EP4630138A1

EP4630138A1 - Separation process of alcohol mixtures with high water content by liquid-liquid extraction

Info

Publication number: EP4630138A1
Application number: EP23817545.9A
Authority: EP
Inventors: Laura Annamaria PELLEGRINI; Elvira SPATOLISANO; Samuele Gori; Giacomo FILIPPINI
Original assignee: Eni SpA
Current assignee: Eni SpA
Priority date: 2022-12-06
Filing date: 2023-12-04
Publication date: 2025-10-15
Also published as: IT202200025071A1; WO2024121703A1; CN120583985A; MX2025006308A

Abstract

The present invention relates to a proces s for separating alcohol mixtures with high water content using an extraction solvent. In particular, the present invention is directed to a proces s for the liquid-liquid extraction of a mixture of C2-C3 or C2-C4 alcohols from a mixture of C2-C3 or C2- C4 alcohols and water with an extraction solvent, where the extraction solvent is selected from isobutyraldehyde and an n-pentane/dimethyl acetal mixture.

Description

SEPARATION PROCESS OFALCOHOLMIXTURES WITH HIGH WATER CONTENT BY LIQUID-LIQUID EXTRACTION

Description

Technical field of the invention

The present invention relates to a process for separating alcohol mixtures with high water content.

Background art

While water-alcohol separation is a widely known and studied problem in the literature, the separation of alcohol mixtures from water (i.e., when multiple alcohol species are simultaneously present in an aqueous solution) is a more rarely addressed problem due to the complexity thereof. The difficulty in the separation lies in the identification of a solvent or solvent mixture capable of simultaneously extracting all the alcohols, separating them from the significant water content in which they are dissolved, while minimizing the energy demands of the process.

Ethylene glycol is one of the most widely used solvents in the context of extractive water-alcohol distillation. The high boiling point and high water affinity make ethylene glycol an excellent dehydrating agent. The solution usually adopted to promote the dehydration of a stream of alcohols can be summarized as follows: the added solvent (i.e., ethylene glycol) is fed, together with the water-alcohol mixture to be separated, to an extractive distillation column. Such a column carries out the fractionation of the alcohol stream, obtained as a top product, from the water-ethylene glycol stream, recovered from the bottom of the aforesaid column. The latter stream must be further fractionated in a downstream distillation column, so that the solvent can be regenerated and then recycled to the extractive distillation column. Extractive distillation with ethylene glycol as a solvent usually requires a non- negligible thermal load to be provided to the reboiler of the solvent regeneration column.

In fact, despite the chemical-physical features of ethylene glycol making it particularly suitable for applications as a dehydrating agent, the high boiling character thereof is responsible for the high temperatures reached during regeneration, which evidence weighs down the energy requirements of the process itself (i.e., the thermal loads of the column bottom reboilers and the related operating temperatures).

As an alternative to extractive distillation, liquidliquid extraction using toluene, iso-octane or other hydrocarbon species as solvents is reported in the literature. However, the suggested solutions concern negligible amounts of mixed water and ethanol. Therefore, they are not flexible as for the distribution of alcohols in the feed and are not suitable for the separation of alcohol mixtures particularly diluted in water.

Various known processes provide mixtures of alcohols, in particular C2-C3 or C2-C4 alcohols, in water as products or by-products. An example is the glycerol hydrogenation process to give propanols described in copending patent application No. 102021000016859 filed on 06/28/2021.

In particular, the use of propanols in biofuels is recommended. Propanols have the same positive features as ethanol with reference to the ignition qualities and calorific value thereof, but have higher energy density (+12%), lower volatility, and lower latent vaporization heat (-8%). In principle, a good source of propanols can be glycerin.

Propanols can also be produced using noble metalbased catalysts, modified by dopants and promoters, in particular metal oxides (groups 4-7) (Shinmi, Y., Koso, S., Kubota, T., Nakagawa, Y., Tomishige, K., Modification of Rh/SiCh catalyst for the Hydrogenolysis of Glycerol in Water, Appl. Catal. B Environ. 2010, 94 (3-4), 318-326).

These processes produce mixed propanols with a high amount of water which can reach up to 50% by weight and, as such, cannot be advantageously used in a biofuel. Therefore, there is a need to provide a simple and cost-effective process for isolating a substantially water-free mixture of propanols.

Summary of the invention

The inventors of the present patent application started from the consideration that the added solvent must have very precise features to promote the separation of the solute (i.e., the alcohols) from the original solvent (i.e., water), such as:

- appearing as a heterogeneous azeotrope with water, so that a miscibility gap is established resulting in the formation of an additional liquid phase (see Errore. L'origins riferimento non e stata trovata.A, 2B, 2C by way of example, in which the immiscibility region between the two liquid phases is highlighted in grey);

- appearing as a preferably positive slope of the conodal lines (i.e., the segments identifying the thermodynamic equilibrium between aqueous phase and organic phase), to minimize the flow rate of the solvent added circulating in the process.

Once the most suitable solvent or solvent mixture for promoting the separation of the aqueous phase (raffinate) from the organic phase (extract, i.e., the extraction solvent-solute mixture) has been identified, it is necessary to proceed with the regeneration of the solvent, so that the latter can be recycled to the extractor while the alcohol mixture can be separated as a pure mixture. Therefore, for the aforesaid separation to be feasible without particularly high energy costs, the extraction solvent must:

-not have azeotropes present with each of the alcohols present in the mixture;

- have a normal boiling point low enough not to result in high temperatures in the distillation step.

The present invention is based on the discovery that treating a mixture of C2-C3 or C2-C4 alcohols and water with an extraction solvent selected from isobutyraldehyde or an n-pentane/dimethyl acetal mixture, yields a mixture of C2-C3 or C2-C4 alcohols and extraction solvent from which the latter can be regenerated with a low energy expenditure .

Therefore, the present invention relates to:

1) a process for the liquid-liquid extraction of a mixture of C2-C3 or C2-C4 alcohols from a mixture of C2- C3 or C2-C4 alcohols and water with an extraction solvent, where the extraction solvent is selected from isobutyraldehyde and an n-pentane/dimethyl acetal mixture.

The invention is also directed to: 2) a process according to 1), where the mixture of C2-C3 alcohols consists of ethanol, n-propanol, and isopropanol;

3) a process according to 1) or 2), where the water content in the mixture of C2-C3 alcohols and water is up to 50% by weight;

4) a process according to any one of 1) to 3), comprising or consisting of the following steps: a) contacting the water-alcohol mixture with an extraction solvent selected from isobutyraldehyde and an n-pentane/dimethyl acetal mixture to give an extraction solvent/alcohol mixture; b) separating the extraction solvent;

5) a process according to 4), where step b) is conducted in a distillation column and where the overhead recovered extraction solvent is recycled to step a);

6) a process according to any one of 1) to 5), where the weight ratio of extraction solvent to water-alcohol mixture is between 1:1 and 5:1;

7) a process according to 6), where, when the extraction solvent is isobutyraldehyde, the weight ratio of extraction solvent to water-alcohol mixture is between 1:1 and 3:1, or between 1.5:1 and 2.5:1;

8) a process according to 6), where, when the extraction solvent is an n-pentane-dimethyl acetal mixture, the weight ratio of extraction solvent to water- alcohol mixture is between 2:1 and 5:1, or between 2.5:1 and 4:1;

9) a process according to 8), where the n- pentane/dimethyl acetal weight ratio is between 1:1 and 2:1, or of about 1.5:1;

10) a process according to any one of 1) to 9), where the recovery of C2-C3 alcohols is greater than 99% by weight, or when the extraction solvent is isobutyraldehyde, preferably greater than 99.9% by weight;

11) a process according to any one of 1) to 10), where the purity of the mixture of recovered C2-C3 alcohols is greater than 98.5% by weight, or greater than 99% by weight, or when the extraction solvent is an n-pentane- dimethyl acetal mixture, greater than 99.9% by weight;

12) a process according to any one of 1) to 11), where the liquid-liquid extraction is conducted at ambient pressure and temperature;

13) a process according to any one of 1) to 12), where step a) is conducted in a liquid-liquid separator in which the extraction solvent is fed into the lower section and the mixture of C2-C3 alcohols and water is fed into the upper section, and where the extracted mixture of extraction solvent and C2-C3 alcohols is removed in the upper section and the separated water is removed in the lower section. Brief description of the drawings

Figure 1 shows a diagram showing a plant for carrying out the process of the invention in a preferred embodiment;

Figures 2A, 2B, and 2C show the triangular diagrams for the systems: A) water - ethanol - isobutyraldehyde; B) water - isopropanol - isobutyraldehyde; C) water - 1- propanol - isobutyraldehyde.

Detailed description of the invention

The invention aims to perform a separation of a mixture of short-chain primary alcohols (mainly C2-C3 or C2-C4) containing a high water content (up to about 50% by weight) using an extracting agent selected from isobutyraldehyde and an n-pentane/dimethyl acetal mixture.

Isobutyraldehyde has proved to be an ideal solvent to perform the required separation, since:

- it has an extended miscibility gap with water- alcohol mixtures (see the triangular diagrams in figures 2A, 2B and 2C, where the grey area represents the immiscibility area between the two liquid phases);

- it does not have azeotropes with the alcohols present in the feed mixture (ethanol, n-propanol, and iso-propanol);

- it has a normal boiling temperature of 64°C, lower than that of ethanol (about 78°C).

The thermodynamic behavior of isobutyraldehyde as a solvent, which is difficult to replicate considering a single species, can be approximated by mixing together several solvents with different features. The present inventors have found that n-pentane-dimethyl acetal mixtures can be advantageously used for the liquid-liquid extraction according to the invention.

The present invention relates to a process for the liquid-liquid extraction of a mixture of C2-C4 or C2-C3 alcohols from a mixture of C2-C4 or C2-C3 alcohols and water (hereinafter also referred to as the "water-alcohol mixture") with an extraction solvent, where the extraction solvent is selected from isobutyraldehyde and an n-pentane/dimethyl acetal mixture.

More specifically, the invention is directed to a process for the liquid-liquid extraction of a mixture of C2-C4 or C2-C3 alcohols from a mixture of C2-C4 or C2-C3 alcohols and water (hereinafter also referred to as the "water-alcohol mixture") with an extraction solvent, comprising or consisting of the following steps: a) contacting the water-alcohol mixture with an extraction solvent selected from isobutyraldehyde and an n-pentane/dimethyl acetal mixture to give an extraction solvent/alcohol mixture; b) separating the extraction solvent. For the purposes of the present invention, the term "comprising" means a process where the characterizing steps consist of steps a) and b), but which can also include other steps such as a pre-treatment of the mixture of C2-C3 alcohols in water or the final addition of additives to the organic mixture forming the extract in order to be used in a biofuel.

For the purposes of the present invention, the term "consisting of" means a process where no further pre- or post-processing of the extraction mixture is included.

The invention allows obtaining a mixture of pure alcohols usable as an octane booster directly in the biofuel mixture with improved motor properties (e.g., octane number).

The mixture of C2-C3 alcohols comprises ethanol, n- propanol, and isopropanol as main components.

Typical compositions of a water-alcohol mixture to be subjected to the process of the invention are shown in tables 1 and 4.

The extraction solvent, selected from isobutyraldehyde and an n-pentane/dimethyl acetal mixture, is fed into a liquid-liquid extraction column (C-100 in figure 1) operating at atmospheric pressure and ambient temperature, in which it is contacted with the water-alcohol feed to be extracted. Such an apparatus allows the separation of the aqueous phase {Raffinate in figure 1) from the organic phase {Extract in figure 1). The organic phase contains the extraction solvent in which almost all of the alcohols present in the mixture are dissolved, while the aqueous phase consists of water containing traces of alcohols and extraction solvent. The organic phase is sent, after heating in E-l, to subsequent distillation in column C-101f to allow the regeneration of the solvent and the consequent recycling thereof to the liquid-liquid extraction column. The distillation column C-101, provided with a total condenser and a partial reboiler, carries out the fractionation of the tail stream {Product in figure 1), consisting of the alcohols originally fed at the battery limits, from the overhead stream, consisting of the heterogeneous solvent-water azeotrope which is separated by decantation in T-100. The organic phase leaving the decanter T-100 forms the recycling solvent to the extractor C-100. The aqueous phase is instead mixed with the raffinate leaving the column C-100 and further treated in C-102. The optional equipment C-102 represents the possible finishing stage for the aqueous phase leaving the process, so that the latter complies with the ThOD specifications imposed.

The equipment C-102 is similar to a standard distillation column or a flash stage, depending on the degree of purity desired for the water leaving the process.

The weight ratio of extraction solvent to water- alcohol mixture is between 1:1 and 5:1. Preferably, when the extraction solvent is isobutyraldehyde, the weight ratio of extraction solvent to water-alcohol mixture is between 1:1 and 3:1, more preferably between 1.5:1 and 2.5:1. Preferably, when the extraction solvent is an n- pentane dimethyl - acetal mixture, the weight ratio of extraction solvent to water-alcohol mixture is between 2:1 and 5:1, more preferably between 2.5:1 and 4:1.

When the extraction solvent is an n-pentane dimethyl acetal mixture, the n-pentane/dimethyl acetal weight ratio is preferably between 1:1 and 2:1, or of about 1.5:1.

By way of example, stream specifications according to an embodiment of the process of the invention using isobutyraldehyde as an extraction solvent are shown in Table 1.

Table 1. Stream specifications for isobutyraldehyde as a solvent

Table 2 instead shows the thermal loads for the same exemplary embodiment in Table 1.

The thermal loads in the following tables derive from the heats at the reboilers and are equivalent to the difference in Enthalpies between the streams leaving and those entering the reboilers, where the enthalpy of each stream is the sum of the enthalpies of the individual components weighed based on the composition of each stream. Table 2. Thermal loads for the example in Table 1 (isobutyraldehyde as a solvent) and with reference to the equipment in Figure 1

The results obtained with such an exemplary embodiment are shown in Table 3.

Table 3. Final results related to the example in Table 1 (isobutyraldehyde as a solvent)

In such an experiment, the weight ratio of extraction solvent/alcohol mixture and water was about 1.9. Table 4 shows the flow rates of the material flows according to a different exemplary embodiment of the process of the invention using an n-pentane-dimethyl acetal mixture as the extraction solvent (n- pentane/dimethyl acetal weight ratio = 1.48).

Table 4. Specifications of the streams highlighted in 5 Figure 1, in the case of n-pentane-dimethyl acetal as a solvent mixture

Table 5 instead shows the thermal loads for the same exemplary embodiment in Table 4.

10

Table 5. Thermal loads for the example in Table 4 (n-pentane- dimethyl acetal mixture as a solvent) and with reference to 15 the equipment in Figure 1 E-5 57.16 57.15 -53.42

E-6 86.24 98.53 573.30

The results obtained with this exemplary embodiment are shown in Table 6.

Table 6. Final results for the example in Table 4 (n-pentane- dimethyl acetal mixture as a solvent)> In such an experiment, the weight ratio of extraction solvent /alcohol mixture and water was greater than 3.

Table 7. Comparison of contributions to the exergy balance In order to compare the two suggested solutions and prior art, it is necessary to consider the thermal loads of the reboilers with the related temperature levels. For this reason, it is useful to introduce an overall exergy balance, expressed as: where : expresses power requests and expresses the thermal loads associated with the temperatures at which the latter are supplied.

By comparing the three cases under examination, the results shown in Table 7 are obtained, from which it is apparent that:

- the process with isobutyraldehyde leads to a reduction in exergy terms of 58% with respect to the prior art (ethylene glycol);

- the process with n-pentane/dimethyl acetal leads to a reduction in exergy terms of 58% with respect to the prior art (ethylene glycol).

The process of the invention generally allows achieving several advantages. In fact:

- the water-alcohol separation is promoted by the liquid-liquid extraction. Such a solution shows lower energy costs if compared to extractive distillation, since the exchange of material between the phases occurs in the liquid phase only, thus not requiring the vaporization of part of the liquid circulating in the column;

- the solvent regeneration is less wasteful, involving lower boiling compounds.

Claims

1. A process for the liquid-liquid extraction of a mixture of C2-C4 or C2-C3 alcohols from a mixture of C2- C4 or C2-C3 alcohols and water with an extraction solvent, wherein the extraction solvent is selected from isobutyraldehyde and a mixture of n-pentane/dimethyl acetal.

2. The process according to claim 1, wherein the mixture of C2-C3 alcohols consists of ethanol, n-propanol and isopropanol.

3. The process according to claim 1 or 2, wherein the water content in the mixture of C2-C3 alcohols and water is up to 50% by weight.

4. The process according to any one of claims 1 to 3, comprising or consisting of the following steps: a) contacting the water-alcohol mixture with an extraction solvent selected from isobutyraldehyde and an n-pentane/dimethyl acetal mixture to give a mixture of extraction solvent/alcohol; b) separating the extraction solvent.

5. The process according to claim 4, wherein step b) is conducted in a distillation column and wherein the overhead recovered extraction solvent is recycled to step a).

6. The process according to any one of claims 1 to 5, wherein the weight ratio of extraction solvent to water- alcohol mixture is between 1:1 and 5:1.

7. The process according to claim 6, wherein, when the extraction solvent is isobutyraldehyde, the weight ratio of extraction solvent to water-alcohol mixture is between 1:1 and 3:1, or between 1.5:1 and 2.5:1.

8. The process according to claim 6, wherein, when the extraction solvent is an n-pentane-dimethyl acetal mixture, the weight ratio of extraction solvent to water- alcohol mixture is between 2:1 and 5:1, or between 2.5:1 and 4:1.

9. The process according to claim 8, wherein the n- pentane/dimethyl acetal weight ratio is between 1:1 and 2:1, or of about 1.5:1.

10. The process according to any one of claims 1 to

9, wherein the recovery of C2-C3 alcohols is greater than 99% by weight or, when the extraction solvent is isobutyraldehyde, preferably greater than 99.9% by weight.

11. The process according to any one of claims 1 to

10, wherein the purity of the mixture of recovered C2-C3 alcohols is greater than 98.5% by weight, or greater than 99% by weight or, when the extraction solvent is an n- pentane-dimethyl acetal mixture, greater than 99.9% by weight.

12. The process according to any one of claims 1 to 11, wherein the liquid-liquid extraction is conducted at ambient pressure and temperature.

13. The process according to any one of claims 1 to 12, wherein step a) is conducted in a liquid-liquid separator where the extraction solvent is fed into the lower section and the mixture of C2-C3 alcohols and water is fed into the upper section, and wherein the extracted mixture of extraction solvent and C2-C3 alcohols is removed in the upper section and the separated water is removed in the lower section.