Method and Apparatus for Minimizing Steam Consumption in the Production of Pulp for Fiberboard and the Like

The present invention relates to a method and apparatus for producing pulp for fiberboard and the like, in which steam discharged at the end of the defibrating or refining process is used to presteam the raw pulp material.

The lignocellulose material in the process of the present invention is mechanically defibrated to fiberpulp and may consist of wood, straw or bagasse, and will be generally referred to as wood or, disintegrated in form of pieces, as chips or raw chips.

The method of the present invention is applicable to all defibering processes in which the defibration of chips takes place in an atmosphere of saturated steam at temperatures exceeding 100.degree. C., usually between 130.degree. C. and 200.degree. C. and corresponding steam pressures of 294 kiloPascals-1569 kiloPascals. The method provides a considerable saving of steam relative to that at present consumed in order to carry through the following conventional cycle, namely, presteaming of incoming raw chips to 90.degree. C.-100.degree. C. with steam at atmospheric pressure, defibration of the presteamed chips after heating with steam to the defibration temperature, blowing out fiberpulp and steam to a cyclone under atmospheric pressure and use of the separated 100.degree. C. steam for presteaming of the raw chips. The pulp constitutes the basic material for the manufacture of fiberboard and is diluted and if necessary refined before forming into wet laps, which are pressed and/or dried to finished product. In particular, the present method is suitable when so-called disc refiners are used and provides a nearly 100% utilization of the heat developed by the defibration for increasing the temperature of the chips before entering the defibration zone.

Heretofore, the chips have been heated with fresh steam to the defibration temperature in a preheater directly connected to the defibrator, at which the heat generated in the defibration zone has only to a small extent been utilized for increasing the temperature of the chips entering the defibration zone.

As energy consumed for the defibration of the chips to a considerable extent, probably more than 80%, is transformed into heat, this means that at an energy consumption of 150 kwh per ton of pulp (dry) from wood chips with a dryness of 50% and the incoming chips preheated to 95.degree. C., developed heat is sufficient to increase the temperature of the chips to 170.degree. C. (defibration temperature) and, theoretically, no extra steam has to be added for increasing the temperature of the incoming chips to the defibration temperature. For carrying through the whole process, only such an amount of low pressure steam will be consumed as is required for increasing the temperature of the raw chips to 95.degree. C.

It is known, as, for example, from U.S. Pat. No. 4,012,279, to preheat the incoming raw chips to the defibration unit with steam, which is released at the discharge of the pulp at atmospheric pressure. In the defibration process carried out up to this point, however, liberated steam contains considerably more heat than that consumed for the preheating of the raw chips to 90.degree. C.-100.degree. C., and this represents the main source of heat energy loss in the process.

In order to attain the lowest possible heat consumption in the defibration process, the released steam should not contain more heat than is consumed for preheating the incoming raw chips to 90.degree.-100.degree. C. This implies, generally, that all heat generated in the defibration zone has to be used for heating the chips before reaching the defibration zone. The generated heat, however, is usually not sufficient to bring the chips to the defibration temperature and, therefore, addition of fresh steam is required.

The present invention contemplates a method and apparatus for minimizing steam consumption in the production of pulp for fiberboard and the like from lignocellulosic material by using refiners or defibrators operating at a temperature above 100.degree. C., preferably at temperatures ranging between 130.degree. C. and 200.degree. C., and corresponding steam pressures ranging between 3 kgs/cm.sub.2 and 16 kgs/cm.sub.2 or 294-1569 kiloPascals. The object of the invention is achieved by separating a portion of the steam discharged at the outlet side of the defibrator and recirculating it to the inlet side under increased pressure above the discharge pressure to impart to the separated portion of the steam a heating capacity to heat the chips to the desired defibrating temperatures at the inlet side.

By recirculation of steam from the discharge side of the defibrator to the feed-in side, with a low pressure compressor or some other type of blowing machine, heat is forced back to the feed-in side for heating the chips before reaching the defibration zone. The additional heat in form of fresh steam, which must be imparted to the chips in order to raise the temperature in the defibration zone, can be supplied either before or after the low pressure compressor. The compressor operates with a slightly increased pressure above the pressure prevailing at the discharge side of the defibrator, so that the circulating steam can overcome the resistance which arises during the passage of the steam through the chips and the defibration zone, and thus allows the steam to accompany the chips and pulp through the defibrator. In this manner, the steam which is generated in the defibration zone is also prevented from blowing backwards against the incoming chips, which, among other things, can disturb an even supply of chips.

Also, in the present case, when operating with a closed white water system, for example, white water can be added to the defibrator as described in U.S. Pat. No. 4,012,279.

The drawing is a flow sheet illustrating schematically how the process according to an embodiment of the invention may be carried out in order to minimize the comsumption of energy.

Referring to the drawing, the incoming chips are continuously fed by the conveyor 1 to the chip presteamer 2 where the chips meet in counterflow discharged steam of 100.degree. C., which imparts to the chips a temperature of 90.degree. C.-100.degree. C. when they pass through the feeder 3. From the feeder 3, the hot chips are passed to the screw conveyor 4, which feeds the chips into the defibrator preheater 6 and simultaneously dewaters the chips by squeezing out water so that they acquire dryness of at least 50%, suitably 55%-65%. The squeezed-out warm water is discharged through the pipe 5. Steam is passed to the preheater 6 partly through recirculation of process steam from the pressure vessel 9 via the steam compressor 10 and the pipe 11, and partly by addition of fresh steam through the pipe 18, so that the temperature of the incoming chips to the defibrator 8 via the feeder 7 will correspond to the defibration temperature. The pressure vessel 9 should preferably be cylindrical and designed so that the pulp is introduced tangentially and the steam drawn off from the center of the vessel as in a cyclone. If necessary, a filter can be fitted in the circulation pipe before the compressor 10 in order to free the steam from accompanying fibers. The amount of fresh steam is regulated so that the chips after having passed through the presteamer 2 will have a temperature between 90.degree. C. and 100.degree. C., and this is achieved by means of a temperature indicator 20 located in the feeder 3 which governs the valve 19 for supplying fresh pressurized steam to the pipe 11. By forced circulation, the generated steam in the defibration zone will be conveyed along with the pulp to the pressure vessel 9. Pulp and excess steam are blown out through the discharge valve 12 and the pipe 13 to the cyclone 14, where steam and pulp are separated. The pulp may be diluted with so-called white water and is discharged through the pipe 15 for the manufacture of board sheets, and the steam is blown by the fan 16 through the pipe 17 to the chip preheater 2. White water can suitably be supplied to the defibrator through the pipe 21.

Sufficient fresh pressurized steam should be supplied to the defibrator preheater so that the amount of released steam at the discharge of the pulp has a heat content sufficient to increase the temperature of the raw chips to 90.degree. C.-100.degree. C. This creates an excess of steam in the defibrator system, and such excess must be blown off. The formation of excess steam in the defibrator unit also enhances the discharge of the pulp from the pressure vessel.

In order to explain the advantages resulting from recirculation of steam, a specific example is given below for a pulp production of 5000 kg/h dry fiber pulp from wood under following conditions: yield by weight 99%, dryness of the raw chips 50% and entrance temperature +5.degree. C., power consumption 625 Kwh and defibration temperature 171.degree. C. corresponding to 785 kiloPascals (8kg/cm.sub.2) steam pressure.

Preheating of incoming raw chips from 5.degree. C. to 95.degree. C. consumes 1233 kg steam of atmospheric pressure and produces a corresponding amount of condensate. The chips will then attain a dryness of 44.5% and 2153 kg water is squeezed out when the chips are fed by the screw feeder 3 into the defibrator preheater 6. The entering chips have then a dryness of 55% and a temperature of 95.degree. C. In order to increase the temperature in the defibrator preheater to 171.degree. C., 492,000 Calories are consumed, which corresponds to 1004 kg steam of 785 kiloPascals absolute pressure. 80% of added defibration energy is converted into heat, which corresponds to 430,000 Calories or 877 kg steam from water of 171.degree. C. Therefore, 127 kg steam must be added in order to provide a sufficient amount of heat available for increasing the temperature of the chips to the defibration temperature. This is the theoretically lowest possible steam consumption with which the defibration can be carried out when the temperature of entering chips to the defibrator preheater is 95.degree. C.

At the discharge of the finished pulp under these conditions, only 823 kgs steam of atmospheric pressure are released, and, consequently, 410 kg must be added in order to make it possible to preheat the raw chips to 95.degree. C. It is, however, more advantageous to supply the heat which is lacking for the preheating of the raw chips in the form of high pressure steam to the defibrator preheater, as thereby the discharge of the pulp is enhanced. Therefore, a total amount of 522 kgs of fresh steam of 785 kiloPascals has to be supplied to the defibrator preheater, which, together with the heat generated in the defibration zone, produces 1399 kgs of steam at a pressure of 785 kiloPascals. Of this amount of steam, 1004 kgs are recirculated, and the balance of 395 kgs is discharged with the pulp, which, when reduced to atmospheric pressure, produces so much steam that incoming raw chips can be preheated to 95.degree. C.

Under the aforementioned conditions, 125 Kwh and 105 kgs of steam at a pressure of 785 kiloPascals per ton of produced pulp are consumed. As far as we know, this implies an energy consumption considerably lower than that which, up to date, is used by the defibration of wood.

At the operation of the defibration process with the lowest possible total energy consumption, the amount of released steam is in normal cases fully sufficient for the transportion of the discharged pulp to the cyclone separator. In some cases, depending on the length of the blow pipe, level differences or production capacity, the released steam may, however, be insufficient. In such cases, an extra amount of steam must be added to the defibration system, which is most easily achieved by increasing the supply of fresh steam to the defibrator. In some cases, it is also possible to add steam to the blow pipe after the discharge valve and in this way increase both the amount and pressure drop of the transporting steam.