Cellulosic hollow fibers, solute

Ultrafiltration Rates and Rejection of Solutes by Cellulosic Hollow Fibers... 

Preparation of Hollow Fiber Membrane. CTA (Cellulose Tri-Acetate) hollow fiber membranes were prepared by aplnning a dope solution of CTA followed by soaking and anealing. 

An alternate procedure used in a few specialty applications is the cuprammonium process. This involves stabilization of cellulose in an ammonia solution of cupric oxide. Solubilization occurs by complex formation of cupric ion with ammonia and the hydroxyl groups of cellulose. Regeneration of cellulose, after formation of the desired products, is accomplished by treatment with acid. The main application of the cuprammonium process is for the synthesis of films and hollow fibers for use in artificial kidney dialysis machines. The cuprammonium process yields products with superior permeability and biocompatibility properties compared to the xanthation process. Less than 1% of all regenerated cellulose is produced by the cuprammonium process. 

Reverse osmosis is nsed as a method of desalting seawater, recovering wastewater from paper mill operations, pollution control, industrial water treatment, chemical separations, and food processing. This method involves application of pressure to the surface of a saline solution, thus forcing pure water to pass from the solution through a membrane that is too dense to permit passage of sodium and chlorine ions. Hollow fibers of cellulose acetate or nylon are used as membranes, since their large surface area offers more efficient separation. 

Hollow fine fiber modules made from cellulose triacetate or aromatic polyamides were produced in the past for seawater desalination. These modules incorporated the membrane around a central tube, and feed solution flowed rapidly outward to the shell. Because the fibers were extremely tightly packed inside the pressure vessel, flow of the feed solution was quite slow. As much as 40-50 % of the feed could be removed as permeate in a single pass through the module. However, the low flow and many constrictions meant that extremely good pretreatment of the feed solution was required to prevent membrane fouling from scale or particulates. A schematic illustration of such a hollow fiber module is shown in Figure 3.47. 

Concurrently with the work on carbon dioxide and hydrogen sulfide at General Electric, Steigelmann and Hughes [27] and others at Standard Oil were developing facilitated transport membranes for olefin separations. The principal target was the separation of ethylene/ethane and propylene/propane mixtures. Both separations are performed on a massive scale by distillation, but the relative volatilities of the olefins and paraffins are so small that large columns with up to 200 trays are required. In the facilitated transport process, concentrated aqueous silver salt solutions, held in microporous cellulose acetate flat sheets or hollow fibers, were used as the carrier. 

Suppose we place a semipermeable membrane between a saline (salt) solution and pure water. If the saline solution is pressurized under a greater pressure than its osmotic pressure, the direction of flow can be reversed. That is, the net flow of water molecules will be from the saline solution through the membrane into the pure water. This process is called reverse osmosis. The membrane usually consists of cellulose acetate or hollow fibers of a material structurally similar to nylon. This method has been used for the purification of brackish (mildly saline) water. It has the economic advantages of low cost, ease of apparatus construction, and simplicity of operation. Because this method of water purification requires no heat, it has a great advantage over distillation. 

Hughes immobilized AgN03 solutions in cellulose acetate hollow fibers to prepare immobilized liquid membranes for ethylene and propylene transport. 

The removal of water from aqueous salt notations by reveres osmosis, as in seawater desaiiontion with cellulose acetate menibmues or aylon hollow fibers, is believed to occur primarily by a diffusive transport mechanism for both water and solutes. On the other hand, in the use of membranes for die removal of water from aqueous solutions containing higher molecular weight solutes, such as die ultrafiltration of protein solutions, die solvent is believed transported by a viscous How mechanism within the pores of the membrane and nolute molecules are corrected with the solvent in die larger pores.66-66... 

The best-known application of dialysis is the use of artificial kidneys to remove waste products from the blood of persons with kidney disease. Hollow-fiber cellulosic membranes are employed, and blood is passed through the fibers while saline solution is circulated on the outside. Urea and other small molecules diffuse through the membrane to the external solution, while proteins and cells are retained in the blood. The dialyzing solution has added salts and glucose to prevent loss of these materials from the blood. 

Several classes of polymeric materials are found to perform adequately for blood processing, including cellulose and cellulose esters, polyamides, polysulfone, and some acrylic and polycarbonate copolymers. However, commercial cellulose, used for the first membranes in the late 1940 s, remains the principal material in which hemodialysis membranes are made. Membranes are obtained by casting or spinning a dope mixture of cellulose dissolved in cuprammonium solution or by deacetylating cellulose acetate hollow fibers [121]. However, polycarbonate-polyether (PC-PE) block copolymers, in which the ratio between hydrophobic PC and hydrophilic PE blocks can be varied to modulate the mechanical properties as well as the diffusivity and permeability of the membrane, compete with cellulose in the hemodialysis market. 

A suitable polymer material for preparation of carbon membranes should not cause pore holes or any defects after the carbonization. Up to now, various precursor materials such as polyimide, polyacrylonitrile (PAN), poly(phthalazinone ether sulfone ketone) and poly(phenylene oxide) have been used for the fabrication of carbon molecular sieve membranes. Likewise, aromatic polyimide and its derivatives have been extensively used as precursor for carbon membranes due to their rigid structure and high carbon yields. The membrane morphology of polyimide could be well maintained during the high temperature carbonization process. A commercially available and cheap polymeric material is cellulose acetate (CA, MW 100 000, DS = 2.45) this was also used as the precursor material for preparation of carbon membranes by He et al They reported that cellulose acetate can be easily dissolved in many solvents to form the dope solution for spinning the hollow fibers, and the hollow fiber carbon membranes prepared showed good separation performances. 

The fact that silver ions form complexes with olefins has led to at least one patent for separation of unsaturated from saturated hydrocarbons (13). Very recently, however, Amoco was forced by economics to shelve plans to replace distillation columns in olefin plants with membrane-based separators involving hollow fibers of cellulose acetate saturated with solutions of silver nitrate (37). 

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