Effects of oxygen enrichment on natural gas consumption and

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Kinetics, Catalysis, and Reaction Engineering

Effects of oxygen enrichment on natural gas consumption and emissions of toxic gases (CO, aromatics, and SO2) in Claus process Mohammad Al Hamadi, Salisu Ibrahim, and Abhijeet Raj Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03408 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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Industrial & Engineering Chemistry Research

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Effects of oxygen enrichment on natural gas consumption and emissions of

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toxic gases (CO, aromatics, and SO2) in Claus process

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Mohammad Al Hamadi, Salisu Ibrahim, Abhijeet Raj*

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Department of Chemical Engineering, The Petroleum Institute, Khalifa University, Abu Dhabi,

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UAE

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ABSTRACT

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The contaminants of acid gas feed to the Claus process plants such as benzene, toluene,

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ethylbenzene, and xylene (BTEX) increase the operational cost through catalyst deactivation and

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high fuel gas consumption and impact the sulfur recovery efficiency and the emission of toxic

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gases (such as CO and SO2). In this study, a detailed and validated reaction mechanism for Claus

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feed combustion is utilized to simulate the Claus process plant by integrating Chemkin Pro and

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Aspen HYSYS software. The effect of oxygen enrichment of air on sulfur recovery, BTEX

13

destruction, toxic gas emissions, and fuel gas consumption is studied. Upon increasing the oxygen

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concentration in air, BTEX concentration decreased substantially due to their enhanced oxidation

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by SO2 and O2. An increase in oxygen concentration resulted in (a) increased SO2 emission and

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decreased CO2 emission from the incinerator, and (b) decreased fuel gas consumption in the

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incinerator. Interestingly, CO emission increased with increase in oxygen concentration in air as

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the furnace temperature increased up to 1350°C, but it decreased with the further increase in the

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furnace temperature at higher oxygen concentrations. The reaction path analysis is presented to

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understand this decrease in CO emissions at high oxygen concentrations. The results demonstrate

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that a high oxygen concentration in air can be utilized to decrease fuel gas consumption and CO

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and CO2 emissions in the Claus process. The oxygen concentration, required to minimize the

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emission of aromatics, SO2, CO, and CO2 depended on the feed composition, and the developed

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Industrial & Engineering Chemistry Research 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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reaction mechanism can assist in optimizing the oxygen enrichment level required for a given feed

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in a Claus process plant.

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Keywords: Oxygen enrichment; Claus Process; BTEX; CO; Reaction Kinetics; Reaction

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mechanism.

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Corresponding author: Abhijeet Raj ([email protected]). Phone: +971-2-6075738.

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1. INTRODUCTION

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The regulatory agencies worldwide are enacting very strict legislations on the maximum allowable

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quantities of sulfur in gasoline and sales gas. In recent years, the United States Environmental

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Protection Agency (US EPA) has decreased the maximum allowable sulfur content in gasoline

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from 30 to 10 ppm 1. In addition, gas refineries must ensure that the H2S content in natural gas

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does not exceed a maximum of 4 ppmv 2. To achieve these low concentrations, more efficient

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desulfurization processes are being utilized 3, leading to the production of a large volume of acid

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gas (a gas rich in H2S and CO2). The treatment of acid gas occurs mostly in the Claus process unit,

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where elemental sulfur and high-pressure steam are recovered from H2S. The Claus process,

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depicted in Figure 1, consists of thermal and catalytic sections and a tail gas incinerator. The

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thermal section consists of a reaction furnace and a waste heat boiler (WHB). In the reaction

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furnace, one-third of the H2S is combusted to SO2 and H2O through reaction 1, and then H2S reacts

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with SO2 to produce sulfur, S2 (reaction 2) at high temperatures above 1000°C.

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𝐻2𝑆 + 1.5𝑂2→𝑆𝑂2 + 𝐻2𝑂 (𝑅1)

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2𝐻2𝑆 + 𝑆𝑂2⇄1.5𝑆2 + 2𝐻2𝑂 (𝑅2)


2

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The effluent gases are allowed to cool down to a temperature of 315°C in the WHB to recover

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thermal energy and elemental sulfur. Subsequently, in the catalytic section, elemental sulfur is

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produced through reactions 3 and 4 at a much lower temperature (160-400°C) in the presence of

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alumina and/or titanium catalysts 4.

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2𝐻2𝑆 + 𝑆𝑂2⇄0.5𝑆6 + 2𝐻2𝑂 (𝑅3)

49

3 2𝐻2𝑆 + 𝑆𝑂2⇄ 𝑆8 + 2𝐻2𝑂 (𝑅4) 8

50 51

Figure 1: Process Flow Schema of a three-stage Claus Process.

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The typical sulfur recovery efficiencies (SRE) of 90-96% and 95-98% are achieved with two and

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three catalytic sequences, respectively 3. The gases exiting the catalytic section are further treated

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in the incinerator, where remaining H2S is converted to SO2 and CO to CO2 at temperatures above

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600°C. The Claus process plants are required to achieve SRE above 98% in order to meet the

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environmental regulations 3. The SRE in Claus process is mostly dependent on the operating

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temperature of the Claus process reactors

5

and the feed conditions (concentrations of H2S and


3


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associated contaminants in acid gas) that often fluctuate 6. The feed to the Claus process usually

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have undesired characteristics such as low concentrations of H2S and the presence of contaminants

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such as CH4, CS2, CO2, aromatics, and mercaptans that cause severe operating problems, leading

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to a decrease in the SRE and an increase in the operating costs 7. To mitigate the impact of these

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contaminants and increase the SRE, an optimum temperature in the Claus furnace is required. The

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numerical 5 and experimental 8 studies show that SRE increases at temperatures up to 1400 K, and

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decreases with further increase in it. An increase in CO2 concentration in acid gas decreases SRE

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and increases the production of CO, COS, and CS2, but enhances BTEX destruction in the Claus

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furnace.

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To increase the SRE, process modifications have been proposed, which include the installation of

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additional catalytic units 9, addition of a tail gas treatment unit (TGTU) 10, acid gas enrichment to

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increase feed quality 7, optimization of process variables11, moisture removal to overcome the

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equilibrium constraints of Claus reactions

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additional catalytic unit or an acid gas enrichment unit increases the capital and operating costs to

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a process that is already economically deficient due to the low sale price of sulfur14. The process

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optimization approach is usually deployed to obtain the optimum operating conditions to maximize

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sulfur recovery and steam production and decreases fuel gas consumption and the operating cost,

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though it only provides a marginal increase in the SRE (≈ 0.5%) 15.

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Oxygen enrichment is a commercial and cost-effective technique that enhances the flame stability

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and the processing capacity of the Claus furnace, and promotes the destruction of hydrocarbon

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contaminants present in the feed, which helps to increase the SRE depending on the utilized oxygen

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concentration

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accompanied by a decrease in nitrogen flow to the furnace. This increases the furnace temperature

13, 16.

12,

and oxygen enrichment

13.

The installation of an

During oxygen enrichment, an increase in oxygen concentration is


4

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13

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and gas residence time that favors BTEX destruction

and decreases the feed flowrate, which

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helps to reduce the equipment size (capital cost and plant footprint) and the pressure drop through

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the unit. For new units, up to 50% of flowrate reduction can be achieved by oxygen enrichment

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and this translates to capital cost savings of up to 70% excluding the oxygen generation unit cost

85

16.

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to increase the acid gas processing capacity by 20–25% without major modifications to the existing

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equipment in the conventional Claus process plant. To increase the acid gas processing capacity

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by up to 75%, oxygen concentrations in the range of 28–45% of O2 in air is required in addition to

89

the installation of special burners designed for direct oxygen injection and to withstand high

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temperature

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temperature was compared to some commonly practiced techniques such as fuel gas and acid gas

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co-firing in the furnace, preheating of air and acid gas feed to the furnace, and acid gas enrichment

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to increase the H2S concentration in it. The oxygen enrichment technique was found to be most

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effective in increasing the Claus furnace temperature. Similar findings were reported in 19, where

95

simulations were conducted using a simplified kinetic mechanism for the Claus furnace that

96

accounted for the reactions of H2S, BTEX, CO2, CS2 and COS. However, it was noted that high

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oxygen concentrations (up to 100% O2) and H2S concentrations above 50% in the acid gas stream

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triggered excessive temperature excursions (>1500°C) that can damage the in-house furnace

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refractory linings and WHB tubing. Besides, excessive furnace temperatures could also decrease

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the production of sulfur due to the increased production of COS, as CO production from CO2

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increases in the furnace. In such cases, furnace temperature can be regulated through staged

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combustion 16, effluent gas recirculation 19, and by using additives (e.g. H2O) to the furnace 12.

In 16, it was reported that oxygen concentrations up to 28% in the combustion air can be used

17.

In

18,

the use of oxygen enrichment technique to increase the Claus furnace


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Some numerical and experimental studies on the effect of oxygen enrichment on the increased

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production of undesired by-products that include CO, SO2, COS and CS2 in the Claus furnace have

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been reported. Li et al. 20 reported the lab-scale experimental data on the structure of rich diffusion

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flames of H2S/CO2 with different oxygen concentrations in air (21, 30 and 50%). They found that

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oxygen enrichment increases COS formation through the reactions between CO and sulfur species.

108

The production of CS2, which significantly relied on CO2 reactivity, increased with the higher

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concentrations of oxygen due to increase in the formation of CS radicals that reacted with several

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sulfur species (S, S2, SH, SO and SO2) to form CS2. Rahman et al.

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mechanism and acid gas feed containing 62.5% H2S to investigate the effect of oxygen enrichment

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on the furnace temperature, BTEX destruction, residence time, and sulfur production. It was found

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that oxygen concentration higher than 25% was required for complete BTEX destruction, while

114

maintaining the high levels of sulfur production. However, higher oxygen concentrations above

115

25% increased the production of CO and COS, but the production of CS2 decreased. Using an acid

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gas feed consisting of 69% H2S, Abdoli et al.

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sulfur production and contaminant destruction in the Claus furnace. They used an industrial plant

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data and a 6-step global reaction model for the Claus furnace to conduct CFD simulations with

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due consideration to the turbulence, combustion, and radiation effects in the Claus furnace. They

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found that the higher oxygen concentrations increased the production of S2, while the production

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of COS and CS2 decreased and CO remained almost constant, even though the Claus furnace

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temperature increased up to 1500°C. The trends of CO and COS contradicts the observations from

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experiments 20, 22 and simulations 17, 19.

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It is evident from literature that high temperature, which accompanies oxygen enrichment,

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increases CO, CS2, and COS production in the Claus furnace due to the faster CO2 decomposition

21

17

used a detailed reaction

evaluated the effect of oxygen enrichment on


6

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17.

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the SRE 23 and increase sulfur emissions in the tail gas 9. Although CO can be oxidized in the tail

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gas treatment unit (TGTU) and/or the incinerator to meet the environmental standards, such

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practice involves firing a large volume of natural (or fuel) gas that increases the operating cost and

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the carbon footprint of the Claus plants. It is, therefore, very important to optimize oxygen

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enrichment in the Claus furnace, while monitoring the performance of the catalytic units and the

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incinerator to minimize the emissions of undesired byproducts, operating costs, and to maximize

133

the SRE. The effect of oxygen enrichment on the production of important species such as SO2,

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CO, and COS can alter the incinerator’s fuel gas consumption and the H2S/SO2 ratio required to

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maintain optimal sulfur recovery in the catalytic units. Most studies focus on the Claus furnace,

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rather than the entire Claus plant to investigate the role of oxygen enrichment on the process

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performance. The standalone effects of oxygen concentration, temperature, and volumetric

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flowrate on the complex reactions occurring in the Claus furnace are also not fully understood.

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The changes in these parameters can have divergent effects on the reactions in the furnace to

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increase the emissions of CO, COS, and CS2 and the fuel gas consumption in the incinerator.

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In this study, a detailed reaction mechanism for sulfur recovery unit (SRU) feed combustion is

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utilized to quantify the effects of oxygen concentration in air on toxic gas emission, BTEX

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destruction, sulfur recovery efficiency, and fuel gas consumption in the incinerator. The standalone

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effects of oxygen concentrations at different furnace temperatures is conducted for better

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understanding of the role of oxygen enrichment on the production pathways of sulfur and

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undesired byproducts. Simulations are conducted using CHEMKIN and Aspen HYSYS software

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using an industrial feed from a UAE Claus process plant. The simulation model developed in this

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work can be used for the optimization of oxygen enrichment in the Claus furnace to increase

The presence of COS and CS2 in the catalytic section can cause sulfate formation to decrease


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processing capacity, decrease toxic gas emissions and save on the operating cost of the Claus

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process.

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2. SIMULATION METHODOLOGY

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The kinetic simulation software, CHEMKIN PRO [12] is used to model the Claus furnace and the

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WHB, while the chemical process simulator, Aspen HYSYS 24 is deployed for the catalytic section

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and the incinerator simulation, and the model is validated using the experimental data from the

155

literature. The previously developed detailed reaction mechanism for acid gas combustion

156

adopted to account for the complex radical chemistry occurring in the Claus furnace and the WHB.

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The mechanism consists of 290 chemical species and 1900 reactions and has been validated

158

previously in

159

results.

17.

17

is

The scheme depicted in Figure 2 shows the procedure followed to obtain the


8

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161 162

Figure 2: Schema of the procedure and steps used for the SRU simulations.

163

The furnace was modelled as a steady-state plug flow reactor (PFR) due to the high Reynolds

164

number of the process gas, and its further justification can be found in

165

surrounding in the PFR was neglected due to the presence of refractories in the industrial Claus


25.

The heat loss to the

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furnaces. Since the temperature of the gas decreases in the WHB via heat exchange with cooling

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water, the WHB was modelled as a PFR with heat exchange. The WHB’s cooling water enters at

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115°C and 41.5 barg and exits as high-pressure steam at 253.5°C. The ambient temperature is the

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average temperature of the heating/cooling fluid which was calculated to be equal 184°C.

170

The catalytic section (consisting of two catalytic converters, heaters, and sulfur condensers) and

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the incinerator were modelled in Aspen HYSYS (Sulsim). The properties of the WHB's effluent

172

stream from CHEMKIN are defined as the inlet stream for the catalytic section in HYSYS. The

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stream is initially cooled in a condenser to a temperature of 135°C before it gets reheated to a

174

temperature of 254°C in a heater upstream the first catalytic converter. The effluent of the first

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catalytic converter is cooled to a temperature of 135°C in a condenser before it gets reheated to a

176

temperature of 155°C upstream the second catalytic unit, where H2S/SO2 ratio is maintained to a

177

value of two. The catalytic converters in HYSYS are simulated through free energy minimization

178

calculation based on the inlet composition and temperature and the outlet pressure. The reactions

179

of COS and CS2 hydrolysis occur in the catalytic reactors and they are quantified using empirical

180

data, while the species, CO, H2, and hydrocarbons are constrained from reacting. The first catalytic

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converter is a regular catalytic bed equipped with alumina catalyst and the second converter is a

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sub-dew point catalytic converter (CBA) equipped with alumina catalyst. The catalyst is assumed

183

to be fresh, and HYSYS’s default value of the pressure drop (0.5 psi) through both catalytic

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converters is used. The incinerator was fired through fuel gas combustion (using normal air) with

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a composition of 95% methane, 4% ethane and 1 % propane. The temperature target at the

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incinerator’s exit was set to 650°C, while the targeted excess oxygen concentration was around

187

3% in the incinerator’s stack. The fuel gas and air flow rates to the incinerator is dependent on the


10

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properties of the process gas flow from the catalytic section and the targeted exiting temperature

189

and excess oxygen concentration at the incinerator exit.

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3. MODEL VALIDATION

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Since the major objective of this work was to utilize a detailed reaction mechanism to evaluate

192

oxygen enrichment effects on the industrial Claus process plants, it was necessary to validate it

193

with the experimental data. The detailed mechanism used for the Claus furnace simulations has

194

been validated using several experimental data from the lab-scale setups and the industrial Claus

195

process plant in the UAE 17. Its sub-mechanisms on H2S, CO2, COS, CS2 and hydrocarbon were

196

validated successfully in

197

before using the adopted mechanism to substantiate the role of oxygen concentrations on the

198

emissions of toxic byproducts such as CO and SO2 in the Claus furnace, it was necessary to validate

199

the CO sub-mechanism using the recent experimental data on CO oxidation in the presence of

200

CO2, N2, and SO2 in order to enhance reliability of the simulation results.

201

In

202

investigated using lab-scale flow reactor within a temperature range of 527–1527°C. They used a

203

micro gas chromatograph equipped with a TCD detector to measure the concentration of CO with

204

an uncertainty of ±5%, while the SO2 composition was measured using Fourier Transform Infrared

205

(FTIR) spectrometer that had an uncertainty of ±10%, but not less than ±10 ppm. The inlet

206

conditions used for the simulation are shown in Table 1. It was noted that the SO2 concentration

207

did not show any noticeable changes for the entire temperature range analyzed in all the

208

experiments, and were not reported. The experimentally observed data on the CO concentrations

209

with the changes in the reactor temperature with/without SO2 addition under CO2 and N2 diluted

210

atmosphere were collected and compared with the simulated data. Figure 3 shows a comparison

28,

17, 26,

while the H2O sub-mechanism was validated in

12, 27.

However,

the effect of SO2 on the oxidation of CO under CO2 and N2-diluted conditions was


11


211

of the experimental data with model predictions, depicting the effect of SO2 on the conversion of

212

CO in CO2 and N2 diluted reactor.

213

Table 1. Experimental conditions obtained from

214

pressure and total gas flow rate, 16.7 cm3(STP)/s, are constant.

No. 1 2 3 4

𝜆=

[𝑂2] [𝐶𝑂] ([𝑂2] [𝐶𝑂])𝑠𝑡𝑜𝑖𝑐 0.7 0.7 0.7 0.7

28.

Residence time, tr [s] = 3100/T [K], and

[CO] (ppm)

[O2] (ppm)

[SO2] (ppm)

[H2O] (%)

[CO2] (%)

[N2] (%)

2000 2000 2000 2000

700 700 700 700

1000 0 1000 0

0.6 0.6 0.6 0.6

75 75 0 0

24.0 24.1 99.0 99.1

215 Simulation 1.0

Experimental

With SO2 (CO2 Dilution)

With SO2 (N2 Dilution)

0.8 0.6 0.4

CO/COinlet

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

Page 12 of 34

0.2 0.0 1.0

Without SO2 (CO2 Dilution)

Without SO2 (N2 Dilution)

0.8 0.6 0.4 0.2 0.0

216

1000

1200

1400

1600

1800

1000

1200

1400

1600

1800

Reactor Temperature (K)

217

Figure 3: CO oxidation in a CO2 or N2 atmosphere in the presence and absence of SO2 at different

218

reactor temperatures. The experimental data is from 28.

219

A good agreement can be observed between the model prediction and the experimental data. In

220

the CO2 atmosphere, the temperatures below 1127°C favored CO conversion to CO2, but the higher

221

temperatures hindered CO conversion due to its production from CO2 decomposition. Both in the


12

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222

presence and the absence of SO2, the oxidation of CO is observed to be inhibited in the CO2

223

atmosphere compared to the N2 case. For both the CO2 and N2 atmospheres, the addition of SO2

224

inhibited the oxidation of CO due to the reaction of HOSO (intermediate radical formed from the

225

reaction of SO2 with H radicals) with oxygen to produce SO2 and HO2. The radical, HOSO is an

226

intermediate product formed from the initial decomposition of SO2 via its reaction with H radicals

227

under the oxygen-deficient condition. The adopted reaction mechanism reliably captured all the

228

observed trends, which highlight its accuracy and reliability.

229

The mechanism was also validated using experimental data from the industrial Claus process

230

plants operating in Iran and Canada, in order to enhance the model reliability under a wide range

231

of conditions. The plant data, alongside the inlet feed conditions and geometrical parameters of

232

the furnace (shown in Tables 2 and 3) were reported in 29 for South Pars’s plant in Iran and in 30

233

for Ultramar Refinery plant in Canada. The plant data were only available at the exit of the furnace.

234 235

Table 2: Inlet streams properties and physical parameters of the South Pars's SRU furnace. Streams Properties Streams Composition (mol%)

Pressure (Pa) Temperature (°C) CO2 N2 CH4 C2H6 C3H8 H2S O2 H2O Molar Flow (mole/s) Length (m) Furnace Specifications Diameter (m) Gas residence time (s) Gas superficial velocity (m/s) Gas residence time in boiler (s)

Acid Gas Air Fuel gas 177000 168000 600000 218 220 40 53.16 0.00 1.03 0.00 73.00 3.68 0.90 0.00 89.46 0.00 0.00 5.66 0.00 0.00 0.17 36.04 0.00 0.00 0.00 19.50 0.00 9.90 7.50 0.00 171.11 181.50 3.24 6.5 3.4 2.0 62.44 0.48

236


13


237

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Table 3: Inlet feed properties and geometrical parameters of Ultramar Refinery's SRU. Stream Properties Stream Composition (mol%)

Pressure (kPa) Temperature (K) CH4 CO2 H2O H2S N2 O2 Ar C2H4 C2H6 Mass Flow (kg/s) Furnace Length (m) Specifications Diameter (m)

Feed 182.3 1497 0.0117 4.24 8.585 32.5 46.3 12.3 0.624 0.00977 0.01134 0.0135 8 0.05

238

239 240

Figure 4: Comparison of predicted species compositions at the exit of Claus furnace with plant

241

data from (a) South Pars’s SRU in Iran 29, and (b) Ultramar Refinery’s SRU in Canada 30.

242

Figure 4 shows a comparison between the observed species composition at the exit of the furnace

243

with their predicted values. A good agreement can be observed between the experimental data and

244

the model simulations for most species. The mechanism under-predicted the mole fraction of S2 at

245

the exit of the furnace, when compared to the plant data from South Pars’s refinery. Sulfur exists


14

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246

in various isomeric forms (S1-S8) and their relative concentration is highly temperature-dependent,

247

which makes their accurate quantification in the experiments difficult. While S2 is the most stable

248

one at high temperatures above 350°C, the stability of S8 is enhanced at low temperature. Thus,

249

the gas sampling technique and the quenching procedure have significant effect on the detection

250

and the quantification of sulfur, which, unfortunately, were not made available. For this reason,

251

the discrepancy between the predicted S2 concentration and the plant data could be attributed to

252

the experimental error. These validations highlight the accuracy and the reliability of the adopted

253

reaction mechanism and the model developed for the furnace simulations.

254

4. RESULTS AND DISCUSSION

255

The simulations of the thermal, catalytic, and incinerator sections of the Claus process plant were

256

carried out to evaluate the effect of oxygen enrichment on the major performance indices that

257

include, sulfur recovery, processing capacity, fuel gas consumption, and the emission of harmful

258

gases including BTEX, CO, SO2, and COS. The entire Claus process plant simulations were

259

conducted using the practical feed conditions and the physical specifications shown in Table 4,

260

which were obtained from an industrial Claus process plant operating in the UAE.

261

Table 4: Properties of feed streams and physical parameters of furnace and WHB in UAE's Claus

262

process plant.

Streams Properties

Streams Composition (mole fraction)

Temperature (°C) C6H6 C7H8 C2H6 C3H8 C4H10 C6H5CH3 CH4 CO2

Acid Gas 230°C 0.000314 1.5E-5 0.000345 0.000292 0.000103 0.000231 0.003024 0.0 to 0.645


Air 0 0 0 0 0 0 0 0

15


263 264 265 266 267 268

Furnace and WHB Specifications

Page 16 of 34

H2S 0.99 to 0.345 0 N2 0.00159 0.759 S 1.0E-5 0 iC4H10 3.4E-5 0 o-C8H10 4.2E-5 0 AR 0 0 H2O 0 0.0399 O2 0 0.201 Flowrate (kg/s) 12.4 Furnace: Length (12m); Diameter (3.3m); Pressure (1.64bar). WHB: Length (10m); Pressure (1.54bar); Number of Tubes (2500); Diameter of each tube (58.93 mm).

269

To investigate the effect of oxygen enrichment with the changes in H2S content in the acid gas

270

stream, the concentrations of H2S and CO2 were varied accordingly while the concentration of all

271

the other species were maintained. It was necessary to ensure that the adiabatic temperature in the

272

Claus furnace did not exceed the maximum value of 1527°C (1800 K) that is required to protect

273

the refractory linings from damage. Thus, for a given H2S concentrations in the acid gas stream,

274

simulations were conducted to determine the oxygen concentration at which the maximum

275

allowable temperature of 1527°C in the Claus furnace is reached. Figure 5 shows the concentration

276

of oxygen in air that is required to attain the maximum permissible temperature in the Claus

277

furnace with different compositions of H2S in the acid gas stream. It can be observed that the

278

increase in H2S concentrations in the acid gas decreased the amount of oxygen required to achieve

279

the maximum allowable temperature in the Claus furnace. For high H2S concentration (above

280

90%) in the acid gas, only a low-level oxygen enrichment is permissible, while for the acid gas

281

with less than 60% H2S, the entire combustion air can be replaced with oxygen without exceeding

282

the temperature limit of the furnace. The results have also been validated using the literature-based

283

information for rich acid gas streams. In 20, for an acid gas with 60% H2S and an equivalence ratio


16

Page 17 of 34 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


284

of 3, 100% oxygen enrichment level could be used without exceeding the furnace temperature of

285

1800 K, which agrees with our results (presented as point (a) in the figure). The results in their

286

study were obtained using Gibbs reactor in Aspen Plus software for a simplified Claus furnace

287

with heat recovery unit through equilibrium calculations by considering the species, H2S, CO2, O2,

288

N2, SO2, COS, CS2, S2, S6, S8, H2, CO, C, and H2O. In 19, the feed had 85% H2S, and the required

289

oxygen enrichment level was 34.69% to reach the furnace temperature close to 1800 K (presented

290

as point (b) in the figure). Our results predict the permissible oxygen enrichment level of 34.3%

291

for such feed, which is close to their results. In 31, for 90% H2S feed, it is mentioned that only 30

292

to 32 mol% oxygen in airstream could be used to hold the furnace temperature below 1811 K

293

(presented as point (c) in the figure). Our results predict a permissible oxygen enrichment level of

294

31.3% for a feed containing 90% H2S, and is in between their range.

295 296

Figure 5. The oxygen enrichment level required to achieve the maximum permissible temperature

297

of 1527°C in the Claus furnace for acid gas feeds with varying H2S concentration. For acid gas


17


Page 18 of 34

298

with < 60% H2S (grey dotted line), an enrichment level of 100% can be applied without exceeding

299

1527°C. For validation, reported data from (a) 20, (b) 19, and (c) 31 are also provided here.

300

The species composition and the properties of the process gas such as the temperature and the flow

301

rates were collected for the further analysis to quantify the effect of oxygen enrichment and

302

changes in acid gas composition on toxic gas emissions, fuel gas consumption, processing capacity

303

and sulfur recovery. Three different composition of acid gas were selected for analysis, which

304

include 34.8% (≈35%) H2S, referred to as Case 1, 54.8% (≈55%) H2S, referred to as Case 2, and

305

74.8% (≈75%) H2S, referred to as Case 3. Note that the acid gas feed obtained from the plant in

306

UAE contained 54.8% H2S, but the scope of the oxygen enrichment study was extended to higher

307

and lower H2S concentrations (74.8% and 34.8% H2S) due to the frequent fluctuations in acid gas

308

composition and to observe the effect of H2S concentration on the results of oxygen enrichment.

309

The higher H2S concentrations will lead to a much higher adiabatic temperature in the Claus

310

furnace, which could alter the rates of several reactions governing the production of undesired

311

byproducts (such as CO, SO2 and BTEX). The simulations were conducted using the feed

312

conditions given in Table 4. For all the simulation cases (1, 2 and 3) and the oxygen concentrations

313

in air, the flow rate of acid gas entering the Claus furnace was kept constant. Overall sulfur

314

recovery efficiency (SRE) was computed using the widely used formula [15]:

315

SRE =

Mass of Sulfur Produced × 100 Mass of Sulfur in Inlet H2S

316

Here, the mass of sulfur produced is calculated from the amount of sulfur flowing from the

317

condensers. The results obtained for cases 1-3 are presented in Figures 6-8, which show the effect

318

of oxygen concentration on furnace temperature, processing capacity, steam production, the

319

incinerator’s fuel gas consumption, and the exiting molar flow rates of CO2, CO, SO2, and BTEX.


18

Page 19 of 34

35% H2S in Acid Gas (Case 1) (a)

Sulfur Recovery Efficiency Steam Production in WHB

Capacity Increase Incinerator's Fuel Gas Consumption

Furnace Maximum Temperature (°C) 81 10

02 11

16 11

25 28 11 11

98.8

30

20

10

0

Sulfur Recovery Efficiency (%)

Capacity Increase (%)

40

28.5

98.6

28.0 98.4

27.5

98.2

27.0 26.5

98.0

26.0 97.8

Steam Production in WHB (103 kg/hr)

29.0

25.5 20

30

40

50

60

70

80

90

900

800

700

600

500

400

Incinerator's Fuel Gas Consumption (kg/hr)

40 10

1 4 95 99

100

Oxygen Enrichment Level (%)

(b)

SO2 Flowrate at Exit of Incinerator CO Flowrate at Exit of Incinerator

CO2 Flowrate at Exit of Incinerator BTEX's Flowrate at Exit of Incinerator

Furnace Maximum Temperature (°C) 40 10

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

20 18 16 14 12 10 8 6 4 2 0

780

760

740

720

SO2 Flowrate at Exit of Incinerator (kgmol/hr)

0.16

CO2 Flowrate at Exit of Incinerator (kgmol/hr)

1 4 95 99

CO Flowrate at Exit of Incinerator (kgmol/hr)

BTEX's Flowrate at Exit of Incinerator (kgmole/hr)

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


81 10

02 11

16 11

25 28 11 11

9

8

7

6

5 20

30

40

50

60

70

80

90

100


320

Figure 6: Case 1 (35% H2S): (a) sulfur recovery efficiency, steam production, processing capacity, and fuel gas

321

consumption, and (b) flowrates of SO2, CO2, CO and BTEX at the exit of the incinerator at different O2 concentrations.


19


55% H2S in Acid Gas (Case 2) (a)

Sulfur Recovery Efficiency Steam Production in WHB


Furnace Maximum Temperature (°C)

30

20

10

67 13

03 14

32 39 14 14

98.2



40

20 13

45.0

98.1

44.5

98.0

44.0

97.9

43.5

97.8

43.0

0 97.7

1000 900 800 700 600 500 400 300

20

30

40

50

60

70

80

90


50

41 12


13 58 11 11

100


(b)



0.0005

0.0000

32 30 28 26 24 22 20 18 16 14

580

560

540

520


0.0010

34


0.0015


14 3 14 2 39

14 03

13 67

13 20

12 41

11 1113 58



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

Page 20 of 34

14

13

12

11

20

30

40

50

60

70

80

90

100


322


323



20

Page 21 of 34

75% H2S in Acid Gas (Case 3) Sulfur Recovery Efficiency Steam Production in WHB


30

20

10



40

0

69.0

98.3

68.5 98.2 68.0 98.1

67.5

98.0

67.0

97.9

66.5 66.0

97.8


1200 69.5

98.4

65.5 20

25

30

35

40

45

1100 1000 900 800 700 600 500


15 68

10 15

14 38

13 52

12 37


50


(b)



0.00008

0.00004

0.00000

26

24

22

20

360

350

340

330

320


0.00012

28


0.00016


15 68

15 10

14 38

13 52

12 37



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


20 19 18 17 16 15 20

25

30

35

40

45

50


324


325



21


Page 22 of 34

326

For cases 1-2, shown in Figures 6 and 7, an increase in oxygen concentrations from 20 to 100 %

327

(and the corresponding reduction in inert N2 concentration) increased furnace temperatures to

328

1129°C in case 1 and 1439°C in case 2, which is below the refractory limit of the Claus furnace.

329

The increase in gas processing capacity of the furnace is defined as:

330

Capacity increase (%) Vol.flow rate of nonenriched feed ― Vol. flow rate of enriched feed = × 100 Vol. flow rate of nonenriched feed

331

Increasing the oxygen enrichment level to 100% increased the acid gas processing capacity by

332

35% (in case 1) and 45% (in case 2). However, despite the increase in the furnace temperature, the

333

production of high-pressure steam decreased due to the reduction in the volumetric flow of air.

334

The increase in oxygen enrichment causes a rapid decrease in the emission of BTEX and their

335

molar flow rates remained almost zero at oxygen concentrations above 30% in all cases. This was

336

found to be due to the faster oxidation of BTEX through the combined efforts of oxygen and SO2

337

as the Claus furnace temperature increased with increase in oxygen concentration. For example,

338

near the burner in the furnace, benzene reacted with oxidants such as OH and O atom to form

339

phenyl (C6H5) and phenoxy (C6H5O) radicals, and in the latter part of the furnace, phenyl radicals

340

got oxidized by SO2 and phenoxy radicals decomposed to C5H5 and CO. Toluene and xylenes had

341

similar fates, where oxidants, O and OH formed benzyl radicals that got oxidized by SO2 in the

342

latter part of the furnace. It should be noted that the production of higher aromatics (PAHs) from

343

the hydrocarbon contaminants (such as aliphatics and BTEX) present in the Claus feed also

344

occurred in the furnace, but their flow rates at the exit of WHB were found to be negligible. For

345

example, with increasing oxygen concentrations, the molar flow rate of PAHs at the WHB exit

346

increased from 0.0005 to 0.0069 kmol/h in case 1, but remained below 0.0001 kmol/h in cases 2

347

and 3 due to their higher furnace temperatures. The production of PAHs in the furnace is dependent


22

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348

on the operating temperature and the inlet concentration of hydrocarbons 6. The rapid oxidation of

349

BTEX due to the favorable furnace temperature and low concentration of small hydrocarbons,

350

particularly methane (due to zero fuel gas injection) helped to suppress the reactions of PAH

351

production. The incinerator’s fuel gas consumption decreased by up to 40% in case 1 (Figure 6)

352

and 60% in case 2 (Figure 7) due to the decrease in the volumetric flow rate of gas with increase

353

in oxygen concentration. The decrease in the fuel gas consumption in the incinerator was found to

354

be primarily responsible for the decrease in the molar flow rates of CO2 from the incinerator’s exit.

355

The molar flow rates of the undesired species (BTEX, CO, and SO2), mainly controlled by the

356

chemistry occurring in the Claus furnace, were also analyzed. Note that the species CO, H2, and

357

hydrocarbons (including BTEX) are constrained from reacting in the catalytic reactors, hence they

358

were analyzed at the exit of the Claus furnace and the WHB. The results suggest that the use of

359

high oxygen concentration (above 30%) in the Claus furnace will have a negative impact on the

360

SRE, which leads to an increase in SO2 emissions. This effect was more pronounced in the results

361

of case 2 (Figure 7) and case 3 (Figure 8) with higher H2S concentrations due to the higher furnace

362

temperatures. In case 2, it can be observed that SO2 emission at the incinerator’s exit decreased

363

due to the increase in sulfur production as the furnace temperature increased (from 1113 to

364

1200°C) at oxygen concentrations below 25%, but with further increase in the oxygen

365

concentration and the furnace temperature, a decrease in sulfur production occurred that caused

366

the SO2 molar flow rate to increase. The decrease in sulfur production at higher furnace

367

temperature was mainly due to the oxidation reaction of sulfur to SO2. The following reactions in

368

the mechanism were responsible for sulfur oxidation in the high oxygen enrichment cases: 𝑆2

369

+𝑂⇌𝑆𝑂 + 𝑆, 𝑆2 +𝑂𝐻⇌𝑆2𝑂 + 𝐻, and 𝑆3 +𝑂𝐻⇌𝑆𝑂 + 𝐻𝑆𝑆. The molar flow rate of CO emission

370

increased up to a maximum value corresponding to about 50% O2 concentrations and furnace


23


Page 24 of 34

371

temperatures around 1054°C, but surprisingly decreased with the further increase in oxygen

372

concentration. This suggest that a further increase in furnace temperature up to the permissible

373

limit of 1530°C could decrease CO emission. This was verified by increasing the H2S

374

concentration in acid gas up to 74.8% (case 3) and increasing the oxygen concentration until the

375

furnace refractory limit is attained.

376

In case 3 (shown in Figure 8), the maximum furnace temperature (1568°C) exceeded refractory

377

design temperature at 50% O2 concentrations, and thereby, oxygen concentrations above 50% were

378

not considered. An increase in oxygen concentration decreased the incinerator fuel gas

379

consumption that caused a decrease in the molar flow rates of CO2 in the incinerator’s stack, which

380

agrees with the trends observed previously in cases 1 and 2. The high furnace temperatures near

381

1200-1500°C caused a marginal decrease in sulfur recovery efficiency (by up to 0.4%), which

382

increased the SO2 molar flowrate from the incinerator stack. The steam production in the WHB

383

increased, despite the decrease in the volumetric flow with higher oxygen concentrations in air due

384

to the significant increase in furnace temperature that rose up to 1527°C with 50% O2

385

concentration. Also, an increase in oxygen concentrations from 20 to 28% caused the CO molar

386

flow rates to increase to a maximum value, but the CO molar flow rate decreased rapidly with the

387

further increase in oxygen concentration. Compared to cases 1 and 2, the CO molar flow rates

388

showed a more severe decrease due to the higher furnace temperatures in case 3. The Claus furnace

389

is characterized by complex radical chemistry of several competing reactions that could lead to

390

CO production and consumption. An increase in furnace temperature could increase the rates of

391

several reactions that consume CO (such as CO oxidation by oxygenated sulfur radicals) or cause

392

the reaction of CO production (such as CO2+H=CO+OH) to occur in the reverse direction.

393

Nevertheless, the reason for this drastic decrease in CO at high temperatures has been explored


24

Page 25 of 34 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


394

further, as this can provide insight into the favorable means of decreasing CO emissions during

395

the combustion with oxygen-enriched air.

396

To understand the chemistry of CO production and consumption in the Claus furnace, the

397

fundamental studies were conducted to investigate the standalone effect of oxygen concentrations

398

on the reactions of CO production at fixed furnace temperatures (isothermal conditions) ranging

399

from 950 to 1500 °C. The use of isothermal regime in the Claus furnace allowed us to decouple

400

the competing effects of temperature and oxygen concentrations on the radical reaction chemistry.

401

The catalytic section was allowed to operate normally with a controlled H2S/SO2 ratio of 2

402

upstream of the 2nd catalytic unit. Throughout the simulations, the species molar flow rates in Claus

403

furnace, WHB, and the incinerator’s stack and the reaction rates of the most important reactions

404

occurring in the Claus furnace and the WHB were collected and analyzed.

405

Figure 9 shows the effect of oxygen concentrations and fixed furnace temperatures on the molar

406

flow rates of CO exiting the Claus furnace and the WHB. It can be observed that for a fixed oxygen

407

concentration, an increase in furnace temperature increased the emissions of CO due to its faster

408

production from CO2 decomposition (mainly CO2+H=CO+OH). But interestingly, upon

409

increasing the oxygen concentration in air and maintaining fixed furnace temperatures below

410

1100°C, the CO molar flow rates from the furnace exit increased, while it decreased with further

411

increase in the furnace temperatures up to 1500°C. At the WHB exit, CO molar flow rate decreased

412

with the increase in oxygen concentrations, and a more significant decrease was seen at the

413

temperatures above 1100°C. In order to understand the reactions responsible for the observed

414

trends of CO production, molar flow rates and the rates of several competing reactions of CO were

415

analyzed. The cases of 20% and 100% oxygen concentration at fixed temperatures of 1050°C and

416

1400°C were chosen for the further investigation and comparative analysis.


25

(a) 250 200 150 100

CO Flowrate at Exit of WHB (kgmole/hr)

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

CO Flowrate at Exit of Furnace (kgmole/hr)


Base 24% O2 34% O2 50% O2 65% O2 80% O2 95% O2 100% O2

50 0 1000

1100

1200

1300

1400

1500

(b)

140 120 100 80 60

Page 26 of 34

Base 24% O2 34% O2 50% O2 65% O2 80% O2 95% O2 100% O2

40 20 0 1000

Furnace Temperature (°C)

1100

1200

1300

1400

1500

Furnace Temperature (°C)

417

Figure 9: Effect of fixed furnace temperature and oxygen concentrations on the molar flow rates

418

of CO at the exit of (a) furnace and (b) WHB.

419

Figure 10 shows the molar flowrate of CO along the Claus furnace and the WHB at two different

420

oxygen concentrations (20% and 100% O2 in air) and fixed furnace temperatures (1050°C and

421

1400°C). At 1050°C, while a high oxygen concentration (100%) increased CO production in the

422

furnace, the oxidation of CO in the WHB occurred at a faster rate and its exiting molar flow rate

423

decreased when compared to the base case with 20% oxygen concentration in air. In the initial

424

parts of the Claus furnace, the production of CO mainly occurred through CO2 chemical

425

decomposition with H radicals (reaction 5), but in the latter parts, its contribution to CO production

426

becomes insignificant due to the consumption of H radicals by other reactions of COS

427

decomposition (reaction 6) and the formation of H2 and H2O. Subsequently, the CO production

428

proceeds through the reactions of COS with H, S and HS radicals (reactions 6-8). The COS

429

production, shown in Figure 11, was found to be occurring through reaction 9, which is the major

430

reaction of CO conversion in the WHB.


26

Page 27 of 34

(a)

(b)

Furnace ← → WHB

11

Furnace ← → WHB

Base Case Enriched Case

CO Flowrate (mole s-1)

9 8 7 6 5 4 3

CO Flowrate (mole s-1)

10

2 1 0 0

65 60 55 50 45 40 35 30 25 20 15 10 5 0


0

200 400 600 800 1000 1200 1400 1600 1800 2000 2200

200 400 600 800 1000 1200 1400 1600 1800 2000 2200

Distance (cm)

Distance (cm)

431

Figure 10: CO Flowrate in the furnace and the WHB at 20% and 100% O2 enrichment levels for

432

fixed furnace temperature of (a) 1050 °C and (b) 1400 °C. 𝐶𝑂2 + 𝐻⇌𝐶𝑂 + 𝑂𝐻

Reaction (5)

𝐶𝑂𝑆 + 𝐻⇌𝐶𝑂 + 𝑆𝐻

Reaction (6)

𝐶𝑂𝑆 + 𝑆⇌𝐶𝑂 + 𝑆2

Reaction (7)

𝐶𝑂𝑆 + 𝐻𝑆⇌𝐶𝑂 + 𝐻𝑆𝑆

Reaction (8)

𝐶𝑂 + 𝑆3⇌𝐶𝑂𝑆 + 𝑆2

Reaction (9)

(a)

(b)

Furnace ← → WHB

11

Furnace ← → WHB

22 20

10

18

COS Flowrate (mole s-1)

9

COS Flowrate (mole s-1)

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



8 7 6 5 4 3 2 1

16 14


12 10 8 6 4 2

0

0 0

200 400 600 800 1000 1200 1400 1600 1800 2000 2200

0

200 400 600 800 1000 1200 1400 1600 1800 2000 2200

Distance (cm)

Distance (cm)


27


Page 28 of 34

433

Figure 11: COS flowrate in the furnace and the WHB at base case conditions and 100% O2

434

concentration for fixed furnace temperatures of (a) 1050°C and (b) 1400°C.

435

The increase in oxygen concentration increased the rate of CO production from reactions 5-8 due

436

to the faster rates of H2S oxidation that enhanced the radical pool. The initial oxidation of H2S is

437

characterized by the formation of several active radicals that include S, HS, HSS, HSO, and SO

438

species, and the rates of their production and consumption increased with increasing oxygen

439

concentrations. At fixed temperature of 1050°C and with the increase in O2 concentration (from

440

20 to 100%), the rates of CO production from reactions 6 and 8 were found to be 3 times faster (at

441

locations closer to the furnace exit) due to the enhancement in radical concentrations of H and HS

442

species. This justifies the observed increase in CO production with increasing oxygen

443

concentrations at temperatures below 1100°C in the Claus furnace.

444

The increase in temperature from 1050 to 1400°C caused CO production in the Claus furnace to

445

reach equilibrium value due the emergence of reverse reaction 5 (responsible for the initial

446

production of CO). Interestingly, an increase in oxygen concentration from 20 to 100% decreased

447

the equilibrium value of CO molar flow rate by up to 10% as indicated in Figure 10, even though

448

its production in the initial parts of the furnace was faster at higher oxygen concentration. It was

449

observed that the higher furnace temperatures (above 1100°C) triggered CO production from

450

reaction 10 in addition to reaction 5, and the rate of reaction 10 increased at higher oxygen

451

concentration. 𝐶𝑂2 + 𝑆⇌𝐶𝑂 + 𝑆𝑂

Reaction (10)

452

While the increase in oxygen concentrations increased the rate of CO production from reactions 5,

453

6, and 8, reactions 5 (in reverse direction) and 9 consumed CO at much faster rates causing a

454

decrease in its molar flow rate in the furnace and the WHB. Reaction 7, whose contribution to CO


28

Page 29 of 34 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


455

production was noticeably high at temperatures below 1100°C, was found to be negligible at higher

456

temperatures. Thus, the observed decrease in CO production with increase in oxygen concentration

457

at temperatures above 1100°C is not only due to the emergence of reverse reaction 5 but also due

458

to slower rates of other reactions leading to CO production.

459

At all temperatures and oxygen concentrations examined, reaction 9 was the more predominant

460

pathway for CO conversion to COS in the Claus furnace and WHB, though the reverse of reaction

461

5 and reaction 11 to a lesser extent contributed to CO oxidation to CO2 at temperatures above

462

1100°C. With the increase in oxygen concentration, CO conversion via reactions 9 and 11 was

463

found to be more dominant in the WHB than in the furnace, thus causing a more significant

464

decrease in the exiting molar flow rate of CO at higher temperatures. 𝐶𝑂 + 𝑆𝑂2⇌𝐶𝑂2 + 𝑆𝑂

Reaction (11)

465

The above analysis suggests that, provided the Claus furnace temperatures remain below 1100°C,

466

increase in oxygen concentrations could result in higher CO production in the furnace but faster

467

conversion to COS in the WHB. In contrast, at higher furnace temperatures above 1100°C,

468

increasing concentrations of oxygen could decrease CO production in the Claus furnace and

469

increase its conversion to COS and CO2 in the WHB. The production of radical species such as H,

470

S, HS, and SO radicals that governs the initial oxidation of H2S plays a very critical role in the

471

conversion of CO to COS and CO2. Clearly, the conversion of CO is expected to occur at faster

472

rates with high concentrations of H2S in the acid gas stream as the production of active radicals

473

will increase. Hence, the reduction of CO at high O2 concentrations (seen in figure 7 and more

474

significantly in figure 8) that is accompanied with higher furnace temperatures occurred due to its

475

lower production in the furnace and faster conversion to COS and CO2 in the WHB. The COS

476

exiting the WHB is hydrolyzed and converted to sulfur in the catalytic reactors.


29


Page 30 of 34

477

5. CONCLUSION

478

The effect of oxygen concentration in air (20 to 100%) in an industrial SRU on the incinerator’s

479

fuel gas consumption and the emission of harmful aromatics, CO, and SO2 was studied using a

480

detailed reaction mechanism. The oxygen enrichment effects were investigated for three feed

481

conditions, consisting of about 35, 55, and 75% H2S in the acid gas stream. For all the feed

482

conditions, the increase in oxygen concentration decreased significantly the fuel gas consumption

483

in the incinerator and the CO2 emission, while the SO2 emission increased at the high enrichment

484

levels. The increase in oxygen concentration increased CO emissions (which was mainly produced

485

in the Claus furnace) as the furnace temperatures remained below 1350°C, while the CO emissions

486

decreased with the further increase in the oxygen concentration and the furnace temperature. The

487

decrease in CO emission occurred very rapidly with higher concentrations of H2S in the acid gas,

488

where higher furnace temperatures up to 1527 °C could be achieved. The effect of furnace

489

temperatures at different oxygen concentrations on the major reactions of CO in the Claus furnace

490

and the WHB was investigated. It was observed that, while an increase in the furnace temperature

491

(within 950-1500°C) increased CO production at fixed oxygen concentration at the WHB exit, a

492

decrease in CO production occurred with increase in oxygen concentration at fixed temperatures.

493

At furnace temperatures below 1100°C, the increase in oxygen concentration resulted in higher

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CO production in the furnace but faster conversion to COS in the WHB. However, higher furnace

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temperatures above 1100°C decreased CO production in the Claus furnace and increased the rates

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of CO conversion to COS and CO2 in the WHB (through the reactions CO+S3=COS+S2,

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CO+OH=CO2+H and CO+SO=CO2+S). The production of radical species such as H, S, HS, and

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SO radicals that governs the initial oxidation of H2S played a very critical role in the conversion

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of CO to COS and CO2. Hence, the decrease in CO emissions observed at high oxygen


30

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500

concentration (accompanied with higher furnace temperatures) occurred due to its lower

501

production in the furnace and faster conversion to COS and CO2 in the WHB. COS, thus produced,

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can be hydrolyzed in the catalytic section to produce H2S and CO2. Clearly, the oxygen

503

concentration required to destruct the aromatics and minimize the production of CO depends on

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the feed composition. For the UAE’s SRU feed, an enrichment of above 50% was found to be

505

suitable to achieve the two targets.

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ACKNOWLEDGMENT

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The authors acknowledge the financial support by the Petroleum Institute Gas Processing and

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Materials Science Research Centre under grant number GRC17004.

509

REFERENCES

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Table of Contents graphic

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Effects of oxygen enrichment on natural gas consumption and

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