Method and apparatus for purifying fatty acid isopropyl esters and for producing pure fatty acid isopropyl esters

Abstract

A process for purifying a fatty acid isopropyl ester comprising the steps of: a) providing a crude fatty acid isopropyl ester composition comprising a fatty acid isopropyl ester to be purified, at least one fatty acid and optionally at least one fatty acid ester different from the fatty acid isopropyl ester to be purified, and b) distilling the crude fatty acid isopropyl ester composition provided in step a) in a divided wall column to produce an overhead stream, a bottoms stream and a side stream, wherein the side stream is a purified fatty acid isopropyl ester composition.

Description

[0001] The present invention relates to a method for purifying fatty acid isopropyl esters. The present invention also relates to a method for preparing pure fatty acid isopropyl esters and an apparatus for preparing pure fatty acid isopropyl esters.

[0002] The fatty acid ester of most methyl alcohol and acetate has the smell and taste of characteristic, and therefore only has limited purposes, for example, is used for the solvent or the dispersant of medicine etc., fatty acid isopropyl ester is widely used in industry.For example, fatty acid isopropyl ester is as the raw material of various products (for example cosmetics, food and medicine), as the composition of spices, as wetting agent, as emulsifying agent, as plasticizer, as thickener and other.More specifically, isopropyl laurate is the widely used composition of spices and cosmetics (for example lotion, soap and shampoo).In addition, isopropyl myristate is a kind of polar softening agent (emollient), and is often used as the disinfecting composition of mouthwash, and isopropyl palmitate is then as softening agent, wetting agent, thickener and antistatic agent.Usually, fatty acid isopropyl ester becomes corresponding fatty acid isopropyl ester (for example isopropyl laurate) and water and produces by the catalyst-assisted condensation reaction between fatty acid (for example lauric acid) and the isopropyl alcohol. In order to increase the yield of this reaction, in reaction process, at least some of the water that generates can be removed with some from the isopropyl alcohol of mixture, for example, by distillation or stripping.The reaction mixture of gained comprises target fatty acid isopropyl ester, unreacted lipid acid, residual water, isopropyl alcohol, other impurity and by product.Most isopropyl alcohol and water are removed from reaction mixture by separation (for example, by vaporization step), and the remaining crude mixture that comprises target fatty acid isopropyl ester, unreacted lipid acid, residual water, residual isopropyl alcohol, other impurity and by product is further purified.This further purification is carried out by two follow-up distillation steps usually.When in the first distillation step that carries out in the first distillation tower, the low-boiling-point component of mixture (promptly having the composition of the boiling point lower than target fatty acid isopropyl ester to be purified) is removed as overhead stream, bottoms is fed to the second distillation tower, and wherein the high-boiling-point component of mixture (promptly having the composition of the boiling point higher than target fatty acid isopropyl ester to be purified) is removed as bottoms stream. The purified target fatty acid isopropyl esters are obtained during the second distillation step as an overhead fraction or side draw, which can subsequently be purified further, for example by a stripping step with nitrogen.

[0003] Although the purified fatty acid isopropyl ester composition obtained in this way has a high content of fatty acid isopropyl ester, for example, more than 99% by weight, it still contains some key impurities of excessive amount. These key impurities are mainly unreacted fatty acids, and not only have the fatty acids corresponding to the fatty acids contained as components in the purified fatty acid isopropyl ester, but also similar fatty acids. Similar fatty acids refer to fatty acids different from the fatty acids contained as components in the target fatty acid isopropyl ester, only in that it contains one to four methylene groups or one to four methylene groups. Such similar fatty acids are also referred to as adjacent fatty acids. These similar fatty acids are usually derived from the starting fatty acid composition, because the purified fatty acid will not be 100% pure, but still contain a certain amount of similar fatty acids. Therefore, for example, palmitic acid usually contains a small amount of myristic acid, and myristic acid usually contains a small amount of lauric acid. A part of these included similar fatty acids will be esterified to their corresponding fatty acid isopropyl esters in the condensation reaction process, and the remainder of these similar fatty acids in the reaction mixture remains unesterified. The present invention relates to the purification of fatty acid isopropyl ester.Other impurities are non-even-numbered fatty acids (for example undecanoic acid, tridecanoic acid, pentadecanoic acid (pentacylic acid) etc.), palmitoleic acid and leaches (leaches) from acidic reaction catalyst.Unreacted fatty acid remaining in the purified fatty acid isopropyl ester causes the relatively high acid number of fatty acid isopropyl ester composition, for example higher than the acid number of 0.5mg KOH / g sample.Yet, for application (for example in personal care products, cosmetics and pharmaceutical products, as the purposes in soap, perfume, medicine etc.), except other purity standards (for example absolute purity, water content, alcohol content and smell), extremely low acid number is an important purity standard.Yet, with known two-step distillation purification process, it is impossible to further reduce the acid number (i.e. the content of free fatty acids and corresponding similar free fatty acids) from purified fatty acid isopropyl ester.Other shortcoming of known two-step distillation purification process is their high running cost (OPEX) and the required relatively high capital cost (CAPEX) of equipment.

[0004] In view of this, it was an object of the present invention to provide a process for the purification of crude fatty acid isopropyl ester compositions which overcomes the aforementioned disadvantages, i.e. to provide a process for the purification of crude fatty acid isopropyl ester compositions which produces a purified fatty acid isopropyl ester composition having an improved purity and, in particular, a reduced acid number, wherein, moreover, the process is continuous, has reduced energy consumption and requires equipment with relatively low investment costs.

[0005] According to a first aspect of the present invention, this object is met by providing a method for purifying fatty acid isopropyl esters, comprising the steps of:

[0006] a) providing a crude fatty acid isopropyl ester composition comprising fatty acid isopropyl ester to be purified, at least one fatty acid and optionally at least one fatty acid ester different from the fatty acid isopropyl ester to be purified, and

[0007] b) distilling the crude fatty acid isopropyl ester composition provided in step a) in a dividing wall column to produce an overhead stream, a bottom stream, and a side stream, wherein the side stream is a purified fatty acid isopropyl ester composition.

[0008] This solution is based on the following discovery: by using a distillation step in a dividing wall (distillation) column, not only can the operating costs and the relatively high capital costs required for the equipment be reduced, since only one distillation column, not two, needs to be operated, but it is particularly surprising and unexpected that the purity of the resulting purified fatty acid isopropyl ester composition is also significantly improved, in particular by significantly reducing its acid number, compared to a corresponding method comprising two subsequent conventional distillations in a non-dividing wall column. More specifically, the free fatty acid content in the purified fatty acid isopropyl ester composition obtained according to the inventive method can be reduced to about one-third compared to the content of free fatty acids in the purified fatty acid isopropyl ester composition obtained based on the aforementioned prior art method of two subsequent distillation steps not carried out in a dividing wall column. The present inventors have found that the behavior of fatty acid isopropyl esters and their corresponding fatty acids and similar fatty acids (e.g., adjacent fatty acids) is not ideal. For example, isopropyl palmitate and myristic acid form an azeotrope, while isopropyl palmitate and palmitic acid exhibit a non-ideal gas / liquid equilibrium. Thus, a further reduction in the free fatty acid content of the purified fatty acid isopropyl esters, and in particular in the content of very similar free fatty acids, is only possible with known purification processes based on two subsequent distillation steps not carried out in a dividing wall column, with a considerable reduction in capacity. However, the use of a dividing wall column surprisingly allows a further reduction in the content of such free fatty acids, presumably because the total reflux ratio of the dividing wall column is increased compared to conventional distillation columns, because of the ratio of the reflux rate divided by the side stream rate, and because the separation limitations due to the non-ideal gas / liquid equilibrium between the isopropyl ester and its corresponding or similar free fatty acid are in fact overcome.

[0009] The present invention is not particularly limited to the type of fatty acid component of the fatty acid isopropyl ester contained in the crude fatty acid isopropyl ester composition and that must be purified. Preferably, the fatty acid component of the fatty acid isopropyl ester contained in the crude fatty acid isopropyl ester composition and that must be purified is a saturated fatty acid, which refers to a fatty acid with a saturated aliphatic chain. More preferably, the fatty acid component of the fatty acid isopropyl ester contained in the crude fatty acid isopropyl ester composition and that must be purified is an unbranched saturated fatty acid, still more preferably an even-numbered unbranched saturated fatty acid, and even more preferably an even-numbered C 6-22Unbranched saturated fatty acids. The fatty acid component of the fatty acid isopropyl esters contained in the crude fatty acid isopropyl ester composition and that must be purified may not be a naturally occurring fatty acid. However, it is preferred that it is a naturally occurring fatty acid. Most preferably, the fatty acid component of the fatty acid isopropyl esters contained in the crude fatty acid isopropyl ester composition and that must be purified is lauric acid, myristic acid, palmitic acid, stearic acid, or any mixture of two or more of the foregoing fatty acids.

[0010] Therefore, it is particularly preferred that the crude fatty acid isopropyl ester composition provided in step a) contains isopropyl laurate, isopropyl myristate, isopropyl palmitate, isopropyl stearate or any mixture of two or more of the aforementioned fatty acid isopropyl esters as the fatty acid isopropyl ester to be purified.

[0011] Preferably, the crude fatty acid isopropyl ester composition provided in step a) is a reaction mixture obtained by a catalyst-assisted condensation reaction between a fatty acid (preferably one of the aforementioned fatty acids) and isopropyl alcohol, or a pre-purified reaction mixture obtained by a catalyst-assisted condensation reaction between the fatty acids and by subsequent removal of a portion of isopropyl alcohol and water from the reaction mixture. This crude fatty acid isopropyl ester composition contains the target fatty acid isopropyl ester to be purified, more or less isopropyl alcohol, more or less water, a small amount of other fatty acid isopropyl esters and a small amount of residual fatty acids, in particular fatty acids corresponding to the fatty acid component of the target fatty acid isopropyl ester and fatty acids similar thereto.

[0012] For example, the crude fatty acid isopropyl ester composition provided in step a) contains, based on the total weight of the crude fatty acid isopropyl ester composition, 20 to 80 wt.-% and more preferably 30 to 70 wt.-% of fatty acid isopropyl ester to be purified, 30 to 80 wt.-% and more preferably 30 to 70 wt.-% of one or more fatty acids, 0.2 to 2.0 wt.-% and more preferably 0.1 to 1.0 wt.-% of one or more fatty acid isopropyl esters other than the fatty acid isopropyl ester to be purified, 0.001 to 1.0 wt.-% and more preferably 0.01 to 0.2 wt.-% of isopropyl alcohol, and up to 0.5 wt.-% and more preferably 0.01 to 0.2 wt.-% of water.

[0013] According to the present invention, in step b), the crude fatty acid isopropyl ester composition provided in step a) is distilled in a dividing wall column. The present invention is not particularly limited by the type of dividing wall column. Thus, in principle, any dividing wall column can be used, i.e., any distillation column comprising at least one wall that divides at least one axial section of the interior of the distillation column into two different subsections.

[0014] In a further development of the concept of the invention, it is proposed that the dividing wall column used in step b) comprises a dividing wall which extends at least substantially vertically downwards. At least substantially vertically downwards in this context means that the angle between the dividing wall and the longitudinal axis of the dividing wall column is at most 20°, preferably at most 10°, more preferably at most 5° and most preferably 0°.

[0015] The dividing wall column used in step b) can be a top dividing wall column, an intermediate dividing wall column, or a bottom dividing wall column. In a top dividing wall column, the dividing wall is contained in the top section of the distillation column, i.e., the dividing wall extends from the top of the distillation column (to which the dividing wall is attached) downward in the direction of the bottom of the distillation column to a certain extent, for example, up to 90%, up to 80%, up to 70%, up to 60%, up to 50%, or up to 40% of the height of the distillation column, as viewed from the top to the bottom of the distillation column. Thus, the bottom section of the top dividing wall column remains undivided. Thus, the dividing wall ends at a height above the bottom of the distillation column, as viewed from the top to the bottom of the distillation column, of at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, or at least 60% of the height of the column.

[0016] Similarly, the dividing wall of a bottom dividing wall column is contained in the bottom section of the distillation column, i.e., the dividing wall extends from the bottom of the distillation column (to which it is attached) upward in the direction of the top of the distillation column to a certain extent, for example, up to 90%, up to 80%, up to 70%, up to 60%, up to 50%, or up to 40% of the height of the distillation column, as viewed from the bottom to the top of the distillation column. Thus, the dividing wall ends at a point at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, or at least 60% of the height of the distillation column, as viewed from the bottom to the top of the distillation column. Thus, the top section of the distillation column remains undivided.

[0017] Similarly, the dividing wall of the intermediate dividing wall column is contained in the middle section of the distillation column, i.e., the dividing wall does not contact either the top or bottom of the dividing wall column, but rather extends so that the top and bottom sections of the distillation column remain undivided. Preferably, the dividing wall of the intermediate dividing wall column extends from at least 10% to at most 90% of the height of the dividing wall column, preferably from at least 20% to at most 80%, and more preferably from at least 25% to at most 25%. In other words, the dividing wall of the intermediate dividing wall column is at a distance from the top and bottom of the intermediate dividing wall column that is, independently, at least 10%, at least 20%, or at least 25% of the height of the dividing wall column. For example, the dividing wall of the intermediate dividing wall column extends from 10% to 70%, 20% to 75%, or 25% to 75% of the height of the dividing wall column, viewed from top to bottom along the longitudinal axis of the intermediate dividing wall column.

[0018] Regardless of whether a top dividing wall column, an intermediate dividing wall column, or a bottom dividing wall column is used, it is preferred that, as viewed in cross section of the distillation column, the dividing wall divides the axial portion of the distillation column into approximately two equally sized subsections or halves, respectively. This means that the dividing wall is arranged, based on the cross section of the dividing wall column, so as to divide the cross section of the dividing wall column into two subsections, wherein each of the two subsections covers at least 40%, more preferably at least 45%, and most preferably 50%, of the total area of ​​the cross section of the dividing wall column, with the total area covered by the two subsections naturally being 100%. For example, one subsection may cover 43% of the total area of ​​the cross section and the other may cover 57% of the total area of ​​the cross section, or both subsections may each cover exactly 50% of the total area of ​​the cross section.

[0019] According to a particularly preferred embodiment of the present invention, the dividing wall column used in step b) is an intermediate dividing wall column comprising:

[0020] an undivided top section for concentrating low-boiling components, comprising an outlet for an overhead stream of low-boiling components, wherein the low-boiling components have a lower boiling point than the fatty acid isopropyl esters to be purified,

[0021] an undivided bottom section for concentrating high-boiling components, comprising an outlet for a bottom stream of high-boiling components, wherein the high-boiling components have a higher boiling point than the fatty acid isopropyl esters to be purified, and

[0022] - a middle section arranged between the top section and the bottom section, having at least one dividing wall for dividing the middle section into at least one feed sub-section and at least one product withdrawal sub-section, the feed sub-section having an inlet for feeding the crude fatty acid isopropyl ester composition, and the product withdrawal sub-section having a side withdrawal outlet for the purified fatty acid isopropyl ester composition.

[0023] In this context, a "segment" refers to an axially extending section of the interior of a distillation column, wherein the sum of all three segments (i.e., the top segment, the bottom segment, and the middle segment) constitutes the entire interior of the distillation column. An undivided segment is a segment that does not contain any walls, let alone any walls that separate the segment into two or more sub-segments.

[0024] According to another particularly preferred embodiment of the present invention, the side take outlet for the purified fatty acid isopropyl ester composition is arranged in the middle section of the intermediate dividing wall column, above the inlet for the feed of the crude fatty acid isopropyl ester composition. This means that the side take outlet for the purified fatty acid isopropyl ester composition is arranged in the product take-off subsection, vertically above the position where the inlet for the feed of the crude fatty acid isopropyl ester composition is arranged in the feed subsection of the dividing wall column, as seen from the bottom to the top in the length direction of the distillation column. The inventors of the present invention have found that a particularly effective separation of free fatty acids, and in particular fatty acids corresponding to the fatty acid component of the target fatty acid isopropyl ester, and fatty acids similar thereto is achieved thereby.

[0025] In a further development of the concept of the invention, it is proposed that the feed subsection and the product removal subsection of the middle section of the dividing wall column each contain at least two packed beds, wherein adjacent packed beds are arranged on top of each other. Such packed beds are commonly used in distillation columns to increase the contact area between the liquid phase descending the distillation column and the gas phase ascending the distillation column during operation. Preferably, a distributor is arranged on top of each packed bed to evenly distribute the liquid phase over the cross section of the bed while leaving sufficient space for the gas phase to ascend through it. Furthermore, a grid-like retaining device and a collector are preferably arranged below each packed bed, wherein the grid-like structure holds the packed bed in place and the collector collects the liquid phase dripping from the packed bed while leaving sufficient open space in the collector for the gas phase to ascend.

[0026] Each packed bed can contain one or more structured packing elements or random packing. The structured packing bed contains one or more layers of structured packing elements, which are stacked vertically on top of each other, wherein each layer consists of one or more structured packing elements. The larger the diameter of the structured packing bed, the greater the number of structured packing elements per layer. The structured packing elements contain a plurality of different sheets, each sheet providing a surface area for the liquid phase that drips and spreads along the surface of the sheet. In addition, open spaces are provided between the different sheets of the structured packing elements, which are filled with a gas phase and provide an upward path for the gas phase, while the gas phase is driven by a pressure gradient. By allowing the liquid phase to diffuse on the surface of the structured packing elements, an interface is created between the liquid phase and the rising gas phase so that efficient heat and mass transfer between the two phases is established at the interface.

[0027] Preferably, each structured packing element is a cross-channel corrugated sheet packing, which is assembled from a plurality of corrugated sheets arranged in parallel and in contact with each other. Typically, the corrugated metal sheets are fixed to each other by means of a number of rods penetrating the corrugated sheets perpendicular to the longitudinal sections of the corrugated sheets, wherein the rods are fixed with the first and last corrugated sheets by means of washers and nuts or by bending the rods. Each corrugated sheet comprises a plurality of alternatingly oriented peaks and valleys, wherein adjacent corrugated sheets are oriented so that the corrugations of adjacent corrugated sheets intersect in a cross manner with the corrugations of the corrugated sheets extending obliquely with respect to the vertical or longitudinal direction, thereby forming inclined channels that continuously intersect with each other. These inclined channels actively influence the flow of gas and liquid phases within the packing and promote mass and / or heat transfer between the phases.

[0028] Good results are particularly achieved when the feed subsection and the product removal subsection of the middle section of the dividing wall column each contain at least three packed beds, more preferably at least four packed beds and most preferably four packed beds, wherein adjacent packed beds are stacked one on top of another and wherein each packed bed preferably contains one or more structured packing elements or random packing. All packed beds completely cover the surface area of ​​the respective middle section, i.e. the sum of the feed subsection and the product removal subsection. Each structured packing element may have a surface area of ​​125 to 900 m 2 / m 3 , preferably 200 to 750m 2 / m 3 and more preferably 250 to 500 m 2 / m 3 The specific surface area is within a range of 100 Å. All the packed beds may have the same height or different heights. In addition, the packed bed of the feed sub-section and the packed bed of the withdrawal sub-section may be arranged parallel to each other, that is, the lower end or upper end of the packed bed of the feed sub-section is at the same height as the corresponding end of the packed bed of the withdrawal sub-section, as viewed in the length direction of the distillation column.

[0029] In a further development of the concept of the present invention, it is recommended that the inlet for the feed of the crude fatty acid isopropyl ester composition is located between two adjacent packed beds of the feed subsection, and the outlet for the purified fatty acid isopropyl ester composition is located between two adjacent packed beds of the product withdrawal subsection, wherein the side withdrawal outlet for the purified fatty acid isopropyl ester composition is preferably arranged in the withdrawal subsection of the middle section of the intermediate dividing wall column, above the inlet for the feed of the crude fatty acid isopropyl ester composition.

[0030] In a preferred embodiment of the present invention, the feed subsection and the product take-out subsection of the middle section of the intermediate dividing wall column each comprise four packed beds, wherein, from the bottom of the dividing wall column to the top, the inlet for the feed of the crude fatty acid isopropyl ester composition is located between the first and second adjacent packed beds of the feed subsection, and the outlet for the purified fatty acid isopropyl ester composition is located between the third and fourth adjacent packed beds of the product take-out subsection of the dividing wall column. Furthermore, all packed beds can have the same height or different heights. In addition, the packed beds of the feed subsection and the packed beds of the take-out subsection can be arranged parallel to each other, i.e., from the length direction of the distillation tower, the lower end or upper end of the packed bed of the feed subsection is at the same height as the corresponding end of the packed bed of the take-out subsection.

[0031] In an alternative preferred embodiment of the present invention, the feed subsection and the product removal subsection of the middle section of the dividing wall column each comprise four packed beds, wherein, viewed from the bottom to the top of the dividing wall column, the inlet for the feed of the crude fatty acid isopropyl ester composition is located between the second and third adjacent packed beds in the feed subsection, and wherein the outlet for the purified fatty acid isopropyl ester composition is located between the second and third adjacent packed beds in the product removal subsection, wherein the upper end of the second packed bed in the product removal subsection is above the lower end of the third packed bed in the feed subsection of the dividing wall column. In other words, in this embodiment, the outlet for the purified fatty acid isopropyl ester composition is also located above the inlet for the feed of the crude fatty acid isopropyl ester composition.

[0032] The top and bottom sections of the intermediate dividing wall column (which are located above and below the middle section) need not include packed beds. However, preferably, the top section of the intermediate dividing wall column contains at least one, and preferably one, packed bed and / or the bottom section of the intermediate dividing wall column contains at least one, and preferably one, packed bed. More preferably, the top section of the intermediate dividing wall column contains one packed bed and the bottom section of the intermediate dividing wall column contains one packed bed. Furthermore, each packed bed may contain one or more structured packing elements or random packing.

[0033] When dividing wall column has at least 40, preferably at least 45, more preferably at least 50 and more preferably at least 60 theoretical stages in feed subsection and product taking out subsection respectively during its operation, especially good results are obtained aspect the separation efficiency between target fatty acid isopropyl ester to be purified and the fatty acid corresponding to the fatty acid component of target fatty acid isopropyl ester and the fatty acid similar thereto. The feed subsection can have the theoretical stage or the theoretical stage of different numbers of taking out subsection same number as the product. For example, the feed subsection can have 50 theoretical stages and the product takes out subsection has 42 theoretical stages, or the feed subsection and the product take out subsection have 50 theoretical stages respectively. Preferably, the feed subsection has the theoretical stage of taking out subsection same number as the product.

[0034] Preferably, during operation of the dividing wall column, the pressure and temperature at the bottom of the dividing wall column are in the range of 5 to 100 mbar and 150 to 300° C., more preferably in the range of 10 to 60 mbar and 180 to 280° C., and still more preferably in the range of 30 to 50 mbar and 220 to 240° C. Furthermore, it is preferred that during operation of the dividing wall column, the pressure and temperature at the top of the dividing wall column are in the range of 1 to 100 mbar and 100 to 280° C., more preferably in the range of 12 to 30 mbar and 150 to 220° C., and still more preferably in the range of 10 to 20 mbar and 180 to 200° C., wherein the temperature at the top of the dividing wall column is lower than the temperature at the bottom of the dividing wall column.

[0035] In a further development of the inventive concept, it is proposed that the dividing wall column used in step b) has a top condenser in fluid communication with the top portion and a reboiler in fluid communication with the bottom portion of the intermediate dividing wall distillation column. More specifically, it is preferred that the dividing wall column used in step b) has a top condenser having an inlet connected via a line to the top portion or another portion of the top section of the dividing wall column and an outlet connected to a line for condensed liquid, the line branching into a liquid removal line and a recycle line for recycling a portion of the liquid condensed in the condenser to the top section of the dividing wall distillation column. Similarly, it is preferred that the dividing wall column used in step b) has a reboiler having an inlet connected via a line to the bottom portion or another portion of the bottom section of the dividing wall column and an outlet connected to a line for a gas stream, the line branching into a gas removal line and a recycle line for recycling a portion of the gas generated in the reboiler to the bottom section of the dividing wall distillation column.

[0036] According to particularly preferred embodiments of the present invention, the reflux ratio is 200 to 1,000, preferably 300 to 900 and more preferably 500 to 800. The reflux ratio is the ratio of the amount of liquid condensed in the overhead condenser and recycled to the dividing wall column divided by the amount of liquid condensed in the overhead condenser and withdrawn from the distillation column (e.g., via a liquid removal line).

[0037] Furthermore, it is preferred that the ratio of the reflux rate divided by the side stream rate is from 4 to 10, more preferably from 5 to 8 and most preferably from 6 to 7. The ratio of the reflux rate divided by the side stream rate is the amount of liquid condensed in the overhead condenser and recycled to the dividing wall column divided by the amount of the side stream removed from the dividing wall column.

[0038] Taking into account the separation efficiency of the dividing wall column used in step b), the process according to the invention does not require any further distillation steps to be carried out in addition to the distillation step carried out in the dividing wall column.

[0039] Optionally, the purified fatty acid isopropyl ester composition obtained in step b) of the process can, but need not necessarily, be subjected to a further stripping step, for example with nitrogen, in step c) .Thus, potential compounds causing residual odors can be completely removed.

[0040] As described above, the process according to the invention is characterized in that the separation efficiency of fatty acids, and in particular fatty acids corresponding to the fatty acid component of the target fatty acid isopropyl ester and fatty acids similar thereto, from the target fatty acid isopropyl ester is particularly good. Therefore, it is preferred that the purified fatty acid isopropyl ester composition obtained as a side stream in step b) of the process according to the invention contains at least 99.00% by weight, preferably at least 99.30%, more preferably at least 99.50%, even more preferably at least 99.60% and most preferably at least 99.70% of the target fatty acid isopropyl ester.

[0041] Similarly, it is preferred that the purified fatty acid isopropyl ester composition obtained as a side stream in step b) of the process according to the present invention contains at most 0.070 wt.-%, preferably at most 0.060 wt.-%, more preferably at most 0.050 wt.-%, even more preferably at most 0.040 wt.-% and most more preferably at most 0.030 wt.-% of impurities comprising free fatty acids and other fatty acid isopropyl esters than the target fatty acid isopropyl esters.

[0042] Thus, a purified fatty acid isopropyl ester composition having an acid value of less than 1.0 mgKOH / g sample, preferably less than 0.5 mgKOH / g sample, more preferably less than 0.4 mgKOH / g sample and most preferably less than 0.3 mgKOH / g sample is obtained. The acid value of the fatty acid isopropyl ester composition is determined according to the present invention by titration or by gel chromatography. And more preferably by AOCSTe 1a-64.

[0043] Preferably, the overhead stream obtained in step b) contains at most 80% by weight and more preferably at most 70% by weight of the target fatty acid isopropyl ester to be purified. Conversely, the overhead stream obtained in step b) preferably contains at least 20% by weight and more preferably at least 30% by weight of the sum of fatty acids, water, isopropanol and fatty acid isopropyl esters other than the target fatty acid isopropyl ester to be purified.

[0044] Preferably, the bottom stream obtained in step b) contains at most 30% by weight, more preferably at most 20% by weight, even more preferably at most 15% by weight and most preferably at most 10% by weight of the target fatty acid isopropyl ester to be purified. Conversely, the bottom stream obtained in step b) preferably contains at least 70% by weight, more preferably at least 80% by weight, even more preferably at least 85% by weight and most preferably at least 90% by weight of the sum of fatty acids and fatty acid isopropyl esters other than the fatty acid isopropyl ester to be purified.

[0045] According to a second aspect, the present invention relates to a process for the preparation of pure fatty acid isopropyl esters comprising the following steps:

[0046] i) providing a starting mixture containing fatty acids and isopropanol,

[0047] ii) reacting the starting mixture, preferably in the presence of a catalyst, to obtain a reaction mixture containing fatty acid isopropyl ester, unreacted fatty acid, unreacted isopropanol and optionally water, by-products and other impurities,

[0048] iii) optionally, removing at least a portion of the isopropanol from the reaction mixture by vaporization to obtain a modified reaction mixture, and

[0049] iv) carrying out the method for purifying fatty acid isopropyl esters as described above, wherein the crude fatty acid isopropyl ester composition provided in step a) is the reaction mixture obtained in step ii) or the modified reaction mixture obtained in step iii).

[0050] All features described above in relation to the first aspect of the invention (ie in relation to the process for purifying fatty acid isopropyl esters) are also preferred for the second aspect of the invention (ie in relation to step iv) of the process for preparing pure fatty acid isopropyl esters).

[0051] In particular, the purified fatty acid isopropyl ester composition obtained in step b) of step iv) may be further subjected to a stripping step, for example a stripping step with nitrogen.

[0052] According to a third aspect, the present invention relates to an apparatus for preparing pure fatty acid isopropyl esters, comprising:

[0053] i) at least one reactor comprising one or more inlets for fatty acids and isopropanol or a mixture of fatty acids and isopropanol, and an outlet for a reaction mixture containing fatty acid isopropyl esters, unreacted fatty acid, unreacted isopropanol and optionally water, by-products and other impurities,

[0054] ii) optionally, a separation device for separating at least a portion of the isopropanol from the reaction mixture to obtain a modified reaction mixture, wherein the separation device comprises an inlet connected to the outlet of the reactor for the reaction mixture, an outlet for isopropanol and an outlet for the modified reaction mixture, and

[0055] iii) A dividing wall column comprising an inlet which is connected to the outlet of the reactor for the reaction mixture and / or to the outlet of the separation device for the modified reaction mixture.

[0056] For example, reactor a) is a vessel containing the catalyst.

[0057] In an alternative embodiment of the present invention, at least one reactor a) is a distillation column, it comprises outlet and the overhead outlet for the reaction mixture as bottom outlet, wherein the overhead outlet is connected with a water-removing unit by a pipeline, wherein the water-removing unit comprises the outlet for water and the outlet for Virahol, the outlet wherein being used for Virahol is connected by a recirculation line with reactor a) for the inlet for Virahol or (indirectly) with reactor a) for the inlet of the mixture of lipid acid and Virahol. In the latter's alternative, the outlet for Virahol is connected with the container for mixing Virahol and lipid acid, wherein this container has the outlet line connecting this container with the inlet for lipid acid and Virahol mixture. Therefore, can from mixture, constantly remove Virahol and reaction product water in reaction process, so that reaction equilibrium is moved to the product side.

[0058] In another embodiment of the present invention, this equipment can comprise two reactors, wherein the first reactor is as pre-reactor, and is the container that comprises one or more inlets that are used for lipid acid and isopropyl alcohol or the mixture of lipid acid and isopropyl alcohol and the outlet for pre-reaction mixture, described pre-reaction mixture contains fatty acid isopropyl ester, unreacted lipid acid, unreacted isopropyl alcohol and optionally water, by product and other impurity.The second reactor can be a distillation column, it comprises the inlet that is connected with the outlet for pre-reaction mixture of the first reactor, as the outlet and the overhead outlet for (final) reaction mixture of bottom outlet, wherein this overhead outlet is connected with the water-removing unit by pipeline.The water-removing unit comprises the outlet for water and the outlet for isopropyl alcohol, wherein the outlet that is used for isopropyl alcohol is connected with the inlet for isopropyl alcohol of the first reactor by recirculation line or (indirectly) with the inlet for lipid acid and the mixture of isopropyl alcohol of the first reactor.In the latter's alternative, the outlet that is used for isopropyl alcohol is connected with the container that is used to mix isopropyl alcohol and lipid acid, wherein this container has the outlet pipeline that connects this container with the inlet for lipid acid and isopropyl alcohol mixture. Therefore, isopropyl alcohol and reaction product water can be continuously removed from the mixture during the reaction to shift the reaction equilibrium toward the product side.

[0059] The water removal unit may be a vaporizer, a distillation column and preferably a pervaporation unit.

[0060] According to a further preferred embodiment of the present invention, the apparatus comprises an optional separating device b), wherein the outlet for isopropanol of the separating device b) is connected via a recirculation line to the inlet for isopropanol of the reactor a) or to the inlet for the mixture of fatty acids and isopropanol of the reactor a), wherein the outlet for isopropanol is connected to a vessel for mixing isopropanol and fatty acids, the outlet line connecting the vessel to the inlet for the mixture of fatty acids and isopropanol.

[0061] The separation device may be a distillation column and is preferably a vaporizer.

[0062] All of the features described above with respect to the first aspect of the present invention (i.e., with respect to the method for purifying fatty acid isopropyl esters) are also preferred for the third aspect of the present invention. Specifically, all of the features described above with respect to the dividing wall distillation column of the first aspect of the present invention are also preferred for the dividing wall distillation column iii) of the third aspect of the present invention.

[0063] Therefore, the dividing wall column is preferably an intermediate dividing wall column comprising a dividing wall which extends at least essentially vertically downwards, i.e. wherein the angle between the dividing wall and the longitudinal axis of the dividing wall column is at most 20°, preferably at most 10°, more preferably at most 5° and most preferably 0°.

[0064] Preferably, the dividing wall is contained in the middle section of the distillation column, i.e., the dividing wall does not contact either the top or the bottom of the dividing wall column, but rather extends such that the top and bottom sections of the distillation column remain undivided. Preferably, the dividing wall of the intermediate dividing wall column extends, viewed along the longitudinal axis of the intermediate dividing wall column, for at least 10% to at most 90%, preferably at least 20% to at most 80%, and more preferably at least 25% to at most 25% of the height of the dividing wall column.

[0065] Furthermore, it is preferred that the dividing wall divides the axial section of the distillation column into two subsections, viewed in the cross section of the distillation column, wherein the two subsections each cover at least 40%, more preferably at least 45% and most preferably 50% of the total area of ​​the cross section of the dividing wall column, the sum of the coverage of the two subsections being of course 100%.

[0066] According to a particularly preferred embodiment of the present invention, the dividing wall column iii) is an intermediate dividing wall column comprising:

[0067] an undivided top section for concentrating low-boiling components, comprising an outlet for a top stream of low-boiling components, wherein the low-boiling components have a lower boiling point than the fatty acid isopropyl esters to be purified,

[0068] an undivided bottom section for concentrating high-boiling components, comprising an outlet for a bottom stream of high-boiling components, wherein the high-boiling components have a higher boiling point than the fatty acid isopropyl esters to be purified, and

[0069] - a middle section arranged between the top section and the bottom section, which has at least one dividing wall for dividing the middle section into at least one feed sub-section and at least one product withdrawal sub-section, the feed sub-section having an inlet for feeding the crude fatty acid isopropyl ester composition and the product withdrawal sub-section having a side withdrawal outlet for the purified fatty acid isopropyl ester composition.

[0070] According to a further particularly preferred embodiment of the present invention, the side draw outlet for the purified fatty acid isopropyl ester composition is arranged in the product withdrawal subsection of the middle section of the intermediate dividing wall column, above the inlet for the feed of the crude fatty acid isopropyl ester composition.

[0071] In a further development of the concept of the invention, it is proposed that the feed subsection and the product withdrawal subsection of the middle section of the dividing wall column each comprise at least two, preferably at least three, more preferably at least four, and most preferably four, packed beds arranged one above the other. Each packed bed preferably comprises one or more structured packing elements, more preferably cross-channel corrugated sheet packing.

[0072] In a particularly preferred embodiment of the present invention, the inlet for the feed of the crude fatty acid isopropyl ester composition is located between two adjacent packed beds of the feed subsection, and the outlet for the purified fatty acid isopropyl ester composition is located between two adjacent packed beds of the product withdrawal subsection, wherein if the middle section of the intermediate dividing wall column is above the inlet for the feed of the crude fatty acid isopropyl ester composition, the side outlet for the purified fatty acid isopropyl ester composition is arranged in the product withdrawal subsection.

[0073] In a preferred embodiment of the present invention, the feed subsection and the product take-out subsection of the middle section of the intermediate dividing wall column each comprise four packed beds, wherein from the bottom of the dividing wall column to the top, the inlet for the feed of the crude fatty acid isopropyl ester composition is located between the first and second adjacent packed beds of the feed subsection, and wherein the outlet for the purified fatty acid isopropyl ester composition is located between the third and fourth adjacent packed beds of the product take-out subsection of the dividing wall column. All packed beds can have the same height or different heights. In addition, the packed beds of the feed subsection and the packed beds of the take-out subsection can be arranged parallel to each other, i.e., in the length direction of the distillation tower, the lower end or upper end of the packed bed of the feed subsection is at the same height as the corresponding end of the packed bed of the take-out subsection.

[0074] In an alternative preferred embodiment of the present invention, the feed subsection and the product removal subsection of the middle section of the dividing wall column each comprise four packed beds, wherein the inlet for the feed of the crude fatty acid isopropyl ester composition is located between the second and third adjacent packed beds in the feed subsection, as viewed from the bottom to the top of the dividing wall column, and wherein the outlet for the purified fatty acid isopropyl ester composition is located between the second and third adjacent packed beds in the product removal subsection, wherein the upper end of the second packed bed in the product removal subsection is above the lower end of the third packed bed in the feed subsection of the dividing wall column. In other words, the outlet for the purified fatty acid isopropyl ester composition is located above the inlet for the feed of the crude fatty acid isopropyl ester composition.

[0075] The top and bottom sections of the intermediate dividing wall column located above and below the middle section need not include a packed bed. However, preferably, the top section of the intermediate dividing wall column includes at least one, and preferably one, packed bed and / or the bottom section of the intermediate dividing wall column includes at least one, and preferably one, packed bed. More preferably, the top section of the intermediate dividing wall column includes one packed bed and the bottom section of the intermediate dividing wall column includes one packed bed.

[0076] According to yet another aspect, the present invention relates to an apparatus for preparing pure fatty acid isopropyl esters, comprising:

[0077] i) at least one reactor, one or more inlets for fatty acids and isopropanol or a mixture of fatty acids and isopropanol, and an outlet for a reaction mixture containing fatty acid isopropyl esters, unreacted fatty acid, unreacted isopropanol and optionally water, by-products and other impurities,

[0078] ii) optionally, a separation device for separating at least a portion of the isopropanol from the reaction mixture to obtain a modified reaction mixture, wherein the separation device comprises an inlet connected to the outlet of the reactor for the reaction mixture, an outlet for the isopropanol and an outlet for the modified reaction mixture, and

[0079] iii) a dividing wall column comprising an inlet connected to the outlet of the reactor for the reaction mixture and / or to the outlet of the separation device for the modified reaction mixture,

[0080] Wherein the dividing wall column is an intermediate dividing wall column, which comprises:

[0081] - an undivided top section,

[0082] - undivided bottom section and

[0083] - a middle section arranged between the top section and the bottom section, which has at least one dividing wall for dividing the middle section into at least one feed sub-section and at least one product withdrawal sub-section, the feed sub-section having an inlet for the feed of the crude fatty acid isopropyl ester composition and the product withdrawal sub-section having a side withdrawal outlet for the purified fatty acid isopropyl ester composition, wherein the feed sub-section and the product withdrawal sub-section of the middle section of the dividing wall column each contain at least two packed beds, wherein adjacent packed beds are stacked on each other, and wherein the inlet for the feed of the crude fatty acid isopropyl ester composition is located between two adjacent packed beds of the feed sub-section, and the outlet for the purified fatty acid isopropyl ester composition is located between two adjacent packed beds of the product withdrawal sub-section, wherein the side withdrawal outlet for the purified fatty acid isopropyl ester composition is arranged in the middle sub-section of the intermediate dividing wall column, above the inlet for the feed of the crude fatty acid isopropyl ester composition.

[0084] Preferably, the feed subsection and the product withdrawal subsection of the middle section of the dividing wall column each comprise at least three and most preferably four packed beds, wherein adjacent packed beds are arranged one above the other.

[0085] Furthermore, it is preferred that each packed bed comprises one or more structured packing elements or random packing.

[0086] The invention is then described by means of the accompanying drawings, which are illustrative and non-limiting, in which:

[0087] Figure 1shows a schematic longitudinal section of a dividing wall column suitable for use in the method for purifying fatty acid isopropyl ester according to one embodiment of the present invention, and

[0088] Figure 2 A schematic diagram of an apparatus for preparing pure fatty acid isopropyl ester according to another embodiment of the present invention is shown.

[0089] Figure 1 The dividing wall column 10 schematically shown in FIG. 1 is an intermediate dividing wall column 10 that includes a dividing wall 12 extending vertically downwardly, as viewed through the height of the dividing wall column 10. More specifically, the dividing wall 12 extends vertically downwardly between the top and bottom of the dividing wall column 14 for approximately 25% to approximately 75% of the height of the intermediate dividing wall column. Thus, the dividing wall 12 includes an undivided top section 14, an undivided bottom section 16, and a middle section 18 disposed between the top section 14 and the bottom section 16. The dividing wall 12 extends through the entire height of the middle section 18, but not within the top section 14 and not within the bottom section 16. Furthermore, the dividing wall 12 divides the middle section 18 of the dividing wall column 12 into a first, feed subsection 20 and a second, product removal subsection 22.

[0090] The feed subsection 20 of the middle section 18 of the dividing wall column 10 comprises four packed beds 24, 24', 24", 24", which are arranged one above the other and each of which comprises several structured packing elements. The product withdrawal subsection 22 of the middle section 18 of the dividing wall column 10 also comprises four packed beds 26, 26', 26", 26'", which are arranged one above the other and each of which comprises several structured packing elements. The packing beds 24, 24', 24", 24", 26, 26', 26", 26'" have different heights and the packing beds 24, 24', 24", 24" of the feed subsection 20 are not parallel to the packing beds 26, 26', 26", 26'" of the product withdrawal subsection 22, but are offset to a certain extent. A distributor 28 is provided above each of the packed beds 24, 24', 24", 24", 26, 26', 26", 26'", and a collector 30 is provided below each of the packed beds 24, 24', 24", 24". The dividing wall column 10 also includes an inlet connected to a feed inlet line 32 and a side draw outlet connected to a product draw line 34. While the feed inlet line 32 is located at a height between the first packed bed 24 and the second packed bed 24' of the feed sub-section 20 of the middle portion 18 of the dividing wall column 10, the product draw line 34 is located at a height between the third packed bed 26" and the fourth packed bed 26'" of the product draw sub-section 22 of the middle portion 18 of the dividing wall column 10. Therefore, as viewed from the bottom to the top of the dividing wall column 10, the product draw line 34 is located above the feed inlet line 32. The undivided top section 14 of the dividing wall column 10 contains a packed bed 36 and the undivided bottom section 16 of the dividing wall column 10 also contains a packed bed 38 .

[0091] Furthermore, the dividing wall column 10 includes a top condenser 40 having an inlet connected to the top of the top section 14 of the dividing wall column 10 via a line 42, and an outlet for condensing liquid connected to a line 44, which branches into a liquid removal line 46 and a recycle line 48 for recycling a portion of the liquid that has condensed in the condenser 40 to the top section 14 of the dividing wall column 10. Furthermore, the dividing wall column 10 includes a reboiler 50 and a bottom outlet line 52, which branches into a line 54 connected to the inlet of the reboiler 50 and a removal line 56. The reboiler 50 also has an outlet connected to a recycle line 58, which is used to recycle a portion of the gas that has been generated in the reboiler 50 to the bottom section 16 of the dividing wall column 10.

[0092] During operation, a crude fatty acid isopropyl ester composition containing fatty acid isopropyl ester to be purified, at least one fatty acid, and optionally at least one fatty acid ester different from the fatty acid isopropyl ester to be purified is fed to the feed section 20 of the middle section 18 of the dividing wall column 10 via the feed inlet line 32 and distilled. Low-boiling point components of the crude fatty acid isopropyl ester composition (i.e., components having a lower boiling point than the target fatty acid isopropyl ester to be purified) are removed as an overhead stream via line 42. The overhead stream is condensed in the condenser 40, and a portion of the condensed liquid of the low-boiling point components is withdrawn from the dividing wall column 10 via line 46, while the remaining portion thereof is recycled to the top section 14 of the dividing wall column 10 via the recycle line 48. High-boiling point components of the crude fatty acid isopropyl ester composition (i.e., components having a higher boiling point than the target fatty acid isopropyl ester to be purified) are removed from the dividing wall column 10 as a bottom stream via line 52. A portion of the bottom stream is introduced into the reboiler 50 through the line 54 and evaporated therein and recycled through the recycle line 58, while another portion of the bottom stream of the high-boiling point component is withdrawn from the dividing wall column 10 through the line 56. The resulting purified fatty acid isopropyl ester composition is withdrawn from the dividing wall column 10 through the product withdrawal line 34.

[0093] Figure 2 6. The equipment 60 for preparing pure fatty acid isopropyl ester shown in the figure comprises first reactor 62 or pre-reactor 62, second reactor 64 and separating device 66 respectively.Pre-reactor 62 is a container comprising an inlet line 68 for fatty acid, an inlet line 70 for isopropyl alcohol and an outlet line 72 for pre-reaction mixture, and said pre-reaction mixture comprises fatty acid isopropyl ester, unreacted fatty acid, unreacted isopropyl alcohol and optionally water, by product and other impurity.Second reactor 64 is a distillation tower, which comprises an inlet being connected with the outlet line 72 for pre-reaction mixture of pre-reactor 62, an outlet line 74 and a tower top outlet line 76 as bottom outlet for (final) reaction mixture, wherein tower top outlet line 76 is connected with water-removing unit 80 by pipeline 78.Water-removing unit 80 is actually a pervaporation unit, which comprises an outlet line 82 for water and an outlet line 84 for isopropyl alcohol, and wherein the outlet 84 for isopropyl alcohol is connected with the inlet line 70 for isopropyl alcohol of pre-reactor 62 by recirculation line 86. Separation unit 66 is a distillation column having an overhead line 88 for isopropyl alcohol leading to recycle line 86 and a bottoms line 90 for removing the altered reaction mixture and feeding it to dividing wall column 10 via feed inlet line 32, and is implemented as described above with respect to Figure 1 A recycle line 92 leads from the bottom removal line 56 of the dividing wall column 10 back to the inlet line 68 for fatty acids.

[0094] In the running process of equipment 60, fatty acid and isopropyl alcohol are fed to pre-reactor 62 by inlet lines 68 and 70, and these are reacted into fatty acid isopropyl ester therein.Pre-reaction mixture takes out from pre-reactor 62 and is fed to second reactor 64 by pipeline 72, and the reaction of fatty acid and isopropyl alcohol is completed therein.The mixture of isopropyl alcohol and water is continuously removed from second reactor 64 by overhead outlet line 76 and is introduced into water-removing unit 80, wherein isopropyl alcohol is separated from water.When taking out water by outlet line 82, isopropyl alcohol is recycled in pre-reactor 62 by recirculation line 86 and by isopropyl alcohol inlet line 70.Reaction mixture takes out from second reactor 64 and is fed to separator 66 by outlet line 74.In separator 66, the major part of residue isopropyl alcohol is removed from reaction mixture, and it takes out from separator 66 by overhead outlet line 88 and is fed back in the pre-reactor by recirculation line 86 and by isopropyl alcohol inlet line 70. The modified reaction mixture thus obtained is removed from the separation device 66 via the bottom line 90, fed to the dividing wall column 10 via the feed inlet line and treated as described above with respect to Figure 1 The purification is described in a dividing wall column 10 .

[0095] Reference numerals

[0096] 10 Dividing Wall Column

[0097] 12 Dividing wall of dividing wall column

[0098] 14 Undivided top section

[0099] 16 Undivided bottom segment

[0100] 18 Central Section

[0101] 20 Feed sub-section of the middle section

[0102] 22 Product removal sub-section of the middle section

[0103] 24, 24', 24", 24"' packed bed in the feed subsection

[0104] 26, 26', 26", 26"' product removal sub-section packed bed

[0105] 28 Distributor

[0106] 30 Collector

[0107] 32 Feed inlet pipeline

[0108] 34 Product extraction pipeline

[0109] 36 Undivided top section packed bed

[0110] 38 Undivided bottom section packed bed

[0111] 40 Tower top condenser

[0112] 42 Pipeline leading to the inlet of the condenser

[0113] 44 Pipeline from the outlet of the condenser

[0114] 46 Liquid removal line

[0115] 48 Recirculation line for liquids

[0116] 50 Reboiler

[0117] 52 bottom outlet pipeline

[0118] 54 Pipeline to the inlet of the reboiler

[0119] 56 Remove pipeline

[0120] 58 Recirculation line for gas

[0121] 60 Equipment for preparing pure fatty acid isopropyl esters

[0122] 62 First reactor / pre-reactor

[0123] 64 Second Reactor

[0124] 66 Separation device

[0125] 68 Inlet line for fatty acids

[0126] 70 Inlet line for isopropyl alcohol

[0127] 72 outlet line for pre-reaction mixture

[0128] 74 Outlet line for (final) reaction mixture

[0129] 76 Second reactor top outlet pipeline

[0130] 78 pipeline

[0131] 80 Water removal unit

[0132] 82 Export line for water

[0133] 84 Outlet line for isopropyl alcohol

[0134] 86 Recirculation line

[0135] 88 Top outlet pipeline of separation unit

[0136] 90 Bottom line of separation unit

[0137] 92 Recirculation line

Claims

1. A method for purifying fatty acid isopropyl esters, comprising the steps of: a) providing a crude fatty acid isopropyl ester composition comprising fatty acid isopropyl ester to be purified, at least one fatty acid and optionally at least one fatty acid ester different from the fatty acid isopropyl ester to be purified, and b) distilling the crude fatty acid isopropyl ester composition provided in step a) in a dividing wall column to produce an overhead stream, a bottom stream and a side stream, wherein the side stream is a purified fatty acid isopropyl ester composition.

2. The method according to claim 1, wherein the crude fatty acid isopropyl ester composition provided in step a) contains isopropyl laurate, isopropyl myristate, isopropyl palmitate, isopropyl stearate or any mixture of two or more of the foregoing fatty acid isopropyl esters as the fatty acid isopropyl esters to be purified.

3. The method according to claim 1 or 2, wherein the crude fatty acid isopropyl ester composition provided in step a) contains 20 to 80 wt. % of the fatty acid isopropyl ester to be purified, 30 to 80 wt. % of one or more fatty acids, 0.2 to 2.0 wt. % of one or more fatty acid isopropyl esters different from the fatty acid isopropyl ester to be purified, 0.01 to 0.2 wt. % of isopropyl alcohol and up to 0.5 wt. % of water, based on the total weight of the crude fatty acid isopropyl ester composition.

4. The process according to claim 1 , wherein the dividing wall column used in step b) comprises a dividing wall which extends at least substantially vertically downwards, wherein substantially vertically downwards means that the angle between the dividing wall and the length axis of the dividing wall column is at most 20°, preferably at most 10°, more preferably at most 5° and most preferably 0°, wherein the dividing wall extends from 10% to 90%, preferably from 20% to 80% and more preferably from 25% to 75% of the height of the dividing wall column as seen along the length axis of the dividing wall column.

5. The process according to any one of the preceding claims, wherein the dividing wall column used in step b) is an intermediate dividing wall column comprising: an undivided top section for concentrating low-boiling components, comprising an outlet for an overhead stream of said low-boiling components, wherein said low-boiling components have a lower boiling point than said fatty acid isopropyl esters to be purified, an undivided bottom section for concentrating high-boiling components, comprising an outlet for a bottom stream of said high-boiling components, wherein said high-boiling components have a higher boiling point than said fatty acid isopropyl esters to be purified, and - a middle section arranged between the top section and the bottom section, having at least one dividing wall for dividing the middle section into at least one feed sub-section and at least one product withdrawal sub-section, wherein the feed sub-section has an inlet for feeding the crude fatty acid isopropyl ester composition and the product withdrawal sub-section has a side withdrawal outlet for the purified fatty acid isopropyl ester composition.

6. The process according to claim 5, wherein the side draw outlet for the purified fatty acid isopropyl ester composition is arranged in the middle section of the intermediate dividing wall column, above the inlet for the feed of the crude fatty acid isopropyl ester composition.

7. The process according to claim 5 or 6, wherein the feed subsection and the product withdrawal subsection of the middle section of the dividing wall column each comprise at least two packed beds, more preferably at least three packed beds and most preferably four packed beds, wherein adjacent packed beds are stacked on top of each other, and wherein each packed bed preferably comprises one or more structured packing elements or random packing.

8. A method according to claim 7, wherein the inlet for the feed of the crude fatty acid isopropyl ester composition is located between two adjacent packed beds of the feed sub-section, and the outlet for the purified fatty acid isopropyl ester composition is located between two adjacent packed beds of the product withdrawal sub-section, wherein the side withdrawal outlet for the purified fatty acid isopropyl ester composition is arranged in the middle sub-section of the intermediate dividing wall column, above the inlet for the feed of the crude fatty acid isopropyl ester composition.

9. The process according to any one of the preceding claims, wherein the reflux ratio is from 200 to 1,000, preferably from 300 to 900 and more preferably from 500 to 800, and / or wherein the ratio of the reflux rate divided by the side stream rate is from 4 to 10, preferably from 5 to 8 and more preferably from 6 to 7.

10. The process according to any of the preceding claims, wherein no further distillation is carried out apart from the distillation carried out in the dividing wall column.

11. A process according to any one of the preceding claims, wherein the purified fatty acid isopropyl ester composition obtained as a side stream in step b) comprises at least 99.00 wt. %, preferably at least 99.30 wt. %, more preferably at least 99.50 wt. %, even more preferably at least 99.60 wt. % and most preferably at least 99.70 wt. % of the target fatty acid isopropyl ester, wherein the purified fatty acid isopropyl ester composition obtained as a side stream in step b) comprises at most 0.070 wt. %, preferably at most 0.060 wt. %, more preferably at most 0.050 wt. %, even more preferably at most 0.040 wt. % and most preferably at most 0.030 wt. % of impurities, said impurities comprising free fatty acids and other fatty acid isopropyl esters other than the fatty acid isopropyl ester to be purified.

12. A method for preparing pure fatty acid isopropyl esters, comprising the steps of: i) providing a starting mixture containing fatty acids and isopropanol, ii) reacting the starting mixture, preferably in the presence of a catalyst, to obtain a reaction mixture containing the fatty acid isopropyl ester, unreacted fatty acid, unreacted isopropanol and optionally water, by-products and other impurities, iii) optionally, removing at least a portion of said isopropanol from said reaction mixture by vaporization to obtain a modified reaction mixture, and iv) carrying out the process for purifying the fatty acid isopropyl esters according to any one of the preceding claims, wherein the crude fatty acid isopropyl ester composition provided in step a) is the reaction mixture obtained in step ii) or the altered reaction mixture obtained in step iii).

13. An apparatus for preparing pure fatty acid isopropyl esters, comprising: i) at least one reactor, one or more inlets for fatty acids and isopropanol or a mixture of fatty acids and isopropanol, and an outlet for a reaction mixture containing said fatty acid isopropyl esters, unreacted fatty acids, unreacted isopropanol and optionally water, by-products and other impurities, ii) optionally a separation device for separating at least a portion of the isopropanol from the reaction mixture to obtain a modified reaction mixture, wherein the separation device comprises an inlet connected to the outlet for the reaction mixture of the reactor, an outlet for isopropanol and an outlet for the modified reaction mixture, and iii) A dividing wall column comprising an inlet which is connected to the outlet of the reactor for the reaction mixture and / or to the outlet of the separation device for the modified reaction mixture.

14. The apparatus according to claim 13, wherein the reactor a) is a distillation column comprising the outlet for the reaction mixture as a bottom outlet and a top outlet, wherein the top outlet is connected via a recirculation line to a water removal unit, wherein the water removal unit comprises an outlet for water and an outlet for isopropanol, wherein the outlet for isopropanol is connected via a recirculation line to the inlet for isopropanol of the reactor a).

15. The apparatus according to claim 13 or 14, wherein the separation device b) is present, wherein the outlet for isopropanol of the separation device b) is connected to the inlet for isopropanol of the reactor a) via a recirculation line.

16. An apparatus for preparing pure fatty acid isopropyl esters, comprising: i) at least one reactor, one or more inlets for fatty acids and isopropanol or a mixture of fatty acids and isopropanol, and an outlet for a reaction mixture containing said fatty acid isopropyl esters, unreacted fatty acids, unreacted isopropanol and optionally water, by-products and other impurities, ii) optionally a separation device for separating at least a portion of the isopropanol from the reaction mixture to obtain a modified reaction mixture, wherein the separation device comprises an inlet connected to the outlet for the reaction mixture of the reactor, an outlet for isopropanol and an outlet for the modified reaction mixture, and iii) a dividing wall column comprising an inlet which is connected to the outlet of the reactor for the reaction mixture and / or to the outlet of the separation device for the modified reaction mixture, wherein the dividing wall column is an intermediate dividing wall column comprising: - an undivided top section, - an undivided bottom section and - a middle section arranged between the top section and the bottom section, having at least one dividing wall for dividing the middle section into at least one feed sub-section and at least one product withdrawal sub-section, the feed sub-section having an inlet for a feed of a crude fatty acid isopropyl ester composition and the product withdrawal sub-section having a side withdrawal outlet for a purified fatty acid isopropyl ester composition, wherein the feed sub-section and the product withdrawal sub-section of the middle section of the dividing wall column each comprise at least two packed beds, wherein adjacent packed beds are stacked one on top of another, and wherein the inlet for the feed of the crude fatty acid isopropyl ester composition is located between two adjacent packed beds of the feed sub-section, and the outlet for the purified fatty acid isopropyl ester composition is located between two adjacent packed beds of the product withdrawal sub-section, wherein the side withdrawal outlet for the purified fatty acid isopropyl ester composition is arranged in the middle sub-section of the intermediate dividing wall column, above the inlet for the feed of the crude fatty acid isopropyl ester composition.

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