SRU simulation getting the properties right

Reliable, predictive simulation software for SRUs requires recognition of many of the unique properties and behaviours of sulphur.

Optimized Gas Treating, Inc.

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Article Summary

There are several commercially available tools for simulating the performance of sulphur recovery units (SRUs). The general basis for these simulators is often regression to a collection of plant performance data. However, the results are only as good as the data on which they are based.

Collecting accurate SRU performance data is fraught with difficulty, much of it associated with the collection and analysis of samples. For example, gas samples from the reaction furnace must be very rapidly quenched, otherwise hydrogen keeps reacting. Samples containing water must have the water quickly removed to prevent further reaction. Careful attention must be paid to the metallurgy of sample containers because they can have catalytic activity — early catalysts were iron based, and even stainless steel is reactive towards sulphur dioxide. Material balances around SRUs require special techniques to work around analytical data limitations. Even when the data are accurate, simulators that depend on this kind of regression have only limited reliability when extrapolated to conditions outside the range of the basic data.

We have recently completed development of a simulation module for SRUs that has a more fundamentals oriented basis, including reaction kinetics of COS and CS2 formation and ammonia, BTEX, and hydrocarbon destruction in the furnace. The furnace model includes non-equilibrium conversion of hydrocarbons to COS, CO, and other species as well as burner mixing characteristics, which affect ammonia destruction. The Sulphur Converter model predicts profiles of dew points and conversion through converter beds as well as COS and CS2 destruction profiles. Sulphur condensers have rigorous sizing and rating integrated into their simulation. The solubility of H2S, H2SX and SO2 in all liquid sulphur streams is calculated, with particular application to sulphur pits where air or another carrier can be used to sparge and sweep these volatile species from liquid sulphur. The SRU model can be fully integrated with upstream acid gas removal and enrichment processes, and with downstream TGTU quench and tail gas amine treating. Through the use of detailed, highly non-ideal aqueous ionic chemistry, it includes the effects of SO2, heat stable salts, and monomethyl-monoethanolamine (MMEA, a degradation product of MDEA) on the performance of these units.

Developing reliable, predictive, simulation software for SRUs necessarily requires recognition of many of the unique properties and behaviours of sulphur. To a large extent, the uniqueness of these properties stems from the fact that sulphur occurs predominantly in three forms: S2, S6, and S8. In liquid form, as temperature increases, the S8 allotrope will polymerise to higher molecular weight forms. Its distribution amongst these allotropes is temperature dependent, and transition between forms occurs spontaneously, accompanied by substantial enthalpies of reaction. This article addresses several properties of sulphur.

Molecular formula and molecular weight of sulphur vapour
The most fundamental property of any compound is its molecular weight. Sulphur exists in several forms under conditions prevalent in SRUs. In the temperature range of interest in SRUs, sulphur vapour exists predominantly in the allotropic forms S2, S6, and S8 with temperature dependent distribution amongst these forms. Thus, its molecular weight is temperature dependent and can be expressed in terms of the average number of sulphur atoms per molecule of sulphur. The enthalpy changes associated with the conversion of one species form into another makes the Claus reaction endothermic in the reaction furnace where the S2 form dominates, but exothermic in converter beds where the S6 and S8 forms dominate. Conversion between forms also makes the molar flow rate of sulphur bearing streams variable because sulphur is not simply ‘elemental sulphur’ – it is actually a mixture of the different forms with variable molecular weight. This effect, however, is normally a minor one because sulphur concentrations in gas streams usually are not high, with vapour composition dominated by water and nitrogen from the air used in the combustion of H2S to SO2 in the furnace.

Figure 1 shows a comparison between the temperature dependence of the average number of atoms of elemental sulphur per mole of molecular sulphur as reported in the GPSA Data Book,1 the experimental data2,3 and the simulator model. The model is based on the reaction equilibria:

The reaction equilibrium constants have the following form where, because pressures are low, vapour phase mole fractions rather than fugacities can be used with equal validity:

Here P is the total pressure and the yi are mole fractions. The temperature dependence of the equilibrium constants obeys a van’t Hoff type of equation with temperature dependent Gibbs free energies of formation regressed from tabular data4.

Without looking at the original references, it is not commonly known that Figure 1 is at a pressure equal to the vapour pressure of sulphur at temperatures below its normal boiling point, and equal to one atmosphere at higher temperatures. The points in Figure 1 have been interpolated from the experimental data of Figure 2. The data in Figure 2 are from two sources2,3 and are the result of P-V-T measurements of a known mass of sulphur to determine the average molecular weight of the sample of gaseous sulphur. These measurements were not included in the original tabular data4 that form the basis for the ProTreat model. Thus, the lines in Figure 2 are model predictions and have been made independently from the data plotted in the figure. The simulation model is in reasonably close agreement with the measured data.

Liquid sulphur
Liquid sulphur is normally dominated by S6 and S8 rings, but at higher temperatures (above ~160°C, 320°F) the rings open and the short linear chains begin to polymerise. As will become apparent, the opening of these rings and the subsequent polymerisation of liquid sulphur has a significant effect on H2S and H2SX solubility, viscosity, and heat capacity. Dissolved H2S and H2SX also affect viscosity.

Vapour pressure of liquid sulphur
An important property, and one from which the latent heat of vaporisation can be derived, is vapour pressure. Figure 3 shows vapour pressure data from several sources, together with calculations from the simulator.

Latent heat of vaporisation
Heat of vaporisation is an important property in sulphur condenser calculations. Data have been taken5 and regressed for use in the simulation model. The data (Figure 4) form an unusual curve, first decreasing and then increasing with temperature. This is the result of the changing distribution of the S2, S6, and S8 allotropes of sulphur with temperature. In reality, what is measured and presented as latent heat of vaporisation also includes the heats of reaction associated with the chemical reactions that occur as the sulphur composition changes between its allotropic forms.

Viscosity of pure liquid sulphur

Pure liquid sulphur exists mostly in the forms of S6 and S8 rings, but around 160°C the ring structure opens and the sulphur polymerises. This is reflected graphically in Figure 5 where viscosity data5 are compared with simulation results. In the vicinity of the transition temperature, the viscosity undergoes a three to four orders of magnitude change, going from being relatively easy to pump to almost non-pumpable. The simulator accurately represents this behaviour.

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