WO2017114831A1 - Process for converting alkanes and/or alkenes to alkenes and carboxylic acids - Google Patents

Abstract

The present invention provides a process for oxidatively converting alkanes and/or alkenes containing 2 to 6 carbon atoms ("C₂-C₆") to C₂-C₆ alkenes and C₂-C₆ carboxylic acids, comprising a reaction step which comprises contacting a stream comprising oxygen and said C₂-C₆ alkane and/or alkene with a mixed metal oxide catalyst, to produce a stream comprising C₂-C₆ alkene, water and one or more C₂-C₆ carboxylic acids, a carboxylic acid separation step which comprises contacting said stream comprising alkene, water and one or more C₂-C₆ carboxylic acids with a selective solvent, resulting in absorption or extraction of the C₂-C₆ carboxylic acids by the solvent, a solvent recovery step which comprises separating said absorbed or extracted C₂-C₆ carboxylic acids from the solvent, resulting in a product stream comprising said one or more C₂-C₆ carboxylic acids and a stream comprising recovered solvent, and optionally a recycling step which comprises recycling at least a portion of the stream comprising recovered solvent to the carboxylic acid separation step, wherein the selective solvent is an oxygen-containing organic compound having (i) a Hansen solubility parameter distance R_a with respect to the C₂-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 one or more C₂-C₆ carboxylic acids.

Description

PROCESS FOR CONVERTING ALKANES AND/OR ALKENES TO ALKENES AND

CARBOXYLIC ACIDS

Field of the invention

The present invention relates to an integrated process for converting alkanes to alkenes and carboxylic acids and/or converting alkenes to carboxylic acids.

Background of the invention

It is known to oxidatively dehydrogenate alkanes, such as alkanes containing 2 to 6 ("C2-C6") carbon atoms, for example ethane or propane resulting in ethylene and propylene,

respectively, in an oxidative dehydrogenation

(oxydehydrogenation; ODH) process. Examples of alkane ODH processes, including catalysts and other process conditions, are for example disclosed in US7091377, WO2003064035,

US20040147393, WO2010096909 and US20100256432. Mixed metal oxide catalysts containing molybdenum (Mo) , vanadium (V) , niobium (Nb) and optionally tellurium (Te) as the metals, can be used as such oxydehydrogenation catalysts. 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 acetic acid and acrylic acid, respectively.

In the above processes, the carboxylic acids thus produced are generally considered as undesired by-products because of their low concentration and the difficulty and cost of recovery from the effluent stream. 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 distillative separation of carboxylic acid and water troublesome, as this would require very large condensate recycle and/or separation trains. Accordingly, these processes are typically carried out under conditions that limit the formation of carboxylic acids.

However, C₂-C₆ carboxylic acids are valuable ingredients and building blocks for use in the chemical industry. For example, 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, dilute 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. 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 an integrated process for converting alkanes to alkenes and carboxylic acids and/or converting alkenes to carboxylic acids by means of oxidative dehydrogenation (ODH) , wherein carboxylic acids are recovered from the ODH product stream in a

technically advantageous, efficient and affordable process. Summary of the invention

It was surprisingly found that the above-mentioned objective can be attained by means of an alkane oxidative dehydrogenation and/or alkene oxidation process, wherein carboxylic acids are selectively absorbed or extracted from the reactor effluent to produce a product stream comprising alkene and another

concentrated product stream comprising carboxylic acid.

Accordingly, in a first aspect the present invention pertains to a process for oxidatively converting alkanes and/or alkenes containing 2 to 6 carbon atoms ("C₂-C₆") to C₂-C₆ alkenes and C₂-C6 carboxylic acids, comprising

a reaction step which comprises contacting a stream comprising oxygen and said C₂-C6 alkane and/or alkene with a mixed metal oxide catalyst, to produce a stream comprising C₂- Ce alkene, water and one or more C₂-C6 carboxylic acids,

a carboxylic acid separation step which comprises

contacting said stream comprising alkene, water and one or more C₂-C6 carboxylic acids with a selective solvent, resulting in absorption or extraction of the C₂-C6 carboxylic acids by the solvent ,

a solvent recovery step which comprises separating said absorbed or extracted C₂-C6 carboxylic acids from the solvent, resulting in a product stream comprising said one or more C₂-C₆ carboxylic acids and a stream comprising recovered solvent, and optionally a recycling step which comprises recycling at least a portion of the stream comprising recovered solvent to the carboxylic acid separation step,

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

(i) a Hansen solubility parameter distance R_a with respect to the C₂-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 one or more C₂-C₆ carboxylic acids.

An advantage of the present invention is that by proper valorization of C2-C6 carboxylic acids produced as side

products it allows relaxing carboxylic acid specifications, thereby widening the operating window to accommodate e.g.

higher pressure, lower temperature and/or easier management of heat release and explosion risks in the ODH process. For example, relaxing the carboxylic acid (such as acetic acid) specification to 10 mol% selectivity could deliver an aqueous side-stream containing 20 wt% carboxylic acid. Brief description of the drawings

Figure 1 shows an embodiment of the present invention, wherein a vaporous effluent from an alkane oxidative

dehydrogenation process is subjected to absorption of the carboxylic acid by a selective solvent, and wherein the

absorbed carboxylic acid is separated from the solvent by distillation to provide a concentrated carboxylic acid stream.

Figure 2 shows an embodiment of the present invention, wherein a liquid effluent from an alkane oxidative

dehydrogenation process is subjected to liquid-liquid

extraction of the carboxylic acid by a selective solvent, and wherein the extracted carboxylic acid is separated from the solvent by distillation to provide a concentrated carboxylic acid stream.

Fig. 3 shows an embodiment of the present invention, wherein a liquid or vaporous effluent from an alkane oxidative dehydrogenation process is subjected to extractive distillation of the water/carboxylic acid mixture using a selective solvent, and wherein the extracted carboxylic acid is separated from the solvent by distillation to provide a concentrated carboxylic acid stream.

Detailed description of the invention

In the reaction step of the present invention, an alkane and/or alkene containing 2 to 6 carbon atoms is contacted with a catalyst suitable for oxidative dehydrogenation . Typical process conditions and catalyst compositions for the alkane oxidative dehydrogenation and/or alkene oxidation process have been described in detail in WO2015082602, US7091377, WO2003064035, US20040147393 , W02010096909 , WO2013021034 and US20100256432, as well as in Wu, Shujie et al, Reaction

Kinetics, Mechanisms and Catalysis 2012, 106(1), pp. 157-164 and Jing, Fangli et al . , Catalysis Today 2013, 203, 32-39, the disclosures of which are herein incorporated by reference.

Suitably, in the present invention, the catalyst is a mixed metal oxide catalyst containing molybdenum, vanadium, niobium and optionally tellurium as the metals, which catalyst may have the following formula:

M_OlV_aTe_bNb_c0_n

wherein :

a, b, c and n represent the atomic ratio of the of the element in question with respect to molybdenum (Mo) ;

a (for V) is from 0.01 to 1, preferably 0.05 to 0.60, more preferably 0.10 to 0.40, more preferably 0.20 to 0.35, most preferably 0.25 to 0.30;

b (for Te) is 0 or from >0 to 1, preferably 0.01 to 0.40, more preferably 0.05 to 0.30, more preferably 0.05 to 0.20, most preferably 0.09 to 0.15;

c (for Nb) is from >0 to 1, preferably 0.01 to 0.40, more preferably 0.05 to 0.30, more preferably 0.10 to 0.25, most preferably 0.14 to 0.20; and

n (for 0) is a number which is determined by the valency and frequency of elements other than oxygen.

Further, in the alkane oxidative dehydrogenation process and/or alkene oxidation process of the present invention, the temperature is of from 300 to 500 °C, preferably of from 310 to 450 °C, more preferably of from 320 to 420 °C, most preferably of from 330 to 420 °C.

Preferably, said temperature is at least 310 °C, more preferably at least 320 °C, more preferably at least 330 °C, more preferably at least 340 °C, more preferably at least 345 °C, more preferably at least 350 °C, more preferably at least 355 °C, most preferably at least 360 °C.

Further, preferably, said temperature is at most 480 °C, more preferably at most 460 °C, more preferably at most 450 °C, more preferably at most 440 °C, more preferably at most 430 °C, more preferably at most 420 °C, more preferably at most 410 °C, most preferably at most 400 °C.

Still further, in the alkane oxidative dehydrogenation process and/or alkene oxidation process of the present

invention, typical pressures are 0.1-20 bara (i.e. "bar

absolute") . Further, in a preferred embodiment of the present invention, the pressure is from 0.1 to 15 bara, more preferably of from 0.5 to 10 bara, most preferably of from 1 to 5 bara.

Generally, in the present invention, the volume ratio of oxygen to the C₂-C6 alkane and/or C₂-C6 alkene may be in the range of from 0.1:1 to 10:1, more suitably 0.3:1 to 7:1, most suitably 0.5:1 to 5:1.

Said ratio of oxygen to the C₂-C6 alkane and/or C₂-C6 alkene is the ratio at the entrance of a reactor, which reactor may comprise a catalyst bed. Obviously, after entering the reactor, at least part of the oxygen and C₂-C6 alkane and/or alkene gets converted .

As mentioned above, in the present invention, a gas stream comprising oxygen and C₂-C₆ alkane and/or C₂-C₆ alkene may be contacted with a suitable catalyst. The amount of such catalyst is not essential. Preferably, a catalytically effective amount of the catalyst is used, that is to say an amount sufficient to promote the alkane oxydehydrogenation and/or alkene oxidation reaction.

Preferably, in an oxidative alkane conversion process as described herein, the alkane containing 2 to 6 carbon atoms is a linear or branched alkane in which case said alkane may be selected from the group consisting of ethane, propane, butane, pentane and hexane . Further, preferably, said alkane contains 2 to 4 carbon atoms and is selected from the group consisting of ethane, propane, n-butane and iso-butane. More preferably, said alkane is ethane or propane. Most preferably, said alkane is ethane .

Further, preferably, in an alkene oxidation process as defined herein, the alkene containing 2 to 6 carbon atoms is a linear or branched alkene in which case said alkene may be selected from the group consisting of ethylene, propylene, n- butene, iso-butene, pentene and hexene . Further, preferably, said alkene contains 2 to 4 carbon atoms and is selected from the group consisting of ethylene, propylene and iso-butene. More preferably, said alkene is ethylene or propylene, most preferably ethylene.

The product of said alkane oxidative dehydrogenation process may comprise the dehydrogenated equivalent of the alkane, that is to say the corresponding alkene. For example, in the case of ethane such product may comprise ethylene, in the case of propane such product may comprise propylene, in the case of butane such product may comprise butene (butylene) and/or butadiene and so on. Such dehydrogenated equivalent of the alkane is initially formed in said alkane oxidative

dehydrogenation process. However, in said same process, said dehydrogenated equivalent 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. As mentioned above, it is preferred that the alkane containing 2 to 6 carbon atoms is ethane or propane. In the case of ethane, the product of said alkane oxidative dehydrogenation process may comprise ethylene and acetic acid. In the case of propane, the product of said alkane oxidative dehydrogenation process may comprise propylene and acrylic acid. Further, in the case of butane, the product of said alkane oxidative dehydrogenation process may comprise butylene, butadiene, vinylacetic acid, crotonic acid and/or methacrylic acid. In a preferred embodiment, the carboxylic acid product of said alkane oxidative dehydrogenation process comprises acetic acid, acrylic acid, methacrylic acid and combinations thereof. More preferably, the carboxylic acid product of said alkane oxidative dehydrogenation process predominantly comprises acetic acid, most preferably essentially consists of acetic acid .

The product of said alkene oxidation process comprises the oxidized equivalent of the alkene. Preferably, said oxidized equivalent of the alkene is the corresponding carboxylic acid. 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 2 to 6 carbon atoms is ethylene or propylene. In the case of ethylene, the product of said alkene oxidation process may comprise acetic acid. In the case of propylene, the product of said alkene oxidation process may comprise acrylic acid. Further, in the case of butylene (1- butene, 2-butene, iso-butylene and combinations therof) , the product of said alkene oxidation process may comprise

vinylacetic acid, crotonic acid and/or methacrylic acid. In a preferred embodiment, the carboxylic acid product of said alkene oxidation process comprises acetic acid, acrylic acid, methacrylic acid and combinations thereof. More preferably, the carboxylic acid product of said alkene oxidation process predominantly comprises acetic acid, most preferably consists of acetic acid.

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.

Herein, unless specified otherwise, the term "C₂-C6

carboxylic acid" and any amounts or concentrations specified in connection therewith refers to the sum of all saturated and unsaturated carboxylic acids having 2, 3, 4, 5 and 6 carbon atoms that are present.

Thus, the product of the alkane oxidative dehydrogenation and/or alkene oxidation step is a vaporous stream typically comprising C₂-C6 alkane and/or C₂-C6 alkene and one or more of the corresponding carboxylic acids, as well as water (H₂0) and optionally carbon dioxide (C0₂) . Typically, reaction conditions are controlled such that selectivity towards carboxylic acid (on carbon basis) produced from C₂-C₆ alkane and/or alkene is in the range of 1-99%, preferably 1-50 %, more preferably 1-20 %. For example, formation of carboxylic acid is favoured by the addition of steam to the reactor.

In the carboxylic acid separation step of the process of the present invention, said vapour stream is contacted with a selective solvent for selectively absorbing or extracting the carboxylic acid from the stream. This results in a so-called

"fat" solvent stream, comprising the selective solvent and the absorbed or extracted C₂-C₆ carboxylic acid, or a combination of C₂-C₆ carboxylic acids.

In order to effectuate selective absorption or extraction of carboxylic acid from the vaporous effluent of the oxidative dehydrogenation and/or alkene oxidation process, in the

carboxylic acid separation step of the process of the present invention, the selective solvent has an affinity for any C₂-C6 carboxylic acid present that is higher than that for water, and for any of the other components of the effluent of the alkane oxidative dehydrogenation and/or alkene oxidation step.

This means that under the conditions applied in said carboxylic acid separation step, including pressure and

temperature which are further defined herein below, said selective solvent has an affinity for the C₂-C6 carboxylic acids as defined herein which is higher than that for water (and other components of the effluent stream of the alkane oxidative dehydrogenation and/or alkene oxidation step). It is further preferred that the selective solvent is a low-volatile or non-volatile compound, having a relative volatility with respect to water and carboxylic acid under the absorption, extraction and solvent separation (recovery) conditions as defined herein that is substantially below unity. The selective solvent having a low volatility and low water miscibility offers the advantage of minimizing selective solvent losses as vapor in the effluent stream and subsequent dissolution in water. Additionally, it allows an easy and energy efficient recovery (separation) of absorbed or extracted C₂-C₆ carboxylic acid by preferably distillation, said

carboxylic acid being obtained as the top product.

Furthermore, in order to minimize the cost of

(distillative) separation of the carboxylic acid from the selective solvent, an appreciable boiling point difference between these compounds is desirable.

The present inventors have found that oxygen-containing solvents being characterized by (i) a short Hansen solubility parameter distance R_a with respect to the C₂-C6 carboxylic acid, notably an R_a with respect to the carboxylic acid as determined at 25 °C of 15 MPa^1/2 or less, (ii) a partition logP₀w 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 the C₂-C6 carboxylic acids to be separated, are excellent selective solvents for use in a process for absorbing or extracting C₂-C₆ carboxylic acids acid from aqueous liquid or vapour effluent streams comprising C₂-C₆ carboxylic acids. 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)

5di, δ_ρι 5_hi = Hansen (or equivalent) parameter for compound 1 (in MPa^0'5)

&d2, δ_ρ2 , 5_h2 = 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 C₂-C₆ carboxylic acid to be recovered (i.e., the C₂-C₆

carboxylic acid being compound 1 and the solvent being compound 2, or vice versa) , the higher the affinity of this solvent for the C₂-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 solubility

parameters have been derived by alternative methods to the original Hansen method, resulting in likewise useful parameters such as Hoy's cohesion parameters for liquids.

It is preferred that the Hansen solubility parameter distance R_a with respect the C₂-C₆ carboxylic acid 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 1 /7 or less.

It was further found by the present inventors that

excellent recovery of C2-C6 carboxylic acids from aqueous solutions is obtained when the 1-octanol/water partition coefficient of the selective solvent is relatively high. The 1- octanol/water partition coefficient, commonly expressed as its logarithmic value logP₀w_? 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 = ^lulog[Coctanol/C_w 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 a 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 C₂-C₆ carboxylic acid-water mixture .

Suitable selective 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 selective 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, 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 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 facilitate effective separation (recovery) of the selective solvent from the C₂-C₆ carboxylic acid by e.g. distillation, it is preferred that the selective solvent has a boiling point at atmospheric pressure that is at least 10 °C higher, preferably at least 20 °C higher, more preferably at least 30 °C higher, even more preferably at least 40 °C higher, most preferably at least 50 °C higher than the boiling point (s) of the C₂-C₆ carboxylic acid(s) to be separated.

For example, acetic acid has a boiling point of about 117 °C. Typically, the selective solvent has a boiling point of at least 125 °C. Preferably, it has a boiling point of at least 140 °C, more preferably at least 160 °C, even more preferably at least 170 °C, yet even more preferably at least 180 °C most preferably at least 200 °C.

From an economic perspective, it is preferred that the selective 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, 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 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 selective 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 -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 oxidatively converting alkanes and/or alkenes containing 2 to 6 carbon atoms ("C₂-C₆") to C₂-C₆ alkenes and C₂-C₆ carboxylic acids, comprising

a reaction step which comprises contacting a stream comprising oxygen and said C₂-C₆ alkane and/or alkene with a mixed metal oxide catalyst, to produce a stream comprising C₂- Ce alkene, water and one or more C₂-C6 carboxylic acids,

a carboxylic acid separation step which comprises

contacting said stream comprising alkene, water and one or more C₂-C6 carboxylic acids with a selective solvent, resulting in absorption or extraction of the C₂-C6 carboxylic acids by the solvent,

a solvent recovery step which comprises separating said absorbed or extracted C₂-C6 carboxylic acids from the solvent, resulting in a product stream comprising said one or more C₂-C6 carboxylic acids and a stream comprising recovered solvent, and optionally a recycling step which comprises recycling at least a portion of the stream comprising recovered solvent to the carboxylic acid separation step,

wherein the selective 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 alcohols,

1- phenyl ethanol, 2-phenyl ethanol, cumyl alcohol (2-phenyl-2- propanol) , xylenols (such as 2 , 6-xylen-l-ol ) , guaiacol (2- methoxyphenol ) , creosol, cresols such as m-cresol, phenoxy ethanol and naphthol; examples of suitable linear alcohols include those having the general formula C_nH_n+1OH, 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 selective 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 selective solvents for use

according to the invention, including their Hansen solubility parameter distance R_a with respect to acetic (C₂) acid and acrylic (C₃) 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 and 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 acetic acid (5_d = 14.5; δ_ρ =8.0; 5_h = 13.5; all in MPa^0'5) and acrylic 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 ) .

Ra Ra

(MPa^{0 5}) (MPa^{0 5})

bp

Solvent w. r . t . w. r . t . LogPow

(°C) acetic acrylic

acid acid

acetic acid 0 7 -0.13 117 acrylic acid 7 0 0.28 138

2-ethylbutanol 5 5 1.78 124 n-butyl acetate 9 10 1.77 125 2-methyl-3-pentanol 1 7 2.98 127

3-methyl-l-butanol 2 6 1.14 131 iso-pentyl alcohol 1.33 131

4 4

(calc) n-butyl acrylate 9 11 2.39 145

2-butyl 1-octanol 7 7 5.05 145

5-methyl-2-hexanol 2 7 1.97 148 iso-hexanol (4-methyl-l- 1.6 152

4 4

pentanol )

anisole 10 9 2.11 153 cyclohexanone 11 10 0.81 155 cyclo-hexanol 7 3 1.32 161 furfural 14 13 0.41 162 n-butyl butyrate 10 11 2.83 165

2-butoxy-ethanol 4 4 0.8 171 cyclohexyl acetate 9 9 2.29 173 benzofuran 12 10 2.67 174 di-isobutyl carbinol 6 8 3.31 178

2-octanol 4 6 2.90 179 iso-octanol (2-ethyl 2.72 180

6 5

hexanol )

phenol 7 1 1.46 181 pentanoic acid 5 7 1.39 185

1-octanol 5 5 3.05 195 methyl benzoate 12 11 2.2 199

2-ethylhexyl acetate 10 11 3.71 200

2 , 6-xylenol 6 6 2.4 201 acetophenone 14 12 1.58 202 cresol (m) 8 2 1.94 203 octyl acetate 3.84 203

10 11

(calc) guaiacol 4 4 1.34 205 benzyl alcohol 8 2 1.1 205 hexanoic acid 4 4 1.84 206 triethyl phosphate 7 8 1.08 215 isophorone 7 8 2.07 215

2-propyl heptanol 3 8 4.4 218 iso-decanol 3.62 220

3 8

(calc)

1-decanol 4.2 220

9 6

(calc) isopropyl acetate 6 9 1.28 220

2-undecanol 4 9 4.4 229 octanoic acid 3.32 237

7 9

(calc) butyl diglycol acetate 7 8 1.1 238 propylene glycol phenyl 1.41 241

7 4

ether

1-undecanol 4.66 243

3 6

(calc)

1-phenoxy ethanol 7 3 1.1 245

2-phenoxy ethanol 7 1 1.2 247

2-dodecanol 5.02 257

5 10

(calc)

1-dodecanol 4 6 5.13 259

2-benzyloxy ethanol 7 3 1.17 265 (calc) tributyl phosphate 10 11 2.5 289

The oxygen-containing solvents as defined herein are characterized by having very good selectivity, as compared to water, for C2-C6 carboxylic acids. Furthermore, these solvents have relatively high boiling points and low volatility, thus minimizing their loss as vapour in the top stream of any distillation equipment employed and allowing efficient

separation from the carboxylic acid as the bottom stream using in a subsequent distillative solvent recovery step.

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

In order to avoid loss of solvent with C₂-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 the one or more C₂- C₆ carboxylic acids to be recovered.

In one embodiment, the solvent mixture may comprise one or more organic alcohols as defined herein and additionally one or more of the corresponding C2-C6 carboxylic - such as acetate or acrylate - esters, which may form during extraction and/or regeneration of the selective alcohol solvent. If this is undesirable, these esters may at least partially be hydrolyzed, for example by feeding steam to the bottom of the absorption or (extractive) distillation column, in the carboxylic acid separation or solvent regeneration step.

The carboxylic acid separation step may be carried out using any absorption or extraction system available in the art that is suitable for contacting a gaseous, vaporous or liquid feed stream comprising carboxylic acid with a selective solvent to result in in absorption or extraction of carboxylic acid by the selective solvent.

The C₂-C₆ carboxylic acid may be absorbed by the selective solvent from the vaporous effluent of the oxidative

dehydrogenation (ODH) process or alkene oxidation process as described herein. Accordingly, in one embodiment, the C₂-C₆ carboxylic acid separation step is carried by absorption from a vaporous stream comprising C₂-C₆ alkene, water and one or more C₂-C₆ carboxylic acids resulting from the oxidative conversion of C₂-C₆ alkanes and/or C₂-C₆ alkenes. Suitably, the C₂-C₆ carboxylic acid separation step is carried out in an absorption column having inlets for receiving a vaporous feed stream and for selective solvent, wherein the vaporous stream comprising carboxylic acid is fed into the lower zone of a packed or tray absorption column and the liquid selective solvent is fed into the upper zone of the absorption column, and wherein the carboxylic acid is absorbed by the selective solvent via direct contact of the rising vapour stream and the falling selective solvent .

In another embodiment, the carboxylic acid separation step is performed by means of liquid-liquid extraction (LLE) of the liquid effluent of an oxidative dehydrogenation (ODH) process or alkene oxidation process using the selective solvent as extractive solvent. In such case, the vaporous effluent of the ODH process is condensed by reducing its temperature to provide a liquid aqueous effluent comprising the carboxylic acid.

In another preferred embodiment, the carboxylic acid separation step is performed by means of extractive

distillation (ED) from a vaporous stream comprising C₂-C6 alkene, water and one or more C₂-C6 carboxylic acids resulting from the oxidative conversion of C₂-C6 alkanes and/or C₂-C6 alkenes. Extractive distillation is a distillation process wherein a selective ("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.

It is preferred that the feed stream of the carboxylic acid absorption, extraction or extractive distillation step

comprises C₂-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%.

A concentration step, for example of a dilute aqueous liquid or vaporous process effluent comprising C₂-C6 carboxylic acid, may be applied prior to contacting the carboxylic acid with the selective solvent in the absorption or extraction unit. Such concentration step may comprise any suitable method for removing excess water from an aqueous carboxylic acid stream, including reverse osmosis, liquid-liquid extraction (LLE) , adsorption or carboxylic acid-selective pervaporation .

In a preferred embodiment of the invention, a dilute liquid aqueous stream comprising C₂-C₆ carboxylic acid is subjected to liquid-liquid extraction (LLE) using an extractive solvent as defined herein to obtain a more concentrated stream comprising C₂-C₆ carboxylic 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 C₂-C₆ carboxylic acid is treated using carboxylic acid-selective pervaporation to produce a concentrated C₂-C₆ carboxylic acid/water vapour stream, which is subsequently separated using absorption or extractive distillation as described herein. In yet another embodiment, a vaporous effluent comprising C₂-C6 carboxylic acid is concentrated by adsorption onto a solid, followed by desorption of a more concentrated C₂-C6 carboxylic acid/water vapour stream

subsequently separated using absorption or extractive

distillation as described herein.

Typically, such a concentration step yields a liquid or vaporous aqueous feed stream comprising C₂-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 amount of selective solvent to be used in the

carboxylic acid separation step may vary within wide ranges, depending on the concentration of carboxylic acid present in the vaporous feed stream as well as the extraction or absorption method employed, each having its own other process parameters, including temperature and pressure.

Typically, the ratio (wt/wt) of selective solvent to carboxylic acid supplied in the carboxylic acid separation step 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.

The temperature in the carboxylic acid separation step may vary within wide ranges. Typically, the temperature in the carboxylic acid separation step is in the range of of from room temperature to 300 °C, more preferably 50 to 280 °C, even more preferably 90 to 260 °C, most preferably 100 to 250 °C.

The pressure in the carboxylic acid separation step may also vary within wide ranges. Typically, the pressure in the carboxylic acid separation step is in the range of of from 0.1 to 20 bar, more preferably 1 to 10 bar, most preferably 2 to 6 bar. It is especially advantageous that the pressure that may be needed in said carboxylic acid separation step may be the same as the pressure in the ODH and/or alkene oxidation

reaction step. In the latter case, there would be no need at all for any compression of said vapour stream in order to carry out the carboxylic acid separation step.

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 C₂-C₆ carboxylic 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 acid 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 C₂-C₆ carboxylic 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 C₂-C₆ carboxylic acid present in the vapour or liquid stream that is subjected to the carboxylic acid separation step is absorbed or extracted by the selective solvent, resulting in a fat selective solvent stream. Furthermore, it is preferred that the selective solvent extracts or absorbs substantially none of the water present in the vapour stream originating from the alkane oxidative

dehydrogenation and/or alkene oxidation step.

Preferably, the fat solvent stream comprises water and carboxylic 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 step, the absorbed or extracted carboxylic acid is removed from the selective solvent resulting in a product stream comprising C2-C6 carboxylic acid and another stream comprising the selective solvent now depleted of C₂-C₆ carboxylic acid. Preferably, in the solvent recovery step, separation of the solvent, and of optional other solvents present, from the C₂-C₆ carboxylic acid is effected by distilling the fat selective solvent comprising carboxylic acid, resulting in a top stream comprising carboxylic acid and a bottom stream comprising the selective solvent. Distillation may be carried out in any distillation unit known to the skilled person that is suitable for separating the fat selective solvent comprising carboxylic acid in a carboxylic acid tops stream and a bottom stream comprising the selective 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 C₂-C6 carboxylic acid present in the stream fed to the solvent recovery unit

comprising C₂-C6 carboxylic acid and fat selective 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 C₂-C6 carboxylic acid and fat selective solvent is recovered.

In a preferred embodiment, at least a portion of the bottom stream comprising the selective solvent is recirculated to the carboxylic acid separation step. 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 carboxylic acid separation step. In one embodiment, the entire bottom stream comprising the selective solvent is recirculated to the carboxylic acid separation step. In one embodiment, the entire bottom stream comprising the selective solvent is recirculated to the carboxylic acid separation step.

The carboxylic acid separation step further produces a vapour stream comprising alkene, water and carbon dioxide and optionally unconverted alkane. Typically, this vapour stream comprising alkene, water, carbon dioxide and optionally alkane is obtained as a top stream in an absorption column. Water may be removed from said vapour stream using a condensation step, for example by cooling down the vapour stream from the

carboxylic acid separation step to a lower temperature, for example room temperature, so that the water condenses and can then be removed from the vapour stream.

The vapour stream comprising alkane and/or alkene, water and optionally carbon dioxide resulting from a vapour phase carboxylic acid separation step may further comprise entrained selective solvent. Typically, said vapour stream resulting from the carboxylic acid separation step 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 selective solvent. Said entrained selective solvent is suitably condensed with water in the aforementioned condensation step and may be recovered by liquid-liquid

separation from the liquid water. Advantageously, such liquid- liquid separation occurs spontaneously upon condensation due to the preferred poor miscibility of water and the selective solvent. In a preferred embodiment, the selective solvent thus recovered is at least partially recirculated to the C₂-C₆ carboxylic acid separation step either as a separate stream or by mixing with a recirculated selective solvent stream from the solvent recovery step.

The resulting gas stream comprising alkane and/or alkene and carbon dioxide is suitably further treated to remove water, optionally to allow recovery and recycle of entrained solvent. For example, such drying may be carried out by contacting the gas stream with an absorbent which has a high affinity for water, such as for example triethylene glycol (TEG) , for example at a temperature in the range of from 30 to 50 °C, suitably about 40 °C. Alternatively, such drying may be carried out by contacting the gas stream with molecular sieves (or "mol sieves") , suitably at a relatively low temperature in the range of from 10 to 25 °C. Using molecular sieves is preferred in a case where the remaining water content should be as low as possible. Other treatments could involve removal of carbon dioxide, typically by absorption e.g. in amine or caustic bath or on solid adsorbent, deep drying of resulting gas, and hydrocarbon purification such as C1/C2/C3+ split and dedicated C2/C2= split.

Detailed description of the drawings

In Figure 1, a gas stream 1 comprising C₂-C₆ alkane and/or alkene containing 2 to 6 carbon atoms and a gas stream 2

comprising oxygen are fed to an oxidative dehydrogenation reactor 3 containing an ODH catalyst and operating under ODH conditions. ODH effluent stream 4, comprising vaporous water, C₂-C₆ alkene, carbon dioxide and C₂-C₆ carboxylic acid is fed to an absorption column 5 to which further an absorbing solvent 6 is fed. C₂-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 C₂-C6 carboxylic acid is fed supplied to a solvent regeneration unit, comprising a distillation unit 9 equipped with condenser section 9a and reboiler section 9b. Desorbed C₂-C6 carboxylic acid leaves distillation unit 9 as stream 10, while absorbing solvent now depleted of absorbed C₂-C6 carboxylic acid exits desorption unit 9 as stream 11. The C₂-C₆ carboxylic acid- depleted absorbing solvent stream 11 may be partially

recirculated to absorption column 5 as absorbing solvent recirculation stream 12. C₂-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.

In Figure 2, effluent stream 1 comprising water, C₂-C6 alkene, carbon dioxide and C₂-C₆ carboxylic 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. C₂-C6 carboxylic 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 C₂-C6 carboxylic acid is fed supplied to a solvent regeneration unit, comprising a distillation unit 6 equipped with condenser section 8a and reboiler section 8. C₂-C₆ carboxylic acid leaves distillation unit 6 as top stream 9, while extractive solvent now depleted of C₂-C₆ carboxylic acid exits distillation unit 6 as bottom stream 10. The C₂-C₆ carboxylic acid-depleted

extractive solvent stream 10 may be partially recirculated to extractive distillation column 2 as extractive solvent

recirculation stream 11. C₂-C₆ carboxylic 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 at least partially be recirculated to extractive distillation column 2 via recirculation stream 15.

In Figure 3, a condensed liquid ODH effluent stream 1

comprising water and C₂-C₆ carboxylic acid is fed to a liquid- liquid extraction column 2 to which further an extractive solvent 3 is fed. C₂-C6 carboxylic acid is absorbed by the extractive solvent, which exits the liquid-liquid extraction column as "fat" solvent stream 5. A liquid stream comprising water exits the liquid-liquid extraction column as stream 7.

Stream 5 comprising fat extractive solvent and extracted

C₂-C6 carboxylic acid is fed supplied to a solvent regeneration unit, comprising a distillation unit 6 equipped with a

condenser section 8a and reboiler section 8. C₂-C₆ carboxylic acid leaves desorption unit distillation unit 6 as stream 9, while extractive solvent now depleted of C₂-C6 carboxylic acid exits distillation unit 6 as stream 10. The C₂-C₆ carboxylic acid-depleted selective solvent stream 10 may be partially recirculated to liquid-liquid extraction column 2 as extractive solvent recirculation stream 11. C₂-C₆ carboxylic acid stream 9 may be further purified downstream.

The invention is further illustrated by the following

Examples . EXAMPLES

Conversion of ethane to ethylene and recovery of acetic acid.

(A) Preparation of the catalyst

A mixed metal oxide catalyst containing molybdenum (Mo) , vanadium (V) , niobium (Nb) and tellurium (Te) was prepared, for which catalyst the molar ratio of said 4 metals was

MolVO .29NbO .17TeO .12.

Two solutions were prepared. Solution 1 was obtained by dissolving 15.8 g of ammonium niobate oxalate and 4.0 g of oxalic acid dihydrate in 160 ml of water at room temperature. Solution 2 was prepared by dissolving 35.6 g of ammonium heptamolybdate, 6.9 g of ammonium metavanadate and 5.8 g of telluric acid (Te(OH)6) in 200 ml of water at 70 °C. 7.0 g of concentrated nitric acid was then added to solution 2. The 2 solutions were combined which yielded an orange gel-like precipitate. The mixture was evaporated to dryness with the aid of a rotating evaporator ("rotavap") at 50 °C.

The dried material was further dried in static air at 120 °C for 16 hours, milled to a fine powder and then calcined in static air at a temperature of 325 °C for 2 hours. After the air calcination, the material was further calcined in a

nitrogen (N2) stream at 600 °C for 2 hours. Then the material was treated with an aqueous 5% oxalic acid solution at 80 °C and filtered and dried at 120 °C.

The dried catalyst powder was pressed into pills which pills were then milled. The milled material was then sieved using a sieve having a mesh size of 40-80 mesh. The sieved material, having a size of 40-80 mesh and composed of porous catalyst particles, was then used in the ethane oxidative dehydrogenation experiments described below.

(B) Catalytic oxidative dehydrogenation of ethane

The catalyst thus prepared was used in experiments

involving ethane oxidative dehydrogenation (ethane ODH) within a small-scale testing unit comprising a vertically oriented, cylindrical, quartz reactor having an inner diameter of 3.0 mm. 0.65 g of the catalyst was loaded in the reactor. The catalyst bed height was 6 cm. On top of the catalyst bed, another bed having a height of 8 cm was placed which latter bed contained inert silicon carbide (SiC) particles having an average

diameter of 0.8 mm.

In these experiments, a gas stream comprising 63 vol.% of ethane, 21 vol.% of oxygen (02) and 16 vol.% of nitrogen (N2) was fed to the top of the reactor and then sent downwardly through the catalyst bed to the bottom of the reactor. Said gas stream was a combined gas stream comprising a flow of ethane having a rate of 3.00 Nl/hr, a flow of oxygen having a rate of 1.00 Nl/hr and a flow of nitrogen having a rate of 0.77 Nl/hr. "Nl" stands for "normal litre" as measured at standard

temperature and pressure, namely 32 °F (0 °C) and 1 bara (100 kPa) . The pressure in the reactor was at least 2.3 bara. The reactor was heated such that the temperature of the catalyst (at the end of the catalyst bed) was 370 °C.

The conversion of ethane and the product composition were measured with a gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) and with another GC equipped with a flame ionization detector. (C) 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 was 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 that was used for chemical process optimization, while parameter

libraries are Shell proprietary.

Example 1: Conversion of ethane to ethylene and acetic acid absorption from vapour phase

(A) Effluent composition

The composition of the vaporous effluent of the oxidative dehydrogenation reactor as described above is as follows:

(B) Vapour-phase absorption of acetic acid

The vapour-phase effluent obtained in (A) is directly supplied to a counter-current absorption column in which the acetic acid is absorbed from the vapour phase by a liquid solvent consisting of 1-decanol. The temperature of the vaporous effluent supplied to the absorption column is 120 °C, the 1-decanol solvent is supplied at a temperature of 50 °C. The pressure of the vaporous effluent is 3.7 bar, the pressure of the 1-decanol solvent is 5.0 bar. The volume ratio of solvent to vaporous effluent is 1.74xl0^~3.

The top vapour phase 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 absorption column.

At the bottom of the absorption column an acetic acid and solvent mixture ("fat solvent") is withdrawn, which is subsequently supplied to an 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 absorption 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 Vaporous Aqueous Solvent

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

(wt %) (wt %)

Water 37.86 2.07 99.83 0.00

Acetic 61.93 0.00 0.15 0.01 acid

1-decanol 0.12 0.02 0.00 99.99

Other 0.09 97.90 0.02 0.00 components 99 % of the acetic acid in the effluent of the ethane oxidative dehydrogenat ion process is recovered as a

concentrated (> 60 wt%) acetic acid stream.

Example 2: Conversion of ethane to ethylene and acetic acid extraction from vapour phase

(A) Effluent composition

Example 1 is repeated, with the distinction that ethane oxidative dehydrogenat ion step (A) is performed under pressure conditions allowing increased formation of acetic acid.

The composition of the vaporous effluent of the oxidative dehydrogenat ion reactor is as follows:

(B) Vapour-phase absorption of acetic acid

Example 1 (B) is repeated, with the distinction that the volume ratio of solvent to vaporous effluent is 3.03xl0^~3. The composition of the various streams produced in this process is as follows: Product streams

Stream Acetic acid [solvent Vaporous Aqueous Solvent recovery top] [VLL [VLL [purge] (vol%) decanter decanter (vol%)

] ]

(vol%) (vol%)

Water 22.43 2.08 99.54 0.00

Acetic 77.39 0.01 0.44 0.01 acid

1-decanol 0.07 0.02 0.00 99.99

Other 0.12 97.90 0.02 0.00 component

s

99 % of the acetic acid in the effluent of the ethane oxidative dehydrogenation process is recovered as a

concentrated (> 75 wt%) acetic acid stream.

Example 3: Conversion of ethane to ethylene and acetic acid extraction from liquid phase

(A) Catalytic oxidative dehydrogenation of ethane

The ethane oxidative dehydrogenation process of Example 1 is repeated. The vaporous effluent is condensed at a temperature of 50 °C, to produce a condensate stream having the composition as follows:

(B) Liquid-liquid extraction of acetic acid

The liquid-phase effluent obtained in (A) is supplied to a counter-current extraction column in which the acetic acid is extracted from the liquid phase by a recirculated liquid phase having the composition as follows:

Component Concentration (wt%)

Water 0

Acetic acid 0.01

Propionic acid 0.02

1-decanol 99.98

Other components 0 The temperature of the condensed liquid effluent supplied to the extraction column is 50 °C, the recirculated liquid solvent comprising 1-decanol is supplied at a temperature of 50 °C. The pressure of the condensed liquid effluent is 3.0 bar, the pressure of the 1-decanol solvent stream is 5.0 bar. The volume ratio of solvent to liquid effluent is 0.83.

The composition of the aqueous liquid top stream and the fat solvent bottom stream obtained from the extraction column is as follows:

The fat solvent withdrawn at the bottom of the extraction column is subsequently supplied to a solvent recovery column as described in Example 1, 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 extraction column. A minor solvent purge is performed in order to minimize solvent losses while avoiding potential build-up of impurities.

Example 4: Conversion of ethane to ethylene and acetic acid extraction from liquid phase

(A) Catalytic oxidative dehydrogenation of ethane

Example 3 is repeated, with the distinction that ethane oxidative dehydrogenation step (A) is performed under pressure conditions allowing increased formation of acetic acid. A condensate stream is produced having the composition as follows :

(B) Liquid-liquid extraction of acetic acid

The liquid-phase effluent obtained in (A) is supplied to a counter-current extraction column in which the acetic acid is extracted from the liquid phase by a recirculated liquid phase having the composition as follows: Component Concentration (wt%)

Water 0

Acetic acid 0.02

Propionic acid 0.02

1-decanol 99.96

Other components 0

The volume ratio of solvent to liquid effluent is 0.64. The composition of the aqueous liquid top stream and the fat solvent bottom stream thus obtained is as follows:

Product streams (wt%)

Component Solvent Aqueous

(bottom) (top)

Water 6.37 99.55

Acetic acid 14.62 0.44

Propionic acid 0.02 0.01

1-decanol 78.97 0.00

Other components 0.03 0.00

Example 5. Vapour-phase extractive distillation of acetic acid

An aqueous vapour-phase ODH 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, C0₂, CO, 0₂) 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 2.04xl0^~3.

The conditions of the extractive distillation column are as follows :

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:

99 % of the acetic acid present in the vaporous aqueous ODH effluent is recovered as a pure (> 99 wt%) acetic acid stream. Example 6. Vapour-phase extractive distillation of acetic acid

The process of Example 5 is repeated, with the distinction that an aqueous vapour-phase ODH 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, C0₂, CO, 0₂) 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:

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

Claims

C L A I M S

1. A process for oxidatively converting alkanes and/or alkenes containing 2 to 6 carbon atoms ("C₂-C₆") to C₂-C₆ alkenes and C₂-C₆ carboxylic acids, comprising

a reaction step which comprises contacting a stream comprising oxygen and said C₂-C₆ alkane and/or alkene with a mixed metal oxide catalyst, to produce a stream comprising C₂- C₆ alkene, water and one or more C₂-C₆ carboxylic acids,

a carboxylic acid separation step which comprises

contacting said stream comprising alkene, water and one or more C₂-C₆ carboxylic acids with a selective solvent, resulting in absorption or extraction of the C₂-C₆ carboxylic acids by the solvent ,

a solvent recovery step which comprises separating said absorbed or extracted C₂-C6 carboxylic acids from the solvent, resulting in a product stream comprising said one or more C₂-C6 carboxylic acids and a stream comprising recovered solvent, and optionally a recycling step which comprises recycling at least a portion of the stream comprising recovered solvent to the carboxylic acid separation step,

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

(i) a Hansen solubility parameter distance R_a with respect to the C₂-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 one or more C₂-C₆ carboxylic acids.

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

3. Process according to any one of the preceding claims, wherein the alcohol is an aromatic or aliphatic, branched or linear, primary, secondary or tertiary alcohol having 4-20, 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 oxidatively converting alkanes and/or alkenes containing 2 to 6 carbon atoms ("C₂-C₆") to C₂-C₆ alkenes and C₂-C6 carboxylic acids, comprising

a reaction step which comprises contacting a stream comprising oxygen and said C₂-C6 alkane and/or alkene with a mixed metal oxide catalyst, to produce a stream comprising C₂- C₆ alkene, water and one or more C₂-C₆ carboxylic acids, a carboxylic acid separation step which comprises

contacting said stream comprising alkene, water and one or more C₂-C₆ carboxylic acids with a selective solvent, resulting in absorption or extraction of the C₂-C₆ carboxylic acids by the solvent,

a solvent recovery step which comprises separating said absorbed or extracted C₂-C₆ carboxylic acids from the solvent, resulting in a product stream comprising said one or more C₂-C₆ carboxylic acids and a stream comprising recovered solvent, and optionally a recycling step which comprises recycling at least a portion of the stream comprising recovered solvent to the carboxylic acid separation step,

wherein the selective 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.

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- 1-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 the C₂-C₆ carboxylic acid separation step is carried by absorption from a vaporous stream comprising C₂-C₆ alkene, water and one or more C₂-C₆ carboxylic acids resulting from the oxidative conversion of C₂-C₆ alkanes and/or C₂-C₆ alkenes.

7. Process according to any one of the preceding claims, wherein the C₂-C6 carboxylic acid separation step is carried by extractive distillation from a vaporous stream comprising C₂-C6 alkene, water and one or more C₂-C6 carboxylic acids resulting from the oxidative conversion of C₂-C6 alkanes and/or C₂-C6 alkenes.

8. Process according to any one of the preceding claims, wherein the C₂-C6 carboxylic acid separation step is carried by liquid-liquid extraction (LLE) from a liquid stream comprising C₂-C6 alkene, water and one or more C₂-C6 carboxylic acids resulting from the oxidative conversion of C₂-C6 alkanes and/or C₂-C₆ alkenes.

9. Process according to any one of the preceding claims, wherein before the C₂-C₆ carboxylic acid separation step, the stream comprising C₂-C₆ alkene, water and one or more C₂-C₆ carboxylic acids is concentrated using reverse osmosis, carboxylic acid-selective pervaporation, adsorption-desorption using a solid adsorbent or liquid-liquid extraction (LLE) , preferably liquid-liquid extraction (LLE) .

10. Process according to any one of the preceding claims, wherein the carboxylic acid separation step further results in a vapour stream comprising alkene, water and optionally carbon dioxide, and wherein water is recovered from said vapour stream by condensation.

11. Process according to claim 10, wherein said vapour stream further comprises entrained selective solvent, and wherein said entrained selective solvent is recovered by liquid-liquid separation from water upon condensation.

12. Process according to any one of the preceding claims, wherein the process is a process of the oxidative

dehydrogenation of a C₂ or C3 alkane, preferably ethane or propane, more preferably ethane.

13. Process according to any one of the preceding claims, wherein the process is a process of the oxidation of a C₂ or C3 alkene, preferably ethylene or propylene, more preferably ethylene .

14. Process according to any one of the preceding claims, wherein the product of the oxidative dehydrogenation of an alkane and/or the oxidation of an alkene comprises acrylic and/or acetic acid, preferably acetic acid.