Kinetic Simulations of H2 Production from H2S Pyrolysis in Sulfur

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Kinetic Simulations of H2 Production from H2S Pyrolysis in Sulfur Recovery Units using a Detailed Reaction Mechanism Arjun Ravikumar, Abhijeet Raj, Salisu Ibrahim, Ramees K Rahman, and Ahmed Sultan Al Shoaibi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01549 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on November 4, 2016

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Energy & Fuels

Kinetic Simulations of H2 Production from H2S Pyrolysis in Sulfur Recovery Units using a Detailed Reaction Mechanism Arjun Ravikumar1,2, Abhijeet Raj*,1, Salisu Ibrahim1, Ramees K. Rahman1, Ahmed Al Shoaibi1 1

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Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, UAE

Department of Chemical Engineering, National Institute of Technology, Trichy, India

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Abstract

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Acid gas (H2S and CO2) is produced in large volumes worldwide from the desulfurization of

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hydrocarbon fuels, and is utilized in Sulfur Recovery Units (SRU) to produce sulfur. However,

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the hydrogen content of acid gas is wasted as low-grade steam, which highlights the need for the

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efficient utilization of this resource. The production of H2 from acid gas is desired, as it is an

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inexpensive feedstock. In this work, a kinetic study is conducted on H2 production from acid gas

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in an industrial SRU to utilize its built-in inertia, while saving on the capital cost and enhancing

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the processing capacity of SRU. The thermal energy generated during the combustion of acid gas

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in the Reaction Furnace (RF) is used for acid gas pyrolysis in the waste heat boiler (WHB) of

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SRU. While this technique has been investigated previously, its realization at industrial scale is

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hindered by low H2 yield. This paper presents suitable means of enhancing H2 production via

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operational modifications in RF and WHB. A detailed reaction mechanism, developed for acid

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gas combustion and pyrolysis and validated using experimental data from industrial furnaces and

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reactors, is used for the kinetic simulations of the SRU thermal unit. The results show that RF

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operational changes such as the extent of H2S oxidation and feed preheating can increase H2

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yield from 3% to 38% in the WHB without changing the composition of the acid gas stream.

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This significant improvement in H2 yield can help in realizing its production from acid gas in

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SRU.

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Key words: H2S; H2 production; Complete combustion; SRU; Claus process; Simulation.

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*

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Introduction

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Hydrogen (H2) is an important feedstock in the petroleum industry, and a significant amount of

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money is spent to produce it from conventional sources such as steam reforming of natural gas,

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partial oxidation of residual oil [1], gasification of coal [2] and biomass[3], and water electrolysis

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[4]. In recent years, several researchers have identified the dissociation of hydrogen sulfide (H2S)

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to produce H2 [5] and the simultaneous destruction of acid gas (H2S and CO2) to produce syngas

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(H2 and CO) [6], as potentially economical methods to produce H2. For instance, while water has

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a dissociation energy of 2.9 eV/molecule, the production of H2 and sulfur from H2S requires only

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0.2 eV/ molecule [7]. Ideally, a low cost method of H2 production from H2S can benefit the

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petroleum refineries that utilize it in significant amounts in hydro-treating processes such as

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hydro-desulfurization (HDS). In HDS process, H2 is used to convert the sulfur species present in

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the crude oil to H2S, which is subsequently separated from the oil in the form of acid gas [8].

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This process results in acid gas production in significant quantities that is currently under-

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utilized in Sulfur Recovery Units (SRU) to produce sulfur, while its H2 component is lost as low-

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grade steam [9].

Corresponding author. E-mail address: [email protected]. Phone: +971-2-6075738

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The SRU consists of a high-temperature reaction furnace (RF), a waste heat boiler

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(WHB), low-temperature multiple catalytic reactors, and sulfur condensers [10]. In the thermal

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section (i.e., RF and WHB), H2S undergoes partial oxidation in air to produce SO2 and H2O

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through reaction R1, and the unreacted H2S further reacts with SO2 to produced sulfur (R2). The

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air to fuel ratio in the RF is carefully controlled to ensure that the molar ratio of H2S to SO2 at


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the exit of the furnace is 2:1. This ratio is required for the further reactions in the catalytic

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reactors.

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H2S + 1.5O2 → SO2 + H2O

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2H2S + SO2 → 3S + 2H2O

(R1) (R2)

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Currently, a high sulfur recovery efficiency (>99%) is required to comply with the tight

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environmental regulations on the emission of toxic gases [11]. The conventional SRU does not

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achieve such a high efficiency without modifications due to technical difficulties arising from

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non-uniformity of acid gas feed. It is very difficult to achieve high furnace temperatures required

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to ensure the destruction of impurities (such as hydrocarbons, mercaptans and ammonia), if the

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acid gas feed contain less than 40% H2S [10]. These impurities then promote byproducts

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formation (such as COS and CS2) that reduce sulfur production [12], and cause catalyst

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deactivation in catalytic reactors via soot formation and deposition [10]. While conventional

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SRUs have undergone several process modifications by previous researchers to increase its

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efficiency and to meet the emission standards, a high operational cost is often incurred [8]. With

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the low selling price of sulfur in the international market (owing to its low demand) [13], the

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operation of SRU is not driven by profit, but rather, by the need to comply with the

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environmental standards on H2S emissions [14].

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In the near future, an increase in the production of acid gas is expected, due to highly

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stringent environmental regulations and due to the increasing dependency on sour reservoirs

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(containing high amount of acid gas with natural gas). The processing capacity of existing SRU

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may be insufficient to meet future environmental regulations [11]. Thus, an urgent need exists to

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seek cost-effective and alternative ways of recovering valuable products from acid gas, while 3 ACS Paragon Plus Environment

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utilizing the built-in inertia of the existing SRU to save capital cost and increase their processing

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capacity.

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The direct conversions of H2S to produce H2 [15] and acid gas (H2S and CO2) to syngas

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(H2 and CO) [16] have been examined as viable means of recovering value-added products from

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acid gas. While most of these studies have focused on understanding the reaction chemistry

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involved in the acid gas conversion process [17], some authors have investigated the operational

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conditions that can increase H2 [18] and syngas [19] yield. A reaction mechanism for H2S

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pyrolysis, consisting of 22 pyrolysis reactions, was proposed in [20] to provide an accurate tool

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for optimizing reactor operational conditions to increase H2S conversion. The reaction rate

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parameters in this mechanism were optimized in [21], where the revised mechanistic model,

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containing 20 reactions, provided about 10-20% improvements in the model predictions, when

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compared to the experimental data and the previous kinetic modeling results. However, even the

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improved kinetic scheme of [21] could not predict the experimental data satisfactorily at high

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temperatures above 1223 K. In [5], a detailed reaction mechanism, consisting of 432 reactions

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was proposed. This mechanism showed a good match between the experimental data and the

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model predictions for a temperature range of 850 K - 1300 K under various pyrolysis conditions

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and reactor types.

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In order to evaluate economical means of producing H2 from H2S, numerous approaches

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have been published in the literature, and the most notable ones include catalytic and non-

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catalytic thermal decomposition [14], direct electrochemical, indirect photochemical, and plasma

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systems [22]. However, none of these techniques has been realized on the commercial scale due

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to the lingering limitations that are yet to be addressed [23]. Generally, the reaction is highly

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endothermic and the equilibrium conversion even at high temperatures is low [7]. 4 ACS Paragon Plus Environment

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Thermodynamic equilibrium calculation indicates that less than 20% conversion is achieved

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even at high temperature (1300K), while the energy cost is rather high (4eV per H2 molecule)

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[7].

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In [9] and [10, 22] the comparative advantages of existing methods and their limitations

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were evaluated. The photochemical methods have high energy costs and low conversion [22].

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The electrolysis methods encounter some challenges such as sulfur passivation of anode [23],

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and the requirements of high electrical energy and chemical oxidants [24]. The catalytic or non-

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catalytic thermal decomposition of H2S is considered as the most direct process for H2

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production, for which a pilot plant-scale has been built and operated successfully [25]. However,

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in this process, a low H2 yield, a requirement of an economical heat source, and the limitations in

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heat transfer to the acid gas are the major drawbacks [26].

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In order to address the issue of energy requirement for thermal cracking, some

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modifications to the existing SRU have been proposed by Reed et al. [25] in which the energy

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released in the RF is supplied for H2S cracking. A key feature of this technique is retrofitting an

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existing SRU with an acid gas cracking technology. In [25], the catalytic acid gas decomposition

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using the heat generated from the RF was studied. A ceramic tube containing cobalt-

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molybdenum catalyst was mounted along the length of RF, through which a portion of acid gas

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feed to the RF was injected. In the tests performed, 10% by volume of acid gas feed was injected

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through the tubes, and a fairly low yield of H2 (3.6% by volume) was obtained in the tail gas at

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RF temperature of 1403 K.

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In [9], two different reactor configurations for the thermal cracking of acid gas were

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studied: (i) a modified methane reformer, and (ii) a tubular heat exchanger in the place of WHB


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section in SRU. In the first scheme, the heat energy from the hot product gases of methane

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oxidation was used for H2S thermal decomposition in tubes installed inside the RF. In the second

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scheme, acid gas for cracking was passed through the tubes in a heat exchanger-type reactor. The

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heat exchanger tube bundle was located inside a horizontal single-pass shell in which hot gas

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from the RF would flow. Using a single-step kinetic model, the H2S conversion within 22-26%

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was computed with the RF temperature of 1473 K. A comparison of the H2 production costs in

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the two schemes showed that costs were greater in the second scheme due to low H2 yield in it.

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While the H2S conversion obtained using a simplified model and used in calculations may be

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less reliable, the study presented an important finding that higher H2 yield from acid gas would

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be required for this H2 production process to be economically attractive.

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Dowling et al. [26] demonstrated the use of H2S thermal cracking concept in a pilot scale

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Claus furnace. The ceramic cracking coils, placed in the reaction furnace/WHB were used to

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decompose a fraction of the acid gas feed. The product gas from cracking coils was channeled to

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a separation unit for H2 removal, while the effluent was recycled to the main inlet of RF. The

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published test data showed that conversions of 26% and 28% can be achieved at typical Claus

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RF temperatures of 1373 K and 1473 K, respectively, with 90% H2S and 10% CO2 in acid gas

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feed. However, lower H2 yield were obtained with higher concentrations of CO2 in acid gas due

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to decrease in the RF temperature. In [[22] and refs. therein], cost estimates showed that the

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capital investment for H2 production unit in SRUs could be recovered in less than four years, if a

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high yield is achieved. While this technique seems promising, heat transfer limitations and low

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yield of H2 still need to be addressed before its commercialization. As mentioned before, the

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yield of H2 has a significant impact on the economic feasibility of its production from acid gas

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on an industrial scale. 6 ACS Paragon Plus Environment

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In this paper, kinetic simulations are conducted to investigate the process conditions that

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enhance heat transfer and stimulate H2 production in the WHB of SRU using the hot exhaust

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gases from the RF as the heat source. The simulations of the RF and the WHB are conducted

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using an industrial feed composition and a detailed and well-validated reaction mechanism.

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Figure 1 shows a schematic of the modified SRU, wherein water, used to cool down the hot

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furnace gases in WHB, is replaced by acid gas. The heat transfer between the injected acid gas

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and the hot furnace gas is used to crack H2S and produce H2 in the WHB. A quenching and

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separation unit is used to recover H2, while the rest of the gas stream is sent to the RF. This

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configuration allows the production of H2 as a valuable product along with increasing the acid

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gas processing capacity of SRU.

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Reaction Mechanism

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A detailed reaction mechanism for acid gas combustion and pyrolysis along with the

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thermodynamic and transport properties of the chemical species were adopted from our previous

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study [5]. This mechanism consists of 258 species and 1695 reactions for the pyrolysis and

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oxidation of H2S, as well as the reactions of other impurities that are often present in the Claus

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feed. The important species including intermediate radicals, formed during the pyrolysis of H2S

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are H2S, H, H2, S, S2, HS, HSSH, HSS, and H2S2. The species, HSSH and H2S2, are isomers with

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molecular structures, H-S-S-H and HH>S=S, respectively. Most of the reactions of HSS and HSSH

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were derived from [21] and [27], and those involving oxidation of sulfur species were adopted

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from [28]. Since CO2 is a major component of acid gas, its decomposition to CO and the further

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reactions of CO leading to the formation of species such as COS and HCO were included. For


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the thermal and oxidative destruction of hydrocarbon contaminants present in the acid gas, the

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reactions related to C1-C4 fuels and monocyclic (benzene, toluene, and xylene) and polycyclic

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aromatics hydrocarbons up to coronene (C24H12) were added to the acid gas mechanism. Some

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species, generating from the interaction of sulfurous compounds and hydrocarbons such as

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CH3SH, CH2S, HCS, HOCS2, and OCS2, were also included in the mechanism. In [5], the

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mechanism was extensively validated with experimental data of premixed flames and tubular

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reactors, but for relatively low concentration of H2S in the acid gas. However, the experimental

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data were mainly from lab-scale reactors [5]. The mechanism files used in this work for

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simulations are provided in the Supplementary material.

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Results and Discussions

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Mechanism validation

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Since this paper deals with acid gas combustion and pyrolysis in SRU, the experimental data on

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H2S thermal dissociation and combustion in industrial RF and WHB were used for mechanism

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validations, which would enhance the model reliability under wide range of conditions. The

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kinetic simulations using a detailed reaction mechanism were conducted using CHEMKIN PRO

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software [29], where Claus RF was modelled as a plug flow reactor under steady state condition

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[30]. In a Claus RF, Reynolds and Peclet numbers are very high. Thus, the flow inside it is fully

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developed, and the molecular diffusion is neglected [31]. This justifies the use of plug flow

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reactor as a model for Claus RF, which is also in line with the justifications provided in [31]. The

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presence of refractory linings on the RF ensures negligible heat loss. For this reason, the RF was

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assumed to be adiabatic. The gas-phase energy equation was solved in the model to determine

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the temperatures at different locations varying due to combustion.


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Figure 2 presents a comparison between the measured data on chemical species from [32]

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and the simulation results for a Claus RF with a length of 6.5 m, diameter of 3.5 m, and pressure

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of 1.5 atm. The geometrical and inlet parameters of the simulated reactor are provided in Table

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1. The experimental data in [32] were only available at the end of the furnace. It can be observed

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that the reaction mechanism predicted the industrial data with a good accuracy.

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In [31], the gas composition at the exit of the WHB of an industrial SRU was provided

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alongside the operating conditions and geometry parameters of the RF and the WHB. The

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geometrical and inlet parameters of the simulated reactor are provided in Table 2. Figure 3

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shows the comparison of the simulation results with the industrial data, alongside modelling

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results from Manenti et al. [31]. A good match is observed between them for all the species

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measured except S2. It is important to note that, in the work by Manenti et al., gas quenching was

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considered at an approximate distance of 750 cm (corresponding to the temperature of 950 ºC) in

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the WHB tubes, after which they assumed gas composition to be constant. Sulfur exists in the

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form of eight isomers (S1-S8), with S2 and S8 being the most stable ones, but their relative

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concentrations are highly temperature-dependent. The stability (and the concentration) of S2

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among the sulfur isomers is highest at temperatures above 600 K, but below this temperature, S8

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is dominant. Since the gas sampling/quenching procedure and the temperature, at which

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measurements were made, was not clear, gas quenching was not considered in this work. Thus,

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some difference between the computed and observed values for sulfur is expected. The predicted

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S2 concentration at the WHB exit in this figure corresponds to a temperature of 590 K (i.e. WHB

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outlet temperature).

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In [33], a SRU, present at Ultramar Refinery, Wilmington, California was used to

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measure the concentrations of chemical species at the end of the reaction furnace with an aim to 9 ACS Paragon Plus Environment

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study H2 formation from H2S in the RF. The geometrical and inlet parameters of the simulated

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reactor are provided in Table 1. Figure 4 presents a comparison between the experimental data

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for different species and the simulation results. An excellent match was found between the two

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for all the species, which indicates that the mechanism can capture the H2S combustion and

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pyrolysis chemistry very well.

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In [15], Hawboldt et al. studied H2S conversion (defined below) to H2 and S2 in a plug

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flow reactor within a temperature range of 1123 –1473 K and residence times of 0.05 – 1.5 s. ‫ܪ‬ଶ ܵ ܿ‫ ݊݋݅ݏݎ݁ݒ݊݋‬ሺ%ሻ =

݉‫ܪ ݂݋ ݏ݈݁݋‬ଶ ܵ ܽ‫ ݐ݈݁݊݅ ݐ‬− ݉‫ܪ ݂݋ ݏ݈݁݋‬ଶ ܵ ܽ‫ݐ݅ݔ݁ ݐ‬ × 100 ݉‫ܪ ݂݋ ݏ݈݁݋‬ଶ ܵ ܽ‫ݐ݈݁݊݅ ݐ‬

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A feed containing 2.5 mol% H2S and 97.5 mol% N2 at 1 atm pressure was used. Figure 5(a)

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presents a comparison between the experimental data and the simulation results at different

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temperatures. A good match between the two was found, which indicates a good predictive

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capability of the model at these temperatures. In [20], the dissociation of H2S in the presence of

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S2 was experimentally studied in a continuous perfectly-mixed quartz reactor with a feed

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containing 3.34% H2S, 1.67% S2, and 95% Ar at residence times of 0.4s –1.6s in the temperature

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range of 1073-1373 K and at a constant pressure of 1.5 atm. Figure 5(b) shows a comparison

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between the experimental data and the simulated results on H2S conversion, where some

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difference (a maximum of 5% in H2S conversion) was seen. Due to the absence of pressure

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information in the reactor in the experimental work [20], simulations were carried using a

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homogeneous batch reactor model for the specified residence times and a fixed pressure of 1.5

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atm. Thus, some differences are expected. Another reason for the difference could be the low

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predictive capability of the model at low temperatures. However, the dearth of experimental data

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in the literature at low temperatures limit the model validation in that regime. At all the 10 ACS Paragon Plus Environment

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temperatures in this figure, the H2S conversion became independent on the residence time

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beyond 900 ms.

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Dowling et al. [26] reported experimental data on H2 yield from pure H2S in a ceramic thermal cracker. The H2 yield was calculated using the following formula: ‫ܪ‬ଶ ‫ ݈݀݁݅ݕ‬ሺ%ሻ =

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௠௢௟௘௦ ௢௙ ுమ ௣௥௢ௗ௨௖௘ௗ ௠௢௟௘௦ ௢௙ ுమ ௌ ௔௧ ௜௡௟௘௧

× 100.

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In Figure 6, a comparison of the simulated and experimentally observed profiles of H2 yield is

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shown at different temperatures at a fixed residence time of 0.2 s, wherein a satisfactory

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agreement between them is observed. It can be seen in this figure that the decomposition of H2S

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is energy intensive, and high temperatures (above 1473 K) are required to obtain a H2 yield

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above 30%.

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A limited number of validation studies have been presented above, which is restricted by

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the availability of measured species profiles from commercial acid gas reactors and furnaces in

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the literature. A further validation of the reaction mechanism using experimental data from

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several lab-scale flow reactors, batch reactors, premixed laminar flames, and shock tubes with

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varying feed compositions and temperatures can be found in a previous study [5], where

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excellent predictions of the species profiles at different reactor lengths, temperatures, and

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residence times were found.

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Modified SRU simulations

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With an aim to propose SRU process modifications to enhance H2 yield in it and to make H2

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production cost-effective, kinetic simulations were conducted by varying SRU reactor


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conditions. For the simulations, the feed and reactor conditions were taken from a SRU, being

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operated in the UAE, and are provided in Tables 3 and 4. However, before conducting the SRU

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simulations, it was important to understand the role of residence time and temperature of the

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reactor on H2 yield from the given SRU feed (Table 3). Figure 7 presents the variation in H2

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yield from the SRU feed at different temperatures and residence times. With reactor temperatures

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below 1000 K, H2 production is insignificant even at high residence times. On increasing the

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reactor temperature to 1200 K, H2 yield increased to a steady state value with increasing

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residence time. It can be observed that H2 yield of about 30-45% can be achieved in a

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temperature range of 1600 to 2000 K and at residence times of up to 0.25 s. At high temperatures

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(1400-2000 K), with rapid H2 production, H2 yield was found to reach a maximum value, and

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then it decreased slightly before reaching a steady state value at higher residence times.

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Evidently, long residence times in the reactor did not support high H2 yield, which could be due

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its decomposition at high temperatures. To understand this, reaction path analysis was conducted

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to determine the reactions responsible for H2 production and consumption, which is shown in

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Figure 8. The most significant reactions involving H2, which occur during the cracking of H2S,

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are listed below as R3 to R7. The decrease in H2 yield at high residence times was due to the

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reverse reactions involving H2 that were triggered at high concentrations of H2 to form H2S.

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H2S ↔ H2 + 0.5S2

(R3)

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H2S + H ↔ SH + H2

(R4)

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HSS + H ↔ S2 + H2

(R5)

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2SH ↔ S2 + H2

(R6)


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It is clear from the above simulations that high temperatures above 1600 K are mandatory to

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have an appreciable H2 yield. The direct thermal splitting of H2S to H2 and S within the WHB

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section of SRU was investigated using the heat energy generated from the combustion of acid

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gas feed in the RF section. The WHB in a SRU is used to cool down the hot process gas stream

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from RF before injecting it to the low-temperature catalytic reactors. Instead of using water as a

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coolant, acid gas was injected in the WHB to achieve thermal cracking of H2S and produce H2 in

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it. The Claus feed to the RF of SRU plant in Abu Dhabi consisted of acid gas, air, fuel gas, and

274

vent gas streams (as shown in Table 3). These streams are premixed and preheated to a

275

temperature of 511 K prior to combustion to ensure sustained acid gas combustion (i.e. no flame

276

extinction) and to obtain sufficiently high temperature in the furnace to destruct feed

277

contaminants.

278 279 280 281 282

The WHB was modelled as a heat exchanger due to heat transfer between hot process gas and coolant gas, and based on the justifications provided in [31]. In WHB, the overall heat transfer coefficient, U, was calculated using the energy balance equation, as shown below. Q = MhotCP,hot(THin-THout) = McoldCP,cold(TCout-TCin) = UA∆Tlmtd

283

Here, Q is the amount of heat transferred, Mhot and Mcold are the mass flow rates of hot and cold

284

fluids, CP,hot and CP,cold are the heat capacities of the hot and cold fluids, THin and THout are the

285

inlet and outlet temperatures of the hot fluid, TCin and TCout are the inlet and outlet temperatures

286

of the cold fluid, A is the heat transfer area, and ∆Tlmtd is the log mean temperature difference. In

287

this energy balance equation, U, THout, and TCout were not known, and were calculated in this

288

work. The overall heat transfer coefficient, U is given by the following equation: 13 ACS Paragon Plus Environment

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1 1 dx୵ 1 = + + UA h୲ A୲ kA hୱ Aୗ 289

Here, ht and hs are the individual heat transfer coefficients on tube and shell sides, respectively,

290

dxw and k are tube wall thickness and thermal conductivity, respectively, and At and As are the

291

surface areas for heat transfer on tube and shell sides, respectively.

292 293

The tube-side heat transfer coefficient, ht is calculated using Dittus- Boelter correlation [34], as shown below. h୲ D୧୬ = 0.023 Re଴.଼ Pr ଴.ସ k ୮୰୭ୡୣୱୱ

294

Here, Din is the inside tube diameter, kprocess is the thermal conductivity of the process gas, Re is

295

the Reynolds number and Pr is the Prandtl number. The correlation is applicable for Reynolds

296

number from 10,000 to 120,000, and has an accuracy of ±15%.

297

The shell-side heat transfer coefficient, hs is calculated using the following equation [34]: ଴.଺

hୱ Dୣ ඥGୠ Gୡ d୭ = 0.02 ቜ ቝ k ୌమ ୗ μ 298

Pr ଴.ଷଷ

The required mass velocities, Gb and Gc are given as [34]: Mୱ୦ୣ୪୪ Gୠ = π ଶ ଶ 4 ൫F୆ Dୗ − N୲ d୭ ൯ Gୡ =

Mୱ୦ୣ୪୪

d P୆ Dୗ ൤1 − P଴ ൨ ୲


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299

In the above equations, De is the equivalent diameter on the shell side, kH2S is the thermal

300

conductivity of the acid gas stream, µ is the viscosity of the fluid, Pr is the Prandtl number, Ds is

301

the shell inner diameter, Mshell is the mass flow rate through the shell, FB is the fraction of the

302

shell cross-section that makes up the baffle window, Nt is the number of tubes in the baffle

303

window (usually approximated by FB × number of tubes), PB is the baffle pitch (spacing), Pt is

304

the tube pitch, and do is the tube outside diameter.

305

When a portion of the total acid gas fed into the RF was injected into the shell-side of the

306

WHB (with the exhaust gas from the RF flowing in the tubes), an overall heat transfer coefficient

307

(U) of 20 W/m2K was obtained in a cross flow configuration.

308

Figure 9 presents the temperature profiles in the RF, and Figure 10 shows the temperature

309

profiles and H2 yield on the shell-side of the WHB. At the SRU feed conditions (referred to as

310

actual feed conditions, and represent by blue lines in the figures), the temperature of the exhaust

311

gas from the RF was 1490 K before it entered the tube-side of the WHB. The initial temperature

312

of the acid gas, which was fed to the shell-side of the WHB, was 511 K. Because of heat transfer

313

and pyrolysis reactions in the WHB, the acid gas temperature increased from 511 to 1140 K

314

along the length of the WHB (blue line in Figure 10). The profiles, shown by red and green lines

315

in Figure 9 and 10, will be discussed later. Despite a high residence time of 1.8 s in the WHB, a

316

very low H2 yield (2-3%) was obtained at the actual SRU conditions. This result highlights the

317

importance of kinetic simulations. The simulations results on Claus furnace, presented before,

318

suggest that the equilibrium calculations may be suitable for it. However, in some feasibility

319

studies on H2 production in SRU, equilibrium was assumed in the pyrolytic reactor, where H2S

320

cracking was taking place [14]. Clearly, since H2 yield in the WHB is highly dependent on the

321

temperature (and the rate of heat transfer in the WHB), equilibrium may not be attained in a 15 ACS Paragon Plus Environment

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322

residence time of 1.8 s, when gas temperature is low. The low H2 yields also suggests that higher

323

exhaust gas temperature from the RF (i.e. above 1490 K) would be required to increase H2 yield

324

beyond 3%.

325

In the view of the above argument, two modifications in the RF feed conditions were

326

studied in this paper that could increase the RF temperature: (a) complete combustion of the acid

327

gas in the RF, and (b) feed preheating to increase adiabatic flame temperature. The former can be

328

achieved by changing the flow rates of air and/or acid gas streams to ensure H2S:O2 molar ratio

329

of 1:1.5 in the reactant mixture (i.e. stoichiometric combustion). For the latter case, the feed inlet

330

temperature can be increased from 511 K to a higher value by preheating the inlet acid gas

331

and/or air streams. In practice, acid gas is preheated by either using steam generated from the

332

WHB or using fired heaters [8]. The latter one of preferred when feed preheating to a high

333

temperature is desired, where natural gas is burnt to produce heat. The hot process gas exiting the

334

WHB can be sent to the feed preheater to heat the feed streams entering the furnace. Thereafter,

335

it can be sent to a condenser to recover sulfur prior to the first catalytic reactor. The inlet flow

336

rates and gas composition for the case of complete feed combustion are given in Table 5. Figure

337

11 presents the effects of feed preheating on flame temperature with actual feed composition as

338

well as with modified feed composition to ensure complete combustion of H2S. Clearly,

339

complete combustion ensures much higher flame temperatures than the actual feed composition

340

at all feed inlet temperatures, and provides an ideal way to increase the RF exhaust gas

341

temperature to enhance the rate of heat transfer in the WHB for H2S cracking.

342

It is important to understand the implication of complete combustion on the species

343

present in the RF. Figure 12 shows the species profiles in the RF and the WHB (tube-side) at

344

actual feed and at complete combustion conditions. As expected, SO2 concentration increases 16 ACS Paragon Plus Environment

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Page 17 of 48

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Energy & Fuels

345

significantly, while S2 concentration reduces due to unavailability of H2S to react with SO2 to

346

produce S2. The required ratio of H2S to SO2 (2:1) in the product gas exiting the WHB is also

347

not maintained, which is required for the catalytic reactors. However, this can be resolved by

348

injecting a defined amount of acid gas to the SO2-rich gas exiting the WHB. Moreover, if the

349

acid gas feed contains NH3 and/or considerable amounts of aromatics such as benzene, toluene,

350

ethylbenzene, and xylenes (BTEX) and other hydrocarbons as contaminants, then injecting a

351

portion of acid gas into the process stream after WHB is not feasible. For such an acid gas feed,

352

two approaches are suggested. (a) A portion of acid gas required to balance the H2S to SO2 ratio

353

can be bypassed and injected in the Claus furnace after certain distance from the furnace inlet.

354

This technique is commonly practiced in industry while processing ammonia-contaminated acid

355

gas in SRU. It divides the furnace into two reaction zones, wherein complete combustion occurs

356

in the first zone near the furnace inlet, while NH3 and other impurities are destroyed at high

357

temperatures in the second zone, which is further away from the furnace inlet. The flow rate of

358

bypassed acid gas can be adjusted to ensure that required H2S to SO2 ratio is maintained. (b)

359

Some existing SRU (such as those implementing ProClaus technology [35]) does not require the

360

H2S to SO2 ratio in the Claus furnace to be maintained. The process consists of a thermal stage, a

361

selective reduction step (for SO2 conversion to sulfur in the presence of reducing agents such as

362

CO and H2) in the first catalytic reactor, and a selective oxidation step (for direct H2S oxidation

363

to sulfur) in the last catalytic reactor. This SRU configuration is achieved with minimum

364

modifications, low capital and operating costs, and ease of operation for existing plants [35]. The

365

complete acid gas combustion generates sufficient concentrations of reducing agents, CO and H2

366

in the Claus furnace to support SO2 conversion. ProClaus technology takes advantage of the CO


Energy & Fuels

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Page 18 of 48

367

and H2 produced in the furnace to reduce SO2 to elemental sulfur [36], as shown by Reactions

368

R8-R9 [35].

369

SO2 + 2H2 = 1/x Sx + 2H2O

(R7)

370

SO2 + 2CO = 1/x Sx + 2CO2

(R8)

371

An advantage of the complete combustion in the RF is the reduced levels of COS and CS2 in the

372

exhaust gas, which are pollutants for the environment. Note that these changes in operational

373

procedures of RF are feasible with the current SRU in industry.

374

The effect of complete combustion in the RF on H2 yield in the WHB (on the shell-side)

375

is shown in Figure 10 by red lines. A sharp increase in H2 yield to about 23% can be seen. The

376

complete combustion of acid gas resulted in an increase in the temperature of the gas exiting the

377

RF to 1850 K (as compared to 1490 K at the actual feed condition). This increased the energy

378

input to the WHB, and the temperature on the shell-side of the WHB increased (as shown in

379

Figure 10). However, higher temperatures led to shorter residence time in the WHB due to gas

380

expansion that increased the gas velocity.

381

In Figure 10, it is also shown that the production of H2 in the WHB can be further

382

enhanced with a combination of feed preheating to a higher temperature (from 511 to 800 K) and

383

complete combustion of acid gas in the RF (shown by green lines). With this modification, the

384

RF exhaust gas temperature increased from 1490 (for actual feed condition) to 2050 K.

385

Correspondingly, H2 yield increased to 28%. While higher RF temperatures are desired for H2

386

production in the WHB, the refractory materials of the RF in some conventional SRU are limited

387

to a temperature of 1800 K, and changes in refractory lining may be required for higher RF


Page 19 of 48

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388

temperature. However, the modified SRU built with oxygen enrichment technology are equipped

389

to handle high RF temperatures near 2000 K [37].

390

To determine the effect of the change in the flow configuration in the WHB on H2

391

production, the hot gases from the RF were now allowed to pass through the shell-side, while the

392

acid gas was injected in the tubes of the WHB for cracking. A new value of 25 W/m2K for U was

393

found for this flow configuration (higher than the value of 20 W/m2K for the previous

394

configuration). Figure 13 shows the profiles of H2 yield and gas temperatures on the tube-side of

395

the WHB for this modified flow configuration. An increase in heat transfer in the WHB caused a

396

reduction in the residence time of gas in the tubes due to higher gas velocities. For actual feed

397

conditions, despite a decrease in the residence time from 1.8 to 1 s, H2 yield increased from 3%

398

in the previous WHB configuration to 8% in the present one. The maximum gas temperature in

399

the tubes of the WHB increased from 1100 to 1240 K. For complete combustion case

400

(represented by red line), H2 yield increased from 23% in previous configuration to 31% in the

401

present one, while the residence time decreased from 1.5 s to 0.85 s. For the case involving feed

402

preheating along with complete combustion (represented by green line), H2 yield increased from

403

28 to 36%, while the residence time decreased from 1.3 s to 0.74 s. Clearly, an improved heat

404

transfer in this configuration led to better H2 yields.

405

The changes in the physical parameters such as the internal diameter and the length of the

406

tubes in the WHB may further help in increasing the H2 yield by changing the residence time and

407

the value of U in the WHB. Figure 14 shows the effect of the change in tube internal diameter in

408

WHB on H2 yield and gas temperature. While changing the tube diameter, the number of tubes

409

were also changed to keep the total surface area for heat transfer to be constant. Table 6 provides

410

the values of the number of tubes, U, and the gas velocities for different tube diameters that were 19 ACS Paragon Plus Environment

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Page 20 of 48

411

used in the simulations. The efficient flow configuration involving exhaust gas from the RF on

412

the shell-side and acid gas on the tube-side of the WHB was used. The value of U reduced with

413

increasing diameter. The residence time increased from 0.18 to 1.45 s (as the gas velocity

414

reduced) with increase in tube diameter from 2 to 6 cm. This enhanced the amount of heat

415

transferred in the WHB, and led to an increase in the acid gas temperature in the tubes (though

416

the value of U decreased with increasing diameter). While H2 yield increased with increasing

417

tube diameter up to 5 cm, a further increase in the diameter led to a slight reduction in H2 yield

418

due to the reactions of H2 and sulfur species to form H2S. A H2 yield of 37 % at 5 cm tube

419

diameter could be obtained, and this diameter was used to investigate the effect of tube length on

420

H2 yield in the WHB.

421

Figure 15 presents the variation in H2 yield with the change in the length (between 6-12

422

m) of the WHB tubes with a fixed diameter of 0.05 m. It can be observed that an increase in tube

423

length increased the maximum acid gas temperature and the residence time in the WHB. As a

424

result, H2 yield increased from 36 to 38% on increasing tube length from 6 to 10 m, but with

425

further increase in the tube length to 12 m, H2 yield reduced to 36%. The reason for this

426

reduction has been explained before in the discussion of Figure 7, where H2 yield has been

427

shown to go through a maximum value with increasing residence time. These results suggest that

428

high temperatures and optimum residence time in the WHB (by varying process and/or

429

geometrical parameters) can help in achieving a high H2 yield from acid gas pyrolysis.

430

The study, presented in this paper, contributes to the fundamental understanding of the

431

pyrolysis of H2S to produce economically valuable H2 in non-isothermal conditions by utilizing

432

the thermal energy generated from H2S combustion. It also suggests that the variation in the

433

operating conditions and physical parameters can have a significant effect on temperature 20 ACS Paragon Plus Environment

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Energy & Fuels

434

distribution in the furnace and heat exchange in the waste heat boiler, which can also affect H2

435

production rate from H2S.

436

While the increased H2 yield with the process modifications described in the paper is

437

encouraging, and may help in realizing H2 production from acid gas, it is important to understand

438

the economic aspects of the process modifications. For example, complete combustion in the

439

furnace may require new refractory linings that can sustain high temperatures close to 2000 K,

440

and their installation may increase the capital cost. Moreover, the arrangements to carry hot

441

process gas exiting the WHB to the feed preheaters would be required. The complete combustion

442

of H2S would reduce sulfur yield in the furnace section, which would lead to a higher load to the

443

catalytic units for sulfur production. In the catalytic sections, if reducing catalysts (such as

444

sulfide CoMo/y-Al2O3) for converting SO2 to sulfur are used, it may add up to the operational

445

cost, since such catalysts may be more expensive than traditional alumina (Al2O3) and silica

446

(SiO2) catalysts. If the traditional catalysts were to be used, additional acid gas injection to the

447

furnace (away from the furnace inlet) to obtain desired H2S to SO2 ratio in the catalytic units

448

would be required. A detailed economic analysis in the future can help in understanding the net

449

profit in producing H2 and sulfur though these modifications.

450

Conclusions

451

With an aim to investigate the possibility of H2 production in Sulfur Recovery Units (SRU) from

452

acid gas (H2S and CO2), a modified SRU, where a portion of acid gas is injected in the waste

453

heat boiler (WHB) to produce H2 from acid gas cracking by using thermal energy of the exhaust

454

gas from the reaction furnace (RF), is studied. A detailed reaction mechanism, developed in a

455

previous work for acid gas combustion and pyrolysis, was firstly validated using experimental


Energy & Fuels

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Page 22 of 48

456

data from various industrial SRU and lab-scale reactors. Thereafter, the mechanism was used for

457

kinetic simulations of the RF and the WHB of the modified SRU. For simulations, the feed

458

conditions and physical parameters of an industrial SRU in the UAE was used. The results

459

revealed that the rate of heat transfer and energy input to the WHB greatly influence H2

460

production from acid gas in the WHB. The SRU operations under actual feed conditions led to a

461

H2 yield of only 3% in the WHB. Therefore, the following process modifications were tested to

462

increase H2 yield: (a) complete combustion of H2S in the RF to increase gas temperature (and

463

energy input to the WHB), (b) feed preheating to increase adiabatic flame temperature (or, gas

464

temperature in the RF), (c) changing flow configurations in the WHB (exhaust gas from the RF

465

on the tube-side and acid gas on the shell-side, and vice versa), and (d) changes in tube diameter

466

and length of the WHB. The complete combustion of H2S in the RF (by using additional air to

467

have H2S:O2 ratio of 1:1.5 in the feed) provided sufficiently high temperature in the WHB to

468

increase the H2 yield to 23%. Moreover, feed preheating to 800 K in combination with complete

469

combustion could increase this yield to 28%. It was found that a flow configuration involving RF

470

exhaust gas in the shell and acid gas in the tubes of the WHB with complete H2S combustion in

471

the RF could further increase the H2 yield to 36% due to enhanced heat transfer. H2 yield firstly

472

increased with increasing tube diameter, and after reaching a maximum value of 37%, its value

473

reduced slightly. The variation in tube length did not have any significant effect on H2 yield

474

beyond 8 m for the given feed conditions, and a H2 yield of 38% could be achieved with a tube

475

length of 10 m. Thus, an optimal combination of RF temperature, residence time in the WHB,

476

and geometrical parameters is required for high hydrogen production from a given acid gas

477

stream. The improvements in H2 yield from H2S in conventional SRU could provide an

478

economical path to produce H2 from industrial waste streams rich in acid gas.


Page 23 of 48

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479

Acknowledgments

480

This work has been financially supported by The Petroleum Institute Research Centre and the

481

Petroleum Institute Gas Processing and Materials Science Research Centre (GRC), Abu Dhabi,

482

UAE.

483

Supplementary material

484

The reaction mechanism, the thermodynamic data, and the transport data files, used for the

485

simulations in this work, are provided in the supplementary material.

486 487

References

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Bartels, J.R., M.B. Pate, and N.K. Olson, An economic survey of hydrogen production from conventional and alternative energy sources. International journal of hydrogen energy, 2010. 35(16): p. 8371-8384. Wang, X., et al., Experimental Study on H2S Release during Coal Gasification. Energy & Fuels, 2011. 25(10): p. 4865-4865. Chaudhari, S., A. Dalai, and N. Bakhshi, Production of hydrogen and/or syngas (H2+ CO) via steam gasification of biomass-derived chars. Energy & fuels, 2003. 17(4): p. 1062-1067. Yu, G., H. Wang, and K.T. Chuang, Upper bound for the efficiency of a novel chemical cycle of H2S splitting for H2 production. Energy & Fuels, 2009. 23(4): p. 2184-2191. Cong, T.Y., et al., A detailed reaction mechanism for hydrogen production via hydrogen sulphide (H 2 S) thermolysis and oxidation. International Journal of Hydrogen Energy, 2016. 41(16): p. 6662-6675. El-Melih, A., et al., Experimental examination of syngas recovery from acid gases. Applied Energy, 2016. 164: p. 64-68. Moghiman, M., et al., A numerical study on thermal dissociation of H2S. World Academy of Science, Engineering and Technology, International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering, 2010. 4(2): p. 244-249. Gupta, A., S. Ibrahim, and A. Al Shoaibi, Advances in sulfur chemistry for treatment of acid gases. Progress in Energy and Combustion Science, 2016. 54: p. 65-92. Cox, B.G., P.F. Clarke, and B.B. Pruden, Economics of thermal dissociation of H 2 S to produce hydrogen. International journal of hydrogen energy, 1998. 23(7): p. 531-544. 23 ACS Paragon Plus Environment

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Mohammed, S., A. Raj, and A. Al Shoaibi, Effects of fuel gas addition to Claus furnace on the formation of soot precursors. Combustion and Flame, 2016. 168: p. 240-254. Wall, D., A Higher Tier: Tier 3 Low Sulfur Gasoline Regulations and Their Effects. Hydrocarbon Engineering, 2013: p. 30-36. Rhodes, C., et al., The low-temperature hydrolysis of carbonyl sulfide and carbon disulfide: a review. Catalysis Today, 2000. 59(3): p. 443-464. Harrison P., Global Sulphur Outlook.

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http://www.optimin.co.za/assets/documents/Sulphur-Market-Outlook.pdf. Last accessed on 10th Septermber 2016. Luinstra, E., Hydrogen from H2S: technologies and economics. 1995: Sulfotech

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Research. Hawboldt, K., W. Monnery, and W. Svrcek, New experimental data and kinetic rate expression for H 2 S pyrolysis and re-association. Chemical Engineering Science, 2000. 55(5): p. 957-966. Bassani, A., et al., Novel coal gasification process: improvement of syngas yield and reduction of emissions. Chem. Eng. Trans, 2015. 43(1): p. 1483-1488. Ibrahim, S. and A. Raj, Kinetic Simulation of Acid Gas (H2S and CO2) Destruction for Simultaneous Syngas and Sulfur Recovery. Industrial & Engineering Chemistry Research, 2016. Palma, V., et al., H 2 production by thermal decomposition of H 2 S in the presence of oxygen. international journal of hydrogen energy, 2015. 40(1): p. 106-113. Li, Y., et al., Equilibrium prediction of acid gas partial oxidation with presence of CH 4 and CO 2 for hydrogen production. Applied Thermal Engineering, 2016. 107: p. 125-134. Binoist, M., et al., Kinetic study of the pyrolysis of H2S. Industrial & engineering chemistry research, 2003. 42(17): p. 3943-3951. Manenti, F., D. Papasidero, and E. Ranzi, Revised kinetic scheme for thermal furnace of sulfur recovery units. Chemical engineering transactions, 2013. 32: p. 1185-1290. Reverberi, A.P., et al., A review on hydrogen production from hydrogen sulphide by chemical and photochemical methods. Journal of Cleaner Production, 2016. Luinstra, E., H~ 2S: A potential source of hydrogen. SULPHUR-LONDON-, 1996: p. 37-47. Kalina, D. and E. Maas, Indirect hydrogen sulfide conversion—I. An acidic electrochemical process. International Journal of Hydrogen Energy, 1985. 10(3): p. 157-162. Reed, R.L., Modified claus furnace. 1986, Google Patents. Dowling, N., et al. An H2S thermal cracking claus furnace modification for the production of hydrogen. in Annual Technical Meeting. 1995. Petroleum Society of Canada.


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Sendt, K., M. Jazbec, and B. Haynes, Chemical kinetic modeling of the H/S system: H 2 S thermolysis and H 2 sulfidation. Proceedings of the Combustion Institute, 2002. 29(2): p. 2439-2446. Zhou, C., K. Sendt, and B.S. Haynes, Experimental and kinetic modelling study of H2S oxidation. Proceedings of the Combustion Institute, 2013. 34(1): p. 625632. Design, R., Chemkin-Pro Release 15131. Reaction Design, San Diego, CA, 2013. Kee, R., et al., CHEMKIN Release 4.0, Reaction Design. Inc., San Diego, CA, 2004. Manenti, G., et al., Design of SRU thermal reactor and waste heat boiler considering recombination reactions. Procedia Engineering, 2012. 42: p. 376383. Nabikandi, N.J. and S. Fatemi, Kinetic modelling of a commercial sulfur recovery

unit based on Claus straight through process: Comparison with equilibrium model. Journal of Industrial and Engineering Chemistry, 2015. 30: p. 50-63. Sames, J., et al. Field measurements of hydrogen production in an oxygenenriched claus furnace. in Proceedings Sulfur 1990 International Conference. 1990. Welty, J.R., et al., Fundamentals of momentum, heat, and mass transfer. 2009: John Wiley & Sons. Rameshni, M. and R. Street. PROClaus: The new standard for claus performance. in Sulfur Recovery Symposium, Canmore, Alberta. 2001. Zhao, H., et al., Recovery of elemental sulphur via selective catalytic reduction of SO 2 over sulphided CoMo/γ-Al 2 O 3 catalysts. Fuel, 2015. 147: p. 67-75. Goar, B.G., Temperature moderation of an oxygen enriched Claus sulfur plant. 1985, Google Patents.

582


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nomenclature Q M Cp T A ∆Tlmtd U, h dxw k Din Re Pr De µ Ds FB Nt PB Pt

Heat flux (W/m2) Flow rate (kg/hr) Specific heat at constant pressure (J/kg.K) Temperature K Heat transfer area (m2) Log mean temperature difference Overall Heat transfer coefficient (W/m2 K) Heat transfer coefficient (W/m2 K) Thickness (m) Thermal Conductivity (W/m.K) Inside tube diameter (m) Reynolds Number Prandtl Number Equivalent diameter on the shell side Dynamic viscosity (kg/s.m) Shell inner diameter m Fraction of the shell cross-section that makes up the baffle window Number of tubes in the baffle window (FB × number of tubes) Baffle pitch Tube pitch 26 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

do

Tube outside diameter (m)

Table 1. Geometrical parameters and inlet conditions from[32] used for mechanism validation. Parameters

Furnace Length

Inlet operating conditions and Geometry parameters from [32] 6.5 m

Inlet operating conditions and Geometry parameters from [33] 8m

Furnace Diameter

3.4 m

0.05 m

Velocity/mass flow rate

62.4 m/s

0.0135 kg/s

Pressure

150 kPa

182.3 kPa

Temperature

1273 K

1497 K


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Inlet composition (mol%)

C2H6 (0.515), CH4 (1.24), CO2 (25.57), H2O (8.585), H2S (17.33), N2 (37.26), O2 (9.96)

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CH4 (0.0117), CO2 (4.24), H2O (8.585), H2S (32.5), N2 (46.3), O2 (12.3), Ar (.624), C2H4 (0.00977), C2H6 (0.01134)

Table 2. Geometrical parameters and inlet conditions from[31] used for mechanism validation.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Acid gas composition (vol%)

Acid gas flow rate Acid gas temperature (oC) Combustion air composition (Vol%) Combustion air flow rate Combustion air temperature (oC) Operating pressure Reactor length Reactor Diameter WHB tube length WHB tube diameter

C2H6 (1.40), CH4 (2.1), CO2 (6.62), H2O (6.40), H2S (79.55), H2 (3.7), NH3 (0.48), C3H8 (1.98), BUT12 (0.11), CO (0.32), H2O (6.4) 4230.5 kg/h 125 H2O (9.70), N2 (71.38), O2 (18.92) 8907.1 kg/h 45 159 kPa 6.5 m 1.55 m 6.00 m 0.050 m

Table 3. Geometrical parameters and inlet conditions of the reaction furnace (RF) used in the simulation of SRU. 29 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Furnace Length (m) Furnace diameter (m) Burner diameter (m) Pressure (kPa) Inlet temperature (K) Inlet mixture velocity (m/s) Inlet acid gas flow rate (kmol/h) Inlet oxidizer flow rate (kmol/h) Inlet fuel gas flow rate (kmol/h) Inlet vent gas flow rate (kmol/h) Inlet acid gas composition (mol%)

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12.6 5.2 1.82 177 511 2.58 2820 4946 70 403 H2S (62.53), CO2 (26.58), H2O (9.63), H2 (0.01), CH4 (0.6918), C2H6 (0.25), C3H8 (0.23), N2 (0.003), CS2 (0.0004), Xylene (0.0088), Toluene (0.02859), Benzene (0.0318), Ethylbenzene (0.0073) Inlet oxidizer composition (mol%) O2 (19.76), N2 (74.54), H2O (5.7), Ar (0.86) Inlet fuel gas composition (mol%) C2H4 (98.68), C3H8 (0.02),C2H6 (1.3) Inlet vent gas composition (mol%) O2 (10.88), N2 (41.49), H2O (5.7),Ar (0.11), H2S (0.15), CO2 (0.065), H2O (47.29),SO2 (0.07) Inlet mixture (composition (mol%) H2S (21.40), O2 (12.1), N2 (45.34), CO2 (9.1), H2O (10), H2 (0.00034), CH4 (1.07), C2H6 (0.09642), C3H8 (0.078), Ar (0.521), CS2 (0.0000136), SO2 (0.0034) Xylene (0.00303), Toluene (0.00978), Benzene (0.0108), Ethylbenzene (0.0025)

Table 4. Geometrical parameters and inlet conditions for the waste heat boiler (WHB). 30 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Shell diameter I.D No of tubes O.D x Length Tube Thickness Tube Pitch Boiler feed water/Steam flow rate Process gas flow rate

5000 mm 4196 50.8 x 7925 mm 4.572 mm (7 BWG) 69.85 mm 167200 kg/hr 273852.7 kg/hr

Table 5. Modified inlet feed conditions to ensure complete H2S combustion in the Reaction Furnace. 31 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Inlet mixture velocity (m/s) Inlet acid gas flow rate (kmol/h) Inlet oxidizer flow rate (kmol/h Inlet fuel gas flow rate (kmol/h) Inlet vent gas flow rate (kmol/h) Inlet mixture composition (mol%)

Page 32 of 48

2.22 1218 5405 70 403 H2S (10.01), O2 (15.06), N2 (57.32), CO2 (4.58), H2O (10.1), H2 (0.00171), CH4 (1.00514), C2H6 (0.0557), C3H8 (0.0396), Ar (0.66), CS2 (0.0000686), SO2 (0.0039), Xylene (0.00152), Toluene (0.0049), Benzene (0.0054), Ethylbenzene (0.00125)

Table 6. The number of tubes, overall heat transfer coefficients, and gas velocity in the tubes of the WHB for different tube diameters. Diameter, m

No. of tubes

Overall heat transfer 32

ACS Paragon Plus Environment

Gas velocity,

Page 33 of 48

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Energy & Fuels

0.02 0.03 0.04 0.05 0.06

6924 5227 4265 3507 3012

coefficient, W/m2K 60 42 24 22 19

m/s 29.59 13.15 6.93 4.73 3.28

SULPHUR AND UNREACTED GAS RECYCLED BACK TO THE FURNACE

WASTE HEAT BOILER FURNACE AIR ACID GAS

TO CATALYTIC STAGES

Combustion

QUENCHING

H2


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. A simplified schematic diagram of SRU with modified thermal section for H2 production. A portion of the acid gas is sent to the waste heat boiler (a heat exchanger) for H2S cracking to form H2 using the heat energy of the gas exiting the furnace.


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Mole fraction

0.4

0.3

0.2

0.1

0

N2 (exp) CO2 (exp) H2O (exp)

1

0.12

10

N2 (calc) CO2 (calc) H2O (calc)

100 H2 (exp) S2 (exp) H2S (exp) SO2 (exp) CS2 (exp)

0.1

Mole fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.08

1000 H2 (calc) S2 (calc) H2S (calc) SO2 (calc) CS2 (calc)

0.06 0.04 0.02 0 1

10

100

1000

Distance, cm Figure 2. Experimentally observed (exp) and calculated (calc) gas-phase species composition at the exit of the RF of an industrial SRU. The experimental data was taken from [32]. 35 ACS Paragon Plus Environment

Energy & Fuels

0.12 H2S (exp) CO (exp) H2 (exp) S2 (exp) SO2 (exp)

0.1

Mole fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.08

H2S (calc) CO (calc) H2 (calc) Sx (calc) SO2 (calc)

RF

H2S (Manenti) CO (Manenti) H2 (Manenti) S2 (Manenti) SO2 (Manenti)

WHB

0.06 0.04 0.02 0

0

200 400 600 800 1000 1200 1400

Distance, cm Figure 3. Experimentally observed (exp) and calculated (calc) gas-phase species composition at the exit of the WHB of an industrial SRU with the experimental data was taken from Sames et al. [33] and model predicted data from Manenti et al.[31].


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0.25 H2 (calc) H2O (calc) CO (calc) CO2 (calc) H2S (calc) S2 (calc) SO2 (calc)

0.20

Mole fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.15

H2 (exp) H2O (exp) CO (exp) CO2 (exp) H2S (exp) S2 (exp) SO2 (exp)

0.10 0.05 0

0

200

400

600

Distance, cm

800

Figure 4. Experimentally observed (exp) and calculated (calc) gas-phase species composition at the exit of the RF of an industrial SRU. The experimental data was taken from Sames et al. [33].


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80 70 60 50 40 30 20 10 0

1000 ºC

1050 ºC

1150 ºC

0

200

400

600

800

1000

Residence time, ms

H2S Conversion, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Conversion,%

Energy & Fuels

35 30 25 20 15 10 5 0

970 ⁰C

940 ⁰C

900 ⁰C

850 ⁰C

400

600

800

1000

1200

1400

1600

Residence time, ms Figure 5. (a) A comparison of simulated and experimentally observed H2S conversion data from Hawboldt et al. [15]. (b) A comparison of simulated and experimentally observed H2S conversion data from Binoist et al. [20]. The lines are simulation results, while the dots are experimental data.


Page 39 of 48

50 45 40

H2 Yield ,%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

35 30 25 20 exp simulated

15 10 1000

1100

1200

1300

Temperature, ºC Figure 6. Simulated and experimentally (exp) observed H2 yields from Dowling et al. [26] at different temperatures.


Energy & Fuels

800 K

1000 K

1200 K

1400 K

1600 K

1800 K

2000 K

50

H2 Yield, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 30 20 10 0 1E-5

1E-4

1E-3

1E-2

1E-1

1E+0

Residence time, s Figure 7. Thermal Decomposition of H2S at different reactor temperatures in between 800 and 2000 K and residence time up to 2.4 s.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 8. Reaction path analysis of H2 production from H2S.


Energy & Fuels

2000

Temperature, K

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1600 1200

Actual feed Complete comustion Preheated + Complete combustion

800 400 0

200

400 600 800 Distance, cm

1000

1200

Figure 9. Temperature profiles in the RF at actual feed condition (blue line), at modified feed composition for complete H2S combustion (red line), and at a combination of high feed temperature (800 K) and complete H2S combustion (green line).


30 25 20 15 10 5 0

Actual feed Complete combustion Preheated + complete combustion

0

0.3

0.6

0.9

1.2

1.5

1.8

Residence time in WHB, s 1600

Temperature, K

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

H2 Yield, %

Page 43 of 48

1400 1200 1000 Actual feed

800

Complete combustion

600

Preheated + complete combsution

400 0

100 200 300 400 500 600 700 800

Distance in WHB, cm Figure 10. H2 Yield and temperature profiles on the shell-side of WHB (where acid gas cracking is taking place). The blue line refers to actual SRU feed conditions. The red line refers to a modified SRU feed composition to achieve complete H2S combustion in the RF. The green line refers to a case where, along with complete combustion of H2S, the feed was preheated to a temperature of 800 K. 43 ACS Paragon Plus Environment

Energy & Fuels

2400

Flame temperature, K

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Actual feed

complete combustion

2200 2000 1800 1600 1400 1200 300

400

500

600

700

800

900

1000

Feed inlet temperature, K Figure 11. Flame temperature in the RF at different feed inlet temperatures for actual SRU feed condition and for the modified feed composition to ensure complete combustion of H2S.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 12. Species profiles in the RF and the WHB (tube-side) at actual feed condition (blue lines) and at modified feed composition for complete H2S combustion (orange lines). The results refer to a simulation where a portion of acid gas was sent to the shell-side of the WHB, when hot exhaust gas from the RF was flowing on the tube-side of the WHB.


40 35 30 25 20 15 10 5 0

Page 46 of 48

Actual feed Complete combustion Preheated + complete combustion

0.2

0.4

0.6

0.8

1


Temperature, K

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

H2 Yield, %

Energy & Fuels

1400 1200 1000 800 600 400

0 100 200 300 400 500 600 700 800 Distance in WHB, cm Figure 13. H2 Yield and temperature profiles on the tube-side of WHB (where acid gas cracking is taking place). The blue line refers to actual SRU feed conditions. The red line refers to a modified SRU feed composition to achieve complete H2S combustion in the RF. The green line refers to a case where, along with complete combustion of H2S, the feed was preheated to a temperature of 800 K.


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H2 Yield, %

40 35 30 25 20 15 10 5 0

D=.02m

D=.03m

D = .04 m

D = .05 m

D = .06 m

0

0.5

1

1.5


Temperature, K

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1400 1200

D=2cm D = 4cm D=6cm

1000

D= 3cm D =5cm

800 0

100 200 300 400 500 600 700 800

Distance in WHB, cm Figure 14. Effect of the change in tube diameter (from 2 cm to 6 cm) on H2 yield and temperature on the tube-side of WHB.


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40 35 30 25 20 15 10 5 0

L=6m L=8m L = 10 m L = 12 m

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Residence time, s

Temperature, K

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

H2 Yield, %

Energy & Fuels

1600 1500 1400 1300 1200 1100 1000 900 800

L=6m L=8m L = 10 m L = 12 m

0

200

400

600

800

1000

1200

Distance, cm Figure 15. Effect of the change in tube length on H2 yield and temperature on the tube-side of WHB.


Kinetic Simulations of H2 Production from H2S Pyrolysis in Sulfur

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