Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C263/00—Preparation of derivatives of isocyanic acid
  • C07C263/10—Preparation of derivatives of isocyanic acid by reaction of amines with carbonyl halides, e.g. with phosgene

Definitions

• the present invention relates to a process for the phosgenation of amines in the gas phase, in which a specific type of heat exchanger is used for vaporizing the amines.
• EP-A 928 785; EP-A 1 319 655; EP-A 1 555 258; EP-A 1 275 639; EP-A 1 275 640; EP-A 1 403 248; and EP-A 1 526 129 each describes a specific embodiment of this technology, but these disclosures relate to the reactor itself and the reaction conditions without going into details about the vaporizer technology used for pre-treatment of the starting materials.
• Shell-and-tube heat exchangers, plate heat exchangers or falling film evaporators, preferably with a pumped circuit, are customarily used for heating and vaporizing the starting materials used, i.e., amines and phosgene.
• Heater coils matrices operated electrically or by means of heat transfer fluids are used for heating the gaseous amines.
• these apparatuses have the disadvantage that the relatively high film thicknesses which occur adversely affect mass transfer and heat transfer, so that an increased residence time is required.
• decomposition with elimination of ammonia occurs, particularly in the vaporization and superheating of aliphatic amines. This not only reduces the yield but also causes the formation of deposits of ammonium chloride in pipes and the reactor in the subsequent phosgenation reaction. The plants therefore have to be cleaned relatively frequently, resulting in corresponding losses of production.
• Suitable aliphatic or cycloaliphatic amines include: 1,4-diaminobutane; 1,6-diaminohexane (HDA); 1,11-diaminoundecane; 1-amino-3,5,5-trimethyl-5-aminomethylcyclohexane (IPDA); 4,4′-diaminodicyclohexylmethane; 2,2-bis(4-aminocyclohexyl)propane; and 1,8-diamino-4-(aminomethyl)octane(triaminononane).
• diamines and/or triamines of the above-mentioned type which have exclusively aliphatically or cycloaliphatically bound amino groups, e.g. isophoronediamine (IPDA), hexamethylenediamine (HDA), bis(p-aminocyclohexyl)methane (PACM 20) and 1,8-diamino-4-(aminomethyl)-octane(triaminononane).
• IPDA isophoronediamine
• HDA hexamethylenediamine
• PAM 20 bis(p-aminocyclohexyl)methane
• 1,8-diamino-4-(aminomethyl)-octane(triaminononane 1,8-diamino-4-(aminomethyl)-octane(triaminononane
• Such stacked channel heat exchangers are suitable both as milli heat exchangers and as micro heat exchangers for the process of the invention.
• the hydraulic diameter (D) is the characterizing parameter for the purposes of the present invention.
• Such stacked channel micro heat exchangers are marketed, for example, by the Klastechnik Düsseldorf and are described in K. Schubert, J. Brandner, M. Fichtner, G. Linder, U. Schygulla, A. Wenka, “Microstructure devices for applications in thermal and chemical process engineering, Heat and Transport Phenomena in Microsystems”, Proc. Of the Internat. Conf ., Banff, October 15-20, 2.000.
• channel tube heat exchangers have one or more parallel tubes for the flow of the amines arranged in an enclosed surrounding space instead of stacked channels.
• the heat transfer medium flows through the surrounding space.
• Such specific tube heat exchangers corresponding to the above-mentioned criteria can have one or more channel tubes arranged in a parallel fashion.
• the surrounding space of such tube heat exchangers is preferably provided with deflection plates which improve the flow conditions and thus the heat transfer.
• the heat transfer medium can flow through the surrounding space either in co-current or in counter-current.
• the channel tubes used in such specific tube heat exchangers each usually have a length of from 10 cm to 400 cm, preferably from 30 to 150 cm.
• the wall thickness of the tubes is usually from 0.5 to 6 mm.
• the hydraulic diameter of the channels for conveying the amine stream is preferably at least 5 ⁇ m but less than 1.000 ⁇ m, more preferably from 30 to 500 ⁇ m.
• the heat exchange area per unit volume of the amine channels is preferably from 1 ⁇ 10 3 to 1 ⁇ 10 5 m 2 /m 3 in micro heat exchangers of the above-described type, more preferably from 2 ⁇ 10 3 to 1 ⁇ 10 5 m 2 /m 3 and in milli heat exchangers of the above-described type preferably from 1 to 2 ⁇ 10 3 m 2 /m 3 .
• the channels for conveying the heating medium preferably have a hydraulic diameter of from 5 to 10.000 ⁇ m, more preferably from 5 to 1.000 ⁇ m, most preferably from 30 to 500 ⁇ m.
• the flow channels can in principle also contain internals. This increases heat transfer compared to systems in which no such internals are present.
• the internals can also be fixed to the channels. In this case, the internals additionally act as heat transfer fins by means of which heat transfer is additionally added.
• Such internals can, for example, be layer structures.
• Such structures are generally made up of at least three layers, with each structured layer in the installed state having a multiplicity of openings which are arranged in at least one longitudinal row and the openings of a middle layer intersecting with at least three openings of an adjacent layer so that the sequence of the intersecting openings forms a flow channel in the longitudinal direction or transverse direction of the layers.
• Such structures can be formed by use of metal sheets having a sequence of obliquely arranged openings, as described in EP-A 1 284 159. Instead of metal sheets with openings, it is also possible to use comb profile layers as described in EP-A1 486 749.
• the openings of the metal sheet structures or the comb teeth of the comb structures are arranged at an angle of from 5 to 85° , preferably from 30 to 60°, to the main flow direction.
• the number of openings or comb teeth in a structured layer to form a series of openings is preferably at least 50, more preferably at least 200, most preferably at least 500.
• a micro or milli heat exchanger channel filled with structured layers is particularly advantageous in terms of back mixing and the temperature profile when the ratio of channel length (L) to the hydraulic diameter of the channel (D) (the L/D ratio) is greater than 10, preferably greater than 100 and more preferably greater than 500.
• Micro and milli channels having a rectangular or oval cross section are particularly well-suited to the use of layer structures.
• stacked channel micro heat exchangers and stacked channel milli heat exchangers not only the channels for the flow of the amines but also channels through which the heating medium is conveyed can be configured in this way. This can be useful in order to improve heat transfer to the heat transfer side, too.
• micro or milli heat exchangers or micro or milli vaporizers can be made of any metallic material, e.g. steel, stainless steel, titanium, Hastelloy, Inconel or other metallic alloys.
• heating medium it is possible to use the customary heating media such as steam, pressurized water or heat transfer fluids.
• the temperature at which the heater heat exchanger or vaporizer heat exchanger used according to the invention is operated depends on the boiling point of the amine to be vaporized.
• the aim is for the temperature after passage through the heater heat exchanger to be just below the boiling point of the amine and for all the previously liquid amine to be brought into the gas phase after passage through the vaporizer and, if appropriate, for the gaseous amine to be superheated in the same heat exchanger or a further heat exchanger.
• Circulating flows through the apparatuses are deliberately dispensed with, so that the amine passes through the apparatuses only once. This has the advantage that the volume of pump reservoirs which are otherwise necessary can also be dispensed with and the residence time at high temperatures is reduced further.
• the precise pressure and temperature conditions can easily be determined by a person skilled in the art by means of routine experiments.
• the mean residence time of the amines in the heater is preferably from 0.001 to 60 s, more preferably from 0.01 to 10 s.
• the mean residence time of the amines in the vaporizer is preferably from 0.001 to 60 s, more preferably from 0.01 to 10 s.
• An advantage of the process of the invention is that, due to the short residence times and therefore low integral temperature stresses in the milli and micro structural components, decomposition of temperature-sensitive aliphatic amines is reduced compared to conventional vaporizers or is avoided completely.
• the surface-to-volume ratio is increased in the vaporization due to the geometrically dictated formation of small bubbles, so that very efficient vaporization is possible.
• the feed streams can also be passed over internals which enable better mixing of the reactants in the gas space to be achieved.
• Similar measures can also be taken in the reactor itself in order to improve the mixing of amine and phosgene and thus ensure substantially trouble-free continuous operation. Examples of such measures are the installation of swirl-inducing internals in the feed lines or a tapering diameter of the reactor tube downstream of the confluence of the amine stream and the phosgene stream. Further suitable measures may be found in the published patents and applications discussed herein.
• the feed streams can also be diluted with inert diluents before being fed into the reaction space.
• a preferred inert gas for dilution is nitrogen.
• Suitable inert solvents whose vapors can likewise be used for diluting diamine are, for example, chlorobenzene, o-dichlorobenzene, xylene, chloronaphthalene, deca-hydronaphthalene and mixtures thereof.
• the amount of any inert gas or solvent vapor used as diluent is not critical, but can be utilized to reduce the vaporization temperature of the amine.
• the molar excess of phosgene per amino group is usually from 30 to 300%, preferably from 60 to 170%.
• Suitable cylindrical reaction spaces are, for example, tube reactors without internals and without moving parts in the interior of the reactor.
• the tube reactors are generally made of steel, glass, alloy steel or enamelled steel and have a length which is sufficient to allow complete reaction of the amine with the phosgene under the process conditions.
• the gas streams are generally fed into the tube reactor at one end of the reactor, for example, through nozzles installed at one end of the tube reactor or through a combination of nozzle and an annular gap between nozzle and a mixing tube.
• the mixing tube is likewise maintained at a temperature within the range from 200 to 600° C., preferably from 300 to 500° C., with this temperature being maintained, if necessary, by heating of the reaction tube.
• the pressure in the feed lines to the reaction space is generally from 200 to 3.000 mbar and that at the output from the reaction space is generally from 150 to 2.000 mbar, with care being taken to ensure a flow velocity within the reaction space of at least 3 m/s, preferably at least 6 m/s and more preferably from 10 to 120 m/s, by maintaining an appropriate differential pressure. Under these conditions, turbulent flow generally prevails within the reaction space.
• Solvents of the types which have been mentioned by way of example above in particular technical-grade dichlorobenzene, which are maintained at a temperature of from 120 to 200° C., preferably from 120 to 170° C., are particularly well-suited for the selective recovery of the isocyanate from the mixture leaving the reaction space in gaseous form.
• Conceivable methods of selectively condensing the isocyanate formed from the gas mixture leaving the reactor using such solvents are, for example, passing the gas mixture through the respective solvent or spraying the solvent (solvent mist) into the gas stream.
• the gas mixture passing through the condensation stage for recovering the isocyanate is subsequently freed of excess phosgene in known manner.
• This can be effected by means of a cold trap, absorption in an inert solvent (e.g., chlorobenzene or dichlorobenzene) maintained at a temperature of from ⁇ 10° C. to 8° C. or by adsorption and hydrolysis on activated carbon.
• the hydrogen chloride gas which passes through the phosgene recovery stage can be recycled in a manner known to those skilled in the art for recovery of the chlorine required for the phosgene synthesis.
• a plurality of milli heat exchangers each having rectangular flow channels were connected in series and were in each case used for heating, vaporization and superheating.
• the flow channels had an internal height of 3.1 mm, an internal width of 18 mm and were filled with a layer structure. This filling was made up of three layers each of which had a height of 1 mm.
• the total length of the channels per vaporizer was 300 mm.
• the heat transfer area (arithmetic mean of internal and external wall area) per channel was 156 cm 2 and the free internal volume was 12.8 cm 3 .
• MHE 1-MHE 3 countercurrent heat exchanger
• All milli heat-exchanger apparatuses had an interior shell diameter of about 40 mm and were provided with a plurality of deflection plates in the volume within the shell through which heat transfer medium flowed.
• the amines were heated from 60° C. to the boiling point in the first heat exchanger series (MHE 1-MHE 3) and then vaporized and superheated in the second heat exchanger series (MHE 4-MHE 5).
• the amine was condensed in the downstream condenser, fed into the receiver and subsequently pumped around the circuit again.
• samples were analyzed by gas chromatography and ammonia analysis at regular intervals.
• HDA was heated to 217° C. at a pressure of 2.3 bara (pressure in bar absolute) in the MHE 1-MHE 3 heated to 224° C. and then vaporized and superheated to 305° C. at a pressure of 1.0 bara in the MHE 4-MHE 5 heated to 307° C.
• the mean residence time in MHE 1-MHE 3 was 4.7 s and in MHE 4-MHE 5 was 9.4 s, assuming complete liquid flow as far as the outlet.
• the real residence time was significantly below this value because of vaporization.
• the concentration of secondary components increased from 170 ppm to 270 ppm.
• IPDA was heated to 260° C. at a pressure of 1.6 bara in the MHE 1-MHE 3 heated to 277° C. and then vaporized and superheated to 302° C. at a pressure of 1.0 bara in the MHE 4-MHE 5 heated to 305° C.
• the mean residence time in MHE 1-MHE 3 was 5.2 s and in MHE 4-MHE 5 was 10.5 s, assuming complete liquid flow as far as the outlet.
• the real residence time was significantly below this value because of vaporization.
• the concentration of secondary components increased from 1.300 ppm to 2.200 ppm.
• PACM 20 was heated to 327° C. at a pressure of 1.2 bara in the MHE 1-MHE 3 heated to 338° C. and then vaporized and superheated to 335° C. at a pressure of 1.0 bara in the MHE 4-MHE 5 heated to 352° C.
• the mean residence time in MHE 1-MHE 3 was 7 s and in MHE 4-MHE 5 was 14 s, assuming complete liquid flow as far as the outlet.
• the real residence time was significantly below this value because of vaporization.
• the concentration of secondary components increased from 3.900 ppm to 4.400 ppm.
• Heat transfer coefficients determined were: from 350 to 1.850 W/(m 2 K) for heating to the boiling point at pump circulation rates of from 10 to 100 kg/h, 900 W/(m 2 K) for vaporization at a pump circulation rate of 15 kg/h and 250 W/(m 2 K) for superheating at pump circulation rates of 15 kg/h.

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• Chemical & Material Sciences (AREA)
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• Chemical Kinetics & Catalysis (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
• Silicon Compounds (AREA)
• Vaporization, Distillation, Condensation, Sublimation, And Cold Traps (AREA)

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• 2005
  • 2005-08-02 DE DE102005036870A patent/DE102005036870A1/de not_active Withdrawn
• 2006
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