Optimization Process of MDEA Degradation by using UVH

88 Figure 4. 14 Effect of temperature on TOC removal.

4.1.7 Optimization Process of MDEA Degradation by using UVH

2 O 2 The optimization of MDEA degradation process was initially conducted by screening of factors that influence the degradation process. Factors without optimum values during preliminary experiments were screened based on the limitation of the present equipment capability. The factors screened were UV intensity and initial MDEA concentration. UV intensity at 12.06 mWcm 2 was the highest intensity of UV lamp available in the laboratory. The initial concentration of MDEA solution at 1000 ppm of organic carbon was the best concentration to monitor the degradation process. Therefore, these values were chosen for further experiments to screen the other factors. RSM is a well known process to evaluate the effect of each factor and their interaction. The factors screened using RSM were initial concentration of H 2 O 2 , initial pH, and temperature of reaction. Screening of independent factors affecting the TOC removal was carried out using Box-Behnken design. Three level factors consisting of 2 2 full factorials with 3 center point were created. The three levels that was chosen 89 based on the preliminary study was coded as low -1, middle 0, and high +1 are presented in Table 4.1. Table 4. 1 The three levels chosen from the preliminary study. Factors Levels Low -1 Middle 0 High +1 A: H 2 O 2 0.12 M 0.18 M 0.24 M B: Temperature 30 ºC 40 ºC 50 ºC C: pH 7 9 11 By applying the Box-Behnken design, the generated matrix design of the experiments is shown in Table 4.2. The combination of three level factors resulting in thirteen 13 experiments, with different possible combination to cover the entire range of variable. Table 4. 2 Box-Behnken matrix design. Experiment Operational parameter pH H 2 O 2 M Temperature ºC 1 9.0 0.12 50.0 2 11.0 0.18 50.0 3 7.0 0.18 30.0 4 11.0 0.18 30.0 5 9.0 0.24 30.0 6 9.0 0.12 30.0 7 11.0 0.12 40.0 8 7.0 0.18 50.0 9 7.0 0.24 40.0 10 11.0 0.24 40.0 11 7.0 0.12 40.0 12 9.0 0.18 40.0 13 9.0 0.24 50.0 90 Based on the matrix design, experiments were conducted and the percentages of TOC removal obtained from the experiments were fitted to the second order polynomial equation to represent the present experimental results on TOC removal with other parameters Equation 4.2. TOC removal = -511.106 + 56.2002pH + 7.76336Temperature + 2212.35H 2 O 2 - 2.16741pH 2 + 20.9167pHH 2 O 2 - 0.620375pHTemperature - 3954.27H 2 O 2 2 - 21.2887H 2 O 2 Temperature + 0.00447125Temperature 2 4.2 The predicted values according to Equation 4.2, are compared with those obtained from experiments Table 4.3. Table 4. 3 Box-Behnken matrix design with experimental and fitted value. Experiment Operational parameter Response TOC removal pH H 2 O 2 Temperature Observed value Fitted value 1 9.0 0.12 50.0 43.92 42.71 2 11.0 0.18 50.0 22.14 22.90 3 7.0 0.18 30.0 65.00 64.24 4 11.0 0.18 30.0 73.63 73.60 5 9.0 0.24 30.0 82.45 83.66 6 9.0 0.12 30.0 43.47 43.05 7 11.0 0.12 40.0 23.08 23.53 8 7.0 0.18 50.0 63.14 63.17 9 7.0 0.24 40.0 54.49 54.04 10 11.0 0.24 40.0 44.79 43.61 11 7.0 0.12 40.0 42.82 44.00 12 9.0 0.18 40.0 64.07 64.20 13 9.0 0.24 50.0 31.81 32.23 91 The experimental value R 2 = 0.9980 of TOC removal satisfactory agree with those predicted using Equation 4.2, which are shown in Figure 4.15, proving the validity of the second order polynomial model Equation 4.1 for representing the present degradation of MDEA using UVH 2 O 2 oxidation process. Figure 4. 15 Relation between experimental value and fitted value of TOC removal. Pareto chart of the standardized effect at P = 0.05 Figure 4.16 clearly shows the standardized effects of factors and interaction between each factor affecting TOC removal. Figure 4. 16 Pareto chart of the standardized effect for percentage TOC removal for screening of significant factors for the degradation of MDEA. 92 The order of the most significant factor for increasing TOC removal is: initial concentration of H 2 O 2 interaction between pH and H 2 O 2 quadratic temperature. Meanwhile, the order of the most significant factor to reduce the TOC removal is: temperature quadratic H 2 O 2 interaction between H 2 O 2 and temperature interaction between pH and temperature pH quadratic pH. ANOVA analysis was performed to identify the significant factors affecting the TOC removal at P-value = 0.05 and the results of ANOVA analysis is presented in Table 4.4. A significant factor affecting the TOC removal was indicated by P-value of less than 0.05. Table 4.4 shows that the quadratic factor of temperature gave insignificant effect P-value = 0.5489 and the other factors and their interaction gave a significant effect for TOC removal P-value 0.05. Table 4. 4 ANOVA analysis for TOC removal of synthetic MDEA waste. Source Sum of Squares Df Mean Square F-Ratio P-Value A:pH 477.56 1 477.56 267.01 0.0000 B:H 2 O 2 453.713 1 453.713 253.68 0.0000 C:Temperature 1340.14 1 1340.14 749.29 0.0000 AA 277.523 1 277.523 155.17 0.0001 AB 25.2004 1 25.2004 14.09 0.0132 AC 615.784 1 615.784 344.29 0.0000 BB 748.231 1 748.231 418.35 0.0000 BC 652.624 1 652.624 364.89 0.0000 CC 0.738169 1 0.738169 0.41 0.5489 Total error 8.94274 5 1.78855 Total corr. 4551.51 14 In order to examine the effect of factors towards the responses, a graphical representation known as contour plots of regression model obtained using Equation 4.2 is shown in Figure 4.17. As illustrated in Figure 4.17, increasing concentrations of H 2 O 2 to a certain level causes an increase in TOC removal, due to the increasing sources of available hydroxyl radicals. 93 Further increase of H 2 O 2 will decrease TOC removal, which could be related to the scavenging effect of H 2 O 2 itself, leading to a decrease in H 2 O 2 concentration in the system. In addition, by increasing the pH to a certain level will increase the removal of TOC, which might be due to the reason that hydroxyl radical oxidation mechanism for compounds containing nitrogen atoms are favorable at higher pH levels [89]. However, further rise in pH lead to a reduction in TOC removal, due to the decomposition of H 2 O 2 itself at higher pH. Figure 4. 17 3D Contour plots of TOC removal for MDEA. Based on the contour plots of TOC removal Figure 4.17, the optimum conditions for the degradation of MDEA using UVH 2 O 2 process at initial concentration of MDEA = 2000 ppm = 1000 ppm of TOC and UV intensity = 12.06 mWcm 2 are: temperature = 30 ºC, pH = 9.76, and the concentration of H 2 O 2 = 0.22 M Table 4.5, in the other words, the molar ratio between MDEA TOC M to H 2 O 2 M = 1:2.56. 94 Table 4. 5 Optimum condition for the degradation of MDEA by using UVH 2 O 2 at initial TOC = 1000 ppm; UV light intensity = 12.06 mWcm 2 . Factor Optimum condition H 2 O 2 0.22 M pH 9.76 Temperature 30 ⁰C Experiments were conducted in duplicate, using the obtained optimum condition and the estimated TOC removal was 85.74, which satisfactory agree with the predicted value of 86.05 Figure 4.18 indicating the validity of the present proposed Equation 4.2. The deviation between the experimental value and the predicted value was estimated to be 0.31. Figure 4. 18 Comparison of predicted and experimental TOC removal at optimum condition. 95

4.2 Degradation Intermediates Identification and Development of