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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.

The feasibility of performing the SO2/NOx adsorption process in a gas-solid vortex reactor (GSVR) is examined and compared with the more traditional riser technology. The multiphase reacting flow is modeled using the Eulerian-Eulerian two-fluid model. Models of nonreacting flows were validated using data from a bench-scale experimental setup. The GSVR has the potential to significantly improved heat/mass transfer between phases, as compared to more conventional fluidization technologies. Process intensification opportunities are investigated. The model predicts continuous removal efficiencies greater than 99% for SO2 and approximately 80% for NOx. The gas-solid slip velocity and convective mass transfer coefficient for the riser were 0.2-0.5 and 0.06-0.12 m/s, respectively, whereas the values for the GSVR were 6-7 and 1.0-1.1 m/s, respectively. This order of magnitude increase in the external mass transfer coefficient highlights the potential intensification opportunities provided by the GSVR.

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...

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.

When coarse-grid calculations in two-phase flow are performed, mesoscale fluctuations, such as clusters, cannot be explicitly captured. Their impact on macroscale fluctuations, however, has to be taken into account by the introduction of an appropriate closure model. A new closure model to describe gas−solid drag is introduced. The effect of particle clustering on the interphase momentum-transfer coefficient is taken into account by introducing the concept of effective drag. Clustering results in a decreasing value of the interphase momentum-transfer coefficient because the contribution of particles in the cluster is considered to be negligible in dilute phase flow. For solids fractions greater than 1%, clustering phenomena become increasingly important, resulting in an important decrease of the interphase momentum-transfer coefficient. Calculation results for a riser reactor, both neglecting and including the effects of clustering on the gas−solid drag, are presented and validated with experimental data....

The three-dimensional simulation of an industrial-scale fluid catalytic cracking riser reactor is performed using a novel density-based solution algorithm. The particle-level fluctuations are modeled in the framework of the kinetic theory of granular flow. The reactor model includes separate continuity equations for the components in the bulk gas and inside the solid phase. The results show a core-annular flow pattern in the major part of the riser with a significant densification of the core region halfway up the riser. The higher solid fraction and a lower solid velocity result in a higher conversion in the annulus region than in the core. However, the concentration profiles of the gaseous components are relatively flat due to the excellent radial mixing in the riser. The radially averaged axial solid velocity shows a considerable slip from the gas phase, amounting to a typical slip factor (ratio of gas/solid axial velocities) of about 2. The high slip factor results from the radial segregation of parti...

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 transient kinetic model has been developed for the simultaneous adsorption of SO2−NOx on Na/γ-Al2O3 sorbent, using step response experimental data from a fixed-bed microreactor. The reactor temperature ranges from 367 to 407 K, and the molar SO2/NO ratio from 2 to 8 with and without NO2. SO2 and NO2 readily adsorb on the sorbent surface, producing SO2* and NO2* species. However, NO and O2 only adsorb simultaneously and in the presence of sufficient SO2* on the surface. The proposed mechanism consists of chemisorption steps for SO2 and NO2, whereas NO and O2 are adsorbed simultaneously via an Eley−Rideal step involving a surface species derived from SO2*. The latter is followed by several consecutive steps involving more SO2* species as well as O2, leading to the formation of a complex with a stoichiometry of SO2/O2/NO of 10/5.5/1. The enhanced SO2 sorption capacity in the presence of NO2 is described adequately by considering that the adsorbed NO2* opens a new reaction path for SO2 adsorption. Both the ...

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.

In this paper, a new thermal cracking technology using shell-and-tube type pyrolysis reactors, in which the accent is shifted from radiative to convective heat transfer, is introduced. A smooth temperature distribution can be achieved with the proposed reactor geometry. This lowers the coking rate and increases the run length of the reactor and the lifetime of the tubes. A reactor model accounting in great detail for the reactor geometry and the different heat-transfer mechanisms is combined with a rigorous kinetic model based on radical reaction mechanisms to predict the temperature distribution in the reactor, the heat transfer from flue gas to process gas, and the naphtha conversion, together with the associated product yields for the various tubes.

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.