DESCRIPTION

BACKGROUND In the pulp and paper industry, wood chips and other comminuted cellulosic feedstocks are usually processed in slurry form, mixed with water or other chemical process liquids. The slurried feedstock undergoes different thermo-chemical process step, usually under a steam pressure greater than atmospheric pressure and at a temperature greater than 100°C. As the slurried feedstock is handled and processed at very diluted conditions, the slurried feedstock is subjected to a liquid removal step to increase the solid content. The removal step is conducted typically by means of a drainer apparatus, which removes a portion of the liquids from the slurried feedstock under the action of gravity, even if other mechanical forces may also be applied. The drainer may be operated continuously, thereby draining liquids from the slurried feedstock are removed while the slurried feedstock moves through the drainer. Typically, the drainers in pulp industry are operated with the main axis oriented in the vertical direction. In Figure 5 of US6,451 , 172 a conventional drainer for draining liquid from a cellulosic slurry is described. The figure illustrates a conventional In-line Drainer as sold by Ahlstrom Machinery having an inlet for a particulate- bearing liquid to be strained, an outlet for liquid that has been strained, and an outlet for the strained liquid. Drainer includes a cylindrical housing, having a cover plate at a first or inlet end having the inlet opening and a second or outlet end having end cover plate. The cover plate typically includes a lifting eye and appropriate mounting hardware, for example, threaded studs and nuts. The drainer includes a cylindrical screen basket positioned in the housing. The upper end the screen basket is mounted to the housing by means of a annular mounting flange on the housing and appropriate mounting hardware, for example, threaded screws. The lower end of the screen basket is snugly fit into a machined surface in the inlet. The screen basket is positioned in the housing so that an annular cavity is created between the outside surface of the screen basket and the inside surface of the housing. The screen basket may also include a lifting eye for removing the basked for replacement or servicing. The housing also typically includes a gusseted mounting flange for installing the drainer in the desired location and a steam purge inlet for introducing steam for periodic steam cleaning of the drainer.

In the disclosed conventional drainer, the screen basket is fabricated from a series of evenly-spaced vertical bars supported by a series of external annular rings so that a straining surface is provided having a series of vertical slots between the bars. The bars are typically welded to the rings and to support rings and located at either end of the basket. The screen basket also typically includes unperforated cylindrical sections and at each end of the screen basket.

The lower cylindrical section of the basket includes a helical baffle, which is typically referred to as the "flight". The flight induces a helical flow to the liquid introduced under pressure to inlet so that the orientation of any chips, pins or fines that may be present in the slurry is less likely to be aligned with the vertical slots of the screen basket.

US6,451 , 172 also discloses an invention which is an improvement of the conventional drainer mentioned above. The disclosed invention consists or comprises a liquid separating device having a cylindrical housing elongated in a direction of elongation having an inlet at or adjacent a first end of the housing, an outlet at or adjacent a second end, opposite, the first end, and an inside surface; a cylindrical screen assembly centrally mounted in the housing having a plurality of elongated apertures having an angle of orientation and an outside surface; an annular cavity formed by the outside surface of the screen and the inside surface of the housing; and an outlet for separated liquid located in the housing and communicating with the annular cavity; wherein the angle of orientation of the screen assembly apertures is oblique to the direction of elongation of the housing. The angle of orientation is preferably at least 5° to the direction of elongation of the housing or screen basket, but is typically between about 10° to 80°, preferably about 30° to 60°, most preferably 40° to 50° to the direction of the elongation of the housing or screen basket. For example, the orientation of the slots relative to the elongation of the housing is about 45°. The drainer slots may be continuous slots or they may be interrupted by unperforated "land" areas. These land areas may be uniformly located throughout the screen basket so that a uniform pattern of slots and land areas is provided or the slots and land areas may be distributed non- uniformly. The orientation of the slots may also vary, for example, the angle of orientation of the slots at one elevation in the direction of elongation of the screen basket may be different from the orientation of the slots at second or an adjacent elevation. The orientation of slots at one elevation in the direction of elongation of the screen basket may also vary, for example, producing a "herring bone"-type pattern of slots.

The conventional drainer and the drainer disclosed in US6,451 , 172 present many drawbacks. Some of these drawbacks are independent from the kind of slurried feedstock, while others are specific to a slurried thermally treated ligno-cellulosic feedstock, specifically a slurried thermally treated straw, such as wheat straw. Namely, both the disclosed drainers comprise a unique basket. Increasing the length of the screen basket, it is difficult to reach manufacturing tolerances, and the screen basket will be deformed to a certain extent. This feature is critical in the case that a screw conveyor is present and rotates inside the screen basket. The deformation is expected to be more pronounced in the case of inclined slots, wherein it is needed to preserve a fixed curvature to the inclined slots. In the disclosed drainers, the elongated slots have the same inclination with respect to the drainer axis, or may have different orientations provided they are interrupted by unperforated "land" areas, the latter solution increasing the manufacturing costs. A long unique basket is moreover difficult to be transported and installed. Moreover, if a damage occurs in a localized area of the basket, the whole basket must be replaced.

The apparatuses disclosed in the prior art have been developed for being used in the pulp industry, specifically for draining liquids from a very diluted wood chip slurry, while they fail to operate in the case of a slurry of a low density feedstock composed mainly by elongated, flexible fines which have been subjected to a thermal treatment, for instance a thermally treated straw biomass, in particular a high consistency thermally treated straw biomass. Namely, inventors have found that the apparatuses of the prior art are clogged in a very short time and fail to work, when operated to drain a liquid from a thermally treated straw biomass. There is thereby the need of an apparatus for draining a liquid from a slurried thermally treated ligno-cellulosic biomass, specifically a straw feedstock, which improves the drawbacks of the prior art. SUMMARY

It is disclosed an inclined drainer for continuously draining a liquid from a slurried thermally treated ligno-cellulosic biomass stream moving in an axial flow direction positively inclined relative the horizontal plane perpendicular to the force of gravity to create a drained slurried thermally treated ligno-cellulosic biomass stream. Said drainer comprises an external housing elongated in the axial flow direction , said external housing having a top and a bottom relative to gravity and a drainer axis in the axial flow direction , and comprising: a slurry inlet located below the center of the external housing relative to gravity; a liquid outlet , located at a height lower than the slurry inlet with respect to gravity; an outlet of the drained slurried thermally treated ligno-cellulosic biomass stream, located above the center of the external housing relative to gravity. The disclosed drainer further comprises an annular screen system comprising two or more coaxially disposed annular screen baskets , each of said annular screen baskets having a plurality of screen slots , wherein the annular screen system is coaxially disposed to the drainer axis, thereby defining a slurry flow volume internal to the annular screen system connected to the slurry inlet, a draining shell connected to the liquid outlet and a discharge chamber of the a drained slurried thermally treated ligno-cellulosic biomass stream which is in direct communication with the slurry flow volume. The disclosed drainer further comprises a screw conveyor located in the slurry flow volume having a rotation axis coaxial to the drainer axis, having a gap between the screw conveyor and the annular screen system. It is also disclosed that the axial flow direction may have an positive inclination a from the horizontal plane perpendicular to the force of gravity which is a value in a range selected from the group consisting of 5° to 60°, 10° to 45°, 15° to 40°, and 20° to 30°.

It is further disclosed that the plurality of screen slots of at least one screen basket may be elongated in the axial flow direction.

It is also disclosed that the plurality of screen slots of all the screen baskets may be elongated in the axial flow direction.

It is further disclosed that the plurality of screen slots of the screen basket located close to the bottom relative to gravity may be elongated in a direction perpendicular to the axial flow direction .

It is also disclosed that the annular screen baskets have a top region and a bottom region relative to gravity, and in at least one screen basket the total area of the screen slots in the top region may be less than the total area of the screen slots in the bottom region.

It is further disclosed that in at least one screen basket the top region may not contain screen slots.

It is also disclosed that the top region has an angular aperture β which may be greater than 10° and less than a value selected from the group consisting of 180°, 150°, 120°, 90° and 60°.

It is further disclosed that the external housing may further comprise a steam inlet located in the discharge chamber.

It is also disclosed that the gap between the screw conveyor and the annular screen system may be greater than 2 mm and less than a value selected from the group consisting of 100 mm, 50mm, 30mm, 20mm, 15mm and 10mm.

It is further disclosed that the screw conveyor may consist of or comprise a double flight screw section.

It is also disclosed that the annular screen system has a length in the axial flow direction which may be at least a percent value selected from the group consisting of 50% 60% , 70%, and 75% of the length of the external housing and less than 95% .

It is further disclosed that the annular screen system has a total draining surface which may be at least a percent value selected from the group consisting of 30%, 40%, 50%, and 60% of the lateral surface of the external housing .

It is also disclosed that the total length of the annular screen system may be less than 20m and greater than a value selected from the group consisting of lm, 2m, 3m, 5m, and 10m. It is further disclosed that the annular screen system may comprise four screen baskets (1 18).

It is also disclosed that the annular screen baskets have a basket length which may be greater than 0.3m and less than a value selected from the group consisting of 5m, 3m, 2.5m, 2m, and 1.5m. It is further disclosed that the ligno-cellulosic biomass has a bulk density and the bulk density may be greater than 10Kg/m ³ and less than a value selected from the group consisting of 300kg/ m ³ , 250 kg/m ³ , 200 kg/m ³ , 150 kg/m ³ , 100 kg/m ³ , 75 kg/m ³ , and 50 kg/m ³ .

It is also disclosed that the ligno-cellulosic biomass may be selected from the group consisting of switchgrass, Mischantus, Arundo Donax, sugar cane straw, bagasse, wheat straw, barley straw, and rice straw.

It is further disclosed that the ligno-cellulosic biomass may be a straw.

It is also disclosed that the ligno-cellulosic biomass may have been thermally treated in the presence of a soaking liquid comprising water for a soaking time between 1 minute and 24 hours and a soaking temperature between 120°C and 210°C.

It is further disclosed that the slurried thermally treated ligno-cellulosic biomass has a dry matter content by weight which may be greater than 5% and less than a value selected from the group consisting of 18%, 15%, 12%, 10%.

It is also disclosed that the liquid drained from the drainer may be greater than a value selected from the group consisting of 70%, 80%, 85%, 90% and 95% by weight of the free liquid.

BRIEF DESCRIPTION OF FIGURES Figure 1 is a cross-sectional view of a first embodiment of the disclosed drainer.

Figures 2 and 3 are longitudinal and transversal cross-sections of a preferred draining basket respectively.

Figure 4 is a cross-sectional view of a second embodiment of the disclosed drainer.

Figure 5 is a cross-sectional view of a third embodiment of the disclosed drainer.

DETAILED DESCRIPTION

It is disclosed an inclined drainer for draining a liquid from a slurried thermally treated ligno-cellulosic biomass stream.

Even if the inclined drainer may be used for draining a slurry of any ligno- cellulosic biomass, including slurried wood chips typically used in pulp industry, it's use is specifically recommended in the case of a slurry of ligno-cellulosic feedstock, which has been previously subjected to a thermal treatment, which has the effect to change the elastic properties of the solids in the slurry. The disclosed drainer works properly in the case of a slurry of a thermally treated low density feedstock, specifically in the case that the feedstock is composed mainly by elongated flexible particles, or fines, preferably straw, more preferably wheat straw. Moreover, the disclosed drainer is able to work also in the case of high consistency slurry.

Namely, inventors have discovered that drainers for wood chips as disclosed in the prior art fail to work in the case of a slurried thermally treated straw. Without being limited by any theory and interpretation, inventors believe that a slurried thermally treated straw feedstock has rheological properties which are different from a wood chips slurry due to the fact that the single straw particles aggregate to form a sort of an elastic network. Moreover, the slurried thermally treated straw feedstock easily clog the draining surface of the drainer. The disclosed drainer has an increased draining surface with respect to the drainers of the prior art and may be manufactured to very large size preserving the strict tolerances required by the preferred application to a slurried thermally treated straw feedstock.

Moreover, the disclosed drainer presents simplified transportation and installation procedures and maintenance and replacement costs are reduced with respect to the prior art drainers. The disclosed drainer is used to drain a liquid from a slurry, which is a mixture of liquid and undissolved solid particles. By the expression "drain a liquid" it is meant the removal of at least a portion of the liquid from the slurry preferably under the action of the force of gravity. As detailed in the following of the present specification, other forces, preferably mechanical forces, may promote the separation of liquids from the slurry. Thereby, the slurry of the thermally treated ligno-cellulosic biomass entering the drainer is separated into at least two streams, a first stream comprising at least a portion of the liquid of the incoming slurry and a second stream comprising at least a portion of the solids of the incoming slurry. The disclosed drainer is intended to be used for continuous draining a liquid from the slurry. In order for the draining to be continuous, it is not necessary that the slurried thermally treated ligno-cellulosic biomass stream is continuously introduced into the drainer, but it can be introduced at steady aliquots or pulses. Thus there are moments when there is no slurried thermally treated ligno-cellulosic biomass stream entering the drainer. But, over time, the total mass introduced into the drainer equals the total mass removed from the drainer. That is, the draining is occurring or progressing at the same time that either the slurried thermally treated ligno-cellulosic biomass stream is introduced into drainer and/ or the drained slurried thermally treated ligno-cellulosic biomass stream is removed from the drainer. Another way to state this is that the draining in the drainer occurs while simultaneously, or at the same time, removing the drained slurried thermally treated ligno-cellulosic biomass stream from the drainer. Such removal is done in a continuous manner which includes an aliquot or pulse removal.

In the drainer, the slurried thermally treated ligno-cellulosic biomass stream flows in an axial direction (101). It is noted that the axial flow direction is intended to be an average direction. Being the slurry transported by means of a screw conveyor, the flow direction will be different at different positions and it may not be locally oriented in an axial direction. Namely, this is the typical flow generated by a screw conveyor. The flow direction is the average of the slurry flow over the whole slurry flow volume. With reference to Figure 1 , a drainer according to a first embodiment of the invention is disclosed which is indicated, as a whole, with the reference number 100. The drainer 100 comprises three main components: an external hollow housing 102, an annular screen basket system 103, and a screw conveyor 104. The external hollow housing 102 is extended along an axial flow direction 101 and comprises a lateral surface 102A, a closed upper 102B and a closed bottom 102C. The annular screen basket system 103 is located in the external housing 102 and extended along the axial flow direction 101 , defining a slurry flow volume 105 internal to the annular screen system and a draining gap (or cavity) 106 comprised between the outer side of the screen system 103 and the inner side of the external housing. The screw conveyor 104 is located in the slurry flow volume 105 and extended along the axial follow direction 101.

The external housing 102 has a geometric shape having a rotational symmetry around an axis which is the drainer axis. Preferably, the external housing has a cylindrical shape, but it may have also a trunked conic shape.

The annular screen system 103 has a geometric shape having a rotational symmetry around an axis which is the annular screen system axis. Preferably, the annular screen system 103 has a cylindrical shape, but it may have also a trunked conic shape. More preferably, both the external housing 102 and the annular screen system 103 have a cylindrical shape. In one embodiment, the external housing 102 has a cylindrical shape and the annular screen system 103 has a trunked conic shape. In another embodiment, the external housing 102 has a trunked conic shape and the annular screen system 103 has a cylindrical shape.

The shape of the screw conveyor 104 can be inscribed in a geometrical shape having a rotational symmetry around an axis which is the screw conveyor axis. The screen conveyor 104 can rotate around the screen axis, which is thereby a rotation axis. Preferably, the screw conveyor can be inscribed in a cylindrical shape.

The annular screen system 103 is located inside the external housing 102, and the screw conveyor 104 is located inside the annular screen system 103, and the three main components are coaxially disposed; the common axis is the axis of the drainer, which is parallel to the axial flow direction 101.

The annular screen system 103 comprises a plurality of baskets 1 18 disposed coaxially in sequence and the upper end 107 and the bottom end 108 of the annular screen system 103 are mounted in the internal of the housing 101 so that a draining gap 106 is created between the outside surface of the annular screen system 103 and the inside surface of the housing 101. The drainer 100 also includes a discharge chamber 109 which is defined in the upper portion of the housing 101 between the upper end of the annular screen system 103 and the closed upper 102B of the housing 101. The discharge chamber 109 is in direct communication with the slurry flow volume 105 internal to the annular screen system 103 . Preferably, the annular screen system 103 is mounted within the housing 101 in such a way that the upper end 107 of the screen system 103 is fit into a machined surface to the housing by means of an annular mounting flange 1 10 on the housing 101, while the bottom end 107 is fixed to the bottom 102C of the housing 101 by means of appropriate mounting hardware, for example threaded screws.

The disclosed drainer 100 is intended to be used in a specific inclination a. The inclination a of the drainer is defined with respect to a plane perpendicular to the force of gravity, which is an horizontal plane 1 1 1. The axial flow direction 101 has an inclination a from the horizontal plane 1 1 1 perpendicular to the force of gravity which may be from 5° to 55°, preferably from 10° to 45°, more preferably from 15° to 40°, and most preferably from 20° to 30°. In pulp industry, the drainers are usually oriented in a vertical direction, while it is very difficult to convey vertically the preferred slurry of a thermally pretreated straw. The inclination angle a of the drainer 100 is defined from the horizontal plane to the flow direction, that is counterclockwise in the Figures 1. Thereby, it is possible to identify a top and a bottom of the drainer and its components with respect to gravity.

The center of the external housing is defined as the point on the drainer axis located at a height which is the mean of the top height and the bottom height of the external housing.

The closed top 102B and the closed bottom 102C are preferably realized by means of separated flanges, which are connected to the external housing 102 by means suitable hardware mounting, such as screws.

The external housing 102 comprises a slurry inlet 1 12 which is located below the center of the external housing 102 with respect to gravity, preferably on the lateral surface 102 A of the external housing 102, more preferably close to the bottom 102C of the external housing 102. In another embodiment, the slurry inlet 1 12 is located at the bottom 102C of the external housing 102. In a further embodiment, the external housing 101 comprises more than one slurry inlets 1 12, provided that at least one slurry inlet 1 12 is located below the center of the external housing 102. The slurry inlet 1 12 is connected to the slurry flow volume 105 internal to the annular screen system 103, preferably by means of a hollow inlet tube 1 13 extending from the slurry inlet 1 12 to the annular screen system 103. In the case that the slurry inlet 1 12 is positioned on the lateral surface 102 A of the external housing 102, the hollow inlet tube 1 13 is fixed on the annular screen system 103 and the annular screen system is configured to provide an unobstructed flow of the slurry into the slurry flow volume 105. Preferably, the hollow inlet tube 1 13 is welded on the lateral surface of the lowermost screen basket 1 18, and the portion of the screen slots 120 intercepting the incoming slurry flow is removed.

The external housing 102 further comprises a liquid outlet 1 14 which is located at a height lower than the slurry input 1 12, preferably on the lateral surface 102 A of the external housing 102, more preferably close to the bottom 102C of the external housing 102. In an embodiment, the liquid outlet 1 14 is located on the bottom 102C of the external housing 102. In another embodiment, the external housing 102 comprises more than one liquid outlets 1 14, provided that at least one liquid outlets 1 14 is located a height lower than the slurry inlet 1 12, or at a height lower the slurry inlet 1 12 closest to the bottom 102C in the case that more slurry inlets 1 12 are present.

The external housing further comprises an outlet 1 15 of the drained slurried thermally treated ligno-cellulosic biomass stream which is located above the center of the external housing 102 relative to gravity, more preferably close to or at the top 102B of the external housing 102. In an embodiment, the outlet 1 15 of the drained slurried thermally treated ligno-cellulosic biomass stream is located on the top 102B of the external housing 102. In another embodiment, the external housing 102 comprises more than one outlets of the drained slurried thermally treated ligno- cellulosic biomass stream. The outlet 1 15 of the drained slurried thermally treated ligno-cellulosic biomass stream is located in a position such that the feedstock may be removed from the inclined drainer 100 under the action of gravity force. The external housing 102 may further comprise an optional steam inlet 1 16 located at or close to the top 102B of the external housing 102, which is in direct communication with the slurry flow volume 105 and the steam entering the drainer flows into the draining gap 106 through the slurry flow volume 105 and the screen slots 120 of the annular screen system 103. Optionally, another fluid in vapor phase at the drainer operating conditions may be used. By means of this configuration, the slurry may be drained under the action of the pressure imposed by the steam, provided that the slurry fills the slurry flow volume 105. Moreover, steam may be used also to purge the annular screen system 103. The external housing 102 may further comprise an optional gas inlet 1 17, preferably located at or close to the top 102B of the external housing 102, which is in direct communication with the slurry flow volume 105 and the steam entering the drainer flows into the draining gap 106 through the slurry flow volume 105 and the screen slots 120 of the annular screen system 103. By gas it is intended a fluid in gas phase at the drainer operating conditions, or a mixture of gases. Preferred gases are inert gases, such as Nitrogen or Argon. By means of this configuration, the slurry may be drained under the action of the pressure imposed by inserted gas, provided that the slurry fills the slurry flow volume 105. Moreover, the added gas may be used also to purge the annular screen system 103.

The disclosed inclined drainer 100 is preferably operated at a pressure greater than 1 bar, being preferably the pressure a steam pressure or a gas pressure, or a mixture thereof. Thereby, the external housing 102 is designed and realized with a material or materials to withstand a pressure which is greater than 1 bar, more preferably greater than 3 bar, even more preferably greater than 5 bar, even more preferably greater than 8 bar steam, and most preferably greater than 10 bar, and less than 50 bar. Preferably, the material or materials used can be operated for a long time without corrosion problems due to chemical effects and mechanical frictions. A suitable material is stainless steel. The inlets 1 12, 1 16 and 1 17 and outlets 1 14 and 1 15 of the external housing, as well all the connections between separated components, are realized according to well known in the art principle and techniques to withstand at the operating conditions.

While the drainers of the prior art are single basket drainers, in the disclosed drainer the annular screen system 103 comprises two or more screen basket 1 18, preferably the annular screen system comprises four screen baskets ad depicted in Figure 1. The screen baskets 1 18 are coaxially disposed. By the expression "coaxially disposed" it is meant that the angle between the axes of two adjacent screen baskets is less than 5°, preferably less than 3°, more preferably less than 2°, and most preferably less than 1°. Inventors have found that by assembling two or more screen baskets 1 18 to form the annular screen system 103, it is possible to realize an annular screen system with very strict tolerances. Namely, the single basket screen system of the prior art, as reported e.g. in US 6,451 , 172 considered above, cannot be manufactured over a long length without avoiding deformations of the lateral surface due to accumulated tensile stresses. In the disclosed multi-basket screen system, mechanical stresses are released over the length of the single baskets, causing small mechanical deformation. Moreover the screen baskets 1 18 may be connected together in such a way to correct, or minimize, the overall deformation of the annular screen system 103, for instance by introducing a slight misalignment between the axes of adjacent screen baskets during the assembly of the annular screen system. The misalignment may be introduced by means of adjustable connection means such as screws or pins to provide reciprocal regulation of screen baskets axes, or by inserting a suitable gauge between the adjacent ends of the screen basket. Gaps which are eventually introduced between adjacent ends may be filled, or sealed, to prevent the lateral flow of the slurry. The correction of the reciprocal basket alignment may be introduced during installation phase, or may be done to correct deformations generated over a long operating run.

Figures 2 and 3 show a longitudinal and transversal cross-sections respectively of a screen basket 1 18. The screen baskets 1 18 have a annular shape, preferably they have a hollow cylindrical shape. For sake of clarity, the baskets are substantially the lateral surface of hollow cylinders. The screen baskets comprise a draining surface or surfaces, which is a void area on the lateral surface of the screen basket represented in black in Figures 2 and 3, and a connection system to connect to the basket to an adjacent basket or adjacent baskets or to the housing 102 of the drainer 100. The connection system is preferably located at the opposed ends 1 19A and 1 19B of the basket, and may comprise an annular mounting flange and appropriate mounting hardware, such as threaded screws. In the case that a basket is slightly deformed, the connection system may be regulated or tuned to compensate the deformation along the axis of the drainer.

Namely, inventors have discovered that the preferred slurried thermally treated straw stream tends to be accumulated in the volume between the screw conveyor 104 and the internal surface of the annular screen system 103, thereby clogging the screen baskets 1 18 and strongly reducing the draining efficiency. This effect is greatly enhanced with respect to a slurried thermally treated wood chips stream. In the disclosed drainer, the gap between the screw conveyor 104 and the annular screen system 103 may be strongly reduced with respect to the prior art drainers, and the accumulated straw particles are scratched out by the rotating screw. One of the improvements of the disclosed prior art is the self-cleaning effect obtained which reduces maintenance stops. The gap between the screw conveyor 104 and the annular screen system 103 may be greater than 2 mm and less than 100 mm, preferably less than 50mm, more preferably less than 30mm, even more preferably less than 20mm, even yet more preferably less than 15mm, and most preferably less than 10mm. The screen baskets 1 18 are characterized by having a plurality of screen slots 120 on the lateral surface of each screen. The screen slots may be of any shapes, such as a for instance a circular shape, but preferably they have an elongated shape, as depicted in Figure 2. By elongated shape it is meant that the screen slot may be inscribed in a rectangular shape having an aspect ratio greater than 1 , preferably greater than 2, more preferably greater than 5, even more preferably greater than 10, and most preferably greater than 20. The screen slots may be realized by perforating a cylindrical surface. The length of the perforated slots may be from 1mm to lm, or from 1 cm to 50 cm, or from 5cm to 20cm. Unperforated sections may also be included between adjacent baskets or at the ends of the screen system.

In a preferred embodiment, the screen basket 1 18 is fabricated from a series of bars 121 which are preferably evenly- spaced. The bars are typically welded to the support rings 122 which are located at either end 1 19A or 1 19B of the basket. The bars may be supported by one or more external annular rings 122, preventing the deformation of the bars under the action of the slurry flow, so that a straining surface is provided having a series of slots between the bars. The one or more external annular rings 122 may be welded to the bars, or tightly fixed by means of screw fixing mountings. In one embodiment the slots have a width of between about 1 - 4 mm, and the slots are substantially evenly spaced by about 2-4 mm. In another embodiment the slots have a width of between about 5-7 mm, and the slots are substantially evenly spaced by about 4-8 mm. The elongated slots 120 in a screen basket 1 18 are preferably oriented according to a common orientation with respect to the axis of the screen basket. Preferably, the slots are elongated in the direction of the screen basket axis, which is the axial flow direction 101. In one embodiment, the screen slots 120 are oriented in an oblique direction with respect to the screen basket axis. The angle between the screen basket axis and the elongation direction may be between 5° and 90°, preferably between 10° and 80°, more preferably between 30° and 60°. In one embodiment, at least a screen baskets contains screen slots having different shapes, for instances not elongated and elongated screen slots. Elongated screen slots in a screen basket may then have different orientations. The annular screen system 103 is realized coaxially assembling two or more screen baskets 1 18, wherein each screen basket may have slots having in principle any shape and preferred orientation. That is the annular screen system may be realized by combining any arbitrary sequence of screen baskets 1 18. So there may be screen baskets having elongated screen slots and not elongated screen slots, such as for instance perforated circular screen slots. In one embodiment, the screen slots of at least one screen basket are elongated in the direction of the axis drainer, which is the axial flow direction 101. In another preferred embodiment, the screen slots of all the screen baskets are elongated in the direction of the axis drainer, which is the axial flow direction as depicted e.g. in Figure 1.

Figures 4 and 5 show respective drainers according to second and third preferred embodiments of the invention wherein the drainers are indicated, as a whole, with the reference number 200 and 300 respectively. Components of the drainers 200 and 300 which are structurally and/ or functionally the same or equivalent to those of the drainer 100 illustrated in Fig. 1 are indicted with the same reference numbers of the latter. In the drainer 200, depicted in Figure 4, the screen slots 120 of the screen basket located close to the bottom 102C relative to gravity (i.e. the lowermost basket 1 18) are elongated in a direction perpendicular to the drainer axis. In this embodiment, the slurry inlet 1 12 is preferably located on the lateral surface 102 A of the external housing 102, and a connection guide 1 13, such a tube, connects the slurry inlet 1 12 to the internal volume 105 of the screen basket 1 18 located close to the bottom 102C. The connection guide 1 13 may be welded on the lateral surface of the screen slot 120, preventing that the slurry flows in the draining gap 106. As the slurry entering the slurry flow volume will change from a flow direction normal to the drainer axis to a flow direction parallel to the drainer axis in a region close to the bottom of the slurry flow volume, it is believed that this configuration prevents the clogging of the screen slots in the lowermost basket 1 18.

In a preferred embodiment, the screen slots 120 are be distributed uniformly on the surface of the screen basket 1 18. In Figures 1 and 4, two exemplary drainers comprising screen baskets having uniformly distributed screen slots are depicted.

In another embodiment, the screen slots are not uniformly distributed on the basket surface, preferably the screen slots have not a uniform radial distribution, as represented in Figures 3 and 5. Since the screen basket 1 18 is intended to be used under the action of gravity force, it is possible to define a top region 123 and a bottom region 124 of the screen basket relative to gravity. The top region 123 and bottom region 124 may be defined considering a section of the screen basket perpendicular to the axis of the screen basket and a vertical line passing through the axis of the screen basket, as represented in Figure 3. The angular aperture β of the top region 123 is preferably less than 180°, more preferably less than 150°, even more preferably less than 120° even yet more preferably less than 90°, and most preferably less than 60°. In one embodiment, the total area of the screen slots in the top region of the screen basket is less than the total area of the screen slots in the bottom region of the screen basket. Stated in another way, the draining surface in the top region 123 is less than the draining surface in the bottom region 124. In a preferred embodiment, the top region 123 of the screen basket does not contain screen slots, as depicted in Figures 2-3.

Similarly, it is possible to define a top region and a bottom region of the annular screen system 103. In an embodiment, in at least one screen basket 1 18 the total area of the screen slots 120 in the top region of the screen basket 1 18 is less than the total area of the screen slots 120 in the bottom region of the screen basket. In a preferred embodiment, the total area of the screen slots 120 in the top region of the screen system 103 is less than the total area of the screen slots 120 in the bottom region screen system. In another preferred embodiment, the top region of at least a screen basket 1 18 does not contain screen slots 120. In yet another preferred embodiment, the top region of the screen system 103 does not contain screen slots 120, as in the drainer 300 depicted in Figure 5. Preferably, the screen slots in the bottom region 124 are uniformly distributed.

Namely, inventors have found that, because the disclosed drainers are to be used in an inclined position with respect to an horizontal plane, the screen slots located in the top region are less effective in draining liquids from the slurry, or not effective at all. Thereby, in a preferred embodiment, at least one screen basket of the screen system is realized by means of a plain top region 123, which is essentially a section of a tube, and a plurality of preferably evenly- spaced bars, which are welded to form the draining surface. One advantage of this configuration is that the screen basket may be manufactured with very strict tolerances, being the rigidity of the basket assured by the plain tube section. A further advantage is the reduction of manufacturing costs related to bars assembly and welding. As previously stated, the disclosed drainers may be realized to have a bigger size and with an increased draining surface than the drainer of the prior art.

In particular, the total length of the annular screen system 103 may be greater than 1 m, preferably greater than 2 m, more preferably greater than 3 m, even more preferably greater than 5 m, and most preferably greater than 10m. The total length is preferably less than 20 m, based on practical and engineering considerations.

The screen basket length is preferably less than 5m, more preferably less 3m, even more preferably less than 2.5m, even yet more preferably less than 2m, and most preferably less than 1.5m. The basket length is preferably greater than 0.3m for cost and installation reasons.

The annular screen system 103 may have a total draining surface which at least 30%, preferably at least 40%, more preferably at least 50%, and most preferably at least 60% of the lateral surface of the external housing. The annular screen system 103 has a length in the axial flow direction which may be at least 50%, preferably at least 60%, more preferably at least 70%, and most preferably at least 75% of the length of the external housing. The screen system is preferably less than 95% .

In the disclosed drainers, the screw conveyor 104 is mounted by means of appropriated mounting hardware to permit low friction rotation and taking into account thermal deformation, such as bearings and bearing systems, or one or more bushings. The screw conveyor 104 is basically constituted by helical screw blade or blades, called flights, rotating around the conveyor shaft. The screw conveyor is designed according to well known in the art design rules, taking into account rheological properties of the slurry and the conveyed flow to be obtained. The shaft is then typically connected to an electric motor which provides to rotate the shaft at the desired rotation speed.

In an embodiment, the screw conveyor 104 is designed to have a certain compression rate, typically by reducing the flights volume along the profile of the conveyor. The compression rate may be between 1 : 1 to 3: 1 ; preferably between 1 : 1 to 2: 1 , more preferably between 1 : 1 to 1.5: 1. Thereby, a certain compression may also be imparted to the slurry while it is conveyed by the screw conveyor, provided that the slurry fills the conveyor.

Preferably, the upper portion of the screw conveyor 104 does not contain flights, that is the screw conveyor ends with a bar portion of the shaft. The bar portion of the shaft preferably is located in the discharge chamber 109, and may present one or more radially disposed pole to fragment the drained slurried thermally treated ligno-cellulosic biomass stream into smaller pieces. Optionally, one or more fixed poles 124 are also located in the discharge chamber 109, fasten to the external housing 102, to improve fragmentation of the drained slurried thermally treated ligno- cellulosic biomass stream. Inventors have found that the presence of poles feature is particularly useful in the case of a drained slurried thermally treated straw stream, which forms a compact block at the end of the screw conveyor 104.

In a preferred embodiment, at least a portion of the screw conveyor 104 is designed to have a double flights profile comprising two independent helixes. The design rules of double flights profile are well known in the art. Basically, in a double flights profile two independent helical profiles having the same pitch are superimposed and shifted by a distance which is lower than the pitch. The portion of the screw conveyor having the double flights profile is preferably at the end of the screw profile. The double flights profile conveys the slurry into two separated flow channels and inventors have found that this profile greatly enhances fragmentation of the drained slurried thermally treated ligno-cellulosic biomass stream into small blocks.

The disclosed inclined drainers may drain a slurry of any kind of ligno- cellulosic biomass stream which has been preferably thermally treated. The advantages are evident in the case of a ligno-cellulosic biomass comminuted in chips, wherein the chips are characterized by a low bulk density, as determined according to the standard ASABE S 269.4 DEC91 (ASABE standards, American Society of Agricultural and Biological Engineers), which defines methods and procedures for measuring unit density, bulk density, durability, and moisture content of various densified products composed mainly of forage, woody crops, or other fibrous and non-fibrous material for bulk handling in the feed and non- feed industries. The bulk density of the ligno-cellulosic biomass may be less than 300kg/ m ³ , preferably less than 250 kg/m ³ , more preferably less than 200 kg/ m ³ , even more preferably less than 150 kg/ m ³ , even yet more preferably less than 100 kg/m ³ , most preferably less than 75 kg/m ³ , being less than 50 kg/m ³ the even most preferred value. The bulk density may be greater than 10kg/m ³ , preferably greater than 15 kg/m ³ , more preferably greater than 20 kg/m ³ . The bulk density is measured at a moisture content of 10%. Even if the ligno-cellulosic biomass may be composed by chips of any shape, the advantages are evident in the case of elongated chips, being favorable of the formation of bridging effects. The ligno-cellulosic biomass, which has been eventually comminuted, may be characterized by the mean aspect ratio of the chips, wherein the aspect ratio of a chip is defined as the ratio of its longest size and the mean size in the section transversal to the longest size. The average is done on a sampling of the ligno-cellulosic biomass having a statistical relevance. As an example, in the case of wheat straw, the chip may be as long as some tens of centimeter and the mean transversal size is typically a few millimeters. The mean aspect ratio may be more than 3: 1 , preferably more than 5: 1 , more preferably more than 10: 1 , even more preferably more than 15: 1 , even yet more preferably more than 20: 1 , most preferably more than 30: 1 , being more than 40: 1 the even most preferred value.

Preferably, the ligno-cellulosic biomass is selected from the group consisting of switchgrass, Mischantus, Arundo Donax, sugar cane straw, bagasse, wheat straw, barley straw, and rice straw. In another embodiment, the ligno-cellulosic biomass is a straw.

As it is known in the art that a ligno-cellulosic biomass, comprising insoluble carbohydrates and lignin, is recalcitrant to the action of biological catalyst such as enzymes, it is subjected to a thermal treatment, or pre-treatment, which increases the enzymatic accessibility to carbohydrates and results in lower molecular weight polysaccharides and depolymerized lignin. Thermal treatment is conducted in the presence of hot water in liquid and or vapor phase and optionally of chemicals such as acids or bases.

In the integrated second generation industrial operations, pre-treatment is often used to ensure that the structure of the ligno-cellulosic content is rendered more accessible to the catalysts, such as enzymes, and at the same time the concentrations of harmful inhibitory by-products such as acetic acid, furfural and hydroxymethyl furfural remain substantially low. There are several strategies to achieve increased accessibility, many of which may yet be invented.

The current pre-treatment strategies imply subjecting the ligno-cellulosic biomass material to temperatures between 1 10-250°C for 1-60 min e.g.: Hot water extraction

Multistage dilute acid hydrolysis, which removes dissolved material before inhibitory substances are formed

Dilute acid hydrolyses at relatively low severity conditions

Alkaline wet oxidation Steam explosion.

A preferred pretreatment of the ligno-cellulosic biomass includes a soaking of the ligno-cellulosic biomass feedstock. The soaked ligno- cellulosic biomass may then be steam exploded after the soaking step.

The soaking occurs in a substance such as water in either vapor form, steam, or liquid form or liquid and steam together, to produce a product. The product is a soaked ligno-cellulosic biomass containing a liquid, with the liquid usually comprising water in its liquid or vapor form or some mixture and polysaccharides which have been solubilized in the soaking process in oligomeric and monomeric form, such as glucose and xylose and their oligomers.

This soaking can be done by any number of techniques that expose the ligno-cellulosic biomass to water, which could be steam or liquid or mixture of steam and water, or, more in general, to water at high temperature and high pressure. The temperature should be in one of the following ranges: 145 to 165°C, 120 to 210°C, 140 to 210°C, 150 to 200°C, 155 to 185°C, 160 to 180°C. Although the time could be lengthy, such as up to but less than 24 hours, or less than 16 hours, or less than 12 hours, or less than 9 hours, or less than 6 hours; the time of exposure is preferably quite short, ranging from 1 minute to 6 hours, from 1 minute to 4 hours, from 1 minute to 3 hours, from 1 minute to 2.5 hours, more preferably 5 minutes to 1.5 hours, 5 minutes to 1 hour, 15 minutes to 1 hour.

If steam is used, it is preferably saturated, but could be superheated. The soaking step can be batch or continuous, with or without stirring. A low temperature soak prior to the high temperature soak can be used. The temperature of the low temperature soak is in the range of 25 to 90°C. Although the time could be lengthy, such as up to but less than 24 hours, or less than 16 hours, or less than 12 hours, or less than 9 hours or less than 6 hours; the time of exposure is preferably quite short, ranging from 1 minute to 6 hours, from 1 minute to 4 hours, from 1 minute to 3 hours, from 1 minute to 2.5 hours, more preferably 5 minutes to 1.5 hours, 5 minutes to 1 hour, 15 minutes to 1 hour.

Either soaking step could also include the addition of other compounds, e.g. H2SO4, NH3, in order to achieve higher performance later on in the process. However, it is preferred that acid, base or halogens not be used anywhere in the process or pre-treatment. The feedstock is preferably void of added sulfur, halogens, or nitrogen. The amount of sulfur, if present, in the composition is in the range of 0 to 1% by dry weight of the total composition. Additionally, the amount of total halogens, if present, are in the range of 0 to 1% by dry weight of the total composition. The soaked ligno-cellulosic biomass is in a slurry form of a thermally treated solid ligno-cellulosic biomass in a soaking liquid. Optionally, the soaked ligno-cellulosic biomass to additional process step before being inserted into the disclosed drainer. The disclosed drainer may be used to remove liquid from a high consistency slurried thermally treated ligno-cellulosic biomass. The consistency may be conveniently expressed in terms of dry matter of the slurry. Preferably, the slurried thermally treated ligno-cellulosic biomass has a dry matter content which is greater than 5% and less than 18%, more preferably 15%, 12%, 10% by weight.

The slurry of the pre-treated ligno-cellulosic biomass enters the disclosed inclined drainer from the slurry inlet 1 12. The slurry may enter under the action of gravity force, or it may be forced to enter by means of a suitable pump or pumping system. The selection of the pump or pumping system is done taking into account the rheological properties and the consistency of the specific slurry to be pumped. The slurry is conveyed from the slurry inlet 1 12 to the outlet 1 15 of the drained slurried thermally treated ligno- cellulosic biomass stream by means of the rotating screw conveyor 104, and the slurry flow is regulated by the rotation speed of the screw conveyor 104. The flow of the slurry entering the drainer and the rotation speed of the screw conveyor define the level of the slurry in the slurry flow volume, in particular whether the slurry flow volume is completely full or partially empty.

The liquid is drained from the slurry into the draining gap 106 while the slurry is moving in the axial direction. The draining of the liquid form the slurry occurs under the action of gravity force. Eventually a certain compression may also be imparted to the slurry by the screw conveyor, in the case that the screw conveyor is designed to have a compression rate greater than 1 : 1. A certain compression may also be imparted to the slurry by a fluid in vapor or gas phase, such as steam, inserted from the steam inlet 1 16 in the discharge chamber 109.

The liquid drained from the slurry flows to the bottom portion on the draining gap 106 under the action of gravity force and it is removed from the drainer through the liquid outlet 1 14. The removal may occur under the gravity force or being driven by an external force.

The disclosed drainers remove at least a portion of the free liquid of the slurried thermally treated ligno-cellulosic biomass. Preferably, the liquid drained from the drainer is greater than 70%, more preferably greater than 80%, even more preferably greater than 85%, even yet more preferably greater than 90%, and most preferably greater than 95% by weight of the free liquid. The free liquid may be determined as the liquid which is separated by decanting an aliquot of the slurried thermally treated ligno-cellulosic biomass for 1 hour in a decanter under the force of gravity at 25°C. The disclosed drainer may also remove a portion of the liquid which is soaked in the thermally treated ligno-cellulosic biomass, as it may occur in the case that a certain compression is impressed to the biomass.