HYDROMECHANICAL METHOD T O INCREASE EFFICIENCY OF ALGAL PHOTOSYNTHESIS R . L. M I L L E R , ’ A. G. FREDRICKSON, A. H. BROWN,2 AND H . M . T S U C H I Y A University of Minnesota, Minneapolis, Minn.
Results of some experiments on a method to increase the efficiency of light utilization b y dense algal cultures are given. The method involves subjecting an algal cell suspension to a time-dependent pattern of light and dark so that advantage i s taken of “flashing light effects.” Theoretical studies show that controlled patterns of light and dark are needed to yield appreciable increases in efficiency. Algal cell suspensions were ptaced in the annular space between two concentric cylinders; the inner cylinder was rotated and the outer cylinder was fixed. The resulting fluid motion-Taylor vortices-and the inhomogeneous light field in dense suspensions provided the controlled, time-dependent pattern of light and dark. Experiments indicated that rate of photosynthesis increases with increasing rotor speed, and that flashing light effects rather than enhanced mass transfer rates cause the observed increase. Highest efficiencies were found at high suspension densities, low incident light intensity, and high rotor speeds.
in which men or animals are essential parts of a closed or partially closed ecological system require adequate and reliable life support systems. Moreover, limitations on the mass, power, and volume dictate that life support systems be light, compact, and energetically efficient. I t has been suggested that the best life support systems for long-term missions will be miniaturized versions of the solarterrestrial ecological system. I n particular, the simplest miniaturized scheme which has been proposed is composed of men and a photosynthetic organism, usually a unicellular alga. Ecological and engineering problems of fundamental nature must be solved before a n algal life support system can be made operational and reliable. This paper presents a possible solution to one problem from the latter class. Algae in such life support systems will probably be cultured in liquid medium. The energy source for algal photosynthesis will be either solar radiation or a power source carried on board. Whatever the source of energy, the algal system will have to make the most efficient use of that source, consistent with reliability requirements. O n the other hand, efficiency in photosynthesis is usually achieved a t the cost of a low photosynthetic rate, which makes a large culture volume necessary to meet load requirements. This paper gives the results of some experiments designed to improve both efficiency and rate in photosynthesizing algal cultures. The results described have application not only in life support systems, but also in certain terrestrial processes. Thus, cultivation of algae for food is being considered in various countries--see (27)--and algal fermentations have been suggested for the production of biochemicals ( 6 , 7, 70, 24). Improvement of efficiency and rate of algal growth may improve the current economic status of such processes. PERATIONS
S t a t e m e n t of P r o b l e m
When a single algal cell is subjected to continuous irradiation, a t very low light intensities, the rate of photosynthesis is proportional to the intensity, and independent of temperature (within limits). At high light intensities, photosynthesis is Present address, California Research Gorp., Richmond, Calif Present address, Department of Biology, University of Pennsylvania, Philadelphia, Pa. 1
2
134
l & E C PROCESS D E S I G N A N D D E V E L O P M E N T
“light-saturated”-that is, independent of light intensity (but highly temperature-dependent) . A useful and perhaps not too badly oversimplified kinetic model for photosynthesis was proposed by Lumry and Rieske ( 7 7 ) . They suggested a two-step process:
The first stage involves the primary light absorption and the migration of electronic excitation energy to a “trapping center” (denoted by T ) . The rate of this reaction is temperatureindependent. The second step, Reaction 2, is the transfer of excitation energy to a water ,molecule, with subsequent splitting of the molecule into a reducing agent, { H ], and a n oxidizing agent, { O H ) . This step is temperature-dependent, with a n activation energy of about 13,000 cal. per mole (77) in isolated chloroplasts; other values are obtained when whole cells are used. Subsequent reactions include the formation of water and molecular oxygen from the oxidizing agent, and the reduction of carbon dioxide by the reducing agent. Although many more reactions occur in actively metabolizing algae, the two described above appear to limit the rate of photosynthesis under usual conditions of algal propagation. Alternative schemes are discussed by Rabinowitch (76). Clearly, it is desirable to irradiate a single algal celi such that the incident intensity seen by it is near the point where the curve of photosynthetic rate us. intensity begins to flatten out. Unfortunately, under practical conditions it is impossible to provide optimal illumination for every cell in the culture. Cells near the light source see a high intensity, but cells far from the source see a low intensity, because the intensity of a light beam decreases rapidly (approximately exponentially) as it traverses the culture. In other words, such cells are “shaded” by cells between the light source and themselves. Hence, one could achieve a high photosynthetic rate by operating at a high incident intensity, so that most cells receive saturating illumination. Under such conditions, much of the incident energy must be converted into heat, rather than into free energy, since energy absorbed in excess of that required to saturate the photosynthetic rate is not used in the chemical activities of algae. On the other hand, excessive energy losses could be avoided by using low incident intensity, but now efficiency is achieved with a very low rate of photosynthesis.
quences of light and darkness of appropriate duration (ca. 1 and 10 milliseconds, respectively). Achievement of this ideal situation is not possible in any practical apparatus, but it can be approximated by several methods. The method chosen here is that of the rotating, concentric cylinder (Couette or Taylor) apparatus. In a Couette apparatus, the cell suspension to be studied is placed in the annular space between the cylinders. Rotntion of one or the other, or both of the cylinders, then produces a tangential motion (primary motion) in the suspension. The paths of cells in suspension are concentric circles about the axis of the apparatus, if there is no secondary motion. Taylor (23) showed by theoretical analysis that the primary fluid motion is stable-that is, there is no secondary motionif only the outer cylinder is rotated. O n the other hand, he predicted that the primary motion would be unstable if the outer cylinder is fixed and the inner cylinder is rotated faster than some critical value-which he calculated. Taylor’s predictions have been amply verified by experiments. The form of secondary motion which occurs is a series of toroidal vortices (Taylor vortices) filling the gap between the cylinders. A schematic diagram of the motion is shown in Figure 1 ; Figure 2 is a photograph of the paths of aluminum particles suspended in water in the Couette apparatus used in this work. Clearly, algal cells suspended in fluid medium in the Couette apparatus will participate in vortex motion. Hence, in a dense suspension where there are large variations in light intensity across the suspension thickness, a regular pattern of light and (relative) darkness will be seen by individual algal cells. Of course the time scale (angular speed of the vortices) must be of the right order of magnitude, if flashing light effects are to be significant. This can be accomplished by proper choice of apparatus dimensions and rotor speed. The flow which prevails in a Couette apparatus is likely to be somewhat more complicated than indicated above. Appel ( 7 ) found three regimes of flow: laminar Couette flow below the critical rotational speed, laminar vortices a t intermediate speeds, and turbulent motion a t high speeds. A vortex pattern is present even in turbulent motion, however. Appel found a linear relationship between vortex rotational speed and rotor speed in laminar vortex motion. In turbulent motion, the speed of the vortices reached a maximum, and then decreased with further increases in rotor speed. Kaye and Elgar ( 8 ) studied flow patterns in the Couette apparatus when an axial motion was superimposed on the tangential motion. They found that Taylor vortices persisted even when there was a considerable axial flow. In the experiments described below, there was axial flow of a second phase-gas-through the apparatus. Few data are available on two-phase flow in the Couette apparatus; Olin et d.(74) found that axial flow of a second phase through the apparatus did not destroy the vortex pattern. This was confirmed in the present study. Experimental
Materials a n d Methods. The necessary components for the Taylor PGE (photosynthetic gas exchanger) were designed and constructed with the objective of making batch, short-term experiments of 2- t d 6-hour length rather than continuous long-term studies. A schematic of the apparatus is shown in Figure 3. The body of the experimental apparatus consisted of three concentric glass cylinders 25 inches long with a stainless steel rotor in the center. The three annular chambers thus formed contained, from the outside, a ferrous ammonium sulfate 136
l&EC PROCESS DESIGN A N D DEVELOPMENT
-H
T
+G
E L
S
Figure 3. A.
Schematic diagram of Taylor apparatus
Algol chamber Drain hole G. Gas outlet H. Seal 1. lights M . Gas manifold
D.
P.
Photocell Rotor S. Infrored-absorbing solution T. Thermocouple hole W. Water jacket
R.
solution of 3 cm. which filtered approximately 80% of the infrared radiation incident on the apparatus, cooling water circulated from a constant temperature bath. and the algal suspension. The algal chamber was bounded on the outside by a precision-bore glass tube, 2.000 inches in inside diameter, and on the inside by the rotor. The annular gap was 0.223 inch. The algal suspension was admitted and removed through a small drain hole in the bottom plate. All tubing, valves, and fittings which came in contact with the algae were made of either glass or stainless steel. The stainless steel rotor, 1.554 inches in diameter, was coated with a black Teflon film to minimize the reflection of any residual radiation which penetrated through the algal suspension. The rotor was turned by a variable-speed d.c. motor. The rotational speed was thus variable from 0 to approximately 3000 r.p.m. The lighting system consisted of 32 300-watt photoflood lamps located in a circular bank around the apparatus. T h e light bank was 26 inches high by 30 inches in diameter and was backed with a chromium-plated steel reflecting shield. Power to all the lamps was supplied by a variable transformer. The level of light intensity was adjusted by the transformer setting and monitored by two photovoltaic cells located on diagonally opposite sides of the lamp reflector shield. T h e transformer settings were calibrated in terms of radiant flux by means of a vacuum thermocouple, calibrated against radiation standards provided by the National Bureau of Standards. This procedure of altering light intensity has the disadvantage that the spectral qualities of light differ a t varying intensities, but the advantage that light intensity can be varied continuously. Figure 4 is a schematic of the complete gas flow system. T h e influent gas in our experiments was always a mixture of C O ?
LNeedle Figure 4.
Valves
Schematic diagram of gas flow system
and N2 rather than C O Zand air. This eliminated the necessity of a prerun 0 2 determination, a potential source of experimental error. The input COZ concentration was varied from 4 to 15% by volume, depending upon the algal cell suspension density employed for a given experiment. The influent gas was passed through three stages of regulation, a flow controller, and an upstream rotameter before reaching the inlet manifold a t the base of the experimental apparatus. From each of eight outlets on the manifold, the gas was regulated by a needle valve to a n inlet orifice located in the bottom plate of the apparatus. The manifold pressure was monitored on a mercury manometer to correct for gas flow rate through the upstream rotameter. As a check against system leaks, the effluent gas from the algal chamber was passed through a downstream rotameter and finally exhausted through a gas collection bottle. The oxygen content of the effluent gas was monitored by a Chemtronics, Model GP-10 gas phase oxygen transducer. At selected times after steady state had been attained during each experiment. a sample of the effluent gas was collected in the gas sample tube and the 0 2 and CO, content was accurately determined by gas chromatography. This procedure provided another method for oxygen determination and calibration of the oxygen transducrr. It also served to monitor the COZ content of the effluent gas. Usually six to 10 determinations were made in this manner during each experiment. The algal strain employed in our experiments was Chlorella pyrenoidosa, C-37-2. This organism, which was obtained from Jack Myers of the University of Texas, was originally isolated by H. Tamiya of the Cniversity of Tokyo. Cells for each experiment were propagated through tivo liquid, batch cultures under lighted conditions a t 37’ C. before final harvest. A heterotrophic medium of tryptone, glucose, and yeast extract (TGY) was used in each culture. This medium was employed instead of a n inorganic medium because it produced a higher yield of cells in a shorter incubation period. The final culture flasks were incubated for approximately 42 hours a t 37” C. on a reciprocating shaker (83 21/2-inch strokes per minute) under an irradiation of about 450 foot-candles of light (as measured by a Westori Model 756 lightmeter at the level of the bottom of the flasks). The number of culture flasks inoculated depended upon the suspension density required for each experiment; 500 ml. of culture in a I-liter flask yielded about 1.8 ml. of wet-packed cells in 42 hours. The cells were harvested by centrifugation and washed twice with an 0.05M KHzPO, solution of pH 6. After final centrifugation, the cells were resuspended in 500 ml. of the same buffer and transferred to the apparatus. The phosphate buffer suspension medium was used to keep growth (cell
division) a t an absolute minimum. For our experiments, the rate of photosynthesis was taken as the rate of oxygen production. Prior to introduction of the cell suspension into the apparatus, small aliquots of the suspension were removed to determine the cell suspension density (by centrifugation for 20 minutes a t 800 g), and the cell size distribution (Model B Coulter counter). T h e cell suspension was introduced into the appariitus through a drain hole in the bottom plate of the apparatus The light intensity and inner cylinder rotational speed were set a t the desired starting values and the system was allowed to equilibrate with respect to oxygen production. After a steady-state condition was attained. the rotor speed was increased to a new value and the oxygen production again allowed to reach a steady state. The usual procedure was to start a t 0 or 200 r.p.m. and increase by increments of 200 r.p.m. u p to a maximum speed of 2000 r.p.m.
Determination of Rate and Efficiency of Photosynthesis. Unless otherwise noted, the term “rate of photosynthesis’’ used in this paper means net rate of photosynthesis. uncorrected for respiration. The parameters necessary for the determination of the rate of photosynthesis were the gas flow rate (corrected to standard condition) and the concentration of oxygen in the effluent gas. Gas flow rate was obtained from the upstream and downstream rotameter readings. Agreement between the two rotameters was always within the limits that each could be read. Output from the oxygen transducer was continuously recorded. The oxygen concentration of the effluent gas was checked by gas chromatography from time to time. Rates of photosynthesis are reported as either Over-all rate of
0
2
3,?Xstd. ml. 0 , production = V hr. liter
or Rate of
0
2
production per unit volume of cells = gsXstd. ml. 0 2 Vp hr. ml. cells
where qs = gas flow rate, std. ml. per hr. X = mole fraction of 0 2 in effluent gas VOL. 3
NO. 2
APRIL 1964
137
Results and Discussion
V
= volume of suspension, liters
p
= packed cell volume, ml. of cells per liter
The dimensional eficiency of photosynthesis was defined as the amount of oxygen produced per unit of energy expended. Efficiency of light utilization was based upon the power required for illumination only. This was obtained from light intensity measurements made a t the outer surface of the algal chamber with a correction made for the irradiation removed by the infrared filter material. Thus
p40 ? xX
Dimensional efficiency =
106
gram-moles 0 kw.-hr.
2
This can be converted into a dimensionless efficiency by multiplying by the free energy change of the photosynthetic reaction. Total efficiency was based on power required for both illumination and rotation of the inner cylinder. The power for the latter was obtained from a wattmeter connected to the motor. Total efficiency =
x
qsx IoA ~
-
P
+
gram-moles 0
106,
2
kw.-hr.
Pm
In the above equations
Io A
P P
= incident intensity, mw. per sq. cm. = area of illuminated surface, sq. cm. = power required for motor, mw. = fraction of incident radiant energy passed by infrared
A value of 0.185 was used
filter.
I,= 20 MW/SQ. CM P = 24 ML/LITER
\
f = 1/4
IOoo:
2k
800-
-I I
d
‘ r
700-
i
f.112
d
c v)
600-
0 c 0 3
8
5000 ROTOR SPEED
0“
A
INCREASING
ROTOR SPEED DECREASING
400 so
2000
1600
1200
ROTOR
800
IS0
400
SPEED, RPM
Figure 5. Rate of oxygen production per unit volume of algal suspension as function of rotor speed -Calculated
138
from Equations 5 and 6 for two different f values
I&EC PROCESS DESIGN A N D DEVELOPMENT
Effect of Rotational Speed. Figure 5 shows the results of a typical experimental run. Oxygen production in standard (0’ C., 1 atm.) milliliters per hour per liter of suspension is shown as a function of inner cylinder rotational speed in revolutions per minute. Z represents the incident light intensity in milliwatts per square centimeter of illuminated surface, and p the suspension density in milliliters of cells per liter of suspension. Experimental data are indicated by the circles or triangles; circles refer to data obtained by increasing rotor speed and triangles refer to those found upon decreasing rotor speed. The solid lines represent the calculated rate of photosynthesis for two different modes of intermittent illumination (see Appendix). The parameter used to characterize each theoretical t d ) , where t , is the flash time curve is defined by f = t,!(t, and t,j is the dark time. Hencef = ’/? corresponds to the situation in which each cell in the suspension is exposed to the incident light intensity for one fourth of a cycle. The curve for f = 1!2 corresponds to the situation in which each cell is exposed to an average intensity for one half a vortex cycle and in complete darkness for the other half. The “average” intensitv is th;tt estimated a t l / , of the culture depth, by means of Beer’s law. The flash times corresponding to each theoretical curve are shoivn on the abscissa. and are inversely proportional to,rotor speed. This correlation is based on the assumption of a linear relationship between the rotational speed of the inner sylinder and that of the Taylor vortices. This assumption &as been shown to be correct (7, 3) a t rotor speeds where the vortex flow is laminar. In the region of turbulent vortex flow, Lowever, it may not be valid, as shown by .4ppel (I). In our apparatus, the transition to turbulent flow appeared to begin a t about 200 r.p.m,; it was complete a t about 600 r.p.m., as determined from photographs of the fine fluid motion. Therefore: most of the experimental data were obtained in the region where the vortex flow was apparently turbulent. Hence, the flash times shown in the figure have only a relative significance. ‘The trend shown in Figure 5 indicates that the increases in the rate of oxygen production are more pronounced in the rotor speed range from 200 to about 1400. Above 1400 r.p.m. the rate appears to approach a constant value. Qualitatively, the experimental curves are in approximate agreement with what the theory predicts, and also with the predictions of Kok (9). The experimental data appear to follow the theoretical curve forf = somewhat better than that forf = This indicates that the light and dark periods actually obtained in our apparatus were approximately equal. Efficiency of photosynthetic gas exchange in gram-moles of 0 2 per kilowatt hour for the same experiment is shown as a function of rotor speed in Figure 6. The two sets of points in this figure represent efficiencies based on t\vo different considerations of power requirements. Circles refer to the efficiency of light utilization-that is: efficiency based on power required for illumination only. Triangles refer to the total efficiency, as based on power required for both illumination and rotation of the inner cylinder. Obviously, any increase in the rate of photosynthesis by agitation must increase the efficiency of light utilization. The total efficiency is a more valid criterion upon which to assess the merit of apparatus designed to increase the rate of photosynthesis by hydromechanical motion. This efficiency shows a maximum with increasing rotor speed. The optimal rotor speed will, of course, depend upon the light intensity, since the power required for
+
I=2OMW./SQ.
CM P= 24 MLJLITER
a 0.12
e
iooeo 0
5
O 0
0.10
0
2-
X 0
O
0
0 0 0 0
3 0.000
I >0 z 0.06-
w
A
A
4.
A
0
CIRCLES : EFFICIENCY TRIANGLES : TOTAL EFFICIENCY
A
LL LL
w
0.04
0
I
1
1600 1200 800 ROTOR SPEED, R.RM.
2000
400
Figure 6. Efficiency and total efficiency of photosynthesis based on incident energy flux as function of rotor speed
0
Speed decreasing Speed increasing
illumination is proport.iona1 to the intensity. At low intensities, the power required for rotation is a larger portion of the total and hence the optimal rotor speed is somewhat smaller than that shown in Figure 6 In general, the increases in the rate of photosynthesis with increase in rotor speed are more pronounced a t higher suspension densities, in agreement with the predictions of Kok ( 9 ) and Phillips and Myers (75). In our experiments, the fraction of the total incident energy absorbed in the first 10% of the culture depth varied from about 0.75 a t the highest suspension density of 28 ml. per liter, to about 0.2 a t the lowest suspension density of 3.5 ml. per liter. The highest value of 0.75 is somewhat less than the value of 0.9 that has been postulated ( 9 ) as a minimal absorption value for 10 % of the culture depth, in order to realize gains by turbulence. However, suspension densities higher than 28 ml. per liter were difficult to obtain in our \vork brcausc of the limited source of algae. It is of interest to coniparr the increases in the rate of photosynthesis Lvith iricrcasrd rotor speed in our experiments with those observed by Davis et a / . (4). Culture density employed
(L
I
I,
P = 18 ML/LITER
w
5 600-
= 2 0 MW./CM.
0
I
0
e
0
a
0
I \
A
z w
e 0
(3
t.
A
X
0
0
A
c
2
1
0
A
4001
1
I
A
0 CONTROL
A
.A
f BAFFLED ANNULUS I
2000
I
1200 800 ROTOR SPEED, RPM
1600
I
400
0
Figure 7. Effect of random mixing on photosynthesis rate compared with effect of Taylor mixing
by Davis was 150 grams per liter, fresh weight, or approximately 140 ml. of cells per liter. This value is more than four times the highest suspension density used in our study. The maximum total gain in rate of oxygen production between 0 and 2000 r.p.m. in our investigation was approximately 100%. Davis studied the rate of growth of Chlorella for 12-hour periods in a similar type of apparatus, and found a total gain in grolvth rate of 71.47, between 0 and 475 r.p.m., and a gain of 69.8% between 0 and 208 r.p.m. In our experiments. the increase in rate of oxygen evolution between 0 and 200 r.p.m. \vas usually less than 20%, and in some cases negative. These cliscrepancies may be due to the exceedingly dense culture employed by Davis, the strain of alga employed, the fact that Davis performed his experiments \vith proliferating cells~\\-hereas washed cells were employed in our studies, and the fact that Davis determined the rate of groivth over 12-hour periods, Lvhereas in our study the rate of oxygen production with \vashed cell suspensions was determined over intervals of 10 to 30 minutes. Comparison with Completely Stirred Case. In an attempt to compare the rate of photosynthesis obtained in apparatus utilizing Taylor vortex flow with the rate in a completely (randomly) stirred suspension, several experiments were performed with the annular chamber partially obstructed. The baffle arrangement for this purpose consisted of four stainless steel rods (obstructing some 40% of the annular gap) centered in the annulus 90 degrees apart by two rings. T h e baffles extended over the entire length of the algal chamber. With each experiment conducted in this fashion? a "control" experiment (without baffles) was performed under identical conditions. using fresh cells from the same culture. Thus, the length of the light path was the same for each method of agitation. Figure 7 shows the results of one such pair of experiments with rate of oxygen production plotted against rotor speed. At rotor speeds below 1200 r.p.m., the increase in the rate of photosynthesis was greater with the nonbaffled system in which a definite vortex pattern existed. With the baffled system, the increase was small a t low speeds and larger a t higher speeds. HoLvever, it was apparent from visual observation that at high rotor speeds, the flow in the baffled annulus displayed a regular pattern that \cas nearly identical to the flow pattern in the clear annulus a t high rotor speeds. Hence, the increases in the rate of photosynthesis which were obtained with the baffled system at rotor speeds above 1200 r.p.ni. were due to the formation of a regular vortex type flow and not to increased (random) agitation. The experiments and observations of the flow patterns in the baffled annulus are further evidence of the stability of secondary flow in rotating cyclinder apparatus. Effect of Temperature. Figure 8 shows the results of three experimental runs in which all conditions were identical except the suspension temperature. This parameter \vas held constant for each experiment a t 37 ', 27 ', and 17' C.?respectively. The cells for these experiments were grown a t 37 ' C. from a single inoculum culture in 12 1-liter flasks, four of which were harvested for each experiment. The entire study was completed in about 8 hours to minimize cell age effects. The primary significance of Figure 8 is to show that the rate of photosynthesis in our experiments \vas not limited by mass transfer. The argument is as follows. According to the interpolated rate constant data of Tamiya et al. (22) and Sorokin and Myers (ZO)?the ratio of photosynthetic rate a t 37' C. to photosynthetic rate a t 27' C. for the alga Chlorella is about 2.0. Hence, from temperature effects, the rate of photosynthesis a t 37' C. should be about twice that a t 27' C. However, VOL. 3
NO. 2
APRIL
1964
139
.
e
I
A m
I,= 9.2MWISQCM.~ P = 13 ML./LITER
J I
35 MW./SQ. CM. 20 9.2 ALL POINTS AT 1000 R W . 4.5
d
r
>
f
400-
I =2 0
37 "C. e m
d Icn
$ 300F-
0 3 0 0
200-
27 "C. m B
z W
(3
>
:: 100-
17 "C.
A
A 10
20
30
1 )
SUSPENSfON DENSITY, MLAITER
0 2000
1600
1200
800
400
0
ROTOR SPEED, RPM
Figure 9. Rate of oxygen production per unit volume of suspension at 1000-r.p.m. rotor speed as function of suspension density
Figure 8. Effect of temperature and rotor speed on photosynthetic rate ALL POINTS AT 1000 R.RM.
if mass transfer were limiting at 37' C., the limitations would be less a t 27' C., owing to the slower thermochemical activity. and the ratio of the photosynthetic rates at these two temperatures would be close to unity. The actual ratio of the rates a t 1000 r.p.m. was 1.85. This. in addition to other experiments where different concentrations of C 0 2 yielded approximately the same rate of photosynthesis, indicated that light, rather than mass transfer, was the limiting factor in our experiments. Hence, the increases in the rate obtained by increasing rotor speed \vex indeed due to improved flash patterns. Effect of Cell Suspension Density. Figures 9 and 10 show the rate of 0 2 production as a function of cell suspension density at a rotor speed of 1000 r.p.m. The parameter in these figures is the incident light intensity in milliwatts per square centimeter of illuminated surface. The rate of oxygen production in the two figures is given by different units. I n Figure 9 the units are milliliters per hour per liter of suspension, \vhereas in Figure 10 the units are milliliters per hour per milliliter of cells. Hence Figure 9 gives the over-all rate of 0 2 production and Figure 10 gives the rate per unit cell volume. As shown in Figure 9, the rate of 0 2 production per unit volume of suspension appears to approach a limiting valuedependent on incident intensity-as the suspension density is increased. The data shown in Figure 9 are further evidence that flashing light effects were occurring. If such were not the case, the curves in Figure 9 would pass through m a x h a . Such maxima are observed for growth (as distinguished from photosynthesis) in unstirred algal cultures--e.g., see (13). Probably, these are due principally to a rapid decrease of photosynthetic rate per unit cell volume, without a corresponding decrease in respiration rate per unit cell volume, as suspension density is increased. Figure 10 shows that the photosynthetic rate per unit cell volume decreases with increasing suspension density even with flashing light effects, but clearly, the rate of decrease with increasing suspension density must be much less than in the unstirred case. Figure 11 shows the efficiency of light utilization as a func140
l & E C PROCESS D E S I G N A N D DEVELOPMENT
60
.
A
i
35 MW/SQ.CM. 20 9.2 4.5
I
40
7a
I
2 30 (3
>. X
0
2 20 w'
ti a 10 I
I
OO
10
20
30
40
SUSPENSION DENSITY, ML./LITER Figure 10. Rate of oxygen production per unit volume of cellular material at 1000r.p.m. rotor speed as function of suspension density
tion of algal cell suspension density. Efficiency in this figure increases with suspension density in much the same manner as the over-all rate does. However, a comparison of Figures 9 and 11 indicates that the effect of increasing light intensity is essentially reversed. Hence, it is important to note that the highest over-all rate of photosynthesis does not yield the highest efficiency. The more conventional method of reporting maximum photosynthetic rate per unit packed cell volume can be
35
0 20 A 9.2
a I
ALL POINTS AT 1000 R.RM.
t
1.4.5
Y
\
6
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800
600
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0 P= 2 5 ML/LITER 14 6.5 0 3.1 ALL
A
K W
x
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0
/ -
0 0
20
IO
40
30
INTENSITY, MW/SQ. CM. I
IO
20
30
4
SUSPENSION DENSITY, ML./LITER
Figure 12. Over-all rate of oxygen production as function of incident intensity a t 1000 r.p.m.
Figure 11. Efficiency of light utilization as function of algal cell suspension density and incident intensity a t 1000 r.p.m.
P = 2 4 ML./LITER 14 A 6.5 0 3.5
0.2
8
d
r
zz I
misleading in the case of optically dense suspensions. For example, the maximum rate per unit cell volume in Figure 10 does not correspond to either the highest over-all rate in Figure 9, or the highest efficiency of light utilization in Figure 11. Effect of Light Intensity. T h e over-all rate of photosynthesis and the efficiency of light utilization a t 1000 r.p.m. are shoLvn as functions of light intensity in Figures 12 and 13, respectively. The parameter in these experiments is the cell suspension density. As can be seen, there is an apparent maximum in the over-all rate of photosynthesis in Figure 12. ‘This may be a consethe inhibition of photoquence of light “solarization”-i.e., synthesis caused by high radiant intensities. According to Rabinowitch (761, this phenomenon occurs most easily in algae adapted to \Teak ligli~t. The cells for our experiments were cultured under an intensity of 2.1 mw. per sq. cm., which in comparison to the optimal values shown in Figure 12 is relatively small. As can be seen, the optimal light intensity appears to increase some\\hat lvith increasing cell density. A comparison of Figures 12 and 13 shoLvs clearly that the over-all rate is greatest a t relatively high intensities, whereas the efficiency of light utilization is greatest at low intensities. Although it is not shown, the efficiency in Figure 13 must undergo a maximum and in fact become negative a t extremely low intensities, owing to respiration effects. The net rate of oxygen production and thus the efficiency are both zero at the compensating intensity, Lvhere photosynthesis and respiration become equal. In general, the efficiency appears to increase ivith increasing suspension density and decreasing light intensity. It is significant that high efficiency with Taylor apparatus corresponds to conditions where the suspension density is relatively high and i.he incident intensity relatively low. It seems likely that pho.tosynthetic gas exchangers illuminated by artificial light will he operated under conditions \vhere the suspension density is high in order to yield a plentiful supply of oxygen and (possibly) foodstuffs? and the light intensity is
\
ALL POINTS AT 1000 R.PM.
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> X
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LL
W
L o - 3 . 5
c
IO
20
30
1
.INCIDENT INTENSITY, MW./SQ. CM. Figure 13. Efficiency of light utilization as function of incident intensity a t 1000 r.p.rn.
low because of power limitations. In either a n unstirred or a completely stirred culture, these conditions would lead to low efficiencies, whereas in a culture exposed to programmed light (such as by means of Taylor vortices), these conditions correspond to the point of highest efficiency. There are a t least two possibilities for improving the performance of the rotating cylinder gas exchanger. In the first place, the apparatus could be constructed to minimize loss of light to the surroundings. Most of the power supplied to the lamp bank in the present experiments was lost to the surroundings by reflection from the apparatus. This does not appear in the values of efficiency cited, of course, since the power used in making calculation of efficiency was taken to he that actually incident upon the algal suspension. This represents only about 0.5% of the total power supplied to the VOL. 3
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lights. Much more efficient use of power could be obtained by mounting the light source inside the rotor (which would have to be transparent) and silvering the outer annulus cylinder. In the second place, the apparatus could be modified to permit continuous propagation of algal cells. Conditions used in the experiments described above were such that growth was minimal-that is. the liquid solution had no added nitrogen source. Again, cells used ivere cultivated at low light intensities (450 foot-candles or about 2 mw. per sq. cm.) but subjected to intensities as high as 35 mw. per sq. cm. in the test apparatus. Hence, cells may not have had a maximal potential for photosynthesis. The data of Kok ( 9 ) show that Chlorella cells cultured in strong light contain less chlorophyll and have a lower saturated photosynthetic rate per unit of cell volume than cells cultured in weak light. The saturated photosynthetic rate per unit of chlorophyll was found to be higher in the lightadapted cells, however. Hence, there is a rather complex optimization problem, involving adaptation phenomena, flashing light effects, and absorbance of culture, which remains to be investigated. The continuous propagation principle is the feasible means of studying the problem and is currently being studied. Finally, there are other hydrodynamic situations giving rise to secondary flows which could be used to take advantage of the Aashing light effect. Secondary Aow in a pipe bend (79) is one such example. These and other hydromechanical methods may yield better performance, in terms of efficiency and/or reliability, than the situation described herein. Conclusions
A photosynthetic gas exchanger designed to take advantage of the flashing light effect has been tested. The flow pattern in the exchanger subjected a strain of the green alga Chlorella to different patterns of intermittent light. Rate and efficiency of photosynthetic oxygen production were determined in batch experiments. Measurements were made over 10- to 30-minute periods, a t apparatus rotor speeds from 0 to 2000 r.p.m. in 200-r.p.m. increments. Effects of light intensity, cell suspension density. temperature, and gas flow rate were studied. 1he following conclusions were drawn : Rate of oxygen production increases with increasing rotor speed. The increase is due to flashing light effects. and not to enhanced mass transfer. Efficiency of light utilization also increases with rotor speed. However, efficiency based on total power (illumination plus agitation) yields an optimum rotor speed. In this case, the optimum rotor speed increases with increasing light intensity. Over-all rate of oxygen production increases with increasing suspension density, and exhibits a maximum with increasing light intensity. Rate of oxygen production per unit cell volume exhibits a maximum with both suspension density and light intensity. Efficiency increases with increasing suspension density and decreasing light intensity, except a t extremely low intensities, where a maximum efficiency must exist. Appendix
Order-of-magnitude predictions of photosynthetic rate in the Taylor apparatus can be made if some mechanism of photosynthesis is assumed. For instance, assume that Equations 1 and 2 describe the mechanism, Then i n a single algal cell, the rate of production of activated trapping centers, per unit cell volume, is 142
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
where k L ' and kD' are the rate constants for Reactions 1 and 2, IT*] is the concentration of activated trapping centers, To is the total concentration of trapping centers, and l i s the incident light intensity. I n the Taylor apparatus, I is not a constant for a single cell, but is a (periodic) function of time. By combining expressions for the variation of light intensity across the annular gap (Beer's law), and the rotational speed of particles along a given radius of a vortex. i t is possible to obtain an expression for the time dependence of the light intensity seen by a given algal cell. This may be substituted into Equation 3 and the resulting linear differential equation integrated to obtain the time dependence of IT*]. The arbitrar) constant which results from integration is evaluated b) requiring [ T * ]to be periodic, as is I . The rate of photosynthesis per unit cell volume is
and this may be averaged over all cells present to yield the over-all average rate of oxygen production per unit volume of cells. Details are given by Miller (72). The expression which results is too formidable for hand calculation, and a simpler, less rigorous treatment is needed. The picture of the situation in the Couette apparatus outlined above may be replaced \\ith the following idealized one: During a fractionfof thr time required for an algal cell to make one circuit of a Taylor vortex, the cell sees a light intensity gI0, where 0 < g < 1 and IOis the light intensity incident upon the cell suspension. During the remaining fraction (1 - f ) of the circuit time, the cell is in total darkness. Equation 3 is readily solved for this case. and the instantaneous rate can be calculated from Equation 4. If one averages the rate over the total cycle time, the mean rate of photosynthesis. per unit cell volume. results and is ~
in which k D = k,'To, k L = kL'To, and 7 is the cycle time. The cycle time is related to the angular speed of the vortices, w , by 1
=
2R/W
(6)
Clearly, different particle paths in the annular growth chamber require different values o f f and g ; particles near the annulus walls will have values of g near unity and small values of f; particles near the center of a vortex \vi11 have small values of g and f values approaching unity. Hence. Equation 5 should be averaged over all values off and g . The required averaging destroys the simplicity sought for, however, so one simply assumes that with appropriate average values o f f and g , Equation 5 describes the behavior of cells a t all positions in the annulus. Various values o f f and g can then be substituted into Equation 5 and the resulting curves compared with experimental data. This has been done in Figure 5 ; the solid lines represent values computed for f
and f = ’/q. The value of g was chosen so that glo equaled the calculated intensity one quarter of the distance into the suspension. Kinetic constants and the extinction coefficient, e, for Beer’s law were taken from the literature ( 7 2 ) . The lines in Figure 5 show that Equation 5 is sufficient for estimating order-of-magnitude effects of rotor speed ; the value f = ‘ 1 2 seems to be somewhat better than the value f = ‘ / 4 ; in more dense suspensions than used here, perhaps the latter value woiild be more appropriate. =
‘/2
Literoture Cited
(1) Appel, D. W., Tappi 42,767-73 (1959). (2) Brown, H. T., Esconibe, F., Proc. Roy. SOC.(London) B76, 29-111 (1905). (3)‘ Crookewit, P., Honiig, C. C., Kramers, H., Chem. En,y. Sci. 4, 111-18 (1955). (4) Davis, E. A,, Dedrick, J., French, C. S., Milner, H. W., Myers, J., Smith, J. H. C., Spoehr, H. A,, Chap. 9, pp. 135-8, in “.41gal Culture from 1,aboratory to Pilot Plant,” J. S. Burlew, ed., Carnegie Institution of iVashington, IVashington, D. C., Publication 600, 1953. (5) Fredrickson, A. G., Brown, A. H., Miller, R. L., Tsuchiya, H. M.,Am. Rocket Soc. J . 31, 1429-35 (1961). (6) Guehler, P. F., Doclson, K. M., Tsuchiya, H. M., Proc. A’atl. Acad. Sci. 48, 377-9 (1962). (7) Hutner, S. H.. Division of Microbial Chemistry and Technology, 142nd meeting, ACS, Atlantic City, N. J., September 1962. (8) Kaye, J., Elgar, E. C., Trans. A S W E 80,753-65 (1958). (9) Kok, B., Chap. 6, p!). 63-75, in “Algal Culture from Laboratory to Pilot Plant,” .1. S. Biirlew, ed., Carnegie Institution of LYashington, Washington, D. C., Publication 600, 1953.
(10) Krauss, R. W., Division of Microbial Chemistry and Technology, 142nd meeting, ACS, Atlantic City, N. J., September 1962. (11) Lumry, R., Rieske, J. S., Plont Physiol. 34, 301-5 (1959). (12) M.iller, R. L., Ph.D. thesis in chemical engineering, University of Minnesota, Minneapolis, 1962. (13) Myers, J., Graham, J. R., Plant Physiol. 34, 345-52 (1959); 36, 342-6 (1961). (14) Olin, J., Koenig, W. W.,Babb, A. L., McCarthy, J. L., Chem. Eng. Progr. Symp. Ser. 50, 103-10, 111-21 (1954). (15) Phillips, J. N., .Jr., Myers, J., Plant Physzol. 29, 152-61 (1954). (16 ) Rabinowitch. E. I.. “Photosvnthesis and Related Processes.” ‘ V o l . I, pp. 532-6, Vol. 11. p.’l, Chap. 28, Interscience, New York, 1945, 1951. (17) Rieske, J. S., Lumry, R., Spikes, J. D., Plant Physiol. 34, 293-300 (1959). (18) Sasa. T.. J . Gen. Abbl. ,‘ Microbiol. ( T o k v o ) 3. 121-4 (1957). (19) Schlichting, H., “Boundary Laye; Theory,” pp. 426-7, McGraw-Hill, New York, 1955. (20) Sorokin, C., Myers, J.?Science 117, 330-1 (1953). (21) Tamiya, H., Ann. Rei,. Plant Physiol. 8 , 309-34 (1957). (22) Tamiya, H., Hase, E.: Shibata, K., Mituya, A., Iwamura, T., Nihei, T., Sasa, T., Chap. 16, pp. 204-32, in “Algal Culture from Laboratory to Pilot Plant,” J. S. Burlew, ed., Carnegie Institution of LVashington, N‘ashington, D. C., Publication 600, 1953. (23) Taylor, G. I., Phil. Trans. Roy. SOC.A223, 289-343 (1923). (24) Van Baalen, C., Division of Microbial Chemistry and Technology, 142nd meeting, ACS, Atlantic City, 3’.J., September 1962. RECEIVED for review May 20, 1963 ACCEPTED October 28, 1963 Work supported in part by the National Aeronautics and Space .4dministration. Grant NS G 79-60.
INORGANIC ION EXCHANGE SEPARATION OF CESIUM FROM PUREX-TYPE HIGH-LEVEL RADIOACTIVE WASTES JACK L. NELSON, GEORGE J. A L K I R E , AND BASIL W . M,ERCER Hanford Atomic Products Operation, General Electric Co., Richland, TVash.
A flowsheet is presented which includes cesium loading, sodium removal with oxalic acid, water wash, and acid removal with NH40H, all a t low temperatures, and elution with 2M (NHJzC03 a t elevated temperature. The clinoptilolite column can b e re-used with bed make-up of about 2% per cycle. A relatively pure
(CS)~CO solution ~ is obtained b y volatilization of the (NH&C03 from the eluate. Data on the effects of varied flow rate, temperature, waste dilution, exchanger particle size, and cesium concentration are given. The flowsheet is adaptable to removal of small amounts of cesium from various solutions high in salts or acid.
and fission products are produced by the irradiaof uranium in nuclear reactors. These materials are chemically separated from each other by the Purex process, with the gross fission product portion becoming the socalled high-level radioactive wastes. These wastes can be treated directly for long-term confinement or first treated to separate certain long-lived fission products such as cesium-1 37 and strontium-90. Research on the use of minerals and other inorganic ion exchangers for treatment of low-level wastes has been in progress a t General Electric, Hanford Laboratories, and other sites for several years l(7, 5, 6 ) . Recently this work has been extended toward separation, purification, and packaging of specific radionuclides from high-level wastes. Mineral and inorganic ion exchangers show considerable promise in this type of application because of their ionic selectivity. thermal IJJTONITIM
Ption
stability, and radiation stability. Work by Hanford Laboratories personnel has shown that the natural zeolite, clinoptilolite, has unique properties that make it especially attractive for use in separation of cesium from many types of wastes. The objective of the investigation reported in this paper was a chemical flowsheet for the separation of cesium from acid Purex formaldehyde-treated waste (FTW) as a step in a highlevel radioactive waste treatment program. The flowsheet presented in Figure 1 was conceived as a part of a n over-all process to separate and concentrate cesium from high salt wastes on an exchanger of high cesium selectivity. followed by elution and reloading on a n exchanger of high capacity. The cesium on the high capacity exchanger could be used as such, stored, shipped, or removed a t any time for additional processing. Zeolites of highest cesium selectivity generally d o not also VOL. 3
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