Using controlled vacuum as a tool for use in chemical laboratories

Various methods for application related control of vacuum are compared with regard to the value for the user. Important criteria are process time, solvent recovery rate and ease of use and handling. Modern speed controlled chemistry diaphragm pumping units with fully automatic controller offer many advantages. Dr Juergen Dirscherl reports.

Many processes profit from operation under a controlled vacuum. The best example is the rotary evaporator. By application of a suitable low pressure ­ as close as possible to the boiling point of the solvent to be evaporated ­ distillations can be run at low temperatures. If the applied pressure is too high the evaporation takes unnecessarily long. If it is too low the solvents tend to foam over spoiling the experiment. The technique of controlling the process pressure is decisive for performance and economic as well as ecological aspects.

In a modern lab nobody would regulate the heating power of a water bath or a drying oven manually. Despite the fact that a skilled user could do it as well as an electronic temperature controller this would be waste of money and time.

In recent years many water-jet pumps and oil-sealed rotary vane pumps have been replaced by modern chemistry diaphragm pumps. But with regard to regulation of the vacuum level there are still many manual systems in use. Early systems employed air admittance valves to "use up“ some of the pumping speed with external air. This is disastrous as regards solvent recovery and operating noise. A better solution are tube valves in the suction line to regulate the pumping action. More modern systems rely on PTFE valves mounted directly at the pumping unit (Fig. 1). But one should keep in mind that such a valve just regulates the pumping speed by throttling ­ not the pressure level!

With some patience a skilled user can achieve good results with such manual systems. But most users don't have the time to regulate the optimum vacuum level regularly ­ often over hours depending on the application. The evaporation itself causes the solvent temperature and composition to change continuously. As a consequence the boiling point drifts and the user has to adjust the valve in order to keep the process running. In a modern lab with its high cost and time constraints this does not make sense any more ­ researchers should have better things to do!

For many years there have been automatic vacuum controllers on the market which keep the pressure level constant. A common method is to regulate the pressure using an electro-magnetic 2-point valve (open-close). High quality systems of this kind offer true pressure regulation and allow many processes to be operated within reasonable time scales and with minimum effort from the user (Fig. 2).

The pump runs at full speed and the controller measures the pressure in the chamber. As soon as the pressure drops below a pre-set level the valve between pump and chamber is closed. The valve is opened as soon as the pressure rises above the pre-set level plus a hysteresis (necessary to prevent the valve from flickering).

Though offering good performance the pressure varies between the upper and lower set point. As the system often works far above the optimum pressure (boiling point) the process time is relatively long. During phases with an open valve full vacuum is applied to the liquid surface and fluid drops could be sucked into the pump due to the high pumping speed operating at this time. During phases with a closed valve the evaporation more or less stops completely. Simple low-cost controllers tend to under- or overshoot making process control very difficult. Non vacuum-specialists ­ particularly users but also manufacturers ­ often cannot cope with such problems. The user needs to know sensible values for the pressure set point and hysteresis. But these values are different for every solvent and each bath temperature.

As soon as the evaporation starts the solvent temperature drops due to aevaporative cooling'. The solvent temperature is then mainly governed by the heating power of the bath. If the heating power is insufficient, the bath temperature decreases. Some manufacturers try to overcome this problem by offering the possibility to program pressure ramps into the controller. But this increases the number of free parameters further. How shall the user know the optimum parameters? Such systems are not able to detect and follow boiling points automatically.

The solution to this common problem is for continuous vacuum control using a variable speed motor to drive the chemistry diaphragm pump. Though it sounds straightforward the technical realisation to accomplish a usable and economical speed control from >2000rpm down to a few rpm is rather demanding. And only the combination with a fully automatic controller gives the Vacuubrand VARIO-system (Fig. 3) its full performance: Under- and over-shooting of the pressure are diminished and the pressure is controlled with high precision without hysteresis.

Fully automatic vacuum controllers with fuzzy logic find the boiling pressure(s) of the solvent (mixtures) by analysing the pressure characteristic during pump down. The boiling point is approached carefully to prevent foaming. The pressure is regulated and follows the boiling pressure in case of drifting ­ fully automatically, without any need for adjusting parameters or pressure levels!

The controller adjusts the pressure exactly to the boiling point in order to achieve high evaporation rates and shortest possible process time without foaming. There is no better way available than this combination of proven mechanics and modern electronics and software.

Fig. 4 schematically shows pressure-over-time characteristics of an evaporation with 2-point control and automatic speed control. Speed control offers unmatched pressure control precision without any hysteresis.

Faster processing speeds up through-put, reducing laboratory operating costs. If costs for the working day are taken into account in a cost of ownership calculation then VARIO-pumping units are the clear winners as compared with conventional pumps. This holds even more for modern parallel evaporation units with their high operating cost per hour.

With VARIO systems distillations are performed with optimum efficiency resulting not only in fast evaporation times but also high solvent recovery rates of close to 100percent.

Additional benefits are the low noise and vibration level as the pump runs most of the time at very low speeds.

Enquiry No 66

Dt Juergen Dirscherl is with Vacuubrand GmbH & Co KG, wertheim, Germany. www.vacuubrand.de

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