Pyrolysis of LPG In a Fast-Mixing Reactor

Pyrolysis of LPG in a fast-mixing reactor

М.G. Ktalkherman, Khristianovich Institute of Theoretical and Applied Mechanics, Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russia

I. G. Namyatov, Institute of Chemical Kinetics and Combustion, Siberian Branch of Russian Academy of Sciences,

Novosibirsk 630090, Russia

V. А. Emel’kin, Khristianovich Institute of Theoretical and Applied Mechanics, Siberian Branch of Russian Academy of Science, Novosibirsk 630090, Russia

We analyzed the method of hydrocarbon pyrolysis in a flow of high-enthalpy hear carrier with the feedstock jets injected normally to the reactor axis. The experiments were carried out in the laboratory-scale device, the pyrolyzed feedstock was LPG and combustion products of a stoichiometric mixture H₂ – O₂ – air as a heat carrier. A mixer installed at the reactor inlet provided the fast (» 0.05 ms) and qualitative mixing of the heat carrier and feedstock. Under such conditions, as the mixture temperature rose at the reactor inlet up to 1,400 K, the ethylene yield reached 47%, which is much higher than in the conventional method of pyrolysis. The experimental data are generally in good agreement with the results of numerical simulation of the process within the inlet temperature range 1,350 – 1,500 K.

 In [1] the authors propose the model of the high-temperature pyrolysis of hydrocarbons based on the possibility to realize the ultra-fast feedstock / heat carrier mixing. In this procedure, the combustion chamber is a heat-carrier generator. In this chamber, the products of oxygen-fuel mixture combustion mix with the dilution steam in order to reach the preset temperature. The feedstock (or mixed feedstock and part of the dilution steam) mixes quickly with the heat-carrier flow in the mixer attached to the combustion chamber; then the mixture enters at the reactor inlet. The products are cooled quickly at the reactor outlet. The present work differs from the known researches in the method of working mixture formation, namely it results from the mixing of feedstock jets (+steam) injected normally to the reactor axis, and heat carrier. Since the reaction time decreases dramatically due to the higher process temperature, the challenge of feedstock / heat carrier mixing is critical. To solve this problem, the experiments were carried out in a model gas-dynamic device [2] in order to study the quality of mixing of colliding radial jets with a transversal flow in the channel. The results of these experiments [2] enabled to determine the optimal mixer geometry which corresponded to the operational conditions of the experimental facility. This facility was utilized to study experimentally the LPG pyrolysis in the high temperatures range unachievable in the conventional method of furnace pyrolysis. The model experiments were performed in a heat-carrier flow which presented the products of combustion of the stoichiometric mixture H₂ – O₂ – air. The temperature at the mixer inlet was controlled by the oxygen / air ratio. The reactor contained two sections of 40 and 80 mm in diameter respectively. In our experiments we used two geometrically similar mixer designs with the channel diameter of 40 and 15 mm. Both mixers provided high-quality mixing within the length equal to one diameter of the mixing chamber. The area-averaged deviation of the injected substance concentration from the average value did not exceed 2.5 – 3.5 % at the mixer outlet. The time of feedstock and heat carrier mixing was ~ 1 ms and 0.05 ms at D = 40 and 15 mm respectively. The rate of feedstock heating in the mixer reached ~10⁷ K/s. The temperature of the feedstock / heat carrier mixture at the reactor inlet varied during the experiments within the range 1,350 – 1,500 K, the pressure in the reactor was close to atmospheric. Water-cooled samplers and thermocouples were located along the reactor axis. Temperature distribution in the reactor was used to analyze numerically the process on the base of a detailed kinetic model of hydrocarbons pyrolysis. In general the calculation results are in quite good agreement with the experiment in such cases when the speed of feedstock and heat carrier mixing is high enough to provide the minimal influence of the reaction in the mixing zone on the process. In these experiments, the agreement between the calculation and experiment was reached at the mixing time of 0.05 ms even at the reactor inlet temperature 1,500 K. The highest ethylene yield (47%) was reached in our experiments at the reactor inlet temperature of 1,400 K. This value is approximately 1.2 times higher than in the conventional pyrolysis method. The ethylene yield reaches 45% at the residence time of 50 ms.

The developed mathematical model was used to analyze the effectiveness of the proposed pyrolysis method with a real heat carrier – the products of combustion of the steam-diluted stoichiometric mixture CH₄ – O₂. Propane was taken as a pyrolyzed feedstock. The pressure in the reactor was 0.1 MPa, the reactor inlet temperature varied within the range of 1,200 – 1,500 K. No reactions were assumed in the area of feedstock / heat carrier mixing. This condition is quite well met in our experiments right up to the temperature at the reactor inlet being of 1,500 K. The calculations showed the relatively weak influence of the initial temperature on olefins yield. The maximum concentrations of C₂H₄ and C₃H₆ do not coincide in time, and the maximum total single-pass yield of these olefins (0.577) corresponds to the residence time of 13 ms. On the other hand, the total concentration of C₂H₄ and C₃H₆ in the products with the account the recycle of propane and ethane, resting in the pyro-gas, slightly rises with the temperature growth: from 0.62 to 0.65 within the temperature range of 1,200 – 1,500 K. In general, the theoretical and experimental results demonstrate good potentiality of the considered pyrolysis method from the viewpoint of higher olefins yield. The realizability of this method in the large-scale industry depends in particular on how well the pyrolysis unit is incorporated into the existing systems of heat utilization and pyrolysis products separation.

1. Ktalkherman MG, Namyatov IG. Pyrolysis of hydrocarbons in a heat-carrier flow with fast mixing of the components. Combustion, Explosion & Shock Waves. 2008; 44: 529-534.
2. Ktalkherman MG, Emel’kim VA, Pozdnyakov BA. Influence of the geometrical and gas-dynamic parameters of a mixer on the mixing with of radial jets colliding. Journal of Engineering Physics and Thermophysics. 2010; 83: 539-548.