Membrane fouling in reverse osmosis RO systems is as all resin injection

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Membrane fouling in reverse osmosis RO systems is as all resin injection Powered By Docstoc
					Summary

Appreciation of a membrane's surface chemistry and steric exclusion character is needed to
truly understand and predict membrane performance for specific industrial separations. The
interpreter of membrane characterization data must consider both factors in the assessment of
separation potential. The test conditions employed will strongly influence the separation data
outcome, as will inherent surface force interactions between the membrane and solution
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components. Sepa CF cell solute challenges and affinity chromatography methods are useful
tools to characterize the pore size and surface character of ultrafiltration and nanofiltration
membranes.

The characterization data presented demonstrate the separation potential of B-type
membranes. For example, the combination of the B-type membrane surface charge and pore
size affords economical separations of salts from organics. The anionic surface charge of B-
type membranes also makes them competitive for high fouling applications. Dye concentration,
paper pulp waste treatment and similar applications appear promising for B-type membranes
where traditional membranes are not well suited.

Key words: Anionic membrane, diafiltration, molecular weight cutoff, nanofiltration, ultrafiltration

Introduction

Membrane separations in the ultrafiltration (UF) and especially the nanofiltration (NF) regions of
the filtration spectrum are governed by a complex combination of both steric exclusion and
surface force interactions. UF and NF membranes bearing formal surface charges display
unusual selectivity behavior not predicted on the basis of physical pore size alone. Therefore,
practical characterization should employ several techniques to gain insight on membrane
function.

In this work we elucidated the separation characteristics of a novel, anionically charged
membrane with UF and NF capabilities. This study included three distinct techniques to
characterize the separation potential solute challenges to demonstrate the apparent molecular
weight cutoff (MWCO) of the membrane; a special affinity chromatography technique intended
to qualitatively demonstrate surface force interactions between the solutes and the membrane
resin; and pilot scale operation of actual industrial applications.

Experimental

Molecular weight cutoff

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The membrane characterized in this work is a novel thin layer composite. (TLC ). The
permselective barrier layer is produced from a unique polymer material. The commercial
membrane manufactured from this proprietary resin is referred to as B-type membrane. B-type
resin features a formal negative surface charge which is responsible for unique separation
characteristics of B-type membranes.

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Solute challenges were completed using a bench-top laboratory test device known as a Sepa
CF cell (Figure 1). This device is unique among test cells: it is designed to model the fluid
dynamics of commercial spiral-wound membrane elements ("sepralators"). Like the larger
sepralators, the Sepa CF cell functions in a true cross-flow or tangential flow mode. Challenge
solutions are pumped under pressure through feed channel spacers. and permeate is routed
through a woven fabric spacer called the permeate carrier. Both of these spacers are the same
materials used in a spiral-wound element (sepralator). The presence of the mesh spacer in the
feed channel in Figure 1 is designed to promote turbulence in the flow channel to reduce the
build-up of solute near the membrane surface.
                           Figure 1. Sepa-CF test cell and cell holder.

For the present work cell pressures of 50 and 100 psi (3.5 and 6.9 bar) were used, each in
combination with average crossflow velocities of both 0.1 and 0.2 m/s. These operating
conditions are typical of those used in actual industrial processes employing sepralators [1]. A
schematic diagram of the test apparatus is shown in Fig. 2.
                       Figure 2. Schematic of Membrane Test Apparatus




    A variety of organic solutes and inorganic salts were used to characterize membrane
   separation. Challenge concentration ranged from 500-2000 ppm. Solute detection in the
permeate was measured by refractive index for the organic solutes and by conductivity for salts.

                                  Process application studies

 Process application tests were conducted on-site with the actual process solution using pilot-
    scale equipment. Separation objectives included product concentration or reclamation,
               diafiltration for desalting and waste discharge volume reduction.

             The operating parameters used in these applications were as follows:

Applied pressure:           100-300 psig (6.9-20 bar)
Crossflow velocity:         0.1 and 10.0 m/s
                                  °          °
Process temperature:        20-50 C (68-120 F)
Mesh spacer flow channel: 0.86 mm (0.034")



                             Membrane surface force experiments

     Qualitative measures of membrane-solute surface force interactions were determined by
 injecting solutes into a chromatography column filled with the B membrane polymer. A relative
    scale of adsorption affinity for each of the solutes was constructed by comparing retention
                                               volumes.

To prepare a column B-type resin was pulverized into a fine powder with a ball mill, then sieved
   with mechanical agitation to obtain 38-45 µ size particles. The particulate B-type resin was
 packed dry into a stainless steel chromatography column (25 cm long x 4.6 mm ID). Sintered
 stainless steel 2 µ frits were used on the ends of the column to contain the particular packing.
 The column was run on a high performance liquid chromatography (HPLC) system using pure
  water (18.2 µS-cm treated via reverse osmosis, then deionization) as the mobile phase. The
 system was allowed to equilibrate for approximately 6 hours prior to solute injection to ensure
                                 proper bed settling in the column.
Solutes analyzed included monovalent and divalent salts, polysaccharides, polyethylene glycols
  (PEGs) and alcohols. All solutes were tested at 200 ppm concentration with the exception of
 deuterium oxide, a reference solute, which was tested at 1%. Sample injection volumes were
 100 µL, and detection was accomplished using differential refractive index measurement. The
   retention volume of each solute was used to calculate the degree to which the solutes were
  partitioned between the bulk fluid flowing through the particle interstices and the thin layer of
    interfacial fluid surrounding the particles of packing material. The equation describing this
                                              partition is:

                                KA = (Vsolute - Vmin) ÷ (Vwater – Vmin)

where the Vmin is the lowest retention volume among all solutes tested and Vwater is the retention
                         volume of water as measured by D2O injection.

                                    Results and Discussion

                         Retention characteristics and apparent MWCO

  Commercial UF and NF membranes are typically rated according to a nominal solute retention
 level (e.g., 90%) commonly termed MWCO. Comparison of MWCO from various manufacturers
   can be difficult when different test methods are used to determine the MWCO value. This is
frequently the case in practice because a standard test method has not yet been adopted by the
                                              industry.

 The MWCO rating of a UF or NF membrane can vary widely depending upon the operating
parameters and the chemical interactions or surface forces that occur between the solvent, the
  test solute and the membrane. A membrane user should consider an MWCO rating in the
context of the chemistry, the separation device configuration and the particular test conditions
   used to produce the rating. These same parameters will affect the usefulness of a given
                              membrane for specific applications.

                                        Operating parameters

   Solute separation can be influenced by changing cross-flow rate and operating pressure in
 many instances [2]. This is illustrated in Table I. Variation of the operating parameters resulted
in significantly different solute separation in the particular case of B-type membrane operated in
                                           a Sepa CF cell.

  The differences in solute retention shown in Table 1 can be understood using the concept of
 concentration polarization, the concentration gradient of solute molecules near the membrane
     surface. Increasing cross-flow velocities through the Sepa CF cell generally reduced
concentration polarization, resulting in higher separation and higher permeate flux. Lowering the
 operating pressures also decreased concentration polarization, resulting in higher separation
and higher flux in proportion to the operating pressure. Thus, high cross-flow and low operating
                      pressure provided the most economical separations.

      Table I. Effect of Sepa CF Operating Parameters on B-Type Membrane Separation


                              Operating      Crossflow       Permeate       Solute
                 Solute
                              Pressure        Velocity         Rate       Separation
               (1000 ppm)                                           a
                                (bar)          (m/s)          (LMH)          (%)

                 Na2SO4           3.5             0.1            12.4        50

                                  3.5             0.2            13.0        57

                                  6.9             0.1            24.2        33
                                6.9            0.2            24.7       43

               Raffinose        3.5            0.1            13.5       17

                                3.5            0.2            14.1       24

                                6.9            0.1            23.6        8

                                6.9            0.2            17.7       12

               PEG 3350         3.5            0.1            6.5        30

                                3.5            0.2            5.9        40

                                6.9            0.1            11.8       22

                                6.9            0.2            11.2       38




 These trends between fluid dynamics and membrane results are commonly observed for UF
  membranes [3]. However, the extent to which concentration polarization affects a particular
   membrane separation is not readily predicted. Accurate assessment of optimal operating
 conditions is usually possible only by means of empirical studies with actual process streams
                                        and membranes.

                                      Chemical interactions

     The membrane user should not presume that all separations effected by an ultrafilter or
  nanofilter occur by size exclusion mechanisms alone. separation data for B-type membranes
  with large nonionic organics are shown in Figure 3. The PEG and polysaccharide solutes are
typical of those used to characterize the size exclusion properties of UF membranes. The data
    suggest that the B-type membrane is best described as a "tight" ultrafilter with an MWCO
 ranging from about 2k000 daltons for polysaccharides to around 5,000 daltons for PEG's. The
MWCO result for PEG's is higher than that for polysaccharides because PEG's are adsorbed by
   the B-type membrane while polysaccharides are repelled by the membrane. The effects of
   adsorption and repulsion on apparent MWCO are addressed further in the later section on
                             membrane surface force characteristics.
 Figure 3: B-type membrane retention of nonionic organics. All solutes tested individually.
Saccharides: Sepa ST stirred cell, 100 psig (6.9 bar), 150 rpm (0.4 m/s at outer edge). PEG's:
                Sepa CF radial flow cell, 50 psig (3.4 bar), 0.9 m/s crossflow.




Figure 4: B-type membrane salt retention test conditions. 0.32 m/s crossflow velocity; 250 psig
                         (17 bar), 2000 ppm salt concentration.

Figure 4 demonstrates that B-type membranes can function as an NF membrane in the context
   of salt separations despite high MWCO ratings for uncharged organics. Charge repulsion
between salt anions and the membrane surface complement separation due to physical sieving.
  Note, for example, the membrane's ability to discriminate between NaCI and Na 2SO4. Sulfate
 has twice the charge of chloride and is also much larger. Charge repulsion occurs further from
   the membrane surface and hence sulfate is more easily excluded from membrane pores.

   Another notable feature is the effect of calcium and magnesium on salt separation. These
   results suggest that polyvalent cations effectively mask the charge character of the B-type
 membrane barrier layer. Thus, these salts are not rejected as effectively by means of Donnan
    exclusion. Additionally, Figure 5 shows that higher salt concentrations also decrease salt
 separation. This phenomenon improves the efficiency of salt removal by B-type membranes,
especially in diafiltration and concentration of very salty process streams. Mass transfer of both
      salt and solvent water benefit. Increased salt passage reduces the difference in salt
concentrations across the membrane, consequently reducing osmotic pressure and increasing
  the available effective pressure. The effective pressure driving permeate flux is the result of
 osmotic pressure subtracted from applied pressure as measured by a gauge. Higher effective
                            pressure results in higher permeate water flux.




                   Figure 5: Effect of salt concentration on retention.
    B-Type membrane. Radial flow cell test. 5.2 m/s average crossflow, 100 psig (6.9 bar)

              Table II. B-Type Membrane Retention of Ionically Charge Organics


                            Solute
                                            Molecular weight     % Separation
                          (2% feed)

                   FD&C red dye #40                480             98.0-99.5

                  FD&C yellow dye #5               534             94.4-98.4

                         Blue textile
                                                   826               >99.99
                         dye (19%)

                         Tannic acid              1700                99.5



                   a
                       192T Sepralators, 0.1 m/s crossflow, 100 psig (6.9 bar).

 Table II exemplifies the NF character of B-type membrane challenged to several commercial
 anionic dyes and tannic acid, a model compound for the humic acid contaminants common to
    many surface water supplies. High separation of these charged organic compounds was
 achieved, yet their molecular weights are well below the B-type membrane's MWCO obtained
for uncharged solutes. Separation appears analogous to that for inorganic salts due primarily to
                          charge repulsion at the membrane surface.

                                     Application case studies

 Two examples of successful applications illustrate the important separation characteristics of
 the versatile B-type membranes. Pilot scale application tests with actual process streams are
 always advised for UF and NF separations because most of these applications have complex
                       solutions and hence it is difficult to predict results.

The first case involved a textile dye of a nominal 7,000 molecular weight in a process solution
   contaminated by high concentrations of salts. The B-type membrane sepralators in this
application passed essentially no dye and all of the salts in the feed stream. B-type membrane
       effected the separation at moderate operating pressures and high product rates.

   Table III shows the performance of the B-type sepralator in a 30 L (8 gal) batch pilot test
   process with diafiltration followed by concentration. This application was also tried with a
   polysulfone UF membrane with larger pore size, but the surface character of the B-type
  membrane afforded less fouling and higher steady-state processing flux. The application is
                           under development with B-type membranes.

           Table III. Diafiltration and Concentration of Textile Dye (MW 6000-8000)

                                                      End of           End of
                                      Initial
                                                    diafiltration   concentration
                     Dye
                                                                         25.3
                 concentration,        8.6              8.6
                                                                     (approx. 3X)
                     wt%
                      Dye
                                      99.84            99.92            99.87
                 retention, wt%
                      Salt
                 concentration
                                       2.7              0.26               0.15
                 in feedstock,
                      wt%
                              b
                  Flux, LMH            12.4            28.9                14.1


             a
                 192T sepralator, 290 psig (20 bar) operating pressure, 30°C (86°F).
                                 b
                                   Liters per square meter per hour.

  A second application involved waste treatment of paper and pulp production effluent [4]. A B-
     type membrane was effective at removal of lignosulfates from kraft E-stage wastewater
   streams. Results showed a high reduction in waste stream color and COD in the permeate
 stream. As in the dye processing example, the separation is effected with high product rates at
a moderate operating pressure. The data, summarized in Table IV, show the B-type membrane
   competitive advantages over typical UF membranes. The B-type membrane effects greater
removal of environmental pollutants than UF membranes tested in previous work at comparable
    or higher permeate flux rates [5, 6]. Membrane processing costs for paper and pulp waste
    treatment have been historically regarded as prohibitive. Recent environmental concerns
together with improved membranes may offer new opportunities for treatment of paper and pulp
  wastes. Membrane improvements in the case of B-type membrane include increased fouling
                          resistance and greater separation capability.

                          Table IV. Kraft e-stage Liquor Waste Treatment
                                                               Competitor's
                                            B-type TLC           modified
                                            membrane           polysulfone
                                                                membrane
                          Operating
                          pressure,           6.9 (100)           9.3 (135)
                          bar (psig)
                                     b
                         Flux, LMH            33.0-105           23.0-59.5
                      COD rejection, %           88              50.0-65.0
                       Color rejection,
                                                 97              75.0-90.0
                              %


a
     50°C (120°F), pH = 10, ABB CR-250 stirred cell, 10 m/s at end of blade; feed stream rate 7.5
                                          1pm (2 gpm).
                               b
                                 Liters per square meter per hour

                              Membrane surface force characteristics

   Solute retention data from HPLC adsorption affinity experiments can help demonstrate the
  surface force mechanisms which contribute to the separation characteristics of a membrane.
 Sourirajan et al. [7] developed an interpretation of solute retention as a measure of the degree
to which the solutes interact with the membrane polymer. This interaction is described either as
  adsorption or repulsion. The theory allows for the calculation of a partition coefficient, KA, for
 each solute injected into the column. Values of KA are measured relative to that of water (D2O
injection); values less than one indicate a net repulsion, and values greater than one indicate a
   net adsorption. The general trends in resin solute partition data are shown in Figure 6 for a
                                         variety of solutes.




                       Figure 6. Solute partition coefficients for B-type resin.

       The partition coefficients are useful for predicting separation capabilities of UF and NF
          membranes with various challenge solutes. In the case of B-type membranes, the
    chromatography data show that alcohols and PEG's are adsorbed by the membrane polymer
      while saccharides and salts are repelled by the polymer. This suggests that separation of
    saccharides and salts by a B-type membrane will be higher than separation of similarly sized
        alcohols and PEG's. This hypothesis was supported experimentally by comparing the
    saccharide and PEG separation of B-type membrane depicted in Figure 3. The B-type
 membrane rejected more polysaccharide than PEG's at any given molecular weight. Higher
saccharide separation occurred despite the fact that the saccharides are spatially smaller than
                          PEG's at equivalent molecular weights.

                                            Alcohols

The grouping of "higher alcohols" in Figure 6 refers to ethanol, propanol, isopropanol, n-butanol
 and n-pentanol. HPLC injection data suggest that these compounds are strongly adsorbed by
  the B-type polymer packing material. Peak response for these materials was poorly defined,
 typically appearing as an extended drift in the baseline of the chromatogram. The adsorption
noted in this case most probably results from hydrogen bonding between the alcohol hydroxyl
                groups and hydrophilic functional groups on the B-type polymer.

                                      Polyethylene glycols

Adsorption of PEG solutes by a B-type resin was similar to that of alcohols. Again the hydroxyl
 end groups on these molecules probably hydrogen bond with the hydrophilic moieties on the
   polymer. We also conjecture that the long linear conformation of these molecules allows
  hydrophobic interactions between hydrophobic regions of the B-type polymer and the PEG
                                           solutes.

                                          Saccharides

Results show that saccharide retention time decreases with increasing MW. This behavior is not
  completely understood at present. It is believed that glucose coordinates with metal cations
contained in the B-type polymer. Larger saccharides may be unable to coordinate as effectively
                         with metallic moieties in the B-type polymer.

                                       Ionic compounds

 The HPLC column was unable to discriminate between monovalent and divalent salts on the
 basis of charge repulsion; both were strongly repelled. In contrast, the B-type membrane did
  separate monovalent and divalent salts; separation of the divalent species was higher. This
suggests that the membrane discriminates between monovalent and divalent salts on the basis
                            of both ionic size and charge repulsion.

                                          References

     1. M. Cheryan, Ultrafiltration Handbook, Technomic Publishing, Pennsylvania, 1986.
  2. B.J. Rudie, R.A. Condiff and P.W. Kariniemi, The Influence of Operating Parameters on
    Ultrafiltration Membrane Dextran Rejection, International Congress on Membranes and
                              Membranes Processes, Chicago, 1990.
      3. S. Bruin, A. Kikkert, J.A.G. Weldring and J. Hiddink, Desalination, 35 (1980) 223.
4. H. Buisson, Personal Communication, Wastewater Technology Centre, Burlington, Ontario,
                                          Canada, 1991.
  5. M.K. Hill, UF of Krafi Black Liquor--Final Report, Phase I, DOE Contract No. DE-ACO2-
                               82CE40606, modification A003, 1985.
 6. D.J. Paulson and D.D. Spatz, RO/UF Membrane Applied to the Pulp and Paper Industry,
Proceedings, Tech. Assoc. of TAPPI International Dissolving and Specialty PuIps Conference,
                                          Atlanta, 1983.
      7. T. Matsuura, Y. Taketani and S.J. Sourirajan, Colloid Interface Sci., 98 (1983)10.