Countercurrent absorption extraction

Dynantics of Heat Exchangers, Simple Batch Extraction, Multi-Solute Batch Extraction, Multistage Countercurrent Ctiscade, Extraction Cascade with Backmixing, Countercurrent Extraction Cascade with Reaction, Absorption with Chemical Reaction, Membrane Transfer Processes... 

In contrast to continuous packed bed columns, each stage, whether cocurrent or countercurrent, can be considered to be at equilibrium for many multi-phase mass-transfer processes such as distillation, absorption, extraction etc. Such stages are usually called ideal stages . 

CO is extracted by countercurrent absorption in a column operating at about 2.10 Pa absolute, with a feedstock inlet temperature of about 40 C and effluent exit around 65 C. This is accompanied by the physical dissolution of small amounts of other constituents, including hydrogen, which are salted out by cooling and expansion of the extract at 0.5, 10 Pa absolute Thecomple.x obtained is then preheated to 100 to 105 C and sent to a regeneration column. In this column, operating at 0.15.10 Pa absolute, CO is liberated... 

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. 

Among the unit operations, adsorption may be considered a prototype for all fluid-solid separation operations. When it is conducted under countercurrent conditions, the calculation methods required are entirely analogous to those for countercurrent absorption or extraction (H3). Often, however, it is most economical to conduct adsorption in a semi continuous arrangement, in which the solid phase is present as a fixed bed of granular particles. The fluid phase passes through the interstices of this bed at a constant flow rate and for an extended period of time. The concentration gradients in the fluid and solid phases display a transient or unsteady-state behavior, and their evolution depends upon the pertinent material balances, rates, and equilibria. 

Countercurrent multistage extraction is the application of SFE to the fractionation of liquid mixtures and the purification of liquid substances. SFE in the modification of countercurrent multistage operation extends the possibilities of separation processes, such as fractional distillation, absorption and liquid-liquid extraction to the isolation and purification of components of low volatility. The separation of volatile liquid components is also possible at tempera-... 

The terms "number of stages N" and "height equivalent to a theoretical plate HETP" are used in chromatography however, their meanings are quite different in comparison to continuously operated countercurrent columns (absorption, extraction, rectification). Here, the column length or height must be sufficient to draw apart the bands of components or fractions. This reqnires favorable eqnUibria, certain retardation differences, and limited axial dispersion. 

These operations are all similar in that the mixture to be separated is brou t into contact with another insoluble phase, the adsorbent solid, and the unequal distribution of the ori al constituents between the adsorbed phase on the solid surface and the bulk of the fluid then permits a s aration to be made. All the techniques previously found valuable in the contact of insoluble fluids are useful in adsorption. Thus we have batchwise single-stage and continuous multistage separations and separations analogous to countercurrent absorption and stripping in the field of gas-liquid contact and to rectification and extraction with the use of reflux. In addition, the rigidity and immobility of a bed of solid... 

Figure 8.1.2. (a) Type (2) systems. Countercurrent flow of a gas stream and an absorbent liquid in a vertical column for absorption of a species from the gas countercurrent separation system without recycle or reflux, (b) Countercurrent solvent extraction column lighter liquid introduced at column bottom by dispersing it as droplets as it rises through a continuous heavier liquid, which flows downward through the column. In aqueous-organic extraction systems, usually the aqueous phase is heavier. 

For ail species moving in a given direction in a coiumn, the solution of this equation wiii indicate that, at that coiumn exit, aii such species wiii appear/exist therefore multicomponent separation is not possibie. Oniy a binary separation is possible with one species moving in the opposite direction in the column and therefore available as a pure species. This is the primary reason why we will see that a countercurrent colutnn used for steady state processes such as distillation, absorption, extraction, crystallization, etc., separates a binary mixture only. For temany mixture separation, two columns are needed. Three columns are employed to separate a four-component mixture (see Chapter 9 for various schematics). However, if a feed sample injection is made, as in elution chromatography, into a mobile phase in countercurrent flow vis-k-vis another mobile phase, transient multicomponent separation would appear to be feasible. If pulse injection of one phctse containing feed is introduced countercurrent to the other phase, it may be possible to achieve a multi-component separation capability (as is tme for cocmrent flow, considered in Section 8.2). 

Readers should consult the extensive study by Macias-Salinas and Fair (2002) on axial mixing effects in packed gas-liquid contactors. Various types of situations have been considered neglect of gas-phase dispersion (desorption of O2 from water) dispersion in both phases (SO2 absorption by water) neglect of liquid-phase dispersion (water cooling with air). They have employed the rigorous analytical solution of Hartland and Mecklenburgh (1966) developed for countercurrent liquid extraction and applied it to gas absorption with gas as the raffinate phase and the liquid as the extract phase by a trial-and-error algorithm. 

The efficiencies which may be obtained can consequently be calculated by simple stoichiometry from the equilibrium data. In the ease of countercurrent-packed columns, the solute can theoretically be completely extracted, but equilibrium is not always reached because of the poorer contact between the phases. The rate of solute transfer between phases governs the operation, and the analytical treatment of the performance of such equipment follows closely the methods employed for gas absorption. In the ease of two immiscible liquids, the equilibrium concentrations of a third component in each of the two phases are ordinarily related as follows ... 

This section concerns the modelling of countercurrent flow, differential mass transfer applications, for both steady-state and non-steady-state design or simulation purposes. For simplicity, the treatment is restricted to the case of a single solute, transferring between two inert phases, as in the standard treatments of liquid-liquid extraction or gas absorption column design. 

As Sherwood and Pigford(3) point out, the use of spray towers, packed towers or mechanical columns enables continuous countercurrent extraction to be obtained in a similar manner to that in gas absorption or distillation. Applying the two-film theory of mass transfer, explained in detail in Volume 1, Chapter 10, the concentration gradients for transfer to a desired solute from a raffinate to an extract phase are as shown in Figure 13.19, which is similar to Figure 12.1 for gas absorption. 

Systems that exhibit behavior of the type illustrated in Fig. 4 cannot be purified in a single crystallization stage. They represent situations in which multiple stages or continuous-contacting devices may be useful. The principles of such operations are analogous to those of other countercurrent contacting operations—for example, distillation, absorption, and extraction. 

An absorption zone in which the acetylenic compounds, butadiene and a certain quantity of butenes are extracted from the feedstock in countercurrent gas/Iiquid treatment with N methylp TTolidone, containing about 8 per cent weight water. This operation takes place in a column with about 80 trays operating betw een 45 and 55 at M to 0.6.10 Pa. on a feedstock previously vaporized by heat exchange with the hot solvent recycle and introduced at the bottom, llte N-methylpynolidone is inuodoced at the top, where the unabsorbed gases, chiefly butanes and butenes, leave the column. 

A rectification zone in which the dissolved bntenes fraction is displaced from the solvent in countercurrent flow with butadiene t apdr, which is obtain by controlled reboiling of the extract. These butenes, which contain butadiene, are returned to the absorption zone. A sidestream is drawn off at a level where the olefin content is practically nil, the acenlene content low, and the 1,3-butadiene content a maximum. Separation takes place in a column with about 45 travs operatins at the bottom around 75 C at 0.7. 10 Pa. 

In principle, the flow sheet of such a facility (Fig 5.14) comprises the cooling of the C cut in gaseous form, followed by its countercurrent pass into an absorption column (20 trays), where the solvent flows, and which operates at about 2.10 Pa absolme. The extract, containing nearly all the initial acetylene, is sent to a stripping column (25 trays), operating at 0.105.10 Pa absolute to remove the other absorbed gases, which are recycled to the previous column after recompresston. A desorber (25 trays) then separates the dimethylfonnamide at the bottom and the acet ene at the top (Fig 5.11). 

Acid Treatment. Under most conditions, some of the conjunct polymer is removed by the isobutane in the extraction step. Just how much and whether it will be removed depends on the specific composition of the absorption mixture and the extraction conditions. In the pilot unit, relatively weak make-up acid of 97.5% was charged to the alkylation step, so the recycle acid charged to the absorber was relatively high in water. If fresh make-up acid of 99.5% concentration, the usual or better drying of alkylation streams, and a strictly countercurrent absorber were... 

The selective leaching of the nickel and cobalt from the gangue material is carried out in Pachuca-type leaching vessels with 98% sulfuric acid at temperatures between 450 and 500°F and with equivalent pressures between 400 and 600 psi. A typical leach liquor contains 6 gm/liter Ni, 0.6 gm/liter Co, 0.6 gm/liter Fe, 2.3 gm/liter Al, and 67 gm/liter SO4. Each batch is leached for about 90 min to extract 95% of the metal values. Preheating and heating of the ore slurry is by direct steam absorption in the reaction vessel. The barren residue is separated from the leach liquor by countercurrent decantation to a density of 55-60% solids. The solids are washed with very little dilution and about 99.5% of the soluble value are recovered. 

In a similar manner to that for the absorption column a linear relationship between the compositions of the two phases can be found for extraction and rectification. To illustrate this we will look at a rectification column. The basic process of rectification is when boiling a multicomponent mixture the vapour generated flows upwards countercurrent to the condensate which falls down the column. As the condensate is colder than the vapour, the components with higher boiling points, the least volatile, condense. They release their enthalpy of condensation to the components with the lower boiling points, the so called more volatile components, which are vaporized. This causes the vapour to become rich in the more volatile components while the less volatile components make up the liquid. The... 

Spray or packed extraction towers, as shown in Figure 5.2, are similar to those used in gas absorption with the exception that two liquids, rather than a gas and a liquid, are flowing countercurrent to each other. The less dense, or lighter, liquid enters the tower bottom through a distributor, which creates small drops flowing upwards. These rise through the... 

Some extraction systems are such that the solvent and diluent phases are almost completely immiscible in each other. Hence, separation yields an extract phase essentially free of diluent and a raffinate phase that is almost pure diluent. This greatly simplifies the characterization of the system. When partial miscibility for an extraction process is very low, the system may be considered immiscible and application of McCabe-Thiele analysis is appropriate. It is important to note that McCabe-Thiele analysis for immiscible extraction applies to a countercurrent cascade. The McCabe-Thiele analysis for immiscible extraction is analogous to the analysis for absorption and stripping processes. Consider the flow scheme shown in Figure 5.23,... 

The emergence of chemical engineering as a professional field of specialized knowledge was catalyzed to a major extent by the systematic classification of apparatus in terms of the Unit Operations. With further progress, the design methods evolved for particular apparatus types have proved equally applicable to other unit operations similar in physical arrangement, material and energy balances, rate behavior, and phase equilibrium. Thus there has been a very extensive development of parallel calculation methods for the separation operations conducted under countercurrent flow conditions—the fluid-fluid operations of distillation, absorption, and extraction. 

Identified by Andrade-Aispuro and Crouzet (1983) using original laboratory and pilot units allowing recovery of the volatiles produced during coffee roasting, with a system based on an absorption column equipped with Raschig rings and a countercurrent flow of water. The condensate was fractionated by chemical extraction into basic, acid, neutral and phenolic fractions. The constituents were separated and... 

Since most continuous extraction methods use countercurrent contacts between two phases, one a light liquid and the other a heavier one, many of the fundamentals of countercurrent gas absorption and of rectification carry over into the study of liquid extraction. Thus questions about ideal stages, stage efficiency, minimum ratio between the two streams, and size of equipment have the same importance in extraction as in distillation. 

We begin the development of graphical design procedures by deriving generalized material balance equations for countercurrent cascades. These are first applied to simple, single-section distillation, extraction, absorption, and... 

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