Dialysis convective

Solute flux within a pore can be modeled as the sum of hindered convection and hindered diffusion [Deen, AIChE33,1409 (1987)]. Diffusive transport is seen in dialysis and system start-up but is negligible for commercially practical operation. The steady-state solute convective flux in the pore is J, = KJc = where c is the radially... 

Dialysis continues to meet certain specialized applications, particularly those in biotechnology and the life sciences. Delicate substances can be separated without damage because dialysis is typically performed under mild conditions ambient temperature, no appreciable transmembrane pressure drop, and low-shear flow. While slow compared with pressure-driven processes, dialysis discriminates small molecules from large ones reliably because the absence of a pressure gradient across the membrane prevents convective flow through defects in the membrane. This advantage is significant for two... 

All separation procedures rely on some element of differential transport the separation may be based on differences in phase equilibria, as in chromatography, or on the kinetics of transport, as in electrophoresis and centrifugation. Precipitation procedures, filtration and dialysis are also members of the broad dass of separation methods these have been discussed in an earlier chapter (Sect. 3.4). The transport processes involved in separating components are often opposed by dispersion processes such as diffusion and convection, which have to be minimised to achieve the best separation results. 

Hemodiafiltration (HDF) is a modahty in which diffusive and convective methods are combined to increase removal of large molecular weight intoxicants. If this ultrafiltration dialysis is conducted at lower blood and dialysate flow rates, convection can provide equivalent total clearance with less hemodynamic perturbance and dialysate wastage. 

The utility of continuous renal replacement therapies (CRRT) such as continuous venous-venous hemodialysis (CWHD) in the treatment of poisoning is uncertain. As CRRT provides slower clearance than conventional hemodialysis it may not be appropriate for drug removal in acute intoxications [25]. However, the lower blood flow rates and longer treatment times of continuous modalities may be desirable for vulnerable, hemodynamically unstable, patients who are not candidates for conventional hemodialysis [7]. Unlike hemodialysis, CRRT can give effective clearances in hypotensive patients. If the clinical condition of the patient requires a low intensity treatment that will necessarily decrease diffusive clearance, slow extended dialysis (SLED) or continuous treatment times with additional convective clearance (CVVHF and CVVHDF) can likely provide adequate total drug clearance [24]. 

The URR does not take convective removal of urea or RRF into account. Its accuracy is lower than K /V measured by formal UKM, particularly at high values of URR and Observational studies in populations of dialysis... 

The URR is an easy calculation and thus is frequently used to measure the delivered dialysis dose. However, the URR does not account for the contribution of convective removal of urea. The Kt/V is the dialyzer clearance of urea K) in L/h multiplied by the duration of dialysis (/) in hours, divided by the urea distribution volume of the patient (V) in liters. Kt/Vi a unitless parameter that quantitates the fraction of the patient s total body water that is cleared of urea during a dialysis session. Urea kinetic modeling, using special computer software, is the optimal means to determine the Kt/V. Kt/V can also be calculated by using the following equation. ... 

For small-scale synthesis enclosure of enzymes in dialysis tubes has been described for several systems (membrane-enclosed enzyme catalysis or the MEEC technique 127 ). In this case mass transport of the low-molecular-weight substrates and products across the membrane becomes rate limiting because mass transport only occurs by diffusion and not by convection as described below. 

Dialysis is a diffusion-based separation process that uses a semipermeable membrane to separate species by vittue of their different mobilities in the membrane. A feed solution, containing the solutes to he separated, flows ou one side of the membrane while a solvent stream, die dialysate, flows on die other side (Fig. 21. -1). Solute transport across the membrane occurs by diffusion driven by the difference in solme chemical potential between the two membrane-solution interfaces. In practical dialysis devices, no obligatory transmembrane hydraulic pressure may add an additional component of convective transport. Convective transport also may occur if one stream, usually the feed, is highly concentrated, thus giving rise to a transmembrane osmotic gradient down which solvent will flow. In such circumstances, the description of solute transport becomes more complex since it must incorporate some function of die trans-membrane fluid velocity. 

As solute is removed from ihe feed-side membrane-solution interface by dialysis, the layer is depleted and its concentration must he restored from the bulk solution. In laminar flow, which is usual In smallbore hollow-fiber and thin-film plate-and-frame devices, there is no convection and repletion of the interfacial layer is solely by diffusion from the bulk solution. As such diffusion occurs, the concentration gradient from the bulk solution to the imeriacial layer decmases, Tims, the rate of restoration of the inierfacia solute concentration is a fuactlon of the solute size, the transmembrane flux, and the rate of solute supply, that is, the axial feed flow rate. 

In the past, the most common method used for microsolute removal has been batch dialysis. The solution to be dialyzed was placed in seamless regen-erated-cellulose tubing ("sausage bags") and suspended in the dialysate allowing salts to diffuse across the membrane. With diafiltration, the same degree of salt removal can be accomplished much more rapidly with smaller volumes of dialysate. The pressure driven convective transport of solutes across the membrane is much faster than concentration driven diffusion (particularly at low salt concentrations). In addition, with diafiltration, all solutes (saltsand alcohol) are removed at the same rate independent of the size and diffusivity of the various species this makes the process more predictable and controllable. 

J.E. DiNunzio and M. Jubara, Donnan dialysis preconcentration for ion chromatography, Anal. Chem., 1983, 55, 1013 J.A. Cox and G.R. Litwinski, High sample convection Donnan dialysis, Anal. Chem., 1983, 55, 1640-1642 J.A. Koropchak and L. Allen, Flow infection Donnan dialysis preconcentration of cations for flame atomic absorption spectrophotometry, Anal. Chem., 1989, 61, 1410. 

Hollow-fiber membrane modules are the mass-transfer equivalent of shell-and-tube heat exchangers. As fluids flow through the shell and lumen, mass is transferred from one stream to the other across the fiber wall. In contrast to heat exchangers, though, mass transfer may involve a combination of diffusion and convection, depending on the nature of the membrane. These modules are used for a wide range of membrane processes, including gas separation, reverse osmosis, filtration, and dialysis. 

Synthetic separation membranes are either nonporous or porous. For nonpor-ous membranes, permeability and selectivity are based on a solution-diffusion mechanism examples for technical membrane separations are gas separation, reverse osmosis, or pervaporation. For porous membranes, either diffusive or convective How can yield a selectivity based on size, for larger pore sizes typically according to a sieving mechanism examples for technical membrane separations are dialysis, ultrafiltration, or microfiltration. It is important to note that additional interactions between permeand and membrane, e.g., based on ion exchange or affinity, can change the membrane s selectivity completely membrane adsorbers with a pore structure of a microfiltration membrane are an example. 

In all electrofocusing, some method must be used to stabilize the liquid against convection currents. The usual methods employ sucrose (density gradient) or polyacrylamide (gel). These substances are present in much larger quantities than the actual substance being tested. This always involves the risk of disturbances. Sucrose must be removed by dialysis at the end of the fractionation. It is also difficult to separate the protein from the polyacrylamide. 

In practice a combination of dialysis and filtration is used, haemodiafiltration. Haemodiafiltration (HDF) is a combination of diffusion and convection. Diffusion is mainly effective for the removal of small waste molecules such as urea and creatinine. Larger molecules, for example beta-2-microglobuline, may only be removed from the blood by convection. For sufficient convective transport per HDF treatment an equivalent of 60 L of plasma is filtrated. At the same time the same volume is given back to the patient in the form of substitution solution. The substitution solution enters the circulation of the patient. This is the same process as the administration of an infusion, which is why some European Inspectorates regards solutions for HDF as parenterals. 

In many processes, including those in nature, transport proceeds via diffusion rather than convection. Substances diffuse spontaneously from a high to a low chemical potential. Processes which make use of a concentration difference as the driving force are gas separation, vapour permeation, pervaporation, dialysis, diffusion dialysis, carrier mediated processes and membrane contactors (In pervaporation, gas separation and vapour permeation it is preferred to express the driving force as a partial pressure difference or an activity difference rather than concentration difference). On the basis of differences in structure and functionality it is possible to distinguish between processes that use a synthetic solid (polymeric or sometimes ceramic or zeolitic) membrane (gas separation, dialysis and pervaporation) and those that use a liquid (with or without a carder) as the membrane. 

During the sampling step, ionic analytes are continuously transferred to the acceptor solution through the membrane. Unless the analytes are immediately replenished at the membrane interface, their concentration falls to an adverse level affecting dramatically the dialysis efficiency. By the use of probes, this problem can be avoided by keeping the unknown solution under convective mixing (e.g. with a magnetic stirrer) in order to ensure a constant supply to the membrane surface. 

Dialysis Symmetric microporous membrane, 0.1 to 10 pA pore size Concentration gradient Diffusion in convection-ffee layer Separation of salts and microsolutes from macromolecular solutions... 

Most microfluidic microdialysis systems consist of a two-compartment system with a sample flow channel and perfusion flow channel separated by the microdialysis membrane. A two-compartment cocurrent mass transport model of microdialysis is shown in Fig. 3. For this micro-dialysis system, molecules inside the sample channel are dialyzed across the membrane into the perfusion flow channel. Again, this system may be modeled by balancing the sample and perfusion convective fluxes with the diffusion of analyte across the membrane. Assuming the overall permeability is constant, K, over the diffusion path, the mass transfer along the membrane diffusional area. A, can be described as... 

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