Reverse Osmosis and Ultrafiltration - American Chemical Society

24 Ultrafiltrative Solute Rejection Behavior of Black Liquor RAJNISH and P. K. BHATTACHARYA

Downloaded by YORK UNIV on July 2, 2012 | http://pubs.acs.org Publication Date: January 1, 1985 | doi: 10.1021/bk-1985-0281.ch024

Department of Chemical Engineering, Indian Institute of Technology, Kanpur-20816, India

Experimental studies were being carried out on the solute rejection behaviour of black liquor through ultrafiltration using a stirred batch cell. Influences of operating parameters viz. pressure (3-5 atm.), concentration of feed black liquor (1-5% t.d.s.) and stirrer speed (300-900 rpm) were studied on solute rejection behaviour at ambient temperature using different pore size (0.02 μm to less than 0.005 μm) cellulose acetate membranes. Concentration polarization model was used to evaluate the wall concentration by estimating the mass transfer coefficient through velocity variation method. The real rejection was always found to be more than the observed rejection and both vary similarly with pressure and concentration of black liquor. Recovery of inorganic chemicals from spent liquor, termed black liquor, is an integral part of the Kraft Pulping Processes. Infact, the economy of the Kraft Pulping Process is tied up with the efficiency of recovery cycle. With rising energy costs, increasing capital investment and environmental regulations, it has become more and more important to find energy efficient alternative methods to concentrate and fractionate industrial effluents. Ultrafiltration (UF) is one such favourable process. The present work was undertaken to study the separation of black liquor using UF as membrane process technique with the following objectives : i) To study the rejection behaviour of membranes towards black liquor/ ii) To analyse both flux and rejection data, iii) Effect of concentration polarization on transmembrane flux. 0097-6156/85/0281-O313$06.00/0 © 1985 American Chemical Society

In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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REVERSE OSMOSIS AND ULTRAFILTRATION

The development of high flux UF membranes (1-2) has resulted in increased importance of fluid-phase mass transfer resistances. Goldsmith (3») pointed out the importance of considering osmotic pressure effect at the membrane surface and analyzed the experimental data by solving equations from film theory :


He calculated mass transfer coefficient k using the equation obtained from phenomenological principles/ Equation 2, and then compared with theoretical values.

The difference between these two values was attributed to the concentration dependency of viscosity and diffusion coefficient. Membrane rejection/ or the ability to reject or retain a given solute is defined as (_4) :

The concentration at membrane surface C' is always higher than that in bulk/ Cl, because of concentration polarization. Therefore, corrected rejection characteristics of the membrane are described by real rejection/ R r e a l / a s •

Combining Equations 1 and 4 leads to :

Concentration polarization modulus/ (CJ/CJ)* ratio of wall to bulk concentration/ signifies the extent of concentration polarization present in the system. Velocity variation method is used to determine the mass transfer coefficient (5-6) . The value of k is usually a function of Reynolds number and can be expressed as:

In the stirred batch system, the velocity can be varied by changing stirrer speed and k can be found (!5) /

Using relation (Equation 7 ) , Equation 1 is rewritten as :


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RAJNISH AND BHATTACHARYA

Ultwfiltrative

Solute Rejection Behavior

315


for stirred batch cell, Equation 1 becomes

True rejection is given by extrapolation to axis of In [ (l-R0bs)/^obs] vs J2/4&0*7 plot. R real' ^ e m a s s transfer coefficient, k, is equation 1 and the effects of concentration can be corrected.

an ordinate Using this calculated by polarization

Experimental Concentrated Kraft black liquor was obtained from Central Pulp Mills, Surat and was kept in air-tight bottle. It was diluted with distilled water accordingly to get required concentration. Ultrafiltration cell, with magnetic stirrer and other accessories were imported from M/s. Schleicher and Schull, West Germany. Following specifications were provided for the cell : Model UP 110/0 Useful volume Filter diameter Maximum working Pressure Venting Valve

: 500 ml; Residual Volume : 107.5 mm; Effective filter : 7 atms; area Temperature : Set stability for between 5-7 atms.

: 0.5 ml : 70 cm 2 : 130°C

Strobotac (General Electric, U.S.A.) was used to find stirrer speed. It measured the rpm by matching the frequency of its flash light with stirrer speed. Moist type cellulose acetate membranes were obtained from M/s. Schleicher and Schull, Jest Germany with following specifications : Specifications :

Type

Cellulose Acetate -do-do-

AC-63 AC-62 AC-61

Pore size /am 0.020-0.010 0.010-0.005 0.005-0.002

Cut-off size 70,000 20,000 10,000

As recommended by manufacturer/ unused membranes were kept in a 25-30% solution of ethanol.



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Procedure. Membrane was put on a supporting porous plate/ placed at the lower part of the cell. A 200 ml of black liquor of known concentration was introduced into the cell/ simultaneously starting the magnetic stirrer. Black liquor feed was pressurized by pump through pressure storage vessel and was maintained/ for a particular set/ a constant value by adjusting the regulating valves in the pump. Permeate was withdrawn at atmospheric pressure and the volumes were collected and noted at different time intervals. After completion of the filtration/ permeate concentration was determined. Cell was dismantled/ washed and assembled for next run. i-Iembranes were always thoroughly washed with distilled water to avoid clogging of pores . Pressure (3-5 atm), concentration of black liquor (1-5/u total dissolved solids) and stirrer speed (300-900 rpm) were chosen as operating parameters and were varied to study trie influences on various characteristics of membrane transport along with different membranes. All the experimental data are available (2)• Results and Jiscussion Selection of Black Liquor Concentration. Black liquor concentration is an important consideration to study the solute rejection behaviour through microporous membranes. It is known (j8) that as the concentration of a solution increases, its activity and thus the osmotic pressure of the solution increases, resulting in lowering the solvent flux. It is also known (9.) that viscosity of black liquor increases significantly beyond 5p t.d.s./ which restricts the effective use of magnetic stirrer, used in the system. Black liquor, for its typical characteristics as macromolecular solutes of various sizes in alkaline solution/ is a difficult solution to handle for experimental work. A little higher concentration (more than 5% t.d.s.)/ gave experimental difficulties like choice of membranes/ clogging of pores and reuse of membranes. Hence, it was decided to use dilute black liquor of less than 5% t.d.s. (in the range of 1 to 5%) for experimental work. Selection of Aembranes. For ultrafiltrative studies/ the prime consideration in selection of membranes is to obtain a balance between the characteristics of feed liquor and separation ability of membrane. Laboratory membranes (2) gave casting problems viz. non-uniformity of membrane layer, lack of reproducibility of experimental data and low solute rejection (around 32%). All the experimental work were carried out with commercially available ultrafilters for .heir improved solute rejection and reproducibility of results.



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Ultrafiltrative Solute Rejection Behavior

317

affect of Feed Liquor Concentration and Applied Pressure on Solute Rejection, The objective of this study was to establish an analytical method of ultrafiltrative rejection data. A plot (Figure 1) was made between change in solute rejection of black liquor to pressure gradient for different feed concentrations with different membranes at a stirring speed of 300 rpm and ambient temperature, observed solute rejection (R0bs) w a s calculated according to liquation 3. It was observed (Figure 1) that with increase in pressure gradient/ solute rejection increases. However, with increase in feed concentration the solute rejection decreases significantly. It is known \10) that with increase in feed concentration/ wall concentration increases resulting in more solute flux. 3o the phenomena gets varified by experimental observation. However, increase in solute rejection with pressure contradicts the usual pattern (10) as rejection .should decrease with pressure. At higher pressure, accumulation of solute at the membrane surface increases/ raising the concentration gradient across the membrane/ resulting in more permeation of solute along with higher solvent flux, thus reducing the rejection. Therefore/ some additional mechanism is desired to explain such experimental observation. However/ this behaviour is not typical to black liquor feed only. Goldsmith (3) had also reported such behaviour for Cabowax (polyethylene glycol) solutions. It was explained (3) that the concentrated solution at membrane surface may act as a dynamic membrane. Since/ black liquor constitutes organics containing lower and higher molecular weight (1/000-20,000) macromolecules of alkali legnins, a slight increase in concentration of higher molecular weight species at the membrane surface may result in increased rejection for lower molecular weight solutes. Alternatively/ it can be assumed that an increase in operating pressure may lead to compaction of the membrane skin. Hence, the resultant decrease in pore size gave rise to increased rejection. Correction of Concentration Polarization. The observed rejection does not take into account the concentration polarization as this only considers bulk and permeate concentrations. Therefore, determination of wall concentration becomes an important aspect for the correction of concentration polarization. It was essential to evaluate the mass transfer coefficient, k, to determine the wall concentration, C^w, (Equation 1 ) . It was decided to compute k by the method of variation of velocity (ljO / as no diffusion data was available for black liquor because of its complexity in nature. For the evaluation of constant solvent flux, J2# experiments were carried out for small amount of permeate. Around 30 ml of permeate solution was collected from a 200 ml of feed solution. It was observed that for a



318


Figure 1.

Effect of pressure on observed rejection at 300 rprn.



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319

particular feed concentration and pressure/ the variations in solvent flux were insignificant even with varying stirring speed. Therefore/ solvent flux was assumed to be constant for a set of feed concentration/ pressure and type of membrane. A different set of experimental data at 5 atm. were obtained to compute the mass transfer coefficient/ k, for velocity variation method. Equation 9fwas utilized for the purpose. A plot (Figure 2) of In [(l-Robs^/^obsJ v s 1/ta0*7. was made. A linear relationship was observed and real rejection value/ Rreal' w a s evaluated from the intercept of this plot (Figure 2) . This value v/ould be independent of stirring speed and hence would represent the true rejection for a membrane. Mass transfer coefficient can now be evaluated by using Equation 1 after determining the wall concentration/ c { w / from Equation 4. Once the relationship between k anciO was found for different feed concentrations and membranes/ the values of Rreal w e r e calculated for all sets of data. The variation of Rreal with pressure for different concentrations is shown in Figure 3. The real rejection was always found to be more than the observed which signifies wall concentration more than the bulk concentration. Hence/ it concludes the presence of concentration polarization. It was also (Figure 3) clear that the real rejection increased with pressure but decreased with increase in feed concentration. This further verified the results obtained earlier (Figure 1 ) . Effect of Agitation on Solute Rejection, The tendency of accumulation of solute at membrane surface results in decrease in rejection. Therefore/ an increase in rejection can be obtained by increasing dissipation of concentrated boundary layer into bulk solution. The effect of agitation on solute rejection is shown in Figure 4 by generating data points from Figure 2. The trend of curves indicate the increase in rejection with stirring speed. Increased agitation caused the gradual dissolution of gel layer# with subsequent increase in solute rejection. Figure 4 shows that the slopes of curves are more for high feed concentrations (4.06% t.d.s.). This specifies the pronounced effect of stirring on solute rejection for concentrated solution as compared to dilute solutions (0.869% t.d.s.). Further# observed rejection does not change appreciably with stirrer speed beyond 800 rpm (Figure 4 ) . This specifies that at higher stirring speed R 0 bs approaches R r e al• Concentration Polarization Modulus. In order to study the concentration polarization further* variations in polarization modulus ^cj_,Vci) with stirrer speed were plotted (Figure 5 ) . Higher values of polarization modulus were observed at low stirrer speeds and with increase in



320


?igure 2.

Figure 3.

Relationship between observed rejection and stirrer speed.

Effect of pressure on real rejection at 3 00 rpm.



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Figure 4.

Figure 5.


Effect of variation in stirrer speed observed rejection at 5 atrn.

321

on

Effect of variation in stirrer speed on concentration polarization modulus at 5 atrn.


322


speed it decreased gradually. On extrapolation of curves for infinite stirrer speed, the modulus would approach to unity or in otherwords the wall concentration would come closer to the value of bulk concentration. It was also observed that polarization modulus increased with feed concentration for a particular stirrer speed. This signifies the prominence of concentration polarization at higher feed concentrations. Thus verifying the inference obtained from variation of observed rejection with stirrer speed (Figure 4 ) . Conclusions


i)

The real rejection was always found to be more than the observed rejection, signifying wall concentration to be higher than the bulk concentration, h e n c e , specifies the presence of concentration polarization. ii) Observed rejection does not change appreciably with stirring speed beyond 800 rpm, signifying closer approach to real rejection at higher speed. iii) A t higher feed concentration/ the prominence of concentration polarization was observed from the variation of polarization modulus with feed concentration. iv) Further work is being recommended for higher solute rejection for black liquor/ using low cut-off membranes with high pressure (upto 15 atm.) ultrafiltration cell. Mass transfer properties with diffusion data estimation is to be the aim of further work on ultrafiltration studies with black liquor. Use of model compounds(globular proteins or dextrans) is suggested for the initial characterization of the membranes as a quality control measure.

Nomenclature a,b Constants ~ « A Pure water permeability/ cm /cm .sec.atm. CJ Hi^h pressure side concentration of solute/ w t % C* Low pressure side concentration of solute/ w t % C.Vail concentration of solute, w t L ~ J2 Solvent flux through membrane/ cm /cm .sec. k Mass transfer coefficient/ cm/sec. P Pressure, atm. U Feed velocity, cm/sec. Ti Osmotic pressure, atm. O Stirrer speed/ rpm. Acknowledgments One of the authors (Rajnish) feels pleasure to acknowledge the co-operation of chemical engineering departmental staff for their assistance during laboratory work and to M r . Raj Khanna for efficient typing work.


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Literature Cited 1. 2. 3.


4. 5. 6. 7. 8. 9. 10. 11.

VanOss, J. In "Ultrafiltration Membranes"; Perry, E.S., Ed.; PROGRESS IN SEPARATION AND PURIFICATION Vol. III; Wiley-Interscience : New York, 1970. Kutowy, 0.; Thayer, W.L.; Sourirajan, S. Desalination 1978, 26, 195. Goldsmith, R.L. Ind. Eng. Chem. Fundam. 1971, 10, 113. Kimura, S.; Sourirajan, S. AIChEJ. 1967, 13, 497. Jonsson, G.; Boesen, C.E. Desalination 1977, 21, 1. Kimura, S. Bull, Soc. Sea Water Sci. Jpn. 1974, 27, 295. Rajnish, M.Tech. Thesis, I.I.T., Kanpur, 1983. Barrow, G.M. "Physical Chemistry"; McGraw-Hill, 1966. Swartz, J.N. In "Alkaline Pulping"; Macdonald, R . G . , Ed.; PULP AND PAPER MANUFACTURE Vol. I; McGraw-Hill, 1969. Baker, R.W.; Strathmann, H. J. Appl. Polym. Sci. 1970, 14, 1197. Nakao, S.; Kimura, S. J. of Chem. Eng. of Jpn. 1981, 14, 32.

RECEIVED February 22, 1985


Reverse Osmosis and Ultrafiltration - American Chemical Society

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