Journal o f Radioanalytical Chemistry, Vol. 8 (1971) 33 38

ETHANE PREPARATION FROM ATMOSPHERIC HUMIDITY WITH NATURAL TRITIUM CONTENTS

M. S E L I G A

Comenius University, Department o f Nuclear Physics, Bratislava (Czechoslovakia)

(Received January 5, ]971)

Atmospheric humidity in the surroundings of A-1 atomic power station is taken off by means of an absorpt ion equipment in order to measure natural tr i t ium con- centrat ion in atmosphere. Atmospheric water is reduced with Mg at 580 ~ and hydrogen obtained is used for catalytic hydrogenation of acetylene. Opt imum condi- tions of ethane preparat ion in dependence of temperature, molar ratio of the compo- nents, flow rate and the number of hydrogenation cycles when using the new selective nickel catalyst 40-01 are reported.

Introduction

Difficulties with tritium detection are due to its low fl-energy (Ep . . . . = 18.64 keV) and its long half-life, 12.26 years. It is therefore inevitable to use internal methods of registration by which the investigated radionuclide is brought into the detector in a suitab!e form. It was found that ethane was the most suitable gas for filling the proportional counter, and the preparation of ethane by the catalytic hydro- genation of acetylene is a rather simple procedure.

Catalytic hydrogenation of acetylene has been studied using Rh, Ir, Pd and Pt catalysts, 1 and also by means of other elements of Ru and Os. 2 BOND et al) dealt with acetylene hydrogenation on alumina-supported Ru and Os catalysts, and their work was aimed to compare the catalytic activity of both elements. Pd and Pt catalysts, according to CHOUDA 4 catalyze the hydrogenation reaction, but also as by-products polymers of acetylene occur. Alumina-supported Ni catalyst 40-01 was found to be the most suitable one.

Materials and methods

Acetylene (12/Ill-I, tech. gas), Argon (CSN 6543), Nalsit 13X (product of Slovnaft n. p., Bratislava), Porapak Q Mesh 100- 120 (product of Waters Assoc., Framingham, Mass., USA), metallic Mg, pure powder (product of Lachema n. p., Brno), Ni catalyst 40-01 (product of Chemick6 zfivody SCSP, Zfilu~i, CSSR), spring water (from Sz6ch6ny, Hungary), alumogel catalyst 44-00 (product of Chemick6 zfivody SCSP, Zgtlu~i, CSSR).

3 J. Radioanal. Chem. 8 (1971)

34 M. SELIGA: ETHANE PREPARATION

Hydrogen preparation

Atmospheric humidity was taken off and processed on an absorbing equipment described earlier, s

The water sample was reduced to H 2 by metallic magnesium in a dehydrogena- tion equipment. Then it was purified and filled into an evacuated balloon or it was used directly for hydrogenation. In the first experiments the reduction was performed in silica tubes. The reduction was quantitative at 580 ~ but the silica tubes were broken on cooling. Therefore a special stainless steel tube was used in the reduction zone of the dehydrogenation equipment.

Preparation of ethane

To measure tritium contents by means of a proportional counter, ethane and its preparation by means of acetylene hydrogenation on Ni catalysts 40-01 was found to be suitable, and also ethane was prepared by the action of the spring water f rom Sz6ch6ny (Hungary) on calcium carbide. Hydrogen from this water was used again for the hydrogenation. The proper active elements were not prepared by this method because of isotopic fractionation which takes place in the step of the acetylene preparation. A detailed description of the apparatus for the hydro- genation and purification is given elsewhere. 6

Analytical methods

The entire kinetics of hydrogenation reaction as well as the purification of ethane were investigated by means of a gas chromatograph which was a component of the whole apparatus, and so it was possible to measure directly the reaction mixture even between the particular cycles] The gas-carrier was hydrogen at 400 torr. Measurements were performed at 120 mA (6 u with a Kipp Micrograph BD-5 at 20 #V sensitivity. Double Pt and P t - R h kataro- meter was used as detecting unit and the flow rate of hydrogen was 50 ml/min at 20 ~

Nalsit 13X, Porapak Q and fi,fi'-oxidipropionitril (15~), respectively, were used as dividing fillings and di-2-ethylhexyl-sebacate (3 ~ ) as anchored phase on N - A W chromaton (0 .16-0 .2 mm) from the D-7-2/69 charge (produced by Lachema, Brno). The samples produced by Slovnaft, Bratislava, were used as ethane and ethylene standards.

Apparatus

Tritium concentration of ethane obtained was measured by means of a pro- portional counter with a built-in anticoincidence counter. The sensitive volume of the inner counter was 0.5 1 and the sensitive volume of the anticoincidence counter was 0.82 1. s

3. Radioanal. Chem. 8 (1971)

M. S E L I G A : E T H A N E P R E P A R A T I O N 35

Results and discussion

The catalytic hydrogenation of acetylene under flow conditions on the 40-01 catalyst depends essentially on the working temperature, reactants and the flow rate. These variables were taken into consideration when investigating the kinetics of the reaction in order to reach opt imum reaction condition for obtaining ethane.

Dependence on the reaction conditions of the amount of ethane obtained was determined by differentiation, and kinetic dependences of ethane amount on time at various working temperatures were obtained. Integral kinetic dependences of ethane formation were ascertained at constant reaction time (working cycles) when 100 ~ conversion, related to acetylene, was obtained at opt imum conditions.

1 ..e. 100

"2

i ' u

50

0 ~ 0 5 10

% rain

Fig. l. Dependence of the reacting component's concentration of the basic mixture on time. l -- C2H2, 2 -- C.,H4, 3 -- CzH6, 4 -- higher hydrocarbons

Concentration dependence of the reacting components is given in Fig. 1 : that of the basic mixture (acetylene and hydrogen) on time in case of two consequent irreversible reactions with formation of an intermediate product (ethylene) and final products, i.e. ethane and hydrocarbonic butane fraction (containing n-butene, l-butene, etc.). 9 The reaction took place at 220 ~ the flow rate being 3 l/rain and the volume ratio of the basic mixture acetylene : hydrogen 1 : 2.5. Further experiments showed that the working temperature is not optimum for the reac- tion, what is evident f rom the plot in Fig. 2, where integral dependence of ethane on working temperature is plotted at constant flow rate and cycles of hydrogena- tion, respectively, and on the ratio of the basic reaction components.

From the plot it follows unambiguously that the optimum temperature of the reaction lies between 90 and 150 ~ and at higher temperatures it is necessary to increase the number of hydrogenation cycles to obtain a higher conversion of ethane, although up to 5 ~ acetylene dimers are formed above 200 ~ and car- bonation occurs at about 400 ~ resulting in a marked decrease in the ethane yield.

3* J. Radioanal. Chem. 8 (1971)

36 M. S E L I G A : E T H A N E P R E P A R A T I O N

100 o

O O

2 o t.)

50

L ] I , . .

50 150 250 350 T,':'C

F i g . 2 . Dependence of ethane conversion on the temperature. 1 - - e t h a n e , 2 - - ethylene

.-e"

l j

I 100 - -

50

O 0

t 2 3

10 20

Number of CLJctes

F i g . 3 . Dependence of ethane conversion on the number of hydrogenation cycles. 1 - - 2 l/min, 2 -- 81/min, 3-- 16 l/min

Dependence of ethane conversion on the number of hydrogenation cycles is shown in Fig. 3. The dependence was determined at constant temperature and component ratio (105 ~ and C2H 2 : H2 = 1 : 2.5). It follows from this integral dependence that the rate of acetylene hydrogenation is indirectly proportional to the flow rate of the reaction mixture, which corresponds unambiguously with contact time of the hydrogenation mixture with the catalyst. To obtain optimum ethane conversion eight hydrogenation cycles are necessary at a flow rate of

J. Radioanal. Chem. 8 (197l)

M. SELIGA: ETHANE P R E P A R A T I O N 37

3 l/min (Curve 1), 15 cycles at 8 l/min (Curve 2 )and up to 20 hydrogenation cycles at a flow rate of 16 l/rain (Curve 3).

The opt imum course of the reaction mixture was determined from the ethane conversion dependence on the flow rate shown in Fig. 4. The dependence was obtained at constant temperature and ratio o f reaction components (105 ~ and C2H 2 : H 2 = 1 : 2.5), making use of 10 hydrogenation cycles. Under these con- ditions it fol lows from the plot that more than 10 hydrogenation cycles suffice only for the flow rates up to 3 l/rain. The dependence of ethane conversion at different ratios o f the basic components (C2H 2 : H2) is shown in Fig. 5. The dependence was obtained at constant temperature, flow rate and number of

100

(_/

o

0 5 10 15

Flow rate, t/rain

Fig. 4. Dependence of ethane conversion on the flow rate. 1 -- C2H6, 2 -- C2H 4

~ =-

t.;

50

o �9 o o

2

i i �9 2:1 1:1 1:2 1:3 1:4

C2H2:H2 ratio

Fig. 5. Dependence of ethane conversion on the basic component ratio. 1 -- C2H~, 2 -- C2H~

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38 M. SELIGA: ETHANE PREPARATION

hydrogenation cycles (105 ~ 3 1/min and 10 cycles). The minimum stoichiome- tric ratio for the hydrogenation reaction is C2H 2 : H a = 1 : 2.5 (vol.). At a volume ratio higher than 2.5 a decrease of ethane yield occurs because ethylene is formed in the first step. At higher ratios the percentage of the conversion does not change.

In all the cases considered, because of the two-step hydrogenation reaction, we measured also the ethylene contents.

The catalyst regeneration as well as ethane purification were described earlier)

References

1. G. C. BOND, P. B. WELLS, J. Catalysis, 6 (1966) 397. 2. J. SHERIDAN, W. D. REID, J. Chem. Soc., (1952) 1962. 3. G. C. BOND, G. WEBB, P. B. WELLS, J. Catalysis, 12 (1968) 157. 4. M. CHOUDA, Curr. Sei., 36 (1967) 171. 5. M. SELIGA, P. POVINEC, M. CHUDY, Collection Czech. Chem. Commun., (in the press). 6. P. POVINEC, M. CHUDY, M. SELIGA, ~. S,~R(), Tech. Rept. K J F - U K 10/70, 1970. 7. M. SELIGA, P. POVlNEC, M. CHUD(', Collection Czech. Chem. Commun., 35 (1970) 1278. 8. P. POVINEC, M. CHUBS', M. ~ELIGA, S. S/,R6, lsotopenpraxis, 7 (1971) 54. 9. F. ZEMAN, private communication.

J. Radioanal. Chem. 8 (1971)