Mass transfer dialysis

In H dialysis, mass transfer is executed under reproducible flow conditions which may be interrupted sometimes, but also in a reproducible manner. H on-line dialysis typically yield reproducibilities of 1-2% r.s.d. Although air-segmented continuous flow systems have also been used for on-line dialysis purposes, the introduction of air segments into the system defied the possibility of producing the highly reproducible conditions required for a non-equilibrated separation, and worse performances were obtained than those for non-segmented FLA systems. 

An artificial kidney is used to remove toxic materials from the blood stream by the process of dialysis (mass transfer) across porous cellophane membranes (shown in Figure 6.9). 

Electrodialysis. In reverse osmosis pressure achieves the mass transfer. In electro dialysis (qv), dc is appHed to a series of alternating cationic and anionic membranes. Anions pass through the anion-permeable membranes but are prevented from migrating by the cationic permeable membranes. Only ionic species are separated by this method, whereas reverse osmosis can deal with nonionic species. The advantages and disadvantages of reverse osmosis are shared by electro dialysis. 

Jacobson I, Sandberg M, Hamberger A. 1985. Mass transfer in brain dialysis devices-a new method for the estimation of extracellular amino acids concentration. J Neurosci Methods 15(3) 263-268. 

Dialysis, which involves mass transfer between two miscible liquids separated by a membrane, has been widely used in continuous systems [1], particularly in clinical analysis as a means for separating macromolecules present in biological fluids from the species of interest (of generally low molecular weights). 

For a detailed description of the separation processes that may take place at the sensing microzone, the foundation of which is closely related to non-chromatographic continuous separation techniques based on mass transfer across a gas-liquid (gas diffusion), liquid-liquid (dialysis, ultrafiltration) or liquid-solid interface (sorption), interested readers are referred to specialized monographs e.g. [3]). 

Dialysis procedures are relatively slow when mass transfer is based only on diffusion. These procedures do not offer particular selectivity when they are concurrently used for extraction and cleanup purposes, because many low-molecular-weight sample components along with the analyte can pass through the membrane. Dialysis systems must be renewed frequently, automation is difficult except for the continous-flow systems, and there is a significant temperature dependence. 

Lysaght, M.J., Ford, C.A., Colton, C.K. et al. (1977) Mass transfer in clinical blood ultrafiltration devices, in Technical Aspects of Renal Dialysis (ed. T.M. Frost), Pitman Medical, London, UK. 

De Lange ECM, De Boer AG, Breimer DD (2000) Methodological issues in microdialysis sampling for pharmacokinetic studies. Adv Drug Deliv Rev 45 125-148 Elmquist WF, Sawchuk RJ (1997) Application of microdialysis in pharmacokinetic studies. Pharm Res 14 267-288 Evrard PA, Deridder G, Verbeeck RK (1996) Intravenous microdialysis in the mouse and the rat development and pharmacokinetic application of a new probe. Pharm Res 13 12-17 Jacobson I, Sandberg M, Hamberger A (1985) Mass transfer in brain dialysis devices a new method for the estimation of extracellular amino acids concentration. J Neurosci Methods 15 263-268... 

There are a number of different membrane techniques which have been suggested as alternatives to the SPE and LLE techniques. It is necessary to distinguish between porous and nonporous membranes, as they have different characteristics and fields of application. In porous membrane techniques, the liquids on each side of the membrane are physically connected through the pores. These membranes are used in Donnan dialysis to separate low-molecular-mass analytes from high-molecular-mass matrix components, leading to an efficient cleanup, but no discrimination between different small molecules. No enrichment of the small molecules is possible instead, the mass transfer process is a simple concentration difference over the membrane. Nonporous membranes are used for extraction techniques. 

Hollow fiber contactors use membranes to separate two phases and transport is due to diffusion, chemical reaction, or chemical potential rather than pressure. The main examples of hollow fiber contactors are found in dialysis, gas adsorption/deadsorption, and solvent extraction. Use of hydrophilic and hydrophobic fiber materials controls the wetting of the pores. Typically, the phase that has higher mass transfer is allowed to wet the pores in order to minimize overall mass transfer resistance. 

Membrane operation is a specific, but not exotic, operation. In fact it is a hybrid of classical heat and mass transfer processes (Figure 4.1). Direct contact mass transfer operations tend to reach equilibrium due to a difference of chemical potential between two phases that are put into contact. In the same way, temperature equilibrium is aimed at during heat transfer operations, for which driving force is a temperature gradient. In contrast, for membrane operations, by using the specific properties of separation of the thin layer material that constitutes the membrane, under the particular driving force that is applied, it is possible to deviate from the equilibrium that prevails at fluid-to-fluid interphase with classical direct contact mass exchange systems and to reorientate the mass transfer properties. In particular, this is the case with classical operations such as microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), gas separation (GS), pervaporation (PV), dialysis (DI) or electrodialysis (ED), for which a few characteristics are recalled in Table 4.1. 

However, the use of permeable and semipermeable membranes in microfilters, ultrafilters, osmosis, reverse osmosis, dialysis (which are comparatively newer methods of separation) has problems like high capital costs, low mass transfer rate, low selectivity, and large equipment size. 

As was shown in the BAHLM model for transport kinetics, the values of the overaU mass-transfer coefficients govern the location (i ax) and the maximum quantity (Ofemax) of the metal species in the LM phase (see Eqs (27) and (28)). At Ormax Qf, the BAHLM is working mainly as a Donnan dialysis system, in which the loaded carrier solution is a treated feed. In this... 

Ktari T, Larchet C, Auclair B. Mass transfer characterization in Donnan dialysis. J Membr Sci 1993 84 53-61. 

Lake MA, Melsheimer SS. Mass transfer characterization of Donnan dialysis. AIChE J 1978 24(1) 130-138. 

The field of membrane separations is radically different from processes based on vapor-liquid or fluid-solid operations. This separation process is based on differences in mass transfer and permeation rates, rather than phase equilibrium conditions. Nevertheless, membrane separations share the same goal as the more traditional separation processes the separation and purification of products. The principles of multi-component membrane separation are discussed for membrane modules in various flow patterns. Several applications are considered, including purification, dialysis, and reverse osmosis. 

On one side of the membrane is the solvent, and on the other is the solution to be separated. The particles will pass from the solution side to the solvent side, in the direction of decreasing solute concentration. In a batch dialysis process, the mass transfer of solute passing through the membrane at a given time is ... 

One real application of membrane dialysis is the recovery of sodium hydroxide from a textile mill. The percent recovery was reported to be between 87.3% and 94.6%. However, dialysis is limited to smaller flowrates since the mass transfer coefficient (K) is relatively small. See Refs. [11,13] for additional information. 

The main parameters influencing mass transference through membranes in flow analysis have been discussed in specialised texts that generally deal with both gas diffusion and dialysis [254—257],... 

Hydrophobic membranes, e.g., PTFE, permit the efficient removal of volatile analytes from the sample matrix by diffusion though the micropores [257]. As these membranes have a high diffusion efficiency for many gaseous species, selectivity is usually low. For hydrophilic porous membranes, mass transference usually relies on dialysis, provided differences in donor and acceptor stream pressures are low [258] the chemical species originally in the donor stream migrate through the solvent in the interstitial volume of the membrane. Ionic species are therefore efficiently separated from the macromolecules in the sample matrix. Increasing the difference in pressures of both streams favours the micro-filtration process therefore, filtration and dialysis may occur simultaneously [259,260]. 

Dialysis involves the mass transference between two miscible liquid phases (the donor and acceptor solutions) separated by a liquid membrane through which some chemical species are likely to pass. Miscibility between the donor and acceptor solutions is inherent to dialysis and distinguishes it from e.g., liquid—liquid extraction, osmosis and ultrafiltration [271], These latter two membrane-based separation approaches tend to occur concomitantly with dialysis and involve the solvent rather than the solute crossing the membrane. In osmosis, the driving force towards separation is the concentration difference involved whereas in ultrafiltration, also called reverse osmosis, the driving force is an applied pressure that forces the solution across the membrane. 

Electrodialysis. The driving force is an external electric field that accelerates mass transference across the membrane. Charged compounds are transferred, whereas uncharged compounds are retained. As the dialysis efficiency depends on the external applied electric field, a potentiostat should be linked to both sides of the membrane. Details are given elsewhere [277,278]. 

This approach was further exploited in the spectrophotometric flow-injection determination of chloride in industrial effluents involving a passive neutral membrane [288]. The advantages inherent to passive dialysis were maintained and the mass transference was improved. Wide-range spectrophotometry was achieved by varying the applied voltage and/or the flow rates of donor and acceptor streams. 

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