Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
2002-06-24
2004-03-09
Hightower, P. Hampton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S170000, C528S312000, C528S319000, C528S320000, C528S322000, C528S323000
Reexamination Certificate
active
06703476
ABSTRACT:
The present invention relates to a process for preparing polyamides from aminonitriles and water.
The reaction of aminonitriles with water to prepare polyamides is known for example from DE-A-197 09 390, where water—aminonitrile reaction mixtures are reacted in a multistep process at high temperatures and pressures in the presence of heterogeneous metal oxide fixed bed catalysts. The catalyst used improves the viscosity buildup and increases the carboxyl end group number in the polyamide. The catalyst can be separated from the reaction product, so that the product properties are not adversely affected by it.
Compared to polyamides polymerized from caprolactam in a conventional manner, the carboxyl end group number of a polyamide prepared from aminocapronitrile (ACN) is frequently significantly lower. In addition, long total reaction mixture residence times are frequently needed to obtain a prepolymer melt that can be pelletized, extracted and dried. As a result of this and because of the large number of reaction stages, the process is not always economically advantageous or is associated with relatively high capital expenditure costs.
It is an object of the present invention to provide a process for preparing polyamides from aminonitriles and water that requires fewer reaction stages and has an improved space-time yield. The process may also preferably lead to an increased carboxyl end group content in the product.
We have found that this object is achieved according to the invention by the process for preparing a polyamide from an aminonitrile and water by
(1) reacting the aminonitrile with water at from 180 to 350° C. and such a pressure in the range from 30 to 120 bar that a gaseous phase is present as well as a liquid phase, in a first reaction stage,
(2) expanding the reaction mixture obtained in the first reaction stage via an evaporator zone or adiabatically with removal of water and ammonia into a second reaction stage, and
(3) postcondensing in the second reaction stage at from 0.1 mbar to 5 bar and from 230 to 320° C.
In one embodiment of the invention, the reacting in the first reaction stage is effected in the presence of a heterogeneous catalyst.
In a further embodiment of the invention, the reaction in the first recation stage is carried out without catalyst and instead the reaction mixture obtained in the first reaction stage is reacted in the presence of a heterogeneous catalyst at from 200 to 320° C. and a pressure at which the reaction mixture is present as a single liquid phase in a further reaction stage between the first reaction stage and the expanding step.
The embodiments thus comprise 2 or 3 process stages, which can be operated continuously or batchwise.
It is common to both the embodiments that, in a first process stage, aminonitriles are reacted with water in a reactor that contains a gas phase as well as a liquid phase, the reaction mixture, and that the components of the gas phase can be separated from the liquid phase via a column.
Useful catalysts for the purposes of the invention include known heterogeneous catalysis metal oxides, such as zirconium oxide, aluminum oxide, magnesium oxide, cerium oxide, lanthanum oxide and preferably titanium dioxide and also beta-zeolites and sheet-silicates. Particular preference is given to titanium dioxide in the anatase form. Preferably the titanium dioxide is at least 70% by weight, particularly preferably at least 90% by weight, especially essentially completely, in the anatase form. We have further found that silica gel, zeolites and doped metal oxides, doped with ruthenium, copper or fluoride for example, distinctly improve the reaction of the reactants mentioned. Useful catalysts are notable in particular for being slightly Brönsted acidic and having a large specific surface area. According to the invention, the heterogeneous catalyst has a macroscopic form permitting mechanical removal of the polymer melt from the catalyst, for example by means of sieves or filters. For example, the catalyst can be used in extrudate pellet form or in the form of a coating on packing elements.
The two embodiments of the present invention will now be more particularly described with reference to the drawing, where
FIG.
1
and
FIG. 2
are schematics illustrating the two embodiments of the invention. The reference numerals have the following meanings:
1:
aminonitrile feed
2:
water feed
3:
first reaction stage
4:
column
5, 6:
internals coated with the catalyst
7:
evaporator zone
8:
polycondensation stage
9:
column
10:
pump
11:
polyamide exit stream
12:
pump
13:
second reaction stage
Two-stage Embodiment (see
FIG. 1
)
The process of the invention is characterized by various reaction zones. The reaction of aminonitrile with water (
1
) takes place in a first reaction stage (
3
) at from 180 to 350° C., preferably at 230 to 290° C. The pressure chosen is such that, as well as a liquid phase, there is a gaseous phase which includes especially ammonia and water and can be removed via a column (
4
). Particularly preferably the pressure is adjusted in such a way that the water content of the reaction mixture remains constant and very large amounts of ammonia can be withdrawn from the gas phase. In a preferred embodiment, the reaction mixture in the reaction stage is continuously supplied with water (
2
) and continuously dewatered via the gas phase or the column (
4
). The reaction stage therefore has high pressures in the range from 30 to 120 bar.
According to the invention, the reaction volume contains heterogeneous metal oxide catalysts or internals (
5
,
6
) coated with the metal oxide catalyst. If desired, the reaction zones through which water flows continuously are spatially separated from those reaction zones which contain the catalyst material.
Transfer from the High Pressure into the Low Pressure Stage (Separator or Polycondensation Stage)
The pressurized reaction mixture is subsequently expanded into a polycondensation stage (
8
) either adiabatically or via an evaporator zone.
Adiabatic Expansion
Adiabatic expansion is preferable when the water content of the reaction mixture is not more than 10% by weight, based on the total mass.
The expansion results in a flash evaporation of the water still present in the polymer through utilization of the heat of reaction or enthalpy previously stored in the polymer melt. In contrast to a conventional evaporation of water on a heat exchanger surface, it is impossible for precipitations onto heat exchanger surfaces and other apparatus surfaces to take place from the polymer matrix in the course of a flash evaporation. Fouling due to organic or inorganic precipitations is avoided. In addition, the heat released in the process is directly utilized for water evaporation, yielding a further energy and cost saving. Moreover, it is desirable for the reaction mixture to cool down, since lowering the temperature will shift the polycondensation equilibrium to the side of the higher molecular weight product. The water vapor released in the course of the expansion includes volatile constituents such as aminonitrile monomer and oligomer. Rectification by a column (
9
) can be used to remove the water vapor from the system and to recycle the organics back into the process.
Entry into the Second Reaction Stage Via an Evaporator Zone
When the reaction mixture to be introduced into the second polycondensation stage has a high water content (>10% by weight), the use of an evaporator zone (
7
) will be advantageous. The volatile, low molecular weight components such as water and ammonia in the reaction mixture can transfer into the gas phase there. In addition, the evaporation zone ensures a sufficient input of heat into the reaction mixture to compensate the cooling of the mixture due to water evaporation.
The temperatures in the evaporator zone range from 230 to 350° C., preferably from 250 to 290° C., and the residence time is customarily less than 5 minutes, preferably less than 60 seconds. The evaporator zone is advantageously configured as a tube bundle (
7
) in which the tubes, if de
Krauss Dieter
Mohrschladt Ralf
Winterling Helmut
Hampton Hightower P.
Keil & Weinkauf
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