WO2003008333A1

WO2003008333A1 - Method for the separation of ammonia from a mixture

Info

Publication number: WO2003008333A1
Application number: PCT/EP2002/007486
Authority: WO
Inventors: Karl-Heinz Walczuch; Axel Polt; Frank Poplow; Hermann Luyken; Stefan Meckl; Heinz Auer
Original assignee: Basf Aktiengesellschaft
Priority date: 2001-07-16
Filing date: 2002-07-05
Publication date: 2003-01-30
Also published as: DE10133788A1; AR034755A1

Abstract

The invention relates to a method for the separation of ammonia from a mixture which contains ammonia and whose boiling point in an ammonia-free state is more than 50 °C higher than the boiling point of pure ammonia, characterized in that said method comprises n stages, n being a natural number of at least 2, wherein a) said mixture is fed to stage 1; b) at each stage m, m being a natural number of 1 to n-1, part of the ammonia is separated from the educt mixture fed from stage 1 at a pressure Pm by means of evaporation in order to obtain a product mixture which has an ammonia content corresponding to pressure Pm and temperature tm, and gaseous ammonia, c) the product mixture obtained in stage m is used in step m+1 as an educt mixture, with the exception of stage n, according to step b), d) in step n, the ammonia is separated at a pressure Pn and temperature tn either partially or fully from the educt mixture fed from stage n-1 by means of evaporation in order to obtain a product and gaseous ammonia, each pressure Pm being greater than Pm+1.

Description

Process for the separation of ammonia from a mixture

description

The present invention relates to a process for separating ammonia from a mixture which contains ammonia and whose boiling point in the ammonia-free state is more than 50 ° C. higher than the boiling point of pure ammonia, characterized in that in a process with n stages, with n is a natural number at least 2,

a) feeds this mixture to a stage 1,

b) in each stage m, with m a natural number from 1 to n-1, part of the ammonia at a pressure p _m from the educt mixture fed from stage m-1 separated by evaporation to give a product mixture which one Pressure p _m and the temperature t _m corresponding ammonia content, and of gaseous ammonia,

c) the product mixture obtained in stage m in stage m + 1, with

Exception of stage n, used as starting material mixture according to step b),

d) in stage n the ammonia at a pressure p _n and one

Temperature t _{n is} partially or completely separated from the educt mixture fed from stage n-1 by evaporation to give a product and gaseous ammonia,

where each pressure p _{m is} greater than p _{m +} .

Mixtures that contain ammonia and whose boiling point in the ammonia-free state is more than 50 ° C higher than the boiling point of pure ammonia, arise as direct reaction mixtures in numerous industrial processes. For example, hydrogenations are carried out in ammonia as a liquid diluent or in liquid diluents in the presence of ammonia. The ammonia used in excess or as a liquid diluent is usually separated off from the reaction mixture before the product of value is obtained, in order to be able to recycle the separated ammonia into the reaction for reasons of cost savings and environmental protection. As a rule, distillative processes are used to remove the ammonia. The generally large boiling point difference between pure ammonia and the reaction mixture in the ammonia-free state is problematic here. On the one hand, there is an attempt to carry out the distillation at high pressure in order to condense the distilled ammonia in an economical manner at temperatures above 20 ° C. using cooling water as the cooling medium. Condensation at lower temperatures requires the use of refrigeration systems, the operation of which is associated with high costs. On the other hand, a high pressure during the distillation requires high bottom temperatures during the distillation, which can damage the product of value. In addition, energy sources, such as steam, and the corresponding distillation devices generally become more expensive with increasing temperature.

DE-A-195 00 222 describes the separation of ammonia from a mixture which contains ammonia, hexamethylene diamine, 6-aminocapronitrile and adiponitrile and was obtained in the partial hydrogenation of adiponitrile. In order to be able to separate the ammonia at high pressure and at the same time not too high bottom temperature, the use of an intermediate boiler is disclosed. Disadvantages here are the high costs for the device and the large circuit currents which are required for effective ammonia separation.

The object of the present invention was to provide a process which removes ammonia from a mixture which contains ammonia and whose boiling point in the ammonia-free state is more than 50 ° C. higher than the boiling point of pure ammonia, enabled in a technically simple and economical manner while avoiding the disadvantages mentioned.

Accordingly, the process defined at the outset was found.

According to the invention uses a mixture containing ammonia and having a boiling point in ammonia-free state by more than 50 ° C higher than is the boiling point of pure ammonia (boiling point of rei ^¬ nem ammonia -33 ° C), a. So far, no restrictions are known with regard to the mixture, its preparation or composition. In a preferred embodiment, such mixtures come into consideration whose ammonia content is 1 to 99, preferably 5 to 99% by weight, based on the total weight of the mixture. At lower ammonia contents, the process according to the invention begins to become uneconomical, without however affecting its basic technical applicability. Such mixtures can be obtained, for example, in the partial hydrogenation of adiponitrile in ammonia to adiponitrile, hexamethylenediamine and 6-aminocapronitrile. Such mixtures usually have contents of adiponitrile in the range from 5 to 80% by weight, hexamethylenediamine in the range from 5 to 80% by weight, 6-aminocapronitrile in the range from 5 to 80% by weight, ammonia in the range from 10 to 90% by weight and other impurities which are customary per se, such as catalyst or promoters or impurities in the starting materials, with the proviso that the sum of the stated values does not exceed 100% by weight. Such mixtures and their preparation are known per se. The boiling point of the ammonia-free mixture is significantly more than 50 ° C above the boiling point of ammonia.

This mixture is used in the first of the n stages, where n is a natural number of at least 2, preferably 2, 3, 4, 5, 6, 7, 8, 9, 10, in particular 2, 3, 4, 5, 6 ,

According to the invention, in step b), in each stage m, with m a natural number from 1 to n-1, part of the ammonia at a pressure p _{m is} removed from the starting material mixture fed from stage m-1 by evaporation to give one Product mixture which has an ammonia content corresponding to the pressure p _m and the temperature t _m , and of gaseous ammonia.

For the separation, customary apparatuses are considered, as described, for example, in: Kirk-Oth er, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 7, John Wiley & Sons, New York, 1979, pages 870-881 , Distillation devices with separation stages, such as distillation columns with trays or packings, preferably sieve tray columns, bubble tray trays, packing columns or packed columns, are suitable. Separation devices without separation stages, such as bubbles with or without an evaporator, or natural or forced-flow evaporators are also suitable.

The separation can be carried out continuously or batchwise.

During the separation, heat can be supplied to the mixture using suitable devices, for example using an evaporator. Likewise, heat can be added to the feed to the distillation stage in question before or after the expansion, that is to say at the pressure of the corresponding stage or the pressure of the subsequent stage. Likewise, the evaporation of the ammonia by adding heat or by cooling the Product mixture from the temperature t _m -ι to the temperature T _m during the expansion of the pressure p _m p _m _ι be achieved on the pressure.

The optimum pressure p _m for the respective stage m can easily be determined by a few simple preliminary tests, depending on the mixture used. In response to this pressure p _m results in the required liquid-phase temperature and the ammonia content of the residual product mixture in the respective stage m.

According to the invention, the product mixture obtained in stage m is used in stage m + 1, with the exception of stage n, as the starting material mixture according to step b). For the purposes of the present invention, this means that the parameters already described for step m according to step b) are considered for carrying out step m + 1.

According to the invention, step d) in step n) partially or completely separates the ammonia at a pressure p _n from the educt mixture fed from step n-1 by evaporation to obtain a product and gaseous ammonia. For carrying out step d), the parameters already described for carrying out step m in step b) are also suitable.

According to the invention, each pressure p _{m is} greater than p _{m +} ι. This means that relaxation takes place from each level to each subsequent level m + 1.

The temperature t _m in the respective stage m results from the composition of the mixture in the respective stage m and the respective pressure p _m .

The temperature t _n in stage n results from the composition of the mixture in stage n and the pressure p _n .

It is also possible to set the respective concentration in stage m by means of pressure p _m and temperature t _m , the temperature t _{m being able to be set} in particular by supplying it with heat.

It is also possible to set the respective concentration in stage n by means of pressure p _n and temperature t _n , the temperature t _{n being able to be set} in particular by supplying it with heat. In a preferred embodiment, the gaseous ammonia obtained in stage m or stage n can be compressed. This compression can be done so that the ammonia remains gaseous or liquefied.

In a further preferred embodiment, the gaseous ammonia obtained in stage m + 1 can be compressed and combined with the gaseous ammonia obtained in stage m. The ammonia can advantageously be liquefied after the combination. In an advantageous embodiment, the pressures of the relevant stages m and m + 1 can be selected so that the gaseous ammonia from stage m + 1 can be brought to the pressure of the gaseous ammonia from stage m in a one-stage compressor.

In a further preferred embodiment, the gaseous ammonia obtained in each stage m + 1 can be compressed and combined with the gaseous ammonia obtained in the respective stage m. The ammonia after the combination with the gaseous ammonia of the first stage can advantageously be liquefied. In an advantageous embodiment, the pressures of the relevant stages m and m + 1 can be selected so that the gaseous ammonia from stage m + 1 can be brought to the pressure of the gaseous ammonia from stage m in a one-stage compressor. In a particularly advantageous embodiment, the compressors of stages 2 to n can be driven by the same shaft.

In an advantageous embodiment, the gaseous stream from stage m + 1 can be cooled before compression, it being possible to separate off condensable fractions. In a particularly advantageous embodiment, the condensable components of the gaseous stream can be returned from stage m + 1 to stage m + 1.

example

1,000 kg / h of a mixture containing 70% ammonia, 10% hexamethylenediamine, 10% 6-aminocapronitrile and 10% adiponitrile were released from a pressure of 40 bar to a pressure of 20 bar and in a bubble Bl with evaporator VI ammonia distilled off. The ammonia aml distilled off was liquefied at 45 ° C. using a condenser K 1. An ammonia content of about 5% was set in the bottom of the bladder Bl, resulting in a bottom temperature of 175 ° C. The bottom draw from bladder Bl was expanded to a pressure of 8 bar in a bladder B2 without an evaporator. The ammonia which is released by the expansion process am2 was compressed with a compressor V2 to a pressure of 20 bar and condensed together with ammonia aml in the condenser Kl. The bottom draw from bubble B2 was depressurized to a pressure of 3 bar in a column K3 and slightly heated with a heat exchanger W3 before being introduced into the column. The ammonia outgassing by the expansion process was removed overhead and cooled with a cooler W4. The condensate formed was returned to column K3 as reflux. The gaseous ammonia am.3 from the cooler W4 was compressed with a compressor V3 from 3 bar to 8 bar, combined with the stream am2 and further compressed in the compressor V2 to 20 bar and then condensed with the stream aml in the condenser W2. The bottom draw from column K3 contained less than 0.2% ammonia.

Claims

claims

1. A process for the separation of ammonia from a mixture which contains ammonia and whose boiling point in the ammonia-free state is more than 50 ° C higher than the boiling point of pure ammonia, characterized in that in a process with n stages, with n a natural number at least 2,

a) feeds this mixture to a stage 1,

c) the product mixture obtained in stage m is used in stage m + 1, with the exception of stage n, as starting material mixture in accordance with step b),

d) in stage n the ammonia at a pressure p _n and a temperature t _n partially or completely from the

Stage n-1 feed mixture separated by evaporation to obtain a product and gaseous ammonia,

where each pressure p _{m is} greater than p _{m +} ι.

2. The method according to claim 1, wherein the gaseous ammonia obtained in a stage m or in stage n is compressed.

3. The method according to claim 1, wherein the gaseous ammonia obtained in a stage m + 1 is compressed and combined with the gaseous ammonia obtained in stage m.

4. The method according to claim 1, wherein the gaseous ammonia accumulating in each stage m + 1 is compressed and combined with the gaseous ammonia accumulating in the respective stage m.

5. The method according to claims 1 to 4, wherein the gaseous ammonia is then liquefied.

6. The method according to claims 1 to 5, wherein the pressures p _m and p _{n of} the individual stages m and n are selected so that a single-stage compression takes place in the compression of the ammonia.

7. The method according to claims 1 to 6, wherein no heat is supplied to one or more of stages 2 to n.