DOI: 10.3303/CET25117145 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 17 December 2024; Revised: 15 March 2025; Accepted: 17 May 2025 
Please cite this article as: Silva C.C.G., Silva L.H.S., Guirardello R., 2025, Synthesis of Global Reaction Routes Using Thermodynamic Evaluation, 
Chemical Engineering Transactions, 117, 865-870  DOI:10.3303/CET25117145 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 117, 2025 

A publication of 

 

The Italian Association 

of Chemical Engineering 

Online at www.cetjournal.it 

Guest Editors: Fabrizio Bezzo, Flavio Manenti, Gabriele Pannocchia, Almerinda di Benedetto 

Copyright © 2025, AIDIC Servizi S.r.l. 

ISBN 979-12-81206-17-5; ISSN 2283-9216 

Synthesis of Global Reaction Routes using Thermodynamic 

Evaluation 

Consuelo C.G. Silvaa , Luiz Henrique S. Silvab, Reginaldo Guirardelloc 

a,b Federal University of Recôncavo Bahia – UFRB – Brazil  
c School of Chemical Engineering – Unicamp, Av. Albert Einstein 500, Campinas–SP, 13083-852, Brazil 

guira@feq.unicamp.br 

The global demand for sustainable, energy-efficient chemical processes has accelerated innovations in 

methane (CH4) utilization. Methane, a versatile and abundant resource, offers dual benefits: meeting energy 

demands while reducing environmental impacts through advanced conversion methods. However, existing 

approaches often face the challenge of being endothermic, consuming substantial energy and limiting industrial 

feasibility. This study introduces as a goal an exothermic process design to optimize energy use, while achieving 

sustainability goals such as avoiding the emission of carbon dioxide. Through thermodynamic pathway 

evaluations, one case study, the production of urea, was identified as a viable product of CH4 conversion, 

alongside with hydrogen generation, in an overall energetically favourable process. Rigorous simulations 

analyze the process feasibility, emphasizing carbon emission reductions and resource efficiency. By integrating 

advanced modelling tools, this work aligns with global efforts to develop green chemical technologies, 

highlighting its role in industrial innovation and environmental responsibility. 

1. Introduction 

Many processes aiming to use CH4 without emitting CO2 have the disadvantage of being endothermic, that is, 

they consume energy, so the motivation for this work is from the very beginning the process to use reagents 

and products in such a way that the overall process is exothermal or autothermal, so that it does not consume 

energy. The thermodynamic evaluation of a global reaction route is performed in order to determine whether 

the overall reaction is thermodynamically favourable or not. This step is important, because it predicts whether 

the process will consume energy or not. From the formation enthalpy, the enthalpy variation ΔH of the global 

reaction is calculated. In addition, ΔG provides important information regarding the spontaneity of the process 

or individual steps. If ΔG is negative the reaction route is thermodynamically favourable.  Therefore, the 

thermodynamic evaluation of a global reaction route involves the evaluation of the formation enthalpy of 

reagents and products, the determination of variations of enthalpy and Gibbs energy, and the interpretation of 

the results to determine whether the reaction route is thermodynamically favourable or not. 

The thermodynamics of a global reaction route can also be evaluated by considering the variation of Gibbs 

energy ΔG. The Gibbs energy is the available energy to perform work in a system and is related to variations of 

enthalpy and entropy. If ΔG is negative, it means that the reaction is spontaneous and the reaction route is 

thermodynamically favourable. If ΔG is positive, it means that the reaction is not spontaneous and the reaction 

route is thermodynamically unfavourable. As a case study, the proposed approach was applied in a 

thermodynamic analysis for the production of urea, which is exothermic, while avoiding the production of CO2. 

For thermodynamic analysis, it is considered how many reaction routes are possible for the same reagent and 

the same product. The data obtained was handled in graphical and numerical computational simulations with 

the help of the software Aspen, which is a process modelling tool used for process monitoring, optimization and 

simulation. Also, this particular case study (urea production) uses the CO2 generated in one reaction step to 

produce the final product, thus preventing carbon dioxide from being released into the atmosphere. Finally, 

hydrogen is also produced as a by-product in this particular process. 

 

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2. Methodology - Linear Programming Model 

This is a synthesis strategy, since the process is not known in advance, not even the products and reactants, 

except methane as raw material, hydrogen as a by-product and carbon dioxide is not desired as a product 

(maybe only as an intermediate). Also, it is desirable that the global process be exothermic. 

The first step is the definition of the products and the reactants. They are not specified in advance, they will be 

determined from a list of possible substances that may exist or not. These substances have their enthalpies 

evaluated at the conditions of the feed and exit of the process (reference state), to calculate the total enthalpy 

variation, so that the whole process is exothermic, to avoid energy consumption. This problem can then be 

formulated as a linear programming model, whose resolution will define the overall process, and is given by:

max 
𝑛𝑖

 𝑛desired product  (1) 

 

 𝑎𝑚𝑖 ∙ 𝑛𝑖
products

=

𝑁𝑝

𝑖=1

 𝑎𝑚𝑖 ∙ 𝑛𝑖
reactants

𝑁𝑟

𝑖=1

                (atomic balance) (2) 

 

 −∆𝐻𝑅
global

 ≥ given value ≥ 0 (3) 

 

𝑛𝑖 ≥ 0 (4) 

 

where: 

∆𝐻𝑅
global 𝑇0,𝑃0 =  𝑛𝑖

products
∙ ℎ 𝑖(𝑇0,𝑃0)

𝑁𝑝

𝑖=1

−  𝑛𝑖
reactants ∙ ℎ 𝑖(𝑇0,𝑃0)

𝑁𝑟

𝑖=1

 

 

(5) 

 

and where the decision variables are 𝑛𝑖: if 𝑛𝑖 > 0 the substance exists in the global mass balance, while if 𝑛𝑖 =

0 the substance is not present in the global balance (it may only exist as an intermediate). One critical step is 

the definition of the set 𝑖 ∈ {list of possible substances}. 

Once the raw materials and products have been determined, a viable reaction route must be found to obtain the 

desired products from the raw materials. This step involves an analysis of the Gibbs energy of each intermediate 

reaction involved, so that each reaction has a favourable equilibrium at the temperature, pressure and 

composition of each reaction step in real operating conditions (since intermediate steps may use different 

temperatures and pressures than the reference conditions). This stage of the project can then be carried out 

using a commercial software, such as Aspen, to simulate the entire process with the routes considered. 

3. Case Study - Global Route 

This is just an illustrative example, since the proposed methodology is general. The following conditions were 

considered: 

- potential raw materials: CH4, O2, N2, H2O 

- potential products: (NH2)2CO (urea), C2H4 (ethylene), C6H6 (benzene), H2O, H2 

Note that in this step water can be either a raw material or a product, it is not yet known. 

- calculation basis: 100 mol CH4 

- reference conditions for the beginning and end of the overall process: 25 ºC and 1 atm. 

The enthalpy of formation data were obtained from the thermodynamics literature (Reid et al, 1987). The 

calculations were performed with an Excel spreadsheet, using Solver to find the solution of the LP model. 

Using this small list of substances, the substance that leads to more hydrogen is benzene (burning part of the 

hydrogen for ∆𝐻global > 0), but the difference with urea is small. Also, urea is useful as a fertilizer. Therefore, 

this case study was set to the production of urea. The raw materials were CH4, N2 and H2O, while the products 

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were (NH2)2CO and H2. However, this step does not provide the reaction routes from the reactants to the final 

products. Therefore, the next step is finding an adequate route, which in this case is well known (Figure 1): 

Step1: CH4 + 2 H2O  CO2 + 4 H2 (6) 

Step2: N2 + 3 H2 2 NH3 (7) 

Step3: CO2 + 2 NH3  CO(NH2)2 + H2O (8) 

global: CH4 + H2O + N2  CO(NH2)2 + H2 (9) 

 

Font: Developed by the author 

Figure 1 : Process Flow Diagram 

Then, after choosing a route, this study evaluates the reaction routes' thermodynamic and energetic feasibility 

using Aspen Plus simulations and thermodynamic analyses to optimize operating conditions and maximize 

efficiency. Simulations modelled reaction conditions and calculated mass-energy balances. Thermodynamic 

analyses determined key quantities (ΔH, ΔS, ΔG) to assess reaction spontaneity and favourable conditions. 

Integrating these results provides an approach for developing efficient, sustainable processes. 

4. Case Studies - Routes of Reactions 

4.1. Methane and Oxygen Reaction 

This case study analyzes the chemical reaction between methane (CH4) and oxygen (O2), with the aim of 

evaluating the products formed, the energy involved and the associated flows. The analysis was carried out 

using Aspen Plus software. The reaction between methane and oxygen can take place via different chemical 

routes, depending on the conditions of temperature, pressure and the ratio between the reactants. These 

conditions were chosen to evaluate the behaviour of the different chemical routes, providing a basis for future 

adjustments. The main reactions include: 

1. Complete Combustion (highly exothermic): 

CH4 + 2 O2 → CO2 + 2 H2O (10) 

2. Incomplete Combustion (exothermic): 

CH4 + 3/2 O2 → CO + 2 H2O (11) 

3. Partial Oxidation Reaction (slightly exothermic): 

CH4 + 1/2 O2 → CO + 2 H2 (12) 

4. Solid Carbon Formation (if oxygen is extremely limited, endothermic):  

CH4 → C + 2 H2 (13) 

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The products depend on the prevailing thermodynamic conditions and chemical routes, which are: carbon 

dioxide (CO2), water (H2O), carbon monoxide (CO), hydrogen (H2), solid carbon (C). For preliminary analysis, it 

was considered at the feed: molar ratio 1 CH4 : 2 O2, T = 25oC, P = 1 atm. 

4.2. Methane and Water Reaction 

This case study analyzes the interaction between methane (CH4) and water (H2O), focusing on the 

thermodynamic analysis and feasibility of the process. The simulation, carried out in the Aspen Plus software, 

investigates the energy flows, the molar and volumetric fractions of the products, as well as the overall efficiency 

of the reactions, allowing a detailed assessment of the ideal conditions for industrial applications. The reaction 

between CH4 and H2O follows various chemical routes, which are widely used in industrial processes, such as 

the production of synthesis gas (syngas) and hydrogen. The products vary according to the predominant 

chemical route and thermodynamic conditions. The main reactions include: 

1. Methane Steam Reform (endothermic): 

CH4 + H2O → CO + 3 H2 (14) 

This is the main reaction in industrial processes for synthesis gas production (mixture of CO and H2). 

2. Gas-water shift reaction (slightly exothermic): 

CO + H2O → CO2 + H2 (15) 

3. Solid Carbon Formation (exothermic): 

2 CO → C + CO2 (16) 

The products depend on the prevailing thermodynamic conditions and chemical routes, which are: hydrogen 

(H2), carbon monoxide (CO), carbon dioxide (CO2), water (H2O) and solid carbon (C). For preliminary analysis, 

it was considered at the feed: molar fractions yCH4 = 0.5289 and yH2O = 0.4710, T = 25oC, P = 1 atm. 

4.3. Converting Intermediate Products into Value Added Products 

Using the results of the previous case studies, which demonstrate the thermodynamic feasibility of the reactions 

between methane and oxygen and methane and water, it is proposed a process to use these reactions in the 

production of more valued products, such as urea. This is based on the proposed approach of focusing on the 

synthesis of high value-added chemical compounds, integrating sustainable technologies and optimized 

processes. In the case of urea, it can be obtained from the reaction between ammonia (NH3) and carbon dioxide 

(CO2), by developing and evaluating thermodynamically viable and economically attractive routes. For example, 

integrating with methane reforming, using the hydrogen (H2) produced in steam reforming as raw material for 

the synthesis of ammonia (NH3) and evaluating the energy efficiency of the ammonia synthesis process 

considering the use of nitrogen from the air. For the conversion of CO2 and NH3 into urea, the CO2 captured in 

the combustion and the reforming can be used to produce urea (NH2CONH2), a product used as a fertilizer and 

industrial additive. The final process will be found by carrying out simulations to determine the optimal conditions 

(temperature, pressure and molar ratios) to maximize the urea yield. 

5. Numerical Results 

The thermodynamic analysis of global reaction routes plays a fundamental role in the development of 

sustainable processes for utilizing methane (CH4) without emitting carbon dioxide (CO2). This study is especially 

relevant to overcome the limitation of many endothermic processes, which consume energy and reduce 

economic and environmental viability. This work focused on evaluating different reaction routes for the 

production of urea, as a case study, avoiding the emission of CO2, with a focus on identifying a global exothermic 

process. Thermodynamics was considered by calculating the enthalpy variations (ΔH) and Gibbs energy (ΔG) 

of the reactions. These analyses made it possible to predict the spontaneity and thermodynamic feasibility of 

the processes studied. The initial conditions for the reactants and the final conditions for the products in the 

global process were considered at the reference state of 25°C and 1 atm. 

In order to achieve the proposed objectives, the Aspen software was used to carry out graphical and numerical 

simulations, using a conversion reactor. The input data, which included CH4, O2, and specific temperature and 

pressure conditions, was parameterized to evaluate the thermodynamic behaviour of different combinations of 

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reactants and products. The simulations carried out were fundamental in determining global enthalpy variations 

and identifying potentially favourable reaction routes. The work also considered the need for the reactions to be 

sufficiently exothermic to compensate for losses in the process, ensuring greater efficiency and sustainability. 

The results presented below provide an analysis of the routes evaluated, taking into account the thermodynamic 

criteria established and the practical feasibility of implementation of the proposed strategy. 

Table 1: Case Study 1 – CH4 + O2 

Compound Molar Ratio 

CH4 1 

O2 2 

Table 2: Case Study 1 – Flows 

Energy Molar Flow Mass Flow Enthalpy 

-4646.14kW 62.35 mol/s 1kg/s -15.50 kJ/kmol 

 

As shown in Tables 1 and 2, the negative variation in enthalpy indicates that the process is exothermic, which 

is in agreement that the simulated reaction route meets the objective of compensating for energy losses, making 

the process thermodynamically favourable in terms of energy released. Also, all the reaction products are in the 

gas phase at the end of the process. 

The increase in molar flux suggests a reaction with efficient conversion of reactants into products, since for an 

initial mole of CH4 and 2 moles of O2, an increase in molar flow indicates the formation of several products per 

mole of reactant, such as CO2 and H2O. On the other hand, a constant mass flow (not molar flow) just reflects 

the conservation of mass in the process. The conversion of CH4 and O2 into products maintains the same initial 

mass flow, with the difference concentrated in the composition of the products. 

The negative enthalpy reinforces the exothermic nature of the reaction. This value reflects the release of heat 

in the process, contributing to the goal of minimizing the need for external energy. 

Table 3: Case Study 2 – CH4 + H2O 

Compound Molar Faction Input 

CH4 0.5289 

H2O 0.4710 

Table 4: Case Study 2 – Flows 

Energy Molar Flow Mass Flow Enthalpy 

-20699kW 117.84 mol/s 2kg/s 22332.9 kJ/kmol 

As shown in Tables 3 and 4, this process is endothermic, which suggests that the chemical reactions between 

CH4 and H2O involve the formation of energetic by-products. Possible answers include: 

Steam reforming (endothermic): 

CH4 + H2O → CO + 3 H2 (17) 

The reforming event is endothermic, but the overall process can be exothermic due to other interactions. 

Gas-water displacement reaction (exothermic): 

CO + H2O → CO2 + H2 (18) 

The following observations can also be made, at the end of the overall process: 

Product Phase 

● Approximately half of the products are in the vapour phase, while the rest remain in the liquid phase. 

● The presence of the liquid phase can be attributed to excess (unconverted) water or to the partial 

condensation of water vapour under reference conditions (25°C, 1atm) at the end of the process. 

● Components in the vapour phase predominantly include H2, CO, and possibly residual CH4. In the 

liquid phase, water dominates. 

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Physical flows 

● The molar amount of products is significantly greater than the input of reactants, suggesting the 

formation of multiple molecules of gaseous products, such as H2. 

● Volumetric expansion is associated with the production of gases, such as H2, which occupy a larger 

volume than liquids. 

Enthalpy variation 

● A negative value indicates an exothermic system. This is in line with the energy released in secondary 

reactions, such as the gas-water displacement reaction. 

The results are in line with advanced studies on methane reforming and CO2 capture. Aasberg-Petersen et al. 

(2003) highlighted that autothermal reforming improves energy efficiency. Adhikari et al. (2007) demonstrated 

that the combination of reformers and displacement reactions improves the conversion of CO to H2. 

6. Conclusions 

This study contributes with optimized strategies for urea and H2 production from methane, as a case study, 

while avoiding CO2 release. Based on a list of possible products and raw materials, with the goal of a global 

process that is exothermic and produces a valuable product and hydrogen, without releasing carbon dioxide, 

the product chosen was urea and hydrogen, using methane, water and air (N2 and O2) as reactants. The 

preliminary results indicated the viability of the proposal. However, this global route requires some reactions to 

achieve the main goal, so some of these individual reactions of the global route were studied in this work. 

Analysis of the CH4 + O2 route indicated that it is exothermic and produces carbon dioxide, which can be used 

later in the production of urea. Analysis of the CH4 + H2O route highlights a thermodynamically promising 

process for generating useful gases such as H2 and CO, with strategic potential for meeting the demand for 

chemical and energy inputs in the transition to more sustainable technologies. The coexistence of liquid and 

steam phases at the end of the process reflects the presence of unconverted residual water, opening up the 

opportunity to optimize the process through precise control of operating variables such as temperature and 

pressure, as well as the introduction of more selective catalysts. In addition, partial conversion observed suggest 

that fine adjustments to the reaction design can significantly increase the efficiency of the process. The 

integration of optimized reforming and water-gas shift steps can increase H2 production, consolidating the 

system as an attractive solution for generating clean energy and high value-added chemical inputs. In this way, 

this study provides a solid basis for the development of promising technologies in the use of CH4, with direct 

implications for environmental sustainability and the advancement of a low-carbon economy. 

Other steps of reactions to produce urea have to be evaluated, such as the production of NH3 and the reaction 

of CO2 and NH3, using real process conditions (temperature and pressure at operating conditions), simulated 

with a software such as Aspen. It will be a part of this work as future research, as well as the scale-up and 

technical economic analysis of the process. 

References 

Aasberg-Petersen K., Christensen T.S., Nielsen C.S., Dybkjaer I., 2003, Recent developments in autothermal 

reforming and pre-reforming for synthesis gas production in GTL applications. Fuel Processing Technology, 

Amsterdam, v. 83, n. 1- 3, p. 253- 261. 

Adhikiri S., Fernado S., Gwaltney S., 2007, A thermodynamic analysis of hydrogen production by steam 

reforming of glycerol. International Journal of Hydrogen Energy, p. 2875 – 2880. 

BOROS L.A.D., Mathematical and Thermodynamic Modeling of Solid-Liquid Equilibrium of Fatty Systems, 2005, 

Master's Thesis in Chemical Engineering – Unicamp – Brazil.  

Chaubey R., Sahu S., James O.O., Maity S., 2013, A review on development of industrial processes and 

emerging techniques for production of hydrogen from renew able and sustainable sources. Renewable and 

Sustainable Energy Review s, Oxford, v. 23, p. 443-462. 

Edgar T.F., Himmelblau D.M., Lasdon L.S., 2001, Optimization of Chemical Processes, McGraw-Hill, 2nd ed., 

New York. 

Freitas A.C.D, Guirardello R., 2014, Thermodynamic analysis of methane reforming with CO2, CO2 + H2O, CO2 

+ O2 and CO2 + air for hydrogen and synthesis gas production. Journal of CO2 utilization, p 30-38. 

Reid R.C., Prausnitz J.M., Poling B.E., 1987, The Properties of Gases & Liquids, 4th Edition, McGraw-Hill, New 

York. 

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