Liquid H2S Research Articles

Understanding why constant energy or constant temperature may affect nucleation behavior in MD simulations: A study of gas hydrate nucleation.

Molecular dynamics simulations have been widely used in exploring the nucleation behavior of many systems, including gas hydrates. Gas hydrates are ice-like solids in which gas molecules are trapped in water cages. During hydrate formation, a considerable amount of heat is released, and previous work has reported that the choice of temperature control scheme may affect the behavior of hydrate formation. The origins of this effect have remained an open question. To address this question, extensive NVE simulations and thermostatted (NPT and NVT) simulations with different temperature coupling strengths have been performed and compared for systems where a water nanodroplet is immersed in a H2S liquid. Detailed analysis of the hydrate structures and their mechanisms of formation has been carried out. Slower nucleation rates in NVE simulations in comparison to NPT simulations have been observed in agreement with previous studies. Probability distributions for various temperature measures along with their spatial distributions have been examined. Interestingly, a comparison of these temperature distributions reveals a small yet noticeable difference in the widths of the distributions for water. The somewhat reduced fluctuations in the temperature for the water species in the NVE simulations appear to be responsible for reducing the hydrate nucleation rate. We further conjecture that the NVE-impeded nucleation rate may be the result of the finite size of the surroundings (here the liquid H2S portion of the system). Additionally, a local spatial temperature gradient arising from the heat released during hydrate formation could not be detected.

A Hybrid Data-Physics Framework for Reservoir Performance Prediction with Application to H2S Production

Summary Model-based reservoir management workflows rely on the ability to generate predictions for large numbers of model and decision scenarios. When suitable simulators or models are not available or cannot be evaluated in a sufficiently short time frame, surrogate modeling techniques can be used instead. In the first part of this paper, we describe extensions of a recently developed open-source framework for creating and training flow network surrogate models, called FlowNet. In particular, we discuss functionality to reproduce historical well rates for wells with arbitrary trajectories, multiple perforated sections, and changing well type or injection phase, as one may encounter in large and complex fields with a long history. Furthermore, we discuss strategies for the placement of additional network nodes in the presence of flow barriers. Despite their flexibility and speed, the applicability of flow network models is limited to phenomena that can be simulated with available numerical simulators. Prediction of poorly understood physics, such as reservoir souring, may require a more data-driven approach. We discuss an extension of the FlowNet framework with a machine learning (ML) proxy for the purpose of generating predictions of H2S production rates. The combined data-physics proxy is trained on historical liquid volume rates, seawater fractions, and H2S production data from a real North Sea oil and gas field, and is then used to generate predictions of H2S production. Several experiments are presented in which the data source, data type, and length of the history are varied. Results indicate that, given a sufficient number of training data, FlowNet is able to produce reliable predictions of conventional oilfield quantities. An experiment performed with the ML proxy suggests that, at least for some production wells, useful predictions of H2S production can be obtained much faster and at much lower computational cost and complexity than would be possible with high-fidelity models. Finally, we discuss some of the current limitations of the approach and options to address them.

Born-Oppenheimer molecular dynamics and electronic properties of liquid H2S: The importance of a non-local approach to dispersion interactions

The dynamics and electronic properties of liquid H2S were investigated by Born–Oppenheimer molecular dynamics (BOMD). The Vydrov and Van Voorhis VV10 exchange–correlation functional [J. Chem. Phys. 133 (2010) 244103] that includes non-local correlation was adopted for generating the dynamics. This functional predicts the binding energy of the (H2S)2 dimer in very good agreement with experiment. Emphasis was placed on the structure, polarisation effects, and electronic absorption in liquid phase. The dipole moment in liquid H2S increases by 0.2D relative to the gas phase. A very small change (∼0.1 eV) in the position of the first maximum of the electronic absorption spectrum is observed when we compare liquid H2S and the isolated molecule. This is in contrast to liquid water where a [0.4-0.7] eV blue shift in the first peak of the absorption spectrum relative to the gas phase is observed. Exciton binding energies of H2S are calculated and compared with results for water. Their importance for explaining the difference between the blue-shift in H2S and water is discussed. The results indicate that although the (H2S)2 dimer is HB, some specific features characterising the structure and electronic properties of other HB liquids are not observed in liquid H2S. The interest of a non-local treatment of van der Waals interactions in liquid H2S is stressed.

Water content of liquid H2S in equilibrium with the hydrate phase

Acid gas injection containing H2S is one strategy for managing atmospheric sulfur emissions where sulfur recovery may not be feasible or economical in some locations. Apart from its toxicity and corrosivity in wet environments, H2S forms a high temperature hydrate in the presence of water, which can cause issues such as plugging of transport pipelines or injection wellheads. Before transportation to injection, liquid water is removed from an injectate within the interstage suction scrubbers before each compression cycle and/or by further dehydration. Water content data are important in order to determine and define the dehydration requirements of acid gas injectates. Prior to this work, there were no water content data for H2S at hydrate forming conditions because of the difficulties in measuring low concentration of water above hydrates and high H2S toxicity. In this study, we report the water content of pure H2S in the hydrate forming regions from p = 4.178 to 20.572 MPa and T = 247.65 to 298.28 K using tunable diode laser spectroscopy. The measured data show a relative water content difference of ca. 2.5% from the model reported in this work using the Sloan et al. (1976), and van der Waal & Platteeuw (1959) equations for the hydrate phase, and the fluid phase model of Bernard et al. (2012).

Liquid-Redox Process Used for Sulfur Removal on an FPSO

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper 2019-1016 OMC, “Sulfur Removal on an FPSO: A Liquid-Redox-Process Case Study,” by William I. Echt, Merichem, prepared for the 2019 Offshore Mediterranean Conference and Exhibition, Ravenna, Italy, 27-29 March. The paper has not been peer reviewed. Eni began producing oil reserves from the Aquila reservoir in the Adriatic Sea soon after its discovery in the early 1980s. As primary production decreased, a decision was made to begin enhanced recovery with artificial gas lift. With the play in deep water (815 m) and 46 km off the southern coast of Italy, a floating production, storage, and offloading (FPSO) vessel was needed. After a 5-year run, the Firenze halted operations in 2018 because of low oil production. The complete paper examines the decision to use hydrogen-sulfide- (H2S) removal technology, the cost of operation, and the unit’s availability over its lifetime. Introduction As the industry searches for reserves in ever-deeper formations, the requirement of contending with sulfur increases. Several H2S-removal technologies are available, including nonregenerative liquid scavengers (triazine-based), nonregenerative solid-bed absorbents, and the regenerative liquid-reduction/oxidation (redox) process. These technologies remove sulfur from associated gas streams and do not release them to the environment. The nonregenerative technologies are often referred to as scavengers. Process Evaluation During the initial design phase, several H2S-removal technologies were evaluated per the following criteria: Turndown capability H2S-removal efficiency Degree of operator involvement required Maintenance requirement and waste material produced Proven reliability in marine conditions The evaluation led to the selection of the liquid-redox process after it received the highest marks in four of the five criteria. Both liquid and solid H2S scavengers were considered as alternatives to the liquid-redox process for this installation. The considerations for each criterion are detailed in the complete paper.

The H2S dimer revisited – Insights from wave-function and density functional theory methods. Ab initio molecular dynamics simulations of liquid H2S

We performed an investigation of the hydrogen bonding in H2S dimers in gas-phase using HF, MP2, PBE, PBE0 and BLYP. A series of symmetry operations such as translations and proper and improper rotations were performed to the donor and acceptor molecules, and the energetic dependency of the H-bond as a function of these transformations was obtained. We found that the DFT methods predict a repulsive region around 5 to 6 times more repulsive that that obtained with the wave-function methods. Additionally, we determined the Lennard-Jones potential for the translations and derived a Morse potential for the different rotations and ab initio MD simulations were performed for liquid H2S. We found that remarkably, the length of the H-bonds and height of rotational barriers in liquid H2S are similar to the ones of the dimer. This is because of the weak and highly directional H-bonds in both the liquid and H2S dimer.

Atomic Layer Deposition of Tin Monosulfide Using Vapor from Liquid Bis(N,N'-diisopropylformamidinato)tin(II) and H2S.

The oxide and sulfide of divalent tin show considerable promise for sustainable thin-film optoelectronics, as transparent conducting and light absorbing p-type layers, respectively. Chemical vapor deposition (CVD) and atomic layer deposition (ALD) provide attractive routes to these layers. The literature on volatile tin(II) compounds used as CVD or ALD precursors shows that new compounds can provide different growth rates, film morphologies, preferred crystallographic orientations, and other material properties. We report here the synthesis and characterization of a new liquid tin(II) precursor, bis(N,N'-diisopropylformamidinato)tin(II) (1), which is effective in ALD of SnS in combination with H2S between 65 and 180 °C. Like other highly reactive tin(II) precursors, the growth per cycle linearly decreases from 0.82 Å/cycle at 65 °C to 0.4 Å/cycle at 180 °C. This is obviously different from the case of previously reported SnS ALD using bis(2,4-pentanedionato)tin(II), Sn(acac)2, and H2S; films grow at 0.22-0.24 Å/cycle almost independent of the substrate temperature (125-225 °C, J. Phys. Chem. C 2010, 114, 17597). Quartz crystal microbalance (QCM) experiments for SnS ALD using 1 at 80, 120, and 160 °C were carried out to study the linear decrease of the growth per cycle with increasing substrate temperature. On the basis of these QCM studies, although the mechanism of chemisorption-loss of one ligand or two-can be manipulated by changing the exposure of 1, the purging time, or the temperature, only the temperature changes the growth per cycle. We therefore attribute the decreasing growth per cycle with increasing temperature to a decreasing surface thiol density. Photovoltaic devices prepared from 1-derived SnS have a performance similar to those of the best devices prepared from other precursors, and the device yield and replicability of J-V properties are substantially increased by using 1.

Effect of liquid surface area on hydrogen sulfide oxidation during micro-aeration in dairy manure digesters

Although there are a variety of commercially available biological and chemical treatments for removal of hydrogen sulfide (H2S) from biogas, managing biogas H2S remains a significant challenge for agricultural digesters where labor and operational funds are very limited compared to municipal and industrial digesters. The objectives of this study were to evaluate headspace aeration for reducing H2S levels in low cost plug flow digesters and to characterize the relationship between the liquid surface area and H2S oxidation rates. Experiments with replicate field scale plug flow digesters showed that H2S levels decreased from 3500 ppmv to <100 ppmv when headspace oxygen levels were 0.5 to 1%. Methane production was not affected by aeration rates that resulted in headspace oxygen levels of up to 1%. Pilot scale experiments using 65 to 104 L desulfurization units showed that H2S oxidation rates increased with increases in liquid surface area. These results support the hypothesis that H2S oxidation rates are limited, in part, by the surface area available for oxygen transfer, and can be increased by growth of biofilms containing H2S oxidizing bacteria. Maximum removal rates corresponded to 40 to 100 g S m-2 d-1 of liquid surface area at biogas retention times of 30 to 40 min.

Simulation of Liquid and Supercritical Hydrogen Sulfide and of Alkali Ions in the Pure and Aqueous Liquid

A polarizable model for hydrogen sulfide (H2S) is optimized based on the experimental properties of the monomer and of the bulk liquid. The model is characterized by rigid SH bonds but flexible HSH angle and the polarizability is based on the Drude oscillator model. Bonded parameters and atomic charges are based on the experimental properties of the gaseous monomer. Atomic Lennard-Jones (LJ) parameters are adjusted based on the density of H2S around the critical point (in the temperature range 363-393 K and pressure range 8.023-10.013 MPa). The model gives binding energies for H2S dimers, trimers, and tetramers in good agreement with ab initio MP2(full)/6-311++G(d,p) results. It shows a liquid structure in very good agreement with neutron diffraction data. The model also gives density, self-diffusion coefficient, heat of vaporization, and dielectric constant of liquid hydrogen sulfide at the normal boiling point in good agreement with experimental data. In addition, the model is transferable to high temperature and pressure conditions, as evidenced from simulations up to 542.2 K and 40 MPa. The model is used in combination with the SWM4-NDP water model, with LJ parameters between the S and O atoms adjusted to reproduce the experimental hydration free energy of H2S. Simulations suggest that, in its first solvation shell, a single H2O molecule is solvated by 10 H2S molecules while a single H2S molecule is solvated by 20.5 H2O molecules. Pair-specific LJ parameters between alkali ions (Li(+), Na(+), K(+), Rb(+), Cs(+)) and the S atom are adjusted to reproduce ab initio binding energies of the ion-H2S pairs at the CCSD(T) level. Simulations based on these parameters show that alkali ions have higher coordination numbers and lower solvation free energies in liquid H2S than in liquid water or liquid ammonia. The model is also used to investigate the preferential solvation of the ions in aqueous solutions with a 10% H2S mole fraction. Results show that the ions are preferentially solvated by water in their first solvation shell but have no significant selectivity to either ligands in their second shells.

Simulation of hydrogen sulphide absorption in alkaline solution using a packed column

In this work, a simulation tool was developed for hydrogen sulphide (H2S) removal in an alkaline solution in packed columns working at countercurrent. Modelling takes into account the mass-transfer enhancement due to the reversible reactions between H2S and the alkaline species (, and HO−) in the liquid film. Many parameters can be controlled by the user such as the gas and liquid inlet H2S concentrations, the gas and liquid flow rates, the scrubbing liquid pH, the desired H2S removal efficiency, the temperature, the alkalinity, etc. Since the influence of the hydrodynamic and mass-transfer performances in a packed column is well known, the numerical resolutions performed were dedicated to the study of the influence of the chemical conditions (through the pH and the alkalinity), the temperature and the liquid-to-gas mass flow rate ratio (L/G). A packed column of 3 m equipped with a given random packing material working at countercurrent and steady state has been modelled. The results show that the H2S removal efficiency increases with the L/G, the pH, the alkalinity and more surprisingly with the temperature. Alkalinity has a very significant effect on the removal efficiency through the mass-transfer enhancement and buffering effect, which limits pH decreasing due to H2S absorption. This numerical resolution provides a tool for designers and researchers involved in H2S treatment to understand deeper the process and optimize their processes.

Rate of H2S and CO2 attack on pozzolan-amended Class H well cement under geologic sequestration conditions

Experiments were conducted to investigate the rates of H2S and CO2 alteration of pozzolan-amended wellbore cement (35vol% pozzolan–65vol% cement), so as to evaluate the potential impact of H2S and CO2 induced degradation of existing cemented wells present at acid gas co-sequestration sites. In the exposure experiments, pozzolan-amended cement samples were mixed, cured and exposed to mixtures of H2S and CO2 under lab-simulated geologic sequestration conditions (50°C and 15.2MPa) for 2.5, 9, 28 and 90 days. Measurement of the carbon alteration front was used to calculate the rate of CO2 alteration of pozzolan-amended cement exposed to a mixture of 79mol% CO2 and 21mol% H2S under geologic sequestration conditions in exposure periods of 0–90 days. Average CO2 alteration rates (rate of movement of CaCO3 precipitation front) were 3.3×10−3mm/day and 3.2×10−3mm/day for cement samples exposed to a 1% NaCl solution saturated with CO2 and H2S, and those in contact with a supercritical mixture of CO2 and H2S, respectively. Two scenarios were considered for measuring and quantifying alteration caused by H2S over the exposure periods of 0–90 days: sulfur-rich zone thickness, and sulfur alteration index. The average rate of H2S alteration determined by sulfur-rich zone thickness divided by exposure duration was 4.3×10−3mm/day for cement exposed to 1% NaCl solution saturated with CO2 and H2S, and the average rate of H2S alteration determined by sulfur alteration index divided by exposure duration was 8.2×10−3day−1. Cement exposed to a supercritical mixture of CO2 and H2S result in H2S alteration rates determined by sulfur-rich zone thickness divided by exposure duration of 3.1×10−3mm/day, and average rates of H2S alteration determined by sulfur alteration index divided by exposure duration of 6.3×10−3day−1. Sulfur alteration index results also show that H2S was able to penetrate to the core of pozzolan-amended wellbore cement after 2.5 days of exposure, though this was not readily apparent in the sulfur-rich zone thickness results. Sulfur-rich zone thickness is best used to describe high-level sulfur alteration in the rim of samples. The results indicate that (1) an aqueous environment is more favorable for H2S attack on pozzolan-amended cement than a supercritical CO2 and H2S environment; (2) for 90 days of exposure significant alteration induced by H2S and CO2 occurs at regions very close to the fluid/cement interface; (3) H2S penetrates pozzolan-amended cement more rapidly than CO2 in aqueous contact environments. In contrast, under supercritical liquid environment, H2S and CO2 have similar penetration rates in pozzolan-amended cement.

A Drude Polarizable Model for Liquid Hydrogen Sulfide

A polarizable force field for liquid hydrogen sulfide (H2S) has been developed based on the Drude oscillator model. This force field has been designed to be analogous to the SWM4-NDP water model; the model is rigid with point charges assigned to the H and S atoms and a lone pair on the bisector of ∠HSH in the molecular plane. Positions of the lone pair and the charges have been defined such that the model has a static dipole moment of 0.98 D, equal to the experimental value. Polarizability is incorporated by a charged (Drude) particle attached to the S atom through a harmonic potential. Intermolecular nonbonded forces are included by use of a Lennard-Jones potential between S atoms. The model was parametrized to reproduce the density, enthalpy of vaporization, and dielectric constant of pure H2S at 212 K and 1 atm. The calculated density, enthalpy of vaporization, shear viscosity coefficient, and self-diffusion coefficient are in good agreement with experiment over the temperature range 212-298 K along the liquid-vapor coexistence curve of liquid H2S. The radial distribution function calculated from this model is in good agreement with experimental diffraction data and ab initio molecular dynamics simulations.

Study on the wellbore flow and phase transition during formation H&lt;SUB align="right"&gt;2S invasion

Considering the special physical properties of hydrogen sulphide (H2S) and its phase transition in the wellbore, a mathematical model for wellbore flow and heat transfer during formation H2S invasion was established. Heat transfer and pressure were calculated along with H2S physical properties parameters, and a solution method of the model was proposed. The results indicate that liquid H2S transforms into gas state during its rising process in the annulus when choke pressure is low; the flow above phase transition point is two-phase flow while the flow below phase transition point is single-phase flow in the annulus. The paper exhibits the results and the reasons of the bottom-hole pressure and the depth of phase transition varying with different H2S influx rate, injection rate, choke pressure, and land surface temperature. The study is of certain significance for improving the safety during the exploration of acidic oil and gas reservoirs.

Are rocks still water‐wet in the presence of dense CO2 or H2S?

AbstractThe various modes of acid gas storage in aquifers, namely structural, residual, and local capillary trapping, are effective only if the rock remains water‐wet. This paper reports an evaluation, by means of the captive‐bubble method, of the water‐wet character in presence of dense acid gases (CO2, H2S) of typical rock‐forming minerals such as mica, quartz, calcite, and of a carbonate‐rich rock sampled from the caprock of a CO2 storage reservoir in the South‐West of France. The method, which is improved from that previously implemented with similar systems by Chiquet et al. (Geofluids 2007; 7: 112), allows the advancing and receding contact angles, as well as the adhesion behavior of the acid gas on the mineral substrate, to be evaluated over a large range of temperatures (up to 140°C), pressures (up to 150 bar), and brine salinities (up to NaCl saturation) representative of various geological storage conditions. The water‐receding (or gas‐advancing) angle that controls structural and local capillary trapping is observed to be not significantly altered in the presence of dense CO2 or H2S. In contrast, some alteration of the water‐advancing (or gas‐receding) angle involved in residual trapping is observed, along with acid gas adhesion, particularly on mica. A spectacular wettability reversal is even observed with mica and liquid H2S. These results complement other recent observations on similar systems and present analogies with the wetting behavior of crude oil/brine/mineral systems, which has been thoroughly studied over the past decades. An insight is given into the interfacial forces that govern wettability in acid gas‐bearing aquifers, and the consequences for acid gas geological storage are discussed along with open questions for future work.

Degradation Kinetics of Fe-EDTA in Hydrogen Sulfide Removal Process

Available data on the degradation of Fe-EDTA liquid redox H2S removal processes are reviewed, and the effect of H2S molar flow rate, the initial concentration of Fe(III)EDTA, and the presence of sodium citrate in Fe-EDTA solution were investigated in this study. The semibatch with continuous flow of H2S containing biogas was used under a wide range of experimental conditions; , H2S molar flow rate, (1.08 × 10−3–3.40 × 10−3 mol/h), the initial concentration of Fe(III)EDTA, (2.17–8.16 mol/m3), and the concentration of sodium citrate, (0–300 mol/m3). The result showed that sodium citrate acted as stabilizer with a good ability to reduce the degradation rate. The degradation rate of Fe-EDTA was found to follow pseudo first-order kinetics. Empirical correlations expressed the degradation rate constant as a function of significant H2S molar flow rate, and the initial Fe(III)EDTA and sodium citrate concentration were successfully developed for the prediction of Fe-EDTA degradation rate. Moreover, the precipitated solid, called sulfur cake, was recovered, and its composition was investigated. The result revealed that the sulfur cake contained more than 98% sulfur element and almost balances with iron, and no significant EDTA was degraded into the solid form.

Macroscopic Nanotemplating of Semiconductor Films with Hydrogen‐Bonded Lyotropic Liquid Crystals

AbstractAqueous gel‐like lyotropic liquid crystals with extensive hydrogen bonding and nanoscale hydrophilic compartments have been used to define the growth of macroscopic nanotemplated CdS and CdTe thin films. These mesoporous semiconductor films contain a hexagonal array of 2.5 nm pores, 7 nm center‐to‐center, that extend in an aligned fashion perpendicular to the substrate. The CdS is deposited on a polypropylene substrate by a reaction between Cd(NO3)2 dissolved in the liquid crystal and H2S transported via diffusion through the substrate. The CdTe is electrodeposited on indium‐tin‐oxide‐coated glass from TeO2 and Cd(NO3)2, both of which are dissolved in the liquid‐crystal template. The porous nature of the CdTe films enables chemical transformations of the entire bulk of the film. As electrodeposited, the CdTe films are Te rich and, in contrast to a non‐templated film, the excess Te could be removed via a chemical treatment, proving the continuity of the pores in the nanotemplated films. These results suggest that liquid‐crystal lithography with hydrogen‐bonding amphiphiles may be a useful approach to create materials with nanoscale features over macroscopic dimensions.

Densities of Toluene, Carbon Dioxide, Carbonyl Sulfide, and Hydrogen Sulfide over a Wide Temperature and Pressure Range in the Sub- and Supercritical State

Densities of toluene, carbon dioxide (CO2), carbonyl sulfide (COS), and hydrogen sulfide (H2S) have been measured with a new computer-controlled high-temperature high-pressure vibrating tube densimeter system (DMA-HDT) for temperatures from 273 K up to 623 K and pressures up to 40 MPa in the sub- and supercritical state. With respect to accuracy, reliability, suitability, and time consumption this system represents an optimum for measuring PρTproperties in the compressed liquid and supercritical state. Densities of liquid toluene, COS, and H2S as a function of temperature and pressure were correlated with the new three-dimensional density correlation system (TRIDEN). The PρT data from CO2 and the data from COS and H2S in the supercritical state and around the critical point were correlated with a virial-type equation of state. For checking the accuracy and suitability of the vibrating tube densimeter system the experimental densities of toluene and CO2 were compared with the results obtained using referen...

Synthesis and Structure of a S4Si4 Cage Compound

The reaction of the tetraaminodisilane R2Si2(NH2)4 (R = CH(SiMe3)2) with liquid H2S at −70 °C resulted in the formation of a R4S4Si4 cage. The core of the molecule consists of four five-membered rings. Two disilane units are bridged cross over by four sulfur atoms. This is a new structural type in the group of silicon−sulfur ring and cluster compounds.

Voronoi polyhedra analysis of the local structure of water from ambient to supercritical conditions

The changes of the local structure of water are investigated with increasing temperature and decreasing density from ambient conditions up to the critical point. The local structure around the molecules is analyzed in terms of the properties of the Voronoi polyhedra (VP) of the molecules. For reference, the entire analysis is also performed in liquid H2S. In this way, the effect of the hydrogen bonding on the local structure can also be studied. In determining the VP of the molecules an exact algorithm is used. In analyzing the local structure it is found that the decrease of the density not only leads to the increase of the free volume available for the molecules, but it also increases the volume of the voids present in the system. A linear relation is found between the average free volume of the molecules and the average volume of the vacancies, the latter being always about the double of the former. The comparison of the results concerning the shape of the VP of the molecules obtained for water at different thermodynamic state points and for liquid H2S clearly reveals that even above the critical point, hydrogen bonds are still playing a very important role in forming the molecular level structure of water. It is also found that the tetrahedral arrangement of the nearest-neighbor molecules, which is a characteristic feature of the water structure under ambient conditions, already disappears at temperatures well below the critical point, and the relative arrangement of the first four nearest-neighbor molecules becomes rather similar to that in liquid H2S.

Orientational correlations in liquid hydrogen sulphide

Neutron diffraction data on liquid H2S, D2S and an equimolar mixture of the two are presented for two thermodynamic states along the liquid—vapour coexistence curve. Comparison of the site-site radial distribution functions obtained with those of a model fluid where orientational correlations are absent and with those of water demonstrates that, although hydrogen sulphide does not form hydrogen bonds, nevertheless weak orientational correlations between first neighbouring molecules are detectable in both thermodynamic states. An analysis of the molecular configurations compatible with the experimental data, as calculated by applying the empirical potential structural refinement, is presented also.