the sulfur recovery process as well as an indication on the purity levels of the gas. Other techniques that are currently available for this application are indirect, require consumables such as reagents, or paper tape.

In order to accurately measure the H2S absorbance spectra a high-resolution high signal- to- noise low- stray light spectrophotometer is needed. The AAI H2S analyzer is a UV-VIS diode array process spectrophotometer, contains no moving parts and is designed to address on-line applications that require fast, reliable, and maintenance-free monitoring of concentrations, including both single- and multi-components.

H2S can be measured in the presence of up to 500-PPM aromatics. The AAI H2S analyzer is operating at high pressure (up to 100 Bars) to simplify the sampling system and increase S/N.

With analyzers currently operating on six continents, in some of the world’s harshest conditions, AAI provides a track record you can trust.

Mercaptans and H2S in Natural gas

Mercaptans with low molecular weight are particularly volatile, toxic, and corrosive. Mercaptans are frequently removed from natural gas by caustic washing over a catalyst bed. The catalyst needs to be regenerated every several hours by heating and purging with a dry carrier gas stream.  

Mercaptans have the general formula R – S – H, where R represents an aliphatic or cyclic radical. The process for the removal of RSH from the natural gas is optimized by first eliminating the H₂S, which interferes with the mercaptan extraction efficiency and regeneration of the circulating caustic. A caustic regeneration system converts the extracted mercaptans to disulfide oil that is separated and removed, while the regenerated caustic is returned to the extraction section. The disulfides are separated and the caustic reused for extraction. The catalyst is regenerated by removal of the mercaptans from the molecular sieve bed by heating and purging with a carrier gas. Every several hours the system switches between catalysts, where one is used for the oxidation and the other goes through regeneration. For optimal results, the concentrations of RSH and H2S during the catalyst regeneration need to be monitored. 

This application requires the simultaneous monitoring of percent RSH and low PPM  H₂S.  Due to the large dynamic range, a Gas Chromatograph (GC) is traditionally applied to this application. Instead of the GC a UV process diode array analyzer is now utilized, where both components are simultaneously monitored. The UV technique, with the use of robust analysis procedures, provides a long-term, low-maintenance solution.  

The advantages of the direct spectroscopic method over the traditional gas chromatographic method include: no separation, instantaneous reading, solid state: very low maintenance

Total sulfur by a direct method 

For total sulfur analysis the cross-interference of H2S, mercaptans (the functional S-H group), SO2, and other sulfur compounds is a concern. AAI’s innovative multi-component analysis corrects for this interference, mathematically de-convoluting the interfering compounds and allowing simultaneous measurement of the total sulfur without the need for physical separation..

 In the direct method, the analyzer mathematically derives the concentration of each sulfur compound from the full, high-resolution absorbance spectra it obtains from the original process stream. There is no need to mechanically consolidate all sulfur into one specific component, as by combustion in the standard method. The concentrations of sulfur based components like H2S, COS, and mercaptans are computed individually from the spectrum and added together, such that the computer outputs a total sulfur reading. The direct method effectively measures total sulfur in the original process stream as multiple components. The direct method is less expensive, requiring neither combustion nor the costs associated with it, and demonstrates the long-term benefits of incorporating AAI’s flagship technology into your process.

 View PDF