WO2017114829A1

WO2017114829A1 - Carboxylic acid absorption from vaporous aqueous streams

Info

Publication number: WO2017114829A1
Application number: PCT/EP2016/082706
Authority: WO
Inventors: Kai Jürgen FISCHER; Jean Paul Andre Marie Joseph Ghislain LANGE; Guus VAN ROSSUM; Ronald Jan Schoonebeek; Carmelo PEREZ GOLF
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Oil Company
Priority date: 2015-12-29
Filing date: 2016-12-27
Publication date: 2017-07-06

Abstract

The invention provides a process for the recovery of C₃-C₆ carboxylic acid from a vaporous aqueous stream, comprising providing a vaporous aqueous stream comprising C₃-C₆ carboxylic acid, contacting said vaporous aqueous stream comprising C₃-C₆ carboxylic acid with an absorbing solvent in an absorption unit, to produce a first stream comprising absorbing solvent and C₃-C₆ carboxylic acid and a second stream comprising water vapour, feeding said first stream comprising absorbing solvent and C₃-C₆ carboxylic acid to a solvent recovery unit, to produce a third stream comprising C₃-C₆ carboxylic acid and a fourth stream comprising absorbing solvent, and optionally recycling at least a portion of the fourth stream comprising absorbing solvent to the absorption unit, wherein the absorbing solvent is an oxygen-containing organic compound having (i) a Hansen solubility parameter distance R_a with respect to C₃-C₆ carboxylic 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_OW 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 the C₃-C₆ carboxylic acid.

Description

CARBOXYLIC ACID ABSORPTION FROM VAPOROUS AQUEOUS STREAMS

Field of the invention

The present invention relates to a process of recovering C C6 carboxylic acids from vaporous aqueous streams by means absorption .

Background of the invention

Several oxidative chemical conversion processes known in the art produce aqueous streams comprising carboxylic acids as a side product. For example, it is known to oxidatively dehydrogenate alkanes having 3 to 6 carbon atoms ("C₃-C₆ alkanes") such as propane or butane resulting in propylene and butylene, respectively, in an oxidative dehydrogenation (oxydehydrogenation; ODH) process. The dehydrogenated equivalent of the alkane may be further oxidized under the same conditions into the corresponding carboxylic acid, which may or may not contain one or more unsaturated double carbon- carbon bonds, such as acrylic acid and methacrylic acid.

Other examples include the dehydrogenation of alcohols, the oxidation of aldehydes and the conversion (fermentation, pyrolysis, liquefaction) of biomass.

In the above process as well as in other oxidative conversion process, the carboxylic acids thus produced are generally considered as waste products. Although they could be condensed together with water from the reactor effluent as an aqueous carboxylic acid (ca. 10 wt%) stream, the low relative volatility of carboxylic acids to water renders ordinary distillative separation of carboxylic acid and water troublesome, as this would require very large condensate recycle and/or separation trains. However, C₃-C₆ carboxylic acids are valuable ingredients and building blocks for use in the chemical industry. For example, the global demand for acrylic acid is around 5 million tonnes per year (Mt/a), with applications as

superabsorbent in e.g. incontinence and personal care products, in surface coatings, adhesives and sealants, in textiles, in the water treatment industry, in mineral processing and numerous other applications in the form of acrylate esters.

It is an objective of the present invention to provide a technically advantageous, efficient and affordable process for recovering C3-C6 carboxylic acids from vaporous aqueous streams .

Summary of the invention

It was surprisingly found that the above-mentioned objective can be attained by means of an absorption process for recovering C₃-C₆ carboxylic acid from a mixture of C₃-C₆ carboxylic acid and water, wherein the absorbing solvent is characterized by a small Hansen solubility parameter distance R_a with respect to the one or more C₃-C₆ carboxylic acids to be recovered, a relatively high 1-octanol/water partition coefficient logP₀w and a boiling point exceeding that of the C₃-C₆ carboxylic acids to be recovered.

Accordingly, in a first aspect the present invention pertains to a process for the recovery of C₃-C₆ carboxylic acid from an aqueous stream, comprising

providing a vaporous aqueous stream comprising C₃-C6 carboxylic acid,

contacting said vaporous aqueous stream comprising C₃-C₆ carboxylic acid with an absorbing solvent in an absorption unit, to produce a first stream comprising absorbing solvent and C₃-C₆ carboxylic acid and a second stream comprising water vapour,

feeding said first stream comprising absorbing solvent and C₃-C₆ carboxylic acid to a solvent recovery unit, to produce a third stream comprising C₃-C₆ carboxylic acid and a fourth stream comprising absorbing solvent,

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

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

(i) a Hansen solubility parameter distance R_a with respect to C₃-C₆ carboxylic 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 logP₀w 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 the C₃-C₆ carboxylic acid to be recovered.

The invention further relates to a process for the recovery of C₃-C6 carboxylic acid from a vaporous aqueous stream, comprising

providing a vaporous aqueous stream comprising C₃-C6 carboxylic acid,

contacting said aqueous stream comprising C₃-C₆

carboxylic acid with an absorbing solvent in an absorption unit, to produce a first stream comprising absorbing solvent and C₃-C₆ carboxylic acid and a second stream comprising water vapour,

wherein the absorbing 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 the C₃-C₆ carboxylic acid as determined at 25 °C of 15 MPa^{1 2} or less, preferably 12 MPa^{1 2} or less, more

1

preferably 10 MPa or less;

(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 absorbing C₃-C₆ carboxylic acid from a water-containing vapour stream. Brief description of the drawings

Figure 1 shows an embodiment of the present invention, wherein an aqueous vapour stream comprising C3-C6 carboxylic acid is contacted with an absorbing solvent in an absorption unit, to produce a top stream comprising water vapour and a bottom stream comprising absorbing solvent and C₃-C₆

carboxylic acid, and wherein said bottom stream comprising absorbing solvent and C₃-C₆ carboxylic acid is fed to a solvent recovery unit to produce a top stream comprising C₃- C₆ carboxylic acid and a bottom stream comprising absorbing 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.

The aqueous stream comprising C₃-C₆ carboxylic acid used as feed stream for the absorption process 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 C₃-C₆ carboxylic acid. Herein, unless specified otherwise, the term "C₃-C₆ carboxylic acid" and any amounts or concentrations specified in connection therewith refers to the sum of all saturated and unsaturated carboxylic acids having 3, 4, 5 and 6 carbon atoms that are present. Typically, said aqueous stream comprising one or more C₃-C₆ carboxylic acid originates from an oxidative chemical conversion process of C₃-C₆ alkanes and/or C₃-C₆ alkenes, wherein the C₃-C₆ carboxylic acid is obtained as a side product. It is preferred that the aqueous feed stream of the absorption process comprises C₃-C₆ carboxylic 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%.

In one embodiment, the aqueous stream comprising

carboxylic acid originates from the oxidative dehydrogenation ("ODH") of an alkane containing 3 to 6 carbon atoms,

preferably propane or butane. This oxidative alkane

dehydrogenation process typically produces a product stream comprising the corresponding alkene and carbon dioxide, as well as water and carboxylic acid. In case of propane, the product of said alkane oxidative dehydrogenation process may comprise propylene and acrylic acid. Further, in case of butane, the product of said alkane oxidative dehydrogenation process may comprise butylene, butadiene, vinylacetic acid, crotonic acid and/or methacrylic acid.

In another embodiment, the aqueous stream comprising carboxylic acid originates from the oxidation of an alkene containing 3 to 6 carbon atoms, preferably propylene, in the presence of water to produce a product stream comprising the corresponding carboxylic acids . Said carboxylic acid may or may not contain one or more unsaturated double carbon-carbon bonds. As mentioned above, it is preferred that the alkene containing 3 to 6 carbon atoms is propylene, butylene or butadiene. In case of propylene, the product of said alkene oxidation process may comprise acrylic acid. Further, in case of butylene and/or butadiene, the product of said alkene oxidation process may comprise vinylacetic acid, crotonic acid and/or methacrylic acid.

In a preferred embodiment, the C₃-C₆ carboxylic acid is a C₃ or a C₄ carboxylic acid, or a combination thereof.

Preferably, the carboxylic acid to be recovered is acrylic acid, propionic acid, vinylacetic acid, iso-crotonic acid, n- crotonic acid, iso-butyric acid, n-butyric acid, methacrylic acid, or a combination thereof. In a particularly preferred embodiment, the carboxylic acid to be recovered is

methacrylic acid and/or acrylic acid, most preferably acrylic acid.

A concentration step, for example of a dilute aqueous gaseous process effluent comprising C₃-C₆ carboxylic acid, may be applied prior to contacting the C₃-C₆ carboxylic acid with the absorbing solvent in the absorption unit. Such concentration step may comprise any suitable method for removing excess water from an aqueous C3-C6 carboxylic acid stream, including reverse osmosis or carboxylic acid- selective pervaporation, liquid-liquid (L/L) extraction or adsorption on a solid adsorbant .

In one embodiment of the invention, a vaporous effluent comprising C₃-C₆ carboxylic acid is treated using carboxylic acid-selective pervaporation to produce a concentrated C₃-C₆ carboxylic acid/water vapour stream, which is subsequently separated using an absorption process as described herein. In another embodiment, a vaporous effluent comprising C₃-C₆ carboxylic acid is concentrated by condensation followed by L/L extraction with a high-boiling solvent and distillation of the more concentrated C₃-C₆ carboxylic acid/water vapour stream from the high boiling solvent and, finally, separation using absorption from the vapour phase as described herein. In another embodiment, a vaporous effluent comprising C₃-C₆ carboxylic acid is concentrated by adsorption onto a solid, followed by desorption of a more concentrated C₃-C₆

carboxylic acid/water vapour stream subsequently separated using absorption from the vapour phase as described herein.

Typically, such a concentration step yields an aqueous feed stream comprising C₃-C₆ carboxylic 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%.

The vaporous phase stream comprising water and C₃-C₆ carboxylic acid may be the effluent stream from a gas-phase (oxidative) conversion process of C₃-C₆ alkane and/or C₃-C₆ alkene . By directly subjecting the vaporous effluent

comprising C₃-C₆ carboxylic acid and water of such process to the absorption step, capital and operating expenditure on excessive condensation and reheating steps can be avoided.

In the absorption process of the invention, the gaseous aqueous stream comprising C₃-C₆ carboxylic acid is contacted with an absorbing solvent in a suitable absorption unit in order to separate the carbocyclic acid from water. Several absorption methods for vaporous streams are available in the art. For example, absorption may suitably be performed in an absorption column, typically a multi-stage countercurrent packed or tray absorption column having inlets for receiving a vaporous feed stream and for absorbing solvent, wherein absorbing solvent is continuously fed at an upper stage of the absorption column, and wherein the C₃-C₆ carboxylic acid is absorbed by the solvent via direct contact of the rising vapour stream and the falling solvent.

Generally, choice of absorbing solvent is of high importance in the absorption process, since suitable

absorbing solvents can decrease the solvent ratio and/or the liquid load of the absorption unit, thus rendering an easy and more economical implementation of the absorption set-up, such as an absorption 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 C₃-C₆ carboxylic acid, (ii) a partition logPOW 3-S 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 C₃-C₆ carboxylic acid, are excellent absorbing solvents for use in a process for recovering C₃-C₆ carboxylic acid from aqueous vapour streams comprising C₃-C₆ carboxylic 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_di _f δ_ρΐ , 5_hi = Hansen (or equivalent) parameter for compound 1 (in MPa^0-5)

5_d2, 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 C₃-C₆ carboxylic acid to be recovered (i.e., the C₃-C₆ carboxylic acid being compound 1 and the solvent being compound 2, or vice versa) , the higher the affinity of this solvent for the C₃-C₆ carboxylic 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 the one or more C₃-C₆ carboxylic acids to be recovered 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 or less.

It was further found by the present inventors that selective recovery of C₃-C₆ carboxylic acid from aqueous streams is obtained when the 1-octanol/water partition coefficient of the absorbing 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 = ¹⁰log[C_octanoi/C_water] (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 absorbing 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 absorption of water from the C₃-C₆ carboxylic acid-water mixture .

Suitable absorbing solvents for use as described herein have a partition coefficient logP₀w as determined at 25 °C and pH 7 of at least 0. Typically, the absorbing solvent for use as described herein has a logP₀w of at least 0.5,

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

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 logP₀w 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 absorbing solvent from C₃-C₆ carboxylic acid by e.g. distillation, advantageously the absorbing 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 the C₃-C₆ carboxylic acid(s) .

For example, for the recovery of acrylic (C₃) acid, which has a boiling point of ca. 140 °C at atmospheric pressure, it is preferred that the absorbing solvent has a boiling point of at least 145 °C. Preferably, it has a boiling point of at least 150 °C, more preferably at least 160 °C, even more preferably at least 170 °C.

For the recovery of methacrylic (C₄) acid, which has a boiling point of ca. 160 °C at atmospheric pressure, it is preferred that the absorbing solvent has a boiling point of at least 165 °C. Preferably, it has a boiling point of at least 170 °C, more preferably at least 180 °C, even more preferably at least 200 °C.

From an economic perspective, it is preferred that the absorbing 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 and eventual thermal degradation of the solvent .

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, depending on the carboxylic acid to be recovered, higher-boiling

carboxylic acids 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 the carboxylic acid to be recovered, the skilled person will be capable of selecting suitable absorbing 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 process for the recovery of C₃-C₆ carboxylic acid from a vaporous aqueous stream, comprising

providing a vaporous aqueous stream comprising C3-C6 carboxylic acid,

contacting said aqueous stream comprising C₃-C₆

carboxylic acid with an absorbing solvent in an absorption unit, to produce a first stream comprising absorbing solvent and C₃-C₆ carboxylic acid and a second stream comprising water,

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_nH_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 absorbing 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 absorbing solvents for use according to the invention, including their Hansen solubility parameter distance R_a with respect to C₃-C₆ carboxylic 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 acrylic 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 acrylic acid acid (5_d = 17.7; δ_ρ =6.4; 5_h = 14.9; 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. LogP₀w 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

(MPa^{0 5})

bp

Solvent . r . t . LogPow

(°C)

acrylic

acid

acrylic acid 0 0.28 138

n-butyl acrylate 11 2.39 145

2-butyl 1-octanol 7 5.05 145

5-methyl-2-hexanol 7 1.97 148

iso-hexanol (4-methyl-l- 1.6 152

4

pentanol )

anisole 9 2.11 153

cyclohexanone 10 0.81 155 cyclo-hexanol 3 1.32 161 furfural 13 0.41 162 n-butyl butyrate 11 2.83 165

2-butoxy-ethanol 4 0.8 171 cyclohexyl acetate 9 2.29 173 benzofuran 10 2.67 174 di-isobutyl carbinol 8 3.31 178

2-octanol 6 2.90 179 iso-octanol (2-ethyl 2.72 180

5

hexanol )

phenol 1 1.46 181 pentanoic acid 7 1.39 185

1-octanol 5 3.05 195 methyl benzoate 11 2.2 199

2-ethylhexyl acetate 11 3.71 200

2 , 6-xylenol 6 2.4 201 acetophenone 12 1.58 202 cresol (m) 2 1.94 203 octyl acetate 3.84 203

11

(calc) guaiacol 4 1.34 205 benzyl alcohol 2 1.1 205 hexanoic acid 4 1.84 206 triethyl phosphate 8 1.08 215 isophorone 8 2.07 215

2-propyl heptanol 8 4.4 218 iso-decanol 3.62 220

8

(calc)

1-decanol 4.2 220

6

(calc) isopropyl acetate 9 1.28 220

2-undecanol 9 4.4 229 octanoic acid 3.32 237

9

(calc)

butyl diglycol acetate 8 1.1 238

propylene glycol phenyl 1.41 241

4

ether

1-undecanol 4.66 243

6

(calc)

1-phenoxy ethanol 3 1.1 245

2-phenoxy ethanol 1 1.2 247

2-dodecanol 5.02 257

10

(calc)

1-dodecanol 6 5.13 259

2-benzyloxy ethanol 1.17 265

3

(calc)

tributyl phosphate 11 2.5 289

The oxygen-containing solvents as defined herein are characterized by having very good selectivity, as compared to water, for C₃-C₆ carboxylic acid. Furthermore, these solvents have relatively high boiling points and low volatility, thus minimizing their loss as vapour in the top stream of an absorption unit and allowing efficient separation from C₃-C₆ carboxylic acid as the bottom stream using in a subsequent distillation (solvent recovery) step.

It is possible to combine the absorbing solvent with one or more other solvents. In one embodiment, a mixture of two or more absorbing solvents as defined herein are used. In another embodiment, an absorbing solvent as defined herein is combined with one or more solvents selected from carboxylic esters, ethers, aldehydes or ketones. When one or more absorbing solvents as defined herein are used in admixture with another solvent not according to the invention, it is preferred that the one or more absorbing 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 absorbing solvents as defined herein are used in the absence of amine compounds. In one embodiment, the absorbing solvent is employed in the absence of any other solvent not according to the invention. In order to avoid loss of solvent with C₃-C₆ carboxylic 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 C₃-C₆

carboxylic 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 absorption and/or regeneration of the absorbing alcohol solvent. If this is undesirable, these esters may at least partially be hydrolyzed, for example by feeding steam to the bottom of the column in the absorption or solvent regeneration (desorption) step.

The invention further relates to the use of an oxygen- containing organic compound as fully defined above as a solvent for absorbing C₃-C₆ carboxylic acid from a water- containing vapour stream. Depending on, among others, the concentration of C₃-C₆ carboxylic acid in the aqueous feed stream, the amount of absorbing solvent employed in the absorption process may vary within wide ranges, for example in a ratio (wt/wt) of absorbing solvent to C₃-C₆ carboxylic acid supplied to the absorption 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.

The temperature in the absorption 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 absorption 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 absorption unit may also vary within wide ranges. Typically, the pressure in the absorption 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 C₃-C₆ carboxylic acid(s) to be recovered 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 C₃-C₆ carboxylic acid(s) to be recovered 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 C₃-C₆ carboxylic acid present in the vaporous aqueous feed stream of the absorption unit exits said absorption unit in the absorbing 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 C₃-C₆ carboxylic acid present in the feed stream of the absorption unit is recovered in the absorbing solvent stream of said absorption unit. Furthermore, in order to avoid the need for any further water removal steps, it is preferred that the absorbing solvent entrains substantially none of the water present in the gaseous aqueous feed stream of the absorption unit. Preferably, the absorbing solvent effluent stream of the absorption unit comprises water and C₃-C₆ carboxylic acid in a weight 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, C₃-C₆ carboxylic acid is removed (desorbed) from the absorbing solvent resulting in a product stream comprising C₃-C₆ carboxylic acid and another stream comprising the absorbing solvent now depleted of C₃-C₆ carboxylic acid.

In the solvent recovery unit, recovery of the absorbing solvent, and of optional other solvents present, is typically effectuated by distilling the effluent stream of the

absorption unit comprising C₃-C₆ carboxylic acid and absorbing solvent, resulting in a top stream comprising C₃-C₆ carboxylic acid and a bottom stream comprising the absorbing solvent. Distillation may be carried out in any distillation unit known to the skilled that is suitable for separating absorbing solvent from C₃-C₆ carboxylic 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 the carboxylic 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 absorbing 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 absorbing 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 the carboxylic acid at operating temperature. In one embodiment, steam is fed at the bottom of the solvent regeneration (desorption) unit to hydrolyze any esters that may have been formed in the C₃-C₆ carboxylic 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 C₃-C₆ carboxylic acid present in the stream fed to the solvent recovery unit comprising C₃-C₆ carboxylic acid and absorbing 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 C₃-C₆ carboxylic acid and absorbing solvent is recovered .

Typically, the C₃-C₆ carboxylic acid product stream of the solvent recovery unit comprises C₃-C₆ carboxylic 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 C₃-C₆ carboxylic acid present in the aqueous stream provided to the absorption 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 C₃-C₆ carboxylic 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 absorbing solvent, typically the bottom stream of a distillation unit, is recirculated to the absorption 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 absorption unit. In one embodiment, the entire bottom stream comprising the absorbing solvent is recirculated to the absorption unit.

In the absorption 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 absorption 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 absorption unit may further comprise entrained absorbing solvent. Typically, said top stream of the absorption 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 absorbing solvent. Said entrained absorbing 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 absorbing solvent. In a preferred embodiment, the absorbing solvent thus recovered is at least partially recirculated to the absorption unit either as a separate stream or by mixing with a recirculated absorbing solvent stream from the solvent recovery unit .

The top stream comprising C₃-C₆ carboxylic 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 C₃-C₆ carboxylic acid products according to market requirements.

Detailed description of the drawing

In Figure 1, a vapour stream 4 comprising water and C₃-C₆ carboxylic acid is fed to an absorption column 5 to which further an absorbing solvent 6 is fed. C₃-C₆ carboxylic acid is absorbed by the absorbing solvent, which exits the absorption column as "fat" solvent stream 7. A vapour stream comprising water and other gaseous compounds exits the absorption column as stream 8.

Stream 7 comprising fat absorbing solvent and absorbed C₃-C₆ carboxylic acid is fed supplied to a solvent recovery (desorption) unit, comprising a distillation unit 9 equipped with condenser section 9a and reboiler section 9b. Desorbed C₃-C₆ carboxylic acid leaves distillation unit 9 as stream 10, while absorbing solvent now depleted of absorbed C₃-C₆ carboxylic acid exits distillation unit 9 as stream 11. The C₃-C₆ carboxylic acid-depleted absorbing solvent stream 11 may be partially recirculated to absorption column 5 as absorbing solvent recirculation stream 12. C₃-C₆ carboxylic acid stream 10 may be further purified downstream.

The vapour stream 8 comprising water and other gaseous compounds obtained as a top stream from absorption column 5 is fed to a condensation unit 13, where water is removed via stream 14. A product stream comprising gaseous compounds is removed via stream 15, from where it may undergo further separation and/or purification further downstream.

In condensation unit 13, spontaneous separation from the condensed water of absorbing solvent entrained in vapour stream 8 originating from absorption column 5 may occur. This separated absorbing solvent stream 16 may at least partially be recirculated to absorption column 5 via recirculation stream 17.

Claims

C L A I M S

1. A process for the recovery of C₃-C₆ carboxylic acid from a vaporous aqueous stream, comprising

providing a vaporous aqueous stream comprising C3-C6 carboxylic acid,

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

(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 C₃-C₆ carboxylic acid.

2. Process according to claim 1, wherein the oxygen- containing absorbing solvent is a compound selected from the group consisting of alcohols, ethers, esters and acids, more preferably alcohols.

3. Process according to claim 1 or 2, wherein the alcohol is an aromatic, olefinic or aliphatic, branched or linear, primary, secondary or tertiary alcohol having 5-20,

preferably 6-18, more preferably 6-16 carbon atoms, most preferably 8-16 carbon atoms, preferably 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.

4. A process for the recovery of C₃-C₆ carboxylic acid from a vaporous aqueous stream, comprising

providing a vaporous aqueous stream comprising C3-C6 carboxylic acid,

contacting said aqueous stream comprising C₃-C₆

5. Process according to any one of claims 2-4, wherein the alcohol is selected from the group consisting of phenol, benzyl alcohol, alkyl phenols, creosol, xylenols, guaiacol ( 2-methoxyfenol ) , cresols, phenoxy ethanol, naphthol, 1- hexanol, 2-hexanol, 3-hexanol, 1-octanol, 2-octanol, 1- decanol, 2-decanol, 1-dodecanol, 2-dodecanol, 2-methyl-2- pentanol, 2-methyl-3-pentanol, 3-methyl-3-pentanol, 2-methyl- 2-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,

methylisobutyl carbinol, pinacolyl alcohol, 2-propyl

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

cyclopentanol , trimethyl cyclohexanols , cyclohexanemethanol, methyl cyclohexanemethanol, pinacol, citronellol, menthol and isoborneol .

6. Process according to any one of the preceding claims, wherein before contacting the vaporous aqueous stream comprising C₃-C₆ carboxylic acid with an absorbing solvent in the absorption unit, said aqueous stream is concentrated using reverse osmosis, carboxylic acid-selective

pervaporation, L/L extraction or adsorption-desorption using a solid adsorbent.

7. Process according to claim 5 or 6, which further comprises condensing the second stream comprising water from the absorption unit to produce liquid water,

optionally separating entrained absorbing solvent from said top stream,

and optionally recirculating at least a portion of the separated entrained absorbing solvent to the absorption unit.

8. Process according to any one of the preceding claims, wherein the aqueous stream comprising C₃-C₆ carboxylic acid has a concentration of at least 0.1 wt%.

9. Process according to any one of the preceding claims, wherein the ratio (wt/wt) of absorbing solvent to C₃-C₆ carboxylic acid supplied to the absorption unit is 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.

10. Process according to any one of the preceding claims, wherein the aqueous stream comprising C₃-C₆ carboxylic acid originates from the oxidative dehydrogenation of an alkane containing 3 to 6 carbon atoms, preferably propane.

11. Process any one of the preceding claims, wherein the aqueous stream comprising C3-C6 carboxylic acid originates from the oxidation of an alkene containing 3 to 6 carbon atoms, preferably propylene.

12. Process according to any one of the preceding claims, wherein the aqueous stream comprising C₃-C₆ carboxylic acid comprises acrylic acid or methacrylic acid, preferably acrylic acid.

13. Process according to any one of the preceding claims, wherein the C₃-C₆ carboxylic acid product stream of the solvent recovery unit comprises C3-C6 carboxylic acid in a concentration of at least 70 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%.

14. Process according to any one of the preceding claims, wherein 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 C₃-C₆ carboxylic acid is recovered, based on the amount of C₃-C₆ carboxylic acid present in the aqueous effluent stream provided to the absorption step.

15. Use of an oxygen-containing organic compound having

(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 "Cat atmospheric pressure

as a solvent for absorbing C₃-C₆ carboxylic acid from a water-containing vapour stream.