Modeling the evaporation of a hydrocarbon feedstock in the convection section of a steam cracker

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Abstract

A model has been developed allowing to simulate the flow boiling process of a hydrocarbon feedstock. For the calculation of the different horizontal two-phase flow regimes that evolve during flow boiling, use is made of the Volume Of Fluid (VOF) model that uses a Piecewise Linear Interface Calculation (PLIC) method to reconstruct the interface between both phases in each computational cell. In-house developed codes calculate the mass and energy transfer phenomena occurring during this flow boiling process. As such, an existing CFD code is completed with a newly developed complete evaporation model.

The developed model is used to simulate the flow boiling process of a hydrocarbon feedstock in the tubes of a convection section heat exchanger of a steam cracker. The simulation results show a succession of horizontal two-phase flow regimes in agreement with the literature.

Introduction

Steam cracking of hydrocarbons is one of the most important processes in petrochemical industry converting a hydrocarbon feedstock into more valuable products such as ethylene and propylene.

A steam-cracking furnace, as can be seen in Fig. 1, consists of two sections: the radiation section and the convection section. The radiation section is heated with burners supplying the necessary heat for the endothermal cracking process. In this section, over 90% of the heat transfer from the flue gas and the furnace walls to the reactor coils is radiative. Hence, the radiation section is an empty volume with at most two rows of vertically suspended tubular reactor coils in the middle of this section. The flue gas flows along these coils. A complete detailed simulation of the radiation section involves a coupled simulation of both the furnace and the reactor coils as the conditions in the reactor coils and the flue gas temperature influence one another (Heynderickx, Oprins, Dick, & Marin, 2001; Stefanidis, Heynderickx, & Marin, 2006).

In the convection section, the remaining energy of the flue gas leaving the radiation section is used to preheat and vaporize the hydrocarbon feedstock and to overheat steam. The hydrocarbon–steam mixture, leaving the convection section, is then introduced in the reactor coils of the radiation section to be cracked. In the convection section, the major part of the heat transfer is due to convection, as the flue gas temperature is much lower. As a result, the convection section is filled with a large number of horizontally suspended heat exchanger tubes over which the flue gas flows. As for the radiation section, a correct simulation of the convection section will require a coupled simulation of the heat transfer from the flue gas side to the heat exchanger tubes and of the phenomena of heating and evaporation inside these tubes.

Comparable to the coking problems in the tubes of the radiation section (Cai, Krzywicki, & Oballa, 2002; Wauters & Marin, 2002), fouling problems can occur in the tubes of the convection section. To analyze and solve these problems, a coupled simulation of the flue gas side and the process gas side of the convection section is needed. Such a simulation can, e.g. reveal the presence of hot spots on the convection section tubes.

The first step in unraveling the convection section coking problem is a correct calculation of the phenomena in the lowest temperature heat exchanger of the convection section where the hydrocarbon feedstock is heated and partially vaporized. Indeed, a correct simulation of this heat exchanger is felt throughout the complete convection section simulation as the different heat exchangers in this section are coupled. Thus, during the flow boiling process, a two-phase vapor–liquid mixture flows through the tubes (Dhir, 1998). The liquid-to-vapor ratio changes continuously, but, even more important, the flow regime of the two-phase mixture changes as well. Although commercial CFD software packages dispose over several models to simulate the two-phase vapor–liquid flow (Ghorai & Nigam, 2006), it needs to be checked whether the predicted two-phase flow regime corresponds with the actual flow regime for the given liquid-to-vapor ratio. For the commercial CFD software package Fluent Inc. (2006) that has been used in this work, correct results are obtained (De Schepper, Heynderickx, & Marin, 2008). On the other hand, the software package does not dispose over a model that is able to simulate the evaporation process itself. In order to overcome this, an in-house developed code will be used to complete the available CFD code. The results of this work, in particular the phenomena of heat and mass transfer encountered during the flow boiling process in the tubes, are presented in this paper.

Section snippets

Flow boiling

Flow boiling inside tubes is a considerably more complex process than pool boiling. The continuous evaporation of liquid results in a vapor volume fraction increase, causing in turn an acceleration of both phases. Flow boiling studies have shown that several flow regimes are observed, depending on the amount of vapor phase that has already been formed (Thome, 2004). Next to inertia, viscous and pressure forces influencing single-phase flow, the two-phase vapor–liquid flow regime is also

CFD modeling of flow boiling

The computation of boiling flows remains a challenge in computational fluid dynamics. These flows are characterized by the discontinuity of many of the flow variables across the phase interface. These discontinuities pose several computational difficulties requiring special treatment. In addition, the location of the phase interface is not known a priori and must be determined as a part of the solution procedure.

The simulation of boiling phenomena using CFD is possible by applying

Tube geometry and operating conditions

The flow boiling of a hydrocarbon feedstock in the convection section heat exchanger of a steam cracker is simulated. One tube in this heat exchanger is made out of four horizontal passes, each with a length of 11.3 m and a diameter of 0.0525 m. These four passes are connected by two vertical bends and one horizontal bend. The horizontal bend is located between the second and third pass. The complete 3D-computational domain is divided into 1,993,648 hexahedral cells. Near the tube wall, three

Conclusions

The main purpose of the presented work is the development of a model that allows to perform 3D-simulations of the evaporation process of a hydrocarbon feedstock in a heat exchanger of the convection section of a steam cracker. The calculation of the evaporation process is one of the steps required to complete the research on convection section modeling.

The heating, evaporation and boiling phenomena occurring at conditions typical for the convection section of a steam cracker could be modeled

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