Ind. Eng. Chem. Fundam. 1984, 23, 243-252
- = membrane anions
243
Meares, P.; Thain, J. P.; Dawson, D. G. "Membranes-A Series of Advances", G. Eisenman, Ed.; Marcel Decker, Inc.: New York, 1972; pp 55-124. Michaels, A. S.; Bixler, H. J.; Hausslein, R. W.; Fleming, S. M. US. Department of the Interior. Office of Saline Water, Research and Development Progress Report No. 149, Washington, DC, 1965. Newman, J. "Electrochemical Systems"; PrenticaHail; Englewood Cliffs, NJ, 1973; pp 239-250. Plntauro, P. N. Dlssertetion, University of California, Los Angeles, 1980. Pintauro, P.N.; Bennlon. D.N. Ind. Eng. Chem. Fundem. preceding paper in thls issue. Robinson, R. A.; Stokes, R. H. "Electrolyte Solutions"; Butterworth Scientific Publlcations: London, 1959; pp 102-104. Scattergood, E. M.; Lightfoot, E. N. Tf8nS. Faraday Sm. 1968, 6 4 , 1135. Sinha, M. M.S. Thesis, University of California, Los Angeles, 1977. Sinha, M.; Bennlon, D. N. J. Electrochem. SOC. 1978, 125, 556. Yeager, H. L. University of Calgary, Alberta, Canada, private communication, 1980. Yeager, H. L.; Klpling, B.; Dotson, R. L. J. Nectrochem. SOC. 1980, 127, 303.
Superscripts 0 = initial or reference state F = final * = radiotracer species Registry No. NaC1, 7647-14-5; Nafion 110, 61261-17-4.
Literature Cited Bennion, D. N.; Pintauro, P.N. AICh€ Symp. Ser. 1981, (204), 190. Choi. K. W.; Bennion, D. N. Ind. Eng. Chem. Fundam. 1975, 14, 296. Helfferich, F. "Ion Exchange," McGraw-Hill: New York, 1962; pp 339-420. Hooke, R.; Jeeves, T. A. J. ASSOC. Comput. Mech. 1961, 8, 212. Kamo, N.; Toyoshima, Y.; Nozaki, H.; Kabatake, Y. KollOM-2. 2. Pdym. 1971, 248, 914. Keller, K. H.; Canales, E. R.; Yum. S.1. J. Phys. Chem. 1971, 75. 379. Kressman, T. R. E.; StanbrMge, P. A,; Tye, F. L.; Wilson, A. G. Trans. Faraday SOC. 1983, 5 9 , 2133. Lonsdale, H. K. "Desalination by Reverse Osmosis", U. Merten, Ed.; The M. I.T. Press: Cambridge, MA, 1966; pp 93-160.
Received for review March 7, 1983 Accepted December 23, 1983
EXPERIMENTAL TECHNIQUES Gradientless Reactor for Gas-Liquid Reactions Yonatan Manor Department of Chemlcal Englneerlng, Unlverslty of Illlnols, Urbana, Illinois 6 180 1
Roger A. Schmltr' Department of Chemlcal Englneerlng, Unlverslty of Notre Dame, Notre Dame, Indiana 46556
A continuous-kw gradientless reactor was designed in this research to facilitate the study of relatively fast gas-liquid reactions under Isothermal conditions and in the absence of transport limitations. The essential component of the reactor was a multi-bladed rotor which contacted the gas-liquid interface directly. The mixing characteristics were tested, and mass transfer studies yielded rates greater than 30 times those of previously employed reactors. To demonstrate the utility of a reactor of this type, kinetic studies of the oxidation of sulfite and of propionaldehyde were conducted under kinetically controlled conditions at rates not previously attainable under such conditions.
Introduction Studies of the kinetics of fast gas-liquid reactions are generally complicated because their rates are usually retarded by transport limitations, and isothermality is difficult to maintain. Most prior research has been aimed at trying to model mathematically the simultaneous processes of reaction and transport or at designing equipment for improved parameter measurement and control. Little attention has been given to the development of gas-liquid contactors which would eliminate transport interferences over the range of reaction rates of interest. In the commonly employed sparged reactor, mass transfer rates are increased by adding extra stirring through an impeller submerged in the liquid phase. The energy introduced by the impeller is transferred through the liquid bulk to the gas-liquid interface where it is utilized for mixing near the interface and for bubble re-
dispersion. Most of the energy is dissipated through internal friction and liquid turbulence. Once the boundaries of the dispersed gas phase become rigid, as in the case of small bubbles or in the presence of impurities at the interface, additional energy input by increased stirring has little or no effect. The present work was aimed at designing and testing a continuous-flow reactor in which the energy for increasing the rates of interphase transport is imparted by a multi-bladed rotor directly at the gas-liquid interface. The primary utilization of the reactor would be for studies of intrinsic chemical kinetics at the laboratory scale although it is possible that a similar design could be considered for use at the commercial scale. The results of laboratory tests reported in this paper show that significantly higher values of kL are attainable than have been reported for other types of reactors, and that the bulk fluid 0
1984 American Chemical Society
2U
Ind. Eng. chan.Fundarn.. Vol. 23. No. 2. 1984
7-
\
Fisurn 3. Schematic diagram of flow patterns induced by the rotor. Letters are defined in Table 1. Table 1. Identification of Symbols in Fimres 1-4 Figures 1-3 Figurn 1. Vertical nor seetion of the mador. Letters are defined in Table 1.
A
sealing arrangement
B gasphase C
rotor
D sealing E bearings F
excessgas flow
G gas collected from the liquid exit
H overflow vessel I J
liquid exit rotor blades
K gas feed L adjustable shaft M baffles
N reactor shell 0 cooling jacket P coolinfi water feed and exit Q sampling port
R
grooves
S liquid phase T gas-liquid interface
U top and bottom plates
Figure 4 AA
PID complroller
BB
thermocouple speed controlled motor DD reactor EE overflow vessel F F condenser GG excess gas flow rotameter HH gas feed rotameter I1 liquid flow rotameter JJ blanketing gas rotameter K K blanketing gaa valve LL pressure regulator M M gas reed valve N N bulk solution 00 blanketing gas feed PP sampling porl
CC
QQ ss Tr
float level controller
R R receiver
uu
cooling water pump cold water positive displacement pump lor the liquid feed
V liquid feed Figure 2. H o h n t a l defined in Table 1.
&ion#
of the reactor. Letters are
phases are gradientless. Furthermore, owing to the deaign of the reactor, very efficient heat removal is achieved to a cooling jacket, and good temperature control is possible even for highly exothermic reactions. Description of t h e Reactor The reactor designed for this study belongs to the thin-film evaporator t y p e a type in which a thin Iayer of liquid is continuouslyspread on the inside wall of a heating or cooling jacket (Mutzenburg et el., 1965: Penney and Bell. 1967). T w o views of the reactor are shown in Figures 1 and 2. The liquid film, however, is not spread on the wall by the rotor while flowing downward by gravitation as in the thin film evaporators; rather it is forced to the reactor wall by the rotor against a back pressure created on the liquid outlet. Figure 3 shows a schematic representation of the liquid layer formation on the reactor wall. The liquid phase (S) is confined to the gap between the rotor blades (J) and the cooling jacket (0). I t rotates around the circumference of the cooling jacket wall while b e i i vigorously mixed. The liquid is continuously fed (V, in Figure 2) and withdrawn from the reactor (I). The
reactor is therefore operated as a continuous-flow backmixed devioe as oppoeed to the plug flow of the thin-film evaporators. (See Table I for a list of the symbols in Figures-1-4.) The reactor was fabricated from type 304 stainless steel. The bearing a t the bottom of the rotor (E) and the sealing at the top (D)were made from graphite. The reactor was composed from a shell (N) which also served as a cooling jacket, top and bottom cover plates (U),a rotating a w m bly-rotor (C). and baffles (M). The reactor shell was built from double walls. The outside wall was '/,in. thick and the inside wall was only '/a in. thick for achieving sufficient heat transfer through the wall. Coolant was pumped (P) through the space between the walls. in. Semicircular grooves '/a in. wide (R)were cut deep into the inside wall of the cooling jacket. The grooves were oriented diagonally from the bottom to the top of the jacket while turning half the circumference. They were separated by 1 / 2 in. from one another and were cut in a criss-crow fashion. The grooves were added to the wall in order to facilitate liquid mixing in the vertical direction. The rotor (C) was cylindrically shaped with top and bottom plates connected to one another by 24 blades (J). The blades were fixed facing toward the cooling jacket a t
Ind. Eng. Chem. Fundam.. Vol. 23. No. 2. 1984 245
an angle of 20' to the tangent. The rotor was mounted on an adjustable and hollow stationary post (L) inside the reactor. It was driven hy a speed-controlled motor through the upper shaft. Baffles were mounted on the bottom post (L) inside the rotor to promote mixing in the bulk gas phase. The rotor blades were 2 in. long and the rotor diameter was 4.5 in. Above a minimum rotational speed the rotor caused a liquid layer to form on the cooling jacket wall. The centrifugal force of the circulating liquid together with the radial momentum acquired from the rotor blades held the liquid layer to the wall and confined it to the space between the rotor blades (J)and the cooling jacket (0). The gas phase filled the rest of the reactor, occupying mainly the inner space of the rotor. A thin sheet of stainless steel sliding on a graphite disk served as a seal (A) to prevent the liquid from overflowing into the rotor. The liquid was fed into the reactor through a 2 in. long groove in the shell (V) and exited the reactor through an identical groove on the opposite (I) side into an overflow vessel (H). The overflow vessel sewed to create a small back pressure (about 4 in. of water) on the exiting liquid. The higher the back pressure, the faster the rotor must be turned in order to form the desired liquid layer. A short tube (Q) (16-gauge) leading directly from the liquid layer was used for quick liquid sampling. Bubbles entrained in the discharged liquid (I) were collected in the overflow vessel and combined ( G ) with the excess gas flow (F). The gas entered through the lower shaft (L) and left through the top plate. A small flow of gas was forced through the bottom plate (K)to prevent the liquid from penetrating under the rotor. The gas phase was mixed by the baffles (M)which prevented the gas from turning with the rotor, while producing flow currents between the rotor blades. The high absorption rate of the gaseous reactant into the liquid was achieved by the unique formation and stirring of the liquid phase. The rotor blades were in contact with the liquid only at the interface, as illustrated schematically in Figure 3. The strong shear forces, in addition to the actual slicing of the interface by the blades, provided a very high surface renewal rate and consequently high mass transfer rate. In preliminary tests, a clear plexiglass shell was installed in the reactor for visual studies of the liquid layer formation using water as the liquid phase. Observation with a stroboscope showed the formation of a small wave in front of each blade. Taylor vortices, marked by the concentration of small gas bubbles in the center of the vortices, were observed. These vortices promoted mixing of the liquid phase in the vertical direction beyond that which was caused by the grooves in the cooling jacket wall. The reactor was designed to operate at temperatures up to 300 " C and pressures in the range of 0.5 to 5 atm. The rotor speed was limited by the motor limit specification, which was 2500 rpm. The reaction between the gaseous and liquid reactants was assumed to take place only in the liquid phase which was shown in teats described below to be well-mixed. The reaction rate was therefore calculated from the following equation.
The liquid phase volume was measured independently, as explained later, for any specific rotor speed and liquid phase flow. The gaseous reactant consumption was directly measured as the difference between the inlet and outlet flow rates. The liquid phase concentrations Cth and
Figure 4. The experimental apparatus. Letters me defined in Table I.
CtoUtwere obtained by direct analysis of the feed and effluent solutions, respectively. The technique of creating a thin film of liquid on a heating jacket has been reviewed elsewhere (Mutzenburg et al., 1965; Penney and Bell, 1967). It has been applied principally to evaporators and concentrators. Very little information can be found in the literature about its application for the absorption of gases or as a reactor. Reactors of the rotator-type which belong to the thin-film class have proved suitable for continuous gas-liquid processes which require the removal of a large amount of heat and which involve highly viscous products (Kulov and Malyusov, 1966; Brostrom, 1975). Experimental System The complete experimental assembly is shown in Figure 4. A prepared liquid solution (NN) was transported by a calibrated positive displacement pump (UU)through a rotameter (11) into the reactor. The discharged liquid was collected in a receiving vessel (RR)which was kept under the same pressure as the reador. The liquid was released from the receiving v e l by a level control valve (QQ). The gas feed flow rate was measured by a rotameter (HH). The unreacted gas collected from the reactor and the overflow vessel was passed through a condenser (FF) to remove organic vapors and measured in a rotameter (GG)identical with the gas feed rotameter. The pressure in the system was controlled by keeping a back pressure on the gas outlet. For operation below atmospheric pressure the liquid and gas outlets were kept under vacuum. The reactor was cooled by pumping (SS) ice water through the reactor cooling jacket. A feedback control system was used to keep the liquid phase at a constant temperature. In this system the temperature of the liquid solution was measured at the liquid outlet (I, Figures 1 and 2), and a controller (AA) automatically adjusted the flow of ice water through the reactor to keep the desired temperature. Tracer Tests of Mixing and Liquid Volume Obviously, the volume of the liquid phase in the reactor depends on the thickness of the liquid film and the extent of the penetration of the liquid between the blades of the rotor. We expected the penetration to depend on the rotor speed, liquid flow rate, and the physical properties of the liquid. To find these dependencies, we carried out experiments in which the reactor response to an impulse input of tracer in the liquid feed was measured and analyzed mathematically. For a perfectly mixed vessel, the logarithm of the exit tracer concentration in response to
Ind. Eng. Chem. Fundam., Vol. 23, No. 2, 1984
246
35
-
30
+
- 2oL -E
m
>
0
I
I5
65
I ME
aec
IO
Figure 5. Experimental tracer response (a), and least-squares curve (b) for u / V = 0.1542 s-l (Le., for V = 32.45 cm3). 1
I
37
t
34
[ I
I
1 1800
I
T
I 2200
I
2000
1
]
3 58
I I
I 2
3
4
5
6
v (cm3/sec)
i
Figure 7. Liquid volume dependence on liquid feed rate.
i
reason for this was that gas bubbles which formed in the reacting liquid interfered with the measurement of the light absorbance. (The data points shown in Figure 7 for mixtures other than ink and water are discussed later.) The information in Figures 6 and 7 was used to obtain a value for V in eq 1 for studies of reaction rates. Tests of the micromixing in the liquid phase were conducted during studies of the oxidation of sodium sulfite. They are described in a later section. We note that the rotor assembly was designed to produce vigorous mixing also of the gas phase. No tests were conducted of this mixing, however, because a pure gas was used throughout this work.
1
2400
RPM
Figure 6. Liquid volume dependence on rotor speed for u = 5 cm3/s. Data points represent the average of 40 experimental runs.
this input should follow a straight line when plotted vs. time. The slope of the straight line is the reciprocal of the average liquid residence time, and hence it gives the liquid volume for a given liquid volumetric flow rate. In the first series of tracer experiments, 1 cm3 of radioactive H3P04was injected through a plastic tube directly into the reactor. The response was measured in the effluent liquid stream by means of a scintillation detector. An on-line digital computer was used to store the data and to calculate the least-squares slope of the aforementioned straight line. The responses verified that the liquid in the reactor was well-mixed on a macroscopic scale as shown by the comparison in Figure 5 of the acutal response and that which follows the least-squares curve for perfect mixing. Furthermore, we found that the response was not sensitive in any way to the location in the reactor of the tracer injection. The experimental data presented in Figure 6 relates the liquid volume to the rotor speed and shows that the volume decreased with increasing rotor speed as expected. At very high rotational speeds, the volume probably approaches a minimum value corresponding to the condition at which the liquid layer barely contacts the rotor blades and penetration between the blades does not occur. As Figure 6 indicates, the data from these tracer studies were scattered. In fact, we found it impossible to use these data for correlating the liquid volume to the liquid flow rate. For this reason, we conducted another series of tests in a manner similar to those described above except that an ink dye was used as the tracer. In these tests, the response was measured directly in the liquid phase in the reactor by a light absorption technique using a fiber optic device. The data from these experiments were much smoother, and led to the liquid volume correlations shown in Figure 7. One disadvantage of the ink tracer studies was that we found it necessary to use pure water as the liquid medium in place of the reacting mixture. The
Mass Transfer Study A chemical technique in which a gaseous reactant is absorbed into a solution where it undergoes a fast pseudo-first-order reaction was used in this work to measure a, the interfacial area per unit reactor volume, and k,, the mass transfer coefficient. This technique has been used in the past by many investigators to measure these same quantities in different types of reactors (Linek and Mayrhoferova, 1970; Charpentier and Laurent, 1974; Laurent and Charpentier, 1974; Robinson and Wilkie, 1974; Prasher, 1975; Beenacker and Van Swaaij, 1976; Spidharam and Sharma, 1976; Ganguli and VanDen Berg, 1978). The following analytical solution, obtained from penetration theory (Astarita, 1967),describes the chemical absorption in the fast-reaction regime where the absorbed gas is completely consumed before it can penetrate to the bulk solution.
Q, = AC*[D(S + k)]'I2
(2)
In this technique the interfacial parameters, the surface area, and the surface renewal rate, are calculated from a plot of the measured absorption rate, Qg,vs. the kinetic constant, k , at constant hydrodynamic conditions, that is, at constant values of S and A. The reaction rate constant k is most conveniently varied by adjusting the concentration of a homogeneous catalyst. The plot of Q,2 vs. k provides a straight line where (AC*)2D is the slope and (AC*)*DSis the intercept at k = 0. The interfacial area, A , and the surface renewal rate, S, can be calculated directly from the slope and intercept if values for the gas solubility and diffusivity in the liquid solution are available. The chemical absorption of O2 into a concentrated sodium sulfite solution, with eo2- as a homogeneous catalyst, was chosen for this study. The dependence of k on catalyst concentration and the liquid physical
Ind. Eng. Chem. Fundam., Vol. 23, No. 2, 1984
25
c
Ternperoture
[so31
:
247
30%
* 0.5g rnole/liler
1400
1600
1800 2000 2200 2400
RPM
Figure 9. Mass transfer rate versus rotor speed. Table 11. Mass Transfer Coefficients and Effective Interfacial Areas in Reported Studies of Gas-Liquid Reactors (Charpentier, 1978)
4
8
12
16
20
[ c o + ~xI 1 0 - ~ g( m o I e / I i t e r ) Figure 8. Effect of the catalyst concentration on the rate of oxygen consumption.
properties were taken from the work of Linek and Mayrhoferova (1970), who studied this reaction using a wetted-wall column. The values of this mass transfer coefficient, k,, is given simply by kL = (DS)'I2. Astarita (1967) gives conditions involving kL and a, which, when satisfied, provide assurance that reaction is in the kinetic regime. In that regime, a highly desirable one for studies of basic kinetics, mass transfer processes have no effect on the rate of chemical conversion, and the interface concentration is the same as the bulk. The oxidation of the sulfite ions to sulfate results in a drop in the pH as indicated by the following reaction.
co2+
S032-(weak acid) + 1/202
Sod2(strong acid)
In order to keep the pH level constant throughout the experiments, the conversion of SO?- was adjusted to 30% by manipulating the liquid flow rate. At this conversion, the pH dropped from 8.5 to 8.25 between the feed and effluent. The value of k corresponding to the latter value was used. A plot of the square of the oxygen consumption rate, measured as the difference between the inlet and outlet gas flow rate, vs. catalyst concentration is presented in Figure 8. The overall mass transfer rate was calculated from eq 2 by extrapolating the linear portion of the curves to zero catalyst concentration. At high catalyst concentration, the data appear to fit straight lines, but the lines deflect at high rpm or low catalyst concentration. The departure from the straight lines at high mass transfer rates (high rpm) or low reaction rates (low catalyst) is expected because under these conditions the absorbed gas penetrates into the liquid bulk. The expression in eq 2 of the chemical absorption in the fast reaction regime becomes invalid. Measurements of the mass transport properties that can be reached in the reactor were conducted below a rotor speed of 2200 rpm due to the difficulties in keeping the conversion below 30% at higher speeds. As shown in Figure 9, a clear trend of a continuous increase of the overall mass transfer rate with the rotor speed was found. The value of kLa shown for 2200 rpm in Figure 9 represents an improvement by a factor of thirty over that reported in other studies (Table 11). In the chemical kinetics studies described below, the reactor was operated with a rotor speed of 2500 rpm. An
type of reactor packed columns, countercurrent packed columns, cocurrent plate columns, bubble cap plate columns, sieve plates bubble columns packed bubble columns tube reactors, horizontal and coiled tube reactors, vertical spray columns agitated bubble reactors submersed and plunging jet hydrocyclone ejector reactor venturi Manor and Schmitz (this study)
m ax kL x 100, cm/s 2
6 5 20 4 4 10
5 1.5 4 0.5 30
-
10 53
max a,
cm-'
max k L a X 100, s- '
7
3.5
17 4 2 6 3 7 10 1 20 1.2 0.5 20 25 51
102 20 40 24 12 70
-
50 1.5 80 0.6 15
250 2700
extrapolation to this speed shows the mass transfer rate to be greater by a factor of about 50 over any previous design. As shown in Table I, high values of kLa were achieved in other types of reactors by providing a large effective interfacial area, such as in packed column reactors, or by having a short exposure time of the liquid to the gas phase, such as in jet reactors, resulting in a high mass transfer coefficient. Increasing only the interfacial area does not necessarily eliminate a mass transfer limitation, and providing a high mass transfer coefficient and a small contact area makes the reaction conversion difficult to detect. The reactor developed in this work provides both the large surface area and a high mass transfer coefficient as shown in Table I. Further, it is operated as a continuous-flow reactor, which makes it well suited for kinetic studies of chemical reactions. Micromixing of the liquid was tested during the oxidation of the sodium sulfite solution where the reaction was mass-transfer controlled and the oxygen absorption rate was proportional to the square root of the catalyst concentration. The catalyst was separately fed into the reactor where it was mixed with the sulfite solution. No oxygen absorption variations were observed due to a different location of the catalyst feed. This indicates strongly that the liquid phase mixing was faster than the gas absorption rate, and once the gas-liquid transfer limitation is overcome, gradientless operational conditions are reached. Kinetics of the Sulfite Oxidation Reaction Though the sulfite oxidation reaction has been studied and used extensively through recent decades, there is disagreement as to the kinetics of the reaction system
248
Ind. Eng. Chem. Fundam., Vol. 23, No. 2, 1984
---0.06 C
z 0 003 $7 PS X L
0:
0.04
-
(4
1
I
Temperature * 30°C [GO++] = 1 0 - 5 q m o l e / l i t e r
1
pH.85
y Iotm
A
4
LEa 1
; m s I-
- 0 7 3 7 otm
--
$zoo2t
-1
W E
G a D
L
g 0.02 '1,
L 0
1
02
01
SO^]
m 0
if 0.01
03 i
04
j
g m o l e / irter )
Figure 11. Effect of sulfite concentration on the oxidation rate at different oxygen pressures.
a I
C 0.2 0.4 0.6 0.8
-Li---L1
1.0 1.2 1.4 1.6 OXYGEN PRESSURE i o t r n )
1.8
Figure 10. Sulfite oxidation dependence on oxygen pressure.
(Astarita, 1967; Danckwerts, 1970). It seemed appropriate, therefore, to conduct a short kinetics study using our reactor to test the validity of the rate expression derived by Linek and Mayrhoferova (1970),who obtained data using a wetted column device under mass transfer control. The kinetic study of the sulfite oxidation reaction in our reactor was possible only at low catalyst concentrations where the reaction is slow enough to proceed in the kinetic regime. That regime was determined by the measured kLa values. A prepared sodium sulfite solution containing the catalyst at the desired pH of 8.5 was fed continuously into the reactor where the sulfite ions were oxidized. When the system reached a steady state, as determined from m8asurements of oxygen consumption and temperature, pH and temperature readings in the effluent were taken. The oxidation rate wm calculated from the oxygen consumption as the reading difference between the inlet and outlet rotameters and from the sulfite conversion. The system parameters, such as O2pressure and liquid flow rate or the solution concentration, were then changed and a new set of data was recorded. The sulfite concentration was measured by collecting a liquid sample of 20 cm3through the sampling port and adding it to an excess of a 0.1 N solution of iodine-iodide. The mixture was then back titrated with a 0.1 N solution of solium thiosulfate using starch as an indicator. The dependence of the sulfite oxidation reaction on the oxygen pressure was measured in the range of 0.6 to 1.75 atm. The reaction rate appears to obey a first-order dependence on the oxygen pressure over the entire range of operation as shown by the plotted data in Figure 10. Linek and Mayrhoferova (1970) found a sharp change from first to second order at a pressure below 0.625 atm, which is near the low end of the pressure range used in this study. Further, the data in Figure 11show the reaction to be zero order in the sulfite concentration, in agreement with the results of DeWall and Okeson (1966) and Linek and Mayrhoferova (1970). The reaction rate dependence on the pH was measured by oxidizing separately prepared solutions with different pH levels between 7.75 to 8.75 and keeping the sulfite conversion constant. The rate decreased sharply at the lower pH values, but the range tested was too narrow for the specification of a form of pH dependence. During the oxygen dependence test, the pH dropped to different values depending on the sulfite conversion. The reaction rate was corrected to a pH base of 8.5 from the
independent test of the pH effect on the reaction rate. Measurements of the rate dependence on the catalyst concentration were limited by the presence of other ions in the solution whkh have been reported to catalyze the sulfite oxidation. A concentration order of g-mol/L for copper and iron was measured in the solution that we used. Although the reaction hardly proceeded without the cobalt catalyst and adding small amounts of cobalt substantially enhanced the reaction rate, the combined effect of copper, iron, and cobalt was not obvious. The catalyst concentration could not be increased above 2 X gmol/L where the reaction became too fast to be operated in the kinetic regime. The catalyst concentration used in the mass transfer study was about 10 times larger than that of the iron and copper. Therefore, only a small interference was expected. Oxidation of Propionaldehyde Many kinetic studies have been performed on the oxidation of low aliphatic aldehydes. The high rate of the catalyzed oxidation reaction at moderate conditions forced researchers to conduct experiments under mass transfer rate limitations (Ladhabhoy and Sharma, 1969; Hobbs et al., 1972; Gurumurthy and Govindarao, 1974). Others preferred to reduce the reactant concentration in order to slow the reaction rate (Bawn and Williamson, 1951; Bawn and Jolley, 1956; Carpenter, 1965; Venugopal et al., 1967; Hendricks, 1978). Our reactor facilitates the study of the aldehyde oxidation reaction in the kinetic regime a t high catalyst and aldehyde concentrations. The oxidation reaction of propionaldehyde was chosen to demonstrate this capability. The oxidation reaction of the propionaldehyde is generally assumed :o follow a chain mechanism where the only primary molecular product is the corresponding peroxy acid. The formation of acid as a final product takes place by oxidation of the aldehyde by the peroxy acid-the reaction of Baeyer and Villiger (1899). Free radicals can be formed in many ways. For the catalyzed reactions, the following initiation steps have been proposed (Gurumurthy, 1973; Gurumurthy and Govidarao, 1974; Hendricks et a]., 1978)
-
Mn3+ + RCHO Mn3+ + RCOOOH Mn2++ RCOOOH
-
Mn2++ RCO.
+ H+
-+
[Mn3+- RCOOOH] Mn2++ RC000.
Mn3+ + RCOO.
Mn3++ [intermediate peroxide]
(R = CH,CH,-)
+ OH-
-
(I)
H+ (11) (111)
radicals (IV)
Ind. Eng. Chem. Fundam., Vol. 23, No. 2, 1984 249
Reaction I is considered to be the main source of radicals at low catalyst and peroxides concentration, while reaction I11 is fast and returns the catalyst to its high oxidation state. The decomposition of peroxides in reactions I1 and IV may form significant amounts of radicals (Spidharam and Sharma, 1976). Propagation takes place in two steps RCO. RC000.
+02
+ RCHO
+
+
RC000.
RCOOOH
(V)
+ RCO.
(VI)
where reaction VI is considered to be sluwer than the radical combination with oxygen (reaction V). Three possible termination reactions can take place between the two radicals, RCOO., RCO. (Gurumurthy and Govindarao, 1974; Hendriks et a1.,1978)
+ RCO. R C 0 0 0 . + RCO. RC000. + RC000. RCO.
(VII)
4
(VIII)
+
+
(IX)
The catalyst may also undergo termination reaction with radicals (Hendriks et al., 1978) RC000. RCO.
+ Mn2+
-
RC000-
+ Mn3+
+ Mn3+H,O- RCOOH + Mn2++ H+
(X) (XI)
The peroxy acid molecule formed in reaction 6 oxidizes another aldehyde molecule in a two-step reaction to form two acid molecules. Bawn and Williamson (1951) showed the first step to be very fast and to reach equilibrium. RCOOOH
+ RCHO 2 intermediate peroxide
(XII)
It is followed by the catalyzed decomposition of the intermediate peroxide to the acid product and radicals as shown below. intermediate peroxide
8
(XIII)
ZRCOOH radicals
(reaction Iv)
Experimental Section Kinetic data were collected for the oxidation of propionaldehyde to perpropionic acid and its subsequent decomposition to the propionic acid. The controlled parameters in the experiments were the oxygen pressure, reactor temperature, catalyst concentration, and the concentration of propionaldehyde in acetic acid in the feed. Response measurements were made of the oxygen uptake and concentrations of propionaldehyde, propionic acid, and peroxides in the liquid effluent. All experiments were conducted with pure oxygen as the gas feed component. The oxygen partial pressure in the reactor was corrected for the presence of liquid vapors in the gas phase which were assumed to be in equilibrium with the liquid phase. (The equilibrium concentration was measured separately in an inert atmosphere by use of a gas chromatograph.) This assumption was justified because the gas effluent rate was kept very low with a minimum removal of liquid vapors from the reactor. Pre-prepared solutions of propionaldehyde in the range of 1 2 to 60% by weight and catalyst manganese acetate gmol/L in acetic acid were allowed to stand of 5 x after preparation for a few hours until all the manganic ions returned to the lower oxidation state, Mn2+,and the solutions became clear. Eecause the solutions were fed into the reactor with the manganese ions in the lower oxidation state, a special start-up procedure was required. The re-
actor was filled with solution and then started without liquid flow and with a large excess of oxygen flow. In less than a minute, the oxygen consumption sharply increased, indicating the beginning of the oxidation reaction. The liquid flow was then started. The reaction rate was calculated from the oxygen consumption and compared to the rate calculated from the aldehyde conversion to give a difference of less than 6%. Liquid samples were taken from each run for the analysis of the aldehyde and peroxides concentration. Separation of the three major liquid phase components, propionaldehyde, propionic acid, and acetic acid, was accomplished with a gas chromatograph using a 9-ft glass column filled with a polar liquid phase (10% AT-1200 1%H3P04on Chrom. W-AW 80/100, Alltech Association). Since the peroxides in the liquid phase continued to decompose after leaving the reactor, a very fast sampling and analysis technique was devised to measure their concentrations. An exact time for collecting a 5-cm3sample directly from a constant flow at the sampling port was measured. A 5-cm3sample was then collected directly into a 50-cm3 solution of acetic acid containing iodine. The peroxides reacted immediately with the iodine, rather than the aldehyde. The solution was titrated after 15 min with sodium thiosulfate (Carpenter, 19fi5). The time required for the liquid to reach the iodine solution was about 1.3 s, which permitted a maximum peroxides conversion of 10% at the reactor temperature. The sampled liquid was cooled immediately to 0 "C within the sampling port in order to decrease the rate of the peroxides decomposition. Therefore, the peroxides conversion and the resulting error were kept below 10%.
+
Results a n d Discussion In the kinetics study of the propionaldehyde solution, the oxygen absorption rate was measured for each reactant and catalyst concentration at different liquid flow rates. It was observed that the oxyge.1absorption rate increased with higher liquid flow rate more than expected from the liquid phase volume change as measured with the dye tracer. This difference was attributed to the different properties of the propionaldehyde solution from the water employed with tracer studies, in particular the tendency to form more gas bubbles a t low liquid flow rates and therefore reduce the effective volume. The actual liquid phase volume was therefore corrected according to the observed oxygen absorption rate with liquid flow rate. The corrected curve so obtained is shown in Figure 7. Manganese acetate (Mn(C2H302)2-2H20) was used in this study as the catalyst. The reaction rate was found to be strongly dependent on the catalyst in the low concentration range up to lo4 g-mol/L. At higher concentrations, the increased rate of the oxidation declined up to 5 X lo4 g-mol/L of the catalyst where the reaction reached a limiting rate as shown in Figure 12. The kinetic study was then pursued in the range of zero-order dependence on the catalyst. The reaction rqte dependence on the oxygen pressure was investigated in the range of 1 to 1.7 atm. The results are plotted in Figure 13. The reaction appears to show a first-order dependence on the oxygen pressure although the straight line fitted to the experimental points does not exactly cross the origin. The reaction dependence on the propionaldehyde concentration in acetic acid solution was measurcd in the range of 12 to 60% by weight and at 1atm of oxygen. The results are plotted in Figure 14. The experimental points fall on a straight line, the slope of which appears to be independent of the catalyst concentration. The intersection point of the extrapolated lines and the zero aldehyde
250
Ind. Eng. Chem. Fundam., Vol. 23,No. 2 , 1984 I
'
I
i
C
_-L--i 31 32 33 34 IOOO/T ( O K - ' )
Lu
c c: r
35
Figure 15. Arrhenius plot for propionaldehyde oxidation. Temperature
25OC
0 2 gmole/liter
P
Figure 12. Catalyst effect on oxygen consumption in propionaldehyde oxidation at 25 "C, 1 atm, with 20 wt % propionaldehyde in the feed.
5
a - [Peroxides; =
0107 g m o l e / l i t e r
$006
2
E
6
4
-
I 0 1 2
yMnt3] X , O ~ (grnoie~iter)
Figure 16. Dependence of peroxides oxidation on catalyst concentration with 16 wt 70aldehyde in the feed.
0 i-
f 0041
2
1
,
50 0 0 3 5w
~
Temperature
'
1
i
Mni3
004k
002c
I
,
25OC =
(grnole/liler:~
Figure 13. Effect of oxygen pressure on the oxidation of propionaldehyde with 20 wt 70propionaldehyde in the feed. a
001
-
(r
I-
~
-1-
i-
J
01 02 03 PEROXIDES CONCENTRATION i g mole / liter)
Figure 17. Peroxides oxidation dependence on peroxides concentration with 16 wt 70propionaldehyde.
kb, the propionaldehyde oxidation was found to be slightly sensitive to the temperature.
!L-
Temperature = 2 5 %
0 W
+
n
003-
[02] =
I
atm
J
1 L _i_-IO
30 40 50 f i t % PROP'ONAL3EhYDE 20
60
Figure 14. The effect of propionaldehyde concentration.
concentration axis increased with the catalyst concentration up to the limiting rate. The oxidation rate of the propionaldehyde can therefore be represented by the equation
The reaction dependence on the temperature was measured in the range of 15 to 35 "C. The results are given in Figure 15 as Arrhenius plots for the rate constants k , and kh. As shown by the following expressions for k , and
k , = 0.474e-EalRT kb = 0.29eTEbIRT(wt % propionaldehyde x s)-l
(44 (4b)
where E, = 1.44 kcal/g-mol
Eb = 1.62 kcal/g-mol The peroxides decomposition rate was calculated as the difference between the propionaldehyde oxidation rate, translated to peroxides formation, and the measured amount of peroxides contained in the liquid effluent. The decomposition rates taken from separate experiments of the same peroxide concentration and different catalyst concentration are plotted vs. the catalyst concentration in Figure 16 and appear to obey a first-order
Ind. Eng. Chem. Fundam., Vol. 23, No. 2, 1984 0.041
I
E
I
1 31
I
1
I
I
32 33 34 IOOO/T (OK-')
I
J
35
Figure 18. Arrhenius plot for peroxides oxidation.
reaction. At low peroxides concentration, the decomposition rate was first order in its concentration while, toward higher concentrations, the rate reached a limit as shown in Figure 17. The peroxides decomposition rate depends also on the propionaldehydeconcentration with a tendency to decrease with an increase of the propionaldehyde concentration. This behavior was observed before and was attributed to the solvent effect (Kagan and Lyubarskii, 1935). The decomposition rate at a constant concentration of the acetic acid solvent can be expressed as (5) rateperoxides decomp = kc[Mnl [peroxides] with a limiting rate at high peroxides concentration ratemax= kd[Mn] (6) The activation energy measured for the decomposition of the peroxides (Figure 18) was 10.7 kcal/g-mol. The decomposition rate constants, k, and kd, are given by
k, = 1.94 X 10'O exp(-E,/RT) L/g-mol s kd
= 3.38 x
io9 exp(-Ed/RT)
s-'
(7a)
(7b)
where
E , = E d = 10.6 kcal/g-mol Gurumurthy (1973) measured the rate of the perpropionic acid decomposition in the homogeneous phase by rapidly mixing a preparared peroxide solution with another solution containing the catalyst. He found the activation energy for this reaction to be 10.2 kcal/g-mol. The oxidation of aliphatic and aromatic aldehydes is well established to be a free radical chain reaction (Denisov et al., 1977). At moderate conditions, many studies have demonstrated the reaction rate to be zero order in oxygen and to obey a rate equation of the form rateoridation = k[RCH0I3/' (cat.)'/2
At high catalyst concentration, on the other hand, the agreement is less obvious although a general trend toward zero-order dependence on the catalyst and a varying dependence on the oxygen concentration emerges (Venugopal et al., 1967; Denisov et al., 1977). A possible distinction can be made between the reaction studies conducted at moderate conditions and those at high oxygen absorption rates. The former include low concentrations of catalyst (Marta et al., 1968),aldehyde (Bawn and Jolley, 19561, and peroxides (Gurmurthy and Govindarao, 1974), and oxygen dispersion intensity; the latter require high catalyst (Marta et al., 1968) or aldehyde (Venugopal et al., 1967) concentration and may result in high peroxides accumulation. The oxidation of propionaldehyde in our study is distinguished by the very fast reaction rate achieved in our reactor. The reaction was studied at high catalyst and aldehyde concentrations in addition to the high peroxides
251
concentration of 0.1 to 0.3 g-mol/L. Gurumurthy and Govindarao (1974) found only traces of peroxides at the same aldehyde and catalyst concentrations. The high peroxides concentration drives the manganese to the higher oxidation state and therefore increases the role of reaction XI in the termination. A t high peroxides concentration, the peroxide decomposition was found to depend on the catalyst concentration, but not on the peroxide concentration. The same dependence is assumed for the radicals formation in reaction IV. Radicals formed in reaction I and as a byproduct from the peroxides decomposition in reaction IV initiate the propagation reactions V and VI. If the termination reaction VI is applied, a rate expression which agrees with the form of eq 3 is obtained for the oxidation of propionaldehyde. The decomposition of the peroxides has been reported previously to be first order in the catalyst and peroxides (Bawn and Williamson, 1951). The shift to zero-order dependence at high peroxides concentration in our study may be explained by the formation of an intermediate complex of the Mn3+ with the peroxides (Gurumurthy, 1973). A t a high peroxides concentration, the amount of the complex depends only on the availability of Mn3+,and the decomposition becomes insensitive to the peroxides. The wide range of kinetic data found in the literature for the oxidation of aldehyde suggests the many possible combinations of initiation and termination reactions that may control the overall oxidation rate. In the fast-reaction regime the oxidation reaction proceeds in the small volume of the interface while peroxides decompose in the liquid bulk. Because of the large ratio of the liquid bulk volume to the interface volumes, peroxides hardly accumulate and their effect remains unnoticed. In the kinetic regime, the contribution of peroxides to initiation and termination may conceal other possible processes. The importance of these reactions on the overall oxidation rate and their dependence on the experimental conditions are probably the source of the contradicting results found in the literature. The present reactor facilitates the study of this reaction at conditions not available before and may contribute to a better understanding of its kinetics.
Conclusions The reactor described in this work achieved an interfacial mass transfer rate at least 30 times that reported previously for other types of reactors. The sharp increase in the mass transfer rate is the result of directly mixing the liquid near the gas-liquid interface by mechanical means as opposed to other mechanically agitated reactors where the mixing is provided through the liquid bulk. The reactor is of the gradientless type which makes it suitable for studies of chemical reaction kinetics. It also provides for adequate heat removal so that isothermal operation can be maintained over a wide range of conditions. A kinetic study of a highly exothermic reaction was conducted for the demonstration of the reactor's capability. The oxidation of propionaldehyde in acetic acid with high catalyst concentration was studied at rates 100 times faster than other studies which used different reactor designs. As a result of the high oxidation rate, the peroxides, which are the direct product of the propionaldehyde oxidation, accumulated to a high concentration where they began to play a major role in the initiation and termination of the chain oxidation reaction of the propionaldehyde. The reactor of the type tested here is well-suited for studies of the kinetics of fast gas-liquid reactions. With some modification, such as a catalytic wall, it could be employed for laboratory studies of gas-liquid-solid reactions.
Ind. Eng. Chem. Fundam. 1984, 23, 252-256
252
Acknowledgment This work received financial support from the Petroleum Research Fund (Grant No. 11309-AC7).
Nomenclature a = interfacial area per unit volume of liquid phase, A / V, cm-'
A = interfacial area, cm2 Ce = concentration of liquid reactant, g-mol/L
C* = equilibrium concentration of dissolved gas in liquid, g-mol/L D = diffusion coefficient in the liquid, cm2/s E,, Eb = activation energies in eq 4 E,, Ed = activation energies in eq 7 k = reaction rate constant, s-l k,, kb = kinetic constants in eq 3 and 4 k,, kd = kinetic constants in eq 5, 6, and 7 k L = mass transfer coefficient in the liquid phase, (DS)1/2, cm/s Q, = rate of gas absorption, g-mol/s S = surface renewal rate, s-l V = volume of the liquid phase in the reactor, cm3 u = liquid flow rate, cm3/s Registry No. Propionaldehyde, 123-38-6. Literature Cited Astarita, G. "Mass Transfer with Chemical Reaction"; Elsevier: New York, 1967. Baeyer, A,; Villiger, V. Ber. 1899, 3 2 8 , 3625.
Bawn, C. E. H.; Wllliamson, J. B. Trans. Faraday SOC. 1951, 4 7 , 721. Bawn, C. E. H.; Jolley, J. B. R o c . R . SOC.London, Ser. A 1956, 327, 296. Beenacker, A. A.; Van Swaaij, W. P. M. R o c . Eur. Chem. Eng. Symp. Hiedelberg, 1976. Brostrom, A. Trans. Inst. Chem. Eng. 1975, 53, 26. Carpenter, B. Ind. Eng. Chem. Process Des. Dev. 1985, 4 , 105. Charpentier, J. C.; Laurent, A. AIChE J. 1974, 2 0 , 1029. Charpentier, J. C. Chem. Reactlon Eng. Rev. Houston 1978, 223. Danckwerts, P. V. "Gas-Liquid Reactions"; McGraw-Hill: New York, 1970. Denisov, E. T.; Mitskevlch, N. F.; Agabekov, V. F. "Liquid Phase Oxidation of Oxygen Containing Compounds"; Consult. Bureau: New York, 1977. DeWall, K. J. A.; Okeson, J. C. Chem. Eng. Sci. 1966, 21, 559. Ganguli, K. L.; Van Den Berg, H. Chem. Eng. Sci. 1978, 3 3 , 27. Gurumurthy, C. V. J. Appi. 8iotechnol. 1973, 2 3 , 769. Gurumurthy, C. V.; Govindarao, V. M. H. Ind. Eng. Chem. Fundam. 1974, 73,9. Hendriks, C. F.; Hendrik, C. A.; Heertjes. P. M. Ind. Eng. Chem. Prod. Res. Dev. 1978, 1 7 , 261. Hobbs, C. C.; Drew, E. H.; Van7 Hof, H. A,; Mesich, F. G.; Onore. M. J. Ind. Eng. Chem. Prod. Res. Dev. 1972, 1 7 , 220. Kagan, M. I.; Lyubarskii, G. D. Zh. Fir. Khim. 1935, 6 , 536. Kulov, N. N.; Malyusov, V. A. Doki. Akad. Nauk SSSR 1966, 177(6), 1288. Ladhabhoy, M. E.; Sharma, M. M. J. Appl. Chem. 1989, 19, 267. Laurent, A.; Charpentier, J. C. J. Chim. Phys. 1974, 77, 613. Llnek, V.; Mayrhoferova, J. Chem. Eng. SCi. 1970, 25. 787. Marta, F.; Boga. E.; Matok, M. Discuss. Faraday SOC. 1988, 46, 173. Mutzenburg, A. 6.; Parker, N.; Fischer, R. Chem. Eng. Sept 13, 1965, 175. Penney, W. R.; Bell, K. J. Ind. Eng. Chem. 1987, 59(4), 40. Prasher, B. D. AIChE J. 1975, 27,407. Robinson, C. W.; Wilke, C. R . AIChE J . 1874, 2 0 , 285. Spidharam, K.; Sharma, M. M. Chem. Eng. Sci. 1976, 31, 767. Venugopal, E.; Kumar, R.; Kuloor, N. R. Ind. Eng. Chem. Process Des. Dev, 1967, 6 , 139.
Received for review January 10, 1983 Accepted November 22, 1983
On-Line Analysis of Fischer-Tropsch Synthesis Products Ronald A. Dlctor and Alexis T. Bell' Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Department of Chemical Engineering, University of California, Berkeley, California 94720
A gas chromatographic system has been developed for the rapid, on-line analysis of products produced during Fischer-Tropsch synthesis. The system utilizes a single chromatograph fitted with two columns. A packed column containing Chromosorb 106 is used to separate C,-C5 hydrocarbons, low molecular weight oxygenated compounds, C02 and H20. All C5+ products are separated using a capillary column coated with OV-101. Complete analysis of products containing up to 32 carbon atoms can be achieved in 2.5 h.
Introduction Interest in Fischer-Tropsch synthesis for the production of liquid fuels and chemicals has motivated the development of improved methods for product analysis. Primary attention has focused on gas chromatography, which in some instances has been combined with other analytical methods such as mass spectrometry. In most investigations, the unconsumed reactants and noncondensable products (e.g., C1-CB hydrocarbon, low molecular weight oxygenated compounds, and COP) are analyzed on-line, whereas the condensable products (e.g., C6+hydrocarbons and oxygenated compounds, and HzO) are collected, separated into an organic and aqueous phase, and then analyzed off-line (Bauer and Dyer, 1982; Deckwer et al., 1982; Huff et al., 1982; Feimer et al., 1981; Atwood and Bennett, 1979). This approach, while capable of providing good product identification, suffers from a number of disadvantages. The principal ones are the following: (1) Quantitation: Accurate volumetric measurements of each phase must be made. Components usually appear in more 0 196-43 1318411023-0252$01.50f 0
than one fraction (or phase), and fractions must be added to yield the full product spectrum. (2) Handling: Problems of vaporization and oxidation of products during product storage and handling can be severe (Huff et al., 1982). It is also possible that reactions may occur in the condensate (e.g., hydrolysis of esters to acids and alcohols). (3) Conversion: Reactors must be run at high conversion in order to accumulate sufficient amounts of condensate in short periods of time (typically, 5 to 10 h). Low conversion studies, ideal for determining kinetics, are impractical. (4) Analysis turnaround: One complete analysis necessarily involves many hours of collection plus analysis time. This makes it difficult to follow the dynamics of catalyst deactivation or changes in reaction conditions. Many of the difficulties cited can be overcome by complete on-line analysis of all products, and several groups have reported on efforts to develop chromatographic systems for this purpose. Everson et al. (1978) used a column packed with Chromosorb 102 to analyze the complete product spectrum. The mole fractions of CH,, CO, G 1984 American Chemical Society
