Use of Detailed Kinetic Models for Multiscale Process Simulations of Sulfur Recovery Units

The modeling of thermal reaction furnaces of sulfur recovery units is a quite cumbersome problem since it involves different modeling scales such as the kinetic/molecular micro-scale, the reactor design meso-scale, and the chemical process macro-scale. The present paper proposes preliminary results of a multiscale approach to model the thermal furnaces and waste heat boiler based on detailed kinetics and reactor network analysis (RNA). The main kinetic mechanisms are discussed and validated using experimental data; industrial data is used to validate the RNA layout. Introduction Process simulation is nowadays supported by many tools and commercial flowsheeting packages involving unit operations, reactors, thermodynamic libraries, and property databases. These tools make possible the simulation of complex processes and overall plants, but they still have certain key-open-issues to be handled to perform accurate simulations and deepen the process understanding. One of the hardest problems is the simulation of non-ideal reactors via detailed kinetic schemes. This lack in the current process simulators is mainly due to: (I) the need of complex and well-established kinetic schemes to characterize the reaction environment; (II) the need to face simulation issues at different scales (kinetic and plant scales); and (III) the need of powerful solvers to handle the resulting largescale stiff nonlinear systems coming from kinetic modeling. The paper investigates the possibility to bridge the gap in process modeling by coupling OpenSMOKE and BzzMath libraries, two freely downloadable tools. OpenSMOKE [1] allows to simulate non-ideal reactors by solving complex networks of ideal elements. It is based on consolidated kinetic schemes [2]. BzzMath library [3, 4] is a numerical library for scientific computing. Specifically, it includes very performing and robust solvers for several numerical areas. It is worth underlining that these tools can be fully integrated in the most widespread commercial packages as discussed elsewhere [5-7]. For its well-known difficulties and renewed academic and industrial interest, the validation case is the thermal furnace of Claus processes, XXXV Meeting of the Italian Section of the Combustion Institute 2 designed to remove sulfur from acid gas streams. Kinetics (microscale) The kinetics of thermal reaction furnace of sulfur recovery units is very complex and not yet completely understood. The kinetics governing the transformations of sulfur compounds has been studied by Mueller et al. [8], who described the main the oxidation mechanisms, and Dagaut et al. [9], who highlighted the inhibition effects of SO2 on the radical pool. The pyrolysis of hydrogen sulfide, H2S, has been defined in detail by different authors [10, 11]. Other authors focused their research on the formation mechanisms of a specific species such as the CS2 and the COS, but they are not considered in this work for the sake of conciseness, although also the model previsions of these species are in good agreement with the experimental sets that we analyzed. The kinetic mechanisms are collected in an overall kinetic scheme containing 800+ reactions [2], for which the key-reactions only are reported hereinafter as validation. Although in presence of oxygen, the pyrolysis of H2S is particularly important in the Claus furnaces, looking forward to its high reactivity at the typical operating conditions and the non-stoichiometric inflow of combustion air. According to the Binoist’s reactor and conditions [11] the key steps for the H2S pyrolysis are: 2 H S M SH H M + = + + (1) 2 2 H S M H S M + = + + (2) Binoist’s kinetic parameters are used for (2), whereas Arrhenius parameters 14 0 2 10 k = ⋅ mol/l/s and 66000 E = cal/mol are proposed for (1). A selection of model previsions related to the reactions above is given in Figure 1 and Figure 2. 0.00% 10.00% 20.00% 30.00% 40.00% 50.00% 60.00% 0.00E+00 5.00E-01 1.00E+00 1.50E+00 2.00E+00 Residence Time (s) C on ve rs io n (H 2S ) Polimi @ 900°C Polimi @ 940°C Polimi @ 970°C Polimi @ 1000°C Polimi @ 1050°C Polimi @ 850°C Exp. Data @940°C Exp. Data @970°C Exp. Data @1000°C Exp. Data @1050°C Exp. Data @850°C Exp. Data @900°C 0.00E+00 5.00E-03 1.00E-02 1.50E-02 2.00E-02 2.50E-02 3.00E-02 0 0.5 1 1.5 2 Residence Time (s) H 2 M ol e Fr ac tio n Polimi @ 850°C Polimi @ 900°C Polimi @ 940°C Polimi @ 970°C Polimi @ 1000°C Polimi @ 1050°C Exp. Data @ 850°C Exp. Data @ 900°C Exp. Data @ 940°C Exp. Data @ 970°C Exp. Data @ 1000°C Exp. Data @ 1050°C Figure 1. Pyrolysis (Data: Binoist et al., 2003): H2S conversion. Figure 2. Pyrolysis (Binoist et al., 2003): H2 formation. Under the combustion regime of Claus furnaces, the H2S is partially (one third) oxidized to SO2. The partial oxidation allows to achieve the optimal ratio XXXV Meeting of the Italian Section of the Combustion Institute 3 2 2 / 2 H S SO = at the catalytic reactors (Claus converters) to maximize the yield of the overall SRU: 2 2 2 2 3 / 2 x H S SO x S H O + = + (3) and thus to maximize the sequestration of elemental sulfur. x accounts for the sulfur equilibrium ( 1, 2, 4,6,8 x = ). More details on the Claus process can be found elsewhere [6, 12]. The oxidation of sulfur compounds can be conveniently described using the analysis of the H2S explosion diagram to give SO2. The sensitivity analysis in correspondence with the slow-oxidation region highlighted the following predominant reactions ordered by relevance: 2 2 SH O M HSO M + + = + (4) 2 2 2 O H S HO SH + = + (5) 2 2 SO O SO O + = + (6) Conversely, in the explosion region, the reaction (6) is the most important one. The second limit in the explosion diagram defines the passage from the low to the high pressure mechanisms. It is determined by the following competing reactions: 2 SH O SO OH + = + (7) 2 2 SH O M HSO M + + = + (4) The ratio 7 4 / 1 r r = describes the transition from low to high pressure mechanism. It is possible to evaluate the explosion diagram using the corresponding constants: ( ) [ ][ ] ( ) [ ][ ]( ) ( ) [ ][ ] ( ) [ ][ ] 10 7 2 7 2 7 8 4 4 2 4 2 exp / 10 exp 12350 / / exp / 3 10 / SH O E RT SH O RT r P T r SH O P RT E RT SH O P RT α α − − = = = − ⋅ (8) ( ) 10 7 4 7 8 4 1