Apr-2000

Cracking furnace design by computer application

Examples of how a technique known as Computational Fluid Dynamics (CFD) has been employed to improve and optimise the design of the cracking furnace. The authors describe simulation used for the radiant and heat recovery sections.

Dieter Kaufmann and Dieter Mihailowitsch
Linde AG

Article Summary

A modern cracking furnace within a steam cracker is a highly complex unit. It consists of various components which are optimised in terms of process, energy, mechanics and price. The interaction of these components is not always obvious to an observer (Figure 1). In the convection section the hydrocarbon feedstock is preheated against flue gas, mixed with dilution steam and superheated. The cracking itself – the transformation of the large hydrocarbon molecule chains into small olefin molecules, preferably ethylene and propylene – takes place within the cracking tubes. These tubes are hung within the radiant section and are fired from the outside.

In the following quench exchangers the product of the cracking reactions is rapidly cooled down to stop the chemical reactions. The resultant cracked gas is then routed to the fractionation section via the cracked gas line.

In addition to the hydrocarbon feedstock preparation, part of the flue gas heat is typically recovered to produce superheated high pressure steam. Therefore, boiler feed water is preheated in the convection section and sent to the steam drum. The boiling water is continuously extracted in a natural cycle to the above-mentioned quench exchangers where the water is evaporated. The produced saturated steam is collected again in the steam drum and superheated in the convection section.

After this final step, which is typically temperature controlled, the produced high pressure steam is routed to the high pressure steam line of the plant.

In order to meet the increasing demands and requirements of this product especially regarding environmental aspects or cost reduction, it is necessary to improve the furnace design continuously. An effective tool to obtain this goal is the Computational Fluid Dynamics (CFD) approach. With its potential to visualise complex processes within this unit, it facilitates a better understanding of its functioning and the opportunity to perform design optimisations.

This article uses design examples to demonstrate how the CFD-technique is used by Linde to improve and optimise the cracking furnace design. The following examples are discussed in detail:
— Radiant section simulation of a mix-fired furnace
— Substitution of mix-firing by pure floor firing
— Comparison between operational data and CFD-simulation
— Convection section design for large cracking furnaces.

Simulation of radiant section
The simulation of a cracking furnace radiant section by CFD is a challenging task. Within this radiant section, the radiant coils which are substantial for the cracking process, are installed. In these tubes the cracking process itself takes place where the long hydrocarbon chains of the feedstock are split up into short, mainly olefinic, fractions. This process is highly endothermic.

The required energy is transferred to the radiant tubes from the firing which surrounds these tubes. The heat transfer is dominated by radiation, but up to a certain extent convective heat transfer also takes place. The simulation of this process is mainly complicated by the fact that the heat sink (the radiant tubes) shows a gradient not only along the furnace length but also along the furnace height. The firebox temperature shows a similar behaviour.

Experience shows that the difficult accessible heat transfer from the combustion process to the radiant coils for this firebox geometry under consideration is predominantly dependant on the gradients in furnace height. These gradients are mainly influenced by the burner arrangement and the corresponding flame profile. Unlike this behaviour, the gradients in furnace length are of minor importance due to overall radiant box sizes which are typically long and narrow (Figure 2).

That is why only a thin slice along the total furnace height is selected as calculation domain instead of the whole firebox (shown in Figure 2). This slice is selected in such a manner that a representative combination of floor and side wall burners and the corresponding section of radiant coils will be simulated. By definition, the calculation domain considers only a small percentage of the furnace’s heat exchange and neglects any interaction with adjacent gas regions or with front and back walls.

So this approach actually simulates an “infinite” box length with a properly even heat distribution in furnace length which is justified from the overall box dimensions.

The advantage of this calculation domain selection lies in the high local grid resolution achievable with acceptable efforts. By this approach it can be guaranteed that especially the burners, the corresponding flames and the radiant tubes can be simulated in great detail. An example of the grid resolution in the area of the floor burners is shown in Figure 3.

The burner head is modelled in great detail with the actual dimensions of the fuel nozzle, the primary air swirler and the secondary air channels on the outer ring of the burner stone. The figure also shows part of the radiant coil bottom section which is geometrically simplified in the area of the return bend. The actual coil surface, which determines the heat transfer from the firing to the cracking process is kept constant.

Such a balance element is typically simulated with approximately one million grid elements, and for every element each relevant conservation equation has to be solved, ie the conservation equations for mass, impulse, energy and species conservation for the compounds of the simplified combustion mechanism as well as turbulence simulation, approaches radiation transport and soot formation.

As a result, the detailed simulation of the burners allows a very precise prediction of the fluid dynamics within the firebox, the flame sizes, the flame profiles and, finally, a very exact prediction of the total heat transfer within the radiant box.