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Flash evaporation intensified by microwave energy and performance analysis
Journal Pre-proofs Flash evaporation intensified by microwave energy and performance analysis Tian Shihong, Ju Shaohua, Zhang Lihua, Peng Jinhui, Guo lei, Liu Xiao ling PII: DOI: Reference:
S1359-4311(19)34323-6 https://doi.org/10.1016/j.applthermaleng.2019.114471 ATE 114471
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
24 June 2019 28 September 2019 30 September 2019
Please cite this article as: T. Shihong, J. Shaohua, Z. Lihua, P. Jinhui, G. lei, L. Xiao ling, Flash evaporation intensified by microwave energy and performance analysis, Applied Thermal Engineering (2019), doi: https:// doi.org/10.1016/j.applthermaleng.2019.114471
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Flash evaporation intensified by microwave energy and performance analysis Tian Shihong a, b, c, d, Ju Shaohua a, b, c, d,*, Zhang Lihua a,b,c,d, Peng Jinhuia,b,c, Guo lei a,b,c,d,
a.
Liu Xiao linga,b,c,d
National Local Joint Laboratory of Engineering Application of Microwave Energy and Equipment Technology, Kunming, Yunnan 650093, China
b.
Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming University of Science and Technology, Kunming, Yunnan, 650093, China
c.
Yunnan Provincial Key Laboratory of Intensification Metallurgy, Kunming, Yunnan 650093, China
d.
Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan, 650093, China
Abstract： A novel microwave flash evaporation (MFE) system is presented in this paper first time, it may solve the inherent problems of conventional flash evaporation (CFE) system, such as high energy consumption, scaling and long stages et al. In order to demonstrate the performance of such a novel system, some single factors experiments have been carried out , and the influence of some parameters such as microwave power, initial temperature, and liquid flow rate have been studied. Results shows that the evaporation quantity increase with the increase of microwave power, flow rate and *
Corresponding author. Tel.: +86 15587107838
E-mail addresses : [email protected] (Shaohua Ju); [email protected] (Shihong Tian)
initial temperature, the optimized operation parameter is gained at the flow-rate of 40 L/h, initial liquid temperature of 50 ℃ and microwave power of 1.35 kW, the evaporation quantity of MFE can be as high as 1.35 L in 20 min, but the evaporation rate of CFE is only 0.74 L in 20 min. The ascendant performance of MFE system is shown through comparing with CFE system under same parameters. In conclusion, one stage of MFE could take the place of two stages of CFE. Moreover, the energy efficiency is calculated and represented by a dimensionless parameter GOR meaning Gain Output Ratio, for the optimized operation parameters, the maximum energy efficiency for MSF system is about 79.38%. Key words： Microwave heating; Energy conversion; Flash evaporation; Vapor-liquid two phase;
1. Introduction Evaporation process has been applied in many industry fields, such as desalination, solution concentration, waste water purification, powder materials preparation, drying, cooling, and food engineering. The typical method for realizing evaporation is consuming thermal energy to heat liquid or material such as multi-effect evaporation (MEE) and multi-stage flash (MSF) [1]. However, those conventional method face the following inherent problems including high complex of the whole equipment, scaling on the heat exchanger surface, high investment cost and long stages et al. [2,3,4]
In most cases, the MEE and MSF is always combined and used in a plant, the flash evaporation is usually used to realize the vaporization of solution and recycle the thermal energy from flash steam as the secondary heat source simultaneously [5]. Such a type of flash system is composed of a series of stages, the system approaches ideal total latent heat recovery through fractionating the overall temperature differential between the heat source and liquid into a large number of stages, accordingly, the pressure gradients in the system among every stage is required [6]. As a result, those systems as conventional flash evaporation (CFE) system were researched and developed for a long time, including formation, theories, and process, etc. furthermore, a lot of fundamental studies on flash evaporation were done by researchers, mainly focus on film flash evaporation [7], droplet flash evaporation [8] and spray flash evaporation [9], and in terms of its mechanism, heat transfer and mass transfer, and process research. For example, Miyatake et al [10,11] carried out a flash evaporation experiment on superheated pipe water, a non-equilibrium temperature difference(NETD) and non-equilibrium fraction(NEF) was proposed to express the principle of the flash evaporation of water film. Besides, D. Saury et al [12] determined the mass of water evaporated during a sudden pressure drop, the temperature of the water film corresponding to the phenomenon of pressure drop as well as the mass flow rates was measured. A correlation between the mass of water evaporated by flash and the superheat was then obtained. Those work explained the rules and heat & mass transfer process of water film flash evaporation. The droplet flash evaporation and spray flash evaporation is also focused due to
its advantages of high specific surface area, small size, high heat flux and high heat & mass transfer efficiency. This process is widely used in cooling and freezing, Cheng Wen-long et al [13] conducted a vacuum spray flash evaporation cooling (VSFEC) system to carry out the influence of spray flow rate and spray height on the heat transfer characteristics, results shown that the heat flux can be removed efficiently from the heat source with a very small spray flow rate, which is only one third of that of conventional spray cooling under the same heat flux. This effectively proves the droplet flash and spray flash under vacuum conditions have good heat transfer performance and evaporation effect. Zhou et al. [15] studied the two-phase flashing spray of volatile liquids through photography and phase Doppler Particle Analyzer (PDPA), the phase change process, temperature distribution and dynamic characteristics of two-phase liquid spray flash was revealed, as a result, the superheated liquid formed of a jet or droplets will lead to explosive atomization, quickly evaporation and phase change which is shown fine droplet and a short spray distance. The conventional flash evaporation (CFE) process is also widely researched, including the apparatus of CFE, the evaporation phenomenon in CFE, and the process optimization of CFE. Taking seawater desalination as an example. Muthunayagam A.E. et al [15] conducted some experiments on seawater desalination in a pilot with a superheating between 26 ℃ and 32 ℃ and a pressure ranging from 1.3kPa to 2.3kPa. A new concept for designing desalination systems was set up by them, and they done a good work on seawater desalination. Furthermore, Sebastian P et al. [16] used a
mono-stage falling jet flash evaporator to observe the evolution of pressures, temperatures and flows inside the system, for the network model, the steady-state behavior and the influence from another factor were validated by them, and the simulation results of the process behavior of vintage flash evaporator was also discussed. However, the superheat degree of liquid is determined by the pressure difference between ambient pressure and the saturated vapor pressure of liquid, the temperature of liquid would decrease quickly in conventional process after flash evaporation occur. Consequently, the sensible heat of liquid could not sustain the persistence of evaporation. Meanwhile, the traditional heat transfer method could not feed thermal energy to liquid in vacuum flash system [17, 18], whatever the heat-pipe or high temperature steam, such a phenomenon limits the development of flash evaporation. There should have a new method to be considered to intensify flash evaporation process, so as to increase the productivity of flash evaporation system. It is well known that microwave heating as one type of heating method which is quickly, effectively and easy to control, microwave can induce the temperature raise of material through the interaction and dielectric loss with polar material [19, 20]. Due to its long wavelength, microwave owns a better penetrability. Whatever in vacuum environment or not, it can quickly transfer energy into material on site. [21] Water solution can absorb the microwave strongly because water molecule own large dielectric constant and large dissipation factor. Yousefi T et al [22] reported a numerical investigation of the effect of microwave heating continuous flowing water,
the effects of inlet velocity, applicator height and applicator diameter on the temperature field was examined, through comparing the assumptions in this work with the constant dielectric properties case. Average outlet temperature decreases with inlet velocity increasing, and heat absorption drops significantly with raising the applicator diameter more than a critical value. Cherbański R et al.[23] investigated the processes of microwave induced natural convection in water through researched a numerical model about microwave heating water in a monomode applicator, Results shown that microwave was chiefly induced in the strongest hot spot in water. As a result, these work provide a theoretical foundation to utilize microwave heating strengthen the flash evaporation of aqueous solution. However, the application of microwave heating in flash evaporation process intensification have not been reported on literatures and industries until now. A novel concept, design and process of microwave heating flash evaporation system (MFE) are invented for the first time in this paper. The design of the MFE system is introduced, and its ascendant performance is presented through comparing with CFE system by single factor experiments with same parameters. The effects of various operation parameters on the evaporation quantity are investigated, such as microwave power, flow-rate and initial temperature. In addition, the energy efficiency of MFE under an optimized operation condition is discussed. 2. Experimental apparatus and instrumentation 2.1 MFE experiment system As shown in Fig.(1), the apparatus mainly contains four parts: a microwave
generation part, a flash evaporation chamber, a vapor and liquid collected part and a system control part. The microwave generation part contains a magnetron of 1.5 kW, a wave guide and a quart glass window compressed with silica gel cushion which can not only seal the vacuum flash chamber, but also act as a wave-transmitting material. Vapor and liquid collected part includes a condenser and two tanks, the system control part is formed of a control panel, connecting line and parameter measurement meter and its display head. Specificly, the structure of flash evaporation chamber is shown in Fig.2.
1
2
3
4 6
7
8 10
5
14 8 9
11 12 13
1. Feed tank 2. Flow-meter 3. Thermocouples 4. Piezometer 5.Flash evaporation chamber 6. Magnetron 7.Wave guide 8. Thermometer 9. Concentrated tank 10. Cooling tank 11. Atmospheric valve 12.Distilled water tank 13. Vacuum pump 14. Central control box Fig.1 The schematic of experimental apparatus for MFE`
4 1 1
1
2
5
3
6
(a) The schematic diagram of flash evaporation chamber 1. Magnetron 2. Rectangular waveguide 3. Trapezoidal waveguide 4. Inlet 5. SiC perforated plate 6.Outlet
(b) The structure of SiC perforated plate Fig.2 The schematic diagram of flash chamber
The cylindrical flash evaporation chamber coupled with microwave cavity is made of stainless steel with a height of 0.4 m and a diameter of 0.3 m, there are one inlet of target liquid at the top of chamber and one outlet of the finish liquid at its bottom. In the flash evaporation chamber, there are two circular SiC perforated plates with a diameter of 22.6 cm and a height of 0.6 cm. There are a number of hole with a diameter of 0.6 cm setting on the plates to increase the heat transfer area between the
plate and passing liquid. 2.2. Test procedure and measurement Tape water is used as the initial experiment liquid in this study. It is heated to a certain initial temperature by an electrical heater setting at the bottom of the initial solution tank. and it is spraied into flash chamber through a nozzle setted on the top cap. Before experiment, the flash evaporation chamber is preheated by hot water at the initial temperature about 10 min, afterward, open the valve and put those water out. And then suck the air inside the evaporation chamber by a liquid ring vacuum pump. Once the vacuum degree of chamber reached the range of 0.065-0.075 MPa. Open the cooling water system for the magnetron and the steam condensation part. After that, adjust the flow control valve to a flow-rate required. Meanwhile, turn on the electric switch of microwave magnetron to a given electric currency. Then, the microwave heating flash evaporation (MFE) process is started and continues for about 20 minutes. The operation data including microwave power, flow-rate, pressure, initial temperature were recorded in every 5 minutes. During experiment, The liquid and steam temperature is measured by thermocouples which is made of stainless-steel with a diameter 10 mm, a precision of 1 ℃ , and a range of 0-300 ℃ . The pressure of the flash evaporation chamber is measured by a piezometer with a measurement rang of -0.1MPa to 1Mpa and a precision is 0.02kPa. The flow rate can be controlled by a valve and a flow-meter with range of 0-60L/h and a precision is about 2L/h. Considering the capacity volume of concentrated tank, the total experiment
time is set as 20minutes. After 20minutes, stop the experiment by the following process: stopping the microwave energy feeding, closing the flow control valve and cooling water; opening the atmospheric valve to make the chamber pressure return to ambient pressure, at last, open the water valve underneath of the condensation tank to give off the condensed water, then measure and record its volume and temperature. The power consumed by microwave during each experiment is also recorded by an electricity meter setting in central control box. For conventional experiments, the procedure is almost the same as above and the only difference is without the step of opening and closing of the microwave. 2.3 Experimental design and uncertainty analysis In our experiments, some key factors of flow-rate, microwave power, inlet temperature of water are investigated during the MFE and CFE processes, the range of the operation parameters are designed as showed in Table.1. For comparison, a serials of conventional experiments without microwave energy input are also conducted with the same operation parameters in Table.1. The uncertainty of the parameters, calculated using the Moffat method [24], is also shown in Table.2. Table 1 The controlled operation parameters Parameter
Value
Microwave power (kW)
0.81, 0.9, 0.99, 1.08, 1.17, 1.26, 1.35
Flow-rate (L/h)
10, 20, 30, 40
Initial temperature of water (℃)
40-50
Table 2 Uncertainty analysis.
Parameter
Absolute uncertainly
Minimal measured value
Uncertainty
Power (kW)
0.1
0.81
1.2346×10-01
Temperature (℃)
1
31
3.2258×10-02
Flow rate(L/h)
2
10
2.0000×10-01
Volume (L)
1×10-3
0.4
2.5000×10-03
Pressure(KPa)
0.02
65
3.0769×10-04
In addition, all of the experiments are done twice, the temperature of flash evaporation chamber under the condition of microwave power of 1.35 kW is randomly selected and set as an example to illustrate the variance of experimental data. The result is shown in Fig.3. 48
Average temperature (℃)
46 44 42 40 38 36 34 32
Average temperature 0
5
10
15
20
Time (min)
Fig.3 The variance of experimental data
3. Result and discussion 3.1 Influence of microwave power Under experimental conditions of the inlet flow-rate of 40 L/h, the initial temperature is among 40 ℃ to 50 ℃ and the initial pressure of 5.8 kPa. According to the transformation among the latent heat and sensible heat of water in flash chamber and the heat transfer amount from microwave, the energy balance of CFE
and MFE can be expressed as following Eq.(1).
mh l l mc hc ms hs Q l l Qmicrowave mc hc ms hs Q mh
(1)
Assuming the energy loss of CFE and MFE are the same, according to Eq.(1), the enthalpy of water and steam will down to the saturated value which is corresponding to the pressure of flash chamber. Considering the researches of Miyatake
[3,4].
The flash evaporation is decided by superheat degree and the
evaporation quantity would increase with the increase of superheat degree. In our research, microwave is used as the external heat source to reheat the liquid in flash evaporation chamber. Thus, the liquid would be in the form of overheat due to the energy from microwave. Then the rate of boiling of liquid would become more strongly, thus, more liquid water molecules would get rid of the bondage from surface tension and intermolecular force and turn into gaseous vapor. Evidently, the temperature raise of water is corresponding to the amount of energy transformation from microwave. So, more microwave energy would lead to more evaporation and achieve better evaporation quantity. The experimental results for different microwave power are shown in Fig.4.
1.50 Evaporation quantity
Evaporation quantity (ml)
1.35
1.20
1.05
0.90
0.75
0.60 0.72
0.81
0.90
0.99
1.08
1.17
1.26
1.35
1.44
Microwave power (kW)
Fig.4 Effect of microwave power on evaporation quantity
It is shown that with the increase of microwave power, the evaporation quantity have different raise tendency. The increase rate of evaporation quantity is slow when microwave power is less than 1.17 kW, but when microwave power is increase, the raise rate of evaporation quantity turn into quickly. Because the change of electro-magnetic field intensity in the flash chamber is influenced by the microwave power, the energy generation transform intensity between water and electro-magnetic field can be expressed as Eq.(2). q q e q h 2 f 0 r ' tan E 2 f 0 r ' tan H 2
(2)
Table 3 The experimental data for MFE with different microwave power and CFE
Experiment
Microwave
Tc
Tf (℃)
Ts
Vd
Vl(L) Tl(℃) groups
MFE
CFE
Power(kW)
(℃)
TⅠ
TⅡ
(℃)
(L/20min)
1.35
13.3
50
42
41
40
45
1.35
1.26
13.3
50
38
38
37
42
1.14
1.17
13.3
47
41
35
34
40
0.94
1.08
13.3
48
35
35
34
39
0.89
0.99
13.3
48
37
36
35
40
0.85
0.9
13.3
50
37
35
34
40
0.8
0.81
13.3
48
38
35
33
42
0.7
-
13.3
47
34
33
31
35
0.74
In addition, microwave not only enhance the quantity of flash evaporation, but also raise the temperate of concentrated solution. According to the basic thermodynamic parameter of water, the temperature of liquid is the same as the saturated temperature of vapor which is decided by saturation vapor pressure in flash chamber. The Antoine equation [25] is the method that is frequently used to calculate saturation vapor pressure in engineering, it is defined as ln p sat A
B C T
(3)
For water, when temperature is among 10 ℃ to168 ℃. It is defined as ln p sat 16.37379
3876.659 229.73 T
From Eq.(4), under the flash pressure of 5.8 kPa, the saturation temperature of water during flash evaporation is 35.5 ℃. Table.3 shows the temperature of
(4)
concentrated solution, vapor and flash chamber under the condition of different microwave power, Therefore, the heat effect of microwave is testified by the temperature raise of finishing liquid and increased production during MFE process. 3.2 Influence of initial temperature In this section, the influence of initial temperature for flash evaporation quantity of MFE and CFE are analyzed. Under the condition for microwave of 1.35 kW, flow rate of 40 L/h and initial pressure of 5.8 kPa, with different initial temperature, a single factor experiment is developed, the result is shown in Fig.5. 1.5 1.4
Evaporation quantity (L)
1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5
MFE CFE
0.4 0.3 39
40
41
42
43
44
45
46
47
48
49
50
51
Initial temperature (℃)
Fig.5. Effect of different initial temperature for MFE and CFE
It is indicated that with the increase of initial temperature during CFE, the evaporation quantity is raised. According to the previous study of Miyatake [3,4], superheat degree is the heat source of flash evaporation, the superheat degree is defined as
T Tl Tsat
(5)
According to Eq.(5), There are some methods to increase evaporation quantity, such as, raising initial temperature, decreasing pressure of flash evaporation chamber
and vaporizer, or enlarging the heat transfer area for flash evaporation, increasing the initial temperature of liquid and changing the thermal equilibrium. In our experiment, the pressure of flash evaporation chamber is as low as 5.8 kPa, in order to enhance the flash evaporation quantity, microwave is used to reheat the liquid during flash process until liquid flow out of flash chamber, meanwhile, the SiC perforated plate is heated by microwave, they can transfer heat to the liquid and steam, too. In addition, SiC perforated plate make the flow of steam and water retard, thus, the flash time and heating time become longer. As a result, the thermal equilibrium during flash evaporation is broken by microwave heating, the thermal disturbance of flash chamber is enhanced, and the boiling of liquid in chamber become more acutely. As Fig.6 shown, the quantity of flash evaporation of MFE process is more than CFE process under the same initial chamber pressure under different inlet temperature. On the one hand, the flash evaporation quantity is raised under the microwave power of 1.35 kW, under the initial temperature of 50 ℃, the evaporation capacity of MFE is almost twice of CFE. On the other hand, the heat effect of microwave is also proven by comparing the temperature variety of flash chamber and vapor for MFE and CFE. The experiment data for different initial temperature is shown in Table.4. From Table.4, the temperature of flash chamber and steam for MFE is higher than CFE during flash evaporation experiment. The latent heat of vaporization of water is decreasing with the increase of temperature. This phenomenon makes the germination of vaporization become more easily. As a result, microwave raises the temperature of liquid and vapor in flash chamber, which means the evaporation effect of liquid is
strengthen by microwave. Table 4 The experimental data of MFE and CFE for different initial temperature Experiment Tl (℃)
Vl(L/h)
Tc (℃)
groups
MFE
CFE
Vd
Tf (℃) Ts(℃) TⅠ
TⅡ
(L/20min)
50
40
41
40
39
43
1.35
48
40
37
37
37
42
0.96
45
40
38
40
40
44
0.85
43
40
39
39
38
42
0.77
40
40
40
41
40
43
0.7
50
40
39
35
33
39
0.74
48
40
35
34
35
36
0.8
45
40
34
33
32
36
0.55
43
40
34
34
33
36
0.48
40
40
38
37
36
40
0.47
3.2 Influence of flow rate The pressure difference between inlet valve and flash chamber is the most important factor which decides the intension of flash evaporation, the initial pressure of flash chamber is kept at 5.8 kPa. According Eq.(1), the total quantity of flash evaporation is decided by the inlet mass of liquid and enthalpy of liquid, it is known that the quantity of flash evaporation is raised with the increasing of flow rate. Under the microwave heating, the relationship between the absorb of microwave and energy transform quantity is identified as
cp (
T T ) ( k T ) q ( x , y , z , t ) t z
(6)
The temperature distribution of water in flash chamber can be inferred by Eq.(6). Assuming all of the energy from microwave is used to raise liquid temperature in flash chamber, the thermal variation of water is calculated as Eq.(7)
Qm mc l p T ml vt
(7)
The enthalpy of liquid is arising with temperature. Microwave heating raises the temperature of liquid in flash chamber, so the demand of energy for evaporation for unit quantity is decrease. As a result, the flash evaporation quantity is enhance by the heating of microwave. As shown in Fig.6, the evaporation quantity of MFE process is more than CFE process under the same flow rate. 1.5 1.4 1.3
Evaporation quantity (L)
1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3
MFE CFE
0.2 0.1 0.0
10
20
30
40
Flow rate (L/h)
Fig.6 Effect of flow rate on MFE and CFE
As shown in Table.5, under the condition of the different flow rate, the distilled water quantity is increasing with the raise of flow-rate. On the one hand, because the temperature of flash chamber is enhanced by the microwave. The transitive energy from microwave to liquid droplets is fed on site, this part of energy transfers into the sensible heat of water droplets. On the other hand, microwave changes the heat
transfer process of liquid in flash chamber, the core of liquid droplets is also heated by microwave, all of the water droplets in the flash chamber are superheated. The increase of flow rate and microwave wave are both enhance the heat disturbance in flash chamber, so the heat and mass transfer is increasing in flash chamber. Hence, the productivity of MFE is arise comparing to CFE. Table 5 The experimental data of MFE and CFE under different flow rate Experimen Flow rate Tl(℃) Tc(℃) t groups
Vd
Tf (℃)
(L/h)
Ts(℃) TⅠ
TⅡ
Vl(L) (L/20min)
40
50
36
40
39
44
13.3
1.35
30
48
34
38
37
41
10
0.85
20
46
34
38
37
41
6.7
0.74
10
44
34
40
40
45
3.4
0.64
40
50
35
38
37
41
13.3
0.74
30
46
33
33
32
39
10
0.6
20
44
34
38
37
42
6.7
0.37
10
40
30
35
34
39
3.4
0.4
MFE
CFE
4. The energy efficiency of microwave heating flash evaporation (MFE) system The energy efficiency for MFE system is calculated, and a dimensionless radio of Gained Output Ratio (GOR) is used to evaluate the effectiveness of distilled water production [26]. As an energy ratio for the total latent heat of flash evaporation of the water to the difference between total inlet heat and outlet heat, it is defined as an
index for the energy efficiency of MFE and CFE, the GOR is defined as:
GOR
md c
(8)
ml hl mc hc
A larger value of GOR in an evaporation system represents better distilled system. In order to calculate the heat difference, an average temperature of experiment is used to reckon the enthalpy of water and steam. The values of GOR under different experimental conditions and parameters are calculated and illustrated in Fig.7. 3.5
3.0
GOR
2.5
2.0
1.5
GOR 1.0
0.8
0.9
1.0
1.1
1.2
1.3
1.4
Microwave power (kW)
(a) Different microwave power
5.0
CFE MFE
4.5 4.0
GOR
3.5 3.0 2.5 2.0 1.5 1.0 39
40
41
42
43
44
45
46
47
48
49
Initial temperature (℃)
(b) Different initial temperature
50
51
7.0
CFE MFE
6.5 6.0
GOR
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0
10
20
30
40
Flow rate (L/h)
(c) Different flow rate Fig.7 The value of GOR for different experiment condition and parameter
Fig.7(a) shows the effect of the microwave power on GOR, with the increase of microwave power, the value of GOR is arising. Under the condition of the microwave power of 1.35 kW, the flow rate of 40L/h and the initial inlet temperature of water is 50 ℃ . The maximum value of GOR for MFE is 3.26, but for CFE process, the maximum value of GOR is 1.61. Simultaneously, the enthalpy of unit mass steam is rising with the temperature. Such a phenomenon of MFE makes the value of GOR is greater than the value of GOR of CFE. Fig.7(b) and Fig.7(c) shows the effect of the initial temperature and the flow rate on GOR. Clearly, the value of GOR is reductive with the increase of initial temperature. Meanwhile, the value of GOR has a trend of decrease with the increasing of flow rate. For these two different operating conditions, the value of GOR for MFE is about 1 to 2 higher than the value of GOR for CFE. The reason is that the difference between total latent heat of vaporization and the total enthalpy different of inlet to outlet leads to the variety of GOR.
In order to evaluate the energy efficiency of total MFE system, the ratio between requisite vaporation heat for distilled water and the total heat difference for inlet and outlet of liquid is calculated. Such a ratio is used to characterize the energy efficiency of total system, it is designed as follow:
md c ms hs mc hc ml hl md hd
(9)
According the experimental data shown in Table.3, the energy efficiency for different microwave power is calculated by Eq.(9) and illustrated in Fig.8.
Fig.8 The energy efficiency of MFE and CFE system
As a result, the energy efficiency for MFE system is among 60% to 80%. the quantity of distilled water of MFE system is raise with the microwave power, identically, the energy efficiency have a trend of raise with the increase of microwave power, it is indicated that the higher microwave power would get a better effect in MFE system, Specially, the maximum energy efficiency is 79.38%, and the value is gained under the condition of microwave power of 1.35 kW. However, the energy efficiency of CFE system is only about 59.42%, the energy efficiency of MFE system is higher than CFE system. According to Eq.(9), it is indicated that
microwave heating enhances the heat and mass transfer in evaporation process. 5. Conclusion In this work, a novel microwave heating flash evaporation (MFE) equipment. Significant effects of microwave power, initial temperature and flow rate of productivity and energy efficiency are observed through present experiment, the energy efficiency of MFE system is also presented in our study, and a dimensionless number of GOR is used to evaluate the energy efficiency of MFE system. The following points are concluded:
The microwave power, initial temperature and flow rate have important effect on flash evaporation quantity, energy efficiency and GOR.
The productivity of MFE system and CFE system are compared. As the experimental data shown. MFE has as twice as the approximately efficiency of CFE under various operation parameters, this illustrates that one stage of MFE can take place of two stages of conventional flash evaporation system.
The increase in microwave power, initial temperature of liquid and flow rate increases the productivity under the same initial pressure of vacuum tank. MFE have a greater productivity than CFE under the same condition.
According to the value of GOR which is influenced by the temperature difference of liquid of inlet and outlet and the production of distilled water as well as temperature, MFE have a better Gain Output Ratio than CFE. The value of GOR for MFE is about 1 to 2 higher than the value of GOR for CFE.
The maximum energy efficiency of MFE system is 79.38%.
Nomenclature
𝜀0
Permittivity: the real part
m
Mass, (kg)
𝜀′𝑟
Permittivity: the imaginary part
h
Enthalpy, (kJ/kg)
𝛿
Dielectric loss angle
∆Q
Energy loss (kJ)
f
Frequency, (Hz)
Qmicrowave
Microwave energy, (kJ)
π
Circumference ratio
v
Flow rate, (V/L)
V
Volume, (L)
E
Electric field intensity, (V/m)
H
Magnetic field intensity, (A/m)
Subscripts
q
Heat quantity
l
Liquid of inlet
A,B,C Empirical constant number of c
Liquid of concentrated
Antoine equation
f
Flash chamber
∆T
Superheated degree, (℃)
s
Steam
T
Temperature, (℃)
d
Distilled water
t
Time, (min)
sat
Saturated
e
Electric
h
Magnetic
η
energy efficiency
Greek symbols c𝜌
Specific heat, (kJ/kg ∙ ℃)
(x,y,z)
Cartesian coordinate system
k
Thermal conductivity
Abbreviations
ω
Axial vacuity of fluid
GOR
Gain output ratio
ρ
Material density
MFE
Microwave flash evaporation
CFE
Conventional flash evaporation
Acknowledgment This research is supported by basic research project for application in Yunnan province. 2015FA017. Reference [1] Nafey A S , Fath H E S , Mabrouk A A . Thermo-economic investigation of multi effect evaporation (MEE) and hybrid multi effect evaporation—multi stage flash (MEE-MSF) systems[J]. Desalination, 2006, 201(1-3):241-254. [2] Dae Hyun Kim. A review of desalting process techniques and economic analysis of the recovery of salts from retentates. Desalination 270 (2011) 1–8 [3] Druetta P, Aguirre P, Mussati S. Minimizing the total cost of multi effect evaporation
systems
for
seawater
desalination,
Desalination,
2014,
344(9):431-445. [4] A novel flash boosted evaporation process for alumina refineries[J]. Applied Thermal Engineering, 2016, 94:375-384. [5] Mabrouk A A , Nafey A S , Fath H E S . Thermoeconomic analysis of some existing desalination processes[J]. Desalination, 2007, 205(1-3):354-373. [6] Kalogirou S A. Solar Energy Engineering (Second Edition)[M]. 2014. [7] Yu W , Qi H , Qingzhong Y , et al. Energy and exergy analyses of circulatory flash evaporation of aqueous NaCl solution[J]. Desalination. [8] Chen Q , Thu K , Bui T D , et al. Development of a model for spray evaporation based on droplet analysis[J]. Desalination, 2016, 399:69-77.
[9] Gao W , Li C , Xu C , et al. Experimental study on water separation process in a novel spray flash vacuum evaporator with heat-pipe[J]. Desalination, 2016, 386:39-47. [10]Miyatake, Osamu, et al. Fundamental experiments with flash evaporation.Heat Transfer - Japanese Research2.4 (1973):89-100. [11]Miyatake, Osamu, et al. "Flash evaporation phenomena of pool water."Heat Transfer - Japanese Research6.2 (1977):13-24. [12]D. Saury *, S. Harmand, M. Siroux. Experimental study of flash evaporation of a water film. International Journal of Heat and Mass Transfer 45 (2002) 3447–3457 [13]Cheng W L, Chen H, Hu L, et al. Effect of droplet flash evaporation on vacuum flash evaporation cooling: Modeling, International Journal of Heat & Mass Transfer 2015, 84:149-157. [14]Z.F. Zhou, W.T. Wu, B. Chen, et al., An experimental study on the spray and thermal characteristics of R134a two-phase flashing spray, Int. J. Heat Mass Transfer 55 (15) (2012) 4460–4468. [15]Muthunayagam A E, Ramamurthi K, Paden J R. Low temperature flash vaporization for desalination, Desalination 2005, 180(1-3):25-32. [16]Sebastian P, Nadeau J P. Experiments and modeling of falling jet flash evaporators for vintage treatment, International Journal of Thermal Sciences, 2002, 41(3):269-280. [17]C.C. Tseng, R. Viskanta. Enhancement of water droplet evaporation by radiation absorption, Fire Safety Journal 2006, 41 (3): 236-247
[18]Hongtao Zhang, Vasudevan Raghavan, George Gogos. Subcritical and supercritical droplet evaporation within a zero-gravity environment: Low Weber number relative motion, International Communications in Heat and Mass Transfer 2008, 35 (4): 385-394 [19]Ganesapillai
Magesh,
Regupathi
Iyyaswami,
Murugesan
Thanapalan.
Characterization and process optimization of microwave drying of plaster of Paris, Drying Technology 2008, 26(12): 1484-1496, [20]Zheng Xianzhe, Wang Yingkuan, Liu Chenghai, Sun Jingkun, Liu Bingxin, Zhang Baohui, Lin Zhen, Liu Haijun. Microwave Energy Absorption Behavior of Foamed Berry Puree Under Microwave Drying Conditions, Drying Technology 2013, 31(7): 785-794 [21]Ming Huang, Jinhui Peng, Jing JingYang. Jiaqiang Wang. A New Equation for the Description of the Dielectric Losses Under Microwave Irradiation, J. Phys. D: APPl. Phy 2006, 39: 2255-2258 [22]Yousefi T, Mousavi S A, Saghir M Z, et al. An investigation on the microwave heating of flowing water: A numerical study [J]. International Journal of Thermal Sciences, 2013, 71(71):118-127 [23]Cherbański R, Rudniak L. Modelling of microwave heating of water in a monomode applicator – Influence of operating conditions [J]. International Journal of Thermal Sciences, 2013, 74(6):214-229 [24]Moffat R J. Contributions to the Theory of Single-Sample Uncertainty Analysis [J]. Journal of Fluids Engineering, 1982, 104(2):250--260.
[25]Surhone L M, Tennoe M T, Henssonow S F, et al. Antoine Equation[M]. 2010. [26]El-Agouz S A, El-Aziz G B A, Awad A M. Solar desalination system using spray evaporation [J]. Energy, 2014, 76:276-283.
1
2
3
4 6
7
8 10
5
14 8
11 12
9
13
1. Feed tank 2. Flow-meter 3. Thermocouples 4. Piezometer 5.Flash evaporation chamber 6. Magnetron 7.Wave guide 8. Thermometer 9. Concentrated tank 10. Cooling tank 11. Atmospheric valve 12.Distilled water tank 13. Vacuum pump 14. Central control box Fig.1 The schematic of experimental apparatus for MFE` 4 1 1
1
2
5
3
6 (a) The schematic diagram of flash evaporation chamber 1. Magnetron 2. Rectangular waveguide 3. Trapezoidal waveguide 4. Inlet 5. SiC perforated plate 6.Outlet
(b) The structure of SiC perforated plate Fig.2 The schematic diagram of flash chamber
1.50 Evaporation quantity
Evaporation quantity (ml)
1.35
1.20
1.05
0.90
0.75
0.60 0.72
0.81
0.90
0.99
1.08
1.17
1.26
1.35
1.44
Microwave power (kW)
Fig.3 Effect of microwave power on evaporation quantity
1.5 1.4
Evaporation quantity (L)
1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5
MFE CFE
0.4 0.3 39
40
41
42
43
44
45
46
47
48
49
50
51
Initial temperature (℃)
Fig.4. Effect of different initial temperature for MFE and CFE
1.5 1.4 1.3
Evaporation quantity (L)
1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3
MFE CFE
0.2 0.1 0.0
10
20
30
40
Flow rate (L/h)
Fig.5 Effect of flow rate on MFE and CFE
3.5
3.0
GOR
2.5
2.0
1.5
GOR 1.0
1.0
0.9
0.8
1.3
1.2
1.1
1.4
Microwave power (kW)
(a) Different microwave power
5.0
CFE MFE
4.5 4.0
GOR
3.5 3.0 2.5 2.0 1.5 1.0 39
40
41
42
43
44
45
46
47
48
49
Initial temperature (℃)
(b) Different initial temperature
50
51
7.0
CFE MFE
6.5 6.0
GOR
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0
10
20
30
40
Flow rate (L/h)
(c) Different flow rate Fig.6 The value of GOR for different experiment condition and parameter
Fig.7 The energy efficiency of MFE and CFE system
Fig.8 Physical model of MFE process
Table 1 The controlled operation parameters
Parameter
Value
Microwave power (kW)
0.81, 0.9, 0.99, 1.08, 1.17, 1.26, 1.35
Flow-rate (L/h)
10, 20, 30, 40
Initial temperature of water (℃)
40-50
Table 2 Uncertainty analysis. Parameter
Absolute uncertainly
Minimal measured value
Uncertainty
Power (kW)
0.1
0.81
1.2346×10-01
Temperature (℃)
1
31
3.2258×10-02
Flow rate(L/h)
2
10
2.0000×10-01
Volume (L)
1×10-3
0.4
2.5000×10-03
Pressure(KPa)
0.02
65
3.0769×10-04
Table 3 The experimental data for MFE with different microwave power and CFE
Experimen t groups
MFE
CFE
Microwave Power(kW)
Vl(L)
1.35 1.26 1.17 1.08 0.99 0.9 0.81 -
13.3 13.3 13.3 13.3 13.3 13.3 13.3 13.3
Tl(℃) Tc (℃) 50 50 47 48 48 50 48 47
42 38 41 35 37 37 38 34
Tf (℃) TⅠ 41 38 35 35 36 35 35 33
TⅡ 40 37 34 34 35 34 33 31
Ts(℃)
Vd (L/20min)
45 42 40 39 40 40 42 35
1.35 1.14 0.94 0.89 0.85 0.8 0.7 0.74
Table 4 The experimental data of MFE and CFE for different initial temperature Experiment groups
MFE
CFE
Tl (℃)
Vl(L/h)
Tc (℃)
50
40
48
Tf (℃)
Vd
Ts(℃)
(L/20min)
TⅠ
TⅡ
41
40
39
43
1.35
40
37
37
37
42
0.96
45
40
38
40
40
44
0.85
43
40
39
39
38
42
0.77
40
40
40
41
40
43
0.7
50
40
39
35
33
39
0.74
48
40
35
34
35
36
0.8
45
40
34
33
32
36
0.55
43
40
34
34
33
36
0.48
40
40
38
37
36
40
0.47
Table 5 The experimental data of MFE and CFE under different flow rate
Experimen Flow rate Tl(℃) Tc(℃) t groups (L/h) MFE
CFE
40 30 20 10 40 30 20 10
50 48 46 44 50 46 44 40
36 34 34 34 35 33 34 30
Tf (℃) TⅠ 40 38 38 40 38 33 38 35
TⅡ 39 37 37 40 37 32 37 34
Ts(℃)
Vl(L)
Vd (L/20min)
44 41 41 45 41 39 42 39
13.3 10 6.7 3.4 13.3 10 6.7 3.4
1.35 0.85 0.74 0.64 0.74 0.6 0.37 0.4
HIGHLIGHTS
Microwave flash evaporation system (MFE) used for evaporation is introduced.
The production of MFE is approximately twice as much as the conventional.
MFE could reduce effective number, shorten process and reduce investment.
The maximum energy efficiency of MFE system is 79.38%.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S1359-4311(19)34323-6 https://doi.org/10.1016/j.applthermaleng.2019.114471 ATE 114471
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
24 June 2019 28 September 2019 30 September 2019
Please cite this article as: T. Shihong, J. Shaohua, Z. Lihua, P. Jinhui, G. lei, L. Xiao ling, Flash evaporation intensified by microwave energy and performance analysis, Applied Thermal Engineering (2019), doi: https:// doi.org/10.1016/j.applthermaleng.2019.114471
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Flash evaporation intensified by microwave energy and performance analysis Tian Shihong a, b, c, d, Ju Shaohua a, b, c, d,*, Zhang Lihua a,b,c,d, Peng Jinhuia,b,c, Guo lei a,b,c,d,
a.
Liu Xiao linga,b,c,d
National Local Joint Laboratory of Engineering Application of Microwave Energy and Equipment Technology, Kunming, Yunnan 650093, China
b.
Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming University of Science and Technology, Kunming, Yunnan, 650093, China
c.
Yunnan Provincial Key Laboratory of Intensification Metallurgy, Kunming, Yunnan 650093, China
d.
Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan, 650093, China
Abstract： A novel microwave flash evaporation (MFE) system is presented in this paper first time, it may solve the inherent problems of conventional flash evaporation (CFE) system, such as high energy consumption, scaling and long stages et al. In order to demonstrate the performance of such a novel system, some single factors experiments have been carried out , and the influence of some parameters such as microwave power, initial temperature, and liquid flow rate have been studied. Results shows that the evaporation quantity increase with the increase of microwave power, flow rate and *
Corresponding author. Tel.: +86 15587107838
E-mail addresses : [email protected] (Shaohua Ju); [email protected] (Shihong Tian)
initial temperature, the optimized operation parameter is gained at the flow-rate of 40 L/h, initial liquid temperature of 50 ℃ and microwave power of 1.35 kW, the evaporation quantity of MFE can be as high as 1.35 L in 20 min, but the evaporation rate of CFE is only 0.74 L in 20 min. The ascendant performance of MFE system is shown through comparing with CFE system under same parameters. In conclusion, one stage of MFE could take the place of two stages of CFE. Moreover, the energy efficiency is calculated and represented by a dimensionless parameter GOR meaning Gain Output Ratio, for the optimized operation parameters, the maximum energy efficiency for MSF system is about 79.38%. Key words： Microwave heating; Energy conversion; Flash evaporation; Vapor-liquid two phase;
1. Introduction Evaporation process has been applied in many industry fields, such as desalination, solution concentration, waste water purification, powder materials preparation, drying, cooling, and food engineering. The typical method for realizing evaporation is consuming thermal energy to heat liquid or material such as multi-effect evaporation (MEE) and multi-stage flash (MSF) [1]. However, those conventional method face the following inherent problems including high complex of the whole equipment, scaling on the heat exchanger surface, high investment cost and long stages et al. [2,3,4]
In most cases, the MEE and MSF is always combined and used in a plant, the flash evaporation is usually used to realize the vaporization of solution and recycle the thermal energy from flash steam as the secondary heat source simultaneously [5]. Such a type of flash system is composed of a series of stages, the system approaches ideal total latent heat recovery through fractionating the overall temperature differential between the heat source and liquid into a large number of stages, accordingly, the pressure gradients in the system among every stage is required [6]. As a result, those systems as conventional flash evaporation (CFE) system were researched and developed for a long time, including formation, theories, and process, etc. furthermore, a lot of fundamental studies on flash evaporation were done by researchers, mainly focus on film flash evaporation [7], droplet flash evaporation [8] and spray flash evaporation [9], and in terms of its mechanism, heat transfer and mass transfer, and process research. For example, Miyatake et al [10,11] carried out a flash evaporation experiment on superheated pipe water, a non-equilibrium temperature difference(NETD) and non-equilibrium fraction(NEF) was proposed to express the principle of the flash evaporation of water film. Besides, D. Saury et al [12] determined the mass of water evaporated during a sudden pressure drop, the temperature of the water film corresponding to the phenomenon of pressure drop as well as the mass flow rates was measured. A correlation between the mass of water evaporated by flash and the superheat was then obtained. Those work explained the rules and heat & mass transfer process of water film flash evaporation. The droplet flash evaporation and spray flash evaporation is also focused due to
its advantages of high specific surface area, small size, high heat flux and high heat & mass transfer efficiency. This process is widely used in cooling and freezing, Cheng Wen-long et al [13] conducted a vacuum spray flash evaporation cooling (VSFEC) system to carry out the influence of spray flow rate and spray height on the heat transfer characteristics, results shown that the heat flux can be removed efficiently from the heat source with a very small spray flow rate, which is only one third of that of conventional spray cooling under the same heat flux. This effectively proves the droplet flash and spray flash under vacuum conditions have good heat transfer performance and evaporation effect. Zhou et al. [15] studied the two-phase flashing spray of volatile liquids through photography and phase Doppler Particle Analyzer (PDPA), the phase change process, temperature distribution and dynamic characteristics of two-phase liquid spray flash was revealed, as a result, the superheated liquid formed of a jet or droplets will lead to explosive atomization, quickly evaporation and phase change which is shown fine droplet and a short spray distance. The conventional flash evaporation (CFE) process is also widely researched, including the apparatus of CFE, the evaporation phenomenon in CFE, and the process optimization of CFE. Taking seawater desalination as an example. Muthunayagam A.E. et al [15] conducted some experiments on seawater desalination in a pilot with a superheating between 26 ℃ and 32 ℃ and a pressure ranging from 1.3kPa to 2.3kPa. A new concept for designing desalination systems was set up by them, and they done a good work on seawater desalination. Furthermore, Sebastian P et al. [16] used a
mono-stage falling jet flash evaporator to observe the evolution of pressures, temperatures and flows inside the system, for the network model, the steady-state behavior and the influence from another factor were validated by them, and the simulation results of the process behavior of vintage flash evaporator was also discussed. However, the superheat degree of liquid is determined by the pressure difference between ambient pressure and the saturated vapor pressure of liquid, the temperature of liquid would decrease quickly in conventional process after flash evaporation occur. Consequently, the sensible heat of liquid could not sustain the persistence of evaporation. Meanwhile, the traditional heat transfer method could not feed thermal energy to liquid in vacuum flash system [17, 18], whatever the heat-pipe or high temperature steam, such a phenomenon limits the development of flash evaporation. There should have a new method to be considered to intensify flash evaporation process, so as to increase the productivity of flash evaporation system. It is well known that microwave heating as one type of heating method which is quickly, effectively and easy to control, microwave can induce the temperature raise of material through the interaction and dielectric loss with polar material [19, 20]. Due to its long wavelength, microwave owns a better penetrability. Whatever in vacuum environment or not, it can quickly transfer energy into material on site. [21] Water solution can absorb the microwave strongly because water molecule own large dielectric constant and large dissipation factor. Yousefi T et al [22] reported a numerical investigation of the effect of microwave heating continuous flowing water,
the effects of inlet velocity, applicator height and applicator diameter on the temperature field was examined, through comparing the assumptions in this work with the constant dielectric properties case. Average outlet temperature decreases with inlet velocity increasing, and heat absorption drops significantly with raising the applicator diameter more than a critical value. Cherbański R et al.[23] investigated the processes of microwave induced natural convection in water through researched a numerical model about microwave heating water in a monomode applicator, Results shown that microwave was chiefly induced in the strongest hot spot in water. As a result, these work provide a theoretical foundation to utilize microwave heating strengthen the flash evaporation of aqueous solution. However, the application of microwave heating in flash evaporation process intensification have not been reported on literatures and industries until now. A novel concept, design and process of microwave heating flash evaporation system (MFE) are invented for the first time in this paper. The design of the MFE system is introduced, and its ascendant performance is presented through comparing with CFE system by single factor experiments with same parameters. The effects of various operation parameters on the evaporation quantity are investigated, such as microwave power, flow-rate and initial temperature. In addition, the energy efficiency of MFE under an optimized operation condition is discussed. 2. Experimental apparatus and instrumentation 2.1 MFE experiment system As shown in Fig.(1), the apparatus mainly contains four parts: a microwave
generation part, a flash evaporation chamber, a vapor and liquid collected part and a system control part. The microwave generation part contains a magnetron of 1.5 kW, a wave guide and a quart glass window compressed with silica gel cushion which can not only seal the vacuum flash chamber, but also act as a wave-transmitting material. Vapor and liquid collected part includes a condenser and two tanks, the system control part is formed of a control panel, connecting line and parameter measurement meter and its display head. Specificly, the structure of flash evaporation chamber is shown in Fig.2.
1
2
3
4 6
7
8 10
5
14 8 9
11 12 13
1. Feed tank 2. Flow-meter 3. Thermocouples 4. Piezometer 5.Flash evaporation chamber 6. Magnetron 7.Wave guide 8. Thermometer 9. Concentrated tank 10. Cooling tank 11. Atmospheric valve 12.Distilled water tank 13. Vacuum pump 14. Central control box Fig.1 The schematic of experimental apparatus for MFE`
4 1 1
1
2
5
3
6
(a) The schematic diagram of flash evaporation chamber 1. Magnetron 2. Rectangular waveguide 3. Trapezoidal waveguide 4. Inlet 5. SiC perforated plate 6.Outlet
(b) The structure of SiC perforated plate Fig.2 The schematic diagram of flash chamber
The cylindrical flash evaporation chamber coupled with microwave cavity is made of stainless steel with a height of 0.4 m and a diameter of 0.3 m, there are one inlet of target liquid at the top of chamber and one outlet of the finish liquid at its bottom. In the flash evaporation chamber, there are two circular SiC perforated plates with a diameter of 22.6 cm and a height of 0.6 cm. There are a number of hole with a diameter of 0.6 cm setting on the plates to increase the heat transfer area between the
plate and passing liquid. 2.2. Test procedure and measurement Tape water is used as the initial experiment liquid in this study. It is heated to a certain initial temperature by an electrical heater setting at the bottom of the initial solution tank. and it is spraied into flash chamber through a nozzle setted on the top cap. Before experiment, the flash evaporation chamber is preheated by hot water at the initial temperature about 10 min, afterward, open the valve and put those water out. And then suck the air inside the evaporation chamber by a liquid ring vacuum pump. Once the vacuum degree of chamber reached the range of 0.065-0.075 MPa. Open the cooling water system for the magnetron and the steam condensation part. After that, adjust the flow control valve to a flow-rate required. Meanwhile, turn on the electric switch of microwave magnetron to a given electric currency. Then, the microwave heating flash evaporation (MFE) process is started and continues for about 20 minutes. The operation data including microwave power, flow-rate, pressure, initial temperature were recorded in every 5 minutes. During experiment, The liquid and steam temperature is measured by thermocouples which is made of stainless-steel with a diameter 10 mm, a precision of 1 ℃ , and a range of 0-300 ℃ . The pressure of the flash evaporation chamber is measured by a piezometer with a measurement rang of -0.1MPa to 1Mpa and a precision is 0.02kPa. The flow rate can be controlled by a valve and a flow-meter with range of 0-60L/h and a precision is about 2L/h. Considering the capacity volume of concentrated tank, the total experiment
time is set as 20minutes. After 20minutes, stop the experiment by the following process: stopping the microwave energy feeding, closing the flow control valve and cooling water; opening the atmospheric valve to make the chamber pressure return to ambient pressure, at last, open the water valve underneath of the condensation tank to give off the condensed water, then measure and record its volume and temperature. The power consumed by microwave during each experiment is also recorded by an electricity meter setting in central control box. For conventional experiments, the procedure is almost the same as above and the only difference is without the step of opening and closing of the microwave. 2.3 Experimental design and uncertainty analysis In our experiments, some key factors of flow-rate, microwave power, inlet temperature of water are investigated during the MFE and CFE processes, the range of the operation parameters are designed as showed in Table.1. For comparison, a serials of conventional experiments without microwave energy input are also conducted with the same operation parameters in Table.1. The uncertainty of the parameters, calculated using the Moffat method [24], is also shown in Table.2. Table 1 The controlled operation parameters Parameter
Value
Microwave power (kW)
0.81, 0.9, 0.99, 1.08, 1.17, 1.26, 1.35
Flow-rate (L/h)
10, 20, 30, 40
Initial temperature of water (℃)
40-50
Table 2 Uncertainty analysis.
Parameter
Absolute uncertainly
Minimal measured value
Uncertainty
Power (kW)
0.1
0.81
1.2346×10-01
Temperature (℃)
1
31
3.2258×10-02
Flow rate(L/h)
2
10
2.0000×10-01
Volume (L)
1×10-3
0.4
2.5000×10-03
Pressure(KPa)
0.02
65
3.0769×10-04
In addition, all of the experiments are done twice, the temperature of flash evaporation chamber under the condition of microwave power of 1.35 kW is randomly selected and set as an example to illustrate the variance of experimental data. The result is shown in Fig.3. 48
Average temperature (℃)
46 44 42 40 38 36 34 32
Average temperature 0
5
10
15
20
Time (min)
Fig.3 The variance of experimental data
3. Result and discussion 3.1 Influence of microwave power Under experimental conditions of the inlet flow-rate of 40 L/h, the initial temperature is among 40 ℃ to 50 ℃ and the initial pressure of 5.8 kPa. According to the transformation among the latent heat and sensible heat of water in flash chamber and the heat transfer amount from microwave, the energy balance of CFE
and MFE can be expressed as following Eq.(1).
mh l l mc hc ms hs Q l l Qmicrowave mc hc ms hs Q mh
(1)
Assuming the energy loss of CFE and MFE are the same, according to Eq.(1), the enthalpy of water and steam will down to the saturated value which is corresponding to the pressure of flash chamber. Considering the researches of Miyatake
[3,4].
The flash evaporation is decided by superheat degree and the
evaporation quantity would increase with the increase of superheat degree. In our research, microwave is used as the external heat source to reheat the liquid in flash evaporation chamber. Thus, the liquid would be in the form of overheat due to the energy from microwave. Then the rate of boiling of liquid would become more strongly, thus, more liquid water molecules would get rid of the bondage from surface tension and intermolecular force and turn into gaseous vapor. Evidently, the temperature raise of water is corresponding to the amount of energy transformation from microwave. So, more microwave energy would lead to more evaporation and achieve better evaporation quantity. The experimental results for different microwave power are shown in Fig.4.
1.50 Evaporation quantity
Evaporation quantity (ml)
1.35
1.20
1.05
0.90
0.75
0.60 0.72
0.81
0.90
0.99
1.08
1.17
1.26
1.35
1.44
Microwave power (kW)
Fig.4 Effect of microwave power on evaporation quantity
It is shown that with the increase of microwave power, the evaporation quantity have different raise tendency. The increase rate of evaporation quantity is slow when microwave power is less than 1.17 kW, but when microwave power is increase, the raise rate of evaporation quantity turn into quickly. Because the change of electro-magnetic field intensity in the flash chamber is influenced by the microwave power, the energy generation transform intensity between water and electro-magnetic field can be expressed as Eq.(2). q q e q h 2 f 0 r ' tan E 2 f 0 r ' tan H 2
(2)
Table 3 The experimental data for MFE with different microwave power and CFE
Experiment
Microwave
Tc
Tf (℃)
Ts
Vd
Vl(L) Tl(℃) groups
MFE
CFE
Power(kW)
(℃)
TⅠ
TⅡ
(℃)
(L/20min)
1.35
13.3
50
42
41
40
45
1.35
1.26
13.3
50
38
38
37
42
1.14
1.17
13.3
47
41
35
34
40
0.94
1.08
13.3
48
35
35
34
39
0.89
0.99
13.3
48
37
36
35
40
0.85
0.9
13.3
50
37
35
34
40
0.8
0.81
13.3
48
38
35
33
42
0.7
-
13.3
47
34
33
31
35
0.74
In addition, microwave not only enhance the quantity of flash evaporation, but also raise the temperate of concentrated solution. According to the basic thermodynamic parameter of water, the temperature of liquid is the same as the saturated temperature of vapor which is decided by saturation vapor pressure in flash chamber. The Antoine equation [25] is the method that is frequently used to calculate saturation vapor pressure in engineering, it is defined as ln p sat A
B C T
(3)
For water, when temperature is among 10 ℃ to168 ℃. It is defined as ln p sat 16.37379
3876.659 229.73 T
From Eq.(4), under the flash pressure of 5.8 kPa, the saturation temperature of water during flash evaporation is 35.5 ℃. Table.3 shows the temperature of
(4)
concentrated solution, vapor and flash chamber under the condition of different microwave power, Therefore, the heat effect of microwave is testified by the temperature raise of finishing liquid and increased production during MFE process. 3.2 Influence of initial temperature In this section, the influence of initial temperature for flash evaporation quantity of MFE and CFE are analyzed. Under the condition for microwave of 1.35 kW, flow rate of 40 L/h and initial pressure of 5.8 kPa, with different initial temperature, a single factor experiment is developed, the result is shown in Fig.5. 1.5 1.4
Evaporation quantity (L)
1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5
MFE CFE
0.4 0.3 39
40
41
42
43
44
45
46
47
48
49
50
51
Initial temperature (℃)
Fig.5. Effect of different initial temperature for MFE and CFE
It is indicated that with the increase of initial temperature during CFE, the evaporation quantity is raised. According to the previous study of Miyatake [3,4], superheat degree is the heat source of flash evaporation, the superheat degree is defined as
T Tl Tsat
(5)
According to Eq.(5), There are some methods to increase evaporation quantity, such as, raising initial temperature, decreasing pressure of flash evaporation chamber
and vaporizer, or enlarging the heat transfer area for flash evaporation, increasing the initial temperature of liquid and changing the thermal equilibrium. In our experiment, the pressure of flash evaporation chamber is as low as 5.8 kPa, in order to enhance the flash evaporation quantity, microwave is used to reheat the liquid during flash process until liquid flow out of flash chamber, meanwhile, the SiC perforated plate is heated by microwave, they can transfer heat to the liquid and steam, too. In addition, SiC perforated plate make the flow of steam and water retard, thus, the flash time and heating time become longer. As a result, the thermal equilibrium during flash evaporation is broken by microwave heating, the thermal disturbance of flash chamber is enhanced, and the boiling of liquid in chamber become more acutely. As Fig.6 shown, the quantity of flash evaporation of MFE process is more than CFE process under the same initial chamber pressure under different inlet temperature. On the one hand, the flash evaporation quantity is raised under the microwave power of 1.35 kW, under the initial temperature of 50 ℃, the evaporation capacity of MFE is almost twice of CFE. On the other hand, the heat effect of microwave is also proven by comparing the temperature variety of flash chamber and vapor for MFE and CFE. The experiment data for different initial temperature is shown in Table.4. From Table.4, the temperature of flash chamber and steam for MFE is higher than CFE during flash evaporation experiment. The latent heat of vaporization of water is decreasing with the increase of temperature. This phenomenon makes the germination of vaporization become more easily. As a result, microwave raises the temperature of liquid and vapor in flash chamber, which means the evaporation effect of liquid is
strengthen by microwave. Table 4 The experimental data of MFE and CFE for different initial temperature Experiment Tl (℃)
Vl(L/h)
Tc (℃)
groups
MFE
CFE
Vd
Tf (℃) Ts(℃) TⅠ
TⅡ
(L/20min)
50
40
41
40
39
43
1.35
48
40
37
37
37
42
0.96
45
40
38
40
40
44
0.85
43
40
39
39
38
42
0.77
40
40
40
41
40
43
0.7
50
40
39
35
33
39
0.74
48
40
35
34
35
36
0.8
45
40
34
33
32
36
0.55
43
40
34
34
33
36
0.48
40
40
38
37
36
40
0.47
3.2 Influence of flow rate The pressure difference between inlet valve and flash chamber is the most important factor which decides the intension of flash evaporation, the initial pressure of flash chamber is kept at 5.8 kPa. According Eq.(1), the total quantity of flash evaporation is decided by the inlet mass of liquid and enthalpy of liquid, it is known that the quantity of flash evaporation is raised with the increasing of flow rate. Under the microwave heating, the relationship between the absorb of microwave and energy transform quantity is identified as
cp (
T T ) ( k T ) q ( x , y , z , t ) t z
(6)
The temperature distribution of water in flash chamber can be inferred by Eq.(6). Assuming all of the energy from microwave is used to raise liquid temperature in flash chamber, the thermal variation of water is calculated as Eq.(7)
Qm mc l p T ml vt
(7)
The enthalpy of liquid is arising with temperature. Microwave heating raises the temperature of liquid in flash chamber, so the demand of energy for evaporation for unit quantity is decrease. As a result, the flash evaporation quantity is enhance by the heating of microwave. As shown in Fig.6, the evaporation quantity of MFE process is more than CFE process under the same flow rate. 1.5 1.4 1.3
Evaporation quantity (L)
1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3
MFE CFE
0.2 0.1 0.0
10
20
30
40
Flow rate (L/h)
Fig.6 Effect of flow rate on MFE and CFE
As shown in Table.5, under the condition of the different flow rate, the distilled water quantity is increasing with the raise of flow-rate. On the one hand, because the temperature of flash chamber is enhanced by the microwave. The transitive energy from microwave to liquid droplets is fed on site, this part of energy transfers into the sensible heat of water droplets. On the other hand, microwave changes the heat
transfer process of liquid in flash chamber, the core of liquid droplets is also heated by microwave, all of the water droplets in the flash chamber are superheated. The increase of flow rate and microwave wave are both enhance the heat disturbance in flash chamber, so the heat and mass transfer is increasing in flash chamber. Hence, the productivity of MFE is arise comparing to CFE. Table 5 The experimental data of MFE and CFE under different flow rate Experimen Flow rate Tl(℃) Tc(℃) t groups
Vd
Tf (℃)
(L/h)
Ts(℃) TⅠ
TⅡ
Vl(L) (L/20min)
40
50
36
40
39
44
13.3
1.35
30
48
34
38
37
41
10
0.85
20
46
34
38
37
41
6.7
0.74
10
44
34
40
40
45
3.4
0.64
40
50
35
38
37
41
13.3
0.74
30
46
33
33
32
39
10
0.6
20
44
34
38
37
42
6.7
0.37
10
40
30
35
34
39
3.4
0.4
MFE
CFE
4. The energy efficiency of microwave heating flash evaporation (MFE) system The energy efficiency for MFE system is calculated, and a dimensionless radio of Gained Output Ratio (GOR) is used to evaluate the effectiveness of distilled water production [26]. As an energy ratio for the total latent heat of flash evaporation of the water to the difference between total inlet heat and outlet heat, it is defined as an
index for the energy efficiency of MFE and CFE, the GOR is defined as:
GOR
md c
(8)
ml hl mc hc
A larger value of GOR in an evaporation system represents better distilled system. In order to calculate the heat difference, an average temperature of experiment is used to reckon the enthalpy of water and steam. The values of GOR under different experimental conditions and parameters are calculated and illustrated in Fig.7. 3.5
3.0
GOR
2.5
2.0
1.5
GOR 1.0
0.8
0.9
1.0
1.1
1.2
1.3
1.4
Microwave power (kW)
(a) Different microwave power
5.0
CFE MFE
4.5 4.0
GOR
3.5 3.0 2.5 2.0 1.5 1.0 39
40
41
42
43
44
45
46
47
48
49
Initial temperature (℃)
(b) Different initial temperature
50
51
7.0
CFE MFE
6.5 6.0
GOR
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0
10
20
30
40
Flow rate (L/h)
(c) Different flow rate Fig.7 The value of GOR for different experiment condition and parameter
Fig.7(a) shows the effect of the microwave power on GOR, with the increase of microwave power, the value of GOR is arising. Under the condition of the microwave power of 1.35 kW, the flow rate of 40L/h and the initial inlet temperature of water is 50 ℃ . The maximum value of GOR for MFE is 3.26, but for CFE process, the maximum value of GOR is 1.61. Simultaneously, the enthalpy of unit mass steam is rising with the temperature. Such a phenomenon of MFE makes the value of GOR is greater than the value of GOR of CFE. Fig.7(b) and Fig.7(c) shows the effect of the initial temperature and the flow rate on GOR. Clearly, the value of GOR is reductive with the increase of initial temperature. Meanwhile, the value of GOR has a trend of decrease with the increasing of flow rate. For these two different operating conditions, the value of GOR for MFE is about 1 to 2 higher than the value of GOR for CFE. The reason is that the difference between total latent heat of vaporization and the total enthalpy different of inlet to outlet leads to the variety of GOR.
In order to evaluate the energy efficiency of total MFE system, the ratio between requisite vaporation heat for distilled water and the total heat difference for inlet and outlet of liquid is calculated. Such a ratio is used to characterize the energy efficiency of total system, it is designed as follow:
md c ms hs mc hc ml hl md hd
(9)
According the experimental data shown in Table.3, the energy efficiency for different microwave power is calculated by Eq.(9) and illustrated in Fig.8.
Fig.8 The energy efficiency of MFE and CFE system
As a result, the energy efficiency for MFE system is among 60% to 80%. the quantity of distilled water of MFE system is raise with the microwave power, identically, the energy efficiency have a trend of raise with the increase of microwave power, it is indicated that the higher microwave power would get a better effect in MFE system, Specially, the maximum energy efficiency is 79.38%, and the value is gained under the condition of microwave power of 1.35 kW. However, the energy efficiency of CFE system is only about 59.42%, the energy efficiency of MFE system is higher than CFE system. According to Eq.(9), it is indicated that
microwave heating enhances the heat and mass transfer in evaporation process. 5. Conclusion In this work, a novel microwave heating flash evaporation (MFE) equipment. Significant effects of microwave power, initial temperature and flow rate of productivity and energy efficiency are observed through present experiment, the energy efficiency of MFE system is also presented in our study, and a dimensionless number of GOR is used to evaluate the energy efficiency of MFE system. The following points are concluded:
The microwave power, initial temperature and flow rate have important effect on flash evaporation quantity, energy efficiency and GOR.
The productivity of MFE system and CFE system are compared. As the experimental data shown. MFE has as twice as the approximately efficiency of CFE under various operation parameters, this illustrates that one stage of MFE can take place of two stages of conventional flash evaporation system.
The increase in microwave power, initial temperature of liquid and flow rate increases the productivity under the same initial pressure of vacuum tank. MFE have a greater productivity than CFE under the same condition.
According to the value of GOR which is influenced by the temperature difference of liquid of inlet and outlet and the production of distilled water as well as temperature, MFE have a better Gain Output Ratio than CFE. The value of GOR for MFE is about 1 to 2 higher than the value of GOR for CFE.
The maximum energy efficiency of MFE system is 79.38%.
Nomenclature
𝜀0
Permittivity: the real part
m
Mass, (kg)
𝜀′𝑟
Permittivity: the imaginary part
h
Enthalpy, (kJ/kg)
𝛿
Dielectric loss angle
∆Q
Energy loss (kJ)
f
Frequency, (Hz)
Qmicrowave
Microwave energy, (kJ)
π
Circumference ratio
v
Flow rate, (V/L)
V
Volume, (L)
E
Electric field intensity, (V/m)
H
Magnetic field intensity, (A/m)
Subscripts
q
Heat quantity
l
Liquid of inlet
A,B,C Empirical constant number of c
Liquid of concentrated
Antoine equation
f
Flash chamber
∆T
Superheated degree, (℃)
s
Steam
T
Temperature, (℃)
d
Distilled water
t
Time, (min)
sat
Saturated
e
Electric
h
Magnetic
η
energy efficiency
Greek symbols c𝜌
Specific heat, (kJ/kg ∙ ℃)
(x,y,z)
Cartesian coordinate system
k
Thermal conductivity
Abbreviations
ω
Axial vacuity of fluid
GOR
Gain output ratio
ρ
Material density
MFE
Microwave flash evaporation
CFE
Conventional flash evaporation
Acknowledgment This research is supported by basic research project for application in Yunnan province. 2015FA017. Reference [1] Nafey A S , Fath H E S , Mabrouk A A . Thermo-economic investigation of multi effect evaporation (MEE) and hybrid multi effect evaporation—multi stage flash (MEE-MSF) systems[J]. Desalination, 2006, 201(1-3):241-254. [2] Dae Hyun Kim. A review of desalting process techniques and economic analysis of the recovery of salts from retentates. Desalination 270 (2011) 1–8 [3] Druetta P, Aguirre P, Mussati S. Minimizing the total cost of multi effect evaporation
systems
for
seawater
desalination,
Desalination,
2014,
344(9):431-445. [4] A novel flash boosted evaporation process for alumina refineries[J]. Applied Thermal Engineering, 2016, 94:375-384. [5] Mabrouk A A , Nafey A S , Fath H E S . Thermoeconomic analysis of some existing desalination processes[J]. Desalination, 2007, 205(1-3):354-373. [6] Kalogirou S A. Solar Energy Engineering (Second Edition)[M]. 2014. [7] Yu W , Qi H , Qingzhong Y , et al. Energy and exergy analyses of circulatory flash evaporation of aqueous NaCl solution[J]. Desalination. [8] Chen Q , Thu K , Bui T D , et al. Development of a model for spray evaporation based on droplet analysis[J]. Desalination, 2016, 399:69-77.
[9] Gao W , Li C , Xu C , et al. Experimental study on water separation process in a novel spray flash vacuum evaporator with heat-pipe[J]. Desalination, 2016, 386:39-47. [10]Miyatake, Osamu, et al. Fundamental experiments with flash evaporation.Heat Transfer - Japanese Research2.4 (1973):89-100. [11]Miyatake, Osamu, et al. "Flash evaporation phenomena of pool water."Heat Transfer - Japanese Research6.2 (1977):13-24. [12]D. Saury *, S. Harmand, M. Siroux. Experimental study of flash evaporation of a water film. International Journal of Heat and Mass Transfer 45 (2002) 3447–3457 [13]Cheng W L, Chen H, Hu L, et al. Effect of droplet flash evaporation on vacuum flash evaporation cooling: Modeling, International Journal of Heat & Mass Transfer 2015, 84:149-157. [14]Z.F. Zhou, W.T. Wu, B. Chen, et al., An experimental study on the spray and thermal characteristics of R134a two-phase flashing spray, Int. J. Heat Mass Transfer 55 (15) (2012) 4460–4468. [15]Muthunayagam A E, Ramamurthi K, Paden J R. Low temperature flash vaporization for desalination, Desalination 2005, 180(1-3):25-32. [16]Sebastian P, Nadeau J P. Experiments and modeling of falling jet flash evaporators for vintage treatment, International Journal of Thermal Sciences, 2002, 41(3):269-280. [17]C.C. Tseng, R. Viskanta. Enhancement of water droplet evaporation by radiation absorption, Fire Safety Journal 2006, 41 (3): 236-247
[18]Hongtao Zhang, Vasudevan Raghavan, George Gogos. Subcritical and supercritical droplet evaporation within a zero-gravity environment: Low Weber number relative motion, International Communications in Heat and Mass Transfer 2008, 35 (4): 385-394 [19]Ganesapillai
Magesh,
Regupathi
Iyyaswami,
Murugesan
Thanapalan.
Characterization and process optimization of microwave drying of plaster of Paris, Drying Technology 2008, 26(12): 1484-1496, [20]Zheng Xianzhe, Wang Yingkuan, Liu Chenghai, Sun Jingkun, Liu Bingxin, Zhang Baohui, Lin Zhen, Liu Haijun. Microwave Energy Absorption Behavior of Foamed Berry Puree Under Microwave Drying Conditions, Drying Technology 2013, 31(7): 785-794 [21]Ming Huang, Jinhui Peng, Jing JingYang. Jiaqiang Wang. A New Equation for the Description of the Dielectric Losses Under Microwave Irradiation, J. Phys. D: APPl. Phy 2006, 39: 2255-2258 [22]Yousefi T, Mousavi S A, Saghir M Z, et al. An investigation on the microwave heating of flowing water: A numerical study [J]. International Journal of Thermal Sciences, 2013, 71(71):118-127 [23]Cherbański R, Rudniak L. Modelling of microwave heating of water in a monomode applicator – Influence of operating conditions [J]. International Journal of Thermal Sciences, 2013, 74(6):214-229 [24]Moffat R J. Contributions to the Theory of Single-Sample Uncertainty Analysis [J]. Journal of Fluids Engineering, 1982, 104(2):250--260.
[25]Surhone L M, Tennoe M T, Henssonow S F, et al. Antoine Equation[M]. 2010. [26]El-Agouz S A, El-Aziz G B A, Awad A M. Solar desalination system using spray evaporation [J]. Energy, 2014, 76:276-283.
1
2
3
4 6
7
8 10
5
14 8
11 12
9
13
1. Feed tank 2. Flow-meter 3. Thermocouples 4. Piezometer 5.Flash evaporation chamber 6. Magnetron 7.Wave guide 8. Thermometer 9. Concentrated tank 10. Cooling tank 11. Atmospheric valve 12.Distilled water tank 13. Vacuum pump 14. Central control box Fig.1 The schematic of experimental apparatus for MFE` 4 1 1
1
2
5
3
6 (a) The schematic diagram of flash evaporation chamber 1. Magnetron 2. Rectangular waveguide 3. Trapezoidal waveguide 4. Inlet 5. SiC perforated plate 6.Outlet
(b) The structure of SiC perforated plate Fig.2 The schematic diagram of flash chamber
1.50 Evaporation quantity
Evaporation quantity (ml)
1.35
1.20
1.05
0.90
0.75
0.60 0.72
0.81
0.90
0.99
1.08
1.17
1.26
1.35
1.44
Microwave power (kW)
Fig.3 Effect of microwave power on evaporation quantity
1.5 1.4
Evaporation quantity (L)
1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5
MFE CFE
0.4 0.3 39
40
41
42
43
44
45
46
47
48
49
50
51
Initial temperature (℃)
Fig.4. Effect of different initial temperature for MFE and CFE
1.5 1.4 1.3
Evaporation quantity (L)
1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3
MFE CFE
0.2 0.1 0.0
10
20
30
40
Flow rate (L/h)
Fig.5 Effect of flow rate on MFE and CFE
3.5
3.0
GOR
2.5
2.0
1.5
GOR 1.0
1.0
0.9
0.8
1.3
1.2
1.1
1.4
Microwave power (kW)
(a) Different microwave power
5.0
CFE MFE
4.5 4.0
GOR
3.5 3.0 2.5 2.0 1.5 1.0 39
40
41
42
43
44
45
46
47
48
49
Initial temperature (℃)
(b) Different initial temperature
50
51
7.0
CFE MFE
6.5 6.0
GOR
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0
10
20
30
40
Flow rate (L/h)
(c) Different flow rate Fig.6 The value of GOR for different experiment condition and parameter
Fig.7 The energy efficiency of MFE and CFE system
Fig.8 Physical model of MFE process
Table 1 The controlled operation parameters
Parameter
Value
Microwave power (kW)
0.81, 0.9, 0.99, 1.08, 1.17, 1.26, 1.35
Flow-rate (L/h)
10, 20, 30, 40
Initial temperature of water (℃)
40-50
Table 2 Uncertainty analysis. Parameter
Absolute uncertainly
Minimal measured value
Uncertainty
Power (kW)
0.1
0.81
1.2346×10-01
Temperature (℃)
1
31
3.2258×10-02
Flow rate(L/h)
2
10
2.0000×10-01
Volume (L)
1×10-3
0.4
2.5000×10-03
Pressure(KPa)
0.02
65
3.0769×10-04
Table 3 The experimental data for MFE with different microwave power and CFE
Experimen t groups
MFE
CFE
Microwave Power(kW)
Vl(L)
1.35 1.26 1.17 1.08 0.99 0.9 0.81 -
13.3 13.3 13.3 13.3 13.3 13.3 13.3 13.3
Tl(℃) Tc (℃) 50 50 47 48 48 50 48 47
42 38 41 35 37 37 38 34
Tf (℃) TⅠ 41 38 35 35 36 35 35 33
TⅡ 40 37 34 34 35 34 33 31
Ts(℃)
Vd (L/20min)
45 42 40 39 40 40 42 35
1.35 1.14 0.94 0.89 0.85 0.8 0.7 0.74
Table 4 The experimental data of MFE and CFE for different initial temperature Experiment groups
MFE
CFE
Tl (℃)
Vl(L/h)
Tc (℃)
50
40
48
Tf (℃)
Vd
Ts(℃)
(L/20min)
TⅠ
TⅡ
41
40
39
43
1.35
40
37
37
37
42
0.96
45
40
38
40
40
44
0.85
43
40
39
39
38
42
0.77
40
40
40
41
40
43
0.7
50
40
39
35
33
39
0.74
48
40
35
34
35
36
0.8
45
40
34
33
32
36
0.55
43
40
34
34
33
36
0.48
40
40
38
37
36
40
0.47
Table 5 The experimental data of MFE and CFE under different flow rate
Experimen Flow rate Tl(℃) Tc(℃) t groups (L/h) MFE
CFE
40 30 20 10 40 30 20 10
50 48 46 44 50 46 44 40
36 34 34 34 35 33 34 30
Tf (℃) TⅠ 40 38 38 40 38 33 38 35
TⅡ 39 37 37 40 37 32 37 34
Ts(℃)
Vl(L)
Vd (L/20min)
44 41 41 45 41 39 42 39
13.3 10 6.7 3.4 13.3 10 6.7 3.4
1.35 0.85 0.74 0.64 0.74 0.6 0.37 0.4
HIGHLIGHTS
Microwave flash evaporation system (MFE) used for evaporation is introduced.
The production of MFE is approximately twice as much as the conventional.
MFE could reduce effective number, shorten process and reduce investment.
The maximum energy efficiency of MFE system is 79.38%.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: