How to upgrade your steam cracker.

Some olefin plants have been running continuously since the 1960s, which is why their performance, efficiency and often the safety of start-up and operation leaves much to be desired. They probably do not meet the current emission standards of your countries either, which exposes your company to additional costs and reducing your margin.

Many of you are asking yourself what can be done and how, so here I will try to describe possible scenarios.

Step 1 - Technology

The furnace is an integral part of the installation and any interference in its technological parameters should not take place without an analysis of the entire process, so you should start by contacting the technology provider. Here, however, you need to know what you expect, whether you just want to improve the energy efficiency of the plant or increase its performance as well. With the current technology, by replacing a very old furnace you can get up to 30% more efficiency (on the same foundations), so e.g. instead of 8 steam crackers you can have only 6 new ones. The technology company will check the whole unit starting from the feed supply (including boiler water and steam circuit) and covering downstream equipment (transfer lines, quench column, compressor etc.).

Step 2 – Solutions for Furnace

With the knowledge gained from technology analysis you have to make a decision about the furnace, and here you have two options

1.??????????? Building a new furnace

The best solution regardless of the installation method. Particularly attractive if the location allows the old furnace to be pulled out and replaced by a new one completely pre-assembled on temporary foundations.

Even if it is not possible to transport the old furnace with trolleys or SPMTs and replace it with a new one in the same way, advanced modularization must be taken into account from the beginning of the steel structure and piping design. It is a mistake to repeat the solutions of the existing furnace, designed for the traditional way of assembly, where the modular construction was in fact convection section only. Already when designing radiant section, it is worth avoiding gunning the refractory lining, bearing in mind that the insulating concrete static cast in the workshop is homogenous and has better insulation parameters. It can also be pre-dried in the required time from the moment of application without fear of losing the warranty.

The steel structure around the convection should form modules together with pipelines, preferably in transport dimensions (workshop preassembly), and if it is not possible, maximum preassembly on the construction site. I mean here the division of columns and beams and strength calculations taking into account the method of lifting together with the weight of the installed equipment.

2.???????????Revamp of the old one

The modernization of the furnace from the process side usually involves replacement of the radiant coil, TLE, expansion of the convection part and replacement of external piping, burners and automation. For this to be possible, it is necessary to make sure that the foundations and steel structure of the radiant coil will carry new loads, and if not, to make the appropriate reinforcement.

Here you should start with a reliable assessment of the technical condition of the radiant box and the refractory lining. If advanced corrosion of the casing is found (at least partial replacement is necessary) combined with bad condition of the refractory lining, we can assume that the renovation of the radiation itself combined with its reinforcement will take up to 3 months. Installation of a new box, where integrated reinforcements can be designed, takes up to 2 weeks! Quite recently we as a the company had the opportunity to carry out a deep modernization of the first two identical steam crackers (out of four in total) with both methods. At the remaining two ones that we are upgrading no one has considered the first method anymore.

The steel structure, if it is completely redesigned and replaced, should be designed from the beginning with modularization for easier and shorter installation time.

In every revamp, the time of shutting down the device from production and the time of performing works in the explosion hazard zone is crucial.

Burner is a very important element affecting the furnace operation. Modernization, in addition to

a possible increase in efficiency is aimed at reducing emissions. However, let us remember that in Ultra LowNOx burners the construction is subordinated to the emission, the flame has a different shape and is less stable than in conventional constructions. It is worth and even necessary carrying out CFD analysis of the combustion chamber and test one burner on the target fuel. This will help to avoid problems with combustion instability and flame impingement.

Control and safety instrumentation - well-designed control systems allow to optimize work and save fuel and there is a place for the designers to show their skills (subject to separate discussion).

As far as safety instrumentation is concerned, country-specific standards should be applied, taking into account the specificity of the device's operation. I am thinking especially of the time delays on interlocks, which effectively filter out short-term signal deviations eliminating unnecessary shutdowns and wide use of 2oo3 logic.

Remember that most of the furnace explosions took place during start up! The safety interlock system must effectively prevent unlimited number of ignition attempts without an adequate and still valid purge or limit the fuel supply to prevent explosive concentrations in the radiant chamber.

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The above considerations were the result of observations of the stem cracker revamps (already 3 over a period of 4 years conducted by our company - NAFTO). I invite you to discuss and share your experience.