WO2013168113A1

WO2013168113A1 - Method for preparing alkali metal alkoxides

Info

Publication number: WO2013168113A1
Application number: PCT/IB2013/053730
Authority: WO
Inventors: Clarisa MOCCIARO; Carlos H. MENDIONDO; Gabriel I. HOROWITZ; Rodolfo R. CARLEVARIS; Horacio ABADIAS
Original assignee: Ypf Sociedad Anonima; Invap S.E.
Priority date: 2012-05-10
Filing date: 2013-05-08
Publication date: 2013-11-14
Also published as: AR093211A1

Abstract

The present invention relates to a process for preparing solutions of alkali metal alkoxides, using a reactive distillation column and an alcohol dehydration column. The process is characterized by the use of a thermocompressor, which allows a considerable saving of energy and operative cost in the process, and optionally an alkaline mist separator located at the top of the reactive distillation column, which avoids contamination in the process.

Description

METHOD FOR PREPARING ALKALI METAL ALKOXIDES

FIELD OF THE INVENTION

The present invention relates to the preparation of alkali metal alkoxides starting from alcohols and hydroxides of corresponding alkali metals.

BAKGROUND OF THE INVENTION

The term "metal alkoxide" is used to designate chemical compounds in which a metal is bonded to one or more alkyl groups by an oxygen atom. In particular "alkali metal alkoxide" refers to a metal alkoxide where the metal is a metal of the group of alkali metals.

Metal alkoxides may be used for a variety of applications. They are useful for obtaining metals in organic solutions, for example in a homogeneous catalytic reaction. Compared to other inorganic metal compounds such as metal carboxylates , alcoxides are advantageous due to their catalytic properties, ease of hydrolysis, solubility in organic solvents and are easily distillable. They are used as catalysts (e.g., Ziegler-Natta polymerizations, transesterifications and condensation) for partial or total hydrolysis, alcoholysis, transesterification, in coatings for plastics, textiles, glass and metals, and additives for adhesives and paints, sol-gel applications, synthesis of minerals capable of containing radioactive waste and for crosslinking or hardening of natural or synthetic materials. In particular, alkali metal alkoxides, hereinafter called simply "alkoxides" are mainly used in the synthesis of numerous organic compounds, where they act as strong bases.

Alkali metal alkoxides may be obtained by various processes. In particular, they can be produced from the reaction between an alkali metal hydroxide (MOH) and an alcohol (ROH) . Said reaction can be written

MOH + ROH <→ MOR + H₂0 wherein MOR represents the alkali metal alkoxide.

This reaction is usually an equilibrium reaction, whereby the produced water must be removed by physical or chemical processes so as to displace the equilibrium reaction towards the production of products .

For this reason, most industrial processes for obtaining alkoxides combine the chemical reaction with various separation technologies. One of the processes most commonly used is the reactive distillation, in which the alkali metal hydroxide is contacted countercurrently with the alcohol in a separation column. The mixture of alcohol vapor and water vapor must be separated in a dehydration process.

The basic principles of this process are described for example in U.S. Patent 2,877,274. This patent describes a method for preparing an alkali metal methoxide in a distillation column, using only a metal hydroxide aqueous solution and methanol gas in a continuous distillation process . Other processes for preparing alkali alkoxides are described in other patent documents such as U.S. Patent 1,910,331 which discloses the reaction of an alkali hydroxide in alcoholic solution with an alcohol stream in vapor phase, and U.S. Patent 4,895,989 which describes separating the alcohol-water mixture through a pervaporation process with a membrane, after the formation reaction of the alkali alkoxide.

In U.S. patent 6, 759, 560 B2 it is disclosed a method of obtaining alkali metal methoxides from alkali metal hydroxides and methanol, by means of a reactive distillation step and a fractionation step, which can be carried out in two separate columns or in a single outer column wall in which the lower region of the column is divided by a vertical dividing wall installing between column walls In the fractionation step the mixture of alcohol and water vapor leaving the top of the column reactor is fed to a dehydration column. The alcohol obtained is recycled to the reactive distillation column. In this configuration, the reaction column operates without reflux and overhead vapors obtained are brought to the fractionating column using a partial condenser and a mechanical vapor compressor.

U.S. patent 7,847,133 B2, equivalent to patent application AR 066773 Al describes a method of preparing alkali metal alkoxides by means of a reaction step and a fractionation step in two separate columns. In the reaction column, the top stream is split and a fraction thereof is condensed and returned as reflux to the column, while the other fraction is fed to a fractionation column using a mechanical vapor compressor . It is known that compression, when compared to condensation, of the alcohol vapor which leaves the reactive distillation column (to be sent to the rectification column alcohol) significantly reduces the power consumption of the process.

It is also known that in industrial practice, the use of mechanical compressors for these applications increases the capital and maintenance costs, because of the complex large rotating machinery involved, that show a relatively short mean time between failures due to normal wear.

Moreover, it is known that the alcohol vapor leaving the last plate of the reactive distillation column carries large amounts of alkaline mist consisting of mixtures of alcohol, water, alkali hydroxides and alkoxides. These mists, formed by droplets, carry pollutants into the rest of the plant and can compromise the performance of other equipment.

There is thus a need to improve the process of obtaining metal alkoxides, reducing overall energy consumption, and eliminating alkaline droplets of mist. Furthermore, the use of equipment with no moving parts instead of mechanical compressors can significantly reduce the capital and maintenance costs of the plant.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention a method for preparing alkali metal alkoxides comprising:

• a reaction step, carried out in a reactive distillation column, where an aqueous solution of metal hydroxide and an alcohol are contacted in countercurrent, which operates without reflux and without condensation of the overhead vapors . A mixture of water vapor and unreacted alcohol is obtained at the top of the column. The metal alkoxide solution is obtained at the bottom of the column;

• a separation step, carried out in a fractionation column in which water is separated from the vapors containing the alkoxide and the unreacted alcohol, where said vapors obtained in the reactive distillation column are fed to the fractionation column using a thermocompressor; and

• a step of recycling of alcohol from the fractionation column to the reactive distillation column.

The present invention describes the use of thermocompressors instead of mechanical compressors. The thermocompressors utilize the principle of drag produced by a jet of high pressure steam, which compresses the alcohol vapor stream. This process is carried out in equipment called ejectors, involving very simple construction with no moving parts. The use of these devices dramatically reduces capital costs, shutdowns and maintenance costs .

This kind of equipment are based on fully mature technology with decades of experience in industrial applications, giving them a uniquely high degree of reliability. They need minimal maintenance, reduced to the mere change of the steam nozzle after ten or more years of operation. This change does not require skilled labor as is the case for complex machines like mechanical compressors.

The use of steam as the "motive fluid" produces an increased concentration of water vapor in the vapor stream of water and alcohol which is sent to the rectification column. This increase in the concentration of water has no adverse effect on the performance of the alcohol rectification column, due to the high purity required in its top and bottom ends .

In a preferred embodiment of the method of the present invention, the use of thermocompression reduces power consumption in a range between 30% and 60% compared to processes where the alcohol vapor is condensed before being sent by pumps to the column rectification of alcohol. This is due to a reduced consumption of steam by the reboiler of the fractionating column and the elimination of the need to use a condenser for overhead vapors from the reactive distillation column, which in turn reduces the corresponding consumption of cooling water, and the corresponding costs.

In another preferred embodiment of the present invention, an alkaline mist separator located inside the reactive distillation column, between the last plate and the vapor outlet, is used. The use of alkaline mist separator is aimed to retain with high efficiency droplets from the last plate of the reactive distillation column, thereby preventing them from being dragged out to other equipment within the plant. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a process flow diagram of the method of the present invention.

Figure 2 shows a process flow diagram of a method of the traditional configuration.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in greater detail with reference to the accompanying figures, which illustrate a preferred embodiment of the invention.

Figure 1 shows a process in which an aqueous solution of alkali metal hydroxides fed through the pipe (8) is previously heated in the equipment (9) and introduced through the pipe (10) to the top of the reactive distillation column (12), which carries an alkaline mist separator (11) near its upper end.

An aqueous solution of alkali metal alkoxide (14) is obtained from the bottom, and a stream (18) is obtained from the top of the reactive distillation column (12), being stream 18 a mixture of water vapor and unreacted alcohol.

The vapors contained in stream (18) leaving the column (12) are compressed by means of a thermocompressor (1) which operates by means of a jet of steam at high pressure. This compression process allows the introduction of the vapors in the alcohol rectification column (2) through the stream (19), in order to remove process water and purify the alcohol that is recycled to the reactive distillation column (12) . A water stream (3) is obtained from the bottom of the rectification column (2), which stream is separated from the unreacted alcohol. Said purified alcohol is obtained from the top of the column in stream (7), which is recycled to the reactive distillation column. Part of this stream from the top, stream (6), is condensed and recycled to the tower.

Reactive distillation column (12) operates at a pressure lower than that of the alcohol rectification column (2) .

Columns (12) and (2) are equipped at their bottom ends with reboilers (16) and (4) respectively. A stream (17) is obtained from the reboiler (16) of column (12) which stream is reintroduced into column (12) . A stream (5) is obtained from the reboiler (4) which stream is introduced into column (2) .

Column (2) is also equipped with a partial condenser which allows recirculation of part of the alcohol as a liquid and sends the rest in vapor form to the reactive distillation column ( 12 ) .

Fresh alcohol can be introduced upstream of the reboiler (16), through the stream (15a) or in the recycle stream of column 2 through the stream (15b) .

Example

The following is an example in which steam consumption is compared using two different plant configurations. One of the configurations corresponds to a preferred embodiment of the invention and the other to the traditional configuration. On one hand, a traditional configuration, which is described in U.S. patent 2, 877, 274, is shown in Figure 2. In this configuration the reactive column (12) top vapor condenses, and this stream is fed in liquid state to the dehydration column ( 2 ) .

On the other hand, a plant corresponding to the present invention in which a thermocompressor (1) is used, is shown in Figure 1, so that steam from top of column (12) is recompressed and injected in gaseous state to the dehydration column ( 2 ) .

In both cases components available on the market are used in the systems. In particular, the thermocompressor of the plant schematized in Figure 1 consists of a steam ejector with commercial characteristics .

In this example the steam consumption is evaluated in both configurations, when similar amounts of reagents are fed, and the same amount of alkoxide is obtained as a product.

The results of this evaluation are shown in Table 1.

Table 1

From this example it can be seen that for the above mentioned conditions, using a thermocompressor between reactive and dehydration columns (Figure 1) gives a steam consumption saving of 43% with regard to an installation of traditional configuration (Figure 2) . This steam consumption saving represents a significant energy saving in the overall process .

Claims

1. A method for preparing alkali metal alkoxides, from an aqueous solution of an alkali metal hydroxide and an alcohol, which comprises the following steps:

• reactive distillation of the alcohol and the alkali metal hydroxide in a distillation column wherein said column is operated without reflux, and in which the overhead vapors are introduced into a thermocompressor to be fed to a fractionation step, and

• fractionation on a second distillation column further comprising recycling a fraction of the overhead vapors to the reactive distillation column.

2. Method according to claim 1, wherein the thermocompressor comprises an ejector where water steam is the motive fluid.

3. Method according to any one of claims 1 and 2, wherein the reactive distillation column comprises an alkaline mist separator .

4. Method according to claim 3, wherein the alkaline mist separator is located at the top of the reactive distillation column.