Ind. Eng. Chem. Res. 1994,33, 277-284
277
Kinetics, Mass Transfer, and Palladium Catalyst Deactivation in the Hydrogenation Step of the Hydrogen Peroxide Synthesis via Anthraquinone E. Santacesaria,’J M.D i SerioJ R. Velotti? a n d U. Leonet Dipartimento di Chimica, Uniuersith di Napoli, Via Mezzocannone 4, 80134 Napoli, Italy, and Ausimont SPA-CERZOS, Bussi (Pescara),Italy
Hydrogenation of 2-ethyl-5,6,7,8-tetrahydroanthraquinone is a key step in the industrial production of hydrogen peroxide via anthraquinone with the process named all-TETRA. This reaction on palladium-supported catalysts is very fast; consequently, the chemical regime can hardly be achieved and kinetics is always masked by mass-transfer limitations. Nevertheless, it is possible to demonstrate that the reaction occurs with a zero- and first-order kinetics with respect to hydrogen and to the organic reagent, respectively. These reaction orders can be explained on the basis of reasonable reaction mechanisms described and discussed in this paper. Kinetics has been studied by performing runs in two different laboratory reactors: a semibatch and a continuous stirred tank reactor. In particular, the continuous reactor has been used for studying catalyst deactivation. Two types of catalyst poisoning have been recognized, a reversible one, related to the presence of water, and a permanent, not yet explained one. A kinetic expression is given also for deactivation. The kinetic parameters obtained from the experimental runs have been verified by simulating the behavior of an industrial reactor also considering catalyst deactivation. Introduction In the process named all-TETRA (see Powell, 1968;Kirk and Othmer, 1981; Ulmann’s, 1989) for the industrial production of hydrogen peroxide, the hydrogenation of 2-ethyl-5,6,7,8-tetrahydroanthraquinone (THEAQ) is a fundamental step. In fact, reactions occurring in the process are the following:
II
?H
0
OH
(THEAQ)
(THEAOH,)
OH
0
0 (THEAQ)
Hydrogen peroxide is then extracted with a slightly acidic solution. Although the working solution normally contains 30% ethylanthraquinone (EAQ) and 70% THEAQ (Powell, 1968;Kirkand Othmer, 1981;Ulmann’s, 1989),THEAQ only participates in the production cycle because hydrogenation is usually kept at about 70% of conversion and the following equilibrium is operative: EAQH, + THEAQ F? EAQ + THEAQH, (3) The equilibrium is completely shifted to the right, as reported from Berglin and Shoon (1983).Hydrogenation of THEAQ occurs both directly and via EAQ. However, the kinetic behavior of the two mentioned anthraquinones in the hydrogenation is quite similar. Therefore, their mixtures can be considered as a single pure component.
* To whom correspondence should be addressed.
+ Universita di Napoli.
* Ausimont SPA-CERIOS.
Hydrogenation is normally performed in slurry reactors, in the presence of a palladium-supported catalyst. Very few papers have been devoted to the kinetics of this reaction. Kirdin et al. (1970)and Berglin and ShMn (1981), for example, studied the reaction in the presence of RaneyNi catalyst. Santacesariaet al. (1988)studied the kinetics and mass transfer in the presence of a palladium-supported catalyst and concluded that palladium is extremely active and the reaction is always controlled by mass transfer. Therefore, kinetic data were interpreted in that work by neglecting the chemical reaction rate contribution. This assumption is a rough approximationwhich can reasonably be accepted only for runs performed with fresh catalysts. However, it was observed that palladium catalysts are susceptible to deactivation by poisoning. Therefore, the importance of the chemical reaction rate contribution can be estimated by observing the effect of the catalyst poisoning. The present work must therefore be considered an improvement of the one published from Santacesaria et al. (1988).Reaction orders and kinetic parameters will be determined, the kinetic law found will be discussed, and the related reaction mechanism will be suggested. Kinetic parameters have been determined from runs performed in laboratory slurry semibatch and continuous reactors. Mass-transfer parameters have newly been estimated, and the obtained values are in good agreement with those reported in the previous work (Santacesaria et al., 1988). Catalyst deactivation kinetics has been studied in a laboratory continuous reactor. Two deactivation mechanisms have been recognized, a reversible and fast one due to the presence of water and a permanent and slow one probably due to the formation of bulky molecules on the palladium surface as a consequence of anthraquinone molecules condensation. The kinetic model developed in this work, also considering catalyst deactivation, has successfully been tested in an industrial plant. Experimental Section Apparatus, Methods, and Reagents. Kinetic runs were performed in a slurry semibatch reactor. The scheme
0888-5885/94/2633-0277$04.50/0 0 1994 American Chemical Society
278 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994
w Automatic
Figure 1. Scheme of the slurry semibatch reactor. A = hydrogen feed, tube is connected to a mass flowmeter and to the hydrogen bottle; B = exit for liquid sample withdrawing: C = magnetic drive stirrer; D = baffles; E = inlet of thermostatic fluid; F = outlet of thermostatic fluid; G = thermal insulation; TC = thermocouple.
Figure 2. Scheme of the continuous reactor. A = hydrogen feed; B = feed of solution; C = magnetic drive stirrer; D = baffles; E = inlet of thermostatic fluid; F = outlet of thermostaic fluid; G = wire cloth; H = solution outlet; LC = liquid level automatic control: S = sensing element. cm3 of HZ adsorbed
of the reactor is reported in Figure 1. A solution containing the mixture of EAQ (30%) and THEAQ (70%) commonly used in industry has been employed in all the experiments. The concentrationsof the reactants have been determined by gas chromatographic analysis performed on a Cp-Sil19capillary column of 25-m length and 0.25-mm diameter. As the resulting kinetic behavior of the two mentioned substances was quite similar, we will treat the mixture as a unique active component called "anthraquinone". The stainless steel reactor was jacketed and equipped with a magnetically driven stirrer consisting of a turbine connected, as in Figure 1, to a hollowed cylinder and able to develop a great interfacial area. Teflon baffles were put inside the reactor. The internal size of the reactor was exactly the same as that of the glass reactor described in a previous paper by Santacesaria et al. (1988). Therefore, the reactor fluid dynamics was well-known. Hydrogen consumption was directly measured with a mass flowmeter connected to a computer. Thus, we evaluated the instantaneous hydrogen flow rates, that is, the reaction rates and the total volume of reacted hydrogen at different times. Kinetic runs were performed by changing catalyst concentration, rotating speed of the stirrer, anthraquinone concentrations, and temperature. Other kinetic runs were performed in a laboratory continuous reactor whose scheme is reported in Figure 2. This reactor has the same fluid dynamic characteristics of the already described semibatch reactor of Figure 1, being identical in the design of the stirrer, the baffles, and the internal size. A closely woven wire net keeps the catalyst confined inside the reactor. Again, reaction rates were determined by measuring hydrogen consumption rates with a mass flowmeter connected with a computer. The kinetic and mass-transfer parameters determined in this work reproduce well the data obtained with both the semibatch and continuous reactors. However, the continuous reactor has mainly been used to study catalyst deactivation in long time runs. The catalyst employed and solvent characteristics are the same as reported by Santacesaria et al. (1988).
3500 1
1
I
I-
~
2ooo 1500-
.:::I
0
0
500 1000 1500 2000 2500 3000 3500 4000 4500 time
(s)
Figure 3. Kinetic run performed at 70 O C , speed = lo00 rpm, m = 0.005 g/cm3, [THEAQ] = 4.7 X le mol/cm3,p H , ' 1 atm, V ~ = 2 0 0 cm3. Table 1. Main Properties of the Catalyst Used average diameter specific surface area metallic surface area specific weight bulk density porosity
0.013 cm 178 m2/g 149 m2/g 2.25 g/cm3 0.77 g/cm3 0.80 cm3/g
However, some catalyst properties are summarized in Table 1. All the reagents were provided by Ausimont SPA, while the catalyst was supplied by Montecatini Tecnologie SPA. The stainless steel reactors were built by INOX Impianti Co. Results and Discussion Kinetic Runs in the Semibatch Reactor and Their Interpretation. Figure 3reports the example of a hydrogenation run performed at 70 "C and 1000 rpm with a 0.0055 g/cm3 catalyst concentration. In this run the hydrogen consumption trend during time is nearly linear
Ind. Eng. Chem. Res., Vol. 33, No.2, 1994 279 cm3 of H, adsorbed
%JHr
(5)
I 500
40
0
180
120
60
time
3 20
so0
240
700 rmn/
o
y
(5)
Figure 4. Kinetic N ~ Sperformed at different catalyst concentrations. Catalyst hold-up is reported on the plot in gIcm3. Temperature=% 'C, speed = loo0 rpm, [THEAQ1=0.88 x lo-' mollcms, PH,=1 atm, V~=300 ems. Symbols correspond to experimentaldata; lines are obtained by calculations.
10
0 0
100
200
300
4w
500
l / m (cm Yg) Figure 6. Trend of pHJ" against l/m at different stirring ratea. Temperature=%°C,[THEAQI =O.&?X l ( r m o l l c m s , p ~ ~ l a t m .
vL=300 cm3.
300
*.-'
-
200
d d
,.*."
60
120
loo"d",,'./ o,
