Pulp and Paper Industry (2 Ov)

Pulp and Paper Industry (3 credits)

Exercise 4: Beating of Chemical Pulp

(1) Introduction

1.1. Schopper-Riegler number (SR) SR number is a relative value for the filtration property of pulp. It is based on the reduction of filtration rate due to the formation of a fiber pad on the wire of the apparatus. The filtration rate decreases as the thickness of the fiber pad on the wire increases. Therefore, the faster the filtration rate decreases, the larger is the SR number of the pulp.

1.2. Specific Filtration Resistance (SFR) SFR number of the pulp may be determined by filtrating a slurry through a wire in a cylinder. The test is carried out by measuring the time required for a known volume of slurry to fitrate through a wire. The filtration rate decreases as a function of time as the thickness of the pulp pad increases and as the hydrostatic pressure caused by the slurry decreases.

1.2.1. Equation for the Specific Filtration Resistance

According to Darcy’s law, a general equation for a filtration process may be written as:

dV AdP   (1) d WR in which a liquid with the volume V filtrates in time  through a wire with a area A. dP is pressure drop through the wire,  is the viscosity of the liquid, W is the basis weight of fiber pad and R is a filtration resistance.

The hydrostatic pressure, dP, may be written as

dP=gh=gV/A (2) in which  is the density of the slurry and g is the gravitation constant (9.81 m/s2). Thus, Equation (1) may be written as:

dV gV   (3) d WR

For the basis weight, W, we may write:

(V V )C W  0 (4) A

dV AgV    (5) d (V0 V )CR

V Ag dV V0   (6) d  V  1 CR  V0 

V V V d Ag g V0 V0 V0   (7) d  V   V      V0  1CR  1Ch0 R V0  V0 

 V   V   1Ch R 0   0 1 Ch0 R d V0   V    (8) V V g d g V0 V0

a d V a  1  Ch R V   d  1  0 d a  V V   V /V  g V (9) 1 d 0 1  0  0 V0

Ch R a V V Ch R   0  ln  0 a  ln a 1 ln(1) a    (10) g 1 V0 V0 g  a g R  (11) Ch0 (a  ln a 1)

(2) Measurements

2.1. Beating time as a variant

In this exercise the effect of beating time on the filtrating properties of the pulp will be clarified. Using chemical pulp, prepare a slurry with a volume of 23 litres and a concentration of 15.7 g oven-dry pulp/l (See Exercise 1 for reference). Pour the slurry into the beater and start the beating. Take samples for the determination of SR and SFR after 5, 15, 30 and 60 minutes from the beginning of the beating. The required volume is 2 x 1.0 litres (concistency 2.0 g/l) for the determination of SR number and 250 ml (concistency 0.5 g/l) for the SFR. SR number is then reported as an average value for the two measurements.

For the determination of SFR a volume corresponding 0.5 g oven-dry pulp/l is needed. Pour water into the cylinder until the level is ca 10 cm above the wire. Pour the slurry into the cylinder and add water until the total volume above the wire is 1000 ml. The time required for a known volume to filtrate through the wire will then be measured.

2.2. Beating consistency as a variant

Prepare a slurry with a concistency of 7.85 g oven-dry pulp/l and with a volume of 23 litres. Pour the slurry into the beater and start the beating. Samples for the SR and SFR determinations will be taken after 5, 15, 30 and 60 minutes from the beginning of the beating. For the SR determinations the slurry must be diluted to the concistency of 2.0 g/l.

(3) Report

Beating time as a variant

Show graphically the evolution of SR as a function of SFR. How are the SR and the SFR related?

Beating consistency as a variant

Show graphically the evolution of SR as a function of SFR for the slurries with two different concentrations.

Show graphically the evolution of SR and SFR number as a function of time as well as Specific Energy Consumption, SEC, for the slurries with two different concentrations. In which way do the curves (function of time and function of SEC) differ from each other?