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Experimental investigation and cost assessment of the salt production by solar assisted evaporation of saturated brine
Accepted Manuscript Experimental investigation and cost assessment of the salt production by solar assisted evaporation of saturated brine
Jing Zhao, Huaigang Cheng, Xiao Wang, Wenting Cheng, Fangqin Cheng PII: DOI: Reference:
S1004-9541(17)30396-8 doi:10.1016/j.cjche.2017.08.016 CJCHE 916
To appear in: Received date: Revised date: Accepted date:
1 April 2017 22 August 2017 22 August 2017
Please cite this article as: Jing Zhao, Huaigang Cheng, Xiao Wang, Wenting Cheng, Fangqin Cheng , Experimental investigation and cost assessment of the salt production by solar assisted evaporation of saturated brine. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Cjche(2017), doi:10.1016/j.cjche.2017.08.016
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ACCEPTED MANUSCRIPT Experimental investigation and cost assessment of the salt production by solar assisted evaporation of saturated brine* ZHAO Jing(赵静)1,2, CHENG Huaigang(成怀刚)1,#, Wang Xiao(王晓)2, CHENG Wenting(程文婷)1, and CHENG Fangqin(程芳琴)1,* *
Institute of Resources and Environmental Engineering, State Environment Protection Key Laboratory of Efficient
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1
Utilization of Coal Waste Resources, Shanxi University, Taiyuan 030006, China College of Chemical Engineering, Qinghai University, Xining 817000, China
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2
Abstract The technical feasibility and economy of solar heat collection-forced evaporation process are the keys to
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its practicality, especially its application in strong brine treatment. The operation cost of applying solar collection
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in salt manufacturing through depth evaporation of brine has been studied. For Na+,K+,Mg2+//Cl-,SO42--H2O
salt–water system, most of the NaCl and all of the Carnallite were separated. The operation cost reached the
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optimum when the heat collection and evaporation were controlled at 75 and 55 °C, respectively. When the solar
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radiation amount was 19557 kJ·m-2·d-1, the solar collector area for producing Carnallite was about 34.27 m2·t-1salts,
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and the operation cost was $13 t-1salts. The energy consumption of salt manufacturing is at least 25% higher than
that of natural evaporation. Regarding the economy, the solar assisted salt manufacturing process is recommended
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to be performed at a production scale of more than 20 tons per day.
Keywords solar collection, evaporation, brine, salt manufacturing, economy
_______________________________ * Supported by National Natural Science Foundation of China (51674162), the Tri-Jin Scholars and Natural Science Foundation of Shanxi Province (201601D102058) and the Provincial Research Projects (2016JD06, 2016-HZ-803). ** Corresponding author. Tel.: +86-351-7018553; Fax: +86-351-7018553. E-mail: [email protected] (F.Cheng). # ZHAO Jing and CHENG Huaigang ([email protected]) contribute equally to this work.
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1 INTRODUCTION Utilizing solar heat energy for forced evaporation is common in brine treatment. Moreover, in most cases, it is used in the desalination technology of seawater or brackish water to separate fresh
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water from salt brine with 3.5% or lower salinity. When applying this method in the evaporation of higher-concentration brine, the economy of solar energy is not easily determined. The economy
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of solar energy depends on two factors: the other products from the evaporation besides fresh water (whether the values of the products are more than the cost of the consumed tremendous
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energy or not); and the feasibility of forced evaporation (the viscosity of the strong brine is higher
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and the evaporation is slower). The use of renewable energy may ease the pressure of energy consumption, as long as this process is characterized by technical feasibility and economic
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feasibility.
The major salt components of seawater and salt lakes, e.g., NaCl and KCl, can saturate and
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separate only when the total concentration is more than 25%. Under most circumstances, natural evaporation method is used to obtain the solid phase salt from the liquid. In the salt pan, it
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naturally evaporates through solar radiation, which concentrates the brine and separates the salt. No fossil energy is needed in this method; it is also easy to apply. Therefore, natural evaporation has been used for hundred years. However, solar thermal power is always considered a low-grade energy source and it is difficult to convert into other forms of energy. Thus, extracting salt from the brine usually involves a very time-consuming process. Nearly six months are required to prepare the solid salt deposit by evaporation in a solar salt pan[1], which requires a large area of
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land. Although improving the efficiency of solar energy is challenging, many new evaporation methods have been developed. For instance, the concept of the solar pond has been accepted for
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years[2, 3]. A salinity gradient solar pond is an artificially constructed pond that reaches high temperatures due to the accumulation of solar thermal energy and the simultaneous prevention of
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convection in the pond. The main function of solar ponds is to accumulate heat, which can promote evaporation by incorporating a combination of evaporation technologies, including
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evaporation ponds[4] and membrane distillation techniques[5]. The solar still, a simple device
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used to distill water, has also been used to process brine[6], but it is mainly used to produce potable water rather than capture salt. The solar dryer, a more intense form of the solar still, was
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also developed to promote brine concentration with the ultimate goal of salt recovery, and has made some contributions to the salt production industry[7]. One of the other methods that can be
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used to improve the water evaporation process is a solar-driven humidification-dehumidification process. In such processes, the vapor removed from the brine feed during evaporation is directed
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from the humidification unit into the dehumidification unit, and then condenses to form potable water[8].
The direct methods for accelerating evaporation involve forced evaporation methods, such as spray evaporation[9], direct-contact evaporation[10] and multiple-effect distillation[11, 12]. In the study of heat source for forced evaporation, heat collectors with efficiencies that are higher than the solar pond are common. In terms of the cost of solar heat collection and evaporation, many
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studies suggest that solar heat collection is categorized under capital intensive enterprise. The service cost of a solar heat collector and the related capital costs have been estimated, including the plate collector[13], the evacuated tube collector, and the solar-assisted steam generation
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system[14]. One study[15] suggests that the investment in collectors accounts for the maximum proportion of all expenses, which is about one third of the total investment. For the processes of solar pond coupling and multi-stage desalination, water cost is estimated to be $1.80 m-3 to $4.55
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m-3[16], which is more expensive than the desalination process with coal or oil as energy. Another
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conclusion is that the scale enlargement of solar heat collection-evaporation desalination
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contributes to the cost reduction of the products. For example, when the scale of the desalination water is 15 m3·d-1, the water production cost is $5.48 m-3; whereas when the scale of the
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desalination water is 300 m3·d-1, the cost of water production is $2.39 m-3[17]. In a multi-effect
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distillation plant whose size is less than 100m3·d-1, the water has to be produced at a cost of €2-€8
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m-3, but the water cost for a 91,000 m3·d-1 desalination plant is only €0.42-€0.81 m-3[18]. Many studies on solar collection methods for forced evaporation have been conducted.
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However, most of the collected heat energy is used for seawater desalination but not salt manufacturing. Most salt manufacturing processes have large evaporation capacity and low salt price. Therefore, the profits generated by forced evaporation process are relatively minimal. In most cases, based on the profit motive, forced evaporation is used primarily in the production of easily salting products[19] or high-profit salts. The production of Glauber's salt[20], which is made using a multiple-effect evaporation process, is a thermally efficient and economical example.
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Another example of using forced evaporation is in the Searles Lake of the Mojave Desert[21], where a three-effect evaporation method is used to process brine and prepare salt. Most of the forced evaporation processes involve taking coal, oil, and other fossil resources as energy source.
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To rapidly provide the latent heat of vaporization, the forced evaporation process requires enough heat energy for a short time. Evidently, it is difficult for natural solar radiation to efficiently
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provide intensive heat, which is also a bottleneck issue in applying solar energy in the forced evaporation process of strong brine.
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The current study aimed to use solar heat collectors to concentrate solar heat energy on the
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brine to carry out depth evaporation and manufacture industrial salt, as well as to analyze the energy consumption and operation cost of the process. Salt manufacturers would significantly
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benefit if a fast and efficient method of brine evaporation and salt production was developed. The
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selected brine systems in this study were the Na+,K+,Mg2+//Cl-,SO42--H2O salt–water systems,
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which are the major normal components of seawater, and a subsystem of many brine systems of salt lakes. These brine systems can be found in the salt lakes of China, Israel, and Jordan. In
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addition, other types of salt lakes exist, such as the Great Salt Lake and the Searles Salt Lake in the USA, where the brine also contains the components of this subsystem. Thus, this kind of salt–water system may be used in the salt-manufacturing process of strong brine evaporation.
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2 EXPERIMENTAL DETAILS As shown in Figure 1, a forced evaporator/crystallizer combined with a 2 m2 plate solar collector was built in Qinghai Province, northwest China, at an altitude of 2700 meters, where the
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average annual solar radiation intensity is about 19557 kJ·m-2·d-1. The plate solar collector was used to replenish heat for the brine in a timely manner during the evaporation process. A solar
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power meter (Model SM206) was used to record the radiation intensity of sunlight.
Figure 1 Schematic diagram and photos of experimental set-up (1-Plate solar collector; 2-Brine tank; 3-Vapor-liquid separator; 4-evaporator (crystallizer); 5-Heat exchanger (condenser); 6-Water tank; 7-Solar collector (preheater) )
Brine was obtained by dissolving the solid potash mine of the earth’s surface using fresh water and was used as the feed brine after mixed with a certain amount of Bischofite
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(MgCl2·6H2O). Before entering the plate solar collector and the evaporator, brine was stored in two 140 L tanks and preheated for one day by two preheaters composed of 33 solar evacuated tubes whose upper ends were connected to the tanks. The preheater was used in the intermittent
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operation mode. The evacuated tubes converted the solar radiation to heat energy and the brine in tanks was heated continuously from morning till evening, and then the brine reached the
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maximum temperature. Hot brine was pumped out of the storage tanks on the next day and then transferred into the plate solar collector and the evaporator.
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During the daytime, the plate collector and the evaporator were operated in the continuous
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operation mode. The hot brine was introduced into the evaporator coupled with a vacuum pump for concentration and flash evaporation. The secondary steam from the evaporation was separated
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in the separator and then condensed to the recycled fresh water in the condenser. The recycled freshwater was used again to dissolve the solid potash mine of the earth’s surface. The solid phase
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salt precipitated and was separated at the bottom of the evaporator, and the supernatant at the upper part of the evaporator continues to cycle to the plate solar collector. During the whole
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process, the brine was constantly concentrated until the desired concentration was achieved before it was discharged.
The evaporation of high concentration brine solutions requires high density thermal energy. However, the grade of the secondary steam is usually low. Thus, the secondary steam was not reused for evaporation, resulting in a relatively low efficiency of heat utilization. The recycled freshwater was about 40 °C in temperature, whose sensible heat can enhance the dissolution of the
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solid potash mine of the earth’s surface. The preheating and heating to brine were carried out from 08:00 to 18:30 every day. The data were got from August 6th to 30th. The evaporation ratio was defined to be the ratio of the amount
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of water evaporated to the amount of water in the feed, and was used to measure the evaporation process. Analyses of the components of the liquid and solid phases were performed using atomic
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absorption spectroscopy (Shimadzu AA 6650) for cations and the titration method for anions. Components of the solid phase were further confirmed using X-ray diffraction (XRD, Bruker AXS
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D8).
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3 THEORETICAL CALCULATION
3.1 Evaporated water and solid salt production
F
o
75
C
I C (MgCl2) IV
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B (KCl)
(Car)
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III
F
C (MgCl2)
E
E0 II
E1 IV
A (NaCl) O1
O
O0
― 75°C, ······ 15°C Figure 2 Phase diagram of Na+,K+,Mg2+//Cl-,SO42--H2O salt–water system (Crystalline phase area: I-KCl, II-NaCl, III-Carnallite (KCl·MgCl2·6H2O), IV- Bischofite (MgCl2·6H2O);
Phase points: O - feed brine, E - co-saturated solution of KCl, NaCl and Carnallite, F – co-saturated solution of NaCl, Carnallite and Bischofite)
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Ignoring the low content of sulfate, the analysis of evaporation process of Na+,K+,Mg2+//Cl--H2O system according to its dry phase diagram is shown in Figure 2. Assuming that A, B and C represent NaCl, KCl and MgCl2, and O, E and F represent the liquid phase points,
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respectively. In practice, the brine-mixing operation is usually carried out so that the phase point of feed brine (O) locates on the line AE, which can ensure that all of the KCl is precipitated in the
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form of Carnallite rather than Sylvinite. As a result, NaCl is separated first during the evaporation process and then the liquid phase moves from the point O to the point E, followed by the
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precipitation of Carnallite when the liquid phase finally reaches point F. Thus, the evaporation of
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this kind of brine can be divided into two stages, i.e., NaCl precipitation stage and Carnallite precipitation stage. In the salt lake industry, Carnallite can be used as the raw material of
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potassium fertilizer.
Generally, the calculations can be carried out using the salt–water system phase diagram.
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However, this research adopted one more direct method. In the precipitation stage of A, B remains
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unseparated and the equation below holds,
wo c B ,o = wE c B , E
(1)
w A = wo c A , o − w E c A , E
(2)
wvapor , E = wo − w E − w A
(3)
where w represents the mass of brine or salt, depending on the subscript of phase point. The suvscript vapor means the vaporation amount of water. c represent the mass percentage of salt components in brine.
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In the Carnallite precipitation stage, the brine at point E would finally reach the point F and separate major salt product xB·yC·zH2O and other by-product salts, then the equation below applies.
wE c B , E − w F c B , F
wE c C , E − w F c C , F =
yM C
(4)
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xM B
where M means the molecular weight. In the above equations, wo, cB,O, wE, cB,E, cC,F and cC,E
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can be seen as known through phase diagram or through experiment. Then, the mass of
w E c B , E − wF c B , F xM B
(xM B + yM C + zM H 2O )
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wBC =
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xB·yC·zH2O (wBC) and by-product salts (wby-producted,j) can be calculated,
wby − product , j = w E c j , E − w F c j , F
j=1,2,3…
(5)
(6)
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where j represents the mass percentages of by-product salt component which can also be determined through phase diagram or experimental analysis. Thus, the mass of all the precipitated
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solid-phase salt and the evaporated water amount are, m
wsalts = wBC + ∑ wby − product, j
(7)
j =1
wvapor,F=wE-wF-wsalts
(8)
At this time, the grade of the salt product of Carnallite can be defined as wBC/wsalts×100%.
3.2 Energy consumption It is assumed that T represents the brine temperature, and subscript in and out represent the brine before and after the flash evaporation, respectively, and r means the latent heat of water
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vaporization. By ignoring heat loss, the amount of flashed water can be estimated as
winc p (Tin − Tout ) r
; where cp represents the specific heat of brine. Thus, the mass of the
precipitated salt from the brine can be expressed as
winc p (Tin − Tout ) wsalts . Usually the brine r wvapor
win c p (Tin − Tout ) 1 + wsalts w r vapor
second flash to the initial feeding brine is
c p (Tin − Tout ) 1 + wsalts w r vapor
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ϕ = 1−
. The mass ratio of the brine for
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can be expressed as win −
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needs to be flashed circularly, thus the mass of the brine in the recirculation for the second flash
(9)
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When the circulation times is n which can be determined through trial, the following equation
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is used,
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n win ∑ ϕ i −1 c p (Tin − Tout ) = wvapor r i =1
(10)
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The released heat of crystallization in the salt precipitation process was roughly excluded
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from the calculation, then the equation below can be applied,
qAη = win c p (Tin − Tout )
(11)
where q represents the daily solar heat on the unit collector area, A represents the area of plate solar collector, respectively. η represents the collector’s work efficiency, taking the empirical value of 50%. If the unit price of the solar heat collector is a $·m-2, its total investment is Aa $. When production cost is calculated, this amount can carry out the depreciation according to a five-year depreciation periods[13], represented by F in this study. At present, the average unit
ACCEPTED MANUSCRIPT price of solar collectors is about 161 $·m-2 and the lowest enquiry is about 80.6 $·m-2. During flash evaporation, the secondary steam volume per unit time can be approximately
calculated as
wvapor ρ vapor t work
, where ρ represents the density and twork represents the operation time of
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the evaporator. According to the steam volume and vacuum degree, the required power Nvacuum can be determined based on the type of injection pump for the vacuum generation available in the
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market. Empirically and approximately, when the steam volume falls between 20 m3·h-1 to 80
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m3·h-1, Nvacuum approximately presents a linear increase from 1.5 kW to 5.5 kW; when the steam
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volume falls between 80 m3·h-1 and 500 m3·h-1, Nvacuum approximately presents a linear increase n
win ∑ ϕ i −1
from 5.5 kW to 15 kW. The average flow rate of brine circulation is about
i =1
. Based on
t work
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this data, the type of circulating pump in the market was selected, and its power Npump was
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determined. When the flow rate falls between 5.0 m3·h-1 to 200 m3·h-1, Nvacuum roughly presents a
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linear increase from 3.3 kW to 48 kW. The electricity cost is relevant to the local electricity price Ψ, i.e., about 0.10 $·kW-1·h-1 for industrial uses.
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The total energy consumption of the whole process can be obtained as below,
W = win c p (Tin − Tambience ) + wvapor r + (N vacuum + N pump )t work
(12)
where Tambience represents the ambient temperature. It is assumed that wvapor,15 and r15 are the evaporated water amount and the latent heat of vaporization in natural evaporation at 15 °C, respectively. Thus, the energy consumption can be contrasted to evaluate the extra energy consumption of flashing with that of natural evaporation, as shown in equation (13)
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W / W Natural =
win c p (Tin − Tambience ) + wvapor r + (N vacuum + N pump )t work wvapor ,15 r15
(13)
If only the energy-related cost including the collector investment and energy consumption is considered, the operation cost for producing per unit ton of salt can be estimated as below,
A ⋅ a / (F ⋅ t workdays ) + Ψ (N vacuum + N pump )t work / day
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J=
wsalts / day
(14)
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where tworkdays is the operating days, which is 270 days per year in this calculation; twork/day and wsalts/day represents the working hours and the salts production per day, respectively.
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4 RESULTS AND DISCUSSION
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4.1 Pretreatment and characteristics of brine under high temperature
The evaporation temperature of salt-pan natural evaporation generally is about 15 °C, while it
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is approximate 75 °C for solar energy-forced evaporation process. As shown in Figure 2, the feed brine initially precipitates NaCl out at any temperature during evaporation because it is located in
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the crystalline phase area of NaCl. However, in the subsequent evaporation, the precipitation laws of salt at different temperatures vary. For example, at the evaporation temperature of 75 °C, the
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phase point of the liquid phase shifts to point E, which reaches the area of Carnallite precipitation. However, at the evaporation temperature of 15 °C, the phase diagram indicates that the precipitation area of Sylvinite (mainly composed of KCl and NaCl) is significantly large. This result demonstrates the considerable precipitation amount of Sylvinite and reduced precipitation amount of Carnallite. In the actual production process, the lesser the Sylvinite precipitate, the better the result is. Figure 2 shows that when the phase point of brine is close to the precipitation
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area of KCl or Carnallite, the mass ratio of MgCl2/KCl in the feed brine determines the precipitation of Sylvinite or Carnallite. At the evaporation temperature of 75 °C, the initial ratio of MgCl2/KCl should be controlled to approximately 6 according to Figure 2. The phase point of
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brine obtained by dissolving the solid potash mine is located in point O0 in Figure 2, where the MgCl2/KCl ratio is 4.07. The expected Carnallite requires that the point O0 moves toward MgCl2.
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Thus, Bischofite, i.e., the residue after brine evaporation in salt pan operations, is usually used in brine mixing operation and added into the brine to increase the MgCl2/KCl ratio. Figure 2
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indicates the changes in brine components caused by the different additions of Bischofite, i.e., the
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points O and O1. In natural evaporation, the addition of 200 kg·m-3brine of Bischofite led to the MgCl2/KCl ratio of 9.84 (see point O1), which ensures that separation of Carnallite (see point E1).
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In forced evaporation at 75 °C, the addition of 50 kg·m-3brine is enough, which nearly results in a MgCl2/KCl ratio of 6 (see point O). As indicated in Figure 2, evaporation at that time ensures that
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the separated Carnallite is in solid phase (see point E1). Figure 3 shows the changes in KCl content of the separated salt during continuous
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evaporation of feed brine at point O in Figure 2. When in solid phase salt, the evaporation ratio reached approximately 58% and the KCl content suddenly increased but gradually decreases after 62%. The NaCl, the Carnallite with Sylvinite and the Bischofite without potassium is successively separated as the evaporation continues, which causes the KCl content in the solid phase salt to initially increase and then decrease. Thus, brine is in the separation interval of Carnallite when the evaporation ratio is between 58% and 62%. Generally, the large perturbation of the liquid phase
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during the rapid evaporation could influence salt crystallization and even cause non-crystallization. In this work, evaporation occurred on the liquid phase surface through the evaporator. The bulk liquid under its surface, which was where the crystallization occurred, was undisturbed. Thus, the
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20 18 16 14 12 10 8 6 4 2 0
6
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4 2 0
0
Figure 3
Viscosity (mPa.s)
8
10
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KCl (wt%)
crystallization can be ensured.
20 30 40 50 60 Evaporation ratio (%)
70
Viscosity of liquid phase and change of KCl content in solid phase during the evaporation process
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(Viscosity of liquid phase: ◊ 35°C, □ 45°C, Δ 55°C, ○ 65°C, × 75°C KCl content in solid phase: ● the experimental, ― the calculated)
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Although the evaporation ratio could reach up to 62%, some difficulties occurred in in-depth
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evaporation. Figure 3 shows that the liquid viscosity increases along with evaporation when the evaporation ratio is greater than 50%, which is unfavorable for the flow in the evaporator,
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indicating that a thorough evaporation is difficult to achieve in forced evaporation. Therefore in
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this study NaCl and Carnallite were extracted when the evaporation was completed, meanwhile the remaining liquid phase containing a large amount of MgCl2 flowed into the salt pan and was naturally evaporated. The main difference between natural evaporation and forced evaporation is that forced evaporation cannot be conducted during the Bischofite separation period.
4.2 The solar assisted evaporation of brine For the evaporation of brine, the higher the heating temperature and the lower the flashing
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temperature are, the larger the evaporation capacity of flashing is. However, the total operating conditions may not be quite simple. Figure 4 shows the brine circulation times n, collector area A, the power consumption Nvacuum+Npump, and the estimated operating cost J in the solar
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collection-forced evaporation process under different Tin and Tout when one ton of salt is produced. The circulation times of the brine show decreasing and increasing trends along with the increase in
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Tin and Tout, respectively, and the increase of brine temperature can ease the load of the pumps, whereas the collector areas show contrary variation trends. As a result, the data of operating cost
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show that for Tin, the temperature should be lower than 75 °C, whereas for Tout, the higher the
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temperature, the better. Considering the circulation times of the brine, controlling Tout at 55 °C is
50
60
III
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16.0 70
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60
II
50
I
20
30
10
20
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0
65
70
75
80 85 o Tin ( C)
120
40
IV
2
30
A (m /tsalts)
n
40
125
90
95
10
15.5 15.0
115
14.5
110
14.0
J ($/tsalts)
70
Npump+Nvacuum (kW.h/tsalts)
better.
(a) wsalts(Carnallite)-1.5 t·d-1, Tout-55 °C, a-80.6$·m-2, electricity price-0.10 $·kW-1·h-1, q-19557kJ·m-2·d-1
40
II
140
50
130
40
2
30 20
IV
10 0
60
III 45
50 Tout
55 ( C)
60
30
120
20
110
10 65
100
20 18 16
J ($/tsalts)
I
150 Npump+Nvacuum (kW.h/tsalts)
50
70
A (m /tsalts)
60
n
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70
14 12 10
o
(b) wsalts(Carnallite)-1.5 t·d-1, Tin-75 °C, a-80.6$·m-2, electricity price-0.10 $·kW-1·h-1, q-19557kJ·m-2·d-1 I-Circulation times, n; II-Solar collector area, A; III-Electric power consumption, Npump+Nvacuum; IV-Operation cost, J Figure 4
Operating parameters in the solar heat collection-forced evaporation process
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70 60
2000
50 1500
40 30
1000
20 500
10
0
0 9
11 13 15 Time (hour)
17
19
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7
Figure 5
Temperature (°C)
2
Solar radiation (w/m )
80 2500
Temperature of brine heated by solar collectors
(○ Solar radiation; □ Temperature of brine heated by the preheater; solar collector)
Temperature of brine heated by the plate
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To obtain a reasonable evaporation program, the feed temperature and evaporation ratio at
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solid–liquid separation should be constant, and the evaporation time or evaporation rate must be determined during brine evaporation. Figure 5 shows the preheating and heating to brine. In the
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preheating process, experimental results showed that the collector tubes could provide almost 280
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L of brine at 75 °C. In the evaporation process, the brine circularly flows through the plate
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collector to maintain the temperature at 55 °C or above, which can ensure the smooth operation of the forced evaporation process. Figure 6 shows the evaporation rate (speed) comparison between
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forced evaporation and natural evaporation, both with an initial inventory of 12.5 L and liquid
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surface evaporation area of 0.03 m2. Under natural evaporation conditions, the evaporation ratio reached 45% after 300 h, and the evaporation rate seemed gradually slow down because the increasing solution concentration resulted in higher vapor pressure. The decrease in evaporation rate was also found in forced evaporation, but the evaporation ratio took only 5 h to reach above 60%, which is 100 times faster than 20 to 30 days in natural evaporation.
Evaporation ratio (%)
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2
0
50
4
6
8
10
12
80 70 60 50 40 30 20 10 0
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100 150 200 250 300 Time (h)
Figure 6 Evaporation ratio in forced evaporation and natural evaporation (■ 15°C, natural evaporation; ▼ 75°C without vacuum;○ 55°C with vacuum; Δ 75°C with vacuum)
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As shown in the above analysis and the data in Figure 3 and Figure 6, the feed brine in the experiment were preheated to 75 °C. Meanwhile, the solid–liquid separation was carried out at the
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evaporation ratio of 58% and 62%, respectively; accordingly, the evaporation was divided into
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three stages, i.e., the stages of solid NaCl separation, the Carnallite/NaCl mixture separation, and the discharging liquid phase, respectively. When the temperature of discharging liquid phase drops
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from Tout to Tambience, Bischofite is in paste form, which contains certain moisture. This paste was piled up and dried for brine mixing. Table 1 shows the product components of the three stages.
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The solid phase product of the first stage is confirmed as NaCl by the XRD analysis shown in
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Figure 7. The K+ and Mg2+ contents of solid salt obtained in the second stage reached 16.62%, which was of better quality than that of natural evaporation. Moreover, the XRD results shown in Figure 7 indicate that K and Mg are mainly crystallized in the form of Carnallite. The brine in the third stage mostly consisted of MgCl2 with 36.60% content.
ACCEPTED MANUSCRIPT Table 1
The chemical composition of the solid salts /wt% Mg2+
Cl-
SO42-
H 2O
Stage 1
33.42
0.96
2.16
57.3
1.77
4.39
Stage 2
5.52
9.12
7.50
33.20
4.09
40.57
Stage 3
0.48
0.53
9.86
26.74
6.04
56.35
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K+
14000 12000 10000 8000 6000
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Intensity (Counts) .
Na+
4000 2000 0 20
30
40
50
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10
2θ (°)
60
70
80
(a) Salt of stage 1 (NaCl)
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5000 4000 3000 2000 1000
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Intensity (Counts) .
6000
0
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(b) Salt of stage 2 (Carnallite & NaCl) Figure 7
XRD results of deposit solid in brine evaporation
1000
40
800
I
1.00
Figure 8
III 0
10
20
400
10
200
V
IV
20 wsalts (ton/day)
30
600
2
30
1.25
0.75
A (m /tonsalts)
1.50
0 40
0
75
50
25
J ($/tsalts)
1.75
50
Npump+Nvacuum (kW.h/tsalts)
2.00
II W/Wnatural
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4.3 Consumption of thermal energy
0
Operating parameters under various production scales
(Tin-75°C, Tout-55°C, a-80.6$·m-2, electricity price-0.10 $·kW-1·h-1, q-19557kJ·m-2·d-1, I-W/Wnatural; II-Solar collector area, A; III-Electric power consumption, Npump+Nvacuum; IV-Operation cost for Carnallite production, V-Operation cost for NaCl production, J)
Owing to the cost of thermal collector and evaporator, forced evaporation is less economical
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compared with natural evaporation. Nevertheless, profits can still be realized because of the increased production efficiency. However, the biggest problem for the forced evaporation might be the estimation of the cost of energy. It is well known that the forced evaporation technology is
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economically competitive only for production on a large scale (thousands of m3·d-1). Figure 8 shows the results under the assumed condition of daily eight-hour and annual nine-month operation of the collector-evaporator equipment. The solar collector area for producing one ton of
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Carnallite is constant, which is about 34.27 m2·t-1salts. The energy consumption of the pump
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gradually decreases with the expansion of the production scale. Therefore, the total operation cost
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also decreases, which makes the energy consumption of forced evaporation closer to that of natural evaporation. Their ratio W/Wnatural also decreases continuously until finally approaching
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1.25. This observation indicates that the energy consumption of forced evaporation is at least 25% higher than that of natural evaporation. When the production scale of Carnallite is over 20 tons per
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day, its operation cost can reach a stable value. The cost at this time is equivalent to 13 $·t-1salts. When the production scale of Carnallite is over 50 tons per day, the operation cost can be
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decreased to 10 $·t-1salts. For the solar collection-forced evaporation process, we can evaluate the operation cost by comparing the price of solid phase salt product. The price of local Carnallite is about 65 $·t-1salts. Considering that the equipment investment (evaporator, heat storage tank, and other commonly used equipments) is usually equivalent to two-thirds of investment in solar collectors[15], the price of 13 $·t-1salts for the operation cost is acceptable. However, in brine evaporation, the amount of evaporated water in the precipitation stage of
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NaCl is large, whereas the precipitation amount of salt is relatively low; thus, its operation cost is higher than that of Carnallite. Apparently, the solar collection-forced evaporation process is not suitable for NaCl production in terms of economy.
J ($/tsalts)
22
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24 IV
20 II
18
III
16
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I
14 65
70
75
80 85 o Tin ( C)
90
95
Figure 9
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Tout-55°C, wsalts(Carnallite)-1.5 t·d-1
Operation cost under various prices for energy consumption
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(I: a-80.6$·m-2, Ψ-0.10$·kW-1·h-1, q-19557kJ·m-2·d-1; II: a-80.6$·m-2, Ψ-0.10 $·kW-1·h-1, q-9778.5kJ·m-2·d-1; III: a-242$·m-2, Ψ-0.10$·kW-1·h-1, q-19557 kJ·m-2·d-1; IV: a-80.6$·m-2, Ψ-0.16 $·kW-1·h-1, q-19557 kJ·m-2·d-1)
The operating cost of the solar collection-forced evaporation process is also influenced by the
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price of energy, which always fluctuates. Therefore, the economy of solar collection-forced evaporation process also changes. Figure 9 shows the operation costs of the solar collection-forced
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evaporation process after changes in collector price, power price, solar radiation intensity, and Tin. The influence of solar collection cost on the total operating cost is different from that of the power
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cost. As long as the unit price of power increases, no matter which temperature the evaporation process is at, the operating cost will increase as well. After the solar collection cost increases, regardless of whether it is caused by the collector price or the increasing collector area caused by the reducing radiation strength, the influence of high temperature manifests more significantly than that of low temperature. Therefore, the higher the temperature, the greater the increase in the operating cost. Under the condition of high solar collection cost, implementation of the whole
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process at low temperature is more adequate.
5 CONCLUSIONS The solar collection-forced evaporation experiment applied in the treatment of saturated brine
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of Na+,K+,Mg2+//Cl-,SO42--H2O system was designed and studied. The evaporation temperature after solar heating is higher than that of natural evaporation, which results in different changes of
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physical properties of the brine. The evaporation rate of the brine is 100 times faster than that of natural evaporation. NaCl, Carnallite and Bischofite precipitated in sequence, and the control
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points of solid-liquid separation during the evaporation process are determined by a phase diagram
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and experiments. Due to the viscosity increase, the precipitation stage of Bischofite is not suitable for forced evaporation. To obtain more Carnallite, the MgCl2/KCl ratio in the feed brine should be
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controlled at 5.88 instead of 9.84 in the natural evaporation. The current research estimated the solar collector area, power consumption, and total
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operation cost. Results indicated that the economy is at optimum when the feed brine is heated to 75 °C and evaporated at 55 °C in temperature. The larger the production scale of solar
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collection-forced evaporation, the lower the operation cost. The recommended production scale is at least 20 tons per day, in which the operation cost is 13 $·t-1salts, and the solar collector area for producing one ton of Carnallite is about 34.27 m2. Given such production scale, the energy consumption of forced evaporation is at least 25% higher than that of natural evaporation. Furthermore, the higher the operation temperature, the more significant the influence of collector cost on the operation cost will be.
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REFERENCES (1) Tijani, M. N., Loehnert, E. P., “Exploitation and traditional processing techniques of brine salt in parts of the Benue-Trough”, Nigeria. Int. J. Miner. Process, 74(1–4), 157–167(2004). (2) El-Sebaii, A.A., Ramadan, M.R.I., Aboul-Enein, S., Khalla, A.M., “History of the solar ponds: A review study”,
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Renew. Sust. Energ. Rev., 15(6), 3319-3325(2011). (3) Nie, Z., Bu, L.Z., Zheng, M.P., Huang, W.N., “Experimental study of natural brine solar ponds in Tibet”, Sol. Energy, 85(7), 1537-1542(2011).
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(4) Agha, K.R., Abughres, S.M., Ramadan, A.M., “Maintenance strategy for a salt gradient solar pond coupled with an evaporation pond”, Sol. Energy, 77(1), 95-104(2004).
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(5) Suárez, F., Tyler, S.W., Childress, A.E., “A Theoretical Study of a Direct Contact Membrane Distillation
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System Coupled to a Salt-Gradient Solar Pond for Terminal Lakes Reclamation”, Water Res., 44(15), 4601-4615(2010).
(6) Xiao, G., Wang, X.H., Ni, M.J., Wang, F., Zhu, W.J., Luo, Z.Y., Cen, K.F., “A review on solar stills for brine
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desalination”, Appl. Energy, 103, 642-652(2013).
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(7) Pereira, M.C., Mendes, J.F., Horta, P., Korovessis, N., “Final design of an advanced solar dryer for salt recovery from brine effluent of an MED desalination plant”, Desalination, 211(1-3), 222-231(2007).
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(8) Hou, S.B., “Two-stage solar multi-effect humidification dehumidification desalination process plotted from pinch analysis”, Desalination, 222(1-3), 572-578(2008).
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(9) Hou, J.W., Cheng, H.G., Wang, D., Gao, X.L., Gao, C.J., “Experimental investigation of low temperature distillation coupled with spray evaporation”, Desalination, 258(1-3), 5-11(2010). (10) Ribeiro, Jr. C.P., Lage, P.L.C., “Experimental study on bubble size distributions in a direct-contact evaporator”, Braz. J. Chem. Eng., 21(1), 69-81(2004). (11) Boopathy, R., Gnanamani, A., Mandal, A. B., Sekaran, G., “A First Report on the Selective Precipitation of Sodium Chloride from the Evaporated Residue of Reverse Osmosis Reject Salt Generated from the Leather Industry”, Ind. Eng. Chem. Res., 51(15), 5527-5534(2012). (12) Boopathy, R., Sekaran, G., “First Report on Separation of Sulfate Ions from Evaporated Residue of RO Reject
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Stream Generated from the Leather Industry”, Ind. Eng. Chem. Res., 52(26), 9239−9246(2013). (13) Tyagi, S.K., Wang, S.W., Park, S.R., Sharma, A., “Economic considerations and cost comparisons between the heat pumps and solar collectors for the application of plume control from wet cooling towers of commercial buildings”, Renew. Sust. Energ. Rev., 12(8), 2194–2210(2008).
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(14) Brunet, R., Antipova, E., Guillén-Gosálbez, G., Jiménez, L., “Reducing the Environmental Impact of Biodiesel Production from Vegetable Oil by Use of a Solar-Assisted Steam Generation System with Heat Storage”, Ind. Eng. Chem. Res., 51 (51), 16660-16669(2012).
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(15) Liu, X.H., Chen, W.B., Gu, M., Shen, S.Q., Cao, G.J., “Thermal and economic analyses of solar desalination system with evacuated tube collectors”, Sol. Energy, 93, 144–150(2013).
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(16) Agha, K.R., “The thermal characteristics and economic analysis of a solar pond coupled low temperature multi stage desalination plant”, Sol. Energy, 83(4), 501-510(2009).
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( 17 ) Szacsvay, T., Hofer-Noser, P., Posnansky, M., “Technical and economic aspects of small-scale solar-pondpowered seawater desalination systems”, Desalination, 122(2-3), 185-193(1999).
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223(1-3), 448-456(2008).
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(18) Karagiannis, I.C., Soldatos, P.G., “Water desalination cost literature: review and assessment”, Desalination,
( 19 ) Bayuadri, C., Verrill, C.L., “Rousseau R.W. Stability of sodium sulfate dicarbonate (similar to
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2Na2CO3·Na2SO4) crystals obtained from evaporation of aqueous solutions of Na2CO3 and Na2SO4”, Ind. Eng. Chem. Res., 45(21), 7144-7150(2006).
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(20) Garrett, D.E., “Sodium Sulfate: Handbook of Deposits, Processing, and Use”, Academic Press, pp. 262(2001). (21) Raymond, R.E., Albert, K.J., “Riegel's Handbook of Industrial Chemistry”, Springer, pp. 430(2003).
Jing Zhao, Huaigang Cheng, Xiao Wang, Wenting Cheng, Fangqin Cheng PII: DOI: Reference:
S1004-9541(17)30396-8 doi:10.1016/j.cjche.2017.08.016 CJCHE 916
To appear in: Received date: Revised date: Accepted date:
1 April 2017 22 August 2017 22 August 2017
Please cite this article as: Jing Zhao, Huaigang Cheng, Xiao Wang, Wenting Cheng, Fangqin Cheng , Experimental investigation and cost assessment of the salt production by solar assisted evaporation of saturated brine. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Cjche(2017), doi:10.1016/j.cjche.2017.08.016
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ACCEPTED MANUSCRIPT Experimental investigation and cost assessment of the salt production by solar assisted evaporation of saturated brine* ZHAO Jing(赵静)1,2, CHENG Huaigang(成怀刚)1,#, Wang Xiao(王晓)2, CHENG Wenting(程文婷)1, and CHENG Fangqin(程芳琴)1,* *
Institute of Resources and Environmental Engineering, State Environment Protection Key Laboratory of Efficient
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1
Utilization of Coal Waste Resources, Shanxi University, Taiyuan 030006, China College of Chemical Engineering, Qinghai University, Xining 817000, China
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2
Abstract The technical feasibility and economy of solar heat collection-forced evaporation process are the keys to
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its practicality, especially its application in strong brine treatment. The operation cost of applying solar collection
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in salt manufacturing through depth evaporation of brine has been studied. For Na+,K+,Mg2+//Cl-,SO42--H2O
salt–water system, most of the NaCl and all of the Carnallite were separated. The operation cost reached the
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optimum when the heat collection and evaporation were controlled at 75 and 55 °C, respectively. When the solar
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radiation amount was 19557 kJ·m-2·d-1, the solar collector area for producing Carnallite was about 34.27 m2·t-1salts,
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and the operation cost was $13 t-1salts. The energy consumption of salt manufacturing is at least 25% higher than
that of natural evaporation. Regarding the economy, the solar assisted salt manufacturing process is recommended
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to be performed at a production scale of more than 20 tons per day.
Keywords solar collection, evaporation, brine, salt manufacturing, economy
_______________________________ * Supported by National Natural Science Foundation of China (51674162), the Tri-Jin Scholars and Natural Science Foundation of Shanxi Province (201601D102058) and the Provincial Research Projects (2016JD06, 2016-HZ-803). ** Corresponding author. Tel.: +86-351-7018553; Fax: +86-351-7018553. E-mail: [email protected] (F.Cheng). # ZHAO Jing and CHENG Huaigang ([email protected]) contribute equally to this work.
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1 INTRODUCTION Utilizing solar heat energy for forced evaporation is common in brine treatment. Moreover, in most cases, it is used in the desalination technology of seawater or brackish water to separate fresh
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water from salt brine with 3.5% or lower salinity. When applying this method in the evaporation of higher-concentration brine, the economy of solar energy is not easily determined. The economy
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of solar energy depends on two factors: the other products from the evaporation besides fresh water (whether the values of the products are more than the cost of the consumed tremendous
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energy or not); and the feasibility of forced evaporation (the viscosity of the strong brine is higher
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and the evaporation is slower). The use of renewable energy may ease the pressure of energy consumption, as long as this process is characterized by technical feasibility and economic
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feasibility.
The major salt components of seawater and salt lakes, e.g., NaCl and KCl, can saturate and
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separate only when the total concentration is more than 25%. Under most circumstances, natural evaporation method is used to obtain the solid phase salt from the liquid. In the salt pan, it
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naturally evaporates through solar radiation, which concentrates the brine and separates the salt. No fossil energy is needed in this method; it is also easy to apply. Therefore, natural evaporation has been used for hundred years. However, solar thermal power is always considered a low-grade energy source and it is difficult to convert into other forms of energy. Thus, extracting salt from the brine usually involves a very time-consuming process. Nearly six months are required to prepare the solid salt deposit by evaporation in a solar salt pan[1], which requires a large area of
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land. Although improving the efficiency of solar energy is challenging, many new evaporation methods have been developed. For instance, the concept of the solar pond has been accepted for
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years[2, 3]. A salinity gradient solar pond is an artificially constructed pond that reaches high temperatures due to the accumulation of solar thermal energy and the simultaneous prevention of
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convection in the pond. The main function of solar ponds is to accumulate heat, which can promote evaporation by incorporating a combination of evaporation technologies, including
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evaporation ponds[4] and membrane distillation techniques[5]. The solar still, a simple device
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used to distill water, has also been used to process brine[6], but it is mainly used to produce potable water rather than capture salt. The solar dryer, a more intense form of the solar still, was
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also developed to promote brine concentration with the ultimate goal of salt recovery, and has made some contributions to the salt production industry[7]. One of the other methods that can be
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used to improve the water evaporation process is a solar-driven humidification-dehumidification process. In such processes, the vapor removed from the brine feed during evaporation is directed
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from the humidification unit into the dehumidification unit, and then condenses to form potable water[8].
The direct methods for accelerating evaporation involve forced evaporation methods, such as spray evaporation[9], direct-contact evaporation[10] and multiple-effect distillation[11, 12]. In the study of heat source for forced evaporation, heat collectors with efficiencies that are higher than the solar pond are common. In terms of the cost of solar heat collection and evaporation, many
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studies suggest that solar heat collection is categorized under capital intensive enterprise. The service cost of a solar heat collector and the related capital costs have been estimated, including the plate collector[13], the evacuated tube collector, and the solar-assisted steam generation
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system[14]. One study[15] suggests that the investment in collectors accounts for the maximum proportion of all expenses, which is about one third of the total investment. For the processes of solar pond coupling and multi-stage desalination, water cost is estimated to be $1.80 m-3 to $4.55
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m-3[16], which is more expensive than the desalination process with coal or oil as energy. Another
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conclusion is that the scale enlargement of solar heat collection-evaporation desalination
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contributes to the cost reduction of the products. For example, when the scale of the desalination water is 15 m3·d-1, the water production cost is $5.48 m-3; whereas when the scale of the
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desalination water is 300 m3·d-1, the cost of water production is $2.39 m-3[17]. In a multi-effect
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distillation plant whose size is less than 100m3·d-1, the water has to be produced at a cost of €2-€8
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m-3, but the water cost for a 91,000 m3·d-1 desalination plant is only €0.42-€0.81 m-3[18]. Many studies on solar collection methods for forced evaporation have been conducted.
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However, most of the collected heat energy is used for seawater desalination but not salt manufacturing. Most salt manufacturing processes have large evaporation capacity and low salt price. Therefore, the profits generated by forced evaporation process are relatively minimal. In most cases, based on the profit motive, forced evaporation is used primarily in the production of easily salting products[19] or high-profit salts. The production of Glauber's salt[20], which is made using a multiple-effect evaporation process, is a thermally efficient and economical example.
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Another example of using forced evaporation is in the Searles Lake of the Mojave Desert[21], where a three-effect evaporation method is used to process brine and prepare salt. Most of the forced evaporation processes involve taking coal, oil, and other fossil resources as energy source.
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To rapidly provide the latent heat of vaporization, the forced evaporation process requires enough heat energy for a short time. Evidently, it is difficult for natural solar radiation to efficiently
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provide intensive heat, which is also a bottleneck issue in applying solar energy in the forced evaporation process of strong brine.
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The current study aimed to use solar heat collectors to concentrate solar heat energy on the
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brine to carry out depth evaporation and manufacture industrial salt, as well as to analyze the energy consumption and operation cost of the process. Salt manufacturers would significantly
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benefit if a fast and efficient method of brine evaporation and salt production was developed. The
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selected brine systems in this study were the Na+,K+,Mg2+//Cl-,SO42--H2O salt–water systems,
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which are the major normal components of seawater, and a subsystem of many brine systems of salt lakes. These brine systems can be found in the salt lakes of China, Israel, and Jordan. In
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addition, other types of salt lakes exist, such as the Great Salt Lake and the Searles Salt Lake in the USA, where the brine also contains the components of this subsystem. Thus, this kind of salt–water system may be used in the salt-manufacturing process of strong brine evaporation.
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2 EXPERIMENTAL DETAILS As shown in Figure 1, a forced evaporator/crystallizer combined with a 2 m2 plate solar collector was built in Qinghai Province, northwest China, at an altitude of 2700 meters, where the
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average annual solar radiation intensity is about 19557 kJ·m-2·d-1. The plate solar collector was used to replenish heat for the brine in a timely manner during the evaporation process. A solar
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power meter (Model SM206) was used to record the radiation intensity of sunlight.
Figure 1 Schematic diagram and photos of experimental set-up (1-Plate solar collector; 2-Brine tank; 3-Vapor-liquid separator; 4-evaporator (crystallizer); 5-Heat exchanger (condenser); 6-Water tank; 7-Solar collector (preheater) )
Brine was obtained by dissolving the solid potash mine of the earth’s surface using fresh water and was used as the feed brine after mixed with a certain amount of Bischofite
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(MgCl2·6H2O). Before entering the plate solar collector and the evaporator, brine was stored in two 140 L tanks and preheated for one day by two preheaters composed of 33 solar evacuated tubes whose upper ends were connected to the tanks. The preheater was used in the intermittent
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operation mode. The evacuated tubes converted the solar radiation to heat energy and the brine in tanks was heated continuously from morning till evening, and then the brine reached the
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maximum temperature. Hot brine was pumped out of the storage tanks on the next day and then transferred into the plate solar collector and the evaporator.
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During the daytime, the plate collector and the evaporator were operated in the continuous
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operation mode. The hot brine was introduced into the evaporator coupled with a vacuum pump for concentration and flash evaporation. The secondary steam from the evaporation was separated
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in the separator and then condensed to the recycled fresh water in the condenser. The recycled freshwater was used again to dissolve the solid potash mine of the earth’s surface. The solid phase
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salt precipitated and was separated at the bottom of the evaporator, and the supernatant at the upper part of the evaporator continues to cycle to the plate solar collector. During the whole
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process, the brine was constantly concentrated until the desired concentration was achieved before it was discharged.
The evaporation of high concentration brine solutions requires high density thermal energy. However, the grade of the secondary steam is usually low. Thus, the secondary steam was not reused for evaporation, resulting in a relatively low efficiency of heat utilization. The recycled freshwater was about 40 °C in temperature, whose sensible heat can enhance the dissolution of the
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solid potash mine of the earth’s surface. The preheating and heating to brine were carried out from 08:00 to 18:30 every day. The data were got from August 6th to 30th. The evaporation ratio was defined to be the ratio of the amount
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of water evaporated to the amount of water in the feed, and was used to measure the evaporation process. Analyses of the components of the liquid and solid phases were performed using atomic
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absorption spectroscopy (Shimadzu AA 6650) for cations and the titration method for anions. Components of the solid phase were further confirmed using X-ray diffraction (XRD, Bruker AXS
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D8).
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3 THEORETICAL CALCULATION
3.1 Evaporated water and solid salt production
F
o
75
C
I C (MgCl2) IV
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B (KCl)
(Car)
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III
F
C (MgCl2)
E
E0 II
E1 IV
A (NaCl) O1
O
O0
― 75°C, ······ 15°C Figure 2 Phase diagram of Na+,K+,Mg2+//Cl-,SO42--H2O salt–water system (Crystalline phase area: I-KCl, II-NaCl, III-Carnallite (KCl·MgCl2·6H2O), IV- Bischofite (MgCl2·6H2O);
Phase points: O - feed brine, E - co-saturated solution of KCl, NaCl and Carnallite, F – co-saturated solution of NaCl, Carnallite and Bischofite)
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Ignoring the low content of sulfate, the analysis of evaporation process of Na+,K+,Mg2+//Cl--H2O system according to its dry phase diagram is shown in Figure 2. Assuming that A, B and C represent NaCl, KCl and MgCl2, and O, E and F represent the liquid phase points,
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respectively. In practice, the brine-mixing operation is usually carried out so that the phase point of feed brine (O) locates on the line AE, which can ensure that all of the KCl is precipitated in the
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form of Carnallite rather than Sylvinite. As a result, NaCl is separated first during the evaporation process and then the liquid phase moves from the point O to the point E, followed by the
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precipitation of Carnallite when the liquid phase finally reaches point F. Thus, the evaporation of
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this kind of brine can be divided into two stages, i.e., NaCl precipitation stage and Carnallite precipitation stage. In the salt lake industry, Carnallite can be used as the raw material of
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potassium fertilizer.
Generally, the calculations can be carried out using the salt–water system phase diagram.
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However, this research adopted one more direct method. In the precipitation stage of A, B remains
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unseparated and the equation below holds,
wo c B ,o = wE c B , E
(1)
w A = wo c A , o − w E c A , E
(2)
wvapor , E = wo − w E − w A
(3)
where w represents the mass of brine or salt, depending on the subscript of phase point. The suvscript vapor means the vaporation amount of water. c represent the mass percentage of salt components in brine.
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In the Carnallite precipitation stage, the brine at point E would finally reach the point F and separate major salt product xB·yC·zH2O and other by-product salts, then the equation below applies.
wE c B , E − w F c B , F
wE c C , E − w F c C , F =
yM C
(4)
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xM B
where M means the molecular weight. In the above equations, wo, cB,O, wE, cB,E, cC,F and cC,E
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can be seen as known through phase diagram or through experiment. Then, the mass of
w E c B , E − wF c B , F xM B
(xM B + yM C + zM H 2O )
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wBC =
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xB·yC·zH2O (wBC) and by-product salts (wby-producted,j) can be calculated,
wby − product , j = w E c j , E − w F c j , F
j=1,2,3…
(5)
(6)
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where j represents the mass percentages of by-product salt component which can also be determined through phase diagram or experimental analysis. Thus, the mass of all the precipitated
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solid-phase salt and the evaporated water amount are, m
wsalts = wBC + ∑ wby − product, j
(7)
j =1
wvapor,F=wE-wF-wsalts
(8)
At this time, the grade of the salt product of Carnallite can be defined as wBC/wsalts×100%.
3.2 Energy consumption It is assumed that T represents the brine temperature, and subscript in and out represent the brine before and after the flash evaporation, respectively, and r means the latent heat of water
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vaporization. By ignoring heat loss, the amount of flashed water can be estimated as
winc p (Tin − Tout ) r
; where cp represents the specific heat of brine. Thus, the mass of the
precipitated salt from the brine can be expressed as
winc p (Tin − Tout ) wsalts . Usually the brine r wvapor
win c p (Tin − Tout ) 1 + wsalts w r vapor
second flash to the initial feeding brine is
c p (Tin − Tout ) 1 + wsalts w r vapor
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ϕ = 1−
. The mass ratio of the brine for
SC
can be expressed as win −
RI PT
needs to be flashed circularly, thus the mass of the brine in the recirculation for the second flash
(9)
MA
When the circulation times is n which can be determined through trial, the following equation
D
is used,
PT E
n win ∑ ϕ i −1 c p (Tin − Tout ) = wvapor r i =1
(10)
CE
The released heat of crystallization in the salt precipitation process was roughly excluded
AC
from the calculation, then the equation below can be applied,
qAη = win c p (Tin − Tout )
(11)
where q represents the daily solar heat on the unit collector area, A represents the area of plate solar collector, respectively. η represents the collector’s work efficiency, taking the empirical value of 50%. If the unit price of the solar heat collector is a $·m-2, its total investment is Aa $. When production cost is calculated, this amount can carry out the depreciation according to a five-year depreciation periods[13], represented by F in this study. At present, the average unit
ACCEPTED MANUSCRIPT price of solar collectors is about 161 $·m-2 and the lowest enquiry is about 80.6 $·m-2. During flash evaporation, the secondary steam volume per unit time can be approximately
calculated as
wvapor ρ vapor t work
, where ρ represents the density and twork represents the operation time of
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the evaporator. According to the steam volume and vacuum degree, the required power Nvacuum can be determined based on the type of injection pump for the vacuum generation available in the
SC
market. Empirically and approximately, when the steam volume falls between 20 m3·h-1 to 80
NU
m3·h-1, Nvacuum approximately presents a linear increase from 1.5 kW to 5.5 kW; when the steam
MA
volume falls between 80 m3·h-1 and 500 m3·h-1, Nvacuum approximately presents a linear increase n
win ∑ ϕ i −1
from 5.5 kW to 15 kW. The average flow rate of brine circulation is about
i =1
. Based on
t work
D
this data, the type of circulating pump in the market was selected, and its power Npump was
PT E
determined. When the flow rate falls between 5.0 m3·h-1 to 200 m3·h-1, Nvacuum roughly presents a
CE
linear increase from 3.3 kW to 48 kW. The electricity cost is relevant to the local electricity price Ψ, i.e., about 0.10 $·kW-1·h-1 for industrial uses.
AC
The total energy consumption of the whole process can be obtained as below,
W = win c p (Tin − Tambience ) + wvapor r + (N vacuum + N pump )t work
(12)
where Tambience represents the ambient temperature. It is assumed that wvapor,15 and r15 are the evaporated water amount and the latent heat of vaporization in natural evaporation at 15 °C, respectively. Thus, the energy consumption can be contrasted to evaluate the extra energy consumption of flashing with that of natural evaporation, as shown in equation (13)
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W / W Natural =
win c p (Tin − Tambience ) + wvapor r + (N vacuum + N pump )t work wvapor ,15 r15
(13)
If only the energy-related cost including the collector investment and energy consumption is considered, the operation cost for producing per unit ton of salt can be estimated as below,
A ⋅ a / (F ⋅ t workdays ) + Ψ (N vacuum + N pump )t work / day
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J=
wsalts / day
(14)
SC
where tworkdays is the operating days, which is 270 days per year in this calculation; twork/day and wsalts/day represents the working hours and the salts production per day, respectively.
NU
4 RESULTS AND DISCUSSION
MA
4.1 Pretreatment and characteristics of brine under high temperature
The evaporation temperature of salt-pan natural evaporation generally is about 15 °C, while it
PT E
D
is approximate 75 °C for solar energy-forced evaporation process. As shown in Figure 2, the feed brine initially precipitates NaCl out at any temperature during evaporation because it is located in
CE
the crystalline phase area of NaCl. However, in the subsequent evaporation, the precipitation laws of salt at different temperatures vary. For example, at the evaporation temperature of 75 °C, the
AC
phase point of the liquid phase shifts to point E, which reaches the area of Carnallite precipitation. However, at the evaporation temperature of 15 °C, the phase diagram indicates that the precipitation area of Sylvinite (mainly composed of KCl and NaCl) is significantly large. This result demonstrates the considerable precipitation amount of Sylvinite and reduced precipitation amount of Carnallite. In the actual production process, the lesser the Sylvinite precipitate, the better the result is. Figure 2 shows that when the phase point of brine is close to the precipitation
ACCEPTED MANUSCRIPT
area of KCl or Carnallite, the mass ratio of MgCl2/KCl in the feed brine determines the precipitation of Sylvinite or Carnallite. At the evaporation temperature of 75 °C, the initial ratio of MgCl2/KCl should be controlled to approximately 6 according to Figure 2. The phase point of
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brine obtained by dissolving the solid potash mine is located in point O0 in Figure 2, where the MgCl2/KCl ratio is 4.07. The expected Carnallite requires that the point O0 moves toward MgCl2.
SC
Thus, Bischofite, i.e., the residue after brine evaporation in salt pan operations, is usually used in brine mixing operation and added into the brine to increase the MgCl2/KCl ratio. Figure 2
NU
indicates the changes in brine components caused by the different additions of Bischofite, i.e., the
MA
points O and O1. In natural evaporation, the addition of 200 kg·m-3brine of Bischofite led to the MgCl2/KCl ratio of 9.84 (see point O1), which ensures that separation of Carnallite (see point E1).
PT E
D
In forced evaporation at 75 °C, the addition of 50 kg·m-3brine is enough, which nearly results in a MgCl2/KCl ratio of 6 (see point O). As indicated in Figure 2, evaporation at that time ensures that
CE
the separated Carnallite is in solid phase (see point E1). Figure 3 shows the changes in KCl content of the separated salt during continuous
AC
evaporation of feed brine at point O in Figure 2. When in solid phase salt, the evaporation ratio reached approximately 58% and the KCl content suddenly increased but gradually decreases after 62%. The NaCl, the Carnallite with Sylvinite and the Bischofite without potassium is successively separated as the evaporation continues, which causes the KCl content in the solid phase salt to initially increase and then decrease. Thus, brine is in the separation interval of Carnallite when the evaporation ratio is between 58% and 62%. Generally, the large perturbation of the liquid phase
ACCEPTED MANUSCRIPT
during the rapid evaporation could influence salt crystallization and even cause non-crystallization. In this work, evaporation occurred on the liquid phase surface through the evaporator. The bulk liquid under its surface, which was where the crystallization occurred, was undisturbed. Thus, the
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20 18 16 14 12 10 8 6 4 2 0
6
SC
4 2 0
0
Figure 3
Viscosity (mPa.s)
8
10
NU
KCl (wt%)
crystallization can be ensured.
20 30 40 50 60 Evaporation ratio (%)
70
Viscosity of liquid phase and change of KCl content in solid phase during the evaporation process
MA
(Viscosity of liquid phase: ◊ 35°C, □ 45°C, Δ 55°C, ○ 65°C, × 75°C KCl content in solid phase: ● the experimental, ― the calculated)
D
Although the evaporation ratio could reach up to 62%, some difficulties occurred in in-depth
PT E
evaporation. Figure 3 shows that the liquid viscosity increases along with evaporation when the evaporation ratio is greater than 50%, which is unfavorable for the flow in the evaporator,
CE
indicating that a thorough evaporation is difficult to achieve in forced evaporation. Therefore in
AC
this study NaCl and Carnallite were extracted when the evaporation was completed, meanwhile the remaining liquid phase containing a large amount of MgCl2 flowed into the salt pan and was naturally evaporated. The main difference between natural evaporation and forced evaporation is that forced evaporation cannot be conducted during the Bischofite separation period.
4.2 The solar assisted evaporation of brine For the evaporation of brine, the higher the heating temperature and the lower the flashing
ACCEPTED MANUSCRIPT
temperature are, the larger the evaporation capacity of flashing is. However, the total operating conditions may not be quite simple. Figure 4 shows the brine circulation times n, collector area A, the power consumption Nvacuum+Npump, and the estimated operating cost J in the solar
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collection-forced evaporation process under different Tin and Tout when one ton of salt is produced. The circulation times of the brine show decreasing and increasing trends along with the increase in
SC
Tin and Tout, respectively, and the increase of brine temperature can ease the load of the pumps, whereas the collector areas show contrary variation trends. As a result, the data of operating cost
NU
show that for Tin, the temperature should be lower than 75 °C, whereas for Tout, the higher the
MA
temperature, the better. Considering the circulation times of the brine, controlling Tout at 55 °C is
50
60
III
PT E
16.0 70
D
60
II
50
I
20
30
10
20
CE
0
65
70
75
80 85 o Tin ( C)
120
40
IV
2
30
A (m /tsalts)
n
40
125
90
95
10
15.5 15.0
115
14.5
110
14.0
J ($/tsalts)
70
Npump+Nvacuum (kW.h/tsalts)
better.
(a) wsalts(Carnallite)-1.5 t·d-1, Tout-55 °C, a-80.6$·m-2, electricity price-0.10 $·kW-1·h-1, q-19557kJ·m-2·d-1
40
II
140
50
130
40
2
30 20
IV
10 0
60
III 45
50 Tout
55 ( C)
60
30
120
20
110
10 65
100
20 18 16
J ($/tsalts)
I
150 Npump+Nvacuum (kW.h/tsalts)
50
70
A (m /tsalts)
60
n
AC
70
14 12 10
o
(b) wsalts(Carnallite)-1.5 t·d-1, Tin-75 °C, a-80.6$·m-2, electricity price-0.10 $·kW-1·h-1, q-19557kJ·m-2·d-1 I-Circulation times, n; II-Solar collector area, A; III-Electric power consumption, Npump+Nvacuum; IV-Operation cost, J Figure 4
Operating parameters in the solar heat collection-forced evaporation process
ACCEPTED MANUSCRIPT
70 60
2000
50 1500
40 30
1000
20 500
10
0
0 9
11 13 15 Time (hour)
17
19
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7
Figure 5
Temperature (°C)
2
Solar radiation (w/m )
80 2500
Temperature of brine heated by solar collectors
(○ Solar radiation; □ Temperature of brine heated by the preheater; solar collector)
Temperature of brine heated by the plate
SC
To obtain a reasonable evaporation program, the feed temperature and evaporation ratio at
NU
solid–liquid separation should be constant, and the evaporation time or evaporation rate must be determined during brine evaporation. Figure 5 shows the preheating and heating to brine. In the
MA
preheating process, experimental results showed that the collector tubes could provide almost 280
D
L of brine at 75 °C. In the evaporation process, the brine circularly flows through the plate
PT E
collector to maintain the temperature at 55 °C or above, which can ensure the smooth operation of the forced evaporation process. Figure 6 shows the evaporation rate (speed) comparison between
CE
forced evaporation and natural evaporation, both with an initial inventory of 12.5 L and liquid
AC
surface evaporation area of 0.03 m2. Under natural evaporation conditions, the evaporation ratio reached 45% after 300 h, and the evaporation rate seemed gradually slow down because the increasing solution concentration resulted in higher vapor pressure. The decrease in evaporation rate was also found in forced evaporation, but the evaporation ratio took only 5 h to reach above 60%, which is 100 times faster than 20 to 30 days in natural evaporation.
Evaporation ratio (%)
ACCEPTED MANUSCRIPT 0
2
0
50
4
6
8
10
12
80 70 60 50 40 30 20 10 0
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100 150 200 250 300 Time (h)
Figure 6 Evaporation ratio in forced evaporation and natural evaporation (■ 15°C, natural evaporation; ▼ 75°C without vacuum;○ 55°C with vacuum; Δ 75°C with vacuum)
SC
As shown in the above analysis and the data in Figure 3 and Figure 6, the feed brine in the experiment were preheated to 75 °C. Meanwhile, the solid–liquid separation was carried out at the
NU
evaporation ratio of 58% and 62%, respectively; accordingly, the evaporation was divided into
MA
three stages, i.e., the stages of solid NaCl separation, the Carnallite/NaCl mixture separation, and the discharging liquid phase, respectively. When the temperature of discharging liquid phase drops
PT E
D
from Tout to Tambience, Bischofite is in paste form, which contains certain moisture. This paste was piled up and dried for brine mixing. Table 1 shows the product components of the three stages.
CE
The solid phase product of the first stage is confirmed as NaCl by the XRD analysis shown in
AC
Figure 7. The K+ and Mg2+ contents of solid salt obtained in the second stage reached 16.62%, which was of better quality than that of natural evaporation. Moreover, the XRD results shown in Figure 7 indicate that K and Mg are mainly crystallized in the form of Carnallite. The brine in the third stage mostly consisted of MgCl2 with 36.60% content.
ACCEPTED MANUSCRIPT Table 1
The chemical composition of the solid salts /wt% Mg2+
Cl-
SO42-
H 2O
Stage 1
33.42
0.96
2.16
57.3
1.77
4.39
Stage 2
5.52
9.12
7.50
33.20
4.09
40.57
Stage 3
0.48
0.53
9.86
26.74
6.04
56.35
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K+
14000 12000 10000 8000 6000
SC
Intensity (Counts) .
Na+
4000 2000 0 20
30
40
50
NU
10
2θ (°)
60
70
80
(a) Salt of stage 1 (NaCl)
MA
5000 4000 3000 2000 1000
D
Intensity (Counts) .
6000
0
PT E
(b) Salt of stage 2 (Carnallite & NaCl) Figure 7
XRD results of deposit solid in brine evaporation
1000
40
800
I
1.00
Figure 8
III 0
10
20
400
10
200
V
IV
20 wsalts (ton/day)
30
600
2
30
1.25
0.75
A (m /tonsalts)
1.50
0 40
0
75
50
25
J ($/tsalts)
1.75
50
Npump+Nvacuum (kW.h/tsalts)
2.00
II W/Wnatural
AC
CE
4.3 Consumption of thermal energy
0
Operating parameters under various production scales
(Tin-75°C, Tout-55°C, a-80.6$·m-2, electricity price-0.10 $·kW-1·h-1, q-19557kJ·m-2·d-1, I-W/Wnatural; II-Solar collector area, A; III-Electric power consumption, Npump+Nvacuum; IV-Operation cost for Carnallite production, V-Operation cost for NaCl production, J)
Owing to the cost of thermal collector and evaporator, forced evaporation is less economical
ACCEPTED MANUSCRIPT
compared with natural evaporation. Nevertheless, profits can still be realized because of the increased production efficiency. However, the biggest problem for the forced evaporation might be the estimation of the cost of energy. It is well known that the forced evaporation technology is
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economically competitive only for production on a large scale (thousands of m3·d-1). Figure 8 shows the results under the assumed condition of daily eight-hour and annual nine-month operation of the collector-evaporator equipment. The solar collector area for producing one ton of
SC
Carnallite is constant, which is about 34.27 m2·t-1salts. The energy consumption of the pump
NU
gradually decreases with the expansion of the production scale. Therefore, the total operation cost
MA
also decreases, which makes the energy consumption of forced evaporation closer to that of natural evaporation. Their ratio W/Wnatural also decreases continuously until finally approaching
PT E
D
1.25. This observation indicates that the energy consumption of forced evaporation is at least 25% higher than that of natural evaporation. When the production scale of Carnallite is over 20 tons per
CE
day, its operation cost can reach a stable value. The cost at this time is equivalent to 13 $·t-1salts. When the production scale of Carnallite is over 50 tons per day, the operation cost can be
AC
decreased to 10 $·t-1salts. For the solar collection-forced evaporation process, we can evaluate the operation cost by comparing the price of solid phase salt product. The price of local Carnallite is about 65 $·t-1salts. Considering that the equipment investment (evaporator, heat storage tank, and other commonly used equipments) is usually equivalent to two-thirds of investment in solar collectors[15], the price of 13 $·t-1salts for the operation cost is acceptable. However, in brine evaporation, the amount of evaporated water in the precipitation stage of
ACCEPTED MANUSCRIPT
NaCl is large, whereas the precipitation amount of salt is relatively low; thus, its operation cost is higher than that of Carnallite. Apparently, the solar collection-forced evaporation process is not suitable for NaCl production in terms of economy.
J ($/tsalts)
22
RI PT
24 IV
20 II
18
III
16
SC
I
14 65
70
75
80 85 o Tin ( C)
90
95
Figure 9
NU
Tout-55°C, wsalts(Carnallite)-1.5 t·d-1
Operation cost under various prices for energy consumption
MA
(I: a-80.6$·m-2, Ψ-0.10$·kW-1·h-1, q-19557kJ·m-2·d-1; II: a-80.6$·m-2, Ψ-0.10 $·kW-1·h-1, q-9778.5kJ·m-2·d-1; III: a-242$·m-2, Ψ-0.10$·kW-1·h-1, q-19557 kJ·m-2·d-1; IV: a-80.6$·m-2, Ψ-0.16 $·kW-1·h-1, q-19557 kJ·m-2·d-1)
The operating cost of the solar collection-forced evaporation process is also influenced by the
PT E
D
price of energy, which always fluctuates. Therefore, the economy of solar collection-forced evaporation process also changes. Figure 9 shows the operation costs of the solar collection-forced
CE
evaporation process after changes in collector price, power price, solar radiation intensity, and Tin. The influence of solar collection cost on the total operating cost is different from that of the power
AC
cost. As long as the unit price of power increases, no matter which temperature the evaporation process is at, the operating cost will increase as well. After the solar collection cost increases, regardless of whether it is caused by the collector price or the increasing collector area caused by the reducing radiation strength, the influence of high temperature manifests more significantly than that of low temperature. Therefore, the higher the temperature, the greater the increase in the operating cost. Under the condition of high solar collection cost, implementation of the whole
ACCEPTED MANUSCRIPT
process at low temperature is more adequate.
5 CONCLUSIONS The solar collection-forced evaporation experiment applied in the treatment of saturated brine
RI PT
of Na+,K+,Mg2+//Cl-,SO42--H2O system was designed and studied. The evaporation temperature after solar heating is higher than that of natural evaporation, which results in different changes of
SC
physical properties of the brine. The evaporation rate of the brine is 100 times faster than that of natural evaporation. NaCl, Carnallite and Bischofite precipitated in sequence, and the control
NU
points of solid-liquid separation during the evaporation process are determined by a phase diagram
MA
and experiments. Due to the viscosity increase, the precipitation stage of Bischofite is not suitable for forced evaporation. To obtain more Carnallite, the MgCl2/KCl ratio in the feed brine should be
PT E
D
controlled at 5.88 instead of 9.84 in the natural evaporation. The current research estimated the solar collector area, power consumption, and total
CE
operation cost. Results indicated that the economy is at optimum when the feed brine is heated to 75 °C and evaporated at 55 °C in temperature. The larger the production scale of solar
AC
collection-forced evaporation, the lower the operation cost. The recommended production scale is at least 20 tons per day, in which the operation cost is 13 $·t-1salts, and the solar collector area for producing one ton of Carnallite is about 34.27 m2. Given such production scale, the energy consumption of forced evaporation is at least 25% higher than that of natural evaporation. Furthermore, the higher the operation temperature, the more significant the influence of collector cost on the operation cost will be.
ACCEPTED MANUSCRIPT
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RI PT
Renew. Sust. Energ. Rev., 15(6), 3319-3325(2011). (3) Nie, Z., Bu, L.Z., Zheng, M.P., Huang, W.N., “Experimental study of natural brine solar ponds in Tibet”, Sol. Energy, 85(7), 1537-1542(2011).
SC
(4) Agha, K.R., Abughres, S.M., Ramadan, A.M., “Maintenance strategy for a salt gradient solar pond coupled with an evaporation pond”, Sol. Energy, 77(1), 95-104(2004).
NU
(5) Suárez, F., Tyler, S.W., Childress, A.E., “A Theoretical Study of a Direct Contact Membrane Distillation
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D
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PT E
(7) Pereira, M.C., Mendes, J.F., Horta, P., Korovessis, N., “Final design of an advanced solar dryer for salt recovery from brine effluent of an MED desalination plant”, Desalination, 211(1-3), 222-231(2007).
CE
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AC
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(14) Brunet, R., Antipova, E., Guillén-Gosálbez, G., Jiménez, L., “Reducing the Environmental Impact of Biodiesel Production from Vegetable Oil by Use of a Solar-Assisted Steam Generation System with Heat Storage”, Ind. Eng. Chem. Res., 51 (51), 16660-16669(2012).
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(16) Agha, K.R., “The thermal characteristics and economic analysis of a solar pond coupled low temperature multi stage desalination plant”, Sol. Energy, 83(4), 501-510(2009).
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( 17 ) Szacsvay, T., Hofer-Noser, P., Posnansky, M., “Technical and economic aspects of small-scale solar-pondpowered seawater desalination systems”, Desalination, 122(2-3), 185-193(1999).
PT E
223(1-3), 448-456(2008).
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(18) Karagiannis, I.C., Soldatos, P.G., “Water desalination cost literature: review and assessment”, Desalination,
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AC
(20) Garrett, D.E., “Sodium Sulfate: Handbook of Deposits, Processing, and Use”, Academic Press, pp. 262(2001). (21) Raymond, R.E., Albert, K.J., “Riegel's Handbook of Industrial Chemistry”, Springer, pp. 430(2003).