Search

Performing an adequate fouling study for the heat exchangers in the convection section of a steam cracker requires reliable data on circumferential tube wall temperature profiles. A hybrid Computat...

This work evaluates the feasibility of biogas air-fuel combustion and natural gas oxy-fuel in steam cracking furnaces. Four cases, namely air-fuel combustion of pure natural gas, 20% CO2, 40% CO2 diluted natural gas, and oxy-fuel combustion of natural gas are investigated both experimentally and numerically. The John Zink Hamworthy Combustion test furnace, representing a section of a steam cracking furnace, is used for experimental studies. A three-dimensional steady-state CFD model is also developed to simulate the test furnace. The simulation results of the air-fuel combustion scenarios are in good agreement with the experimental data, with the maximum and average relative errors of furnace temperature of 3.86% and 1.78%, respectively. The reduction of flame length with increasing CO2 mole fraction in the fuel is observed in both experiments and simulations. It is shown that CO2 dilution has minor effect on the overall heat flux profile, which is beneficial for retrofitting existing furnaces. On the other hand, the oxy-fuel combustion simulation using default EDC model predicts a significant flame lift-off and incident radiative heat flux shift towards the higher elevations which was not observed in the experiments. This can be mainly attributed to the reduced reaction rate in a CO2 and H2O enriched combustion environment. Adjusting the EDC model parameters helps to achieve better agreement between simulation results and experimental data, while additional lab-scale experiments are essential for further validation of the numerical model. Moreover, it is of particular interest to study the optimal mole fraction of O2 in oxy-fuel combustion scenario.

To study fouling in steam cracker convection section tubes, accurate tube wall temperature profiles are needed. In this work, tube wall temperature profiles are calculated using a hybrid model, combining a one-dimensional (1D) process gas side model and a computational fluid dynamics (CFD) flue gas side model. The CFD flue gas side model assures the flue gas side accuracy, accounting for local temperatures, while the 1D process gas side model limits the computational cost. Flow separation in the flue gas side at the upper circumference of each tube suggests the need for a compartmentalized 1D approach. A considerable effect is observed. The hybrid CFD-1D model provides accurate tube wall temperature profiles in a reasonable simulation time, a first step towards simulation-based design of more efficient steam cracker convection sections.

The radiation and convection section of a steam cracker are thermally coupled. Optimization and design requires a coupled simulation of both sections. In this work a 1D model for the convection section, CONVEC-1D, is developed. Several models for the different heat transfer phenomena are implemented and evaluated. For flow boiling, an empirical and a mechanistic model are developed and compared for both single- and multicomponent hydrocarbon feeds. The latter is performing best over a wide range of operating conditions, taking into account the different two-phase flow regimes. The coupled iterative procedure is demonstrated for an n-pentane steam cracker convection section.

This work discusses fouling in the vapor–steam mixture overheater in the convection section of an industrial steam cracker due to the thermal degradation of heavy hydrocarbon droplets deposited on the tube wall. A spray of heavy hydrocarbon multicomponent droplets is injected in a tube of the vapor–steam mixture overheater and the path of the droplets through the tube is followed by an Eulerian–Lagrangian computational fluid dynamics simulation. To study tube fouling, the droplet impingement behavior on the wall, the evaporation of the deposited liquid, and a coking model describing thermal coke formation due to degradation of heavy hydrocarbons are required. To describe the droplet impingement behavior, a regime map for single component millimeter-sized droplets is taken from the literature. Two simulations are performed to study fouling problems in a vapor-mixture overheater tube. Simulation results are found to be grid sensitive. By analyzing and comparing simulation results it is concluded tha...

Účinek vysoceemisivních povlaků na radiační přenos tepla v parních krakovacích pecích není zdaleka znám. Chybí experimentální data popisující emisivní vlastnosti materiálů, které se používají v parních krakovacích pecích. Z tohoto důvodu se provádí měření spektrální normálové emisivity na zvýšených teplotách, přičemž se hodnotí emisivní vlastnosti žáruvzdorných šamotových cihel před a po depozici vysoceemisivního povlaku. Po nanesení povlaku s vysokou emisivitou na základní šamotový materiál se výrazně zvýší jeho emisivita. Provedené experimenty s parní krakovací jednotkou ukazují 5% snížení rychlosti spalování topného plynu po nanesení vysoceemisivního povlaku na žáruvzdorný materiál krakovací komory. Parametrické studie zabývající se účinkem emisivních vlastností reaktorové cívky a stěny pece na radiační přenos tepla ve spalovací komoře potvrzují, že pro přesné modelování chování povlaků s vysokou emisivitou je vyžadován jiný model než model šedého plynu. Přestože model šedého plynu postačuje k zachycení chování chladiče reaktorové cívky, je nezbytný jiný, nešedý model, který zobecňuje absorpci a opětovné vyzařování plynu ve spcifických spektrálních pásmech, aby mohl přesně zachytit výhody využití vysoceemisivního povlaku na stěně pece. The effect of high emissivity coatings on the radiative heat transfer in steam cracking furnaces is far from understood. To start, there is a lack of experimental data describing the emissive properties of the materials encountered in steam cracking furnaces. Therefore, spectral normal emissivity measurements are carried out, evaluating the emissive properties of refractory firebricks before and after applying a high emissivity coating at elevated temperatures. The emissive properties are enhanced significantly after applying a high emissivity coating. Pilot unit steam cracking experiments show a 5 % reduction in fuel gas firing rate after applying a high emissivity coating on the refractory of the cracking cells. A parametric study, showing the effect of reactor coil and furnace wall emissive properties on the radiative heat transfer inside a tube-in-box geometry, confirms that a non-gray gas model is required to accurately model the behavior of high emissivity coatings. Even though a gray gas model suffices to capture the heat sink behavior of a reactor coil, a non-gray gas model that is able to account for the absorption and re-emission in specific bands is necessary to accurately model the benefits of applying a high emissivity coating on the furnace wall.

Regime maps that predict the heavy hydrocarbon droplet impingement behavior on a heated wall (Weber number of the impinging droplet v/s wall temperature) are constructed based on CFD simulations using the Volume of Fluid model with the geo-reconstruct scheme. Based on the simulation results, maps are constructed for single-component droplets with a diameter of 50 and of 100 µm. The applied CFD model is validated by comparing these with regime maps available in literature, constructed based on experimental data for model liquids and liquid mixtures. The impingement regimes of Splash, Stick, Rebound and Breakup are well-predicted. Two distinct types of Splash (Splash with ligament formation and Splash with ring detachment) are reported for the first time. Using the validated CFD model, regime maps are constructed for multi-component heavy hydrocarbon droplets with a diameter of 50 and of 100 µm. Significant differences between the single-component and the multi-component droplet impact behavior are observed. Improved and new correlations for regime transitions, droplet stretching on the wall, droplet rebounding velocity and number of splashed droplets are derived based on energy balances. They are found to correlate well with CFD predictions.

IMPROOF will develop and demonstrate the steam cracking furnace of the 21st century by drastically improving the energy efficiency of the current state-of-the-art, in a cost effective way, while simultaneously reducing emissions of greenhouse gases and NOX per ton of ethylene produced by at least 25%. Therefore, the latest technological innovations in the field of energy efficiency and fouling minimization are implemented and combined, proving that these technologies work properly at TRL 5 and 6 levels. The first steps to reach the ultimate objective, i.e. to deploy the furnace at the demonstrator at commercial scale with the most effective technologies, will be discussed based on novel pilot scale data and modeling results.

As large floor-fired furnaces have many applications in refinery and (petro-) chemical units and about 80% of heat transfer in these furnaces is by radiation, the accurate description of radiative heat transfer is of the most importance for accurate design and optimization. However, the impact of using different radiation models in coupled furnace/reactor simulations has never been evaluated before. Therefore, coupled furnace/reactor simulations of an industrial naphtha cracking furnace with a 130 kt/a capacity have been conducted. Computational fluid dynamics simulations were performed for the furnace side, while the one-dimensional reactor model COILSIM1D was used for the reactor simulations. The Adiabatic, P-1, discrete ordinates model (DOM), and discrete transfer radiation model (DTRM) were evaluated for modeling the radiative heat transfer. The results with DOM and DTRM are very similar both on the furnace and the reactor sides. The flue gas temperature using DOM is higher than when using the P-1 radiation model, resulting in higher incident radiation. Comparing the simulated results of all radiation models to the industrial product yields and run lengths shows that DOM and DTRM outperform the others. As DOM has a broader application range than DTRM, and because the current implementation of DTRM in FLUENT/14.0 cannot be run in parallel yet, DOM is the recommended radiation model for run length simulations of steam cracking furnaces.

Coke formation in the convection section of a steam cracker occurs when heavy feeds are cracked. This work presents CFD simulations of coke formation in the mixture superheater tubes in the convection section of a steam cracker. The hydrocarbon feed used for the simulations is a gas condensate. Eleven representative chemical species are selected, based on their boiling points, to mimic the entire range of feed components. The liquid–vapor spray flow in the mixture superheater tube is simulated based on an Eulerian–Lagrangian approach using ANSYS FLUENT 13.0. Evaporation of multicomponent droplets suspended in the vapor phase or deposited on a tube wall is considered. The mixture superheater tubes make three horizontal passes (11.3 m long and 0.077 m diameter) through the convection section. The droplet–wall interaction model considers ‘Splash’, ‘Rebound induced breakup’, ‘Rebound’ and ‘Stick’. The amount of liquid deposited on the mixture superheater tube wall is obtained by simulating the spray flow. The amount of coke formed from the liquid deposited on a wall is based on the phase separation model of (Wiehe, 1993). Industrial & Engineering Chemistry Research 32, 2447–2454. Spatial variations of the coke layer formed in the mixture superheater tubes as a function of outer tube wall temperatures and initial droplet diameter are presented. For outer tube wall temperatures lower than the boiling point of the highest-boiling species in the feed a 1 mm thick coke layer is formed over a period of 1 month. For outer tube wall temperatures higher than the boiling point of the highest boiling component in the feed no coke is formed in the mixture superheater tubes. This work provides guidelines to minimize the extent of coke formation in the steam cracker convection section when a heavy feed is cracked. It also provides possible remedies to completely eliminate the coking problem when cracking heavy feeds.

The presence of liquid hydrocarbon droplets in the convection section of a steam cracker may cause serious fouling problems due to coke formation, especially in high temperature zones. In order to investigate these fouling problems, a model has been developed and implemented in a CFD code to accurately simulate the behavior of a hydrocarbon droplet impinging on a hot surface. The impact energy of the droplet and the hot surface temperature are found to determine the impact behavior. On the basis of the newly developed model, the positions where droplets preferentially collide with the convection section tube walls and liquid material is deposited are determined. Furthermore, a kinetic model describing coke formation out of liquid hydrocarbon droplets in the temperature range of 450−700 K has been developed to calculate the rate of coke formation in each zone of the convection section tube. The calculated coke layer thickness on the most vulnerable tube locations and the industrially available values corre...

A method is proposed to simulate reactive flow, fully taking into account the effect of the reactions on the flow. Operator splitting is used to separate the computation of convection and reaction. A fast solver for mildly stiff ordinary differential equations and parallelization of the reaction term evaluation have been implemented to reduce the CPU time. The proposed method is applied to the steady-state, two-phase gas–solid simulation of a Fluid Catalytic Cracking riser reactor. A relatively simple kinetic model with four lumped components is used to demonstrate the feasibility of the method. The results show that the method is able to handle reactive flow with significant feedback of the reactions on the flow.

The efficiency of the application of high-emissivity coatings on the furnace walls in steam cracking technology can only be evaluated on the basis of a description of radiative heat transfer distinguishing between the frequency bands. To this end, a non-grey gas radiation model based on the exponential wide band model (EWBM) has been developed and applied in the context of three-dimensional CFD simulations of an industrial naphtha cracking furnace with side-wall radiation burners. Applying a high-emissivity coating on the furnace wall decreases the net outgoing radiation from the furnace wall in the absorption bands and increases the net outgoing radiation from the furnace wall in the clear windows. Since radiation that is emitted by the furnace wall and travels through the flue gas in the clear windows can reach the reactor tubes without partially being absorbed by the flue gas, contrary to radiation that is emitted by the furnace wall and travels through the flue gas in the absorption bands, the thermal efficiency of the furnace increases. It was found that application of a high-emissivity coating on the furnace walls improves the thermal efficiency of the furnace (∼1%), the naphtha conversion (∼1%) and the ethylene yield (∼0.5%). These differences are small but, considering the industrial importance and scale of the steam cracking process, significant.

The endothermic thermal cracking process of hydrocarbons takes place in tubular reactor coils suspended in large gas-fired pyrolysis furnaces. Heat transfer to the reactor tubes is mostly due to radiation from the furnace refractory walls and the flue gas. A three-dimensional (3-D) simulation of the flow in an industrial scale steam cracking furnace is performed. The renormalization group (RNG) k − ɛ turbulence model is used. The combustion kinetics is modeled by a three-step reaction mechanism, while turbulence–chemistry interaction is taken into account through the Finite Rate/Eddy-Dissipation model. The Discrete Ordinates model (DOM), the P-1 and the Rosseland Radiation model are used for modeling of the radiative heat transfer. The results of using the different radiation models are compared mutually with adiabatic simulation results. The absorption coefficient of the gas mixture is calculated by means of a Weighted-Sum-of-Gray-Gas model (WSGGM). The effect is discussed of the use of different radiation models on the predicted wall, tube skin and flue gas temperature profiles and heat fluxes towards the reactor tubes, as well as on the predicted species concentration profiles and structure of the furnace flames under normal firing conditions.

A constant composition gray gas and a constant composition nongray gas radiation model are developed and applied in computational fluid dynamic simulations of an industrial scale steam cracking furnace. Both models are based on the exponential wide band model. The gray gas model simplification, commonly used for simulations of industrial applications, is found to have an effect on predicted variable fields like flue gas flow, temperature, and heat flux to the reactor tubes. When the nongray gas model is used, higher energy absorption by the flue gas in the furnace and lower energy transfer to the process gas in the reactor tubes is calculated because of the high absorption coefficients in the strongly absorbing bands of 2.7 and 4.3 mm. Thus, the calculated thermal efficiency increases from 37.5% when using the nongray gas model to 42.6% when using the gray gas model. A 5% difference in the thermal efficiency is large considering the scale and the importance of the process and should be taken into account by the furnace designer. It is also shown that although both models reproduce the basic characteristics of the flow pattern in the furnace, quantitative differences in the flue gas speed are predicted in some regions of the furnace domain. 2007 American Institute of Chemical Engineers AIChE J, 53: 1658–1669, 2007

Using a heterogeneous plug flow model for the reactor tubes and a general gas−solid reaction/diffusion model with intrinsic reaction kinetics for coke combustion and coke gasification, an in-depth study on the influence of seven operating parameters of the decoking procedure of an ethane cracking furnace is performed. On the basis of the results of this study, the air and steam flow to the reactor tubes during the decoking procedure are adapted to propose an optimized decoking procedure that allows reducing the required decoking time of an industrial ethane cracking furnace by a factor of 3. In the new decoking conditions for the ethane cracker, the air flow rate is raised by a factor of 3, while the steam dilution is lowered by a factor of 4.5.

A combined furnace and reactor calculation is performed for the simulation of the decoking of an ethane cracker with a steam/air mixture. Different gas–solid reaction/diffusion models with a corresponding texture model describing the solid phase changes during the decoking operation are combined with kinetics for the decoking by combustion and gasification, determined based on experimental data [Heynderickx, G.J., Schools, E.M., Marin, G.B., 2005. Coke combustion and gasification kinetics in ethane steam crackers. A.I.Ch.E. Journal 51 (5), 1414–1428]. By comparing the calculated coil outlet temperatures and the calculated partial pressures of carbon dioxide at the coil outlet with the measured values in an industrial unit, the need for a general gas–solid reaction/diffusion model and the corresponding decoking kinetics is confirmed. The decoking time for an industrial unit is simulated adequately.

A three-dimensional mathematical model has been developed for the simulation of flow, temperature and concentration fields in the radiation section of industrial scale steam cracking units. The model takes into account turbulence–chemistry interactions through the Eddy Dissipation Concept (EDC) model and makes use of Detailed Reaction Kinetics (DRK), which allows the detailed investigation of the flame structure. Furthermore, simulation results obtained with the EDC-DRK model are compared with simulation results obtained with a simplified model combining the Eddy Break Up (EBU)/finite rate formulation with Simplified Reaction Kinetics (SRK). When the EBU-SRK model is used, much faster fuel oxidation and products formation is predicted. The location of the peak temperature is shifted towards the burner, resulting in a smaller flame and the confinement of the combustion process into a smaller area. This is most likely because of the inherent deficiency of the simplified model to correctly predict the overall (effective) burning rate when the turbulent mixing rate and the reaction rate are comparable. It is shown that when neither the “fast-chemistry” nor the “slow-chemistry” approximation is satisfied, the overall burning rate is overpredicted. The smaller flame volumes obtained with the EBU-SRK model have important effects on the predicted temperature distribution in the furnace as well as on other significant design parameters like the refractory wall and tube skin temperatures. It is suggested that more sophisticated turbulence–chemistry interaction models like the EDC model and more Detailed Reaction Kinetics should be used for combustion modeling in steam cracking furnaces under normal firing conditions.

A systematic reduction of the detailed combustion chemistry based on the application of quasi steady state (QSS) approximation for some species leads to several reduced mechanisms (7- to 12-step) for a hydrocarbon-hydrogen fuel with a composition representative for industrial steam cracking furnaces. The basis for the construction of all reduced mechanisms is a skeletal mechanism obtained from the detailed GRI-Mech 3.0 and consisting of 223 elementary reaction steps. The performance of reduced chemistry was assessed under different flame regimes and over a wide range of operating conditions. An eight-step mechanism provides satisfactory temperature and major and minor species concentration predictions including NO. Therefore, it is put forward for combustion simulations in steam cracking furnaces.

A simulation model for the development of the optimal decoking procedure of a steam cracker is constructed. It is based on experimental data for the process gas side coke combustion with oxygen and the process gas side coke gasification with steam, using coke samples taken from an industrial steam cracker. The texture changes of the coke during the combustion and gasification are accounted for. Different kinetic models are combined with a coke porosity model and different gas–solid reaction/diffusion models. The model parameters are estimated from the experimental data and model discrimination is performed. For the coke combustion the kinetic model is based on the dissociative chemisorption of oxygen on the active coke sites, whereas a general gas–solid reaction/diffusion model is required. The growth of the pores and the overlap of the pores during the coke combustion are found to compensate each other. For the coke gasification, the kinetic mechanism is based on the dissociative chemisorption of steam, whereas a uniform gas–solid reaction/diffusion model is used. © 2005 American Institute of Chemical Engineers AIChE J, 2005

The efficiency of the application of high-emissivity coatings on reactor tubes and furnace walls in steam-cracking technology can be evaluated only on the basis of a description of radiative heat transfer in frequency bands. For the flue gases in the furnace, a radiation model is introduced in which the gas emits and absorbs radiation in discrete bands only; for the surfaces a nongray surface approach is applied. First, the influence of the model changes on the calculated radiative energy exchange is evaluated theoretically. Next, different furnace simulations are performed to do a numerical evaluation of the introduction of banded radiation models. The changes in the furnace thermal efficiency, attributed to the application of high-emissivity coatings on furnace walls and tube skins in steam-cracking furnaces, are calculated. © 2005 American Institute of Chemical Engineers AIChE J, 2005

The efficiency of the application of high-emissivity coatings on reactor tubes and furnace walls in steam cracking technology can only be evaluated based on a correct description of the radiative heat transfer in the furnace. For the flue gases, a banded gas radiation model is introduced; for the surfaces a non-grey surface approach is applied. For the same total heat input, a thermal efficiency rise of over 5% is calculated for a naphtha cracking furnace under normal operating conditions.

Radial temperature profiles during steam cracking result in radial nonuniformities in the product yields due to radial variations in the concentration of the radicals. The effect of using a 1-D or a 2-D reactor model on the calculated product yields is evaluated for the cracking of ethane. With a 2-D reactor model the simulated ethylene yield decreases. Ethylene formed at the high-temperature zone near the hot wall diffuses to the center where secondary reactions are favored, generating C3 and C4 olefins. This effect is confirmed by the calculation of a reactor of a Kellogg Millisecond Furnace. In this small-diameter reactor the 1-D behavior is more pronounced, resulting in higher ethylene yields at comparable conversions. The effect of the radial gradients on the coking rate calculated with a fundamental kinetic coking model based on elementary reaction steps is even more pronounced. Only when the coke model is coupled to a 2-D reactor model, a good agreement with the reference data is observed. In order to obtain accurate simulation results the more detailed 2-D reactor model is required, even if this increases the computational effort. © 2004 American Institute of Chemical Engineers AIChE J, 50: 173–183, 2004

Three-dimensional flue gas flow fields and temperature profiles are calculated in a 4/2/1 split coil naphtha-cracking furnace with long flame burners and an asymmetric flue gas outlet. The heat streams in the furnace connected to the asymmetric flue gas flow fields are found to be compensated for by a net radiative heat transfer in the opposite direction. As a result, the temperature distribution in the furnace remains nearly symmetrical. A furnace simulation under conditions of nonuniform heating confirms the compensating nature of radiative heat streams in the furnace. Furthermore, nonuniform heating was found to improve the thermal efficiency of the furnace and the cracking results. Optimization of heating conditions remains a point of interest.

A 3-D computational fluid-dynamics model containing equations for mass, heat and momentum transfer and using a k- ∈ closure model, was used to calculate the 3-D flue-gas flow pattern and the corresponding 3-D temperature field for a pyrolysis furnace. The computational fluid dynamics model is combined with a reactor model for the cracking tubes and a radiation model for the radiative heat transfer in the furnace box. Detailed reaction kinetics for the naphtha cracking reactions in the tubes and a five-step reaction mechanism for the combustion of methane in the flames were used. This complete model reveals asymmetric flow patterns in a naphtha-cracking furnace with 4/2/1 split-coil reactors and fired with long-flame burners.

The on-stream time of cracking furnaces is limited by the formation of coke on the internal skin of the reactor tubes. Rigorous simulations of an ethane cracking furnace, coupling reactor and furnace models with the kinetics of coke formation, prove that the run length of the furnace can be augmented by more than 40% by replacing the cracking tubes of circular cross section by cracking tubes of elliptical cross section. The increase of furnace heat-transfer efficiency and the more uniform circumferential heat fluxes and temperatures favor the run length of the furnace with tubes of elliptical cross section.

Circular tubes suspended in pyrolysis furnaces suffer from significant nonuniformities in heat flux, tube skin temperature, and coking rate profiles around the tube perimeter, due to the presence of front sides and shadow sides on the tubes. Simulation results for an ethane cracker with cracking tubes of elliptical cross section reveal that smoother circumferential heat flux, tube skin temperature, and coking rate profiles are obtained as the eccentricity of the elliptical tubes increases. The circumferential maximal values in coking rates are reduced by 30%. The more uniform tube skin temperature distributions and coke layers in tubes of elliptical cross section favor the run length of the furnace and the tube metal life.

A new steam cracking technology has been developed that uses honeycomb-type ceramic reactors with high surface to volume ratios and is heated by flue gas. "Rectangular" process gas temperature profiles are possible with such a reactor. An optimal combination of temperature profile and residence time leads to a very significant increase in olefins yield with respect to conventional cracking units. Bench-scale results are compared with predictions based on a reactor model, which accounts in great detail for the geometry of the structure and the associated heat transfer, combined with a rigorous kinetic model based on radical reaction mechanisms.

To assess the effect of applying a high emissivity coating to the reactor coils in a steam cracking furnace, a complete energy balance was made for two cases based on simulations of the radiant section, reactors, convection section, and transfer line exchanger. A base case with a typical emissivity spectrum for a generic high-alloy steel was compared to a case with an artificially increased emissivity corresponding to a high emissivity coating. At the same cracking severity, coating the radiant coils increases the radiant section efficiency by 0.70% absolute, reduces the required furnace firing rate by 1.73%, and reduces the flue gas bridge wall temperature by 14 K. Minor changes to the convection section layout are required to compensate for the shift in duty to the radiant section: the reactor feed is still fully preheated to the targeted crossover temperature, but the production of high pressure steam is reduced.