The present invention relates to a process and a plant for producing fatty alcohols from a reaction mixture containing at least one fatty acid ester and at least one free fatty acid, wherein the at least one fatty acid ester is hydrogenated with hydrogen to at least one fatty alcohol, wherein the hydrogenation is effected on a catalyst at a temperature of 1 50 to 300 °C and a pressure of 50 to 250 bar.

Fatty alcohols chiefly are linear, monofunctional, terminal alcohols with alkyl radicals with a chain length of 8 to 1 8 C atoms. In general, fatty alcohols are obtained from renewable raw materials by hydrogenation of the corresponding esters, in particular of the methyl esters, of fatty acids or fatty acid mixtures. This production process for fatty alcohols is described for example in " Ullmanns Enzyklopadie der Technischen Chemie", third edition, vol. 7, page 440 ff. Such catalyzed reaction proceeds according to the following reaction scheme:

Due to their production, an esterification of the free fatty acids, the fatty acid esters used as educts still contain residual quantities of free fatty acids. The content of free fatty acids can be determined titrimetrically by the consumption of the alkaline titration agent potassium hydroxide (KOH). Its consumption in mg based on the amount of fatty acid ester used provides the so-called acid number in the unit mg(KOH)/g(fatty acid ester). The technical purity of the fatty acid esters lies in the range of an acid number from 0.5 to 1 .0 mg(KOH)/g(fatty acid ester). US 6,187,974 describes a process for producing fatty alcohols from lauric oils. Laurie oils are the vegetable oils coconut oil and palm kernel oil, since they have a high content of lauric acid. The lauric acid derived from the alkane n-dodecane is deacidified and in a basically catalyzed transesterification step subsequently converted with methanol to the corresponding fatty acid methyl ester and puri- fied. The purified fatty acid methyl ester is subjected to a selective hydrogenation, so that the corresponding fatty alcohols are obtained.

This and similar hydrogenation processes have in common that the catalyst exhibits a decreasing catalyst activity in the course of the process.

Such a reduced activity leads to the fact that the residence time of the fatty acid esters in the reactor must be prolonged distinctly to achieve high conversions, or complete conversions no longer are achieved at all. In particular in operation on an industrial scale the problem arises that the catalyst must be replaced fre- quently, which on the one hand involves high costs for a new catalyst or for the catalyst regeneration and on the other hand results in long shut-down periods of the plant.

Therefore, it is the object of the present invention to provide a process with which the catalyst for the hydrogenation of the fatty acid esters stably remains active over an extended period.

This object is solved by a process with the features of claim 1 . In the process, a reaction mixture which contains at least one fatty acid ester and at least one non-esterified free fatty acid is hydrogenated to the corresponding fatty alcohols on a catalyst. The hydrogenation is effected at a temperature of 1 50 to 300 °C and a pressure of 50 to 300 bar. According to the invention, the amount of free fatty acids contained in the reaction mixture is lowered before the hydrogenation, whereby longer service lives of the catalyst and an improved yield with respect to the fatty alcohols can be achieved.

The invention is based on the finding that free fatty acids contained in the mixture in principle likewise are converted to the corresponding alcohols according to the following reaction

with R = C _n H ₂ n ₊ i with n = 7 - 1 7, but as long as they are contained in the system, they occupy and block free activity centers of the catalyst used.

In particular when the hydrogenation is effected at a temperature between 170 and 220 °C and/or at a pressure of 50 to 100 bar, this effect is decisive for the resulting conversion figures, as under these conditions the conversion of the free fatty acids is effected at a rate distinctly slowed down and the free fatty acids thus are present in the system for a correspondingly long time.

The previously observed effect that in hydrogenation experiments in the medium-pressure range a distinct reduction of the catalytic activity of the catalyst heterogeneously introduced into the reaction mixture and an increased occurrence of the so-called catalyst leaching, i.e. a discharge of the active centers occurs, can be prevented by lowering the amount of free fatty acids contained in the reaction mixture. Thus, a medium-pressure synthesis of fatty alcohols also can be carried out without an increased consumption of catalyst and with similar process costs concerning the ongoing operation. This is of economic interest in particular because the investment costs for such a plant are distinctly lower than for high-pressure plants, since not all plant sections must be designed for the high-pressure range. Calculations reveal an about 15-20 % saving on the investment costs for a medium-size plant with a production capacity of 100 kt/a.

In particular when the catalyst is present in solid form, an at least partial removal of the free fatty acids contained in the reaction mixture is required, as in the case of a solid catalyst system these fatty acids temporarily occupy the catalytic surfaces for a long time and thus a reduced catalyst activity occurs.

Preferably, the catalyst used is a metallic catalyst, particularly preferably a cop- per-based catalyst, preferably on the basis of copper chromite. A copper- containing catalyst has the advantage that it converts the fatty acid esters to fatty alcohols with great selectivity at a high space/time yield. Catalysts on the basis of copper and copper chromite with varying dopings of iron or manganese are preferred particularly. The corrosive properties of the free fatty acids are revealed particularly strongly when metallic catalysts are used, in particular when copper-containing catalysts are used. In laboratory experiments it is found that the degradation of the metallic surface proceeds so strongly that when a copper-containing catalyst is used, such degradation even becomes visually visible in the reaction product in the form of a copper-reddish discoloration. In particular when using a metallic catalyst, the free fatty acids therefore must at least partly be removed or neutralized before the actual hydrogenation in the way according to the invention, in order to achieve acceptable conversion figures. Furthermore, it was found to be particularly favorable to lower the amount of free fatty acids contained in the reaction mixture to an acid number of < 0.1 mg(KOH)/g(fatty acid ester). The acid number hence is so low that the described effect, namely the occupation of active catalytic centers, virtually can be excluded completely.

Lowering the amount of free fatty acids contained in the reaction mixture can be achieved by adding a base. By forming the corresponding salts, the free fatty acids can be removed in the form of their soaps and the amount of acidic groups present in the reaction mixture can be lowered distinctly.

As base, NaOH or KOH preferably are used, since these bases are substances available at low cost, which have such a high basicity that a complete neutralization reaction virtually occurs.

Such neutralization is followed by a separation of the soaps, a water wash preferably effected in a column, and drying.

Furthermore, the amount of free fatty acids contained in the reaction mixture can be lowered by using an ion exchanger. This has the advantage that the number of the acidic groups damaging the catalyst is reduced without salts being formed in the reaction mixture. In principle, basic anion exchangers in their OH ^" form are employed, which is why during the exchange merely an amount of water corresponding to the fatty acid concentration is released to the fatty acid ester to be treated. The fatty acids are retained in the ion exchanger.

An advantageous aspect of the invention provides to use fatty acid methyl ester as fatty acid ester, since the resulting short-chain alcohol methanol can be separated easily. On a technical scale, fatty acid methyl esters are produced almost exclusively by transesterification of triglycerides with methanol. Beside fatty acid methyl esters the feedstocks for the hydrogenation process according to the invention to the same extent also are fatty acid esters of the fatty alcohols (so-called wax esters). In principle, however, other fatty acid esters, in particular esters of short-chain alcohols such as ethanol or propanol, also are conceivable.

When fatty acid methyl esters are used, methanol is obtained during their hydrogenation beside the corresponding fatty alcohol. When the pressure of the reaction product (fatty alcohol) at the outlet of the reactor is lowered to a value below the partial pressure of methanol at the respective temperature existing in the reactor, the residual quantity of methanol remaining in the product stream can evaporate and a product almost free from methanol can be obtained.

The invention furthermore also comprises a plant for producing fatty alcohols from a reaction mixture containing at least one fatty acid ester and at least one free fatty acid, preferably according to the described process, with the features of claim 10. Such plant provides a reactor for the hydrogenation of fatty acid esters on a catalyst at a temperature of 1 50 to 250 °C and a pressure of 50 to 300 bar. The reactor also includes a gas introduction device for introducing hydrogen. According to the invention, the plant comprises a means for lowering the amount of free fatty acids contained in the reaction mixture before the hydrogenation. With such a plant set-up it can reliably be prevented that the catalytic activity of the catalyst is lowered strongly by existing free fatty acids.

In a preferred aspect of the invention, the reactor is designed for a hydrogena- tion at a temperature of 170 to 220°C and a pressure of 50 to 100 bar. Such design has the advantage that the investment costs for a plant for producing fatty alcohols are distinctly lower than in known plants which are designed for the high-pressure range of about 200 to 250 bar. Furthermore, it was found to be particularly favorable to operate the reactor with a hydrogen recycle. Thus, it is possible to withdraw the short-chain alcohols (e.g. methanol when using fatty acid methyl esters) obtained during the hydro- genation beside the fatty alcohols, which due to a phase transition are shifted from the liquid phase into the gas phase, in gaseous form along with the circulated hydrogen.

It is preferred particularly to provide a cooling system within the recirculation of the hydrogen, so that here the short-chain alcohol withdrawn is condensed and can be discharged from the process. By shifting the chemical equilibrium, this in turn additionally promotes the production of fatty alcohols.

Preferably, the plant also includes a measuring device in a container upstream of the reactor, in a conduit upstream of the reactor or in the reactor itself, in order to determine the acid number in mg(KOH)/g(fatty acid ester). The measuring device for example is a gas chromatograph (GC) operating in situ or an infrared spectrometer e.g. for carrying out NIT or NI R, in which the acid number is inferred from the measured parameter. By the value thus determined, a dosing device for introducing a base or the residence time in an ion exchanger can be controlled such that the acid number is lowered to a value of < 0.1 mg(KOH)/g(fatty acid ester).

In a preferred aspect, the reaction mixture also is stripped before the hydro- genation, preferably after lowering the content of the free fatty acids in the reac- tion mixture. Activity losses of the catalyst due to contained water thereby can be avoided.

Further features, advantages and possible applications of the invention can be taken from the following description of the drawing and the exemplary embodi- ments. All features described and/or illustrated form the subject-matter of the invention per se or in any combination, independent of their inclusion in the claims or their back-reference.

In the drawing:

Fig. 1 shows a plant according to the invention for producing fatty alcohols.

Fig. 1 shows a plant 1 according to the invention for producing fatty alcohols from fatty acid esters. Via conduit 10, a reaction mixture containing at least one fatty acid ester, preferably fatty acid methyl ester, and at least one free fatty acid is passed into a container 1 1 , before it is supplied to a reactor 20 for hydrogen- ating the fatty acid esters. The container 1 1 includes a means 1 2 by which the amount of the at least one free fatty acid contained in the reaction mixture is reduced. In the present configuration, this is an ion exchanger, but it can also be a device for adding a base. The means 1 2 not only can be provided in the container 1 1 , as shown, but also in a conduit 10 or 14 upstream of the reactor 20 or directly in connection with the reactor 20.

The acid number of the reaction mixture is determined by means of a measuring device 13, wherein the acid number of the reaction mixture before the hydro- genation should be about < 0.1 mg(KOH)/g(fatty acid ester). To achieve better intermixing, the container 1 1 can be stirred.

Via conduit 14, the reaction mixture with the lowered acid number is supplied to a stripper 1 5 in which the water content of the reaction mixture is reduced. A water content of > 100 ppm shows no distinct ester cleavage when heating the ester to 220 °C. In the presence of a copper catalyst, for example the catalyst CU860 (BASF, Cu on Si0 ₂ ), this catalyst shows a distinct activity in the ester cleavage at a temperature of 220 °C. Via conduit 1 5, the reaction mixture then is introduced into the reactor 20. The reactor 20 is designed as batch reactor and favorably is stirred with a stirrer 21 . For the hydrogenation, which preferably is heterogeneously catalyzed, particularly preferably is effected on a copper catalyst at temperatures between 1 70 and 220 °C and a pressure of 50 to 100 bar, hydrogen is supplied. The supply of hydrogen is effected via conduits 27 and 28 and a gas introduction device 29 into the reactor 20.

The hydrogen itself is circulated via conduit 24, a cooling system 25 and con- duits 26 and 28. This has the advantage that short-chain alcohols, preferably methanol, which are obtained during the hydrogenation and partly are present in gaseous form, can be withdrawn together with the hydrogen via conduit 24 and can be supplied to the cooling system 25. In the cooling system 25 the short- chain alcohols, in particular methanol, then condense out, so that the corre- spondingly purified hydrogen can be guided back into the reactor 20 via the conduits 26 and 28 and the gas introduction device 29. Thus, short-chain alcohols can be removed already during the reaction and the reaction proceeds particularly favorably. Short-chain in the sense of the application refers to alcohols with a chain length of not more than five C atoms.

After completion of the reaction, the system can be depressurized in the reactor 20, so that when using fatty acid methyl esters, i.e. methanol, the short-chain alcohols obtained during the reaction are present in gaseous form and can be withdrawn in gaseous form via conduit 22. Via conduit 23, the resulting fatty alcohols are withdrawn in liquid form. The separation of the alcohols thereby is simplified distinctly. Experiment examples

Example 1 All experiments were carried out in a batch reaction in a stirred autoclave, wherein a catalyst/educt ratio of 0.273 ml(catalyst)/ml(educt) or of 0.253 g(catalyst)/g(educt) was employed. The catalyst mass and the catalyst volume refer to the unreduced, dry form of the used copper catalyst of the type CU860 (BASF) in extruded form. The educt volume refers to the density of the raw material at 1 5 °C. Before the first use of the catalyst for producing fatty alcohols, the catalyst was subjected to a reduction procedure, in order to put it into the active state.

The used raw material was a fatty acid methyl ester of technical purity of the firm BASF (trade name Synative ES 8309®), which in the composition chiefly consisted of lauric acid methyl ester (about 75%) and myristic acid methyl ester (about 20%) and small amounts of lower and higher fatty acid methyl esters. The density of this commercial product was 860 kg/t at 1 5 °C. The acid number of the used fatty acid methyl ester was determined to be 0.6 mg(KOH)/g(fatty acid methyl ester). The moisture content was about 300 ppm.

For providing a deacidified raw material for comparative experiments, the above-mentioned commercial product Synative ES 8309® was treated with the ion exchanger Lewatit MP 62 (BASF), in order to achieve a reduction of the acid number to a value of 0.1 mg(KOH)/g(fatty acid methyl ester). By a downstream stripping process for removing the reaction water with nitrogen, a moisture content of about 1 00 to 1 50 ppm is obtained. The hydrogenation process was carried out for seven hours at 200 °C and a hydrogen pressure of 75 bar, wherein a sample each was taken from the reaction mixture after 4 and after 7 hours. Example 1 revealed that in untreated fatty acid methyl esters it takes distinctly longer, until a stationary, i.e. uniform catalyst activity of a previously untreated catalyst is achieved, and it never is possible to achieve the conversion figures which are achieved with a previously deacidified reaction mixture. Table 1 distinctly shows a delay in achieving a stationary catalyst activity when using untreated raw materials with an acid number of 0.6 mg(KOH)/g(fatty acid methyl ester). When using deacidified raw material, a constant activity has been achieved already after three experiments (7 hours of experiment each = 21 catalyst operating hours), whereas in the case of untreated raw material a con- stant activity only could be observed after eight experiments (7 hours of experiment each = 56 catalyst operating hours).

In addition, when using a reaction mixture pretreated according to the invention, a distinctly higher catalyst activity can be observed during the first four hours in the form of an increased conversion (U):

Table 1 : Conversion (U) of fatty acid methyl ester (FAME) in dependence on the reaction time

The catalyst leaching observed during the hydrogenation of the untreated raw material in the form of a copper-reddish discoloration of the product could be observed over the complete series of experiments with untreated fatty acid methyl esters, whereas it could not observed at all in the series of experiments with deacidified raw material. Example 2

All experiments in Example 2 were carried out as batch reaction in a stirred autoclave, wherein a catalyst/educt ratio of 0.08 ml(catalyst)/ml(educt) or of 0.1 g(catalyst)/g(educt) was employed. The catalyst mass and the catalyst volume refer to the unreduced, dry form of the used copper catalyst of the type CU860 (BASF) in extruded form. The educt volume refers to the density of the raw material at 1 5 °C. Before the first use of the catalyst for hydrogenating the fatty acid esters, the same was subjected to a reduction procedure, in order to put it into the active state.

As educt, octyl octanoate (98%) was used (Sigma-Aldrich W281 1 07). For this educt an acid number of 0.05 mg(KOH)/g(fatty acid ester) and a moisture content of about 200 ppm was determined. The density of the commercial product is 870 kg/t at 1 5 °C.

For producing an acid-containing raw material for comparative experiments octanoic acid was added to the above-mentioned commercial product W281 1 07, in order to set an acid number in the range of 0.5 mg(KOH)/g(fatty acid methyl ester). During the hydrogenation process carried out for seven hours at 200 °C and a hydrogen pressure of 75 bar, a sample was taken from the reaction mixture each at the start and after 1 , 3, 5 and 7 hours.

Table 2: Time course of the conversion on fatty acid ester (FAE) in dependence on the acid number (SZ)

Table 2 distinctly shows a delay in the reaction rate in the experiments with added fatty acid and correspondingly increased acid number. Due to a reduced catalyst activity as compared to the educt with an acid number of 0.05 mg(KOH)/g(fatty acid ester) the conversion achieved falls distinctly. When using this practically acid-free raw material, a conversion of fatty acid esters to the corresponding fatty alcohols of 18% can be observed already after one hour, whereas in the comparative experiment with added fatty acid no significant conversion can be measured with 1 .5%. It also is observed that the acid number in the reaction mixture distinctly decreases over time, i.e. the contained free fatty acids disappear in the course of the reaction. Coupled to a reduced catalyst activity the finding underlying the invention also is confirmed here, according to which the fatty acids occupy catalytically active centers and thus an at least temporary catalyst inhibition occurs.

List of Reference Numerals

1 plant

10 conduit

1 1 container

12 ion exchanger

13 measuring device

14 conduit

15 stripper

1 6 conduit

20 reactor

21 stirrer

22-24 conduit

25 cooling system

26-28 conduit

29 gas introduction device