EP1445299B1 - Process for obtaining an etherification feedstock - Google Patents

Description

The present invention relates to a process for obtaining a hydrocarbon fraction that can be used as a feedstock of an etherification unit and that contains a reduced quantity of diene, nitrogen and sulfur compounds, from an initial charge of hydrocarbons.

More particularly, the present invention finds application in the treatment of gasolines with a view to obtaining a fraction comprising olefins containing from 4 to 6 carbon atoms, and having a reduced content of diene, nitrile, nitrogen and sulfur compounds. this fraction can then be used to supply etherification units, for example in order to produce methyl tertiary butyl ethers (MTBE), ethyl tertiary butyl ethers (ETBE), tertiary amyl methyl ether (TAME) or tertiary amyl ethyl ether ( TAEE)

It is known that the activity of acid catalysts generally used in etherification units can be greatly reduced due to the presence in the section to be treated of undesirable compounds. Of these compounds, diolefins (dienes) may be the source of polymer deposition on the catalyst, and the nitriles result in progressive deactivation of the catalyst. On the other hand, certain compounds such as light sulfides and amines have a certain basicity and can also progressively degrade the activity of acid catalysts.

The disadvantages associated with these pollutants are for example described in the application for WO 96/00714 .

Many studies to remove such compounds from the cut used as a filler in the etherification units have been proposed. Most advocate solvent washing, usually water to at least partially remove nitriles. However, the relatively low solubility of certain nitriles such as propionitrile in polar solvents requires large quantities of solvent which will then have to be reprocessed, these processes thus generating a significant additional cost.

• le brevet US 5,569,790 qui divulgue un procédé visant à éliminer les traces d'acétone, d'acétonitriles ou de propionitriles dans une coupe d'hydrocarbures comprenant 4 à 6 atomes de carbone. Les nitriles sont finalement éliminés par lavage à l'eau. L'eau usée est régénérée avant d'être recyclée vers le procédé.
• le brevet US 5,675,043 qui propose un procédé pour éliminer les composés azotés de type nitriles d'une coupe d'hydrocarbures par extraction à l'aide d'un solvant de type glycol.
  Ces deux procédés présentent cependant l'inconvénient de générer un solution polaire contenant les composés azotés extraits de la coupe hydrocarbure. Cette solution polaire doit être retraitée et régénérée pour être recyclée dans le procédé, ce qui entraîne une complexité accrue de l'unité.
• le brevet US 5,300,126 qui divulgue un procédé d'élimination des dioléfines les coupes C4/C5 sont mises en contact avec un diénophile tel que l'anhydride maléique. Les composés azotés basiques et les dioléfines sont extraits après qu'ils aient réagi avec l'anhydride maléique.

Various processes have been proposed to facilitate the removal of pollutants. By way of example, mention may be made of:

• the US Patent 5,569,790 which discloses a process for removing traces of acetone, acetonitriles or propionitriles in a hydrocarbon section comprising 4 to 6 carbon atoms. The nitriles are finally removed by washing with water. Wastewater is regenerated before being recycled to the process.
• the US Patent 5,675,043 which proposes a method for removing nitrile nitrogen compounds from a hydrocarbon cut by extraction with a glycol solvent.
  These two methods, however, have the disadvantage of generating a polar solution containing the nitrogen compounds extracted from the hydrocarbon fraction. This polar solution must be reprocessed and regenerated for recycling into the process, resulting in increased complexity of the unit.
• the US Patent 5,300,126 which discloses a process for removing diolefins C4 / C5 cuts are contacted with a dienophile such as maleic anhydride. The basic nitrogen compounds and diolefins are extracted after they have reacted with maleic anhydride.

The request for WO 96/00714 proposes a method for reducing the content of pollutants present in the charges for etherification units, by selective hydrogenation. The pollutants eliminated by this process are nitriles and diolefins. The diolefins are initially hydrogenated to olefins, and then the nitriles are hydrogenated to amines. The catalyst used in this step is a catalyst comprising cobalt. Since amines are basic compounds, they are then easily extracted by washing with water. This method therefore also requires the implementation of a washing step to remove the nitrogen compounds in the form of amines and thus has the aforementioned drawback of the reprocessing of the washing solutions. The US Patent 6,469,223 describes a process for the selective hydrogenation of dienes in a hydrocarbon feedstock.

The present invention provides a cost-effective and simple way of eliminating, at least partially, the dienes, the nitrile-type nitrogen compounds and the light sulfur compounds present in the initial gasoline. The method used according to the invention comprises at least two stages: a first stage of selective hydrogenation of the whole of the initial hydrocarbon feedstock and a second stage of fractionation of said hydrogenated feedstock.

During said hydrogenation it was found by the applicant that, under the conditions indicated, not only are the dienes converted into olefins without these being substantially hydrogenated, but surprisingly and unexpectedly that a substantial amount or even all of the Nitrile-type nitrogen compounds are converted to heavier nitrogen compounds, and some or all of the light sulfur compounds are converted to heavier sulfur compounds.

According to the invention, it is thus possible during a subsequent fractionation step to concentrate, by simple distillation, the nitrogenous and sulfur compounds in the heavier fractions resulting from said fractionation.

1. a) une hydrogénation sélective de ladite charge initiale d'hydrocarbures, en présence d'un catalyseur du groupe VIII de la classification périodique ,
2. b) un fractionnement par distillation des effluents issus de l'étape a), dans des conditions permettant d'obtenir au moins deux coupes dont
   • ladite fraction hydrocarbonée présentant un point d'ébullition supérieur qui est inférieur à 60°C et comprenant une quantité réduite de composés diéniques, azotés et soufrés et
   • une fraction lourde contenant les hydrocarbures lourds et la majorité des composés azotés et soufrés issu de l'hydrogénation de l'étape a).

More specifically, the present invention relates to a process for obtaining a hydrocarbon fraction that can be used as a feedstock of an etherification unit and containing a reduced quantity of diene, nitrogen and sulfur compounds, from an initial feedstock. hydrocarbons comprising a mixture of olefins, dienes, nitriles and sulfur compounds, said process comprising at least the following successive steps:

1. a) a selective hydrogenation of said initial hydrocarbon feed, in the presence of a catalyst of group VIII of the periodic table,
2. b) a fractionation by distillation of the effluents resulting from stage a), under conditions making it possible to obtain at least two slices of which
   • said hydrocarbon fraction having an upper boiling point which is less than 60 ° C and comprising a reduced amount of diene, nitrogen and sulfur compounds and
   • a heavy fraction containing the heavy hydrocarbons and the majority of nitrogen and sulfur compounds derived from the hydrogenation of step a).

The invention will be better understood on reading the preferred embodiment of the invention which follows, given for purely illustrative and in no way limiting.

According to this embodiment, the following steps are carried out:

1 °) Selective Hydrogenation (step a):

This step is intended to at least partially remove the diolefins present in the initial hydrocarbon feed, such as preferably a gasoline cut. Diolefins are precursors of gums which polymerize in the etherification reactors and limit their life as well as nitrile-type nitrogen compounds (whose formula is given below). It has indeed been found by the applicant that under the experimental conditions mentioned in the rest of the description, said nitriles are converted into heavy nitrogen compounds which respond positively to the measurement of basic nitrogen.

Nitrite compounds: R - C ≡ N, where R comprises from 1 to 6 carbon atoms.

This step also makes it possible to convert light sulfur compounds such as mercaptans, sulphides and CS2, whose boiling point is generally lower than that of thiophene, into heavier sulfur compounds whose boiling point is higher than that of thiophene.

Said selective hydrogenation step generally takes place in the presence of a catalyst comprising at least one metal of group VIII of the periodic table, preferably chosen from the group formed by platinum, palladium and nickel, and a support. Without departing from the scope of the invention, the cobalt may also be chosen from said group. For example, a catalyst containing 1 to 20% by weight of nickel deposited on an inert support, such as, for example, alumina, silica, silica-alumina, a nickel aluminate or a support containing at least 50% alumina. Another Group VIB metal of the Periodic Table can be combined with this group VIII metal to form a bimetallic catalyst, such as, for example, molybdenum or tungsten. This group VIB metal is preferably deposited at a level of 1% by weight at 20% by weight on the support.

The choice of operating conditions is particularly important according to the invention. The operation will generally be carried out under pressure in the presence of a quantity of hydrogen in small excess relative to the stoichiometric value necessary for hydrogenating the diolefins. hydrogen and the feedstock to be treated are injected in ascending or descending streams into a preferably fixed catalyst bed reactor. The temperature is most generally between 50 and 300 ° C, and preferably between 80 and 250 ° C, and very preferably between 120 and 210 ° C.

The pressure is chosen to be sufficient to maintain more than 80%, and preferably more than 95% by weight of the gasoline to be treated in liquid form in the reactor; it is most generally between 0.4 and 5 MPa inclusive, and preferably greater than 1 MPa. An advantageous pressure is between 1 to 4 MPa inclusive.

The space velocity is, in these conditions of the order of 1 to 12 h ^-1 , preferably of the order of 2 to 10 h ^-1 .

The initial hydrocarbon feedstock such as a petrol cut may contain up to a few% by weight of diolefins. After hydrogenation, the diolefin content is most often reduced to less than 5000 ppm, preferably less than 2500 ppm, or even less than 1500 ppm.

According to a possible embodiment of the invention, step a) takes place in a catalytic hydrogenation reactor which comprises a catalytic reaction zone traversed by the entire charge and the amount of hydrogen necessary to effect the desired reactions. .

2 °) Separation of Nitrogen Compounds from Step a) (Step b)

It was surprisingly found by the applicant that all the nitrogenous compounds derived from hydrogenation step a) had a boiling point above 55 ° C. and could therefore be separated by simple distillation from the fraction comprising olefins containing from 4 to 6 carbon atoms, and intended to supply etherification units.

Thus, the nitrogen compounds derived from step a) are compounds with a boiling point greater than those of hydrocarbons comprising 5 carbon atoms. In addition, these compounds react with the measurement of basic nitrogen according to the ASTM4739 method.

Step b) consists of a separation of the nitrogen compounds by distillation. The effluents resulting from stage a) are fractionated in order to produce at least two sections, a light fraction comprising the majority of the hydrocarbons, and mainly olefins containing 4 or 5 carbon atoms and free of nitrogen compounds, and a heavy fraction concentrating heavy hydrocarbons and nitrogen compounds weighed up during step a). In addition to the nitrogen compounds, this preferred embodiment also makes it possible to at least partially eliminate the sulfur compounds of said light fraction.

The nitrogen compound content of the light fraction of the gasoline from step b) generally contains less than 20 ppm nitrogen, preferably less than 10 ppm nitrogen, and very preferably less than 5 ppm nitrogen.

The content of light sulfur compounds in the light fraction of gasoline is generally less than 200 ppm, preferably less than 100 ppm, and very preferably less than 50 ppm sulfur.

In order to better understand the advantages associated with the present invention, the following examples are provided without implied limitation.

Example 1 (comparative):

• Azote basique : mesure de l'azote sous forme basique selon la méthode ASTM 4739
• Azote total : mesure de l'azote total selon la méthode ASTM4629
• MAV : mesure de la teneur en dioléfines
• IBr: Indice de brome, mesure de la teneur en oléfines
• S : mesure de la teneur en soufre élémentaire

A gasoline A from a catalytic cracking unit is distilled in a preparative three-slice distillation column: a light end-point gasoline 55 ° C, an intermediate gasoline of 55 ° C initial point and end point 140 ° C, and a heavy essence of initial point 140 ° C. Each cut thus produced is analyzed. The analyzes performed are detailed below:

• Basic nitrogen: measurement of nitrogen in basic form according to ASTM 4739
• Total nitrogen: measurement of total nitrogen according to ASTM4629
• MAV: measurement of diolefin content
• IBr: Bromine index, measurement of olefin content
• S: measurement of elemental sulfur content

Table 1 summarizes the characteristics of gasoline A as well as the 3 fractions obtained by distillation. <u> Table 1 </ u> Essence A PI-55 55-140 140+ Density 0.716 .6504 .7382 0.842 Basic nitrogen (ppm) 9 1 1 71 Total nitrogen (ppm) 21 15 13 110 MAV (mg / g) 14 6.5 15.5 30 HBr 101 130 79 48 S (ppm) 780 92 926 2563 Yield (% wt) 100 29.59 64.27 6.14

The light gasoline corresponding to the PI-55 cut concentrates the majority of the light olefins (C4 and C5). It therefore constitutes the etherification charge. This fraction comprises 15 ppm of nitrogen, 92 ppm of sulfur and a MAV (maleic acid value) of 6.5, which corresponds to almost 0.8% by weight of diolefins. This gasoline treated on an acidic etherification catalyst would cause a premature deactivation thereof.

Example 2 (according to the invention):

The example below is in accordance with the present invention. The initial charge used is similar to that of Example 1.

After a first selective hydrogenation treatment in a first step a), the gasoline produced (gasoline B) is distilled in three sections (step b)).

Step a) is carried out as follows: gasoline A is treated on a fixed bed reactor charged with HR845® catalyst based on nickel and molybdenum marketed by Axens, in the presence of hydrogen. The reaction is carried out at 165 ° C. under a pressure of 20 bar (2 MPa) and a space velocity of 4 h ^-1 . The H ₂ / charge ratio expressed in liter of hydrogen per liter of feedstock is 6. The gasoline thus produced during step a) is gasoline B, the characteristics of which are given in Table 2.

Gasoline B is then fractionated by distillation into 3 fractions whose cutting points are identical to the fractions of Example 1.

The characteristics of Gasoline B and the three final cuts are summarized in Table 2. <u> Table 2 </ u> gasoline
B PI-55 55-140 140+ Sp Gr .7166 .6518 .7375 .8474 basic nitrogen, ppm 13 <1 6 173 Total nitrogen, ppm 21 <1 12 230 MAV (mg / g) 0.8 0.2 7 8.1 HBr 98 128.7 93 47 S, ppm 782 <1 928 2719 Yield (wt%) 100 27.4 66.7 5.4

It is found that the fraction P1 (initial point of distillation) -55 ° C of gasoline B which concentrates the C4 and C5 olefins intended to supply the etherification unit is strongly depleted in diolefins and free of nitrogen compounds and sulfur. This cut can therefore be used directly as etherification unit load, without it being necessary to implement a complementary step of extraction of nitrogen compounds.

This example shows that it is possible, according to the invention, to produce a C4-C5 fraction depleted of diolefins and free of nitrogen compounds without resorting to a step of removing the nitrogen compounds by washing but a simple distillation. Likewise, thanks to the present process, the sulfur compounds have also been greatly reduced or completely eliminated from the PI-55 ° C. fraction of the gasoline.

Claims (8)

1. Process for obtaining a hydrocarbon fraction that can be used as a feedstock of an etherification unit and that contains a small amount of diene compounds, nitrogen-containing compounds and sulfur-containing compounds, starting from an initial hydrocarbon feedstock that comprises a mixture of olefins, dienes, and nitriles as well as sulfur-containing compounds, whereby said process comprises at least the following successive stages:

   a) a selective hydrogenation of said initial hydrocarbon feedstock in the presence of a catalyst that comprises at least one metal of group VIII and another metal of group VIB of the periodic table,

   b) a fractionation by distillation of the effluents that are obtained from stage a) under conditions that make it possible to obtain at least two fractions including

   -- said hydrocarbon fraction presenting a higher boiling point that is less than 60 °C and that comprises a small amount of diene compounds, nitrogen-containing compounds and sulfur-containing
   compounds, and

   -- a heavy fraction that contains heavy hydrocarbons and the majority of the nitrogen-containing and sulfur-containing compounds obtained from the hydrogenation of stage a).
2. Process according to claim 1, in which the metal of group VIII is selected from the group that consists of platinum, palladium and nickel.
3. Process according to claim 2, in which the catalyst contains 1% by weight to 20% by weight of nickel that is deposited on an inert substrate.
4. Process according to claim 1, in which the metal of group VIII is cobalt.
5. Process according to one of the preceding claims, in which said catalyst comprises 1% by weight to 20% by weight of metal of group VIB.
6. Process according to one of the preceding claims, in which the metal of group VIB is molybdenum or tungsten.
7. Process according to one of the preceding claims, in which said catalyst operates under a pressure of 0.4 to 5 MPa, at a temperature of 50 to 300°C with an hourly volumetric flow rate of the feedstock of 1 h^-1 to 12 h^-1.
8. Process according to one of the preceding claims, in which said hydrogenation is carried out in the presence of an amount of hydrogen that slightly exceeds the stoichiometric value that is necessary for hydrogenating all of the dienes that are present in the initial hydrocarbon feedstock.