Minimum solvent requirement

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 oxime is freely soluble in water and in most organic liquids. Recrystallise the crude dry product from a minimum of 60-80 petrol or (less suitably) cyclohexane for this purpose first determine approximately, by means of a small-scale test-tube experiment, the minimum proportion of the hot solvent required to dissolve the oxime from about 0-5 g. of the crude material. Then place the bulk of the crude product in a small (100 ml.) round-bottomed or conical flask fitted with a reflux water-condenser, add the required amount of the solvent and boil the mixture on a water-bath. Then turn out the gas, and quickly filter the hot mixture through a fluted filter-paper into a conical flask the sodium chloride remains on the filter, whilst the filtrate on cooling in ice-water deposits the acetoxime as colourless crystals. These, when filtered anddried (either by pressing between drying-paper or by placing in an atmospheric desiccator) have m.p. 60 . Acetoxime sublimes rather readily when exposed to the air, and rapidly when warmed or when placed in a vacuum. Hence the necessity for an atmospheric desiccator for drying purposes. 

Dissolve 5 g. of phenol in 75 ml. of 10 per cent, sodium hydroxide solution contained in a wide-mouthed reagent bottle or conical flask of about 200 ml. capacity. Add 11 g. (9 ml.) of redistilled benzoyl chloride, cork the vessel securely, and shake the mixture vigorously for 15-20 minutes. At the end of this period the reaction is usually practically complete and a sohd product is obtained. Filter oflf the soUd ester with suction, break up any lumps on the filter, wash thoroughly with water and drain well. RecrystaUise the crude ester from rectified (or methylated) spirit use a quantity of hot solvent approximately twice the minimum volume required for complete solution in order to ensure that the ester does not separate until the temperature of the solution has fallen below the melting point of phenyl benzoate. Filter the hot solution, if necessary, through a hot water funnel or through a Buchner funnel preheated by the filtration of some boiling solvent. Colourless crystals of phenyl benzoate, m.p. 69°, are thus obtained. The yield is 8 g. 

If the feed, solvent, and extract compositions are specified, and the ratio of solvent to feed is gradually reduced, the number of ideal stages required increases. In economic terms, the effect of reducing the solvent-to-feed ratio is to reduce the operating cost, but the capital cost is increased because of the increased number of stages required. At the minimum solvent-to-feed ratio, the number of ideal stages approaches infinity and the specified separation is impossible at any lower solvent-to-feed ratio. In practice the economically optimum solvent-to-feed ratio is usually 1.5 to 2 times the minimum value. 

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. 

PFE is based on the adjustment of known extraction conditions of traditional solvent extraction to higher temperatures and pressures. The main reasons for enhanced extraction performance at elevated temperature and pressure are (i) solubility and mass transfer effects and (ii) disruption of surface equilibria [487]. In PFE, a certain minimum pressure is required to maintain the extraction solvent in the liquid state at a temperature above the atmospheric boiling point. High pressure elevates the boiling point of the solvent and also enhances penetration of the solvent into the sample matrix. This accelerates the desorption of analytes from the sample surface and their dissolution into the solvent. The final result is improved extraction efficiency along with short extraction time and low solvent requirements. While pressures well above the values required to keep the extraction solvent from boiling should be used, no influence on the ASE extraction efficiency is noticeable by variations from 100 to 300 bar [122]. 

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. 

Because of the small ionic radius of lithium ion, most simple salts of lithium fail to meet the minimum solubility requirement in low dielectric media. Examples are halides, LiX (where X = Cl and F), or the oxides Li20. Although solubility in nonaqueous solvents would increase if the anion is replaced by a so-called soft Lewis base such as Br , I , S , or carboxylates (R—C02 ), the improvement is usually realized at the expense of the anodic stability of the salt because these anions are readily oxidized on the charged surfaces of cathode materials at <4.0 V vs Li. 

Obviously, no solvent satisfies all these requirements, and the selection of a desirable solvent involves a compromise between these and other factors. When a preliminary selection has been made, a secondary screening can be performed based on simplified calculations of minimum energy requirement, since the capital costs for similar process configurations will not vary too much. 

In summary, it is possible to predict for a given stock the yield which can be expected for any given viscosity index of the finished oil. Given this yield and a minimum of experimental data, it is possible to predict the necessary solvent requirements given the solvent requirements, material balance relationships may be obtained from correlation which give the concentration of the solvent in the raffinate layer. Experience has indicated that the weakest link in the chain is the prediction of solvent requirement for treatment to a given yield or viscosity index. 

For each series of measurements about 50 g of solvent was transferred quantitatively in the dry box to the cell by pouring it into the dilution bulb this was the minimum amount required to fill the cell bulb. The cell was removed from the dry box, placed in the oil bath, and connected to the bridge. Time was allowed for the attainment of thermal equilibrium then at least three resistance measurements were made at five-min intervals, and the average value was calculated. The cell was removed from the bath and returned to the dry box. Dilute stock solution was quantitatively added to the cell by means of a weighing buret. The contents of the cell were carefully mixed, and the resistance of the solution was measured as before. The procedure just described was repeated several times with the dilute stock solution and then with the concentrated stock solution. About ten concentrations with a hundredfold range were obtained. A portion of the final solution in the cell (the most concentrated solution) was removed, and the infrared spectrum taken no absorption band indicative of traces of water was observed at 3600 cm-1. It was necessary to obtain the densities of... 

An alternative approach is to measure the minimum temperature required to bring about dissolution of the polymer, again in a series of solvents of known solubility parameters. This is illustrated in Figure 2.25. The solubility parameter of the polymer is taken to be the value of Si that corresponds to the smallest required increase in temperature. 

Thus the extractor column raffinate outlet rate and the solvent inlet rate are approximately equal. This is indeed the minimum solvent rate allowed, since a lower rate will overload the solvent, referencing the plait point. This rate will also set the required minimum extractor column diameter. For some refinery-type extraction operations, such as lube oil extractors, where relatively much larger solvent-raffinate rates apply, this method for determining minimum solvent rate is very economical and desirable. 

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 actual solvent rate specified for the separation must exceed the minimum solvent rate, or an infinite number of stages will be required. For a contactor with a finite number of stages, this means that the separation will not be achieved unless actual solvent rate exceeds the minimum. The higher the solvent rate specified, the greater is the distance between the operating line and the equilibrium curve, and the smaller is the number of stages required. 

In order for a coal liquefaction process to be viable two requirements must be met. First, the process must be self-sufficient in solvent, in other words that the quantity of recycle solvent be greater than or equal to the initial solvent used to initiate the process, and second, the solvent quality must remain sufficiently high to perform desired conversion of the coal while keeping to a minimum solvent consuming reaction, charing, and gas formation. 

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. 

Minimum and Maximum Solvent-to-Feed Ratios Normally, it is possible to quickly estimate the physical constraints on solvent usage for a standard extraction application in terms of minimum and maximum solvent-to-feed ratios. As discussed above, the minimum theoretical amount of solvent needed to transfer a high fraction of solute i is the amount corresponding to % =. In practice, the minimum practical extraction factor is about 1.3, because at lower values the required number of theoretical stages increases dramatically. This gives a minimum solvent-to-feed ratio for a practical process equal to... 

Lastly, we would like to point out that the head group of the ionic surfactant have to be hydrated by a minimum amount of water in order to dissolve into a low polarity solvent (e.g. short chain alcohols). In the hydrocarbon oil rich corner of a microemulsion phase diagram, micellization occurs as long as the minimum water required to hydrate the ionic head group is added (5). Hence the minimum water to surfactant molar ratio required for such hydration can be determined by light scattering measurement. The ratio has been found to be 10 for sulfate surfactants in toluene and 8 for carboxy-... 

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