Solvent recyclability

As a matter of fact, the main advantage in comparison with HPLC is the reduction of solvent consumption, which is limited to the organic modifiers, and that will be nonexistent when no modifier is used. Usually, one of the drawbacks of HPLC applied at large scale is that the product must be recovered from dilute solution and the solvent recycled in order to make the process less expensive. In that sense, SFC can be advantageous because it requires fewer manipulations of the sample after the chromatographic process. This facilitates recovery of the products after the separation. Although SFC is usually superior to HPLC with respect to enantioselectivity, efficiency and time of analysis [136], its use is limited to compounds which are soluble in nonpolar solvents (carbon dioxide, CO,). This represents a major drawback, as many of the chemical and pharmaceutical products of interest are relatively polar. 

A related study used the air- and moisture-stable ionic liquids [RMIM][PFg] (R = butyl-decyl) as solvents for the oligomerization of ethylene to higher a-olefins [49]. The reaction used the cationic nickel complex 2 (Figure 7.4-1) under biphasic conditions to give oligomers of up to nine repeat units, with better selectivity and reactivity than obtained in conventional solvents. Recycling of the catalyst/ionic liquid solution was possible with little change in selectivity, and only a small drop in activity was observed. 

Transition metal oxides or their combinations with metal oxides from the lower row 5 a elements were found to be effective catalysts for the oxidation of propene to acrolein. Examples of commercially used catalysts are supported CuO (used in the Shell process) and Bi203/Mo03 (used in the Sohio process). In both processes, the reaction is carried out at temperature and pressure ranges of 300-360°C and 1-2 atmospheres. In the Sohio process, a mixture of propylene, air, and steam is introduced to the reactor. The hot effluent is quenched to cool the product mixture and to remove the gases. Acrylic acid, a by-product from the oxidation reaction, is separated in a stripping tower where the acrolein-acetaldehyde mixture enters as an overhead stream. Acrolein is then separated from acetaldehyde in a solvent extraction tower. Finally, acrolein is distilled and the solvent recycled. 

Figure 5.2 Environmental scores of reduction steps in routes B and C (Scheme 5.1) according to the Eco-lndicator95 evaluation method (Figure 2 in reference [11 ]). Ml = catalyst, M2 = reduction, M3 = catalyst removal, M4 = extraction, M5 = solvent drain off, M5 = rectification, M7 = enantiomeric purification, M8 = solvent recycling. Reproduced from Jodicke [11 ], Copyright 1 999, with permission from Elsevier.

Figure 5.6 Cost Indices Cl for the biocatalytic (a) and chemical catalytic (b) synthesis of (S)-styrene oxide (Scheme 5.3), using the software EATOS. The bottom figure shows the cost savings achieved by solvent recycling.

These findings support the general usefulness of efforts to increase yield and solvent recycling rates, as documented in Case study 4 (see also Geisler et al. [9]). However, producing an LCA requires a substantially higher effort for data acquisition as compared to mass balancing. 

If it is already known at the pilot stage in which plant the final operation will be situated, solvent recycling, waste treatment, and utility supply processes should be modelled according to that specific plant. This reduces uncertainty in the scale-up from pilot to operation. 

Hydrolysis of products Phase separation Distillation organic phase Solvent recycle 85-95% yield... 

J I Solvent recycle-I- S° Ven, Stage 3 Solvent 8 a aqueous ll —Aqueous ... 

The solvents are mixed in a contactor, to effect the transfer of solute, and then the phases separated. The depleted feed solvent leaving the extractor is called the raffinate, and the solute rich extraction solvent, the extract. The solute is normally recovered from the extraction solvent, by distillation, and the extraction solvent recycled. 

Liquid-liquid extraction. With liquid-liquid extraction, wastewater containing organic waste is contacted with a solvent in which the organic waste is more soluble. The waste is then separated from the solvent by evaporation or distillation and the solvent recycled. 

The general flow scheme of a production liquid chromatograph is similar to that of the corresponding GC unit, shown in Figure 19.5, with four main differences. First, thermostatting requirements for the column are less strict, and may sometimes even be dispensed with. Secondly, the feed is injected as a liquid, and not vaporised. Thirdly, if the product is to be separated from the mobile-phase solvent, distillation or evaporation and solvent recycle are incorporated in the loop(28,41,42). Finally, the liquid streams are filtered to ensure column longevity, and de-aerated to prevent air bubbles forming. 

Prior to the study of Jacobs, Song et al. [87] had already reported monomeric Cr-salen 63 catalyzed ARO of epoxides with TMSN3 in ionic liquid l-butyl-3-methylimidazolium salts [bmim][X]. The ARO reaction proceeded readily at room temperature with easy catalyst/solvent recycling and does not include hazardous workup stages such as distillation of the azide product. 

As shown in Figure 3.7, since 1995 there has been a large reduction in the amount of solvent wastes directly released, treated, and used for energy recovery on-site. There have also been moderate reductions in the amount of wastes used for energy recovery and treatment off-site. However, there were increases in every other form of on- and off-site waste treatment. In 2006, about 70% of solvent waste was treated for disposal or recycled. The remaining 30% was either directly released or treated for energy recovery. The fractions of solvent wastes treated and recycled are very close to the values reported earlier by Lee-Jeffs and Constable in 2008 (A. Lee-Jeffs, private communication D.J.C. Constable, private communication). This shows an increasing trend in the amount of solvents recycled in order to reduce... 

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