Conceptual Design and Process Feasibility Analysis of a Novel

Research Article pubs.acs.org/journal/ascecg

Conceptual Design and Process Feasibility Analysis of a Novel Ammonia Synthesis Process by Efficient Heat Integration Chunfeng Song,*,†,‡ Qingling Liu,† Na Ji,† Yingjin Song,† and Yutaka Kitamura§ †

Tianjin Key Laboratory of Indoor Air Environmental Quality Control, School of Environmental Science and Technology, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, China ‡ Key Laboratory of Efficient Utilization of Low and Medium Grade Energy, Tianjin University, Ministry of Education, Tianjin 300072, China § Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8572, Japan S Supporting Information *

ABSTRACT: Ammonia synthesis by hydrogen and nitrogen is an important pathway for ammonia production. However, design of an energy efficient and environmentally friendly route for ammonia synthesis is still a challenge and needs to be overcome. Performance and economic feasibility of ammonia synthesis loop processes significantly depend on not only configuration arrangement but also on operating conditions. Thus, a novel ammonia synthesis route with exergy recovery and heat integration was designed by process simulation in this work. The energy and material balance of the proposed process was investigated and compared with the conventional process. The heat integration performance and its influence on total energy consumption were also evaluated. The investigation results showed that the energy consumption of the proposed process was reduced to 16.72 MW, which equaled 38.18% of the conventional process with the feed natural gas of both processes set at 0.083 kmol/s. Approximately 57.9 MW could be recovered in the proposed ammonia synthesis process by heat exchanger networks. KEYWORDS: Ammonia synthesis, Reforming, Shift, Heat integration, Heat exchange

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INTRODUCTION Ammonia is a bulk chemical with a wide range of applications, such as fertilizer production, chemical production, and environmental protection.1,2 In 2014, total worldwide NH3 production exceeded 140 million tons, and the demand of ammonia showed an increasing trend.3 China is one of the largest ammonia production countries, and around 48 million tons was produced in 2014, which accounted for 34.3% of the world total.4 Usually, ammonia is produced by a synthesis reaction in the Haber− Bosch process.5−7 The source of hydrogen can be provided by methane steam reforming (MSR) followed by a water−gas shift reaction (WGS), which consumes about 2% of the world’s natural gas.8 Nitrogen is typically sourced from atmospheric air. In the last decades, significant progress has been made in the ammonia synthesis field, including novel catalysts, cogeneration, process optimization, and integration.3,9,10 In 2011, Siddiq et al. reviewed the existing models and computational schemes that have been used to simulate the industrial ammonia synthesis processes. Due to process efficiency related to the laws of conservation of mass, momentum, and energy for multispecies mixtures, optimized models were thus nonlinear, coupled, partial differential equations which required numerical computing methods to solve for the process variables.11 In 2014, Andersson © 2017 American Chemical Society

and Lundgren performed a techno-economic evaluation of ammonia production via integrated biomass gasification in an existing pulp and paper mill. The simulation results indicated that the overall energy efficiency of the integrated system was increased by 10% compared to a traditional stand-alone mill in parallel with the operation of ammonia production plant.12 In 2013, Sahafzadeh et al. attempted to integrate a gas turbine with an ammonia synthesis loop to reduce the exergy loss and produce electricity. The investigation results showed that total amount of exergy loss could be saved by 3.32 MW, which indicated a 19% reduction compared to the conventional ammonia synthesis process.13 In 2017, Arora et al. compared the economic and environmental potential of the novel biomassbased ammonia production processes in Australia, Brazil, and India. They used the Multi-Objective Optimization (MOO) approach to minimize the manufacturing cost and the environmental impact of the biomass-to-ammonia processes. The results demonstrated that both the economic and environmental profiles of each process were strongly related to the location.14 According to the European Roadmap of Process Intensification Received: June 12, 2017 Revised: July 13, 2017 Published: July 19, 2017 7420

DOI: 10.1021/acssuschemeng.7b01887 ACS Sustainable Chem. Eng. 2017, 5, 7420−7432

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic of conventional ammonia synthesis loop route.

Figure 2. Process flow diagram of typical ammonia synthesis process. 7421


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Figure 3. Schematic of heat integration schemes in proposed ammonia synthesis loop route.

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AMMONIA SYNTHESIS ROUTES Figure 1 depicts the schematic of the typical ammonia synthesis loop process, including reforming, water−gas shift (WGS), methanation, purification, compression, and synthesis. Generally, the feedstock is primary heated and compressed to the required condition of catalytic reaction. During the reforming, shift, methanation, and synthesis reaction, substantial sensible and latent heat is wasted. Furthermore, an amount of external heat is necessary in the syngas purification stage for CO2 sorbent regeneration by temperature swing. For compression treatment, an amount of compression heat is discharged by intercooling. The detail configuration of the conventional ammonia synthesis process is shown in Figure 2. In order to obtain an efficient reaction rate, the reforming is industrially conducted in the presence of a catalyst at the temperatures between 250 and 500 °C and pressures between 150 and 250 bar.23 Therefore, the feedstock (i.e., natural gas/S1, water/S5, and air/S9) is first compressed (by compressor 1 and 2) and heated (by heater 1 to 3) before entering reforming reactors. To improve the reforming efficiency, two serial reactors (R1 and R2) are arranged. The reforming product (S14) is then cooled by cooler-4. In the shift and methanation stage, the concentration of carbon monoxide in the syngas is reduced by the water−gas shift (WGS) and methanation reaction (R3 to R5). The waste reaction heat of R3 and R5 is recovered by heat exchangers 1 (S34 → S35) and 2 (S33 → S34). To enhance synthesis efficiency, the generated CO2 in catalysis reactions should be removed by purification units.24 In this work, the typical temperature swing adsorption (TSA) approach is used to separate CO2 from syngas.25 The CO2 capture units include two parts, adsorption (R6) and desorption (R7). The exothermic heat released in the adsorption column is removed by cooler-8, and endothermic heat for sorbent regeneration is provided by heater-4. The captured CO2 can be transported to a storage site or reused for enhanced oil recovery etc.26 The refreshed sorbent (S32) is sent to an adsorption column to capture CO2 again. Meanwhile, the purified syngas (S33) is pumped to R-5 for further treatment. In the compression stage, three-stage compression (compressors 3 to 5) is carried out to increase the pressure of reactants (S40). The condensate water (S39, S43, S47, and S51) associated with a pressure increase is removed from the syngas stream by phase separators (F-4 to F-7). The obtained dry syngas (S52) is pumped to conversion units. In the ammonia synthesis stage, the product stream is chilled to 4 °C (S54 → S55) to liquefy and separate ammonia (S59).

(PI-PETCHEM), the potential benefits in the ammonia production sector are significant: 5% higher overall energy efficiency for the short/midterm (10−20 years) and 20% higher (30−40 years) for the long-term.15 Therefore, more efforts should be made on further improvement of process efficiency. Efficient ammonia synthesis loop process design has a significant influence on techno-economic performance, environmental impact, operation flexibility, etc.16,17 Pinch technology and exergy analysis would be effective tools for thermodynamic understanding of ammonia synthesis loop.18,19 Kirova-Yordanova has applied an exergy method to estimate the effect of critical process parameters on the exergy efficiency of industrial ammonia synthesis. The estimation results indicated that utilization of the reaction heat at a higher temperature level for HP steam generation and superheating would be an efficient way to improve the overall exergy efficiency of ammonia plants.20 Florez-Orrego and de Oliveira Junior presented an exergy and environmental assessment of a 1000 metric t/day ammonia production plant. A breakdown of the total exergy destruction rate (136.5 MW) showed that around 59% corresponded to the catalytic reforming stage followed far behind by ammonia synthesis and condensation (18.3%) and gas purification units (13.2%).21 Ghannadzadeh and Sadeqzadeh performed exergy analysis on the advanced ammonia production process. The total internal and external exergy losses were calculated as 3152 and 6364 kJ/kg, respectively. Hereinto, catalytic reforming accounted for the largest exergy loss (3098 kJ/kg) and thus had the largest potential for waste heat recovery.22 The aim of this work is to design a novel ammonia synthesis process by waste heat recovery and integration. To minimum the exergy destruction and additional heat utility, the reaction heat of reforming is recovered to preheat feedstock (natural gas and water). The waste sensible heat of shift and methanation reaction is recycled for steam generation and CO2 sorbent regeneration (by temperature swing adsorption). In the compression stage, heat exchangers are used to heat water via compression heat. In addition, the waste heat of ammonia conversion is recovered to preheat unreacted gas. The energy and material balance of the proposed ammonia synthesis route is investigated and compared with the conventional process. Meanwhile, the heat integration performance of heat exchanger networks in the different processes is also studied. Finally, the total energy consumption and efficiency of the proposed process is evaluated. 7422


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Figure 4. Process flow diagram of proposed ammonia synthesis process based on heat integration.

Figure 5. Energy and material balance of conventional ammonia synthesis process. 7423


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Figure 6. Energy and material balance of proposed ammonia synthesis process.

Figure 7. T-Q diagram of heat exchangers in the conventional ammonia synthesis loop process.

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HEAT INTEGRATION SCHEMES To improve the efficiency, waste heat recovery and integration are designed to decrease exergy destruction in the proposed process. As shown in Figure 3, the sensible and latent heats associated with reforming products can be recovered to preheat natural gas, airs and water. The endothermic heat of the CO2 desorption sorbent can be also provided by the waste heat from the catalytic stage. Part of the compression heat is recovered for steam generation. In addition, the waste heat of the synthesis stage is exchanged with the recycle gas to reduce external heat utility.

The detailed configuration of the proposed ammonia synthesis process is presented in Figure 4. In the advanced reforming stage, the waste heat of reacted stream (S12, syngas) from a reformer (R-2) is serially exchanged (HX-1 and HX-2) with natural gas (S1) and water (S3). In the shift and methanation stage, the waste reaction heat is recovered by heat exchangers 3 and 4 (HX-3 and HX-4) to preheat the feedstock (S35 → S36) of reactor 5 (R5), and then, the residual heat is used to generate steam (S39 → S42) for sorbent regeneration (HX-8) in the syngas purification stage. In the CO2 removal unit, the advanced temperature swing adsorption (ATSA) process is designed. 7424


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Figure 8. continued

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Figure 8. continued

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Figure 8. T-Q diagram of heat exchangers in proposed ammonia synthesis loop process.

respectively. The sorbent used for CO2 capture is selected as zeolite. PRO/II 9.3, Inversy, is used to carry out simulation work. The Soave−Redich−Kwong property model is selected as the thermodynamic method for the processes. Most of the components for the simulation are available in the PRO/II database. Zeolite and its property for syngas purification are entered by the user-defined method. Reforming reactors 1 and 2 (R-1 and R-2) are simulated as the Gibbs reactor, and the other reactors (R-3 to R-6) are set at the equilibrium reactor. To simplify the simulation, the following assumptions are used: (1) The minimum temperature approach is set at 10 °C. (2) The isentropic efficiency of compressors is set at 75%. (3) The conversion rate of ammonia in reactor-8 (R-8) is set at 30%. (4) The heat exchangers are counter-current type and formulated using pinch analysis and the specified minimum temperature approach.27 (5) There is no heat loss in the heat exchangers.

Different from the conventional TSA route, the sensible heat for sorbent regeneration is provided by waste sensible and latent heats from the catalytic shift and methanation reaction. Thus, the external heat utility is avoided. In the compression stage, the waste heat of compressed streams (S47 and S57) is recovered for steam generation (S61 → S65), which can obviously decrease the additional steam utility in the catalytic reforming stage. In the ammonia synthesis stage, unreacted feedstock (S73) can be recycled to catalytic reforming for further reaction. It can be found that there is an amount of waste heat discharged associated with off gas (S78) in the synthesis reactor (R8). Therefore, part of the sensible heat can be recovered by heat exchangers 14 and 15 (HX-14 and HX-15) to preheat feedstock (S76 → S77) of R8 and recycle gas (S73 → S74). As a result, the exergy destruction of the proposed ammonia synthesis loop route can be obviously reduced by efficient sensible and latent heats integration.

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METHODOLOGY

RESULTS AND DISCUSSION Energy and Material Balance. The energy and material balance of the conventional ammonia synthesis process is shown in Figure 5. The critical stream summary of the conventional ammonia synthesis loop, including reforming, shift and methanation, purification, compression, and synthesis, is listed in Table S1. Before the catalytic reforming stage, the feedstock (natural gas/S1, water/S5, and air/S9) needs to be preheated

The feedstock in both the conventional and proposed ammonia synthesis processes consists of natural gas, water, and air. The composition of natural gas is assumed to be CH4 (80.75 mol %), C2H6 (7.45 mol %), C3H8 (3.25 mol %), C4H10 (2.31 mol %), C5H12 (0.24 mol %), CO2 (2.95 mol %), and N2 (3.05 mol %). Air is defined by consisting of N2 (78.05 mol %), O2 (21 mol %), and Ar (0.95 mol %). The flow rate of natural gas, feedwater, and air in both processes is set at 0.083, 0.498, 0.127 kmol/s, 7427


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Figure 9. Grid diagram of heat exchanger networks in conventional ammonia synthesis process.

separate liquid NH3 (S72) from unreacted gas (S73). The wasted reaction heat (17.58 MW) from a synthesis reactor (R-8) is recovered by HX-14 to HX-15, and the temperature of the recycle gas (S74) can be increased to promote subsequent catalytic reaction. Compared with the conventional process, the total energy input of the proposed process can be reduced to 23.16 MW. Approximately 57.9 MW waste sensible and latent heats are recycled in the process by heat integration. Heat Integration Performance. The heat integration performances of the conventional and proposed processes are shown in Figures 7 and 8, respectively. As depicted in Figure 7, there are four heat exchangers in the conventional ammonia synthesis process. In the shift and methanation stage, HX-1 and HX-2 are utilized to recover the waste reaction heat from reaction products. The sensible heat (1.55 MW) of reaction product (S15 → S16) in R-3 is recovered by HX-1 to heat the feed (S34 → S35) of R-5. The waste reaction heat (2.34 MW) in R-5 is also recovered by HX-2 and used to preheat the feed stream (S33 → S34). In the ammonia synthesis stage, the feed stream (S60) of a synthesis reactor (R-8) is first heated (1.44 MW) by S53 in HX-3 and then heated (16.08 MW) by the reaction product (S63) in HX-4. From the temperature−heat (T-Q) diagrams, it can be observed that the heat coupling between sensible and latent heats in HX-1 to HX-4 is well established. However, the waste heat in conventional reforming, CO2 purification, and compression stages is discharged to the environment without efficient reutilization. In the designed ammonia synthesis route, 15 heat exchangers are utilized to facilitate waste heat integration. As shown in Figure 8, a temperature−heat (T-Q) diagram of each heat exchanger in the proposed process is presented. Different from the conventional process, the sensible heat (17.08 MW) of a reaction product (S12) in the reforming stage (R-2) is recovered by HX-1 (1.66 MW) and HX-2 (15.42 MW) to heat natural gas and water, respectively. In the advanced shift and methanation

(24.77 MW) and compressed (11.56 MW). In detail, the feed natural gas (S1) is first heated (1.67 MW) to the reaction temperature (393.33 °C). The feed H2O (S5) is heated (23.1 MW) to steam (S6) and compressed (9.54 MW) to the required pressure (2404.18 kPa). In addition, the feed air (S9) is also compressed (2.02 MW) to 2093.91 kPa for reforming. The sensible heat (17.2 MW) of S13 in the reforming sections (R-1 and R-2) is removed by cooling water before entering the shift and methanation reactors. In the WGS and methanation stages, part of the reaction heat (3.89 MW) is recovered by HX-1 and HX-2. Then, CO2 in the syngas is removed by temperature swing adsorption (2.28 MW). The purified syngas is further compressed by three-stage compression (4.00 MW), and the waste compression heat (4.10 MW) is removed by cooling water. Finally, in the ammonia synthesis stage, HX-3 and HX-4 is used to recover 17.52 MW waste heat from the synthesis product (S60 and S63). As shown in Figure 6, the energy and material balance of the proposed ammonia synthesis process is illustrated. The critical stream summary of the proposed ammonia synthesis loop route, including reforming, shift and methanation, purification, compression, and synthesis, is listed in Tables S2−S6. To integrate the waste heat, two heat exchangers (HX-1 and HX-2) are used to recover the reaction heat (17.08 MW) from the reforming product (S12). In the shift and methanation stage, the catalytic reaction heat (14.89 MW) is also recovered by a heat exchanger network (HX-3 to HX-7) to evaporate water (S39) to steam (S42). The sensible and latent heats of the obtained hot steam can be used by HX-8 to provide desorption heat (2.28 MW) for CO2 sorbent regeneration (temperature swing). In the advanced compression stage, the compression heat (6.07 MW) and part of the reaction heat (4.07 MW) from the catalytic shift and methanation stage are also recuperated (HX-9 to HX-12) to generate steam (S65). During the ammonia synthesis stage, the crude product stream (S68) is chilled (to 4.44 °C) by cooler-8 to 7428


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Figure 10. Grid diagram of heat exchanger networks in proposed ammonia synthesis process.

In the compression stage, the waste compression heat (2.00 MW) is recovered to heat water (S61 → S64) by HX-9 to HX-11, and the hot water (S64) is further heated (4.07 MW) by HX-12 to generate hot steam (S64 → S65). In the ammonia synthesis stage, except for reaction heat (17.52 MW) recovery (HX-13 and HX-14), part of the latent heat (0.056 MW) of the synthesis product (S79) is recovered by HX-15 to heat recycle gas (S73 → S74). The heat pairing curves in the heat exchangers (HX-1 to HX-15) present the efficient sensible and latent heats integration. As a result, the exergy destruction of the designed ammonia synthesis route could be minimized compared to the conventional process, leading to a significant reduction in energy consumption. The grid diagrams of the conventional and proposed ammonia synthesis processes are presented in Figures 9 and 10. As shown in Figure 9, four heaters (H-1 to H-4) are used in the conventional process to adjust the temperature of feedstock streams (natural gas, water, and air). Meanwhile, four heat exchangers (HX-1 to HX-4) are involved to form the heat recovery network. Fourteen coolers (C-1 to C-14) are used to adjust the temperature of hot streams. In the proposed process (Figure 10), the number of heaters and coolers can be obviously decreased due to efficient heat recovery. For heat utility, only one heater (H-1) is necessary. For cold utility, coolers 1 to 8 (C-1 to C-8) are required to remove the low grade waste heat. Totally, 15 heat exchangers are used to integrate waste sensible and latent heats of each stage to optimize the heat coupling arrangement.

Figure 11. Energy consumption in each stage of conventional and proposed ammonia synthesis processes.

stage, HX-3 and HX-4 are used to recover the waste sensible heat (3.89 MW) from R-3 and R-5, which is the same with conventional process. In addition, HX-5 to HX-7 are used to recuperate the waste sensible and latent heats (totally 11.00 MW) and generate steam (S39 → S42). In the syngas purification stage, the obtained hot steam (S42) is sent to HX-8 to provide desorption heat (2.28 MW) for CO2 sorbent regeneration. Therefore, the additional heat utility is avoided in this section. 7429


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Figure 12. Recovered energy by heat integration in each stage of conventional and proposed processes.

Figure 13. Comparison of exergy loss in existing ammonia synthesis loops.

Performance Comparison. To evaluate the technoeconomic feasibility, comparison of energy consumption between the conventional and proposed ammonia synthesis routes is carried out and illustrated in Figure 11. As shown in the results, the energy consumption of the conventional ammonia synthesis process mainly consisted of four parts: catalytic reforming (natural gas pretreatment, 1.67 MW; air pretreatment, 2.02 MW; and steam generation, 32.64 MW), syngas purification (sorbent regeneration, 2.28 MW), compression (three-stage compression, 4.00 MW), and ammonia synthesis (chilling, 1.16 MW). Therefore, it needs to consume totally 43.77 MW for the conventional process. By contrast, the energy consumption of the proposed process is composed of three parts: catalytic reforming

(air pretreatment, 2.02 MW; and steam generation, 9.54 MW), compression (three-stage compression, 4.00 MW), and ammonia synthesis (chilling, 1.16 MW). Due to an effective heat paring between hot and cold streams, most of the waste sensible and latent heats is reused in the proposed process, as presented in Figure 12. In the conventional ammonia synthesis process, approximately 3.89 and 17.52 MW can be recycled in the catalytic shift and ammonia synthesis stages, respectively. Compared with the energy recovery capacity of the conventional process, around 17.08, 14.89, 2.28, 6.07, and 17.58 MW can be recovered in the advanced reforming, shift and methanation, purification, compression, and synthesis stages, respectively. Thus, the total energy requirement of the designed ammonia 7430


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(2) Penkuhn, M.; Tsatsaronis, G. Comparison of different ammonia synthesis loop configurations with the aid of advanced exergy analysis. Energy 2017, na DOI: 10.1016/j.energy.2017.02.175. (3) Shipman, M. A.; Symes, M. D. Recent progress towards the electrosynthesis of ammonia from sustainable resources. Catal. Today 2017, 286, 57−68. (4) U.S. Geological Survey. Mineral Commodity Summaries, Nitrogen (Fixed)Ammonia. January 2015; pp 112−113. (5) Jennings, J. R., Ed.; Catalytic Ammonia Synthesis: Fundamentals and Practice; Plenum, New York, 1991. (6) Erisman, J. W.; Sutton, M. A.; Galloway, J.; Klimont, Z.; Winiwarter, W. How a century of ammonia synthesis changed the world. Nat. Geosci. 2008, 1, 636−639. (7) Tanabe, Y.; Nishibayashi, Y. Developing more sustainable processes for ammonia synthesis. Coord. Chem. Rev. 2013, 257, 2551− 2564. (8) Ammonia, 1. Introduction. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley, 2011. (9) Bicer, Y.; Dincer, I.; Zamfirescu, C.; Vezina, G.; Raso, F. Comparative life cycle assessment of various ammonia production methods. J. Cleaner Prod. 2016, 135, 1379−1395. (10) Kyriakou, V.; Garagounis, I.; Vasileiou, E.; Vourros, A.; Stoukides, M. Progress in the electrochemical synthesis of ammonia. Catal. Today 2017, 286, 2−13. (11) Siddiq, S.; Khushnood, S.; Koreshi, Z. U.; Shah, M. T. Process simulation of ammonia synthesis for increasing heat recovery in a thermal storage plant: a review. Technical Journal; University of Engineering and Technology: Taxila, Pakistan, 2011. (12) Andersson, J.; Lundgren, J. Techno-economic analysis of ammonia production via integrated biomass gasification. Appl. Energy 2014, 130, 484−490. (13) Sahafzadeh, M.; Ataei, A.; Tahouni, N.; Panjeshahi, M. H. Integration of a gas turbine with an ammonia process for improving energy efficiency. Appl. Therm. Eng. 2013, 58, 594−604. (14) Arora, P.; Hoadley, A. F. A.; Mahajani, S. M.; Ganesh, A. Multiobjective optimization of biomass based ammonia production- Potential and perspective in different countries. J. Cleaner Prod. 2017, 148, 363− 374. (15) Dopper, J. G. European Roadmap for Process Intensification; Ministry of Economic Affairs, Delft, The Netherlands. 2007; pp 53. (16) Arora, P.; Hoadley, A. F. A.; Mahajani, S. M.; Ganesh, A. SmallScale Ammonia Production from Biomass: A Techno-Enviro-Economic Perspective. Ind. Eng. Chem. Res. 2016, 55 (22), 6422−6434. (17) Martínez, I.; Armaroli, D.; Gazzani, M.; Romano, M. C. Integration of the Ca−Cu Process in Ammonia Production Plants. Ind. Eng. Chem. Res. 2017, 56 (9), 2526−2539. (18) Panjeshahi, M. H.; Langeroudi, E. G.; Tahouni, N. Retrofit of ammonia plant for improving energy efficiency. Energy 2008, 33, 46−64. (19) Bicer, Y.; Dincer, I.; Vezina, G.; Raso, F. Impact Assessment and Environmental Evaluation of Various Ammonia Production Processes. Environ. Manage. 2017, 59 (5), 842−855. (20) Kirova-Yordanova, Z. Exergy analysis of industrial ammonia synthesis. Energy 2004, 29, 2373−2384. (21) Florez-Orrego, D.; de Oliveira Junior, S. On the efficiency, exergy costs and CO2 emission cost allocation for an integrated syngas and ammonia production plant. Energy 2016, 117, 341−360. (22) Ghannadzadeh, A.; Sadeqzadeh, M. Diagnosis of an alternative ammonia process technology to reduce exergy losses. Energy Convers. Manage. 2016, 109, 63−70. (23) Ammonia, 2. Production processes. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley, 2011. (24) Nikačević, N.; Jovanović, M.; Petkovska, M. Enhanced ammonia synthesis in multifunctional reactor with in situ adsorption. Chem. Eng. Res. Des. 2011, 89, 398−404. (25) Song, C.; Kansha, Y.; Fu, Q.; Ishizuka, M.; Tsutsumi, A. Reducing energy consumption of advanced PTSA CO2 capture process―Experimental and numerical study. J. Taiwan Inst. Chem. Eng. 2016, 64, 69−78.

synthesis loop process can be reduced to 16.72 MW. The comparison of exergy loss between the proposed and existing processes is also carried out, as shown in Figure 13. The exergy loss of the designed process is the lowest (4.96 MJ/kg-NH3) compared with the conventional (12.98 MJ/kg-NH3) and reference routes22 (9.52 MJ/kg-NH3). That is because the waste heat of shift and methanation, syngas purification, and conversion stages can be effectively recovered in the designed ammonia synthesis route.

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CONCLUSION A novel ammonia synthesis loop route was designed and optimized by heat integration in this work. The energy and material balance of the designed process was investigated and compared with the conventional process. The investigation results indicated that the energy requirement of the proposed process could be reduced to 16.72 MW, which accounted for 38.18% of the conventional process. The heat integration potential of the proposed process was validated by the T-Q and grid diagram analysis. In the advanced catalytic reforming and shift stages, 17.08 and 14.89 MW of reaction heat were recovered, respectively, to preheat feedstock (natural gas and water). In the syngas purification stage, the CO2 desorption heat (2.28 MW) was provided by the waste reaction heat from shift and methanation reactors. In the three-stage compression units, the compression heat (6.07 MW) was recovered by boiler to generate steam. In the ammonia synthesis stage, the reaction heat (totally 17.58 MW) was exchanged with recycle gas to decrease the exergy destruction. As a result, approximately 57.9 MW of waste sensible and latent heat could be recovered in the designed ammonia synthesis process. Although the additional heat exchangers would lead to more investment than the conventional route, the waste heat recovery efficiency of the proposed process obviously increased, which presented the energy-savings potential in the commercial application.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01887. Properties of critical streams in the conventional and proposed ammonia synthesis loop routes: Tables S1−S6. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 02287401255. Fax: +86 02287401255. E-mail: [email protected]. ORCID

Chunfeng Song: 0000-0002-9617-8297 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by National Natural Science Fund of China (Grant No. 51506147). REFERENCES

(1) Rafiqul, I.; Weber, C.; Lehmann, B.; Voss, A. Energy efficiency improvements in ammonia productionperspectives and uncertainties. Energy 2005, 30, 2487−2504. 7431


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ACS Sustainable Chemistry & Engineering (26) Song, C.; Liu, Q.; Ji, N.; Deng, S.; Zhao, J.; Li, Y.; Kitamura, Y. Reducing the energy consumption of membrane-cryogenic hybrid CO2 capture by process optimization. Energy 2017, 124, 29−39. (27) Song, C.; Liu, Q.; Ji, N.; Kansha, Y.; Tsutsumi, A. Optimization of steam methane reforming coupled with pressure swing adsorption hydrogen production process by heat integration. Appl. Energy 2015, 154, 392−401.

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Conceptual Design and Process Feasibility Analysis of a Novel

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