PROCESS FOR THE PREPARATION OF BIODIESEL

The present invention concerns a process for the preparation of biodiesel. In particular, the invention relates to a process for the preparation of biodiesel which comprises esterification and/or trans-esterification of fatty acids and/or glycerides contained in vegetable oils and /or animal fats in the presence of an alkyl alcohol and a catalyst, in order to obtain a mixture of methyl esters and glycerin, subsequently separated.

At present vegetable oils and animal fats cannot be economically transformed into methyl esters if their free acidity content is greater than a few weight percentage points. The neutralisation of these free fatty acids (FFA) with alkaline compounds, according to a known technique, produces soaps which hinder and make the methylation process unmanageable due to the presence of solids and emulsions. The elimination of these soaps, before or after the trans-esterification reaction with - in general - methanol, by washing with water furthermore entails the formation of liquid effluents which have to be treated. In practice, when the FFA content exceeds 3%, these neutralisation processes become uneconomic and industrially unmanageable.

Removal of the FFAs by means of another known technique of distillation is an alternative process but it has to be performed under a high vacuum and is energy - intensive; again, it is industrially feasible for FFA content below 3%, due to the energy consumption required for evaporation of the FFA which has to be performed under a high vacuum. The majority of the cheapest and therefore most interesting raw oils for the production of biodiesel have an acidity content ranging from 5% for raw palm oil to 14% for jatropha oil and even higher values for recycled oils /fats and for oleins.

Given the importance of biodiesel as an alternative fuel for automotive purposes and for generating electricity with large high-performance diesel engines, there is an obvious need for economically advantageous technologies able to transform the acids and glycerides contained in oils and fats of any origin and type, raw or refined, including waste or recycled oil, into methyl esters.

According to a first embodiment, the invention concerns a continuous process for the preparation of biodiesel which comprises the esterification and/or trans- esterification of fatty acids and/or glycerides contained in vegetable oils and/or animal fats in order to obtain methyl esters and glycerin, which entails: a) bringing into contact, for 10-90 minutes at a temperature of 40-120 ⁰ C, the vegetable oils and/or animal fats with a mixture consisting of water and at least

one non-polar, aprotic, non-halogenated organic solvent having boiling point below 100 ⁰ C, in order to obtain an aqueous phase and an organic phase; b) separation of the aqueous phase from the organic phase; c) recovery of the organic solvent of the separated organic phase; d) bringing into contact, at a temperature of 120-250 ⁰ C and for a contact time of 15-90 minutes, the organic phase resulting from step c with at least one C1-C4 alkyl alcohol in a quantity of 20-60% in weight with respect to the quantity of the fatty acids and/or glycerides present in said organic phase, in the presence of a solid heterogeneous catalyst, based on MgO and AI2O3, where the MgO/Abθ3 molar ratio is greater than 4, said catalyst being suspended in the organic phase, and where the quantity of the catalyst with respect to the volume of the organic phase is equal to 10%-20%, in order to obtain a mixture of methyl esters and glycerin; e) separation of the methyl esters from the glycerin of the mixture obtained in step d.

The process of the invention permits the use of vegetable oils and animal fats of any origin and type, raw or refined, including waste or recycled oils; preferred oils are vegetable oils selected from the group comprising palm oil, jatropha oil, rapeseed oil, sunflower oil, soybean oil and acid oils, for example deriving from the hydrolysis of soaps produced in the refinement processes of oils for use in the food industry.

Furthermore, vegetable oils and/or animal fats having any free fatty acid content, even above 3% in weight, can be used via the process of the present invention; in particular, the process of the invention is industrially advantageous in the presence of a free fatty acid content of 5-30% in weight.

In step a, the temperature is preferably 100-120 ⁰ C, while the organic solvent is preferably hexane and/or heptane while the contact time is, in particular, 20 minutes; it is furthermore preferable to add to the water, in step a, at least one water-soluble biodegradable carboxylic acid. It is also preferable to carry out purification of the aqueous phase resulting from step b.

Preferably, in step d, the temperature is 50-70 ⁰ C, in particular 65 ⁰ C, and the contact time is 60 minutes; it is also preferable to perform step d under agitation. In particular, step d is carried out in a fluidised bed reactor. In particular, the alkyl alcohol is selected from the group consisting of methanol, ethanol, propanol and butanol. The MgO /AI2O3 molar ratio is preferably 5-70.

According to a further preferred embodiment, the mixture resulting from step d undergoes, in sequence: fj dehydration; g) treatment with sodium methylate in a quantity of 1-10 per thousand with respect to the glycerides and 15-20% with respect to the fatty acids present in said mixture, in order to obtain a mixture enriched in methyl esters and glycerin; h) neutralisation of the enriched mixture.

In particular, step g is performed at a temperature of 70- 120 ⁰ C, preferably 100 ⁰ C, and the contact time of the sodium methylate with the mixture resulting from step f is 5-90 minutes, preferably 30 minutes.

It is also preferable to recover the alkyl alcohol at the end of steps d or e and the glycerin resulting from steps e or h.

According to the process of the present invention, the oils and the fats are continuously in contact with a mixture of solvent and water, substantially in order to eliminate rubbers and other extraneous substances, preferably under agitation, at a temperature of 40-120 ⁰ C, in particular 100-120 ⁰ C. The solvents that can be advantageously used are hexane, heptane and any low-boiling organic solvent (with boiling point below 100 ⁰ C) provided it is not halogenated and is substantially insoluble in water. Additives such as water-soluble biodegradable dicarboxylic acids can be added to the water to facilitate separation of any phosphorous-based compounds present in excessive quantities in the starting oils/fats.

The mixing contact time varies between 10 and 90 minutes, preferably 20 minutes. The organic and aqueous phases deriving from the mixing are separated continuously by means of the methods normally used in the sector, as will be evident to a person skilled in the art. The aqueous phase, which contains all or most of the saline, proteic or polar impurities generally present in the starting oil/fat, can also, optionally, be purified by permeation on membranes, for example, and reintroduced into the process in step a. The organic phase containing the solvent, the glycerides and the free fatty acids is sent to an evaporation/distillation unit for integral recovery of the solvent. Together with the solvent, the water entrained in the separation between the organic and the aqueous phases also evaporates; the water is then decanted from the solvent and discharged. The organic phase without solvent and water is fed to the esterification/trans- esterification reactor, preferably of the fluidised bed type, where the catalyst is kept suspended in the reaction mixture by insufflation into the reactor of vapours

of - preferably - methanol, taken from the gaseous phase of the reactor and recirculated in the reactor itself by blower or compressor.

Said three-phase fluidised bed reactor (gaseous phase of methanol which keeps the solid catalyst in suspension in the liquid reaction phase) ensures optimal turbulence of the solid particles of catalyst in the liquid bed, favouring the reaction kinetics.

It has surprisingly been found that catalysts based on magnesium oxide and aluminium oxide promote the simultaneous reaction of esterification and trans- esterification with methanol (or other C1-C4 alkyl alcohols) without being affected by the initial presence of FFA in the starting oils /fats. Said catalysts are obtained by mixing two solutions: a first solution containing MgtNCbh and AltNCbh 1.0 molar in (Mg + Al) at the desired Mg/Al atomic ratios(for example 1 , 2, 4, 20) and a second solution prepared by dissolving NaOH and Na2CO3 according to the description in A.L. McKenzie, CT. Fishel, R.J. Davis, J. Catal. 138 (1992) 547. The reaction temperature can vary from 12O ⁰ C to 25O ⁰ C and the contact time between 15 and 90 minutes, preferably 60 minutes.

The reactor generally typically consists of a cylindrical body with jacket: the heating fluid (in general diathermic oil or vapour) flows inside the jacket. The oils and/or the fats together with the alkyl alcohol are introduced via an inlet located at the bottom of the reactor, in a liquid state, via a first grille having the purpose of retaining the particles of catalyst located above, and move through the reactor until they meet and cross a second grille having the purpose of containing the catalyst, which is thus fluidised in the space between the two grilles. After crossing the second grille, the liquid phase leaves from a lateral duct situated higher up than the second grille with level control which maintains a liquid head above the lateral duct. Above the liquid head is the last section of the reactor occupied by the gaseous phase of the alkyl alcohol, balanced with the reaction mixture. Said gaseous phase is sucked from the head of the reactor by a compressor (or recirculation blower) and reintroduced, by means of a diffuser or other analogous and homologous dispersion system, into the section of reactor between the two grilles, where the catalyst is located, just above the first grille. The flow of vapours of the alkyl alcohol keeps the catalyst particles in suspension and agitation. The flow rate of said vapours is regulated so that the catalyst is fluidised or suspended in the liquid phase but not entrained. The quantity of catalyst contained in the fluidised bed reactor is equal to 10%-20% of the incoming volume of oils and /or fats.

The purity of the methyl esters and the residual acidity measured at the end of the process of the present invention fall within the limits required for renewable fuels suitable for use in static diesel engines for generating electricity. The alkyl alcohol found in the reaction mixture at the outlet of the fluidised bed reactor is recovered by evaporation, for example, and the condensed methanol is dehydrated by distillation, for example.

The glycerin separated by decantation (or similar methods) has a high purity (> 98.5%) since in the reaction mixture, at the outlet of the fluidised bed reactor, there are no residues of catalyst to be neutralised, thus avoiding the formation of salts.

According to a further aspect, the process of the present invention comprises further processing steps for the production of high purity biodiesel, for example for automotive use (in compliance with the EN 14214 standard). In this case, the reaction mixture resulting from step d is dehydrated by - for example - percolation in a bed of calcium oxide (to remove as calcium hydrate the esterification water that has formed in the fluidised bed reactor). The dehydrated mixture is sent, without any treatment or separation, to another reactor, preferably of tubular type, positioned in series with the previous one, into which sodium methylate is introduced - for example - as co-catalyst; the calcium hydrate acts as a co-catalyst. The sodium methylate catalyst (expressed as 100%) is added in quantities of 1- 10 per thousand with respect to the mass of glycerides not yet reacted and equal to 15-20% of the residual free acidity. The reaction temperature can vary between 70 and 12O ⁰ C, and the residence time between 5 and 90 minutes (preferably 30 minutes at 100 ⁰ C). The final mixture resulting from the treatment described consisting substantially of methyl esters, glycerin and basic residues of catalyst (sodium methylate and calcium hydrate) is neutralised with an acid such as, for example, carbonic acid, phosphoric acid or acetic acid. The excess methanol is recovered - for example - by means of evaporation and the final separation between methyl esters and raw glycerin can be performed - for example - by decantation and subsequent centrifugation or other method known in the sector, as will be evident to a person skilled in the art according to the description of the present invention. The following examples illustrate the invention without limiting it. Example 1 10 kg/hour of raw palm oil having 5.5% of FFA were treated with a mixture of 2 kg/hour of heptane and 1 kg/hour of water under strong agitation in a container of suitable dimensions, for 15 minutes. The organic and aqueous phases were

then decanted and the organic phase was fed continuously into a fluidised bed reactor containing a catalyst with MgO and AI2O3 base and molar ratio of 20, between the oxides, prepared by mixing a first solution containing Mg(NO3)2 and A1(NO3)2 1.0 molar in (Mg + Al), in an atomic ratio of Al/Mg from 1 to 3, with a second solution prepared by dissolving NaOH and Na2CO3 in equal ponderal parts at 10% in water. The first solution was fed at a speed of 1 cm ³ /min for 4 hours under vigorous agitation, while the second solution was fed in order to maintain the pH constant at 10. The gels obtained were kept at 65 ⁰ C for 24 hours, then filtered and washed to pH 7. Lastly, the solids obtained were first dried at 85 ⁰ C for 14 hours and then calcined in air at 500 ⁰ C for 14 hours.

The reactor was brought to 15O ⁰ C and the residence time set to 60 minutes. The methanol and the glycerin were separated from the reaction mixture, the first by evaporation and the second by centrifugation. The analysis of the methyl esters obtained gave the following results:

The quality of the product obtained complies with the requirements established by the manufacturers of large static diesel engines (for example WARTSILA and

CATERPILLAR) for the use of renewable fuels in their engines.

Example 2

10 kg/hour of reaction mixture obtained as in example 1 and taken directly at the outlet of the fluidised bed reactor were sent to a second reactor, after percolation through a bed of calcium oxide, adding 1 g/h of sodium methylate (as 100%). The second reactor, of the tubular type with double pipe, had an inner pipe with diameter 0.375 cm and an outer pipe with diameter 2.54 cm for an overall length of 1.3 metres.

The resulting methanol and glycerin were separated from the reaction mixture after neutralisation with phosphoric acid, the first by evaporation and the second by centrifugation. Analysis of the methyl esters obtained gave the following results:

The quality of the methyl esters contained in the product obtained via the process of the invention therefore complies with the EN 14214 standard.