JP3166295B2 - Acetal separation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[Industrial application] Di-lower alkyl acetals such as acetaldehyde dimethyl acetal, diethyl acetal, dipropyl acetal and dibutyl acetal are used as various industrial raw materials, especially as organic solvents, synthetic fragrances, synthetic resins and adhesives. Industrially useful as synthetic intermediates such as alkyl vinyl ethers to be used and N-vinyl carboxylic acid amide which is a hydrophilic monomer.

[0002]

2. Description of the Related Art In order to produce these di-lower alkyl acetal, it is generally known to react acetaldehyde with a lower alcohol in the presence of an acid catalyst.
It is said that there is no particularly significant difference between the reactions depending on the type of lower alcohol as a unit reaction. However, it goes without saying that the types of products and by-products in these reactions differ depending on the individual starting materials, and that the products such as distillation characteristics, crystallization, compatibility with the solvent, and partition ratio are different. The physical properties that greatly affect the separation / purification means also largely depend on what is used as the starting material. In particular, in actual industrial production, in addition to the reactivity of starting materials, the cost of raw materials, the ease of acquisition and handling, the final yield of the target product after separation and purification, and the overall production process including the separation and purification system Simplicity, manufacturing equipment construction costs, ease of operation and management, and utility costs are also extremely important factors.
Therefore, it is necessary to determine the process including the raw material source in view of all these factors.

Various methods for producing di-lower alkyl acetals from aldehydes and lower alcohols have been conventionally reported. For example, JP-A-3-246247 discloses a method for producing an acetal using allyl alcohol. However, since this reaction is an equilibrium reaction, the disclosed method requires an equilibrium conversion rate equal to or higher than the equilibrium conversion rate. No results were obtained. Japanese Patent Application Laid-Open No. 62-116534 describes a method for producing an acetal using a C 4 alcohol as a raw material. However, in order to obtain satisfactory reaction results, a large amount of an acetal is required as a catalyst and a dehydrating agent. In addition, extra steps such as removal of the catalyst after the reaction and washing of the reaction solution with water are required, which cannot be said to be a satisfactory production method.

Various studies have also been made on the production of dimethyl acetal from acetaldehyde and methanol. In general, an inorganic solid acid such as hydrochloric acid, sulfuric acid, organic sulfonic acid and zeolite, and an acid catalyst such as an ion exchange resin are generally used. Although a reaction method is known, this reaction is an equilibrium reaction similarly to the acetalization reaction with other lower alcohols, and therefore, the conversion is limited as it is.
Therefore, in order to increase the yield of the target product, for example, it is necessary to take measures such as using one of the reaction raw materials in a large excess, or taking out the reaction product quickly out of the reaction system. This reaction-finished solution usually contains by-produced water and unreacted acetaldehyde.If the reaction solution is purified by distillation after neutralization or removal of the catalyst, decomposition of the target compound dimethyl acetal or undesired undesired acetaldehyde is undesirable. There is a problem that impurities are produced as by-products.

In the case of dimethyl acetal, calcium chloride or the like is also used as a catalyst and a dehydrating agent to remove water generated by the reaction and increase the conversion, but a large amount of calcium chloride is required. Disadvantages such as extra cost and complicated handling. In addition, there is a method of removing water generated by the reaction using an inert solvent such as normal heptane or toluene by azeotropic distillation, but this method is not a useful method because the number of steps for separating and recovering the solvent increases. Further, an appropriate combination of these methods has been proposed, but does not essentially solve the problem.

On the other hand, in general, as a reaction operation method for compensating for the drawbacks such as the limit of the conversion rate accompanying the equilibrium reaction, a separation operation by distillation is carried out while the reaction is being carried out, and a reaction distillation method or an apparatus thereof for improving the equilibrium conversion rate is used. For example, as for equilibrium reaction using an acid catalyst, esterification reaction (JP-A-63-277645 and the like), etherification reaction (JP-A-1-316337 and the like),
Many examples are known, such as an acetalization reaction (JP-B-62-29419, JP-A-3-56134). In this reactive distillation method, since the reaction is carried out with a distillation operation, in addition to various problems in the reaction, the distillation characteristics of the raw material system including reaction raw materials, products, by-products, and various substances of the production system are greatly affected. . Therefore, what kind of catalyst using any raw material,
It is particularly important to comprehensively select the reaction conditions, reaction operation, and whether to perform the reaction in the separation step.

For example, Japanese Patent Publication No. 62-29419 discloses a method for producing an acetal of an unsaturated alcohol having 3 or more carbon atoms by reactive distillation using nitric acid having a relatively low boiling point as a catalyst. . However, when this method is used for the synthesis of acetaldehyde dimethyl acetal obtained from methanol and acetaldehyde, which are the cheapest alcohols and aldehydes, both the target acetal and nitric acid flow out from the top of the column, which is not preferable for this mixture. It produces high-boiling by-products and has practically no practical use.

According to Japanese Patent Application Laid-Open No. 3-56134, there is proposed a reactive distillation apparatus characterized in that in an equilibrium reaction using a solid acid or a solid base as a catalyst, a reaction solution is forcibly circulated to a reactor. ing. This apparatus uses a formalin aqueous solution containing a large amount of water which is a main cause of catalyst deterioration as a raw material, that is, regeneration of a catalyst which frequently occurs when producing methylal from methanol and formalin aqueous solution,
Although it is a useful device because it is easy to replace, it requires additional equipment such as an extra pump as compared with a normal reactive distillation device, and the disadvantage of increasing the construction cost and operating cost by that amount is inevitable.

Further, in addition to the above, when a mixture of acetaldehyde dimethyl acetal and methanol formed by the reaction is separated by distillation, the mixture having an azeotropic composition close to the azeotropic composition of acetal and methanol forms an azeotropic mixture. There is a problem that it is extremely difficult to separate

Several proposals have been made for the separation of acetal and methanol. For example, JP-A-58-103331 discloses that the azeotropic distillation in a system of a methanol-methylal mixture utilizes the fact that the ratio of both components in the azeotropic composition is different by changing the operating pressure. A method has been proposed in which pressure distillation is combined with atmospheric or vacuum distillation. However, even if these pressure conditions are changed, the boiling points are close to each other, so that the number of stages and the reflux ratio of each distillation column must be made very large, so that the operation cost and high equipment cost of many distillation columns are required. Becomes

DE 1007311 proposes extractive distillation with water in a methanol-dimethyl acetal system. However, apart from the fact that it is difficult to completely remove methanol with this method, the obtained acetal contains water,
Since the water must be further removed by azeotropic distillation or by a desiccant or the like, it is troublesome and requires a loss due to the operation.

Furthermore, Japanese Patent Publication No. 38-19707 proposes the separation of an acetal-alcohol system by extractive distillation using an alcohol or an amino compound, but when such a reactive compound is added to the system and heated. Undesirable side reactions such as decomposition of the target substance and generation of impurities occur.
This leads to a decrease in purity and yield.

In general, when a mixture having a composition close to an azeotropic composition is separated, it is known that an azeotropic solvent is added as a third component and distillation is performed. 1) The azeotropic temperature is lower than the azeotropic temperature of methanol and acetal, and the temperature difference is large in order to improve the separation of acetal. (2) The content of methanol in the azeotrope is high from the viewpoint of energy. , (3) inertness to acetal, and the like. However, no azeotropic solvent satisfying these selection conditions has been proposed.

[0014]

SUMMARY OF THE INVENTION An object of the present invention is to develop a process for producing a di-lower alkyl acetal simply and with good final yield in an industrially advantageous manner. It is an object of the present invention to find a method for efficiently separating acetal when separating a mixture having a composition close to an azeotropic composition by distillation. More specifically,
The azeotropic temperature is lower than the azeotropic temperature of methanol and acetal, the temperature difference is large, the methanol content in the azeotrope is large, the azeotrope is inert to acetal, and the azeotrope can be distilled at lower temperature. An object of the present invention is to find a boiling solvent and to find a method of effectively circulating the used azeotropic solvent.

[0015]

Means for Solving the Problems The present inventors have conducted intensive studies in view of such circumstances, and as a result, among many organic solvents, normal pentane or cyclopentane forms an azeotrope with methanol and a small amount of acetal, It has been found that its boiling point and azeotropic composition are relatively effective for the above purpose and are effective azeotropic solvents. That is, the azeotropic temperature of normal pentane with methanol and acetal is 30 ° C. at normal pressure, the weight composition of normal pentane / methanol / acetal is about 92/7/1, and the azeotropic temperature of cyclopentane with methanol and acetal is The boiling temperature was 41 ° C. at normal pressure, and the weight composition of cyclopentane / methanol / acetal was about 79/18/3. By utilizing this characteristic, an effective process was successfully developed and the present invention was completed.

That is, according to the present invention, 1) when distilling and separating a mixed solution containing methanol and acetaldehyde dimethyl acetal as main components, using normal pentane or cyclopentane as an azeotropic solvent and converting methanol in the mixed solution into normal pentane or A method for separating acetaldehyde dimethyl acetal, comprising separating an azeotropic mixture of cyclopentane and a small amount of acetaldehyde dimethyl acetal from an upper portion of a distillation column, and distilling a fraction containing acetaldehyde dimethyl acetal as a main component from a lower portion of the distillation column. And 2) a distillate containing methanol and normal pentane or cyclopentane as main components and a small amount of acetaldehyde dimethyl acetal as an azeotropic component is brought into countercurrent contact with water in an extraction column, and methanol is extracted and separated from the bottom of the column. And on the tower The normal pentane or cyclopentane separating a fraction composed mainly from this is to provide cents a way to reuse as an azeotropic solvent.

Hereinafter, the present invention will be described in more detail. The mixed solution containing methanol and acetaldehyde dimethyl acetal as the main components may be any solution, and there is no particular limitation. Usually, a molar excess of about a few moles of methanol and acetaldehyde is added to sulfuric acid, phosphoric acid, p-toluenesulfonic acid, etc. In the presence of a solid acid catalyst such as a liquid or strongly acidic ion exchange resin, mordenite, zeolite, etc.
Those obtained by reacting at a temperature of 0 to 70 ° C. for about 0.1 to 60 minutes, for example, those obtained from the top of a reactive distillation apparatus by the method of the applicant of the present invention on April 19, 1994 Used.

According to the literature, the boiling point and the composition of the compound and the azeotrope used in the present invention at normal pressure are defined as methanol 6
4.7 ° C, dimethyl acetal 64.3 ° C, normal pentane 36 ° C, normal pentane-methanol 31 ° C
(Methanol 15% by weight), methanol-acetal 5
It is said that 7.5 ° C. (methanol 24% by weight) and cyclopentane are 49 ° C., and cyclopentane-methanol 39 ° C. (methanol 14% by weight). Further, according to the findings of the present inventors, normal pentane forms an azeotrope with methanol and a small amount of acetal, the azeotropic temperature of which is about 30 ° C., and the weight composition of normal pentane / methanol / acetal is about 92 / 7/1. Cyclopentane also formed an azeotrope with methanol and a small amount of acetal, the azeotropic temperature was about 41 ° C., and the weight composition of cyclopentane / methanol / acetal was found to be about 79/18/3. .

The amount of normal pentane or cyclopentane to be added to the separation system is such that the methanol contained in the dimethyl acetal-methanol mixture to be treated is entirely azeotropic with normal pentane or cyclopentane. Adding too much increases the load on the distillation system and is not preferred. Incidentally, from the original purpose, it is to obtain as much dimethyl acetal as possible,
From this viewpoint, it can be said that it is preferable to use normal pentane having a lower dimethylacetal content than cyclopentane as the three-component azeotropic composition.

The distillation apparatus used in the method of the present invention is not particularly limited, but usually a rectification column having a so-called theoretical plate number of about 1 to 100 plates, preferably 5 to 50 plates is used. Any structure can be used for the rectification column.
There is no strict requirement for its structural style. For example, tray towers include bubble bell trays, uniflux trays, flexi trays, natter float trays, ballast trays, perforated trays, cascade trays, venturi trays, kittel trays, recycling trays, chimney trays,
A tray tower using a jet tray, a turbo grid tray, a ripple tray, a dual flow tray, a baffle tray, or the like can be given. In addition, ring-type packing, saddle-type packing, spray pack, Panapack, Goodroy packing, Steadman packing,
Packing towers using Dickson rings, McMahon packing, sluser packing, helix, vertical plate packing, and the like.

The pressure in the column may be any of reduced pressure, normal pressure and pressurized pressure, but usually normal pressure is desirable. The present invention can be carried out continuously or discontinuously, but continuous operation is preferred in terms of productivity, operation stability, and the like. The reflux ratio is not particularly limited, and depends on the performance of the rectification column and the specifications of the dimethyl acetal to be separated, but about 1 to 10 is sufficient.

The distillate containing methanol and normal pentane or cyclopentane as main components and a small amount of dimethyl acetal as an azeotropic component is subjected to an extraction column to separate and recover normal pentane or cyclopentane as an azeotropic solvent. , Almost all of methanol and most of dimethyl acetal and water are withdrawn from the bottom of the column, and a liquid substantially consisting of normal pentane or cyclopentane is recovered from the top of the column, and again Used as an azeotropic solvent. Since dimethyl acetal is extremely unstable to acids, a small amount of a basic substance, for example, sodium carbonate, sodium hydrogen carbonate, sodium phosphate, etc., is added to the water used for extraction so that the pH of the water does not drop below 7. It may be. However, the use of a large amount of these inorganic salts, including neutral inorganic salts such as sodium chloride, must be avoided because the solubility of methanol and the like in the aqueous layer is reduced due to the salting out effect.

The extraction temperature is not necessarily limited, but is usually 1
It is desirable that it is 0-50 degreeC, Preferably it is 15-45 degreeC. If the extraction temperature is lower than 10 ° C., the oil layer of the raw material separates into two layers, while if it is higher than 50 ° C., the vapor pressure of the oil layer components increases, and neither is preferable. The pressure may be any of reduced pressure, normal pressure, and pressurized pressure, but usually, normal pressure may be used. The method of contact between the water and the oil layer may be a batch type or a continuous type, and among these, the continuous type is preferably used in terms of productivity and operational stability. Specifically, such an extraction operation is preferably performed using an extraction column, and the oil layer is supplied from the lower portion of the extraction column, and water is supplied from the upper portion of the extraction column, and the extraction operation is performed by a countercurrent contact method. It is desirable. It is desirable that the number of theoretical plates of this extraction column is about 2 to 10 plates.

The extraction column may have any structure, and there is no strict condition on the structure. For example, a spray tower, a packed tower, a pulsating packed tower, etc. as a countercurrent differential extraction tower, a perforated plate extraction tower, a baffle tower, etc. as a non-stirred staged extraction tower,
Various types of agitation type extraction towers such as Scheiber tower, rotating disk extraction tower, Ordoshu-Rashton tower, Graesser extractor, ARD tower, Kuni tower, pulsating perforated plate tower, pulsating plate tower alternate pulsating flow type extraction tower, etc. Things are used.

The amount of water and oil layer supplied to the extraction tower per unit time is determined according to the volume of the extraction tower, the extraction capacity, and the like. １ ： 1: 5
0, preferably 1: 5 to 1:30. If the water supply ratio is too low, methanol cannot be sufficiently extracted, and the amount of residual methanol in the oil layer increases. If the water supply ratio is too high, methanol is recovered from the aqueous layer after extraction or used as a raw material for acetalization. When it is used, the load of the utility increases, which is not economically preferable.

The oil layer after extraction is normal pentane or cyclopentane containing a trace amount of dimethyl acetal,
Since it does not substantially contain water or methanol, it can be used as it is as an azeotropic solvent in the previous step. If a large amount of water is mixed into the oil layer after the extraction, the water content of the acetal obtained from the bottom of the distillation column in the previous step increases, and the hydrolysis of the acetal gradually proceeds, resulting in a decrease in the acetal yield. . Further, if a large amount of methanol is mixed in the oil layer after the extraction, the amount of methanol circulating in the process increases, and the load on the apparatus such as the distillation column in the previous step unnecessarily increases, which is not economically preferable. Since the aqueous layer after extraction contains methanol as a main component, it may be used as a raw material for acetalization as it is. However, it is distilled once to remove water, and the methanol concentration is further increased before being used as a raw material in the process. You may.

Next, as a typical example of the method of the present invention, the separation of high-purity dimethyl cetal using sulfuric acid as a catalyst and normal pentane as an azeotropic solvent will be described more specifically based on a flow diagram. . FIG.
In the above, A represents a reactive distillation column, B represents a distillation column, and C represents an extraction column, and solid lines and numbers 1 to 9 represent the flow of substances.

First step (acetalization reaction): Newly required methanol 1 is supplied to methanol 9 containing a necessary amount of raw material acetaldehyde 2 and a small amount of acetal and water recovered from countercurrent extraction column C. It is continuously introduced into the reactive distillation column A. This methanol contains the required amount of acid catalyst,
For example, sulfuric acid is dissolved and mixed. Water 5 generated by reaction with water previously contained in the raw material is discharged from the bottom of the column. The sulfuric acid of the catalyst is dissolved in this water, and if necessary, it is subjected to appropriate neutralization and drainage treatment before being discarded.

Second step (separation of high-purity acetal): The reaction solution 4 consisting of methanol and dimethyl acetal is introduced into the distillation column B together with the light liquid of the countercurrent extraction column C which is normal pentane containing a trace amount of dimethyl acetal, It is separated by distillation under normal pressure, and high-purity acetal 5 is obtained from the bottom of the column. Normal pentane is distilled from the top of the column as a three-component azeotrope 6 of normal pentane-methanol-dimethyl acetal. Normal pentane is supplied in an amount necessary to produce a three-component azeotrope of normal pentane-methanol-dimethyl acetal.

Third step (recovery of normal pentane): A three-component azeotrope 6 of normal pentane-methanol-dimethyl acetal extracted from the top of the distillation column B is countercurrently flowed with a small amount of water 7 in the countercurrent extraction column C. Make contact. The heavy liquid 9 obtained by extracting almost all of methanol and most of the dimethyl acetal from the light liquid is withdrawn from the bottom of the column and recovered as a part of the raw material for the reactive distillation in the first step. The light liquid 8 consisting essentially of normal pentane is recovered from the top of the column and reused as an entrainer in the second step.

[0031]

EXAMPLES Hereinafter, typical examples of the method of the present invention will be shown as examples, and will be described in more detail together with comparative examples.
These are exemplifications for explanation, and therefore, the present invention is not limited by the following examples. Example 1 In a 25-stage glass Aldershaw-type rectification tower having a tower diameter of 30 mm and a first stage from the top, 184 g of normal pentane per hour was placed on the first stage.
At the top of the 10th stage, 14 g of methanol per hour,
Dimethyl acetal was introduced at a rate of 34 g per hour, and a reflux ratio of 6,
Heating was performed so as to maintain the tower top temperature of 31 ° C. 500 g of dimethyl acetal was placed in the lower part of the rectification column.
The flask was immersed in a 90 ° C. oil bath and heated, and the flask contents were withdrawn at 32 g / h. The liquid extracted from the flask was substantially free of normal pentane, and was dimethyl acetal containing 0.3% of methanol. A 200 g / h dimethyl acetal-methanol-normal pentane mixture was withdrawn from the top of the column. Both the distillate and the bottoms contained substantially no water or acetaldehyde.
Similar results were obtained even when the introduction position of normal pentane was changed to the tenth stage from the top.

Example 2 Cyclopentane was introduced at the top of the same rectification column as in Example 1 at 87 g / h, methanol was introduced at the 10th stage from the top at 20 g / h, and dimethyl acetal was introduced at 51 g / h. Heating was performed so as to maintain a reflux ratio of 6 and a top temperature of 42 ° C. 100 g of dimethyl acetal at the bottom of the rectification column
Was placed in a 90 ml oil bath and heated, and the contents of the flask were withdrawn at 48 g / h.
The flask withdrawal liquid is substantially free of cyclopentane,
It was a dimethyl acetal containing 0.5% of methanol. A 110 g / h dimethyl acetal-methanol-cyclopentane mixture was withdrawn from the top of the column. Distillate,
Both bottoms contained substantially no water or acetaldehyde. Similar results were obtained even when the introduction position of cyclopentane was changed to the tenth stage from the top.

Example 3 [First step]; methanol containing 0.5% by weight of sulfuric acid at a rate of 180 g / h was introduced into the fifth stage from the top of a 25-stage glass Oldershaw rectification column, and 15 stages from the top Acetaldehyde was introduced into the eyes at 72 g / h. At the bottom of the rectification column, a 500 ml flask containing 100 g of water was set up.
And the contents of the flask were withdrawn at 29 g / h. The liquid discharged from the flask was substantially free of organic matter. A dimethyl acetal-methanol mixture at a reflux ratio of 2 and 221 g / h was withdrawn from the top of the tower. The distillate was substantially free of water and acetaldehyde. The acetaldehyde conversion was 100% and the dimethyl acetal yield was 100%.

[Second step]: Normal pentane is introduced into the first stage from the top of a 25-stage glass Aldershaw type rectification column at 184 g / h, and the 10th stage from the top is dimethyl acetal containing 28% by weight of methanol. Was introduced at 48 g / h. Heating was performed so as to maintain a reflux ratio of 6 and a tower top temperature of 31 ° C. 10 dimethyl acetal was placed at the bottom of the rectification column.
A 500 ml flask containing 0 g was provided, immersed in an oil bath at 90 ° C. and heated, and the contents of the flask were withdrawn at 32 g / h. The liquid extracted from the flask was substantially free of normal pentane, and was dimethyl acetal containing 0.3% of methanol. A 200 g / h dimethyl acetal-methanol-normal pentane mixture was withdrawn from the top of the column. Both the distillate and the bottoms contained substantially no water or acetaldehyde.

[Third step]: 30 baffle plates are 25 m long
In a vertically moving liquid-liquid countercurrent extraction device having a column inner diameter of 50 mm attached at intervals of m, the distillate of the second step was lightly liquefied in the second step from the bottom of the extraction column at 2370 g / h, and heavy liquid was water from the top of the extraction column. 153 g / h was fed and countercurrent extraction was performed. The baffle plate reciprocated up and down for 150 cycles with a stroke of 12.5 mm. The light liquid after extraction contained almost no water and methanol, and was normal pentane containing 2% by weight of dimethyl acetal. The heavy liquid was methanol 80% by weight, dimethyl acetal 5% by weight, and the others were water.

[0036]

According to the present invention, methanol and dimethyl whose boiling points are very close to each other in a mixed solution containing methanol and acetaldehyde dimethyl acetal as main components obtained by reacting acetaldehyde and methanol in the presence of an acid catalyst. The acetal is separated by distillation using normal pentane or cyclopentane as an azeotropic solvent, methanol is distilled off as a mixture of normal pentane or cyclopentane and a small amount of dimethyl acetal, and dimethyl acetal is efficiently separated from the bottom of the column. In particular, since the target dimethyl acetal is not exposed to an unnecessary high temperature for a long time, there is no possibility that undesired impurities are produced as by-products.
Further, the mixture of the distilled normal pentane or cyclopentane, methanol and dimethyl acetal is countercurrently extracted with water to separate normal pentane or cyclopentane, and the whole is extremely efficiently used as an azeotropic solvent again. It was possible to operate, and as a result, it was possible to produce dimethyl acetal industrially easily and with a good final yield.

[Brief description of the drawings]

The drawing is a flow diagram showing a typical example of the separation of high-purity dimethyl acetal using normal pentane for the method of the present invention. In FIG. 1, A indicates a reactive distillation column, B indicates a distillation column, and C indicates a water extraction column, and solid lines and numerals 1 to 9 indicate the flow of substances.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-58-32838 (JP, A) JP-A-3-56134 (JP, A) JP-A-58-36425 (JP, A) 29419 (JP, B2) (58) Field surveyed (Int. Cl. ⁷ , DB name) C07C 43/00-43/32 C07C 41/58-41/60

Claims (2)

(57) [Claims] When a mixed solution containing methanol and acetaldehyde dimethyl acetal as main components is separated by distillation, normal pentane or cyclopentane is used as an azeotropic solvent, and methanol in the mixed solution is mixed with normal pentane or cyclopentane and a small amount of A method for separating acetaldehyde dimethyl acetal, comprising separating an azeotrope with acetaldehyde dimethyl acetal from an upper portion of a distillation column and distilling a fraction containing acetaldehyde dimethyl acetal as a main component from a lower portion of the distillation column. 2. A fraction containing methanol and normal pentane or cyclopentane as the main components and containing a small amount of acetaldehyde dimethyl acetal as an azeotropic component is brought into countercurrent contact with water in an extraction column, and methanol is removed from the bottom of the column. 2. The separation method according to claim 1, wherein the separation is carried out by extraction, and a fraction containing normal pentane or cyclopentane as a main component is separated from the upper part of the column and reused as an azeotropic solvent.