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.
Crude oil and natural gas liquids are most often regarded as “energy” products substances that fuel cars, homes, factories and offices. However, they are basically mixtures of thousands of different hydrocarbons that can be rearranged to make petrochemical products one of which is ethylene.
Ethylene is used in the production of a variety of everyday articles that are used around the home, such as plastics, textiles, toiletries, detergents, paints, antifreeze, lightweight car components and medical supplies. It can also be used to help seedlings, vegetables and fruit trees ripen and grow faster
These are the step Ethylene production
Step 1: Preparation and storage
When the ethane arrives at FEP it still contains carbon dioxide, which must be removed before the feedstock can be used in the production of ethylene. This part of the process – which is known as feed treatment – uses an amine-based solution to wash the ethane and remove the carbon dioxide. Once treated, the ethane can be processed immediately or temporarily stored in the ethane tank. Propane can be used to supplement the ethane if supplies are low
Step 2: Steam cracking
The next part of the process is the actual cracking, where the ethylene is produced. The ethane feedstock is fed into seven furnaces, where it is mixed with steam and heated until it reaches temperatures above 800ºC. Under these conditions, the ethane breaks down into ethylene and small amounts of other gases, including hydrogen. When the maximum amount of ethylene as been produced, the cracking reaction is stopped by quickly cooling the furnace process stream. Steam generated as the furnace process stream is cooled is used to power machinery at the plant.
Step 3: cooling
The ethylene mix is then cooled to 25ºC in a series of heat exchangers and a piece of equipment called a quench tower, where cold water is cascaded down over the gas. During this process some of the by-products, such as tar, condense and are removed. Once it has been through the tower, the water is cooled again, cleaned and returned to the top of the tower to be re-used.
Step 4: Compression
Next, the gas is fed into a process compressor, driven by a powerful combination of gas and steam turbines. Once compressed, it is cooled again in what is known as the “cold box”, using a series of heat exchangers.
Step 5: Separation
When it reaches very low temperatures, the gas mixture becomes a liquid. The different components making up the gas are then separated by carefully boiling them off in a process known as distillation (or fractionation). This takes place in a series of three tall towers. In the first tower, known as the “de-methaniser”, hydrogen and methane are removed from the top. What is left is then fed into the second tower, the “de-ethaniser”. From the bottom of the de-ethaniser, a mixture called C5+ is drawn off. This is a valuable hydrocarbon, which is hipped to plant for processing aromatics plant, and ultimately used to make products like synthetic fibres, packaging and insulation materials. The ethylene, acetylene and any remaining ethane are removed from the top of the de-ethaniser and they are routed to the acetylene converters where hydrogen is added to convert acetylene to ethylene over a catalyst. The ethane and ethylene mix then enters the final tower – the “ethylene splitter”. Here the gases separate and any ethane that hasn’t been cracked is recycled back to the furnaces to be processed again.