"Process for producing biodiesel from vegetable oils"

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The present invention relates to a process for producing biodiesel from vegetable oils.

In particular, the present invention relates to a process for producing biodiesel from vegetable oils, the virgin or exhausted ones, of the type realized in small size plants .

As it is known, in the last two decades, global climate change and the awareness of the existence of environmental issues related to anthropogenic factors has led to the search of the causes of environmental degradation and to possible solutions. In particular, it was recognized that the increase of some gases named "greenhouse effect gases" (Greenhouse Gases or GHG) , mainly due to the sectors of transport, has greatly contributed to an increase in the average temperature of the Earth (EEA, 2009) . To overcome this problem, the main International policies (Kyoto Protocol) and especially the European policies (Directive 2009/28 / EC) have tried to set limits on emissions of these gases through the incentives for the production of biofuels, the consumption of which has become mandatory in blending with fossil fuel. Among the agro- energy sources, biodiesel is today the source of renewable energy more widely available on the European market, as well as the biofuel that can satisfy the principles of sustainability currently debated. Many biofuels, such as, ethanol, bio-methanol , biodesel and bio-hydrogen appear to be attractive alternatives for the future in the transport sector and thus a significantly increase of their production is expected in the coming decades.

The production of biodiesel at industrial level requires the use of a reaction called trans-esterification, in which a triglyceride molecule reacts with a short-chain alcohol at the presence of a catalyst. More in detail, the reaction of trans-esterification is the reaction between a triglyceride and an alcohol leading to the formation of a mixture of fatty acids and glycerin, and is the process that leads to the formation of biodiesel. The reaction of trans-esterification is an equilibrium reaction that is carried out by means of reagent mixing and requires an excess of alcohol to get high conversions. In addition to the desired products also glycerol is obtained, which is insoluble in the methyl ester, thus causing the formation of two phases, which must be separated. Being this reaction reversible, high amounts of alcohol are required to obtain maximum conversion, but that makes difficult the final separation of the biodiesel and of the glycerol from alcohol .

The steps for production of biodiesel from vegetable oils are numerous. First of all it is necessary to verify the content of free fatty acids in the origin oil, for example by titration: if this content is less than about 2,5% wt (by weight), then it's possible to carry out the trans-esterification, otherwise a previous acid esterification is performed. Then, a pre-treatment step follows, namely the oil refining to obtain a raw material with constant characteristics. Obviously if the origin oil is a refined oil there is no pre-treatment need; otherwise the oil must be refined by a process of preliminary purification to remove impurities and possibly the water present in oil. This process is critical when oil from waste frying and recycling is used. The third step is the mixing of alcohol and catalyst (almost always KOH, potassium hydroxide) , realized in a suitable container in a controlled environment to obtain a homogeneous mixture. The catalyst (KOH) is supplied from a storage tank and added in a predetermined amount to the methanol. The next step is the mixing reaction of oil with the alcohol / catalyst, normally using a 1:6 ratio of alcohol with respect to oil, by reacting the mixture oil / methanol at a temperature of 50-60 ⁰ C for about 90-120 min. The separation step is the next one: the methylester-glycerin mixture is transferred to a unit for decanting or centrifuging where, the glycerine phase is separated and transferred to the storage tank for its eventual neutralization with water and phosphoric acid. The purification step of the methyl ester follows to eliminate the traces of the hydrophilic phase (traces of glycerol remained in suspension, excess of methanol, catalyst), performing washing the product with water (liquid-liquid extraction), which is then dried and finally stored. In a later step a vacuum distillation is performed to remove the water. Dry cleaning are also made using special filters with ion exchange resins. It's possible to delete the methanol from the crude biodiesel using a vacuum distillation. The aqueous phase coming from the washing of biodiesel and the aqueous phase coming from the eventual neutralization of the glycerine are separately transferred to dedicated distillation columns. The methanol, which is recovered for subsequent trans- esterification, and the glycerin with required purity originate from the distillation columns.

Recent studies try to optimize the different steps illustrated above, in particular the separation of glycerine and methanol and the removal of the catalyst, in order to be able to reuse these substances, thus lowering the overall costs of production of biodiesel. Other studies are going to improve the mixing to increase yields and optimize energy consumption.

In fact reactors equipped with static mixers, inserted in the pipes for loading the reagents are being studied; the goal of these devices is to create and maintain the dispersion of a liquid in another liquid. In this way the energy consumption of the reactor reduces, since there is no longer need to create the dispersion inside with high speed agitation, but must stir the reaction mixture, at a lower speed, only to ensure that the reaction temperature is constant and there are no thermal gradients in the mixture. A further development of this solution is to insert the solid catalyst within static mixers. All these innovations have a positive impact on the global energy balance of the production of biodiesel: it seeks to reduce the energy expenditure for the production for the benefit of energy from fuel obtained. The recent research aims, therefore, to seek innovative solutions for the various units of the process, solutions that will make machines, systems and processes more efficient. However, in the case of small size plant the problems of high operating and plant cost in front of a small quantity of biodiesel produced and of the cost of excise duty to be charged on the final product have not been solved in the prior art.

Purpose of the present invention is to provide a process for producing biodiesel from vegetable oils that is economical and efficient, thus overcoming the limitations of previously described process with reference to the known technique.

According to the present invention, a process for producing biodiesel from vegetable oils is provided, as defined in claim 1.

For a better understanding of the present invention a preferred embodiment is now described, purely as non- limiting example, with reference to the accompanying drawings, in which:

- Figure 1 shows a scheme of a process for producing biodiesel from vegetable oils, according to the invention.

With reference to such figures, and in particular to the figure 1, a process for producing biodiesel from vegetable oils is shown, according to the invention. In particular, the process for producing biodiesel from vegetable oils, virgin or exhausted one, comprises: - An initialization step, or Start-up;

- A step of stripping the alcohol and a simultaneous primary trans-esterification step at room temperature inside a first trans-esterification reactor RT1;

- a secondary decantation step;

- A step of purification and storage of crude biodiesel ;

- A step of secondary trans-esterification at the reaction temperature inside a second trans-esterification reactor RT2;

- A step of decantation, purification and storage of crude biodiesel.

Advantageously according to the invention, the startup step comprises:

- A step of providing vegetable oil;

- A step of mixing a catalyst and an alcohol;

- A step of trans-esterification in a single step; and

- A step of purification of crude biodiesel from glycerin by means of decantation.

More in detail, the step of providing vegetable oil, virgin or exhausted one, comprises the step of filtering the oil, through a filter plates pump, to remove carbonaceous residues and small percentages of water, so as to obtain an oil with moisture, ability to be unsaponifiable and insoluble products (MIU) to the maximum of 3% and an acidity of 5% and the absence of impurities. The start-up step involves a trans- esterification in a single step where the filtered oil is pumped into the second secondary trans-esterification reactor RT2, inside of which it is heated and recirculated via a pump.

The mixing step of the catalyst with the alcohol comprises mixing potassium hydroxide with methanol to obtain a mixture, improperly called "methoxide", which is added to the heated oil, as soon as the oil temperature reaches 55 °C - 60 °C. In particular, methanol is in the ratio of 6:1 with respect to oil and the potassium hydroxide is about 1% w/w with respect to the oil and mixing is deemed complete after about 5 - 10 minutes. The step for preparing the methoxide, as described, is followed by the single step transesterification, and, more specifically, the methoxide is sent into the reactor RT2 by means of the suction circuit of said pump. The line of methoxide is inserted in the suction circuit of said pump by means of a beveled nozzle, defined as a "whistle" system. The mixing of methoxide with oil thus realized allows oil and methoxide intimately blend both thanks to the "whistle" system that at the passage of the mixture in the pump helping to create a high turbulence in the fluid and reacting to form the mixture of methyl esters. Once all the methoxide has been mixed with the oil, the reagent mixture continues to be mixed by means of a circulation pump in the reactor RT2 for about 90-120 minutes, at the reaction temperature.

The purification step of the crude biodiesel from the glycerin is made by means of decantation inside the reactor RT2, and is favored by the conformation of the reactor RT2 which has a conical bottom. At the end of the decantation process that proceeds for about two hours, the glycerin is removed by means of a drain valve on the bottom of the reactor. Finally, the glycerin is transferred to the storage tank. The crude biodiesel is present in the reactor RT2 with a high percentage of methanol that will be recovered by means of a stripping step simultaneously with the step of production of biodiesel in two steps.

The alcohol stripping step and the simultaneous primary transesterification step at room temperature in a first transesterification reactor RT1 comprises the steps of:

- Pouring the oil from its storage tank to the reactor RT1; - At the same time bringing the biodiesel in the reactor RT2 at a temperature of about 65-70 °C, which is the evaporation temperature of the methanol, where the biodiesel is recirculated by means of a suitable pump to a stripping column CS1, placed above the reactor RT2 to which it is connected. The filling allows to increase the exchange surface and allow evaporation of methanol;

- Once finished the transfer of the oil inside the reactor RT1, allowing the circulating of oil from the reactor RT1, via a pump, to an ejector ET1 placed on top of the reactor RT1 and put into communication with the stripping column CS1 through the methanol line;

- To flowing simultaneously, inside the reactor RT1 used to recycle oil, the methoxide in proportions from ¾ to ¾ to let occur the primary transesterification step that will last at least for the time required for stripping.

Advantageously according to the invention, the methoxide, once prepared as already described above, is sent into the reactor RT1 through the suction circuit of the pump that is recirculating oil in the reactor RT1. The line of methoxide also in this case is grafted onto the suction circuit of said pump through the "whistle" system.

Advantageously according to the invention, the ejector ET1 provides to lower the pressure, lowering the vapor pressure inside the packed column thus favoring the evaporation of the methanol. In fact, in the process according to the invention, an optimal mixing between methoxide and oil is obtained in the reactor RT1 thanks to the strong speed present inside the tubes connected to the ejector and to the good turbulence that is generated inside the ejector to ensure a high degree of vacuum in the packed column, such as to promote trans-esterification at lower temperatures and lower reaction times.

Still advantageously according to the invention, the oil is used as motor fluid recirculating from the reactor RT1 to the ejector ET1, thus retrieving the stripped methanol, which is recovered from the top of the stripping column CS1, condensed through double threaded tube water cooled condensers, aspirated from the ejector ET1 through its suction nozzle, which sends the methanol in the diffuser where it mixes with the oil for finally relapsing in the reactor RT1.

Advantageously according to the invention, the primary is carried out at a temperature inferior compared to the secondary transesterification . In fact, thanks to the better mixing inside the reactor RT1, it is possible to realize a high conversion to methylesters at temperatures of 30-40 °C, during time periods fairly according to the times of stripping and less than 8 hours normally necessary with such temperatures. Therefore, in addition to recover methanol from the crude biodiesel, the process according to the invention advantageously improves the mixing between the oil and other reagents (methoxide added to the aspirated methanol) .

Advantageously according to the invention, the stripping step ends when the pressure indicator placed on the reactor RT2 doesn't detect pressure changes and is below 0.2 bara. At this point by means of an opening and closing valves system, the flow is blocked in the ejector ET1 and the reactor RT2 is brought to atmospheric pressure.

Advantageously according to the invention, the purification step of the crude biodiesel in the reactor RT2 comprises the steps of:

- Dry cleaning of biodiesel using a tower filled with ion exchange resin, for example Purolite PD206, suitably sized;

- Filtration of biodiesel; - The stabilization of the biodiesel using appropriate additives mixed in the storage tank SB1, by means of a pump.

According to an aspect of the invention, the filtering of biodiesel takes place by sending the biodiesel from a tower of filtration FP1 to another battery of filters designed to remove any suspended solids (of the order of microns) and traces of water present in the biodiesel, and, more specifically, a mechanical filter FS1, which holds particles of diameter greater than 0.7 microns, and a water filter cartridge FW1.

Advantageously according to the invention, for the whole duration of the methanol stripping step, the primary transesterification reaction proceeds in the reactor RT1. When the stripping is completed, if necessary, the mixture will continue the transesterification inside the reactor RT1. The biodiesel produced in the primary transesterification is separated from the glycerine by means of decantation inside the reactor RT1 and the separation is favored by the conformation of the conical bottom of the reactor RT1. At the end of the decantation process that lasts about two hours, the glycerin is removed through a drain valve on the bottom of the reactor. The secondary transesterification step at the reaction temperature in the second transesterification reactor RT2 comprises the steps of:

- terminating the steps of primary transesterification and of separation of the glycerine, obtained by decantation, sending the mixture present in the reactor RT1 to the reactor RT2 by means of a pump;

- carrying out the secondary transesterification inside the reactor RT2 introducing the remaining methoxide quantities and taking into account the quantity of stripped methanol previously introduced into the reactor RT1.

Advantageously according to the invention, the steps of preparation of methoxide and of mixing are identical to those of the primary transesterification .

According to an aspect of the invention, the step of secondary transesterification comprises the steps of bringing the mixture to the reaction temperature and of injecting the methoxide continuing to mix for about 90 -120 minutes .

Advantageously according to the invention, the step of secondary transesterification is followed by:

- a separation step of the glycerin from the biodiesel as already described for the step of primary transesterification; - a stripping step occurring simultaneously with the primary transesterification;

- a refining step.

Advantageously according to the invention, the whole process is calibrated taking into account that the process is suitable for small plants with capacity between 0.15 to 5 ton/day.

Therefore, the process for producing biodiesel from vegetable oils, according to the invention allow to realize the transesterification reaction in two different steps contemporary purifying biodiesel, using a single reactor for the purification and for the first trans- esterification.

Another advantage of the process for producing biodiesel from vegetable oils according to the invention consists in the saving of reactors, time, cost of plant and operating costs.

Another advantage of the process for producing biodiesel from vegetable oils according to the invention consists in the possibility of working with excess of methanol allowing to maximize the reaction.

Another advantage of the process for producing biodiesel from vegetable oils according to the invention consists in the possibility to completely recover the reagent and transfer it directly to the oil avoiding the inclusion of an intermediate step, such as washing with water which determines most losses, both in terms of reagent and in terms of energy to be used to retrieve the methanol .

Finally, the process for producing biodiesel from vegetable oils according to the invention allows to recover methanol without creating wastewater.

Finally it is clear that the process for producing biodiesel from vegetable oils described and illustrated here can be modified and varied without departing from the protective scope of the present invention, as defined in the appended claims.