Ethylene and propylene are important chemicals for use in the production of other useful materials, such as polyethylene and polypropylene. Polyethylene and polypropylene are two of the most common plastics found in use today and have a wide variety of uses for, for example, a material for fabrication and as a material for packaging. Other uses for ethylene and propylene include the production of vinyl chloride, ethylene oxide, ethylbenzene and alcohol.
The great bulk of the ethylene consumed in the production of the plastics and petrochemicals such as polyethylene is produced by the thermal cracking of higher molecular weight hydrocarbons. Steam is usually mixed with the feed stream to the cracking reactor to reduce the hydrocarbon partial pressure and enhance olefin yield and to reduce the formation and deposition of carbonaceous material in the cracking reactors. The process is therefore often referred to a steam cracking or pyrolysis.
The composition of the feed to the steam cracking reactor affects the results. A fundamental basis of this is the propensity of some hydrocarbons to crack more easily than others. The normal ranking of tendency of the hydrocarbons to crack to ethylene is normally given as: normal paraffins; iso-paraffins; olefins; naphthenes; and, aromatics. Benzene and other aromatics are particularly resistant to steam cracking and undesirable as cracking feed stocks, with only the alkyl side chains being cracked to produce the desired product.
The feed stream to a steam cracking unit can be quite diverse and can be chosen from a variety of petroleum fractions. The feed stream to the subject process preferably has a boiling point range falling within the naphtha boiling point range or about 36° C. to 205° C. It is preferred that the feed stream does not contain appreciable amounts, e.g. more than 5 mole %, of C12 hydrocarbons. A representative feed stream to the subject process is a C5-C11 fraction produced by fractional distillation of a hydrotreated petroleum fraction. Hydrotreating is desired to reduce the sulfur and nitrogen content of the feed down to acceptable levels. A second representative feed is a similar fraction comprising C5 through C9 hydrocarbons.
The feed to a steam cracking unit is also normally a mixture of hydrocarbons varying both by type of hydrocarbon and carbon number. This variety results in it being very difficult to separate less desirable feed components, such as naphthenes and aromatics, from the feed stream by fractional distillation. The hydrocarbons that are not the normal paraffins can be removed by solvent extraction or adsorption. These hydrocarbons can be upgraded to improve the feedstock to the steam cracking unit.
One way to upgrade these hydrocarbons is to pass the non-normal paraffins to an isomerization zone. In the isomerization zone, the non-normal paraffins are converted, in the presence of a catalyst, into normal paraffins.
Many isomerization zones utilize a catalyst which contains, among other things, chloride. It is common for the catalyst to lose small amounts of the chloride into the effluent from the isomerization zones. Steam crackers, however, are very sensitive to the presence of chloride in the product stream. Accordingly, any product streams from isomerization zones which have a chlorided-catalyst must be further treated in order to minimize most, if not all, of the chloride from the product stream as it is passed into the steam cracker. One such method involves an adsorption zone which can remove most of the chloride from the effluent streams from the isomerization zone.
It would be desirable to have a process which produces a feed stream for a steam cracker which does not require a treatment to remove chloride prior to passing to the steam cracker.