In the laboratory, we chemists don’t think much about how many different vessels the phases that have held our product have occupied at one point or another during a reaction step. For example, we might perform a reaction in one flask, reverse quench into another, extract and cut a phase into a third, etc. We might do counter-current extraction requiring numerous separators funnels and numerous beakers. All the dirty glassware just gets rinsed, washed, and dried.
But in the plant and pilot-plant settings, once used, each vessel and all the pumps, filters, and transfer lines need to be cleaned, disassembled, and analyzed for trace impurities before they can be stored for their next application. Transfers back and forth between vessels need to be pumped. If vessels are not available, phases must be drummed off and then returned for further processing. Such complications increase processing costs.
Therefore, at scale in the pilot plant or plant, there can be a choice to be made between using extra time, extra labor, and extra vessels or instead changing the density of one of the liquid phases in the reactor during the work-up so that the phase that you wish to cut away to waste (the phase without product) becomes the lower phase.
First Situation #1
The reaction workup is from an organic solvent that is less dense than and immiscible with water. By appropriate usually pH adjustments, by-products, coproducts, and processing chemicals are extracted into a lower, mostly aqueous phase which is drawn off through the bottom reactor exit port, leaving the desired product in the upper less-dense organic phase.
This is the more frequent and easiest situation. Aqueous waste can be drummed off without complications. No second vessel is needed.
Second Situation #2
The workup is from an organic solvent less-dense than water as in situation #1 but in this case, the desired product can be phase shifted into the aqueous lower phase. Now rather than drawing off this lower product-rich phase into another vessel, an organic solvent denser than water is mixed into the upper phase until its overall graded density becomes greater and the phases invert bringing the aqueous product-bearing phase to the top layer. Cutting away the mixed organic layer removes coproduce, byproduct, or processing chemicals leaving the desired product in the water layer still in the first reactor. Now treating his aqueous phase properly (such as neutralizing a salt derivative) makes the product switch phases back to whatever water-immiscible new less-dense than water organic solvent that is added. Now the lower aqueous phase, empty of product, can be withdrawn from the bottom valve of the reactor leaving the desired product dissolved in the new organic solvent.
Denser-than-water co-solvents that can be used to invert the overall density of the organic reaction solvent relative to water could be dichloromethane, chloroform, chlorobenzene, 1,2-dichlorobenzene, trichloroethylene or tetrachloroethylene.
By introducing the added expense of a denser-than-water organic cosolvent you have avoided whatever extra expense is attributable to the time and additional costs related to unloading and temporarily storing the aqueous phase and then returning it to the processing vessel.
Notice that there is no such choice required in the laboratory. Transfers are rapid and extra glassware is freely available.
Third Situation #3
The workup is from an organic solvent more dense than water such as dichloromethane, chloroform, chlorobenzene, 1,2-dichlorobenzene, trichloroethylene, tetrachloroethylene, ⍺,⍺,⍺-trifluorotoluene, 3-chloro-⍺,⍺,⍺-trifluorotoluene or liquid sulfur dioxide. Since the product will remain in the organic layer a less-dense cosolvent must be mixed into the more-dense reaction solvent to cause the relative densities to flip and the organic layer rise to the top of the reactor. Then the aqueous wash can be drawn off leaving the product in the mixed organics in the reactor.
Fourth Situation #4
The workup is from an organic solvent denser than water such as dichloroethane, chloroform, chlorobenzene, 1,2-dichlorobenzene, trichloroethylene, or tetrachloroethylene wherein the desired product can be phase-switched into an upper aqueous layer. When this is done the more-dense reaction solvent can be drawn off and a new second organic solvent immiscible with water added to the reactor . The desired product is switched back to this organic phase and the water wash is removed.
Situations #1 and #4 are the most preferred since no extra added solvent type is required to change the organic phase density. So, if the desired reaction product is extractable into water, the reaction solvent should be preferably denser than water suggesting chlorinated solvents. When the desired product is essentially neutral and cannot be made to switch into an aqueous phase, a reaction solvent lighter than water is preferred.
Where an extra organic solvent is needed to avoid using an extra vessel, the comparative costs need to be considered.
Removing Dense Chlorinated Solvents
Getting Product out of:
Chlorobenzene. Distil with 2-methyl -1-propanol (density 0.802 g/ml) to remove chlorobenzene as its azeotrope bp.108.4 C. Then you can get it into toluene using another azeotrope bp. 101.2 C. Other switches from Isobutanol are possible.
1,2-Dichlorobenzene can be removed with an azeotrope with glycol (d=1.113 g/ml.) bp. 165.8 C. A product in ethylene glycol can then be extracted into any one of a number of immiscible organic solvents.
Trichloroethylene forms lower boiling azeotropes with methanol, ethanol, acetic acid, and water.
Tetrachloroethylene forms lower boiling azeotropes with butanol, ethanol, acetic acid, and isopropanol.
No comments:
Post a Comment