GB1600570A

GB1600570A - Method and apparatus for thermomechanical pulping

Info

Publication number: GB1600570A
Application number: GB24145/78A
Authority: GB
Original assignee: Beloit Corp
Current assignee: Beloit Corp
Priority date: 1977-06-08
Filing date: 1978-05-30
Publication date: 1981-10-21
Also published as: BR7803644A; DE2825069A1; FI781703A; ES470572A1; CA1074606A; FR2393880B1; DE2825069C3; SE7806552L; JPS5418901A; DE2825069B2; FR2393880A1

Description

(54) METHOD AND APPARATUS FOR THERMO-MECHANICAL PULPING (71) We, BELOW CORPORATION, a corporation organised and existing under the laws of the State of Wisconsin, United States of America, of Beloit, Wisconsin 53511, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method of, and apparatus for, thermo-mechanical pulping wood chips.

Historically, the two most common methods of pulping wood chips have been the mechanical pulping operation in which wood chips are mechanicall abraded, usually by means of a stone wheel, and a chemical treatment wherein the wood chips are processed with sulphate or sulphite baths. The mechanical system, while quite inexpensive, leaves too much lignin in the resulting pulp, and the fibres are shorter than would be optimum for various types of paper making. The chemical process on the other hand requires the use of large amounts of equipment such as high pressure tanks which are quite expensive. The high initial cost of installation and the cost of operation of the chemical processes are not always justified.

In more recent times, a new technique known as thermo-mechanical pulping and refining has become commercially accepted. In a typical thermo-mechanical pulping operation, wood chips from a chip bin are fed through a chip washing system and then by means of a feed screw into a rotary feeder valve which feeds the chips to a steaming tube operating at superatmospheric pressure. The material is introduced into one end of the tube and is conveyed by means of a screw type conveyor to the outlet end. During their travel through the tube, the wood chips are contacted with steam from multiple steam jets which maintain a uniform steam pressure throughout the length of the tube.

Following the treatment in the steaming tube which brings the temperature of the material above the lignin softening temperature, the material passes through a first stage pressurised refiner to which there is coupled a blow valve. The mixture of steam and pulp is then passed to a cyclone separator, and a conveyor system delivers the pulp freed from the steam in the separator to a second stage refiner which is non-pressurised. The pulp produced in the second stage refiner is then passed into a stock chest and then goes through the various other stages which are common to any procedure for making up a suspension of fibres suitable for use in a headbox.

The expense of a pressurised steam vessel which has a screw or other discharge mechanism is high and the operation is further complicated by the need for expansion joints, pressure seals, and other pressure resistant equipment which significantly raises the cost of the thermal-mechanical pulping system.

U.S. Patent Specification No. 3,921,918 describes a method of mechanically refining which involves pretreating fibrous material with steam in a preheater followed by refining the steam treated material in a disc refiner under conditions which generate steam, with at least a part of the developed steam being directed back to the preheater.

U.S. Patent Specification No. 3,661, 328 describes a pulp refining system utilising multistage disc refining. In the first stage, the refining is carried out in a pressurised environment under moderately elevated temperatures. Subsequent disc refining steps are carried out under atmospheric pressure conditions. The multistage refining process is said to result in a reduction in the bulk of the fibre furnished.

There are commercial machines on the market which involve continuously feeding wood chips to a steam heated chamber where they are compressed and then passed to a screw conveyor and ultimately into a grinding apparatus.

There is also a chip refiner process utilising pre-steaming of the chips described in the 'Pulp and Paper International' for January 1977.

One of the difficulties with thermomechanical pulping has been the fact that the chips are darkened by exposure to high pressure steam. What is more, the steaming is not always too effective to remove substantial amounts of pitch from the wood.

The present invention provides an improved method and apparatus for thermo-mechanical pulping wherein the wood chips are contacted with steam at ambient pressure conditions to raise the temperature to approximately that of the steam, that is approximately 212 F (100 "C). The steam treated chips are then compacted severely, at compaction ratios of at least 2.5 to 1 and preferably of about 2.7 to 1, contacted with pressurised steam for a time sufficient to raise the temperature to at least the lignin softening point, and thereafter refined.

By using a pre-steaming step ahead of the compaction, up to five times as much pitch is removed as without the steaming treatment. The compaction serves to reduce the moisture content to at least as low as 30% usually on the order of 25% whereas previously it was common to leave moisture contents up to 50%. The higher compaction also fractures the fibre bonds in the chips to a greater extent, thereby reducing the amount of power required in the first stage of refining.

In a preferred embodiment of the present invention the chips are treated with a steam at atmospheric conditions for about 1 to 5 minutes and then are conveyed to a pressurised refiner operating at a pressure of about 20 to 80 psi gauge (138 to 552 kPa). After compaction, it takes only a few seconds to heat the chips to the temperature necessary for proper treatment. This is accomplished by contacting the compacted chips with pressurised steam for a time sufficient to raise the temperature to at least the lignin softening point, whereupon the chips which now have significant amounts of fractured bonds between fibres therein are refined in a refining stage operating at a pressure of 20 to 80 psi gauge (138 to 552 kPa).After treatment in the refiner, the mixture of pulp and steam may be passed through a cyclone separator from which the steam is vented and the pulp may be passed to a dilution chest.

Upon dilution with water, the pulp suspension preferably goes to a disc type refiner arranged to refine further the pulp thus produced before being passed on into the ancillary equipment preceding the headbox.

The single figure in the drawings illustrates somewhat shcematically a thermo-mechanical pulping and refining system according to the present invention.

In the single figure of the drawings, there is illustrated a chip bin 10 which is open to the ambient atmosphere and into which there extends a manifold 11 carrying a plurality of steam jet lines 12. Steam at substantially atmospheric pressure is received into the manifold 11 from a line 13 under the control of a gate valve 14. The residence time of the wood chips in the chip bin during steaming is typically of the order of 1 to 5 minutes, or at least sufficient to bring the chips to a temperature approximating the temperature of the steam (212 F, 100 "C).

After atmospheric steaming, the chips are delivered to a plug screw feeder 15 which isolates the atmospheric pressure of the chip bin 10 from the superatmospheric pressure existing in a first refiner stage 16. The plug screw feeder 15 delivers the chips through a vertically positioned tube 17 and then into an inlet conveyor 18 of the first refiner 16.The retention time of the steamed chips in the plug screw feeder 15 and the tube 17 is of the order of only about 10 to 30 seconds but that time is sufficient to bring the temperature of the steamed chips rapidly to the softening range for lignin which is about 2300 to 245 "F (1100 to 118 "C). By heating the chips up close to 212 "F (100 "C) in an environment that does not darken the chips, it only takes a few seconds to heat them to the lignin softening temperature. Consequently, the retention time at the higher pressure with correspondingly higher temperatures is greatly reduced, and this contributes to less darkening of the chips.

By steaming at atmospheric pressure ahead of the plug screw feeder 15, up to 5 times as much pitch is removed from the wood chips as occurs when no steaming is used.

Steamed chips are subjected to substantial compression whereby the moisture content is reduced to as low as 30% or preferably as low as 25%, in contrast to conventional plug feeder operation which reduces the moisture content to about 50%. The higher compaction used, of the order of at least 2.5 to 1 or preferably 2.7 to 1 as compared to the normal compression ratio of 1.9 to 1 in these devices, fractures bonds between fibres in the chips to a greater extent than usual, enabling the first stage of refining to be operated at a reduced power level.

The first stage refiner 16 operates at a pressure of about 20 to 30 psi gauge (138 to 552 kPa). The refiner itself is preferably a horizontal single disc refiner of the type available commercially from the Beloit Corporation under their trademark 'Uni-Mount'. Basically, this type of refiner has a single dynamically balanced disc driven by a synchronous or induction motor. During operation of the refiner, steam is generated and this steam can be conveniently used as a source for the steam in the line 13, by providing a line 19 extending from the pressurised tube 17 into the gate valve 14.

The refined material in the first refiner 16 is then passed by means of a line 21 to a blow cyclone separator 22. In passing through the blow cyclone separator 22, the material flow spirals downwardly, releasing steam which is vented by means of a line 23. The pulp may be passed by means of a discharge line 24 into a dilution chest 25 into which dilution water is introduced by means of a line 26. A pump 27 discharges the stock solution from the dilution chest 25 through a line 28 under the control of a valve 29 into an optional refiner feed chest 30 which is essentially a storage device. Pulp constituents from screens or cleaners or other stages of the paper making process may be introduced into the refiner feed chest 30 by means of a line 31.

Periodically, a pump 32 delivers the pulp to a second refiner 33 which is preferably a double disc type refiner wherein a pair of refining discs cooperate with a rotor such that half of the incoming stock passes through the rotor into the second set of discs. The stock passes in parallel across both sets of discs, merges and then leaves the unit through a single outlet. This type of refiner is available commercially from the Beloit Corporation under the trademark 'DD' refiners. Finally, the pulp after treatment in the second refiner 33 passes into a line 34 under the control of a valve 35 from where it can be sent to screens, cleaners, and the rest of the auxiliary equipment preceding the headbox.

The inclusion of dilution chest 25 and feed chest 30 is not essential for all types of installations. Instead, the pulp may conveniently be passed directly from the cyclone separator to a counter-rotating refiner with atmospheric discharge.

The generation of steam in the first refiner 16 is usually adequate to keep the inlet pressure to at least 20 psi gauge with the excess steam being vented back to the atmospheric chip bin or other points in the process. Thus, the plug screw feeder of the present invention eliminates the necessity for the inlet rotary valve, steaming tube, cross conveyor, and the other special expansion and support connections in a typical pressurised system for thermo-mechanical pulping and refining.

WHAT WE CLAIM IS: 1. A method of thermo-mechanical pulping wood chips which comprises: contacting wood chips with steam at ambient pressure conditions thereby to raise the temperature to approximately 212 "F (100 "C), compacting the steam treated chips with a compaction ratio of at least 2.5 to 1, contacting the compacted chips with pressurised steam for a time sufficient to raise the temperature to at least the lignin softening point, and thereafter refining the resulting chips.

2. A method according to claim 1, wherein said refining is carried out at a pressure of 20 to 80 psi gauge (138 to 552 kPa).

3. A method according to claim 1 or claim 2, wherein said contacting with pressurised steam is carried out for 10 to 30 seconds.

4. A method according to any of claims 1 to 3, wherein steam generated during said refining is recycled into contact with said chips.

5. A method according to any of claims 1 to 4, wherein said chips are compacted to a moisture content at least as low as 30%.

6. A method according to any of claims 1 to 4, wherein said chips are compacted to a moisture content of about 25%.

7. A method according to any of claims 1 to 6, wherein said chips are compacted with a compaction ratio of about 2.7 to 1.

8. An apparatus for thermo-mechanical pulping of wood chips comprising: a vessel for holding said chips, said vessel being operable at atmospheric pressure, means for introducing steam into said vessel for steaming the wood chips therein, a pressurised refiner, a plug screw feeder for delivering steamed chips from said vessel to said pressurised refiner, separator means for receiving the pulp-steam mixture from said pressurised refiner, and a second refiner arranged to further refine the pulp thus produced.

9. An apparatus according to claim 8, wherein said separator is a cyclone separator.

10. A method of thermo-mechanical pulping wood chips according to claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

11. An apparatus for thermo-mechanical pulping of wood chips according to claim 8 and substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Periodically, a pump 32 delivers the pulp to a second refiner 33 which is preferably a double disc type refiner wherein a pair of refining discs cooperate with a rotor such that half of the incoming stock passes through the rotor into the second set of discs. The stock passes in parallel across both sets of discs, merges and then leaves the unit through a single outlet. This type of refiner is available commercially from the Beloit Corporation under the trademark 'DD' refiners. Finally, the pulp after treatment in the second refiner 33 passes into a line 34 under the control of a valve 35 from where it can be sent to screens, cleaners, and the rest of the auxiliary equipment preceding the headbox. The inclusion of dilution chest 25 and feed chest 30 is not essential for all types of installations. Instead, the pulp may conveniently be passed directly from the cyclone separator to a counter-rotating refiner with atmospheric discharge. The generation of steam in the first refiner 16 is usually adequate to keep the inlet pressure to at least 20 psi gauge with the excess steam being vented back to the atmospheric chip bin or other points in the process. Thus, the plug screw feeder of the present invention eliminates the necessity for the inlet rotary valve, steaming tube, cross conveyor, and the other special expansion and support connections in a typical pressurised system for thermo-mechanical pulping and refining. WHAT WE CLAIM IS:

1. A method of thermo-mechanical pulping wood chips which comprises: contacting wood chips with steam at ambient pressure conditions thereby to raise the temperature to approximately 212 "F (100 "C), compacting the steam treated chips with a compaction ratio of at least 2.5 to 1, contacting the compacted chips with pressurised steam for a time sufficient to raise the temperature to at least the lignin softening point, and thereafter refining the resulting chips.