Puu : Conventional and Non- Conventional Pulping as a Basis for Biorefinery (7 cr)

Transcription

1 Puu : Conventional and Non- Conventional Pulping as a Basis for Biorefinery (7 cr) Lecture 12: Conventional Alkaline Processes III Washing Equipment in Kraft Pulp Mills

2 Learning Objectives After this lecture the student *understands, and is able to explain, the phenomena taking place during pulp washing recognises and is able to describe equipment used in kraft pulp washing can explain how the theoretical washing principles apply to the described equipment understands the advantages and disadvantages of the different washing equipment and can make conclusions on why certain equipment is used at a certain point of the fibreline Puu

3 Modelling the mass transfer of washing displacement The above presented E-value concept is a kind of black box model that defines the washing process as a number of completely mixed ideal stages Washing can be considered by means of dispersion mass transfer Washing can be considered by semi-batch model, where the cake is divided several layers (thickness direction).

4 Mass transfer in pulp washing Dispersion mass transfer in pulp washing: modelling the displacement phenomena In the displacement washing, the cleaner washing liquid pushes (displaces) the more unclean liquid from the pulp mat This is the main washing mechanism in practically all pulp washing equipment During the displacement, there is always some mixing of the washing liquid and the unclean washable liquid (axial mixing) The axial mixing makes the washing response curve look like a skiing hill instead of the a sharp step-wise change The displacement washing can be modelled either by means of the E-factor concept or by using dispersion models

5 Mass transfer in pulp washing Dispersion mass transfer in pulp washing: modelling the displacement phenomena

6 Mass transfer in pulp washing The basic dispersion model of pulp washing: c t = D L 2 c 2 x - u c x Time-dependent term Axial dispersion c = concentration of the washable component D L = dispersion coefficient u = velocity of the displacing flow x = spatial coordinate t = time coordinate Convection term

7 Mass transfer in pulp washing Dispersion models: Introducing dimensionless parameters Dimensionless concentration C: C= Conc. of wash liquid Dimensionless length Z: Z = z L Initial conc. Dimensionless time, : Peclet number, Pe: Q = Pe = ut L ul D L Thickness of the pulp mat

8 Mass transfer in pulp washing Dimensionless dispersion model where the mass transfer between the liquid and the fibers has been omitted C Q = 1 Pe 2 Z C 2 - C Z Initial condition: Boundary conditions: C = 1, Q = 0 CPe C - = 0, Z = Z 0 (On the mat surface) C Z = 0, Z = 1 (At the bottom of the mat from where the filtrate escapes)

9 Mass transfer in pulp washing Breakthrough curves describe the solution of the dispersion model: 1 Dimensionless concentration, C 0,8 0,6 0,4 Pe = 2 Pe = 6 Pe =10 Pe = 20 0, ,5 1 1,5 2 2,5 3 3,5 Dimensionless time, Q

10 Segregated filtrate circulation: 2-stage, 3-sections (example)

11 Solute from the cake, 2-stage washing 1.00 Fraction of solute removed from cake P=7.5 P=2.0 P=1.0 P= Wash ratio

12 Change, 2-stage, P = Change in fraction of solute removed from cake Case 2 Case Wash ratio

13 W = Fraction of solute in the cake after washing Number of washing stages C1, P = 1 C2, P = 1 C3, P = 1 C1, P = 2 C2, P = 2 C3, P = 2

14 Observation of segregation Segregation of filtrates intensify displacement washing Improvement is the largest, when wash ratio is around one (DF=0) With segregations of filtrates less washing stages is needed in some cases

15 Mass transfer in pulp washing The significance of the Peclet number: The smaller the Peclet number, the more axial mixing and the lower washing efficiency The higher the Peclet number, the more sharp displacement and the higher washing efficiency The E-factor and the Peclet number are related to each other

16 Mass transfer in pulp washing The higher the wash liquid velocity => the higher dispersion coefficient => the lower Peclet number => the lower washing efficiency

17 Other mass transfer issues: Sorption Sorption, for instance for sodium Langmuir isotherm A*B*c/(1 + B*c) A unit kgna/bdt (for instance A= 4.04) B unit l/mg (for instance B = ) c unit mg/l A is maximum amount of sorped sodium. Parameters depends on ph, wood species, Kappa-number etc.

18 Other mass transfer issues: Leaching Organic material is leached out from a fiber phase to external liquid phase Rapid in the beginning, but can take several hours Depends on temperature, ionic strength of the liquid, wood species, kappa-number etc.

19 Properties of fibers and the fiber cake Charge of fibers pk a = 3.1 uronic acids HA 1 = 118 mmol kg -1 dry fiber pk = 5.6 carboxyls in lignin a and in extractives HA 2 = 21 mmol kg -1 dry fiber pk a = 9.0 phenolic hydroxyls in lignin HA 3 = 20 mmol kg -1 dry fiber Fiber Saturation Point (FSP) V f = 1.45 dm 3 kg -1 dry fiber Cake dimension area A = m 2 height L = 0.10 m Table 1. Pulp and cake properties (Räsänen, 2003).

20 Donnan equilibrium (idea) M + M 2+ I - M 2+ OH - M 2+ M + I - I - M + f M + M + H + s I -

21 Donnan equilibrium (Basic concept) )) 1 ( ( ) (,,,,,,. - + = = + = + = + «- + z f T s T k eq e eq e z eq f f e T s f k f e k e T k j j V V m n c c c V V V m c V c V n A H HA l l

22 Washing and Donnan equilibrium Advection-dispersion equation is applied to all solute components in the external liquid phase of the cake Fiber charges and acid groups and volume of the fiber phase (FSP) Electro neutrality in the fiber phase and in the external phase Donnan equilibrium included between the fiber phase and the external phase A time-dependent mass transfer included between the fiber phase and the external phase

23 Leaching and displacement washing test Leaching test: Oxygen delignified hardwood pulp. Sulfuric acid was added and the filtrate samples were taken in the different time. Displacement test: Oxygen delignified hardwood pulp. Wash liquid (ph = 1.8) was applied. The samples from the leaving filtrate were taken in the different time.

24 Leaching test 60 Na 0.4 Sodium/Magnesium/Calcium (mg/l) Ca Mg Mn Manganese (mg/l) Time (min) 0.0

25 Displacement washing: ph, filtrate ph Calculated Measured Dimensionless volume (V E /V E0 )

26 Displacement washing: Na, filtrate Na (mmol/l) Calculated Measured Dimensionless volume (V/V E )

27 Mg, filtrate Mg (mmol/l) Calculated Measured Dimensionless volume (V/V E )

28 ph inside the cake (simulation) ph V/VE=0.5 V/VE=1.0 V/VE=1.5 V/VE= Dimensionless thickness (x/l)

29 Washing equipment and their operational principles Digester washing: Continuous cooking Hi-Heat washing Displacement batch cooking (SuperBatch, RDH) terminal displacement Diffuser washing: Single stage atmospheric diffuser Two stage atmospheric diffuser Pressure diffuser

30 Washing equipment and their operational principles Drum filters: Vacuum filters Pressure filters Multistage displacement washers: DD-washers Flat wire washers Wash presses: Dilution-thickener presses (screw or roll presses) Displacement presses(twinroll-, Compact Presses, AWP)

31 Continuous cooking Hi Heat washing > 120 o C - Wash temperature o C, washing time 2 4 h, -E 10 -value at best > 10, typically 6 8, high capacity 3-5

32 The terminal displacement of the displacemen batch cooking Wash temperature o C Cooling of the wash liquid Washing time min E 10 -value = 2 3 for the dissolved dry solids The main purpose of the terminal displacement is the heat recovery which is clearly more efficient than the actual washing phenomenon (E 10 =5-6)

33 Diffuser washers: atmospheric diffuser - Feed consistency 8 12 %; oulet consistency about the same - The washing is pure displacement - Stepwise operation: - Screens up = washing stage -Fast return down when the screens are cleaned -E 10 -values: - One stage Two stages 6 8,5

34 Construction of the atmospheric diffuser: screen assembly

35 Continuous digester and a diffuser washer

36 Pressure diffuser - Washing can be performed in o C - Displacement in overpressure - Small footprint, stands outside - Otherwise the same operational principle as with atmospheric diffusers

37 Pressure diffuser: construction

38 Drum washers - Traditonal pulp washer type, still used in older pulp mills - Washing is based on thickening and displacment - Outlet consistency % - The required pressure difference either with vacuum or overpressure -E 10 -values typically 2-4

39 Multistage washing: DD-washer In the same washer there can be 1 4 Washing stages

40 Multistage washing: DD-washer In the same washing apparatus 1 4 countercurrent washing stages can be performed Feed consistency either 3 4 % (LCDD) tai 8 10 % (MCDD) Max. capacity in the range of 6000 ton/d (one stage) E 10 -value= (4-stages) = 5 10 (1 2-stages) Washing efficiency can be improved with the so called segregated washing

41 Typical layout of a DD-washer Pulp storage tower Deknotting Washer Filtrate tanks Dilution line Feed pump

42 Wash presses Twin roll presses (Andritz, Valmet, GL&V) Screw presses (eg. Andritz,Thune, Krima) Belt presses (Andritz)

43 TwinRoll TM Family Washed pulp Feed pulp Feed pulp Filtrates out Wash liquid

44 TwinRoll-press Washed pulp Feed pulp Wash liquid Filtrate

45 Compact Press (GL&V)

46 TwinRoll Evolution (Valmet)

47 AWP (Andritz Wash Press)

48 Wash presses One-stage displacement-thickening Feed consistency typically 3 7 % Outlet consistency % E 10 -value = A press is well applicable especially at the end of the washing line where the clean washing water is introduced. The demand of washing water is small due to the high discharge consistency. Water lock.

49 Summary of the washing equipment Washer type Feed cons., % Outlet cons., % E 10 -value Supplier Continuous cooking Hi- Heat washing Andritz, Valmet Displacement batch Terminal displacement Atmospheric diffuser (1-stage) GL&V, Valmet Andritz, Valmet Atmospheric diffuser (2-stage) Andritz, Valmet Pressure diffuser ,5 5,5 Andritz, Valmet DD-washer (4-stage) DD-washer (1,0-2-stage) Twinroll-press. Compact press 4 4, Andritz Andritz GL&V, Valmet Dewatering Press Andritz, Valmet Drum filter Many suppliers

50 Washing systems Suggested values for the Total washing efficiency: E 10 -value = before the O 2 -stage (depends on wood) E 10 -value = 6-8 after the O 2 -stage COD-wash loss suggested values: Before the oxygen stage < 100 kg/ton pulp Into bleaching < 10 kg COD/ton pulp Dilution factor: 2 3 m 3 /ton pulp The selection of the washing equipment is based on many factors: investment costs vs. washing performance, capacity, layout, operational availability etc.