The contact time effect of lower alkane pyrolysis process on target product yield in the presence of high-silicon zeolite retaining catalysts

ABSTRACT

The article studied the catalytic changes of lower alkanes. As a raw material has used a fraction of lower alkanes with the following composition (mass.%): СН₄ – 1,12; С₂Н₆ – 5,73; С₃Н₈ –75,16; i-С₄Н₁₀ –7,80; n-С₄Н₁₀ – 10,19. The experiments were carried out in a flow-type device at atmospheric pressure in the temperature range of 600-800 °C, water vapour: the ratio of raw materials was 0.4: 1. The duration of the experiments was 1 hour. The interval of change of contact time of the raw material with the catalyst is 0.1-0.5 s. The gaseous phase analysis was performed by chromatographic method.

At the same time, the yield of lower olefins increased in the temperature range of 600-750 °C. The subsequent increase of the contact time to 0.5 s leads to an increase in the concentration of raw materials, but at the same time, the yield of lower olefins decreases and the yield of by-products increases significantly due to the increase in the rate of lateral reactions.

АННОТАЦИЯ

В статье изучены каталитические превращения низших алканов. В качестве сырья использовалась фракция низших алканов со следующим составом (масс. %): СН₄ – 1,12; С₂Н₆ – 5,73; С₃Н₈ –75,16; i-С₄Н₁₀ –7,80; n-С₄Н₁₀ – 10,19. Эксперименты проводились в аппарате проточного типа при атмосферном давлении в диапазоне температур 600-800 °C, когда соотношение водяного пара: сырья было равно 0,4:1. Продолжительность опытов-1 час. Интервал изменения времени контакта сырья с катализатором составляет 0,1-0,5 °С. Газофазный анализ выполнен хроматографическим методом.

В то же время выход низших олефинов увеличивается в интервале температур 600-750 °C. Последующее увеличение времени контакта до 0,5 с приводит к увеличению концентрации сырья, но при этом снижается выход низших олефинов и значительно увеличивается выход побочных продуктов за счет увеличения скорости боковых реакций.

Keywords: methane, ethane, propane, butane, conversion, contact time, activation energy.

Ключевые слова: метан, этан, пропан, бутан, конверсия, время контакта, энергия активации.

Introduction. One of the main processes in the production of low molecular weight olefins is the pyrolysis process. At present, in many devices, the hydrocarbon feedstock is thermally ignited. In recent years, much attention has been paid to improving the pyrolysis process, as the demand for low molecular weight olefins in petrochemistry C₂-C₄ is expected to increase year by year [1-5].

The development of the pyrolysis process goes in different directions. One of the promising directions in the development of the pyrolysis process is the use of heterogeneous catalysts to increase the yield of target products and reduce the temperature and consumption coefficients of raw materials and energy carriers. As effective catalysts for the pyrolysis process, it is proposed to use zeolites of the pentacil family, which are sufficiently resistant to the coking process and the effect of water vapour [6-11].

The purpose of this work is to study the effect of contact time on the yield of the target products of the pyrolysis of low alkanes in the presence of high-silicon zeolite-containing catalysts.

Experimental part. As a raw material was used a fraction of lower alkanes with the following composition (mass.%): СН₄ – 1,12; С₂Н₆ – 5,73; С₃Н₈ –75,16; i-С₄Н₁₀ –7,80; n-С₄Н₁₀ – 10,19 [12-16]. The experiments were carried out in a flow-type device at atmospheric pressure in the temperature range of 600-800 °C, water vapour: when the ratio of raw materials was 0.4: 1. The duration of the experiments was 1 hour. The interval of change of contact time of the raw material with the catalyst is 0.1-0.5 s. The gaseous phase analysis was performed by the chromatographic method [17-20].

Analysis of gases at the entrance and exit of the reactor was carried out on a chromatograph LXM-80 with a thermal conductivity detector, the separation of H₂, O₂, N₂, CH₄, CO was carried out in a column filled with zeolite 5A, length 2 m, inner diameter 3 mm. The temperature regime of separation is isothermal, the temperature is 70 °C, the carrier gas is argon, the gas flow rate is -20 ml/min. Separation of C₂-hydrocarbons and CO₂ was carried out in a column filled with Porapak Q, 2 m long and 3 mm in inner diameter. The temperature regime of separation is isothermal, the temperature is - 80 °C, the carrier gas is helium, and its flow rate is 20 ml/min. Separation of C₃-C₄-hydrocarbons was carried out in a column with Al₂O₃ modified with Vaseline oil, 4 m long and 3 mm in diameter. The temperature regime of separation was isothermal, the temperature was 20 °C, the carrier gas was helium, the consumption was -20 ml/min, and the liquid products of the reaction were carried out on a chromatograph 5000.1 with a detector crystal on thermal conductivity.

Products of cracking reaction of propane-butane fraction “Column filled with silica gel Tsvet-500, KSK-2,5, length 3 m, thermostat temperature 500 °C, detector-catarometer, carrier gas (helium) consumption 30 ml/min) in chromatography method was analyzed. The quantitative analysis was carried out by the absolute calibration method using the calibration schedule of the standard for individual alkanes. For this purpose, a quantitative relationship was established between the amount of the component being determined (mol) and its peak surface. The number of moles is calculated using the Mendeleev-Clapeyron equation:

PV = nRT

The analysis of the composition of the reaction products was carried out as follows:

1) The surface (or height) is recalculated to the amount of substance:

k₁ - ranking multiplier (angle coefficient)

Table 1.

Levelling coefficients of components

2) Then mol,% were converted to mass:

m_i = M_i ∙ X_i

3) the mass concentration (yield) of the i-component was calculated according to the following formula:

4) Productivity and conversion of starting materials were calculated from the following formula:

5) The selectivity of the process by components was calculated according to the following formula:

In addition to the composition of the products in the gas mixture, the formation of coke is also an important characteristic. For quantitative evaluation of coke, the weight of the reaction site was measured before and after the reaction. The mass of coke was then determined. The volume of hydrocarbon feedstock passing through the reaction zone was calculated according to the following formula:

Where  - free volume, , cm³; t - reaction time (average 5 hours); - is the time of gas passage from the reactor;

- Catalyst capacity;

 - Volume of the reaction tube, cm³;

- is the inner diameter of the tube, cm; H is the length of the pipe, cm.

Then the amount of gas was found:

The molar mass of C in the primary raw material was found by the following formula:

Where,  is the molecular mass of the carbon in the sh-component in the mol component. Mass of carbon in all components of the starting material:

Mass fraction of coke:

Results and discussion. The results of the study of the effect of the contact time of the raw material with the catalyst on the yield of the target products of the pyrolysis process of the propane-butane fraction are shown in Figures 1-4.

Figure 1. Dependence of raw material conversion on contact time in pyrolysis of propane-butane fraction in the presence of HSZ catalyst. Contact time 0.1 (1), 0.25 s (2), 0.5 s (3)

Figure 1 shows the data characterizing the dependence of the PBF (propane-butane fraction) on the conversion time of the raw material as a result of high-temperature changes in the catalyst containing HSZ. The data depicted in Figure 1 show an increase in raw material conversion with increasing contact time from 0.1 s to 0.5 s. Increasing the process temperature also allows for an increase in raw material conversion. The maximum value of PBF conversion is 90.03 mass. % was recorded at 0.5 s contact time and 800 °C.

Figures 2-4 show the data characterizing the dependence of the yield of the sum of ethylene, propylene and C₂-C₄ unsaturated hydrocarbons on the contact time as a result of high-temperature changes of PBF in the catalyst containing HSZ.

Figure 2. Dependence of ethylene yield on contact time in pyrolysis of propane-butane fraction in the presence of HSZ catalyst. Contact time 0.1 (1), 0.25 s (2), 0.5 s (3).

The data shown in Figure 2 show that increasing the contact time from 0.1 to 0.5 s leads to an increase in ethylene yield in the temperature range of 600–750 °C. At the same time, the ethylene yield was higher at 0.25 s of contact time than at 0.5 s of contact time. The maximum yield of ethylene at a temperature of 800 °C: during contact 0.1 s - 35.78 mass.%; During contact 0.25 s - 35.50 mas.%; During 0.5 s contact - 32.3 mass.% formed.

From the data described in Figure 3, the maximum yield of propylene in the temperature range of 600–750 °C was recorded at a contact time of 0.25 s. The maximum yield of propylene at 800 °C was reached at 0.1 s of contact time. And amounted to 18.0 mass.%.

At the same time, the propylene formed 15.11% by mass at 0.25 s of contact time and 12.21 mass.% at 0.5 s contact time.

Figure 3. Dependence of propylene yield on contact time in pyrolysis of propane-butane fraction in the presence of HSZ catalyst. Contact time 0.1 (1), 0.25 s (2), 0.5 s (3).

The data in Figure 4 indicate that the yield of the sum of C₂ – C₄ unsaturated hydrocarbons was higher at 0.25 s contact time compared to the results at 0.1 and 0.5 s contact time in the 600-750 °C temperature range. When the temperature was raised to 800 °C, the maximum yield of the sum of C₂ – C₄ unsaturated hydrocarbons was reached at 0.1 s of contact time and amounted to 57.48 mass.%. At the same time, the maximum yield of the amount of unsaturated hydrocarbons C₂ – C₄ was 52.70% by mass at 0.25 s of contact time, and 46.13% by mass at 0.5 s of contact time.

Figure 4. Dependence of the yield of the amount of C₂ – C₄ unsaturated hydrocarbons in the pyrolysis of the propane-butane fraction in the presence of the catalyst HSZ on the contact time. Contact time 0.1 (1), 0.25 s (2), 0.5 s (3).

It should be noted that the HSZ pentacyl storage catalyst showed the greatest catalytic activity at a contact time of 0.25 s in the temperature range of 600–750 °C. An increase in the catalytic activity of HSZ was observed during 0.1 s of contact time when the temperature was raised to 800 °C. Increasing the contact time from 0.25 to 0.5 does not lead to an increase in the yield of the sum of ethylene, propylene and C₂ – C₄ unsaturated hydrocarbons. Increasing the contact time also allows increasing the yield of by-products of the reaction - hydrogen, methane, resin and coke resins.

1. Initiation

C₃H₈ ↔

C₃H₈ ↔  (surface)

2. Continuation of the chain

3. ..H₃ →

The pyrolysis reaction is one of the first-order reactions. Therefore, ln, c^-1   is appropriate.

Where α is the rate of change of the initial hydrocarbon; t-contact time (determined from the ratio of the volume of gas flow through the reactor to the volume of the reactor). The activation energy was calculated from the experimental data using the small squares method. For this, the logarithmic form of the Arrhenius equation was used:

А and  were found on the graph of ln k_eff and  coordinate dependencies. The calculation of the velocity constant was performed according to the 1-order reaction equation. This is confirmed by the relationship between the contact time τ and ln 1/1-α shown in Figure 5:

Figure 5. Dependence of the rate of change of hydrocarbons on the contact time

The activation energy of the decomposition reaction of propane and the accumulation of ethylene and methane were calculated on the basis of the relationship between ln k and  according to the Arrhenius equation:

Figure 6. The relationship between the activation energy ln k and 1 / T.

Figure 7. The dependence of the rate of change of butane on the contact time .

Conclusion. Thus, the results obtained show that an increase in the contact time from 0.1 to 0.25 s leads to an increase in raw material conversion as a result of high-temperature changes in PBF in the pentacyl-containing HSZ catalyst. At the same time, the yield of lower olefins increased in the temperature range of 600–750 °C. A subsequent increase in contact time to 0.5 s leads to an increase in raw material concentration, but at the same time, the yield of lower olefins decreases and the yield of by-products increases significantly due to the increase in the rates of side reactions.

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