Production of acetylene concentrates from the gaseous products of the electrocracking of liquid hydrocarbons by the thermal displacement method

PRODUCTION OF ACETYLENE CONCENTRATES FROM THE GASEOUS PRODUCTS OF THE ELECTROCRACKING OF LIQUID HYDROCARBONS BY THE THERMAL DISPLACEMENT METHOD * O. P. KR]VORUCHKO,

A. L. LAPIDUS, and

M. V. Lomonosov

Ye. A. SAMOILENKO

M. I. YANOVSKII

Moscow Institute of Fine Chemical Technology;

Institute of Chemical Physics, U.S.S.R. Academy of Sciences (Received 16 January 1963) THE electrocracking of liquid hydrocarbons [112, 3] generally gives a mixture of gaseous products containing 25-32% of acetylene, 45-55% of hydrogen, 5 - 1 2 ~ of olefins, 2-5% of methane, about 1% of divinyl, and 1-2% of homologues and derivatives of acetylene. A method has been described [4, 5] for the isolation of acetylene by the hypersorption method on activated carbon from the gaseous products formed in the partial oxidation of methane. In this way, acetylene was recovered from the bottom of the column in the form of a 99% concentrate. The absence of heavier products lightened the task of separation. For the mixtures of gaseous products formed in the electrocracking of liquid hydrocarbons the presence of compounds heavier a n d lighter than acetylene is characteristic. In this case, it appeared to be advantageous to use the method of thermal displacement [6, 7], which should apparently give a final product of high purity. EXPERIMENTAL

In order to study the mechanism of the separation processes in the thermal displacement method in more detail, a specially constructed column was used with a device for taking samples of the gas phase along the length of the layer (Fig. 1). The fifteen sections of the stainless steel column, each 120 m m long and 15 m m in internal diameter were joined through intermediate teflon t sleeves with the aid of flanges. The samples were taken off through side-tubes mounted in the intermediate sleeves; the external orifice of each tube was sealed with a silicone rubber packing with the aid of a threaded cap with an orifice. The samples were taken with a medicinal syringe. Each section was heated by * Neftekhimiya 3, No. 4, 523-530, 1963. were used.

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means of an electrical winding. For rapid cooling, a jacket was provided, with a right-angled screw thread. The temperature in the sorbent layer was measured by thermocouples placed in a metal pocket. The experiments were carried out

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FIG. 1. Construction of the separating column: 1--body of the section; 2--intermediate sleeve; 3--flange; 4--side-tube for taking samples; 5--packing; 6--threaded cap; 7--thermocouple pocket; 8--lid. in the following way. The mixture to be separated was passed into the bottom of the column at a constant rate measured by a flow-meter. The process of movement and the formation of fronts could be observed by taking samples at various parts of the column. The analysis was carried out by a specially developed chromatographic method. After the distribution of the sorbed substances in the layer had been established, the feed of the mixture was stopped, the inlet cock was closed, and thermal displacement was carried out, whereupon the distribution of the sorbed substances gradually changed. ANALYTICAL PROCEDURE

Some model experiments were performed with propane-propy]ene mixtures, the analysis of which was carried out on a 0.5-0.25 mm fraction of K S K silica gel. The diameter of the column was 4 m m and its length 400 ram. The carrier gas was helium and its rate 15-30 ml/min. Sometimes the analysis of the CaHs+CsH e mixture was carried out by gas-liquid chromatography on an 8-m column filled with INZ-600 support, 0-5-0.25 ram fraction, impregnated

Production of acetylene concentrates

211

with 2 0 ~ (by weight) of adipodinitrile. In both cases, a thermal-conductivity detector was used with tungsten spirals with a resistance of 60~ [8]. Analysis of the mixture of gaseous products from the electrocracking of liquid hydrocarbons was carried out on three separate columns at room temperature. Identification was effected from the residence times of the pure components. A calibration graph obtained for the pure substances was used for the quantitative calculation of the chromatograms. RESULTS AND DISCUSSION

Two groups of experiments were carried out on the sectional column. In the first, the column was saturated with a mixture of 56.7% of propane and 43.3 o/o of propylene until the issuing gas had the composition of the initial mixture. The mixture was passed at the rate of 50 ml/min, which permitted, with the method of analysis developed, a sufficient amount of data on the movement of the components along the layer to be obtained. Samples were taken after sections 1, 3, 5, 7, and so on. Fig. 2, a, b, c gives the distribution of the components at various times and the curve (Fig. 2, d) shows the r~te of movement of the propylene front. The zone of pure propane (Fig. 2, a, b, and c), as was to be expected, broadens as it moves.

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FIG. 2. Layer saturation of the separating column: a--distribution of propane and propylene by layers 45 rain after the beginning of saturation; b--distribution of the sorbed materials after 165 rain; axis of abscissae--section numbers; c--ditto, after 225 min; d--rate of movement of the propylene front: /--propane; 2--propylene. As can be seen from Fig. 2, d, the propylene front moves with a constant velocity, since the dependence of the path travelled on the time is close to linear and, consequently, in our case the Shilov-Dubinina "parallel migration" conditions are realized. The process of thermal desorption was carried out by

212

O.P. KRIVORUCH:KOet al.

successively switching on the heaters of the sections. The volume of desorbed gas was measured at the outlet of the column. The heating of each successive section was begun after the evolution of gas had ceased. The thermal displacement separation was carried out at 130 ° . During the separation process, curves of the dependence of the change of concentration of the components on the time at 15 points of the column were recorded. Then, at a predetermined time from the beginning of the experiment, the distribution of the components in the layer in the gas phase was determined. In this way, we obtained, as it were, photographs of the motion and redistribution of the components of the mixture (Fig. 3, a, b, c). C, % bg vol. 90

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FIG. 3. Gradual change in the distribution of propane and propylene on thermal displacement -- heating bylayers: a--after 90 min; b--after 210 min; c--after 330 rain; 1--propane; 2-- propylene. On heating the lower sections, the mixture of gases was desorbed and passed along the column into the cold part of the sorbent. Because of the difference in the adsorption coefficients of propane and propylene, the latter became concentrated in front of the hot zone and, after m a n y repetitions of the desorption and adsorption process a zone of pure propylene was formed. As the zone of pure propylene broadened, part of the unseparated mixture was displaced from the column. After a predetermined time, the band of pure propylene was distributed over the whole length of the column. The liberation of gas ceased. I t is apparent t h a t the selected experimental temperature was sufficient to separate the mixture but was low for the complete desorption of the pure propylene. I t must also be noted t h a t in the first three sections a certain a m o u n t of propane was retained for quite a long time. In the other series of experiments, thermal displacement was achieved by heating the whole layer simultaneously. In these experiments, only l l sections of the column were saturated at 20 ° . Then the whole column was slowly heated. The distribution of the propane and propylene along the column at various temperatures is shown in Fig. 4, a, b, c, d.

Production of acetylene concentrates

213

I t follows from a consideration of these results t h a t when the whole layer is gradually heated pure propane is displaced from the column and the mixture is enriched with propylene up to a definite limit depending on the temperature

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FIo. 4. Change in the distribution of propane and propylene by layers on thermal displacement-gradual heating of the whole column: a-- 20°; b-- 50°; c-- 100°; d -- 140°; a, b -- 1-propane, 2--propylene; c, d-- 1--propylene; 2--propane. of the experiment (in our case, from 43.3 to 84% at 140°), after which the enriched mixture of propane and propylene is displaced from the column without further change of composition. The results of the latter experiments also permit the presence of propane in the first few sections of the column in the preceding experiments to be explained. A necessary condition for the formation of a zone of pure substances in thermal displacement is the m a n y times repeated performance of the desorption and adsorption processes during the heating of the column by layers. I t is evidently disadvantageous to saturate the whole column with the mixture. Consequently, the separation of the mixture of products from the electrocracking of liquid hydrocarbons under investigation was subsequently carried out with layer-heating of the column and incomplete saturation of it with the mixture. The experiments were carried out in a glass column 15 m m in diameter and 1.5 m long with a thermocouple pocket on the axis of the column. The

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F~(L 5. Chromatograms obtained in the complete analysis of the gaseous products of the electrocracking of liquid hydrocarbons: a--column with BAU activated carbon, analysis for hydrogen and methane; b--8-m column with adipodinitrile, analysis of the main products; c--8-m column with adipodinitrile, analysis of the micro constituents: d--2-m colunm with adipodinitrile, analysis of homologues and derivatives of acetylene /--hydrogen; 2--methane; 3--ethylene; g--propane; 5--isobutane; 6--propylene; 7--n-butane; 8--acetylene; 9--but-l-cue; 10--Jsobutylene; 11--trans-but.2-ene; 12--cis-but-2.ene; 13--buta-l,3-diene; /g--methylacetylene; 15--vinylaeetylene; 16--diacetylene.

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Production of acetylene concentrates

sorbent used was ShSM silica gel, 1-0-5 m m fraction. Before each experiment a complete analysis of the initial mixture was carried out by the m et hod described below. Figure 5, a, b, c, and d gives the four chromatograms generally t aken in a complete analysis of the mixture. To determine the hydrogen and the methane (Fig. 5, a) a column 5.2 m long with an internal diameter of 4 m m filled with BAU activated carbon with a grain size of 0.5-1 m m was used. The carrier gas (N2) was passed at the rate of 30 ml/min. The chromatogram shows t h a t only H 2 and CH 4 issued from the column, the remaining components being firmly sorbed by the carbon. In spite of this, in prolonged operation the residence times of H 2 and CH 4 on the column remained practically constant, although the column was never regenerated. Figure 5, b, c, shows chromatograms tak en with a column 4 m m in diameter and 8 m long filled with the standard support INZ-600 impregnated with 20% by weight of adipodinitrile. The carrier gas (He) was passed at the rate of 24 ml/min. To determine the main components, from 0.3 to 0.5 m] of sample was introduced into the column (Fig. 5, b). To determine the micro components, 3-5 m] of sample was used (Fig. 5, c). The heavier components (diacetylene and vinylaeetylene) were analysed on a 2-m column filled with the same support (Fig. 5, d). As indicated above, the m e t hod of absolute calibration was used for quantitative calcula4

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FIG. 6. FIG. 7. FIG. 6. Thermal displacement chromatogram in the production of an acetylene concentrate methane -- • ; ethylene-- × ; acetylene-- C); propylene-- A FIG. 7. Thermal displacement chromatogram of a fraction of pure acetylene: a--ehromatogram corresponding to a volume of 950 ml of gas issued; b--the same, volume 1630 ml; c--the same, volume 1700 ml; /--ethylene; 2--propane; 3--propylene; 4--acetylene; I - - moment of introduction.

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R E S U L T S O F T H E A N A L Y S I S OF T H E G A S E O U S P R O D U C T S O F T H E E L E C T R O C R A C K I N G OF L I Q U I D H Y D R O C A R B O N S

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Production of acetylene concentrates

217

tions, this obviously being better in our case since the complete analysis of the mixture was carried out on three columns. Table 1 gives typical results of the analysis of a mixture, showing satisfactory reproducibility. To isolate the acetylene concentrate, a glass column of the dimensions given above was filled with 185 g of ShSM silica gel. At its inlet was arranged a column containing 39 g of ShSM silica gel, 1-0.5 mm fraction. Before the experiment, the separating and preliminary columns were dried in a current of nitrogen at 150 ° for 3 hours. The sequence of issue of the components on ShSM silica gel was established in special dynamic experiments: H2, air, methane, ethylene, acetylene, propylene, butane, and so on. Saturation of the separating column with the mixture was carried out until traces of butane appeared at the outlet of the first column. Thus, in the separating column the "heaviest" product was propylene. After the start of the mixture the first column was disconnected, the inlet was closed, and thermal desorption by layers was carried out. The volume of the desorbed gas and its composition were determined at the outlet. The desorption temperature was 300 °. Table 2 gives data on the change in the contents of the components in the gas issuing from the column on thermal displacement ~,eparation of the mixture. Figure 6 shows the thermal displacement ehromatogram on isolating acetylene from the mixture. The results obtained show t h a t on desorption by layers a fairly sharp separation of the lighter (H2, air, CH 4, C2Ha) and heavier (Carte) components from the acetylene takes place and the method of thermal displacement can give relatively pure acetylene concentrates.

TABLE2.

C O N T E N T S OF THE COMPONENTS IN THE GAS ISSUING FROM THE SEPARATING

COLUMN(% BYVOL.)

Volume of issuing gas, ml 500 570 600 645 700 850 930 950 1000

CH4 CIH~ Call3 C2H2 C3H6 100 100 84 0 0 0 0 0 0

0 0 16 100 100 100 84"5 5"5 2.5

0 0 0 0 0 0 0 0 0 0 0 0 15"5 0 4"0 90"5 1"5 96

Volume of I issuing CH4 gas, ml 1290 1500 1590 1630 1640 1700 1750 1765 1780

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C3He

98

Z

0 98 0 99 0 100 0 98 2 91 9 19"5 80.5 7"5 92.5 0 100

Figure 7, a, b, c, shows chromatograms corresponding to definite volumes of gas issued from the column on thermal displacement.

218

O. P. KRIVORUCHKO et al. SUMMARY

1. A n a p p a r a t u s h a s b e e n c o n s t r u c t e d which p e r m i t s t h e m e c h a n i s m of t h e processes o f a d s o r p t i o n s e p a r a t i o n of gaseous h y d r o c a r b o n s b y the t h e r m a l d i s p l a c e m e n t m e t h o d to be studied. 2. T a k i n g t h e m i x t u r e C3Hs-~CsHs as a model, the o p t i m u m conditions for t h e r m a l d i s p l a c e m e n t s e p a r a t i o n h a v e b e e n selected. 3. T h e possibility h a s b e e n s h o w n of o b t a i n i n g a c e t y l e n e c o n c e n t r a t e s f r o m t h e gaseous p r o d u c t s o f t h e e l e c t r o c r a c k i n g of liquid h y d r o c a r b o n s a n d a c h r o m a t o g r a p h i c m e t h o d for t h e i r a n a l y s i s h a s been w o r k e d out. Translated by B. J. HAZZARD

REFERENCES 1. V. V. TATARINOV, U.S.S.R. Authors' Certificate No. 39904, 1933 2. A. F. DOBRYANSKII and A. D. KOKURIN, Zh. prikl, khim. 20, 10, 995, 1947 3. N. S. PECHURO and A. N. MERKUR'EV, Sb. Problemy elektricheskoi obrabotki materialov. (In Symposium: Problems of the Electrical Treatment of Materials.) Izd. Akad. Nauk SSSR, 181, 1962 ~. N. V. KEL'TSEV and A. Ya. KHALIF, Khim. i tekh. topliv., No. 12, 17-22, 1956 5. P. BENEDEK, L. SEPESHI and Z. NAD', Gaz. prom., No. 2, 30, 1958 6. M. I. YANOVSI~II, S. N. OZIRANER and LU PEI-CHZHAN, Zh. prikl. Khim. 33, No. 5, 1084, 1960 7. O, V. AL'TSHULER, O. M. VINOGRADOVA, S. Z. ROGINSKII and M. I. YANOVSKII, Dokl. Akad. Nauk SSSR, 140, 1307, 1961 8. A. S. PONOMAREV, Zavodsk. lab., No. 5, 634, 1960