Kinetics studies of ketazine formation: effect of temperature and catalyst concentration.

Kaur, Raminder ; Machiraju, Ramakrishna ; Nigam, K.D.P. 等

INTRODUCTION

Gas-liquid-liquid reactions have a prominent role in chemical reaction engineering. But unfortunately these have hardly enticed researchers for the past one and a half decades. Due to the advent of homogeneous biphasic catalysis in various reaction systems, such as hydroformylation, carbonylation, hydrogenation, and oligomerization (Herrmann and Kohlpaintner, 1993; Chaudhari et al., 1995; Cornil, 1999), the gas-liquid-liquid systems have emerged as a challenging field to pursue research. For homogeneous catalyzed reactions, the catalyst is usually totally soluble in the liquid phase and the reactions can be classified into the categories of gas-liquid, liquid-liquid, and gas-liquid-liquid reactions. Most of the pharmaceutical and fine chemical processes can be categorized according to these classifications.

Gas-liquid-liquid reactions mainly comprise reaction systems that inherently consist of three phases due to two (or more) immiscible reactants, reaction products or catalyst, or are the systems, in which an additional inert liquid phase is intentionally added to a gas-liquid system to increase the mass transfer rate. Koth synthesis (Bahrmann, 1980) represents a very good example of the former type, in which all three reactants originate from different phases. The later concept is employed in a few biochemical applications (Rols et al., 1990). However, the addition of a second liquid phase can also impede the gas-liquid mass transfer (Yoshida et al., 1970). Cents et al. (2001) used the Danckwerts-plot in gas-liquid-liquid systems for the analysis of mass transfer parameters and concluded that two types of systems exist, systems that enhance mass transfer and systems that do not enhance mass transfer. Major part of the research in gas-liquid-liquid systems has been found to be conducted on the effect of the dispersed phase on the mass transfer and the interfacial area and a number of mass transfer models had been presented, e.g. homogeneous models developed by Bruining et al. (1986), Mehra (1988), Littel et al. (1994), Nagy and Moser (1995), Van Ede et al. (1995) and heterogeneous models by Holstvoogd et al. (1988), Brilman (1998), Brilman et al. (2000), Zhang et al. (2006), Karve and Juvekar (1990).

The formation of methyl ethyl ketazine is a gas-liquid-liquid reaction, whose mechanism is highly complicated. It is fascinating to note that the system initially consisted of only two phases, i.e., reactants gas (ammonia) and liquid (mixture of 46% aqueous hydrogen peroxide, 70% solution of the catalyst acetamide in DM water, and methyl ethyl ketone, all present in a single phase) becomes a gas-liquid-liquid reaction system, after the instigation of product. Although methyl ethyl ketone is organic in nature, it has good solubility in the aqueous mixture of hydrogen peroxide and catalyst acetamide. The reaction system thus contains three phases: reactant gas phase, lighter organic phase containing the main product, i.e., ketazine (and unreacted ketone that got transferred from the single phase present initially to organic layer formed during the course of the reaction), and heavier aqueous phase containing regenerated catalyst and water formed during the reaction. It is imperative that the parameterc study of the ketazine formation should be carried out extensively, for the commercialization of such a process. The parameters investigated are effect of agitation, temperature, catalyst concentration, ammonia flow rate, and MEK/peroxide ratio. In this paper, the effects of temperature and catalyst concentration on the formation of methyl ethyl ketazine have been reported.

Formation of ketazine is a special case of homogeneous catalyzed reactions following intrinsic systems in reaction process. In the literature, no information on kinetics studies for methyl ethyl ketazine reaction system is reported. Only a few references are available regarding either the formation of ketazines or for the production of hydrazine hydrate through ketazine route, mainly using hydrogen peroxide including patents (Eichenhofer and Sehliebs, 1976, 1977; Schirmann et al., 1976; Maekawa and Kume, 1979; Kazuhiko et al., 1983; Kuriyama et al., 1983; Schirmann et al., 1988; Schirmann and Tellier, 1993; Zhao et al., 1993; Kuriyama et al., 1997; Schirmann, 2002, 2003). These surveys give no pertinent information for the effects of temperature and catalyst concentration on the product formation.

Ketazines are the azines of ketones with the generic formula: [R.sub.2] C=N-N=[CR.sub.2]. Methyl ethyl ketazine is a paramount member of the ketazine family consisting of -N-N- bond. It is an intermediate in the formation of hydrazine hydrate that is one of the most versatile chemicals finding use in a wide spectrum of applications such as rocket fuel, energy source in fuel cell, blowing agent in plastic industry, in synthesis of organic nitrogen compounds, and in making herbicides and pesticides for agricultural use. Ketazine process involves oxidation of ammonia by hydrogen peroxide in the presence of methyl ethyl ketone. The azine thus formed can be easily hydrolyzed under pressure (0.8-10 MPa) to give concentrated aqueous hydrazine and regenerate ketone. Most hydrazine now-a-days is being produced by ketazine process. Overall reaction for the methyl ethyl ketazine formation, as reported by Kirk and Othmer (2004) is:

[FORMAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The reaction steps are given below:

One molecule of ammonia reacts with one molecule of MEK to form Schiff base.

[FORMAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

Hydrogen peroxide reacts with acetamide to form iminoperacetic acid.

[FORMAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

Iminoperacetic acid then oxidizes the Schiff base to give the oxaziridine and regenerates acetamide.

[FORMAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

The oxaziridine oxidizes a second molecule of ammonia to form a hydrazone.

[FORMAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

Hydrazone with excess ketone forms the azine.

[FORMAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

In the present investigation, the effect of temperature and catalyst concentration on yield of ketazine has been studied. In these kinetics studies, experiments were conducted at temperatures 30, 40, 50, 60, and 70[degrees]C and catalyst to peroxide ratio equal to 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0. It however becomes very important to define the term percentage yield, as different criteria are used by different research workers. In this paper, percentage yield of ketazine is calculated as:

Percentage yield of ketazine = Moles of ketazine formed x 100 / Moles of peroxide reacted

(6)

One another important calculation is carried out for the percentage conversion of peroxide and is calculated from the relation:

Percentage conversion of peroxide = Moles of peroxide reacted / Moles of peroxide fed

(7)

[FIGURE 1 OMITTED]

EXPERIMENTAL

Chemicals

Hydrogen peroxide (46%) was purchased from Gujarat Alkalis and Chemicals Limited, Baroda, India. Ammonia (100%) was supplied by Ranga Chemrkach, Secunderabad, India, methyl ethyl ketone (>99%) was supplied by Saumar Specialty Chemicals Limited, Kancheepuram, India. Catalyst, acetamide (99%) was purchased from Otira Pharmaceuticals Pvt. Limited, Bonthapally, India. Disodium salt of ethylene dinitrilo tetra acetic acid (EDTA), G.R. Grade, acting as peroxide stabilizer is purchased from Merck Limited, Mumbai, India, Ammonium acetate, (G.R. Grade); cocatalyst was purchased from Merck Limited, Mumbai. DM water was used in all experiments supplied by Discovery Lab, IICT, Hyderabad, India. Acetamide is used as a 70% solution in DM water.

Experimental Set-up

The set-up used for experimentation is as shown in Figure 1. The set-up consists of a four-necked glass reactor of 500 ml, capacity with 180 mm height, 90 mm diameter equipped with a mechanical stirrer. The stirrer was mounted on an overhung shaft, i.e., shaft supported from above, along the axis of the reactor, with a clearance from the bottom equal to one-third of the diameter of the reactor. The shaft is driven by a 1/8 H P motor which is controlled through a dimmerstat. Heating is carried out by a well-controlled water bath within [+ or -] 0.1[degrees]C. Reactor is supported on a circular copper plate within the water bath. Capillary flow meters with 0.5 mm nominal capillary (which are calibrated in advance) are used to measure the flow rate of ammonia. One of the necks of the reactor is equipped with a reflux condenser, to minimize the carry over of the MEK vapours. Cooling is carried out by circulating methanol at -5[degrees]C from a cryostat (Type: FC 600, 230 V, 50 Hz, 0.45 KW), manufactured by Julabo Labortechnik, GmbH, Eisenbahnstrasse, Seelbach, Germany.

Experimental Procedure

To study the effects of temperature on percentage yield of ketazine, experiments were conducted at 30, 40, 50, 60, and 70[degrees]C. The initial charge of the reactor consisted of 0.75 mol of peroxide and 3.0 mol of methyl ethyl ketone. The catalyst concentration was maintained at 2.0 mol/mol of peroxide. The ammonia flow rate was maintained at 0.375 mol/h. The reactor was placed in a hot water bath. The stirrer was connected to a motor, regulated by a dimmerstat. The speed of the stirrer was measured by a tachometer (DT 2001 B, Electronic Automation Pvt. Ltd., Bangalore, India). Temperature of the reaction mixture was measured by a long-stem thermometer inserted in a thermowell, whose tip was properly located in the reactor. Ammonia at required flow rate was passed through a buffer vessel before being fed to the reactor to smoothen the flow fluctuations. The outlet from the reactor was connected to a condenser and then to a HCl trap to absorb unreacted ammonia.

To carry out the reaction, required moles of hydrogen peroxide were mixed with EDTA. EDTA (2 g) was used per mole of HZOZ. EDTA acts as peroxide stabilizer and thus forms complexes with metallic ions, if present, which otherwise form metallic oxides by reacting with peroxide. Also, acetamide solution was mixed with ammonium acetate, which acts as a co-catalyst. Ammonium acetate (1.5 g) was added per mole of acetamide. The purpose of adding ammonium acetate is that, after the recovery of ketazine, if reconstituting of acetamide has to be carried out for the next run, this initially added ammonium acetate gets converted into the acetamide by simple heating.

All the reactants, except ammonia in the required mole ratio, were charged into the reactor. Ammonia gas was fed according to the required flow rate. The stirrer was set at required rpm. Water bath and cooling systems were switched on. When the required temperature was reached, further heating was stopped and was maintained for next 6 h. The samples from the reaction mass were collected after every quenched by keeping in ice for freezing of the reaction and separating the lighter organic and heavier aqueous layers. Samples from both phases were then analyzed for ascertaining the percentage of peroxide and percentage of ketazine in the reaction mixture. The percentage yield of ketazine and percentage conversion of peroxide were then calculated according to the Equations (6) and (7), respectively.

Similarly, to study the effects of catalyst concentration, the catalyst to peroxide mole ratios selected were 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0. The initial charge of the reactor consisted of 0.75 mol of peroxide and 3.0 mol of methyl ethyl ketone. The temperature was maintained at 50[degrees]C. The ammonia flow rate was maintained at 0.375 mol/h.

The reaction medium initially consisted of a single liquid phase and a gas phase. However, as the reaction proceeded and some ketazine was formed, a second liquid phase appeared due to the low miscibility of ketazine in aqueous phase. There was a continuous exchange of ketazine from the reaction phase to the organic phase containing unreacted methyl ethyl ketone and ketazine. The whole reaction system then became a gas-liquid-liquid system. The rate of the dissolution of ammonia into the aqueous phase was limited by the temperature and the pressure of the system. In the liquid phase (aqueous phase), where the reaction was taking place, ammonia was consumed, the rate of the reaction was limited by the concentration of ammonia, peroxide, acetamide, and methyl ethyl ketone in the liquid phase (aqueous phase).

Analysis

Both methyl ethyl ketazine and hydrogen peroxide in the reaction mixture have been estimated by using iodate method, because various applications of titrations using iodate gave results of good accuracy. The organic layer has been analyzed for the amount of methyl ethyl ketazine by titrating against 0.1 M (0.4 N) potassium iodate, KI[O.sub.3] solution (Kirk and Othmer, 2004) and the aqueous layer had been analyzed for the peroxide present by titrating against standard 0.1 N KMn[O.sub.4] solution.

[FIGURE 2 OMITTED]

RESULTS AND DISCUSSION

In the interpretation of reaction kinetics, it is necessary to conduct experiments in the kinetic regime where mass transfer resistance between the phases is completely eliminated. To isolate kinetic factors entirely from interactions with transport phenomena, the conventional approach is to strengthen the agitation to minimize the relative importance of interfacial resistance to mass transfer as far as possible. To ensure the reaction is in the chemical reaction controlling region, preliminary experiments were carried out at a temperature of 50[degrees]C with different stirring speeds. It was observed that the reaction rate was enhanced greatly with the increase in the agitation speed at first and then approached to a constant value. The desired level of agitation for methyl ethyl ketazine production was found to be 600 rpm (Kaur et al., 2007). Hence, studies are carried out at 600 rpm.

Effect of Temperature

The effect of temperature on the yield of ketazine has been studied for 30, 40, 50, 60, and 70[degrees]C at 600 rpm. Yield of ketazine versus time plots for these experiments are given in Figure 2. From Figure 2, it is clear that as temperature increases, the yield of ketazine increases. When the reaction is carried out at 60[degrees]C for 6 h, the percentage yield obtained is 96% which is comparable to 94% ketazine obtained for a 6 h reaction at 60[degrees]C by Myauchi et al. (1995) using silica gel (obtained by gelation of silicic acid; or its condensation products) as catalyst.

The behaviour of conversion of peroxide versus time at this temperature range is shown in Figure 3. It is clear from Figure 3 that conversion of hydrogen peroxide increases with increase in temperature. It is indicated by Figure 3 that, for a 10[degrees]C rise, i.e., from 40 to 50[degrees]C the initial conversion of peroxide increased nearly 1.4 times in 30 and 60 min, i.e., from 3.23 to 4.60 and from 8.07 to 10.99. Similarly for 90 and 120 min of reaction, conversion increases nearly 1.3 times, i.e., from 12.31 to 15.57 and from 17.14 to 22.69. Also 1.3 to 1.4 times increase in conversion is observed for 50 to 60[degrees]C rise for 30 to 120 min of the reaction. Earlier experimental studies into the kinetics of the chemical reactions have revealed that an increase of 10[degrees]C in temperature brings about nearly twofold increase in the reaction rate. In our system also, the observed increase in conversion is significantly high (1.3 to 1.4 times), indicating a possible kinetic controlled mechanism. However, the curve for 60 and 70[degrees]C almost overlap. For such reaction system, it is expectable that the yield of product increases with the increase in temperature and then levels off. The reason is that the solubility of ammonia in liquid is decreased with the increase in temperature.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The percentage yield of ketazine versus temperature is shown in Figure 4. From Figure 4, it is clear that yield of ketazine increases with temperature up to 60[degrees]C only, and beyond 60[degrees]C, as indicated by Figure 2 and Figure 4, there is no appreciable change in the yield. In other words, with the increase in temperature, yield of ketazine increases and reaches a saturation value at 60[degrees]C and above this temperature the effect of temperature on yield of ketazine is negligible. This behaviour is similar to that shown by Jeffrey and Wharton (1965) for the kinetics studies of the formation of hydrazine hydrate by raschig process where the yield at first increased rapidly with temperature from 75 to 160[degrees]C and then levels off at 160[degrees]C. Also, the curve given by Reed (1957) for hydrazine production was similar in shape. On further increase in the temperature, i.e., beyond 60[degrees]C, yield of ketazine becomes almost constant and there is no significant change in yield for 60 to 70[degrees]C rise (Figure 4). Reaction beyond 70[degrees]C could not be carried out, as carry over losses were more. When the reaction was tried at 80[degrees]C and atmospheric pressure, methyl ethyl ketone started evaporating and there was no product formation. Thus, desired temperature for carrying out the reaction at atmospheric pressure is found to be 60[degrees]C.

As can be seen from these studies, beyond 60[degrees]C, there is no significant improvement either in conversion of peroxide or in the yield of ketazine. Up to 60[degrees]C, the reaction is controlled by chemical kinetics as indicated by the experimental results. Beyond 60[degrees]C, no appreciable increase has been observed, thus indicating the reaction is mass transfer controlled. However, as temperature is increased, the complex nature of the process dominates, where the two rate processes, i.e., diffusion of ammonia into the reaction mixture and consumption of peroxide and ammonia by chemical reaction, compete. Furthermore, with increase in temperature, solubility of ammonia decreases. This decreases the ammonia mass transfer rate into the reaction medium and so decreases the reaction rate. The net effect observed is a commutative effect of all these processes, showing increase in upto 60[degrees]C and further no significant change.

[FIGURE 5 OMITTED]

It can be seen that the conversion rate of peroxide increases with reaction temperature. The temperature dependence of the reaction can be determined using Arrhenius equation. The activation energy of the reaction can be calculated from the initial rates at different temperatures, i.e., 30, 40, 50, and 60[degrees]C. The initial rates were calculated from hydrogen peroxide concentration versus time plots during the initial period of the reaction. By plotting the concentration of peroxide versus time and finding the initial reaction rates (slope of concentration vs. time) at different temperatures and plotting the initial reaction rate versus 1/T (Figure 5), we can get the activation energy. The activation energy calculated was found to be 24.5 kJ/mol for the peroxide conversion. An activation energy of 45 kJ/mol was obtained by Abilov and Golodov (1987) when oxidative dimerization of ammonia was carried out by [S.sub.2] [O.sub.8.sup.2-] in the presence of ketones and Ag (I) salts, i.e., AgN[O.sub.3] as catalyst.

Effect of Catalyst Concentration

The effect of catalyst concentration on yield of ketazine has been studied for the catalyst to peroxide mole ratios equal to 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0. The behaviour of yield of ketazine versus time and conversion of peroxide versus time are shown in Figures 6 and 7, respectively. The yields as well as conversions are found to increase with increase in catalyst to peroxide ratios. With increase in catalyst to peroxide ratio from 0.5 to 2.5 mol/mol, the yield is increased from 42.33 to 74.58 for 5 h of reaction and 56.90 to 86.03 for 6 h of reaction. Similarly, the increase in values for conversion for 5 h of reaction is from 49.65 to 76.28 and for 6 h reaction is from 56.24 to 88.81. Thus, with the catalyst concentration, the rate of reaction is enhanced. This type of behaviour is expected as far as mass transfer limitations are negligible and any increase in the catalyst concentration will proportionally enhance the concentration of the active catalytic species and hence the rate. From Figure 6, it is clear that yield of ketazine increases with increase in catalyst to peroxide ratio up to a value equal to 2.5 mol/mol and there is no significant increase in the yield at a mole ratio of 3.0 mol/mol. Hence, with the increase in catalyst concentration, the yield increases and reaches a saturated value at catalyst to peroxide ratio equal to 2.5 mol/mol and on further addition, the effect of catalyst concentration on ketazine formation is negligible. A similar result for the effect of catalyst concentration on yield of azine was reported by Hayasi et al. (1976) in the production of benzophenone azine from benzophenone by passing ammonia and oxygen in the presence of zinc chloride and cuprous chloride. The yield was reported to increase linearly up to a certain amount of catalyst and then flattened. In our studies, it has been revealed that the required catalyst to peroxide ratio is 2.5, beyond which no appreciable effect on the yield has been observed.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

CONCLUSIONS

The effects of temperature and catalyst concentration on the methyl ethyl ketazine formation have been studied. These studies have not been reported in literature so far. The conversion of peroxide is found to increase with increase in temperature thus indicating it a kinetics-controlled reaction. The yield at first increases rapidly with increase in temperature and then levels off at 60[degrees]C. Ketazine yield is found to increase with catalyst to peroxide ratio up to the ratio equal to 2.5 and showed negligible change afterwards. Thus, the desired temperature for carrying out the reaction is 60[degrees]C and the required catalyst to peroxide ratio is 2.5. The activation energy calculated for the reaction is 24.5 kJ/mol.

NOMENCLATURE

[E.sub.a] activation energy, kJ/mol
R the universal gas constant (8.314 J/mol K)
T temperature, K

Manuscript received March 1, 2007; revised manuscript received June 21, 2007; accepted for publication June 22, 2007.

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Raminder Kaur (1), Ramakrishna Machiraju (1) and K.D.P. Nigam (2) *

(1.) Indian Institute of Chemical Technology, Hyderabad 500007, AP, India

(2.) Indian Institute of Technology, Delhi, Houz Khas, New Delhi 110016, India

* Author to whom correspondence may be addressed. E-mail addresses: nigamkdp@gmail.com,mrkiict@hotmail.com

DOI 10.1002/cjce.20013