Performance analysis of a mechanical vapor recompression zero-emission system with water-injected compressor

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Applied Energy Symposium and Forum 2018: Low carbon cities and urban energy systems, CUE2018-Applied Energy andLow Forum 2018: Low carbon andsystems, Applied Energy Symposium andSymposium Forum 2018: carbon cities and urbancities energy CUE2018, 5–7 June 2018, Shanghai, China CUE2018, 5–7 June 2018, Shanghai, China urban energy systems, 5–7 June 2018, Shanghai, China

Performance analysis of a mechanical vapor recompression zeroThe 15th International on District Heating and Cooling zeroPerformance analysis of a Symposium mechanical vapor recompression emission system with water-injected compressor emission system with water-injected compressor Assessing the feasibility of using the*, heat demand-outdoor a,b,c a,b,c, Hanzhi Wanga,b,c , Shuaiqi Li , Chong Huang Shihui Hea,b,c , Wenji Songa,b,c , a,b,c a,b,c a,b,c, a,b,c a,b,c Hanzhi Wang function , Shuaiqi Li Huang *, Shihui He , Wenji Song , a,b,c temperature for ,aChong long-term district heat demand forecast Ziping Feng Ziping Fenga,b,c a a Academy of Sciences, b Guangzhou 510640, Guangdong, c Guangzhoua,b,c Institute of Energy Conversion, Chinese I. Andrić *, A. PinaConversion, , P. Ferrão ,Academy J. Fournier ., Guangzhou B. Guangdong, Lacarrière , O. LeChina. Correc CAS Key Laboratory of Renewable Guangzhou 510640, China. Guangzhou Institute of Energy ChineseEnergy, of Sciences, 510640, Guangdong, China. a a

c c

b

Guangdong Provincial KeybCAS Laboratory of New and and510640, Development, Guangzhou Key Laboratory of Renewable Energy Energy,Research Guangzhou Guangdong, China.510640, Guangdong, China. a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, Guangdong, China. b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France

Abstract Abstract Wastewater in cities and industry sectors is always characterized with high-salinity. In this paper, a novel mechanical vapor Wastewater in evaporative cities and industry sectors system is always In this paper, novel mechanical vapor recompression crystallization wascharacterized proposed to with treat high-salinity. high-salinity wastewater anda realize zero-emission. A Abstract recompressionmodel evaporative crystallization system was proposed to treat high-salinity wastewater and realize A mathematical was developed using the software Engineering Equation Solver (EES). The effects of the zero-emission. flashing pressure mathematical modelwastewater was developed using the software Engineering Equation Solver (EES). The the effects ofinjection the flashing pressure (P ), the feeding salinity (c ), the first-effect evaporation concentration (c ) and water ratio (β ) on flash 0 evap v District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the (Pgreenhouse ), the feeding wastewater salinity (c0), the first-effect concentration (cevapwere ) and the water injection ratio (β flash v) on the coefficient of performance (COP), compression work (Wevaporation ) and require circulation (CR) studied detail. The results show comp gas emissions from the building sector. These systems highratio investments which areinreturned through the heat the coefficient performance (COP), compression work (Wcomp ) renovation and ratioto(CR) were studied in detail. results show that the Due cevap has significant on the CR, especially when thecirculation cevap ispolicies, close c0; the water injected MVRThe system obtains sales. toofthe changedinfluence climate conditions and building heat demand in the future could decrease, that theCOP, cevap the has significant influence the CR, whenimproved the cevap isnearly close 5.0% to c0;for thevarious water injected MVR system obtains higher andinvestment specifically, withperiod. the on increase of especially the βv the COP operation conditions. prolonging return higher COP,scope and specifically, increase of the βv the COP the improved nearly –5.0% for various operation conditions. The main of this paperwith is tothe assess the feasibility of using heat demand outdoor temperature function for heat demand forecast. © The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 Copyright 2018 Elsevier Ltd. All rights reserved. Copyright © 2018 Elsevier Ltd. All rights reserved. Copyright ©that 2018 Elsevier Ltd. All rights reserved. buildingsand vary in both construction period andscientific typology.committee Three weather scenarios (low, medium, and high)Forum and three Selection peer-review under responsibility of the of Applied Energy Symposium 2018:district Low Selection and peer-review under responsibility of the scientific committee of the CUE2018-Applied Energy Symposium and Selection and peer-review under responsibility of the scientific committee of Applied Energy Symposium and Forum 2018: Low renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were carbon cities and urban energy systems, CUE2018. Forum 2018: Low carbon cities and urban energy systems. carbon citieswith andresults urban energy systems, heat CUE2018. compared from a dynamic demand model, previously developed and validated by the authors. Keywords: MVR, high-salinity wastewater, zero-emission, water injection, superheat. The results showed that when only weather changeenhanced is considered, the margin of error could be acceptable for some applications Keywords: high-salinity wastewater, zero-emission, enhanced water injection, superheat. (the errorMVR, in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the 1.The Introduction 1.decrease Introduction in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). On thedistributed other hand,infunction intercept increased 7.8-12.7% per decade (depending on the High-salinity wastewater is widely cities and industry sectorsforwhich is difficult to treat. Evaporative coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and High-salinity wastewater is widely distributed in cities and industry sectors which is difficult to treat. Evaporative crystallization technologies, including multiple effect evaporation (MEE), multi-stage flash (MSF), thermal vapor improve the accuracy of heat demand estimations. crystallization technologies, including multiple effect evaporation (MEE), multi-stage flash (MSF), thermal vapor © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * Corresponding author. Tel.: +86 20 87048514; fax: +86 20 87048514. Cooling.

E-mail address:author. [email protected] * Corresponding Tel.: +86 20 87048514; fax: +86 20 87048514. E-mail address: [email protected] Keywords: Heat demand; Forecast; Climate change 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection peer-review under responsibility the scientific 1876-6102and Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the Applied Energy Symposium and Forum 2018: Low carbon cities and urbanand energy systems, under CUE2018. Selection peer-review responsibility of the scientific committee of the Applied Energy Symposium and Forum 2018: Low carbon cities and urban energy systems, CUE2018. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the CUE2018-Applied Energy Symposium and Forum 2018: Low carbon cities and urban energy systems. 10.1016/j.egypro.2018.09.191

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compression (TVC) and mechanical vapor recompression (MVR), are very promising in treating high-salinity wastewater and realizing zero-emission. Comparing to MEE, MSF and TVC technologies, MVRs have no requirement for condenser and external heat source, thus are more compact and efficient [1-3]. Recently, studies on the MVR technology are mainly focused on the system performance and optimization. Zhou et al. [4] presented a comprehensive design model of single-effect MVR system with a falling film evaporator to treat highly concentrated Na2SO4 wastewater. Liang et al. [5] analyzed a double-effect MVR system for highly concentrated (NH4)2SO4 wastewater and the optimum first effect emission concentration was obtained at which a minimum power consumption is needed. Liu et al. [6] proposed a hierarchical compression MVR evaporation system to deal with the solutions with high boiling point elevation, and the optimum compression ratio and the first stage effluent concentration were derived by using NaOH solution as an example. Han et al. [7] compared the single-stage and multistage MVR systems to achieve zero-emission in desalination systems. The results from the energy and exergy analysis shown that MVR are available for zero-emission evaporation systems. Onishi et al. [8,9] introduced a rigorous optimization model for the single-effect and multi-effect MVR systems design to achieve zero liquid discharge for the desalination of high-salinity produced water from shale gas hydraulic fracturing. Performance improvement potential in the compression processes are also investigated. Wang et al. [10] and Yang et al. [11] carried out the experimental studies on MVR systems with single screw compressor. Water is injected to the compressor to cool down the overheated vapor, and the performance of the MVR system is significantly improved. Shen et al. [1,12] analyzed a single-effect MVR desalination system with water injected twin screw compressor and designed a double-effect MVR system. The results from the above researches demonstrated that water injection could make the vapor compression process close to a saturation vapor compression, thus increasing the compressor isentropic efficiency and reducing the power consumption. In order to improve system efficiency in high-salinity wastewater treatment, a novel mechanical vapor recompression zero-emission system with water injected compressor is proposed in this study. A mathematical model has been developed using the engineering equation solver (EES). The effects of the flashing pressure (Pflash), the feeding waste water salinity (c0), first-stage evaporation concentration (cevap) and water injection ratio (βv) on the circulation ratio (CR), coefficient of performance (COP) and compression work (Wcomp) are studied in detail. 2. System description The schematic diagram of the mechanical vapor recompression zero-emission system is illustrated in Fig. 1. As shown in Fig.1, the proposed system mainly consists of an evaporator, main heat exchanger, compressor, flash tank, circulation pump and crystallizer. In the operation of this system, high-salinity wastewater is concentrated in the evaporator, and then circulated to the main heat exchanger mixing with saturated wastewater from the flash tank. The wastewater is heated by the high temperature vapor in the main heat exchanger and then separate vapor and crystal in the flash tank. The vapor compressor is used to lift pressure and temperature so that the vapor latent heat can be reused.

Fig. 1. Schematic diagram of a mechanical vapor recompression zero-emission system



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3. Mathematical modelling and numerical calculation To clarify the thermodynamic performances of the mechanical vapor recompression zero-emission system under different feeding and operating conditions, a mathematical model is developed based on the balance equations. The following assumptions are made to simplify the analysis: (a) Steady-state operating conditions are assumed for the calculation; (b) Thermal and pressure losses in pipes and each component are ignored; (c) The vapor and solution leaving the flash tank are saturated respectively; (d) The crystallization heat of sodium chloride is ignored; (e) Isentropic efficiencies for compressor and circulation pump are assumed to be 0.8 and 0.75 respectively. With the above assumptions, the mathematical model of each component can be developed using balance equations, including overall mass balance equations, solute balance equations and energy balance equations. Overall mass balance: 0 (1)  min −  mout = Solute balance: 0  min cin −  mout cout =

(2)

Energy balance for each component: 0 (  min hin + Qin ) − (  mout hout + Qout ) + W =

(3)

where W represents compression work and pumping power. The compression work is calculated using isentropic efficiency: Wcomp = Wise / comp

(4)

where Wise is the isentropic work; ηcomp is the compressor isentropic efficiency, which is assumed to be 0.8. The superheat at the outlet compressor is defined as: (5) tsuph =t10 − tsat,10 where tsat,10 is the saturated temperature under outlet pressure. The middle pressure in the water injection process is given as follows: (6) Pmid = Pflash n where π is the compression ratio; the index n is chosen as 0.5 in the calculation. The water injection ratio βv is defined as the injection mass flow rate to the compressor inlet mass flow rate. The equation is defined as: (7)  =m9 / m8 The circulation ratio is calculated by Eq. (8): CR=m13 / m1 (8) The power consumption of the circulation pump is defined as: m v (P − P ) Wpump = 4 4 5 4 (9)

pump

The heat loads in the main heat exchanger and evaporator are as follows:

Qi = (UA)i LMTDi

(10)

where LMTD is the log mean temperature differences; subscript i represents evap and mhex. The energy balance equation in the flash tank is defined as:

(m6cp6t6 − m13cp13t13 ) − m7 h7 = 0

(11) Based on the first law of thermodynamics, the coefficient of performance (COP) is evaluated by the following equation: Qevap + Qmhex (12) COP = Wcomp + Wpump

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4. Results and discussion To clarify the effects of the water injection ratio (βv), evaporation concentration (cevap), feed concentration (c0) and flashing pressure (Pflash) on the circulation ratio (CR), coefficient of performance (COP) and compression work (Wcomp), the mathematical model of the proposed system is developed and calculated using Engineering Equation Solver (EES). The main parameters used in the simulation inputs are given in Table 1, and the results of parametric study is presented in Fig. 2 and 3. Table 1 Main parameters and typical values Main parameters

Typical values

Feed mass flow rate, m0 Feed concentration, c0 Feed temperature, t0 Evaporation concentration, cevap Flashing pressure, Pflash Main heat exchanger pinch temperature difference, tp,mhex Pressure ratio, π Water injection ratio, βv Compressor isentropic efficiency, ηcomp Pumping efficiency, ηpump

1 kg/s 5.0% 25 oC 20% 30 kPa 3.0 oC 2.0 0.02 0.8 0.75

As shown in Fig. 2, the cevap have great influence on the CR, and as the cevap increases from c0 to 24%, the CR decreased by 96.6%, 89.7% and 82.6% respectively with the c0 under 2%, 4% and 6%. Moreover, it also can be seen that the CR is more sensitive to the variation of cevap when it approaches c0. It is clear that lower circulation ratio means lower capacity demand for circulation pump and more compact equipment, thus increasing evaporation concentration is useful to improve system performance. However, in order to avoid crystallization in the evaporator and main heat exchanger, the evaporation concentration should be lower than the saturation concentration. Meanwhile, the influences of the c0 and Pflash are also given in Fig. 2(a) and Fig. 2(b) respectively. It can be seen that the CR decreases with c0 while increases with Pflash, and the impacts of the c0 is more prominent than the Pflash. (a)

(b)

100 80

Pflash=30kPa

c0=4%

60

CR (-)

CR (-)

Pflash=45kPa

70

c0=6%

40

Pflash=60kPa

50 30

20 0

90

c0=2%

0

5

10

15

cevap (%)

20

25

10

5

10

15

cevap (%)

20

25

Fig. 2. CR vs. cevap under various (a) c0, and (b)Pflash.

The variations of COP and Wcomp with different water injection ratio are given in Fig. 3. Water injection can reduce the vapor superheat, thus can greatly improve the performance of the compressor and decrease the compression work. As it presented in Fig. 3, the COP increases and Wcomp decreases with the water injection ratio. The COP improved about 5.0% with water injection ratio increasing from 0 to 0.03 under different operating conditions. Moreover, the impacts of the feed mass concentration and flashing pressure are also given in Fig. 3. As shown in Fig. 3(a), the COP decreases with the c0, and that is mainly caused by the rising in pumping power since the mass flow rate at the outlet evaporator is increased linearly when the cevap is a constant. The variation of COP and Wcomp



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under different flashing pressure is given in Fig. 3(b). The COP decreases and Wcomp increases with the flashing pressure, and that is because the latent heat decreases with the vapor temperature. This is consistent with the results obtained by Ettouney[13].

c0=6%

c0=6%

COP (-)

c0=4%

17.5

c0=4%

155

140 135

16.5 0.00

0.01

0.02

130 0.03

βv (-)

Pflash=30kPa

Pflash=30kPa

Pflash=60kPa

Pflash=60kPa

Pflash=45kPa

150 145

17.0

(b) 18.0 17.5

Pflash=45kPa

155 150

Wcomp (kW)

c0=2%

COP (-)

c0=2%

Wcomp (kW)

(a) 18.0

145 17.0

140 16.5 16.0 0.00

135

0.01

βv (-)

0.02

130 0.03

Fig. 3. COP and Wcomp vs. βv under various (a) c0, and (b)Pflash.

5. Conclusions A mathematical model of a mechanical vapor recompression zero-emission system is developed and evaluated using Engineering Equation Solver (EES). The effects of the flashing pressure (Pflash), the feeding wastewater salinity (c0), the first-effect evaporation concentration (cevap) and the water injection ratio (βv) on the circulation ratio (CR), coefficient of performance (COP) and compression work (Wcomp) are studied in detail. The derived conclusions are as follows: (1) The cevap has significant influence on the performance of CR, especially when the cevap is close to c0. When cevap increases from c0 to 24%, the CR decreased by 96.6%, 89.7% and 82.6% respectively with the c0 under 2%, 4% and 6%. (2) Water injected MVR system obtains higher COP and lower Wcomp, and the COP increased about 5% with water injection ratio increased from 0 to 0.03 under different operating conditions. (3) With the increase of c0, the CR increased and the COP decreased; with the increase of Pflash, the CR and COP decreased simultaneously. It is clear that the proposed system is suitable to treat high-salinity wastewater and realize zero-emission. The conclusions from cevap and βv can provide some useful information for system design and optimum operation. Acknowledgements This research was funded by the Science and Technology Planning Project of Guangdong Province (No.: 2014B050505014) and the Key Laboratory of Renewable Energy, Chinese Academy of Sciences (No. y807j11001). Appendix A. Thermodynamic property correlations The relation between saturated mass concentration of the sodium chloride solution and boiling temperature is given as follows [14]: csat =0.2573 + 0.0055exp(tb / 62.2565) tb  [0,108.8 C] (A.1) where tb = 26.36ln Pflash − 12.92 Pflash [20,101.3 kPa] (A.2) Boling point elevation [15]: (A.3) ' = tb − tv,sat

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 ' 0.0162

(tv,sat + 273)2



(72.99c 2 + 8.875c + 0.01525)

(A.4)

where c is the mass concentration. The specific heat capacity of the sodium chloride wastewater is given as follows [5]: (A.5) cp = cp,water(1 − ci ) + cp,NaCl(1 − ci ) where cp,water and cp,NaCl are the specific heat capacity of the water and sodium chloride, respectively, under specific temperature; ci is the mass concentration of the sodium chloride. References [1] SHEN J B, XING Z W, ZHANG K, et al. Development of a water-injected twin-screw compressor for mechanical vapor compression desalination systems [J]. Applied Thermal Engineering, 2016, 95:125-135. [2] HAN D, HE W F, YUE C, et al. Analysis of energy saving for ammonium sulfate solution processing with self-heat recuperation principle [J]. Applied Thermal Engineering, 2014, 73(1): 641-649. [3] WALMSLEY T G, ATKINS M J, WALMSLEY M R W, et al. Appropriate placement of vapour recompression in ultra-low energy industrial milk evaporation systems using Pinch Analysis [J]. Energy, 2016, 116:1269-1281. [4] ZHOU Y S, SHI C J, DONG G Q. Analysis of a mechanical vapor recompression wastewater distillation system [J]. Desalination, 2014, 353:9197. [5] LIANG L, HAN D, MA R, et al. Treatment of high-concentration wastewater using double-effect mechanical vapor recompression [J]. Desalination, 2013, 314(4): 139-146. [6] LIU Yan, PEI Chenglin, WANG Jianda, et al. Design and analysis of an evaporation system of solutions with high boiling point elevation [J]. The Chinese Journal of Process Engineering, 2017, 17(4):859-865. (in Chinese) [7] HAN D, HE W F, YUE C, et al. Study on desalination of zero-emission system based on mechanical vapor compression [J]. Applied Energy, 2017, 185:1490-1496. [8] ONISHI V C, CARRERO-PARRENO A, REYES-LABARTA J A, et al. Shale gas flowback water desalination: Single vs multiple-effect evaporation with vapor recompression cycle and thermal integration [J]. Desalination, 2017, 404(C): 230-248. [9] ONISHI V C, CARRERO-PARRENO A, REYES-LABARTA J A, et al. Desalination of shale gas produced water: A rigorous design approach for zero-liquid discharge evaporation systems [J]. Journal of Cleaner Production, 2017, 140:1399-1414. [10] WANG Liwei, ZHUANG Jiangfa, YANG Luwei, et al. Experimental study on performance of MVR system driven by single screw water vapor compressor [J]. Journal of University of Chinese Academy of Sciences, 2015, 32(1):38-45. (in Chinese) [11] YANG J L, ZHANG C, ZHANG Z T, et al. Study on mechanical vapor recompression system with wet compression single screw compressor [J]. Applied Thermal Engineering, 2016, 103:205-211. [12] SHEN J B, XING Z W, WANG X L, et al. Analysis of a single-effect mechanical vapor compression desalination system using water injected twin screw compressors [J]. Desalination, 2014, 333(1): 146-153. [13] ETTOUNEY H. Design of single-effect mechanical vapor compression [J]. Desalination, 2006, 190(1-3): 1-15. [14] LIU Guangqi, MA Lianxiang, LIU Jie. Chenmical property date handbook (Inorganic volume) [M]. Beijing: Chemical Industry Press, 2002. (in Chinese) [15] Dalian university of technology. Principles of Chemical Industry[M]. Beijing: Higher Education Press, 2002. (in Chinese)