STREAMS

Field of the invention

The present invention relates to a process of recovering acetic acid from liquid or vaporous aqueous streams by means of extractive distillation.

Background of the invention

Several oxidative chemical conversion processes known in the art produce aqueous streams comprising acetic acid as a side product. For example, it is known to oxidatively dehydrogenate ethane to produce ethylene in an oxidative dehydrogenation (oxydehydrogenation; ODH) process. The ethylene thus produced may be further oxidized under the same conditions into acetic acid. Other examples include the dehydrogenation of alcohols, the oxidation of aldehydes and the conversion (e.g. fermentation, pyrolysis, liquefaction) of biomass .

In the above process as well as in other oxidative conversion process, the acetic acid thus produced is

generally considered as a waste product. Although the acetic acid could be condensed together with water from the reactor effluent as an aqueous acetic acid (ca. 10 wt%) stream, the low relative volatility of acetic acid to water renders ordinary distillative separation of acetic acid and water troublesome, as this would require very large condensate recycle and/or separation trains.

However, acetic acid is a valuable ingredient and building block for use in the chemical industry. Acetic acid is used in the production of cellulose acetate for

photographic film and polyvinyl acetate for wood glue, as well as synthetic fibers and fabrics. In households, diluted acetic acid is often used in descaling agents. In the food industry, acetic acid is an approved food additive for use as an acidity regulator and as a condiment. The global demand for acetic acid is around 6.5 million tonnes per year (Mt/a), of which approximately 1.5 million tonnes is met by

recycling; the remainder is manufactured from petrochemical feedstock. As a chemical reagent, biological sources of acetic acid are of interest, but generally uncompetitive.

It is an objective of the present invention to provide a technically advantageous, efficient and affordable process for recovering acetic acid from vaporous and/or liquid aqueous streams.

Summary of the invention

It was surprisingly found that the above-mentioned objective can be attained by means of an extractive distillation process for recovering acetic acid from a mixture of acetic acid and water, wherein the extractive solvent is

characterized by a small Hansen solubility parameter distance R _a with respect to acetic acid, a relatively high 1- octanol/water partition coefficient logP ₀ w and a boiling point exceeding that of acetic acid.

Accordingly, in a first aspect the present invention pertains to a process for the recovery of acetic acid from an aqueous stream, comprising

providing a liquid or vaporous aqueous stream comprising acetic acid,

contacting said aqueous stream comprising acetic acid with an extractive solvent in an extractive distillation unit, to produce a first stream comprising extractive solvent and acetic acid and a second stream comprising water, feeding said first stream comprising extractive solvent and acetic acid to a solvent recovery unit, to produce a third stream comprising acetic acid and a fourth stream comprising extractive solvent,

optionally subjecting the second stream comprising water to a condensation step to allow liquid-liquid separation of entrained extractive solvent and water and optionally recycling at least part of said entrained extractive solvent to the extractive distillation unit,

and optionally recycling at least a portion of the fourth stream comprising extractive solvent to the extractive distillation unit,

wherein the extractive solvent is an oxygen-containing organic compound having

(i) a Hansen solubility parameter distance R _a with respect to acetic acid as determined at 25 °C of 15 MPa ^{1 2} or less, preferably 12 MPa ^{1 2} or less, more preferably 10 MPa ^{1 2} or less;

(ii) a 1-octanol/water partition coefficient logP ₀ w as determined at 25 °C and pH 7 of at least 0, preferably at least 0.5, more preferably at least 1.0, even more preferably at least 1.5, yet even more preferably at least 2.0, most preferably at least 3.0; and

(iii) a boiling point at atmospheric pressure that is at least 5 °C higher, preferably at least 10 °C higher, more preferably at least 20 °C higher than the boiling point of acetic acid.

The invention further relates to a process for the recovery of acetic acid from a liquid or vaporous aqueous stream, comprising

providing a liquid or vaporous aqueous stream comprising acetic acid, contacting said aqueous stream comprising acetic acid with an extractive solvent in an extractive distillation unit, to produce a first stream comprising extractive solvent and acetic acid and a second stream comprising water,

feeding said first stream comprising extractive solvent and acetic acid to a solvent recovery unit, to produce a third stream comprising acetic acid and a fourth stream comprising extractive solvent,

and optionally recycling at least a portion of the fourth stream comprising extractive solvent to the extractive distillation unit,

and optionally subjecting the second stream comprising water to a condensation step to allow liquid-liquid

separation of entrained extractive solvent and water and optionally recycling at least part of said entrained

extractive solvent to the extractive distillation unit,

wherein the extractive solvent is a cyclic or aromatic alcohol having 6 to 20 carbon atoms, a linear aliphatic alcohol having 6 to 14 carbon atoms or a branched aliphatic alcohol having 5 to 14 carbon atoms.

In another aspect, the invention relates to the use of an oxygen-containing organic compound having

(i) a Hansen solubility parameter distance R _a with respect to acetic acid as determined at 25 °C of 15 MPa ^{1 2} or less, preferably 12 MPa ^{1 2} or less, more preferably 10 MPa ^{1 2} or less;

(ii) a 1-octanol/water partition coefficient logP ₀ w as determined at 25 °C and pH 7 of at least 0, preferably at least 0.5, more preferably at least 1.0, even more preferably at least 1.5, yet even more preferably at least 2.0, most preferably at least 3.0; and

(iii) a boiling point of at least 125 °C, preferably at least 140 °C, more preferably at least 160 °C, even more preferably at least 180 °C, most preferably at least 200 °C at atmospheric pressure

as a solvent for extracting acetic acid from a water- containing vapour or liquid stream using extractive

distillation.

Brief description of the drawings

Figure 1 shows an embodiment of the present invention, wherein a stream comprising acetic acid is contacted with an extractive solvent in an extractive distillation unit, to produce a top stream comprising water vapour and a bottom stream comprising extractive solvent and acetic acid, and wherein said bottom stream comprising extractive solvent and acetic acid is fed to a solvent recovery unit to produce a top stream comprising acetic acid and a bottom stream comprising extractive solvent.

Detailed description of the invention

While the process of the present invention and the streams used in said process are described in terms of

"comprising", "containing" or "including" one or more various described steps and components, respectively, they can also "consist essentially of" or "consist of" said one or more various described steps and components, respectively.

In the process of the present invention, the term

"aqueous stream" may refer both to a water-containing stream in the liquid phase and to a water-containing stream in the vapour phase, said aqueous stream further comprising acetic acid in the liquid or vapour/gas phase, respectively. The aqueous stream comprising acetic acid may be any stream comprising at least 0.1, or at least 1 wt%, more preferably at least 3 wt%, even more preferably at least 5 wt%, yet even more preferably at least 10 wt% or 15 wt%, most preferably at least 20 wt% wt% of acetic acid. Typically, said aqueous stream comprising acetic acid originates from an oxidative chemical conversion process of ethane and/or ethylene, wherein the acetic acid is obtained as a side product. It is preferred that the aqueous feed stream of the extractive distillation process comprises acetic acid in a concentration of at least 1 wt%, more preferably at least 3 wt%, even more preferably at least 5 wt%, yet even more preferably at least 10 wt%, most preferably at least 20 wt%.

A concentration step, for example of a dilute aqueous liquid or gaseous process effluent comprising acetic acid, may be applied prior to contacting the acetic acid with the extractive solvent in the extractive distillation unit. Such concentration step may comprise any suitable method for removing excess water from an aqueous acetic acid stream, including reverse osmosis, liquid-liquid extraction,

adsorption or carboxylic acid-selective pervaporation.

In a preferred embodiment of the invention, a dilute liquid aqueous stream comprising acetic acid is subjected to liquid-liquid extraction (LLE) using an extractive solvent as defined herein to obtain a more concentrated stream

comprising acetic acid and water, which is subsequently used as the feed stream of an extractive distillation process as described herein in order to remove entrained water. In another embodiment of the invention, a gaseous or vaporous effluent comprising acetic acid is treated using carboxylic acid-selective pervaporation to produce a concentrated acetic acid/water vapour stream, which is subsequently separated using extractive distillation as described herein. In yet another embodiment, a vaporous effluent comprising acetic acid is concentrated by adsorption onto a solid, followed by desorption of a more concentrated acetic acid/water vapour stream subsequently separated using extractive distillation as described herein.

Typically, such a concentration step yields an aqueous feed stream comprising acetic acid in a concentration of at least 5 wt%, more preferably at least 10 wt%, even more preferably at least 15 wt%, most preferably at least 20 wt%.

In a particularly preferred embodiment, the aqueous stream comprising acetic acid is in the vapour phase. Such a vaporous phase stream comprising water and acetic acid may be the effluent stream from a gas-phase (oxidative) conversion process of ethane and/or ethylene. By directly subjecting the vaporous effluent comprising acetic acid and water of such process to the extractive distillation step, capital and operating expenditure on excessive condensation and reheating steps can be avoided.

In one embodiment, the aqueous stream comprising acetic acid originates from the oxidative dehydrogenation ("ODH") of ethane. This oxidative alkane dehydrogenation process typically produces a product stream comprising ethylene and carbon dioxide, as well as water and acetic acid. In another embodiment, the aqueous stream comprising acetic acid originates from the oxidation of ethylene. In the extractive distillation process of the invention, the gaseous or liquid aqueous stream comprising acetic acid is contacted with an extractive solvent in a suitable extractive distillation unit in order to separate the carbocyclic acid from water.

Extractive distillation is a distillation process wherein an extractive solvent is added in order to modify the relative volatility of the components that need to be separated, thus enabling a larger degree of separation or requiring less effort to effect the same separation. The extractive solvent is typically a high-boiling, relatively non-volatile compound. The extractive solvent typically boils at a higher temperature than any of the close-boiling components being separated. In this way the lower-boiling component of the resulting mixture is obtained at the top of the extractive distillation column and the other component along with the extractive solvent is obtained from the bottom section. Owing to the high boiling point of the extractive solvent, this bottom stream can then be separated in a secondary distillation (or rectification) column in order to provide a purified product and recover the extractive solvent. Extractive distillation should be distinguished from the best-known form of azeotropic distillation, i.e. wherein the solvent (or entrainer) forms a low-boiling azeotrope with the compound to be separated, and is thus vaporized into the top rather than collected at the bottom of the distillation column.

In the present invention, any suitable extractive distillation unit available in the art may be employed.

Typically, such extractive distillation unit comprises a tray (plate) column having an inlet for receiving an aqueous feed stream comprising the component to be separated (such as acetic acid) , wherein the extractive solvent is fed to a tray above this feed stream. The extractive solvent preferentially associates with the component to be separated, taking it down the column where it is obtained as a bottom stream, whereas the lower-boiling water component of the resulting mixture is obtained as the top distillate stream.

Generally, choice of extractive solvent is of high importance in the extractive distillation process, since suitable extractive solvents can decrease the solvent ratio and/or the liquid load of the extractive distillation unit, thus rendering an easy and more economical implementation of the extractive distillation column in a process line-up. The present inventors have now surprisingly found that certain oxygen-containing solvents being characterized by (i) a short Hansen solubility parameter distance R _a with respect to acetic acid, (ii) a partition coefficient logP ₀ w as determined at 25 ° C and pH 7 of at least 0 , and (iii) a boiling point at atmospheric pressure that is at least 5 °C higher, preferably at least 10 °C higher, more preferably at least 20 °C higher than the boiling point of acetic acid, are excellent extractive solvents for use in a process for recovering acetic acid from aqueous liquid or vapour streams comprising acetic acid.

Hansen solubility parameters (HSP) can be used as a means for predicting the likeliness of one compound (solvent) dissolving in another. More specifically, each compound is characterized by three Hansen parameters, each generally expressed in MPa ⁰ ' ⁵ : 5 _d , denoting the energy from dispersion forces between molecules; δ _ρ , denoting the energy from dipolar intermolecular forces between molecules; and 5 _h , denoting the energy from hydrogen bonds between molecules. The affinity between compounds can be described using a multidimensional vector that quantifies these solvent atomic and molecular interactions, as a Hansen solubility parameters (HSP) distance R _a which is defined in Equation ( 1 ) : (R _a ) ² = 4 ( 5 _d2 - 5 _dl ) ² + (δ _ρ2 - δ _ρ1 ) ² + ( 5 _h2 - 5 _hl ) ² ( 1 ) wherein

R _a = distance in HSP space between compound 1 and compound 2 (MPa ⁰ - ⁵ )

5 _d i , δ _ρ ι , 5 _h i = Hansen (or equivalent) parameter for compound 1 (in MPa ^0-5 )

5 _d 2, δ _ρ2 , 5h2 = Hansen (or equivalent) parameter for compound 2 (in MPa ^0-5 ) Thus, in the context of the present invention, the smaller the value for R _a for a given solvent calculated with respect to the acetic acid to be recovered (i.e., acetic acid being compound 1 and the solvent being compound 2, or vice versa), the higher the affinity of this solvent for acetic acid to be recovered will be.

Hansen solubility parameters for numerous solvents can be found in, among others, CRC Handbook of Solubility

Parameters and Other Cohesion Parameters, Second Edition by Allan F.M. Barton, CRC press 1991; Hansen Solubility

Parameters: A User's Handbook by Charles M. Hansen, CRC press 2007. It is also explained in these handbooks how analogous, equivalent solubility parameters have been derived by alternative methods to the original Hansen method, resulting in similarly useful parameters such as Hoy' s cohesion parameters for liquids .

It is preferred that the Hansen solubility parameter distance R _a with respect to acetic acid as determined at 25 °C is 12 MPa ^{1 2} or less, preferably 10 MPa ^{1 2} or less, more preferably 8 MPa ^{1 2} or less, most preferably 5 MPa ^{1 2} or less.

It was further found by the present inventors that excellent recovery of acetic acid from aqueous streams is obtained when the 1-octanol/water partition coefficient of the extractive solvent is relatively high. The 1- octanol/water partition coefficient, commonly expressed as its logarithmic value logPow^ represents the relative

concentrations of a compound when dissolved in a mixture of 1-octanol and water at equilibrium, according to the

following expression: lOgPow = "log [Coctanol/C _wa ter] (2) wherein Coctanoi = concentration of the compound in 1-octanol

Coctanoi = concentration of the compound in water

As such, in the context of the present invention, the partition coefficient is a measure for the hydrophobicity of an extractive solvent. Without wishing to be bound by theory, it is the inventors' belief that solvents having a suitably high partition coefficient are effective in minimizing the extraction of water from the acetic acid-water mixture.

Suitable extractive solvents for use as described herein have a partition coefficient logP ₀ w as determined at 25 °C and pH 7 of at least 0. Typically, the extractive solvent for use as described herein has a logP ₀ w of at least 0.5,

preferably at least 1.0, more preferably at least 1.5, even more preferably at least 2.0, yet even more preferably at least 3.0, most preferably at least 4.0.

Experimentally determined 1-octanol/water partition coefficients for several organic solvent classes are listed in, for example, James Sangster, Octanol-Water Partition Coefficients of Simple Organic Compounds, J. Phys . Chem. Ref. Data, Vol.18, No. 3, 1989. Where experimentally determined partition coefficients are not accessible, several

established reliable methods for calculating logP ₀ w values are available; these include the proprietary methods ClogP (Bio-Loom; BioByte Corp. /Pomona College) and miLogP

(Molinspiration Cheminformatics ) (see also Mannhold, M. et al. Calculation of Molecular Lipophilicity: State-of-the-Art and Comparison of Log P Methods on more than 96, 000

compounds. J. Pharm. Sci. 2009, 98, 861-893) .

In order to realize cost-effective separation (recovery) of the extractive solvent from acetic acid by e.g.

distillation, advantageously the extractive solvent has a boiling point at atmospheric pressure that is at least 5 °C higher, preferably at least 10 °C higher, more preferably at least 20 °C higher than the boiling point of acetic acid.

Thus, for the recovery of acetic acid, which has a boiling point of 117 °C at atmospheric pressure, it is preferred that the extractive solvent has a boiling point of at least 122 °C, or 125 °C. Preferably, it has a boiling point of at least 130 °C, more preferably at least 150 °C, even more preferably at least 170 °C.

From an economic perspective, it is preferred that the extractive solvent has a boiling point that does not exceed 300 °C, more preferably not exceeds 280 °C, even more preferably not exceeds 250 °C, most preferably not exceeds 225 °C, at atmospheric pressure, in order to avoid excessive heating expenditure.

Suitable oxygen-containing compounds having a Hansen solubility parameter distance R _a , partition coefficient and boiling point ranges as defined herein can be found in the classes of carboxylic acids, esters of carboxylic acids, ethers, aldehydes, ketones, alcohols and organic phosphates. These oxygen-containing component may be linear, branched or cyclic, saturated or unsaturated, and may be aliphatic or contain aromatic rings. Examples of such compounds include organic phosphates such as triethyl phosphate and tributyl phosphate, heterocyclic hydrocarbons such as benzofuran, carboxylic esters such as methyl benzoate, n-butyl butyrate, n-butyl acrylate, 2-ethylhexyl acetate, diethyl phthalate, isopropyl acetate, octyl acetate and cyclohexyl acetate, ketones such as acetophenone, dipropyl ketone and 5-ethyl-2- nonanone, high-boiling functionalized ethers such as anisole, diethylene glycol ethyl ether, diethylene glycol monobutyl ether, propylene glycol phenyl ether, 2-butoxy ethanol, 2- phenoxy ethanol and butyl diglycol acetate and carboxylic acids having at least 5 carbon atoms such as pentanoic acid, hexanoic acid, heptanoic acid and octanoic acid.

Based on the criteria as provided herein for the Hansen solubility parameter distance R _a , partition coefficient and boiling point, and taking into account the boiling point of acetic acid, the skilled person will be capable of selecting suitable extractive solvents from each of these classes of oxygen-containing organic compounds .

Particularly preferred oxygen-containing compounds having a Hansen solubility parameter distance R _a , partition

coefficient and boiling point as defined herein are selected from the class of protic oxygenates, i.e. containing hydroxyl (-OH) group such as acids and alcohols and more preferably organic alcohols. Herein, organic alcohols are understood to organic compounds wherein one or more hydroxyl functional groups (-OH) are bound to a carbon atom. This includes linear, branched and cyclic alcohols, saturated and

unsaturated alcohols, primary, secondary or tertiary

alcohols, and aromatic as well as aliphatic alcohols. The alcohol may contain one hydroxyl group, or may contain two (diol) or more (triol, etc.) hydroxyl groups, provided that any surplus of hydroxyl groups does not result in an

undesirably high affinity for water. The alcohols for use according to the invention may further contain other

functional groups, such as oxygen-containing groups such as carbonyl, acid-, ether- or ester functional groups.

Preferred alcohols for use according to the invention are cyclic or aromatic alcohols having 6 to 20 carbon atoms, linear aliphatic alcohols having 6 to 14 carbon atoms and branched aliphatic alcohols having 5 to 14 carbon atoms.

In one aspect, the invention relates to a process for the recovery of acetic acid from a liquid or vaporous aqueous stream, comprising providing a liquid or vaporous aqueous stream comprising acetic acid,

contacting said aqueous stream comprising acetic acid with an extractive solvent in an extractive distillation unit, to produce a first stream comprising extractive solvent and acetic acid and a second stream comprising water,

feeding said first stream comprising extractive solvent and acetic acid to a solvent recovery unit, to produce a third stream comprising acetic acid and a fourth stream comprising extractive solvent,

optionally subjecting the second stream comprising water to a condensation step to allow liquid-liquid separation of entrained extractive solvent and water and optionally recycling at least part of said entrained extractive solvent to the extractive distillation unit,

and optionally recycling at least a portion of the fourth stream comprising extractive solvent to the extractive distillation unit,

wherein the extractive solvent is a cyclic or aromatic alcohol having 6 to 20 carbon atoms, a linear aliphatic alcohol having 6 to 14 carbon atoms or a branched aliphatic alcohol having 5 to 14 carbon atoms.

Examples of cyclic alcohols include unsubstituted and alkyl-substituted cyclohexanols and cyclopentanols, such as cyclohexanol, methyl cyclohexanol , methyl cyclopentanol, trimethyl cyclohexanols and (4-methylcyclohexyl) methanol; examples of aromatic alcohols include phenol, benzyl alcohol,

1- phenyl ethanol, 2-phenyl ethanol, cumyl alcohol (2-phenyl-

2-propanol) , xylenols (such as 2, 6-xylen-l-ol) , guaiacol (2- methoxyfenol ) , creosol, cresols such as m-cresol, phenoxy ethanol and naphthol; examples of suitable linear alcohols include those having the general formula C _n H _n+ iOH, wherein n is in the range of 6 to 14, preferably in the range of from 8 to 12, such as 1-hexanol, 2-hexanol, 3-hexanol, 1-octanol and 2-octanol, 1-decanol, 2-decanol, 1-dodecanol and 2-dodecanol; examples of suitable branched alcohols include those having in the range of 5 to 14, preferably in the range of 6 to 12 carbon atoms, such as 2-methyl-2-pentanol, 2-methyl-3- pentanol, 3-methyl-3-pentanol, 2-methyl-l-pentanol, 2,3- dimethyl-l-butanol , 2, 2-dimethyl-l-butanol, 2, 3-dimethyl-2- butanol, 3, 3-dimethyl-2-butanol, 4-methyl-l-pentanol (iso- hexanol) , 4-methyl-2-pentanol, 2-ethyl-l-butanol , 5-methyl-2- hexanol, 3-methyl-2-hexanol, 2, 2-dimethyl-l-pentanol, 4,4- dimethyl-l-pentanol, 2-ethyl-l-hexanol (iso-octanol ) , di- isobutyl carbinol (2, 6-dimetyl-4-heptanol) , 2-propyl

heptanol, 3-methyl-l-butanol (isopentyl alcohol) , 2-methyl-l- butanol, 2-benzyloxy-ethanol , 2-phenoxy ethanol and 2-butoxy- ethanol.

Examples of alcohols containing other functional groups, such as oxygen-containing groups like aldehyde, ether- or ester groups, are diacetone alcohol and methyl salicylate. Other suitable alcohols include terpene-based alcohols such as pinacol, citronellol, menthol, and isoborneol.

Particularly preferred extractive solvents for use according to the invention are 1-hexanol, 1-octanol, 1- decanol, 1-dodecanol, 2-ethyl-hexanol, diisobutyl carbinol, cresols, xylenols, anisole, butyl butyrate and 2-ethyl-hexyl- acetate.

An overview of suitable extractive solvents for use according to the invention, including their Hansen solubility parameter distance R _a with respect to acetic acid, 1- octanol/water partition coefficient and boiling point is provided in Table 1. Table 1. Values for Hansen solubility parameter distance Ra with respect to acetic acid at 25 °C, 1-octanol/water partition coefficient at 25 °C and pH 7, and boiling point at atmospheric pressure. Values for Hansen solubility parameter distance R _a have been calculated from the known values of 5 _d , δ _ρ , and 5 _h for acetic acid (5 _d = 14.5; δ _ρ =8.0; 5 _h = 13.5; all in MPa ^0'5 ) , and of the solvent using Equation (1) as provided above. Hansen solubility parameters are taken from CRC

Handbook of Solubility Parameters and Other Cohesion

Parameters, Second Edition by Allan F.M. Barton, CRC press 1991; Hansen Solubility Parameters: A User's Handbook by Charles M. Hansen, CRC press 2007. LogP ₀ w values are taken from James Sangster, Octanol-Water Partition Coefficients of Simple Organic Compounds, J. Phys . Chem. Ref. Data, Vol.18, No. 3, 1989, from technical data sheets supplied by solvent manufacturers or calculated using miLogP software

(Molinspiration Cheminformatics ) .

R _a bp

Solvent LogPow

(MPa ^{0 5} ) (°C)

acetic acid 0 -0.13 117

2-ethylbutanol 5 1.78 124

n-butyl acetate 9 1.77 125

2-methyl-3-pentanol 1 2.98 127

3-methyl-1-butanol 2 1.14 131

1.33

iso-pentyl alcohol 4 131

(calc)

n-butyl acrylate 9 2.39 145

2-butyl 1-octanol 7 5.05 145

5-methyl-2-hexanol 2 1.97 148

iso-hexanol (4-methyl-l-

4 1.6 152

pentanol )

anisole 10 2.11 153 cyclohexanone 11 0.81 155 cyclohexanol 7 1.32 161 furfural 14 0.41 162 n-butyl butyrate 10 2.83 165

2-butoxy-ethanol 4 0.8 171 cyclohexyl acetate 9 2.29 173 benzofuran 12 2.67 174 di-isobutyl carbinol 6 3.31 178

2-octanol 4 2.90 179 iso-octanol (2-ethyl

6 2.72 180 hexanol )

phenol 7 1.46 181 pentanoic acid 5 1.39 185

1-octanol 5 3.05 195 methyl benzoate 12 2.2 199

2-ethylhexyl acetate 10 3.71 200

2 , 6-xylenol 6 2.4 201 acetophenone 14 1.58 202 cresol (m) 8 1.94 203

3.84 octyl acetate 10 203

(calc) guaiacol 4 1.34 205 benzyl alcohol 8 1.1 205 hexanoic acid 4 1.84 206 triethyl phosphate 7 1.08 215 isophorone 7 2.07 215

2-propyl heptanol 3 4.4 218

3.62 iso-decanol 3 220

(calc)

4.2

1-decanol 9 220

(calc) isopropyl acetate 6 1.28 220 2-undecanol 4 4.4 229

3.32

octanoic acid 7 237

(calc)

butyl diglycol acetate 7 1.1 238

propylene glycol phenyl

7 1.41 241

ether

4.66

1-undecanol 3 243

(calc)

1-phenoxy ethanol 7 1.1 245

2-phenoxy ethanol 7 1.2 247

5.02

2-dodecanol 5 257

(calc)

1-dodecanol 4 5.13 259

1.17

2-benzyloxy ethanol 7 265

(calc)

tributyl phosphate 10 2.5 289

The oxygen-containing solvents as defined herein are characterized by having very good selectivity, as compared to water, for acetic acid. Furthermore, these solvents have relatively high boiling points and low volatility, thus minimizing their loss as vapour in the top stream of an extractive distillation unit and allowing efficient

separation from acetic acid as the bottom stream using in a subsequent distillation (solvent recovery) step.

It is possible to combine the extractive solvent with one or more other solvents. In one embodiment, a mixture of two or more extractive solvents as defined herein are used. In another embodiment, an extractive solvent as defined herein is combined with one or more solvents selected from

carboxylic esters, ethers, aldehydes or ketones. When one or more extractive solvents as defined herein are used in admixture with another solvent not according to the invention, it is preferred that the one or more extractive solvents with Hansen solubility parameter distance R _a , partition coefficient and boiling point as defined herein are present in a concentration of at least 40 wt%, more

preferably at least 50 wt%, even more preferably at least 70 wt%, most preferably at least 80 wt% or 90 wt% based on total weight of the solvent mixture. In one embodiment, the solvent mixture contains less than 40 wt%, preferably less than 30 wt%, more preferably less than 20 wt%, even more preferably less than 10 wt% of amine. In one embodiment, the one or more extractive solvents as defined herein are used in the absence of amine compounds. In one embodiment, the extractive solvent is employed in the absence of any other solvent not according to the invention. In order to avoid loss of solvent with acetic acid, it is further preferred that if a mixture of solvents is used, that such mixture contains less than 20 wt%, more preferably less than 10 wt%, even more preferably less than 5 wt%, most preferably less than 2 wt%, based on total weight of the solvent mixture, of a solvent having a boiling that is less than 5 °C higher than the boiling point of acetic acid.

In one embodiment, the solvent mixture may comprise one or more organic alcohols as defined herein and additionally one or more of the corresponding acetate esters, which may form during extraction and/or regeneration (recovery) of the extractive alcohol solvent. If this is undesirable, these esters may at least partially be hydrolyzed, for example by feeding steam to the bottom of the (extractive) distillation column, in the extraction or solvent regeneration step.

The invention further relates to the use of an oxygen- containing organic compound as fully defined above as a solvent for extracting acetic acid from a water-containing vapour or liquid stream using extractive distillation. Depending on, among others, the concentration of acetic acid in the aqueous feed stream, the amount of extractive solvent employed in the extractive distillation process may vary within wide ranges, for example in a ratio (wt/wt) of extractive solvent to acetic acid supplied to the extractive distillation unit in the range of from 100:1 to 0.1:1, preferably in the range of from 50:1 to 0.25:1, more

preferably in the range of from 40:1 to 0.5:1, most

preferably in the range of from 10:1 to 1:1.

The temperature in the extractive distillation step may vary within wide ranges due to the selection of different mixtures of acid and solvents and operation pressures. It is within the ability of one skilled in the art to select appropriate operating temperature for a given mixture at a given pressure.

Typically, the temperature in the extractive distillation unit as described herein is in the range of of from 80 to 300 °C, more preferably 90 to 260 °C, most preferably 100 to 250 °C. The pressure in the extractive distillation unit may also vary within wide ranges. Typically, the pressure in the extractive distillation unit is in the range of of from 0.1 to 20 bar, more preferably 1 to 10 bar, most preferably 2 to 6 bar .

In one embodiment, the temperature is at most 50 °C, preferably at most 20 °C, more preferably at most 10 °C, most preferably at most 5 °C higher than the condensation

temperature of the acetic acid at operating pressure. In one embodiment, the temperature is at least 0 °C, preferably at least 10 °C, more preferably at least 20 °C, most preferably at least 30 °C above the condensation temperature of water at operating pressure.

In one embodiment, the pressure is at least 50 %, preferably at least 80 %, more preferably at least 100 %, most preferably at least 120 % of the condensation pressure of the acetic acid at operating temperature. Furthermore, the pressure is typically at most 99 %, preferably at most 90 %, more preferably at most 80 %, even more preferably at most 70 %, most preferably at most 50 % of the condensation pressure of water at operating temperature.

Advantageously, substantially all of the acetic acid present in the vaporous or liquid aqueous feed stream of the extractive distillation unit exits said extractive

distillation unit in the extractive solvent stream.

Typically, at least 90 wt%, preferably at least 95 wt%, more preferably at least 99 wt%, even more preferably at least 99.5 wt%, yet even more preferably at least 99.8 wt%, most preferably at least 99.9 wt% of acetic acid present in the feed stream of the extractive distillation unit is recovered in the extractive solvent stream of said extractive

distillation unit. Furthermore, in order to avoid the need for any further water removal steps, it is preferred that the extractive solvent entrains substantially none of the water present in the gaseous or liquid aqueous feed stream of the extractive distillation unit. Preferably, the extractive solvent effluent stream of the extractive distillation unit comprises water and acetic acid in a ratio of less than 1:1, more preferably less than 0.5:1, even more preferably less than 0.1:1, yet even more preferably less than 0.05:1, most preferably less than 0.01:1 or about zero.

In the solvent recovery unit, acetic acid is removed from the extractive solvent resulting in a product stream

comprising acetic acid and another stream comprising the extractive solvent now depleted of acetic acid.

In the solvent recovery unit, recovery of the extractive solvent, and of optional other solvents present, is typically effectuated by distilling the effluent stream of the extractive distillation unit comprising acetic acid and extractive solvent, resulting in a top stream comprising acetic acid and a bottom stream comprising the extractive solvent. Distillation may be carried out in any distillation unit known to the skilled that is suitable for separating extractive solvent from acetic acid, and it is within the ability of one skilled in the art to select appropriate operating conditions for obtaining a desired degree of product purity and/or solvent recovery.

Typically, the temperature in the solvent recovery unit would vary depending on the solvent/mixture of solvents selected and is in the range of of from 80 to 300 °C, more preferably 100 to 250 °C, most preferably 110 to 200 °C. The pressure in the solvent recovery unit is suitably in the range of of from 0.1 to 10 bar, more preferably 0.5 to 5 bar, most preferably 1 to 3 bar.

In one embodiment, the temperature in the solvent recovery unit is at least 0 °C, preferably at least 10 °C, more preferably at least 20 °C, most preferably at least 30 °C above the condensation temperature of acetic acid at operating pressure. In one embodiment, the temperature in the solvent recovery unit is at most 20 °C, preferably at most 10 °C, more preferably at most 5 °C, most preferably at most 0 °C below the condensation temperature of the extractive solvent at operating pressure.

Typically, the pressure is at least at least 100 %, more preferably at least 110 %, even more preferably at least 120 %, most preferably at least 130 % of the condensation pressure of the extractive solvent at operating temperature. Typically, the pressure is at most 100 %, preferably at most 90 %, more preferably at most 80 %, even more preferably at most 70 %, most preferably at most 50 % of the condensation pressure of acetic acid at operating temperature. In one embodiment, steam is fed at the bottom of the solvent regeneration unit to hydrolyze any esters that may have been formed in the acetic acid/solvent mixture.

It is preferred that at least 80 wt%, more preferably at least 90 wt%, even more preferably at least 95 wt%, yet even more preferably at least 98 wt% of the acetic acid present in the stream fed to the solvent recovery unit comprising acetic acid and extractive solvent is recovered.

It is further preferred that at least 80 wt%, more preferably at least 90 wt%, even more preferably at least 95 wt%, yet even more preferably at least 98 wt% of the solvent present in the stream fed to the solvent recovery unit comprising acetic acid and extractive solvent is recovered..

Typically, the acetic acid product stream of the solvent recovery unit comprises acetic acid in a concentration of at least 70 wt%, preferably at least 80 wt%, more preferably at least 90 wt%, more preferably at least 95 wt%, even more preferably at least 99 wt%, yet even more preferably at least 99.5 wt%, most preferably at least 99.9 wt%.

Based on the amount of acetic acid present in the aqueous stream provided to the extractive distillation unit, at least 50 wt%, more preferably at least 75 wt%, even more preferably at least 90 wt%, yet even more preferably at least 95 wt%, most preferably at least 99 wt% of acetic acid is recovered in the process as defined herein.

In a preferred embodiment, at least a portion of the stream of the solvent recovery unit comprising the extractive solvent, typically the bottom stream of a distillation unit, is recirculated to the extractive distillation unit.

Typically, at least 20 wt%, preferably at least 50 wt%, more preferably at least 70 wt%, most preferably at least 90 wt% of the recovered solvent stream is recirculated to the extractive distillation unit. In one embodiment, the entire bottom stream comprising the extractive solvent is

recirculated to the extractive distillation unit.

In the extractive distillation column typically a top stream comprising or substantially consisting of water vapour, and optionally other gases lighter than water, is produced. Water may be recovered from this top stream using a condensation step, for example by cooling down the top stream of the extractive distillation unit to a lower temperature, for example room temperature, so that the water can be recovered as a liquid stream.

The water vapour top stream of the extractive

distillation unit may further comprise entrained extractive solvent. Typically, said top stream of the extractive distillation unit comprises no more than 3 vol%, preferably at most 1 vol%, more preferably at most 0.3, even more preferably at most 0.1, most preferably at most 0.01 vol% of entrained extractive solvent. Said entrained extractive solvent may be recovered by liquid-liquid separation from the liquid water formed in the aforementioned condensation step. Advantageously, such liquid-liquid separation occurs

spontaneously upon condensation due to the preferred poor miscibility of water and the extractive solvent. In a preferred embodiment, the extractive solvent thus recovered is at least partially recirculated to the extractive

distillation unit either as a separate stream or by mixing with a recirculated extractive solvent stream from the solvent recovery unit .

The top stream comprising acetic acid originating from the solvent recovery unit may be further treated downstream, for example to further remove water by (azeotropic)

distillation, pervaporation, etc., and/or other purification methods available in the art to obtain the purity and specifications for acetic acid products according to market requirements .

Detailed description of the drawing

In Figure 1, a stream 1 comprising water and acetic acid is fed to an extractive distillation column 2 equipped with reboiler section 4 and condenser section 4a to which further an extractive solvent 3 is fed. Acetic acid is extracted by the extractive solvent, which exits the extractive

distillation column as "fat" solvent stream 5. A vapour stream comprising water and other gaseous compounds exits the extractive distillation column as stream 7.

Stream 5 comprising extractive solvent and extracted acetic acid is fed supplied to a solvent regeneration

(recovery) unit, comprising a distillation unit 6 equipped with condenser section 8a and reboiler section 8. Acetic acid leaves distillation unit 6 as top stream 9, while extractive solvent now depleted of acetic acid exits distillation unit 6 as bottom stream 10. The acetic acid-depleted extractive solvent stream 10 may be partially recirculated to extractive distillation column 2 as extractive solvent recirculation stream 11. Acetic acid stream 9 may be further purified downstream .

The vapour stream 7 comprising water and other gaseous compounds obtained as a top stream from extractive

distillation column 2 is fed to a condensation unit 12, where water is removed via stream 13. A product stream comprising gaseous compounds is removed via stream 16, from where it may undergo further separation and/or purification further downstream .

In condensation unit 12, spontaneous separation from the condensed water of extractive solvent entrained in vapour stream 7 originating from extractive distillation column 2 may occur. This separated extractive solvent stream 14 may least partially be recirculated to extractive distillation column 2 via recirculation stream 15.

The invention is further illustrated by the following Examples .

EXAMPLES

General: Modeling of process thermodynamics

Modeling of the thermodynamics of the carboxylic acid separation and solvent recovery processes was performed using a Shell proprietary PSRK (Predictive Soave-Redlich-Kwong) - UNIFAC (UNIQUAC Functional-group Activity Coefficients) method which uses a modified Soave-Redlich-Kwong (SRK) cubic equation of state as the basis for both gas/vapour and liquid phases, connected to the UNIFAC group contribution activity coefficient model through an excess free energy mixing rule. The activity coefficient model is used to adapt the equation of state parameters and calculate the excess properties of vapour and liquid phases. The PSRK-UNIFAC framework is integrated in the Aspen Plus (AspenTech) process simulation software used for chemical process optimization, while parameter libraries are Shell proprietary.

Example 1. Vapour-phase extractive distillation of acetic acid

An aqueous vapour-phase effluent stream comprising 25.96 wt% water, 2.89 wt% acetic acid (10 wt% on gas-free basis), 33.28 wt% of ethylene and 37.87 of other compounds (including ethane, C0 ₂ , CO, 0 ₂ ) is supplied to an extractive

distillation column in which the acetic acid is extracted from the vapour phase by 1-decanol as the extractive solvent. The volume ratio of solvent to vaporous effluent is 2.04x10 ^"

3

The conditions of the extractive distillation column are as follows : Extractive distillation column pressure 3

(bar)

Number of stages (including reboiler) 15

Feed stage for aqueous acetic acid stream 10

Aqueous acetic acid feed temperature (°C) 120

Feed stage for solvent 1

Solvent feed temperature (°C) 50

Solvent purge ratio (%) 3

The top vapour phase of the extractive distillation unit is condensed in a heat exchanger and sent to a vapour-liquid- liquid decanter. The vapour phase containing the ethylene product and other gases unconverted or co-produced in the oxidative dehydrogenation reactor are supplied to an ethylene purification unit. A high purity (about 99.9 wt%) water phase is obtained at the vapour-liquid-liquid decanter and sent to a water treatment unit and the small solvent phase is refluxed back to the extractive distillation column.

At the bottom of the extractive distillation column an acetic acid and extractive solvent mixture ("fat solvent") is withdrawn, which is subsequently supplied to a solvent recovery column, wherein acetic acid is obtained at the top. An acetic acid-depleted ("lean solvent") stream is withdrawn at the bottom of the solvent recovery column, which is subsequently cooled and recycled to the extractive

distillation column. A minor solvent purge is performed in order to minimize solvent losses while avoiding potential build-up of impurities .

The composition of the various streams produced in this process is as follows: Product streams

Stream Acetic acid Gaseous Aqueous Solvent

[solvent [VLL [VLL [purge] recovery] decanter] decanter] (wt %)

( t%) ( t %) ( t %)

Water 0.09 2.06 99.87 0.00

Acetic acid 99.9 0.00 0.11 0.05

Propionic 0.01 0.00 0.01 0.01

acid

Decanol 0.00 0.02 0.00 99.94

Other 0.06 97.91 0.02 0.00

components

99 % of the acetic acid present in the vaporous aqueous effluent is recovered as a pure (> 99 wt%) acetic acid stream .

Example 2. Vapour-phase extractive distillation of acetic acid

The process of Example 1 is repeated, with the

distinction that an aqueous vapour-phase effluent stream comprising 24.47 wt% water, 8.25 wt% acetic acid (25 wt% on gas-free basis), 28.02 wt% of ethylene and 39.26 wt% of other compounds (including ethane, C0 ₂ , CO, 0 ₂ ) is supplied to the extractive distillation column. The volume ratio of solvent to vaporous effluent is 3.85xl0 ^~3 .

The composition of the various streams produced in this process is as follows: Product streams

Stream Acetic acid Gaseous Aqueous Solvent

[solvent [VLL [VLL [purge] recovery] decanter] decanter] (wt %)

( t%) ( t %) ( t %)

Water 0.03 2.06 99.65 0.00

Acetic acid 99.9 0.00 0.33 0.09

Propionic 0.022 0.00 0.00 0.01

acid

Decanol 0.05 0.02 0.00 99.91

Other 0.00 97.92 0.02 0.00

components

99 % of the acetic acid present in the aqueous effluent stream is recovered as a pure (> 99 wt%) acetic acid stream.