Ind. Eng. Chem. Res. 1996, 35, 1257-1262
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An Experimental Study on the Kinetics of the Formation and Decomposition of Sulfanes in the Sulfur/H2S System Ingrid Winter* and Volker Meyn German Petroleum Institute, Walther-Nernst Strasse 7, D-38678 Clausthal-Zellerfeld, Germany
A gas chromatographic technique has been worked out which allows one to determine the kinetics of the formation and decomposition of sulfanes in the sulfur-hydrogen sulfide system at high temperature. Quantitative kinetic data are of great importance not only for research in that field but also for various practical applications like, for example, the sulfur deposition problem in sour gas reservoirs. Kinetic measurements have been carried out at different constant temperatures in the range of 120-270 °C in a chromatographic reactor, containing sulfur-coated glass beads and quartz powder, respectively. In order to determine the kinetic parameters, a numerical model of the gas chromatographic reactor has been developed. Based on the results obtained by fitting the experimental chromatograms to the model, a simplified reaction scheme is proposed. Preliminary results show that sulfane decomposition in the sulfur/H2S system is a rather slow reaction. Introduction Sulfanes are polymeric sulfur compounds of the composition H2Sx (with x g 2). At room temperature sulfanes are thermodynamically unstable with respect to H2S and sulfur. Since the decomposition is catalyzed by many substances, e.g., alkali, water, dust, and rough surfaces, pure sulfanes are extremely difficult to prepare and to handle. The decomposition without catalysts requires rather high activation energies (105 kJ/mol for H2S2) (Fehe´r and Weber, 1957). The formation of sulfanes in the sulfur/H2S system was first postulated by Fanelli, who investigated the solubility of hydrogen sulfide in liquid sulfur. He showed that the H2S solubility increases sharply between 120 and 180 °C, proceeds through a broad maximum, and then decreases at temperatures above 385 °C (Fanelli, 1949). He interpreted these observations by the suggestion that a chemical reaction between sulfur and H2S takes place, resulting in the formation of sulfanes. In later years this prediction was confirmed experimentally by using infrared (Wiewiorowski and Touro, 1966) and NMR techniques (Hyne et al., 1966). Only little is known about the reactivity of the sulfur/ H2S system. Nevertheless, quantitative kinetic data are of great interest not only for research in that field but also for various practical reasons. One important example is the sulfur deposition problem associated with the exploitation of natural gas fields containing H2S. Production of such “sour gases” is often severely hampered due to sulfur depositing in the reservoir, producting tubing, and surface equipment (Hyne, 1968). It is believed that sulfur, which is present in the reservoir in the elemental form, is transported by sour gas not only as elemental sulfur but also in the form of sulfanes, resulting from a chemical reaction between sulfur and H2S (Hyne, 1986). It was shown that the partial pressuresor the “physical solubility”sof sulfur in sour gases strongly increases with increasing pressure, temperature, and H2S content (Brunner and Woll, 1980). During the production process, the equilibrium concentration in the gas phase decreases as a result of the temperature and pressure drops existing between the reservoir and the well head, and sulfur is deposited in the liquid or solid state. An experimental proof of sulfane formation at reservoir conditions is still missing. Nevertheless, higher 0888-5885/96/2635-1257$12.00/0
pressures will force the chemical equilibrium to higher sulfane concentrations, and it can be assumed that in sour gas reservoirs considerable amounts of these compounds are present in the gas phase, which decompose back into sulfur and hydrogen sulfide in the pressure drop-off zone of production facilities (Hyne, 1986). A reference to the existence of sulfanes in the gas phase of the liquid sulfur/H2S system at high pressures (8 MPa) was obtained by a mass spectrometric method. To this, the high-pressure gas phase was introduced through a cooled aperture system directly into the ionization chamber of a mass spectrometer. Sulfanes up to H2S6 were detected (Meyn, unpublished). The question whether the sulfur transport in sour gases proceeds via physically solved elemental sulfur or the formation of volatile sulfanes is of great practical importance. If the sulfur is dissolved physically, its solubility will respond virtually instantaneously to changes in pressure and temperature; i.e., sulfur deposition will occur as soon as saturation conditions are reached along the production flow path. If, however, the sulfur is bonded chemically, the attainment of saturation conditions involves a chemical decomposition which may be associated with a delay in sulfur deposition. By choosing appropriate production conditions, deposition might be controlled so far that most of the sulfur deposits beyond the well head, thus being easier to handle. For an effective control, first of all, more about the reactivity of the sulfur/H2S system must be known. Experimental Method For the study of the sulfur/H2S system the kinetic gas chromatographic technique in combination with a mass spectrometric detection was used. In this method, a pulse of hydrogen sulfide is fed into the helium carrier gas stream at the inlet of a chromatographic column containing liquid sulfur as the stationary phase. During the passage of the H2S pulse through the column, chemical reactions take place. Simultaneously, partial separation of the reaction mixture occurs, the extent of which depends on the differences in the partition coefficients of the individual compounds. In kinetic measurements, residence or reaction times can be © 1996 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996
readily varied by changing the carrier gas flow rate. A mathematical analysis of the elution profile yields the kinetic parameters and the partition coefficients of mobile compounds. One of the advantages of the gas chromatographic method is that volatile reactants, like, for example, the very poisonous hydrogen sulfide, are easy to handle. Only small quantities of sample are required, and the amount of material introduced to the column and eluted from it is easy to measure. With regard to the formation of sulfanes in sour gas reservoirs, it is also important that measurements at high pressure can be carried out. By a variation of the sulfur support, the influence of the reservoir rock on the formation and decomposition of sulfanes may be investigated. To develop a chromatographic column suitable for the purposes of this study, first of all, a material had to be identified which can be coated with liquid sulfur and which is inert to such an extent that it does not react either with sulfur or with any other compound of the reaction mixture. As quartz is a rather inert material, we first tried to coat a fused silica capillary column with liquid sulfur. Unfortunately, this attempt failed because pure sulfur does not wet SiO2 surfaces. The next try to deposit sulfur on the porous layer of a commercial Al2O3 capillary column was successful, and sulfur layers at the range of a few micrometers could be obtained. Experiments using this setup showed, however, that even small changes in the thickness of the liquid sulfur phase lead to a large acceleration of the reaction; i.e., Al2O3 acts as a strong catalyst. Since with other commercially available porous layer columns similar effects are to be expected, we dropped the idea of using capillary columns and began to test various packed columns. As support of the liquid sulfur phase, we used deactivated materials, namely, silane-treated glass beads and an acid-washed quartz powder. The column material itself was Pyrex glass and quartz glass, respectively. To keep the influence of side effects as small as possible, the transfer lines between the chromatographic column and the detector were made of fused silica tubings. Vespel graphite was used as the sealant. As reagents, sulfur, purified by the method of Wartenberg (1956), with a carbon content of less than 10-4% and hydrogen sulfide with a purity of >99.7% were employed. Before each run, the chromatographic column was conditioned at high temperatures, applying a predetermined helium flow. This procedure removes traces of sulfanes and hydrogen sulfide which could be dissolved in the liquid sulfur phase from previous runs. Independent of the reactor type chosen, there remains the problem that the stationary liquid sulfur phase tends to bleed from the column as temperature is increased. To avoid a reduction of detector sensitivity due to the resulting high sulfur background, special devices were used. In one experimental setup, the eluted sulfur was trapped by a cartridge filled with a porous polymer. This cartridge was installed between the chromatographic column and the detector. Since volatile sulfanes could also be held back, in another setup the sorbent cartridge was exchanged for a so-called life valve. A cross-sectional diagram of this device is shown in Figure 1. Essentially, it consists of a column connector, equipped with two additional gas inlet and outlet ports. By giving a continuous helium stream to the gas inlet port and periodically shutting the gas outlet port, well-defined pulses of the column effluent are given to
Figure 1. Cross-sectional diagram of the life valve.
Figure 2. Kinetic gas chromatography: experimental setup with life valve.
Figure 3. Chromatograms (ion 34) of the reaction S(fl) + H2S(g) f H2Sx f H2S: column, quartz; length, 150 cm; inner diameter, 3 mm; support, acid-washed quartz powder (particle diameter: 60-200 µm); sulfur, 4.5 g; H2S, 1.28 × 10-6 mol; carrier gas, helium (2.8 mL/min).
the detector at predetermined intervals of time. In order to achieve a more complete separation of the reactants in the pulse, a short analytical column (length ) 11 m, i.e., retention times < 30 s) was inserted between the life valve and detector. The whole experimental setup was constructed in such a way that the analytical column and reactor can be operated at different temperatures (Figure 2). Figure 3 shows typical chromatograms of the sulfurhydrogen sulfide reaction which were obtained at different temperatures on a column containing sulfurcoated quartz powder. In each case the track of the ion 34 (H2S) is shown. The narrow-shaped peak, at a retention time only slightly larger than the dead time
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developed. To this, the approximation is made that the transport of components takes place solely in the gas phase and the chemical reaction solely in the liquid phase. Assuming the reactor to be ideal, i.e., isothermal, homogeneous, and without diffusion processes resulting in peak broadening, the change of the concentration of a component i in a volume element of the reactor due to the transport in the gas phase and the chemical reaction in the liquid phase can be represented by the partial differential equation (1) (see also Langer and Patton, 1973): Figure 4. Chromatograms (ion 34) of the reaction S(fl) + H2S(g) f H2Sx f H2S: column, glass; length, 100 cm; inner diameter, 3 mm; support, silane-treated glass beads (average diameter: 200 µm); sulfur, 0.5 g; H2S, 6.2 × 10-7 mol; carrier gas, helium (1.8 mL/min).
of the column, is assigned to unconverted H2S which presumably elutes first. At temperatures above 200 °C the tailing of this reactant peak broadens drastically with increasing temperature, indicating that a chemical reaction between sulfur and hydrogen sulfide takes place. The H2S eluted with the tailing is attributed to the decomposition of the primary formed sulfanes. A mass balance using the area of the chromatograms shows that all of the hydrogen sulfide given to the reactor is eluted as H2S. This agrees with the fact that the sulfanes themselves could not be detected by any of our experimental setups. This finding corresponds also with results reported by Hyne and co-workers, who used the NMR technique to study the nature of the molecular species present in solutions of H2S in liquid sulfur (Hyne et al., 1966). They showed that at a temperature of 180 °C mainly, if not exclusively, sulfanes with a chain length g 6 are formed. It can be supposed that we could not detect the sulfanes, because their vapor pressure is too low under the operating conditions. According to the data of Brunner and Woll, the solubility of the sulfanes can be increased in the gas phase by applying higher H2S pressures. Thus, their detectability might be enlarged. Figure 4 shows typical reaction chromatograms which were obtained on a column containing sulfur-coated glass beads. An appreciable conversion of H2S is observed even at temperatures slightly higher than 150 °C; i.e., on this column type the reaction rate is considerable larger than on the quartz powder reactor. At 250 °C almost all of the hydrogen sulfide input seems to be consumed. However, integration of the chromatogram reveals that, in contrast to the reaction carried out at lower temperature, sulfane decomposition is not complete within the 50 min interval. This behavior points to a complicated reaction mechanism. Chromatograms obtained with the glass beads and quartz powder reactor, respectively, show different shapes of the elution curves. Using the glass beads column, the reaction between sulfur and H2S always results in a single, broad-shaped peak; i.e., there is no significant separation of products and reactant. Employing the quartz powder column, a narrow-shaped peak with a half-width that is rather constant at all temperatures is eluted first. This peak is attributed to unconverted H2S. At higher temperature a partially resolved product peak is eluted behind. Model of the Gas Chromatographic Reactor In order to evaluate the kinetic data, a numerical model of the gas chromatographic reactor has been
(
)
δci Vv δ(uxciv) ) δt V δx
+ flow
( )
Vl δcil V δt
(1)
Equation 1 is transformed to give the rate of change of the mole fraction of the component i in the gas phase. To this, the mole fraction in the gas phase (yi) and in the liquid phase (xi) and the partition coefficient (Ki) are used:
yi ) niv/ngv
(2)
xi ) nil/ngl
(3)
Ki ) yi/xi
(4)
The total amount of component i can be written as:
(
)
ngl ni ) ying + xing ) yi ng + Ki v
l
v
(5)
The total amount of the gas phase is given by:
ngv )
Pg(x) Vv RT
(6)
with
[
Pg(0)2 - Pg(L)2 Pg(x) ) Pg(0) x L 2
]
1/2
(7)
With eqs 2-6 and the equation for the linear velocity of the carrier gas
ux )
n˘ RT
(8)
Pg(x)A
the differential equation (1) can be transformed to give the change of the mole fraction of the component i in the gas phase with time:
(
)
δyi δyi 1 ) n˘ + νi δt δξ g Kgi
(9)
with
Kgi )
ngv ng
l
+
1 Ki
(10)
Equation 9 can be used to set up a system of partial differential equations, the concrete form of which depends on the reaction scheme chosen. For the reaction scheme presented in this paper (Figure 8), the following
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Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996
differential equations have to be taken into account (Chart 1):
Chart 1 ν1 ) -k1x1 + k2x2 + k4x3
(11)
ν2 ) k1x1 - k2x2 - k3x2
(12)
ν3 ) k3x2 - k4x3
(13)
Figure 5. Block system used in model calculations: L ) 150 cm, Lv ) 17 cm, Ni ) 264.
In order to solve the differential equation system, it has to be minded that in a chromatographic column flow of components over the boundary at the outlet of the reactor takes place. This can be accounted for by introducing an additional source term qi into eq 9. For this system, the initial and boundary conditions listed in Chart 2 can be formulated:
Chart 2 yi(0exeL,t)0) ) 0; δyi (x)0,t) ) 0; δx
i * H2S
i * H2S
(14)
Figure 6. Simulation results: distribution of components in the glass beads reactor at various times (reaction scheme see Figure 8; with K1 ) 2000 (H2S), K3 ) 10 (H2S2), k1 ) 0.1 s-1, k2 ) 10-3 s-1 at T ) 200 °C).
(15)
n˘ s(x,t) ) 0
(16)
δns (x,t) ) 0 δx
(17)
qi(0ex
