GB2181166A - Preparing a pumpable slurry of fibrous lignocellulosic material - Google Patents

Abstract

A method of preparing a pumpable slurry of fibrous lignocellulosic material and water comprises passing particles of fibrous lignocellulosic material into a mixing chamber and heating together with water at a temperature in the range 100 to 300 DEG C and at a pressure which is above the vapour pressure of water at that temperature, and passing the resulting mixture through a restricted opening under pressure. The pumpable slurry may be readily passed through a pipeline for further processing as a feedstock or for use in preparing moulded products. Binders and dyestuffs may be added to the lignocellulosic material either before or during mixing with water in the mixing chamber. The lignocellulosic material may be wood in the form of chips, pellets, or sawdust.

Description

SPECIFICATION Method of preparing a pumpable slurry The present invention relates to a method of preparing a pumpable slurry of fibrous cellulosic material and water.

Fibrous lignocellulosic materials such as wood may be used as feedstocks for a variety of processes such as the fermentation of wood to sugars and to ethanol. They may also be used as base materials from which useful compounds like lignin and cellulose are produced, and in the production of paper and related materials. Fibrous lignocellulosic materials include wood, straw and other lignocellulosic agricultural waste such as bagasse.

In order to introduce wood into a reactor through restrictions such as pipes, valves and heat exchangers it is necessary to comminute the wood and mix it with water. The wood/water slurries thus formed can only pass adequately through pipes of reduced diameter if the wood concentrations are relatively low. If wood concentrations are increased the pumpability of wood/water slurries deteriorates quickly. It is the tendency of the wood fibres to entangle and aggregate which can lead to the formation of wood plugs from which water separates and passes on, if the wood concentration in the wood/water slurry is too high. Eventually pipe blockages will occur.

The paper industry uses so called "open vane centrifugal" pumps and "moyno" pumps which have the advantage of high capacity although the pressure at which these pumps can be operated is limited. The above problems of segregation and blockage are avoided by dilution of the wood/water mixture with 10 to 20 parts by weight of water per part by weight of wood and further by milling of the wood.

According to the present invention there is provided a method of preparing a pumpable slurry of fibrous lignocellulosic material and water which comprises passing particles of fibrous lignocellulosic material into a mixing chamber and heating together with water at a temperature in the range 100 to 300"C and at a pressure which is above the vapour pressure of water at that temperature, and passing the resulting mixture through a restricted opening under pressure.

Water has substantially neutral pH (i.e.

about pH 7). No acid or alkali is added in the method of the invention.

Using the method of the invention it is possible to prepare pumpable slurries containing less than 10 parts by weight of water per part by weight of lignocellulosic material. Preferably the amount of water is 1 to 4 parts by weight per part by weight of lignocellulosic material. It is very advantageous to be able to prepare slurries wherein the amount of water is about 1 part by weight of lignocellulosic material.

Temperatures above 300"C are not suitable because of the likely occurence of pyrolysis of the biomass above that temperature. The temperature is preferably in the range 150 to 250"C, more preferably 200 to 250"C. Temperatures of 220 to 230"C would be very suitable.

In a preferred embodiment of the invention, the mixing chamber is cylindrical, the restricted opening is circular and the diameter of the restricted opening is 5 to 15% of the diameter of the mixing chamber. Such a mixing chamber may be provided with a reciprocating piston for pushing the resulting mixture towards the restricted opening, and the mixture may conveniently pass from the mixing chamber through a substantially conical restriction opening.

The lignocellulosic material used in the method of the invention is preferably wood, conveniently in the form of sawdust (particle size less than about 3mm), or in the form of milled wood, chips or pellets (particle size in the range 3 to 30mm). Wood pellets may be in the form of cylindrical particles having diameters in the range 3 to 10mm and lengths in the range 5 to 30mm.

Pumpable slurries prepared by the method according to the present invention can suitably be pumped to a reactor in which biomass is physically treated or chemically converted, e.g.

a fermentation reactor or a pulp extractor. The pressure which is created in the mixing chamber may suitably be in the range of 10-30 bar (10 to 30x 105 Pa) if the biomass has to be passed directly from the mixing chamber via one or more restrictions into a reactor for pulping or hydrolysis (e.g. for subsequent fermentation). These pumpable slurries are also capable of being pumped through pipelines to reactors for thermochemical conversion which operate at relatively high pressures to convert biomass into oil and gas. Such a process is normally operated at a pressure of around 200 bar (200x 105 Pa). The pressure under which the pumpable slurry would pass from the mixing chamber into the conversion reactor should be sufficient to compensate for pressure drops which occur in equipment such as heat exchangers which are connected to the reactor.Therefore pressures as high as 400 bar (400x 105Pa) may have to be applied.

The residence time of the lignocellulosic material in the mixing chamber can suitably vary from 2 minutes to 2 hours depending on the subsequent process in which the resulting pumpable slurry is to be used as feedstock.

For example if the pumpable slurry has to be used in a fermentation process the residence time is suitably in the range of 5-10 minutes.

The pumpable slurry according to the present invention can also be used in a process for the production of moulded articles. In such a process the pumpable slurry is extruded into a mould. Therefore the present invention further provides the use of a pumpable slurry prepared according to the present method in preparing a moulded product Various kinds of consumer products which are at present made from papier mache or synthetic polymers may be moulded from biomass which has been made pumpable with the method according to the present invention. Examples are cups, plates, packing materials, sheets, trays, parts for furniture such as chair legs and the like. In order to ensure that the extrusion products do not disintegrate easily one or more binders can be added to the lignocellulosic material either before or during mixing with water in the mixing chamber.

At the same time, if desired, dyestuff can be added. The present invention therefore also provides a product moulded from a pumpable slurry prepared according to the present method.

A distinct advantage of use of pumpable slurry in preparing moulded products is that packing material can be produced in a sterile form because of the heat treatment it undergoes in the mixing chamber, wherein microorganisms which are normally present in wood and which might cause decay are killed in the process. Due to the heat treatment the most vulnerable components for decay (sugars, proteins) degrade partly. Thus products which are made with the above process and which may have to be used outdoors may need less extensive preservative treatment. Another advantage of the present method is that very small amounts of water can be present in the slurry, which reduces the requirement for (expensive) drying of the extruded products. Deformation of the products by shrinking will also be reduced.Therefore the present method may be used for the production of relatively inexpensive building materials.

The present invention will be further understood with reference to the following Examples.

Example 1 A wood/water mixture containing 1 part by weight of water on 1 part by weight of wood chips of particle size (diameter) in the range 3 to 30mm was heated in a cylindrical mixing chamber having a diameter of 50mm and a length of 500mm to a temperature of 200-250"C under a pressure of about 25 bar (25x 105 Pa) for times ranging from 2 to 120 minutes. Then the water-containing wood plug thus formed was pressed through a conical restriction in which the diameter gradually decreased from 50mm to 15mm hence through an outlet (restricted opening) of 6mm diameter attached at the narrow end of the conical restriction. The pressure drop between the chamber and the outlet was about 10 bar (10 x 105 Pa).

It is surprising that the wood plug could be pressed without segregation through an outlet of 6mm under a pressure drop of only 10 bar (10x105 Pa) over the restriction.

Comparative Example To assess the pumpability through pipes of restricted diameter of wood/water mixtures which had not been preheated at elevated temperature as in the method according to the present invention attempts were made to press water/wood chips (1:1 w/w) of particle size (diameter) in the range 3-30mm and water/sawdust (particle size less than 3mm) (1:1 w/w) through a pipe with a diameter which decreased from 50 to 45mm. 500 bar (500x 105 Pa) was needed to get the water/ wood mixtures through the restriction. When sawdust was used, even at pressures of 2500 bar (2500x 105 Pa) the water/wood could not be pressed through a conical pipe of which the diameter decreased from 50-15mm. When the wood was ground to wood pulp i.e. particles with a maximum length of 2mm and a diameter of 0.1 mum, 1000 bar was required to press the water/ wood through a 15mm diameter pipe.

It will be clear from the above Examples that the present method offers a solution to the problem of pumpability of biomass, which is remarkably effective.

Claims (9)

1. A method of preparing a pumpable slurry of fibrous lignocellulosic material and water which comprises passing particles of fibrous lignocellulosic material into a mixing chamber and heating together with water at a temperature in the range 100 to 300"C and at a pressure which is above the vapour pressure of water at that temperature, and passing the resulting mixture through a restricted opening under pressure.

2. A method according to Claim 1 wherein the temperature is in the range 150 to 250"C.

3. A method according to Claim 2 wherein the temperature is in the range 200 to 250"C.

4. A method according to Claim 1, 2 or 3 wherein the amount of water is 1 to 4 parts by weight per part by weight of lignocellulosic material.

5. A method according to Claim 1 wherein the mixing chamber is cylindrical, the restricted opening is circular, and the diameter of the restricted opening is 5 to 15% of the diameter of the mixing chamber.

6. A method according to Claim 1 substantially as hereinbefore described with reference to Example 1.

7. Pumpable slurry prepared by a method according to any one of Claims 1 to 6.

8. Use of a slurry according to Claim 7 in preparing a moulded product.

9. Product moulded from a slurry according to Claim 7.