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Catalytic Wet Oxidation of Stripped Sour Water from an Oil-Shale Refining Process Jaidev Prasad,† James Tardio,† Deepak B. Akolekar,† Suresh K. Bhargava,*,† and Stephen C. Grocott‡ Department of Applied Chemistry, RMIT University, P.O. Box 2476V, Melbourne, Victoria 3001, Australia, and BHP Billiton (Worsley Alumina Pty Ltd.), P.O. Box 344, Collie, Western Australia 6225, Australia
The catalytic wet oxidation of an organics-laden wastewater produced during oil-shale refining (stripped sour water) was investigated using a high-pressure reaction system. The effects of different catalysts and catalyst loads were investigated under standard optimal reaction conditions that had been previously determined. Tests were also done on monometallic and bimetallic catalytic systems. It was found that Cu(NO3)2 exhibited the best catalytic properties at a load of 33.3 mmol/L. The removal of catalyst, odor, and residual organics from treated solutions by adsorption using activated carbon was also studied to assess whether the environmental acceptability of the process could be improved. It was found that 64% of the Cu used could be taken up by adsorption alone, and up to 99.8% Cu removal could be achieved through a combination of precipitation, filtration, and absorptive techniques. Odor was also significantly reduced, and total organic carbon removal could be improved from 65% using CWO alone to 83.7% after adsorption. 1. Introduction
Table 1. Selected Identified Components of Stripped Sour Watera,b
The refinement of oil shale is a multibillion dollar industry that produces an assortment of petroleum products. As part of the refining process, oil shale has to be retorted at high temperature to liberate target hydrocarbons from the clay mineral matrix of shale oil rock. This produces a wastewater at the rate of approximately 20 m3/kt of product per hour, which has a high concentration of phenols, organic acids, amines, ketones, aldehydes, and polyphenols resembling fulvic and humic acids as well as residual hydrocarbons. It also contains high concentrations of ammonia and hydrogen sulfide, which are removed together with some light hydrocarbons by steam stripping. After this, the water is known as stripped sour water (SSW). Some of the identified constituents of SSW are presented in Table 1. SSW has a total organic carbon (TOC) concentration of about 9 g/L and a chemical oxygen demand (COD) of 18 g/L, which does not satisfy environmental discharge requirements. Therefore, treatment of SSW to reduce organic loading is necessary before it can be discharged into receiving water bodies. When considering various treatment options, factors such as the organic and inorganic species in the wastewater and their concentrations and toxicities have to be assessed. Most treatment methods today are based on single or hybrid techniques that rely on one or a combination of chemical or physical treatments including adsorption-separation, reverse osmosis or distillation, aerobic and anaerobic biological treatments, wet air oxidation, and incineration. Wet oxidation (WO) is an established liquid-phase process for the aerobic oxidation of an organics-laden wastestream under high temperatures (200-315 °C) * To whom correspondence should be addressed. Tel.: +61 3 9925 3365. Fax: +61 3 9639 1321. E-mail: suresh.bhargava@ rmit.edu.au. † RMIT University. ‡ BHP Billiton (Worsley Alumina Pty Ltd.).
analyte
SSW combined base/neutral/acid (mg/L)
pyridine 1-H-pyrrole 2-picoline 3- and 4-picoline 2,6-dimethyl pyridine other C2-alkyl pyridine isomers C3-alkyl pyridines cyclopentanone cyclohexanone 3-methyl-2-cyclopentanone other methyl cyclopentanone isomers C2-alkyl cyclopentanone isomers phenol o-cresol m- and p-cresol C2-alkyl phenol isomers aniline propanenitrile methyl ethyl ketone butanenitrile pentanone isomers pentanenitrile acetic acid n-propanoic acid n-butanoic acid n-pentanoic acid n-hexanoic acid n-heptanoic acid n-octanoic acid
87 130 87 99 120 200 390 55 40 160 100 190 350 110 120 57 47 140 53 34 36 33 60 220 100 160 150 110 47
a
Practical quantitation limit: 0.5. b USEPA GC/MS 8270D.
and pressures (2-21 MPa).1 Although the major products are carbon dioxide and water from the mineralization of the organics, some large organic molecules are not completely oxidized and are instead broken down to smaller intermediates and products (typically lowmolecular-weight organic acids). Because no hazardous chemicals are required to kickstart the WO process and the major products generated are CO2 and H2O, which are relatively harmless, a
10.1021/ie0400705 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/25/2004
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Figure 1. High-pressure wet oxidation reaction system.
major advantage of the WO process is its cleanliness. Another benefit is its ability to treat large volumes of effluent. The main disadvantage of WO is that high temperatures (and consequently pressures) are usually required to achieve high removal efficiencies. However, the application of catalysts in catalytic wet oxidation (CWO) allows the use of more moderate conditions, which, in turn, lowers operating costs and safety risks. WO and CWO processes have so far been successfully applied to the treatment of wastewaters from the textile/ paper,2-5 food, chemical, pharmaceutical,6-8 and alumina industries.9,10 The literature also shows that CWO using homogeneous transition metal catalysts has performed well with simple solutions of organic compounds, especially phenol,11,12 which is a major constituent of SSW. Homogeneous catalysts usually offer the advantage of easy preparation. However, these are also usually difficult to recover. Some might be recoverable through ion exchange or might be cheap throwaway catalysts, whereas others that pose environmental problems might need to be captured using adsorbents prior to discharging the treated wastewater. This paper focuses on the application of the catalytic wet oxidation process to SSW. It details the screening and selection of catalysts, examines the effect of catalyst loading, and also investigates the viability of posttreatment solution polishing and catalyst removal from the treated liquor through the use of activated carbon. 2. Experimental Section All CWO reactions were carried out in a continuously stirred, 1.2-L, stainless steel 316 autoclave (Parr Autoclave, Moline, IL) that had attachments for gas addition to the liquid phase and liquid sample withdrawal. A schematic diagram of the autoclave assembly used is presented in Figure 1. In the standard reaction, a specific amount of test catalyst was placed in the reactor vessel, followed by 0.6 L of SSW acidified to the appropriate pH. The headspace area was evacuated to ∼14 kPa, and the vessel and its contents were heated to 200 °C with the stirrer set at 800 rpm. Once the
target temperature was reached, oxygen was introduced and maintained at a constant partial pressure of 0.5 MPa (PO2) for the duration of the test, which was 3 h. Because chloride salts (used as test catalysts) would, at low pH, have caused corrosion in stainless steel, the autoclave wall was protected by means of a removable glass lining and its internal components were coated with Teflon. The above values of oxygen partial pressure, temperature, and stirrer speed used were chosen on the basis of previous studies conducted in a batch system.13 Under these conditions, the oxygen partial pressure was well above the stoichiometric requirement. In addition, the stirrer rate at 800 rpm was sufficiently high to ensure that reactions were carried out in a kinetically controlled regime, where no mass-transfer limitations exist and the chemical reactions themselves are rate controlling. Given that the partial pressure of oxygen was maintained constant and in excess of stoichiometric requirements, the concentration of dissolved oxygen can be considered to be constant in the system. These standard conditions thus act as a platform for comparing the efficiencies of different catalysts. Effective catalysts that were used in these experiments were bismuth(III) nitrate, purity 98%; copper(II) nitrate 3-hydrate, purity 99.5%; and iron(II) chloride 4-hydrate, purity 98% (all from BDH Chemicals Ltd). Also used were cerium(III) nitrate hexahydrate, purity 99%; cobalt(II) nitrate hexahydrate, purity 98%; and iron(III) nitrate nonahydrate, purity >98% (all from Aldrich Chemical Co.). Manganese(II) nitrate 4-hydrate, purity 98%, was supplied by Riedel-de Haen, and 100% Pd metal was supplied by Golden West Refining Ltd. The pure Pd metal was dissolved in HCl (36%) produced by Ajax Finechem prior to use. TOC was analyzed using an O.I. Analytical model 1010 total carbon analyzer. pH measurements were taken using a Metrohm 620 pH meter. The activated carbon (RB4, steam activated) used in copper removal tests was supplied by Norit. The activated carbon, which was supplied as ∼3-mm extrudates,
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Figure 2. Influence of different catalysts at various pHs. Conditions: 3 h, 0.5 MPa O2 pressure, 200 °C, 800 rpm, 20 mmol of metal catalyst.
was crushed (mortar and pestle) to a smaller particle size prior to use, to allow improved solid-solution contact. The 0.5-1.4-mm particle size fraction was collected by dry sieving and used in all tests. The initial pH of post-catalytically wet-oxidized SSW was altered using a concentrated NaOH solution. Experiments were conducted in a sealed round-bottom flask with 20 mL of treated solution being stirred using a stirrer flea and magnetic stirrer for 2 h with 0.5 g of activated carbon. All tests were conducted at room temperature. Copper concentrations were measured using atomic absorption spectroscopy (Perkin-Elmer model PE 3110). 3. Results and Discussion Monometallic Catalyst Systems: Catalyst Screening and Selection. The catalytic performance of the chloride and nitrate salts of several transition metals was investigated. The results of some promising catalysts are shown in Figure 2. Tests were carried out using the three initial pHs 3.5, 6, and 13.6, the last being the as-received pH of SSW. Cu2+ salts were found to be effective at all initial pHs, especially pH 3.5 where the greatest TOC removal of approximately 65% was obtained. The results obtained with wet oxidation indicate that an increase in TOC reduction of about 14% can be expected as the pH is decreased from 13.6 to 3.5. However, the Cu(NO3)2, Co(NO3)2, CuCl2, and Mn(NO3)2 catalysts all showed a much greater increase in activity than this. This might have been the result of a number of factors. First, these transition metal salts have higher solubilities in acidic conditions, being completely soluble at pH 3.5,14 which would have led to high activities at this pH. The nitrates of Mn2+ and Co2+ were also found to be more effective catalysts than those of Cu2+ at pH 6 and 13.6 but not as effective as Cu2+ at pH 3.5. The better performance of Mn and Co at higher pHs can be attributed to their greater solubility than Cu at these pH values, as Cu2+ precipitates above pH 5.5. The superior performance of Cu2+ at low pH is most likely due to the stronger oxidation potential of Cu2+ compared to Mn2+ and Co2+.14 Another effect solution pH could have had on the activity of the transition metals tested involves its effect on the oxidation states of the metal species. Of the transition metals used in this study, only the oxidation state of Fe is significantly affected by solution pH in the pH range used. In a system containing Fe, Fe2+ is predominantly present at low pH and Fe3+ at high pH.
Figure 3. Effect of increasing Cu(NO3)2 loading. Conditions: pHi 3.5, 3 h, 0.5 MPa O2 pressure, 200 °C, 800 rpm.
This could partly explain why Fe(NO3)3, although promising, as it performed better than Cu2+ at pH 6 and 13.6, did not compare favorably against Cu2+ at pH 3.5. This accounts for the different trend observed for the CWO of SSW using the Fe catalyst compared to that of the Cu-, Co-, and Mn-based catalysts. The better results at low pH might also be due to the catalysts studied being present predominantly as soluble or insoluble hydrolyzed species at higher pH values. This would make it more difficult for the metal ions to form complexes with the organic species available. Hence, if a complexation-based reaction mechanism were operating, then it would be hindered at higher pH, as complex formation would most likely involve displacement of hydroxide.15 The high activity of Cu2+ could also be due to Cu being capable of promoting the oxidation of organics through homolytic bond cleavage, which involves the promotion of free-radical reactions. This occurs via a catalytic cycle involving the reduction-oxidation homolytic reactions of hydroperoxides, which are formed in the chemical reaction step of WO. This is shown in eqs 3.1 and 3.2 below.
ROOH + Cu+ f RO• + Cu2+ + OH-
reduction (3.1)
ROOH + Cu2+ f ROO• + Cu+ + H+
oxidation (3.2)
Cu+ is easily oxidized back to Cu2+ by oxygen as well. The alkyl peroxy radicals (ROO•) produced in the oxidation reaction then react with the starting organic compound(s) to produce more hydroperoxides.
ROO• + RH f ROOH + R•
(3.3)
The superior performance of Cu2+ at pH 3.5 compared to the other transition metals tested is thus most likely due to a combination of the chemical (oxidation strength, ability to form coordination complexes, ability to decompose hydroperoxy intermediates, ease of regeneration of Cu2+ by oxygen) and physical properties (solubility of Cu2+ at low pH) of this catalyst. Catalyst Loading Experiments. Having determined the most effective catalyst, it was necessary to investigate the optimal load at which the Cu(NO3)2 should be used. A series of experiments was carried out to study the effect of Cu2+ concentration on the CWO of SSW. The percent TOC reductions that were obtained from these experiments are presented in Figure 3. As can be seen, there was a significant (16%) increase in
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Table 2. Bimetallic Catalyst Screening Resultsa,b no.
catalyst combination
TOC reduction (%)
1 2 3 4 5 6 7 8
Cu/Fe Cu/Mn Cu/Co Co/Bi Pd/Mn Cu/Ce Cu/Pd Mn/Ce
66.4 57.1 57.9 52.3 57.8 61.9 72.6 53.6
a Metal concentration ) 20 mmol (10 mmol each). b Conditions: pHi 3.5, 3 h, 0.5 MPa O2 pressure, 200 °C, 800 rpm.
performance with even small amounts (5 mmol) of catalyst as compared to the uncatalyzed run. Also, the performance steadily increased to a limiting value of about 75% TOC reduction that was reached at a catalyst load of 40 mmol. This level of efficiency remained steady with increasing concentrations of Cu(NO3)2 even up to concentrations of 120 mmol, which is still well below the solubility limit of Cu(NO3)2 at pH 3.5. Catalyst performance increases with metal load because, initially, with increases in metal concentration, the amount of soluble metal in solution increases, thus promoting the rate of the homogeneous catalytic reaction. Usually though, performance keeps increasing until the solubility limit of the catalyst is reached, at which point it levels off as there can be no further increase in soluble metal concentration. This is different from our observation though, because the leveling-off effect was found to occur even as soluble metal concentrations were increasing, i.e., even at catalyst concentrations far below the solubility limit. This was attributed to the reaction being rate limited by the amount of organics susceptible to Cu-catalyzed oxidation. Thus, once a critical concentration of Cu is reached, the reaction is going “as fast as it can” such that any further increase in soluble metal availability does not help. The optimal catalyst quantity was chosen as 20 mmol, at which about 65% TOC removal was obtained. Although this is below the 75% achieved at 40 mmol, 20 mmol was chosen as the optimal catalyst concentration because the costs associated with purchasing and recovering twice the amount of Cu in exchange for an additional 10% TOC reduction would most likely not be cost-effective from an industrial point of view. Bimetallic Catalyst Systems. Tests on bimetallic catalysts were carried out with various metal combinations, and the results are shown in Table 2. Only two of the bimetallic systems studied, Cu/Fe and Cu/Pd, were capable of achieving a higher percent TOC reduction than the best monometallic catalyst, Cu2+. The extra TOC reduction achieved however, was minor (1.8% for Cu/Fe and 8.1% for Cu/Pd). However, as Cu/Fe and Cu/Pd did yield promising results the effect of varying the metal combinations was investigated to determine whether this had an effect on TOC destruction. The results of these tests are illustrated in Figures 4 and 5, and they show that, for both systems, the 1:1 ratio gave the best results (approximately 66% for Cu/Fe and 73% for Cu/Pd). The purpose of exploring homogeneous bimetallic systems was two-fold. If metals were able to catalyze the oxidation of differing groups of organics with varying efficiencies, then the combined use of two metals should have a positive synergistic effect on TOC destruction. The second possible benefit is that the presence of one metal could influence the chemical redox properties of
Figure 4. Effect of Cu/Fe bimetallic catalytic system. Conditions: pHi 3.5, 3 h, 0.5 MPa O2 pressure, 200 °C, 800 rpm, total metal concentration ) 20 mmol.
Figure 5. Effect of Cu/Pd bimetallic catalytic system. Conditions: pHi 3.5, 3 h, 0.5 MPa O2 pressure, 200 °C, 800 rpm, total metal concentration ) 20 mmol.
the other, such as in Wacker (Cu/Pd) catalyst systems, so that a more potent oxidizing system is developed. Because the results obtained with the Cu/Fe were not appreciably different from the performance of monometallic Cu, it can be concluded that Fe targets organic compounds that are very similar to those targeted by Cu. By comparing the individual actions of Cu and Fe at pH 3.5, however, it is apparent that Cu catalyzes the WO of more of the organics in SSW than Fe under the reactions conditions used (pH 3.5, 200 °C, 0.5 MPa O2, 800 rpm, 3 h). The catalytic mechanism of the Cu/Fe system differs from that of the Cu/Pd system. The improvement of the 1:1 Pd/Cu system over monometallic Cu (73% compared to 65%) suggests that in addition to Pd and Cu possibly targeting different groups of organic compounds, they could be influencing each other synergistically so that the combination is a beneficial one. However, from an industrial viewpoint, the use of Pd as a catalyst is unlikely to prove cost-effective. Although there is limited literature on the treatment of similar industrial wastewaters by catalytic wet oxidation, the results that have been obtained thus far with monometallic and bimetallic catalysts are comparable to results obtained in this study in terms of the superior activity of copper-based catalysts compared to other transition-metal-based catalysts. Copper-based catalysts have also been shown to be more effective at catalyzing TOC removal from Bayer liquor compared to other transition metal catalysts. Yamada et al.16 reported CuSO4 to be capable of >20% more TOC removal than the sulfate salts of Zn, Mn, Ni, Fe and Co under the conditions used in their study. Chang et al.17 also reported CuSO4 to be a better catalyst (∼10% or more COD removal) then MnO2 and Co2O3 in their studies on phenol and p-chlorophenol synthesized wastewaters.
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Figure 6. Effect of pH on removal of Cu2+ by activated carbon. Adsorption conditions: 2 h, room temperature, 25 g of carbon/L of solution. Inset: Precipitation and filtration carried out at pH 6.
A study on the CWO of industrial pulp and paper waste by Akolekar et al.18 has also found that TOC was more effectively removed (by about 7%) by a Cu/Pd catalyst than by a Mn/Pd one. Further Treatment with Activated Carbon. Treated SSW, despite having a significantly reduced TOC, continued to pose environmental challenges because it had a Cu(II) loading of about 2 g/L, which is much higher than the legally allowable limit. It also still had a very strong odor and a residual COD of 6 g/L. Treatment with activated carbon was investigated to test its ability to improve the above situations. The CWO-treated SSW was further treated with activated carbon (RB4, steam activated supplied by Norit) for 2 h between pH 3 and pH 5, with the results for copper adsorption shown in Figure 6. As can be seen, there is a definite trend within this pH range with Cu adsorption increasing linearly with increases in pH. This is believed to be due to the chemisorption of Cu being dependent on the availability of active sites on the activated carbon surface. At lower pHs, the increased concentration of H+ ions increases competition for these sites, resulting in less Cu(II) being effectively removed. Because the maximum removal achievable was only about 64%, which falls short of environmental regulations, precipitation of the copper prior to treatment with activated carbon was attempted. The results of this study are shown as an inset in Figure 6. At pH 6, the copper precipitates as Cu(OH)2, which can be filtered off, allowing 94% of the Cu(II) to be removed. Further treatment of the filtrate with activated carbon improves the Cu(II) removal to 99.8%. In addition to copper removal, a major benefit that was observed after treatment with activated carbon was the removal of odor from all samples. When the TOC results after treatment with activated carbon were analyzed (presented in Figure 7), it was found that, corresponding with the loss of odor, significant organic adsorption had occurred with the adsorption, boosting the overall percent TOC reduction to 83.7%, an increase of ∼18% over CWO, when treated at pH 4.6. The COD of the solution was also reduced to 3.3 g/L. 4. Conclusions This work investigated several different catalysts of which Cu2+ at a loading of 33.3 mmol/L, (20 mmol per
Figure 7. Contribution of treatment with activated carbon to Total TOC Reduction. CWO conditions: pHi 3.5, 3 h, 0.5 MPa O2 pressure, 200 °C, 800 rpm, 20 mmol Cu2+. Adsorption conditions: 2 h, room temperature, 25 g of carbon/L of solution.
600 mL) was found to be the most effective monometallic catalyst, giving 65% TOC removal at 200 °C at a starting pH of 3.5. Among bimetallic catalysts, the Cu/ Pd combination was also found to be effective, possibly working on separate organic groups to achieve ∼73% TOC removal. However, this combination cannot be used on a large scale because of cost considerations. The environmental acceptability of CWO-treated solutions was improved by further treatment with activated carbon, which was able to reduce the Cu(II) concentration by up to 64% using adsorption alone and by up to 99.8% when used in tandem with precipitation/ filtration. Odor abatement was also accomplished with a corresponding improvement in TOC reductions to 83.7%. The spent activated carbon generated, however, also might require treatment (cementation, solidification) before it can be disposed. Although these improvements make the CWO-treated solution less environmentally damaging, there is a need to study how the current CWO/adsorption process can be optimized to enable the treated effluent to meet environmental discharge guidelines, which typically allow only 100 mg/L COD. Also, there is a need to assess the costs associated with adsorption using activated carbon and those of filtration of the precipitated Cu(OH)2 to determine the industrial viability of such methods. This will determine whether these combined methods can yield an industrially and environmentally viable, cost-effective method for treating SSW.
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Received for review March 1, 2004 Resubmitted for review June 1, 2004 Accepted July 1, 2004 IE0400705
