Pyrolysis gasoline; production mix, at plant
Disclaimer: The LCI and LCIA results of this dataset can vary from the published Eco-profile on PlasticsEurope website due to the conversion of the original LCI to an ILCD compliant LCI, and some updates of characterization factors used in the Eco-profile since the time of the publication. This was done in agreement with PlasticsEurope. null
The world-wide demand for lower olefins, i.e. ethylene, propylene and butadienes is higher than for any other chemical as they are the primary feedstock for most plastics, polymers and man-made fibres. But lower olefins are only found in very low concentrations in crude oil due to their high reactivity. It is thus necessary to split up longer, saturated hydrocarbons into shorter, unsaturated compounds using the large-scale cracking process.
The chemical reaction for the cracking process is a dehydrogenation and can be affected either catalytically or thermally. In the European Union the steam cracking process which thermally induces the reaction accounts for the lion's share of the ethylene, propylene and butadiene production.
Due to the rising demand for ethylene and propylene as precursors for the polymer production not only naphtha, but also gas fractions are used as feedstock for steam cracking. In the European Union they play a minor role, whereas in the USA even most crackers use gas feedstock.
In the steam cracking process suitable hydrocarbons are heated to temperatures of up to 800 °C or even higher in the presence of steam to crack the modules into the desired products - lower olefins.
Only a limited number of international technology contractors licenses the equipment used for crackers. The generic design of the machines is quite similar. Little modifications help to optimize the plant performance according to local conditions. Besides differences in the furnace, pressure and temperature of the fractionation columns and refrigeration systems may also vary or turbo expanders may be in use.
Regardless of feedstock or contractor a cracker may be separated into three sections namely pyrolysis, primary fractionation/compression and product fractionation. In the pyrolysis section the hydrocarbon feedstock is preheated and then vaporised with superheated steam before passing into long and narrow tubes arranged in a cracking furnace. The hydrocarbon feedstock is cracked into smaller molecules by controlling residence time, temperature profile and partial pressure. This process is highly endothermic and therefore requires a high energy input. Therefore the tubes of the furnace are heated to 750 - 875 °C by oil or gas fire burners. To reduce the partial pressure of the hydrocarbon mixture and to minimise coke formation high-pressure steam is injected which gives the process the name steam cracking. To quickly quench the product gases to 550 - 650°C and to recover heat for internal use, transfer line exchangers (TLEs) may be used.
The primary fractionation and compression section consists of the primary fractionator (naphtha and gas oil feed only), quench tower, gas compressor and gas cleanup facilities. The primary fractionator is used to condense out and fractionate fuel oil streams produced from naphtha and gas oil fed crackers. The gases are de-superheated in the quench tower by a circulating oil or water stream. The circulating oil or water stream is used as a medium level heat source for the rest of the plant. Product gases from the quench tower .are condensed by four or five stages of gas compression. The gas is cooled after each stage and passed through a liquid knock-out drum. Finally, acid gases and carbon dioxide are removed from the cracked gas.
The chilling train usually consists of four or five successive stages of chilling, incorporating ethylene and propylene refrigeration as well as an elaborate self-refrigeration system. This produces hydrogen which is used for downstream hydrogenation, hydrotreating of the heavier products or sold as a product. The exact process flow sequence varies according to the feedstock and the design arrangement, but various fractionation towers are used to separate the desired products. This may include a sequence of de-methaniser, followed by a de-ethaniser . Bottoms from the de-ethaniser are directed to the de-propaniser and the de-butaniser. The lighter the feedstock, the fewer fractions need to be separated and the separation system may be constructer simpler. After separation the ethylene still contains undesirable acetylene and ethane. Acetylene is removed either by selective catalytic hydrogenation or by extractive distillation. After separation from ethylene ethane is recycled back to the cracker. Similarly the C3 fraction contains methyl acetylene and propadiene after separation. Selective hydrogenation is used to convert this into propylene and propane prior to separation in a C3 splitter.
In the European Union crackers are basically fed with Naphtha and condensates, also called natural gas liquids (NGL). Both sorts of feedstock are very similar mixtures of hydrocarbons. Liquid feedstocks have a high share as they are transported easily. Other important feedstock for crackers in the EU are gas oil, butane, propane, refinery gas and ethane. Ethane mainly comes from North Sea gas fields whereas other feedstock gases come from refineries.