Conceptual Design and Process Feasibility Analysis of a Novel

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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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b01887 • Publication Date (Web): 19 Jul 2017 Downloaded from http://pubs.acs.org on July 22, 2017

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ACS Sustainable Chemistry & Engineering

Conceptual design and process feasibility analysis of a novel ammonia synthesis process by efficient heat integration Chunfeng Song †, ‡ *, Qingling Liu †, Na Ji †, Yingjin Song †, 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

* Corresponding author. Tel.: +86-022-2740-1255 E-mail address: [email protected]

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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 needs to be overcome. Performance and economic feasibility of ammonia synthesis loop processes significantly depends on not only configuration arrangement but also operating condition. 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 to 38.18 % of 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 application, 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 the synthesis reaction in the Haber-Bosch process.5-7 The source of hydrogen can be provided by methane steam reforming (MSR) followed by 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, co-generation, process optimization and integration.3,9-10 In 2011, Sadaf 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 multi-species mixtures, optimized models were thus non-linear, coupled, partial differential equations which required numerical computing methods to solve for the process variables.11 In 2014, Jim Andersson and Joakim 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

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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, Meysam Sahafzadeh et al. attempted to integrate gas turbine with 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 19% reduction compared to the conventional ammonia synthesis process.13 In 2017, Pratham Arora et al. compared the economic and environmental potential of the novel biomass-based ammonia production processes in Australia, Brazil, and India. They used 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 (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 paid 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 Zornitza Kirova-Yordanova has applied

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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 Daniel Florez-Orrego and Silvio 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 the ammonia synthesis and condensation (18.3%) and the gas purification units (13.2%).21 Ali Ghannadzadeh and Majid 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

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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.

AMMONIA SYNTHESIS ROUTES

Figure 1 depicts the schematic of 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, amount of external heat is necessary in the syngas purification stage for CO2 sorbent regeneration by temperature swing. For the compression treatment, amount of compression heat is discharged by inter-cooling.

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Figure 1. Schematic of conventional ammonia synthesis loop route.

The detail configuration of the conventional ammonia synthesis process is shown in Figure 2. In order to obtain 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 firstly 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 water gas shift (WGS) and methanation reaction (R3 to R5). The waste reaction heat of R3 and R5 is recovered by heat exchanger 1 (S34→S35) and 2 (S33→S34). To enhance synthesis efficiency, the generated CO2 in catalysis reactions should be removed by purification unit.24 In this work, the typical

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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 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 storage site or reused for enhanced oil recovery etc.26 The refreshed sorbent (S32) is sent to adsorption column to capture CO2 again. Meanwhile, the purified syngas (S33) is pumped to R-5 for further treatment. In compression stage, three-stage compression (compressor 3 to 5) is carried out to increase the pressure of reactants (S40). The condensate water (S39, S43, S47 and S51) associated with pressure increase is removed from syngas stream by phase separators (F-4 to 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).

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

HEAT INTEGRATION SCHEMES

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

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

The detail configuration of proposed ammonia synthesis process is presented in Figure 4. In the advanced reforming stage, the waste heat of reacted stream (S12, syngas) from reformer (R-2) is serially exchanged (HX-1 and 2) with natural gas (S1) and water (S3). In shift and methanation stage, the waste reaction heat is recovered by heat

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exchanger 3 and 4 (HX-3 and 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 syngas purification stage. In CO2 removal unit, advanced temperature swing adsorption (ATSA) process is designed. Different with conventional TSA route, the sensible heat for sorbent regeneration is provided by waste sensible and latent heat from catalytic shift and methanation reaction. Thus, the external heat utility is avoided. In compression stage, the waste heat of compressed streams (S47 and S57) is recovered for steam generation (S61→S65), and which can obviously decrease the additional steam utility in catalytic reforming stage. In ammonia synthesis stage, unreacted feedstock (S73) can be recycled to catalytic reforming for further reaction. It can be found that there is amount of waste heat discharged associated with off gas (S78) in synthesis reactor (R8). Therefore, part of sensible heat can be recovered by heat exchanger 14 and 15 (HX-14 and 15) to preheat feedstock (S76→S77) of R8 and recycle gas (S73→S74). As a result, the exergy destruction of proposed ammonia synthesis loop route can be obviously reduced by efficient sensible and latent heat integration.

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

METHODOLOGY

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The feedstock in both of conventional and proposed ammonia synthesis processes is consisted of natural gas, water and air. The composition of natural gas is assumed as CH4 (80.75 mol.%), C2H6 (7.45 mol.%), C3H8 (3.25 mol.%), C4H10 (2.31 mol.%), C5H12 (0.24 mol.%), CO2 (2.95 mol.%), 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, feed water and air in the both processes is set at 0.083 kmol/s, 0.498 kmol/s, 0.127 kmol/s. 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 thermodynamic method for the processes. Most of the components for the simulation are available in PRO/II database. Zeolite and its property for syngas purification are entered by the user-defined method. The reforming reactor 1 and 2 (R-1 and R-2) are simulated as Gibbs reactor, and the other reactors (R-3 to R-6) are set at Equilibrium reactor. To simplify the simulation, the following assumption is conducted: 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 specified minimum temperature approach.27 5) There is no heat loss in the heat exchangers.

RESULTS AND DISCUSSION

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Energy and material balance. The energy and material balance of 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 catalytic reforming stage, the feedstock (natural gas/S1, water/S5, and air/S9) needs to be preheated (24.77 MW) and compressed (11.56 MW). In detail, the feed natural gas (S1) is firstly 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 reforming section (R-1 and 2) is removed by cooling water before entering shift and methanation reactors. In WGS and methanation stage, part of reaction heat (3.89 MW) is recovered by HX-1 and 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 ammonia synthesis stage, HX-3 and 4 is used to recover 17.52 MW waste heat from synthesis product (S60 and S63).

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

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As shown in Figure 6, the energy and material balance of 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 Table S2 to S6. To integrate the waste heat, two heat exchangers (HX-1 and 2) are used to recover the reaction heat (17.08 MW) from reforming product (S12). In the shift and methanation stage, the catalytic reaction heat (14.89 MW) is also recovered by heat exchanger network (HX 3 to 7) to evaporate water (S39) to steam (S42). The sensible and latent heat of 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 reaction heat (4.07 MW) from catalytic shift and methanation stage is also recuperated (by HX 9 to 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 separate liquid NH3 (S72) from unreacted gas (S73). The wasted reaction heat (17.58 MW) from synthesis reactor (R-8) is recovered by HX-14 to 15, and the temperature of recycle gas (S74) can be increased to promote subsequent catalytic reaction. Compared with conventional process, the total energy input of proposed process can be reduced to 23.16 MW. Approximately 57.9 MW waste sensible and latent heat is recycled in the process by heat integration.

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

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Heat integration performance. The heat integration performance of the conventional and proposed processes is shown in Figure 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 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 ammonia synthesis stage, the feed stream (S60) of synthesis reactor (R-8) is firstly 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 heat in HX-1 to 4 is well established. However, the waste heat in conventional reforming, CO2 purification and compression stages is discharged to the environment without efficient reutilization.

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

In the designed ammonia synthesis route, fifteen heat exchangers are utilized to facilitate waste heat integration. As shown in Figure 8, temperature-heat (T-Q) diagram of each heat exchanger in the proposed process is presented. Different with the conventional process, the sensible heat (17.08 MW) of reaction product (S12) in

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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 stage, HX-3 and 4 are used to recover the waste sensible heat (3.89 MW) from R-3 and 5, which is the same with conventional process. In addition, HX-5 to 7 are used to recuperate the waste sensible and latent heat (totally 11.00 MW), and generate steam (S39→S42). In 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. In compression stage, the waste compression heat (2.00 MW) is recovered to heat water (S61→S64) by HX-9 to 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 reaction heat (17.52 MW) recovery (by HX-13 and 14), part of latent heat (0.056 MW) of 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 15) present the efficient sensible and latent heat integration. As a result, the exergy destruction of designed ammonia synthesis route could be minimized compared to the conventional process, leading to a significant reduction in energy consumption.

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350 300 250 △Tmin=192.47 oC

200 150

HX-14

100

10

11

Duty 16.08 MW

50

Duty 1.44 MW 1


450


30

0

HX-12


Zones [-]

Temperature (oC)

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


12

0

1

2

3

4

5

6

Zones [-]

Zones [-]

23


7

8

9

10

11


300 250

Temperature (oC)

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


150

△Tmin=17.41 oC

100 HX-15 50 0

Duty 0.056 MW 1

2

3

4

5

6

7

8

9

10

11

Zones [-]

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

The grid diagram of conventional and proposed ammonia synthesis processes is presented in Figure 8 and 9. As shown in Figure 8, four heaters (from H-1 to 4) are used in the conventional process to adjust temperature of feedstock streams (natural gas, water and air). Meanwhile, four heat exchangers (from HX-1 to 4) are involved to form heat recovery network. Fourteen coolers (from C-1 to 14) are used to adjust temperature of hot streams. In the proposed process (as shown in Figure 9), 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, cooler 1 to 8 (C-1 to 8) is required to remove the low grade waste heat. Totally, fifteen heat exchangers are used to integrate waste sensible and latent heat of each stage to optimize heat coupling arrangement.

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

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

Performance comparison. To evaluate the techno-economic 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 conventional ammonia synthesis process is mainly consisted of four parts: catalytic reforming (natural gas pretreatment, 1.67 MW, air pretreatment, 2.02

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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 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 heat is reused in the proposed process, as presented in Figure 12. In the conventional ammonia synthesis process, approximately 3.89 MW and 17.52 MW can be recycled in the catalytic shift and ammonia synthesis stage. Compared with the energy recovery capacity of conventional process, around 17.08 MW, 14.89 MW, 2.28 MW, 6.07 MW and 17.58 MW can be recovered in the advanced reforming, shift and methanation, purification, compression and synthesis stage, respectively. Thus, the total energy requirement of the designed ammonia 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 designed process is the lowest (4.96 MJ/kg-NH3) compared with the conventional (12.98 MJ/kg-NH3) and reference route

22

(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.

27



40 Air pretreatment

Reforming Shift & Methanation Purification Compression Synthesis

35 Energy consumption (MW)

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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30

NG pretreatment

25 Air pretreatment

20 15 10

Steam generation

Steam generation

Desorption 3-stage compression

3-stage compression

Chilling

5

Chilling

0 Proposed

Conventional

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

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20


HX-15

18 HX-3

16

Recovered energy (MW)

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


14

HX-3

12

HX-4 HX-2

10

HX-5

HX-14

HX-4

8

HX-11

6 HX-6

4

HX-12

HX-10

HX-1

2

HX-8

HX-2

HX-1

0 Conventional

HX-7

HX-9

HX-13

Proposed

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

29



14000


Exergy loss (kJ/kg-NH3)

12000 10000 398 8000

3430.5

10770.8 6000

1779.4 825

4000

3427.2 2000

343.9

676.0 1185.9

3082.7

343.9 1185.9

0

r fe e

22

s es

c en

oc pr

ed os op ss Pr oce pr

Re

na tio en nv s s Co oce pr l

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

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 proposed process could be reduced to 16.72 MW, which accounted for 38.18 % that 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 stage, 17.08 MW and 14.89 MW

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reaction heat was recovered to preheat feedstock (natural gas and water). In 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 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 conventional route, the waste heat recovery efficiency of proposed process obviously increased, which presented the energy-saving potential in the commercial application.

ACKNOWLEDGEMENT

This research was financially supported by National Natural Science Fund of China (grant no. 51506147).

ASSOCIATED CONTENT

Supporting Information Available The properties of critical streams in the conventional and proposed ammonia synthesis loop routes are listed in Table S1 to S6. This material is available free of charge via the internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author *Phone: Tel.: +86 02287401255; Fax: +86 02287401255. E-mail address: [email protected]

REFERENCES

(1) Rafiqul, I.; Weber, C.; Lehmann, B.; Voss, A. Energy efficiency improvements in ammonia production—perspectives

and

uncertainties.

Energy.

2005,

30,

2487–2504.

https://doi.org/10.1016/j.energy.2004.12.004. (2) Penkuhn, M.; Tsatsaronis, G. Comparison of different ammonia synthesis loop configurations with

the

aid

of

advanced

exergy

analysis.

Energy.

2017,

http://dx.doi.org/10.1016/j.energy.2017.02.175. (3) Shipman, M. A.; Symes, M. D. Recent progress towards the electrosynthesis of ammonia from

sustainable

resources.

Catalysis

Today.

2017,

286,

57–68.

http://dx.doi.org/10.1016/j.cattod.2016.05.008. (4) U.S. Geological Survey, Mineral Commodity Summaries, Nitrogen (Fixed)—Ammonia. January 2015, pp112-113. (5) Jennings (Ed.), J.R. Catalytic Ammonia Synthesis: Fundamentals and Practice, Plenum, New York, NY, USA, 1991. (6) Erisman, J. W.; Sutton, M. A.; Galloway, J.; Klimont, Z.; Winiwarter, W. How a century of

32


Page 33 of 36

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


ammonia

synthesis

changed

the

Nature

world.

Geoscience.

2008,

1,

636–639.

DOI:10.1038/ngeo325. (7) Tanabe, Y.; Nishibayashi, Y. Developing more sustainable processes for ammonia synthesis.

Coordination

Chemistry

Reviews.

257,

2013,

2551–2564.

https://doi.org/10.1016/j.ccr.2013.02.010. (8) Appl M. Ammonia, 1. Introduction. Ullmann's Encyclopedia of Industrial Chemistry. Wiley;

2011a. (9) Bicer, Y.; Dincer, I.; Zamfirescu, C.; Vezina, G.; Raso, F. Comparative life cycle assessment of various ammonia production methods. Journal of Cleaner Production. 2016, 135, 1379-1395. https://doi.org/10.1016/j.jclepro.2016.07.023. (10) Kyriakou, V.; Garagounis, I.; Vasileiou, E.; Vourros, A.; Stoukides, M. Progress in the electrochemical

synthesis

of

ammonia.

Catalysis

Today.

2017,

286,

2–13.

https://doi.org/10.1016/j.cattod.2016.06.014. (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, 2011. (12) Andersson, J.; Lundgren, J. Techno-economic analysis of ammonia production via integrated biomass

gasification.

Applied

Energy.

2014,

130,

484–490.

https://doi.org/10.1016/j.apenergy.2014.02.029. (13) Sahafzadeh, M.; Ataei, A.; Tahouni, N.; Panjeshahi, M. H. Integration of a gas turbine with an ammonia process for improving energy efficiency. Applied Thermal Engineering. 2013, 58,

33



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

594-604. https://doi.org/10.1016/j.applthermaleng.2013.05.006. (14) Arora, P.; Hoadley, A.F.A.; Mahajani, S. M.; Ganesh, A. Multi-objective optimization of biomass based ammonia production- Potential and perspective in different countries. Journal of

Cleaner Production. 2017, 148, 363-374. https://doi.org/10.1016/j.jclepro.2017.01.148. (15) Dopper, J.G. European roadmap for process intensification. Delft, The Netherlands: Ministry of Economic Affairs; 2007, pp 53. (16) Arora, P.;

Hoadley, A. F. A.; Mahajani, S. M.;

Ganesh, A. Small-Scale Ammonia

Production from Biomass: A Techno-Enviro-Economic Perspective. Ind. Eng. Chem. Res. 2016, 55 (22), 6422–6434. DOI: 10.1021/acs.iecr.5b04937. (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. DOI: 10.1021/acs.iecr.6b04615. (18) Panjeshahi, M.H.; Langeroudi, E. G.; Tahouni, N. Retrofit of ammonia plant for improving energy efficiency. Energy. 2008, 33, 46–64. https://doi.org/10.1016/j.energy.2007.08.011. (19) Bicer, Y.; Dincer, I.; Vezina, G.; Raso, F. Impact Assessment and Environmental Evaluation of Various Ammonia Production Processes. Environmental Management. 2017, 59 (5), 842–855. DOI:10.1007/s00267-017-0831-6. (20) Kirova-Yordanova, Z. Exergy analysis of industrial ammonia synthesis. Energy. 2004, 29, 2373–2384. https://doi.org/10.1016/j.energy.2004.03.036. (21) Florez-Orrego, D.; Junior, S. O. On the efficiency, exergy costs and CO2 emission cost allocation for an integrated syngas and ammonia production plant. Energy. 2016, 117, 341-360.

34


Page 34 of 36

Page 35 of 36

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


https://doi.org/10.1016/j.energy.2016.05.096. (22) Ghannadzadeh, A.; Sadeqzadeh, M. Diagnosis of an alternative ammonia process technology to reduce

exergy

losses.

Energy

Conversion

and

Management.

2016,

109,

63–70.

https://doi.org/10.1016/j.enconman.2015.11.040. (23) Appl, M. Ammonia, 2. Production processes. Ullmann's Encyclopedia of Industrial Chemistry. Wiley; 2011b. (24) Nikačević, N.; Jovanović, M.; Petkovska, M. Enhanced ammonia synthesis in multifunctional reactor with in situ adsorption. Chemical Engineering Research and Design. 2011, 89, 398–404. https://doi.org/10.1016/j.cherd.2010.08.011. (25) Song, C.; Kansha, Y.; Fu, Q.; Ishizuka, M.; Tsutsumi, A. Reducing energy consumption of advanced PTSA CO2 capture process―Experimental and numerical study. Journal of the

Taiwan

Institute

of

Chemical

Engineers.

2016,

64,

69–78.

https://doi.org/10.1016/j.jtice.2015.12.006. (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. https://doi.org/10.1016/j.energy.2017.02.054. (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.

Applied Energy. 2015, 154, 392–401. https://doi.org/10.1016/j.apenergy.2015.05.038.

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TOC Art A novel ammonia synthesis process has been designed and presented energy saving potential due to optimal waste heat integration.

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Conceptual Design and Process Feasibility Analysis of a Novel

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