Pump/refill cycles

Charge a two-necked, round-bottomed flask (50 mL) with copper(l) cyanide (0.1 g, 1.1 mmol), lithium bromide (0.11 g, 1.3 mmol), and a stirring bar. Fit the flask with a gas adapter (with stopcock) and a septum. Connect the flask to a vacuum/inert atmosphere manifold, and subject it to three pump/refill cycles. Add dry, distilled THF (15 mL) to the flask by use of a dry, glass syringe. Stir this mixture, and then cool it in an ice-bath. 

Rapid entry or removal of materials and supplies from the box are facilitated by the use of a large (845i/min) vacuim pump distinct from the smaller pimp discussed previously. The rapid punping speed allows three ante-chamber pump-refill cycles to be completed in just over three minutes total time. 

Syringe pumps driven by screw mechanisms were popular in the 1960s because of their inherent precision and pulseless flow characteristics. Their disadvantages are higher manufacturing costs and the problems associated with syringe refill cycles. Syringe pumps are currently used in specialized systems for microbore and capillary HPLC. 

Pump and Purge. A key objective of cannula techniques is to achieve very low partial pressures of oxygen without the use of time consuming high vacuum techniques, and this is achieved by repeated pump and purge cycles. Even a heavily used rotary pump will give an ultimate vacuum of 1 x 10 2 mm Hg, and will rapidly reduce the pressure in a small reaction vessel to 1 m Hg. In a typical example a 224 mL reaction vessel contains approximately 2 mmol of O2, and evacuation to 1 mm Hg reduces the quantity of 02 to 2 x 10 3 mmol. If the vessel is refilled and the cycle repeated the quantity of O2 reduces to 2 x 10 mmol, and it is clear that relatively poor vacuums are sufficient to provide very low effective partial pressures of O2 after 2 or 3 pump/purge cycles. 

After three freeze/pump/thaw cycles, the reaction vessel is fitted with hoses supplying liquid from a constant temperature bath and allowed to equilibrate to the desired temperature. Concurrently, coolant is supplied to the reflux condenser. Upon temperature equilibration, the reaction vessel valve, A4, is opened and the vessel is refilled with hydrogen, through valve Al. Again, excess pressure is released through the pressure release bubbler. The reactant gas, valve, Al, and the pressure release bubbler valve, A6, are then... 

In single-headed reciprocating pumps, the eluent is delivered to the column for only half of the pumping cycle. A pulse dampener is used to soften the spike of pressure at the peak of the pumping cycle and to provide a eluent flow when the pump is refilling. Use of a dual head pump is better because heads are operated 180° out of phase with each other. One pump head pumps while the other is filling and vice versa. 

At the end of the regeneration step, the CO2 flow is stopped, the vessel is opened to allow the CO2 to depressurize, and the activated carbon is removed via slurry pumps. The vessel is then refilled with spent activated carbon, the water drained, and the desorption process repeated. Incidentally, the CO2 left in the vessel at the end of the regeneration cycle is lost in this procedure. Although the cost of CO2 is rather low, about 5-10 cents per pound depending upon the volume used, its consumption should be considered in evaluating the viability of a process, especially if the cost of the material being extracted is low. The CO2 that is lost represents an economic penalty for this process, but part or all of it can be recovered with the installation of an additional CO2 surge tank system. 

All experiments are to be carried out in a well-ventilated fume hood. The imperative exclusion of air and moisture is ascertained by the appropriate Schlenk and glovebox techniques. Solvents are dried by distillation from sodium/benzophenone (dme) or potassium (hexane) and degassed by three pump—freeze—thaw cycles prior to use. All glass equipment is stored at 120°C overnight, assembled and subjected to at least two cycles of evacuation and refilling with argon while still hot. 

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