GB2087260A - Improving heat utilization in a pulp grinding process - Google Patents

Abstract

A method of improving the utilization of the heat energy of a process for making groundwood pulp in which wood is ground in a grinding space (3) under an absolute pressure exceeding the atmospheric pressure and heated shower water (G) is fed into the grinding space. Steam (H) is released in a steam separator (11) from the groundwood pulp (A) discharged from the grinding space, and replacement water (E) is added to the shower water for replacing of shower water losses. The amount and the temperature of the replacement water are chosen so that a desired energy distribution is obtained between the heat energy of the steam (H) released in the steam separator and the heat energy of the groundwood pulp (B) discharged from the steam separator. By adjusting the temperature of the groundwood pulp discharged from the grinding space either by means of the amount and/or by means of the temperature of the replacement water added to the pulp, it is possible to adjust the distribution of the heat energy generated in the grinding process between the consumption needs for steam and the consumption needs for warm water. <IMAGE>

Description

SPECIFICATION A method of improving the utilization of the heat energy of a process for making groundwood pulp This invention relates to a method of improving the utilization of the heat energy of a process for making groundwood pulp, according to which method -wood is ground by a rotating grinding member in a grinding space, in which an absolute pressure exceeding the atmospheric pressure is maintained, - warm shower water is sprayed into the grinding space, - the groundwood pulp is fed from the grinding space to a steam separator, in which the heat energy of the groundwood pulp is released as steam, - the groundwood pulp is fed from the steam separator to a thickener, from which water released from the pulp is returned into the grinding space as shower water, - groundwood pulp from the thickener is recovered for utilization of its heat energy.

- the steam released from the steam separator is recovered for utilization of its heat energy, and - groundwood pulp from the thickener is recovered by utilization of its heat energy.

A pressurized grinding method is earlier known (Fl-patent application 782414, 780514 and 780515, SE-patent application 7411949-6, SE patents 318178 and 336952), in which grinding takes place in a pressurized grinding space under a pressure above the atmospheric pressure. Wood is fed into the pressurized grinding space by means of pressure equalizing chambers mounted above the grinding pockets of the grinder.

The grinding space, defined by gates and a pulp chest, is pressurized preferably with air or steam. The wood is defibrated by pressing blocks of wood by means of a hydraulic piston against the grinding stone.

The vibration caused by the grinding stone, the heat caused by the friction, and the shower water separate the fibers from the wood material. It has been found that in pressurized grinding the teperature of the shower water has a greater influence of the defibration than in grinding under atmospheric pressure. The warmer the shower waters are, the longer and more unbroken are the fibers separated from the wood material, and the stronger is the paper made of these fibers. Thus it is the better for the pressurized grinding, the warmer the shower waters are when returned to the grinder.

Afterthe defibration the pulp suspension flows from the grinding space through a pipe in which sticks and bigger slabs of wood are cut into pieces by a stick crusher before adjusting the flow rate. The temperature of the pulp discharged from the grinding space normally exceeds 100 C. In practice the pulp temperature may rise up to 145 C, depending on the temperature of the shower water and on the pressure of the grinding space. Thereby the temperature of the shower water sprayed into the grinding space must be 130 - 135"C and the pressure in the grinding space must be 3 bar.The heat energy contained in the pulp suspension is released in the form of steam in a steam separator within which the pressure is decreased to atmospheric pressure, because the temperature of the pulp must be below the boiling point of water after the steam separator. From the steam separator the pulp can flow directly into a thickener in which the hot shower water is separated from the pulp and returned to the grinding space. The pulp can also be discharged from the steam separator into a water tank from which it is pumped to different kinds of screening: pressurized screening or hydro cleaning before the pulp enters a thickener in which the hot shower water is separated from the pulp. From the thickener the pulp is discharged with a consistency of 5 - 33 %.

Together with the discharged pulp the system looses 2-20% of the warm shower water. This amount and other possible losses have to be replaced with replacement water which is added to the shower water tank after the thickener. In the known systems the replacement water amount added to the system compensates in the long run the water amount escaping from the pulp, and the temperature of the replacement water added to the system is held as high as possible in order to keep the decrease of the temperature of the shower water fed into the grinding space as small as possible.

The electrical energy supplied to the grinder is altered in the grinder nearly one hundred percent to heat energy bount to the pulp and the shower water. When the grinder operates in balance, i.e. the temperatures in different points of the grinder are constant, as much energy escapes from the grinder as is supplied therein. The energy has in principle three routes of escape: 1. together with the steam which is released in the steam separator, 2. together with the pulp suspension fed to the paper machine, 3. as losses through vanishing and through the walls to the surroundings.

The extent of the losses depends mainly on the construction of the plant and on the difference between the temperature of the process and that of the surroundings.

The energy per time unit which escapes together with the pulp suspension can be calculated from the following formula: p = cp Cp. rh (t1 - tt) where p = escaping energy, (kW) specific heat of the pulp suspension, (kJ/kg."C) rh= flow of the pulp suspension, (kg/sec) tt = average temperature of the incoming replacement water, ("C) t I = average temperature of the outgoing pulp suspension, ("C) The remaining energy is released as steam from the steam separator.

A disadvantage of the earlier known system is the fact that it is not possible to influence on the heat energy distribution between the steam and the pulp suspension.

Often it would be advantageous to obtain more energy than normally from the groundwood plant together with the pulp suspension fed to the paper machine. On the other hand, the water and pulp systems of a paper mill are often continuously in an unstable state, and the temperature of the pulp fed to the paper machine varies for several different reasons. A constant temperature would, however, be advantageous for the operation of the paper machine. A constant energy flow/unit weight of the pulp suspension from the grnundwood plant, however, cannot prevent a fluctuation of the pulp temperature at the paper machine.

In wintertime there may be profitable use for the released steam, while in summertime the steam may go to waste. By decreasing and increasing the temperature level of the paper machine head box according to the seasons, the energy economy of the mill could be influenced considerably without a decrease in production and without a deterioration of the product quality. This is not possible to obtain with the earlier known systems.

An object of this invention is to eliminate the above mentioned disadvantages and to afford better possibilities for an economic choice of the place of consumption of the heat energy generated during the pressure grinding process. This purpose is obtained according to the invention by a method characterized in that replacement water is added to the pulp suspension for adjusting the energy distribution between the heat energy of the steam released from the steam separator and on the heat energy of the pulp discharged from the steam separator.

The invention is based on the idea that by altering the temperature of the pulp discharged from the grinding space either by means of the amount and/or by means of the temperature of the replacement water added to the system, it is possible to influence on the distribution of the heat energy generated in the process between the energy of the steam released from the steam separator and the energy of the pulp and the hot water discharged from the process.

The invention will be described in the following in more detail with reference to the accompanying drawing which illustrates schematically a pressure grinding process according to the invention.

The drawing illustrates a grinder 1 comprising a rotating grinding stone 2 operating in a pressurized grinding space 3. Two grinding pockets 4 are provided in the grinding space. Pressure equalizing chambers 5 closed by gates are mounted above said pockets. On opposite sides of the grinding stone there are hydraulic pistons 6 for pressing blocks of wood, dropped into the grinding pockets, against the grinding stone. In the grinding space a number of shower pipes 7 are provided for spraying warm shower water on the grinding stone. A pulp pit 8 for collecting of the groundwood pulp is provided in the grinding space.

A pipe 9 forthe pulp A leads from the pulp pit of the grinder through a stick crusher 10 and a blow valve 10a to a steam separator 11 which is provided with a steam exhaust pipe 12 for the steam H released from the pulp. From the steam separator the groundwood pulp B is relieved of steam flows through a pipe 13to a tank 14.

From the tank leads to a pipe 15 through a pump 16 and a pressure screen 17 to a thickener 18 for the groundwood pulp C to be thickened. The thickener is provided with an outlet 19 for the thickened groundwood pulp D.

In the thickener there is a vat 20 for water F which is released from the groundwood pulp C. From the vat leads a pipe 21 to a vat 22. From the vat leads a pipe 23 through a pump 24 to shower pipes 7 in the grinding space for feeding warm shower water G to said shower pipes.

When groundwod pulp is ground, the shower water circulates continuously through a circulation system formed by the pipes 9-13-15-21-23. A part of the shower water escapes together with the thickened groundwood pulp and due to other losses of water in the process, exhausting steam etc., more water must be fed into the circulating system than the amount of water which escapes from the thickener together with the pulp.

Replacement water E is supplied by a pipe 25 to the pipe 9 after the stick crusher 10 but before the steam separator 11. The amount of replacement water is regulated by a valve 26 adjusted by a regulator 27. The regulator can receive its operation signal 28 e.g. from the head box temperature of the paper machine, from the temperature of the high density stock chest of the paper machine, from the temperature of the storage chest of the paper machine or from the temperature of pressure of some place of consumption of the steam released from the steam separator.

For securing of the level of the lowest surface of thé vat 22 replacement water is fed into the system also through a valve 29. The position of the valve is adjusted by a regulator 30 according to a signal from a level transmitter 31.

Heated replacement water J supplied to the process is pumped by a pump 32 through a pipe system 33 to the final stages of a paper making process as dilution and replacement water.

The earlier known system operates in the following way: The temperature of the groundwood pulp discharged from the grinder 1 is 1 15"C in the pipe 9 and the flow amount is 80 kg/sec. A replacement water amount of 10 kg/sec is fed by the pipe 25 and the temperature is 50"C. The temperature of the groundwood pulp A before reaching the steam separator is 80.115C+ 10.500C = 107,8 C (80 + 10) In the steam separator the groundwood pulp is cooled to a temperature of 100 C and 1,30 kg/sec of steam is released. The heat energy transferred with the steam is 2,9 MW. Now, if more energy supplied bathe steam is required for the paper machine for raising the temperature of the head box, this is not possible to obtain in the known system without structural changes.

The system according to the invention operates as follows: A regulator 27 receives an adjustment signal 28 and the valve 26 opens. Consequently the replacement water amount E increases.

At the beginning the valve 26 is opened so much that the water steam E increases 100 %. The temperature of the pulp A before the steam separator is 80.115"C + 20. 50"C = 102 C 80 + 20 Thereby the steam amount which is released decreases to 0,37 kg/sec and the heat energy transferred with the steam is then only 0,84 MW. The operation of the regulator can be controlled either so that a change in the valve position is directly comparable to the difference between the measured value and a set value or so that the whole regulating system operates according to a logic program of a computer.

A replacement water amount of 20 kg/sec is added to the process, although only kg/sec can be allowed to escape together with the pulp from the thickener in order to maintain the desired pulp consistency level. The added replacement water J is collected by the thickener 18 into the vat 22 from which the water is pumped by a pump 32 along a pipe system 33 e.g. as dilution water for the final stages of the paper making princess.

The same result can be obtained by reducing the temperature tt of the replacement water E in the above mentioned formula.

Alternatively, it might be desirable to reduce the energy amount supplied to the paper machine e.g.

because the temperature of the head box is rising too high or because it is preferred to utilize the energy in the points of consumption of the steam H released from the steam separator. In the known system even this is not possible.

In the method according to the invention this can be realized by reducing the amount of the added replacement water E. If the amount of the replacement water E is equal to the water amount, which escapes together with the groundwood pulp D from the thickener and due to other losses, the amount of the replacement water cannot be decreased. The formula indicates that it is only possible to decrease the temperature difference (t - tt).

In the process, for example, it is possible to reduce the temperature difference by means of raising the temperature tt of the added replacement water E. If e.g. the temperature of the replacement water is + 60"C instead of earlier + 50"C, the temperature of the water which enters the steam separator will be: 80. 1150C+ 10.600C = 108,9"C 90 The amount of steam released is 1,48 kg/sec and then the heat energy transferred with the steam is 3,3 MW.

In this example it is according to the method of the present invention possible to adjust the steam amount released in the steam separator between 0,37-1,48 kg/sec and correspondingly the heat energy released with the steam between 0,84 MW - 3,3 MW.

The drawing and the accompanying description shall only illustrate the principle of the invention. In its details the method may vary within the scope of the claims. The replacement water E can be added also at other points of the shower water circuit 9-13-15-21-23 than before the steam separator 11, e.g. between the steam separator 11 and the thickener 18, to the thickener 18, to the water tank 22 or the shower water pipe 23.

In such a case the replacement water is added to the groundwood pulp A and influences the energy distribution in the steam separator 11 only in the following circulation, i.e. after the replacement water has passed through the grinding space 3.

Claims (7)

1. A method of improving the utilization of the heat energy of a process for making groundwood pulp, according to which method - wood is ground by a rotating grinding member in a grinding space in which an absolute pressure exceeding the atmospheric pressure is maintained, - warm shower water is sprayed into the grinding space, - the groundwood pulp is fed from the grinding space to a steam separator in which the heat energy of the ground wood pulp is released as steam, - the groundwood pulp is fed from the steam separator to a thickener from which water released from the pulp is returned to the grinding space as shower water, - replacement water is added to the shower water for replacing shower water losses, - the steam released from the steam separator is recovered for utilization of its heat energy, and - the groundwood pulp from the thickner is recovered for utilization of its heat energy, characterized in that replacement water is added to the grounded pulp for adjusting the energy distribution between the heat energy of the steam which is released from the steam separator and the heat energy of the groundwood pulp discharged from the steam separator.

2. A method according to claim 1, characterized in that the amount of the replacement water added to the groundwood pulp is adjusted.

3. A method according to claim 1, characterized in that the temperature of the replacement water added to the groundwood pulp is adjusted.

4. A method according to claim 2 or 3, characterized in that the temperature or the amount of the replacement water is adjusted in accordance with the temperature of the groundwood pulp discharged from the thickener and the temperature at the point of consumption of hot water.

5. A method according to claim 2 or 3, characterized in that the temperature or the amount of the replacement water is adjusted in accordance with the temperature and/or the pressure at the point of consumption of steam released from the steam separator.

6. A method according to claim 1, characterized in that the replacement water is added to the groundwood pulp before the steam separator.

7. A method of improving utilization of heat energy of a process for making groundwood pulp, substantially as herein described.