Polyvinyl alcohol recovery by ultrafiltration: effects of membrane type and operating conditions

Separations Technology 5 (1995) 97-103

Polyvinyl alcohol recovery by ultrafiltration: effects membrane type and operating conditions

S.H. Lin*, W.J. Lan

of

Department of Chemical Engineeting, Yuan Ze Institute of Technology, Neili, Taoyuan 320, Taiwan. ROC

Received 11 January 1995; accepted 13 March 1995

Abstract

Recovery of polyvinyl alcohol (PVA) by ultrafiltration (UF) from simulated desizing wastewater is investigated. Batch experiments were conducted to examine the performance characteristics of the UF membranes of the hydrophilic and hydrophobic types and of different pore sizes. The performances of those membranes were compared using several characteristic parameters of the ultrafiltration process. The experimental results have indicated that the ultrafiltration performances of different membranes are not be easily judged by a single characteristic parameter alone. Instead, several characteristic parameters must be considered simultaneously. In the context of overall performances, the hydrophilic membranes are found to be superior to the hydrophobic types. The UF membrane of a lower molecular weight cut-off (MWCO) is definitely a better choice than that of a higher MWCO. The effects of operating pressure, temperature and mixer speed on the ultrafiltration performances were also explored. Reduction of the pollution strength of the permeate in terins of chemical oqgen demand (COD) was also considered.

Keywords: Polyvinyl alcohol; Ultrafiltration; Desizing wastewater; Membrane

1. Introduction

In a modern textile plant, high-speed looms are widely used in weaving the chemical or natural fibers into cloths. Before the fibers are woven into cloths in a high-speed loom, they invariably pass through a sizing step. In this sizing operation, the fibers are coated with a layer of sizing agents which may consist of polyvinyl alcohol (PVA) alone or a mixture of PVA, corn starch and carboxymethyl cellulose (CMC) surfactants. The sizing agents serve to smooth and strengthen the fibers, preventing them from being broken and getting tangled during the weaving process. After the weaving operation, the sizing agents need to be removed by hot water before the cloths can be further processed. The washing step using hot water is professionally known as the desizing step.

* Corresponding author.

The hot wastewater generated in this washing step is usually mixed with the wastewaters from other sources and goes to the wastewater treatment plant [l-3].

The hot desizing wastewater containing the sizing agents has a very high polluting strength which con- tributes considerably to the high temperature and high chemical oxygen demand (COD) concentration of the textile wastewater [l-31. Aside from being highly polluting itself, the major component of the sizing agents, PVA, is a valuable material. It goes through the sizing and desizing processes with very little change in its physical and chemical properties. Hence from the standpoint of economics and pollution control, it is highly sensible to recover PVA for reuse.

The purpose of this study is to experimentally in- vestigate the PVA recovery from the simulated desizing wastewater by ultrafiltration (UF). Ultrafiltration is an important separation method, finding wide applications in various chemical industrial processes [4]. It

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98 S. H. Lin, KJ. Lan /Separations Technology 5 (I 995) 97-103

has been employed for recovering metals from waste effluents [7] and organic pollutants from aqueous solution [8]. Bodzek et al. [9] succeeded in recovering mineral oil by ultrafiltration. Nystrom [lo] treated the oil/water emulsion systems by ultrafiltration. Those investigators had found ultrafiltration to be an economical and effective means for their applications. Ultrafiltration recovery of PVA was mentioned in a book by Cheryan [5], but a search of literature found no previous investigations of this process. Experi- ments were conducted in the present work to look specifically into the effects of different types of membrane and operating conditions on the ultrafiltration performances for the PVA recovery. The experimental data could provide information of much academic and practical significance.

2. Experimental studies

The batch experimental setup is shown in Fig. 1. The system consisted of an UF cell with a diameter of 6 cm and a cell volume of 300 cm3. An outside water heating jacket allowed the UF cell be controlled to a desired constant temperature. The pressure in the UF cell was maintained using nitrogen gas. To keep the PVA concentration uniform in the UF cell, a stirrer was provided. Three types of the Amicon (Amicon, Inc., Beverly, Massachusetts, USA) UF membranes were employed. They were hydrophilic Amicon YM30 and YM 100 and hydrophobic PM30 membranes. The YM series was a cellulose acetate membrane while the PM one was a polysulfone type and both are able

Nz gas

to stand operating temperatures over 100°C. The membrane had an effective filtration surface area of 28.7 cm’. The nominal molecular weight cut-offs (MWCO) of the three membranes were represented by the numerals 30 000, 100000 and 30000, respectively. The difference in MWCO permitted examination of the effects of the membrane type and MWCO on the UF performance.

An initial solution containing 1140 mg/l PVA (GR grade as supplied by E. Merck, Germany) was prepared. The PVA concentration was within the range of PVA level normally found in the hot desizing wastewater of a dyeing process. An initial amount of 240 ml of the prepared PVA solution was pumped into the UF cell by a micropump and the pump was then turned off. After heating to 80°C or other desired temperatures, the UF cell containing the PVA solution was sealed and pressurized using the nitrogen gas to start the filtration process. The amount of permeate and its PVA concentration were taken peri- odically. The PVA concentration was measured using the iodometrical method and a Hitachi UV (Model 3410, Hitachi, Inc., Japan) with wavelength of 670 nm. The chemical oxygen demand (COD) concentration, which represents the pollutant strength of the permeate, was determined by the standard methods ml.

3. Discussion of results

As mentioned earlier, the two UF membranes tested here are the hydrophilic cellulose acetate type (YM30

UF

Desizing Micropump Wastewater

.___I_. ;_*_kembrane

Reservoir

k Permeate

Fig. 1. Batch experimental setup of the ultrafiltration system.

i Temperature : Controller I I I

_______ -:

$Thermocouple : c e Retentate

:

S.H. Lin, WJ. Lan /Separations Technology 5 (1995) 97-10.1

and YMlOO) and the hydrophobic polysulfone type (PM301 with two molecular weight cut-offs (MWCOs) of 30000 and 100000. The membrane with large MWCO enables more solution containing a small amount of PVA to pass through, as reflected by Fig. 2 which shows the largest permeation for the hydrophilic YMlOO membrane. The opposite extreme goes with the hydrophobic PM30 membrane which has significantly lower permeate and retentate PVA concentrations and low permeation. It is noted that the retentate PVA concentrations pertaining to the three membranes increases fairly rapidly with time, as shown in Fig. 3, while the permeate PVA concentrations remained below 200 mg/l.

According to Fig. 3, for the same MWCO, the hydrophobic membrane (PM30) appears to be more effective than the hydrophilic type (YM30) in retain- ing PVA due, primarily, to its low permeation. To quantify the effectiveness of these two UF membranes in a better prospective, a rejection coefficient (R) is defined as [5]:

in which C, and C, are, respectively, the permeate and retentate PVA concentrations. Accordingly, a rejection coefficient closer to one denotes a better ultrafiltration efficiency. Fig. 4 compares the rejection coefficients for the three UF membranes. This figure surprisingly shows that the hydrophobic PM30 membrane has the biggest rejection coefficient among the three membranes. Although the PM30 membrane has the lowest retentate concentration, as shown in Fig. 3,

I, I I I I I I I I I I

10 20 30 40 50 60 ’

Time, min

Fig. 2. Effect of different membrane types on the permeation at Fig. 4. Effect of different membrane types on rejection coefficient 8O”C, 0.135 MPa and 500 rpm mixer speed. at SOT, 0.135 MPa and 500 rpm mixer speed.

/

/ PM30

Time, min

Fig. 3 Effect of different membrane types on the retentate PVA concentration at 80°C. 0.135 MPa and 500 rpm mixer speed.

it also has the lowest permeate concentration among the three membranes tested here. This could be due to the fact that PVA has a certain affinity toward the PM30 membrane. Such membrane/solute interaction results in a higher rejection coefficient for PM30. However, because of its low permeation and retentate concentration, PM30 is obviously not a better membrane than YM30 for the present PVA recovery purpose. Hence, the rejection coefficient alone does not adequately reflect the performance characteristics of a membrane.

An alternative for assessing the performances 01 the three UF membranes can be based on the recov-

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100 S. H. Lin, WV. Lan /Separations Technology 5 (1995) 97-103

ery factor CR,) which is defined as:

R, =; 0

where W, and W, are, respectively, the weights of PVA in the initial retentate and at time t. By this definition, R, is a measurement of the PVA retention by the membrane during the ultrafiltration process and hence a membrane with a high R, is preferable. Fig. 5 shows the recovery factors for the three UF membranes. The hydrophilic YMlOO is the worst one among the three as would be expected because of its larger pore size which leads to more PVA passage through the membrane. It is also clear here that the recovery factor for the hydrophobic PM30 is significantly better than that of the hydrophilic YM30.

The above discussion does not necessarily lead to the conclusion that the hydrophobic PM30 is a better choice than the hydrophilic YM30 for the present PVA recovery by ultrafiltration. In practice one also needs to consider the performance of an UF membrane after backwash because the UF membranes are normally backwashed several times before they are replaced. To do so, a sequence of experiments was devised to test the reusability of the three membranes after backwash. The experiments consisted of four steps. An initial ultrafiltration using distilled water was performed and it was followed by a l-h ultrafiltration of PVA solution. The membrane was then thoroughly washed following the cleaning procedure and finally a repeated ultrafiltration using distilled water was conducted. The cleaning procedure, as recommended by the manufacturer, includes 30 min of ultrasonic cleaning followed by 30 min immersion of

p 0.88

5 d 0.84

Time, min

Fig. 5. Effect of different membrane types on recovery factor at S(lT, 0.135 MPa and 500 rpm mixer speed.

the UF membrane in a 0.1 M NaOH solution and then in a 100 mg/l NaClO solution. The membrane finally went through another 30 min of ultrasonic cleaning. Such a cleaning procedure is considerably more rigorous than that employed in a practical ultrafiltration backwash operation. During the two ultrafiltration tests using distilled water, the accumulated volumes were registered as a function of time. A recovery coefficient (R,) is defined here as the ratio of the accumulated volumes of the two ultrafiltration runs:

R, = e, Qo (3)

where Q, is the accumulated volume of distilled water of the first distilled water ultrafiltration run using fresh membrane and Q, is that of the second distilled water ultrafiltration run using the membrane after cleaning. According to this definition, a recovery coefficient of 1 means complete removal of all impuri- ties by the cleaning and the UF membrane has been restored to the original condition. This is apparently an ideal case which is difficult to achieve in practice. The recovery coefficients for the three UF membranes as a function of time are displayed in Fig. 6. It is obvious from this figure that the YM30 membrane has a high recovery coefficient which stayed above 0.95. The recovery coefficient for the YMlOO membrane decreases slightly with time and stayed at around 0.8 which is deemed acceptable. The hydrophobic PM30 membrane shows a rather low recovery coefficient below 0.4. Such a poor recovery coefficient clearly indicates that the hydrophobic membrane is not suitable for the repeated PVA recovery applications. The low reusability of PM30 will lead to a high membrane replacement cost in practical applications.

The sizing and desizing operations are conducted at a temperature near 90°C - primarily because of the high viscosity of the sizing mixture at a lower temperature. The high viscosity of the sizing agents at low temperature would render the sizing and desizing operations rather difficult to perform. Hence the desizing wastewater invariably has a temperature of around 80°C. This is the reason the majority of the present experiments were conducted at 80°C. How- ever, it would be of much practical interest to see to what extent a lower temperature of the desizing wastewater affects the ultrafiltration operation. Figs. 7 and 8 compare the permeate and retentate PVA concentrations at 80 and 60°C respectively. Also shown in Fig. 8 is the effect of the mixer speed. The temperature effect on the PVA concentrations is seen to be quite strong. Both the permeate and retentate PVA

S.H. Lin, W.J. Lan /Separations Technology 5 (1995) 97-103

0.0 I I I I I 0 2 4 6 8 10

Time, min

Fig. 6. Effect of different membrane types on recovery coefficient at 80°C 0.135 MPa and 500 rpm mixer speed.

concentrations at 60°C are seen to vary essentially linearly with time and the nonlinear temperature effect on the PVA concentration is apparent at 80°C. Although higher temperature causes more PVA passage through the membrane, it does help considerably in elevating the retentate PVA concentration and facilitating the ultrafiltration operation. Hence maintaining the ultrafiltration at 80°C is strongly favored.

Fig. 8 also reveals that an increase in the mixer speed has a significantly beneficial effect on the retentate concentration enhancement. The similar effect of mixer speed on the permeate PVA concentration is demonstrated in Fig. 9 for the YM30 membrane. High mixer speed will prevent PVA accumula-

01 ’ “1 1 1” fl “‘I 1 0 10 20 30 40 50 60 70

Time, min Fig. 7. Effect of temperature on the permeate PVA concentration of YM30 membrane at 0.135 MPa and 500 rpm mixer speed.

c Tempratur$C) 1 Mixer Spmd(rpa) t

01 “I ’ ” I ’ ” I ’ I 0 10 20 30 40 50 60 ’

101

Time, min

Fig. 8. Effects of temperature and mixer speed on the retentate PVA concentration of YM30 membrane at 0.135 MPa.

tion near the membrane surface, which, in many situations, can cause PVA polarization (Cheryan, 1986). Hence maintaining a uniform PVA concentration within the UF cell can be highly conducive to PVA ultrafiltration. Such an advantage is clearly reflected in Figs. 8 and 9. Especially in Fig. 8, the retentate PVA concentration is seen to be markedly improved as the mixer speed increases.

Figs. 10 and 11 show the pressure effect on the permeation for the PM30 and YM30 membranes, respectively. Fig. 10 indicates that there is only a little improvement in the PVA permeations for PM30 for an applied pressure exceeding 0.202 MPa, reflecting a similar effect observed in a previous figure. For the hydrophilic YM30, the pressure effect is relatively

01 ‘1 I ’ I”’ ” ‘1’ 0 10 20 30 40 50 60

Time, min

0

Fig. 9. Effect of mixer speed on the permeate PVA concentration of YM30 membrane at 8O”C, 0.135 MPa and 500 rpm mixer speed.

102 S.H. Lin, WJ. Lan /Separations Technology 5 (1995) 97-103

10 20 30 40 50 60 70

Time, min

Fig. 10. Effect of pressure on the permeation of PM30 membrane at 8O”C, 0.135 MPa and 500 rpm mixer speed.

mild, with 0.202 MPa pressure having a slightly better PVA permeation for YM30. The observations made here reinforce the previous conclusion that the applied pressure for the present PVA recovery by ultrafiltration does not need to exceed 0.2 MPa.

Pertaining to those in Fig. 8, the effects of temperature and mixer speed on the PVA permeation for YM30 are seen in Fig. 12. It is apparent that for efficient PVA ultrafiltration, maintaining a constant temperature at 80°C and high mixer speed at 500 rpm is highly desirable.

The chemical oxygen demand (COD), which is an index representing the strength of a pollutant in the

Time, min

Fig. 11. Effect of pressure on the permeation of YM30 membrane at 8O”C, 0.135 MPa and 500 rpm mixer speed.

10 Tqanhn=&) I Miwr Speed(rpn)

0 : 801500 8- 8: 601500

A : 80/200

6-

I II I I I I I I I I 20 40 60 SO 100 120

Time, min

Fig. 12. Effects of temperature and mixer speed on the permeation of YM30 membrane at 0.135 MPa.

wastewater, was also measured. The safe discharge COD requirement by the Taiwan EPA for the industrial wastewaters is 200 mg/l. The initial solution containing 1140 mg/l PVA was found to have a COD concentration of 1650 mg/l. At a higher initial PVA concentration of 2420 mg/l, the COD concentration was elevated to 3810 mg/l. The pollutant strength of the initial solutions exceeds the safe discharge requirement by a wide margin. At the operating conditions of 0.135 MPa and 80°C the measured COD concentration of the permeate collected within 70 min of ultrafiltration was 106 mg/l using the hydrophobic PM30 and 207 mg/l using the hydrophilic YM30. These represent 93.6 and 87.5% COD reduc- tions, respectively, which are excellent. Considering the very low biodegradability of PVA (Lin and Lin, 1993; Lin and Peng, 19941, ultra-filtration appears to be an excellent choice for PVA removal from desizing wastewater. The advantage of ultrafiltration is further amplified by the fact that the recovered PVA is highly valuable.

4. Conclusions

The ultrafiltration method has been employed in the present study to recover polyvinyl alcohol (PVA) from the stimulated desizing wastewater of a dyeing and finishing mill. Batch experiments have been conducted to examine the effects of the membrane type, pressure, temperature and mixer speed on the ultrafiltration efficiency. Several indices are proposed for evaluating the ultrafiltration performances using a specific type of membrane. It has been difficult to judge the ultrafiltration performance of a particular

S. H. Lin, W.J. Lan /Separations Technology 5 (I 995) 97-l 03 103

membrane by a single index alone. Very often different criteria need to be considered to come to the final conclusion. The present experimental results indicate that the hydrophobic membrane performs well in terms of PVA retention and fluid permeation. How- ever, it has a very low recovery coefficient, implying low reusability of this membrane after backwash. This makes the hydrophilic membrane a better choice for the present purpose. A hydrophilic membrane with a proper, low molecular weight cut-off has been found to be a better UF performer.

An 80°C temperature, which is the temperature of the desizing wastewater, is necessary for efficient operation of the ultrafiltration process. A decrease in the operating temperature would lead to a significant decrease in the ultrafiltration permeation and is not recommended for practical applications. A pressure of about 0.2 MPa was found sufficient to provide efficient ultrafiltration operation. Maintaining the PVA concentration relatively uniform in the UF cell at a proper mixer speed is beneficial in preventing the blocking of the membrane pores and the polarization caused by the concentration gradient of PVA near the membrane surface. In general, the ultrafiltration using a hydrophilic membrane with a proper MWCO has been observed to be very efficient for PVA recovery with good overall performances.

Acknowledgements

The authors are very grateful to the National Sci- ence Council, Taiwan, ROC (under the grant NSC84- 01 150155-Ol-015E) for their financial support of this project.

Notation

permeate PVA concentration retentate PVA concentration

QO

Qt

accumulated volume fresh UF membrane accumulated volume

of distilled water of

at time t of distilled water after PVA ultrafiltration and membrane cleaning

R rejection coefficient R, recovery coefficient R, recovery factor w, weight of PVA in the initial retentate w, weight of PVA in the retentate at time t

References

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Polyvinyl alcohol recovery by ultrafiltration: effects of membrane type and operating conditions