Minimum solvent requirement flow

The reader will recall that in the discussion of the countercurrent gas scrubber, Section 7.1.4, we mention the limiting case that arises when the slope of the operating line and the associated solvent flow is progressively reduced until it intersects the equilibrium curve. This results in a condition termed a pinch and corresponds to a cascade with an infinite number of stages and a minimum solvent requirement. Any solvent flow rate below this value causes a rise in the effluent concentration and can therefore no longer meet the specified solute recovery. 

The variable that has the most significant impact on the economics of an extractive distillation is the solvent-to-feed (S/F) ratio. For closeboiling or pinched nonazeotropic mixtures, no minimum-solvent flow rate is required to effect the separation, as the separation is always theoretically possible (if not economical) in the absence of the solvent. However, the extent of enhancement of the relative volatihty is largely determined by the solvent concentration and hence the S/F ratio. The relative volatility tends to increase as the S/F ratio increases. Thus, a given separation can be accomplished in fewer equihbrium stages. As an illustration, the total number of theoretical stages required as a function of S/F ratio is plotted in Fig. 13-75 7 for the separation of the nonazeotropic mixture of vinyl acetate and ethyl acetate using phenol as the solvent. 

If an extended tie-line passes through the pole point P, an infinite number of stages will be needed. This condition sets the minimum flow of extraction-solvent required. It is analogous to a pinch point in distillation. 

When the operating line and the equilibrium curve intersect, an infinite number of stages is required to achieve the separation (Fig. 11). The intersection point is called the pinch point and may occur at the bottom (Fig. 11a), at the top (Fig. lib), or at a tangent point (Fig. lie). The solvent rate leading to this intersection is the minimum solvent flow required to absorb the specified amount of solute. 

The normal flow of solvent into the solution (osmosis) can be prevented by applying an external pressure to the solution. The minimum pressure required to stop the osmosis is equal to the osmotic pressure of the solution. 

The amount of solvent required depends strongly on the extraction mode used. In the static mode, the sample is extracted with a minimum volume of solvent (usually < 15 ml), with no outflow. When the solvent volume used in the static mode does not ensure quantitative extraction of the target analytes, several extraction cycles or the dynamic mode must be used. In the dynamic mode, the extractant flows continuously through the extraction cell, and so the volume of solvent that comes into contact with the sample is a direct function of both the flow rate of the circulating extractant and the extraction time. Obviously, the dynamic mode uses larger volumes of solvent than the static mode, and so it is less well suited for trace analysis, although there are various ways of minimizing this dilution effect, as shown below. 

New capillary columns have to be conditioned to remove residual traces of solvent and lower relative molecular mass fractions of the liquid phase. Carrier gas should flow at room temperature for some time to remove oxygen the column is then eiqiosed to moderate temperatures (80-100 °C) for some hours before the temperature is increased to a value that must be a compromise between minimum time required to achieve a stable baseline and maximum column life time. The normal temperature is the maximum temperature required for the analysis. To avoid destruction of the column at higher temperatures, a sufficient flow of carrier gas through the column should be maintained. During conditioning, the column should be left disconnected from the ECD so as to minimize detector contamination. 

The designer first must select the solvent liquid to be used and then specify its circulation rate. The greater the solubility of solute in the solvent, the lower will be the necessary liquid rate. The minimum solvent rate possible is that flow which produces a concentration of solute in the effluent liquid which is in equilibrium with the solute concentration in the entering gas stream. Of course, an infinite number of theoretical stages is required at minimum solvent flow. The economic optimum design results from a balance between the solvent circulation rate and the depth of packing in the absorber. 

In considering the functionality, therefore, it is necessary to consider not only the number of required solvent inlets, fraction outlets, pump and valve types, sensors, detectors, etc., but also the pipework size and configuration to ensure that they are optimal for the required flow and that dead legs and dilution zones are kept to a minimum. 

The value of K is one of the main parameters used to establish the minimum ratio of extraction solvent to feed solvent that can be employed in an extraction process. For exanmle, if the partition ratio K is 4, then a countercurrent extractor woula require 0.25 kg or more of extraction-solvent flow to remove all the solute from 1 kg of feed-solvent flow. 

The maximum and minimum flow rate available from the solvent pump may also, under certain circumstances, determine the minimum or maximum column diameter that can be employed. As a consequence, limits will be placed on the mass sensitivity of the chromatographic system as well as the solvent consumption. Almost all commercially available LC solvent pumps, however, have a flow rate range that will include all optimum flow rates that are likely to be required in analytical chromatography... 

Safety. As well as the reduced inventory of chemicals and solvents, highly exothermic reactions can be safely handled because of the excellent heat flow properties of a flow reactor and the fact that smaller quantities of reactants are present at any time. Similarly, toxic reagents are present in smaller quantities and require minimum handling. 

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