Process for purifying isopropanol

Abstract

Provided herein is a method for purifying isopropanol, comprising (a) providing a feed comprising at least 99.0 wt% isopropanol; and (b) subjecting said feed to two or more distillation steps, including a higher-pressure distillation at a pressure of at least 6 bar; and a 5 lower-pressure distillation at a pressure below bar; thereby obtaining purified isopropanol.

Description

[0001] PROCESS FOR PURIFYING ISOPROPANOL

[0002] FIELD OF THE INVENTION

[0003] The present disclosure relates to processes for purifying isopropanol.

[0004] BACKGROUND OF THE INVENTION

[0005] Isopropanol (also known as “isopropyl alcohol” or “2 -propanol”, and also referred to herein as “IP A”) is a fluid widely used for various purposes such as coatings, industrial processes, household uses, and personal care products. Its ability to evaporate quickly, miscibility with water, and ability to dissolve a wide range of non-polar compounds also makes it particularly useful in semiconductor manufacturing processes, in particular for wafer cleaning to remove trace organics, water, and particles. Proper cleaning is extremely important in the manufacture of complex semiconductor chips, as residues can result in defects and pattern collapse. In order to avoid such residues, high-purity isopropanol is required.

[0006] Typical isopropanol purification processes remove impurities via a combination of purification methods such as distillation, ion exchange, and membrane filtration. International patent application publication WO 01 / 94284 describes a process for producing ultra-high purity isopropanol via a separation column such as a distillation column. US Patent US 10,647,648 describes a method for purifying organic solvents such as isopropanol with a monolithic organic porous cation exchanger.

[0007] As semiconductor chips become more advanced and miniaturized, the removal of contaminants becomes even more critical, and an isopropanol of increased purity is required for cleaning the substrates. There remains a need for processes for making ultra-pure isopropanol suitable for use in the semiconductor industry.

[0008] SUMMARY OF THE INVENTION

[0009] It is an object of the present application to provide methods for the purification of isopropanol. In particular, provided herein is a method for purifying isopropanol, comprising:

[0010] (a) providing a feed comprising at least 99.0 wt% isopropanol; and

[0011] (b) subjecting said feed to one or more distillation steps, including a higher-pressure distillation at a pressure of at least 6 bar; preferably followed by a lower-pressure distillation at a pressure below 5 bar; thereby obtaining purified isopropanol. The present inventors surprisingly found that the distillation at higher pressure can allow for the effective removal of impurities such as tert-butyl alcohol (also known as “2- methylpropan-2-ol” or “tert-butanol” and referred to herein as “TBA”), which is difficult to separate from IPA given the near-identical boiling points and similar chemical nature of IPA and TBA.

[0012] The independent and dependent claims set out particular and preferred features of the invention. Features from the dependent claims may be combined with features of the independent or other dependent claims, and / or with features set out in the description above and / or hereinafter as appropriate.

[0013] The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description which illustrates, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention.

[0014] BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The following figures are included to illustrate certain aspects of the present disclosure and should not be viewed as exclusive embodiments. The subject matter disclosed is capable of considerable modifications, alterations, combinations, and equivalents in form and function, as will occur to one having ordinary skill in the art and having the benefit of this disclosure.

[0016] Fig- 1 illustrates an isopropanol purification process according to a particular embodiment of the method described herein. Fig- 2 illustrates an isopropanol purification process according to a further embodiment of the method described herein.

[0017] DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described with respect to particular embodiments.

[0019] It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, steps or components as referred to, but does not preclude the presence or addition of one or more other features, steps or components, or groups thereof. Thus, the scope of the expression " a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. Throughout this specification, reference to "one embodiment" or "an embodiment" are made. Such references indicate that a particular feature, described in relation to the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, though they could. Furthermore, the particular features or characteristics may be combined in any suitable manner in one or more embodiments, as would be apparent to one of ordinary skill in the art.

[0020] All numerical values within the detailed description herein are modified by “about” the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.

[0021] The following terms are provided solely to aid in the understanding of the invention.

[0022] Unless specified otherwise, “isopropanol having a purity of x %”, refers to a composition containing x percent by weight (wt%) isopropanol based on the total weight of the composition excluding water. Isopropanol purity can be measured via gas chromatography with flameionization detection (GC-FID). The water content in isopropanol may be determined via Coulometric Karl Fischer reagent titration according to ASTM E1064-16.

[0023] Unless specified otherwise, the term “ppm” means weight-based parts per million and is identical to mg / kg. Similarly, the term “ppb” means weight-based parts per billion and is identical to pg / kg; and the term “ppf ’ means weight-based parts per trillion and is identical to ng / kg.

[0024] Unless specified otherwise, the term “organic impurities” refers to organic molecules other than isopropanol, such as TBA, 1 -propanol, and the like. For the avoidance of doubt, water is not an organic molecule.

[0025] Unless specified otherwise, all pressures expressed in “bar” are absolute pressures. Only pressures expressed in “barg” are gauge pressures.

[0026] Provided herein is a method for purifying isopropanol comprising (a) providing an isopropanol feed having a purity of at least 99.0%; and (b) subjecting the feed to one or more distillation steps, including a higher-pressure distillation at a pressure of at least 6 bar and a lower-pressure distillation at a pressure below 5 bar; thereby obtaining purified isopropanol. More details about the method and embodiments thereof will be discussed further below. In a first step of the method, an isopropanol feed is provided, having an isopropanol purity of at least 99.0 % i.e., the isopropanol feed comprises at least 99.0 percent by weight isopropanol based on the total feed weight excluding water. In preferred embodiments, the isopropanol feed has a purity of at least 99.5%, at least 99.8%, or at least 99.9%.

[0027] As stated above, the isopropanol purity is based on the total feed excluding water. In preferred embodiments, the isopropanol feed contains 0.2 wt% or less water, preferably 0.1 wt% or less, for example about 0.025 wt%.

[0028] In preferred embodiments, the isopropanol feed contains less than 0.1 wt% metals (metal ions), preferably less than 0.01 wt%, more preferably less than 1 ppm, for example 1-10 ppb. The metal content can be determined using inductively coupled plasma mass spectrometry (ICP- MS).

[0029] The feed, i.e. isopropanol having a purity of at least 99.0% is widely commercially available from various manufacturers. Isopropanol can be prepared via any method suitable for preparing isopropanol. Various methods are known in the art, including:

[0030] - Indirect hydration, wherein propylene (propene) is reacted with sulfuric acid thereby forming sulfate esters, followed by hydrolysis of the esters;

[0031] - Direct hydration, wherein propylene is reacted with water at high pressure in the presence of an acid catalyst; and

[0032] - Hydrogenation of acetone.

[0033] Whereas these methods alone may not directly result in isopropanol having a purity of 99.0 % or more, methods for purifying isopropanol to obtain such purity are well known in the art. Examples include distillation (e.g., as described in WO 01 / 94284 and the references discussed therein), extractive distillation (see e.g., US 5,897,750), ion exchange (e.g., as described in US 10,647,648), membrane separation (e.g., as described in US 5,585,527), solvent extraction (e.g., as described in US 4,399,000), and combinations thereof. One example of a method for manufacturing 99.9% pure isopropanol is explained in WO 01 / 94284 (Fig. 6 and page 5, line 14 to page 6, line 5).

[0034] In particular embodiments, the method described herein comprises the step of preparing the isopropanol feed, preferably via one of the three methods described above, followed by purification. In other embodiments, the isopropanol may be obtained from a waste stream, for example a waste isopropanol stream obtained from semiconductor substrate cleaning. The waste stream may need to be treated to increase the purity to 99.0 % isopropanol or higher, using one or more of the purification methods known in the art. The present method is characterized in that the isopropanol feed is subjected to two or more distillation steps, of which at least one is a higher-pressure distillation at a pressure of at least 6 bar. The present inventors found that at such pressures, heavier molecules such as TBA and butanone (methyl ethyl ketone or MEK) can be separated from IPA via distillation. Since boiling points are pressure-dependent, the pressure will determine the temperature needed for the distillation, but can be determined by the skilled person without undue burden. Typically, the higher-pressure distillation is done at a pressure ranging from 6 bar to 20 bar, preferably from 6 to 15 bar, more preferably from 8 bar to 12 bar, for example at about 10 bar. The present inventors surprisingly found that pressures below 6 bar result in less effective TBA removal, and higher pressures lead to more effective TBA removal. On the other hand, higher pressures require high temperatures which are energy-intensive and may lead to excessive formation of impurities.

[0035] The higher-pressure distillation is performed using a distillation column (also referred to herein as “high-pressure distillation column”) as known in the art. The high-pressure distillation column can be a tray column (also known as plate column) configured with at least 30 trays (plates), preferably at least 60 trays, at least 80 trays, more preferably at least 100 trays, such as about 110 trays. Such high number of plates facilitates efficient separation of the heavier molecules such as TBA from IPA.

[0036] The isopropanol feed typically enters the high-pressure distillation column near the bottom of the column, preferably at least below tray 20, or below tray 10 (wherein trays are counted from bottom to top). The skilled person will understand that similar results may be obtained using packed columns having a similar theoretical plate number. Moreover, the skilled person will understand that similar results may be obtained with columns having a lower theoretical plate number, by appropriate recirculation of top fraction to the column. As described above, the present inventors surprisingly found that higher pressures lead to more effective TBA removal. Accordingly, the skilled person will understand that the required number of (theoretical) trays also depends on the applied pressure, wherein higher pressures require fewer trays than lower pressures.

[0037] The bottom fraction of the high-pressure distillation column is enriched in heavier molecules such as TBA, and may still be used as technical grade isopropanol. Alternatively, the bottom fraction may be recycled and purified in order to form new isopropanol feed for the process. The overhead fraction of the column contains a reduced amount of heavier molecules, and is also referred to herein as “high-pressure overhead fraction”. Typically, the isopropanol feed is separated in 40 to 60 wt% of bottom fraction and 40-60 wt% of high-pressure overhead fraction. The terms “bottom fraction” and “overhead fraction” as used herein are relative and mean that the overhead fraction is collected at a higher point in the column than the bottom fraction. In particular embodiments, the bottom fraction is not necessarily collected at the very bottom of the column, and / or the overhead fraction is not necessarily collected at the very top of the column. Analysis of the (heavier) impurities present in the isopropanol can provide guidance regarding at which points in the column the bottom fraction and overhead fraction can be collected. Samples may be taken throughout the process. In preferred embodiments, the overhead fraction is collected at or near the top of the column. In preferred embodiments, the bottom fraction is collected at or near the bottom of the column.

[0038] The present method further includes a lower-pressure distillation, performed at a pressure below the pressure used for the higher-pressure distillation. Typically, the lower-pressure distillation is performed at a pressure below 5 bar. The present inventors found that at such pressures, molecules such as acetone and ethanol can be separated from IPA via distillation. Preferably the lower-pressure distillation is done at a pressure ranging from 0.5 bar to 5 bar, preferably from 0.5 bar to 3 bar, for example about 2 bar.

[0039] The lower-pressure distillation is performed after the higher-pressure distillation step. The present inventors found that some acetone may be formed during the higher-pressure distillation step. In particular embodiments, this can result in a high-pressure overhead fraction having a lower overall isopropanol content compared to the feed. Accordingly, the bottom fraction of the higher-pressure distillation step may be “purified” in the sense that the concentration of at least one organic impurity (such as tert-butyl alcohol) is lower than the concentration of said organic impurity in the feed, but said fraction does not necessarily have a higher isopropanol content than the feed. However, the purified isopropanol obtained via the present method (including both the higher-pressure and lower-pressure distillation steps) has a higher isopropanol content than the feed.

[0040] Acetone formed during the higher-pressure distillation can be removed via the lower-pressure distillation performed after the higher-pressure distillation. The lower-pressure distillation can occur immediately after the higher-pressure distillation, but in certain embodiments, the high- pressure overhead fraction may be subjected to one or more other purification steps (such as other distillation steps and / or ion exchange) prior to the lower-pressure distillation.

[0041] The lower-pressure distillation is performed using a distillation column (also referred to herein as “low-pressure distillation column” or “low-pressure column”) as known in the art. The low- pressure distillation column can be a tray column (also known as plate column) configured with at least 30 trays (plates), preferably at least 60 trays, more preferably at least 75 trays, even more preferably at least 90 trays, such as about 100 trays. Such high number of plates facilitates efficient separation of the lighter molecules such as acetone from the IP A. The isopropanol feed (i.e., the high-pressure overhead fraction, optionally purified via other purification steps) typically enters the column near the top of the column, preferably between trays 80 and 100 (wherein trays are counted from bottom to top). As also indicated above for the higher-pressure distillation, the term “bottom fraction” and “overhead fraction” are relative and mean that the overhead fraction is collected at a higher point in the column than the bottom fraction. The skilled person will understand that similar results may be obtained using packed columns having a similar theoretical plate number. Moreover, the skilled person will understand that similar results may be obtained with columns having a lower theoretical plate number, by appropriate recirculation.

[0042] The top fraction of the low-pressure distillation column is enriched in lighter molecules such as acetone, and may still be used as lower grade isopropanol, or in the preparation thereof. Alternatively, the top fraction may be recycled and purified in order to form new isopropanol feed for the process. The bottom fraction of the column is enriched in isopropanol and is also referred to herein as “low-pressure isopropanol-enriched fraction”.

[0043] As described above, a bottom fraction may be collected at or near the bottom of a distillation column. Accordingly, the low-pressure isopropanol-enriched fraction may be collected at or near the bottom of the low-pressure distillation column.

[0044] The present inventors have found that the bottom of the low-pressure distillation column is not only enriched with isopropanol, but can also have a (slightly) higher inorganic content (e.g., metal content) compared to other fractions of the column. Therefore, in particular embodiments, the low-pressure isopropanol-enriched fraction may be collected as a vapor side stream above (but near) the bottom of the distillation column; generally also (well) below where the feed enters the low-pressure distillation column. In this way, a stream may be collected having an improved balance of high isopropanol content and low metal content. In particular embodiments, the low-pressure column has at least 75 trays, wherein the vapor side stream is collected between plates 2 and 10 of the low-pressure column, preferably between plates 2 and 5. The product collected at the bottom of the low-pressure distillation column (i.e., below the side stream) may still be suitable for use as lower grade (e.g., technical grade) isopropanol.

[0045] Additionally or alternatively, the low-pressure isopropanol-rich fraction may be a vapor side stream collected from a reboiler associated with the low-pressure column. Reboilers are used to heat the bottom of a distillation column, by boiling a bottom fraction of a distillation column to generate vapors which are returned to the column to drive the distillation separation. The reboiler vapor side stream can be expected to have a lower metals content compared to the distillation bottoms stream.

[0046] Typically, the lower-pressure distillation results in 5 to 15 wt% of top fraction and 85 to 95 wt% of low-pressure isopropanol-enriched fraction. In case the low-pressure isopropanol- enriched fraction is collected as a vapor side stream above the bottom of the lower-pressure column, the lower-pressure distillation may result in 5-15 wt% of top fraction, 70-85 wt% of side stream, and 5-15 wt% of bottom fraction.

[0047] In particular embodiments, the method described herein may contain further purification steps in addition to the one or more distillation steps. In particular embodiments, the method may comprise an ion exchange step using an ion exchange resin. This can be used to (further) reduce the metal content in the isopropanol, for examples to a metal content of 100 ppt or lower. The metal content can be determined via ICP-MS.

[0048] Suitable ion exchange resins for reducing metal content in isopropanol are well known in the art and may comprise a solid phase containing bound groups that carry a positive or negative ionic charge, and exchangeable counterions that can be displaced.

[0049] Metal impurities include all metal ions, such as sodium, potassium, calcium, and iron. Preferably, the ion exchange resin comprises a cationic resin, an anionic resin, or mixtures thereof. An example of a suitable resin is Amber, et™ UP6040 resin sold by Rohm and Haas. If an acidic resin is used, cations are removed, which includes most metals expected to be present in the isopropanol. In this case, the isopropanol may be subjected to additional treatment with an anionic resin to further remove anions. The cation metal absorption frees up anion counterions that can be absorbed by the anion resin. In addition, the anion resin can be capable of removing trace acidic compounds that can cause cation release. The resins can be chosen on the basis of which species are to be removed. Single resin beds can be used alone or in various combination. Moreover, the ion exchange resin can be a hybrid device comprising resin- impregnated filters or membranes.

[0050] The ion exchange step may be performed at any stage of the method, e.g. before or after the higher-pressure distillation, between the higher-pressure distillation and lower-pressure distillation, or after the higher- and lower-pressure distillations. In preferred embodiments, an ion exchange step is performed after the lower-pressure distillation step. In particular embodiments, the isopropanol-enriched fractions after higher-pressure and lower-pressure distillation may be subjected to further purification.

[0051] In particular embodiments, the method described herein may further contain a filtering step, wherein isopropanol at any stage of the process is filtered using a filtering device. The filtering step can be used to remove particles (if present). The filtering step may occur at any stage of the method. The filtering device can be any suitable device capable of removing particles ranging in size from 0.05 pm (micrometer) to 10 pm. Suitable filtering devices include a membrane (e.g., a ceramic, polymeric, or metallic membrane), a microfiltration device or cartridge, an ultra-filtration device, or a combination thereof.

[0052] The purified isopropanol obtained by the present method preferably has a higher isopropanol content than the isopropanol feed. In particular embodiments, the purified isopropanol obtained via the method may have a purity of at least 99.995%, preferably at least 99.999%. This can make the purified isopropanol suitable for even the most demanding semiconductor applications. Moreover, the purified isopropanol may contain less than 100 ppm water. In particular embodiments, the purified isopropanol contains 40 to 100 ppm water. The water content in isopropanol may be determined via Coulometric Karl Fischer reagent titration according to ASTM E1064-16.

[0053] In particular embodiments, the isopropanol feed comprises tert-butyl alcohol (TBA). Because TBA and IPA have very similar properties, it is extremely difficult to remove small quantities of TBA from IPA via conventional purification methods. The present inventors have found that the present method is particularly suitable for reducing the TBA content of isopropanol, to levels below 2 ppm, or even lower. Accordingly, in specific embodiments, the method described herein may be used for reducing the TBA content of an isopropanol feed. In these or other embodiments, the isopropanol feed may comprise at least 5 ppm TBA, at least 10 ppm TBA, or even at least 20 ppm TBA. In such embodiments, the purified isopropanol can contain less than 5 ppm TBA, for example less than 2 ppm TBA, or even less than 1.5 ppm TBA.

[0054] As described above, the method described herein is particularly suitable for removing TBA impurities from isopropanol. The present inventors believe that the method allows for reducing TBA content in isopropanol to low amounts which are extremely difficult to obtain, if at all, using other purification methods such as extractive distillation, membrane filtration, adsorption, and crystallization. Accordingly, provided herein is the use of a higher-pressure distillation step as described herein to reduce TBA content in IP A.

[0055] Fig. 1 illustrates an isopropanol purification process according to a particular embodiment of the method described herein. A feed (1) of technical grade IPA having a purity of at least 99.0 wt% enters a 110-plate distillation column (2) at a location ranging from plates 7-11 for distillation at a pressure of at least 6 bar. The bottom fraction (3) is enriched in heavier molecules such as TBA which may be used as technical grade IPA and is partially returned to the distillation column (2) after passing a reboiler (4), thereby supplying heat to the column. The top fraction (6) passes a condenser (7), is partially recirculated to the high-pressure column (2) and the remainder moves to a 100-plate distillation column (9) via a drum (8). Fraction (6) enters the low-pressure column (9) at a location ranging from plates 90-95 for distillation at a pressure below 5 bar. The top fraction (10) is enriched in low-boiling components such as acetone and moves to a drum (12) after passing a condenser (11). The top fraction may then be recirculated back to the column (9) and / or used as lower grade IPA. The bottom fraction (14) is enriched in IPA and depleted of organic impurities. The fraction (14) may be partially recycled to the column (2) after passing a reboiler (19). The rest of the bottom fraction (14) passes a cooler (15) and optionally moves through an ion extraction unit (16) for removing metals and a filter (17) for removing particulate matter, resulting in purified IPA (18).

[0056] Fig. 2 illustrates an isopropanol purification process according to a further embodiment of the method described herein. This embodiment is identical as the one shown in Fig. 1, except that the IPA-rich fraction is not collected at the bottom of the low-pressure column (9). Instead, the IPA-rich fraction is collected as a vapor side stream slightly above the column bottom (around plate 3), but well below the point of entry of the feed (6) into the column (9). The vapor side stream passes a condenser / cooler (20) and is optionally further purified in an ion exchange unit (21) and filter (22), resulting in purified IPA (23). The use of a vapor side stream is particularly useful to obtain IP A with further reduced metal content. The actual bottom fraction (14) may still be used as lower grade IPA (24) and is partially recycled to the column (9) after passing a reboiler (19).

[0057] Further disclosed are the following embodiments of the methods described herein: Embodiment 1. A method for purifying isopropanol, comprising: (a) providing a feed comprising at least 99.0 wt% isopropanol based on the total feed weight excluding water; and (b) subjecting said feed to two or more distillation steps, including a higher-pressure distillation at a pressure of at least 6 bar; and a lower-pressure distillation at a pressure below 5 bar, wherein said higher-pressure distillation is performed prior to said lower-pressure distillation; thereby obtaining a purified isopropanol.

[0058] Embodiment 2. The method of embodiment 1, wherein said purified isopropanol obtained after said higher-pressure distillation and said lower-pressure distillation is from 30 % to 50 % of the weight of the feed stream.

[0059] Embodiment 3 The method embodiment 1 or 2, wherein said lower-pressure distillation results in 85-95 wt% of a low-pressure isopropanol-enriched fraction and 5-15 wt% of a low-pressure top fraction.

[0060] Embodiment 4. The method of any one of embodiments 1 to 3, wherein said lower-pressure distillation is performed at a pressure from 0.5 bar to 5 bar.

[0061] Embodiment 5. The method of embodiment 4, wherein said lower-pressure distillation is performed at a pressure from 0.5 bar to 3 bar.

[0062] Embodiment 6. The method of any one of embodiments 1 to 5, wherein said feed comprises at least 99.5 wt% isopropanol based on the total weight of said feed excluding water.

[0063] Embodiment 7. The method of any one of embodiments 1 to 6, wherein said purified isopropanol comprises at least 99.8 wt%, and preferably at least 99.9 wt% isopropanol based on the total weight of said purified isopropanol excluding water.

[0064] Embodiment 8. The method of embodiment 7, wherein said purified isopropanol comprises at least 99.99 wt% isopropanol based on the total weight of said purified isopropanol excluding water.

[0065] Embodiment 9. The method of embodiment 8, wherein said purified isopropanol comprises at least 99.999 wt% isopropanol based on the total weight of said purified isopropanol excluding water. Embodiment 10. The method of any one of embodiments 1 to 9, wherein said feed contains tert-butyl alcohol in an amount of at least 5 ppm.

[0066] Embodiment 11. The method of embodiment 10, wherein said purified isopropanol contains tert-butyl alcohol in an amount of 2 ppm or less.

[0067] Embodiment 12. The method of any one of embodiments 1 to 11, wherein said feed contains at least 20 ppm of organic content other than isopropanol.

[0068] Embodiment 13. The method of any one of embodiments 1 to 12, wherein said higher-pressure distillation is performed at a pressure ranging from 8 bar to 20 bar.

[0069] Embodiment 14. The method of any one of embodiments 1 to 12, wherein said higher-pressure distillation is performed at a pressure ranging from 6 bar to 15 bar.

[0070] Embodiment 15. The method of any one of embodiments 1 to 14, wherein said higher-pressure distillation is performed at a pressure ranging from 8 bar to 12 bar.

[0071] Embodiment 16. The method of any one of embodiments 1 to 15, wherein said purified isopropanol contains less than 100 ppm water.

[0072] Embodiment 17. The method of any one of embodiments 1 to 16, further comprising a metal removal step via ion exchange.

[0073] Embodiment 18. The method of any one of embodiments 1 to 17, wherein said higher-pressure distillation results in 40-60 wt% of a high-pressure overhead fraction and 40-60 wt% of a high- pressure bottom fraction.

[0074] Embodiment 19. The method of any one of embodiments 1 to 18, including: obtaining a top fraction from said higher-pressure distillation; entering said top fraction, optionally after further purification, as a low-pressure feed into a distillation column for said lower-pressure distillation; collecting a side stream, preferably a vapor side stream, from said distillation column above the bottom of said distillation column but below where said low-pressure feed enters said distillation column, and optionally further purifying said side stream; thereby obtaining said purified isopropanol.

[0075] Embodiment 20. A composition comprising obtainable via the method of any one of embodiments 1 to 19, said composition comprising at least 99.999 wt% isopropanol based on the total organic weight of said composition; less than 2 ppm tert-butyl alcohol; and optionally 40 ppm to 100 ppm water. EXAMPLES

[0076] The following examples are provided for the purpose of illustrating the present invention and by no means are meant and in no way should be interpreted to limit the scope of the present invention.

[0077] An isopropanol feed having a purity of 99.99% was provided. The feed was prepared via indirect hydration. Sulfuric acid was removed via distillation, followed by water removal via azeotropic distillation. Further impurities are removed via additional distillation, resulting in 99.99% pure isopropanol. The isopropanol feed contained about 130 ppm water, and 10 ppm TBA. A pilot study was performed wherein the feed was first distilled at a pressure of about 9 bar, via 3 batches on a packed distillation column, equivalent to a single-pass distillation using a plate column containing about 110 plates. The final overhead fraction from the 3rdbatch was subsequently distilled at a pressure of about 2 bar, via 2 batches over a packed distillation column, equivalent to a single-pass distillation using a plate column containing about 70 plates. A bottom fraction was collected having an isopropanol purity of more than 99.999%, a water content of less than 150 ppm, and a TBA content of 1 ppm. The relatively high water content can be explained by the highly hygroscopic nature of isopropanol, as no specific precautions were taken to avoid contamination of the sample.

[0078] In a further experiment, an isopropanol feed having a purity of 99.99% was provided, containing about 150 ppm water, and 8.5 ppm TBA. The feed was first distilled at a pressure of about 16.5 bar, via distillation over a packed distillation column (equivalent to a 35-plate column). An overhead fraction collected from the column had an isopropanol purity of 99.99%, a water content of 250 ppm, and a TBA content of 1.3 ppm. The overhead fraction was subsequently distilled at a pressure of about 1.7 bar, via distillation over a packed distillation column, equivalent to distillation using a plate column containing about 35 plates. A bottom fraction was collected having an isopropanol purity of 99.999%, a water content of less than 100 ppm, and a TBA content of 2 ppm.

[0079] Any alterations, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description without departing from the spirit or scope of the present disclosure and that when numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated.

Claims

CLAIMS1. A method for purifying isopropanol, comprising:(a) providing a feed comprising at least 99.0 wt% isopropanol based on the total feed weight excluding water; and(b) subjecting said feed to two or more distillation steps, including a higher-pressure distillation at a pressure of at least 6 bar; and one or more further purification steps including a lower-pressure distillation at a pressure below 5 bar, wherein said higher-pressure distillation is performed prior to said lower-pressure distillation; thereby obtaining purified isopropanol.

2. The method of claim 1, wherein said lower-pressure distillation results in 85-95 wt% of a low-pressure isopropanol-enriched fraction and 5-15 wt% of a low-pressure top fraction.

3. The method of claim 1 or 2, wherein said purified isopropanol comprises at least 99.9 wt% isopropanol based on the total weight of said purified isopropanol excluding water.

4. The method of any one of claims 1 to 3, wherein said feed contains tert-butyl alcohol in an amount of at least 5 ppm.

5. The method of claim 4, wherein said purified isopropanol contains tert-butyl alcohol in an amount of 2 ppm or less.

6. The method of any one of claims 1 to 5, wherein said feed contains at least 20 ppm of organic content other than isopropanol.

7. The method of any one of claims 1 to 6, wherein said higher-pressure distillation is performed at a pressure ranging from 8 bar to 20 bar.

8. The method of any one of claims 1 to 6, wherein said higher-pressure distillation is performed at a pressure ranging from 8 bar to 12 bar.

9. The method of any one of claims 1 to 8, wherein said purified isopropanol contains less than 100 ppm water.

10. The method of any one of claims 1 to 9, wherein said purified isopropanol obtained after said higher-pressure distillation and said lower-pressure distillation is from 30 % to 50 % of the weight of the feed stream.

11. The method of any one of claims 1 to 10, further comprising a metal removal step via ion exchange.

12. The method of any one of claims 1 to 11, wherein said higher-pressure distillation results in 40-60 wt% of a high-pressure overhead fraction and 40-60 wt% of a high-pressure bottom fraction.

13. The method of any one of claims 1 to 12, wherein said lower-pressure distillation is performed at a pressure from 0.5 bar to 5 bar.

14. The method of any one of claims 1 to 13, including: obtaining a top fraction from said higher-pressure distillation; entering said top fraction, optionally after further purification, as a low-pressure feed into a distillation column for said lower-pressure distillation; collecting a side stream from said distillation column above the bottom of said distillation column but below where said low-pressure feed enters said distillation column, and optionally further purifying said side stream; thereby obtaining said purified isopropanol.

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