WO1997045394A1

WO1997045394A1 - Distillation process

Info

Publication number: WO1997045394A1
Application number: PCT/CA1997/000346
Authority: WO
Inventors: Karl Tze-Tang Chuang; Zhanping Xu
Original assignee: Governors Of The University Of Alberta
Priority date: 1996-05-24
Filing date: 1997-05-22
Publication date: 1997-12-04
Also published as: AU2760597A

Abstract

Carboxylic acid in dilute aqueous solution is reacted to convert it into its ester having a boiling point different from water and is more easily separated by distillation.

Description

DISTILLATION PROCESS

Field of Invention

The present invention relates to separation of chemical compounds by distillation, more particularly, to a conversion distillation process particularly adapted to the separation of acetic acid or the like from dilute aqueous solutions.

Background of the Invention

Dilute aqueous solutions containing chemicals with a boiling point higher than that of water, such as acetic acid, acrylic acid, propionic acid or formic acid, are produced in many industrial applications. The common problems for the recovery of these chemicals involve their separation from relatively large amounts of water.

Dilute acetic acid solution is produced in many chemical and petrochemical plants involving the production or the use of acetic acid. To date, there exist five recovery processes: (1) azeotropic distillation, (2) simple distillation, (3) liquid-liquid extraction, (4) chemical treatment, and (5) adsorption. Among these processes, simple or azeotropic distillation facilities are more competitive. Since acetic acid has a higher boiling point than water, the consumption of energy (steam) is very high by conventional distillation because a large amount of water must be vaporized. Also, with the increase in acetic acid concentration downward the column, the solution becomes more and more corrosive. Therefore, cost in the manufacture and maintenance of the column is very high.

Although there are many catalytic distillation processes, none have been reported to separate from water chemicals with a boiling point higher than that of water.

Brief Description of the Present Invention It is an object of the present invention to provide a new process of separation by catalytic distillation, including reacting one of the components to be separated to produce a different composition having a lower boiling point that is distilled off during the distillation operation.

Broadly, the present invention relates to a method of distilling to separate a material having a first boiling point from a second material having a second boiling point comprising reacting in a distillation column said first material with a third material to form a fourth material having a fourth boiling point said fourth boiling point being significantly different from said second boiling point and separating said second material from said fourth material in said column.

Preferably, said fourth boiling point will be lower than the said second boiling point.

Preferably, said second material is water.

Preferably, said first material is. acetic acid and said third material is selected from the group consisting of methanol and ethanol which reacts with said first material to produce said fourth material as an acetate of said third material. Preferably, said first material is acrylic acid, and said third material is methanol which reacts with said acrylic acid to produce said fourth material as an acrylate of said third material.

Preferably, said first material is formic acid and said third material is selected from the group consisting of methanol, ethanol and n-propanol which reacts with said first material to produce said fourth material as a formate of said third material.

Preferably, said first material propionic acid and said third material is methanol which reacts with said first material to produce said fourth material as a propinate of said third material.

Preferably, a solid acid catalyst is present to expedite said reacting in said distillation column.

Preferably, said solid acid catalyst selected from the group consisting of Amberlyst 15 and Amberlyst 35 produced by Rohm and Haas, USA. Brief Description of the Drawings

Further features, objects and advantages will be evident from the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which;

Figure 1 shows a schematic diagram of removing dilute acetic acid from water by catalytic distillation. Description of the Preferred Embodiments In the system illustrated in Figure 1 a stream of an organic alcohol (third material) selected from the group consisting of methanol, ethanol or n-propanol is introduced into a distillation column adjacent to the bottom of the column and the waste water (second material) containing or contaminated with an organic acid of the group consisting of acetic acid acrylic acid formic acid or propionic acid (first material) at low concentration generally below about 10% by weight of the contaminated water (second material) adjacent to the top of the column. In the column, the acid (first material) in the presence of a suitable catalyst reacts with alcohol (third material) to form an ester and at the same time and space, the so formed ester (fourth material) is separated from water.

Finally, the ester (and excess alcohol) is distilled as top product and the cleaned water is discharged from the bottom of the column. Other solid acid catalysts, such as Amberlite IR-20, other cation ion exchange resins, acid clay, etc., can also be used for the catalysis of the reaction in this process.

Either pure methanol or aqueous methanol solution can be used for the process. Other alcohols like ethanol may also be used.

The catalytic distillation column can be divided into three zones as indicated in Figure 1 with a reaction/separation zone in the middle and top and bottom separation zone on opposite sides of the reaction/separation zone and at the top and bottom of the column respectively. The reaction/separation zone consists of separation device and reaction bed filled with solid acid catalyst.

As above described the dilute acid stream (contaminated waste stream) is introduced to the column adjacent to the top i.e. in the upper portion of the column while alcohol stream introduced in a lower portion of the column. When large excess alcohol is used, the unreacted alcohol will be separated from the ester and recycled back to the catalytic distillation process.

In the above described system a solid acid catalyst selected from the group consisting of Amberlyst 15 and Amberlyst 35 produced by Rohm and Haas, USA has been found to be effective in aiding the reaction particularly of methanol with acetic acid to produce methyl acetate.

Other solid acid catalysts, such as Amberlite IR-20 also produced by Rohm and

Haas, USA, other cation ion exchange resins, acid clay, etc., can also be used for the catalysis of the reaction in the process of separating the acid from a dilute solution in water. The alcohol may be added in the gaseous form or in aqueous solution, however if in solution the alcohol must be released from the solution to more effectively react and form the ester.

The amount of alcohol such as methanol added to the column is dependent on many factors such as column height, the purity of top and bottom products. The optimum amount of methanol is determined by computer simulation based on the design specifications. The simulation can be conducted by well known commercial simulation package, Aspen Plus (Aspen Technology, Inc., Massachusetts, USA), incorporated with the kinetics and vapor-liquid equilibrium models developed at the University of Alberta. The basic concept of the invention of reacting one of the components to be separated to convert it with or without a catalyst into a different compound having a boiling point different from the other component from which it is to be separated so that the components may more easily be separated by distillation may be applied to a number of different chemical systems. The principle for the determination of what acids can be removed and what alcohols should be added in the catalytic distillation processes is that the boiling point of resulting reaction product, ester, should be lower than that of water for easy separation and breaking reaction equilibrium. The Table 1 shows some examples of the carboxylic acids which can be removed and the alcohols which may be added. The selection of alcohols is determined by the usefulness of reaction product and the reaction rate.

Table 1

It is possible to apply the method to mixtures of acids provided the different esters formed may, if desired, be separated at different levels in the column.

The invention is particularly suited for dilute systems wherein the minor component intended to be separated by distillation has a boiling point about the same as or higher than that of the major component and the reaction converts the rninor component to a compound having a boiling point significantly lower than the major component so that the amount of energy to boil off the compound having the lower boiling point is significantly reduced.

The invention is illustrated in the following examples. In these examples the catalyst used was Amberlyst 15 produced by Rohm and Haas, USA. Example 1

A column with a diameter of 100 mm of a height of 1.5 m was used to remove acetic acid from water. Included in the column were specially designed five catalytic reaction/separation units and two separation trays. A mixture of acetic acid/water containing 5% by weight of acetic acid and methanol were fed to the column at rates of 140 g/min and 13.3 g/min respectively (the mixture adjacent to the top and the methanol adjacent to the bottom of the column). 11.6 g/min of product was withdrawn from the column top and the rest was discharged from the column bottom. The reflux ratio was 30.6. The column was operated under atmospheric pressure. Temperatures in the column ranged from 61.7 to 97.8°C and 54% by weight of acetic acid was removed and converted to methyl acetate. Example 2

With the same column as stated in Example 1, a mixture of acetic acid/water containing 2.5% by weight of acetic acid and methanol were fed to the column, as above described, at rates of 140 g/min and 7.0 g/min respectively. 5.3 g/min of product was withdrawn from the column top and the rest was discharged from the column bottom. The reflux ratio was 64.5. The column was operated under atmospheric pressure. Temperatures in the column ranged from 62.3 to 98.7°C and 51% by weight of acetic acid was removed and converted to methyl acetate. Example 3

Computer simulation was conducted with commercial process simulation software package, Aspen Plus (Aspen Technology, Inc., Massachusetts, USA), incorporated with the kinetics and vapor-liquid equilibrium models developed at the University of Alberta. The simulation results were verified with experimental results as shown in Examples 1 and 2. Then, a column with 25 reaction and/or separation stages was simulated for removal of acetic acid from water. A mixture of acetic acid/water (HAc/Water) containing 5% by weight of acetic acid (HAc) and methanol (MeOH) are fed to the column at stage 3 and stage 18 (counted from column to bottom) at rates of 10,000 kg/hr and 500 kg/hr respectively. 600 kg/hr of product is withdrawn from the column top and the rest is discharged from the column bottom. The reflux ratio is 24. The column is operated under atmospheric pressure. Temperature in the column range from 54 to 96°C and more than 90% by weight of acetic acid can be removed and converted to methyl acetate (Me Ac). The simulation results are summarized in Table II .

Table II Summary of Simulation Results

Example 4 A column with seven separation/reaction equilibrium stages was simulated for the removal of formic acid from water. A mixture of formic acid/water containing 5% by weight of formic acid and methanol are fed to the column at stage 2 and stage 6 (counted from column top) at a rate of 10,000 kg/hr and 500 kg/hr respectively. The product at a rate of 700 kg/hr is withdrawn from the column top and the rest is discharged from the column bottoms. The reflux ratio is 15. The column is operated under atmospheric pressure. Temperatures in the column range from 35 to 99°C and more than 89% by weight of formic acid can be removed and converted to methyl formate.

Having described the invention, modifications will be evident to those skilled in the art without departing from the scope of the invention as defined in the appended claims.

Claims

Claims We claim

1. A method of distilling to separate a material having a first boiling point from a second material having a second boiling point comprising reacting in a distillation column said first material with a third material to form a fourth material having a fourth boiling point said fourth boiling point being significantly different from said second boiling point and separating said second material from said fourth material in said column.

2. A method as defined in claim 1 wherein said fourth boiling point is lower than the said second boiling point.

3. A method as defined in claim 2 wherein said second material is water.

4. A method as defined in claim 3 wherein said first material is. acetic acid and said third material is selected from the group consisting of methanol and ethanol which reacts with said first material to produce said fourth material as an acetate of said third material.

5. A method as defined in claim 3 wherein said first material is acrylic acid, and said third material is methanol which reacts with said acrylic acid to produce said fourth material as an acrylate of said third material.

6. A method as defined in claim 3 wherein said first material is formic acid and said third material is selected from the group consisting of methanol, ethanol and n-propanol which reacts with said first material to produce said fourth material as a formate of said third material.

7. A method as defined in claim 3 wherein said first material propionic acid and said third material is methanol which reacts with said first material to produce said fourth material as an propionate of said third material.

8. A method as defined in any one of claims 1 to 7 inclusive wherein a solid acid catalyst is present to expedite said reacting in said distillation column.