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Ultrafiltration of bleach effluents in cellulose production
Desalination,
115
79 (1990) 115-124
Ekvier SciencePublishersB.V.,Amsterdam
Ultrafiltration of Bleach Effluents in Cellulose Production hL4RIA DINAAFONSOand h4ARIANORBERTADE PINHO Chemical Engineeting Department, Instituto Supetior Tkcnico, 1094 Lisboa Codtx (Pomgal)
(ReceivedOctober29,1989;in revisedformApliill2,1990)
SUMMARY Ultrafiltration performance for colour removal in the effluents of the first caustic extraction stage in cellulose production has been investigated. The pH of the effluents is adjusted to 8.65. The operation ,is optimized with respect to polysulfone membrane cut-off, operating pressure and feed circulation rate. The dependence of product rate with pressure and of colour rejection with feed circulation rate is quantified. For a membrane of 10,000 cut-off, the colour removal is maximal and nearly independent of the feed circulation rate.
INTRODUCl-ION In the production of cellulose from wood the most commonly used process today is the kraft process, which produces a pulp that in most cases has to be bleached. In the bleaching plant the first alkali extraction stage (El) produces a dark NaCl brine with a high biological oxygen demand. Because of its high lignin content (about 60-70% of the total discharge of lignin derivatives [l] in the form of strongly coloured chlorinated lignin compounds), it is unsuitable for biological sewage treatment. Among the various separation processes for treatment of these bleach plant effluents, continuous multi-stage ultrafiltration (UF) is a technically and economically feasible colour removal process due to its simplicity, low energy requirements and specificity for removing high molecular weight coloured compounds [2-4].
OOll-9164/90/$03.50
0 1990 Elsevier Science Publishers B.V.
116
However, the UF of these effluents was only possible with the development of alkali-resistant membranes and among these the polysulfone membranes have been the ones most commonly used. However, in the case of eucalyptus pulping, very insoluble metal-ellagic acid complexes are always present in the effluents, leading to severe problems of membrane fouling [5]. The physico-chemical complexity of the effluents from the first caustic extraction and the great number of possible interactions with polysulfone membranes renders it practically impossible to predict the UF performance from results relative to UF of model systems like dextrans. Laboratory scale investigations of the optimal operating conditions should therefore be carried out with the industrial effluents so that an efficient scale-up can be made. In a previous work [7], it was concluded that membrane fouling could be reduced by solubilization of metal-ellagic acid complexes through a pH adjustment to the value of 8.65. In this work the effect of membrane cut-off and of operating conditions on the performance of ultrafiltration of the effluents from the first caustic extraction of a kraft pulping mill is studied.
EXPERIMENTAL
Efluent E, In the cellulose production utilizing eucalyptus wood, the tanin extracts (polifenols, sugars, etc.) susceptible to acid hydrolysis produce water soluble compounds like galic acid, glucose, elagic acid, pirogalol, etc. The bleach sequence consisting mainly of chlorination of residual l&in, alkali extraction with NaOH of chlorinated lignin and oxidation and whitening with chlorine dioxide, yields in a first alkali extraction the effluent Et composed of inorganic materials, chlorophenols, lignosulfonates, galic acid, elagic acid, chebulinic acid, pirogalol, etc. The type of bonding between these compounds and subsequently their solubilities are strongly dependent on the pH. In fact the elagic acid at pH > 10.8 forms very insoluble metal complexes. At pH between 7.5-10.8 the OH groups of the elagic acid become activated due to the destruction of the metal complexes. In a previous study [7], it was found that the performance of polysulfone, membranes is optimal at a pH of 8.65. Our experiments are conducted at this pH with the effluent having around 3400 unities of platinum colour.
117
The effluents are subjected to filtration with Millipore filters of 0.45 c in pore diameter. Membranes The commercial membranes are sulfonated polysulfone membranes with molecular weight cut-off of 10,000, 20,000 and 40,000, supplied by RhonePoulenc. They are characterized through a pure water permeability shown in Table I and a pore size distribution shown in Table II. The determination of the pore size distribution is described by Brites et al. [6]. TABLE I Membrane pure water permeability Membrane cut-off
Pure water permeability, Lp (kg h-’ m-* bar-l)
10,000
67.7
20,000
92.3
40,000
110.9
TABLE II Membrane average pore radius and standard deviation cut-off
RpX 10’
10,000
1.66
0.02
20,000
1.78
0.01
40,000
2.35
0.03
(m)
c&
Ultrafiltration The experimental set-up shown in Fig. 1 and procedure have been described previously [7]. A cross-section of the flat plate ultrafiltration cell is shown in Fig. 2.
118
-Fig. 1. Experimental set-up. 1. feed tank, 2. pump; 3. thermostated water bath; 4. flowmeter; 5. and 6. pressure gauge; 7. ultrafiltration cell; 8. product outlet.
Fig. 2. Ultrafiltration cell cross-section. 1. feed inlet; 2. feed outlet; 3. permeate outlet; 4. porous stainless steel plate (membrane support).
119
The hydrodynamic conditions are characterized through a Reynolds number (Re) defined with an average velocity at the outlet of the feed chamber (v& PQf Re = -=-
PVfsD
prh
(1)
cr
where
Q, = Vfs -
AS P I-1 Experiments
I;/f,4
average velocity at the outlet of feed chamber surface area of feed chamber outlet with diameter, D = 41.6 mm and height, h = 0.7 mm (Fig. 2) feed density feed viscosity are carried out at 25°C.
In each experimental run determinations are made of the pure water permeation rate (PWP) and of the product rate (PR), and feed and permeate samples are analysed for colour determination. Each experimental run for a given feed circulation rate lasted for four hours. In between experimental runs the membranes are washed with distilled water at 25°C till the PWP is regained within 5% of the initial value. When these values were not obtained, the membranes were washed with a solution of 250 ppm in NaOCl at a temperature of 25°C. Washing for four hours always resulted in the PWP recuperation within 5% of the initial value. The colour determination is based on Hazen definition of a unit colour for aqueous solutions of K,PtCl, with a concentration of 1 mg/l. The wave length of 400 nanometers is used for the absorbance measurements. The colour rejection is defined byf = [l - (C,/C,)] x 100 with Cf and Cp being the number of colour unities in feed and permeate, respectively.
RESULTS
The experimental data for the performance of membranes with a cut-off of 10,000, 20,000 and 40,000 are ‘ven in Tables III and IV. Feed circulation rates of 81, 102 and 123 1 h-!l corresponding respectively to Reynolds numbers above defined of 10.3 x 103, 12.9 x ld and 15.5 x lo3 were tested. In this range the permeate flux did not depend on the Reynolds number.
120 50
1.5
1.0
2.0
Prmssura (bar1
Fig. 3. Variation of permeate flux with transmembrane 10,000 - 0,20,ooo - v, 40,000 - cl.
pressure. Membrane cut-off:
In Fig. 3 the variation is shown of the permeate flux, J,, with the transmembrane pressure, AP. For each membrane the experimental points are fitted by a curve of the type
J” = [A+BApy-I’*
(2)
In Table V the fitting parameters A, B and n are shown for each membrane. Table VI shows the values of the ratios (aJ,/aAP)xl/Lp for the three membranes and at three different pressures. The experimental colour rejections are independent of the transmembrane pressure in the range of 1.0-2.0 bar. The variation of colour rejection with the feed circulation rate is shown in Fig. 4. The experimental data are fitted for each membrane by a straight line, F=C+DRe
with the values of C and D given in Table VII.
(3)
121
TABLE III Permeate fluxes Membrane cut-off
Pressure (bar)
Permeate flux (kg mm2h-‘)
10,000
1.0 1.5 2.0
43.3 60.0 68.6
20,000
1.0 1.5 2.0
46.4 54.5 58.5
40,000
1.0 1.5 2.0
56.2 65.5 69.7
Re
Colour rejection coefficient (!%)
10,000
10,300 12,900 15,500
65.8 63.5 66.7
20,000
10,300 12,900 15,500
59.9 61.5 63.8
40,000
10,300 12,900 15,500
56.6 56.4 58.5
TABLE IV Colour rejection coefficients Membrane cut-off
122
TABLE V Fitting parameters for the variation of flux with pressure Fitting parameters Membrane cut-off 10,000
A x 10’
B x 10’
n
2.6
2.7
2.5
20,000
29.2
11.8
2.0
40,000
20.6
8.1
2.0
TABLE VI Variation of flux with pressure Membrane cut-off
Pressure (bar)
10,000
1.0 1.5 2.0
0.163 0.074
20,000
1.0 1.5 2.0
0.154 0.059 0.028
40,OQO
1.0 1.5 2.0
0.150 0.057 0.027
0.389
For membranes with a cut-off of 10,000, there is nearly no variation of the colour rejection with the feed circulation rate. The same is not true for the other membranes.
123 TABLE VII Fitting parameters for the variation of &our
rejection with Reynolds number
Fitting parameters Membrane cut-off
c
D x lo4
10,000
63.32
1.56
20,000
52.22
7.37
40,000
52.41
3.66
‘; ..? .u = x 0
0
10000
66
.-s E z
Cut-off
0
2?
60
; S 8
2ooo
1
66 I
10000
12000
I
I
14000
16000
Roynoldn number
Fig. 4. Variation of colour rejection coefficient with Reynolds number. Membrane cut-off: lO,ooo- 0,20,ooo- v,40,000-0.
CONCLUSIONS
The membranes with a cut-off of 40,000 and 20,000 have low colour rejection factors, and membrane 20,000 already shows concentration polarization effects for the pressure of one bar.
124
In contrast with this situation and besides the expected higher colour rejections, membrane 10,000 presents good behaviour with respect to variation of permeate fluxes with pressure showing a more efficient increase of the flux with pressure (Table VI). For all these reasons, membrane 10,000 is chosen to operate at the pressure of one bar [(aJ,/aAP)/LP = 0.3891 and feed circulation rate of 81 l/h as the colour rejections did not show variation with tested feed circulation rates.
REFERENCES 1 2 3 4 5 6
0. Olsen, Desalination, 35 (1980) 291. E. Muratore, M. Pichon and P. Monzie, Revue A.T.I.P., 350 (1981) 367. H. Lundahl and I. Mansson, Tappi, 63(4) (1980) 97. J. Dorica, A. Wong and B.C. Garner, Tappi, May (1986) 122. A.W. Mckenzie, AJ. Pearson and J.R. Stephens, Appita, 21 (1967) 2. A.M. Brites and M.N. Pinho, Adsorption in Polysulfone Membranes, Proc. of 2nd Intern. Conf. on Separation Science & Technology, Hamilton, Canada (1989). 7 C.M. Silvestre, M.T. Pires, M.D. Afonso and M.N. de Pinho, The Use of Ultrafiltration for Color Removal from Bleach Plant Effluents, Proc. of 2nd Intern. Conf. on Separation Science & Technology, Hamilton, Canada (1989).
115
79 (1990) 115-124
Ekvier SciencePublishersB.V.,Amsterdam
Ultrafiltration of Bleach Effluents in Cellulose Production hL4RIA DINAAFONSOand h4ARIANORBERTADE PINHO Chemical Engineeting Department, Instituto Supetior Tkcnico, 1094 Lisboa Codtx (Pomgal)
(ReceivedOctober29,1989;in revisedformApliill2,1990)
SUMMARY Ultrafiltration performance for colour removal in the effluents of the first caustic extraction stage in cellulose production has been investigated. The pH of the effluents is adjusted to 8.65. The operation ,is optimized with respect to polysulfone membrane cut-off, operating pressure and feed circulation rate. The dependence of product rate with pressure and of colour rejection with feed circulation rate is quantified. For a membrane of 10,000 cut-off, the colour removal is maximal and nearly independent of the feed circulation rate.
INTRODUCl-ION In the production of cellulose from wood the most commonly used process today is the kraft process, which produces a pulp that in most cases has to be bleached. In the bleaching plant the first alkali extraction stage (El) produces a dark NaCl brine with a high biological oxygen demand. Because of its high lignin content (about 60-70% of the total discharge of lignin derivatives [l] in the form of strongly coloured chlorinated lignin compounds), it is unsuitable for biological sewage treatment. Among the various separation processes for treatment of these bleach plant effluents, continuous multi-stage ultrafiltration (UF) is a technically and economically feasible colour removal process due to its simplicity, low energy requirements and specificity for removing high molecular weight coloured compounds [2-4].
OOll-9164/90/$03.50
0 1990 Elsevier Science Publishers B.V.
116
However, the UF of these effluents was only possible with the development of alkali-resistant membranes and among these the polysulfone membranes have been the ones most commonly used. However, in the case of eucalyptus pulping, very insoluble metal-ellagic acid complexes are always present in the effluents, leading to severe problems of membrane fouling [5]. The physico-chemical complexity of the effluents from the first caustic extraction and the great number of possible interactions with polysulfone membranes renders it practically impossible to predict the UF performance from results relative to UF of model systems like dextrans. Laboratory scale investigations of the optimal operating conditions should therefore be carried out with the industrial effluents so that an efficient scale-up can be made. In a previous work [7], it was concluded that membrane fouling could be reduced by solubilization of metal-ellagic acid complexes through a pH adjustment to the value of 8.65. In this work the effect of membrane cut-off and of operating conditions on the performance of ultrafiltration of the effluents from the first caustic extraction of a kraft pulping mill is studied.
EXPERIMENTAL
Efluent E, In the cellulose production utilizing eucalyptus wood, the tanin extracts (polifenols, sugars, etc.) susceptible to acid hydrolysis produce water soluble compounds like galic acid, glucose, elagic acid, pirogalol, etc. The bleach sequence consisting mainly of chlorination of residual l&in, alkali extraction with NaOH of chlorinated lignin and oxidation and whitening with chlorine dioxide, yields in a first alkali extraction the effluent Et composed of inorganic materials, chlorophenols, lignosulfonates, galic acid, elagic acid, chebulinic acid, pirogalol, etc. The type of bonding between these compounds and subsequently their solubilities are strongly dependent on the pH. In fact the elagic acid at pH > 10.8 forms very insoluble metal complexes. At pH between 7.5-10.8 the OH groups of the elagic acid become activated due to the destruction of the metal complexes. In a previous study [7], it was found that the performance of polysulfone, membranes is optimal at a pH of 8.65. Our experiments are conducted at this pH with the effluent having around 3400 unities of platinum colour.
117
The effluents are subjected to filtration with Millipore filters of 0.45 c in pore diameter. Membranes The commercial membranes are sulfonated polysulfone membranes with molecular weight cut-off of 10,000, 20,000 and 40,000, supplied by RhonePoulenc. They are characterized through a pure water permeability shown in Table I and a pore size distribution shown in Table II. The determination of the pore size distribution is described by Brites et al. [6]. TABLE I Membrane pure water permeability Membrane cut-off
Pure water permeability, Lp (kg h-’ m-* bar-l)
10,000
67.7
20,000
92.3
40,000
110.9
TABLE II Membrane average pore radius and standard deviation cut-off
RpX 10’
10,000
1.66
0.02
20,000
1.78
0.01
40,000
2.35
0.03
(m)
c&
Ultrafiltration The experimental set-up shown in Fig. 1 and procedure have been described previously [7]. A cross-section of the flat plate ultrafiltration cell is shown in Fig. 2.
118
-Fig. 1. Experimental set-up. 1. feed tank, 2. pump; 3. thermostated water bath; 4. flowmeter; 5. and 6. pressure gauge; 7. ultrafiltration cell; 8. product outlet.
Fig. 2. Ultrafiltration cell cross-section. 1. feed inlet; 2. feed outlet; 3. permeate outlet; 4. porous stainless steel plate (membrane support).
119
The hydrodynamic conditions are characterized through a Reynolds number (Re) defined with an average velocity at the outlet of the feed chamber (v& PQf Re = -=-
PVfsD
prh
(1)
cr
where
Q, = Vfs -
AS P I-1 Experiments
I;/f,4
average velocity at the outlet of feed chamber surface area of feed chamber outlet with diameter, D = 41.6 mm and height, h = 0.7 mm (Fig. 2) feed density feed viscosity are carried out at 25°C.
In each experimental run determinations are made of the pure water permeation rate (PWP) and of the product rate (PR), and feed and permeate samples are analysed for colour determination. Each experimental run for a given feed circulation rate lasted for four hours. In between experimental runs the membranes are washed with distilled water at 25°C till the PWP is regained within 5% of the initial value. When these values were not obtained, the membranes were washed with a solution of 250 ppm in NaOCl at a temperature of 25°C. Washing for four hours always resulted in the PWP recuperation within 5% of the initial value. The colour determination is based on Hazen definition of a unit colour for aqueous solutions of K,PtCl, with a concentration of 1 mg/l. The wave length of 400 nanometers is used for the absorbance measurements. The colour rejection is defined byf = [l - (C,/C,)] x 100 with Cf and Cp being the number of colour unities in feed and permeate, respectively.
RESULTS
The experimental data for the performance of membranes with a cut-off of 10,000, 20,000 and 40,000 are ‘ven in Tables III and IV. Feed circulation rates of 81, 102 and 123 1 h-!l corresponding respectively to Reynolds numbers above defined of 10.3 x 103, 12.9 x ld and 15.5 x lo3 were tested. In this range the permeate flux did not depend on the Reynolds number.
120 50
1.5
1.0
2.0
Prmssura (bar1
Fig. 3. Variation of permeate flux with transmembrane 10,000 - 0,20,ooo - v, 40,000 - cl.
pressure. Membrane cut-off:
In Fig. 3 the variation is shown of the permeate flux, J,, with the transmembrane pressure, AP. For each membrane the experimental points are fitted by a curve of the type
J” = [A+BApy-I’*
(2)
In Table V the fitting parameters A, B and n are shown for each membrane. Table VI shows the values of the ratios (aJ,/aAP)xl/Lp for the three membranes and at three different pressures. The experimental colour rejections are independent of the transmembrane pressure in the range of 1.0-2.0 bar. The variation of colour rejection with the feed circulation rate is shown in Fig. 4. The experimental data are fitted for each membrane by a straight line, F=C+DRe
with the values of C and D given in Table VII.
(3)
121
TABLE III Permeate fluxes Membrane cut-off
Pressure (bar)
Permeate flux (kg mm2h-‘)
10,000
1.0 1.5 2.0
43.3 60.0 68.6
20,000
1.0 1.5 2.0
46.4 54.5 58.5
40,000
1.0 1.5 2.0
56.2 65.5 69.7
Re
Colour rejection coefficient (!%)
10,000
10,300 12,900 15,500
65.8 63.5 66.7
20,000
10,300 12,900 15,500
59.9 61.5 63.8
40,000
10,300 12,900 15,500
56.6 56.4 58.5
TABLE IV Colour rejection coefficients Membrane cut-off
122
TABLE V Fitting parameters for the variation of flux with pressure Fitting parameters Membrane cut-off 10,000
A x 10’
B x 10’
n
2.6
2.7
2.5
20,000
29.2
11.8
2.0
40,000
20.6
8.1
2.0
TABLE VI Variation of flux with pressure Membrane cut-off
Pressure (bar)
10,000
1.0 1.5 2.0
0.163 0.074
20,000
1.0 1.5 2.0
0.154 0.059 0.028
40,OQO
1.0 1.5 2.0
0.150 0.057 0.027
0.389
For membranes with a cut-off of 10,000, there is nearly no variation of the colour rejection with the feed circulation rate. The same is not true for the other membranes.
123 TABLE VII Fitting parameters for the variation of &our
rejection with Reynolds number
Fitting parameters Membrane cut-off
c
D x lo4
10,000
63.32
1.56
20,000
52.22
7.37
40,000
52.41
3.66
‘; ..? .u = x 0
0
10000
66
.-s E z
Cut-off
0
2?
60
; S 8
2ooo
1
66 I
10000
12000
I
I
14000
16000
Roynoldn number
Fig. 4. Variation of colour rejection coefficient with Reynolds number. Membrane cut-off: lO,ooo- 0,20,ooo- v,40,000-0.
CONCLUSIONS
The membranes with a cut-off of 40,000 and 20,000 have low colour rejection factors, and membrane 20,000 already shows concentration polarization effects for the pressure of one bar.
124
In contrast with this situation and besides the expected higher colour rejections, membrane 10,000 presents good behaviour with respect to variation of permeate fluxes with pressure showing a more efficient increase of the flux with pressure (Table VI). For all these reasons, membrane 10,000 is chosen to operate at the pressure of one bar [(aJ,/aAP)/LP = 0.3891 and feed circulation rate of 81 l/h as the colour rejections did not show variation with tested feed circulation rates.
REFERENCES 1 2 3 4 5 6
0. Olsen, Desalination, 35 (1980) 291. E. Muratore, M. Pichon and P. Monzie, Revue A.T.I.P., 350 (1981) 367. H. Lundahl and I. Mansson, Tappi, 63(4) (1980) 97. J. Dorica, A. Wong and B.C. Garner, Tappi, May (1986) 122. A.W. Mckenzie, AJ. Pearson and J.R. Stephens, Appita, 21 (1967) 2. A.M. Brites and M.N. Pinho, Adsorption in Polysulfone Membranes, Proc. of 2nd Intern. Conf. on Separation Science & Technology, Hamilton, Canada (1989). 7 C.M. Silvestre, M.T. Pires, M.D. Afonso and M.N. de Pinho, The Use of Ultrafiltration for Color Removal from Bleach Plant Effluents, Proc. of 2nd Intern. Conf. on Separation Science & Technology, Hamilton, Canada (1989).