Article pubs.acs.org/EF
Exergy Analysis of Waste Heat Recovery Section in Steam-Natural Gas Reforming Process Mohammad H. Shariati and Fatollah Farhadi* Sharif University of Technology, Department of Chemical and Petroleum Engineering, P. O. Box 11155-3465, Tehran, Iran ABSTRACT: In this work, an exergy analysis is performed for the waste heat recovery section (WHRS) of the steam-natural gas reforming (SNGR) process as a major energy intensive process. Two alternate conditions are investigated to evaluate the required thermodynamic parameters: normal operating condition and increase of C2+ components in the process feed stream. At normal operating condition, the exergy efficiency of WHRS amounts to 0.58 while some 17.2 kJ energy is destructed for each mole of H2 produced. If heavier than methane components are increased in the feed up to 8.5 mole %, despite the increase of H2 production, the exergy efficiency decreases down to 0.54 and exergy destruction is reduced to 13.8 kJ for each mole of H2 produced. It is concluded that waste heat recovery has an important role on the exergy parameters of steam reforming processes, and an efficient heat exchange mechanism should be designed for this purpose. In addition, a change of reforming feed composition has a significant effect on the process efficiency, which implies its control for an efficient operation.
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INTRODUCTION Hydrogen (H2) is traditionally an important feedstock in chemical process industries and oil refineries.1 Processing of fossil fuels and production of ammonia, methanol, and some other petrochemical products are main consumers of hydrogen.2,3 Steam reforming of hydrocarbons is the most common process for industrial production of hydrogen.4 This process is a large energy consumer due to its high temperature reactions (700−1100 °C).5 Therefore, analysis and optimization of energy consumption in this process is a major concern. Recently, exergy analysis, as a powerful tool for analysis, evaluation, and improvement of thermal processes, is used to identify the location, magnitude, and sources of thermodynamic inefficiencies, and to optimize the usage of energy resources with economic and environmental aspects.6,7 Sensitivity analysis by fixing the level of all variables except one and measuring the response for several values is a theoretical rule for evaluation of the steam-methane reforming (SMR) process. A factorial DOE (design of experiment) method is used to evaluate the sensitivity of SMR performance, and a threevariable equation is reported that represents the process exergy efficiency in terms of steam reformer temperature, pressure, and steam-to-carbon ratio.8 Exergy parameters of the steam reforming processes can be improved by incorporation of an added heat exchanger for better heat recovery.9 In addition, exergy analysis and optimization of the autothermal reforming (ATR) process is studied in the literature.10 In order to investigate the SMR process from either energy or environmental aspects, exergy efficiency and CO2 emission are evaluated for different reforming variables.2,4 Therefore, exergy analysis of the reforming processes in hydrogen or methanol plants is reported in many studies, but the WHRS of the steam reforming unit in these plants is not considered separately as a main energy consuming unit. In this work, an exergy analysis is presented for the WHRS of the methanol production process via SNGR with emphasis on exergy flows, destruction, waste, and efficiency. At first, the SNGR unit and its WHRS are described and simulated. The © 2015 American Chemical Society
exergy analysis at two practical conditions is presented in the Results and Discussion section.
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PROCESS DESCRIPTION AND SIMULATION
The steam reforming unit of the considered methanol plant in Iran is designed to produce syngas (a mixture of steam, hydrogen, and carbon mono/dioxide) for methanol synthesis. Reforming Unit. A recent development in large methanol plants has been to adjust the syngas composition for higher methanol yields and higher carbon efficiencies. This has led to adding an autothermal reforming or a partial oxidation process unit for the adjustment of the stoichiometric number (2 < SN < 2.1) for methanol synthesis. The CO2-methane reforming is also another initiative for this adjustment.11 The reforming operation in the present case is accomplished in two stages: steam reforming and autothermal reforming, arranged in series. The process feed is the pipeline natural gas from South Pars gas refineries off the Persian Gulf. The natural gas feed stream is mixed with steam. One part of this mixture is going to the steam reforming unit; the second part is mixed with the reforming products to feed the autothermal reactor (Figure 1). Product stream with the adjusted desired ratios, from the autothermal reactor, containing 48.3 mol % H2, 6% CO2, and 16.1% CO will go to water gas shift reactors and then to the methanol synthesis reactor.11 The steam reforming unit (Figure 1) includes three main parts: WHRS, also called the feed preheater section, prereformer reactor, and reformer reactor. The main process feed and the combustion air are preheated in WHRS by using the waste heat of flue gases, which are leaving the reformer furnace at 935 °C. The prereformer reactor is converting heavier than methane hydrocarbons, inevitably present in the natural gas.11 Received: December 18, 2014 Revised: April 21, 2015 Published: April 23, 2015 3322
DOI: 10.1021/ef5027983 Energy Fuels 2015, 29, 3322−3327
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Energy & Fuels
and 4. For a steady-state system, the exergy destruction is the difference between the total amount of exergy into and out of the system. The unused exergy or total exergy loss of the system is defined as the sum of the exergy destroyed within the system (ED) and the exergy wasted in the exhaust stream (EL). Exergy of the exhaust stream is theoretically recoverable.13 Ein = E43 + E10a + E08a + E06 + E04a + E36a + E38
(1)
Eout = E44 + E11 + E08b + E07 + E04b + E36c
(2)
ED = Ein − Eout
(3)
EUnused = ED + EL = Ein − Eout + E44
(4)
Exergy efficiency of the WHRS is defined as the ratio of product exergy to fuel exergy (eq 5). This equation can be rewritten by using unused exergy. The fuel and product exergies are defined as eqs 6 and 7. Figure 1. Block flow diagram of the steam reforming unit (DeSu: desulfurization unit, Sat: saturator, PreR: prereformer, SR: steam reformer, ATR: autothermal reformer, WHRS: waste heat recovery section, FH-02: auxiliary burner, E: heat exchanger).
ηExergy =
E EP = 1 − Unused EF EF
(5)
EF = E43 + E38
(6)
EP = E11 − E10a + E08b − E08a + E07 − E06 + E04b
Heat Recovery Section. WHRS of the steam reforming unit consists of five heat exchangers and one auxiliary burner, placed all in series in a huge horizontal channel, located downstream of hot flue gases exhausted from the reforming furnace. These five-coiled heat exchangers are preheating the main feed, reformer combustion air, autothermal feed, steam prereformer, and reformer feed streams, respectively. The auxiliary burner is used to supplement the WHRS heat duty and boost the heat content of the flue gas. The heat exchangers are shown schematically in Figure 1, and their design specifications are listed in Table 1. Process Simulation. A process simulation for exergy analysis of the WHRS is performed in Aspen-Hysys V7.3 simulator linked to MS Excel. For thermo-physical properties prediction in the process simulation, the SRK (Soave− Redlich−Kwong) equation of state is suggested to be a suitable thermodynamic model.9,12 This simulation includes WHRS, steam/natural gas mixing vessel, and prereforming and reforming reactors (Figure 2). Operating parameters such as temperature, pressure, flow rate, and composition of streams are obtained from simulation and are compared to the plant design original simulation.
− E04a + E36c − E36a
(7)
All exergy terms in eqs 1−7 are the sum of the chemical and physical exergies: Ei = EiCh + EiPh = ṁ i(Ei̅ Ch + Ei̅ Ph)
(8)
In this work, it is assumed that all streams are an ideal gas mixture. The molar chemical exergy of an ideal mixture of N ideal gases is determined from eq 9.13 N
Ei̅ Ch = RT0 ∑ xj ln(xj) + j=1
N
∑ xj E̅jCh,ref (9)
j=1
13
E̅ Ch j,ref
is the standard molar chemical exergy of substance j. xj denotes the mole fraction of substance j in stream i. The molar physical exergy of stream i is determined from eq 10 Ei̅ Ph = (H̅ i − H̅ 0) − T0(Si̅ − S0̅ )
(10)
where H̅ and S̅ denote the molar enthalpy and entropy, respectively. The subscript 0 denotes property values at the temperature and pressure of the environment.
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EXERGY ANALYSIS AND EVALUATION METHODS Exergy analysis focuses on a system’s exergy flows, destruction, waste, and efficiency. The potential and kinetic exergies are neglected in this evaluation because of their small values compared to physical and chemical exergies. The steady-state exergy flows into and out of the WHRS are defined in eqs 1 and 2. Destructed exergy and unused exergy are defined in eqs 3
RESULTS AND DISCUSSION The simulation results are used to evaluate the exergy parameters such as exergy efficiency, exergy destruction, and exergy loss. This evaluation is prepared for two different practical conditions: normal operating condition and increasing of the C2+ component in the main feed.
Table 1. Design Specification of Heat Exchangers Located in the WHRS heat exchanger
heat duty (MW)
total area (m2)
log mean temp. diff. (°C)
overall U (W/m2·K)
E-10 E-12 E-09 E-13 E-14
10.25 6.64 28.92 5.89 15.08
304 308 2010 1996 4795
372.50 269.16 162.02 96.89 101.26
90.52 80.08 88.80 30.45 31.06
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DOI: 10.1021/ef5027983 Energy Fuels 2015, 29, 3322−3327
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Figure 2. Simulated process flow diagram of SNGR unit.
Normal Operating Condition. The SNGR unit is designed to produce syngas with 0.93 kmol/s H2 at normal condition. Mass flow rate, specific chemical and physical exergy, and total exergy flow rate of streams into and out of the WHRS are shown in Table 2. Exergy efficiency result and percent of Table 2. Specific Exergy and Exergy Flow Rate of Streams at Normal Operating Condition
stream
mass flow rate (kg/h)
specific chemical exergy (kJ/kg)
specific physical exergy (kJ/kg)
total specific exergy (kJ/kg)
total exergy flow rate (MW)
10a 11 08a 08b 6 7 04a 04b 36a 36b 37 36c 38 43 44
155491.1 155491.1 103165.2 103165.2 142591.3 142591.3 131859.0 131859.0 175005.0 175005.0 37328.0 137677.0 1767.0 158021.1 197116.1
16988.50 16988.50 25310.61 25310.61 25310.61 25310.61 46764.05 46764.05 2.61 2.61 2.61 2.61 47619.46 83.42 78.30
1081.89 1215.91 985.26 1109.83 985.26 1404.00 833.74 917.51 1.51 101.84 101.84 101.84 235.45 670.17 62.89
18070.39 18204.41 26295.87 26420.44 26295.87 26714.61 47597.79 47681.55 4.12 104.46 104.46 104.46 47854.91 753.58 141.19
780.50 786.28 753.56 757.13 1041.54 1058.13 1743.39 1746.46 0.20 5.08 1.08 3.99 23.49 33.08 7.73
Figure 3. Exergy analysis results for WHSR components at normal condition.
total destroyed exergy for heat exchangers in the WHRS are shown in Figures 3 and 4, respectively. The FH-02 in results of this work represents the auxiliary burner combined with mixing of its flue gas and reformer flue gas. A large amount of exergy is destroyed in FH-02 due to irreversibility of combustion reactions and mixing of two streams at its outlet. Also, the heat exchangers E-09 and E-14 have large exergy destruction because of large heat transfer area and high heat duty. Results of the overall exergy analysis around the WHRS are shown in Table 3. According to this analysis, exergy efficiency of the WHRS at normal operating condition is about 58%. Total exergy destroyed in this section is about 17.21 kJ/mol H2, which amounts roughly to an equivalent of 12.23 million cubic meters of natural gas losses per year. Exergy loss from the steam reforming unit exhaust stream is about 8.3 kJ/mol H2, which is quite lower than values reported by Hajjaji and co-workers,8,9
Figure 4. Percent of total destroyed exergy in WHSR components at normal condition.
who reported 19.76 kJ/mol H2 at the best condition. It is because of using more heat exchangers combined with an auxiliary burner in the WHRS and a better design of heat exchangers train based on the maximum energy recovery from the flue gas. Increase of C2+ Component in the Natural Gas Feed Stream. A common challenge is the increase of the C2+ component of the natural gas fed to the plant. At the normal design condition, the C2+ components’ mole percent in the reforming unit feed stream (stream 04a) is 0.66%. However, at 3324
DOI: 10.1021/ef5027983 Energy Fuels 2015, 29, 3322−3327
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amount of required fuel should be increased about 1.61 times of the normal operating value. This big increase of the required fuel in the reformer is due to higher endothermic reforming reactions of C2+ that occur in the prereformer and cause a big reduction of the reformer feed temperature. Heat of steam reforming reactions of C2, C3, and n-C4 is 1.82, 2.42, and 3.16 times that of the C1 reaction heat, respectively.1 Increasing the reformer fuel causes variation of the flue gas flow rate and WHRS streams temperatures. These temperature variations are shown in Figure 6. The temperatures of other streams, coming from other units, are remaining almost the same as their normal operating condition.
Table 3. Overall Exergy Analysis Results for WHRS at Normal Condition parameter
value
total exergy in (MW) total exergy out (MW) total exergy destruction (MW) total exergy loss (MW) total fuel exergy (MW) total product exergy (MW) exergy efficiency
4375.76 4359.73 16.03 7.73 56.57 32.81 0.58
the abnormal condition, this value rises to 8.5%, which is beyond the maximum design condition of the plant. The feed natural gas composition at normal condition, at the worst case (8.5% of C2+ existence), and the C2+ composition of the feed natural gas are shown in Table 4. Therefore, it is necessary to investigate the C2+ change impact on the temperatures, amount of the hydrogen produced in the reformer, and the WHRS exergy parameters. Table 4. C2+ Composition of the Main Process Feed Stream (04a) to the Reforming Unit component
normal condition state (1) (mol %)
worst condition of C2+ content state (2) (mol %)
C2+ composition (mol %)
CO2 N2 H2O CH4 C2H6 C3H8 C4H10 C5H10 total
0.71 3.84
