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Modifying Effects of Hydrogen Sulfide When Contemplating Subsurface Injection of Sulfur

Mitchell J. Stashick, Gabriel O. Sofekun and Robert A. Marriott*

Department of Chemistry, University of Calgary, Calgary, Alberta, Canada

Abstract

Transportation and handling of molten sulfur is inevitably challenging due to the anomalous rheological behaviour of sulfur with changing temperature. After melting at 115 °C, sulfur's viscosity remains close to 10 cP. The onset of the ?-transition region is observed at T ~ 160 °C. The viscosity drastically rises in this region, increasing to a maximum of about 93000 cP at 187 °C. This anomaly is due to the cleavage of sulfur rings and production of reactive diradical sulfur species that concatenate to create sulfur polymers. The entanglement of these sulfur chains causes the dramatic rise in viscosity. Within this work, new data is reported that examines the modifying effects of hydrogen sulfide (H2S) within liquid sulfur. This may be used when considering the potential injection of liquid sulfur into depleted reservoirs for safe long-term storage. H2S, when present in liquid sulfur, can either physically dissolve or chemically react to generate polysulfane. The chemical reaction causes significant changes in the sulfur chain distribution and consequently changes the viscosity curve of liquid sulfur as a function of temperature. This study reports viscosity measurements performed from 120 < T < 280 °C with concentrations of H2S in sulfur ranging from 0 ppmw to 284 ppmw.

Keywords: Rheology, viscosity, ?-transition, sulfur, polymers, hydrogen sulfide

1.1 Introduction

Sulfur requires transportation and handling on an industrial scale. After acknowledging that there were approximately eighty million metric tons of elemental sulfur produced globally in 2018, this becomes abundantly clear [1]. Sulfur recovery from oil refineries and sour gas treatment plants (sour gas is natural gas with hydrogen sulfide (H2S) > 5.7 mgm-3) has continued to increase, bringing about more deliberation on the utilization and/ or storage of sulfur product. When the value of sulfur is not high enough to economically justify transportation and sale of the material, it is often common to block pour for long-term storage. Block pouring sulfur is a relatively inexpensive method where sulfur is solidified within retaining walls on large plots of land. However, there are some disadvantages to this technique. The land on which the blocks are poured must be leased or owned and this can cause some financial strain. Also, bacteria classified as Thiobacilli are known to metabolize sulfur by oxidizing the material to sulfate and in doing so produce sulfuric acid (H2SO4). This along with rain and further weathering results in the need for water treatment of the acid runoff, incurring additional cost. Lastly, large amounts of weathering sustained by sulfur blocks can yield high content of materials such as dirt, sand and moisture. This can create issues of purity if the price of sulfur rises and it makes economic sense to sell the product again [2]. Alternatively, producers have chosen to inject H2S (acid gas injection) to mitigate stranded sulfur. With acid gas injection, one needs to consider the energy required for compression of gas, storage under pressure and the loss of energy due to eliminating the Claus plant.

In future operations, an alternative to block pouring or acid gas injection could include the potential injection of liquid elemental sulfur into depleted reservoirs for safe long-term storage. This method would diminish some of the issues experienced with block pouring, but would likely sustain other costs needed for infrastructure and operation. Also, other fundamental understanding is currently lacking in order to pursue this approach. The conditions under which an injection such as this would take place must be known and understood in how they will impact the rheological behavior of sulfur.

One of several conditions to consider is that all sulfur produced in Sulfur Recovery Units at refineries and gas processing facilities will contain some residual H2S. This is because sulfur is recovered before complete conversion from H2S. Generally, recovered sulfur from a plant process can have up to 380 ppmw of dissolved H2S depending on the facility set up [3]. Another condition to consider is the temperature of the depleted underground reservoir. For this, the range of interest for potential injection was found to be around 120 < T < 280 °C. It should be recognized that this temperature range includes the ?-transition region of liquid sulfur. Also, high shear must be considered as this will be encountered in sulfur pumps and the near-wellbore region. Studying sulfur's shear flow with and without H2S over the specified temperature range could therefore greatly improve the fundamental understanding needed for injection of molten sulfur.

The viscosity temperature dependence of sulfur within the ?-transition region can be explained by a scission-recombination equilibrium for polymers and the term reptation. In this occurrence, reptation is defined as the thermal motion of entangled macromolecular sulfur chains. At T > 160 °C, cleavage of sulfur S8 rings results in the generation of reactive diradical sulfur species that combine to create sulfur polymers [4]. The average polymer chain lengths increases as a function of temperature leading to more entanglement and higher viscosity values. At 187°C, the maximum polymer chain length is reached, along with the maximum viscosity. Beyond this temperature, the viscosity decreases due to the thermal scission of polymer chains; however, if H2S is present, the viscosity profile of sulfur changes depending on concentration. H2S is known to chemically react with diradical sulfur species during the ?-transition to produce polysulfane, as shown in Eq. (1-1). This reaction terminates the polymerization, which reduces the average sulfur polymer chain length. Therefore, a reduction in the viscosity of the liquid is observed [5-7].

1.2 Experimental

1.2.1 Materials

Oxygen-free, nitrogen gas (N2; 99.998% as per certificate of analysis) was purchased from Praxair Technology, Inc. Hydrogen sulfide gas (H2S; 99.6% containing N2, Ar and trace CO2 as per certificate of analysis and in-house gas chromatography analysis) was purchased from Praxair Technology, Inc.

Sulfur used for this study was air prilled and supplied by Keyera Energy from their Strachan, Alberta, gas plant in 2017. Analyses of the air prills were performed and reported in Table 1.1. Moisture (volatiles) content, total nonpolymer sulfur content/overall purity, and ash content were determined by methods used at ASRL, reported by Sofekun et al. [4]. Total carbon content and H2S content were also performed on the sulfur prills. Additionally, it was necessary to perform analyses of H2S content before and after rheometric measurements. Total carbon content was measured using an analysis method established by Dowling et al. [8]. H2S content was measured using an analysis method detailed by Sofekun et al., Marriott et al., and Tuoro and Wiewioroski [3, 4, 9, 10]. Large sulfur samples that had been equilibrated with H2S were partially used to record infrared spectra (six total measurements) prior to each set of rheometric measurements. All samples were analyzed using a Nicolet 380 FT-IR (Thermo Electron Corp.) spectrometer.

Table 1.1 Analytical results of the Keyera air prilled sulfur.

1.2.2 Rheometer

The experimental system was composed of an Anton-Paar MCR 302 rheometer with a custom built gravity-fed charging rig, a high-pressure N cylinder and a thermostated Fourier Transform Infrared (FT-IR) cell used2 fromprevious studies [4, 9, 10]. A head pressure tee for inducing H2S partial pressures along with a purging setup connected to KOH traps for removal of H2S before venting was added. This permitted the safe introduction of differing concentrations of H2S in liquid sulfur samples. All valves and tubing used for both the pressure tee and gravity-fed charging rig were made of 316L stainless steel. The system was housed inside a negative-pressure bay equipped with automatic toxic gas detectors.

Geometry and measuring limitations for the Anton-Paar MCR 302 were detailed by Sofekun et al. [4]. Air checks and motor adjustments were performed at a rotational speed of 0.3 min-1 at set time intervals as recommended by the instrument manufacturer. This confirmed on a regular basis that the rheometer was in good working condition.

The procurement of viscosity data sets corresponding to each differing H2S concentration in sulfur were carried out using the following experimental procedure: Two hundred grams of high purity sulfur prills were placed in an oven overnight at 135 °C. The experimental system was prepared for rheometric measurements of liquid elemental sulfur by inputting set points for the temperature controllers on the gravity-fed charging rig and rheometer cylinder jacket of 135 °C. After...