Countercurrent processes Absorption Distillation

To remove traces of the absorbent always present in the overhead product of the absorber, some of the ethanol is recycled as reflux at the top of the absorber A-1. Analogously, a small part of the loaded absorbent is boiled up at the bottom of A-1 to remove most of the coabsorbed ethanol. However, reboil and reflux rates are rather small. Thus, the countercurrent flow within the column A-1 is dominated by the vapor feed (low boiling fraction) at the bottom and the liquid feed (high boiling fraction) at the top of the column what is characteristic of absorption columns. Thus, the process should better be called absorptive distillation instead of extractive distillation. 

Gas extraction extends the possibilities of separation processes like distillation, absorption and liquid-liquid extraction to the isolation and purification of components of low volatility. Furthermore, it enables separation of components with very similar properties if used in the countercurrent mode. Process temperatures in gas extraction are determined by the critical temperature of the solvent and not, as is the case of distillation of any kind, by the liquid-vapor transition of the feed mixture. As compared to liquid-liquid extraction, gas extraction makes easily possible to operate a two cascade separation column, applying a stripping and an enriching section. Combined, these possibilites allow gas extraction to be operated at very moderate temperatures and as a separation process for difficult separations. 

Conventional packed towers are used for different mass transfer operations such as distillation and absorption. The tower consists of a vertically positioned cylindrical shell filled with packing. Liquid enters at the top of the packed column and flows downward, and gas enters at the bottom and flows upward through the packing. Thus, a countercurrent process takes place, and the packing provides for a high area for mass transfer contact (see Fig. 6.18). The efficiency of the packed tower for mass transfer depends upon the specific area of packing and liquid irrigation rates. It has... 

In the following part of this section, we provide simple mathematical descriptions of a few common features of two-phase/two-region countercurrent devices, specifically some general considerations on equations of change, operating lines and multicomponent separation capability. Sections 8.1.2, 8.1.3, 8.1.4, 8.1.5 and 8.1.6 cover two-phase systems of gas-Uquid absorption, distillation, solvent extraction, melt crystallization and adsorption/SMB. Sections 8.1.7, 8.1.8 and 8.1.9 consider the countercurrent membrane processes of dialysis (and electrodialysis), liquid membrane separation and gas permeation. Tbe subsequent sections cover very briefly the processes in gas centrifuge and thermal diffusion. 

Adsorption is very different from absorption, distillation, and extraction. These three processes, detailed in the five previous chapters, typically involve two fluids flowing steadily in opposite directions. In absorption, a gas mixture flows upward through a packed column while an absorbing liquid trickles down. In distillation, a liquid mixture is split into a more volatile liquid distillate and a less volatile bottoms stream. In extraction, two liquid streams move countercurrently to yield an extract and a raffinate. To be sure, in some cases, the contacting may involve near-equilibrium states, and in other cases it may be described with nonequilibrium ideas like mass transfer coefficients. Still, all three units operations involve two fluids at steady state. 

Many chemical and biological systems include multistage processes rather than only continuous contact ones. The most common multistage systems are absorption and distillation columns. Most of these systems involve more than one phase and they therefore fall under the category of heterogeneous multistage systems. Multistage systems can be cocurrent or countercurrent. 

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 . 

Many chemical and biological processes are multistage. Multistage processes include absorption towers, distillation columns, and batteries of continuous stirred tank reactors (CSTRs). These processes may be either cocurrent or countercurrent. The steady state of a multistage process is usually described by a set of linear equations that can be treated via matrices. On the other hand, the unsteady-state dynamic behavior of a multistage process is usually described by a set of ordinary differential equations that gives rise to a matrix differential equation. 

Equipment Absorption, stripping, and distillation operations are usually carried out in vertical, cylindrical columns or towers in which devices such as plates or packing elements are placed. The gas and liquid normally flow countercurrently, and the devices serve to provide the contacting and development of interfacial surface through which mass transfer takes place. Background material on this mass transfer process is given in Sec. 5. 

Reactive absorption can be realized in a variety of equipment types, e.g., in him absorbers, plate columns, packed units, or bubble columns. This process is characterized by independent how of both phases, which is different from distillation and permits both cocurrent (downflow and uphow) and countercurrent regimes. 

Figure 6.2 shows a basic flow diagram for either an absorption or a stripping column. The process differs from distillation in that one stream enters with a contaminant and exits clean while a second enters clean and exits with the contaminant. Hence, there are two feed streams, the solute carrier and the mass-separating agent, which enter the column at opposite ends. This creates a flow pattern similar to distillation in which a gas phase flows countercurrent to a liquid phase. An absorption column is equivalent to the rectifying section of a distillation column. 

Countercurrent Distillation (Rectification) 193 Table 2-24. Determination of the point efficiency Eg in absorption and rectification processes [2.73],... 

There is complete formal analogy between continuous countercurrent adsorption and other continuous countercurrent operations such as distillation or gas absorption. Such processes are normally operated at steady state and, for an isothermal system containing only a single adsorbable species, the analysis is straightforward. 

To design such a process, the McCabe-Thiele method may be used to determine the number of theoretical separation stages, as examined in Sections 3.3.2-3.3.4 for distillation, absorption (gas scrubbing), and liquid-liquid-extraction. Thus, we obtain the number of theoretical extraction stages of a countercurrent extraction column based on the equilibrium curve (solubility of extract in the solvent for a given content in the solid) and the operating line. The latter depends on the extract content of the solid feed and residue, and on the in- and outlet extract concentration in the solvent The extract content of the feed is fixed, and the value of the residue is specified by the required degree of extraction. The inlet content of the extract in the solvent is also fixed, as either pure solvent is used or the value is specified by separation of the extract from the used solvent after the extraction. Therefore, the only parameter that is left is the outlet concentration of the extract in the solvent, which depends on the ratio of the solvent flow to the feed rate of the solid feedstock (mass balance). 

Most mass transfer equipments consist of gas (vapor) and liquid two-phase flow, for instance, vapor-liquid two-phase cross-current flow is undertaken in tray distillation column gas-liquid two-phase countercurrent flow is taken place in packed absorption column. Some processes may also include solid phase, such as adsorption or catalytic reaction. Thus, the fluid system may contain gas and liquid two phases, or gas, liquid single phase besides solid phase. 

Chapter 8 deals with the configuration of bulk flows of two phases/regions (one of which may be solid) perpendicular to the direction of force(s). The directions of motion of the two phases may be parallel to each other in either countercurrent or cocurrent fashion, or they may be in crossflow. Figures 8.1.1-8.1.4 illustrate the counter-current flow vs. force configuration for all three classes of separation systems. Conventional countercurrent devices/ processes of gas absorption/stripping, column distillation... 

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). 

Many topics could not be covered in this book. A much abbreviated list includes the molecular basis of equilibrium partitioning of molecules between different phases enthal-pic and entropic contributions to partitioning/selectivity the molecular basis of afBnity binding in bioseparations nonisothermal analysis of absorption columns, adsorption beds, distillation columns, etc. multicomponent multistage separations in distillation columns numerical methods for multicomponent multistage countercurrent separation processes experimental methods in separation studies hybrid separation processes selection of separation processes for solving a separation problem reaction-separation/ separation-reaction/reaction-separation-reaction processes and devices. 

The distillation shown in Fig. 12.1-1(a) is a close parallel to the process of gas absorption. The two other forms of distillation shown in Fig. 12.1-1 are similar. In Fig. 12.1 -1 (b), a saturated liquid feed enters the top of the column and flows downwards into a heated kettle, called a reboiler. The reboiler produces a vapor, which passes up the column, countercurrently to the falling liquid. The liquid bottoms product drawn out of the reboiler is normally in thermodynamic equilibrium with the vapor leaving the reboiler to go up the column. This type of distillation would be effective in removing a trace of impurity, like a volatile organic compound (VOC) from a less volatile solvent like water. 

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