Porous barriers

Semicontinuous and continuous systems are, with few exceptions, practiced in columns. Most columnar systems are semicontinuous since flow of the stream being processed must be intermpted for regeneration. Columnar installations almost always involve the process stream flowing down through a resin bed. Those that are upflow use a flow rate that either partially fluidizes the bed, or forms a packed bed against an upper porous barrier or distributor for process streams. 

Filtration is the separation of a fluid-solids mixture involving passage of most of the fluidthrough a porous barrier which retains most of the solid particulates contained in the mixture. This subsec tion deals only with the filtration of solids from liquids gas filtration is treated in Sec. 17. Filtration is the term for the unit operation. A filter is a piece of unit-operations equipment by which filtration is performed. The filter medium or septum is the barrier that lets the liquid pass while retaining most of the solids it may be a screen, cloth, paper, or bed of solids. The hquid that passes through the filter medium is called the filtrate. 

Self-Test 4.15A It takes 30. mL of argon 40. s to effuse through a porous barrier. 

Sflf-Test 4.15B It takes a certain amount of helium atoms 10. s to effuse through a porous barrier. How long does it take the same amount of methane, CH4, molecules to effuse through the same barrier under the same conditions ... 

Because the ratio is so close to 1, the vapor must be allowed to effuse repeatedly through porous barriers consisting of screens with large numbers of minute holes. In practice, it is allowed to do so thousands of times. 

A using a porous barrier to separate a solid from a liquid... 

Figure 11.1 shows one example of a galvanic cell, called the Daniell cell. One half of the cell consists of a piece of zinc placed in a zinc sulfate solution. The other half of the cell consists of a piece of copper placed in a copper(II) sulfate solution. A porous barrier, sometimes called a semi-permeable membrane, separates these two half-cells. It stops the copper(II) ions from coming into direct contact with the zinc electrode. 

The Daniell cell is named after its inventor, the English chemist John Frederic Daniell (1790-1845). In the photograph shown here, the zinc sulfate solution is placed inside a porous cup, which is placed in a larger container of copper sulfate solution. The cup acts as the porous barrier. 

A typical galvanic cell, such as the Daniell cell shown here, includes two electrodes, electrolyte solutions, a porous barrier, and an external circuit. Electrons flow through the external circuit from the negative anode to the positive cathode. 

The separator between the half-cells does not need to be a porous barrier. Figure 11.5 shows an alternative device. This device, called a salt bridge, contains an electrolyte solution that does not interfere in the reaction. 

Like a galvanic cell, an electrolytic cell includes electrodes, at least one electrolyte, and an external circuit. Unlike galvanic cells, electrolytic cells require an external source of electricity, sometimes called the external voltage. This is included in the external circuit. Except for the external source of electricity, an electrolytic cell may look just like a galvanic cell. Some electrolytic cells include a porous barrier or salt bridge. In other electrolytic cells, the two half-reactions are not separated, and take place in the same container. 

Each cell of a lead-acid battery is a single compartment, with no porous barrier or salt bridge. Fibreglass or wooden sheets are placed between the electrodes to prevent them from touching. 

During World War II, plastics and lubricating compounds of unusual chemical and thermal stability were required for many applications, in particular for pumping apparatus used to separate 235U from 23SU by diffusion of corrosive uranium hexafluoride through porous barriers. It was natural to consider the use of substances made only of carbon and fluorine (fluorocarbons) for such purposes, and considerable effort was spent on methods of preparing compounds such as Today, many such substances are in common use. 

Effusion is the escape of a gas through a small hole into a vacuum (Fig. 4.21). Effusion occurs whenever a gas is separated from a vacuum by a porous barrier—a barrier that contains microscopic holes—or a single pinhole. A gas escapes through a pinhole because there are more collisions with the hole on the high-pressure side than on the low-pressure side, so more molecules pass from the high-pressure region into the low-pressure region than pass in the opposite direction. 

In an experiment to determine the molar mass of a newly synthesized chlo-rofluorocarbon (CFC) gas for use in a refrigeration system, it was found that. 25 mL of the gas effused through a porous barrier in 65 s. The same volume... 

A dry cell is the workhorse of primary cells.1 Its familiar cylindrical zinc container serves as the anode and in the center is the cathode, a carbon rod. The interior of the container is lined with paper that serves as the porous barrier. The electrolyte is a moist paste of ammonium chloride, manganese(IV) oxide, finely granulated carbon, and an inert filleq usually... 

This may work well if the process involves only electrically neutral species. However, when ions are discriminated on the basis of size, the partitioning process is affected by the Donnan potential. This potential, which we discuss more fully in Chapter 6, develops at the membrane/electrolyte interface. Another possibility is to discriminate on the basis of charge, as shown in Fig. 7.10 (see Chapter 7). Again, a porous barrier membrane is used, although here it would contain fixed, electrically charged moieties. When placed in front of the transducer, it rejects the like-charged species by electrostatic repulsion. In other words, it is a form of ion exchange membrane. 

Acrylic emulsion - The emulsion consisted of suspended crosslinked (gel) particles that are not water-soluble and form a film upon evaporation of the aqueous phase. However, the water did not evaporate quickly enough to form a continuous film on agar because agar is 95% water, and it continuously provided moisture that prevented film formation. The result was a porous barrier, but a continuous film was later obtained by dissolving dried emulsion solids in ethanol. 

Because the mechanisms are based on pore flow and size exclusion (cf. Section 2.2), the polymer material itself does not have direct influence on flux and selectivity in U F. The U F membranes usually have an integrally asymmetric structure, obtained via the NIPS technique, and the porous selective barrier (pore size and thickness ranges are 2-50 nm and 0.1-1 (im, respectively) is located at the top (skin) surface supported by a macroporous sublayer (cf. Section 2.4.2). However, the pore-size distribution in that porous barrier is typically rather broad (Figure 2.6), resulting in limited size selectivity. 

Rather than using a diagram such as that in Fig. 5, to describe an electrochemical cell, a standard simplified diagram is used. Vertical lines separate the various phases in the cell. For the separation between two liquid phases (by a porous barrier), a dotted or dashed vertical line is used. The terminals of the cells are placed on the ends of the diagram, with the anode on the left. Any metals attached to the terminals are written next to them. Gas or insoluble materials in contact with the metals are written next, and the electrolytic solution of the cell is described in the center of the diagram. To completely define the cell, the concentrations or activities of solutions and the pressures of gases are included. The simplified diagram for the cell illustrated in Fig. 5 is therefore... 

This cell has two electrolytic solutions, aqueous CuS04 and aqueous ZnS04, which are separated by a porous barrier. The barrier stops convective mixing of the two solutions. With the circuit open, there can be no net ionic flow through this barrier. However, ions of different size (e.g., Cu2+ and Zn2+) have different rates of diffusion through the pores of the barrier. A junction potential must therefore develop at the barrier to counter the tendency for diffusion to produce a net current. Liquid junctions are indicated by broken vertical lines in the cell... 

The two compartments of this cell are separated by a porous barrier that allows ions to pass through while preventing gross mixing of the two solutions. When the two electrodes are connected, charges flow in the directions indicated. Notethatthe buildup of positive charge on the left side can be offset either by diffusion of Zn2+ to the right or (less efficiently) by Cl to the left. 

This arrangement is called a galvanic cell. A typical cell might consist of two pieces of metal, one zinc and the other copper, each immersed each in a solution containing a dissolved salt of the corresponding metal. The two solutions are separated by a porous barrier that prevents them from rapidly mixing but allows ions to diffuse through. 

In the simplest cells, the barrier between the two solutions can be a porous membrane, but for precise measurements, a more compbcated arrangement, known as a salt bridge, is used. The salt bridge consists of an intermediate compartment filled with a concentrated solution of KC1 and fitted with porous barriers at each end. The purpose of the salt bridge is to minimize the natural potential difference, known as the junction potential, that develops (as mentioned in the previous section) when any two phases (such as the two solutions) are in contact. This potential difference would combine with the two half-cell potentials so as introduce a degree of uncertainty into any measurement of the cell potential. With the... 

A voltaic, or galvanic, cell allows the oxidation and reduction of substances to be physically separated. This is accomplished by allowing the electrons to pass from one location to the other by way of an external path (such as a wire). The circuit in a voltaic cell must be completed using a salt bridge or another porous barrier that allows for the transfer of anions between the two half-cells. 

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