Recovering wastewater in cooling water system with thermal

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Recovering wastewater in cooling water system with thermal membrane distillation Jiaze Ma, Hafiz Muhammad Irfan, Yufei Wang, Xiao Feng, and Dongmei Xu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00317 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 15, 2018

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Industrial & Engineering Chemistry Research

Recovering wastewater in cooling water system with thermal membrane distillation Jiaze Ma1, Hafiz M. Irfan1, Yufei Wang ∗1, Xiao Feng 2, Dongmei Xu3 1

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, China

2

School of Chemical Engineering & Technology, Xi'an Jiaotong University, Xi'an 710049, China

3

College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao,

266590, China

Abstract: A cooling water system is widely used in industry, where cooling towers consume large amount of fresh water each year. In this study, to reduce fresh water consumption of the system, thermal membrane distillation (TMD) is used to treat blowdown water of cooling tower, permeate water is sent back to tower as makeup water. TMD is driven by waste heat of process stream which is initially cooled by cooling water system. To obtain the optimal framework with minimum cost, the design and operation of cooling towers and TMD is optimized simultaneously. A case study taken from industry is employed to express the effectiveness of proposed model. Results show that optimization model can obtain up to 29.4% reduction on fresh water consumption in comparison with the system without wastewater recovery. Results also indicate that when water price is higher than 1.067$/ton, the structure with TMD unit is profitable. Key work: cooling water system, cooling tower, thermal membrane distillation, wastewater recovery.

1. Introduction A cooling water has suitable thermal properties and a non-harmful chemical composition, a cooling water system by far, has been widely used and thoroughly

∗

Corresponding author: [email protected]

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

studied. Since cooling water system is one of the most common methods for rejecting waste heat, water consumption and energy consumption of cooling towers are extremely high. Therefore, in the past decades, many models have been proposed for reducing energy and water consumption of the cooling towers. The model proposed for optimizing cooling tower can be roughly classified into two categories: 1. Intensifying heat transfer of cooling towers. 2. Reducing fresh water consumption of cooling towers. Intensifying heat transfer of cooling tower is an effective approach for reducing the energy consumption of cooling tower. Xie et al. studied the effects of using nanofluid1 and geometry2 of heat exchanger on the performance of heat transfer of wet cooling tower. Serna-González et al.3 and Rubio-Castro et al.4 studied the detailed design of mechanical draft cooling tower and formulated mathematical model for optimizing operating conditions. Bornman et al.5 proposed a hybrid model is to be optimized by bulk air cooling tower and this model can reduce 13% energy consumption of the cooling tower. Shen et al.6 discussed the effect of enhanced tube on the heat transfer of cooling tower. Rahmati et al.7 studied the effect of the number of stage on the thermal performance of cooling tower. Other researchers studied the internal or external effects on thermal performance of cooling tower, e.g. atmospheric condition8, type of fills9, or fouling behavior of the cooling tower10.


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The second class of method for optimizing cooling tower focuses on reducing fresh water consumption of cooling tower. It has been reported that 60-70% of industrial fresh water is consumed by cooling water system each year11. With the increasing awareness of water saving and environment protection, many works have been conducted to explore the strategies of water saving in a cooling water system. First, several water treatment techniques are employed. Altman et al.

12

employed pressure

driven membrane to purify side stream of recirculating cooling water, 49% of reduction on discharge was yielded. Rahmani et al.13 reduce water consumption by increasing the cycles of concentration, scaling inhibitor is also used to prevent scaling of coolers and reduce blowdown of cooling tower. Walker et al.14 they utilized treated municipal wastewater as a source of tower makeup water, and reduced fresh water consumption of the system. Zhang et al.15 employed microfilter (MF) and ultrafilter (UF) to dispose blowdown water of cooling tower. Both of them are capable to produce high and consistent quality of permeated water. Except water filtration and chemical treatment, reducing cooling load of tower can also reduce fresh water consumption of the system. Installing air cooler is a commonly used method to reduce heat load of cooling tower. Some models of formulating cooling water system with air coolers were reported 16-17. The literatures cited above can reduce fresh water consumption to the some extent. However, using water treatment techniques e.g. reverse osmosis, ultrafiltration or using chemical inhibitors are expensive. Normally, the purifying one ton of wastewater by reverse osmosis requires 2-3kWh electricity18. The high energy and



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cost demand of water treatment techniques made those methods unprofitable. Therefore, in this work, we used thermal membrane distillation that can be driven by industrial waste heat, to treat wastewater of cooling tower and reduce fresh water consumption of the cooling tower. Thermal membrane distillation has been studied from different perspectives, e.g. heat transfer and mass transfer19-20, energy demand 21, optimal operation conditions22. Ding et al.

23

explored the feasibility of using solar

energy to power the membrane distillation. The pilot unit was able to produce permeate water from brackish water consistently. Yu et al.24 purified simulated cooling tower blowdown water in laboratory with membrane distillation. The optimal working condition of membrane distillation unit was discussed. Warsinger et al.25 gave a review study on scaling and fouling behavior of membrane distillation. The effect of flow rate, temperature, and salt concentration of feeds on the performance of membrane distillation were studied. Moreover, many researchers designed and integrated the membrane distillation with process streams. Bamufleh et al.26 combined multi-effect distillation and membrane distillation for treating seawater. Membrane distillation can be used to treat the brine water from multi-effect distillation unit. Elsay el at.27 and González-Bravo et al.28 utilized waste heat from process hot streams to drive thermal membrane distillation for desalination of seawater. Thermal membrane distillation can be driven by process waste heat, and it generates high quality permeate water (almost 100% salt separation factor

29

).

Furthermore, TMD

is employed in this work to treat the blowdown water of cooling tower and reduce fresh water consumption. One industrial case is defined to show the effectiveness of


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proposed model. And sensitivity analysis is conducted to study the effect of water price on the performance of system.

2. Problem statement

Figure 1. Air and water coolers coupling structure

Figure 1 shows the conceptual design of cooling water system with TMD unit. Given a cooling water system with certain heat load on the cooling tower, the discharged wastewater from cooling tower is purified and recovered by thermal membrane distillation. The purified wastewater of membrane distillation unit is then transported back to cooling tower as a makeup water. A part of heat from process hot streams in cooling water system is used as heat resource for membrane distillation unit. The design and operation of cooling tower as well as membrane distillation unit are optimized simultaneously. The objective is to find the most optimal structure that satisfies the required cooling services with minimum total annual cost. Besides, the influence of water price on the performance of system is discussed.



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3. Model formulation 3.1 Cooling tower formulation There are many factors influence the performance and cost of the cooling towers, including local air wet bulb temperature, air humidity, atmospheric pressure, air flow rate, water flow rate and inlet and outlet temperature etc. In order to formulate the heat and mass transfer of cooling towers, the following definitions are necessary: First, operational cost of cooling tower is consisted of fan cost, make-up cooling water cost, chemical treatment cost and the blowdown treatment cost. During the working process, minerals and impurities accumulate at the bottom of cooling tower. To maintain the normal working condition of the cooling tower, a part of cooling water has to be discharged. Furthermore, a part of cooling water evaporates during the working process. In order to keep fixed water flow rate, equivalent makeup water should be supplemented to the system. AOC tower = OC fan − tower + ct ⋅ ft + w ⋅ h ⋅ Mmakeup + d ⋅ h ⋅ Bblowdown

(1)

In Eq.(1), AOCfan-tower is tower fan operational cost. ct is the cost of chemical treatment. ft is total flow rate of cooling water. Mmakeup is flow rate of makeup water. Bblowdown is flow rate of water blowdown. w, h are price of fresh water and annual operation time, d denotes the unit cost of disposing rejected water16. AOCfan − tower =

e ⋅ h ⋅ Cfactor ⋅ Fair − tower

η fan − tower


(2)

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According to Evans30, 1kw of electricity is required for drafting 18216.44m3/h of air, the power consumption of tower fan can be deduced. In Eq.(2), Cfactor is fan factor, Fair-tower is air mass flow rate of cooling tower, ηfan-tower is fan efficiency. The air flow rate in the cooling tower is related to amount of water evaporation Evap, inlet air humidity win and outlet air humidity wout. Fair =

Evap wout − win

(3)

The amount of water evaporation is related to the difference between inlet and outlet temperature of cooling tower, and the water flow rate. In Eq.(5), Tcin and Tcout are cooling tower inlet and outlet temperature31. Range denotes the temperature difference between inlet and outlet temperature of cooling tower.32 Evop = 0.00153 ⋅ ft ⋅ Range

(4)

Range = T cin − T cout

(5)

In addition, the amount of blowdown water and makeup water are related to evaporation and cycle of concentration (πc). Bblowdown =

Evap

(6)

π c −1

M makeup = E vap ⋅

πc

(7)

π c −1

The cooling tower inlet air humidity is local air humidity. Outlet air humidity depends on the vapor pressure.

Wout =

MWw Ps ⋅ MWair Pa − Ps

(8)

In Eq.(8), MWw and MWair are water and air molecular weight, Ps and Pa are vapor pressure and local atmospheric pressure. Vapor pressure is function of mean temperature Tmean,ct in cooling tower.



LnPs = 23.1 −

Tmean, ct =

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4111 Tmean , ct + 237.7

(9)

Tcout + Tcin 2

(10)

In addition, capital cost of cooling tower is determined by many factors33. Eq.(11) is used for calculating the annualized capital cost of cooling tower. Approach32 represents the temperature difference between cooling tower outlet temperature and air wet bulb temperature Twb. AFCtower = Af ⋅ (746.74 ft 0.79 ⋅ Range0.57 ⋅ Approach −0.9924 + ( 0.022Twb + 0.39 )

2.447

)

Approach = T cout − T wb

(11) (12)

The relationship between cooling tower inlet and outlet temperature is shown as Eq.(13). Qtower is total heat load of cooling tower, cp is heat capacity of cooling water.

Qtower = (Tcin − Tcout ) ⋅ cp ⋅ ft

(13)

3.1 Direct contact membrane distillation formulation 3.1.1 Mass transfer through the membrane In this work, direct contact membrane distillation (DCMD) is used. DCMD is the most used membrane distillation. It has been reported that 63.3% membrane distillation units used in industry are DCMD29, due to the simple operation mode. In DCMD process, water vapor transfers from the feed side to permeate side, owning to the pressure difference between two sides of the membrane. The permeate flux Jw depends on pressure difference and coefficient of permeability Bw. The permeate flux can be

shown as follows:

Jw = Bw ⋅ ∆Pw

(14)


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Jw = Bw( pwo , f ⋅ γ w, f ⋅ xw, f − pwo , p ⋅ γ w, p ⋅ xw, p)

(15)

In Eq.(15), pwo , f and pwo , p are the water vapor pressure of feed and water vapor pressure of permeate. γw,f and xw,f are the activity coefficient of water in feed and mole fraction of water in feed. Water vapor pressure of feed and permeate are obtained from Antoine equation, Tm,f and Tm,p are the temperatures of feed and permeate on both sides of membrane: 3816 .44 ) T m , f − 46 .13 3816 .44 ) = exp( 23 .1964 − T m , p − 46 .13

p wo , f = exp( 23 .1964 −

(16)

p wo , p

(17)

The separation factor of membrane distillation is closed to 100%29. The permeated aqueous solution is usually used as drinkable water or distilled water. Since 99% of the salt in feed water can be removed through membrane distillation, the salt concentration of permeate water can be regarded as zero. Therefore, Eq.(15) can be simplified as follow:

Jw = Bw( pwo , f ⋅ γ w, f ⋅ xw, f − pwo , p )

(18)

Figure 2 shows the process configuration of DCMD. The vapor pressure difference is induced by transmembrane temperature difference. Consequently, volatile molecules evaporate at the hot liquid/vapor interface, cross the membrane pores in vapor phase and condense in the cold liquid/vapor interface inside the membrane module. The temperature of hot feed water decreases through the membrane module, and the temperature of permeate side increases through the module. Since the pressure difference is driven by temperature difference, logarithmic mean pressure difference can also be used to represent the pressure difference throughout the membrane29:



∆Pw =

∆Pw,1 − ∆Pw,2 ∆P ln( w,1 ) ∆Pw,2

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(19)

Figure 2. Direct contact membrane distillation process configuration

The area of membrane is related to the flow rate of permeate and unit flux rate across the membrane: Am =

Wpermeate Jw

(20)

3.1.2 Heat transfer through the membrane

Figure 3. Heat transfer through membrane of DCMD

Figure 3 displays the heat transfer through the membrane. There are three steps of heat transferring. Heat transfers through feed boundary layer, membrane, and permeate boundary layer. The temperature gradient exists in the module, and the ratio


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of temperature difference on both sides of membrane and bulk are expressed as polarization coefficient:

θ=

Tm , f − T m, p Tb , f − T b, p

(21)

Tm,f, Tm,p, Tb,f, Tb,p are the temperature at membrane of feed, membrane of permeate, bulk of feed, bulk of permeate. The experiments on different types of membrane indicate that polarization coefficient exhibit linear dependence on bulk feed temperature29. The polarization coefficient is a function of feed bulk temperature: θ = α − β ⋅ Tb , p

(22)

For TMD with laminar flows and comparable flows of the feed and the sweeping liquid, the temperature difference between the bulk and membrane on each side of membrane is roughly the same34: Tb , f − Tm , f ≈ T m , p − Tb , p

(23)

The permeability of membrane is related to membrane material and mean membrane temperature. Parameter Bwb depends on the pore radius, pore tortuosity, thickness of membrane etc27. The mean temperature of membrane is given by Eq.(25).

Bw = Bwb ⋅ Tm1.334 Tmean , tmd =

(24)

Tb , f − Tb , p 2

(25)



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Figure 4. Heat transfer resistance of DCMD

Figure 4 shows the heat transfer resistance of DCMD. The total heat flux through membrane is sum of the heat used in vaporizing flux (qv) and conduction loss (qc). In Eq.(27), km is conductivity of membrane and δ is the thickness of membrane. km is related to mean membrane temperature35.

qv = JwHvw km

(26)

(Tm , f − Tm , p )

(27)

km = 1.7 ×10−7 Tm − 4.0 ×10−5

(28)

qc =

δ

In Eq.(29), η is the thermal efficient of membrane. The value of η is the ratio between RAW the heat used for vaporizing flux and heat provided for TMD. Wf is flow rate of

discharged water from cooling tower. Cp,f is heat capacity of feed bulk water. Tb,f is s temperature of feed bulk, Tf is temperature of supplied wastewater. ∆Hvw denotes

latent heat of vaporization.

ηthermalWfRAW Cp, f (Tb, f − Tfs ) = Wpermeate∆Hvw

(29)

Latent heat of vaporization exhibits linear dependence on membrane feed temperature: ∆Hvw = 3190 − 2.5009T m , f

(30)


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Apart from conduction heat loss, the rest of heat losses are assumed to account for 50%27 of conduction heat loss. Therefore, the thermal efficient of membrane can be expressed as Eq.(31).

η thermal = 1 −

(1 + 0.5) ⋅ qc qc + qv

(31)

3.1.3 Water recovery rate and recycle ratio The wastewater recovery rate is the ratio between permeate flux and flow rate of feed water27: s Wpermeate η thermalCp , f (Tb , f − t f ) = ξ= ∆Hvw W fRaw

(32)

In this work, the recovery rate is set as 0.8. When water recovery rate is lower than the target rate, a part of reject water has to be recycled and mixed with the raw feed water. The mixed water is then preheated and sent to the membrane for filteration. The salt concentration of reject water is obtained through Eq.(33). In the equation, y represents the salt concentration.

yreject =

W fraw y raw f (W fraw − W permeate )

=

y raw f

(33)

1− ξ

The salt concentration of the feed water is calculated through the following equation and v denotes the recycle ratio: y TMD = f

y raw + vyreject f

(34)

1+ v

The flow rate of feed water in TMD is the sum of the raw feed water discharged by cooling tower, and the recycled water.

WTMD = W fraw + Wrecycle

(35)



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The temperature of the supplied feed water in TMD is obtained as follow: T fs =

W fraw ⋅ Traw + Wrecycle ⋅ Trecycle

(36)

WTMD

The rejected water has to be disposed by outer treatment unit, and its flow rate is calculated as Eq.(37). Wdis is the flow rate of rejected water of TMD.

Wdis = W fraw ⋅ (1 − ξ )

(37)

The annualized capital cost of TMD is related to membrane cost and non-membrane cost.

AFCTMD = 45 Am + 1115WTMD

(38)

The operation cost of TMD is consisted of pumping cost, chemical treatment and labor cost. The operation cost is given as follow:

AOCTMD = 928.8WTMD + 1486 ⋅Wfraw

(39)

Finally, the objective is to minimize TAC (total annual cost). Total annual cost includes capital and operation cost of both cooling tower and thermal membrane distillation. The cost related to water makeup and disposing rejected water is also included in operation cost of cooling tower.

TAC = AOCTMD + AFCTMD + AOCCT + AFCCT

(40)

4. Case study In this study, to verify the effectiveness of proposed model, a case study taken from a refinery in Nanjing is employed. In this paper, NLP (non-linear programming) model is employed and the model is implemented in software GAMS, solvers BARON and SCIP are used to solve the NLP problem. There are 29 variables and 24 constraints.


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The calculation time of SCIP solver is 2.5993s. The calculation time of BARON solver is 0.6213s. The same results are yielded by two solvers. The refinery has one centralized cooling tower that supplied cooling water to each plant in factory. The discharge rate (blowdown) of cooling tower is 11.17kg/s. The water quality of cooling tower blowdown is shown in Table 1. Table 1. Water quality of cooling tower blowdown36 Item

Concentration (mg/L)

Calcium

481.7

Magnesium

63

Chloride

500

Bicarbonate

0

Silica

0.9

Nitrate

86.7

Physical and economic Properties and constraints of the case study are listed in Table 2 and Table 3 respectively. Table 2. Physical

Properties and constraints of the case study

Items

Data

Wet bulb temperature Twb

10°C

Feed water specific heat capacity Cp,f

4.18kJ/(kg °C)

Cycle of concentration π

3

Plant annual operation time

8600hr

Water recovery rate ξ

27

0.65mm 37

Polarization coefficient θ

θ=1.104-0.00086Tb,f (Tb,f in K)

Bw,b37

7.5×10-11

Concentration of salt

0.113wt%

Inlet temperature of recirculating water

308K

Inlet temperature of recirculating water

298K

Table 3. Economic Items

data of the case study Cost data

Pumping cost of TMD Labor cost of TMD

27

37

0.03$/m3 0.03$/m3

Chemical treatment cost of TMD37

0.015$/m3

Electricity price

0.1$/kWh



Membrane cost29

70 $/m2

Lang factor38

5.0 27

Membrane lifespan

4 years

Non-membrane lifespan27

10 years

Chemical treatment index ct (cooling tower) 33

110

The unit price of membrane using polypropylene hollow fiber membrane is estimated as 70 $/m2. Lang factor is taken as 5.0, therefore the membrane module cost is estimated as 350 $/ m2. The membrane is supposed as 4 years lifespan without salvage value, and non-membrane equipment have 10 years lifespan without salvage value. In order to illustrate the necessity of coupling TMD with cooling water system, the base case is optimized without TMD. Then we optimize same case with TMD for wastewater recovery, and compare corresponding data with the data of base case. The case is optimized under different water price, and we have studied the effect of water price on the performance of the system. When the price of water is set as 1.067 $/t, the configuration with TMD and system without TMD have roughly same total annual cost. The corresponding configurations are

as presented in Figure 5 and Figure 6. In

figure 5 shows the configuration of base case. The heat load of cooling tower is 61033kW. The evaporation rate of cooling tower is 22.31kg/s. With the evaporation of recirculating water inside the cooling tower, the rest of cooling water is cooled down from 308 K to 298 K. Meanwhile, 11.17kg/s wastewater is discharged by cooling tower. Without any wastewater recovery, the rejected water (blowdown water) has to be sent to centralized treatment unit where reverse osmosis is employed to dispose the wastewater. Typical energy demand for desalination by reverse osmosis is 2-3kWh/m3 18, in this work, we assumedthe energy demand is 2.5kWh/ m3. The recirculation ratio (cycle of concentration) of system is a fixed parameter. The cycle


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of concentration in this case is 3. To reduce complexity of the model, the cycle of concentration is not an optimized variable in this model. In base case, the electricity input for reverse osmosis is 101kW.In figure 6 is the optimized configuration with wastewater recovery.

Figure 5 Cooling water system without wastewater recovery

For the case with TMD, the heat load of cooling tower is the sum of heat load of hot process streams and cooling load of TMD. Therefore the heat load on cooling tower is 58797kW, which is slightly lower than the heat load of cooling tower in the base case. The reason is that 22489kW of heat is supplied for TMD by hot process streams. During the process of membrane distillation, a part of heat is loss due to the conduction and convection. Therefore the cooling load of TMD is 20253kW and it is 2236kW lower than the heat input from process streams. Since the heat load of cooling tower reduced, the amount of evaporation and blowdown also decreased. The discharged water from cooling tower is 10.74kg/s. The wastewater is sent to TMD for filteration. To prevent from fouling on membrane, the maximum feed water temperature of TMD is set as 363K. In this case, 22489kW of heat from process hot streams is used to heat up the raw and recycled salt water. The optimal feed temperature of membrane obtained in this case is 363K. Water recovery rate is set as



0.8 and part of reject water of membrane has to be reused and mixed with raw salt water. In this case, the recycle ratio is 8.91, the flow rate and temperature of recycle water is 106.5kg/s and 313K. On the permeate side, the inlet and outlet temperature of permeate cold water are 303K and 353K. The flow rate of reject water of TMD is 2.15kg/s. The reject water is sent to the reverse osmosis unit which consumes 19.3kW electricity for disposing the reject water from TMD. Finally, 8.59kg/s permeate clean water is yielded and it is used as makeup water for cooling tower.

Figure 6 Cooling water system with wastewater recovery

Table 4 and Table 5 shows comparison between the physical and economic results of two configurations. In figure 7 the cost distribution of the two systems as well as the comparisons between them. As shown in Table 4, for configuration with TMD, with the recovery of wastewater and reduction on tower cooling load, 29.4% of fresh water consumption can be saved. Two systems have roughly same amount of total annual cost when water price is 1.067 $/kWh. As shown in Table 5, with the significant reduction on fresh water consumption, 326445$ fresh water cost is saved in system


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with TMD. Meanwhile, 69830$ of wastewater disposal cost is saved in system with TMD. Table 4. Comparison of physical results Items

CT without TMD

CT with TMD

Water evaporation in tower (kiloton/year)

691.6

665.0

Reject waste water (kiloton/year)

345.8

66.6

Recovered waste water (kiloton/year)

-

265.9

Makeup fresh water (kiloton/year)

1037.4

731.6

Heat load of cooling tower (kW)

61033

58683

Table 5. Comparison of economic results Items

CT without TMD

CT with TMD

Tower operation cost ($)

253,905

244,131

Tower capital cost ($)

51,934

50,349

Makeup water cost ($)

1,106,980

780,535

Reject disposal cost ($)

86,456

16,626

TMD capital cost ($)

-

293,146

TMD operation cost ($)

-

114,909

TAC($)

1,499,431

1,499,803

Figure 7 Cost comparisons and distributions of two systems

In Figure 7, the cost distribution of two systems indicates that the fresh water cost accounts for major part of TAC. Recovering wastewater and reducing fresh water consumption can bring both environmental benefits and profits.



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Finally, a sensitivity analysis is used to study the effect of water price on the performance of the system. With 0.05$ incremental of water price, the system is optimized under seven different water prices. The corresponding cost is shown in Table 6 and Figure 8. It is observed that when water price is higher than 1.067$/t, the system with TMD unit is more economical than system without waste water recovery. Given the emphasis on water saving and environmental protection, the system with TMD is an effective configuration for saving water and reducing cost. Table 6. Influence of water price on system performance Water Price ($/t)

TAC of CT without TMD (k$)

TAC of CT with TMD (k$)

0.917

1343

1389

0.967

1395

1426

1.017

1447

1463

1.067

1499

1499

1.117

1551

1536

1.167

1603

1572

1.217

1654

1609

Figure 8 Effect of water price on system performance


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5. Conclusion In this study, we present a method for recovering wastewater in cooling system by employing thermal membrane distillation. After introducing thermal membrane distillation into cooling water system, fresh water consumption for cooling service reduced significantly as a result of reduction of heat load on cooling tower and waste water recovery. The cost of wastewater disposal is also reduced, because TMD unit has converted 80% of reject water (blowdown) into useful makeup water. In case study, 305.8kt of fresh water can be saved each year by using TMD. We also realized that fresh water price exerts great influence on the configuration. When the fresh water price is higher than 1.067$/t, employing TMD unit to recover waste water can bring both environmental benefits and profits. The fresh water consumption accounts for major part of cost of cooling water system. More than half of TAC of system is on fresh water consumption.

Furthermore, recovering wastewater in cooling water

system with TMD, and utilizing the heat from process hot streams, is an effective approach for saving cost and reducing environmental impact.

Acknowledgements Financial support from the National Natural Science Foundation of China under Grant No. 21576286 and Science Foundation of China University of Petroleum, Beijing (No. 2462017BJB03 and 2462018BJC004) are gratefully acknowledged.



Nomenclature AOC AFC Bblowdown Bw cp Cfactor d Evap e Fair-tower ft h ∆Hvw Jw km Mmakeup MWw MWair △p w

annualized operation cost annualized capital cost flow rate of water blowdown coefficient of permeability specific heat capacity of cooling water fan factor of cooling tower unit cost of disposing rejected water water evaporation unit cost of electricity air mass flow rate of cooling tower flow rate of recirculating water annual operational time latent heat of flux vaporization permeate flux conductivity of membrane flow rate of makeup water water molecular weight air molecular weight pressure difference of membrane

pwo , f

water vapor pressure of the feed

pwo , p

water vapor pressure of permeate

Qtower qv qc Tambient Tcin Tcout Tm,f Tm,p Tb,f Tb,p Tmean,ct Tmean,tmd △Tmin Twb

heat load on cooling tower heat used in vaporizing flux conduction loss local ambient temperature cooling tower inlet temperature cooling tower outlet temperature the temperature at membrane of feed the temperature at membrane of permeate the temperature at bulk of feed the temperature at bulk of permeate Mean temperature of cooling tower Mean temperature of membrane face velocity of air cooler local wet bulb temperature

Tfs

temperature of supplied wastewater

v w win

recycle ratio water price inlet air humidity of cooling tower


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wout W x y Greek Letter η ηfan-cooler ρ γ θ α β δ ξ

outlet air humidity of cooling tower water flow rate in TMD mole fraction of water in the feed salt concentration thermal efficiency of membrane cooler’s fan efficiency density of cooling water activity coefficient polarization coefficient parameter of polarization coefficient parameter of polarization coefficient thickness of membrane water recovery rate

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Recovering wastewater in cooling water system with thermal

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