- Email: [email protected]
A mechanical swirling device for vacuum evaporation
Vacuum, VoL I V No. 1
LETTERS
TO
January, 1954
THE
EDITOR !
A Mechanical Swirling Device for Vacuum Evaporation Sommaire D E S C R I P T I O N EST DONNI~E
d'un m&anigme qui communique
un mouvement de rotation fi un ou plusieurs flacons, simultan~ment. Ce syst~me remplace le m&anisme /t main employd auparavant dans la distillation sous vide de solvants volatiles, emp&he les chocs et permet de distiller des produits/t taux d'dvaporation ~levr. IT IS OFTEN NECESSARYin chemical laboratories to free a solution from its solvent by evaporation and this operation is facilitated by the use of reduced pressure. However, unless some precautions are taken, frothing and bumping will occur with resultant loss of material. It is usual for the operator to swirl the evacuated flask of solution by hand in or over a steam-bath, so that local over-heating cannot take place and frothing is partly suppressed by centrifugal action. I f the solution holds solvent tenaciously--as in the case of many oils--it may be necessary to swirl the flask manually all day to remove the last traces of solvent, even when very low pressures are employed. Clearly this procedure is tedious and time consuming. On a small scale, mechanical stirring is not very satisfactory as it involves loss of solute to the stirring mechanisms and this may be a disadvantage where a dry residue is to be weighed in the flask after evaporation. Further, many of the vacuum sealing materials used for shaft seals are attacked by the common laboratory solvents. The commercially available flask shakers, whilst producing very good turbulence in the contents of the flask, are not very suitable for solvent evaporation since they are likely to induce frothing under vacuum conditions. Hand swirling, on the other hand, produces a vortex in the flask contents which has a large surface area and by its centrifugal action tends to suppress foaming. This is the normal laboratory method and occupies one operator per flask during the period of evaporation. We have arranged a simple mechanism which moves the flask in a manner similar to that produced
58
Fig. I. A detailed drawing of the drive unit: Legend: A.--Off-centre, off-axis spigot, producing a conical
orbit. A cylindrical orbit is produced if this portion of the shaft is turned parallel with the upper section. B.--Metal bellows which keeps the mechanism clean and prevents rotation of the drive boss. C.--Drive boss. D.--To geared motor drive. by wrist action and so dispenses with any internal rotating parts. By driving from above the level of the flask, we leave the bottom of the flask itself clear, so that it may be held in or over the source of heat whilst evaporation is taking place. T h e action is identical with that produced by hand swirling, but being mechanical it is more regular and can be adapted to greater rates of evaporation by increasing the speed of swirling. Moreover, one drive may be used to swirl a number of flasks at once, so that one operator can attend to four or six flasks at a time. T h e mechanism of the drive unit is as follows : A shaft is rotated in ball-races by means of a variablespeed geared motor. One end of this shaft projects from the ball-race housing and has an eccentric spigot, turned parallel to the main shaft but displaced from it by a small distance. On this spigot is mounted a second ball-race housing of a similar diameter to that in which the first shaft runs and the two housings are connected with a length of metallic bellows tubing, (see Fig. 1). T h e bellows serves the double purpose of preventing the revolution of one housing with respect to the other and at the same time excluding corrosive vapours from the bearings. When the drive shaft is rotated, the lower ball-race
January, 1954
Vacuum, Vol. I V No. I
Fig. 2. A view of the complete assembly of the swirling device. The flask in the upper left corner is parked in the upper clamp after completion of the solvent removal.
housing describes small circles but remains in a constant orientation. Consequently any apparatus connected to the lower housing will also describe small circles and simulate wrist action or swirling. Such a drive unit is in use in these laboratories for routine evaporation of light solvent from up to six flasks and is shown in Fig. 2. This drive unit has ½-inch shafts at each end, and is driven by a variable speed, ~ h.p., geared motor with a maximum speed of 250 r.p.m. A length of ½inch diameter, alloy rod is fixed horizontally to the drive end of the unit and carries standard laboratory clamps. A trough shaped steam bath is arranged underneath and in front of this is situated a vacuum manifold with cocks at intervals. Above the drive unit is another row o f clamps into which the flasks may be placed one by one as they reach dryness, so that overheating of the residues is avoided. As the amplitude is small, it is easy to remove flasks from the clamps whilst the system is in motion. For 250-ml, flasks an amplitude of about ~-inch to a~-inch is quite sufficient and control is obtained by varying the speed of swirling.
Legend:
A-Geared Motor B-Drive Unit C-Support Framework D-Driving Rod E-Overflow F-Steam Inlet G-Vacuum Manifold.
Once made, the unit itself can also be used for agitating titration flasks, for hydrogenations and for similar laboratory duties. For small flasks up to 250 ml. the motion may be transmitted directly through a standard cone connection fitting into the neck of the flask and, where this method is applied, any number of flasks can be swirled and pumped from common pumping and driving systems. A modification of this device can be used to provide a flexible wide-bore link between a swirling distillation flask and a static glass fractionating and condensing assembly. This takes the place of the conventional air or nitrogen leak in vacuum distillations, where recovery of both distillate and residue is important. This work is the subject of a patent application. P. R. WATT, Walton Oaks Experimental Station, Vitamins Ltd., Tadworth, Surrey, England. 1st January, 1956.
59
LETTERS
TO
January, 1954
THE
EDITOR !
A Mechanical Swirling Device for Vacuum Evaporation Sommaire D E S C R I P T I O N EST DONNI~E
d'un m&anigme qui communique
un mouvement de rotation fi un ou plusieurs flacons, simultan~ment. Ce syst~me remplace le m&anisme /t main employd auparavant dans la distillation sous vide de solvants volatiles, emp&he les chocs et permet de distiller des produits/t taux d'dvaporation ~levr. IT IS OFTEN NECESSARYin chemical laboratories to free a solution from its solvent by evaporation and this operation is facilitated by the use of reduced pressure. However, unless some precautions are taken, frothing and bumping will occur with resultant loss of material. It is usual for the operator to swirl the evacuated flask of solution by hand in or over a steam-bath, so that local over-heating cannot take place and frothing is partly suppressed by centrifugal action. I f the solution holds solvent tenaciously--as in the case of many oils--it may be necessary to swirl the flask manually all day to remove the last traces of solvent, even when very low pressures are employed. Clearly this procedure is tedious and time consuming. On a small scale, mechanical stirring is not very satisfactory as it involves loss of solute to the stirring mechanisms and this may be a disadvantage where a dry residue is to be weighed in the flask after evaporation. Further, many of the vacuum sealing materials used for shaft seals are attacked by the common laboratory solvents. The commercially available flask shakers, whilst producing very good turbulence in the contents of the flask, are not very suitable for solvent evaporation since they are likely to induce frothing under vacuum conditions. Hand swirling, on the other hand, produces a vortex in the flask contents which has a large surface area and by its centrifugal action tends to suppress foaming. This is the normal laboratory method and occupies one operator per flask during the period of evaporation. We have arranged a simple mechanism which moves the flask in a manner similar to that produced
58
Fig. I. A detailed drawing of the drive unit: Legend: A.--Off-centre, off-axis spigot, producing a conical
orbit. A cylindrical orbit is produced if this portion of the shaft is turned parallel with the upper section. B.--Metal bellows which keeps the mechanism clean and prevents rotation of the drive boss. C.--Drive boss. D.--To geared motor drive. by wrist action and so dispenses with any internal rotating parts. By driving from above the level of the flask, we leave the bottom of the flask itself clear, so that it may be held in or over the source of heat whilst evaporation is taking place. T h e action is identical with that produced by hand swirling, but being mechanical it is more regular and can be adapted to greater rates of evaporation by increasing the speed of swirling. Moreover, one drive may be used to swirl a number of flasks at once, so that one operator can attend to four or six flasks at a time. T h e mechanism of the drive unit is as follows : A shaft is rotated in ball-races by means of a variablespeed geared motor. One end of this shaft projects from the ball-race housing and has an eccentric spigot, turned parallel to the main shaft but displaced from it by a small distance. On this spigot is mounted a second ball-race housing of a similar diameter to that in which the first shaft runs and the two housings are connected with a length of metallic bellows tubing, (see Fig. 1). T h e bellows serves the double purpose of preventing the revolution of one housing with respect to the other and at the same time excluding corrosive vapours from the bearings. When the drive shaft is rotated, the lower ball-race
January, 1954
Vacuum, Vol. I V No. I
Fig. 2. A view of the complete assembly of the swirling device. The flask in the upper left corner is parked in the upper clamp after completion of the solvent removal.
housing describes small circles but remains in a constant orientation. Consequently any apparatus connected to the lower housing will also describe small circles and simulate wrist action or swirling. Such a drive unit is in use in these laboratories for routine evaporation of light solvent from up to six flasks and is shown in Fig. 2. This drive unit has ½-inch shafts at each end, and is driven by a variable speed, ~ h.p., geared motor with a maximum speed of 250 r.p.m. A length of ½inch diameter, alloy rod is fixed horizontally to the drive end of the unit and carries standard laboratory clamps. A trough shaped steam bath is arranged underneath and in front of this is situated a vacuum manifold with cocks at intervals. Above the drive unit is another row o f clamps into which the flasks may be placed one by one as they reach dryness, so that overheating of the residues is avoided. As the amplitude is small, it is easy to remove flasks from the clamps whilst the system is in motion. For 250-ml, flasks an amplitude of about ~-inch to a~-inch is quite sufficient and control is obtained by varying the speed of swirling.
Legend:
A-Geared Motor B-Drive Unit C-Support Framework D-Driving Rod E-Overflow F-Steam Inlet G-Vacuum Manifold.
Once made, the unit itself can also be used for agitating titration flasks, for hydrogenations and for similar laboratory duties. For small flasks up to 250 ml. the motion may be transmitted directly through a standard cone connection fitting into the neck of the flask and, where this method is applied, any number of flasks can be swirled and pumped from common pumping and driving systems. A modification of this device can be used to provide a flexible wide-bore link between a swirling distillation flask and a static glass fractionating and condensing assembly. This takes the place of the conventional air or nitrogen leak in vacuum distillations, where recovery of both distillate and residue is important. This work is the subject of a patent application. P. R. WATT, Walton Oaks Experimental Station, Vitamins Ltd., Tadworth, Surrey, England. 1st January, 1956.
59