In the Laboratory
Tangential Ultrafiltration of Aqueous Saccharomyces cerevisiae Suspensions Carlos M. Silva,* Patrícia S. Neves, Francisco A. Da Silva, and Ana M. R. B. Xavier Departamento de Química, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal; *
[email protected] M. F. J. Eusébio Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Monte de Caparica, 2829-516 Caparica, Portugal
Membrane separation systems have a wide range of applicabilities, from purifications to bulk separations and have been used extensively in the chemical process industry. Their use in the food industry is also becoming more common. Some examples of commercial uses of ultrafiltration (UF) processes are cited to illustrate their importance: (i) concentration and fractionation of proteins; (ii) preconcentration of milk before making cheese; (iii) clarification of wine and fruit juices; (iv) recovery of vaccines and antibiotics from fermentation broth; (v) separation of wax components from lower molecular weight hydrocarbons; (vi) separation of oil–water emulsions; (vii) removal of polymer constituents from wastewaters; (viii) pre-treatment step for reverse osmosis and nanofiltration; (ix) maintain high cell-density in continuous fermentation broth by cell recycling; (x) color removal from Kraft black liquor in paper-making; and (xi) yeast removal after beer fermentation, commonly accomplished by sterilizing filtration. There are some educational publications concerning membrane process engineering; however, they usually focus on design projects, theoretical analysis, problem solving, and homework exercises (1, 2). We present an experiment on UF especially designed to illustrate its practical and theoretical principles. The laboratory exercise comprises experiments with pure water and aqueous Saccharomyces cerevisiae (Baker’s yeast) suspensions. An UF setup and an experimental procedure to measure important process variables and to detect the characteristic phenomena is described. The laboratory takes six hours. Students are divided into groups of three. In the first week students carry out the lab exercise and in the second week students work on the calculations and simulations, which usually require computational support. Theoretical Background In a membrane separation process, a mixture consisting of two or more components is partially separated by means of a semipermeable barrier (membrane) that rejects some solutes or through which one or more species move faster than other species. This operation is shown schematically in Figure 1 where a feed mixture is separated into a retentate and a permeate: streams retained by and passing through the membrane, respectively. The sweep, which is optional, is a liquid or gas used to help remove the permeate. The transmembrane flux of species i, Ni , is given by (3, 4):
130
Ni
PM, i lM
PM,i t driving t driving force force
(1)
where PM,i is the permeance, PM,i is the permeability, and lM is membrane thickness. Ultrafiltration is primarily a size-exclusion separation process whose driving force is transmembrane pressure (Δ P). UF membranes normally have pore sizes in the range from 10 to 1000 Å and are able to retain components in the mass range of 300–500,000 Da. Typical values for fluxes are 10–50 L∙(m2 h) for applied pressures of 1.0–5.0 bar. Inserting ΔP in eq 1 and subscript “w” for water (assuming that only solvent permeates freely) gives (3, 4):
Nw
PM,w %P PM,w %P lM
(2)
The performance of a membrane system is often described in terms of retention factor, separation factor, and molecular weight cutoff. In the case of dilute liquid mixtures consisting of solvent and solute it is more convenient to express selectivity in terms of the retention towards the solute, Rf, because solvent molecules pass freely through the membrane. Accordingly,
Rf
C F CP CF
(3)
where CF and CP are the solute concentration in the feed and permeate streams. Rf varies between 1 (perfect solute retention) and 0 (both components permeates membrane freely). When we feed a suspension or a macromolecular solution and apply a small pressure across a UF membrane, we get a low flux. Increasing ΔP increases Nw , but only to a certain point. It rapidly levels off to a maximum, Nw,∞, that can be one or two orders of magnitude smaller than would be obtained with pure water at the same pressure. As Figure 2 shows, such penalization is more pronounced at higher solute concentrations in the bulk feed side of membrane, Cb. This phenomenon is called polarization and is a common feature of all pressure-driven membrane processes. Polarization causes solute concentration near the membrane to rise sharply until a gel layer of concentration Cg is formed, at membrane module feed
retentate
sweep
permeate
Figure 1. Schematic representation of a membrane process.
Journal of Chemical Education • Vol. 85 No. 1 January 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory Nw
pure water (Cb = 0)
thermometer PF
Cb1
Nw,e1
feed
membrane module
PR VR
VF
thermostatic peristaltic bath pump PP
Cb2
Nw,e2 Cb2 > Cb1
VP
%P Figure 2. Relationship between flux and transmembrane pressure in ultrafiltration.
which point the flux shows no further increase with pressure. The limiting flux, Nw,∞, depends exclusively on Cb and on feed flow outside and along the membrane. It is calculated by
N w , e k f ln
Cg CP C b CP
(4)
where kf is the convective mass transfer coefficient. For turbulent flow, the mass transfer coefficient, and therefore Nw,∞, is more-orless proportional to 0.8 power of outside velocity. Experiments under tangential flow achieve steady-state conditions since the gel layer does not builds up indefinitely as in a deadend pattern. The high shear exerted by the suspension flowing tangential to the membrane surface sweeps the deposited particles toward the capillary exit so that the gel layer remains constant and relatively thin. For complete retention of solute, Cp = 0 and eq 4 simplifies to the trivial form: Cg N w , e k f ln (5) Cb According to eq 5, Cg and kf may be computed from nonlinear regression of (Cb, Nw,∞) data points. For the particular case of only two measurements, the following analytical solution applies:
kf
Cg
N w , e1 N w , e 2 ln C b 2 C b1
C b 2 N w , e1
1 N w , e1 N w , e 2
(6)
In this article we present a laboratory work on UF of aqueous Saccharomyces cerevisiae suspensions that comprises two different runs: 1. Experiments with pure water to determine solvent permeance.
2. Experiments using commercial Baker´s yeast suspensions of different concentrations to detect polarization and
electronic balance
Figure 3. Schematic of the experimental setup.
gel layer formation and to observe the existent relation between plateau Nw,∞ and Cb.
Experimental Section The schematic of the experimental setup is shown in Figure 3. Students start the experiments with pure water, measuring Nw for several fixed transmembrane pressures (ΔP) to determine water permeance. They then prepare two Saccharomyces cerevisiae suspensions of known concentrations to carry out experiments to detect polarization and gel layer formation and calculate Cg and kf . Several runs must be performed to study the effect ΔP has upon permeate flux, Nw. At the end of lab session the UF membrane must be cleaned with Ultrasil to prevent membrane fouling. The detailed experimental procedure is available in the online supplement. Hazards There are no significant hazards associated with this experiment. Saccharomyces cerevisiae (Baker’s yeast) is a food product and Ultrasil is a liquid combination of stabilized enzymes and surfactants for membrane cleaning in food industry. It is recommended to use gloves during the experimental work to protect skin. Results and Discussion
C b1 N w , e 2
computer and data acquisition system
The volume of water collected, Vp, as function of time for the pure water experiment (Cb = 0 kg∙m3) and transmembrane pressure ΔP = 0.2694 ± 0.0010 bar is shown in Figure 4. The permeate flow is calculated from the data. In this case linear regression with zero intercept gives the volumetric flow dVp∙dt = (3.770 ± 0.035) × 10‒8 m3∙s. The permeate flux as function of ΔP for pure water and for the two biosuspensions (1.500 ± 0.002 kg∙m3 and 3.000 ± 0.002 kg∙m3) are shown in Figure 5. Permeance of water, obtained from linear fitting of experimental data with pure water, gives PM,w = (4.463 ± 0.032) × 10‒5 m3∙(m2 s bar). The data in Figure 5 also show that hydrodynamic resistance is a membrane constant independent of the applied pressure as eq 1 points out.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 1 January 2008 • Journal of Chemical Education
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In the Laboratory 7
Vp / (10ź6 m3)
20
Nw / (10ź5 m3/m2 s)
25
Vp (3.770 q 0.035) t 10ź8 t
15
10
5
0 0
100
200
300
400
500
6
Nw (4.463 q 0.032) t 10ź5 %P
5 4 3 2 1 0
600
0
Time / s
0.3
0.6
0.9
1.2
1.5
%P / bar
Figure 4. Permeated volume versus time for pure water and transmembrane pressure ΔP = 0.2694 ± 0.0010 bar.
Figure 5. Permeate flux versus transmembrane pressure for feed concentrations: × = 0.0, ■ = 1.500 ± 0.002 and △ = 3.000 ± 0.002 kg/m3.
The results achieved with yeast suspensions (Figure 5) show that polarization phenomenon occurs and gives rise to the formation of a gel layer in the operating conditions. From the spectrophotometer analysis of the permeate, students conclude this UF membrane system performs ideally, since CP = 0 and Rf = 1. Gel layer formation is easily detected by the appearance of a plateau on Nw versus ΔP curves (dashed lines in Figure 5): Nw,∞ = (1.180 ± 0.0129) × 10‒5 m3∙(m2 s) for Cb = 1.500 ± 0.002 kg∙m3 and Nw,∞ = (0.6500 ± 0.0062) × 10‒5 m3∙(m2 s) for Cb = 3.000 ± 0.002 kg∙m3. It is important students realize, during experimental work, that at this point increasing driving force does not have any further effect upon permeate flux, after the new stationary state is attained. They should justify this observation via the gel layer model: When transmembrane pressure increases, permeate flux jumps immediately (see eq 2) giving rise to higher yeast deposition and so the gel layer becomes thicker and eventually more compact. Therefore, the resistance to solvent transport will rise until the gel layer gets to a new steady-state thickness, causing Nw to drop to the initial undisturbed value. Students should also recognize from Figure 5 that polarization has a stronger punitive action at higher bulk concentrations of yeast: for the same ΔP, the flux of permeated water varies inversely with Cb, as eqs 4 or 5 predict. For the two pairs of limiting values (Cb, Nw,∞) found in this work, the gel layer concentration and convective mass transfer coefficient are Cg = 7.02 ± 0.19 kg∙m3 and kf = (7.650 ± 0.207) × 10‒6 m∙s. Students are encouraged to compare their results with those obtained by other groups, as well as predict them with their own parameters whenever it is theoretically possible. Instructors should ask students to discuss the effect some operation variables and parameters would have upon their results, for instance, (i) velocity along the membrane, with direct impact on the Reynolds number; (ii) pulsating flow; (iii) membrane fouling from (ir)reversible deposition of retained particles, pore blocking, adsorption, and so forth; (iv) the use of turbulence promoters; and (v) feed temperature. Temperature is particularly interesting, since its increase will reduce concentration polarization as a result of kf enhancement, while causing an increment in the permeate flux, which opposes the influence of the improved mass transfer. Comments from students who performed this lab exercise are frequently positive, especially when they detect gel layer formation. They are often surprised when observing the cycle of instantaneous increase in permeate flux and consequent
decline to the previous steady state, although such trend is reported in the literature and taught in lecture. Instructors found some (very few) students who measured the water flow during transient state. This experiment helps students to assimilate and understand concepts on pressure-driven membrane processes, especially polarization, gel layer formation, and the influence of hydrodynamic conditions upon permeation fluxes.
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Conclusions This laboratory exercise explains membrane separation fundamentals and introduces students to the experimental determination of ultrafiltration parameters. Solvent permeance is calculated from pure water experiments. Polarization phenomenon and gel layer formation are detected from experiments with different concentrations of suspensions. Both the gel layer concentration and convective mass transfer coefficient can be determined. The undesirable effect of polarization is more pronounced at higher feed bulk concentrations. Students become familiar with the potential and limitations of ultrafiltration, especially with the delicate trade-off between large driving force (ΔP) and long-term flux. Literature Cited 1. Lipscomb, G. G. Chem. Eng. Educ. 2003, 31 (1), 46–51. 2. Zydney, A. L. Chem. Eng. Educ. 2003, 36 (1), 33–37. 3. Mulder, M. Basic Principles of Membrane Technology, 2nd ed.; Kluwer Academic Publishers: Dordrecht, 1996. 4. Wankat, P. C. Rate-Controlled Separations; Chapman & Hall: Glasgow, 1994.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Jan/abs130.html Abstract and keywords Full text (PDF) Supplement
A list with chemicals, materials, and equipment used The detailed experimental procedure The sequence of calculations that students carry out Some tips and warnings for instructors A calibration curve of optical density to determine the concentration of Saccharomyces cerevisiae suspensions
Journal of Chemical Education • Vol. 85 No. 1 January 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
