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Cavitational decontamination of unsymmetrical dimethylhydrazine waste water
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Cavitational decontamination of unsymmetrical dimethylhydrazine waste water Mahmood Torabi Angaji∗, Reza Ghiaee School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 21 August 2014 Revised 27 October 2014 Accepted 14 November 2014 Available online xxx Keywords: Unsymmetrical dimethylhydrazine Nitrosodimethylamine Cavitation reactor Advanced oxidation
a b s t r a c t An advanced oxidation process based on using a combination of hydrodynamic cavitation, and acoustic cavitation has been investigated for decontamination of unsymmetrical dimethylhydrazine (UDMH) waste water. The effect of various operating parameters such as velocity, inlet pressure, temperature, and pH on the extent of UDMH conversion has been studied with the aim of maximizing the extent of decontamination. It has been observed that an optimum pressure and operating temperature in a moderately acidic medium are more favorable for a rapid removal of UDMH. No toxic byproduct was observed in the decontaminated samples, although the oxidation process eliminated up to 97% of UDMH. A synergistic effect has been obtained by a hybrid method of acoustic and hydrodynamic cavitation. The total and per pass, synergistic effect achieves 16% and 68%, respectively. Due to a high optimization potential, this hybrid method can be visualized as a new step in wastewater treatment facilities involving UDMH or other refractory toxic materials. © 2015 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.
1. Introduction Unsymmetrical dimethylhydrazine (UDMH) is primarily used as a high-energy propellant. It is also used in manufacturing a plant growth regulator, chemical synthesis, photographic chemicals, as a stabilizer for fuel additives and as an absorbent for acid gases. UDMH is known as a primary eco-toxicant with the maximum permissible concentration in ambient water as low as 0.02 mg/l [1]. Once occurred in the soil or water under natural conditions, UDMH is spontaneously oxidized to form nitrosodimethylamine (NDMA). NDMA is a probable human carcinogen with 10−6 lifetime cancer risk associated with a drinking water concentration of 0.7 ng/l determined by the U.S. Environmental Protection Agency (USEPA) [2]. Common water treatment methods, such as chlorination, oxidation with ozone, oxygen or hydrogen peroxide is often insufficient for the UDMH removal [3]. Furthermore, NDMA is an ineluctable byproduct of these methods [4]. Advanced oxidation processes (AOPs), such as photocatalysis, Fenton, photo-Fenton, and UV/H2 O2 have been investigated and reported by some researchers as efficient methods for degradation of UDMH, and minimizing the formation of NDMA [5–7]. However, the continuous need for an oxidizing agent (hydrogen peroxide, ozone . . . ) and complexity of the regeneration cycle of the catalyst make these methods inefficient for industrial applications. In this study we propose a method for resolving this insufficiency with the help of a cavitation reactor. ∗
Corresponding author. Tel.: +982166498982; fax: +982166987784. E-mail address: [email protected], [email protected] (M. Torabi Angaji).
A cavitation reactor can be used for in situ production of oxidant in an aqueous solution. In a cavitation reactor, a liquid undergoes a dynamic pressure reduction while under constant temperature. The pressure reduction causes gas bubbles to explosively develop, grow and then collapse. Cavitation decomposes water into extremely reactive hydrogen atoms and hydroxyl radicals, which recombine to form hydrogen peroxide and molecular hydrogen (Eqs. (1)–(3)) [8]. Hydroxyl radicals and hydrogen peroxide molecules are responsible for the oxidation of organic and inorganic compounds (Eqs. (5) and (9)). Therefore, it can be (theoretically) concluded that; cavitational AOT does not require any chemical agent as an oxidant. Cavitation
H2 O −−−−−→ H˙ + OH˙
(1)
2OH˙ −−−−−→ H2 O2
(2)
2H˙ −−−−−→ H2
(3)
2H2 O2 −−−−−→ 2H2 O + O2
(4)
˙ −−→ Oxidation products Organic/Inorganic molecule + OH
(5)
Cavitatin O2 −−−−−→ O˙ + O˙
(6)
OH˙ + H2 O2 −−−−−→ HOO˙ + H2 O
(7)
2HOO˙ −−−−−→ H2 O + O2
(8)
http://dx.doi.org/10.1016/j.jtice.2014.11.008 1876-1070/© 2015 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.
Please cite this article as: M. Torabi Angaji, R. Ghiaee, Cavitational decontamination of unsymmetrical dimethylhydrazine waste water, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2014.11.008
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˙ ˙ O) ˙ + Organic compoundes −−→ Degradation Radical (OH,HO O, (9) products There are four general methods of reducing dynamic pressure in a cavitation reactor, which differ in their manner of energy input (acoustic, hydrodynamic, particle induced and optical). Many researchers have studied cavitational advanced oxidation processes having the help of an acoustic cavitation (AC) reactor or a hydrodynamic cavitation (HC) reactor [9]. However, few studies on the combination of these two kinds of reactors in a hybrid system have been reported [10]. It is likely that; such a hybrid system will lead to a more intense production of cavitation bubbles (HC advantage), and more violent bubble collapsing (AC advantage) [11]. Another advantage is ease of scale up due to characteristic numbers, describing the hydraulic system [11,12]. Besides, the hybrid method works continuously, which is also a benefit for industrial applications. The aim of our study is to investigate the capabilities of such a hybrid method for decontamination of an aqueous solution of UDMH. The results are unique, because this is, to the best of our knowledge, the first time that a hybrid cavitation reactor is used to treat UDMH contaminated wastewater. 2. Material and methods 2.1. Chemicals Unsymmetrical dimethylhydrazine (C2 H8 N2 , CAS-No.: 57-14-7, 99%), formaldehyde (CAS-No.: 82115-62-6, 2%), sulfuric acid (97% Merk), hydrochloric acid (37% Merk), disodium salt of the chromotropic acid dihydrate (Aldrich), and distilled water were used in the study.
Fig. 2. Venturi tube dimensions. Table 1 Reactor different modes and respective positions of flow control valves.
1 2 3
Working mode of the reactor
Valves positions Open
Closed
Partially open
AC HC AHC
V1 , V3 , V5 , V6 V1 , V4 , V7 V1 , V4 , V5 , V6
V4 , V7 V3 , V5 , V6 V3 , V7
V2 V2 V2
(upstream diameter 40 mm, throat diameter 10 mm) as shown in Fig. 2. By-pass 1 is provided to control the flow rate of the liquid through the main line. By-pass 2 is used when the ultrasonic reactor must be investigated individually. The main line itself is branched into two lines; one is by-pass 3, which makes it possible to work on AHC, AC or HC modes, and the other one ends to the acoustic section of the reactor. The acoustic section is equipped with a double-walled glass ultrasonic reactor of 1 liter volume, an ultrasonic transducer (MEINHARDT Ultrasonic Technology, E/805/T02) and a power generator (850 kHz, 120 W electrical power, 40 W acoustic power). Piezoelectric pressure gauges are provided to measure the pump outlet pressure (P1 ), inducer inlet pressure (P2 ) and the fully recovered downstream pressure (P3 ). To measure the flow velocity in the system, an electromagnetic flow meter (Iran Madar Co., MagAB, 20–400 lit/min, Pmax = 200 bar) is used.
2.2. Experimental setup 2.3. Experimental procedure A schematic representation of the experimental setup is shown in Fig. 1. It is essentially a closed-loop acoustic-hydrodynamic-cavitation (AHC) reactor that operates in re-circulation mode. The reactor comprises a holding tank of 200 liter volume, a centrifugal pump (MOTTAHED Industrial and Manufacturing Co., MCP 50–170, 3.0 kW, 2850 rpm), (V1 –V7 ) flow control valves and pipes. The used pipes (stainless steel, 316/316L) have an inner diameter of 38 mm and an external diameter of 40 mm. The tank is provided with a jacket, to adjust and control the temperature (T1 ). The suction side of the centrifugal pump is connected to the bottom of the holding tank and the discharge line of the pump branches into three lines. The main line includes a hydrodynamic cavitation inducer, which is a nozzle-type venturi tube
UDMH solution (150 liter) of 0.015 M was prepared. The initial pH of the solution was adjusted within the range of 2–7.4 (see Section 3.6) by using hydrochloric acid 1 M. The holding tank was filled with the solution. The pump could be switched on after holding tank, and ultrasonic reactor was closed. Depending on the chosen mode of the experiment, the hydrodynamic/ultrasound section of the reactor could be in or out of service. Table 1 represents all selectable working modes of the reactor and the positions of flow control valves in each mode. If AHC mode was chosen, the ultrasound generator should be set on the maximum power input (120 W electrical power, 40 W acoustic power and 2 W/cm2 acoustic power density). The solution temperature was kept constant by the cooling system during the experiments. Samples were taken and analyzed for UDMH and NDMA content at defined time intervals. As the main objective of the experiment was complete oxidation of UDMH, the recirculation time of the solution (number of passes) was optimized to reach the maximum conversion of UDMH. All experiments were carried out in duplicate to estimate the repeatability (test–retest reliability) of the obtained data, which were analyzed by statistics tools provided by Excel, MS Office 2013. The graphs were plotted using mean values obtained from the data. 2.4. Analysis
Fig. 1. Schematic diagram of the experimental setup: (1) holding tank; (2) cooling system; (3) pump; (4) hydrodynamic cavitation inducer (venturi tube); (5) ultrasonic reactor; (6) ultrasonic transducer; (7) flow meter; (8) data acquisition system; T1 –T2 : thermocouples; P1 –P3 : piezoelectric pressure gauges; V1 –V7 : valves.
Concentrations of UDMH and NDMA in aqueous solutions were determined using a spectrophotometer (Perkin-Elmer, Lambda 35). The UDMH concentration was determined using the absorption band of the formaldehyde hydrazone (ε = 5000 M−1 cm−1 at λ = 235 nm), the product of the reaction between UDMH and formaldehyde [6]. The NDMA concentration was determined using the absorption band of this substance in the UV range (ε = 90 M−1 cm−1 at λ = 330 nm).
Please cite this article as: M. Torabi Angaji, R. Ghiaee, Cavitational decontamination of unsymmetrical dimethylhydrazine waste water, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2014.11.008
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Fig. 4. Effect of inlet pressure (P2 ) on UDMH overall conversion.
velocity, the overall conversion of UDMH increases by trend within 60 min. Similar behavior has been reported by Hatanaka et al. [13] for an aqueous luminol solution. The mechanism of the enhancement is that fluid flow prevents bubbles from coalescing and clustering, which are responsible for the quenching of cavitation. For AC mode, the higher velocity of fluid, the more bubbles are available to collapse and the higher overall conversion of UDMH will be reached. As shown in Fig. 3b, the conversion of UDMH per pass decreases with an increasing fluid velocity. This result is not surprising, because the residence time per pass in the ultrasonic field is much lower with a high fluid velocity. Braeutigam et al. [14] have studied the effect of fluid velocity on hydrodynamic cavitation performance. They have reported that a minimum velocity must exist to create hydrodynamic cavitation. Instead of repeating their studies for the present case, we investigated the effect of inlet pressure on cavitation intensity and UDMH removal. The results are presented in the next section. Fig. 3. (a) UDMH overall conversion in dependence of the fluid velocity in AC system. (b) UDMH conversion per pass in dependence of the fluid velocity in AC system.
Formaldehyde concentration was determined using the absorption band of the species (ε = 12,600 M−1 cm−1 at λ = 570 nm) formed during the reaction between formaldehyde and chromotropic acid. An aliquot of the solution to be analyzed (0.05–1.0 ml) was brought to 1.0 ml with distilled water. Then 1.0 ml of 0.5% chromotropic acid solution and 8 ml of 81% sulfuric acid were added. The solution was heated at 60 °C for 20 min. After the solution cooled down to the ambient temperature, the absorbance at 570 nm was measured relative to the solution prepared in the same way with the distilled water used instead of the formaldehyde solution sample. Quantitative analysis of the solution was conducted with a chromato-mass-spectrometer (gas chromatograph CP-3800 with a column Paraplot-Q, D = 0.25 mm in combination with a quadruple ion trap mass-spectrometer (Varian, Saturn 2000)). The chromato-mass-spectrometry was conducted with the data acquisition time of 22.5 min, heating from 120 to 220 °C with the temperature gradient of 8 °C/min. An identification of the solution components was performed following the analysis of pure substances using the mass-spectra library. 3. Results and discussion 3.1. Influence of the fluid velocity The influence of the fluid velocity on the acoustic field of the reactor was investigated. As a result of choosing the AC mode, there was no hydrodynamic cavity. The flow in the ultrasonic section was regulated with the help of bypass lines (see Table 1). The results are shown in Fig. 3a and b. It is observed that with an increasing fluid
3.2. Effect of inlet pressure (P2 ) The influence of the inlet pressure on the hydrodynamic cavitation intensity and its effect on the conversion of UDMH was investigated. As a result of choosing the HC mode, there was no acoustic cavitation. The flow in the venturi tube (HC-inducer) was regulated with the help of by-pass lines (see Table 1). The investigated results are shown in Fig. 4. UDMH overall conversion increases with the inlet pressure at first, and decreases when P2 reaches 7 bar. This phenomenon is attributed as follows: when P2 is lower than 7 bar, the throat pressure will decrease with increasing of P2 , which resulted in the increase of a pressure gradient at the expansion section of the venturi and ˙ the concentration of hydroxyl free radical (OH). However, when P2 exceeds 7 bar, the throat pressure will increase proportionally to P2 , and this causes a decline in the overall conversion of UDMH. The increase in extent of UDMH degradation with increasing pressure can be attributed to the enhanced cavitation activity at higher pressures. Bubble dynamics studies [15] have indicated that the cavitational intensity generated by the collapse of the cavity increases with higher inlet pressure of the system up to a limited event. The increase in the cavitational collapse intensity generates higher temperatures and pressures resulting in enhanced dissociation of the water molecules trapped in the cavity thereby leading to a higher concentration of hydroxyl radicals. Also, higher cavitational activity is expected to have better activation effects on the advanced oxidation process in terms of greater bubble size reduction leading to a larger surface area for enhanced rates of reactions. The UDMH overall conversion reduction from 37 to 11 is corresponded to inlet pressure increment from 7 to 8 bar. This is attributed to the cavity cloud formation wherein larger bubbles escape the liquid without collapsing or result into an incomplete and/or cushioned
Please cite this article as: M. Torabi Angaji, R. Ghiaee, Cavitational decontamination of unsymmetrical dimethylhydrazine waste water, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2014.11.008
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Fig. 5. UDMH conversion per pass, AHC method.
collapse. The results on the effect of inlet pressure up to 7 bar are in accordance with many of the studies reported earlier, though conducted on different cavitation devices, materials and conditions. Saharan et al. [16] have investigated the use of hydrodynamic cavitation reactors for degradation of Reactive Red 120 dye solution (34 μM) using venturi as cavitating device and H2 O2 as an oxidant. They have reported that the rate of degradation increases with increased inlet pressure. However, there is no sign of any decline in the extent of dye degradation in their study. Pradhan and Gogate [17] have studied the removal of p-nitrophenol using venturi and orifice plates as cavitation device and Fenton chemistry. They have stated that there exists a critical operating pressure, which maximized the removal of p-nitrophenol. In Fig. 5, considering the UDMH conversion per pass, a similar result is shown for the hybrid system. An increasing inlet pressure, after reaching an optimum value, leads to a decrease in the conversion. This result can be explained as follows; the total fluid velocity in the system increases with higher P2 and as a consequence the residence time in the ultrasonic reactor decreases. Thus, the acoustic irradiation time per pass, and the conversion decreases, because the ultrasonic irradiation has the dominant influence on the conversion of UDMH. Due to high velocity, the number of passes per unit time increases and that is why the conversion per pass has an optimum in AHC mode as shown in Fig. 5. 3.3. Effect of operating temperature Operating temperature is another key parameter, which affects the intensity of cavitation as well as efficacy of UDMH oxidation reaction. Normally, cavitation intensity decreases with an increase in temperature due to the formation of vapor cavities that collapse less violently. However, the oxidation reaction, in terms of generation of hydroxyl radicals, is significantly enhanced at higher operating temperatures. Kavitha and Palanivelu [18] have reported that the maximum extent of degradation of cresols occurs at an operating temperature of 30 °C. Sun et al. [19] have reported that the rate constant for degradation of p-nitroaniline increased by almost 100% with a rise in temperature from 20 °C to 30 °C. The effect of operating temperature was investigated at six constant temperatures ranging from 15 to 40 °C and was performed with precise temperature control and the extent of variation in these sets was ±1 °C. The extent of UDMH conversion at the end of 60 min increases with temperature and reaches to maximum at 25 °C. The conversion decreases smoothly with increasing the temperature higher than 25 °C. The phenomena can be explained as follows: when the temperature is lower than 25 °C, the cavitation intensity and the concentration of hydroxyl free radical will increase with the increase of temperature and results in the increase of the oxidation rate of
Fig. 6. Effect of operating temperature on UDMH overall conversion.
Fig. 7. Effect of number of passes on UDMH overall conversion.
UDMH. Increase in the temperature more than 25 °C, increases vapor pressure and reduces the viscosity and surface tension. A rise in temperature reduces the gas solubility which is the chief source of cavity nuclei and reduces the rate of occurrence of cavitation events. Thus the overall number of the cavities formed decreases thereby reducing the efficiency of cavitation phenomena [20]. Fig. 6 shows the effect of operating temperature on the conversion of UDMH for AHC-reactor at its optimized inlet pressure while the ultrasonic transducer has been working with 40 W acoustic power. Due to these results we have set the temperature of the cooling system at 25 ± 1 °C which gives us the optimum operating temperature and the best efficacy of the UDMH removal in the AHC mode.
3.4. Effect of number of passes (time of operation) Fig. 7 shows the effect of the number of passes on the conversion of UDMH for AHC-reactor at its optimized inlet pressure and temperature. The number of passes (Np ), is expressed as:
Np =
volumetric flow rate × time of operation total volume of solution in holding tank
(10)
The extent of UDMH degradation increases with increasing number of passes. However, it was found that there is an optimum number of passes (56 passes or 40 min of circulation for this set of operational parameters) that would yield acceptable conversion. It is also to be noted that this factor is crucial in determining the cost of operation, fewer numbers of passes lower the cost.
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Fig. 8. Possible pathways of UDMH oxidation initiated by hydroxyl radicals. Table 2 Possible oxidation products of UDMH.
1 2 3 4 5 6 7 8 9 10
The substance name
The substance formula
Retention time in GC/MS (min)
Observed products
Methanol Dimethylamine Formaldehyde hydrazone Formic acid Nitromethane Acetic acid N,N-Dimethylhydroxylamine Formaldehyde dimethylhydrazone Nitrosodimethylamine Monomethylhydrazine
CH3 OH (CH3 )2 NH NH2 NCH2 HCOOH CH3 NO2 CH3 COOH (CH3 )2 NOH (CH3 )2 NNCH2 (CH3 )2 NNO CH3 NHNH2
4.6 6.8 7.7 8.5 9.4 11.0 11.1 11.6 14.8 15.1
Not observed Not observed Not observed + + + Not observed Not observed Not observed Not observed
Table 3 Data of product analysis dependent on pH0 .
3.5. Products of the UDMH oxidation The attack of hydroxyl radicals to the UDMH molecules in the presence of H+ (acidic medium) can be done through its methyl group or amino group according to the following reactions (Eqs. (11) and (12)).
˙ H+ (CH3 )2 NNH2 + OH˙ −→ H2 O + H2 C NNH2 H3 C
H+
˙ (CH3 )2 NNH2 + OH˙ −→ H2 O + (CH3 )2 NNH
(11)
(12)
As it is shown in Fig. 8, these two reactions can be continued in many ways. And variety of intermediate products may be listed for each pathway. However, instead of the detailed study of the intermediate products, we gave a careful attention to the formation of oxidation end-products of UDMH, since some of them such as NDMA, formaldehyde and methanol can be toxic. Table 2 represents the list of substances that can potentially appear as oxidation products of UDMH and their retention time in the chromato-mass spectrometer columns under the conditions
No.
1
2
3
4
5
6
7
pH0 [HCOOH] (mM) [CH3 NO2 ] (mM) [CH3 COOH] (mM) [NO3 − ] (mM)
1.0 14.5 0.2 1 2.4
2.0 2 0.2 1.5 1.9
5.0 3.5 0.1 0.1 1.9
7.4 0.44 0.2 1.1 1
8.4 11 0.3 1.5 1.8
9.3 7 0.2 0.2 1.6
9.7 3.4 0.2 0.2 1.5
applied. Nitromethane, formic and acetic acid were the only carboncontaining products, which observed within 40–60 min reaction time, at moderate acidic medium (pH ࣈ 3) and initial UDMH concentration of 0.015 M. 3.6. Effect of initial pH The influence of the initial pH value, pH0 , of the solution on the UDMH oxidation was studied for the pH0 range from 1.0 to 9.7 (Table 3). All experiments were carried out at a constant temperature
Please cite this article as: M. Torabi Angaji, R. Ghiaee, Cavitational decontamination of unsymmetrical dimethylhydrazine waste water, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2014.11.008
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M. Torabi Angaji, R. Ghiaee / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–6 Table 4 Comparison of UDMH conversion of individual methods and hybrid method. Working mode of the reactor
Conversion per pass (%) Conversion after 40 min (%)
HC
AC
AHC
0.63 37
0.84 47
1.73 97.8
(25 ± 1 °C) and AHC mode. The circulation time of solution in the reactor was 40 min. Hydrochloric acid 1 M solution was used to adjust the pH0 . The rate of UDMH oxidation reaction was low in a strong acidic medium. As a result, the formic acid accumulation was observed. The pH0 values from 2 to 7.4 appeared to be the optimum for the total UDMH oxidation since the reaction rate was relatively high as well as the UDMH conversion. At pH0 8.4, the rate of UDMH oxidation decreased. 3.7. Comparison of the individual methods and the hybrid method Investigations were performed with HC, AC or the AHC system. In the case of HC, ultrasound was not used and the solution flows through by-pass 3 line. For acoustic method by-pass 2 line was used. In the case of the hybrid method, both by-pass 2 and 3 lines were closed, and ultrasound generator was operated with its maximum power. In Table 4, the overall conversions after 40 min and the conversions per pass are listed. Best results obtained by combining the acoustic and hydrodynamic cavitation modes, which led to an overall UDMH conversion equal to 97.8% and a conversion per pass equal to 1.73%. The results achieved by ultrasound treatment were 47% and 0.84% respectively. Working in the hydrodynamic cavitation mode, the reactor reached to an overall UDMH conversion up to 37% and a conversion per pass equal to 0.63%. The synergistic effect can be calculated as:
synergistic effect =
X(HC+AC) × 100% X(HC) + X(AC)
(13)
where X(HC+AC) is the conversion with the hybrid method, X(HC) and X(AC) are the conversions with the individual methods. It was calculated that the synergistic effect is about 16% with regard to an experimental time of 40 min. For the synergistic effect per pass, a much better result of 68% was obtained. This means that the hybrid method is 68% better than the sum of the individual methods per pass. 4. Conclusions In this study, hydrodynamic and acoustic cavitation and a combination of both as a hybrid method of advanced oxidation have been investigated. The study was focused on the oxidation of aqueous solution of UDMH and was done by an experimental setup designed for this case. The setup made it possible to study the impact of any one of the variety of cavitation modes on the oxidation reaction of UDMH individually or at the same time. All experiments were performed with a model aqueous solution. The concentrations of UDMH and its possible oxidation products in the aqueous solutions were determined using spectrophotometry technique. Quantitative analyses of the solution were conducted with a chromato-mass-spectrometer. No NDMA or any other toxic by-product was observed in the investigated samples. UDMH was oxidized and removed from the solution after 40 min of the cavitational oxidation. The rates of UDMH oxidation were low in the strong acidic and basic medium. A range of pH values from 2 to 7.4 appeared to be optimum. The extent of the overall conversion of UDMH increased with an increasing fluid velocity when the acoustic irritation was the only source of cavitation. For the AHC and HC modes an optimum inlet pressure, which maximized the overall conversion
Synergistic effect (%)
68 16
of UDMH, was detected. An optimal operating temperature was found for the AHC mode. Comparing the AHC mode with the individual modes, a synergism was observed. The combination achieved the best conversion rates, besides synergies of 16% for a 40 min treatment and 68% in a single pass. Oxidation under optimum reaction conditions resulted in the UDMH mineralization to the extent of 97.8%, having formic acid, acetic acid, and nitromethane the only carbonaceous residues of the UDMH. We conclude that the hybrid method of combination of hydrodynamic and acoustic cavitation is a novel method for mineralization and decontamination of UDMH aqueous solution. Important advantages of this method is the high optimization potential of the process, and no necessity of any chemical agents, which make it possible to realize a mobile unit for treatment of UDMH at a distant location. So that it could exclude hazardous transportation to the centralized treatment facilities. References [1] Fedorov LA. Liquid missile propellants in the former Soviet Union. Environ Pollut 1999;105:157–61. [2] USEPA integrated risk information system home page. Office of Research and Development (ORD), National Center for Environmental Assessment. http://www.epa.gov/iris/subst/0045.htm [accessed 26.03.13]. [3] Mitch WA, Gerecke AC, Sedlak DL. A N-nitrosodimethylamine (NDMA) precursor analysis for chlorination of water and wastewater. Water Res 2003;37:3733–41. [4] Schreiber IM, Mitch WA. Influence of the order of reagent addition on NDMA formation during chloramination. Environ Sci Technol 2005;39:3811–18. [5] Gou XL, Lv XM, Cui H, Li X. Treatment of UDMH wastewater by microwave catalytic oxidation process. Appl Mech Mater 2013;295–298:1486–9. [6] Makhotkina OA, Kuznetsova EV, Preis SV. Catalytic detoxification of 1,1-dimethylhydrazine aqueous solutions in heterogeneous Fenton system. Appl Catal B-Environ 2006;68:85–91. [7] Pestunova OP, Elizarova GL, Ismagilov ZR, Kerzhentsev MA, Parmon VN. Detoxification of water containing 1,1-dimethylhydrazine by catalytic oxidation with dioxygen and hydrogen peroxide over Cu- and Fe-containing catalysts. Catal Today 2002;75:219–25. [8] Kumar PS, Kumar MS, Pandit AB. Experimental quantification of chemical effects of hydrodynamic cavitation. Chem Eng Sci 2000;55:1633–9. [9] Parsa JB, Zonouzian SAE. Optimization of a heterogeneous catalytic hydrodynamic cavitation reactor performance in decolorization of Rhodamine B: application of scrap iron sheets. Ultrason Sonochem 2013;20:1442–9. [10] Franke M, Braeutigam P, Wu ZL, Ren Y, Ondruschka B. Enhancement of chloroform degradation by the combination of hydrodynamic and acoustic cavitation. Ultrason Sonochem 2011;18:888–94. [11] Mitch WA, Sharp JO, Valentine RL, Alvarez-Cohen L, Sedlak DL. Nnitrosodimethylamine (NDMA) as a drinking water contaminant: a review. Environ Eng Sci 2003;20:389. [12] Schreiber MI, Mitch WA. Influence of the order of reagent addition on NDMA formation during chloramination. Environ Sci Technol 2005;39:389. [13] Hatanaka SI, Mitome H, Yasui K, Hayashi S. Multibubble sonoluminescence enhancement by fluid flow. Ultrasonics 2006;44:e435–e8. [14] Braeutigam P, Franke M, Wu ZL, Ondruschka B. Role of different parameters towards the optimization of hydrodynamic cavitation. Chem Eng Technol 2010;33:932–40. [15] Gogate PR, Pandit AB. Engineering design methods for cavitational reactors. II. Hydrodynamic cavitation. AIChE J 2000;46:1641–9. [16] Saharan VK, Badve MP, Pandit AB. Degradation of Reactive Red 120 dye using hydrodynamic cavitation. Chem Eng J 2011;178:100–7. [17] Pradhan AA, Gogate PR. Removal of p-nitrophenol using hydrodynamic cavitation and Fenton chemistry at pilot scale operation. Chem Eng J 2010;156:77–82. [18] Kavitha V, Palanivelu K. Destruction of cresols by Fenton oxidation process. Water Res 2005;39:3062–72. [19] Sun JH, Sun SP, Fan MH, Guo HQ, Qiao LP, Sun RX. A kinetic study on the degradation of p-nitroaniline by Fenton oxidation process. J Hazard Mater 2007;148:172–7. [20] Gogate PR, Pandit AB. Hydrodynamic cavitation reactors: a state of the art review. Rev Chem Eng 2001;17:1.
Please cite this article as: M. Torabi Angaji, R. Ghiaee, Cavitational decontamination of unsymmetrical dimethylhydrazine waste water, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2014.11.008
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Cavitational decontamination of unsymmetrical dimethylhydrazine waste water Mahmood Torabi Angaji∗, Reza Ghiaee School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 21 August 2014 Revised 27 October 2014 Accepted 14 November 2014 Available online xxx Keywords: Unsymmetrical dimethylhydrazine Nitrosodimethylamine Cavitation reactor Advanced oxidation
a b s t r a c t An advanced oxidation process based on using a combination of hydrodynamic cavitation, and acoustic cavitation has been investigated for decontamination of unsymmetrical dimethylhydrazine (UDMH) waste water. The effect of various operating parameters such as velocity, inlet pressure, temperature, and pH on the extent of UDMH conversion has been studied with the aim of maximizing the extent of decontamination. It has been observed that an optimum pressure and operating temperature in a moderately acidic medium are more favorable for a rapid removal of UDMH. No toxic byproduct was observed in the decontaminated samples, although the oxidation process eliminated up to 97% of UDMH. A synergistic effect has been obtained by a hybrid method of acoustic and hydrodynamic cavitation. The total and per pass, synergistic effect achieves 16% and 68%, respectively. Due to a high optimization potential, this hybrid method can be visualized as a new step in wastewater treatment facilities involving UDMH or other refractory toxic materials. © 2015 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.
1. Introduction Unsymmetrical dimethylhydrazine (UDMH) is primarily used as a high-energy propellant. It is also used in manufacturing a plant growth regulator, chemical synthesis, photographic chemicals, as a stabilizer for fuel additives and as an absorbent for acid gases. UDMH is known as a primary eco-toxicant with the maximum permissible concentration in ambient water as low as 0.02 mg/l [1]. Once occurred in the soil or water under natural conditions, UDMH is spontaneously oxidized to form nitrosodimethylamine (NDMA). NDMA is a probable human carcinogen with 10−6 lifetime cancer risk associated with a drinking water concentration of 0.7 ng/l determined by the U.S. Environmental Protection Agency (USEPA) [2]. Common water treatment methods, such as chlorination, oxidation with ozone, oxygen or hydrogen peroxide is often insufficient for the UDMH removal [3]. Furthermore, NDMA is an ineluctable byproduct of these methods [4]. Advanced oxidation processes (AOPs), such as photocatalysis, Fenton, photo-Fenton, and UV/H2 O2 have been investigated and reported by some researchers as efficient methods for degradation of UDMH, and minimizing the formation of NDMA [5–7]. However, the continuous need for an oxidizing agent (hydrogen peroxide, ozone . . . ) and complexity of the regeneration cycle of the catalyst make these methods inefficient for industrial applications. In this study we propose a method for resolving this insufficiency with the help of a cavitation reactor. ∗
Corresponding author. Tel.: +982166498982; fax: +982166987784. E-mail address: [email protected], [email protected] (M. Torabi Angaji).
A cavitation reactor can be used for in situ production of oxidant in an aqueous solution. In a cavitation reactor, a liquid undergoes a dynamic pressure reduction while under constant temperature. The pressure reduction causes gas bubbles to explosively develop, grow and then collapse. Cavitation decomposes water into extremely reactive hydrogen atoms and hydroxyl radicals, which recombine to form hydrogen peroxide and molecular hydrogen (Eqs. (1)–(3)) [8]. Hydroxyl radicals and hydrogen peroxide molecules are responsible for the oxidation of organic and inorganic compounds (Eqs. (5) and (9)). Therefore, it can be (theoretically) concluded that; cavitational AOT does not require any chemical agent as an oxidant. Cavitation
H2 O −−−−−→ H˙ + OH˙
(1)
2OH˙ −−−−−→ H2 O2
(2)
2H˙ −−−−−→ H2
(3)
2H2 O2 −−−−−→ 2H2 O + O2
(4)
˙ −−→ Oxidation products Organic/Inorganic molecule + OH
(5)
Cavitatin O2 −−−−−→ O˙ + O˙
(6)
OH˙ + H2 O2 −−−−−→ HOO˙ + H2 O
(7)
2HOO˙ −−−−−→ H2 O + O2
(8)
http://dx.doi.org/10.1016/j.jtice.2014.11.008 1876-1070/© 2015 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.
Please cite this article as: M. Torabi Angaji, R. Ghiaee, Cavitational decontamination of unsymmetrical dimethylhydrazine waste water, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2014.11.008
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˙ ˙ O) ˙ + Organic compoundes −−→ Degradation Radical (OH,HO O, (9) products There are four general methods of reducing dynamic pressure in a cavitation reactor, which differ in their manner of energy input (acoustic, hydrodynamic, particle induced and optical). Many researchers have studied cavitational advanced oxidation processes having the help of an acoustic cavitation (AC) reactor or a hydrodynamic cavitation (HC) reactor [9]. However, few studies on the combination of these two kinds of reactors in a hybrid system have been reported [10]. It is likely that; such a hybrid system will lead to a more intense production of cavitation bubbles (HC advantage), and more violent bubble collapsing (AC advantage) [11]. Another advantage is ease of scale up due to characteristic numbers, describing the hydraulic system [11,12]. Besides, the hybrid method works continuously, which is also a benefit for industrial applications. The aim of our study is to investigate the capabilities of such a hybrid method for decontamination of an aqueous solution of UDMH. The results are unique, because this is, to the best of our knowledge, the first time that a hybrid cavitation reactor is used to treat UDMH contaminated wastewater. 2. Material and methods 2.1. Chemicals Unsymmetrical dimethylhydrazine (C2 H8 N2 , CAS-No.: 57-14-7, 99%), formaldehyde (CAS-No.: 82115-62-6, 2%), sulfuric acid (97% Merk), hydrochloric acid (37% Merk), disodium salt of the chromotropic acid dihydrate (Aldrich), and distilled water were used in the study.
Fig. 2. Venturi tube dimensions. Table 1 Reactor different modes and respective positions of flow control valves.
1 2 3
Working mode of the reactor
Valves positions Open
Closed
Partially open
AC HC AHC
V1 , V3 , V5 , V6 V1 , V4 , V7 V1 , V4 , V5 , V6
V4 , V7 V3 , V5 , V6 V3 , V7
V2 V2 V2
(upstream diameter 40 mm, throat diameter 10 mm) as shown in Fig. 2. By-pass 1 is provided to control the flow rate of the liquid through the main line. By-pass 2 is used when the ultrasonic reactor must be investigated individually. The main line itself is branched into two lines; one is by-pass 3, which makes it possible to work on AHC, AC or HC modes, and the other one ends to the acoustic section of the reactor. The acoustic section is equipped with a double-walled glass ultrasonic reactor of 1 liter volume, an ultrasonic transducer (MEINHARDT Ultrasonic Technology, E/805/T02) and a power generator (850 kHz, 120 W electrical power, 40 W acoustic power). Piezoelectric pressure gauges are provided to measure the pump outlet pressure (P1 ), inducer inlet pressure (P2 ) and the fully recovered downstream pressure (P3 ). To measure the flow velocity in the system, an electromagnetic flow meter (Iran Madar Co., MagAB, 20–400 lit/min, Pmax = 200 bar) is used.
2.2. Experimental setup 2.3. Experimental procedure A schematic representation of the experimental setup is shown in Fig. 1. It is essentially a closed-loop acoustic-hydrodynamic-cavitation (AHC) reactor that operates in re-circulation mode. The reactor comprises a holding tank of 200 liter volume, a centrifugal pump (MOTTAHED Industrial and Manufacturing Co., MCP 50–170, 3.0 kW, 2850 rpm), (V1 –V7 ) flow control valves and pipes. The used pipes (stainless steel, 316/316L) have an inner diameter of 38 mm and an external diameter of 40 mm. The tank is provided with a jacket, to adjust and control the temperature (T1 ). The suction side of the centrifugal pump is connected to the bottom of the holding tank and the discharge line of the pump branches into three lines. The main line includes a hydrodynamic cavitation inducer, which is a nozzle-type venturi tube
UDMH solution (150 liter) of 0.015 M was prepared. The initial pH of the solution was adjusted within the range of 2–7.4 (see Section 3.6) by using hydrochloric acid 1 M. The holding tank was filled with the solution. The pump could be switched on after holding tank, and ultrasonic reactor was closed. Depending on the chosen mode of the experiment, the hydrodynamic/ultrasound section of the reactor could be in or out of service. Table 1 represents all selectable working modes of the reactor and the positions of flow control valves in each mode. If AHC mode was chosen, the ultrasound generator should be set on the maximum power input (120 W electrical power, 40 W acoustic power and 2 W/cm2 acoustic power density). The solution temperature was kept constant by the cooling system during the experiments. Samples were taken and analyzed for UDMH and NDMA content at defined time intervals. As the main objective of the experiment was complete oxidation of UDMH, the recirculation time of the solution (number of passes) was optimized to reach the maximum conversion of UDMH. All experiments were carried out in duplicate to estimate the repeatability (test–retest reliability) of the obtained data, which were analyzed by statistics tools provided by Excel, MS Office 2013. The graphs were plotted using mean values obtained from the data. 2.4. Analysis
Fig. 1. Schematic diagram of the experimental setup: (1) holding tank; (2) cooling system; (3) pump; (4) hydrodynamic cavitation inducer (venturi tube); (5) ultrasonic reactor; (6) ultrasonic transducer; (7) flow meter; (8) data acquisition system; T1 –T2 : thermocouples; P1 –P3 : piezoelectric pressure gauges; V1 –V7 : valves.
Concentrations of UDMH and NDMA in aqueous solutions were determined using a spectrophotometer (Perkin-Elmer, Lambda 35). The UDMH concentration was determined using the absorption band of the formaldehyde hydrazone (ε = 5000 M−1 cm−1 at λ = 235 nm), the product of the reaction between UDMH and formaldehyde [6]. The NDMA concentration was determined using the absorption band of this substance in the UV range (ε = 90 M−1 cm−1 at λ = 330 nm).
Please cite this article as: M. Torabi Angaji, R. Ghiaee, Cavitational decontamination of unsymmetrical dimethylhydrazine waste water, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2014.11.008
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Fig. 4. Effect of inlet pressure (P2 ) on UDMH overall conversion.
velocity, the overall conversion of UDMH increases by trend within 60 min. Similar behavior has been reported by Hatanaka et al. [13] for an aqueous luminol solution. The mechanism of the enhancement is that fluid flow prevents bubbles from coalescing and clustering, which are responsible for the quenching of cavitation. For AC mode, the higher velocity of fluid, the more bubbles are available to collapse and the higher overall conversion of UDMH will be reached. As shown in Fig. 3b, the conversion of UDMH per pass decreases with an increasing fluid velocity. This result is not surprising, because the residence time per pass in the ultrasonic field is much lower with a high fluid velocity. Braeutigam et al. [14] have studied the effect of fluid velocity on hydrodynamic cavitation performance. They have reported that a minimum velocity must exist to create hydrodynamic cavitation. Instead of repeating their studies for the present case, we investigated the effect of inlet pressure on cavitation intensity and UDMH removal. The results are presented in the next section. Fig. 3. (a) UDMH overall conversion in dependence of the fluid velocity in AC system. (b) UDMH conversion per pass in dependence of the fluid velocity in AC system.
Formaldehyde concentration was determined using the absorption band of the species (ε = 12,600 M−1 cm−1 at λ = 570 nm) formed during the reaction between formaldehyde and chromotropic acid. An aliquot of the solution to be analyzed (0.05–1.0 ml) was brought to 1.0 ml with distilled water. Then 1.0 ml of 0.5% chromotropic acid solution and 8 ml of 81% sulfuric acid were added. The solution was heated at 60 °C for 20 min. After the solution cooled down to the ambient temperature, the absorbance at 570 nm was measured relative to the solution prepared in the same way with the distilled water used instead of the formaldehyde solution sample. Quantitative analysis of the solution was conducted with a chromato-mass-spectrometer (gas chromatograph CP-3800 with a column Paraplot-Q, D = 0.25 mm in combination with a quadruple ion trap mass-spectrometer (Varian, Saturn 2000)). The chromato-mass-spectrometry was conducted with the data acquisition time of 22.5 min, heating from 120 to 220 °C with the temperature gradient of 8 °C/min. An identification of the solution components was performed following the analysis of pure substances using the mass-spectra library. 3. Results and discussion 3.1. Influence of the fluid velocity The influence of the fluid velocity on the acoustic field of the reactor was investigated. As a result of choosing the AC mode, there was no hydrodynamic cavity. The flow in the ultrasonic section was regulated with the help of bypass lines (see Table 1). The results are shown in Fig. 3a and b. It is observed that with an increasing fluid
3.2. Effect of inlet pressure (P2 ) The influence of the inlet pressure on the hydrodynamic cavitation intensity and its effect on the conversion of UDMH was investigated. As a result of choosing the HC mode, there was no acoustic cavitation. The flow in the venturi tube (HC-inducer) was regulated with the help of by-pass lines (see Table 1). The investigated results are shown in Fig. 4. UDMH overall conversion increases with the inlet pressure at first, and decreases when P2 reaches 7 bar. This phenomenon is attributed as follows: when P2 is lower than 7 bar, the throat pressure will decrease with increasing of P2 , which resulted in the increase of a pressure gradient at the expansion section of the venturi and ˙ the concentration of hydroxyl free radical (OH). However, when P2 exceeds 7 bar, the throat pressure will increase proportionally to P2 , and this causes a decline in the overall conversion of UDMH. The increase in extent of UDMH degradation with increasing pressure can be attributed to the enhanced cavitation activity at higher pressures. Bubble dynamics studies [15] have indicated that the cavitational intensity generated by the collapse of the cavity increases with higher inlet pressure of the system up to a limited event. The increase in the cavitational collapse intensity generates higher temperatures and pressures resulting in enhanced dissociation of the water molecules trapped in the cavity thereby leading to a higher concentration of hydroxyl radicals. Also, higher cavitational activity is expected to have better activation effects on the advanced oxidation process in terms of greater bubble size reduction leading to a larger surface area for enhanced rates of reactions. The UDMH overall conversion reduction from 37 to 11 is corresponded to inlet pressure increment from 7 to 8 bar. This is attributed to the cavity cloud formation wherein larger bubbles escape the liquid without collapsing or result into an incomplete and/or cushioned
Please cite this article as: M. Torabi Angaji, R. Ghiaee, Cavitational decontamination of unsymmetrical dimethylhydrazine waste water, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2014.11.008
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Fig. 5. UDMH conversion per pass, AHC method.
collapse. The results on the effect of inlet pressure up to 7 bar are in accordance with many of the studies reported earlier, though conducted on different cavitation devices, materials and conditions. Saharan et al. [16] have investigated the use of hydrodynamic cavitation reactors for degradation of Reactive Red 120 dye solution (34 μM) using venturi as cavitating device and H2 O2 as an oxidant. They have reported that the rate of degradation increases with increased inlet pressure. However, there is no sign of any decline in the extent of dye degradation in their study. Pradhan and Gogate [17] have studied the removal of p-nitrophenol using venturi and orifice plates as cavitation device and Fenton chemistry. They have stated that there exists a critical operating pressure, which maximized the removal of p-nitrophenol. In Fig. 5, considering the UDMH conversion per pass, a similar result is shown for the hybrid system. An increasing inlet pressure, after reaching an optimum value, leads to a decrease in the conversion. This result can be explained as follows; the total fluid velocity in the system increases with higher P2 and as a consequence the residence time in the ultrasonic reactor decreases. Thus, the acoustic irradiation time per pass, and the conversion decreases, because the ultrasonic irradiation has the dominant influence on the conversion of UDMH. Due to high velocity, the number of passes per unit time increases and that is why the conversion per pass has an optimum in AHC mode as shown in Fig. 5. 3.3. Effect of operating temperature Operating temperature is another key parameter, which affects the intensity of cavitation as well as efficacy of UDMH oxidation reaction. Normally, cavitation intensity decreases with an increase in temperature due to the formation of vapor cavities that collapse less violently. However, the oxidation reaction, in terms of generation of hydroxyl radicals, is significantly enhanced at higher operating temperatures. Kavitha and Palanivelu [18] have reported that the maximum extent of degradation of cresols occurs at an operating temperature of 30 °C. Sun et al. [19] have reported that the rate constant for degradation of p-nitroaniline increased by almost 100% with a rise in temperature from 20 °C to 30 °C. The effect of operating temperature was investigated at six constant temperatures ranging from 15 to 40 °C and was performed with precise temperature control and the extent of variation in these sets was ±1 °C. The extent of UDMH conversion at the end of 60 min increases with temperature and reaches to maximum at 25 °C. The conversion decreases smoothly with increasing the temperature higher than 25 °C. The phenomena can be explained as follows: when the temperature is lower than 25 °C, the cavitation intensity and the concentration of hydroxyl free radical will increase with the increase of temperature and results in the increase of the oxidation rate of
Fig. 6. Effect of operating temperature on UDMH overall conversion.
Fig. 7. Effect of number of passes on UDMH overall conversion.
UDMH. Increase in the temperature more than 25 °C, increases vapor pressure and reduces the viscosity and surface tension. A rise in temperature reduces the gas solubility which is the chief source of cavity nuclei and reduces the rate of occurrence of cavitation events. Thus the overall number of the cavities formed decreases thereby reducing the efficiency of cavitation phenomena [20]. Fig. 6 shows the effect of operating temperature on the conversion of UDMH for AHC-reactor at its optimized inlet pressure while the ultrasonic transducer has been working with 40 W acoustic power. Due to these results we have set the temperature of the cooling system at 25 ± 1 °C which gives us the optimum operating temperature and the best efficacy of the UDMH removal in the AHC mode.
3.4. Effect of number of passes (time of operation) Fig. 7 shows the effect of the number of passes on the conversion of UDMH for AHC-reactor at its optimized inlet pressure and temperature. The number of passes (Np ), is expressed as:
Np =
volumetric flow rate × time of operation total volume of solution in holding tank
(10)
The extent of UDMH degradation increases with increasing number of passes. However, it was found that there is an optimum number of passes (56 passes or 40 min of circulation for this set of operational parameters) that would yield acceptable conversion. It is also to be noted that this factor is crucial in determining the cost of operation, fewer numbers of passes lower the cost.
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Fig. 8. Possible pathways of UDMH oxidation initiated by hydroxyl radicals. Table 2 Possible oxidation products of UDMH.
1 2 3 4 5 6 7 8 9 10
The substance name
The substance formula
Retention time in GC/MS (min)
Observed products
Methanol Dimethylamine Formaldehyde hydrazone Formic acid Nitromethane Acetic acid N,N-Dimethylhydroxylamine Formaldehyde dimethylhydrazone Nitrosodimethylamine Monomethylhydrazine
CH3 OH (CH3 )2 NH NH2 NCH2 HCOOH CH3 NO2 CH3 COOH (CH3 )2 NOH (CH3 )2 NNCH2 (CH3 )2 NNO CH3 NHNH2
4.6 6.8 7.7 8.5 9.4 11.0 11.1 11.6 14.8 15.1
Not observed Not observed Not observed + + + Not observed Not observed Not observed Not observed
Table 3 Data of product analysis dependent on pH0 .
3.5. Products of the UDMH oxidation The attack of hydroxyl radicals to the UDMH molecules in the presence of H+ (acidic medium) can be done through its methyl group or amino group according to the following reactions (Eqs. (11) and (12)).
˙ H+ (CH3 )2 NNH2 + OH˙ −→ H2 O + H2 C NNH2 H3 C
H+
˙ (CH3 )2 NNH2 + OH˙ −→ H2 O + (CH3 )2 NNH
(11)
(12)
As it is shown in Fig. 8, these two reactions can be continued in many ways. And variety of intermediate products may be listed for each pathway. However, instead of the detailed study of the intermediate products, we gave a careful attention to the formation of oxidation end-products of UDMH, since some of them such as NDMA, formaldehyde and methanol can be toxic. Table 2 represents the list of substances that can potentially appear as oxidation products of UDMH and their retention time in the chromato-mass spectrometer columns under the conditions
No.
1
2
3
4
5
6
7
pH0 [HCOOH] (mM) [CH3 NO2 ] (mM) [CH3 COOH] (mM) [NO3 − ] (mM)
1.0 14.5 0.2 1 2.4
2.0 2 0.2 1.5 1.9
5.0 3.5 0.1 0.1 1.9
7.4 0.44 0.2 1.1 1
8.4 11 0.3 1.5 1.8
9.3 7 0.2 0.2 1.6
9.7 3.4 0.2 0.2 1.5
applied. Nitromethane, formic and acetic acid were the only carboncontaining products, which observed within 40–60 min reaction time, at moderate acidic medium (pH ࣈ 3) and initial UDMH concentration of 0.015 M. 3.6. Effect of initial pH The influence of the initial pH value, pH0 , of the solution on the UDMH oxidation was studied for the pH0 range from 1.0 to 9.7 (Table 3). All experiments were carried out at a constant temperature
Please cite this article as: M. Torabi Angaji, R. Ghiaee, Cavitational decontamination of unsymmetrical dimethylhydrazine waste water, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2014.11.008
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M. Torabi Angaji, R. Ghiaee / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–6 Table 4 Comparison of UDMH conversion of individual methods and hybrid method. Working mode of the reactor
Conversion per pass (%) Conversion after 40 min (%)
HC
AC
AHC
0.63 37
0.84 47
1.73 97.8
(25 ± 1 °C) and AHC mode. The circulation time of solution in the reactor was 40 min. Hydrochloric acid 1 M solution was used to adjust the pH0 . The rate of UDMH oxidation reaction was low in a strong acidic medium. As a result, the formic acid accumulation was observed. The pH0 values from 2 to 7.4 appeared to be the optimum for the total UDMH oxidation since the reaction rate was relatively high as well as the UDMH conversion. At pH0 8.4, the rate of UDMH oxidation decreased. 3.7. Comparison of the individual methods and the hybrid method Investigations were performed with HC, AC or the AHC system. In the case of HC, ultrasound was not used and the solution flows through by-pass 3 line. For acoustic method by-pass 2 line was used. In the case of the hybrid method, both by-pass 2 and 3 lines were closed, and ultrasound generator was operated with its maximum power. In Table 4, the overall conversions after 40 min and the conversions per pass are listed. Best results obtained by combining the acoustic and hydrodynamic cavitation modes, which led to an overall UDMH conversion equal to 97.8% and a conversion per pass equal to 1.73%. The results achieved by ultrasound treatment were 47% and 0.84% respectively. Working in the hydrodynamic cavitation mode, the reactor reached to an overall UDMH conversion up to 37% and a conversion per pass equal to 0.63%. The synergistic effect can be calculated as:
synergistic effect =
X(HC+AC) × 100% X(HC) + X(AC)
(13)
where X(HC+AC) is the conversion with the hybrid method, X(HC) and X(AC) are the conversions with the individual methods. It was calculated that the synergistic effect is about 16% with regard to an experimental time of 40 min. For the synergistic effect per pass, a much better result of 68% was obtained. This means that the hybrid method is 68% better than the sum of the individual methods per pass. 4. Conclusions In this study, hydrodynamic and acoustic cavitation and a combination of both as a hybrid method of advanced oxidation have been investigated. The study was focused on the oxidation of aqueous solution of UDMH and was done by an experimental setup designed for this case. The setup made it possible to study the impact of any one of the variety of cavitation modes on the oxidation reaction of UDMH individually or at the same time. All experiments were performed with a model aqueous solution. The concentrations of UDMH and its possible oxidation products in the aqueous solutions were determined using spectrophotometry technique. Quantitative analyses of the solution were conducted with a chromato-mass-spectrometer. No NDMA or any other toxic by-product was observed in the investigated samples. UDMH was oxidized and removed from the solution after 40 min of the cavitational oxidation. The rates of UDMH oxidation were low in the strong acidic and basic medium. A range of pH values from 2 to 7.4 appeared to be optimum. The extent of the overall conversion of UDMH increased with an increasing fluid velocity when the acoustic irritation was the only source of cavitation. For the AHC and HC modes an optimum inlet pressure, which maximized the overall conversion
Synergistic effect (%)
68 16
of UDMH, was detected. An optimal operating temperature was found for the AHC mode. Comparing the AHC mode with the individual modes, a synergism was observed. The combination achieved the best conversion rates, besides synergies of 16% for a 40 min treatment and 68% in a single pass. Oxidation under optimum reaction conditions resulted in the UDMH mineralization to the extent of 97.8%, having formic acid, acetic acid, and nitromethane the only carbonaceous residues of the UDMH. We conclude that the hybrid method of combination of hydrodynamic and acoustic cavitation is a novel method for mineralization and decontamination of UDMH aqueous solution. Important advantages of this method is the high optimization potential of the process, and no necessity of any chemical agents, which make it possible to realize a mobile unit for treatment of UDMH at a distant location. So that it could exclude hazardous transportation to the centralized treatment facilities. References [1] Fedorov LA. Liquid missile propellants in the former Soviet Union. Environ Pollut 1999;105:157–61. [2] USEPA integrated risk information system home page. Office of Research and Development (ORD), National Center for Environmental Assessment. http://www.epa.gov/iris/subst/0045.htm [accessed 26.03.13]. [3] Mitch WA, Gerecke AC, Sedlak DL. A N-nitrosodimethylamine (NDMA) precursor analysis for chlorination of water and wastewater. Water Res 2003;37:3733–41. [4] Schreiber IM, Mitch WA. Influence of the order of reagent addition on NDMA formation during chloramination. Environ Sci Technol 2005;39:3811–18. [5] Gou XL, Lv XM, Cui H, Li X. Treatment of UDMH wastewater by microwave catalytic oxidation process. Appl Mech Mater 2013;295–298:1486–9. [6] Makhotkina OA, Kuznetsova EV, Preis SV. Catalytic detoxification of 1,1-dimethylhydrazine aqueous solutions in heterogeneous Fenton system. Appl Catal B-Environ 2006;68:85–91. [7] Pestunova OP, Elizarova GL, Ismagilov ZR, Kerzhentsev MA, Parmon VN. Detoxification of water containing 1,1-dimethylhydrazine by catalytic oxidation with dioxygen and hydrogen peroxide over Cu- and Fe-containing catalysts. Catal Today 2002;75:219–25. [8] Kumar PS, Kumar MS, Pandit AB. Experimental quantification of chemical effects of hydrodynamic cavitation. Chem Eng Sci 2000;55:1633–9. [9] Parsa JB, Zonouzian SAE. Optimization of a heterogeneous catalytic hydrodynamic cavitation reactor performance in decolorization of Rhodamine B: application of scrap iron sheets. Ultrason Sonochem 2013;20:1442–9. [10] Franke M, Braeutigam P, Wu ZL, Ren Y, Ondruschka B. Enhancement of chloroform degradation by the combination of hydrodynamic and acoustic cavitation. Ultrason Sonochem 2011;18:888–94. [11] Mitch WA, Sharp JO, Valentine RL, Alvarez-Cohen L, Sedlak DL. Nnitrosodimethylamine (NDMA) as a drinking water contaminant: a review. Environ Eng Sci 2003;20:389. [12] Schreiber MI, Mitch WA. Influence of the order of reagent addition on NDMA formation during chloramination. Environ Sci Technol 2005;39:389. [13] Hatanaka SI, Mitome H, Yasui K, Hayashi S. Multibubble sonoluminescence enhancement by fluid flow. Ultrasonics 2006;44:e435–e8. [14] Braeutigam P, Franke M, Wu ZL, Ondruschka B. Role of different parameters towards the optimization of hydrodynamic cavitation. Chem Eng Technol 2010;33:932–40. [15] Gogate PR, Pandit AB. Engineering design methods for cavitational reactors. II. Hydrodynamic cavitation. AIChE J 2000;46:1641–9. [16] Saharan VK, Badve MP, Pandit AB. Degradation of Reactive Red 120 dye using hydrodynamic cavitation. Chem Eng J 2011;178:100–7. [17] Pradhan AA, Gogate PR. Removal of p-nitrophenol using hydrodynamic cavitation and Fenton chemistry at pilot scale operation. Chem Eng J 2010;156:77–82. [18] Kavitha V, Palanivelu K. Destruction of cresols by Fenton oxidation process. Water Res 2005;39:3062–72. [19] Sun JH, Sun SP, Fan MH, Guo HQ, Qiao LP, Sun RX. A kinetic study on the degradation of p-nitroaniline by Fenton oxidation process. J Hazard Mater 2007;148:172–7. [20] Gogate PR, Pandit AB. Hydrodynamic cavitation reactors: a state of the art review. Rev Chem Eng 2001;17:1.
Please cite this article as: M. Torabi Angaji, R. Ghiaee, Cavitational decontamination of unsymmetrical dimethylhydrazine waste water, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2014.11.008