CHAPTER 3 EXPERIMENTAL METHODOLOGY

Transcription

1 Chapter 3: EXPERIMENTAL METHODOLOGY 37 CHAPTER 3 EXPERIMENTAL METHODOLOGY 3.1 Chip Preparation and Screening White spruce chips, Picea glauca, obtained from Weyerhaeuser s forest management lands near Grande Prairie, Alberta were used in this study. The white spruce logs were run through the mill and the chips were collected at a conveyor belt waterfall right after the chipper. These chips were made with a Chip-N-Saw chipper (a chipping canter with 3 knife segments) which produces thin curved chips as shown in Figures 3.1 and 3.2. The chips were air dried (to approximately 90% solids content). They were classified using a ChipClass classifier, shown in Figure 3.3, as follows: oversized chips (chips that did not pass through 45 mm holes), overthick chips (retained on 10 mm slots), accept chips (retained on 7 mm holes), pin chips (retained on 3 mm holes) and fines (passed through 3 mm holes). Figure 3.4 shows the chip size distributions of this screening. The accept chips were further classified using a slotted screen to produce accepts of 2-6 mm in thickness. For each chip fraction, the average thickness, length and width of the chips were measured by randomly selecting 250 individual chips for measurement by hand, as shown in Appendix A. Only the accept (2-6 mm thickness) and pin fractions were used in our tests, as shown in Table 3.1. The oversize fraction would cause non-uniform packing in our small test vessel. The undersize fraction (fines) could not be cooked in the laboratory digester due to problems maintaining uniform liquor circulation and the risk of plugging the digester circulation loop.

2 Chapter 3: EXPERIMENTAL METHODOLOGY 38 Figure 3.1: The curvature and irregular thickness of the Chip-N-Saw chips. Figure 3.2: Side view of a piece of accept chip.

3 Chapter 3: EXPERIMENTAL METHODOLOGY 39 Flow resistance tests were made on four chip size distributions having varying quantities of pin chips, as shows in Table 3.2. This covered the range of 10-25% pin chips typically found in industrial operations. The mixture furnishes were obtained by blending the required mass fraction of accept and pin chips and mixing for 20 minutes. Over-sized (45 mm holes) Over-thick (10 mm slots) Accepts (7 mm holes) Pins Fines (3 mm holes) Figure 3.3: Laboratory chips size distribution analysis. Figure 3.4: Feed chip size distributions received from Weyerhaeuser.

4 Chapter 3: EXPERIMENTAL METHODOLOGY 40 Table 3.1: Average dimensions of accept and pin chips used in tests. Standard deviations are given. Screening (retained on) 100% accept fraction Average thickness (mm) Average width (mm) Average length (mm) Weight (%) 6 mm slot 6.2 ± ± ± mm slot 4.0 ± ± ± mm slot 2.7 ± ± ± % pin chips 3 mm hole 1.8 ± ± ± Table 3.2: Four Chip size distributions used in the experimental tests. Sauter mean diameter was measured for each mixture (see Appendix B). Weight percentage (%) Sauter mean Furnish accepts pins diameter (mm) Preparation of Cooking Wood Chips Standard bomb cooks (50 O.D. grams) were conducted to determine the kappa number vs. H- factor relationship for the white spruce chips with the pulping conditions listed in Table 3.3. The bomb cooks were done in six autoclave digesters. At the desired H-fa ctor, an individual autoclave was stopped and the cooked chips w ere removed and w ashed. The cooked chips were disintegrated in a Wennberg disintegrator for 10 m inutes and the pulp was screened using an 8- cut vibrating flat screen. The screen accepts were washed and de-watered in a vacuum box. The dry weight of both screened accepts and rejects were calculated to determine the pulp yield. Finally, the residual effective alkali (REA) of the liquor, the pulp screened yield and kappa number were measured. The r esults are shown in Appendix D. The kappa number of the

5 0 Chapter 3: EXPERIMENTAL METHODOLOGY 41 screened pulp was determined using modified procedure CPPA G.18. The average test repeatability is 1.2% for kappa numbers between 20 and 190. Using the above information, cooks of each furnish (Table 3.2) were prepared at kappa numbers of 24, 48 and 70 (which represents the variation of kappa number during a cook) using a 28 L Werverk laboratory digester (Paprican, Vancouver). We used conventional kraft cooking conditions to model those in the mill being studied, as shown in Table 3.3. Usually the values fall within the following ranges: 17-19% effective alkali on wood, 25-35% sulfidity, liquor-to- wood ratio of 3.5:1 to 4.5:1, and a min period of increasing temperature to a maximum cooking temperature of C (Smook, 1992). Cooking times are suitable to reach the target kappa numbers. After the cook, cooked chips were washed and removed from the digesters and transferred into a basket. The transferring process was done very carefully in order to protect the dimensions of the cooked chips. Then, the cooked chips were refrigerated at 4 C prior to tests. Figures 3.5 to 3.8 compare the chips before and after cooking to different kappa numbers. Some cooked chips were taken for kappa number determination by using CPPA G.18 method. According to CPPA G.18, with unbleached pulps, the average repeatability is 1.2% for kappa numbers between 20 and Table 3.3: Cooking conditions used in laboratory cooks. Wood species White spruce Wood chips charge 50 O.D grams (for autoclave small digester) 3.5 O.D. kg (for 28 L Werverk digester) Liqour to Wood ratio 4.5:1 Effective Alkali charge (EA) 17% as Na 2 O Sulfidity 29.4% Heating rate 1.9 o C/min Maximum cooking temperature 170 o C Time to Temperature 90 minutes Time at Temperature To desired H-factor

6 Chapter 3: EXPERIMENTAL METHODOLOGY 42 Before cooked, kappa number = 180 After cooked to kappa number = 70 After cooked to kappa number = 48 After cooked to kappa number = 24 Figure 3.5: Comparison of 100% accept chips before and after cooking to different kappa numbers. The measured Sauter mean diameter of the chips are 6.78 mm. Scale in the picture is in cm.

7 Chapter 3: EXPERIMENTAL METHODOLOGY 43 Before cooked, kappa number = 180 After cooked to kappa number = 69 After cooked to kappa number = 47 After cooked to kappa number = 24 Figure 3.6: Comparison of 100% pins chips before and after cooking to different kappa numbers. The measured Sauter mean diameter of the chips are 3.78 mm. Scale in the picture is in cm.

8 Chapter 3: EXPERIMENTAL METHODOLOGY 44 Before cooked, kappa number = 180 After cooked to kappa number = 65 After cooked to kappa number = 43 After cooked to kappa number = 23 Figure 3.7: Comparison of 75% accepts + 25% pins before and after cooking to different kappa numbers. The measured Sauter mean diameter of the chips are 5.66 mm. Scale in the picture is in cm.

9 Chapter 3: EXPERIMENTAL METHODOLOGY 45 Before cooked, kappa number = 180 After cooked to kappa number = 66 After cooked to kappa number = 42 After cooked to kappa number = 22 Figure 3.8: Comparison of 87.5% accepts % pins before and after cooking to different kappa numbers. The measured Sauter mean diameter of the chips are 6.17 mm. Scale in the picture is in cm.

10 Chapter 3: EXPERIMENTAL METHODOLOGY Equipment Experimental Setup A schematic diagram of the test apparatus for pressure drop measurements is shown in Figures 3.9 and The column has an inside diameter of 15 cm, a height of 45 cm and was made from 1.25 cm (0.5 inch) thick Plexiglas. It was equipped with a perforated plate (having an open area of 40% using 3 mm diameter holes), as shown in Figure 3.11, to compress the chip column and create uniform liquor flow through the chips. The system also consisted of a pneumatic cylinder (American Cylinder Co., Inc., Model 1500DVS-1½", Peotone, Illinois), a centrifugal pump (Cole-Parmer Inc., Model P , Concard, Ontario), magnetic flowmeter, two differential pressure transducers and a 25 L reservoir tank. Pressure drop and flow rate were recorded using an automated data acquisition system. It is assumed that there is no channeling or other wall effect throughout the tests. If the diameter of the packed column is greater than 10 times the diameter of particles, wall effects can generally be neglected (Winterberg and Tsotsas, 2000). In our studies, the larger possible Sauter mean diameter of particle is m and column diameter is 0.15 m. Therefore, the ratio is 22 which greater than reported value of 10.

11 Chapter 3: EXPERIMENTAL METHODOLOGY 47 Figure 3.9: Photograph of experimental setup.

12 Chapter 3: EXPERIMENTAL METHODOLOGY 48 air to pneumatic cylinder Vent valve overflow Magnetic flow meter Packed column 8L dp 32.5 cm Three-way valve 25 L Bypass Reservoir Pump Piping ½" Figure 3.10: Schematic of equipment setup.

13 Chapter 3: EXPERIMENTAL METHODOLOGY mm (1/2 in) 3 mm Inlet flow 12 mm 145 mm screw Figure 3.11: Schematic of a perforate plate at top view.

14 Chapter 3: EXPERIMENTAL METHODOLOGY Measurement Devices and Data Acquisition Pressure drop and flow rates were logged on a computer. The pressure transducers (Omega Engineering Inc., model PX26-001DV, Laval, Quebec) were connected to the data acquisition board. A power supply (Omega Engineering Inc., model PST-4130, Laval, Quebec) was used as a power source for the pressure transducers. A magnetic flowmeter transmitter was used to measure the flowrate (Rosemount Measurement, model 8712, Irvine, CA). Signal from the pressure transducers and from the magnetic flowmeter were acquired by a data acquisition card (National Instruments, PCI-1200 PCI board, Austin, TX) installed in a personal computer. Data acquisition software (National Instruments, LabView 5, Austin, TX) performed the data collection, averaging and logging. Readings were taken and averaged over ten second intervals. These averages were logged to the computer. Refer to Table 3.4 for addition information about the measurement devices and data acquisition system. 3.4 Experimental Procedure Cooked wood chips were filled carefully in the test column to an initial height of about 45 cm (e.g. Figures 3.12 and 3.13). The packing was done in a normal fashion, it means without applied any vibrator or shaken the column. Any of these disturbances could compact the bed. At the start of an experiment, water (average 23 0 C) from the 25 L reservoir was pumped upward through the column to remove any trapped air and to fill the column with liquid. Flow was then shifted through the flow-meter to the top of the column and allowed to stabilize before taking pressure drop data. During an experiment, the packed bed could be compressed by the piston to give compaction loads up to 18 kpa on the chip column. The applied load was kept constant during a test while the flowrate was changed in increments up to 10 L/min. These ranges covered those

15 Chapter 3: EXPERIMENTAL METHODOLOGY 51 encountered in operating digesters. Data were taken with increasing flow rate to avoid any hysteresis effects. Data Acquisition Magnetic Flowmeter Pressure Transducers Power Supply Table 3.4: List of measuring devices. Description The PCI-1200 PCI board has 4 differential analog input channels and 2 analog outputs. The voltage input range is software programmable for 0-10 V (unipolar) or ±5 V (bipolar). The single-channel sampling rate of the ADC is 100 ks/s. LabView 5 was used to log the data. A complete magnetic flowmeter system consists of two components: the Rosemount Model 8712C microprocessorbased magnetic flowmeter transmitter, and a Rosemount Model 8711 flowtube. The flowtube (0.3") is installed vertically in-line with process piping. Coils located on opposite sides of the flowtube create a magnetic field, and conductive liquid moving through the magnetic field generates a voltage that is detected by two electrodes. The transmitter controls the generation of the magnetic field and senses the voltage detected by the electrodes. Based on the sensed voltage, the transmitter calculates a flow rate and produces analog and frequency output signals proportional to this flow rate. The output signals are 4 to 20 ma. The system accuracy is ±0.5% of 25 L/min (±0.125 L/min). OMEGA s pressure sensors are four-active piezoresistive bridge devices. When pressure is applied, a different output voltage proportional to the pressure is produced. The voltage output from the transducer is 16.7 mv/v. The excitation voltage is 10 VDC. The accuracy is ±1% of 2 psid (±0.02 psid). PST-4140 is an AC line powered adjustable output power source. The voltage input is 115 Vac and the voltage output is 4 to 15 Vdc (adjustable).

16 Chapter 3: EXPERIMENTAL METHODOLOGY 52 Figure 3.12: 100% accept chips in test column ( kappa 48) at Pc = 0 kpa and void fraction of 0.60.

17 Chapter 3: EXPERIMENTAL METHODOLOGY 53 Figure 3.13: 100% pin chips in test column (kappa 48) at Pc = 0 kpa and void fraction of 0.59.

18 Chapter 3: EXPERIMENTAL METHODOLOGY Void Fraction Measurement Two methods were employed to determine the void fraction in the test column. There are the density method and the water displacement method Density Method In the test column (with no gas present) cooked wood chips occupy the volume fraction,, with the remaining fraction filled with water (external fluid), ε l, as shown graphically in Figure Here, ε c ε c + ε l =1 (3.1) Flow direction ε c + ε l =1 Represents wood chips Figure 3.14: Schematic of a representative volume element. Prior to cooking, the chip density, ρ c, was determined by using the water displacement method (CPPA standard A.8P) and found to be 368 ± 3 (O.D kg)/(m 3 green volume). When the

19 Chapter 3: EXPERIMENTAL METHODOLOGY 55 chips w ere cooked, density decreases due to removal of lignin and carbohydrate. To account for the decreased density following a cook, we multiply the uncooked chip density by the yield, ρ = Yρ ' c c (3.2) where Y is the fractional screened yield measured. It is noted that the chips were well washed to remove dissolved lignin and carbohydrate. of When the cooked wood chips are added to the column, they have a bulk packing density 0 ρ b (volume occupied by a given mass of ovendry cooked wood chips) (see Appendix I for sample calculation). Since the cooked chip column is compressed during a test, the bulk cooked p chip density under compressi on,, varies according to the equation, ρ b ρ P b 0 = H 0 ρ b (3.3) H p where H o is the initial height of the uncompressed column and H p is the height of the column when compressed. The volume fraction of the cooked chips in the column now becomes: ρ ε c = ρ P b ' c (3.4) where P ' ρ b is the bulk packing density of cooked chips under loading and ρ c is the density of cooked chips (Equation 3.2). Substituting Equation 3.4 into Equation 3.1 and rearranging, we get, ρ ε l = 1 ρ p b ' c (3.5) By substituting Equation 3.2 and 3.3 into Equation 3.5 we obtain:

20 Chapter 3: EXPERIMENTAL METHODOLOGY 56 H o ρ 0 b H p ε l =1 ( 3.6) Yρ c Sample calculation for determining the void fraction can be found in Appendix I Water Displacement Method First, the test column was filled with cooked wood chips. Water was added to the column. The column was divided into four zones of equal height, each having a volume, V t, of 1767 ml (see Appendix G). For each section, water was drained out and volume was recorded, V. The void fraction of the liquid is computed from: f V ε l = V f t (3.7) The comparison between the density and water displacement methods is found in Section Superficial Velocity Determination Superficial velocity is expressed as follows: Q U = (3.8) A where Q is the volumetric liquid flowrate (m 3 /s) and A is total cross-sectional area of the chip column (m 2 ). Sample calculatio n can be found in Appendix I.

21 Chapter 3: EXPERIMENTAL METHODOLOGY Pressure Drop Measurement The procedure used to measure pressure drop across the bed is as follows. The pre ssure drop in the bed is not measured over the whole depth, but between small tubes inserted a short distance from each end, their open ends facing at right angles to flow axis. This avoids errors due to the supporting grid, and other end effects, and enables the length over which the pressure loss determined to be accurately measured. Before the test run, we needed to determine pressure drop across the screen plate and was found by measuring the screen plate resistance at empty column with different flowrate. The screen plate resistance as a function of superficial velocity was shown in Figure During the test, the differential pressure transducer measures the pressure drops including: screen plate, water and bed, as shown in Figure In order to obtain the pressure drop across the bed only, we needed to subtract pressure drop across screen plate and pressure drop of water section from the measured pressure drop. Then, bed pressure drop is divided by the height of the bed, as follows: dp dl bed bed P corrected water = (3.9) L P bed where P corrected = Pmeasured Pscreen (3.10) and P screen = U (3.11) where Pmeasured is the measured pressure drop from the pressure transducer, Pscreen is the pressure dro p across the screen plate, Pwater is the pressure drop of water section, Pcorrected is

22 Chapter 3: EXPERIMENTAL METHODOLOGY 58 the pressure drop after subtracted the screen plate s pressure drop from measured pressure drop (kpa) and U is superficial velocity in mm/s. The pressure drop of water is calculated using the height of water, h water, between the upper pressure port and the chip bed, as shown in Figure 3.16, and the equation is P = ρgh (3.12) water water The height of the water section can be determined as h water ( m ) = X H (3.13) where X is 0.1 m as shown in Figure Equation 3.13 is only valid at 0 m H 0.35 m. Therefore, the height of the measured bed can be determined as L Bed ( m) = h (3.14) water Sample calculation for determining the pressure drop can be found in Appendix I.

23 Chapter 3: EXPERIMENTAL METHODOLOGY 59 1 Pressure Dr op acr oss S cr een P lat e (kp a ) y = x R 2 = Superficial Velocity (mm/s) Figure 3.15: Pressure drop across screen plate as a function of superficial velocity.

24 Chapter 3: EXPERIMENTAL METHODOLOGY 60 X h water L Bed H Figure 3.16: Pressure drop measurement technique.