阅读说明：本技术 用于从粗混合物提纯异构单体诸如4,4’-亚甲基二苯基二异氰酸酯的设备和方法 (Apparatus and method for purifying isomeric monomers such as 4,4' -methylene diphenyl diisocyanate from crude mixtures ) 是由 P.费斯勒 M.施特潘斯基 H.延森 于 2018-11-26 设计创作，主要内容包括：本发明涉及一种用于从不同异构单体的混合物制备提纯的异构的亚甲基二苯基二异氰酸酯单体的设备,其中该设备包含蒸馏装置,该蒸馏装置包含：a)包括结构化填料的蒸馏塔,b)不同异构的亚甲基二苯基二异氰酸酯单体的混合物的来源,c)蒸发器,d)顶部蒸气冷凝器,e)可选地,顶部真空系统,和f)流量控制的回流系统,其中顶部蒸气冷凝器包含壳管式布置结构,并且被实施以便将冷凝水直接过冷至低于47°C,并且其中流量控制的回流系统包含加热器,该加热器被实施以便将在顶部蒸气冷凝器中形成的冷凝物的部分流再加热至高达190℃。(The invention relates to a device for producing purified isomeric methylene diphenyl diisocyanate monomers from a mixture of different isomeric monomers, wherein the device comprises a distillation apparatus comprising: a) a distillation column comprising structured packing, b) a source of a mixture of different isomeric methylene diphenyl diisocyanate monomers, C) an evaporator, d) an overhead vapor condenser, e) optionally, an overhead vacuum system, and f) a flow-controlled reflux system, wherein the overhead vapor condenser comprises a shell-and-tube arrangement and is implemented to subcool condensed water directly to less than 47 ℃, and wherein the flow-controlled reflux system comprises a heater implemented to reheat a partial stream of condensate formed in the overhead vapor condenser up to 190 ℃.)

1. An apparatus for producing purified isomeric monomers from a mixture of different isomeric methylene diphenyl diisocyanate monomers, wherein said apparatus (10) comprises a distillation apparatus (12), said distillation apparatus (12) comprising:

a) a distillation column (14) comprising structured packing (24),

b) a source (13) of a mixture of different isomeric methylene diphenyl diisocyanate monomers,

c) an evaporator (16) for the liquid to be evaporated,

d) a top vapor condenser (18),

e) a top vacuum system (20), and

f) a flow-controlled return system (22),

wherein the overhead vapor condenser (18) comprises a shell-and-tube arrangement and is implemented so as to subcool the condensate directly to below 47 ℃, and

wherein the flow-controlled reflux system (22) comprises a heater (23), the heater (23) being implemented so as to reheat a partial stream of condensate formed in the overhead vapor condenser (18) up to 190 ℃.

2. The apparatus according to claim 1, wherein the apparatus (10) does not comprise any further distillation column.

3. The apparatus according to claim 1 or 2, wherein the apparatus (10) further comprises a dynamic crystallization device, preferably a falling film crystallization device or a suspension crystallization device (29), downstream of the distillation device (12), and more preferably a suspension crystallization device (29) comprising a suspension crystallizer (54).

4. The apparatus according to any one of the preceding claims, wherein the source (13) of the mixture of different isomeric monomers comprises different isomers of methylene diphenyl diisocyanate comprising: 4,4' -methylene diphenyl diisocyanate and at least one of 2,2' -methylene diphenyl diisocyanate, 2,4' -methylene diphenyl diisocyanate and dimers, oligomers and polymers thereof.

5. The plant according to any of the preceding claims, wherein the overhead vapour condenser (18) is implemented so as to subcool the condensate directly to below 46 ℃, preferably to at most 45 ℃, and most preferably to at most 42 ℃.

6. The apparatus according to any one of the preceding claims, wherein the evaporator (16) is a falling film evaporator or a thin film evaporator, and preferably a falling film evaporator.

7. A process for preparing purified methylene diphenyl diisocyanate from a crude mixture comprising different isomeric methylene diphenyl diisocyanate monomers and dimers, oligomers and polymers thereof, wherein said process is performed in an apparatus (10) according to any of the preceding claims.

8. A process according to claim 7, wherein the process is for preparing 4,4 '-methylene diphenyl diisocyanate from a crude methylene diphenyl diisocyanate mixture comprising different isomers, dimers, oligomers and polymers of methylene diphenyl diisocyanate, wherein the mixture contains 4,4' -methylene diphenyl diisocyanate and at least one of 2,2 '-methylene diphenyl diisocyanate, 2,4' -methylene diphenyl diisocyanate and oligomers thereof.

9. Process according to claim 7 or 8, wherein the pressure at the top of the distillation column (14) is set to 2 to 10mbar, preferably at most 3 mbar.

10. Process according to any of claims 7 to 9, wherein the average residence time of the liquid in the tubes of the overhead vapour condenser (18) is adjusted to 30 seconds to 5 minutes, and preferably less than 1 minute.

11. Process according to any one of claims 7 to 10, wherein the process is performed such that the dimer content of condensed liquid withdrawn from the overhead vapor condenser (18) is less than 200 ppm, preferably at most 150 ppm, more preferably at most 100 ppm, even more preferably at most 70ppm, and most preferably at most 60 ppm.

12. A process according to any one of claims 7 to 11, wherein the ratio of reflux to condensate is set to 0.05 to 0.25, preferably 0.10 to 0.20, and most preferably about 0.15.

13. A process according to any one of claims 7 to 12, wherein the condensate withdrawn from the overhead vapour condenser (18) has the following characteristics:

i) dimer content is less than 200 ppm, preferably at most 150 ppm, more preferably at most 100 ppm, even more preferably at most 70ppm, and most preferably at most 60 ppm, and/or

ii) the content of polymeric methylene diphenyl diisocyanate is at most 2000 ppm, preferably at most 1500 ppm, more preferably at most 1000 ppm, and most preferably at most 800 ppm, and/or

iii) a color of at most 100 APHA, preferably at most 50 APHA, more preferably at most 40 APHA, and/or

iv) a hydrolysable chloride content of 10 to 150 ppm, preferably 30 to 90 ppm, more preferably 50 to 80 ppm, and most preferably 65 to 75 ppm, and/or

v) a content of 4,4' -methylenediphenyl diisocyanate of 88 to 96% by weight, preferably 90 to 95% by weight, and more preferably 92 to 94% by weight,

of these, all of the criteria i) to v) are preferably satisfied.

14. Process according to any one of claims 7 to 13, wherein the condensed liquid withdrawn from the overhead vapour condenser (18) is subsequently fed as a liquid feed into a dynamic crystallisation device, preferably a falling film or suspension crystallisation device (29), and more preferably a suspension crystallisation device (29) comprising a suspension crystalliser (54).

15. A purified 4,4' -methylene diphenyl diisocyanate component having the following characteristics:

i) a dimer content of less than 200 ppm, preferably at most 150 ppm, more preferably at most 100 ppm, even more preferably at most 70ppm, and most preferably at most 60 ppm,

ii) the content of polymerized methylene diphenyl diisocyanate is at most 2000 ppm, preferably at most 1500 ppm, more preferably at most 1000 ppm, and most preferably at most 800 ppm,

iii) a color of at most 100 APHA, preferably at most 50 APHA, more preferably at most 40 APHA,

iv) a hydrolysable chloride content of 10 to 150 ppm, preferably 30 to 90 ppm, more preferably 50 to 80 ppm, and most preferably 65 to 75 ppm, and

v) the content of 4,4' -methylenediphenyl diisocyanate is from 88 to 96% by weight, preferably from 90 to 95% by weight and more preferably from 92 to 94% by weight.

Technical Field

The present invention relates to an apparatus for preparing purified isomeric monomers from a mixture of different isomeric monomers, and in particular to an apparatus for preparing purified 4,4' -methylene diphenyl diisocyanate from a crude mixture of methylene diphenyl diisocyanate, said crude mixture comprising therein: different isomers of methylene diphenyl diisocyanate, i.e., at least one of 4,4' -methylene diphenyl diisocyanate and 2,2' -methylene diphenyl diisocyanate and 2,4' -methylene diphenyl diisocyanate, as well as dimers of methylene diphenyl diisocyanate, oligomers of methylene diphenyl diisocyanate, polymers of methylene diphenyl diisocyanate, and light materials such as dichlorobenzene. Furthermore, the present invention relates to a process for preparing purified isomeric monomers from a crude mixture comprising different isomeric monomers and dimers, oligomers and polymers thereof, and in particular to a process for purifying 4,4 '-methylene diphenyl diisocyanate from a crude methylene diphenyl diisocyanate mixture comprising different isomers, dimers, oligomers and polymers of methylene diphenyl diisocyanate, which mixture comprises 4,4' -methylene diphenyl diisocyanate and at least one of 2,2 '-methylene diphenyl diisocyanate and 2,4' -methylene diphenyl diisocyanate, wherein the process is carried out in the above-described apparatus.

Background

Methylene diphenyl diisocyanate (abbreviated to MDI) exists in three isomeric forms, namely in the form of 2,2' -methylene diphenyl diisocyanate, in the form of 2,4' -methylene diphenyl diisocyanate and in the form of 4,4' -methylene diphenyl diisocyanate. To date, the most widely used of these three isomers is 4,4' -methylene diphenyl diisocyanate. In particular, 4,4' -methylene diphenyl diisocyanate is an important compound for producing rigid polyurethanes. More specifically, rigid polyurethanes are prepared by mixing 4,4' -methylene diphenyl diisocyanate with one or more polyols and optionally a blowing agent. Rigid polyurethanes are in turn important materials for their excellent thermal insulation properties, for example in refrigerators, refrigerators and buildings.

Currently, methylene diphenyl diisocyanate is prepared by phosgenation of methylene diphenyl diamine. More specifically, methylene diphenyl diisocyanate is typically produced by a process comprising the steps of: i) by reacting aniline and formaldehyde in the presence of an acid catalyst to form polyamines of the methylene diphenyl diamine and diphenylmethane series, and b) phosgenating the methylene diphenyl diamine and polyamines of the diphenylmethane series to produce a mixture of MDI isomers and polymeric MDI. Subsequently, the resulting crude mixture of methylene diphenyl diisocyanate, including MDI isomers, MDI dimers, MDI oligomers, MDI polymers and light substances such as dichlorobenzene (hereinafter referred to as "crude MDI mixture"), is purified so as to obtain the desired purified MDI isomers, such as in particular purified 4,4' -methylene diphenyl diisocyanate.

A typical method for purifying a crude MDI mixture is distillation using two or more distillation columns. In the first column, commonly referred to as the crude MDI distillation column, the crude MDI mixture is separated into a bottom fraction rich in polymeric MDI and a top fraction rich in 2,2' -MDI, 2,4' -MDI and 4,4' -MDI. The top fraction enriched in 2,2'-MDI, 2,4' -MDI and 4,4'-MDI is fed to a second column (commonly referred to as an isomer distillation column) where the mixture is separated into a bottom fraction enriched in polymeric MDI and MDI dimers, a lower middle fraction enriched in 4,4' -MDI, a higher middle fraction enriched in 2,4'-MDI and 2,2' -MDI and a top fraction enriched in light materials such as dichlorobenzene.

It is also known to carry out melt crystallization with the 4,4 '-MDI-rich fraction obtained in the second isomer distillation column in order to increase the 4,4' -MDI content, for example 95% by weight, to more than 99% by weight.

US 4,189,354 discloses a process for producing MDI isomers with an adjusted chlorine compound content comprising the steps of: (a) subjecting the MDI mixture to a first distillation stage, thereby obtaining MDI isomers as top product, (b) subjecting the top product to a second distillation stage using a recycle ratio of from 0.1 to 10, and wherein from 0.5 to 10% by weight of the feed to said second distillation stage is removed as sump product, (c) subjecting the top product of the second distillation stage to a third distillation stage, thereby removing volatile impurities therefrom, and (d) treating the sump product of said third distillation stage to obtain purified 2,4 '-and 4,4' -diisocyanodiphenylmethane.

However, the main disadvantage of the aforementioned MDI isomer purification processes which require multiple distillation columns is that they are expensive to operate and require very complex equipment. Furthermore, the purification process of MDI isomers by melt crystallization has the following disadvantages: melt crystallization does not significantly reduce the MDI dimer content. However, high MDI dimer content can negatively impact melt crystallization for a number of reasons. First, a high MDI dimer content results in the formation of very fine crystals, resulting in a cloudy liquid during crystallization. Secondly, the high MDI dimer content affects the further growth rate and size of the monomeric MDI, which in turn affects the quality of the final product. High MDI dimer levels above 200 ppm can also affect distillation equipment, since such high MDI dimer levels can lead to fouling of, for example, condensers and are therefore difficult to clean.

Disclosure of Invention

In view of this, it is an object of the present invention to provide an apparatus and a process for preparing purified isomeric monomers from a crude MDI mixture, which results in a reduced concentration of MDI dimers in the purified product and which can be operated at relatively low operating costs.

According to the invention, this object is achieved by providing an apparatus for preparing purified isomeric methylene diphenyl diisocyanate monomers from a mixture of different isomeric monomers, wherein the apparatus comprises a distillation apparatus comprising:

a) a distillation column comprising a structured packing,

b) a source of a mixture of different isomeric methylene diphenyl diisocyanate monomers,

c) an evaporator, a water-cooling device and a water-cooling device,

d) a top part steam condenser,

e) optionally, a top vacuum system, and

f) a flow-controlled return system for the flow of water,

wherein the overhead vapour condenser comprises a shell and tube arrangement and is preferably implemented to subcool the condensate directly to below 47 ℃, and wherein the flow-controlled reflux system comprises a heater which is preferably implemented to reheat a partial stream of the condensate formed in the overhead vapour condenser up to 190 ℃.

This solution is based on the following findings: by a combination of the above measures, the dimer concentration in the condensed top product is significantly reduced, and in particular to less than 200 ppm. This is achieved in particular by carrying out the condensation of the overhead vapour to subcool the condensate directly to below 47 ℃. It has been found that this reduces dimer formation. Due to the low MDI dimer content, fouling in condensers implemented as shell-and-tube condensers is reliably avoided, so that the condensers can be cleaned easily. Furthermore, due to the low dimer content, the condensed top product is an ideal starting material for further purification in a subsequent dynamic crystallization device (such as a falling film or suspension crystallization device, preferably a suspension crystallization device), since-due to the low dimer content-the formation of very fine crystals during crystallization is reliably avoided, leading to turbid liquids, and further influencing the growth rate and size of the monomeric MDI which has an influence on the quality of the final product.

Since the partial stream of the condensate, which is fed back into the distillation column via the flow-controlled reflux system, is heated to temperatures of up to 190 ℃, precipitation of the dimers in the reflux liquid is reliably avoided. Precipitation of such dimers in the reflux liquid will result in blockage of the dispenser opening. Furthermore, heating the partial stream of condensate formed in the overhead vapor condenser, which is fed back into the distillation column via a flow-controlled reflux system, to a temperature of up to 190 ℃, reliably avoids condensation of the vapor in the distillation column due to the entry of cold liquid and thus avoids high energy losses and achieves high reflux rates. A high reflux rate has a positive effect on the opening of the dispenser, since the opening does not have to be very small and therefore the risk of clogging by precipitates (such as MDI dimers) is significantly reduced. Furthermore, heating the partial stream of condensate formed in the overhead vapor condenser (which is fed back into the distillation column via a flow-controlled reflux system) to temperatures of up to 190 ℃ increases the allowable reflux rate.

In summary, the apparatus and process according to the invention allow obtaining, after a distillation column, as condensed top product of the distillation column (i.e. as condensate or condensed phase, respectively), a purified isomeric methylene diphenyl diisocyanate monomer with a low MDI dimer content, such as a purified 4,4' -methylene diphenyl diisocyanate having, for example, the following characteristics:

i) dimer content is less than 200 ppm, preferably at most 150 ppm, more preferably at most 100 ppm, even more preferably at most 70ppm, and most preferably at most 60 ppm, and/or

ii) the content of polymeric methylene diphenyl diisocyanate is at most 2000 ppm, preferably at most 1500 ppm, more preferably at most 1000 ppm, and most preferably at most 800 ppm, and/or

iii) a color of at most 100 APHA, preferably at most 50 APHA, more preferably at most 40 APHA, and/or

iv) a hydrolysable chloride content of 10 to 150 ppm, preferably 30 to 90 ppm, more preferably 50 to 80 ppm, and most preferably 65 to 75 ppm, and/or

v) the content of 4,4' -methylenediphenyl diisocyanate is from 88 to 96% by weight, preferably from 90 to 95% by weight and more preferably from 92 to 94% by weight.

Although the dimer content is determined by FTIR spectroscopy, for example according to ASTM D8036, the content of 4,4' -methylenediphenyl diisocyanate is determined by HPLC-UV analysis, for example according to ASTM D7252, the hydrolysable chloride content is determined by HCl acidity measurement, for example by argentity determination according to ASTM D5523, and the APHA color is measured, for example, according to the visual ASTM D1209 APHA/Pt-Co color scale or according to the instrumental ASTM D5386 method.

Preferably, the condensed phase meets at least two, more preferably at least three, even more preferably at least four, most preferably all of the above criteria i) to v).

Finally, the plant according to the invention requires less operating costs compared to a classical purification plant comprising two or more distillation columns.

According to a particularly preferred embodiment of the present invention, the apparatus according to the present invention does not comprise any further distillation column in addition to the above-described distillation column, i.e. preferably the apparatus according to the present invention comprises only one distillation column. In particular, the apparatus of the present invention preferably does not contain an isomer distillation column.

According to another particularly preferred embodiment of the invention, the apparatus according to the invention further comprises a dynamic crystallization device downstream of the distillation device. Preferably, the dynamic crystallization device is a falling film crystallization device or a suspension crystallization device. More preferably, the dynamic crystallization device is a suspension crystallization device. The dynamic crystallization device is preferably connected to the distillation device in such a way that the main partial stream of condensed and supercooled liquid obtained in the overhead vapor condenser, which is withdrawn from the overhead vapor condenser and thus from the distillation device, is subsequently fed as a liquid feed into the suspension crystallization device. This allows further purification of the desired isomer, such as in particular 4,4' -methylene diphenyl diisocyanate. In addition to producing a product with reduced dimer content, this embodiment also requires lower operating costs than a classical purification apparatus comprising two or more distillation columns.

Preferably, the suspension crystallization device further comprises a wash column device. This allows for efficient separation of the purified crystals of the isomeric monomer from the mother liquor.

For example, a scrubber unit may comprise:

-a cylindrical container, wherein the cylindrical container comprises:

a piston having a piston head and a piston rod, wherein the piston is arranged to be reciprocally movable in the cylindrical container, wherein the piston delimits a washing chamber inside the cylindrical container below the piston head, and wherein the piston head comprises at least one filter device,

an inlet for feeding a crystal suspension mixture consisting of crystals and mother liquor into a cylindrical container,

an outlet for discharging mother liquor from the cylindrical vessel,

an outlet for discharging the crystals and/or the crystal melt from the cylindrical container,

a circulation conduit for circulating the melt, arranged outside the cylindrical container, communicating with the washing chamber,

-means arranged in the washing chamber for limiting the movement of the crystal bed that has been compacted in the washing chamber by the piston and for guiding the washing liquid from the circulation conduit into the cylindrical container so as to distribute it evenly over the entire cross-section of the cylindrical container, wherein the means are preferably rotating scrapers or static grids.

For the sake of clarity, it should be noted that the inlet for supplying the crystal suspension mixture consisting of crystals and mother liquor into the cylindrical container may be a line directly connected to the housing of the cylindrical container, or the inlet may be inside a piston rod which is open at the end of the piston rod which ends in a piston head.

As mentioned above, the apparatus according to the invention is particularly suitable for purifying 4,4' -MDI from a crude MDI mixture. Thus, the source of the mixture of different isomeric monomers is preferably a source of a mixture of different isomers of methylene diphenyl diisocyanate comprising 4,4' -methylene diphenyl diisocyanate and at least one of 2,2' -methylene diphenyl diisocyanate, 2,4' -methylene diphenyl diisocyanate, as well as MDI dimers, MDI oligomers, MDI polymers and light materials. Thus, it is preferred that the source of the mixture of different isomeric monomers comprises 4,4' -methylene diphenyl diisocyanate and at least one of 2,2' -methylene diphenyl diisocyanate, 2,4' -methylene diphenyl diisocyanate, and oligomers thereof. The oligomer preferably comprises 3 or more rings, such as 3 to 5 rings. For example, the source of the mixture of different isomeric monomers may be a reactor, reactor train, system or apparatus for phosgenating methylene diphenyldiamine.

For example, the crude MDI mixture may comprise 30 to 70% by weight, preferably 40 to 60% by weight, and more preferably 52 to 58% by weight of 4,4' MDI, 0.5 to 10% by weight, preferably 1 to 7% by weight, and more preferably 3 to 5% by weight of 2,4' MDI, 0.01 to 1% by weight, preferably 0.05 to 0.2% by weight, and more preferably 0.075 to 0.125% by weight of 2,2' MDI, 0.1 to 10% by weight, preferably 0.5 to 2% by weight, and more preferably 0.75 to 1.25% by weight of MDI dimers, and 10 to 70% by weight, preferably 20 to 60% by weight, and more preferably 30 to 50% by weight of MDI oligomers comprising 3 or more rings.

A particularly important feature of the present invention is the implementation of the overhead vapor condenser as a shell and tube arrangement to subcool the condensate directly to below 47 ℃. This is one of the main measures to ensure a low dimer content of less than 200 ppm in the purified isomeric monomers. Preferably, the overhead vapor condenser is implemented as a shell and tube arrangement and so as to subcool the condensate directly to below 46 ℃, more preferably to at most 45 ℃, and most preferably to at most 42 ℃.

In a further development of the inventive concept it is proposed that the evaporator is a falling film evaporator or a thin film evaporator, and more preferably a falling film evaporator. In this embodiment, the average residence time of the liquid and vapor in the evaporator is relatively short, which reduces or even completely avoids dimer formation in the evaporator. Preferably, the evaporator is arranged outside the distillation column, and more preferably between the source for the mixture of different isomeric monomers and the distillation column.

According to another particularly preferred embodiment of the invention, the structured filler has a specific surface area of from 100 to750 m²/m³Preferably 150 to 350 m²/m³And more preferably 200 to 300 m²/m³And most preferably 225 to 275 m²/m³. This leads to a particularly efficient mass and heat transfer between the descending liquid phase and the ascending gas phase and thus to a complete or at least almost complete removal of dimers from the ascending gas, since dimers are effectively washed away by the descending liquid. This contributes to a low dimer content in the overhead fraction of the distillation column and thus also to the liquid condensed in the overhead vapor condenser.

Furthermore, it is preferred that the distillation column comprises only one structured packed bed and not more than one structured packed bed. The presence of one structured packed bed results in a shorter average residence time of liquid, and in particular vapor, in the structured packing than if two or more structured packed beds were present. This is due to the fact that: in the case of two or more beds, the residence time of the vapor and liquid is increased due to the resistance between the transition zones of the two beds.

Reducing the average residence time in the distillation column helps to reduce dimer formation in the structured packing and thus reduces fouling in the structured packing and improves operational safety.

Preferably, the distillation column comprises an open channel splash distributor above the structured packing. Such open channel splash distributors allow for uniform distribution of liquid across the cross-section of the structured packing and allow for precise adjustment of the volume of liquid distributed onto the structured packing, thereby precisely controlling the reflux ratio.

The flow-controlled return system is preferably arranged downstream of the overhead vapor condenser and in particular between the overhead vapor condenser and the distributor arranged above the structured packing. More specifically, it is preferred that the flow-controlled reflux system comprises a line connected to the outlet line of the overhead vapor condenser through which condensed liquid is withdrawn from the overhead vapor condenser. Through this line of the flow-controlled reflux system, a partial stream of condensed liquid withdrawn from the overhead vapor condenser is dispersed and transferred to a distributor arranged above the structured packing. According to the invention, the flow-controlled reflux system further comprises a heater by means of which the reflux liquid can be heated to a temperature of up to 190 ℃, preferably to 150 ℃ to 180 ℃, and preferably to about 180 ℃ in the case of purifying an MDI mixture. Thus, fouling in the dispenser can be reliably avoided.

Further, it is preferred that the apparatus comprises a cold trap between the overhead vapour condenser and the overhead vacuum system.

In a further development of the inventive concept, it is proposed that the distillation column further comprises a sump, the diameter of which is smaller than the diameter of the section of the distillation column above the sump. This results in a reduction in the residence time of the liquid in the sump and thus helps to reduce dimer formation in the distillation column.

Preferably, the different parts of the device according to the invention are connected to each other in the following way: the source of the mixture of different isomeric monomers (e.g. as preferred for the crude MDI mixture) is fluidly connected via a line to an evaporator where the mixture is evaporated. In turn, the evaporator is fluidly connected via a line to an inlet of the distillation column, wherein the inlet is arranged at a lower end of the distillation column. The top of the distillation column is fluidly connected via a line to an overhead vapor condenser in which the vapor is condensed. The condensed liquid outlet line arranged at the lower end of the overhead vapor condenser is fluidly connected via one line with the inlet of the flow-controlled reflux system and thus with the heater of the flow-controlled reflux system, wherein the lower end of the overhead vapor condenser is preferably further fluidly connected via a different line with the inlet of the optional suspension crystallization device. In turn, the heater of the flow-controlled reflux system is fluidly connected via a line to a distributor disposed above the structured packing of the distillation column. Furthermore, the overhead vapor condenser is provided with a vapor outlet line which is fluidly connected with an overhead vacuum system which in turn is fluidly connected via a line with an inlet at the upper part of the distillation column. At the sump of the distillation column, a further outlet line is provided which is in fluid connection via a line with the inlet line into the evaporator and which is further connected with the outlet line for withdrawing a fraction enriched in polymeric MDI from the apparatus. The suspension crystallization device is provided with two outlet lines, one for withdrawing the fraction with purified 4,4' MDI and the other for withdrawing the other fraction.

According to a second aspect, the present invention relates to a process for the preparation of a purified isomeric methylene diphenyl diisocyanate monomer from a crude mixture comprising therein different isomeric monomers and dimers, oligomers and polymers thereof, wherein the process is carried out in an apparatus as described above.

Preferably, the process is used for the preparation of 4,4 '-methylene diphenyl diisocyanate from a crude MDI mixture comprising different isomers of methylene diphenyl diisocyanate, i.e. 4,4' -methylene diphenyl diisocyanate and at least one of 2,2 '-methylene diphenyl diisocyanate, 2,4' -methylene diphenyl diisocyanate, as well as MDI dimers, MDI oligomers, MDI polymers and light materials.

In addition, the condensate is preferably subcooled in an overhead vapour condenser to 47 ℃, preferably to below 45 ℃, more preferably to at most 43 ℃ and most preferably to at most 42 ℃. The closer the cooling temperature is to 42 ℃, the more reliably the dimer formation in the overhead vapor condenser loop is minimized and in particular a condensed liquid with a dimer content of less than 200 ppm is obtained.

In a further development of the inventive concept, it is proposed to set the pressure at the top of the distillation column to 2 to 10mbar, and preferably at most 3 mbar. This reliably allows the bottom temperature of the distillation column to be kept well below 210 ℃, which helps to minimize dimer formation.

In order to further reduce the dimer content in the condensed liquid, it is proposed to set the average residence time of the liquid in the tubes of the overhead vapor condenser (which is subsequently also referred to as "overhead vapor condenser loop") to 30 seconds to 5 minutes, and preferably to less than 1 minute. The shorter the average residence time of the liquid in the overhead vapor condenser loop, the lower the tendency to form dimers.

In summary, it is particularly preferred according to the present invention to carry out the process in such a way that the apparatus is operated such that the dimer content of the condensed liquid withdrawn from the overhead vapor condenser is less than 200 ppm, preferably at most 150 ppm, more preferably at most 100 ppm, even more preferably at most 70ppm and most preferably at most 60 ppm.

Preferably, the ratio of reflux to condensate is set to 0.05 to 0.25, preferably to 0.10 to 0.20, and most preferably to about 0.15. The difference in the ratio of reflux to condensate results in a tendency to more effectively wash out dimer from the rising vapor by the descending liquid in the distillation column and thus results in a reduction in dimer content.

According to another preferred embodiment of the present invention, the condensed liquid withdrawn from the overhead vapor condenser is subsequently (i.e. downstream of the distillation column) fed as a liquid feed to a dynamic crystallization device, preferably a falling film crystallization device, and more preferably a suspension crystallization device.

As mentioned above, the condensed top product of the distillation column, i.e. the liquid feed to the dynamic crystallization unit, is preferably an isomeric monomer with a low MDI dimer content having the following characteristics:

i) dimer content is less than 200 ppm, preferably at most 150 ppm, more preferably at most 100 ppm, even more preferably at most 70ppm, and most preferably at most 60 ppm, and/or

ii) the content of polymeric methylene diphenyl diisocyanate is at most 2000 ppm, preferably at most 1500 ppm, more preferably at most 1000 ppm, and most preferably at most 800 ppm, and/or

iii) a color of at most 100 APHA, preferably at most 50 APHA, more preferably at most 40 APHA, and/or

iv) a hydrolysable chloride content of 10 to 150 ppm, preferably 30 to 90 ppm, more preferably 50 to 80 ppm, and most preferably 65 to 75 ppm, and/or

v) the content of 4,4' -methylenediphenyl diisocyanate is from 88 to 96% by weight, preferably from 90 to 95% by weight and more preferably from 92 to 94% by weight.

Preferably, the liquid feed into the dynamic crystallization device meets at least two, more preferably at least three, even more preferably at least four, most preferably all of the above criteria i) to v).

Furthermore, it is preferred that the refluxed liquid is heated to a temperature of 150 to 190 ℃, preferably to a temperature of 150 to 180 ℃, and more preferably to a temperature of about 180 ℃, in a controlled flow reflux system, in particular downstream of the overhead vapor condenser and between a distributor arranged above the structured packing. Thereby, fouling in the dispenser can be reliably avoided.

During crystallization in a suspension crystallization device, the liquid feed is cooled at the surface of the crystallization zone to a temperature below the equilibrium freezing temperature of the liquid, such that a crystal layer enriched in the isomeric monomer to be separated and purified is deposited on the surface, thereby forming a mother liquor having a lower concentration of the isomeric monomer to be separated and purified than the liquid feed, wherein the liquid feed comprises from 88 to 96% by weight, preferably from 90 to 95% by weight, and more preferably from 92 to 94% by weight of 4,4' -methylenediphenyl diisocyanate.

The crystals formed during the crystallization are preferably separated from the mother liquor in a wash column unit in order to obtain the separated and purified isomeric monomer.

According to a third aspect, the present invention relates to a purified methylene diphenyl diisocyanate component obtained as a condensation top product of a distillation column after the distillation column (i.e. as condensate or condensed phase, respectively), having the following characteristics:

i) a dimer content of less than 200 ppm, preferably at most 150 ppm, more preferably at most 100 ppm, even more preferably at most 70ppm, and most preferably at most 60 ppm,

ii) the content of polymerized methylene diphenyl diisocyanate is at most 2000 ppm, preferably at most 1500 ppm, more preferably at most 1000 ppm, and most preferably at most 800 ppm,

iii) a color of at most 100 APHA, preferably at most 50 APHA, more preferably at most 40 APHA,

iv) a hydrolysable chloride content of 10 to 150 ppm, preferably 30 to 90 ppm, more preferably 50 to 80 ppm, and most preferably 65 to 75 ppm, and

v) the content of 4,4' -methylenediphenyl diisocyanate is from 88 to 96% by weight, preferably from 90 to 95% by weight and more preferably from 92 to 94% by weight.

According to a fourth aspect, the present invention relates to a purified 4,4 '-methylene diphenyl diisocyanate obtainable by a process comprising the above described crystallization stage, wherein the purified 4,4' -methylene diphenyl diisocyanate has at least one of the subsequent features, and preferably all of the subsequent features:

i) a color of at most 100 APHA, preferably a color of at most 50 APHA, more preferably a color of at most 30 APHA, and most preferably a color of at most 20 APHA,

ii) a hydrolysable chloride content of at most 100 ppm, preferably at most 50 ppm, more preferably at most 30 ppm, and most preferably at most 15 ppm, and

iii) the content of 4,4' -methylenediphenyl diisocyanate is greater than 98.4% by weight, preferably at least 98.5% by weight, and more preferably greater than 98.6% by weight.

Preferably, the purified 4,4' -methylenediphenyl diisocyanate according to the invention satisfies at least two, and more preferably all three, of the above-mentioned characteristics i) to iii).

Drawings

The invention will be explained in more detail hereinafter with reference to the accompanying drawings, in which:

FIG. 1 shows schematically an apparatus according to the invention for preparing purified 4,4' MDI from a crude MDI mixture,

FIG. 2 schematically shows a suspension crystallization device included in the apparatus shown in FIG. 1, and

fig. 3 schematically shows a wash column unit of the suspension crystallization unit shown in fig. 2.

Detailed Description

The apparatus 10 shown in fig. 1 comprises a distillation apparatus 12 comprising a source 13 for a crude MDI mixture, a distillation column 14, an evaporator 16, an overhead vapor condenser 18, an overhead vacuum system 20, and a flow-controlled reflux system 22. The flow-controlled reflux system 22 comprises a heater 23 and the distillation column 14 comprises structured packing 24, with an open channel splash distributor 26 disposed above the structured packing 24. In addition, a cold trap 27 is included between the overhead vapor condenser 18 and the overhead vacuum system 20, and a vacuum source 28 is included in the overhead vacuum system 20. Furthermore, the apparatus 10 comprises a suspension crystallization device 29 downstream of the distillation device 12. .

The source 13 for the mixture of different isomeric monomers is connected via feed line 30 and line 32 to an evaporator 16, which evaporator 16 is embodied as a falling-film evaporator. Via line 34, the falling-film evaporator 16 is connected to the lower part of the distillation column 14. Distillation column 14 comprises sump 36, the diameter of sump 36 being smaller than the diameter of the section of distillation column 14 above sump 36. The sump 36 is connected to a line 37, which line 37 divides into lines 37' and 37 ". Line 37 "is an outlet line when line 37' is connected to line 32 leading to evaporator 16.

Line 38 leads from the top of the distillation column 14 to an overhead vapor condenser 18, which overhead vapor condenser 18 is embodied as a shell-and-tube evaporator. The lower end of the overhead vapor condenser 18 is connected to a withdrawal line 40, which withdrawal line 40 divides into a return line 42 and a line 44. When the return line 42 is connected to the heater 23 of the return system 22 and downstream thereof to the open channel splash distributor 26, the line 44 leads to the suspension crystallization device 29. The suspension crystallization device 29 has two outlet lines 46, 48.

The lower end of the overhead vapor condenser 18 is further connected to a vapor line 50, through which vapor line 50 the remaining vapor is withdrawn from the overhead vapor condenser 18. Vapor line 50 leads to cold trap 27, vacuum line 52 connects from cold trap 27 to vacuum source 28, and line 52 returns from vacuum source 28 into the upper end of distillation column 14.

During operation, a crude MDI mixture containing different isomeric MDI monomers, such as 2,2' -MDI, 2,4' -MDI and 4,4' -MDI, is fed from the source 13 via lines 30, 32 into the evaporator 16 where the liquid is evaporated. The resulting vapor flows via line 34 into the lower portion of distillation column 14 where it rises through structured packing 24. In structured packing 24, the ascending vapor is in intimate contact with the descending liquid, and the descending liquid washes out a major portion of the dimer formed in evaporator 16 and the lower portion of distillation column 14 from the ascending vapor. Vapor is withdrawn from the top of the distillation column 14 via line 38 and passed to the overhead vapor condenser 18 where the vapor is liquefied and the condensate is directly subcooled to about 42 ℃. Further, the flow rate of the vapor and the flow rate of the piping between the top of the distillation column 14 and the overhead vapor condenser 18 are adjusted such that the average residence time of the vapor in the overhead vapor condenser 18 is about 30 seconds to 5 minutes, and preferably less than 1 minute. Due to the direct subcooling of the condensed liquid and the short residence time of the vapor in the tube, only a trace amount of dimer is formed in the tube. Thus, the dimer content of the condensed liquid withdrawn from overhead vapor condenser 18 is less than 100 ppm, i.e., about 60 ppm, and the concentration of 4,4' -MDI is about 92.5% by weight. A portion of this condensed liquid is fed via line 44 to the suspension crystallization device 29 for further purification, while the remaining portion of the condensed liquid is returned to the open channel splash distributor 26 of the distillation column 14 via the flow-controlled reflux system 22. This backflow descends down through structured packing 24 as liquid or wash liquid, respectively. A fraction enriched in polymeric MDI is withdrawn from the distillation column 14 via line 37 with a portion of the stream being returned thereto via lines 37', 32 and 34 with the feed and the remainder of the fraction being withdrawn from the apparatus 10 via line 37 ".

The condensed liquid is further purified in the suspension crystallisation device 29 and leaves the apparatus via an outlet line 46, while the mother liquor of the crystalliser is withdrawn via a residue outlet line 48. The purified product leaving the apparatus via product outlet line 46 has a 4,4' -MDI content of at least 98.5% by weight.

As shown in fig. 2, the suspension crystallization device 29 comprises a suspension crystallizer 54 and a wash column device 56. The suspension crystallizer 54 is provided with a cooling system 58, which cooling system 58 allows cooling of the suspension crystallizer 54. The condensed liquid obtained in the overhead vapor condenser 18 is fed via line 44 into a circulation line 60, which circulation line 60 connects an outlet at the lower end of the suspension crystallizer 54 and the upper end of the suspension crystallizer 54. A transfer line 62 leads from the recycle line 60 to the scrub column unit 56. The scrubber unit 56 comprises a cylindrical vessel 63, which cylindrical vessel 63 is provided with a circulation line 64 at its lower end, which circulation line 64 in turn comprises a heater 66.

During operation, condensed liquid obtained in the overhead vapor condenser 18 is fed via line 44 into the recycle line 60 and from there into the upper end of the suspension crystallizer 54. The suspension crystallizer 54 is cooled via a cooling system 58 such that the temperature of the mixture contained therein is below the equilibrium freezing temperature of the liquid. Accordingly, pure or at least substantially pure 4,4' MDI crystals are formed in the suspension crystallizer 54, while 2,2' MDI and 2,4' MDI remain substantially in the mother liquor. A portion of the crystal suspension suspended in the mother liquor is continuously withdrawn from the suspension crystallizer 54 via line 60, into which a feed is introduced via line 44. A portion of the mixture so formed is returned to the suspension crystallizer 54 via line 60, while the remaining portion of the stream is transferred to the scrub column unit 56 via transfer line 62. As an alternative to the embodiment shown in fig. 2 and 3, the delivery line 62 may be a line terminating within the piston rod and opening at the end where the piston rod terminates in the piston head. In the cylindrical vessel 63 of the wash column unit 56, the crystals are separated from the suspension, thereby obtaining a crystal fraction and a mother liquor fraction. While the mother liquor fraction is withdrawn from the scrub column unit 56 via the residual outlet line 46, the crystal fraction is withdrawn from the cylindrical vessel 63 of the scrub column unit 56 into a recycle line 64 and directed through a heater 66 where the crystal fraction is heated to ensure all of the crystals are melted. A portion of the crystal fraction is recycled to the cylindrical vessel 63, while the remaining portion of the crystal fraction is withdrawn from the scrubber unit 56 via product outlet line 48.

The wash column unit 56 of the suspension crystallisation apparatus 29 is shown in more detail in figure 3. The scrub column apparatus 56 comprises a cylindrical vessel 63 in which a piston 72 is arranged so as to be reciprocally movable in the cylindrical vessel 63. Piston 72 includes a piston head 74 and a piston rod 76, wherein piston 72 defines a washing chamber 78 inside cylindrical container 63 above piston head 74. Piston head 74 contains a filter device 80, and when piston 72 moves downward, filter device 80 allows the mother liquor to flow through piston head 74, while the crystals cannot flow through piston head 74. The mother liquor passes through the filtration device 80 of the piston head 74 and is discharged from the cylindrical vessel 63 via the residual outlet line 46.

In the lower part of the cylindrical container 63 a scraper 82 is provided, which contains two rotating discs 84.

The circulation line 64 is arranged in the lower part of the cylindrical container 63 below the scraper 82. The recycle line 64 includes a heater 66.

In operation, a suspension comprising crystals suspended in mother liquor is fed via transfer line 62 into the washing chamber 78 of the cylindrical vessel 63 of the wash column apparatus 29, where the crystals are separated from the mother liquor and washed. In the cylindrical container 63, a piston 72 moves up and down in a controlled manner. When the piston 72 performs a suction stroke, i.e. when the piston moves upwards, a certain amount of suspension is introduced from the delivery line 62 into the washing chamber 78. When a predetermined amount of suspension is introduced into the washing chamber 78, the piston 72 is controlled to perform a compression stroke, i.e. a downward movement, which results in a compression or compaction, respectively, of the suspension, as the suspension mixture moves towards the scraper 82, wherein the high resistance to further vertical movement is affected by the suspension. Thus, a compacted crystal bed is formed in the lower portion of the washing chamber 78. During the compression stroke of the piston 72, most of the mother liquor contained in the crystal suspension mixture is pressed through the filter means 80 of the piston head 74 and leaves the cylindrical container 63 via the residual outlet line 46.

The lowermost portion of the crystal bed formed near the upper end of the scraper 82 is crushed by the scraper 82 and is pressed by the pressure generated by the downwardly moving piston 72 into the circulation line 64 where it is pumped by means of a pump and heated via the heater 66 to melt the crystal to generate a crystal melt. A portion of the crystal melt is removed from the apparatus via product outlet 48, with the remaining portion of the circulating crystal melt being reintroduced into cylindrical vessel 63 via the outlet of circulation line 64. The crystal melt or the washing liquid, respectively, flows upwards through the crystal bed and exchanges mother liquor present between the crystals of the crystal bed and thus serves to wash out the crystal bed. Thus, the crystal melt actually acts as a washing liquid. During the upward movement of the crystal bed and the displacement of the mother liquor, a washing front is formed at the phase boundary between the crystal melt and the mother liquor.

In summary, the wash column unit 29 results in an efficient separation of mother liquor and crystals and also allows for obtaining very pure 4,4' MDI crystals due to efficient washing of the crystals from the mother liquor before the molten crystals are discharged from the apparatus 10 as product.

Reference numerals

10 device

12 distillation device

13 for the source of the mixture of different isomeric monomers

14 distillation column

16 evaporator

18 overhead vapor condenser

20 top vacuum system

22 flow controlled reflux system

23 Heater

24 structured packing

26 open channel splash distributor

27 cold trap

28 vacuum source

29 suspension crystallization device

30 feed line

32 pipeline

34 pipeline

36 storage tank

37. 37', 37' ' line

38 pipeline

40 draw line

42 return line

44 pipeline

46 remainder outlet line

48 product outlet line

50 vapor line

52 vacuum line

54 suspension crystallizer

56 washing tower device

58 cooling system

60 circulation line of suspension crystallizer

62 transfer line

63 cylindrical container

64 circulation line of a scrubber unit

66 heater

72 piston

74 piston head

76 piston rod

78 washing chamber

80 filter device

82 scraper

84 scraper rotary disk