The process to produce ethylene is one of the most complex processes in industry. Due to the huge amount of components which are produced by thermal cracking of the feedstock various separation steps are required to produce the desired quality of ethylene and of other possible byproducts.
Linde's ethylene process has been optimized for various feeds - gas crackers working with ethane, propane, butane - liquid crackers consuming feedstock like naphtha, LPG, AGO, HGO – dual feed crackers being flexible to handle 100% gaseous or 100% liquid feedstocks or any mixtures of the two.
Concerning plant size, Linde steam cracking technology has been successfully applied for small plants with 200 KTA ethylene production up to mega plants with more than 1500 KTA ethylene production. Extensive design studies have been performed for mega olefin plants with capacities as high as 2000 KTA ethylene which proved that the technology principles of the Linde steam cracking technology must not be compromised due to safety, mechanical or constructability reasons even at high plant capacities.
The block flow diagram shows the typical Linde process sequence which is further individually optimized depending on the existing design basis.
The main highlights of the Linde process are:
Proven Cracking Furnace Design
Front End Deethanizer
Front End Hydrogenation or Raw Gas Hydrogenation
Integrated Cold Train and Demethanizer
Heat pumped low pressure C2 Splitter with open Ethylene Refrigeration Cycle
The cracked gas is cooled down to a temperature of -15 °C to -30 °C after compression in the cracked gas compressor. Non-condensed cracked gas and collected condensates are fed to the deethanizer, where the total cracked gas stream is separated into a C2 minus and a C3 plus fraction.
The C2 minus stream is fed to the hydrogenation, where acetylene reacts with part of the hydrogen still present in the cracked gas.
The acetylene-free cracked gas is further cooled and partially condensed in the low temperature section until the non-condensed gas has achieved the required hydrogen product and/or tailgas purity where the C2 content is reduced to zero.
The collected condensates are fed to the demethanizer, where methane is separated from the C2 fraction.
The bottom product of the methane column, consisting of the pure ethylene/ethane stream, is fed to the C2 splitter after vaporization and recovery of chilling duty, where the ethylene product is separated from ethane. Since the C2 fraction fed to the C2 splitter is not contaminated with methane or hydrogen, the C2 splitter can be operated via a heat pump. The heat pump compressor is integrated into the ethylene refrigeration cycle, where it is combined with the third compressor stage.
A comparison of the Linde process with other processes like the demethanizer first, or the depropanizer first process results in a number of advantages for the Linde process. The main ones are as follows:
Investment costs are lower, since the number of equipment items and the total heat exchanger surface area is smaller; mainly due to the open ethylene refrigerant cycle
The power consumption of the three large compressors delivering the separation duty for the gas separation is approximately 5 % lower
All heavy hydrocarbons present in the cracked gas stream are separated upstream of the low temperature section. Thus the low temperature section and the demethanizer are not loaded with these components. This is of special importance from a safety point of view, if the cracked gas contains traces of NOx introduced by off gas which is fed between the cracked gas compressor stages to the ethylene process.This means that the reaction of NOx with butadiene or C5-dienes, which would result in the formation of explosive resins, can be practically excluded.
Because of the more favorable partial pressure conditions of acetylene and hydrogen, the hydrogenation that is used in this process in the total C2 minus stream leads to a catalyst cycle length of 5 years and more, compared with a cycle length of 6 to 12 months in a tail end hydrogenation. In addition, the formation of green oil is greatly reduced.
The hydrogenation does not require the addition of purified hydrogen, so that the start-up of the plant can be performed more quickly. There is no need for the removal of methane from the C2 fraction, which results in the saving of one process step.
The aforementioned advantages are one reason why Linde has gained worldwide a reputation as leading company in ethylene plant technology.