The effect of short air exposure periods on the performance of cellulose acetate membranes from casting solutions with high cellulose acetate content

Sirkar, Kamalesh K.; Agarwal, Nirmal K.; Rangaiah, Gade Pandu

doi:10.1002/app.1978.070220714

Cited by 21 publications

(8 citation statements)

References 22 publications

(4 reference statements)

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“…Figure 3b shows that such pores are formed instantaneously. This is keeping with the results of Sirkar et al (1978) who allowed very small exposure timesdown to 0.01 s a n d still obtained good desalination membranes.…”

Section: Resultssupporting

confidence: 89%

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Mechanism of pore formation in reverse osmosis membranes during the casting process

Neogi

1983

AIChE Journal

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The process of pore formation during the casting of reverse osmosis membranes is analyzed. The process consists of two steps. The first step is the evaporation step, where the cast polymer solution is allowed to dry for 1 -100 s. The second step is the gel formation step, where the cast is soaked in water leaving behind the membrane in form of a gel. The evaporation step gives rise to a thin (-0.1 gm)skin of high density and very small pores which is chiefly responsible for desalination. The gel forms the backing (-100 to 250 pm) and contains large pores.It is shown that low evaporation rates accompanied by shrinkage during evap oration gives rise to an instability leading to the formation of the skin region. The evaporation effect is fast, is confined to the skin region, and gives rise to very small pores. The gel formation is shown to be a very slow process which cannot interfere with the skin formation due to the vast differences in their rates of formation. It also gives rise to larger pores. All key features of the above experimental observations are explained.The kinetics of the process depend on the diffusion coefficients D and Dp of the solvent and the polymer. However, the main factor is the solution chemistry of the polymersolvent system which controls both the effectiveness of the skin and the gel formed. For the first time, the relevant thermodynamic parameters which determine the extent and sizes of pore formation have been obtained. P. NEOGI Department of Chemical EngineeringUniversity of Missouri Rolla, MO 65401 SCOPESeparation with membranes is not new, but it still holds promise for wider use in the future. Only asymmetric polymeric membranes are discussed here. Reverse osmosis membranes are made of ionizable polymers like cellulose acetate. In presence of water, the walls of the membrane pores electrify, setting up an electrical field inside the pores. Under these conditions, it is possible to hinder the transport of ions through these pores, and at the same time allow the passage of water. Desalination is thus effected, the success of which depends on the ability of the polymer to electrify as well as on the sizes of the pores. The latter also plays a crucial role in ultrafiltration since the pores do not permit the passage of particles larger than their diameters. Filtration or separation of macromolecules, biological cells, and colloidal particles are thus possible. Consequently, membranes find important application in separation of proteins and enzymes, particularly as these materials are heat-sensitive and the process requires neither heating nor cooling. They also find application in biomedical engineering like in fabrication of artificial kidneys. It is noteworthy that these membranes often have very high separation or rejection efficiencies.It is evident that the separation takes place due to the existence of pores of desirable radii. Although extensive research has been done on the membrane (and pore) forming properties of a number of polymers, no reliable design criteria have emerged, ...

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Section: Resultssupporting

confidence: 89%

“…In Loeb-Sourirajan membranes, this is done by decreasing the casting temperatures. In Sirkar et al (1978) type membranes, this is done by increasing the polymer content. A practical limitation exists in either method; viz., the viscosity of the solution can become too high for the membrane to be cast.…”

Section: Further Discussion On Membrane Castingmentioning

confidence: 99%

Mechanism of pore formation in reverse osmosis membranes during the casting process

Neogi

1983

AIChE Journal

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“…Dioxane content higher than 40% leads to somewhat inferior performance, as reported earlier. 5 The PWP constants of membranes from 32% CA solution S-4 with 40% dioxane undergo smaller reductions with increasing pressure than those from similar casting solutions with 30% CA. The PWP constants of membranes from casting solutions with 60% dioxane decrease faster with increasing pressure than those of membranes from identical casting solutions with 40% dioxane.…”

Section: Discussionmentioning

confidence: 96%

“…Membranes prepared from such casting compositions with very low air-evaporation times (0.05-2 sec) perform the desalination of 0.5 w t % NaCl feed a t 600 psig as efficiently as the best known CA membranes in the literature. 5 The objective of this study was therefore to find out the productivity of similar membranes under considerably lower pressures of operations for brine feeds encountered in the desalination of brackish and sea waters. RangaiahI2 had tested a few of his membranes a t 250 psig with a 5000-ppm feed.…”

Section: Introductionmentioning

confidence: 99%

Low‐pressure reverse osmosis desalination with improved cellulose acetate membranes

Ghosh

Sirkar

1979

J of Applied Polymer Sci

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SynopsisThe advantages of carrying out reverse osmosis desalination with improved membranes at low pressures have been pointed out. Cellulose acetate (CA) semipermeable membranes developed with low air-exposure periods by Sirkar et al. from novel Manjikian-type casting solutions having high CA content, partial replacement of acetone by dioxane, and small amounts of ZnClz (or none) have been tested for the following combinations of operating pressure (lb/in?)/feed brine concentration (in ppm): 250/5000; 300/7500; 400/10,000; 600/30,000; 700/40,000; 750/50,000; 800/60,000. In the pressure range of 250-600 lb/in.2 (psig), the pure water permeability (PWP) constants of membranes for a given salt transport parameter are greater than those of similar membranes tested earlier by Sirkar e t al. a t 600 psig with a 5000-ppm brine feed. The PWP constants decrease faster with pressure as the casting solution CA concentration is reduced from 32% to 30% and the dioxane content of the solvent is raised from 40% to 60%. The intrinsic performances of membranes from a 32% CA casting solution with 40% dioxane improve as the pressure is increased from 250 to 600 psig. The solution structure-rate of evaporation concept along with the phenomenon of fingerlike cavities and protrusions in membranes from high dioxane casting solutions explain most of the observations of this work. The superior low-pressure performance of these CA membranes provide a basis for systematically lowering the operating pressure in reverse osmosis desalination.

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“…Pinnau and Koros [2] have shown that asymmetric membranes with ultrathin and defect free skin layers can only be manufactured by dry/wet phase inversion processes. A number of other studies have reported that membrane structures and performances can be significantly influenced by evaporation times on the order of few seconds before wet-casting step [3,4]. These observations clearly indicate that a mathematical model for combined dry/wet phase inversion process needs to be developed and applied to industrial practice.…”

Section: Introductionmentioning

confidence: 92%

Modeling of asymmetric membrane formation by a combination of dry/wet phase inversion processes

Altınkaya

2006

Desalination

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The invention of asymmetric polymer membranes by Loeb and Sourirajan has caused an enormous growth in the field of membrane science and technology. Application areas range from chemical, biomedical to biotechnology industries. Asymmetric membranes are mostly fabricated by a process called phase inversion which can be achieved through four principal methods: immersion precipitation (wet phase inversion), vapor-induced phase separation, thermally induced phase separation and dry-casting. In these techniques, an initially homogeneous polymer solution becomes thermodynamically unstable and phase separates into polymerlean and polymer-rich phases. Structure of these types of membranes is greatly influenced by even slight change in membrane preparation conditions

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The effect of short air exposure periods on the performance of cellulose acetate membranes from casting solutions with high cellulose acetate content

Cited by 21 publications

References 22 publications

Mechanism of pore formation in reverse osmosis membranes during the casting process

Mechanism of pore formation in reverse osmosis membranes during the casting process

Low‐pressure reverse osmosis desalination with improved cellulose acetate membranes

Modeling of asymmetric membrane formation by a combination of dry/wet phase inversion processes

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