An Annular Center-Line Light Source Photochemical Reactor. Design Equations and Experimental Results Haya Atlas, David Hasson, Mordecai B. Rubin, and William Resnick' Department of Chemical Engineering, Israel Institute of Technology, Haifa, Israel
The photochemical reaction between 9,lO-phenanthrenequinoneand 1,4-dioxane, in which the product species as well as the reactant absorb light in the wavelength range of the light source, was studied theoretically and experimentally for an annular reactor with a polychromatic center-line light source. The reaction rate expressions developed for this case of reactant and product absorption predict conversions as a function of time similar to those obtained experimentally.The experimental results were characterized by a high, constant initial reaction rate followed by a falling rate which reached zero at high conversions. Deviations between the experimental and predicted results can be explained by the hydrodynamic regime in the reactor and the presence of low concentrations of air or water in the system.
Introduction Photochemical reactions are attractive for a number of reasons. Among these reasons are the specificity of the reaction, the possibility of performing reactions whose equilibrium yields would be low if activated by thermal means and, in some cases, the absence of side reactions. Although photochemical reactions are usually studied with a monochromatic light source (Calvert and Pitts, 1966), industrial reactors would, in all probability, employ a polychromatic source. If monochromatic light would be required it would be possible to employ an appropriate filter; otherwise the polychromatic light source could be used as such. I t would then be necessary, for reactor design purposes, to calculate the individual contributions of each wavelength to the reaction. Among the research in which polychromatic light was employed, the work by Huff and Walker (1962), Gaertner and Kent (1958), and Boval and Smith (1973) can be mentioned. The gas-phase chlorination of chloroform was studied by Huff and Walker in a tubular reactor with an external light source. The light source was located a t one focus of an elliptical reflector with the tubular reactor a t the other. Reaction kinetics as a function of flow rate, concentration, light source, and reactor diameter and length were studied. A summation procedure was used to calculate the radiation absorbed. The intensity of radiation that impinged on the reactor wall was determined by employing the photochemical reduction of oxalic acid as an actinometer. The latter reaction was also used by Boval and Smith for actinometric purposes. Gaertner and Kent studied the photolysis of oxalic acid in an annular flow reactor with an external polychromatic light source. They determined the conversion as a function of velocity and of size of the annular chamber. An average attenuation coefficient based on the lamp characteristics was calculated in order to obtain an expression involving the conversion. They found a linear relationship between irradiation time and conversion for low conversions. Boval and Smith studied the kinetics of the photolysis and oxidation of 2,4-dichlorophenoxy acetic acid in three types of reactor operation: flow-differential, flow-integral, and batch-recycle. They used an annular reactor with a polychromatic lamp located on the axis of the annulus. They found that the initial reaction rate was a function of the light absorbed and independent of the reactant concentration at the low concentration levels investigated. Radiation intensity a t any particular point in a photo68
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. I , 1976
chemical reactor is a function of the distance from the light source and the absorbing characteristics of the medium. Concentration gradients in the medium can result in mixing and diffusion effects which can, in turn, affect the light intensity decay characteristics. Criteria for evaluating these effects were studied in a number of researches. Hill and Felder (1965) studied three types of flow reactors: tubular with external radiation source, flat plate reactor irradiated from one side, and an annular reactor with internal radiation. The case studied involved chain-reaction kinetics with the stationary-state hypothesis. One of their important conclusions was that the concentration of the active radical behaved as ( R * ) N M I( R * ) L M
5
(R*)CM
where NM refers to the non-mixed case, LM to the lateral mixing case, and CM to the completely mixed situation. Shah and Felder (1971) studied, by theoretical considerations, a photosensitized reaction in an annular-flow reactor with a center-line monochromatic light source. They analyzed models that assumed plug-flow with perfect radial mixing, no-mixing, and dispersion and determined the criteria for the regions of validity of one or more of the models. The parameters studied, in the form of dimensionless groups, were optical thickness, the ratio of the mean residence time and a characteristic reaction time, and the ratio of the radii of the annulus. Felder and Hill (1969) calculated fractional conversion for continuous tubular and batch reactors. They found that when radial mixing in a tubular reactor does have an effect, it is to enhance the fractional conversion. Also, for the batch reactor, mixing either increases the fractional conversion a t a given time or else has no effect. Shendelman and Hill (1971) also studied mixing and dispersion effects in bleachable and photosensitized reactions. They concluded that mixing effects in bleachable systems were smaller than in photosensitized reactions. A convenient industrial technique for carrying out a photochemical reaction would be an annular reactor with a fluorescent lamp as the center-line light source. By appropriate choice of phosphor, lamps which emit a large proportion of their output in the near-ultraviolet region are readily produced. They have a relatively constant output over useful lives in the 1000-hr range and are reasonably cheap. In this work models were derived to describe this type of reactor-an annular reactor with a polychromatic centerline light source. The system chosen for the experimental
L\
i
of the particular light source employed. The volumetric light absorption rate will, therefore, be a function of the light source, of the absorbing species present, and of their absorption characteristics as well as of the reactor geometry. The equations for the monochromatic case can now be extended to the case of a polychromatic light source by appropriate averaging of the parameters that vary with wavelength.
\\
1 1 c
r1
I , = CUCZ, - exp[-Ec(r - rl)] r
\
I
(4)
I
0.0
8
8
350
1
1
'
L
'
'
'
'
where
'
La)
453
500
WAVE LENGTH, nm
Figure 1. Absorption curves for 9,lO-phenanthrenequinone and adduct.
study was the reaction of 9,lO-phenanthrenequinone(PQ) with 1,4-dioxane (solvent) to give the 1:l adduct
I,
=c I w x A
The summation on A is over the absorption range of the chemical species involved and/or the radiation range of the lamp. This averaging would be suitable for the case where is essentially constant; otherwise the reaction rate fpr a polychromatic light source could be written as
solvent 0
PQ 1:l adduct
(5)
This reaction has been studied previously with a monochromatic light source by Rubin (1963, 1969) and Rubin and Zwitkowitz (1964) and is known to be a clean reaction with a quantum efficiency of unity in the absence of oxygen. Absorbance curves for PQ and adduct are shown in Figure 1. The solvent absorbance is nil in the wavelengths of interest. Polychromatic Light Absorption The situation prevailing for a photochemical reaction taking place with polychromatic light can be deduced by appropriate extension of the monochromatic case. For a photochemical reaction in a constant volume batch reactor with monochromatic light the reaction rate can be written as dc = -dxI,x dt where c refers to the concentration of reacting species and Zx, is the volumetric light absorption rate by the reacting species. The Beer-Lambert equation for light absorption in an annulus with a monochromatic center-line source can be easily derived for the case of radial light emission
where dAaXIwX cy($=
c
Iwx
It should be noted that eq 5 is approximate because the exponential term contains an averaged value for a. For the particular reaction studied in this work previous research using monochromatic light had shown that the quantum yield was unity over a wide range of wavelength (Rubin and Zwitkowitz). Development of Model Equations Batch Annular Reactor. The following basic assumptions were made: (1) isothermal reactor; (2) no dark reactions; (3) negligible reflected radiation; (4) constant physical properties; and ( 5 ) radial emission. The reaction rate can be expressed by D = $Za
and the rate of change of reactant concentration will be
(3)
with the initial and boundary conditions
Although Jacob and Dranoff (1970) have developed expres(r,O) = ci sions in which the radial emission assumption can be reac laxed, the small improvement in accuracy which would be - (t,r = r1,r2) = 0; D, # 0 ar obtained for the geometry studied experimentally in this work did not justify the additional mathematical complexiThe following dimensionless groups are defined ty. c =C I C i The attenuation coefficient, fib, as well as the molar absorptivity, a h , are characteristics of the absorbing species and their numerical values are dependent upon the wavelength as is the numerical value for the quantum yield, d ~ . Upon substitution of the dimensionless groups and I, In addition, the incident light intensity will vary with the wavelength and is a function of the emission characteristics eq 4
(sa) (6b) (74
from
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 1, 1976
69
aC
JC
at
P
- = avP2C(p,t)- - exp[-D(p
- k)C]
(8)
where
would represent the attenuation coefficient due to both absorbing species. The average molar absorptivity for the product species, 2, is calculated by eq 4a. Equation 9 can now, by inspection, be modified to account for light absorption by both species dE 1 _ dt
D 3 (Ycirz with initial and boundary conditions
(84
- (t,p = 1,k) = 0; a # 0
(8e)
aC aP
In order to calculate the overall conversion, eq 8 is integrated over the reactor cross-sectional area. I. Batch Annular Reactor with Perfect Radial Mixing. Preliminary measurements in the reactor system used in the present work indicated that a radial concentration gradient was not established and that the practical situation approached the case of perfect radial mixing. The diffusion term in eq 8 drops out with the assumption of perfect radial mixing and the surviving terms are then integrated.
-
sy
exp[-D(p
- k ) C ] dA
SdA SdA As a result of the assumption of perfect radial mixing c(P,t)lDp-- = C P M ( t ) E c(t) we can write -dC =
-
Jc
P
exp[-D(p
+ 6pi.p)
X
[exp[-(&lEl
C(P,O) = 1
dA =
2Zwr1cult.1 (rz2 - r12)(61E1
- k)C]p dp
s
(12)
From mole balance considerations for the particular reaction investigated in this work ci = c1
+
c2
and eq 12 becomes, upon elimination of cp dE 1 2Z,rliil~~ _ X dt (rZ2- r12)(62ci+ ~ . ~ ( C U I- 0 1 2 ) ) [exp[-(bzci
+ E1(&1 - iiz))(rp- r d ] - 11
(13) Equation 13 can be readily evaluated by numerical integration. 111. Plug Flow Reactor with Perfect Radial MixingOnly Reactant Absorbs Light. The following assumptions are made in addition to those assumed in the previous development: (1)negligible axial diffusion; (2)steady state; and (3) flat velocity and concentration profiles. As a result of these assumptions the flow reactor equations can be obtained immediately by replacing the reaction time in the batch case by the average residence time for the plug flow case. The average residence time for an axial distance of z will be
'=;
dB
+ &2~p)(r2- rl)] - 11
z
which, upon substitution into eq 9 yields
dt SSPdPd~ Integrating the right-hand side between the limits p = 1 and k yields
de_
2JL [exp[-DC(l - k)] - 11 (15) dZ - D ( l - k 2 ) a z/L. Equation 15 is integrated between the limwhere Z its Z = 0 to 2 = 1to yield the model equation 2JL 1 - exp(-a) (16) a 1 - exp(-ac) D ( l - k2)a IV. Plug Flow Reactor with Perfect Radial Mixing-. Reactant and Product Species Absorb Light. Equation 15 for the case of light absorption by both the reactant and product species will become, in dimensional form l - @ + - l1n
which, in turn, upon integration between the limits t = 0 to t yields the model equation 1
- In a
1 -exp(-a) 1 - exp(-aC)
- c + 1 =D ( l2J- k2)t
(10)
where
21,r
u
D(l- k)
(104
The average reduced concentration of the surviving reactant species can be obtained as a function of time from eq 10 and the fractional conversion of reactant to product will be (111. Batch Annular Reactor with Perfect Radial Mixing-Reactant and Product Species Absorb Light. Equation 10 will be valid for the case where only the reactant absorbs light and is converted to product in accordance with the quantum efficiency. In the event that both the reactant and product species absorb light, only light absorbed by the reactant will produce product while the light absorbed by the product cannot result in reaction. For the case where the system includes two absorbing species, eq 4 can be written rl 1,1 = Culcll, - [exp[-(rulcl + fizcz)(r - rl)J] (11) r where Z,1 would represent the light absorbed by reactant species 1, whereas (filcl + 6 2 ~ 2 )in the exponential term
e).
70
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 1, 1976
a
16 1 E l
di.1z - (r22 - r12)(61i.1+
X iipE2)
[exp[-(61F1
+ &2i.2)(r2- r d ] - 11
(17)
By mole balance consideration, as in the batch model, eq 17 can be put into its working form
Experimental Section Batch Reactor. The batch reactor consisted of two concentric horizontal Pyrex tubes with the reactant contained in the annular space. The inner diameter of the outer tube was 82 mm and the outer diameter of the inner tube was 52 mm, with 2.5 mm wall thickness. The volume of the annular reactor chamber was 3.6 l. A Philips Super Actinic fluorescent lamp was located along the center line of the inner pipe and cooling water was circulated through the inner an-
ABSOLUTE SPECTRAL ENERGY DISTRIBUTION FOR T L " A 14 W/oS')
-A
Figure 2. Absolute spectral energy distribution curve for light source.
Ill Figure 3. Schematic diagram of batch-recycle system: 1, annular reactor chamber; 2, fluorescent light source; 3, annular cooling water chamber; 4, inlet and outlet distributors. nulus between the lamp and inner reactor tube. Cooling water was circulated to maintain a temperature 43 f 2"C, the temperature at which the lamp manufacturer claimed maximum lamp efficiency is obtained. Although the total lamp length was 150 cm only 114 cm of lamp was exposed in the reactor itself. The spectral energy distribution curve for the lamp, as supplied by the manufacturer, is shown in Figure 2. The outer pipe was equipped with capillary glass sampling taps along its length and samples were removed by a hypodermic syringe. Samples were analyzed by spectrophotometric methods. The reaction quantum efficiency and product composition are affected by the presence of oxygen and the reactor was, therefore, purged with nitrogen prior to each run and a nitrogen blanket was maintained. Batch-Recycle System. The same reactor that was used in the batch experiments was also used for the batchrecycle system. The reactor was equipped with distributors at each end through which the reactant solution entered and left the reactor. Each distributor had a volume of approximately 7 1. The exit stream from the reactor entered a 33-1. mixing-recycle vessel equipped with temperature control to maintain 43°C. The volume of solution circulating in the system during a run was approximately 35.5 1. and it was recycled to the reactor through a rotameter by a stainless steel ECO gear pump. Although a continuous stream of nitrogen swept the mixing vessel the possibility exists that the free-board over the solution was not completely free of oxygen because the openings in the vessel top for the mixing shaft, inlet, and outlet pipes and control devices could have permitted air to enter. The batch-recycle system is shown schematically in Figure 3. Prior to each run in either system, the equipment was
first flushed with acetone and dried by vacuum after the acetone was drained. It was then flushed with dioxane, drained, and charged with the reactant solution. In the batch-recycle runs the solution was charged to the mixing vessel and circulated through the system until the desired circulation rate was established. Samples for determining initial concentration were taken and only then was the lamp turned on. The lamp start-up period was very short so that a time zero was taken as the time the switch was closed. Samples for analysis were taken at predetermined intervals during the course of each run. Two spectrophotometers were used for analysis. Absorption a t 396 nm, at which wavelength only PQ absorbs, was measured in a Beckman DB spectrophotometer. The course of the reaction could then be followed by the disappearance of PQ. Complete spectra for some of the samples were measured in a Cary 15 spectrophotometer over the range of 330-500 nm. The range of variables studied was as follows. In the batch runs initial concentration of reactant ranged from 4 X M to 8 X M with reaction times up to 1 hr. In the batch-recycle runs initial concentrations ranged from to 1 X M with circulation rates from 3.6 to 4.75 X 16.1 l./min. Run times were from 1 to 4 hr dependent upon the initial reactant concentration. Several batch runs were made with long irradiation times of up to 20 hr in order to study some of the chemical aspects and effects of long irradiation periods. Materials Used. Fluka purum grade 9,lO-phenanthrenequinone was purified by recrystallization from distilled dioxane. The melting point of recrystallized PQ was 202-204"C, identical with the literature value. Frutarom analytical grade 1,4-dioxane was purified by distillation and the bp of the redistilled dioxane was 101-103°C. Solutions of PQ in dioxane were prepared by adding a weighed quantity of PQ to a stirred solution of dioxane while dried nitrogen was bubbled through the solution. The concentration of PQ used in the analysis of the experimental runs was that determined by light absorption measurements. Analysis of Experimental Results Calculations made for fractional light absorption by the reactant solution for the given reactor geometry showed that more than 99% of the light emitted was absorbed a t solution concentrations of 1.5 x mol of PQ/l. or greater. Because all of the batch runs were made at reactant concentrations greater than 1.5 X mol/l., it was possible to use the experimentally obtained values for initial reaction rate to provide actinometric data and to compare them with data supplied by the lamp manufacturer. The initial reaction rate was 4.46 X 10-5 mol/sec equivalent to 4.46 X 10-5 einstein/sec for the reaction with unity quantum efficiency. When converted to a radiation intensity basis over the inner reactor surface, with radius 2.6 cm and length 114 einstein/sec cm2. The data supcm, I , equals 2.39 X plied by the lamp manufacturer yield a calculated value for I , of 2.48 X einstein/sec cm2. This calculated value, however, assumed that the inner Pyrex tube did not absorb any radiation whereas some absorption would occur. The value for I , as obtained in the actinometric measurements was used in all of the subsequent calculations and analyses. Values for molar absorptivity, h, as calculated by eq 4a for PQ and adduct were 1316 and 518 l./mol cm, respectively. Batch System. Experimental results for the batch runs are presented in graphical form in Figures 4 and 5. The experimental curves, the solid lines, are characterized by a constant, high, initial reaction rate which then decays and reaches zero in high conversion. The initial rate correInd. Eng. Chem., Process Des. Dev., Vol. 15, No. 1, 1976
71
t.min
Figure 4. Reactant concentration as function of time, batch operaM. tion, initial concentration of 3.9 X
4 0
6-
A
EXPERIV,ENTA~ PREDICTED A: PREDICTED A I
%&-
E
n -
3 3d
21-
0
5
10
15
20
25
30
35
40
45
50
I
55
t.min
Figure 5. Reactant concentration as function of time, batch operaM. tion, initial concentration of 8.16 X sponds, of course, to the maximum rate permitted by the light emitted because all of the emitted light is absorbed a t the initial reactant concentration. The obvious explanation for the decay in rate is the increase in light transmission out of the reactor as the reactant concentration decreases due to reaction. T o examine this explanation, eq 10, which assumes perfect radial mixing with no product species absorption, was solved. The results are plotted in Figures 4 and 5 as the dash-dot curves. The initial characteristics are noted to be similar to the experimental results. Over part of the reaction period, however, higher reactant concentrations are predicted by the model than were obtained experimentally. In other words, the predicted conversion is lower than the experimental conversion. Equation 12, which takes product absorption into account, was solved numerically and the results are shown as the dashed curves in Figure 4. Again the predicted curve is similar to the experimental curve but it also intersects the experimental curve. In other words, over part of the reaction period the predicted rate was lower than the experimentally measured rate and this occurred a t concentrations where product light absorption was non-negligible as compared to reactant absorption. This predicted behavior, as compared to the observed behavior, would indicate that the model which assumes radial mixing does not describe the true physical situation. The question that must now be answered is: would a 72
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 1, 1976
model that assumes a radial concentration gradient give, better predicted -results? Model equations were derived for this situation but they were not soluble by analytical means. A qualitative evaluation was then made. If we assume, for example, that the solution adjacent to the inner reactor annulus is rich in product whereas the solution more remote from the light source is rich in reactant, it is easily shown that practically all of the light emitted will be absorbed by the product species without, of course, resulting in additional product formation. Only a small fraction of the light survives to enter the reactant-rich zone and to produce more product. In other words, a model assuming a concentration profile with reactant concentration increasing radially outward from the light source would result in an even lower reaction rate than that predicted by the perfect-radial mixing model. Conversely, if the model were such that the reactant concentration decreased radially outward from the lamp, a high reaction rate, with rates approaching the maximum permitted by the light output, would be obtained. The latter case could occur in the physical situation only if unexpected flow gradients would be present in the annular chamber of the reactor. In preliminary work with undistilled dioxane which contained small, particulate contaminants these particles were observed to flow from the outer wall of the annular chamber to the inner wall. This flow pattern evidently resulted from thermal convection currents produced by the lamp in spite of the cooling water circulation. Visual and photographic observation of the reaction with purified materials which are reported herein showed the following characteristic behavior. As the reaction proceeded the typical yellow color of the solution of PQ in dioxane gradually disappeared and the solution became colorless until fluorescence was obtained after extremely long irradiation periods. Shortly after irradiation was begun, however, alternating vertical bands of yellow and almost colorless reactant were observed. In addition, some patches of reactant retained their color for relatively long periods after adjacent material had become almost colorless. An axial yellowish region was also observed to persist along the bottom of the reactor. This could indicate that solution was circulating radially a t the outer radius of the reactor to the bottom of the reactor and then moving vertically up to the lamp. Unfortunately, the reactor design did not permit visual observation of the radial cross-section. These observations, however, do tend to confirm the existence of unexpected flow gradients in the reactor. These gradients would tend to confirm the explanation that, a t least in part of the reactor, the reactant concentration decreases radially outward from the light source and would explain the high reaction rates and deviations observed as compared to the predictions of the various models. Although all of the models predict a decreasing reaction rate as the reaction proceeds beyond a certain time, the experimental results showed a constant, low concentration after decay of the initial, high constant rate. Two explanations for the measured results are possible. The first explanation lies with a minor side reaction (Rubin, 1969), disproportion of two radicals derived from PQ yielding 9,lO-dihydroxyphenanthrene. Although the latter is stable in a nitrogen atmosphere, the presence of air results in its rapid oxidation to yield PQ. During a run samples, which were withdrawn from the reactor by a hypodermic syringe, were ejected into small unpurged sample bottles for later analysis. Contact with air, therefore, took place during the entire period from sample withdrawal until analysis so that the oxidation reaction could take place yielding additional P Q as compared to the actual concentration that prevailed a t the time of sampling. The net
Table I. Effect of Reactant Concentration on Initial Reaction Rate, Batch-Recycle Operation
Run
Initial reaction rate, mol/sec x l o 5
Initial PQ concn, M X 103
Maximum theoreta
05
Predicted
I
I
\
B-Zmin
oh:\
\
\
-
I
Q :16 1 Lrt min"
1
L 3 3 -
Exptl
b,
$ 02-
x
EXPERIMENTAL PREDICTED C'
A
PREDICTED CI
0
\,
Figure 8. Reactant concentration as function of time, batch-recyM, 16.1 l./min. cle operation, initial concentration of 1.45 X
4
0
0 6
0
-
i
01-
3
EXPERIMENTAL PREDICTED C ! A PREDICTED ' ' : 8 o
t , min
0
1
0
2
0
3
0
4
0
~
t , min
Figure 7. Reactant concentration as function of time, batch-recyM , 16.1 l./min. cle operation, initial concentration of 9.5 X result would be that the measured surviving reactant concentration would be higher than that actually existing in the reactor a t the time of sampling a t very low PQ concentration. The second explanation involves the possible hydrolysis of the adduct after its formation. The hydrolysis of adduct results in the reformation of P Q and this hydrolysis could occur either in the reactor because dioxane was not dried before use or else in the sample bottle or during analysis from contact with moisture vapor in the air. Again the net result would be a higher measured value for the PQ concentration as compared to the concentration actually existing in the reactor during sampling. In support of these two hypotheses it can be mentioned that the measured concentration of PQ in solution which had been in the reactor for some time after irradiation after the run was terminated, was significantly higher than the measured concentration in a sample withdrawn immediately after cessation of irradiation. The two explanations could indicate that although the measured reaction rate was essentially constant when extremely low concentrations of reactant were present, the actual reaction decreased with time as expected from the models.
Figure 9. Reactant concentration as function of time, batch-recyM, 16.1l./min. cle operation, initial concentration of 2.5 X Batch-Recycle System Results In the batch-recycle system the light intensity, reactor geometry, and total volume of reactant solution were maintained constant. The effect of initial reactant concentration and recycle flow rate on the reaction rate were studied. Reactant concentrations ranged from 4.75 X lop4 to 1 X lo-' M with recycle rates of 16.1, 7.2, and 3.6 l./min. Effect of Concentration. Experimental results are presented in Table I with some typical time-concentration behavior shown in Figures 6-9 for a constant recycle rate of 16.1 l./min. The results can be characterized by linear change in concentration of PQ with time, i.e., constant initial reaction rate, which decays to a low constant concentration. From Table I it can be seen that the initial reaction rate is high and approaches the maximum rate permitted by the lamp when the initial reactant concentration is high and decreases as the initial reactant concentration decreases. This reduction is due to the fact that not all of the light is absorbed a t the lower initial concentrations. At the lowest initial concentration 15% of the light emitted by the lamp is transmitted out of the reactor. These rates are compared with those predicted from a model based on the assumption that the recycle system behaved as a perfectly mixed vessel with an average residence time, 8. The comparison is reasonable except for two cases that will now be discussed. Ind. Eng. Chem., Process Des. Dev., Vol. 15,No. 1, 1976
73
Table 11. Effect of Recycle Rate on Initial Reaction Rate, Batch-Recycle Operation Circulation rate, l./min
Initial PQ concn, M X lo3
16.1 7.2 3.6
2.5 2.35 2.65 0.95 0.925
16.1 7.2
Residence Initial reaction rate, time in mol/sec X l o 5 recycle system, min Predicted Exptl 2 4.47 8.94 2 4.47
4.34 4.22 4.27 3.5 3.31
4.07 4.22 4.27 3.05 3.31
0.3 6 ilt m i h ' EXPERIMENTAL PREOCTED C! A PREDICTED CII o x
2.0
t,min
Figure 10. Reactant concentration as function of time, batch-recycle operation, initial concentration of 2.65 X low3M , 3.6 l./min. Presence of air in the system can result in values for quantum yield that are variable and less than unity (Rubin, 1969). If dissolved air is present initially it will disappear either due to reaction or to degassing in the mixing vessel. In this event, a delay in the beginning of the reaction would be noted as in Figures 6 and 8. If large quantities of air are present or if air enters the system continuously the measured reaction rate would be lower than expected. In some runs air entered the system through the pump seal and, in the event that air bubbles were observed, the run was terminated. The possibility always exists, however, that air entered the system through the seal but the bubbles either were too small to be visible or else the air dissolved in the circulating stream. The low initial rate observed in run 4 (Figure 8) 1.02 X 10-5 mol/sec obtained as can be explained by the prescompared with 3.89 X ence of air. In this run, although no air was observed initially, after some time small bubbles of air were seen and then large bubbles as the seal failed completely. At this point the run was terminated. The data are, nevertheless, presented as they give definite proof of the effect of air on the measured rate. Two additional attempts a t replicate runs were made and in both cases the same seal failure occurred. As these runs were the last runs scheduled the research was terminated before a new pump was installed. Effect of Recycle Rate. Experimental results are presented in Table I1 with typical time-concentration behavior shown in Figures 9 and 10. The characteristic behavior of reaction rate with time is the same as described earlier for the effect of concentration, an initial constant high rate followed by decay and zero reaction rate. From Table I1 it can be noted that the initial reaction rates are close to the predicted values. Discussion of Batch-Recycle System Results The predicted values for concentration as a function of time are shown in Figures 7-10 with the dash-dot lines rep74
Ind. Eng. Chern., Process Des. Dev., Vol. 15, No. 1, 1976
resenting the case where the batch-recycle predictions assume no product absorption, 111, and the dashed line the case with product absorption, IV. If the first few minutes of the reaction stabilization period are neglected the following conclusions can be drawn. Predicted values assuming product light absorption are closer to the experimental values than those predicted assuming that only the reactant absorbs. The predicted values are closer to the experimental values at the higher recirculation rates indicating, as one would expect, that high flow velocity through the reactor chamber would result in a flow regime closely approaching perfect radial mixing. However, in practically every case, the measured reactant concentration was higher or, conversely, the conversion was lower than predicted by the models. The most likely explanation is that small quantities of air, unobserved by the naked eye, continuously entered the system through the pump seal, which, as mentioned previously, occasionally failed completely. In addition, air could have entered the circulating stream in the mixing vessel even though nitrogen was continuously fed into the vessel freeboard. Both models predict a monotonically decreasing reactant concentration with time whereas a constant, low concentration was obtained experimentally. The same explanation as postulated for batch operation applies here with the added condition that the solution volume in the batch-recycle case is much larger than for batch operation so that small concentrations of air or water would have a greater proportional effect. Run 4 showed extremely large deviations from the predicted values although the general shape of the curve is similar. This run, in which air was most definitely present, was mentioned earlier in connection with the discussion on initial reaction rates. It represents an extreme example of the effect of the presence of air on measured reaction rates and is an additional indication that low measured rates are, in all probability, due to the presence of air in the circulating stream. Nomenclature a = defined in eq 10a A = reactor cross-section, cm2 c = concentration of absorbing or reacting species, M ci = initial reactant concentration co = reactant species concentration in mixing vessel c1 = reactant species concentration cp = product species concentration C = dimensionless concentration, c/ci D = defined in eq 8c D, = radial diffusivity I, = volumetric light absorption rate, einsteins/sec cm3 or einsteins/sec 1. I, = incident light intensity a t reactor wall, einstein/sec cm2 J = defined'in eq 8b k = dimensionless group, r l / r p L = reactor length, cm Q = volumetric flow rate, l./min r = radius, cm r1 = inner annular radius r2 = outer annular radius ; = time, sec t = average residence time, sec u = linear velocity, cm/sec CL = average linear velocity U = dimensionless velocity, u/n z = length along reactor 2 = dimensionless length, t / L Greek Letters CYA = molar absorptivity, l./mol cm
weighted average for ax 0 = mixing vessel residence time, sec p = attentuation coefficient, cm-l A = wavelength p = dimensionless group, r / n CT = definedineq8a = quantum efficiency, molleinstein 9 = weighted average value for 9~ n = reaction rate, mol/sec cm3 & =
Wiley, New York, N.Y., 1966. Felder. R. M., Hill, F. 6.. Chem. Eng. Sci., 24, 385 (1969). Gaertner, R. F., Kent, J. A., hd. Eng. Chem., 50, 1223 (1958). Hill, F. B., Felder, R. M.. A./.Ch.f. J.. 11, 873(1965). Huff, J. E., Walker, C. A,, A.I.Ch.E. J., 8, 193 (1962). Jacob, S.M., Dranoff, J. S., A.1.Ch.E. J., 16, 359 (1970). Rubin, M. E., Top. Current Chem., 13, 251 (1969). Rubin. M. B., J. Org. Chem., 28, 1949 (1963). Rubin, M. E., Zwitkowitz, P., J. Org. Chem., 29, 2362 (1964). Shah, M. A., Felder, R. M.,Paper No. 69C, 64th Annual Meeting, A.I.Ch.E., San Francisco, Calif., 1971. Shendelman, L. H.. Hill, F. B., Chem. Eng. J.. 2, 261 (1971).
L i t e r a t u r e Cited Received for review January 3, 1975 Accepted August 22,1975
Eoval, E., Smith, J. M.,Chem. Eng. Sci., 28, 1661 (1973). Calvert. J. G.,Pitts, J. N., "Photochemistry", pp 19-22, 189-196. 682-750,
Drop Formation at Sieve Plate Distributor Yellamraju P. Saradhy and Rajinder Kumar. Depaflment of Chemical Engineering, hdian lnsritute of Science, Bangalore 5600 12, M i a
Drop formation from a distributor of sieve plate type is investigated. Data are collected over a wide range of variables for formation of drops in both Newtonian and power law fluids. The drop size is found to be strongly influenced by the number of operating orifices whereas the inter-orifice distance seems to have insignificant effect except in deciding the condition of coalescence. The existing concepts on drop formation from single orifi ces are extended to the case of formation of drops over a sieve plate in the absence of coalescence.
Drop formation from isolated submerged orifices has been reported extensively (Hayworth and Treybal, 1950; Null and Johnson, 1958; Rao et al., 1966; Scheele and Meister, 1968; Kumar, 1971; Heertjes et al., 1971; Kumar and Saradhy, 1972) in the literature. However, single orifices are seldom employed in industry, the most common being sieve plates. The available information on spray columns (Johnson and Bliss, 1946; Markowitz and Bergles, 1970) essentially pertains to the overall phenomenon and relatively little attention has been paid to the formation region. In view of its importance in industrial operations, this investigation was undertaken. The objectives were to ascertain whether drop is influenced by the presence of other orifices and if so to develop expressions to account for the changes. Experimental S e t u p a n d P r o c e d u r e The experimental setup is schematically represented in Figure 1. The orifice plates, each having different inter-orifice distance but uniform orifice diameter, are made of brass. Each one of them is 1.2 cm thick, half of which is threaded to fix it into the orifice plate holder (8). Each plate contains 25 holes drilled in a square pitch and having uniform inter-orifice distance. While working with lower number of orifices, the undesired holes were plugged with cork. If the holes are numbered as shown in Figure 2, the plugged holes while working with 16, 9, and 4 orifices are 1 to 9, 1 to 16, and 1 to 21, respectively. A calibrated rotameter (5) was used to measure the flow rate. During a typical run, the quiescent continuous phase was taken in the metal tank (9) after the pump (1)pumping the dispersed phase was switched on. The bypass valve (14A) and the needle valve (6) were adjusted to get the desired flow rate when desired number of orifices were functioning.
The frequency of drop formation is then determined using the strobometer (Sankara Srinivas et al., 1969). Drop volumes were calculated from total flow rate, number of orifices functioning, and frequency. Experiments were repeated with various continuous phases for different combinations of operating orifices and different inter-orifice distances. The respective continuous and dispersed phases were mutually saturated before each run to avoid mass transfer during formation. While working with Newtonian continuous phases, in some of the runs, the number of operating orifices is deliberately limited to four and the transition flow rate where shift in the number of orifices functioning occurs is recorded. Though all the four orifices were open, as expected not all of them were functioning all the time. The number of operating orifices was dependent on the upstream pressure and the pressure drop across the operating orifices. An interesting observation when two or three orifices were working was that no particular combination of orifices had worked all the time. Instead, the combination was continuously changing. Either benzene or xylene (B.D.H. analytical grades) formed the dispersed phase in all the experiments. Water, glycerol (commercial), and sugar solutions were employed as Newtonian continuous phases. CMC (carboxyl methyl cellulose) and sodium silicate solutions were used as power law fluids. Properties of the continuous phase were further changed by changing the concentration. The viscosities of Newtonian liquids were measured with a Hoppler viscometer (falling ball type). The power law parameters were determined using a capillary tube viscometer. The absence of viscoelasticity in CMC solutions was tested by the method attributed to Philippoff (Kumar and Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 1, 1976
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