In the furnace, the temperature range is between 1223 and 1573 K and the pressure varies from 1.01 to 1.51 bar, along with the waste heat boiler is simulated at 640 K It is well known that acid gases contain a considerable amount of CO₂. In order to estimate the reuse of CO₂, the models were also run with different percentages of CO₂ inlet compositions varying from 25% to 44% at 1373 K and 640 K in the thermal furnace and WHB respectively. The innovative reaction can be used when streams with both CO₂ and H₂S are presented. Temperature and pressure ranges were chosen according to the average operation temperatures reported in the literature.

In a typical Claus process, the thermal part consists of a burner, a thermal furnace (TR) and a waste heat boiler (WHB). In this study, the burner and thermal furnace were modelled as one plug flow reactor, and the waste heat boiler was simulated as a bundle of several PFRs as shown in Figure 1.

Figure 1

Thermal section of a typical Claus process

The horizontal furnace reactor was modelled by assuming:

•Steady-state operation condition;

•Adiabatic condition (well-isolated furnace);

•Ideal gas mixture (due to high temperature and low pressure);

•Fully developed flow both in TR and WHB;

•Absence of fouling;

•Inlet streams are acid gas and air. Air can be enriched by oxygen supply.

In this work, the air was not enriched and was carried with 21% of oxygen. Table 1 shows the properties of inlet streams of sulfur recovery unit (SRU) obtained from industrial plants and used also for simulating the novel technology, suitably separating the main flow from the impurities.

The oxidation reactions take place in the flame zone that is 0.1 - 0.5 m of the furnace. The other reactions occur along the thermal section of the reactor.

At high operating temperatures, S₂ is the dominant allotrope of the elemental sulfur. Therefore, in this work, elemental sulfur was mentioned as S₂, even if, it has allotropes such as S₁ to S₈.

The thermal section of AG2S^TM process is made by regenerative thermal reactor (RTR), which has a different configuration compared with Claus process. It has a thermal reactor, waste heat boiler and a heat exchanger. The main point is to feed the preheated acid gas with an optimal H₂S/CO₂ ratio. Therefore, the demanding oxygen or air stream is much lower than the one required in Claus process. Thermal reactor and WHB were simulated considering them as an adiabatic and a non-isothermal PFRs, respectively.

Figure 2

AG2S^TM process scheme

AG2S^TM process was studied using the Simulation Suite PRO/II® by Schneider-Electric Simulation Science, using SRK (Soave-Redlich-Kwong) equation of state as thermodynamic model. The process scheme of the AG2S^TM is shown in Figure 2 . In order to use this technology at industrial level, it is necessary to revamp existing Claus plants, modifying some of the current used unit operations, in particular the thermal furnace and keeping unchanged the catalytic section. The design of the new reactor has allowed the overcoming of the problem: the implemented solution consists in a thermal regenerative reactor (RTR). In this configuration the efficiency of the process is improving by burning a proper amount of H₂S in order to minimize generation of steam and obtained an autothermal reactor.

Fresh reactants (CO₂ and H₂S coming from upstream plants, assisted by air or oxygen) are injected along with recycled reactants into RTR. The recycle can be also mixed with fresh reactants before pre-heating and injection into the reactor recycle for improving self-sustainability of the plant from process standpoint, that is, for reduction or elimination of exhausts.

For assuring a proper production yield, it is essential to prevent any possible recombination effects, which have been proven to be significant during relatively slow cooling; for this purpose, a Waste Heat Boiler (WHB) is installed just at outlet of RTR to quench the reactions.

The WHB and recycle pre-heating equipment play a key role in the regenerative process,

therefore, they could be considered a portion of the RTR. On the whole, the RTR could be also seen as a system constituted of a “regenerative” and a “recuperative” section, and not just the reaction chamber itself.

As mention the aim of this process is to recover as much as possible hydrogen from the H₂S molecule, in order to be able to produce a huge amount of syngas. In order to allow the pyrolysis of H₂S high temperatures (1273 - 1473 K) [25] are necessary to: (i) activate the reactive system from chemical-thermodynamics standpoint, (ii) quicken kinetics and (iii) reduce by-products.

The outflow of the catalytic reactor, which includes a certain amount of syngas, must be purified from the unreacted acid gas (H₂S and CO₂). The simulation of the catalytic reactor is carried out using conversion reactor in Pro/II. Al₂O₃ sites hydrolyse COS and CS₂ and support the dispersed elements that allows to increase the conversion up to 70% in the new technology and it is about 99% for Claus reaction [26].

The AG2S^TM technology approach at real plant results to be a novelty, both processes were simulated with several field data using DSMOKE. The software is a general framework developed by our research group for numerical simulations of reacting systems with detailed kinetic mechanisms including thousands of chemical species and reactions [19], involving radicals and complex mechanism to explain all the main reactions of the process.

For each reactor, 146 species mass balances, one global mass balance and one energy balance were solved, using standard material (eq. (2)) and energy balances (eq. (3)) of plug flow reactor:

dwidt= ∑j=1NRφi,j Rj Wj i=1, … , NC (2)

cpdTdt= ∑j=1NR-ΔHj Rj+ Uext SV Text-T (3)

The evaluation of the potential application of AG2S^TM technology on sulfur recovery is related to the quantity of H₂S presents in the feedstock. In this work to prove the validity of the process a feedstock containing 23% of H₂S was chosen. The feed composition is reported in Table 1. Because Claus process operates at quite high temperatures, the temperature range was chosen as 1223 - 1573 K in this work.

According to the simulation results, Figure 3 shows the dependence of H₂S conversion, of both processes, in function of the temperature. The conversion of H₂S is increasing gradually with an increase of temperature and achieving 70% and 83% for AG2S^TM and Claus processes, respectively.

The acid gas and air meet in the burner, which is the first step of the Claus furnace. In that part, hydrogen sulfide conversion increases fast depends on the oxidation reactions, in fact the conversion at all temperatures are in a range of 70 - 83%.

Instead, in AG2S^TM technology the feed is preheated in order to achieve the maximum hydrogen sulfide conversion. The reaction between H₂S and CO₂ (Equation 4) occurs immediately due to high activation energy. The hydrogen sulfide conversion achieve minimum ≈ 30% at 1223 K and maximum ≈ 70% at 1573 K . These results show that the conversion is strongly dependent of operating temperature.

Figure 3

Conversion of H₂S in Claus (a) and AG2S^TM (b) in function of temperature (K)

Together with decreasing H₂S content, one of the main purposes of AG2S^TM process is to quantify the production of syngas. It is a valuable commodity for use as a fuel in gas engines or to produce valuable chemicals such as ammonia and liquid fuels.

Yield of syngas is an important indicator of process feasibility. It is defined as ratio between moles of syngas and moles of reagents:

${\eta }_{\mathrm{s}\mathrm{y}\mathrm{n}\mathrm{g}\mathrm{a}\mathrm{s} }=\frac{F}{F^{\mathrm{o}}} \frac{x_{\mathrm{C}\mathrm{O}}+x_{{\mathrm{H}}_2\mathrm{ } }}{x_{ \mathrm{C}{\mathrm{O}}_2}^0+x_{{\mathrm{H}}_2\mathrm{S}}^0}$ (4)

where x_i is the molar fraction, F is the molar flow rate and F^o the initial conditions.

The yield of syngas producing respect to initial reagents is shown in Figure 4. Hydrogen and syngas production have been widely studied. According to [27], the temperature is the most influent factor for pyrolysis, and increasing the temperature, resulted in increase in gas yield and more hydrogen production. The optimization of temperature is around 1473 K for the process. The results illustrated that higher temperature was needed in the process up than 1373 K to maximize syngas yield.

The low temperatures are not be able to activate the reactive system from chemical-thermodynamics standpoint; hence, at the temperatures lower than 1373 K, the syngas yield is very low and the time which is needed for the occurrence of main reactions is longer. Besides, the side reactions completely occur along with the main reactions at 2.5 m, 0.4 m, and 0.25 m at the temperatures 1373 K, 1473 K, and 1573 K, respectively. The peaks show that the reactions take place quicker and the yield rises by increasing the temperature, as expected.

Figure 4

Syngas yield AG2S^TM technology in function of operating temperature (K) and reactor length

Table 2 shows the molar fraction of CO₂ in function of operating temperature. As seen in table CO₂ conversions, obtained from Claus furnace are almost equal at any considered temperature. Instead, the new technology is strongly dependent on the temperature and achieves the best CO₂ conversion up than 1473 K.

The hydrogen sulfide decomposition reaction plays an important role in the formation of CO and COS. In fact, the recombination reactions (eqs. (5)-(7)) that occur at the front of the WHB are:

H₂ + ½ S₂ ⇌ H₂ S (5)

CO + ½ S₂ ⇌ COS (6)

CH₄ + 2S₂ → CS₂ + 2H₂S (7)

These reactions not only influence sulfur recovery, air demand, and hydrogen production in the SRU, but they also affect the performance of the WHB.

The COS and CS₂ are formed in the front-end units, i.e. the TR and the WHB. They can be converted to H₂S and CO via hydrolysis reactions in the catalytic converters. The main reaction occurring in the converters is favoured thermodynamically at lower temperatures (623 K). At these temperatures the hydrolysis reactions are kinetically limited which often results in little conversions of COS and CS_2. The unconverted COS/CS₂ fend up in the tail gas where they represent a large proportion of the sulfur content. The temperature profile’s trend shows a great stability of the COS species at high temperature (1273 - 1573 K) while it is possible to note an inverse trend for the CS₂ species which is more stable at low temperature. The formation of these species is mainly due to the presence, albeit in small quantities, of H₂S in the WHB unit in which the reactions eq. (6) and eq. (7) take place.

Figure 5

Recombination products concentrations of WHB in function of temperature

The investigation of CO₂ effect was done by taking into account three feed based on different CO₂ content, because it is well-known that AG2S^TM achieves its better performance with high CO₂ content. The data of two Oil Refinery Companies located in Mesaieed, Qatar and one in Nanjing, China are used as references. They were mentioned in this study as case study 1, 2 and 3 which contain 25 mol %, 39 mol % and 44 mol % of carbon dioxide respectively.

As seen in Figure 6, CO₂ effect are strongly influence of feed composition, this shows that efficiency of AG2S^TM process is strongly dependent on inlet CO₂.

The results illustrate that the novel technology has a significant predominance comparing with Claus process in terms of CO₂ conversion. Following the graph in Figure 6 (straight line), there is a linear dependence between the CO₂ content and its conversion, in fact the process achieves 27% conversion when the molar composition is 44%. In the Claus process in Figure 6 (dotted line) the trend is the reverse and the conversion is low (1.5 - 6%), due to a not optimal figuration; in fact, reducing CO₂ is not the main target of this process. However, the reactions occur in the furnace are not able to eliminate carbon dioxide sufficiently, a further step is required.

Figure 6

CO₂ conversion in AG2S^TM technology (straight line) and Claus process (dotted line) in function of carbon dioxide inlet composition and reactor length

Previous data have demonstrated the functioning of AG2S^TM in a specific range of applicability. In this section the first basic economic evaluation is performed.

Chemical plants are, of course, built to create value, that is – in a medium-term – make profits; thus, the estimates of the investment required (CAPEX, Capital Expenditure) and of the annual Cash Flows (CF, embedding the Operational Expenditure, OPEX) are needed so that the profitability of any initiative can be assessed. The items to be estimated so as to sum up to the cash flow are the variable costs of production, the fixed costs of production (overall, OPEX) and the revenues from sales, including both main products and by-products considering green-field scenarios (Table 3).

In this section, the economic assessment was carried out referring to Chemical Engineering Design [28]. The cost of the plant was established for the CAPEX (which includes also Inside Battery Limits (ISBL), cost of the plants itself, Offsite Battery Limits (OSBL), cost of modification and improvements to be made to the site infrastructure, engineering and construction costs and, working capital and contingency charges); as for the revenues’ and costs’ estimation, the following parameters have been included: the prices of raw materials such as hydrogen, carbon monoxide; carbon dioxide taxation at 20%; utilities’ costs and exploited production capacity.

A study on greenfield case is made in order to have an early stage of the project design. The results show that there is an increase in the revenues greater than the one of investment costs. Even though a lot less impacting on the global level, aiming to reduce CO₂ emissions which come from refineries, the development of the technology as an alternative to the Claus process needs to be considered.

In this section, the estimation of the CAPEX, the revenues and the variable and fixed costs of production are presented. In fact, in this case, the designed process section needs to be built from scratch, so the case needs to be managed as a project on its own. All costs related to the construction of a whole new plant are scaled on the single section and so a project profitability study can be carried [29].

The added expenditures to be considered are show in Table 4. These are fixed costs of production, which means independent of the fact that the plant is producing and from the production volumes themselves.

Considering that the plant needs to be started for the first time, the Working Capital contribution needs to be accounted for. It is assumed to be 5% of ISBL+OSBL and of course, it is added to the CAPEX since it is not an annual cost.

In this analysis, the cost of the furnace and waste heat boiler are taken in account and added to the previously estimated ISBL. The geometry of the thermal furnace, the waste heat boiler and catalytic bed is maintained unchanged.

For the catalyst characteristics, previous work of [30] has been followed, assimilating their catalyst features (bed density and volume of catalyst per volume of reactor) as the standard ones for the Claus process. All calculations made for the case study are reported in Table 4 together with details of CAPEX contributions.

The total ISBL results to be 3 750 000 UDS with 1 - 150 000 USD to be accounted for depreciation. This cost appears to be very low and so the greenfield case very promising. All costs and revenues for the study are resumed in Table 5.

The CO₂ abatement, the syngas production and the exploitation of this promising technology in a greenfield case were considered. The greenfield results allow to certainly state that investment was worth taking.