*******

DESCRIPTION

FIELD OF THE INVENTION

The present invention relates to a plant and a method for the treatment/regeneration of spent bleaching sands/earths and/or acid water from plants for the production of biofuels. The plant and method of the invention simultaneously enable the production of biogas.

STATE OF THE ART

In the context of CO2 emission reduction projects, biofuels are fuels that have become increasingly popular in recent years. They are obtained by processing organic substances such as plants or waste of animal origin and are today used to fuel combustion engines, generate electricity and produce natural gas.

The term biofuel identifies all fuels derived by processing biomasses, i.e. organic substances of both plant and animal origin. In simpler terms, they are fuels obtained from the direct cultivation of several plants, agricultural or animal husbandry waste, wood processing and algae.

These fuels are used as an alternative to fossil fuels for wholly similar purposes. They can in fact fuel combustion engines, generate natural gas or be exploited to produce electricity. Precisely because they are obtained from both plant and animal organic substances, biofuels are considered renewable sources; unlike oil, coal and other minerals, their availability is not limited either in amount or over time.

The development of biofuels began in the second half of the last century, mainly in response to two challenges: to find a valid alternative to oil, given that worldwide deposits were already being depleted, and to discover fuels that were less polluting on an environmental level. The aim was to define a virtuous circle where the waste products of certain human activities, such as agriculture or animal husbandry precisely, could be reused as economical, highly efficient fuels with a low CO2 impact and free of heavy metals that are harmful to health.

Today it is possible to obtain biofuels from a whole variety of sources. As for fuel obtained in the agricultural sector, plants such as maize, soybean, rape and sunflower are preferred, but there is also the recovery of vegetable oils of industrial origin, wood processing waste, several marine algae, as well as the recycling of manure and other fertilisers of animal origin.

Biofuels are produced by means of:

• a complex fermentation process in special facilities: over time, a controlled microclimate and the action of anaerobic bacteria - i.e. bacteria able to survive in the absence of oxygen - bring about the decomposition of organic substance, which generates an inflammable vegetable oil and natural gas;

• a process first of deoxygenation and then isomerisation, to render the feedstocks wholly similar to the diesel fuel currently on the market.

With regard to the starting raw material (feedstock), a radical change has been observed in recent years:

• It started around the beginning of the 2000s with first-generation feedstocks, mainly palm oil, which could be sent directly to diesel desulfurization plants without producing processing waste, initially in blends with classic hydrocarbon feedstocks.

• When the construction of plants dedicated to the treatment of exclusively green feedstocks began, blending was integrated with second-generation feedstocks, mainly of plant origin, such as palm oil mill effluent (POME), palm oil fatty acid distillate (PFEAD), other virgin vegetable oils (rapeseed, maize, etc.) or oils already used in the food industry, such as UCO (used cooking oil), and it was necessary to introduce a pre-treatment of feedstocks to remove heavy metals (bleaching pre-treatment).

• At present, thanks also to the political will to eliminate plant feedstocks, such as palm oil, that are a dietary staple of some populations, mainly in Central America, and the deforestation phenomena associated therewith, we are observing a rapid transition to third-generation feedstocks (animal fats). This change has made it necessary to add a further degumming pre-treatment upstream of the bleaching treatment.

At first, the limit to the modification of feedstocks was the content of heavy metals and phosphatides. The former impacted on the catalytic activity of the catalysts of the treatment units (deoxygenation and isomerisation), whilst the latter resulted in problems in the plant loading filters and a difficult management of pressure drops in catalytic reactors (need for frequent shutdowns for skimming). For this reason, the first pre-treatment unit, called bleaching unit, was introduced.

Subsequently, when some thought of introducing, among feedstocks, those of animal origin, it was realised that a bleaching treatment was no longer sufficient on its own to manage phosphatides and a degumming treatment was added upstream of the bleaching treatment.

The degumming process is necessary for “soft” oils, such as sunflower, maize, soybean and rapeseed oils, waste oils originating from food preparation (UCOs) and fats of animal origin (AF), and is composed of:

■ Acid reaction to render the gums hydratable (soluble);

■ Base reaction to agglomerate the gums;

■ Separation of gums from the oil by centrifugation;

■ Acid washing to further reduce the content of phosphorus and metals in the oil;

■ Drying by means of a vacuum treatment to reduce residual moisture.

The bleaching process is a cold process consisting in the absorption of gums or phosphatides still present in the oil on the bleaching earths.

The bleaching treatment is the only treatment necessary when refining oils with a small content of gums and metals, such as palm oil, palm kernel oil, and coconut oil, since the gum content in these oils is limited.

The first step of the bleaching process, which consists in adding acid and a small amount of bleaching earth to the oil, is followed by filtration. This treatment reduces not only the content of residual phosphatides, but also the content of impurities and metals.

After the treatment, the bleaching earth is removed from the oil by filtration with a filter press, whereby an oil with a very low content of impurities and metals is obtained.

At present, spent bleaching earths (SBE) are sent to a storage tank and disposed of as industrial waste.

As industrial waste it is considered “dangerous goods” according to UN 3088- code 4.2 (spent Bleaching Earths) with waste code 07 06 10.

With the transition from second-generation feedstocks to third-generation feedstocks we are observing an increase in the production of industrial waste (spent bleaching earths, gummy water and soapstock) which needs to be treated outside the refinery, with a negative impact both in terms of CO2 emission and the economic returns in relation to biofuel production costs.

There is thus a greatly felt need in the sector to provide a plant and a method for the treatment/regeneration of spent bleaching earths and/or acid water from plants for the production of biofuels which enable biogas to be produced at the same time.

SUMMARY OF THE INVENTION

The present invention relates to a method of treating or regenerating spent bleaching earths from a plant for the production of biofuels, in particular from the bleaching pretreatment of feedstocks of plant origin that are used for the production of biofuels.

The method comprises a first pre-treatment step in which the bleaching earths mixed with neutral water or with gummy acid water originating from the same plants for the production of biofuel, in particular from the degumming treatment carried out upstream of the bleaching treatment when the feedstocks are also of animal origin, are neutralised by adding a base, preferably calcium hydroxide, in order to bring the pH to neutrality.

Subsequently, the neutralised bleaching earths are subjected to anaerobic digestion by a biomass comprising anaerobic bacteria. During the digestion step a biogas, which may optionally be subjected to purification, and a solid/liquid suspension are generated.

The solid/liquid suspension is subsequently separated into a solid component comprising the bleaching earths and a liquid component comprising the digestate.

The bleaching earths are then subjected to a heat treatment, at a temperature of between 400°C and 600°C. The combustion fumes produced are subsequently treated at a high temperature, preferably between 800°C and 1200°C, preferably for a time of at least 3 seconds to remove the dioxins.

Then follows reactivation and removal of the metals from the heat-treated bleaching earths with the aid of an acid solution and drying of the regenerated earths to render them usable in a plant for the production of biofuels.

The plant of the invention comprises: a) a pre-treatment section where the bleaching earths from a plant for the production of biofuel, in a mixture with neutral water or gummy acid water, are subjected to a base treatment; b) an anaerobic digestion section comprising at least two digester bioreactors wherein the mixture comprising the neutralised bleaching earths is brought into contact with a biomass; c) a dehydration/separation section comprising at least one separator for separating the bleaching earths from the digestate; d) a regeneration section comprising at least one combustion unit for carrying out a heat treatment of the bleaching earths from the dehydration section; e) a reactivation section comprising at least one reactor wherein the bleaching earths from the heat treatment are brought into contact with an acid solution; f) a drying section wherein the acidified bleaching earths are heated to remove excess moisture.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows a block diagram of a preferred embodiment of the method according to the invention;

Figure 2 shows a preferred embodiment of a plant according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The method of treatment/regeneration of the spent bleaching earths from plants for the production of biofuels comprises the following steps: a) pre-treatment of the spent bleaching earths; b) anaerobic digestion of the spent bleaching earths and production of biomethane; c) dehydration or separation of the spent bleaching earths from the digestate; d) a heat treatment of the spent bleaching earths separated from the digestate at a temperature between 400°C and 600°C (regeneration step). e) an acid treatment for reactivating the bleaching earths (step of reactivation and removal of the absorbed heavy metals); f) a drying heat treatment.

The bleaching earths that are treated/regenerated with the method of the invention come from plants for the production of biofuels that use, as raw material (feedstock), plant or animal sources, such as, for example, crops grown for the purpose (maize, rape, soybean, sunflower), vegetable oils, such as palm, rape, or maize oil, used cooking oil (UCO), agricultural or animal husbandry waste, wood processing waste, algae, or animal fats.

If the feedstocks comprise oils with a low gum and metal content, such as palm oil, palm kernel oil or coconut oil, it is necessary to carry out only the so-called “bleaching” pre-treatment, in which the bleaching earths are used to adsorb the phosphatides and the small amount of gums present in these oils. The bleaching earths consist of clays, in particular bentonites, which adsorb the gums and/or phosphatides present in these vegetable oils.

If, on the other hand, the feedstocks are spent oils of food origin (UCO), fats of animal origin and oils such as sunflower, maize, soybean and rapeseed oil, with a high gum and metal content, it is necessary also to carry out a “degumming” treatment upstream of the bleaching treatment. In this case, waste acid water is generated, and it can also be treated with the method of the invention.

With reference to the flow diagram in figure 1 , in the pre-treatment step a), the bleaching earths from the bleaching pre-treatment are mixed with neutral water and/or gummy acid water from a degumming treatment, realising in the latter case a method of treatment of the acid water from a plant for the production of biofuels, in addition to a method of treatment/regeneration of the bleaching earths.

After the bleaching earths have been mixed with water, the mixture undergoes homogenisation.

Subsequently, the pre-treatment envisages the neutralisation of the mixture/homogenate with a base, preferably calcium hydroxide, caustic soda and/or milk of lime.

The mixture/homogenate is heated to a temperature below/equal to 100°C, preferably between 50°C and 80°C. Heating preferably takes place by means of a heating fluid.

The pH the mixture/homogenate must reach is a neutral pH, preferably between 6 and 7.5.

Once neutralised, the mixture/homogenate is sent to the anaerobic digestion step b), in which it is brought into contact with a bacterial biomass at a temperature below 100°C, preferably between 50°C and 80°C. The bacterial biomass comprises anaerobic bacteria, preferably recycled/recovered from other digesters. The heating is carried out by means of a heating fluid or with other heating systems.

The anaerobic digestion takes place for a time between 21 and 28 days, preferably between 22 and 25 days.

The pH is preferably adjusted in a range of values between 7 and 8 to favour digestion.

During the anaerobic digestion step the following are produced: biogas, which after purification in biomethane will be collected and in part reused as fuel and a solid/liquid suspension (digestate) that is sent to the separation/dehydration step c)

A part of the digestate leaving the digesters is not sent to the subsequent separation/dehydration section, but is rather fed back into the digesters for the purpose of re-inoculating the anaerobic bacteria present in the digestor.

In a preferred embodiment, the biogas produced is sent to a purification step b1) in order to remove H2S and siloxanes (not shown in figure 1 ). The purification step comprises: an abatement of H2S by passing the biogas through a catalytic iron filter (CIF), preferably through at least 2, 3 or 4 filters in series, and/or further abatement of the residual H2S by passing it through at least one activated carbon absorber.

The catalytic iron filter allows up to about 75% of the H2S content to be removed, whereas the treatment with a catalytic absorber allows the concentration of H2S in the biogas to be reduced to values below 200 ppm.

The two purification treatments can be applied in series, one after the other, for example first the treatment with the CIF filter and subsequently the treatment with an activated carbon absorber. Alternatively, the two treatments can be applied individually according to the biogas purification requirements.

The purification step also comprises a further treatment to remove siloxanes by passing the biogas through at least one catalytic absorber, preferably through at least two catalytic absorbers.

This further treatment can be alternative to the first two or carried out in addition to the first two. It can take place upstream of the first two or after the first two.

In the separation/dehydration step c) the liquid phase of the digestate is separated from the solid component comprising the bleaching sands, preferably by gravity, for example using a deep cone thickener, and/or by centrifugation. The liquid phase comprising the digestate is sent off for waste water treatment or directly to the sewer system, without prior treatment, if the wastewater is sufficiently pure to be discharged into the sewer.

In the regeneration step d), the bleaching earths separated from the digestate are subjected to a heat treatment at between 400°C and 600°C, preferably between 400 and 500°C. The combustion fumes are preferably sent to a post-combustion treatment at a temperature between 800°C and 1200°C, preferably between 800°C and 1100°C.

The post-combustion treatment is applied for at least 3 seconds and serves to remove the dioxins.

In the reactivation step e), the bleaching earths are brought into contact with an acid solution at a temperature between 60°C and 100°C for a time between 20 and 100 minutes.

The acid solution is then separated from the bleaching earths and reused at least in part for the reactivation of the bleaching earths and in part sent off for disposal.

Finally, the bleaching earths are subjected to a heat/drying treatment in step f), at a temperature between 600 and 700°C, in order to ready them for reuse in a plant for the production of biofuel.

In a preferred embodiment, the method envisages recycling energy by using the fumes from the bleaching earths regeneration step d) and from the drying heat treatment of step f). After removal of the dioxin, the fumes from step d) are used for the heat/drying treatment of step f), before being released into the atmosphere. The fumes from step d) and/or f) are also used to generate a hot fluid, preferably tempered/hot water, which can be used to heat the reaction mixture of the pretreatment step a) and/or introduced into the digestion reaction.

With reference to figure 2, the invention also relates to a plant for treating or regenerating spent bleaching sands/earths from the production of biofuel, in particular from the bleaching pre-treatment. The plant also serves to treat the acid water from the production of biofuel, in particular from the degumming pre-treatment. The plant provides for simultaneous production of biogas.

The plant of the invention comprises a pre-treatment section comprising a neutralisation reactor (100) and at least a first line (101 ) connected to a plant for the production of biofuel in order to feed, into the neutralisation reactor, the spent bleaching earths from the bleaching pre-treatment to which the raw materials used to produce biofuel are subjected.

In one embodiment, the pre-treatment section comprises at least a second line (102) for feeding, into the neutralisation reactor, neutral water or acid water from a plant for the production of biofuel, in particular from the degumming pre-treatment of the raw materials used to produce biofuel. In the latter case the line (102) is connected to a plant for the production of biofuel.

The neutralisation reactor (100) comprises a base, for example calcium hydroxide, and a stirrer for maintaining the aqueous suspension comprising the bleaching earths under stirring.

The neutralisation reaction of the suspension of bleaching earths takes place at a temperature below/equal to 100°C, preferably between 50°C and 80°C, by means of a heat exchange with a hot fluid, for example tempered water from a tank (not shown in the figure). A line (103) coming from the tank creates a closed circuit with tempered water that is used to recover part of the energy developed in the heat regeneration step. In one embodiment, the line (103) serves both the reactor (100) and the digesters (105, 106). In this manner, the digesters are also heated by means of a heat exchange with the hot fluid transported by the line (103). In this case, the temperature of the fluid must be kept below 100°C to safeguard the bacterial biomass contained in the digesters.

In one embodiment, the hot fluid is heated using the heat originating from the regeneration step d) and drying step f). This embodiment is not shown in figure 2.

Present in one embodiment there is a heat exchanger (104) through which the heating fluid of the line (103) is conveyed to the neutralisation reactor (100) and optionally to the digesters (105, 106). There is a possible variant of the plant in which the heating fluid of the neutralisation reactor and of the digesters comes from different lines and from different heated tanks. Alternatively, it is possible that the suspension in the neutralisation reactor is heated with an alternative system that does not envisage a heat exchange with a heating fluid, for example by electricity, preferably self-produced.

The neutralisation reactor comprises a means for checking the pH, for example an inline pH meter or analysis performed by an operator. The bleaching earth suspension must be brought to a neutral pH, preferably to between 7 and 7.5.

Downstream of the pre-treatment section, the plant comprises an anaerobic digestion section which comprises at least two anaerobic digesters (105, 106) into which the neutral suspension of bleaching earths from the neutralisation reactor is conveyed. The neutral suspension of bleaching earths is conveyed from the neutralisation reactor through a line not shown in the figures. The lines are provided with a system of pumps for drawing in the neutral suspension and pushing it towards the digesters. There are at least 2 anaerobic digesters, but they may also be present in a larger number, and they comprise a bacterial biomass that serves to digest the substances adsorbed onto the bleaching earths and present in the neutral suspension.

The digesters are equipped with systems for controlling the pH and temperature. The temperature is controlled by using a heating fluid transported by the line (103) or with other systems, as explained above.

The pH is adjusted by adding calcium hydroxide, milk of lime or caustic soda.

Most of the digestate leaving the digesters is sent to the subsequent separation section through the line (107), whilst a small part of the digestate is recirculated to the digesters through the line (108).

During the digestion, biogas is formed; it is collected in at least two gasometers (109, 110), each connected to a digestor through the lines (111 ).

In one embodiment, the gasometers comprise at least 3 membranes and a compressor which serves to draw in the biogas and bring it to a pressure compatible with the plant network.

The gasometers are connected to a biogas purification section, if present. This purification section is optional and may also not be present.

The purification section comprises at least two reactors for the abatement/removal of H2S (112, 113) and at least one reactor for the removal of siloxanes (114). These reactors can be operated in series, one after the other, or they can function alternately. The reactors for the abatement of H2S are preferably catalytic iron filters (CIF) and/or activated carbon filters. The reactor for the removal of siloxanes is preferably a catalytic absorber.

The line (115) connects the gasometers to the purification section, if present, or directly to the collection of the biogas for other uses. If the purification section is present, the line (115) transports the biogas to the purification reactors and subsequently conveys the purified biogas to be collected for other uses.

In a preferred embodiment, the biogas, purified or not purified, is partially sent to the regeneration section (first and second heat treatment) through the line (116) and thus used to maintain/adjust the regeneration temperature.

Downstream of the digestion step, there is a dehydration/separation section that comprises at least one separation unit (117), connected to the digesters by means of the line (107), which serves to transport the solid/liquid suspension (digestate).

The separation unit (117) preferably comprises a solid/liquid separator, for example a deep cone thickener, which works by gravity, and/or a centrifuge (117a).

The liquid phase (liquid digestate) leaving the separation unit (117 and/or 117a) is sent off for waste water treatment or to the sewer drain through a line (118) coming out of the separation unit (117 and/or 117a).

The solid phase comprising the bleaching earths is sent to the regeneration section through the line (119).

The regeneration section comprises a first heating unit (120) and preferably a second heating unit (121 ).

The first heating unit (120) is preferably a rotary oven that reaches temperatures between 400°C and 600°C, preferably between 400 and 500°C, and into which biogas transported by the line (116) coming from the gasometers and/or from the purification section may be blown. In addition or as an alternative to the biogas, the temperature is maintained/controlled by blowing in hot air transported by the line (122) coming from a special generator and/or recycled from the hot air leaving the second heating unit (121 ). A heat exchanger (122a) is optionally present on the line (122).

The second heating unit (121 ) is preferably an afterburner where the fumes from the heat treatment of the solid step comprising bleaching earths are treated at a temperature between 800°C and 1200°C, preferably between 800°C and 1100°C so as to remove the dioxins. Biogas and/or hot air from the line (116) and/or (122) is also blown into the afterburner in order to maintain/increase the combustion temperature.

Downstream of the regeneration section there is a reactivation section comprising a reactor (123) into which the regenerated bleaching earths coming from the heat regeneration through the line (124) and an acid solution coming through the line (125) are introduced.

The reactor (123) is preferably provided with a thermostatic jacket that has the function of maintaining a constant temperature. The reactor is preferably equipped with a mixer to favour the reactivation reaction, at the end of which an acidic solid/liquid suspension is formed.

The acidic solid/liquid suspension is sent from the reactor (123) to a separator (124), preferably a filter press, through the line (125), wherein the acid solution is separated from the solid part comprising the bleaching earths.

The acid solution is partially recirculated to the reactor (123) and partially sent to the drain through the lines (126 and 127), respectively. The solid part is sent to the heat treatment/drying section.

The drying section, located downstream of the reactivation section, comprises a heating unit (128), for example a rotary oven, preferably heated by means of the hot air flow coming from the regeneration section through the line (122). Alternatively, the heating unit is heated with other means.

The regenerated bleaching earths leave the heating unit ready for use in a plant for the production of biofuel.