south african journal of chemical engineering 24 (2017) 1e7
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Analysis of potassium nitrate purification with recovery of solvent through single effect mechanical vapor compression Kiprotich E. Kosgey*, Sammy L. Kiambi, Peter T. Cherop Durban University of Technology, Department of Chemical Engineering, 1334, Durban, 4000, South Africa
article info
abstract
Article history:
Analysis of purification of potassium nitrate with incorporation of single effect mechanical
Received 16 November 2016
vapor compressor for solvent recovery was done. Analysis focused on the effect of concen-
Received in revised form
tration and temperature of mother liquor on the energy efficiency of the process and the
24 May 2017
amount of recovered solvent. Performance coefficient of mechanical vapor compressor
Accepted 25 May 2017
ranged between 1.5 and 7.5 depending primarily on the temperature of mother liquor. It was found that with increase in temperature of mother liquor through pre-heating, the power of
Keywords:
the compressor, compression ratio and amount of heat supplied to the evaporator decrease.
Performance coefficient
For a 40% concentrated feed solution and mother liquor temperature above 80 C, perfor-
Mother liquor
mance coefficient is higher than 4. It is therefore concluded that preheating mother liquor
Concentrated solution
and reduction of the effect of concentration of both mother liquor and concentrated waste
Recovered solvent
stream through other methods reduces the power consumption of purification process.
Boiling point elevation
© 2017 The Authors. Published by Elsevier B.V. on behalf of Institution of Chemical Engi-
Mechanical vapor compressor
neers. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
1.
Introduction
Potassium nitrate (KNO3) is used as a food preservative, fertilizer and heat transfer agent in chemical industries. KNO3 is also essential in the production of explosives, glass and steel (Abidaud, 1991; Freilich, 2005; Jaroszek et al., 2016). The only known ore of KNO3 is caliche mined in Chile (Freilich, 2005; Velasco, 1992). Potassium chloride (KCl), also known as potash, is another mined compound of potassium. To meet the high demand of KNO3, the mineral deposits are supplemented with the manufactured compound. The following chemical processes have been developed for its production: (i) Reaction of potassium chloride with nitric acid at elevated temperatures (Eq. (1)) (Abidaud, 1991; Freilich, 2005). 100 C
3KCl þ 4HNO3 ! 3KNO3 þ Cl2 þ NOCl þ 2H2 O
(1)
(ii) Potassium chloride react with hot aqueous sodium nitrate to produce sodium chloride (NaCl) and potassium nitrate (Eq. (2)) (Jaroszek et al., 2016). NaCl is less soluble at elevated temperatures and thus crystallizes out of the hot solution rich in KNO3 (Abidaud, 1991; Freilich, 2005; Jaroszek et al., 2016). NaNO3 ðaqÞ þ KCl ðsÞ/NaCl ðsÞ þ KNO3 ðaqÞ
(2)
(iii) Nitric acid is reacted with potassium chloride to produce potassium nitrate and hydrochloric acid which is extracted with an organic solvent (Eq. (3)) (Freilich, 2005; et al., 2013). Jurisova HNO3 þ KCl /HCl þ KNO3
(3)
(iv) Electrodialysis e cation exchange membranes (CEM) and anion exchange membranes (AEM) alternate in a
* Corresponding author. E-mail address:
[email protected] (K.E. Kosgey). http://dx.doi.org/10.1016/j.sajce.2017.05.004 1026-9185/© 2017 The Authors. Published by Elsevier B.V. on behalf of Institution of Chemical Engineers. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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south african journal of chemical engineering 24 (2017) 1e7
Nomenclature Roman letters and abbreviations P Solvent, kg/s E evaporator W concentrated solution, kg/s t Temperature, C S crystals, kg/s L mother liquor, kg/s p Pressure, kPa h Enthalpy, kJ/Kg D Dissolution SP separator H heat exchanger EV Expansion valve Cr crystallizer K Ebullioscopic constant, C kg/mol MVC Mechanical vapor compressor Q Quantity of heat, kJ F Feed, kg/s N Power of compressor, kW x Concentration, % w/w C Concentration, mol/m3 c Specific heat capacity, kJ/kg- C n Compression ratio r Latent heat, kJ/Kg i Van't Hoff factor Subscripts v1 Inlet vapor v2 Outlet vapor o Fresh solvent cr Fractionation temperature eb Ebullioscopic T Solid feed compr compressor adi adiabatic A solute mex mechanical cond condensation sol solvent Greek symbols h efficiency r Density, kg/m3 d Boiling point elevation, C ε Performance coefficient
compartment with external electric field. The ion exchange membranes keep ions of the same charge but allow ions of opposite charge to pass through (Jaroszek et al., 2016; Nagarale et al., 2006). Jaroszek et al. (2016) fed K2SO4 into the two outermost cells in a four cell compartment and NaNO3 in the central cell leading to separation of Naþ and NO3 ions and subsequent production of sodium sulfate and potassium nitrate (Eq. (4)). Potassium and sodium ions easily pass through negatively charged CEM but are retained by positively charged AEM while sulfate and nitrate anions easily pass through positively charged AEM but are retained by negatively charged CEM (Nagarale et al., 2006).
NaNO3 þ K2 SO4 /Na2 SO4 þ KNO3
(4)
(v) Ion exchange e nitric acid is passed through cationic exchange resin loaded with potassium leading to the production of potassium nitrate (Eq. (5)). Potassium hydroxide (KOH) solution is added to neutralize unreacted nitric acid (0.5% by weight) subsequently producing pure potassium nitrate (Nagarale et al., 2006). HNO3 þ RK /HR þ KNO3
(5)
Each method of KNO3 production has its advantages and disadvantages and the choice of one method over another is influenced by the availability of raw materials, purity of KNO3, et al., 2013). waste treatment and cost of production (Jurisova In Freilich (2005) it was reported that sodium chloride is the main contaminant in all the ores of potassium. Other contaminants include clay, silica, kieserite (MgSO4$H2O), anhydrite (CaSO4) (Freilich, 2005), perchlorates (Urbansky et al., 2001) and iodine (Velasco, 1992). The ores are purified through froth flotation, heavy media separation processes, screening and fractional crystallization (Freilich, 2005). Jaroszek et al. (2016) produced KNO3 through electrodialysis of the highest reported purity of 99.9% but transportation of ions of impurities with potassium and nitrate ions still presented a challenge. Machuca and Cernı´n (2014) reported molar percentage composition of chlorides at 2% of KNO3 produced through electrodialysis. Impurities in KNO3 especially for use in agriculture and food industry can be hazardous. Caliche deposits are natural sources of perchlorates (Urbansky et al., 2001) and it is highly likely that mined deposits are contaminated with it. Perchlorates are known to affect the uptake of iodine in the thyroid gland (Urbansky et al., 2001). Other contaminants could also present other health issues and there is therefore need to purify KNO3. One of the ways of its purification is recrystallization. Crystallization techniques are advantageous over other purification methods because they are cheap, highly efficient and simple (Lu et al., 2017; Mullin, 2001). The possibility of solvent recirculation and self-heat recuperation greatly improves the prospects of the process. Multi stage flash distillation, single and multi-effect evaporation, mechanical vapor compression (MVC), thermal vapor compression (Alasfour and Abdulrahim, 2011; Han et al., 2015), membrane technology (Lu et al., 2017; Han et al., 2015) ion exchange technology and nanotechnology (Lu et al., 2017) have been widely researched on for water recovery. Compression of vapors has made thermal processes of water recovery common especially because of low cost, flexibility, high purity of distillate and thermodynamic efficiency (Alasfour and Abdulrahim, 2011). Of the thermal processes, MVC is highly efficient (Alasfour and Abdulrahim, 2011; Zhou et al., 2014). Extensive research in the use of mechanical vapor compressors in water recovery from waste water and saline water has been done. Zhou et al. (2014) investigated power consumption through reduction of temperature difference between the compressed steam in the evaporator tubes and the saline wastewater. They concluded that power consumption increase linearly with temperature difference. These findings were in line with (Alasfour and Abdulrahim, 2011) conclusions on their study in temperature drop in MVC unit. Treatment of high strength wastewater in double effect mechanical vapor recompression unit was studied by Liang et al. (2013) with
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south african journal of chemical engineering 24 (2017) 1e7
PO, xO, tO
FT, xT, tT
D
F, xF, tF
S+L
Cr
S, xS, tS
SP
tcr
L, xL, tL
P, x2, t2
H1
L, xL, tL1 V, hv1, p1 tv1 H2
Compr tv2, p2 ,hv2
E
W, xW, tW2
L, xL, tL2
EV P, h4, t1, p4
h3, p3 W, xW, tw1
Fig. 1 e Schematic diagram of re-crystallization in which mother liquor L is pre-heated by recovered solvent P and concentrated solution W.
2.
Principle of operation
The feed, is dissolved in the recovered solvent P with the addition of a small amount of fresh solvent PO (Fig. 1). The amount of PO depends on the loss of solvent with concentrated solution W. Energy required for dissolution in the process can be supplied by solvent P coming from evaporator E. Solution F of KNO3 is fed to crystallizer Cr where it is cooled to temperature tcr which will change with the concentration of KNO3 in the solution (Fig. 2). As a result of crystallization, magma S þ L is produced which is fed to separator SP (Fig. 1). Mother liquor L at temperature tL, after its separation from
crystals S, is fed into evaporator E where most of the solvent is recovered, leaving concentrated solution W. Vapors P of the solvent are compressed from pressure p1 to pressure p2 raising the enthalpy of the vapors from hv1 to hv2. The compressed vapors are then fed into the heating chamber of evaporator E where upon condensation boils the mother liquor L. The condensate P with enthalpy h3 and the concentrated solution
120 100 80
tF , оС
focus on decrease in total heat transfer area with decrease in temperature difference between condensing steam and boiling solution. Han et al. (2014) applied self-heat recovery theory in evaporative crystallization of ammonium sulfate. Han et al. (2017) demonstrated that in double effect mechanical vapor compression, the total power consumption is less than in a single effect mechanical vapor compression system but compression stages higher than two do not guarantee reduction in total power consumption with increase in the number of stages. In this paper, design for purification process incorporating single effect mechanical vapor compression for solvent recovery is developed with analysis of performance based on energy efficiency. This is because purification of KNO3 is necessary to remove harmful contaminants like perchlorates. In addition, water recovery is essential especially with the common problem of water scarcity and strict regulations concerning wastewater treatment and disposal.
60 40 20 0
-20
0
20
40
хF , % КNО3
60
Fig. 2 e T-X diagram for KNO3 solution in water (Pavlov et al., 1987).
80
4
south african journal of chemical engineering 24 (2017) 1e7
W which is rich in impurities exit the evaporator. Pressure of the condensate P drops from p3 to p4 on passing through expansion valve EV. Recovered solvent P from the evaporator E is hot and if its heat content is higher than the heat needed in dissolution stage then it is necessary to cool to the required temperature t2. For this purpose, the hot solvent is fed to heat exchanger H1 to pre-heat mother liquor to temperature tL1. The temperature of mother liquor can further be raised to tL2 by passing it through heat exchanger H2 which is heated by hot concentrated solution W from the evaporator. Heating mother liquor L before feeding to the evaporator, can significantly reduce the amount of heat needed in the evaporator.
2.1.
Basis for calculation
2.1.1.
Material balance
The material balance for stage D (dissolution) in Fig. 1 can be determined as: P þ P O þ FT ¼ F
(1a)
Px2 þ PO xO þFT xT ¼ FxF
(2a)
where P, PO, FT and F are the mass flow rates of recovered solvent, fresh solvent, feed (solid) and solution of KNO3 respectively; x2, xO, xT and xF are the solute concentrations in the recovered solvent, fresh solvent, solid feed and solution respectively. From Eqs. (1a) and (2a), solute concentration in solution F can be determined as: xF ¼
Px2 þ PO xO þ FT xT P þ PO þ FT
(3a)
Without a trace of the solute in the recovered solvent P and PO (x2 ¼ 0 and xO ¼ 0), Eq. (3a) simplifies to: xF ¼
FT xT P þ PO þ FT
(4a)
LxL ¼ WxW þ Px2
(5a) (6)
where xL, xW and x2 are solute concentrations in mother liquor, concentrated solution and recovered solvent respectively. From Eqs. (5a) and (6), P can be determined as: P¼L
xW xL xW x2
t2 ¼
FcF tF þ FT ðrD cT tT Þ PO xO tO PCP
(7)
(10)
The amount of energy QE that need to be supplied to evaporator E can be determined from Eq. (11). LcL tL þ QE ¼ WcW tW1 þ Phv1
(11)
where cL and cW are the specific heats of mother liquor and concentrated solution respectively; tL and tW are temperatures of mother liquor at entry into the evaporator and concentrated solution at exit from the evaporator respectively. QE transferred from compressed vapors P to the boiling solution P can also be determined as: QE ¼ Pðhv2 h3 Þ ¼ Prcond
(12)
where rcond is the latent heat of condensation of solvent vapors in the evaporator. The specific heat capacity of solutions L and W can be determined using the method of mixtures as (Qiao et al., 2017): cL ¼ cA xL þ cP ð1 xL Þ
(13)
cW ¼ cA xW þ cP ð1 xW Þ
(14)
where cA and cP are specific heat capacities of the solute (KNO3 ¼ 1.39 kJ/kg- C (Takahashi et al., 1988)) and solvent (water ¼ 4.18 kJ/kg- C (Han et al., 2014)) respectively. The heat content of the recovered solvent may be more than the heat necessary to dissolve the solid feed. In this case, the recovered solvent is cooled down to temperature t2 in heat exchanger H1 and in the process transferring heat Q1 to mother liquor L. Q1 ¼ PcP ðt2 t1 Þ ¼ LcL ðtL1 tL Þ
Material balance for evaporator E is: L¼WþP
solution respectively; rD is the heat of dissolution of the solid feed. From Eq. (9), it is possible to define temperature t2 at which it is necessary to return the recovered solvent to stage D for dissolution of the solid feed so that solution F exits at temperature tF:
(15)
where t1 and t2 are the temperature of the solvent before and after passing through the heat exchanger H1. If mother liquor L is fed to the evaporator at the boiling temperature, tL2 can be determined from Eq. (16) (Alasfour and Abdulrahim, 2011; R. N. G. Pavlov and Noskov, 1987). tL2 ¼ tP þ dL
(16)
where tP is the boiling temperature of pure solvent at pressure p1 and dL is boiling point elevation of mother liquor with concentration xL. dL can be determined as (Gazagnes et al., 2007): i:CKNO3 rsol
And since there is no solute in the recovered solvent (x2 ¼ 0), Eq. (7) becomes:
dL ¼ Keb
xL P¼L 1 xW
where Keb is ebullioscopic constant of water (0.51 C.kg/mol), i is the Van't Hoff factor of 2 for KNO3, CKNO3 is the concentration of KNO3 (mol/m3) and rsol is the density of water (kg/m3) (Gazagnes et al., 2007). In heat exchanger H2, Q2 transferred from concentrated solution W to mother liquor L can be determined as:
2.1.2.
(8)
Energy balance
The amount of heat required for dissolution of the feed can be determined as: FT cT tT þ PcP t2 þ PO cO tO ¼ FT rD þ FcF tF
(9)
where cT, cP, cO and cF are the specific heats of solid feed, recovered solvent, fresh solvent and solution respectively; t2, tT and tF are temperatures of recovered solvent, solid feed and
Q2 ¼ LcL ðtL2 tL1 Þ ¼ WcW ðtw1 tw2 Þ
(17)
(18)
where tW1 and tW2 are the inlet and exit temperature of concentrated solution before and after passing through the heat exchanger.
5
south african journal of chemical engineering 24 (2017) 1e7
0.6
tW ¼ tP þ dW
0.5
(19)
where dW is boiling point elevation of solution W with concentration xW and can be calculated using Eq. (17). Heat exchange between streams P, W and L can reduce energy demand for solvent recovery. If the energy supplied with recovered solvent is not enough for dissolution of solid feed then fresh solvent should be heated up before supplying for dissolution. Heat transfer area for heat exchangers and evaporator can be calculated as described in Ettouney (2006).
3
0.4
1
0.3 10
20
30
40
хL , % KNO3 2
1 1.5
Performance coefficient of the heat pump
The theoretical power of compressor can be determined from Eq. (20) (Zhou et al., 2014).
1
2
(20)
0.5
3
Accordingly, the actual compressor capacity is (Zhou et al., 2014; Nosov et al., 2009):
0
Ncompr ¼ Pðhv2 hv1 Þ
Pðhv2 hv1 Þ hadi hmex
(21)
QE h h ðhv2 h3 Þ ¼ ad mex hv2 hv1 Ncompr
2.1.4.
(22)
Process parameters
Process parameters were based on data from previous study (Nosov et al., 2009): solid feed rate FT ¼ 1 kg/s, KNO3 concentration in the solution xF ¼ 20e50%, range of fractionation temperature tcr ¼ 0e30 C. The following assumptions have been made: 1) There is no heat loss in the process apparatuses 2) The effect of impurities on process thermal parameters will be negligible compared to the effect of KNO3 in mother liquor 3) Only pure crystals of KNO3 will be produced
3.
20
b
30
40
хL , % KNO3
where hadi and hmex are adiabatic and mechanical efficiencies of the compressor respectively. Performance coefficient ε of mechanical vapor compressor is (Zou and Xie, 2017): ε¼
10
Results and discussion
Calculated results for elevation of boiling point of the solvent are presented in Table 1: With increase in concentration of KNO3, boiling point of the solution rises. This directly affects the amount of vapors generated in the evaporator and subsequently flow rate of the recovered solvent (Fig. 3(a)). More solvent will be recovered in the evaporator from a more concentrated feed solution to the crystallizer than a less concentrated feed solution to the crystallizer. Gazagnes et al. (2007) established that with
Fig. 3 e Change in flow rate ratio of recovered solvent to flow rate of KNO3 solution (a) and flow rate ratio of concentrated solution to concentration of KNO3 in mother liquor (b) tL ¼ 90 С: xF ¼ 1 e 20%; 2 e 30%; 3 e 40%. increase in NaCl concentration, the performance of a desalination unit diminished because less water vaporized at the membrane surface. The amount of exiting concentrated solution, however, increases with elevation of boiling point as less water evaporates from mother liquor with increase in its concentration (Fig. 3(b)). Recycled solvent P supplies the required heat for dissolution in stage D and as a result, reduces demand for additional heat at this stage. Mathematical analysis has shown that concentration xF depends primarily on temperature t2 (Fig. 4). This is because the recovered solvent supplies heat of dissolution of the fresh solid. Thus, with increasing xF, temperature t2 at which recovered solvent is supplied to stage D increases. A low fractionation temperature leads to a lower t2 than a high fractionation temperature. t2, also increases with concentration xL of mother liquor and xF of solution (Fig. 5). This is due to the elevation of boiling point of the solvent. Another important factor influencing the process of recrystallization of KNO3, is energy consumption at the recovery of solvent. In this case, thermal energy consumption
100
3
80
t2, °C
Ncompr ¼
a
2
FRWFW/F
2.1.3.
FRPF=P/F
Boiling temperature of W can be determined as (Alasfour and Abdulrahim, 2011; Pavlov et al., 1987):
2
1 60 40 20 20
Table 1 e Boiling point elevation of water at different concentrations of potassium nitrate. Concentration, % Boiling point elevation, C
20
30
40
50
2.02
3.03
4.04
5.05
30
xF, %
40
Fig. 4 e Change in temperature of recovered solvent fed to stage D with the concentration of KNO3 solution: tcr ¼ 1 e 0 С; 2 e 10 С; 3 e 20 С.
50
6
south african journal of chemical engineering 24 (2017) 1e7
100
50
4
60
2
40
a
40
3 n = P2 /P1
t2 , °С
80
1
20
3
30
2
20
1
10 0 10
20
30
40
0
хL , % KNO3
0
QE /F, kJ/Kg
1600
a
1 1500
2 3
1400
1300
1200 0
10
20
30
40
50
60
70
80
tL , °C
1
1800
40
50
60
Ncompr /F, KJ/Kg
1 2
600
80
b
3 400 200
0
10
20
30
40
50
60
70
80
tL , °C Fig. 7 e Relationship between the level of compression of vapors (a) and power of compressor (b) with temperature of mother liquor. xF ¼ 40%, xL ¼ 12%: xW ¼ 1 e 75%; 2 e 65%; 3 e 55%. increase in temperature is small. Aybar (2002) concluded that the minimum temperature at which water should be fed into the evaporator is 75 C as latent heat of condensation of steam does not give enough heat for evaporation at lower temperatures. Lower temperatures lead to a larger temperature difference between compressed superheated steam and mother liquor in the evaporator. According to Zhou et al. (2014), larger temperature difference leads to a higher compression ratio and subsequently higher energy consumption by the compressor. High power consumption reduces the energy efficiency of the process. At tL below 10 C, performance coefficient is extremely low, in the range ε ¼ 1.5e1.7 (Fig. 8). Performance coefficient at temperatures of mother liquor above 70 C rises rapidly with increase in temperature. For a 55% w/w concentrated solution, performance coefficient is double that of 75% w/w concentrated solution with tL above 80 C. An increase in concentration of concentrated waste steam leads to an increase in the pressure ratio. This is as a result of the effect of boiling point elevation (Han et al., 2015).
b 8
3
6 1400
70
0
2
1600
30
800
3
ε
QE /F,KJ/Kg
2000
20
tL , °C
Fig. 5 e Variation of temperature of recovered solvent supplied for dissolution of solid feed with concentration of mother liquor xF ¼ 1 e 20%; 2 e 30%; 3 e 40%; 4 e 50%. QE in the evaporator, depends on parameters of dissolution of solids and subsequent crystallization. It was found that QE is mainly dependent on the concentration xL of mother liquor and its temperature tL (Fig. 6(a) and (b)). On increasing concentration xL there is a reduction in the amount of heat QE per unit mass of the solution F since generation of vapors P decreases and mass flow rate of concentrated solution W increases. An increase in temperature tL through preheating with recovered solvent P and waste stream W also leads to a reduction of QE. If mother liquor L is supplied to evaporator E without preheating, that is at tL ¼ tcr ¼ 0 C, consumption of heat in the evaporator is high and the compression ratio of the compressor reaches very high values of n ¼ 40e45 (Fig. 7a). Failure to preheat mother liquor leads to a significant increase in the power of the compressor per unit mass of solution (Fig. 7b). Lower than 70 C, compression ratio rapidly decreases with increase in temperature of mother liquor and at higher temperatures the change in compression ratio with
10
2
4
1
1200
2
1000 10
20
30
xL , % Fig. 6 e Variation of heat QE in evaporator E with temperature of mother liquor (a) and its concentration (b): xF ¼ 1 e 20% KNO3; 2 e 30%; 3 e 40%.
40
0 0
10
20
30
40
50
60
70
80
tL , °C Fig. 8 e Variation of performance coefficient with temperature of mother liquor xF ¼ 40%: xW ¼ 1 e 75%; 2 e 65%; 3 e 55%.
south african journal of chemical engineering 24 (2017) 1e7
4.
Conclusion
Production of KNO3 through conventional methods with subsequent purification through recrystallization with solvent recovery and incorporation of mechanical vapor compressor is energy efficient and it presents good economic prospects. Analysis has shown that in the considered design, the performance coefficient, depending on the temperature of mother liquor, is between 1.5 and 7.5. Without pre-heating mother liquor, COP is very low. Above 80 C, power consumption by the compressor is low and COP is higher than 4. Concentration of mother liquor influences solvent recovery through elevation of boiling point. A more concentrated mother liquor boils at a higher temperature than a pure solvent and thus consumes more power. As a result, other than preheating mother liquor, the rise in power consumption as a result of boiling point elevation should be minimized through other methods. In instances where KNO3 concentration is high and impurities are low, recirculation of mother liquor can be considered whereas if concentration of impurities is high, an additional crystallizer and separator to reduce concentration of impurities before solvent recovery can be installed.
Acknowledgement The authors acknowledge the support given in sourcing some of the articles given by Jane Avenal Finlayson of Durban University of Technology Steve Biko campus library.
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