Statistical Analysis Techniques for High Quality Water Iman Morsi1, Amr El Zawawi2, Mostafa Amin3
1,3Faculty of Engineering, Arab Academy for Science and Technology, Alex, Egypt, [email protected], [email protected]
2Faculty of Engineering, Alexandria University, Alex, Egypt, E-mail: [email protected]
Abstract: Water quality standards usually changes from application to another. For industrial application high water quality is required for boilers and turbines protection. Habitual desalinated planets water is used, but by the time it creates some problems because of salts and silica concentrations. Some important parameters have been studied for desalinated water which are Sodium (Na), Calcium (Ca), Magnesium (Mg), Potassium (k), Sulfate (So), Silica (Sio2), Chloride (Cl), and Fluoride (F-); in addition to PH and conductivity. The linear regression model, based on these parameters, is extracted and its coefficients and error are calculated using Regstat method. The principle component analysis (PCA) is applied for clustering water parameters according to the value of measured parameters; the result indicates that it is important to follow the desalination process by another purification process to improve water quality. The demineralization process can decrease ionic impurities significantly, so it is recommended to be used to protect industrial component from deposits and corrosion.
Keywords: Desalination, Demineralization, Regstat method, Principle component analysis (PCA).
1. Introduction
Lately Egypt has been trying to expand in the industrial field, especially in the field of electricity generation, oil refining and petrochemicals production. It is indispensable using water in industrial application to generate steam, closed cooling systems and water injection combustion system. The water quality required changes depending on its purpose [1]. Sea water can be used for cooling systems but it cannot be use for steam generation because of high salts and silica concentration, so desalinated water has been used. In the present paper ten parameters are measured using different equipment.
These parameters are Sodium (Na), Calcium (Ca), Magnesium (Mg), Potassium (k), Sulfate (So), Silica (Sio2), Chloride (Cl) and Fluoride (F-); in addition to PH and conductivity. It is found that desalinated water contains high salts level especially sodium and calcium salts, follow desalination process by demineralization plant that improves water quality to be combatable with its purpose.
2. DemineralizationSystem Description
The desalinated water is passed through a polishing mixed bed consisting of cation and anion resin. The two resin components are kept in the same vessel and intermixed by agitation with the compressed air.
The grains of resin are thus arranged side by side and the whole bed behaves like an infinite number of anion and cation exchangers in series, thus producing treated water of high quality. When salts or silica concentrations increases, the regeneration process shall be started [2]. To carry out regeneration two resins are separated hydraulically as the anion resin is lighter; it rises to the top while heavier cation resins falls to the bottom. When the resins are separated each of them is simultaneously regenerated with caustic and acid. Cation resin is generated with sulfuric acid 4% and anion resin with 3.0% caustic.
Any excess regeneration is removed by rinsing each bed separately. After partial emptying of the vessel, the two resins are remixed with air supplied by the blower. After thorough air mixing, the vessel is refilled. Rinsing is completed and the vessel is then ready to put in service cycle.
3. Instrumentation
To detect the level of different parameters several samples have been taken from the Mediterranean Sea and the desalination plant, and the new demineralization plant in Sidi krir power station for electricity production from October 2011 to July 2012. More than 150 samples for ten variables are used in the data analysis.
3.1 Measurement of Conductivity
The Pur-sense model 410vp is used for the automatic, continuous measurement of the conductivity. It can be used in power plants, boiler feed water and steam. Conductivity indicates the sum of all ions in the water. Metal plates are dipped into the solution and a known alternating voltage is applied; a certain current results proportional to the resistance. Measurements range 1 - 1400 mS/cm and accuracy (+) or (-) 4 % [3].
A four-wire resistance measuring method is used. The four-wire technique uses four conductors to connect the resistance to the measuring instrument. Only the outer two conductors carry substantial current. The inner two conductors connecting the voltmeter to the test resistance carry negligible current and; therefore, drop negligible voltage along their lengths. Voltage dropped across the current carrying wires is irrelevant, since that voltage drop is never detected by the voltmeter. Since the voltmeter only measures voltage dropped across the resistor and not the resistance plus wiring resistance, the resulting resistance measurement is much more accurate. In the case of conductivity measurement, it is not wire resistance that we care to ignore, but rather the added resistance caused by plating of the electrodes. By using four electrodes instead of two, we are able to measure voltage dropped across a length of liquid solution only, and completely ignore the resistive effects of electrode plating as shown in Fig 1.
Fig 1- Four wire conductivity sensor. Source: Ref 3 3.2 Measurement of PH
Color change is a common pH test method used for manual laboratory analyses, but it is not well suited to continuous process measurement. By far the most common pH measurement method in use is an electrochemical special pH sensitive electrode inserted into an aqueous solution that generates a voltage dependent upon the pH value of that solution. Electrochemical pH measurement is based on the Nernst equation, which describes the electrical potential by ions migrating through a permeable membrane [4].
(1) Where:
V = Voltage produced across membrane due to ion exchange, in volts (V) R = Universal gas constant (8.315 J/mol・K)
T = Absolute temperature, in Kelvin (K)
n = Number of electrons transferred per ion exchanged.
F = Faraday constant, in coulombs per mole (96,485 C/mol e−).
C1and C2 = Concentration of ion in measured and reference solution respectively, in moles per liter.
The Nernst equation describes the amount of electrical voltage developed across a special glass membrane due to hydrogen ion exchange between the process liquid solution and a buffer solution inside the bulb formulated to maintain a constant pH value of 7.0 pH. Special pH-measurement electrodes are manufactured with a closed end made of this glass; a small quantity of buffer solution contained within the glass bulb as shown in Fig 2. If the ionic concentrations on both sides of the membrane are equal, no Nernst potential will develop.
Fig 2- PH sensor with thin glass bulb. Source: Ref 4
3.3 Measurement of Anion andCation
The Dionex DX-120 ion chromatograph is a liquid chromatography technique using ion exchange mechanisms and suppressed conductivity detection for the separation and determination of anions and cations. According to Kohlraush’s law of independent migration, conductivity is directly proportional to concentration. The conductivity of a dilute solution is the sum of the individual contributions to conductivity of all the ions in the solution multiplied by their concentration [5].
(2) Where:
K Is the measured conductivity in S/cm
Ci Is the concentration of the ions in equivalents/L
The ionic limiting equivalent conductivity, λi, is specific for each ion. It is the conductivity of the ion divided by the concentration and extrapolated to infinite dilution. Table 1 lists limiting equivalent conductivities for a number of organic and inorganic ions.
Table 1- Limiting equivalent conductivities at 25 C 0. Source Ref 6
Anions λi Cations λi
OH- 198 H+ 350
F- 54 Li+ 39
Cl- 76 Na+ 50
Br- 78 K+ 74
I- 77 NH4+ 73
NO3- 71 Mg2+ 53
HCO3- 45 Ca2+ 60
SO4- 80 Sr2+ 59
Acetate 41 CH3NH3+ 58
Benzoate 23 N(CH3Ch2)4+ 33
4. Statistical Analysis 4.1 Regstat method
Figures 3-9 show the measurements data for 150 samples of Sodium (Na), Calcium (Ca), Magnesium (Mg), Potassium (k), Sulfate (So), Silica (SiO2), Chloride (Cl) and Fluoride (F-). Each figure contains the results of both types of waters, which are compared with the permissible limit of this parameter as used in Metito (Overseas) LTD, Sharjah, UAE [5].
Fig 3- Comparison between sodium concentrations with standard limits for concentration
Fig 4- Comparison between calcium concentrations with standard limits for concentration
Fig 5- Comparison between Magnesium concentrations with standard limits for concentration
Fig 6- Comparison between Potassium concentrations with standard limits for concentration
Fig 7- Comparison between Fluoride concentrations with standard limits for concentration
Fig 8- Comparison between Sodium concentrations with standard limits for concentration
Fig 9- Comparison between Sulfate concentrations with standard limits for concentration
A linear regression model is used to create the relation between the measured parameter as shown in equation (3) and β in which regression coefficients are calculated using regstat method, where calcium, chloride, fluoride, magnesium, potassium, sodium and sulfate are represented by variable X from X1 to X7 [7].
Y=β1+ β2X1+ β3X2+ β4X3+ β5X4+ β6X5+ β7X6+ β8X7 (3) Beta coefficients are calculated using regstat method as shown in table 2.The total concentration (Y) is calculated by taking a sample from the measured data with regression coefficients and the result is shown also in table (2).
Table 2- Beta coefficient for desalinated and demineralized water
The measurements of different parameters for both types of waters are indicated in table 3 with average, standard deviation and error.
Beta coefficients
Desalinated water
Demineralized water
β 1 46.4542 -29.7014
β 2 48.1988 -665.023
β 3 -2.0692 323.9808
β 4 -5.3931 1473.230
β 5 -7.7259 -2127.47
β 6 -10.487 -1621.62
β 7 -1.3765 1475.750
β 8 -1.1727 1879.275
Y 7.0614 2.0218
Table 3- Statistical analysis results of measured parameters
Type of water Parameters Range Average Standard value ׀Error׀
Desalinated water Calcium (ppm) 0.27 - 0.61 0.44 0.3 0.14
Demin water Calcium (ppb) 11.14 - 18.01 0.014 0.3 0.286
Desalinated water Sodium (ppm) 5.21 – 10.35 7.78 0.01 7.77
Demin water Sodium (ppb) 3.01 – 7.25 0.00513 0.01 0.048
Desalinated water Magnesium (ppm) 0.42 – 0.98 0.7 0.3 0.4
Demin water Magnesium (ppb) 1.01 – 3.15 0.0028 0.3 0.297
Desalinated water Sulfate (ppm) 1.15 - 3.96 2.55 0.1 2.45
Demin water Sulfate (ppb) 13.70 - 20.41 0.0176 0.1 0.082
Desalinated water Potassium (ppm) 0.21 - 0.85 0.53 0.1 0.43
Demin water Potassium (ppb) 2.41 - 7.92 0.0056 0.1 0.0944
Desalinated water Chloride (ppm) 11.25 - 17.32 14.29 0.01 14.19
Demin water Chloride (ppb) 14.20 - 18.25 0.0162 0.01 0.006
Desalinated water Flurried (ppm) 0.10 - 0.21 0.15 0.1 0.05
Demin water Flurried (ppb) 7.81 - 13.72 0.017 0.1 0.083
Desalinated water Silica(ppm) 0.01- 0.02 0.015 0.01 0.005
Demin water Silica( ppb) 10- 20 0.015 0.01 0.005
Desalinated water PH 6.5 - 7.8 7.15 7 0.15
Demin water PH 5.5 - 7.5 6.50 7 0.5
Desalinated water Conductivity(µs/cm) mmmmmm)
10 - 25 17.50 0.1 17.4
Demin water Conductivity(µs/cm) <0.2 0.2 0.1 0.1
4.2 Principal Component Analysis (PCA)
The principle component analysis (PCA) is used to create uncorrelated variable from the original variable and make clustering for seven different parameters for desalinated water and demineralized water .PCA is a technique that reduces the data, performing a variance analysis between factors and projects data to line, which minimizes the projection error [9]. The eigenvector which is used for projection lies in the direction of the largest variance [8]. In this paper the data are expressed from seven different parameters which are: Sodium (Na), Calcium (Ca), Chloride (Cl), Potassium (K), Magnesium (Mg), Sulfate (So4) and Fluoride (F-).The data are illustrated in two dimensional spaces as shown in figures 10, and 11.
Different colors indicate different parameters concentrations in each type of water. Calcium (Ca), Sodium (Na), Magnesium (Mg), Sulfate (So4), Potassium (k), Chloride (Cl), Fluoride (F) are indicated from 1 to 7 respectively. Figure (10) shows that desalinated water contains high value of concentration of the measured parameter compared to figure (11), which illustrates the concentration of demineralized water. The distinguishing between the characteristics of each type of water is due to the different location of parameters around the axes of PCA. The concentration of calcium is located on the upper right in the desalinated water, while it is located on the upper left in the demineralized water. The concentration of sodium is located on the upper right in the desalinated water, while it is located on the lower right in the demineralized water. The concentration of magnesium is located on the middle lower in the desalinated water, while it is located on the lower left in the demineralized water. The concentration of sulfate is located on the lower left in the desalinated water, while it is located on the upper middle in the demineralized water. The concentration of potassium is located on the upper left in the desalinated water, while it is located on the upper right in the demineralized water. The
concentration of chloride is located on the lower left in the desalinated water; while it is located on the upper left in the demineralized water .The concentration of fluoride is located on the upper left in the desalinated water, while it is located on the lower left in the demineralized water. Clustering is based on different locations and different scales which changes with the concentration of each parameter.
Fig 10- The clustering using PCA for desalinated water parameters
Fig 11: The clustering using PCA for demineralized water parameters
5. Conclusion
Improving the quality of water which is used in industrial application is very important to protect industrial components. New technologies have been used like DX120 ion chromatograph and conductivity and PH sensors. Analysis of these parameters using the regstast method to find out the change of parameters concentration and using principle components analysis (PCA) for clustering these component .According to the above measurements and analysis the demineralized water proves that it treats desalinated water by removing ionic impurities, which are naturally present in water, so that it can meet high water quality standards to be combatable with its purpose.
Reference
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