EP1445299A1 - Process for obtaining an etherification feedstock - Google Patents

Abstract

Method for preparing a hydrocarbon feedstock (A) for etherification, where (A) has reduced contents of dienes (I), and nitrogen or sulfur compounds (II), from an initial fraction (B) containing olefins (III), (I), nitriles and sulfur compounds. It comprises (a) selective hydrogenation of (B) in presence of a Group VIII metal catalyst and (b) fractional distillation of the product to give (A) and a heavy fraction (C) containing heavy hydrocarbons and most of (II).

Description

The present invention relates to a process for obtaining a usable hydrocarbon fraction as charge of an etherification unit and containing a reduced amount of compounds diene, nitrogen and sulfur, from an initial charge of hydrocarbons.

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

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

The drawbacks linked to these pollutants are for example described in the patent application WO 96/00714.

Numerous studies aimed at eliminating such compounds from the cut used as filler in etherification units have been proposed. Most recommend washing with solvent, most often with water to at least partially remove the nitriles. However the relatively low solubility of certain nitriles such as propionitrile in polar solvents require significant quantities of solvent which must then be reprocessed, these processes thereby 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 entraíne 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 methods have been proposed to facilitate the elimination of pollutants. By way of example, we can cite:

• US Patent 5,569,790 which discloses a process aimed at eliminating traces of acetone, acetonitriles or propionitriles in a cut of hydrocarbons comprising 4 to 6 carbon atoms. The nitriles are finally removed by washing with water. The waste water is regenerated before being recycled to the process.
• US Pat. No. 5,675,043 which proposes a process for removing nitrogenous compounds of the nitrile type from a hydrocarbon fraction by extraction using a solvent of the glycol type.
  These two methods however have the drawback of generating a polar solution containing the nitrogenous compounds extracted from the hydrocarbon fraction. This polar solution must be reprocessed and regenerated to be recycled in the process, which leads to increased complexity of the unit.
• US Patent 5,300,126 which discloses a process for removing diolefins, the C4 / C5 cuts are brought into contact with a dienophile such as maleic anhydride. The basic nitrogen compounds and the diolefins are extracted after they have reacted with maleic anhydride.

Patent application WO 96/00714 provides a method for reducing the content of pollutants present in the charges for etherification units, by selective hydrogenation. The pollutants removed by this process are nitriles and diolefins. The diolefins are in initially hydrogenated to olefins, then the nitriles are hydrogenated to amines. The catalyst used in this step is a catalyst comprising cobalt. Amines being basic compounds, these are then easily extracted by washing with water. This process therefore also requires the implementation of a washing step in order to remove the compounds nitrogen in the form of amines and therefore has the aforementioned drawback of the reprocessing of washing solutions.

The present invention provides an economical and simple to implement means for remove, at least partially, dienes, nitrile nitrogen compounds and light sulfur compounds present in the original gasoline. The process implemented according to the invention comprises at least two stages: a first stage of selective hydrogenation of the entire initial hydrocarbon charge and a second stage of fractionation of said hydrogenated charge.

During said hydrogenation it was found by the applicant that, under the conditions indicated, not only the dienes are transformed into olefins without these being substantially hydrogenated, but surprisingly and unexpectedly that a quantity substantial or all of the nitrogenous compounds of the nitrile type are transformed into heavier nitrogen compounds, as well as some or all of the sulfur compounds light is transformed into heavier sulfur compounds.

According to the invention, it is thus possible during a subsequent step of fractionation of concentrating nitrogen and sulfur compounds by simple distillation in the most fractions heavy from said fractionation.

a) une hydrogénation sélective de ladite charge initiale d'hydrocarbures, en présence d'un catalyseur du groupe VIII de la classification périodique ,

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 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 usable as feedstock of an etherification unit and containing a reduced amount of diene, nitrogen and sulfur compounds, from an initial feedstock d hydrocarbons comprising a mixture of olefins, dienes, nitriles and also sulfur-containing compounds, said process comprising at least the following successive steps:

a) a selective hydrogenation of said initial charge of hydrocarbons, in the presence of a catalyst from group VIII of the periodic table,

b) fractionation by distillation of the effluents from step a), under conditions allowing at least two cuts to be obtained,

• said hydrocarbon fraction and comprising a reduced amount of diene, nitrogen and sulfur compounds and
• a heavy fraction containing the heavy hydrocarbons and the majority of the nitrogen and sulfur compounds resulting from the hydrogenation of stage a).

In general, said hydrocarbon fraction has a higher cutting or boiling point which is less than 100 ° C, preferably less than 80 ° C and very preferably less than 60 ° C.

The invention will be better understood on reading the preferred embodiment of the invention which follows, given purely by way of illustration 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 eliminate the diolefins present in the initial hydrocarbon charge, such as preferably a petrol cut. Diolefins are gum precursors which polymerize in etherification reactors and limit their lifespan as well as nitrogenous nitrile compounds (the formula of which is given below). It was indeed found by the applicant that under the conditions mentioned in the rest of the description, said nitriles are transformed into heavy nitrogen compounds which respond positively to the measurement of basic nitrogen.

Compounds of the nitrile type: R - C ≡ N, where R comprises from 1 to 6 carbon atoms.

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

Said selective hydrogenation stage generally takes place in the presence of a catalyst comprising at least one metal from group VIII of the periodic table, preferably chosen from the group formed by platinum, palladium and nickel, and a support. Without leaving the within the framework of the invention, cobalt may also be chosen from said group. We will use by example a catalyst containing 1 to 20% by weight of nickel deposited on an inert support, such that for example alumina, silica, silica-alumina, a nickel aluminate or a support containing at least 50% alumina. Another metal from group VIB of the classification periodic can be associated 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 to 20% by weight on the support.

The choice of operating conditions is particularly important according to the invention. We will operate most generally under pressure in the presence of a small amount of hydrogen compared to the stoichiometric value necessary to hydrogenate the diolefins. hydrogen and the charge to be treated are injected in updrafts or downdrafts in a reactor preferably with a fixed catalyst bed. 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 as sufficient to maintain more than 80%, and preferably more 95% by weight of the gasoline to be treated in liquid form in the reactor; she is the most generally between 0.4 to 5 MPa, limits included, and preferably greater than 1 MPa. Advantageous pressure is between 1 to 4 MPa, limits included.

The space speed 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 charge such as a petrol cut can contain up to a few% 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 reactor catalytic hydrogenation which includes a catalytic reaction zone crossed by the total charge and the quantity of hydrogen necessary to carry out the desired reactions.

2 °) Separation of the nitrogenous compounds resulting from stage a) (stage b))

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

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

Step b) consists of a separation of the nitrogen compounds by distillation. The effluents from from step a) are split to produce at least two cuts, including a slight fraction comprising the majority of hydrocarbons, and mainly olefins with 4 or 5 atoms of carbon and free of nitrogen compounds, and a heavy fraction concentrating the hydrocarbons heavy and the nitrogen compounds weighed down during step a). In addition to nitrogen compounds, this preferred embodiment further allows at least partial removal of the compounds sulfur of said light fraction.

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

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

In order to better understand the advantages related to the present invention, are provided as nonlimiting the examples which follow

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 distillation column in three sections: a light gasoline of end point 55 ° C, an intermediate gasoline of initial point 55 ° C and end point 140 ° C, and a heavy gasoline with an initial point of 140 ° C. Each section thus produced is analyzed. The analyzes carried out are detailed below:

Basic nitrogen: measurement of nitrogen in basic form according to method ASTM 4739

Total nitrogen: measurement of total nitrogen according to the ASTM4629 method

MAV: measurement of the diolefin content

IBr: Bromine index, measurement of olefin content

S: measurement of elemental sulfur content

Table 1 brings together the characteristics of essence A as well as the 3 fractions obtained by distillation. 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 essence corresponding to the PI-55 cut concentrates most of the olefins light (C4 and C5). It therefore constitutes the etherification charge. This fraction includes 15 ppm nitrogen, 92 ppm sulfur and an AVM (maleic acid value) of 6.5, which corresponds to nearly 0.8% by weight of diolefins. This gasoline treated on an acid etherification catalyst would result in premature deactivation of it.

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 (essence B) is distilled in three sections (step b)).

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

Essence B is then fractionated by distillation into 3 fractions, the cutting points of which are identical to the fractions of Example 1.

The characteristics of essence B and of the three final cuts are gathered in table 2. 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

We note that the fraction Pl (initial point of distillation) -55 ° C of gasoline B which concentrates the olefins in C4 and C5 intended to supply the etherification unit is greatly depleted in diolefins and free of nitrogen and sulfur compounds. This cut can therefore be directly used as an etherification unit charge, without the need to using an additional step for extracting nitrogen compounds.

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

Claims (10)

Process for obtaining a hydrocarbon fraction usable as feedstock of an etherification unit and containing a reduced quantity of diene compounds, nitrogen and sulfur, from an initial feedstock of hydrocarbons comprising a mixture of olefins, dienes, nitriles as well as sulfur compounds, said process comprising at least the following successive steps: a) a selective hydrogenation of said initial charge of hydrocarbons, in the presence of a catalyst comprising at least one metal from group VIII and another metal from group VIB of the periodic table, b) fractionation by distillation of the effluents from step a), under conditions allowing at least two cuts to be obtained, said hydrocarbon fraction and comprising a reduced amount of diene, nitrogen and sulfur compounds and a heavy fraction containing the heavy hydrocarbons and the majority of the nitrogen and sulfur compounds resulting from the hydrogenation of stage a). The method of claim 1 wherein the Group VIII metal is selected from the group consisting of platinum, palladium, nickel. Process according to Claim 2, in which the catalyst contains 1% by weight to 20% by weight of nickel deposited on an inert support. The method of claim 1 wherein the Group VIII metal is cobalt. Method according to one of the preceding claims, in which said catalyst comprises 1% by weight to 20% by weight of metal from group VIB. Method according to one of the preceding claims, in which the group VIB metal is molybdenum or tungsten. Method 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 space velocity of the charge from 1 h ^-1 to 12 h ^{- 1} . Method according to one of the preceding claims, in which said hydrogenation is carried out in the presence of a quantity of hydrogen in small excess relative to the value stoichiometric necessary to hydrogenate all of the dienes present in the initial charge of hydrocarbons. Method according to one of the preceding claims, in which said fraction hydrocarbon has a higher boiling point which is less than 100 ° C. The method of claim 6 wherein said boiling point is less than 60 ° C.