(51e) Process of Cracking Hydrocarbon Feedstock with a Heavy Tail

Process of cracking hydrocarbon feedstock with a heavy tail

Cracking furnaces are the heart of the ethylene plant. These furnaces convert hydrocarbon feedstock into ethylene, propylene and other cracked gas products by thermal cracking in the presence of steam. Typical examples of hydrocarbon feeds are ethane, propane, butanes, naphtha's, kerosenes and heavy atmospheric and vacuum gas oils.

It could also be economical to crack low quality feedstock with a heavy tail. This is a feedstock with a fraction which is vaporised at a much higher temperature compared to rest of the feedstock. Sometimes the fraction is even not vaporisable at all. Examples of such components are tar, solid particles and the residuum of high boiling fractions.

If a feedstock with a heavy tail is cracked in a cracking furnace, the heavy tail usually causes fouling in the convection section, radiant section and the transfer line exchangers. This fouling results in short on stream time and results in an uneconomical operation.

However, the low quality feedstock could be cracked without fouling issues if the heavy tail is separated in the convection section. In this case the heavy tail needs to be separated from the gas stream in a separation device. Consequently, the heavy tail bypasses the rest of the cracking furnace. In practice it is difficult to effectively control the separation of the heavy tail, because the operating parameters often have a limited flexibility.

This paper presents a cracking furnace design which has to possibility to crack a hydrocarbon feedstock with a heavy tail. The design has the flexibility to effectively control the temperature of the separation device without penalties on the furnace efficiency. The temperature of the separation device is controlled by regulating the heat exchange capacity of the feed preheater upstream the separation device.

Johan van der Eijk

13.10.2011