WO2010140037A1

WO2010140037A1 - Process for the drying of algal biomass

Info

Publication number: WO2010140037A1
Application number: PCT/IB2010/001230
Authority: WO
Inventors: Gianni Pandolfi; Roberta Miglio; Lino Carnelli
Original assignee: Eni SpA
Current assignee: Eni SpA
Priority date: 2009-06-05
Filing date: 2010-05-18
Publication date: 2010-12-09
Anticipated expiration: 2011-12-05
Also published as: ITMI20090984A1; IT1394412B1

Abstract

A process for the drying of algal biomass, comprising: - producing an aqueous suspension of algal biomass; concentrating said aqueous suspension of algal biomass in order to obtain a concentrated aqueous suspension of algal biomass; depositing a layer of said concentrated aqueous suspension of algal biomass on a hydrophobic surface, in order to obtain a layer having a thickness lower than or equal to 1 cm, preferably ranging from 0.2 to 0.6 cm,- putting this layer in contact with hot air, said hot air being at a temperature ranging from 40°C to 110°C, preferably ranging from 60°C to 9O°C in order to obtain a layer of dried algal biomass. The dried algal biomass obtained by means of the above-mentioned process can be advantageously used for the production of lipids. Said lipids can be used, in turn, for the production of biodiesel or green diesel. Alternatively, said dried algal biomass can be advantageously used for the production of bio-oil or bio- crude. Alternatively, said dried algal biomass can be advantageously used for the production of energy through thermal treatments such as, for example, combustion or gasification.

Description

PROCESS FOR THE DRYING OF ALGAL BIOMASS

The present invention relates to a process for the drying of algal biomass.

More specifically, the present invention relates to a process for the drying of algal biomass, which comprises depositing a layer of a concentrated aqueous suspension of algal biomass on a hydrophobic surface and putting said layer in contact with hot air.

The dried algal biomass obtained through the above process can be advantageously used in the production of lipids. Said lipids, in turn, can be used in the production of biodiesel or green diesel.

Alternatively, said dried algal biomass can be advantageously used for the production of bio-oil or bio-crude.

Alternatively, said dried algal biomass can be advantageously used for the production of energy through thermal treatments such as, for example, combustion or gasification. Algae, in particular microalgae, are currently cultivated for the production of valuable compounds such as, for example, poly-unsaturated fatty acids [for example, eicosapentaenoic acid (EPA) , docosahexaenoic acid (DHA) , and the like] , vitamins (for example, β- carotene, and the like) and gelling agents, which are included in the nutritional, pharmaceutical and cosmetic fields.

The cultivation of microalgae for the above field is characterized by rather limited production capacities (in the order of hundreds-thousands of tons per year) and by a high added value of the compounds obtained (hundreds-thousands of euro per kilogram) . This is why complex and expensive production systems, particularly with respect to the concentration and drying of the biomass and to the extraction of products of interest, which must satisfy the strict regulations of the sanitary and nutritional type, typical of the above-mentioned fields, can be tolerated. The shift from the above-mentioned fields, wherein the microalgae are traditionally used, to the energy field, requires the development of technologies which lead to considerable increases in the production capacity (in the order of hundreds-thousands per year to millions of tons per year) and a large reduction in the production costs due to the limited added value of the products destined for the energy field (hundreds of euro per ton) .

Processes relating to the extraction of compounds from algal biomasses, wherein said algal biomasses are dried, are described in the art.

Molina Grima et al . , for example, describe the recovery of algal biomass and metabolites in the following article "Recovery of microalgal biomass and metabolites: process options and economics", published in "Biotechnology Advances" (2003), 20, pg. 491-515. In said article, it is specified the necessity of treating the algal biomass recovered, for example, by centrifugation, rapidly, as said biomass can be seriously damaged in a few hours in hot climates. In order to keep said algal biomass for longer periods of time, it is therefore useful to subject said biomass to drying. Suggested drying methods include: spray drying, drum drying, freeze drying, and sun drying.

United States Patent US 5,539,133 describes a method for obtaining lipids with a high quantity of poly-unsaturated fatty acids having from 20 to 22 carbon atoms through the extraction of a raw material of an animal or vegetable origin, said raw material having a water content of from 5% by weight to 50% by weight and a particles size of from 0.01 mm to 50 mm, said extraction being carried out using an organic solvent, preferably miscible with water (e.g., ethanol) , or through liquefied compressed gas (e.g., carbon dioxide, propane, or blends thereof) . As raw material which can be used for this purpose, unicellular algae (microalgae) , or macroalgae belonging to the family of red, brown o green algae, are also indicated, said algae being previously dried. As an alternative, partially dried products ("algae flour") based on red or brown algae available on the market, can be used as raw material . Said polyunsaturated fatty acids such as, for example, docosahexaenoic acid (DHA) , arachidonic acid, are particularly used in the food industry.

Miao X. et al . describe the production of biodiesel from algae in the following article: "Biodiesel production from heterotrophic microalgal oil" , published in "Bioresource Technology" (2006), 97, pg. 841-846. In said article, algae of the species Chorella protothecoides, grown under heterotrophic conditions, are collected by centrifugation, washed with distilled water and subsequently dried in a freeze drier. The freeze-dried algae are subsequently pulverized in a mortar and subjected to extraction with hexane in order to extract the oil. The extracted oil is subsequently .subjected to acid transesterification in the presence of methanol and sulphuric acid as transesterification catalyst in order to obtain biodiesel.

The production of biodiesel from algae is also described by Hossain S. et . al . in the following article: "Biodiesel Fuel Production from Algae as Renewable Energy" , published in "American Journal of Biochemistry and Biotechnology" (2008), 4 (3), pg. 250- 254. In said article, algae of the species Oedogonium and Spirogyra, after being ground and chopped as much as possible, are dried in a thermostatic oven at 80⁰C, for 20 minutes, in order to eliminate the water. Subsequently, a solution of hexane and ether is mixed with the dried algae to extract the oil. The oil extracted is recovered by means of evaporation, under vacuum, from said hexane and ether solution. The transesterification reaction of the oil thus obtained, for the purpose of obtaining biodiesel, is carried out in the presence of a mixture of sodium hydroxide as transesterification catalyst and methanol (basic transesterification) . United States Patent Application US 2006/0275863 describes a method for preserving the xanthophylls in algal cells, comprising: growing algal cells containing xanthophylls and subsequently recovering said algal cells; and drying the algal cells in order to obtain algal cells having a humidity content lower than or equal to 3% by weight. No particular limitations are indicated with respect to the drying process . For example, said algal cells can be dried through drum drying, hot air drying, freeze drying, spray drying. Spray drying is preferred and the temperature of the air at the outlet of the equipment used for said spray drying is regulated at 125⁰C or higher.

In the "Proceedings of the 14th International Drying Symposium (IDS 2004)", Sao Paulo, Brazil, 22-25 August 2004, Vol. B, pages 813-820, the drying process is described, which can also be applied to algal biomass, known by the trade-name of Refractance Window^® of Desert Lake Technologies. In said process, once the water suspension of algal biomass has been collected and filtered, it is introduced, by means of small jets, onto a conveyor belt consisting of a transparent polyester material (for example, Mylar^® of DuPont) in order to form a thin layer, of a thickness generally ranging from 0.2 mm to 1 mm. Under the conveyor belt, there is a chamber with hot water at about 70⁰C, said water being heated by means of an external oven. Said conveyor belt slides on the water, the heat passes underneath the belt and is absorbed by the water which represents up to 95% by weight with respect to the total weight of the above-mentioned aqueous suspension of algal biomass, thus evaporating the water in the environment. At the end of this process, the dried algal biomass appears in the form of a thin sheet.

The above processes can have various critical aspects, however. During the drying of the algal biomass by means of spray drying, for example, the mixture air/algal biomass powder which is formed, can be explosive under certain conditions (temperature, dimension and concentration of the powder, residual humidity, etc.) which imposes particular restrictions for the spray drying equipment, with consequent economical expenses . Furthermore, the algal biomass powder obtained by spray drying generally has fine dimensions and can therefore be potentially inhalable and harmful for the respiratory system of the operators. Consequently the installation of suitable filters is generally required in order not to harm the health of the operators with consequent additional economical expenses .

In addition, although the freeze drying process is a drying process capable of giving high-quality dried algal biomass, it is extremely onerous in energy and economic terms.

Furthermore, the use of dried algal biomass powder in the subsequent processes for the production of lipids, for example, by means of solvent extraction as described above, can cause problems of entrainment of said powder into the liquid phase, which can make the separation and purification of said lipids more complex and, consequently, decrease the extraction yield.

The Applicant has now found that the drying of algal biomass can be advantageously obtained by depositing a layer of a concentrated aqueous suspension of algal biomass on a hydrophobic surface and putting said layer in contact with hot air. Said process allows dried algal biomass to be obtained avoiding the formation of algal biomass powder and, consequently, the optional problems mentioned above. This process also allows dried algal biomass to be obtained having a high quality, equal to that obtained by means of freeze drying, but avoiding the costs of the freeze drying process .

Furthermore, this process allows to avoid the use of heated water and, consequently to simplify the drying process also with respect to the process known as Refractance Window^® described above, with a consequent economical advantage.

An object of the present invention therefore relates to a process for the drying of algal biomass, comprising: producing an aqueous suspension of algal biomass; - concentrating said aqueous suspension of algal , biomass in order to obtain a concentrated aqueous suspension of algal biomass; depositing a layer of said concentrated aqueous suspension of algal biomass on a hydrophobic surface, in order to obtain a layer having a thickness lower than or equal to 1 cm, preferably ranging from 0.2 to 0.6 cm; putting this layer in contact with hot air, said hot air being at a temperature ranging from 4O⁰C to

HO⁰C, preferably ranging from 60⁰C to 9O⁰C in order to obtain a layer of dried algal biomass.

For the purposes of the present description and of the following claims, the term "hydrophobic surface" means a surface on which a drop of water forms a contact angle higher than 90° .

For the purposes of the present description and of the following claims, the definitions of the numerical intervals always include the extremes unless otherwise specified.

In accordance with a preferred embodiment of the present invention, the production of said aqueous algal biomass can derive from the cultivation of algae, preferably unicellular (microalgae) , carried out in wastewater coming from industrial and/or civil wastewaters. In this case, the algal cultivation metabolizes the substances containing nitrogen and/or phosphorous contained therein, contributing to their purification. As CO₂ necessary for algal growth, that contained in industrial combustion gases (refineries, thermoelectric plants, hydrogen generation plants, etc.), can be used.

In accordance with a further preferred embodiment of the present invention, the production of said aqueous suspension of algal biomass can derive from the cultivation of algae, preferably unicellular (microalgae) , carried out in sea water.

The cultivation of said algae can be carried out under photo-autotrophic conditions, heterotrophic conditions, or under mixotrophic conditions.

In accordance with a preferred embodiment of the present invention, the concentration of said aqueous suspension of algal biomass can be carried out through various processes such as, for example: gravitational separation by means of sedimentation and/or thickening apparatuses, typically used in the water treatment plants; flotation; - gravimetric separation by means of cyclones or spirals ; centrifugation,- filtration by means of membranes for ultra- or micro- filtration, or vacuum filtration; - treatment by means of filter presses or belt presses .

In order to facilitate the above-mentioned concentration, said aqueous suspension of algal biomass can be subjected to flocculation. Said flocculation can be carried out by means of various processes, such as, for example : bio-flocculation (for example, by cultivating algae in culture mediums having a low nitrogen concentration) ; addition of at least one flocculating agent to said aqueous suspension of algal biomass. According to a preferred embodiment of the present invention, said flocculation can be carried out by the addition of at least one flocculating agent. Said flocculating agent is preferably selected from: metal salts such as, for example, iron chloride, aluminium sulphate, iron sulphate; cationic polyelectrolytes such as, for example, polyethylene imines, polyacrylamides, chitosane; or mixtures thereof.

For example, the concentration of algal strains of fresh water, such as, for example, the strain Shenedesmus sp. , can be particularly facilitated by the use of cationic polyelectrolytes, preferably polyacrylamides, adopted in a quantity of 2 ppm - 10 ppm (passage of the algal concentration from 0.2 g/1 to 30 g/1 - 40 g/1 in a few hours) .

According to a preferred embodiment of the present invention, said concentrated aqueous suspension of algal biomass can have a concentration of dry algal biomass ranging from 5% by weight to 25% by weight, more preferably from 8% by weight to 20% by weight, with respect to the total weight of the concentrated aqueous suspension of algal biomass . According to a preferred embodiment of the present invention, said concentrated aqueous suspension of algal biomass can have a concentration of water ranging from 75% by weight to 95% by weight, more preferably ranging from 80% by weight to 92% by weight, with respect to the total weight of the concentrated aqueous suspension of algal biomass .

The water released from the concentration of said aqueous suspension of algal biomass, can be recovered in a large quantity and re-used in the above-mentioned process as water in the production of said aqueous suspension of algal biomass (i.e. as cultivation water of the algae) .

In accordance with a preferred embodiment of the present invention, said hydrophobic surface can include at least one hydrophobic polymer having a melting point higher than or equal to HO⁰C, preferably ranging from 120⁰C to 380⁰C.

In accordance with a further preferred embodiment of the present invention, said hydrophobic polymer can be selected from: polyolefins, such as, for example, polypropylene, polyethylene, ; polyesters, such as, for example, polyethylene terephthalate (e.g. Mylar of DuPont) ; halogenated polymers, such as, for example, polyvinylchloride, polytetrafluoroethylene (e.g. Teflon⁰ of DuPont); polystyrenes; polysulfones , such as, for example, polyethersulfone; or mixtures thereof. Polypropylene, polyethylene, polyethylene terephthalate (e.g., Mylar* of DuPont), or mixtures thereof, are preferred.

In accordance with a preferred embodiment of the present invention, said hydrophobic surface can have a thickness ranging from 0.2 mm to 5 mm, preferably ranging from 0.5 mm to 3 mm. Said hydrophobic surface can be smooth or wrinkled.

In order to automate the depositing operations of said concentrated aqueous suspension of algal biomass on said hydrophobic surface, a conveyor belt can be advantageously used.

In accordance with a preferred embodiment of the present invention, said hydrophobic surface can be a conveyor belt .

In accordance with a preferred embodiment of the present invention, said hydrophobic surface can be applied on a conveyor belt. In this case, said conveyor belt can be of polymeric material such as, for example, polyamides (e.g., nylon), rubbers, or mixtures thereof.

Said conveyor belt can operate batchwise, when the conveyor belt is kept in movement both during the depositing operations of the concentrated aqueous suspension of algal biomass on said conveyor belt and during the discharge of the dried algal biomass from said conveyor belt and is stopped during the drying operations of the algal biomass; or in continuous, when the conveyor belt is kept running during the whole process .

In accordance with a preferred embodiment of the present invention, said conveyor belt can move at a rate ranging from 0.1 m/min to 130 m/tnin, preferably ranging from:

90 m/min to 120 m/min when operating batchwise; 0.2 m/min to 1 m/min when the operating in continuous . In accordance with a preferred embodiment of the present invention, said hot air can be heated using solar energy, or waste vapours coming from other processings. The use of solar energy is preferred. It should be noted that if waste vapours are used, it is advantageous to adopt a heat exchanger.

In accordance with a preferred embodiment of the present invention, said layer of concentrated suspension of algal biomass can be put in contact with hot air for a time ranging from 30 minutes to 10 hours; preferably from 1 hour to 6 hours.

In accordance with a preferred embodiment of the present invention, said hot air can be anhydrous air.

For the purposes of the present description and of the following claims, the term "anhydrous air" refers to air having at room temperature (i.e. 25°C) , a relative humidity content lower than or equal to 70%, preferably ranging from 0% to 50%.

In order to avoid optional movements of the layer of dried algal biomass, and to avoid the optional entrainment of the same into the air, above said hydrophobic surface can be placed a metal grid, or a net of polymeric material such as, for example, polyvinylchloride, said grid or net preferably with meshes having a dimension lower than 2 cm x 2 cm (h x

1) , or a sheet of paper, for example blotting paper. It has to be noted that said grid, net or sheet of paper, must not come into contact with the layer of concentrated aqueous suspension of algal biomass deposited on said hydrophobic surface.

In accordance with a preferred embodiment of the present invention, said grid, net or sheet of paper can be placed at a distance, with respect to said hydrophobic surface, lower than or equal to 5 cm, preferably ranging from 1 to 4 cm.

The change of hot air in contact with said layer of concentrated aqueous suspension of algal biomass takes place naturally, due to the change of air density caused by both the heating and humidity formed.

Alternatively, for the purpose of conveying and moving the hot air which heats said layer of concentrated aqueous suspension of algal biomass and of continuously removing the damp air formed, it can be advantageous to effect said procedure inside a substantially closed structure.

A further object of the present invention therefore relates to a process for the drying of algal biomass inside a substantially closed structure, said substantially closed structure including at least one hydrophobic surface, comprising: producing an aqueous suspension of algal biomass; concentrating said aqueous suspension of algal biomass in order to obtain a concentrated aqueous suspension of algal biomass; depositing a layer of said concentrated aqueous suspension of algal biomass on a hydrophobic surface, in order to obtain a layer having a thickness lower than or equal to 1 cm, preferably ranging from 0.2 cm to 0.6 cm; putting this layer in contact with hot air, said hot air being at a temperature ranging from 40⁰C to 110⁰C, preferably ranging from 60⁰C to 90⁰C in order to obtain a layer of dried algal biomass .

For the purposes of the present description and of the following claims, the term "substantially closed structure" means a structure equipped with a cover, which can optionally be opened, wherein the air enters from one side and exits from the opposite side.

According to a preferred embodiment of the present invention, said substantially closed structure can be equipped with slanting side panels (hut covering) , or by at least one upper panel (flat covering) and at least one side panel. Said panels can be selected, for example, from glass; plastic materials such as, for example, plexiglass, polyacrylates, polycarbonates, polystyrene, polyolefins (e.g. polyethylene, polypropylene) ; composite materials such as, for example, polyolefins reinforced with glass fibres; said plastic materials and/or said composite materials preferably being stabilized to UV rays by the addition of UV stabilizing agents, such as, for example, benzophenones, benzotriazoles, sterically hindered amines (HALS), or mixtures thereof. Preferably, said panels are made of UV-stabilized polyethylene.

In accordance with a preferred embodiment of the present invention, said hydrophobic surface can be a conveyor belt. In accordance with a further preferred embodiment of the present invention, said hydrophobic surface can be applied on a conveyor belt. In this case, said conveyor belt can be made of polymeric materials, such as, for example, polyamides (e.g., nylon), rubbers, or mixtures thereof .

In accordance with a preferred embodiment of the present invention, said conveyor belt can move at a rate ranging from 0.1 m/min to 130 m/min, preferably ranging from:

90 m/min to 120 m/min, when operating batchwise; 0.2 m/min to 1 m/min when operating in continuous . The movement of said hot air inside said substantially closed structure can be carried out by means of a chimney, or by means of a fan.

In accordance with a preferred embodiment of the present invention, said hot air can move inside said substantially closed structure at a flow-rate ranging from 0.2 m/min to 10 m/min, preferably ranging from 0.3 m/min to 4 m/min.

In accordance with a further preferred embodiment of the present invention, said hot air can move inside said substantially closed structure at a flow-rate ranging from 15 Nm³/ⁿ to 250 Nm³/h, preferably from 18 Nm³/h. and 240 Nm³/h

It has to be noted that said layer of dried algal biomass can generally have a thickness lower than or equal to 1/10 with respect to the thickness of the layer of the concentrated aqueous suspension of algal biomass deposited on said hydrophobic surface.

In accordance with a preferred embodiment of the present invention, said layer of dried algal biomass can have a thickness lower than or equal to 0.1 cm, preferably ranging from 0.02 cm to 0.06 cm.

In accordance with a preferred embodiment of the present invention, said layer of dried algal biomass can have a residual humidity content lower than or equal to 5% by weight, preferably ranging from 0.5% to 3% by weight, with respect to the total weight of said layer of dried algal biomass .

The present invention will now be illustrated in greater detail through an illustrative form with reference to Figure 1 provided hereunder. According to a typical embodiment of the process object of the present invention, the aqueous suspension of algal biomass is produced through the cultivation of microalgae, carried out in wastewater coming from industrial discharges, or in sea water. The production of microalgae, for example, can be suitably carried out by combining cultivation systems such as photoreactors and open ponds .

The aqueous suspension of algal biomass thus obtained, is concentrated by gravitational separation, preferably in the presence of a flocculating agent, obtaining a concentrated aqueous suspension of algal biomass (2) .

The water released from the concentration of the aqueous suspension of algal biomass can be mostly recovered and re-used in the above process as water in the production of said aqueous suspension of algal biomass (i.e. as cultivation water of algae) (not represented in Figure 1) . The concentred aqueous suspension of algal biomass (2) is fed through a feeding pump (not represented in Figure 1) to a hopper (3) situated inside a substantially closed structure (1) , said substantially- closed structure comprising a frame (6) , sloping side panels (hut covering) (7) (the sloping is not shown in Figure 1) and supports (8) . Said panels are preferably made of polyethylene stabilized to ultraviolet rays . Said panels can be opened (not represented in the figure 1) . A concentrated aqueous suspension of algal biomass, having a concentration of dry algal biomass preferably ranging from 8% by weight to 20% by weight with respect to the total weight of the concentrated aqueous suspension of algal biomass, is then fed to said hopper (3) .

The concentrated aqueous suspension of algal biomass leaving the hopper, is deposited on a conveyor belt, preferably of Mylar* of DuPont, in order to obtain a layer of concentrated aqueous suspension of algal biomass having a thickness preferably ranging from 0.2 cm to 0.6 cm.

Air is introduced into this substantially closed structure (1) , in countercurrent with respect to the conveyor belt (4) , by means of the fan (13) through the duct (14) . As mentioned above, the air can be heated with the use of solar energy or, as alternatively, it can be preheated using waste vapours coming from other processing, through the heat exchanger (12) . Alternatively, in order to reduce the consumption of electric energy, as represented in Figure 1, said fan (13) can be activated using the energy [represented with the dashed line (17)] provided by a solar panel (16) . Said solar panel (16) can include a photovoltaic device capable of collecting and converting the solar radiations into electric energy.

Inside said substantially closed structure (1) , the air is heated to a temperature ranging from 6O⁰C to 90⁰C, using solar energy. As a further alternative, in order to reduce the consumption of electric energy, the fan (13) can be stopped, and the air circulation inside said substantially closed structure (1) , can be allowed by the presence of a chimney (5) . Said chimney (5) also serves for eliminating the water vapour deriving from the drying of the concentrated aqueous suspension of algal biomass. Said water vapour, in fact, leaves the chimney (5) as humid air. As represented in Figure 1, a metal net (15) is positioned above said conveyor belt (4), said net preferably having meshes with dimensions lower than 2 cm x 2 cm (h x 1) , at a distance preferably ranging from 1 cm to 4 cm with respect to said conveyor belt (4) .

At the end of the drying, a thin layer of dried algal biomass is obtained, having a thickness preferably ranging from 0.02 cm to 0.06 cm, which is detached from the conveyor belt (4) either naturally, or by means of a scraper (not represented in Figure 1) which can be situated at the end of the conveyor belt (4) , on the opposite side with respect to the hopper (3), fragmenting into pieces ("leaves") . Said pieces ("leaves") are subsequently collected in a discharge hopper (9) . Said pieces ("leaves") can be directly sent for further treatment, such as, for example, lipid extraction, biodiesel or green diesel production, bio- oil or of "bio-crude" production, energy production, or they can be collected into sacks (not represented in Figure 1) and sent to storage. Said sacks can be stored for long periods (in the order of months) .

As mentioned above, the dried algal biomass obtained according to the process of the present invention, can be subjected to further treatment.

Said dried algal biomass, for example, can be subjected to lipid extraction. Said extraction can be carried out by means of processes known in the art, such as, for example, mechanical extraction; extraction in the presence of organic solvents (for example, C₆-C₈ hydrocarbons, alcohols or mixtures thereof, etc.) operating in liquid phase, or operating under supercritical conditions (for example, in the presence of carbon dioxide, propane, or mixtures thereof, etc.) . The extracted lipids can be subjected to transesterification in the presence of an alcohol having from 1 to 4 carbon atoms, preferably methanol, ethanol, and a catalyst, preferably an acid catalyst, in order to produce glycerol and alkyl esters, in particular methyl esters or ethyl esters (biodiesel) .

Alternatively, the extracted lipid can be subjected to hydrogenation/deoxygenation in the presence of hydrogen and a catalyst for the production of green diesel. Hydrogenation/deoxygenation processes are known in the art and are described, for example, in European patent application EP 1,728,844.

As a further alternative, said dried algal biomass can be used for the production of bio-oil or bio-crude which can be carried out by means of thermal treatments such as, for example, liquefaction or pyrolysis.

Finally, said dried algal biomass can be used in the production of energy which can be carried out by means of thermal treatments such as, for example, combustion or gasification.

Some illustrative and non-limiting examples are provided hereunder for a better understanding of the present invention and for its embodiment.

EXAMPLE 1 Drying of algal biomass

An aqueous suspension of algal biomass was used for this purpose, obtained from the algal strain of Scenedesmus sp. grown in a culture medium M4N indicated in literature for the cultivation of microalgae . The aqueous suspension was subjected to determination of the concentration of algal bioraass which proved to be equal to 0.92 g/1.

Said aqueous suspension of algal biomass was then centrifuged in a disk centrifuge (Alfa Laval) obtaining an aqueous suspension of algal biomass having a concentration of dry algal biomass equal to 10 g/litre.

Said aqueous suspension of algal biomass was subsequently subjected again to centrifugation in a rotor centrifuge (Sorvall) until a concentrated aqueous suspension of algal biomass was obtained, having a concentration of dry algal biomass equal to 18% by weight with respect to the total weight of said concentrated aqueous suspension of algal biomass. A sample of said concentrated aqueous suspension of algal biomass was poured onto a tray having dimensions equal to 50 cm x 50 cm covered by a polyester film

(Mylar^® of DuPont) , in order to obtain a layer of concentrated aqueous suspension of algal biomass having a thickness of 0.5 cm.

The tray was then covered by a metal grid having meshes with dimensions equal to 0.8 cm x 0.8 cm (h x 1) positioned at a distance of 2 cm with respect to said layer of concentrated aqueous suspension of algal biomass and was placed in a ventilated oven under the following operative conditions : air temperature 65°C; residence time: 4 hrs; air flow-rate: 20 Nm³/h.. At the end of the drying, the tray was extracted from the oven: the layer of dried algal biomass was in the form of "leaves" detached from the polyester film.

Said "leaves" were collected and subjected to thermogravimetric analysis, with an operating temperature of 120⁰C, in order to determine the content of residual humidity which proved to be lower than 1.5% by weight with respect to the total weight of said layer of dried algal biomass. EXAMPLE 2

Solvent extraction

A sample of algal biomass, dried as described in Example 1, was subjected to solvent extraction (sample D • For comparative purposes, a sample of algal biomass obtained as described in Example 1, but dried through freeze drying, at -5O⁰C, for about 20 hrs, was subjected to solvent extraction (sample 2) .

For this purpose, both samples were subjected to extraction in a Soxhlet apparatus, using n-octane as extraction solvent, operating under the following conditions : n-octane/dried algal biomass ratio: 25 w/w,- temperature: 125⁰C; - time: 20 cycles of 15 minutes each.

At the end of the extraction, the octane containing the fraction extracted from the algal biomass was subjected to vacuum distillation, at 53⁰C, obtaining a quantity of "bio-crude" (weight % with respect to the total weight of the dried algal biomass subjected to extraction) equal to: sample 1 (invention) : 13.5% by weight sample 2 (comparative): 13.0% by weight. The above reported data therefore demonstrate that, by operating in accordance with the process of the present invention a dried algal biomass is obtained which allows an extraction yield of "bio-crude" equal to that obtained by the algal biomass dried through freeze drying. EXAMPLE 3

Drying of algal biomass

For this purpose, a concentrated aqueous suspension of algal biomass of the algal strain Nannochloropsis sp. (Instant Algae^® produced by Reed Mariculture) , was used.

Said concentrated aqueous suspension was diluted with deionized water in order to obtain a concentration of algal biomass equal to 0.200 g/1. Said aqueous suspension of algal biomass was placed in a settler and left to settle for 2 hrs, after the addition, under strong stirring (500 rpm) , of 10 mg/1 of polyacrylamide . The solid phase, comprising a concentrated aqueous suspension of algal biomass having a concentration of dried algal biomass equal to 33 g/litre, was collected from the bottom of the settler.

Subsequently, said concentrated aqueous suspension of algal biomass was subjected to vacuum filtration and a concentrated aqueous suspension of algal biomass was obtained having a concentration of dried algal biomass equal to 10% by weight with respect to- the total weight of the concentrated aqueous suspension of algal biomass . A sample of said concentrated aqueous suspension of algal biomass was poured onto a tray having dimensions equal to 50 cm x 50 cm, covered by a film of corrugated polypropylene, in order to obtain a layer of concentrated aqueous suspension of algal biomass having a thickness of 0.3 cm.

The tray was then covered with a metal grid having meshes with dimensions equal to 0.8 cm x 0.8 cm (h x 1) positioned at a distance of 2 cm with respect to said layer of aqueous suspension of algal biomass and was placed in a ventilated oven, under the following operative conditions: air temperature 75⁰C; residence time: 2 hrs ; At the end of the drying, the tray was extracted from the oven: the layer of dried algal biomass was in the form of "leaves" detached from the polyester film. Said "leaves" were collected and subjected to thermogravimetric analysis, with an operating temperature of 120⁰C, in order to determine the content of residual humidity which proved to be lower than 3% by weight with respect to the total weight of said layer of dried algal biomass.

Claims

1. A process for the drying of algal biomass, comprising-. producing an aqueous suspension of algal biomass; - concentrating said aqueous suspension of algal biomass in order to obtain a concentrated aqueous suspension of algal biomass; depositing a layer of said concentrated aqueous suspension of algal biomass on a hydrophobic surface, in order to obtain a layer having a thickness lower than or equal to 1 cm; putting this layer in contact with hot air, said hot air being at a temperature ranging from 40⁰C to HO⁰C in order to obtain a layer of dried algal biomass .

2. A process for the drying of algal biomass inside a substantially closed structure, said substantially closed structure comprising at least one hydrophobic surface, including: - producing an aqueous suspension of algal biomass; concentrating said aqueous suspension of algal biomass in order to obtain a concentrated aqueous suspension of algal biomass; - depositing a layer of said concentrated aqueous suspension of algal biomass on a hydrophobic surface, in order to obtain a layer having a thickness lower than or equal to 1 cm; putting this layer in contact with hot air, said hot air being at a temperature ranging from 40⁰C to 11O⁰C in order to obtain a layer of dried algal biomass .

3. The process according to claim 1 or 2, wherein said layer has a thickness ranging from 0.2 cm to 0.6 cm.

4. The process according to any of the previous claims, wherein said hot air is at a temperature ranging from 60⁰C to 90⁰C.

5. The process according to any of the previous claims, wherein said aqueous suspension of algal biomass derives from the cultivation of unicellular algae (microalgae) , carried out in wastewaters coming from industrial and/or civil wastewaters .

6. The process according to any of the claims from 1 to 4, wherein said aqueous suspension of algal biomass comes from the cultivation of unicellular algae (microalgae) carried out in sea water.

7. The process according to any of the previous claims, wherein the concentration of the aqueous suspension of algal biomass is carried out by means of: gravitational separation by means of sedimentation and/or thickening apparatuses typically used in the water treatment plants; flotation; gravimetric separation by means of cyclones or spirals ; centrifugation,- filtration by means of membranes for ultra- or micro-filtration, or vacuum filtration; treatment by means of filter presses or belt presses .

8. The process according to any of the previous claims, wherein said aqueous suspension of algal biomass is subjected to a flocculation treatment carried out by means of : bioflocculation; - the addition of at least one flocculating agent to said aqueous suspension of algal biomass.

9. The process according to claim 8, wherein said flocculating agent is selected from: metal salts, such as iron chloride, aluminium sulphate, iron sulphate; cationic polyelectrolytes such as polyethylene imines, polyacrylamides ; chitosane; or mixtures thereof .

10. The process according to any of the previous claims, wherein said concentrated aqueous suspension of algal biomass has a concentration of dry algal biomass ranging from 5% by weight to 25% by weight with respect to the total weight of the concentrated aqueous suspension of algal biomass .

11. The process according to claim 10, wherein said concentrated aqueous suspension of algal biomass has a concentration of dry algal biomass ranging from 8% by weight to 20% by weight with respect to the total weight of the concentrated aqueous suspension of algal biomass.

12. The process according to any of the previous claims, wherein said concentrated aqueous suspension of algal biomass has a concentration of water ranging from 75% by weight to 95% by weight with respect to the total weight of the concentrated aqueous suspension of algal biomass .

13. The process according to claim 12, wherein said concentrated aqueous suspension of algal biomass has a water concentration ranging from 80% by weight to 92% by weight with respect to the total weight of the concentrated aqueous suspension of algal biomass.

14. The process according to any of the previous claims, wherein said hydrophobic surface comprises at least one hydrophobic polymer having a melting point higher than or equal to 110⁰C.

15. The process according to claim 14, wherein said hydrophobic polymer has a melting point ranging from 120⁰C to 380⁰C.

16 The process according to claim 14 or 15, wherein said hydrophobic polymer is selected from: polyolefins such as polypropylene, polyethylene; polyesters, such as polyethylene terephthalate; halogenated polymers such as polyvinyl chloride, polytetrafluoroethylene; polystyrenes; polysulfones or mixtures thereof .

17. The process according to any of the previous claims wherein said hydrophobic surface has a thickness ranging from 0.2 mm to 5 mm.

18. The process according to claim 17, wherein said hydrophobic surface has a thickness ranging from 0.5 to 3 mm .

19. The process according to any of the previous claims wherein said hydrophobic surface is a conveyor belt.

20. The process according to one of the claims from 1 to 18, wherein said hydrophobic surface is applied on a conveyor belt.

21. The process according to any of the previous claims wherein said conveyor belt moves at a rate ranging from 0.1 m/min to 130 m/min.

22. The process according to claim 21, wherein said conveyor belt moves at a rate ranging from: - 90 m/min to 120 m/min, when operating batchwise; 0.2 m/min to 1 m/min, when operating in continuous .

23. The process according to any of the previous claims wherein said hot air is heated using solar energy, or waste vapours coming from other processings.

24. The process according to claim 23, wherein said hot air is heated using solar energy.

25. The process according to any of the previous claims wherein said layer of concentrated aqueous suspension of algal biomass is put in contact with hot air for a time ranging from 30 minutes to 10 hours .

26. The process according to claim 25, wherein said layer of concentrated aqueous suspension of algal biomass is put in contact with hot air for a time ranging from 1 hour to 6 hours .

27. The process according to any of the previous claims, wherein said hot air is anhydrous air.

28. The process according to any of the previous claims wherein a metal grid, a net of polymeric material or a sheet of paper, are placed at a distance lower than or equal to 5 cm with respect to said hydrophobic surface .

29. The process according to claim 28, wherein said distance ranges from 1 to 4 cm.

30. The process according to any of the claims from 2 to 29, wherein said substantially closed structure is equipped with tilted side panels (hut covering) , or with at least one upper panel (flat covering) and at least one said side panel.

31. The process according to any of the claims from 2 to 30, wherein said hot air moves inside said substantially closed structure at a flow-rate ranging from 0.2 m/min to 10 m/min.

32. The process according to claim 31, wherein said hot air moves inside said substantially closed structure at a flow-rate ranging from 0.3 m/min to 4 m/min.

33. The process according to any of the claims from 2 to 32, wherein said hot air moves inside said substantially closed structure at a flow-rate ranging from 15 Nm³/h to 250 Nm³/h.

34. The process according to claim 33, wherein said hot air moves inside said substantially closed structure at a flow-rate ranging from 18 Nm³/h to 240 Nm³/h.

35. The process according to any of the previous claims, wherein said layer of dried algal biomass has a thickness lower than or equal to 0.1 cm.

36. The process according to claim 35, wherein said layer of dried algal biomass has a thickness ranging from 0.02 cm to 0.06 cm.

37. The process according to any of the previous claims, wherein said layer of dried algal biomass has a content of residual humidity lower than or equal to 5% by weight with respect to the total weight of said layer of dried algal biomass.

38. The process according to claim 37, wherein said layer of dried algal biomass has a content of residual humidity ranging from 0.5% by weight to 3% by weight with respect to the total weight of said layer of dried algal biomass.

39. Use of the dried algal biomass obtained according to any of the previous claims for the production Of: lipids; biodiesel or green diesel; - bio-oil or bio-crude; energy.