- Email: [email protected]
Dual-purpose chemical desalination process
DESALINATION Desalination 113 (1997) 19-25
Dual-purpose chemical desalination process H.K. Abdel-Aal*, A.A. Ibrahim, M.A. Shalabi and D.K. Al-Harbi Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Tel. +966 (3) 860-2205; Fax +966 (3) 860-4234; E-mail: [email protected]
Received 12 April 1996; accepted 13 May 1997
Abstract The separation of sodium chloride from sabkha water was carried out using a chemical conversion approach. A sdium cation and chloride anion were converted into sodium bicarbonate, which is precipitated under experimental conditions, and ammonium chloride, which can be separated by crystallization, respectively. Concentrated brine solutions of 25% salt content were the feedstock used. In addition to partially desalted water, chemical products of economic potential values were produced, namely soda ash and ammonium chloride. Utilization of the brine exit from MSF desalination plants as feed stock is another alternative. Keywords:
???
1. Introduction Mixing processes are common in nature. This entails the reverse procedure of separation. The separation process is difficult to achieve because it is the opposite of mixing, a process favored by the second law of thermodynamics. Consequently, separation often accounts for major production costs in the chemical and petrochemical industries. Sabkhas are saline flats that are underlain by silt, clay and sand, and often encrusted with salt. *Corresponding author.
They are equilibrium surfaces whose level is largely controlled by hydrological and climatic conditions [ 11. The objective of this study is to apply separation methods to desalt indigenous sabkha deposits found in the Kingdom of Saudi Arabia, using a chemical approach. In Saudi Arabia, sabkhas exist either along the coast of the Red Sea and the Arabian Gulf as coastal sabkha or inland as continental sabkha. Fig. 1 summarizes the sabkhas in the coastal plains of the eastern province as described by Johnson et al. [2]. The ground water is rich in salt and is usually less
00 1 l-9 164/97/$09.50 0 1997 Elsevier Science B.V. All rights reserved PZI 001 l-9164(97)001 11-2
20
H. K. Abdel-A al et al. / Desalination I13 (1997) 19-25
Table 1 SaWCA/EDCplants (SADAF, Jubail, 1991)
Fig. 1. Sabkha flat along the east coast of Saudi Arabia. than 1 meter below the surface. The NaCl concentration reaches up to 25% by weight. Sabkha is characterized by high a content of cations such as Mg++, but it is not normally saturated in these ions. In the Arabian gulf, the exploitation of these sabkha resources has attracted many researchers and companies. For instance, SADAF, of Jubail in Saudi Arabia, obtains raw brine from a salt plant at Riyas as shown in Table 1. The brine is used in the chlor-alkali plant to produce H,, Cl,, and caustic soda.
2. Objectives of the study The main objective of this work is to apply the chemical separation method to desalt indigenous sabkha resources, which leads to the recovery of chemical products as well as partially desalted obtained are sodium water. The products bicarbonate (NaHCO,), soda ash (Na,CO,), and water rich in fertile salts, namely ammonium chloride (NH&l).
Annual capacity (metric tons)
Daily capacity (metric tons)
Salt plant
Salt
556,100
1,700
Chlor-alkali plant
Chlorine Hydrogen Caustic soda
333,000 9,800 367,000
1,000 29.6 1,100
Ethylene dichloride plant
EDC
454,000
1,363
The chemical separation method is not to be considered as an alternative to the existing conventional desalination processes. The proposed method uses saturated brines at least five times more concentrated than seawater and produces valuable chemical products in addition to partially desalted water. In this respect, it is regarded as a supplement to the physical multistage flash desalination processes. The economics of the proposed chemical process as compared to the existing ones cannot be easily evaluated. This requires comparing the value of sales of products vs. the total production costs including cost of raw materials of chemical feed.
3. Apparatus and procedure Bubble columns are frequently used as absorbers and reactors. Their widespread use is due to ease of construction, maintenance and absence of moving parts. In addition, solids can be handled without any erosion and plugging problems [3]. Moreover, slow reactions can be carried out due to high liquid residence time and high values of effective interfacial area, and overall mass transfer can be obtained [4]. The absorption of CO, in ammoniated sabkha water involves both chemical reaction and mass
H. K. Abdel-A al et al. /Desalination
113 (1997) 19-25
21
Table 2 Typical experimental chemical compositions of seawater and sabkha water, ppm Sample location
Arabian Gulf
seawater
Fig. 2. Sketch of the batch gas bubbler.
transfer. The primary reaction which consumes CO, is a fast reaction, while the hydrolysis of the carbamate formed is slow. To provide sufficient residence time of liquid for the slow hydrolysis of carbamate and to reduce the plugging problems due to solid precipitation, a bubble column was used in this study. The schematic diagram of the batch gas bubbler, as given in Fig. 2, comprises an absorption glass vessel of about 40cm long and a 5cm inside diameter, fitted with socket, inlet cone and a long inlet tube which passes almost to the bottom of the absorption vessel. The unabsorbed gas is vented to the atmosphere. The first phase of this study involved the selection of the site of sabkha in the Eastern Province. The site considered in this study is located at the King Fahd Military Hospital. Samples of the sabkha brine were taken for chemical analysis in order to compare the ionic phases at site with those in the literature. Table 2 provides the results. A brine solution was treated with ammonia to give an ammoniated brine of known and fixed concentration of salt and ammonia. The resulting solution was placed in the batch gas bubbler. Then carbonation of the ammoniated brine was carried out using pure carbon dioxide gas. The gas passed through the gas meter to set the required flow rate. The temperature of the system was regulated by using a cooling water jacket, and the carbonation time was recorded during the operation. The carbonation of the solution was suspended after specified periods and the solution
Na’ Ca++ Mg++ K+ ClSO,Total alkalinity PH TDS
20,650 420 1,550 660 35,000 3,300 52.1
Eastern Province sabkha 58,410 1,433 4,810 2,396 102,130 6,118 564 8.3 182,488
6,150
is passed through a filter to separate the precipitate formed from the mother liquor (filtrate). In the filtrate, analysis of carbonates, chloride, NaCl and NH&l, is accomplished, while the precipitate analysis of carbonates, chloride and NaCl is carried out. The Na+ was determined by an atomic absorption spectrophotometer, model 3 100 Perkin Elmer. The separation method of NaCl, the major constituent, in the sabkha is carried out using a series of chemical reactions involving the conversion of both Na’ and Cl- into NaHCO, and NH,Cl, respectively. Fig. 3 shows an outline of the method of separation. The main chemical reactions are as follows: CO, + 2NH, = NH,COONH,COO-
+ NH;
+ H,O = NH, + HOCOO-
(1) (2)
NH, + HCO, = NH,HCO,
(3)
2 NH,HCO, + 2 NaCl = 2 NaHCO, + 2 NH&l
(4)
Table 3 presents the consumption analysis with the main reactions responsible for the separation of NaCl from sabkha. A preliminary economic assessment could be initiated using the data presented in Table 3. This
22
H K. Abdel-Aal et al. /Desalination
Precipitation
113 (1997) 19-25
would require, however, assigning the dollar value for each of the species entering the reactions as well as the products produced by the process. For an input of (1 mole NH, + 1 mole CO, + 1 mole NaCl), the products are: 1 mole of NaHCO, + 1 mole of NH&l + an equivalent amount of partially desalted water. The difference between the revenue sales of the three products and the costs of the raw materials should be a gauge for the economic feasibility of the proposed process. Reactions: 9 carbamate formation hydrolysis of the carbamate . formation of ammonium bicarbonate . conversion of sodium chloride
and Separatio
+
??
Fig. 3. Outline of a method of separation.
Table 3 Production consumption NaCl from Sabkha Species
NH3
Reactions 1
2
-2
+1 -1
“20 co2
NaCl NH,’ NH,COO“CO; NH,HCO, NaHCO, NH&l
analysis for the separation of
3
4
-1 -1 +1 +1
-1 -1 +1
-1 +1
-1 +1 +1
Net -1 -Ia -1 -1 0 0 0 0 +1 +lb
waterC aWater entering chemical reaction. bNH,Cl is separated by crystallization or could be let? in solution as fertilizer. “Desalted water is produced by physical separation after precipitation of NaHCO,.
The parameters taken into account in this study include the concentrations of ammonia in the sabkha and the extent of carbonation. The purpose of parametric investigation is to work out and enhance optimum chemical conversion of NaCl in the sabkha into NaHCO, and NH&l. The conversion was progress of the chemical monitored by determining the concentration of unreacted NaCl as well as the NH&l formed in the solution. The latter was determined by a gravimetric method. The accuracy and the precision in the determination of Na were greater than any other species present in the solution. Effect of carbonation: the absorption of CO, in ammoniated sabkha is a diffusion controlled process where the resistance lies in the liquid forms phase. Initially, the CO, absorbed carbonate of NH, which transforms to bicarbonate that reacts with NaCl in the sabkha. The transformation reaction is as follows: (NH&CO,
+ CO, + H,O = 2NH,HCO,
(5)
This causes the pH of the solution to change appreciably in the initial period of carbonation and then remains almost constant in the value range of S-10. The absorption of CO, and the
H.K. Abdel-Aal et al. /Desalination
following reactions indicated the separation is less efficient at the low temperatures due to poor reaction rates and at high temperatures due to the decrease of solubility and absorption of gas in the liquid. An optimum temperature of 22°C was used in this study.
113 (1997) 19-25
23
5
Temp 4
8
. .
:
. .
4. Results and discussion The percentage conversion of sodium and chloride ions in the sabkha for various conditions was studied. The carbonation of the ammoniated sabkha brine is carried out at specified conditions. Fig. 4 shows the results when 2.5 and 3.5 molar ammonia solutions were carbonated at the rate of 2Vmin. The results indicate that the rate of sodium ion removal increases with higher carbonation time and tend towards a constant value. The percent of removal of sodium ion was 43% and 56%, respectively. The enhancement of the conversion of sodium ion in the case of 3.5 molar ammonia solution is due to the increase of ammonia concentration that favors the formation of ammonium bicarbonate. When 4.5 molar and 7.5 ammonia solutions were used, keeping all other conditions constant, the conversions of sodium ions were increased as shown in Fig. 5. The results of these two figures the role of ammonia in turn emphasize concentration in the overall efficiency. Indeed, the conversion increased to 58% and 67%, respectively, when synthetic saline water prepared from pure NaCl dissolved in deionized water is used. The conversion of NaCl in the synthetic saline was higher than corresponding sabkha using similar conditions as shown in Table 4. Other conditions are reported by Ibrahim
[51* The presence of the other elements and impurities in the sabkha will contribute to the inhibition of absorption of CO,, or consumption of NH, such as the reactions of NH, with ions of Mgff and Ca++. Ca++ + (NH& CO,-CaCO,
= 22’C
Flow =2l/m1n
8
+ 2 NH:
(6)
.
.
.
.
I 200
I 250
I4 ?? 3.5 M NH3
.
25M
0
NH3 I 100
I 50
00
I 150
Carbonation
Time
3
0
( hr.I
Fig. 4. Reduction of sodium ions in sabkha for 3.5 M and 2.5 M NH,.
Temp Flow
= 22°C = 2 I /mm
.
c ii
.
3-
’
5 i;
8
. .
2-
. . .
:
z
.
0
.
7.5M NH,
.
45MNH,
.oo
I
I
50
100
I 150
200
I 2.50
3 )O
Fig. 5. Reduction of sodium ions in sabkha for 7.5 M and 4.5 M NH,. Mg++ + (NH&
CO,-M&O,
+ 2NH,i
(7)
M&O, + (NH& CO, + 4 $0 - M&O, (NH& CO, 4 H,O
(8)
Mg++ + 2NH,OH
(9)
- Mg(OH), + 2 NH,+
The experimental data can be correlated statistically using regression analysis. For instance, the case 7.5M NH,, the separation using the batch glass bubbler may be considered. Since
24
H. K. Abdel-A al et al. /Desalination
I 13 (I 997) I9-25
Table 4 Filtrate analysis (batch gas bubbler). Feed: synthetic saline water; flow: 2 Vmin; temp: 22°C Ini. NH, M
Ini. NaCl, g
Time, h
Flow, l/m
CO,, wt%
HCO,, wt%
Cl, wt%
TAa, wt%
NH&l, g/l
Filt. NaCl, 8/l
Conv., %
2.5 2.5 3.5 3.5 4.5
30 30 25 25 25
1.5 2 1.5 2 2.5
2 2 2 2 3
2.13 0.48 2.09
2.68 2.19 3.98 4.77 5.33
15.1 16 14 14.1 14.4
18.7 15.3 47.3 38.5 46.3
57.34 46.1 26.7 26.3 52.1
13.5 12.9 8.6 6.2 5.4
54.9 56.8 65.4 75 78.4
M M M M M
“Total alkalinity.
in this study it was important to know the mathematical correlation between carbonation time and Na+ concentration remaining in the solution, it was necessary that the conditions for efficient operation of the experiment be handled in such a way that the data were capable of providing the desired information from the adopted statistical model. The statistical analysis used serves as a basis for the formulation of a relationship between the physical quantities involved in the experiment. Various statistical models were tested, and their accuracy was checked using the parameters such as the coefficient of determination and the coefficient of variation. The best fit was achieved using the exponential model, and the corresponding values of parametric coefficients are shown in Fig. 6. This model Iit most of the data except in the initial range of carbonation time. Sodium ion concentration can be expressed as a function of the carbonation time. The best fit is indicated by the closeness of the coefficient of determination to unity. Fig. 6 has a coefficient of determination (R2) of 0.99 which indicated that 99% of the data is accounted for by the regression equation: y = 0.376x2 - 2.025x + 4.034
(10)
where y is the Na+ concentration after a carbonation time, x. As can be seen from the figure, the best fit of the experimental data is represented by a parabolic model.
5 ,
I
75
NH3
Flow
Zl/min
y -
0.376
x2-
2025x
+ 4.034
‘b -
Theor
.
0, 00
Exwr , , 20 40
, 60
(
,
80
loo
Carbonation
,
,
,
Time
,
180
140
,
I
,
2 20
I hr. 1
Fig. 6. Comparison between experimental and statistical results.
5. Conclusions 1. Separation of sabkha water of high salinity
was carried out in a batch gas bubbler. The conversion of NaCl found in ammoniated sabkha brine into sodium bicarbonate and ammonium chloride was investigated. The highest conversion of NaCl achieved was 67%. The effect of temperature and initial concentrations of NH, showed that an optimum temperature of separation is 22°C the higher concentrations of salts in the feed reducing the conversion. A parabolic type exponential model was correlated with the experimental data. 2. Indigenous resources of raw materials are abundant which can be utilized for testing the separation approach. They also can be used as a
H. K. Abdel-A al et al. / Desalination I1 3 (1997) 19-25
source of extraction of useful minerals. Ground water RO treatment plants, as well as MSF plants, result in a brine stream of 10% to 40% of the feed. In inland areas, disposal of brine streams to open lands or dry stream beds is environmentally unacceptable [6]. This method can serve as a remedy to such problems. The proposed method could be extended for pollution control applications. Brine solutions to be disposed of either from desalination plants or oil fields are excellent feed stocks for the purification of many industrial gas effluents that contain CO, and NH, gases. The purity of separated salts such as NaHCO, depends on the type of feed used in the separation process. 3. The production-consumption analysis presented for the separation process along with the experimental data are to be used in a future study for economic feasibility analysis. As stated earlier, the proposed method uses saturated brines at least five times more concentrated than seawater and produces some chemical products of potential value on top of partially desalted water. In this regard, the method should be considered as a novel approach rather than a competitive one to the traditional thermal, chemical and membrane processes.
25
This work is an extension of the research initiated and published by the authors on the conversion of highly saline water [7]. Acknowledgments The authors are grateful to KFUPM for the support provided throughout this work.
References [II
121 t31 [41 151 161
[71
H. Johnson, M.R. Kamal, G.O. Pierson and J.B. Ramsay, in: S.S. Al-Sayyari and J.G. Zolt, eds.. Quaternary Period in Saudi Arabia, Springer-Verlag, Austria, 1978, p. 84. P.G. Fookes, W.J. French and S.M.M. Rice, Q.J. Eng. Geol., London, 18 (1985) 101. R.A. Mashelkar an M.M. Sharma, Trans. Inst. Chem. Engrs., 48 (1970) T162. R.H. Perry and C. Chilton, eds., Chemical Engineers’ Handbook, 5th ed., McGraw-Hill, Japan, 1983. A.A. Ibrahim, Ph.D. dissertation, KFUPM, Dhahran, Saudi Arabia, 1993. A.J. Al-Saati and Al-Abdual’aly, Brine production. recovery and disposal in groundwater RO plants in the central region of Saudi Arabia, abstract presented to symposium on desalination processes in Saudi Arabia, KSU, 1994. H.K. Abdel-Aal, A.A. Ibrahim, M.A. Shalabi and D.K. Al-Harbi, I&EC Res., 35 (1996) 3.
Dual-purpose chemical desalination process H.K. Abdel-Aal*, A.A. Ibrahim, M.A. Shalabi and D.K. Al-Harbi Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Tel. +966 (3) 860-2205; Fax +966 (3) 860-4234; E-mail: [email protected]
Received 12 April 1996; accepted 13 May 1997
Abstract The separation of sodium chloride from sabkha water was carried out using a chemical conversion approach. A sdium cation and chloride anion were converted into sodium bicarbonate, which is precipitated under experimental conditions, and ammonium chloride, which can be separated by crystallization, respectively. Concentrated brine solutions of 25% salt content were the feedstock used. In addition to partially desalted water, chemical products of economic potential values were produced, namely soda ash and ammonium chloride. Utilization of the brine exit from MSF desalination plants as feed stock is another alternative. Keywords:
???
1. Introduction Mixing processes are common in nature. This entails the reverse procedure of separation. The separation process is difficult to achieve because it is the opposite of mixing, a process favored by the second law of thermodynamics. Consequently, separation often accounts for major production costs in the chemical and petrochemical industries. Sabkhas are saline flats that are underlain by silt, clay and sand, and often encrusted with salt. *Corresponding author.
They are equilibrium surfaces whose level is largely controlled by hydrological and climatic conditions [ 11. The objective of this study is to apply separation methods to desalt indigenous sabkha deposits found in the Kingdom of Saudi Arabia, using a chemical approach. In Saudi Arabia, sabkhas exist either along the coast of the Red Sea and the Arabian Gulf as coastal sabkha or inland as continental sabkha. Fig. 1 summarizes the sabkhas in the coastal plains of the eastern province as described by Johnson et al. [2]. The ground water is rich in salt and is usually less
00 1 l-9 164/97/$09.50 0 1997 Elsevier Science B.V. All rights reserved PZI 001 l-9164(97)001 11-2
20
H. K. Abdel-A al et al. / Desalination I13 (1997) 19-25
Table 1 SaWCA/EDCplants (SADAF, Jubail, 1991)
Fig. 1. Sabkha flat along the east coast of Saudi Arabia. than 1 meter below the surface. The NaCl concentration reaches up to 25% by weight. Sabkha is characterized by high a content of cations such as Mg++, but it is not normally saturated in these ions. In the Arabian gulf, the exploitation of these sabkha resources has attracted many researchers and companies. For instance, SADAF, of Jubail in Saudi Arabia, obtains raw brine from a salt plant at Riyas as shown in Table 1. The brine is used in the chlor-alkali plant to produce H,, Cl,, and caustic soda.
2. Objectives of the study The main objective of this work is to apply the chemical separation method to desalt indigenous sabkha resources, which leads to the recovery of chemical products as well as partially desalted obtained are sodium water. The products bicarbonate (NaHCO,), soda ash (Na,CO,), and water rich in fertile salts, namely ammonium chloride (NH&l).
Annual capacity (metric tons)
Daily capacity (metric tons)
Salt plant
Salt
556,100
1,700
Chlor-alkali plant
Chlorine Hydrogen Caustic soda
333,000 9,800 367,000
1,000 29.6 1,100
Ethylene dichloride plant
EDC
454,000
1,363
The chemical separation method is not to be considered as an alternative to the existing conventional desalination processes. The proposed method uses saturated brines at least five times more concentrated than seawater and produces valuable chemical products in addition to partially desalted water. In this respect, it is regarded as a supplement to the physical multistage flash desalination processes. The economics of the proposed chemical process as compared to the existing ones cannot be easily evaluated. This requires comparing the value of sales of products vs. the total production costs including cost of raw materials of chemical feed.
3. Apparatus and procedure Bubble columns are frequently used as absorbers and reactors. Their widespread use is due to ease of construction, maintenance and absence of moving parts. In addition, solids can be handled without any erosion and plugging problems [3]. Moreover, slow reactions can be carried out due to high liquid residence time and high values of effective interfacial area, and overall mass transfer can be obtained [4]. The absorption of CO, in ammoniated sabkha water involves both chemical reaction and mass
H. K. Abdel-A al et al. /Desalination
113 (1997) 19-25
21
Table 2 Typical experimental chemical compositions of seawater and sabkha water, ppm Sample location
Arabian Gulf
seawater
Fig. 2. Sketch of the batch gas bubbler.
transfer. The primary reaction which consumes CO, is a fast reaction, while the hydrolysis of the carbamate formed is slow. To provide sufficient residence time of liquid for the slow hydrolysis of carbamate and to reduce the plugging problems due to solid precipitation, a bubble column was used in this study. The schematic diagram of the batch gas bubbler, as given in Fig. 2, comprises an absorption glass vessel of about 40cm long and a 5cm inside diameter, fitted with socket, inlet cone and a long inlet tube which passes almost to the bottom of the absorption vessel. The unabsorbed gas is vented to the atmosphere. The first phase of this study involved the selection of the site of sabkha in the Eastern Province. The site considered in this study is located at the King Fahd Military Hospital. Samples of the sabkha brine were taken for chemical analysis in order to compare the ionic phases at site with those in the literature. Table 2 provides the results. A brine solution was treated with ammonia to give an ammoniated brine of known and fixed concentration of salt and ammonia. The resulting solution was placed in the batch gas bubbler. Then carbonation of the ammoniated brine was carried out using pure carbon dioxide gas. The gas passed through the gas meter to set the required flow rate. The temperature of the system was regulated by using a cooling water jacket, and the carbonation time was recorded during the operation. The carbonation of the solution was suspended after specified periods and the solution
Na’ Ca++ Mg++ K+ ClSO,Total alkalinity PH TDS
20,650 420 1,550 660 35,000 3,300 52.1
Eastern Province sabkha 58,410 1,433 4,810 2,396 102,130 6,118 564 8.3 182,488
6,150
is passed through a filter to separate the precipitate formed from the mother liquor (filtrate). In the filtrate, analysis of carbonates, chloride, NaCl and NH&l, is accomplished, while the precipitate analysis of carbonates, chloride and NaCl is carried out. The Na+ was determined by an atomic absorption spectrophotometer, model 3 100 Perkin Elmer. The separation method of NaCl, the major constituent, in the sabkha is carried out using a series of chemical reactions involving the conversion of both Na’ and Cl- into NaHCO, and NH,Cl, respectively. Fig. 3 shows an outline of the method of separation. The main chemical reactions are as follows: CO, + 2NH, = NH,COONH,COO-
+ NH;
+ H,O = NH, + HOCOO-
(1) (2)
NH, + HCO, = NH,HCO,
(3)
2 NH,HCO, + 2 NaCl = 2 NaHCO, + 2 NH&l
(4)
Table 3 presents the consumption analysis with the main reactions responsible for the separation of NaCl from sabkha. A preliminary economic assessment could be initiated using the data presented in Table 3. This
22
H K. Abdel-Aal et al. /Desalination
Precipitation
113 (1997) 19-25
would require, however, assigning the dollar value for each of the species entering the reactions as well as the products produced by the process. For an input of (1 mole NH, + 1 mole CO, + 1 mole NaCl), the products are: 1 mole of NaHCO, + 1 mole of NH&l + an equivalent amount of partially desalted water. The difference between the revenue sales of the three products and the costs of the raw materials should be a gauge for the economic feasibility of the proposed process. Reactions: 9 carbamate formation hydrolysis of the carbamate . formation of ammonium bicarbonate . conversion of sodium chloride
and Separatio
+
??
Fig. 3. Outline of a method of separation.
Table 3 Production consumption NaCl from Sabkha Species
NH3
Reactions 1
2
-2
+1 -1
“20 co2
NaCl NH,’ NH,COO“CO; NH,HCO, NaHCO, NH&l
analysis for the separation of
3
4
-1 -1 +1 +1
-1 -1 +1
-1 +1
-1 +1 +1
Net -1 -Ia -1 -1 0 0 0 0 +1 +lb
waterC aWater entering chemical reaction. bNH,Cl is separated by crystallization or could be let? in solution as fertilizer. “Desalted water is produced by physical separation after precipitation of NaHCO,.
The parameters taken into account in this study include the concentrations of ammonia in the sabkha and the extent of carbonation. The purpose of parametric investigation is to work out and enhance optimum chemical conversion of NaCl in the sabkha into NaHCO, and NH&l. The conversion was progress of the chemical monitored by determining the concentration of unreacted NaCl as well as the NH&l formed in the solution. The latter was determined by a gravimetric method. The accuracy and the precision in the determination of Na were greater than any other species present in the solution. Effect of carbonation: the absorption of CO, in ammoniated sabkha is a diffusion controlled process where the resistance lies in the liquid forms phase. Initially, the CO, absorbed carbonate of NH, which transforms to bicarbonate that reacts with NaCl in the sabkha. The transformation reaction is as follows: (NH&CO,
+ CO, + H,O = 2NH,HCO,
(5)
This causes the pH of the solution to change appreciably in the initial period of carbonation and then remains almost constant in the value range of S-10. The absorption of CO, and the
H.K. Abdel-Aal et al. /Desalination
following reactions indicated the separation is less efficient at the low temperatures due to poor reaction rates and at high temperatures due to the decrease of solubility and absorption of gas in the liquid. An optimum temperature of 22°C was used in this study.
113 (1997) 19-25
23
5
Temp 4
8
. .
:
. .
4. Results and discussion The percentage conversion of sodium and chloride ions in the sabkha for various conditions was studied. The carbonation of the ammoniated sabkha brine is carried out at specified conditions. Fig. 4 shows the results when 2.5 and 3.5 molar ammonia solutions were carbonated at the rate of 2Vmin. The results indicate that the rate of sodium ion removal increases with higher carbonation time and tend towards a constant value. The percent of removal of sodium ion was 43% and 56%, respectively. The enhancement of the conversion of sodium ion in the case of 3.5 molar ammonia solution is due to the increase of ammonia concentration that favors the formation of ammonium bicarbonate. When 4.5 molar and 7.5 ammonia solutions were used, keeping all other conditions constant, the conversions of sodium ions were increased as shown in Fig. 5. The results of these two figures the role of ammonia in turn emphasize concentration in the overall efficiency. Indeed, the conversion increased to 58% and 67%, respectively, when synthetic saline water prepared from pure NaCl dissolved in deionized water is used. The conversion of NaCl in the synthetic saline was higher than corresponding sabkha using similar conditions as shown in Table 4. Other conditions are reported by Ibrahim
[51* The presence of the other elements and impurities in the sabkha will contribute to the inhibition of absorption of CO,, or consumption of NH, such as the reactions of NH, with ions of Mgff and Ca++. Ca++ + (NH& CO,-CaCO,
= 22’C
Flow =2l/m1n
8
+ 2 NH:
(6)
.
.
.
.
I 200
I 250
I4 ?? 3.5 M NH3
.
25M
0
NH3 I 100
I 50
00
I 150
Carbonation
Time
3
0
( hr.I
Fig. 4. Reduction of sodium ions in sabkha for 3.5 M and 2.5 M NH,.
Temp Flow
= 22°C = 2 I /mm
.
c ii
.
3-
’
5 i;
8
. .
2-
. . .
:
z
.
0
.
7.5M NH,
.
45MNH,
.oo
I
I
50
100
I 150
200
I 2.50
3 )O
Fig. 5. Reduction of sodium ions in sabkha for 7.5 M and 4.5 M NH,. Mg++ + (NH&
CO,-M&O,
+ 2NH,i
(7)
M&O, + (NH& CO, + 4 $0 - M&O, (NH& CO, 4 H,O
(8)
Mg++ + 2NH,OH
(9)
- Mg(OH), + 2 NH,+
The experimental data can be correlated statistically using regression analysis. For instance, the case 7.5M NH,, the separation using the batch glass bubbler may be considered. Since
24
H. K. Abdel-A al et al. /Desalination
I 13 (I 997) I9-25
Table 4 Filtrate analysis (batch gas bubbler). Feed: synthetic saline water; flow: 2 Vmin; temp: 22°C Ini. NH, M
Ini. NaCl, g
Time, h
Flow, l/m
CO,, wt%
HCO,, wt%
Cl, wt%
TAa, wt%
NH&l, g/l
Filt. NaCl, 8/l
Conv., %
2.5 2.5 3.5 3.5 4.5
30 30 25 25 25
1.5 2 1.5 2 2.5
2 2 2 2 3
2.13 0.48 2.09
2.68 2.19 3.98 4.77 5.33
15.1 16 14 14.1 14.4
18.7 15.3 47.3 38.5 46.3
57.34 46.1 26.7 26.3 52.1
13.5 12.9 8.6 6.2 5.4
54.9 56.8 65.4 75 78.4
M M M M M
“Total alkalinity.
in this study it was important to know the mathematical correlation between carbonation time and Na+ concentration remaining in the solution, it was necessary that the conditions for efficient operation of the experiment be handled in such a way that the data were capable of providing the desired information from the adopted statistical model. The statistical analysis used serves as a basis for the formulation of a relationship between the physical quantities involved in the experiment. Various statistical models were tested, and their accuracy was checked using the parameters such as the coefficient of determination and the coefficient of variation. The best fit was achieved using the exponential model, and the corresponding values of parametric coefficients are shown in Fig. 6. This model Iit most of the data except in the initial range of carbonation time. Sodium ion concentration can be expressed as a function of the carbonation time. The best fit is indicated by the closeness of the coefficient of determination to unity. Fig. 6 has a coefficient of determination (R2) of 0.99 which indicated that 99% of the data is accounted for by the regression equation: y = 0.376x2 - 2.025x + 4.034
(10)
where y is the Na+ concentration after a carbonation time, x. As can be seen from the figure, the best fit of the experimental data is represented by a parabolic model.
5 ,
I
75
NH3
Flow
Zl/min
y -
0.376
x2-
2025x
+ 4.034
‘b -
Theor
.
0, 00
Exwr , , 20 40
, 60
(
,
80
loo
Carbonation
,
,
,
Time
,
180
140
,
I
,
2 20
I hr. 1
Fig. 6. Comparison between experimental and statistical results.
5. Conclusions 1. Separation of sabkha water of high salinity
was carried out in a batch gas bubbler. The conversion of NaCl found in ammoniated sabkha brine into sodium bicarbonate and ammonium chloride was investigated. The highest conversion of NaCl achieved was 67%. The effect of temperature and initial concentrations of NH, showed that an optimum temperature of separation is 22°C the higher concentrations of salts in the feed reducing the conversion. A parabolic type exponential model was correlated with the experimental data. 2. Indigenous resources of raw materials are abundant which can be utilized for testing the separation approach. They also can be used as a
H. K. Abdel-A al et al. / Desalination I1 3 (1997) 19-25
source of extraction of useful minerals. Ground water RO treatment plants, as well as MSF plants, result in a brine stream of 10% to 40% of the feed. In inland areas, disposal of brine streams to open lands or dry stream beds is environmentally unacceptable [6]. This method can serve as a remedy to such problems. The proposed method could be extended for pollution control applications. Brine solutions to be disposed of either from desalination plants or oil fields are excellent feed stocks for the purification of many industrial gas effluents that contain CO, and NH, gases. The purity of separated salts such as NaHCO, depends on the type of feed used in the separation process. 3. The production-consumption analysis presented for the separation process along with the experimental data are to be used in a future study for economic feasibility analysis. As stated earlier, the proposed method uses saturated brines at least five times more concentrated than seawater and produces some chemical products of potential value on top of partially desalted water. In this regard, the method should be considered as a novel approach rather than a competitive one to the traditional thermal, chemical and membrane processes.
25
This work is an extension of the research initiated and published by the authors on the conversion of highly saline water [7]. Acknowledgments The authors are grateful to KFUPM for the support provided throughout this work.
References [II
121 t31 [41 151 161
[71
H. Johnson, M.R. Kamal, G.O. Pierson and J.B. Ramsay, in: S.S. Al-Sayyari and J.G. Zolt, eds.. Quaternary Period in Saudi Arabia, Springer-Verlag, Austria, 1978, p. 84. P.G. Fookes, W.J. French and S.M.M. Rice, Q.J. Eng. Geol., London, 18 (1985) 101. R.A. Mashelkar an M.M. Sharma, Trans. Inst. Chem. Engrs., 48 (1970) T162. R.H. Perry and C. Chilton, eds., Chemical Engineers’ Handbook, 5th ed., McGraw-Hill, Japan, 1983. A.A. Ibrahim, Ph.D. dissertation, KFUPM, Dhahran, Saudi Arabia, 1993. A.J. Al-Saati and Al-Abdual’aly, Brine production. recovery and disposal in groundwater RO plants in the central region of Saudi Arabia, abstract presented to symposium on desalination processes in Saudi Arabia, KSU, 1994. H.K. Abdel-Aal, A.A. Ibrahim, M.A. Shalabi and D.K. Al-Harbi, I&EC Res., 35 (1996) 3.