EP0201955B1 - Treatment of a fuel of a mixture of hydrocarbons and alcohols, and product obtained therefrom by selective adsorption of water - Google Patents

Description

The invention relates to a method of treating a fuel composed of a mixture of hydrocarbon (s) and alcohol (s) having an alcoholic content by volume of less than 10%; it extends to a product for selective adsorption of water in the presence of polar compounds.

Alcohols, and in particular methanol and ethanol, are alternative fuels which, added in small percentage to hydrocarbons, have the advantage of providing a fuel mixture requiring no modification or specific adjustment of traditional petrol engines. However, it has been observed that these mixtures are very sensitive to the presence of traces of water which cause a phenomenon of demixing leading to separation of the liquid into two phases of different densities: an upper phase containing the majority of the hydrocarbons and a phase polar lower rich in alcohol; this demixing phenomenon is all the more marked when the temperature of the mixture is low. However, in practice, it is impossible to rigorously avoid the presence of water in this type of mixture because of the inevitable contact of the latter with more or less humid atmospheres during storage, transport and distribution. For example, a content of 500 p.p.m. of water is sufficient to produce the demixing at 11 ° C of a mixture composed of 5% methanol and 95% of premium fuel.

US Pat. No. 4,279,620 describes a process for stabilizing a premium fuel / ethanol mixture, which consists in drying it by contact with a saturated solution of Ca C1 ₂ and with solid Ca CI ₂ . The implementation of such a two-step process is relatively delicate; moreover, the calcium chloride which becomes deliquescent after hydration is the source of serious difficulties in the case of a continuous industrial implementation in a fixed bed.

On an industrial scale, the water adsorbents commonly used in a fixed bed are essentially aluminas, molecular sieves and silica gel. However, the regeneration of the first two adsorbents takes place at high temperatures (250 ° to 300 ° C) and leads to significant energy consumption, so that their use would make the overall energy balance of alcohol fuels unfavorable. Silica gel can be regenerated at a more moderate temperature but is mechanically fragile, which makes it difficult to use.

In addition, the inventors were able to observe in the laboratory that the kinetics of water adsorption of these adsorbents is slow in the hydrocarbon / alcohol medium, which makes them difficult to use industrially continuously in a fixed bed.

The present invention proposes to provide a new process for the treatment of fuels containing alcohols in order to prevent their demixing and to stabilize their homogeneity.

An essential objective of the invention is to provide an economical process from an energy point of view.

Another objective is to ensure rapid treatment making it possible to treat large quantities of fuels with moderate quantities of adsorbents.

Another objective is to authorize continuous industrial implementation.

Furthermore, the invention also aims to provide a new adsorption product having a preferential affinity with respect to water, which makes it possible to carry out drying in the presence of polar compounds.

The treatment process targeted by the invention applies to fuels composed of a mixture of hydrocarbon (s) and alcohol (s) having an alcohol content by volume of less than 10%; this process consists in bringing the mixture into the presence of at least one ion-exchange resin, cationic, capable of dissociating strongly in ionic form in an aqueous medium, so as to adsorb part of the water dissolved in said mixture in order to to limit its water content to a value below the limit threshold for demixing of said mixture at the minimum temperature for use.

Experiments have shown that the ion-exchange resins mentioned above were capable of selectively fixing the water dissolved in the hydrocarbon / alcohol mixture, with good efficiency, making it easy to lower the water content of the fuel, below the demixing limit threshold (even in the case of a low value threshold corresponding to low operating temperatures expected for the fuel). These resins can be regenerated at low temperature (of the order of 120 ° C.) so that the process of the invention uses low levels of calories.

It should be noted that ion exchange resins have so far been used on an industrial scale for the exclusive purpose of demineralization or softening by ion exchange; however, certain scientific publications mention the adsorbent properties of these resins with regard to various compounds and in particular water (CE WYMORE, “Sulfonic-type cation-exchange resins as desiccants” Ind. Eng. Chem. Prod. Res. .Development 1962, Vol. 1 n ° 3, p. 173 to 178; JA BOHORQUEZ et al, "Application of strong cationic resins to the drying of organic solvents", Bull. Soc. Chim. De France, 1982, n ° 5 -6 Part I, p. 193 to 196 and p. 197 to 201). However, it is well known to scientists specializing in this type of resins that their field of effectiveness is limited to apolar organic media.

• - p. 173 : « les matériaux très polaires tels que les alcools inférieurs sont difficiles à sécher »,
• - p. 175 « avec l'éthanol, la performance des résines est en rapport avec leur cinétique relative d'adsorption de l'eau, car l'éthanol est difficile à sécher, et le facteur cinétique devient plus important dans la performance globale de la résine »... « L'alcool qui a envahi la résine apparaît entrer en compétition avec l'eau vis-à-vis des ions hydrogènes ».

Thus, the tests carried out in polar organic medium, in particular in alcoholic medium, have shown that these resins have only a very low selectivity with respect to water: they fix both the alcohol molecules and the water molecules and are unable to remove traces of water dissolved in an alcohol, because they are very quickly saturated by the alcohol molecules in the medium, which bind on them. Thus in the first publication mentioned above, it is mentioned (translation):

• - p. 173: "very polar materials such as lower alcohols are difficult to dry",
• - p. 175 "with ethanol, the performance of resins is related to their relative kinetics of water adsorption, since ethanol is difficult to dry, and the kinetic factor becomes more important in the overall performance of the resin" ... "The alcohol which has invaded the resin appears to compete with water against hydrogen ions".

• - p. 195 : « Malgré une mise en contact de plus de 24 heures et l'emploi de quantités de résine sèche notablement supérieures à celles utilisées avec le benzène, nous n'avons constaté aucune adsorption sélective de l'eau contenue dans ces solvants. On peut penser qu'il y a, en ce qui concerne la fixation des molécules d'eau, compétition entre la résine et le solvant ; de plus, étant donné le caractère polaire des molécules de solvant, c'est l'ensemble solvant-eau dissoute qui pénètre dans les pores de la résine et provoque son gonflement. De ce fait la quantité d'eau fixée par la résine est très faible. Par conséquent, nous avons admis que les résines échangeuses d'ions de type cationique forte ne sont pas efficaces pour le séchage des liquides polaires ».

Similarly, in the second publication mentioned above, it is indicated with regard to polar solvents and in particular ethanol:

• - p. 195: “Despite having been in contact for more than 24 hours and the use of quantities of dry resin considerably greater than those used with benzene, we have not found any selective adsorption of the water contained in these solvents. One can think that there is, as regards the fixing of water molecules, competition between the resin and the solvent; moreover, given the polar nature of the solvent molecules, it is the solvent-dissolved water unit which penetrates into the pores of the resin and causes it to swell. Therefore the amount of water fixed by the resin is very low. Therefore, we have admitted that ion exchange resins of strong cationic type are not effective for the drying of polar liquids ”.

Thus, the prior art teaches a person skilled in the art that the targeted resins are incapable of selectively fixing water and would preferentially fix alcohol in the application concerned, taking into account the high percentage of alcohol relative to water. eliminate that the hydrocarbon / alcohol mixtures contain. The inventors rejected this prejudice and demonstrated experimentally that the targeted resins had, in a mixed hydrocarbon / alcohol medium, a selective power for adsorption of water, making it possible to eliminate most of the water initially present; this unexpected result is currently difficult to explain. Tests have shown that this selective water adsorption remains effective for fuels containing an alcohol content of less than about 10%; this area of efficiency covers the legal area of composition of hydrocarbon / alcohol fuel mixtures (decree of October 9, 1983 in the Official Journal FR).

According to a preferred embodiment, a cationic resin (s) is used, packaged in the form of alkaline or alkaline-earth salts. This type of resin has the advantage of not undergoing any degradation and therefore no loss of adsorption capacity, during the regeneration phases.

In addition, the said resins are advantageously packaged in the form of potassium or in the form of magnesium; it is possible to use both ionic forms of resins at the same time.

The resin packaged in potassium form has the advantage of having a very rapid adsorption kinetics and is therefore particularly well suited to continuous processing, the fuel being brought through a fixed bed of resin. The resin packaged in the form of magnesium has a much slower kinetics but a very high adsorption capacity (of the order of 5 times greater than the first); Consequently, this resin is more particularly suitable for batch processing in which it remains in situ in the fuel for long periods. The combination of the two resins and of the two implementations can make it possible, in certain applications, to cope, at the same time, with a rapid increase in the water content of the fuel (requiring rapid trapping), and with a slow evolution of this content (generally requiring the fixing of large quantities of water)

Experiments seem to show that strong cationic sulfonic resins are preferable; however other resins also give good results and in particular weak cationic carboxylic resins.

The process of the invention can in particular be applied to hydrocarbon (s) / methanol, or hydrocarbon (s) / ethanol mixtures, optionally containing a third solvent consisting of an alcohol of higher molecular weight, in particular tert-butanol; the hydrocarbon can as well consist of a super-fuel as an ordinary fuel.

The comparative examples provided below relate to a fuel of known formulation (usually designated by “M3B2”), containing by volume (to within approximately 1%) 95% of super-fuel, 3% of methanol and 2% of tert-butanoi.

The invention extends, as such, to a product for the selective adsorption of water in the presence of polar compounds, comprising at least one cationic resin, capable of dissociating strongly in ionic form in aqueous and conditioned medium (s ) so as to contain on its ionic sites, either K┬ counterions, or Mg┬┬ counterions, or a coupling of two K┬, Mg┬┬ counterions.

The resin or resins of said adsorption product are in particular constituted by strong cationic sulfonic resins or weak cationic carboxylic resins.

Example 1 - Treatment in a fixed bed

In a column with an internal diameter D = 1.5 cm, the dry adsorbent is introduced over a height of ₁₀ cm. The fuel M3B2 is brought to pass through this bed, from top to bottom with a flow rate of 0.42 l / h (passage speed: 0.066 cm / s); the initial water content of this fuel is in the example of 720 mg / l. Content water is measured at the end of the bed.

• - adsorbant constitué par de l'alumine activé type « Gamma » (granulométrie : 2 à 5 mm),
• - adsorbant constitué par du silicagel (granulométrie : 3 à 6 mm),
• - adsorbant constitué par un tamis moléculaire 3.10^-10 m (extrudé 1,6 mm),
• - adsorbant conforme à l'invention.

The experiment is carried out in the following four cases:

• - adsorbent consisting of activated alumina type "Gamma" (particle size: 2 to 5 mm),
• - adsorbent consisting of silica gel (particle size: 3 to 6 mm),
• - adsorbent consisting of a 3.10 ^-10 m molecular sieve (1.6 mm extruded),
• - adsorbent according to the invention.

The adsorbent according to the invention is, in this example, a cationic sulfonic resin packaged in potassium form, having a structure composed of the styrene-divinylbenzene copolymer, type "X8 (bridging rate: 8% of divinylbenzene); the particle size of this resin is between 50 and 100 mesh (resin "DOWEX 50 W manufactured by the company Dow Chemical).

The curves A, B, C, D of FIG. 1 illustrate the results obtained respectively for these four adsorbents (on the abscissa is the amount of cumulative fuel treated, and on the ordinate, the water content of the fuel at the outlet).

It can be seen that the adsorbent targeted by the invention proves by far the most effective and makes it possible to fix considerably greater amounts of water than the others and, therefore, to treat larger volumes of fuel for the same volume. adsorbent involved.

Example 2 - Batch processing

In a series of bottles, there is a volume of 250 cm ³ of M3B2 fuel having a water content of 650 mg / 1. An increasing quantity of dry adsorbent is added to each of these flasks; each bottle is sealed and shaken until the liquid / solid balance is obtained. The residual water concentration of each of the flasks is then measured by the "Karl Fischer _"method; the points thus defined make it possible to draw the isotherm of the adsorbent concerned.

• - tamis moléculaire 3.10^-10 m, identique au précédent,
• - adsorbant conforme à l'invention.

This experiment is carried out for the following two adsorbents:

• - molecular sieve 3.10 ^-10 m, identical to the previous one,
• - adsorbent according to the invention.

The adsorbent according to the invention is, in this example, a cationic sulfonic resin packaged in magnesium form, having the same supporting structure as above.

The curves E and F of FIG. 2 correspond respectively to the isotherms of these two adsorbents (on the abscissa is plotted in mg / 1 the water content of the fuel at equilibrium, and on the ordinate, in mg of water per g. D dry adsorbent, the water content of the adsorbent at equilibrium).

The resin according to the invention has an adsorption capacity equivalent to, or even greater than, that of the molecular sieve which is considered to be remarkable.

The essential advantage of the resin lies, on the one hand, in its much lower price, on the other hand, in its ease of regeneration which operates from low-level calories (120 to 140 ° C), while the regeneration of the molecular sieve requires temperatures of the order of 250 ° C to 300 ° C.

From the isotherm F relating to the adsorbent according to the invention, it is possible in each case to deduce the amount of resin to be used.

For example, for an M3B2 fuel intended to be used at a minimum temperature of -24 ° C., the demixing threshold is of the order of 800 p.p.m. of water by weight.

If we consider a storage tank of 50,000 liters of M3B2 having a water content of 1,500 ppm by weight, it is necessary to have approximately 360 kg of the abovementioned resin to reach at equilibrium a content equal to 400 ppm (safety factor equal to 2).

If we limit ourselves to an equilibrium content of 800 p.p.m., the quantity of resin to be used is only about 120 kg.

By deducing from the experimental data an analytical expression of the isotherm, the mass M (in kg) of resin to be used to lower the content of 1 m ³ of mixture M3B2, from the Co value to the C value (in mg / 1), is given by the following relation:

Claims (13)

1. Process for the treatment of fuel composed of a mixture of hydrocarbon(s) and alcohol(s) with a volume content of alcohol amounting to less than 10 % for stabilising the homogeneity of said mixture at its minimum temperature of utilisation, characterised in that it consists in bringing the mixture together with at least one cationic ion exchange resin capable of intensive dissociation in ionic form when in the presence of water, so as to adsorb a part of the water dissolved in said mixture with a view to limiting the water contant of the latter to a value lower than the demixing threshold of said mixture at the minimum temperature of utilisation.

2. Process for treatment according to claim 1, characterised in that the fuel is brought together with at least one cationic resin conditioned in the form of alkaline or earth alkaline salts.

3. Process for treatment according to claim 2, characterised in that the fuel is brought together with a resin conditioned in the form of potassium.

4. Process for treatment according to claim 2, characterised in that the fuel is brought together with a resin conditioned in the form of magnesium.

5. Process for treatment according to claim 2, characterised in that the fuel is brought together with two resins, one conditioned in the form of potassium, and the other in the form of magnesium.

6. Process for treatment according to claims 2, 3, 4 or 5, characterised in that use is made of one or several strong cationic sulpho-resins.

7. Process for treatment according to claims 2, 3, 4 or 5, characterised in that use is made of one or several weak cationic carboxylic resins.

8. Process for treatment according to claims 1, 2, 3, 4, 5, 6 or 7, in which the fuel is caused to pass through a fixed bed of resin(s).

9. Process according to claims 1 to 8 for the treatment of a fuel composed of a mix of hydrocarbon(s) and methanol.

10. Process according to claims 1 to 8 for the treatment of a fuel composed of a mix of hydrocarbon(s) and ethanol.

11. Process according to claims 9 or 10 for the treatment of a fuel containing, in addition, a third solvent consisting in an alcohol of higher molecular weight, in particular tertiary butanol.

12. Process according to claims 1 to 11 for the treatment of a fuel consisting of a mixture of knock-resistant fuel and alcohol(s).

13. Process according to claims 9, 11 and 12 jointly for the treatment of a fuel consisting in terms of volume (with an accuracy of about 1 %) of 95 % knock-resistant fuel, 3 % methanol and 2 % tertiary butanol.

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