DETERMINING THE DEGREE OF COOKING IN A SULPHITE DIGESTER DELIGNIFICATION

Abstract: To determine the degree of cooking in a sulphite digester used for delignification, the digester temperature (T) and pres­ sure (P d ) are sensed (12, 15) and utilised in a unit comprising a plurality of function blocks to calculate the digester reaction rate (k) according to the Arrhenius equation. This output is mul­ tiplied by a value proportional to the partial pressure (p SO₂) of sulphur dioxide in the digester to obtain a delignification rate value. The delignification rate value is integrated over time to determine the degree of cooking or Kappa Number. This num­ ber is compared to a desired set point value for the Kappa Num­ ber. When the actual Kappa number reaches the desired set point, a signal is produced which is provided to an operator. Function blocks spread over two controller modules are utilised to achieve the needed calculations in a simple manner without the use of an analog or digital computer.

Description: DETERMINING THE DEGREE OF COOKING IN A SULPHITE DIGESTER FOR DELIGNIFICATION This invention relates to methods of and apparatus for determining the degree of cooking in a sulphite digester for delignification. The invention can be used, for example, in the pulp industry. Lignin is the major noncarbohydrate constituent of wood. It functions as a natural plastic binder for the cellulose fibres. Its exact formula is unknown. Lignin is removed from wood by both the sulphite and sulphate pulp processes. It is known that the rate of delignification is a function of cooking liquor composition and cooking temperature, and that this rate increases rapidly with temperature and is affected by acid concentration. It is further known that the rate of delignification for acid pulping varies with temperature in accordance with the known Arrhenius equation. The velocity factor reaction rate (k) can be determined from this equation and subsequently substituted in the standard rate of delignification equation to determine this latter rate. The prior art is primarily concerned with processes associated with wood pulping delignification. US Patent No. 3 523 060 (Herdel et al) discloses a modification of the sulphite pulping processes wherein a very large quantity of sulphur dioxide is utilised and the delignification step is forced by utilising a very high temperature. US Patent No. 4 295 929 (Leithem) is directed to the same process, but in this case the proportion of combined sulphur dioxide used in the digestion process is varied as a function of the rate of heating. In essence, US 4 295 929 teaches that an increase in the proportion of sulphur dioxide used in the digestion process combined with an increase in the heating rate allows a considerable shortening of the total digestion time. US 3 523 060 and US 4 295 929 are thus directed to variations of the sulphite digestion process in order to shorten the total digestion time. US Patent No. 2 545 389 (Somer et al) discloses apparatus for increasing the sulphur dioxide content of the cooking acid used in the foregoing process. There is an inverse relation between sulphur dioxide content and total digestion time. US 2 545 389 is thus directed to the apparatus for increasing the sulphur dioxide content of the cooking acid rather than to the process itself. According to the invention there is provided a method of determining the degree of cooking in a delignification sulphite digester, the method being characterised by sensing the digester temperature, sensing the digester pressure, providing values corresponding to a plurality of constants including an energy of activity constant for the digester reaction and the gas constant, continuously calculating the digester reaction rate of the sulphite digester as a function of digester temperature and pressure and the constants, to obtain a value thereof over time, obtaining a value corresponding to a partial pressure of sulphur dioxide in the digester as a function of digester pressure and temperature, multiplying the partial pressure of sulphur dioxide value by the reaction rate value to obtain a delignification rate value, and integrating the delignification rate value over a time to establish a value corresponding to the degree of cooking in the digester. The invention also provides apparatus for accomplishing the foregoing method including individual function blocks which are connected together to achieve the various calculations. According to another aspect of the invention there is provided apparatus for determining the degree of cooking in a sulphite digester for delignification, the apparatus being characterised by: a first sensor for sensing a digester temperature; a second sensor for sensing a digester pressure; a first controller module connected to the first and second sensors and having means for providing a plurality of constants including an energy of activity constant for the digester reaction and the gas constant; means in the first controller module for continuously calculating a digester reaction rate of the digester as a function of digester temperature, pressure, and the plurality of constants to obtain values therefor over time; ; means in the first controller module for obtaining a value corresponding to the partial pressure of sulphur dioxide in the digester as a function of the digester temperature and pressure; at least one multiplier in the first controller module for multiplying the partial pressure value by the digester reaction rate value to obtain a delignification rate value; and a second controller module connected to the first controller module for receiving the delignification rate value and integrating the delignification rate value over time to obtain a Kappa value of cooking in the digester. A preferred embodiment of the present invention described hereinbelow provides a method and apparatus for calculating sulphite digester rate of delignification and determining the completion of cooking, utilising function blocks to continuously solve the so-called Arrhenius equation for the velocity factor reaction. This calculation takes place in the first controller module with the second controller module being provided with function blocks to obtain a value corresponding to the rate of delignification. The delignification rate is continuously computed and monitored, and when the cook has been brought up to a desired set point, an indication, such as an alarm, is provided to an operator to terminate the cooking process. By using simple function blocks, a digital computer, with its corresponding expensive programming and equipment, is avoided. An analog computer or device, known to have accuracy and flexibility problems, is also avoided. The preferred apparatus for and methods of determining the degree of cooking in a sulphite digester are simple in design, rugged in construction, and economical to manufacture. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of one embodiment of the invention showing the use of two modules having plural function blocks; Figure 2 is a block diagram of an Arrhenius equation module shown in the diagram of Figure 1; and Figure 3 is a curve showing reaction rate plotted against time, graphically illustrating how the area under this curve is calculated for establishing a degree of cooking in a sulphite digester. The drawings show apparatus embodying the Invention for determining the degree of cooking in a delignification sulphite digester. The rate of delignification is primarily a function of the cooking liquor composition and cooking temperature. Since there are established mathematical expressions for the rate of delignification, it is possible to determine how much cooking time is required based upon the cooking temperature for a particular pulp quality. See Pulp and Paper Manufacture, 2nd Ed., Volume I, The Pulpinq of Wood, pp 282 to 285. The rate of delignification increases rapidly with increasing temperature, but the effect is altered by the acid concentration. The rate of delignification for acid pulping varies with temperature in accordance with the Arrhenius equation: E log k = A - 2.303 RT (1) where: k = measure of the reaction rate; A = constant; E = energy of activity (approximately 21 Kcal); T = digester temperature (Kelvins); and R = gas constant (1.987 Cal/ C). The rate of reaction is a little more than doubled by an increase of 10 0C in temperature. Cooking is extremely slow at temperatures below 100 C. Figure 1 illustrates how calculation of the digester rate of delignification and Kappa (K) Number can be implemented in a Bailey NETWORK 90 (Trade Mark) system. The entire delignification rate and Knumber calculation can be accomplished using two NETWORK 90 controller modules 10, 30 for a particular digester. Information concerning the NETWORK 90 can be found in Bailey Controls Company Application Guide 260-2 and Bailey Controls Company Product Specification E93-906. In the first controller module 10, the Arrhenius equation can be continuously solved for the velocity factor reaction rate (k), by means of a calculator unit or module 23 shown in greater detail in Figure 2. The calculation is based upon digester temperature measurement as determined by a digester temperature transmitter 12 which provides one of a plurality of inputs to the unit 23. The rate of delignification (dL/dt) at any instant for a sulphite batch digester is expressed as follows: Rate = k (p 5 2) n (2) where: k = velocity factor reaction rate varying with temperature according to the Arrhenius equation; n = constant less than unity; and P SO2 = partial pressure of sulphur dioxide in the digester (digester pressure minus steam pressure at digester temperature). The partial pressure of sulphur dioxide in the digester is approximated by measuring the difference between steam pressure at the digester temperature and the actual digester pressure. This partial pressure is linearised and corected for the assumption that the digester pressure is that of only steam and sulphur dioxide in equilibrium with the digester acid. Steam pressure is calculated as a function of temperature in a unit 14 and the actual pressure is supplied by a digester pressure transmitter 15. The difference between the steam pressure and digester pressure is taken in a comparator unit 16. The n power factor in Equation (2) is provided by a function generator 17 and provides a means for compensating for the foregoing assumption. The constants A, E, 2.303 and R of Equation (1) are provided to the unit 23 by units 18, 19, 20 and 21, respectively. An output (log k) from the unit 23 is integrated to provide k and multiplied by the partial pressure output value from the function generator 17 to provide an output from the controller module 10 which represents the rate of delignification at any instant for the sulphite digester, in accordance with Equation (2), and is directed to the second controller module 30. The digester cook operator manually enters total 502 and free 502 acid strength chemical tests as inputs to the controller module 30 over units 32 and 33, respectively. The controller automatically subtracts the difference, which become the combined 502 content, in a comparator unit 34. The delignification rate can then have acid strength S02 correction applied over a function generator 35 and multipliers 36, 37. The delignification rate will be integrated for each cook, in a unit 38, with respect to time, for totalising the area under the reaction curve of Figure 3. This area totalisation is related to a degree of cooking (Kappa Number) set point for the particular grade. When the cook has been brought up to the desired set point, an alarm will sound for the digester cook operators final decision on when to terminate the cook. The equation for this calculation is as follows: e (A(E/2.303)RT) [Pd -Ps]n................... (3) where: Pd = digester pressure; and P5 = steam pressure. The deqree of cookinq (CD) is derived as follows: EMI6.1 (A-E/2.303)RT) EMI6.2 <tb> [Pd-Ps]n <SEP> # <SEP> <SEP> dt <SEP> .................... <SEP> (4) <tb> The equation used in actually obtaining the total area under the curve as shown in Figure 3 is as follows: EMI6.3 <tb> <SEP> (CD)N <SEP> = <SEP> S <SEP> (rate) <tb> N <SEP> (rate). <SEP> N <SEP> (5) <tb>

Claims: CLAIMS 1. A method of determining the degree of cooking in a sulphite digester for delignification reactions, the method being characterised by: sensing (12) the digester temperature (T); sensing (15) the digester pressure (P & providing values corresponding to a plurality of constants including an energy of activity constant (E) for the digester reaction and the gas constant (R); continuously calculating the digester reaction rate (k) of the sulphite digester as a function of digester temperature (T), digester pressure (Pd) and the plurality of constants, to obtain values there for over time; obtaining a value corresponding to the partial pressure of sulphur dioxide as a function of digester temperature (T) and pressure multiplying the partial pressure value by the reaction rate value to obtain a delignification rate value; and integrating the delignification rate value over time to obtain a Kappa number for cooking in the digester which corresponds to the degree of cooking (CD) therein. 2. A method according to claim 1, including calculating the digester reaction rate (k) according to the equation: E log k A 2.303 where A is constant. 3. A method according to claim 1 or claim 2, wherein steam pressure (P8) in the digester is calculated (14) as a function of digester temperature (T) and a difference is taken (16) between the steam pressure (P ) and the digester pressure (Pd), which difference is proportional to the partial pressure of sulphur dioxide. 4. A method according to claim 1, claim 2 or claim 3, including selecting a desired Kappa Number for a desired degree of cooking in the sulphite digester, comparing the desired Kappa number with the obtained Kappa number, and, when the desired Kappa number corresponds to the obtained Kappa number, generating a signal. 5. Apparatus for determining the degree of cooking in a sulphite digester for delignification, the apparatus being characterised by: a first sensor (12) for sensing a digester temperature (T); a second sensor (15) for sensing a digester pressure a first controller module (10) connected to the first and second sensors (12, 15) and having means (18 to 20) for providing a plurality of constants including an energy of activity constant (E) for the digester reaction and the gas constant (R); means in the first controller module (10) for continuously calculating a digester reaction rate (k) of the digester as a function of digester temperature (T), pressure (Pd), and the plurality of constants to obtain values therefor over time; ; means in the first controller module (10) for obtaining a value corresponding to the partial pressure of sulphur dioxide in the digester as a function of the digester temperature (T) and pressure at least one multiplier in the first controller module . (10) for multiplying the partial pressure value by the digester reaction rate value to obtain a delignification rate value; and a second controller module (30) connected to the first controller module (10) for receiving the delignification rate value and integrating the delignification rate value over time to obtain a Kappa value of cooking in the digester. 6. Apparatus according to claim 5, wherein the first and second controller modules (10, 30) include a plurality of function blocks each for conducting one of a comparison function, an arithmetic function, and a plurality of arithmetic functions. 7. Apparatus according to claim 5 or claim 6, wherein the first controller module (10) includes a first function generator (14) for receiving a signal from the first sensor (12) and generating a value corresponding to a steam pressure (pus) of the digester, a comparator (16) connected to the first function generator (14) and the second sensor (15) for obtaining a difference between the steam pressure (P ) and digester pressure (Pd), a second function generator (17) connected to the comparator (16) for generating a value corresponding to the partial pressure of sulphur dioxide in the digester, the second function generator (17) being connected to said at least one multiplier of the first controller module (10), and an Arrhenius equation unit (23) connected to the first sensor (12) and connected to the means (18 to 20) for providing the plurality of constants for generating the digester reaction rate value. 8. Apparatus according to claim 5, claim 6 or claim 7, wherein the second controller module (30) includes means (32, 33) for manually setting a percent total sulphur dioxide value and a percent-free sulphur dioxide value and a comparator (34) connected to the total and free sulphur dioxide setting means (32, 33) for obtaining a difference therebetween, the comparator (34) being connected in the second controller module (30) through a multiplier (37) for multiplying said difference with the delignification rate.

Inventors: JOHNSON, RALPH K., KAYA, AZMI, KEYES, MARION A. IV
Applicants: THE BABCOCK & WILCOX COMPANY
Application year: 1983
Main classification: D21C7
IPC codes: G01N31, D21C3