Comments on a modified tutwiler method for the determination of H2S and SO2 in gases

A modified form of the Tutwiler analysis which was proposed a few years ago and has to some extent, been used as a method for controlling the Claus sulphur recovery process, has been examined as to its exactitude for this purpose, viz. for, determining both H₂S and SO₂ in gas mixtures.It has been found suitable for the determination of H₂S alone, and also (with a small modification) of SO₂ alone, but not for both compounds if they occur together.These disadvantages are not possessed by an alternative method of analysis, recently published.     

ADVANCES IN CLAUS REACTION AND RELATED REACTIONS

Abstract

During the sweetening of sour natural gas H2S and other contaminants are separated from natural gas. The conversion of H,S and other sulfur conipounds to sulfur is accomplished by the well known Claus process in which H2S and SOz are allowed to react catalytically over an alumina-based catalyst at around 250°C. The Claus reaction is thermodynamically limited so that 2–4 catalytic stages, with intervening sulfur removal, are required to achieve total conversions of 95–98%. There is considerable research activity into all phases of sulfur recovery operations with the major emphasis on maximizing the overall sulfur recovery. This report summarizes the developments in Claus reaction and attempts to focus attention on potential areas for future research.     

Field technology for desulfurization of associated petroleum gas

Applicability of gas desulfurization technologies for sulfur-containing gases under conditions of the limited choice is analyzed. Data on efficiency of a new process for catalytic desulfurization of sulfur-containing gases through H2S and RSH conversion in sulfur and disulfides, respectively, is demonstrated. The technology is approved in pilot projects of gas desulfurization at a flare line, on a gas-processing plant. The equipment is developed and certificated. Installations for catalytic desulfurization are intended to be equipped with a complete set of blocks for gas preparation and power-generating equipment directly at deposits. This solution substantially decreases both capital and operational expenditures compared to the traditional multi-stage absorption/desorption technologies such as expensive Claus process.

     

A gas chromatographic analysis for SO2 oxidation

Summary An improved method of determining yields of sulphur trioxide in sulphur dioxide oxidations is reported. The method relies on the absorption of SO₃ from the reaction gases. An all-teflon system incorporating a gas density balance is used. Separation of SO₂ and O₂ is effected on a Porapak PS column at ambient temperature.     

Identification des allotropes S6 et S7 du soufre dans le trophosome du vestimentifère Riftia pachyptila

The two sulphur allotropes S₆ and S₇ were characterised by gas chromatography/mass spectrometry coupling experiments in trophosomes of Riftia pachyptila (Vestimentifera), which inhabit hydrothermal vents. Both allotropes (approximately 3% for both) are associated with the usual form S₈ that represents about 97% of the total sulphur fraction. These new natural varieties of sulphur could be due to bacterial oxidation of hydrogen sulphide in extreme conditions (about 300 kg cm^–2). However, the occurrence of the same allotropes in mineral sulphur from different origin, terrestrial and extraterrestrial as well, could indicate that natural sulphur is a mixture of these three allotropes, with proportions depending on its geographical origin.     

Microbial reduction of SO2 as a means of byproduct recovery from regenerable,dry scrubbing processes for flue gas desulfurization

A project is under way at the University of Tulsa to investigate the reduction of SO₂ to H₂S by sulfate reducing bacteria (SRB) in co-culture with mixed fermentative heterotrophs. We have previously demonstrated that SO₂ is completely reduced to H₂S (contact times of 1–2 s) in cultures in which no redox poising agents were required and glucose served as the ultimate source of carbon energy. We have proposed that such a microbial process could be coupled with a Claus reactor to recover elemental sulfur as a byproduct of regenerable, dry scrubbing processes for flue gas desulfurization. The development of this process concept has continued with a study of the use of molasses as a source of carbon and reduced nitrogen, identification of important non-SRB heterotrophs in process cultures, and the identification of the end products of carbohydrate fermentation that serve as carbon and energy sources for the SRB and identification of the end products of SRB metabolism.     

An FT-IR study of adsorption of sulfur dioxide on alpha- and gamma-alumina

In the Claus process hydrogen sulfide reacts to elemental sulfur. Because the Claus reaction is thermodynamically limited, sulfur compounds are still present in Claus tailgas. To avoid air pollution, the tailgas has to be treated.Alfa- and gamma-alumina are being used either as a catalyst or as a support for an active component in the Claus process and some tailgas treatment processes. In order to elucidate the mechanism of the Claus reaction, the adsorption of sulfur dioxide on both of the above aluminas was investigated using Fourier transform infrared spectroscopy.Different adsorbed species displaying a different heat of adsorption were detected. A broad band near 3500 cm^–1 is associated with the basic hydroxyl groups. This band is assigned to a hydrogen bond between the surface of alumina and a bisulfite species. As bisulfite species are reactive towards hydrogen sulfide, we assume that bisulfite species are active intermediates on alumina in the Claus reaction.     

Determination of sulphur in uranium matrix by total reflection X-ray fluorescence spectrometry

The possibility of sulphur determination in uranium matrix by total reflection x-ray fluorescence spectrometry (TXRF) has been studied. Calibration solutions and samples of sulphur in uranium matrix were prepared by mixing uranium in form of a standard uranyl nitrate solution and sulphur in the form of Na₂SO₄ standard solution, prepared by dissolving Na₂SO₄ in Milli-Q water. For major element analysis of sulphur, it was determined without separation of uranium whereas for the trace level determinations, uranium was first separated by solvent extraction using 30% tri-n-butyl phosphate (TBP) in dodecane as an extractant. In order to countercheck the TXRF results, a few samples of Rb₂U(SO₄)₃, a chemical assay standard for uranium, were diluted to different dilutions and sulphur content in these solutions were determined. The TXRF determined results for trace determinations of sulphur in these diluted solutions were counterchecked after addition of another uranium solution, so that sulphur is at trace level compared to uranium, separating uranium from these solution mixtures using TBP extraction and determining sulphur in aqueous phase by TXRF. For such TXRF determinations, Co was used as internal standard and W Lα was used as excitation source. The precision and accuracy of the method was assessed for trace and major element determinations and was found to be better than 8% (1σ RSD) and 15% at a concentration level of 1 μg/mL of sulphur measured in solutions whereas for Rb₂U(SO₄)₃, these values were found to be better than 4 and 13%, respectively.     

Integration of Nine Steps into One Membrane Reactor To Produce Synthesis Gases for Ammonia and Liquid Fuel

The synthesis of ammonia and liquid fuel are two important chemical processes in which most of the energy is consumed in the production of H₂/N₂ and H₂/CO synthesis gases from natural gas (methane). Here, we report a membrane reactor with a mixed ionic‐electronic conducting membrane, in which the nine steps for the production of the two types of synthesis gases are shortened to one step by using water, air, and methane as feeds. In the membrane reactor, there is no direct CO₂ emission and no CO or H₂S present in the ammonia synthesis gas. The energy consumption for the production of the two synthesis gases can be reduced by 63 % by using this membrane reactor. This promising membrane reactor process has been successfully demonstrated by experiment.     

Gas-permeation continuous flow coulometric analysis: determination of sulphur dioxide

 A gas permeation system using two gaseous streams flowing on both sides of a membrane is developed. This gas permeation device and a coulometric detector are adapted for the continuous measurement of relatively high concentrations of sulphur dioxide. The interferences of other gases (NO₂, NO and CO₂) can be eliminated by using a scrubber behind the gas permeation device in the acceptor stream. The effects of the donor flow rates and gas pressure as well as the membrane thickness on the signal are discussed. The relative standard deviation is 1.3% (n=7) for 2.002×10^-3 mol/mol certified sulphur dioxide. Received: 19 July 1996/Revised: 22 October 1996/Accepted: 29 October 1996     

Studies of the soluble part of oxidised coals

Soluble products obtained from the oxidation of four types of coal, each characterised by different degree of coalification and different degree of sulphur content, are studied. The coals are oxidised with peracetic acid (PAA) and nitric acid. Analyses are performed by Atmospheric Pressure-Temperature Programmed Reduction (AP-TPR) and Fourier Transform Infrared Spectroscopy (FTIR). The soluble products contain much more sulphur than the insoluble products of oxidation. The products obtained from the reaction with HNO₃ contain higher amounts of inorganic sulphur compounds, while those obtained from the reaction with PAA are characterised by an increased content of organic sulphur species.     

Al-doped B80 fullerene as a suitable candidate for H2, CH4, and CO2 adsorption for clean energy applications

Dispersion-corrected density functional theory method was performed to report on a high-performance adsorbent for removal of CO₂ from the precombustion and natural gases. At first, the effect of Al atom impurity on the structural and electronic properties of B₈₀ fullerene is studied. Then, the adsorption geometries and energies of gases (H₂, CH₄, or CO₂) on the B₈₀ and AlB₇₉ (amphoteric adsorbents) are explored. The Al atom enhances reactivity of the cage toward the gases and the adsorption processes are more exothermic with low and high energy barriers for chemisorption of H₂ and CO₂, respectively. Stable chemisorption of CO₂ on the AlB₇₉ is validated by the high adsorption energy and large charge transfer, while the CH₄ is just physically adsorbed on the AlB₇₉. Further, the physisorbed gases can enhance field emission current of the AlB₇₉ and in the continuous capturing of the gases, the magnetic moment of the cage is quenched. Furthermore, dependency of the electronic structure of the adsorbent on the gas adsorption is intensively studied. We suggest that the AlB₇₉ could be a promising material for capture, storage, and separation of the gases and as a novel material for sustainable energy and sweetening process in the petroleum industry.     

Influence of residence time and catalyst regeneration on the pyrolysis–zeolite catalysis of oil shale

Oil shale from the Kark region of Pakistan has been pyrolysed in a fixed bed batch reactor and the properties of the derived shale oil determined. The reactor system was then modified to incorporate a second reactor where the derived vapours from oil shale pyrolysis were passed directly to the second reactor containing zeolite ZSM-5 catalyst. The influence of the process parameters of vapour residence time (VRT) over the catalyst and the regeneration of the catalyst were examined. The yield and composition of the derived gases before and after catalysis were determined. In addition, the yield and composition of the derived oil in terms of total nitrogen and sulphur content and the content of aromatic hydrocarbons in the oils was investigated. The results showed that the yield of oil after catalysis was reduced with a consequent higher yield of gases and formation of coke on the catalyst. The main gases from the pyrolysis of oil shales were CO₂, CO, H₂, CH₄, C₂H₄, C₂H₆ and C₃H₆, C₃H₈ and minor concentrations of other hydrocarbon gases. The main role of catalysis was to convert the long chain alkanes and alkenes in the oil to lower molecular weight, short chain, alkyl substituted and iso species and high concentrations of aromatic hydrocarbons. Total nitrogen and sulphur contents in the oils were markedly reduced after catalysis. This reduction was reflected in the reduced concentration of nitrogen and sulphur containing aromatic hydrocarbons. The influence of longer VRTs was to increase the formation of aromatic hydrocarbons, reduce the nitrogen, and sulphur compounds in the oils. The influence of catalyst regeneration, involving five regenerations was not significant on the yield and composition of the derived catalytically upgraded oils.     

Low temperature degradation and characterization of natural rubber

Low temperature degradation of natural rubber was performed with potassium persulfate (K₂S₂O₈, KPS) in the latex stage at 30 °C to accomplish a good processability of the rubber. Various grades of natural rubbers were used as a source rubber. Gel content, molecular weight and chemical structure of the rubbers were characterized by swelling method, size exclusion chromatography and ¹H NMR spectroscopy, respectively. The well characterized natural rubber was subjected to oxidative degradation with KPS at 30 °C. Mooney viscosity decreased when the latex was degraded with 1.0 phr of KPS and it was dependent upon the amount of KPS. Molecular weight and gel content of the degraded natural rubber were about one-half as low as those of the source rubber. Chemical structure of the rubber was analyzed through Fourier transform infrared and ¹H NMR spectroscopic methods. The degraded natural rubber was found to contain carbonyl and formyl groups as an evidence of the oxidative degradation. Tensile strength of a vulcanizate prepared from the degraded natural rubber was the same as that prepared from the source rubber, even though the gel content and the molecular weight of the degraded rubber were distinguished from those of the source rubber.     

Sampling of tar from sewage sludge gasification using solid phase adsorption

Sewage sludge is a residue from wastewater treatment plants which is considered to be harmful to the environment and all living organisms. Gasification technology is a potential source of renewable energy that converts the sewage sludge into gases that can be used to generate energy or as raw material in chemical synthesis processes. But tar produced during gasification is one of the problems for the implementation of the gasification technology. Tar can condense on pipes and filters and may cause blockage and corrosion in the engines and turbines. Consequently, to minimize tar content in syngas, the ability to quantify tar levels in process streams is essential. The aim of this work was to develop an accurate tar sampling and analysis methodology using solid phase adsorption (SPA) in order to apply it to tar sampling from sewage sludge gasification gases. Four types of commercial SPA cartridges have been tested to determine the most suitable one for the sampling of individual tar compounds in such streams. Afterwards, the capacity, breakthrough volume and sample stability of the Supelclean? ENVI-Carb/NH₂, which is identified as the most suitable, have been determined. Basically, no significant influences from water, H₂S or NH₃ were detected. The cartridge was used in sampling real samples, and comparable results were obtained with the present and traditional methods.     

Hydrate capture CO2 from shifted synthesis gas,flue gas and sour natural gas or biogas

CO₂ capture by hydrate formation is a novel gas separation technology, by which CO₂ is selectively engaged in the cages of hydrate and is separated with other gases, based on the differences of phase equilibrium for CO₂ and other gases. However, rigorous temperature and pressure, high energy cost and industrialized hydration separator dragged the development of the hydrate based CO₂ capture. In this paper, the key problems in CO₂ capture from the different sources such as shifted synthesis gas, flue gas and sour natural gas or biogas were analyzed. For shifted synthesis gas and flue gas, its high energy consumption is the barrier, and for the sour natural gas or biogas (CO₂/CH₄ system), the bottleneck is how to enhance the selectivity of CO₂ hydration. For these gases, scale-up is the main difficulty. Also, this paper explored the possibility of separating different gases by selective hydrate formation and reviewed the progress of CO₂ separation from shifted synthesis gas, flue gas and sour natural gas or biogas.     

Plasma-Chemical Treatment of Process Gases with Low-Concentration Fluorine-Containing Components

The problems of recycling of the gases with low concentration of fluorine-containing components have been considered. The model of their plasma-chemical treatment with the heat recovery of effluent gases is proposed. The process thermodynamics for various compounds of reacting gases is considered. The heat- and mass transfer processes are simulated in the plasma-chemical reactor. Temperature and velocity profiles of treated gas flows have been obtained within the recovery zone, mixing area with the plasma jet as well as in the reactor working zone. The compliance of the resulting distributions and the temperature range needed for the full conversion is demonstrated. The experiments on the plasma-chemical treatment of carbon tetrafluoride and nitrogen trifluoride have been conducted. The resulting degrees of CF₄ and NF₃ decomposition turned out to be above 90% providing more than double reduction of specific energy cost as compared with the available facilities of similar purposes. An approach is proposed to reduce nitrogen oxide content in effluent gases at reactor outlet and to decrease in future the specific energy cost of conversion of halogen-containing gases.     

A new simple numerical model based on experimental scorch curve data fitting for the interpretation of sulphur vulcanization

Sulphur was the first agent used to vulcanize commercial elastomers (e.g. natural rubber) and allows meaningful cost reductions during the industrial process (production cost ratio between peroxides and accelerated sulphur is around 5). Therefore, accelerated sulphur vulcanization is the most popular technique for the production of polydiene and EPDM elastomers items. At present, crosslinking mechanisms are not analytically known in detail, therefore reticulation kinetic has to be deduced from mechanical properties obtained during standardized tests, as for instance the oscillating disc rheometer. In the present paper, we propose a numerical model to fit experimental rheometer data based on a simple composite three functions curve, able to describe the increase of the viscosity at successive curing times at different controlled temperature to use during the production of thick items vulcanized with sulphur. It is believed that rheometer curve is able to give an indirect information on the rubber reticulation kinetic at different temperatures, to use in a successive step to establish simplified analytical kinetic formulas to adopt in the accelerated sulphur vulcanization of polydiene and EPDM elastomers. In the model, it is necessary to collect rheometer curves at different specimen temperatures, because vulcanization in industrial practice occurs at variable temperatures during curing, with considerable differences from the core to boundary of the item. Once that rheometer curves are suitably collected in a database, they are used to predict the optimal vulcanization of real items industrially produced. Finally, a so called alternating tangent approach (AT) is implemented to determine optimal input parameters (curing external temperature T _n and rubber exposition time t) to use in the production process. Output mechanical property (objective function) to optimize is represented by the average tensile strength of the item. A meaningful example of engineering interest, consisting of a thick 2D EPDM cylinder is illustrated to validate the model proposed.     

Chemistry and concentration levels of free radicals in the stratosphere

Summary Free radicals in the atmosphere are the result of a thermodynamic non-equilibrium maintained by the presence of sunlight. For the stratosphere (15–50 km) the most important photochemical active region is that between 200–300 nm, involving O₂, O₃ and a small number of other source gases as the main photochemical source gases of free radicals. In this paper we briefly review the chemistry (sources and sinks) of the most important radical groups. Their associated concentration levels are derived from an one-dimensional (vertical) coupled chemical/dynamical model calculation. A comparison with measured concentrations is also made.     

Fast non-destructive determination of iron and sulphur in coal by means of neutron radiative capture

The possibility of determination of iron and sulphur in large-scale samples of coal /20–50 kg/, based on process /n, /, was evaluated. The spectral lines of the doublet 7631 keV and 7645 keV were used for the determination of iron, while the line at 5421 keV was used for the determination of sulphur. The neutron source was²⁵²Cf /total neutron emission at 2.5×10⁷ s^–1/ located additionally in D₂O moderator. A Ge/Li/ detector was used for gamma radiation detection. The calibration dependencies of the analyzer were linear. In exposure times of up to 1 h, the detection limits of 0.34% and 0.64% and accuracies of 0.25% and 0.4% have been achieved in case of iron and sulphur, respectively.