What is vacuum filtration?

Vacuum filtration (or suction filtration) is an indispensable technique for distillation, extraction, and purification. Compared with traditional, gravity-assisted filtration, adding a vacuum to your system greatly increases speed and efficiency in your laboratory.

Vacuum filtration apparatus, adapted for laboratory work, is often used to isolate the product of synthesis of a reaction when the product is a solid in suspension. The product of synthesis is then recovered faster, and the solid is drier than in the case of a simple filtration. Other than isolating a solid, filtration is also a stage of purification: the soluble impurities in the solvent are eliminated in the filtrate (liquid).

This apparatus is often used to purify a liquid. When a synthesised product is filtered, the insolubles (catalysers, impurities, sub-products of the reaction, salts, …) remain in the filter. In this case, vacuum filtration is also more efficient that a simple filtration: there is more liquid recovered, and the yield is therefore better.

The working principle of vacuum filtration is to form negative pressure at the outlet of filtrate and use it as the driving force of filtration. The vacuum filtration equipment with intermittent operation is mainly introduced below. This equipment can filter suspensions of various concentrations.

Intermittent operation of vacuum filtration equipment has been developed because of its ability to achieve automated operation. The working principle of this equipment is as follows:

The containers are separated into upper and lower chambers by filter media, which constitutes a simple suction and filtration equipment. The suspension is added to the upper chamber. Under the action of pressure, the filter medium enters the lower chamber and becomes the filtered liquid. The solid particles are intercepted on the surface of the filter medium to form the filter residue.

During the filtration process, the vacuum filtration residue layer accumulated on the surface of the filter medium gradually thickens, and the resistance of liquid passing through the filter residue layer increases, and the filtration speed decreases. When the filter chamber is full of filter residue or the filtering speed is too small, stop filtering, remove the filter residue, and regenerate the filter medium to complete a filtering cycle.

After prefreezing the product, conditions must be established in which ice can be removed from the frozen product via sublimation, resulting in a dry, structurally intact product. This requires very careful control of the two parameters, temperature and pressure, involved in the freeze drying system. The rate of sublimation of ice from a frozen product depends upon the difference in vapor pressure of the product compared to the vapor pressure of the ice collector. Molecules migrate from the higher pressure sample to a lower pressure area. Since vapor pressure is related to temperature, it is necessary that the product temperature is warmer than the cold trap (ice collector) temperature. It is extremely important that the temperature at which a product is freeze dried is balanced between the temperature that maintains the frozen integrity of the product and the temperature that maximizes the vapor pressure of the product. This balance is key to optimum drying. The typical phase diagram shown in Figure 1 illustrates this point. Most products are frozen well below their eutectic or glass transition point (Point A), and then the temperature is raised to just below this critical temperature (Point B) and they are subjected to a reduced pressure. At this point the freeze drying process is started.