Membrane materials cellulose acetate membranes

MEMBRANE MATERIAL CELLULOSE ACETATE (E-398) OPERATING PRESSURE 1724 kPog (ZSOpsig)... 

In the last years, the selection of polymers had been extended to most of the commonly used membrane materials cellulose acetate [25], polyamide [34,36], polyacrylonitrile [37], polysulfone [26,37], and modified polysulfone [23], but also including polystyrene and PVC [37], or PVC-co-PAA [38]. The exceptions are the hydrophobic—and almost nonfunctional— polymers (polyolefines, PVDF, or Teflon). However, because both recognition sites and pore structure are fixed at the same time within the same material, a comparison of the efficiency of different MIP membranes, and thus polymer materials, was rather complicated [26]. 

The preferential sorption-capillary flow mechanism of reverse osmosis does that. In the NaCl-H20-cellulose acetate membrane system, water is preferentially sorbed at the membrane-solution Interface due to electrostatic repulsion of ions in the vicinity of materials of low dielectric constant (13) and also due to the polar character of the cellulose acetate membrane material. In the p-chlorophenol-water-cellulose acetate membrane system, solute is preferentially sorbed at the interface due to higher acidity (proton donating ability) of p-chlorophenol compared to that of water and the net proton acceptor (basic) character of the polar part of cellulose acetate membrane material. In the benzene-water-cellulose acetate membrane, and cumene-water-cellulose acetate membrane systems, again solute is preferentially sorbed at the interface due to nonpolar... 

Figure 3. Potential functions for surface ( and friction ( forces as a function of the distance d front cellulose acetate membrane material for different solution...

With particular reference to reverse osmosis systems involving cellulose acetate membranes and aqueous solutions, the membrane material has both polar and nonpolar character, and the solvent, of course, is polar. When these two components of the reverse osmosis system are kept constant, preferential sorption at the membrane-solution interface, and, in turn, solute separation in reverse osmosis, may be expected to be controlled by the chemical nature of the solute. If the latter can be expressed by appropriate quantitative physicochemical parameters representing polar-, steric-, nonpolar-, and/or ionic-character of the solutes, then one can expect unique correlations to exist between such parameters and reverse osmosis data on solute separations for each membrane. Experimental results confirm that such is indeed the case (18). 

In 1966, Cadotte developed a method for casting mlcroporous support film from polysulfone, polycarbonate, and polyphenylene oxide plastics ( ). Of these, polysulfone (Union Carbide Corporation, Udel P-3500) proved to have the best combination of compaction resistance and surface microporosity. Use of the mlcroporous sheet as a support for ultrathin cellulose acetate membranes produced fluxes of 10 to 15 gfd, an increase of about five-fold over that of the original mlcroporous asymmetric cellulose acetate support. Since that time, mlcroporous polysulfone has been widely adopted as the material of choice for the support film in composite membranes, while finding use itself in many ultrafiltration processes. 

Negative rejections were consistently measured for several compounds by using the cellulose acetate membrane system. Compounds of this nature must possess a strong affinity for the membrane material and have relatively high transport rates through the membrane. 

The SEPAREX system will recover over 90% of the hydrogen at a purity of 96+% for recycle, while increasing the heating value of the fuel gas from -550 BTU/SCF to -950 BTU/SCF. The projected flow rates and gas purities for the membrane separation are shown in Table II. Under the bone-dry feed conditions the cellulose acetate membrane is not affected by HCl. Special materials of construction and adhesives have been used in the fabrication of the spiral-wound elements to ensure their resistance to HCl in the gas streams. 

Figure 2.15 Measurements of Rosenbaum and Cotton [20] of the water concentration gradients in a laminated reverse osmosis cellulose acetate membrane under applied pressures of 68 and 136 atm. Reprinted from Steady-state Distribution of Water in Cellulose Acetate Membrane, S. Rosenbaum and O. Cotton, J. Polym. Sci. 7, 101 Copyright 1969. This material is used by permission of John Wiley Sons, Inc.

Figure 3.10 The porosity of cellulose acetate membranes cast from 15-wt% solutions with various solvents. The same trend of high porosity and rapid precipitation with high solubility-parameter solvents was seen with a number of other membrane materials [25]...

Figure 7.16 An illustration of the efficiency of back-pulsing in removing fouling materials from the surface of microfiltration membranes. Direct microscopic observations of Mores and Davis [9] of cellulose acetate membranes fouled with a 0.1 wt% yeast suspension. The membrane was backflushed with permeate solution at 3 psi for various times. Reprinted from J. Membr. Sci. 189, W.D. Mores and R.H. Davis, Direct Visual Observation of Yeast Deposition and Removal During Microfiltration, p. 217, Copyright 2001, with permission from Elsevier...

S.Y. Lee, B.S. Minhas and M.D. Donohue, Effect of Gas Composition and Pressure on Permeation through Cellulose Acetate Membranes, in New Membrane Materials and Processes for Separation, K.K. Sirkar and D.R. Lloyd (eds), AIChE Symposium Series Number 261, AIChE, New York, NY, Vol. 84, p. 93 (1988). 

The process shown in Figure 9.21 was first developed by Separex, using cellulose acetate membranes. The separation factor for methanol from MTBE is high (>1000) because the membrane material, cellulose acetate, is relatively glassy and hydrophilic. Thus, both the mobility selectivity term and the sorption term in Equation (9.5) significantly favor permeation of the smaller molecule, methanol, because methanol is more polar than MTBE or isobutene, the other feed components. These membranes are reported to work well for feed methanol concentrations up to 6%. Above this concentration, the membrane is plasticized, and selectivity is lost. More recently, Sulzer (GFT) has also studied this separation using their plasma-polymerized membrane [56],... 

A similar application is the processing of fuel gas, whose major components are hydrogen (about 80%) and methane (about 20%). Asymmetric cellulose acetate membranes have been used successfully to extract the more valuable hydrogen at high purity. New membrane materials more resistant to harsh conditions will accelerate the application of other H2 recovery schemes for... 

Bacterial production was estimated by incorporation of 3H-thymidine (Fuhrman and Azam 1982). Duplicate 10 ml subsamples were incubated for 1 h in the dark at 2°C in the presence of 20 nM of 3H-thymidine (Amersham 40-50 Ci mol ), filtered on 0.2-pm cellulose acetate membrane filters (Sarto-rius) and extracted with ice-cold 5% Trichloroacetic acid (TCA) (Becquevort and Smith 2001). Thymidine incorporation was converted into bacterial production using conversion factors of Ducklow et al. (1999) established for the Ross Sea bacterial communities (i.e., 8.6 X 1017 bacteria produced per mole of thymidine incorporated in the cold TCA insoluble material). For bacterial production, the relative standard deviation was 9.3% (n = 20). The specific growth rate was estimated from bacterial production and bacterial biomass. 

Hon-celluloslc Membranes. Despite an Intensive search for more favorable membrane polymers, cellulose acetate remained the best material for reverse osmosis until 1969 when the first B-9 permeator for brackish water desalination was Introduced by Du Font. Richter and Hoehn ( ) Invented aromatic polyamide asymmetric hollow-fiber... 

The method has already been employed for polymeric membranes by several authors (14-16). Although there are some limitations for using this technique, for example, cylindrical pores are assumed and the membranes have to be dried without damaging the pore structure before the measurements can start, results were obtained for UF membranes made from different polymeric materials (Cellulose Acetate (CA), Poly-2,6-dimethy1-1,4-Phenylene Oxide (PPO) and some other non cellulosic materials). 

The breakthrough for UF was made when we could replace the cellulose-acetate membranes by more resistant materials. We expect the same breakthrough for RO in the years to come. 

A typical example is the negative separation observed for aqueous phenol solutions when using cellulose acetate membranes i.e., the permeate stream is more concentrated in phenol than is the feed stream. This anomalous behavior can be accounted for by postulating that solutes such as phenol are preferentially attracted to or preferentially sorbed by the cellulose acetate membrane material. 

At this point, it is important to describe exactly what is meant by solute preferential sorption and the consequences that result from this situation. Consider first the classical case of the separation of aqueous NaCl solutions by cellulose acetate membranes. In this instance the membrane material has a stronger affinity for the solvent than it has for the solute. The result... 

On the other hand, for a hydrocarbon solute such as benzene, the nonpolar forces predominate. The cellulose acetate membrane material has a nonpolar character due to its carbon backbone. 

Reverse osmosis separations of 12 alkali meteil halides in methanol solutions have been studied using cellulose acetate membranes of different surface porosities. Data for surface excess free energy parameters for the ions and ion pairs Involved have been generated for the above mend>rane material-solution systems. These data offer a means of predicting the performance of cellulose acetate membranes in the reverse osmosis treatment of methanol solutions involving the above ions from only a single set of experimental data. 

RO is well-suited for use in treating secondary wastewater effluents, even though there will be some organic fouling. Cellulose-acetate membranes are especially useful for this purpose, since they can reject 90-99% of all salts and 90% of all organic material - all of this can be accomplished in one unit. 

In general, adsorption is achieved by applying a solution of the molecule to be immobilized to a membrane or him on the sensor transducer and allowing the molecule to adsorb to the transducer over a specified time period. The membrane or film may be hydrophilic or hydrophobic or may contain ionic groups depending on the molecule to be immobilized. Various support/surface materials have been used for adsorption but the most used are silica, cellulose acetate membranes, and polymers such as PVC and polystyrene. As shown in table 8.5, adsorption is still used in the fabrication of many chemical sensors and biosensors. 

The cellulose acetate membranes are asymmetric and fabricated from a single polymer. The use of electron microscopy in the 1960s demonstrated that the cellulose acetate membranes consisted of a relatively thin dense layer and a thicker porous layer of the same material. The membrane thickness is usually about 100 micrometers with the dense layer accounting for about 0.2% of the thickness and the remainder being an open cell porous matrix (see Figure 4.5). 

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