Contact scheme

Preparative chromatographic processes are of increasing importance particularly in the production of fine chemicals. A mixture of compounds is introduced into the liquid mobile phase, and this then flows through a packed column containing the stationary solid phase. The contacting scheme is thus differential, but since the adsorption characteristics of the compounds in the mixture are similar, many equivalent theoretical stages are required for their separation. Chromatographic processes are mostly ran under transient conditions, such that... 

From the standpoint of favorable product distribution, order the contacting schemes of Fig. 7.2, from the most desirable to the least desirable. 

Comment. Example 7.2 verifies these qualitative findings. We should also note that there are still other contacting schemes which are superior to the best found in this example. For example, if we can use an excess of a reactant, or if it is practical to separate and recycle unconverted reactant, then vastly improved product distribution is possible. 

These fractional yield expressions allow us to relate the product distribution from different types of reactors and to search for the best contacting scheme. However, one condition must be satisfied before we can safely use these relationships We must truly have parallel reactions in which no product influences the... 

The key to optimum design for multiple reactions is proper contacting and proper flow pattern of fluids within the reactor. These requirements are determined by the stoichiometry and observed kinetics. Usually qualitative reasoning alone can already determine the correct contacting scheme. This is discussed further in Chapter 10. However, to determine the actual equipment size requires quantitative considerations. 

Two-Phase Systems. A stream of solids always behaves as a macrofluid, but for gas reacting with liquid, either phase can be a macro- or microfluid depending on the contacting scheme being used. The sketches of Fig. 11.3 show completely opposite behavior. We treat these two phase reactors in later chapters. 

The Contacting Scheme. In gas-liquid systems semibatch and countercurrent contacting schemes predominate. In liquid-liquid systems mixed flow (mixer-... 

The four versus sketches of Fig. 24.2 represent various possible ideal contacting schemes of gas with liquid. Sketch the contacting scheme for straight physical absorption corresponding to the versus operating lines XY shown in Fig. P24.1. 

In Example 291e we could have used any one of the three contacting schemes shown in Fig. P29.19. We stated without proof that the bypass scheme was best. Show that this is so by calculating C out for the three contacting schemes of Fig. P29.19. 

Fig. 1.23. Contacting schemes to match possible concentration levels required for high relative yields in parallel reactions...

Fluidization refers to the state of solid particles in a suspended condition owing to the flow of fluid, gas, and/or liquid. Contact schemes of fluidized bed systems can be classified on the basis of the states of solid motion. For a batch-solids system, the fluid at a low velocity merely percolates through the voids between packed particles, while the particles remain motionless. The solids in this case are in the fixed bed state. With an increase in the fluid velocity, particles move apart and become suspended the bed then enters the fluidization state. The fluidization characteristics vary, depending on whether gas, liquid, or gas-liquid is the fluidizing medium. 

Fig. 10.4. Two scanner realizations tripod and tube, (A) contact scheme with the z-signal added to x- and y-signal, (B) contact scheme with z-electrode in the middle.

In an isolation step, where yield and concentration are more important than purity, the adsorption mechanism can be considered an on/off process, and several alternative contacting schemes can be used. Ligands have been bound to magnetized particles (137, 138) for continuous countercurrent adsorption in magnetically stabilized fluidized beds. Ligands attached to liquid perfluorocarbons (143), to dextran and related polymers (144), or incorporated into liposomes (145), or reversed micelles (146) may be used for biospecific liquid-liquid extraction or "affinity partitioning". Ligands have also been attached to surfactants and biopolymers for selective precipitation of dilute protein species (147, 148). 

Inactivation of spores (a 6 log reduction in B. subtilis spores in 1 h) at moderate temperatures (40-55°C) was achieved by Ishikawa and coworkers (60) using a contacting scheme referred to as the SCF CO2 microbubble... 

The existence of mercaptans, thiophenes, and other organic sulfur compounds in (gasifier) product gas will probably be a function of the degree of severity of the process, contacting schemes, and heat-up rate. Those processes that tend to produce tars and oils may also tend to drive off high-molecular-weight organic sulfur compounds into the raw product gas. 

Fig. 10.3 Generation of high dynamic pressures with chemical explosives (a) cylindrical scheme, (b) planar direct-contact scheme, (c) planar scheme with explosively accelerated impactor plate. 1 detonator, 2 explosive, 3 ampoule, 4 sample, 5 plane wave generator, 6 layer of inert material, 7 impactor plate...

Important factors to consider in selecting gas-solid reactors include gas-solid contact schemes, nature of the reactions, temperature, and pressure among many other factors. Of particular pertinence in determining desirable reactor performance can be proper selection of particles, flow regime, and system configurations. 

He used the theory of elasticity to study when two elastic bodies are in contact by accounting for the displacement and stresses in the x-, y-, and z-directions and furthermore accounting for what happens both within and outside of the surface of contact (Scheme 5). 

The calculation of the interaction modes between two Schiff base molecules showed the preference for the formation of (R2 (18)) dimers, stabilized by the O-H- O hydrogen bond, thus leaving the pyridine nitrogen without any significant hydrogen bond contacts (Scheme 13.8a). When one carries in mind the robustness of O-H- -N(py) synthon [83], such stabilization is not intuitively expected. However, the same dimeric substructure already occurred in similar metastable keto-amino tautomers of nonmethoxy Schiff base derivative IX [45]. To accomplish these dimers, it would be energetically favorable for both participating Schiff base molecules to take the keto-amino form. 

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