WO2001094691A1

WO2001094691A1 - Method for batch cooking

Info

Publication number: WO2001094691A1
Application number: PCT/FI2001/000523
Authority: WO
Inventors: Päivi UUSITALO
Original assignee: Metso Paper, Inc.
Priority date: 2000-06-06
Filing date: 2001-06-01
Publication date: 2001-12-13
Also published as: AU6398101A; FI20001351A0

Abstract

In a batch cooking process for pulp production, the number of tanks required for storing black liquor at various temperatures is reduced by using heat exchange in the flow path from tank to digester, independently from the digester heating devices. Thus, black liquor of a desired temperature can be introduced to the digester at any stage in the process, whereby the need for heating in the digester circulations is reduced, and heat exchangers may be eliminated.

Description

METHOD FORBATCH COOKING

Field of tine imvei-tioiπi

T e invention relates to batch pulping, More particularly, the invention is related to a s method of designing a batch pulping process in order to save investment costs by using a limited number of tanks in combination with heat exchangers.^'

With the event of displacement batch cooking processes in alkaline pulping applications, 0 batch pulping became a competitive technology in comparison with continuous processes. In displacement cooking, heat energy from previous cooks is transferred to the raw material of a subsequent cook by initially filling the chip-charged batch digester with spent (black) liquor that has been discharged from a completed cook batch and kept at its high temperature in an accumulating tank. By using sequences of spent liquors at increasing s temperatures before hot fresh cooking (white) liquor is introduced for the final cooking stage, a significant amount of energy is recycled instead of being expelled from the cooking plant with each completed batch. Such a system is disclosed in e.g. US patent 5,578,149 (Fagerlund). The process has been further refined by taking into account the chemical processes at play in different stages of the process, whereby limits are set for the 0 temperature, alkalinity, sulfidity and solids content of the various spent liquors introduced in sequence during the treatment of each chip charge, as disclosed in FI patents 92224 and 93866. The process can also be adapted to suit different sources of raw material by adding more sulfidity early in the process (FI pat. appl. 962260), low pH precleaning (FI pat. appl 974455), or calcium-rich raw material (FI pat. appl. 992132). Thus, great flexibility is 5 available to handle situations where the process must be adjusted because of variations in the quality of the lignocellulosic material being used as a starting point.

However, displacement batch cooking is technically complicated and requires a significant amount of equipment in addition to digesters: heating devices, pumps, pipes, valves, o instruments and accumulator tanks. A number of tanks must be pressure vessels in order to store liquors having temperatures clearly above atmospheric boiling point. This tends to lower the attractivity of the process. Pressure vessel codes are strict, and any alterations are connected with time-consuming certifications. In a typical process of the kind described above, the cooking line requires a number of digesters, two pressure tanks for holding black liquor at different temperatures above the atmospheric boiling point (herein called "hot" black liquor) but having different chemical properties, at least one atmospheric tank for holding black liquor at a temperature below its s cooking point (herein called "warm" black liquor), a pressurized white liquor tank, plus tanks for holding wash filtrate and possible other required liquors. Pumps and heat exchangers are provided for transferring energy between streams of liquors to provide the correct temperatures in different stages of the process. In a typical process utilizing black liquor impregnation and pretreatment, warm black liquor is introduced first into the 0 digester, followed by hot black liquor displacing the warm black liquor, and finally white liquor at or near the cooking temperature. Black liquor displaced during these fillmg stages may be collected for further recycling or passed on to the evaporation plant. After the main cooking stage, the resulting black liquor is displaced from the digester by wash filtrate, and may be divided into several fractions depending on its physical and chemical properties. s These fractions may then be used in an appropriate sequence for impregnation and pretreatment of a subsequent chip charge.

As the technology of automation, instruments and on-line monitoring of pulping liquors has developed, it has become possible to control the properties of the liquors in a more refined 0 manner, and thus the need for fractioning different categories of spent liquor into separate tanks has diminished.

Siiiπmary øf t e mvrøtfion

In accordance with the present invention, a method is provided for producing chemical pulp 5 by means of a batch cooking process, wherein a heating device is provided in the piping connecting the pressurized hot liquor tanks with the digester for obtaining the desired temperatures in the liquors entering the digester. Thus, heating of the liquors conducted to the digester takes place on the flow path from tank to digester. The heating device may be included in a circulation loop drawing liquor from a tank and returning it to the same tank. o From such a loop, liquor may be withdrawn after the heat exchanger and conducted to the digester. However, the heating device according to the present invention is not the heating device found in digester-specific circulation loops according to the prior art. On the contrary, according to the present invention heat exchangers in the digester circulation are eliminated in favor of direct steam injection as described below. Preferably, the hot black liquor displaced at a temperature above atmospheric boiling temperature is collected into a single hot black liquor accumulation tank. White liquor is s supplied from a hot white liquor tank. By means of the steam operated heat exchangers, indirect steam heating may then be used to bring the entering liquors to temperature; as the hot liquor fill with hot black and white liquor thus brings the digester temperature up to nearly the cooking temperature, direct steam heating can be used in the digester circulation for supplying the additional heat required, instead of digester-specific heat exchangers. As lo one tank line serves a number of digesters, the total number of required heat exchangers decreases.

Brief description of tine drawings

Figure 1 shows a prior art displacement batch cooking system with two pressurized hot 75 black liquor tanks, with the tank farm and a schematic of the digester filling sequence during one batch;

Figure 2 shows another prior art displacement batch cooking system using one pressurized hot black liquor tank only but without the characterizing features of the present invention;

Figure 3 shows a displacement batch cooking system according to the present invention; 20 Figure 4 shows another embodiment of the present invention, having a piping design different from the system of Figure 3;

Figure 5 shows an embodiment of the present invention which provides for handling of calcium-rich raw material; and

Figure 6 shows an embodiment of the present invention, in which no impregnation step is 25. used, but instead pre-steaming of the chips is included.

Detailed disclosure of the imvemtiom

For clarity, in the different figures, functionally corresponding tanks carry the same number although the process varies. Valves, instruments and control loops are not shown, but it is 3o obvious to the person skilled in the art that these may be provided at any location to enable any desired flow path. Figure 1 shows a displacement batch cooking system according to the prior art. The required tanks are shown at the top, while the sequence of steps carried out in the digester during one batch is diagrammatically shown at the bottom, with the corresponding liquor flows indicated. After chip charge, the digester is filled with impregnation black liquor A from atmospheric tank 5. Next, the impregnation black liquor is displaced by hot black liquor B from tank 1 and is transferred to receiving tank 4 and further to the evaporation and recovery facilities. The hot black liquor is in turn displaced by white liquor C from pressurized tank 3 and a smaller amount of hot black liquor B from pressurized tank 1. The displaced liquor from the impregnation liquor and hot black liquor treatment, Al and A2, below about atmospheric boiling point is conducted to black liquor tank 4. The displaced liquor above about atmospheric boiling point B2 is conducted to hot black liquor tank 2. • The digester temperature is brought to the desired cooking temperature by direct steam heating, i.e. steam is injected into the digester circulation loop. After completed cooking, the resulting black liquor is displaced using wash filtrate E from tank 6 into tanks 1 and 2 as fractions Bl and Dl respectively, depending on the quality of the liquor leaving the digester. The displacement with wash liquor brings the temperature of the digester content below about the boiling point at atmospheric pressure. Then the cooked raw material is discharged from the digester. In the cooking sequence the liquor displacements typically occur from bottom to top of the digester. From tank 2, hot black liquor is conveyed to tank 5 to generate new impregnation black liquor, its heat content being partly transferred to white liquor or to a water stream through heat exchangers. Tank 5 is provided with soap separation. Tanks 1 and 3 are further provided with circulation loops and heat exchangers for adjusting temperatures.

Figure 2 shows another, less sophisticated system according to the prior art. Following chip charge, the digester is filled with impregnation liquor AA from tank 5. Hot black liquor B from tank 2 displaces the impregnation liquor A2 and the displaced liquor AB is led to receiving tank 5 and further to the evaporation and recovery facilities. White liquor and a small amount of hot black liquor displaces the hot black liquor. The displaced liquor above about atmospheric boiling point is conducted to hot black liquor tank 2. In this design, the heat content of the liquors charged to the digester is not sufficient to bring the digester contents close to cooking temperature prior to the cooking step, but a circulation loop with heat exchangers is used in the digester to supply the required additional heat. After the cook, the resulting black liquor is displaced with wash liquor E to a single pressurized black liquor tank 2, which step brings the temperature of the digester contents to below the boiling point at atmospheric pressure. When comparing the processes of figure 1 and 2, it is evident that the temperature within tank 2 in figure 2 is approximately the weighed average temperature of tank 1 and tank 2 in figure 1. The temperature of the hot black liquor conducted to the digester is therefore lower in the process shown in figure 2, compared to the process shown in figure 1.

Figure 3 shows a system according to the present invention. Compared to the system of Figure 1, tank 1 has been eUminated and displaced hot black liquors B frorr different stages all enter the same pressure tank, tank 2. In this embodiment of the present invention, this tank is equipped as shown with a heating circuit comprising a steam operated heat exchanger, and piping to draw off heated black liquor from the circuit to the digester. Thus, the temperature of hot black liquor to be charged to the reactor is adjusted in said heat exchanger, and any additional heating in the digester after filling can be carried out with direct steam injection. As heat adjustment takes place in the transfer line, less heating capacity is required for heating the digester contents in the digester itself. The number of digesters being larger than the number of tank sets as shown in the figure, a corresponding amount of heat exchange equipment can be eliminated in the initial investment.

An advantage of the heat adjustment in the transfer line is that no essential temperature increase will occur of the liquor stored in tank 2 as higher temperature will result in higher amounts of hot water produced. Impregnation black liquor is generated by conveying hot black liquor from tank 2 to the atmospheric impregnation liquor tank 5 via a heat exchanger, whereby the liquor transfers part of its heat to white liquor entering white liquor tank 3. The displaced liquor in the impregnation and hot fill stages below about atmospheric boiling point is conducted to black liquor tank 4 and above about atmospheric boiling point is conducted to hot black liquor tank 2.

Figure 4 shows a system corresponding to that shown in Figure 3, but with piping interconnecting the heating circuits of the hot black liquor tank and of the white liquor tank. This embodiment provides great flexibility, since only two heat exchangers are required in, for example, a batch digester system with multiple lines used, typically in plants at higher pulp production levels. For example, a set-up of eight digesters is typically dived into two lines, i.e. four digesters per line. Each line in a displacement batch digester system comprises a number of digesters, e.g. typically three to five digesters, served by line-specific pumps and pipes connecting the digesters and the tanks, but all digesters in the plant (all lines) are served by the same set of tanks. In a batch digester system with, for example, two lines, the embodiment shown in figure 4 therefore provides further simplification in comparison to a system as shown in figure 3. In a batch system according to figure 3 with two lines, two heat exchanger circuits of the hot black liquor tank and one heat exchanger circuit of the white liquor tank 3 would be required. In a batch system according to figure 4 with two lines, only two heat exchanger circuits woulr* be r quirecs for both the hot black and white liquor tanks 2 and 3, thus, a saving of one heat exchanger in comparison to figure 3. Furthermore, the embodiment requires a lower number of pumps in a multiple line digester system. The heating circuits of the hot black liquor tank and of the white liquor tank can be used to heat the content of the tanks 2 and 3, especially when the circuit B and/or C connected to the digesters is not in use in the batch sequence. This flexibility is especially important for heating of the white liquor in tank 3 and for steam leveling in the system. The process of figure 4 can also be used in a batch digester system with a single line.

Figure 5 shows a batch cooking system according to the present invention, in which provisions are made for handling raw material rich in calcium. As described in Finnish patent application 992132, particularly the first portion of black liquor displaced from the digester during a batch, following the impregnation stage, may contain a large amount of calcium bound to dissolved organic material. As this organic material is broken down at high temperatures during later stages of the process, calcium is released and may cause severe scaling if allowed to precipitate as carbonate on heat exchange surfaces in the black liquor evaporation plant of the cooking chemicals recovery system. For this reason, it may be advantageous to use such calcium-rich liquors for final displacement of the cooking liquor after a batch. The calcium is thereby released in the liquor due to the high temperature, but will not encounter any heat exchange surfaces and no scaling occurs. The arrangement of hot black liquor tank and white liquor tank is identical to that of Figure 4. No impregnation liquor tank is provided, but the first liquor introduced during the batch may originate from the countercurrent wash process. After the impregnation stage, the liquor displaced during the next liquor introduction is rich in calcium, and is stored in an atmospheric tank until it is reintroduced after the cook to displace the spent cooking liquor, which is at cooking temperature. Breakdown of organic material and calcium release then takes place where it cannot cause scaling. To further simplify the tank farm arrangement, tanks 4 and 6 of Figure 5 may be combined into one.

Figure 6 shows a further embodiment of the present invention, wherein the impregnation stage is replaced with a steaming stage. Such a process may be used for treating the raw material with steam for packing and removing gases in the digester whereby measures for removal of so-called non-process compounds are taken during the steaming step (FI appl 982411). An atmospheric impregnation black liquor tank is not required, but the temperature of the black and white liquor fed to the digester is regulated using indirect heat exchange according to the present invention, as described above.

In the various embodiments of the invention described in figures 3-6, tank 2 is designed to enable uniform physical and chemical properties of the liquor conducted from the tank to the next stage. Thus, the tank is designed to dampen the swings in liquor properties of the inlet to the tank. Such internal tank arrangements are for example described in FI 992802. For example, the inlet to the tank can be separated from the outlet to avoid "channelling" through the tank. The inlet and outlet can be separated by internal separator elements, interconnected internal chambers separating the inlet and outlet of the tank, long enough distance between tank inlet and tank outlet, mixing arrangement in the tank, and proper size of the tank. The alkali content of the hot black liquor conducted from tank 2 in the various embodiments of the invention may be adjusted with white liquor to a pre-set constant value before introduction to the next stage.

Claims

1. A process for producing chemical pulp in a batch digester, said process comprising the steps of s a) charging lignocellulose-containing material to the digester, b) treating the Hgnocellulose-containing material in sequence with steam and/or an impregnation liquor and subsequently with hot black liquor and with hot white liquor, displacing spent liquor from the digester during said sequence, c) heating and cooking the lignocellulose-containing material so as to produce cooked lo lignoceUulose-containing material and hot black liquor, d) displacing said hot black liquor from the digester, characterised in that at least one hot liquor transferred to the digester is heated to a desired temperature in a heating device provided in the flow path from the tank to the digester, said heating device being located outside the digester circulation circuit.

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2. The process of claim 1, characterised in that hot black liquor is stored in a single tank above atmospheric pressure.

3. The process of claim 1 or 2, characterised in that the heating device is provided in a 20 circuit with inlet from and outlet to the same liquor tank.

4. The process of any claim 1 -3, cliaracteraed in that digester heating in stage c) is carried out with direct steam injection.