Patent ID: 12214522

DETAILED DESCRIPTION

Numerical attributes such as first, second, third, and so on are used in this description and the appended claims for the purpose of giving unambiguous names to concepts. They do not refer to any particular order, unless otherwise explicitly stated.

In the context of this description the term wood particles refers to a material that consists mainly of pieces of wood formed by cutting or chipping larger pieces of wood such as trees, branches, logging residues, stumps, roots, and wood waste. The size of the wood particles may vary in a wide range from a few millimetres to a few centimetres, so the wood particles meant here are typically larger than those meant with the term sawdust. The wood used to make wood particles may be debarked or it main contain bark. For a wood-to-sugar process the preferred raw material is broadleaf wood due to its relatively high inherent sugar content, but the use of other kinds of wood is not excluded. The terms wood chips, wooden chips, or just chips can be used to mean the same thing as wood particles. The term chips is used in the appended drawing because it is short.

FIG.1illustrates schematically how in a method and arrangement for treating wood particles the wood particles may go to pretreatment, schematically illustrated as101. The purpose of the pretreatment101is to prepare the incoming wood particles for efficient use in the process, by removing some unwanted impurities, by compensating for some of the natural fluctuations in the characteristics of the material, and by breaking down the natural structure of the wood material. Hemicellulose (C5 sugars) can be collected from the pretreatment101, and cellulose (or lignocellulosic material) can be taken further to a hydrolysis102to produce carbohydrates of desired kind.

FIG.2illustrates an example of a product flow through various stages that all belong to the pretreatment101ofFIG.1. Washing201is done with water, removing some mainly inorganic impurities such as sand. Washed wood particles are taken to steam treatment202for the purpose of removing air from inside the wood particles and to preheat them to an elevated temperature. Steam-treated wood particles are taken to dilute acid treatment203for impregnating them with a dilute acid solution. The aim of the dilute acid treatment203is to make the dilute acid solution penetrate into the wood particles as evenly as possible.

The acid-impregnated wood particles are taken to hemihydrolysis (not separately shown inFIG.2) where they are under elevated pressure and temperature. At the output of the hemihydrolysis the wood particles undergo a steam explosion204that breaks their structure. The output stream from the steam explosion204goes through steam separation (not separately shown) to mixing205where water is added and the resulting mass is homogenized mechanically to break up agglomerates. Solids and liquids may then be separated at206for feeding into later process stages.

It has been found that in a pretreatment process of a commercially viable scale the use of a known plug screw feeder to feed wood particles into an impregnating stage may result in less than optimal impregnating. This is an important finding, because spatially even absorption of the dilute acid solution in the wood particle would be highly desirable: it affects the quality of the product later in the process. The relatively large flow rates of a commercially viable wood-to-sugar process mean that it is not feasible to assume that a wood particle would spend more than some minutes in the impregnating vessel401. Aiming at longer times would mean that the impregnating vessel401would need to be larger than is practical to build. However, after only some minutes under the influence of the acidic solution the wood particle may not have been fully penetrated, or at least the spatial distribution of acid inside the wood particle is not completely even. This problem is made even worse if the wood particle was not in optimal condition (shape, size, amount of bark in unbarked feedstock, not compressed enough) for absorbing the dilute acid solution when it came to the impregnating stage.

The compression ratios of commercially available plug screw feeders are between 1.5 and 6, and the compression ratio tends to become lower when one goes towards larger equipment and larger material flows. This is an underlying cause of the problem that in commercially viable, industrial scale processes the work done on the wood particle in the compressing conveyor before an impregnating stage may not be sufficient for effective impregnation.

It has been found that more efficient compressing, and consequently a better initial condition of the wood particles for impregnating, can be achieved by using two or more conveyors between the pre-steaming silo and the impregnating vessel(s). This way the compressing work done per wood particle by an individual conveyor becomes larger.

FIG.3illustrates schematically a part of an arrangement for pretreating wood particles in a wood treatment process. As a part thereof there is an arrangement for feeding the wood particles into an impregnating stage of the wood treatment process. The arrangement comprises a feed silo, which in the embodiment ofFIG.3is a pre-steaming silo301for treating the wood particles with steam. The arrangement comprises an impregnating vessel305for receiving wood particles in the impregnating stage that is discussed here. The impregnating vessel305comprises an input for a dilute solution of an acid, such as sulphuric acid, for soaking the received wood particles in said dilute acid solution in order to prepare them for hemihydrolysis and steam explosion in a reactor307further downstream in the process. Other acids that could be used for said impregnation comprise—but are not limited to—nitric acid, phosphoric acid, lactic acid, acetic acid, formic acid and carbonic acid.

The arrangement comprises two or more conveyors302,303, and304between the pre-steaming silo301and the impregnating vessel305for transferring wood particles from the pre-steaming silo301to the impregnating vessel305. Each of these conveyors is a compressing conveyor for applying pressure to the wood particles on their way to through the respective conveyor.

Concerning the feed silo, which inFIG.3is the pre-steaming silo301, it is not a necessary requirement that it is used for pre-steaming. As was described above with reference toFIG.2, advantageous effects of pre-steaming include removing air from inside the wood particle and preheating. These or corresponding effects can be achieved otherwise than by applying steam; for example various vacuum and microwave related techniques can be used. If the silo is a presteaming silo, the steam used therein may be pure steam of water or it may contain additives, such as some acid, to enhance the desired effect. The fact that the conveyors302to304are fed from a silo has an advantageous effect, because stable operation and constant effect on the transferred material of plug screw feeders of force feed screws (which are examples of compressing conveyors) are easy to achieve by using a silo for feeding and by maintaining the surface level of the material inside the silo sufficiently high.

Taken the relatively large flow rates of a commercially viable wood-to-sugar process it is not feasible to assume that a wood particle would spend more than some minutes in the impregnating vessel305. Aiming at longer times would mean that the impregnating vessel305would need to be larger than is practical to build. However, after only some minutes under the influence of the acidic solution the acid may not have fully penetrated the wood particle, or at least the spatial distribution of acid inside the wood particle may not be completely even. It has been found advantageous to provide a soaking silo as a temporary storage in which the spatial distribution of acid inside the wood particle has time to even out. In the schematic presentation ofFIG.3the soaking silo would appear in block306. The residence time in the soaking silo502may be in the orders of some tens of minutes. In an embodiment the residence time in the soaking silo502is not more than 60 minutes. In another, preferred embodiment the residence time in the soaking silo502is not more than 30 minutes.

In the arrangement ofFIG.3the two or more conveyors operate in parallel. That is, each of the two or more conveyors302to304comprises a respective conveyor input and a respective conveyor output, and the conveyor inputs are coupled to the presteaming silo301in parallel for receiving a respective component stream of wood particles from the pre-steaming silo301. There is only one, common impregnating vessel305, which is coupled to at least two (here: all) of the conveyor outputs for receiving at least two (here: all) of the component streams.

Taken that there is a certain gross material flow through the process, each of the N parallel compressing conveyors (N=2, 3, conveys an 1/N fraction of the gross material flow. In total, N times the work of an individual compressing conveyor is performed on the material flow. This ensures much more thoroughly conditioned wood particles at the beginning of the impregnating stage in the impregnating vessel305than if only one compressing conveyor would be used for transferring.

FIG.4illustrates an embodiment that is otherwise similar to that ofFIG.3but there is provided a divider401between the feed silo (the pre-steaming silo301) and the conveyor inputs for dividing wood particles from the feed silo into the component streams. InFIG.3it was assumed that the conveyor inputs were simply installed at the bottom of the presteaming silo301. The divider401may be a static, mechanical divider like a wedge-shaped barrier that mechanically guides wood particles from the pre-steaming silo301to the conveyor inputs. Additionally or alternatively it may comprise conveyors of its own, like a double screw feeder that receives a flow of wood particles from the silo in the middle and that exhibits helixes of opposite handedness towards its two ends.

FIG.5illustrates an embodiment in which the compressing conveyors operate in series. That is, each of the two or more conveyors501to503comprises a respective conveyor input and a respective conveyor output, of which only the conveyor input of a first conveyor501is coupled to the feed silo (the pre-steaming silo301) for receiving wood particles from the feed silo. The conveyor output of the first conveyor501is coupled to the conveyor input of the second conveyor502for transferring the wood particles through said first501and second502conveyors in sequence on their way from the feed silo to the impregnating stage.FIG.5illustrates schematically the possibility of having more than two compressing conveyors in series (see block503inFIG.5).

In an embodiment where two or more compressing converters operate in series, in order to fully achieve the advantageous effect of double compressing work on the wood particle the compression stages should follow each other directly enough, with minimum delay in between so that the mechanical deformation achieved in a preceding compression does not relax too much before the next compression. Therefore the conveyor output of the preceding conveyor is most advantageously directly coupled to the conveyor input of the next conveyor, with few or no intermittent pieces of apparatus therebetween.

FIG.6illustrates an embodiment that is otherwise similar to that ofFIG.3orFIG.4(the divider401being optional) but there are separate impregnating vessels coupled to at least two of the compressing conveyors302to304. That is, a first impregnating vessel601is coupled to the conveyor output of a first conveyor302to receive a first component stream, and a second impregnating vessel602is coupled to the conveyor output of a second conveyor303to receive a second component stream. The arrangement comprises a combiner stage after said first601and second602impregnating vessels for combining outputs of the first601and second602impregnating vessels to a later stage of the wood treatment process. In the schematic presentation ofFIG.6the combiner stage is included in block306.

An additional advantageous effect of using at least two compressing conveyors before the impregnating stage has been found when some further stages of the process are considered in more detail.FIG.7illustrates schematically an apparatus for pretreating wood particles. A pre-steaming silo701is provided for treating the wood particles with steam, i.e. implementing at least part of the stage shown as stage202inFIG.2. At least two compressing conveyors702are provided for transferring steam-treated wood particles from the pre-steaming silo701to one or more impregnating vessels703in which the dilute acid treatment stage203ofFIG.2is implemented. Downstream from the one or more impregnating vessels703there may be provided one or more soaking silos704for giving the dilute acid solution more time to penetrate into the inner parts of the wood particles. One or more further compressing conveyors705may be used to transfer the acid-impregnated wood particles into a reactor706in which the hemihydrolysis reaction takes place and at the output of which the steam explosion takes place.

The dry matter content and acid content of the material that goes into the reactor706are important process parameters, because they have an effect on how the desired reactions proceed in the reactor706and downstream from it in the process. The reactor706is pressurized, and the compressing capability of the immediately preceding compressing conveyor705may be used to aid the feeding of the material flow into the reactor706.

Now when two or more compressing conveyors702have been used before the impregnating vessel(s)703, the penetration of dilute acid solution into the wood particle may be more complete than if only one compressing conveyor had been used at stage702; the material may be “wetter” of dilute acid. The extent to which this occurs can be controlled by controlling, among others, the operation of the compressing conveyors702. In order to achieve the desired dry matter content and acid content at the input of the reactor706, the operation of the later compressing conveyor705can be controlled. There is a wider control window, i.e. more accurate controlling possibilities, for the last-mentioned due to the fact that two or more compressing conveyors were used at stage702.

It is obvious to a person skilled in the art that with the advancement of technology, the ideas explained above may be implemented in various ways. The claimed scope is thus not limited to the examples described above.