Raw material pre-treatment process

The purpose of the reforming feedstock pretreatment is to cut fractions that meet the requirements of reforming and to remove impurities and water that are harmful to the reforming catalyst, meeting the requirements for fractions, group composition and impurity content of the reforming feedstock. The process consists of pre-fractionation, top hydrogenation and dehydration of the feedstock.

1. Pre-fractionation

Different products for different purposes in the reforming unit require different feed flows. In addition, the reforming feed should not contain light hydrocarbons below C5, as light hydrocarbons not only do not produce aromatics, but also increase the energy consumption of the unit, reduce the purity of hydrogen production, etc. Therefore, in order to improve the economics of the reforming unit, a feedstock pre-fractionation is required to select the right fraction.

The pre-fractionation unit for feedstock consists of a fractionation tower and its associated systems. Depending on the position of the fractionation tower in the pre-treatment system, it can be divided into a pre-fractionation process and a post-fractionation process. The so-called pre-fractionation process is, as the name suggests, fractionation followed by hydrogenation, while the post-fractionation process is hydrogenation followed by fractionation

2. Pre-hydrogenation

Pre-hydrogenation is the removal of impurities from the raw material under molybdenum-cobalt-nickel catalyst and hydrogen pressure.

① Compounds containing sulphur, nitrogen and oxygen are hydrogenated under pre-hydrogenation conditions to produce hydrogen sulphide, ammonia and water, which are separated out by a pre-hydrogenated stripper or dehydration tower.

② Olefins are hydrogenated to produce saturated hydrocarbons. The degree of olefin saturation is expressed in terms of bromine or iodine valence, and generally requires a bromine or iodine valence of <1g/100g of oil for the reformed feedstock.

③Metal compounds such as arsenic, lead and copper are decomposed under pre-hydrogenation conditions to monomeric metals which are then adsorbed onto the catalyst surface.

3. Pre-arsenicisation

A very small amount of arsenic compounds in the feed can cause permanent catalyst deactivation, particularly in reforming catalysts where the platinum is more sensitive to arsenic and can combine with As to form PtAs, permanently deactivating the catalyst. Typically catalytic reformers require less than 1ng/g of arsenic in the feed.

There are three methods of pre-arsenic removal: adsorption, oxidation and hydrogenation. Currently, hydrogenation is the main method used in industry.

The hydrogenation method uses a hydrogenated pre-arsenic reactor in series with a pre-hydrogenated refining reactor, where the reaction temperature, pressure and hydrogen-to-oil ratio of the two reactors are basically the same. The catalyst used for pre-arsenic removal is a nickel tetramolybdate hydrorefining catalyst.