Energy consumption and exergy analyses of a supercritical water oxidation system with a transpiring wall reactor

Energy Conversion and Management 145 (2017) 82–92

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Energy consumption and exergy analyses of a supercritical water oxidation system with a transpiring wall reactor Fengming Zhang a,c,⇑, Boya Shen a, Chuangjian Su a, Chunyan Xu b, Jianan Ma a, Yun Xiong a, Chunyuan Ma b a Guangdong Key Laboratory of Membrane Materials and Membrane Separation, Guangzhou Institutes of Advanced Technology, Chinese Academy of Sciences, 511458 Guangzhou, China b National Engineering Laboratory for Coal-fired Pollutants Emission Reduction, Shandong University, 250061 Jinan, China c Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, 518055 Shenzhen, China

a r t i c l e

i n f o

Article history: Received 6 February 2017 Received in revised form 16 April 2017 Accepted 25 April 2017

Keywords: Supercritical water oxidation Energy consumption Exergy Transpiring wall reactor Aspen plus Optimization

a b s t r a c t A supercritical water oxidation system with a transpiring wall reactor was simulated by Aspen Plus, and simulation model was validated by comparisons with experimental reactor outlet temperatures and product properties (total organic carbon and CO). Energy and exergy analyses were conducted to reduce energy consumption and exergy loss of the system. It is indicated that the system’s energy efficiency and exergy efficiency are 97.73% and 13.28% at typical operating conditions, respectively. The exergy loss of electric heater, heat exchanger and reactor account for 39.89%, 26.64%, and 17.23% of the system’s total exergy loss, respectively. The process optimization is conducted by preheating the middle branch of transpiring water with the heat of the reaction products to reduce energy consumption, and the net electric cost is reduced from 7.43 ¥/h to 5.96 ¥/h. It can be observed that when the split coefficient for split 2 equals to 0.6, the minimum electricity input is required for the system. When the feed concentration is increased from 2 wt.% to 10 wt.%, net electric cost per COD significantly decreases from 14.05 ¥/kg to 1.31 ¥/kg, which indicates that higher feed concentrations are beneficial for reducing the cost of energy consumption. What is more, when feed flow rate increases from 6 kg/h to 16 kg/h, net electric cost per COD increases from 3.21 ¥/kg to 4.61¥/kg, which shows that higher energy consumption costs will be required at higher feed flow rates. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Treating high concentration and refractory organic wastewater is a tough problem. Supercritical water oxidation (SCWO) uses the excellent properties of supercritical water (SCW) to achieve rapid and complete degradation of organic waste, and it has been proven to be a promising technology to treat organic waste [1–3]. SCW refers to water under high temperature and high pressure (P > 22.1 MPa, T > 647 K), which acts as a non-polar solvent with high diffusivity and excellent transport properties [4,5]. When pressurizing as well as heating waste water to its supercritical state, organic compounds can be fully oxidized within a singlephase mixture and become non-toxic products, such as CO2, H2O,

⇑ Corresponding author at: Guangdong Key Laboratory of Membrane Materials and Membrane Separation, Guangzhou Institutes of Advanced Technology, Chinese Academy of Sciences, 511458 Guangzhou, China. E-mail address: [email protected] (F. Zhang). http://dx.doi.org/10.1016/j.enconman.2017.04.082 0196-8904/Ó 2017 Elsevier Ltd. All rights reserved.

etc [6,7]. Only a residence time of a few seconds to 1 min is required to fully destroy the organic compounds under a fast reaction rate, and thus the reactor requires a very small volume. Although SCWO has plenty of unique advantages in treating wastewater, some technical problems such as corrosion, salt plugging have blocked its development for many years. The inorganic acid (such as HCl, H2SO4, etc.) combining with high temperature and high concentration of oxygen can cause severe corrosion of the reactor and other devices [8]. The inorganic salt is hardly soluble in supercritical water, and thus leading to the plugging of the reactor, as well as the preheating and cooling section [9]. For now, an effective solution to solve both corrosion and salt plugging is the usage of a transpiring wall reactor [10,11]. The transpiring wall reactor usually consists of a dual shell with an outer pressure-resistant vessel and an inner porous tube. Transpiring water at subcritical temperatures passes through the porous pipe to form a protective film on its inner surface. This water film can prevent the reactants spreading to the porous wall and can dissolve some of the salt, thus reducing corrosion and avoiding salt

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Nomenclature Abbreviations COD chemical oxygen demand CWR cooling water revenue, ¥/h EC electrical cost, ¥/h EH1 electric heater 1 EH2 electric heater 2 EH3 electric heater 3 EI electricity input, kW ex specific exergy, kW/kg F mass flow rate, kg/h F⁄cw the modified value for Fcw, kg/h FINAL final products FLASH flash drum g gravitational acceleration, m/s2 h specific enthalpy, kJ/kg h0 specific enthalpy at reference state, kJ/kg HE1 heat exchanger 1 HE2 heat exchanger 2 HE3 heat exchanger 3 HE4 heat exchanger 4 M molar mass, kg/mol M1 mixer 1 M2 mixer 2 M3 mixer 3 M4 mixer 4 NEC difference between electric cost and cooling water revenue, ¥/h NECPC net energy cost per COD, ¥/kg P pressure, MPa P1 pump 1 P2 pump 2 P3 air compressor PLUG plug flow reactor qr specific reaction heat, kW/kg Qr reaction heat, kW Q energy, kW Qu recovered energy, kW Qall refers to the system’s overall energy input, kW r reaction rate R transpiration intensity; universal gas constant, 8.3145 kJ/mol

plugging [12]. Large quantities of researches certify that the transpiring wall reactor has an effective role in resisting corrosion and salt plugging [10–12]. Pressurization and heating are the essential steps for the SCWO process, and thus the requirement of energy consumption is considerably high. Energy recovery from the effluent is the leading method to reducing energy consumption of the system. It has been reported that autothermal operation can be achieved when feed concentration reaches 2 wt.% [13]. What is more, many theoretical studies suggest that power generation is an efficient solution for energy recovery [14–18]. However, we should remind that large amounts of transpiring water at relatively lower temperatures (20–350 °C) will be injected into the reactor to protect the transpiring wall, and the temperature of reactor effluent ranging from 300 °C to 350 °C is much lower than that of the traditional SCWO process, which ranges from 400 °C to 650 °C [19]. Therefore, lower-grade energy of the reactor effluent makes power generation or autothermal operation not applicable for a supercritical water oxidation system with a transpiring wall reactor, and a cascade

4H0f SCW SCWO s s0 t tw tw1 tw2 tw3 T TOC v z W

standard enthalpy of formation, kJ/mol supercritical water supercritical water oxidation specific entropy, kJ/(kg K) specific entropy at reference state, kJ/(K kg) time, s; temperature °C transpiring water the upper branch of transpiring water the middle branch of transpiring water the lower branch of transpiring water temperature, K total organic carbon velocity, m/s elevation, m work, electricity, kW

Greek letters x the concentration of methanol g energy efficiency w exergy efficiency n exergy loss coefficient Subscripts 0 environmental state cw cooling water cold cold stream hot hot stream in inlet ox oxygen out outlet p pressurization system r reaction split2-1 the split coefficient for the first stream in split 2 split2-2 the split coefficient for the second stream in split 2 w organic waste water Superscripts ch chemical ph physical

utilization of heat energy recovery will be more effective and feasible [19]. Besides, an exergy analysis is frequently used in the process optimization to reduce energy consumption [20,21]. However, few publications have focused on a SCWO system [6,22,23], and no publication focuses on a SCWO system with a transpiring wall reactor yet. In this paper, a supercritical water oxidation system with a transpiring wall reactor is simulated by Aspen Plus. Experimental data are provided to compare with simulation results. Firstly, exergy analysis of the system is conducted to obtain the exergy loss distribution. Then a process optimization is performed to reduce energy consumption. What is more, the influences of operating parameters (such as split coefficient, feed concentration, and feed flow rate) on the cost of energy consumption are analyzed. 2. Experimental setup During early studies, a SCWO system (Fig. 1a) with a transpiring wall reactor was built and successfully operated, and plenty of

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F. Zhang et al. / Energy Conversion and Management 145 (2017) 82–92

Fig. 1. The diagram of the SCWO pilot plant with a transpiring wall reactor (a), and the scheme of the transpiring wall reactor (b).

experiments were conducted [19,24–26]. Oxygen was used as the oxidant in the experiment. Desalinated water was used as transpiring water and a desalinated water-methanol mixture was used as artificial wastewater which is called as ‘‘feed” in this paper. Five streams were introduced into the reactor, feed and oxygen were injected from the top of the reactor. The transpiring water (tw) was divided into three branches, namely, the upper branch of transpiring water (tw1), the middle branch of transpiring water (tw2), and the lower branch of transpiring water (tw3). These three branches were injected from the side of the reactor. The reactor effluent was cooled and introduced into a gas-liquid separator. The aqueous effluent was analyzed by a TOC analyzer (Shimadzu, TOC 5000A), and the gaseous effluent was analyzed by a gas chromatograph (Agilent 6890 GC). The transpiring wall reactor consists of the outer vessel and the inner porous tube (Fig. 1b). The inner and outer diameters of the outer vessel are 80 and 114 mm, respectively. And the inner and outer diameters of the inner porous tube are 55 and 60 mm, respectively. The transpiring wall reactor has an effective reaction volume of about 1.8 L [14]. The temperatures of each stream flowing into the reactor and reactor outlet were measured by PT100 resistance thermometer. Oxygen and tw3 were fed into the reactor without preheating process. Detailed measuring points can be seen in Fig. 1a. In order to maximize energy recovery and reduce energy consumption, cascade utilization of thermal energy was designed in this SCWO system, and three tubular countercurrent heat exchangers were arranged within the system to recover heat from reactor effluent. Base on the previous energy balance calculation [25], the reactor effluent was initially split into two streams: one was used to preheat feed stream in heat exchanger 1 (HE1), the other one was used to preheat transpiring water in heat exchanger 2 (HE2). The remaining energy in the reaction products can be recovered by heat exchanger 3 (HE3) to supply hot water externally. These three heat exchangers had the same configuration, but their lengths were different: 30 m for EH1 and EH2, and 20 m for EH3. The central tube of the heat exchanger had internal and external diameters of 5 and 10 mm, respectively. The outer tube had internal and external diameters of 13 and 19 mm, respectively. Further

details about the SCWO system and analytical method can be found elsewhere [19,24–26]. 3. Methodology 3.1. Reaction kinetics A desalinated water-methanol mixture is also used as the artificial wastewater in Aspen Plus. It has been proven by our previous experimental results [24,26] and other publications [27,28] that CO is the major intermediate during supercritical water oxidation of methanol, thus a two-step mechanism based on Arrhenius law is created and implemented in the simulation as Eqs. (1)–(4).

CH3 OH þ O2 ¼ CO þ H2 O

ð1Þ

CO þ 0:5O2 ¼ CO2

ð2Þ

rCH3 OH ¼ 

d½CH3 OH dt

¼ 2:0  1021  exp

rCO ¼ 


 303:85 kJ=mol ½CH3 OH RT


 d½CO 88 kJ=mol ¼ 3:16  106  exp ½CO dt RT

ð3Þ

ð4Þ

where d[CH3OH]/dt is the reaction rate of methanol, d[CO]/dt is the reaction rate of CO, R is universal gas constant, 8.3145 kJ mol1, T is the temperature (K). 3.2. Transpiring wall reactor The transpiring wall reactor is the most important equipment of the system, and it can be separated into three sections: mixing section, adiabatic reacting section and cooling section according to its reaction and flow characteristics [12]. However, it is too complicated to directly draw up the heat and mass transfer process in the reactor, so a simplified model is proposed to simulate the transpiring wall reactor (Fig. 2a). Based on the structure size of the

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85

Fig. 2. Aspen Plus model for transpiring wall reactor (a) and SCWO process (b).

reactor as shown in Fig. 1b, mixing section with a length of 100 mm provides a sufficient mixing space for reactants. Among the three branches of transpiring water, the upper branch is the only one which can directly influence the reaction [19], and thus the feed, oxygen and the upper branch of transpiring water firstly flow into a mixer to get fully mixed as a simplification. The adiabatic reacting section is simulated by a plug flow reactor (PLUG) with an effective length of approximately 275 mm long based on experimental results [24,26]. When reaction finishes, the product flows into cooling section, which has a length of 375 mm in the lower part of the reactor. This is the place where the middle branch as well as the lower branch of transpiring water is injected into the reactor sequentially, and two mixers are used to simulate the mixing process. Finally, the hot effluent flows out of the reactor. The highest temperature of the system mainly concentrates in the reactor, and thus reactor usually contributes the largest amount of heat dissipation [19]. In order to simulate the heat loss, the heat transfer coefficient of PLUG is set to be 3 W/m2 according to the experimental data of reactor thermal insulation material, and heat loss is usually less than 5% for most conditions.

3.3. Process flow The simulation process is presented in Fig. 2b. After the feed is pressurized by pump 1 (P1), it firstly flows into HE1 to be heated up by one branch of the final products (FINAL), and then flows into electric heater 1 (EH1) to be further heated. At the same time, oxygen is pressurized by the air compressor (P3) and then flows into mixer 1 (M1) to fully mix with the feed as well as the tw1. After the transpiring water (tw) is pressurized by pump 2 (P2), it splits into three branches (tw1, tw2, and tw3). Before tw1 reaches M1, it firstly flows into HE2 to be preheated and then flows to electric heater 2 (EH2) to be further heated. tw2 is heated by electric heater 3 (EH3) and then it mixes with the effluent in mixer 2 (M2). tw3 mixes with the effluent in mixer 3 (M3) to form the final products (FINAL). FINAL is split into two branches in split 2 and they are treated as hot streams to preheat the feed and tw1. After that, the two branches of FINAL reunite in mixer 4 (M4) and are cooled down to a temperature between 40 °C and 60 °C by exchanging heat with cooling water in HE3. Besides, the heated cooling water that has a temperature higher than 60 °C can be used as domestic hot water.

Finally, the cooled product is separated into gas and liquid in the flash drum (FLASH) before being discharged. 3.4. Other consideration The property method of fluid is set to be PRMHV2, which is commonly used for multiple interactions at high temperature and high pressure. The efficiencies for P1, P2, and P3 are set to be 0.9, 0.9, and 0.75, respectively. The operating pressure of the system is set to be 23 MPa. 4. Parameters definition 4.1. Energy efficiency System’s overall energy efficiency is defined as follows.

gall ¼ ¼

Qu Q all F cw ðhHE3;cold;out  hHE3;cold;in Þ Q w þ Q tw þ Q ox þ Q r þ Q cw þ W P1 þ W P2 þ W P3 þ W EH1 þ W EH2 þ W EH3 ð5Þ

In SCWO system, recovered energy (Qu) is the thermal energy passed from the FINAL to cooling water within HE3, and Qall is the system’s overall energy input. Enthalpies carried by feed (Qw), oxygen (Qox), transpiring water (Qtw) and cooling water (Qcw) are quite small due to their temperature and pressure at normal state and thus can be neglected. Therefore, energy input for the system mainly includes thermal energy released during oxidation reaction (Qr), electricity consumed by P1 (WP1), P2 (WP2), P3 (WP3), EH1 (WEH1), EH2 (WEH2), and EH3 (WEH3). Since mixer, heat exchanger and electric heater are assumed to be adiabatic in the simulation, energy loss is mainly caused by heat dissipation of the reactor. Its energy efficiency can be calculated by Eq. (6).

gPLUG ¼

ðF w þ F ox þ F tw1 ÞhPLUG;out ðF w þ F ox þ F tw1 ÞhM1;out þ Q r

ð6Þ

where Fw, Fox, and Ftw1 are the mass flow of feed, oxygen, and tw1, respectively. The thermal energy released during methanol oxidation (Qr) can be calculated by Hess’s law, which is shown as follows.

Q r ¼ F w xqr

ð7Þ

86

qr ¼

F. Zhang et al. / Energy Conversion and Management 145 (2017) 82–92

i 1h ðDH0f ÞCO þ 2ðDH0f ÞH O  ðDH0f ÞCH O  1:5ðDH0f ÞO 2 2 4 2 M

ð8Þ

where x is the concentration of methanol in the feed, M is the

There are three electric heaters in the system, and the exergy efficiency of EH1, EH2, and EH3 can be calculated by Eqs. (18)– (20), respectively.

molar mass of CH3OH, and DH0f is standard enthalpy of formation.

wEH1 ¼

  ph F w exph EH1;out  exEH1;in W EH1

4.2. Exergy analysis Exergy refers to the maximum useful work that can be obtained from a system. The exergy of a flow at a specific state consists of two parts: physical exergy and chemical exergy. Physical exergy can be calculated by Eq. (9).

exph ¼ ðh  h0 Þ  T 0 ðs  s0 Þ þ

v2 2

þ gz

ð9Þ

wEH2 ¼

wEH3 ¼

  ph F tw1 exph EH2;out  exEH2;in W EH2   ph F tw2 exph EH3;out  exEH3;in W EH3

ð18Þ

ð19Þ

ð20Þ

For our SCWO system, the specific kinetic exergy (v2/2) and specific potential exergy (gz) of the fluid are equal to zero. The temperature (T0) and pressure (P0) of reference environment are 288.15 K and 0.1 MPa, respectively. The chemical exergy (exch) for a fluid is defined with respect to the true dead state, which considers the composition of chemicals at the reference environment [29,30]. The values of chemical exergies for reactants as well as products are taken from the literature [31]: methanol (718.0 kJ/mol, liquid); oxygen (3.97 kJ/kg, gas); carbon dioxide (19.6 kJ/kg, gas); water (0.77 kJ/kg, liquid). The change in chemical exergy due to methanol oxidation of can be calculated by Eq. (10).

The exergy efficiency for the PLUG is calculated by using Eq. (21).

 1  ch ch ch exch ¼ exCH3 OH þ 1:5exch O2  exCO2  2exH4 O M

wM2 ¼

ð10Þ

Similar to energy efficiency, the overall exergy efficiency of the system is calculated by Eq. (11).

wall ¼

  ph F cw exph HE4;cold;out  exHE4;cold;in

F w xexch þ W P1 þ W P2 þ W P3 þ W EH1 þ W EH2 þ W EH3

wP2 ¼

wP3 ¼

ph F w ðexph P1;out  exP1;in Þ

ð12Þ

W P1 ph F tw ðexph P2;out  exP2;in Þ

ð13Þ

W P2 F ox ðexph P3;out



wHE2 ¼

wHE3

wM1 ¼

wM3 ¼

F tw1 exph HE2;cold;out

ðF w þ F ox þ F tw1 Þexph M1;out F w exph EH1;out

wM4 ¼

ph þ F ox exph P1;out þ F tw1 exEH2;out

ðF w þ F ox þ F tw1 þ F tw2 Þexph M2;out ph ðF w þ F ox þ F tw1 Þexph PLUG;out þ F tw2 exEH3;out

ðF w þ F ox þ F tw1 þ F tw2 þ F tw3 Þexph M3;out ph ðF w þ F ox þ F tw1 þ F tw2 Þexph M2;out þ F tw3 exP2;out

F FINAL exph M4;out F Split21 exph HE2;hot;out

þ F Split22 exph HE1;hot;out

ð22Þ

ð23Þ

ð24Þ

ð25Þ

There is no change in physical state when fluid flows through Split 1 and Split 2, thus exergy loss can be ignored for these two devices. When mixture finally flows into FLASH, gas and liquid are separated and discharged. The exergy input for FLASH equals to its exergy loss. Exergy loss coefficient which can show the exergy loss distribution of the system is defined as the exergy loss of each device divided by the overall exergy input of the system as follows.

exergy loss of each dev ice exergy input of the system

ð26Þ

4.3. Cost analysis of energy consumption

  ph F w exph HE1;cold;out  exHE1;cold;in   ¼ ph F Split22 exph HE1;hot;in  exHE1;hot;out 

ð21Þ

ð14Þ

W P3

There are three heat exchangers in the system, the exergy efficiency of HE1, HE2, and HE3 can be calculated by Eqs. (15)–(17), respectively.

wHE1

ch ðF w þ F ox þ F tw1 Þexph M1;out þ F w xex

The exergy efficiency of M1, M2, M3 and M4 can be calculated by Eqs. (22)–(25), respectively.

n¼ exph P3;in Þ

ðF w þ F ox þ F tw1 Þexph PLUG;out

ð11Þ

In this process, pressurization system contains three parts: P1, P2 and P3. All of them are aimed to raise the pressure of fluids to 23 MPa, and their exergy input refer to the electricity input. The exergy efficiency of P1, P2, and P3 can be calculated by Eqs. (12)–(14), respectively.

wP1 ¼

wPLUG ¼

exph HE2;cold;in

ð15Þ



   ph F Split21 exHE2;hot;in  exph HE2;hot;out

  ph F cw exph HE3;cold;out  exHE3;cold;in   ¼ ph F FINAL exph HE3;hot;in  exHE3;hot;out

ð16Þ

ð17Þ

The cooling water coming out of HE4 usually has a temperature higher than 60 °C, and it can bring some revenue when used as domestic hot water. Therefore, net electric cost (NEC) is the difference between electric cost (EC) and cooling water revenue (CWR), which can be calculated by the following Eqs. (27)–(29).

NEC ¼ EC  CWR

ð27Þ

EC ¼ 0:886EI

ð28Þ

EI ¼ W P1 þ W P2 þ W P3 þ W EH1 þ W EH2 þ W EH3

ð29Þ

In above equations, the unit price for industrial electricity in Guangdong Province of China is 0.886 ¥/kW h [32], and electricity input (EI) includes the total amount of the electricity consumed by pumps, compressor, and electric heaters.

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F. Zhang et al. / Energy Conversion and Management 145 (2017) 82–92

The hot water temperature ranges from 60 °C to 100 °C at different operating conditions. In order to calculate the cooling water revenue more accurately, the hot water flow rate is converted to a modified value as shown in Eq. (30).

F cw ¼ F cw ð0:025t cw  0:502Þ

ð30Þ

This equation is calculated based on the relationship between enthalpy for water and temperatures, which is obtained from NIST database [33], detailed information about the modified flow for cooling water can be found in the supplementary document [Fig. S1, and Eqs. (S1–S4)]. And thus cooling water revenue is calculated as follows:

CWR ¼

17F cw ¼ 0:017F cw 1000

ð31Þ

where 17 refers to the unit price of domestic hot water (¥/t) which has a temperature of 60 °C [32]. Since the treatment capacity of organic wastewater can be characterized by total COD, net electric cost per COD (NECPC) is used as the criteria to compare the system’s energy consumption cost at different operating conditions.

NECPC ¼

ECC EC  CWR ¼ F COD F COD

ð32Þ

where FCOD is the mass flow of COD, which can be calculated by Eq. (33) according to Eqs. (1) and (2).

F COD ¼ 1:5F w x

ð33Þ

5. Results and discussion 5.1. Model validation A comparison between experimental data and simulation results is summarized in Table 1. When increasing the feed temperature from 369 °C to 419 °C and keeping other parameters constant, the temperature of FINAL calculated by simulation increases from 336 °C to 352.6 °C. At the same time, the reactor outlet temperature increases from 286 °C to 299 °C in the experiment. The temperature calculated by simulation is approximately 50 °C higher than the temperature in the experiment. Besides, when increasing the temperature of tw1 from 200 °C to 340 °C, the temperature of FINAL increases from 315.5 °C to 351.6 °C in the simulation, and the reactor outlet temperature in the experiment increases from 236 °C to 293 °C. The comparison shows that the trends of the reactor outlet temperatures at different conditions between simulated and experimental results are consistent. Although the temperature calculated by simulation is higher than the temperature shown in the experiment, it is actually quite reasonable when considering heat dissipation of all devices.

Because of ideal mixing and underestimation of heat loss in the simulation, the reactor outlet concentrations of TOC are shown to be zero, which are usually lower than the experimental ones. And comparing the simulation results with their corresponding experimental data at different feed temperatures (Table 1, A1A4), it is obvious to see that CO concentration in final products is lower than 0.5%. When the temperature of feed exceeds 390 °C, the concentrations of TOC and CO change very slowly. When the temperature of tw1 reaches 340 °C, the concentration of TOC and CO will be negligible (Table 1, B1-B4). So the temperatures of feed and tw1 are set to be 400 °C and 350 °C in the following simulation, respectively, in order to ensure the full feed degradation. 5.2. Base case In order to reduce energy consumption and exergy loss of the system, energy and exergy analysis at typical operating conditions (Table 1, A3) are performed and results are summarized in Table 2A. The system’s overall energy efficiency and exergy efficiency are 97.73% and 13.28%, respectively. The high energy efficiency is due to the ideal hypothesis in the simulation, and the low exergy efficiency is due to the irreversibility of energy conversion and transmission in the SCWO process. Besides, the energy imported into the system belongs to high-grade energy (work, electric energy, and chemical energy), but they are all converted into low-grade thermal energy in reacting fluid and cooling water. In order to improve the system’s energy consumption, exergy loss of each piece of equipment is calculated and shown in Fig. 3. Exergy loss coefficient of reactor is the sum of PLUG, M1, M2, and M3 according to model simplification. It is observable that heat exchanger, electric heater and reactor contribute to the main exergy loss of the system. For three heat exchangers, the sum of their exergy loss accounts for 26.64% of the system’s overall exergy input. Their exergy efficiencies are around 50% and their exergy loss is mainly caused by large temperature difference heat exchange between cold and hot streams. For three electric heaters, their exergy loss accounts for 39.89% of the overall exergy input. The exergy efficiency for EH1 as well as EH2 reaches 50%, but that of EH3 is only around 30% due to the conversion of electric energy to thermal energy at low temperature. The exergy loss caused by reactor accounts for 17.23% of the overall exergy input and it mainly contains two parts, the first part is due to the transformation of chemical energy into thermal energy during oxidation reaction. Besides, when mixing two kinds of liquids, their temperature difference as well as strong friction causes the second part of irreversible exergy loss. The reactor’s inlet temperatures for all streams are usually kept constant in the simulation to ensure the water film protection and feed degradation based on previous studies [12,24], and thus improvement for reactor is not the focus in this paper. However, some improvements still can be done to lower the exergy loss

Table 1 Operating parameters and results for experiments and simulation. NO. x/wt.% Fw/kg h1 Ftw1/kg h1 Ftw2/kg h1 Ftw3/kg h1 Tw/°C Ttw1/°C Ttw2/°C TOCout,sim/ppma TOCout,exp/ppmb COout,sim/% COout,exp/% Tout,sim/°C Tout,exp/°C A1 A2 A3 A4 B1 B2 B3 B4 a b

6 6 6 6 6 6 6 6

10 9.9 9.9 9.9 9.2 9.1 9 9.1

19.33 19.28 19.65 18.83 14.55 14.59 14.03 14.17

6.44 6.43 6.55 6.28 4.85 4.86 4.68 4.72

12.89 12.85 13.1 12.55 9.7 9.73 9.35 9.45

369 390 410 419 429 428 428 429

363 363 363 362 200 240 285 340

269 271 268 272 251 250 249 254

0 0 0 0 0 0 0 0

34.3 17.7 17.3 17.0 88.6 77 63 28.8

0 0 0 0 0.44 1.11E-03 0 0

0.24 0.14 0.15 0.05 0.28 0.12 0.03 0.04

336 346.2 348.4 351.6 315.5 324.1 337.5 351.6

TOCout,sim, COout,sim, and Tout,sim represent the TOC concentration, CO concentration, and temperature of reactor outlet in the simulation, respectively. TOCout,exp, COout,exp, and Tout,exp represent the TOC concentration, CO concentration, and temperature of reactor outlet in the experiment, respectively.

286 294 297 299 236 256 272 293

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F. Zhang et al. / Energy Conversion and Management 145 (2017) 82–92

Table 2 Comparison of energy and exergy analyses between base case and optimized case without EH3.

A

B

a b

c

a

Energy input/kW Exergy input/kW W/% g/%c Exergy loss/kW

Energy input/kW Exergy input/kW W/% g/% Exergy loss/kW

P1

P2

P3

HE1

HE2

HE3

HE4

EH1

EH2

EH3

M1

PLUG

M2

M3

M4

FLASH

Overall

0.11 0.11 66.97 88.61 0.04

0.44 0.44 68.23 89.88 0.14

0.37 0.37 71.44 0.55 0.12

2.77 1.38 57.72 100 0.58

5.12 2.52 55.92 99.97 1.11

14.79 4.43 46.11 100 2.39

– – – – –

4.43 4.43 53.45 100 2.06

4.55 4.55 50.53 100 2.25

1.92 1.92 30.41 100 1.34

17 9.29 97.21 100 0.19

18.12 9.1 90.11 98.98 0.91

18.96 8.31 96.46 100 0.58

19.21 7.89 85.61 100 1.02

16.26 4.76 99.79 100 0.01

2.71 0.52 0 0 0.52

15.29 15.44 13.28 97.73 13.26

P1

P2

P3

HE1

HE2

HE3

HE4b

EH1

EH2

EH3

M1

PLUG

M2

M3

M4

FLASH

Overall

0.11 0.11 66.97 88.61 0.04

0.44 0.44 68.23 89.88 0.14

0.37 0.37 71.44 0.55 0.12

2.68 1.36 57.63 99.98 0.58

4.94 2.47 56.04 100 1.09

12.36 3.61 46.12 99.89 1.95

1.55 0.7 59.13 100 0.29

4.46 4.46 53.38 100 2.08

4.61 4.61 50.41 100 2.28

– – – – –

17 9.29 97.94 100 0.19

18.12 9.1 90.0 99.44 0.91

18.96 8.31 88.79 100 0.58

19.21 7.89 93.5 1005 1.01

16.01 4.64 58.28 100 0

2.56 0.51 0 0 0.51

13.46 13.61 12.18 98.74 11.77

The energy input is as same as the exergy input for pumps, electric heaters, and energy input and energy. The exergy efficiency of HE4 can refer to Eqs. (15)–(17). The energy efficiencies for heat exchanger, electric heater, and mixer are equal to nearly 100% for the ideal condition.

4.10¥/h after the optimization, system’s NEC is still reduced to 5.96¥/h, which is 1.47 ¥/h cheaper than that of the old version. Thus the NECPC decreases from 6.63¥/kg to 4.99¥/kg. It can be concluded that the optimized process is more energy saving and economical than the old one, and thus the optimized process is used for further studies. 5.4. Effect of the operating parameters

Fig. 3. Exergy loss distribution of the system in the base case.

and energy consumption of the system. Heaters are the easiest ones to be manipulated, and their exergy loss can directly be reduced by decreasing the electrical input. This can be achieved by adjusting the split coefficients of SPLIT 2 to change the outlet temperature of feed in HE1 and the outlet temperature of tw1 in HE2. Details discussion will be shown in Section 5.4.1. Besides, it is found that the reaction effluents’ temperature before HE3 is usually above 250 °C, which is much higher than the setting temperature for EH3. This process is considered to be optimized by preheating tw2 with the heat of the reaction products, and thus EH3 can be deleted. This hypothesis is tested in Section 5.3. 5.3. Process optimization In this section, the process is optimized by preheating tw2 with the heat of the reaction products as shown in Fig. 4. Energy as well as exergy analysis is performed, and results are also summarized in Table 2B to compare with the base case (Table 2A). It can be seen the overall energy efficiency and exergy efficiency for the optimized process are 98.74% and 12.18%, respectively, and it doesn’t change much comparing with the former one. However, the exergy input and exergy loss are significantly lowered from 15.44 kW and 13.26 kW to 13.61 kW and 11.77 kW, respectively. Energy consumption analysis is also performed for both processes and results are plotted in Fig. 5. It is observable that the optimized process is more energy saving than the previous one. The EI for the optimized process is 9.98 kW, which is 1.84 kW lower than that of the old version. Correspondingly, EC decreases from 10.47 ¥/h to 8.59 ¥/h. Although CWR slightly decreases from 4.51¥/h to

5.4.1. Split coefficient As shown in Fig. 4, the reactor effluent is divided into two streams (SPLIT2-1 and SPLIT2-2) by SPLIT 2 to preheat tw1 and feed, respectively. The split coefficient SPLIT2-1 has a direct influence on the outlet temperatures of feed and tw1 in the heat exchangers, thus indirectly affects the electricity input of EH1 as well as EH2. The split coefficient SPLIT2-1 is tested in this section with operating conditions listed in Table 3(A1-A7). Energetic analysis is performed and the results are summarized in Fig. 6 and Table S1(A1–A7) in the supplementary document. It is observable that system’s overall energy efficiency and exergy efficiency are around 95% and 10.5% (Fig. 6b), respectively. The reaction heat (Qr) is fixed at 4.3 kW with constant feed flow and concentration. When the split coefficient SPLIT2-1 is increased from 0.2 to 0.8, more of the hot reactor effluent flows to HE2 to preheat tw1, thus WEH1 increases from 4.1 kW to 4.99 kW, and WEH2 decreases from 5.71 kW to 3.66 kW (Fig. 6a). System’s EI firstly decreases and then increases when the SPLIT2-1 is increased from 0.2 to 0.8 as a superposition of WEH1 and WEH2. And EI reaches its smallest value (9.01 kW) when SPLIT2-1 equals to 0.6, which is 1.49 kW lower than the result at SPLIT2-1 equals to 0.2. Besides, CWR and NECPC have a same tread with EI when the SPLIT2-1 is increased from 0.2 to 0.8 (Fig. 6c and d), and the NECPC reaches its smallest value (3.44¥/kg) when SPLIT2-1 equals to 0.6. Thus the optimized value for the split coefficient SPLIT2-1 is 0.6, which is kept constant in the following studies. 5.4.2. Feed concentration Feed concentration between 2 wt.% and 10 wt.% are tested in this section with operating conditions and detailed results listed in Table 3(B1-B5) and Table S1(B1–B5) in the supplementary document, respectively. When feed concentration increases, both of oxygen and transpiring water flow rates will also be increased for feed degradation and reactor protection. As shown in Fig. 7a, when feed concentration increases from 2 wt.% to 10 wt.%, Qr significantly increases from 1.16 kW to 5.84 kW, electrical input for WP1 + WP2 + WP3 increases from 0.58 kW to 1.02 kW for the increase of oxygen and transpiring water flow, WEH1 decreases

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Fig. 4. The optimized energy recovery process by deleting EH3.

Fig. 5. Energy consumption comparison between the base case and the optimized case without EH3.

from 4.69 kW to 3.85 kW for more energy recovery from Qr, but WEH2 shows a firstly increasing and then a decreasing trend because of the above two effects (Fig. 7a). And thus, the system’s EI changes very little when feed concentration increases from 2 wt.% to 10 wt.%. A highest EI of 9.11 kW is present when feed concentration equals to 6 wt.%. System’s overall energy efficiency increases from 89.42% to 93.77% and its exergy efficiency slightly increases from 10.33% to 11.44% (Fig. 7b), and it indicates that higher energy and exergy efficiencies are present at higher feed concentrations. When feed concentration increases from 2 wt.% to 10 wt.%, EC slightly decreases from 8.04 ¥/h to 7.40 ¥/h, but CWR significantly increases from 3.82 ¥/h to 5.44 ¥/h, thus NEC decreases from 4.22 ¥/h to 1.96 ¥/h (Fig. 7c). On the other hand, COD treatment capacity will be increased with increasing feed concentration. Therefore, NECPC decreases significantly from 14.05 ¥/kg to 1.31 ¥/kg when feed concentration increases from 2 wt.% to 10 wt.%. We should remind that chemical energy in the wastewater is costless, and more chemical energy will be provided at higher feed concentrations, which are quite beneficial to reducing the cost of energy consumption.

Table 3 Operating parameters for studying split coefficient, feed concentration, and feed flow rate.

a b

c

No.a

Fw/kg h1

Fox/kg h1

x/wt.%

FCOD/kg h1

Ftw1/kg h1b

Ftw2/kg h1

Ftw3/kg h1

Fcw/kg h1c

Split coefficient

Tw/°C

Ttw1/°C

A1 A2 A3 A4 A5 A6 A7 B1 B2 B3 B4 B5 C1 C2 C3 C4 C5 C6

10 10 10 10 10 10 10 10 10 10 10 10 6 8 10 12 14 16

1.35 1.35 1.35 1.35 1.35 1.35 1.35 0.45 0.9 1.35 1.8 2.25 0.81 1.08 1.35 1.62 1.89 2.16

6 6 6 6 6 6 6 2 4 6 8 10 6 6 6 6 6 6

0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.3 0.6 0.9 1.2 1.5 0.54 0.72 0.90 1.08 1.26 1.44

18.57 18.57 18.57 18.57 18.57 18.57 18.57 17.10 17.84 18.57 19.31 20.05 11.14 14.86 18.57 22.29 26.00 29.72

6.19 6.19 6.19 6.19 6.19 6.19 6.19 5.70 5.95 6.19 6.44 6.69 3.71 4.95 6.19 7.43 8.67 9.91

12.38 12.38 12.38 12.38 12.38 12.38 12.38 11.40 11.89 12.38 12.87 13.46 7.43 9.91 12.38 14.86 17.33 19.81

150 150 150 150 150 150 150 150 150 150 150 150 150 150 150 200 300 300

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6

400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400

350 350 350 350 350 350 350 350 350 350 350 350 350 350 350 350 350 350

A5, B3, and C3 are the same operating condition. Transpiring water flow are calculated by transpiring intensity which is consistent with the experimental conditions [19]. Fcw is adjusted to keep hot water ranges between 60 °C and 100 °C.

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Fig. 6. The effect of split coefficient on energy input (a), system efficiency (b), electric cost (c), and net electric cost per COD (d).

Fig. 7. The effect of feed concentration on energy input (a), system efficiency (b), electric cost (c), and net electric cost per COD (d).

5.4.3. Feed flow rate System’s performance at different feed flow rates is discussed in this section with operating parameters and detailed results listed in Tables 3(C1-C5) and S1(C1-C6) in the supplementary document,

respectively. Flow rate of cooling water injected into HE4 is set to be 150 kg/h for C1-C3, 200 kg/h for C4 and 300 kg/h for C5, respectively to adjust output temperature of the cooling water. According to Fig. 8a, when increasing the feed flow rate from 6 kg/h to 16 kg/

F. Zhang et al. / Energy Conversion and Management 145 (2017) 82–92

91

Fig. 8. The effect of feed flow on energy input (a), system efficiency (b), electric cost (c), and net electric cost per COD (d).

h, electricity input required by pumps (WP1 + WP2 + WP3) and heaters (WEH1 + WEH2) grow sustainably from 0.49 kW and 4.75 kW to 1.28 kW and 14.05 kW, respectively. This is due to the increase in oxygen input and transpiring water input with increasing feed flow. And thus system’s EI significantly increases from 5.24 kW to 15.33 kW. Besides, Qr increases from 2.1 kW to 5.78 kW when the feed flow rate is increased from 6 kg/h to 16 kg/h. System’s overall energy efficiency shows a continuously decreasing trend since more heat is dissipated in the reactor (Fig. 8b). When feed flow rate increases from 6 kg/h to 16 kg/h, CWR increases from 2.90 ¥/h to 6.92 ¥/h, EC increases from 4.63 ¥/h to 13.56¥/h, NEC increases from 1.73¥/h to 6.64¥/h, and thus NECPC increases from 3.21 ¥/kg to 4.61¥/kg. So in conclusion, higher energy consumption cost will be needed at higher feed flow rates, and lower feed flow rate is recommended for this system.

but the optimized process can lower the electricity input, which economically reduces its NEC from 7.43 ¥/h to 5.96 ¥/h. In order to analyze the influences of operating parameters on the system’ performance, split coefficient SPLIT2-1, feed concentration, and feed flowrate are studied based on the optimized process. It can be observed that when SPLIT2-1 equals to 0.6, the minimum electricity input is required for the system. When feed concentration is increased from 2 wt.% to 10 wt.%, its NECPC significantly decreases from 14.05 ¥/kg to 1.31 ¥/kg, which indicates that higher feed concentrations are beneficial for reducing the cost of energy consumption. What is more, the NECPC increases from 3.21 ¥/kg to 4.61¥/kg when feed flow rate is increased from 6 kg/h to 16 kg/h, which shows that higher energy consumption costs will be needed at higher feed flow rates.

Acknowledgement 6. Conclusion In this paper, a supercritical water oxidation system with a transpiring wall reactor was simulated by Aspen Plus, and simulation model was validated by comparisons with experimental reactor outlet temperatures and product properties (total organic carbon and CO). It can be seen that the system’s energy efficiency and exergy efficiency are 97.73% and 13.28% at typical operating conditions, respectively. The exergy loss of electric heater, heat exchanger and reactor accounts for 39.89%, 26.64%, and 17.23% of the system’s exergy loss, respectively. It is found that the reaction effluents’ temperature before HE3 is usually above 250 °C, which is much higher than the setting temperature for EH3. Thus, an optimized process is designed by preheating tw2 with the heat of the reaction products. Although the energy efficiency and exergy efficiency of the optimized process are similar to the former one,

This work is supported by Youth Innovation Promotion Association CAS (No. 2017412), PhD Start-Up Fund of Natural Science Foundation of Guangdong Province (No. 2014A030310203), Guangdong Province Science and Technology Plan Project (No. 2013B091100003; No. 2016B090918037), Nansha District technology development project (No. 2015KF013), Science research project of Guangzhou City (201707010407), and Shenzhen Basic Research Project (No. JCYJ20150316144639927), China.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.enconman.2017. 04.082.

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