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ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL No 1 2020 ISSN 0005-2531 (Print)
UDC 544.478.7
SOME ASPECTS OF OXYCRACKING CATALYSTS REGENERATION K.Y.Ajamov, E.A.Huseynova, L.A.Mursalova, *S.R.Safarova
Azerbaijan State Oil and Industry University (ASOIU), Research Institute for geotechnological problems of oil and gas and chemistry
Received 24.04.2019 Accepted 04.09.2019
On the basis of the results of derivatographic studies of catalyst samples for oxycracking of vacuum gas oil, it is establi shel that the surface of the spent catalyst is covered with 2 types of seal products: oxygen-containing organic oxidative seal products (OSP) and more stable oxygen-free high-molecular seal products (SP) having characteristic temperature anomalies. A staged mechanism of surface products decomposition has been proposed. The Kissenger method is used to estimate the activation energy for the destruction of surface seal products. It is noted that an increase in the duration stay in the conditions of regeneration leads to a decrease in the activation energy.
Keyword: oxycracking, differential-thermogravimetric analysis, zeolite-containing catalyst, regeneration.
doi.org/10.32737/0005-2531-2020-1-77-81
A wide range of physico-chemical and operational requirements are imposed to industrial catalysts for catalytic cracking, among which the structure, strength, state of elements and composition, structure of active centers, production cost, and of course, activity and selectivity should be noted [1-5]. Besides, industrial catalysts of catalytic cracking should guarantee a high yield of gasoline fraction with a high octane number, the production of secondary products in the form of diesel fraction and wet gas, as well as maintain these indices after regeneration. The latter requirement is also one of the most important demand, since during the work, operationregeneration cycle takes place from 10.000 to 100.000 times.
In the works [6-8] the results of the investigation of the process of oxidative catalytic cracking (oxycracking) of vacuum gas oil in the presence of a zeolite-containing catalyst OMNICAT are presented. The influence of the main technological parameters of oxycracking (temperature, oxidation degree of the oil fraction, flow rate) on the degree of vacuum gas oil conversion, selectivity, yield and composition of the reaction products were studied. In the course of the research, it was found that a higher catalytic activity of bizeolite catalysts in the process of oxycracking is associated with a change in the
structure of active centers - the formation and participation of surface oxygen centers embedded in the structure of oxidative seal products, serving as additional basic centers.
The purpose of this work was thermogra-vimetric studies of oxycracking catalysts subjected to regeneration with various duration of the process.
Experimental part
The process of oxycracking was carried out in a flow reactor with a fixed bed of an industrial zeolite-containing catalytic cracking catalyst OMNIKAT-340 at a temperature of 5 000C, atmospheric pressure and a feed volumetrik velocity of 2 h-1. The air supply to the reaction zone made up 0.5% per raw materials. The duration of the process fasted for 15 minutes. Vacuum gas oil was used as a raw material (i.b.p. 2700C - f.b.p. 5000C).
The regeneration process of the spent catalyst was carried out in a flow reactor at a temperature of 5500C and atmospheric pressure at an air supply rate of 2.63 l/h, varying the duration of the samples being in an oxidizing environment from 3 to 9 minutes.
The characteristics of the samples used in the thermogravimetric comparative analysis are shown below:
Sample designation 1 2
3
4
Characteristics of the sample A spent sample of the catalyst before regeneration A sample subjected to regeneration within 3 minutes A sample subjected to regeneration within 6 minutes A sample subjected to regeneration within 9 minutes
Phase transformations and mass changes during heat treatment of the samples were studied on a CD STA 429 F3A-0836-M installation of NETZSCH firm. Sample weights were placed in platinum crucibles. For the analysis, a linear-polythermal heating mode was chosen at a rate of 100C/min in a nitrogen atmosphere. Air speed 80 ml/min. The maximum temperature of heating reached 9000C. The sensitivity of the device was 0.025 p,g, the calorimetric sensitivity varied from 0.4 to 15 ^V/mW; error of - 0.2%.
To calculate the activation energy, the
Kissinger equation was used [9]:
f \
ln
V
i t^dta y
y max }
<r
= ln A- —
R
1
A
j^DTA V max J
where V is the heating rate (K/min); Tmax is the
temperature corresponding to the maximum deviation on the DTA curve; A is a constant; E is the effective activation energy of the process; R is the universal gas constant.
Results and discussion
First of all, it should be noted that at 20-160(181)0C on all 4 thermograms the beginning of mass loss was recorded, that was not accompanied by thermal effects and was caused by the conditions of the analysis, as a cjnsequence of which these data were not taken into account. Thermogravimetric profiles of the studied samples (Table) are characterized by three areas (Table 1): the first thermal effect is associated with the removal of chemically bound water -with its sufficiently rapid desorption, which is followed by a symmetrical endothermic effect
with a pronounced maximum (marked temperature area indicates this type of moisture; I-endo, Table 1); exothermic effects are presented in two different intervals (250-555.7 and 385.9-831.20C) what end indicates different qualitative composition of components end, are probably associated with the removal of volatile oxygen-containing organic oxidative seal products (OSP) (II exo-, Table 1) and more stable oxygen-free high-molecular seal products (SP), respectively (III exo-, Table 1); exo-effects peaks have a growing and asymmetrical form that indicates slow processes, while in exo II these features are more pronounced. It should be noted that II-exo-thermic peak of the sample 4 is expressed as an inflection, not a maximum, "blurring" as a whole in terms of temperature and does not have a clear localization [9, 10].
Sample 1, which was not subjected to regeneration after participating in the oxycracking process, is characterized by the widest temperature range of the endothermic peak: the initial temperature is lower and the final temperature is higher than in other samples. For the samples 24, after participating in the regeneration process, the following changes take place: the temperature interval of the endothermic peak gradually narrows; the end of the first and second exothermic effects, as well as the maxima of the latter, are shifted to a lower temperature area and the longer the samples stay in the oxidizing environment during the regeneration process, the stronger this parameter drifts in this direction. However, the temperature of the onset of III exothermal anomalies (peaks) in samples sub-
Table 1. Characteristic of the effects on the basis of results of the differential thermal analysis
Sample Thermal effect
I (endo-) II (exo-) III (exo-)
begin. max. end begin. max. end begin. max. end
1 162 223.1 278 278 476 555.7 385.9 663 831.2
2 170 223.3 278 278 482.5 531.8 391.8 631 726.6
3 170 231.4 276 276 471.3 522.7 439.7 620 821.6
4 181 221.2 250.6 250.6 - - - 602 788.4
jected to regeneration, on the contrary, becomes higher, as longer the sample was in an oxidizing environment. These temperature changes are most likely associated with changes in the composition and ratio of SP and OSP under the conditions of the regeneration process.
The total mass loss values decrease in the following sequence:
sample 1 > sample 2 > sample 3 > sample 4 (26.2 mass %) (23.1 mass %) (14.4mass %) (14 mass %)
In so doing, the mass loss values, associated with dehydration (I endo-) among all the samples have an identical dependence what is associated with the removal of water during the regeneration process, whereas with the decomposition of the OSP and SP (II and III exo-) are characterized by another sequence, and this is probably due to changes in the composition of the OSP and SP under the influence of an oxidizing environment and the rate of the reactions taking place.
Taking into account the furnace atmosphere (nitrogen) when conducting deriva-tographic studies, it can be assumed that the above three thermal effects are associated with the transformations of surface products along the following routes:
HCK] • [CxHy] • [CxHyOz](S)^ [H20] ^ [ЦСК] • [CxHy] • [CxHy0z]+[H20] (g) (a)
HCK] • [CxHyOz] ^ [ЦCК] • [Cx-nHy]+[COz]n (g) (b) MCK] • [CxHy] ^ [ЦCК] • [CxHy_m] + [H2]m (g) (c)
^CKHCxHyMO^ ^ MCK][CxHy02j ^ MCK][Cx_zHy] + [C02jz (g) (d)
The evolution of the chemical composition of the surface seal products of the samples 1-4 in accordance with the routes (a-d) depending on the duration of stay in the regeneration process conditions is confirmed by the decrease in mass loss during water desorption (route a, Figure) and the flow of solid-phase decomposition reactions with evolution of gaseous products (route b, c, Figure). Estimation of mass loss in accordance with routes a-d allowed to determine that destruction processes of surface compounds and change in their composition under the influence of oxidizing environment have the greatest impact: after the first 3 minutes, the contribution of - decomposition of OSP (route b see temperature interval II of the exo-
effect of the sample 2, it narrows; table 1) - is predominant, the amount of these products is 2.0% less than before regeneration, besides, a 1.4% increase in the SP content is observed, which in our opinion is a consequence of oxygen compounds to unsaturated surface compounds that are part of SP (d); this assumption is confirmed by the predominant loss of mass of SP (route c, sample 3) - their content is reduced by 5.2%, whereas OSP is only 3.9%. It is interesting that a further increase in the duration of stay in the conditions of regeneration does not contribute to a decrease or change in the nature of the surface products.
It is possible that the regeneration of the sample within 3 minutes leads to certain changes in the structure of SP, an increase in the proportion of less condensed and possibly the formation of newly formed oxygen-containing SP (see extremum on the route c, Figure). This is confirmed by the shift of the maximum and the end of the III-exo-thermal effect to the left, to the region of low temperatures, as well as the formation of new oxygen - SP bonds will favorably affect the reduction of SP connectivity with the surface, reducing the activation energy of the destruction process of this complex (see below). The influence of the oxidizing environment on the resistance of the surface OSP and SP was estimated by calculating the corresponding activation energies E, determined by the Kis-Sindzher method (Table 2), however, taking into account the fact that the exo-anomalies have a wide temperature range, and therefore the complex composition of surface compounds, the total process will be considered, and the estimated activation energy of destruction will be apparent. Note that for all the studied samples 2-4, which were subjected to regeneration, the values of activation energies are significantly lower than for the initial sample 1.
Based on the obtained values, we can conclude that at the first stage of a 3-minute stay in the oxidizing environment of the regeneration process, surface OSPs begin to undergo degradation more easily: the energy related to the thermal effect of their decomposition (sample 2) decreases by almost 5.3%, while for high-molecular SP, by 5%.
The effect of the duration of the sample regeneration process on the total mass loss and the ratio of wind thermal transformations (a-d):
| | - *sample 1 (before regeneration); - sample 2; - sample 3; - sample 4.
Table 2. Values of activation energies and peak amplitudes
Sample Activation energy, kcal/mole Amplitude (height of the peak), mkV/mg
II (exo-) III (exo-) I (endo-) II (exo-) III (exo-)
1 47.609 60.351 1.093 1.773 1.632
2 45.125 (-5.28%) 57.331 (-5.0%) 1.071 1.699 1.794
3 44.573 (-6.38%) 55.772 (-7.59%) 1.036 1.563 1.456
4 - 52.680 (-12.71%) 1.032 1.558 1.452
With an increase in duration of stay under the conditions of the regeneration process up to 6 minutes (sample 3), the picture changes to the opposite: SPs become less resistant to oxidative degradation than OSP, and the maximum decrease in the total activation energy of decomposition of surface compaction products was also noted for this sample. These data correlate well with the mass loss results shown in the figure. The amplitude of the peak (height) characterizing the intensity of the thermal process also varies symbatically to values of the activation energy (Table 2).
Thus, the study of the influence of the duration of the process of oxidative regenera-
tion of a catalyst for oxycracking vacuum gas oil has established that:
- the surface of the spent catalyst is covered with 2 types of seal products: oxygen-containing organic oxidative seal products (OSP) and more stable oxygen-free high-molecular seal products (SP) with characteristic temperature anomalies;
- routes for the transformation of the corresponding surface products are proposed;
- it has been revealed that the course of destruction of SP is preceded by an incubation period what may indicate that their conversion takes place after certain structural changes (the addition of oxygen to
unsaturated surface compounds that are part of SP);
- the activation energy of the destruction process of the corresponding surface products has been calculated.
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OKSiKREKiNQ KATALiZATORLARIN REGENERASiYASININ BOZÏ ASPEKTLORÏ
K.Y.0camov, E.A.Huseynova, L.A.Mursalova, S.RSafarova
Derivatoqrafik analiz malumatlarina asaslanaraq oksidlaçdirici regenerasiya prosesinda içtirak edan vakuum qazoylun oksikrekinq prosesinda içtirak edan katalizator numunalari tadqiq edilarak muayyan olunmuçdur ki, katalizatorun sathinda xarakterik temperatur anomaliyalari ila olan 2 nov sixlaçma mahsulu movcuddur: oksi-gen tarkibli sixlaçma mahsullar (OSM) va daha sabit oksigen olmayan yuksakmolekullu sixlaçma mahsullari (SM). Sathi mahsullann tadrici parçalanma mexanizmi irali surulub. Kissinger metodu ila sath sixlaçma mahsul-lannin destruksiya edilmasi uçun aktivasiya enerjisini qiymatlandirilmiçdir. Qeyd olunmuçdur ki, regenerasiya çaraitinda qalma muddatinin artmasi aktivasiya enerjisinda azalmaya sabab olur.
Açar sozlar: oksikrekinq, diferensial-termoqravimetrik analiz, seolittarkibli katalizator, regenerasiya.
НЕКОТОРЫЕ АСПЕКТЫ РЕГЕНЕРАЦИИ КАТАЛИЗАТОРОВ ОКСИКРЕКИНГА
К.Ю.Аджамов, Э.А.Гусейнова, Л.А.Мурсалова, С.Р.Сафарова
На основании результатов дериватографических исследований образцов катализатора оксикрекинга вакуумного газойля установлено, что поверхность отработанного катализатора покрыта 2-мя типами продуктов уплотнения: кислородсодержащими органическими продуктами окислительного уплотнения (ПОУ) и более устойчивыми бескислородными высокомолекулярными продуктами уплотнения (ПУ), имеющими характерные температурные аномалии. Предложен поэтапный механизм разложения поверхностных продуктов. Методом Киссен-джера проведена оценка энергии активации деструкции поверхностных продуктов уплотнения. Отмечено, что увеличение длительности пребывания в условиях регенерации приводит к уменьшению энергии активации.
Ключевые слова: оксикрекинг, дифференциально-термогравиметрический анализ, цеолитсодержащий катализатор, регенерация.
