WO2000058420A1

WO2000058420A1 - Hydrotreating

Info

Publication number: WO2000058420A1
Application number: PCT/GB2000/001059
Authority: WO
Inventors: Mark Sean Doran
Original assignee: Imperial Chemical Industries Plc
Priority date: 1999-03-30
Filing date: 2000-03-21
Publication date: 2000-10-05
Also published as: AU3310500A; GB9907191D0

Abstract

A process for the removal of impurities including organic sulphur and nitrogen compounds from a hydrocarbon feedstock containing at least 2 ppm by weight of organic nitrogen compounds comprising adding hydrogen to the feedstock and passing the resultant mixture over a bed of a hydrotreating catalyst at an elevated temperature sufficient to decrease the organic nitrogen content to below 0.2 ppm by weight, cooling the resultant mixture, separating the cooled mixture into a liquid stream comprising the hydrotreated feedstock and a gaseous stream containing residual hydrogen, and hydrogen sulphide and ammonia formed by hydrogenation of the organic sulphur and nitrogen compounds, and passing the hydrotreated feedstock through a bed of an absorbent for mercaptans.

Description

Hydrotreating

This invention relates to hydrotreating and in particular to hydrotreating hydrocarbon feedstocks preparatory to catalytic reforming

Hydrocarbon feedstocks, such as naphtha, contain a variety of impurities including organic sulphur and nitrogen compounds In order to decrease these impurities to acceptable levels, it is commonplace to subject the feedstock to a hydrotreating step wherein the feedstock, in admixture with hydrogen, is passed at an elevated temperature over a bed of a suitable catalyst, such as a supported, sulphided, cobalt or nickel/molybdenum composition The sulphur compounds are converted to hydrogen sulphide and the nitrogen compounds to ammonia. The mixture is then cooled and passed to a stripping column where the treated feedstock is separated as a liquid phase and the light components, including hydrogen, hydrogen sulphide and ammonia, is separated as a gaseous stream. The resultant treated feedstock stream typically has organic sulphur and organic nitrogen contents each in the range 0 2 to 0.5 ppm by weight. The treated hydrocarbon feedstock is then often subjected to catalytic reforming to increase the aromatics content of the hydrocarbon stream. The catalysts employed for catalytic reforming are often noble metals, such as platinum and/or rhenium on a suitable support. Generally the catalytic reforming catalyst is used in the form of a chloride and indeed it is often desirable to add chlorine compounds to maintain the reforming catalyst in the active state. The catalytic reforming reaction produces hydrogen and any residual nitrogen compounds in the feed will tend to be hydrogenated by the reforming catalyst to give ammonia, and likewise hydrogen chloride will be formed by reaction of the hydrogen with the catalyst. The hydrogen chloride and ammonia will tend to combine to form ammonium chloride and it has been found that this can lead to fouling and blockages in the catalytic reforming unit stabiliser section and hydrogen make-gas systems.

The amount of residual nitrogen in the hydrotreated feedstock can be decreased by increasing the temperature of the hydrotreating stage. However, at the temperatures necessary to obtain a hydrotreated feedstock having a low organic nitrogen content, e.g below 0 2 ppm, and especially below 0.1 ppm, by weight, re-combination of the hydrogen sulphide with the hydrocarbons is liable to occur giving rise to mercaptans and hence the organic sulphur content of the hydrotreated feedstock may not be as low as is desired During a subsequent catalytic reforming operation, such mercaptans are liable to decompose poisoning the reforming catalysts

In the present invention these problems are overcome by operating the hydrotreating stage under conditions such that the organic nitrogen content of the hydrotreated feedstock is low, and passing the hydrotreated feedstock, after separation of the light components as aforesaid, through a bed of an absorbent capable of removing mercaptans Accordingly the present invention provides a process for the removal of impurities including organic sulphur and nitrogen compounds from a hydrocarbon feedstock containing at least 2 ppm by weight of organic nitrogen compounds comprising adding hydrogen to the feedstock and passing the resultant mixture over a bed of a hydrotreating catalyst at an elevated temperature sufficient to decrease the organic nitrogen content to below 0.2 ppm by weight, cooling the resultant mixture, separating the cooled mixture into a liquid stream comprising the hydrotreated feedstock and a gaseous stream containing residual hydrogen, and hydrogen sulphide and ammonia formed by hydrogenation of the organic sulphur and nitrogen compounds, and passing the hydrotreated feedstock through a bed of an absorbent for mercaptans.

The hydrocarbon feedstock will depend on the desired products but may be, for example, naphtha (e.g. containing hydrocarbons having 5 or more carbon atoms and a final atmospheric pressure boiling point of up to 204°C), middle distillate or atmospheric gas oil (e.g. having an atmospheric pressure boiling point range of 177-343°C), vacuum gas oil (e.g. atmospheric pressure boiling point range 343-566°C), or residuum (atmospheric pressure boiling point above 566°C). The nature and amount of the impurities will depend on the feedstock, but the feedstock will generally contain significant amounts of organic sulphur and nitrogen compounds and may also contain metals such as vanadium and nickel. Such metals can also be removed by the hydrotreating process. The catalyst employed for the hydrotreating stage will also depend on the nature of the feedstock and on the impurities to be removed. Examples of suitable hydrotreating catalysts are compositions comprising a sulphided composition containing cobalt and/or nickel plus tungsten and/or molybdenum, e.g. cobalt molybdate or tungstate or nickel molybdate or tungstate, on a support which is often alumina. The catalyst often also contains a phosphorus component. Examples of hydrotreating catalysts are described in US 4014821 ,

US 4392985, US 4500424, US 4885594, and US 5246569. The hydrotreating conditions will depend upon the nature of the hydrocarbon feedstock and the nature and amount of impurities and on the catalyst employed. Generally the hydrotreating will be effected under superatmospheπc pressure, e.g. 5 to 150 bar abs., and at a temperature in the range 300- 500°C with a hydrogen to hydrocarbon ratio of 50 to 2000 litres of hydrogen (at STP) per litre of hydrocarbon liquid (at STP). The feedstock and hydrotreating conditions are preferably such that the feedstock/hydrogen mixture is a single, i.e. gaseous, phase under the hydrotreating conditions, although a mixed phase system may alternatively be employed. The hydrotreating conditions should be such that the hydrotreating is effective to reduce the organic nitrogen content to below 0.2 ppm by weight, and preferably to below 0.1 ppm by weight, and in particular to below 0 05 ppm by weight To this end the hydrotreating temperature is preferably above 310°C, particularly above 320°C As indicated above, under such hydrotreating conditions the organic sulpnur content of the hydrotreated feedstock may not be as low as is desirable as a result of the formation of mercaptans. Thus whereas it may be desirable for the organic sulphur content of the hydrotreated feedstock to be below 0.5 ppm by weight, and in some cases below 0.2 ppm by weight, under the conditions necessary to achieve the desired level of organic nitrogen, i.e less than 0.2 ppm, the organic sulphur content of the treated feedstock may be above the desired limit, and may be 1 ppm by weight or more

In the present invention, after hydrotreating, the mixture of hydrotreated feedstock and hydrogen is cooled to a sufficient temperature to condense most, if not all, of the hydrocarbon components and then separated into a gaseous stream containing the light components, e.g. hydrogen, hydrogen sulphide and ammonia, and a liquid stream containing the hydrotreated feedstock. This separation is conveniently effected in a column with the liquid hydrotreated feedstock being withdrawn from the lower part of the column and the gaseous stream taken as overheads. The liquid stream will contain some or all of any mercaptans formed, or remaining, as a result of the hydrotreating stage. In order to decrease the organic sulphur content further, the hydrotreated feedstock is then passed through a bed of a suitable mercaptan absorbent. This will also remove any hydrogen sulphide present, e.g. dissolved in the hydrotreated feedstock. While zinc oxide, and/or basic zinc carbonate, compositions may be employed as the mercaptan absorbent, it is preferred to employ an absorbent containing copper or nickel compounds, especially copper oxide. The copper compounds may be employed in combination with zinc compounds. Particularly suitable absorbents containing copper oxide and zinc oxide are described in US 4871710.

The conditions under which the hydrotreated feedstock is contacted with the mercaptan absorbent will depend on the nature of the hydrotreated feedstock and on the nature of the mercaptan absorbent. The temperature of the liquid hydrotreated feedstock at the lower part of the column used to effect separation of thereof from the gaseous stream is typically in the range 150 to 200°C. Since copper-containing compositions are effective as mercaptan absorbents at such temperatures, the absorbent bed may be disposed in the lower part of the separator column so that the liquid stream passes through the absorbent bed prior to discharge from the separator column. Alternatively the absorbent may be disposed in a separate vessel to which the liquid hydrotreated feedstock is fed. The use of a separate vessel of course enables the mercaptan removal to be effected at a temperature different to, e.g greater than, that prevailing in the lower part of the separation column, e.g up to about 200°C and possibly up to 300°C

As a result of passage of the hydrotreated feedstock through the mercaptan absorbent, the organic sulphur content of the hydrotreated feedstock can be decreased to the desired level, e g to below 0.2 ppm by weight

Claims

A process for the removal of impurities including organic sulphur and nitrogen compounds from a hydrocarbon feedstock containing at least 2 ppm by weight of organic nitrogen compounds comprising adding hydrogen to the feedstock and passing the resultant mixture over a bed of a hydrotreating catalyst at an elevated temperature sufficient to decrease the organic nitrogen content to below 0 2 ppm by weight, cooling the resultant mixture, separating the cooled mixture into a liquid stream comprising the hydrotreated feedstock and a gaseous stream containing residual hydrogen, and hydrogen sulphide and ammonia formed by hydrogenation of the organic sulphur and nitrogen compounds, and passing the hydrotreated feedstock through a bed of an absorbent for mercaptans

A process according to claim 1 wherein the feedstock is hydrotreated under such conditions that the organic nitrogen content of the hydrotreated feedstock is below 0 1 ppm by weight

A process according to claim 1 or claim 2 wherein the hydrotreating stage is effected at a temperature above 310°C

A process according to any one of claims 1 to 3 wherein the hydrotreating is effected using a catalyst comprising a supported sulphided cobalt and/or nickel molybdate and/or tungstate

A process according to any one of claims 1 to 4, wherein the mercaptan absorbent comprises at least one copper compound

A process according to claim 5 wherein the mercaptan absorbent comprises copper oxide

A process according to any one of claims 1 to 6 wherein cooled mixture is separated into the liquid hydrotreated feedstock stream and the gaseous stream in a column and the bed of mercaptan absorbent is disposed in the lower part of the column so that the liquid hydrotreated feedstock passes through the bed of mercaptan absorbent prior to discharge rrom the column A process according to any one of claims 1 to 7 wherein, after passage through the mercaptan absorbent bed, the hydrotreated feedstock is subjected to catalytic reforming using a chlorine containing catalyst