Utiii lil 11 iiiffl
J III f 111
Ill llll i o w o BIBLIOTHEEK TU Delft P 1258 3163 C 353354CATALYTIC AROMATIZATION AND DEALKYLATION OF BICYCLIC HYDROCARBONS
DEALKYLATION OF BICYCLIC
HYDROCARBONS PRESENT IN
KEROSENE FRACTIONS
PROEFSCHRIFT
TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAPPEN AAN DE TECHNI-SCHE HOGESCHOOL TE DELFT, OP GEZAG VAN DE RECTOR MAGNIFICUS IR. H. J. DE WIJS, HOOGLERAAR IN DE AFDELING DER MIJNBOUWKUNDE, VOOR EEN COMMISSIE UIT DE SENAAT TE VERDEDIGEN OP DON-DERDAG 23 FEBRUARI 1967 TE 16.00 UUR
DOOR
MAHMOUD IBRAHIM ALLAM
SCHEIKUNDIG INGENIEUR
GEBOREN TE ALEXANDRIË
DIT PROEFSCHRIFT IS GOEDGEKEURD DOOR DE PROMOTOR
PROF. DR. IR. J. C. VLUGTER
C O N T E N T S
INTRODUCTION
Page
1 General 1 2 Production and consumption of
naph-thalene 2 3 Types and mechanisms of catalytic
re-forming reactions 5 4 Aromatization and hydrodealkylation of
the bicyclonaphthenes in kerosene fractions . 9
CHAPTER I A P P A R A T U S , MATERIALS, AND A N A L Y T I C A L METHODS
I-l General 11 1-2 The semi-technical unit 11
1-3 Experiments using the semi-technical unit . 16 1-4 Raw materials and chemicals used . . . . 1 8
1-5 Catalysts used 19 1-6 Analysis 21 CHAPTER II D E H Y D R O G E N A T I O N OF D E C A L I N ON P L A T I N U M REFORMING CATA-LYST CK303 II-l Introduction 28 II-2 Orientation Experiments 30
II-3 Effect of total pressure on reactions of
decalin 30 II-4 Changes due to variation of hydrogen
partial pressure 34 II-5 Influence of temperature and catalyst on
reactions of decalin 36 11-6 The variation of product distribution -with.
contact time 40 II-7 Reactions of decalin on dechlorinated
catalyst 48 II-8 Reactions of decalin on a catalyst with
higher platinum content 50 II-9 The change of catalyst activity with time . 55
CHAPTER III D E H Y D R O G E N A T I O N O F T E T R A L I N OVER P L A T I N U M REFORMING CA-T A L Y S CA-T CK303
Page
I I I - l Introduction 57 III-2 Influence of total pressure on product
distribution 58 III-3 Effect of hydrogen partial pressure on
reactions of tetralin 60 III-4 Influence of temperature on reactions of
tetralin 61 III-5 Dependence of product distribution on
contact time 63 III-6 Thermodynamics of the
decalin-tetralin-naphthalene system 66
CHAPTER IV REACTIONS OF M E T H Y L I N D A N ON P L A T I N U M REFORMING CATA-LYST CK303
IV-1 Introduction 74 IV-2 Preparation of methylindan 74
IV-3 Analysis of the products of reactions of
methylindan 78 IV-4 Influence of temperature on reactions of
methylindan 79 IV-5 Influence of hydrogen to hydrocarbon
ratio 82 IV-6 Influence of contact time 84
JV-7 Conclusions 86
CHAPTER V SUMMARY OF REACTIONS OF DE-CALIN, T E T R A L I N , N A P H T H A L E N E AND M E T H Y L I N D A N ON P L A T I -N U M REFORMI-NG C A T A L Y S T CK303 . 89
CHAPTER VI REACTIONS OF ALKYL D E R I V A T -IVES OF D E C A L I N ON P L A T I N U M REFORMING C A T A L Y S T CK303
VI-1 Introduction 96
A REACTIONS OF B E T A M E T H Y L -D E C A L I N
VI-2 Influence of temperature on reactions of
vin
Page
VI-3 Influence of contact time 100 VI-4 Changes following variation of hydrogen
partial pressure 101 VI-5 Scheme of reactions 102
B REACTIONS OF 1,6 D I M E T H Y L -D E C A L I N
VI-6 Thermodynamic considerations 103 VI-7 Influence of temperature on reactions of
1,6-dimethyldecalin 105 VI-8 Influence of contact time 109 VI-9 Reactions of dimethyldecalins on a
dechlorinated catalyst 113 VI-10 Influence of pressure on reactions of
dimethyldecalins 115
CHAPTER VII H Y D R O D E A L K Y L A T I O N OF 1,6-DIMETHYLNAPHTHALENE
VII-1 Introduction 118 VII-2 Experiments on hydrodealkylation of
1,6-dimethylnaphthalene 119 VII-3 Kinetics of the reaction 126
CHAPTER VIII A R O M A T I Z A T I O N A N D DEALKY-L A T I O N OF I N D U S T R I A DEALKY-L OIDEALKY-L FRACTIONS
VIII-1 General 132 VIII-2 Analysis of the tar distillate 134
VIII-3 Aromatization - Dealkylation of the tar
distillate 138 VIII-4 Analysis of the bicyclonaphthene rich
kerosene 140 VIII-5 Aromatization of the naphthenic kerosene
fraction 143 VIII-6 Hydrodealkylation of the aromatized
kerosene fraction 147
SUMMARY 1 4 9
SAMENVATTNG 1 5 3
INTRODUCTION 1. General.
Up till the beginning of the twentieth century, nature was the source of most chemicals or intermediates which man needed for his daily use. Simple physical separation and purification methods were used to obtain final products from the raw materials. With the discovery of the importance of petroleum as a source of fuels and chemical products, a new era of technology was started. Groups of chemists and engineers were, and are still, buisy with the newly opened fields of investigations. They first considered the black oil, produced from the deep earth, as a fuel that could compete with coal or even replace it. A new phase followed when new operations were developed for separation of highly refined fractions, such as extractive distillation and crystallization. The third phase, which continued between the years 1930-1940, represented a drastic change when the separation and purification operations were preceded by thermal treatment processes. The start-up of the thermal cracking processes-followed by the discovery of the cracking catalysts — not only represented a major change in refinery techniques, but also was the first step when the scientists started to show an utmost skill to compete with nature for the production of final products.
The production of aromatic intermediates was a privilege of the coke oven and tar distillation industries. Within the last 20 years, the source of these materials has undergone a fundamental change in Europe and the United States. Petroleum and natural gas have steadily ousted coal and its derivatives from their position as the basic raw materials for the chemical industry. For example, in 1965 the petrochemical processes accounted for 87"/o of the aromatics and derivatives produced in the United States. This figure is expected to exceed 91''/o by 1970. In Europe, there is a similar rapid increase in the production and consumption of aromatics from petroleum. There are several reasons for this switch from coal to petroleum as the raw material for the production of aromatics:
1) The petrochemical process produces a number of individual chemicals at one time, using considerable facilities for separat-ion, while a limited number can be produced from coal
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(benzene, naphthalene, xylene, and phenols are the main products from coal besides coke).
2) Easiness of handling and transportation of petroleum.
3) The aromatics produced from petroleum contain small amounts of sulphur, nitrogen, and oxygen compounds which may be as low as 10 ppm. The products from the coal industries contain 0.1-0.2''/o sulphur.
4) The increased tonnage of refined petroleum makes it possible to rely on as a stable source of raw materials.
The production of coal is mainly dependent on the requirements of the steel industry of coke. The many technical developments in the steel industry caused the consumption of coke first to fluctuate and later to drop gradually. On the records are now patents pertaining to injection of heavy naphtha or sulphur-free fuel oils in place of coke in the blast furnaces.
As a result of this change-over from coal to petroleum, the production of coal in the United States and in Europe has been halted. In the last few years, the prices of petroleum and nphtha have been showing a big decline while the price of coal has been increasing gradually". However, in some countries — like England and Japan — this change-over is slow and must be carefully planned to avoid several economical and social problems. In Hol-land, Germany, and Belgium, plans are being carefully designed to substitute other industries for those dependent on coal production from coal mines. In England the change-over is, however, expected to proceed with wide steps after the production of natural gas from the North Sea basin has achieved its goal.
2. Production and consumption of naphthalene.
Naphthalene is the parent hydrocarbon for a great number of compounds which are chemically and industrially important. They include pharmaceuticals, resins, surface-active agents, agricultural chemicals, flavours, perfumes, and insecticides. Naphthalene is also the basic raw material for the manufacture of phthalic acid and anthranilic acid, which are intermediates to indigo, indanthracene and triphenylmethane dyes. It is also the nucleus of an array of intermediates used in the manufacture of Azo-dyes. As a moth repellent, naphthalene has fallen out with the introduction of chloro-compounds of benzene and phthalic anhydride. The chief
use of naphthalene is in the manufacture of phthalic anhydride. This chemical is used extensively in the plastics and plasticizers industry. It finds wide use as a vinyl plasticizer, for alkyd resins, and polyester resins. The chlorine derivatives of phthalic anhydride are described as fire-retardants for alkyds, plasticizers, and resins*.
Naphthalene was one of the chemicals produced exclusively by the coke oven and tar distillation industries. Therefore, its production had been fluctuating according to fluctuations in these two industries. In the United States there were periods of shortage of naphthalene and the consumers were forced to import from the European and Japanese markets^-'. The increased consumption of naphthalene in Europe — as a result of the multifold growth in the plastics industry — had hardened the situation for the ameri-can importers. Since the consumption of naphthalene by the chemical industry was, and still is, increasing, it was necessary to switch to petroleum as a reliable source which is constantly available. The following table shows the production and import figures of naphthalene in the United States from 1953-1970.
U.S. PRODUCTION AND IMPORTS OF NAPHTHALENE.
Year Production, million pounds/year Imports, From Petroleum From Coal Total million pounds/year
1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1970 (Est.) 0 0 0 0 0 0 0 0 50 167 288 312 340 419 276 295 477 491 420 345 425 517 447 424 339 415 460 650 276 295 477 491 420 345 425 517 497 591 627 272 800 960 83 67 122 79 93 81 59 41 102 39 28 20 20 Nil.
The situation in Europe is now similar to that in the United States. The European production of phthalic anhydride is estimated at360.000 tons/year'', and sharp increases in demand were reported. Phthalic anhydride was until few years ago produced only by
oxidation of naphthalene using air and a vanadium oxide catalyst. Ortho-xylene has been competing with naphthalene in the last two years, as a raw material for the oxidation process. Until now, the preference of one over the other depends mainly on the cost of the raw material and its reliability as a stable source. Although the changeover to o-xylene looks economically attractive, the complete takeover is not foreseen for a long time to come. In England, Japan, Italy, Sweden, Australia, and the U.S.S.R., are announced plans for phthalic anhydride plants based on naphthalene". It seems, anyhow, that the future production of naphthalene will depend on the competitive price of o-xylene. To hold its firm stand against o-xylene, either the costs of production of napthalene must be lowered, or the existing prices be lowered as long as the producers can afford.
The use of the hydrodealkylation processes for the manufacture of naphthalene was started in 1961 in the United States. Several processes have since been patented for the manufacture of naph-thalene by catalytically dealkylating aromatic extracts, light crack cycle oils, heavy naphthas, heavy platformates or residue fractions, from the pyrolysis processes for the production of ethylene'•^•^•"'. Although the process as such proved to be an economical success, it has many drawbacks which are outlined in the following points:
1) A considerable portion of the compounds which are potential sources of naphthalene and which form between 20-30 wf/o of the feed, is lost as alkylbenzenes.
2) A considerable fraction of the catalytic surface area available is used by the cracking reactions instead of the desirable hydrodealkylation reactions.
3) The cracked products can dehydrogenate at these high temperatures and, further, condense or polymerize ultimately forming "coke" which deactivates the catalyst.
4) During hydrodealkylation, the saturates may cause reactor temparature control problems, due to cracking and subsequent exothermic hydrogenation reactions.
5) To avoid these effects, rather large hydrogen to hydrocarbon ratios are generally used, firstly for the hydrogenation of "coke" precursors and secondly for the large consumption of hydrogen by the cracking reactions. Since the cost of hydrogen forms a considerable part of the total costs of production, the process would be more profitable if the hydrogen consumption could be limited.
It is believed that if the bicyclonaphthenes can be converted in a first step to the more stable aromatics under less severe con-ditions, the drawbacks outlined above would decrease to a minimum and the economics of the process would, accordingly improve. The conversion of the bicyclonaphthenes to aromatics liberates hydrogen which can be further used in the hydrodeal-kylation step; this means a significant decrase in the hydrogen requirement for the process. Moreover, there are kerosenes from special crudes which contain high percentages of alkyl derivatives of decalin (see chapter VIII). At the high temperature severity of the hydrodealkylation process, these kerosenes can hydrocrack to alkylbenzenes and gases. Other fractions contain high concen-trations of compounds with the indene or indan structures. These structures must be isomerized to the 6-6 ring system before they can be dehydrogenated to naphthalene. An aromatization step must be developed whereby these bicyclonaphthenes are converted to the naphthalene system before the hydrodealkylation process.
A study was carried out on this aromatization process and it was started on model hydrocarbons of the decalin, tetralin, and methylindan types.
The catalyst to be used for the aromatization step must be capable of performing hydrogenation-dehydrogenation activity as well as acid function properties. The combination of these functions can be best found in a platinum reforming catalyst. An industrial catalyst was used, namely the CK 303 from "Ketjen Zwavelzuur fabrieken v/h Ketjen N.V. Amsterdam".
In the following part are described briefly the types and the mechanism of reactions on such a catalyst.
3. Types and mechanism of the catalytic reforming reactions.
The processing of petroleum fractions had undergone a fun-damental change by the discovery of the cracking catalysts. Reactions which yield large quantities of high octane gasolines have been extensively investigated, and they were found to b e " ' ^ : cracking of heavy long chain molecules, the skeletal isomerization of 5-ring compounds to alkylbenzenes, isomerization of straight chain compounds, and removal of compounds of oxygen, sulphur and nitrogen by hydrogenation. Side reactions of condensation and polymerization of intermediate olefinic compounds cause coke deposition on the catalyst and, therefore, result in a gradual
— 6 —
deactivation of the catalyst. Such a catalyst would be suitable for treatment of huge quantities of straight-run gasolines if it was not so rapidly deactivated. The oxides of the transition metals of group VI were found to possess (de)hydrogenation activity in combination with their acidic function properties. Because of their hydrogenation activity they were capable of hydrogenating the coke precursors. The oxides of chromium, molybdenum, and cobalt were used in processes where the catalyst was periodically regenerated.
The metals of group VIII were found to have superior (de)-hydrogenation properties than those of group VI due to the high dispersability of these metals on the carriers, and their favourable crystalline structures. Platinum was found to be the most active of the metals of this group. Ciapetta'' gave the following table for the relative activities of the transition metal oxides and the noble metals of group VIII.
Catalyst Dehydrogenation Activity. 34 »/o Cr^Os 10 "/o MoO, 5 »/o Ni 5 o/o Co 0.5 »/o If 1 «/o Pd 5 Vo Ni 1 Vo Rh 0.5 Vo Pt on ALO3 on AI2O3 on AI2O3 on AI2O3 on AI2O3 on AI2O3 on Si O2 on ALO3 on AI2O3 0.5 3 13 13 190 200 320 890 1400-400
By combining the acid characters of the cracking catalysts with the (de)hydrogenation activity of platinum, the so-called bi-functional catalysts were obtained. These catalyst were used widely in processes wich produce gasolines of superior knocking qualities from straight-run gasolines or naphthas of suitable boiling range. The reactions typical of these processes are:
1) Dehydrogenation of alkylcyclohexanes to benzene derivatives. 2) Dehydroisomerisation of alkylcyclopentanes to alkyl benzenes. 3) Isomerization of paraffins to isoparaffins.
4) Dehydrocyclization of paraffins to aromatics. 5) Dealkylation of long side chains of aromatics.
6) Hydrocracking of rather long chains to gasoline constituents and gases.
reactions of the hydrocarbons. Since the start of the first plat-forming unit developed by U O P in 1949, there were published results of a vast amount of research on the bifunctional catalyst. An extensive list of literature is given by Ciapetta^'^ and by Connor^*.
From the beginning of the platinum reforming processes, the investigators were well aware of the complexity of the reactions which take place on the bifunctional catalysts. Ciapetta^^'^^, Mills'", H i n d i n " , and Weisz'* have shown experimentally that two distinct types of reactions occur on the double site catalyst. The two sites mechanism suggested by most of the authors proceeds in the following steps:
1) Dehydrogenation of the naphthene or paraffin to an inter-mediate olefin at the metal site.
2) Release and migration of the olefin in the gas phase to the acid site of the catalyst.
3) Addition of a proton at the acid site with subsequent formation of a carbonium ion.
4) Skeletal rearrangement of the carbonium ion.
5) Release and migration of the new olefin from the acid site through the gas phase to the metal site.
6) Further hydrogenation or dehydrogenation of the olefin and release of a product olefin, paraffin, or aromatic molecule.
Simply these steps can be schematically reprecented as:
. M ^ . . t i t M A ^ ^A' y B > C — » D ' ^ ^: D
The equilibrium between A and D and their conjugates A and D is set at the metallic function M, and the isomerization occurs at the acid site i; t represents the transfer step from one site to the other. It was proved by Weisz and others^'-^-^^ that the rate controlling step is the isomerization at the acid site. The dehy-drogenation and hydehy-drogenation reactions at the metal site are extremely rapid. It was also proved by Weisz that the two catalytic sites need not be in neighbouring positions, on the other hand, they may be several Angstrom units apart. Therefore, the intermediate olefins must be of sufficient stability in order to diffuse from one site to the other.
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Several other mechanisms were suggested such as that by Bond^* who suggested that the reaction may occur at the atomic interface between the metal and the acid parts of the catalyst. Khoobiar^^ has suggested that radicals are formed at the metal sites of the catalyst, and they migrate to the catalyst-diluent interface where they further react. Hydrogen radicals were supposed to be formed by platinum; the hydrogen radicals migrate to the interface and further react with cyclohexane to form olefins. McHenry^" has found that platinum-alumina catalysts which are most active in dehydrocyclization contain a large percentage of their platinum in a form which is soluble in hydrofluoric acid in acetyl acetone. The soluble platinum is neither the platinum oxide nor finely divided platinum. It has a positive valency of 4 and is believed to be present as a complex formed by the reaction of chloroplatinic acid and the alumina support. The complex is only slowly reduced by hydrogen.
The dehydrogenation activity of platinum in case of cyclonaph-thenes is attributed to its favourable crystalline structure-". The cyclonaphthene molecule, for example : cyclohexane, is adsorbed and is lying flat on the (III) phase of the platinum crystallite. The extraction of 6 hydrogen atoms is affected simultaneously by 6 platinum atoms, and after breaking of bonds the hydrogen is liberated as molecular hydrogen while the hydrocarbon is released as an aromatic molecule. This mechanism had been approved by several authors-'; they also suggested that the molecule can be adsorbed edgewise, 2 hydrogen atoms are extracted at once, and an olefin is released. The olefin is readsorbed to complete the dehydrogenation at the catalyst surface.
The platinum reforming catalysts which contain acidic alumina as a support are suggested to have two types of acid sites:
1) The first type contributes to the acidity by reacting with water to produce protons. These are known as Lewis acid sites. 2) The second type of acid centres, known as Brönsted acid sites,
has readily available protons.
The two centres are interconvertable since a Lewis acid site may be hydrated by a water molecule and be converted to a Brönsted acid site.
The carbonium ions are formed on these acid sites, thus:
R—CH = CH2 + H — y R—CH — C H E (Brönsted acid site) R-CH2-CH3 + L — > R—CH^—CH3 + H L (Lewis acid site)
The acidity of the platinum reforming catalyst may be strengthened by other acid groups like chlorine or fluorine.
It was suggested by Leftin^'^ that the main action on saturated compounds is through the interaction of Lewis acid sites with the molecule, but in the presence of olefins, free protons available from the Brönsted acid sites clearly reveal themselves.
The platinum reforming catalysts are sensitive to poisoning, and the poisoning effect is in most cases cumulative. Compounds of oxygen, sulphur, and nitrogen may have a temporary or permanent poisoning effect, but compounds of base metals, such as arsenic, molybdenum, vanadium, and lead are permanent poisons. Coke deposition during operation has also a permanent poisoning effect. An effect which causes catalysts activity decline is the platinum crystallite growth at high temperatures above 600°C'".
Before use, the platinum reforming catalyst is activated in an atmosphere of hydrogen at temperatures as high as 520-540°C in order to reduce the platinum oxide to platinum which exists then in tinely distributed crystallites at the catalyst surface.
4. Aromatization and hydrodealkylation of the bicyclonaphthenes
in kerosene fractions.
From the previous review it was concluded that a pretreatment of the kerosene fractions in an aromatization step could increase the yield of naphthalene from the hydrodealkylation process. The platinum reforming catalyst was used to study the dehydrogenation reactions of model hydrocarbons of the decalin, tetralin, and methylindan types. The results of these investigations were then used to determine the conditions suitable for aromatizing the kerosene fractions, i.e. to change them to fractions rich in tetralin and naphthalene.
The aromatized kerosenes were dealkylated using conditions which were less severe than those used in the industrial hydrodeal-kylation processes. The two step process of aromatization and dealkylation yielded a product of high naphthalene content. The feasibility of the two step process was found to depend on the type of feed to be treated.
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Literature:
1) Stabough, R. B., Hydr. Proc. Petrol, Ref., 45, (5), 250 (1966). 2) Steinhofer, A., Hydr. Proc. Petrol. Ref., 44, (8), 134 (1965).
3) Ballard Jr., H. D., Adv. in Petrol. Chem. And Ref., Interscience 1965, Vol. X .
4) European Chemical News, 9, (217), 25 (1966).
5) Graham, J. J., European Chemical News, Large Plant Subliment, (1966). 6) News Tips From European Chemical News, volumes 7 to 10.
7) Asselin, G. F., and E. R. Erikson, Chem. Eng, Progr., 58, (4) 47 (1962). 8) Emmerson, H. R., et al. Oil Gas J., 61-2, (22), 123 (1963).
9) Annon, Chemical Week, 88, (9), 45 (1961). 10) Staff, Ind. Eng. Chem., 54, (2), 28 (1962).
11) Thomas, C. L., Ind. Eng, Chem., 41, (2), 2564 (1949).
12) Greensfelder, B. S., et al, Ind. Eng. Chem., 41, (2), 2537 (1949). 13) Ciapetta, F. G., et al. Catalysis by Emmctt, Reinhold 1958, Vol. VI. 14) Connor, H., Chemistry and Industry, Nov. 26th, 1960, P-1454. 15) Ciapetta, F. G., Ind. Eng. Chem. 45, 162 (1953).
16) Mills, G. A., et al, 2nd W o r l d Congress On Catalysis, Paris 1960, Paper 113. 17) Hindin, S. G., et al, J. Phys. Chem. 62, 244.
18) Weisz, P. B., and E. W . Swegler, Science, 126, 31 (1957),
19) Schuiken, N. I., Advances In Catalysis, Academic Press 1957, Vol. IX, 783. 20) Trapnell, B. M., Advances in Catalysis, Academic Press 1951, Vol. I l l , P - 1 . 21) Weisz, P. B., and C. D. Prater, Advances In Catalysis, Academic Press,
1957, Vol. I X , P-582.
22) Weis, P. B., Advances In Catalysis, Academic Press 1962, Vol. 13, P-137. 23) Sinfelt, J. H., and H. Harwitz, J. Phys. Chem., 64, 892, (1962).
24) Bond, C. J., Discussions Part 3, Advances In Catalysis, Academic Press 1957, Vol. IX, P-639.
25) Khoobiar, S., et al. Paper Presented To The " T h i r d International Congress On Catalysis", Amsterdam 1964.
26) McHenry, K. W., et al, "Second W o r l d Congress On Catalysis, Paris 1960, P-2295.
27) Balandin, A. A., and Brussow J. J., Z. Phys. Chem.. B34, 96 (1936). 28) Leftin, H. P., and W . K. Hall, "Second World Congress On Catalysis".
CHAPTER I
A P P A R A T U S , MATERIALS, A N D A N A L Y T I C A L M E T H O D S
I - l . General.
The reactions which take place on a platinum reforming catalyst are of both the exothermic and the endothermic types. The dehydrogenation and the isomerization reactions are endothermic, and the cracking and the hydrogenation reactions are exothermic. In an industrial platinum reforming process the reactions mostly encountered are the dehydrogenation of naphthenes, the isomerization of paraffins, the dehydrocyclization of paraffins, and the dehydroisomerization of alkylcyclopentanes to benzenes. The overall result is an endothermic process which necessiates that heat is added intermittantly or continuously to the reacting stream. Most of the industrial processes operate with adiabatic type reactors. The longitudinal temperature drop in these reactors is in the order of 50-100°C depending on the severity of the process, feed stock, and the formulation of the catalyst. The feed enters the first reactor at a temperature of 490-530°C''^'^. The effluent from the first reactor is reheated and is fed to the second reactor at a temperature slightly higher than that at the inlet to the first reactor. The endothermicity of the dehydrogenation reaction causes a tremendous radial temperature profile. Dijkstra* reported a tem-perature drop of 50-100°C between the centre of the catalyst bed and the catalyst near the walls of the reactor, a distance of about 9 mm. The reaction was the dehydrogenation of cyclohexane on an industrial platinum reforming catalyst. Exploratory runs were carried out using decalin as feed to a reactor which contained an industrial platinum reforming catalyst and the temperature drop from the wall of the reactor to the centre of the reactor tube, a distance of 4 mm, was found to be 36-40°C.
1-2. The semi-technical unit.
It was desirable to study the reactions of model substances of the bicyclonaphthene type on an industrial catalyst before aromatizing a whole fraction. It was decided to construct an apparatus which would have approximately the same features as the reactors in an industrial process. Moreover, such a reactor must be operated
— 12 —
isothermally so that it might be possible to carry out kinetic studies on the reactions of the model hydrocarbons. It was also intended to use two reactors in series to perform an aromatization step followed by a hydrodealkylation step. For this purpose an apparatus was designed and constructed which has the following features:
1. The reactor is of the plug flow type with a fixed catalyst bed. It is made of a stainless steel 316 tube A, see fig. 1-1, of 15 mm internal diameter and a total length of 260 mm. The tube has a total capacity of 46.5 cc. At the bottom of the tube a sieve plate is placed which supports the contents of the reactor. The reactor tube can be screwed on a flange and an effluent exit tube B.
2. Through a system of sealing o-rings, C, are passed three ther-mocouples of the thermocoax type made by Philips N.V. of Eindhoven. Each thermocouple has a diameter of IV2 mm. The position of the hot junction of each thermocouple can be changed along the length of the reactor tube by moving the thermocouples through the o-rings to the required height. During the experiment the position of the thermocouple cannot be changed. In order to get a controllable reaction system it is necessary to measure the temperature at several points along the length of the reactor. For this purpose 5 thermocouples are required. One thermocouple measures the temperature of the feed stream before it enters the catalyst section, the other four thermocouples measure the temperature of the reaction mixture at selected points. Another tube, similar to that of the reactor tube and only 14 cm. long is placed over the first one. Through a similar system of o-rings at the top of the upper tube pass the other two thermocouples. These measure the temperature of the reaction mixture just before it enters the catalyst section and the temperature at the top of the catalyst bed. The two tubes are placed above each other in another tube D made of Nymonic 80 steel which has the composition : 20 "/o Cr, 2.3 "/o Ti, 1.3 "/o Al, 5 "/o Fe, and the rest is Ni. This kind of steel can be used for reactor operation at a temperature severity of about 750°C for a long time, or 815°C for a short time.
3. The outer Nymonic steel tube is heated by 5 sections of heating wires which are wound uniformly around the tube. The first heating coil supplies heat to the upper tube of the reactor which
REACTORTUBE
FLANGE AND OUTLET-CONNECTIONS O-RINGS SEALING
REACTOR OUTERTUBE THERMOCOUPLE CONNECTORS
— 14 —
is usually used as a preheater to the catalyst secetion. The length of the catalyst bed is divided into 4 sections : 2 sections each 3 cm long, followed by one section 6 cm long, and the last is 14 cm long. Each section is heated by a separately control-led heating coil. The amount of heat delivered by each coil is controlled by a variac which operates from the 220 V mains. This system of heating and the 5 thermocoples in the reactor permit a rather good temperature control of the catalyst section.
The response to changes in heating is gradual and uniform. Stable conditions of temperature for the part under control are obtained in a rather short time.
4. Three reactors which have identical dimensions are connected in series to form the whole reactor system. The connections between the reactors are short and are heated by infra-red radiator heaters, of 1000 W each, provided by "Ebstein Works in Germany". The three reactors system is chosen in order to have a good temperature control of each one apart, and for use of the three reactors for different functions. For the first purpose the heating and control system proved to be extremely efficient. The hydrodealkylation process requires a temperature severity of 640-750°C while the reforming process requires a temperature of 450°C or less. It is necessary to separate the reactors which operate at so different temperature severities. 5. A calibrated reservoir delivers the liquid feed to a metering pump which pumps it to the inlet of a stainless steel 316, 3 mm diameter pipe which is used as a preheater. A lock system controls the stroke of the pump so that the volume of feed can be varied. The preheater tube is connected by a flange system to the top of the first reactor. A thermocouple placed at the end of the preheater tube measures the temperature of the feed stream to the reactors.
6. The product from the last reactor flows through a heated tube to a high pressure seperator, which is heated to about 100°C. High pressure gas is separated from the liquid product in the high pressure separator. The gases flow to a knock out drum, operating at the same pressure, then through a cooler to the seperating drum of a gas circulation compressor. The com-pressor circulates the dry gas back to the inlet of the preheater tube where it mixes with the liquid feed.
SEMI TECHNICALPLANT SCHEME FIG. 1-2 A REACTOR 1 B REACTOR 2 C REACTOR 3 D PREHEATER E S E P A R A T O R ! F SEPARATOR 2 G COOLER H PRODUCT RECEIVER K GASFLOW RATE METER L GAS RECYCLE COMPRESSOR M F E E D P U M P S N F E E D RESERVOIRS P BACKPRESSURE VALVE H W E T G A S M E T E R S Y H P REDUCING VALVE Z H . P H j B O T L L E
— 16 —
The gas flow rate is measured in a flowmeter which operates
on the principle of displacement of a cetrain volume of oil, of suitabel viscosity, by the flowing gas. The whole system is operated under pressure. A back pressure valve is placed on the gas line to the compressor so that any gas liberated by the reaction can be released from the system through the valve. The amount of the gas released is measured by a wet gasometer.
7. The liquid product from the high pressure separator flows through a needle valve to a heated glass product receiver. Low pressure gas is separated in the glass receiver and is measured by another wet gasometer.
It is necessary to keep the effluent stream from the reactors to the receiving sample flasks above 80°C to avoid condensation and solidification of naphthalene; hot water jackets, heating tapes, and infra red lamp are used for this purpose. Fig. 1-2 shows a schematic diagram of the semi-technical plant.
1-3. Experiments using the semi-technical unit.
The semi-technical plant proved to be extremely efficient for carrying out experiments under isothermal conditions. During reactions using decalin as feed to the reactors, it was necessary to dilute the catalyst to get rid of the large radial temperature dif-ference caused by the reaction endothermicity. The experiments were always performed using a fixed volume of feed while changing the volume of catalyst if different liquid hourly space velocities (LHSV) were required. Dilution of the catalyst was done using steatit Raschig rings broken to a size comparable to thai of the catalyst. A dilution of one volume of the catalyst by three volumes of the crushed steatit lowered the temperature difference between the wall and the centre of the catalyst from about 36°C to 1°C. Because of the limited volume of one reactor, the volume of the catalyst in the first reactor was always 10 cc and that of steatit was 30 cc. The reaction endothermicity dropped tremend-ously after the first reactor, so that a dilution of only 1 : 1 was sufficient to keep the reaction under isothermal conditions. Thus, the second reactor could be filled with 20 cc catalyst and 20cc crushed steatit. After the second reactor no more dilution was required for further increments of catalyst volume. Dilution was
necessary in all experiments using decalin, /^-methyldecalin, or dimethyldecalin as feed to the reactor. In the cases of tetralin and methylindan the temperature difference between the wall and the centre of the reactor was so small that dilution was not necessary. The positions of the five thermocouples in the reactor were adjusted before each experiment according to the amount of catalyst put into the reactor. The points of measurement were chosen in a way that accurate use could be made of the heating coils. Due to the flexibility of the heating system and the accuracy of the thermocouples, the experiments were performed under isothermal conditions with a great deal of accuracy.
Experimental procedure :
1. The reactor is dismantled and cleaned. The position of the thermocouples are adjusted to the required height, and the o-rings are tightened.
2. The thermocouples are adjusted along the reactor axis by help of a narrow tube and the reactor is filled with the platinum reforming catalyst. Tapping is gently applied to the walls while filling the reactor. The tube is withdrawn gradually until the whole reactor is filled.
3. The reactor is fixed in position in the plant and the system is closed and flushed 3 times with hydrogen. The system is then pressurized to 50 atm. with hydrogen. Tests for leaks are carried out after each filling.
4. The hydrogen is circulated over the catalyst by the compressor and the reactor is gradually heated to a temperature of 520-530°C. The heating is continued for 8 hours. In that way the catalyst is activated (reduced) with hydrogen at a temperature higher than the reaction temperature. T h e same treatment is given to the catalyst before each experiment.
5. The catalyst is then cooled to about 380°C. Feed is started and adjusted at the required rate, and hydrogen circulation is adjusted to obtain the required hydrogen to hydrocarbon ratio.
— 18 —
6. The temperature and pressure are controlled and adjusted to the required level for V2 hr. before a sample is collected.
7. If several samples are to be collected using the same catalyst while changing one of the variables, the change is gradually done over a period of ^hz hr. The sample is collected under constant conditions.
8. After the last sample is collected, the reactor is cooled and is emptied from the catalyst of which a sample is taken for analysis.
Note: Several experiments were carried out to measure the influence of temperature on the reactions, using different amounts of catalyst. The timing of the samples for each temperature level was always the same. Thus samples at each temperature level were collected using catalysts which had the same life time.
9. The activation temperature for the chromia-alumina hydro-dealkylation catalyst was 690-700° C. The same procedure was used for experiments on the hydrodealkylation reaction.
1-4. Raw materials and chemicals used.
Several bicyclonaphthenes were used as feed to the reactor. The products contained, besides the unconverted naphthenes, the partially as well as the completely dehydrogenated derivatives. The isomerization, cracking, and dealkylation reactions produced a considerable number of compounds in addition to the main products of dehydrogenation. Only a small number of bicyclo-naphthenes and aromatic compounds was available in the pure state. The others had to be prepared either from the parent naphthene or aromatic by partial (de)hydrogenation. In the following table are listed the compounds which were used as feed to the reactor or produced by the reactions.
Table 1-1
Chemical Boiling point, °C (760 mm) Purity, mole O/o source
Naphthalene 217.96 100 a Tetralin 207.20 96.5 (A) a Decalin 98.5 (B) a (SOO/o Trans-, 217.96 207.20 186.7 194.6 187.5 186.0 205-216 200/o Cis) Methylindan 9G.5-98.0(C) 1 ^ „ 2— „ Methyldecalin 205-216 100.0 (D) c Dimethyldecalin 216-218 100.0 (E) d Dimethylnaphthalens 260-270 100.0 e
a- Stores of the "Afdeling Chemische Technologie" of the University. b- Prepared by isomerization of decalin following the method which is
described in section IV-2.
c , d - P r e p a r e d by hydrogenation of the methylnaphthalenes following the method in section V I - 1 .
e- The dimethylnaphthalene was obtained from "Rütgerswerke U n d Teer-verwerkung Aktiengesellschaft "Castrop-Rauxel". The mixture contained 87.5''/o as 1.6 dimethylnaphthalene.
A- Impurities : CS'/o naphthalene, 3 % decalin B- „ : iVo Tetralin, 0.5»/o o-xylene
C-, D-, and E: T h e products were free of aromatics and alkylbenzenes.
1-5. Catalysts used.
It is rather difficult to choose a catalyst for the aromatization step since the reactions to be catalyzed in one step require several functions at a time. A pure dehydrogenation catalyst will produce an aromatic fraction from a naphthene rich distillate. Condensation reactions will take place on the same catalyet unless the reaction is carried out under conditions which will not favour formation of these condensation products. These conditions give also a smaller yield of aromatics. Several distillates contain rather large per-centages of compounds which have the indene structure. Under normal dehydrogenation conditions, these indenes will condense and deactivate the catalyst at an increased rate. It is necessary to remove such structures from the oil fractions before processing or convert them to the 6-6 ring compounds. The removal of these
— 20 —
compounds is almost impossible due to their number and wide boiling range. To accomplish the purpose of (de)hydrogenation and isomerization, a bifunctional catalyst is the most suitable for the purpose. The platinum reforming catalyst presents itself at the top of a list of catalysts that can be used.
No experimental screening of the different catalysts was made. An industrial platinum reforming catalyst, the CK 303 of Cyanamide Ketjen N.V. of Amsterdam, was used. It was of the extruded type with alumina as support and had the following properties:
Composition
Pt 0.299 »/o by weight Fe 0.0084 «/o by weight Cl 0.69 "/o by weight Na 0.0014 »/o by weight Cu 0.0021 Vo by weight Si 0.0065 «/o by weight B- Catalytic Properties Surface area 170 m-/g Pore volume 0.48 ml/g Average length 3.9 mm.
Another catalyst of Cyanamide Ketjen, the CK 306, was also used for some experiments. The catalyst contained 0.582 wt Vo Pt and 0.69 wt "/o Cl; all other properties were almost the same as CK 303.
For special runs, both catalysts were dechlorinated by soda wash as described in section II-7. The product catalyst had no isomerization activity and was used to study the dehydrogenation reactions without interference of side reactions.
For the dealkylation of the alkylaromatics, produced from the aromatization step, an industrial catalyst was used. The catalyst was the Harshaw Cr 0205 T ; it was of the tabletted form with 19 wt "/o CroO:! mounted on alumina, and had a surface area of 60 m7g.
The platinum reforming catalysts were activated in the reactors at 520-530°C under 50 atm. hydrogen. The hydrogen was cir-culated over the catalyst for 8 hours. The same treatment was always given to the catalyst before use. Similarly, the hydrodeal-kylation chromia catalyst was activated at 690-700°C before use.
1-6. Analysis.
In fig. 4-1 are shown the bicyclonaphthenes which were used as feed to the reactors or produced during reactions of the model compounds on the platinum reforming catalysts. Most of the compounds listed are found in straight run or tar distillates as well as in catalytically and thermally processed oil fractions. The parent compounds are : naphthalene, tetralin, decalin, indene, and hydrindan. Addition of alkyl groups to the parent hydrocarbons gives rise to a large number of compounds depending on the position of branching, length of the side chains, conformation of the parent hydrocarbon and axial and equatorial configurations of the resulting derivative. For example, hydrindan has two forms, the cis- and the trans-, which on methylation in the 1-position
.•CH3
tó
- • 2 i s o m e r s - • 2 i s o m e r s CH, C + - C H 3 •^^3 ( 2 - p o s i t i o n ) -»• 2 i s o m e r s -+ 2 i s o m e r s Fig. 1-3— 22 —
gives rise to 4 isomers; the addition of another group to the 2 position causes the number of isomers to multiply, fig. 1-3.
The introduction of the methylgroup into other positions yields 48 compounds with different conformations.
It will be shown later on that a naphthenic straight run kerosene fraction which has a boiling range of 190-235°C, contained more than 100 different compounds. The aromatization of that fraction produced a mixture of more than 300 compounds. The separation of these compounds and their identification in a complex mixture is a tedious and time consuming work even by use of the modern tools of analysis. Simple distillation, distillation under reduced pressure, azeotropic distillation, molecular distillation, thermal diffusion, and separation by sulfonation or aducts formation had all been tried on fractions of different nature ^•'*. Coupled to these were the modern analytical tools such as mass-spectroscopy, infra-red, and ultra-violet apparatuses. All these analytical tools could yield only a fairly satisfactory amount of knowledge on distribution of different hydrocarbon groups in oil fractions. Correlation of physical properties was very useful in this direction''". Thanks to the extensive and patient investigations of several experienced analysts who established the definite existence of hundreds of compounds in several distillates'''^''. The results of these invest-igations were in some cases extrapolated to high boiling range fractions.
The development of gas liquid chromatography (GLC) provided the analysts with a magnificent tool for separation and iden-tification of compounds from complex mixtures. In many laboratories, preparative GLC apparatus have been used for separation of compounds and collection of minute quantities of an unknown material which was further identified by spectral analysis or nuclear magnetic resonance (NMR). However, the problem of separation and identification of a great number of compounds in a mixture is still worked on by a number of laboratories throughout the whole world.
In the following few pages is described the method of analysis of reaction products of model bicyclonaphthenes on platinum reforming catalyst. In chapter VIII, on the aromatization and dealkylation of a kerosene and a tar distillate, the analysis of complex mixtures will be described.
The number of products of reactions of decalin or tetralin on a platinum reforming catalyst is comparatively small so that a simple analysis by GLC can be performed. The dehydrogenation reactions give a mixture of decalin, tetralin, and naphthalene. The isomerization reactions yield methylhydrindan which can be dehydrogenated to methylindan and methylindene. Many alkyl-benzenes are produced by the cracking reactions. The product mixture thus contains bicycloaromatics, bicyclonaphthenes, and alkylbenzenes. The separation of the compounds of this mixture by GLC was tried on several columns under different conditions. Chromosorb white was used as support for the stationary phases which were tested; these were Apizon L, Apizon M, and Apizon N.
The percentage of the stationary phase on the support was varied between 3 and 15 "/o. These GLC columns are usually operated on the basis of differences in boiling points of the com-pounds to be separated. The resolution and separation of the peaks for the compounds of the above mentioned mixture were exceptionally good, specially when using Apizon M columns, except for the peaks of trans-decalin and the methylindans, and for those of the methylhydrindans and indene and indan. All these compounds have very near boiling points; for example the boiling points of trans-decalin and I-methylindan are 186.7 and 187.5C respectively. The methylhydrindans boiling range interferes with that of indan and indene and higher alkylbenzenes. It was necessary to separate these compounds in order to obtain chromato grams which could be quantitatively measured with a reasonable degree of accuracy. The silicon rubber GE SE 52 was the most suitable stationary phase which separated trans-decalin from the methylindan; it could even resolve the 1- and 2-methylindan peaks but no good separation of them was, however, possible. The separation of methylhydrindans was not possible using a simple GLC aparatus. However, these compounds were separated and identified using a Golay column and a flame ionization detector of a Carlo-Erba apparatus, as will be described in chapter IV.
An 8 meter column of 4 mm diameter was prepared for analysis of products of reactions of model bicyclonaphthenes. The filling was 10 "/o silicon rubber GE SE 52 on chromosorb white. Table 1-2 gives the conditions of separation, the retention times and cor-rection factors of the compounds of the mixture relative to naph-thalene.
Table 1-2.
Gas-Liquid Chromatograjjhic Analysis of Aromatic and Naphthenic Hydro-carbons boiling between 140 and 220°C:
Apparatus: Recorder: Detector: Column:
Locally built in the electronic and instrument work-shops of the department.
Honeywell, of response range form 0 to 2.5 mV. chart speed = 1 cm/min.
Standard thermal conductivity cell, bridge current = 200 mA.
8 meters long, 4 mm internal diameter. Filling is 10 "/o GE SE 52 on chromosorb white of mesh size 60-80; number of theoretical plates (Naphthalene) = 2960. H E T P = 0.27 cm. Temperature of analysis: Carrier gas: Inlet pressure: Outlet pressure: Carrier gas flowrate:
Compound o-xylene Propylbenzene Butylbenzene Ethylbenzene Indan Indene m-hydrindan* Trans-decalin 2-methylindan 1-methylindan Cis-decalin 1-methylindene 2-methylindene Tetralin Naphthalene 175°C Hydrogen 2,05 kg/cm2 atmospheric 3.6 1/hr. Results of Analysis : Relative Retention (Naphthalene) 0.161 0.369 0.393 0.315 0.434 0.445 — 0.493 0.552 0.558 0.687 0.724 0.761 0.873 1.000 Correction Factor at 175°C (Naphthalene) 1.01 1.01 1.02 1.02 0.98 0.99 — 0.80 0.96 0.96 0.80 1.13 1.13 0.93 1.00
* The amount of methylhydrindans produced during reactions of decalin and tetralin were substantially small (not more than O.s'/o) and they were included in the alkylbenzene fraction.
Note: The correction factor was calculated on the basis of molar fractions. Fig. 1-4 shows a chromatogram of the reaction products of decalin on the platinum reforming catalyst.
Another GLC-apparatus was used for the analysis of products of reactions of the methyldecalin on platinum reforming catalyst and the dealkylation of the methylnaphthalenes. These compounds have a boiling range of 180-275°C (including the alkylbenzenes). Table 1-3 gives the conditions of separation, the relative retention and correction factors of the compounds relative to naphthalene.
Fig. 7-1 shows a typical chromatogram for products of reactions of 1,6 dimethylnaphthalene, separated by that column.
The peaks, separated by these two columns were in most cases regular and completely separated. Therefore the peak area for each compound was determined by multiplying the peak height and breadth at half height (hw'/a). In case of the dimethylderi-vatives, the peak areas were determined for all the compounds using a planimeter.
— 26 —
Table 1-3
Gas-Liquid Chromatographic Analysis of Aromatic and Naphtenic Hydrocarbons boiling between 180-275°C.
Apparatus: Recorder: Detector: Column: Temperature of Analysis: Carrier gas: Inlet pressure: Outlet pressure: Carrier gas flowrate:
Locally built.
Hitachi-Perkin and Elmer of response range from 0-2.5 mV, chart speed' can be varied between 1,4, and
10 cm/min.
Standard thermal conductivity cell, bridge current 200 mA.
8 meters long, 4 mm diameter. Filling is 5''/o GE SE 52 on chromosorb white of 60-80 mesh size.
182°C hydrogen 1.25 kg/cm^ atmospheric 2.4 1/hr. Results of Analysis:
Compound Relative Retention (Naphthalene) Correction Factor (Naphthalene) AB (a) Decalin ctralin B-Mcthyldecalin* J3-Methyltetralin* Naphthalene B-Methylnaphthalene a-Mcthylnaphthalcne Dimethyldecalin* Dimcthyltetralin* 1,6-DimethylnaphthaIene 1.067 2.082 — — ^ — 1.000 1..506 1.623 1.000, 1.726, 2,464 0.922 1.568, 1.370, 1.252 0.800 0.860 0.980 0.815 0.955 1.000 0.907 0.907 0.739 0.729 0.734
m'xturcs of isomers of these compounds.
" correction factor is an average for a number of alkylbenzenes at this temperature.
Literature:
1) Haensel, V., and C. V. Berger, Adv. Petrol. Chem. Ref., Interscience 1958, P-386.
2) White, A. C , et al, Proc. Am. Petrol. Inst., 36 (III), 265 (1956) 3) Berg, C , Petrol. Ref., 31, (12), 131 (1952).
4) Dijkstra, F., Thesis, University Of Delft, Holland, 1966. 5) ONeal, M. J., T. P. Wies, Anal. Chem., 23, 830 (1951).
6) Young, W . S., R. A. Brown, and F. W . Melpolder, Proc. Third Inter-national Congress On Petroleum, T h e Hague 1951, P-9I.
7) Sullivan, J. et al, Ind. Eng. Cem., 49, 110 (1957).
8) Brown, F. A., and F. W . Melpolder, Anal. Chem, 26, 1904 (1954). 9) Vlugter, J. C , H. A. van Westen, and H. Waterman. J. of The institution
of Petrol. Tech., 18, 73 (1932).
10) Van Nes, K., and H. A. van Westen, "Aspects Of The Constitution Of Mineral Oils", Elsevier N.Y., 1951.
11) Rossini, F. D., et al, "Hydrocarbons From Petroleum", A.P.I. Proj. 46, 1953.
12) Lumpkin, H. E., et al. Anal. Chem., 26, 1719 (1954). 13) Hastings, S. H., et al. Anal. Chem., 28, 1243 (1956).
— 28 — CHAPTER II
D E H Y D R O G E N A T I O N OF D E C A L I N ON P L A T I N U M REFORMING C A T A L Y S T CK303
I I - l . Introduction
Decalin is a cyclonaphthene formed of two cyclohexane rings fused together in the 1-2 position (4,4,0-dicyclodecane). Due to the nonplanar structure of the cyclohexane ring, decalin can exist in several strainless configurations of the: chair-chair, chair-boat or boat-boat combinations. Moreover, the replacement of the two hydrogen atoms from the 1-2 position of one ring by the other ring gives rise to a cis- or a trans-form. Theoretically, all possible com-binations can exist, but calculation of the interaction energy^ indicated that the most stable combinations are the cis- and the trans-forms of the chair-chair configurations, a and b, as in fig. 2-1.
CO
T r a n s - d e c a l i n C i s - d e c a l i n
F i g . 2 - 1
In a system that involves reactions of decalin, there is an interconversion between the two isomers. For example, during hydrogenation of tetralin, the one first formed is cis-decalin which isomerizes to the thermodynamically more stable trans-decalin. Physical properties of the two decalins, and the mixture of them which was used in the experimental work, are given in table 2-1.
Table 2-1
Physical Properties Of Decalin
Trans-decalin Cis-decalin Decalin Used
(80»/o trans-, 20''/o cis-)
<
"I'
B.P., °C at 1 atm. M.P., °C at 1 atm. Impurities, mole Vo 0.872 1.4701 186.7 —30.7 0.895 1.4828 194.6 —43.3 0.879 1.4735 0.25Vo as tetralinCis- and trans-decalin exist both in several petroleum distillates, specially in the heavy naphthas, kerosenes, and straight run gas-oils. The methyl derivatives of decalin form a large percentage of some kerosene fractions (see section VIII-4).
Decalin has two tertiary carbon atoms, and its alkyl derivatives contain increasing numbers of these tertiary carbon atoms. Such compounds can easily form tertiary carbonium ions on acidic catalysts, and further isomerize or crack to the monocyclic compounds. They yield ultimately paraffins and isoparaffins. The rate of cracking of these compounds at 650-750°C is very high^'*. This is the temperature range used in the industrial hydrodeal-kylation processes.
A systematic study was carried out aiming at converting the decalins at less severe temperature conditions to the aromatics which are more stable and refractive at high temperature range. The process variables studied were:
1) Effect of total pressure.
2) Changes due to variation of hydrogen partial pressure. 3) Influence of temperature on product distribution. 4) Influence of contact time on the catalytic equilibrium.
5) Reactions of decalin on dechlorinated platinum reforming catalyst.
6) Effect of the platinum content of the catalyst.
— 30 —
II-2. Orientation experiments
Several orientation experiments were carried out using decalin as feed and the undiluted catalyst CK303. The reaction indicated a high degree of endothermicity. At 400-450°C and under a hydrogen to hydrocarbon ratio of 10, a temperature difference of 36-40°C was measured between the centre of the catalyst bed and the inner wall of the reactor (a distance of 4 mm). It was necessary to dilute either the feed or the catalyst in order to obtain a controllable isothermal reaction. The number of the reaction products was rather high (at least 15), and because it was difficult to find a feed diluent that would not poison the catalyst or complicate the reaction overall paths, it was, therefore, decided to dilute the catalyst. The catalyst was diluted with steatit Raschig rings which were broken and sieved to a size comparable to that of the catalyst. A blanc test showed that the steatit particles were inert to the reaction for the range of 400-450°C. Dilution of one volume of the catalyst with an equal volume of steatit decreased the forementioned temperature difference to 20°C. Further dilutions were tested and the best result was obtained by diluting one volume of the catalyst with three volumes of steatit; the temperature difference dropped to 1-2°C. No more dilution was used in order to avoid effects of excessive dilution, such as short-circuiting. For tests which required use of more than 10 cc catalyst (the capacity of one reactor using a dilution ratio of 3 : 1 ) , the next 20 cc portion was diluted with an equal volume of steatit because of the decrease in the reaction endothermicity after the first reactor. No further dilution was necessary for additional quantities of catalyst. As another precaution, the temperature at the centre of the catalyst bed for the first 10 cc was kept at 1°C less than the experimental temperature assuming the mean value between the wall temperature and the centre to be the reaction temperature. For all experiments with decalin, a fixed feed rate was used, namely 83 ml/hr (0.522 mole/hr). A fixed hydrogen to hydrocarbon ratio was used at the inlet to the reactor so that the initial flow conditions were always the same for all experiments.
11-3. Effect of total pressure on reactions of decalin.
After activating the catalyst for 8 hours under a hydrogen pressure of 50 atm at 520-530°C, the temperature was lowered to
430°C and kept constant for sometime. The decalin was fed at the predetermined rate and mixed with hydrogen then passed through the preheaters before entering the catalyst section. The temperature of the catalyst was adjusted to 430°C and kept at that level during the whole experiment. The pressure was adjusted to 60 atm and the system was kept at constant conditions for V2 hr. before collecting a sample. The pressure was then lowered a step of 10 atmospheres by opening the back pressure valve. The conditions were readjusted over a period of '/2 hr. and the system was kept at constant conditions for another '/2 hr. before the second sample was collected. Seven samples were collected in this manner over a period of 9 hours. Results of the experiment are given in table 2-2 and fig. 2-2.
Table 2-2
Effect of Pressure On Reactions Of Decalin Temperature = 430°C , H^ : HC = 10 Exp. Pressure, Conversion, Product Distribution, Mole "/o NO D H 2 0 - 1 2 3 4 5 6 7 atm. 60 50 40 35 30 25 20 mole Vo 80.3 85.1 89.5 91.2 92.3 90.0 87.5 N 17.4 27.2 40.4 47.6 53.7 59.5 63.4 T 38.6 44.4 39.1 33.4 27.4 21.4 15.8 D 19.7 14.9 10.5 9.8 7.7 11.3 12.5 M I = 13.1 8.5 7.1 7.3 7.0 6.2 6.3 MI 4.7 2.5 1.2 0.9 1.0 0.9 1.1 1 + 1 = 2.5 1.2 0.8 0.5 0.5 0.3 0.3 AB 3.9 1.5 0.9 0.5 0.8 0.5 0.5
The abbreviations used for the tables and figures are:
N = Napthalene, T = Tetralin, D = Decalin (Cis -and Trans-) MI = Methylindan, MI= = Methyllindene, I-|-I= = Indan + Indene, AB = Alkylbenzenes.
The dehydrogenation of decalin under hydrogen pressure can be represented by equation 2 - 1 :
C O * '° «2
- = - CXD
* 13 H2
^ = -
Q Q . ,5
H,
2 - 1— 32 — D e c a l i n „ TGmperature = 430 C 100 90 80 70 01 o 50 /.O 30 • 20 10 O O 10 20 30 i.0 50 60 70 Pressure, a t m . F i g . 2 _ 2
Equation 2-1 predicts that an increase of the total pressure sup-presses the dehydrogenation in favour of first tetralin and second decalin. This is clearly demonstrated in fig. 2-2. It was noticed, however, that decalin conversion increased in the range of 20-35 atm. At low pressure the napthalene produced by the dehydrogen-ation reaction formed an insulating layer on the catalyst surface and the increase of pressure increased the diffusion of decalin through this layer and thus increased the conversion before the kinetic effect could develop. The increase of conversion in this pressure range might also be due to a direct hydrogen dispro-portionation reaction between decalin in the bulk gas and the
_ j LHSV = 0.39 hr. D e c a l i n Conversion N a p h t h a l e n e T e t r a l i n M I A
strongly adsorbed naphthalene. Tetralin was formed as a result of this reaction; being less adsorbed on the catalyst, tetralin escaped to the bulk gas as product. Fig. 2-2 demonstrated that the concentration of the byproducts did not increase with the increase of pressure from 20 to 40 atm, the pressure range through which the concentration of tetralin increased and that of decalin decreased. On the other hand, the concentration of the byproducts increased markedly above 40 atm when the decalin concentration increased due to the effect of pressure. From this experiment and a similar one starting from tetralin as feed (section III-2), it was concluded that the byproducts were mostly formed from decalin. Decalin is distinguished from tetralin and naphthalene by having two tertiary carbon atoms which can easily form carbonium ions and isomerize as by the following mechanism:
CO^CO^CO
CH3 CH-,
Oh^Q^^CX
The forming of a secondary carbonium ion is more difficult and yields 2-methylindan, as follows:
CO^00"^^CO*
The methylhydrindans can further dehydrogenate as in equations 2-2 and 2-3:
l""' CH3 CH3 CH3
CO—Oö-=-Oü ^ 0Ö
The intermediate carbonium ion can also crack and yield alkylbenzenes: r—^^CH3 ^ C H - C H ,
I
"CHj 2-ia
CH-CHjThe mixture of products contained a higher percentage of 1-methylindan than 2-1-methylindan, and contained also a higher percentage of 1- and 3-methylindene than 2-methylindene. This showed that the formation of carbonium ions was much easier from the tertiary carbon atoms at the bridge than from the secondary carbon atoms on the rings. Therefore, the tetralin molecule is more stable and refractive than the decalin molecule; also the naphthalene molecule can not isomerize or crack unless it is partially hydrogenated. At a pressure higher than 35 atm, the concentration of decalin was high enough to support the isomer-ization and cracking reactions.
11-4. Changes due to variation of hydrogen pratial pressure.
The experiment was performed by increasing in steps the hydrogen to hydrocarbon ratio at the inlet to the reactor and keeping all other conditions constant. The change was done over a period of V2 hr. and the system was kept for at least */2 hr. at constant conditions before a sample was withdrawn. A repro-ducibility experiment was carried out at the end and the result demonstrated that the catalyst had a constant activity throughout
the experiment. Results are given in table 2-3 and fig. 2-3. An increase of hydrogen partial pressure had a pronounced effect on the dehydrogenation of decalin to tetralin and a slight effect on the dehydrogenation of tetralin to naphthalene. The equilibrium constants for equation 2-1 are given by:
•^pl K o ^ p 2 — Temperature = Exp. NO DH22-1 2 3 4 5 6 7 Molar H . : H C 3.7 6.0 8.9 10.0 14.5 19.8 26.0 P p-' T X ^H2 % p p'' N X *^H2 P
T
Table 2-3 Effect Of Hyd atm.' J. 2 atm.''rogen Partial Pressure On Reactions Of Decalin 130°C, Pressure = Conversion, mole Vo N 89.5 47.6 87.3 46.8 85.4 46.1 84.7 45.8 82.4 45.5 80.7 45.4 78.7 44.4 30 atm., 1 LHSV Product Distribut T 27.6 25.4 25.7 25.3 24.3 23.1 22.4 D 10.5 12.7 14.6 15.3 17.6 19.3 21.3 = 0.27 ion , MI 9.2 9.1 9.3 8.9 9.0 8.2 7.8 hr. MI = 2.4 3.0 2.2 2.3 1.7 1.9 1.9 2-4 2-5 mole "/o 1 + 1 = AB 0.9 1.8 0.9 2.1 0.7 1.4 0.9 1.7 0.6 1.3 0.7 1.3 0.8 1.3 Equations 2-4 and 2-5 indicate that the concentration of tetralin is inversely proportional to the third power of the hydrogen partial pressure, and the concentration of naphthalene to only the second power of hydrogen partial pressure. The increase of hydrogen partial pressure had also influenced the true contact time of the hydrocarbons with the catalyst by dilution. If the bond strength between the hydrocarbons and the catalyst surface, and their rate of adsorption was in the order:
naphthalene > tetralin > decalin (see section II-6)
then the difference between the rates of adsorption of tetralin and decalin would add to affect the first step of dehydrogenation more than the second step. This effect was clearly demonstrated on the
— 36 —
change in the byproducts concentration. The formation of these compounds depends mainly, as was shown in section II-3, on the concentration of decalin and its contact time with the catalyst (see also II-6). Although the concentration of decalin almost doubled between H2 : H C = 4 and 26, the concentration of the byproducts dropped gradually with the increase of the hydrogen partial pressure. This could have been caused by a decrease of the contact time on diluting the reaction mixture with more hydrogen.
F i g , 2 - 3 90 80 70 60 50 0 0 o^O 2: 30 20 10
n
.,,_____ - ^ ^^-"^ ^^ 1 _ ________ • • - ^ ^- - ^
_xj
D e c a l i n Temperature = 430°C Pressure = 30 atm. L H S V - ° - ^ ^ ^ ' N « . Li, — 1, • u V.h\ A . MIA 10 12 U 16 18 20 22 24 26 Molar H , : HC11-5. Influence of temperature and quantity of catalyst
on reactions of decalin.
The effect of temperature was studied using 9 different quantities of catalyst. The experiments were carried out with temperature steps of 10°C between 410 and 450°C. A start-up period of about one hour preceded each run to allow for the establishment of thermal and chemical equilibrium. Each succeed-ing larger quantity of catalyst was composed totally of fresh catalyst. Results are shown in table 2-4a to 4i and fig. 2-4 a to 4i.
a) 1 LHSV
Table 2-4
Effect Of Temperature On Reactions Of Decalin H.j : HC = , Pressure = 30 atm. = 0.036 hr. Temperature, °C Conversion, mole Vo
Product Distribution , mole Vo
N D MI M I = I + I = + A B 410 420 430 440 450 28.3 30.4 36.4 39.3 45.6 13.1 11.0 71.7 2.4 1.7 0.1 17.1 10.4 69.6 1.4 1.4 0.1 21.9 10.0 63.6 2.5 1.8 0.2 25.5 9.0 60.7 2.9 1.5 0.3 30.2 8.7 54.4 3.5 2.4 0.5 b) 1 LHSV = 0.15 hr. 410 420 430 440 450 61.1 67.0 72.2 76.6 79.1 31.8 39.0 45.2 51.5 54.3 26.0 23.4 21.6 18.2 16.1 38.9 33.0 27.8 23.7 20.9 2.0 2.9 3.0 4.7 5.5 0.7 1.2 1.8 1.1 1.7 0.4 0.5 0.6 0.8 1.4 c) 1 LHSV 0.181 hr. 410 420 430 440 450 70.4 73.3 78.1 81.9 85.2 37.4 28.9 29.6 3.3 0.4 0.4 43.4 26.0 26.7 3.3 0.3 0.4 49.6 23.4 21.9 3.8 0.8 0.5 57.6 18.0 18.1 5.1 0.6 0.6 60.9 15.3 14.8 6.8 1.4 0.9 d) 1 LHSV = 0.225 hr. 410 420 430 440 450 79.5 85.0 89.6 91.7 93.5 42.6 33.7 20.5 2.7 0.3 0.3 51.5 29.4 15.0 3.5 0.2 0.4 61.2 23.8 10.4 3.9 0.2 0.5 66.0 19.2 8.3 5.2 0.5 0.8 69.4 15.6 6.5 6.3 1.4 0.9 e) 1 LHSV 0.361 hr. 410 420 430 440 450 84.6 89.6 92.7 94.6 96.1 43.7 51.2 58.8 65.3 34.9 30.8 25.5 19.9 15.4 11.0 7.3 5.4 5.1 5.5 6.5 7.5 0.2 0.5 0.8 1.3 0.8 0.9 1.1 0.8 68.5 16.0 3.9 8.7 1.9 1.1
a) — 38 — Fig 2.L D ecaL I n Pressure = 30 atm. H -1 „ . „ ' 100 90 • 0 70 o so ü 50 o ^ 1 0 30 20 10 O LHSV -0.03 6 hr b) HC < 10 , 1 "• LHSV "^ ^ M I A ^ _ _ ^ N ^ _ -LHSV = 0.181 hr 100 10 20 d) 30 10 SO 100 10 20 30 10 SO 100 10 20 30 10 SO Temperature . °C 1 n 1 100 90 iO n . • 60 e « 50 O ^ 1 0 30 20 10 O LHSV -0.225 hr e) LHSV =0.361 hr f) LHSV •0536 hr MIA 10 20 30 10 SO 100 10 30 30 10 SO 100 10 30 30 10 SO Temperature . °C 100 90 >0 70 10 30 20 10 n g' LJSV - o - e ^ ^ ^
