阅读说明：本技术 干燥装置及其用途以及用于在使用该干燥装置的情况下制备异氰酸酯的方法 (Drying device, use thereof and method for producing isocyanates using said drying device ) 是由 J·泽赫林 A·普卢姆 M·库尔曼 于 2020-03-31 设计创作，主要内容包括：本发明涉及一种用于从有待干燥的原材料蒸发挥发性的成分的干燥装置、一种用于在使用这种干燥装置的情况下制备异氰酸酯的方法以及像所述干燥装置的用途：用于对蒸馏釜底液、含油的废物、染料或漆料废物、污泥、被有机化合物污染的矿物材料和煤浆进行干燥。在所述干燥装置中,将被蒸发的成分(余气)通过余气穹起部和余气管路被导引至冷凝器中。所述干燥装置的特征在于,在该干燥装置的运行中故意允许或者引起所述余气在所述余气穹起部中和/或在所述余气管路中的部分冷凝并且所述余气的已冷凝的成分通过为此目的而被安置在余气穹起部和/或余气管路中的机构被从所述干燥装置中排出。(The invention relates to a drying device for evaporating volatile components from a raw material to be dried, to a method for producing isocyanates using such a drying device, and to the use of such a drying device: for drying still bottoms, oily wastes, dye or paint wastes, sludges, mineral materials contaminated with organic compounds and coal slurries. In the drying device, a component to be evaporated (residual gas) is guided into the condenser through the residual gas dome and the residual gas pipe. The drying device is characterized in that, during operation of the drying device, partial condensation of the residual gas in the residual gas dome and/or in the residual gas line is deliberately allowed or caused and the condensed components of the residual gas are discharged from the drying device by means of a device which is arranged for this purpose in the residual gas dome and/or in the residual gas line.)

1. Drying apparatus set up for carrying out a drying process on a raw material to be dried while obtaining a material to be dried and a vapor phase, the drying apparatus having:

a dryer comprising a heatable drying chamber with an inlet for raw material, an outlet for the dried material and a channel for the vapour phase, wherein the channel opens into a residual gas dome having an outlet for the vapour phase;

a condenser connected after the discharge port for the vapor phase;

a residual gas line connecting the discharge port for the vapor phase with the condenser;

wherein

(i) The residual-gas dome, instead of the residual-gas line, is set up in such a way that partial condensation of the vapor phase takes place in the residual-gas dome during the drying process, or

(ii) The residual gas line, instead of the residual gas dome, is set up in such a way that partial condensation of the vapor phase takes place in the residual gas line during the drying process, or

(iii) The residual gas dome and the residual gas line are set up such that a partial condensation of the vapor phase takes place during the drying process in the residual gas dome and the residual gas line,

wherein

In case (i) inside the residual air dome,

in case (ii) inside the residual gas line and

in case (iii) inside the residual gas dome and inside the residual gas line

A discharge for the components of the vapor phase which are liquefied in the partial condensation is arranged, wherein the discharge is set up such that the components which are liquefied in the partial condensation are discharged (i) from the residual gas dome, (ii) from the residual gas line or (iii) from the residual gas dome and from the residual gas line via a discharge opening and are separated in this way from the components of the vapor phase which are not liquefied in the partial condensation.

2. The drying apparatus of claim 1, wherein said dryer is selected from the group consisting of a vacuum dryer with product tumbling with horizontal axis, a drum, a pan dryer, a belt dryer, and a pelletizing screw.

3. The drying apparatus of claim 2, wherein the dryer is a product tumbling vacuum dryer selected from the group consisting of kneading dryers, paddle dryers, and paddle dryers.

4. Drying apparatus according to claim 3, wherein the dryer is a paddle dryer having an interior space in which a rotor shaft is arranged which can be rotatably driven about its axis, which rotor shaft is set up for distributing the raw material during the drying process on the solid material swirled by means of rotor blades arranged at the rotor shaft and conveying the raw material from an inlet opening towards an outlet opening, wherein the solid material is dried material or inert solid material.

5. Drying apparatus according to any one of the preceding claims, wherein part of the vapour phase is condensed

(a) By releasing or insulating the residual gas dome and/or the residual gas line

(b) By omitting or heating the residual gas dome and/or the residual gas line

(c) By means for cooling the residual gas dome and/or the residual gas line or

(d) By a combination of two or more of the above measures

To be implemented.

6. Drying apparatus according to any one of the preceding claims, wherein a drain mechanism is present in the residual gas dome and is selected from the group consisting of a drain trough with a connected drain line and a droplet collector with a connected drain line.

7. Drying apparatus according to any one of the preceding claims, wherein a drain mechanism is present in the surplus air line and is selected from the group consisting of a drain funnel with a connected drain line, a low point in the surplus air line with a connected drain line, a drain trough in the surplus air line with a connected drain line.

8. Drying apparatus according to any one of the preceding claims, wherein the condenser is selected from the group consisting of a tube bundle heat exchanger, a plate heat exchanger and a spray condenser.

9. A method for producing isocyanates by phosgenating primary amines corresponding to the isocyanates to be produced while obtaining a liquid crude process product comprising the isocyanates to be produced, which method comprises subjecting this liquid crude process product to a distillation treatment while obtaining a still bottoms;

also included is the treatment of such still bottoms, wherein such treatment comprises the steps of:

1) optionally preconcentrating the still bottoms in an evaporator by partial evaporation of the isocyanate to be prepared contained in the still bottoms, wherein a preconcentrated liquid stream depleted in isocyanate to be prepared is obtained;

2) drying the still bottoms or the pre-concentrated liquid stream depleted in isocyanate to be produced obtained in step 1) in a drying apparatus according to any of the preceding claims, wherein the isocyanate to be produced is obtained as a vapour phase with formation of the process product as a solid of the dried material and is liquefied in the condenser.

10. The method of claim 9, wherein the phosgenation is performed in the liquid phase in the presence of a solvent.

11. The process according to claim 9, wherein the phosgenation is carried out in the gas phase, wherein a gaseous process product comprising the isocyanate to be prepared is produced, and wherein this gaseous process product is cooled by contact with a quench liquid selected from the group consisting of solvent, isocyanate to be prepared and mixture of isocyanate to be prepared and solvent and thereby a liquid crude process product comprising the isocyanate to be prepared is obtained.

12. The method according to any one of claims 9 to 11, wherein the drying in step 2) is at a temperature in the range of 150 ℃ to 500 ℃ and at 20mbar_{(Absolute value)}To 200mbar_{(Absolute value)}Under a pressure within the range of (1).

13. The process according to any one of claims 9 to 12, comprising step 1), wherein the pre-concentration is at a temperature in the range of 120 ℃ to 180 ℃ and at 10mbar_{(Absolute value)}To 60mbar_{(Absolute value)}Pressure in the range ofIs carried out in the case of (1).

14. Use of a drying apparatus according to any one of claims 1 to 8 for drying raw materials selected from the group consisting of still bottoms, oily wastes, dye or paint wastes, sludge, mineral materials contaminated with organic compounds and coal slurries.

15. The use of claim 14, comprising drying of still bottoms, wherein the still bottoms

(a) In the preparation of isocyanates or

(b) In the preparation of primary amines which can be converted into isocyanates or

(b) In the refining of petroleum

And (4) generating.

Technical Field

The invention relates to a drying device for evaporating volatile components from a raw material to be dried, to a method for producing isocyanates using such a drying device, and to the use of such a drying device: for drying still bottoms, oily wastes, dye or paint wastes, sludges, mineral materials contaminated with organic compounds and coal slurries. In the drying device, the evaporated component (residual gas) is guided into the condenser via the residual gas dome and the residual gas line. The drying device is characterized in that in its operation a partial condensation of the residual gas in the residual gas dome and/or in the residual gas line is deliberately allowed or caused, and the condensed components of the residual gas are discharged from the drying device via means which are arranged for this purpose in the residual gas dome and/or in the residual gas line.

Background

Such devices are used in many technical fields for the following purposes: volatile compounds are removed by evaporation from the (usually already highly viscous but still flowable) starting material, wherein a material depleted of volatile compounds ("dried") is obtained, which is produced as a solid material or as a highly viscous liquid. Such devices are often referred to as dryers or drying reactors, even though they have not always been used for a long time for "drying" in the sense of dewatering. Known types of dryers are vacuum dryers with product tumbling with horizontal axis (see e.g. EP 0626368 Al), drum, tray dryers (see e.g. EP 0289647 Al), belt dryers, granulating screws and fluid bed dryers (see e.g. WO 2012/159736 Al).

For example, in many branches of industry, distillation still bottoms are produced which, in addition to the high-boiling constituents of the raw mixture to be distilled, also contain a proportion of light volatile components which are often deliberately not distilled off completely in order to keep the distillation still flowable even at moderate temperatures, which is absolutely necessary for a continuously carried out distillation.

Examples for this are found in the preparation of isocyanates by phosgenation and in the preparation of their precursor compounds, i.e. the corresponding primary amines. Specifically, a distillation bottom liquid produced in the production of p-tolylene diisocyanate (hereinafter referred to as TDI) and its amine precursor tolylenediamine (hereinafter referred to as TDA) is exemplified. In both cases, such still bottoms usually contain a considerable portion of the valuable products (TDI or TDA) in addition to high-boiling additional components, often referred to as distillation residues or simply as residues. Patent applications DE 102012108261 Al (kneading dryer), EP 2540702A 2 (fluidized bed dryer) and WO 2018/114846 Al (various types of dryers) are mentioned by way of example, which are directed to the treatment of TDI residues. As an example of the application of a vacuum dryer for product tumbling with a horizontal axis, european patent application EP 0626368 Al is cited. This application relates to a process for preparing pure distilled isocyanates by converting the corresponding amines with phosgene in a suitable solvent and working up by multistage distillation into pure isocyanate, pure solvent and a portion of the residue. The process is characterized in that the residue obtained from the distillation process is continuously fed together with 2 to 50 wt.% high-boiling hydrocarbons, preferably bitumen which is inert under distillation conditions, to a heated, product-reversing vacuum dryer with a horizontal axis, the proportion of isocyanate still present in the residue is continuously distilled off at a temperature of 160 to 280 ℃ and a pressure of 2 to 50mbar, and the remaining residue is continuously discharged as freely flowing, non-dusting particles, optionally cooled and, optionally after grinding, fed to a combustion process. In this example, a liquid mixture of 67.4% by weight of TDI, 29.1% by weight of polymer residue and 3.5% by weight of solvent was fed at three locations from above to a dryer of the type mentioned which was heated to an internal temperature of 240 ℃ and evacuated to a pressure of 12 mbar. At the end of the dryer opposite the introduction point, the dried pellets are discharged at the lower part via an impeller gate, while the mixture of TDI and solvent is discharged at two locations above. In this patent application, no targeted partial condensation of the residual gas in the residual gas dome and/or in the residual gas line is disclosed. As an additional aspect, it is merely mentioned that the condensate produced in the connected residual air system is separately discharged in order to remove any dust deposits that may occur on the walls of the residual air system. From this it is not known: at which position of the residual air system and under which conditions condensate is produced (or whether condensate is produced completely regularly).

After separation of distillable petroleum fractions for the production of valuable substances such as gas, Liquefied Petroleum Gas (LPG), motor fuel, solvents and kerosene, residues also remain in the refining of crude oil. Such residues can be routed, for example, to a coking plant and "cracked", i.e. chemically decomposed at high temperatures (e.g. 500 ℃) into substances having a lower molar mass. As an alternative thereto, it is proposed in german patent application DE 102017103363 Al (also published as WO 2018/149951a 1) to expose such residues in a kneader dryer (also referred to as mixing kneader or kneader reactor) to a vacuum having a pressure of 10mbar or less and a temperature of at least 300 ℃ in order to separate volatile constituents (including valuable products) which cannot be removed by the preceding distillation step via a steam connection and to discharge the remaining non-volatile substances through a discharge device. International patent application WO 2016/078994 Al also addresses the treatment of refinery residues with the use of mixing kneaders.

The drying device as described at the outset is likewise used for the treatment of residual materials such as oily waste, dye or paint waste and sludge. The same applies to the dewatering of coal slurries, which are obtained, for example, by hydrothermal carbonization of biomass. German patent application DE OS 3912586 describes a method and a regeneration mechanism for the thermal treatment of pasty or slurried substances, such as, for example, wet coal, in heated reactors, such as, for example, fluidized bed reactors, such as, for example, drying, low-temperature dry distillation or gasification. The medium flowing out of the reactor as a gas phase has condensable vapors, which are condensed, whereby the medium can be reused and fed back to the reactor via a recycle blower (3, see the drawing). A regeneration heat exchanger (4) connected as a superheating regulator or reheater is connected upstream of the two condensers (2) arranged in series, so that the condensers are charged with saturated medium and the circulation fan (3) is flowed through by the dried medium. In the heat exchanger (4), the gaseous phase flowing out of the reactor is cooled by heat exchange with the cold residual gas leaving the condenser (in the case of a gaseous phase which has no material contact with the cold residual gas; also referred to as indirect heat exchange), which can thus be quasi superheated and in this way can be fed back to the reactor without the risk of droplet formation. The portion of the gas phase flowing out of the reactor that is in the liquid state and that may form during this cooling of the gas phase is supplied to the condenser together with the remaining portion of the gas phase, rather than being discharged via a discharge device. In this patent application, no targeted partial condensation of the residual gas in the residual gas dome and/or in the residual gas line is disclosed in order to avoid deposits.

Another field of application for such a device is the thermal treatment of organically contaminated mineral materials, in particular correspondingly contaminated soils, in order to discharge the contaminated material. A corresponding method is described in german patent application DE 4200890 Al. A drying unit is described (see fig. 3), in which the dryer itself is configured as a tray dryer. The dryer is supplied with the necessary thermal energy via a hot oil system (10). The individual chemical products and water vapor leave the dryer (3) via the residual gas dome and are supplied from there via lines (33) to a plurality of series-connected condensation devices (27). These condensation devices are formed by condensers (28, 29) and heated collecting containers (30, 31), wherein a vacuum unit (32) is connected to the last condenser (29). Between the condensers 28 and 29 there is also a steam generator (35) and a hot water generator (36), in which steam generator (35) and hot water generator (36) of course also cooling of the residual gas takes place. In this patent application, no targeted partial condensation of the residual gas in the residual gas dome and/or in the residual gas line between the residual gas dome and the first condenser (28) in the flow direction is disclosed.

In general, in the prior art, efforts are made to discharge evaporated volatile constituents (residual gases) of the raw mixture to be dried from the dryer as far as possible without condensation, since for example the return of condensed constituents at relatively cold surfaces into the drying material is undesirable. Furthermore, there is a concern that solid deposits may form at such relatively cold surfaces. As a result, generally all surfaces that may come into contact with the residual air produced (in particular the residual air dome arranged on the actual drying chamber and the discharge line for the residual air) are at least well insulated and often even concomitantly heated. This is also the case in the already mentioned german patent application DE 4200890 Al, in which the track is explicitly written in column 6, columns 47 to 51: the volatile components … … discharged from the drying unit, i.e. the dryer (3), are conducted in a heated line system, i.e. the line (33), to the respective condensation device (27).

However, operational practices have shown that the insulation or even heating of these mechanisms has not always shown the desired success for a long time, but nevertheless deposits may form, which in extreme cases may even result in the drying process being interrupted at regular intervals and the apparatus having to be cleaned repeatedly. Accordingly, there is a need for further improvements in this area of technology.

Disclosure of Invention

In view of the described requirements, one subject of the invention is a drying device (1000) which is set up for (continuously) carrying out a drying process on a raw material (10) to be dried while obtaining a dried material (20) and a vapor phase (30, = residual gas), having:

dryer (100), the dryer (100) comprising a heatable drying chamber (110), the drying chamber (110) having (at least one) inlet (120) for raw material (10), (at least one) outlet (130) for dried material (20), and (at least one) channel (140) for a vapor phase (30), wherein the channel opens into (at least one) residual gas dome (150), the residual gas dome (150) having an outlet (160) for the vapor phase;

(at least one) condenser connected after the discharge for the vapor phase;

a residual gas line (200), the residual gas line (200) connecting a discharge (160) for the vapor phase (30) with the condenser (300);

wherein

(i) Instead of the residual gas line (200), the residual gas dome (150) is designed in such a way that a partial condensation of the vapor phase takes place (continuously) therein (i.e. in the residual gas dome (150)) during the drying process, or

(ii) Instead of the residual gas dome (150), the residual gas line (200) is designed in such a way that a partial condensation of the vapor phase takes place (continuously) during the drying process (i.e. in the residual gas line (200)), or

(iii) The residual-gas dome (150) and the residual-gas line (200) are designed in such a way that a partial condensation of the vapor phase takes place in them (i.e. in the residual-gas dome (150) and in the residual-gas line (200)) during the drying process,

wherein

In the interior of the residual air dome (150) in case (i),

in case (ii) in the interior of the residual gas line (200) and

in case (iii) in the interior of the residual gas dome (150) and in the interior of the residual gas line (200)

A discharge device (151, 201) for the partially condensed liquefied component (31, 32) of the vapor phase (30) is arranged, wherein the discharge device is designed such that the partially condensed liquefied component (31, 32) is discharged through a discharge opening (152, 202)

(ii) discharging from the residual air dome (in case (i)),

Is discharged from the residual gas line (in case (ii)) or

(in case (iii)) from the residual gas dome and the residual gas line

And is thus separated from the components of the vapor phase which are not liquefied in the partial condensation (the components (31, 32) which are liquefied in the partial condensation are thus conducted away from the drying device (1000) and not, for example, back into the drying chamber (110).

Another subject matter of the invention relates to a method for producing isocyanates by phosgenation of primary amines corresponding to the isocyanates to be produced, while obtaining a liquid crude process product comprising the isocyanates to be produced, which method comprises a distillative treatment of this liquid crude process product while obtaining a still bottoms;

also included is the treatment of such still bottoms, wherein such treatment comprises the steps of:

1) optionally preconcentrating the still bottoms in an evaporator by partial evaporation of the isocyanate to be prepared contained in the still bottoms, wherein a preconcentrated liquid stream depleted in isocyanate to be prepared is obtained;

2) the still bottoms or the preconcentrated liquid stream obtained in step 1) depleted in the isocyanate to be produced is dried in the drying apparatus according to the invention, wherein the isocyanate to be produced is obtained as a vapor phase in the case of a solid process product as dried material and is liquefied in a condenser.

Another subject matter of the invention relates to the use of a drying device according to the invention: for drying raw materials selected from the group consisting of still bottoms, oily wastes, dye or paint wastes, sludge, coal slurry and mineral materials contaminated with organic compounds, in particular correspondingly contaminated soils, preferably from the group consisting of still bottoms, oily wastes, dye or paint wastes, sludge and coal slurry.

Within the scope of the invention, the term "residual gas dome" refers to an arch on the dryer, in which the vapor phase flowing upward from the drying chamber (i.e. the residual gas flowing upward) accumulates. By "residual gas line" is meant the following lines: the vapor phase (residual gas) flows from the residual gas dome through the line into a condenser. If a plurality of condensers (for example 2 to 4 or precisely 2) are connected in series, the term "residual gas line" is used in the present description to denote a line which connects the residual gas dome to the first condenser in the flow direction of the vapor phase (residual gas).

The term "drain" is understood to mean an insert or a line low point for trapping the draining condensate, which is set up to collect the condensed draining condensate largely up to completely on the inner wall of the residual gas dome and/or of the residual gas line and to discharge it in a targeted manner from the residual gas dome or from the residual gas line.

It was found, completely surprisingly, that, in direct contrast to the observations which have hitherto been prevailing in the prior art, (partial) condensation of the vapor phase formed in the dryer in the residual gas dome and/or in the residual gas line is not only disadvantageous, but can even be advantageous, since the tendency to form deposits is reduced by the condensate flowing down at the inner wall of the residual gas dome or of the residual gas line and the deposits which may have formed can be continuously washed away. It has therefore been found entirely surprisingly that what is generally regarded in the prior art as an important insulation or even heating for the residual gas dome and the residual gas line can even be the cause of the formation of deposits. The invention is therefore characterized by a targeted partial condensation of the vapor phase in the residual-gas dome and/or in the residual-gas line, which takes place during the entire drying process, i.e. continuously (and not, for example, only in special operating states differing from normal operation). Backflow into the substance to be dried is prevented to the greatest possible extent up to completely by the discharge device provided according to the invention.

Drawings

In the drawings: fig. 1 shows a possible embodiment of a drying device (1000) according to the invention; fig. 2 shows a further possible embodiment of a drying device (1000) according to the invention.

Reference numerals have the same meaning in both figures.

Detailed Description

The following is a brief summary of various possible embodiments.

In a first embodiment of the drying device according to the invention, which can be combined with all other embodiments, the dryer is selected from the group consisting of a vacuum dryer with product tumbling with horizontal axis, a drum, a tray dryer, a belt dryer and a pelletizing screw.

A second embodiment of the drying device according to the invention, in which the dryer is a product-reversing vacuum dryer selected from the group consisting of kneading dryers, paddle dryers and paddle dryers, is a special embodiment of the first embodiment.

A third embodiment of the drying device according to the invention, in which the dryer is a paddle dryer which has an interior in which a rotor shaft is arranged which can be driven rotatably about its axis and which is designed to distribute the raw material during the drying process to the solid material swirled by means of rotor blades arranged on the rotor shaft and to convey the raw material from the inlet opening toward the outlet opening, is a special configuration of the second embodiment, wherein the solid material is dried material or inert solid material.

In a fourth embodiment of the drying device according to the invention, which can be combined with all other embodiments, the partial condensation of the vapor phase is achieved by the isolated removal of the residual gas dome and/or the residual gas line.

In a fifth embodiment of the drying device according to the invention, which can be combined with all other embodiments, the partial condensation of the vapor phase is achieved by the elimination of the heating of the residual-gas dome and/or the residual-gas line.

In a sixth embodiment of the drying device according to the invention, which can be combined with all other embodiments, the partial condensation of the vapor phase is achieved by means of a means for cooling the residual gas dome and/or the residual gas line.

In a seventh embodiment of the drying appliance according to the invention, which can be combined with all other embodiments, a discharge means is present in the residual air dome and is selected from the group consisting of a discharge trough with a connected discharge line (also referred to as a discharge flange) and a droplet collector with a connected discharge line.

In an eighth embodiment of the drying appliance according to the invention, which can be combined with all further embodiments, a discharge means is present in the residual gas line and is selected from the group consisting of a discharge funnel with a connected discharge line, a low point in the residual gas line with a connected discharge line and a discharge trough in the residual gas line with a connected discharge line.

In a ninth embodiment of the drying apparatus according to the invention, which can be combined with all other embodiments, the condenser is selected from the group consisting of a tube bundle heat exchanger, a plate heat exchanger and a spray condenser.

In a first embodiment of the process according to the invention for preparing isocyanates, which can be combined with all other embodiments (as long as it does not relate exclusively to gas-phase phosgenation), the phosgenation is carried out in the liquid phase in the presence of a solvent.

In a second embodiment of the process according to the invention for preparing isocyanates, which can be combined with all other embodiments (as long as it does not relate exclusively to liquid-phase phosgenation), the phosgenation is carried out in the gas phase, wherein a gaseous process product containing the isocyanates to be prepared is produced, and wherein this gaseous process product is cooled by contact with a quench liquid selected from the group consisting of solvents, isocyanates to be prepared and mixtures of isocyanates to be prepared and solvents and a liquid crude process product containing the isocyanates to be prepared is obtained.

The third embodiment of the inventive method for producing isocyanates, in which the quenching liquid is selected from the group consisting of solvents and mixtures of the isocyanates to be produced and solvents, is a special embodiment of the second embodiment.

A fourth embodiment of the inventive method for producing isocyanates is a special embodiment of the first and third embodiment, wherein the solvent is selected from the group consisting of chlorobenzene, o-dichlorobenzene, p-dichlorobenzene, isomers of trichlorobenzene, toluene, isomers of xylene and mixtures of the aforementioned solvents.

In a fifth embodiment of the process according to the invention for preparing isocyanates, which can be combined with all further embodiments, the process is carried out at a temperature in the range from 150 ℃ to 500 ℃ and at 20mbar_{(Absolute value)}To 200mbar_{(Absolute value)}At a pressure in the range of 185 ℃ to 320 ℃ and at a temperature of 50mbar_{(Absolute value)}To 180mbar_{(Absolute value)}In the case of a pressure in the range of (1), particularly preferably in the case of a temperature in the range of from 250 ℃ to 310 ℃ and at 80mbar_{(Absolute value)}To 150mbar_{(Absolute value)}The drying in step 2) is carried out under a pressure within the range of (1).

In a sixth embodiment of the process according to the invention for preparing isocyanates, which can be combined with all further embodiments, step 1) is included, wherein at a temperature in the range from 120 ℃ to 180 ℃ and at 10mbar_{(Absolute value)}To 60mbar_{(Absolute value)}At a pressure in the range of from 130 ℃ to 175 ℃ and at a temperature of from 25mbar_{(Absolute value)}To 45mbar_{(Absolute value)}The pre-concentration is carried out at a pressure in the range of (a).

In a seventh embodiment of the process according to the invention for preparing isocyanates, which can be combined with all other embodiments, the isocyanate to be prepared is selected from the group consisting of toluene diisocyanate, naphthyl diisocyanate, 1, 5-pentane diisocyanate, 1, 6-hexamethylene diisocyanate, isophorone diisocyanate, xylene diisocyanate and the diisocyanate dicyclohexylmethyl ether.

The eighth embodiment of the process according to the invention for preparing isocyanates, in which the isocyanate to be prepared is toluene diisocyanate, is a special configuration of the seventh embodiment.

In a first embodiment of the use according to the invention of the drying apparatus according to the invention, the use relates to the drying of a still bottoms, which is produced in the production of isocyanates or in the production of primary amines which can be converted into isocyanates.

In a second embodiment of the use according to the invention of the drying device according to the invention, the use relates to the drying of a still bottoms, wherein the still bottoms are produced in the refining of petroleum.

The embodiments of the invention briefly described above and other possible embodiments are explained in detail below. The embodiments can be combined with one another as desired, provided that no different conclusions are drawn from the context.

Suitable dryers (100) according to the invention are those of the type known to the person skilled in the art, such as, inter alia, vacuum dryers with product tumbling with horizontal axes (in particular kneading dryers, paddle and blade dryers), drums, tray dryers, belt dryers and pelletizing screws. Particularly preferred are paddle dryers in which the material which has been dried (or other inert solid material particles) is brought into motion at least close to the fluidized layer (or in the ideal case and preferably the fluidized layer) by a particularly fast rotating paddle system and the raw material to be dried is conveyed onto this quasi-fluidized layer (or actually the fluidized layer). The raw material to be dried is thus applied here to an at least preliminarily (ideally and preferably practically) fluidized bed. Such dryers, which are known in the concept of combined fluidized bed (CFT) dryers, are particularly suitable for drying viscous products. In contrast to the convective fluidized bed dryer, the swirling takes place purely mechanically here. CFT dryers are described, for example, in WO 2012/159736 a1 and EP 2540702 a 2. These dryers are characterized in that a rotor shaft (not shown in the figures) which can be rotatably driven about its axis is arranged in the interior of the dryer and which is set up to distribute the raw material during the drying process to the solid material, which is dried or inert solid material, swirled by means of rotor blades arranged on the rotor shaft and to convey said raw material from the inlet opening towards the outlet opening. In the operation of these dryers, it is provided, as mentioned above, that a bed of swirled material is already present when the raw material to be dried is first introduced into the dryer. For this purpose, the already dried material (from an earlier drying process) or the solid material which is inert under the present conditions (preferably inorganic spherical particles such as spheres, in particular of aluminum oxide) is introduced into a dryer and is set in motion by means of a rotor shaft. After the start-up phase, a stable operating state occurs in which the starting material is solidified by drying and is discharged partly as solid material and partly as a fluidized bed of solid material particles for further drying processes. Of course, it is then no longer necessary to add further already dried material from another drying process or to add inert solid material. It is also possible within the scope of the invention to use the calming zone described in WO 2012/159736 Al between the fluidized-bed region and the dry matter discharge (in the terminology of the invention, the discharge opening (130) for the dried material (20)).

As shown in fig. 1, the drying chamber (110) preferably has the shape of a horizontally arranged (substantially) cylindrical body. The internals in the interior of the drying chamber (110) for conveying the material to be dried from the inlet opening (120) to the outlet opening (130) are not shown in fig. 1 (and in fig. 2) for the sake of clarity. Such mechanisms, like for example the previously mentioned rotor shafts or other mechanisms listed in the documents mentioned at the outset, are sufficiently well known to the person skilled in the art that they are not discussed here. The residual air dome (150) preferably has the shape of a vertically arranged (substantially) cylindrical body. In the prior art, the residual gas dome is used for gravity separation of small droplets that may be entrained. This is achieved by: the cross section of the residual air dome is chosen sufficiently large according to the remaining dimensions of the dryer and the kind of material to be dried, which is a conventional design for the person skilled in the art. Within the scope of the invention, the residual air dome also fulfills this function. However, the partial condensation of the vapor phase according to the invention exceeds this in that way: a portion of the evaporated component of the raw material to be dried is liquefied again on the inner surface of the dome. Preferably, the dryer has (exactly one) residual air dome. However, embodiments with more than one (in particular 2) residual air dome are also conceivable.

The dryer (100) according to the invention is operated continuously, and more precisely preferably at a temperature in the range from 150 ℃ to 500 ℃ and at 20mbar_{(Absolute value)}To 200mbar_{(Absolute value)}At a pressure in the range of 185 ℃ to 320 ℃ and at a temperature of 50mbar_{(Absolute value)}To 180mbar_{(Absolute value)}In the case of pressures in the range of (1), particularly preferably in the case of temperatures in the range of from 250 ℃ to 310 ℃ and at 80mbar_{(Absolute value)}To 150mbar_{(Absolute value)}The dryer (100) according to the invention is operated with a pressure in the range of (1). It is particularly preferred to operate the dryer (100) such that the dried material is produced as a solid material.

Depending on the boundary conditions (type of raw material to be dried, operating conditions, etc.), partial condensation of the vapor phase can already be achieved in the sense of the present invention by the isolated omission of the residual gas dome or residual gas line. Preferably, the residual gas dome or the residual gas line is not heated. Depending on the boiling point of the volatile component, it may also be necessary to actively cool the residual gas dome or the residual gas line. This can be done in a manner known per se to the person skilled in the art, in particular by flowing through a cooling jacket or an applied cold coil with a medium which is colder than the boiling temperature of the vapour phase.

In the above-mentioned cases (i) and (iii), a discharge mechanism (151) is present in the residual air dome (150). The preferred discharge means (151) are a horizontally circumferential, inclined or spirally downwardly oriented discharge groove (which can also be referred to as a discharge flange) which is fitted on the inner wall of the residual gas dome with a connected discharge line and a droplet collector with all connected discharge lines (which are not shown in the figure). Such mechanisms are known per se to the person skilled in the art. The drainage channel covers preferably 75% to 100%, particularly preferably 90% to 100%, very particularly preferably 100%, of the circumference of the inner residual air dome, and preferably has a height of 1cm to 50 cm and a depth of 1cm to 50 cm, particularly preferably a height of 10cm to 30cm and a depth of 10cm to 30 cm.

In the above-mentioned cases (ii) and (iii), there is a discharge mechanism (201) in the residual gas line (200). The preferred discharge means (201) are a discharge funnel in the low point of the pipe with a connected discharge line, a low point in the residual gas line with a connected discharge line, and a horizontally circumferential, inclined or spirally downward directed discharge groove (the discharge line is not shown in the figures) mounted on the inner wall of the residual gas line in the residual gas line with a connected discharge line. Fig. 2 shows an exemplary embodiment with a discharge funnel (201) in the low point of the residual gas line (200). The drainage channel covers preferably 75% to 100%, particularly preferably 90% to 100% and very particularly preferably 100% of the circumference of the pipe and has a height of 1cm to 50 cm and a depth of 1cm to 20 cm, preferably a height of 10cm to 30cm and a depth of 5 cm to 15 cm.

The uncondensed components of the vapor phase (30) pass to a condenser (300) where they are liquefied and discharged from the drying apparatus (1000) as stream 33. That is to say, two to three streams of liquefied vapor phase, in case (i) streams 31 and 33, in case (ii) streams 32 and 33, and in case (iii) streams 31, 32 and 33, which are shown in fig. 1 and are particularly preferred, are generated in the apparatus according to the invention. Mechanisms known to the person skilled in the art, such as, inter alia, tube bundle heat exchangers, plate heat exchangers and spray condensers, are suitable as condensers (300). A plurality of condensers, in particular 2 to 4 condensers, preferably (exactly) 2 condensers, can be connected in series one after the other. However, the use of one single condenser is in many cases sufficient and is therefore also the most preferred embodiment.

The drying apparatus according to the invention can be used with particular advantage in the treatment of isocyanates, preferably p-tolylene diisocyanate, naphthylene diisocyanate, 1.5-pentane diisocyanate, 1.6-hexamethylene diisocyanate, isophorone diisocyanate, xylene diisocyanate or the diisocyanate dicyclohexylmethane, particularly preferably in the treatment of p-tolylene diisocyanate. Therefore, a further subject matter of the present invention is a process for preparing isocyanates by a) phosgenation of primary amines corresponding to the isocyanates to be prepared, i.e. in particular TDA, in the case of obtaining a liquid crude process product comprising the isocyanates to be prepared, which process comprises b) subjecting this liquid crude process product to a distillation treatment in the case of obtaining a distillation still bottoms;

further comprising c) a treatment of the still bottoms, wherein the treatment comprises the following steps:

1) optionally preconcentrating the still bottoms in an evaporator by partial evaporation of the isocyanate to be prepared contained in the still bottoms, wherein a preconcentrated liquid stream depleted in isocyanate to be prepared is obtained;

2) drying the still bottoms or the preconcentrated liquid stream depleted in isocyanate to be produced obtained in step 1) in a drying apparatus according to any of the preceding claims, wherein the isocyanate to be produced is obtained as a vapour phase and liquefied in a condenser with the formation of the process product as a solid of the dried material.

The phosgenation in step a) is carried out here in the liquid phase or in the gas phase in the presence of a solvent. The reaction is carried out in the gas phase, the gaseous process product initially produced, which contains the isocyanate to be prepared, is cooled by contact with a quench liquid, a liquid crude process product being obtained, which contains the isocyanate to be prepared. Suitable quench liquids are, in particular, organic solvents, the isocyanate to be prepared itself or mixtures composed of the isocyanate to be prepared and organic solvents. The quenching liquid mostly contains an organic solvent. Suitable solvents for this purpose (and as solvents in liquid-phase processes) are, in particular, chlorobenzene, o-dichlorobenzene, p-dichlorobenzene, isomers of trichlorobenzene, toluene, isomers of xylene and mixtures of the aforementioned solvents. Step a) can be carried out in particular as described in WO 2018/114846 Al page 19, line 7 to page 25, line 33, with particular preference being given to gas-phase phosgenation (see WO 2018/114846 Al page 21, line 24 to page 25, line 33).

After phosgenation in step a), the liquid crude process product obtained in step b), which comprises the isocyanate to be prepared, is treated. The treatment of the crude isocyanate can be carried out according to known methods. Examples are described in EP-A-1413571, U.S. Pat. No. 3, 2003/0230476 Al (TDI) and EP 0289840 Bl (HDI, IDPI and H12-MDI).

Alternatively, in a separate step b.1), dissolved phosgene and dissolved hydrogen chloride are first separated off from the liquid crude process product obtained in step a). This process embodiment is particularly preferred when the phosgenation in step a) is carried out in the liquid phase, since the liquid crude process product obtained in the liquid-phase phosgenation tends to contain a significantly greater proportion of dissolved phosgene and dissolved hydrogen chloride than the proportion obtained in the gas-phase phosgenation. Step b.l) can in principle be carried out in various ways known to the person skilled in the art, in particular by distillation, adsorption or a combination of both. In the following, possible embodiments are shown with the aid of different variants.

Following step b.l), or in particular in the case of carrying out step a) in the gas phase, directly following step a), it is possible to add the separation of the solvent in a separate step b.2). Step b.2) can be carried out in various ways known to the person skilled in the art, in particular by distillation. In the following, possible embodiments are shown with the aid of different variants.

In step b.3) of the process according to the invention, the isocyanate to be prepared is isolated by distillation. This can in principle be carried out in various ways known to the person skilled in the art for this purpose. In the following, possible embodiments are shown with the aid of different variants.

In detail, various embodiments are possible for the design of the process according to step b). Preferred variants are described below by way of example of TDI:

modification 1

Variant 1 is particularly suitable when step a) is carried out in the liquid phase, variant 1 being described essentially in the PERP report of the Chem System for TDI/MDI (Chem System, Process Evaluation Research Planning TDI/MDI 98/99S 8, Tarrytown, N.Y., USA: Chem Systems 1999, pages 27-32). In this variant, after the distillative separation of hydrogen chloride and phosgene (corresponding to step b.l in the terminology of the present invention)) has been effected, the liquid reaction mixture still comprises a solvent fraction of more than 50 mass%, preferably from 51 mass% to 85 mass%, particularly preferably from 55 mass% to 65 mass%, relative to its total mass. This mixture is fed to a solvent separation process (corresponding to step b.2 in the terminology of the present invention) in which the solvent TDI mixture is first distilled off in a pre-evaporator to a solvent distillation column. In the solvent distillation column, the solvent is distilled off and fed back to the process. The bottoms of this solvent distillation relative to the total mass of the bottoms, in addition to TDI, particularly preferably also comprises from 15 mass% to 25 mass% of solvent relative to the total mass of this bottoms. The liquid stream is conducted to a so-called intermediate column, where further solvent is distilled off and the bottoms product freed of solvent is fed to a final distillation column for the purification of TDI. This last distillation column is operated at subatmospheric pressure and provides as distillate stream purified, marketable isocyanate TDI (corresponding to step b.3) in the terminology of the present invention). A part of the TDI was left in the bottom liquid of the distillation still of this last distillation column. The tasks of the intermediate column and the distillation column for the purification of TDI can also be combined in a dividing wall column as described in US 2003/0230476 Al, in which a residual gas stream consisting of low boilers and solvent, a fraction of pure TDI as a distillate stream taken off in the region of the dividing wall and a product stream comprising TDI and high boilers (distillation residue) as a distillation bottom are obtained. In order to recover TDI, the bottom liquid of a distillation column used for the purification of TDI or the bottom liquid of a dividing wall column which unites an intermediate column and a TDI purification column is treated. For this purpose, the liquid stream can be conducted to a pre-evaporator of the solvent distillation as shown in the insert ii.a.5 reported in the referred PERP system. The bottom product of this pre-evaporator is then directed to a process for recovering the TDI contained therein. The Process in "TDI residual Processing Facility" shown in inset II.A.6 in the Chem System, Process Evaluation Research Planning TDI/MDI 98/99S 8, Tarrytown, N.Y., USA: Chem Systems 1999, pages 27-32, of the Chem System for TDI/MDI report (Chem System), can be replaced by step c) of the present invention. Since the raw materials fed to step c) in this embodiment still contain solvent (that is to say especially from 2.0 to 10 mass percents of solvent relative to the total mass of the raw materials) as a result of the feed of the still bottoms from step b.3) into the pre-evaporator of the solvent separation process from step b.2), it is preferred to carry out step 1) and to separate this solvent there before the drying in step 2). It is of course also possible to dispense with the feeding of the still bottoms from step b.3) into the pre-evaporator and instead to feed the still bottoms directly to the treatment in step c).

Modification 2

In contrast to variant 1, in this embodiment, after the distillative separation of hydrogen chloride and phosgene has been effected, the liquid reaction mixture still contains a solvent fraction of only 50% by mass or less relative to its total mass. The mixture is conveyed to a pre-evaporator from which the solvent TDI mixture is distilled off to a distillation column. In this variant, TDI has already been removed from the solvent in the last-mentioned distillation column, so that the bottoms of this distillation column can be conducted to a TDI purification column, and therefore in this variant there is one less column than in variant 1. The TDI purification column operates at subatmospheric pressure and provides as a distillate stream purified, marketable isocyanate TDI. The tasks of the TDI purification column and the distillation column connected upstream thereof can also be combined in a dividing wall column as described in EP 1413571 A1, in which a residual gas stream consisting of low boilers and solvent, a fraction of pure TDI as distillate stream taken off in the region of the dividing wall and a product stream comprising TDI and higher boiling components (distillation residue) as distillation still bottoms are obtained. In order to recover the TDI, the bottom liquid of the distillation still of the TDI purification column or the bottom liquid of the dividing wall column which unites the TDI purification column and the distillation column connected in front thereof is treated. In variant 2, such a treatment according to step c) of the invention can also be carried out. For this purpose, the liquid flow can be conducted into the above-mentioned pre-evaporator. The bottom product of this pre-evaporator is then directed to a process for recovering the TDI contained therein. Since in this embodiment the raw material fed to step c) still contains solvent (i.e. especially 2.0 to 10 mass% solvent with respect to the total mass of the raw material) as a result of the still bottoms being fed to the pre-evaporator of the solvent separation process, it is preferred to carry out step 1) and to separate the solvent there before the drying in step 2). It is of course also possible to dispense with the feeding of the still bottoms from step b.3) into the pre-evaporator and instead to feed the still bottoms directly to the treatment in step c).

Modification 3

Variant 3 comprises the distillation sequence described in variants 2 and 1, but without a separate pre-evaporator which feeds the liquid bottom discharge to the process according to step c). In this case, in the distillation sequence described, a portion of the distillation residue is entrained via the liquid stream up to the respective last TDI purification column. Such a method is also known in principle (EP 1717223 a 2). In this case, complete removal of the distillation residue is achieved via the still bottoms of the last distillation column (which is assigned to step b.3 in the terminology of the present invention)). The treatment of the still bottoms according to step c) of the invention can also be carried out in variant 3.

Modification 4

This variant is used in particular when step a) is carried out in the gas phase. Since the liquid crude process product obtained in the gas-phase phosgenation contains at best dissolved phosgene and dissolved hydrogen chloride in relatively small amounts (i.e. compared to the liquid-phase phosgenation), the separate separation of hydrogen chloride and phosgene in step b.l) can be dispensed with. Either the liquid crude process product is fed directly to a solvent separation process (corresponding to step b.2)), in which the solvent and possibly dissolved hydrogen chloride and possibly dissolved phosgene are separated off via a top distillation, or, when the solvent is sufficiently low, the liquid crude process product is fed directly to a TDI purification column. The TDI purification column is in both cases preferably designed as a dividing wall column. The low boilers, i.e.the by-products having a lower boiling point than TDI, any hydrogen chloride present and any phosgene present, any solvent and any inert gases, are removed as residual gas via the top. The purified TDI is removed as a distillate stream in the region of the partition. The resulting still bottoms comprise distillation residues and an amount of TDI which is not distilled off in order to maintain the handleability of the still bottoms, and possibly a trace portion of solvent. Of course, instead of a dividing wall column, two series-connected non-dividing wall distillation columns can also be used.

In this variant, the solvent separation process according to step b.2), if carried out, is preferably carried out at a temperature in the range from 160 ℃ to 200 ℃ and at a pressure in the range from 160 mbar to 220 mbar, wherein both data relate to the bottom of the distillation column used. In this way a still bottoms is obtained which comprises, relative to its total mass, preferably from 9 to 20 mass percents of solvent, from 79 to 90 mass percents of TDI and from 1 to 5 mass percents of compounds boiling higher than TDI.

The TDI purification according to step b.3) is preferably carried out at a temperature in the range from 160 ℃ to 200 ℃ and at a pressure in the range from 50mbar to 100mbar, in particular when carried out in a dividing wall column, wherein both data relate to the bottom of the distillation column used. In this way a distillation still bottoms is obtained which comprises, relative to its total mass, preferably from 0.00 to 1.00 mass%, of solvent, from 80.0 to 95.0 mass%, of TDI and from 4.00 to 20.0 mass%, of compounds boiling higher than TDI.

Without depending on the exact design of step b), in step b.3) in all possible process implementations (at least one) distillate stream comprising the first portion of the isocyanate to be prepared and (at least one) still bottoms comprising the second portion of the isocyanate to be prepared and the distillation residue are obtained. The treatment of the still bottoms is the subject of step c) of the process according to the invention. As explained in variants 1 and 2, further still bottoms can also be fed to step c) together with the still bottoms from step b.3).

Apart from the proportion of the isocyanate to be prepared, which should be recovered as completely as possible, and the proportion of the solvent, the still bottoms consist of distillation residues.

It is preferred that the still bottoms are first preconcentrated in step 1), i.e. the isocyanate to be prepared has been partially separated by evaporation without the remaining liquid stream becoming solid. Such a preconcentration by partial evaporation can in principle be carried out in all evaporators known to the person skilled in the art. It is particularly preferred that step 1) is carried out in an evaporator selected from the group consisting of thin-layer evaporators, climbing-layer evaporators, falling-film evaporators, long-tube evaporators, spiral-tube evaporators, forced circulation reduced pressure evaporators and combinations of these apparatuses. Falling film evaporators are particularly preferred here. It is also possible to connect a plurality of evaporators in series. Preferably, the preconcentration according to step 1) is carried out at a temperature in the range of 120 ℃ to 180 ℃ and at 10mbar_{(Absolute value)}To 60mbar_{(Absolute value)}In the case of a pressure in the range of (1), particularly preferably at a temperature in the range of from 130 ℃ to 175 ℃ and at 25mbar_{(Absolute value)}To 45mbar_{(Absolute value)}Under a pressure within the range of (1). Step 1) can be carried out continuously or intermittently. Continuous process practice is preferred.

In step 2), the preconcentrated liquid stream obtained in step 1) depleted in the isocyanate to be prepared or the still bottoms obtained in step b.3) when step 1) is dispensed with is now dried in the drying apparatus according to the invention. The drying is preferably carried out at a temperature in the range from 150 ℃ to 500 ℃ and at 20mbar_{(Absolute value)}To 200mbar_{(Absolute value)}In the case of a pressure in the range of (1), particularly preferably in the case of a temperature in the range of from 185 ℃ to 320 ℃ and at 50mbar_{(Absolute value)}To 180mbar_{(Absolute value)}Under a pressure in the range of (1), very particularly preferably at a temperature in the range of from 250 ℃ to 310 ℃ and at 80mbar_{(Absolute value)}To 150mbar_{(Absolute value)}Under a pressure within the range of (1).

During drying, the isocyanate to be prepared is evaporated and liquefied in a condenser, whereby the isocyanate initially present in the liquid stream to be dried is largely recovered. Solid material remains which is composed almost exclusively of distillation residues and contains the isocyanate to be prepared at best also in trace amounts (preferably a maximum of 1.0 mass% of the isocyanate to be prepared, particularly preferably a maximum of 0.1 mass% of the isocyanate to be prepared, in each case relative to the total mass of the solid material obtained in step 2). The solid material is preferably discharged continuously from the drying device according to the invention.

In addition to the particularly preferred use of the drying device according to the invention, it is of course also conceivable and preferred to use other drying devices. A further subject of the invention is therefore the use of a drying device according to the invention: for the treatment of still bottoms, oily wastes, dye or paint wastes, sludges, mineral materials contaminated with organic compounds (in particular correspondingly contaminated soils) or coal slurries.

As already discussed above, the still bottoms obtained in the preparation of isocyanates, in particular TDA, are particularly suitable for treatment with the apparatus according to the invention. The same applies to the precursor amine, in particular also to TDA.

However, the still bottoms produced in the refining of petroleum can also be advantageously treated with the drying apparatus according to the invention.

The following examples illustrate the use of the drying apparatus according to the invention in the treatment of TDI. It is immediately obvious here to the person skilled in the art, however, that the invention is by no means limited thereto.

Example (b):

the equipment used was: vane dryers, in which the raw material to be dried is conveyed onto a heated bed consisting of mechanically agitated and swirled granules consisting of dried raw material (from an earlier drying process).

The residual air dome part of the dryer is provided with a discharge groove. And a discharge funnel is arranged at the lower point of the residual air pipeline of the dryer.

Heating temperature: 280 ℃ to 305 ℃;

the process pressure: 90 mbar_{(Absolute value)}To 120 mbar_{(Absolute value)}；

Raw materials to be dried: 800kg/h to 1100 kg/h of a mixture of TDI and high-boiling secondary components (residuum) in a substantial mass ratio of 1: 1;

rotor speed: 5 to 42 Upm.

Example 1 (control)

The residual air dome and the residual air line are completely insulated (40 mm mineral wool plus applied metal sheet for moisture protection). Furthermore, the residual air dome, including the residual air discharge pipe connection, is heated via the heat jacket with a heat transfer medium having a temperature of 300 ℃ in order to prevent condensation.

The dripping TDI was not visible through the sight glass arranged at the residual gas dome and at the residual gas line. After 5 days of run time, the dryer had to be shut down due to the pressure difference between the dryer chamber and the TDI condenser. In a subsequent visual inspection it was found that the residual gas outlet pipe connections and residual gas lines were largely occupied by solid material. At a plurality of locations, the entire cross-section of the residual air path is blocked. The inner surface of the residual air dome is occupied by a layer of a brown solid material of about 10cm thickness; the TDI discharge slot disappears completely in the solid material layer.

Example 2 (according to the invention)

The residual air pipeline and the residual air dome of the vane type dryer are not isolated at all. The supply of heat to the residual air dome and the cover of the residual air discharge pipe joint is prevented.

The continued outflow of TDI can be observed via sight glasses positioned at the TDI condensate collection location at the residual air dome drain and at the drain funnel of the residual air line. After 14 days of operation, the dryer was shut off and the residual air path was visually inspected. It has been found that the inner surface of the residual gas discharge pipe fitting and the residual gas pipe are completely free of solid material. The inner surface of the residual air dome is free of solid material deposits except for a very thin black layer. The TDI discharge slot is completely clean except for some fines and the discharge port is unoccupied. Thus, the entire residual air path of the dryer is unobstructed and the dryer can continue to operate without creating a pressure differential.