Cracking hydrocarbon feedstock with a heavy tail
The introduction of a vapour/liquid separation step allows for the efficient removal of a heavy tail from the furnace convection section
Johan van der Eijk
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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, naphthas, 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 that vaporises at a much higher temperature compared to the rest of the feedstock. Sometimes the fraction is not even vaporisable. Examples of such components are tar-like material, 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 causes fouling in the convection section, radiant section and the transfer line exchangers. This fouling results in a short on-stream time and an uneconomical operation.
However, the low-quality feedstock can be cracked without fouling issues if the heavy tail is separated out at a suitable location in the convection section. Consequently, the heavy tail bypasses the remaining non-fouling part of the cracking furnace. Special attention has to be given to effectively control the separation of the heavy tail, because the operating parameters often have a limited flexibility.
Steam cracking is a process for the thermal conversion of hydrocarbon feedstock into ethylene, propylene and other cracked gas products by thermal cracking in the presence of steam. Various types of feedstock are used for steam cracking, such as ethane, propane, butanes, straight-run refinery fractions (naphtha, kerosene, atmospheric gas oil and vacuum gas oil) and hydrotreated feedstock (HVGO and hydrowax). The common denominator among these different feedstock types is that the feedstock is a distilled product. This means that the feedstock is clean with no heavy ends (in the form of residue, tar, solid particles or very high-boiling fractions) present.
Nowadays, there is an increasing interest among ethylene producers to process natural gas condensates. These are basically unrefined hydrocarbons that are obtained as a byproduct from natural gas production. As these condensates have a wide boiling range, they are not directly suitable as motor fuel. This implies that their price level is lower than that of straight-run or hydrotreated feedstock and explains their popularity as petrochemical feedstock.
The condensates available on the market originate from a wide variety of sources, and so vary in their qualities and price levels. Some of them are directly suitable as cracker feedstock, but others contain heavy ends or impurities, which have adverse effects on the run length of the cracking furnace. The heavy tail and/or impurities may cause increased fouling in the following furnace parts:
• The convection section, due to the formation of deposits
• The radiant section, due to increased catalytic coking activity caused by the presence of metals
• Transfer line exchangers, due to increased tar formation.
On-line removal of the heavy tail inside the cracking furnace makes it feasible to crack low-quality and low-cost feedstock without expensive off-site pretreatment steps.
In the convection section of a cracking furnace, heat is recovered from the hot flue gases leaving the radiant section where the actual steam cracking takes place. The recovered heat is used for high-pressure steam generation, and for preheating the feedstock and process steam. In the case of liquid feedstock, vaporisation takes place in the convection section.
An innovation concerns the process for the removal of the heavy tail from the furnace convection section by the introduction of a vapour/liquid separation step at a temperature where the mixture of feed and process steam is not yet fully vaporised and the heavy tail is still present in the liquid phase. As the amount of heavy tail is very small, a high vaporisation degree (in the order of 97–99.5%) is required as an operating point for the separator.
Having the correct operating temperature is crucial. A too low temperature implies a loss of valuable feedstock, whereas a too high temperature causes a slip of heavy tail to the downstream furnace sections with subsequent run length problems. Part of the innovation was to develop a scheme for the control of the separation temperature that does not interfere with the normal furnace control parameters, such as capacity, severity and dilution steam ratio. To this purpose, an extra degree of freedom was created by having a controllable duty of the feed preheater (FPH) upstream of the separator.
A typical approach to controlling heat exchanger duty is to apply bypass control: by bypassing part of the stream to be heated, the heat pick-up can be reduced. However, a bypass control scheme for the FPH is not the most suitable option for a cracking furnace, as a very high vaporisation degree in the FPH may result in subsequent fouling problems. Also, without further precautions, a lower FPH duty results so that not all flue gas heat is recovered. Consequently, the stack temperature increases and overall furnace efficiency drops.
An innovative design concept was developed to overcome the problems linked to a conventional bypass control scheme, allowing FPH duty to be controlled while maintaining full flow through it without furnace efficiency loss. To this purpose, the FPH is placed between two economiser banks, heating the boiler feed water used for the generation of high-pressure steam (HP steam generation is the common way of heat recovery in a cracking furnace). Usually, a single economiser bank is applied to the furnace HP steam system. However, in the new concept, the boiler feed water is routed under split flow control over the two economisers. As a result, the flue gas temperature at the inlet of the FPH can be varied, and so too its duty.
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