Process for preparing fatty acids by ester hydrolysis

Abstract

A process and a plant are specified, with which free fatty acids can be obtained in a simple manner by hydrolysis of fatty acid alkyl esters, especially fatty acid methyl esters (FAME), or alternatively of fatty acid triglycerides present in oils and fats of vegetable and animal origin. According to the invention, a portion of the free fatty acids already produced is recycled back into the reaction mixture, which results in self-acceleration of the hydrolysis reaction.

Claims

1. A process for preparing fatty acids by hydrolysis of fatty acid alkyl esters, the process comprising the following steps: a) providing the fatty acid alkyl esters; b) reacting the fatty acid alkyl esters with water under hydrolysis conditions at temperatures of at least 200 C., where the pressure is chosen such that the water is in the liquid phase and where no external substance extraneous to the process is added as homogeneous or heterogeneous catalyst; c) discharging a hydrolysis product comprising free fatty acids (FFA), water, unconverted fatty acid alkyl esters and methanol; d) feeding the hydrolysis product to a phase separation apparatus and separating the hydrolysis product under phase separation conditions into a light phase comprising free fatty acids and unconverted fatty acid alkyl esters and a heavy phase comprising water and methanol; e) feeding the light phase into a first separation apparatus that works by a thermal separation process and separating the light phase into a first separation product enriched in free fatty acids and a second separation product enriched in unconverted fatty acid alkyl esters, the separation being conducted in such a way that the second separation product further comprises a proportion of free fatty acids; f) discharging the first separation product as FFA product; and g) recycling at least a portion of the second separation product to reaction step b).

2. The process according to claim 1, wherein feeding step e) and/or recycling step g) are effected in such a way that, during reaction step b), the proportion of free fatty acids, based on the proportion of fatty acid alkyl ester, is >0% to 10% by weight.

3. The process according to claim 1, wherein reaction step b) is conducted at a temperature of at least 220 C.

4. The process according to claim 1, wherein the methanol-comprising heavy phase obtained in step d) is fed to a second separation apparatus that works by a thermal separation process and separated into a methanol-enriched third separation product and a water-enriched fourth separation product, the third separation product being discharged from the process as methanol product and the fourth separation product being at least partly recycled to reaction step b).

5. The process according to claim 1, wherein the hydrolysis product obtained in step b) is first fed to the second separation apparatus in which a methanol-enriched top product is selectively separated from the hydrolysis product and discharged from the process as methanol product.

6. The process according to claim 5, wherein the second separation apparatus is configured as a flash stage which is preferably configured and operated in an adiabatic manner.

7. The process according to claim 5, wherein the methanol-depleted hydrolysis product is fed to the phase separation apparatus and separated therein under phase separation conditions into a light phase comprising free fatty acids and unconverted fatty acid alkyl esters and a heavy phase comprising water and methanol, the heavy phase being at least partly recycled to reaction step b) and the light phase being fed to the first separation apparatus.

8. The process according to claim 1, wherein the phase separation conditions comprise the cooling of the hydrolysis product or of the methanol-depleted hydrolysis product to a temperature of 220 C.

9. The process according to claim 8, wherein the cooling is brought about by means of a cooling apparatus upstream of the phase separation apparatus and/or by virtue of the separation of the methanol-enriched top product from the hydrolysis product being conducted adiabatically.

10. The process according to claim 1, wherein the phase separation conditions comprise the cooling of the hydrolysis product or of the methanol-depleted hydrolysis product to a temperature of 180 C.

11. The process according to claim 1, wherein the ratio of water to fatty acid methyl ester in the reaction of fatty acid methyl ester with water in step b) is at least 2 mol/mol.

12. A plant for preparation of fatty acids by hydrolysis of fatty acid alkyl esters, comprising the following plant components: a) means of providing the fatty acid alkyl esters; b) at least one hydrolysis reactor for reacting the fatty acid alkyl esters with water under hydrolysis conditions at temperatures of at least 200 C., suitable for establishing a pressure at which the water is in the liquid phase at the reaction temperature; c) means of discharging a hydrolysis product comprising free fatty acids (FFA), water, unconverted fatty acid alkyl esters and methanol; d) a phase separation apparatus suitable for separating the hydrolysis product under phase separation conditions into a light phase comprising free fatty acids and unconverted fatty acid alkyl esters and a heavy phase comprising water and methanol, means of feeding the hydrolysis product to the phase separation apparatus, means of discharging the light phase, means of discharging the heavy phase; e) a first separation apparatus which works by a thermal separation process, suitable for separating the light phase into a first separation product enriched in free fatty acids and a second separation product enriched in unconverted fatty acid alkyl esters, the second separation product further comprising a proportion of free fatty acids, means of feeding the light phase into the first separation apparatus, means of discharging a first separation product from the first separation apparatus, means of discharging a second separation product from the first separation apparatus; f) means of discharging the first separation product as FFA product; and g) means of recycling at least a portion of the second separation product to the at least one hydrolysis reactor.

13. The plant according to claim 12, further comprising a second separation apparatus suitable for separating the heavy phase into a methanol-enriched third separation product and a water-enriched fourth separation product, means of feeding the heavy phase into the second separation apparatus, means of discharging the third separation product from the second separation apparatus and of discharging it from the plant as methanol product, means of discharging the fourth separation product from the second separation apparatus, means of recycling at least a portion of the fourth separation product to the at least one hydrolysis reactor.

14. The plant according to claim 13, further comprising means of feeding the hydrolysis product obtained in the at least one hydrolysis reactor to the second separation apparatus, means of selectively separating a methanol-enriched top product from the hydrolysis product, means of discharging the methanol-enriched top product from the plant as methanol product.

15. The plant according to claim 14, wherein the second separation apparatus is configured as a flash stage, preferably as an adiabatic flash stage.

16. The plant according to claim 14, further comprising means of feeding the methanol-depleted hydrolysis product to the phase separation apparatus, means of recycling at least a portion of the heavy phase to the at least one hydrolysis reactor, means of feeding the light phase to the first separation apparatus.

17. The process according to claim 1, wherein the fatty acid alkyl esters comprise fatty acid methyl esters (FAME).

18. The plant according to claim 13, wherein the fatty acid alkyl esters comprise fatty acid methyl esters (FAME).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Developments, advantages and possible uses of the invention will also be apparent from the description of working and numerical examples and of the drawings which follows. In this context, all features that have been described and/or represented in images, in themselves or in any combination, form the invention, irrespective of their recapitulation in the claims or the dependency references thereof.

(2) The figures show:

(3) FIG. 1 the schematic representation of the process according to the invention or of the plant according to the invention in a first configuration,

(4) FIG. 2 the schematic representation of the process according to the invention or of the plant according to the invention in a second configuration.

DETAILED DESCRIPTION OF THE INVENTION

(5) In the schematic flow diagram, shown in FIG. 1, of a first configuration of the process according to the invention or of the plant according to the invention, the fatty acid methyl ester (FAME) and water (H.sub.2O) are fed to the hydrolysis reactor 3 via conduits 1 and 2. The hydrolysis reactor, indicated merely in schematic form, works continuously with vigorous backmixing and is configured, for example, as a continuous stirred tank reactor. A portion of the water required for the ester hydrolysis can also be introduced into the hydrolysis reactor as steam. Preferably, this is done in such a way as to additionally contribute to the mixing of the liquid reaction mixture, i.e., for example, by blowing it into the liquid mixture. It may also be the case that the steam serves as heat carrier for heating of the contents of the reactor.

(6) The reactor pressure is chosen such that the reaction mixture remains in the liquid phase at the reaction temperature established by a heating apparatus which is not shown in the diagram. The pressure is adjusted in a known manner via the vapour pressure of the components involved and optionally additionally by addition of an inert gas.

(7) On attainment of a particular final conversion, the hydrolysis product leaves the hydrolysis reactor via conduit 4, is cooled in the cooling apparatus 5 and is then fed to the phase separation apparatus 7 via conduit 6. The phase separation apparatus in the example shown is a simple vessel with overflows and outlets for a heavy liquid phase and a light liquid phase, in which the phases are separated under gravity owing to the different density of the two liquid phases.

(8) From the phase separation apparatus, the light nonpolar phase comprising the free fatty acid product (FFA) and unconverted fatty acid methyl ester is removed via conduit 8 and introduced into the first separation apparatus which, in the example shown, is configured as a distillation. In the distillative separation of the light phase, a fraction enriched in free fatty acids is obtained (first separation product), which is discharged from the process as FFA product via conduit 10. The remaining fraction (second separation product) which is recycled via conduits 11 and 1 to the hydrolysis reactor 3 comprises, as well as unconverted fatty acid methyl ester, also traces of methanol and significant proportions of free fatty acid. The latter, after being recycled into the hydrolysis reactor, acts as catalyst for the conversion of further fatty acid methyl ester to free fatty acid.

(9) The heavy polar phase comprising unconverted water and methanol as co-product of the ester hydrolysis is removed from the phase separation apparatus 7 via conduit 12 and introduced into the second separation apparatus 13 which, in the example shown, is likewise configured as a distillation. In the distillative separation of the heavy phase, the top product obtained from distillation is a methanol product (MeOH) (third separation product), which is discharged from the process via conduit 14 and optionally sent to further workup. The bottom product obtained is a water-enriched fraction (fourth separation product), which is recycled via conduits 15 and 2 to the hydrolysis reactor 3.

(10) In the schematic diagram of a second configuration of the process according to the invention or of the plant according to the invention shown in FIG. 2, the process procedure as far as reference numeral 3 corresponds to that in FIG. 1. On attainment of a particular final conversion, the hydrolysis product leaves the hydrolysis reactor via conduit 4, but is then subjected to adiabatic (flash) expansion by means of expansion valve 16 and introduced via conduit 17 into the second separation apparatus 13a, which is configured here as a simple phase separation apparatus for separation of a gaseous, methanol-enriched phase (third separation product) from a methanol-depleted liquid phase (fourth separation product). The top product obtained from the phase separation apparatus 13a is a methanol product (MeOH) (third separation product), which is discharged from the process via conduit 14 and optionally sent to further workup.

(11) On account of the adiabatic expansion, the temperature of the fourth separation product is lower than that of the hydrolysis product leaving the hydrolysis reactor 3. As a result, the cooling apparatus 5 to which the methanol-depleted liquid phase is applied via conduit 18 can be designed in a smaller size in terms of the cooling output required to establish a defined temperature in the phase separation apparatus 7.

(12) Via conduit 6, the methanol-depleted liquid phase is applied to the phase separation apparatus 7, the properties and mode of operation of which correspond substantially to those that have been elucidated in FIG. 1. However, the phase separation proceeds more easily or quickly compared to the configuration shown in FIG. 1, since methanol, which acts as a solubilizer between the polar and nonpolar phase and hence makes the phase separation more difficult, has been removed from the liquid phase beforehand. Because of the more rapid phase separation, the phase separation apparatus 7 in the configuration shown in FIG. 2 can thus have a smaller design.

(13) In the distillative separation in the first separation apparatus 9 to which the light phase is applied via conduit 8, a fraction enriched in free fatty acids is obtained (first separation product), which is discharged from the process as FFA product via conduit 10. The remaining fraction (second separation product) which is recycled to the hydrolysis reactor 3 via conduits 11 and 1 comprises, as well as unconverted fatty acid methyl ester, also traces of methanol and significant proportions of free fatty acid. The latter, after being recycled into the hydrolysis reactor, acts as catalyst for the conversion of further fatty acid methyl ester to free fatty acid.

(14) The heavy polar phase discharged from the phase separation apparatus 7 via conduit 12, comprising unconverted water and methanol as co-product of the ester hydrolysis, is recycled via conduit 12 and conduit 2 to the hydrolysis reactor 3.

(15) Of all the recycle streams in the working examples shown in FIG. 1 and FIG. 2, small proportions can be discharged and discarded (purged), in order to prevent the accumulation of impurities and other unwanted components.

(16) In both working examples discussed, it is possible to discharge a methanol-rich stream as top product from the reaction apparatus via a conduit which is not shown. In this way, the reaction equilibrium is shifted in the direction of the hydrolysis products and hence the hydrolysis reaction is promoted.

NUMERICAL EXAMPLES

(17) Reaction Parameters

(18) To demonstrate the hydrolysis reaction, experiments were conducted in an autoclave with various experimental parameters and FAME chain lengths. The reaction mixture was stirred here at a stirrer rotation speed of 500 min.sup.1. The experimental results obtained are compiled in Table 1.

(19) Within experiment series 1, a distinct acceleration of the conversion profile with increasing temperature becomes apparent. At 240 and 260 C. an identical final state is achieved, whereas at 220 C. the observation time was insufficient to attain this state.

(20) The effect of methanol discharge by means of flash evaporation during the reaction is shown by the comparison between Examples 1c and 2a. The conversion achieved is about 5% higher in the final state of the reaction mixture if methanol has been removed from equilibrium.

(21) TABLE-US-00001 TABLE 1 FAME conversion as a function of reaction time, temperature and water/FAME ratio Experiment No. 1a 1b 1c 2a 2b 2c 3a 3b 3c 4a 4b FAME C8 C8 C8 C8 C8 C8 C8 C8 C8 C10 C10 chain length Water/FAME* 16 16 16 16 8 2 24 16 8 16 8 T/ C. 220 240 260 260 260 260 240 240 240 260 260 MeOH yes yes yes no no no no no no no no discharge** Reaction time/h FAME conversion 0.5 h 2% 5% 24% 21% 12% 5% 7% 10% 3% 12% 10% 1.0 h 7% 23% 60% 63% 43% 32% 26% 33% 13% 44% 41% 1.5 h 19% 61% 74% 73% 61% 44% 52% 58% 37% 68% 58% 2.0 h 35% 76% 78% 75% 63% 45% 72% 72% 52% 71% 62% 3.0 h 58% 80% 80% 75% 63% 45% 77% 75% 62% 72% 63% 4.0 h 67% 80% 80% 75% 63% 45% 79% 75% 62% 72% 63% *water/FAME ratio [mol of water per mole of FAME] **methanol was evaporated (flashed) out of the reaction mixture by lowering the pressure during the reaction

(22) The effect of the water/FAME ratio is shown by experiment series 2 and 3. The magnitude of the conversion attained in the final state increases with an increased amount of water. In the case of identical water/FAME ratios, an increase in temperature brings about a shortening of the reaction time needed to attain this final state.

(23) The effect of the FAME chain length becomes clear in the comparison of experiments 2a and 2b with experiments 4a and 4b. In this case, under otherwise identical conditions, conversions at a comparable level are attained after an identical reaction time.

(24) The experiments were repeated with different stirrer rotation speeds. Within the first two hours of the experiment, faster rises in the conversion curve against time were found at higher stirrer rotation speeds. After 2 h in each experiment, however, an identical final state of the conversion was attained.

(25) Catalytic Effect of Free Fatty Acids (FFA)

(26) To demonstrate the catalytic effect of free fatty acids on the hydrolysis reaction, further experiments were conducted in an autoclave at different temperatures under otherwise identical conditions both with and without addition of FFA. The amount of the FFA added was 5.28% g/g, based on the amount of C.sub.10-FAME used, which corresponded to an FFA concentration of 5% by weight in FAME, or about 3% by weight, based on the overall reaction mixture. The reaction mixture was stirred here at 500 min.sup.1. The results obtained here are compiled in Table 2 below.

(27) Experiment series 6 served as reference, since addition of FFA was dispensed with here. Within experiment series 6, a distinct retardation in the conversion profile became apparent with decreasing temperature. In contrast to the results with C.sub.8-FAME (cf. Table 1, experiment series 1a to 1b), an identical final state was not attained at the varying temperature of 240 to 260 C.

(28) In the comparison with experiment series 7 (with addition of FFA), the catalytic effect of the free fatty acid at the start of the reaction becomes clear. Here, a constant final state was attained after only 2 h, whereas this was not attained in experiment series 6 until after 3 h (experiment 6b+6c).

(29) The magnitude of the FAME conversion in the final state in the experiments with addition of FFA decreases proportionally with the concentration of the FFA added in the FAME, since the FFA concentration in the final state of the reaction is established in an equilibrium and hence leads to a restriction of the maximum FAME conversion.

(30) TABLE-US-00002 TABLE 2 FAME conversion as a function of the reaction time and temperature with and without FFA as catalyst Experiment No. 6a 6b 6c 7a 7b 7c FAME C10 C10 C10 C10 C10 C10 chain length Water/FAME* 9.5 9.5 9.5 9.5 9.5 9.5 T/ C. 260 250 240 260 250 240 MeOH no no no no no no discharge** FFA no no no yes yes yes addition Reaction time/h FAME conversion 0.5 h 7% 4% 1% 45.1% 30% 23% 1.0 h 41% 23% 8% 62.0% 53% 46% 1.5 h 64% 52% 27% 64.1% 58% 54% 2.0 h 68% 61% 48% 64.4% 60% 57% 3.0 h 68% 64% 60% 64.4% 60% 57% 4.0 h 68% 64% 60% 64.4% 60% 57% *water/FAME ratio [mol of water per mole of FAME] **methanol was evaporated (flashed) out of the reaction mixture by lowering the pressure during the reaction

(31) Phase Separation of the Reaction Mixture in the Final State from Experimental Example 2a

(32) The reaction mixture from experimental example 2a (see Table 1) with a water/FAME ratio of 16 mol/mol was produced in an autoclave equipped with a sightglass. Thus, the observation of phase volumes and the controlled sampling of the individual phases was enabled. As a result of the good solubility ratios of the relatively short-chain reactants and products in one another (in this case C.sub.8-FAME as reactant), a homogeneous reaction mixture formed in the final state of the reaction. In the course of cooling of this homogeneous reaction mixture, commencement of phase formation was observed from 224 C. (cloud point). The cooling was continued gradually and the phases that formed were each determined volumetrically and analysed (see Table 3).

(33) TABLE-US-00003 TABLE 3 Phase formation and phase composition; reaction mixture from experimental example 2a Ex. No. 2a/1 2a/2 2a/3 Upper phase 50% vol 46% vol 43% vol component FFA + FAME-rich light phase % w/w % w/w % w/w Water 33.8 26.2 18.6 Methanol 4.3 4.3 3.8 FAME 18.2 20.9 23.2 FFA 43.7 48.6 54.4 Reaction mixture in Cooling to 217 C. Cooling to 199 C. Cooling the final state of the to hydrolysis reaction 177 C. % w/w Water 64.2 Methanol 5.3 FAME 8.3 FFA 22.2 Lower phase 50% vol 54% vol 57% vol component Water- + methanol-rich heavy phase % w/w % w/w % w/w Water 89.1 92.1 93.2 Methanol 5.9 5.7 5.9 FAME 1.1 0.4 0.2 FFA 3.9 1.8 0.7

(34) Formation of an FFA- and FAME-rich light phase and of a water- and methanol-rich heavy phase was observed. With decreasing phase separation temperature (2a/1>2a/2>2a/3), the separation was completed such that there was further enrichment of FFA and FAME in the light phase and further enrichment of water and methanol in the heavy phase.

(35) Phase Separation of the Reaction Mixture in the Final State from Experimental Example 2b

(36) The reaction mixture from experimental example 2b (for preparation see Table 1) with a water/FAME ratio of 8 mol/mol was produced in an autoclave equipped with a sightglass. Thus, the observation of phase volumes and the controlled sampling of the individual phases was enabled. As a result of the good solubility ratios of the relatively short-chain reactants and products in one another (in this case C.sub.8-FAME as reactant), a homogeneous reaction mixture formed here too in the final state of the reaction. In the course of cooling of this homogeneous reaction mixture, commencement of phase formation was observed from 227 C. (cloud point). The cooling was continued gradually and the phases that formed were each determined volumetrically and analysed (see Table 4).

(37) Again, the formation of an FFA- and FAME-rich light phase and of a water- and methanol-rich heavy phase was observed. Here too, the separation was completed with decreasing phase separation temperature (2b/1>2b/2>2b/3) in such a way that there was further enrichment of FFA and FAME in the light phase and further enrichment of water and methanol in the heavy phase. An exception here is the water- and methanol-rich heavy phase in Example 2b/3. In the course of cooling to 180 C., cloudiness (an emulsion) was observed, which explains the deviation in its composition.

(38) TABLE-US-00004 TABLE 4 Phase formation and the phase composition thereof; reaction mixture from experimental example 2b Ex. No. 2b/1 2b/2 2b/3 Upper phase not determined 52% vol 61% vol component FFA + FAME-rich light phase % w/w % w/w % w/w Water 30.5 22.6 17.5 Methanol 6.3 5.7 5.4 FAME 28.8 29.3 33.5 FFA 34.3 42.5 43.5 Reaction mixture in Cooling to 215 C. Cooling to 197 C. Cooling the final state of the to hydrolysis reaction 180 C. % w/w Water 43.1 Methanol 6.8 FAME 19.9 FFA 30.1 Lower phase not determined 48% vol 39% vol component Water- + methanol-rich heavy phase % w/w % w/w % w/w Water 86.9 89.5 84.6 Methanol 8.1 8.1 8.3 FAME 1.8 0.7 3.7 FFA 3.2 1.7 3.4

(39) Industrial applicability

(40) The invention provides a process and a plant with which free fatty acids can be obtained in a simple manner by hydrolysis of fatty acid alkyl esters, especially fatty acid methyl esters (FAME), or alternatively of fatty acid triglycerides present in oils and fats of vegetable and animal origin. Since the process does not require the use of external substances extraneous to the process as homogeneous or heterogeneous catalysts, particular economic and ecological advantages are obtained, since there is no need for any catalysts to be recovered from the hydrolysis product and subsequently regenerated or disposed of in a costly and inconvenient manner. The autocatalytic action of the free fatty acids added to the reaction mixture permits a reduction in size of the reaction apparatuses used for achievement of a fixed production rate.

(41) While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

(42) The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

(43) Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of comprising). Comprising as used herein may be replaced by the more limited transitional terms consisting essentially of and consisting of unless otherwise indicated herein.

(44) Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

(45) Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

(46) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

(47) All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.

LIST OF REFERENCE NUMERALS

(48) [1] conduit [2] conduit [3] hydrolysis reactor [4] conduit [5] cooling apparatus [6] conduit [7] phase separation apparatus [8] conduit [9] first separation apparatus (distillation) [10] conduit [11] conduit [12] conduit [13] second separation apparatus (distillation) [13a] second separation apparatus (phase separation apparatus, flash) [14] conduit [15] conduit [16] expansion valve [17] conduit [18] conduit