Dialysis

Dialysis is liquid permeation where solutes with different permeabilities and film resistances are transported at different fluxes through the membrane, resulting in separation of the solutes. The driving force for each solute is its concentration gradient across the membrane, and the flux is given by Equations 18.11 through 18.14. 

Dialysis has many applications, perhaps the most important of which is blood purihcation with the artificial kidney, where urea and other impurities are removed from the blood by dialysis. Applications in the chemical process industry include the removal of metal ions from acid solutions, and the purification of many other products such as polymers and pharmaceuticals. 

In many applications dialysis is carried out in countercurrent fiow, flat sheet equipment, where laminar fiow is assumed. Countercurrent fiow calculations similar to those discussed in Section 18.2 could be applied to countercurrent dialysis. Alternatively, a simplified model may be used where the driving force is expressed as a log mean average of the concentration differences, as shown in Example 18.5. 

An aqueous solution of H2SO4 (1) and CUSO4 (2) is fed to a dialyzer to remove some of the CUSO4 from the acid. The feed flow rate is 0.30 mVh and the concentrations of the solutes in the feed are 0.2 kmol/m H2SO4 and 0.1 kmol/m CUSO4. The solvent is pure water with a flow rate of 0.32 mVh, and is sent to the dialyzer in countercurrent flow to the feed. The permeances and mass transfer coefficients (assumed equal on both sides of the membrane) are given below for the solutes  

It is required to lower the concentration of CUSO4 in the diffusate to 0.075 kmol/m. Calculate the required membrane area and the compositions of the diffusate and dialysate. It may be assumed that no water permeation occurs. 

Dialysis is attractive when the concentration differences for the main diffusing solutes are large and the permeability differences between those solutes and the other solutes and/or colloids is large. Although commercial applications of dialysis do not rival reverse osmosis and gas permeation, it has been applied to a number of important separation processes, such as purification of pharmaceuticals, production of a reduced-alcohol beer, and hemodialysis (described in detail in Example 1.3). 

Topical microporous-membrane materials used in dialysis are hydrophilic, including cellulose, cellulose acetate, and various acid-resistant polyvinyl copolymers, typically less than 50 pm thick and with pore diameters of 15 to 100 A. Dialysis membranes can be thin because pressures on either side of the membrane are essentially equal. The most common membrane modules are plate-and-frame and hollow-fiber. Compact hollow-fiber hemodialyzers, which are widely used, typically contain several thousand 200-pm-diameter fibers with a wall thickness of 20-30 pm and a length of 10 to 30 cm (Seader and Henley, 2006). 

In a plate-and-frame dialyzer, the flow pattern is nearly countercurrent. Because total flow rates change little and solute concentrations are typically small, it is common to estimate the solute transport rate by assuming a constant overall mass-transfer coefficient with a log-mean concentration driving force. Thus, 

Although the transport of solvents, such as water, which usually occurs in a direction opposite the solute, could be formulated in similar terms, it is more common to report the so-called water-transport number, which is the ratio of the water flux to the solute flux. A negative value of the water-transport number indicates transport of solvent in the same direction as the solute. Ideally, the absolute value of the water transport number should be less than 1.0. 

A countercurrent plate-and-frame dialyzer is to be sized to process 1.0 m3/h of an aqueous solution containing 25 wt% H2S04 and smaller amounts of copper and nickel sulfates. A wash water rate of 1000 kg/h is to be used, and it is desired to recover 60% of the acid at 298 K. From laboratory experiments with an acid-resistant vinyl membrane, a permeance of 0.03 cm/min for the acid and a water-transport number of +0.8 were reported. Transmembrane transport of copper and nickel sulfates is negligible. For these operating conditions, it has been estimated that the combined external mass-transfer coefficients will be 0.02 cm/min. Estimate the membrane area required. 

Dialysis separations are often used for removal of interferents in the sample matrix. The technique is based on differences in mobility of ionic or molecular constituents in a liquid phase during their transport across a semi-permeable membrane into a second liquid phase which need not be immiscible with the first. Mass transfer occurs between a donor phase and an acceptor phase separated by a membrane which selectively allows penetration of solutes by blocking the passage of macro-molecules or by differences in molecular diffusivities. The driving force of the mass transfer is the existence of a concentration gradient of the transferable solute between the two phases. 

The first application of on-line dialysis to a flow system seems to be that made by Skeggs [ 1 ] in his pioneering work on segmented continuous flow analysis. The first report on using on-line dialysis in a non-segmented flow system was that made by Kadish and Hall [2], whereas Hansen and Ruzicka [3] were the first to report such applications in FIA. Despite its early implementation in HA, apyplications of on-line dialysis in this field have been rather few compared to other separation techniques, and mostly dedicated to the analysis of blood serum. This may be due to the fact that dialysis is a slow separation procedure compared to the speed of most FI procedures, and the dialysis efficiencies are usually quite low. 

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. 

Owing to the short time available for solute transfer in FLA, under normal experimental parameters the solute transfer is, at best, usually less than 15%, occasionally less than 1%. This becomes a serious limitation when sensitivity of the method is of concern 

Dialysis membranes can be animal membranes and cellophane, but mostly used membranes are made of cellulose. Now, various-sized dialysis tubes, made by the American Union Carbide and American medical spectrum, are commonly used. The MWCO of the tubes are usually around 10,000. In order to improve the efficiency of dialysis, a variety of devices can also be used, including various types of Zeineh dialyzer (Biomed Instruments Inc. US), by which the speed and efficiency of dialysis can be greatly increased. 

In the preparation of CGTase, dialysis can often be used for desalination. Most of the preparation processes use ammonium sulfate precipitation method, and some of the preparation processes use buffer solution such as Tris-HCl buffer or acetate buffer fluid, introducing a number of ions to the system. In order to obtain pure CGTase, dialysis approach is mostly employed to remove salts, which is an effective way. 

In dialysis, liquid phases containing the same solvent are present on both sides of the membrane in the absence of a pressure difference. The pressure terms can therefore be neglected and the following equation may be obtained from eq. V - 152 if oq = 1. 

This simple equation describes the solute flux in dialysis indicating that it is proportional to the concentration difference. Separation arises from diffraences in permeability coefficients thus macromolecules have much lower diffusion coefficients and distribution coeffients than low molecular weight components. 

FIA systems comprising dialysis (or microfiltration) employ modules similar to those used for gas diffusion, the hydrophobic microporous membrane being replaced by a hydrophilic dialysis membrane (such as cu-prophan, cellulose aceate, or cellulose nitrate). Basically, the incorporation of a dialysis unit into a FIA manifold is to serve one (or several) of the following objectives (1) separation of an analyte species from unwarranted matrix constituents, (2) as an exact and reproducible means of dilution, or (3) microfiltration performed continuously by transferring species from one stream (the donor) to another stream (the acceptor). 

Within the last few years FIA has found increased application in process monitoring, such as in biotechnology. A common feature of these procedures is that they require (a) representative samples to be drawn from a process solution and (b) the samples have to be purified from 

It is sometimes advantageous to take advantage of membrane hydrolysis and this is used to convert proteins into acidic forms without recourse to conventional chemical means which might interfere with the system. Consider dialysis into pure water of a salt NaR from a solution through a membrane which allows passage of Na but is impermeable to R  

Sodium ions from II diffuse into I along with an equivalent number of hydroxyl ions. These latter arise from the dissociation of water which is necessary to maintain electroneutrality of I. The hydrogen ions produced by this process then associate with anions R to form the weak acid RH and 

At equilibrium we may write, in accordance with Equation (7.66) therefore, 

Although the number of sodium ions passing into I to meet the equilibrium conditions is not large, continuous replacement of solution I by pure water forces the process to continue by encouraging a continuous movement towards equilibrium. In this way the hydrolysis of the species NaR may be effected to a significant extent. 

For a flat membrane, the mass transfer fluxes through the two liquid films on the membrane surfaces and through the membrane should be equal to JA. 

kL1 and kL2 are the liquid film mass transfer coefficients (mT1) on the membrane surfaces of the feed side and the dialysate side, respectively CMi and 

CM2 are the solute concentrations (kmol m-3) in the feed and dialysate at the membrane surfaces, respectively and Cand C 2 are the solute concentrations in the membrane (kmol m-3) at its surfaces on the feed side and the dialysate side, respectively. The relationships between CM1 and Cj, and between Cj 2 and CM2 are given by the solute solubility in the membrane. kM is the diffusive membrane permeability (m h 1), and should be equal to DM/xM, where DM is the diffusivity of the solute through the membrane (m2h 1) and xM is the membrane thickness (m). Dm varies with membranes and with solutes for a given membrane, DM usually decreases with increasing size of solute molecules and increases with temperature. 

In the case where the membrane is flat, the overall mass transfer resistance -that is, the sum of the individual mass transfer resistances of the two liquid films and the membrane - is given as  

Dialysis is used on large scale in some chemical industries. In medicine, blood dialyzers, which are used extensively to treat kidney disease patients, are discussed in Chapter 14. 

If cells (including blood cells) are immersed in solutions with a higher concentration of materials, then the osmotic pressures causes the water to pass from the cells and they shrivel. This is because water passes out of the cells through the cell walls, which are semi-permeable, into the more concentrated solution. This is called crenation and can be disastrous for the cells. Food preservation processes can use this to advantage, for example, if meat is treated with salt then any bacteria cells on the surface shrivel and die. Similarly fruit can be covered with sugar with the same effect, and candied fruit is formed. 

People who eat a lot of very salty food experience water retention in tissue cells, because water taken in as drinks to try to compensate for this enters the cells, which have more concentrated salt solutions, resulting in the appearance of puffiness which is called oedema . It is worth looking up the remedy for this condition and possible medication. 

Dialysis waste products dialyse out into the washing dialysing solution 

Haemodialysis usually takes about 4-7 h and the dialysing solutions are changed periodically. Ask in your hospital about the exact procedures used for dialysis, usually organized by the renal unit. You also might ask what stringent safety and health risk precautions the workers must undertake when working in the unit. 

Division of Nmhmiogy School of Medicine University of Louisville Louisville, Kentucky 

Dialysis first was reported in 1861 by Graham, who used parehment paper as a membrane. His experiments were based on the observations of a school teacher, W. G. Schmidt, that animal membranes were less permeable to colloids than to sugar or salt. Over the next 100 years, dialysis became widely used as a laboratory technique for the purification of small quantities of solutes but, with minor exceptions, it realired no large-scale industrial applications. In the last 20 years, development of dialysis for the treatment of kidney fulure has brought about a resurgence of interest in dialysis for a wide range of separations. 

Dialysis is a difliiaon-based separation process that uses a semipermeable membrane to separate species by virtue of thmr ditferem mobilities in the membrane. A feed solution, containing the solutes to be separated, flows on one side of flie membrane while a solvent stream, the dialysate, flows on the other si (Hg. 21.1-1). Solute transport across the membrane occurs by diffusion driven by the difference in solute chemical potential between the two membrane-solution interfaces. In practical dialysis devices, an 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 Iransmrenbrane osmotic gradient down which solvent will flow. In such circumstances, the description of solute transport becomes ttwre complex since it must incorporate some function of the trans-membrane fluid velocity. 

The relative transfer of two solutes across a dialysis membrane is a function of both their diffusivities in the mmnbtane and their driving forces. Separations will be efficient only for species that differ signifi-catttly in diffusion coefficient. Since diffusion coefficients are a relatively weak function of molecular size, 

FIGURE 21.1 1 Schematic lepiesentation of dialysis. Small, highly mobile species (O) diffuse across the membrane, from feed to dialysate, while larger molecules ( ), to which the membrane is relatively im-permeaUe, remain in the feed stream. 

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A diffusion mechanism is also used in dialysis as a means of separating colloids from crystalloids. The rate of diffusion of molecules in gels is practically the same as in water, indicating the continuous nature of the aqueous phase. The diffusion of gases into a stream of vapour is of considerable importance in diffusion pumps. 

This is an analysis frequently conducted on oil lubricants. Generally, the additive is known and its concentration can be followed by direct comparison of the oil with additive and the base stock. For example, concentrations of a few ppm of dithiophosphates or phenols are obtained with an interferometer. However, additive oils today contain a large number of products their identification or their analysis by IR spectrometry most often requires preliminary separation, either by dialysis or by liquid phase chromatography. 

After preparation, colloidal suspensions usually need to undergo purification procedures before detailed studies can be carried out. A common technique for charged particles (typically in aqueous suspension) is dialysis, to deal witli ionic impurities and small solutes. More extensive deionization can be achieved using ion exchange resins. 

Illustration of a dialysis membrane in action. In (a) the sample solution is placed in the dialysis tube and submerged in the solvent, (b) Smaller particles pass through the membrane, but larger particles remain within the dialysis tube. 

In one version of the urea electrode, shown in Figure 11.16, an NH3 electrode is modified by adding a dialysis membrane that physically traps a pH 7.0 buffered solution of urease between the dialysis membrane and the gas-permeable... 

Directions are provided for constructing and characterizing an ammonium ion-selective electrode. The electrode is then modified to respond to urea by adding a few milligrams of urease and covering with a section of dialysis membrane. Directions for determining urea in serum also are provided. 

Separation Modules Incorporating a separation module in the flow injection manifold allows separations, such as dialysis, gaseous diffusion, and liquid-liquid extraction, to be included in a flow injection analysis. Such separations are never complete, but are reproducible if the operating conditions are carefully controlled. 

Separation module for a flow Injection analysis using a semipermeable membrane for dialysis and gaseous diffusion. 

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