Countercurrent Foam Fractionation at High Rates of Throughput by

Countercurrent Foam Fractionation at High Rates of Throughput by Means of Perforated Plate Columns Guillermo A. Aguayo’ and Robert Lernlich*

Downloaded by UNIV OF NEBRASKA-LINCOLN on August 26, 2015 | http://pubs.acs.org Publication Date: April 1, 1974 | doi: 10.1021/i260050a010

Department of Chemical and Nuclear Engmeenng, University of Cincinnati Cincinnati, Ohio 45227

In a foam fractionation column of 6.35 cm i.d. that separated Triton X-100 (and in some runs also Methyl Orange) from water, the installation of suitable perforated plates was found to reduce the overall adverse effect of channeling at high rates of surface and liquid throughput, and to improve considerably the corresponding column performance under conditions of enriching, stripping, and combined operation. At total reflux and a relatively high superficial gas velocity of 8 cm/sec (based on the cross section of the empty column), the total number of theoretical stages was as much as tripled, and if the liquid pool is excluded, was as much as quintupled through the use of these plates. Simultaneously, the rate of liquid overflow in !he foam was tripled. The advantage of using plates was also demonstrated in terms of the quantity N,U/V which is a simple size-free criterion of overall column performance that accounts for both the ability to concentrate material and the ability to handle throughput. In stripping operation, a high liquid feed rate of 0.58 cm/sec superficial velocity (again based on the empty column cross sectign) was well tolerated by appropriate plates. These findings point the way toward countercurrent columns of greater capacity than heretofore, and so should allow wider industrial application of foam fractionation.

Introduction Foam fractionation is based on the selective adsorption of one or more solutes on the surfaces of gas bubbles that rise through a solution. These bubbles then form a foam which is relatively rich in adsorbed material. By removing this foam, a partial separation of components results. Principles are now fairly well established and the subject has been periodically reviewed alone (Shedlovsky, 1948; Cassidy, 1957; Rubin and Gaden, 1962; Lemlich, 1968a) as well as within the context of the other adsubble techniques (Lemlich, 1968b, 1972a,b, 1973, in press). Foam fractionation can be operated continuously in the simple mode, the stripping mode, the enriching mode, or the combined mode (Lemlich, 1 9 6 8 ~ )At . low superficial gas velocities, that is, below roughly 2.5 cm/sec, foam produced by a spinneret moves substantially in plug flow (Hoffer and Rubin, 1969). However, as the superficial gas velocity is raised above that figure, deviations begin to appear. Above 4 cm/sec, channeling becomes excessive, the bubbles become quite mobile relative to one another, and countercurrent contact becomes poor. When the superficial gas velocity is raised sufficiently high the interface between the pool and the foam atop it disappears, and the system becomes simply a gas-in-liquid dispersion which is sometimes called a gas emulsion (Bikerman, 1965).This superficial gas velocity for flooding depends on the bubble size distribution and hence on the particular sparger employed (Hoffer and Rubin, 1969). In the present study it was about 6 cm/sec. Thus the high rates of foam production which are generated by high gas rates are unsuited to the counterflow operations of stripping, enriching, and combined operation. Poor countercurrent contact may also result from excessive liquid feed to the foam. So for high throughput in an ordinary column, a large cross sectional area must be used in order to keep the superficial velocity low. In order to obviate such needs for large cross sections, the present study was undertaken to examine the feasibility of modifying the interior of a foam fractionation column so as to handle larger throughputs in the counterflow modes,

’ Present adaress. Department of CPem c a Engmeer ng. ,n vers t y of PJerto R.co. MayagLez. Pberto RICO 00708

Leonard and Lemlich (1965a) made an attempt along this line by using perforated plates at low velocities, but found that the overall separation was poorer with the plates than without them. Wace and Banfield (1966) employed bubble cap plates and reported “plate efficiencies of up to 30%,” but presented little related operational data. The a p p r o p h of the present study is to devise and examine several types of perforated plates a t high throughput and then to test the best more extensively. Success with plates should open the door to greater industrial acceptance of foam fractionation and other adsubble techniques. Theory Some theory for foam fractionation columns has been developed by Lemlich (1968b,c). Some of its pertinent features are as follows. The downflowing stream within the column is considered to be the downflowing interstitial liquid. However, the upflowing stream is viewed as consisting of the ascending bubble surfaces plus the interstitial liquid that is carried up by the bubbles. This ascending liquid is taken to be in equilibrium with the corresponding ascending surface, so that the combined upflowing stream has an effective concentration of

C=C+-

GSI‘ U

with r in equilibrium with C. S can be conveniently approximated as 6.3/d32 for counterflow operation or as 6/d32 for simple mode operation, where d32 is Znldl3/ Znld12 (Leonard and Lemlich, 1965b; Fanlo and Lemlich, 1965). If the foam is stable, the foam density under steady continuous operation is essentially uniform along the column within any section that is uninterrupted by inlet or outlet streams, at least for plug flow (Leonard and Lemlich, 1965a,b; Fanlo and Lemlich, 1965; Shih and Lemlich, 1967, 1971; Jashnani and Lemlich, 1974) so that the flow rates of the upflowing and downflowing streams are also uniform within any such column section. Accordingly, the operating lines are straight. For simplicity, the internal flow rates are equated to the corresponding exterInd. Eng. Chem., Process Des. Develop., Vol. 13, No. 2 , 1974

153

Table I. Expressions for a , b, CT, and CB for Various Modes of Continuous Column Operation with Liquid Feedd c---

Combined modes Individual modes Parameter Enricher Stripper

Enriching section

Stripping section

R RS-1 CD 1-

R

F

R R + 1

a

Q

CD - wcw b R + l ;Q CW CD

CB

CT

I

I

--wcw Q

CF CD

CF

+

F/D R S 1

CW CF

a For CT and Cg of a combined column, the feed is assumed t o enter a t the optimum location.

Table 11. Expressions for 01 and p for Major Surfactant and Trace Colligend, with Liquid Feed

r


Parameter

Surfactant = constant

a

1

B

GST/Q

Colligend r = KC

+ (GSK/Q) 0 1

nal flow rates such as feed, reflux, and foam overflow, or to their appropriate sums or differences. If the solute of interest is the major (or only) surfactant present, its r in the foam is likely to be substantially constant. This near constancy of I' holds with even greater certainty when, as was the case in the present investigation, the range of concentrations in the liquid does not extend much below the critical micelle concentration. On the other hand, if the solute of interest is present at very low concentrations (a trace colligend) and is adsorbed a t the bubble surfaces which are created through the presence of the surfactant (the collector), then the linear iso= KC, may apply to the colligend. Either way, therm, us. C will be the effective equilibrium curve of straight, and the number of transfer units involved in the separation can be readily calculated (Lemlich, 1968b,c; Jashnani and Lemlich, 1973). The number of theoretical stages can be found graphically (Lemlich, 1968a,b). However, the present authors found it more convenient and precise to calculate the number analytically, as follows. Let eq 2 and 3 represent the linear operating line and linear effective equilibrium curve, respectively.

e*

E,

-

Cnfi

= aCnfl

+b +P

(3) Combining eq 2 and 3, and solving the resulting first-order linear difference equation, yields

N,

= In

=

[(c, - -),/(c. b - Pa a

d " + 1

./I)-

-a - a

(4)

Expressions for a, b, CT,and C, are given in Table I, and for a and /3 in Table 11. For constant F at total reflux, eq 4 is 010 indeterminate and, through the consideration of limits, is replaced by

N , = (CD - Cw)/P (5) Where appropriate, the pool taken as a theoretical stage can be discounted by subtracting 1 from the right-hand sides of eq 4 and 5 to give N , for the foam alone. These equations are analogous to those previously obtained by Martin (1963) for distillation and extraction. Further details have been placed on file (Aguayo, 1972). By allowing Np in eq 4 to approach infinity, the expressions which result agree with those for infinitely tall columns derived 154

Ind. Eng.

Chem., Process Des. Develop., Vol. 13, No. 2, 1974

1-

rh -

I A

Figure 1. Schematic diagram of the apparatus, showing the following: A. prepurified nitrogen; B. control valves; C. humidifying column; D. psychrometer; E. mercury manometer; F. needle valve; G. gas rotameter; H. sparger (spinneret); I. bottoms product; J. foam column; K. feed distributor; L. reflux distributor; M. overhead bend; N. variable speed motor; 0. foam breaker; P. reflux splitter; Q. electromagnet; R. top product; s. timer; T. pump; U. feed tank; V. feed rotameter; W. liquid pool; X. removable plate.

previously in a different way (Brunner and Lemlich, 1963; Leonard and Lemlich, 1965b). The rate of upflow can be predicted from theory for interstitial flow (Leonard and Lemlich, 1965b; Fanlo and Lemlich, 1965) for a column that is free of obstructions provided the liquid content of the foam is not overly high. However, these two exclusions do not apply to most of the present study because plates and very wet foams are involved. Accordingly, no attempt was made to apply the said theory here.

Experimental Section Apparatus. Figure 1 shows the overall experimental setup. The foam fractionation column was constructed from flanged lengths of 6.35 cm i.d. Plexiglas pipe of 0.318 cm wall thickness. The bottom length was 43.2 cm, and the next five lengths were each 12.7 cm. All lengths were detachable so as to permit easy insertion and removal of plates. Five identical plates were used at a time. Prehumidified nitrogen gas from a pressure cylinder passed in turn through control valves, a packed humidifying column to avoid any spurious evaporative effects, a micrometer needle valve, a calibrated rotameter, and a submerged gold-platinum spinneret with orifices of 0.05 mm diameter. The bubbles that resulted produced a foam which ascended through the column and exited out the top through an overhead bend, of 12.7 cm maximum height, that was made of the same type and size of Plexiglas pipe as the vertical column. The foam discharged into a spinning-basket foam breaker. The resulting foamate (collapsed foam) passed through an automatic electromagnetic reflux splitter. The refluxed portion (if any) was pumped back to the top of the vertical column through a four-pronged glass spider which served as distributor. The unrefluxed foamate and the bottoms stream were sewered. Feed was pumped from a covered 170-1. polyethylene tank into the column through the reflux distributor when stripping (and therefore no reflux) operation was em-


ployed, or through a similar distributor located 25.4 cm below the reflux distributor and 73.7 cm above the bottom of the column when combined operation was employed, or directly into the liquid pool when enriching operation was employed. The interface between pool and foam was maintained approximately 30 cm above the spinneret. All lines were of Tygon or glass. All gaskets were of Neoprene rubber or Teflon. Flow rates of liquid other than feed were measured by collection and timing. Bubble sizes at various levels were measured photographically. These were corrected for statistical bias (de Vries, 1957, 1972) by equating d32 for the foam to d23 as seen by the camera a t the column wall, where dzi is Zn,d12/Znld,(Lemlich, 1 9 7 2 ~ ) . Surface tension was measured with a du Nuoy ring tensiometer. Operating temperature was maintained at 25 f 1" by room air conditioning. Plates. Three types of perforated plate were investigated. All were of 0.635-cm thick Plexiglas with holes of 0.318 cm diameter spaced on an equilateral triangular pattern with center-to-center distances of 0.635 cm. The first type was a plain perforated plate. The second had a segment removed as shown in Figure 2a. The third was equipped with weirs and downcomer, as shown in Figure 2b. A fourth type, namely, screen plates with a rectangular mesh of 12.6 x 15.8 apertures per cm, was also studied. Solutes. Solutions, in distilled water, of the familiar nonionic surfactant Triton X-100 (Rohm and Haas, 10070 active ingredient by label, average molecular weight approximately 633) were used for all foam fractionation runs. In addition, the dye Methyl Orange (Fisher, sodium salt, minimum purity 99% by label, molecular weight 32'7.3) was included in the feed solutions for all but the total reflux runs and some of the equilibrium (simple mode) runs. Concentrations were measured with a Beckman DU spectrophotometer. Triton X-100 was measured at a wavelength of 275.5 mp. Methyl Orange was measured a t 504 mp and a pH of 1.0 in order to avoid interference by the Triton X-100. The Methyl Orange did interfere with the measurement of the Triton X-100, but fortunately this interference proved to be additive and so could be compensated (Aguayo, 1972). Precautions. Inasmuch as the solutes adsorb on glass and plastic, when changing systems a long dummy run of 8- to 10-hr duration was conducted in order to saturate (age) the apparatus properly before actual runs were begun again. Continuous operation was employed for all runs. The time required to attain essentially steady state, as determined by hourly measurements of concentration, ranged from 3 hr with the simple mode to 12 hr for total reflux at low gas velocities. The results reported are for steady state. Self-consistency was checked by material balance. For Triton X-100 the average absolute deviation was 3.870 and the average algebraic deviation was 1.1%. For Methyl Orange the corresponding deviations were 3.8 and -3.5%, respectively. The small loss represented by the latter may be due to adsorption on glass and plastic. For the ordinary runs conducted in the simple mode, the plates and the five column lengths above the pool were removed. The overhead bend was then attached directly to the remaining length which contained the pool. In addition, the liquid level was raised by about 13 cm. Thus the height of the vertical column of foam did not exceed about 5 cm. In this way, the residence time and hence any coalescence in the vertical foam was mini-

54 crn

(b) Figure 2. Perforated plates showing (a) top view of segment plate and (b) side view of weirs-and-downcomerplate.

mized, thus assuring more accurate determinations of (Brunner and Lemlich, 1963).

r

Results a n d Discussion Equilibrium (Simple Mode). Fifteen runs in the continuous mode a t u p of 0.66-1.25 cm/sec with pool concentrations of Triton X-100 ranging from 1.8 x lo-" to 10.1 x g mol/cc yielded, by application of eq 1, an average r that leveled off above the critical micelle concentration to a value of 2.62 x 10-10 g mol/cm2. This agrees fairly well with previous determinations (Leonard and Lemlich, 1965a; Fanlo and Lemlich, 1965; Jashnani and Lemlich, 1973) as well as with an estimate of 2.65 x g mol/cm2 obtained uia the simplified Gibbs adsorption equation (Glasstone, 1946) and based on present measurements of surface tension below the critical micelle concentration of roughly 3 x g mol/cc. Twenty-three runs a t ug of 0.82-1.26 cm/sec, with pool concentrations of Triton X-100 ranging from 1.6 x lo-' to 10.3 X 10-7 g mol/cc and with 4.7 x to 1.0 X g mol/cc of Methyl Orange also present in the pool, yielded a n average r of Triton X-100 that leveled off to 2.42 x 1 0 - 1 O g mol/cm2, which shows that the presence of the Methyl Orange did not much affect r of Triton X-100. These same 23 runs yielded a K of 3.8 X 10-5 cm for the linear adsorption isotherm of the Methyl Orange. This value of K was substantially independent of the concentration of Triton X-100, which indicates that the micelles of collector did not compete against the surface for the adsorption of the colligend. In other words, the nonionic collector apparently did not attract the colligend ions to any significant degree but simply provided a surface (through foaming) at which the colligend could be adsorbed. Without the correction for statistical bias of bubble sizes which was mentioned earlier, the corresponding values of r would have been 2.81 x g mol/cm2 in the absence of Methyl Orange and 2.71 x 10-lO g mol/ cm2 in its presence, and the corresponding value of K would have been 4.2 x 10-5 cm. Comparison of Plates. Runs were carried out a t total reflux, using only Triton X-100 in water, in order to examine the effectiveness of the several types of plate relative to one another and relative to the plateless (blank) column. Ind.

Eng. Chem.,

Process Des. Develop., Vol. 13, No. 2 , 1974

155

100 80

60 40

' 0 20

2. IO

-

250

l2-

- I

.a8

- 0.6

v

3 - 0.4 ' 8

-

-0.2

4

8 8 .

0.1

4 .

1

2

,

,

3

4

.

,

5

8

.

,

f

8

1

i

9

l

O


vp, cm/sec Figure 3. Effect of plate design and superficial gas velocity on the separation ratio and foamate rate at total reflux with Triton X-100 in water. Type of Symbol Curve perforated plate A

A

0.

v v O.

A

B C D

Plain Segment Weirs and downcomer Plateless (blank)

With the screen plates a t u p > 4 cm/sec the foam became very sparse and was torn apart as it passed through the screens. At ug > 8 cm/sec, liquid accumulated very heavily on the top screen plate and flooded through the overhead bend. Accordingly, the screen plates were abandoned. Twenty-eight runs a t total reflux were conducted with the three types of perforated plate and the plateless column. The concentration of Triton X-100 in the pool ranged from 2.2 x 10-7 to 2.7 x 10-7 g mol/cc. The selection of a fairly narrow range of pool concentrations was made to assure better the comparability of performance. Results are shown in Figure 3. At low up the separation ratios were high (some exceeding 100) and the plateless column gave the best separation. This agrees with the aforementioned earlier investigation of low up (Leonard and Lemlich, 1965a). The suction within the Plateau borders of a plug foam of low liquid content helps to distribute the downflowing liquid evenly over the entire cross section of the column. The presence of plates disrupts the plug flow of the foam and so disrupts the good distribution of liquid flow. On the other hand, a t high cp the separation ratios were lower and the plateless column gave the poorest separation; all the perforated plate columns performed better. This can be explained by noting that a higher up increases the liquid content in the foam. This increase not only makes for a more dilute foamate but also decreases the interstitial suction within the foam, thus increasing interbubble movement which destroys the pattern of counterflow. The plates act as baffles which curb this undesirable movement and divide the column into more or less discrete stages. Thus the plates help preserve the integrity of the upflowing and downflowing streams. The plates also dampen any pulsation and promote coalescence which makes for larger bubbles, more drainage, and more induced internal reflux. Of equal importance with the effect of the plates on the 156

Ind. Eng. Chem., Process Des. Develop., Vol. 13,No. 2, 1974

3

s : p IO

.

.-

=

;,e4:.

.

2-

0 1

2 1 , . 0

1'""

- 2

-

2 3 4 5 6 7 8 9 1 0

vg ,cm /sac Figure 4. Effect of plate design and superficial gas velocity on the number of theoretical plates and column efficiency, at total reflux with Triton X-100in water. Symbols and curve letters are the same as in Figure 3.

separation ratio is the effect on the foam overflow rate, as shown in Figure 3. At high up the plain perforated plates and the plates with weirs and downcomers delivered much higher Q than did the plateless column. This is attributed, a t least in part, to the apparent reduction or elimination of large slugs of gas which formed in the pool at high ug and rose rapidly up the plateless column without forming foam. (The formation of gas slugs a t high up also implies that, contrary to earlier suggestions (Lemlich, 1968b,c), r from simple mode operation is perhaps better determined at more moderate ug. The present authors found a up of about 1 cm/sec to be satisfactory for this purpose.) The local action of the plates was not quite analogous to that of distillation. Some foam flowed up the downcomers and the downcomer segments, as well as up through the perforations. Stagnant foam was observed at plate comers (edges). Finally, some mass transfer occurred in the foam between the plates and elsewhere within the column, especially a t low u g where the aforementioned interstitial suction was strong enough to distribute the downflowing liquid well. As a result, some average column efficiencies in excess of 100% were found. The number of theoretical plates (theoretical stages) and the corresponding efficiencies are shown in Figure 4. With r taken as constant, N , was calculated from eq 5 and includes the pool. The efficiency was calculated as ( N , 1)/5; that is, the pool was assumed to be one theoretical stage and was excluded. Short circuiting of gas in the pool and column was taken into account by first calculating an effective G via eq 1 from some auxiliary simple-mode runs a t various u p with the complete column, and then assuming that the same ratio of effective'G to total G holds when operating in the counterflow modes at the same up. It is clear from Figure 4 that at high u g the column with plain plates, or with' plates fitted with weirs and downcomers, is distinctly better than the plateless column in terms of theoretical stages or column efficiency. If the correction for the short circuiting of gas and the correction for the statistical bias in measuring bubble sizes are omitted, the resulting Np will be lowered but the clear superiority of the said two types of plates over the plateless column will still be evident. Figure 5 shows a plot of N , U / V against uE. (At total reflux, U = 8 . ) The quantity N , U / V is a simple size-free criterion of the overall performance of the column in terms of both its ability to concentrate material and its ability to handle throughput of foam liquid. Based on this

3.0

-

Pm0

2.5

' 0 0

"- 2.0

+;

m

2

X

x

3

1.5

'

;.I.

2

1.0

I 0.5

0 I

0

2

3

4

5

6

7

8

9

o

vs , cm/sec


Figure 5. Effect of plate design and superficial gas velocity on a criterion of column performance at total reflux with Triton X-100 in water. Symbols and curve letters are the same as in Figure 3. .

5

'

"

"

"

'

R-0

-o

02

as

a4

a8

1.0

R R+ I

Figure 6. Enriching mode for Triton X-100. Solid curves and solid points show the number of theoretical plates for the column of five actual plates with weirs and downcomers. Dashed curves and hollow points show the corresponding performance for the column without plates. Feed concentraSuperficial tion of Triton X-100, (g mol/ gas velocity, Symbol 0

0

A

h

m o

v v

CC)

x 107 2.6 5.0 2.6 5.0

cm/sec 4 4 8 8

criterion, the plain plates and those with weirs and downcomers are again very much superior to the plateless column. Of these two favorable types of plate, the plain plate performed the best, as may be seen from Figu'res 3-5. However, high liquid holdup was encountered with this type of plate. Furthermore, the liquid seal was observed to dump cyclically from plate to plate down to the pool. Such holdup and instability might make for some difficulties in future practical applications insofar as startup and control are concerned. Accordingly, a conservative approach was adopted, that is, the plain plates were set aside and only the plates with weirs and downcomers were employed for the remainder of the investigation. Plate with Weirs a n d Downcomer. This type of plate was further studied and compared against the plateless column under conditions of stripping and partial reflux with u p of 4 and 8 cm/sec for each mode. The feed rate

a2

a4

R+ I

as

QO

LO

R

Figure 7. Enriching mode for Triton X-100. Performance of the column with five actual plates with weirs and downcomers compared against performance without plates. Symbols and curves are as defined for Figure 6. was approximately 5.5 cc/sec, which corresponds to a superficial velocity of about 0.17 cm/sec based on the cross sectional area of the empty column. Solute in the feed was 5 x 10-9 g mol/cc of Methyl Orange plus either about 2.6 x 10-7 or 5 x 10 -7 g mol/cc of Triton X-100. Figures 6 and 7 show the results (for Triton X-100) of operating the column as an enricher, and Figures 8 and 9 show the results of operation as a stripper. As before, at high u R the performance of the column with the plates is distinctly superior to the performance of the column without plates. The same is noted for the results of combined operation (omitted for the sake of brevity) even though they are approximate due to the fact that for this mode the feed entered on the third actual plate rather than on the optimum theoretical plate. The results for Methyl Orange are not shown here. They were somewhat erratic. This is attributed, at least partly, to the low range of I' for this solute which in turn makes the calculation of N , rather sensitive to error. Nevertheless, for all modes of counterflow operation at u g of 8 cm/ sec, N p with the plates was always greater than N , without them. Auxiliary Runs and Checks. Eight additional runs were carried out at a higher feed rate of about 11 cc/sec (which corresponds to 0.35 cm/sec) in a 2 x 2 x 2 factorial set, the independent variables of which were stripping us. combined operation, up of 4 us. 8 cm/sec, and plates with weirs and downcomers us. the plateless column. The results in terms of N,U/V confirmed the superiority of the plate column over the plateless column, even at this higher feed rate. Furthermore, as with the results at the lower feed rate, the superiority of the plate column in terms of NpU/V was generally more evident at the higher u p of 8 cm/sec than at the lower ug of 4 cm/sec. This held for stripping as well as combined operation. In an effort to extend the range even further, a dummy stripping run was conducted with the plates with weirs and downcomers at a still higher feed rate of 18.3 cc/sec (which corresponds to 0.58 cm/sec). Results were very satisfying in that there was still no flooding or dumping with this type of plate. However, no quantitative data were taken because the very high feed rate depleted the feed tank too soon to reach steady state. Two auxiliary runs were conducted in the combined mode with the anionic colligend Methyl Orange at a feed Ind. Eng. Chem., Process Des. Develop., Vol. 13, No. 2, 1974

157

4,

r

1

1

X

0

2

4

Vg

6

0

,cm/sec


Figure 8. Stripping mode for Triton X-100. Solid curves and solid points show the number of theoretical plates for the column of five actual plates with weirs and downcomers. Dashed curves and hollow points show corresponding performance for the column without plates. Feed concentration of Triton X-100, Symbol (g mol/cc) x 107

e o A

A

2.6 5.0

concentration of 5 x 10-9 g mol/cc, but with 4 x lo-' g mol/cc of the cationic collector hexadecyltrimethylammonium bromide in place of the nonionic collector Triton X-100. The resulting CD/CF - 1 for Methyl Orange was 3.03 at u p of 4 cm/sec, and 0.99 at u p of 8 cm/sec. This decrease in separation accords with the aforementioned increase in channeling and deterioration in counterflow as v g is increased. The corresponding values of CD/CF - 1 for Methyl Orange when Triton X-100 was the collector were merely 0.44 and 0.13, respectively. The higher values with hexadecyltrimethylammonium bromide are attributed to the better adsorption of an anionic colligend by a surface of cationic collector than by a surface of nonionic collector. The ratios.are 3.03/0.44 = 6.9 at 4 cm/sec, and 0.99/0.13 = 7.6 at 8 cm/sec. The near equality between the two ratios suggests that the adverse hydrodynamic effect of the higher gas velocity on the separation may be fundamentally the same for the two collectors. Of course, further work would be required to confirm this. Some runs were also conducted with the related adsubble technique of bubble fractionation (Dorman and Lemlich, 1965; Lemlich, 1966, 1972a,d; Cannon and Lemlich, 1972) as distinct from foam fractionation. The column was filled with a dilute aqueous solution of the surface-active dye Crystal Violet Chloride (Eastman, 99% dye by label), which is a familiar system (Harper and Lemlich, 1966; Shah and Lemlich, 1970), and nitrogen was bubbled through. Operation was in the combined, enriching, and batch modes. However, unlike foam fractionation, no improvement was found with the weirs-and-downcomer plates, and only a small improvement was found with the plain plates. In view of this feeble performance, bubble fractionation was not pursued any further here. Details are on file (Aguayo, 1972). The conclusions below apply to foam fractionation. Major Conclusions At low gas velocities, the plateless column generally performed the best. However, at high gas velocities, which gave high rates of surface throughput, columns with plates performed the best. Of the designs tested, the plain perforated plate yielded the best quantitative results. At total reflux and a high superficial gas velocity of 8 cm/sec (based on the empty 158

Ind. Eng. Chern., Process Des. Develop., Vol. 13, No. 2 , 1974

0

2

4

~

6

8

vg ,cm/sec Figure 9. Stripping mode for Triton X-100. Performance of the column of five plates with weirs and downcomers compared against the performance without plates. Symbols and curves are as defined for Figure 8. column cross section) the total number of theoretical stages ( N , ) was approximately tripled, the number of theoretical stages excluding the pool ( N , - 1) was approximately quintupled, and NpU/V was approximately octupled, all in comparison with the plateless column. However, the plain perforated plate exhibited high liquid holdup and cyclic plate-to-plate dumping. The perforated plate with weirs and downcomer was second in quantitative performance and did not show the possibly undesirable behavior just mentioned. Also, it was demonstrated that this type of plate can tolerate high liquid feed rates, superficial feed velocities of up to 0.58 cm/sec (based on the empty column cross section) having been employed in the stripping mode. Over all, only a few types of plate were tested, which means the design can undoubtedly be improved further. Nevertheless, the results that were obtained are very encouraging and show that with the use of plates the range of counterflow foam fractionation can be extended to high rates of throughput. This new capacity for higher throughput should allow greater industrial application of foam fractionation. Nomenclature a = slope of the operating line

b = ordinate-intercept of the operating line, g mol/cc C = concentration in the liquid, g mol/cc Cg = concentration in the downflow at the bottom, g mol/cc C D = concentration in the foamate (collapsed foam), g mol/cc C F = concentration in the feed, g mol/cc Cn+1 = concentration in the downflow from the theoretical plate numbered (n+ 1)from the bottom, g mol/cc C T = concentration in the downflow at the top, g mol/cc Cw = concentration in the bottoms stream, g mol/cc = effective concentration in the upflow, g mol/cc (T, = effective concentration in the upflow from the theoretical plate numbered n from the bottom, g mol/cc cn+l= effective concentration in the upflow from the theoretical plate numbered (n 1) from the bottom, g moljcc = effective equilibrium concentration in the upflow, g mol/cc D = flow rate of the foamate product which is withdrawn, cc/sec di = diameter of individual bubble, cm dZl = bubble diameter averaged by second and first moments, cm

+

c*

= bubble diameter averaged by third and second moments, cm F = flow rate of feed, cc/sec G = flow rate of gas, cc/sec K = linear adsorption equilibrium constant, cm Np = number of theoretical plates (theoretical stages) n' = number of bubbles of diameter d' Q = foam overflow rate on a gas-free (collapsed) basis, cc/sec R = refluxratio S = surface-to-volume ratio of bubbles, cm-1 U = rate of upflowing liquid, ccfsec V = volume of column, cc u g = superficial velocity of gas based on cross section of empty column, cm/sec W = flow rate of bottoms stream, cc/sec d32

Greek Letters = slope of effective equilibrium line (? = ordinate-intercept of effective equilibrium line, g mol/cc r = solute surface excess (concentration at surface), g mol/cm* Z = summation over all bubble sizes


cy

Literature Cited Aguayo, G. A.,Ph.D. Dissertation, University Cincinnati, Ohio, 1972. Brunner, C. A., Lemich, R.. Ind. Eng. Chern., Fundam., 2, 297 (1963). Bikerman, J. J., lnd. Eng. Chern., 57 ( l ) , 56 (1965). Cannon, K. D.. Lemlich, R . , Chern. Eng. Progr. Symp. Ser.. 68 (124). 180 (1972). Cassidy, H. G., "Fundamentals of Chromatography (Technique of Organic Chemistry)," Vol. 10, A. Weissberger, Ed., Wiley-lnterscience, New York, N. Y., 1957, p 327. de Vries, A. J., "Foam Stability." Rubber Stichting, Delft, 1957, p 24. de Vries, A. J., in "Adsorptive Bubble Separation Techniques," R . Lemlich, Ed., Academic Press, New York, N. Y., 1972, Chapter 2, pp 7-31,

Dorman, D. C., Lemlich, R., Nature (London), 207. 145 11965). Fanlo, S., Lemlich, R., AIChE-lnst, Chern. €ng. Syrnp. Sef., 9, 75, 85 (1965). Glasstone, S.,"Textbook of Physical Chemistry,'' Van Nostrand, New York, N. Y., 1946, p 1208. Harper, D. O., Lemlich, R., A l C h E J . . 12, 1220 (1966). Hotter, M. S.,Rubin. E., Ind. Eng. Chern. Fundarn., 8, 483 (1969). Jashnani, I. L., Lemlich, R., J. coll. lnterface Sci., in press, 1974. Jashani, I. L., Lemlich, R., Ind. Eng. Chem., Process Des. Develop.. 12, 312 (1973). Lemlich, R., AlChEJ., 12, 802 (1966). Erratain 13, 1017 (1967). Lemlich, R., Chern. Eng.. 75 (27), 95 (1968a). Errata in 76 (6), 5 (1969). Lemlich, R., Ind. Eng. Chern., 60 ( l o ) , 16 (1968b). Lemlich, R . , in "Progress in Separation and Purification," Vol. 1, E. S. Perry, Ed., Wiley-lnterscience, New York, N. Y., 1968c, Chapter 2, pp 1-56. Lemlich, R . , Ed., "Adsorptive Bubble Separation Techniques," Academic Press, New York, N. Y., 1972a. Lemlich, R., "Recent Developments in Separation Science," Vol. 1, N. N. Li, Ed., Chemical Rubber Publishing Co., Cleveland, Ohio, Chapter 5, 1972b, pp 113-127. Lemlich, R.. J. Cosmet. Chern., 23, 299 ( 1 9 7 2 ~ ) Lemiich, R.. J. Geophys. Res., 77, 5204 (1972d), Lemlich, R., "The Adsorptive Bubble Separation Techniques," Proceedings of the Conference on Traces of Heavy Metals in Water: Removal Processes and Monitoring, Nov 15-16, 1973, Center for Environmental Studies, Princeton, University, Princeton, N. J.. in press, 1973. Leonard, R . A,, Lemlich, R.,AlChEJ., 11, 25 (1965a) Leonard, R . A,, Lemlich, R . , AlChEJ.. 11, 18 (1965b). Martin, J. J.,AiChEJ.. 9, 646 (1963). Rubin, E., Gaden. E. L., Jr., in "New Chemical Engineering Separation Techniques," H. M. Schoen, Ed., Wiley-lnterscience, New York, N. Y . . 1962, Chapter 5, pp 319-383. Shah, G. N.,Lemlich, R . , lnd. Eng. Chern., Fundarn.. 9, 350 (1970). Shedlovsky, L., Ann. N. Y . Acad. Sci., 49, 279 (1948). Shih, F. S., Lemlich, R . , AiChEJ., 13, 751 (1967). Shih, F. S., Lemlich. R . , lnd. Eng. Chem.. Fundarn.. 10, 254 (1971). Wace, P. F., Banfield, D. L., Chem. Proc. €ng., 47 ( l o ) , 70 (1966).

Receiued for reuiew August 15, 1973 Accepted December 26, 1973

This study was i n d i r e c t l y supported in p a r t by t h e University of Puerto Rico.

Hydrogen from Coal Char in a Continuous Electrofluid Reactor Justin L. Beeson, Allen H. Pulsifer,* and Thomas D. Wheelock Department of Chemical Engineering and Engineering Research Institute, lowa State University, Arnes, lowa 5001 0

The production of a hydrogen-rich synthesis gas from coal char and steam was investigated in a 12-in. diameter, continuous electrofluid reactor. The reactor has been operated at temperatures approaching 2000°F and using both single and three-phase ac power. Data on the gasification rate and the electrical characteristics of the system were taken and several different electrode materials were tested. A large part of the interelectrode resistance seemed to be due to contact resistance. Electrode life remains a problem.

Introduction Synthesis gas containing various amounts of hydrogen and carbon monoxide may be processed directly to synthetic natural gas or liquid fuels, or may be used to hydrogasify coal to obtain methane. A process for producing a hydrogen-rich synthesis gas from coal char in an electrofluid reactor has been investigated a t Iowa State University over the past several years. In this process the char is gasified with steam a t high temperatures in a fluidized bed reactor which is heated by passing an electric current through the bed of conducting particles. The steam reacts with the carbon in the char to produce a gas

containing hydrogen, carbon monoxide, and carbon dioxide, with the amounts of the individual components being dependent upon the operating conditions of the electrofluid reactor (Pulsifer and Wheelock, 1972). A basic understanding of the behavior of the electrofluid reactor is needed for its optimum design and application, and this is one of the objectives of the work at Iowa State University. To this end, experiments have been carried out in a 4-in. diameter batch reactor (Pulsifer, et d., 1969), and preliminary results were obtained from operation of a 12-in. diameter continuous reactor (Beeson, et al., 1970). Typical results of gasification runs and inforInd. Eng. Chem.,Process Des. Develop., Vol. 13, No. 2, 1974

159

Countercurrent Foam Fractionation at High Rates of Throughput by

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