Process for producing poly-alpha-olefins

Abstract

A process for the production of a poly-alpha-olefin is described. The process includes providing at least one alpha-olefin monomer and a catalyst, and reacting the at least one alpha-olefin monomer in the presence of the catalyst to form a mixture of poly-alpha-olefins (PAOs). The obtained poly-alpha-olefin mixture is fractionated into two PAO fractions, wherein the two PAO fractions are a first PAO fraction having a kinematic viscosity of 5 cSt or less and a second PAO fraction having a kinematic viscosity of greater than 5 cSt. Optionally, a recycle fraction is also obtained in the fractionation, and at least a portion of the recycle fraction comprising the dimer of the alpha-olefin can be recycled back to the reaction step. The process further comprises a step in which the two PAO fractions obtained are directly subjected to separate hydrogenation to obtain a hydrogenated PAO fraction.

Description

Technical Field

[0001] The present invention relates to a process for producing poly-alpha-olefins from alpha-olefin monomers. More particularly, the present invention relates to a process for producing poly-alpha-olefins wherein a product mixture is fractionated into two product fractions and the product fractions are hydrogenated separately.

[0002] The present invention thus provides a simple process, which can be continuous, for producing high quality poly-alpha-olefins with good yields and with optimal process and equipment set-up. Background Art

[0003] The oligomerization of alpha-olefins to form components used in the production of various grades of synthetic lubricants is well known. The production process generally involves oligomerizing alpha-olefin monomers using a catalyst or catalyst complex, removing the catalyst or catalyst complex from the reaction product, and subjecting the oligomerization product to various post-reaction treatments. The oligomerization product is generally referred to as a poly-alpha-olefin (PAO) and can be classified based on the product viscosity (measured in cSt) in a typical range of 2 to 100 cSt. Typical low viscosity PAO products have viscosities of 2, 4, 6, or 8 cSt.

[0004] In the oligomerization of alpha-olefins, dimers, trimers, tetramers, pentamers, etc. are formed. Heavier products (i.e., having a higher number of monomer units in the oligomerized product) naturally have higher viscosities than lighter products. PAO products are useful in a variety of applications, and a variety of viscosities and other properties are required for PAO products depending on the use. Different oligomers can also be mixed to form products with desired properties.

[0005] Patent publication US 4,434,309 describes the oligomerization of low molecular weight alpha-olefins to produce synthetic lubricant base stocks. The oligomerization is carried out in the presence of a catalyst, which may be boron trifluoride, and a protic promoter.

[0006] A key aspect of the production process is the separation of the catalyst or catalyst complex from the oligomerization product. Patent publication EP1694439 describes a typical PAO production process and the separation of a catalyst complex containing boron trifluoride (BF3) and an alcohol from the oligomerization product by distillation under reduced pressure.

[0007] The oligomeric products produced generally require hydrogenation to saturate any double bonds still present in the poly-alpha-olefin (PAO). The typical process sequence for hydrogenating unsaturated products is to hydrogenate the product intermediate directly after the process reaction stage, and the hydrogenation is followed by at least one separation step to obtain the desired saturated product fraction. Alternatively, the product is first separated into multiple product streams with the desired properties and then the product streams are hydrogenated. However, this process sequence requires intermediate storage capacity and batch hydrogenation per activity.

[0008] There remains a need to develop an optimal process sequence in PAO production to enable production of high quality products with adequate and economical equipment and process setup, high plant throughput and low downtime. Summary of the invention

[0009] Therefore, it is an object of the present invention to provide a robust process for obtaining poly-alpha-olefins with an optimized product fractionation and hydrogenation steps. The object of the present invention is achieved by a process characterized by what is stated in the independent claim. Preferred embodiments of the present invention are disclosed in the dependent claims.

[0010] Therefore, the present invention provides a process for producing poly-α-olefins, the process comprising:

[0011] a) providing at least one α-olefin monomer and a catalyst,

[0012] b) reacting at least one alpha-olefin monomer in the presence of a catalyst to form a mixture of poly-alpha-olefins (PAOs),

[0013] c) fractionating the obtained poly-α-olefin mixture into two PAO fractions, wherein the two PAO fractions are a first PAO fraction having a kinematic viscosity of 5 cSt or less and a second PAO fraction having a kinematic viscosity of greater than 5 cSt; and optionally a recycle fraction, at least a portion of which is recycled to the reaction step, and the recycle fraction contains dimers of α-olefins,

[0014] d) hydrogenating the two obtained PAO fractions separately to obtain hydrogenated PAO fractions, and

[0015] e) optionally further fractionating at least one of the hydrogenated PAO fractions to obtain hydrogenated PAO subfractions.

[0016] An advantage of the present invention is that it provides a process for producing poly-alpha-olefins with an optimal process and equipment set-up. In one embodiment, the process of the present invention provides the possibility to carry out the oligomerization and hydrogenation steps continuously with little or no intermediate storage of non-hydrogenated products. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Hereinafter, the present invention will be described in more detail by means of preferred embodiments with reference to the accompanying drawings, in which:

[0018] Figure 1 is a schematic process description of one embodiment of the present invention, wherein the oligomerization, fractionation and hydrogenation steps are described.

[0019] Figure 2 The cracking of various hydrogenated PAO products at different temperatures is shown.

[0020] Figure 3 The desired fractionation temperatures for various PAO products are shown. DETAILED DESCRIPTION

[0021] It is usually optimal to hydrogenate the entire product stream obtained from the process before separation into the final products, especially when the final products are not sensitive to the separation conditions. However, poly-alpha-olefin products (PAO) have strict product specifications which cannot be met if product degradation is caused in the final process steps. Therefore, the typical process sequence for poly-alpha-olefin production provides a hydrogenation step as the last required process step for poly-alpha-olefin treatment, which leads to the difficulty that all fractionated intermediate products have to be hydrogenated separately. This can be arranged in separate parallel and continuous units for each product or in one unit operated batchwise on an activity basis.

[0022] Beneficial production process concepts are based on an optimal equipment arrangement with a minimum amount of equipment required, which efficiently and continuously performs the desired once-through processing while minimizing reprocessing of raw materials and intermediate process streams. A continuous process concept will enable continuous full use of the equipment under designed and optimally controlled operating conditions, thereby also minimizing unnecessary process interruptions and equipment maintenance.

[0023] The batch process concept has the advantage of enabling the processing of multiple high-quality products in the same minimal amount of equipment. However, the equipment is laborious to operate and is only fully utilized part of the time. In addition, the transient mode in batch production reduces the total output of the product and leads to the need for reprocessing. Therefore, batch production is usually only beneficial in small-scale production processes and when several types of products need to be processed in the same batch equipment in active mode.

[0024] It is therefore an object of the present invention to provide a solution to the problems described, which are related to continuous processes on the one hand, and to batch processes on the other hand. It is therefore an object of the present invention to provide a robust process for obtaining poly-alpha-olefins, which has an optimal product fractionation and hydrogenation step. The present invention relates to a process for producing poly-alpha-olefins (PAOs) from alpha-olefins. The term "alpha-olefin" herein means the same as alpha-olefins (or α-olefins) and is a 1-olefin, i.e. having a double bond in the primary carbon, also called the alpha carbon. The carbon chain of the alpha-olefin is preferably linear, i.e. an n-olefin. If there is no specific mention that the alpha-olefin is branched, it is considered to mean a linear alpha-olefin. In one embodiment of the present invention, the alpha-olefin monomers include C4-C 20 α-olefins, preferably C8-C 12 α-olefins, and more preferably, the monomers include C 10 α-olefin (1-decene). It should be noted that even if the feedstock for the oligomerization reaction contains primarily (for example) C10-α-olefin (1-decene), the feedstock is not completely pure 1-decene and always contains various amounts of other olefins. Other olefins will also react in the oligomerization reaction. Therefore, the poly-α-olefin (PAO) product formed in the oligomerization reaction always contains a mixture of products with different degrees of oligomerization (dimers, trimers, tetramers, etc.) and oligomers from olefin monomers other than the primarily selected α-olefin of the feedstock.

[0025] The products of the oligomerization reaction of alpha-olefin monomers are called poly-alpha-olefins (PAO), or poly-alpha-olefin, poly-alpha-olefin, polyalphaolefin, or polyalfaolefin. Different oligomerization products can be characterized based on the number of monomer units in the oligomer, i.e., dimer, trimer, tetramer, etc. The oligomerization products can also be characterized based on the viscosity value of the product in cSt. Typical viscosities of light PAO products include (but are not limited to) kinematic viscosities of 2, 4, 6, 8, etc. in cSt at 100°C.

[0026] PAO products with a specific kinematic viscosity, such as 4 cSt or 6 cSt, are always a mixture of oligomers. This is because a product with a specific kinematic viscosity is desired and the goal is not to obtain a product with only one type of oligomer. It is the properties of the product that are important, not the chemical structure itself. Products with specific properties, such as kinematic viscosity, are desirable.

[0027] Therefore, the formed PAO product mixture usually needs fractionation to obtain the product with the desired properties. The oligomerization reaction always produces a mixture of oligomers, and the final PAO product is usually formed by fractionating the PAO reaction mixture to obtain the product with the desired properties (such as viscosity and volatility). Usually, fractionation is also needed to separate unreacted monomers and / or low-level oligomerization products, such as dimers, which should be separated from the product mixture and usually recycled to the oligomerization step. In addition, it is necessary to remove the catalyst of the catalyst complex from the product mixture, which is usually carried out before the PAO fractionation.

[0028] The oligomerization reaction of α-monomers also requires the presence of a catalyst. In the present method, the catalyst can be any suitable catalyst capable of catalyzing the oligomerization of α-monomers to form poly-α-oligomers. In one embodiment of the present invention, the catalyst is a catalyst complex comprising a main catalyst and a co-catalyst. The term "catalyst complex" is used herein to refer to a combination of a catalyst and a co-catalyst (also referred to as a proton promoter). The catalyst complex can be a physical mixture, or the catalyst and the co-catalyst can be combined with each other, or it can be a combination of the two. The main catalyst can be an aluminum halide or a boron halide, preferably the main catalyst is boron trifluoride (BF3). The co-catalyst can be a C1-C 10 Alcohol or C1-C5 alcohol, preferably the co-catalyst is n-butanol.

[0029] The performance and advantages of the present invention are not dependent on the selected α-olefin or catalyst or alternatively catalyst complex used for oligomerization. However, in one embodiment of the present invention, the monomer is linear 1-decene and the catalyst complex is formed by BF3 as the main catalyst and n-butanol as the cocatalyst.

[0030] The method according to the invention comprises the following steps: reacting at least one alpha-olefin monomer in the presence of a catalyst in an oligomerization reaction to form a mixture of poly-alpha-olefins (PAOs), and fractionating the mixture obtained into two PAO fractions and a possible recycle fraction. At least a portion of the recycle fraction is recycled to the reaction step for further oligomerization. The recycle fraction generally contains at least dimers, but may also contain unreacted monomers. The recycle fraction may also be more than one stream, and the separation of the recycle fraction may include more than one separation step. Before the fractionation step of obtaining the PAO fraction, the catalyst or catalyst complex and possible unreacted monomers are removed from the product mixture outlet of the oligomerization reaction.

[0031] The mixture of poly-α-olefins obtained in the oligomerization reaction is fractionated in a fractionation step to obtain two PAO fractions.

[0032] The two PAO fractions are a first PAO fraction having a kinematic viscosity of 5 cSt or less and a second PAO fraction having a kinematic viscosity greater than 5 cSt. The first PAO fraction having a kinematic viscosity of 5 cSt or less may also be referred to as a lighter fraction, and the second PAO fraction having a kinematic viscosity greater than 5 cSt may also be referred to as a heavier fraction. In one embodiment, more than two PAO fractions, such as three, four or five PAO fractions, are obtained in the fractionation of a mixture of poly-alpha-olefins.

[0033] In this article, the fraction (or PAO fraction) of PAO with a kinematic viscosity of 5 cSt or less refers to the fraction containing a PAO product with a kinematic viscosity of 5 cSt or less measured at 100°C, which means that the kinematic viscosity can be up to 5 cSt, such as 4.5 cSt, 4 cSt or 3.5 cSt, etc. In one embodiment, the first fraction of the PAO product has a kinematic viscosity of 2 cSt to 5 cSt. Similarly, the fraction (or PAO fraction) of PAO with a kinematic viscosity greater than 5 cSt refers to a PAO product wherein the measured kinematic viscosity of the PAO product is higher than 5 cSt (such as 5.5 cSt, 6 cSt or 6.5 cSt, etc.) in this article. In one embodiment, the second fraction of the PAO product has a kinematic viscosity greater than 5 cSt to 10 cSt, or greater than 5 cSt to 12 cSt. The fraction of the PAO product separated in fractions has a measured kinematic viscosity, and the range of the kinematic viscosity of the first and second fractions refers to the measured kinematic viscosity of the fraction within the range.

[0034] Alternatively, in one embodiment, the first PAO fraction has a kinematic viscosity of 4 cSt or less, and the second PAO fraction has a kinematic viscosity greater than 4 cSt. Still alternatively, in one embodiment, the first PAO fraction has a kinematic viscosity of 6 cSt or less, and the second PAO fraction has a kinematic viscosity greater than 6 cSt.

[0035] Typically, in the present method for producing poly-alpha-olefins according to the present invention, three different PAO products with different kinematic viscosities are produced. In one embodiment of the present invention, the three PAO products are 2 cSt products, 4 cSt products, and 6 cSt products or 8 cSt products. Therefore, the first fraction (lighter fraction) of the PAO product includes two PAO products, i.e., 2 cSt and 4 cSt products, and the second fraction (heavier fraction) of the PAO product includes a PAO product, i.e., 6 cSt products or 8 cSt products. Therefore, the first PAO fraction with a kinematic viscosity of 5 cSt or less generally includes two PAO products, i.e., 2 cSt and 4 cSt products. Similarly, the second PAO fraction with a kinematic viscosity greater than 5 cSt generally includes a PAO product, i.e., 6 cSt or 8 cSt products.

[0036] The present method comprises that the mixture of poly-alpha-olefin obtained is fractionated into two PAO fractions and optionally a recycle fraction, a part of which can be recycled to the reaction step, and the recycle fraction comprises the dimer of alpha-olefin. It is beneficial to carry out fractionation so as to obtain two fractions with roughly equal volumes. This means that if a separate hydrogenation unit is used for two PAO fractions, the hydrogenation unit can have the same size, and the stream and pipeline of the hydrogenation unit are interchangeable. Two fractions with roughly equal scale can be obtained by fractionating the PAO product so that a fraction contains a PAO product with a kinematic viscosity of 5 cSt or lower and a second fraction of a PAO with a kinematic viscosity greater than 5 cSt.

[0037] Obtaining two fractions is also beneficial, since only the lighter fraction, i.e. the fraction with a kinematic viscosity of 5 cSt or less, needs to be further separated after hydrogenation. This further separation can be carried out under mild processing conditions and therefore does not cause product degradation. On the other hand, the heavier fraction, i.e. the fraction with a kinematic viscosity greater than 5 cSt, will be a completely ready-made product after hydrogenation and no further product degradation will occur. The separation of the heavy product requires more stringent operating conditions, which will result in a high risk of product degradation.

[0038] It has been unexpectedly found that fractionation into two PAO products, i.e. a first PAO fraction having a kinematic viscosity of 5 cSt or less and a second PAO fraction having a kinematic viscosity greater than 5 cSt is beneficial. This is because further fractionation of the lighter PAO fraction into two different PAO products (typically 2 cSt and 4 cSt products) can be performed at lower temperatures without product degradation (see the examples provided herein). Typically, the heavier PAO fraction is also a ready-made product after hydrogenation, and further fractionation of the heavier PAO product of hydrogenation is not required.

[0039] When a heavier PAO product, such as an 8 cSt product, is required, the total product volume is reduced and the operating time is extended. However, obtaining two PAO fractions is still beneficial because the desired heavy fraction can be hydrogenated in one hydrogenation unit, while another lighter fraction, which is significantly smaller in volume, can be stored in an intermediate tank when a heavier product, such as an 8 cSt product, is required, and the other hydrogenation unit can be repaired and maintained. Therefore, obtaining two PAO fractions and hydrogenating the PAO fractions separately in a batch manner or having two separate hydrogenation units for the two PAO fractions enables an end-to-end process with optimal product fractionation and hydrogenation steps for obtaining poly-alpha-olefins.

[0040] In one embodiment of the invention, obtaining two PAO fractions enables two hydroprocessing units to have separate lines that can be interchanged and thus enables catalyst replacement in one hydrogenation unit during continuous end-to-end processing mode in the other hydrogenation unit.

[0041] The kinematic viscosity of a mixture or fraction of a PAO product is a typical method for characterizing PAO products and various poly-alpha-olefin mixtures. Those skilled in the art are very familiar with measuring the kinematic viscosity of products such as PAO products. Kinematic viscosity is measured according to ASTM D-445 (latest version) and is typically measured at least at 100°C, 40°C and 0°C. If not otherwise specified, the kinematic viscosity used herein is measured at 100°C. Kinematic viscosity is given in cSt (centistokes), which corresponds to the SI unit mm 2 / s.

[0042] In this article, a PAO fraction with a kinematic viscosity of, for example, 5 cSt or less means that the kinematic viscosity value of the fraction at 100° C. is 5 cSt or less. A PAO fraction is typically a mixture of oligomers, and it is therefore useful to define the fraction based on the kinematic viscosity of the fraction, rather than, for example, using the number of monomer units in the oligomer. Those skilled in the art are very familiar with obtaining a PAO fraction with a specific kinematic viscosity, and therefore a mixture of PAO products can be fractionated to obtain a PAO fraction with a specific kinematic viscosity. Fractionation can be performed by any well-known distillation or fractionation procedure, such as vacuum distillation.

[0043] The fractionation into two PAO fractions can be carried out as a vacuum distillation, such that the desired temperature is, for example, up to 280° C., up to 300° C., or up to 330° C. to separate a first PAO fraction having a kinematic viscosity of 5 cSt or less and a second PAO fraction having a kinematic viscosity of greater than 5 cSt. The distillation temperature depends on the kinematic viscosity of the bottom fraction and the reduced pressure. The reduced pressure at the top of the distillation column can be 0.2 kPa to 1 kPa.

[0044] The method according to the present invention also includes the step of hydrogenating the two PAO fractions obtained to obtain a hydrogenated PAO fraction. The hydrogenation step of the two PAO fractions occurs directly after the fractionation, and therefore the method includes the step of directly hydrogenating the two PAO fractions obtained (hydrogenated separately) to obtain a hydrogenated PAO fraction. The direct hydrogenation of the two PAO fractions means that before the hydrogenation step is performed, the PAO fractions are not further oligomerized, recycled to the oligomerization or other chemical modifications of the fractions. The hydrogenation of the PAO fractions is performed separately and can be performed in a separate hydrogenation unit. Therefore, in one embodiment, there are at least two separate hydrogenation units working in parallel.

[0045] The hydrogenation of PAO products is well known to those skilled in the art, and the present method is not limited to any particular hydrogenation method. Typically, the hydrogenation is carried out in the presence of a noble metal or nickel catalyst, preferably a nickel catalyst. The hydrogenation can be carried out in a fixed bed reactor. In one embodiment, the hydrogenation is carried out at a hydrogen pressure of 2 MPa to 10 MPa, preferably 2.5 MPa to 8 MPa, more preferably 3 MPa to 6 MPa; at a temperature of 100°C to 320°C, preferably 120°C to 280°C, more preferably 130°C to 250°C. Typically, the hydrogenation temperature is 25 to 500 Nm 3 Hydrogen / m 3 As the amount of hydrocarbon, hydrogen was used, and the LHSV was 0.2 to 10 1 / h.

[0046] In one embodiment of the present invention, the method further comprises a further fractionation step after hydrogenation of the PAO fraction, wherein sub-fractions of the hydrogenated PAO fraction are obtained. In one embodiment, a hydrogenated PAO sub-fraction is obtained from a hydrogenated PAO fraction having a kinematic viscosity of 5 cSt or less, and further fractionation to sub-fractions is performed to obtain a first hydrogenated PAO sub-fraction having a kinematic viscosity of about 2 cSt and a second hydrogenated PAO sub-fraction having a kinematic viscosity of about 4 cSt.

[0047] Figure 1A schematic process of one embodiment of the present invention is shown. In an oligomerization unit 10, oligomerization reaction and catalytic separation of alpha-olefin monomers are carried out to produce a PAO product. The PAO product formed is subjected to fractionation, wherein two PAO product fractions are obtained. Prior to the fractionation step, the catalyst complex is removed from the product stream (not shown in the figure). A first fraction containing PAO products having a kinematic viscosity of 2 cSt and 4 cSt and a second fraction containing PAO products having a kinematic viscosity of 6 cSt or 8 cSt are obtained. In separate hydrogenation units, the two PAO fractions are hydrogenated, a first hydrogenation unit 30 for 2 cSt and 4 cSt PAO product fractions, and a second hydrogenation unit 35 for 6 cSt or 8 cSt PAO products. An optional dimer recycle stream 25 can also be discharged from the fractionation unit 20 and at least partially recycled back to the oligomerization unit 10. After the first hydrogenation unit 30, the hydrogenated PAO fraction of the 2 cSt and 4 cSt PAO products is passed to a second fractionation unit 40 to separate the 2 cSt and 4 cSt PAO products.

[0048] It has been unexpectedly found that it is beneficial to fractionate two PAO product fractions before hydrogenation, and the hydrogenation of at least two PAO fractions is carried out separately or in a separate hydrogenation unit. This is because the first fractionation of the recycle fraction that separates the dimer that mainly contains alpha-olefins usually needs to be separated from the PAO product mixture before hydrogenation. In addition, the fractionation of the PAO product usually leads to cracking and the formation of possible double bonds or the like, and a hydrogenation step is required after the fractionation step. Particularly advantageously, the PAO product mixture is fractionated into two fractions with a kinematic viscosity of just 5 cSt, and these fractions are directly hydrogenated separately. It is beneficial to fractionate and directly hydrogenate the two PAO fractions obtained at this specific kinematic viscosity, because this ensures that each fraction is hydrogenated under the conditions that are most suitable for its respective properties. Specifically, the PAO fraction with a kinematic viscosity of 5 cSt or less can undergo a further fractionation step to obtain two PAO sub-fractions with a kinematic viscosity of 2 cSt and 4 cSt, respectively. This further separation can be carried out under mild processing conditions, thereby preventing product degradation, as shown in the following examples.

[0049] Fractionation into more than two fractions can be beneficial, but generally the cost of having more than two hydrogenation units outweighs the benefit of obtaining more than two PAO fractions. If more than two PAO fractions are obtained prior to hydrogenation, the process should include intermediate storage of more PAO fractions and conduct the hydrogenation in a semi-batch mode of operation.

[0050] Example 1

[0051] Two PAO products with different kinematic viscosities, 4 cSt PAO and 8 cSt PAO grades, were tested for thermal stability. The PAO products were loaded into similar autoclaves and placed in an oven maintained at a constant temperature. The autoclaves were placed in the oven for several hours. The amount of product left in the autoclave after the test was recorded. Liquid samples were analyzed by GC to quantify the cleavage products. The test was repeated for two groups at temperatures of 280°C, 300°C, and 320°C. All tests had some color change. Figure 2 The measured cracking rates are provided.

[0052] The results clearly show that decreasing the temperature decreases the cracking rate almost linearly. It can also be observed that the heavier 8 cSt PAO product has a higher tendency to crack. In other words, the longer the oligomer chain, the more sensitive it is to thermal degradation.

[0053] exist Figure 3 In, typical maximum processing temperatures required for fractionation of various PAO products as a function of product grade are provided. It can be concluded that fractionating heavy PAO grades (e.g., 6 cSt and 8 cSt) presents a higher risk of product degradation during processing due to the higher temperatures required for fractionation.

[0054] It is obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways.The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1. A method for producing poly-α-olefins, the method comprising: a) providing at least one α-olefin monomer and a catalyst, b) reacting said at least one alpha-olefin monomer in the presence of said catalyst to form a mixture of poly-alpha-olefins (PAOs), c) fractionating the obtained mixture of poly-alpha-olefins into two PAO fractions and optionally a recycle fraction, wherein the two PAO fractions are a first PAO fraction having a kinematic viscosity of 5 cSt or less and a second PAO fraction having a kinematic viscosity of greater than 5 cSt; at least a portion of the recycle fraction is recycled back to the reaction step, and the recycle fraction contains dimers of alpha-olefins, d) subjecting the two PAO fractions obtained to separate hydrogenation to obtain hydrogenated PAO fractions, and e) optionally further fractionating at least one of the hydrogenated PAO fractions to obtain hydrogenated PAO subfractions.

2. The process according to claim 1, wherein the two PAO fractions obtained are hydrogenated in separate hydrogenation units.

3. The method according to claim 1, wherein the method comprises a further fractionation step e), and the hydrogenated PAO fraction having a kinematic viscosity of 5 cSt or less is subjected to a further fractionation step e) to obtain a first hydrogenated PAO sub-fraction having a kinematic viscosity of about 2 cSt and a second hydrogenated PAO sub-fraction having a kinematic viscosity of about 4 cSt.

4. The process of claim 1, wherein the at least one α-olefin monomer comprises C4-C 20 α-olefins, preferably C8-C 12 α-olefins, and more preferably C 10 α-Olefins.

5. The method according to claim 1, wherein the catalyst is a catalyst composite comprising a main catalyst and a co-catalyst, and the main catalyst is preferably an aluminum halide or a boron halide, more preferably the main catalyst is BF3, and the co-catalyst is preferably selected from C1-C 10 Alcohol, and more preferably the co-catalyst is n-butanol.

6. The process according to claim 1, wherein the hydrogenation is carried out in a fixed bed reactor in the presence of a noble metal or nickel catalyst.

7. The process according to claim 1, wherein the hydrogenation is carried out at a pressure of 2 MPa to 10 MPa, preferably 2.5 MPa to 8 MPa, more preferably 3 MPa to 6 MPa, at a temperature of 100 to 320°C, preferably 120 to 280°C, more preferably 130 to 250°C.

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