Separation Processes with Flow Reversal

A general scheme for the countercurrent ion-exchange separation process with flow reversal is analogous to the schemes for such two-phase separation processes as rectification, chemical exchange, etc. (Fig.l). 

The process in question is performed in two countercurrent columns. In both columns, ion-exchange resin moves down, i.e., firom top to bottom and solution moves in the opposite direction, i.e., firom bottom to top. 

A solution of the mixture of ions to be separated is fed into the bottom of column I. At the top end of the column flow reversal is affected, i.e., ions to be separated are transferred from the solution to the ion-exchange resin and directed back into the column. Leaving the column the ion-exchange resin, saturated with the mixture of ions to be separated, is moved into the top part of column II. At the bottom part of column II flow reversal is performed, i.e., ions to be separated are transferred from the ion-exchange resin to the solution and directed back into the column. Leaving the top part of column II the solution is moved at once, or after a particular treatment, for example after a concentration change, to the bottom part of column I. 

As a result of the fact that the selectivity of the ion exchanger toward mixture components is different the more weakly sorbed component is gradually concentrated in the top of column I while the more strongly sorbed one is concentrated at the bottom of column II. Once the required degree of enrichment is achieved it is possible to remove the product while adding the original mixture of components at the bottom of column I. 

Flow reversal, i.e., the transfer of the mixture of ions from solution to ion exchanger in the top portion of the column (sorption) and from ion exchanger into solution in the bottom portion (desorption) is effected with the help of specially selected auxiliary ions which differ in their affinity for the ion exchanger from the ions separated. 

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Figure 2 A scheme for process with flow reversal inside separation columns upon applying two auxiliary ions flow reversal.

Figure 4 A scheme for a process with flow reversal outside separation columns.

This article reviews the phase behavior of polymer blends with special emphasis on blends of random copolymers. Thermodynamic issues are considered and then experimental results on miscibility and phase separation are summarized. Section 3 deals with characteristic features of both the liquid-liquid phase separation process and the reverse phenomenon of phase dissolution in blends. This also involves morphology control by definite phase decomposition. In Sect. 4 attention will be focused on flow-induced phase changes in polymer blends. Experimental results and theoretical approaches are outlined. 

Comparison of the different schemes employable in various ion-exchange processes shows that the most suitable and simplest are those with flow reversal carried out directly in the column using one auxiliary ion. In the processes employed with this scheme the number of supplementary operations is kept to a minimum since the ion exchanger leaving column II can be used at once to sorb separated ions in column I. 

In the mode of minimum reflux adiabatic sections trajectories intersect reversible distillation trajectories in points Therefore, the separation process between product point and point can be carried out in principle, maintaining phase equilibrium between meeting flows of vapor and liquid in the cross-section at the height of the colunm by means of differential input or output of heat. We call such a separation process, with the same product compositions as at adiabatic distillation, a partially reversible one. A completely reversible process is feasible only for the preferable split that is rarely used in practice. Nonadiabatic distillation used in industry is a process intermediate between adiabatic and partially reversible distillation. Summary input and output of heat at nonadiabatic and adiabatic distillation are the same, and the energetic gain at nonadiabatic distillation is obtained at the transfer of a part of input or output heat to more moderate temperature level, which uses cheaper heat carriers and/or coolants. 

When a membrane is placed between pure water and an aqueous sodium chloride solution, water flows from the chamber filled with pure water to that filled with the sodium chloride solution, whereas sodium chloride does not flow (Figure 1.2a). As water flows into the sodium chloride solution chamber, the water level of the solution increases until the flow of pure water stops (Figure 1.2b) at the steady state. The difference between the water level of the sodium chloride solution and that of pure water at the steady state, when converted to hydrostatic pressure, is called osmotic pressure. When a pressure higher than the osmotic pressure is applied to the sodium chloride solution, the flow of pure water is reversed the flow from the sodium chloride solution to the pure water begins to occur. There is no flow of sodium chloride through the membrane. As a result, pure water can be obtained from the sodium chloride solution. The above separation process is called reverse osmosis. 

Capillary electrochromatography (CEC) is a rapidly emerging technique that adds a new dimension to current separation science. The major "news" in this method is that the hydrodynamic flow of the eluting liquid, which is typical of HPLC, is replaced by a flow driven by electro-endoosmosis. This increases considerably the selection of available separation mechanisms. For example, combinations of traditional processes such as reversed-phase- or ion-exchange- separations with electromigration techniques are now possible. Also, CEC is opening new horizons in the separation of non-polar compounds, and thus represents an alternative to the widely used micellar electrokinetic chromatography. 

Two fundamental principles applicable to the design of continuous countercurrent separation processes are known. They are identifiable with (i) flow reversal of the separated mixture at the edge of the contact sys-... 

On this basis, the automation of countercurrent column operation is easier to accomplish by flow reversal in the separation column than outside the column. However, it is not always possible to select an auxiliary ion which permits the flow reversal operation to occur solely in the column. This necessitates conduct of this operation outside the apparatus as well. If ions with similar properties are to be separated or the desired substances are to be purified by separating them from a small amount of impurity, difficulty in automating the column operation arises. Under these conditions it is expedient to carry out partial flow reversal inside the column and to complete the process outside the column. 

It is to be noted that with complete flow reversal the separation process in column I is the analogue of the frontal separation process or the frontal analysis in a flxed bed of the sorbent the separation process in column II is the analogue of the reverse frontal analysis and the process in both columns, with conditions, phase composition, and temperature unchanged in passing from one column to the other is the analogue of displacement chromatography. The analogy is related only to the... 

In mass transfer apparatus one of two processes can take place. Multicomponent mixtures can either be separated into their individual substances or in reverse can be produced from these individual components. This happens in mass transfer apparatus by bringing the components into contact with each other and using the different solubilities of the individual components in the phases to separate or bind them together. An example, which we have already discussed, was the transfer of a component from a liquid mixture into a gas by evaporation. In the following section we will limit ourselves to mass transfer devices in which physical processes take place. Apparatus where a chemical reaction also influences the mass transfer will be discussed in section 2.5. Mass will be transferred between two phases which are in direct contact with each other and are not separated by a membrane which is only permeable for certain components. The individual phases will mostly flow countercurrent to each other, in order to get the best mass transfer. The separation processes most frequently implemented are absorption, extraction and rectification. 

If a pressure higher than the osmotic pressure difference is applied to the side with high salt concentration, the water flow can be reversed. This process is termed reverse osmosis (RO), also known as hyperfiltration. This phenomenon makes the separation of... 

Reverse osmosis is a cross-flow membrane separation process which separates a feed stream into a product stream and a reject stream. The recovery of a reverse osmosis plant is defined as a percentage of feedwater that is recovered as product water. As all of the feedwater must be pretreated and pressurized, it is economically prudent to maximize the recovery in order to minimize power consumption and the size of the pretreatment equipment. Since most of the salts remain in the reject stream, the concentration of salts increases in that stream with increased recovery. For instance, at 50% recovery, the salt concentration in the reject is about double that of the feed and at 90% recovery, the salt concentration in the reject is nearly 10 times that of the feed. In cases of sparingly soluble salts, such as calcium sulfate, the solubility limits may be exceeded at a high recovery. This could result in precipitation of the salt on the membrane surface resulting in decreased flux and/or increased salt passage. In addition, an increase in recovery will increase the average salt concentration in the feed/reject stream and this produces a product water with increased salt content. Consequently, the recovery of a reverse osmosis plant is established after careful consideration of the desired product quality, the solubility limits of the feed constituents, feedwater availability and reject disposal requirements. 

Mass transfer rates attainable In menbrane separation devices, such as gas permeators or dlalyzers, can be limited by solute transport through the menbrane. The addition Into the menbrane of a mobile carrier species, which reacts rapidly and reversibly with the solute of Interest, can Increase the membrane s solute permeability and selectivity by carrier-facilitated transport. Mass separation is analyzed for the case of fully developed, one-dimensional, laminar flow of a Newtonian fluid in a parallel-plate separation device with reactive menbranes. The effect of the diffusion and reaction parameters on the separation is investigated. The advantage of using a carrier-facilitated membrane process is shown to depend on the wall Sherwood number, tfrien the wall Sherwood nunber Is below ten, the presence of a carrier-facilitated membrane system is desirable to Improve solute separation. 

Membrane separation processes such as gas permeation, pervaporation, reverse osmosis (RO), and ultrafiltration (UF) are not operated as equilibrium-staged processes. Instead, these separations are based on the rate at which solutes transfer though a semipermeable membrane. The key to understanding these membrane processes is the rate of mass transfer not equilibrium. Yet, despite this difference we will see many similarities in the solution methods for different flow patterns with the solution methods developed for equilibrium-staged separations. Because the analyses of these processes are often analogous to the methods used for equilibrium processes, we can use our understanding of equilibrium processes to help understand membrane separators. These membrane processes are usually either conplementary or conpetitive with distillation, absorption, and extraction. 

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