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Ind. Eng. Chem. Res. 1991,30, 2583-2586
RESEARCH NOTES Advanced Oxidation Processes for Destruction of Dissolved Organics in Process Wastewater: Statistical Design of Experiments A unique design of experiments is presented as an aid to study the destruction of dissolved organic matter in process wastewater. The chief dissolved components of the wastewater were formaldehyde, methanol, 2-propanol, ethanol, acetic acid, formic acid, and unknowns. The unknowns were assumed to be C5alcohols or, specifically, pentanol. Taguchi methods were applied in the design of experiments for each different experimental variable to determine its percentage contribution to the destruction of the dissolved organic matter. This technique helped us gain an insight into the relative significance of the various experimental factors under actual operating levels. The utility of this approach is attractive particularly because AOP (advanced oxidation processes) is a developmental technology, and hence, there is scant documentation or experience on the destruction of a complex mixture of dissolved organic compounds. When there are a large number of experimental variables, and there exists indecision as to which can be manipulated in order to achieve higher removal efficiencies of the dissolved organics, then this technique can provide an extremely effective structured approach. Introduction The technique of defining and investigating all possible conditions in an experiment involving multiple factors is known as the design of experiments. This technique is also referred to as factorial design (Barad et al., 1989). The method of choosing a limited number of experiments that provide the most information is known as a partial factorial experiment. Although this short-cut method is logical, no guidelines exist for ita application by choosing the best set of experiments. Taguchi's approach complements this idea. Taguchi methods have been developed in order to establish guidelines for the systematic investigation of only a fraction of all the possible combinations of variables and to determine the effect of each factor on the overall objective of the experiment. This technique simplifies the experimental design and saves considerable time and expense. Furthermore, Taguchi's methodology has helped simplify and standardize the fractional factorial designs in such a manner that designs for the same experiment by different experimenters should yield similar data and similar conclusions (Roy, 1990; Lin et al., 1990). The standard experimental techniques and the analysis methods used in this approach produce consistency and reproducibility rarely found in any other statistical method. Because of the large number of experimental variables in this study (Tock et al., 19911, the design of this particular experiment was begun by arranging the experiments with an ordered plan in which all the relevant factors are varied in a regular routine. Through the factorial approach, a more complete variable-response picture was efficiently obtained, as compared to the alternative of varying each of the factors one at a time while keeping the others constant. Moreover, in this experimental design it was assumed that the factors were linear and independent of each other. In other words, the outcome was assumed to be directly proportional to the linear combinations of individual factor main effects (Roy, 1990; Barad et al., 1989). Steps Involved in Designing Factorial Experiments 1. Definition of the Problem. The problem in this instance was to study the effect of different variables in-
Table I. Description of Exwrimental Levels temp, platinum level O C air oxidizer catal no air no oxidizer no platinum 1 98 2 25 with air with oxidizer with platinum
~
time, h 1 2
volved in the destruction of dissolved organic matter in a process wastewater stream, i.e., a distillation bottoms. The measured response was the percentage removal of organic matter which was calculated by measuring the decrease in the chemical oxygen demand (COD). The COD of each wastewater sample was obtained in triplicate before and after treatment, and then the average of the triplicate values was used as the response. The average value method was used because the COD value for the replicated samples was found to differ by about 10% from the actual value of a standard sample. The large experimental error was due to the complex multicomponent nature of the wastewater. 2. Identification of Factors To Be Investigated and Their Levels. A review of the literature (Glaze and Kang, 1988, 1989a,b; Leavitt et al., 1990; Staehlin and HoignB, 19851 led to the following as different factors to be tested in the destruction of COD by chemical oxidation: (a) treatment time; (b) temperature; (c) amount of air (oxygen) fed into the treatment vessel; (d) type of oxidizer added in addition to the oxygen in the air stream; (e) the presence of a platinum (Pt) catalyst as a solid surface on which oxidation could occur. It was decided to test the above-mentioned factors at two working levels. The description of the two experimental levels chosen is detailed in Table I. Two liquid-phase oxidizers, namely, a 3% solution of hydrogen peroxide and an aqueous solution (lo00 g/L) of ammonium nitrate, were to be tested for differences in treatment results. The liquid peroxide solution was added at 50 mL/h, while the nitrate solution was added at 12.5 mL/h at level 2. Air, when used, was fed at 2 L/min at level 2. 3. Selection of Orthogonal Array. An LBarray was used in the design of the experiment. This is an array with eight rows and seven columns and is used to design experiments up to seven, two-level factors. An L8array was
0888-5885191/2630-2583$02.50JO 0 1991 American Chemical Society
2584 Ind. Eng. Chem. Res., Vol. 30, No. 12, 1991 Table 11. LBArray Used in (ROY.1990) expt no. 1 2 1 1 1 2 1 1 3 1 2 4 1 2 5 2 1 6 2 1 7 2 2 8 2 2
Modeling Process Equipment 3 1 1 2 2 2 2 1 1
4 1 2 1 2 1 2 1 2
5 1 2 1 2 2 1 2 1
6 1 2 2 1 1 2 2 1
7 1 2 2 1 2 1 1 2
Table 111. Description in Words of the Runs Required expt no. descrbtion of exptl run 1 no air, no oxidizer, no platinum, 98 "C,1 h 2 no air, oxidizer, platinum, 98 "C, 2 h 3 air, no oxidizer, platinum, 98 "C, 2 h 4 air, oxidizer, no platinum, 98 "C,1 h 5 no air, no oxidizer, no platinum, 25 "C, 2 h 6 no air, oxidizer, platinum, 25 "C, 1 h 7 air, no oxidizer, platinum, 25 O C , 1 h 8 air, oxidizer, no platinum, 25 "C, 2 h Cooling
.ter
Th
Percentage Removal of COD
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30t
I
,
I
0
00
1.0
2.0
3.0
4.0
5.0
6.0
70
Time (hours)
* % Removal (25 C) 4% Removal (98 C) Figure 2. Effect of temperature on COD reduction. The experimental data were obtained by treating 450 mL of the process wastewater with 2 L/min of air, 50 mL/h of lo00 g/L aqueous ammonium nitrate solution, and 0.127-mm-diameterwire in a 52-mesh platinum gauze catalyst. Table IV. Interaction between Two Columns in a n LB Array (Roy, 1990) column 1 2 3 4 5 6 7 (1) 3' 2 5 4 7 6 ( 2 ) l 6 7 4 5 (3) 7 6 5 4 (4) 1 2 3' (5) 3' 2 (6) 1
(7)
Three
mantle
Figure 1. Experimental setup used in the design of experiments to destroy dissolved organics in process wastewater.
chosen because this is the lowest order array which can accommodate five two-level factors. If a detailed analysis of interaction parameters was desired, then a higher order array (LIBor LS2)would have to be chosen. The L8array is illustrated in Table 11. Each row represents a trial condition with factor levels indicated by numbers in the row. The vertical columns correspond to the variables specified in the study. Each column contains four level 1 and four level 2 conditions for each factor assigned to the column. Since each column contains the same number of 1's and 2'5, the array is called an orthogonal array (OA). 4. Assignment of Factors to Columns. It was decided to assign the five factors involved in the study, namely, temperature, air, oxidizer, platinum catalyst, and treatment time to columns 1,2,4,6,and 7. The choice of the column assignments to the experimental variables was arbitrary. Table 11, therefore, identifies eight trial runs needed to complete the experimental design at the working levels chosen for each trial run. The verbal description of each experimental run is shown in Table 111. All experiments were carried out in a 500-mL flask. When it was required in an experiment, the solid platinum gauze (52-mesh, 0.127-mm-diameter wire) was submerged in the wastewater solution, and air, when used, was passed through the gauze (Figure 1). Analysis of Results In this particular design, it was desired to compare the
two liquid oxidizer solutions, 3% hydrogen peroxide and lo00 g/L ammonium nitrate. It was desired to compare their effectiveness in addition to finding the relative significance of each of the experimental variables. In this case, the experimental measurement was the total COD of the wastewater. These data were used to calculate the percentage removal of organic compounds, which was used as the criterion in the statistical analysis. The oxidizer producing the higher percentage of removal of the organic compounds is preferred. This quality characteristic is referred to as "the bigger the better". [Suppose the experimental response was the COD value of the wastewater. Then, the oxidizer producing lower COD following treatment is the better one. This quality characteristic is referred to as "the smaller the better". When the object or process has a target value, such as a final COD value of 500 ppm, then the quality characteristic is known as "the nominal the best".] An analysis of variance (ANOVA) procedure was used to determine the percentage contribution of each of the factors involved. For the overall scope of our experiment, it was found that temperature had the most significant effect on the percentage removal of organic matter, contributing 95% to the total COD destruction (Figure 2). The effect of temperature is manifested in two ways. First, an increase in temperature enhances removal by deaeration of the more volatile components in the contaminated water by increasing their vapor pressures. Second, an increase in temperature accelerates the rate of chemical reaction by oxidation. Finally, on the basis of the average response values for the two oxidizers tested, hydrogen peroxide was found to be better than ammonium nitrate as the oxidizing species over the temperature range investigated. A study of four interaction parameters was carried out. Taguchi has determined the relationships for interacting columns. These are presented in a table called the triangular table of interactions (Table IV).
Ind. Eng. Chem. Res., Vol. 30, No. 12, 1991 2585 ~~~~~
~
Ammonium nitrate as oxldizer. 49.8
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t
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45 49
t
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0
E
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*,
Hydrogen peroxide
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Oxidizer (Level 1 )
(Level 1)
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Figure 4. Response curves for oxidizer-time interaction. sponse is the percentage removal of COD.
Oxidizer (Love1 2)
24
13.6
Time (Level 1)
Oxidizer (Love1 1)
Air
2
I Time (Level 1)
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-9.3
Figure 5. Response curves for temperature-oxidizer interaction. response is the percentage removal of COD.
(Level
I
0
Oxidizer (Level 1)
Hydrogen peroxide as oxidizer
HI
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Temperature (Level 1 )
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Figure 3. Response curves for temperature-air interaction. *, response is the percentage removal of COD.
a
\ Temperature (Level
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0
-
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Figure 6. Response curves for air-time interaction. the percentage removal of COD.
*, response is
The number in parentheses at the bottom of each column identifies the column. To find the factors which interact to yield column 3, we locate 3* in the table and move horizontally across to the left and vertically down to find (1)-(2), (5)-(6), and (4)-(7) in Table IV. Thus, the interaction effects of columns (1)-(2), (5)-(6), and (4)-(7) can be studied in column 3. Table IV was thus used to determine the possible interaction effects that could be studied in the two vacant columns, namely, column 3 and column 5 of Table 11. If columns 6 and 7 had been left blank, the possible interaction effects that could be studied (using Table IV) are (2)-(4), (3)-(5), and ( 0 4 7 ) in column 6 and (3)-(4), (2)-(5), and (1)-(6) in column 7. A detailed analysis of interaction parameters was not desired in this preliminary design. Hence, the effect of the choice of experimental variables in other columns was not investigated. A study of four two-factor interaction effects, temperature-air and oxidizer-time in column 3, and temperature-oxidizer and air-time in column 5, was carried out. The response curves for these interaction effects are illustrated in Figures 3-6. The response curves which do not intersect (Figures 3,5, and 6) imply that the interactions of those parameters were not significant over the range of levels investigated. The response curves for the interaction parameters oxidizer-time (Figure 4) do cross, thus indicating an interaction between these two factors.
guidelines as to which factor (temperature) had to be varied in order to successfully achieve the desired result. If a more complete study of interaction parameters was desired, a higher order orthogonal array (LI6or L3J would have to be used, and more experiments would have to be carried out. A large number of observations (26 = 32) would have been needed by equivalent one-factor experiments in order to demonstrate the strong effect of temperature. The results of this fractional factorial design must be used with caution, however. The results are valid only for the range of variable levels indicated. We believe that at higher temperatures the catalyst activity may be even more enhanced, and that the decomposition of ammonium nitrate into more active oxygen species will be favored. Hence, at higher temperatures, the percentage contributions of the different variables could be significantly different from the results just obtained with the variable levels used in this study.
Conclusions The statistical analysis procedure suggested by Taguchi was extremely useful in determining the percentage contribution of the factors to be studied in the experimental design. The design was preliminary, however, and not intended to carry out a detailed study of the interaction parameters. Even so, the technique helped to establish
Literature Cited
Acknowledgment We acknowledge Hcechst Celanese Corporation, Pampa, for their assistance with GC/MS analytical work on this research project and the Texas Tech University Water Resources Center for financial support. Registry No. H3N.HNO3,6484-52-2; HzOz, 7722-84-1; Pt, 7440-06-4.
Barad, M.; Bezalel, C.; Goldstein, J. R. Prior Research is the Key to Fractional Factorial Design. Qual. B o g . 1989,22 (3), 71-75. Glaze, W. H.; Kang, J. W. Advanced Oxidation Processes for Treating Groundwater Contaminated with TCE and PCE Laboratory Studies. J. Am. Water Works Assoc. 1988,80 (5), 57-63. Glaze, W. H.; Kang, J. W. Description of a Kinetic Model for the Oxidation of Hazardous Materials in Aqueous Media With Ozone
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Ind. Eng. Chem. Res. 1991,30, 2586-2592
and Hydrogen Peroxide in a Semibatch Reactor. Ind. Eng. Chem. Res. 1989a, 28 (111, 1573-1580. Glaze, W. H.; Kang, J. W. Test of a Kinetic Model for the Oxidation of Organic Compounds With Ozone and Hydrogen Peroxide in a Semibatch Reactor. Ind. Eng. Chem. Res. 1989b, 28 ( l l ) , 1580-1587. Leavitt, D. D.; Horbath, J. S.; Abraham, M. A. Homogeneously Catalyzed Oxidation for the Destruction of Aqueous Organic Wastes. Enuiron. Prog. 1990, 9 (4), 222-228. Lin, P. K. H.; Sullivan, L. P.; Taguchi, G. Using Taguchi Methods in Quality Engineering. Qual. Prog. 1990, 23 (9), 55-59. Roy, R. K. A Primer On The Taguchi Method; Van-Nostrand Reinhold: New York, 1990, pp 40-71. Staehelin, J.; Hoign6, Decomposition of Ozone in Water in the Presence of Organic Solutes Acting as Promoters and Inhibitors of Radical Chain Reactions. Enuiron.Sci. Technol. 1985,19 (121, 1206-1213.
Tock, R. W.; Rege, M. A.; Bhojani, S. H. Simultaneoue Air Stripping and Advanced Oxidation Processes (AOP) for Process Water Treatment. Presented at the Summer Meeting of the American Institute of Chemical Engineers, Pittsburgh, PA, 1991; paper 28H.
* Author to whom correspondence should be addressed. Mahesh A. Rege,* Sanjay H.Bhojani Richard W.Tock, Raghu S. Narayan Department of Chemical Engineering Texas Tech University Lubbock, Texas 79409 Received for review April 8, 1991 Revised manuscript received September 11, 1991 Accepted September 27, 1991
Effects of Catalytic Hydrotreating on Light Cycle Oil Fuel Quality A pilot plant study was conducted to evaluate three commercial catalysts for hydrotreating of light cycle oil to reduce its aromatic content and improve the cetane index. The operating parameters were varied between 325 and 400 "C,1and 3 h-l, and 4 and 10 MPa at 535 L/L. The data showed that, in general, the product density and aromatic content decreased as the temperature or pressure increased or space velocity decreased. The cetane index improvement ranged from 7.3 to 10.0 for the Ni-W/A1203 catalyst and from 6.1 to 10.1 for the Ni-Mo/A1,03 catalysts. The catalyst performance was evaluated in terms of hydrodesulfurization, hydrodenitrogenation, hydrogenation, aromatic saturation, and hydrogen consumption. This study confirms that light cycle oil can be hydrotreated to improve its cetane quality, thus increasing the extent of its blending ratio into the diesel pool. Introduction The demand for high-quality middle distillates, transportation diesel, and jet fuel has grown significantly over the past decade. Currently, the diesel fuel consumption worldwide is growing at the rate of 5 % . The increased demand for diesel fuel occurred at the expense of heavy fuel oil consumption in most parts of the world. The decline in fuel oil demand, which is expected to continue in the 199Os,was a result of higher crude prices in the 19708 (Denny et al., 1990). In order to meet this increased demand for diesel fuel and to dispose off fuel oil surplus, refiners are increasing the severity of FCC operation. This would increase production of light cycle oil (LCO), which can be used as a blending component into the diesel pool. The LCO, however, has a low cetane index and higher density, sulfur, unsaturates, and aromatics. These properties adversely affect the quality of the resulting diesel fuel thus limiting its blending ratio. The refining industry faces tighter specifications in the 1990s. Current regulation in the US calls for an 80% reduction in sulfur content (from 0.25 to 0.05 w t %) of highway diesel fuel by 1993. The US has also set a minimum cetane index of 40 to maintain the aromatic content of diesel fuel at its current level of 31-34%. A t the same time, the demand for high-quality middle distillates, transportation diesel, and jet fuel is growing significantly (Denny et al., 1990, Miller, 1991). Refiners are looking for a hydrotreating strategy that will meet near-term sulfur requirement and also accommodate future aromatic specification. This would require active research in the development of selective catalysts for deep desulfurization, aromatic saturation, and cetane improvement (Wilson and Kriz, 1984a,b; Wilson et al., 1985, 1986; Nooy et al., 1986; Lee, 1991). This paper aims to determine the improvement of LCO quality by hydrotreating using commercially available catalysts under a wide range of operating conditions. The 0888-5885/91/2630-2586$02.50/0
Table I. Analysis of Light Cycle Oil DroDertv analvtical method density, g/cm8 ASTM D-1298 AP! gravity ASTM D-287 refractive index ASTM D-1218 elem analysis carbon, w t % elem analysis hydrogen, w t % 13C NMR aromatic carbon, w t % aniline point, OC ASTM D-611 Dhormann nitrogen, ppm Dhormann sulfur, wt % ASTM D-976-80 cetane index ASTM D-3710 simuld distilln, OC
IBP" 10% 30% 50% 70% 90% FBP
value 0.933 21.2 1.531 86.8 10.4 53.1 21.6 42 2.6 28.9 208 244 263 273 286 311 338
IBP, initial boiling point; FBP, final boiling point.
effect of catalyst type and operating conditions on desulfurization, denitrogenation, aromatic saturation, and cetane index improvement will be investigated. The results are discussed in light of the thermodynamic equilibrium of aromatic saturation.
Experimental Section The LCO feedstock used in this study is a representative middle distillate produced from FCC of vacuum gas oil obtained from a refinery in Saudi Arabia. The analytical results of LCO, given in Table I, shows that it contains high aromatic carbon (53.1%) and sulfur (2.6 w t % ) content, while it has low hydrogen content and cetane index. It should be noted that the aromatic content and sulfur are much higher than the diesel specifications of 31-34% and 0.25%, respectively. Three commercial hydrotreating catalysts were selected for the screening tests on the basis of recommendations 0 1991 American Chemical Society
