Solvent Exchange Drying Process

For the solvent exchange, methanol and hexane are used as the first and the second solvent, respectively. The hollow fiber is removed from the water bath, and subjected to successive immersion into a methanol bath and a hexane bath for two days in each solvent. Then, the hollow fiber membrane is allowed to dry at room temperature while both ends of the fiber are fixed to prevent serious shrinkage. 

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Using inorganic salts as precursors, alcohol as the supercritical drying agent, and a batch process a solvent-exchange step was necessary to remove water from the gel. 

Most gas separation processes require that the selective membrane layer be extremely thin to achieve economical fluxes. Typical membrane thicknesses are less than 0.5 xm and often less than 0.1 xm. Early gas separation membranes [22] were adapted from the cellulose acetate membranes produced for reverse osmosis by the Loeb-Sourirajan phase separation process. These membranes are produced by precipitation in water the water must be removed before the membranes can be used to separate gases. However, the capillary forces generated as the liquid evaporates cause collapse of the finely microporous substrate of the cellulose acetate membrane, destroying its usefulness. This problem has been overcome by a solvent exchange process in which the water is first exchanged for an alcohol, then for hexane. The surface tension forces generated as liquid hexane is evaporated are much reduced, and a dry membrane is produced. Membranes produced by this method have been widely used by Grace (now GMS, a division of Kvaemer) and Separex (now a division of UOP) to separate carbon dioxide from methane in natural gas. 

Under similar acetylation conditions [153], the acetyl content of cotton (4.3%) was more than twice that of ramie (2.1%) or linen (1.3%) cellulose. Mercerization [8,154,155] greatly enhances the reactivity (or accessibility) of cellulose if it is maintained in a never-dried state. Drying of mercerized cellulose considerably reduced its reactivity to even less than half that of the unmercerized samples [8]. However, its impact, can be alleviated by dehydration through a solvent exchange process. The reactivity of mercerized samples expressed by acetyl content increased from 1.7% to 17.7% if water-washed sample was solvent-exchanged with pyridine and to 29% if washing directly with absolute ethanol and a subsequent pyridine exchange. 

The current version ( ) of the fiber treatment process employs a drawing rig equipped with a solvent treatment tube where an as-spun PET monofilament briefly contacts a swelling agent, causing rapid crystallization of surface layers. Solvent exchange and drying steps quench the crystallization before the fiber is drawn between feed and take-up rolls. A skln/core morphology results... 

The wet cellulose acetate membranes prepared for reverse osmosis purposes can be used for gas separation when they are dried. The water in the cellulose acetate membrane cannot be evaporated in air, however, since the asymmetric structure of the membrane will collapse. Instead, the multi-stage solvent exchange and evaporation method is applied. In this method, a water-miscible solvent such as ethanol first replaces the water in the membrane. Then, a second volatile solvent such as hexane replaces the first solvent. The second solvent is subsequently air-evaporated to obtain a dry membrane [13,14]. The reason for replacing water with hexane is to reduce the capillary force inside the pore so that it will not collapse during the drying process. 

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