MXPA01000748A - Method for distilling ammonia - Google Patents

Abstract

The invention concerns an improved method for distilling ammonia from a mixture, more particularly a mixture derived from the reaction between an aminonitrile and water (reaction also referred to as cyclizing hydrolysis). More precisely, the invention concerns a method for distilling ammonia contained in an aqueous caprolactam solution, characterised in that it consists in distilling by means of a column, having a base temperature not more than 160°C and under absolute pressure not more than 5 bar, the distilled ammonia at the column top being compressed under pressure not less than 10 bar, then condensed at a temperature ranging from 25°C to 60°C.

Description

PROCESS TO DISTILATE AMMONIA DESCRIPTION OF THE INVENTION The present invention relates to an improved process for distilling ammonia from a mixture, more particularly from a mixture resulting from the reaction of an aminonitrile and water (this reaction is called cyclization or cyclization hydrolysis). During the cyclizing hydrolysis of an aminonitrile in order to form a lactam, an ammonia molecule is also formed per lactam olecule. At the end of the reaction, the reaction mixture therefore contains at least one lactam produced, the excess being water and ammonia. This may contain minor amounts of aminonitrile that has not reacted, or possible by-products of the reaction. This may also comprise a solvent possibly used in the cyclizing hydrolysis reaction. For the description of the various processes for the preparation of a lactam by means of hydrolysis of an aminonitrile, reference can be made, for example, to European Patent EP-A-0,659,741 or to International Patent Application WO-A-96/22974 or to International Patent Applications WO-A-95/14664 and 95 / 14665. Cyclizing hydrolysis can be carried out in the vapor phase or in the liquid phase. The process of the invention can be applied to mixtures resulting from one or other of these preparation methods. Among the solutions resulting from the cyclizing hydrolysis of an aminonitrile in a lactam, those corresponding to the preparation of caprolactam from 6-aminocapronitrile and water, are the most important from an industrial point of view, since said caprolactam, after polymerization, it leads to nylon 6. The process of the invention will therefore more specifically refer to the distillation of ammonia from aqueous solutions of caprolactam, but can also be extrapolated to aqueous solutions of other lactams. When it is desired to separate the dissolved ammonia in an aqueous solution of caprolactam, it is necessary to prevent the degradation of the caprolactam as much as possible. This means that, during the In the case of distillation, the temperature at the bottom of the column will preferably be maintained at a value less than or equal to 160 ° C and more preferably less than or equal to 145 ° C. This constraint means that the absolute pressure must be limited to a value less than or equal to 5 bar and preferably less than or equal to 3.5 bar. As a consequence of the above, the temperature of the ammonia leaving the top of the column is, depending on the previous absolute pressure limit, adopted, less than or equal to + 5 ° C and preferably less than or equal to -6 ° C. . In an industrial process, the effective condensation of the ammonia in the upper part of the column requires a cooling unit of considerable power. For example, a refrigeration unit of 300 kw will be necessary to distill approximately 1200 kilograms / hour of ammonia. The object of the present invention is a process for the separation, by distillation, of the ammonia contained in an aqueous solution of caprolactam, by maintaining the temperature of said solution at the bottom of the column, at a value less than or equal to 160 °. C and preferably less than or equal to 145 ° C, under conditions that make it possible to reduce the consumption of necessary energy, especially electricity, for the same yield of distilled ammonia. More specifically, the invention consists of a process for distilling the ammonia contained in an aqueous solution of caprolactam, characterized in that the distillation is carried out using a column with a bottom temperature less than or equal to 160 ° C and at a lower absolute pressure that or equal to 5 bar, the distilled ammonia in the upper part of the column is compressed to a pressure greater than or equal to 10 bar and then condensed at a temperature of 25 ° C to -60 ° C. Preferably, the temperature at the bottom of the column is less than or equal to 145 ° C and the absolute pressure is less than or equal to 3.5 bar. The distilled ammonia in the upper part of the column is preferably compressed at a pressure between 10 bar and 25 bar. After condensation, the liquid ammonia obtained at a pressure of 10 to 25 bar is expanded to the pressure of the column in a liquid / vapor separator. During the expansion, the ammonia vaporizes partially and cools, and the steam generated is sent to the top of the distillation column, and then recompressed by the compressor at the top of the column. The liquid recovered in the liquid / vapor separator is pumped and then part of it is returned to the top of the column as reflux, the other part, which constitutes the pure ammonia recovered, is sent as usable product. This solution does not require the presence of an external refrigeration unit to condense the distilled ammonia. The energy needed for compression is of the order of approximately 70% of the energy that would be consumed by the refrigeration unit. In addition, the investment needed for the process is less because the way in which currents flow is simplified. The aqueous caprolactam solution, which contains ammonia, has a concentration of caprolactam and an ammonia concentration that vary widely depending on the conditions under which caprolactam is prepared, especially the initial molar ratio of water / aminonitrile and if present or not an organic solvent. The concentration of caprolactam it may in general vary from 5% to 80%, and preferably from 20% to 75%, by weight with respect to the total weight of the solution. The concentration of ammonia can vary in general from 0.85% to 15%, and preferably from, 3.5% to 14%, by weight with respect to the total weight of the solution. Since the ammonia to be removed has physical characteristics that are very different from the other compounds in the aqueous solution that contains it, it is not necessary to use a column that has a large number of theoretical plates. The packed columns having approximately ten theoretical plates are very suitable for implementing the process of the invention. However, this indicative value does not constitute an absolute limit in the context of the invention. Higher performance columns only have a higher investment cost, while less effective columns can still be used, although they are not preferred. The packaging used can be of the loose packing type or ordered packing, as proposed by the various manufacturers and adjusted to size according to the rules of the technique. It is also possible to use columns with plates, although this is less advantageous from the point of view of head losses. The air steam compressor may, depending on the applications and the processing capabilities, be of the positive displacement compressor type such as, for example, an alternating motion compressor or a screw compressor, or of the centrifugal type, without These technologies are exhaustive with respect to the previously mentioned process. The following example illustrates the invention. This example and the comparative test correspond to a caprolactam production of 8 t / hour.

EXAMPLE 1: Distillation with air vapor compression Distillation of a product having the following composition: ammonia: 9% by weight water: 30% by weight - caprolactam: 61% by weight.

The characteristics of the distillation column are as follows: - - number of distillation stages: 10 reflux ratio: 0.30 - absolute operating pressure in the upper part of the column: 3.5 bar. upper temperature: 35 ° C bottom temperature: 145 ° C heater heat load: 1.1 Gcal / hour air compressor absolute distribution pressure: 14 bar energy consumed by the air compressor: 160 kw - heat load of the condenser in the Compressor supply: 0.7 Gcal / hour.

The caprolactam solution that was obtained after the distillation of the ammonia has the following composition: ammonia: 0.01% by weight water: 33% by weight caprolactam: 67% by weight.

COMPARATIVE TEST 1: Conventional distillation using a refrigeration unit at the top of the column The distillation of a product having the following composition: ammonia: 9% by weight water: 30% by weight caprolactam: 61% by weight The characteristics of the distillation column are as follows: number of distillation stages: 10 reflux ratio : 0.35 absolute operating pressure at the top of the column: 3.5 bar top temperature: -9 ° C bottom temperature: 145 ° C heater heat load: 1.1 Gcal / hour - condenser heat load: 0.5 Gcal / hour energy consumed by the refrigeration unit: 270 kw, with condensation at 35 ° C.

The caprolactam solution that was obtained after the distillation of the ammonia has the following composition: ammonia: 0.01% by weight - water: 33% by weight caprolactam: 67% by weight.

The performance of the distillation in Example 1 is strictly identical, in terms of the separation efficiency of the ammonia, to that in the comparative test. The energy consumption to achieve these results is markedly lower in the process according to the invention.

Claims (6)

1. A process for separating, by distillation, the ammonia contained in an aqueous solution of caprolactam, characterized the process because the distillation is carried out using a column with a bottom temperature less than or equal to 160 ° C and a lower absolute pressure than or equal to 5 bar, the distilled ammonia in the upper part of the column is compressed to an absolute pressure greater than or equal to 10 bar and then condensed at a temperature of 25 ° C to 60 ° C.

2. The process according to claim 1, characterized in that the temperature at the bottom of the column is less than or equal to 145 ° C and the absolute pressure is less than or equal to 3.5 bar.

3. The process according to any of claims 1 and 2, characterized in that the ammonia distilled in the upper part of the column is compressed to an absolute pressure of between 10 bar and 25 bar.

4. The process according to any of claims 1 to 3, characterized in that the pressure of the distilled ammonia in the upper part of the column is increased by means of a compressor of the positive displacement type or of the centrifugal type.

5. The process according to any of claims 1 to 4, characterized in that the aqueous solution used contains from 5% to 80% by weight of the caprolactam with respect to the total weight of the solution and from 0.85% to 15% by weight of ammonia. eon with respect to the total weight of the solution.

6. The process according to any of claims 1 to 5, characterized in that the aqueous solution used contains from 20% to 75% by weight of the caprolactam with respect to the total weight of the solution, and from 3.5% to 14% by weight of the ammonia with respect to the total weight of the solution.