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Separation of iodine and steam at elevated temperatures
Reactor Science and Technology (Journal of Nuclear Energy Parts A/B)
SEPARATION
1963, Vol. 17, pp. 377 io 384.
Pel-gamon Press Ltd.
OF IODINE AND STEAM TEMPERATURES
Printed in Northern Ireland
AT ELEVATED
P. J. KREIJGER* and TH. VAN DER PLAS Euratom--R.C.N.-Kema Reactor Development Group, Arnhem, The Netherlands (First received 15 February 1963 and injtml form 8 May 1963)
Abstract-The separation of small amounts of iodine and steam in a stainless-steel rectifying column is studied between 120” and 275°C. It is found that under the experimental conditions at high temperatures (275°C) a nearly complete separation between iodine and water can be obtained, whereas this separation becomes more difficult at lower temperatures. At 120°C no separation is possible. The mechanism leading to the separation is a combined effect of chemical reaction and distillation. In connexion with this work a direct determination of the phase equilibrium constant of 1%and water at 100°C is carried out.
1. INTRODUCTION
THE volatility of iodine at low concentration in an aqueous solution at temperatures of 250-300°C is of importance for the removal of neutron absorbing fission products from nuclear reactors with an aqueous fuel, of both the solution and the suspension type. The problems connected with iodine removal from a solution have been discussed (LANE, 1958) and for the removal of iodine from a nuclear reactor of the suspension type a flow sheet has been presented (KREIJGER, 1958) that requires a rectification column made of stainless steel for the separation of iodine and steam. In the solution an appreciable part of the iodine formed is found in the molecular state. In the suspension reactor the reducing conditions obtained by means of an important hydrogen overpressure decrease the possibility of occurrence of molecular iodine. However, in the evaporator discussed (KREIJGER, 1958) oxidizing conditions might be present because hydrogen is stripped from the feed solution by boiling and an appreciable radiolysis of water might be expected there. In this case volatile iodine may be formed as a consequence of which I, is swept out from the evaporator together with steam. Therefore an experimental study has been made of the separation of trace amounts of iodine from steam in a stainless-steel rectifying column at elevated temperatures. Before describing the experimental procedure (Section 3) and the results (Section 4) the volatility of molecular iodine (I,) dissolved in water will be * This work forms part of the Ph.D. thesis of P. J. KREIJGER, presented to the University of Technology, Delft, The Netherlands. 1
discussed in Section 2. Finally, the mechanism leading to the separation will be treated theoretically in Section 5. 2. THE BEHAVIOUR
OF IODINE
IN WATER
The properties of iodine in an aqueous solution at higher temperatures (>lOO”C) are not well known. This is probably due to the fact that the determination of the chemical state of iodine at high temperatures is difficult. In stainless steel molecular iodine in an aqueous solution is reduced rapidly in absence of oxygen. If oxygen is present a certain distribution over the several oxidation states, i.e. I-, I, and IO,-, will occur. Of these three states only I, can be considered to be appreciably volatile. The volatility of I, in water at lower temperatures (< 1OO’C) has been investigated by TAYLOR (1958) at 2O”C, KEELER et al. (1957) and the present authors at 100°C (see Appendix). To facilitate the comparison of the volatility of iodine with that of other less soluble gases, Henry’s constant, N, will be used here. At low concentrations of the solute, H is the proportionality factor between the partial vapour pressure of the solute above the solution, p, and the mole fraction of the solute in the solution, X. Thus: p = TABLE
Temp.(“C) 20 100 100
I .-HENRY’S
Hx.
CONSTANT, H,
H(atm) 11 0.09 38
Reference TAYLOR KEELERet al. This work (see Appendix)
Water vapour at temperatures considered an ideal gas. The 377
OF I, IN WATER
below 100°C can be relative volatility of
P. J. KREIJGER and TH. VAN DER PLAS
378
Jti(rtm)
2
lo* 9 e 7 6 5 1
I
I
300 ZM) 150 2
I
I 100
50
3
I
I
25 0 oc 4
I ~10yw’
FIG. l.-Henry’s constant for several gasesas function of temperature.
5
tained to prevent reduction of I,. [The determination of K at 100°C mentioned earlier by KEELERet al. was carried out in glass apparatus (see also Appendix).] In view of the discrepancy between the K-values at 100°C of KEELERet al. and the present authors a confirmation of the high temperature values of KEELER et al. seems desirable. In our opinion the hightemperature value is in line with the measurements on other gases. Unfortunately we had no opportunity to carry out this work. In our case an oxidizing agent is added to the evaporator contents. I, is swept with steam into the column where no oxidizing circumstances are present. Here, I, is reduced to I- and the volatility of I, in water at high temperatures is probably small, so an efficient reduction of the iodine concentration in the top of the column is to be expected. A separation carried out at lower temperatures (e.g. 100°C) may become difficult. Although the volatility of I, is higher than that of water at these temperatures, the apparent volatility may become nearly equal to that of water due to the reduction of I, to non-volatile I-.
3. EXPERIMENTAL PART In Fig. 2 a diagram is shown of the evaporator, the iodine with respect to water, u, can be given by: rectifying column and the auxiliary parts. The apparatus is made of stainless steel. u = H/pQ The evaporator has a volume of about 40 1. Heat is supplied to it by low-frequency induction heating of wherep” is the pressure of the saturated water vapour. From the data of TAYLORand of this work it can be the wall. To this end the walls of the evaporator are constructed of carbon steel, clad with stainless steel at seen that cc > I for T G lOO”C,hence in this temperathe inner side. ture region I, is more volatile than water. According On the top of the evaporator vessel a rectifying to KEELERet al. water is more volatile than I, at 100°C. Because this is in contradiction with general experi- section is built. This column contains ten bubble cap ence at this temperature this value of H is, in our trays, each containing three bubble caps. The diameter of the trays is 119 mm, that of the bubble caps is opinion, doubtful. In order to extrapolate H to higher temperatures, we 32.5 mm. A reflux condenser is mounted above the column. have to assume a certain degree of analogy between the behaviour of the value of H for iodine in water The reflux is taken off partially from the 4th plate and with that of other gases. In Fig. 1 Henry’s constant of is returned directly to the still. This enables the several gases in water has been plotted against tem- adjustment of the partial reflux in the lower part of the perature as done by HIMMELBLAU and ARENDS(1959). column. The trays are fitted in the column by means of a It appears that above about 100°C the value of H drops rapidly with increasing temperature. The same Teflon-asbestos sealing. Temperatures, pressures and flow rates are measured behaviour is to be expected for I, in water. Since the saturated steam pressure rapidly increases with in- at several places by conventional means. The system creasing temperature, above a certain temperature I, is pressurized with nitrogen. The column is provided with four vapour sample may become less volatile than water. This trend of the volatility is confirmed by the value of K m 0.3 at connexions and three connexions for the sampling of temperatures between 200-330°C reported by KEELER liquid. The mass of liquid taken as a sample of the liquid et al. These experiments, were carried out in a stainless-steel autoclave. Oxygen overpressure was main- (ca. 40 g) is smaller than the mass on the tray (ca.
Separation of iodine and steam at elevatedtemperatures
RECEIVER
379
U
N2-CVUNDERS L= LIQUID
SAMPLE
V= VAPOUR
CONNEXION
SAMPLE
CONNEXION
COOLING WATER
FIG. 2.-Diagram of column and the auxiliary apparatus. 250 g), but the vapour sample contains more material (cd. 40 g) than there is vapour present over a tray (ca. 15 g). For this reason, the vapour samples are considered to be less reliable. An exception can be made for the vapour sample from the evaporator. In the evaporator much more steam is present than is removed by sampling. It is found to be necessary to keep oxidizing conditions in the evaporator to ensure a steady feed of T, to the column. Potassium bichromate and sulphuric acid are added therefore to the iodine solution in the evaporator. The iodine concentration was about 5. 1O-6 molar, half a millicurie of 1311was used as a tracer per experiment. Measurements of the distribution of I, between the liquid and vapour in the evaporator have been made. These values cannot be used to determine the equilibrium constant for pure water because of the acidity of the solution in the evaporator. The samples are taken if flow and temperature conditions throughout the column have become steady. This has been usually the case about four hours after
the temperature in the evaporator has reached the desired value. The sample receivers are charged with KI carrier solution before sampling. The samples are taken in such a manner that each is taken upstream with respect to the preceding one, the liquid samples being taken first. The determination of iodine was carried out by determining the 1311with a scintillation counter. It is found that the adsorption of iodine on the walls of the apparatus is negligible and also that the volatility of HI and H,SO, can be neglected with respect to the volatility of iodine. 4. RESULTS AND DISCUSSION From the corrected counting rates, the values of the iodine concentrations in the vapour, yn, and in the liquid, x,, of the sampled trays are found. The values of the ratio i = y/x for one tray are calculated under the assumption that this ratio is constant for all trays by using the relations: S”y,,,V
= X$(Sn+l - l)/(S - 1) + x,L
P. J. KREIJGERand TH. VAN DER PLAS
380
and:
S”x,L
.= x,D(S”
where:
S is the separation D is the liquid
V is the vapour
-
l)/CS -
1) + x,L
factor (S =
at the top (kg/hr),
flow (kg/hr).
The 5 values are calculated from each possible combination of two concentrations measured at different places in the column. In Table 2 the values for 5 are collected for several values of total reflux (L = V) and several temperatures. The c values obtained from the experiments with partial reflux are collected in Table 3. In both tables the accuracy of the [ value (givingY/x for one tray) is indicated by an upper and lower limit, with a probability of 70 per cent that the true value is within these limits (i.e. the 70 per cent confidence limits). We have found that S is approximately 0.45 in the temperature range of 268-275°C. Therefore, an efficient separation of iodine from steam can be obtained at this temperature. However, at lower temperatures, the value of the separation factor, S, rises, while at 120°C practically no separation is obtained (S m 1). The results given in the tables confirm the expectation that the vapour samples are less reliable. In experiments with larger concentrations of iodine no influence of this concentration was found. We restrict the discussion now to the results obtained at temperatures of approximately 270°C. In Fig. 3 the values of 5, calculated from the concentrations Yll (vapour in the evaporator) and x7 (liquid on the 7th tray) are plotted against the reflux L. This is done for both total reflux and partial reflux with V = 16 kg/hr. To explain the dependence of 5 on L we will consider in more detail the combined mechanism of distillation and chemical reaction in Section 5. 5. THEORY IODINE
OF THE FROM
WITH
SEPARATION
STEAM
CHEMICAL
BY
OF
surfaces of the apparatus, e.g. trays and column walls. This idea is formulated in this Section. We consider a rectification column of identical plates all at the same temperature and the same conditions of fluid flow etc. This is possible in the present case because the concentration of iodine is always small and hence the bulk properties of the steam and water are not influenced by the iodine concentration. Every tray of the column is supposed to be a perfectly mixed continuous flow reactor as the mixing time on the trays can be assumed to be smallcompared with the time of residence of the liquid on a single tray. The reaction occurring on the tray is the reduction of I, to I- by the metal of the tray. It is assumed that the conversion rate can be expressed by: rn = k xno
where k is the reaction rate constant and x0 is the mass fraction of I2 in the solution on the tray. The index IZ identifies the tray. This form of the rate equation is probable because the amount of metal available for reaction is large compared with the amount of iodine. We assume furthermore that the small amount of iodide formed does not influence the volatility of the of the iodide concenmolecular iodine (K independent tration, x’) and that the iodide does not enter the vapour in any form (for example as HI). The latter assumption has been verified experimentally. We define : 5, = Y?lIxn~
FREE
yn+lV=x,L
DISTILLATION
REACTION
The magnitude of a separation in a distillation column normally is described by a separation factor S = 5 V/L = yV/xL. In normal distillation practice the value of Y/x is a constant determined by pressure and temperature only. There is no dependence on the liquid flow L if the column is operated properly. In our case a strong dependence of 5 = y/x on L is found. To explain this we have considered a mechanism that competes with the distillation of the iodine. It is a chemical reaction of the iodine with the metal
(11
where x, is the mass fraction of total iodide in the liquid on the n-th tray and yn. the mass fraction of iodine in the vapour on the n-th tray. Our aim is to relate 5, with the other variables, i.e. the phase equilibrium constant, K, the Murphree plate efficiency, E, and the reaction rate constant k. Denoting the vapour and liquid flow by V and L respectively and the liquid take off at the top D, a mass balance for iodine for the part of the column above the (n + 1)-st tray is (see Fig. 4 for notation):
and the mass balance
+ x,D
for the n-th tray is:
X,-l L + yn+1v=
x,L +y,E
At total reflux D = 0, while for partial reflux we will restrict this derivation to those cases where x,,D is small with respect to x,L. This is the case in the lower section of the column. Hence we may write: Y +Iv Combination
= x,L.
(2)
of (1) and (2) gives : YJY nil = LVIL.
(3)
no precision
* Where
Reflux Uklhr)
270 270 273 271 270 200 200 121
I
-__I 26.4 19.9kO.4 19.4+0.7 12.9&0.2 3.7 14561.5 6.5 3.1+0,1
Tempera ture (“C)
I
limits
0~54fO~lO 0.48+0.04 0.55t0.06 0.44t0.04 0.36*0.01 0.91&0.02 0.88 1.10*0.12
yI1 and y7 __-~ 0.521_0~02 044+0.14 0.50*0.05 0.34%0.06 0.32_t0.07 0.93_+0.06 0.88 0.95*0.05
are indicated,
yI1 and x7
and y4
only
0.46+0.06 0.95iO.06 0.77 0.99&0.03
0.54mt0.04 0.53’0.08 0.52t0.04
yI1 and yz 0.5 $0.4 0.3810.24 0,43$0.03 0.1610.03 0.25&0,10 1.01 I 0.02 0.90 0.62+0,09
x, and y,
The precision
of < was available.
0~59-_10~01 0.5OiO.08 0.48+0.03 0.39&0.03 0.45+0.03 0.91+0.03 0.81 0.98f0.05
yI1 and x2
one measurement
0.95~t0.04 0.85 0,95_t0.05
0.39_+0.07
yll
tray.
OF [ FOR TOTALKEFLUX,AT
Z = z for one x
TARLE 2.-VALUES
0,93_;kO.O2 0.84 0,87_+0.06
0.35iO.08
xv and ,va
is indicated
THEREFLUX
x, and x2
0.62 to.10 0~55jO~ll 0.44hO.02 0.3810.02 0.52*0.05 0.91 kO.03 0.77 0.91j_0.05
1
xi. and yz
0.53hO.14 0.9210.02 0.72 0.94*0.04
0.55iO.02 0.5 +0.3 0.5OiO.09
i~__I
and y4
and x2
0.66+0.02 0.58jO.06 0.45kO.06 0.48 i 0.08 0.6710.19 0.89+0.02 0.75 1.02iO.13
YT
TEMPERATURE
0.92 J:O.O7 0.82 0.9X+0.03
y7
limits*
FLOW LAND
by the 70 per cent confidence
DIFFERENTVALUESOF
T
0.8710.04 0.54 1.12,0.04
Y, and,J% -___-___
0.43+0-13 1.07+0.06 O-50 1.15+0.20
0.31+0.24 0.9 io.4 0.79+0.16
sf and
y,
P. J. KREIJGER and TH.VAN DER PLAS
382
V AND THEREFLLJXL
TABLE 3.-VALUES OF 5 FOR PARTIAL REFLUX,AT DIFFERENTVALUES OF THEVAPOURFLOW 5 = Temperature
Vapourflow
(“C)
The precision is indicated by the 70 per cent confidence limits*
Reflux
V(kg/hr)
Ukglhr)
s, and xq
yll and x7 17.0 11.2 + 0.6 10.9 12.0 11.9 15.5 i 0.4 15.9 i 0.2 14.1 -+ 0.8 14.5 15.5 & 0.6 16.1 f 0.2 15.6 15.4 16.6 5 0.8 17.0 18.4 24.6 5 0.1
275 213 273 273 273 275 271 274 273 275 214 268 270 270 270 268 215
0.9 1.8 zt 0.4 1.4 2.3 3.6 3.5 f 0.3 3.8 i 0.2 4.5 f 0.5 4.8 5.2 * 0.3 6.3 IO.2 6.9 7.3 8.3 * 0.7 8.7 10.3 11.2 * 0.1
zfor one tray y7 and x4
x7 and y7 0.020
0.023 0.055 * 0.01
0.12 & 0.02 0.13 & 0.02 0.15 & 0.02
0.063 0.070
0.031 0.038
0.12 5 0.01
0.18 * 0.10
0.10
0.22 0.20 0.083 0.10 * 0.01
0.15
0.13
0.11 & 0.02 0.21 f 0.01 0.23 0.08 0.24 & 0.03 0.17 0.15 0.13 f 0.1
0.23 & 0.02
* See remark to Table 2.
The Murphree
plate efficiency, E, is (PERRY, 1953)
E = On+1 - Yn)itin+~ - J%O)
(4)
YnlYnH = (1 - E) + KE x~/Y%+~.
(5)
or Because the total iodine concentration, x,, is the sum of the iodine and iodide concentrations :
On tray n, iodide ions are produced according to: Y, = kx,O. Then, the mass balance for the iodide on tray n is: x;_~ L + kxnOm = x,‘L, where m is the mass of the liquid on the tray. balance can be written as:
This
xn’ - x,‘_~ = kx,OmlL.
x, = x,’ + xno,
(2) can be written as: x,o/yn+l = V/L(l -i_ xlL’/x,O)P1*
(6)
By summing over all trays above tray y1and assuming that in the condenser no iodide is formed, as x0 is considered to be small here, we find:
Substitution of (3) and (6) into (5) gives: 5, = (1 - E) L/V + KE/(l
+ x,‘/x,o).
X/l’=
(7)
oao-
oJO 0
0 24
FIG.
3.-The
40
SO
1 60
l@O
12.0
VI.0
16.0
e
TOTALREFLUX
0
PARTIAL
REFLUX
18.0
200 -
m/L ki
xio.
(8)
i=O
V -
Wkg/h
22.0 REFLUX
24.0 L (kg/h
26.0 1
ratio 5 = U/X for one plate calculated from y 11and X, as a function of the reflux L.
Separation of iodine and steam at elevated temperatures
383
The experimental points for 5 at partial reflux show a linear dependence of 5 on L. This means that (11) can be written as: 5 w (1 - E) L/V + KE/{mk/L(l = (1 - E) L/V + KE L(l -
-
5 V/L)}
[V/L)mk.
(12)
Introducing the experimental values E = 0.68, m = 0.38 kg, V = 16 kg/hr and 5 = 0.15, L = 5 kg/hr it follows that K/k = 0.01 hr.
This is the only result that follows directly from the experiments. Using the value for K = O-3 determined by KEELER et al. (4) the value for the reaction rate constant is found to be : k = 30 hr-l.
FIG. 4.-Schematic
This means that roughly every one and a half minutes half the amount of the I, present on a tray would be reduced if there was no transport of iodine to and from the tray.
drawing of the column.
This expression introduced into (7) gives: 1, = (1 - E) L/V + KE/(l
+ m/L . ki
6. CONCLUSIONS
xt/xno).
(9)
i=O
If no chemical reaction took place (k = 0), 5 at total reflux would be constant, which is contrary to experiment. Because ~~_r/y~ is smaller than unity we can simplify (9). To do this we will write (4) as: X”,_Jx,o = tin-1 - ~n(l - L)}/IY~ - Y,+X(1 - E)> (10)
= Y12-1IYn.
Substitution of (10) and (3) into (9) gives, after summation over a large number of trays: [ w (I - E) L/V + KE/(l
+ mk/L . l/(1 -
< V/L)). (11)
As 5 is not linear in the variables E, K and k, a direct determination of the numerical values of these quantities by applying the method of least squares is impossible. An examination of the results plotted in Fig. 3 leads to an estimation of the value for E and the ratio K/k. Assuming that E is not very much dependent on V in the region of V-values investigated, E can be estimated by extrapolation of the straight line through the points for total reflux to L = 0. Thus: E cw 0.68.
It is shown that by means of an evaporator-rectifier combination as used here, iodine can be concentrated in the evaporator, and iodine-free water delivered at the top. Moreover, depending on circumstances, the molecular iodine will be converted to iodide ion-a form with a much lower volatility. The apparatus described can therefore be used as an efficient hold-up for iodine. To a large degree this is due to the transformation of molecular iodine into iodide ions. We have postulated that this reaction takes place on the stainless-steel walls of the apparatus, but we have not proved this by experiment. The application of our results to a solution- or slurry-type liquid fuel reactor is obvious. They might also be used to estimate the carry-over of iodine in the case of a direct boiling water reactor. In that case the separate values of K and k, given in Section 5 should be used. It would then be desirable to confirm the value of K (or k, or both) in separate experiments. It should be noted however that in the case of a boiling water reactor the carry-over by entrainment might be more important. Acknowledgments-The authors are indebted to Prof. ir. H. KRAMERS, Delft, and ir. K. A. WARSCHAUERfor their kind interest and advice, to irs. D. G. H. LATZKO, A. SPRUYTand the
N.V. Comprimo for the design, construction and operation of the system and to Mr. H. J. van REMMENand MR. W. J. de BRUIN for their assistance with the experiments.
P. J. KREIJGERand TH. VANDERPLAS
384
TABLE4.-DETERMINATIONOF PHASEEQUILIBRIUM CONSTANT AT 100°C Time
2
2
4
5
7
7
7
I
16
16
16
K
10.0
11.2
16.3
28.9
31.2
32.2
33.4
26.1
38.4
31.5
38.2
so
NOTATION Description Liquid taken off from the 4th plate Murphree plate efficiency on the vapour side Henry’s constant Phase equilibrium constant: Mass fraction camp. in vapour Mass fraction camp. in liquid Liquid reflux Molecular weight of the solvent Separation factor: S =
r
Conversion rate:
Symbol D E H K
L M s T V k m n
X
x0
x’
kg 12
kg liquid. set Iodine concentration” kg iodine in the liquid: kg Molecular iodine concenkg I2 tration in the liquid: kg kg IIodine concentration in the liquid : kg Iodine concentration kg iodine in the vapour : kg vapour Summation operator Relative volatility Ratio of the iodine concentration in the vapour and in the liquid,
REFERENCES Dimension kg/set
atm
kg/set “C
kg/set set-l kg
atm atm set-l
X
fndex
i 2 etc.
Conference
kg/m3
on the Peaceful
Uses of Atomic Energy,
Geneva,
Vol. 9, p. 427. United Nations N.Y. APPENDIX Determination qf the phase equilibrium constant qf1, in water at 100°C
The glass apparatus used by KEELERet al. (19.57) for the same purpose was copied. The experiments were carried out at a higher iodine concentration however in order to get an iodine concentration in the condensate of ca. 10e3 mols per mole of water. Iodine in the sample is determined by adding an excess of sodium thiosulphate solution and back titration with standard potassium iodate, using an amperometric end point (KNOWLES and LONDON,1953). More than 7 hr were needed to establish the steady state with a circulation of 60 ml/hr, which means that every 2 hours the entire contents are circulated. The approach to the steady state can be seen in Table 4, where time is the time in hours before sampling, x iodine concentration in the still pot, y iodine concentration in condensate and K = y/x.
for one plate c = r
Density Description Index for indicating a certain experiment or a certain plate The numbers indicate a specific plate in the column.
HIMMELBLAU D. M. and ARENDS E. (1959) Chem. In,. Techn. 31, 12 791. KEELER R. A., ANDERSONC. J., KLACH S. J. and CHAPPEL R. M. (1957) KLX-10080. KNOWLESG. and LONDONG. F. (1953) Analyst 78,159. KREIJGER P. J., PLASTH. VANDER, SCHEEB. L. A. VANDERand LANE J. A., MACPHERSONA. G. and MASLAN F. (1958) Fluid Fuel Reactors Addison-Wesley. PERRY J. H. (1953) Chemical Engineers Handbook, 3rd edn. p. 552. TAYLOR R. F. (1958) AERE CE/R 2469. WENT J. J. (1958) Proceedings of the Second International
SEPARATION
1963, Vol. 17, pp. 377 io 384.
Pel-gamon Press Ltd.
OF IODINE AND STEAM TEMPERATURES
Printed in Northern Ireland
AT ELEVATED
P. J. KREIJGER* and TH. VAN DER PLAS Euratom--R.C.N.-Kema Reactor Development Group, Arnhem, The Netherlands (First received 15 February 1963 and injtml form 8 May 1963)
Abstract-The separation of small amounts of iodine and steam in a stainless-steel rectifying column is studied between 120” and 275°C. It is found that under the experimental conditions at high temperatures (275°C) a nearly complete separation between iodine and water can be obtained, whereas this separation becomes more difficult at lower temperatures. At 120°C no separation is possible. The mechanism leading to the separation is a combined effect of chemical reaction and distillation. In connexion with this work a direct determination of the phase equilibrium constant of 1%and water at 100°C is carried out.
1. INTRODUCTION
THE volatility of iodine at low concentration in an aqueous solution at temperatures of 250-300°C is of importance for the removal of neutron absorbing fission products from nuclear reactors with an aqueous fuel, of both the solution and the suspension type. The problems connected with iodine removal from a solution have been discussed (LANE, 1958) and for the removal of iodine from a nuclear reactor of the suspension type a flow sheet has been presented (KREIJGER, 1958) that requires a rectification column made of stainless steel for the separation of iodine and steam. In the solution an appreciable part of the iodine formed is found in the molecular state. In the suspension reactor the reducing conditions obtained by means of an important hydrogen overpressure decrease the possibility of occurrence of molecular iodine. However, in the evaporator discussed (KREIJGER, 1958) oxidizing conditions might be present because hydrogen is stripped from the feed solution by boiling and an appreciable radiolysis of water might be expected there. In this case volatile iodine may be formed as a consequence of which I, is swept out from the evaporator together with steam. Therefore an experimental study has been made of the separation of trace amounts of iodine from steam in a stainless-steel rectifying column at elevated temperatures. Before describing the experimental procedure (Section 3) and the results (Section 4) the volatility of molecular iodine (I,) dissolved in water will be * This work forms part of the Ph.D. thesis of P. J. KREIJGER, presented to the University of Technology, Delft, The Netherlands. 1
discussed in Section 2. Finally, the mechanism leading to the separation will be treated theoretically in Section 5. 2. THE BEHAVIOUR
OF IODINE
IN WATER
The properties of iodine in an aqueous solution at higher temperatures (>lOO”C) are not well known. This is probably due to the fact that the determination of the chemical state of iodine at high temperatures is difficult. In stainless steel molecular iodine in an aqueous solution is reduced rapidly in absence of oxygen. If oxygen is present a certain distribution over the several oxidation states, i.e. I-, I, and IO,-, will occur. Of these three states only I, can be considered to be appreciably volatile. The volatility of I, in water at lower temperatures (< 1OO’C) has been investigated by TAYLOR (1958) at 2O”C, KEELER et al. (1957) and the present authors at 100°C (see Appendix). To facilitate the comparison of the volatility of iodine with that of other less soluble gases, Henry’s constant, N, will be used here. At low concentrations of the solute, H is the proportionality factor between the partial vapour pressure of the solute above the solution, p, and the mole fraction of the solute in the solution, X. Thus: p = TABLE
Temp.(“C) 20 100 100
I .-HENRY’S
Hx.
CONSTANT, H,
H(atm) 11 0.09 38
Reference TAYLOR KEELERet al. This work (see Appendix)
Water vapour at temperatures considered an ideal gas. The 377
OF I, IN WATER
below 100°C can be relative volatility of
P. J. KREIJGER and TH. VAN DER PLAS
378
Jti(rtm)
2
lo* 9 e 7 6 5 1
I
I
300 ZM) 150 2
I
I 100
50
3
I
I
25 0 oc 4
I ~10yw’
FIG. l.-Henry’s constant for several gasesas function of temperature.
5
tained to prevent reduction of I,. [The determination of K at 100°C mentioned earlier by KEELERet al. was carried out in glass apparatus (see also Appendix).] In view of the discrepancy between the K-values at 100°C of KEELERet al. and the present authors a confirmation of the high temperature values of KEELER et al. seems desirable. In our opinion the hightemperature value is in line with the measurements on other gases. Unfortunately we had no opportunity to carry out this work. In our case an oxidizing agent is added to the evaporator contents. I, is swept with steam into the column where no oxidizing circumstances are present. Here, I, is reduced to I- and the volatility of I, in water at high temperatures is probably small, so an efficient reduction of the iodine concentration in the top of the column is to be expected. A separation carried out at lower temperatures (e.g. 100°C) may become difficult. Although the volatility of I, is higher than that of water at these temperatures, the apparent volatility may become nearly equal to that of water due to the reduction of I, to non-volatile I-.
3. EXPERIMENTAL PART In Fig. 2 a diagram is shown of the evaporator, the iodine with respect to water, u, can be given by: rectifying column and the auxiliary parts. The apparatus is made of stainless steel. u = H/pQ The evaporator has a volume of about 40 1. Heat is supplied to it by low-frequency induction heating of wherep” is the pressure of the saturated water vapour. From the data of TAYLORand of this work it can be the wall. To this end the walls of the evaporator are constructed of carbon steel, clad with stainless steel at seen that cc > I for T G lOO”C,hence in this temperathe inner side. ture region I, is more volatile than water. According On the top of the evaporator vessel a rectifying to KEELERet al. water is more volatile than I, at 100°C. Because this is in contradiction with general experi- section is built. This column contains ten bubble cap ence at this temperature this value of H is, in our trays, each containing three bubble caps. The diameter of the trays is 119 mm, that of the bubble caps is opinion, doubtful. In order to extrapolate H to higher temperatures, we 32.5 mm. A reflux condenser is mounted above the column. have to assume a certain degree of analogy between the behaviour of the value of H for iodine in water The reflux is taken off partially from the 4th plate and with that of other gases. In Fig. 1 Henry’s constant of is returned directly to the still. This enables the several gases in water has been plotted against tem- adjustment of the partial reflux in the lower part of the perature as done by HIMMELBLAU and ARENDS(1959). column. The trays are fitted in the column by means of a It appears that above about 100°C the value of H drops rapidly with increasing temperature. The same Teflon-asbestos sealing. Temperatures, pressures and flow rates are measured behaviour is to be expected for I, in water. Since the saturated steam pressure rapidly increases with in- at several places by conventional means. The system creasing temperature, above a certain temperature I, is pressurized with nitrogen. The column is provided with four vapour sample may become less volatile than water. This trend of the volatility is confirmed by the value of K m 0.3 at connexions and three connexions for the sampling of temperatures between 200-330°C reported by KEELER liquid. The mass of liquid taken as a sample of the liquid et al. These experiments, were carried out in a stainless-steel autoclave. Oxygen overpressure was main- (ca. 40 g) is smaller than the mass on the tray (ca.
Separation of iodine and steam at elevatedtemperatures
RECEIVER
379
U
N2-CVUNDERS L= LIQUID
SAMPLE
V= VAPOUR
CONNEXION
SAMPLE
CONNEXION
COOLING WATER
FIG. 2.-Diagram of column and the auxiliary apparatus. 250 g), but the vapour sample contains more material (cd. 40 g) than there is vapour present over a tray (ca. 15 g). For this reason, the vapour samples are considered to be less reliable. An exception can be made for the vapour sample from the evaporator. In the evaporator much more steam is present than is removed by sampling. It is found to be necessary to keep oxidizing conditions in the evaporator to ensure a steady feed of T, to the column. Potassium bichromate and sulphuric acid are added therefore to the iodine solution in the evaporator. The iodine concentration was about 5. 1O-6 molar, half a millicurie of 1311was used as a tracer per experiment. Measurements of the distribution of I, between the liquid and vapour in the evaporator have been made. These values cannot be used to determine the equilibrium constant for pure water because of the acidity of the solution in the evaporator. The samples are taken if flow and temperature conditions throughout the column have become steady. This has been usually the case about four hours after
the temperature in the evaporator has reached the desired value. The sample receivers are charged with KI carrier solution before sampling. The samples are taken in such a manner that each is taken upstream with respect to the preceding one, the liquid samples being taken first. The determination of iodine was carried out by determining the 1311with a scintillation counter. It is found that the adsorption of iodine on the walls of the apparatus is negligible and also that the volatility of HI and H,SO, can be neglected with respect to the volatility of iodine. 4. RESULTS AND DISCUSSION From the corrected counting rates, the values of the iodine concentrations in the vapour, yn, and in the liquid, x,, of the sampled trays are found. The values of the ratio i = y/x for one tray are calculated under the assumption that this ratio is constant for all trays by using the relations: S”y,,,V
= X$(Sn+l - l)/(S - 1) + x,L
P. J. KREIJGERand TH. VAN DER PLAS
380
and:
S”x,L
.= x,D(S”
where:
S is the separation D is the liquid
V is the vapour
-
l)/CS -
1) + x,L
factor (S =
at the top (kg/hr),
flow (kg/hr).
The 5 values are calculated from each possible combination of two concentrations measured at different places in the column. In Table 2 the values for 5 are collected for several values of total reflux (L = V) and several temperatures. The c values obtained from the experiments with partial reflux are collected in Table 3. In both tables the accuracy of the [ value (givingY/x for one tray) is indicated by an upper and lower limit, with a probability of 70 per cent that the true value is within these limits (i.e. the 70 per cent confidence limits). We have found that S is approximately 0.45 in the temperature range of 268-275°C. Therefore, an efficient separation of iodine from steam can be obtained at this temperature. However, at lower temperatures, the value of the separation factor, S, rises, while at 120°C practically no separation is obtained (S m 1). The results given in the tables confirm the expectation that the vapour samples are less reliable. In experiments with larger concentrations of iodine no influence of this concentration was found. We restrict the discussion now to the results obtained at temperatures of approximately 270°C. In Fig. 3 the values of 5, calculated from the concentrations Yll (vapour in the evaporator) and x7 (liquid on the 7th tray) are plotted against the reflux L. This is done for both total reflux and partial reflux with V = 16 kg/hr. To explain the dependence of 5 on L we will consider in more detail the combined mechanism of distillation and chemical reaction in Section 5. 5. THEORY IODINE
OF THE FROM
WITH
SEPARATION
STEAM
CHEMICAL
BY
OF
surfaces of the apparatus, e.g. trays and column walls. This idea is formulated in this Section. We consider a rectification column of identical plates all at the same temperature and the same conditions of fluid flow etc. This is possible in the present case because the concentration of iodine is always small and hence the bulk properties of the steam and water are not influenced by the iodine concentration. Every tray of the column is supposed to be a perfectly mixed continuous flow reactor as the mixing time on the trays can be assumed to be smallcompared with the time of residence of the liquid on a single tray. The reaction occurring on the tray is the reduction of I, to I- by the metal of the tray. It is assumed that the conversion rate can be expressed by: rn = k xno
where k is the reaction rate constant and x0 is the mass fraction of I2 in the solution on the tray. The index IZ identifies the tray. This form of the rate equation is probable because the amount of metal available for reaction is large compared with the amount of iodine. We assume furthermore that the small amount of iodide formed does not influence the volatility of the of the iodide concenmolecular iodine (K independent tration, x’) and that the iodide does not enter the vapour in any form (for example as HI). The latter assumption has been verified experimentally. We define : 5, = Y?lIxn~
FREE
yn+lV=x,L
DISTILLATION
REACTION
The magnitude of a separation in a distillation column normally is described by a separation factor S = 5 V/L = yV/xL. In normal distillation practice the value of Y/x is a constant determined by pressure and temperature only. There is no dependence on the liquid flow L if the column is operated properly. In our case a strong dependence of 5 = y/x on L is found. To explain this we have considered a mechanism that competes with the distillation of the iodine. It is a chemical reaction of the iodine with the metal
(11
where x, is the mass fraction of total iodide in the liquid on the n-th tray and yn. the mass fraction of iodine in the vapour on the n-th tray. Our aim is to relate 5, with the other variables, i.e. the phase equilibrium constant, K, the Murphree plate efficiency, E, and the reaction rate constant k. Denoting the vapour and liquid flow by V and L respectively and the liquid take off at the top D, a mass balance for iodine for the part of the column above the (n + 1)-st tray is (see Fig. 4 for notation):
and the mass balance
+ x,D
for the n-th tray is:
X,-l L + yn+1v=
x,L +y,E
At total reflux D = 0, while for partial reflux we will restrict this derivation to those cases where x,,D is small with respect to x,L. This is the case in the lower section of the column. Hence we may write: Y +Iv Combination
= x,L.
(2)
of (1) and (2) gives : YJY nil = LVIL.
(3)
no precision
* Where
Reflux Uklhr)
270 270 273 271 270 200 200 121
I
-__I 26.4 19.9kO.4 19.4+0.7 12.9&0.2 3.7 14561.5 6.5 3.1+0,1
Tempera ture (“C)
I
limits
0~54fO~lO 0.48+0.04 0.55t0.06 0.44t0.04 0.36*0.01 0.91&0.02 0.88 1.10*0.12
yI1 and y7 __-~ 0.521_0~02 044+0.14 0.50*0.05 0.34%0.06 0.32_t0.07 0.93_+0.06 0.88 0.95*0.05
are indicated,
yI1 and x7
and y4
only
0.46+0.06 0.95iO.06 0.77 0.99&0.03
0.54mt0.04 0.53’0.08 0.52t0.04
yI1 and yz 0.5 $0.4 0.3810.24 0,43$0.03 0.1610.03 0.25&0,10 1.01 I 0.02 0.90 0.62+0,09
x, and y,
The precision
of < was available.
0~59-_10~01 0.5OiO.08 0.48+0.03 0.39&0.03 0.45+0.03 0.91+0.03 0.81 0.98f0.05
yI1 and x2
one measurement
0.95~t0.04 0.85 0,95_t0.05
0.39_+0.07
yll
tray.
OF [ FOR TOTALKEFLUX,AT
Z = z for one x
TARLE 2.-VALUES
0,93_;kO.O2 0.84 0,87_+0.06
0.35iO.08
xv and ,va
is indicated
THEREFLUX
x, and x2
0.62 to.10 0~55jO~ll 0.44hO.02 0.3810.02 0.52*0.05 0.91 kO.03 0.77 0.91j_0.05
1
xi. and yz
0.53hO.14 0.9210.02 0.72 0.94*0.04
0.55iO.02 0.5 +0.3 0.5OiO.09
i~__I
and y4
and x2
0.66+0.02 0.58jO.06 0.45kO.06 0.48 i 0.08 0.6710.19 0.89+0.02 0.75 1.02iO.13
YT
TEMPERATURE
0.92 J:O.O7 0.82 0.9X+0.03
y7
limits*
FLOW LAND
by the 70 per cent confidence
DIFFERENTVALUESOF
T
0.8710.04 0.54 1.12,0.04
Y, and,J% -___-___
0.43+0-13 1.07+0.06 O-50 1.15+0.20
0.31+0.24 0.9 io.4 0.79+0.16
sf and
y,
P. J. KREIJGER and TH.VAN DER PLAS
382
V AND THEREFLLJXL
TABLE 3.-VALUES OF 5 FOR PARTIAL REFLUX,AT DIFFERENTVALUES OF THEVAPOURFLOW 5 = Temperature
Vapourflow
(“C)
The precision is indicated by the 70 per cent confidence limits*
Reflux
V(kg/hr)
Ukglhr)
s, and xq
yll and x7 17.0 11.2 + 0.6 10.9 12.0 11.9 15.5 i 0.4 15.9 i 0.2 14.1 -+ 0.8 14.5 15.5 & 0.6 16.1 f 0.2 15.6 15.4 16.6 5 0.8 17.0 18.4 24.6 5 0.1
275 213 273 273 273 275 271 274 273 275 214 268 270 270 270 268 215
0.9 1.8 zt 0.4 1.4 2.3 3.6 3.5 f 0.3 3.8 i 0.2 4.5 f 0.5 4.8 5.2 * 0.3 6.3 IO.2 6.9 7.3 8.3 * 0.7 8.7 10.3 11.2 * 0.1
zfor one tray y7 and x4
x7 and y7 0.020
0.023 0.055 * 0.01
0.12 & 0.02 0.13 & 0.02 0.15 & 0.02
0.063 0.070
0.031 0.038
0.12 5 0.01
0.18 * 0.10
0.10
0.22 0.20 0.083 0.10 * 0.01
0.15
0.13
0.11 & 0.02 0.21 f 0.01 0.23 0.08 0.24 & 0.03 0.17 0.15 0.13 f 0.1
0.23 & 0.02
* See remark to Table 2.
The Murphree
plate efficiency, E, is (PERRY, 1953)
E = On+1 - Yn)itin+~ - J%O)
(4)
YnlYnH = (1 - E) + KE x~/Y%+~.
(5)
or Because the total iodine concentration, x,, is the sum of the iodine and iodide concentrations :
On tray n, iodide ions are produced according to: Y, = kx,O. Then, the mass balance for the iodide on tray n is: x;_~ L + kxnOm = x,‘L, where m is the mass of the liquid on the tray. balance can be written as:
This
xn’ - x,‘_~ = kx,OmlL.
x, = x,’ + xno,
(2) can be written as: x,o/yn+l = V/L(l -i_ xlL’/x,O)P1*
(6)
By summing over all trays above tray y1and assuming that in the condenser no iodide is formed, as x0 is considered to be small here, we find:
Substitution of (3) and (6) into (5) gives: 5, = (1 - E) L/V + KE/(l
+ x,‘/x,o).
X/l’=
(7)
oao-
oJO 0
0 24
FIG.
3.-The
40
SO
1 60
l@O
12.0
VI.0
16.0
e
TOTALREFLUX
0
PARTIAL
REFLUX
18.0
200 -
m/L ki
xio.
(8)
i=O
V -
Wkg/h
22.0 REFLUX
24.0 L (kg/h
26.0 1
ratio 5 = U/X for one plate calculated from y 11and X, as a function of the reflux L.
Separation of iodine and steam at elevated temperatures
383
The experimental points for 5 at partial reflux show a linear dependence of 5 on L. This means that (11) can be written as: 5 w (1 - E) L/V + KE/{mk/L(l = (1 - E) L/V + KE L(l -
-
5 V/L)}
[V/L)mk.
(12)
Introducing the experimental values E = 0.68, m = 0.38 kg, V = 16 kg/hr and 5 = 0.15, L = 5 kg/hr it follows that K/k = 0.01 hr.
This is the only result that follows directly from the experiments. Using the value for K = O-3 determined by KEELER et al. (4) the value for the reaction rate constant is found to be : k = 30 hr-l.
FIG. 4.-Schematic
This means that roughly every one and a half minutes half the amount of the I, present on a tray would be reduced if there was no transport of iodine to and from the tray.
drawing of the column.
This expression introduced into (7) gives: 1, = (1 - E) L/V + KE/(l
+ m/L . ki
6. CONCLUSIONS
xt/xno).
(9)
i=O
If no chemical reaction took place (k = 0), 5 at total reflux would be constant, which is contrary to experiment. Because ~~_r/y~ is smaller than unity we can simplify (9). To do this we will write (4) as: X”,_Jx,o = tin-1 - ~n(l - L)}/IY~ - Y,+X(1 - E)> (10)
= Y12-1IYn.
Substitution of (10) and (3) into (9) gives, after summation over a large number of trays: [ w (I - E) L/V + KE/(l
+ mk/L . l/(1 -
< V/L)). (11)
As 5 is not linear in the variables E, K and k, a direct determination of the numerical values of these quantities by applying the method of least squares is impossible. An examination of the results plotted in Fig. 3 leads to an estimation of the value for E and the ratio K/k. Assuming that E is not very much dependent on V in the region of V-values investigated, E can be estimated by extrapolation of the straight line through the points for total reflux to L = 0. Thus: E cw 0.68.
It is shown that by means of an evaporator-rectifier combination as used here, iodine can be concentrated in the evaporator, and iodine-free water delivered at the top. Moreover, depending on circumstances, the molecular iodine will be converted to iodide ion-a form with a much lower volatility. The apparatus described can therefore be used as an efficient hold-up for iodine. To a large degree this is due to the transformation of molecular iodine into iodide ions. We have postulated that this reaction takes place on the stainless-steel walls of the apparatus, but we have not proved this by experiment. The application of our results to a solution- or slurry-type liquid fuel reactor is obvious. They might also be used to estimate the carry-over of iodine in the case of a direct boiling water reactor. In that case the separate values of K and k, given in Section 5 should be used. It would then be desirable to confirm the value of K (or k, or both) in separate experiments. It should be noted however that in the case of a boiling water reactor the carry-over by entrainment might be more important. Acknowledgments-The authors are indebted to Prof. ir. H. KRAMERS, Delft, and ir. K. A. WARSCHAUERfor their kind interest and advice, to irs. D. G. H. LATZKO, A. SPRUYTand the
N.V. Comprimo for the design, construction and operation of the system and to Mr. H. J. van REMMENand MR. W. J. de BRUIN for their assistance with the experiments.
P. J. KREIJGERand TH. VANDERPLAS
384
TABLE4.-DETERMINATIONOF PHASEEQUILIBRIUM CONSTANT AT 100°C Time
2
2
4
5
7
7
7
I
16
16
16
K
10.0
11.2
16.3
28.9
31.2
32.2
33.4
26.1
38.4
31.5
38.2
so
NOTATION Description Liquid taken off from the 4th plate Murphree plate efficiency on the vapour side Henry’s constant Phase equilibrium constant: Mass fraction camp. in vapour Mass fraction camp. in liquid Liquid reflux Molecular weight of the solvent Separation factor: S =
r
Conversion rate:
Symbol D E H K
L M s T V k m n
X
x0
x’
kg 12
kg liquid. set Iodine concentration” kg iodine in the liquid: kg Molecular iodine concenkg I2 tration in the liquid: kg kg IIodine concentration in the liquid : kg Iodine concentration kg iodine in the vapour : kg vapour Summation operator Relative volatility Ratio of the iodine concentration in the vapour and in the liquid,
REFERENCES Dimension kg/set
atm
kg/set “C
kg/set set-l kg
atm atm set-l
X
fndex
i 2 etc.
Conference
kg/m3
on the Peaceful
Uses of Atomic Energy,
Geneva,
Vol. 9, p. 427. United Nations N.Y. APPENDIX Determination qf the phase equilibrium constant qf1, in water at 100°C
The glass apparatus used by KEELERet al. (19.57) for the same purpose was copied. The experiments were carried out at a higher iodine concentration however in order to get an iodine concentration in the condensate of ca. 10e3 mols per mole of water. Iodine in the sample is determined by adding an excess of sodium thiosulphate solution and back titration with standard potassium iodate, using an amperometric end point (KNOWLES and LONDON,1953). More than 7 hr were needed to establish the steady state with a circulation of 60 ml/hr, which means that every 2 hours the entire contents are circulated. The approach to the steady state can be seen in Table 4, where time is the time in hours before sampling, x iodine concentration in the still pot, y iodine concentration in condensate and K = y/x.
for one plate c = r
Density Description Index for indicating a certain experiment or a certain plate The numbers indicate a specific plate in the column.
HIMMELBLAU D. M. and ARENDS E. (1959) Chem. In,. Techn. 31, 12 791. KEELER R. A., ANDERSONC. J., KLACH S. J. and CHAPPEL R. M. (1957) KLX-10080. KNOWLESG. and LONDONG. F. (1953) Analyst 78,159. KREIJGER P. J., PLASTH. VANDER, SCHEEB. L. A. VANDERand LANE J. A., MACPHERSONA. G. and MASLAN F. (1958) Fluid Fuel Reactors Addison-Wesley. PERRY J. H. (1953) Chemical Engineers Handbook, 3rd edn. p. 552. TAYLOR R. F. (1958) AERE CE/R 2469. WENT J. J. (1958) Proceedings of the Second International