Process for the production of 2-alkylalkanol

Abstract

Disclosed is a process for the production of 2-alkylalkanol comprising: providing a mixed aldehyde stream 10 comprising iso-aldehyde and normal-aldehyde through a process comprising hydroformylation o

Description

Field of the Invention The present invention relates to processes for the production of 2-alkylalkanol. In particular, but not exclusively, the present invention relates to processes for the production of 2-ethylhexanol. Background The general process for the production of oxo alcohols by hydroformylation, optionally aldolization, and hydrogenation is well-known. The feedstocks to the process are syngas and olefin. Those feedstocks can be obtained from any source. For example, olefin can be obtained from Fischer Tropsch reactions, methanol-to-olefin process, propane dehydrogenation, from a refinery, from sustainable sources or from any other source of olefin. The syngas can, for example be obtained from reforming (of coal, natural gas, or any other suitable feedstock) or gasification. The syngas can also be obtained from sustainable feedstocks. An example is capturing carbon dioxide and producing hydrogen from electrolysis, or another source, before using the reverse water gas shift reaction to convert the carbon dioxide and hydrogen to syngas. Where there are contaminants present in the feedstocks these can be removed in a feedstock purification section. Hydroformylation is the reaction of carbon monoxide with olefin to form aldehyde. The aldehyde has one more carbon that the olefin. Hydroformylation is typically carried out using a homogeneous rhodium catalyst with an organophosphorus ligand, such as those listed in US4769498, US4885401, US5113022, US5202297, US5235113, US4668651, US4748261, US5254741, US5391801, US5059710, US3527809, US4283562, US4400548, US4482749, US4599206, US4716250, US4717775, US4731486, US4737588 or WO8001690. Typically, the hydroformylation reaction is carried out in two or more reactors. Aldehyde product and the rhodium catalyst are passed from the reactors to a catalyst recovery section which separates the aldehyde product from the rhodium catalyst and its solvent. The solvent may typically be the aldehyde, heavies formed in the reaction or any other suitable solvent. Examples of techniques to separate the rhodium catalyst, or to stabilise the rhodium catalyst can be found in US4774361, US5874640, US5892119, US6090987, US6294700, US6100432, US5114473, US4148830 and US4247486. The aldehyde will contain a mix of normal and iso aldehyde. The aldehyde may be hydrogenated to both normal and iso alcohol, or aldolized and hydrogenated to a mixture of branched alcohols, such as Cio alcohols in the form of a mixture of 2-propylheptanol isomers. However, the aldehyde is typically sent to an isomer column to substantially separate the normal and iso aldehyde. Downstream of the isomer column, normal aldehyde, such as n-butyraldehyde, is typically then either hydrogenated to normal alcohol, such as n-butanol, or passed to an aldolization section. The iso-aldehyde may be used as a product or may, in some cases, such as when 2-propylheptanol is being produced, be passed with some or all of the normal aldehyde to an aldolization section. In aldolization the aldehyde reacts to form an acrolein (also known as an alkenal), which can then be hydrogenated to an alcohol. Examples include aldolization of n-butyraldehyde to form ethyl-propyl-acrolein (also known as 2-ethylhexenal), which is then hydrogenated to 2-ethylhexanol, or aldolization of valeraldehyde to propyl-butyl-acrolein (also known as 2-propylheptenal), which is then hydrogenated to 2-propylheptanol. The hydrogenation, whether of the normal- or iso-aldehyde coming from the isomer column, or of the acrolein from the aldolization, may be carried out in a reactor such as that disclosed in WO2012 / 110775 or WO2023 / 161612. The product alcohols may be refined, typically in one or more distillation columns, to achieve desired purities. Examples of refining schemes are disclosed in WO2018 / 069714 and WO2019 / 197831 . This invention relates particularly to the production of 2-alkylalkanol, for example 2-ethylhexanol or 2-propylheptanol and preferably 2-ethylhexanol. In accordance with the above general process, the olefin and syngas are reacted via hydroformylation to produce normal- and iso-aldehyde. The normal- and iso-aldehyde are separated in an isomer column. The normal-aldehyde is aldolized to produce 2-alkylalkenal and the 2-alkylalkenal is hydrogenated to 2-alkylalkanol. The iso-aldehyde is typically either hydrogenated to iso-alkanol or sold as iso-aldehyde. In some processes, only some of the normal-aldehyde is used for 2-alkylalkanol production and some of the normal-aldehyde is hydrogenated to normal-alkanol. As a specific example, propene and syngas may be hydroformylated to form a mix of iso-butyraldehyde and normal-butyraldehyde. Following separation of the isobutyraldehyde and normal-butyraldehyde in an isomer column at least some of the normal-butyraldehyde is aldolized to 2-ethylhexenal, which is then hydrogenated to 2-ethylhexanol. The iso-butyraldehyde may be hydrogenated to iso-butanol or sold as iso-butyraldehyde. Some of the normal-butyraldehyde may be hydrogenated to normal-butanol. The isomer column is typically a large distillation column with a large reflux ratio, defined as the ratio of reflux to the column feed. The large reflux ratio is required because it is separating normal-aldehyde from iso-aldehyde, which have similar boiling points, and a high level of purity is required for both the normal- and iso-aldehyde. The normal-aldehyde has the higher boiling point and is therefore withdrawn at or near the bottom of the isomer column. The iso-aldehyde is withdrawn at or near the top of the isomer column. In addition to the normal- and iso-aldehyde, there will be heavy contaminants, so-called "heavies", in the isomer column. Because the heavies have a higher boiling point than the normal- and iso-aldehyde, the heavies are concentrated in the bottom of the column, where they can contaminate the normal-aldehyde. Contamination of the normal-aldehyde with heavies may be problematic where the plant is producing 2-alkylalkanol because the heavies, which are typically dimers and trimers of the aldehyde chain length (i.e., if the aldehyde is butyraldehyde, the heavies will be C4 dimers and trimers), will have similar boiling points to the 2-alkylalkanol and will thus be difficult to separate from the 2-alkylalkanol product. Thus, the process requires costly equipment to remove the heavies from the 2-alkylalkanol product. Also, some heavies may react in the downstream aldolization and hydrogenation to form new contaminants that have boiling points very close to the 2- alkylalkanol and are not practical to remove from the 2-alkylalkanol product. An example is the creation of 2-ethyl-4-methyl-l-pentanol (EMPOH), which is practically inseparable from the 2-ethylhexanol. The normal-aldehyde can alternatively be processed in a heavies removal section after the isomer column, but this also adds equipment items, and hence cost, to the process. An example of such a system is US6511583, where, from a first distillation column, a gaseous side draw of n-butyraldehyde is taken and a bottoms stream comprising heavies and n-butyraldehyde is sent to a second distillation column where the n-butyraldehyde is separated from the heavies and returned to the first distillation column. Such a system may require high sump temperatures in the first distillation column, and even higher sump temperatures in the second distillation column, in order to achieve sufficient concentration of the heavies, which can in turn cause the formation of more heavies and a loss of efficiency. The removal of the n-butyraldehyde as a vapour, for example in the gaseous side draws, can also incur a significant energy cost and increase the column duty. Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art. In particular, preferred embodiments of the present invention seek to provide improved isomer column arrangements to further improve the cost and energy efficiency of 2-alkylalkanol production. Summary of Invention According to a first aspect of the invention, there is provided a process for the production of 2-alkylalkanol comprising: providing a mixed aldehyde stream comprising iso aldehyde and normal-aldehyde through a process comprising hydroformylation of an olefin; passing the mixed aldehyde stream to an isomer column for separation of iso-aldehyde from normal-aldehyde; withdrawing a first liquid normal-aldehyde stream from a first position in the isomer column; withdrawing a second liquid normal-aldehyde stream from a second position in the isomer column such that the second liquid normal-aldehyde stream has a greater concentration of heavies than the first liquid normal-aldehyde stream; and sending at least part of the first liquid normal-aldehyde stream to an 2-alkylalkanol production process comprising aldolization of normal-aldehyde to 2-alkylalkenal and hydrogenation of the 2-alkylalkenal to 2-alkylalkanol. By withdrawing two liquid normal-aldehyde streams, with the second liquid normal-aldehyde stream being withdrawn from a second position such that the second liquid normal-aldehyde stream has a greater concentration of heavies than the first liquid normal aldehyde stream, the present invention allows the first liquid normal-aldehyde stream to have a lower heavies concentration that would be the case for a single normal-aldehyde stream withdrawn at or near the bottom of the column in the conventional way. That first liquid normal-aldehyde stream may then be used for 2-alkylalkanol production, with the advantage of reduced heavies content in the final 2-alkylalkanol product. The second liquid normal-aldehyde stream may be sent to a heavies removal column. The heavies removal column may be an existing upstream heavies removal column, i.e. a heavies removal column that is upstream of the isomer column, such as a column for separation of mixed aldehyde from heavies upstream of the isomer column. However, the heavies removal column is preferably a separate heavies removal column. However, the second liquid normal-aldehyde stream is preferably hydrogenated to alkanol. Since that alkanol will have the same chain length as the aldehyde, the heavies may be readily separated from the alkanol product. Such a process therefore offers a significant advantage over simply splitting a single normal-aldehyde stream between 2-alkylalkanol and alkanol production, by instead increasing the heavies content in the second liquid normal-aldehyde stream used for alkanol production and reducing the heavies content in the first liquid normal-aldehyde stream used for 2-alkylalkanol production. The high reflux ratio in the isomer column may allow a significant difference in heavies concentration even if the first and second liquid normal-aldehyde streams are withdrawn from positions only a stage or two apart in the isomer column. An advantage of the invention is that it significantly concentrates the heavies in the isomer column into a stream, the second liquid normal-aldehyde stream, from which they can be removed, for example in a smaller, less utility-intensive heavies removal column or, preferably, by using the second liquid normal-aldehyde stream to produce a product from which the heavies are readily separated. Heavies in the isomer column may include heavies entering the isomer column in the mixed aldehyde stream and heavies made in the isomer column. The total heavies leaving the isomer column may be defined as the sum of the molar flowrate of heavies in the first liquid normal-aldehyde stream and the molar flowrate of heavies in the second liquid normal-aldehyde stream. The proportion of the heavies in the first liquid normal-aldehyde stream may be defined as the ratio, on a molar basis, of the heavies molar flowrate in the first liquid-normal aldehyde stream to the total heavies leaving the column. Similarly, the total liquid normal-aldehyde leaving the isomer column may be defined as the sum of the molar flowrate of liquid normal-aldehyde in the first liquid normal-aldehyde stream and the molar flowrate of liquid normal-aldehyde in the second liquid normal-aldehyde stream. The proportion of the liquid normal-aldehyde in the first liquid normal-aldehyde stream may be defined as the ratio, on a molar basis, of the liquid normal-aldehyde molar flowrate in the first liquid-normal aldehyde stream to the total liquid normal-aldehyde leaving the column. Preferably the ratio of the proportion of the liquid normal-aldehyde in the first liquid normal-aldehyde stream to the proportion of the heavies in the first liquid normal-aldehyde stream is at least 2, more preferably at least 3, yet more preferably at least 4 and even more preferably at least 5. This advantageously means that the first liquid normal-aldehyde stream contains a lower proportion of the heavies and thus 2-alkylalkanol produced from the first liquid normal-aldehyde stream will have a lower level of heavies and costly heavies removal equipment and utility, typically steam, costs may be avoided. Concentrating the heavies in the second liquid normal-aldehyde stream means that the first liquid normal-aldehyde stream can be used for production of 2-alkylalkanol, where low heavies content is more important, and the second liquid normal-aldehyde stream can be either treated to remove the heavies or, preferably, used to produce normal-alkanol from which the heavies are more readily separated. It will be appreciated that, because the second liquid normal-aldehyde stream is not the only normal-aldehyde stream withdrawn from the isomer column, it can be treated to remove heavies in a heavies column from which a normal-aldehyde product stream is taken or by recycling it upstream to a column upstream of the isomer column. That is not possible if there is only a single normal-aldehyde stream withdrawn from the isomer column. Thus, the second liquid normal-aldehyde stream may be sent to a heavies removal column from which normal-aldehyde is recovered. The heavies removal column may be an upstream heavies removal column, such as a column for separation of mixed aldehyde from heavies upstream of the isomer column. In that way the normal aldehyde is effectively recovered through its reintroduction as part of the mixed aldehyde stream fed to the isomer column. However, preferably the heavies removal column is a separate heavies removal column and the normal-aldehyde recovered in the heavies removal column is mixed with the first liquid normal-aldehyde stream. Such an arrangement advantageously has a reduced utility, typically steam, duty compared to taking only a single liquid normal-aldehyde stream from the isomer column. Where the second liquid normal-aldehyde stream is to be sent to heavies removal, the flowrate of the second liquid normal-aldehyde stream is preferably a small fraction of the flowrate of the first liquid normal-aldehyde stream. In that way the heavies are concentrated in a relatively small stream, which therefore requires smaller equipment items to process and fewer utilities such as steam. However, there is a balance to be struck because the smaller the flowrate of the second liquid normal-aldehyde stream, the higher the temperature required in the reboiler, which may increase the formation of further heavies and require the use of higher-grade steam, which is more expensive. The flowrate of the second liquid normal-aldehyde stream is thus preferably chosen so as to obtain the benefits of a smaller flowrate in that stream without too greatly increasing the temperature of the isomer column reboiler. Preferably, and especially preferably when the second liquid-normal aldehyde stream is being sent to a heavies removal column, the liquid normal-aldehyde flowrate in the second liquid normal-aldehyde stream is at least 3 wt% of the liquid normal-aldehyde flowrate in the first liquid normal-aldehyde stream. Preferably it is also not more than 30 wt% and more preferably not more than 15 wt% of the liquid normal-aldehyde flowrate in the first liquid normal-aldehyde stream. The invention may be especially beneficial in a process that is producing both 2-alkylalkanol, by aldolization of the normal-aldehyde and then hydrogenation of the resulting 2-alkylalkenal, and normal-alkanol by hydrogenation of the normal-aldehyde. In such a process, the first liquid normal-aldehyde stream can be sent to the aldolization and on to 2-alkylalkanol production, where the low heavies content of the first liquid normal-aldehyde stream is particularly beneficial. The second liquid normal-aldehyde stream can be sent to hydrogenation to normal-alkanol, a process in which a higher heavies content can be tolerated because the heavies are more readily separated from the normal-alkanol product. Thus, in a particularly preferred aspect of the invention at least part of the second liquid normal-aldehyde stream is sent to an alkanol production process comprising hydrogenation of the normal-aldehyde to normal-alkanol. The heavies concentration in the first liquid normal-aldehyde stream is preferably not more than 0.5 wt%, and more preferably not more than 0.25 wt%. Such concentrations may be sufficiently low to permit the production of on-specification 2-alkylalkanol with reasonably-sized downstream separation equipment. While the process may be applicable to the production of various 2-alkylalkanols, particularly important 2-alkylalkanols are Cg-Cio 2-alkylalkanols. Thus, the normal aldehyde is preferably normal-butyraldehyde or normal valeraldehyde or mixture of normal-butyraldehyde and normal-valeraldehyde and the iso-aldehyde is preferably isobutyraldehyde or iso-valeraldehyde or mixture of isobutyraldehyde and iso-valeraldehyde. In other words, the isoaldehyde and normal-aldehyde are preferably iso- and normalisomers of butyraldehyde, valeraldehyde or a mixture of butyraldehyde and valeraldehyde. The skilled person will understand which C4 and / or C5 normal-alkanols are produced from the hydrogenation of normal-butyraldehyde and / or normal valeraldehyde, which Cg-Cio 2-alkylalkenals are produced from the aldolization, including cross-aldolization, of normal-butyraldehyde and normal-valeraldehyde, and which Cg-Cio 2-alkylalkanols are produced from the subsequent hydrogenation of those Cg-Cio 2-alkylalkenals. Thus, the olefin is preferably a C3 to C4 olefin, the iso-aldehyde is preferably a C4 to C5 iso-aldehyde, the normal-aldehyde is preferably a C4 to C5 normal-aldehyde, the 2-alkylalkenal is preferably a Cs to C10 2-alkylalkenal, the 2-alkylalkanol is preferably a Cs to C10 2-alkylalkanol, and, where present, the normal-alkanol is preferably a C4 to C5 normal-alkanol. In a most preferred aspect, the olefin is propene, the iso-aldehyde is iso-butyraldehyde, the normal-aldehyde is normal-butyraldehyde, the 2-alkylalkenal is 2-ethylhexenal, the 2- alkylalkanol is 2-ethylhexanol, and, where present, the normal-alkanol is normal-butanol. In some embodiments, the isomer column preferably comprises a falling film reboiler. A falling film reboiler may advantageously reduce the residence time in the reboiler, and thus reduce for example the formation of new heavies. Such an arrangement may be particularly beneficial if only a small second liquid normal-aldehyde stream is being withdrawn and sent to heavies removal, for example by recycling to an upstream column. In such an arrangement, the first liquid normal-aldehyde stream may preferably be taken from the tray 2 liquid of the isomer column, which may advantageously further reduce the heavies content of the first liquid normal-aldehyde stream by reducing the effect of heavies in the reboiler vapour due to the heavies concentration in the reboiler. Such an arrangement may benefit from an extra stage in the isomer column compared to a column in which the first liquid normal-aldehyde stream is taken from the tray 1 liquid, and a falling film reboiler may be particularly beneficial in permitting such an arrangement while reducing the residence time at the high reboiler temperatures that may be required. The isomer column will be sized and operated such that sufficient separation of iso- and normal-aldehyde is achieved. Preferably the iso-aldehyde concentration in the first liquid normal-aldehyde stream is not more than 0.5 wt%, more preferably not more than 0.2 wt% and most preferably not more than 0.1 wt%. It will be appreciated that the isoaldehyde concentration in the second liquid normal-aldehyde stream will be lower than in the first liquid normal-aldehyde stream because the second liquid normal-aldehyde stream is taken from a second position such that the concentration of heavies in the second liquid normal-aldehyde stream is higher than the concentration of heavies in the first liquid normal-aldehyde stream. Such a second position will also therefore have a lower concentration of iso-aldehyde than the first position. The skilled person will understand that the first and second liquid normal-aldehyde streams are streams that are removed from the isomer column and sent to another part of the process. A stream that is merely fed from one part of the isomer column to another part of the isomer column, perhaps via some heating or cooling apparatus, such as a reboiler, would not be considered a normal-aldehyde stream within the meaning of this invention, because it is not conveying normal-aldehyde to another part of the process. The first and second liquid normal-aldehyde streams may comprise 90 wt% or more, preferably 95 wt% or more, normal-aldehyde. The skilled person will understand that the position from which a stream is drawn in the isomer column affects the composition of the stream. Additionally, in the region of the bottom of the isomer column the vapour may contain light components, such as iso-aldehyde, that are travelling up the column. Thus, it is advantageous to withdraw liquid normal-aldehyde streams, as opposed to vapour normal-aldehyde streams, as liquid streams are not contaminated by such light vapour components. For example, the presence of isobutyraldehyde in a first normal-butyraldehyde stream would be undesirable as it can also react in a downstream aldolization step to form EMPOH, which is practically inseparable from 2-ethylhexanol, which is the desired product of the normal-butyraldehyde aldolization and hydrogenation. In the present invention the second liquid normal-aldehyde stream has a greater concentration of heavies than the first liquid normal-aldehyde stream. This is preferably achieved by withdrawing the first liquid normal-aldehyde stream from a first position such that the first liquid normal-aldehyde stream does not comprise reboiler liquid and withdrawing the second liquid normal-aldehyde stream from a second position such that the second liquid normal-aldehyde stream comprises reboiler liquid. This may be advantageous because a large amount of liquid is vaporised in the reboiler and it is thus possible to get a step increase in heavies concentration in the reboiler liquid. Preferably the first liquid normal-aldehyde stream is withdrawn from the tray 1, 2 or 3 liquid of the isomer column. In some embodiments, the second liquid normal-aldehyde stream is preferably withdrawn from the bottoms liquid of the isomer column. It will be understood that the bottoms liquid comprises reboiler liquid and liquid descending the isomer column, for example descending from tray 1. In some embodiments, the second liquid normal-aldehyde stream is preferably withdrawn from the reboiler liquid of the isomer column. It will be appreciated that features described in relation to one aspect of the invention may be equally applicable in another aspect of the invention. Some features may not be applicable to, and may be excluded from, particular aspects of the invention. Description of the Drawings Embodiments of the present invention will now be described, by way of example, and not in any limitative sense, with reference to the accompanying drawings, of which: Figure 1 is a block diagram of a process for the production of 2-alkylalkanol according to the present invention; Figure 2 is a block diagram of a process for the production of 2-alkylalkanol according to the present invention; Figure 3 is a view of the bottom of an isomer column for use in the present invention; and Figure 4 is a view of the bottom of an isomer column for use in the present invention. Detailed Description In figure 1 olefin 1 and syngas 2 are fed with a catalyst 3 to an oxo reaction section 4. The catalyst 3 typically comprises Rhodium with an organophosphorus ligand, often dissolved in the aldehyde being produced in the oxo reaction section 4. The oxo reaction section 4 typically comprises 2 reactors, but may comprise more or fewer, and produces a reactor effluent 5 comprising normal- and iso-aldehyde, catalyst, unreacted syngas and olefin and other contaminants. The reactor effluent 5 is passed to a separation section 6. The separation section 6 typically comprises one or more vaporisers and may also comprise distillation columns and other separations such as membranes. In the separation section 6 the catalyst 3 is separated and recycled to the oxo reaction section 4, with make-up catalyst added as required. Light components, such as unreacted olefin and syngas, are also removed. The separation section 6 may, but does not necessarily, also include an upstream heavies removal column 8, in which heavies 9 are separated from a mixed aldehyde stream 10 comprising the iso- and normal-aldehyde. The present invention may, for example, mean that such an upstream heavies removal column 8 can be avoided. When present, the upstream heavies column 8 may be downstream of the separation section 6, as shown in Figure 1, or it may be at a position within the separation section 6. While the mixed aldehyde stream 10 will have a lower heavies content than the feed 7 to the upstream heavies removal column 8, the mixed aldehyde stream 10 will still include some heavies and heavies may still be formed in the isomer column, thus the desirability of the present invention remains even when upstream heavies removal column 8 is present. It will be appreciated that a variety of technologies are available for the oxo reaction section 4 and the separation section 6. Examples of those technologies include the LP-OxoSM process available from Johnson Matthey Davy and Dow. Aqueous technologies and technologies using cobalt-based hydroformylation are also available. The invention is not particularly limited to any one technology for those sections. Instead, the invention can operate with any olefin hydroformylation technology resulting in a mixed aldehyde stream 10 comprising normal-aldehyde and iso-aldehyde. The mixed aldehyde stream 10 is fed to an isomer column 11. At or near the top of the isomer column 11 an iso-aldehyde stream 12 is withdrawn. The iso-aldehyde stream 12 may be used as a product or may be passed to a hydrogenation 14 to make useful products such as neo pentyl glycol, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, or iso-alkanols. Toward the bottom of the isomer column 11 a first liquid normal-aldehyde stream 15 is withdrawn at a first position. At least some of the first liquid normal-aldehyde stream 15 is fed to an 2-alkylalkanol production process 16 in which the normal-aldehyde is aldolized to an 2-alkylalkenal, which is the hydrogenated to the 2-alkylalkanol. A second liquid normal-aldehyde stream 17 is withdrawn from a second position in the isomer column 11 such that the heavies concentration in the second liquid normal-aldehyde stream 17 is higher than the heavies concentration in the first liquid normal-aldehyde stream 15. In this embodiment, the second liquid normal-aldehyde stream 17 is sent to a hydrogenation 18 in which the normal-aldehyde is hydrogenated to normal-alkanol. The isomer column 11 comprises a reboiler 20. A bottoms stream 19 from the isomer column is passed to the reboiler 20 where it is heated and returned as a reboiler stream 21 to the isomer column 11. This embodiment may therefore be advantageous because the heavies are concentrated in the second liquid normal-aldehydes stream 17, which is hydrogenated to normal-alkanol from which the heavies can be readily separated. The first liquid normal-aldehyde stream 15, which is being used for 2-alkylalkanol production where heavies are more problematic, has a lower concentration of heavies. In effect therefore, the invention has preferentially sent the heavies to the downstream process that is more able to tolerate their presence and thus achieves on-specification products at efficient isomer column duties and temperatures. In Figure 2, where like numbered items are as in Figure 1 and not described again, the second liquid normal-aldehyde stream 17' is fed to a separate heavies removal column 22. From the heavies removal column 22, a recovered normal-aldehyde stream 24 with the heavies removed is combined with the first liquid normal-aldehyde stream 15 being fed to the 2-alkylalkanol production process 16. A heavies waste stream 23 is taken from the bottom of the heavies removal column 22. This embodiment is preferred for a plant that is not producing normal-alkanol. The advantage of such an arrangement is that the heavies are concentrated in a relatively small second liquid normal-aldehyde stream 17' that can then be processed in smaller and more cost-effective equipment for heavies removal than would be the case for removing heavies from the entirety of a single normal-aldehyde stream withdrawn from at or near the bottom of the isomer column 11. The invention therefore permits more efficient removal of the heavies even if there is no normal-alkanol production to send them to. The second liquid normal-aldehyde stream 17' could also be fed to the feed 7 of upstream heavies removal column 8. In Figure 3, the bottom of an isomer column 100, suitable for use in any of the processes described in relation to Figures 1 and 2 above, contains a series of trays 101, 102, 103, 104, 105. The first tray 101 is located at the bottom of the series and the trays are counted upwards, so the second tray 102 is above the first tray 101, the third tray 103 is above the second tray 102 and so on. The isomer column 100 includes a reboiler 142. Liquid bottoms stream 141 is withdrawn from the isomer column 100 and passed to the reboiler 142. Steam 144 is also fed to the reboiler 142, which condenses heating the liquid bottoms stream 141. The condensed steam leaves as condensed stream 145 and the heated liquid bottoms stream 141 leaves as reboiler stream 143, which is fed back to the isomer column 100. Liquid normal-aldehyde streams can be withdrawn from a number of positions 121, 122, 123, 131 in the isomer column 100. Position 131 is a bottoms stream, which comprises reboiler liquid from the reboiler stream 143 and liquid descending from the first tray 101. Position 121 is liquid from the first tray 101, position 122 is liquid from the second tray 102 and position 123 is liquid from the third tray 103. The concentration of heavies in a stream withdrawn from position 131 will be higher than in a stream withdrawn from position 121, position 122, or position 123 because the reboiler 142 effects a step increase in the heavies concentration in reboiler stream 143. Thus, in an example of the invention, the first liquid normal-aldehyde stream is withdrawn from position 121 and the second liquid normal-aldehyde stream is withdrawn from position 131. 4000 kmol / hr of liquid is falling off the first tray 101. About 190 kmol / hr of liquid normal-butyraldehyde is withdrawn at position 121 in the first liquid normal-aldehyde stream and sent to 2-ethylhexanol production. About 460 kmol / hr of liquid normal-butyraldehyde is withdrawn at position 131 in the second liquid normal-aldehyde stream and sent to normal-butanol production. The remaining flowrate is circulated via reboiler 142. The vapour flowrate up from the first tray 101 is 3350 kmol / hr. The first liquid normal-aldehyde stream has a heavies concentration that is reduced to 16.3% of the heavies concentration of a single normal-aldehyde stream withdrawn from the bottom of the same isomer column 100 (that is, from position 131). 29.2 mol% (190 kmol / hr / (190 kmol / hr + 460 kmol / hr)) of the liquid normal-aldehyde leaving the isomer column is in the first liquid normal-aldehyde stream, while only 4.7 mol% of the total heavies leaving the isomer column are in the first liquid normal-aldehyde stream. The ratio of the proportion of the liquid normal-aldehyde in the first liquid normal-aldehyde stream to the proportion of the heavies in the first liquid normal-aldehyde stream is 6.2. The first liquid normal-aldehyde stream withdrawn at position 121 has an increased concentration of iso-butyraldehyde compared to a single normal-aldehyde stream withdrawn from position 131 of the same isomer column 100, but the increase is small (0.089 wt% increased from 0.07 wt%). The increase in iso-butyraldehyde can be mitigated by adding one more tray to the isomer column 100, with an 0.6% increase in reboiler duty. That would result in an iso-butyraldehyde concentration of around 0.069 wt% in the first liquid normal-aldehyde stream at position 121 and an iso-butyraldehyde concentration of around 0.054 wt% in the second liquid normal-aldehyde stream at position 131, while retaining the beneficial reduction in heavies concentration in the first liquid normal-aldehyde stream withdrawn at position 121. The advantage of that reduced heavies concentration in the normal-aldehyde stream being sent to 2-ethylhexanol production outweighs the cost of the extra tray or increased reboiler duty and results in a more beneficial and efficient process. Although this example has been described in the context of 2-ethylhexanol and normal-butanol production, similar benefits can be obtained for other 2-alkylalkanol and normal-alkanol production. In Figure 4, the bottom of an isomer column 100', similar to isomer column 100 and also suitable for use in any of the processes described in relation to Figures 1, 2 or 3 above, includes a reboiler 142 that operates as described for Figure 3 with like numbered items being the same and not described again here. In Figure 4 only the first tray 101 and the corresponding position 121 from which a liquid normal-aldehyde stream may be withdrawn are shown. However, the skilled person will appreciate that the other trays and corresponding positions would also be present. In Figure 4 a weir 150 is provided onto which the liquid 51 from the reboiler stream 143 is fed. The vapour 52 from the reboiler stream 143 passes up through the first tray 101. Some of the liquid 51 from the reboiler stream is withdrawn at position 131' as a liquid normal-aldehyde stream, while some flows 53 over the weir and back into the bottoms liquid. The concentration of heavies in that liquid normal-aldehyde stream at position 131' will be higher than that in a liquid normal-aldehyde stream withdrawn at position 121, or indeed withdrawn from the bottoms liquid of the isomer column 100'. Thus, in an example embodiment, a first liquid normal-aldehyde stream may be withdrawn at position 121 and a second liquid normal-aldehyde stream, having a higher heavies concentration, may be withdrawn at position 131'. The second liquid normal-aldehyde stream comprises reboiler liquid 51 from reboiler stream 143, while the first liquid normal-aldehyde stream does not. The embodiment thus takes advantage of the step increase in heavies concentration in the reboiler stream 143 due to the large amount of vaporisation in the reboiler 142 . In an example embodiment of the invention, the heavies concentration at position 131' is about 7 wt% and the flowrate of the second liquid normal-aldehyde stream withdrawn at position 131' is about 5% of the flowrate of the first liquid normal-aldehyde stream withdrawn at position 121. The second liquid normal-aldehyde stream is sent to a small separate heavies removal column sized for that flowrate. Such an example results in a reduction of heavies concentration in the first liquid normal-aldehyde stream withdrawn at position 121 to about 16% of the heavies concentration that would be present in a single normal-aldehyde stream withdrawn from the bottom of the isomer column 100 operating at the same conditions. Thus, even without any normal-alkanol production present, a large reduction in the heavies concentration in the normal-aldehyde stream being sent to 2-alkylalkanol production can be achieved with the withdrawal of only a small second liquid normal-aldehyde stream that can be processed in existing or 5 relatively small additional equipment. It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the 10 scope of the invention as defined by the appended claims. For example, although the examples have been described in relation to 2-ethylhexanol production, they may apply equally to 2-propylheptanol production or production of other 2-alkylalkanols. While the isomer column has been described in 15 relation to a trayed column, other column configurations, for example using packings, are also possible.

Claims

1. A process for the production of 2-alkylalkanol comprising: providing a mixed aldehyde stream comprising iso-aldehyde and normal-aldehyde through a process comprising hydroformylation of an olefin; passing the mixed aldehyde stream to an isomer column for separation of iso-aldehyde from normal-aldehyde; withdrawing a first liquid normal-aldehyde stream from a first position in the isomer column; withdrawing a second liquid normal-aldehyde stream from a second position in the isomer column such that the second liquid normal-aldehyde stream has a greater concentration of heavies than the first liquid normal aldehyde stream; and sending at least part of the first liquid normal aldehyde stream to an 2-alkylalkanol production process comprising aldolization of normal-aldehyde to 2-alkylalkenal and hydrogenation of the 2-alkylalkenal to 2-alkylalkanol.

2. A process according to claim 1, wherein the concentration of heavies in the first liquid normal-aldehyde stream is less than 50% of the concentration of heavies that would be present in a single normal-aldehyde stream withdrawn from the bottom of the same isomer column operated at the same conditions, and preferably wherein the concentration of heavies in the first liquid normal-aldehyde stream is less than 25% of the concentration of heavies that would be present in a single normal-aldehyde stream withdrawn from the bottom of the same isomer column operated at the same conditions .

3. A process according to claim 1 or claim 2, wherein the ratio of the proportion of the liquid normal-aldehyde in the first liquid normal-aldehyde stream to the proportion of the heavies in the first liquid normal-aldehyde stream is at least 2.

4. A process according to any preceding claim, wherein the second liquid normal-aldehyde stream is sent to a heavies removal column from which normal-aldehyde is recovered.

5. A process according to claim 4, wherein normal-aldehyde recovered in the heavies removal column is mixed with the first liquid normal-aldehyde stream.

6. A process according to any of claims 4 to 5, wherein the normal-aldehyde flowrate in the second liquid normal-aldehyde stream is at least 3 wt% and not more than 30 wt% of the normal-aldehyde flowrate in the first liquid normal-aldehyde stream.

7. A process according to any of claims 1 to 3, wherein at least part of the second liquid normal-aldehyde stream is sent to an alkanol production process comprising hydrogenation of the normal-aldehyde to normal-alkanol.

8. A process according to any preceding claim, wherein the heavies concentration in the first liquid normal-aldehyde stream is not more than 0.5 wt%.

9. A process according to any preceding claim, wherein the iso-aldehyde and normal-aldehyde are iso- and normalisomers of butyraldehyde, valeraldehyde or a mixture of butyraldehyde and valeraldehyde.

10. A process according to any preceding claim, wherein the olefin is a Ca to C4 olefin, the iso-aldehyde is a C4 to C5 iso-aldehyde, the normal-aldehyde is a C4 to C5 normal-aldehyde, the 2-alkylalkenal is a Cs to C10 2-alkylalkenal, the 2-alkylalkanol is a Cs to C10 2-alkylalkanol, and, where present, the normal-alkanol is a C4 to C5 normal-alkanol.

11. A process according to any preceding claim, wherein the olefin is propene, the iso-aldehyde is isobutyraldehyde, the normal-aldehyde is normal-butyraldehyde, the 2-alkylalkenal is 2-ethylhexenal, the 2-alkylalkanol is 2-ethylhexanol, and, where present, the normal-alkanol is normal-butanol.

12. A process according to any preceding claim wherein the isomer column comprises a falling film reboiler.

13. A process according to any preceding claim wherein the iso-aldehyde concentration in the first liquid normal-aldehyde stream is not more than 0.5 wt%.

14. A process according to any preceding claim wherein the second liquid normal-aldehyde stream comprises reboiler liquid and the first liquid normal-aldehyde stream does not comprise reboiler liquid.

15. A process according to any preceding claim wherein the second liquid normal-aldehyde stream is withdrawn from the reboiler liquid of the isomer column.

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