Technical Field

The present invention relates to a process for preparing sweetened hydrocarbon liquid compositions with an olefin content of 5 vol.% or more with reduced gum formation, to the product so obtained and to the scavenger composition which may be used in the process.

Background Art

The expression "sweetened" versus "sour" results from the removal of odorous mercaptans and the like from a hydrocarbon liquid composition, as explained hereafter.

An example of a hydrocarbon liquid composition with an olefin content of 5 vol.% or more is shale oil, which is a synthetic crude oil produced by retorting oil shale. Oil shale is an organic-rich fine-grained sedimentary rock containing kerogen (a solid mixture of organic chemical compounds). Although the present invention is described in respect of shale oil, it is to be understood that the invention concerns any hydrocarbon liquid composition with an olefin content greater than 5 vol.%.

From "The Chemistry of Shale Oil and its Refining", by S.H. Guo (Encyclopedia of Life Support Systems), it is known that shale oil derived from oil shale does not directly substitute for crude oil in all applications. Compared with petroleum crude, shale oil is heavy, viscous, and is high in nitrogen and oxygen compounds. Other characteristic properties of shale oils are: (1 ) high levels of olefins and diolefins -which are not present in petroleum crudes- requiring special care during processing due to their tendency to polymerize and form gums; (2) high levels of aromatic compounds, deleterious to kerosene and diesel cuts; (3) high carbon/hydrogen ratio; (4) low sulfur levels, compared with most crudes available in the world (though for some shale oils from the retorting of marine oil shale, high sulfur compounds are present); (5) suspended solids (finely divided rock) which cause process problems chiefly if a first step of processing is hydrotreating; (6) moderate levels of metals. The hydrocarbons making up shale oil can be classified as being about 20 vol.% alkanes, 20 vol.% aromatics, 25 vol.% aromatic resins, and 35 vol.% olefins and naphthenes. By contrast, a typical crude would contain about 15 vol.% alkanes, 50 vol.% aromatics, and 35 vol.% naphthenes with hardly any olefins or resins. In other words, shale oil may be distinguished from typical petroleum crudes by their content of olefins constituting (significantly) more than 5 vol.% of the make-up. Estonian shale oil tends to have an olefin content of more than 50 vol.%.

A process that may be used to upgrade shale oil is the removal of mercaptans by catalytic oxidation (so-called MEROX process, also known as "sweetening") as discussed hereafter. However, it has been found that the removal of mercaptans, or sweetening of the shale oil, results in an increased tendency of gum formation of the so produced sweetened shale oil. Thus, as mentioned by Mr. Gue, a particular problem with shale oil concerns gum formation. This problem has been described also by Fathoni, A. Z.; Batts, B. D. A literature review of fuel stability studies with a particular emphasis on shale oil. Energy Fuels 1992, 6, 681 -693, and G. U. Dinneen, W. D. Bickel,. Ind. Eng. Chem. , 1951 , 43 (7), pp 1604-1607.

Gum formation is part of a larger problem concerning storage and thermal stability. Storage stability concerns the ability of a hydrocarbon liquid composition, stored over extended periods of time, to remain unchanged or without appreciable deterioration under ambient conditions. Thermal stability concerns the ability of the hydrocarbon liquid composition to suffer relatively high-temperature stress for short periods of time, without appreciable deterioration. Such changes or degradations include color change, development of soluble and/or insoluble gum, development of particulate matter followed by sediment/deposit, development of coke and fouling materials, change in global physicochemical properties, and other undesired effects. Gums, in case the hydrocarbon liquid composition is used as fuel, may stick to metal surfaces along a vehicle-fuel system, from the tank to the

combustion chamber. Accumulation of these products can cause engine wear and can have adverse effects on the engine efficiency, performance, emission, and durability. In addition, The gum formation leads to an increase of the fuel density, distillation temperatures, aromatics, and oxygen concentration and a decrease of the concentration of olefins.

Shale oil, in particular the Estonian shale oil, suffers from rapid formation of gums which problem is actually amplified upon catalytic oxidative removal of mercaptans, such as in the MEROX reaction.

Mercaptan is the generic name for a family of organic compounds where sulfur and a hydrogen atom (SH) are bonded to one of the carbon atoms in the molecule. The hydrogen atom in the SH radical can ionize and produce a mildly acidic environment, which may lead to corrosion. The most noticeable characteristic of mercaptan is their strong, unpleasant odor even when their concentration is only a few parts per million. Processes within oil refineries or natural gas processing plants that remove mercaptans and/or hydrogen sulfide (H2S) are commonly referred to as sweetening processes because they results in products which no longer have the sour, foul odors of mercaptans and hydrogen sulfide. The liquid hydrocarbon disulfides may remain in the sweetened products, they may be used as part of the refinery or natural gas processing plant fuel, or they may be processed further. Merox is an acronym for mercaptan oxidation. It is a catalytic chemical process developed by UOP and used in oil refineries and natural gas processing plants to remove mercaptans from LPG, propane, butanes, light naphthas, kerosene and jet fuel by converting them to liquid hydrocarbon disulfides. The Merox process requires an alkaline environment which, in some process versions, is provided by an aqueous solution of sodium hydroxide (NaOH), a strong base, commonly referred to as caustic. In other versions of the process, the alkalinity is provided by ammonia, which is a weak base. The Merox process is for instance disclosed in GB1240727. This specific document concerns the loss of catalyst. In a preferred embodiment it describes the sweetening of a sour organic stream, whereby the latter is contacted in an oxidation zone with an oxidizing agent and with an alkaline solution of a phthalocyanine catalyst to convert at least a portion of any mercaptan component present in it to disulphide. The effluent from the oxidation (sweetening) zone is then passed to a separation zone from which separated, sweetened organic phase and separated alkaline phase containing pthalocyanine catalyst are withdrawn, while liquid phthalocyanine catalyst complex is withdrawn from the interfacial zone between said phases and admixed with withdrawn alkaline phase thereby increasing the concentration of catalyst therein before recycling the same to the oxidation zone. In the examples commercial light naphtha having a mercaptan sulfur content of 150 to 200 ppm by weight is contacted in a sweetening zone with air and a caustic alkali solution containing 125 ppm by weight of cobalt phthalocyanine sulfonate. The effluent from the sweetening zone was passed to a separate zone from which an organic phase was withdrawn and found to be sweet to the doctor test.

In US2921021 and many later patents, the sweetening of sour gasoline and other petroleum distillates are described. The problem of excessive and augmented gum formation when applying the Merox process for sweetening of shale oil or other sour hydrocarbon liquid compositions with an olefin content of 5 vol.% or more has not been addressed.

In other words, a process for sweetening shale oil or similar sour hydrocarbon liquid composition with an olefin content of 5 vol.% or more, with reduced gum formation and therefore better storage and thermal stability, would be highly desirable. Surprisingly, it has now been found how to reduce gum formation for shale oil or similar sour hydrocarbon liquid compositions that are sweetened by applying the Merox process.

Summary of the Invention

Accordingly, the invention provides a process for preparing a sweetened hydrocarbon liquid composition with reduced tendency to form gums from a sour hydrocarbon liquid

composition containing mercaptans and with an olefin content of 5 vol.% or more, by reacting the mercaptans in the sour hydrocarbon liquid composition into disulfides with an oxidizing agent in the presence of a scavenger composition comprising a cobalt complex as catalyst and a strong base, wherein the strong base consists of KOH. Also provided is a scavenger composition comprising the catalyst and the base that may be used in the process of the invention, and a sweetened hydrocarbon liquid composition with reduced tendency to form gums. The latter is of particular importance as it allows the use of said sweetened hydrocarbon liquid compositions as fuel or as part of a blend that is used as fuel.

Description of the embodiments

The expression "reduced tendency to form gums", is relative to the same process using NaOH as base in the same reaction. It is preferred to keep unwashed gum, pursuant to ASTM D381 at less than 120, preferably less than 100 mg/100ml. More preferably, the unwashed gum is not increased vis-a-vis the sour hydrocarbon liquid composition by more than 50%, more preferably still, not increased at all. Ideally, the tendency to form gums , pursant to ASTM D381 , is less than 30 mg/ml, which may require blending the sour or sweetened hydrocarbon liquid composition with another hydrocarbon liquid composition that is already low in the tendency to form gums.

The Merox process is known from many art references. For instance, in both US2921021 and GB1240727 said process is described for petroleum distillates. It involves treating a sour hydrocarbon liquid composition with an oxidizing agent, generally air, in the presence of a catalyst and a strong base.

In the process of the present invention, conventional equipment may be used, as well as conventional conditions. For instance, the sour hydrocarbon liquid composition may be treated at ambient to elevated temperatures, e.g., from 5 to 200°C, typically from 10 to 50°C. Atmospheric pressure can be employed, although super-atmospheric pressure up to 70 MPa, e.g., up to 5 MPa may be used, if desired. The time of contact of the sour hydrocarbon liquid composition with the oxidizing agent and the scavenger composition containing the catalyst and strong base is generally set to achieve the desired reduction in mercaptan level and may range from 1 minute to 5 days or more. This also depends on the amount of catalyst used. The catalyst may be used in a range of 1 to 15, preferably 5 to 10 ppm of catalyst by weight to 1 ppm of mercaptan and hbS (as S) by weight. For instance, at a dosage scavenger composition to mercaptan and h S of 5: 1 , the duration of the treatment at ambient temperature and atmospheric pressure may last from 6 hours to 3 days. The ratio for shale oil tends to be higher than the common ratio used for petroleum cuts, as shale oil tends to comprise more "heavy" mercaptans than found in petroleum cuts.

The result of the process of the invention results in a new product, a sweetened

hydrocarbon liquid composition with a reduced tendency to form gums, despite the high initial olefin content. In particular, the present invention results in a new shale oil-based fuel composition with reduced tendency to form gums. Scavenger compositions for use in Merox processes are known. The present composition comprising cobalt(phthalocyanine) and KOH dissolved in ethanol, preferably at a

concentration of 150-250 ppm of catalyst, more preferably about 200 ppm catalyst, dissolved in 20 vol.% of a 50% KOH solution with 80 vol.% ethanol. Lower amounts of KOH may be used, but this affects the efficiency of the scavenger composition during the conversion of the mercaptans to disulfides. Higher amounts of KOH may be used, but may run into solubility issues. The scavenger composition of the present invention is believed to be novel. Also the use of the scavenger composition for sweetening sour hydrocarbon compositions with the specific aim to reduce the tendency of gum formation is believed to be novel, and therefore claimed.

As indicated, the process is particularly suitable for shale oil, with an olefin content that is 40 vol.% or higher, even 50 vol.% or higher. Using a sour hydrocarbon liquid composition with a high olefin content is the preferred embodiment of the present invention. However, the present process may also be used for sour hydrocarbon liquid compositions with a lower content of olefins, or blends with a lower content of olefins.

In order to sweeten a petroleum product, a caustic solution, such as sodium hydroxide or potassium hydroxide, is generally first used to convert the mercaptan compounds to the ionic state, RS ^~ . The caustic solution is also helpful in that it removes naphthenic acids and other organic acids in general such as phenolic acids, and other sulfur compounds from refined petroleum products and petroleum distillate. Various processes for regenerating the caustic solutions and apparatus for same are disclosed in the prior art. For example, U.S. Pat. Nos. 8,597,501 and 7,326,333 disclose such exemplary processes and apparatus. In various documents processes and equipment are described that allow the regeneration of the caustic streams.

Pursuant to a preferred embodiment of the present invention, the sour hydrocarbon liquid composition containing mercaptans and with an olefin content of 5 vol.% or more prior to the MEROX reaction is first subjected to a caustic wash. Moreover, it has been found that if said caustic wash is performed using KOH, rather than NaOH or ammonia and the like, the tendency of gum formation is even further reduced. In other words, surprisingly a synergy has been found, when subjecting a sour hydrocarbon liquid composition containing mercaptans and with an olefin content of 5 vol.% or more first to a caustic wash using KOH, and then reacting the mercaptans in said sour hydrocarbon liquid composition into disulfides with an oxidizing agent in the presence of a scavenger composition comprising a cobalt complex as catalyst and a strong base, wherein the strong base consists of KOH.

By way of example, the washing may be carried out by mixing the sour hydrocarbon liquid composition with a 50% (w/w) potassium hydroxide solution, allowing the mixture to settle, and removing the caustic solution. Typically, the 50% solution is used in an amount of 5% by weight vis-a-vis the sour hydrocarbon liquid composition. However, greater and smaller amounts may be used, as well as caustic solutions that are greater or lower in concentration. A simple wash step typically will reduce the content of hydrogen sulfide and mercaptans by 90% or more.

The invention is illustrated by the examples following hereafter. The examples are provided to illustrate the process of the present invention and indicate the benefits to be afforded by the present invention. It is understood that the examples are given for the sole purpose of illustration and are not considered to limit the invention.

Experiments

Test methods

• UOP 163 is used for the mercaptan and H2S contents, as S, expressed in mg/kg.

Typically, the reaction is monitored and analysed at 6 or 12 hour intervals. (Analysis is stopped if the desired reduction of the mercaptan level has been achieved.)

• ASTM D381 is used for the unwashed and washed gum contents, in mg/100 ml

The experiments are carried out with shale oil-based gasoline from Eesti Energia, the Estonian state-owned company, internationally known as Enefit. Products from different units were used, Enefit-140 and Enefit-280. As shale-oil is a product from nature, the composition varies. Generally, it has an olefin content of at least 50 vol.%. The shale oil- based gasoline used in Examples 1-3 were first washed with an NaOH solution.

Example 1

Comparative experiments 1-4 and experiment 5 were carried out with a batch of Enefit-280 with an H2S value (as S) of 184 ppm, a mercaptan content (as S) of 1331 ppm, and a washed gum content of 46,6 mg/100ml. It was treated for 48 hrs with 4 commercial scavenger compositions (I to IV) and the scavenger composition according to the present invention (A). Only the scavenger composition according to the present invention contained KOH as base. The dosage was 1 :5 (5 ppm of catalyst to 1 ppm mercaptans). The result of the tests are illustrated in Table 1. Table 1

The above results clearly show that the inventive scavenger composition clearly outperforms commercial scavenger compositions when used to sweeten a sour hydrocarbon liquid composition based on shale oil (and thus containing more than 5 vol.% olefins).

Example 2

Experiment 6 and comparative experiments 7-9 were carried out with a mixture of batches from the units E140 + E280 in a volume ratio of 1 : 1. This mixture had an h S value (as S) of 193 ppm, a mercaptan content (as S) of 1429 ppm, an unwashed gum content (72hr) of 101 mg/100 ml and a washed gum content (72hr) of 99.8 mg/100 ml. The mixture was treated for 72 hrs with the scavenger composition according to the invention (A), and a similar scavenger composition containing NaOH instead of KOH (composition B). Scavenger composition (B) is commercially available as HFA 6126M-10. The dosage was varied from 1 :5 (5 ppm of catalyst to 1 ppm mercaptans) to 1 :7. The result of the tests are illustrated in Table 2.

Table 2

Exp. Sampl 6 7 8 9

e

Sample E140 + E280 in a volume ratio of 1 : 1

Scavenger A B B B

Ratio - 1 :5 1 :5 1 :6 1 :7

H2S, mg/kg UOP 163 193 <1 ,0 <1 ,0

Mercaptan, mg/kg UOP 163 1429 135 129 1 16 129

(24hr) (48hr) (24hr)

Unwashed gums ASTM 101.0 94.5 103.5 1 13.3 130.5 mg/ml D381

washed gums mg/ml ASTM 99.8 91.0 99.5 109.5 126.0 The above results clearly show that the inventive scavenger composition clearly outperforms a comparable scavenger composition that employs NaOH as base when used to sweeten a sour hydrocarbon liquid composition based on shale oil (and thus containing more than 5 vol.% olefins). Whereas NaOH increases the tendency of gum formation, as is seen when the ratio is increased, KOH actually and surprisingly reduces the tendency of gum formation (at least vis-a-vis scavenger compositions based on NaOH).

Example 3

Experiment 10 and comparative experiment 1 1 were carried out with Enefit-140, with an H2S value (as S) of 208 ppm, a mercaptan content (as S) of 2033 ppm, an unwashed gum content of 74.3 mg/100 ml and a washed gum content of 72.5 mg/100 ml was treated for 72 hrs with the scavenger according to the invention (A), and a similar composition, containing NaOH instead of KOH (B). The dosage was 1 :8 (8 ppm of catalyst to 1 ppm mercaptans). The result of the tests are illustrated in Table 3.

Table 3

The above results clearly show that the inventive scavenger composition clearly outperforms a comparable scavenger composition that employs NaOH as base when used to sweeten a sour hydrocarbon liquid composition based on shale oil (and thus containing more than 5 vol.% olefins). The tendency of gum formation of the sweetened hydrocarbon composition using scavenger composition B, using NaOH as base, has almost doubled. The tendency of gum formation of the sweetened hydrocarbon composition using scavenger composition A, using KOH as base, is significantly less. This reduction in gum formation, when substituting NaOH by KOH, is entirely unexpected. This effect is not seen with any other (organic or inorganic) base either.

Example 4 Experiments similar to those in Examples 1-3 were performed, now using shale oil-based gasoline first washed with a a 50% KOH solution. This led to a further reduction in gum formation.

10 and comparative experiment 1 1 were carried out with Enefit-140, with an H2S value (as S) of 208 ppm, a mercaptan content (as S) of 2033 ppm, an unwashed gum content of 74.3 mg/100 ml and a washed gum content of 72.5 mg/100 ml was treated for 72 hrs with the scavenger according to the invention (A), and a similar composition, containing NaOH instead of KOH (B). The dosage was 1 :8 (8 ppm of catalyst to 1 ppm mercaptans). The result of the tests are illustrated in Table 3.