EP0589112A1 - Process for the preparation of a fraction rich in tertiary amyl methyl ether free from olefins and of a paraffinic fraction rich in n-pentane - Google Patents

Abstract

Simultaneous production of a cut rich in a tert-amyl alkyl ether free from olefins and of a paraffinic cut rich in n-pentane. The invention is characterised in that a feedstock based on isopentenes is hydrogenated under severe conditions to reach a distribution of the methylbutenes which is close to the thermodynamic equilibrium and then in that the hydrogenation effluent, which is practically free from diolefins, cyclopentene and n-pentenes, is subjected to an etherification with an alcohol followed by a separation of the said alcohol from the product obtained containing the tert-amyl alkyl ether, the fraction thus obtained is then distilled as an n-pentene-rich light cut and a heavier cut rich in tert-amyl ethyl ether and cyclopentene. <IMAGE>

Description

The invention relates to a process for the simultaneous production of a cut rich in tertioamyl ether (TAME) in particular) and a cut rich in n-pentane, from a C₅ cut containing isopentenes, cyclopentene and cyclopentadiene.

Cracking processes such as steam cracking, visbreaking, coking, catalytic cracking provide C₅ cuts rich in olefins. Some of them may contain significant proportions of methyl butenes (isopentenes).

This is particularly the case for C₅ steam cracking cuts which may contain up to 15% of the mixture of 2-methyl-butene-1, 2-methyl-butene-2, 3-methyl-butene-1. On the other hand, this cut can simultaneously contain up to 30% of diolefins in the form of isoprene, pentadiene and cyclopentadiene. A typical composition of this cut is given in Table 1. Table 1 (% weight) C₄- 1 nC₅ 26 isoC₅ 24 nC₅ = 4.5 Methylbutenes 12.0 Cyclopentene 1.5 Isoprene 13.5 Pentadiene 9.0 Cyclopentadiene 7.5 C₆ + 1.0

From a C₅ catalytic cracking cut it is possible to hydrogenate the diolefins to olefins (with the exception of methylbutenes) as described in patent US-A-4,724,274 of the applicant. The reaction takes place by passing the feed to be treated (C₅ cracking cut) with hydrogen and with 2 to 50 ppm by weight (expressed as sulfur relative to the feed) of at least sulfur compounds, contact with a supported catalyst containing at least one noble metal from group VIII, at a temperature of 20 to 150 ° C and a pressure of 5 to 100 bar.

In addition, during this hydrogenation stage, it is possible with regard to isopentenes, to isomerize 3-methyl-butene-1 and 2-methyl-butene-1 to 2-methyl-butene-2, in order to achieve a distribution of these products in proportions close to thermodynamic equilibrium. Table 2 gives this composition at equilibrium.

The C₅ cuts of steam cracking differ from the C₅ cuts of catalytic cracking by the presence of cyclopentadiene and cyclopentene in significant quantities (see Table 1). It has been found that, surprisingly, the use of a C₅ cut of steam cracking instead of a C₅ cut of catalytic cracking makes it possible to obtain an isopentene distribution even closer to the thermodynamic equilibrium conditions.

It is also known that cuts rich in methyl butenes (isopentenes) are feedstocks of choice for the processes for the etherification of iso-olefins with 5 carbon atoms also called isoamylenes by an alcohol (for example methanol) to produce a tertioamylalkyl ether (e.g. tertioamylmethylether) (TAME). This ether could advantageously be used as a mixture in automotive fuels to improve the research octane (NOR) and engine (NOM) indices. These etherification processes are described in various patents, for example US-A-4,336,407.

In general, before the etherification step, the olefinic compounds are not separated from the paraffinic compounds, in particular because of the high cost of separation.

After etherification the product obtained is a mixture of paraffins, olefins, and tertioamylalkylether (TAME, for example).

It will be noted that only the isolated fines present in the feed to be etherified are transformed into ether. Indeed, linear and cyclic olefins as well as saturated molecules are refractory to the etherification reaction.

US Pat. No. 4,361,422 describes such a process, in which more precisely a charge constituted by a C₅ cut, and more particularly a C₅ cut from steam cracking, is subjected to advanced hydrogenation so as to hydrogenate the diolefins (and in particular the cyclopentadiene) and to substantially conserve the etherifiable mono-olefins. The catalyst used contains 0.2 to 2% by weight of palladium on a support, for example alumina or silica, and the operation is carried out between 50 and 200 ° C, with an outlet temperature of at least 120 ° C, under a pressure of 5 to 6 bars and at an hourly liquid charge rate of 0.50 to 10 volumes per volume of catalyst.

The effluent obtained, practically free of diolefins, but containing mono-olefins, is subjected to etherification by an excess of alcohol (methanol for example).

The product obtained containing up to 10% alcohol is sent to the fuel pool. After adding 0.15 g / l of Pb, it has a NOM index of 87.5 to 88.

The mixture obtained, which went directly into the composition of an automotive fuel given its properties, however, had the drawback of introducing olefinic compounds into the final fuel. However, it is known that if the olefins generally have fairly high research octane numbers (NOR) their motor octane numbers (NOM) are very low. The development and marketing of increasingly efficient and sophisticated engines, as well as the elimination of lead in fuels, made necessary by the introduction of catalytic converters, have led to a tightening of the octane index specifications, in particular engine automotive fuels. The constraints linked to environmental protection require a reduction in the olefin content of fuels. Thus it becomes disadvantageous to introduce these residual olefins into the fuels.

This development has led to the appearance of separation columns downstream of the etherification units so as to separate the olefins and paraffins from the ether produced. In such a scheme, the current practice consists in using ether for fuels and in recycling the mixture of paraffins and olefins towards the steam cracking unit so as to produce other valuable compounds such as ethylene and propylene. Nevertheless, it is known to those skilled in the art that the yields of ethylene and propylene obtained during steam cracking depend eminently on the quality of the charge to be cracked.

It is generally accepted that the yields of ethylene and propylene are clearly better when the cracked charge is composed of saturated molecules such as isoparaffins, or better still normal paraffins.

We have now found a new process, which by combining steps of hydrogenation, etherification and separation advantageously improves the enhancement of the C₅ cut. More specifically, the invention relates to a process for the simultaneous production of a fraction rich in tertioamylalkyl ether and cyclopentene and of a paraffinic fraction rich in n-pentane, from a charge constituted by a fraction containing C₅ hydrocarbons, rich in olefins and containing isopentenes, cyclopentene and cyclopentadiene, process in which the charge is subjected to an isomerizing hydrogenation by passage of said charge with hydrogen in contact with at least one supported catalyst containing at least one noble metal of GVIII , at a temperature of 20 ° C to 150 ° C, a pressure of 5 to 100 bars, the effluent obtained is then subjected to etherification by an alcohol, process in which typically the hydrogenation is carried out in the presence of 2 at 400 ppm of at least one sulfur compound (ppm by weight of sulfur relative to the feed) and of hydrogen in the molar ratio H2 / HC = 0.55 to 1, the effluent obtained which is practically free of diolefins, cyclopentene and n-pentenes is subjected to etherification with an alcohol followed by separation of said alcohol from the product containing ether, the fraction thus obtained is then distilled under 1 to 8 bars, in a section light paraffinic containing n-pentene and a heavier cut containing more than 90% by weight of tertioamylalkylether and cyclopentene. Figures 1 and 2 show the two variants of this association.

In the version illustrated in FIG. 1, the very advanced hydrogenation stage of the section C coupe arriving via the line (4) makes it possible to supply the etherification unit (2) with a charge free from diolefins and linear olefins (5) as described in Examples 3 and 4 below. In addition, the isomerization of methyl-butenes-1 carried out during this step (1) leads to the production of a large amount of methyl-2-butene-2.

Surprisingly, the performances of the etherification unit (2) are improved by the use of such a cut both from the point of view of the ether yield and from the point of view of the lifetime of the catalyst.

This catalyst is advantageously constituted by 0.2 to 2% by weight of palladium deposited on a support, which is for example an alumina or a silica. Another particularly interesting catalyst (described in US Pat. No. 4,490,481) consists of 0.03 to 1% by weight of gold on a support, it is resistant to sulfur.

The product (6) obtained in this etherification step is sent to a separation column (3) operating under a pressure of 1 to 8 bars. At the head is obtained a section essentially containing saturated molecules (7) which will constitute a better quality filler for steam cracking compared to a section still containing olefins.

The bottom product of column (8) consisting of a mixture of TAME and cyclopentane has, surprisingly, an improved quality compared to a mixture containing residual olefins, in particular as regards the octane numbers sought ( NOR) and motor (NAME).

In the second version illustrated in FIG. 2, the hydrogenation step is carried out on a cut C₅ - 200 ° C which, after depentanization by line (9) in column (10), will provide a cut C₅ free of diolefins and d 'linear olefins (5) to the etherification unit (2) which will be followed by a separation column (3). Via line (11), a product C₆ + (generally C₆ - 200 ° C) will be drawn from column 10.

The following nonlimiting examples illustrate the invention:

EXAMPLE 1: (Comparison)

In this example, we propose to describe the methoxylation reaction of a raw C₅ cut from a steam cracking unit. The composition of this cup is given below: (% weight) C-₄ 1 nC₅ 24 iso C₅ 22 nC = ₅ 5 Me-3 butene-1 6 Me-2 butene-1 4.8 Me-2 butene-2 2.4 Cyclopentene 2 Isoprene 14 Pentadiene 10 Cyclopentadiene 8 C + ₆ 0.8

This cut also has a sulfur content of 10 ppm. To carry out this methoxylation, the mixture cuts C coupe and methanol is passed from bottom to top on a fixed bed of catalyst of the ion exchange resin type in its acid form; these are crosslinked sulfonic polystyrene resins which are in the form of micro-beads from 0.15 to 0.40 mm in diameter. The fixed catalyst bed is placed in a tubular reactor maintained under substantially isothermal conditions.

Before use, the catalyst was impregnated with methanol.

The conditions for processing this mixture are as follows: Pressure: 5 to 8 bar Temperature : 65 ° C Charge volume flow rate per catalyst volume: 1 Methanol flow rate in moles per mole of reactive isoamylene: 1

By reactive isoamylene means the sum of 2-methyl-butene-1 and 2-methyl-butene-2.

The operation is carried out continuously for a hundred hours. The conversion of reactive isoamylenes is 65%, but there is a gradual increase in the pressure drop in the reactor. When this pressure drop reaches 3 bar, the test is stopped and the catalyst discharged: the catalyst grains are agglomerated; in fact these grains are embedded in a gangue formed by the polymerization of the diolefins of the filler.

The following table gives the typical average compositions (in% by weight) of the feedstock and of the reactor effluent during the experiment. Charge Effluent C-₄ 1.0 1.0 nC₅ 23.2 23.2 Cyclopentane - - iso C₅ 21.3 21.3 nC = ₅ 4.8 4.8 Me-3 butene-1 5.8 5.8 Me-2 butene-1 4.7 0.2 Me-2 butene-2 2.3 2.2 Cyclopentene 1.9 1.9 Isoprene 13.6 13.6 Pentadiene 9.7 9.7 Cyclopentadiene 7.8 7.8 C + ₆ 0.8 0.8 Methanol 3.1 1.1 TAME - 6.6

EXAMPLE 2: (Comparison)

In this example, the same C₅ cut of steam cracking is methoxylated after having been freed by selective hydrogenation of its diolefinic compounds. This selective hydrogenation is carried out as follows: the raw C₅ cut is passed over a fixed bed of catalyst consisting of 0.3% by weight of palladium deposited on a tetragonal gamma alumina in the form of beads; the specific surface of this alumina is 60 m² / g. The fixed catalyst bed is placed in a tubular reactor maintained under substantially isothermal conditions. Before use, the catalyst is reduced at atmospheric pressure under a stream of hydrogen at 100 ° C for 2 hours. The load processing conditions are as follows: Pressure: 25 bar Temperature : 80 ° C Charge volume flow rate per catalyst volume and per hour: 5 Flow rate of hydrogen in moles per mole of hydrocarbon charge: 0.5

The hydrogenated product with the following composition (in% by weight): % weight C-₄ 1 nC₅ 25 Cyclopentane 1 isopentane 22.5 nC = ₅ 14 Me-3 butene-1 1.6 Me-2 butene-1 3.4 Me-2 butene-2 7.7 Cyclopentene 9 Isoprene - Pentadiene - Cyclopentadiene - C + ₆ 1

Methanol is added to this product and the reactive olefins are methoxylated under the conditions already described in Example 1.

It is noted during this experiment which lasted 500 hours, without sign of deactivation of the catalyst, that the pressure drop in the reactor has not changed: when the reactor is unloaded, there is no trace of the polymers described in the example 1.

Furthermore, the conversion of reactive isoamylenes is very close to that already given in Example 1. The composition in TAME of the effluent is 19.8% by weight whereas it was only 6.6% by weight in Example 1.

The following table gives the typical composition (in% by weight) of the feedstock and of the reactor effluent during the experiment. Charge Effluent C-₄ 0.9 0.9 nC₅ 22.6 22.6 Cyclopentane 0.9 0.9 iso C₅ 20.3 20.3 nC = ₅ 12.7 12.7 Me-3 butene-1 3.1 3.0 Me-2 butene-1 6.6 0.7 Me-2 butene-2 14.4 6.7 Cyclopentene 8.1 8.1 Isoprene - - Pentadiene - - Cyclopentadiene - - C + ₆ 0.9 0.9 Methanol 9.6 3.4 TAME - 19.8

The effluent mixture from the reactor is then washed with water to remove the residual methanol and then separated by distillation in two sections, the compositions (in% by weight) are as follows: Charge Distillate Residue C-₄ 0.9 1.2 - nC₅ 23.6 31.5 - Cyclopentane 0.9 0.3 2.8 isopentane 21.0 28.1 - nC = ₅ 13.1 17.5 - Me-3 butene-1 3.1 4.1 - Me-2 butene-1 0.7 0.9 - Me-2 butene-2 6.9 9.2 - Cyclopentene 8.4 7.2 11.9 Isoprene - - - Pentadiene - - - Cyclopentadiene - - - C + ₆ 0.9 - 3.6 Methanol - - - TAME 20.5 - 81.7

A cut rich in TAME is thus obtained which can be directly incorporated into a gasoline pool, its octane numbers NOR and NOM are respectively: NOR = 105 NAME = 95.4

The distillate contains 38.9% olefins, which makes it unattractive to use as a steam cracker filler.

EXAMPLE 3 (According to the invention)

In this example, the same C₅ cut of steam cracking gasoline is always methoxylated after having been rid of it by a thorough hydrogenation of its linear and cyclic olefins and diolefins. The isoamylenes are not affected by this hydrogenation but for these products a mixture is obtained, the composition is close to that expected under thermodynamic equilibrium conditions. The treatment conditions for this hydrogenation, which is carried out in the same apparatus and with the same catalyst as those already described in Example 2, are as follows: Pressure: 25 bar Temperature : 120 ° C Volume flow rate of the charge per volume of catalyst and per hour: 4 Flow rate of hydrogen in moles per mole of hydrocarbon charge: 0.7

The hydrogenated product has the following composition (in% by weight): % weight C-₄ 1.0 nC₅ 39.0 Cyclopentane 9.8 isopentane 23.0 nC = ₅ <10 ppm Me-3 butene-1 0.2 Me-2 butene-1 4.0 Me-2 butene-2 22.0 Cyclopentene - Isoprene - Pentadiene - Cyclopentadiene - C + ₆ 1

Methanol is added to this product and the reactive olefins are methoxylated under the conditions described in Example 1.

It is noted during this experiment which lasted 500 hours without sign of deactivation of the catalyst, that the pressure drop in the reactor has not changed which was already the case in Example 2. On unloading, the catalyst is found in the same state as that described in Example 2, there is no trace of the polymers described in Example 1.

Furthermore, the conversion of reactive isoamylenes is very close to that already given in Examples 1 and 2.

The TAME composition of the effluent is 22.0%, which is higher than those which resulted from the conditions described in Examples 1 (6.6% by weight) and 2 (19.8% by weight).

The following table gives the typical composition (in% by weight) of the feedstock and of the reactor effluent during the experiment. Charge Effluent C-₄ 0.9 0.9 nC₅ 34.9 34.9 Cyclopentane 8.8 8.8 isopentane 20.6 20.6 nC = ₅ - - Me-3 butene-1 0.2 0.2 Me-2 butene-1 3.6 0.7 Me-2 butene-2 19.7 7.4 Cyclopentene - - Isoprene - - Pentadiene - - Cyclopentadiene - - C + ₆ 0.9 0.9 Methanol 10.4 3.6 TAME - 22.0

The effluent mixture from the reactor is then washed with water to remove the excess methanol, then separated by distillation into two sections, the compositions (in% by weight) are as follows: Charge Distillate Residue C-₄ 5 0.9 1.3 - nC₅ 36.3 53.2 - Cyclopentane 9.1 1.5 25.5 isopentane 21.4 31.4 - nC = ₅ - - - Me-3 butene-1 0.2 0.3 - Me-2 butene-1 0.7 1.0 - Me-2 butene-2 7.7 11.3 - Cyclopentene - - - Isoprene - - - Pentadiene - - - Cyclopentadiene - - - C + ₆ 0.9 - 2.8 Methanol - - - TAME 22.8 - 71.7

A cut rich in TAME and cyclopentane is thus obtained, the octane numbers of which are excellent (NOR = 105.3 and NOM = 96) and which can be directly valued in a petrol pool. The distillate which contains 87.4% of saturated compounds including 53.2% of n-pentane is an excellent filler for the steam cracker.

EXAMPLE 4 (According to the invention)

In this example, we start from the total cut of steam cracking gasoline comprising the cut C₅ but with a final boiling point equal to 200 ° C. This essence is first treated in a hydrogenation step, the aim of which is to eliminate all of the diolefinic and styrenic compounds as well as the pentenes and the cyclopentenes. This hydrogenation is carried out in the same apparatus as in Examples 2 and 3 and using the same catalyst but the operating conditions chosen are as follows: Pressure: 28 bar Temperature : 140 ° C Charge volume flow per catlyser volume and per hour: 2 Flow rate of hydrogen in moles per mole of hydrocarbon charge: 1

The product obtained is then sent to a distillation column from which the following cuts are drawn:
- A C₆ cut - 200 ° C at the bottom.
- A C₅ cut at the top, the composition (in% by weight) is as follows: (% weight) C-₄ 1.2 nC₅ 38.8 Cyclopentane 10.8 isopentane 22.0 nC = ₅ - <10 ppm Me-3 butene-1 0.3 Me-2 butene-1 3.9 Me-2 butene-2 21.5 Cyclopentene - Isoprene - Pentadiene - Cyclopentadiene - C + ₆ 1.5

Methanol is added to this product and the reactive olefins are methoxylated under the conditions described in Example 1.

It is noted during this experiment, which lasted 500 hours without sign of deactivation of the catalyst, that the pressure drop in the reactor has not changed, which was already the case in Examples 2 and 3.

On unloading, the catalyst is found in the same state as that described in Examples 2 and 3, there is no trace of polymers as described in Example 1.

The conversion of reactive isoamylenes is very close to that already given in Examples 1, 2 and 3.

The TAME composition of the effluent is 21.6% which is higher than those which resulted from the conditions described in Examples 1 (6.6% by weight) and 2 (19.8% by weight).

The following table gives the typical composition (in% by weight) of the feedstock and of the reactor effluent during the experiment. Charge Effluent C-₄ 1.1 1.1 nC₅ 34.8 34.8 Cyclopentane 9.7 9.7 isopentane 19.7 19.7 nC = ₅ - - Me-3 butene-1 0.3 0.3 Me-2 butene-1 3.5 0.7 Me-2 butene-2 19.5 7.3 Cyclopentene - - Isoprene - - Pentadiene - - Cyclopentadiene - - C + ₆ 1.3 1.3 Methanol 10.1 3.5 TAME - 21.6

The effluent mixture from the reactor is then washed with water to remove the excess methanol, then separated by distillation into two sections, the compositions (in% by weight) are as follows: Charge Distillate Residue C-₄ 1.1 1.6 - nC₅ 36.1 53.7 - Cyclopentane 10.1 1.5 27.7 isopentane 20.4 30.4 - nC = ₅ - - - Me-3 butene-1 0.3 0.4 - Me-2 butene-1 0.7 1.1 - Me-2 butene-2 7.6 11.3 - Cyclopentene - - - Isoprene - - - Pentadiene - - - Cyclopentadiene - - - C + ₆ 1.3 - 4.0 Methanol - - - TAME 22.4 - 68.3

A cut rich in TAME and cyclopentane is thus obtained, the octane numbers of which are excellent (NOR = 105 and NOM = 95.5) and which can be directly valued in a petrol pool. The distillate which contains 87% of saturated compounds including 53.7% of n-pentane is an excellent filler for the steam cracker.

Claims (6)

Process for the simultaneous production of a fraction rich in tertioamylalkyl ether and cyclopentene and of a paraffinic fraction rich in n-pentane, from a charge consisting of a fraction containing C₅ hydrocarbons, rich in olefins and containing isopentenes, cyclopentene and cyclopentadiene, process in which the charge is subjected to an isomerizing hydrogenation by passage of said charge with hydrogen in contact with at least one supported catalyst containing at least one noble metal of GVIII, at a temperature of 20 ° C. at 150 ° C, a pressure of 5 to 100 bars, the effluent obtained is then subjected to etherification with an alcohol, said process being characterized in that the hydrogenation is carried out in the presence of 2 to 400 ppm of at least one sulfur compound (ppm by weight of sulfur relative to the feed) and hydrogen in the molar ratio H2 / HC = 0.55 to 1, the effluent obtained which is practically free of dio olefins, cyclopentene and n-pentenes are subjected to etherification with an alcohol followed by separation of said alcohol from the product containing ether, the fraction thus obtained is then distilled under 1 to 8 bars, in a light paraffinic cut containing n -pentane and a heavier cut containing more than 90% by weight of tertioamylalkylether and cyclopentane. Process according to claim 1, in which the catalyst consists of 0.2 to 2% by weight of palladium on a support. Process according to claim 1, in which the catalyst consists of 0.03 to 1% by weight of palladium, 0.003 to 0.3% by weight of gold and a support. Method according to one of the preceding claims, in which the charge consists of a C₅ steam cracking cut. Method according to one of the preceding claims, in which the charge subjected to hydrogenation consists of a total cut of steam cracking gasoline containing the cut C₅, and the hydrogenated effluent obtained is distilled into a fraction and a fraction C₆ + and a C₅ fraction which is free of diolefins, cyclopentene and n-pentenes. Method according to one of the preceding claims, in which the alcohol is methanol and the tertioamylalkyl ether tertioamylthyl ether.

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