MXPA01002578A - Method for producing highly pure monoethylene glycol - Google Patents

Abstract

The invention relates to a method for obtaining highly pure monoethylene glycol from the product of hydrolysis of ethylene oxide by distillation, by means of dehydration under pressure, preferably in a cascade, vacuum dehydration and subsequent pure distillation. According to the inventive method, the pressure dehydration column or at least the first pressure dehydration column of the cascade (2, 3, 4) has a stripping section with at least one separation stage, preferably 2-10 separation stages, especially 3-6 stages, and in that part of the top flow of the dehydration column(s) (2, 3, 4) is sluiced out with the stripping section.

Description

PROCESS FOR OBTAINING HIGH PURITY MONOETHYLENGLICOL.

Monoethylene glycol is produced industrially by hydrolysis of ethylene oxide, removal of water and purification by distillation. In order to improve the selectivity of the hydrolysis of ethylene oxide (hereinafter abbreviated as EO), the reactor for hydrolysis is operated using a large excess of water (weight ratio water = 4: 1 to 15: 1). This makes it possible to suppress the fraction of higher glycols, especially diethylene glycol, triethylene glycol, etc. The hydrolysis reactor is usually operated at temperatures between 120 and 250 ° C and pressures of 30-40 bar. The hydrolysis product is first released from water, to a residual aqueous content of 100-200 ppm, and then the different glycols are separated in their pure state. The elimination of water is usually carried out in a battery of columns with stepped pressure, ordered at descending pressure. For reasons of thermal integration, usually only the bottom reheater of the first column under pressure is heated with external steam, while all other columns under pressure are heated with the vapors of the preceding column. Feeding enters each column at a point below the first plate, since no extractor (or desorption) section is required to separate water and glycols. Depending ** && , ** of the water content of the hydrolysis reactor effluent and of the pressure / temperature level of the external steam used in the bottom reheater of the first column, the pressure water eliminating battery comprises 2 to 7 columns. The stage of elimination of water under pressure is followed by a stage of elimination of water under vacuum, which usually takes place in a column equipped with an extraction section. The water obtained from the water removal processes is recycled at a point in the upstream of the hydrolysis reactor. The water-free glycol solution is separated into its pure components into a plurality of columns. The onoethylene glycol, diethylene glycol and triethylene glycol are each extracted as top column products, while all the other higher glycols are obtained as a mixture, known as polyethylene glycols, as the bottom product of the last column. Conventional glycol plants, in addition to the product streams, usually have only one additional outlet, the acetaldehyde purge of the reheater of the lower head of the second pressurized water scavenger column. There, the non-condensed fraction of the vapors in the first column used for heating is removed from the system. In this way, the secondary components, either introduced to the glycol plant by the water / EO stream or formed in the glycol plant as a consequence of secondary reactions, can only be extracted from the system by means of the acetaldehyde purge or by of product streams. This last modality affects the quality of the products and, therefore, is not the desirable one. Until now, glycol plants were optimized only with respect to their main functions, especially "energy and capital cost reduction for water removal and purification by distillation." Lately, increasingly rigid requirements are being demanded on Product quality in the case of mono-ethylene glycol, especially with reference to the content of secondary components There are two qualities of monoethylene glycol: the technical grade (antifreeze grade), with lower requirements, to use as a refrigerant, and fiber quality , with strict requirements, used in the manufacture of fibers.The exact specification of fiber quality varies with the client, but as regards the free aldehydes, reported as acetaldehyde, and -determined spectrophotometrically as MBTH blue complex, in General covers the range of 7 to 20 ppm, and for the minimum transmittance in UV it generally covers the range 76% -80% at 220 nm and 90% -95% at 275 nm. The substances that contribute to the determination of aldehydes are, in particular, formaldehyde, acetaldehyde and gualcolaldehyde. The active substances in the UV, known as UV pollutants, are largely unknown, but they cause the products to fall out of specification even at concentrations of less than 1 ppm. Examples are acrolein and crotonaldehyde. Patent JP-A-60,089,439 describes a process for purifying monoethylene glycol by means of vacuum distillation with inert gas supply. The nitrogen stream desorbs most of the secondary components, giving rise to a glycol of high purity that is suitable for the manufacture of fibers. However, the process has the drawback that large amounts of nitrogen are required for the effective removal of the secondary components. This leads to the undesirable loss of product in the effluent gas and to an excessively large fluid dynamic load in the distillation column. DE-A-1,942,094 describes a process for purifying monoethylene glycols by means of steam distillation in a desorption column, the vapor increasing the volatility of the impurities with respect to the monoethylene glycol. CA-C-133050 discloses a process for purifying monoethylene glycol by adding bisulfite ions and subsequent treatment with ion exchange resins. There are also procedures for the purification of monoethylene glycol in which it is stated that the formation of secondary components is reduced by special devices in the area of the construction of the apparatuses and in the construction materials used for the apparatuses. DE-A-19602116 discloses a purification process for monoethylene glycol in an apparatus whose surface has been treated with reducing phosphorus compounds. However, the aforementioned processes have the disadvantage of requiring additives or additional artifacts in the equipment to obtain a high purity monoethylene glycol. It is an object of the present invention to provide a distillation process for obtaining high purity monoethylene glycol, without the use of additives or specific construction materials. Secondary components that would cause the product to fall out of specification must be removed from the system in predominantly aqueous waste streams that do not have glycol contents greater than 1% by weight, and the secondary components of the waste streams must be concentrated in one factor of 10-100, since otherwise too much waste water would be produced. We have discovered that this objective is achieved by a process for the recovery of high purity monoethylene glycol by distillation, obtained by hydrolysis of ethylene oxide, by means of elimination of water under pressure, preferably in a battery, vacuum water removal and subsequent purification by distillation, comprising the removal of water under pressure carried out in a column for eliminating water having a desorption section with at least one separation step, particularly preferably from 2 to 10 separation steps, ideally from 3 to 6 stages, and a portion of the upper stream of the water removal column (s) having a desorption section is removed from the system. It was determined that the elimination of secondary components that would cause the product to fall out of specification is particularly efficient at certain points in the process. The identification of these points of the process is not a trivial matter, since the complex equilibria of phases have until now been impossible to arrive at a sufficiently reliable evaluation of the behavior of the secondary components. For this reason, the large conventional industrial processes only have a very rough outlet for the extremely low boiling secondary components: the acetaldehyde purge in the reheater of the lower head of the second pressurized water removing column. This output is not optimized, since the behavior of the secondary components was mostly unknown and was not taken into account in the process design stage. ^^^^^^^^^^ a¡g > jg | ^^ g ^ a ^ l ^ j The components are subdivided here into three categories according to their boiling range: 1. low boiling point, with a lower volatility than water (especially acetaldehyde, formaldehyde in water pure, acrolein); 2. medium boiling point, with an intermediate volatility between that of water and that of monoethylene glycol (especially formaldehyde in aqueous solutions containing glycols, formaldehyde in anhydrous monoethylene glycol, glycoaldehyde, crotonaldehyde); 3. High boiling point, with a lower volatility than that of monoethylene glycol (especially relatively high molecular weight aldehydes, "UV contaminants.) According to the invention, the removal of secondary components, especially low-point ones, is improved. of boiling, in the stage of elimination of water under pressure For this purpose, the column of elimination of water under pressure or at least the first column of elimination of water under pressure of the battery, has a section of desorption with at least one separation step, particularly preferably from 2 to 10 separation steps, ideally from 3 to 6 stages, and a portion of the upper head stream of the water removing column (s) being removed from the system. possess a section of desorption.

Major conventional industrial processes use a purge of acetaldehyde in the reheater of the lower head of the second column of elimination of water under pressure: this is where they essentially condense the vapors of the first column of elimination of water under pressure, being the non-condensed fraction , about 1-5% by weight of the total vapors, removed from the system. If desired, the remaining vapors can be condensed in an additional heat exchanger, and the condensation heat can be used at a suitable point in the overall process. However, this conventional solution will eliminate through the purge of acetaldehyde only the secondary components that leave the first column of elimination of water under pressure, as part of the vapors. This is inadequate in the case of formaldehyde in particular, since the volatility of formaldehyde in aqueous solutions of glycols decreases with the increasing content of glycols, especially as a result of chemical reactions of formaldehyde with water and glycols. In order to separate formaldehyde from the glycolic product of the lower head of the pressurized water removal column, the pressurized water removal column itself or at least the first column of pressurized water removal from a battery requires a section of desorption of at least one separative step, preferably 2 to 10 separating steps, or, better still, of 3 to 6 separating steps. Only when the formaldehyde has been extracted to the purely aqueous vapors of the first column can it be purged from the system together with the acetaldehyde. The removal efficiency of the formaldehyde in the desorption section is improved with the temperature and correspondingly with the pressure in the column of elimination of water under pressure, or in the first column of elimination of water under pressure of the battery, and with the aqueous content of the reactor effluent. Two of the additional plates of the desorption section can be saved if the lower head reheater is constructed as a "divided base" as described in DE-C-3338488. The amount of secondary components, especially acetaldehyde or formaldehyde, extracted However, it must be borne in mind that the quantity of steam not condensed in the reheater of the lower head of the second column of water removal can not be increased by the system. integrated energy and by virtue of the control engineering constraints The inventors discovered a particularly preferred version of the process, by which the further removal of the secondary components of the condensed steam by means of steam desorption is possible. with secondary components can later be used by its energy content at some appropriate point in the process. The desorption of steam, therefore, does not require additional energy, only an additional device. The removal of the secondary components of the system is particularly effective when the effluent from the desorber is recycled to the first column of water removal, since this recycling will ince the aldehyde content in the upper head of the first column of water removal to pressure and in the desorbedor and, consequently, it will also ince the speed of removal 10. • It will be advantageous if the temperature below the feed point is above 80 ° C, but preferably in the range between 100 ° C and 250 ° C, or, better still, in the range between 115 ° C and 230 ° C. The pressure in the desorption section will not be less than 1 bar, preferably within the range of between 2 and 30 bar. It will be advantageous if the upper st of the water removal column (s) with a desorption section is introduced into a partial condenser and / or a desorber, especially a steam desorber, and the (s) Enriched current (s) in the secondary components are removed from the system. It is convenient that the partial condenser and / or the desorber be operated above 90 ° C, preferably 25 between 120 ° and 250 ° C.

The variants of the invention will be described below in more detail by means of examples, with reference to illustrations, in which: Figure 1 shows an industrial scale process scheme for the recovery of glycols according to the prior art; Figure 2 shows the schematic of a particularly preferred process for the recovery of glycols according to the invention; Figure 3 shows an illustrative example of a process of the invention, characterized by a column of elimination of water under pressure with a desorption section and an outlet for secondary components in the form of upper current and also the subsequent concentration in a partial condenser and a desorbedor. Figure 1 shows an industrial scale process scheme for the recovery of glycols according to the prior art. The hydrolysis tor 1 is fed with a water / ethylene oxide mixture having a water: ethylene oxide weight ratio ranging from 4: 1 to 15: 1 and then a pressure water removal stage, represented by a battery of three columns with stepped pressure 2, 3 and 4.

The feeding point of columns 2, 3 and 4 is located in the bottom ain each case. The current of -%. ^ to < ** ^. ^ ^^ ilt ^ J ^ é ^^? ii? ^^ steam from the first pressure water removal column 2 condenses in the bottom reheater of the second pressurized water removal column 3 and the non-condensed fraction is removed from the system under the name of acetaldehyde purge (/ ACH, ie water / acetaldehyde). The condensed vapors coming from the pressurized water removal columns 2, 3 and 4 are recycled at a countercurrent point of the hydrolysis tor 1. The lower st of the last pressure water removal column 4 is introduced into the section medium of a vacuum water removal column 5. The steam, of predominantly aqueous content, coming from the vacuum water removal column 5, is similarly condensed and recycled at a countercurrent point of the hydrolysis tor 1. The effluent of the vacuum water removal column 5 feeds into a distillation column for purification of monoethylene glycol 6, from which more secondary components, especially formaldehyde, glycoaldehyde and UV contaminants, are extracted as products of the monoethylene glycol top head. The lower effluent from the distillation column for purification of monoethylene glycol 6 feeds a distillation column for purification of diethylene glycol., from which pure diethylene glycol is extracted as a product from the upper head, and whose lower effluent feeds an additional column, the distillation column for purification of triethylene glycol 8. The product of the upper head of the distillation column for purification of triethylene glycol is triethylene glycol pure, and the lower effluent 5 from column 8 contains a mixture of higher glycols, known as polyethylene glycol. Figure 2, on the other hand, shows the scheme of an industrial scale process for the recovery of high purity monoethylene glycol according to the invention. Compared with the process diagram of Figure 1, the feed is introduced into the first pressure water removal column 2 at a point higher than the height of this column, and this pressure water removal column 2 possesses a Desorption section between 2 and 6 dishes. 15 An additional difference with respect to the process of Figure 1 is that the vapor of the first column of elimination of water under pressure 2, after a partial condensation in the reheater of the head of the bottom of the column of elimination of water under pressure 3, submit to The desorber effluent is a gaseous stream of secondary components (W / ACH / FA, ie water / acetaldehyde / formaldehyde) that leaves the system. Figure 3 shows an example of proposed modification for the invention of a water removal column to pressure 2 with desorption section and also with a desorber 9 to concentrate the secondary components prior to their removal from the system. The feed 21 of the glyco-Jf loaded stream to be separated is found in the 5th plate of a pressure water removal column 2 consisting of 20 bubble plates. Its upper stream 23 is introduced, after a partial condensation, as stream 26 in a desorber 9 consisting of 10 bubble plates, and is subjected to desorption, remaining free of components Secondary, by means of the countercurrent vapor 29. The gas streams 25 and 27, which contain secondary components, are removed from the system. The fraction 24 of the lower effluent of the desorber 9 constitutes the reflux that enters the elimination column of water 2. The The composition of streams 21-29 is shown in Table la for a process of the invention. For the purposes of comparison, the composition of the streams 21-29 in Table lb is dumped for a process according to the prior art, that is to say with a column for eliminating water-pressure, without section desorption and without desorbedor. t ^. ^^. ^ A ^^^, - ^ - i? L * "IÜITÍ - lüMM * 1 ^ ^. ^ T tveiK.M? Is? Aamm * tsj An O LP NJ t P o o The process of the invention provides a product stream 22, obtained from the first column of elimination of water under pressure 2, which has a lower level of impurities (0.0 g / h of acetaldehyde and 2.0 g / h of formaldehyde ) than the previous art (0.3 g / h of acetaldehyde and 4.6 g / h of formaldehyde). The secondary components removed from the system by the process of the invention are 1.1 g / h of acetaldehyde and 0.7 g / h of formaldehyde in stream 25, and 1.6 g / h of acetaldehyde and 1.4 g / h. h of formaldehyde in stream 27, compared to only 1.2 g / h of acetaldehyde and 0.6 g / h of formaldehyde in stream 25 according to the prior art process.

Claims (4)

1. A process for the recovery of high purity monoethylene glycol by distillation, obtained by hydrolysis of ethylene oxide, by means of elimination of water under pressure, elimination of water under vacuum and subsequent purification by distillation, characterized in that it comprises the columns for eliminating pressurized water or at least the first pressure water removal column of a battery (2, 3, 4) having a desorption section with at least one stage 10 separative, preferably with between 2 and 10 separation stages, or, better still with between 3 and 6 stages, and removing from the system a portion of the upper stream of the water elimination column (s) (2, 3). , 4) that has (n) a desorption section.

2. A process according to claim 1, characterized in that the temperature below the feed point of the pressurized water removal column (2) or the first pressure water removal column of the battery (2, 3, 4) is above 80 ° C, but 20 preferably within the range between 100 ° C and 250 ° C, or, better yet, within the range between 115 ° C and 230 ° C, and the pressure in the desorption section is not less than 1 bar, preferably within the range of 2 to 30 bar.

3. A process according to claim 1 or 2, characterized in that the upper stream of the column (s) having a section of desorption, is introduced into a partial condenser and / or into a desorber, especially a steam desorber, and the current (s) (s) gas (s) - # enriched (s) with secondary components is (are) removed from the system.

4. A process according to any of claims 1 to 3, characterized in that the partial condenser and the desorber are operated above 90 ° C, 10 preferably between 120 ° C and 250 ° C. fifteen twenty 25 ^ Mg ^ gJBgeys ^^? ^ S »SUMMARY OF THE INVENTION. A process for the recovery of high purity monoethylene glycol by distillation, obtained by hydrolysis of ethylene oxide, by means of elimination of water under pressure, elimination of water under vacuum, and subsequent purification by 5 distillation, comprising the pressure water removal columns or at least the first elimination column . of pressurized water of a battery (2, 3, 4) having a desorption section with at least one separative stage, preferably with between 2 and 10 separative stages, or, better 10 still with between 3 and 6 stages, and a portion of the upper stream of the water removal column (s) having (n) a desorption section is removed from the system. The temperature below the feed point of the pressurized water removal column (2) or the first column of the battery pressure water removal (2, 3, 4) is above 80 ° C and the pressure in the section I f of desorption is not less than 1 bar. The upper stream of the water removal column (s) having a desorption section is introduced into a partial condenser. 20 and or a desorber, especially a steam desorber, and the (e) gaseous stream (s) enriched with secondary components is (are) removed from the system. Said partial condenser and said desorber ee operate above 90 ° C. Glycol plants are optimized only with respect to their 25 main functions, energy and cost reduction for the elimination of water and purification by distillation. technical (antifreeze grade), with lower requirements, to use as a refrigerant, and? w quality fiber, with strict requirements, used in the manufacture of fibers. The specification of fiber quality in what makes aldehydes free, informs < G? P as acetaldehyde, and determined spectrophotometrically as MBTH blue complex, generally ranges from 7 to 20 ppm, and for minimum UV transmittance it generally ranges from 76% -80% to 220 nm and 90% -95% at 275 nm. The substances that contribute to the determination of aldehydes are, in particular, formaldehyde, acetaldehyde and glycoaldehyde. The active substances in the UV, contaminants of the UV, are generally unknown, but they make fall to the products out of specification even in concentrations less than lppm, hence the importance of this process.