ManMont Engineering

Modern refineries are based on maximisation of conversion and require vacuum distillation operation to fit with the flow diagram of the overall process. This means that the vacuum tower operation must be steady without impacts coming from fired heater fluctuation.
The design has to guarantee that the profiles of pressure, temperature and vaporisation from the inlet to the heater to the tower will be achieved during operation independently of feed variation and fuel fluctuation. Fluid-dynamic simulation of the outlet tubes, manifold and transfer line are to be carried out to have the highest possible symmetry of the different flow streams resulting in equal individual flows properties.
A similar DCS control as described for the crude distillation unit is suitable for vacuum distillation as well and is necessary to keep on the long run the above conditions.
Considering the layout of the furnace, capital cost consideration are the same as for crude distillation with smaller heaters being designed as vertical cylindrical and larger one being done to the “high intensity concept”.
Vertical tube heaters will be distributed in the minimum number of streams suitable for the allowable pressure drop even if resulting in large outlet tube diameter. In our experience such tubes, in order to stay safely below the critical velocity, may have diameter as large as 350 mm.

The main characteristic of fired heaters for catalytic reforming unit is the very high gas feed flows that need to be heated and then reheated several times for the different adiabatic reaction steps.

This implies very large compressors being used to give the system the necessary flowrate. Operating as well as capital cost consideration are, therefore, in favour of low pressure drop multi-pass heater design.

The use of parallel inlet/outlet manifold and low pressure drop tubes hairpins having very large radius return bends located in the firebox is the most common solution. In order to further minimise the coil tube length, the double fired arrangement is preferred when high duties are involved even though other consideration may time to time suggest different solutions.

Manifolds may be top (fig.5) or bottom installed, with the last option being preferred in our experience since the load of the radiant coil may be taken by the concrete foundation instead of needing unnecessary additional structure in the radiant steelwork envelope.

Due to the very tight layout of the multistage reactors special experience is needed in the design of the manifold expansion which has to be fully taken inside the radiant box by the movements of the radiant coil. A complex system of spring supports and guides has to be studied to achieve this result.

Modern heaters are based on low pressure drop design therefore can not accept the high pressure drop of conventional tube bundle convection coil. As a consequence, the process service in the heaters is segregated in the radiant section and has very low thermal efficiency with flue gas leaving radiant section typically around 800°C. Since the catalytic reformers size is in general very large, it is imperative to recover the waste heat up to an overall efficiency of around 90% with or without air preheating. This means that the raising steam is an unavoidable solution but it is still possible to use part or all of the waste heat to preheat the charge to the different re-boiling columns connected to the unit (splitter, debutanizer etc.) bearing in mind the heat transfer optimization concept of pinch technology as well as the needs of flexibility in start up and operation.

Among the various kind of fired heaters in refinery service, the steam reforming furnaces are the ones that have been subject to more improvements in the past years. These improvements were achieved in the areas of major concern:

○ Tube metallurgy
○ Catalyst
○ Coil configuration and firing arrangement
○ Inlet and outlet headers system

Tube metallurgy improved to the latest generation HP 35 Modified Microalloys grade (25 Ni 35 Cr + Nb + micro quantities of special elements). The implementation of such new materials has led to much thinner tube thickness at same design conditions (typically 9 mm).

At same time the new generation of catalysts developed by the leading catalyst manufacturers has allowed to design at higher heat flux still avoiding carbon formation and or catalyst damages (typically 70.000 – 80.000 kCal/m2 hr).

The coil arrangement has evolved from the old side fired layout to the most modern top fired one where vertical lanes of tubes fed from the top are fired on both sides by burners installed in the arch of the furnace. This layout allows:

○ Modularity of size: more lanes of tubes may be added in one single radiant box up to Hydrogen production of 150.000+ Nm3/hr.
○ Optimized heat flux: the major part of the heat is given where required, i.e. at the top cold section of the tubes
○ Optimized use of tube material: due to the fairly flat tube skin temperature profile the minimum required thickness is almost the same in each section along the tube length.

Very important improvements have been achieved also in the field of mechanical design of the outlet headers where, in order to avoid any mechanical stress to the catalyst tube, long flexible tubing is used to connect the inlet and the outlet headers to each of the individual catalyst tubes. The load of the tubes, therefore, is easily taken by the floor of the furnaces and by the top spring supports with no movement other than vertical expansion and no external load other than the dead weight of the catalyst tube themselves.
The high temperatures involved in the process of steam reforming result in very high temperature of the flue gases leaving the radiant section. Several solutions are available to recover the heat from such gases:

○ Hydrocarbon feed preheating
○ Hydrocarbon/steam mixed flow preheating
○ Boiler feed water preheating
○ Steam generation
○ Steam superheating
○ Combustion air preheating

Depending on the economic value given to steam production in the overall refinery balance, such production may be maximized or limited to the steam flow required for the steam reforming reaction only. The unit will then be either a net steam exporter or a self sufficient steam producer. In any case the approach of the pinch technology has to be followed in order to have the best utilization of the heat source at the various temperatures. which in turn will allow to optimize the selection of metallurgy for the tubes of the different streams.

Fired heaters in Visbreaking service (as well as in any cracking application) are to be considered as reactor furnaces and their design needs all necessary knowledge of cracking kinetics in general and Visbreaking process specifically.

Based on the feed composition and the coil geometry, Manmont takes the advantage of simulation programs which enables designers to predict the conversion rate and product composition at different temperature / pressure / heat flux conditions. The model will also predict the tube skin and film temperature profiles allowing to avoid film overheating and keeping control on the formation of undesired by-products (like the asphaltenes) that may have negative impact on the stabilization of the fuel oil produced in the Visbreaking unit.

The program takes also into account both liquid phase and vapour phase reactions and contains a model for the prediction of coke build up at a given forecast of running conditions.

As far as heater coil geometry is concerned, the simulation program shows no particular need for any specific lay out, therefore the most economical arrangement may be preferred, like the vertical cylindrical one.

All considerations about the need for a rigorous kinetics modelling program, Visbreaking unit, are in general valid also for the Coking unit heaters as well. However, the higher criticality of Coking versus Visbreaking reaction result in some specific differences:

○ Faster coke build-up means shorter run time and requires more than one heater (typically two) to be installed in the unit. This allows to run at reduced capacity during the decoking operation.
○ Much lower residence time / higher mass velocity is required to have the smallest possible difference between bulk and film temperature, thus allowing for the required conversion rate at minimal coke build-up on the inner tube surface.

Simulation software and field feedback data show that the preferable configuration is the horizontal tube lay out with two heaters running in parallel.