42 1
Ind. Eng. Chem. Process Des. Dev. 1082, 21, 421-426
Separation of Volatile Materials by Gas Membranes Masanao Imal, Shlntaro Furusakl, and Terukatsu Mlyauchl’ Department of Chemical Engineering, Universlty of Tokyo, Tokyo 113, Japan
Separation of NH, and I2 from aqueous solution by use of gas membranes is presented. Two types of gas membranes, Le., tubular and flat membranes including spiral modules, are investigated. The Wilson plot to obtain mass transfer resistance In both gas phase and liquid boundary film is presented. From an analysis of the experimental data, the spiral-module membrane showed the smallest gas phase resistance and thus should have good prospects for practical applications.
Table I. Properties of the Shell-Tube Module
Introduction Production of synthetic polymer membranes and their applications to separation have been developed recently. Many types of polymer membranes have been applied to several purposes. Besides solid polymeric membranes, liquid and gas membranes have also been developed. The term “gas membrane” is used to describe a gas film which separates two liquids. This separation is accomplished by means of a liquid-repulsing porous membrane, such as porous polytetrafluoroethylene (PTFE) for aqueous solutions. Is this case, PTFE membrane repulses the liquid phase so that the gas phase is confined in the pores inside the membrane. Thus, a stable gas membrane can be established inside a hydrophobic membrane as a separating diaphragm for aqueous solutions. The gas membrane can be utilized to separate volatile materials from aqueous solutions as well as enabling compact apparatuses to be designed. Both stripping and absorption of the volatile component can be conducted simultaneously in the membrane in a single apparatus. A study of mass transfer processes in the gas membrane is important for the investigation of industrial applications. Watanabe and Miyauchi (1976) studied the separation of iodine by a gas membrane plus some preliminary experiments on multi-tubular gas membranes (Miyauchi et al., 1978), but these seem to be the only reports available on the application of gas membranes. The purpose of this work is to investigate process characteristics of gas membranes and to seek a practical method for their industrial use. Theory on mass transfer through the membrane is presented, and important factors which affect the mass transfer characteristics are studied. Gas Membrane and Transfer Process The concept of a “gas membrane” is a very recent idea presented by Watanabe and Miyauchi in 1976. The concept is very attractive since diffusivity in the gas phase is much larger than that in the liquid or solid phases. Any porous hydrophobic membrane can be used as a structural frame to hold the gas. For example, a porous PTFE membrane may be used. The pressure to expel gas or air from the pore by permeating liquid is a function of pore diameter, surface tension, and angle of contact. In this research GORE-TEX (Hosokawa, 1979), which is the product of W. L. Gore & Associates, Inc., was used. A microscopic photograph of GORE-TEX is shown in Figure 1. The struc;ture is rabher similar to the network of PTFE textiles. The expel pressure of air by water for the membrane (water entry pressure) wae measured as 7.5 X lo4
inside diameter of shell, m outside diameter of membrane tube, m thickness of membrane, m void fraction maximum pore diameter, pm effective length of tube, m effective area, m’
Pa, from which the maximum pore diameter is calculated as shown in Table I. When the hydrophobic membrane is placed between different aqueous solutions as shown in Figure 2, the two solutions cannot mix together unless the pressure difference across the membrane is more than the expel pressure. Thus, the membrane works as a separating diaphragm between aqueous solutions. In this system, mass transfer of a volatile material will proceed when a concentration difference exists across the membrane, Mass transfer resistance can be considered in the usual way as confined to boundary films in both liquids and in the gas membrane itself. If side 1 of Figure 2 is an aqueous solution containing a volatile material such as C 0 2 and side 2 contains an absorbent solution, e.g., NaOH solution, then the C 0 2 will transfer across the membrane. Thus, stripping of C 0 2 and absorption by NaOH solution are simultaneously carried out by the gas membrane system. In this paper, separations of NH3 by HzS04solution and of I2 by NaOH solution are described. Mass transfer flux is expressed by eq 1and 2, which are obtained by considering the three resistances mentioned above. (1) N = koL(CL1 - CL,)
N = kL1(CC1 - C L ~ , ) = ~
-
( c G CG,) ~
= kL2(CL2, - CLJ
(2)
The equilibrium is assumed to obey Henry’s law, i.e., cG* = mccb When chemical absorption occurs as in this study, the mass transfer resistance (kL2-l) of the absorbing solution can be neglected as shown later. Thus, the overall resistance is given by eq 3. 1 _1 -_ - 1 (3)
+-
koL
kL1
mckm
This equation will be used throughout this study. Experimental Section The experimental study was based on an aqueous ammonium solution-dilute sulfuric acid system and also on an iodine solution-Na0H solution system. That is, ammonia was stripped out of the aqueous solution and absorbed by H2S04 aqueous solution. The concentration of NH, in the feed solution was 0.01-0.1 N, and the concentration of HzS04was 0.01-0.1 N. Iodine was stripped
*Address correspondence to this author at Department of Indqstrial Chemistry, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo, 162, Japan. 0196-4305/82/1121-0421$01.25/0
4.20 x 10-3 8.00 x 1.05 x 10-3 0.6 2.6 0.46 0.232
@
1982 American Chemical Society
422
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 3, 1982
Figure 3. Experimental apparatus for the tube-bundle module system.
Figure 1. Microscopic photograph of the porous PTFE membrane (X;r,Oo).
Figure 2. Concentration profile across a gas membrane.
from an 0.01 N aqueous solution and absorbed in 0.01-1 N NaOH solution. For this dilute feed solution, Henry’s law may be considered applicable. A change of concentration of the absorbent solution by a factor of 10 did not affect the overall mass transfer resistance. A change of rate of impeller rotation also did not affect it (see Figures 7 and 8). These facts show that the resistance of the absorbent solution is negligible and eq 3 is applicable. Experiments were carried out a t 298 K except for the transfer of I2 using a tube bundle, which was carried out a t 303 K. Two types of gas membrane module were studied, namely (i) a tube-bundle and (ii) a flat membrane including a spiral module variant. Concentration was measured by ordinary titration. Samples of NH, solutions were mixed with the standard HCl solution and the excess HCI was back-titrated with 0.01 or 0.1 N NaOH solution (Kamata, 1970) using methyl red-methylene blue indicator. Samples of I2solutions were directly titrated by 0.01 or 0.1 N Na2S203solution with starch indicator (Basset et al., 1979). Experiments with the Tube-Bundle Type Gas Membrane Apparatus. A tube-bundle module was designed after several preliminary experiments with a single shell and tube system (Imai, 1980). The experimental apparatus is shown in Figure 3. Physical properties of the membrane
used are given in Table I. The module is just like the tube-bundle-type heat exchanger with semicircular baffle plates. The shell is made of glass so that the inside of the module is visible. The baffles and end plates are made of stainless steel. In the experiments the absorbent solution was introduced into the tube-side conduit and the feed solution containing the transferred materials was introduced into the shell side. This arrangement was chosen since cross flow in the shell side can provide a larger mass transfer coefficient than the tube side. The tube-bundle module is mechanically strong because the cylindrical tube is able to withstand pressure and the support baffles prevent deformation of the tubes. Thus, this type of module is suitable for membranes which are not necessarily mechanically strong. Liquid Film Mass Transfer Coefficient. In order to obtain mass transfer characteristics of the gas membrane, it is necessary that the liquid film mass transfer coefficient of the feed solution be evaluated correctly. For cross flows to a staggered tube bundle, the following equation has been > ZOO0 (McAdams, 1954; Colbum, proposed for D+&/p 1933).
Using the heat and mass transfer analogy, this equation is transformed into eq 5 to calculate mass transfer coefficient kk Sh = 0.33Re0.6Sc1/3 (5) where Sh = k f l T / D L ,Re = D u m a ~ / pSc, = c(/pDk Therefore, kL can be considered to be proportional to u-O.~. This relation holds approximately for the case of Re > 200 according to McAdams (1942, p 232). The relationship by Donohue (1949) for baffled shell and tube heat exchangers also says that h is proportional to u,’.~. Thus, eq 3 is modified to eq 6, where /3 is a constant. -1 =-
1
+-
1
(6) pum,o*6 m&m From eq 6, (m&J1 can be obtained from the intercept of the plot of koL-*vs. u-+*~. This type of the plot is called the Wilson plot. Results. The Wilson plot for NH, removal is shown in Figure 4 from which the value of mJzmis found to be 4.76 X lo+ m/s. Knowing the value of void fraction, c, of the membrane, its tortuosity, x , can be obtained from eq 7. ~
O
L
The value of x is thus found to be 2.02. Figure 4 shows that the contribution of the gas membrane to the mass transfer resistance is considerable. Results for the transfer
Ind. Eng. Chem. Process Des. Dev., Vol.
I
2
21, No. 3, 1982
423
I
x
0 4
12
60
5
58
'
uk% [(misjo6 I Figure 4. Wilson plot for NHS removal, shell-tube module.
l
I
I
0
3
2
1
u:ix
[(m/sP
I
case
NH,
I2 average
X
2.02 2.22 2.1
Figure 6. Experimental apparatus for the flat membrane. Dimensions are in mm. Table 111. Specification of Gas Membrane
Figure 5. Wilson plot for Iz removal, shell-tube module. Table 11. Comparison of the Constant
b
CY
CY
0.15 0.19 0.17
of I2 are given in Figure 5 from which the value of m,k, is found to be 1.25 X m/s and the value of x is 2.22. In the above calculation, the value of m, for the case of I2 removal at 303 K is estimated from data at 298 K by Watanabe (1976) and Nishizawa et al. (1969) using Eggleton's chart (1967) for temperature correction. The estimated value is 0.0572, which is almost 100 times larger than that of NH,, i.e. m, = 6.46 X lo4 (Sherwood, 1925). Thus most of the mass transfer resistance exists in the liquid boundary film for the case of I, absorption. Agreement between the values of x for the two cases is satisfactory (Table 11). The constant a in the equation, Sh = aRe09Sc'f3,can be determined from experimental results. It is also shown in Table 11. The value of CY is smaller than that in eq 4 or 5. This is possible because eq 4 is based on a staggered arrangement of an infinite number of tubes. Fully developed cross flow would not be obtained in the small experimental module used in this study. Mizushina et al. (1978) have suggested for empirical reasons that shell-side film coefficients estimated by eq 4 should be multiplied by 0.6. The results in Table I1 appear to support this. By employing a thinner section it should be possible to improve the mass transfer characteristics of a gas membrane. However, the manufacture of thin porous PTFE membrane tube is not easy and furthermore it is very difficult to obtain. On the other hand, flat porous thinsection PTFE membranes are readily available and this form was used for the experiments described below. Experiments with Flat Gas Membrane Apparatus. The experimental apparatus shown in Figure 6 comprises two Pyrex transfer cells faced together with a flat porous PTFE membrane located between the
effective area, m 2 effective diameter, m liquid volume, m 3 thickness, p m void fraction
2.39 x 10-3 5.52 X 2.00 x 10-4 4.00 x 0.75
cells. The cells were immersed in a water bath which was controlled at 298 K. Solutions in the two cells were stirred by motors at constant rotation using six-blade impellers made of stainless steel. Table I11 shows the physical properties of the membrane. Film Mass Transfer Coefficient. The film mass transfer coefficient in baffled stirred tanks can be estimated from the equation of Johnson and Huang (1956)
-kLDT - - 0.0924( DL
d2np
y
)"(L ) 5 PDL
(8)
The transfer cell in Figure 6 is not the same as that used by Johnson and Huang. However, eq 8 has been found to be applicable to the transfer cell used in this study although the factor 0.0924 can vary (Watanabe and Miyauchi, 1976; Kojima et al., 1979). Thus, kL is considered to be proportional to Hence, if (kOL)-lis plotted against n4.'l then the value of m&, can be obtained from the intercept. Results. The thickness of the membrane investigated was 40 pm for one type and 97 pm for another. The latter membrane was used for the spiral module (type B) which will be described later. Figure 7 shows the Wilson plot for NH3 removal for both membrane thicknesses. Figure 8 shows I2 removal by the 40-pm membrane. In this case, 1, was dissolved in 0.1 N KI solution. The value of the apparent m, is affected by the equilibrium, I- + I, 2 13-. Rigourous calculations were impossible because of lack of data of m,. By using the same value of x as in the case of NH3 absorption, i.e. 2.28, the apparent m, is estimated to be 2.58 X lo4. The value of x for the 97-pm membrane, 2.90, is a little larger probably due to a different manufacturing conditions. From the Wilson plot, the liquid-film mass transfer coefficients are also obtained. Their dimensionless forms
424
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 3, 1982
,.’
u 00
E .
P.7
/
L YI i
4x lo4
72 Y
ZXIOL
/
./.
/ I
I
A
nls 1 l(pm)
0 0
6 3 97 63
40
C,
33 83
40
B _-
0
02
01
“071
03
A Feed Solution
a -
B
/-
L 0
Mdv~ - -
40
Absobent Solution
Figure 10. General scheme of the spiral module.
05
04
rcs-l)-071]
Figure 7. Wilson plot for NH3 removal, flat membrane.
/*
L
4
1XlOo
5
15 ’00s
[(misi
1
I
Figure 11. Wilson plot for the spiral module membrane, type A. 0.3
no’’
05
I
Table IV. Specification of Spiral Module ~-
~
[(jl)-G71]
Figure 8. Wilson plot for I, removal, flat membrane thickness = 40
m. I
,
I
1001
membrane spacer in module effective area, m z thickness, km void fraction
type A doublea fixed 1.14 36a 0.690
type B single unfixed 0.99 97 0.694
a Two membranes are stuck together including honeycomb lattice with adhesive.
0
5
3
Re
7
104
I 2
C-I
Figure 9. Boundary film mass transfer coefficient derived from Wilson plot.
are given in Figure 9. The constant y in the equation Sh = yRe0.71Sc0.5 is found from the plot to be 0.116. Miyauchi et al. (1978) report y = 0.071 for a similar cell with smaller impeller (d = 0.019). The difference may be due to the influence of turbulence at the membrane surface. Thus mass transfer coefficients at the membrane surface can be different even at an identical Re if the ratio of cell diameter to impeller diameter is not the same. Turbulent eddies on the membrane surface will be smaller for a smaller impeller because the distance between the impeller and wall is large. The value of mckmof NH3 absorption by the flat gas membrane is 1.33 x m/s, which is of an equivalent order to that for the liquid film. Therefore, the flat gas membrane is more suited than the tubular type for mass transfer of volatile materials.
Experiment with a Spiral Module Membrane From the results of the flat gas membrane, a spiral module shown in Figure 10 was investigated for transfer of NH3. Physical properties of the spiral membrane are given in Table IV. The membranes were formed into a roll and inserted into an outer shell made of polymethyl methacrylate resin. In order to prevent choking of the space between membranes, a resin net spacer was sandwiched between them. Two types of membrane construction were used. For type A, the spacer was attached to the membrane by a polymeric adhesive. The membrane was composed of two sheets stuck together. For type B a thicker membrane material was used without adhesive. The module was installed in place of the tube bundle in the shell and tube module of Figure 3. Experimental Results For the Spiral Module Type A. Calculations were based on procedures given earlier. Ignoring centrifugal effects, mass transfer correlations for flat plates can be considered applicable. Figure 11 shows the Wilson plot based on the correlation for boundary layers on flat plates, S h = 0.036Re0%3c’’3 (Colburn, 1933). The plot yielded a value of mckmequal to 6.06 X m/s. Also, the value of x is calculated to be 5.08. This tortuosity value is large compared with previous values and may be due to the adhesive used to attach the spacer to the membrane surface and due to the layered membrane.
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 3, 1982 425
Table V. Mass Transfer Coefficient for the Gas Phase, at 298 K membrane apparatus migrant thickness, p m single tube tube bundle
700 1000 1000 40 97 40 36 97
NH3 NH3
I, a
flat
NH3 NH3 I, NH3 NH3
spiral (type A)
(type B) Single membrane thickness.
a At 303K. KI solution; m, = 2.58 X
-
mF - = 6.46 X
6x10‘ 0 . 0
E
0.
0 .
4x104
Y_
_ _ _ _ 9_ _ _ _Q Ta- - - - - -0
0 0
: 7
----2x10~
L
1 -
X
1.90 2.02 2.22 2.26 2.85 (2.26) 5.08 (2.85)
E
0.69 0.60 0.60 0.75 0.69 0.75 0.69 0.69
for NH,; 0.0572 for I, (in H,O).
m P m ,m/sC 8.33 x 4.76 X 1.25 x 1.33 x 4.00 x 1.73 x 6.06 x
10-4 10-4 10-5 10-~d
10-5
(4.00x 10-5) I, is dissolved in 0.1 N
(2) For the spiral module, the mass transfer resistance of the gas membrane is small, being about the same magnitude as the boundary film resistance in a turbulent media. The tubular module showed large mass transfer resistance in the gas phase. Acknowledgment Authors would like to express their thanks to Mr. Takana0 Hosokawa of Junko-sha co. for supplying the porous PTFE membrane and manufacturing the experimental apparatuses. Nomenclature c = concentration, moi/m3 CL = concentration in liquid phase, mol/m3 CG = concentration in gas phase, mol/m3 cG* = concentration in gas phase in equilibrium to cL, mol/m3 c = heat capacity, J/kg K L& = diffusivity of gas, mz/s D L = diffusivity of liquid, mz/s DT = diameter of pipe or tank, m d = diameter of impeller, m h = heat transfer coefficient, W M-2 K-’ k = thermal conductivity, W m-l K-’ k, = mass transfer coefficient in gas phase, m/s kL = liquid boundary film mass transfer coefficient, m/s koL = overall mass transfer coefficient, m/s 1 = thickness of membrane, m m, = distribution factor, dimensionless n = rate of rotation, s-l n, = rate of rotation in absorbent solution, s-l Re = Reynolds number, dimensionless Sc = Schmidt number, dimensionless Sh = Sherwood number, dimensionless u = average flow rate, m/s ,u = maximum flow rate, m/s Greek Letters a,8, 7.= constants t = void fraction in porous membrane, dimensionless p = viscosity, Pes p = density, kg/m3 x = tortuosity, dimensionless Subscripts 1, 2 = liquid phase (refer to Figure 2) Literature Cited Basset, J.; Denney, R. C.; Jeffery, G. H.; Mendham, J. “Textbook of Quantitative Inorganic Analysis”, 4th &.; Longmans: New York, 1979. 1933, 29, 174. Colburn. A. P. Trans. AI= Donohue, D. A. Ind. €47. Chem. 1949, 4 1 , 2499. Eggleton, A. E. J. U.K. Atomic Energy Res. Establishment Report 1987, R4887. Hosokawa, T. Sen-I Gekkaishi 1979. 35, 69. Imai, M. B.S. Thesis, Department of Chemical Engineering University of Tokyo, Japan, 1980. Johnson, A. I.; Huang, C. J. AIChEJ. 1968, 2, 412. Kamata, H. “Analytical Chemistry”, Corona Pub.: Tokyo, 1970; Vol. I, p 93. Kojima, T.; Tomb, J.; Miyauchl. T. Kag8ku Kogaku Ronbunshu 1979, 5, 476. McAdams, W. H. “Heat Transmlsslon”, 2nd ed.; McGraw-Hill: New York, 1942; p 229.
Ind. Eng. Chem. Process Des. Dev. 1982, 21, 426-431
426
McAdams, W. H. "Heat Transmission", 3rd ed.; McGraw-Hill: New York, 1954; p 272. Mlyauchi, T.; Morita, K.; Kakegawa, T.; Watanabe. H. Rep. Asahi Glass found. Ind. Techno/. 1978, 33, 295. Mlzushlna. A.; Kunltomo, T.; Suzukl, M.; Nakajima, B.; Nishlkawa, K.; Wada, T. "Kagaku Kogaku Binran" (Chem. Eng. Hankbook), Vol. 4, 1978; p 280. Nlshizawa. Y.; Kigoshi, Y.; Oshima, S.; Osawa, Y.; Maekawa, T. J . At. Energy SOC.Jpn 1960, 7 1 , 205.
Sherwood, T. K. Ind. Eng. Chem. 1025, 77, 745. Watanabe, H. D.Eng. Thesis, Unlverslty of Tokyo, Tokyo, 1976. Watanabe, H.; Miyauchl. T. kagaku Kogeku Ronbunshu 1976, 2 , 262.
Received for review April 4, 1981 Revised manuscript received December 8, 1981 Accepted January 14, 1982
Selective Oxidation of Dibenrothiophene by Peroxybenzoic Acid Formed in Situ All Paybarah, Russell L. Bone, and Wllllam H. Corcoran' California Institute of Technology, Pasadena, Callfornia 9 1 725
Peroxybenzoic acid (POB) was formed by oxidation of benzaklehyde in the presence of uttraviolet light. The POB which was formed selectively oxidized dlbenzothiophene (DBT) to sulfoxide (DBTO) and sulfone (DBTO,) under experimental conditions. DBT sulfoxide was converted to sulfone at a substantially greater rate than DBT to sulfoxide.
Introduction The principal objective of this research was to examine a system to oxidize selectively the thiophenic compounds, important sulfur-containing constituents in fuel oil, as an initial step in desulfurization. Final removal of the oxidized sulfur may be accomplished by pyrolysis as demonstrated by Wallace and Heimlich (1968) and LaCount and Friedman (1977). There have been several successful attempts in oxidizing sulfur in organic sulfides such as by Attar (1977), Dankleff et al. (1968), Ibne-Rasa et al. (1974), and Kharasch (1961), but selective oxidation of sulfur in thiophenic compounds has been more difficult. According to Ford and Young (1965), the stability of the thiophenic compounds and their resistance to oxidation can be explained by: (1)the slightly strained thiophenic nucleus that opposes further strain caused by oxidation, and (2) the delocalization of the lone-pair electrons of the sulfur because of the aromatic structure of the thiophene as discussed by Schomaker and Pauling (1939). The resonance character of the ring becomes more restricted by adding a conjugated substituent to the molecule. For this reason, dibenzothiophene is oxidized more easily than thiophene. Furthermore, selectivity of the oxidation of the thiophenic compounds can also be explained in terms of the electron delocalization of the aromatic rings due to their resonance character. The validity of this interpretation is demonstrated by addition of an electron-donating group such as a methyl moiety to an aromatic ring. This addition promotes more electron localization which makes the molecule more susceptible to an electrophilic attack by an oxidizing agent. In general, to reduce the difficulty of oxidation of thiophenic compounds, a very strong oxidizing agent is required, and peroxyacids show promise as noted by Ford and Young (1965) and Overberger and Cummins (1953). Peroxyformic and peroxyacetic acids, although the two most powerful oxidizers of their kind, were rejected because of their extreme explosive potential when subjected to shock or strong light. Peroxybenzoic acid, a weaker oxidizer, and accordingly a safer one, was considered instead. If thiophenic compounds in fuel oil were oxidized on a commercial scale by addition of per0196-4305/82/1121-0426$01.25/0
oxyacid, an enormous amount would be required, which in turn would necessitate great care in storage and handling. Therefore, for the sake of safety and economy, it was decided to consider oxidation by way of peroxybenzoic acid formed in situ as a substitute for addition of a concentrated peroxyacid. Liquid-phase oxidation of aldehydes by O2to the peroxyacid and then the selective oxidation of dibenzothiophene by peroxybenzoic and peroxyacetic acids have been treated separately in the past by Cooper and Melville (1951), Ford and Young (1965), Greco et al. (1960), and Ingles and Melville (1953). The combination of these two processes, however, has not been studied. In the study by Ingles and Melville (1953), oxidation of benzaldehyde in the presence of UV light gave a substantial conversion to POB as the primary product. A free-radical mechanism gave the best fit for their rate data. The sequence may be written as initiation RCHO
-
RCHO
+ O2
I
+ H.
photochemical effect
(1)
RCO.
+ HOP. thermal effect
(2)
RCO.
propagation
RCO.
+ o22RCO,.
RC03. + RCHO -% RC03H + RCO. termination
(3)
(4)
k4
2RC03inert product (5) Besides the free-radical mechanism, there are other reactions that take place in the oxidation of benzaldehyde, such as RC0,H
+ RCHO 22RC02H
2RC03H 0
1982 American Chemical Society
2ROH
+ O2
(6)
(7)
