EP0416979A1 - Process for fixed bed sweetening of petroleum fractions - Google Patents

Abstract

The invention relates to a process for softening an oil cut in the presence of an oxidizing agent, by catalytic oxidation of the mercaptans which it contains, in the presence of a fixed bed support impregnated with a metal chelate and in l absence of an aqueous base. According to the invention, the water content of the support is maintained within a range of predetermined values by acting on the solvent power of the filler with respect to the water of the support, as a function of the temperature. No abstract figure.

Description

The present invention relates to the softening in a fixed bed of petroleum fractions, by catalytic oxidation to disulphides of the mercaptans which they contain.

In principle, such an oxidation can be obtained simply by mixing the petroleum fraction to be treated and an aqueous solution of an alkaline base, to which a catalyst based on a metal chelate is added, in the presence of an oxidizing agent. The petroleum cut and the aqueous solution of the alkaline base are not miscible and it is at the interface of the two liquid phases that the mercaptans are converted to disulfides (see French patent No. 1,249,134).

With mercaptans which are more difficult to oxidize, it is preferable to treat the petroleum fraction using a supported catalyst, in the presence of an alkaline base and an oxidizing agent, and this process is designated by the designation "fixed bed softening process". The oxidizing agent, generally air, is mixed with the cut to be softened. The alkaline base, usually an aqueous sodium hydroxide solution, is introduced either continuously or intermittently into the reaction medium to maintain the alkaline conditions and the aqueous phase necessary for the oxidation reaction. The metal chelate, used as a catalyst, is generally a metal phthalocyanine (see French patent n ° 1,301,844). The reaction is carried out at a pressure generally between 5 and 30.10⁵ Pascals, at a Gomprise temperature between 20 and 40 ° C. However, it is well known to those skilled in the art that, when such a reaction is carried out in an oxidizing medium with aqueous sodium hydroxide (or a strong base), but at a temperature higher than room temperature, the stability metal chelate catalyst decreases rapidly at the expense of the softening reaction.

In addition, it is advisable to renew the sodium solution which runs out, on the one hand, because of the impurities from the load, which dissolve in this solution and make it unsuitable for recycling and, on the other hand, due to the variation in the concentration of the base, which decreases due to the supply of water by the load and the transformation of mercaptans into disulfides.

To overcome this drawback, it has been proposed (see patents FR 2,343,043, U.S. 4,498,978 and U.S. 4,502,949) to eliminate the use of aqueous sodium hydroxide (or base). However, for the reaction to proceed normally, the active sites of the support must then be in contact with the mercaptans present in the petroleum charge, which supposes a homogeneous medium and therefore the absence of an aqueous solution. However, it appears that the water molecules already present in the feed and especially those produced during the reaction favor the appearance on the surface of the catalyst of such an aqueous solution, the maintenance of which exceeds a certain threshold. leads to a lowering of the catalytic activity. It has therefore been proposed either to integrate into the support a solid desiccant (US patent 4,498,978), or to periodically resorb this aqueous phase by drying the catalyst using a polar solvent miscible with water such as 'an alcohol (patent FR 88.16.907). However, these solutions, if they prove to be effective, necessarily lead to relatively high operating costs.

The present invention aims to remedy all these drawbacks by proposing a process for softening an oil cut, by catalytic oxidation of mercaptans, requiring neither the use of an inorganic or organic anhydrous base, nor the use of a desiccant mixed with the support, nor the periodic drying of the support using a solvent.

The Applicant has, in fact, observed that, since the oxidation reaction of the mercaptans can take place in the absence of a basic solution, it becomes possible to carry out this reaction without damage to the catalyst at a higher temperature than according to prior art, that is to say at a temperature generally above 40 ° C. This action on the temperature (action which can be continuous or momentary) results in a modification of the solubility of the water in the charge and, consequently, a modification of the quantity of water present on the surface of the catalytic support. It is then possible to maintain the catalytic properties at their optimum level.

The subject of the invention is therefore a process for softening an oil cut in the presence of an oxidizing agent, by catalytic oxidation of the mercaptans which it contains, in the presence of a support in a fixed bed impregnated with a metal chelate. and in the absence of an aqueous base, this process being characterized in that the water content of the support is maintained within a range of predetermined values by action on the solvent power of the filler with respect to the water of the support, depending on the temperature, and in that the temperature of the charge is fixed at a value sufficient to dissolve the reaction water resulting from the transformation of the mercaptans into disulfides.

The temperature of the load is thus chosen so as to maintain the water content of the support between between 0.1 and 50% by weight of the support and, preferably, between 1 and 25% by weight of the latter.

This range of predetermined values of water contents of the support will depend, of course, on the very nature of the catalytic support used during the softening reaction. Indeed, the Applicant has established that, if many catalytic supports are capable of being used without aqueous sodium hydroxide (or without base), their activity will only manifest themselves when their water content (also called hydration level of the support) is kept within a relatively narrow range of values, variable depending on the supports, but apparently linked to the content of the silicate support and to the structure of its pores.

Thus, with catalytic supports of the type described in European patent application No. 252 953, good conversion of the mercaptans to disulfides is observed, without base and without aqueous solution, when the hydration level of the support is included in an interval of 1 to about 11% by weight (below 1%, the support is too dry, while above 11% an aqueous phase appears on the support).

With other supports of the basic aluminosilicate type, one can observe, depending on the type of catalyst, a widening or, on the contrary, a restriction of this interval. The same phenomenon seems to have been observed with supports based on activated carbon (see U.S. Patent 4,498,978). However, it appears that the range of values of the hydration rate of such a support based on activated carbon is too narrow for industrial application.

Once the optimum value of the hydration rate of the support determined experimentally, it is possible, by means known in the art, to adapt the initial water content of the support to this value:
- to lower the initial rate of hydration of the support, it is possible, among other means, either to raise the temperature of the charge in the reactor, or to inject continuously or discontinuously a quantity of polar solvent miscible with water, or else circulating in the reactor a hot fluid such as air;
- Conversely, to increase this rate, it is possible either to lower the temperature of the reactor, or to send a determined quantity of water into it.

During its work on the softening of mercaptans in the absence of any base and any aqueous solution, the Applicant has now established that once the desired rate of hydration of the catalytic support in the catalytic bed has been obtained, this rate could be kept substantially constant during the softening reaction, despite
- the water supplies by the load, regular or accidental, especially when the softening unit is located downstream of a prewash unit,
- the appearance of reaction water (two molecules of mercaptans giving rise to the formation of a molecule of disulfide and a molecule of water).

In fact, despite the poor affinity of water for hydrocarbons, the simple fact of raising the temperature of the charge in the softening reactor makes it possible to reduce the hydration rate of the support, despite the affinity apparent of it for water molecules. In addition, when the quantities of water resulting from the reaction are large, it is possible to maintain the hydration of the support at a determined value by regulating the reaction temperature to values considered hitherto considered impracticable, due to the instability at these temperatures of the catalyst in the presence of an aqueous base. Thus, as soon as the water content of the support becomes too high, the temperature of the load is increased until this content returns to the set value; conversely, when said content tends to decrease below this value, the temperature of the charge is consequently reduced.

The temperature of the petroleum cut will generally be kept above 30 ° C and preferably between 40 and 140 ° C; the necessary heat can be provided by any means of a type known per se, but preferably it will be provided by heat exchange upstream of the reaction zone.

When the petroleum charge to be treated is, for example, a cut containing between 20 and 300 ppm of mercaptans and less than 150 ppm of water soluble in the charge to be treated, the catalytic oxidation reaction will be advantageously carried out at a higher temperature. at 30 ° C and preferably between 40 and 120 ° C. These conditions are aimed in particular at the softening of petroleum fractions such as kerosene or so-called catalytic or direct distillation gasolines.

Similarly, when the petroleum charge to be treated is for example a cut containing between 300 and 3000 ppm of mercaptans and quantities of soluble water greater than 100 ppm, the catalytic oxidation reaction will be carried out at a temperature higher than 40 ° C and preferably between 50 and 140 ° C. These conditions are aimed in particular at the softening of petroleum cuts such as light petrol.

In the two previous cases, the optimum level of hydration of the support on which the catalytic activity depends will be maintained at a determined value, generally between 1 and 25% by weight, despite possible variations in the charge, for example by fluctuating the temperature of a few degrees around an equilibrium temperature, depending on whether the support itself tends to hydrate or dehydrate.

This rate can also be maintained by substantially raising the reaction temperature and by injecting a certain amount of water, which can vary depending on the water and mercaptan contents of the feed.

According to a particularly advantageous embodiment of the method according to the invention, it is therefore possible to inject water into the petroleum cut, upstream of the reaction zone, in order to more easily adjust the water content of the support in the predetermined range of water contents.

A possible alternative will consist, for example, in carrying out the reaction at a temperature a few degrees lower than the equilibrium temperature mentioned above and in sending, periodically, or continuously, a determined quantity of poorly hydrated filler to a higher temperature.

In order to adapt the temperature of the mercaptan oxidation reaction, means will be used to measure the water content of the feed and of the effluent, or of the catalytic support, as well as the mercaptan contents of the feed. and effluent.

Among the means for measuring the water content of the support, one can use, on the one hand, systems allowing, in a manner known per se, the sampling of catalytic support samples without stopping the unit. From these samples, it is easy to measure, by the Karl FISCHER method, the hydration rate of the support and then to reduce this rate to its optimal value by temperature regulation, in accordance with the present invention. It is also possible to use a probe system placed directly inside the catalytic bed.

Among the means for measuring the mercaptan content of the feed, both at the inlet of the reactor and at its outlet, it is possible to use chromatographic analyzers of a type known per se or, failing this, to carry out periodic samples for analysis according to a method such as that called silver nitrate.

Another means, easy to implement, consists in using two probes for continuous measurement of the water content of the feed, the first being located upstream of the catalytic bed, the second downstream of the latter. Among the probes making it possible to continuously measure the water content of the feed, it is possible to use commercial probes, such as those sold by the Endress Hauser Company, or capacitive probes of the type described in French patent application No. 2.512 .958. The difference between the content measured downstream and that measured upstream then makes it easy to determine, taking into account the quantity of mercaptans transformed into disulphides, by how much increases or decreases the water content of the support. A modification of the reaction temperature (or of the quantity of additional water injected) then makes it possible to make the water content of the catalytic support in the reactor substantially constant.

A first advantage of the present invention follows from the very principle of the reaction, which is linked to the absence of base or of caustic solution in the feed and in the effluents: on the one hand, there is no longer any need to separate the bases, neither to reprocess them, nor to have an expensive unit of destruction of these; on the other hand, the absence of base, even in trace amounts in the treated petroleum charges, makes the latter excellent products for subsequent direct use.

A second advantage of the present invention results from the fact that the reaction water is entrained by the petroleum charge itself during the softening. There is therefore no longer any need for costly drying of the support by injecting solvents into the feed, or by using a desiccant in the catalytic bed.

A third advantage of the present invention results from the fact that the softening reaction is carried out at a higher temperature than according to the prior art: the kinetics of the reaction are improved, and it becomes possible to carry out the reaction continuously at a hourly space velocity greater than that of the prior art. This hourly space speed may be of the order of 1 to 8 v.v.h. (charge volume per catalyst volume and per hour) for kerosene type charges and of the order of 1 to 10 v.v.h. for petrol type loads. This results in a significant saving on the construction costs of the softening unit, since the size thereof can be reduced accordingly. Finally, the reaction kinetics being improved, it is also possible to treat more refractory or heavier fillers which are difficult to soften according to the prior art, or to carry out the softening of the mercaptans continuously where only discontinuous techniques were applicable. until now.

A fourth advantage of the present invention finally results from the fact that the softening reaction is carried out in homogeneous phase, without the formation of gums, which leads to a reduction in washing and maintenance costs, and a simplification of the equipment located downstream. of unity.

The process according to the invention is well suited to the softening of all petroleum cuts and, in particular, the softening of petrol and kerosene. Indeed, these petroleum fractions contain only very little water (an amount generally less than 500 ppm) and, consequently, the rise in temperature necessary to dissolve the water molecules generated in situ during the reaction of softening will be kept within acceptable limits.

In addition, when the petroleum charge to be treated contains relatively large quantities of mercaptans, for example for charges whose mercaptan content is greater than 300 ppm, it will be possible to compensate for the formation in situ of a corresponding quantity of molecules d water, for example by drying the charge beforehand on molecular sieves, (or by cooling it, then by decanting it), in order to limit as much as possible its initial water content.

Except for the absence of an aqueous base and the higher temperature and hourly space velocity conditions than according to the prior art, the other conditions of the oil charge softening reaction according to the invention are generally the same as those described in prior art. These reaction conditions could, for example, be as follows: - temperature : 40 to 140 ° C, - pressure: 10⁵ to 30.10⁵ Pascals, - quantity of oxidizing agent (air): 1 to 31 / kg of mercaptans, - hourly space velocity in vvh (charge volume per catalyst volume and per hour): 1 to 10, - water content of the support (% by weight): 1 to 25.

Among the supports which can be used in accordance with the present invention, it is possible to use, in a manner known per se, supports based on activated carbon, on alumina, of clay, aluminosilicates, silicates or mixtures of these.

As the metal chelate, any chelate used for this purpose in the prior art, especially phthalocyanines, porphyrins or metallic corrines, can be deposited on the support. Particularly preferred is cobalt phthalocyanine and vanadium phthalocyanine. Preferably, metallic phthalocyanine is used in the form of a derivative of the latter, with particular preference for their commercially available sulfonates, such as, for example, cobalt phthalocyanine disulphonate and mixtures thereof. this.

An embodiment of the invention will be described below in detail, by way of nonlimiting example, with reference to the single figure of the accompanying drawing.

This figure represents a diagram of continuous implementation of the method according to the invention.

In this embodiment, the supply of the reactor 1 in petroleum cut to be softened is carried out by line 2, into which the oxidizing agent, for example air, is introduced directly by line 3. The treated petroleum cut is evacuated by line 4, which feeds a filter system 5, intended to remove traces of water and nascent sulfur often produced during the oxidation of mercaptans and not retained by the support. The treated charge is then transferred by line 6 to a storage enclosure 7.

In accordance with the invention, measurement probes 8 and 9, placed respectively upstream and downstream of the reactor, make it possible to permanently determine the water and mercaptan contents at the inlet and at the outlet of the reactor 1. It is thus it is possible to continuously check whether the water content of the catalytic support increases or decreases. A corrective action can then be carried out by modifying the quantity of heat supplied to the load by a heat exchanger 10 placed on the line 2 upstream of the reactor 1. Part of the heat provided by the exchanger 10 can then be recovered using the exchanger 11 placed on the line 4 downstream of the reactor 1 and from the water formed eliminated at 5.

Water can optionally be injected into the feedstock through line 12, placed here between the temperature regulation phase at 10 and the catalytic oxidation phase at 1 of the petroleum cut. In this embodiment of the invention, the temperature of the charge at the outlet of the exchanger 10 will be slightly higher than that required to keep the water content of the support equal to its set value, and an additional d water will, for example, be introduced via line 12 to make this content equal to the set value, despite fluctuations in the contents of water and mercaptans or those of the temperature of the charge.

This embodiment of the invention has the advantage that it is easier to ensure regulation by varying the amount of water injected than by varying the temperature of the charge.

As will be shown by the examples below, which are not limiting either, the implementation of the invention proves to be particularly effective in softening petroleum fractions, even those deemed to be difficult to treat.

EXAMPLE 1

The catalytic support used in this example was prepared as described in European patent application No. 252,853. After impregnation of the support with a sulfonated cobalt phthalocyanine of the type marketed by the French company PROCATALYSE under the name "LCPS", this catalyst is in the form of an aggregate (with a specific surface of approximately 50 m² / g, which contains mainly about 1.5 kg of chelate per m³ of support This support contains approximately 10% by weight of carbon, 20% by weight of silicon, and 8 to 9% by weight of potassium salts in the form of insoluble salts.

Studies of this catalytic support on pilot show that the optimum water content is between 1 and about 10% by weight: below 1% by weight of water content, the catalyst deactivates quickly, while deactivation is much slower beyond 1 % and up to 10 or 11% by weight.

Two identical batches of catalyst, L₁ and L₂, are prepared, which are placed directly in a pilot reactor, the ratio of the height to the diameter here being around 5. We then make sure of the correct initial rate of hydration of the catalyst (5 to 6% by weight of water), by passing over it, at ambient temperature, an equal volume of industrial ethanol at an hourly space speed of 1 vvh

There are two charges C₁ and C₂:
the charge C₁ is a charge of the kerosene type, resulting from a mixture of isthmus-cactus crudes and Mayan crude, containing approximately 80 ppm of mercaptans and 100 ppm of water;
- The charge C₂ is a gasoline type charge, containing approximately 300 ppm of mercaptans and 150 ppm of water.

These charges have the following characteristics: C₁ C₂ - aromatic compounds (% by volume): 20 60 - olefins (% by volume): <5 20 - saturated hydrocarbons (% by volume): 78 20 - mercaptans content (ppm by volume): 80 300 - water content at 40 ° C (ppm by weight): 100 * 150 * The load coming here from a prewash unit, this value corresponds to the maximum solubility of water in the load at this temperature.

Air is used as the oxidizing agent, and no basic aqueous solution is used. We pass the load C₁ on the first batch L₁ under operating conditions which are as follows: T1 T2 T3 T4 - reaction temperature (° C): 40 80 45 80 - pressure (pascals): 6.10⁵ 6.10⁵ 6.10⁵ 6.10⁵ - air flow (1) 1.8 1.8 1.8 1.8 - initial water content (ppm): 100 100 100 677 - water content of effluents (ppm): 100 700 120 700 - hourly space velocity (vvh): 0.8 4 1 4 - initial hydration rate (% by weight): 5 10 2 5 - final hydration rate (% by weight): 10 2 5 7 - cycle time (days): 90 1 > to 300 > to 300

During the T1 test, it is noted that the mercaptan content of the treated charge C₁ is less than 5 ppm during the first sixty days approximately, then that the mercaptan content of the effluents rises gradually and exceeds 10 ppm after 90 days, due to the progressive saturation in water of the catalytic support; it is then easy to see that the reaction water has been captured by the support, the hydration rate of which has risen to more than 10%.

One then proceeds to the T2 test, by raising the temperature of the charge to 80 ° C. and passing the hourly space speed to 4 vvh. It is noted that the mercaptan content of the treated charge C₁ becomes again less than 5 ppm, during the First 6 days, then this content rises sharply and exceeds 10 ppm after 7 days, this time due to dehydration of the catalytic support, the hydration rate of which has become less than 1%.

The operating conditions are then modified to carry out the T3 test in accordance with the method according to the invention: it is found that the mercaptan content of the treated charge C₁ always remains below 5 ppm, after 300 days of operation. Indeed, the hydration rate of the support has remained substantially constant, due to the temperature rise from 40 to 45 ° C, which allows to remove with the charge all the reaction water from the conversion of mercaptans to disulfides .

The reaction conditions are again modified to carry out the T4 test, also in accordance with the method according to the invention: it is also found that the mercaptan content of the treated C₁ charge always remains less than 5 p.p.m., even after 300 days of operation. Indeed, the hydration rate of the support is maintained at a constant value of around 6%, despite the sharp increase in temperature from (from 45 to 80 ° C) and in hourly space speed (from 1 to 4 vvh) thanks to a sufficient injection of water into the feed (+ 577 ppm) so that, despite the significant increase in its solvent power at 80 ° C, the feed can no longer dissolve in the reactor except the amount of reaction water produced.

Another test is then carried out on the charge C₂, which is this time a species much richer in water and especially in mercaptans. The operating conditions are then as follows: T5 T6 - reaction temperature (° C): 40 70 - pressure (pascals): 6.10⁵ 6.10⁵ - air flow (1): 1.8 1.8 - initial water content of the charge: 150 815 - water content of effluents (ppm): 150 900 - hourly space velocity (vvh): 0.8 5 - initial hydration rate (% by weight of): 5 11 - final hydration rate (% by weight of): 11 6 - cycle time (days): 28 > to 300

First, on batch L₂ with charge C₂, a test is carried out under the conditions described in T₅: it can be seen that under temperature conditions little different from those practiced in the prior art, the amount of reaction water due to the relatively large quantity of mercaptans in gasoline C₂ requires to stop the test after 28 days.

The reaction conditions are therefore modified to use those described in T₆, and it can be seen that, as with kerosene feedstocks, the process according to the invention makes it possible to keep the mercaptan content constantly below 10 ppm, by an action on the solvent power of the load as a function of the temperature, similar to the T4 test described previously.

Claims (12)

1.- Process for softening an oil cut in the presence of an oxidizing agent, by catalytic oxidation of the mercaptans it contains, in the presence of a support in a fixed bed impregnated with a metal chelate and in the absence of an aqueous base, this process being characterized in that the water content of the support is maintained within a range of predetermined values by action on the solvent power of the filler with respect to the water of the support, depending on the temperature and in that the temperature of the feed is fixed at a value sufficient to dissolve the reaction water resulting from the transformation of the mercaptans into disulfides. 2.- Method according to claim 1, characterized in that the heat necessary to bring the charge to the desired temperature to dissolve the reaction water is provided to it by heat exchange upstream of the reaction zone. 3. - Method according to one of claims 1 and 2, characterized in that the temperature of the load is chosen so as to maintain the water content of the support between 0.1 and 50% by weight of the support and, preferably , between 1 and 25% by weight thereof. 4.- Method according to one of claims 1 to 3, characterized in that the temperature of the load to be treated is maintained above 30 ° C and, preferably, between 40 and 140 ° C. 5.- Method according to one of claims 1 to 4, characterized in that the temperature of the charge is fixed at a determined value, higher than that which is necessary to dissolve the reaction water, and in that the the water content of the support is maintained within a range of predetermined values, by injecting water, continuously or not, into the load. 6.- Method according to one of claims 1 to 5, applied to the softening of a load containing between 20 and 300 ppm of mercaptans, characterized in that the temperature of the load to be treated is maintained higher than 30 ° C and preferably between 40 and 120 ° C. 7.- Method according to one of claims 1 to 5, applied to the softening of a load containing between 300 and 3000 ppm of mercaptans, characterized in that the temperature of the c \ iarge to be treated is maintained above 40 ° C and preferably between 50 and 140 ° C. 8.- Method according to one of claims 1 to 7, characterized in that the hourly space velocity of the charfe is between 1 and 10 v.v.h. (volume of charge per volume of catalyst and per hour). 9.- Method according to one of claims 1 to 8, characterized in that the regulation of the temperature of the load to be treated is controlled by a system for measuring the difference in the water content of the load upstream and in downstream of the catalytic oxidation phase. 10.- Method according to one of claims 1 to 8, characterized in that the regulation of the temperature of the load to be treated is controlled by a means of measuring the water content of the support in fixed bed. 11.- Method according to one of claims 1 to 10, characterized in that the support in a fixed bed is chosen from the group consisting of active carbon, aluminas, clays, aluminosilicates, silicates and their mixtures. 12.- Method according to one of claims 1 to 11, characterized in that the metal chelate is chosen from the group consisting of phthalocyanines, porphyrins and metal corrines.

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