- Email: [email protected]
Use of carbon-14 to measure carbon dioxide removal in a synthetic ammonia plant
:nternational
Journal
of Applied
Radiation
and
kotopes,
1965,
Vol.
16, pp.
561. 564.
Pergaman
Press
Ltd.
Printed
in Northern
Ireland
Use of Carbon-14 to Measure Carbon Dioxide Removal in a Synthetic Ammonia Plant B.
A.
FRIES,
D.
E.
California
HULL, Research
R.
J.
DUPZYK
Corporation,
and D. J. LAMOREE
Richmond,
California
(Received 13 April 1965) Use of the total-sample method was extended to measurement of the leakage of carbon dioxide through an oil-filled knockout trap in a large synthetic ammonia plant. Approximately 1 mc of C?40, was injected into the plant, and gas samples were withdrawn from points ahead of and beyond the trap. The C140, was recovered from the samples and counted in a liquid The high sensitivity of the method is illustrated by the detection of scintillation counter. approximately 1 /i’ccof Cl” m . a gas stream flowing at over 6 x 106 standard ft3/hr. The trap was shown to be 99.9 percent efficient in CO, removal. L’EMPLOI BIOXYDE
DU C’IRBONE-14 POUR MESURER L’EXTRACTION DU DE CARBONE DXNS UNE INST.4LL.2TION ;Z AMMONIAC SYNTHEI’IQUE
L’emploi de la mtthode d’Cchantillon total fut Ctendu z?~ la mesure de la fuite du bioxyde de carbone & travers un purgcur rempli d’huile dans une grande installation B gaz ammoniac synthktique. Environ 1 mc de C140, fut lance dans l’installation et on retirait des echantillons de gaz & des points avant et aprhs le purgeur. Le C’“O, fut extrait des kchantillons et comptt dans un compteur de scintillation liquide. La grande scnsibilitt de la mCthode se dCmontre par la ditection d’environ 1 /tc de C l4 dans un courant de gaz coulant B plus de 6 x lo6 pieds cubiques/hr (environ 170,000 m3/hr). 11 se montra que le purgeur avait une efficacite d’extraction de CO, de 99.9”,.
DER
GEBRAUCH IN
VON EINEM
Cl4 ZUR MESSUNG SYNTHETISCHEN
DER KOHLENDIOXYDTRENNUNG I\MMONIAK WERK
Der Gebrauchder Gesamtprobenmethode wurde zur Messung dcs Leckens von Kohlendioxyd durch eine mit ii1 gefiillte Sperr-Falle in einem Grossbetrieb fiir synthtischen Ammonik erweitert. Ungefghr 1 mc von C l4 0, wurde in den Kreislauf injizicrt und Gasproben an verschieden Stellen vor und nach der Falle entnommen. Das C140, wurde aus den Proben wiedergewonnen und mit einem Fliissigkeitsszintillationsz2hler gemessen. Die hohe Empfindlichkeit der Methode wird angezeigt durch den Nachweis von ungefshr l/kc von Cl4 in einem Gasstrom von mehr als 6 x lo6 Standard Kubik Fuss pro Stunde. Die benutzte Falle war zu 99,976 bei der Beseitigung von CO, wirksam. 1
561
562
B. A. Fries, D. E. Hull, R. J. Du;bzyk and D. J. Lamr-ee
INTRODUCTION the total-sample method in flow measurements has recently been described. In this technique a portion of a single-pulsed tracer wave passing a sampling point is withdrawn at a uniform rate over a period of time long enough to include the whole tracer wave. The flow is computed from the equation, & == AFIRT, where il is the amount of tracer injected (mc), R is the sample count rate (cps), T is the sample collection time (SK), and F is the usual counting sensitivity factor of the total-count method (cps/ me/ml) (I). In fluid flow measurements the application of the total-sample method leads to improved precision’“) by making it possible to count as long as necessary to achieve the desired statistical accuracy. In the conventional total-count method a limit is set on the statistics of counting the tracer wave by quantities of radioactivity that are both economic and feasible to handle. The application of the total-sample method to steam flow measurement(“) is based on the use cf tritium tracer, an isotope which can be counted in the field only with great difficulty but which can be counted quite simply in the laboratory. Its application to another kind of problem, measuring the efficiency of a process unit in a large synthetic ammonia plant, is dcscribed here. The unit, a knockout pot, removes traces of carbon dioxide from the synthesis gas. Carbon dioxide must be removed because it poisons the synthesis catalyst, and, if present in the final liquid ammonia product, may cause plugging of agricultural spray equipment. Carbon dioxide enters the system at about 10 ppm where it reacts with ammonia to form an aerosol of ammonium carbamate. The aerosol is removed by a packed oil trap. Lsfeasurement of the efficiency of the trap by chemical analysis of its contents was not feasible because of decomposition of the carbamate and difficulty of quantitative recovery during a cleanout of the trap. An estimate of the efficiency of the trap, based on an analysis of ammonia for CO, at about 1 ppm concentration, was about 96 percent. Kadioactivity techniques for a more precise measurement were then explored. These require an isotopic tracer for CO, because a USE
OF
chemical separation is involved in the process. Carbon-14 is the only possible tracer. The total-sample technique appeared to be the most practical method in view of the large flow rates and the difficulty of counting soft Cl* P-rays. A test was deemed feasible with only a few millicuries of Cl* if the collected samples were counted at high efficiency in a liquid scintillation counter. DESIGN
OF
EXPERIMENT
A simplified schematic diagram of the synthesis section of the ammonia plant is shown in Fig. 1. The synthesis gas, a mixture of N, and H,, enters the plant at 2200 psig. It is compressed to 4500 psig, then it enters the synthesis loop where it combines with recycle gas. The combined gases pass through the knockout trap, where CO, is removed, then to the synthesis converter, and to the KH, separator. Lnconverted gases plus NH, not removed by the separator are then recycled. A portion of the recycle gases is vented to the atmosphere to remove argon which slowly accumulates in the The C1*02 injection and sampling system. points for the test are also shown in Fig. 1. The principle to be employed in the test would be to inject a single pulse of C?*Og into the synthesis gas line and to sample both ahead of and beyond the knockout trap. Since the tracer would be highly diluted, a concentrating step would be made involving the absorption of (10, in NaOH solution and recovery by precipitation as BaCO,. The radioactivity would bc measured by regeneration of CO, and reabsorption in a small volume of a base for liquid scintillation counting. Calculations showed that even with several millicuries of Cl*, the process stream must be sampled at a rate of about 10 ft3/min to gel an adequate amount of activity for counting. F~I absorbing CO, at this rate, a large absorber was built and tested, It was made from a 5-ft length of 3-in. diameter glass tubing which was packed with 3 ft of broken glass tubing and filled with 1 1. of 1 R/IKaOH. 6Ve measured its efficiency by injecting with a syringe a known quantity of high specific activity C?*Oz into the rubber hose leading to the absorber while compressed air flowed at a rate of 10 ft3/min. The recol-cry of C?*O‘,. was 80 percent.
Use of carbon-14
to measure carbon dioxide removal
563
VENT -
RECYCLE COMPRESSOR m
4 SAMPLE POINT
5050 MSCFH 7% NH,
F40 _
KNOCKOUT ptAJ
INE!ON m
e
HIGH s.s.
PRESSURE TUB'NG
SYNTHESIS CONVERTER
J
SAMPLE POINT
FIG. 1 PRELIMINARY TEST KRYPTON-85
WITH
It was desirable to fix the time for collecting the tracered sample so as to avoid the possible overlapping of a second passage of tracer not removed in the first passage. To achieve this, a preliminary test was made with a gas-phase tracer to determine the duration of the tracer wave at the sample points and to test the high pressure injection equipment. For the tracer injection, the high-pressure gas (4500 psig) from the compressor was used to force 190 mc of Krs5 into the low-pressure side of the compressor. The tracer was measured and handled in the manner described for Krs5 gas flow measurementst4). The Krs5 activity at the sample points was detected with gas counting chambers from sample streams drawn from the two lines. A single peak was found on the injection line about 10 set after the injection. After the peak, the count rate fell off very rapidly. On the process line, a peak appeared 60 set after the
injection, then a second and third peak were found before the count rate leveled off as a result of dispersion of the tracer in the recirculating system. The interval between these recurring peaks was about 150 sec. From these data we decided to sample the injection line for 1 min and the process line for 3 min after the injection of Cl40 2’ PLANT TEST The test with Ci40s was made starting with approximately 1 mc. This was injected into the plant in the same way as the Krs5. On the injection line, 0.020 ft3 of gas was passed into a small NaOH absorber over a 1 min period. Several cubic feet per minute of gas was passed out an adjacent valve to keep the sample line leading from the main line flushed. On the process line 12.75 ft3 of gas was passed through the large absorber over a 3 min period. The true rate was somewhat larger than this because this stream contained about 6 percent ammonia, part or most of which dissolved in the
564
B. A. Fries, D. E. Hull, R. .I. Duflzyyh-and D. J. Lamoree
'rABLE
BaCO, ‘I’otal
Sample Background Process line
482
Injection
348
line
(mg) Samplr
1
Gross (cpsi 1.80 2.03 2.22 9.85 9.40
207 207 144 140
.t 0.04 0.05 0.05 0.18 0.18
Injection stream
efficiency
6,510,OOO
1,460,000
recovered
Sampling time, T(min) Sample volume, V( ft3) Sample activity, R/ V( cps/ft3) Counler sensitivity,
3.0
activity
through
1260
equation; by
Cl4
to
added
(spike
absorber
solutions
were
to the gas
the laboratory
for radioassay.
the
volume
1 1. initial absorber.
added
d =
tate
of
BaCO,
added.
for
Two
precipitate were
taken
in 4 ml All
and of
then
BaCl,
diffusion
cell,
the CO,
liberated
was
1 M Hyamine
of the absorbing
counter. Table
The
counted
These
counting.
into a 60 ml counting and
was
BaCO,
Conway
hydroxide* solution
with
was
cell containing
toluene-paraterphenyl-POPOP
solution
was
precipi-
dried
scintillation a Tracerlab
counting
data
of Hyamine
10-X
are
CE-1
shown
in
1.
* Hydroxide ment Co.).
(Packard
activities
process
and
Instru-
The The
trap
was recovered
calculation
from
for the process
the
stream
i: 3.0
1.6 /.K.
was found
remarkable
ft3/hr.
other
prohibitively
in Table
to be about
sensitivity a total
of the total-sample
quantity
To obtain
large
99.9 percent
by the fact that we were
in a stream
methods
2.
CO,.
is highlighted
to measure
standard
of
samples
>: 106/60) x 0.094
1 PC of Cl4 dispersed with
was
quantity
; X was used in the form cps/ft”, activity
in removing
technique able
as cps/pc,
scintillation
the results are shown
effective
passing the total-
18,700 =
All
from
a known
liquid
(6.51
QZZT
to the
of Na,CO,
each
for
in a small
in methanol. transferred
of
from
sufficient
handling,
was added,
absorbed
to give
portions were
placed
HClO,
a
The
820 ml of
recovered
150 mg
to each solution
back
About
was
About
brought
on
average
and the activity calculated
F
The
the
F, expressed the
the total
samples. is:
the gas went
on
counting
method)
since
0.13
injected
determined
meter.
large
19.2 cps for the total
the trap were
sample
18,700
before
gave
lines, respectively.
The
through trap,
absorber
samples
trapping
recovery
RESULTS
960
1.6
liquid
1
& 0.15 & 0.15 -1: 0.44 + 0.44
for 80 percent
large
0.020
0.094
solution,
the
from
injection
0.54 0.!)8 19.4 18.9
80 percent
\,alues of 1.2 and
A,IAl(“b)
absorber
for
sample
(cps
a correction and
drainage
1.0
12.75
F(CPSl/iC) Cl4 in plant stream, A(w! Leaking
Applying
Process stream
Q(fi3/hr)
Total
0.23 ?m0.06 0.42 _i 0.06 8.05 : 0.18 7.60 i 0.18
:I: + -i: i
TAHIX 2
Plant flow rate,
Dj,t (CPSJ
equally
would
quantities
of only
about
of over 6 x 10” precise results have
required
of the isotope.
REFERENCES 1. HULL D. E. Znt.J. a#Z. Rad. Zxotol,es 4, 1 (1958!. 2. HULL I>.E. Sytnposium on Radiotracer A&lications in the Petroleum and Chemical Industries, American Chemical Society, Philadelphia, ,\pril6-10 (1964). 3. FKIES B. ‘-1. Jnt. .I. af$l. Rad. Irotopes 16, 35 (196.5). 4. FRIES B. A. Int. .I. a(,i,l. Rad. Iroto(,e.c 13, 277 i 1962 I.
Journal
of Applied
Radiation
and
kotopes,
1965,
Vol.
16, pp.
561. 564.
Pergaman
Press
Ltd.
Printed
in Northern
Ireland
Use of Carbon-14 to Measure Carbon Dioxide Removal in a Synthetic Ammonia Plant B.
A.
FRIES,
D.
E.
California
HULL, Research
R.
J.
DUPZYK
Corporation,
and D. J. LAMOREE
Richmond,
California
(Received 13 April 1965) Use of the total-sample method was extended to measurement of the leakage of carbon dioxide through an oil-filled knockout trap in a large synthetic ammonia plant. Approximately 1 mc of C?40, was injected into the plant, and gas samples were withdrawn from points ahead of and beyond the trap. The C140, was recovered from the samples and counted in a liquid The high sensitivity of the method is illustrated by the detection of scintillation counter. approximately 1 /i’ccof Cl” m . a gas stream flowing at over 6 x 106 standard ft3/hr. The trap was shown to be 99.9 percent efficient in CO, removal. L’EMPLOI BIOXYDE
DU C’IRBONE-14 POUR MESURER L’EXTRACTION DU DE CARBONE DXNS UNE INST.4LL.2TION ;Z AMMONIAC SYNTHEI’IQUE
L’emploi de la mtthode d’Cchantillon total fut Ctendu z?~ la mesure de la fuite du bioxyde de carbone & travers un purgcur rempli d’huile dans une grande installation B gaz ammoniac synthktique. Environ 1 mc de C140, fut lance dans l’installation et on retirait des echantillons de gaz & des points avant et aprhs le purgeur. Le C’“O, fut extrait des kchantillons et comptt dans un compteur de scintillation liquide. La grande scnsibilitt de la mCthode se dCmontre par la ditection d’environ 1 /tc de C l4 dans un courant de gaz coulant B plus de 6 x lo6 pieds cubiques/hr (environ 170,000 m3/hr). 11 se montra que le purgeur avait une efficacite d’extraction de CO, de 99.9”,.
DER
GEBRAUCH IN
VON EINEM
Cl4 ZUR MESSUNG SYNTHETISCHEN
DER KOHLENDIOXYDTRENNUNG I\MMONIAK WERK
Der Gebrauchder Gesamtprobenmethode wurde zur Messung dcs Leckens von Kohlendioxyd durch eine mit ii1 gefiillte Sperr-Falle in einem Grossbetrieb fiir synthtischen Ammonik erweitert. Ungefghr 1 mc von C l4 0, wurde in den Kreislauf injizicrt und Gasproben an verschieden Stellen vor und nach der Falle entnommen. Das C140, wurde aus den Proben wiedergewonnen und mit einem Fliissigkeitsszintillationsz2hler gemessen. Die hohe Empfindlichkeit der Methode wird angezeigt durch den Nachweis von ungefshr l/kc von Cl4 in einem Gasstrom von mehr als 6 x lo6 Standard Kubik Fuss pro Stunde. Die benutzte Falle war zu 99,976 bei der Beseitigung von CO, wirksam. 1
561
562
B. A. Fries, D. E. Hull, R. J. Du;bzyk and D. J. Lamr-ee
INTRODUCTION the total-sample method in flow measurements has recently been described. In this technique a portion of a single-pulsed tracer wave passing a sampling point is withdrawn at a uniform rate over a period of time long enough to include the whole tracer wave. The flow is computed from the equation, & == AFIRT, where il is the amount of tracer injected (mc), R is the sample count rate (cps), T is the sample collection time (SK), and F is the usual counting sensitivity factor of the total-count method (cps/ me/ml) (I). In fluid flow measurements the application of the total-sample method leads to improved precision’“) by making it possible to count as long as necessary to achieve the desired statistical accuracy. In the conventional total-count method a limit is set on the statistics of counting the tracer wave by quantities of radioactivity that are both economic and feasible to handle. The application of the total-sample method to steam flow measurement(“) is based on the use cf tritium tracer, an isotope which can be counted in the field only with great difficulty but which can be counted quite simply in the laboratory. Its application to another kind of problem, measuring the efficiency of a process unit in a large synthetic ammonia plant, is dcscribed here. The unit, a knockout pot, removes traces of carbon dioxide from the synthesis gas. Carbon dioxide must be removed because it poisons the synthesis catalyst, and, if present in the final liquid ammonia product, may cause plugging of agricultural spray equipment. Carbon dioxide enters the system at about 10 ppm where it reacts with ammonia to form an aerosol of ammonium carbamate. The aerosol is removed by a packed oil trap. Lsfeasurement of the efficiency of the trap by chemical analysis of its contents was not feasible because of decomposition of the carbamate and difficulty of quantitative recovery during a cleanout of the trap. An estimate of the efficiency of the trap, based on an analysis of ammonia for CO, at about 1 ppm concentration, was about 96 percent. Kadioactivity techniques for a more precise measurement were then explored. These require an isotopic tracer for CO, because a USE
OF
chemical separation is involved in the process. Carbon-14 is the only possible tracer. The total-sample technique appeared to be the most practical method in view of the large flow rates and the difficulty of counting soft Cl* P-rays. A test was deemed feasible with only a few millicuries of Cl* if the collected samples were counted at high efficiency in a liquid scintillation counter. DESIGN
OF
EXPERIMENT
A simplified schematic diagram of the synthesis section of the ammonia plant is shown in Fig. 1. The synthesis gas, a mixture of N, and H,, enters the plant at 2200 psig. It is compressed to 4500 psig, then it enters the synthesis loop where it combines with recycle gas. The combined gases pass through the knockout trap, where CO, is removed, then to the synthesis converter, and to the KH, separator. Lnconverted gases plus NH, not removed by the separator are then recycled. A portion of the recycle gases is vented to the atmosphere to remove argon which slowly accumulates in the The C1*02 injection and sampling system. points for the test are also shown in Fig. 1. The principle to be employed in the test would be to inject a single pulse of C?*Og into the synthesis gas line and to sample both ahead of and beyond the knockout trap. Since the tracer would be highly diluted, a concentrating step would be made involving the absorption of (10, in NaOH solution and recovery by precipitation as BaCO,. The radioactivity would bc measured by regeneration of CO, and reabsorption in a small volume of a base for liquid scintillation counting. Calculations showed that even with several millicuries of Cl*, the process stream must be sampled at a rate of about 10 ft3/min to gel an adequate amount of activity for counting. F~I absorbing CO, at this rate, a large absorber was built and tested, It was made from a 5-ft length of 3-in. diameter glass tubing which was packed with 3 ft of broken glass tubing and filled with 1 1. of 1 R/IKaOH. 6Ve measured its efficiency by injecting with a syringe a known quantity of high specific activity C?*Oz into the rubber hose leading to the absorber while compressed air flowed at a rate of 10 ft3/min. The recol-cry of C?*O‘,. was 80 percent.
Use of carbon-14
to measure carbon dioxide removal
563
VENT -
RECYCLE COMPRESSOR m
4 SAMPLE POINT
5050 MSCFH 7% NH,
F40 _
KNOCKOUT ptAJ
INE!ON m
e
HIGH s.s.
PRESSURE TUB'NG
SYNTHESIS CONVERTER
J
SAMPLE POINT
FIG. 1 PRELIMINARY TEST KRYPTON-85
WITH
It was desirable to fix the time for collecting the tracered sample so as to avoid the possible overlapping of a second passage of tracer not removed in the first passage. To achieve this, a preliminary test was made with a gas-phase tracer to determine the duration of the tracer wave at the sample points and to test the high pressure injection equipment. For the tracer injection, the high-pressure gas (4500 psig) from the compressor was used to force 190 mc of Krs5 into the low-pressure side of the compressor. The tracer was measured and handled in the manner described for Krs5 gas flow measurementst4). The Krs5 activity at the sample points was detected with gas counting chambers from sample streams drawn from the two lines. A single peak was found on the injection line about 10 set after the injection. After the peak, the count rate fell off very rapidly. On the process line, a peak appeared 60 set after the
injection, then a second and third peak were found before the count rate leveled off as a result of dispersion of the tracer in the recirculating system. The interval between these recurring peaks was about 150 sec. From these data we decided to sample the injection line for 1 min and the process line for 3 min after the injection of Cl40 2’ PLANT TEST The test with Ci40s was made starting with approximately 1 mc. This was injected into the plant in the same way as the Krs5. On the injection line, 0.020 ft3 of gas was passed into a small NaOH absorber over a 1 min period. Several cubic feet per minute of gas was passed out an adjacent valve to keep the sample line leading from the main line flushed. On the process line 12.75 ft3 of gas was passed through the large absorber over a 3 min period. The true rate was somewhat larger than this because this stream contained about 6 percent ammonia, part or most of which dissolved in the
564
B. A. Fries, D. E. Hull, R. .I. Duflzyyh-and D. J. Lamoree
'rABLE
BaCO, ‘I’otal
Sample Background Process line
482
Injection
348
line
(mg) Samplr
1
Gross (cpsi 1.80 2.03 2.22 9.85 9.40
207 207 144 140
.t 0.04 0.05 0.05 0.18 0.18
Injection stream
efficiency
6,510,OOO
1,460,000
recovered
Sampling time, T(min) Sample volume, V( ft3) Sample activity, R/ V( cps/ft3) Counler sensitivity,
3.0
activity
through
1260
equation; by
Cl4
to
added
(spike
absorber
solutions
were
to the gas
the laboratory
for radioassay.
the
volume
1 1. initial absorber.
added
d =
tate
of
BaCO,
added.
for
Two
precipitate were
taken
in 4 ml All
and of
then
BaCl,
diffusion
cell,
the CO,
liberated
was
1 M Hyamine
of the absorbing
counter. Table
The
counted
These
counting.
into a 60 ml counting and
was
BaCO,
Conway
hydroxide* solution
with
was
cell containing
toluene-paraterphenyl-POPOP
solution
was
precipi-
dried
scintillation a Tracerlab
counting
data
of Hyamine
10-X
are
CE-1
shown
in
1.
* Hydroxide ment Co.).
(Packard
activities
process
and
Instru-
The The
trap
was recovered
calculation
from
for the process
the
stream
i: 3.0
1.6 /.K.
was found
remarkable
ft3/hr.
other
prohibitively
in Table
to be about
sensitivity a total
of the total-sample
quantity
To obtain
large
99.9 percent
by the fact that we were
in a stream
methods
2.
CO,.
is highlighted
to measure
standard
of
samples
>: 106/60) x 0.094
1 PC of Cl4 dispersed with
was
quantity
; X was used in the form cps/ft”, activity
in removing
technique able
as cps/pc,
scintillation
the results are shown
effective
passing the total-
18,700 =
All
from
a known
liquid
(6.51
QZZT
to the
of Na,CO,
each
for
in a small
in methanol. transferred
of
from
sufficient
handling,
was added,
absorbed
to give
portions were
placed
HClO,
a
The
820 ml of
recovered
150 mg
to each solution
back
About
was
About
brought
on
average
and the activity calculated
F
The
the
F, expressed the
the total
samples. is:
the gas went
on
counting
method)
since
0.13
injected
determined
meter.
large
19.2 cps for the total
the trap were
sample
18,700
before
gave
lines, respectively.
The
through trap,
absorber
samples
trapping
recovery
RESULTS
960
1.6
liquid
1
& 0.15 & 0.15 -1: 0.44 + 0.44
for 80 percent
large
0.020
0.094
solution,
the
from
injection
0.54 0.!)8 19.4 18.9
80 percent
\,alues of 1.2 and
A,IAl(“b)
absorber
for
sample
(cps
a correction and
drainage
1.0
12.75
F(CPSl/iC) Cl4 in plant stream, A(w! Leaking
Applying
Process stream
Q(fi3/hr)
Total
0.23 ?m0.06 0.42 _i 0.06 8.05 : 0.18 7.60 i 0.18
:I: + -i: i
TAHIX 2
Plant flow rate,
Dj,t (CPSJ
equally
would
quantities
of only
about
of over 6 x 10” precise results have
required
of the isotope.
REFERENCES 1. HULL D. E. Znt.J. a#Z. Rad. Zxotol,es 4, 1 (1958!. 2. HULL I>.E. Sytnposium on Radiotracer A&lications in the Petroleum and Chemical Industries, American Chemical Society, Philadelphia, ,\pril6-10 (1964). 3. FKIES B. ‘-1. Jnt. .I. af$l. Rad. Irotopes 16, 35 (196.5). 4. FRIES B. A. Int. .I. a(,i,l. Rad. Iroto(,e.c 13, 277 i 1962 I.