Solvent To Feed Ratio Uin Extractive Distillation

Distillation Range For Vinyl Acetate

Solvent/Feed Ratio

FIG. 13-75 Number of theoretical stages versus solvent-to-feed ratio for extractive distillation. (a) Close-boiling vinyl acetate-ethyl acetate system with phenol solvent. (b) Azeotropic acetone-methanol system with water solvent.

no longer be achieved and the distillate purity actually decreases for a given number of stages [LaRoche et al., AIChE J., 38, 1309 (1992)]. The difference between Rmin and Rmax increases as the S/F ratio increases. Large amounts of reflux lowers the solvent concentration in the upper section of the column, degrading rather than enhancing column performance. Because the reflux ratio goes through a maximum, the conventional-control strategy of increasing reflux to maintain purity can be detrimental rather than beneficial. However Rmax generally occurs at impractically high reflux ratios and is typically not of major concern.

The thermal quality of the solvent feed has no effect on the value of (S/F)min, but does affect the minimum reflux to some extent, especially as the (S/F) ratio increases. Rmax occurs at higher values of the reflux ratio as the upper-feed quality decreases; a subcooled upper feed provides additional refluxing capacity and less external reflux is required for the same separation. It is also sometimes advantageous to introduce the primary feed to the extractive distillation column as a vapor to help maintain a higher solvent concentration on the feed tray and the trays immediately below.

Robinson and Gilliland (Elements of Fractional Distillation, McGraw-Hill, New York, 1950), Smith (Design of Equilibrium Stage Processes, McGraw-Hill, New York, 1963), Van Winkle (Distillation, McGraw-Hill, New York, 1967), and Walas (Chemical Process Equipment, Butterworths, Boston, 1988) discuss rigorous stage-by-stage design techniques as well as shortcut and graphical methods for determining minimum stages, (S/F)min, minimum reflux, and the optimum locations of the solvent and primary feed points. More recently Knapp and Doherty [AIChE J., 40, 243 (1994)] have published column-design methods based on geometric arguments and fixed-point analysis. Most commercial simulators are capable of solving multiple-feed extractive distillation heat and material balances, but do not include straightforward techniques for calculating (S/F)min, minimum or maximum reflux.

Solvent Screening and Selection Choosing an effective solvent can have the most profound effect on the economics of an extractive distillation process. The approach most often adopted is to first generate a short list of potential solvents using simple qualitative screening and selection methods. Experimental verification is best undertaken only after a list of promising candidate solvents has been generated and some chance at economic viability has been demonstrated via preliminary process modeling.

Solvent selection and screening approaches can be divided into two levels of analysis. The first level focuses on identification of functional groups or chemical families that are likely to give favorable solventkey component molecular interactions. The second level of analysis identifies and compares individual-candidate solvents. The various methods of analysis are described briefly and illustrated with an example of choosing a solvent for the methanol-acetone separation.

First Level: Broad Screening by Functional Group or Chemical Family

Homologous series. Select candidate solvents from the high-boiling homologous series of both light and heavy key components. Favor homologs of the heavy key, as this tends to enhance the natural relative volatility of the system. Homologous components tend to form ideal solutions and are unlikely to form azeotropes [Scheibel, Chem. Eng. Prog. 44(12), 927 (1948)].

Robbins chart. Select candidate solvents from groups in the Rob-bins Chart (Table 13-15) that tend to give positive (or no) deviations from Raoult's law for the key component desired in the distillate and negative (or no) deviations for the other key.

Hydrogen-bonding characteristics. Select candidate solvents from groups that are likely to cause the formation of hydrogen bonds with the key component to be removed in the bottoms, or disruption of hydrogen bonds with the key to be removed in the distillate. Formation and disruption of hydrogen bonds are often associated with strong negative and positive deviations, respectively from Raoult's law. Several authors have developed charts indicating expected hydrogen bonding interactions between families of compounds [Ewell et al., Ind. Eng. Chem., 36, 871 (1944), Gilmont et al., Ind. Eng. Chem., 53, 223 (1961), and Berg, Chem. Eng. Prog., 65(9), 52 (1969)]. Table 13-20 presents a hydrogen-bonding classification of chemical families and a summary of deviations from Raoult's law.

Polarity characteristics. Select candidate solvents from chemical groups that tend to show higher polarity than one key component or lower polarity than the other key. Polarity effects are often cited as a factor in causing deviations from Raoult's law [Hopkins and Fritsch, Chem. Eng. Prog., 51(8), (1954), Carlson et al., Ind. Eng. Chem., 46, 350 (1954), and Prausnitz and Anderson, AIChE J., 7, 96 (1961)]. The general trend in polarity based on the functional group of a molecule is given in Table 13-21. The chart is best for molecules of similar size. A more quantitative measure of the polarity of a molecule is the polarity contribution to the three-term Hansen solubility parameter. A tabulation of calculated three-term solubility parameters is provided by Barton (CRC Handbook of Solubility Parameters and other Cohesion Parameters, CRC Press, Boca Raton, 1991), along with a group-contribution method for calculating the three-term solubility parameters of compounds not listed in the reference.

Second Level: Identification of Individual Candidate Solvents

Boiling point characteristics. Select only candidate solvents that boil at least 30-40°C above the key components to ensure that the sol

Class

Chemical family

H-Bonding, Strongly Associative (HBSA)

Water

Primary amides Secondary amides

Polyacids Dicarboxylic acids Monohydroxy acids

Polyphenols Oximes

Hydroxylamines

Amino alcohols Polyols

H-Bond Acceptor-Donor (HBAD)

Phenols Aromatic acids Aromatic amines Alpha H nitriles

Imines

Monocarboxylic acids Other monoacids Peracids

Alpha H nitros Azines

Primary amines Secondary amines

n-alcohols Other alcohols Ether alcohols

H-Bond Acceptor (HBA)

Acyl chlorides Acyl fluorides Hetero nitrogen aromatics Hetero oxygen aromatics

Tertiary amides Tertiary amines Other nitriles Other nitros Isocyanates Peroxides

Aldehydes Anhydrides Cyclo ketones Aliphatic ketones Esters Ethers

Aromatic esters Aromatic nitriles Aromatic ethers Sulfones Sulfolanes

n-Bonding Acceptor (n-HBA)

Alkynes Alkenes

Aromatics Unsaturated esters

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Responses

  • alice
    What is a solvent to feed ratio?
    2 years ago
  • Louis
    How do you figure a solvent to feed ratio?
    8 months ago

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