Process for Producing a Purified 1,4-Butanediol Stream

Abstract

A process for producing a purified 1,4-butanediol stream is disclosed that includes hydrocracking a dialkyl succinate in one or more mixed gas / liquid phase reaction steps to form a crude 1,4-butanediol stream containing 1,4-butanediol, gamma-butyrolactone, tetrahydrofuran, and an alkanol, transferring the crude 1,4-butanediol stream to a purification process where at least a portion of the gamma-butyrolactone, tetrahydrofuran, and the alkanol are removed from the 1,4-butanediol, and recovering a purified 1,4-butanediol stream from the purification process having a higher concentration of 1,4-butanediol than the crude 1,4-butanediol stream. The purification process includes a polishing section in which an intermediate stream comprising 1,4-butanediol and 2-(4'-hydroxybutoxy)-tetrahydrofuran is passed through a catalyst bed to reduce the 2-(4'-hydroxybutoxy)-tetrahydrofuran content of the intermediate stream.

Description

[Technical field]

[0001] The present invention relates to a process for producing a purified 1,4-butanediol stream. Specifically, but not by way of limitation, the present invention relates to a process for producing a purified 1,4-butanediol stream after hydrogenolysis of a dialkyl succinate in one or more mixed gas / liquid phase reaction stages. [Background technology]

[0002] Butane-1,4-diol is used as a monomer in the production of plastics such as polybutylene terephthalate, polybutylene succinate (PBS), and polybutylene adipate terephthalate (PBAT), and as an intermediate for the production of gamma-butyrolactone and the important solvent tetrahydrofuran.

[0003] One route to butane-1,4-diol involves reacting acetylene with formaldehyde via the Reppe reaction to give butyne-1,4-diol, which is then hydrogenated to produce butane-1,4-diol.

[0004] Another process for producing butane-1,4-diol uses maleic anhydride as a starting material, which is esterified with an alkanol, usually a C1-C4 alkanol such as methanol or ethanol, to give the corresponding dialkyl maleate, which can then be subjected to hydrogenation to give dialkyl succinate and hydrogenolysis to give butane-1,4-diol and an alkanol, which can be recycled to produce further dialkyl maleates. Processes and plants for producing dialkyl maleates from maleic anhydride are described, for example, in U.S. Pat. No. 4,795,824 and WO 90 / 08127. Gas-phase hydrogenation of dialkyl maleates to give butane-1,4-diol is further discussed in U.S. Pat. Nos. 4,584,419, 4,751,334, and WO 88 / 00937.

[0005] In the hydrogenolysis of dialkyl succinates, such as dimethyl succinate or diethyl succinate, certain amounts of valuable by-products, γ-butyrolactone and tetrahydrofuran, may also be produced. These by-products have a ready market, so their simultaneous production with butane-1,4-diol is not a disadvantage. In addition, the hydrogenolysis product mixture usually contains small amounts of the corresponding dialkyl succinates, n-butanol, the corresponding dialkyl alkoxy succinates, such as diethyl ethoxy succinate, and water.

[0006] Another minor by-product has been identified as a cyclic acetal, namely 2-(4'-hydroxybutoxy)-tetrahydrofuran of the formula: [ka]

[0007] The cyclic acetal by-product, i.e., 2-(4'-hydroxybutoxy)-tetrahydrofuran, is troublesome because its boiling point is very close to that of butane-1,4-diol and they form an azeotrope together. It is therefore difficult, if not impossible, to produce a butane-1,4-diol product essentially free of this cyclic acetal using conventional distillation techniques. Thus, it is said that butane-1,4-diol produced by this hydrocracking route in the prior art typically contains about 0.15% to about 0.20% by weight of cyclic acetals, with other impurities totaling less than about 0.02% by weight. The presence of even trace amounts of the cyclic acetal 2-(4'-hydroxybutoxy)-tetrahydrofuran in butane-1,4-diol is disadvantageous because it is a color-forming substance and will form color in butane-1,4-diol.

[0008] WO 9736846(A1), WO 2006037957(A1) and WO 2013034881(A1) describe methods for purifying butane-1,4-diol.

[0009] WO 9736846 A1 suggests that the cyclic acetal 2-(4'-hydroxybutoxy)-tetrahydrofuran may be formed by reaction of butane-1,4-diol with 4-hydroxybutyraldehyde, a potential intermediate in a series of hydrogenolysis reactions, or by dehydrogenation of butane-1,4-diol itself. Next, WO 9736846 (A1) describes a process for purifying a substantially anhydrous butane-1,4-diol feed containing a small amount of the cyclic acetal 2-(4'-hydroxybutoxy)-tetrahydrofuran, which comprises hydrogenating the butane-1,4-diol feed in a hydrogenation zone in the presence of a hydrogenation catalyst and recovering a butane-1,4-diol product having a reduced content of 2-(4'-hydroxybutoxy)-tetrahydrofuran from the hydrogenation zone, characterized in that the hydrogenation is carried out in the presence of about 0.5 to about 5% by weight of water based on the weight of the butane-1,4-diol feed. In such a process, the amount of water added may correspond to a water:2-(4'-hydroxybutoxy)-tetrahydrofuran molar ratio of about 20:1 to about 500:1.

[0010] WO2006037957A1 suggests that the cyclic acetal 2-(4'-hydroxybutoxy)-tetrahydrofuran may be formed by reaction of 1,4-butanediol with 2-hydroxytetrahydrofuran, a potential intermediate in a series of hydrogenolysis reactions, and / or by dehydrogenation of 1,4-butanediol to hydroxybutyroaldehyde and its cyclization to the more stable 2-hydroxytetrahydrofuran. WO2006037957A1 then describes a process for purifying a crude liquid feed stream containing 1,4-butanediol and small amounts of 2-(4'-hydroxybutoxy)-tetrahydrofuran and / or its precursors, comprising passing the crude feed stream over a heterogeneous liquid-tolerant copper catalyst in the liquid phase at hydrogenation conditions in the presence of hydrogen in a reaction zone, and recovering a purified stream of 1,4-butanediol having a smaller amount of 2-(4'-hydroxybutoxy)-tetrahydrofuran than the crude liquid feed stream.

[0011] WO2013034881A1 identifies problems with the formation of 4-hydroxybutyl (4-hydroxybutyrate) in conventional processes. The formation of 4-hydroxybutyl (4-hydroxybutyrate) is an equilibrium reaction in which 4-hydroxybutyl (4-hydroxybutyrate) can return to 1,4-butanediol and gamma-butyrolactone under certain conditions. WO2013034881A1 reveals that in prior art distillation apparatus, these heavy fractions are fractionated at the bottom of a conventional or divided wall column, and in the high temperature and high residence time regions of the reboiler and sump of the column, components such as 4-hydroxybutyl (4-hydroxybutyrate) react and reform to light fractions, including gamma-butyrolactone. This creates problems with conventional distillation apparatus in that light fractions such as gamma-butyrolactone, which are the result of reactions in the sump, cannot be removed overhead from systems with conventional sidestream apparatus. This is because the lights produced by the reaction of the heavies in the column sump will backflow the column and contaminate the product sidestream, thereby limiting the purity of the product that can be removed in the sidestream. WO2013034881A1 also reveals that in ester hydrogenations such as those described in US4584419, US4751334, and WO88 / 00937, 3-(4-hydroxybutoxy)-tetrahydrofuran is formed as an impurity, and that it is understood to be different from the 2-(4'-hydroxybutoxy)-tetrahydrofuran mentioned above. WO2013034881 discloses that the presence of additional γ-butyrolactone formed in the sump makes it more difficult to remove 3-(4-hydroxybutoxy)-tetrahydrofuran in the final 1,4-butanediol distillation column, thus further limiting the purity of 1,4-butanediol available through conventional separation processes. WO2013034881(A1) then describes a process for purifying a stream containing 1,4-butanediol, comprising: (a) feeding a crude product stream comprising 1,4-butanediol and one or more of gamma-butyrolactone, 2-(4-hydroxybutoxy)-tetrahydrofuran, 4-hydroxybutyl (4-hydroxybutyrate), and 3-(4-hydroxybutoxy)-tetrahydrofuran to a first distillation column; (b) removing a side stream comprising 1,4-butanediol and lights, the lights comprising at least a portion of those produced by the reaction in the first distillation column; (c) transferring said stream to a hydrogenation zone; (d) subjecting the stream from step (c) to hydrogenation in the presence of a hydrogenation catalyst in a hydrogenation zone and recovering from the hydrogenation zone a 1,4-butanediol product stream having a reduced content of 2-(4-hydroxybutoxy)-tetrahydrofuran and optionally further comprising (4-hydroxybutyl)-4-hydroxybutyrate formed by reaction of γ-butyrolactone; (e) transferring the 1,4-butenediol product stream from step (d) to a second distillation column operated such that (4-hydroxybutyl)-4-hydroxybutyrate is removed as a bottoms stream and removing a 1,4-butanediol stream as an overhead; (f) transferring the overhead stream removed in (e) to a third distillation column and recovering a purified 1,4-butanediol stream; A process is disclosed which includes:

[0012] Prior art such as WO2006037957A1 discloses that the amount of 2-(4'-hydroxybutoxy)-tetrahydrofuran and its precursors in at least one C4 compound can be measured using the peak acetal test. The peak acetal test is disclosed as involving removing lights from the 1,4-butanediol crude hydrogenation product at 120°C, followed by further heating at 160°C for 3 hours. Heating, performed using an isomantle heater, round-bottom flask, condenser, and collection pot, is carried out under a nitrogen blanket at atmospheric pressure. Since this procedure allows for the reaction of precursors of acetals, the prior art discloses that it reports the maximum acetal content possible in the product 1,4-butanediol stream if the crude hydrogenation product is subjected to purification by a standard distillation system. The residue is then analyzed by gas chromatography.

[0013] WO 9736846A1 suggests that butane-1,4-diol produced by the hydrogenolysis pathways of U.S. Pat. No. 4,584,419, U.S. Pat. No. 4,751,334, or WO 88 / 00937 typically contains about 0.15% to about 0.20% by weight of cyclic acetals.

[0014] WO2006037957A1 contains the same disclosure, but also discloses a peak acetal test on a crude hydrogenated stream of at least 0.429 wt%, which was reduced to about 0.2 wt% after treating the stream with the invention.

[0015] WO2013034881A1 does not address 2-(4'-hydroxybutoxy)-tetrahydrofuran levels.

[0016] WO 2013076747(A1) in the name of Conser SpA discloses a process for the catalytic hydrogenation of dialkyl maleates to produce 1,4-butanediol and tetrahydrofuran. The process essentially consists of the following steps: a) in a first reaction step, hydrogenating a dialkyl maleate stream over a suitable catalyst to produce a dialkyl succinate; b) Further hydrogenating the dialkyl succinate in a second stage of the reaction by using a different suitable catalyst to produce mainly 1,4-butanediol along with gamma-butyrolactone and tetrahydrofuran as co-products.

[0017] In both stages of the reaction, conditions such as hydrogen / organic feed ratio, pressure and temperature are such as to maintain the reactor in a mixed liquid / gas phase.

[0018] WO2013076747(A1) also discloses, to a surprisingly favorable extent than expected, that tests carried out using the above WO2013076747(A1) reaction in mixed phase and two steps have shown that the formation of the by-product cyclic acetal 2-(hydroxybutoxy)-tetrahydrofuran, which is stated in WO2013076747(A1) to be a particularly undesirable impurity since its boiling point is very close to that of BDO, is significantly reduced compared to other similar processes in the gas phase. It is noted that the 2-(hydroxybutoxy)-tetrahydrofuran referred to in WO2013076747(A1) is the same cyclic acetal as that referred to in other prior art and in the remainder of this document as 2-(4'-hydroxybutoxy)-tetrahydrofuran. WO2013076747(A1) states that, in contrast to US2007 / 0260073 (corresponding to WO2006037957(A1)), this reduction in by-product cyclic acetal formation represents a further and not negligible advantage of the WO2013076747(A1) invention. WO2013076747(A1) states that US2007 / 0260073 teaches that the reduction in acetals can be achieved by contacting a hydrogen stream in the liquid phase with butanediol, which is usually produced in the gas phase, in a separate hydrocracking reactor in the presence of the same type of catalyst as described in the WO2013076747(A1) invention. WO2013076747(A1) states that the invention of WO2013076747(A1) reaches even better results in terms of acetal contamination by simply carrying out the hydrogenolysis reaction in a mixed liquid-gas phase, without the need for any additional purification step in the liquid phase.In other words, WO2013076747A1 states that the mixed liquid-gas phase hydrocracking disclosed in WO2013076747A1 means that polishing hydrogenation as in U.S. Patent Publication No. 2007 / 0230073 is not required due to the reduced formation of cyclic acetals, which are by-products in mixed liquid-gas phase hydrocracking.

[0019] However, applicants have surprisingly found that even when only low levels of the cyclic acetal 2-(4'-hydroxybutoxy)-tetrahydrofuran are present in the crude 1,4-butanediol stream produced by hydrogenolysis of dialkyl succinates, it can still be present at unacceptably high levels in the purified 1,4-butanediol after purification to remove alkanols and valuable by-products such as γ-butyrolactone and tetrahydrofuran.

[0020] Preferred embodiments of the present invention seek to overcome one or more of the above-mentioned deficiencies in the prior art. In particular, preferred embodiments of the present invention seek to provide an improved process for producing purified 1,4-butanediol containing low levels of the cyclic acetal 2-(4'-hydroxybutoxy)-tetrahydrofuran. Summary of the Invention

[0021] According to a first aspect of the present invention, there is provided a process for producing a purified 1,4-butanediol stream, the process comprising hydrocracking a dialkyl succinate in one or more mixed gas / liquid phase reaction steps to form a crude 1,4-butanediol stream comprising 1,4-butanediol, γ-butyrolactone, tetrahydrofuran, and an alkanol, and transferring the crude 1,4-butanediol stream to a purification process, wherein at least a portion of the γ-butyrolactone, tetrahydrofuran, and the alkanol are purified from 1,4-butanediol. and recovering a purified 1,4-butanediol stream having a higher concentration of 1,4-butanediol than the crude 1,4-butanediol stream from the purification process, the purification process including a polishing section passing an intermediate stream comprising 1,4-butanediol and 2-(4'-hydroxybutoxy)-tetrahydrofuran through a catalyst bed to reduce the 2-(4'-hydroxybutoxy)-tetrahydrofuran content of the intermediate stream.

[0022] A specific process in which the present invention may be advantageous is a process involving mixed-phase hydrocracking, such as that disclosed in WO2013076747A1. Such mixed-phase hydrocracking processes are claimed in the prior art to be beneficial due to the low levels of 2-(4'-hydroxybutoxy)-tetrahydrofuran in the hydrocracking product. However, the applicant has found that additional 2-(4'-hydroxybutoxy)-tetrahydrofuran may be produced during the purification of 1,4-butanediol, and therefore the process of the present invention is a valuable addition to mixed-phase hydrocracking processes.

[0023] The dialkyl succinates can be produced by hydrogenation of dialkyl maleates. Preferably, the hydrogenation of dialkyl maleates to dialkyl succinates is carried out in one or more separate reaction steps upstream of the hydrocracking. Preferably, different catalysts are used for the hydrogenation of dialkyl maleates to dialkyl succinates and the hydrocracking of dialkyl succinates to produce a crude 1,4-butanediol stream. Preferably, the reaction steps are mixed gas / liquid phase reaction steps. That is, the conditions in the reaction steps are such as to maintain a mixed liquid / gas phase. That can be achieved, for example, by controlling one or more of the conditions, such as the feed ratio of hydrogen to organic feed, pressure and temperature. However, in some embodiments, the hydrogenation of dialkyl maleates to dialkyl succinates may be carried out in the same one or more reaction steps as the hydrocracking of dialkyl succinates. In such an embodiment, the dialkyl maleate may be fed to one or more of the reaction stages, where it undergoes hydrogenation to dialkyl succinate and then hydrocracking in the same reaction zone to form 1,4-butanediol, typically with co-products γ-butyrolactone and tetrahydrofuran. The dialkyl succinate may therefore be considered an intermediate in the reaction stage. Preferably, the reaction stage is a mixed gas / liquid phase reaction stage. That is, the conditions in the reaction stage are such as to maintain a mixed liquid / gas phase. This can be achieved, for example, by controlling one or more of the conditions, such as the feed ratio of hydrogen to organic feed, pressure and temperature.

[0024] For example, a mixed-phase hydrogenation and hydrocracking of dialkyl maleates to 1,4-butanediol may include a first reaction stage in which the dialkyl maleates are hydrogenated over a catalyst to produce dialkyl succinates, and a second reaction stage in which the dialkyl succinates are further hydrogenated to 1,4-butanediol, typically with co-products γ-butyrolactone and tetrahydrofuran. Preferably, the second reaction stage is carried out over a different catalyst than the first reaction stage, although in some embodiments the catalysts may be the same. For example, the catalyst in the first reaction stage may include palladium supported on a support including, for example, carbon or alumina. For example, the catalyst in the second reaction stage may include copper, such as a copper-chromite catalyst or a copper-zinc oxide catalyst. In each reaction stage, the conditions are such as to maintain a mixed liquid / gas phase. This can be achieved by controlling one or more of the conditions, such as the feed ratio of hydrogen to organic feed, pressure and temperature. Preferably, the first reaction stage takes place in a first reactor and the second reaction stage takes place in a second reactor, although in some embodiments the two stages may take place in a single reactor, e.g., having two or more zones.

[0025] A hydrogen source, typically hydrogen gas, is typically added to the reaction stage where hydrocracking takes place.

[0026] Preferably, the catalyst bed in the polishing section includes a catalyst comprising an active metal. The active metal may include a platinum group metal. The active metal may include nickel or copper. Preferably, the active metal includes at least one of nickel, copper, palladium, platinum, rhodium, and ruthenium. Preferably, the catalyst includes a support. Preferably, the support includes alumina, silica, zirconia, zinc, chromium, carbon, or mixtures thereof, such as silica / alumina or zirconia / alumina. In some cases, silica may be added to the support, which can improve the hydrothermal stability of the support.

[0027] Preferably, the intermediate stream is contacted with hydrogen over a catalyst bed. The hydrogen may be introduced, for example, as a hydrogen gas stream.

[0028] Preferably, the hydrogen pressure in the catalyst bed is between 20 barg and 60 barg, more preferably between 30 barg and 50 barg, most preferably about 40 barg. Preferably, the temperature in the catalyst bed is between 40° C. and 160° C., more preferably between 80° C. and 120° C. Preferably, the intermediate stream further comprises water or an alkanol, most preferably water, and the intermediate stream is contacted with hydrogen over the catalyst bed. Preferably, the water or alkanol is in an amount between 0% and 30% by weight of the intermediate stream, more preferably between 1% and 30% by weight, still more preferably between 5% and 30% by weight, even more preferably between 5% and 20% by weight, most preferably between 10% and 20% by weight.

[0029] Thus, advantageously, 2-(4'-hydroxybutoxy)-tetrahydrofuran is hydrolyzed and then hydrogenated. Water is most preferred, since 2-(4'-hydroxybutoxy)-tetrahydrofuran is advantageously hydrolyzed and then hydrogenated to recover 2 moles of 1,4-butanediol per mole of 2-(4'-hydroxybutoxy)-tetrahydrofuran. Water can then be removed by distillation and recycled.

[0030] Preferably, the polishing section comprises a trickle bed, ie, the catalyst bed is a trickle bed, more preferably, it is a reactor comprising multiple catalyst beds with a flow distributor between each catalyst bed.

[0031] Preferably, the intermediate stream comprises an acid. The acid may be added directly to the intermediate stream. However, preferably, the acid is added by adding an acid-forming species to the intermediate stream, or, most preferably, by not removing or not completely removing the acid-forming species from the intermediate stream. Preferably, the acid-forming species is γ-butyrolactone. For example, the purification process may separate the γ-butyrolactone present in the crude 1,4-butanediol stream as a γ-butyrolactone product, but some of the γ-butyrolactone may remain unseparated and be included in the intermediate stream. The acid may advantageously promote the hydrolysis of the acetal, 2-(4'-hydroxybutoxy)-tetrahydrofuran, to a hemiacetal, which may then undergo hydrogenation more quickly than the acetal itself. Thus, the presence of the acid may remove hydrolysis as a rate-limiting step and improve the overall reaction rate. Forming the acid from γ-butyrolactone may be advantageous since it does not require a separate acid stream to add the acid later to the process.

[0032] Preferably, the polishing section is located toward the downstream end of the purification process. In this way, the polishing section preferably removes 2-(4'-hydroxybutoxy)-tetrahydrofuran formed in hydrogenolysis or from precursors formed in hydrogenolysis, and also any 2-(4'-hydroxybutoxy)-tetrahydrofuran formed in other ways in the purification process. 2-(4'-hydroxybutoxy)-tetrahydrofuran can be formed, for example, by dehydrogenation of 1,4-butanediol, which can be catalyzed due to air intrusion into the purification process, especially into the vacuum tower in the purification process, or in the presence of fines resulting from hydrogenolysis of dialkyl succinates.

[0033] Preferably, the purification process comprises at least one vacuum distillation column, and the polishing section is located downstream of the at least one vacuum distillation column.

[0034] In one embodiment, the polishing section may be the last unit in a refinery process. Such an embodiment may be advantageous when retrofitting a polishing section into an existing refinery process that does not include a polishing section.

[0035] In one embodiment, the crude 1,4-butanediol stream may be transferred to a crude column, preferably operated such that tetrahydrofuran is withdrawn in an overhead stream and γ-butyrolactone and 1,4-butanediol are withdrawn in a bottom stream. Preferably, the overhead stream is transferred to a THF column(s) from which purified tetrahydrofuran is recovered and preferably tetrahydrofuran and alkanol are recycled to the crude column. Preferably, the bottom stream is transferred to a lights column, preferably where alkanol is removed in an overhead stream and γ-butyrolactone and 1,4-butanediol are preferably withdrawn in a further bottom stream, which are transferred to a heavies column. In the heavies column, γ-butyrolactone and dimethyl succinate are preferably removed from the top of the column, 1,4-butanediol is preferably removed as a 1,4-butanediol side stream, and heavies are preferably removed as a heavies bottom stream. One or both of the lights and heavies columns are preferably vacuum distillation columns. γ-butyrolactone is preferably purified via a DMS recycle column that separates dimethyl succinate from γ-butyrolactone for recycle, and a γ-butyrolactone heavies column that preferably removes residual heavies to form a purified γ-butyrolactone stream that is typically removed as a side stream from the γ-butyrolactone heavies column. The 1,4-butanediol side stream is preferably transferred to a polishing section to remove 2-(4'-hydroxybutoxy)-tetrahydrofuran. The polished 1,4-butanediol stream recovered from the polishing section is preferably transferred to a BDO product column from which purified 1,4-butanediol is preferably removed as a side stream. In some embodiments, the side stream may be transferred to a side stripper column to produce purified 1,4-butanediol.

[0036] Preferably, the polishing section includes adding water, or in some embodiments, an alkanol, to the intermediate stream prior to transferring the intermediate stream to the polishing reactor together with the hydrogen-containing stream. A polishing reactor effluent stream having a reduced 2-(4'-hydroxybutoxy)-tetrahydrofuran content compared to the intermediate stream is withdrawn from the polishing reactor. Water is preferably removed from the polishing reactor effluent stream in a water removal tower and is preferably recycled. After removing the water, a polished 1,4-butanediol stream is obtained having a lower 2-(4'-hydroxybutoxy)-tetrahydrofuran content than the intermediate stream.

[0037] In an embodiment of the polishing section, the intermediate stream is preferably fed to a feed drum where it is mixed with water. The feed drum is preferably fed to a polishing reactor that comprises a plurality of catalyst beds, preferably comprising a trickle catalyst bed, more preferably comprising a flow distributor between each catalyst bed. A stream comprising a hydrogen source, preferably hydrogen gas, is also preferably fed to the polishing reactor. The intermediate stream and the stream comprising hydrogen gas are preferably both fed to or near the top of the polishing reactor. For example, both streams may be fed to the headspace of the polishing reactor or above the top catalyst bed in the polishing reactor. A polishing reactor effluent stream having reduced 2-(4'-hydroxybutoxy)-tetrahydrofuran content is withdrawn from the polishing reactor. The polishing reactor effluent stream is preferably withdrawn at or near the bottom of the reactor, for example below the bottom catalyst bed in the polishing reactor. The polishing reactor effluent stream is preferably transferred to a knockout drum, preferably after passing through a filter. A liquid stream, preferably from the knockout drum, is fed to a water removal tower from which a polished 1,4-butanediol stream is preferably withdrawn from the bottom of the water removal tower. The overhead stream from the water removal tower preferably contains water, which is preferably condensed and recycled to the feed drum.

[0038] The 2-(4'-hydroxybutoxy)-tetrahydrofuran concentration in the intermediate stream fed to the polishing section may be at least 1.5 times, preferably at least 2 times, the 2-(4'-hydroxybutoxy)-tetrahydrofuran concentration in the crude 1,4-butanediol stream obtained from the hydrogenolysis of dialkyl succinates. Thus, although the acetal level in the crude 1,4-butanediol stream may appear to be acceptable, the additional 2-(4'-hydroxybutoxy)-tetrahydrofuran produced in the purification process may increase the acetal level, so that, without the polishing section of the present invention, the purified 1,4-butanediol stream would have too high an acetal level. Thus, the present invention may provide significant advantages in processes where acetal removal downstream of the hydrogenolysis of dialkyl succinates is not expected to be necessary.

[0039] Preferably, the purified 1,4-butanediol stream contains less than 0.15 wt.%, more preferably less than 0.1 wt.%, more preferably less than 0.8 wt.%, more preferably less than 0.6 wt.%, more preferably less than 0.4 wt.% 2-(4'-hydroxybutoxy)-tetrahydrofuran.

[0040] The dialkyl succinate preferably comprises a C1-C6 alkyl group, more preferably a C1-C4 alkyl group. Dimethyl succinate and diethyl succinate are particularly preferred, with dimethyl succinate being more preferred. The dialkyl maleate preferably comprises a C1-C6 alkyl group, more preferably a C1-C4 alkyl group. Dimethyl maleate and diethyl maleate are particularly preferred, with dimethyl maleate being more preferred.

[0041] The present invention may be particularly advantageous when the mixed-phase hydrocracking process maintained by the prior art does not require a polishing section, but the present invention may be equally applicable to other hydrocracking methods that would produce only low levels of 2-(4'-hydroxybutoxy)-tetrahydrofuran in the crude 1,4-butanediol stream. The inventors recognize that 2-(4'-hydroxybutoxy)-tetrahydrofuran is not only produced as a by-product of hydrocracking or as a result of precursors formed as by-products of hydrocracking, but also from extra reactions occurring in the purification process. Thus, even if the 2-(4'-hydroxybutoxy)-tetrahydrofuran levels in the crude 1,4-butanediol stream after hydrocracking already appear to be acceptably low, a polishing section according to the present invention is still advantageous for treating new 2-(4'-hydroxybutoxy)-tetrahydrofuran formed in the purification process. Thus, according to a second aspect of the present invention, there is provided a process for producing a purified 1,4-butanediol stream comprising hydrogenolysis of a dialkyl succinate to form a crude 1,4-butanediol stream comprising 1,4-butanediol, γ-butyrolactone, tetrahydrofuran, an alkanol, and less than 0.15 wt. % of 2-(4'-hydroxybutoxy)-tetrahydrofuran, and transferring the crude 1,4-butanediol stream to a purification process, wherein at least one of γ-butyrolactone, tetrahydrofuran, and the alkanol is removed by hydrogenolysis. and recovering a purified 1,4-butanediol stream having a higher concentration of 1,4-butanediol than the crude 1,4-butanediol stream from the purification process, the purification process including a polishing section passing an intermediate stream comprising 1,4-butanediol and 2-(4'-hydroxybutoxy)-tetrahydrofuran through a catalyst bed to reduce the 2-(4'-hydroxybutoxy)-tetrahydrofuran content of the intermediate stream.

[0042] Preferably, the crude 1,4-butanediol stream contains less than 0.1 wt.%, more preferably less than 0.75 wt.% 2-(4'-hydroxybutoxy)-tetrahydrofuran. The crude 1,4-butanediol stream may contain less than 0.7 wt.%, or less than 0.6 wt.%, or less than 0.5 wt.%, or less than 0.4 wt.%, or less than 0.3 wt.%, or less than 0.2 wt.% 2-(4'-hydroxybutoxy)-tetrahydrofuran. Although such low levels would on the surface suggest that there would be no problems with 2-(4'-hydroxybutoxy)-tetrahydrofuran levels in the final product, the inventors recognized that new 2-(4'-hydroxybutoxy)-tetrahydrofuran may be formed in the purification process, and thus a purification process including a polishing section according to the invention may still be advantageous.

[0043] The concentration of 2-(4'-hydroxybutoxy)-tetrahydrofuran in the intermediate stream may be at least 1.5 times, preferably at least 2 times, the concentration of 2-(4'-hydroxybutoxy)-tetrahydrofuran in the crude 1,4-butanediol stream obtained from the hydrogenolysis of dialkyl succinates. For example, the intermediate stream may contain at least 0.2 wt. %, more preferably at least 0.25 wt. %, and even more preferably at least 0.3 wt. % 2-(4'-hydroxybutoxy)-tetrahydrofuran.

[0044] Preferably, the intermediate stream is contacted with hydrogen over a catalyst bed. The hydrogen may be introduced, for example, as a hydrogen gas stream.

[0045] Preferably the hydrogen pressure in the catalyst bed is between 20barg and 60barg, more preferably between 30barg and 50barg, most preferably about 40barg. Preferably the temperature in the catalyst bed is between 40°C and 160°C, more preferably between 80°C and 120°C.

[0046] Preferably, the intermediate stream further comprises water or an alkanol, most preferably water, and the intermediate stream is contacted with hydrogen over a catalyst bed. Preferably, the water or alkanol is in an amount of from 0% to 30% by weight of the intermediate stream, more preferably from 1% to 30% by weight, still more preferably from 5% to 30% by weight, even more preferably from 5% to 20% by weight, most preferably from 10% to 20% by weight.

[0047] Preferably, the intermediate stream comprises an acid. The acid may be added directly to the intermediate stream. However, preferably, the acid is added by adding an acid-forming species to the intermediate stream, or, most preferably, by not removing or not completely removing the acid-forming species from the intermediate stream. Preferably, the acid-forming species is γ-butyrolactone. For example, the purification process can separate γ-butyrolactone present in the crude 1,4-butanediol stream as γ-butyrolactone product, although some of the γ-butyrolactone may remain unseparated and be included in the intermediate stream.

[0048] Preferably, the polishing section is located toward the downstream end of the purification process. In this way, the polishing section preferably removes the 2-(4'-hydroxybutoxy)-tetrahydrofuran formed in the hydrocracking, and also any 2-(4'-hydroxybutoxy)-tetrahydrofuran formed in the purification process. 2-(4'-hydroxybutoxy)-tetrahydrofuran may be formed in the purification process from the precursor formed in the hydrocracking, but additional 2-(4'-hydroxybutoxy)-tetrahydrofuran may be formed in the purification process, for example, by dehydrogenation of 1,4-butanediol, which may be catalyzed due to the ingress of air into the purification process, particularly into the vacuum tower in the purification process, or in the presence of fine particles resulting from the hydrocracking of dialkyl succinates. At least a part of such additional 2-(4'-hydroxybutoxy)-tetrahydrofuran is advantageously removed in the polishing section.

[0049] Preferably, the purification process comprises at least one vacuum distillation column, and the polishing section is located downstream of the at least one vacuum distillation column.

[0050] The amount of 2-(4'-hydroxybutoxy)-tetrahydrofuran is measured using the peak acetal test described above. The peak acetal test involves removing lights from the stream measured at 120°C, followed by further heating at 160°C for 3 hours. Heating, which can be performed using an isomantle heater, a round-bottom flask, a condenser, and a collection pot, is performed under a nitrogen blanket at atmospheric pressure. The residue is then analyzed by gas chromatography to determine the 2-(4'-hydroxybutoxy)-tetrahydrofuran content. This procedure allows for the reaction of precursors of 2-(4'-hydroxybutoxy)-tetrahydrofuran in the stream. In industrial 1,4-butanediol plants, the precursors have reacted to form 2-(4'-hydroxybutoxy)-tetrahydrofuran by the time the purified 1,4-butanediol stream is withdrawn, so the peak acetal test measures the amount of 2-(4'-hydroxybutoxy)-tetrahydrofuran expected in the final 1,4-butanediol product that is due to 2-(4'-hydroxybutoxy)-tetrahydrofuran already present in the measured stream or precursors. Thus, in the prior art, the test is sometimes suggested to represent the maximum 2-(4'-hydroxybutoxy)-tetrahydrofuran content possible in the final 1,4-butanediol stream if the crude hydrocracked product is subjected to purification by a standard distillation system. However, as noted above, the inventors recognized that new 2-(4'-hydroxybutoxy)-tetrahydrofuran is formed during the purification process due to, for example, air ingress or catalyst fines, and that even if the peak acetal test reports that the 2-(4'-hydroxybutoxy)-tetrahydrofuran level in the crude 1,4-butanediol stream appears to be acceptable, the final product may contain unacceptable levels of 2-(4'-hydroxybutoxy)-tetrahydrofuran without the use of the present invention.

[0051] The process of the second aspect of the invention may additionally or alternatively include any of the features, for example those mentioned above in relation to the first aspect of the invention.

[0052] It will be understood that features described in relation to one aspect of the invention may be equally applicable to another aspect of the invention. For example, features described in relation to a first aspect of the invention may be equally applicable to a second aspect of the invention, and vice versa. Some features may not be applicable to or may be excluded from certain aspects of the invention. [Brief description of the drawings]

[0053] Embodiments of the present invention will now be described, by way of example and not limitation, with reference to the accompanying figures, in which:

[0054] [Figure 1] FIG. 1 is an illustration of a process comprising an embodiment of the present invention. [Diagram 2] FIG. 2 is a diagram of a polishing section suitable for use in the process of FIG. 1. [Diagram 3] Graphs of experiments 1, 2, and 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0055] In FIG. 1, a feed of dialkyl maleate is fed to a first reactor 1, to which hydrogen 29 is also fed. At least a portion of the dialkyl maleate is hydrogenated to dialkyl succinate in the first reactor 1. A stream 11 containing the dialkyl succinate is withdrawn from the first reactor 1 and fed to a second reactor 2, to which additional hydrogen 30 is also fed. In the second reactor 2, at least a portion of the dialkyl succinate is converted to 1,4-butanediol by hydrogenolysis. In this embodiment, both reactors 1 and 2 operate in mixed gas / liquid phase, although in other embodiments they may operate in gas or liquid phase. In other embodiments, there may be only one reactor. In such an embodiment, the feed to one reactor may comprise dialkyl succinate produced, for example, from succinic acid, or may comprise dialkyl maleate produced, for example, from maleic anhydride. In the case of dialkyl maleate, the reaction in one reactor includes both hydrogenation of the dialkyl maleate to dialkyl succinate and subsequent hydrogenolysis of the dialkyl succinate to 1,4-butanediol. In other embodiments, hydrogen may be fed only to the first reactor 1, or the hydrogen fed to the second reactor 2 may be at least partially hydrogen recovered from the first reactor 1. A crude 1,4-butanediol stream 12 containing 1,4-butanediol, γ-butyrolactone, tetrahydrofuran, and alkanol is recovered from the second reactor 2 and transferred to a purification process. The crude 1,4-butanediol stream 12 is fed to a crude column 3 operated such that tetrahydrofuran is withdrawn in an overhead stream 14 and γ-butyrolactone and 1,4-butanediol are withdrawn in a bottoms stream 17. The overhead stream 14 is transferred to a THF separation unit 4, typically comprising one or more towers, from which purified tetrahydrofuran 16 is recovered and the tetrahydrofuran and alkanol are recycled 15 to the crude column 3. The bottoms stream 17 is transferred to the lights column 5 where the alkanol is removed in an overhead alkanol stream 18 and gamma-butyrolactone and 1,4-butanediol are withdrawn in a further bottoms stream 19 which is transferred to the heavies column 6.Heavies column 6 removes γ-butyrolactone and dimethyl succinate overhead in crude γ-butyrolactone stream 20, 1,4-butanediol as 1,4-butanediol side stream 25, and heavies as heavies bottoms stream 24. In this embodiment, lights column 5 and heavies column 6 are vacuum distillation columns. γ-butyrolactone in crude γ-butyrolactone stream 20 is purified via DMS recycle column 7 which separates the dimethyl succinate into DMS recycle 21 and γ-butyrolactone heavies column 8 where remaining heavies are removed as removed heavies stream 31. Purified γ-butyrolactone stream 23 is removed as a side stream from γ-butyrolactone heavies column 8. The 1,4-butanediol side stream 25 is transferred as a feed stream to the polishing section 9 where an intermediate stream comprising 1,4-butanediol and 2-(4'-hydroxybutoxy)-tetrahydrofuran is passed over a catalyst bed to remove 2-(4'-hydroxybutoxy)-tetrahydrofuran. An exemplary embodiment of a suitable polishing section 9 is described below and in FIG. 2. The polished 1,4-butanediol stream 26 recovered from the polishing section 9 is transferred to the BDO product column 10 from which a purified 1,4-butanediol stream 27 is removed as a side stream. In some embodiments, the side stream may be transferred to a side stripper column to produce purified 1,4-butanediol. Alternatively, in some embodiments, a 1,4-butanediol pre-column may be present prior to the BDO product column 10. Purified 1,4-butanediol stream 27 has a higher concentration of 1,4-butanediol than crude 1,4-butanediol stream 12. 1,4-butanediol side stream 25 has a higher 2-(4'-hydroxybutoxy)-tetrahydrofuran content than crude 1,4-butanediol stream 12 due to the formation of 2-(4'-hydroxybutoxy)-tetrahydrofuran in one or more of crude column 3, lights column 5, and heavies column 6.Polishing section 9 advantageously reduces the 2-(4'-hydroxybutoxy)-tetrahydrofuran content to an acceptable level such that purified 1,4-butanediol stream 27 has a 2-(4'-hydroxybutoxy)-tetrahydrofuran concentration that is lower than the concentration in 1,4-butanediol side stream 25 and meets specifications for the desired downstream use of 1,4-butanediol. In the embodiment of FIG. 1, polishing section 9 is prior to BDO product column 10. However, polishing section 9 may be present at other points in the purification process, for example downstream of BDO product column 10. Such a configuration may be particularly advantageous when polishing section 9 is retrofitted to a plant.

[0056] An embodiment of the polishing section 9 suitable for use in the process described above in connection with FIG. 1 is shown in FIG. 2. It will be understood that other configurations of the polishing section 9 are possible. In FIG. 2, a 1,4-butanediol side stream 25 containing 1,4-butanediol and 2-(4'-hydroxybutoxy)-tetrahydrofuran is fed as a feed stream to the polishing section 9. In the polishing section 9, the 1,4-butanediol side stream 25 is fed to a feed drum 40 where it is mixed with a water stream 45. An intermediate stream 46 from the feed drum 40 is fed to a polishing reactor 41 that includes multiple catalyst beds with a flow distributor between each bed. A hydrogen feed stream 53 containing hydrogen gas is also fed to the polishing reactor 41. Both the intermediate stream 46 from the feed drum 40 and the hydrogen feed stream 53 are fed near the top of the polishing reactor 41 above the top catalyst bed of the polishing reactor 41. A polishing reactor effluent stream 47 having a reduced content of 2-(4'-hydroxybutoxy)-tetrahydrofuran is collected near the bottom of the polishing reactor 41 below the bottom catalyst bed in the polishing reactor 41. The polishing reactor effluent stream 42 is passed through a filter 42 to form a filtered polishing reactor effluent stream 48 which is transferred to a knockout drum 43. A liquid stream 49 from the knockout drum 43 is fed to a water removal column 44 from the bottom of which a polished 1,4-butanediol stream 26 is withdrawn. The polished 1,4-butanediol stream 26 has a lower 2-(4'-hydroxybutoxy)-tetrahydrofuran content than the 1,4-butanediol side stream 25. An overhead stream 50 from the water removal column 44 is condensed and recycled 52 to the feed drum 40 and a purge 51 is recycled to an upstream point in the purification process.

[0057] experiment Experiment 1 A 500 mL reactor was charged with 400 g of the crude hydrogenation product. The reactor was heated to 120° C. under an inert atmosphere of nitrogen and the lights were removed by distillation. To ensure no further losses occurred, the unit was then changed to operate in reflux mode and the temperature was increased to 160° C., at which point samples were taken over time and analyzed for acetal content by GC.

[0058] The maximum 2-(4'-hydroxybutoxy)-tetrahydrofuran level in this system was found to be 1800 ppm. A graphical summary of this experiment can be found in Figure 3.

[0059] Experiment 2 Run 1 was repeated, except that air was used instead of nitrogen. Results showed that over the first 45 minutes of the test, the rate of 2-(4'-hydroxybutoxy)-tetrahydrofuran formation matched that when nitrogen was used. However, since the initial acetal precursor was consumed as shown by the first run, the presence of air resulted in a significant increase in acetal as the test progressed. A summary of this run compared to run 1 can be found in Figure 3.

[0060] Experiment 3 Run 1 was repeated, except that approximately 10 wt. % of powdered hydrogenation catalyst was added to the reactor. Analysis of samples over time suggests that the high levels of catalyst fines promoted the formation of 2-(4'-hydroxybutoxy)-tetrahydrofuran. A summary of this run compared to run 1 can be found in Figure 3.

[0061] The experiments demonstrate two mechanisms that can increase 2-(4'-hydroxybutoxy)-tetrahydrofuran levels during purification of 1,4-butanediol products. As a result, even crude 1,4-butanediol streams that appear to have acceptably low levels of 2-(4'-hydroxybutoxy)-tetrahydrofuran, including as measured by the peak acetal test, can surprisingly result in excess 2-(4'-hydroxybutoxy)-tetrahydrofuran in the purified 1,4-butanediol stream. The present invention alleviates this problem by including a polishing section in which an intermediate stream containing 1,4-butanediol and 2-(4'-hydroxybutoxy)-tetrahydrofuran is passed through a catalyst bed to reduce the 2-(4'-hydroxybutoxy)-tetrahydrofuran content. If the crude 1,4-butanediol stream is deemed to have acceptably low levels of 2-(4'-hydroxybutoxy)-tetrahydrofuran, it may be counterintuitive to include such a polishing section, especially since adding extra, apparently unnecessary, equipment may increase capital and operating costs. However, applicants have recognized that 2-(4'-hydroxybutoxy)-tetrahydrofuran levels may increase during purification, as demonstrated by Experiments 1-3, and thus, including the polishing section of the present invention may be advantageous. For example, the cost of the polishing section may be advantageous when compared to the cost of measures that would be required to ensure zero undesirable inflow of air or particulates into the purification process.

[0062] It will be appreciated by those skilled in the art that the above embodiments are described by way of example only and not by way of limitation, and that various changes and modifications are possible without departing from the scope of the invention as defined by the appended claims. For example, a different order of separation or a different number of columns in the purification process are possible, and the location of the polishing section within the purification process can also be changed.

Claims

1. A process for producing a purified 1,4-butanediol stream, the process comprising: hydrogenocracking a dialkyl succinate in one or more gas-to-liquid phase reaction steps to form a crude 1,4-butanediol stream containing 1,4-butanediol, γ-butyrolactone, tetrahydrofuran, and alkanol; and transferring the crude 1,4-butanediol stream to a purification process, wherein at least a portion of the γ-butyrolactone, tetrahydrofuran, and alkanol is 1,4-butanediol A process comprising: removing from and recovering from the purification process a purified 1,4-butanediol stream having a higher concentration of 1,4-butanediol than the crude 1,4-butanediol stream, wherein the purification process includes a polishing section that reduces the 2-(4'-hydroxybutoxy)tetrahydrofuran content of the intermediate stream by passing an intermediate stream containing 1,4-butanediol and 2-(4'-hydroxybutoxy)-tetrahydrofuran through a catalyst bed.

2. The process according to claim 1, wherein the intermediate flow further comprises an alkanol or water, and the intermediate flow comes into contact with hydrogen on the catalyst bed.

3. The process according to claim 2, wherein the hydrogen pressure in the catalyst bed is 20 barg to 60 barg.

4. The process according to claim 2 or 3, wherein the intermediate flow further comprises an acid.

5. The process according to claim 4, wherein the acid is formed from γ-butyrolactone in the purification process.

6. The process according to claim 1, wherein the dialkyl succinate is produced by the hydrogenation of a dialkyl maleate.

7. The process according to claim 6, wherein the hydrogenation is carried out in one or more separate gas-to-liquid phase reaction steps.

8. The process according to claim 1, wherein the temperature in the catalyst bed is 40°C to 160°C.

9. The process according to claim 1, wherein the catalyst bed comprises a catalyst containing at least one of an active metal, preferably nickel, copper, palladium, platinum, rhodium, and ruthenium.

10. The process according to claim 9, wherein the catalyst preferably comprises a carrier containing alumina, silica, zirconia, zinc, chromium, carbon, or a mixture thereof.

11. The process according to claim 1, wherein the polishing section is positioned toward the downstream end of the purification process in order to remove 2-(4'-hydroxybutoxy)-tetrahydrofuran formed in the purification process.

12. The process according to claim 1, wherein the purification process comprises at least one vacuum distillation column, and the polishing section is located downstream of the at least one vacuum distillation column.

13. The process according to claim 1, wherein the polishing section includes mixing a feed stream containing 1,4-butanediol and 2-(4'-hydroxybutoxy)-tetrahydrofuran with water to form the intermediate stream, and transferring the intermediate stream, together with a hydrogen-containing stream, to a polishing reactor containing the catalyst bed.

14. The process according to claim 13, wherein a polishing reactor effluent flow having a reduced 2-(4'-hydroxybutoxy)-tetrahydrofuran content compared to the intermediate flow is withdrawn from the polishing reactor, and water is removed from the polishing reactor effluent flow, preferably in a water removal tower, to obtain a polished 1,4-butanediol flow having a lower 2-(4'-hydroxybutoxy)-tetrahydrofuran content than the feed flow.

15. The process according to claim 13, wherein the concentration of 2-(4'-hydroxybutoxy)tetrahydrofuran in the feed stream is at least 1.5 times the concentration of 2-(4'-hydroxybutoxy)tetrahydrofuran in the crude 1,4-butanediol stream.

16. A process for producing a purified 1,4-butanediol stream, the process comprising: hydrocracking a dialkyl succinate to form a crude 1,4-butanediol stream containing 1,4-butanediol, γ-butyrolactone, tetrahydrofuran, alkanol, and less than 0.15% by weight of 2-(4'-hydroxybutoxy)-tetrahydrofuran; and transferring the crude 1,4-butanediol stream to a purification process, wherein at least a portion of the γ-butyrolactone, tetrahydrofuran, and alkanol is A process comprising removing from 1,4-butanediol and recovering from the purification process a purified 1,4-butanediol stream having a higher concentration of 1,4-butanediol than the crude 1,4-butanediol stream, wherein the purification process includes a polishing section that reduces the 2-(4'-hydroxybutoxy)tetrahydrofuran content of the intermediate stream by passing an intermediate stream containing 1,4-butanediol and 2-(4'-hydroxybutoxy)-tetrahydrofuran through a catalyst bed.

17. The process according to claim 16, wherein the concentration of 2-(4'-hydroxybutoxy)-tetrahydrofuran in the intermediate stream is at least 1.5 times the concentration of 2-(4'-hydroxybutoxy)-tetrahydrofuran in the crude 1,4-butanediol stream.

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