WO2008119489A1

WO2008119489A1 - Method for separating product mixtures of transesterification reactions

Info

Publication number: WO2008119489A1
Application number: PCT/EP2008/002361
Authority: WO
Inventors: Michael Traving; Rafael Warsitz; Werner BÄCKER; Johannes-Peter Schaefer; Martina Mutter
Original assignee: Bayer Technology Services Gmbh
Priority date: 2007-04-02
Filing date: 2008-03-26
Publication date: 2008-10-09
Also published as: DE102007016157A1

Abstract

The invention relates to a method for separating product mixtures which are produced by a transesterification reaction and which contain a polar electrolyte-containing organic phase and a nonpolar organic phase. By means of a porous, hydrophobic separating agent, the nonpolar organic phase is separated from the polar electrolyte-containing organic phase, which remains behind.

Description

Process for the separation of product mixtures from transesterification reactions

The present invention relates to a process for the separation of dispersions of polar, electrolyte-containing and non-polar organic phases with the aid of a hydrophobic, porous release agent.

A variety of methods for the separation of dispersions, in particular dispersions of organic and aqueous phases, has long been known. These methods are mostly based on the difference in specific surface tension and density. Generally, immiscible liquids utilize gravity-based phase separation to separate the heavier phase. This procedure can also be applied on an industrial scale, provided that sufficient phase separation occurs.

Another approach to separating organic phases is to use supercritical fluids to extract the product phase. In such a method, the supercritical fluid is used as a gaseous entraining agent, which is fed to the separation stage as a superheated vapor phase - based on the operating conditions of the separation stage - and loaded again with portions of the organic material. As entrainer water or mixtures of water and a short-chain alcohol are often used.

An example of the aforementioned principle based on the purification of fatty acid methyl esters by means of superheated methanol from the glyceride transesterification is described in DE 43 40 093 A1. This application is especially important for the production of biodiesel, as it always results in a separation task for the reaction products of the transesterification step, which must be separated from the glycerol also formed.

However, the disadvantage of this method of separating mixtures of different polarity is precisely the handling of the supercritical vapor and the quantities of entraining agent itself. Especially in view of the ever stricter environmental regulations, the disposal of large amounts of entraining agent in industrial processes is an important aspect. More recently, special release agents have been developed for the above separation steps, showing improved separation at high throughput and greater flow. Among other things, the modern release agents are membranes with hydrophobized surfaces. Examples of such membranes are known from various publications such as DE 33 12 573 Al and DE 37 12 391 Al.

DE 33 12 573 A1 describes a process for dewaxing vegetable oils, in which a special membrane having a critical surface tension of less than 33 mN / ra and a mean pore diameter of 0.05-5 μm is used. This membrane, in conjunction with the choice of a reduced filtration temperature of about -10 ° C-20 ° C, ensures a good separation performance due to the fact that the waxes to be separated are present as solids at this temperature. In this way, unwanted components such as waxes, phospholipids, free fatty acids and water can be separated from a crude vegetable oil.

A similar process for the separation of phospholipids is also known from the documents EP 1 416 037 A1 and DE 32 44 007 C2. Here, too, a filtration is carried out by means of a hydrophobic membrane to improve the quality of vegetable oils.

However, in addition to the above-described refining of natural or recycled oils, the production of biodiesel on an industrial scale has recently gained increasing economic importance. In this preparation, from purified or treated oils by way of a transesterification reaction, preferably methyl esters of the fatty acids contained in the oils are obtained, which are used as fuel. In addition, glycerol is obtained, which is also used as a raw material, for example in the cosmetics industry. The transesterification reaction can be catalyzed either acidic or basic. In addition, enzymatic methods for catalyzing the transesterification reaction are also known. An overview of the processes used hitherto for the large-scale transesterification of oils for obtaining fatty acid esters which can be used as fuel is given, for example, by the publications DE 199 25 871 A1, DE 602 09 028 T2 and DE 10 2004 044 660 A1 together with the references cited therein. All previously known methods are based on the problem that only partially perform the separation of the product mixture of polar, glycerol-containing phase and nonpolar ester phase, so that in particular the ester phase can be obtained only with remaining traces of water, electrolytes and by-products. The known processes are based in particular on an industrial scale on the separation of the heavier polar phase by phase separation by means of gravity.

In the conventional processes for the preparation of fatty acid esters based on a basic transesterification, as described in AT 386222, AT 397966, AT 387399, DE 3727981, DE 3020612, DE 3107318, WO 92/00268 or DE 19925871A1, a separation of intermediates and products described by a phase separation. These phase separations can therefore be realized by standing or lying gravity separators, possibly equipped with internals, or by centrifuges. The methods used can accordingly perform the phase separation with respect to the electrolytes contained insufficient, so that a complex workup of the fatty acid methyl ester must be done. Here, the electrolyte is reacted with an acid to a salt, which is then washed out in a further process step by a water wash.

The phase separation itself is of central importance especially in the economically relevant field of biodiesel production. Phase separation is adversely affected by soaps and mucilage. In order to enable the phase separation, a high expenditure on equipment is driven in the biodiesel process known today, so that free fatty acids, which react in processes of base-catalyzed transesterification of triglycerides to soaps, extracted in upstream process steps or separated in other ways.

Subsequently, in the conventional process, separators or centrifuges are used to separate the reaction products. Disadvantage of this phase separation is as already mentioned above, that they require a complicated separation and neutralization of the basic catalyst and at least one subsequent step in the separation of the salt load from the biodiesel required. The salt load is washed out with an addition of fresh water from the non-polar, organic phase, the biodiesel. This results in high costs due to the necessary disposal or - -

Workup of saline wastewater. At the same time, due to the chemical conversion of the catalyst, it is no longer available for reuse and must be discharged from the process. This represents a further disadvantage with regard to a large-scale use of the known processes, which are preferably designed in a continuous procedure.

In addition, the addition of further polar solvents and in particular water is often necessary to promote the phase separation, so that on the one hand greatly increase the volumes and on the other hand, the necessity of disposing of the solvents used. Both are in the industrial environment of great economic disadvantage.

The object of the present invention is therefore to provide a simplified process for obtaining the non-polar products from the mixture of a transesterification reaction in the highest possible quality.

This object is achieved by a method for the separation of the product mixture of a transesterification reaction with a polar, electrolyte-containing organic phase and a non-polar organic phase in which by means of a porous hydrophobic release agent, the non-polar organic phase of the remaining polar, electrolyte-containing organic phase is separated.

Transesterification reactions, as already indicated above, are an industrially important class of organic reactions in which an ester is converted by exchange of the acid groups or by exchange of the alcoholic groups for another ester. If the transesterification takes place by exchanging the alcoholic groups, this is also called alcoholysis. In alcoholysis, the alcohol to be replaced is generally added in excess to obtain a high yield of the desired ester. Because the transesterification reaction is an equilibrium reaction, which is usually triggered by mixing the reactants. However, the reaction proceeds so slowly that a catalyst for acceleration is required for commercial purposes. An example of an economically important transesterification reaction is the preparation of fatty acid esters of short-chain alcohols, in which by transesterification of natural fats or oils such as rapeseed oil or soybean oil a product mixture usable as a fuel is obtained from various fatty acid esters.

Fats and oils of biological origin consist predominantly of glycerides (mono-, di- and triglyceride). In practice, the Bradshaw process is often used to transesterify

Greasing and oiling with methanol, such as in US Patents 2,271,619 and

2,360,844. But also various modifications of the

Procedure are common. Especially in the field of catalysts used, there is a wide variation. In addition to the base catalyzed processes (see, for example, J. Am. Chem. Soc., 61, 343, or Ullmann, Enzyklopadie der Technischen Chemie, 4th Edition, Vol.

Page 432), acid-catalyzed processes and enzymatic methods are also conceivable.

According to the invention, the term polar, electrolyte-containing organic phase means the proportion of the product mixture from the transesterification reaction which, in addition to the compounds having free hydroxyl groups, also contains those compounds which have one or more free acid groups. As electrolytes, all salt-like organic or inorganic compounds are referred to here and below. These include, for example, the catalysts preferably used in the transesterification reaction, such as sodium or potassium hydroxide, ammonium compounds or sulfuric acid compounds. However, even those salts which enter the reaction mixture as part of the educts used are to be grouped together under the term electrolytes according to the present invention.

According to a preferred embodiment of the invention, the polar, electrolyte-containing organic phase contains methanol, ethanol, isopropanol, water and / or glycerol.

In contrast, the term non-polar organic phase according to the invention means the proportion of the product mixture from the transesterification reaction which essentially contains compounds with non-polar groups such as alkyl groups, alkenyl groups, ester groups and / or functionalities with similarly low polarity.

In a preferred embodiment of the present invention, the nonpolar organic phase contains fatty acid esters. Particularly preferably, the non-polar organic phase may contain methyl, ethyl and / or isopropyl esters of natural fatty acids. - -

With the method according to the invention, it is now possible to provide an industrially usable process for the separation of product mixtures from transesterification reactions which, in addition to a considerably simplified reaction procedure for obtaining the product esters from the mixture, also permits greater contamination of the educts used. These advantages can be achieved in particular due to the good separation effect of the hydrophobic release agent used according to the invention based on the polarity differences of the individual constituents.

As impurities of the starting materials in the process according to the Invention, for example, a high water content of the substances used or a higher proportion of free acids can be tolerated. So far, however, the proportions of the impurities mentioned must not exceed certain limits, as set out in the literature cited above.

With the separation according to the invention of the non-polar ester phase from the remaining polar organic phase by means of a hydrophobic release agent is advantageously achieved that the impurities remain in the remaining polar phase and thus does not interfere with the ester products.

In contrast to the hitherto known methods for the separation of the product mixtures from transesterification reactions, the robustness of the method according to the invention, as compared to a rather poor quality of the reactants, can continue to manifest itself in that mixtures with a higher proportion of soap can now also be separated. The increased proportion of soap results from an increased proportion of free fatty acids in the educt and is converted by the basic catalysts used to soaps. The soaps act in the product mixture in such a way that they stabilize the resulting product mixture as a dispersion or emulsion such that it can not be separated in a conventional manner due to the differences in density and surface tension. This disadvantage is thus overcome by the method according to the invention. Therefore, there is the further process technical advantage that part of the complex purification of the educts can be omitted. The process according to the invention is preferably carried out such that initially a reaction mixture for the transesterification from educt ester, catalyst and alcohol is stirred for a certain time in a reaction vessel. The reaction temperature, pressure and reaction time are not critical and are chosen so that as complete a conversion as possible is achieved with the lowest possible residence time. The resulting product mixture is fed to a separation stage which is provided with a hydrophobic release agent. The hydrophobic release agent allows the non-polar product ester phase to permeate, with the polar organic phase being retained with the alcohol and catalyst components. The thus obtained non-polar phase with the product esters can then be further processed directly, for example, washed again. The remaining polar phase can be worked up in further steps until the desired products, such as, for example, glycerol, are obtained in a purity which allows their use as a raw material for a very wide range of applications. Alternatively, the polar, electrolyte-containing phase can be recycled to the transesterification reaction or act as a starting material in a further transesterification reaction. In this way, the conversion of the reactants can be increased again overall.

The hydrophobic release agent may consist of a hydrophobic membrane of ceramic or a polymer. In particular, the hydrophobic membrane may have a pore size of from 0.05 μm to 10 μm, more preferably from 0.1 μm to 5 μm.

The hydrophobic release agent, by its nature, exhibits a high rate of separation between the polar, electrolyte-containing phase and the nonpolar phase, which can permeate through the membrane. In addition, the release agent should be chosen so that preferably against the background of a large-scale design, a high Permeationsfius and a good resistance of the release agent are given.

In a preferred embodiment of the invention, the release agent consists of a hydrophobic membrane of ceramic, polymer, or composite materials of polymer and ceramic, which have a coating with hydrophobic material, such as perfluorinated polymers (PTFE, PVDF) or hydrophobic polymers (polypropylene), or whose surface has been rendered hydrophobic by the use of isocyanates or silanes. Particularly preferably, the hydrophobic membrane of ceramic, polymer, or corresponding composite materials of polymer and ceramic, which has sufficient hydrophobicity without further modification. - -

In a particular embodiment of the process, the hydrophobic membrane is a ceramic hydrophobized membrane. As ceramic membranes in particular membranes based on Al ₂ O ₃ , ZrO ₂ , TiO ₂ , SiC and combinations of these compounds are used.

In a particular embodiment of the method, the membrane is a hydrophobic polymer membrane. Membranes based on PP, PVDF, PTFE, silicones and combinations of these compounds are used in particular as polymer membranes.

The hydrophobic polymer membrane may have pores having a pore size of from 0.05 μm to 10 μm, more preferably from 0.1 μm to 5 μm.

With particular preference, no further polar organic solvents and / or water are added to the product mixture of the transesterification reaction.

In contrast to known processes in which water and / or alcohols are added in order to improve the phase separation, in the process according to the invention advantageously both an additional part of the customary separation volume to be moved and separated and the subsequent disposal and preparation of the additives can be dispensed with. The reaction, the equipment required and the workup can therefore continue to be considerably simplified.

The process according to the invention can be conducted either as a batch process or as a continuous process. It is particularly preferably conducted as a continuous process.

A process according to the present invention can be used particularly preferably for obtaining fatty acid methyl esters from the transesterification reaction of biological, that is to say vegetable and / or animal, fats or oils. Particularly preferred is a method according to the present invention for the production of biofuels used. - -

Examples:

example 1

The release agent used is a hydrophobic PP polymer membrane. The PP membrane used is tubular with an inner diameter of 5.5 mm and an outer diameter of 8.5 mm, a length of 250 mm and an active inner filtration area of 0.004 m ² . The pore diameter of the membrane is 0.2 μm.

The reaction solution (feed) used resulted from a basic transesterification of rapeseed oil with KOH. In this case, 6950 g of rapeseed oil were mixed with 695 g of methanol in a stirred tank and reacted with 463.3 g of a 25% KOH-methanol catalyst mixture at 35 ° C to implement. After a reaction time of 1 hour, the reaction solution (feed) was passed over the inside of the membrane used by continuously circulating 400 kg / h. In this case, a clear single-phase fatty acid methyl ester phase (permeate) could be deducted. The resulting from the reaction glycerol phase was retained by the PP membrane. The separated permeate phase is single-phase, in the sense that only the physically dissolved glycerol (<0.1% by weight) and / or water (<0.1% by weight) is included. Table 1 shows the phase separation by means of a hydrophobic PP membrane.

Table 1:

Example 2

The release agent used is a hydrophobic PP polymer membrane. The PP membrane used is tubular with an inner diameter of 5.5 mm and an outer diameter of 8.5 mm, a length of 750 mm and an active inner filtration area of 0.012 m ² . The pore diameter of the membrane is 0.2 μm.

6950 g of rapeseed oil and 215 g of oleic acid were mixed with 555 g of methanol in a stirred tank and reacted with 649.9 g of a 25% strength KOH-methanol catalyst mixture at 35 ^° C. After 1 hour reaction time, the reaction solution (feed) was passed through continuous pumping over the inside of the membrane used. A clear fatty acid methyl ester phase (permeate) could be withdrawn. The resulting from the reaction glycerol phase and saponified by the KOH free fatty acids (oleic acid) was retained by the PP membrane. The separated permeate phase is single-phase, in the sense that only the physically dissolved glycerol (<0.1% by weight) and / or water (<0.1% by weight) is included. The free fatty acids (oleic acid) or soaps are also almost completely retained by the membrane.

The reaction solution (feed) described above contains the glycerol phase, which forms a cloudy ester / glycerol phase as a stable dispersion. The complete separation of the glycerol phase from the ester phase by settling of the heavy phase is possible only after a very long period of time (> 24 hours) or in the centrifugal field.

- -

Table 2 shows the phase separation of a hydrophobic PP membrane.

Table 2:

From this result it is clear that even an increased amount of free acid in the reaction mixture of the transesterification reaction with the process according to the invention leads to the separation of this stable dispersion and to the production of a high quality non-polar product. It can thus be shown that the process according to the invention makes it possible to dispense with a corresponding preparation of the educts, which was previously necessary.

Claims

claims

1.) A process for the separation of the product mixture of a transesterification reaction with a polar, electrolyte-containing organic phase and a non-polar organic phase characterized in that by means of a porous hydrophobic release agent, the non-polar organic phase is separated from the remaining polar, electrolyte-containing organic phase.

2.) Process according to claim 1, characterized in that the non-polar organic phase contains fatty acid esters.

3.) Process according to claim 2, characterized in that the non-polar organic phase contains methyl, ethyl and / or isopropyl esters of natural fatty acids.

4.) Method according to one of the preceding claims, characterized in that the polar, electrolyte-containing organic phase contains methanol, ethanol, isopropanol, water and / or glycerol.

5.) Method according to one of the preceding claims, characterized in that the release agent consists of a hydrophobic membrane.

6.) Process according to claim 5, characterized in that the membrane has a pore size of 0.05 .mu.m to 10 .mu.m.

7.) Method according to one of the preceding claims, characterized in that no further polar organic solvents and / or water are added.

8.) Method according to one of the preceding claims, characterized in that it is performed as a continuous process.

9.) Process according to one of the preceding claims for the production of fatty acid methyl esters from the transesterification reaction of vegetable and / or animal fats or oils.

10.) Method according to one of the preceding claims for the production of biofuels.