WO1997039177A1

WO1997039177A1 - Feeding device for moist material at high gas pressure

Info

Publication number: WO1997039177A1
Application number: PCT/SE1996/000487
Authority: WO
Inventors: Kjell Nilsson
Original assignee: Regalco Engineering Ab
Priority date: 1994-10-21
Filing date: 1996-04-16
Publication date: 1997-10-23
Also published as: SE503502C2; SE9403616D0; SE9403616L

Abstract

The invention refers to a feeding device, working under high steam pressure, for wet material for example pre-hydrolysed lignocellulosic material. The device comprises, a pre-feeder (3), a cell-feeder (5), with two counter-rotating wheels with tooth formed dogs, along with one, at the outlet side located, pipe (7) with an outlet opening (11) which has a spring loaded counter weight (8) to maintain a plug of material with a necessary compression in the pipe (7) to achieve a gas-tight sealing.

Description

FEEDING DEVICE FOR MOIST MATERIAL AT HIGH GAS PRESSURE The present invention refers to a feeding device, working under high gas pressure, for wet material for example pre-hydrolysed hgnocellulosic material.

With a device of this kind one should be able to feed the material from a unit with atmospherical pressure to a unit with relatively high pressure, or the other way around, from a pressurized unit to another pressurized unit, with higher or lower pressure. It can for example be a question of feeding pre-hydrolysed hgnocellulosic material, as a 30 % mixture with water, from one unit at a pressure of 1,3 MPa to an other unit with the pressure of 2,8 MPa, which corresponds to temperatures of 190 °C and 228 °C respectively. In that case, when it is a production unit, the material flow can be approximately 23 tons per hour. The volume ofthe two units can be approximately 6 m³. Therefore the quantity of steam, released by a sudden pressure drop or a so called blow-back, due to a leakage over the feeding unit, will be considerable with risk for serious damages and near-accidents.

The goal for the invention is to achieve a feeding system, ofthe type described by way of introduction, whereby the risks for leakage or blow backs have been ehminated in an uncomplicated and effective way.

This has, according to the invention, been achieved by giving the system the properties described in the following claims. In that connection a cell-feeder, of gear wheel type, has been arranged with a continuous feed flow of material so that the cell-feeder is kept filled continuously. In that way the feed out of material from the cell-feeder will also be a smooth, continuous, flow that is then conveyed into a pipe towards its outlet end. This is arranged so that the material will be compressed inside the pipe either by the fact that the pipe is long enough, to give the material column the sufficient compression, or for the reason that the material has to be pressed out between the outlet end ofthe pipe and a flexible counter weight, that aims to keep the outlet opening closed. The material plug, formed in this way, together with the material in the cell-feeder and at its inlet side provides an effective gas-tight sealing.

As long as the feeding device is working normally no leakage can possibly arise. Naturally, discontinuity in the material flow, originating from something that goes wrong with the feeding, could make the material plug insufficient as a gas-tight sealing. The same might happen ifthe differential pressure over the feeding device should suddenly increase considerably or ifthe cell-feeder should happen to stop, resulting in a blow back which, at best, results only in a considerable disturbance in the production. Even such an inconvenience can be ehminated by the system, according to the invention, ifthe earlier mentioned counter weight is arranged with a contact surface against the outlet opening which seals it tight shut when the material flow from the outlet opening stops, or if it is pressed back by a pressure increase at the outlet opening or a pressure drop at the inlet side ofthe cell-feeder. The invention is explained more in detail in the following with reference to enclosed drawings that in a schematic way lengthwise shows three different examples of apphcations according to the invention in which figure 1 shows a system for the feeding of hgnocellulosic material from a pressurized unit to an other pressurized unit, figure 2 shows a system for feeding of material into a pressurized dryer and figure 3 shows a system for discharge of material from a pressurized dryer.

The feeding system, shown in figure 1, is part of a plant for dilute acid hydrolysis of hgnocellulosic material at high temperature. In this case it is desirable to feed pre-hydrolysed hgnocellulosic material, at a consistency of 30 % in water, from a first digester 1 at a pressure of 1,3 MPa, via a shoot 2 equipped with a level control, a pre-feeder 3 driven by a motor 4, a cell- feeder 5, a vertically directed pipe-bend 6, and an inlet pipe 7 to another digester 10 in which a pressure of 2.8 MPa is maintained. Due to the fact that saturated steam exists, the temperature in the two digesters 1 and 10 is 190 °C and 228 °C respectively. In the first digester 1 the material is fed forward by means of a conventional screw and also in the pre-feeder 3 there is a screw, driven by a motor 4, which is arranged to be driven so that the material is always pressed against the inlet opening ofthe cell- feeder 5.

In this case, the inlet pipe 7 to the digester 10 is directed vertically upwards. Ifthe height ofthe material column, from the outlet ofthe cell-feeder to the outlet ofthe pipe 7 in the digester 10, is sufficient, with respect to the property ofthe material and existing operating conditions, to allow the material column, together with the material in the cell-feeder 5 and in the pre-feeder 3, to form a gas-tight sealing even when the cell-feeder 5 is stopped, the pipe 7 can simply end with an opening 11 in the digester 10. If there is a risk for a blow back when the cell- feeder stops the opening 1 1 should be equipped with one flexible sealing 8, working as a non return valve. Further more, ifthe compression ofthe material column in the pipes 6, 7 becomes insufficient to create a gas-tight sealing the flexible sealing 8 has to be equipped with a springy device, for example a pneumatic spring 9, that with the proper force is pressing against the opening 11 creating a counter pressure which the material column has to overcome before it can leave the pipe 7 and be further transported into the digester 10 by the transport screw 13, driven by a motor 12.

The cell-feeder 5, as well as the pre-feeder 3, are constructed for a working pressure, at least at the level ofthe subsequent vessel in the plant. The cell-feeder 5 is, on top of that, made to handle a counter pressure of at least 2,0 MPa, which with a certain margin, in this case 0,5 MPa, overcomes the theoretical counter pressure. This results in special requirements for the design ofthe gear wheel teeth, as well as for the bearings and the sealing. Any leakage of water from the material can also, for several reasons, not be tolerated. The motor 4 should be a hydrauhc motor which, amongst other things, gives such advantages as a simple torque restriction and a precise control ofthe speed over a wide range.

On starting, the full load ofthe pressure difference between the two digesters 10 and 1 is over the sealing ( the counter weight 8 ), that acts as a non return valve, at the opening 11 of the pipe 7. As the rest ofthe system, backwards to the first digester 1, at that moment is not filled with material, a possible small leakage at the counter weight 8 could pass backwards and consequently no pressure built up will occur in these parts. When later, material is introduced to the cell-feeder 5, via the shoot 2 and the pre-feeder 3, it will further be conveyed until the pipes 6, 7 have been filled. Thereafter the pressure will increase, due to the fact that the material will be packed, until the pressure overcomes the pressure from the counter weight 8, whereafter the counter weight will be pressed back, allowing material to pass and enter the digester 10. If later the flow of material should cease, for example due to an electrical power cut, the counter weight will move, with help ofthe force from the hydrauhc spring 9, and press against the opening 11 of the pipe 7 and when the plug of material in the pipes 6, 7, tends to move backwards it will seal the opening.

Figure 2 shows a device for the feeding to a pressurized dryer, which principally has the same design as the device according to figure 1. Therefore the same reference numbers are used for equal components, in both devices. This type of dryers, so called counter pressure dryers, that works with steam at over pressure needs a feeder and a discharger that seals for the over pressure, which means that material should be fed in and out without any leakage of steam. This might in some cases be achieved with ordinary cell-feeders or plug screws. In some apphcations non of those devices are suitable. This might be due to the fact that the material cannot be compressed into a good enough plug, in a plug screw, or that operating problems, that normally are related to cell-feeders, cannot be accepted. The system, described in figure 2, is a better alternative specially when the material has a relatively low dry content, under 40 %, or when the material is amorphous as for example, brown coal, bagasse, sludge from sewage plants, cellulosic pulp, grass or fruit residues or sugar beet molasses.

In the described case sugar beet waste, so called molasses i. e. the sohd rest that is left over when the sugar has been extracted as much as possible, is fed at atmospheric pressure by means of a transport screw 20, via a shoot 2, to a pre-feeder 3, which has a transport screw, driven by a motor 4 at a constant speed which correlates to the maximum capacity ofthe dryer. From this the material is fed into a cell-feeder 5 ofthe type which has two counter rotating wheels with tooth formed dogs designed for the feeding ofthe material in the present case. This is driven with a speed adapted to the capacity ofthe dryer. At the outlet side ofthe cell-feeder a pressure lock is arranged in the form of a short pipe 7 which has an opening 11 against which, by means of a pneumatic spring 9, a flexibly pivoted counter weight 8 is resting and sealing. The counter weight 8 seals before and under the startmg sequence and makes sure that the plug of material, that is formed in the pipe 7, fills it up and gets a predetermined degree of compression. When this has been achieved the material is pressed passed the counter weight 8 and drops down in a transport screw 21 in the counter pressure dryer 22, where the pressure of 0,3 MPa prevails.

Among the advantages with this arrangement can be mentioned a substantial simplicity in design allowing great flexibility. The driven parts can be run at constant speed. When big quantities of material are transported to the feeder they can be received and when modest quantities arrive, smaller quantities are fed in. Ifthe flow of material is discontinued the counter weight will successively get nearer the opening 11 and at the end completely close and seal the opening for the pressure in the dryer 22.

Figure 3 shows a system for the discharge ofthe dried material from the counter pressure dryer 22, shown in figure 2, that is shown here ending with a cyclone 23 through which the material falls down and is collected on top ofthe inlet of a cell-feeder 5, ofthe same type as earlier mentioned. In this case the pressure lock consists of a material column in the pipe 7, which is directed vertically down and which has an opening 11, directed downwards which, as in the previously examples, is equipped with a counter weight 8 that, by means of a pneumatic spring 9, is kept flexibly pivoted and pressed tight shut against the opening 11. This ends in a hood 24, for steam of atmospheric pressure, equipped with a screw conveyor 25 for the discharge ofthe material and an exit pipe for the flash steam, that is released by the rest-moisture in the material in combination with the pressure drop. The energy content in the flash steam can be recovered in a suitable way.

The invention is naturally not restricted to the examples of design shown and described here but can be modified in several ways within the frame ofthe ideas presented in the claims ofthe invention.

Claims

1. A feeding device, working under high steam pressure, for wet material for example pre-hydrolysed hgnocellulosic material, characterised in, that it comprises a material pre-feeder ( 3 ), for example a transport screw, which is arranged to create the most possible homogeneous flow of material at the entry of a cell feeder ( 5 ) with two counter rotating wheels with tooth formed dogs where, at the outlet side, a pipe ( 6, 7 ) is arranged to form one material plug fed against an outlet opening ( 11 ) ofthe pipe ( 7 ) with a degree of compression so adjusted that the material plug together with the material in the cell-feeder ( 5 ) and in the pre- feeder ( 3 ) forms a gas-tight sealing.

2. A feeding device according to claim 1, characterised in, that the pipe ( 6, 7 ), from the outlet ofthe cell feeder ( 5 ) with the outlet opening ( 11 ), essentially is directed vertically upwards.

3. A feeding device according to claim 1 or 2, characterised in, that the material pre-feeder consists of a funnel formed shoot ( 23 ) down to the inlet ofthe cell feeder ( 5 ).

4. A feeding device according to claim 1 or 2 , characterised in, that the material pre-feeder ( 3 ) consists of a transport screw arranged in a pipe.

5. A feeding device according to claims 1 - 4 , characterised in, that the pipe ( 6, 7 ), arranged at the outlet side ofthe cell feeder ( 5 ), has an outlet opening ( 11 ) which is equipped with one, against a springy device ( 9 ), flexible counter weight ( 8 ) against which the material has to press, when passing the outlet opening ( 11 ).

6. A feeding device according to claim 5, characterised in, that the counter weight ( 8 ) has a surface matching the outlet opening ( 11 ) which, when the material flow from the outlet opening is ceasing and the counter weight ( 8 ) is pressed against the outlet opening, leads to a tight shut sealing ofthe outlet opening ( 11 ).