Technical field

The invention refers to a method of controlling a pulping process wherein wood chips are conveyed to an impregnation vessel or to a digester where said wood chips are treated with a liquor comprising chemicals.

Background

In a chemical pulping process, wood chips are treated with chemicals in a digester at enhanced pressure and temperature to dissolve the lignin that binds fibres together. The predominating pulping process today is the Kraft process, wherein wood chips are treated with a mixture of sodium hydroxide and sodium sulphide, known as white liquor. The pulp quality and the consumption of the chemicals used in the pulping process are highly dependent on the raw material, i.e. the wood chips. The moisture content and other properties of the chips may vary due to, e.g., seasonal variations, different chip pile arrangements and the wood species used. The variation of the properties of the raw material makes it difficult to set an accurate dosing of chemicals to the digester. Oftentimes more chemicals than required are used, which is detrimental from both an economical and an environmental perspective. A system for controlling a continuous digester is further disclosed in CA919289.

In conventional chemical pulping processes, the process is controlled on the basis of information obtained from measurements of the properties of the pulp or of the residual alkali content in spent cooking liquor, i.e. the process is controlled in a feed-back manner. Such an approach fails to comply with short-term or sudden

variations of the properties of the chips. In the art, ther are commercial measurement systems available for measuring raw material properties. However, these prior art methods usually involve high investment costs, requires long time for analysis and/or are not accurate enough and are thus no typically used to control the process. Thus, there remains a need for a simple, on-line feed forward control of the pulping process.

Summary of the invention

It is an object of the invention to provide a simple, on- line method for controlling a pulping process. This, and other advantages, is achieved with the method according to the invention.

The invention discloses a method of controlling a pulping process wherein wood chips are conveyed to an impregnation vessel and/or to a digester where said wood chips are treated with a liquor comprising chemicals. The method of the invention comprises the steps of; a . estimating a mass flow of dry wood chips to the impregnation vessel or the digester needed to achieve a desired pulp production, b . measuring the actual mass flow of moisture wood chips conveyed to the impregnation vessel or the digester, c . calculating a ratio between the measured actual mass flow of moisture wood chips and the estimated mass flow of dry wood chips, d . controlling the pulping process based on said

calculated ratio.

The method according to the invention makes it, e.g.

possible to control the supply of chemicals, such as sodium hydroxide and sodium sulphide, to the impregnation vessel or to the digester, the liquor to wood ratio and/or the pulping temperature on-line, in a feed-forward manner. It has been shown, that by controlling the pulping process in accordance with the invention, the stability of the process is improved and a reduction in the variation of the kappa number and the quality of the pulp is achieved. The method is simple and does not require huge investment costs. Further we are using the estimation in a) to calculate the pre-set process parameters. The ratio is then used for fine tuning the process parameters. Thus you create first a set-point which later is used in the regulation of the process.

According to a second aspect of the invention, there is provided a method comprising the steps of; a . estimating a dry density of the wood chips being treated, b . measuring the density of the moisture wood chips conveyed to the impregnation vessel or to the digester, c . calculating a ratio between said estimated dry density of the wood chips and said measured density of the moisture wood chips, and d . controlling the pulping process based on said calculated ratio. According to a third aspect of the invention, there is provided a method comprising the steps of; a . estimating a dry density of the wood chips being treated, b . measuring the density of the moisture wood chips conveyed to the impregnation vessel or to the digester, c . calculating a ratio between said estimated dry density of the wood chips and said measured density of the moisture wood chips, which also involves compensating with a measured dry content, and d. controlling the pulping process based on said calculated ratio.

This may be used when the variations in wood density

dominate. Thus measuring the dry content provides a way to compensate for this variation.

Detailed description of the invention

The invention relates to a method of controlling a pulping process wherein wood chips are treated with chemicals in an impregnation vessel and/or in a digester. The term

"impregnation vessel" as used herein may be any kind of a vessel, pipe or the like suitable for processing of the wood chips. The pulping process may, e.g., be a kraft pulping process, a sulphite pulping process or a chemi-thermo mechanical pulping process. The pulping process may be a batch or a continuous pulping process. The process may, e.g., be a continuous Kraft pulping process, wherein the wood chips are treated with a mixture of sodium hydroxide and sodium sulphide at elevated temperature and pressure.

In accordance with the method of the invention, a ratio between the actual mass flow of fresh, moisture wood chips conveyed to the impregnation vessel or to the digester and an estimated mass flow of dry wood chips to the impregnation vessel or the digester needed to achieve a desired, pre-set, pulp production, is calculated. Said ratio, below referred to as Wood Material Indicator (WMI), may, e.g., be used to control the amount of chemicals, such as sodium hydroxide and sodium sulphide, supplied to the impregnation vessel or the digester and/or to control the liquor to wood ratio in the digester and/or to control the pulping temperature. The WMI may be calculated in accordance with eq. 1.

„ Actual mass flow of moisture chips [kg I s \ ,

WMI = - - - (eq. 1)

Estimated mass flow of dry chips [kg/ s\

Alternatively, the WMI may be calculated by dividing the estimated mass flow of dry wood chips with the actual mass flow of moisture wood chips.

An estimated mass flow of dry wood chips may be calculated by dividing the desired pulp production with the cooking yield in accordance with equation 2. pulp production

Estimated dry mass flow [kg/ s] xlOO (eq. 2)

Cooking Yield The Pulp Production is a targeted, pre-set, value. The Pulp Production in the equation above is expressed in bone dry weight [kg/s], i.e. the weight of 100 % dry pulp. The cooking yield is an estimated value of the cooking yield for the wood species that are being pulped. The estimated cooking yield for kraft pulping of soft wood is, e.g., 46%.

The mass flow of moisture wood chips conveyed to the impregnation vessel or the digester may, e.g., be measured by use of a chip conveyor scale (preferred) , a mass flow scanner (by using X-ray) or a radiating wood flow meter. The measured mass flow of moisture wood varies depending on the moisture content of the raw material and on other

properties, such as the density, of the raw material. In accordance with the second aspect of the invention, the pulping process is controlled by use of a ratio between the actual, measured, value of the density of the moisture fresh chips and an estimated value of the density of dry chips. The estimated value of the density of the dry chips depends on the wood species used. The density of moisture, fresh, chips may be determined by measuring the weight of the chips in the chip silo and dividing said measured value with the volume of the chips in the silo. If the process is a batch pulping process, the density of moisture, fresh, chips may be determined by measuring the weight of the chips in the batch digester and dividing said measured value with the volume of the chips in the batch digester.

The Wood Material Indicator (WMI) may be used to

continuously control the pulping process. This may, e.g., be achieved by regulating the chemical charge to the

impregnation vessel or the digester and/or the Liquor to wood ratio and/or the pulping temperature . The cooking alkali dose may be continuously regulated based on the WMI value. The Liquor to wood ratio may be regulated based on WMI by adjusting the internal circulation of spent cooking liquor or washing liquor in the digester. When the variation in raw material density is higher the wood material indicator can be used in combination with frequent analysis of the dry content of the chips conveyed to the digester as reflected by the third aspect of the present invention.

In such a way the possible variation from humidity and raw material density is separated from each other and this will further improve the precision of the process control if the variation in mass flow is dominated by raw material density variation. This is decided case by case on the conditions in the mill and used raw material.

The estimated dry mass flow is calculated by eq.2. The production in a continuous digester system is normally controlled by a volumetric measurement such as a chip feeding screw or a chip meter. Based on the known feeding volume per revolution, filling degree and estimated chip density the chip feeding device will rotate with a certain speed in order to maintain the target production (estimated dry mass flow) . By utilising the measured wet chip flow measured on the chip conveyor to the digester system in combination with an analysed chip dry content and the known volume (given by the rotation speed of the chip metering device) of chips fed to the digester system it is possible to calculate the measured wet chip density: Wet chip density = WCF / (V x FD x rpm)

Where WCF = wet chip mass flow (kg/min) V = fed volume per revolution (m ³ )

FD = filling degree rpm = rotation speed of the chip metering devise (rpm) The measured dry density is then calculated from:

Measured dry density = wet chip density x dry content.

In this case the WMI is defined as

WMI = Measured dry density/estimated dry density

WMI as described by the ratio above may then be used in the process control. The estimated dry density is dependent on which raw material is used.

Further the method according to the invention is used in a feed forward loop (manner) taking into account what is fed into the system (variation in both dry matter and moisture is combined; in the Nordic conditions where variation in chip bulk density is very low this has worked well) . The response to changes is further relatively fast.

In the following the invention will be described in more details by way of an example with reference to the drawings.

Description of the drawings Fig. 1 shows a schematic view of the flow of wood raw material from a chip pile to an impregnation vessel or to a digester. Total mass flow is measured preferably by using a chip conveyor scale. The raw material of fresh, moisture wood chips is conveyed on a chip conveyor (1) to a chip silo

(2) . The material is further conveyed from the chip silo (2) to an impregnation vessel or a digester by use of a chip feed screw (3) . The screw rotation of the chip feed screw

(3) is controlled based on the calculated estimated dry mass flow, i.e. it depends on the set value of the pulp

production. According to one example of the invention, a chip conveyor scale (4), arranged under the chip conveyor (1), measures the actual flow of fresh, moisture chips continuously. Since the mass of chips in the silo is held substantially constant, measuring of the flow on the chip conveyor gives an accurate value of the actual flow of fresh, moisture chips to the impregnation vessel or the digester. The measured fresh flow value is transmitted to a control unit (5) of a control system, (5) shown

schematically in fig. 3. The estimated dry wood flow is also entered into the control unit (5) . In the control system, a ratio between said flows is calculated (WMI) and used to regulate e.g. the set value for the alkali charge to the impregnation vessel or the digester and the liquid to wood ratio. Preferably, the process is further controlled on the basis of a theoretically calculated flow of wood chips to the impregnation vessel or the digester, i.e. in accordance with the traditional way of controlling a pulping process. Thus, WMI is preferably used to regulate a certain percentage on top of the traditional control of the process. A theoretically calculated set value (set-point) is thus also entered into the system.

Figure 2 shows an example how the WMI as set out above may be determined.

Figure 3 shows a control system in accordance with the invention. The control system may comprise one or several sub control systems for regulating the alkali charge, cooking temperature and/or Liquor to wood ratio for long term, e.g. 24 hours, and/or short term, e.g. 2 hours, variations of the WMI. In the control system (5) shown in fig. 3, the regulation is separated into two circuits; one for long term and one for short term variation of WMI. The system (5) may comprise further control units (6, 7) where further variables (vl, v2, ... Vn) or preset values may be entered for controlling the process, such as the upper and lower limits of alkali charge, the current values of the liquor to wood ratio, cooking temperature (H-factor) and alkali charge etc. Figure 4 shows more schematic how WMI according to the present invention may be used to control alkali dosage, which is only an example of a parameter which may be

controlled by using the WMI according to the present

invention .

Figure 5 shows the WMI control in action (c.f. the example below) . Example

A trial run was performed at the Oulu mill during 4 days where it was shown that WMI control reduces kappa variation.

WMI as a soft sensor appears to be usable and accurate enough to reduce the effects of raw material variations . The results are given below in Table 1.

With

No WMI WMI

control control

Devi Maxi Minim Devi Maxi Mini

Measurement Unit Average ation mum urn Average ation mum mum

Alkali to wood ratio % 19.1 0.10 19.2 19.0 19.0 0.05 19.2 18.8

Residual alkali extraction

[g/l] (average 2h) g/i 6.1 0.27 6.8 5.4 5.8 0.21 6.6 5.2

WMI value as % of DC in

wood (average 2h) % 47.7 0.47 48.7 46.9 48.0 0.41 49.0 47.3

On-line blow kappa value

from analyzer 27.1 1.27 31.5 24.4 26.7 0.74 28.8 25.0

167.

Cooking temperature °c 167.6 0.53 168.6 166.0 169.0 0.72 171.0 0

Cooking production t/d 1 123 6.98 1127 1099 1150 0.16 1150 1149

Table 1. Results from the 4-day Oulu trial. Each case period of 4d running. The raw material was softwood of Scandinavian origin (t/d=tonnes/day) . See also figure which reflects the example. C.f. also figure 2.

The benefits are given below in Table 2.

Benefits of using wood material

indicator (WMI) control

Change in residual alkali variation (black

liquor extraction) 22.1

Change in kappa variation (after digester

blow) 41.7 Table 2. The benefits from the 4-day Oulu trial.

Various embodiments of the present invention have been described above but a person skilled in the art realizes further minor alterations, which would fall into the scope of the present invention. The breadth and scope of the present invention should not be limited by any of the above- described exemplary embodiments, but should be defined only in accordance with the following claims and their

equivalents. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains. Thus the invention is therefore intended to cover various

modifications and equivalent methods included within the spirit and scope of the appended claims.

Any illustration and description in the drawings and in the foregoing description are to be considered exemplary and not restrictive .

Use of the word "comprising" in the claims does not exclude other elements or steps, and use of the article "a" or "an" does not exclude a plurality. Occurrence of features in different dependent claims does not per se exclude a combination of these features. Any reference signs in the claims are for increasing intelligibility and shall not be construed as limiting the scope of the claims.