EP2212637B1 - Method for mechanical dehydration with thermal assistance - Google Patents

Abstract

The method involves heating a product to be dehydrated at temperature lower than the temperature of evaporation of a liquid e.g. water, to be extracted, within a duration ranging between 5 minutes and hours when pressure is lower than 7 bars. The product is heated by conduction from a heated portion of a wall in a compression and/or filtration device. The heated portion of the wall is set to temperature ranging between 40 and 90 degree Celsius.

Description

The present invention relates to a method of thermally assisted mechanical dehydration for the purpose of achieving the solid / liquid separation of a product comprising on the one hand a liquid phase and on the other hand a solid phase.

The separation of a continuous phase (the liquid) and a dispersed phase (the solid) initially mixed can be carried out in various ways: decantation, centrifugation, filtration, compression, magnetic separation, .... One of the techniques of dehydration mechanically conventionally implemented in the industry is the series coupling of a constant pressure filtration and a mechanical compression.

The filtration is applied to suspensions, concentrated or diluted, of coarse particles possibly having a spread particle size distribution as well as colloidal suspensions. The separation is carried out using a filter medium permeable essentially to the liquid phase of the mixture to be separated. The separated liquid is called the filtrate, the effluent or the permeate.

In the case of a supported filtration, a suspension to be filtered is introduced at a constant rate or, more frequently, under constant supply pressure in a closed chamber by a porous support (the filter media). Under the effect of the pressure applied, the suspension migrates to the filter medium, on which the solid particles are deposited and form a filter cake while the filtrate is recovered under the porous support. As in other separation processes, phase separation is never complete: the cake has residual moisture and the filtrate often contains some solids. When the formed filter cakes are compressible, the separation can be continued by compression. The increase in pressure reduces the volume of the cake by expelling the excess liquid contained in the pores, which leads to a decrease in the residual moisture of the cake. For wet split solids, only the compression phase is implemented.

For suspensions, the solid / liquid separation usually begins with a step of thickening the product followed by a mechanical dewatering step during which the liquid contained in the product is separated in liquid form without the addition of heat. During this operation, the liquid content, usually water, can be reduced from 95 to 70% by weight. These figures are variable depending on the dehydrated products and are given here for information only. The efficiency of the mechanical separation is, however, rather poor for colloidal suspensions or pasty products, such as residual sludge. The main advantage of these operations is their low energy consumption. For gravity thickening, consumption between 0.001 and 0.01 kWh is required per cubic meter of water removed. In a mechanical dewatering operation, the energy consumption is of the order of 1 to 10 kWh per cubic meter of water removed.

When a greater reduction in water content is required, thermal drying, which consists of evaporating or vaporizing the water contained in the wet product, can be used. The energy cost of this operation is here of the order of 1000 kWh per cubic meter of water eliminated. The cost of this operation is often prohibitive especially when the operation is practiced on by-products or waste. In addition, the final water content of the dried products is generally less than 5%, although such a low water content is not always essential for the recovery of the waste.

It is known to provide heat input during mechanical dewatering. This is known as thermally assisted mechanical dehydration. The pressure / heat coupling can be considered in different ways: on the one hand according to the mode of contribution of this heat (conduction, convection ...) and on the other hand according to the moment when it is brought (before the filtration stage, during compression or throughout dehydration).

Most often, the pressures used are relatively high and heat gains are such that they allow vaporization of the liquid phase.

A thermally assisted mechanical dehydration process comprising the features of the preamble of claim 1 is known from the document US 4380496 .

These various methods all have the disadvantage of having a relatively poor energy efficiency. In addition, there is sometimes a sticking of the material on the hot walls of the press.

The present invention therefore aims to provide a thermally assisted mechanical dehydration process having an optimized energy balance. Preferably, this method will make it possible to avoid the problems of sticking encountered with certain methods of the prior art.

For this purpose, the present invention provides a thermally assisted mechanical dehydration process in which a product to be dehydrated is introduced into a filtration device and / or compression and wherein a heat input is achieved.

According to the present invention, this method comprises a first step at a first pressure of less than 7 bar during which the product to be dehydrated is heated to a temperature below the evaporation temperature of the liquid to be removed and the liquid to be removed is recovered. progressively, this first step having a duration of between 5 minutes and several hours.

It is proposed here to perform a first treatment (compression and / or filtration) at low pressure and low temperature for a long time. This original approach allows a good separation of the liquid phase from the product to be dehydrated while minimizing the energy consumption to achieve the corresponding dehydration. As the liquid is gradually removed, it is not heated unnecessarily, which limits the energy consumption during the implementation of the process.

According to a preferred embodiment of the process according to the present invention, the first pressure is between 1.5 and 4 bar, preferably around 3 bar for the majority of products for which the process has been tested.

An embodiment of the present invention provides that the filtration and / or compression device comprises an enclosure having a wall of which at least a portion is heated, and that the product to be dehydrated is heated by conduction from the heated portion of the wall. The portion of the wall of the enclosure that is heated may be fixed or movable. If the pressure is achieved by a piston, trays or other elements, it can be provided that the means for exerting pressure on the product to be dehydrated are heated. Heating can be achieved by any means: electrical resistance, heat transfer fluid, radiation etc.

In order to increase the quantity of liquid separated from the product to be dehydrated, the method according to the invention provides that the first low pressure step is followed by a second step at a second pressure, this second pressure being greater than the first pressure and less than 30 bars. An additional separation is thus achieved which makes it possible to increase the efficiency of the process. With some products, such as bentonite for example, this increase in yield is noticeable but it has been noticed with some other products that this second step at higher pressure could extract relatively little liquid. In this second step, the temperature preferably remains lower than the vaporization temperature of the liquid to be extracted. A preferred variant provides that in the second step the heated portion of the wall remains at a substantially constant temperature, this temperature corresponding to the temperature of said heated portion of the wall in the filtration and / or compression device at the end of the first step.

In a process according to the invention in which the device to be dehydrated is disposed in an enclosure, part of the wall of which is heated, then, in the second stage of said process, the heated portion of the wall preferably remains at a substantially constant temperature, this temperature corresponding to the temperature of said heated portion of the wall in the filtering device and / or compression at the end of the first compression step. When the liquid to be removed contains mainly water, the heated portion of the wall is brought to a temperature of, for example, between 40 ° C. and 90 ° C. in the first step.

The method according to the invention may also comprise, before the first step, a dehydration step at room temperature, without the addition of heat, at a pressure corresponding to the first pressure.

The process according to the invention makes it possible to obtain higher yields with a lower energy consumption when it is applied, for example, to the pretreatment of a wet biomass, excluding wood. The wet biomass treated here can be, for example, cereals, forage plants, leguminous plants, fiber plants, oleaginous plants, dye plants, etc. Among these plants, it is for example possible to mention non-limiting examples include alfalfa, ryegrass, soybean and pastel. When the process according to the invention is applied to a pretreatment of wet biomass, this wet biomass, after thermally assisted mechanical dehydration, may for example undergo a final step of thermal drying in a dryer, the dryer then being advantageously equipped with a heat exchanger heat output, and the fog energy recovered in the heat exchanger being at least partially used to achieve heating in the previous steps of the method. In this way, it is possible to optimize the energy balance of the entire treatment chain of the biomass used in the process.

A method according to the invention can also be applied for example to the dehydration of liquid or pasty waste, excluding urban waste sludge and drinking water sludge. This may be industrial sludge such as for example paper sludge or even a waste such as sodium bentonite used.

A method according to the invention is also well suited to the extraction of green juice from a wet biomass, excluding wood. This is for example to extract a high protein juice from a plant such as alfalfa. The liquid discharged during the dehydration is then recovered and is not a waste. To promote in this process the quality of green juice from thermally assisted mechanical dehydration, the temperature of the heated wall advantageously remains below 60 ° C. Preferably, the temperature of the heated walls is of the order of 50 ° C (+/- 2 ° C).

• La figure 1 représente, en perspective avec une coupe partielle, une cellule de filtration/compression utilisée pour la mise en oeuvre d'un procédé selon l'invention,
• La figure 2 illustre l'influence de la température lorsque le procédé selon l'invention est utilisé avec de la bentonite,
• La figure 3 montre l'influence de la pression dans le procédé selon l'invention avec de la bentonite,
• La figure 4 montre l'influence de la température avec le procédé selon l'invention pour de la luzerne,
• La figure 5 montre l'influence de la température dans un procédé selon l'invention appliqué à une boue de papeterie,
• La figure 6 montre l'influence de la pression dans le procédé selon l'invention appliqué à une boue de papeterie,
• La figure 7 illustre la cinétique de production de jus vert lors d'une déshydratation mécanique selon la présente invention, et
• La figure 8 illustre schématiquement un procédé de déshydratation mettant en oeuvre une déshydratation mécanique et un séchage thermique.

Details and advantages of the present invention will emerge more clearly from the description which follows, made with reference to the appended diagrammatic drawings in which:

• The figure 1 represents, in perspective with a partial section, a filtration / compression cell used for carrying out a process according to the invention,
• The figure 2 illustrates the influence of temperature when the process according to the invention is used with bentonite,
• The figure 3 shows the influence of the pressure in the process according to the invention with bentonite,
• The figure 4 shows the influence of temperature with the process according to the invention for alfalfa,
• The figure 5 shows the influence of temperature in a process according to the invention applied to a stationary sludge,
• The figure 6 shows the influence of the pressure in the process according to the invention applied to a stationary sludge,
• The figure 7 illustrates the kinetics of green juice production during mechanical dewatering according to the present invention, and
• The figure 8 schematically illustrates a dehydration process employing mechanical dehydration and thermal drying.

The figure 1 represents a filtration / compression cell that can be used for carrying out a method according to the invention. This cell comprises firstly a cylinder 2 for receiving a product to be dewatered and secondly a piston 4 adapted to the cylinder 2 so as to slide therein. The cylinder 2 is for example machined in polytetrafluoroethylene, known under the brand Teflon and which is a thermal insulating material. The piston 4 can be made of copper, for example. Electrical resistors 6 are integrated in the piston 4 so as to heat it.

An outer casing 8, for example made of steel, provides the mechanical strength of the assembly.

At the bottom of the cylinder 2 is a filter medium 10 which comprises firstly a glass microfiber filter deposited on a grid made for example of a teflon brand material. It is noted under the filter media the presence of a collector 12 of liquid. An outlet 14 is provided for the evacuation of the liquid collected at the collector 12.

A filtration / compression cell such as that shown on the figure 1 is intended to be inserted in a hydraulic press (not shown), for example a Carver hydraulic press.

For the implementation of the process, the product to be dehydrated is poured into the cylinder 2 of the filtration / compression cell. The piston 4 is then introduced into the cylinder 2 and compresses the product to be dehydrated. A constant pressure is then exerted by this piston 4 on the product to be dehydrated in the cylinder 2. A filtrate flows through the filter medium 10 to the collector 12 and is discharged through the outlet 14.

• une première phase de déshydratation à une première pression P1 au cours de laquelle le piston est chauffé pour être à une température T, et
• une seconde phase de déshydratation à pression plus élevée P2 > P1. La température du piston T reste ici identique à sa température à la fin de la première phase.

The thermally assisted mechanical dehydration process according to the invention comprises two main phases:

• a first phase of dehydration at a first pressure P1 during which the piston is heated to be at a temperature T, and
• a second phase of dehydration at higher pressure P2> P1. The temperature of the piston T remains here identical to its temperature at the end of the first phase.

The first phase can be subdivided into two sub-phases: all firstly a first sub-phase where the product to be dehydrated is pressurized at ambient temperature and a second sub-phase where the piston rises in temperature until reaching the temperature T.

The process according to the invention has been applied as non-limiting examples to bentonite, alfalfa and paper mills. For bentonite and alfalfa, the pressure P1 was chosen at 3 bar while, for the paper sludge, the pressure P1 is 1.78 bar. These values correspond to a preferred embodiment and are given here by way of illustration and not limitation.

The pressure P2 during the second phase is chosen to be greater than the pressure P1 while remaining less than 30 bars.

The Figures 2 and 3 show the results obtained with bentonite. This mineral material is used in the field of civil engineering to fix soil and embankments during construction. It is a clay that has been treated to give it special plastic properties. At the end of the worksite, the bentonite is recovered and reused as long as its rheological properties are not altered. At the end of the cycle, the product is stored. It is noted that the bentonite thus stored contains about 90% water. It is therefore interesting to dehydrate this bentonite on the one hand to limit its mass during transport and on the other hand to limit its volume.

The figure 2 illustrates the mass percentage of water removed during the first phase, as a function of the temperature of the piston 4. The duration of this first phase is set here to 3 hours. As well as the other numerical values indicated, this value is given for illustrative and not limiting. With such a duration, a temperature gradient is established in the product to be dehydrated in the filtration / compression cell. The heating of the product to be dehydrated is carried out here by conduction. By having heating resistors in the walls of the cylinder 2, it could be possible to increase the temperature of the product to be dehydrated faster and thus reduce the duration of the phase 1. Other heating modes can be envisaged. In particular, it is possible to heat the product before introducing it into the filtration / compression cell. This solution is not preferred because it requires heating water (or other liquid) which is then removed - hot - during the pressurization of the product to be dehydrated in the filtration / compression cell.

The mass percentage of water removed is calculated in the same way next: it is the ratio between the mass of water separated during a phase of the process and the total mass of water in the material. This ratio is always less than 1 and is expressed as a percentage in the figures.

The figure 2 represents the mass percentage of water removed after 3 hours for different temperatures of the piston 4. The temperature T of the piston varies here between 21 ° C and 90 ° C. It is found that the amount of water removed increases with the temperature of the piston. For temperatures of the order of 80 to 90 ° C, it is noted that 70% of the water contained in the bentonite is removed. Depending on the needs of the user, this mass reduction of the bentonite may be sufficient.

In the case of figure represented on the figure 3 it was chosen to set the temperature of the piston at 80 ° C. As is apparent from the figure 2 70% of the water is removed during the first phase. This value is taken from the figure 3 . This figure shows how much water can be removed during the second phase depending on the pressure applied to the product to be dehydrated during this second phase. The duration of this second phase is here of 2 hours.

On the left of the figure 3 , we chose P2 = 7 bars while the right part of this figure, we chose P2 = 26 bars. It should be noted here that the second phase makes it possible to withdraw a fairly large quantity of water (approximately 25% of the water initially contained in the product). It is also noted that at 26 bar almost all the water contained in the bentonite is removed. It is therefore a priori useless to try to increase the pressure P2.

The present invention can also be implemented with alfalfa. Among the practices involving alfalfa forage conservation, industrial dewatering is an alternative to silage and dehydration in the field. Dehydration allows the best preservation of the initial qualities of the forage. This process of transformation has developed strongly in France, which has become one of the leading producers of dehydrated fodder in Europe. Dehydrated alfalfa production is currently in the range of 12 to 15 tonnes of dry matter per hectare per year. However, the current cost of energy strongly penalizes this mode of treatment.

The dehydration of alfalfa is currently carried out by so-called "dry processes", that is to say by implementing exclusively thermal drying and leading to the development of the meal in animal feed, or processes, called "wet". The latter implement a preliminary step of dehydration by screw press. These "wet" processes make it possible to separate an aqueous phase (green juice), which is rich in proteins and natural pigments. Such processes thus make it possible to obtain, on the one hand, the meal for animal feed and, on the other hand, to enhance the use of green juice in animal or human nutrition.

The process according to the invention makes it possible to increase the yield of the separation.

For the application of the process to alfalfa, the duration of the first phase was fixed at 4 hours. That of the second phase was fixed at 2 hours.

In the drawings, only the results of the separation at the end of phase 1 are shown. It has been noticed that the aqueous phase (the green juice) is mainly removed during the first phase of the process and that only small amounts of water (green juice) are removed during the second phase of the process. For alfalfa, the impact of temperature is preponderant, as shown in particular by figure 4 . We note that for a temperature of 80 ° C, it is possible to extract 80% of the water contained in alfalfa.

Dryness is a parameter used to indicate the percentage by mass of dry matter contained in a product. The dryness of a product is therefore the ratio between the dry mass contained in this product and the total mass of the product. Here too, it is a ratio of less than 1 which is expressed as a percentage. Dryness can also be considered as the ratio of the dry mass of the product to the sum of the dry mass and the water content of this product.

It is interesting here to compare the process according to the invention with a method conventionally used to dehydrate alfalfa "wet".

Alfalfa is assumed to have an initial dryness of 20%. Conventional pressing methods make it possible to obtain a dryness of 32.5%. After drying in a thermal dryer, the final dryness is about 87%.

With the dehydration process according to the invention, it is possible to increase the intermediate dryness of alfalfa, before its introduction into a thermal dryer. The results obtained in the laboratory show that an intermediate dryness of 62.5% can be obtained by using a thermally assisted mechanical dehydration process according to the invention. This makes it possible, on the one hand, to separate a much larger amount of liquid (green juice) and, on the other hand, to limit the energy required to reach the 87% dryness objective in a thermal dryer.

Indeed, if one reasons on the basis of a tonne of alfalfa dehydrated per hour, with the data indicated above, according to a process of the prior art, the amount of liquid (green juice) separated by a route mechanical (to achieve the intermediate dryness of 32.5%) is 380 kg / h and the amount of water evaporated in the heat dryer to achieve a final dryness of 87% is 390 kg / h.

Using the process according to the invention, starting from an initial dryness of 20% to reach a final dryness of 87%, passing through an intermediate dryness of 62.5%, the amount of liquid (green juice) produced is 680 kg / h and the amount of water evaporated in the thermal dryer is 90 kg / h.

This (realistic) numerical example illustrates the gains that can be made for the dehydration of alfalfa. The mass gain in green juice is more than 75% while the amount of water to be evaporated is approximately divided by four.

• déshydratation à faible pression et à température ambiante,
• déshydratation à faible pression et chauffage du piston, et
• incrément de la pression et maintien de la température du piston constante (par rapport à l'étape précédente).

The figure 7 shows the influence of the heating temperature on the extraction kinetics of alfalfa green juice. The process used to obtain the two curves of the figure 7 includes the following steps:

• dehydration at low pressure and at room temperature,
• low pressure dehydration and piston heating, and
• Increasing the pressure and maintaining the constant piston temperature (compared to the previous step).

Experiments have shown that to obtain a protein-rich green juice and a good yield, the temperature of the heated wall is preferably of the order of 50 ° C.

The figure 8 illustrates a dehydration pathway for alfalfa cake. Two stages are represented here: a first stage of mechanical dehydration and a stage of thermal drying. Before the first step, the dryness of the alfalfa is S _initial , between the two stages it is S _intermediate and after the second stage, she is S _final .

It is considered that the energy supplied in the first step is X ₁ , expressed in kWh per ton of separated water and that the energy supplied in the second step is X ₂ , expressed in kWh per ton of separate water.

• une séparation mécanique, pour laquelle il est d'usage de considérer que la consommation énergétique d'un filtre presse est comprise entre 1 et 10 kWh par tonne d'eau séparée, c'est-à-dire que X₁ est compris entre 1 et 10. Une consommation de 10 kWh/t d'eau séparée a été retenue ici (soit X₁ = 10).
• un séchage thermique conventionnellement réalisé dans un sécheur de type four tournant. Le fluide caloporteur entre dans le four à une température de 700 à 800°C et ressort avec une température supérieure à 100°C. La consommation énergétique de ce type d'installation est estimée à environ 800 kWh/t d'eau évaporée, soit X₂ = 800. Il est connu ici d'utiliser par exemple un pré-sécheur à basse température, utilisant de l'air neuf à 70°C préchauffé dans un échangeur air/eau grâce à la condensation des buées sortant du four tournant. Un tel pré-sécheur a été développé pour réduire la consommation énergétique associée à la séparation thermique. La consommation globale du pré-sécheur et du four tournant est alors d'environ 580 kWh/t d'eau évaporée (X₂ = 580), la température d'entrée du fluide caloporteur dans le four tournant devant être abaissée à 400°C.

The conventional die, according to the prior art to the present invention, comprises:

• a mechanical separation, for which it is customary to consider that the energy consumption of a filter press is between 1 and 10 kWh per tonne of water separated, that is to say that X ₁ is between 1 and 10. A consumption of 10 kWh / t of separated water was retained here (ie X ₁ = 10).
• conventional thermal drying in a rotary kiln type dryer. The heat transfer fluid enters the oven at a temperature of 700 to 800 ° C and exits with a temperature above 100 ° C. The energy consumption of this type of installation is estimated at about 800 kWh / t of evaporated water, ie X ₂ = 800. It is known here to use for example a low-temperature pre-dryer, using air Nine at 70 ° C preheated in an air / water heat exchanger due to the condensation of steam coming out of the rotating furnace. Such a pre-dryer has been developed to reduce the energy consumption associated with thermal separation. The overall consumption of the pre-dryer and the rotary kiln is then about 580 kWh / t of evaporated water (X ₂ = 580), the inlet temperature of the heat transfer fluid in the rotary kiln to be lowered to 400 ° C. .

Assuming an alfalfa flow rate of one tonne per hour, an _initial S dryness of the plant of 20%, a dryness after mechanical dehydration S _intermediate of 32.5% and a final dryness S _final after thermal drying of 87%, the power consumed on the conventional sector amounts to 312.25 kW for the sector incorporating the rotary kiln and 227.44 kW for the die integrating the pre-dryer upstream of the rotary kiln. These results on the global sector can also be expressed in terms of specific consumption, which gives 405.45 kWh / t of water for the sector integrating the rotary kiln and 295.33 kWh / t of water for the integrated sector. the pre-dryer upstream of the rotary kiln.

In a die according to the present invention, the filter press is replaced by a device implementing a thermally assisted dehydration process as described for example above. Energy consumption associated with this mechanical assisted mechanical separation was determined by the laboratory pilot described above. The values are shown in the following table for different piston wall temperatures. <b> Table 1 Energy consumption of assisted dehydration process </ b> Wall temperature (° C) Energy consumption (kWh / t separate juice) 50 121.08 70 150.83

The values in this table are probably a bit overvalued.

With an identical calculation base, namely a flow rate of alfalfa of one ton per hour, an initial _initial dryness S of the plant of 20% and a final dryness S _final after thermal drying of 87%, the following consumptions are obtained on the die according to the present invention integrating only the rotary kiln: <b> Table 2 Alternative energy consumption including thermally assisted mechanical dehydration and kiln drying </ b> Wall temperature (° C) Intermediate dryness _intermediate S (%) Power consumption (kW) Specific consumption (kWh / t) Relative gain compared to the conventional sector 50 50.72 204.86 266 34.39 70 56.86 195.24 253.5 37.47

It can be noted here that by recovering the energy contained in the steam coming out of the rotary furnace, by means of a heat exchanger, it is possible to further reduce the energy consumption of the overall system.

The same calculations made for the die according to the invention by integrating the pre-dryer upstream of the rotary furnace lead to: <b> Table 3 Alternative energy consumption including thermally assisted mechanical dehydration and low temperature pre-drying upstream of the rotary kiln </ b> Wall temperature (° C) Intermediate Siccité (%) Power consumption (kW) Specific consumption (kWh / t) Relative gain compared to the conventional sector 50 50.72 168.7 219 25.83 70 56.86 168.4 218.7 25.94

In this configuration, the energy contained in the steam coming out of the rotary kiln has already been enhanced. The figures 5 and 6 relate to the results obtained with stationary sludge using the process according to the invention. These figures correspond to Figures 2 and 3 described in application to bentonite. The duration of the first phase is 30 minutes while the second phase lasts 15 minutes.

The figure 5 shows that, for stationary sludge, the influence of temperature is relatively low for the first phase. Indeed, while at 25 ° C the mass percentage of water removed is of the order of 66%, it is only about 67.5% at 80 ° C.

The second phase is interesting here because it allows to eliminate about 15 to 20% additional water. We note here that the higher the pressure, the more the quantity of separated water is also important. On the figure 6 a variation of a few percent is noted for the passage of a pressure P2 of 7.3 bars at 10.7 bars.

As is clear from the foregoing description, the interest of the thermally assisted mechanical dehydration process according to the invention is proven for the dehydration of bentonite and forage plants, such as alfalfa. Applications in the agri-food field can be envisaged. The method described above can be used for plants, especially woody plants, herbaceous legumes, tomatoes, beets, etc. Agricultural by-products or green waste, can also be treated with a method according to the invention. .

In the mining field, other materials than bentonite can be processed. The process according to the invention can thus be applied to barite, fluorine or various clay minerals.

In general, the present invention can be applied to any biomass but excludes the following definition of wood and sludge as well as drinking water sludge.

• le bois, sous forme de bûches, granulés et plaquettes ;
• les sous-produits du bois qui recouvrent l'ensemble des déchets produits par l'exploitation forestière (branchage, écorces, sciures...), par les scieries (sciures, plaquettes...), par les industries de transformation du bois (menuiseries, fabricants de meubles, parquets) et par les fabricants de panneaux ainsi que les emballages tels que les palettes ;
• les sous-produits de l'industrie tels les boues issues de la pâte à papier (liqueur noire) et les déchets des industries agroalimentaires (marcs de raisin et de café, pulpes et pépins de raisin etc.) ;
• les produits issus de l'agriculture traditionnelle (céréales, oléagineux), résidus tels que la paille, la bagasse (résidus ligneux de la canne à sucre) et les nouvelles plantations à vocation énergétique telles que les taillis à courte rotation (saules, miscanthus, etc.);
• les déchets organiques tels que les déchets urbains comprenant les boues d'épuration, les ordures ménagères, et les déchets en provenance de l'agriculture tels que les effluents agricoles. Dans l'art antérieur, de nombreux équipements ont été développés pour intensifier les procédés de déshydratation conventionnels. Toutefois, dans la pratique, les procédés développés répondent globalement à une problématique différente de celle de la présente invention, puisqu'il s'agit fondamentalement de procédés de séchage. La plupart des procédés connus reposent sur la technologie du filtre presse. Cette technologie facilite la mise en place d'un dispositif de chauffage. En effet, un filtre presse comporte des plateaux creux permettant la circulation d'un fluide caloporteur. Néanmoins, les plateaux du filtre presse étant disjoints, la circulation du fluide caloporteur dans chacun des plateaux rend l'installation compliquée. En outre, les conditions d'utilisation visent à sécher par contact le gâteau après l'avoir déshydraté mécaniquement. Des problèmes de collage (cuisson) du gâteau de filtration apparaissent couramment sur les plateaux chauffants et empêchent, ou ralentissent, la vidange du filtre presse. Pour abaisser alors la température de fonctionnement, un vide partiel doit être réalisé. L'inconvénient majeur de cette technologie de l'art antérieur réside dans son fonctionnement batch. Une technologie mettant en oeuvre un filtre à bandes a été mis au point dans le cadre du séchage impulsionnel par les papetiers qui maîtrisent bien cette technique pour la fabrication du papier. L'apport de chaleur est plus difficile à implémenter car la rotation des rouleaux rend compliquée la circulation de fluides caloporteurs pour les chauffer. Une alternative consiste cependant à réaliser un chauffage par induction. Un autre inconvénient concerne la compacité du procédé puisque le produit est disposé en couche mince d'épaisseur inférieure à 1 mm.

Biomass means:

• wood, in the form of logs, pellets and wafers;
• wood by-products which cover all the waste produced by logging (branching, bark, sawdust ...), sawmills (sawdust, chips, etc.), by the wood-processing industries ( joineries, furniture manufacturers, flooring) and panel manufacturers as well as packaging such as pallets;
• industrial by-products such as sludge from paper pulp (black liquor) and waste from agri-food industries (grape and coffee grounds, pulp and grape seed, etc.);
• products derived from traditional agriculture (cereals, oilseeds), residues such as straw, bagasse (ligneous residues from sugar cane) and new energy plantations such as short-rotation coppices (willow, miscanthus, etc.);
• organic waste such as urban waste including sewage sludge, household waste, and waste from agriculture such as agricultural effluents. In the prior art, many pieces of equipment have been developed to intensify conventional dewatering processes. However, in practice, the processes developed globally respond to a problem different from that of the present invention, since they are basically drying processes. Most known methods rely on press filter technology. This technology facilitates the installation of a heating device. Indeed, a filter press comprises hollow trays for the circulation of a coolant. Nevertheless, the plates of the filter press being disjoint, the circulation of the coolant in each of the trays makes the installation complicated. In addition, the conditions of use are intended to dry contact the cake after having dehydrated mechanically. Problems of sticking (baking) of the filter cake commonly appear on the heating plates and prevent or slow down the emptying of the filter press. To then lower the operating temperature, a partial vacuum must be realized. The major disadvantage of this technology of the prior art lies in its batch operation. A technology using a band filter has been developed in the context of pulse drying by paper manufacturers who are well versed in this technique for the manufacture of paper. The heat input is more difficult to implement because the rotation of the rollers makes it difficult to circulate heat transfer fluids to heat them. An alternative, however, is to provide induction heating. Another disadvantage relates to the compactness of the process since the product is disposed in a thin layer less than 1 mm thick.

In contrast, the present invention, in this preferred embodiment, relies on two thermally assisted separation steps: a first stage, filtration or compression depending on the dehydrated material, at low pressure (of the order of 3 bar in a preferred embodiment) followed by a separation step at higher pressure, the piston temperature remains constant.

In such a method according to the invention, the first step makes it possible to separate a large quantity of liquid at a lower cost. During the second phase of separation, a significant increase in dryness can be expected, depending on the dehydrated products, even at low pressure (of the order of 7 bars). Moreover, the separation is faster than at room temperature.

To proceed to a continuous process, given the pressures to be implemented, the technology of the screw press can be used in the present invention. As already mentioned above, it is possible to heat not only the parts exerting pressure but also the other walls in contact with the product to be dehydrated. This heating can be achieved using electrical resistances but also by using a coolant (thermal oil, hot water, steam, hot air, steam from a dryer ...).

The present invention is not limited to the embodiments described above by way of non-limiting examples and the variants mentioned. It also relates to other variants and applications within the scope of the skilled person within the scope of the claims below.

Claims (12)

1. A method of thermally assisted mechanical dehydration in which a product to dehydrate is introduced into a filtration and/or compression device, in which a provision of heat is supplied, and which comprises a first step at a first pressure of less than 7 bars during which the product to dehydrate is heated to a temperature less than the evaporation temperature of the liquid to eliminate and the liquid to eliminate is progressively collected,
   characterized in that this first step is of a duration comprised between 5 minutes and several hours, and in that this first step at a first pressure is followed by a second step at a second pressure, this second pressure being greater than the first pressure and less than 30 bars.
2. A method according to claim 1, characterized in that the first pressure is comprised between 1.5 and 4 bars.
3. A method according to one of claims 1 or 2, characterized in that the filtration and/or compression device comprises a chamber (2, 4) having a wall at least part of which is heated, and in that the product to dehydrate is heated by conduction from the heated part of the wall.
4. A method according to claims 1 to 3, characterized in that the second step is carried out at a lower temperature than the evaporation temperature of the liquid to extract.
5. A method according to claims 3 and 4, characterized in that in the second step the heated part of the wall remains at a substantially constant temperature, that temperature corresponding to the temperature of said heated part of the wall in the filtration and/or compression device at the end of the first compression step.
6. A method according to one of claims 3 or 5, characterized in that the liquid to eliminate mainly contains water, and in that the heated part of the wall is brought to a temperature comprised between 40°C and 90°C in the first step.
7. A method according to one of claims 1 to 5, characterized in that it comprises before the first step a dehydration step at ambient temperature, without provision of heat, at a pressure corresponding to the first pressure and during which the liquid to eliminate is progressively collected.
8. A method according to one of claims 1 to 7, characterized in that it is applied to the pretreatment of a moist biomass, excluding wood.
9. A method according to claim 8, characterized in that the moist biomass undergoes a final step of thermal drying in a dryer, in that the dryer is provided with a heat exchanger at the output, and in that the energy of the vapors collected in the heat exchanger is at least partially used to perform heating during the preceding steps of the method.
10. A method according to one of claims 1 to 7, characterized in that it is applied to the dehydration of liquid or pasty waste, excluding residual municipal sludge and drinking water sludge.
11. A method according to one of claims 1 to 7, characterized in that it is applied to the extraction of green juice from a moist biomass, excluding wood.
12. A method according to claim 11, characterized in that the temperature of the heated wall remains below 60°C.

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