Solution-diffusion separation, membranes

Mixed-matrix membranes comprising small-pore zeolite or small-pore non-zeolitic molecular sieve materials will combine the solution-diffusion separation mechanism of the polymer material with the molecular sieving mechanism of the zeolites. The small-pore zeolite or non-zeolitic molecular sieve materials in the mixed-matrix membranes are capable of separating mixtures of molecular species... 

The most common separators include the Ryhage or jet diffusion separator (74), the Watson-Biemann or pore diffusion separator (75), and the membrane solution diffusion separator originally developed by Llewellyn (75). The first two separators involve direct passage of the sample into the mass spectrometer the low molecular weight helium diffuses more readily and is pumped away. The membrane separator involves diffusion of the sample through a silicone membrane while the carrier gas vents to the atmosphere carrier gas is thus not confined to helium. There is no best separator the choice depends on the nature of the compounds, the temperature range over which it will be operated, and most usually what is available in a particular laboratory. A convenient configuration for a double-beam mass spectrometer such as the AEI MS-30 is two different separators, one into each beam, which permits rapid evaluation of separator performance. 

As explained in Chapter 5, the transport mechanism in dense crystalline materials is generally made up of incessant displacements of mobile atoms because of the so-called vacancy or interstitial mechanisms. In this sense, the solution-diffusion mechanism is the most commonly used physical model to describe gas transport through dense membranes. The solution-diffusion separation mechanism is based on both solubility and mobility of one species in an effective solid barrier [23-25], This mechanism can be described as follows first, a gas molecule is adsorbed, and in some cases dissociated, on the surface of one side of the membrane, it then dissolves in the membrane material, and thereafter diffuses through the membrane. Finally, in some cases it is associated and desorbs, and in other cases, it only desorbs on the other side of the membrane. For example, for hydrogen transport through a dense metal such as Pd, the H2 molecule has to split up after adsorption, and, thereafter, recombine after diffusing through the membrane on the other side (see Section 5.6.1). 

In the solution-diffusion separation mechanism, the permeating species dissolves in the membrane material and then diffuses responding to the chemical potential gradient. Consequently, the equation governing the solute flux is [24,25]... 

Gas separation is possible even with the two extreme types of membrane considered, i.e. porous and non-porous. The transport mechanisms through these two types of membrane, however, are completely different. Gas separation is performed using membranes based on three general transport mechanisms Knudsen diffusion, solution-diffusion, molecular sieving. Industrially relevant are solution-diffusion based membranes. 

Ultrafiltration separations range from ca 1 to 100 nm. Above ca 50 nm, the process is often known as microfiltration. Transport through ultrafiltration and microfiltration membranes is described by pore-flow models. Below ca 2 nm, interactions between the membrane material and the solute and solvent become significant. That process, called reverse osmosis or hyperfiltration, is best described by solution—diffusion mechanisms. 

FIGt 22-48 Transport mechanisms for separation membranes a) Viscous flow, used in UF and MF. No separation achieved in RO, NF, ED, GAS, or PY (h) Knudsen flow used in some gas membranes. Pore diameter < mean free path, (c) Ultramicroporoiis membrane—precise pore diameter used in gas separation, (d) Solution-diffusion used in gas, RO, PY Molecule dissolves in the membrane and diffuses through. Not shown Electro-dialysis membranes and metallic membranes for hydrogen. 

When paint films are immersed in water or solutions of electrolytes they acquire a charge. The existence of this charge is based on the following evidence. In a junction between two solutions of potassium chloride, 0 -1 N and 0 01 N, there will be no diffusion potential, because the transport numbers of both the and the Cl" ions are almost 0-5. If the solutions are separated by a membrane equally permeable to both ions, there will still be no diffusion potential, but if the membrane is more permeable to one ion than to the other a diffusion potential will arise it can be calculated from the Nernst equation that when the membrane is permeable to only one ion, the potential will have the value of 56 mV. 

Some of the polymeric membranes are suitable for bulk separation of hydrogen from impurities to enrich a dilute hydrogen stream. Dense polymers permeate gases by solution diffusion mechanism. The permeation rate of a gas species through a polymer membrane... 

Let us assume a solution of a non-electrolyte in water, separated from the pure solvent—water—by a semiper-meable membrane forming a piston (Fig. 8). Water enters the solution through the membrane and raises the piston, i.e., the solution can do work or possesses potential energy owing to its osmotic pressure. If the membrane is removed, the osmotic pressure causes diffusion until (if no other forces are active) the solute is uniformly distributed through the solvent. Osmotic pressure is, therefore, a factor tending to bring about uniform concentration. 

De Pinto [90] measured the rate at which available phosphorus is released from various types of particulates suspended in lake water. The equipment consists of two culture vessels separated by a thin membrane filter, thus facilitating the separation of two particulate suspensions, while at the same time permitting their interaction by diffusion of solutes through the membrane. 

First, porous membranes will be discussed. Gases can be separated due to differences in their molecular masses (Knudsen diffusion), due to interaction (surface diffusion, multilayer diffusion and capillary condensation) and due to their size (molecular sieving). All these mechanisms and their possibilities will be discussed. For the sake of simplicity, theoretical aspects are not covered in detail, but examples of separations in literature will be given. The next section deals with nonporous membranes. Here the separation mechanism is solution-diffusion, e.g. solution and diffusion of hydrogen through a platinum membrane. This section is followed by an outline of some new developments and conclusions. 

In dense membranes, no pore space is available for diffusion. Transport in these membranes is achieved by the solution diffusion mechanism. Gases are to a certain extent soluble in the membrane matrix and dissolve. Due to a concentration gradient the dissolved species diffuses through the matrix. Due to differences in solubility and diffusivity of gases in the membrane, separation occurs. The selectivities of these separations can be very high, but the permeability is typically quite low, in comparison to that in porous membranes, primarily due to the low values of diffusion coefficients in the solid membrane phase. 

The cell plasma membrane separates the cell cytoplasm from the external medium. The composition of the cytoplasm must be tightly controlled to optimize cellular processes, but the composition of the external medium is highly variable. The membrane is hydrophobic and impedes solute diffusion. But it also facilitates and regulates solute transfers as the cell absorbs nutrients, expels wastes and maintains turgour. 

Several topics in diffusion have arbitrarily been excluded from the following discussion diffusion in electrolytic solutions (F3, G9, HI, 02, R3, Yl), diffusion in ionized gases (K4), diffusion in macromolecular systems (W2), diffusion through membranes (F14), use of diffusional techniques in isotopic separations (S18), diffusion in metals (S8), and neutron diffusion (F2, G5, H15, W15). 

Transport through the membrane can be considered to occur by a solution-diffusion mechanism under the influence of a chemical potential driving force [48, 49]. The primary benefit of using PV systems is that they are essentially independent of the vapor-liquid equihbrium of solvent mixtures. Therefore PV can be used to overcome the separation barriers created by many azeotropic mixtures [48, 50]. 

Water molecules tend to move from a region of higher water concentration to one of lower water concentration. When two different aqueous solutions are separated by a semipermeable membrane (one that allows the passage of water but not solute molecules), water molecules diffusing from the region of higher water concentration to that of lower water concentration produce osmotic pressure (Fig. 2-12). This pressure, n, measured as the force necessary to resist water movement (Fig. 2-12c), is approximated by the van t Hoff equation ... 

But the activity of C+ in the membrane (dAm) is very nearly constant for the following reason The high concentration of LC+ in the membrane is in equilibrium with free L and a small concentration of free C+ in the membrane. The hydrophobic anion R is poorly soluble in water and therefore cannot leave the membrane. Very little C+ can diffuse out of the membrane because each C+ that enters the aqueous phase leaves behind one R in the membrane. (This separation of charge is the source of the potential difference at the phase boundary.) As soon as a tiny fraction of C diffuses from the membrane into solution, further diffusion is prevented by excess positive charge in the solution near the membrane. 

You can demonstrate the size of colloidal particles with a dialysis experiment in which two solutions are separated by a semipermeable membrane that has pores with diameters of 1—5 nm.3 Small molecules diffuse through these pores, but large molecules (such as proteins or colloids) cannot. (Collecting biological samples by microdialysis was discussed at the opening of Chapter 25.)... 

When solvent and solution are separated by a semipermeable membrane that permits solvent molecules to pass, an osmotic pressure is developed in the solution. This pressure, tt, is defined as the mechanical pressure that must be applied to the solution to prevent solvent molecules from diffusing into it. For water solutions the relationship between tt and the molal concentration m is given by the equation... 

An apparatus lor carrying out a dialysis usually consists of two chambers separated by a semipermeable membrane of parchment paper latex, animal lissue. or oilier colloid. In one chamber the solution is placed, and in Ihc other, the pure solvent. Crystalline substances diffuse from the solution through the membrane and into the solvent much more rapidly than amorphous substances, colloids or large molecules. 

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