Abstract
In the present work, the plug flow reactor of a urea production plant from ammonia produced from natural gas of the Camisea fields (Peru) was designed, comparatively analyzing three commercial processes currently in operation: Stamicarbon, Snamprogetti, and Toyo. ASPEN PLUS software was used as a simulation tool. To carry out the simulation and subsequent comparison of the three urea production patents, it will start from the same feed of ammonia (37,851 kmol/day) and carbon dioxide (18,989 kmol/day) for the three cases. For the design of urea synthesis reactors, reactors of the same size (2.3 m diameter and 29 m length) were designed. The results are graphs that illustrate the variation of the length of the reactor with the reaction temperature, the variation of the length of the reactor with the residence time, and the variation of the conversion with the time for each patent.
1 Introduction
Urea is not only the most produced and used chemical fertilizer in the world but also a multipurpose raw material [1,2]. Since the 1930s and 1940s, urea production has been a major topic among industry researchers [3]. Faced with an increasingly severe energy crisis and a deteriorating environment, researchers continue to analyze their production process in order to reduce energy consumption and obtain cleaner production; that is why process simulation is the primary computer technology for the analysis of process design decisions and their optimization [4]. In addition, the energy demands and environmental challenges for its processing are high, which is why the need for optimization and energy conservation has increased interest in the simulation of plants that produce the said product [5]. More research is currently required in order to solve the industrial challenges that make it difficult to select a more sustainable process design. In previous works, the simulation of urea processes has been developed. However, a limited number of them have been implemented, because the parameters of their quality have not been considered, with the severe operating conditions of the process and the lack of information being the main challenges in the modeling and simulation of such a complex process [6]. Urea production patents (Stamicarbon, Snamprogetti, and Toyo) differ on the separation and recycling of ammonia and carbon dioxide. The optimization of these production technologies aims to maximize the conversion of carbon dioxide and the optimization of heat recovery [7].
Zhang and Yao studied the fluid dynamics of a urea reactor considering a gas–liquid fluid reactor model, in which perforated plates are installed to avoid counter mixing, defining a thermodynamic model, reaction kinetics, and dynamics of fluids and solving the urea reactor model defined by a series of non-linear equations by a numerical method, the solution of which illustrates the whole temperature profile and the concentrations of each component within the reactor. The results of the model were in good agreement with the design data [8].
Rasheed simulated the urea reactor by applying the SR-POLAR equation for the thermodynamic model and proposed a kinetic power model for the formation of ammonium carbamate and urea. The deviations from the industrial data were recorded to be less than 5.0% for the liquid composition at the outlet of the reactor [9].
Zendehboudi et al. proposed a mathematical model for a urea reactor based on the UNIQUAC model; when comparing the results of the model with the industrial data, a standard deviation of less than 2.3% was obtained for the liquid outlet stream [10].
Nawaz evaluated different processes to produce ammonium sulfate and urea with a ratio of 60/40, respectively, from ammonia, whose feed was 550,000 ton/year. It is concluded that the most profitable route to comply with the urea plant design from carbon dioxide was using the Snamprogetti process and ammonium sulfate from sulfuric acid. The mechanical and chemical design of the urea reactor was proposed, determining the following variables: a volume of the pseudo-first-order isothermal plug flow reactor (PFR) of 145.5 m3, a diameter of 2.61 m, a reactor thickness of 100 mm, and a residence time of 16.71 s. A “half-pipe”-type cooling jacket was used to maintain the isothermal conditions of the reactor [11].
Baboo developed a kinetic model for the synthesis of urea from ammonia and carbon dioxide, in which he demonstrated the possibility of increasing the efficiency of the reactor. The optimization of the carbon dioxide conversion is determined by the reduction of energy consumption and recycling and by the increase in the production using the same reactor. The design of the high-efficiency trays in the reactor was developed through the fluid dynamics simulation and the modeling of the physical–chemical equilibria and the heat transfer phenomena, the main one being the reduction of the specific consumption of steam, obtaining reductions of the same amount of 250–300 kg/ton urea and an increase in the capacity of the reactor of 10–20% [12].
Sikder et al. designed a simulation of the Stamicarbon urea production process using the Aspen HYSYS v.7.1 platform to investigate the effect of operating parameters: carbon dioxide temperature, high-pressure (HP) steam temperature, and low pressure (LP) steam, depending on the composition of urea; the following results were obtained: at a range of 357–365°C of HP steam temperature, the 0.055–0.08 urea composition was obtained; at a range of 287–316°C of LP vapor temperature (low pressure), the 0.055–0.782 urea composition was obtained. The heat generated in the urea solution decreased with increasing CO2 temperature and increased with increasing CO2 pressure. The CO2 conversion in the reactor was constant at pressures greater than 210 atm, which was also determined at an NH3/CO2 ratio of 15 [13].
Mane et al. studied the conversion and optimization of the urea reactor by analyzing process parameters such as NH3/CO2 ratio, pressure, and temperature and concluded that these factors intervene in increasing its production by reducing the specific volume of the reactor. The number of trays in the reactor was increased; however, its efficiency was not increased. By modifying the NH3/CO2 feed ratio of the reactor feed, the efficiency of the reactor was increased. From the experimental data, it was concluded that by varying the value of the NH3/CO2 ratio between 3/1 and 3/4, the maximum value of the urea reactor efficiency was obtained [14].
Yoke and Mahadzir proposed mathematical models for the synthesis of urea and ammonia, determining that, for the urea synthesis reaction at optimal operating conditions of 450 K and 12 MPa, the equilibrium carbon dioxide conversion, the maximum molar flux of urea, and its equilibrium concentration were 79%, 410 mol/s, and 750 mol/m3, respectively. The optimal range of the NH3/CO2 feed ratio was 2–3.5. For the ammonia synthesis reaction at optimal operating conditions of 660 K and 23 MPa, the equilibrium nitrogen conversion, the ammonia molar flux, and its equilibrium concentration were 27.5%, 420 mol/s, and 300 mol/m3, respectively. The mean percentage error for the equilibrium nitrogen conversion and the equilibrium ammonia molar flux were 6.154 and 5.932%, respectively, compared to the published data at industrial plant conditions [15].
In this research work, the design and characterization of the PFR of a urea production plant from ammonia produced from natural gas from the Camisea fields (Peru) was proposed, which was defined with the same dimensions for each urea production patent (Stamicarbon, Snamprogetti, and Toyo), carrying out the analysis of residence time, length reactor, temperature, and conversion. The results are graphs that illustrate the variation of the length of the reactor with temperature, the variation of the length of the reactor with the residence time, and the variation of the conversion with the residence time for each patent.
2 Materials and methods
2.1 Thermodynamic package
The modeling of the thermodynamic properties of the ammonia–carbon dioxide–water–ammonium carbamate–nitrogen–oxygen system is based on the SR-POLAR model [16] within ASPEN PLUS, which uses an equation of state suitable for high pressure and temperature conditions of urea synthesis.
The equation for the model is
The temperature dependence of α is given by the following expression, which is a generalization of the correlation already proposed by Mathias [17]
With
Parameters p 1, p 2, and p 3 can be either fitted to experimental vapor pressures or derived from the Antoine parameters of the pure components. They are set to zero for nonpolar molecules.
Mixing rule:
It is suggested to use a linear mixing rule for b and a “pseudo-quadratic” mixing rule for a:
where
Schwartzentruber and Renon proposed the following equation (8) to calculate
where
2.2 Chemical kinetics of reactions
The two main reactions take place in the urea synthesis process:
The first reaction, which takes place in the liquid phase, converts ammonia and carbon dioxide into ammonium carbamate and is very exothermic and fast. The chemical equilibrium is obtained at the operating conditions of the reactor. The second reaction also takes place in the liquid phase and is endothermic; its speed is slow, and equilibrium is not normally reached in the reactor.
Equilibrium has been described in terms of fugacity coefficients since an equation of state is used as a thermodynamic model. The equilibrium constant for Reaction (1), in terms of mole fractions, is written as follows:
The equilibrium constant for Reaction 1 in terms of mole fractions is as follows:
Similarly, equilibrium equations are developed for Reaction 2.
Reaction rates 1 and 2, in units of kmol/s m3, are as follows:
The two velocity equations have been formulated so that they reach equilibrium at long residence times. The rate constant for Reaction (1) (k 1) is set to a large value so that it is in equilibrium. The rate constant for Reaction (2) determines the conversion of urea in the reactor. Only scant information is available to determine k 2 and it is generally advisable to adjust its value to fit the plant data. An approximation for k 2 is the following:
3 Results and discussion
Simulations were carried out from the same feed of ammonia (37,851 kmol/day) from natural gas from the Camisea fields and carbon dioxide (18,989 kmol/day) for each patent (Stamicarbon, Snamprogetti, and Toyo), followed by a subsequent comparison of the three urea production technologies. For the design of urea synthesis reactors, PFRs of the same size (because of having the same feed and because the same chemical reactions involved in the entire process) but with different operating conditions (defined by each patent), were selected.
The dimensions of the reactor were calculated, as shown in Table 1.
Urea synthesis reactor dimensions
| Diameter (m) | Length (m) |
|---|---|
| 2.3 | 29 |
Nawaz, from an ammonia feed of 250,000 ton/year, obtained a PFR volume of 145.5 m3 and a diameter of 2.61 m (corresponding to a length of 27.2 m), with very similar results to those obtained in the present study [11].
The dimensions of the reactors shown in Table 1 will be the same for each patent, each one will be analyzed below.
3.1 Analysis of the reactor temperature with respect to its length
Figure 1 shows the variation of the reactor temperature with respect to its length in the Stamicarbon, Snamprogetti, and Toyo patents.
Reactor length (m) vs temperature (°C).
Table 2 illustrates that in all the patents for obtaining urea, the reactors operate at the same temperatures. In addition, in Figure 1, as the operating pressure of the reactor increases, the length section corresponds to a linear function (which is seen in greater detail in the Toyo patent). In addition, a higher reaction temperature of 190°C is obtained for the Snamprogetti patent, which implies higher energy requirements on its part; on the other hand, almost similar results are obtained for the Stamicarbon and Toyo patents, which are 183 and 182°C respectively.
Operating conditions of the reactors of each patent
| Patent | Temperature | Pressure |
|---|---|---|
| (°C) | (atm) | |
| Urea reactor Stamicarbon | 166 | 136.46 |
| Urea reactor Snamprogetti | 166 | 170.11 |
| Urea reactor Toyo | 167 | 238.16 |
Table 3 illustrates the maximum reaction temperatures of each patent.
Maximum reaction temperature of each patent
| Patent | Temperature reaction |
|---|---|
| (°C) | |
| Urea reactor Stamicarbon | 183 |
| Urea reactor Snamprogetti | 190 |
| Urea reactor Toyo | 182 |
3.2 Analysis of the reaction residence time with respect to the length of the reactor
Figure 2 shows the progression of reaction residence time with respect to the reactor length in the Stamicarbon, Snamprogetti, and Toyo patents.
Residence time (h) vs reactor length (m).
Table 2 shows that in all the patents for obtaining urea, the reactors work at the same pressures.
In Figure 2, it is determined that the higher the operating pressure, the lower the inflection point of the graph (which corresponds to a lower volume range of the reactor); in turn, it will generate a longer residence time.
Table 4 illustrates the residence times of the reactors for each patent.
Residence time of the reactor of each patent
| Patent | Residence time (h) |
|---|---|
| Urea reactor Stamicarbon | 0.42 |
| Urea reactor Snamprogetti | 0.72 |
| Urea reactor Toyo | 1.37 |
3.3 Comparison of patents
Peruvian State has an average demand for urea of 400 kton/year, which is why, for this purpose, it was decided to evaluate each patent to obtain the said production.
It was determined that the feeds of ammonia and carbon dioxide required for this purpose are 235.45 and 305.24 kton/year, respectively, which correspond to the feed flows of the patent with the highest conversion, which is Toyo; this is shown in Figure 3.
CO2 conversion vs time (h).
Table 5 shows a summary of the yields of granulated (solid) urea from ammonia obtained from natural gas.
Summary of urea production of the three patents
| Feed (kton/year) | Product (kton/year) | |||
|---|---|---|---|---|
| Toyo | Stamicarbon | Snamprogetti | ||
| Ammonia | 235.45 | — | — | — |
| Carbon dioxide | 305.24 | — | — | — |
| Solid urea | — | 414.95 | 387.29 | 276.63 |
From Table 5, it can be deduced that the Toyo patent obtains the highest production of urea (414.95 kton/year) from the same supply of ammonia and carbon dioxide; this is due to the fact that the reactor operates at a higher pressure (which is illustrated in Table 2), obtaining a higher conversion (60%), which is illustrated in Figure 3.
When comparing the reactors for the three patents for the production of urea previously mentioned, Figure 3 is obtained.
Figure 3 shows that when comparing the Toyo patent at the same reactor residence time, for example, at 0.42 h (which is the residence time of the Stamicarbon patent, which is the minimum with respect to the three patents), it has the highest CO2 conversion (60%) compared to the Snamprogetti and Stamicarbon patents. This difference in conversions is related to the difference in energy requirement downstream of the respective reactors. A higher conversion of CO2 determines a lower energy consumption in the decomposition process of the non-reactive material.
Binti [18] and Ali and Anantharaman [19] obtained that the conversion of CO2 in the urea reactor was 60%; the conversion obtained by Wang and Li was 58.99% [20]; similar results were obtained in the present research work. The aforementioned authors compared their results with real plant data, obtaining an excellent fit (error ± 5%). In the present study, the SR–POLAR thermodynamic model was used because several authors such as Zahid et al. [5] and Rasheed [9] recommended it for its great adjustment to the thermodynamic system of the present study; however, Ali and Anantharaman used the SRK thermodynamic model, obtaining the same result [19], which shows that this model is also applicable. This result is also the same as predicted by the licensor of the Toyo patent. Yoke Yi and Mahadzir obtained a conversion of carbon dioxide in an equilibrium of 79% [15]; it is very likely that the error they made in their calculations is in the selection of the thermodynamic model, because they do not specify it in their research.
4 Conclusion
The present work developed the design of a reactor for a 400 kton/year urea production, showing that the Toyo patent is the one that obtains the highest conversion (60%), which translates into a lower energy requirement. This result will be obtained independently of the supply of ammonia and carbon dioxide, because the sequence of operations and unit processes of each patent will always be the same for each simulation that is carried out.
Nomenclature
- a
-
attraction parameter (Pa m6/mol2)
- a ij
-
calculation parameter to determine a mix (Pa m6/mol2)
- a mix
-
mix attraction parameter (Pa m6/mol2)
- b
-
repulsion parameter (m3/mol)
- b mix
-
mix repulsion parameter (m3/mol)
- c,d
-
pure-component parameters (for supercritical temperatures)
-
-
Gibbs free energy of the ideal gas of component i for T, P° (J/mol)
- K ij
-
calculation parameter to determine a ij
-
-
parameters of the mixing rule
-
-
parameters for the temperature dependence of
-
-
parameters for the temperature dependence of
-
-
parameters for the temperature dependence of
-
-
acentric factor-dependent function
- p 1,p 2,p 3
-
pure-component parameters in the equation of state
- P
-
pressure (Pa)
- P 0
-
reference pressure (1.01325 × 105 Pa)
- P c
-
critical system pressure (Pa)
- R
-
ideal gas constant (8.3144 J/K mol)
- T
-
absolute temperature (K)
- T c
-
critical system temperatura (K)
- T r
-
pseudo-reduced temperature
-
-
molar volume (m3/mol)
-
-
acentric factor
-
-
mole fraction of component i in the mixture
-
-
mole fraction vector
-
-
correction factor dependent on reduced temperature
-
-
fugacity coefficient of component i for T, P, x i
Acknowledgments
The results presented are part of the doctoral research project sponsored by the National Fund for Scientific, Technological Development and Technological Innovation (FONDECYT), nowadays PROCIENCIA, one of the ten doctoral programs subsidized by PROCIENCIA in Peru – Contract 04-2018-FONDECYT/BM. The authors, therefore, acknowledge with thanks DSR for technical and financial support.
-
Funding information: The results presented are part of the doctoral research project sponsored by the National Fund for Scientific, Technological Development and Technological Innovation (FONDECYT), nowadays PROCIENCIA, one of the ten doctoral programs subsidized by PROCIENCIA in Peru – Contract 04-2018-FONDECYT/BM. The authors, therefore, acknowledge with thanks DSR for technical and financial support.
-
Author contributions: Rommel Ortiz Guzmán – Conceptualization, data curation, formal analysis, funding acquisition, investigation, resources, visualization, writing-original draft; Antonio Bueno Lazo – Methodology, project administration, software, supervision, validation, writing-review and editing.
-
Conflict of interest: The authors declare no conflict of interest.
-
Ethical approval: The conducted research is not related to either human or animal use.
-
Data availability statement: All data generated or analysed during this study are included in this published article.
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© 2022 Rommel Ortiz Guzmán and Antonio Bueno Lazo, published by De Gruyter
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