Membrane separation processes in pulp and paper production

P R O C E E D I N G S OF T H E F I L T R A T I O N S O C I E T Y

Membrane Separation Processes in Pulp and Paper Production By M D Afonso & M N Pinho

InstitutoSuperiorTecnico, 1096LisboaCodex,Portugal This paper was presented to the Vth World Filtration Congt'ess, Nice, France, June, 1990

In cellulose production utilising the kraft process the bleaching plant is the major factor responsible for pollution problems. In fact, the strongly coloured compounds are mainly c o n c e n t r a t e d in the effluents of the first alkali extraction stage (El). T h e large a m o u n t of work devoted to the utilisation of ultraflltraUon (UF) for colour removal from t h e s e effluents is mostly relative to ultrafiltration using organic m e m b r a n e s and more specifically polysulphone m e m b r a n e s . Due to the

fs°ut~l~ognPer°mbelend:Pn~Sedhi: :::s ie°t~Sw:71~ka(dd reW~tsh:~/Ye utilisation of mineral m e m b r a n e s for microfiltration and ultrafiltration of bleach plant effluents. Investigated is the a d v a n t a g e of a s e q u e n c e of microflltration and ultrafiltration steps with respect to the direct use of ultrafiltration.

organic carbon and total solids removal is shown in Table 2, Figs 3 and 4. The optimal colour removal is 75.6% at a feed circulation velocity of 1.26m/s. The pressure ranged from 1 to 5 bar without showinginfluence on the colour removal. For the feed circulation velocities of 1.76m/s and 0.65m/s the TOC removal increases linearly with pressure; f'roc = 4.73 P + 30.53

(1)

For the intermediate velocity of 1.26m/s the TOC removal decreases with pressure: fTOC = -2.88P + 61.30

(2)

but yields higher removal for low pressures (Fig 3). The TS removal is not influenced by the feed circulation velocity and increases linearly with pressure upon the equation: f'rs = 1.86P + 9.59

Experimental Membranes. The Carbosep membranes supplied by Techsep are monotubes made ofporous carbon with an active layer of zirconium oxide. The internal diameter is 6 x 10-3m, the length 0.6m to give a membrane surface area of 0.0120m 2. The membranes used are a microfiltration type with an average pore diameter of 0.141xm and an ultrafiltration type with a cut-off of 10,000. Microfiltration/ultrafiltration. Fig 1 shows a schematic diagram of the experimental set-up. The effluents from the first caustic extraction of a kraft pulping mill are filtered for removal of coarse solids. In each experimental run the permeate fluxes are determined and feed and permeate samples are taken for analysis of colour, total organic carbon (TOC) and total solids (TS). For each of these parameters a separation, f, is defined by:

(3)

according to Fig 4 Tangential microfiltration at the pressure of 1 bar yieldspermeate rates of 28.7kg/m2h with colour removal of 32% and TOC and TS removals of 33% and 14% respectively. The ultrafiltration of the

Table 1. Variation of ultrafiltration permeate fluxes, Jv (kg/m2h), with pressure P (bar) Feed circulation

velocity(m/s)

Curvetiffing

,:o,4,

f = c f - c 0 x 100(%)

1.26

Cf

°"

Jv=(40.O5+2.53P)

[1+

(0'~--73+0.011)°95]

-l°s

]_o,,

where c represents the concentration relative to each of these parameters and the subscripts f and p refer respectively to feed and permeate streams. The colour determination is based on the Hazen definition of a unit colour for aqueous solutions of K2 Pt Cl6 with a concentration of 1mg platinum/I in the form of chloroplatinate ion. The absorbance measurements are made at a wavelength of 445 nanometers. The total organic carbon measurements are made using the equipment Dohrmann model DC/85 A. The total solids content is determined through weight difference of samples evaporated at 100°C. The membranes are cleaned using an alkali-acid sequence proposod by the manufacturer. The experiments are carried out at a pH of 8.6 and temperature of 25°C.

100

m

•

.

v =

1.76

m/s

•

80

Results and Discussion The results relative to ultrafiltration permeate fluxes are shown in Fig 2. There is a strong influence of the feed circulation velocity and, for the low velocity, the limiting flux is attained for very low pressure values. The fitting of the experimental points(a) yields the curves presented in Table 1. The ultrafiltration performance with respect to colour, total

i N

E so

_s-------'-

x ,.I LL kM

I-

40 ~lk

I

I

20

2

I 1

5 8

Fig 1. Experimentalset-up 1, Feed tank, 2, Pump, 3, Valve, 4, Flowmeter, 5, Manometer, 6, Tubular membrane, 7, Concentrate, 8, Permeate

42

v = 0.65 m/s

M,

I 2

I 3

I 4

I ' 5

PRESSURE (bar) Fig 2. Ultraflltrationpermeate fluxes vs pressure • - v = 1.76m/s;• - v : 1.26m/s; • - v : 0.65m/s January/February 1991

Filtration & Separation

PROCEEDINGS

OF THE FILTRATION

SOCIETY

all

Y

what is needed nothing else 1

2

6

4

3

PRESSURE

(bar)

Fig 3. Separation of TOC VI pressure for different feed clrculatlon velocities W - v = 1.76mls; 0 - v = 1.26mk.; A - v = 0.65mls

1

PRESSURE

6

4

3

2

(bar)

Fig 4. Separation of TS vs pressure for different feed circulation velpcities 0 - v = 1.76mls; 0 - \I = 1.26mk.; A - v = 0.65mls

Table 2. Ultrafiltration

colour removal

Feed circulation velocity hllld

Separation I%1

1 76 1 26

62 1 75.6

Table 3. Variation of ultrafiltration permeate (kg/m*h!, with pressure, P (bar) Microfdtration pretreatment Feed circulation velocitv (m/s1

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fluxes, J,

Curve fittina 4.

1.76

J, = (227P + 66.90)

1.26

J, = -2.93P’

0.65

J,=

++

0.052

>1 FILTRATION DIVISION

+ 26.7OP + 2 20

-1.54P’+

-025

APPIAN0 G.- COMO- ITALY

18.1OP+1.79

S.pA. - 22070 Appieno Gentile (Como) - Italy - Vi Milan0 14 Tel. 031/930955 - TX 380192 SAATl - Telefex 0311933392 Table 4. Ultrafiltration colour removal microfiltration pretreatment Separation I%) 36.5 57 6 45.3

1.76 1 26 0.65

Filtration & Separation

January/February 1991

43

PROCEEDINGS

OF THE FILTRATION

SOCIETY

I 1

2

PRESSURE

4

3

I

1

I

I

I

1

2

3

4

5

PRESSURE

(bar

5

j

(bar) Fig 6. Separation

Fig 5. Ultrafiltration permeate fluxes (with MF pretreatment) vs pressure n - v = 1.76mis; 0 - v = 1.26mls; A - v = 0.65mls

permeate of microfiltration yields the results shown in Fig 5. The curves fitted through the experimental points are presented in Table 3. The performance of the sequence of microfiltration and ultrafiltration with respect to colour, TOC and TS removal is shown in Table 4,, Figs 6 & 7. A maximum value is obtained for an intermediate feed cuculation velocity. The TOC removal is lower at the highest feed circulation velocity and varies with pressure upon the linear relation: f.,oc = 1.37P + 23.05

(4)

of TOC vs pressure for different velocities (with MF pretreatment)

feed

circulation

n - v = 1.76m/s; 0 - v = 1.26m/s; A -v = 0.65m/s

_-_-:::II; 1

3

2 PRESSURE

Fig 7. Separation

n- v=

of TS -vs pressure for different velocities (with MF pretreatment) 1.76mk;

0 - v=

1.26m/s;

5

4

(bar) feed

circulation

A - v = 0.65m/s

For the feed circulation velocities of 0.65m/s and 1.26m/s the variation is given by: f.roc = 1.91P + 32.90 (5) The total solids removal increases linearly with pressure, being: fTs = 1.56P + 10.69

(6)

for the feed circulation velocity of 0.65mk and: fTS = l.llP

+ 4.74

(7)

for the feed circulation velocities of 1.26m/s and 1.76m/s.

Conclusions

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44

PARTS

The introduction of a microfiltration step before ultrafiltration results in a better ultrafiltration performance as the limiting fluxes are not reached for the entire pressure range (O-5 bar) and for the tested feed circulationvelocities. The removal of colour and total organic carbon is higher at the intermediate feed circulation velocity of 1.26m/s and the pressure of 4 bar. The removal of the total solids increases with pressure. Although the feed circulation velocity does not influence this removal if ultrafiltration is done directly, the same is not true when microfiltration is introduced as a pretreatment. In this case the removal is improved by lower circulation velocities. REFERENCES I. Silvestre. CM, Pircs, M T. Afonso. M D and Pinho. M N. ‘The USC of Ultrafiltration for Colour Removal from Bleach Plant Effluents’. Proceedings of 2nd International Conference on Separation Science & Technology. Hamilton. Ontario. Canada. 19X9. 2. Afonso. M D and Pinho, M N. ‘Ultrafiltration of Bleach Effluents in Cellulose Production’: Proceedings of 6th International Symposium on ‘Synthetic Membranes in Science and Industry’. Tubingcn. RFA. 19X9. 3. Churchill. S W and Usagi. R. ‘General Expression for the Correlation of Rates of Transfer and Other Phenomena‘; AIChE Journal, Vol IX. No. 6. November 1972.

January/February

1991

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81 Separation