EP3417035B1 - Method for desulfurizing a hydrocarbon mixture - Google Patents

Description

The invention relates to a process for the desulfurization of a hydrocarbon mixture containing organic sulfur compounds according to the preamble of claim 1.

The desulphurization of hydrocarbon mixtures plays an important role in technology primarily because sulfur is a so-called "catalyst poison" and the exceeding of a certain limit value by the sulfur content renders a hydrocarbon mixture unusable for a large number of possible uses. In addition, there are ever stricter legal regulations, which provide maximum values for the sulfur content even in applications in which the hydrocarbons are regularly only burned, in particular to reduce the content of sulfur compounds in the resulting exhaust gases.

According to the state of the art, a large number of desulfurization processes are used. These processes are often carried out as part of the refining of mineral oil, with correspondingly large technical standards being achieved. So-called hydrogenating desulfurization processes are primarily used. In these, the sulfur is released from the organic sulfur compounds in a chemical reaction and combined with hydrogen to form hydrogen sulfide. The hydrogen sulfide is separated from the hydrocarbon mixture and processed regularly in a Claus process.

A disadvantage of this process, however, is that as the sulfur content decreases, the hydrogen requirement for removing the sulfur increases disproportionately to the amount of sulfur removed. This is because some sulfur compounds are less reactive than others in terms of the chemical reactions that play a role in hydrodesulfurization. At the same time, however, other side reactions occur in which hydrogen is consumed. Examples can be the conversion of unsaturated to saturated hydrocarbons or the splitting of hydrocarbons to form hydrocarbons with a higher hydrogen / carbon ratio. Accordingly, the cost of hydrodesulfurization trying to keep the lowest possible To reach residual sulfur levels in the hydrocarbon mixture disproportionately.

With the increasing requirements for low sulfur contents, the interest in alternative desulfurization processes is increasing.

It is known from basic research that there are some desulfurization options for hydrocarbon mixtures that can be implemented at least on a laboratory scale. For example, the generic script reveals GB 759283 A a process in which a sodium dispersion is used to desulfurize a hydrocarbon mixture in an autoclave. It has been shown that it is possible in this way to remove sulfur from the hydrocarbon mixture, but the residual sulfur contents achieved are still above the sulfur content which is regularly required today. Although such attempts to desulfurize hydrocarbon mixtures with the aid of sodium dispersions have been known for many decades, it has never been possible to develop a process which can be meaningfully implemented on an industrial scale on the basis of corresponding attempts.

From Scripture US 7,527,724 B1 A desulfurization process is known in which the sulfur in hydrocarbon mixtures reacts in a tubular reactor with a solution which contains sodium atoms dissolved in the solution. The reaction of sodium with sulfur should take place in a temperature window between - 50 ° C to + 50 ° C.

The invention is therefore based on the object of demonstrating a desulfurization process of the type mentioned at the outset which can be sensibly implemented on an industrial scale.

The object is achieved by a generic method for the desulfurization of a hydrocarbon mixture containing organic sulfur compounds with the characterizing features of claim 1.

According to the invention, the object is achieved in that the reactor in which the reaction takes place is a turbulent-flow reactor, preferably a tubular reactor. Alternatively, a plug reactor with an internal oscillating is preferred driven transport screw are used, in which it is ensured by appropriate technical devices that a turbulent flow arises and the design ensures a sufficient residence time of the hydrocarbon mixture to be desulfurized. It has been shown that the reaction parameters can be set in a reactor, preferably a tubular reactor or a plug reactor, with turbulent flow such that an extremely low residual sulfur content can be achieved when a mixture of a sodium dispersion and a hydrocarbon mixture is passed through this reactor. The term "residual sulfur content" refers to the sulfur which is still present in the form of organic sulfur compounds after the reaction has been carried out. It is understood that for the final removal of the sulfur by sodium, in particular inorganic, preferably in the form of Na2S, bound sulfur still has to be separated from the mixture with the sodium. A number of separation processes known from the prior art are available for this.

Due to the turbulent flow in the reactor, preferably in the tubular reactor or in the plug reactor, sufficient mixing of the reaction mixture can be generated in order to be able to carry out the reaction until low residual sulfur contents are reached, in order to ensure economical desulfurization. In this context, it is particularly advantageous if the tubular reactor has a corresponding length for this. This is preferably at least 100 m, more preferably at least 200 m. It has been shown that with these very long pipe lengths, suitable flow conditions can be realized with a corresponding sufficient dwell time in order to successfully carry out the process on an industrial scale. In this context, it should be mentioned that the tube of the tubular reactor does not have to be straight, it can rather have a meandering, coiled or similar course, in particular in order to enable a space-saving construction of the reactor. The reaction tube of the tube reactor can of course also be composed of a plurality of tubes. The length of the tube reactor therefore means the effective length of the flow path through the tube reactor, which in the present case the medium flowing through the mixture of sodium dispersion and hydrocarbon mixture under reaction conditions. Appropriate technical devices such as the oscillating drive, the turbulent flow and the dwell time are used in the plug reactor guaranteed, and can be regulated if necessary by the speed control of the screw conveyor and / or the duration of the oscillations.
The reactor can advantageously have internals to promote mixing. Such internals are helpful in order to ensure turbulent flow and the associated mixing, which promotes the reaction rate. For this purpose, the reactor can particularly preferably have so-called static mixers, that is to say immovable internals which, due to their geometry, influence the flow accordingly.

It has been found to be advantageous if the temperature in the reactor is at least 250 ° C. It is also advantageous if the temperature in the reactor does not exceed 310 ° C. It has been shown that there are particularly favorable conditions for the desired chemical reactions in this temperature range.

Furthermore, it has been shown that it is beneficial for the reaction if the volume-equivalent spherical diameter of at least 80%, preferably at least 90% of the sodium particles in the dispersion is at most 25 μm, preferably at most 12 μm and particularly preferably at most 5 μm. It has been shown that a sodium dispersion which is both very fine and has the narrowest possible spectrum of particle distribution is particularly advantageous with regard to the reaction rate. The volume-equivalent spherical diameter of a sodium particle is the diameter that a spherical sodium particle of the same volume would have. Due to the low melting point of sodium, this is regularly in liquid form under reaction conditions, as a result of which the sodium particles actually form a spherical shape. Therefore, in the case of sodium, the volume-equivalent sphere diameter is ideal for clearly characterizing the particle size in the suspension. It is also particularly advantageous if at least 90%, preferably at least 95%, of the sodium particles have a volume-equivalent spherical diameter that is less than 5 μm.

In the desulphurization process, preference is given to at least 0.05% by weight of sodium, particularly preferably at least 0.1% by weight of sodium and particularly preferably at least 1.5% by weight of sodium, based on the total weight of the hydrocarbon mixture to be desulfurized, used.

The production of the sodium dispersion according to the rotor-stator principle has proven to be particularly advantageous with regard to the method according to the invention. Here, dispersing devices are used in which the dispersion is generated by a relative movement between a rotor and a stator at a high peripheral speed. It has been found that sodium dispersions produced by such a dispersion process in particular have very narrow particle size distributions and are particularly suitable for the process according to the invention.

It has turned out to be particularly advantageous if the sodium for the preparation of the sodium dispersion is dispersed in an oil, which can be a paraffinic white oil in a particularly advantageous manner. It has been shown that dispersions of sodium in such liquid phases are particularly advantageous for the process according to the invention.

Particularly high levels of desulfurization can be achieved in particular if the proportion of sodium in the dispersion is 1 to 40% by weight, preferably 10 to 33% by weight, based on the total weight of the dispersion. It is particularly advantageous for desulfurization if 1 to 40% by weight of sodium is dispersed in oil and preferably at least 80% of the sodium particles have a volume-equivalent spherical diameter which is less than 25 μm. It is further preferred if 10 to 33% by weight of sodium is dispersed in white oil and preferably at least 90% of the sodium particles have a volume-equivalent spherical diameter which is less than 12 μm, preferably less than 5 μm. The best results for desulfurization can be achieved under the aforementioned conditions.

The liquid phase which is used for dispersing the sodium preferably has a viscosity of at least 4 mm ² / s, particularly preferably at least 12 mm ² / s, and / or at most 20 mm ² / s, particularly preferably at most 17 mm ² / s s. The density of the liquid phase used for the dispersion is preferably at least 0.84 kg / l and or at most 0.89 kg / l. Furthermore, it has proven to be advantageous if the liquid phase used to disperse the sodium has a flash point of at least 150 ° C., preferably at least 200 ° C.

With regard to the present process, it is advantageous if the hydrocarbon mixture to be desulfurized is first treated with a further desulfurization process for the pre-desulfurization. Desulphurization then takes place later by treatment with a desulphurization process according to the invention.

This procedure has the advantage that the advantages of conventional desulfurization processes, in particular hydrogenating desulfurization processes, can be combined with the advantages of desulfurization with a sodium dispersion. It makes sense to first remove that part of the sulfur that can still be removed comparatively easily with a hydrogenating desulfurization process. This will remove most of the sulfur. The process based on the sodium dispersion is then used to further reduce the residual sulfur content. This removes the part of the sulfur that is very difficult to remove with the hydrodesulfurization process, although with such a procedure only a very small part of the total sulfur is removed by means of the sodium dispersion, the overall efficiency of the combined process is significantly increased, since desulfurization using sodium dispersion is used exactly where it is economically superior to hydrodesulfurization, namely when very low residual sulfur levels are reached. At the same time, the advantages of the hydrodesulfurization process can be used to remove comparatively large amounts of sulfur up to a moderate residual sulfur content.

In this context it is of course possible that the desulfurization by means of the sodium dispersion does not have to directly and directly follow the preferably hydrodesulfurization. It is entirely possible in the meantime to subject the resulting hydrocarbon mixture to further process steps, to separate parts of the hydrocarbon mixture or to mix the hydrocarbon mixture with other substances, in particular other hydrocarbon mixtures. This also applies accordingly to the sequence between the preparation of the dispersion and the addition of the sodium dispersion to the hydrocarbon mixture to be desulfurized. In particular, the process variants described above for producing the sodium dispersion have the advantage that the sodium dispersion is correspondingly stable, ie it can easily stored or transported between production sites before it is added to the hydrocarbon mixture.

Another particularly advantageous method provides that the hydrocarbon mixture to be desulfurized is a fuel or a fraction of a hydrocarbon mixture intended for further processing into a fuel. Within refineries in particular, mineral oil fractions for specific later uses - for example the production of diesel fuels - are separated from each other at an early stage and processed further using different process routes. In particular, the fractions used for the use of diesel and / or petrol have a certain proportion of low boilers. In view of this, it is advantageous if the pressure in the reactor is at least 6 bar, preferably at least 8 bar. In this way, a reliable conversion can also be carried out, in particular, of low-boiling constituents in the liquid phase.
Another advantageous process variant, on the other hand, provides that the pressure in the reactor is at most 3 bar, preferably at most 1.5 bar. This process variant is particularly advantageous when the proportion of low boilers in the hydrocarbon mixture is low. Such operation is particularly advantageous when comparatively small and compact systems can be used. Due to the low pressure, the material stresses decrease, in particular the comparatively long tubular reactors can be designed much thinner, which has an extremely positive effect with regard to the size and weight of the plant.

The process according to the invention can be used particularly advantageously, in particular in comparatively compact plants, for the desulfurization of hydrocarbon mixtures which originate from a liquefaction process for the recovery of liquid hydrocarbons from solids. Such methods are used in particular when liquid hydrocarbon mixtures for use as fuels are to be obtained from waste. Low-boiling components often play a subordinate role in these mixtures, whereas the possibility of constructing inexpensive and / or compact plants often represents a considerable economic advantage.

The process according to the invention can likewise be used particularly advantageously for the desulfurization of hydrocarbon mixtures which originate from a process for the preparation of so-called slop oil. Slop oil is a contaminated mixture containing mineral oil, which occurs, for example, and especially when rinsing tanks on ships. These can be, for example, the tanks of mineral oil tankers, but also fuel tanks of ships operated in particular with heavy oil. Cooling circuits and the like on ships of this type, in particular in the area of ship engines, are regularly not completely sealed, so that sea water and other contaminants, including marine life and the like, penetrate into the tanks in question and in the tanks to form the so-called slop oil as residue that is difficult to recover. In particular, the ingredients mineral oil, water and sand in the slop oil form a difficult-to-separate and previously difficult-to-use mixture. It has now been shown that slop oil and / or products which are obtained from a process for the preparation of slop oil, in particular a process in which the first constituents of the slop oil are already separated off, can be treated particularly well using the desulfurization process according to the invention.
The invention is described below with reference to Fig. 1 exemplified schematically.

A hydrocarbon mixture 1 is provided, which originates, for example, from a liquefaction process for the extraction of liquid hydrocarbons or a processing process of slop oil (or a mixture thereof). Furthermore, sodium 2 is dispersed with an oil 3 in the example shown in a process step S1. Dispersion based on the rotor-stator principle is preferably used for this. A paraffinic white oil is preferably used as the oil 1. In a further step S2, the sodium dispersion that was produced in step S1 and the hydrocarbon mixture 1 are mixed. The reaction step S3 is preferably carried out in a tubular reactor with a turbulent flow, the tubular reactor preferably having a length of at least 200 m. In order to enable a compact construction, the reactor can, for example, be designed in a meandering shape. Alternatively, the use of an oscillating plug reactor with an internal transport screw is advantageous. At an advantageous reaction temperature between 280 ° C. and 310 ° C., the process according to the invention occurs in process step S3 underlying chemical reactions, in which the sulfur is dissolved out of the organic sulfur compounds and reacts to inorganic sulfur compounds, in particular to Na2S.

In a further process step S4, the inorganic sulfur-containing constituents, in particular Na2S 5, formed from the desulfurized hydrocarbon product 4 are separated off.

Claims (14)

1. Method for desulphurising a hydrocarbon mixture containing organic sulphur compounds, comprising the following steps:

   a) producing a sodium dispersion,

   b) adding the sodium dispersion to the hydrocarbon mixture to be desulphurised,

   characterized in that the mixture of sodium dispersion and hydrocarbon mixture is passed through a reactor, wherein the reaction conditions, in particular pressure and temperature, are selected such as to bring about a reaction of the sodium with the organic sulphur compounds, during which sulphur atoms are leached out of the organic sulphur compounds and bond with the sodium, the reactor is a turbulent flow reactor, preferably a tubular reactor or an oscillating plug flow reactor having an internal screw conveyor, and the temperature in the reactor is at least 250°C and at most 310°C.
2. Method according to claim 1, characterized in that the reactor is a tubular reactor and preferably has a length of at least 100 m, more preferably at least 200 m.
3. Method according to claim 1 or 2, characterized in that the reactor has internals for aiding the mixing, in particular static mixers.
4. Method according to any one of the preceding claims, characterized in that the volume equivalent spherical diameter of 90% of the sodium particles in the sodium dispersion, in particular of 95% of the sodium particles in the sodium dispersion, is less than 25 µm, preferably less than 5 µm.
5. Method according to any one of the preceding claims, characterized in that the sodium dispersion is produced according to the rotor-stator principle.
6. Method according to any one of the preceding claims, characterized in that the sodium for producing the sodium dispersion is dispersed in an oil as liquid phase, in particular in a paraffinic white oil.
7. Method according to any one of the preceding claims, characterized in that the liquid phase used for dispersing has a viscosity of at least 4 mm²/s, preferably at least 12 mm²/s, and/or at most 20 mm²/s, preferably at most 17 mm²/s.
8. Method according to any one of the preceding claims, characterized in that the liquid phase used for dispersing has a density of at least 0.84 kg/l and/or at most 0.89 kg/1.
9. Method according to any one of the preceding claims, characterized in that the liquid phase used for dispersing has a flash point of at least 150°C, preferably at least 200°C.
10. Method according to any one of the preceding claims, characterized in that the hydrocarbon mixture to be desulphurised is treated first by another, preferably hydrogenating, desulphurisation method for the purpose of pre-desulphurisation and subsequently by a desulphurisation method according to any one of the preceding claims.
11. Method according to any one of the preceding claims, characterized in that the hydrocarbon mixture to be desulphurised originates from a liquefaction process for obtaining liquid hydrocarbons from solids, in particular from waste, and/or is slop oil, and/or originates from a process for treating slop oil, in particular from cleaning processes for ships' tanks.
12. Method according to any one of the preceding claims, characterized in that at least 0.05 wt% sodium, preferably at least 0.1 wt% sodium and particularly preferably at least 1.5 wt% sodium are used, based on the total weight of the hydrocarbon mixture to be desulphurised.
13. Method according to any one of the preceding claims, characterized in that the pressure in the reactor is at most 3 bar, preferably at most 1.5 bar.
14. Method according to any one of claims 1 to 11, characterized in that the hydrocarbon mixture to be desulphurised is a fuel or a fraction of a hydrocarbon mixture, in particular of a mineral oil, that is intended to be further processed to form a fuel, and the pressure in the reactor is at least 6 bar, preferably at least 8 bar.

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