5 The present invention relates to a method for de¬ sulfurization of liquid fuels and petrochemical feedstocks.

All crude petroleum fractions contain sulfur both bound 10 in the carbon chain and as various mercaptans, sulfides, disulfides, S-containing heterocycles (thiophenes) etc. Sulfur also occurs bound in the aromatic rings of hydrocarbon compounds.

15 With the increasing need for environmental protection the sulfur content of fuels has become a problem of major importance. This applies to both the fuels used in energy generation and those consumed in transportation and railroad traffic. Today an increasing number of power

20 plants are desiredly provided with sulfur removal from stack gases, which conventionally takes place through scrubbing of the stack gases with alkalies, generally calcium hydroxide.

25 With the increasing demands for environmental protection, exhaust systems of vehicles are equipped with catalytic conversion systems in which carbon monoxide and nitrogen oxides are converted into carbon dioxide and gaseous nitrogen. The same catalyst converts, however, sulfur

30 contained in the fuel from the sulfur oxide form partially into hydrogen sulfide, which is a toxic gas. f For this reason, requirements concerning the maximum ι allowable sulfur content of fuels used in combustion engines will be tightened in the future.

35

Requirements on sulfur content limits in fuels used in diesel engines will be tightened particularly for ships

employed in seas such as the Baltic Sea, the coastal waters of Europe as well as the Mediterranean Sea.

Mercaptans contained in hydrocarbon feedstock and products are easily removed, since they react with alkalies and are extracted in the water. Methods for removing mercaptans are discussed in, e.g., the U.S. Patent Specification No. 4,705,620. Known in the art are also methods in which the light hydrocarbon fractions are washed with a strong alkali to remove simple sulfur compounds such as sulfuric acid carried over from the pretreatment and hydrogen sulfide residues (U.S. Patent Specification No. 3,438,889) . By the same token, undesirable organic acid residues are removed from the fuel.

Sulfur compounds bound in the actual hydrocarbon chain cannot, however, be removed by these methods, because extraction with alkalis is not capable of breaking a strong bond such as the C-S-C bond. Practical experience shows that the most difficult sulfur atoms to remove from hydrocarbon products are those bound with the aromatic rings and long aliphatic chains.

Besides alkaline extraction, several alternative and complementing methods have been developed for direct desulfurization of petrochemical products without making a distinction between the fuel or non-fuel use of the products.

The most common of such methods is pressurized hydro- genation, known as "hydro-treatment" or "hydro- desulfurization" (HDS) , in which the fuel is hydrogenated with gaseous hydrogen at elevated pressure and in the presence of different catalysts so as to reduce sulfur compounds into hydrogen sulfide, which is further converted into elemental sulfur in the well-known Claus

reaction. The hydrogen required for the reaction is typically produced by electrolytic dissociation of water or, as far as refineries are concerned, in catalytic reforming units, which convert paraffins to aromatics.

A typical desulfurization process based on a conventional HDS process operates at 40 to 100 bar and 350 to 450 °C. Such desulfurization processes can cut the sulfur content from, e.g., 1.9 % to 50 ppm or lower, the reduction of sulfur content being better than 90 %. The yield of desulfurized product (within the same viscosity range) is typically, e.g., 92 %. For the current level of technology, reference is made to the U.S. Patent No. 3,806,444.

As complementary information related to the above methods, the U.S. Patent Specification No. 3,984,305 mentions that the residence time in a catalytic reactor operated at 50 to 200 atm can be 0.5 to 10 hours.

The conventional processes are, however, hampered by several disadvantages. Investments in, e.g., hydrogena- tion methods become expensive due to the high pressures and easily contaminating catalysts used. Such limitations dictate the use of large production units to attain sufficient productivity. The practical size of a profit¬ able unit at current price level is in the order of over a million tons of treated hydrocarbon product per year.

Furthermore, the operating conditions for HDS have to be adjusted to the reactivities of the S-compounds in each of the cuts obtained from the crude distillation.

All above-mentioned processes use a complicated multi- phase distillation method for removing the high molecular weight components which result from polymerization in the reactor. Finally, the current methods fail to completely

desulfurize such heavier components, while the light dis- tillable fractions alone are essentially sulfur-free. Consequently, the actual yield of a desired component remains in the order of 81 to 83 % at best.

The prior art also mentions methods in which a petroleum product is treated at elevated pressure and temperature with potassium sulfide, whereby potassium polysulfide is formed and sulfur can in this manner be removed from the hydrocarbon. The formed polysulfide can be regenerated into potassium sulfide and elemental sulfur (U.S. Patent No. 4,160,721 and U.S. Patent No. 4,210,526) . In these processes the potassium sulfide is kept in molten state at approx. 120 to 325 °C. The sulfur content can by these methods be reduced down to approx. 0.25 % from an initial sulfur content of 10 to 5 %, and moreover, final sulfur content values as low as 0.09 % have been disclosed. To achieve the latter figure, however, requires running the reactions in a hydrogen atmosphere. The reactions described in the above patents are relatively slow necessitating a reaction time up to 60 min.

Known in the art are also electrochemical and bio- electrochemical methods for desulfurization of petroleum products. Thus, Bell et al. (U.S. Patent No. 3,915,819) teach an electrolytic oil purifying method, which comprises mixing the hydrocarbon feedstock with an aqueous alkaline solution to prepare an emulsion, electrolyzing the emulsion thus obtained and, after electrolysis, separating the aqueous phase from the hydrocarbon phase and, when necessary, subjecting it to further treatment. The prior art method comprises discontinuous processing of the hydrocarbon fraction involving prolonged residence times ranging from several hours to many days. As a result of the long residence times, the electrodes will easily be contaminated, which

is evidenced by the sharp rise of voltage shown in the examples of the prior art publication.

EP Patent Specification No. 0,323,748 concerns a bio- electrochemical method according to which processing in a cathodic space, together with Desulfovibrio bacteria achieves a 39 % reduction of sulfur content in six days. This method is not, however, suitable for use on an industrial scale.

It is an object of the present invention to overcome the disadvantages related to the prior art and to provide a novel method for desulfurization of liquid fuels and petrochemical products.

The invention exploits the interesting property of sulfur as an element which distinguishes it from most other elements. Namely, both the oxidized and reduced forms of sulfur react as acids in aqueous solution. While the con- ventional processes have aimed at desulfurization of organic matter by way of reduction through hydrogenation, the present invention aims at oxidizing and reducing the sulfur. This goal is attained relatively simply through electrolytic processing as suggested by Bell et al.

The general oxidation and reduction reactions of organic S-compounds are in summary:

Oxidation: R-S-R > R-S-R > R-S-R > R-S0 ₂ H

II //W 0 0 0

Sulfide Sulfoxide Sulfone Sulfinic acid

Reduction: R-S-R > R-SH > H ₂ S

Sulfide Mercaptan

The bonding energy of carbon and sulfur is 272 kJ/mol, corresponding to 2.75 eV.

Since the dissociation energy of the C=S bond is much higher, the danger of immediate conversion of the C-S bond to a C=S bond is high. To avoid this undesirable occurrence and polymerization of the product into a larger hydrocarbon chain, said electrolytic reaction is performed in an alkaline solution, whereby the formed acid anion can immediately be extracted from both electrodes.

Sulfur and other sulfur compounds bonded to a hydrocarbon chain or aromatic ring are electrolytically oxidized and/or reduced in an aqueous emulsion containing a strong base in dissolved form, whereby the base immediately neutralizes the formed or a forming acid anion. During electrolysis, the sulfur-containing compounds concentrate into the aqueous phase and can be removed after electrolysis by separating the aqueous phase from the electrolyzed oil.

To achieve efficient electrolysis of the hydrocarbon fractions while avoiding excessive contamination of the electrodes, it is, according to the invention, essential to perform the electrolysis in a continuous manner by separately preparing the oil/water emulsion, which is then pumped through the electrolysis cell at such a rate that the emulsion does not have time to settle to any substantial extent before it has passed through the cell.

More specifically, the method in accordance with the invention is principally characterized by

- preparing the emulsion outside of the electrolysis cell, and

- continuously routing the emulsion through the electrolysis cell at such a rate that the residence time of the emulsion in the cell is shorter than the settling time of the emulsion.

An "alkaline" aqueous solution in the sense of this patent application refers to an aqueous solution or suspension of one or more hydroxides of the elements of the period 1A of the Periodic System of elements (Li, Na, K, Rb, Cs) or of the elements of the period IIA: Ca, Sr, Ba. Other basic substances, such as ammonium hydroxide, quaternary ammonium bases etc. can also be used. Particularly preferred alkalies are hydroxides of sodium, potassium, calcium and ammonium. The concentration of the sodium, potassium or ammonium hydroxide solution used is typically 1 to 50 w-%, whereas the concentration of a Ca(OH) ₂ suspension employed is 0.1 to 15 w-%.

The phase ratio of the hydrocarbon-water-emulsion (droplet to continuous phase) is about 1:6 to 1:1, preferably about 1:3. High phase ratios (about 1:1) will lead to situations close to phase inversion, which is highly undesirable, because of very long after-settling times.

As far as the sulfur-removal is concerned, the best results are obtained when the water phase is emulgated in the oil, i.e. the continuous phase of the emulsion is made up of the hydrocarbon fraction. It is, however, possible also to emulgate the hydrocarbon fraction into the water. The latter alternative will increase the conductivity of the emulsion and, thus, give rise to larger current densities during electrolysis. This will then again increase current losses.

High mixing intensity is preferred during emulsification, because it will favour sulfur reduction by decreasing the size of the droplets and thus increasing the mass transfer area.

According to the invention, it is required that the residence time of the emulsion in the cell is

substantially shorter than the emulsion's settling time. Preferably the residence time should be shorter than the time it takes for half of the emulsion to settle.

The settling time of the hydrocarbon-water-emulsion depends on the density, viscosity, composition and temperature of the hydrocarbon fraction and of the mixing intensity of the emulsification (droplet size) . Therefore, the mixing intensity and the residence times are related to each other through the settling time.

Typically, in case of heavy oil fractions, such as those described in Examples 1 and 2, the settling times are 1 to 15 minutes, normally 2 to 5 minutes, whereas lighter fractions (cf. Examples 3 and 4) will settle within about 0.5 to 2 minutes.

Preferably, the emulsion is electrolyzed at a relatively low voltage in the range from about 2.7 to 100 V, in particular from 2.7 to 20 V. The currents used range from about 1 to 200 A, preferably the current is from about 5 to 100 A, current density being preferably from about 0.02 to 0.3 A/cm ² .

After electrolysis the sulfur compounds exist as sulfides, sulfates and sulfites extracted into the alkaline aqueous phase. The concentrations of sulfates are, however, low. If a sodium salt is used as the alkali, the aqueous phase may also contain sodium thiosulfates in minor quantities. All these compounds can be oxidized into sulfites and sulfates, and finally, into sulfates only. Such reactions are preferably performed by blowing in air or oxygen into the warm aqueous phase under alkaline conditions so conveniently available in this case.

A particularly advantageous oxidization reaction of the sulfide into sulfite and sulfate is attained if a suit¬ able oxidization/reduction catalyst is available such as 1,4-naphthokinone or its sulfonic acid (Pulp & Paper Canada, j81, (1980), no. 9, 41...48). The oxidization reaction needs a residence time of 0.5 to 1.5 h and a reaction temperature of approx. 70 to 80 °C, preferably.

A preferred embodiment of the invention uses sodium hydroxide as the alkali. The Na ₂ S0 ₄ solution obtained after electrolysis and oxidization can be converted back to NaOH solution by causticization. The reaction is com¬ pleted quickly, in approx. 5 min, and the equilibrium point attained results in a causticization degree of approx. 35 to 40 %. The theoretical equilibrium point is even higher (Margulis, E.V. et al. , Journal of Applied Chemistry, USSR, no. 3, 1989).

The above-described limitations pose, however, no major technical or economical obstacles to the application of the present desulfurization method, since the method does not generally need the use of high alkali concentrations in the aqueous phase. When necessary, the alkali mixture obtained from causticization can be replenished with fresh alkali.

The salt content of the emulsion routed to electrolysis is kept at the concentration required to attain a sufficient level of conductivity in the emulsion despite the fact that much of the electrode area is covered by an oil film.

The space containing the electrodes and the feed container of the aqueous emulsion are preferably maintained pressurized and heated during the desulfurization process.

According to a first embodiment of the invention, electrolysis is carried out in a cell whose cathode and anode spaces are not separated, but rather communicate freely with each other. This alternative offers the simplest implementation and achieves simultaneous reduction and oxidization of sulfur compounds.

The material of the cathode is selected such as to produce a high discharge overpotential for hydrogen; the material of the anode should produce a high discharge overpotential for oxygen. In this manner, the S will react before the H ₂ or 0 ₂ will start to evolve. A high electrochemical yield will result if only minimal amounts of gaseous H ₂ and 0 ₂ are produced.

It is preferred to have the electrodes shaped as flat or corrrugated plates; flow-through electroodes, such as fixed or fluidized beds, have high efficiency in the treatment of emulsions. They prevent the separation of emulsions into the constituent phases and provide for a good mass transfer at the electrode surface, even at low linear velocities.

The cathodic and anodic spaces are shaped such that by- passing and formation of dead spaces, which result in inefficient treatment and formation of undesirable by¬ products, can be avoided. Surfactants can be used to maintain emulsion.

The electrolysis can also be carried out in a reaction space of the electrolysis cell which is divided in two compartments (the cathode and the anode spaces) by an intermediate wall or membrane impermeable to oil. Semipermeable membranes conventionally employed in other applications serve well for this purpose.

E

B fi¬

O

electrolysis stages and that fresh water is emulsified in the reduced or oxidized organic phase before the second electrolysis stage. This optional procedure is particularly applicable to treatment of oils and similar hydrocarbon products which contain some S-compounds which are readily oxidized (reduced) , while other S-compounds are more easily reduced (oxidized) .

If the S present in the hydrocarbon fraction is readily reduced to form hydrogen sulfide or mercaptans of low molecular weight, the further treatment of the aqueous phase can be simplified. Thus, the reduced S-compounds can be separated from the aqueous phase by acidification and then treated by methods known per se (e.g. converted into elemental sulfur by the Claus reaction) .

A continuous electrolysis process can also be implemented using several electrolysis cells arranged in series. The series-arranged electrolysis cells can have an identical or varying construction. It is possible to run the different cells at different voltages such that the lower the sulfur content of the emulsion to the electrolyzed, the higher the applied cell voltage.

The above-described different embodiments of the inven¬ tion provide an effective treatment method for an oil- containing product. Besides sulfur-containing products, an equivalent treatment can be extended to halogenated hydrocarbons with similar results. Because the bonding strengths of carbon-halogen bonds are very close to those of carbon and sulfur, the method is applicable to the disposal and purification of halogenated hydrocarbons.

To some extent, the secondary reactions which take place during the polarization process can cause contamination of the electrodes. Most of these problems typically associated with the prior art methods are avoided by

virtue of the shear forces created by the high pumping rates. Therefore, in the present invention, problems occur only with high-viscosity bottom residues from fractionating distillation of crude petroleums. When treating light fractions, the electrode surface becomes covered by a protecting polymerized layer of high molecular weight. This layer is, however, easily redissolved into the product being treated provided that the layer remains in contact with the product when current to the cell is temporarily switched off.

According to our tests, contamination of electrodes can be avoided by the following methods:

— Use of rotating electrodes which are continuously wiped clean by means of a mechanical scraper or scrubbing brush

— Use of rapidly vibrating or oscillating electrodes preferably driven by a piezoelectric transducer — Changing the direction of current at short intervals (e.g. at 5 to 60 s intervals)

— Combination of the above methods.

The extracting alkali used for preparing the electrolysis solution can obviously also be calcium hydroxide or ammonium hydroxide or a mixture of alkalis, whereby the causticization reactions commence already during the initial reaction. The presence of solids has, however, detrimental side effects, particularly in their removal from a petroleum product of high viscosity.

In all cases the sulfur removed from the petroleum product is finally advantageously converted into gypsum or a mixture of gypsum and calcium sulfite which with time undergoes a spontaneous oxidation into gypsum.

The invention provides significant benefits, the most important of them being the possibility of reducing the sulfur content of liquid hydrocarbon products such as fuels down to an acceptable level using uncomplicated equipment of low investment cost. The method readily attains a reduction of approx. 90 % in the sulfur content of the products. An additional benefit worth mentioning is that at least 60 % of the chlorine contained in a hydrocarbon feedstock accidentally contaminated with organic halogens is simultaneously removed. The method is equally applicable to the "purification" of chlorinated hydrocarbons, though different regeneration arrangements of recirculated chemicals must be employed.

The invention is next examined with the help of a few application examples.

Example 1

An extremely heavy fuel oil fraction from the CIS, known as "gudron" meaning road tar in Russian, was treated as follows:

Said "gudron" fraction was mixed in 2:1 ratio at 60 °C into a 25 % solution of NaOH and was continuously stirred. The initial sulfur content of the "gudron" fraction was 1.93 % and its viscosity was 135 mPas.

The treatment was carried out as follows:

Table 1. Treatment conditions

** electrodes cleaned

The volume of the electrolysis cell was 2 1 and the elec¬ trode area 0.05 m ² . The electrodes were platinum coated.

As is evident from Table 1, the reduction of sulfur content was from an initial value of 1.93 w-% down to 0.15 w-%, which represents a 92 % reduction in S content and is fully comparable with results attainable by way of conventional pressurized hydrogenation techniques.

Example 2

This example test was carried out using a 10 % solution of NaOH and a feed rate of 80 1/h into an electrolysis cell volume- of 2.0 1 at 35 °C.

The treatment on a relatively heavy fraction called "polygudron" was carried out as follows:

Table 2. Treatment conditions

Each successive line in the table gives the results of a repeated run through the cell using a residence time of approx. 75 s in the electrolysis cell.

Said "gudron" fractions that are the bottom residues from vacuum distillation contain no sulfur in the form of low molecular weight compounds or mercaptans, so their desulfurization by mere alkaline extraction would be ineffective. Therefore, the achieved reduction in sulfur content is rather significant also in this case. The settling time of the gudron-water emulsion is typically about 3 minutes.

Example 3

This example test was carried out on light fuel oil marketed by. Shell, Finland.

Table 3. Treatment conditions

The values of the table confirm that the implementation of the invention achieves an almost complete desulfuriza¬ tion.

Example 4

A light naphta fraction was electrolyzed by running it through an electrolysis cell at two different pumping rates, viz. 200 1/h and 60 1/h, respectively. The volume of the electrolysis cell was 900 ml. The initial S- content of the naphta fraction was 1670 ppm.

The naphta fraction was run through the electrolysis cell four times and subjected to a current of 50 A at 20 °C. By using the pumping rate 200 1/h, the sulfur content of the naphta fraction decreased to 1000 ppm at a voltage in the range from 5 to 6 V.

At the pumping rate 60 1/h, the voltage was unstable and varied between 3 and 9 volts. No decrease of the sulfur content of the naphta fraction could be achieved.

The settling times of the emulsion were about 40 seconds (50 % of the emulsion) and about 70 seconds (about 100 % of the emulsion) .